Thickeners limiting flux

Critical section—Also called terminal section, this is the location in a thickener where the limiting flux is transpiring. 

It is now necessary to determine the limiting total flux 1//77. for a specified concentration Cu of overflow. The required area of the thickener is then obtained by substituting this value into equation 5.43 to give ... 

Limiting solids flux—The solids flux that is equivalent to the rate of mass withdrawal from the bottom of the thickener. 

The osmotic pressure which has to be overcome depends on the concentration of the solution and on the nature of the solutes. As the osmotic pressure increases with concentration, the rate of permeation of the solvent diminishes as the concentrate thickens and it is not economically viable to aim at above a certain concentration (5% for NaCl and 30% for a sugar solution, for example). The usual operating pressures are 40-50 bar giving solvent flux anywhere between 2 and 401m h, depending on the solution and the membrane used. Most membranes are made of cellulose acetate or its close relatives and they usually allow a limited solute passage in order to obtain reasonable permeate flow rates. 

---