Improving selectivity in the butene/isobutane alkylation process

Many chemical reaction systems involve the simultaneous production of desirable and undesirable products. Unless selectivity is large, yield and profitability are adversely affected. Developing a process that balances the many engineering tradeoffs inherent in such a system presents many challenging technical and economic problems. The alkylation process used in petroleum refineries around the world is an important example. Butene and isobutane react to form the desired high-octane-number iso-octane product (RO 100) and the undesired very-low-octane-number dodecane (RO –20). The alkylate mixture is blended into gasoline, and higher selectivity gives more gasoline. A large isobutane recycle is used to adjust reactor composition, which impacts selectivity, but more recycle increases energy consumption in the separation section. Another process feature that increases design complexity is the presence of two inert components in two fresh feed streams. This paper presents a quantitative study of the design of an alkylation process to access the interacting process and economic effects over a wide range of process parameters. Results indicate that economics improve when recycle flowrates are significantly larger than those currently used in existing plants.