How Mist Eliminators Should Be Selected for Biogas and H₂S Scrubber Systems

Biogas treatment systems often include wet scrubbing or chemical absorption stages to remove contaminants such as hydrogen sulfide.

The gas leaving these systems can contain:

• entrained scrubbing liquid;
• condensate;
• reaction products;
• fine droplets.

A mist eliminator protects downstream equipment and reduces loss of scrubbing chemicals.

But biogas service creates several design questions that are not always present in ordinary ventilation scrubbers.

Gas flow can vary significantly.

Condensation can occur as temperature changes.

Sulfur-containing deposits may appear.

The separator must therefore be selected for the complete gas-cleaning system, not merely from vessel diameter.

Why Liquid Carryover Matters in Biogas Systems

Downstream equipment may include:

• blowers;
• compressors;
• dryers;
• activated-carbon beds;
• membranes;
• gas engines.

Excess liquid carryover can interfere with these units.

For example, wet gas entering an adsorption or polishing stage can:

• increase pressure drop;
• reduce media life;
• redistribute contaminants.

The acceptable outlet moisture condition may therefore be determined by downstream equipment rather than by the scrubber itself.

H₂S Scrubbers Can Generate More Than Water Mist

Depending on the treatment process, the scrubbing liquid may contain:

• alkaline solution;
• oxidizing chemicals;
• sulfur species;
• reaction salts.

Captured droplets therefore contain process chemistry.

If the liquid evaporates inside the separator, nonvolatile material can remain.

The demister can experience:

• salt deposition;
• solids accumulation

rather than simply clean-water wetting.

Elemental Sulfur Can Create Fouling Problems

Some H₂S-treatment routes can generate elemental sulfur.

If sulfur-containing droplets or solids reach the mist eliminator, they can accumulate on:

• mesh;
• vane surfaces.

Wet wire mesh can also capture fine particles effectively.

The separator may gradually become an unintended solids filter.

Pressure drop rises.

Drainage deteriorates.

This means sulfur-generation chemistry should be reviewed when selecting separator openness and cleaning strategy.

Biogas Flow Can Vary Widely

Biogas production is often not perfectly constant.

Flow can vary with:

• feed rate;
• biological activity;
• plant operating conditions.

The mist eliminator should therefore be reviewed across:

• minimum;
• normal;
• maximum gas flow.

At high flow, re-entrainment risk may increase.

At low flow, fine-droplet collection behavior may change.

One nominal velocity does not describe the complete operating range.

Condensation Can Occur Easily

Biogas frequently contains substantial water vapor.

As gas cools in:

• piping;
• downstream equipment,

condensate can form.

This creates an important diagnostic distinction.

If liquid appears downstream, determine whether it:

• passed through the mist eliminator;
• formed after the separator.

A demister cannot remove vapor that condenses later.

Temperature profile and dew-point behavior should therefore be reviewed.

Upstream Condensation Can Increase the Demister Duty

The reverse can also occur.

If warm saturated gas cools before reaching the separator, new droplets form upstream.

The demister now receives additional liquid loading that may not be included in the original scrubber spray calculation.

These condensation droplets may also be fine.

The separator duty can therefore change seasonally or after insulation modifications.

Wire Mesh May Be Suitable for Clean Fine-Droplet Service

Where gas is relatively clean and liquid loading is moderate, wire mesh can provide useful fine-droplet removal.

However, the mesh structure should consider:

• sulfur solids;
• salts;
• biological contaminants.

Very dense mesh can become difficult to clean if particulate contamination is significant.

The highest clean-condition efficiency is not always the best lifecycle choice.

Vane Separators Can Offer Greater Fouling Tolerance

Where the liquid loading is heavier or solids are present, open vane geometry can provide:

• larger passages;
• easier drainage;
• better washability.

The tradeoff is fine-droplet performance.

If downstream equipment requires very low carryover, staged separation may deserve consideration.

A first-stage vane can protect a downstream mesh polishing stage.

Material Selection Depends on Actual Chemistry

Biogas itself may contain corrosive contaminants.

The wet scrubbing liquid can create another chemical environment.

Potential materials may include:

• PP;
• PVDF;
• stainless steels

depending on process conditions.

Material selection should consider:

• H₂S;
• pH;
• treatment chemicals;
• temperature.

All frames and support components require the same review.

Biological Fouling May Matter

Some low-temperature gas-treatment systems can develop biological growth or organic contamination in wet areas.

If this reaches the mist eliminator, it can contribute to:

• sticky deposits;
• blocked drainage.

Maintenance design should therefore consider:

• wash access;
• removable sections.

This becomes more important in systems with long operating periods and limited shutdown opportunity.

Pressure Drop Can Affect Blower Performance

Many biogas systems operate at relatively low pressure.

The gas-moving equipment may have limited pressure capability.

An unnecessarily restrictive demister can consume a significant portion of the available pressure.

As fouling develops, the effect becomes greater.

Therefore, low sustainable pressure drop may be almost as important as separation efficiency.

What Data Should Be Provided?

Useful information includes:

• minimum/normal/maximum gas flow;
• operating temperature;
• pressure;
• gas composition;
• scrubbing liquid chemistry;
• solids or sulfur content;
• liquid loading;
• downstream equipment;
• allowable pressure drop.

Historical condensate and fouling behavior can also provide valuable retrofit information.

Final Engineering Perspective

Mist eliminators in biogas and H₂S treatment systems protect downstream equipment from scrubbing liquid and condensate while operating in a variable and potentially fouling gas environment.

The correct design balances droplet removal, pressure drop, sulfur or salt fouling, condensation, gas turndown, drainage, and material compatibility.