Filtration crossflow

Crossflow filtration can be divided into two categories, namely, ultrafiltration if dissolved species such as proteins are filtered or microfiltration if suspended particles are present. 

At steady state, the rate of convective mass transfer of solute is equal to the rate of mass transfer diffusing away from the surface [119] 

Prom Equation 12.21, it can be seen that the polarization modulus Cm/cf, is extremely sensitive to changes in J and k because of exponential functionality involved. For high molecular weight solutes (small k and D) and membrane with high solvent permeability (high J), concentration polarization becomes severe, resulting in precipitation of solute and formation of solids or gel layer on the membrane surface. 

Microfiltration is used when suspended particles are present. For suspended particles, the theory presented for dissolved solids earlier is only valid for very small suspended particles up to approximately 1 /um in size [120]. Two different theories are presented for larger size particles, namely, a shear induced diffusion theory [121] and an inertial lift theory [122]. Details of these theories are beyond the scope of this book. Please refer to [121] [123] for more information. 

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J. Murkes and C. G. Cadsson, Crossflow Filtration, John Wiley Sons, Inc., New York, 1988. 

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. 

P. Zumbusch, W. Kulcke, G. Brunner. Use of alternating electric fields as antifouling strategy in ultrafiltration of biological suspensions. Introduction of a new experimental procedure for crossflow filtration. J Memb Sci 142-.15 (1998). R. L. Rowley, T. D. Shupe, M. W. Schuck. A direct method for determination of chemical potential with molecular dynamics simulations. 1. Pure components. Mol Phys 52 841, 1994. 

D. Jiao and M. M. Sharma. Mechanism of cake buildup in crossflow filtration of colloidal suspensions. J Colloid Interface Sci, 162(2) 454-462, February 1994. 

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)]. 

Regardless of the location of the protein and its state, cell separation needs to be inemensive, simple, and reliable, as large amounts of fermentation-broth dilute in the desired product may be handled. The objectives are to obtain a well-clarified supernatant and solids of maximum dryness, avoiding contamination by using a contained operation. Centrifugation or crossflow filtration is t ically used for cell separation, and both unit operations can be run in a continuous-flow mode [Datar and Rosen, in Stephanopoulos (ed.), op. cit., pp. 369-503]. In recent years, e3q>anded-bea adsorption has become an alternative. It combines broth clarification and adsorption separation in a single step. 

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. 

Cross-flow filters behave in a way similar to that normally observed in crossflow filtration under ambient conditions increased shear-rates and reduced fluid-viscosity result in an increased filtrate number. Cross-microfiltration has been applied to the separation of precipitated salts as solids, giving particle-separation efficiencies typically exceeding 99.9%. Goemans et al. [30] studied sodium nitrate separation from supercritical water. Under the conditions of the study, sodium nitrate was present as the molten salt and was capable of crossing the filter. Separation efficiencies were obtained that varied with temperature, since the solubility decreases as the temperature increases, ranging between 40% and 85%, for 400 °C and 470°C, respectively. These workers explained the separation mechanism as a consequence of a distinct permeability of the filtering medium towards the supercritical solution, as opposed to the molten salt, based on their clearly distinct viscosities. 

If the product is secreted, recovery may involve a simple filtration step to remove any cells and cellular debris. Other clarification techniques include centrifugation and expanded bed adsorption [25], For such intracellular products as recombinant proteins produced in E. coli, the product may be denatured and located in inclusion bodies within the cells [26], Bacterial cells are typically concentrated by centrifugation or crossflow filtration, washed, and then disrupted by homogenization. Inclusion bodies are then isolated, and the protein product extracted and refolded. Validation of recovery operations for an E. coli product is described by Seely et al. [27],... 

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. 

Datar RV, Rosen C-G (1996) Cell and cell debris removal centrifugation and crossflow filtration. In Stephanopoulos G (ed) Bioprocessing. VCH, Weinheim, p 472... 

M.Y. Jaffrin, B.B. Gupta and P. Paullier, Energy Savings Pulsatile Mode Crossflow Filtration, J. Membr. Sci. 86, 281 (1994). 

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

As the liquid passes through the membrane in crossflow filtration, the particles, macromolecules, colloids, and so on, rejected by the membrane will accumulate in... 

The basic hydrodynamic equations are the Navier-Stokes equations [51]. These equations are listed in their general form in Appendix C. The combination of these equations, for example, with Darcy s law, the fluid flow in crossflow filtration in tubular or capillary membranes can be described [52]. In most cases of enzyme or microbial membrane reactors where enzymes are immobilized within the membrane matrix or in a thin layer at the matrix/shell interface or the live cells are inoculated into the shell, a cake layer is not formed on the membrane surface. The concentration-polarization layer can exist but this layer does not alter the value of the convective velocity. Several studies have modeled the convective-flow profiles in a hollow-fiber and/or flat-sheet membranes [11, 35, 44, 53-56]. Bruining [44] gives a general description of flows and pressures for enzyme membrane reactor. Three main modes... 

Asymmetric hollow fibers provide an interesting support for enzyme immobilization, in this case the membrane structure allows the retention of the enzyme into the sponge layer of the fibers by crossflow filtration. The amount of biocatalyst loaded, its distribution and activity through the support and its lifetime are very important parameters to properly orientate the development of such systems. The specific effect that the support has upon the enzyme, however, greatly depend upon both the support and the enzyme involved in the immobilization as well as the method of immobilization used. 

The following processes can be described as selective therapeutic plasmapheresis. In a first step, blood is withdrawn from the patient and separated by crossflow filtration in a hollow-fiber membrane cartridge water and some plasma solutes are transferred through a semipermeable membrane under a convection process. The transmembrane pressure applied from blood to filtrate compartment ensures flow and mass transfers. Then, the filtrate perfuses the adsorption columns where toxins are retained and is finally mixed with blood cells and other plasma components before returning to the patient (Figure 18.11). 

MacDougall J, Krygier V, Sandstrom E. A crossflow filtration system for heavy-metal wastewater treatment. Solid State Technology Mar 2006. 

Membrane processes can be operated in two major modes according to the direction of the feed stream relative to the orientation of the membrane surface dead-end filtration and crossflow filtration (Figure 1.1). The majority of the membrane separation applications use the concept of crossflow where the feed flows parallel to and past the membrane surface while the permeate penetrates through the membrane overall in a... 

Figure 1.1 Schematic diagram of a membrane process (a) crossflow filtration mode and (b) dead-end filtration mode...

Separation of microorganisms. In wastewater U tment utilizing bioreactor systems, there are needs to concentrate and separate microorganisms. During or after the fermentation processes, bacterias or cell debris need to be removed from the system. Crossflow filtration has been investigated for these purposes. An example of this is to treat the evaporator condensate discharged from a Kraft pulp mill. 

Meunier, J.P., 1990, Use of crossflow filtration for process b yeast and tank bottoms, in Proc. 5th World Filtration Congr., Nice, France. 

Burrell K, McKechinie M, and Murray J. Advances in separation technology for the hrewer ceramic crossflow filtration of rough heer. 

Eor a given crossflow filtration, the dominant particle back transport mechanism may depend on the shear rate and the particles size [4]. Brownian diffusion is only important for particles smaller than only a few tenths of a micron in diameter with relative low shear, whereas inertial lift is important for particles larger than several tens of microns with higher shear rates. Shear-induced back transport appears to be important for intermediate particle sizes and shear rates. Li et al. [7] reported that the shear-induced mechanism was able to predict fluxes comparable with the critical fluxes identified by the DOTM. 

Al-Aloum O, Mercier-Bonin M, Ding L, Fonade C, Aptel P, and Jaffrin M, Comparison of three different systems used for flux enhancement Application to crossflow filtration of yeast suspensions. Desalination. 2002 147 31-36. 

Imasaka T, So H, Matsushita HK, Furukawa T, and Kanekuni N, Application of gas-liquid two-phase crossflow filtration to pilot scale methane fermentation, Drying Tech. 1993 11 769. 

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