Chromatography, Radial Flow

The production of a modern biotechnology product, frequently a recombinant protein, requires a multistage downstream process because the feedstock is usually a liquid that is dilute in product concentration and contains many impurities some of which are unknown chemical compounds. Such a process typically centers on two or more liquid chromatography steps in order to achieve the desired purity. As production scales escalate, the chromatography columns used become larger and more expensive. It is not uncommon to have column bed volumes of hundreds of liters at industrial scales. Radial flow columns were first used for gas–solid catalytic reactions in large packed beds. They were designed to increase gas flow rate and reduce pressure by increasing the cross-sectional flow area. Radial flow chromatography (RFC) columns first entered the commercial biotechnology market in the mid-1980s (1). They were marketed as an alternative to the conventional axial flow chromatography (AFC) for preparativeand large-scale applications. These columns were not configured for analytical applications because RFC columns do not offer any advantages for such purposes. In an RFC column (Fig. 1), the mobile phase flows in the radial direction, not in the axial direction. The mobile enters from the outside tube and merges into the center tube (Fig. 2). In comparison to a slim AFC column, an RFC column provides a larger flow area and a shorter flow path (i.e. radial bed length). It allows a higher volumetric flow rate with a lower bed pressure. The effect is equivalent to using a short pancake-like AFC column (Fig. 3). Pancake-like AFC columns are quite common in industrial applications. They are available from most major commercial column vendors. In scale-up to accommodate large feed loads, it is impractical to increase the column height of AFC columns by too much because excessive bed pressure drop will compress the gel too much and make fluid flow more difficult. This is especially troublesome when soft gels are used. Thus, pancake-like AFC columns are used despite their short flow paths. This article is devoted to the discussion of the applications of RFC columns for bioseparations and RFC modeling and scale-up issues. Applications examples will be provided based on the existing open literature. A general rate model will be presented for the modeling of RFC. Experimental and theoretical comparisons between AFC and RFC will be discussed. Figure 1. Diagram of an acrylic Superflo column. (Courtesy of Sepragen Corp.)