Membrane crossflow

Beer clarification by ceramic membrane crossflow microfiluation has not been commercially practiced yet. But the ongoing application research and development efforts in this area and the potential savings in the diatomaceous earth cost will undoubtedly drive the technology to commercialization. 

Crossflow units — Flat sheet membranes - Rotating filter elements... 

Crossflow Filters - These are usually membrane-type filters used for ultrafiltration. In the field of biotechnology these types of filters are used in ultrafiltration devices used in concentrating solutions, and performing buffer exchanges. 

Paulson, David J, Wilson, Richard L. and Spatz, D. Dean, "Crossflow Membrane Technology and Its Applications," Food Technology 38 (12) 11-87, 111(1984). 

Tangential crossflow filtration Process where the feed stream sweeps the membrane surface and the particulate debris is expelled, thus extending filter life. The filtrate flows through the membrane. Most commonly used in the separation of high-and-low-molecular weight matter such as in ultrapure reverse osmosis, ultrafiltration, and submicron microfiltration processes. 

Combining these equations and integrating yield Cf = CioX for a volume reduction factor X = Q/Qo and the observed component passage Si. This allows one to determine either final concentrations from crossflow rates or the reverse. For a fully retained product (Sj= 0), a 10-fold volume reduction (X = 10) produces a 10-fold more concentrated product. However, if the product is only partially retained, the volume reduction does not proportionately increase the final concentration due to losses through the membrane. 

Equations (20-66) and (20-67) present single-pass formulas relating retentate solute concentration, retentate crossflow, permeate flow, and membrane area. For relevant low-feed-concentration applications, polarization is minimal and the flux is mainly a function of pressure. Spiral or hollow fiber modules with low feed channel and permeate pressure drops are preferred. 

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)]. 

Clarification Using Microfiltration Crossflow filtration (microfiltration includes crossflow 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 par lel 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 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. 

S. Yao, A. G. Fane, J. M. Pope 1997, (An investigation of the fluidity of concentration polarisation layers in crossflow membrane filtration of an oil-water emulsion using chemical shift selective flow imaging), Mag. Reson. Imag. 15, 235. 

Figure 5. Crossflow fluid management in tubular membrane conflguration...

Crossflow technology is increasing, as it proves practical. Micioliltration membranes are of an isotropic and homogeneous morphology, i.e., the pore structure is consistent throughout. There is some movement, however, toward ihe use of "skinned" anisotropic membranes. Microliltration membranes are available in a wide variety ol polymers, including some that arc quite chemically inert. They also tire available as tubular, hollow fiber, or capillary fiber elements. 

Membrane reactors allow a different option for the separation of biocatalysts from substrates and products and for retention in the reactor. Size-specific pores allow the substrate and product molecules, but not the enzyme molecules, to pass the membrane. Membrane reactors can be operated as CSTRs with dead-end filtration (Figure 5.5e) or as loop or recycle reactors (Figure 5.5f) with tangential (crossflow) filtration. 

By resorting to the so-called membrane recycle bioreactors (MBR) (Bubbico et al., 1997 Enzminger and Asenjo, 1986), continuous recycling of the culture broth through crossflow MF modules allows removal of the inhibiting metabolites, this helping to maximize cell density in the bioreactor, as well as bioproduct formation rate. Further ED treatment of MF permeates gives rise to two streams, a diluted one to be recycled back into the bioreactor, and a concentrated one to be supplementary refined. 

R. Sondhi and R. Bhave, Role of Backpulsing in Fouling Minimization in Crossflow Filtration with Ceramic Membranes, 7. Membr. Sci. 186, 41 (2001). 

Flux. The film model (Equation 6.6) illustrates that increasing flux has an exponential effect on CP. If we accept that fouling is a consequence of CP the impact of excessive flux is obvious. As a result high flux membranes tend to be short lived and foul unless improved fluid management is able to enhance k. Selection of the appropriate flux and crossflow velocity is a trade-offbetween capital and operating costs (see cost of fouling below). 

The concept of critical flux ( Jcrit) was introduced by Field et al. [3] and is based on the notion that foulants experience convection and back-transport mechanisms and that there is a flux below which the net transport to the membrane, and the fouling, is negligible. As the back transport depends on particle size and crossflow conditions the Jcrit is species and operation dependent. It is a useful concept as it highlights the... 

As noted in Section 6.1.2, in most applications the control of CP, and fouling, dictates the use of crossflow. However, for dilute feeds and low-pressure membranes it has been accepted that batch cycles of deadend operation with solids accumulation removed by periodic backwash requires potentially lower energy. Usually, deadend is at FF and the TM P cycles from a minimum to maximum or over a specified cycle time during the batch. If fouling occurs it is evident through a steady rise in TM Pmin or Rm. Occasional chemical cleaning may restore Rm. 

The processes of interest are NFand RO where the membranes are either nanoporous or essentially nonporous. In these processes the fouling is a surface layer, the effects of which maybe exacerbated by the high retention of solutes by the membrane. Operation is with crossflow and in industry fixed flux is commonly used. This section considers particulate fouling, biofouling and scale formation and then discusses the implications of cake enhanced concentration polarization on fouling outcomes. 

Figure 8.5 Comparison of (a) crossflow, (b) counterflow and (c) counterflow sweep module performance for the separation of water vapor from air. Membrane water/air selectivity = 100, water permeance = 1000 gpu.

In the crossflow module illustrated in Figure 8.5(a), the pooled permeate stream has a water concentration of 1.88%. The counterflow module illustrated in Figure 8.5(b) performs substantially better, providing a pooled permeate stream with a concentration of 3.49%. Not only does the counterflow module perform the separation twice as well, it also requires only about half the membrane area. This improvement is achieved because the gas permeating the membrane at the residue end of the module contains much less water than the gas permeating the membrane at the feed end of the module. Permeate counterflow dilutes the permeate gas at the feed end of the module with low-concentration permeate gas from the residue end of the module. This increases the water concentration driving force across the membrane and so increases the water flux. 

Counterflow modules are always more efficient than crossflow modules, but the advantage is most noticeable when the membrane selectivity is much higher than the pressure ratio across the membrane and a significant fraction of the most permeable component is being removed from the feed gas. This is the case for air-dehydration membrane modules, so counterflow capillary modules are almost always used. With most other gas-separation applications, the advantage offered by counterflow designs does not offset the extra cost of making the counterflow type of module, so they are not widely used. 

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