A simulation model of a reactor for ethylene oxide production

Abstract (Swedish)

This thesis has been performed in the ethylene oxide plant at Akzo Nobel in Stenungsund. Ethylene oxide is an important ethylene based intermediary compound. The primary use for ethylene oxide is in the manufacture of derivatives such as ethylene glycol, surfactants and ethanolamine’s.
In the production of ethylene oxide, ethylene and oxygen reacts over a silver catalyst surface. Mainly two reactions occur, partial oxidation of ethylene to ethylene oxide and total oxidation of ethylene to carbon dioxide and water. The largest cost in production of ethylene oxide is ethylene therefore it’s important to optimize the selectivity towards ethylene oxide and thus reduce the consumption of ethylene.
The aim with this thesis was to create a simulation model of the reactor using Aspen Plus® that correlates with reality. This model is then going to be a tool for future investigations of replacements of catalysts. Initially a literature study was made to increase the understanding of the chemical substance present and the mechanism in the reactor.
Part of this project was to find a thermodynamic model that correlate with the system. Based on the literature study an equation of state model, Peng-Robinson or Soave-Redlich-Kwong was the best choice. Both models were tested and they gave similar results. In this simulation Soave-Redlich-Kwong is used. The reason for that is that Soave-Redlich-Kwong is developed to function well together with water which condenses in the heat exchangers after the reactor (simulation model will incorporate more process equipment in future studies) .
Multiple studies have been done to discover the kinetics of the oxidation reactions of ethylene over a silver catalyst. There has been no general agreement among the kinetics equations and mechanisms described. It is agreed that silver can adsorb oxygen in a number of ways and that this phenomenon is the basis of silver’s efficiency in catalyzing the oxidation of ethylene to ethylene oxide.
The simulation was divided in three various operational conditions, start of run, middle of run and end of run. The selectivity and activity decreases over time and the reason is deactivation of the catalyst that occurs due to poisoning and silver sintering.
In the simulation kinetic parameters from the literature was used as a starting point. These parameters were then adjusted to fit the three various operational conditions. The conclusion was that during operational time the activation energy increased for both reactions which have to be compensated with increasing reactor temperature. At the same time the selectivity for producing ethylene oxide decreases, i.e. more carbon dioxide and water are formed. In future catalyst evaluation it is important to establish that the catalyst can operate within flammable limit and within equipment design.

Popular Abstract

There are high demands on companies today to survive when competition is high and that also applies to companies that manufacture ethylene oxide. The raw material in the production of ethylene oxide is ethylene and oxygen where ethylene is the largest cost. To facilitate the reaction a catalyst, which increases the reaction rate, is used. The catalyst has a major influence on the efficiency and economics of the production of ethylene oxide. Catalysts used in commercial ethylene oxide processes contain silver on an alumina oxide carrier with addition of promoters and moderators to improve selectivity towards ethylene oxide and stabilize activity. For an ethylene oxide company to be competitive in the market, it is important to follow catalyst development and to investigate the possibilities of using catalysts that increase stability and selectivity towards ethylene oxide. When a change of a catalyst to another type of catalyst is considered, it’s important to investigate the change of process parameters that will be a result of another type of catalyst. A simulation model in the commercial program Aspen Plus® has here been performed as a tool for future catalyst evaluations.

Ethylene oxide was first prepared in 1859 by Wurtz using potassium hydroxide solution to eliminate hydrochloric acid from ethylene chlorohydrin. This led to an industrial production of EO (Ethylene Oxide) which began in 1914. The direct catalytic oxidation of ethylene, discovered in 1931 by Leffort, was gradually replaced by the chlorohydrin process. Today EO is produced by direct oxidation of ethylene with air or oxygen.

In the production of ethylene oxide, oxygen and ethylene reacts over a silver catalyst on an alumina oxide carrier and the main reactions are:

C2H4 + ½ O2 → C2H4O (formation of ethylene oxide) (ΔH = -106,7 kJ/mol)
C2H4 + 3 O2 → 2 CO2 + 2 H2O (by products) (ΔH = -1323 kJ/mol)
The reactions are exothermic which means that heat is created. As the catalyst is used, its selectivity and activity decreases due to deactivation of the catalyst. Deactivation occur from impurities in the reactor inlet, changing of silver particles (silver sintering) and blocking of pores in the catalyst. To minimize the deactivation of the catalyst it’s important to control the impurities in the inlet of the reactor and to minimize the temperature in the reactor.
One of the first steps when creating a simulation model of an ethylene oxide reactor in Aspen Plus® is to choose a thermodynamic model that correlate with the system. In this simulation a model by Soave-Redlich-Kwong was used and the reason was that the model is known to give a better fit of a system, which includes condensation of water. This will occur when the simulation is expanded and the heat exchangers downstream of the reactor are included.
In this simulation three operating conditions were studied, start of run (SOR), middle of run (MOR) and end of run (EOR). As a starting point kinetic parameters from the literature were used and fine-tuned to correlate with process parameters for SOR collected from process flow diagrams (PFDs) at AkzoNobel. The second step was to adjust the kinetic parameters from SOR to correlate with process parameter for MOR collected from PFDs at Akzo Nobel. The third step was to adjust the kinetic parameters from MOR to correlate with process parameters from EOR collected from PFDs at AkzoNobel.

As the catalyst ages the selectivity decreases and the reason for this is deactivation of the catalyst. The activation energies for both reactions (1) and (2) are increased over time and this is compensated with higher temperature in the reactor. When selectivity decreases, more ethylene is totally oxidized to CO2 and water and a result from this is that more heat is produced since reaction (2) has a higher enthalpy of reaction.

• author: Perzon, Hanna
• supervisor: Christian Hulteberg; Anders Friborg
• organization: Chemical Engineering (M.Sc.Eng.)
• course: KET920 20131
• year: 2015
• type: H2 - Master's Degree (Two Years)
• subject: Chemistry
• keywords: kemiteknik, chemical engineering
• language: English
• id: 7363439
• date added to LUP: 2015-06-17 11:56:10
• date last changed: 2015-06-18 14:04:28