Desalination and water recycling by air gap membrane distillation

Membrane distillation (MD) is an emerging technology for desalination. Membrane distillation differs from other membrane technologies in that the driving force for desalination is the difference in vapour pressure of water across the membrane, rather than total pressure. The membranes for MD are hydrophobic, which allows water vapour (but not liquid water) to pass. The vapour pressure gradient is created by heating the source water, thereby elevating its vapour pressure. The major energy requirement is for low-grade thermal energy. It is expected that the total costs for drinking water with membrane distillation will be lower than $0.50/m3, even as low as$0.26/m3, depending on the source of the thermal energy required for the evaporation of water through the membrane

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

Air gap membrane distillation: 1. Modelling and mass transport properties for hollow fibre membranes

A predictive model for air gap membrane distillation in a counter current flow configuration using fibre membranes is presented. The water vapour transport across the membrane is described by the dusty-gas model that uses constant membrane mass transport parameters to describe simultaneous Knudsen diffusion, molecular diffusion and viscous flow. This makes the model suitable to describe the membrane distillation process for a wide range of pressures and temperatures. The membrane mass transport properties had to be determined experimentally in separate experiments to obtain a predictive model. The Knudsen diffusion and viscous flow membrane parameters (K0 and B0, respectively) were determined with single gas permeation experiments. The molecular diffusion membrane parameter (K1) was determined with binary gas diffusion experiments. High membrane permeability in combination with small membrane fibre radius, a combination that is advantageous for membrane distillation, made it necessary to pay special attention to effects as pressure drop along the fibre and boundary layer resistances in order to obtain accurate membrane parameters. The gas permeation data show that calculation of K1 from the K0 and B0 values assuming parallel cylindrical pores is accurate within 30% for some membranes but can be wrong by a factor of two for other membranes. This means that (relatively simple) single gas permeation experiments in combination with a cylindrical pore membrane model are, unfortunately, not sufficient to obtain reliable membrane mass transfer properties for model calculations

Air gap membrane distillation. 2. Model validation and hollow fibre module performance analysis

In this paper the experimental results of counter current flow air gap membrane distillation experiments are presented and compared with predictive model calculations. Measurements were carried out with a cylindrical test module containing a single hollow fibre membrane in the centre and a well-defined air gap situated around the fibre. The experimental results show that the previous developed predictive model, with membrane parameters determined from gas permeation experiments, describes correctly the dependence of water vapour flux on temperature level, temperature difference, air gap total pressure, hot water flow and membrane type. At atmospheric air gap pressure, the measured fluxes per saturated water vapour pressure difference between the bulk flows (0.08–0.10 kg/m2h mbar) are comparable with those presented in literature. A reduction of the total air gap pressure to the saturated water vapour pressure of the hot water feed flow temperature of 65 °C, raises the flux by a factor of three. Next to the water vapour flux, the energy efficiency of the process is very important. The measured energy efficiencies (typically 85–90% for a 3 mm air gap and a hot water feed temperature of 65 °C) are slightly below the theoretical values (95–98%), which could be explained by a small heat loss to the surroundings. For air gaps of 1.5 mm or smaller, the energy efficiency is reduced to less than 70%, due to thermal conduction across product water bridges between the membrane fibre and the condenser surface. An optimal air gap is about 3 mm wide and has a total pressure that is equal to or slightly below the saturated water vapour pressure of the hot water entering the hollow fibre membrane