Used styrene plant consisting of 610,000 TPY ethylbenzene (EB) production unit and 530,000 TPY styrene monomer (SM) production unit. The first process section in the SM unit is cracking where EB is in two catalyst filled reactors converted with steam into SM and hydrogen through dehydrogenation. The process operated at high temperature. The second process section in the SM unit is finishing which purifies the SM and recycles unused EB to the feed of the cracking section.