ENVIRONMENTAL REVIEW OF PETROLEUM INDUSTRY EFFLUENTS ANALYSIS

ENVIRONMENTAL REVIEW OF PETROLEUM INDUSTRY EFFLUENTS ANALYSIS
ENVIRONMENTAL REVIEW
OF PETROLEUM INDUSTRY
EFFLUENTS ANALYSIS
Claire Faustine
Master of Science Thesis
Stockholm 2008
Claire Faustine
ENVIRONMENTAL REVIEW OF
PETROLEUM INDUSTRY
EFFLUENTS ANALYSIS
SUPERVISORS:
LENNART NILSON, INDUSTRIAL ECOLOGY
ALAIN MORVAN, AXENS IFP
EXAMINER:
LENNART NILSON, INDUSTRIAL ECOLOGY
Master of Science Thesis
STOCKHOLM 2008
PRESENTED AT
INDUSTRIAL ECOLOGY
ROYAL INSTITUTE OF TECHNOLOGY
TRITA-IM 2008:39
ISSN 1402-7615
Industrial Ecology,
Royal Institute of Technology
www.ima.kth.se
Abstract
The present report deals with environmental issues in refineries and petrochemical processes.
More precisely gaseous, liquid and solid effluents from processes are analysed qualitatively
and quantitatively when possible. Techniques to treat these effluents are reviewed or proposed
when lacking and methods to do not produce these effluents are envisaged.
In the part A of the report general effluents that are released from all types of processes are
studied. These effluents include fugitive emissions, flue gases from process heaters,
blowdown systems emissions and wastewaters. Fugitive emissions, one of the greatest
sources of VOCs can be qualified and quantified by the average emission factor approach and
reduced thanks to the implementation of an LDAR program. Flue gases from process heaters,
which are a major source of NOx, SOx and particulate matters can be characterized with
emission factors and several techniques exist to treat or prevent these emissions. Concerning
blowdown systems emissions, which are difficult to quantify, methods to minimize these
emissions are given. Finally, wastewaters treatment in petroleum industry is shortly described
before best management practices and pollution prevention methods are enounced.
In the part B of the report four families of processes are studied: naphtha hydrotreatment,
naphtha isomerization, catalytic reforming and hydrogenation in olefin plants. Each of these
processes is firstly described, the process flow diagram is explained and continuous and
intermittent effluents are characterized. In addition to general effluents dealt with in part A, it
has been found that processes can produce other effluents such as dioxins in isomerization or
catalytic reforming units or green oils during catalyst regeneration operations.
List of Tables
Table A-2-1: Comparison of available techniques for NOx control for process heaters
Table A-2-2: Comparison of SOx-removal techniques
Table B-1-1: Typical properties of crude oil distillation naphtha
Table B-1-2: Typical properties of naphtha hydrotreating products
Table B-2-1: Typical properties of isomerisation naphtha feed
Table B-3-1: Typical properties of two charges for catalytic reforming unit
Table B-4-1: Typical composition of a raw C3 cut entering a selective hydrogenation unit
Table B-4-2: Compounds possibly present in catalyst regeneration effluents
Table B-4-3: Gaseous effluents during catalyst regeneration in C3 selective hydrogenation
Table B-4-4: Gaseous effluents during catalyst oxidation in C4 selective hydrogenation
Table B-4-5: Gaseous effluents during catalyst regeneration in GHU first reactor
Table B-4-6: Gaseous effluents during catalyst regeneration in GHU first reactor
Table B-4-7: Gaseous effluent during catalyst sulfurization in GHU
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List of Figures
Figure A-2-1: Settling chamber
Figure A-2-2: Baffle chamber
Figure A-2-3: Cyclone
Figure A-2-4: Baghouse
Figure A-2-5: Wet scrubber
Figure A-4-1: Sour waters stripping system
Figure B-1-1: Naphtha hydrotreating process flow diagram
Figure B-1-2: Influents/effluents scheme for naphtha hydrotreating unit in normal operations
Figure B-1-3: Influents/effluents scheme for naphtha hydrotreating unit during catalyst
sulfiding
Figure B-1-4: Influents/effluents scheme for naphtha hydrotreating unit during catalysts
regeneration
Figure B-2-1: Simplified process flow diagram for isomerization with chlorinated Pt/Al2O3
catalyst
Figure B-2-2: IPSORB® isomerization process
Figure B-2-3: HEXORB® isomerization process
Figure B-2-4: Influents/effluents scheme for naphtha isomerisation unit in normal operations
Figure B-2-5: Influents/effluents scheme for naphtha isomerisation unit during dryers
regeneration
Figure B-2-6: 2,3,7,8-Tetrachlordibenzodioxin
Figure B-3-1: Simplified scheme of semi-regenerative process for catalytic reforming
Figure B-3-2: Continuous catalyst regeneration reforming
Figure B-3-4: Influents/effluents scheme for catalytic reforming reaction section
Figure B-3-5: Simplified process flow diagram for CCR regeneration section
Figure B-3-6: Influents/effluents scheme for catalytic reforming in regeneration section
Figure B-4-1: Flow sheet of the C3 selective hydrogenation process
Figure B-4-2: Flow sheet of the C4 selective hydrogenation process
Figure B-4-3: Flow sheet of the gasoline hydrogenation process
Figure B-4-5: Influents/effluents scheme for C3 selective hydrogenation during normal
operations
Figure B-4-6: Influents/effluents scheme for C4 selective hydrogenation during normal
operations
Figure B-4-7: Influents/effluents scheme for gasoline hydrogenation during normal operations
Figure B-4-8: Influents/effluents scheme for C3 selective hydrogenation during catalyst
reduction or reactivation
Figure B-4-9 Influents/effluents scheme for C4 selective hydrogenation during catalyst
reduction, reactivation or stripping
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List of Acronyms
APIBOOSCCRCOCO2DMDSEPAETBEFCCFGFGRFOGHUHAPHCLDARLEALNBLPGMAMTBENOxNO2OFAPDPCDDPCDFPMRPGSCASCRSNCRSOxSO2SRTAMETOCVHAPVOCWI/SI-
American Petroleum Institute
Burner Out Of Service
Continuous Catalytic Reforming
Carbon Monoxide
Carbon Dioxide
Dimethyl Disulfur
United States Environmental Protection Agency
Ethyl Tertiary-Butyl Ether
Fluid Catalytic Cracking
Fuel Gas
Flue Gas Recirculation
Fuel Oil
Gasoline Hydrogenation Unit
Hazardous Air Pollutant
Hydrocarbon
Leak Detection And Repair
Low Excess Air
Low NOx Burner
Liquefied Petroleum Gas
Methyl Acetylene
Methyl Tertiary-Butyl Ether
Nitrogen Oxide
Nitrogen dioxide
Over Fire Air
Propadiene
Polychlorodibenzo-p-dioxin
Polychlorodibenzo-p-furan
Particulate Matter
Raw Pyrolysis Gasoline
Staged Combustion Air
Selective Catalytic Ceduction
Selective Non Catalytic Reduction
Sulfur Oxide
Sulfur dioxide
Semi Regenerative
Tert-Amyl-Methyl-Ether
Total Organic Compound
Volatile Hazardous Aromatic Product
Volatile Organic Compounds
Water Injection / Steam Injection
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Table of contents
INTRODUCTION ................................................................................................................................. 1
AIM AND OBJECTIVES ..................................................................................................................... 1
METHODOLOGY ................................................................................................................................ 1
PART A: MAJOR EMISSION SOURCES OF REFINERIES AND PETROCHEMICAL
INDUSTRY ............................................................................................................................................ 1
1. FUGITIVE EMISSIONS ......................................................................................................................... 1
1.1. AVERAGE EMISSION FACTOR APPROACH ........................................................................................ 1
1.2. IMPLEMENTATION OF A LEAK DETECTION AND REPAIR (LDAR) PROGRAM ................................... 2
1.2.1. Identifying components ............................................................................................................... 2
1.2.2. Leak definition ............................................................................................................................. 2
1.2.3. Monitoring components .............................................................................................................. 2
1.2.4. Repairing components ................................................................................................................ 3
1.2.5. Record keeping ............................................................................................................................ 3
2. FLUE GASES FROM PROCESS HEATERS AND BOILERS ...................................................................... 3
2.1. GENERAL .......................................................................................................................................... 3
2.2. CONTROL TECHNIQUES FOR NOX EMISSIONS REDUCTION ............................................................... 4
2.2.1. Low-NOx burners (LNB) ............................................................................................................ 5
2.2.2. Staged combustion air (SCA) ..................................................................................................... 5
2.2.3. Flue gas recirculation (FGR) ...................................................................................................... 5
2.2.4. Water or steam injection (WI/SI) .............................................................................................. 5
2.2.5. Selective non catalytic reduction (SNCR) ................................................................................. 6
2.2.6. Selective catalytic reduction (SCR) ............................................................................................ 6
2.3. CONTROL TECHNIQUES FOR SOX EMISSIONS REDUCTION ................................................................ 8
2.3.1. Lime and limestone process ........................................................................................................ 8
2.3.2. Dual-alkali scrubbing .................................................................................................................. 9
2.3.3. Activated char process ................................................................................................................ 9
2.3.4. Wellman-Lord process .............................................................................................................. 10
2.4. CONTROL TECHNIQUES FOR PARTICULATE MATTERS EMISSIONS................................................... 10
2.4.1. Inertial collectors ....................................................................................................................... 10
2.4.2. Electrostatic precipitators ........................................................................................................ 11
2.4.3. Fabric filtration ......................................................................................................................... 11
2.4.4. Scrubbing systems ..................................................................................................................... 12
2.4.5. Selection of the control technique for PM emissions .............................................................. 12
2.5. CARBON DIOXIDE............................................................................................................................ 13
3. BLOWDOWN SYSTEMS ...................................................................................................................... 13
3.1. EMISSIONS TO THE FLARE ............................................................................................................... 13
3.2. LIQUID EMISSIONS .......................................................................................................................... 14
4. WASTEWATER .................................................................................................................................. 14
4.1. WASTEWATER TREATMENT TECHNIQUES ....................................................................................... 14
4.1.1. Sour waters stripping ................................................................................................................ 14
4.1.2. Oil water separation .................................................................................................................. 15
4.1.3. Physical and chemical purification .......................................................................................... 15
4.1.4. Biological treatment .................................................................................................................. 15
4.2. BEST MANAGEMENT PRACTICES ..................................................................................................... 15
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4.3. POLLUTION PREVENTION ................................................................................................................ 16
PART B: ANALYSIS OF PROCESSES .......................................................................................... 17
1. NAPHTHA HYDROTREATING UNIT ................................................................................................... 17
1.1. PURPOSE OF THE UNIT..................................................................................................................... 17
1.2. RAW MATERIALS AND RESOURCES INPUT CHARACTERISTICS ........................................................ 17
1.2.1. Naphtha feeds ............................................................................................................................ 17
1.2.2. Hydrogen make-up .................................................................................................................... 18
1.2.3. Catalyst ....................................................................................................................................... 18
1.3. PRODUCTS CHARACTERISTICS ........................................................................................................ 18
1.4. NORMAL OPERATIONS .................................................................................................................... 18
1.4.1. Reaction section ......................................................................................................................... 20
1.4.2. Separation section...................................................................................................................... 20
1.4.3. Influents / effluents scheme ...................................................................................................... 20
1.5. INTERMITTENT OPERATIONS ........................................................................................................... 21
1.5.1. Catalyst sulfiding ....................................................................................................................... 21
1.5.2. Catalyst regeneration ................................................................................................................ 21
1.6. EFFLUENTS CHARACTERIZATION .................................................................................................... 22
1.6.1. Normal operations ..................................................................................................................... 23
1.6.2. Intermittent operations ............................................................................................................. 24
1.6.3. Solid wastes ................................................................................................................................ 24
1.7. EMISSIONS REDUCTION PROPOSALS ............................................................................................... 25
2. NAPHTHA ISOMERISATION .............................................................................................................. 25
2.1. PURPOSE OF THE UNIT..................................................................................................................... 25
2.2. RAW MATERIALS AND RESOURCES INPUT CHARACTERISTICS ........................................................ 25
2.2.1. Naphtha feeds ............................................................................................................................ 25
2.2.2. Hydrogen .................................................................................................................................... 26
2.2.3. Catalyst ....................................................................................................................................... 26
2.2.4. Dryers molecular sieves ............................................................................................................ 26
2.3. PRODUCTS CHARACTERISTICS ........................................................................................................ 26
2.4. NORMAL OPERATIONS .................................................................................................................... 26
2.4.1. Reactions .................................................................................................................................... 26
2.4.2. Influents / effluents scheme ...................................................................................................... 28
2.5. INTERMITTENT OPERATIONS ........................................................................................................... 29
2.5.1. Dryers regeneration .................................................................................................................. 29
2.5.2. Influents / effluents scheme ...................................................................................................... 29
2.6. EFFLUENTS CHARACTERIZATION .................................................................................................... 30
2.6.1. Normal operations ..................................................................................................................... 30
2.6.2. Dryers regeneration .................................................................................................................. 31
2.6.3. Solid wastes ................................................................................................................................ 31
2.7. EMISSIONS REDUCTION PROPOSALS ............................................................................................... 32
2.7.1. Air emissions .............................................................................................................................. 32
2.7.2. Water emissions ......................................................................................................................... 32
2.7.3. Solid wastes ................................................................................................................................ 32
2.8. DIOXINS EMISSIONS ........................................................................................................................ 33
3. CATALYTIC REFORMING ................................................................................................................. 34
3.1. PURPOSE OF THE UNIT..................................................................................................................... 34
3.2. RAW MATERIALS AND RESOURCES INPUT CHARACTERISTICS ........................................................ 34
3.2.1. Naphtha feed .............................................................................................................................. 34
3.2.2. Catalyst ....................................................................................................................................... 35
3.3. PRODUCTS CHARACTERISTICS ........................................................................................................ 35
3.4. REACTION SECTION ........................................................................................................................ 35
3.4.1. Semi-regenerative fixed bed ..................................................................................................... 36
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3.4.2. Continuous Catalyst Regeneration reforming process .......................................................... 36
3.4.3. Influents / effluents scheme ...................................................................................................... 37
3.5. REGENERATION SECTION ................................................................................................................ 37
3.6. EFFLUENTS CHARACTERIZATION .................................................................................................... 39
3.6.1. Reaction section ......................................................................................................................... 39
3.6.2. Regeneration section ................................................................................................................. 40
3.6.3. Solid wastes ................................................................................................................................ 41
3.7. EMISSIONS REDUCTION PROPOSALS ............................................................................................... 41
3.7.1. Air emissions .............................................................................................................................. 41
3.7.2. Solid wastes ................................................................................................................................ 41
3.7.3. Spent caustic .............................................................................................................................. 41
3.8. DIOXINS EMISSIONS ........................................................................................................................ 41
4. HYDROGENATION IN OLEFIN PLANTS ............................................................................................. 42
4.1. PURPOSE OF UNITS .......................................................................................................................... 42
4.2. RAW MATERIALS AND RESOURCES INPUT CHARACTERISTICS ........................................................ 43
4.2.1. Raw C3 cut .................................................................................................................................. 43
4.2.2. Raw C4 cut .................................................................................................................................. 43
4.2.4. Hydrogen make-up .................................................................................................................... 43
4.2.5. Catalyst ....................................................................................................................................... 43
4.3. PRODUCTS CHARACTERISTICS ........................................................................................................ 43
4.4. NORMAL OPERATIONS .................................................................................................................... 44
4.4.1. Selective hydrogenation of C3 ................................................................................................... 44
4.4.2. Selective hydrogenation of C4 ................................................................................................... 45
4.4.3. Gasoline hydrogenation ............................................................................................................ 46
4.4.4. Influents / effluents scheme ...................................................................................................... 47
4.5. INTERMITTENT OPERATIONS ........................................................................................................... 49
4.5.1. Catalyst reduction / reactivation / hot hydrogen stripping.................................................... 49
4.5.2. Catalyst regeneration ................................................................................................................ 50
4.5.3. Catalysts sulfurization in the second reactor of gasoline hydrogenation unit ..................... 53
4.6. EFFLUENTS CHARACTERIZATION .................................................................................................... 53
4.6.1. Normal operations ..................................................................................................................... 54
4.6.2. Intermittent operations ............................................................................................................. 54
4.7. EMISSIONS REDUCTION PROPOSALS ............................................................................................... 55
CONCLUSION .................................................................................................................................... 55
REFERENCES .................................................................................................................................... 56
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Introduction
Refineries and petrochemical processes are responsible for many emissions both into the air
and into the water. Most relevant emissions into the air are nitrous oxides (NOx), sulfur
oxides (SOx), carbon monoxide (CO), methane and volatile organic compounds (VOC).
Waste water from petroleum industry contains organic compounds, phenols, toxic metals and
other pollutants such as iron, dissolved and suspended solids, oil, cyanides, sulfides and
chlorine. In order to reduce these emissions, an accurate analysis of processes is necessary.
The analysis of some processes leads to two conclusions:
On the one hand, we can see that major part of emissions always come from the same sources:
- Fugitive emissions, responsible for VOC releases to the atmosphere.
- Process heaters and boilers, responsible for NOx, SOx and particulate matters releases to the
atmosphere.
- Blowdown systems
For each of these sources, theoretical methods to qualify and quantify pollutants emitted, and
treatment methods available and pollutant production reduction methods are analyzed.
On the other hand, we can see that particular pollutants are emitted from some processes, in
normal or intermittent functioning. For example, dioxins can be produced during catalyst
regeneration of reforming and isomerization units. Usually these kinds of emissions are not
taken into account for different reasons: the formation mechanism of these pollutants is not
well-known (dioxins), the emission occurs rarely (catalyst in-situ regeneration), etc.
Aim and objectives
The aim of this study is to carry out a general environmental assessment of refineries and
petrochemical processes. The first part of this report emphasizes on major emissions sources
and gathered general solutions available and applicable. The second part of this report
lightens particular processes. A methodology to analyze processes is proposed.
Methodology
This report is based on a bibliographic study for general considerations. Process books
produced by Axens are used for the analysis of particular processes. When it comes to
characterize emitted pollutants, only theoretical methods are given, it means that
measurements or monitoring techniques are not taken into account.
Part A: Major emission sources of refineries and
petrochemical industry
1. Fugitive emissions 1,2,3,4
Equipment leaks in refinery processes are responsible for significant amount of emissions.
Even if each individual leak is generally small, according to EPA, it is the largest source of
emissions of volatile organic compounds (VOCs) and volatile hazardous air pollutants
(VHAPs) from petroleum refineries and chemical manufacturing facilities. The US EPA
(United States Environmental Protection Agency) emitted in 1995 a protocol for equipment
leak emission estimates based on emission factors or correlation approaches. The emission
factors approach is the only method available that allows estimation without monitoring. This
method is described below. The implementation of an LDAR (Leak Detection And Repair)
programme will then be dealt with.
1.1. Average Emission Factor Approach
The Average Emission Factor Approach is a combination of average emission factors and
unit-specific data: number of each type of equipment (valves, pump seals, etc.), the service
each equipment is in (gas, light liquid, heavy liquid), the Total Organic Compound (TOC)
concentration of the stream and time period each equipment is in that service. The emission
rate of TOC from all equipment can be calculated with the following formula:
ETOC = FA × WFTOC × N
Where:
ETOC = emission rate of TOC from all equipments in the stream of a given equipment type
(kg/hr)
FA = applicable average emission factor for the equipment type (kg/hr/source)
WFTOC = average weight fraction of TOC in the stream
N = number of pieces of equipment of the applicable equipment type in the stream
Average emission factors are divided into four categories: SOCMI factors, oil and gas
production factors, refinery factors, and factors for petroleum marketing terminals (this last
category is not applicable here). Within each category, factors depend on equipment type and
material in service (light or heavy liquid or gas).Heavy liquid factor is used if the stream's
vapor pressure is less than or equal to 0.003 bars at 20°C. If the vapor pressure is greater than
0.003 bars at 20°C, light liquid factor must be used.
Appendix 1 gathers all the Average Emission Factors and Appendix 2 shows an example of
calculation.
Total TOC fugitive emission from a unit process can be known by summing emissions from
each type of components, from each stream.
Average factors generally determine total hydrocarbon emissions. In order to determine total
VOC emissions, the calculated emission rates must be multiplied by the stream’s weight
percentage of VOC compounds. (Indeed, it can happen that not all organic compounds
present in the stream be classified as VOCs, for instance methane or ethane.
1
If some of the organic compounds in the stream are not classified as VOCs total VOCs
emission can be calculated with the following formula:
EVOC = ETOC × (WPVOC / WPTOC)
Where:
EVOC = the VOC mass emissions from the equipment (kg/hr)
ETOC = the TOC mass emissions from the equipment (kg/hr)
WPVOC = the VOC concentration in the equipment in weight percent
WPTOC = the TOC concentration in the equipment in weight percent
If, for a stream, estimating emissions of a specific VOC in the mixture is necessary, the
following formula can be used:
EX = ETOC × (WPX / WPTOC)
Where:
EX = the organic chemical “X” mass emissions from the equipment (kg/hr)
ETOC = the TOC mass emissions from the equipment (kg/hr)
WPX = the organic chemical “X” concentration in the equipment in weight percent
WPTOC = the TOC concentration in the equipment in weight percent
Three other methods emitted from the protocol for equipment leak emission estimates are
available. However these methods necessitate on-site monitoring so they are not included into
the scope of this study.
1.2. Implementation of a Leak Detection and Repair (LDAR) program
Still according to EPA, the implementation of an LDAR program could lead to a reduction by
63% of emissions from equipment leaks. The following describes the procedure to implement
this program.
1.2.1. Identifying components
Each regulated component must be assigned a unique identification number, recorded and
located in the facility and on the Piping and Instrumentation Diagrams.
1.2.2. Leak definition
Leak definition means the threshold standard (in ppm). It depends on regulation, component
type, service and monitoring interval. Leak definition can also be based on visual inspections
and observations, sound and smell. A leak is detected whenever the measured concentration
(ppm) exceeds the leak definition.
1.2.3. Monitoring components
For many regulations with leak detection provisions, the method for monitoring to detect
leaking components is EPA Reference Method 21. This procedure uses a portable detecting
2
instrument. Monitoring intervals depend on component type and periodic leak rate but are
typically weekly, monthly, quarterly, and yearly.
1.2.4. Repairing components
Components have to be repair as soon as possible after the leak is detected. The following
practices can be applied:
- Tightening bonnet bolts.
- Replacing bonnet bolts.
- Tightening packing gland nuts.
- Injecting lubricant into lubricated packing.
If the repair of any component is technically infeasible without a process unit shutdown, the
component may be placed on the Delay of Repair list.
1.2.5. Record keeping
For each regulated process, a list of ID number for all equipment subject, detailed schematics,
equipment design specifications, piping and instrumentation diagrams and results of
performance testing and leak detection monitoring must be maintain.
For leaking equipment, records, instrument and operator ID numbers and the date the leak
was detected must be maintained. The dates of each repair attempt and an explanation of the
attempted repair method is noted. Dates of successful repairs and results of monitoring tests to
determine if the repair was successful are included.
2. Flue gases from process heaters and boilers 1,5,6,7,8,13
Fuel combustion in process heaters and boilers is an important pollutants and greenhouse
gases emission source. Carbon dioxide (CO2) is the principal gas released but nitrogen and
sulfur oxides (NOx and SOx), carbon monoxide (CO), organic compounds and particulate
matters (PM) are also released in non negligible quantities. In order to reduce the overall air
emissions of a refinery or a petrochemical plant, these emissions must be taken into account.
Several technologies exist to reduce these emissions. The present report synthesizes them.
2.1. General
In process heaters and boilers in refineries and petrochemical plants, two major types of fuel
are burned by combustion sources: fuel gas and fuel oil.
Refinery fuel gas is a collection of light gases generated in a number of processing units in the
refinery. It contains principally hydrogen and methane and variable amounts of light
hydrocarbons such as ethane, ethylene or propane. It can also contain hydrogen sulfide in
trace amounts.
Fuel oil is a fraction obtained from petroleum distillation. It can be divided in two categories:
distillate oils and residual oils, further distinguished by grade numbers with 1 and 2 being
distillate oils and 5 and 6 being residual oils:
- Grade 1: Light domestic fuel oil-distillate.
- Grade 2: Medium domestic fuel oil-distillate.
- Grade 3: Heavy domestic fuel oil-distillate.
- Grade 4: Light industrial fuel oil.
3
- Grade 5: Medium industrial fuel oil.
- Grade 6: Heavy industrial fuel oil.
There are four major types of boilers used in industrial applications: watertube, firetube, cast
iron and tubeless design. Boilers design and size, orientation of heat transfer surfaces and
burner configuration are factors that influence strongly emissions and the potential for
controlling emissions.
Emissions depend also on type and composition of the fuel. Because the combustion
characteristics are different, their combustion can produce significantly different emissions.
Among these emissions can be found:
- Particulate emissions, filterable or condensable, which depends on the completeness of
combustion and the initial fuel ash content.
- Nitrogen oxides emissions, due either to thermal fixation of atmospheric nitrogen in the
combustion air (thermal NOx), or to the conversion of chemically bound nitrogen in the fuel
(fuel NOx).
- Sulfur oxides emissions, that are generated during combustion from the oxidation of sulfur
contained in the fuel.
- Carbon monoxide and organic compounds emissions, which depends on the combustion
efficiency of the fuel.
- Trace metals emissions, which depend on the initial fuel metals content.
All these emissions can be estimated thanks to emission factors available in EPA literature.
Control techniques for the reduction of NOx, SOx and particulate matters are described and
compared below as these three types of emission are the most relevant.
2.2. Control techniques for NOx emissions reduction
NOx reduction in boilers and process heaters can be achieved with combustion modification
and flue gas treatment or a combination of these. The choice of the technique depends on the
type and size of the boiler or heater, the fuel characteristics and the flexibility for
modifications. Practically, NOx reductions consist in thermal NOx* reduction and fuel NOx**
reduction. When fuel with low nitrogen content is used, such as fuel gas or distillate oil,
thermal NOx is the only component that can be controlled.
* Thermal NOx is produced by combination at high flame temperature of nitrogen and
oxygen contained in the combustion air supply. It is produced during the combustion of both
fuel gases and fuel oils.
** Fuel NOx is produced by combination of nitrogen contained in the fuel with excess
oxygen contained in the combustion air. It is only a problem with fuel oils containing bound
nitrogen.
Combustion control involves consequently three main strategies:
- Reducing peak temperatures in the combustion zone.
- Reducing the gas residence time in the high-temperature zone.
- Reducing oxygen concentrations in the combustion zone.
4
These changes can be achieved with process modifications or operating conditions
modifications.
Finally, the flue gas treatment allows reducing NOx emissions.
Here below different technologies are generally and shortly described. The table synthesized
information available concerning efficiency and applicability of these technologies on process
heaters or boilers in petroleum industry, using fuel oil or fuel gas. Only methods that have
been used for industrial process heaters or boilers are considered here but many others
techniques exist.
2.2.1. Low-NOx burners (LNB)
Low-NOx burner is a technology allowing a controlled mixing of fuel and air, resulting in a
cooler flame and consequently less thermal NOx formation. It is designed as a stage
combustion with either staged air or staged fuel. It is applicable to tangential and wall-fired
boilers of various sizes and heaters. It reduces emissions from 40 to 60%.
The basic principle of Low-NOx burner is the separated injection of air and fuel in the furnace
resulting in the destruction of NOx in the flame (fuel-rich combustion zones) and the peak
flame temperature suppression. Moreover the better air flow distribution allows fuel ignition
and flame stability.
2.2.2. Staged combustion air (SCA)
Staged combustion air allows the reduction of fuel NOx by suppressing the amount of air
below that required for complete combustion. It is achieved by injecting a portion of the total
combustion air downstream of the fuel-rich primary combustion zone.
The SCA can be accomplished by several means such as burners out of service (BOOS),
biased firing or overfire air (OFA), depending on the type of boiler. The SCA technique is
highly effective on high nitrogen fuels such as residual oil. It reduces NOx emissions by 20 to
50%.
2.2.3. Flue gas recirculation (FGR)
Flue gas recirculation consists in the rerouting of a portion of flue gases from the stack back
to the furnace. Thus, furnace temperature and oxygen concentration are reduced and so is
thermal NOx formation. Large modifications to the burner and windbox in old boilers are
expensive so this technique is better for new boilers.
2.2.4. Water or steam injection (WI/SI)
Water or steam injection in the flame reduces thermal NOx formation by lowering the peak
temperature of the flame. This technique has a relatively low initial cost so it is considered as
quite efficient for smaller boilers. However this technique can lead to thermal losses and
increase in CO emissions.
5
2.2.5. Selective non catalytic reduction (SNCR)
SNCR is a postcombustion technique consisting in injecting ammonia or urea into combustion
flue gases. A reaction with NOx occurs to produce nitrogen and water. There are not many
experiences to evaluate effectiveness of this technique.
2.2.6. Selective catalytic reduction (SCR)
SCR is another postcombustion technique consisting in injecting ammonia into the
combustion zone in presence of a catalyst to reduce NOx into nitrogen and water. This
method allows achieving NOx emission reduction by 75 to 90%. This technique is rather
common.
Both SNCR and SCR are influenced by sulfur content of the flue gas.
The table below (table 2-1) summarizes available techniques for NOx control for process
heaters. It uses information from the “Alternative Control Techniques Document - NOx
Emissions from Industrial/Commercial/Institutional (ICI) Boilers” published by the US EPA.
This table can help in a first approach for identifying the best technology to use in function of
the type of boiler and the fuel. However, many other parameters are to be taken into account
such as the NOx emissions threshold wanted, the budget, etc.
6
Fuel / boiler
NOx control
Residual oil /
watertube
LNB
% NOx
reduction
20 - 50
FGR
4 - 30
SCA
5 - 50
LNB + FGR
LNB + SCA
SNCR
N.A.
N.A.
40 - 70
SCR
LNB
N.A.
20 - 50
FGR
20 - 68
SCA
LNB + FGR
LNB + SCA
SNCR
17 - 44
N.A.
N.A.
40 - 70
- Most common technique.
- Common technique.
- Commercially offered
WI
SCA
LNB
50 - 77
15 - 50
40 - 85
- Popular technique. Many designs and vendors available.
FGR
LNB + FGR
LNB + SCA
SNCR
SCR
LNB
SCA
LNB
FGR
SCA
LNB
FGR
LNB + FGR
50 - 75
55 - 90
N.A.
10 - 40
80 – 90
30 – 60
50
20 - 50
55 - 75
5
30 - 80
55 - 75
N.A.
Distillate oil
/ watertube
Natural gas /
watertube
Residual oil /
firetube
Distillate oil/
firetube
Natural gas /
firetube
Advantages
Drawbacks
- Relatively inexpensive
- Minimal furnace modification, retrofit feasible for old units
- Many designs and vendors available.
- Available
- Best suited for new units
- Specific emissions data from industrial boilers with LNB are lacking.
- Staged air burners could result in flame impingement on furnace walls of
smaller units.
- Requires extensive modifications to the burner and windbox.
- Possible flame instability at high FGR rates.
- BOOS applicable for boilers with multiple burners only.
- Retrofit is not feasible or not available for all design types.
- Commercially offered.
- Not widely demonstrated on large boilers.
- Elaborate reagent injection, monitoring, and control system required.
- Must have sufficient residence time at proper temperature.
- Available but not widely demonstrated.
- Specific emissions data from industrial boilers equipped with LNB are lacking.
- Applicable to most boiler designs as a retrofit technology .
- New burners generally applicable to all boilers.
- Comercially available.
- Available.
- Best suited to new units.
- Requires extensive modifications to the burner and windbox.
- Possible flame instability at high FGR rates.
- Limited application except BOOS, Bias and OFA for large watertube.
- Not widely demonstrated on large boilers.
- Elaborate reagent injection, monitoring, and control system required.
- Must have sufficient residence time at proper temperature.
- Thermal efficiency loss of 0.5 to 2.5% and CO increase is expected.
- BOOS applicable for boilers with multiple burners only.
- LEA LNBs more applicable to single-burner systems.
- Staged air burners could result in flame impingement on furnace walls.
- Requires extensive modifications to the burner and windbox.
- Most popular technique for clean fuels.
- Applicable principally to multi-burner boilers.
- Several designs are available.
- Effective technique for clean fuels.
- Several designs are available.
- Effective technique used in many applications.
- Most popular technique for very low NOx levels.
- No data available.
- Staged air could result in operational problems.
- Technique not practical unless incorporated in new burner design.
- Specific emissions data from industrial boilers with LNB are lacking.
- Requires extensive modifications to the burner and windbox.
- Technique not practical unless incorporated in new burner design.
- Specific emissions data from industrial boilers with LNB are lacking.
Table A-2-1: Comparison of available techniques for NOx control for process heaters
7
2.3. Control techniques for SOx emissions reduction
On the contrary to NOx, SOx emissions are directly linked to the initial sulfur content of the
fuel and the combustion parameters do not influent on the amount of SOx emitted. Two
strategies can be used to reduce SOx emissions: the formation prevention (low sulfur fuel
usage, fuel desulfurization) or the flue gas desulfurization (wet or dry scrubbing, dual-alkali,
spray drying, Wellman-Lord process, etc.).
There are many postcombustion flue gas desulfurization techniques. Almost all techniques are
based on the acid-alkaline reaction between SO2 (and SO3) and an alkaline agent such as often
lime or limestone, caustic soda, magnesium hydroxide or ammonia. Other techniques are
selective adsorption or absorption.
Flue gas desulfurization is mostly used in thermal power plant. Few refineries have a flue gas
desulfurization, except in Japan where principally dry processes are used. The principles of
four major techniques are given below.
2.3.1. Lime and limestone process
Lime and limestone scrubbings are non-regenerative wet processes producing gypsum. Lime
and limestone scrubbing are very similar. The use of lime (CaO) instead of limestone
(CaCO3) for the slurry preparation is the only difference. The alkaline slurry is sprayed in the
absorber and reacts with the SO2 in the flue gas. Following chemical reactions occur:
SO2 dissociation:
SO2 (gaseous) → SO2 (aqueous)
SO2 + H2O → H2SO3
H2SO3 → H+ + HSO3- → 2H+ + SO3Lime or limestone dissolution:
CaO(solid) + H2O → Ca(OH)2 (aqueous) → Ca2+ + 2HOor
CaCO3 (solid) + H2O → Ca2+ + HCO3- + HOReaction between ions:
Ca2+ + SO32- + 2H+ + 2HO- → CaSO3 (solid) + 2H2O
The following reactions can occur if there is excess oxygen:
SO32- + ½ O2 → SO42SO42- + Ca2+ → CaSO4 (solid)
Lime and limestone processes are the most popular flue gas desulfurization system for utility
boilers. Some system has achieved SO2-removal efficiency greater than 95%. Another
advantage is that these processes produce gypsum, which is saleable. However, these
processes have limited usage in refineries.
8
2.3.2. Dual-alkali scrubbing
Dual-alkali scrubbing is a non-regenerative process using sodium-based solution and lime or
limestone to remove SO2 from flue gases. Following chemical reactions occur:
Main absorption reactions:
2NaOH + SO2 → Na2SO3 + H2O
NaOH + SO2 → NaHSO3
Na2CO3 + SO2 + H2O → 2NaHSO3
Na2CO3 + SO2 → Na2SO3 + CO2
Na2SO3 + SO2 + H2O → 2NaHSO3
2NaOH + SO3 → Na2SO4 + H2O
2Na2SO3 + O2 → 2Na2SO4
Regeneration:
2NaHSO3 + Ca(OH)2 → Na2SO3 + CaSO3. ½ H2O↓ + 3/2 H2O
Na2SO3 + Ca(OH)2 + ½ H2O → 2NaOH + CaSO3. ½ H2O ↓
Na2SO4 + Ca(OH)2 → 2NaOH + CaSO4 ↓
This method is attractive because it has a high SO2 - removal efficiency and it reduces scaling
problems.
2.3.3. Activated char process
Activated char process is the principal dry process used in refineries. The circulating activated
char absorbs SO2 at a temperature comprised between 100 and 200°C. This process has the
advantage to also eliminate NOx present in the flue gases. The following chemical reactions
occur:
Absorption on char and conversion into sulphuric acid:
SO2 + ½ O2 + H2O → H2SO4
NOx reduction with ammonia:
4NO + 4NH3 + O2 → 4N2 + 6H2O
Char regeneration at 400°C:
H2SO4 → H2O + SO3
2SO3 + C → 2SO2 + CO2
After concentration, SO2 is sent to the Claus unit.
This process can achieve an SO2-removal efficiency of 90 % and a NOx-removal efficiency
of 70%.
9
2.3.4. Wellman-Lord process
The Wellman-Lord process consists in the SO2 neutralisation by a sodium-based solution
which is then regenerated. The following chemical reactions occur:
SO2 capture:
SO2 + Na2SO3 + H2O → 2NaHSO3
Na2SO3 + ½ O2 → Na2SO4
Regeneration:
2NaHSO3 → SO2 + Na2SO3 + H2O
SO2-rich gas treatment:
2SO2 + 6H2 → 2H2S + 4H2O
2H2S + SO2 → 3S + 2H2O
The final effluent is sent to the Claus unit.
This process has been often used for utility and industrial boilers. It has the advantage to
regenerate the scrubbing solution and to produce a saleable product. However, installation and
maintenance costs are higher than lime, limestone or dual-alkali systems.
2.4. Control techniques for particulate matters emissions
For large boilers, a good design and good maintenance can minimize soot and condensable
organic compounds emissions. However, fly ash is still emitted, and in this case a
postcombustion PM control is needed. Four common methods are described below.
2.4.1. Inertial collectors
Inertial collectors allow separating particles from gas thanks to mechanical forces such as
centrifugation, gravitation or inertia. The three major types of inertial collectors are settling
chambers, baffle chambers and centrifugal chambers.
2.4.1.1. Settling chambers
A settling chamber is a large box which, by a large size, reduces the speed of the gas stream.
Thus, heavier particles settle down.
Figure A-2-1: Settling chamber
This technique is quite simple and easily manufactured, however it needs a large space and it
has a low efficiency.
10
2.4.1.2. Baffle chambers
In baffle chambers, gas stream changes its direction. Heavier particle do not follow the stream
and settle down.
Figure A-2-2: Baffle chamber
This technique is better used as precleaner.
2.4.1.3. Centrifugal collectors
Centrifugal collectors use the cyclonic action to separate particles from the gas stream.
Particles, which are heavier, are directed towards the wall of the cyclone and fall down.
Figure A-2-3: Cyclone
Single or multicyclones are available.
2.4.2. Electrostatic precipitators
This technique uses electrostatic forces to separate particles from gases. The gas passes
through a passage formed by the discharge and collecting electrodes. Particles receive a
negative charge and are then attracted to a positively charged electrode. Collected particles
are then removed by rapping or vibrating electrodes continuously or intermittently.
2.4.3. Fabric filtration
Fabric filters use filtration to separate particles from gas. The gas stream enters the baghouse
and passes through fabric bags that act as filters. The fabric used can be cotton, synthetic or
glass-fibre materials. This technique is very efficient and cost effective.
11
Figure A-2-4: Baghouse
Fabric filters are classified according to their cleaning methods (mechanical shaking, reverse
air injection).
2.4.4. Scrubbing systems
This technique uses a scrubbing liquid (generally water) that comes into contact with the gas
stream. The three basic operations of wet scrubbers are gas humidification, gas-liquid contact
and gas-liquid separation. The outlet liquid is either cleaned and discharged or recycled into
the scrubber.
Figure A-2-5: Wet scrubber
2.4.5. Selection of the control technique for PM emissions
Design, effectiveness, space requirements, investment, operating, and maintenance costs
differ widely according to the technique. A compromise must be done in function of
advantages and drawbacks of each technique and SOx level emitted. Moreover, general
factors influent the selection of the PM-control technique. These factors are:
- PM concentration and particle size.
- Degree of particle removal required.
- Characteristics of gas stream.
- Characteristics of particles.
- Methods of disposal.
The table below (Table 2-2) indicates some advantages and drawbacks concerning each
technique.
12
Advantages
- Low-cost equipment
- Continuous or batch unloading
Inertial collectors
Electrostatic precipitators
Fabric filtration
Scrubbing systems
- High efficiency
- High efficiency
- Moderate-cost equipment
- High efficiency
- Moderate-cost equipment
- Control also SOx emissions
Drawbacks
- Primary technique
- Abrasion problems for high
particle concentrations
- High-cost equipment
- Can be damaged by high
temperatures or water
- Corrosion problems
- Wet slurry production
- Water pollution
Table A-2-2: Comparison of SOx-removal techniques
2.5. Carbon dioxide
The major gas emitted during combustion is obviously carbon dioxide. These emissions have
generally to be minimised due to actual context. Frequent measures to reduce carbon dioxide
emissions in plants are Energy Management Systems (tool used to control and optimize the
energetic performance) and cogeneration (use of heat engine to produce both electricity and
heat). A good optimization of processes is a way to recover all energy available, thus reducing
CO2 generation.
Conclusion
Many methods exist to reduce emissions from process heaters and boilers. Usage of a cleaner
fuel, better combustion, low-NOx burners, or postcombustion control techniques contribute
all to emit less pollutants into the atmosphere.
3. Blowdown systems 1,14,15
Petroleum industry process units are equipped with a collection unit called the blowdown
system. It allows the safe disposal of liquid and vapor hydrocarbons that are vented in
pressure relief valves or drawn from the unit. This system can also be used to purge the unit in
case of shutdowns. Blowdown materials are partly liquid and partly vapor. The liquid cut is
either recycled into the refinery or sent to the waste water treatment. The vapor cut is either
recycled or discharged directly to the atmosphere or flared. When discharged directly to the
atmosphere, emissions consist principally in hydrocarbons. When flared, sulfur oxides are
emitted. The emission rate of the blowdown system depends on the amount of equipment
considered, the frequency of discharges, and the blowdown system controls.
3.1. Emissions to the flare
Flaring is a safety measure used in petroleum industries to ensure that gases are safely
disposed of. A flare is a device that burn hydrocarbons emitted from emergency process vents
or pressure relief valves. It is usually assumed that flares have a combustion efficiency of at
least 98%.
13
The combustion reaction is:
CxHy + (x + y/4) O2 → x CO2 + y/2 H2O
Principally carbon dioxide is emitted from flares, but also organic compounds and carbon
monoxide, NOx, SOx and soot.
It is actually impossible to estimate flare emissions, however, several measures can be
adopted to minimise these emissions:
- Use of efficient flare tips, and optimization of the size and number of burning nozzles.
- Maximization of flare combustion efficiency by controlling and optimizing flare
fuel/air/steam ratio.
- Minimization of flaring from purge without compromising safety, through measures such as
purge gas reduction devices, flare gas recovery units, inert purge gas.
- Installation of high integrity instrument pressure protection systems, where appropriate, to
reduce over pressure events and avoid or reduce flaring situations.
- Minimization of liquid entrainment in the gas flare stream with a suitable liquid separation
system.
- Implementation of burner maintenance and replacement programs to ensure continuous
maximum flare efficiency.
3.2. Liquid emissions
In order to minimize liquid emissions, it is important to recycle as much as possible drained
liquids. If recycle is not possible, segregation of process drained liquid from relatively clean
water can reduce the quantity of oily sludge generated. Moreover, it is easier to recover oil
from smaller and concentrated streams.
4. Wastewater 12,20
Wastewaters from petroleum industries are various. They can be process waters such as crude
oil desalting waters or sour waters from hydrocracking or hydrotreatment processes, general
effluents such as drained oily waters, washing waters and finally spent caustics. In order to
meet quality requirements about wastewater releases, the best way is to segregate these
different waters. In this chapter, common techniques for wastewater treatments in refineries
are shortly described and then, best management practices for process wastewater are given.
4.1. Wastewater treatment techniques
4.1.1. Sour waters stripping
This operation is a pre-treatment operation before release to the principal wastewater
treatment. It is necessary due to high content of NH4+ and H2S. It consists firstly in an
acidification with a strong acid to dissociate HSNH4 into H2S and (NH4)2SO4 and then in a
vapor stripping of H2S and NH3. This operation results in sulfur elimination of about 90 to
98% and ammonium elimination of about 92 to 97%. Phenols are however not well-stripped
and only 30% of linked ammonia is stripped.
14
Incineration or
Claus unit
Sour waters
Vapor
Desulfured waters
Figure A-4-1: Sour waters stripping system
4.1.2. Oil water separation
Oil water separation is the first step of general treatment of residuals refinery waters. Its
purpose is to eliminate insoluble hydrocarbons and suspended matters. It is classically carried
out by gravity. Several separators are available which can be longitudinal (API separators),
circular, or lamellar.
4.1.3. Physical and chemical purification
This step is necessary before biological treatment. This technique associates one chemical
reaction with a physical separation. Most used techniques are coagulation, flocculation, air
flotation and filtration. It allows elimination of colloidal suspended matters and insoluble
hydrocarbons.
4.1.4. Biological treatment
After physical and chemical treatment, dissolved pollutants are still to be removed. These
pollutants include soluble hydrocarbons, soluble CODs and BODs, phenols and nitrogen
compounds. They are biodegradable and can be removed with biological treatment techniques
such as activated sludge or trickling filters.
4.2. Best management practices
Treatment techniques are quite well-known and widely used in refineries to treat wastewater.
However liquid effluents may also result from accidental releases or leaks. In order to prevent
prevention from these events, management practices can be applied:
- Regularly inspect and perform maintenance of storages and equipment for prevention and
control of accidental releases.
- Maximize recovery into the process and avoid massive discharge of process liquids into the
oily water drainage system.
- Construct storage containment basins with impervious surfaces to prevent contamination of
soil and groundwater.
- Segregate process water from other wastewaters.
- Direct spent caustic soda to caustic oxidation before wastewater treatment system.
- Install a closed process drain system to collect and recover spills of MTBE, ETBE and
TAME.
15
4.3. Pollution prevention
In addition to these management practices, some pollution prevention solutions can be
noticed:
- Control solids entering sewers, which produced more oily sludges.
- Improve recovery of oils from oily sludges.
- Identify benzene sources and install upstream water treatment.
- Recycle and regenerate spent caustics.
- Use oily sludges as feedstock for coking or crude distillation units.
- Recycle lab samples.
16
Part B: Analysis of processes
1. Naphtha hydrotreating unit 11,14,15,18
In gasoline production, naphtha hydrotreating is an essential step. Its purpose is to reduce
sulfur, nitrogen and olefins contents in naphtha before it is fed in paraffin isomerization and
catalytic reforming as catalysts used in these processes are very sensitive to impurities.
Approximately 200 processes have been commercialized by Axens.
1.1. Purpose of the unit
The purpose of the unit is to produce clean desulfurized naphtha cut able to be processed in
isomerization and reforming units. Indeed these processes involve catalysts that are very
sensitive to impurities such as sulfur, nitrogen, water, halogen, diolefins, olefins, arsenic,
mercury and other metals. The high performances of isomerization and reforming units are
very much dependent upon the efficiency of the naphtha pretreater. Naphtha hydrotreating
unit is located after the crude oil distillation and before isomerization and catalytic reforming
units. It pretreats different types of naphtha such as straight run naphthas (paraffinic naphthas
from crude oil distillation), coker naphtha (from coking unit), wild naphtha and naphtha from
hydrocraking unit.
1.2. Raw materials and resources input characteristics
1.2.1. Naphtha feeds
The feed of the naphtha hydrotreating unit is a blend of different raw naphtha feeds. It
contains many different compounds such as paraffins, isoparaffins, olefins, naphtenes and
aromatics, from C1 to C11. Raw naphtha feeds impurities are principally sulphur, nitrogen and
diolefins. Finally, silicon, mercury, lead, arsenic, chlorine, fluorine, oxygenates and oxygen,
and mercapts can be present in trace amounts.
The following table indicates the typical properties of a crude oil distillation naphtha:
Compound
Quantity
Parrafins
55.6 % vol.
Olefins
0.2 % vol.
Naphtenes
37.5 % vol.
Aromatics
6.7 % vol.
Sulfur
500 ppm
Nitrogen
1 ppm
Table B-1-1: Typical properties of crude oil distillation naphtha
Naphtha molecular weight is generally between 100 and 215 g/mol. Its boiling point is
comprised between 80 °C and 180 °C.
17
1.2.2. Hydrogen make-up
The hydrogen make-up is supplied from the isomerization unit or from another unit. It
contains about 95% hydrogen, and hydrocarbons from C1 to C5. Impurities that can be found
are sulphur, nitrogen compounds, carbon oxides, carbonyl sulphide, olefins and chlorides, all
these compounds present in trace amount.
1.2.3. Catalyst
Hydrotreating catalysts are oxide supported (generally Al2O3) and the active phase is
molybdenum or tungsten sulfur with cobalt or nickel.
1.3. Products characteristics
There are two products from naphtha hydrotreating unit: heavy naphtha that goes to catalytic
reforming unit, and light naphtha that goes to isomerization unit.
The following table indicates the typical properties of naphtha hydrotreating products:
Compound
Isomerization product
Reforming product
Parrafins
82.5 % vol.
47.8 % vol.
Olefins
Naphtenes
16.5 % vol.
48.6 % vol.
Aromatics
1 % vol.
8.6 % vol.
Sulfur
< 0.5 ppm
< 0.5 ppm
Nitrogen
< 0.5 ppm
< 0.5 ppm
Table B-1-2: Typical properties of naphtha hydrotreating products
1.4. Normal operations
The figure B-1-1 is an example of a process flow diagram for a naphtha hydrotreating unit in
normal operations. The process flow diagram can be separated into two sections: the reaction
section and the separation section. Before entering the reaction circuit, naphthas from
different sources are mixed together and with hydrogen.
18
Reaction section
off-gas to
saturated gas
recovery
separator purge
to saturated gas
recovery
Separator section
air condenser
flare
stripper
stripper
flare
sulfur
guard
sour
water
oily
water
sewer
12 3 4 5 6
oily
water
sewer
sour
water
water
H2
full range heavy
light
naphtha naphtha naphtha
1, 2, 3, 4, 5, 6 : different sources of naphtha
Figure B-1-1: Naphtha hydrotreating process flow diagram
19
1.4.1. Reaction section
Hydrotreating is performed in two steps: the first one is the partial hydrogenation of diolefins
into olefins, and the second one is hydrogenation of olefins, desulfurization and
denitrification.
Catalyst in the first reactor selectively hydrotreats the naphtha feed. Diolefins and a part of
olefins present in the feed are hydrogenated in liquid phase.
In the second reactor, two catalysts are present: The first one (first bed of the reactor) for
hydrogenation and silica removal and the second one (second and third bed of the reactor) for
aromatic hydrogenation, desulfurization and denitrification.
Reactions occurring within the process are principally desulfurization, denitrification,
hydrogenation and elimination of metals.
Desulfurization
Principal sulfur compounds in naphthas are mercaptans, aliphatic and cyclic sulfides and
disulfides. These compounds react readily with hydrogen to produce the corresponding
saturated compound, releasing H2S.
Denitrification
Typical nitrogen compounds in naphthas are methylpyrrol and pyridine. Nitrogen is removed
by the breaking of the C-N bond producing an aliphatic compound and ammonia.
Hydrogenation
Hydrogenation is the addition of hydrogen to an unsaturated hydrocarbon to produce a
saturated product.
Elimination of arsenic and other metals
In naphthas, arsenic and other metals are usually in organo-metallic form. After
hydrogenation in the hydrotreater reactor, the hydrogenated form reacts with the hydrotreater
catalyst forming a bimetallic compound. Arsenic and other metals are physically adsorbed on
the catalyst.
Prior to the air condenser, water is injected in order to dissolve chloride, sulphide and
ammonium salts, which precipitate at low temperature. Water is recovered in the boot of the
separator drum.
1.4.2. Separation section
The function of this section is to split the full range naphtha into light naphtha, to feed the
isomerization unit, and heavy naphtha, to feed the reforming unit.
1.4.3. Influents / effluents scheme
The following scheme represents simply what enters and what goes out the battery limit
during normal operations.
20
Combustion gas
Naphthas
Hydrotreated naphtas
Naphtha
Hydrotreating Unit
H2
H2O
Sour water
Off-gas
Fugitive emissions
Gas relieves
FG / FO
Figure B-1-2: Influents/effluents scheme for naphtha hydrotreating unit in normal operations
1.5. Intermittent operations
1.5.1. Catalyst sulfiding
The metals of catalysts used in this process are in the oxide form. They must undergo a
treatment to recover the active sulfide form. If sulfiding is not complete, it could lead to metal
sintering resulting in poor activity of the catalyst and heavy coke deposits. This operation is
achieved by injection of the sulfiding agent (dimethyl disulfide (DMDS)) in a circulation of
hydrogen and raw feed. The required amount of DMDS is determined from the decomposition
of DMDS into H2S.
Combustion gas
Inert naphtha
H2
DMDS
H2O
Naphtha
Hydrotreating Unit
Catalysts sulfiding
Water saturated with H2S
Butane
Fugitive emissions
FG / FO
Figure B-1-3: Influents/effluents scheme for naphtha hydrotreating unit during catalyst
sulfiding
1.5.2. Catalyst regeneration
When catalysts activity becomes too low, they must be regenerated. This regeneration can be
in-situ or ex-situ.
If the regeneration is in-situ, the procedure includes:
21
- Commissioning of ammonia injection lines and caustic soda injection lines to the circuit.
- A first coke combustion step with 0.5% vol. oxygen in the reactor inlet gas.
- A second coke combustion step with 1% vol. oxygen in the reactor inlet gas.
- A finishing phase with 1% vol. oxygen.
- The shut-off of ammonia injection and caustic scrubbing.
- Cooling down of the reactor temperature using the recycle gas circulation to prepare the unit
for the new start-up.
Chemical reactions occurring during catalysts regeneration are:
- Coke combustion to produce carbon dioxide and water.
- Oxidation of the metallic sulfides on the catalyst to produce sulfur oxides.
- Neutralization reactions.
SO3 + 2 NH3 + H2O → (NH4)SO4
CO2 + 2 NaOH → Na2CO3 + H2O
SO2 + 2 NaOH → Na2SO3 + H2O
Combustion gas
H2 O
N2
10% wt NaOH
Air
Naphtha
Hydrotreating Unit
Catalysts
regeneration
NH3
Spent caustic stream
Waste vapor
Fugitive emissions
FG / FO
Figure B-1-4: Influents/effluents scheme for naphtha hydrotreating unit during catalysts
regeneration
In case of ex-situ regeneration, the catalyst has to be unloaded from the reactor without
previous coke combustion.
1.6. Effluents characterization
In this part, each effluent (except products from the process) is the most precisely as possible
characterized with data available.
22
1.6.1. Normal operations
Fugitive emissions
Media
Origin
Destination
Quantity
Composition
Gas
Valves, pump seals, flanges, open-ended valves, relief valves,
compressor seals, drains, sample connections.
Diffuse in the atmosphere
Can be estimated with the Average Factor Method
VOCs, sulfur compounds
Fugitive emissions are not negligible. They can be estimated thanks to the Average Factor
Method which is explained in part A of this report.
Gas relieves
Media
Origin
Destination
Quantity
Composition
Gas
Relief valves
Flare and then atmosphere
?
CO2, VOCs, SOx, NOx
Off-gases
Media
Origin
Destination
Quantity
Composition
Gas
Separator and reflux drums
Sour gas treatment or sulfur recovery units
Can be known from material balance
Light fuel gas, H2S (see material balance)
Off-gases are very rich in hydrogen sulphide and light hydrocarbons. It is typically sent to the
sour gas treatment unit and sulfur recovery unit.
Flue gas from furnaces
Media
Origin
Destination
Quantity
Composition
Gas
Fuel oil or fuel gas combustion in heaters
Atmosphere
Can be known from process data
CO2, SOx, NOx, PM, VOCs, metals - Calculated with Emission
Factors
These emissions are indirect emissions from the process. They come from the fuel
combustion in heaters. These emissions can be estimated thanks to Emission Factors. They
depend on the type of fuel burned, firing practice and post combustion controls. The choice of
fuel oil or fuel gas burned in furnaces depends on the fuel available on-site.
23
Sour waters
Media
Origin
Destination
Quantity
Composition
Liquid
Separators
Sour waters treatment
6 to 12 % of the charge
Salts (≈5%)
Sour waters come from stripping steam injected after reactors. Sulfur is converted into H2S
and mercapts, nitrogen compounds are converted into NH3 and cyclic hydrocarbons are
converted into phenols. These waters are sent to sour waters treatment.
1.6.2. Intermittent operations
Spent caustic stream from catalyst regeneration
Media
Origin
Destination
Quantity
Composition
Liquid
Separator drum
Spent caustic treatment unit
Estimated in process book
Estimated in process book – H2O (≈92%), Salts ((≈8%), HC < 50 wt
ppm
During regeneration, a spent caustic stream is drained from the separator drum and sent to the
spent caustic treatment unit.
Waste vapor from catalyst regeneration
Media
Origin
Destination
Quantity
Composition
Gas
Separator drum
Atmosphere
Estimated in process book
N2, CO2, O2 (traces), VOCs (traces)
During regeneration, a waste vapor stream is routed from separator to atmosphere at safe
location under pressure control.
1.6.3. Solid wastes
Catalyst
Media
Origin
Destination
Quantity
Composition
Solid
Reactors
Metals regeneration, reclamation or reuse
Depends on reactor size
Contaminated catalyst
24
1.7. Emissions reduction proposals
Air emissions from naphtha hydrotreating in normal operations arise from process heaters,
vents and fugitive emissions.
In order to reduce fugitive emissions, a leak detection and repair program can be established
(refer to part A of this report).
Concerning process heaters, old furnaces that produce NOx, SOx and particulate matters
should be replaced with emission controls furnaces.
During catalyst regeneration, waste vapor is sent to the atmosphere. These vapors contain
coke, VOC and carbon monoxide in trace amount and carbon dioxide. Before being released
in the atmosphere, this gas should be treated. For example, it could go through a first boiler to
burn carbon monoxide and VOCs, and then through a particulate matters removing apparatus,
such as an electrostatic separator or a cyclone separator.
2. Naphtha isomerization 10,14,15,16,17,18,19
Isomerization is a conversion process which aim is to raise octane number by transforming
straight chains C5-C6 paraffins into branched paraffins (isoparrafins). Axens offers a complete
range of isomerization solutions that can increase C5-C6 naphtha cut octane number up to 92.
Isomerization schemes available are once-through, recycle with diisopentanizer or with
deisohexanizer, advanced recycle (IPSORB®, HEXORB®). The choice of the process depends
on criteria such as feed composition or desired octane number.
2.1. Purpose of the unit
The purpose of the unit is to produce high-octane number isoparaffins by isomerising normal
C5-C6 paraffins. The isomerization unit can also include a benzene hydrogenation step in the
first reactor. Naphtha isomerization unit is located after the hydrotreating unit because the
catalyst is very sensitive to impurities. The product obtained (isomerate) enters in gasoline
composition.
2.2. Raw materials and resources input characteristics
2.2.1. Naphtha feeds
The feed of the naphtha isomerization unit depends a lot on the refinery. It can be either the
hydrotreated naphtha from the crude oil distillation, either light naphtha cut from reforming
unit. A typical isomerization naphtha feed contains principally C5 and C6 paraffins. It contains
also between 0 and 3% of C4 paraffins and other hydrocarbons such as benzene, naphtenes,
olefins and C7+.
25
The following table indicates the typical properties of an isomerization naphtha feed:
Compound
Quantity
Isopentane
20 % wt.
n-Pentane
29 % wt.
Cyclopentane
1 % wt.
2.2-Dimethylbutane
0.3 % wt.
2.2-Dimethylbutane
1.5 % wt.
2-Methylpentane
11 % wt.
3-Methylpentane
8.2 % wt.
n-Hexane
19.5 % wt.
Methylcyclopentane
5 % wt.
Cyclohexane
1.5 % wt.
Benzene
2 % wt.
C7+
1 % wt.
Table B-2-1: Typical properties of isomerisation naphtha feed
Isomerization naphtha feed boiling point is generally comprised between 70 °C and 80 °C.
2.2.2. Hydrogen
Hydrogen is necessary in order to avoid a coke deposit on the catalyst. Moreover, the quantity
of hydrogen must be high enough to hydrogenate benzene and to favorize isomerization
reactions.
2.2.3. Catalyst
Metallic platinum on chlorinated alumina based catalyst, that must be used in inert conditions
and with continuous chlorine injection is the current isomerization catalyst.
2.2.4. Dryers molecular sieves
Synthetic zeolite containing sodium and calcium is used as dryer.
2.3. Products characteristics
High-octane isomerate product is the principal product from this process. By-product is the
fuel gas produced at the head of the stabilization column and washed with caustic soda before
being sent to the fuel gas network.
2.4. Normal operations
2.4.1. Reactions
With platinum on chlorinated alumina based catalyst, the process must include feed and
hydrogen dryers and continuous chlorine injection.
Figure B-2-1 is an example of simplified process flow diagrams for a naphtha isomerization
unit in normal operations.
26
Gas
Naphtha
H2
dryers
Chlorine
Scrubber
Reactor
Stabilization
column
Isomerate
Figure B-2-1: Simplified process flow diagram for isomerization with chlorinated Pt/Al2O3
catalyst
Chlorine injection is necessary to maintain the chlorine amount on the catalyst. Carbon
tetrachloride (CCl4) and tetrachloroethylene (C2Cl4) are usually utilized. A scrubber with
caustic soda and feed water is necessary to eliminate chlorhydric acid present in gases.
Axens offers several types of isomerization processes: one through or recycle. In order to
recycle n-paraffins which have not been transformed, it is necessary to separate it from
isoparaffins. This separation can be achieved either by distillation (diisopentanizer or
deisohexaniser) which is big energy consumer or by adsorption on molecular sieve. A
combination of both can also be designed (IPSORB® and HEXORB®). (See figures B-2-2 and
B-2-3).
Diisopentanizer
Stabilization
column
Reactor
Gas
Light naphtha
(C5/C6)
Molecular
sieve
H2
Figure B-2-2: IPSORB® isomerization process
27
Isomerate
Reactor
Stabilization
column
Separation on
molecular sieve
Deisohexanizer
Gas
Isomerate
H2
Light naphtha
(C5/C6)
Figure B-2-3: HEXORB® isomerization process
Examples of reactions occurring are given below:
CH3
CH3 ─ (CH2)3 ─ CH3
CH3
CH ─ CH2 ─ CH3
CH3
CH3
CH3
CH3 ─ (CH2)4 ─ CH3
CH ─ (CH2)2 ─ CH3
CH3
CH3 ─ C ─ CH2 ─ CH3
CH3 ─ CH2 ─ CH ─ CH2 ─ CH3
+
CH3
CH3 ─ CH ─ CH2 ─ CH3
CH3
2.4.2. Influents / effluents scheme
The following scheme represents simply what enters and what goes out the battery limit
during normal operations.
28
Combustion gas
Light
Naphthas
H2
H2O
Light and heavy Isomerates
Naphtha
Isomerization Unit
Fresh caustic
Fuel gas
Spent caustic
Fugitive emissions
Gas relieves
Chlorine
FG / FO
Figure B-2-4: Influents/effluents scheme for naphtha isomerisation unit in normal operations
2.5. Intermittent operations
2.5.1. Dryers regeneration
Molecular sieves in dryers have to be regenerated on a regular time basis. It is consequently a
cyclic regeneration. For this operation, the deisohexanizer distillate product is generally used
as regenerant. After heating it flows on the sieve and it is recycled after the regenerant
degasser where light components are flared and free water is drained and sent to oily water
sewer.
2.5.2. Influents / effluents scheme
The following scheme represents simply what enters and what goes out the battery limit
during dryers regeneration.
Combustion gas
Isomerate product
Naphtha
Isomerization
Dryers
Regeneration
Isomerate (recycled)
Free H2O
Light components
Fugitive emissions
FG / FO
Figure B-2-5: Influents/effluents scheme for naphtha isomerisation unit during dryers
regeneration
29
2.6. Effluents characterization
In this part, each effluent (except products from the process and by-products) is the most
precisely as possible characterized with data available.
2.6.1. Normal operations
Fugitive emissions
Media
Origin
Destination
Quantity
Composition
Gas
Valves, pump seals, flanges, open-ended valves, relief valves,
compressor seals, drains, sample connections.
Diffuse in the atmosphere
Can be estimated with the Average Factor Method
VOCs, Cl
Fugitive emissions are not negligible. They can be estimated thanks to the Average Factor
Method which is explained in part A.
Chlorine is involved in the process, consequently chlorine can be released in the atmosphere
with fugitive emissions.
Gas relieves
Media
Origin
Destination
Quantity
Composition
Gas
Relief valves, reflux drum
Flare and then atmosphere
?
CO2, VOCs, SOx, NOx, Cl
Chlorine is also emitted in case of gas relieves. This chlorine could be released in form of
dioxins (see specific paragraph later).
Flue gas from furnaces
Media
Origin
Destination
Quantity
Composition
Gas
Fuel oil or fuel gas combustion in heaters
Atmosphere
Can be known from process data
CO2, SOx, NOx, PM, VOCs, metals Factors
Calculated with Emission
These emissions are indirect emissions from the process. They come from the fuel
combustion in heaters. These emissions can be estimated thanks to Emission Factors. They
depend on the type of fuel burned, firing practice and postcombustion controls. The choice of
fuel oil or fuel gas burned in furnaces depends on the fuel available on-site.
30
Spent caustic
Media
Origin
Destination
Quantity
Composition
Liquid
Caustic scrubber
Battery limit
Batch operation (see process book)
2 % wt NaOH, 10% wt NaCl, dissolved HC, H2O
2.6.2. Dryers regeneration
Oily water
Media
Origin
Destination
Quantity
Composition
Liquid
Discharge from regenerant degasser
Oily water sewer
Maximum quantity estimated with maximum water content of feeds
H2O, hydrocarbons
Light components
Media
Origin
Destination
Quantity
Composition
Gas
Regenerant degasser
Flare
?
CO2, VOCs, NOx
2.6.3. Solid wastes
Catalyst
Media
Origin
Destination
Quantity
Composition
Solid
Reactors
Disposal in landfill or regeneration
Depends on reactor size
Contaminated catalyst
Used catalysts from isomerization are either disposed of in landfill or regenerated ex-situ.
These wastes are generally reclaimed due to their precious metal content.
Spent platinum chloride and spent aluminium chloride catalysts are expected to have a small
concentration of contaminants as these catalysts require a clean feed. However, they are
expected to contain dioxins because of the presence of chlorine in the process.
31
Adsorbent
Media
Origin
Destination
Quantity
Composition
Solid
Naphtha and H2 feeds purification, product separation.
Disposal in landfill, recycling / reuse, or regeneration
Depends on dryers / column size
Contaminated molecular sieves
Solid adsorbents are used in three locations in the isomerization process: naphtha feed
purification, hydrogen feed purification and separation of the product. These adsorbents are
regularly regenerated but when they are no longer efficient, they are replaced by fresh
adsorbents. Used adsorbents are either disposed of in landfill or reused in cement plant and
road materials or stored in pile.
2.7. Emissions reduction proposals
2.7.1. Air emissions
Air emissions from isomerization unit in normal operations arise from process heaters, vents
and fugitive emissions.
In order to reduce fugitive emissions, a leak detection and repair program can be established
(see part A of this report).
Concerning process heaters, old furnaces that produce NOx, SOx and particulate matters
should be replaced with emission controls furnaces.
2.7.2. Water emissions
Two types of water emissions occur in this process: spent caustics and oily water.
Spent caustics do not contain sulfur and phenols but about 10 % of NaCl salt and dissolved
hydrocarbons. These hydrocarbons can be removed and recycled. These spent caustics are
free of phenols and sulfur compounds so it should not be mixed with other spent caustics
(which can be sent to phenol recovery units for example).
Oily water removed from the regeneration degasser is also expected to be free from impurities
such as sulfur or nitrogen compounds. It represents a very small amount.
2.7.3. Solid wastes
Concerning the catalyst, source reduction methods are those that extend its life. Its time life is
comprised between three and five years but it can sometimes be replaced after more years.
Currently, recycling of the spent catalyst by sending to metals reclamation is a common
practice since the catalyst is platinum.
Adsorbents are used to extend catalyst life; they are consequently a source reduction
technique for other residuals. They do not have themselves source reduction methods. Their
time life is about three years.
32
2.8. Dioxins emissions
Dioxin compounds are in fact polychlorodibenzo-p-dioxins (PCDD), which include also
furans and pyralens. PCDD are aromatic tricyclic chrorinated molecules. Below is an example
of a compound from this family:
Figure B-2-6: 2,3,7,8-Tetrachlordibenzodioxin
Dioxins and furans are formed only in two refining processes: naphtha catalytic reforming
and isomerization units.
Three major mechanisms are nowadays identified for the formation of dioxins. The first
mechanism involves PCDDs/PCDFs (polychlorinated dibenzo-p-dioxin/polychlorinated
dibenzofuran) contained in the feed and released intact to the environment after combustion,
the second mechanism (precursor mechanism) involves the formation of PCDDs/PCDFs from
the thermal breakdown and molecular rearrangement of aromatic precursors either originating
in the feed or forming as a product of incomplete combustion and the third mechanism
involves the heterogeneous solid-phase formation of PCDDs/PCDFs in the post-combustion
environment on the surface of fly ash.
The formation of dioxins occurs then in case of combustion. It can be supposed nevertheless
that the second mechanism could occur in an isomerization process in the petroleum industry.
An aromatic compound like benzene reacts with chlorine, causing hydrogen abstraction and
the formation of chlorobenzenes and chlorophenols. Homogeneous gas-phase formation of
PCDDs/PCDFs occurs from these precursor compounds at temperatures higher than 500°C,
catalyzed by the presence of copper compounds or other heavy metals. Heterogeneous
formation of PCDDs/PCDFs from gas-phase precursors has been observed at temperature
comprised between 200 and 450°C and by the presence of a transition metal.
Dioxin formation mechanisms are not well-known so we can suppose that the heating of
organic molecules in presence of a chlorine source in industrial processes can produce dioxins
that are released to the atmosphere in case of gas relieves or depressurization.
However, in the literature, source of dioxins concerning petroleum industry is the reforming
and isomerization catalyst regeneration (isomerization catalyst regeneration is very rare).
Nothing either is specified in the European Legislation concerning dioxins emissions in the
petroleum industry.
Conclusion
The isomerization process is different from the others because of the presence of chlorine.
This chlorine is scrubbed in normal operation but can be released in case of gas relieves and
depressurization.
33
3. Catalytic reforming 9,14,15,16,17,18,19
Catalytic reforming is a key process in gasoline production. It allows upgrading naphtha cut
to high-octane products by the obtaining of aromatic products. These compounds are formed
through complex series of reactions such as cyclohexanes dehydrogenation, cyclopentanes
isomerization and dehydrogenation, paraffins isomerization and dehydrocyclization.
Moreover, catalytic reforming is great source of hydrogen. Axens offers several types of
catalytic reforming: semi-regenerative process, cyclic process (Dualforming®) and continuous
process (Octanizing).
3.1. Purpose of the unit
The purpose of the unit is to produce high-octane products thanks to different type of reaction
explained later in this chapter. Catalytic reforming unit is located after the hydrotreating unit
because the catalyst is very sensitive to impurities. The product obtained (reformate) enters in
gasoline composition. It can also be sent to the isomerization unit.
3.2. Raw materials and resources input characteristics
3.2.1. Naphtha feed
The feed of the catalytic reforming unit is the heavy naphtha from the crude distillation. It
often goes through the hydrotreating unit before reforming in order to get naphtha free from
sulfur, nitrogen and olefins. Catalytic reforming naphtha feed contains C5 to C10 paraffins,
naphtenes and aromatics.
Also, gasolines from visbreaking, coking, hydrocracking or fluid catalytic cracking (FCC)
units can be sent to reforming.
The following table indicates the characteristics of two typical charges in weight percent.
.
Paraffins
Naphtenes
Aromatics
Charge A
C5
5.49
2.30
0.41
C6
16.83
5.80
3.18
C7
21.38
8.27
6.80
C8
17.26
5.95
3.08
C9
2.59
0.63
C10
63.55
22.95
13.47
Total
Charge B
C5
0.16
0.27
C6
3.31
5.78
0.20
C7
6.13
14.24
1.20
C8
9.79
14.47
3.54
C9
3.89
17.14
4.29
C10
3.59
11.17
0.88
26.81
63.07
10.10
Total
Table B-3-1: Typical properties of two charges for catalytic reforming unit
34
3.2.2. Catalyst
Catalysts used are bimetallic platinum - rhenium (Pt/Re) or platinum – tin (Pt/Sn) catalysts.
Pt/Re is mostly used for semi-regenerative process and Pt/Sn for circulating bed process.
Reforming catalysts activate many different reactions. However, they are very sensitive to
impurities and require the feed pre-treatment. Another drawback is that these catalysts
produce coke at a non negligible speed. This speed can nevertheless be reduced with high
pressure of hydrogen.
3.3. Products characteristics
Catalytic reforming generates three main products: reformate stream, hydrogen rich gas
stream and LPG stream. There is also a by-product which is a fuel gas stream from the LPG
absorber.
3.4. Reaction section
Reactions occurring in catalytic reforming are numerous. Desired reactions are:
- Dehydrogenation
+ 3 H2
- Isomerization/dehydrogenation
+ 3 H2
- Parrafins isomerization
n-C7H16
i-C7H16
- Dehydrocyclization
n-C7H16
+ 4 H2
Other parasite reactions are promoted with the catalyst. These reactions are dismutation,
alkylation, hydrocracking, hydrodealkylation and coking.
Different technologies exist for catalytic reforming. These differences concern mainly the
reactor type: the catalytic bed can be either fixed or mobile. With fixed bed, two processes
exist: semi-regenerative or cyclic. With a mobile bed, a continuous process is proposed by
Axens. Simplified process schemes and basic information are given below.
35
3.4.1. Semi-regenerative fixed bed
In this process, the catalyst regeneration implies unit shutdown.
Pretreatment
Rectification
Purge
Separating
drum
Purge
Reforming
C4
Separating
drum
Feed
Reformate
Stabilization
Figure B-3-1: Simplified scheme of semi-regenerative process for catalytic reforming
The pre-treatment section is necessary to purify the feed as catalyst is very sensitive to
impurities such as nitrogen, sulfur and oxygenated compounds and metals. A molecular sieve
can also be used in order to eliminate water traces.
Hydrogen recycling (from the stabilization section) is necessary to prevent a rapid catalyst
deactivation.
Finally, light hydrocarbons produced are separated from the product in the stabilization
section.
3.4.2. Continuous Catalyst Regeneration reforming process
Nowadays, units are generally Continuous Catalyst Regeneration (CCR) reforming process
named octanizing®. The scheme of this process is quite similar to semi-regenerative process
except that the catalyst bed is moving from one reactor to another, before regeneration and
introduction back to the first reactor. The scheme below is a simplified representation of the
catalyst circulation in reactors.
36
Catalyst
Regeneration
section
Lift gas
Feed
Effluent
Figure B-3-2: Continuous catalyst regeneration reforming
Two different lift gases are usually used: hydrogen rich gas or nitrogen gas. These gases are
recovered and recycled so are not effluents of the process.
3.4.3. Influents / effluents scheme
The following scheme represents simply what enters and what goes out the battery limit
during normal operations.
Combustion gas
Heavy Naphtha
H2
Reformate
LPG
Catalytic
Reforming Unit
H2
Fuel gas
Fugitive emissions
Gas relieves
FG / FO
Figure B-3-4: Influents/effluents scheme for catalytic reforming reaction section
3.5. Regeneration section
In continuous catalytic regeneration, the process utilizes moving catalyst bed technology. The
purpose is to regenerate continuously the catalyst during normal operation and keep optimum
operating conditions for the unit. The scheme below represents a simplified process flow
diagram for the catalyst regeneration.
37
Catalyst inlet
Purge to the atmosphere
Caustic
Washing drum
Spent
caustic
Dryer
Filter
Compressor
H2O
Air
C2Cl4
Catalyst outlet
Figure B-3-5: Simplified process flow diagram for CCR regeneration section
Before entering the regenerator, the catalyst is sent in a lock hopper where fresh catalyst can
be loaded in case of catalyst attrition. The pressure in this lock hopper is automated and
depressurizations happen during around 5 minutes every 20 to 50 minutes.
The regeneration of catalyst is achieved through the following operations:
- Coke burning.
- Catalyst oxychlorination.
- Catalyst calcination.
- Catalyst cooling.
- Catalyst reduction.
The first three steps are performed in the regenerator, the fourth one in a hopper located up to
the first reactor and the last one in a reduction chamber located between the hopper and the
reactor.
Coke burning, oxychlorination and calcination effluents are all directed to a washing drum
where gases are stripped with caustic soda. Reduction gas (hydrogen from another unit) is
recycled in the reaction section.
The following scheme represents simply what enters and what goes out the battery limit
during regeneration.
38
Combustion gas
Caustic purge
Air
H2 O
Catalytic reforming
Regeneration
C2Cl4
Caustic
soda
Regeneration loop purge
Lock hopper depressurization
Fugitive emissions /
Gas relieves
FG / FO
Figure B-3-6: Influents/effluents scheme for catalytic reforming in regeneration section
3.6. Effluents characterization
In this part, each effluent (except products from the process and by-products) is the most
precisely as possible characterized with data available.
3.6.1. Reaction section
Reaction section of the catalytic reforming process does not have any waste effluent.
Emissions come from fugitive emissions and process heaters.
Fugitive emissions
Media
Origin
Destination
Quantity
Composition
Gas
Valves, pump seals, flanges, open-ended valves, relief valves,
compressor seals, drains, sample connections.
Diffuse in the atmosphere
Can be estimated with the Average Factor Method
VOCs, HAPs, chlorinated compounds
Fugitive emissions are not negligible. They can be estimated thanks to the Average Factor
Method which is explained in part A.
The volatile nature of toluene, xylene, benzene and other HAPs which are formed during
catalytic reforming makes fugitive emissions HAPs largest release source.
Gas relieves
Media
Origin
Destination
Quantity
Composition
Gas
Relief valves, reflux drum
Flare or atmosphere
?
CO2, VOCs, HAPs
39
Flue gas from furnaces
Media
Origin
Destination
Quantity
Composition
Gas
Fuel oil or fuel gas combustion in heaters
Atmosphere
Can be known from process data
CO2, SOx, NOx, PM, VOCs, metals - Calculated with Emission
Factors
These emissions are indirect emissions from the process. They come from the fuel
combustion in heaters. These emissions can be estimated thanks to Emission Factors. They
depend on the type of fuel burned, firing practice and postcombustion controls. The choice of
fuel oil or fuel gas burned in furnaces depends on the fuel available on-site.
3.6.2. Regeneration section
As in reaction section, there are similar fugitive emissions and gas relieves in regeneration
section. Other effluents are described below.
Caustic purge
Media
Origin
Destination
Quantity
Composition
Liquid
Washing drum
Effluent treatment system
Can be found in material balance
H2O, Sodium salts (0.25% wt): NaCl + Na2CO3 + NaOCl, NaOH
(1mg/l max), HCl (1mg/l max), H2CO3, CO2 (3g/l max), dioxins
Trace amounts of dioxins have been observed in catalytic reforming process effluents. This
can be due to coke burning, high temperatures and chlorine presence. More information is
given in paragraph 3.8.
Regeneration loop purge
Media
Origin
Destination
Quantity
Composition
Gas
Washing drum
Atmosphere
Can be found in material balance
N2, CO2, O2, H2O. Possible presence of CO, HCl, SO2.
Lock hopper depressurization
Media
Origin
Destination
Quantity
Composition
Gas
Lock hopper
Atmosphere
Can be found in process book
N2, possible presence of alumina dust
40
3.6.3. Solid wastes
Catalyst
Media
Origin
Destination
Quantity
Composition
Solid
Reactors
Regeneration, metal recovery
Depends on reactor size
Contaminated catalyst
Catalyst from reforming process is regenerate every 6 to 24 months for SR process and
continuously for CCR. Catalysts used are generally very expensive so precautions are taken to
ensure a long lifetime and losses. When the catalyst has lost its activity, metals are recovered
off-site.
3.7. Emissions reduction proposals
3.7.1. Air emissions
Air emissions from isomerization unit in normal operations arise from process heaters, vents
and fugitive emissions.
In order to reduce fugitive emissions, a leak detection and repair program can be established
(see part A of this report).
Concerning process heaters, old furnaces that produce NOx, SOx and particulate matters
should be replaced with emission controls furnaces.
3.7.2. Solid wastes
Concerning the catalyst, source reduction methods are those that extend its life. Currently,
recycling of the spent catalyst by sending to metals reclamation is a common practice since
the catalyst is platinum and other expensive metals.
3.7.3. Spent caustic
In order to minimize spent caustic, contact between caustic and gas must be optimized.
3.8. Dioxins emissions
According to limited testing performed in the United States, catalyst regeneration in the
reforming process is a potential source of PCDDs/PCDFs.
During the reforming process, coke formation onto the catalyst lower its activity. This
can be removed by regeneration via burning at temperatures around 400°C followed
reactivation at temperatures around 500°C using chlorine or chlorinated compounds.
coke burning produces exhaust gases that are vented to the atmosphere or scrubbed
caustic or water.
41
coke
by a
This
with
Studies have been conducted in order to determinate PCDDs and PCDFs concentrations in
waste streams. However no regulation law has been emitted by European or American
legislation concerning dioxins in petroleum industry.
Once dioxins are produced, it is difficult to eliminate them as treatment methods or disposal
systems just transfer dioxins from a medium to another (for example, to scrub a gas transfer
dioxins to liquid). Decomposition of dioxins seems to not be widely used.
A better option would be to prevent dioxins formation, which is also difficult as formation
mechanisms are not well-known and chlorinated compounds are necessary in this process.
4. Hydrogenation in olefin plants
Hydrogenations are simple and relatively similar units present in olefin plants. They are
purification steps whose aim is to selectively hydrogenate dienes, alkynes and olefins which
are unstable compounds into olefins and alkanes. These processes generally do not produce a
lot of effluents. The main issue is basically effluents produced during the catalyst
regeneration. Axens has a strong experience with all types of hydrogenation.
4.1. Purpose of units
The C3 hydrogenation unit is a sub-process in an olefin plant. It is designed to selectively
hydrogenate Methylacetylene (MA) and Propadiene (PD) contained in the C3 stream from the
depropaniser overhead, before it is fed to Propylene Towers. Indeed, in addition to up to 90%
propylene, the raw C3 cut contains a non negligible quantity of MA and PD that have to be
removed in order to meet the propylene product specification. The reactions involved are
hydrogenations of MA and PD with hydrogen. The MAPD hydrogenation to propylene can be
carried out in either the vapour or liquid phase. All modern steam crackers for which the C3
cut is separated before hydrogenation ("tail end hydrogenation") use liquid-phase
hydrogenation as it requires lower investment and has lower operating costs compared to gasphase processing.
The C4 hydrogenation unit is a sub-process in an olefin plant. It is designed to selectively
hydrogenate butadiene contained in the C4 stream from the debutanizer overhead, before it is
fed to isobutylene and butane-1 removal units. The reaction involved is hydrogenation of
butadiene with hydrogen.
The gasoline hydrogenation unit is a sub-process in an olefin plant. It is designed to totally
hydrogenate raw pyrolysis gasoline (RPG) which is the bottom product of the ethylene plant
debutanizer. The purpose of this unit is to eliminate unstable components such as diolefins
and styrenics, and olefins in order to meet the product specification. Indeed cracked gasoline
typically exhibits high aromatics content, about 50% being benzene. It is an ideal feedstock
for benzene production. However, treatment steps with adequate fractionation facilities are
required upstream of the benzene process in order to meet sulphur, olefins and diolefins
content specifications. The treatment process operates in two stages. About 90 first stages and
60 second stages of gasoline hydrogenation have been licensed by Axens.
42
4.2. Raw materials and resources input characteristics
4.2.1. Raw C3 cut
The feed of the C3 selective hydrogenation units is a blend of different C3 compounds with a
major percentage of propane and propylene. Sulfur, nitrogen, arsenic and mercury can be
present in trace amounts.
The following table indicates the typical proportion of the raw C3 cut:
Compound
Quantity
Propane + propylene
90 to 95% wt.
Methylacetylene (MA)
1.5 to 4 % wt.
Propadiene (PD)
1.5 to 3 % wt.
+
(C2 ) + (C4 )
0.25 % wt.
Table B-4-1: Typical composition of a raw C3 cut entering a selective hydrogenation unit
4.2.2. Raw C4 cut
In addition to C4 components, raw C4 cut contains C3 and C5 components in minority
proportions. It can also contain impurities in trace amount such as sulphur, nitrogen, C8+ and
polymers, carbonyl compounds, acetone, mercury or arsenic.
4.2.3. Raw pyrolysis gasoline
Raw pyrolysis gasoline has a high aromatics content, about half of this figure being benzene.
It contains also diolefins, olefins and sulphur in variable amounts. Impurities such as nitrogen,
arsenic, antimony, or mercury can also be present.
4.2.4. Hydrogen make-up
The hydrogen make-up can contain trace amounts of light hydrocarbons (CH4, C2H6), carbon
oxides, chlorhydric and sulfuric acids and mercury. These compounds are in very little
quantity but contaminate the catalyst. They can be consequently present in effluents from
regeneration.
4.2.5. Catalyst
These hydrogenations are promoted with a palladium or nickel-on-alumina catalyst.
4.3. Products characteristics
Hydrogenated C3 cut is mainly composed of propylene (85%) and propane (15%). It shall
have an MAPD content inferior to 500 ppm (vol). It is routed to the next unit of the olefin
plant. There is no by-product from this unit.
C4 product is mainly composed of butenes and routed to the next unit of the olefin plant. It
contains also butane and traces amount of C3 and lighter, C5 dienes and acetylenes,
acetonitrile, sulfur and water. Waste effluents are produced: water and purge gas from recycle
and reflux drums.
43
The hydrogenated raw pyrolysis gasoline should be olefin, diene and sulphur free.
4.4. Normal operations
Simple flow sheets of each process are represented below.
4.4.1. Selective hydrogenation of C3
Hydrogen
Raw C3
cut
hydrogenation
reactors
separator
CW
recycle
pump
Hydrogenated
C3 cut
Figure B-4-1: Flow sheet of the C3 selective hydrogenation process
Feed section
The feed to the reactor consists of both fresh and recycle C3 cut streams. The recycle stream
comes from the separator. The recycle stream and the fresh stream are first mixed together
before being mixed with the hydrogen make-up in the static mixer.
Reactor section
The total stream enters the reactor top and flows downwards through the fixed catalyst bed.
The hydrogenation of MA and PD occurs selectively according to the following reaction:
CH3─ C
CH
+
H2
CH3 ─ CH ═ CH2
CH2 ═ C ═ CH2
+
H2
The reactor effluent is then condensed and cooled down in a cooler.
A spare reactor is foreseen in order to allow the catalyst reduction, reactivation or in-situ
regeneration operations in one reactor while the other one is in operation.
44
For catalyst reduction, reactivation or regeneration operations, the gas is sent into the reactor
at the required temperature and the effluent is sent to battery limit for disposal (flare or
cracker furnace fire box).
Separator section
The reactor effluent is then received in a separator where the off-gas is purged if necessary
(the off-gas purged is recycled). Finally the liquid phase is pumped out and divided into two
streams: the recycle to the reactor and the product routed to battery limit.
4.4.2. Selective hydrogenation of C4
purge
gas
purge
gas
cooler
column
reactor
1
recycle
drum
reflux
drum
reactor
2
WLD
pump
steam
static
mixer
WLD
static
mixer
cooler
cooler
pump
raw C4 cut
hydrogenated C4
Figure B-4-2: Flow sheet of the C4 selective hydrogenation process
Feed section
The feed to the reactor consists of both fresh and recycle C4 cut streams. The recycle stream
comes from the recycle drum. The recycle stream and the fresh stream are first mixed together
before being mixed with the hydrogen make-up.
Reactor section
The total stream enters the first reactor top and flows downwards through the fixed catalyst
bed. The hydrogenation of butadiene occurs selectively according to the following reaction:
CH2 ═ CH ─ CH ═ CH2 + H2 → CH2 ═ CH ─ CH2 ─ CH3
The reactor effluent is then sent in the recycle drum where free water is purged if necessary.
In case of two phase mixture, vapor shall be partially condensed in the condenser. The
resulting liquid falls back to the condenser and the purge gas is sent to the flare. Liquid from
45
the recycle drum is then pumped, cooled split into two streams, the recycle C4 cut and the
stage 2 feed, that goes to the second reactor.
The stage 2 feed is mixed with a second hydrogen make-up and enters the second reactor
bottom and flows upwards through the catalytic bed. The remaining butadiene is
hydrogenates selectively and the effluent is sent to the C4 stabilizer.
Stabilization section
The stage 2 effluent is preheated in a heat exchanger with the stabilizer bottom and feed the
stabilizer. In the stabilizer, light compounds of the products due to the H2 make-up quality are
removed. The column overhead is partially condensed and collected in the reflux drum where
vapor is purged and water drained. The liquid returns back to the column as reflux.
The column bottom stream constitutes the final product. This stream is cooled down and
routed to product storage.
4.4.3. Gasoline hydrogenation
vent to
sour gas
reactor 1
column
reactor 2
compressors
cold
separator
feed
drum
separator
hot
separator
H2O
pump
air cooler
pump
pump
H2O
pump
air cooler
compressor
feed
product
H2
make-up
Figure B-4-3: Flow sheet of the gasoline hydrogenation process
First stage reaction section
The raw pyrolysis gasoline coming from the debutanizer bottom is routed to the feed drum
where a boot allows trapping and draining water if any. It is then mixed with hydrogen make-
46
up and recycle diluent from separators, enters the reactor top and flows downwards through
two beds of the same catalyst. Reactions occur in mixed phase (mainly liquid).
Main reactions happening are:
- Hydrogenation of diolefins.
- Hydrogenation of alkenyl aromatics (hydrogenation of the linear side chain).
- Isomerization of olefins.
- Hydrogenation of olefins
- Thermal and catalytic polymerization of unstable compounds.
The two last reactions must be avoided.
After being cooled, reactor effluent is sent to hot separator. Vapor from the separator, which
contains light hydrocarbons, is partially condensed and sent to cold separator. Vapor from the
cold separator is sent to second stage reaction section as hydrogen make-up. Liquid effluents
from hot and cold separators are mixed together. Part of this liquid is recycled, the remaining
is sent to the second stage reaction section.
Second stage reaction section
The feed coming from the first stage reaction section is mixed with the recycle gas. The
mixture is heated before entering the second reactor top and flows downward through two
types of catalysts: the first one to finish hydrogenation, the second one for desulphurization.
Main desirable reactions occurring are:
- Hydrogenation of olefins.
- Hydrogenation of sulphur compounds.
After consecutive cooling, reactor effluent enters a separator in which the vapour phase is
partly purged, partly recycled, and the liquid phase is partly recycled, partly fed to stabilizer
column.
Stabilization section
The purpose of this column is to eliminate light components such as hydrogen and hydrogen
sulphide dissolved in the gasoline.
The stabilizer overhead is condensed and sent in a reflux drum where vapour and liquids
phases are separated. Vapour phase is routed to sour gas and decanted water is sent to battery
limit.
The column bottom stream constitutes the final product. This desulphurized product is cooled
down and routed to battery limit.
4.4.4. Influents / effluents scheme
The following schemes represent simply what enters and what goes out the battery limit
during normal operations.
47
Raw C3 cut
Hydrogenated C3 cut
C3 selective
hydrogenation
H2
Fugitive emissions
Gas relieves
Figure B-4-5: Influents/effluents scheme for C3 selective hydrogenation during normal
operations
Hydrogenated C4 cut
Raw C4 cut
Recycle C4 cut
C4 selective
hydrogenation
Reflux drum purge gas
Reflux drum water draw-off
Fugitive emissions
H2
Gas relieves
Figure B-4-6: Influents/effluents scheme for C4 selective hydrogenation during normal
operations
Raw pyrolysis
gasoline
Gasoline product
Gasoline
hydrogenation
H2
Off-gas
Fugitive emissions
Gas relieves
Figure B-4-7: Influents/effluents scheme for gasoline hydrogenation during normal
operations
48
4.5. Intermittent operations
4.5.1. Catalyst reduction / reactivation / hot hydrogen stripping
Catalyst reduction
Before the first use of the catalyst, and after regenerations, the catalyst is in the oxide form,
which is inactive. It has to be reduced with hydrogen to recover its active form. The reduction
reaction is the following:
MeO + H2 → Me + H2O
(Me corresponds here to the metal used as catalyst, for instance palladium Pd)
The flow of dry reduction gas through the catalyst allows the elimination of the water
contained in the catalyst.
Catalyst reactivation / hot hydrogen stripping
Reactivation is hot hydrogen stripping used when the catalyst is deactivated by free water
carry-over. These two operations are similar to reduction except the operating temperature.
Reduction /
reactivation
gas
C3 selective
hydrogenation
Catalyst reduction
/ reactivation
Reduction /
reactivation gas
with presence of
HC and H2O
Figure B-4-8: Influents/effluents scheme for C3 selective hydrogenation during catalyst
reduction or reactivation
Gas (H2 (25%)
+ N2 (75%))
C4 selective
hydrogenation
Catalyst reduction
/ reactivation /
stripping
Gas with presence
of HC and H2O
Figure B-4-9 Influents/effluents scheme for C4 selective hydrogenation during catalyst
reduction, reactivation or stripping
49
4.5.2. Catalyst regeneration
The catalyst regeneration can be proceeded in-situ or ex-situ the process. In case of in-situ
regeneration or oxidation, effluents are a non negligible issue even if they do not occur often.
Indeed these gaseous effluents contain a lot of pollutants that have never really been qualified
and quantified and the treatment method currently used seems to not be efficient.
Origin of contaminants
a) Impurities in raw materials
Feed to selective hydrogenation units come from steam cracking or other olefins sources,
consequently many impurities are present, that come from upstream processes. In the same
way, hydrogen contains impurities. Even if these impurities are in very little quantity, they
can fix themselves on the catalyst. When the catalyst is regenerated or oxidised, all these
impurities, which are no longer in trace amount as they concentrated, leave the catalyst and go
out with the gaseous effluent.
These contaminants are various:
- Sulphur compounds (H2S, COS, disulphides and mercaptans)
- Methanol and oxygenated compounds
- Nitrogen compounds (HCN, NH3, amines …)
- Inorganic chlorides and other mineral salts
- Arsenic
- Mercury
- Chlorhydric acid
b) Parasite reactions
As described before, catalyst and operating conditions in selective hydrogenation aim at
privilege the hydrogenation of dienes or alkynes into olefins and disadvantage other chemical
reactions that could occur. However dimerization (chemical union of two identical molecules)
and then oligomerization can occur. It is assumed that 10% of dienes and alkynes are
dimerized. In case of C4 hydrogenation, the product of dimerization is called green-oils. These
molecules (C4 to C20) coat the catalyst and deactivate it but can be removed by regeneration.
Catalyst regeneration principle
During catalyst regeneration procedure, a combustion (or oxidation) is performed in order to
burn deposited coke (or green oils). Other impurities can also be stripped by this process.
Operations of catalyst regeneration are a bit different depending from the process.
Nevertheless, main steps are more or less the same:
- Heating the catalyst bed by circulating heated nitrogen through the reactor.
- Catalyst bed stripping by establishing a steam circulation through the reactor (this step is not
always performed).
- Catalyst pre-oxidation by slowly adding air to the steam.
- Catalyst impurities burning-off by raising temperature and injecting air again.
- Cooling and purge with steam first and then nitrogen.
50
The out coming gas is consequently contaminated with CO2 and CO and also unburned coke
and other impurities. Here below is a list of compounds that can be found in the effluent
regeneration gas:
Compound
Green oils
Maximum content
10 % of the compound that have to be
hydrogenated.
Sulphur (H2S, SO2, SO3)
(1)
Mercury
(1)
Arsenic (AsH3)
(1)
Chloride (HCl)
(1)
(1) The quantity of contaminants deposited on the catalyst can not be known. The only data is
the maximum content of contaminants in the raw materials.
Table B-4-2: Compounds possibly present in catalyst regeneration effluents
Decoking drum
The most usual way to treat in-situ regeneration gas in hydrogenation processes are currently
decoking drum. This decoking drum is basically a drum in which the gas is simply washed
with water before being released in the atmosphere. This washing appears to not be really
efficient and pollutants are consequently released both in the atmosphere and in the waste
water.
The following tables summarize effluents from each step of catalyst regeneration for each
type of process.
Gas
components
Flowrate
Notice
Destination
Heating
Stripping
Pre-oxidation
Burning
Cooling 1
Cooling 2
N2
Steam
Steam + air
Steam + air
Steam
N2
(1)
Presence
of HC
and H2O
(1)
(1)
Presence of
CO2 and HC,
impurities (2)
Atmosphere
via decoking
drum (3)
(1)
Presence of
CO2 and HC,
impurities (2)
Atmosphere
via decoking
drum (3)
(1)
Presence of
air, impurities
(2)
Atmosphere
via decoking
drum (3)
(1)
Flare (3)
Presence
of HC
Flare (3)
Flare (3)
(1) Refer to operating instructions
(2) Impurities correspond to arsenic, mercury, and other compounds present in trace amount
in raw materials and which are trapped on the catalyst.
(3) These are the usual destination but some units can have a special collecting drum.
Table B-4-3: Gaseous effluents during catalyst regeneration in C3 selective hydrogenation
51
Heating
Gas
components
N2
Flowrate
(1)
Notice
Presence of HC
Destination
Flare (3)
Oxidation I
N2 + air
O2 content:
0,3 % vol
(1)
Presence of
CO2 and HC,
impurities (2)
Decoking
drum (3)
Oxidation II
N2 + air
O2 content: 2
% vol
(1)
Presence of
CO2 and HC,
impurities (2)
Decoking
drum (3)
Air purge
Air
(1)
Decoking drum
(3)
(1) Refer to operating instructions
(2) Impurities correspond to arsenic, mercury, and other compounds present in trace amount
in raw materials and which are trapped on the catalyst.
(3) These are the usual destination but some units can have a special collecting drum.
Table B-4-4: Gaseous effluents during catalyst oxidation in C4 selective hydrogenation
First stage reactor:
Gas
components
Flowrate
Notice
Heating
Stripping
N2
Steam
(1)
(1)
Presence of
HC + H2O
Presence of
HC
Preoxidation
Steam +
air
(1)
Presence
of HC,
CO2
Burning
Steam + air
(1)
Presence of
HC, CO2,
impurities (2)
Cooling 1
Cooling 2
Steam
N2
(1)
(1)
-
-
Decoking
Decoking
Decoking
Decoking
Decoking
Decoking
drum or
drum or
drum
drum
drum
drum
flare
flare
(1) Refer to operating instructions
(2) Impurities correspond to arsenic, mercury, and other compounds present in trace amount
in raw materials and which are trapped on the catalyst.
Destination
Table B-4-5: Gaseous effluents during catalyst regeneration in GHU first reactor
52
Second stage reactor:
Gas
components
Flowrate
Notice
Heating
Stripping
N2
Steam
(1)
(1)
Presence of
HC + H2O
Presence of
HC, H2S
Preoxidation
Steam +
air
(1)
Presence
of HC,
H2S, CO2
Burning
Steam + air
(1)
Presence of
HC, H2S,
CO2,
impurities (2)
Cooling 1
Cooling 2
Steam
N2
(1)
(1)
-
-
Decoking
Decoking
Decoking
Decoking
Decoking
Decoking
drum or
drum or
drum
drum
drum
drum
flare
flare
(1) Refer to operating instructions
(2) Impurities correspond to arsenic, mercury, and other compounds present in trace amount
in raw materials and which are trapped on the catalyst.
Destination
Table B-4-6: Gaseous effluents during catalyst regeneration in GHU first reactor
4.5.3. Catalysts sulfurization in the second reactor of gasoline hydrogenation unit
In the gasoline hydrogenation unit, metals of catalysts in the second reactor are in oxide form.
They must undergo a treatment to recover the active sulphide form. The operation consists in
sweeping the catalyst with a hydrogen steam which contains enough sulfiding agent to
sulfurize catalysts.
Sulfurization
H2 + DMDS
(dimethyldisulfide)
(1)
Presence of H2O, H2S
Flare
Gas components
(%mol)
Flowrate
Notice
Destination
Table B-4-7: Gaseous effluent during catalyst sulfurization in GHU
4.6. Effluents characterization
In this part, each effluent (except products from the process) is the most precisely as possible
characterized with data available.
53
4.6.1. Normal operations
Fugitive emissions
Media
Origin
Destination
Quantity
Composition
Gas
Valves, pump seals, flanges, open-ended valves, relief valves,
compressor seals, drains, sample connections.
Diffuse in the atmosphere
Can be estimated with the Average Factor Method
VOCs
Fugitive emissions are not negligible. They can be estimated thanks to the Average Factor
Method which is explained in the part A of this report.
Gas relieves
Media
Origin
Destination
Quantity
Composition
Gas
Relief valves
Flare
?
CO2, VOCs, CO, SOx, NOx
Off-gas
Media
Origin
Destination
Quantity
Composition
Gas
Recycle drums
Sour purge
Estimated in process book
H2, VOC, H2S
4.6.2. Intermittent operations
Reduction / reactivation gas
Media
Origin
Destination
Quantity
Composition
Gas
Reactor
Flare
Flowrate in operating instruction
Presence of HC and H2O
Regeneration
Media
Origin
Destination
Quantity
Composition
Gas
Reactor
Atmosphere, decoking drum
Flowrate in operating instruction
CO2, CO, VOCs, coke, green oils
54
4.7. Emissions reduction proposals
Air emissions from hydrogenation processes in normal operations arise from fugitive
emissions, process vents and process heaters for GHU.
In order to reduce fugitive emissions, a leak detection and repair program can be established
(see part A of this report).
Catalyst regeneration is also responsible for air emissions. In order to minimize emissions,
appropriate gas treatment systems must be used (such as scrubbers). This measure removes
pollutants (green oils) from air to water which still need to be cleaned. Generally it is better to
perform ex-situ regeneration in specific factories which have better techniques for catalyst
regeneration and effluents treatment. Moreover, coke formation should be minimized by
operating conditions control.
Conclusion
This study constitutes a first approach for the development of a complete and precise
environmental study of Axens commercialized processes.
Part A of the report emphasizes on common emissions occurring in all refining processes. It
allows a general point of view on several issues and gives some solutions to these issues.
Fugitive emissions, flue combustion gases, blowdown systems and wastewater emissions are
dealt with. Methods to qualify and quantify effluents are explains and environmental technical
solutions are cited and shortly described. Part B of the report is more process oriented, and
lightens problems concerning particular effluents. Several important processes are analysed.
The methodology used to study each process can besides be widespread to analyse other
processes.
This study is obviously non exhaustive and many points need to be completed and improved,
however it can be used as the starting point to more focused environmental review for Axens
processes.
55
References
1.
U.S. Environmental Protection Agency (EPA). AP 42 Compilation of Air Pollutant
Emission Factors. United States, 1995.
2.
U.S. Environmental Protection Agency (EPA). Protocol for Equipment Leak Emission
Estimates. (EPA-453/R-95-017). United States, 1995.
3.
Texas Commission on Environmental Quality (TCEQ). Emissions Factors for
Equipment Leak Fugitive Components. (RG-360A). United States, 2008.
4.
U.S. Environmental Protection Agency (EPA). Leak Detection and Repair Compliance
Assistance Guidance Best Practices Guide. United States, 2007.
5.
U.S. Environmental Protection Agency (EPA). Alternative Control Techniques
Document: NOx Emissions from Process Heaters. United States, 1994.
6.
U.S. Environmental Protection Agency (EPA). Alternative Control Techniques
Document - NOx Emissions from Industrial/Commercial/Institutional (ICI) Boilers.
(EPA-453/R-94-022). United States, 1994.
7.
California Air Resources Board. Determination of Reasonably Available Control
Technology and Best Available Retrofit Control Technology for Industrial, Institutional,
and Commercial Boilers, Steam Generators, and Process Heaters. United States, 1991.
8.
RTI Health, Social, and Economics Research. Economic Analysis of Air Pollution
Regulations: Boilers and Process Heaters. United States, Research Triangle Park, 2002.
9.
MARTINO G. Réformage catalytique, in Le raffinage du pétrole, tome 3, Procédés de
transformation, Leprince, P. (ed.), Éditions Technip, 1998, p. 105-173.
10.
TRAVERS C. Isomérisation des paraffines légères, in Le raffinage du pétrole, tome 3,
Procédés de transformation, Leprince, P. (ed.), Éditions Technip, 1998, p. 237-264.
11.
HENRICH G., KASZTELAN S. Hydrotraitements, in Le raffinage du pétrole, tome 3,
Procédés de transformation, Leprince, P. (ed.), Éditions Technip, 1998, p. 549-590.
12.
DECOOPMAN F. Traitement des eaux, in Le raffinage du pétrole, tome 3, Procédés de
transformation, Leprince, P. (ed.), Éditions Technip, 1998, p. 657-684.
13.
EC/R Incorporated Timberlyne Center. Stationary Source Control Techniques
Document for Fine Particulate Matter. North Carolina, United States, 1998.
14.
U.S. Environmental Protection Agency (EPA). Profile of Petroleum Refining Industry.
Office of Enforcement and Compliance Assurance, United States, 1995.
15.
World Bank Group. Environmental, Health, and Safety Guidelines for Petroleum
Refining, 2007.
56
16.
United Nations Environment Programme. Standardized Toolkit for Identification and
Quantification of Dioxin and Furan Releases, 2001.
17.
U.S. Environmental Protection Agency (EPA). An Inventory of Sources and
Environmental Releases of Dioxin-Like Compounds in the United States for the Years
1987, 1995, and 2000. United States, 2006.
18.
U.S. Environmental Protection Agency (EPA). Study of selected petroleum refining
residuals - Industry study. United States, 1996.
19.
XP CEN/TS 1948. Stationary source emissions - Determination of the mass
concentration of PCDDs/PCDFs and dioxin-like PCBs. France, 2007.
20.
BEYCHOK M. R. Aqueous wastes from petroleum and petrochemical plants. John
Wiley and Sons, 1967.
57
Appendix 1: Average Emission Factors
SOCMI Average Emission Factors
Equipment type
Service
Emission Factor
(kg/hr/source)
Valves
Gas
Light liquid
Heavy liquid
0.00597
0.00403
0.00023
Pump seals / Agitator seals
Light liquid
Heavy liquid
0.0199
0.00862
Compressor seals
Gas
0.228
Pressure relief valves
Gas
0.104
Connectors
All
0.00183
Open-ended lines
All
0.0017
Sampling connections
All
0.0150
SOCMI factors are used to determine equipment leak emissions from chemical plants or
chemical processes within refineries.
These factors are for total organic compound emission rates.
Refinery Average Emission Factors
Equipment type
Service
Emission Factor
(kg/hr/source)
Valves
Gas
Light liquid
Heavy liquid
0.0268
0.0109
0.00023
Pump seals / Agitator seals
Light liquid
Heavy liquid
0.114
0.021
Compressor seals
Gas
0.636
Pressure relief valves
Gas
0.16
Connectors
All
0.00025
Open-ended lines
All
0.0023
Sampling connections
All
0.0150
58
Refinery factors are used to determine equipment leak fugitive emissions from a refinery
process. For a chemical process located within a refinery that is not specifically considered a
refinery process (for example, an MTBE production unit), the SOCMI factors must be used
rather than the refinery factors.
These factors are for non-methane organic compound emission rates.
Oil and Gas Production Operations Average Emission Factors
Equipment type
Valves
Pump seals
Others
Connectors
Flanges
Open-ended lines
Service
Gas
Heavy oil
Light oil
Water / Oil
Gas
Heavy oil
Light oil
Water / Oil
Gas
Heavy oil
Light oil
Water / Oil
Gas
Heavy oil
Light oil
Water / Oil
Gas
Heavy oil
Light oil
Water / Oil
Gas
Heavy oil
Light oil
Water / Oil
These factors are for total organic compound emission rates.
59
Emission Factor
(kg/hr/source)
4.5 E-03
8.4 E-06
2.5 E-03
9.8 E-05
2.4 E-03
Non available
1.3 E-02
2.4 E-05
8.8 E-03
3.2 E-05
7.5 E-03
1.4 E-02
2.0 E-04
7.5 E-06
2.1 E-04
1.1 E-04
3.9 E-04
3.9 E-07
1.1 E-04
2.9 E-06
2.0 E-03
1.4 E-04
1.4 E-03
2.5 E-04
Appendix 2: Example calculation of fugitive emissions
Assuming a process operating gaseous product, calculation of fugitive emissions can be done
like following.
Data for example calculation:
Equipment type /
service
Valves / gas
Compressors / gas
Relief valves
Open-ended lines
Sampling connections
Equipment count
Hours of operation*
(hr/yr)
8760
8760
8760
8760
8760
272
3
37
489
24
VOC wt. fraction
1
1
1
1
1
* Hours of operation include all time in which material is contained in the equipment.
Source
Valves
Compressor seals
Relief valves
Open-ended lines
Sampling connections
Equipment
count
272
3
37
489
24
Emission factor
(kg/hr/source)
0.0268
0.636
0.16
0.0023
0.015
VOC emissions
rate (kg/hr)
7.2896
1.908
5.92
1.1247
0.36
Total VOC
emissions (t/yr)
VOC emissions rate = equipment count * wt. fraction * emission factor
Total VOC emissions = sum [VOC emissions rate] * hours of operation
60
145.4
TRITA-IM 2008:39
ISSN 1402-7615
Industrial Ecology,
Royal Institute of Technology
www.ima.kth.se
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