Membrane Separation

This Seader & Henley topic covers pressure-driven membrane separations from microfiltration to reverse osmosis. You rank the processes by pore size and molecular-weight cut-off, apply the solution-diffusion or pore-flow flux models, and correct the driving force for concentration polarization and, in RO, for the osmotic pressure that opposes water flow.

(solution-diffusion permeation flux, permeability P_m)

(observed rejection of solute)

(reverse-osmosis water flux, osmotic pressure π)

Relation 1 —

(solution-diffusion permeation flux, permeability P_m)

Write this relation with symbols exactly as in Separation Process Principles — Seader & Henley before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 2 —

(observed rejection of solute)

Write this relation with symbols exactly as in Separation Process Principles — Seader & Henley before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 3 —

(reverse-osmosis water flux, osmotic pressure π)

Write this relation with symbols exactly as in Separation Process Principles — Seader & Henley before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

A membrane lets some species through faster than others, either by size sieving (MF, UF) or by solution-diffusion (NF, RO). In reverse osmosis the applied pressure must first overcome the osmotic pressure of the concentrated feed before any net water permeates, so ΔP − Δπ is the true driving force. Rejected solute accumulates at the membrane surface (concentration polarization), building a local concentration higher than the bulk that both cuts flux and raises solute passage. The stage cut — how much of the feed is taken as permeate — trades throughput against purity, exactly as reflux does in distillation.

State assumptions explicitly before using any relation for membrane separation — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.

Wrong assumptions invalidate the entire solution even when the formula is correct. In Separation Processes viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

1. Read the question and list given data with SI units (common in Separation Processes papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to membrane separation.
4. Use equation 1: .
5. Use equation 2: .
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Membrane Separation appears in refineries and specialty chemicals. In Indian chemical curricula this topic is tested because it connects theory to distillation, extraction, and membranes.

GATE and semester exams often combine membrane separation with earlier units — revise prerequisites before attempting mixed problems.

Industry interview panels sometimes ask: "Where did you use membrane separation?" — answer with a lab, mini-project, or plant visit example if possible.

Students forget to subtract osmotic pressure from applied pressure in RO, ignore concentration polarization, and confuse rejection with recovery. Ranking MF/UF/NF/RO in the wrong pore-size order is a frequent recall error.

Before attempting membrane separation problems, confirm you can:

1. MF, UF, NF, RO ranked by decreasing pore size / MWCO
2. Concentration polarization lowers the effective driving force
3. Stage cut θ = permeate flow / feed flow affects separation

Revise the solved examples in Separation Process Principles — Seader & Henley and one previous-year GATE or university paper for this unit.