Determination of mass transfer resistance during absorption of carbon dioxide by mixed absorbents in PVDF and PP membrane contactor

In this study, the absorption of carbon dioxide by the absorbent which was composed of 2-amino-2-methyl-l-propanol (AMP) + piperazine (PZ) or methyldiethanolamine (MDEA) + piperazine (PZ) in polyvinylidinefluoride (PVDF) and polypropylene (PP) membrane contactors werewas examined. Three resistances were considered in each hollow fiber, i.e., liquid-film diffusion, membrane diffusion, and gas-film diffusion. The mass transfer resistance of membrane k_m was influenced by the wetting ratio using an absorbent with higher reaction rate. The wetting ratio was affected by contact angle between the membrane and absorbent and the viscosity of absorbent. The calculated absorption rates considering wetting ratio of membrane and using the modified correlation equation of gas-phase mass transfer coefficient were reasonably agreeable to those of measured ones (standard deviation, 4%). The fractional resistance of each transport step during the experiments was then determined. The rate-controlling step was dominated by the resistance of gas-film diffusion with mixed absorbents. The absorption rates of CO₂ increase with the increasing of gas flow rates in the most experimental cases. The resistance of liquid-film diffusion was only important using an absorbent with lower reaction rate. The rate-controlling step was the membrane diffusion only at higher gas flow rate with the absorbent composed of AMP and PZ in PVDF hollow fiber membrane contactor.     

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.     

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.     

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 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.     

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.     

CO2 Capture by Liquid Solvents and their Regeneration by Thermal Decomposition of the Solid Carbonated Derivatives

The chemical capture of CO₂ by either aqueous Na₂CO₃ and K₂CO₃ or nonaqueous solutions of the amines 2‐amino‐2‐methyl‐1‐propanol (AMP) or piperazine (PZ) is described. The captured CO₂ is stored as solid NaHCO₃, KHCO₃, and AMP or PZ carbamates. Solid NaHCO₃ and KHCO₃ are decomposed at 200 °C and 250 °C, respectively, to regenerate the carbonates for their reuse. In the experiments with AMP or PZ, the solid carbamates are decomposed at 80 °C–110 °C to regenerate the free amines. The absence of water in the desorption‐regeneration step is intriguing and could have the potential of reducing one of the major disadvantages of aqueous absorbents, namely the energy cost of the regeneration step and amine degradation, yet preserving the efficiency of the absorption in the liquid phase.     

Effects of piperazine concentration and operating conditions on the solubility of CO2 in AMP solution at low CO2 partial pressure

In this study, new equilibrium solubility data for carbon dioxide in aqueous solutions of 2-amino-2-methyl-1-propanol and piperazine (PZ) are provided. The two famous Deshmukh–Mather and Kent Eisenberg thermodynamic models are utilized to predict the CO₂ absorption. The experimental data show that the solubility of CO₂ decreases as the temperature increases. Our data suggest that the addition of PZ has different effects on CO₂ absorption under different partial pressure of the CO₂ in the gas stream. For high partial pressure, the addition of PZ promotes the absorption performance. However, at low CO₂ partial pressure, PZ addition results in less saturated CO₂ loading. The Deshmukh–Mather model can provide an accurate prediction of the experimental data at high partial pressure of CO₂ (i.e. AAD = 3.4%) whereas the modified Kent–Eisenberg model can capture the inverse effects of the PZ at low partial pressure and provides a relatively good approximation of experimental data at low partial pressure (i.e. AAD = 10%).     

Separation of CO2 by 2-amino-2-methyl-1-propanol aqueous solution in microporous hydrophobie hollow fiber contained liquid membrane

The absorption of CO₂ from a mixture of CO₂/N₂ gas was carried out using a flat-stirred vessel and the polytetrafluoroethylene hollow fiber contained aqueous 2-amino-2-methyl-1-propanol (AMP) solution. The reaction of CO₂ with AMP was confirmed to be a second order reversible reaction with fast-reaction region. The mass transfer resistance in the membrane side obtained from the comparison of the measured absorption rates of CO₂ in a hollow fiber contained liquid membrane with a flat-stirred vessel corresponded to about 90% of overall-mass-transfer resistance. The mass transfer coefficient of hollow fiber phase could be evaluated, which was independent of CO₂ loading.     

Carbon dioxide absorption in a cross-flow rotating packed bed

This work investigates the feasibility of applying the cross-flow rotating packed bed (RPB) to the removal of carbon dioxide (CO₂) by absorption from gaseous streams. Monoethanolamine (MEA) aqueous solution was used as the model absorbent. Also, other absorbents such as the NaOH and 2-amino-2-methyl-1-propanol (AMP) aqueous solutions were compared with the MEA aqueous solution. The CO₂ removal efficiency was observed as functions of rotor speed, gas flow rate, liquid flow rate, MEA concentration, and CO₂ concentration. Experimental results indicated that the rotor speed positively affects the CO₂ removal efficiency. Our results further demonstrated that the CO₂ removal efficiency increased with the liquid flow rate and the MEA concentration; however, decreased with the gas flow rate and the CO₂ concentration. Additionally, the CO₂ removal efficiency for the MEA aqueous solution was superior to that for the NaOH and AMP aqueous solutions. Based on the performance comparison with the conventional packed bed and the countercurrent-flow RPB, the cross-flow RPB is an effective absorber for CO₂ absorption process.     

ABSORPTION AND DESORPTION OF CO2 BY AQUEOUS SOLUTIONS OF STERICALLY HINDERED 2-AMINO-2-METHYL-1-PROPANOL IN HYDROPHOBIC MICROPOROUS HOLLOW FIBER CONTAINED CONTACTORS

To understand the behavior of separation of CO₂ from CO₂-N₂ mixtures using a hydrophobic microporous hollow fiber (polytetrafluoroethylene) contained gas-liquid contactor with aqueous solutions of 2-amino-2-methyl-l-propanol (AMP) as liquid media in the shell side, first, the absorption of dilute CO₂ into aqueous AMP solutions and the desorption of CO₂ from CO₂-loaded AMP solutions into N2 stream were investigated separately for various combinations of operational variables. Secondly, the simultaneous absorption and desorption in a single unit was performed to check the possibility of a long-term continuous operation. The resistance to diffusion in the hollow fiber phase during absorption amounted to ca. 86% of the total resistance, and slightly decreased with increasing AMP concentration. The AMP solution partially leaks into pores of the hollow fiber, and both the diffusion and chemical reaction of dissolved CO₂ in the liquid-filled pores under the slow-reaction regime mainly control the overall absorption rate. If the physical diffusion in the liquid-filled part of the pore completely controlled the absorption process in the present hollow fiber contactor, the length of the liquid-filled part would be evaluated to be 72 ~ 108 urn as compared to the total pore length of 500 um. The desorption rate was found to be independent of the gas velocity in the lumen side. The desorption process can be regarded as being controlled by diffusion and chemical reaction in both the stagnant film of the liquid phase and the liquid-filled pore of the hollow fiber phase under the slow or intermediate reaction regime. Simultaneous absorption and desorption process in a single contactor was found to be kept in a stable state at least until 20?h.     

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.     

填料塔中碳酸钾/哌嗪混合吸收液脱除CO2的体积传质系数

The process of removing dilute CO2 from air by using me mixtures of K2CO3 and piperazine (PZ) was conducted in a random packed tower at 25℃. The results showed that PZ increased the absorption rate of CO2 into aqueous K2 CO3 much more effectively than MEA or DEA. The volumetric overall mass transfer coefficient (KGa) of dilute CO2 absorption into K2CO3/PZ was measured. The KGa value was evaluated over the ranges of main operating variables: the concentration of CO2 in inlet gas, gas flow rate, liquid loading, CO2 loading in liquid phase, and the concentrations of K2CO3 and PZ. The test showed that KGa could be remarkably improved by increasing liquid loading and the concentration of PZ, and decreasing the concentration of CO2 in inlet gas, as well as the gas flow rate and CO2 loading in liquid phase.     

Acceleration of the reaction of carbon dioxide into aqueous 2-amino-2-hydroxymethyl-1,3-propanediol solutions by piperazine addition

In this work, the kinetics of the reaction between CO₂ and piperazine-activated aqueous solutions of a sterically hindered alkanolamine, 2-amino-2-hydroxymethyl-1,3-propanediol (AHPD) was studied in a wetted wall column contactor at 303.15, 313.15 and 323.15 K. The AHPD concentration in the aqueous solutions was kept at while the piperazine (PZ) concentration varied in the range . Under pseudo-first-order CO₂ absorption conditions, the overall pseudo-first-order rate constants were determined and reaction rate parameters were calculated with a non-linear regression from the overall reaction rate constant. The ratio of the diffusivity and Henry's law constant for CO₂ in solutions was estimated by applying the N₂O analogy and the Higbie penetration theory, using the physical absorption data of CO₂ and N₂O in water and of N₂O in amine solutions. Piperazine was found to be an effective activator in the aqueous AHPD solutions, as the addition of small amounts of PZ to these solutions has a significant effect on the enhancement of the CO₂ absorption rate for all studied temperatures.     

ABSORPTION OF CO2 INTO AQUEOUS SOLUTIONS OF STERICALLY HINDERED METHYL AMINOETHANOL USING A HYDROPHOBIC MICROPOROUS HOLLOW FIBER CONTAINED CONTACTOR

The absorption of dilute CO₂ into aqueous solutions of sterically hindered 2-methyl aminoethanol (MAE) and the desorption of CO₂ from CO₂-loaded MAE solutions into N₂ stream were investigated separately for the various combinations of operational variables, using a hydrophobic microporous hollow fiber (polytetrafluoroethylene, PTFE) contained gas-liquid contactor with aqueous solutions of MAE as liquid media in the shell side at 30°C. The absorption of CO₂ in this contactor is governed by resistance in the liquid and hollow fiber phases. The resistance to diffusion in the hollow fiber phase amounts to 76–80% of the total resistance. Nevertheless, the absorption rates of CO₂ into aqueous MAE solutions in this contactor are higher than those into aqueous solutions of sterically hindered 2-amino-2-methyl-1-propanol (AMP) in the stirred tank with a plane unbroken gas-liquid interface. The process of desorption of CO₂ from CO₂-loaded MAE solutions can be regarded as being controlled by diffusion and chemical reaction in both the stagnant film of the liquid phase and the liquid-filled pore of the hollow fiber phase under the slow or intermediate reaction regime. Both absorption and desorption rates under the simultaneous absorption-desorption operation in a single unit tend to approach the respective constant values as process time elapses. The total absorption rate here seems to be almost balanced with the total desorpion rate at the constant mass transfer rate periods.     

Removal of CO2 and/or SO2 from gas streams by a membrane absorption method

Studies were made on the membrane absorption of CO₂ and/or SO₂ using hydrophobic microporous hollow-fibre (HF) membrane modules. The absorbent liquids used were aqueous solutions of NaOH, K₂CO₃, alkanolamines and Na₂SO₃, flowing on the lumen side of the HF in laminar flow. A semi-empirical correlation was derived for the gas-phase mass-transfer coefficient on the shell side, by including geometrical factors of the HFs and the shell tube in the general correlation for mass transfer. It was found that the CO₂ absorption rate in various aqueous solutions of alkalis and alkanolamines is successfully described by a model based on gas diffusion through the membrane pores subsequent to gas absorption accompanied by chemical reaction. The simultaneous membrane absorption of SO₂ and CO₂ was also studied using aqueous Na₂SO₃ solution, the selective removal of SO₂ to CO₂ being successfully achieved when both the liquid flow rate and solute concentration are low. This suggests that this membrane absorption method provides an energy saving process for SO₂ removal from flue gases.     

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.     

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.     

Numerical investigation on CO2 absorbed in aqueous N‐methyldiethanolamine + piperazine

A multiphase and multicomponent mass transfer model of CO₂ absorbed in aqueous N‐methyldiethanolamine and piperazine (PZ) was built in the study. In the model, a simple method of mass transfer between phases was proposed. Besides, the hydrodynamics, thermodynamics, and complex reversible chemical reaction were considered simultaneously. The model was validated by comparing with the previous experimental data which showed that simulated results can represent the experimental data with reasonable accuracy. Based on the model, the effects of gas velocity, liquid load and CO₂ loading on the absorption rate, and enhancement factor were analyzed. Model results showed that the enhancement factor increased with a rising gas velocity while decreased with a rising liquid load or CO₂ loading. The change of enhancement factor with CO₂ loading was similar to that of equilibrium concentration of PZ which indicated that PZ was significant to the absorption process. Furthermore, the distributions of specie concentrations were discussed in detail. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2386–2393, 2017     

Enhancement of kinetic rates of CO2 in aqueous solutions of piperazine and methyldiethanolamine with addition of sulfolane

Measurements of kinetics rates of CO₂ in aqueous solutions of methyldiethanolamine (MDEA), piperazine (PZ), and mixtures of (MDEA + PZ), (PZ + sulfolane) and (MDEA + sulfolane) were carried out using the stopped flow technique, and reported in terms of pseudo-first-order rate constants (k₀). When possible, the second-order reaction rate constants (k₂) were regressed from the data. Experiments were performed over new concentration ranges of (10–60), (200–800), (200–800, 10–40), (10–40, 10–200), and (200–800, 10–200) mol/m³ for the above-mentioned five systems, respectively, and at temperatures varying from (298.15–313.15 K). When sulfolane was added to the amine solution, pseudo-first-order rate constants in the mixed solvents were higher than in aqueous MDEA and PZ solutions at all temperatures. The kinetic rates were highest at 298.15 K and decreased at higher temperatures for aqueous (MDEA + sulfolane) solutions but increased with temperature for aqueous (PZ + sulfolane) systems. Reaction orders for both PZ and MDEA were practically one at all sulfolane concentrations and temperatures. The base catalysis mechanism was used to regress very well data for aqueous MDEA and (MDEA + sulfolane + water) and the termolecular mechanism was used for (PZ + sulfolane + water) system. Both the zwitterion and termolecular models were able to fit the experimental data for the aqueous PZ system well. Finally, the termolecular and a hybrid model based on the combination of the Zwitterion and base catalysis mechanisms were able to successfully correlate the experimental data for the mixed aqueous (MDEA + PZ) systems.