Gas Pre-Treatment for Liquefaction: Removing CO₂, H₂S, Moisture, and Heavy Hydrocarbons

Introduction

The growth of small-scale LNG across the US and global markets is directly tied to the expansion of distributed energy, gasification of remote areas, and the shift toward more flexible fuel sources. In these projects, the critical element isn't the liquefaction unit itself — it's what happens before it: gas pretreatment.

A gas pretreatment unit (GPU) before liquefaction is the system that turns raw natural gas or associated gas into a cryogenically safe product. Without it, any LNG plant becomes an unstable technological experiment. The reason is simple: cryogenic temperatures of −162 °C don't forgive mistakes.

If CO₂, H₂S, water, or heavy hydrocarbons remain in the gas, they freeze, form hydrates, plug heat exchangers, destroy valves, and take the cryogenic unit offline. The result: the plant may run for weeks — but not for years.

At the same time, removing CO₂ and H₂S from gas isn't a single technology — it's a whole toolkit: adsorption, amine processes, membrane separation, and cryogenic polishing. Each one fits only a specific gas composition, pressure, and flow rate.

China today offers a wide range of GPU equipment — from adsorption columns to modular membrane units. But quality varies dramatically: from top-tier manufacturers like SASPG and Hangzhou Oxygen to trading companies with no real production capacity.

This article breaks down which technologies actually work, which tolerances are mandatory for LNG, how to build a proper technical specification, and which mistakes most often lead to cryogenic unit failure.

Why Gas Pretreatment Matters: What Happens Without a GPU

Remove the gas pretreatment unit, and the cryogenic module becomes a system with a fast track to failure. The reason: the physics of phase transitions and how impurities behave at low temperatures.

Critical Impurities and Their Behavior

CO₂ — Cryogenics' #1 Enemy

1. freezes at −78 °C
2. forms solid crystals
3. plugs plate-fin heat exchangers
4. causes capacity loss

H₂S

1. toxic
2. causes corrosion
3. destroys aluminum components
4. makes the product off-spec

Water

1. forms hydrates
2. turns to ice
3. blocks pipelines
4. critically reduces system reliability

GOST 5542 sets requirements for natural gas quality, including hydrogen sulfide and other component limits; in industrial practice, these standards serve as a baseline reference for gas pretreatment before processing.

Heavy Hydrocarbons (C5+)

1. condense at higher temperatures
2. freeze in the cryogenic zone
3. create deposits in the cold box
4. reduce liquefaction efficiency

The Bottom Line Without a GPU

Without gas pretreatment, the cryogenic unit runs for weeks, not years.

Gas Treatment Process Configurations (Core Section)

A gas pretreatment unit before liquefaction can be built on four core technologies. In real LNG projects, they're often combined.

1. Adsorption Treatment (Molecular Sieves)

This is the most common configuration for small-scale LNG plants.

Technology Parameters

Advantages

1. high purification depth
2. stable parameters
3. suitable for LNG
4. proven technology

Disadvantages

1. energy costs for regeneration
2. adsorbent replacement required
3. complex automation

Chinese Suppliers

1. Sichuan Air Separation
2. Hangzhou Oxygen
3. integrators Jereh / CIMC Enric

2. Amine (Absorption) Treatment

Used in large-scale gas processing facilities.

Parameters

Advantages

1. continuous process
2. high throughput
3. suitable for large flows

Disadvantages

1. complex amine regeneration
2. equipment corrosion
3. requires additional gas dehydration

Where It's Used

1. large gas processing plants
2. flows from 50,000 m³/h and above

3. Membrane Gas Treatment

Used as a pretreatment stage.

Parameters

Advantages

1. compact footprint
2. no reagents
3. low energy consumption

Disadvantages

1. doesn't reach cryogenic purity
2. requires a cascade
3. membrane instability

4. Cryogenic Separation

Applied in integrated LNG configurations.

Parameters

Advantages

1. combines treatment and cooling
2. improves LNG efficiency
3. removes heavy fractions

Disadvantages

1. high energy intensity
2. requires upstream dehydration
3. complex control

What Your GPU Technical Specification Must Include

Gas pretreatment equipment should never be purchased without a rigorous technical specification.

Input Data

1. gas composition (CH₄, CO₂, H₂S, N₂, C2+);
2. pressure and temperature;
3. flow rate (min/nom/max);
4. seasonal variations;
5. mercury presence;
6. moisture content.

Treated Gas Requirements

Example Wording

The gas pretreatment unit must ensure CO₂ content no higher than 50 ppm, H₂S no higher than 4 ppm, water dew point no higher than −60 °C at 0.1 MPa, and C5+ content no higher than 10 ppm.

Equipment

1. vessel material (09G2S, 12Kh18N10T);
2. adsorbent type (UOP, Grace, or equivalent);
3. membrane type;
4. valves and regulators (brand);
5. automation (Modbus, OPC UA);
6. column redundancy;
7. regeneration system.

Chinese GPU Equipment: Where It Works and Where the Risk Is

Works Well

1. adsorption columns (SASPG, Hangzhou Oxygen);
2. separators and filters;
3. heat exchangers (with material verification);
4. modular GPUs from EPC integrators (Jereh, CIMC Enric).

Weak Spots

1. membranes (inconsistent quality levels);
2. valves (regulators, control valves);
3. automation (SCADA incompatibility);
4. "universal" solutions without gas-specific engineering.

The Biggest Risk

The supplier doesn't request your gas composition → meaning they're designing a one-size-fits-all system → which won't work in real conditions.

Common Mistakes When Buying a GPU

How to Vet a Chinese GPU Supplier (HowTo)

Step 1 — Documentation

1. licenses
2. ISO 9001
3. references

Step 2 — Manufacturing

1. adsorption columns
2. vessel welding
3. non-destructive testing (NDT)

Step 3 — Testing

1. leak-tightness
2. pressure
3. regeneration

Step 4 — Integration

1. interface with cryogenic unit
2. SCADA / PLC
3. protocols