Water-lean Solvent Research Articles

Aerosol Emissions from Water-lean Solvents for Post-combustion CO2 Capture

Advanced water-lean solvents (WLS) for post-combustion CO2 capture have been gaining interest due to their ability to reduce the parasitic penalty from energy needed for solvent regeneration. Commercial implementation of these novel CO2 capture technologies hinges on successful control of amine emissions. RTI conducted a parametric study of fundamental and operational variables influence on overall amine aerosol and vapor emissions from our water-lean solvent eCO2Sol™ using our 6-kW equivalent bench-scale gas absorption system. The parametric testing used a simulated flue gas with 15 % CO2, 2.3–4.2 % H2O, and 0–6 ppm sulfite (SO3) to examine the impact of the presence of aerosols to the capture performance and amine emissions from the system. The SO3 reacts with water in the flue gas to create H2SO4, which forms liquid aerosol droplets and provide nucleation sites for growth of aerosols. Scanning Mobility Particle Sizer and Aerodynamic Particle Sizer instruments monitored the aerosol particle size distribution. Parametric testing results suggested that the presence of the aerosols in the flue gas could increase the overall amine emissions by 10X compared to the baseline emissions from WLS’s vapor pressure. Principal component analysis (PCA) and projection to latent squares (PLS) developed models to predict the aerosol-based amine emissions from process data. The predictive PLS model had a correlation coefficient (Q2) of 0.92 and could predict the aerosol-based emissions from the NAS process with ±15 % accuracy (average absolute deviation, AAD). The PLS regression model also identified key variables affecting aerosol-based emissions from WLS.

Modeling, Design, and Testing of a Novel Biphasic Solvent-Enabled Absorption System for Post-Combustion Carbon Capture

A novel absorption process enabled by a new class of biphasic solvents for post-combustion carbon capture (BiCAP) is presently being developed at the University of Illinois at Urbana-Champaign. The new biphasic solvents are water-lean solvent blends that develop dual liquid phases with the absorbed CO2 highly enriched in one of the phases. These solvents are superior in CO2 capacity and have high thermal and oxidative stability compared to MEA. The BiCAP technology features a unique process configuration of multi-stage CO2 absorption and liquid–liquid phase separation during CO2 absorption, allowing continual separation and removal of the CO2-enriched liquid phase for maintaining low solvent viscosity and a fast absorption rate. A 40 kWe-scale skid has recently been constructed at the Abbott Power Plant in Champaign, IL, to test the BiCAP solvent in continuous operation with a slipstream of real post-combustion flue gas. Along with improvements in the stripping configuration including cold feed bypass, the process is expected to achieve a minimum reboiler duty of 2,210 kJ/kg CO2 captured at a stripper operating pressure of 6 bar.

Non-Aqueous Solvents Unlikely to Present Any Cost Benefit for Post-Combustion CO2 Capture

A water-lean solvent for post-combustion CO2 capture was techno-economically assessed and compared with its parent water-based solvent. At a first glance, the use of nonaqueous solvents, appears to offer lower energy requirements and reduced solvent degradation. However, it is necessary to look at the full picture to avoid rushing to develop a concept that may not deliver all that it promises. A nonaqueous solvent technology requires less regeneration energy compared with using standard aqueous amine solvents, especially if combined with a flash vessel instead of a regeneration column, and has a lower amine degradation rate compared with the aqueous solvent. However, using a nonaqueous formulation offers important technical challenges in terms of operability (increased solvent viscosity) and health, safety and environmental risks (higher amine emissions). The economic evaluation shows a hypothetical reduction of 15% in unit total cost based on savings in energy consumption and solvent refill. However, implementing measures to mitigate viscosity changes and amine emissions nullifies these gains. Therefore, no economic improvement over the benchmark is likely from using the nonaqueous version of a solvent. It is expected that the same risks apply for other nonaqueous technologies, so it is very unlikely that a magic bullet will be found to make nonaqueous solvents an attractive option for CO2 capture.

Thermodynamic modeling and energy analysis of a polyamine-based water-lean solvent for CO2 capture

Amine-based water-lean solvents are considered to be promising energy-saving alternatives to existing aqueous amine solvents for CO2 capture. Herein, we propose water-lean solvents comprising 3,3′-iminobis(N,N-dimethylpropylamine) (60 wt%), water (10–18 wt%), and a non-amine chemical (balance), probe the solubility of CO2 therein, and perform the energy demand analysis using an Aspen Plus®-based simulator. For CO2 solubility modeling, the e-NRTL model and Redlich-Kwong state equation were incorporated with the equilibrium reaction model proposed by previous researchers, and the Henry’s law constant of CO2 was estimated using molecular dynamics simulation as it is difficult to measure experimentally. Experimental data acquired using an equilibrium cell and 13C/1H NMR were used to regress the equilibrium reaction model. The minimum thermal energy demand under desorber pressures of 303 and 505 kPa was calculated to be as low as 1.99 and 1.95 GJ t-CO2−1, respectively, which supports the suitability of the developed WLSs for post-combustion CO2 capture.

Investigating opportunities for water-lean solvents in CO2 capture: VLE and heat of absorption in water-lean solvents containing MEA

A thorough investigation on a set of organic diluents for water-lean solvent formulation has been carried in this work. The performance of aqueous monoethanolamine (MEA) 30 %w/w has been compared to that of mixtures between MEA 30 %w/w and methanol, acetone, monoethylene glycol (MEG), N-methyl-2-pyrrolidone (NMP), tetrahydrofurfuryl alcohol (THFA), sulfolane, cyclopentanone (CC5), furfuryl alcohol and γ-butyrolactone. A small calorimeter was employed to obtain both vapor–liquid equilibrium (VLE) and heat of absorption data. Shifting from an aqueous solvent to a non-aqueous one was observed to have significant implications on the VLE, as all water-lean solvents assessed have lower CO2 solubility than aqueous MEA. However, the heat of absorption is not much affected. The results of these experiments show the limitations of using most ethers, esters and ketones for water-lean solvent formulation, all stemming from the very low dielectric permittivity of these diluents and the difficulty of the stabilization of the intermediary reaction species between MEA and CO2 within these systems. Nevertheless, the low volatility of solvents containing MEG, NMP or THFA could offer opportunities for processes with overall less reboiler heat duties than that of ordinary aqueous MEA.

An experimental based optimization of a novel water lean amine solvent for post combustion CO2 capture process

The development of new amine solvents without the major drawbacks of conventional amines is crucial to industrial applications of CO2 capture. This paper presents a water-lean CO2 capture solvent having a low regeneration energy and low degradation. The water-lean solvent, K2Sol, is a sterically hindered diamine; because of the hindered amine site, K2Sol easily forms bicarbonate, resulting in a high absorption capacity. The minimum solvent regeneration energy is obtained using Gaussian process Bayesian optimization (GPBO) and bench-scale pilot plant experiments. GPBO finds the optimal solution using the input and output relationship of experiments; thus, expensive first-principle model construction can be avoided. According to the pilot plant experiment, the optimal regeneration energies of monoethanolamine (MEA) and K2Sol are 4.3 and 2.8 GJ/t CO2, respectively, indicating that K2Sol requires only 65% of the regeneration energy of MEA. Fewer than 30 experiments are required to find the optimal pilot plant operation for both the MEA and K2Sol experiments. We also describe the superior properties of K2Sol in terms of the CO2 loading, cyclic capacity, regeneration temperature, and degradation.

CO2 solubility and mass transfer in water-lean solvents

Shifting from aqueous to water lean solvents has been evaluated as a mean for increasing the mass transfer rates in regular chemical solvents with amines. An array of amines (monoethanolamine, 2-methylpiperazine and N-methyl diethanolamine) and diluents (sulfolane, ethylene glycol, 1-methylimidazole, dimethyl sulfoxide, N-methyl-2-pyrrolidinone) has been analyzed. Addition of organic diluents seem to generally induce both a shift in chemical equilibrium and an increase in mass transfer rates for a fixed CO2 partial pressure, though not necessarily for a fixed CO2 loading. However, these relative advantages in terms of mass transfer rates decrease the more loaded the water lean solvent is. The equilibrium shift caused by organic diluents has been evaluated in terms of de-stabilization of the species in the solvent, which for monoethanolamine-based solvents can be easily related to a decrease in dielectric permittivity. However, this analysis indicates that such treatment is insufficient for other types of water-lean solvents, suggesting that different kinds of intermolecular interactions should also be considered in future studies.

Lost work: A comparison of water-lean solvent to a second generation aqueous amine process for CO2 capture

Energy use is a critical parameter for CO2 capture from flue gas. Proponents of water-lean solvents claim that they will provide better energy performance than aqueous amine solvents, but papers on water-lean solvents frequently fail to compare their energy performance to second-generation (2 G) aqueous amine systems. These papers claim that the weakest point of aqueous amine systems is the energy loss associated with boiling and condensing water. However, unlike first generation processes, 2 G scrubbing processes use innovative stripper configurations to minimize the loss of heat as excess water vapor from the top of the stripper. Proponents of water-lean solvents also claim that these solvents reduce sensible heat losses because of the lower specific heat of the organics that replace the water. In 2 G aqueous systems, the sensible energy of heating the solvent is minimized by high efficiency plate-and-frame exchangers where the optimized energy and capital cost is proportional not only to the specific heat of the solvent, but also to the viscosity and the thermal conductivity:CAPEX, OPEX,TC∝m ˙kn-12Cp1-n2μm-n2ΔTcrx0.5∝m˙*k-0.325Cp0.825μ0.175*ΔTcrx0.5Because water-lean solvents typically have higher viscosity and lower thermal conductivity, they do not provide significantly better exchanger performance than 2 G aqueous solvents.This paper develops a method to estimate the energy use of 2 G aqueous solvents by summing the minimum work of separation and lost work (irreversibility) of each process unit. This method allows the comparison of energy use for different solvents (aqueous or water-lean) on the same basis without rigorous process simulation. The results demonstrate that a representative water-lean solvent does not provide significantly better energy performance than a representative 2 G aqueous solvent with an advanced stripper configuration.

CO2 Solubility and Mass Transfer in Water Lean Solvents

Abstract Shifting from aqueous to water lean solvents has been evaluated as a mean for increasing the mass transfer rates in regular chemical solvents with amines. An array of amines (monoethanolamine, 2-methylpiperazine and N-methyl diethanolamine) and diluents (sulfolane, ethylene glycol, 1-methylimidazole, dimethyl sulfoxide, N-methyl-2-pyrrolidinone) has been analyzed. Addition of organic diluents seem to generally induce both a shift in chemical equilibrium and an increase in mass transfer rates for a fixed CO2 partial pressure, though not necessarily for a fixed CO2 loading. However, these relative advantages in terms of mass transfer rates decrease the more loaded the water lean solvent is. The equilibrium shift caused by organic diluents has been evaluated in terms of de-stabilization of the species in the solvent, which for monoethanolamine-based solvents can be easily related to a decrease in dielectric permittivity. However, this analysis indicates that such treatment is insufficient for other types of water-lean solvents, suggesting that different kinds of intermolecular interactions should also be considered in future studies.

Attempting to Break the 2 GJ/tonne CO2 Barrier; Development of an Advanced Water-Lean Capture Solvent From Molecules to Detailed Process Design

Solvent-based post-combustion CO2 capture is an energy-intensive process primarily driven by the energy required to regenerate the CO2 capture solvent. Researchers are currently focused on developing drop-in solvent replacements for commercial amine solvents with lower regeneration energies. One approach to reducing the regeneration energy of a solvent is to reduce its water content, thereby reducing unnecessary condensing and consequent boiling in the process. There are a number of water-lean solvent formulations currently under development that allow for water contents below 10% by weight, versus more than 60% for commercial aqueous amines. One solvent class, CO2-Binding Organic Liquids (CO2BOLs), shows promise to reduce the parasitic load to a coal-fired power plant but has been impeded by high viscosities at high CO2 loadings. In this paper, we perform a preliminary modeling study of a new low-viscosity CO2BOL solvent and assess the energetics of different process stripper configurations. By tailoring the process configuration with the unique aspects of the solvent reboiler duties below 2 GJ/tonne CO2 could be achievable. Further, this study suggests that there is no one-size-fits-all process optimum configuration for solvents, and therefore optimal configurations will be solvent specific.

Mesoscopic Structure Facilitates Rapid CO2 Transport and Reactivity in CO2 Capture Solvents.

Mass transfer coefficients of CO2 are anomalously high in water-lean solvents as compared to aqueous amines. Such phenomena are intrinsic to the molecular and nanoscale structure of concentrated organic CO2 capture solvents. To decipher the connections, we performed in situ liquid time-of-flight secondary ionization mass spectroscopy on a representative water-lean solvent, 1-((1,3-Dimethylimidazolidin-2-ylidene)amino)propan-2-ol (IPADM-2-BOL). Two-dimensional (2D) and three-dimensional (3D) chemical mapping of the solvent revealed that IPADM-2-BOL exhibited a heterogeneous molecular structure with regions of CO2-free solvent coexisting with clusters of zwitterionic carbonate ions. Chemical mapping were consistent with molecular dynamic simulation results, indicating CO2 diffusing through pockets and channels of unreacted solvent. The observed mesoscopic structure promotes and enhances the diffusion and reactivity of CO2, likely prevalent in other water-lean solvents. This finding suggests that if the size, shape and orientation of the domains can be controlled, more efficient CO2 capture solvents could be developed to enhance mass-transfer and uptake kinetics.

Are Water-lean Solvent Systems Viable for Post-Combustion CO2 Capture?

We present here an overview of water-lean solvents that compares their projected costs and performance to aqueous amine systems, emphasizing critical areas of study needed to evaluate their performance against their water-based brethren. The work presented here focuses on bridging these knowledge gaps. Because the majority of water-lean solvents are still at the lab scale, substantial studies are still needed to model and assess their performance at full scales. This presents a significant challenge as each formulation has different physical and thermodynamic properties and behavior, and quantifying how these different properties manifest themselves in conventional absorber-stripper configurations, or identifying new configurations that are specific for a solvent's signature behavior. We identify critical areas of study that are needed, and our efforts (e.g. custom infrastructure, molecular models) to predict, measure, and model these behaviors. Such findings are critical for determining the rheology required for heat exchanger design; absorber designs and packing to accommodate solvents with gradient changes (e.g. viscosity, contact angle, surface tension), and stripper configurations without direct steam utilization or water reflux. Another critical area of research need is to understand the molecular structure of the liquid interface and bulk as a function of CO2 loading, and to assess whether conventional film theories accurately quantify solvent behavior, or if thermodynamic models adequately quantify activity coefficients of ions in solution. We conclude with an assessment of our efforts to aid in bridging the knowledge gaps in understanding water-lean solvents, and suggestions of what is needed to enable large-scale demonstrations to meet the United States Department of Energy's goals.

Reinventing Design Principles for Developing Low-Viscosity Carbon Dioxide-Binding Organic Liquids for Flue Gas Clean Up.

Anthropogenic CO2 emissions from point sources (e.g., coal fired-power plants) account for the majority of the greenhouse gases in the atmosphere. Water-lean solvent systems such as CO2 -binding organic liquids (CO2 BOLs) are being developed to reduce the energy requirement for CO2 capture. Many water-lean solvents such as CO2 BOLs are currently limited by the high viscosities of concentrated electrolyte solvents, thus many of these solvents have yet to move toward commercialization. Conventional standard trial-and-error approaches for viscosity reduction, while effective, are time consuming and economically expensive. We rethink the metrics and design principles of low-viscosity CO2 -capture solvents using a combined synthesis and computational modeling approach. We critically study the effects of viscosity reducing factors such as orientation of hydrogen bonding, introduction of higher degrees of freedom, and cation or anion charge solvation, and assess whether or how each factor affects viscosity of CO2 BOL CO2 capture solvents. Ultimately, we found that hydrogen bond orientation and strength is the predominant factor influencing the viscosity in CO2 BOL solvents. With this knowledge, a new CO2 BOL variant, 1-MEIPADM-2-BOL, was synthesized and tested, resulting in a solvent that is approximately 60 % less viscous at 25 mol % CO2 loading than our base compound 1-IPADM-2-BOL. The insights gained from the current study redefine the fundamental concepts and understanding of what influences viscosity in concentrated organic CO2 -capture solvents.

Evaluating Transformational Solvent Systems for Post-combustion CO2 Separations

Abstract Broad research is underway on developing transformational solvents that can capture of CO 2 from flue gas with lower energy compared to aqueous amines. Water-lean, or non-aqueous, solvents are being considered as a class of transformational solvents due to the prospect of lower energy duties by not having to heat and condense water. To date, little is known about the real world performance of water-lean solvent systems compared to commercial aqueous amine technologies, and whether or not they can utilize existing or at least similar processing infrastructure. This paper provides the key results from a comprehensive three-year study of the water-lean CO 2 -Binding Organic Liquids (CO 2 BOL) solvent platform coupled with Polarity-Swing Assisted Regeneration (PSAR). We present here thermodynamic, kinetic, and bench-scale data, followed by Aspen Plus projections of full-scale process performance for three CO 2 BOL/PSAR cases. This paper also provides discussions on materials performance and identifies viscosity as a critical property that most greatly limits the viability of water-lean solvent platforms. We provide results from a new effort spanning molecular modeling and synthesis and experimental testing to decipher the critical material properties needed to address this challenge. We conclude with implications for development of other water-lean solvent systems.