Critical pore diameter (b)



Critical pore diameter (b)

FIGURE 8.6 (a) schematic representation of preferential sorption-capillary flow theory; (b) critical pore diameter for separation; (c) flux decline with time; (d) correction factor for surface area of cellulose acetate; and (e) solute rejection as a function of operating time.

diameters of water molecules. This pore size designated as 2t, where t is the diameter of the water molecule, is called the critical pore diameter. With this configuration, the final separation of the water molecules and the solutes materializes by applying pressure, pushing H2O through the pores (capillary flow).

As the process progresses, solutes build and line up near the membrane surface creating a concentration boundary layer. This layer concentration is much larger than in the bulk solution and, also, much larger, of course, than the concentration in the permeate side. This concentration difference creates a pressure for diffusive transport. The membrane, however, creates a barrier to this diffusion, thus, retaining the solute and not allowing it to pass through easily. Eventually, however, the solute will diffuse out and leak to the permeate side.

8.2.4 Types of Membranes

The first RO membrane put to practical use was the cellulose acetate membrane (CA membrane). The technique of preparation was developed by Sourirajan and Loeb and consisted of casting step, evaporation step, gelation step, and shrinkage step. The casting step involves casting a solution of cellulose acetate in acetone containing an additive into flat or tubular surfaces. The additive (such as magnesium perchlorate) must be soluble in water so that it will easily leach out in the gelation step creating a porous film. After casting, the solvent acetone is evaporated. The material is then subjected to the gelation step where it is immersed in cold water. The film material sets to a gel and the additive leaches out. Finally, the film is subjected to the shrinkage step that determines the size of the pores, depending upon the temperature used in shrinking. High temperatures create smaller pores.

After this first development of the CA membrane, different types of membranes followed: CAB, CTA, PBIL, and PA membranes. CAB is membrane of cellulose acetate butyrate; CTA is cellulose triacetate. The PBIL membrane is a polybenzimidazolone polymer and PA are polyamide membranes. The structure of a PBIL unit is as follows:

Polyethylene amine reacted with tolylene diisocyanate produces the NS-100 membrane (NS stands for nonpolysaccharide). The reaction is carried out as follows:

(CH2CH2NH)nCH2CH2NH + 2[l)

Polyethylene amine N=C=O

Tolylene diisocyanate

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