Solvents For Post-combustion CO2 Capture Research Articles

CO2 solubility and physicochemical properties of 2-amino 2-methyl-1-propanol based solvents for post-combustion CO2 capture. Predictive modeling using machine learning

Global warming, caused by Carbon Dioxide (CO2) emissions from hard-to-abate industries, necessitates CO2 capture as one of the promising alternatives. Post-combustion CO2 capture (PCC) can be retrofitted for industries where chemical absorption using an amine-based solvent is an efficient method for CO2 capture. An essential factor for CO2 absorption in an amine mixture is the study of CO2 solubility and CO2 + Amine physicochemical properties to identify the more efficient solvents. Often experimental data are available in the literature, but precisemodeling is necessary to use such data for further utilization in a process-plant model. In this work, a blended solvent aqueous 2-amine-2methyl-1-Propanal (AMP) with Piperazine (PZ) and 1-(2-aminoethyl) piperazine (AEP) have been identified as a potential candidate for PCC application. The fundamental properties such as the density, viscosity and CO2 solubility in AMP + PZ and AMP + AEP blended solutions have been predicted by employing machine learning to get an exceptionally efficient model. This data-mining technique efficiently calculates solubility, density, and viscosity by using 691, 559, and 559 experimental data sets respectively, from the literature of AMP + PZ. Similarly, 120 data sets of solubility have been collected from the literature for AMP + AEP. Random Forest regression, Artificial Neural Network (ANN), and XGBoost regression algorithms are adapted to predict CO2 solubility in aqueous AMP blended solution. The effectiveness of these models has been confirmed through statistical analysis, leading to the conclusion that XGBoost outperforms the other two approaches. Moreover, an identical approach has been employed to develop a predictive model for the density and viscosity of AMP + PZ solvent. The outcomes imply that the machine learning algorithm performs better with experimental data and may be utilised as an effective simulation tool.

Experimental and theoretical investigation of an ionic liquid-based biphasic solvent for post-combustion CO2 Capture: Breaking through the “Trade-off” effect of viscosity and loading

To break through the “trade-off” effect of biphasic solvents between rich-phase viscosity and CO2 loading, we synthesized a novel ionic liquid (diethylenetriamine-3-hydroxypyridine, [DETA][3HPyr]) with multiple active sites and combined it with the organic solvent diethylene glycol monobutyl ether (DGME) and water to prepare a biphasic solvent for CO2 absorption, the best absorption condition was optimized. The results demonstrated that after absorption, the solvent transformed from homogeneous to biphasic with highly self-concentrated absorption products in the rich phase. The volume of the rich phase accounted for only 36.3 % of the total solvent, while its viscosity decreased significantly to 28.1 mPa·s, with a high blend absorption capacity of 2.67 mol·kg−1. The species of absorption product and absorption mechanism was investigated using 13C NMR. The stable presence of carbamic acid in the system was confirmed, contributing to enhanced absorption capacity. Density functional theory calculations revealed that DGME stabilized carbamic acid through intermolecular hydrogen bonding interactions, and highly polar ions generated during absorption and uneven charge distribution between ions dominated the phase-change process and increased the water content in the rich phase. Thermodynamic energy barrier calculation showed that the solvent effect of DGME reduced the Gibbs free energy barriers of proton transfer and facilitated reaction progress. 3IL4D3H exhibited excellent cyclic performance with low regeneration energy consumption at 1.77 GJ·t−1. This is the first work to revealing the intrinsic dynamics of carbamic acid formation, and provides a new perspective for the phase-change mechanism of ionic liquid-based biphasic solvent.

Nanoporous hypercrosslinked polymers as cost-effective catalysts to significantly promote the CO2 absorption performance of water-lean solvents for post-combustion CO2 capture

In most cases, the slow CO2 sorption kinetics of water-lean solvents—which include low-volatile organic diluents—are the limiting factor when it comes to post-combustion CO2 capture. This study offers a solution by suggesting the use of catalysts based on nanoporous hypercrosslinked polymers (HCPs) to improve the removal efficiency of water-lean solvents based on diethanolamine (DEA) and to increase CO2 mass transfer. Simple and inexpensive monomers were used to prepare three HCPs: HCP-C, HCP-S, and HCP-B, which stand for benzene, polystyrene, and carbazole, respectively. Blends of DEA and Glycerol (Gly) were used to disperse these polymers. Addition of 0.5 wt% HCP to a solvent mixture of 20% DEA and 30% Gly considerably boosts removal efficiency and increases the amount of CO₂ that can be absorbed until the breakthrough point by as much as 3000%, according to the results. Additionally, various gas flow rates, DEA concentrations, and glycerol concentrations were also investigated to determine the HCP-C catalytic effect on the CO2 absorption. A volumetric overall mass transfer coefficient enhancement of up to 319 % is possible in the investigated ranges when using HCP-C, according to the findings. Moreover, a possible catalytic mechanism was suggested.

Enhancing CO2 separation from N2 mixtures using hydrophobic porous supports immobilized with tributyl-tetradecyl-phosphonium chloride [P44414][Cl

The main obstacles in adopting solvent-based CO2 capture technology from power plant flue gases at the industrial scale are the energy requirements for solvent regeneration and their toxicity. These challenges can be overcome using new green and more stable ionic liquids (ILs) as solvents for post-combustion CO2 capture. In the current study, tributyl-tetradecyl-phosphonium chloride [P44414][Cl] as an IL, was immobilized on hydrophobic porous supports of polypropylene (PP), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE) at 298 ± 3 K and pressures up to 2 bar. The surface morphology indicated homogenous immobilization of the IL on the membrane support. Supported ionic liquid membranes (SILMs) were tested for CO2 permeability and CO2/N2 selectivity. None of the SILMs exhibited IL leaching up to 2 bar. The PTFE-based SILM performed better than other supports with minimum loss in water contact angle (WCA) and achieved good antiwetting with a maximum CO2 permeability and selectivity over N2 of 2300 ± 139 Barrer and 31.60 ± 2.4, respectively. This work achieves CO2 permeability about two-fold more than other works having CO2/N2 selectivity range of 25–35 in similar SILMs. The diffusivity of CO2 and N2 in [P44414][Cl] was measured as 3.64 ± 0.18 and 2.01 ± 0.09 [10−8 cm2 s−1] and CO2 and N2 solubility values were 9.79 ± 0.47 and 0.19 ± 0.001 [10−2 cm3(STP) cm−3 cmHg−1], respectively. The high values of Young's modulus and tensile strength of the PTFE support-based SILM (234 ± 12 MPa and 6.07 ± 0.31 MPa, respectively) indicated the long-term application of SILM in flue gas separation. The results indicated phosphonium chloride-based ILs could be better solvent candidates for CO2 removal from large volumes of flue gases than amine-based ILs.

Removal of heat stable salts from degraded amine solvent by “BMED+ED” two-stage electrodialysis unit

Due to the degradation reaction and the presence of acidic impurities (HCl, SO2, etc.) in the flue gas, heat stable salts (HSS) are widely present in the aqueous amine solvent for post-combustion CO2 capture. Accumulation of HSS in the solvent can lead to decreased absorption capacity and even corrosion of equipment in post-combustion capture (PCC) pilot plants. This study presents a novel “BMED+ED” two-stage electrodialysis unit for the efficient removal of HSS, including glycolate, formate, Cl−, and SO42−, from degraded amine solvent with an initial loading of approximately 0.38 mol/mol taken from the solvent loop of a PCC pilot plant after over 1000 hours of operation at real conditions. Experimental results showed that 94.93% of HSSs were removed with an initial HSS concentration of approximately 1920 ppm. The amine loss rate in the two-stage electrodialysis unit was much lower than that in a conventional ED unit, as protonated amine is further converted to organic amines by bipolar membrane electrodialysis. The energy consumption of the two-stage electrodialysis unit was 0.1643 kWh/L amine solvent, and the total cost for amine reclaiming was 1.8145 $/kg amine. This method effectively addressed the challenges of high amine loss rate in the traditional ED treatment process and hazardous wastewater disposal.

Pilot test of water-lean solvent of 2-(ethylamino) ethanol, 1-methyl-2-pyrrolidinone, and water for post-combustion CO2 capture

Water-lean solvent for post-combustion CO2 capture is promising to significantly reduce regeneration energy. It hasn’t been put into commercial application due to unsolved challenges including water balance issue, increased viscosity, and high volatility. Even though numerous water-lean solvents were developed in laboratory, rare of them were tested in pilot scale. Water-lean solvent of 2-(ethylamino) ethanol (EMEA) dissolving into 1-methyl-2-pyrrolidinone (NMP) and water was tested in a CO2 capture pilot with flue gas rate of 280 m3/h. Effects of liquid/gas ratio and reboiler steam rate on CO2 removal efficiency, regeneration energy, lean/rich heat exchanger performance, and volatile amine emission were investigated. Water-lean EMEA achieved CO2 removal efficiency greater than 90 % and the lowest regeneration energy of 2.3 GJ/tCO2, which is 43 % lower than that of 5 M MEA. Water-lean EMEA showed significantly reduced latent heat, greater CO2 capacity, and lower reaction heat (secondary amino) than aqueous MEA and amine blends. It was found that water-lean EMEA regeneration in stripper had extremely lower lean loading (0.1–0.5 mol CO2/L) than aqueous MEA and amine blends, which enables a faster CO2 absorption rate in absorber and a greater CO2 capacity, but also causes larger volatile amine emission. Volatile amine emission after water wash was 110–380 mg/m3. 13C NMR spectrum suggested amine carbamate dominated CO2 absorption products. Due to that increased viscosity decreases heat transfer coefficient (h), lean/rich heat exchanger temperature difference (ΔT) for water-lean EMEA (6.5–7.5℃) was greater than MEA (3.5-5℃) and amine blends (5-6℃), yet smaller than biphasic solvent (8-9℃). A negative power (-0.614) function relation between h and viscosity was fitted and can be further used to normalize sensible heat.

Modeling Differential Enthalpy of Absorption of CO2 with Piperazine as a Function of Temperature.

Temperature-dependent correlations for equilibrium constants (ln K) and heat of absorption (ΔHabs) of different reactions (i.e., deprotonation, double deprotonation, carbamate formation, protonated carbamate formation, dicarbamate formation) involved in the piperazine (PZ)/CO2/H2O system have been calculated using computational chemistry based ln K values input to the Gibbs–Helmholtz equation. This work also presents an extensive study of gaseous phase free energy and enthalpy for different reactions using composite (G3MP2B3, G3MP2, CBS-QB3, and G4MP2) and density functional theory [B3LYP/6-311++G(d,p)] methods. The explicit solvation shell (ESS) model and SM8T solvation free energy coupled with gaseous phase density functional theory calculations give temperature-dependent reaction equilibrium constants for different reactions. Calculated individual and overall reaction equilibrium constants and enthalpies of different reactions involved in CO2 absorption in piperazine solution are compared against experimental data, where available, in the temperature range 273.15–373 K. Postcombustion CO2 capture (PCC) is a temperature swing absorption–desorption process. The enthalpy of the solution directly correlates with the steam requirement of the amine regeneration step. Temperature-dependent correlations for ln K and ΔHabs calculated using computational chemistry tools can help evaluate potential PCC solvents’ thermodynamics and cost-efficiency. These correlations can also be employed in thermodynamic models (e.g., e-UNIQUAC, e-NRTL) to better understand postcombustion CO2 capture solvent chemistry.

Development and testing of a new post-combustion CO2 capture solvent in pilot and demonstration plant

Post-combustion CO2 capture for coal-fired power plants is an important technical option to mitigate greenhouse gas emission from coal-based power generation. CO2 capture based on chemical absorption is currently the dominant technology for large-scale capture unit. The main obstacle of commercial application is the high cost and high energy consumption, innovation of new absorption solvent is one of the keys to the performance improvement. Based on the characteristics of flue gas, combined with the analysis and comparison of potential amine components, the formula of blended amines are developed, which is further screened by true-heat flux calorimetric test combined with vapor-liquid equilibrium test, HNC-5 is selected through the lab test, because of its lower heat of absorption and higher capture performance. The test on a 1000 t/a CO2 capture pilot plant under real flue gas conditions shows that the performance index of HNC-5 is consistent with the lab test results. The CO2 capture ratio is higher than 90%, and the heat consumption during regeneration is reduced by about 20% compared with MEA. The continuous operation results of the 120,000-ton/year CO2 capture unit show that heat duty of regeneration of HNC-5 is about 2.8GJ/tCO2, the cost per ton of CO2 captured is reduced by about 63 CNY, the usage life is longer, helping improve the system availability and reliability.

Performance and sensitivity analysis of packed-column absorption process using multi-amine solvents for post-combustion CO2 capture

Although many novel multi-amine solvents with high CO2 solubility were reported, they have not yet been evaluated for the performance in CO2 absorption processes. In this study, the efficiency and inside profile of the packed-column CO2 absorption process using a novel multi-amine solvent were investigated by using a mathematical model. Three amine solvents were selected: 2-(2-aminoethylamino)-ethanol (AEEA) and diethylenetriamine (DETA) as multi-amine solvents and monoethanolamine (MEA) as a reference. Based on the 90% capture efficiency of flue gas CO2 from a coal-fired power plant, a sensitivity analysis using the liquid-to-gas ratio was conducted to minimize the energy consumption for CO2 capture. Based on the utilization of the same 30 wt% solvent, the lowest reboiler thermal energy achieved by the process using DETA was 3.242 GJ/tonCO2, which was 8.57% lower than that obtained when MEA was employed. In contrast, the process using AEEA required 7.04% higher energy than MEA. In the process modified by rich-solvent split for further enhancing efficiency, the DETA process could achieve 2.962 GJ/tonCO2, which reduced the reboiler duty by 16.8% from the MEA process. The CO2 capture performance of absorbents could not be evaluated not only by the absorption rate and capacity. Owing to the larger molecular weight of multi-amines than MEA, the performance of capture and regeneration was discussed with respect to the amine properties and number of amine moles in the solution. A well-selected multi-amine for a modified process configuration has considerable potential for improving the energy efficiency of CO2 capture.

Development of a rate-based model for CO2 capture using a non-aqueous hydrophobic solvent

Advanced water-lean solvents offer several advantages over aqueous amine solvents for post-combustion CO2 capture, particularly lower parasitic energy penalty, lower regeneration temperatures and lower corrosion. RTI and SINTEF have been working on developing eCO2SolTM, a non-aqueous solvent (NAS) that has shown stripper heat duties of 2.1 - 2.3 GJ/t-CO2 at pilot-scale testing at a 6-kW large bench-scale gas absorption system (BsGAS, RTI International, USA) and at the 60-kW Pilot testing unit (Tiller Plant, SINTEF, Norway). This technology will be demonstrated at the 12 MW scale at Technology Centre, Mongstad (TCM), Norway in early 2022. Development of novel solvent formulations is a time-consuming step, and models are needed to evaluate different process configurations and process conditions, to explore different energy integration strategies to minimize net heat duty and to size equipment and evaluate overall process economics. The non-aqueous solvent formulation includes novel components that have not been studied extensively in the literature. Most of the properties that are needed to develop rate-based kinetic-, thermodynamic and hydrodynamic models are not available and must be generated. This work provides details on the development of a detailed process model including the experimental measurement of properties required for the simulation model. The process model has been tested against quality pilot data gathered from the two pilot plants mentioned. The average absolute deviation (AAD%) between model prediction and experimental data for the specific reboiler duty (SRD) is within 6% for the 6-kW BsGAS at RTI and 7% for 60-kW pilot testing at SINTEF. Mass and heat transfer of the developed rate-based model was validated by predicting the temperature, CO2 loadings (molCO2/molamine) and CO2 concentration profiles of the testing units and compared favourably to experimental data.

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.

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.

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.

Kinetic study of CO2 absorption in aqueous solutions of 2‐((2‐aminoethyl)amino)‐ethanol using a stirred cell reaction calorimeter

AbstractIn the literature, aqueous 2‐((2‐aminoethyl)amino) ethanol (AEEA) is identified as a promising solvent for postcombustion CO2 capture. In this work, the kinetics of CO2 absorption in the aqueous AEEA, containing a primary and a secondary amino group, is studied over a wide temperature range of 303.15‐343.15 K and the amine concentration in the range of 0.47‐2.89 M using the fall‐in‐pressure technique in a stirred cell reaction calorimeter setup with a horizontal gas‐liquid interface. The overall rate constants for (AEEA + H2O + CO2) reaction system are estimated in the pseudo–first‐order reaction regime. The kinetic models based on zwitterion and the termolecular reaction mechanisms are used to predict kinetic rate constants. The experimental kinetic data are better correlated using the zwitterion mechanism (AAD 9.18%) than that of the termolecular mechanism (AAD 10.4%). The density, viscosity, and physical solubility of pure components and aqueous binary mixtures of AEEA are also measured at the similar temperature and concentration ranges of rate kinetics. Empirical models are proposed to predict pure component density and viscosity data with AAD of 0.02% and 7.17%, respectively. The Redlich‐Kister model, the Grunberg‐Nissan model, and the O'Connell's model are used to correlate experimental density, viscosity, and physical solubility data of the binary mixtures with AAD of 0.034%, 4.92%, and 6.5%, respectively. The reaction activation energy (Ea ∼ 32 kJ/mol) of the (AEEA + H2O + CO2) system is calculated from the Arrhenius power‐law model using the zwitterion mechanism, which indicates lower energy barrier than that of the reported value for monoethanolamine (∼44 kJ/mol) in the literature.

Understanding Carbamate Formation Reaction Thermochemistry of Amino Acids as Solvents for Postcombustion CO2 Capture.

The carbamate stability constant for a data set of 10 amino acids, having potential for being postcombustion CO2 capture (PCC) solvents, has been calculated using various implicit and explicit solvation shell models. This work also includes an extensive study of gas-phase free energy and enthalpy for the amino acid carbamate formation reaction with the Hartree Fock method, density functional methods [B3LYP/6-311++G(d,p)], and composite methods (G3MP2B3, G3MP2, CBS-QB3, and G4MP2). Ideal PCC solvent properties require finding a profitable tradeoff between various thermodynamic and system optimization parameters. Benchmark gaseous-phase and solution-phase thermodynamic properties given in this work can help in making informed decisions when choosing promising PCC solvents. The temperature dependency of the carbamate stability constant of amino acids is predicted using PCM and SM8T implicit solvation models. PCC is a temperature swing absorption-desorption process, and the high-temperature sensitivity of the ln KcAmCOO- value is of vital importance in attaining cost-efficient processes.

Evaluating the Performance of Micro-Encapsulated CO2 Sorbents during CO2 Absorption and Regeneration Cycling.

We encapsulated six solvents with novel physical and chemical properties for CO2 sorption within gas-permeable polymer shells, creating Micro-Encapsulated CO2 Sorbents (MECS), to improve the CO2 absorption kinetics and handling of the solvents for postcombustion CO2 capture from flue gas. The solvents were sodium carbonate (Na2CO3) solution, uncatalyzed and with two different promoters, two ionic liquid (IL) solvents, and one CO2-binding organic liquid (CO2BOL). We subjected each of the six MECS to multiple CO2 absorption and regeneration cycles and measured the working CO2 absorption capacity as a function of time to identify promising candidate MECS for large-scale carbon capture. We discovered that the uncatalyzed Na2CO3 and Na2CO3-sarcosine MECS had lower CO2 absorption rates relative to Na2CO3-cyclen MECS over 30 min of absorption, while the CO2BOL Koechanol appeared to permeate through the capsule shell and is thus unsuitable. We rigorously tested the most promising three MECS (Na2CO3-cyclen, IL NDIL0309, and IL NDIL0230) by subjecting each of them to a series of 10 absorption/stripping cycles. The CO2 absorption curves were highly reproducible for these three MECS across 10 cycles, demonstrating successful absorption/regeneration without degradation. As the CO2 absorption rate is dynamic in time and the CO2 loading per mass varies among the three most promising MECS, the process design parameters will ultimately dictate the selection of MECS solvent.

Comprehensive solubility of N2O and mass transfer studies on an effective reactive N,N-dimethylethanolamine (DMEA) solvent for post-combustion CO2 capture

The physical solubility and mass transfer performance of CO2 absorption into aqueous N,N-dimethylethanolamine (DMEA) solution were comprehensively studied in the present work. The physical solubility (Henry’s law constant (He)) was measured using a stirred cell reactor for pure DMEA and aqueous DMEA at temperatures from 293.98 to 333.03 K. Then, the physical solubility of CO2 was obtained based on the “N2O analogy” method. Furthermore, He was correlated successfully by using an empirical polynomial model with an average absolute relative deviation (AARD) of 4.23%. The mass transfer performance was studied in a laboratory scale structured packing absorption column at DMEA concentration of 1–4 kmol/m3, liquid feed temperature of 293.15–333.15 K, liquid flow rate of 3.90–11.7 m3/m2-hr, CO2 partial pressure of 6–20 kPa, lean CO2 loading of 0.050–0.300 mol/mol, over the inert gas flow rate range of 26.11–43.52 kmol/m3-hr. Based on the experimental values of He for CO2 into aqueous DMEA solution, the effect of the gas concentration of A in gas-liquid equilibrium (yA∗) on gas phase volumetric overall mass transfer coefficient (KGav) was considered in this study, but found to be an insignificant influence that can be ignored. Analogously, an attempt to correlate the mass transfer coefficient KGav and outlet concentration of CO2 (yout) using semiempirical models as function of the mentioned operational parameters was conducted. The predictive correlations for KGav and yout of the DMEA-CO2-H2O system in structured packing column are KGav=L0.230.6144aeq-aC/pCO2+0.02327 and lnyout=lnyin-0.77PVGΩL0.230.6144aeq-aC/pCO2+0.02327, with acceptable AARDs of 4.59% and 1.48%, respectively.

Characterization and kinetics of CO2 absorption in potassium carbonate solution promoted by 2-methylpiperazine

In this work, the absorption performance of potassium carbonate (K2CO3) promoted by 2-methylpiperazine (2-MPZ) was examined for CO2 capture by using a stirred cell reactor at temperatures between 303 and 323 K. The density, viscosity and loading capacity of K2CO3 + 2MPZ solutions were experimentally measured. Moreover, Hatta number, enhancement factor, Henry’s constant and diffusivity of CO2 and N2O in K2CO3 + 2MPZ solution was also determined to check the fast pseudo-first-order reaction regime. Furthermore, kinetics of the reaction of CO2 with K2CO3 + 2MPZ solution was studied and described in detail using zwitterion mechanism. The CO2 absorption rate along with the reaction rate constant (k2) for the CO2 + K2CO3 + 2MPZ system were obtained and the results were compared with other systems. It was concluded that the CO2 absorption rate is first order with respect to both 2MPZ and CO2, the activation energy and k2 were found to be 62.45 kJ/mol and8.92×1014exp(−7511.8T), respectively. The results revealed that the K2CO3 + 2MPZ solution could be an attractive solvent for post-combustion CO2 capture due to its excellent characteristics such as the higher CO2 absorption rate and capacity compared to MEA.

Solvent effects on design with operability considerations in post-combustion CO2 capture plants

This work presents an evaluation of the operation of alternative solvents for post-combustion CO2 capture under varying process conditions. The proposed methodology is part of a generalized design and operability framework and relies on the determination of the processes’ steady-state controllability as an inherent system feature, independent of the control structure selection. Assessment of such characteristics can shed light on the effect different capture solvents, process configurations and control structures have on the achieved dynamic responses and further associate them to the physico-chemical properties of the solvents. Three amine based solvents, namely monoethanolamine (MEA), diethanolamine (DEA) and 3-amino-1-propanol (MPA) are introduced to an advanced CO2 capture flowsheet previously designed for optimal steady-state operation. The dynamic characteristics of the investigated solvent-process systems under the influence of disturbances emulating real-time industrial conditions are then assessed and rank-ordered using an index that measures the departure of the process variables from the region of desired operation. As such, for a given disturbance, the drift from the acceptable steady-state operating point and the response of dynamic closed-loop simulations determine the suitability and applicability of each solvent in the employed CO2 capture process configuration. Results reveal the superiority of the dynamic behavior of MPA in terms of economic performance as well as expected process operability for variations in the inlet flue gas flowrate, greatly surpassing the performance of the benchmark solvent, MEA.

Influence of formaldehyde on N-nitrosopiperazine formation from nitrite and piperazine in CO2 capture

Piperazine (PZ) based amine blends are promising solvents for post-combustion CO2 capture, but PZ can form potential carcinogenic nitrosamines from nitrite. In this work, the kinetics of the reaction between nitrite and PZ to form N-nitrosopiperazine (MNPZ) was determined in 0.1–0.5 mol L−1 PZ in the presence of 17–170 mmol L−1 formaldehyde at 60–135 °C. The nitrosation of PZ can be catalyzed by formaldehyde, a primary degradation product of PZ. And the reaction involving nitrite and PZ is first order in nitrite, formaldehyde, and hydronium ion. A kinetic model was established, and the activation energy is 33.6 ± 1.8 kJ mol−1 with a rate constant of 2.3 × 103 ± 0.4 × 103 L2 mol−2 s−1 at 100 °C. The kinetics will be helpful to develop strategies to reduce nitrosamine formation and accumulation in PZ based CO2 capture systems.