Potential of different additives to improve performance of potassium carbonate for CO<Subscript>2</Subscript> absorption

The performance of potassium carbonate (K₂CO₃) solution promoted by three amines, potassium alaninate (K-Ala), potassium serinate (K-Ser) and aminoethylethanolamine (AEEA), in terms of heat of absorption, absorption capacity and rate was studied experimentally. The experiments were performed using a batch reactor, and the results were compared to pure monoethanolamine (MEA) and K₂CO₃ solutions. The heat of absorption of K₂CO₃+additive solution was calculated using the Gibbs-Helmholtz equation. In addition, a correlation for prediction of CO₂ loading was presented. The results indicated that absorption heat, absorption rate and loading capacity of CO₂ increase as the concentration of additive increases. The blend solutions have higher CO₂ loading capacity and absorption rate when compared to pure K₂CO₃. The heat of CO₂ absorption for K₂CO₃+additive solutions was found to be lower than that of the pure MEA. Among the additives, AEEA showed the highest CO₂ absorption capacity and absorption rate with K₂CO₃. In conclusion, the K₂CO₃+AEEA solution with high absorption performance can be a potential solvent to replace the existing amines for CO₂ absorption.     

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.     

The physical solubilities and diffusivities of N<Subscript>2</Subscript>O and CO<Subscript>2</Subscript> in aqueous ammonia solutions on the additions of AMP,glycerol and ethylene glycol

Carbon dioxide (CO₂) is a major greenhouse gas, the emissions of which should be reduced. There are various technologies for the effective separation of CO₂. Of these, chemical absorption methods are generally accepted as the most effective. The monoethanolamine (MEA) process is an effective way to remove CO₂, but is an expensive option for the separation of CO₂ from massive gas-discharging plants. Therefore, ammonia solution, which is less expensive and more effective than MEA, was used for the removal of CO₂. In this study, the physical solubility of N₂O in (ammonia+water), (ammonia+2-amino-2-methyl-1-propanol+water), (ammonia+glycerol+water) and (ammonia+ ethylene glycol+water) was measured at 293, 303, 313, 323 K. Additive concentrations of 1, 3, and 5 wt% AMP, glycerol and ethylene glycol were added for each 9 wt% ammonia solution. A solubility apparatus was used to investigate the solubility of N₂O in ammonia solutions. The diffusivity was measured with a wetted wall column absorber. The “N₂O analogy” is used to estimate the solubility and diffusivity of CO₂ in the aqueous ammonia solutions. OriginPro 7.5 was used to correlate the solubility and diffusivity of N₂O in ammonia solutions. The parameters of the correlation were determined from the measured solubility and diffusivity.     

Characterization of metal corrosion by aqueous amino acid salts for the capture of CO<Subscript>2</Subscript>

We investigated the absorption ability of potassium salts of amino acid solutions for carbon dioxide and compared the results with MEA. The corrosion and degradation behavior were investigated in a CO₂ absorption process using aqueous potassium salts of glycine and taurine. The experimental parameters varied were the concentration, amino acid type, temperature, CO₂ loading, piperazine, and the presence of corrosion inhibitors. The corrosion characteristics of carbon steel were measured with potassium glycinate and potassium taurate solutions over a wide range of concentrations (1.5 to 5.0 M) and temperatures (313.15 to 353.15 K). The corrosion rate was calculated using a weight loss method averaging the results of four specimens. The experimental results indicate that increases in the concentration of the aqueous amino acid salts, solution temperature, CO₂ loading, and piperazine concentration accelerate the corrosion rate. In addition, corrosion inhibitors were proven to be effective in controlling corrosion.     

Simultaneous absorption of CO<Subscript>2</Subscript> and SO<Subscript>2</Subscript> into aqueous AMP/NH<Subscript>3</Subscript> solutions in binary composite absorption system

To enhance the absorption rate for CO₂ and SO₂, aqueous ammonia (NH₃) solution was added to an aqueous 2-amino-2-methyl-1-propanol (AMP) solution. The simultaneous absorption rates of AMP and a blend of AMP+ NH₃ for CO₂ and SO₂ were measured by using a stirred-cell reactor at 303 K. The process operating parameters of interest in this study were the solvent and concentration, and the partial pressures of CO₂ and SO₂. As a result, the addition of NH₃ solution into aqueous AMP solution increased the reaction rate constants of CO₂ and SO₂ by 144 and 109%, respectively, compared to that of AMP solution alone. The simultaneous absorption rate of CO₂/SO₂ on the addition of 1 wt% NH₃ into 10 wt% AMP at a p _A1 of 15 kPa and p _A2 of 1 kPa was 5.50×10⁻⁶ kmol m⁻² s⁻¹, with an increase of 15.5% compared to 10 wt% AMP alone. Consequently, the addition of NH₃ solution into an aqueous AMP solution would be expected to be an excellent absorbent for the simultaneous removal of CO₂/SO₂ from the composition of flue gas emitted from thermoelectric power plants.     

CO2 absorption into a phase change absorbent: Water-lean potassium prolinate/ethanol solution

Phase change solvents are attractive energy-efficient absorbents for carbon dioxide (CO₂) capture due to CO₂-rich phase formation. Potassium prolinate + water + ethanol (ProK/W/Eth) solution has shown good capture characteristics as a promising one in our previous work. In this work, absorption rate of CO₂, solubility of N₂O, and heat of absorption for ProK/W/Eth solution were investigated using a stirred cell reactor and a CPA201 reaction calorimeter and these results were also compared with the aqueous ProK and 30 mass% MEA solutions. Using ethanol as a solvent can substantially increase the CO₂ physical solubility and the absorption rate of CO₂ in ProK/W/Eth solutions is far higher than that in aqueous 30 mass% MEA solutions especially at a low CO₂ loading range. Solid precipitation, obtained from the liquid-to-solid phase change absorption, was analyzed by ¹³C NMR and DSC-TGA. The enthalpy change for ProK/W/Eth solutions at various CO₂ loading was also discussed.     

The effect of relative humidity on CO<Subscript>2</Subscript> capture capacity of potassium-based sorbents

Potassium-based sorbent was prepared by impregnation with potassium carbonate on activated carbon. The role of water and its effects on pretreatment and CO₂ absorption was investigated in a fixed bed reactor. K₂CO₃ could be easily converted into K₂CO₃·1.5H₂O working as an active species by the absorption of water vapor as the following reaction: K₂CO₃+3/2 H₂O→K₂CO₃·1.5H₂O. One mole of K₂CO₃·1.5H₂O absorbed one mole of CO₂ as the following reaction: K₂CO₃·1.5H₂O+CO₂ai2KHCO₃+0.5 H₂O. The K₂CO₃·1.5H₂O phase, however, was easily transformed to the K₂CO₃ phase by thermal desorption even at low temperature under low relative humidity. To enhance CO₂ capture capacity and CO₂ absorption rate, it is very important to maintain the K₂CO₃·1.5H₂O phase worked as an active species, as well as to convert the entire K₂CO₃ to the K₂CO₃·1.5H₂O phase during CO₂ absorption at a temperature range between 50 °C and 70 °C. As a result, the relative humidity plays a very important role in preventing the transformation from K₂CO₃·1.5H₂O to the original phase (K₂CO₃) as well as in producing the K₂CO₃·1.5H₂O from K₂CO₃, during CO₂ absorption between 50 °C and 70 °C.     

Effect of addition of weak acids on CO<Subscript>2</Subscript> desorption from rich amine solvents

Experiments were conducted to study the effect of addition of four weak acids (adipic, suberic, phthalic and sebacic acids) on the regeneration of three types of CO₂-loaded rich solvents (Monoethanolamine (MEA), Diethanolamine (DEA) and Methyldiethanolamine (MDEA)). It was found that CO₂ could be released faster and in a larger quantity when the amount of acid added to the solvent was increased while other desorption conditions were maintained unchanged. Adipic acid appeared to be more effective than phthalic, suberic and sebacic acids in enhancing solvent regeneration rate. Among the three amines investigated, MEA had the highest CO₂ desorption rate, while DEA saved the most energy. The effect of adipic acid residue in the MEA solvent on CO₂ absorption was also investigated. The residue acid reduced the absorption capacity of the MEA solvent significantly when the solvent concentration was low and slightly when the concentration was high.     

Equilibrium solubility of CO<Subscript>2</Subscript> in aqueous binary mixture of 2-(diethylamine)ethanol and 1, 6-hexamethyldiamine

CO₂ solubility data are important for the efficient design and operation of the acid gas CO₂ capture process using aqueous amine mixture. 2-(Diethylamino)ethanol (DEEA) solvent can be manufactured from renewable sources like agricultural products/residue, and 1,6-hexamethyldiamine (HMDA) solvents have higher absorption capacity as well as reaction rate with CO₂ than conventional amine-based solvents. The equilibrium solubility of CO₂ into aqueous binary mixture of DEEA and HMDA was investigated in the temperature range of 303.13-333.13 K and inlet CO₂ partial pressure in the range of 10.133-20.265 kPa. Total concentration of aqueous amine mixtures in the range of 1.0-3.0 kmol/m³ and mole fraction of HMDA in total amine mixture in the range of 0.05-0.20 were taken in this work. CO₂ absorption experiment was performed using semi-batch operated laboratory scale bubble column to measure equilibrium solubility of CO₂ in amine mixture, and CO₂ absorbed amount in saturated carbonated amine mixture was analyzed by precipitation-titration method using BaCl₂. Maximum equilibrium CO₂ solubility in aqueous amine mixture was observed at 0.2 of HMDA mole fraction in total amine mixture with 1.0 kmol/m³ total amine concentration. New solubility data of CO₂ in DEEA+HMDA aqueous mixtures in the current study was compared with solubility data available in previous studies conducted by various researchers. The study shows that the new absorbent as a mixture of DEEA+HMDA is feasible for CO₂ removal from coal-fired power plant stack gas streams.     

Simplified synthesis of K<Subscript>2</Subscript>CO<Subscript>3</Subscript>-promoted hydrotalcite based on hydroxide-form precursors: Effect of Mg/Al/K<Subscript>2</Subscript>CO<Subscript>3</Subscript> ratio on high-temperature CO<Subscript>2</Subscript> sorption capacity

Hydrotalcite was synthesized from hydroxide-form precursors to prepare a novel high-temperature CO₂ sorbent, and the effect of Mg/Al ratio on CO₂ sorption was studied. To enhance the CO₂ sorption capacity of the sorbent, K₂CO₃ was coprecipitated during the synthetic reaction. X-ray diffraction analysis indicated that the prepared samples had a well-defined crystalline hydrotalcite structure, and confirmed that K₂CO₃ was successfully coprecipitated in the samples. The morphology of the hydrotalcite was confirmed by scanning electron microscopy, and N₂ adsorption analysis was used to estimate its surface area and pore volume. In addition, thermogravimetric analysis was used to measure its CO₂ sorption capacity, and the results revealed that the Mg: Al: K₂CO₃ ratio used in the preparation has an optimum value for maximum CO₂ sorption capacity.     

A Novel Process for Renewable Methane Production: Combining Direct Air Capture by K<Subscript>2</Subscript>CO<Subscript>3</Subscript>/Alumina Sorbent with CO<Subscript>2</Subscript> Methanation over Ru/Alumina Catalyst

CO₂ methanation over supported ruthenium catalysts is considered to be a promising process for carbon capture and utilization and power-to-gas technologies. In this work 4% Ru/Al₂O₃ catalyst was synthesized by impregnation of the support with an aqueous solution of Ru(OH)Cl₃, followed by liquid phase reduction using NaBH₄ and gas phase activation using the stoichiometric mixture of CO₂ and H₂ (1:4). Kinetics of CO₂ methanation reaction over the Ru/Al₂O₃ catalyst was studied in a perfectly mixed reactor at temperatures from 200 to 300 °C. The results showed that dependence of the specific activity of the catalyst on temperature followed the Arrhenius law. CO₂ conversion to methane was shown to depend on temperature, water vapor pressure and CO₂:H₂ ratio in the gas mixture. The Ru/Al₂O₃ catalyst was later tested together with the K₂CO₃/Al₂O₃ composite sorbent in the novel direct air capture/methanation process, which combined in one reactor consecutive steps of CO₂ adsorption from the air at room temperature and CO₂ desorption/methanation in H₂ flow at 300 or 350 °C. It was demonstrated that the amount of desorbed CO₂ was practically the same for both temperatures used, while the total conversion of carbon dioxide to methane was 94.2–94.6% at 300 °C and 96.1–96.5% at 350 °C.     

Influence of operating temperature on CO<Subscript>2</Subscript>-NH<Subscript>3</Subscript> reaction in an aqueous solution

Although aqueous ammonia solution has been focused on the removal of CO₂ from flue gas, there have been very few reports regarding the underlying analysis of the reaction between CO₂ and NH₃. In this work, we explored the reaction of CO₂-NH₃-H₂O system at various operating temperatures: 40 °C, 20 °C, and 5 °C. The CO₂ removal efficiency and the loss of ammonia were influenced by the operating temperatures. Also, infrared spectroscopy measurement was used in order to understand the formation mechanism of ion species in absorbent, such as NH₂COO⁻, HCO₃⁻, CO₃²⁻, and NH₄⁺, during CO₂, NH₃, and H₂O reaction. The reactions of CO₂-NH₃-H₂O system at 20 °C and 40 °C have similar reaction routes. However, a different reaction route was observed at 5 °C compared to the other operating temperatures, showing the solid products of ammonium bicarbonates, relatively. The CO₂ removal efficiency and the formation of carbamate and bicarbonate were strongly influenced by the operating temperatures. In particular, the analysis of the formation carbamate and bicarbonate by infrared spectroscopy measurement provides useful information on the reaction mechanism of CO₂ in an aqueous ammonia solution.     

CO<Subscript>2</Subscript> retention ability on alkali cation exchanged titanium silicate,ETS-10

ETS-10 was ion exchanged by various alkali cations (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺) and the BET surface area and pore volume was exactly consistent with cationic size; that is, in the order of Li⁺ > Na⁺ > K⁺ > Rb⁺ > Cs⁺. It was observed that a single point adsorption capacity was inversely proportional to cationic size. The largest CO₂ capacity was observed for Li⁺-ETS-10 and it is attributed to greater cation–quadrupole interactions with CO₂ than larger cation. The results also suggests that as the CO₂ loading is increased, the accessibility of adsorbing CO₂ to framework basic O⁻ sites should have become difficult with the increase in cationic size due to the blocking effect by extra-framework CO₂-M⁺. The slight decrease in the slope of adsorption capacity with temperature, especially beyond 373 K for Li⁺-ETS-10 and K⁺-ETS-10 suggests that the adsorption of CO₂ on small alkali cation exchanged-ETS-10 at high temperature is somewhat associated with basic oxygen anion sites in framework due to the existence of large pore. The CO₂-TPD results show that the amount of desorbed CO₂ at higher temperature was proportionally increased due to the increased basicity of oxygen anions in framework. It also shows that the desorption temperature associated with alkali cations in extra-framework (corresponding to low temperature desorption peak) has been lowered with the increase in cationic size, indicating weak cation–quadrupole interactions with CO₂ for larger cations.     

Simulation of CO<Subscript>2</Subscript> removal in a split-flow gas sweetening process

Split-flow gas sweetening is known to consume less energy than a conventional gas sweetening process when the inlet sour gas contains a high concentration of acid gases. In this work, a computer simulation of a split-flow natural gas sweetening process based on absorption/stripping process with alkanoamine (MEA and DGA) solutions, using Aspen plus, was performed. The input of parameters such as the concentration of sour gases (CO₂, H₂S) in the feed gas has been examined. Simulation results show that the split-flow gas sweetening process can reduce the reboiler duty of a stripping tower better than the conventional gas sweetening process according to the concentration of CO₂ in the feed gas.     

CO2 absorption capacity using aqueous potassium carbonate with 2-methylpiperazine and piperazine

Various amines as a promoter have been added to the K₂CO₃ solution in order to improve the performance of the CO₂ absorption process. Piperazine (PZ) and 2-methylpiperazine (2-MPZ), cyclic diamines were used as promoters in this study. The equilibrium partial pressure of CO₂ $(P_{{\text{CO}}_2}^1)$ and pressure changes were measured at 313, 333 and 353 K condition of flue gas using VLE (vapor–liquid equilibrium) equipment. The results show that 2-MPZ possesses the ability to promote CO₂ absorption capacity. The K₂CO₃/2-MPZ solution had an equilibrium partial pressure of CO₂ that was lower than that of the MEA solution at 333 K.     

Hybrid neural network for prediction of CO<Subscript>2</Subscript> solubility in monoethanolamine and diethanolamine solutions

The solubility of CO₂ in single monoethanolamine (MEA) and diethanolamine (DEA) solutions was predicted by a model developed based on the Kent-Eisenberg model in combination with a neural network. The combination forms a hybrid neural network (HNN) model. Activation functions used in this work were purelin, logsig and tansig. After training, testing and validation utilizing different numbers of hidden nodes, it was found that a neural network with a 3-15-1 configuration provided the best model to predict the deviation value of the loading input. The accuracy of data predicted by the HNN model was determined over a wide range of temperatures (0 to 120 °C), equilibrium CO₂ partial pressures (0.01 to 6,895 kPa) and solution concentrations (0.5 to 5.0M). The HNN model could be used to accurately predict CO₂ solubility in alkanolamine solutions since the predicted CO₂ loading values from the model were in good agreement with experimental data.     

Solubility and diffusivity of CO<Subscript>2</Subscript> in <Emphasis Type="Italic">n</Emphasis>-butanol + N235 system and absorption mechanism of CO<Subscript>2</Subscript> in a coupled reaction-extraction process

The absorption of CO₂ in insoluble organic amine is crucial for understanding the mechanism of coupled reaction-extraction-crystallization process between aqueous chloride and CO₂. In this study, the solubility and diffusivity of CO₂ in n-butanol + N235 system were measured and reported. The absorption of CO₂ in the system is a physical absorption behavior and the solubility of CO₂ decreases with the increase of the mass fraction of N235. The diffusivity of CO₂ increases firstly and then decreases with the increase in the mass fraction of N235. Moreover, the absorption mechanism of CO₂ in the coupled reaction-extraction-crystallization process was investigated and identified by experiments. The results indicated that in the coupled reaction-extraction-crystallization process, CO₂ is absorbed by the aqueous phase rather than by the organic phase and further transferred into the aqueous phase.

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The effect of CO<Subscript>2</Subscript> or steam partial pressure in the regeneration of solid sorbents on the CO<Subscript>2</Subscript> capture efficiency in the two-interconnected bubbling fluidized-beds system

The effect of CO₂ or steam partial pressure in the regeneration of CO₂ solid sorbents was studied in the two-interconnected bubbling fluidized-beds system. Potassium-based dry solid sorbents, which consisted of 35 wt% K₂CO₃ for CO₂ sorption and 65 wt% supporters for mechanical strength, were used. To investigate the CO₂ capture efficiency of the regenerated sorbent after the saturated sorbent was regenerated according to the CO₂ or steam partial pressure in the regeneration, the mole percentage of CO₂ in the regeneration gas was varied from 0 to 50 vol% with N₂ balance and that of steam was varied from 0 to 100 vol% with N₂ balance, respectively. The CO₂ capture efficiency for each experimental condition was investigated for one hour steady-state operation with continuous solid circulation between a carbonator and a regenerator. The CO₂ capture efficiency decreased as the partial pressure of CO₂ in the fluidization gas of the regenerator increased, while it increased as that of steam increased. When 100 vol% of steam was used as the fluidization gas of the regenerator, the CO₂ capture efficiency reached up to 97% and the recovered CO₂ concentration in the regenerator was around 95 vol%. Those results were verified during 10-hour continuous experiment.     

Techno-economic assessment of pulverized coal boilers and IGCC power plants with CO<Subscript>2</Subscript> capture

The current studies on power plant technologies suggest that Integrated Gasification Combined Cycle (IGCC) systems are an effective and economic CO₂ capture technology pathway. In addition, the system in conventional configuration has the advantage of being more “CO₂ capture ready” than other technologies. Pulverized coal boilers (PC) have, however, proven high technical performance attributes and are economically often most practical technologies. To highlight the pros and cons of both technologies in connection with an integrated CO₂ capture, a comparative analysis of ultrasupercritical PC and IGCC is carried out in this paper. The technical design, the mass and energy balance and the system optimizations are implemented by using the ECLIPSE chemical plant simulation software package. Built upon these technologies, the CO₂ capture facilities are incorporated within the system. The most appropriate CO₂ capture systems for the PC system selected for this work are the oxy-fuel system and the postcombustion scheme using Monoethanolamine solvent scrubber column (MEA). The IGCC systems are designed in two configurations: Water gas shift reactor and Selexol-based separation. Both options generate CO₂-rich and hydrogen rich-gas streams. Following the comparative analysis of the technical performance attributes of the above cycles, the economic assessment is carried out using the economic toolbox of ECLIPSE is seamlessly connected to the results of the mass and energy balance as well as the utility usages. The total cost assessment is implemented according to the step-count exponential costing method using the dominant factors and/or a combination of parameters. Subsequently, based on a set of assumptions, the net present value estimation is implemented to calculate the breakeven electricity selling prices and the CO₂ avoidance cost.     

Regenerable potassium-based alumina sorbents prepared by CO<Subscript>2</Subscript> thermal treatment for post-combustion carbon dioxide capture

Potassium carbonate supported on alumina is used as a solid sorbent for CO₂ capture at low temperatures. However, its CO₂ capture capacity decreases immediately after the first cycle. This regeneration problem is due to the formation of the by-product [KAl(CO₃)(OH)₂] during CO₂ sorption. To overcome this problem, a new regenerable potassium-based sorbent was fabricated by CO₂ thermal treatment of sorbents prepared by the impregnation of δ-alumina with K₂CO₃ in the presence of 10 vol% CO₂ and 10 vol% H₂O. The CO₂ capture capacities of the new regenerable sorbents were maintained over multiple CO₂ sorption tests. These results can be explained by the fact that the sorbent prepared by CO₂ thermal treatment did not form any by-product during CO₂ sorption. Based on these results, we suggest that the regeneration properties of potassium-based sorbents using δ-alumina could be significantly improved by the use of the CO₂ thermal treatment developed in this study.