Shell-and-tube modules. The tubular configuration resembles the shell-and-tube heat exchanger except that membranes replace the tubes through which a radial mass flux takes place. This is the most popular configuration for commercial UF, NF, and MF units. A tubular membrane module consists of membrane tubes placed into porous stainless steel fiber glass-reinforced plastic pipes. The diameter of tubular membranes typically varies between 1.0 and 2.5 cm, with a packing density, which is the ratio between the membrane area and the given packing volume of approximately 300 m2 -/m3. In MD operations, such kinds of modules are used for high-viscosity fluids; they also allow the achievement of high feed flow rates that reduce fouling tendency and polarization phenomena. In a capillary membrane module, a large number of membrane capillaries (inner diameter of 0.2-3 mm) are arranged in parallel as a bundle in a shell tube; packing density is in the order of 600-1200 m2/m3 . The biggest disadvantage ofshell-and-tube modules is that damaged membranes cannot be replaced as easily as in flat sheet apparatuses. Therefore, the module use is limited by the membrane life.
Hollow fiber membranes (diameter <0.5 mm) provide high surface area per unit volume, making the flux density greater than in other configurations. However, the softness of the membrane and the small fiber diameter make it susceptible to fouling and damage. The outer diameter typically ranges between 50 and 100 mm, and several thousands of fibers are installed in the vessel. This configuration has the highest packing density (~3000m2/m3). The basic features of thermal MD modules  include that housing and membranes must be resistant to temperature and chemicals, and that capillaries have to be adequately potted free of cracks and with a good adhesion. It must be ensured there is uniform flow through capillaries avoiding dead corners or channel formation.
In MD, it is necessary to achieve high heat transfer coefficients in the tube and on the shell side. Liquid channeling makes this difficult to achieve on the shell side especially when large bundles of fibers are involved. Various modifications have been proposed to the standard shell-and-tube configuration in order to promote mixing so as to reduce fouling and promote turbulence at the membrane surface. Schnider et al. suggested that membrane twisting or braiding promotes mixing . Modifications of module hydrodynamics to reduce fouling are also an active research area .
Flat sheet modules. Flat sheet membranes are used in cross-flow and stirred cells where the membrane needs to be easily removed for replacement and treatment. The packing density is considered low for these modules. Therefore, flat membranes are usually incorporated into plate-and-frame or spiral-wound modules. In plate-and-frame modules, the membranes, the porous support plates, and the spacers are stacked between two endplates and placed in an appropriate housing. In this configuration, the packing density is about 100-400 m2/m3, depending on the number of membrane used. Various cassettes stacked together, each consisting of injection molded plastic frames containing two membranes, intermediate feed channel for warm saltwater, and condensing walls have been used by Andersson et al. (1987) for desalination purposes .
In spiral-wound modules, the feed flow channel spacer, the membrane, and the porous support are enveloped and rolled around a perforated central collection tube. The feed solution moves in axial direction through the feed channel across the membrane surface. The permeate flows radially toward the central pipe. The packing density of this setup is about 3001000 m /m , depending on the channel height. The use of spiral-wound MD modules at industrial level has been proposed for desalination [34,35].
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