Mar-2008

A comparison of physical solvents for acid gas removal

Physical solvents such as DEPG (Selexol or Coastal AGR), NMP or N-Methyl-2- Pyrrolidone (Purisol), Methanol (Rectisol), Propylene Carbonate (Fluor Solvent) and others are becoming increasingly popular as gas treating solvents, especially for coal gasification applications

Barry Burr and Lili Lyddon
Bryan Research & Engineering

Article Summary

The process simulation program ProMax1 is used to perform comparisons for these physical solvents in terms of acid gas removal ability, equipment required and power requirements. Before performing the final comparisons, the simulation results are verified by comparisons with experimental vapour-liquid equilibrium data.

Options for acid gas removal
A number of methods are available for removal of acid gases from product gas streams. Some of the more commonly used methods are chemical solvents, physical solvents, membranes and cryogenic fractionation.2 Ethanolamines (MEA, DEA, MDEA, DGA, etc) and hot potassium carbonate are chemical solvent processes that rely on chemical reactions to remove acid gas constituents from sour gas streams. The regeneration of chemical solvents is achieved by the application of heat, whereas physical solvents can often be stripped of impurities by reducing the pressure without the application of heat. Physical solvents tend to be favoured over chemical solvents when the concentration of acid gases or other impurities is very high. Unlike chemical solvents, physical solvents are non-corrosive, requiring only carbon steel construction.

In general, the economics of CO2 recovery is strongly influenced by the partial pressure of CO2 in the feed gas. At low partial pressures, physical solvents are impractical because the compression of the gas for physical absorption is expensive. However, if the gas is available at high pressure, physical solvents might be a better choice than chemical solvents.

The concentration of heavy hydrocarbons in the feed gas also affects the choice of gas treating solvent. If the concentration of heavy hydrocarbons is high, a physical solvent may not be the best option due to higher co-absorption of hydrocarbons, particularly pentanes plus. Unlike natural gases, where hydrocarbon co-absorption can be a problem for physical solvents, synthesis gases do not contain appreciable quantities of hydrocarbons.³ This makes physical solvents particularly applicable to synthesis gas treating.

The membrane process is applicable for high- pressure gas containing high acid gas concentrations. CO2 recovery is accomplished by pressure-driven mass transfer through a permeable membrane where separation is due to the differences in the permeation rate of different compounds. The acid gas is recovered at low pressure. A high-purity product containing approximately 95% CO2 can be achieved with one or two stages, depending upon feed gas pressure and per cent recovery. Economic considerations may dictate additional capital and incremental energy requirements to increase feed pressure and/or utilise two-stage separation with recompression of gas from the first stage.

Cryogenic fractionation has the advantage that the CO2 can be obtained at relatively high pressure as opposed to the other methods of recovering CO2. This advantage may, however, be offset by the large refrigeration requirement. Special materials are also required for cryogenic service.

Common physical solvents for acid gas removal
A number of physical solvents are available for use in acid gas treating processes. A comprehensive list of common physical solvents may be found in gas purification.⁴ Four of the solvents are considered here: dimethyl ether of polyethylene glycol (DEPG), propylene carbonate (PC), N-Methyl-2-Pyrrolidone (NMP) and methanol (MeOH). An exhaustive literature survey reveals where and how these physical solvents are currently being used.

Dimethyl ether of polyethylene glycol (DEPG)
DEPG is a mixture of dimethyl ethers of polyethylene glycol (CH3O(C2H4O)nCH3 (n is between 2 and 9) used to physically absorb H2S, CO2 and mercaptans from gas streams. Solvents containing DEPG are licensed and/or manufactured by several companies including Coastal Chemical Company (as Coastal AGR), Dow (Selexol) and UOP (Selexol). Other process suppliers such as Clariant GmbH of Germany offer similar solvents. Clariant solvents are a family of dialkyl ethers of polyethylene glycol under the Genosorb name.³

DEPG can be used for selective H2S removal, which requires stripping, vacuum stripping or a reboiler. The process can be configured to yield both a rich H2S feed to the Claus unit as well as bulk CO2 removal. Selective H2S removal with deep CO2 removal usually requires a two-stage process with two absorption and regeneration columns. H2S is selectively removed in the first column by a lean solvent that has been thoroughly stripped with steam, while CO2 is removed in the second absorber. The second-stage solvent can be regenerated with air or nitrogen for deep CO2 removal, or using a series of flashes if bulk CO2 removal is required. DEPG also dehydrates the gas and removes HCN.

Compared to the other solvents, DEPG has a higher viscosity, which reduces mass transfer rates and tray efficiencies and increases packing or tray requirements, especially at reduced temperatures. Since it is sometimes necessary to reduce temperature to increase acid gas solubility and reduce circulation rate, this could be a disadvantage. DEPG requires no water wash to recover solvent due to very low vapour pressure. DEPG is suitable for operation at temperatures up to 347°F (175°C). The minimum operating temperature is usually 0°F (-18°C).

Methanol (MeOH)
There are a number of methanol processes for acid gas removal including the Rectisol process (licensed by Lurgi AG) and Ifpexol (Prosernat). The Rectisol process was the earliest commercial process based on an organic physical solvent and is widely used for synthesis gas applications. The process operates at a very low temperature and is complex compared to other physical solvent processes. The main application for the Rectisol process is purification of synthesis gases derived from the gasification of heavy oil and coal rather than natural gas treating applications.⁴ The two-stage Ifpexol process can be used for natural gas applications. Ifpex-1 removes condensable hydrocarbons and water, and Ifpex-2 removes acid gas.⁵