Cyclohexane is branched chain aliphatic compound. Hydrogen is added to the benzene ring to convert it from aromatic to cyclic aliphatic structure.
Brief description of cyclohexane production by benzene hydrogenation flowsheet
Benzene is vaporized and mixed with hydrogen and passed to the packed bed containing the nickel and platinum catalyst. The operation condition of the reactor should be maintained at the temperatures between 165 to 320 oC and the pressure in the range of 2-3 MPa. Benzene hydrogenation is exothermic process so it is sufficient if the inlet temperature maintained at 150oC to initiate the reaction. Heat generated from reactor used for producing process steam. The outlet of the reactor cooled to 80oC and flashed into the separator. Unreacted gas is recycled to the reactor by the compressor and small liquid portion continuously recycled to the reactor to recover the unreacted benzene. The cooled liquid is complex mixture to separate into individual components by the distillation technique. Carry over gases removed initially by the distillation column and then the bottom of the distilling column containing cyclohexane and fractions of benzene fed to the purification section.
Benzene and cyclohexane separation methods:
Benzene and cyclohexane have their boiling points near 80oC at atmospheric pressure and separating them into pure fractions by distillation technique is quite energy consuming and inefficient in operation. However the molecular size and density differs quite wide that leads to variation in diffusion property and so by membrane separation we can achieve reasonable separation for pure cyclohexane.
Technology to improve the separation process of benzene and cyclohexane:
Pervaporation technique using membrane separation process. Some of the membranes developed are:
- Chitosan membrane built with carbon nanotubes
- Sodium alginate (SA) and sodium carboxymethyl cellulose (CMC) membrane
Extractive distillation: Using an external solvent benzene and cyclohexane can be separated with better efficiency and less operation cost than simple distillation. Compounds like phenol, propylene glycol and aniline are some of the solvent have shown good performance in extractive distillation technique.
Azeotropic distillation: Acetone and water breaks the azeotrope. Cyclohexane is recovered initially by the acetone and benzene is removed from bottom of the first stage and in second stage water recovers the acetone producing the pure cyclohexane. Recovered acetone is recycled again to the first stage. However this process is energy consuming.