Membrane geometry

Correlations for the mass-transfer coefficient, as the Sherwood number for various membrane geometries have been reviewed (39). 

Zeolite/polymer mixed-matrix membranes can be fabricated into dense film, asymmetric flat sheet, or asymmetric hollow fiber. Similar to commercial polymer membranes, mixed-matrix membranes need to have an asymmetric membrane geometry with a thin selective skin layer on a porous support layer to be commercially viable. The skin layer should be made from a zeohte/polymer mixed-matrix material to provide the membrane high selectivity, but the non-selective porous support layer can be made from the zeohte/polymer mixed-matrix material, a pure polymer membrane material, or an inorganic membrane material. 

The mass and heat transfer analogies make possible an evaluation of the mass-transfer coefficient (k) and provide insight into how membrane geometry and fluid-flow conditions can be specified to optimize flux (4). For laminar flow ... 

Membrane Characterization Membranes are always rated for flux and rejection. NaCl is always used as one measure of rejection, and for a very good RO membrane, it will be 99.7 percent or more. Nanofiltration membranes are also tested on a larger solute, commonly MgS04. Test results are very much a function of how the test is run, and membrane suppliers are usually specific on the test conditions. Salt concentration will be specified as some average of feed and exit concentration, but both are bulk values. Salt concentration at the membrane governs performance. Flux, pressure, membrane geometry, and cross-flow velocity all influence polarization and the other variables shown in Fig. 22-63. 

In the process of phase separation by heat treaunent followed by chemical leaching as reviewed in Section 3.2.5, the final membrane geometry (e.g., plate or tube) is... 

As fabricated, a Pd or its alloy membrane suffers from the relatively low catalytic activity of the attached catalyst due to the typical low surface area to volume ratio of the membrane geometry. The catalyst in a dense Pd-based membrane can be the Pd itself or its alloy or some other materials attached to the membrane. Pretreatments to the Pd-based membranes can help alleviate this problem. This and other membrane material and catalysis issues will be further covered in Chapter 9. 

Due to their high stiffness and brittleness, it is not possible to extend to ceramic membranes geometries applicable with organic membranes like spirals, which give rise to high surface volume ratios. The ceramic membranes used for tangential filtration are nsually multichannel tubes or, in some applications, honeycomb monoliths. A very attractive type of membrane is the ceramic hollow fiber with an external diameter of less than 1 mm and ceramic walls with a thickness of a... 

Kq, K, and Bq the constants that are strongly dependent on membrane geometry and the interactions between membrane and molecule... 

In fact, F is also required to be at a minimum with respect to all possible membrane deformations, and this minimization with respect to membrane shape must be carried out self-consistently together with the electrostatic and lipid mixing contributions [27,36]. This presents a challenge, since in principle one has to consider all possible variations in membrane geometry, and these multiple shape deformations generally couple to other degrees of freedom. 

Manufacturer Trade name Membrane material Support material Membrane Geometry of pore membrane diameter element Chaimel inside diameter... 

Way, Noble and Bateman (49) review the historical development of immobilized liquid membranes and propose a number of structural and chemical guidelines for the selection of support materials. Structural factors to be considered include membrane geometry (to maximize surface area per unit volume), membrane thickness (<100 pm), porosity (>50 volume Z), mean pore size (<0.1)jm), pore size distribution (narrow) and tortuosity. The amount of liquid membrane phase available for transport In a membrane module Is proportional to membrane porosity, thickness and geometry. The length of the diffusion path, and therefore membrane productivity, is directly related to membrane thickness and tortuosity. The maximum operating pressure Is directly related to the minimum pore size and the ability of the liquid phase to wet the polymeric support material. Chemically the support must be Inert to all of the liquids which It encounters. Of course, final support selection also depends on the physical state of the mixture to be separated (liquid or gas), the chemical nature of the components to be separated (inert, ionic, polar, dispersive, etc.) as well as the operating conditions of the separation process (temperature and pressure). The discussions in this chapter by Way, Noble and Bateman should be applicable the development of immobilized or supported gas membranes (50). 

Depending upon the membrane geometry, film models can be of two types (a) Uniform flat sheet model, which assumes the membrane to be a planar film and (b) Spherical shell model in which an emulsion globule is characterized as a double shell with the membrane around a single internal phase droplet. Planar geometry models have been used [15-17]. Kremesec and Slattery considered the overall mass transfer resistance as a sum of the resistance through continuous, membrane, and internal phases [17]. 

Two kinds of membrane geometries are predominantly used, the tubular multi-lumen and the multichannel monoliths with circular, hexagonal or honeycomb structures. The number of channels can vary from 1 to 60. 

The coeflBcients and fcj for the feed and product depend on the flow rates, physical properties, and membrane geometry, and they can be predicted using the correlations in Chap. 21. The membrane coeffleient depends on the effective diffusivity and the membrane thickness z ... 

Control of Contact between Catalyst and Reactants - In this use of catalytic membranes, the membrane is porous and in the CMR configuration, and is either intrinsically active or has had a catalyst deposited within the pores. The membrane geometry allows for a degree of control of the contact time. It is operated in the cross-flow mode, in which all of the reactant is forced to flow through the membrane by feeding it to one side with a closed exit. This is illustrated schematically in Figure 19. 

This configuration gives a uniform contact time, which can be tailored to a particular reaction by choice of the membrane thickness and/or reactant flow rate. The pore size of the membrane controls the diffusion regime. Also, the membrane geometry can be used to place a catalyst in the membrane optimally, or to control the partial pressure of the reactants in the phase in contact with the catalyst. 

Membrane geometry Volume concentration factor/diafiltration volumes Protein concentration Yield... 

Although oxygen flux increases inversely with membrane thickness, if membrane thickness becomes smaller than a few hundred microns, it must be supported on a porous substrate. This requires novel methods to fabricate thin membranes mounted on porous supports, membrane sealing materials and techniques, and optimal membrane geometry. 

The two most common membrane geometries are the flat plate and the tube. Single flat plate membranes are usually used in laboratory scale investigations due to their ease of fabrication. Tubular membranes are more and more popular due to their much larger ratio of the membrane surface area to the equipment volume than flat plate membranes [4]. OITM reactor configurations with multi-planar or multi-tubular structures are required for commercial use. 

---