Membrane separation cross-flow filtration

A membrane is defined as an intervening phase separating two phases forming an active or passive barrier to the transport of matter. Membrane processes can be operated as (1) Dead-end filtration and (2) Cross-flow filtration. Dead-end filtration refers to filtration at one end. A problem with these systems is frequent membrane clogging. Cross-flow filtration overcomes the problem of membrane clogging and is widely used in water and wastewater treatment. 

Filtration can be used to separate cells or cell debris, concentrate cells or protein solution, and remove or exchange salts. Two broad categories of filtration are conventional dead-end filtration, where fluid flow is normal to the plane of the membrane, and cross-flow filtration (or tangential flow filtration), where the fluid flows parallel to the membrane surface. 

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. 

Filtration Cross-flow filtration (microfiltration includes cross-flow filtration as one mode of operation in Membrane Separation Processes which appears earlier in this section) relies on the retention of particles by a membrane. The driving force for separation is pressure across a semipermeable membrane, while a tangential flow of the feed stream parallel to the membrane surface inhibits solids settling on and within the membrane matrix (Datar and Rosen, loc. cit.). 

The factors to consider in the selection of cross-flow filtration include the cross-flow velocity, the driving pressure, the separation characteristics of the membrane (permeability and pore size), size of particulates relative to the membrane pore dimensions, and the 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. 

Membrane modules can be configured in various ways to produce a plant of the required separation capability. A simple batch recirculation system has already been described in cross-flow filtration. Such an arrangement is most suitable for small-scale batch operation, but larger scale plants will operate as feed and bleed or continuous single pass operation (Figure 16.20). 

All experimental setups described in the literature for the separation of homogeneous catalysts by membrane filtration technology can be divided into two general classes Dead-end filtration and cross-flow filtration. The first type of unit is characterized by a product flow perpendicular to the surface of the membrane, while the flow in the case of cross-flow filtration is parallel to the membrane surface (see Figure 4.1). 

A continuous cross-flow filtration process has been utilized to investigate the effectiveness in the separation of nano sized (3-5 nm) iron-based catalyst particles from simulated Fischer-Tropsch (FT) catalyst/wax slurry in a pilot-scale slurry bubble column reactor (SBCR). A prototype stainless steel cross-flow filtration module (nominal pore opening of 0.1 pm) was used. A series of cross-flow filtration experiments were initiated to study the effect of mono-olefins and aliphatic alcohol on the filtration flux and membrane performance. 1-hexadecene and 1-dodecanol were doped into activated iron catalyst slurry (with Polywax 500 and 655 as simulated FT wax) to evaluate the effect of their presence on filtration performance. The 1-hexadecene concentrations were varied from 5 to 25 wt% and 1-dodecanol concentrations were varied from 6 to 17 wt% to simulate a range of FT reactor slurries reported in literature. The addition of 1-dodecanol was found to decrease the permeation rate, while the addition of 1-hexadecene was found to have an insignificant or no effect on the permeation rate. 

The objective of the present study is to develop a cross-flow filtration module operated under low transmembrane pressure drop that can result in high permeate flux, and also to demonstrate the efficient use of such a module to continuously separate wax from ultrafine iron catalyst particles from simulated FTS catalyst/ wax slurry products from an SBCR pilot plant unit. An important goal of this research was to monitor and record cross-flow flux measurements over a longterm time-on-stream (TOS) period (500+ h). Two types (active and passive) of permeate flux maintenance procedures were developed and tested during this study. Depending on the efficiency of different flux maintenance or filter media cleaning procedures employed over the long-term test to stabilize the flux over time, the most efficient procedure can be selected for further development and cost optimization. The effect of mono-olefins and aliphatic alcohols on permeate flux and on the efficiency of the filter membrane for catalyst/wax separation was also studied. 

Cross-flow filtration is also referred to as tangential flow filtration or microfiltration, but all three terms refer to a process by which membranes are used to separate components in a liquid solution (or suspension) on the basis of their size. The development of robust membranes in polymeric and ceramic materials has provided a powerful new technology for bioseparations, which is already widespread in the process industries as well as for water treatment processes. 

Cross-flow filter performance is often characterized by a flux rate, which equates to the permeate flow rate per unit area of membrane surface. The flux rate in most biological separations is reduced by a fouling phenomenon called gel polarization, which tends to concentrate material at the surface of membrane to impose an additional resistance to transmembrane flow. The deterioration in flux rate must be well characterized for a commercial bioseparation process to ensure the correct size for the cross-flow filtration unit and avoid hold-ups at this processing stage. 

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... 

There are two different levels where fouling phenomena and related effects may interfere with performance of composite inorganic or hybrid membranes. The first and the more classically reported in literature is the one of the separation process itself, which through various interactions between solution and material (adsorption, surface deposits, pore plugging) generally leads to reduced fluxes and increased retentions. The second, much more less described by authors but of the same nature and with analogous effects, concerns membrane preparation, and the possible interactions between deposited layers. Theses two aspects are linked up with the so-called formed-in-place membranes, obtained by deposition of species onto a ceramic support through cross-flow filtration. In what follows, they will be described in a unified approach. 

M.Y. Jaffrin, R. Ben Amar and B.B. Gupta, Membrane fouling control in cross flow filtration of wine with mineral membranes, in International Technical Conference on Membrane Separation Processes, Brighton, UK, 24-26 May 1989, Paper E2. 

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. 

Table 1 shows the types of separations achievable with MF, UF and RO membranes when operated in cross-flow configuration. For MF or UF application, the choice of membrane materials includes ceramics, metals or polymers, whereas for RO at the present only polymer membranes are predominantly used. Although cross-flow filtration is practiced in all the above three types of membrane applications, the description of membrane... 

The distinction between cross-flow and dead end (also known as through-flow) filtration can be better understood if we first analyze the mechanism of retention. The efficiency of cross-flow filtration is largely dependent on the ability of the membrane to perform an effective surface filtration, especially where suspended or colloidal particles are involved. Table 2 shows the advantages and versatility of cross-flow filtration in meeting a broad range of filtration objectives, Figure 2 illustrates the differences in separation mechanisms of CFF versus dead end filtration. 

Almost all cross-flow filtration processes are inherently susceptible to flux decline due to membrane fouling (a time-dependent phenomenon) and concentration polarization effects which reflect concentration buildup on the membrane surface. This means lower flux (i.e., product output) which could drive the capital costs higher due to the requirement of a larger surface area to realize the desired production rate. In some situations, the lower flux could also result in lower selectivity which means reduced recoveries and/or incomplete removal of impurities from the filtrate. For example, removal of inhibitory metabolites such as lactic acid bacterial or separation of cells from broth while maximizing recovery of soluble products. 1 1... 

The major application today for cross-flow filtration is in the membrane filtration for bioprocessing or fine particle separations. Based on the size of the particles separated, membrane filtrations are categorized as microflltration (MF), ultraflltration (UF), nanoflltration (NF). and reverse osmosis (RO). The ratings of MF membranes are by micron ratings, just like other fabric filter media. In... 

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