However, Figure 1 also shows another important aspect: Scaling down (reducing column length) will result in worse or unchanged
separation because N is reduced. As a consequence it is important during process development to find and operate the shortest possible column
suitable for the separation. This has two benefits: reduced demand for test material during development and a safely up-scalable
To summarize the rules of obtaining flexibility during scaleup:
- Maintain constant flowrate Q in CV/h (not v in cm/h)
- Minimize bed height for successful separation
- Maximize bed height while keeping pressure drop reasonable.
Since the cycle time is constant (constant CV/h) the productivity [g/Lgel/h] is constant, therefore the capacity [g/h] can be adjusted by changing the bed height.
Figure 2. Example of scale up on a volume basis from lab to pilot and in pilot (different bed heights and diameters). Q =
EXTRA COLUMN EFFECTS
An example of scaling up on a volume basis directly from lab-scale to pilot-scale (by a factor 274) is shown in Figure 2.
For all experiments we use columns 20 to 35 cm long. The 20-cm column length was measured as 610 plates at Q = 11 CV/h. As both experiments showed identical elution and impurity profiles we have obtained the same separation and thus
successfully scaled the process. However, the transfer from lab to pilot resulted in a significant larger retention volume.
This deviation arises from extra column effects.
Figure 3. Areas equal the dead volumes of different systems, 0.16 CV for the lab and 1.07 CV for pilot. c/c0 is the scaled concentration
The deviation of 2.3 CV was initially attributed to a difference in the dead volumes. These were calculated by numerical integration
as the area between the nominal gradient (programmed gradient) and the lab or the pilot gradient (Figure 3) and found to be
0.16 CV and 1.07 CV respectively. This difference of 0.91 CV, obviously did not account for the deviation.
Figure 4. The pilot response curve shows that the delay volume (2.6 CV) is made up by two contributions, one from tubing (0.7
CV) and one from mixer (1.9 CV).