Synthesis of Ethylene Glycol Evaluation of Standard Process and Possible Alternatives

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1 Synthesis of Ethylene Glycol Evaluation of Standard Process and Possible Alternatives Miles Beamguard September 4, 2001 Ethylene glycol, a clear, colorless, odorless liquid, is typically synthesized through the catalytic hydration of ethylene oxide. In 1978 the use of ethylene glycol for antifreeze accounted for 48 percent of the total volume produced while polyester film and fibers consisted of 44 percent (Kent). The remaining 8 percent was used for polyester resins for surface coatings and glass fiber luminates, de-icing fluid for airports, and heat transfer agents in refrigeration and electron tubes (Kent). The synthesis of ethylene glycol is usually achieved through feeding an aqueous solution of ethylene oxide into a reactor. The ethylene oxide is hydrated within the reactor to form monoethylene glycol (MEG) as well as smaller amounts of diethylene glycol (DEG) and triethylene glycol (TEG). Upon exiting the reactor the water is evaporated from the glycol-water solution through the use of evaporators and this water is sent back to the head of the system. The concentrated glycol solution is further purified through the use of a stripped and then distilled in a series of vacuum towers to separate the ethylene glycol into MEG, DEG, and TEG. O H H / \ H - C - C - H + H O ---> H - C - C - H 2 H H OH OH ethylene oxide ethylene glycol Another representation of the reaction follows: This reaction proceeds by first protonating the epoxide oxygen, followed by nucleophilic Sn2 displacement. The second step involves nucleophilic Sn2 displacement with H2O as the nucleophile (Wiley).

2 Ethylene Oxide In order to fully understand ethylene glycol synthesis, the production of ethylene oxide should be reviewed. Although ethylene oxide is not the only source for ethylene glycol, it is the preferred method of formation and the greatest use of ethylene oxide is the production of ethylene glycol (Matar). Figure 1: Worldwide Ethylene Oxide Uses (Huang) Ethylene oxide is generally formed in a reactor. This process implements a catalyst, which most commonly is silver based. The reaction takes on the following appearance: H H O H H [O] / \ H+ C = C > - C - C > - C - C - silver water H H catalyst OH OH

3 The act of directly oxidizing ethylene to form ethylene oxide is sometimes called the Lefort process. This process was developed by Union Carbide, and implemented in the world s first plant in Over the course of the next 50 years, several improvements were made to the process. By adding trace chlorides in 1942, Law & Chatwood were able to improve the selectivity. Shell Development Co. improved on direct oxidation by using substantially pure oxygen (Satterfield). Through the many studies and techniques of direct oxidation general guidelines to the process are an optimum reaction temperature between 260 and 280 C when air is used, and 230 C with oxygen. The pressure required is between 1 and 3 MPa with a contact time of approximately 1 second. For any given plant capacities and catalyst, the oxygen-based reactor yields a higher selectivity and requires less catalyst (Kroschwitz). Although the air-based process may have lower operational costs (for small to medium-sized plants), the initial capital cost of the air-based plant is much more than the oxygen-based plant (Kroschwitz). While the air-based process requires more catalyst, more reactors (to achieve a comparable selectivity), a multi-stage compressor, air purification units, a vent gas treating system, and two to three reactor train in series; the oxygen-based process requires a carbon dioxide removal section, more stainless steel, and some expensive instrumentation (McKetta). Regardless of the extra equipment for the air-based process, the level of production for ethylene oxide for an air-based process is still less than the oxygen-based process. The average selectivity ranges from 65-75% compared to 70-80% for the oxygen-based process. Oxygen-based plants can have a yield of up to 0.1 kg ethylene oxide per kg ethylene more than the comparable air-based plants (McKetta). Because the silver catalyst is so expensive, the length of catalyst life is an important factor in process selection. For the oxygen oxidation, less silver is required for the catalyst charge and the catalyst itself last longer (McKetta). The air-based oxidation usually needs 50% more of the catalyst charge than it oxygen counterpart. Process Overview The following design is based around a four-part system developed by the conventional Shell direct oxidation technology. The reactors, which implement a silver/alumina catalyst are feed with ethylene and a high purity oxygen. The simplified and detailed design can be seen in the Figures 2 and 3. (Huang)

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5 Design Details The following table gives the reactor inlet and outlet conditions and compositions: Table 1: Reactor Feed and Effluent Data (Huang) Ethylene Feed Oxygen Feed Reactor Recycle Reactor Effluent 86 o F 86 o F 162 o F 482 o F Temperature 261 psia 261 psia 231 psia 241 psia Pressure Total Molar Flow Component Flows (): Ethylene Oxygen Carbon Dioxide Ethylene Oxide

6 Methane Ethane Nitrogen Argon Water Modeling Through modeling of the ethylene oxide process the cost for synthesis of ethylene glycol could be reduced. Temperature, pressure, oxygen content and ethylene feeds are the driving components of the reaction and should therefore be modeled and compared to existing plants for optimization of the process. Once formed, the ethylene glycol could be more easily separated from its larger glycol counterparts through the use of a nanofiltration membrane. With the proper molecular weight cutoff, this membrane could take the place of the separators. Synthesis of Ethylene Glycol = F (T, P,Ethylene Oxide, Water) Synthesis of Ethylene Oxide = F (T, P, Oxygen, and Ethylene) References: 1. Huang, Janet. jeo & Associates, Ethylene Oxide Reactor System. 10/8/ Kent, James A. Riegel s Handbook of Industrial Chemistry, Van Nostrand Reinhold, New York, 1983, pp Kroschwitz, Jacqueline I. And Howe-Grant, Mary. Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, New York, 1994, pp

7 4. Matar, Sami Catalysis in Petrochemical Processes, Kluwer Academic Publishers, Boston, 1989, p McKetta, John J. and Cunningham, William A. Encyclopedia of Chemical Processing and Design, Marcel Dekker, Inc., New York, 1983, Volume 20, pp Satterfield, Charles N. Heterogeneous Catalysis in Industrial Practice, 2 nd ed, McGraw Hill Inc., New York, 1991, pp Wiley, Dr. Robert, Website