Cell separation membrane filtration

For particulate receptor preparations (intact cells or membranes), it is usual to separate bound from free ligand by either centrifugation or filtration. (For soluble receptor preparations, equilibrium dialysis, using a semipermeable membrane, or gel filtration can be employed.)... 

Unimmobilized Corynebacterium propinquum (CGMCC No. 0886) cells containing a cobalt-dependent NHase were employed in either batch or continuous reactions for the production of nicotinamide from 3-cyanopyridine [24]. In the continuous process, membrane filtration separated precipitated product (>5 wt%) and the microbial cell catalyst from the reaction mixture, where the catalyst was then recovered and returned to the reactor using a continuous addition of aqueous 3-cyanpyridine to maintain substrate concentration at <20% (w/v), a final conversion of >99% was obtained. 

Filtration is a unit operation commonly employed nowadays in biotechnological processes. In this unit operation, a filter medium acts as a physical barrier to particles larger than its pores. Traditional filtration devices such as filter presses and rotary vacuum drum filters have so far found no application for the separation of animal cells. Nevertheless, membrane filters are commonly employed, as well as some alternative filter designs such as spin-filters. In the next sections, the most common types of filters used for animal cell separation will be discussed. 

Microfiltration. Microfiltration, the use of tangential flow anisotropic membranes to permeate the product of choice while retaining solids, can be an attractive cell separation technique because it does not require the use of flocculants or filter aids. It is, in principle, a more technically sophisticated version of classic dead-end filtration processes. Microfiltration yields can be low due to progressive fouling of membranes. Advanced engineering has overcome many of the early... 

At the end of the fermentation, protein is separated from cell mass by filtration, typically with a rotary vacuum filter. The crude enzyme concentration is often lower than suitable for commercial applications, so the concentration of enzyme is increased by ultrafiltration. Most cellulase enzymes have a molecular weight of 25,000 to 75,000 and are retained by ultrafiltration membranes of 5000 molecular weight cutoff. The membranes permit the passage of low molecular weight salts, sugars and other impurities, and are sometimes operated in a diafiltration mode to increase the purity of the enzymes. The crude broth at this point is dark brown. 

Figure 11. A Comparison of Crossflow Membrane Filtration, Precoat Rotary Vacuum Filtration, and Centrifugation, a, Capital Costs for Cell Separation Steps b, Total Manufacturing Costs for Cell Separation Steps (S per Kg of enzyme).

The theory of t angential flow filtration as it applies to cell separations is discussed. Major emphasis, however, is placed on presenting the relationship of experimental results to theoretical performance. Topics highlighted are flux decay with time, effects of operating pressures and flow, membrane fouling, prefiltration requirements and filter geometries. ... 

With the exception of the precoat applications, RVF s do not usually yield absolutely clear filtrate. Although still widely used, rotary vacuum filters are, in some cases, being replaced by membrane separation technology as the method of choice for clarification of fermentation broths and concentrating cell mass. Membranes can yield more complete filtration clarification, but often a wetter cell paste. 

The systems are economically acceptable when separating proteins out of a cell homogenate. On the other hand, when carrying out bioconversion processes, the phase system has to be continuously reused, hence, there must be a method to remove the products from the phase system. This can be performed in a number of ways like adsorption, membrane filtration, etc. 

Plasmapheresis. The separation of plasma from whole blood by continuous membrane filtration represents an improvement over conventional centrifugation techniques in terms of efficiency, safety and cost. In the past, plasmapheresis was carried out with blood donors by collecting their whole blood in plastic bags which were then centrifuged to separate the red cells from the plasma. The supernatant plasma was then decanted and the red cells returned to the donorenabling plasma to be drawn from the same person as frequently as three times per week. Most of this plasma is then processed to yield purified components such as albumin or anti-hemophilic factor (Factor VIII). 

Recovery of cells can be accomplished using one of several alternative unit operations, such as centrifugation, membrane filtration, or settling. Subscripts are employed in the figure to identify the usual process variables at particular points in the diagram. We employ the recycle ratio (R) to represent the ratio of the volumetric flow rate of the suspension of cells leaving the separation device to the volumetric flow rate of the net product stream. The symbol t / represents the ratio of the concentration of biomass in the recycle stream to that in the effluent from the CSTBR. You may assume that the system is operating at steady state and that the feed is sterile. 

The efforts that can be made to restrict the amount of flux decline, i.e. to establish an equilibrium flux rate, can be split into two categories, dependent largely on the scale of operation stirred cells and crossflow filtration. The former is of use in the laboratory for small-scale separations, the latter is more appropriate for process applications. Both stirring and crossflow enq>loy the same princ le hi ear at the sur ce of the deport. Further techniques to restrict deposit thickness, minimise fouling or regenerate flux are discussed in Section. 10.7. All the main membrane arrangements are illustrated in Figure 10.4. 

Figure 5.10 Cell separation using a cross-flow microporous membrane filler. The main product can be either the cell concentrate or the filtrate...

The primary technology for citric acid recovery from fermentation broths is through precipitation with CaCOs, although other technologies include ion exchange, carbon adsorption, membrane filtration, chromatography, and liquid extraction [29]. Unlike other TCA cycle carboxylic acids, citric acid is excreted in the acid form rather than precipitated as citrate salt [30], which has the implication that the cell mass can first be filtered, followed by crystallization with iime with subsequent separation by filtration. The filter cake is next acidified with sulfuric acid, which precipitates the calcium as gypsum and the aqueous citric acid can next be decolourized and recrystallized by evaporation [29]. 

---