CO2 capture solvent selection by combined absorption-desorption analysis

A method was developed for selection of promising solvents based on CO₂ absorption experiments at 40 °C and 9.5 kPa CO₂ partial pressure followed by desorption of the same solvents at 80 °C down to 1.0 kPa CO₂ partial pressure. Experiments conducted on 13 solvent systems under atmospheric conditions revealed the solvents absorption and desorption characteristics and these were compared with 1.0 M, 2.5 M, 5.0 M and 10.0 M MEA. Results showed that absorption or stripping data alone were not sufficient in making robust solvent selection decisions, and that combined data analysis was necessary. 1.0 M tetraethylenepentamine (TEPA) and 5.0 M MEA showed the best performance in terms of absorption rate. 1.5 M Bis-(3-dimethylaminopropyl) amine (TMBPA) was easy to desorb, has high absorption capacity; and when promoted it showed the best performance in terms of CO₂ carrying capacity. At the test conditions, 1.5 M TMBPA promoted with 1.0 M PZ showed the best potential for efficient CO₂ removal at reduced cost of all systems tested. Its cyclic capacity in mol CO₂/mol amine was found to be 70% higher than that of 5 M MEA.     

Landfill gas (LFG) processing via adsorption and alkanolamine absorption

Landfill gas (LFG) was upgraded to pure methane using the adsorption and absorption processes. Different toxic compounds like aromatics and chlorinated compounds were removed using granular activated carbon. The activated carbon adsorbed toxic trace components in the following order: carbon tetrachloride > toluene > chloroform > xylene > ethylbenzene > benzene > trichloroethylene ≈ tetrachloroethylene. After removing all trace components, the gas was fed to absorption apparatus for the removal of carbon dioxide (CO₂). Two alkanolamines, monoethanol amine (MEA) and diethanol amine (DEA) were used for the removal of CO₂ from LFG. The maximum CO₂ loading is obtained for 30 wt.% MEA which is around 2.9 mol L^− 1 of absorbent solution whereas for same concentration of DEA it is around 1.66 mol L ^− 1 of solution. 30 wt% MEA displayed a higher absorption rate of around 6.64 × 10^− 5 mol L^− 1 min^− 1. DEA displayed a higher desorption rate and a better cyclic capacity as compared to MEA. Methane obtained from this process can be further used in the natural gas network for city.     

Reduction of amine mist emissions from a pilot-scale CO2 capture process using charged colloidal gas aphrons

In the CO₂ capture process from coal-derived flue gas where amine solvents are used, the flue gas can entrain small liquid droplets into the gas stream leading to emission of the amine solvent. The entrained drops, or mist, will lead to high solvent losses and cause decreased CO₂ capture performance. In order to reduce the emissions of the fine amine droplets from CO₂ absorber, a novel method using charged colloidal gas aphron (CGA) generated by an anionic surfactant was developed. The CGA absorption process for MEA emission reduction was optimized by investigating the surfactant concentration, stirring speed of the CGA generator, and capture temperature. The results show a significant reduction of MEA emissions of over 50% in the flue gas stream exiting the absorber column of a pilot scale CO₂ capture unit.     

Carbon dioxide absorption into biphasic amine solvent with solvent loss reduction

The main challenge in the CO₂ capture from flue gases is to reduce the energy consumption required for solvent regeneration. Lipophilic amines exhibit a thermomorphic phase transition upon heating, giving rise to autoextractive behaviour, which enhances desorption at temperatures well below the solvent boiling point. The low regeneration temperature of less than 80 °C together with the high cyclic CO₂ loading capacity (c. 0.9 mol-CO₂/mol-absorbent) of such biphasic amine systems permit the use of low temperature and even waste heat for desorption purposes. In order to improve the capture process and reduce the commensurate energy demand still further, desorption experiments were carried out at 70-80 °C and techniques for enhancing CO₂ release without gas stripping were also studied. The comparison of various amines at a concentration of 3 M and for a 15 mol% CO₂ feed gas demonstrates the considerable potential of lipophilic amines for the CO₂ absorption process. Chemical stability is a decisive factor for the industrial application of amine absorbents. Degradation of the novel lipophilic amine absorbents was shown to be minor, while volatility losses represent a major shortcoming of the biphasic solvent systems. Appropriate countermeasures to limit solvent losses were examined experimentally.     

Pilot plant study of two new solvents for post combustion carbon dioxide capture by reactive absorption and comparison to monoethanolamine

Application of new solvents will substantially contribute to the reduction of the energy demand for the post combustion capture of CO₂ from power plant flue gases. The present work describes tests of such new solvents in a gas-fired pilot plant, which comprises the complete absorption/desorption process (column diameters 0.125 m, absorber/desorber packing height 4.25/2.55 m, packing type: Sulzer BX 500, flue gas flow 30–100 kg/h, CO₂ partial pressure 35–135 mbar). Two new solvents CESAR1 (0.28 g/g 2-amino-2-methyl-1-propanol+0.17 g/g piperazine+0.55 g/g H₂O) and CESAR2 (0.32 g/g 1, 2-ethanediamine+0.68 g/g H₂O), which were developed in an EU-project, were systematically studied and compared to MEA (0.3 g/g monoethanolamine+0.7 g/g H₂O). The two new solvents and MEA were studied in the same way in the pilot plant and detailed results are reported for all solvents. In the present study the structured packing Sulzer BX 500 is used. The measurements are carried out at a constant CO₂ removal rate of 90% by an adjustment of the regeneration energy in the desorber for systematically varied solvent flow rates. An optimal solvent flow rate leading to a minimum energy requirement is found from these studies. Direct comparisons of such results can be misleading if there are differences in the kinetics of the different solvent systems. The influence of kinetic effects is experimentally studied by varying the flue gas flow rate at a constant ratio of solvent mass flow to flue gas mass flow and constant CO₂ removal rate. Results from these studies indicate similar kinetics for CESAR1, CESAR2 and MEA. The direct comparison of the pilot plant results for these solvents is therefore justified. Both CESAR1 and CESAR2 show improvements compared to MEA. The most promising is CESAR1 with a reduction of about 20% in the regeneration energy and 45% in the solvent flow rate.     

A comparative study of carbon dioxide capture capabilities between methanol solvent and aqueous monoethanol amine solution

Simulations have been performed to compare the performance of CO₂ capture power between 98.5 wt% methanol solvent and 30 wt% MEA aqueous solution. A general purpose chemical process simulator, PRO/II with PROVISION release 8.3 was used for the modeling of CO₂ capture process. For the simulation of CO₂ capture process using methanol as a solvent, NRTL liquid activity coefficient model was used for the estimation of the liquid phase non-idealities, Peng-Robinson equation of state model was selected for the prediction of vapor phase non-idealities, and Henry’s law option was chosen for the prediction of the solubilities of light gases in methanol and water solvents. Amine special thermodynamic package built-in PRO/II with PROVISION release 8.3 was used for the modeling of CO₂ capture process using MEA aqueous solution. We could conclude that the 30 wt% of MEA aqueous solution showed better performance than the 98.5 wt% methanol solvent in CO₂ capture capability. Through this study, we tried to compare the differences between the two processes from the aspects of capital and operating costs using a commercial process simulator. This will guide the optimal process design in the carbon dioxide capture process.     

Optimization of CO2 capture process with aqueous amines using response surface methodology

Amine is one of candidate solvents that can be used for CO₂ recovery from the flue gas by conventional chemical absorption/desorption process. In this work, we analyzed the impact of different amine absorbents and their concentrations, the absorber and stripper column heights and the operating conditions on the cost of CO₂ recovery plant for post-combustion CO₂ removal. For each amine solvent, the optimum number of stages for the absorber and stripper columns, and the optimum absorbent concentration, i.e., the ones that give the minimum cost for CO₂ removed, is determined by response surface optimization. Our results suggest that CO₂ recovery with 48 wt% DGA requires the lowest CO₂ removal cost of $43.06/ton of CO₂ with the following design and operating conditions: a 20-stage absorber column and a 7-stage stripper column, 26 m³/h of solvent circulation rate, 1903 kW of reboiler duty, and 99°C as the regenerator-inlet temperature.     

Investigation on kinetics of carbon dioxide absorption in aqueous solutions of monoethanolamine + 1, 3-diaminopropane

ABSTRACT

Environmental concerns from global warming and climate change demand carbon dioxide separation from post-combustion gases. Important parameters are involved in choosing the suitable solvent for carbon dioxide separation, including the reaction rate of carbon dioxide and the solvent. In this paper, the kinetics of carbon dioxide (CO₂) absorption in aqueous solutions of Monoethanolamine (MEA) + 1,3-Diaminopropane (DAP), a diamine containing two primary amino group, was developed. The measurements were performed in a stirred cell with a horizontal gas-liquid interface in the temperature range of 313.15–333.15 K and aqueous solutions of 10 wt% MEA + 5 wt% DAP and 12.5 wt% MEA + 2.5 wt% DAP. Experiments were conducted in an isothermal batch reactor with a horizontal gas-liquid interface under pseudo-first-order conditions, enabling the determination of the overall kinetic rate constant from the pressure drop method. Second-order reaction rate constants of CO₂ absorption in amine solutions were estimated using the calculated initial absorption rate. It was found that the rate constants in MEA+ DAP solutions were greater than in MEA solutions which means that DAP increases the reaction rate.     

FAU zeolite membranes for dewatering of amine-based post-combustion CO2 capture solutions

In amine-based CO₂ capture processes, aqueous amine solvent is circulated between absorber (CO₂ absorption) and stripping (solvent regeneration) columns. To reduce solvent regeneration energy demand, a selective membrane can dewater and enrich the CO₂ concentration in solution prior to the stripper, lowering steam requirements for solution heating. In this work, a facile synthesis strategy was developed to prepare faujasite (FAU) zeolite membranes built upon polydopamine (PDA) modified α-Al₂O₃ substrates. PDA facilitated the attachment of zeolite phases onto the substrate surface to form a 3 μm membrane layer. Membrane permeation flux of 4.45 kg m⁻² h⁻¹ and 95% rejection rate calculated by either CO₂ loading or total alkalinity was achieved in dewatering of CO₂ loaded 30 wt% monoethanolamine (MEA) solution. The effects of temperature on membrane dewatering performance and stability were investigated. This study highlights the potential for process integration of membrane technology in amine-based post-combustion CO₂ capture operations.     

Stripper configurations for CO2 capture by aqueous monoethanolamine

Absorption/stripping with amine solvents is a practical tail end technology for CO₂ capture from coal-fired power plants. One of the inhibiting costs of this technology is the energy requirement for solvent regeneration in the stripper, but novel configurations can help reduce this requirement by making the process more reversible. This work looked at several configurations with varying levels of complexity to determine the most useful method for arranging process units. Evaluated configurations included multi-stage flash, multi-pressure columns, and advanced stripping columns. Using a higher number of pressure stages, packing in place of equilibrium flashes, and vapor recompression were all reasonable methods to reduce the overall equivalent work requirement, but the most significant improvement was seen with an interheated column. The interheated column and simple stripper required 33.4 kJ/mol CO₂ and 35.0 kJ/mol CO₂ of work, respectively, at their optimum lean loadings.     

Aqueous piperazine derivatives for CO2 capture: Accurate screening by a wetted wall column

Concentrated aqueous piperazine (PZ) has been identified as a better solvent for CO₂ capture than monoethanolamine (MEA), because it has a greater rate of CO₂ absorption and greater CO₂ capacity. This work evaluates the effect of substitute groups on PZ performance. Many previous screening studies measured absorption/desorption with CO₂/N₂ sparging, which lacks accuracy and cannot be used to estimate actual absorber performance. In this work a wetted wall column was used to accurately measure absorption/desorption rate at typical rich and lean CO₂ loading (α) and simulate performance of real packing. The method also provides accurate measurement of CO₂ solubility at 40-100 °C. This study provided rate and solubility data at 40-100 °C and practical ranges of CO₂ loading for 8 m 1-methylpiperazine (1-MPZ), 8 m 2-methylpiperazine (2-MPZ), 4 m 2-MPZ/4 m PZ, 7.7 m N-(2-hydroxyethyl)piperazine (HEP), 6 m 1-(2-aminoethyl)piperazine (AEP), 8 m 2-piperidine ethanol (2-PE), and 2 m trans-2,5-dimethylpiperazine (2,5-DMPZ). With the measurements of CO₂ flux (NCO₂) and equilibrium driving force, liquid film mass transfer coefficients (k_g′) are calculated. The rate decreases as of 1-MPZ = PZ > 2-MPZ/PZ > 2-MPZ > HEP > MEA > AEP = 2-PE. This method also allows bracketing and determination of equilibrium CO₂ partial pressure (^*PCO₂) at each condition. Semi-empirical solubility models of CO₂ for each amine were regressed from experimental solubility data to find the lean and rich CO₂ loading corresponding to 0.500 kPa and 5 kPa CO₂ partial pressure respectively. Based on the solubility model, the actual operating capacity of the solvents without overstripping decreases in the sequence of 2-PE > 2-MPZ > 2-MPZ/PZ > 1-MPZ > PZ > HEP > AEP > MEA. The enthalpy of CO₂ absorption (ΔH_abs) of all the piperazine derivatives is around 70 kJ/mol CO₂.     

Non-oxidative thermal degradation of amines: GCMS/FTIR spectra analysis and molecular modeling

Aqueous amino solvents, such as monoethanolamine (ETA/MEA), methyl diethanolamine (MDEA) or amine blends, are the most widely used solvents in commercial CO₂ or acid gas separation applications. These commercial solvents have various disadvantages, such as the possibilities of the solvent to be degraded. This research examines the impact of non-oxidative thermal degradations on the performance of the CO₂ absorption and the degradation mechanism of amine solvents. The impact of degradation was conducted by measuring the CO₂ solubility of solvent that had been heated to 120°C for 2 h. Although the performance of CO₂ absorption was not significantly reduced, the degradation of amines was found. Supported by Fourier Transform Infrared (FTIR) and Gas Chromatography/Mass Spectrometer result, the suspected products of non-oxidative thermal degradation of MDEA were MEA and acetone.     

Intensification of low temperature thermomorphic biphasic amine solvent regeneration for CO2 capture

High-energy requirements for solvent regeneration represent one of the main challenges in the conventional post-combustion capture (PCC) process. Thermomorphic biphasic solvent (TBS), comprising lipophilic amines as the active components, exhibit a liquid–liquid phase separation (LLPS) upon heating, giving rise to extractive behaviour, and thus enhancing desorption at temperatures well below the solvent boiling point. The low regeneration temperature of less than 90 °C together with the high cyclic CO₂ loading capacity, 3–4 mol/kg, of such TBS system permits the use of low temperature and even waste heat for desorption purposes. In order to improve the solvent regeneration process and reduce the commensurate energy demand still further, desorption experiments with various techniques for enhancing CO₂ release in place of gas stripping, such as nucleation, agitation, ultrasonic method, etc., were studied at temperatures in the range of 75–85 °C. Nucleation and agitation both accelerate CO₂ desorption, but regenerability by nucleation only achieves 70–85%, while by agitation attains 80–95%. Ultrasonic desorption also intensifies the solvent regeneration and superior to conventional stripping process. The energy consumption for TBS system with those intensification techniques is only half of that for alkanolamine-based process with steam stripping. Extractive regeneration is another potential method to substitute for stripping and reduce the exergy demands. An extraction process using inert solvent was developed for improving the regeneration efficiency and elevated pressures were applied for reducing the significant volatile solvent loss.     

Pilot plant study of post-combustion carbon dioxide capture by reactive absorption: Methodology, comparison of different structured packings, and comprehensive results for monoethanolamine

Post-combustion carbon capture (PCC) from fossil fuel power plants by reactive absorption can substantially contribute to reduce emissions of the greenhouse gas CO₂. To test new solvents for this purpose small pilot plants are used. The present paper describes results of comprehensive studies of the standard PCC solvent MEA (0.3 g/g monoethanolamine in water) in a pilot plant in which the closed cycle of absorption/desorption process is continuously operated (column diameters: 0.125 m, absorber/desorber packing height: 4.25/2.55 m, packing type: Sulzer BX 500, flue gas flow: 30-110 kg/h, CO₂ partial pressure: 35-135 mbar). The data establish a base line for comparisons with new solvents tested in the pilot plant and can be used for a validation of models of the PCC process with MEA. The ratio of the solvent to the flue gas mass flow is systematically varied at constant CO₂ removal rate, and CO₂ partial pressure in the flue gas. Optimal operating points are determined. In the present study the structured packing Sulzer BX 500 is used. The experiments with the removal rate variation are carried out so that the results can directly be compared to those from a previous study in the same plant that was carried out using Sulzer Mellapak 250.Y. A strategy for identifying the influence of absorption kinetics on the results is proposed, which is based on a variation of the gas load at a constant L/G ratio and provides valuable insight on the transferability of pilot plant results.     

Amine modified mesocellular siliceous foam (MCF) as a sorbent for CO2

Adsorption is considered a promising method for carbon capture. CO₂ adsorbents take a variety of forms - but one approach is to fill mesoporous substrates with a polymeric CO₂ selective sorbent. SBA-15 and mesocellular siliceous foam (MCF) are high pore volume, high surface area ordered mesoporous materials for which modification with amine should result in high capacity, highly selective adsorbents. SBA-15 and MCF were separately loaded with approximately one pore volume equivalent of linear polyethyleneimine (PEI) (M_w = 2500) or branched PEI (M_n = 1200). CO₂ adsorption/desorption isotherms under dry CO₂ were obtained at 75, 105 and 115 °C. The CO₂ adsorption/desorption kinetics were improved with temperature, though the CO₂ capacities generally decreased. The adsorption capacity for MCF loaded with branched PEI at 105 and 115 °C were 151 and 133 mg/g adsorbent, respectively (in 50% CO₂/Ar, 20 min adsorption time). These are significantly higher than the adsorption capacity observed for SBA-15 loaded with branched PEI under same conditions, which were 107 and 83 mg/g adsorbent, respectively. Thus the results indicate that, on a unit mass basis, amine modified MCF's are potentially better adsorbents than amine modified SBA-15 for CO₂ capture at modestly elevated temperature in a vacuum swing adsorption process.     

Development of new alumina-modified sorbents for CO2 sorption and regeneration at temperatures below 200 °C

A new regenerable alumina-modified sorbent was developed for CO₂ capture at temperatures below 200 °C. The CO₂ capture capacity of a potassium-based sorbent containing Al₂O₃ (KAlI) decreased during multiple CO₂ sorption (60 °C) and regeneration (200 °C) tests due to the formation of the KAl(CO₃)(OH)₂ phase, which could be converted into the original K₂CO₃ phase above 300 °C. However, the new regenerable potassium-based sorbent (Re-KAl(I)) maintained its CO₂ capture capacity during multiple tests even at a regeneration temperature of 130 °C. In particular, the CO₂ capture capacity of the Re-KAl(I)60 sorbent which was prepared by the impregnation of Al₂O₃ with 60 wt.% K₂CO₃ was about 128 mg CO₂/g sorbent. This excellent CO₂ capture capacity and regeneration property were due to the characteristics of the Re-KAl(I) sorbent producing only a KHCO₃ phase during CO₂ sorption, unlike the KAlI30 sorbent which formed the KHCO₃ and KAl(CO₃)(OH)₂ phases even at 60 °C. This result was explained through the structural effect of the support containing the KAl(CO₃)(OH)₂ phase which was prepared by impregnation of Al₂O₃ with K₂CO₃ in the presence of CO₂.     

CO2 Absorption Mechanism by the Deep Eutectic Solvents Formed by Monoethanolamine-Based Protic Ionic Liquid and Ethylene Glycol

Deep eutectic solvents (DESs) have been widely used to capture CO₂ in recent years. Understanding CO₂ mechanisms by DESs is crucial to the design of efficient DESs for carbon capture. In this work, we studied the CO₂ absorption mechanism by DESs based on ethylene glycol (EG) and protic ionic liquid ([MEAH][Im]), formed by monoethanolamine (MEA) with imidazole (Im). The interactions between CO₂ and DESs [MEAH][Im]-EG (1:3) are investigated thoroughly by applying ¹H and ¹³ C nuclear magnetic resonance (NMR), 2-D NMR, and Fourier-transform infrared (FTIR) techniques. Surprisingly, the results indicate that CO₂ not only binds to the amine group of MEA but also reacts with the deprotonated EG, yielding carbamate and carbonate species, respectively. The reaction mechanism between CO₂ and DESs is proposed, which includes two pathways. One pathway is the deprotonation of the [MEAH]⁺ cation by the [Im]⁻ anion, resulting in the formation of neutral molecule MEA, which then reacts with CO₂ to form a carbamate species. In the other pathway, EG is deprotonated by the [Im]⁻, and then the deprotonated EG, HO-CH₂-CH₂-O⁻, binds with CO₂ to form a carbonate species. The absorption mechanism found by this work is different from those of other DESs formed by protic ionic liquids and EG, and we believe the new insights into the interactions between CO₂ and DESs will be beneficial to the design and applications of DESs for carbon capture in the future.     

Analysis of the heat of reaction and regeneration in alkanolamine-CO<Subscript>2</Subscript> system

The estimation of regeneration heat of absorbent is important because it is a key factor that has an effect on the process efficiency. In this study, thermal stability and regeneration heat of aqueous amine solutions such as monoethanolamine (MEA), 2-amino-2-methyl-1-propanol (AMP), N-methyldiethanolamine (MDEA), and 1,8-diamino-pmenthane (KIER-C3) were investigated by using TGA-DSC analysis. The thermal characteristics of the fresh and CO₂ rich amine solutions were estimated. The CO₂ rich amine solutions were obtained by VLE experiments at T=40 °C. The regeneration heat of aqueous MEA solution was 76.991–66.707 kJ/mol-CO₂, which is similar to heat of absorption. The reproducibility of the results was obtained. The regeneration heat of aqueous KIER-C3 20 wt% solution (1.68 M) was lower than that of aqueous MEA 30 wt% solution (4.91 M). Therefore, the KIER-C3 can be used as an effective absorbent for acid gas removal.     

Carbon dioxide capture by solvent absorption using amino acids: A review

The emission of large amounts of carbon dioxide is of major concern with regard to increasing the risk of climate change. Carbon capture, utilisation and storage (CCUS) has been proposed as an important pathway for slowing the rate of these emissions. Solvent absorption of CO₂ using amino acid solvents has drawn much attention over the last few years due to advantages including their ionic nature, low evaporation rate, low toxicity, high absorption rate and high biodegradation potential, compared to traditional amine solvents. In this review, recent progress on the absorption kinetics of amino acids is summarised, and the engineering potential of using amino acids as carbon capture solvents is discussed. The reaction orders between amino acids and carbon dioxide are typically between 1 and 2. Glycine exhibits a reaction order of 1, whilst, by comparison, lysine, proline and sarcosine have the largest reaction constants with carbon dioxide which is much larger than that of the benchmark solvent monoethanolamine (MEA). Ionic strength, pH and cations such as sodium and potassium have been shown to be important factors influencing the reactivity of amino acids. Corrosivity and reactivity with impurities such as SO_x and NO_x are not considered to be significant problems for amino acids solvents. The precipitation of CO₂ loaded amino acid salts is thought to be a good pathway for increasing CO₂ loading capacity and cutting desorption energy costs if well-controlled. It is recommended that more detailed research on amino acid degradation and overall process energy costs is conducted. Overall, amino acid solvents are recognised as promising potential solvents for carbon dioxide capture.     

Thermodynamic modeling of dealcoholization of beverages using supercritical CO2: Application to wine samples

The supercritical removal of ethanol from alcoholic beverages (brandy, wine, and cider) was studied using the GC-EoS model to represent the phase equilibria behavior of the CO₂ + beverage mixture. Each alcoholic drink was represented as the ethanol + water mixture with the corresponding ethanol concentration (35 wt% for brandy, 9-12 wt% for different wines and 6 wt% for cider). The thermodynamic modeling was based on an accurate representation of the CO₂ + ethanol and CO₂ + water binary mixtures, and the CO₂ + ethanol + water ternary mixture.The GC-EoS model was employed to simulate the countercurrent supercritical CO₂ dealcoholization of the referred beverages; the results obtained compared good with experimental data from the literature. Thus, the model was used to estimate process conditions to achieve an ethanol content reduction from ca. 10 wt% to values lower than 1 wt%. The model results were tested by carrying out several extraction assays using wine, in a 3 m height packed column at 308 K, pressures in the range of 9-18 MPa and solvent to wine ratio between 9 and 30 kg/kg.