Cyclic Membrane Gas Separation Processes Assessment

The analysis of membrane gas separation processes under transient conditions

for practical applications is a rather unexplored domain. One of the main

advantages of membrane processes compared to other separations is their ability

to work under steady conditions, without any separate regeneration step. Nevertheless,

unique separation performances can be obtained in some cases when

a transient regime is applied. Paul explored this issue in a pioneering study,

based on a cyclic process with synchronous valve operation.

Followed by this initial work, more theoretical and experimental studies

about the cyclic process have been performed. Some interests and also some

drawbacks have been defined qualitatively. A general improved selectivity is

highlighted with respect to a conventional process, together with a significant

decrease on productivity. Thus, the trade-off between these two factors is the

key issue to assess whether the cyclic process is competitive with respect to a

conventional process.

Based on Paul's process, a cyclic process for membrane gas separations is

proposed in our study with more available operating possibilities and less

modeling assumptions. The simulation and optimization techniques have been applied

to different gas systems, based on reported permeability data through a given

polymer. By simulating this process, some other unattainable interests with

respect to conventional operations are highlighted besides the improved selectivity,

as well as some important unavoidable drawbacks. The main outcome

of this study is, for each gas pair, a characteristic chart where steady state and

cyclic separation performances (namely selectivity and productivity) are compared.

An example is shown on figure 1 for the He/Ar gas pair. It can be seen

that the cyclic operation offers a very large increase of the process selectivity to

be possibly attained, at the expense of a productivity decrease.