TDLAS. Application Note. Use of TDLAS Based Analyzers for Real-Time Process Control in Ethylene Oxide Production PROCESS INSTRUMENTS

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1 PROCESS INSTRUMENTS Application Note Use of Based Analyzers for Real-Time Process Control in Ethylene Oxide Production Ethylene oxide (EO), which is derived from ethylene, is a high volume chemical feed stock used to make a variety of other chemicals. The most important in terms of volume is the production of ethylene glycol which consumes more than 60% of the ethylene oxide produced. Ethylene glycol is primarily used as an antifreeze agent in motor vehicle radiators and at airports to remove ice from runways and aircraft. Ethylene oxide can also be converted by thermal hydration into mixture of glycols, namely monoethylene glycol, diethylene glycol and triethylene glycol which are used in cosmetic and pharmaceutical industries as a base material to carry active ingredients. These glycols are also used in the ink and printing industry as antifoam agents in distillation processes. In addition ethylene oxide is also used for production of ethoxylates and polyether polyols which are used in the manufacture of biodegradable detergents and polyurethane resins and as a solvent for paints and lacquers. Another use of ethylene oxide is in the production of ethanolamine by reaction with ammonia. Ethanolamine is used as a feedstock in the production of detergents, emulsifiers, polishes, pharmaceuticals, corrosion inhibitors and other chemical intermediates. The Production of Ethylene Oxide To operate with maximum yield and safe conditions ethylene oxide production requires a sophisticated process control system. There are a number of constituents in the process that must be analytically measured in real time. This application note will describe the use of Tunable Diode Laser Absorption Spectroscopy () for the measurement of oxygen, carbon dioxide and ethylene in the ethylene oxide production process. Ethylene oxide is produced by the carefully controlled oxidation of ethylene over a silver catalyst. Controlling the concentrations of the compounds in the reactor and the temperature/ pressure is critical. If the conditions are too mild, little ethylene oxide is produced but if they are too severe, the reaction runs away and carbon dioxide and water are the only products. If silver oxide is used as a catalyst the dominant reaction is formation of ethylene oxide. Some ethylene still ends up being further oxidized (as much as 25% in some processes) resulting in the production of carbon dioxide and water as main by-products through this reaction. Both reactions are exothermic, each gram mole of ethylene converted to ethylene oxide produces 25 kcal of heat and each gram mole of ethylene converted to carbon dioxide and water produces 316 kcal of heat. Temperature is a prime concern in the process design. The temperature of the oxidation is controlled by a heat exchanger built into reactor. The reaction temperature is controlled to ºC and is under a pressure range of atm. The residence time of the ethylene/ oxygen feed in the reactor is only about one second.

2 The ethylene oxide produced represents only a small fraction of the total effluent stream leaving the reactor with the majority being unconverted ethylene. As a result, ethylene oxide manufacturing requires recycle of ethylene back into the reactor. The limit of the conversion of ethylene and oxygen to ethylene oxide during each pass through the reactor is determined by the oxygen concentration in the reactor. Normally, ethylene concentrations in the reactor are in the range 20 40% by volume and oxygen concentration is dictated by the flammable limit. To prevent an explosive condition the oxygen must be limited to less than ~10%. To increase the flammable limit, diluents in the form of nitrogen, argon, methane, ethane or natural gas are added to the reactor mixture. Gas Component, mole% or ppm Reactor Inlet Reactor Outlet Ethylene Oxygen 8 6 Ethylene Oxide Carbon Dioxide 2 3 Water Argon 7 7 Nitrogen 3 3 Methane Ethane Ethylene Dichloride 0.2 ppm 0.2 ppm Vinyl Chloride 1.2 ppm 1.2 ppm Table 1. Typical Stream Composition for the Reactor with Methane Ballast As mentioned previously, carbon dioxide is formed in the reaction. This carbon dioxide is mostly removed in the processing of the recycle stream but it can be partially left in the recycle gas to act as a diluent. To optimize the ethylene oxide yield, catalyst selectivity can be increase by adding moderators such as 1,2-dichloroethane or vinyl chloride. Typical gas stream compositions for the inlet and outlet of the reactor are shown in Table 1. The ethylene oxide production process is shown in Figure 1. The ethylene oxide product is recovered from the reactor effluent gases by absorption in aqueous absorbent. The ethylene oxide is then stripped from the absorbent and separated from the residual water, and light ends by fractionation. The aqueous absorbent from the ethylene oxide stripper is recycled back to ethylene oxide absorber. The overhead stream containing ethylene and carbon dioxide is sent to carbon dioxide absorber where carbon dioxide is partly removed from the recycling gas by absorption in a hot potassium carbonate solution. 3 Methane % Ethylene % Oxygen % Carbon Dioxide % Ethylene Oxygen Diluent Moderator Methane % Ethylene % Oxygen % 1 Reactor 2 Ethylene Recycle Methane % Ethylene % Oxygen % Carbon Dioxide % EO % EO Absorber CO2 Absorber CO2 Stripper EO Desorber 4 5 Methane ppm Ethylene ppm EO ppm Carbon Dioxide Off Gas Methane ppm Ethylene ppm EO ppm Ethylene Oxide Figure 1. Typical ethylene oxide process with key analysis points. 2

3 From the carbon dioxide absorber overhead the recycle ethylene gas is sent back to the reactor for a new cycle. The absorbent from the carbon dioxide absorber is sent to carbon dioxide stripper where carbon dioxide is separated and then vented to atmosphere. The potassium carbonate is returned to the carbon dioxide absorber. The process is controlled to optimize the conversion of ethylene to ethylene oxide but at the same time must assure that there are safe levels of oxygen in the reactor. Carbon dioxide in the reactor outlet is an undesirable side reaction product. The carbon dioxide creation consumes valuable ethylene and needs to be minimized through process control. Optimizing the relative concentrations of oxygen and ethylene also requires real-time process analyzers. Historically ethylene oxide plant analyzers have included paramagnetic oxygen analyzers for oxygen measurements at the inlet and outlet of the reactor, infrared filter photometers for inlet ethylene feed control and Process Gas Chromatographs (PGC s) to measure all of the components of the gas at reactor outlet. Recent advances in Tunable Laser Diode Laser Absorption Spectroscopy () based analyzers can provide measurements for closed loop process control for product yield optimization, plant monitoring and alarms, quality control and environmental safety. experimental protocol is simply a matter of uploading a file. The Model 5100HD offers significant advantages over prior analytical approaches for customers monitoring ethylene, oxygen and carbon dioxide in ethylene oxide manufacturing processes. Oxygen Measurements The purpose of oxygen measurement in the ethylene oxide reactor is to maximize the oxygen concentration while controlling to a limit of around 8% to eliminate the possibility of an explosive condition. The typical oxygen concentration measurement range for this application is 0 12 % by volume. There are two primary locations for the measurement of oxygen in ethylene oxide plants. These locations are inlet and outlet of the ethylene oxide reactor. Paramagnetic oxygen analyzers have been traditionally used in the ethylene oxide manufacturing process. According to Siemens, a leading manufacturer of paramagnetic analyzers, the common practice is to use three paramagnetic analyzers per sample point to assure a 99% up time. based oxygen analyzers can be successfully used for this application without the need for this level of redundancy. is non-contact analysis technique with long term stability, high specificity, selectivity and reliability. analyzers offer the Obviously, control of the ethylene oxide plant performance cannot be solely limited to analyzers. However, there are applications where they can be used very effectively including measurements of ethylene, oxygen and carbon dioxide. AMETEK s Model 5100HD shown in Figure 2 is an extractive type gas analyzer designed for hot/ wet sample analysis. There is no requirement for complicated sample conditioning although a heated transfer line may be required to keep the sample in the gas phase. The 5100HD offers high specificity and sensitivity and uses a sealed reference cell for laser line locking and continuous analyzer performance verification. The analyzer uses a digital implementation of Wavelength Modulation Spectroscopy (WMS), where changing the Figure HD analyzer 3

4 advantages of faster response time and low cost of installation and ownership in comparison with conventional techniques such as paramagnetic devices and PGC s. The AMETEK 5100 HD oxygen analyzer uses a vertical cavity surface emitting diode (VCSEL) laser based source. Like all AMETEK analyzers, it employs an all-digital protocol for the modulation of the laser drive current and the demodulation of the detector response and a reference cell. The reference cell provides laser line lock and allows continuous verification of the performance of the analyzer. The optical set up for the measurements is shown in Figure 3. The beam from the laser diode is collimated with a plano-convex lens, which is used as the window of the sample cell. Since the absorption signal is weak, an extended path length sample cell is used. Multiple reflections of the laser light through the sample cell is achieved using a spherical mirror with a flat mirror at the focal point. Another long focus lens is used as an optical output window of the sample cell and operates as a condenser. On the output of the sample cell, the beam is divided in to two parts by a beam splitter. Signals from sample and reference cell silicon photodiode detectors are input to separate channels of the main electronics unit. sample compartment oven. The laser, reference cell and photo diodes are located in the main electronics compartment, isolated from the heated sample compartment. The analytical performance of the oxygen analyzer has been evaluated at various customer sites within the required specification ranges of pressure and temperature. There are no spectral interferences from other compounds in the measurement of oxygen in ethylene oxide gas production. The data shown in Figure 4 represent the response of the instrument to a series of oxygen challenges in the concentration range of interest. The duration of each of the challenges was 60 minutes or more with return to the 0% gas baseline value between challenges. The speed of the T90 response time was 20 seconds and was determined using a flow rate of 2L/min. The data acquisition Figure 3. Optical Set Up for AMETEK 5100 HD Oxygen Analyzer. The sample cell temperature is controlled with accuracy of +/- 0.1 C and can be set in the range of 40 C 150 C by setting the temperature of the Figure 4. Oxygen in Ethylene Methane Gas Stream. Response to a Series of Concentration Challenges (A), Speed of the Response (B). 4

5 rate was 2 seconds/measurement. The standard deviation of the reading on each of the challenges was between 0.01% and 0.02% of the oxygen concentration. The measured accuracy over the concentration range was better than 0.1%. The AMETEK 5100 HD oxygen analyzer demonstrates a high level of repeatability. Instrumental drift during 15 hours is less than 0.05% of the oxygen concentration. These data are shown in Figure Oxygen, % Average = %, STD = 0.013% Time, hours Figure 5. AMETEK 5100 HD Oxygen Analyzer Drift. It is known that at high concentrations carbon dioxide adversely affects the selectivity of the catalyst used in ethylene oxide production. Despite the fact that the concentration of carbon dioxide in the reactor is relatively low in comparison with ethylene and methane, it is important to monitor the concentration to maximize the yield of ethylene oxide. As undesired product of the reaction, carbon dioxide must be continuously removed from the ethylene recycle stream by the carbon dioxide absorber stripper system in order to control the carbon dioxide levels in the reactor. Process Gas Chromatographs (PGC s) have been used historically to monitor carbon dioxide in the ethylene oxide production process. However, despite the high sensitivity of PGC s the relatively long measurement time (2-3 minutes) is a serious disadvantage. -based measurements provide real time monitoring with data acquisition rate of typically 2 seconds. As was mentioned above the residence time of the feed in the reactor is only about 1 second. It is known that during regular plant startups with a new catalyst the concentration of carbon dioxide can change suddenly. To maximize the yield of ethylene oxide it is important to have a real time monitoring of the carbon dioxide concentration. During one year of field trials in a similar control application under the harsh conditions of a refinery waste water treatment plant oil/water separator head space, the analyzer had zero failures versus nine failures of the paramagnetic analyzer running in parallel. Carbon Dioxide Measurements There are several process control applications for carbon dioxide measurements at ethylene oxide plants: Reactor inlet (0-5% CO 2 volume) Reactor outlet (0-5% CO 2 volume) Ethylene oxide stripper overhead (0 30% CO 2 volume) Carbon dioxide stripper overhead (0 30% CO 2 volume) A schematic representation of the AMETEK 5100 HD analyzer for measurements of carbon dioxide is shown in Figure 6. Examples of the 5100 HD carbon dioxide analyzer s performance for the 0 5% is shown in Figure 7. The data shown in Figure 7 are the responses of the instrument to a series of carbon dioxide challenges over the concentration ranges of interest. The duration for each challenge was several minutes. The response time (T90) was 30 seconds at a flow rate of 2L/min. The data acquisition rate was 2 seconds per measurement. 5

6 Figure HD Optical Set Up Used for Carbon Dioxide Measurements. The standard deviation of the readings was 25ppmv for measurements in the 0 5% range. The accuracy in the range 0 5% was 100ppmv. Ethylene Measurements The optimal conditions for the ethylene oxide production process may be determined by real time monitoring of ethylene concentrations at the different locations of the process. Fast and accurate analysis of feedstock and effluent streams are needed to maximize conversion efficiency and catalyst selectivity ratios. Historically, filter IR photometers have been used to provide ethylene monitoring in the ethylene oxide production process. Filter photomers require frequent zero measuremens which is undesirable for real-time process control. -based measurements provide real time monitoring with a data acquisition and analysis rate of a few seconds. As was mentioned above, the residence time of the feed in the reactor is only about 1 second. Also during normal plant startups with new catalyst and unusual process events, the concentration of ethylene can change suddenly. To optimize the EO yield it is important to have real time monitoring of the ethylene concentration in the process. Taking into account the need to measure ethylene in the inlet and outlet of the reactor AMETEK offers a analyzer with dual sample cells. A photo of the 5100 HD analyzer with two sample cells and two sets of inlet and outlet valves is shown in Figure 8. This configuration allows simultaneous measurements of the ethylene concentration in two separate gas streams with results for each stream reported every two seconds. Figure 7. Response to a series of concentration CO2 challenges in the range 0 5%. Figure 8. AMETEK 5100 HD analyzer with dual sample cells. 6

7 A schematic representation of the dual cell 5100 HD analyzer is shown in Figure 9. The measurement of ethylene is performed with two Distributed FeedBack (DFB) lasers. The outputs of both lasers are coupled into single-mode optical fibers, which in turn are connected to fiber-optic beam splitters. The splitters are used to divide the optical power in a 50/50 ratio for use in the sample and reference measurements, respectively. Reference cell contains a known concentration of ethylene in a non-absorbing matrix. The reference cell is used to lock the output radiation wavelength of the laser for both sample channels. Figure 10. Validation of the ethylene analyzer performance. Response to a series of ethylene challenges. Figure 9. Schematic diagram of the dual cell 5100 HD analyzer for ethylene measurements. The data shown in Figure 10 represents the response of the instrument to a series of ethylene challenges in a concentration range of 0-50%. The duration of each of the challenges was approximately 30 minutes with return to the 0% gas baseline, which was represented by methane. The T90 response time was 20 seconds at a flow rate of 2L/min. A measurement for each stream is completed every two seconds. rate of 2L/min. No significant trends or correlations with the environmental temperature or sample pressure were observed in the data. Over the 24 hour period, a mean value of 0.5% of ethylene, with a standard deviation of 0.01%, was recorded; the value of the drift was less than 0.03% ethylene. The standard deviation of the ethylene readings on each of the challenges was between 0.1 % and 0.3% of the ethylene concentration. The value of the accuracy evaluated at the levels of ethylene from 0 to 50% was in the range % ethylene. Instrumental drift during 24 hours is shown in Figure 18. During the drift test, a sample with 0.5% ethylene in nitrogen was run through the sample cell at a flow Figure 11. Ethylene Analyzer 24 Hours Drift. 7

8 USE OF BASED ANALYZERS FOR REAL-TIME PROCESS CONTROL IN ETHYLENE OXIDE PRODUCTION Conclusion The monitoring of oxygen, carbon dioxide and ethylene in the ethylene oxide production process are critical applications that demand an analyzer with a near 100% availability to maximize the production of ethylene oxide and assure plant safety. The built-in verification feature of the 5100 HD provides a real-time check on the analyzer performance. Fast response, no sample conditioning, and immunity to interference from moisture make laser based AMETEK model 5100 HD a superior and dependable choice. Application Note 150 Freeport Road, Pittsburgh, PA Ph , Fax , by AMETEK, Inc. All rights reserved. Printed in the U.S.A. F-0438 Rev. 1 (0514) One of a family of innovative process analyzer solutions from AMETEK Process Instruments. Specifications subject to change without notice. SALES, SERVICE AND MANUFACTURING: USA - Pennsylvania 150 Freeport Road, Pittsburgh PA Tel: , Fax: USA - Delaware 455 Corporate Blvd., Newark DE Tel: , Fax: Canada - Alberta 2876 Sunridge Way N.E., Calgary AB T1Y 7H2 Tel: , Fax: WORLDWIDE SALES AND SERVICE LOCATIONS: USA - Houston, Texas Tel: , Fax: BRAZIL Tel: CHINA Beijing / Tel: , Fax: Chengdu / Tel: , Fax: Shanghai / Tel: , Fax: USA - Austin, Texas Tel: , Fax: FRANCE Tel: , Fax: GERMANY Tel: , Fax: INDIA Tel: , Fax: SINGAPORE Tel: , Fax: