Surface Tensions of Carbonated 2‐Amino‐2‐methyl‐1‐propanol and Piperazine Aqueous Solutions

Surface tensions of carbonated 2‐amino‐2‐methyl‐1‐propanol (AMP) and piperazine (PZ) aqueous solutions were measured by a surface tension meter which employs the Wilhemy plate principle. A thermodynamic model was proposed to correlate the surface tensions of both CO₂‐unloaded and CO₂‐loaded aqueous solutions by introducing the contribution of CO₂ loading into the formulation of surface tension. Based on experiments and calculations, the effects of temperature, mass fractions of amines, and CO₂ loading on surface tensions of carbonated aqueous solutions were demonstrated.     

Absorption of carbon dioxide in piperazine activated concentrated aqueous 2-amino-2-methyl-1-propanol solvent

In this work new experimental data on the rate of absorption of CO₂ into piperazine (PZ) activated concentrated aqueous solutions of 2-amino-2-methyl-1-propanol (AMP) over the temperature range 303–323 K are presented. The absorption experiments have been carried out in a wetted wall contactor over CO₂ partial pressure range of 5–15 kPa. PZ is used as a rate activator with a concentration ranging from 2 to 8 wt% keeping the total amine concentration in the solution at 40 wt%. The physical properties such as density and viscosity of concentrated aqueous AMP+PZ, as well as physical solubility of CO₂ in concentrated aqueous AMP+PZ, are also measured. New experimental data on vapor liquid equilibrium (VLE) of CO₂ in these concentrated aqueous solutions of AMP+PZ in the temperature range of 303–323 K have also been presented. The VLE measurements are carried out in an equilibrium cell in CO₂ pressure range of 0.1–140 kPa. A thermodynamic model based on electrolyte non-random two-liquid (eNRTL) theory is used to represent the VLE of CO₂ in aqueous AMP+PZ. Liquid phase speciations are estimated considering the nonideality of concentrated solutions of the amines and the calculated activity coefficients by eNRTL model. The CO₂ absorption in the aqueous amine solutions is described by a combined mass transfer-reaction kinetics model developed according to Higbie's penetration theory. The model predictions have been found to be in good agreement with the experimental results of the rates of absorptions of CO₂ into aqueous AMP+PZ.     

Kinetics of the absorption of carbon dioxide into mixed aqueous solutions of 2-amino-2-methyl-l-propanol and piperazine

The reaction kinetics of the absorption of CO₂ into aqueous solutions of piperazine (PZ) and into mixed aqueous solutions of 2-amino-2-methyl-l-propanol (AMP) and PZ were investigated by wetted wall column at 30-40 °C. The physical properties such as density, viscosity, solubility, and diffusivity of the aqueous alkanolamine solutions were also measured. The N₂O analogy was applied to estimate the solubilities and diffusivities of CO₂ in aqueous amine systems. Based on the pseudo-first-order for the CO₂ absorption, the overall pseudo first-order reaction rate constants were determined from the kinetic measurements. For CO₂ absorption into aqueous PZ solutions, the obtained second-order reaction rate constants for the reaction of CO₂ with PZ are in a good agreement with the results of Bishnoi and Rochelle (Chem. Eng. Sci. 55 (2000) 5531). For CO₂ absorption into mixed aqueous solutions of AMP and PZ, it was found that the addition of small amounts of PZ to aqueous AMP solutions has significant effect on the enhancement of the CO₂ absorption rate. For the CO₂ absorption reaction rate model, a hybrid reaction rate model, a second-order reaction for the reaction of CO₂ with PZ and a zwitterion mechanism for the reaction of CO₂ with AMP was used to model the kinetic data. The overall absolute percentage deviation for the calculation of the apparent rate constant k_app is 7.7% for the kinetics data measured. The model is satisfactory to represent the CO₂ absorption into mixed aqueous solutions of AMP and PZ.     

CO2 Absorption into Aqueous Solutions of a Polyamine (PZEA), a Sterically Hindered Amine (AMP), and their Blends

Among numerous techniques existing for reducing CO₂ emissions, CO₂ capture by absorption in aqueous alkanolamine solutions was specifically studied in this work. For the choice of the adequate amine solution, two major criteria must be taken into account: absorption performances (higher with primary and secondary amines) and energy costs for solvent regeneration (more interesting with tertiary and sterically hindered amines). The different types of amines can also be mixed in order to combine the specific advantages of each type of amines, an activation phenomenon being observed. Aqueous solutions of (piperazinyl‐1)‐2‐ethylamine (PZEA, a polyamine known as absorption activator) and 1‐amino‐2‐propanol (AMP, a sterically hindered amine), pure or mixed with other amines, are experimentally compared with respect to CO₂ removal performances by means of absorption test runs achieved in a special gas‐liquid contactor at 25 °C. The positive impact of addition of PZEA to monoethanolamine (MEA), N‐methyldiethanolamine (MDEA), and AMP solutions was clearly highlighted. The absorption performances have also been satisfactorily simulated with coherent physicochemical data.     

Effect of Regeneration Temperature on the Composition and Carbon Dioxide Sorption Ability of a K2CO3/Al2O3 Solid Sorbent in a Bubbling Fluidized Bed Reactor

The effect of the regeneration temperature (150°, 250°, and 350°C) during multiple CO₂ cyclic sorption-regeneration cycles of a K₂CO₃/Al₂O₃ solid sorbent in a bubbling fluidized bed reactor was evaluated in terms of the CO₂ capture capacity and chemical composition of the solid sorbent. The CO₂ capture capacity after regeneration at 150° and 250°C decreased with increasing cycle numbers, reaching approximately 57 and 78%, respectively, and 19.0 and 39.3%, respectively, of the original capacity after one and five regeneration cycles. This decline in the CO₂ capture capacity was due to the accumulation of KHCO₃ (at 150°C) and KAl(CO₃)₂(OH)₂ (150° and 250°C) from their incomplete degradation back to the K₂CO₃/Al₂O₃ solid sorbent. When regenerated at 350°C, the CO₂ capture capacity remained essentially constant in each cycle number because of complete desorption (no residual KHCO₃ and KAl(CO₃)₂(OH)₂). The formation mechanism of complex structure occurred similar to the one in a fixed bed reactor/thermogravimetric analyzer with lower regeneration temperature. The general operation conditions for K₂CO₃/Al₂O₃ solid sorbents are summarized.     

Novel Thermomorphic Biphasic Amine Solvents for CO2 Absorption and Low‐Temperature Extractive Regeneration

The key challenge in amine‐based 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 behavior, which intensifies desorption at temperatures well below the boiling point of aqueous solutions. The low regeneration temperature of less than 80 °C together with the high cyclic CO₂ loading capacity of ～0.9 mol mol_absorbent^–1 of such thermomorphic biphasic solvent (TBS) systems permit the use of low‐value and even waste heat at temperatures below 100 °C for desorption. In order to reduce the exergy demands still further, techniques for enhancing CO₂ release using extractive regeneration with inert solvent addition were investigated at temperatures of 40–50 °C to permit the use of waste heat at temperatures of ～70 °C or less. The regeneration of 3 M absorbed lipophilic amine solutions with 0.93 mol mol^–1 CO₂ loading by using various additional solvents and the evaluation of the extractive performance in multiple‐stage crosscurrent and countercurrent configurations indicated the promise of adding certain hydrophobic solvents to enhance the low‐temperature regeneration of TBS absorbents.     

Reaction kinetics of 2‐((2‐aminoethyl) amino) ethanol in aqueous and non‐aqueous solutions using the stopped‐flow technique

Observed pseudo‐first‐order rate constants (k_o) for the reaction between CO₂ and 2‐((2‐aminoethyl) amino) ethanol (AEEA) were measured using the stopped‐flow technique in an aqueous system at 298, 303, 308 and 313 K, and in non‐aqueous systems of methanol and ethanol at 293, 298, 303 and 308 K. Alkanolamine concentrations ranged from 9.93 to 80.29 mol m^?3 for the aqueous system, 29.99–88.3 mol m^?3 for methanol and 44.17–99.28 mol m^?3 for ethanol. Experimentally obtained rate constants were correlated with two mechanisms. For both the aqueous‐ and non‐aqueous‐AEEA systems, the zwitterion mechanism with a fast deprotonation step correlated the data well as assessed by the reported statistical analysis. As expected, the reaction rate of CO₂ in the aqueous‐AEEA system was found to be much faster than in methanol or ethanol. Compared to other promising amines and diamines studied using the stopped‐flow apparatus, the pseudo‐first‐order reaction rate constants were found to obey the following order: PZ (cyclic‐diamine) > EDA (diamine) > AEEA (diamine) > 3‐AP (primary amine) > MEA (primary amine) > EEA (primary amine) > MO (cyclic‐amine). The reaction rate constant of CO₂ in aqueous‐AEEA was double that in aqueous‐MEA, and the difference increased with an increase in concentration. All reaction orders were practically unity. With a higher capacity for carbon dioxide and a higher reaction rate, AEEA could have been a good substitute to MEA if not for its high thermal degradation. AEEA kinetic behaviour is still of interest as a degradation product of MEA. © 2012 Canadian Society for Chemical Engineering     

CO2 capture into aqueous solutions of piperazine activated 2-amino-2-methyl-1-propanol

In this work, experimental data and a simplified vapor–liquid equilibrium (VLE) model for the absorption of CO₂ into aqueous solutions of piperazine (PZ) activated 2-amino-2-methyl-1-propanol (AMP) are reported. The purpose of the work was to find the AMP/PZ system with the highest concentration and cyclic capacity, which could be used in the industry without forming solid precipitations at operational temperatures. The effect of the AMP/PZ ratio and the total concentration level of amine was studied. The highest possible ratio of AMP/PZ, which does not form solid precipitates during the absorption of CO₂ at 40 °C (40 wt% amine), was identified. Considering the maximum loading found in the screening tests for AMP/PZ (3+1.5 M) and for 30 wt% MEA systems, the AMP/PZ system has about 128% higher specific cyclic capacity if operating between 40 and 80 °C, and almost twice the CO₂ partial pressure at 120 °C compared to MEA.     

Absorption of carbon dioxide by the absorbent composed of piperazine and 2-amino-2-methyl-1-propanol in PVDF membrane contactor

This paper tests the performance of microporous polyvinylidinefluoride (PVDF) hollow fiber in a gas absorption membrane process (GAM) using the aqueous solutions of piperazine (PZ) and 2-amino-2-methyl-1-propanol (AMP). Experiments were conducted at various gas flow rates, liquid flow rates and absorbent concentrations. Experimental results showed that wetting ratio was about 0.036% when used with the aqueous alkanolamine solutions, while that was 0.39% with aqueous piperazine solutions. The CO₂ absorption rates increased with increasing both liquid and gas flow rates at N_Re < 20. The increase of the PZ concentration showed an increase of absorption rate of CO₂. The CO₂ absorption rate was much enhanced by the addition of PZ promoter. The resistance of membrane was predominated as using a low reactivity absorbent and can be neglected as using absorbent of AMP aqueous solution. The resistance of gas-film diffusion was dominated as using the mixed absorbents of AMP and PZ. An increase of PZ concentration, the resistance of liquid-film diffusion decreased but resistance of gas-film increased. Overall, GAM systems were shown to be an effective technology for absorbing CO₂ from simulated flue gas streams, but the viscosity and solvent-membrane relationship were critical factors that can significantly affect system performance.     

Characteristics of absorption and regeneration of carbon dioxide in aqueous 2-amino-2-methyl-1-propanol/ammonia solutions

In this study, the removal efficiency, absorption amount, and loading value of CO₂ into aqueous blended 2-amino-2-methyl-1-propanol (AMP)/ammonia (NH₃) solutions were measured by using the absorption and regeneration continual process. The effect of adding NH₃ to enhance absorption characteristics of AMP was investigated. The performance was evaluated under various operating conditions. As a result, the method of blending AMP and NH₃ was not adequate because of a problem with scale formation. Consequently, NH₃ of 1, 3, 5, and 7 wt% was added to 30 wt% AMP. Of these additions, 5 wt% NH₃ was the optimum concentration because the CO₂ removal efficiency and absorption amount were almost 100% and 2.17 kg CO₂/kg absorbent, respectively. Also, the scale problem was almost absent. As the regenerator temperature varied from 80–110 °C, the loading of rich amine was almost constant, but the loading of lean amine was decreased as the regenerator temperature increased. Thus, the optimum regenerator temperature was 110 °C in this experiment.     

Electrochemical conversion of carbon dioxide to methane in aqueous NaHCO3 solution at less than 273 K

The electrochemical reduction of CO₂ on a Cu electrode was investigated in aqueous NaHCO₃ solution, at low temperature. A divided H-type cell was employed, the catholyte was 0.65 mol dm⁻³ NaHCO₃ aqueous solution and the anolyte was 1.1 mol dm⁻³ KHCO₃ aqueous solution. The temperature during the electrolysis of CO₂ was decreased stepwise to 271 K. Methane and formic acid were obtained as the main products. The maximum Faradaic efficiency of methane was 46% at −2.0 V and 271 K. The efficiency of hydrogen formation, a competing reaction of CO₂ reduction, was significantly depressed with decreasing temperature. Based on the results of this work, the proposed electrochemical method appears to be a viable means for removing CO₂ from the atmosphere and converting it into more valuable chemicals. The synthesis of methane by the electrochemical method might be of practical interest for fuel production and the storage of solar energy.     

Solvent‐free synthesis of glycerol carbonate and glycidol from 3‐chloro‐1,2‐propanediol and potassium (hydrogen) carbonate

BACKGROUND: An indirect solvent‐free synthetic approach for obtaining glycerol carbonate and glycidol from glycerol and CO₂ through their more reactive and easily synthesizable derivatives 3‐chloro‐1,2‐propanediol (HAL) and potassium (hydrogen) carbonate has been studied. RESULTS: The reaction is fast with source of carbonation and temperature having a strong influence on the results. A yield of 80% glycerol carbonate together with a simultaneous substantial production of glycidol (0.56 mol mol^?1 glycerol carbonate) are obtained using K₂CO₃ as the carbonation source at 80 °C, a reaction time of 30 min and a 3:1 HAL/K₂CO₃ molar ratio. A lower yield of glycerol carbonate (60%) is obtained from KHCO₃ after 50 min with the other experimental conditions remaining unchanged. In this case, glycidol formation is zero or insignificant. Glycerol is also obtained in high yields, although in much lower amounts from KHCO₃ (～0.59 mol mol^?1 glycerol carbonate independent of operating conditions) than from K₂CO₃ (0.84–1.1 mol mol^?1 glycerol carbonate, depending on experimental conditions). CONCLUSIONS: The proposed synthetic strategy overcomes the currently difficult direct reaction between glycerol and CO₂, leading to the simultaneous synthesis of two valuable chemicals: glycerol carbonate and glycidol. However, glycerol is also obtained in substantial amounts thus decreasing the overall yield of the process. Thus, methods for preventing its formation must be developed for industrial feasibility. Copyright © 2010 Society of Chemical Industry     

Kinetics of Carbon Dioxide Binding by Promoted Organic Liquids

The reaction kinetics of carbon dioxide and CO₂‐binding organic liquids (CO₂BOLs) promoted by piperazine (PZ) and its derivatives were investigated experimentally by stopped‐flow conductimetry. The study was carried out at 298 K and for a concentration range of 0–0.25 kmol m^?3 of PZ, 1‐(2‐aminoethyl)piperazine (AEPZ) or 1‐(2‐hydroxyethyl)piperazine (NHEPZ) while the superbase (1,1,3,3‐tetramethylguanidine or 1,8‐diazabicyclo[5.4.0]undec‐7‐ene) concentrations were kept constant at 10.0 wt %. Based on pseudo‐first‐order reaction conditions, the intrinsic reaction rate data were analyzed according to a modified termolecular reaction mechanism. The results showed that the rate of reaction between CO₂BOLs and CO₂ could be significantly enhanced by blending with small amounts of PZ, NHEPZ or AEPZ as promoters.     

Solubility of CO2 in Aqueous Piperazine and its Modeling using the Kent‐Eisenberg Approach

A systematic investigation of the equilibrium solubility of CO₂ in aqueous piperazine solutions was conducted in a double‐jacketed stirred cell reactor. The solubilities of CO₂ in the solution were measured at 20, 30, 40, and 50 °C with CO₂ partial pressures ranging from 0.4–95 kPa. Generally the aqueous piperazine solution exhibits the same characteristics as conventional alkanolamines. Increasing the CO₂ partial pressure increases the gas loading, however increasing the temperature or concentration decreases the CO₂ loading. The values of the CO₂ loading obtained confirm that the piperazine forms stable carbamates. The equilibrium solubility data were analyzed using a Kent‐Eisenberg approach. Representation of the model is generally in good agreement with that of the experimental data, especially at high temperature.     

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.     

Absorption of carbon dioxide into aqueous solutions of piperazine activated 2-amino-2-methyl-1-propanol

In this work, new experimental data on the rate of absorption of CO₂ into piperazine (PZ) activated aqueous solutions of 2-amino-2-methyl-1-propanol (AMP) are reported. The absorption experiments using a wetted wall contactor have been carried out over the temperature range of 298-313 K and CO₂ partial pressure range of 2-14 kPa. PZ is used as a rate activator with a concentration ranging from 2 to 8 wt%, keeping the total amine concentration in the solution at 30 wt%. The CO₂ absorption into the aqueous amine solutions is described by a combined mass transfer-reaction kinetics-equilibrium model, developed according to Higbie's penetration theory. Parametric sensitivity analysis is done to determine the effects of possible errors in the model parameters on the accuracy of the calculated CO₂ absorption rates from the model. The model predictions have been found to be in good agreement with the experimental results of rates of absorption of CO₂ into aqueous (PZ+AMP). The good agreement between the model predicted rates and enhancement factors and the experimental results indicates that the combined mass transfer-reaction kinetics-equilibrium model with the appropriate use of model parameters can effectively represent CO₂ mass transfer in PZ activated aqueous AMP solutions.     

CO2 Absorption into Aqueous Solutions of Monoethanolamine,Methyldiethanolamine, Piperazine and their Blends

The removal of carbon dioxide from industrial gases, e.g. in thermal power stations to meet the discharge limits for CO₂ in flue gases, is usually achieved with a reactive absorption technique using aqueous solutions of alkanolamines. From the absorption performance point of view, primary and secondary amines are preferred. However, in case the costs of the solvent regeneration are also taken into account, tertiary amines are much more attractive. In order to combine the specific advantages of tertiary and primary/secondary alkanolamines, both types of solvents are mixed. In this paper, mixtures of monoethanolamine and methyldiethanolamine with piperazine as absorption activator are experimentally compared with respect to CO₂ removal performances at 25 °C. The absorption process in a special packed column has also been simulated with the use of published data on reaction kinetics, physicochemical properties (densities, viscosities, diffusivities, Henry coefficients) of the CO₂‐amines systems, including experimentally determined hydrodynamic and mass transfer characteristics of the CO₂ scrubber.     

The development of kinetics model for CO2 absorption into tertiary amines containing carbonic anhydrase

CO₂ absorption into aqueous solutions of two tertiary alkanolamines, namely, MDEA and DMEA with and without carbonic anhydrase (CA) was investigated with the use of the stopped‐flow technique at temperatures in the range of 293–313 K, CA concentration varying from 0 to 100 g/m³ in aqueous MDEA solution with the amine concentration ranging from 0.1 to 0.5 kmol/m³, and CA concentration varying from 0 to 40 g/m³ in aqueous DMEA solution with the amine concentration ranging from 0.05 to 0.25 kmol/m³. The results show that the pseudofirst‐order reaction rate (k_{0, amine}; s^?1) is significantly enhanced in the presence of CA as compared with that without CA. The enhanced values of the kinetic constant in the presence of CA has been calculated and a new kinetics model for reaction of CO₂ absorption into aqueous tertiary alkanolamine solutions catalyzed by CA has been established and used to make comparisons of experimental and calculated pseudo first‐order reaction rate constant (k_{0, with CA}) in CO₂‐MDEA‐H₂O and CO₂‐DMEA‐H₂O solutions. The AADs were 15.21 and 15.17%, respectively. The effect of pKa on the CA activities has also been studied by comparison of CA activities in different tertiary amine solutions, namely, TEA, MDEA, DMEA, and DEEA. The pKa trend for amines were: DEEA > DMEA > MDEA > TEA. In contrast, the catalyst enhancement in amines was in the order: TEA> MDEA> DMEA> DEEA. Therefore, it can be seen that the catalyst enhancement in the amines decreased with their increasing pKa values. © 2017 American Institute of Chemical Engineers AIChE J, 2017     

Catalytic synthesis of toluene‐2,4‐diisocyanate from dimethyl carbonate

The process for catalytic synthesis of toluene‐2,4‐diisocyanate (TDI) from dimethyl carbonate (DMC) consists of two steps. Starting from the catalytic reaction between toluene‐2,4‐diamine (TDA) and DMC, dimethyl toluene‐2,4‐dicarbamate (TDC) is formed, and then decomposed to TDI. For the first step, the yield of TDC is 53.5% at a temperature of 250 °C, over Zn(OAc)₂/α–Al₂O₃ catalyst. For the second step, the yield of TDI is 92.6% at temperatures of 250–270 °C and under pressure of 2.7 kPa, over uranyl zinc acetate catalyst, when di‐n‐octyl sebacate(DOS) is used as heat‐carrier, and a mixture of tetrahydrofuran (THF) and nitrobenzene is used as solvent. © 2001 Society of Chemical Industry     

A new model for correlation and prediction of equilibrium CO2 solubility in N‐methyl‐4‐piperidinol solvent

In this work, the equilibrium CO₂ solubility in the aqueous tertiary amine, N‐methyl‐4‐piperidinol (MPDL) was measured over a range of temperatures, CO₂ partial pressures and amine concentrations. The dissociation constant of the MPDL solution was determined as well. A new thermodynamic model was developed to predict the equilibrium CO₂ solubility in the MPDL‐H₂O‐CO₂ system. This model, equipped with the correction factor (C_f), can give reasonable prediction with an average absolute deviation of 2.0%, and performs better than other models (i.e., KE model, Li‐Shen model, and Hu‐Chakma). The second‐order reaction rate constant (k₂) of MPDL and the heat of CO₂ absorption (–ΔH_abs) into aqueous MPDL solutions were evaluated as well. Based on the comparison with some conventional amines, MPDL revealed a high‐equilibrium CO₂ loading, reasonably fast absorption rate when compared with other tertiary amines, and a low energy requirement for regeneration. It may, therefore, be considered to be an alternative solvent for CO₂ capture. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3395–3403, 2017