The result is a more compact design plus greater conversion. Removal of a product increases the residence time for a given volume of reactor and drives equilibrium-limited reactions towards completion. This latter advantage is reviewed excellently by Armor 1,2 while the former has been, largely, ignored. In my opinion, a yet-larger advantage of membrane reactors is that they expand the allowed range of temperatures and pressures for a reaction. Membrane reactors fundamentally change the pressure dependence of conversion in gas phase decomposition reactions so that the reactions are preferentially performed at high pressures rather than low.
Higher pressures allow much smaller reactors and more efficient purification.
Membrane reactors can be advantageous also for sequential endothermic and exothermic reactions, by using the product extraction to promote heat transfer. Enhanced heat transfer permits plug flow where CSTR designs would have been necessary otherwise. The net result is smaller reactors, lower capital costs, and often fewer side-reactions. These general benefits will be illustrated for methanol reforming to hydrogen. Without a membrane reactor, the heating and pressure requirements of this process are difficult, requiring a large reactor and significant heat transfer area.