Industrially, cyclohexane was originally obtained by direct distillation of crude oil with a purity of 85%. US Humble Oil Company and Philips Oil Company increased the purity of cyclohexane to 99% by isomerizing methyl cyclopentane in light distillate oil. In the 1960s, with the development of polyamide production, the demand for cyclohexane increased rapidly. Cyclohexane obtained by crude oil separation could not meet the requirements in terms of quantity or quality. Therefore, benzene was used as raw material for hydrogenation and production of rings. The hexane process has developed rapidly. To date, 80% to 85% of the cyclohexane has been obtained by hydrogenation of benzene, and the remainder is still obtained by direct distillation of the crude oil.
Hydrogenation of benzene is a strongly exothermic reaction and the reaction is often carried out under a certain pressure:
Benzene can be completely converted in one pass and high purity cyclohexane can be obtained. The hydrogenation process requires very pure benzene as the raw material (the sulfur content in benzene is less than 1ppm, and benzene and hydrogen-rich products are usually used in the catalytic reforming process as raw materials), which has a good economic effect.
There are many industrial production methods for the hydrogenation of benzene to cyclohexane, and the types of catalysts used, reaction operating conditions, and reactor types are all different. The key is to ensure the complete hydrogenation of benzene, the timely removal of reaction heat, and the control of reaction temperature and retention. At the time, the cyclohexane isoforms constitute methyl cyclopentane. Hydrogenation methods can be divided into liquid-phase and gas-phase methods. The gas-phase reaction pressure is lower, the reaction temperature is higher, the reaction product does not need to separate the catalyst, the liquid phase method has a large production capacity, the reaction conditions are mild, and low-purity (55%) hydrogen gas can be used without a hydrogen circulation system.
Liquid-phase hydrogenation The method developed by the French Petroleum Institute, abbreviated as the IFP method, has a certain representativeness and is widely used in the world. The hydrogenation reactor of the IFP process consists of a main reactor and a final reactor in series (see figure). The main reactor is operated in liquid phase using suspended nickel (see Metal Catalyst) as catalyst. The reaction heat was removed in the reactor by hydrogen bubbling and liquid circulation. The reaction temperature is 200 to 225°C and the pressure is 4.9 MPa. About 95% of the benzene is converted to cyclohexane through the main reactor. The final reactor is operated in a fixed-bed gas phase, where the unconverted benzene is completely converted and the product purity is above 99.8%.
Gas-phase hydrogenation The hydrogen gas mixture of benzene vapor and hydrogen is hydrogenated through a catalyst bed; generally, a fixed-bed reactor in a column can be used to convert benzene in two or three stages in series. Hydraull is the first method developed by U.S. Global Oil Company to realize the industrialization of benzene hydrogenation. It uses a nickel catalyst adiabatic reactor. The last reactor outlet temperature is kept below 275°C, and the pressure is about 3MPa. The rate is above 90% and the purity is greater than 99.9%.