Improving Ethylene Oxide Process Control with the Prima PRO Process Mass Spectrometer

Improving Ethylene Oxide Process Control with the Prima PRO Process Mass Spectrometer
Graham Lewis, Thermo Fisher Scientific, Winsford, Cheshire, United Kingdom
Key Words
• Selectivity
• Carbon Balance
• Oxygen Balance
• Organic Chlorides
• Magnetic Sector
• Rapid Multistream Sampler
• Process Optimization
• Online Gas Analysis
Introduction
Ethylene oxide (EO) is an important chemical intermediate with global production capacity around
30 million tonnes per year. It is extremely reactive because its highly strained ring structure can easily
be opened, and is therefore one of the most useful and versatile chemical intermediates. Products derived
from EO include:
Ethylene glycols: used to produce antifreeze, coolants, polyester and polyethylene terephthalate (PET)
Glycol ethers: used to produce brake fluids, hydraulic fluids, detergents, paints, lacquers and solvents
Ethanolamines: used to manufacture detergents, and to purify natural gas
Ethoxylates: used to manufacture detergents, dispersants, surfactants and emulsifiers
Around 75% of EO is converted to ethylene glycol; as the EO derivatives market is highly competitive,
and feedstock prices are volatile, it is vital that EO process economics are optimized. At the same time,
the EO process is inherently potentially hazardous, so great care must be taken to operate the plant
safely. Process gas analysis plays an important part in both of these areas; this paper will describe the
benefits of process mass spectrometry in general and the Thermo Scientific™ Prima PRO Process Mass
Spectrometer in particular in improving process efficiency while maintaining process safety.
A ppl i cati o n N o tes
Improving Ethylene Oxide Process Control
with the Thermo Scientific Prima PRO
Process Mass Spectrometer
2
The Process
EO is produced commercially by the vapor-phase reaction of ethylene and oxygen over a silver-based catalyst.
There are two possible reactions that can occur:
Main Oxidation Reaction
Ag C2H4 + ½ O2
C2H4O
Side Oxidation Reaction
Ag
C2H4 + 3O2
2CO2 + 2H2O
Both reactions are exothermic. The side reaction is not only completely inefficient, producing no EO, it is over
10 times more exothermic than the main reaction, so much so that large amounts of heat must be removed
from the process to avoid the risk of explosion. The key process control strategy is therefore to maximize the
primary partial oxidation reaction while minimizing or eliminating the competing side reaction to complete
oxidation. This is defined as the selectivity, the number of moles of EO produced in the reactor per 100 moles
of ethylene converted. The key role of the catalyst is to maximize this selectivity.
Process control must ensure that the catalyst maintains its activity for as long as possible, since this is costly
and time consuming to replace. Selectivity is improved by adding various organic chloride inhibitors, designed
to slow preferentially the side oxidation reaction. These must be maintained and monitored at low ppm levels
in the reaction mixture.
As the process generates relatively low levels of EO, process gas is recycled to increase the EO concentration.
In principle, the EO concentration could be increased by increasing the feed gas oxygen content. However,
EO and oxygen can react explosively; the chances of this occurring are minimized by adding methane to the
stream to make it "fuel rich".
EO Process: Analytical Requirements
Gas analysis plays an important part in optimizing the EO process; Figure 1 shows a simplified schematic of
a typical EO process, with gas analysis sample points identified. Table 1 shows typical reactor inlet and outlet
compositions that need to be measured.
Figure 1: Schematic of typical EO process.
Reactor
inlet
A
Organic
chloride
added
Reactor
R1
Reactor
outlets
A Analysis points
A
Reactor
R2
A
Reactor
R3
EO
absorber
A
Compressor outlet
A
C2H4 + O2
introduced
CO2
absorber
Compressor
Reactor
R4
A
A
Recycled gas
3
Table 1: Typical stream compositions
for EO inlet and outlet streams.
Component
Typical Reactor Inlet
Concentration %mol
Typical Reactor Outlet
Concentration %mol
Methane
47
48
Water
0.3
0.8
Nitrogen
1
1
Ethylene
30
28
Ethane
1
1
Oxygen
7.6
5.8
Argon
12
12
Ethylene Oxide
0
2.2
Carbon Dioxide
1
1.5
Organic Chlorides
ppm
ppm
By measuring the inlet and outlet gases, important parameters such as Selectivity, Carbon Balance and
Oxygen Balance can be derived. Analysis speed is critical, as is a wide dynamic range. Additionally, the
analyzers must be able to cope with two different balance gases — during normal operation methane is
used as the bulk gas, but for enhanced safety during plant start-up and shut-down this is replaced by
nitrogen.
Advantages of Mass Spectrometry (MS)
Mass spectrometers offer fast, complete stream analysis; typical analysis time including stream switching
is less than 30 seconds per stream for all components including trace chlorides.
Advantages of Prima PRO Process MS
The ethylene oxide process presents a series of challenges to the Process MS. Prima PRO has been designed
to meet and beat these challenges.
Rapid Multistream Sampling
If the MS is to monitor all EO process streams then a fast, reliable means of switching between streams is
required. Solenoid valve manifolds have too much dead volume and rotary valves suffer from poor reliability
so we developed the unique Rapid Multistream Sampler (RMS). It offers an unmatched combination of
sampling speed and reliability and allows sample selection from 1 of 32 or 1 of 64 streams. Stream settling
times are application dependent and completely user configurable. The RMS includes digital sample flow
recording for every selected stream. This can be used to trigger an alarm if the sample flow drops, for example
if a filter in the sample conditioning system becomes blocked.
The RMS can be heated to 120°C and the stream selector position is optically encoded for reliable, software
controlled stream selection. Temperature and position control signals are communicated via Prima PRO’s
internal network. The RMS has a three year warranty as standard; no other multistream sampling device offers
the same level of guaranteed reliability.
Precision of Analysis
The MS is required to monitor a complex mixture of inorganic and organic compounds over a wide range of
concentrations; if the results are to be used as part of a dynamic plant control strategy they must be reliable
and available.
At the heart of the Prima PRO is a magnetic sector analyzer which offers unrivalled precision and accuracy
compared with other mass spectrometers. We produce both quadrupole and magnetic sector mass
spectrometers; over thirty years of industrial experience have shown the magnetic sector analyzer offers the
best performance for industrial on line gas analysis. Key advantages of magnetic sector analyzers include
improved precision, accuracy, long intervals between calibrations and resistance to contamination. Typically,
analytical precision is between 2 and 10 times better than a quadrupole analyzer, depending on the gases
analyzed and complexity of the mixture.
4
A unique feature of the Prima PRO magnet is that it is laminated. Its analysis times are similar to a quadrupole
analyzer, offering the unique combination of rapid analysis and high stability. This allows the rapid and
extremely stable analysis of an unlimited number of user-defined gases. The scanning magnetic sector is
controlled with 24-bit precision using a magnetic flux measuring device for extremely stable mass alignment.
Our enclosed ion source provides high sensitivity, minimum background interference and maximum
contamination resistance. The high-energy (1000 eV) analyzer offers extremely rugged performance in the
presence of gases and vapors that have the potential for contaminating the analyzer.
Software
Thermo Scientific™ GasWorks™ software supports the analysis of an unlimited number of components per
stream, and an unlimited number of user defined calculations (called Derived Values), such as Selectivity,
Oxygen Balance and Carbon Balance. An unlimited number of analytical methods can be set up, so different
analyses can be defined for different process streams. Analog signals, from temperature and pressure
sensors for example, can also be logged, displayed and used in Derived Value calculations. A range of
industry standard protocols are available for communicating with plant process control systems.
Prima PRO Performance
Table 2 shows Prima PRO’s typical performance specification, covering both methane-rich and nitrogen-rich
process conditions. Analysis time including stream switching time is approximately 25 seconds per stream
for all 14 components. This reduces to 15 seconds if measurement of the chlorides is omitted.
Table 2: Typical Prima PRO
performance specification for EO
inlet and outlet streams.
Measurement
Concentration
Range %mol
Typical Reactor Inlet
Concentration %mol
Typical Reactor Outlet
Concentration %mol
Prima PRO Standard
Deviation (8 hours)
Methane
0 - 70
47
48
≤0.03 %mol or 0.1 % relative*
Water
0-4
0.3
0.8
≤0.01 %mol
Nitrogen
0 - 100
1
1
≤0.03 %mol or 0.1 % relative*
Ethylene
0 - 40
30
28
≤0.03 %mol or 0.1 % relative*
Ethane
0-5
1
1
≤0.005 %mol
Oxygen
0 - 10
7.6
5.8
≤0.005 %mol or 0.1 % relative*
Argon
0 - 20
12
12
≤0.005 %mol or 0.1 % relative*
Ethylene Oxide
0-4
0
2.2
≤0.005 %mol
Carbon Dioxide
0 - 10
1
1.5
≤0.01 %mol or 0.1 % relative*
Methyl Chloride
0 - 5 ppm
1 ppm
1 ppm
≤0.2 ppm
Vinyl Chloride
0 - 5 ppm
1 ppm
1 ppm
≤0.2 ppm
Ethyl Chloride
0 - 5 ppm
1 ppm
1 ppm
≤0.2 ppm
Allyl Chloride
0 - 5 ppm
1 ppm
1 ppm
≤0.2 ppm
1,2
Dichloroethane
0 - 5 ppm
1 ppm
1 ppm
≤0.2 ppm
*whichever is greater
5
Figure 2: EO process reactor outlet
concentrations over 10 hours.
Figure 2 shows 10 hours’ data from an EO reactor outlet, displayed in log scale in Thermo Scientific
GasWorks Data Review software. EO output is just 2%, demonstrating the need to recycle the output
to increase yield.
Carbon & Oxygen Balances
If the gas analyzer together with its associated sample conditioning system is to be used as part of the
plant’s control strategy, it must be able to measure all the carbon and oxygen-containing compounds in
the streams with high accuracy. This can be defined by the Carbon Balance and Oxygen Balance respectively,
the certainty that the analyzer is measuring accurately all the carbon and oxygen atoms in the reactor
inlet and outlet streams. Theoretically both of these should be 100%. Prima PRO’s combination of high
performance RMS and high precision magnetic sector analyzer achieve balances of close to 100%, depending
on the accuracy of the calibration gases. MS provides a considerable improvement on GC analyzers that
provide a slower, partial stream composition — GCs do not measure oxygen, so Oxygen Balance calculation
is not possible with GC alone.
Table 3 shows how these balances can be set up in GasWorks software using the Derived Values feature.
Table 3: Carbon Balance and
Oxygen Balance Derived Values
in GasWorks software.
Carbon Balance:
Oxygen Balance:
100 × {CH 4 + CO 2 + 2 × (C 2 H 4 + C 2 H 6 + EO)} [outlet]
×
100
{CH 4 + CO 2 + 2 × (C 2 H 4 + C 2 H 6 + EO)} [inlet]
×
(100 +0.5EO [outlet])
100 × {2 × (O 2 + CO 2) + EO + H 2 O} [outlet]
×
100
{2 × (O 2 + CO 2) + EO + H 2 O } [inlet]
×
(100 +0.5EO [outlet])
Figure 3 shows Carbon and Oxygen Balances measured over 10 hours with Prima PRO. Both are close
to the theoretical perfect value of 100, with excellent precision, verifying the high degree of confidence
that can be placed in Prima PRO’s results.
6
Figure 3: Carbon & Oxygen Balances
on EO process stream over 10 hours.
In practice, the oxygen concentration is such a critical safety measurement that dedicated paramagnetic oxygen
analyzers will be used in addition to the MS.
Table 4: EO Selectivity Derived
Value in GasWorks software.
Selectivity:
100
×
(EO [outlet]
×
100/(100+0.5 × EO [outlet])
–
EO [inlet])
{C 2 H 4[inlet]
–
C 2H4[outlet]
×
100/(100+0.5 × EO [outlet])}
Figure 4 shows the selectivity of an EO process stream measured over 10 hours with Prima PRO.
Figure 4: EO selectivity on process
stream over 10 hours.
Accuracy of MS during start-up with Nitrogen balance
During normal plant operation, methane is used as the balance gas to ensure the process streams are
‘fuel rich’, thereby minimizing the explosion risk. However, for maximum plant safety, methane is replaced
by nitrogen during plant start-up and shut-down. The gas analyzer is therefore required to analyze two very
different stream compositions. Many analyzers do not have the dynamic range to cope with this requirement
without being calibrated for both balance gases.
7
Methane Balance
Table 5: Prima PRO’s ability
to measure methane- and
nitrogen-balance streams with
single calibration.
Prima PRO
Nitrogen Balance
Certificate
Prima PRO
Certificate
Methane
56.4767
56.44 ±1.13
4.9252
5.17 ± 0.1
Nitrogen
5.8252
5.88 ± 0.12
52.13
51.99 ± 1.04
Ethylene
26.2369
26.2 ± 0.52
25.7779
25.6 ± 0.51
Ethane
0.2014
0.2 ± 0.01
0.1966
0.21 ± 0.01
Oxygen
0.8624
0.87 ± 0.02
5.2183
5.15 ± 0.1
Argon
4.9466
4.95 ± 0.10
4.8151
4.92 ± 0.1
Carbon Dioxide
4.9808
4.99 ± 0.10
4.8682
4.96 ± 0.1
Ethylene Oxide
0.4697
0.47 ± 0.01
2.07
2 ± 0.08
Prima PRO has excellent linearity, and can measure both stream types when calibrated for one
balance gas. Table 5 shows an example of a Prima PRO calibrated with methane as the bulk
gas, then analyzing two cylinders representing methane and nitrogen balance process streams.
The observed differences in measured and certificate values are consistent with the
uncertainties of the cylinder certificate values.
Trace organic chloride analysis
The analysis of ppm levels of organic chlorides in the complex matrix of components present at
percentage levels in the EO process presents challenges even to the best gas analyzer. For
example, ethylene glycol can be produced in the reactor outlet stream by the reaction of EO and
water; this produces a peak at mass 62, where vinyl chloride is measured. The effect must be
minimized by operating the sampling system, inlet system and MS ion source at high
temperatures (80-100 °C for the RMS, 140-180 °C for the ion source) and eliminating cold
spots in the sampling system (the minimum temperature at any point between the process
take-off point and the RMS should be 80 °C).
Percentage level components are measured on the Prima PRO’s Faraday detector; ppm level
components are measured using a Secondary Electron Multiplier (SEM) detector. The ion beam
is directed to the appropriate detector by GasWorks software. Prima PRO uses two Micro
Channel Plate (MCP) detectors as the SEM to measure the ppm level chlorides; this minimizes
noise and maximizes signal to optimize performance.
Figure 5 shows an 8 hour run on four typical organic chlorides measured in an EO calibration
gas. Table 6 shows the standard deviations achieved during the test. These are all well within
the specifications detailed in Table 2.
Figure 5: Repeatability test on four
organic chlorides in calibration gas
over 8 hours.
Methyl
chloride
Vinyl
chloride
Ethyl
chloride
Ethylene
dichloride
Mean
3.51
3.53
4.63
0.61
Standard Deviation
0.04
0.06
0.16
0.05
Summary
Prima PRO is the optimal gas analysis solution for both EO process monitoring and EO
catalyst research & development.
•
•
•
•
Enables process optimization by fast, accurate selectivity measurement
System accuracy is validated by measuring carbon and oxygen balances between
99 and 101%
Inorganic and organic components are monitored on one instrument
Dynamic range enables monitoring of component concentrations from ppm to 100%
Prima PRO’s precision of analysis ensures plant operation and catalyst performance are
optimized and plant safety is maximized.
thermoscientific.com/primapro
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A ppl i cati o n N o tes
Table 6: Standard deviations on four
organic chlorides in calibration gas
over 8 hours.
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