Method for experimental determination of the gas transport properties of highly porous fibre membranes: a first step before predictive modelling of a membrane distillation process

For the predictive modelling of a membrane distillation process, the gas transport properties, defined by the dusty-gas model, of three highly permeable polyethylene and polypropylene fibre membranes have been determined. Single gas permeation experiments were carried out to determine the Knudsen diffusion and viscous flow membrane characteristics (K0 and B0, respectively). Binary gas diffusion experiments were carried out to determine the molecular diffusion membrane characteristic (K1). Because of the high permeability of the fibre membranes, new methods were developed to deal with effects such as pressure drop in the single gas permeation experiments and boundary layer resistance in the binary gas diffusion experiments. The K1 values of the fibre membranes were determined with an inaccuracy of 4–8%. It turned out that calculations of K1 with the values of K0 and B0 assuming cylindrical pores are, for the membranes studied, inaccurate by a factor of two.

For reliable experimental determination of the membrane mass transfer characteristics of three highly permeable fibre membranes, side effects such as pressure drop and boundary layers were taken into account. The effect of pressure drop in the permeable part of the fibre during single gas permeation experiments was excluded by a reduction of the length of the permeable part. The influence of boundary layers in diffusion experiments was reduced to 25–38%, depending on the membrane fibre. K1 values were measured with an inaccuracy of 4–8%. Next to single gas permeation experiments, binary gas diffusion experiments are necessary to determine the value of K1. Calculations of K1 with the values of K0 and B0 assuming cylindrical pores are, for the membranes studied, inaccurate by a factor of two.