Crossflow configuration

The permeate is continuously removed through the membrane from the entrance to the exit of the flow channel of the membrane (in frame-and-plaie, tubular or multichannel monolithic elements) as the process stream flows across the membrane surface. Consequently the tangential velocity decreases along the length of the membrane. The velocity at the entrance, however, normally differs from that at the exit only by a small amount since in one pass the amount of permeate removed is relatively small. The operating pressure determines the tangential velocity. 

The feed stream enters the membrane system which, in one pass, separates the feed into two streams the permeate and retentate (or concentrate depending on the applications). Very often the separation efficiency is not sufficient to raise the concentrate of the 

It is understandable that many types of fluid metering, indicating and controlling equipment are an integral part of the membrane system. This is one of the reasons why the investment cost of the membrane module is not a predominant fraction of the total capital costs. 

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Although most of the discussions on backflushing so far use the crossflow configuration as an example, backflushing also can be and has been employed in through-flow membrane separation applications. 

As most of the membrane separation processes arc operated in the crossflow mode for the reasons discussed earlier (Section 5.5.1 Crossflow Configuration), the crossflow velocity has marked effects on the permeate flux. A higher crossflow velocity typically results in a higher flux. The rate of flux improvement as a function of the crossflow velocity usually can be described by the following equation with specific units ... 

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. 

In the crossflow configuration widely used in the membrane separation industry today, the direction of the feed flow is perpendicular to that of the permeate flow. To increase the driving force for the permeate, a carrier stream to "sweep away the permeate is often used. In most of the membrane configurations such as tubes and multichannel monoliths described in Chapter 5, the carrier stream and the feed stream are generally co urrent or counter-current in direction. 

The configuration of the fuel cells is not limited to the tubular geometry. Michaels el al. [1986] fabricated monolithic fuel cells which basically have a crossflow configuration and all unit cells connected in scries (Figure 11.47) or in parallel (Figure 11.48) for the... 

Microfilters use membranes with pores in the 0.1-1 pm range. They can filter out particles of dust, activated carbon, and ion exchange resin fines, and most microorganisms. Microfilters require low differential pressures (5-20 psi) and are available both as normal flow ( dead end ) and crossflow configurations. In pharmaceutical water purification systems, they are often used as disposable cartridge filters after activated carbon filters, softeners, and ion exchange beds. 

If the gases supplied to the cells are connected manifold externally to the stack, then the crossflow configuration is the only option and gas inlets and outlets for the fuel and oxidant can be located on the four sides of the stack. Figure 2.7 shows the cross-flow configuration adopted by CFC Solutions. 

In this section, we therefore consider the crossflow configuration introduced in Figure 7.0.1(e). The gas flows on the feed side of the membrane under the following conditions ... 

An essential feature of the existing analyses of this crossflow configuration carried out by Weller and Steiner (1950a, b) is that the permeate gas stream emerging from the membrane (at any axial location) does not mix with any other permeates emerging from the membrane at other axial locations. Therefore the membrane permeation flux Ni at any permeator location is given by... 

A major feature of the three techniques of continuous chromatography in the crossflow configuration is that the feed stream is introduced only over a small section of the flow cross section of the carrier fluid. Had the feed stream been introduced throughout the flow cross section in the carrier fluid flow direction, multicomponent separation would not have heen possible feed introduction mode is important. We see in Section 8.3.2 that introduction of the feed fluid across the whole flow cross section for this fluid in a crossflow system is a common feature of separation in a plate in a distillation column which can produce only a hinary separation. 

Cooling towers are broadly classified on the basis of the type of draft natural draft (natural convection), mechanical draft (forced convection) and mechanical and natural. Further distinction is made based on (1) the type of flow i.e. - crossflow, counterflow, cocurrent flow (2) the type of heat dissipation-wet (evaporative cooling), dry, wet-dry and (3) the type of application-industrial or power plant. Each of the major types of cooling towers has a distinct configuration. The major designs are summarized in Figures 1 through 8 and a brief description of each follows. 

Cross-flow is the usual case where cake compressibility is a problem. Cross-flow microfiltration is much the same as cross-flow ultrafiltration in principle. In practice, the devices are often different. As with UF, spiral-wound membranes provide the most economical configuration for many large-scale installations. However, capillary devices and cassettes are widely employed, especially at smaller scale. A detailed description of cross-flow microfiltration had been given by Murkes and Carlsson [Crossflow Filtration, Wiley, New York (1988)]. 

The factors to consider in the selection of crossflow filtration include the flow configuration, tangential linear velocity, transmembrane pressure drop (driving force), separation characteristics of the membrane (permeability and pore size), size of particulates relative to the membrane pore dimensions, low protein-binding ability, and hydrodynamic conditions within the flow module. Again, since particle-particle and particle-membrane interactions are key, broth conditioning (ionic strength, pH, etc.) may be necessary to optimize performance. 

Hyperbolic towers also come in crossflow and counterflow configurations and can be recognized by their shape. Crossflow types have the fill outside... 

Reference A3 details the recommended plate configuration for liquid flowrate versus column internal diameter. This suggests a single-pass crossflow-type sieve plate as shown in Figure 9.1. 

Crossflow membrane reactor. For the same catalytic reaction, various types of reactor configurations discussed in Chapter 8 can result in different performances. For example,... 

The geometry of the membrane reactor and the relative locations and flow directions of the feed, permeate and reientate streams all play important roles in the reactor performance. The simplest, but not efficient, membrane reactors consist of disk or foil membranes with a flow-through configuration [Mischenko et al., 1979 Fumeaux et al., 1987]. The same type of membrane reactor can also be consu cted and operated in the more common crossflow mode. 

Finally, possible causes for deactivation of catalytic membranes are described and severad aspects of regenerating catalytic membrane reactors are discussed. A variety of membrane reactor configurations are mentioned and some unique membrane reactor designs such as double spiral-plate or spiral-tube reactor, fuel cell unit, crossflow dualcompartment reactor, hollow-fiber reactor and fluidized-bed membrane reactor are reviewed. 

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