styrene ak - Review and Screening of Alternative Process 1...

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Review and Screening of Alternative Process 1. Dehydrogenation of ethylbenzene The majority of industrial production of styrene follows from the dehydrogenation of ethylbenzene. The most commonly method for producing styrene involves two steps: the alkylation of benzene with ethylene to produce ethyl benzene (C 8 H 8 ), and followed by catalytic dehydrogenation (hydrogen is removed) of the ethyl benzene to produce styrene (refer to Figure 1). Over almost fifty years of practicing the conventional two step process, refinements have constantly been made to improve conversion and selectivity to ethyl benzene and finally to styrene along with design changes to conserve and utilize the energy in particular from the exothermic alkylation step. The traditional aluminum chloride catalyst used in this alkylation is slowly being replaced by zeolite catalyst technology. Currently the common commercial production of styrene is by dehydrogenation of ethyl benzene in the presence of steam over a catalyst (iron oxide) in either fixed bed adiabatic or tubular isothermal reactors [1]. Figure 1: Dehydrogenation of Ethyl benzene to Styrene Fresh ethylbenzene is mixed with a recycle stream and vaporized. Steam is then added before feeding the effluent into a train of 2 to 4 reactors. Ethylbenzene is mixed in the gas phase with 10–15 times its volume in high temperature steam at 600-650 degrees Celsius and passed over a fixed catalyst bed. The reactors are run adiabatically in multiple reactors with steam added before each stage with typical yields of 88-94%. Most ethylbenzene dehydrogenation catalysts are based on iron (III) oxide and also promoted by several percent potassium oxide or potassium carbonate. On this catalyst, a highly endothermic reaction and reversible chemical reaction takes place. Steam serves several roles in this reaction. It is the source of heat for powering the endothermic reaction, and it removes coke that tends to form on the iron oxide catalyst through the water gas shift reaction. Moreover, it is use to shift equilibrium of the reversible reaction towards the products and to clean the catalyst of any carbon that does form. The potassium promoter enhances this decoking reaction. The steam also dilutes the reactant and products, shifting the position of chemical equilibrium towards products.
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A typical styrene plant consists of two or three reactors in series, which operate under vacuum to enhance the conversion and selectivity. Typical per-pass conversions are ca. 65% for two reactors and 70-75% for three reactors. Selectivity to styrene is 93-97%. The main byproducts are benzene and toluene. Because styrene and ethylbenzene have close boiling points (145 and 136 °C, respectively), their separation requires tall distillation towers and high return/reflux ratios. At its distillation temperatures, styrene tends to polymerize. To minimize this problem, early styrene plants is added with elemental sulfur to inhibit the polymerization. In addition, small residence time, avoidance of high temperature and addition of inhibitor
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This note was uploaded on 06/06/2011 for the course CHEM 3040 taught by Professor Reddy during the Spring '10 term at Taylor's.

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styrene ak - Review and Screening of Alternative Process 1...

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