Explain the basic principles of a membrane separation unit operation

Explanation:

Membrane is a barrier that separates two phases and that totally or partially restricts the transport of one or more chemical species present in the phases.  The membrane separation method is based on the use of chemical potential and/or electrical potential gradient as the driving force for the separation.  The membrane separation  unit operation is based on the use of chemical potential and/or electrical potential gradient as the driving force for the separation.

Given that chemical potential is a function of pressure, temperature and concentration, and that membrane separation processes are generally operated at a constant temperature, both concentration gradients and pressure gradients can act as driving forces capable of promoting the transport of a given species across the membrane.

Examples of processes where the driving force is the pressure gradient are: Microfiltration, Nanofiltration, and Reverse Osmosis. Processes where the driving force are the concentration gradient: gas permeation, dialysis. Processes whose driving force is the electric potential gradient: electrodialysis.

nacl is added to the aqueous of what? ? in order to get a proper answer you have to specify the exact composition of the aqueous solution.