Composite Membranes

The above discussion also applies to the formation of composite or multilayer membranes. Composite membranes can be categorized as either a dense, iso-tropic or asymmetric coating of a high performance separating layer on a microporous substrate. Composite membranes are fabricated in two operations, substrate formation followed by dip coating, allowing for the independent optimization of coating and substrate properties. Gas transport through composite membranes is described by the series resistance model by analogy to an electrical circuit. For a two-layer composite consisting of a thin layer of polymer A on a substrate of polymer B, the flux of gas i though the membrane is given by eqn [17], where l is the thickness of the respective layers:

Figure 18 Hollow-fibre membrane element.

Regardless of their method of formation a critical element of gas separation membranes is that the skin layer must be as thin as possible in order to produce a high flux membrane. The practical limit to the skin layer thickness is thought to be in the range of 500-1000 A. Concurrently, the skin layer must be free of manufacturing defects or pinholes. A defective surface area fraction as low as 10~5% can lower selectivity to an extent that the membrane is not suitable for gas separation. Conventional manufacturing processes are not capable of achieving this level of reliability. The second enabling development in commercializing gas separation membrane technology was the demonstration of a poly(dimethyl-siloxane) defect repair coating to effectively 'plug' manufacturing pinholes and eliminate bulk flow through the defects (Figure 14). Gas permeation through this multicomponent system is described by the series resistance model analogous to an electrical

Figure 18 Hollow-fibre membrane element.

circuit. The total resistance is given by the sum resistances in the coating layer (Rc); the parallel resistance in the defective skin layer; (Rskjd); and the resistance of the substrate Rsub:

After repair, the resistance of the defect, R'd is greater than Rd and also, Rsk > R'd , so that the effective permeability of the composite approaches the intrinsic permeability of a defect-free membrane.

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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