Dialysis countercurrent dialyzer

An industrial application of dialysis is the recovery of caustic from hemi-cellulose solutions produced in making rayon by the viscose process. Flat-sheet membranes are placed parallel to each other in a filter-press arrangement (see Chap. 30, p. 1004) and water is passed countercurrent to the feed solution to produce a dialyzate with up to 6 percent NaOH. Recovery of salts or sugars from other natural products or other colloidal solutions could be achieved by dialysis, but ultrafiltration is more likely to be used because of the higher permeation rates that can be obtained. 

The process of dialysis, in which small solutes diffuse from a feed solution through a microporous membrane into the dialyzing solution, was illustrated in Section 4.3.1 via a membrane kept in a closed vessel To achieve separation in a continuous fashion, we realized there that the dialyzer had to be open. Such a device is schematically shown in Figure 8.1.46(a). A small hollow fiber membrane based dialyzer used as an artificial kidney is illustrated in Figure 8.1.46(b). Both utilize countercurrent flow of feed solution (for example, blood) and the dialyzing solution on two sides of the membrane. 

Example 8.1.20 Countercurrent dialysis may be explored to eliminate substantially 0.05M sodium phosphate from a Img/ml solution of bovine serum albumin (BSA) and 25 mM Tris buffer at a pH of 7 (via NaOH). The dialyzing buffer composition is 25 mM Tris and the pH is 7 (via HCl). The dialysate flow rate is lOOml/min. The membrane module has 0.2 surface area. The value of Kic for the phosphate salt for one membrane module is 0.020 cm/min and that for two modules in series is 0.015 cm/min. Determine the values of (C,yo/C r) for sodium phosphate using the expression developed in Problem 8.1.22 for the following cases (a) feed flow rate, lOcm /min (b) feed flow rate, 20 cm /min (c) feed flow rate, 10 cm /min and two membrane modules in series. Comment on the effects of feed flow rate and putting membrane modules in series. 

In countercurrent dialysis, a particular salt species (i) is to be removed firom the feed solution of a protein into the dialyzing solution, which does not have this salt to start with. Develop an expression for C(yo fCifi in terms of A, Qy-and Qa. (Ans. (Ctfo/CtfL) = [Qa exp (A(l - (Q/Qa))) - QJiHQd- Q O... 

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