Cyclohexane from Benzene and Hydrogen (Liquid-Phase Process)

The process under analysis comprises three major sections: (1) Reaction; (2) Hydrogen Recovery; and (3) Purification.

Reaction. Initially, benzene is fed to the primary reactor alongside fresh and recycled hydrogen. The hydrogenation reaction is carried out in a bubble column reactor, in the presence of a nickel based catalyst. The catalyst is maintained in a suspension with the aid of an external circulation loop. Most of the heat of reaction is removed by the vaporization of the product stream; the remaining reaction heat is removed in the external loop, by passing the reactor reflux stream through a heat exchanger against boiler feedwater, producing low pressure steam. Most of the benzene feed is converted in this step. The top gaseous product stream is directed to a fixed-bed reactor, where the remaining benzene content is converted to cyclohexane. The finishing hydrogenation reaction is conducted in the presence of a solid nickel based catalyst supported on alumina.

Hydrogen Recovery. The product stream is fed to a knock-out drum operated at high pressure. Most of the cyclohexane in the feed condenses producing two streams, one gaseous hydrogen-rich stream and a liquid cyclohexane-rich stream. The hydrogen stream is routed to the Recycle Compressor, where it is compressed to the Primary Reactor reaction pressure and recycled.

Purification. In a distillation column, the liquid cyclohexane-rich stream is stripped of lighter contaminants, such as methane, ethane and soluble hydrogen. Light-ends recovered from column’s top are used for fuel, while a cyclohexane stream with a residual benzene content that is lower than 100 ppm is obtained from column’s bottom.