FIGURE 11 The effect of gas flux on the conversion efficiency of methanol. (Reaction temperature 360 ℃, one sheet of membrane of thickness 1 mm)
The effect of gas flux through the membrane on conversion efficiency of methanol was shown in Figure 11. It could be seen that the conversion efficiency of methanol was decreased from 7.5% to 2.0% with the gas flux through the membrane was increased from 8 m3·m-2·h-1 to 18 m3·m-2·h-1 since the reaction residence time during methanol dehydrogenation was decreased. In addition, the gas flux was changed by adjusting the nitrogen flow rate and higher gas flux with higher nitrogen flowrate would cause the decrease of the methanol concentration. It can be calculated that the concentration of the methanol vapor was decreased from 76% to 37% with the gas flux increased from 8 m3·m-2·h-1 to 18 m3·m-2·h-1. Lower gas flux can promote methanol conversion but leads to lower equipment unitization.
In order to obtain high conversion efficiency of methanol under the condition of higher gas flux, two or three sheets of Cu/ZnO/Ti CMNRs were set up in series for methanol dehydrogenation. As shown in Figure 12, it can be seen that the increasing sheets of membrane resulted in an increase of the conversion efficiency of methanol. Moreover, the conversion efficiency of methanol increased asymptotically and reached a maximum value of 18.5% when three sheets of CMNR applied, which produced logically a pressure drop increase from 2.7 kPa to 6.8 kPa. It can be deduced that higher conversion efficiency of methanol can be expected if more membranes are packed in series. Based upon the current experiments, the preliminary simulation could be proposed: if 25 membranes (packing thickness 25 mm) applied, the conversion efficiency of methanol can up to 80% with the gas pressure drop 58 kPa; if Cu/ZnO/Ti CMNRs increased 70 sheets (packing thickness 70mm), the conversion efficiency of methanol would be expected to over 99% with the gas pressure drop 162 kPa. Compared to general fixed bed reactors (conversion efficiency, bed height, pressure drop), the developed here CMNRs would be of excellent performance.