A membrane reactor for hydrogen storage and transport system using cyclohexane–methylcyclohexane mixtures

Cyclohexane is recognized to be one of the promising hydrogen storage materials but it is pointed out that its freezing point (ca. 6.5 C) is too high to use in the cold district. In this study, therefore, the mixtures of cyclohexane with methylcyclohexane as a hybrid chemical hydride were proposed to lower the freezing point. The hydrogen recovery from the mixtures was testified by being dehydrogenated with a palladium membrane reactor up to 300 C and 4 bar, in which hydrogen generated was selectively separated from the catalyst-packed bed through a palladium membrane and obtained as purified hydrogen on the permeate side. An addition of 20 mol% methylcyclohexane to cyclohexane resulted in about 40 C decrease in the freezing point. The increase in conversion became larger in a region of high reaction pressure because a larger pressure difference increased the amount of hydrogen separated. The conversion of methylcyclohexane was higher than that of cyclohexane at the same condition. In the dehydrogenation of the mixtures, each conversion was found not to be affected by the other very much. In conclusion, the mixtures of methylcyclohexane and cyclohexane can be a hybrid type of chemical hydride, which will be available as a liquid hydrogen carrier at À30 C.

Dehydrogenation of cyclohexane, methylcyclohexane and their mixtures using a palladium membrane eactor up to 300 C and 4 bar, in which the evolved hydrogen was selectively separated from the catalyst-packed reaction zone through the palladium membrane, were conducted. In the dehydrogenation of methylcyclohexane, the conversion with the membrane reactor was higher than that without membrane separation and exceeded the equilibrium conversion, whereby confirming that the membrane reactor was useful to recover hydrogen from the equilibrium limited reaction. It was observed that the percentage of increase in conversion increased in the range of high reaction pressure. This is because the driving force for hydrogen permeation, that is, the difference in the partial pressure of hydrogen, becomes larger up to several bar while the reaction is apt to be somewhat depressed by the chemical equilibrium limitation. The conversion of cyclohexane was lower than that of methylcyclohexane under the same condition, the equilibrium conversion of which was higher than that of cyclohexane. Finally, the dehydrogenation of the mixtures of methylcyclohexane and cyclohexane with the palladium membrane reactor was examined. It was found that each in the mixture showed almost same reactivity with its original one while the conversion of methylcyclohexane was always higher than that of cyclohexane similarly to the separate case. It is concluded that the mixtures as a hybrid type of chemical hydride can be expected to be a candidate for hydrogen storage liquid. The mixture of 20 mol% methylcyclohexane —80 mol% cyclohexane will be available as liquid even at 35 C.