Dialysis coefficient

The dialysis coefficient U for a given component in solution in diffusion dialysis cell is given by the amount of the component that is transported per unit active membrane area, per unit time, and per unit concentration difference of the component. This translates into the following equation ... 

Knowledge of the selectivity and diffusion dialysis coefficient can be used to determine the membrane that offers lower resistance to acid diffusion. It also helps in determining the faster-diffusing acid. 

From the findings of the experiment, it was clear that H2SO4 diffused faster than HCl through Neosepta AFN. On the basis of a 2 h experiment, the diffusion dialysis coefficient U was found as follows ... 

As explained before, because acids selectively adsorb on strongly basic anion exchange membranes and show remarkable permeation through the membranes by concentration gradient, acids can be recovered from waste acid solutions by diffusion dialysis using anion exchange membranes (Figure 6.28, Chapter 6.4.1). Table 3.4 shows dialysis coefficients of various acids and salts, and separation... 

Diffusion dialysis was initially applied in viscose rayon production to recover caustic soda using parchment paper as a membrane. Today the largest industrial utilization of diffusion dialysis is to recover acids or alkalis from waste acids and alkalis using anion or cation exchange membranes. Figure 6.28 shows the principle of diffusion dialysis for acid recovery from waste acid solution. The process and membrane performance are evaluated on the basis of the flux of acids or alkalis (dialysis coefficient) and the ratio of the flux of metal salt to that of acids or alkalis (separation coefficient). The total dialysis coefficient [molh-1 m 2 (moll-1)], Uo, is defined by... 

Figure 6.30 Change in dialysis coefficient of various acids with acid concentration. Anion exchange membrane NEOSEPTA AFN (strong basic anion exchanger).

Table 3.4 shows the dialysis coefficients (Um) and separation coefficients of an anion exchange membrane, NEOSEPTA AFN, for acid recovery in mixed solutions of salts and various acids. Dialysis coefficients of hydrochloric acid and nitric acid are high, which are suitable for recovery by diffusion dialysis. However, results shown in Figure 6.30, Tables 3.4 and Figure 6.7 correspond to Um in Eq. (6.12) and U should be considered in the practical utilization of the membrane. Even though Um of the membrane increases with improvement and modification of the membrane, the total dialysis coefficient, U0, does not always increase due to an increase in Uv... 

Table 6.7 Effect of salts in acid solutions on dialysis coefficient...

Anion exchange membrane NEOSEPTA AFN, prepared by Tokuyama Corp. at 25.0 °C, t4cid dialysis coefficient of acid [molh-1 m 2 (moll-1)] f4lt dialysis coefficient of salt [molh-1 m-2(moII-1)] t/sait/t/acd separation coefficient. 

Dialysis coefficient [molh-1 m2(moll" )]. b Separation coefficient. 

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