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Qwestrum Engineering360 · Chemical Engineering · Mass Transfer

Absorption and Stripping

Absorber design multiplies the number of transfer units (a measure of separation difficulty) by the height of a transfer unit (a measure of packing efficiency); the operating line comes from a solute balance and its minimum slope is set by pinching against the equilibrium line.
Exam tip: keep SI units consistent end-to-end, write the governing relation symbolically before substituting, and sanity-check magnitude and sign.

Key formulas & points

Skim these first — then read the full notes below.

  • Minimum L/G occurs when the operating line touches the equilibrium line
  • Stripping: dilute liquid, high temperature favours desorption
  • Kremser equation for absorption with constant m and L/G

Topic details

Introduction

Treybal treats packed absorption and stripping as complementary operations — one removes solute from gas into liquid, the other the reverse. You draw the operating line from an overall solute balance, locate the minimum liquid rate where it just touches equilibrium, choose an actual rate at 1.2–1.5 times the minimum, and then size the column height as HTU × NTU or use the Kremser equation for dilute, linear systems.

Key relations & formulas

Z=HTU×NTUZ = HTU \times NTU
(packed-column height)
NTUOG=∫dy(y−y∗)NTU_{OG} = \int \frac{dy}{(y - y*)}
(overall gas-phase transfer units)
yn+1−y1=(LG)(x0−x1)y_{n+1} - y_{1} = (\frac{L}{G})(x_{0} - x_{1})
(operating line, dilute)

Notation and sign conventions

Relation 1 —
Z=HTU×NTUZ = HTU \times NTU
Z=HTU×NTUZ = HTU \times NTU
(packed-column height)
Write this relation with symbols exactly as in Mass Transfer Operations — Treybal before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 2 —
NTUOG=∫dy/NTU_{OG} = \int dy/
NTUOG=∫dy(y−y∗)NTU_{OG} = \int \frac{dy}{(y - y*)}
(overall gas-phase transfer units)
Write this relation with symbols exactly as in Mass Transfer Operations — Treybal before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.
Relation 3 —
yn+1−y1=y_{n+1} - y_{1} =
yn+1−y1=(LG)(x0−x1)y_{n+1} - y_{1} = (\frac{L}{G})(x_{0} - x_{1})
(operating line, dilute)
Write this relation with symbols exactly as in Mass Transfer Operations — Treybal before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Concept in depth

NTU quantifies how many equilibrium-driven “stages” of contacting the required change in composition demands — a hard separation needs many transfer units. HTU rolls up the mass-transfer coefficient and interfacial area into a single height per transfer unit, so the column height is their product. The operating line and equilibrium line together define the driving force at every point; as the liquid rate falls, the two lines approach until they touch (a pinch), giving the minimum liquid flow and an infinitely tall column. Stripping simply runs the process in reverse, favoured by high temperature or reduced pressure that lowers solubility.

Assumptions and validity limits

State assumptions explicitly before using any relation for absorption and stripping — 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 Mass Transfer viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

Step-by-step problem approach

1. Read the question and list given data with SI units (common in Mass Transfer 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 absorption and stripping.
4. Use equation 1:
Z=HTU×NTUZ = HTU \times NTU
.
5. Use equation 2:
NTUOG=∫dy/NTU_{OG} = \int dy/
.
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Applications & exam relevance

Absorption and Stripping appears in distillation, absorption, and drying. In Indian chemical curricula this topic is tested because it connects theory to diffusion and interphase transfer.
GATE and semester exams often combine absorption and stripping with earlier units — revise prerequisites before attempting mixed problems.
Industry interview panels sometimes ask: "Where did you use absorption and stripping?" — answer with a lab, mini-project, or plant visit example if possible.

Common mistakes in exams

Frequent errors are picking a liquid rate below the minimum (impossible), inverting the driving force (y − y* versus y* − y), and applying the Kremser equation when the equilibrium line is strongly curved. Confusing absorption factor A = L/(mG) with its reciprocal is another.

Quick revision checklist

Before attempting absorption and stripping problems, confirm you can:
1. Minimum L/G occurs when the operating line touches the equilibrium line
2. Stripping: dilute liquid, high temperature favours desorption
3. Kremser equation for absorption with constant m and L/G
Revise the solved examples in Mass Transfer Operations — Treybal and one previous-year GATE or university paper for this unit.

Worked examples

Try the problem first — open the solution when you are ready to check.

Minimum liquid rate

Problem

A gas with inlet y = 0.05 is to be reduced to 0.005; equilibrium is y* = 2x, and inlet liquid is solute-free. Gas rate G = 100 mol/h. Estimate minimum L.

Solution

At minimum, exit liquid is in equilibrium with inlet gas: x_max = y_in/2 = 0.025. Balance: G(y_in − y_out) = L_min·x_max ⇒ 100(0.045) = L_min(0.025) ⇒ L_min = 180 mol/h.

Conceptual check — Absorption and Stripping

Problem

In a Mass Transfer semester or GATE paper you are asked: "State the main assumption, the governing relation, and one practical consequence of absorption and stripping." What should a complete answer include?

Exams & GATE

McCabe-Thiele / Kremser for absorption when a stepwise or analytical solution is asked.

📖 Standard books (India)

  • Mass Transfer Operations — Treybal

    Read: Syllabus unit

    Absorption, distillation, and extraction

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