Design and Control of the Styrene Process

A recent paper by Vasudevan et al. (Ind. Eng. Chem. Res. 2009, 48, 10941−10961) presented a flowsheet of the styrene process, which has several interesting design and control features. Their study concentrated on a comparison of three plantwide control methodologies. The economic optimum design of the process was given, but no quantitative details were given of how the optimum was obtained. The chemistry of styrene process involves the dehydrogenation of ethylbenzene. The reaction is endothermic, nonequimolar and reversible, so high temperatures and low pressures are conducive to high conversion in the adiabatic vapor-phase reactors. Steam is mixed with the ethylbenzene (EB) fed to the reactors to lower the partial pressure of ethylbenzene and increase conversion. There are also several other side reactions that produce undesirable byproducts (benzene, toluene, ethylene and carbon dioxide), whose reaction rates increase with temperature and partial pressures. The main design optimization variables in this process are the steam-to-EB ratio, reactor inlet temperature, EB recycle flow rate and reactor size. Low reactor temperatures suppress side reactions but require higher EB recycle to achieve the same styrene production rate, which increases separation costs. Higher steam-to-EB ratios also suppress side reactions but increase furnace fuel costs and steam supply costs. The purpose of this paper is to develop a reasonable conceptural design considering capital costs, energy costs and raw material costs and then to develop a plantwide control structure capable of effectively handling large disturbances in production rate. The proposed design is significantly different than the design of Vasudevan et al., featuring higher steam-to-EB ratios, lower reactor temperatures, larger EB recycle flow rates and larger reactors. Styrene yield is improved from 76 to 87%, which results in a 10% reduction in operating costs.