Produce benzene via the hydrodealkylation of C7–C11 aromatics.
Fresh C7–C8+ (to C11) feed is mixed with recycle hydrogen, makeup hydrogen and C7+ aromatics from the recycle tower. The mixture is heated by exchange (1) with reactor effluent and by a furnace (2) that also generates high-pressure steam for better heat recovery. Tight temperature control is maintained in the reactor (3) to arrive at high yields using a multi-point hydrogen quench (4). In this way, conversion is controlled at the optimum level, which depends on reactor throughput, operating conditions and feed composition.
By recycling the diphenyl (5), its total production is minimized to the advantage of increased benzene production. The reactor effluent is cooled by exchange with feed followed by cooling water or air (6) and sent to the flash drum (7) where hydrogen-rich gas separates from the condensed liquid. The gas phase is compressed (8) and returned to the reactor as quench, recycle H2.
Part of the stream is washed counter currently with a feed side stream in the vent H2 absorber (9) for benzene recovery. The absorber overhead flows to the hydrogen purification unit (10) where hydrogen purity is increased to 90%+ so it can be recycled to the reactor. The stabilizer (11) removes light ends, mostly methane and ethane, from the flash drum liquid. The bottoms are sent to the benzene column (12) where high-purity benzene is produced overhead. The bottoms stream, containing unreacted toluene and heavier aromatics, is pumped to the recycle column (13). Toluene, C8 aromatics and diphenyl are distilled overhead and recycled to the reactor. A small purge stream prevents the heavy components from building up in the process.