Note that there is almost a linear relationship between selling price and concentration on this log-log plot for typical products of bioprocessing. The dominance of recovery cost over the bioprocess cost is also apparent because only concentration seems to matter.
It is misleading to present the topic of downstream processing as if you simply select from the various options to put together a recovery process. While each of these options has its devoted followers, the points of interest are not merely the processes themselves but the special features and tricks that match each individual case.
The two guiding principles are purification and concentration. You must reduce the volumes to be handled, especially for those products of bioprocessing that are quite dilute. You strive for some purification while you are concentrating. Options are few for the first steps, and solvent extraction and ion exchange are excellent choices except that they may not work for some compounds.
Recovery of bioproducts tends to be very expensive, so improving the yield just a little bit can repay your salary. There are usually many ways in which to be imaginative and clever in developing a better process. Routine tweaking of conditions to get an incremental increase in yield is not much fun, but replacing an old process with a better one is exciting. Furthermore, you can calculate easily how much you have saved your company and point this out to your boss.
Note from the following applet that it is very difficult to get a good overall yield when you have a multi-step process.
applet added 17-Nov-97