Hot Potassium Carbonate CO2 removal Process

benfield solution composition

A typical Benfield solution composition contains 25-30 equivalent wt% K2CO3, <1 wt% corrosion inhibitor, 3 wt% amine promoter, antifoam, and various contaminants including chlorides, iron, suspended solids, and heat stable salts. For applications where an organic activator can not be utilized a vanadium and boron based inorganic promoter is instead used. The hot potassium carbonate process is a chemical absorption process. The overall reaction occurring in the system is presented below:

K2CO3 + H2O + CO2 ↔ 2KHCO3

co2 removal reaction

Potassium carbonate (K2CO3) is an inorganic salt that does not exert a CO2 vapor pressure when dissolved in a water solution. Potassium bicarbonate (KHCO3) is a weak salt formed after reacting K2CO3 with dissolved CO2 in solution. Potassium bicarbonate is similar to household baking soda (sodium bicarbonate) in that exhibits a CO2 vapor pressure when dissolved in water. When baking soda is dissolved in water the fizzing observed is CO2 vapor leaving the solution.

Plants measure the extent of the overall reaction by looking at the solution conversion or solution fraction. The solution conversion is reported from 0-100% and reports the percent of potassium bicarbonate (KHCO3) in solution. A typical rich solution leaving the bottom of the absorber will have a solution conversion around 80%. While the lean solution leaving the CO2 stripper will typically have a solution conversion around 20-30% as a majority of the KHCO3 will have reacted back to K2CO3.

This reversible reaction makes potassium carbonate a perfect candidate for CO2 removal applications. The reaction is exothermic when absorbing CO2 and endothermic when releasing CO2. In the absorber CO2 under high pressure is dissolved into the water solution where it will react with available K2CO3 forming KHCO3 and releasing heat. Towards the bottom of the CO2 absorber a majority of the potassium sites will have been converted to KHCO3. The loaded rich solution is then sent to the CO2 stripper where the pressure is let down and the KHCO3 molecules will release CO2. Steam is added to the stripper to promote CO2 stripping. At the bottom of the CO2 stripper a majority of the potassium will be in the K2CO3 form and ready for CO2 absorption completing the loop.

The major driving force for CO2 absorption and CO2 stripping is the difference between the vapor pressure of the carbonate solution and the pressure of CO2 in the vapor phase. As, such absorption favors high pressure while stripping favors low pressure. In order to achieve the best CO2 slip in the treated gas from the absorber the lean solution needs to be thoroughly stripped. Most of the various process configurations aim to lower the lean solution vapor pressure sent to the top of the absorber (e.g. lean solution flash tanks, two-stage systems, heat integration, lean solution coolers, etc.) in the most steam efficient way possible. An example process flow diagram for a two-stage HPC system with a semi-lean flash vessel is presented below.

HPC Process Flow Diagram

HPC CO2 Removal Applications

The hot potassium carbonate process is typically utilized in industrial applications where the feed partial pressure of CO2 is around 10-100 psia. HPC systems can typically achieve a CO2 slip from the absorber below 1000 ppmv. Industries where HPC systems are commonly utilized include CO2 removal from steam methane reformed gas at ammonia and hydrogen plants, natural gas, ethylene oxide recycle gas, vinyl acetate monomer recycle gas, and methanol synthesis plants. Additionally, HPC have increasingly been investigated for use in carbon capture plants from fossil fuel power generation, incineration plants, pulp mills, and refinery flue gasses.

Solvents available for industrial CO2 removal applications typically include either amine, alkali salt, or physical solutions. The choice of a CO2 removal solvent during the design phase is typically achieved by looking at the feed CO2 partial pressures and the desired CO2 partial pressure in the treated gas. The graph on the right shows the typical application for each CO2 absorption solvent based on the CO2 partial pressure in the feed and product gasses. The partial pressure ranges are approximations and solvents are commonly applied outside of the typical partial pressure ranges represented.

Graphical representation of CO2 absorption solvent selection. Refernces:

Kidnay, A. J. and Parrish, W. R. Gas treating in Fundamentals of Natural Gas Processing 91–132 (CRC Press, 2006)
DOE/NETL, Carbon Dioxide Capture Handbook 12 (Aug, 2015)
Tennyson, R.N. and Schaaf, R.P., Guidelines can help choose proper process for gas treating plants, Oil Gas J., 75 (2) 78, 1977

History of the Hot Potassium Carbonate Process

Early co2 removal Plants

Aqueous potassium carbonate solutions have been used for CO2 removal applications for over 120 years in a variety of applications. Potassium carbonate solvents were commonly used in the early 1900s for CO2 capture from combustion sources for food grade CO2 production and dry ice production. There have been over 35 companies to issue patentable improvements to the traditional potassium carbonate CO2 removal system. U.S. research performed in the 1940s/1950s led to performance increases for the base process, resulting in a significant increase in installations for traditional oil, gas, and chemical applications.

1929 US patent for CO2 capture from a cement kiln flue gas using potassium carbonate solutions

Bureau of Mines gas purification pilot plant under construction. (photo credit: National Archives and Records Administration)

Bureau of Mines Research

World War II and the corresponding demand placed on the nation’s petroleum reserves lead to interest in synthetic liquid fuel development and the creation of the Liquid Fuels Act of 1944. The Act authorized the U.S. Bureau of Mines, predecessor of the National Energy Technology Laboratory (NETL), to study the production of liquid fuels from coal and oil shale. Research primarily focused on coal-hydrogenation and the Fischer-Tropsch synthesis process. A subset of this research effort was aimed at removing CO2 from the synthesis gas that would damage Fischer-Tropsch catalysts. As a result, an economical process for CO2 removal was developed which utilized a hot potassium carbonate solution as a chemical absorbent. The CO2 removal process was named after its inventors, Homer Benson and Joseph Field.

Benfield Process Commercialization

Benson and Field left the Bureau in the early 1960s to further commercialize the Benfield Process and license the technology under the Benfield Corporation. Since the process originated from a federal research program, the process could not initially be patented in the United States. The Benfield Corporation was also not able to charge a license fee for installations in the United States. As such, a majority of the early patents and applications for this process occurred in Great Britain and Japan. By 1974, there were over 300 operating HPC systems around the world. Benfield Corporation was sold to Union Carbide in 1980. Shortly thereafter, Union Carbide and Allied Signal formed a joint venture, creating UOP, who is currently licensing the process for new applications.

Today, there are over 700 hot potassium carbonate units operating around the world in ammonia plants, hydrogen plants at refineries, vinyl acetate plants, and ethylene oxide plants. As industries look to reduce their carbon emissions, the HPC process has received further interest for CO2 removal in carbon capture and storage applications (CCUS). There have been many technology licensors, process configurations, corrosion inhibitors, and promoters developed for the HPC process; however, all of the modern HPC applications originated from the work done at the U.S. Bureau of Mines.

1957 Bureau of Mines award recipients. Homer E. Benson can be seen in the first row far left. Joseph H. Field is in the first row far right. (photo credit: National Archives and Records Administration)

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