Discontinuous diafiltration

Figure 3.94 Flow schematic for discontinuous diafiltration and concentration of enzymes.

Discontinuous diafiltration Permeable solutes are first ehminated by conventional UF or MF. Water is then added to the concentrated retentate to dilute it back to a certain volume and reprocessed by MF or... 

UF. This process of repetitive MF/UF and dilution to eliminate permeable solutes is called discontinuous diafiltration. 

Figure 6.4.9. Ultrafiltratton in a well-stirred vessel (a) batch ultrafiltration (b) continuous diajUtration/ultrafiltration (c) discontinuous diafiltration using three stages.

An alternative strategy is often adopted it is called discontinuous diafiltration. Consider Figure 6.4.9(c), which shows three identical well-stirred vessels (vessels 1, 2 and 3) having the same ultrafiltration membrane. At time t = 0, a volume Vfo of the feed solution containing a macrosolute i and a microsolute (say, salt) j is present in vessel 1. Assume for the time being that iJj = 1 and / ,= 0. Let the value of the VCR in batch ultrafiltration be 10. Then Cifll, = lOCioandCjfil = Cjo (from equation (6.4.101)). Now let this small volume of concentrate (volume = Vyo/10) be transferred to the next well-stirred vessel (vessel 2), where an amount of fresh buffer solution is added to bring the total solution volume to Vfo- Therefore the two solute concentrations in this vessel 2 are now ... 

This result will allow one to calculate easily the concentration of a small molecular weight impurity after discontinuous diafiltration in n stages. Suppose Rt for the impurity is 0 and V/o/V/r) = 10. Then we see that, for three stages, Cu) , will be (Cffl, /100), a considerable degree of purification indeed. 

Example 6.4.10 Compare the purification achieved for a protein (species i) in relation to a low molecular weight impurity (species j) for the following two processes one-stage discontinuous diafiltration vs. continuous diafiltration. The initial volume of solution is 10 liter the amount of ultrafiltrate produced is 9.5 liter. The UF vessel is well-stirred. The values of i ,- and Rj for the membrane used are flj = 1, Rj = 0. Employ a separation factor, a,j, to compare. 

Obviously batch ultrafiltration or one-stage discontinuous diafiltration is a more efficient technique for puriiying the protein of a low molecular weight impurity. However, one must ensme that the required solvent flux level is achievable in batch UF when the volume is reduced. 

For one-stage discontinuous diafiltration (or batch ultrafiltration) use equation (6.4.101)... 

Diafiltration. Diafiltration refers to the process by which UF can rapidly and efficiently remove salts and/or lower molecular weight species from larger macromolecules. It is commonly used for fractionation as well as desalting and is practiced both as a discontinuous and as a continuous process. 

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. 

Diaiiltration The convective elimination of permeable solutes by the addition of fresh solvent (water) to the retentate. Two modes of operation may be used in diafiltration continuous or discontinuous. 

Diafiltration can be performed in the continuous or discontinuous (batch) mode. In the discontinuous mode, the volume of the retentate is reduced, followed by redilution with water and repeating the UF in consecutive steps. In the continnons mode, water at the processing temperature and pH is added to the feed tank at the same rate as the permeate flux, resulting in a constant feed volume. 

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