Rectisol Process Fundamentals

1. Chilled Methanol as a Physical Solvent

The Rectisol process uses chilled methanol (CH₃OH) at temperatures of −40 to −80°F as a physical solvent for acid gas removal. Unlike chemical solvents such as amines that react with acid gas components, methanol absorbs impurities through physical dissolution governed by Henry’s Law—gas solubility increases with decreasing temperature and increasing pressure. This physical absorption mechanism enables extraordinarily deep removal of multiple contaminants simultaneously.

The process was developed and licensed by Linde AG and Lurgi (now Air Liquide Engineering & Construction). It is the most widely used physical solvent process for syngas purification worldwide, with over 100 operating plants. Methanol’s key advantage as a solvent is its extremely high solubility for H₂S, CO₂, COS, HCN, and mercaptans at cryogenic temperatures—far exceeding the capacity of competing physical solvents such as Selexol (dimethyl ether of polyethylene glycol) or Purisol (N-methyl-2-pyrrolidone).

Deep Removal Capability

Rectisol achieves the deepest acid gas removal of any commercially available gas treating technology. Treated gas specifications can meet the stringent requirements for catalytic chemical synthesis processes where even trace sulfur levels would poison catalysts:

• H₂S removal: <0.1 ppmv (well below the 4 ppmv pipeline specification)
• CO₂ removal: <10 ppmv (orders of magnitude below the 2% pipeline specification)
• COS removal: <0.1 ppmv (critical for catalyst protection in synthesis applications)
• HCN removal: <0.1 ppmv (important for coal gasification syngas cleanup)

Primary Applications

The Rectisol process is predominantly used in applications where ultra-deep purification is required:

• Syngas for methanol synthesis: Methanol catalysts are extremely sensitive to sulfur poisoning; Rectisol provides the sub-ppmv purity required
• Syngas for ammonia synthesis: Iron-based ammonia catalysts require total sulfur below 0.1 ppmv
• Fischer-Tropsch synthesis: Cobalt-based FT catalysts require <10 ppbv total sulfur
• IGCC power plants: Pre-combustion carbon capture with simultaneous sulfur removal
• Coal gasification: Removal of the complex contaminant slate (H₂S, COS, HCN, NH₃, mercaptans) from raw syngas

Rectisol Performance Specifications

These specifications are unattainable with conventional amine treating or other physical solvents operating at ambient temperatures. The combination of cryogenic temperature and methanol’s exceptional solvent properties makes Rectisol the only commercially proven technology capable of simultaneously achieving sub-ppmv specifications for all of these contaminants.

2. Process Description

The Rectisol process operates at cryogenic temperatures (−40 to −80°F) and elevated pressures (400–1,000 psig), where methanol’s physical absorption capacity is maximized. The process is designed with multiple absorption and regeneration stages to achieve selective separation of H₂S and CO₂ into distinct product streams, which is essential for downstream sulfur recovery and carbon management.

Selective Two-Stage Absorption

Most Rectisol installations employ a two-stage absorption scheme:

1. First absorber stage (H₂S removal): The raw syngas contacts lean chilled methanol in the lower section of the absorber column. Because H₂S has approximately 5–7 times higher solubility in methanol than CO₂ at the same conditions, a limited quantity of methanol selectively absorbs essentially all H₂S, COS, and HCN while co-absorbing only a fraction of the CO₂. The H₂S-rich methanol is withdrawn as a separate stream.
2. Second absorber stage (CO₂ removal): The partially treated gas rises into the upper section where it contacts a second stream of lean methanol. This stage removes the bulk of the remaining CO₂ to meet the final product specification. The CO₂-rich methanol is handled separately from the H₂S-rich stream.

Multi-Stage Regeneration

Solvent regeneration in the Rectisol process uses a combination of techniques to minimize energy consumption while producing separate acid gas streams:

• Flash regeneration: The rich methanol is depressurized through multiple flash stages, releasing dissolved gases at progressively lower pressures. Flash gas from the first stage (highest pressure) is often recompressed and recycled to recover valuable syngas components (H₂, CO) that were co-absorbed.
• Nitrogen stripping: Nitrogen gas is used to strip H₂S from the methanol at intermediate temperatures. This produces a concentrated H₂S stream suitable for feed to a Claus sulfur recovery unit.
• Hot methanol regeneration: The final regeneration step heats the methanol to 150–200°F in a distillation column to drive off remaining dissolved gases and achieve the very low lean loading required for deep absorption. The regenerated lean methanol is then re-cooled by heat exchange and refrigeration before returning to the absorber.

Acid Gas Routing

The selective absorption and staged regeneration produce two distinct off-gas streams:

• H₂S-rich stream: Contains concentrated H₂S (typically 30–60 vol%) suitable for Claus sulfur recovery or sulfuric acid production
• CO₂ stream: High-purity CO₂ (typically >95 vol%) suitable for sequestration, enhanced oil recovery (EOR), or urea production

Methanol Recovery

Methanol has significant vapor pressure even at cryogenic temperatures, so a methanol recovery section is required downstream of the absorber to prevent solvent losses in the treated gas. This typically consists of a water wash column where cold treated gas contacts water, which absorbs methanol vapor. The methanol-water mixture is then distilled to recover the methanol for recirculation.

Typical Operating Conditions by Application

3. Refrigeration and Energy Requirements

The most significant operating cost of the Rectisol process is the refrigeration system required to maintain methanol solvent temperature at −40 to −80°F. This cryogenic operation demands a substantial and continuous refrigeration load, which is the primary reason Rectisol has higher energy consumption than ambient-temperature treating processes.

Refrigeration Systems

The refrigeration load is supplied by one or more of the following systems, depending on plant size and the required solvent temperature:

• Propane refrigeration: Single-stage propane compression systems can achieve temperatures down to approximately −40°F. This is the simplest and most common approach for moderate cooling requirements.
• Propane/ethylene cascade: Two-stage cascade systems use propane for the first cooling stage and ethylene or ethane for the second stage, achieving temperatures down to −80°F or lower.
• Mixed-refrigerant systems: A blend of hydrocarbons (methane, ethane, propane, butane) in a single compression loop provides cooling across a wide temperature range. These systems offer better thermodynamic efficiency than cascade systems at the cost of greater operational complexity.

Energy Trade-Off

The Rectisol process consumes approximately 3–5 times more energy per unit of gas treated than Selexol or MDEA amine systems. However, this comparison requires context:

• Lower solvent circulation: Methanol’s high absorption capacity at cryogenic temperatures means significantly less solvent is required per unit of acid gas removed, reducing pumping power
• No reboiler for chemical regeneration: Unlike amine systems that require high-temperature reboiling (240–260°F) to break chemical bonds, Rectisol regeneration uses primarily flash and stripping, with only moderate heating in the hot regeneration step
• Net energy: The refrigeration penalty is partially offset by lower solvent pumping costs and lower regeneration heat duty, but the net energy consumption remains substantially higher than competing processes

Heat Integration

Efficient heat integration is critical to managing the refrigeration load. Standard heat integration strategies include:

• Feed gas precooling: The cold treated gas leaving the absorber is used to precool the incoming feed gas in a gas-gas heat exchanger, reducing the refrigeration duty on the feed
• Solvent cross-exchange: Cold rich methanol from the absorber bottom is used to precool the warm regenerated lean methanol returning to the absorber
• Flash gas cooling: Cold flash gases from low-pressure regeneration stages provide additional cooling to the incoming rich solvent

With proper heat integration, the external refrigeration load can be reduced by 30–40% compared to a non-integrated design. Pinch analysis and detailed heat exchanger network optimization are standard practice in Rectisol plant engineering.

Energy Comparison: Rectisol vs. Selexol vs. MDEA Amine

The energy penalty of Rectisol is justified only when the ultra-deep removal specifications cannot be met by less energy-intensive processes. For conventional natural gas treating to pipeline specifications, MDEA amine or Selexol are far more economical choices.

4. Advantages and Limitations

The Rectisol process occupies a unique niche in gas treating technology. Its extraordinary purification capability comes with significant complexity and cost trade-offs that must be carefully evaluated against project requirements.

Advantages

• Deepest acid gas removal available: No other commercial process achieves sub-0.1 ppmv H₂S and sub-10 ppmv CO₂ simultaneously. This is essential for protecting sensitive downstream catalysts in chemical synthesis applications.
• Simultaneous multi-contaminant removal: A single Rectisol unit removes H₂S, CO₂, COS, HCN, mercaptans, and trace organics in one process step, eliminating the need for separate COS hydrolysis, HCN removal, and mercaptan treatment units.
• Selective H₂S/CO₂ separation: The two-stage absorption scheme produces separate H₂S-rich and CO₂-rich streams, enabling efficient sulfur recovery (Claus unit) and carbon management (sequestration or utilization) from a single treating facility.
• Inexpensive, readily available solvent: Methanol is produced globally at large scale and low cost (approximately $1.00–$1.50 per gallon). Solvent makeup costs are minimal compared to specialty solvents.
• Non-corrosive solvent: Unlike amine solutions that cause significant corrosion in carbon steel equipment (especially at elevated temperatures and high acid gas loadings), methanol is non-corrosive, allowing the use of carbon steel construction throughout much of the plant.
• No chemical degradation: As a physical solvent, methanol does not degrade through reaction with acid gas components, heat-stable salt formation, or oxidative degradation—common problems with amine systems that require expensive solvent reclaiming.

Limitations

• High refrigeration cost: The cryogenic operating temperature requires a large, continuously operating refrigeration system that dominates the plant’s energy consumption and represents the single largest operating cost.
• Complex process scheme: Multi-stage absorption, multi-stage flash regeneration, nitrogen stripping, hot regeneration, and methanol recovery create a process with many more unit operations than a simple amine system. This complexity increases capital cost, plot space, and operator skill requirements.
• Methanol losses in treated gas: Despite methanol recovery systems, some methanol vapor is lost with the treated product gas. Typical losses are 5–20 lb per MMSCF of treated gas, requiring continuous makeup.
• Fire and toxicity hazard: Methanol is flammable (flash point 52°F), toxic by ingestion and inhalation, and burns with an invisible flame. Extensive fire protection, ventilation, and safety systems are required. This is a significant disadvantage compared to water-based amine solvents.
• Large plot space: The multiple process stages, extensive heat exchange network, and refrigeration equipment result in a plant footprint substantially larger than competing treating technologies for the same gas throughput.
• High capital cost: Total installed cost for a Rectisol unit is typically 2–3 times that of an equivalent-capacity amine system, driven by the cryogenic equipment, refrigeration system, and process complexity.

Advantages vs. Limitations Summary

The net assessment is clear: Rectisol is not economical for conventional natural gas treating where pipeline-quality specifications (4 ppmv H₂S, 2% CO₂) are adequate. It is justified only for syngas purification, chemical synthesis feedstock preparation, and pre-combustion carbon capture applications where no other technology can meet the required purity specifications.

5. Applications and Industry Experience

The Rectisol process has over 60 years of commercial operating experience, with more than 100 installations worldwide. The majority of plants are in China and Europe, where coal gasification and chemical synthesis from syngas are major industrial activities. North American installations are fewer but include some of the world’s most notable syngas processing facilities.

Coal Gasification and Syngas

Coal gasification produces a raw syngas containing a complex slate of contaminants including H₂S, CO₂, COS, HCN, NH₃, mercaptans, and trace metals. Rectisol is the preferred treating technology for these applications because it removes all of these contaminants in a single process step, producing a clean syngas suitable for chemical synthesis or power generation.

Fischer-Tropsch and Coal-to-Liquids

Fischer-Tropsch (FT) synthesis converts syngas into liquid hydrocarbons using cobalt or iron catalysts. Cobalt-based FT catalysts are extremely sensitive to sulfur poisoning, requiring total sulfur levels below 10 ppbv—a specification that only Rectisol can reliably achieve on an industrial scale. The Sasol coal-to-liquids complex in South Africa is the world’s largest application of Rectisol for FT synthesis feedstock preparation.

IGCC and Carbon Capture

Integrated Gasification Combined Cycle (IGCC) power plants gasify coal or petroleum coke, clean the syngas, and burn it in a gas turbine for power generation. Rectisol provides pre-combustion carbon capture by removing CO₂ from the shifted syngas at high pressure, producing a hydrogen-rich fuel gas and a concentrated CO₂ stream for sequestration.

Emerging Application: Blue Hydrogen

Blue hydrogen production involves steam methane reforming (SMR) or autothermal reforming (ATR) of natural gas, followed by water-gas shift and CO₂ capture. While most blue hydrogen projects use amine or Selexol for carbon capture, Rectisol is being evaluated for applications requiring very high CO₂ capture rates (>95%) and simultaneous deep desulfurization of the hydrogen product.

Notable Rectisol Installations

The concentration of Rectisol installations in coal gasification and chemical synthesis applications reflects the technology’s core value proposition: it is the only proven commercial solution for achieving the ultra-deep purification required to protect high-value downstream catalysts and meet the most stringent synthesis gas specifications.

References

1. GPSA, Chapter 21 — Hydrocarbon Treating
2. Kohl, A. L. and Nielsen, R. B., Gas Purification, 5th Edition, Gulf Publishing, 1997
3. Ranke, G. and Mohr, V. H., “The Rectisol Wash: New Developments in Acid Gas Removal from Synthesis Gas,” Linde Reports on Science and Technology
4. Burr, B. and Lyddon, L., “A Comparison of Physical Solvents for Acid Gas Removal,” Bryan Research & Engineering Technical Paper