Multicomponent separation performance of MFI-type zeolite membranes

Zeolite membranes are of great interest due to potential ability to separate many industri-ally important molecules with high selectivity. As well as high selectivity, high flux and durabil-ity are essential for practical application of zeolite membranes. Application of zeolite membranes in hydrogen production can be important for the future of hydrogen as a fuel source. Using hydrogen in fuel cells as well as in internal combustion en-gines gives possibility to eventually break the link between transport sector and CO2 emissions and improve energy efficiency. The production of hydrogen by steam reforming could be per-formed more efficiently by the use of a membrane reactor with continuous removal of CO2. Thus the development of membranes which can separate CO2 from synthesis gas is very important. This thesis work has been devoted to investigation of separation performances of MFI-type zeolite membranes for the mixtures related to the steam reforming process. Silicalite-1 and ZSM-5 membranes were tested for separation of the following mixtures: CO2/H2, H2O/H2, and CO2/H2O/H2. Prior to the separation, membranes have been characterized by n-hexane po- rosimetry in order to evaluate membrane quality. The presence of defects in the pore structure can significantly affect separation performances of membranes. The separation experiments with a binary mixture of CO2 and H2 showed that the mem-branes of both types were slightly hydrogen selective. The highest CO2/H2 separation factors were observed at 22 0C and were 0.7 for silicalite-1 and 0.8 for ZSM-5. The hydrogen selectiv-ity could be explained by significant Knudsen diffusion through the support as well as very weak CO2 adsorption. The H2O/H2 separation showed the highest selectivity towards water at 22 0C. The separa-tion factors were 2.2 and 4.0, for silicalite-1 and ZSM-5 respectively. Strong adsorption of water in zeolite pores significantly limits permeation of hydrogen and thus provides effective separa-tion. ZSM- 5 membrane has a higher H2O/H2 separation factor than silicalite-1. This was ex-plained by more pronounced affinity of water to ZSM-5 membrane. When the feed was a ternary mixture of CO2, H2 and H2O, the CO2/H2 separation factors at 22 0C were 2.2 and 3.7 for silicalite-1 membranes S1 and S2 and 4.2 for ZSM-5 membrane. The H2O/H2 separation factors at 22 0C were 2.1 and 4.6 for silicalite-1 membranes S1 and S2 and 4.1 for ZSM-5 membrane. It was seen that at low temperature the CO2/H2 separation factors in a ternary mixture are higher than that observed in a binary mixture of CO2 and H2. Increased CO2/H2 separation factors were obtained due to significant decrease in hydrogen permeance be-cause of blocking effect of water. However, the CO2 permeance was not decreased significantly by the adsorbed water, due to its ability to adsorb in the zeolite pores. H2O/H2 separation factors are not affected by presence of CO2. Results show that for all three mixtures membranes of both types were selective at low temperature, and the selectivity decreased dramatically when the temperature was increased. At temperatures above 100 0C all membranes were selective towards hydrogen. Additionally, separation of ethanol and hydrogen was investigated using both types of membranes. Results show that membranes were very selective towards ethanol at low tempera-ture. The highest separation factors were observed at around 21 0C, and were 19.1 for ZSM-5 and 16.6 for silicalite- 1. Adsorption of ethanol in the zeolite pores drastically decreases hydrogen permeance at low temperature. This may be a promising start to adapt zeolite membranes for ap-plication in ethanol production or separation processes. Separation of hexane isomers such as n-hexane and 2,2-dimethyl-butane was performed using silicalite-1 membrane. The highest selectivity towards n- hexane was achieved at 230 0C and was 130.Validerat; 20101217 (root