High temperature physical absorption

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Category: Carbon capture (mature) Subcategory: Physical absorption Combustion type: Pre

DESCRIPTION

For high-temperature physical absorption, the Selexol process is chosen for the information due to its high commercial readiness. Selexol process is a propriety of the UOP, a Honeywell company.¹ It uses a physical solvent to remove hydrogen sulfide (H₂S) and CO₂ (acid gases) from synthesis or lean natural gas streams (pre-combustion capture) under high pressure, either simultaneously or selectively. The Selexol™ solvent is a mixture of dimethyl ethers of polyethylene glycol (DEPG) and is produced by the Dow Chemical Company.³ The composition of the aqueous solvent reported in the literature by weight is 5% water + 95% DEPG.⁴ The feed gas enters the absorber column, which is made of structured packings. The solvent encounters the feed gas in counter-current flow, resulting in CO₂ and other acid gases dissolving physically into the Selexol solvent due to the high partial pressure, without a chemical reaction (unlike amine systems). The rich solvent leaving the CO₂ column is flashed in a series of drums where pressure is reduced in steps to recover relatively pure CO₂. The regenerated lean solvent is pumped back to the absorber column after cooling and impurity removal. The physical absorption is not economical for gas streams with low CO₂ partial pressure and CO₂ concentration lower than 15%.² Unlike chemical solvents (e.g., amine systems), Selexol does not react with CO₂; it just dissolves it physically. So at low pressures, the solvent has almost no driving force to pull CO₂ out of the gas. At atmospheric pressure, amine systems are usually more practical and cost-effective than pressurizing the gas stream for Selexol due to high compression costs.

High Temperature Physical Absorption Process

TECHNICAL ASPECTS (all % are volume-based)

Point sources: Hydrogen production, ammonia production, cement production, refining and petrochemical, and gas processing, biomass & coal gasification.⁵

CO₂ concentration range: 20-70%⁶

CO₂ capture efficiency: 95%³ – 99%⁵

CO₂ purity: 98%⁷

Min. feed gas pressure: ~21 bar⁸

Max. feed gas temperature: 40 °C⁹

Typical scale: Medium - Very large (7 MtCO₂/yr)¹⁰

Primary energy source: Electricity (& heat for 2-stage)

Impurity tolerance: Solvent is chemically stable and does not form heat-stable salts when in contact with the flue gas.¹¹

FUNCTION IN CCU VALUE CHAIN

Capture CO₂ from flue gases.
Most of the CO₂ is recovered at higher pressures (~6 bar), greatly reducing the CO₂ compressor power and costs.¹²
Another acid gas, H₂S, typically present in synthesis gas, is also removed but requires a thermal stripping step.

LIMITATIONS

Have low affinity for CO₂ compared to chemical solvents and therefore require more solvent.
Process equipment, such as absorbers, pumps, and pipework, is larger.
Weak affinity of CO₂ means removing CO₂ mainly from streams at moderate to high partial pressures and a smaller regeneration energy requirement.
Selexol has a high affinity towards water, requiring an upstream flue gas dehydration step.
Exhibits higher viscosity than those of common physical solvents, particularly at low temperatures, resulting in low mass transfer and high packing requirements.⁹

ENERGY

Electricity is used by solvent pumps, recycle gas compressors, CO₂ compression, and flue gas compression, if the flue gas is not pressurized.
Electricity is used in a refrigeration system if the operating temperature is very low or sub-zero.
Steam may be required for an upstream dehydration step since physical solvents have a high affinity towards water.

CONSUMABLES

Selexol solvent is used to capture CO₂ from flue gas with negligible solvent loss.
Cooling water is used in the H₂S stripping column to condense entrained water and solvent.

Energy and Consumables
Parameter	Value
Solvent make-up (kg/tCO₂)	0.007¹³
Cooling water (t/tCO₂)	~12*
Heat (kWh/tCO₂)	-NA-
Electricity (kWh/tCO₂)	~78*
*VITO TEA study (details below)

COSTS

CAPEX: 3 - 4 €/tCO₂ ¹² (3.4 €/tCO₂ *)

Main CAPEX: absorption column, flash drums and flue gas, and recycle compressors.

OPEX: 21 - 24 €/tCO₂ ¹² (16.1 €/tCO₂ *)

Main OPEX: electricity and cooling water.

CO₂ capture cost: 24-28 €/tCO₂ ¹² (19 €/tCO₂ *)

Depends on absorption temperature, scale, CO₂ concentration, CO₂ partial pressure, etc.

¹² IGCC plant; lifetime – 30 years; CO₂ concentration – 40.4%; CO₂ capture capacity – 3.08 MtCO₂/yr; feed pressure = 29.6 bar; electricity price = 108 €/MWh; includes CO₂ compression to 130 bar; CRF – 0.106; 2018 euros.

* VITO TEA study; IGCC plant; lifetime – 25 years; CO₂ concentration – 40.4%; CO₂ capture capacity – 3.75 MtCO₂/yr; feed pressure = 29.6 bar; electricity price = 100 €/MWh; includes CO₂ compression to 110 bar; WACC = 4.6%; 2022 euros; includes initial solvent inventory in CAPEX.

CO₂ avoidance cost: 21 - 32 €/tCO₂ avoided ¹⁴

¹⁴ IGCC plant; CO₂ compression, transport, and storage cost included; 2000 euros; lifetime – 30 years.

Note: The CO₂ avoidance cost presented here assumes a pressurized feed gas. For atmospheric pressure streams, Selexol is not economical because of low CO₂ partial pressure and the significant cost of compression.

ENVIRONMENTAL

CO₂ footprint: ~17 kgCO₂eq/tCO₂ ¹³

Estimated using only utilities data from literature.¹³ Relatively low footprint due to lower regeneration energy requirements.

Spatial footprint: 178,125 m² (475x375) for 6.02 MtCO₂/yr ¹⁵ (includes two-stage Selexol process and CO₂ compression system)

Environmental issues: Minimal process emissions and effluents.¹

ENGINEERING

Maturity: Commercial (TRL 9)⁵

Most widely used method with a physical solvent.

Retrofittability: Feasible

No significant modifications are required because Selexol operates at relatively low temperatures and pressures, and no thermal regeneration is required.

Scalability: High

Multiple trains can be added for upscaling ^5,7

Process type: Liquid solvent-based without chemical reactions.

Deployment model: Centralized or Decentralized.

Decentralized CO₂ absorption at point sources with centralized desorption.

Technology flexibility: Hybridization with other capture technologies is feasible. Other technologies, such as membranes or PSA, can be used upstream to increase CO₂ concentration.

TECHNOLOGY PROVIDERS

SeparALL™ by HONEYWELL UOP, United States (process)
Selexol™ by DOW, United States (solvent)
Genosorb^® by CALRIANT, Switzerland (solvent)

ALTERNATE PROCESSES

Purisol

Purisol uses n-methyl-2-pyrrolidone (NMP) solvent to remove acid gases CO₂, H₂S, and COS at ambient temperature. Flow schemes used for this solvent are like those used for Selexol. It is particularly well suited to the purification of high-pressure, high CO₂ synthesis gas.

Fluor

Fluor processes use propylene carbonate solvent to remove CO₂ and H₂S from flue gases as well as C3+ hydrocarbons, COS, SO₂, and CS₂. It cannot be used for selective H₂S treatment because it is unstable at high temperatures.

CONTACT INFO

Mohammed Khan (mohammednazeer.khan@vito.be)

Miet Van Dael (miet.vandael@vito.be)

ACKNOWLEDGEMENT

This infosheet was prepared as part of the MAP-IT CCU project funded by VLAIO (grant no. HBC.2023.0544).

REFERENCES

1. UOP. UOP Selexol TM Technology for Acid Gas Removal.; 2009.

2. Songolzadeh M, Soleimani M, Ravanchi MT, Songolzadeh R, Han -W, Rogov VA. Carbon Dioxide Separation from Flue Gases: A Technological Review Emphasizing Reduction in Greenhouse Gas Emissions. Sci World J. 2014;828131:34.

3. Technologies for Efficient Purification of Natural and Synthetic Gases |.

4. Kapetaki Z, Brandani P, Brandani S, Ahn H. Process simulation of a dual-stage Selexol process for 95% carbon capture efficiency at an integrated gasification combined cycle power plant. Int J Greenh Gas Control. 2015;39:17-26.

5. Barlow H, Shahi SSM. State of the Art: CCS Technologies 2024.; 2024.

6. Im D, Roh K, Kim J, Eom Y, Lee JH. Economic assessment and optimization of the Selexol process with novel additives. Int J Greenh Gas Control. 2015;42:109-116.

7. Nelson M, Vimalchand P, Brown R, et al. Carbon Capture at the Kemper IGCC Power Plant. GHGT 2018 - 14th Int Conf Greenh Gas Control Technol. 2018;(October):1-18.

8. Kohl AL, Nielsen R. Gas Purification. 5th ed. Gulf Pub; 1997.

9. Ashkanani HE, Wang R, Shi W, Siefert NS, Hopkinson D, Resnik K. Levelized Cost of CO2 Captured Using Five Physical Solvents in Pre-combustion Applications. Int J Greenh Gas Control. 2020;101:1-46.

10. MIT. LaBarge Fact Sheet: Carbon Dioxide Capture and Storage Project. Carbon Capture and Sequestration Technologies @ MIT. 2016

11. Zaman M, Lee JH. Carbon capture from stationary power generation sources: A review of the current status of the technologies. Korean J Chem Eng. 2013;30(8):1497-1526.

12. Xin K, Hashish M, Roghair I, van Sint Annaland M. Process Simulation and Economic Analysis of Pre-combustion CO2 Capture With Deep Eutectic Solvents. Front Energy Res. 2020;8:323.

13. Singh B, Strømman AH, Hertwich EG. Comparative impact assessment of CCS portfolio: Life cycle perspective. Energy Procedia. 2011;4:2486-2493.

14. Chen C. A Technical and Economic Assessment of Selexol-Based CO2 Capture Technology for IGCC Power Plants. Carnegie Mellon University; 2005.

15. IEA. Retrofit of CO2 Capture to Natural Gas Combined Cycle Power Plants.; 2005

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Infosheet on High Temperature Physical Absorption