Tangential Flow Filtration

Tangential filtration, like NFF, is also a pressure-driven separation process. Fluid flows across membrane surface and only a small fraction of solvent and permeable materials penetrate through the membrane. The fluid circulation minimizes the formation of the filtered solids on the membrane, consequently maintaining flux without increasing pressure. The comparison of TFF vs. NFF is illustrated in Figure 1.10. TFF can be further divided into microfiltration (MF) and UF according to the pore size of the membrane.

Microfiltration is usually used in upstream recovery process to separate intact cells and some cell debris/lysates from the rest of the components in the feed stream. Either the retained cells or the clarified filtrate can be the product stream. Membrane pore size cutoffs used for this type of separation are typically in the range of 0.05 to 1.0 am. Ultrafiltration/diafiltration (UF/DF) is one of the most widely used forms of TFF and is used to separate proteins from buffer components for buffer exchange, desalting, or concentration. Depending on the size and other physicochemical properties of the target protein, membrane NMWL (nominal molecular weight limit) in the range of 1 to 1000 kDa are commonly used.

The driving force through the membrane is determined by TMP which is defined as the difference between the average pressure on the retentate and permeate side:

TMP = Pfeed +2Pretentate - Ppermeate (1.32)

Tangential Flow Filtration Principle
FIGURE 1.9 The mechanism of filter aid assisted depth filtration. (Adapted from Dynamic depth-filtration: Proof of principle. Technical note AMC06 v. 3.1, Advanced Minerals Corporation, 2004.)

where Pfeed is the feed inlet pressure, fWntate is the retentate outlet pressure and, Ppermeate is the permeate pressure. Normally in CFF process, flux initially increases with increasing TMP and then levels off as shown in Figure 1.11.

1.2.2.2.1 Pressure Controlled Model

Several models have been developed to predict flux as a function of process parameters and fluid characteristics. It is generally believed that there is

Normal flow filtration Feed flow Pressure

Normal flow filtration Feed flow Pressure

Tangential flow filtration Pressure

Filtrate Filtrate

FIGURE 1.10 Normal flow filtration vs. TFF. (Courtesy of Millipore Corporation.)

FIGURE 1.11 Effect of TMP on permeate flux, pressure-controlled and pressure-independent region.

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