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Process Analytics in Ethylene Oxide and Ethylene Glycol Plants Chemical Industry

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Process Analytics in Ethylene Oxide and Ethylene Glycol Plants Chemical Industry Case Study Process Analytics in Ethylene Oxide and Ethylene Glycol Plants Chemical Industry Perfect solutions from Siemens Process Analytics Ethylene Oxide (EO) Ethylene oxide, C2H4O, under normal conditions, is a colorless, flammable gas. It is very toxic and carcinogen. Industrial production started in 1925 using the chlorohydrin process and was improved in 1931 by introducing the much more economic direct catalytic oxidation method. Currently, almost all ethylene oxide production plants are based on the direct oxidation process with air or oxygen using a silver based catalyst. CO 2 and water are produced as by-products of the reaction. Because EO reacts readily with many chemicals, it is one of the most versatile intermediates in the production of several industrial chemicals, the most notable of which is ethylene glycol. It is also used as a sterilant for medical equipment. However, most of it is converted into products such as fibers, foils, bottles, plasiticizers, solvents, antifreezes, cosmetics, sport articles, CDs etc. Ethylene Glycol (EG) Ethylene glycol (C2H4(OH)2) is produced from ethylene oxide through a catalytic reaction with water at higher temperature resulting in a yield of mono-ethylene glycol (MEG), known as glycol, and the by-products di-ethylene glycol (DEG) and tri-ethylene glycol (TEG). Ethylene glycol is commonly used as an antifreeze agent in automobile cooling systems. It is also used in deicing solutions for aircraft and boats. Other uses include solvents for the paint and plastic industry, and hydraulic brake fluids. In pure form it is a colorless clear liquid with a sweet taste and a slightly syrupy texture. If ingested, ethylene glycol can damage the kidneys, heart and nervous system. Integrated EO/EG plants Modern EO/EG plants are highly integrated units where part or all of the EO produced in the EO section can be recovered as glycols, if desired, in the glycol section. Plant integration allows for significant savings in utilities as well for the recovery of all bleed streams as high-grade-products instead of lower grade products in the case of non-integrated plants. usa.siemens.com/analyticalproducts Ethylene Oxide and Ethylene Glycols Ethylene oxide production Ethylene The major production steps of the oxygen- 1 Ethylene 7 CO based EO process are: 2 oxide r Oxygen 8 6 • Ethylene (acc. to IUPAC: Ethene) and Mixe oxygen are mixed with recycle gas and, Plant 2 removal removal after adding a moderating substance 2 2 area CO Chloroethane CO such as chloro-ethane, fed into a multi- EO purification 9 tubular reactor. There, ethylene oxide Natural gas 5 Crude EO (EO) is selectively produced utilizing a 3 8 silver-based catalyst at 200 to 300 °C r and 10-20 bar. Along with ethylene oxide (80-85 %), CO2, H2O and heat are Steam r Acqueous EO to Stripper glycol section generated. Reaction heat is recovered by Reacto boiling water at elevated pressure on the Water EO absorbe reactors shellside. It is used at different locations of the plant. 4 + ½ O C H O • EO contained in the reactor product gas 2 2 4 CH 2 = CH 2 enters the EO absorber section where EO Cycle gas + 3 O2 2 CO 2 + 2 H2O is scrubbed from the gas by water. The EO-containing water is concentrated by stripping producing crude EO which is Fig. 1: Ethylene oxide production process with measuring points (see details in table 1) suitable for feeding directly to a glycol producing plant. When pure EO is the desired final product, the crude EO is fed Natural gas, however, is commonly CO2 and water will occur explosively. to a purification column. contaminated with gaseous sulfur Process operation is desired under • The cycle gas leaving the absorber is fed compounds that are known as poison to conditions which will maximize the to the CO2 removal section, where CO2 the silver catalysts. conversion of ethylene to ethylene oxide (a by-product of the EO reaction) is • Catalyst selectivity is an important yet avoid safety problems. In order to find recovered. Some of the CO2 remains in parameter in EO production and should such an optimum, gases such as nitrogen the cycle gas and returns to the EO be as high as possible. Usually, during and methane are fed to the cycle gas and reactor. catalytic processes, other competing mixed with the reaction by-product reactions can take place, and reactants carbon dioxide, and argon, which enters as Process constraints are converted into undesired products. an impurity in the oxygen feed. The goal is The ratio between desired products and to find an optimum mixture which permits Various constraints exist regarding safe the undesired products is called catalyst operation of the process at maximum operation, product quality and plant selectivity. Catalyst selectivity is concentrations of oxygen and ethylene efficiency in running and optimizing the optimized by adding moderating thus increasing the selectivity of the EO production process: substances such as chloro-ethane. ethylene present to ethylene oxide and, on the other side prevents from the danger of • Oxygen is required as reactant to run Control of cycle gas composition explosion. the process and added to the cycle gas. However, at a certain concentration The desired product ethylene oxide Process analyzer are used to find this level (known as flammable limit) in the represents only a relatively small optimum mixture and to keep the gas gas mixture, oxygen will cause the percentage of the total effluent stream concentrations at the predetermined levels danger of a gas explosion. Therefore, leaving the reactor. The remainder of the during the process. See table 1 for the content of oxygen in the cycle gas reactor effluent comprises several diluents measuring details. must be monitored continuously with and reaction by-products. The task of high accuracy and reliability. the diluents is to prevent the gas mixture • Methan is able to increase the to reach unwanted combustibility levels flammable limit (which is treated as during the reaction. If the flammability positive effect) and, hence, is added to limit is reached or exceeded, the cycle gas in the form of natural gas. the complete oxidation of ethylene to 2 Ethylene glycol production Sampling Point Measuring Measuring Meas. Siemens Sampling Stream Task Components Range Analyzer The aqueous EO solution from the 1 Ethylene feed Ethylene purity to avoid C2H2 % MAXUM II ethylene oxide production section is sent catalyst poisoning S compounds to the gylcol section. There, ethylene glycol (MEG, Monoethylene glycol) is 2 Cycle gas Process control CO 0 ... 5 MAXUM II produced from EO by reacting it with 2 after mixer CH 0 ... 80 water at a 10 to 1 ratio of water to EO. 4 C2H4 0 ... 40 This excess of water helps to reduce C2H6 0 ... 2 by-product formation. Some higher glycols C2H4O 0 ... 3 are produced as co-products: diethylene N2 0 ... 20 glycol (DEG) and triethylene glycol (TEG). Ar/O2 0 ... 25 The reactor product is sent to a multi- H2O 0 ... 3 effect evaporation train for removal of the C2H4 0 ... 40 ULTRAMAT 6 water from the glycols in three successive CO2 0 ... 5 stages. The glycols are then sent to the Explosion protection O2 0 ... 12 OXYMAT 6 fractionation train where the MEG, DEG (LEL monitoring) O2 0 ... 12 OXYMAT 6 and TEG products are recovered and O2 0 ... 12 OXYMAT 6 purified. 3 Cycle gas Cycle gas composition Cl-HCs MAXUM II at reactor inlet CI-Hcs Final product samples are collected from the overhead or side stream of the 4 Cycle gas at Process control CO 0 ... 5 MAXUM II purification towers (fractionators). A 2 reactor outlet CH 0 ... 80 variety of process analysis are performed, 4 C2H4 0 ... 40 including water content, MEG, DEG or TEG C2H6 0 ... 2 (see table 2). Other measurements are C2H4O 0 ... 3 color, acidity, aldehyde impurity content N2 0 ... 20 and TOC (fig. 2). The product must meet Ar/O2 0 ... 25 sales specifications prior to being released H2O 0 ... 3 for shipment to customers. Otherwise the Explosion protection O2 0 ... 12 OXYMAT 6 product is normally reprocessed. (LEL monitoring) O2 0 ... 12 OXYMAT 6 O2 0 ... 12 OXYMAT 6 5 EO absorber Cycle gas composition: C2H4 0 ... 40 MAXUM II overhead Process control C2H6 0 ... 2 C2H4O 0 ... 0,05 Ar/O2 0 ... 30 CO2 0 ... 30 H2O 0 ... 10 6 EO product Product quality Formaldehyde MAXUM II Acetaldehyde C2H4, CO2, H2O 7 CO2 absorber Cycle gas composition C2H4 0 ... 40 MAXUM II overhead C2H6 0 ... 2 CvH4O 0 ... 0,05 Ar/O2 0 ... 30 CO2 0 ... 30 H2O 0 ... 10 8 EO to glycol Aldehyde content Aldehyde TPA section 9 Plant area Waste water monitoring EO in water MAXUM II TOC TPA Fig. 2: TOC system (ULTRAMAT 6 Table1: Ethylene oxide process, measuring points (TPA: Third Party Analyzer) based third party analyzer) 3 Process Gas Chromatography The MAXUM gas chromatograph fullfills the requirements for glycols quality control Aqueous Recycle water perfectly: ethylene oxide Steam Water MEG DEG TEG • Accurate analysis by optimized injection: Water Injection module with best evaporization 3 characteristics for high boiling samples (MEG, boiling point 194-205°C, DEG: Plant 242-247°C, TEG: 278°C) avoid area discrimination effect or hydration to r 4 r water. • Repeatable analysis by interference free Reakto Reacto Evaporator Evaporator Evaporator Fractionator Fractionator Fractionator Fractionator separation. Safe separation of trace components is performed by elimination 1 2 of main components by heart-cut. Used analyitcal tools: Siemens live switching in combination with capillary columns. MEG/DEG/TEG: Monoethylene glycol / Diethylene glycol / Triethylene glycol Verification of the results by additional control parameter: cut-rest of main component, GC oven and ambient temperature.
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