Which Physical Solvent Is Best For Acid Gas Removal?

In industries such as refineries, petrochemical plants, oil refining, ammonia plants, and hydrogen plants, acid gas removal is a standard process required for their operations.

Also known as amine scrubbing or amine gas treating, the primary process includes the removal of carbon dioxide (CO2) and hydrogen sulfide (H2S) from gases and vapor streams.

This group of operations uses aqueous solutions and solvents to absorb the gases. Carbon dioxide and hydrogen sulfide are gaseous compounds that form an acidic solution when dissolved in water. There are several reasons why acid gas removal is a necessary process. 

These substances can damage the pipes in varying concentrations by bringing extensive corrosion and rusting. Acid gases can also cause adverse effects on the humans working in the vicinity as prolonged exposure can bring severe illnesses and other adverse health conditions.

The release of these gases in the environment can also cause considerable damage, one of the manifestations including acid rain. However, the industry set standards to regulate acid gas concentrations, which is critical to ensuring an efficient and cost-effective separation process.

Why Need Acid Gas Removal?

For oil and gas industries, acid gas removal is a necessary process that affects and increases the heating value of sales gas. This step can also eliminate the occurrence of crystallization during the liquefaction process. Moreover, amine-based gas absorption is this industry’s most used acid gas removal technology. This technology relies on acid gas absorption into an aqueous amine solution, followed by thermal regeneration.

In this article, you’ll learn which physical solvent you need for acid gas removal.

Physical Solvents And Their Comparison

In high concentrations of acid gases, industries favor physical solvents for this process. However, depending on their purpose and several factors, there are different uses for these aqueous amine solutions.

Here are some of them:

■ Dimethyl Ether of Polyethylene Glycol (DEPG)

DEPG is a compound combined from polyethylene glycol ((CH3O(C2H4O)nCH3) dimethyl ethers and a solvent that can absorb hydrogen sulfide and carbon dioxide from gas streams. Selexol or Coastal AGR is the process that uses DEPG.

DEPG requires a total circulation rate of 15,282 standard gallons per minute (sgpm). These are the physical properties of DEPG:

■ 5.8 viscosity at 25 °C

■ Molecular weight of 280

■ Freezing point at -28 °C

■ Boiling point at 760 mm Hg and 275 °C

For the solubility of selected gases at 25 °C, carbon dioxide is at 3.63 while hydrogen sulfide is at 32.4. Therefore, hydrogen sulfide and carbon dioxide removal are usually done in a 2-stage process with two absorption and regeneration columns.

The first column removes H2S using a lean solvent thoroughly stripped with steam. CO2 is released in the second absorber. In addition, the second-stage solvent can be regenerated with nitrogen or air using a series of flashes, which is especially important when bulk CO2 removal is required. DEPG works by dehydrating the gas.

DEPG has a higher viscosity than other solvents used in acid gas removal, reducing mass transfer rates and tray efficacies and increasing requirements. This quality of DEPG requires a reduction of temperature to increase acid gas solubility and reduce circulation rate. Due to external vapor pressure, water is not necessary for DEPG. Therefore, it is suitable for operation at temperatures ranging from 0 °F (-18 °C) to 347 °F (175 °C).

Several companies that license and manufacture solvents containing DEPG are the Coastal Chemical Company (also known as Coastal AGR), UOP (Selexol), and Dow (Selexol). 

■ N-Methyl-2-Pyrrolidone (NMP)

N-Methyl-2-Pyrrolidone (NMP) has flow schemes similar to DEPG. Ambient temperature or refrigeration down to 5°F (-15°C) are suitable for NMP process operation.

For equipment requirements, (NMP) requires a high total circulation rate of 16,195 sgpm. NMP’s physical properties include the following:

■ 1.65 viscosity at 25 °C

■ The molecular weight of 99

■ A freezing point at -24 °C

■ A boiling point at 760 mm Hg and 202 °C

With carbon dioxide and hydrogen sulfite’s solubility at 25 °C, carbon dioxide has 3.57 while hydrogen sulfide has 36.4.

Compared to DEPG and PC, NMP has a relatively high vapor pressure. Therefore, for solvent recovery, the licensor recommends washing the treated and rejected acid gases, which cannot be used for simultaneous gas dehydration. Moreover, water wash is not required for NMP recovery at sub-ambient temperatures. With physical solvents considered for H2S over CO2, NMP is the most suitable.

Lurgi AG has licensed the Purisol Process, which utilizes NMP.

■ Methanol (MeOH)

Methanol requires processing conditions and equipment that are primarily different from others. The process can be flexible with many frow schemes. 

The optimum flow scheme is defined by product specifications and process objectives. These can be configured to separate synthesis gas into other required final products and components. MeOH for acid gas removal is also known as Rectisol.

The total circulation rate that MeOH requires is 4,623 sgpm. The physical properties of MeOH are:

■ 0.6 viscosity at 25 °C

■ The molecular weight of 32, making it the lightest

■ A freezing point at -92 °C

■ A boiling point at 760 mm Hg at 65 °C, the lowest among others

The selected gases’ solubility at 25°C has carbon dioxide at 13.46 and hydrogen sulfide at 147.

Rectisol is a common MeOH in the industry, with a purification of synthesis derived from the gasification of heavy oil and coal. Ifpexol, another MeOH, has a two-stage process that enterprises can use in natural gas applications. Ifpex-1 filters out hydrocarbons and water, while Ifpex-2 filters out acid gas.

Because methanol has a high vapor pressure under normal conditions, deep refrigeration and other methods are needed to prevent high solvent losses. Water is frequently used to recover methanol by washing three effluent streams. 

Rectisol is typically operated at temperatures below 32 °F (0 °C). The process’s primary advantage is methanol’s high selectivity for H2S over CO2. In addition, it also has the ability to remove COS. Compared to DEPG, MeOH has higher hydrogen sulfate and carbon dioxide solubility. 

On the other hand, the more complex flow scheme and need for refrigeration results in higher capital and operating cost. In addition, compared to other physical solvents, the solvent flow rate for CO2 removal is significantly reduced.

Companies that license the production of this solvent for acid gas removal include the Rectisol process (approved by Lurgi AG) and Ifpexol (Prosernat).

■ Propylene Carbonate (PC) 

PC or Propylene carbonate is the primary solvent in the Fluor process. It is one of the physical solvents used in acid gas removal, where it is primarily used to remove carbon dioxide.

It requires minor equipment, with a total circulation rate requirement of 6085 sgpm. Propylene Carbonate’s physical properties include:

■ 3.0 viscosity at 25 °C

■ The molecular weight of 102

■ A freezing point at -48 °C

■ A boiling point at 760 mm Hg and 240 °C

For carbon dioxide and hydrogen sulfite’s solubility at 25°C, carbon dioxide has 3.41, and hydrogen sulfide has 11.2.

When there is little to no H2S and CO2 removal is critical, Propylene Carbonate (PC) has an advantage over other physical solvents. 

PC has light hydrocarbons in natural gas and hydrogen in synthesis gas, resulting in a lower cycle gas compression requirement. Furthermore, you can use PC at intermediate pressures due to its lower solubility in the purified gas, mainly when applied to gas flashed from rich solvents. 

In addition, PC has lower hydrocarbon losses in CO2 vent gas steam. Companies have improved the process design to include an immediate pressure absorber to remove CO2, significantly lowering operating costs and product losses.

PC can operate at lower temperatures without becoming too dense, allowing for efficient mass transfer. In addition, another innovation involving feed chilling to reduce hydrocarbon absorption is invented. Most of the hydrocarbon condensates when PC cools to 0°F (-18°C). This process reduces the overall solvent circulation rate and plant costs.

Fluor Solvent, a PC-based process, has been in use since the late 1950s. This solvent is licensed by Fluor Daniel, Inc. PC is available as JEFFSOL® PC solvent and is particularly useful in treating syngas.

Conclusion

Some factors to consider in selecting the physical solvent most suitable for the acid gas removal process are the solvent’s purpose, suitability, equipment requirement, and composition. This selection can define the process cost and the outcome of acid gas removal. To cut costs and improve product, acid gas removal and using the proper solvent is vital in removing carbon dioxide and hydrogen sulfide from gases and vapor streams.

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