The plant includes one (1) ethylbenzene (EB) production unit and two (2) styrene monomer (SM) production units. Each unit has reaction and distillation sections. SM Unit 1 has operation capacity of 200,000 TPY; SM Unit 2 operation capacity 300,000 TPY. In the EB reaction, vapor ethylene and liquid benzene are fed into alkylation reactor filled with zeolite catalyst. In the SM reaction, EB and high temperature steam are fed into dehydrogenator with iron(III) oxide catalyst. Unused catalyst is available at additional cost.