Flow Filtration Applications

A flow diagram of a simple cross-flow system is shown in Figure 16.12. This is the system likely to be used for batch processing or development rigs it is in essence a basic pump recirculation loop. The process feed is concentrated by pumping it from the tank and across the membrane in the module at an appropriate velocity. The partly concentrated retentate is recycled into the tank for further processing while the permeate is stored or discarded as required. In cross-flow filtration applications, product washing is frequently necessary and... 

Cross-flow-elec trofiltratiou (CF-EF) is the multifunctional separation process which combines the electrophoretic migration present in elec trofiltration with the particle diffusion and radial-migration forces present in cross-flow filtration (CFF) (microfiltration includes cross-flow filtration as one mode of operation in Membrane Separation Processes which appears later in this section) in order to reduce further the formation of filter cake. Cross-flow-electrofiltratiou can even eliminate the formation of filter cake entirely. This process should find application in the filtration of suspensions when there are charged particles as well as a relatively low conduc tivity in the continuous phase. Low conductivity in the continuous phase is necessary in order to minimize the amount of elec trical power necessaiy to sustain the elec tric field. Low-ionic-strength aqueous media and nonaqueous suspending media fulfill this requirement. 

Cross-flow filtration systems utilize high liquid axial velocities to generate shear at the liquid-membrane interface. Shear is necessary to maintain acceptable permeate fluxes, especially with concentrated catalyst slurries. The degree of catalyst deposition on the filter membrane or membrane fouling is a function of the shear stress at the surface and particle convection with the permeate flow.16 Membrane surface fouling also depends on many application-specific variables, such as particle size in the retentate, viscosity of the permeate, axial velocity, and the transmembrane pressure. All of these variables can influence the degree of deposition of particles within the filter membrane, and thus decrease the effective pore size of the membrane. 

Application of cross-flow filtration for the removal of FT wax products can be a useful technique to maintain a constant catalyst loading in an FTS reactor in continuous operation. Addition of 1-dodecanol (at a concentration of 6 wt%) was found to decrease the permeation rate of the cross-flow filter used for the separation of simulated FT wax and activated iron catalyst slurry. However, additional... 

Aliena, F. W. and Belfort, G. Chem. Eng. Sci. 39 (1984) 343. Lateral migration of spherical particles in porous flow channels application to membrane filtration. 

In summary, the development of inorganic membranes was initially oriented towards uranium enrichment which is still by very far their most significant application. Some of the key participants involved in the nuclear programs further developed them into cross-flow filtration membranes. The recent years have seen the start of a much broader exploration of the manyfold potentialities of inorganic membranes, both in terms of materials and applications. Thus, a multifaceted new field of technology is emerging. 

However, the short lifetime of in-line cartridge filters makes them unsuitable for microfiltration of highly contaminated feed streams. Cross-flow filtration, which overlaps significantly with ultrafiltration technology, described in Chapter 6, is used in such applications. In cross-flow filtration, long filter life is achieved by sweeping the majority of the retained particles from the membrane surface before they enter the membrane. Screen filters are preferred for this application, and an ultrafiltration membrane can be used. The design of such membranes and modules is covered under ultrafiltration (Chapter 6) and will not be repeated here. 

Another already mentioned application of membrane filtration is for the recovery of ionic liquids from wastewaters. Here the challenge is to find appropriate membranes, since rejection values that have been reported to date [136] are too low for industrial application. However, for similar ionic liquids we found a membrane that shows rejection rates above 99% throughout at considerably high permeate flow rates above 50 L m 2 h 1 in cross flow filtration. Such numbers make washing in combination with nanofiltration an interesting option. 

Commercial cross-flow filtration units are often supplied as packaged units where the membrane modules, feed pumps, feed and permeate tanks, heat exchangers, pipes, and controls are all arranged on a frame, or skid, which is preassembled and tested in the fabricator s workshop. It is possible to provide these units with a high level of automation and control, depending on the application. 

When CMP was first emerging as an important process, microporous membranes were tested in a tangential flow filtration (TFF) mode. In typical CMP slurries, TFF was found to be unacceptable due to the high solids content. Also, there was insufficient control of the retentate build-up on the membrane face, called concentration polarization. TFF works best with applications that have lower solids and a greater spread between retained and passed species. The excessive polarization we observed with high solids silica slurries prevented useful fractionation. TFF may be useful for the newer slurries with low solids content. Three critical questions to any filter design in slurry applications are (i) how sharp can the filtration curve be to enable clean fractionation, (ii) how is retained material handled to control filter life, and (iii) how does media selection affect the above two points. 

Graded-density submicron depth filters (Fig. 18.13) are suitable for broader PSD slurry global distribution loop filtration. These filters with large surface area and low face velocity are suitable for high flow POU and POD filtration applications, are typically disposable in nature, and may have nominal ratings of 0.2, 0.3, 0.5, and 1.0 pm. 

In a general way, most of ceramic membrane modules operate in a cross-flow filtration mode [28] as shown in Figure 6.18. However, as discussed hereafter, a dead-end filtration mode may be used in some specific applications. Membrane modules constitute basic units from which all sorts of filtration plants can be designed not only for current liquid applications but also for gas and vapor separation, membrane reactors, and contactors, which represent the future applications of ceramic membranes. In liquid filtration, hydrodynamics in each module can be described as one incoming flow on the feed side gf, which results in two... 

Over the past decade, there has been an upsurge of interest in the use of gas bubbles to enhance membrane processes. The typical applications include two-phase flow filtration with tubular membranes and submerged membrane systems. A major stimulus for the latter has been the development of MBRs. 

Membrane operations are conducted either in a direct flow filtration (also called dead end) mode or in a tangential flow filtration (TFF) mode. Direct flow filtration is simple and easy to implement but has limited capacity for applications with high-solid mass. TFF is capable of processing large-solid masses but is more complex and capital intensive. 

In cross-flow filtration (Fig. IB), shear forces are introduced at the cake surface to reduce cake thickness and total cake resistance. It is exclusively used in membrane separation applications to prevent fouling on membranes. 

In cross-flow filtration (Fig. IB) or delayed cake filtration, the slurry flows parallel to the cake surface with sufficient velocity to prevent partially or entirely the deposition of cake. It is used successfully to increase flow rate in membrane filtration. It is also employed for concentrating and recovering very fine particles in dilute suspensions when deep-bed or cake filtration would not applicable. 

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