Study Of Continuous Dehydrogenation Of Cyclohexane Under Multi-phase Reaction Conditions

Hydrogen is regarded as an ideal new energy resource due to its clean, renewable and high utilization efficient characteristics. However, the volumetric energy storage density of hydrogen is very low, it is very important for practical applications of hydrogen energy to seek an appropriate hydrogen storage carrier to improve its volumetric energy storage density, and settle the absorbing and desorbing hydrogen technologies of the hydrogen storage carrier.The liquid organic hydrocarbon hydrogen storage technology is a promising potential on-board hydrogen storage technology with advantages of high energy storage density, easy to transportation and recyclable, benzene-cyclohexane system is a typical one. The hydrogenation technology of benzene-cyclohexane system, i.e. the hydrogenation of benzene, has fully developed, but the dehydrogenation technology of the system, i.e. the dehydrogenation of cyclohexane, is still not solved, especially for the on-board dehydrogenation technology. The technology aimed at the on-board dehydrogenation of cyclohexane is the bottle-neck of the engineering applications for the benzene-cyclohexane hydrogen storage system.This thesis studied the continuous dehydrogenation technology of cyclohexane under the multi-phase reaction conditions aimed at the on-board dehydrogenation of benzene-cyclohexane system. By means of a home-made experimental apparatus, a feasibility study of the continuous dehydrogenation of cyclohexane was carried out. The experimental results showed that, without any carrier or sweeping gas, the continuous dehydrogenation of cyclohexane catalyzed by Raney-Ni was realized under the multi-phase reaction conditions, and the optimal operation parameters of the reaction system was also obtained. Based on the experiences of the feasibility study, a long time dehydrogenation reaction of cyclohexane was investigated, the deactivation phenomenon of the catalyst was revealed, and the deactivation mechanism of Raney-Ni was also studied. In the first section of the thesis, the feasibility study of the continuous dehydrogenation of cyclohexane under the multi-phase reaction conditions was carried out. By means of a home-made experimental apparatus, without any carrier or sweeping gas, the continuous dehydrogenation of cyclohexane catalyzed by Raney-Ni was realized under the multi-phase reaction conditions. The hydrogen evolution rate, purity of the produced hydrogen and conversion of cyclohexane in the reaction system were significantly influenced by the salt bath temperature, feeding rate of cyclohexane and catalyst dosage. Under the reaction conditions of the salt bath temperature at 370℃, feeding rate of 25 ml-h"1 and catalyst dosage of 3 g, the overall performance of the reaction system achieved its optimal, with the hydrogen evolution rate of 27 ml-min-1, purity of hydrogen 88% and conversion of cyclohexane 9.4%. The apparent reaction kinetic study showed that, within the salt bath temperature ranged from 290℃to 410℃, the apparent activation energy of the continuous dehydrogenation reaction is 12.57 kJ-mol-1.The performance of a long time continuous dehydrogenation reaction of cycloexane under the multi-phase reaction conditions was investigated in the second section of the thesis. Under the conditions of salt bath temperature at 360℃, feeding rate of 28 ml-h-1 and catalyst dosage of 3 g, the reaction system achieved its optimal with the initial hydrogen evolution rate of 37 ml-min-1 and average hydrogen evolution rate of 25 ml-min-1 within 6 hours, as well as the purity of the produced hydrogen over 95%. However, the gradually decrease of the hydrogen evolution rate was observed during the whole reaction process, which indicated the existence of the catalyst deactivation. Under the reaction conditions of the feeding rate of 55 ml-h-1 catalyst dosage of 3 g, and salt bath temperature ranged from 300 to 360℃, the analysis of the catalyst deactivation rate indicated that the order of catalyst deactivation is 4, and the catalyst deactivation rate was nearly a constant under different reaction temperatures, the equation of the catalyst deactivation rate is rd=0.289·(3·0.289·t+1)-4/3. The mechanism of the catalyst (Raney-Ni) deactivation was studied in the final section of the thesis. The experimental research showed that, after the reaction, the particle size distribution of Raney-Ni was not change, while the specific surface area of Raney-Ni was about 70% larger than that of the fresh one, and a lot of substance was found adsorbing on the catalyst. The SEM and Auger electron spectroscopy studies showed that, the surface of the fresh Raney-Ni was clean and flat, after the reaction, the catalyst surface was covered by a large amount of coke deposition, which resulted in the deactivation of the catalyst. The experimental research also indicated that, the simple heating process could not decrease the activity of Raney-Ni, but the ethanol washing treatment in the preparation process of Raney-Ni would decrease the activity of Raney-Ni, because the remained ethanol could result in coke deposition on Raney-Ni surface even the Raney-Ni was only undergone a simple heating.