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Fundamentals

Herb Lutz    EMD Millipore, Biomanufacturing Sciences Network

Abstract

This chapter provides a high-level overview of ultrafiltration. This includes basic terminology, principles of operation, performance characteristics, and applications where it is used. It is intended to provide an overall perspective and furnish a background to enable the reader to more readily understand the subsequent in-depth chapters.

Keywords

cross-flow filtration filtrate flux formulation NFF permeate retentate tangential flow filtration TFF TMP P ultrafiltration UF applications UF membrane UF operation UF processing steps UF system

1.1. Membrane pore sizes

A membrane can be idealized as a film that readily passes solvents and small solutes but retains large solutes above a particular size. UF (Ultrafiltration) membranes retain solutes with hydraulic diameters in the 5-150 nm range. This roughly corresponds to molecular weights in the 1-1000 kDa range covering most proteins, nucleic acids, nanoparticles, viruses, and some polymers. In the field of membranes, the term nanofiltration refers to membranes tighter than ultrafiltration but more open than reverse osmosis (Figure 1.1). However, in biotechnology, it has come to refer to virus-retaining membranes that fall in the open ultrafiltration (roughly 100 kDa) to tight microfiltration range (roughly 0.1 µm). Figure 1.1 Ultrafiltration size range.

1.2. Applications

In a biopharmaceutical manufacturing process, biopharmaceutical products are expressed in a bioreactor (for mammalian cells) or fermentor (for bacterial or yeast cells). Ultrafiltration has been used to retain colloids and cell debris while passing the expressed product. In a similar way, ultrafiltration has been used after a refold pool to retain refold aggregates while passing the refolded product. Following clarification, ultrafiltration has been used to remove the largest contaminant - water - by retaining the product in a concentration step. This can be to reduce the size of subsequent steps, which may have to be sized based on the batch volume rather than on product mass. In addition, volume reduction facilitates transport and storage. This can provide flexibility to a manufacturing operation by decoupling the upstream from downstream operations. Ultrafiltration is used to purify large solutes, such as vaccines, by retaining the desired product and passing through unwanted smaller components. This can include passing unreacted PEG or unconjugated polymers. Ultrafiltration is also used to retain unwanted viruses or aggregates while letting the desired product pass through. When the desired product and unwanted solutes are close in size, this is a challenging fractionation operation. This book will not cover virus ultrafiltration, or nanofiltration as it is frequently called. The most common application is the use of ultrafiltration for final product formulation. This involves retaining the product while concentrating it to the desired target, and conducting a buffer exchange using a diafiltration process. Small contaminants and the old buffer components pass through the membrane.

1.3. Modes of operation

Membranes are encased in modules for ease-of-operation. One could run the membrane in a static or dialysis mode where small buffer solutes can be exchanged by diffusion across the membrane (Figure 1.2). This is convenient at the bench scale but economical commercial operation requires bulk flow. UF can also be run in NFF (normal flow filtration) or TFF (tangential flow filtration) modes. NFF (Figure 1.2a) is the most common type of filtration. NFF mode involves passing the solvent through the filter under pressure where the fluid velocity is perpendicular (or normal) to the plane of the membrane. As the fluid passes through the membrane, it drags solutes with it to the membrane surface where they accumulate and cause the filter hydraulic resistance to increase. For high concentrations of retained solutes, NFF operation leads to relatively rapid plugging and low filter capacities. NFF is used at bench scale where low capacities pose less of a concern and the ease-of-use is convenient. NFF is also used for virus filtration. Figure 1.2 Operating modes. TFF mode (Figure 1.2b) involves adding another fluid velocity component parallel to the plane of the membrane so the net solvent flow strikes the membrane at an angle. The presence of the tangential flow at the membrane surface facilitates backflow of solutes and prevents filter plugging. While there remains a region of high solute concentration at the membrane surface, steady-state operation is reached and TFF operation shows very high filter capacities. TFF is used at manufacturing scale where capacity is important for economics but extra complexity is required to manage the tangential flow.

1.4. Module hydraulics

TFF modules have one entering feed stream with volumetric flowrate qF and two streams leaving - the permeate (or filtrate) stream that has passed through the membrane and represents the normal flow component with volumetric flowrate qP, and the retentate stream that has not passed through the membrane and represents the tangential flow component with volumetric flowrate qR (Figure 1.3). Depending on the application, the product of interest may be retained in the retentate, passed through the membrane into the permeate stream, or separate recoverable products of value may lie in both the retentate and permeate. The ratio of the permeate flow to the feed flow is called the conversion. For dilute feeds, the conversion can be as high as 90%. For most applications with higher solute concentrations, conversions of 10-20% are common. This means that 80-90% of the feed flow winds up in the retentate as the tangential flow component. Figure 1.3 TFF module hydraulics. Along with the two streams leaving the module are two pressure drops. The pressure drop associated with the pressure difference between the retentate PR and the feed PF is commonly referred to as the 'delta P' where

P?=?PF-PR?psid?for?retentate?pressure?PR?psig?and?the?feed?pressure?PF?psig

(1.1) A second pressure drop is associated with the driving force for flow through the membrane between the upstream feed side and the downstream permeate side, commonly referred to as the 'TMP' or transmembrane pressure. For a UF module where the upstream pressure varies along the feed channel, an average upstream pressure of (PF + PR)/2 is used. The permeate side pressure PP is more uniform throughout the module since the permeate flows are much smaller than the feed side flows. The TMP is calculated as

=(PF+PR)/2-PP?psid?for?retentate?pressure?PR?psig,?feed?pressure?PF?psig,?and?permeate?pressure?PP?psig

(1.2) Solvent flow through the membrane is characterized by the flux or J = qp/A, where A is the module membrane area. The flux is the flowrate per unit membrane area with common units of litres/hour-square metre or LMH. High flux means that the membrane module is very productive and can generate a large permeate volume in a small time with a small area. The units of flux can also be interpreted as a velocity of solvent through the membrane. A module with a flux of 36 LMH has a solvent velocity of 0.01 mm/s.

1.5. Basic system and operation

Figure 1.4 shows the components in a batch ultrafiltration system. During processing, the feed solution is contained within a feed tank and pumped by a feed pump into the module. Fluid passing through the membrane is diverted as permeate and fluid retained by the membrane is diverted as retentate. Typically, there is a retentate valve in the retentate line to provide some back pressure and help drive some of the fluid through the module. Depending on the application, the desired product of interest may be retained by the membrane and contained within the retentate stream, or may pass through the membrane and be contained within the permeate stream. It is common that after one pass through the module, the fluid has not been processed sufficiently to use in the next step. Rather, the retentate requires additional processing and is recycled back to the feed tank. The feed solution is repeatedly pumped across the UF modules until processing is complete. Figure 1.4 Batch UF system diagram. The system may be plumbed in different configurations (Chapter 5). For example, diafiltration or buffer exchange takes place when a fresh diafiltration buffer is added to the...