Process History and Technology Variants
Technology Origins
Low-temperature methanol wash is a physical acid gas removal (AGR) process that uses refrigerated methanol as a solvent to separate acid gases—primarily CO₂ and H₂S—from industrial gas streams at subzero temperatures. The technology was pioneered in the 1950s when Linde Aktiengesellschaft (Munich, Germany) and Lurgi Mineralöltechnik (Frankfurt, Germany) independently developed parallel versions marketed under the Rectisol® trademark.
The fundamental principle—cold methanol physical absorption at -40°C to -75°C operating temperatures—has since become the basis for both proprietary licensed technologies and non-proprietary generic implementations worldwide.
Proprietary Rectisol® Technologies
Rectisol® remains a registered trademark held by the original technology developers and their successors:
Linde plc (formed from the 2018 merger of Linde AG and Praxair) continues to independently own, license, and market its Rectisol® technology.
Air Liquide Global E&C Solutions acquired what was originally "Lurgi's Rectisol®" technology in 2007 when Air Liquide purchased the Lurgi engineering firm from GEA Group for approximately €550 million. Air Liquide now licenses its Rectisol® technology and holds the associated trademark rights through its German subsidiary.
While both licensed Rectisol® versions share the same fundamental principle, they are not identical technologies. EPA documentation from the 1980s notes that "the basic processes are similar but with some significant differences" in process configuration, equipment design, and proprietary optimizations. Each licensor offers distinct engineering approaches and process enhancements developed over their independent 70-year technology evolution.
Generic Low-Temperature Methanol Wash Variants
Since methanol is a non-proprietary, inexpensive, and readily available solvent, numerous generic variants of low-temperature methanol wash have been developed, particularly in China for coal gasification applications. These non-licensed implementations use the same fundamental cold methanol physical absorption principle but feature:
- Different process configurations and equipment arrangements
- Varied pressure levels and flash regeneration sequences
- Alternative heat integration schemes
- Customized designs for specific feedstocks and applications
Other Trademarked Methanol-Based Processes
Ifpexol® (licensed by IFP Energies Nouvelles/Axens) is the only other trademarked methanol-based cold absorption process, though it was developed primarily for natural gas treatment with an integrated two-stage configuration combining dehydration and acid gas removal.
Process Summary and Chemistry
Fundamental Principle
Low-temperature methanol wash is a physical acid gas removal (AGR) process that uses methanol (CH₃OH) as a solvent at subzero temperatures (typically -40°C to -75°C) to selectively absorb acid gases from synthesis gas and other industrial gas streams. Unlike amine-based chemical absorption processes, this technology relies on physical solubility governed by Henry's Law, where gas solubility increases with decreasing temperature and increasing pressure.
The process operates at high pressure (typically 2.76 to 6.89 MPa or 400 to 1000 psia) using non-proprietary, inexpensive, and readily available methanol as the absorption medium. This combination of low temperature and high pressure creates highly favorable conditions for acid gas absorption.
Key Chemistry
The process employs physical absorption rather than chemical reaction. The solubility of acid gas components in cold methanol follows the order:
This differential solubility enables selective removal of contaminants, with the process capable of achieving:
- Total sulfur removal to <0.1 ppm (including H₂S and COS)
- CO₂ removal to ppm levels
- H₂S purification down to 50 ppbv in treated syngas
- CO₂ product purity of 97-98.5 mol% suitable for carbon capture and storage (CCS) or enhanced oil recovery (EOR) applications
The process also removes trace contaminants including NH₃, HCN, CS₂, mercaptans, BTX aromatics, and metal carbonyls. This comprehensive contaminant removal capability makes it particularly suitable for syngas from coal or heavy hydrocarbon gasification, which typically contains these impurities.
Selective Separation Capability
Low-temperature methanol wash is particularly valued in syngas-based applications from gasification because it can selectively separate H₂S and CO₂ into two distinct product streams. This selective removal is achieved through multi-stage flash regeneration at different pressure levels, exploiting the differential volatility of absorbed components. The capability enables:
- H₂S-rich stream to be sent to sulfur recovery units (Claus process) for conversion to elemental sulfur
- High-purity CO₂ stream to be compressed for geological storage (CCS), enhanced oil recovery (EOR), or industrial utilization
- Clean syngas product suitable for catalytic downstream processes (Fischer-Tropsch synthesis, methanol production, ammonia synthesis, hydrogen production)
This flexibility in process configuration allows independent management and valorization of each acid gas component, making the technology highly adaptable to different downstream process requirements and environmental constraints.
Detailed Step-by-Step Technology Description
Process Configuration
Low-temperature methanol wash consists of four integrated sections. The description below is based on a representative configuration, though specific implementations vary depending on licensor, application requirements, and engineering preferences.
1. Acid Gas Absorption Section (H₂S and CO₂ Removal)
- Main Absorber Column: the absorber is the primary unit where acid gases are physically absorbed into cold methanol solvent. Typical design parameters include:
- Feed conditions: Raw syngas typically enters at 30-40°C and 30-60 bar
- Number of theoretical stages: 15-25 depending on purity requirements and feed composition
- Operating temperature: -40°C to -75°C in different column sections
- Operating pressure: 35-60 bar (2.76-6.89 MPa)
- Multi-section absorber configuration (from bottom to top):
- H₂S/COS removal section (bottom stages): Preferential absorption of sulfur compounds due to higher solubility
- CO₂ bulk removal section (middle stages): Main CO₂ absorption zone
- CO₂ cooling section: Temperature control for optimal absorption
- CO₂ main wash section: Further CO₂ removal
- CO₂ fine wash section (top stages): Final polishing to achieve ultra-low acid gas concentrations
- Process flow:
- Raw syngas enters the absorber containing typical acid gas concentrations from coal gasification (20-30% CO₂, 0.5-2% H₂S, plus CO, H₂, N₂, and trace contaminants). Regenerated cold methanol solvent is introduced at the column top at temperatures between -40°C and -55°C and pressures of 55-65 bar.
- The absorber includes intercoolers (side coolers or intermediate cooling stages) to remove the heat of absorption and maintain optimal absorption temperatures throughout the column. These coolers are critical for process efficiency as physical absorption is exothermic and temperature-sensitive. Typical cooling duty ranges from 2-5 MW per cooling stage depending on gas throughput.
- Solvent withdrawal: Methanol-rich streams are withdrawn at different points:
- Bottom stream: Contains highest concentration of H₂S and dissolved CO₂ (typically 65-75% methanol, 25-35% acid gases)
- Side stream(s): May be withdrawn at intermediate points for selective regeneration (typically 70-75% methanol, 20-30% CO₂, lower H₂S content)
- Clean syngas product exits the absorber top at cold temperatures (-40°C to -50°C) and high pressure (typically 55-60 bar) with composition achieving:
- CO₂: <0.01-0.1 mol% (depending on specification)
- H₂S: <0.1-1 ppmv (or 50-100 ppbv for ultra-low sulfur applications)
- Overall H₂S removal efficiency: >99.9%
2. CO and H₂ Recovery Section
- Flash Drum System: Physical absorption inevitably dissolves valuable syngas components (CO, H₂) along with acid gases. A flash recovery system recovers these components to improve process economics. This recovery step is essential for maintaining high overall CO and H₂ yields, typically recovering >95% of valuable syngas components.
- Operating principles:
- Rich solvent streams are expanded through pressure reduction valves from absorber pressure (55-60 bar) to intermediate pressure (20-25 bar)
- Flash drums separate vapor and liquid phases at the reduced pressure
- Flash vapor streams recover dissolved CO and H₂ along with some CO₂ (typical composition: 60-85% CO₂, 10-20% CO, 5-15% H₂, minor H₂S)
- Vapor streams are compressed (with isentropic efficiencies of 80-85%) back to absorber feed pressure and recycled to the absorber inlet via a mixer
- Liquid phases proceed to CO₂ desorption (containing 70-75% methanol, 25-30% CO₂, minor H₂S)
3. CO₂ Desorption and Recovery Section
- Multi-stage Flash and Column System: The CO₂ desorption section exploits the differential volatility of absorbed components to produce high-purity CO₂ while recovering methanol solvent. Configurations vary but typically include:
- Low-Pressure Desorption Column:
- Column type: Absorber/stripper with or without reboiler
- Number of stages: 12-20
- Operating pressure: 2-6 bar
- Rich solvent is expanded through multiple pressure reduction valves from intermediate pressure (20-25 bar) to low pressure (2-6 bar)
- Flash evaporation releases CO₂ while H₂S (more soluble) tends to remain in liquid phase
- Overhead vapor: High-purity CO₂ (typically 98-99.6% CO₂) at -40°C to -55°C and 2-6 bar
- Coolers maintain low temperatures (-40°C to -45°C) to condense methanol vapors and return as reflux
- High-Pressure Desorption Column (optional, depending on configuration):
- Number of stages: 15-25
- Operating pressure: 6-20 bar
- Processes different rich solvent stream or intermediate regeneration products
- Overhead vapor: CO₂ product (95-99% purity) at -30°C to -40°C
- Multi-stage flash drums: Bottom liquid from desorption columns may pass through successive flash separators (typically 2-4 stages) at progressively lower pressures to maximize CO₂ recovery and auto-refrigeration.
- Combined CO₂ product stream is compressed and typically achieves:
- Purity: 97-98.5% CO₂ (dry basis, CCS-ready)
- Delivery pressure: 80-150 bar for pipeline transport or storage
- Minor components: Residual methanol vapor, trace H₂S (<5 ppmv), inerts
- Reabsorber Column (for H₂S concentration):
- Processes flash vapors from low-pressure regeneration stages
- Uses lean methanol or nitrogen stripping to remove residual CO₂
- Re-absorbs flashed sulfur compounds (H₂S, COS) for routing to hot regeneration
- Enables separation of H₂S from CO₂ into distinct product streams
4. Methanol Solvent Regeneration Section
- Hot Regeneration and Solvent Recycling: Complete methanol regeneration requires thermal treatment to strip dissolved H₂S and residual CO₂, producing pure solvent for recycle.
- Distillation Column:
- Column type: Conventional distillation with partial condenser
- Number of stages: 8-12
- Operating pressure: Near atmospheric (1-1.5 bar)
- Methanol from CO₂ desorption columns is expanded to atmospheric pressure through expansion valve
- Reboiler duty: Provided by low-pressure steam (2-10 bar saturated steam) or waste heat recovery
- Overhead vapor distillate: H₂S-rich acid gas stream (typical composition: 15-25% H₂S, 75-85% CO₂) at atmospheric pressure
- H₂S/CO₂ ratio designed to meet Claus unit feed requirements (typically >1:2 to 1:3)
- Sent to sulfur recovery unit for H₂S conversion to elemental sulfur
- Bottom product: Regenerated methanol (>99.9% purity) at 60-75°C and atmospheric pressure
- Two-stage regeneration (optional configuration for energy optimization):
- First stage at moderate pressure (2-6 bar) for preliminary stripping
- Second stage at atmospheric pressure for final purification
- Reduces overall reboiler steam consumption by 15-20%
- Methanol Recycle System:
- Regenerated methanol is cooled in heat exchangers to near-ambient temperature (15-25°C)
- Pumped to absorber operating pressure (55-65 bar) using high-pressure centrifugal pumps (typical isentropic efficiency: 75-80%)
- Refrigerated to target absorption temperature (-40°C to -55°C) using refrigeration system
- Recycled to absorber column top
- Methanol make-up: Fresh methanol is continuously added to compensate for losses:
- Solvent losses: 0.04-0.1% of circulation rate due to methanol vapor pressure and mechanical losses
- Losses increase at higher operating temperatures
- Make-up typically ranges from 0.1-0.5 t/h per 400 t/h syngas throughput
5. Refrigeration System
- External Refrigeration (Critical Utility): Since physical absorption in methanol is highly temperature-dependent, substantial refrigeration capacity is required:
- Refrigeration cycle configuration:
- Cascade ammonia-ethane system for multi-level cooling
- Evaporation levels: Typically 2-4 temperature levels (-10°C, -30°C, -50°C, -70°C) depending on process requirements
- Compressor efficiency: Polytropic efficiency 80-84% for refrigeration compressors
- Major refrigeration loads:
- Cooling regenerated methanol from ambient to absorption temperature (-40°C to -55°C)
- Intercooling in absorber column (removing heat of absorption)
- Condensing methanol vapors in desorption column overheads
- Auto-refrigeration integration: Some process variants recover refrigeration duty by flashing high-pressure liquid streams and using the cooling effect to reduce external refrigeration requirements by 10-20%.
Key Process Parameters Summary
| Parameter |
Typical Range |
Unit |
| Absorber pressure |
35-60 |
bar |
| Absorber temperature |
-40 to -75 |
°C |
| Methanol feed temperature |
-40 to -55 |
°C |
| CO₂ desorption pressure |
2-20 |
bar |
| Final regeneration pressure |
1-1.5 |
bar |
| Compressor isentropic efficiency |
80-85 |
% |
| Pump isentropic efficiency |
75-80 |
% |
| Expander isentropic efficiency |
85-90 |
% |
| Methanol circulation rate |
100-200 |
kg/kg acid gas |
| Refrigeration temperature levels |
-10 to -70 |
°C |
Process Flexibility and Efficiency
Process Configurational Flexibility
Low-temperature methanol wash is a highly flexible technology that can be configured in multiple ways depending on application requirements, downstream process needs, and environmental constraints:
Acid Gas Removal Modes:
- Selective H₂S removal: CO₂ remains in treated gas while H₂S is removed to <0.1 ppm; suitable when CO₂ is desired in product gas (e.g., for H₂/CO ratio adjustment in synthesis applications)
- Selective CO₂ removal: Bulk CO₂ removal with minimal H₂S co-absorption; used when separate H₂S treatment is preferred
- Dual removal with separate recovery: Both acid gases removed with H₂S and CO₂ recovered as distinct product streams—the most common configuration for coal gasification
- Total acid gas removal: Combined acid gas stream produced for disposal or single-stage treatment
- Partial CO₂ removal: Controlled CO₂ removal to adjust CO₂/H₂ ratio for downstream synthesis requirements (Fischer-Tropsch, methanol)
Downstream Application Adaptability: The process can be tailored to produce syngas meeting specifications for:
- Hydrogen production (including blue hydrogen with CCS)
- Ammonia synthesis gas
- Methanol synthesis gas
- Fischer-Tropsch synthesis
- Synthetic Natural Gas (SNG) production
- Oxogas/oxochemicals production
- Power generation (IGCC applications)
- Pure CO production
Product Stream Flexibility:
- Clean syngas: Purity specification adjustable from <1 ppmv to <0.1 ppmv total sulfur depending on downstream catalyst requirements
- CO₂ product: Delivery conditions adjustable from low-pressure vent gas to high-pressure supercritical CO₂ (>150 bar) for CCS or EOR
- H₂S-rich stream: H₂S/CO₂ ratio optimized for sulfur recovery unit requirements (Claus or WSA process)
This configurational flexibility makes low-temperature methanol wash particularly suitable for complex gasification-based process schemes where multiple products are needed simultaneously, such as combined production of hydrogen, CO, fuel gas, and chemical feedstocks.
Removal Efficiency and Product Yields
Acid Gas Removal Performance (typical achievable levels):
| Component |
Feed
Concentration |
Product
Concentration |
Removal
Efficiency |
| H₂S |
0.5-2 mol% |
<0.1-1 ppmv (50-100 ppbv) |
>99.95% |
| COS |
50-500 ppmv |
<10 ppbv |
>99.9% |
| Total Sulfur |
1-2 mol% |
<0.1 ppm |
>99.99% |
| CO₂ |
20-35 mol% |
<0.01-0.1 mol% |
70-99%* |
*CO₂ capture level depends on process configuration and optimization objectives
Reference Case Performance (from detailed simulation):
- Raw syngas feed: 28% CO₂, 1.3% H₂S, 23.4% CO, 46.9% H₂, 0.4% N₂
- Clean syngas product: <0.01% CO₂, <0.6 ppmv H₂S (50 ppbv)
- H₂S removal: >99.95% (from 1.3 mol% to 50 ppbv)
- CO₂ capture: 96.4% (from 28 mol% to <0.01 mol%)
Trace Contaminant Removal:
- NH₃ (ammonia): >99% removal
- HCN (hydrogen cyanide): >99% removal
- CS₂ (carbon disulfide): >95% removal
- Mercaptans: >99% removal
- BTX aromatics (benzene, toluene, xylene): >90% removal
- Metal carbonyls (Ni(CO)₄, Fe(CO)₅): >95% removal
- Mercury (Hg): Partial removal depending on speciation
Product Stream Purities:
- Clean syngas: CO₂ <0.01-0.1 mol%, H₂S <0.1-1 ppmv, total sulfur <0.1 ppm
- CO₂ product: 97-98.5% purity (dry basis, CCS-ready with <5 ppmv H₂S)
- H₂S-rich stream to sulfur recovery: H₂S/CO₂ molar ratio 1:2 to 1:5, typically 15-25% H₂S concentration
Syngas Component Recovery (valuable gas conservation):
- CO recovery: >99% (via flash recovery system)
- H₂ recovery: >98% (via flash recovery system)
- Overall carbon efficiency: >98%
Solvent Consumption and Losses
Methanol Consumption:
- Make-up requirement: 0.04-0.1% of total circulation rate
- Typical make-up for 400 t/h syngas throughput: 0.15-0.5 t/h methanol
- Primary loss mechanisms: Vapor losses due to methanol's relatively high vapor pressure, mechanical losses in pumps and seals, carryover in product gas streams
- Losses increase at higher operating temperatures (temperature-dependent vapor pressure)
Methanol circulation rates: Typically 100-200 kg methanol per kg acid gas removed, depending on operating temperature and pressure
Environmental Performance
Indirect CO₂ Emissions (from utility consumption): Process optimization with structured packings reduces indirect CO₂ emissions by 16% through lower steam and electricity consumption
Economic Performance / Commercial Experience
Details are temporarily captured in this note.
References
