Chapter 5: Design of Membrane Modules for Gas Separations
Published:06 Jul 2011
M. Scholz, M. Wessling, and J. Balster, in Membrane Engineering for the Treatment of Gases: Gas-separation Problems with Membranes, ed. E. Drioli, G. Barbieri, E. Drioli, and G. Barbieri, The Royal Society of Chemistry, 2011, vol. 1, ch. 5, pp. 125-149.
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When membranes are required to be applied in processes or any other applications, they must be assembled in so-called modules. The three major module types for gas separation processes are plate and frame, spiral wound, and hollow fiber modules. These designs differ in packing density, manufacturing cost and their range of application. The plate and frame modules, which are highly effective in pervaporation applications, are much less popular than the spiral wound and hollow fiber modules due to the lower area per volume ratio that can be accommodated. The majority of gas separation membranes are arranged in hollow fiber modules due to their high area per volume ratio. This can be achieved by a high packing density which is required since most membranes have low permeabilities and so high membrane areas are necessary. Hollow fibers are easy to manufacture and manufacturing costs are quiet low. In a hollow fiber module the fibers are in parallel and they are placed in a shell. The fibers are connected to the shell by a potting resin which seals the modules' shell side. It allows the modules to be used with shell or bore-side feed.
The objective of designing hollow fiber modules is to minimize non-ideal effects like pressure losses and concentration polarization. Usually there is a trade-off between reducing pressure losses and reducing concentration polarization. The effect of co-current, counter-current and cross-flow configurations on the mass transfer was elaborated.
In order to obtain an optimal separation performance various operational as well as design parameters have to be considered carefully. Here the flow through the module, which is usually counter-current flow, the location of the feed and active layer are the most important parameters. During module manufacturing various detrimental effects can occur, which reduce the module performance, e.g. variations in fiber dimensions and properties (diameter, length, membrane thickness, permeability) and defects (pinholes, blocked fibers). In order to ensure a proper operation of the membrane module these effects have to be avoided.