To produce polymer-grade styrene monomer (SM) by dehydrogenating ethylbenzene (EB) using the Lummus/UOP “Classic” styrene process or the Lummus/UOP SMART process (for revamps involving plant capacity expansion).
In the Classic SM process, EB is catalytically dehydrogenated to styrene in the presence of steam. The vapor phase reaction is carried out at high temperature and under vacuum. The EB (fresh and recycle) is combined with superheated steam, and the mixture is dehydrogenated in a multistage reactor system (1). A heater reheats the process gas between stages. Reactor effluents are cooled to recover waste heat and condense the hydrocarbons and steam. Uncondensed offgas— containing mostly hydrogen— is compressed and is used as fuel or recovered as a valuable byproduct. Condensed hydrocarbons from an oil/water separator (2) are sent to the distillation section. Process condensate is stripped to remove dissolved aromatics and then used internally for steam generation.
A fractionation train (3,4) separates high-purity styrene product; unconverted EB, which is recycled; and the relatively minor byproduct tar, which is used as fuel. In additional columns (5,6), toluene is produced as a minor byproduct and benzene is normally recycled to the upstream EB process.
Typical SM product purity ranges from 99.85% to 99.95%. The process provides high-product yield due to a unique combination of catalyst and operating conditions used in the reactors and the use of a highly effective polymerization inhibitor in the fractionation columns.
The SMART SM process is the same as Classic SM except that oxidative reheat technology is used between the dehydrogenation stages of the multistage reactor system (1). Specially designed reactors are used to achieve the oxidation and dehydrogenation reactions. In oxidative reheat, oxygen is introduced to selectively oxidize part of the hydrogen produced over a proprietary catalyst to reheat the process gas and to remove the equilibrium constraint for the dehydrogenation reaction. The process achieves up to about 75% EB conversion per pass, eliminates the costly interstage reheater, and reduces superheated steam requirements. For existing SM producers, revamping to the SMART process may be the most cost-effective route to increased capacity.
Licensor: Lummus Technology and UOP LLC, A Honeywell Company