In summary, the performance of dense membranes is strictly material-dependent, and it is evaluated by the membrane's permeability and selectivity. On the other hand, the performance of porous membranes is morphology and material-dependent and it is evaluated by parameters such as permeance, retention, and the MWCO. Membranes can be operated in two modes: in the so-called 'dead-end' mode, where a feed stream is completely transported through the membrane, or in continuous mode, in which the feed flows along the membrane. Similar to heat exchange, continuous operations can be performed in co-current, counter-current, and crossflow . Overall, the optimum performance in a membrane-based process comes from the optimization of the membrane module material and morphology as well as the optimization of process conditions.