Membrane module geometry

The function of the gas-separation membrane module is fourfold. First, it must contain the membrane within a pressure housing rated for the apphcation. Second, it must have fittings to introduce feeds and to collect and distribute products leaving the device. Third, it must have internal means for gas-tight sealing between the feed and permeate sides of the membrane and with the containment vessel. Fourth, it must direct the gases in a prescribed manner uniformly over the membrane surfaces. 

GKSS presently employs a flat-film supported on a stack of hoUow ceramic discs in their vapor-recovery devices. Baffling between the discs directs the feed-gas flow across the membrane-covered discs in the stack. The interior of the discs communicates with a central permeate-coUection conduit The short permeate flow path of this design is particularly suited to operating the permeate side under vacuum conditions. 

The more common design employing the flat-sheet is the spiral-wound membrane design shown in Fig. 2.3. 

Permeate flow after passing through membrane 

Residue flow Penneate flow Residue flow 

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Aside from membrane morphology, various membrane module geometries were also studied to determine their appropriateness in various dairy processes. Operation of tubular ceramic membranes usually involves high TMP and pressure drop... 

In Example 9.2, perfect mixing was assumed on both sides of the membrane. Three other idealized flow patterns, common to other mass-transfer processes, have been studied countercurrent flow, cocurrent flow, and crossflow. For a given cut, the flow pattern can significantly affect the degree of separation achieved and the membrane area required. For a given membrane module geometry, it is not always obvious which idealized flow pattern to assume. Hollow-fiber modules are the most versatile since they may be designed to approximate any of the three flow patterns mentioned above. 

Membrane module geometry - Can the membrane formed be incorporated into a module geometry that accommodates conduits for feed and product gases, optimum driving force for the separation, efficient membrane area density, and with minimal pressure head loss (energy) ... 

All UF membranes require periodic cleaning to maintain their hydraulic capacity. Modern systems are designed and built with integral PLC controls to completely automate the cleaning procedures. Selection of the proper membrane module geometry for the specific application will minimize the cleaning frequency and chemical consumption. 

Factors affecting RO membrane separations and water flux include feed variables such as solute concentration, temperature, pH, and pretreatment requirements membrane variables such as polymer type, module geometry, and module arrangement and process variables such as feed flow rate, operating time and pressure, and water recovery. 

Membrane systems consist of membrane elements or modules. For potable water treatment, NF and RO membrane modules are commonly fabricated in a spiral configuration. An important consideration of spiral elements is the design of the feed spacer, which promotes turbulence to reduce fouling. MF and UF membranes often use a hollow fiber geometry. This geometry does not require extensive pretreatment because the fibers can be periodically backwashed. Flow in these hollow fiber systems can be either from the inner lumen of the membrane fiber to the outside (inside-out flow) or from the outside to the inside of the fibers (outside-in flow). Tubular NF membranes are now just entering the marketplace. 

Equation (20-94) gives a permeate concentration as a function of the feed concentration at a stage cut, 0 = 0, To calculate permeate composition as a function of 0, the equation may be used iteratively if the permeate is unmixed, such as would apply in a test cell. The calculation for real devices must take into account the fact that the driving force is variable due to changes on both sides of the membrane, as partial pressure is a point function, nowhere constant. Using the same caveat, permeation rates may be calculated component by component using Eq. (20-86) and permeance values. For any real device, both concentration and permeation require iterative calculations dependent on module geometry. 

The separation efficiency (e.g. permselectivity and permeability) of inorganic membranes depends, to a large extent, on the microstructural features of the membrane/support composites such as pore size and its distribution, pore shape, porosity and tortuosity. The microstructures (as a result of the various preparation methods and the processing conditions discussed in Chapter 2) and the membrane/support geometry will be described in some detail, particularly for commercial inorganic membranes. Other material-related membrane properties will be taken into consideration for specific separation applications. For example, the issues of chemical resistance and surface interaction of the membrane material and the physical nature of the module packing materials in relation to the membranes will be addressed. 

Inorganic membranes commercially available today are dominated by porous membranes, particularly porous ceramic membranes which are essentially the side-products of the earlier technical developments in gaseous diffusion for separating uranium isotopes in the U.S. and France. Summarized in Table 3.1 are the porous inorganic membranes presently available in the market (Hsieh 1988). They vary greatly in pore size, support material and module geometry. 

Design of the membrane module system involves selection of the membrane material the module geometry, eg, spiral-wound or hollow-fiber product flow rate and concentration solvent recovery operating pressure and the minimum tolerable flux (9,11). The effects of these variables can be obtained from laboratory or pilot experiments using different membranes and modules. The membrane module as well as the solvent recovery can be chosen to minimize fouling. Spiral-wound modules are widely used because these offer both high surface area as well as a lower fouling potential. 

A comprehensive presentation of all membrane types, modules and geometries is beyond the scope of this chapter, reference available membrane books for details [12,17, 55, 60, 71, 77,90]. The examples in Figure 16.2 are an illustration of a typical membrane module and installation. The most widespread FS membrane system is mounted as a spiral-wound (SW) unit. In the SW example the actual membrane module is shown together with how they are mounted inside a pressure vessel. A typical installation is shown where several pressure vessels are subsequently mounted in a stack. Pressurized HF units are typically operated as a crossflow system. In the example shown the HF modules are mounted vertically and arranged in a skid. Several variations of the theme can be found depending on the type of module and the manufacturer, where Figure 16.2 is not specific to a particular item. 

There are also techniques involving the use of nonporous, solid or liquid membranes that separate the donor phase from the receiving phase by an evident phase boundary. Most often used are three-phase systems (donor phase, membrane, and acceptor phase) or two-phase systems, in which one of the surrounding phases is the same as the membrane. Solid membranes are made of chemically resistant, hydrophobic polymers (PTFE, PVDF, PS, PP, silicates), metals (Pd alloys), or ceramic materials. Channels of membrane modules have a volume ranging from 10 to 1000 pL and, according to their geometry, can be classified as planar or fibrous. For setting up a membrane system, two modes can be used the membrane can be immersed in a sample (membrane in sample, MIS) or the sample can be introduced into a membrane (sample in membrane, SIM). In both systems, only a small amount of sample is in direct contact with membrane, because ratio of the membrane surface area to the sample volume is small. 

Shown in Table 5.3 is the packing density of the commercially available inorganic membrane modules. The single plate geometry has a very low packing density. Single tubes typically also show a low packing density unless the membrane tube and the module both have small diameters which are not practical other than for laboratory... 

Packing densities of modules containing various inorganic membrane element geometries... 

Membrane element geometry in module Packing density (m /m ) Note... 

Different supports are used, (see Section 10.6.4) with different geometry (discs or tubes), thickness, porosity, tortuosity, composition (alumina, stainless steel, silicon carbide, mullite, zirconia, titania, etc.), and symmetry or asymmetry in its stmcture. Tubular supports are preferable compared to flat supports because they are easier to scale-up (implemented as multichannel modules). However, in laboratory-scale synthesis, it is usually found that making good quality zeolite membranes on a tubular support is more difficult than on a porous plate. One obvious reason is the fact that the area is usually smaller in flat supports, which decreases the likelihood of defects. In Figure 10.1, two commercial tubular supports, one made of a-alumina (left side) and the other of stainless steel (right side) used in zeolite membrane synthesis, are shown. Both ends of the a-alumina support are glazed and both ends of the stainless steel support are welded with nonporous stainless steel to assure a correct sealing in the membrane module and prevent gas bypass. 

For the study of the process, a set of partial differential model equations for a flat sheet pervaporation membrane with an integrated heat exchanger (see fig.2) has been developed. The temperature dependence of the permeability coefficient is defined like an Arrhenius function [S. Sommer, 2003] and our new developed model of the pervaporation process is based on the model proposed by [Wijmans and Baker, 1993] (see equation 1). With this model the effect of the heat integration can be studied under different operating conditions and module geometry and material using a turbulent flow in the feed. The model has been developed in gPROMS and coupled with the model of the distillation column described by [J.-U Repke, 2006], for the study of the whole hybrid system pervaporation distillation. 

Ghogomu, J.N. Guigui, C. Roucha, J.C. Clifton, M.J. Aptel, P. Hollow-fibre membrane module design comparison of different curved geometries with Dean vortices. J. Membr. Sci. 2001, 181, 71-80. 

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