The ethyl benzene (EB) process involves the reaction of benzene with ethylene to form the desired EB product. However, ethylene can also react with EB to form an undesired product of di-ethyl benzene (DEB) if reactor temperatures or ethylene concentrations are high. An unusual feature of the EB process is the ability to recycle "to extinction'' all the DEB formed in the reactor (no net DEB product produced), since DEB reacts with benzene to form EB. Since DEB is the highest-boiling component in the system, it comes out the bottom of the two distillation columns, so there is little energy penalty in having a large DEB recycle. Recycling benzene is more expensive because it goes overhead in the first distillation column. The economic optimum steady-state design is developed that minimizes total annual cost (capital and energy). An effective plantwide control structure is also developed. (C) 2010 American Institute of Chemical Engineers AIChE J, 57: 655-670, 2011