Membranes flow patterns

Staykova, M., Lipowsky, R., and Dimova, R. (2008) Membrane flow patterns in multicomponent giant vesicles induced by alternating electric fields. Sq/t Matter, 4 (11), 2168-2171. 

Improvements ia membrane technology, vahdation of membrane iategrity, and methods to extend filter usage should further improve the performance of membrane filters ia removal of viral particles. Methods to improve or extead filter life and iacrease flow rates by creating more complex flow patterns could possibly be the focus of the next generation of membrane filters designed to remove viral particles. 

Flow inside the capillary membranes, depicted in the lower half of the plot and indicated by positive velocities, shows a regular pattern. The single capillaries are resolved, and flow inside each capillary possesses almost identical maximum velocities. Flow outside the membranes (upper half, negative velocities) reveals a different pattern. Flere, the different flow characteristics between the SMC and SPAN modules become distinct. Obviously, the capillaries in the SMC module are not packed in a regular manner. Large spaces in between the capillaries cause an irregular flow pattern in the dialysate-side with a maximum velocity of about -15 mm s-1 (Figure 4.6.2(a)), which is comparable to the maximum velocity in... 

Figure 10.11 Idealized flow patterns in membrane separation.

The basic assumption of well-mixed fluid on the feed-side of the membrane does not reflect the flow patterns for the configurations used in practice. The assumption simplifies the calculations and allows the basic trends to be demonstrated. 

Gas and liquid flow up along the membrane, then turn to the inlet of a narrow channel at the top of the chamber. This flow pattern enhances continuous replacement of electrolyte over the whole membrane surface. It is especially effective in eliminating gas stagnation at the top zone of the electrolysis area. The DAM-type system ensures that the fine-bubble flow is constant through its narrow channel and that smooth gas separation occurs at the outlet of the channel. Gas and liquid flow separately through an upper duct, an outlet nozzle and an outlet hose, then to a... 

Figure 19.3. Tubular and plate-and-frame membrane modules for reverse osmosis and ultrafiltration, (a) Construction and flow pattern of a single 1 in. dia tube with membrane coating on the inside in Table 19.4, the Ultracor model has seven tubes in a shell and the Supercor has 19 [Koch Membrane Systems (Abcor)]. (b) Assembly of a plate-and-frame ultrafiltration module (Danish Sugar Co.), (c) Flow in a plate-and-frame ultrafiltration module.

Figure 19.5. The Permasep hollow fiber module for reverse osmosis, (a) Cutaway of a DuPont Permasep hollow fiber membrane module for reverse osmosis a unit 1 ft dia and 7 ft active length contains 15-30 million fibers with a surface area of 50,000-80,000 sqft fibers are 25-250 pm outside dia with wall thickness of 5-50pm (DuPont Co.), (b) The countercurrent flow pattern of a Permasep module.

Equation 16 assumes that a concentration gradient exists only in the direction perpendicular to the membrane surface, that is, the flow pattern in the cell is turbulent in such a way that vertical concentration gradients are eliminated, as observed using a tracer dye by Krol et al. (1999). 

The effect of concentration polarization on specific membrane processes is discussed in the individual application chapters. However, a brief comparison of the magnitude of concentration polarization is given in Table 4.1 for processes involving liquid feed solutions. The key simplifying assumption is that the boundary layer thickness is 20 p.m for all processes. This boundary layer thickness is typical of values calculated for separation of solutions with spiral-wound modules in reverse osmosis, pervaporation, and ultrafiltration. Tubular, plate-and-ffame, and bore-side feed hollow fiber modules, because of their better flow velocities, generally have lower calculated boundary layer thicknesses. Hollow fiber modules with shell-side feed generally have larger calculated boundary layer thicknesses because of their poor fluid flow patterns. 

M 20] [P 19] An investigation of the flow patterns in the mixer outlet was made by dilution-type dye imaging [30], On contacting the two streams only in the mixer devices without any membrane actuation (static or passive case), a bi-layered system results with no obvious degree of mixing, as is to be expected. 

For a defect-free ideal membrane, the selectivity is independent of thickness, and either permeability ratios or permeance ratios can be used for comparison of selectivi-ties of different materials. Nonideal module flow patterns, defective separating layers, impurities in feeds, and other factors can lower the actual selectivity of a membrane compared to tabulated values based on ideal conditions (Koros and Pinnau, 1994). 

Other important considerations in process bioseparations are fluid management and membrane rejuvenation methods. Crossflow, or flow tangential to the membrane surface, induces shear at the membrane surface and helps reduce concentration polarization. This flow pattern also creates lift forces that counteract the deposition of particulate matter on the membrane resulting from permeation flow normal to the membrane surface. (See Section I.A.)... 

A number of factors can lead to high pressure drop, including membrane scaling, colloidal fouling, and microbial fouling. These three factors all involve deposition of material onto the surface of the membrane as well as onto components of the membrane module, such as the feed channel spacer. This causes a disruption in the flow pattern through the membrane module, which, in turn, leads to frictional pressure losses or an increase in pressure drop. 

Another source of deviations to the ideal behavior is the smoothness of the channel surface which, in reality, is hardly perfect. The surface quality affects substantially both retention and zone dispersion. Smith et al. [223] illustrated this fact experimentally for Th-FFF. Dilks et al. [458] studied experimentally the effect of sample injection and flow pattern on the zone shape inside the channel by performing measurements in a transparent channel and photographing the colored zones formed under various conditions of injection, flow, and geometric channel irregularities. One important result was that even apparently minor channel irregularities can give rise to considerable distortion of the zone formed. In Fl-FFF, the membrane is the critical parameter as ideally it has to fulfill the requirements of pressure and mechanical stability, even surface, uniform pore size, inert behavior with respect to solvent and samples and sufficient counter pressure to achieve smooth and uniform flow rates. A membrane fulfilling all the above requirements does not exist so that the choice of a membrane for Fl-FFF is always a compromise and depends on the analytical problem. In addition, for all other FFF techniques, the surface quality, in particular the smoothness of the channel accumulation wall, substantially affects both retention and zone dispersion. Smith et al. [223] illustrated this fact experimentally for Th-FFF. 

In a separate parametric study, Mohan and Govind(l)(9) analyzed the effect of design parameters, operating variables, physical properties and flow patterns on membrane reactor. They showed that for a membrane which is permeable to both products and reactants, the maximum equilibrium shift possible is limited by the loss of reactants from the reaction zone. For the case of dehydrogenation reaction with a membrane that only permeates hydrogen, conversions comparable to those achieved with lesser permselective membranes can be attained at a substantially lower feed temperature. 

The driving force for the diffusion of across the solid electrolyte membrane is the difference in O2 concentration between the anode and cathode side. This difference is a result of depletion of O2 by partial oxidation of CzHe on the anode side. Since the concentration of oxygen in the anode has a profound effect on the concentration gradient of 0 across the electrolyte membrane, the flow pattern of fuel/air mixture around the fuel cell has to be carefully controlled through the flow geometry. 

None of the above studies, however, deals with the detailed hydrodynamics in a membrane reactor. It can be appreciated that detailed information on the hydrodynamics in a membrane enhances the understanding and prediction of the separation as well as reaction performances in a membrane reactor. All the reactor models presented in Chapter 10 assume very simple flow patterns in both the tube and annular regions. In almost all cases either plug flow or perfect mixing is used to represent the hydrodynamics in each reactor zone. No studies have yet been published linking detailed hydrodynamics inside a membrane reactor to reactor models. With the advent of CFD, this more complete rigorous description of a membrane reactor should become feasible in the near future. 

The flow patterns of the feed, permeate and retentate streams can greatly influence the membrane reactor performance. First of all, the crossflow configuration distinctly differs from the flow>through membrane reactor. In addition, among the commonly employed crossflow arrangements, the relative flow direction and mixing technique of the feed and the permeate have significant impacts on the reactor behavior as well. Some of these effects are the results of the contact time of the reactant(s) or produces) with the membrane pore surface. 

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