Chapter 1

Phase-Change Solvents for CO2 Capture

Xianfeng Wang,  and Bingyun Li     Department of Orthopaedics, West Virginia University, Morgantown, WV, USA

Abstract

Carbon dioxide (CO2) is a major greenhouse gas responsible for global warming; hence, much effort has been attracted to developing new materials and technologies for CO2 capture from process gas streams. Recently, phase-change solvents have emerged as one of the most promising approaches for CO2 capture. During the phase-change process, an absorbent will typically form two phases (e.g., two liquid phases or liquid-solid phases) after CO2 absorption. These two phases may be defined as CO2-rich and CO2-lean and can easily be separated from each other. The CO2-lean phase can be recycled back to the absorber and the CO2-rich phase can be sent to a stripper for regeneration. By regenerating only the CO2-rich phase, significant energy can be saved. In this chapter, we provide a comprehensive review of the state-of-the-art research activities related to phase-change solvents, including solvent design and characterization, and CO2 capture performance and process design. After a brief introduction of conventional chemical absorption, several new solvents are discussed. Next, the state-of-the-art research activities related to phase-change solvents for CO2 capture are highlighted. Our discussion concludes with some personal perspectives on future development in which phase-change solvents could be further studied.

Keywords

CO2 capture; Energy consumption; Phase-change solvent; Precipitation; Process design

1. Introduction

With the rapid increase of the global population and the industrialization of more and more countries, the consumption of energy is growing explosively. Currently, over 85% of the global energy demand is being supported by the burning of fossil fuels, which releases large amounts of carbon dioxide (CO2) into the atmosphere.1,2 As a result, CO2 concentration in the atmosphere has increased since the beginning of industrialization at an accelerating rate to ~390 ppm in 2010.3,4 The increase of CO2 concentration in the atmosphere and fears of resulting catastrophic global climate change have led to increased demand for CO2 capture and storage (CCS) technologies.5-8 CO2 absorption using chemical reaction is a common process in the chemical industry and, along with other processes, has been applied in the treatment of industrial gas streams containing acid gases like H2S, NOx, and CO2. In these gas-treating processes, aqueous amine solutions are most commonly used, especially monoethnolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), and their mixtures. In principle, these systems could be used for the removal of CO2 from flue gases, such as from power plants in a regenerative absorption-desorption process.9 However, these solvents have a limited cyclic CO2-loading capacity, may lead to high equipment corrosion, require high-energy consumption for regeneration, and suffer from solvent losses by evaporation and/or degradation in an oxygen-rich atmosphere.10,12 To identify cost-effective approaches for CO2 capture, many new materials and techniques have been developed and evaluated.1,9,13-15 A new class of solvents, phase-change solvents, has emerged and been developed into one of the most promising technologies for CO2 capture. Several experimental studies with absorbents that exhibit phase-change features during the absorption or desorption of CO2 have shown promise.16-18 Such phase-change technology removes CO2 from power-plant flue gases using a solvent that, when it reacts with CO2, rapidly forms two distinct phases: a CO2-rich phase and a CO2-lean phase. Only the CO2-rich phase will then undergo regeneration to remove the CO2 and recycle the solvent. By regenerating only the CO2-rich phase, significantly less energy may be needed for regeneration.19,20 In this chapter, we will provide a comprehensive review of the state-of-the-art research activities related to phase-change solvents, including solvent design and characterization, and CO2 capture performance and process design.

2. Conventional Chemical Absorption

Amine scrubbing has been applied to separate CO2 from natural gas and hydrogen since 1930.21 A typical amine developed for this purpose is MEA. The process involves the passage of an aqueous amine solution (typically 30 wt.%) down the top of an absorption tower, while a gaseous stream of flue gas containing CO2 is introduced at the bottom. A blower is required to pump the gas through the absorber. At a temperature of approximately 40 °C, the reaction of CO2 with the amine occurs through a zwitterion mechanism to form carbamates.1 The amine is regenerated by stripping with water vapor at 100-120 °C, and the water is condensed from the stripper vapor, leaving concentrated CO2 that can be compressed to 100-150 bars for geologic sequestration. Although aqueous amine solutions are effective in removing CO2 from natural gas under a variety of conditions, aqueous amine processes often suffer from issues with corrosion, amine oxidative degradation, and solvent losses. Additionally, the use of aqueous amine processes is highly energy intensive, largely because of the thermodynamic properties of water.22 In fact, it is estimated that almost 30% of the energy of the power plant would have to be diverted to run the CO2 capture process, which could result in a doubling of the cost of electricity. Improved strategies for CO2 capture include the use of liquids with lower heat of absorption and increasing the concentration of the absorbent molecules. Other compounds that are often considered are sterically hindered compounds such as 2-amino-2-methyl-1-propanol (AMP), secondary amines such as DEA, and tertiary amines such as MDEA.23 Inorganic solvents such as aqueous potassium and sodium carbonate as well as aqueous ammonia solutions have also been considered for chemical absorption. Note that CO2 capture from ambient air using chemical absorption in aqueous alkali hydroxide solutions has also been proposed.24 Despite the strongly absorbing nature of the solutions, the large energy demands of the regeneration step present a significant challenge in postcombustion CO2 capture from power plants.1

3. New Solvents for CO2 Capture

Numerous technology options exist for postcombustion CO2 capture that are generally compatible with CCS activity. These include solid sorbents, membranes, and new liquid absorbents, as well as processes to directly convert CO2 into carbonates or other manageable species. Solid sorbents like metal organic frameworks represent one type of new materials. However, any industrial process using solid sorbents would likely involve temperature swing. Unfortunately, heat transfer to and from a solid support is challenging. Membranes are attractive, but scaling up to power-plant sizes would be challenging.25,26 Given these considerations, new liquid absorbents, representing the best alternative to aqueous amine technology, are increasingly attracting remarkable interests. Among the emerging solvent technologies for CO2 capture, ionic liquids (ILs) have garnered much attention and are regarded as potential candidates. ILs are commonly defined as liquids, which are composed entirely of ions with a melting point of less than 100 °C. Much of this interest is centered on the possible use of ILs as "green" alternatives to volatile organic solvents. One of the promising advantages of ILs for CO2 capture is their negligible vapor pressure and negligible losses. The lower vapor pressure of ILs may lead to lower energy consumption during CO2 stripping and solvent regeneration.1 In addition to their extremely low vapor pressures, they are nonflammable, environmentally benign, and exhibit exceptional thermal stability. Moreover, numerous combinations of cations and anions can be used to produce new ILs, and this flexibility can be used to tune their chemical and physical properties for CO2 capture.23 Perry and coworkers have employed the concept of combining both physisorbing and chemisorbing components in one molecule to produce an aminosilicone solvent mix for CO2 capture.27 The physical-absorbing portion of the molecule would reside in the backbone and covalent CO2 capture would sit on the termini of tethering groups. They use siloxanes, ethers, perfluoroethers, and amides as physical-absorbing species. Styrene and alkyl derivatives are also considered, based mainly on their cost and availability. Aminoethyl, aminopropyl, aminoethylaminopropyl, and other amine groups are chosen as chemically reactive functional groups. CO2 capacity is related to the number of reactive primary and secondary amines present in the structure. To maintain a liquid state, a hydroxyether cosolvent is employed that allows enhanced physisorption of CO2 in the solvent mixture. Regeneration of the capture solvent system is demonstrated over six cycles and absorption isotherms indicate a 25-50% increase in CO2 capacity over 30 wt.% MEA. In addition, proof of concept for continuous CO2 absorption is verified. Further exploration of this system is in progress with thermal stability and corrosion studies underway as well as further optimization of the amino silicone substrates.

4. Phase Change...