How Engineers Use the Absorption Factor to Evaluate Packed Tower Absorber Performance

Packed tower absorption depends on more than packing efficiency.

Before engineers determine:

• HTU;
• NTU;
• packed height;

they must understand whether the selected gas and liquid flow rates provide a reasonable absorption operating relationship.

One useful engineering concept is the:

Absorption Factor

For a simplified dilute absorption system with an approximately linear equilibrium relationship:

y∗=mxy^*=mx

the absorption factor may be expressed as:

A=LmGA=\frac{L}{mG}

where:

• LL = liquid molar flow on the selected basis;
• GG = gas molar flow on the selected basis;
• mm = slope of the equilibrium relationship.

The absorption factor combines:

Liquid-to-Gas Flow Relationship

with

Gas–Liquid Equilibrium

into one dimensionless process indicator.

The key principle is:

A packed absorber cannot be evaluated from L/G ratio alone. The equilibrium slope determines whether that liquid flow provides strong or weak absorption driving force.

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What Does the Absorption Factor Represent?

The absorption factor compares:

the ability of the liquid flow to carry absorbed material

with:

the equilibrium tendency of the component to remain in the gas phase.

Conceptually:

Higher LL

→ generally increases absorption capability.

Higher GG

→ increases gas load relative to available solvent.

Higher mm

→ generally represents less favorable equilibrium for absorption on the simplified linear basis.

Therefore:

A=LmGA=\frac{L}{mG}

provides more information than simply looking at:

L/GL/G 

---

Why L/G Alone Is Not Enough

Consider two absorption systems.

Both operate at:

L/G=2L/G=2

But:

System A

m=0.5m=0.5

System B

m=2m=2

Their absorption factors are very different.

For System A:

A=20.5=4A=\frac{2}{0.5}=4

For System B:

A=22=1A=\frac{2}{2}=1

The same nominal L/G therefore produces very different absorption behavior because equilibrium is different.

This is why:

Process equilibrium must be considered together with solvent circulation.

---

1. Start With the Equilibrium Relationship

For simplified dilute systems, engineers may approximate:

y∗=mxy^*=mx

where:

• y∗y^* = equilibrium gas composition;
• xx = liquid composition;
• mm = equilibrium-line slope.

The value of mm depends on:

• component;
• solvent;
• temperature;
• pressure.

Therefore the absorption factor is process-specific.

---

2. A Favorable Equilibrium Helps Absorption

If the gas is highly soluble in the liquid:

• equilibrium may favor transfer into the liquid.

The corresponding equilibrium slope may be relatively favorable.

Then a given liquid circulation rate can provide a larger absorption factor.

If equilibrium strongly favors the gas phase:

• more liquid may be required to maintain adequate driving force.

---

3. Temperature Can Change the Absorption Factor

Temperature affects equilibrium.

For many physical absorption systems:

Temperature ↑

↓

Gas solubility ↓

↓

Equilibrium becomes less favorable

↓

mm may increase on the selected linear basis

↓

A=LmGA=\frac{L}{mG}

can decrease.

Therefore the same liquid and gas flows may provide less absorption margin at higher temperature.

This connects directly with #142.

---

4. Pressure Can Change Equilibrium Too

For many physical absorption systems, increased pressure can improve absorption behavior.

Changing pressure may alter:

• equilibrium relationship;
• gas concentration basis;
• actual gas properties.

Therefore absorption factor calculations should use equilibrium data applicable to the actual pressure.

---

5. Liquid Flow Directly Influences A

If all other quantities remain constant:

L↑L \uparrow

causes:

A↑A \uparrow

Increasing solvent flow can therefore move the operating line farther from equilibrium and provide stronger absorption driving force.

But increasing liquid flow also affects:

• pump power;
• distributor loading;
• liquid holdup;
• pressure drop;
• tower hydraulics.

So higher AA is not free.

---

6. Gas Flow Directly Influences A

If liquid flow remains unchanged while:

G↑G \uparrow

then:

A↓A \downarrow

This is important during plant capacity increases.

A tower may originally operate with adequate absorption margin.

After gas throughput rises:

• the same liquid circulation is now spread across more gas flow;
• absorption factor decreases;
• outlet performance may deteriorate.

The packing itself may be unchanged.

---

7. Absorption Factor Can Help Explain Process Changes

Suppose an absorber begins missing its outlet target after a production increase.

Possible causes may include:

• hydraulic capacity;
• mass-transfer limitation;
• reduced absorption factor.

If:

Gas Flow ↑

while:

Liquid Flow remains constant

the process operating line moves.

Therefore separation performance can decline even before flooding occurs.

---

8. A Greater Than One Often Represents a More Favorable Absorption Region

For simplified linear dilute absorption problems, engineers often interpret:

A>1A>1

as a generally favorable absorption condition.

This means liquid capacity relative to equilibrium is comparatively strong.

However:

A > 1 is not a universal guarantee of meeting the outlet specification.

The required:

• NTU;
• HTU;
• solvent inlet condition;
• packing height

still matter.

---

9. A Near One Can Produce Sensitive Design Behavior

When:

A≈1A\approx1

the operating and equilibrium relationships can become relatively close.

The absorber may then become more sensitive to:

• solvent flow;
• temperature;
• equilibrium uncertainty;
• outlet specification.

Small changes in process conditions can produce larger changes in required separation effort.

---

10. A Below One Does Not Automatically Mean “Impossible”

The interpretation depends on:

• boundary conditions;
• required removal;
• solvent inlet composition;
• finite packing height.

For some finite separation duties, absorption can still occur.

However, a low absorption factor can indicate:

• unfavorable operating relationship;
• high NTU requirement;
• reduced process margin.

Therefore it should trigger closer process evaluation.

---

11. Do Not Use One Universal A Target

It would be incorrect to state:

Every packed absorber should operate at A = 1.5.

The appropriate factor depends on:

• process economics;
• solvent availability;
• outlet specification;
• equilibrium;
• packing performance;
• hydraulic limits.

Absorption factor is a design indicator, not a universal specification.

---

12. Absorption Factor Helps Connect Operating and Equilibrium Lines

The slope of the operating relationship depends strongly on:

L/GL/G

while the equilibrium relationship depends on:

mm

The absorption factor effectively compares these slopes.

Therefore it helps engineers understand:

How far is the operating condition from the equilibrium constraint?

This is the underlying reason the parameter is useful.

---

13. Minimum Solvent Flow Is Closely Related

There is a theoretical lower solvent-flow condition for a given absorption duty.

As liquid flow approaches this minimum:

• operating line approaches equilibrium;
• a pinch develops;
• driving force becomes very small;
• required packing height can become very large.

The absorption factor therefore helps understand how close the design may be to an unfavorable solvent-rate limit.

---

14. Designing Exactly at Minimum Solvent Rate Is Usually Impractical

At the limiting condition:

Driving Force→0Driving\ Force \rightarrow 0

at the pinch region.

Then:

NTU→very largeNTU \rightarrow very\ large

conceptually.

Therefore practical design normally requires some operating margin above the theoretical minimum solvent requirement.

How much margin is appropriate depends on:

• process economics;
• tower height;
• solvent cost;
• operating flexibility.

---

15. Increasing Liquid Flow Can Reduce NTU

As liquid flow increases:

• operating line can move farther from equilibrium;
• average driving force can increase.

This can reduce the required NTU.

Therefore an absorber design involves a trade-off:

More Liquid

Potentially:

• lower NTU;
• shorter packing height.

But:

• higher pumping duty;
• greater liquid hydraulic load.

Less Liquid

Potentially:

• lower circulation cost.

But:

• higher NTU;
• greater required packing height;
• less operating margin.

---

16. Tower Design Is an L/G vs Height Trade-Off

This creates a classic engineering optimization:

Higher Solvent Flow

↓

Better Driving Force

↓

Potentially Lower Packing Height

versus

Higher Pumping and Liquid Hydraulic Load

A design should not optimize:

packing height

or:

liquid flow

independently.

---

17. Solvent Inlet Composition Matters

Absorption factor alone does not fully describe absorber performance.

If incoming solvent is already heavily loaded with the transferred component:

• top-of-column driving force may be poor.

Therefore engineers also need:

• liquid inlet composition;
• equilibrium data;
• required outlet specification.

A favorable AA cannot compensate for unusably rich incoming solvent.

---

18. Fresh Solvent Can Provide Stronger Driving Force

A lean solvent generally provides greater capacity to absorb additional material.

Therefore solvent regeneration quality may materially affect absorber performance.

If regenerated solvent becomes progressively richer:

• removal performance can fall

even when:

• gas flow;
• liquid flow;
• packing

remain unchanged.

---

19. Absorption Factor Is Not the Same as Solvent Capacity

The liquid may have a high circulation rate but limited chemical or equilibrium capacity.

Conversely, a highly effective solvent may require less circulation.

Therefore AA describes the simplified relationship between:

• flows;
• equilibrium slope.

It does not replace detailed solvent-loading calculations.

---

20. Reactive Absorption Requires Care

For strongly reactive systems:

• equilibrium may not be represented adequately by one simple linear slope;
• chemical reaction modifies absorption behavior.

Examples can include:

• acid gas + caustic;
• reactive amine systems.

In such cases:

A=LmGA=\frac{L}{mG}

may still provide conceptual understanding in simplified treatment, but detailed design may require:

• reaction equilibrium;
• kinetics;
• enhancement factors;
• rate-based modeling.

---

21. Chemical Solvent Strength Can Act Like an Additional Capacity Constraint

Suppose a caustic scrubber has high liquid circulation.

But:

• available alkalinity is nearly depleted.

The volumetric liquid flow may still look adequate.

Yet chemical absorption capability may be poor.

Therefore reactive scrubbers should not be judged from:

L/GL/G

or AA alone.

Chemical stoichiometry also matters.

---

22. Absorption Factor Does Not Replace Minimum Wetting Rate

These are different.

Absorption Factor

Asks:

Is the liquid flow favorable relative to gas flow and equilibrium?

Minimum Wetting Rate

Asks:

Is enough liquid physically reaching the packing to maintain useful wetting?

A tower can satisfy one and fail the other.

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23. Example of Different Low-Liquid Limits

Suppose liquid circulation is gradually reduced.

Three possible limits may appear:

Process Limit

Absorption factor becomes unfavorable.

Packing Limit

Minimum wetting becomes inadequate.

Distributor Limit

Distributor turndown becomes unacceptable.

The practical minimum circulation rate is controlled by whichever condition becomes unacceptable first.

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24. Absorption Factor Does Not Replace Hydraulic L/G Parameters

#124 dealt with an L/G-related flow parameter used in generalized random-packing hydraulic calculations.

That parameter answers:

How do gas and liquid flow rates interact hydraulically?

#143 answers:

How do liquid flow, gas flow and equilibrium interact from a mass-transfer process standpoint?

These should not be confused.

---

25. Same L/G Can Mean Different Hydraulics and Different Mass Transfer

Two towers can have similar:

L/GL/G

but different:

• gas density;
• liquid density;
• equilibrium slope;
• packing.

Therefore the same numerical L/G does not imply:

• same hydraulic loading;
• same absorption factor;
• same separation performance.

---

26. Absorption Factor Can Change With Operating Load

Consider:

Minimum Gas Case

G=lowG=low

Maximum Gas Case

G=highG=high

If liquid flow does not increase proportionally:

AA

decreases at maximum throughput.

Therefore the high-throughput case can be mass-transfer limiting even if the absorber is still below hydraulic flooding.

---

27. Flow-Control Strategy Matters

Plants may control circulation in different ways.

Fixed Liquid Flow

As gas load rises:

L/G↓L/G \downarrow

and:

A↓A \downarrow

Ratio-Controlled Liquid Flow

Liquid flow rises with gas load.

Then:

L/GL/G

may remain more stable.

This can help maintain absorption behavior across production rates.

---

28. But Constant L/G Does Not Guarantee Constant A

Why?

Because:

mm

can change with:

• temperature;
• pressure;
• composition.

Therefore even when:

L/G=constantL/G = constant

the absorption factor can change if equilibrium changes.

This is especially important in non-isothermal operation.

---

29. Heat Effects Can Reduce A Through Equilibrium

From #142:

absorption heat can cause:

Temperature ↑

which may cause:

m ↑

for some physical absorption systems.

Then:

A=LmGA=\frac{L}{mG}

decreases even though L and G remain unchanged.

This is another reason non-isothermal absorbers need more careful analysis.

---

30. Absorption Factor Can Be Evaluated Across Tower Sections

In a simplified dilute model, flow rates may be treated approximately constant.

In real systems:

• gas molar flow may decrease through absorption;
• liquid composition changes;
• temperature changes;
• equilibrium slope changes.

Therefore a single global AA may be only an approximation.

Rigorous models may evaluate local conditions along the bed.

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31. High Absorption Can Change G

If a substantial portion of gas is absorbed:

GG

may no longer be approximately constant.

Then simplified analytical absorption-factor relationships become less accurate.

Detailed material balances should be used.

---

32. Solvent Evaporation Can Change Both Phases

If solvent evaporates into the gas:

• gas molar flow increases;
• liquid flow changes.

This can alter the local operating relationship.

Hot wet scrubbers may therefore require more rigorous treatment.

---

33. The Kremser Method Uses Absorption Factor

For appropriate simplified systems, absorption factor appears in analytical stage-based relationships such as the Kremser method.

These methods can estimate absorption or stripping performance under assumptions such as:

• dilute solute;
• linear equilibrium;
• approximately constant phase flow rates.

They are useful for engineering understanding and preliminary calculations.

They should not be applied beyond their assumptions without review.

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34. Packed Towers Are Continuous, Not Actual Equilibrium Stages

Even when stage-based analytical relationships are used for preliminary understanding, packed towers provide continuous mass transfer.

Their physical height must ultimately be related to:

• HTU/NTU;
• HETP where appropriate;
• rate-based performance.

Therefore absorption factor does not itself provide physical packed height.

---

35. A Does Not Equal Efficiency

A higher absorption factor generally provides a more favorable process relationship.

But it does not directly specify:

• removal efficiency;
• packing height;
• HTU.

The final performance still depends on:

A / Driving Force

•  

NTU

•  

HTU

•  

Packing Height

---

36. A Does Not Tell You Tower Diameter

Tower diameter depends mainly on:

• actual gas flow;
• liquid load;
• hydraulic capacity;
• flooding constraints.

Absorption factor is primarily a mass-transfer process parameter.

Therefore it should not be used for hydraulic sizing.

---

37. A Strong Absorption Factor Can Still Have Hydraulic Problems

Suppose liquid flow is increased substantially.

The absorption factor improves.

But now:

• liquid loading becomes high;
• pressure drop increases;
• flooding margin decreases.

The process may become thermodynamically easier but hydraulically worse.

Successful design requires both constraints to be satisfied.

---

38. A Weak Absorption Factor Can Make Packing Upgrades Ineffective

Suppose an absorber has poor removal.

A project proposes installing higher-efficiency packing.

But analysis shows the real problem is:

• insufficient solvent flow;
• unfavorable equilibrium.

A better packing may reduce HTU.

But if the process requires an extremely high NTU because the operating line is too close to equilibrium, the upgrade may provide limited benefit.

This is why process feasibility should be checked before blaming the packing.

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39. Existing Tower Troubleshooting Can Use the Same Logic

When an absorber underperforms, compare current operation against the original design:

• gas flow increased?
• liquid flow decreased?
• temperature increased?
• solvent became richer?
• pressure changed?

All of these can change the absorption relationship before any packing damage occurs.

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Example 1 — Production Increase

Original operation:

L=200L=200 G=100G=100 m=1m=1

Therefore:

A=2A=2

Production increases:

G=150G=150

Liquid remains:

L=200L=200

Then:

A=1.33A=1.33

The absorber now has less process absorption margin even though no equipment has changed.

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Example 2 — Increasing Solvent Flow

Current:

A=1.1A=1.1

The tower requires a high NTU because the operating line is close to equilibrium.

Increasing liquid flow increases AA.

This may:

• improve driving force;
• reduce NTU.

However the engineer must verify whether:

• distributor;
• packing;
• tower diameter

can accept the additional liquid load.

---

Example 3 — Higher Temperature

The absorber operates with unchanged:

• gas flow;
• liquid flow.

But process temperature increases.

Equilibrium becomes less favorable, causing mm to increase.

Then:

AA

decreases.

Removal performance falls.

This can occur even without any physical damage to the packing.

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Example 4 — Lean vs Rich Solvent

Two towers have identical:

• L;
• G;
• equilibrium slope.

Their nominal absorption factors are similar.

But Tower A receives lean solvent.

Tower B receives already loaded solvent.

Tower B has less available driving force.

Therefore absorption factor should be used together with actual inlet liquid composition.

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Absorption Factor Evaluation Workflow

Define Transferred Component

↓

Confirm Gas Flow G

↓

Confirm Liquid Flow L

↓

Establish Equilibrium Relationship

↓

Determine Applicable Equilibrium Slope m

↓

A=LmGA=\frac{L}{mG}

↓

Evaluate Minimum / Normal / Maximum Cases

↓

Check Solvent Inlet Composition

↓

Check Proximity to Equilibrium / Pinch

↓

Evaluate Required NTU

↓

Evaluate HTU

↓

Calculate Packed Height

↓

Check Hydraulic Consequences of Selected Liquid Flow

↓

Optimize Process + Tower Design

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Absorption Factor Checklist

Gas

✓ Gas molar flow✓ Solute concentration✓ Temperature✓ Pressure

Liquid

✓ Liquid molar flow✓ Solvent inlet composition✓ Solvent loading

Equilibrium

✓ Equilibrium relationship✓ Applicable slope✓ Temperature dependency✓ Pressure dependency

Process

✓ Required outlet concentration✓ Minimum / normal / maximum cases✓ Physical or reactive absorption

Hydraulic

✓ Liquid loading✓ Distributor turndown✓ Minimum wetting✓ Pressure drop✓ Flooding margin

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Common Absorption-Factor Mistakes

Mistake 1 — Using L/G Alone

Why it fails:

Equilibrium slope also controls absorption difficulty.

---

Mistake 2 — Treating A as a Packing Property

Why it fails:

It is primarily a process operating parameter.

---

Mistake 3 — Assuming A > 1 Guarantees the Outlet Specification

Why it fails:

NTU, HTU, inlet solvent condition and packed height still matter.

---

Mistake 4 — Increasing Liquid Flow Without Checking Hydraulics

Why it fails:

Better process driving force can create higher pressure drop and flooding risk.

---

Mistake 5 — Ignoring Temperature Effects on m

Why it fails:

Equilibrium and therefore A may change during operation.

---

Mistake 6 — Using Simplified A for Strong Reactive Absorption Without Checking Chemistry

Why it fails:

Reaction and solvent depletion can dominate the actual process.

---

Mistake 7 — Confusing Absorption Factor With Hydraulic L/G Flow Parameter

Why it fails:

They answer completely different engineering questions.

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Absorption Factor vs Related Variables

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How the DAIER Engineering Assistant Fits Into Absorption-Factor Evaluation

The DAIER Tower Packing Engineering Assistant can help organize preliminary engineering information including:

• gas flow;
• liquid flow;
• tower diameter;
• packing;
• operating temperature;
• operating pressure.

https://www.pxdaier.com/tower-packing-engineering-assistant.html

For absorption-factor evaluation, engineers additionally need:

• equilibrium data;
• equilibrium slope where the simplified model is applicable;
• liquid and gas molar flow basis;
• solvent inlet condition.

For reactive, strongly non-isothermal or highly nonideal absorption, rigorous process simulation may be more appropriate than a single constant absorption factor.

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Quick Guide

What is the absorption factor?

For a simplified linear dilute absorber, it is commonly expressed as:

A=LmGA=\frac{L}{mG}

It compares solvent flow with gas flow and equilibrium behavior.

Is absorption factor the same as L/G?

No.

Absorption factor also includes the equilibrium slope.

Is a higher absorption factor generally favorable?

Often yes for absorption, because it generally corresponds to a more favorable operating relationship, but it does not guarantee the required removal.

Why does temperature affect the absorption factor?

Because temperature can change gas–liquid equilibrium and therefore the equilibrium slope.

Can increasing liquid flow improve absorption?

Yes, it can improve driving force and reduce NTU, but it also increases hydraulic load and pumping requirements.

Is absorption factor used to size tower diameter?

No.

It is primarily a mass-transfer process parameter, while diameter is primarily determined by hydraulics.

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From L/G to Real Absorption Feasibility

Looking only at:

L/GL/G

is incomplete.

The real engineering sequence is:

Liquid Flow L

•  

Gas Flow G

•  

Equilibrium Slope m

↓

Absorption Factor

↓

Operating Line vs Equilibrium Line

↓

Available Mass-Transfer Driving Force

↓

Required NTU

•  

Applicable HTU

↓

Required Packed Height

↓

Hydraulic Check

The central engineering rule is:

The usefulness of a given solvent flow depends not only on how much liquid is circulating, but on whether that liquid flow is sufficiently favorable relative to the gas flow and the gas–liquid equilibrium of the actual process.