New absorption liquids for the removal of CO2 from dilute gas streams using membrane contactors

A new absorption liquid based on amino acid salts has been studied for CO₂ removal in membrane gas-liquid contactors. Unlike conventional gas treating solvents like aqueous alkanolamines solutions, the new absorption liquid does not wet polyolefin microporous membranes. The wetting characteristics of aqueous alkanolamines and amino acid salt solutions for a hydrophobic membrane was studied by measuring the surface tension of the liquid and the breakthrough pressure of the liquid into the pores of the membrane. The dependence of the breakthrough pressure on surface tension follows the Laplace-Young equation. The performance of the new absorption liquid in the removal of CO₂ was studied in a single fiber membrane contactor over a wide range of partial pressures of CO₂ in the gas phase and amino acid salt concentrations in the liquid. A numerical model to describe the mass transfer accompanied by multiple chemical reactions occurring during the absorption of CO₂ in the liquid flowing through the hollow fiber was developed. The numerical model gives a good prediction of the CO₂ absorption flux across the membrane for the absorption of CO₂ in the aqueous amino acid salt solutions flowing through the hollow fiber.     

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.     

CO2 removal by single and mixed amines in a hollow‐fiber membrane module—investigation of contactor performance

This work investigates CO₂ removal by single and blended amines in a hollow‐fiber membrane contactor (HFMC) under gas‐filled and partially liquid‐filled membrane pores conditions via a two‐scale, nonisothermal, steady‐state model accounting for CO₂ diffusion in gas‐filled pores, CO₂ and amines diffusion/reaction within liquid‐filled pores and CO₂ and amines diffusion/reaction in liquid boundary layer. Model predictions were compared with CO₂ absorption data under various experimental conditions. The model was used to analyze the effects of liquid and gas velocity, CO₂ partial pressure, single (primary, secondary, tertiary, and sterically hindered alkanolamines) and mixed amines solution type, membrane wetting, and cocurrent/countercurrent flow orientation on the HFMC performance. An insignificant difference between the absorption in cocurrent and countercurrent flow was observed in this study. The membrane wetting decreases significantly the performance of hollow‐fiber membrane module. The nonisothermal simulations reveal that the hollow‐fiber membrane module operation can be considered as nearly isothermal. © 2014 American Institute of Chemical Engineers AIChE J, 61: 955–971, 2015     

Simultaneous Absorption of H2S and Co2 Into Aqueous Methyldiethanolamine

Abstract

The tertiary araine methyldiethanolamine (MDEA) is finding increasing application as a chemical solvent for selective absorption of hydrogen sulfide from gases containing hydrogen sulfide and carbon dioxide. Gas streams of this type include some natural gases, synthetic gases from coal and heavy oil gasification and tail gases from sulfur plants. Selectivity for H2S is needed either to enrich Glaus sulfur plant feed in H2S or to remove only H₂S when CO₂ removal is not necessary or economic. For the absorption of hydrogen sulfide into MDEA, the reaction which occurs can be considered to be instantaneous while carbon dioxide undergoes a second-order reaction with MDEA.

In this work, the simultaneous absorption of two gases into a liquid containing a reactant with which both gases react is modelled using the film theory. Physical properties and kinetic rate parameters used in the model have been measured in our laboratory. The model is used to study the effect of process variables on the selectivity of MDEA for H₂S over C0₂. The simultaneous absorption of H₂S and CO₂ gases into aqueous MDEA is studied experimentally using a continuous stirred tank absorber. Experimental absorption rates are compared to predictions based on a multicomponent mass transfer model. The average deviations of the theoretical calculations from the experimental results are 10.2% and 12.9% for C0₂ and H₂S, respectively.     

Rate-based modelling of SO2 absorption into aqueous NaHCO3/Na2CO3 solutions accompanied by the desorption of CO2

A rate-based model of a counter-current reactive absorption/desorption process has been developed for the absorption of SO₂ into NaHCO₃/Na₂CO₃ in a packed column. The model adopts the film theory, includes diffusion and reaction processes, and assumes that thermodynamic equilibrium among the reacting species exists in the bulk liquid. Model predictions were compared to experimental data from literature. For the calculation of the absorption rate of SO₂ into NaHCO₃/Na₂CO₃ solutions and concomitant CO₂-desorption, it is important to take into account all reversible reactions simultaneously. It is clear that the approximate analytical based model cannot be expected to predict the absorption rates under practical conditions because of the complicated nature of the reactive absorption processes. The rigorous numerical approach described here only requires definition of the individual reactions in the system, and subsequent solution is independent of specific assumptions made, or operational variables like pH or compound concentrations. As an example of the flexibility of this approach, additional calculations were conducted for SO₂ absorption in a phosphate-based buffer system.     

Selective absorption of hydrogen sulphide in tertiary amine solutions

An approximate model for the rate of simultaneous absorption of H₂S and CO₂ into aqueous solutions of a tertiary amine is presented, and the model is successfully compared with experimental results of simultaneous absorption. An effect which is not taken into account in the model is shown to become possibly important when the liquid phase is almost a solution of only free amine, and an approximate method of taking such an effect into account is presented. The procedure discussed will be used in a forthcoming work for developing a methodology of designing the required height for a packed absorber for selective removal of H₂S.     

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.     

Simultaneous absorption of carbon dioxide and hydrogen sulfide into aqueous blends of 2-amino-2-methyl-1-propanol and diethanolamine

This work presents an experimental and theoretical investigation of the simultaneous absorption of CO₂ and H₂S into aqueous blends of 2-amino-2-methyl-1-propanol (AMP) and diethanolamine (DEA). The effect of contact time, temperature and amine concentration on the rate of absorption and the selectivity were studied by absorption experiments in a wetted wall column at atmospheric pressure and constant feed gas ratio. The diffusion-reaction processes for CO₂ and H₂S mass transfer in blended amines are modeled according to Higbie's penetration theory with the assumption that all reactions are reversible. The blended amine solvent (AMP+DEA+H₂O) has been found to be an efficient mixed solvent for simultaneous absorption of CO₂ and H₂S. By varying the relative amounts of AMP and DEA the blended amine solvent can be used as an H₂S-selective solvent or an efficient solvent for total removal of CO₂ and H₂S from the gas streams. Predicted results, based on the kinetics-equilibrium-mass transfer coupled model developed in this work, are found to be in good agreement with the experimental results of rates of absorption of CO₂ and H₂S into (AMP+DEA+H₂O) of this work.     

Untersuchung der CO2‐Absorptionskinetik in wässrigen Lösungen von N,N‐Diethylethanolanmin und N‐Ethylethanolamin

N‐Ethylethanolamine (EEA) and N,N‐diethylethanolamine (DEEA) represent promising alkanolamines for CO₂ removal from gaseous streams, as they can be prepared from renewable resources. In this work, the reaction rate constant for the reaction between CO₂ and EEA and the liquid‐side mass transfer coefficient were determined from the absorption rate measurements in a blend comprising DEEA, EEA and H₂O. A stirred‐cell reactor was applied for the absorption studies, whereas a zwitterion mechanism for EEA and a base‐catalyzed hydration mechanism for DEEA were used to describe the reaction kinetics.     

Absorption of carbon dioxide into aqueous blends of 2-amino-2-methyl-1-propanol and diethanolamine

This work presents an experimental and theoretical investigation of CO₂ absorption into aqueous blends of 2-amino-2-methyl-1-propanol (AMP) and diethanolamine (DEA). The CO₂ absorption into the amine blends is described by a combined mass transfer-reaction kinetics-equilibrium model, developed according to Higbie's penetration theory. The model predictions have been found to be in good agreement with the experimental rates of absorption of CO₂ into (AMP+DEA+H₂O). The good agreement between the model predicted rates and enhancement factors and the experimental results indicate that the combined mass transfer-reaction kinetics-equilibrium model with the appropriate use of model parameters can effectively represent CO₂ mass transfer for the aqueous amine blends AMP/DEA.     

Comprehensive reaction kinetics model of CO2 absorption into 1-dimethylamino-2-propanol solution

In the present work, the kinetics of the reactive absorption of CO₂ in 1-dimethylamino-2-propanol (1DMA2P) solution were experimentally measured using a laminar jet absorber over a temperature range of 298–313 K, 1DMA2P concentration range of 0.5–2.0 mol/L, and CO₂ loading range of 0–0.06 mol CO₂/mol amine. The measured kinetics data were then used to develop a comprehensive numerical kinetics model using a FEM-based COMSOL software. The reaction rate model of the CO₂ absorption into 1DMA2P solution were then validated by comparing model rates with the experimental rates. An excellent agreement of model data with experimental data was achieved with an absolute average deviation (AAD) of 6.5%. In addition, vapor–liquid equilibrium plots of all ions in the 1DMA2P-H₂O-CO₂ system were also developed. Further, this work has provided an effective criterion for evaluating CO₂ absorption, that can be used for both the conventional amines and alternative amines for the purpose of providing guidelines or information on how to effectively screen solvents.     

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.     

A study on the reaction between CO2 and alkanolamines in aqueous solutions

Literature data on the rates of reaction between CO₂ and alkanolamines (MEA, DEA, DIPA, TEA and MDEA) in aqueous solution are discussed. These data induced us to carry out absorption experiments of CO₂ into aqueous DEA, DIPA, TEA and MDEA solutions from which the respective rate constantsThe results for DEA and DIPA were analysed by means of a zwitterion-mechanism which was derived from the mechanism originally proposed by Danckwerts [1The reaction rate of CO₂ with aqueous TEA and MDEA solutions shows a significant base catalysis effect which is also reported by Donaldson and Nguy     

Mathematical modelling of mass-transfer and hydrodynamics in CO2 absorbers packed with structured packings

This paper presents a mechanistic model that can predict mass-transfer performance and provide an insight into dynamic behavior within structured packings used for CO₂ absorption. The model was built upon the kinetics and thermodynamics of the absorption system, as well as the liquid irrigation features and the geometry of packing elements. A computer program (Fortran 90) was written to simulate CO₂ absorption into aqueous solutions of sodium hydroxide (NaOH) and monoethanolamine (MEA) in a column packed with Gempak 4A, Mellapak 500Y and Mellapak 500X. The simulation gave essential information, including the concentration of CO₂ in gas-phase, concentration of reactive species in the liquid-phase, system temperature, mass-transfer coefficients (k_G and k_L), and effective interfacial area (a_e) for mass-transfer at different axial positions along the absorption column. The simulation also provided liquid distribution plots representing the quality of liquid distribution or maldistribution across the cross-section of the column. Verification of the model was achieved by comparing simulation results with experimental data. Very good agreement was found for wide ranges of operating and design parameters, including liquid load and initial liquid distribution pattern.     

A study on the reaction between CO2 and alkanolamines in aqueous solutions

Literature data on the rates of reaction between CO₂ and alkanolamines (MEA, DEA, DIPA, TEA and MDEA) in aqueous solution are discussed. These data induced us to carry out absorption experiments of CO₂ into aqueous DEA, DIPA, TEA and MDEA solutions from which the respective rate constants were derived. The experimental technique was similar to that used by Laddha and Danckwerts[30].The results for DEA and DIPA were analysed by means of a zwitterion-mechanism which was derived from the mechanism originally proposed by Danckwerts[16The reaction rate of CO₂ with aqueous TEA and MDEA solutions shows a significant base catalysis effect which is also reported by Donaldson and Nguy     

A Study on CO2 Absorption Kinetics by Aqueous Solutions of N,N‐Diethylethanolamine and N‐Ethylethanolamine

N‐Ethylethanolamine (EEA) and N,N‐diethylethanolamine (DEEA) represent promising candidate alkanolamines for CO₂ removal from gaseous streams, as they can be prepared from renewable resources. In this work, the reaction rate constant for the reaction between CO₂ and EEA was determined from the absorption rate measurements of CO₂ in a blend comprising DEEA, EEA and H₂O. A stirred‐cell reactor with a plane, horizontal gas‐liquid interface was used for the absorption studies. While the DEEA concentration in the formulated solution was varied in the range of 1.5–2.5 kmol/m³, the initial EEA concentration was 0.1 kmol/m³. A zwitterion mechanism for EEA and a base‐catalyzed hydration mechanism for DEEA were used to describe the reaction kinetics. At 303 K, the second‐order reaction rate constant for the CO₂ reaction with EEA was found to be 8041 m³/(kmol s). The liquid‐side mass transfer coefficient was also estimated, and its value (0.004 cm/s) is in line with those typical of stirred‐cell reactors.     

Mass transfer investigation and operational sensitivity analysis of amine-based industrial CO2 capture plant

In this article, the industrial process of CO₂ capture using monoethanolamine as an aqueous solvent was probed carefully from the mass transfer viewpoint. The simulation of this process was done using Rate-Base model, based on two-film theory. The results were validated against real plant data. Compared to the operational unit, the error of calculating absorption percentage and CO₂ loading was estimated around 2%. The liquid temperature profiles calculated by the model agree well with the real temperature along the absorption tower, emphasizing the accuracy of this model. Operational sensitivity analysis of absorption tower was also done with the aim of determining sensitive parameters for the optimized design of absorption tower and optimized operational conditions. Hence, the sensitivity analysis was done for the flow rate of gas, the flow rate of solvent, flue gas temperature, inlet solvent temperature, CO₂ concentration in the flue gas, loading of inlet solvent, and MEA concentration in the solvent. CO₂ absorption percentage, the profile of loading, liquid temperature profile and finally profile of CO₂ mole fraction in gas phase along the absorption tower were studied. To elaborate mass transfer phenomena, enhancement factor, interfacial area, molar flux and liquid hold up were probed. The results show that regarding the CO₂ absorption, the most important parameter was the gas flow rate. Comparing liquid temperature profiles showed that the most important parameter affecting the temperature of the rich solvent was MEA concentration.     

Numerical simulation of CO2 absorption into aqueous methyldiethanolamine solutions

The CO₂ absorption rate into aqueous N-methyldiethanolamine solutions was measured using a stirred cell with a flat gas-liquid interface. The measurements were performed in the temperature range of 293.15 to 333.15 K for various amine concentrations and CO₂ partial pressures. A numerical model of mass-transfer with complex chemical reactions based on the film theory was developed to interpret the experimental results. The model predictions have been found to be in good agreement with the experimental values of CO₂ absorption rates. A comparison is made between the enhancement factor predicted from the detailed model and the approximate solution of mass transfer equations with chemical reaction. The numerical results indicate that under the present experimental conditions, the effect of the reaction between CO₂ and OH^? on the observed mass transfer rates is negligible. The detailed mass transfer model was used for simulating the CO₂ absorption process in terms of the enhancement factor under a variety of operating conditions.     

Absorption of co2 into aqueous tertiary amine/mea solutions

Absorption rates for CO₂ into aqueous solutions of TEA, MDEA and blends of MEA with MDEA and TEA were measured in a stirred cell by a method similar to that used by Laddha and Danckwerts (1981). Second order rate constants for CO₂-TEA and CO₂-MDEA were obtained from the single amine data for temperatures in the range of 25-60°C. A modified pseudo first order model based on the film theory is used to predict the rate of absorption of CO₂ into mixed amine solutions. This model accounts for the variation of amine concentration in the film and assumes a shuttle mechanism for rate enhancement. Bulk liquid concentrations of the various species present are obtained from a simplified thermodynamic model. The model predicts absorption rates that are in agreement with experimental measurements.     

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.