HEAT EFFECT IN GAS PERMEATION, WITH SPECIAL REFERENCE TO SPIRAL-WOUND MODULES

In gas permeation, most gases tend to decrease in temperature, a phenomenon related to the Joule-Thomson effect, which reflects the non-ideal behaviour of the gas. The values for the Joule-Thomson coefficients as usually published in the literature are not correct for use in the context of gas permeation, because (1) in a spiral-wound module the heat transfer and the heat conduction are sufficiently good for the local permeation process to proceed almost isothermally; (2) two different equations of state have to be taken into account, viz. for the retentate and for the permeate composition. The required heat for the local permeation process to proceed isothermally is supplied by the retentate, which cools down accordingly; as a result the overall permeation process (throughout a module) in not isothermal. Since membrane performance depends on the temperature, this has to be accounted for in the design of membrane installations.