C.M. Guijt

Modelling of a transmembrane evaporation module for desalination of seawater

Transmembrane evaporation (often called membrane distillation) carried out in a countercurrent flow module, in which incoming cold seawater is heated by the condensing product water flow, is a promising technology for low-cost seawater desalination. This paper presents a model for preliminary design calculations for such a module. The model calculates temperature profiles alongside and across the module, heat and vapour fluxes through the membrane and the total product flow. Mass transfer of the water vapour through the membrane and the air gap is described with molecular diffusion through stagnant air. Estimated properties and constants have been used to calculate the energy transport. Calculations with variable temperatures, membrane fibre diameter and diffusion distance have been performed. From the calculation results can be concluded that the model is able to describe the process of interest in a qualitative way. The calculations indicate that the highest productivity will be obtained with high temperatures, small membrane fibres and a small air gap.

Determination of membrane properties for use in the modelling of a membrane distillation module

Membrane distillation carried out in a counter current flow module, in which latent heat is recovered by heating the incoming cold seawater with the condensing product water flow, is a promising technology for low cost seawater desalination. The membranes used in this module are hydrophobic (polypropylene, polyethylene) and highly permeable fibre membranes. For modelling purposes the Knudsen diffusion and viscous flow membrane characteristics (K0 and B0 respectively) of five fibre membranes are determined. This paper presents a new, specially developed method for the determination of K0 and B0 values of highly permeable fibre membranes with single gas permeation experiments through a short dead end fibre. In order to be able to make use of a reliable method to determine the values of K0 and B0, it is essential that the pressure inside the permeable part of the membrane is constant. To determine the conditions at which the pressure drop in the permeable part of the membrane fibre is negligible, this part is reduced in length until the values of K0 and B0 become constant. For all membranes the gases He, N2 and CO2 were used. The gases N2 and CO2 lead to consistent values of K0 and B0. Helium gives less accurate results due to its low molecular weight. The three polypropylene membranes have a similar structure and have therefore about the same values for K0 and B0. The same was found for the two polyethylene fibres.

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.