Using Volumetric Flow to Scaleup Chromatographic Processes

Mar 01, 2006

Steffen Kidal
When cGMP material for clinical trials and the market is produced, the lab-scale purification process is scaled up based on methods acceptable to authorities in the various countries, including FDA. The transfer of chromatographic processes from lab scale to pilot and production scale is an important and challenging task in the pharmaceutical industry, and has been dealt with in several more or less obvious ways.

Traditional scaleup only expands the column diameter, keeping column length and linear velocity constant, because it is considered a safe procedure.1 This way of scaling up corresponds to running a number of identical lab columns in parallel, so no wonder it works. However, this principle puts limitations on the development phase, where final production scale demands constraints on column length. Due to wall effects a given minimum column diameter must be maintained.2 The wall effects will be reduced as the number of adsorbent particles across the diameter increase, and 200 particles is recommended as a minimum in general practice.

Steffen Kidal, Camilla Kornbeck, and Thomas Hansen in the pilot plant
To no one's surprise, lab-scale experiments are performed in unnecessarily long columns, demanding relatively large amounts of product. In certain cases this may impose problems such as lack of sufficient product and safety issues in case of hazardous samples. We let equipment control development when we really should have development control the equipment.

At manufacturing scale, integration of the consecutive steps may be prevented by rigid constraints on column lengths. The primary way to adjust column capacity is by changing diameter, which means a wide column; or one can multiply the number of columns. In either case flexibility is limited to choosing an integral number of columns. Furthermore, commercial columns are only available with discrete diameters. One more detail; scaling up column diameter without other adjustments may also result in bed-stability problems due to loss of wall support when exceeding a certain diameter-to-length ratio.3

Figure 1. The dotted line represents the minimum plate number required for the separation. In this example 610 plates are needed. At a flow rate(Q) of 11 CV/h the minimum bed height is 20 cm. Reduce the flowrate to Q = 6 CV/h and the minimum bed height is approximately 10 cm. At Q = 24 CV/h separation is not possible at any bed height. At constant Q (fixed residence time) the plate number is always maintained or increased with increased bed height. Thus, for a fixed flow rate the separation is the same or better when the bed height is increased.
Flexibility in scaleup can be achieved by employing the principle of scaling on a volume basis.4 This principle is based on the theory that the residence time for the target protein controls the separation and thus must be kept constant during scaleup. As a consequence the bed height may be increased as long as the flowrate in units of column-volumes/time and load in g/column-volume are kept constant. Increasing column length by this procedure will result in equal or better separation. Scaling down on a volume basis by reducing bed height will, on the other hand, maintain or reduce the plate numbers and may thus compromise the separation (Figure 1).