Filters tangential flow

A 37 mm diameter filter in a similar holder can be used on-line to filter up to a litre of serum-free cell culture medium. Millipore s Sterivex is a tangential flow filter which can be used with a repeating syringe or on line to filter up to 1 litre of growth medium and Gelman s Acrocap (Fig. 8.3) is similar. 

Figure 1. Schematic of equipment for processing lysates. The tangential flow filter is in the stacked sheet configuration which allows high inlet pressure.

Clarification, or harvest, is typically the first step in downstream processing. During clarification, the cells, cell debris, and other particulates are removed from the cell culture broth, which contains the product. Clarification can be accomplished by depth filtration or by centrifugation followed by some form of filtration. Tangential flow filtration (ITT), sometimes also referred to as cross-flow filtration (CFF) is also used for clarification. Tangential flow filters are designed either in a flat sheet mode or hollow fiber mode. 

ITT systems have been used at all process scales. With the appropriate cleaning validation to demonstrate removal of protein and debris from previous use, tangential flow filters can be cleaned and used multiple times. TFF is usually not appropriate for clarification of a very shear-sensitive cell line or a low viability process as the turbulent flow required to recirculate the cells in the system leads to more cell breakage, resulting in release of more impurities in the clarified product. 

Kempken et al. [113] employed a rotating disc filter to harvest CHO cells, and observed that the filter could be operated at low transmembrane-pressure with high wall shear rates, leading to high filtrate flow rates, high product yields and minimum fouling. They concluded that their system offered a powerful alternative to conventional tangential flow filtration. 

Filtration separates components according to their size. Efficiency depends on the shape and compressibility of the particles, the viscosity of the liquid phase and the driving force, which is the pressure created by overpressure or by vacuum. Filtration can be performed either as dead-end filtration, where the feed stream flows perpendicular to the filter surface (Lee, 1989) or as tangential flow filtration, where the feed stream flows parallel to the filter and the filtrate diffuses across it. Examples of the former are the continuous rotaiy vacuum dram filter, where a rotaiy vacuum filter has a filter medium covering the surface of a rotating drum and the filtrate is drawn through the dram by an... 

Unlike sterilizing and virus removal filters, tangential flow filtration (TFF) filters are often reused. Flow and integrity tests are necessary to ensure the filter remains the same after usage and cleaning. Consistency of filtrate and retentate streams is validated using relevant validated assays that are specific for each process and product. 

Whenever the analyte of interest is soluble in the liquid phase or part of the gas phase, sample removal via a filtering device is the most reasonable solution. Filters mounted in situ are ideal provided they do not foul within an unreasonably short period. If this is the case, a filter operating in bypass must be used because this can be repeatedly exchanged with a freshly prepared one. In our experience, bypass filters should be operated with a high tangential flow, say > 2 m s 1 of superficial liquid velocity. Then, a useful lifetime of a few weeks can be achieved even in cultures of filamentous organisms. 

Ultrafiltration was applied to examine the size fractionation of Al, Ca, Cu, Fe, K, Na, and Pb in white and red wines [91]. Metal determinations were performed on the unfiltered wine, the 0.45 p,m membrane-filtered wine and each ultrafiltrate fraction. Aluminum was determined by ET-AAS, while FAAS was employed for Cu and Fe. An electroanalytical technique, stripping potentiometry, was selected for Pb measurement, whereas flame photometry was chosen for K and Na quantification. Fractionation patterns were evaluated and discussed. Castineira et al. [92] combined on-line tangential-flow multistage ultrafiltration with a home-built carbon analyzer and ICP-MS for size fractionation of nonvolatile dissolved organic compounds and metal species in three German white wines. The study showed that the major part of the elements investigated (up to 25) were dissolved in the size fraction of < 1 kDa, with the exception of Ba, Pb, and Sr, which also appeared in other fractions. 

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... 

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