Selexol Process Fundamentals

1. Introduction to Physical Solvents

Physical solvents absorb acid gases by physical dissolution rather than chemical reaction. The absorption mechanism follows Henry’s Law: the amount of gas dissolved in the solvent is directly proportional to its partial pressure in the gas phase. This is fundamentally different from chemical solvents such as amines, where acid gas absorption occurs through an exothermic chemical reaction between the solvent and the acid gas molecules.

The practical consequence of Henry’s Law behavior is that absorption capacity increases linearly with pressure and decreases with temperature—the opposite trend from chemical solvents, which rely on reaction kinetics that improve at higher temperatures. Physical solvents therefore become increasingly attractive as feed gas pressure increases, particularly above 300–400 psig, where their loading capacity per unit of solvent circulation can exceed that of chemical solvents.

What Is Selexol?

Selexol is the trade name for a mixture of dimethyl ethers of polyethylene glycol (DMPEG), with the general formula CH₃O(CH₂CH₂O)_nCH₃ where n ranges from 3 to 9. The solvent was originally developed by Allied Chemical and is now licensed by UOP (a Honeywell company). It is the most widely used physical solvent in the natural gas and syngas treating industries.

Selexol is best suited for the following applications:

• High-pressure gas streams: Feed pressures above 300 psig where Henry’s Law provides high solvent loading
• High CO₂ content: Gas streams containing >5 mol% CO₂ where bulk removal is required
• Bulk CO₂ removal: Applications where deep CO₂ removal is needed without expensive thermal regeneration
• Selective H₂S removal: Gas streams where H₂S must be removed to pipeline specification while CO₂ can remain in the product
• Syngas treating: Pre-combustion carbon capture and hydrogen purification in IGCC and ammonia plants

Physical vs. Chemical Solvent Comparison

2. Selexol Solvent Properties

The Selexol solvent is a mixture of polyethylene glycol dimethyl ethers with the general chemical formula CH₃O(CH₂CH₂O)_nCH₃, where n typically ranges from 3 to 9. The commercial product is a blend of these oligomers, resulting in an average molecular weight of approximately 280 g/mol. The distribution of chain lengths is optimized to balance viscosity, vapor pressure, and absorption capacity.

Key Physical Properties

• Molecular weight: ~280 g/mol (average of oligomer mixture)
• Density: ~1.03 g/cm³ at 77°F
• Viscosity: ~5.8 cP at 77°F; decreases significantly with temperature
• Vapor pressure: Very low (~0.00073 mmHg at 77°F); negligible solvent losses to treated gas
• Thermal stability: Stable to approximately 350°F; no thermal degradation under normal operating conditions
• Freezing point: −18°F; compatible with chilled operation
• Flash point: ~304°F (closed cup)

Gas Solubility in Selexol

The key advantage of Selexol is its differential solubility for various gas components. Acid gases, particularly H₂S, are much more soluble than light hydrocarbons. The solubility order in decreasing magnitude is:

The H₂S/CO₂ selectivity ratio is approximately 8–9:1 at typical operating conditions. This selectivity enables the process to remove H₂S preferentially from gas streams that also contain significant CO₂, producing a concentrated H₂S stream suitable for Claus sulfur recovery while allowing most of the CO₂ to pass through the absorber.

Relative Solubility of Gases in Selexol

Selexol is non-corrosive to carbon steel, does not foam in the presence of liquid hydrocarbons or well-treating chemicals, and is chemically stable—it does not degrade or form heat-stable salts. These properties result in lower maintenance costs and longer solvent life compared to amine-based systems. Solvent makeup requirements are minimal, typically limited to mechanical losses from pump seals and minor entrainment.

3. Process Configurations

The Selexol process can be configured in several ways depending on the treating objective: selective H₂S removal only, bulk CO₂ removal only, or combined H₂S and CO₂ removal. The choice of configuration is driven by the feed gas composition, product specifications, and downstream processing requirements.

Selective H₂S Removal

In the selective configuration, a single absorber column contacts the sour gas with lean Selexol solvent. Because H₂S is approximately 8 times more soluble than CO₂ in Selexol, H₂S is preferentially absorbed while most of the CO₂ passes through the absorber with the treated gas. This configuration is used when the CO₂ specification on the treated gas is not stringent (for example, when the gas is used as fuel and CO₂ content up to several percent is acceptable), but H₂S must be removed to ≤4 ppmv for pipeline or process requirements.

The rich solvent is regenerated by a combination of multi-stage flash (to recover dissolved hydrocarbons and some CO₂) followed by thermal stripping with steam or hot solvent to drive off the absorbed H₂S. The concentrated H₂S stream is sent to a Claus sulfur recovery unit.

Bulk CO₂ + H₂S Removal

When both H₂S and CO₂ must be removed to low levels, a two-stage absorption system is employed. The first stage selectively absorbs H₂S using lean (thermally regenerated) solvent. The second stage removes bulk CO₂ using semi-lean (flash-regenerated) solvent. This arrangement produces two separate acid gas streams: a concentrated H₂S stream suitable for Claus processing and a CO₂-rich stream that can be vented, compressed for enhanced oil recovery (EOR), or sequestered.

Syngas Treating

In integrated gasification combined cycle (IGCC) and ammonia production plants, the Selexol process treats shifted syngas to remove H₂S and CO₂. Pre-shift configurations treat the raw syngas before the water-gas shift reactor, while post-shift configurations treat the shifted gas after CO has been converted to CO₂ and H₂. Post-shift treating is more common because the higher CO₂ partial pressure after the shift reactor improves Selexol absorption efficiency.

Regeneration Methods

Flash regeneration: Rich solvent is depressurized through 3–5 stages of flash drums at progressively lower pressures. At each stage, dissolved gases desorb from the solvent in approximate reverse order of their solubility. The first flash stages recover dissolved hydrocarbons (which are recompressed and returned to the product gas), while subsequent stages release CO₂. Flash regeneration requires no external heat input and is the primary advantage of physical solvents over chemical solvents in terms of operating cost.

Thermal regeneration: For the H₂S-rich fraction that remains in the solvent after flash regeneration, steam stripping or reboiled stripping is used to drive off the remaining acid gas. The overhead vapor from the stripper is a concentrated H₂S stream (typically 30–60 mol% H₂S) suitable for feed to a Claus unit. Thermal regeneration temperatures are typically 250–300°F, significantly lower than amine reboiler temperatures.

Process Configuration Selection Guide

4. Design Considerations

Selexol process design is governed by the thermodynamic relationship between acid gas partial pressure, solvent temperature, and solvent loading capacity. Because absorption follows Henry’s Law, the key design variables—operating pressure, temperature, and solvent circulation rate—are directly coupled to the gas composition and product specifications.

Operating Pressure

The Selexol process is most effective at elevated pressures, typically above 300 psig. Higher feed gas pressure increases the partial pressure of acid gas components, which directly increases solvent loading per Henry’s Law. At pressures below 200 psig, the solvent loading capacity becomes marginal and chemical solvents (amines) are generally more economical. Most commercial Selexol units operate at feed pressures of 500–1,200 psig.

Operating Temperature

Gas solubility in physical solvents increases as temperature decreases. Selexol units typically operate at 0–40°F to maximize absorption capacity. Chilled operation requires refrigeration, which adds to capital and operating costs but is offset by reduced solvent circulation rates. The solvent freezing point of approximately −18°F sets the lower temperature limit. Warmer operation (60–80°F) is sometimes used when refrigeration is not justified, but at the expense of higher circulation rates.

Solvent Circulation Rate

The required solvent circulation rate is determined by the rich loading at operating conditions. Unlike chemical solvents, where loading is limited by stoichiometry (typically 0.3–0.45 mol acid gas per mol amine), physical solvent loading is limited only by the equilibrium solubility at the operating pressure and temperature. Higher pressures and lower temperatures allow higher rich loadings and therefore lower circulation rates for the same acid gas removal duty.

Absorber Design

The absorber column typically contains 15–25 trays (valve or sieve type) or the equivalent height of structured packing. The number of stages is determined by the required H₂S removal efficiency and the desired CO₂ slip (in selective configurations). For selective H₂S removal, fewer trays are used to limit CO₂ co-absorption, similar to the approach used with selective amines like MDEA.

Flash Drum System

The flash regeneration system typically consists of 3–5 stages of pressure letdown. The first stage flashes at the highest pressure (often 50–60% of absorber pressure) to recover dissolved hydrocarbons, which are recompressed and returned to the product gas. Subsequent stages flash at progressively lower pressures to desorb CO₂. The final flash stage operates near atmospheric pressure or under vacuum.

Hydrocarbon Co-Absorption

Heavier hydrocarbons (C₃+) are significantly more soluble in Selexol than methane and ethane. This co-absorption can result in hydrocarbon losses to the acid gas stream if not properly managed. The first flash stage recovers most co-absorbed hydrocarbons before the acid gas is sent to the Claus unit. Excessive hydrocarbons in the Claus feed can cause operational problems including flame instability and catalyst fouling. Process design must account for hydrocarbon co-absorption to minimize product losses and protect downstream equipment.

Typical Design Parameters

5. Comparison with Other Physical Solvents

Several physical solvents compete with Selexol in the gas treating market. Each has distinct advantages and limitations that determine the most appropriate selection for a given application. The choice among physical solvents depends on the required removal depth, selectivity, operating temperature range, and hydrocarbon co-absorption characteristics.

Rectisol (Chilled Methanol)

The Rectisol process, licensed by Linde and Air Liquide, uses chilled methanol as the physical solvent. It operates at very low temperatures (−40 to −80°F), which provides extremely high acid gas solubility and enables removal of H₂S and CO₂ to sub-ppmv levels. Rectisol is the preferred technology for syngas treating in coal gasification plants, ammonia production, and Fischer-Tropsch synthesis where ultra-deep removal is required. The primary disadvantage is the high refrigeration energy cost and the complexity of handling chilled methanol.

Purisol (N-Methyl-2-Pyrrolidone, NMP)

The Purisol process, licensed by Lurgi, uses N-methyl-2-pyrrolidone (NMP) as the solvent. NMP offers higher H₂S selectivity than Selexol (approximately 10–12:1 versus 8:1 for Selexol) and lower hydrocarbon co-absorption. Purisol is suitable for selective H₂S removal from natural gas at moderate pressures. However, NMP has a higher vapor pressure than Selexol, resulting in greater solvent losses to the treated gas, which increases operating costs and may require a solvent recovery wash section.

Fluor Solvent (Propylene Carbonate)

The Fluor Solvent process uses propylene carbonate and is primarily used for bulk CO₂ removal. It has limited H₂S selectivity and is not suitable for applications requiring tight H₂S specifications. Its main advantage is simplicity—the process typically uses flash regeneration only, with no thermal regeneration. This makes it the lowest-cost option for applications that require only bulk CO₂ removal, such as CO₂ removal from natural gas for pipeline transport.

Selexol Advantages

Selexol occupies a favorable middle ground among physical solvents, offering a good balance of selectivity, absorption capacity, and operational simplicity. Its key advantages include:

• Very low vapor pressure (<0.001 mmHg at 77°F), resulting in negligible solvent losses
• Moderate H₂S/CO₂ selectivity (~8:1) sufficient for most natural gas and syngas applications
• Moderate operating temperatures (0–40°F) requiring less refrigeration than Rectisol
• Non-corrosive, non-foaming, and chemically stable with no degradation products
• Extensive commercial operating history (50+ years) with well-established design practices

Physical Solvent Comparison

References

1. GPSA, Chapter 21 — Hydrocarbon Treating
2. Kohl, A. L. and Nielsen, R. B., Gas Purification, 5th Edition, Gulf Publishing, 1997
3. UOP Selexol Process Technical Bulletins
4. Burr, B. and Lyddon, L., “A Comparison of Physical Solvents for Acid Gas Removal,” Bryan Research & Engineering Technical Paper