Engineering Evaluation Case: A Pressurized Biogas Water Scrubber Removes CO₂ but Also Loses Methane

Water scrubbing is widely used in biogas upgrading because compressed biogas can be contacted with water to remove more soluble components such as:

• CO₂;
• part of the H₂S load.

The packed absorber provides gas-liquid contact.

But the objective is not simply maximum absorption.

The plant wants to remove unwanted gases while keeping as much methane as possible.

That creates a selectivity problem.

Project Situation

Consider a pressurized packed water scrubber treating raw biogas containing:

• methane;
• CO₂;
• residual H₂S;
• water vapor.

The absorber operates above atmospheric pressure to increase gas solubility.

Water flows countercurrently through the packing.

CO₂ removal is strong.

However, the plant observes more methane loss than expected in the water stream or downstream flash gas.

The first instinct may be to reduce packing efficiency.

That is not necessarily the right diagnosis.

Water Does Not Absorb Only CO₂

CO₂ is more soluble than methane under typical water-scrubbing conditions.

Methane still has finite solubility.

When large volumes of water contact pressurized biogas, some methane can dissolve.

The process therefore balances:

• CO₂ removal;
• methane recovery.

Maximum gas-liquid contact is not the only objective.

Pressure Improves Absorption of Both Components

Higher pressure increases the tendency of gases to dissolve.

That helps remove CO₂.

It can also increase methane dissolution.

Therefore, pressure should be optimized at the process-system level rather than raised indefinitely.

Water Flow Has the Same Trade-Off

Increasing water circulation can improve CO₂ capture.

But more water also provides more capacity to dissolve methane.

The process may therefore reach a point where additional water improves product purity but reduces methane recovery.

Packing Influences Approach to Equilibrium

High-efficiency packing increases gas-liquid contact.

This helps the phases approach their equilibrium behavior more closely.

If methane dissolution is thermodynamically possible, better packing cannot distinguish magically between:

• desirable CO₂ absorption;
• undesirable methane absorption.

The process design must manage the equilibrium and water loop.

Pressure Reduction Can Recover Dissolved Gas

After leaving the absorber, loaded water may be depressurized in a flash step.

Dissolved gases then come out of solution.

The flash gas can contain:

• methane;
• CO₂.

Depending on the process, part of this gas may be:

• recycled;
• treated;
• recovered.

The absorber and flash system should therefore be designed together.

Water Regeneration Determines the Returning Liquid Condition

The water may be regenerated using:

• pressure reduction;
• stripping air;
• another process step.

If regeneration is incomplete, returning water contains residual CO₂.

That reduces the driving force in the next pass.

The tower may then need more:

• water;
• packing;
• pressure.

A weak regeneration system can make the absorber appear undersized.

H₂S Changes the Water Chemistry

Raw biogas may contain H₂S.

Some dissolves into the water.

This creates:

• corrosion concerns;
• odor and safety issues;
• downstream water-treatment load.

The H₂S duty should therefore be included rather than treating the tower as a CO₂-only absorber.

Gas Flow Changes Through the Absorber

As CO₂ is removed, total gas molar flow decreases.

Methane fraction rises toward the outlet.

This means gas velocity and composition vary through the packing.

The hydraulic and mass-transfer conditions are not perfectly uniform by elevation.

Methane Recovery Should Be a Defined Performance Metric

A project should not specify only:

• outlet CO₂.

It should also define an acceptable:

• methane loss;
• methane recovery.

Otherwise, one system can achieve excellent CO₂ removal by sacrificing too much valuable product.

Packing Pressure Drop Still Matters

Because the absorber operates under pressure, compression energy is valuable.

Additional pressure drop can increase:

• compressor duty;
• operating cost.

A packing with suitable:

• efficiency;
• capacity;
• low pressure drop

can therefore provide economic benefit.

Water Quality Can Cause Fouling

If water is not clean, the absorber may also face:

• scale;
• biological growth;
• suspended solids.

The water loop should be managed to preserve distributor and packing performance.

What Data Is Needed?

A useful design basis includes:

• raw biogas flow and composition;
• operating pressure;
• temperature;
• water flow;
• required product CO₂;
• methane-recovery target;
• H₂S concentration;
• water regeneration method;
• allowable pressure drop.

Engineering Takeaway

Biogas water scrubbing is not a contest to maximize absorption.

The system must remove CO₂ efficiently while minimizing methane loss and regeneration energy.