New insights into the impurity transport and separation behaviours during metal hydride dehydrogenation for ultra-pure hydrogen

The hydrogen separation and purification method based on metal hydride (MH) is viewed as one of the ideal methods for recycling industrial by-product hydrogen. Usually, self-produce hydrogen purging is conducted to remove the impurity. However, the mechanism of impurity transport and separation under the MH dehydrogenation process is still not clear so that the purity of produced hydrogen or the hydrogen recovery ratio is low. In this paper, a novel poisoning dehydrogenation kinetics considering the effect of passivation layer is proposed to accurately describe the dehydrogenation properties under the impurity poisoning conditions. The poisoning factor is introduced to describe the poisoning effect of impurities on MH materials. More active impurity or higher impurity concentration result in the larger poisoning factor. Based on the poisoning dehydrogenation kinetics, the MH dehydrogenation reaction model for self-produce hydrogen purging process is developed to describe the impurity transport and separation behaviours. The effects of operating parameters on the impurity transport and separation are further investigated for ultra-pure hydrogen. It is found that the low temperature and low outlet flow rate results in the low hydrogen loss for ultra-pure hydrogen. After the optimization, an efficient hydrogen separation and purification with both a hydrogen purity of 99.9999% and a hydrogen recovery ratio of more than 80% is achieved, which shows the potential in the hydrogen separation and purification of industrial by-product hydrogen.