The major difference between a standard packed-bed column and a column used for expanded-bed adsorption is the liquid distribution system. STREAMLINE® columns are specially designed for expanded bed adsorption and have a fixed bottom distributor and a movable top distributor (adaptor). The bottom and top distributors have essentially the same design: a perforated plate with a single-weave screen or net facing the adsorbent. The distributor plate in the adaptor has a lower pressure drop than the bottom distributor. The purpose of the top distributor plate is to create a more even flow (compared with a column with one or a few inlets in the adaptor) when the column is run in downward mode during elution in settled bed mode. The steel screens help to stabilize the flow and confine the adsorbent particles to the column. These columns do not cause cell damage, and it has been shown that shear-sensitive cells pass through the column without breaking (40,42).
Expanded-bed adsorption handles unclarified feedstocks, and for this reason, harsh cleaning procedures are necessary. The columns are constructed of materials that withstand the chemicals used in such procedures and are also designed to have the minimum of stagnant zones where cells could accumulate and risk contaminating the next run. It has been shown that the columns can be efficiently cleaned and sanitized after passage of unclarified feedstocks such as whole cultures of bacteria and yeast (27).
Columns range in size from small lab-scale columns with a 25-mm inner diameter to industrial-scale columns, presently the largest with a 1,200-mm inner diameter. They all have essentially the same design, but differ in the material used for the column tube: the smaller columns have tubes made of glass and industrial columns have tubes made of stainless steel. The larger columns also have a greater number of inlets and a higher pressure drop over the distributors than the smaller columns. One method of verifying successful scale-up and comparing the performance of different sizes of expanded-bed columns is to determine the axial dispersion of a tracer substance (the method is described in "Operation of an Expanded Bed"). Another method is to determine protein breakthrough capacity using a model protein in a buffer system. The results obtained from these methods give a good measure of bed stability, a factor that is crucial for a successful expanded-bed process. The hydrodynamic properties of the columns are tested using the axial-dispersion method, and the "function"—the protein adsorption properties—is evaluated using the protein breakthrough method. The columns have been tested using both methods, and the results demonstrate that the performance of an industrial-sized column is very similar to that of a small lab-scale column (27). Table 1 shows that the number of theoretical plates, and the axial dispersion varies little between the different column sizes. Figure 1 compares protein breakthrough curves (bovine serum albumin [BSA] at 300 cm/h using 15-cm settled bed height), illustrating the maintained protein adsorption characteristics from lab scale to industrial scale.
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