Dialysis:

Dialysis is a membrane separation process wherein solutes diffuse across a membrane barrier by means of a concentration gradient. The rate of diffusion of a solute by the membrane increases linearly along with increase in concentration gradient and the proportionality constant, namely a diffusion coefficient, D, is a features property of the solute for a given membrane. Diffusion coefficients reduce roughly within proportion to the square root of molecular weight of solutes and therefore, solutes along with higher molecular weights diffuse slowly by the membranes.

To achieve selective permeation of solutes through dialysis process, a combination of solute and membrane properties are selected. The pore sizes of membranes are chosen within such a manner to exclude huge size macrosolutes. Also, the procedure is employed for separating those solutes whereas an order of magnitude difference exists between diffusion coefficient values.

Dialysis is a highly constrained process since the molecular diffusional rates are slow and if the receiving solutions (dialysate) are not continuously erased, the solute concentration on the both sides of the membrane will tend to equalise, negating the driving force for separation. A permeant species is not recoverable in pure form and is essentially more dilute in the dialysate than in the starting stream. For these purposes, dialysis has been limited to specialised uses in pharmaceutical and medical reasons whereas partial purification of the feed stream, slightly than recovery of a product is intended.