Mechanism of Membrane Separation

The property of membranes used in separation processes is their ability to control the permeation of different species. Most membranes fall into one of the two broad categories illustrated in Figure 5. In microporous membranes, permeants are separated by pressure-driven flow through tiny pores. A separation is achieved between different permeants because one of the permeants is excluded (filtered) from some of the pores through which the smaller permeants move. In solution-diffusion membranes the membrane material is a dense polymer layer and contains no fixed pores. Permeants dissolve in the membrane material as in a liquid and then diffuse through the membrane down a concentration gradient. Separation of different permeants oc-

Figure 5 Schematic illustrating the two principal types of membrane separation mechanisms. (A) Microporous membranes separate by molecular filtration. (B) Dense solution-diffusion membranes separate because of differences in the solubility and mobility of permeant in the membrane material.

curs because of differences in the solubility of the permeant in the membrane material and the rate at which the permeant diffuses through the membrane.

The difference between the pore-flow and the solution-diffusion mechanisms lies in the relative size and lifetime of pores in the membrane. In dense polymeric solution-diffusion membranes, no permanent pores exist. However, tiny free volume elements, a few tenths of a nanometre in diameter, exist between the polymer chains from which the membrane is made. These free-volume elements are present as statistical fluctuations that appear and disappear on a timescale only slightly slower than the motion of molecules traversing the membrane. Permeating molecules diffuse from free-volume element to free-volume element at a rate determined by the thermal motion of the polymer chains from which the membrane is made. In contrast, in a pore-flow membrane the pores are fixed and do not fluctuate in position or size on the timescale of molecular motion. The larger the individual free-volume elements are, the more likely they are to be present long enough to produce pore-flow characteristics in the membrane. As a rule of thumb the transition between permanent (pore-flow) and transient (solution-diffusion) pores appears to be in the range 0.5-1.0 nm diameter. This means that the processes of gas separation, reverse osmosis and pervaporation, all of which involve separation of permeants with molecular weights of less than 200, use solution-diffusion membranes. On the other hand, microfiltration and ultrafiltration, which involve separation of macromolecular or colloidal material, use finely microporous pore-flow membranes.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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