Transfer across membranes side effects

The ABC materials encourage transfer in numerous instances. Particularly good examples are found in the Challenge sections that conclude the laboratories. The Water unit, for example, includes a laboratory designed to help students understand how molecules move across membranes. The stated purpose of the laboratory is to help students determine the effect of concentration difference on the movement of water and solute across a membrane. The laboratory s stated objective is to enable students to predict the direction of material movement across a membrane based on the concentration of materials on both sides of the membrane. During the laboratory, students measure mass with a balance and work with dialysis bags. At the conclusion of the laboratory, students explore questions designed to help them transfer what they have learned to contexts outside the classroom ... 

This simple formula directly expresses the relationship between water vapour flux and the driving force for mass transfer across the membrane (i.e., the water vapour pressure difference between feed and permeate side). However, in this process heat is also transferred, and thus heat and mass transfer are intimately linked with the consequent development of both concentration and temperature profiles. Moreover, the temperature difference across the membrane while creating a thermal gradient in the fluid phase induces mass transfer, due to the Soret effect. 

An important effect making the MD process different from traditional heat exchanges, the temperature polarisation and concentration polarisation occur in the membrane wall due to the transfer of both water vapour and latent heat. As previously stated, the heat and mass transfer across the membrane move from the hot feed stream to the cold permeate one. The temperature gradients cause a difference in temperature between the Uquid-vapour interfaces and the bulk temperatures on both sides of the membrane. This effect, in membrane science called temperature polarisation, reduces the water vapour flux and in literature it is measured by the so-called temperature polarisation coefficient (t), given by ... 

Pervaporation is characterized by the imposition of a barrier (membrane) layer between a liquid and a vaporous phase, with a mass transfer occurring selectively across the barrier to the vapor side. Separation occurs with the efficacy of the separation effect being determined by the physio chemical structure of the membrane. 

A MC module contains thousands of microporous hollow fibres, which are knitted into a fabric that is wound around a distribution tube with a central baffle as shown in Figure 1.15. The baffle ensures the water is distributed across the fibres, and also results in reduced pressure drop across the contactor. The hollow fibres are packed densely in a membrane module with a surfrce area of up to 4000 n / m. The liquid flows outside (shell side) the membrane, while vacuum is appHed on the inside of the fibre (tube side) forming a film across the pores of the membrane. Mass transfer takes place through this film and the pores due to the difference in the gas partial pressure between the shell side and tube side. Since the membranes are hydrophobic, they are not wetted by water, thereby, efiectively blocking the flow of water through the membrane pores. The membrane provides no selectivity. Rather its purpose is to keep the gas phase and the Hquid phase separated. In effect, the membrane acts as an inert support that allows intimate contact between gas and liquid phases without dispersion. Vacuum on the tube side of the membrane increases the mass transfer rate as in a vacuum tower. The efficiency of the process is enhanced with the aid of nitrogen sweep gas flowing on the permeate side of the membrane. 

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