Batch desalting

Figure 4.12 Batch electrodialysis desalting apparatus. From Schramm [55], Copyright 1980, Laurier L. Schramm.

Because of the favorable sorptive properties of the reversed-phase supports, batch adsorption and desorption can be a very effective way to desalt a chromatographed sample or to partially fractionate a peptide mixture during a purification procedure. For example, 1-2 gm of an oc-tadecyl silica packed into a silanized glass or plastic pipette can be used for the batch fractionation of small amounts of a crude peptide extract from tissues, such as the pancreas or pituitary, or from a synthetic experiment. A number of commercial products, such as the Waters Sep-Pak, have found use in this manner 10) as a purification or sample preparation aid. Protocols for batch extraction procedures on alkyl silicas have been discussed 17a,b) and applied to neuropeptides 10, 158, 166) and other hormonal peptides 88, 162, 167, 168). With these methods recoveries of peptides present in a tissue extract are generally higher than those found with classical fractionation techniques due in part to the fact that proteolytic degradation is minimized. 

Bench-top testing will allow variation in chemical type and dosage, temperature, pressure, agitation, treatment time, electrical input (portable electric desalters only), and wash-water or diluent addition. Variations in temperature and pressure will not allow simulation of high pressures and temperatures. The bench-scale tests imply that a batch treatment of the emulsion is used to determine treating chemicals for a dynamic continuous treating system. Thus, results will have limitations even if the parameters of the test procedure are as accurate as possible. 

Continuous diafiltration is generally more efficient and preferred. In this approach, a batch of the solution to be desalted is maintained at constant volume by adding pure water (dialysate) at the same rate permeate is removed. In this way, the proteins (or other macromolecules retained by the membrane) remain at their initial concentration while the salt concentration decreases continuously. This has been called "constant volume molecular washing" because the salts are washed out of solution. Continuous diafiltration reduces the processing time required in the discontinuous process. 

With feed and bleed or batch recycle systems some or all of the water that has already been processed by the stack is mixed with feed and returned via a recirculation pump. Recycle is inherently less efficient than once-through flow in both stack utilization and in energy consumptions since the same solution must be pumped and desalted repeatedly and then remixed with a more concentrated solution. However, the increased flexibility in process control makes recycle systems attractive for small-scale operations where stacks are over-sized to handle varying loads. Feed and bleed is useful for operations where the enriching stream needs to be as concentrated as possible. Ten-fold enrichment or 90% recovery of some feed waters as diluate can be achieved by concentrate recirculation if solubility limits of the dissolved substances are not exceeded in the concentrate stream. 

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