ABSORPTION AND DESORPTION TO AND FROM LIQUID FILMS ON INCLINED PLANES

Two published theoretical models are examined and applied to experimental results for absorption and desorption. The system used was CO₂/H₂O and studies were made for liquid film flow down inclined planes. Experimental results give “Reduced” values of mass ransfer rates.

Interferometric studies give interfacial concentration, penetration and film depths, and take-up of carbon dioxide. In the case of desorption the interferograms are distorted by “deflections.”

All the experimental values for absorption and desorption differ from those calculated from theoretical models.

Desorption is not a mirror image of absorption, and it is approximately 75% of the transfer rate of absorption over a wide operating range.

A comparison is made of the behaviour of static pools and flowing liquid films.     

Cover Chem. Eng. Technol. 2/2015

CO₂ Capture in a Bubble‐Column Scrubber Bubble columns are widely used in industry, such as on operations of reaction, fermentation, crystallization, desorption, and absorption. They can be operated in batch, continuously, or in semi‐batch, as well as in two or three phases. With the advantages of easy operation, simple structure, high mass transfer efficiency, high absorption factor, and low energy consumption, bubble columns have attracted wide attention in the industry. In recent years, as the carbon dioxide capture, storage, and regeneration are urgent issues, CCS and CCU have been used as the key point to solve greenhouse effect. This plays a great role in CO₂ capture and storage in thermal power plants, in which the CCS capture and regeneration account for 70 % of the power generation cost. How to achieve effective capture and regeneration has become a topical subject in the energy saving and carbon reduction. Among various technologies of CO₂ capture, absorption is the most mature, and MEA is used most widely. Although the capture of acid gases is still dominated by filling towers, many recent studies have confirmed the advantages of bubble towers that prevail over filling towers or other appliances. Thus, bubble columns have been adopted as the absorber and MEA as the absorbent for the new attempt of CO₂ capture. The operation variables include CO₂ concentration, pH, temperature, air flow rate, available gas‐liquid flow rate ratio, absorption efficiency, absorption velocity, overall mass transfer coefficient, and absorption factor, which are the important parameters for the design and operation of absorber. This study adopts the Taguchi experiment design to obtain the priority of parameter type and the optimal parameters of bubble towers for CO₂ capture, so as to achieve energy saving and carbon reduction. DOI: 10.1002/ceat.201400240 CO₂ Capture Using Monoethanolamine in a Bubble‐Column Scrubber Pao‐Chi Chen*, Yi Xin Luo, Pao Wein Cai Chem. Eng. Technol. 2015 , 38 (2), 274–282.     

Kinetics of carbon dioxide absorption and desorption in aqueous alkanolamine solutions using a novel hemispherical contactor—I. Experimental apparatus and mathematical modeling

This two-parts paper summarizes the experimental and theoretical results of a comprehensive and first of its kind study on the kinetics of carbon dioxide (CO₂) absorption and desorption in and from aqueous solutions of monoethanolamine (MEA), diethanolamine (DEA), methyl-diethanolamine (MDEA) and 2-amino-2-methyl-1-propanol (AMP) and their mixtures (i.e., MEA+AMP, MEA+MDEA, DEA+AMP and DEA+MDEA). Part-1 of this paper presents a detailed design of the novel hemispherical apparatus and a rigorous mathematical model applicable to both absorption and desorption conditions with some preliminary results and Part-2 provides detailed results with estimates of kinetic coefficients for CO₂ absorption and desorption for eight different aqueous amine systems.The new hemispherical contactor consists of a 76 mm diameter solid hemisphere housed in a Pyrex glass cylinder with appropriate gas and liquid feed and withdrawal systems. The liquid feed passes through a 4 mm ID tube, which is located in the center of the hemisphere, and discharges at the top. The liquid descends as a well-defined liquid film over the surface of the hemisphere and is collected by a funnel (79 mm ID) at the base of the hemisphere. The interfacial area and hydrodynamics are well defined and the entrance/exit effects as well as surface rippling are minimized. Using this apparatus, the absorption experiments were conducted at near atmospheric pressure with pure CO₂ saturated with water vapor at 293-323 K with initially unloaded solutions and the desorption experiments were performed at 333-383 K for CO₂ loadings between 0.02 and 0.7 moles of CO₂ per mole of amine using humidified nitrogen gas as a stripping medium at total system pressures up to 205 kPa.The new rigorous mathematical model developed to interpret the rate data is based on the principle of diffusional mass transfer accompanied with liquid-phase chemical reactions over a hemispherical liquid film. Also developed in this work is a methodology that uses the rigorous model in conjunction with a non-linear regression technique to estimate kinetic parameters. Preliminary results presented in this part of the paper show that the new experimental apparatus was successful in accurately measuring and the new model and its numerical implementation were successful in accurately predicting both absorption and desorption rates for all aqueous amine systems considered in this study.     

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.     

Carbon Dioxide Desorption from Saturated Organic Solvents

Carbon dioxide removal from various gaseous streams and its subsequent recovery are of considerable industrial importance. The absorption technologies represent the most important operation to separate CO₂. In this work, the mass transfer rates during CO₂ absorption and desorption from propylene carbonate dimethyl ether of polyethylene glycol, and N‐methyl‐2‐pyrrolidone solutions were measured over the temperature range of 293.15–323.15 K in a baffled agitated reactor with a flat gas‐liquid interface operated in a batchwise manner. Two distinct mechanisms of desorption were observed: bubbling and diffusive desorption. The volumetric gas‐liquid mass transfer coefficients for the bubbling desorption were determined from the measured overall and diffusive desorption rates, and were correlated by the semi‐empirical equation. The proposed correlation is a power relationship of supersaturation, Reynolds and Weber numbers, and was found to quantitatively explain the observed phenomena with satisfactory agreement.     

The use of carbonic anhydrase to accelerate carbon dioxide capture processes

The chemical absorption of CO₂ into a monoethanolamine solvent is currently the most widely accepted commercial approach to carbon dioxide capture. However, the subsequent desorption of CO₂ from the solvents is extremely energy intensive. Alternative solvents are more energy efficient, but their slow reaction kinetics in the CO₂ absorption step limits application. The use of a carbonic anhydrase (CA) enzyme as a reaction promoter can potentially overcome this obstacle. Native, engineered and artificial CA enzymes have been investigated for this application. Immobilization of the enzyme within the gas absorber or in a membrane format can increase enzyme stability and avoid thermal denaturation in the stripper. However, immobilization is only effective if the mass transfer of carbon dioxide through the liquid phase to reach the immobilization substrate does not become rate controlling. Further research should also consider the process economics of large‐scale enzyme production and the long‐term performance of the enzyme under real flue gas conditions. © 2014 Society of Chemical Industry     

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.     

Promotion of CO2 mass transfer in carbonate solutions

A general, physico-chemical analysis of mass transfer rate promotion in the system CO₂-potassium carbonate-water-promoter is presented. Different possible mechanisms of promoter action including homogeneous catalysis, “shuttle” mechanism and surface reactions are discussed and classified. A unified picture of promoter chemistry is presented, showing that differences between inorganic and organic promoters are predominently quantitative, not qualitative.The “shuttle” mechanism is analyzed for absorption. The analysis leads to the prediction that the CO₂ mass transfer rate may be influenced by the liquid hold up. This is related to the fact that, although the reaction in the interface region may be fast enough to enhance the mass transfer rate, the different reaction in the bulk may not be fast enough to maintain chemical equlibrium. This complex type of chemical absorption process has not been considered previously in the literature.     

Selective absorption of H2S from CO2 — factors controlling selectivity toward H2S

A novel experimental system was adapted to determine factors controlling selective absorption of hydrogen sulfide (H₂S) from carbon dioxide (CO₂). This work demonstrates that liquid film controlled mass transfer regime and a low value of the liquid side mass transfer coefficient favors selective removal of H₂S from CO₂. By identifying the factor controlling selective removal of H₂S from CO₂, this work lays the basis for the parameter optimization of a process for selective removal of H₂S from CO₂.     

Simultaneous Absorption of Hydrogen Sulfide and Carbon Dioxide into di-isopropanolamine Solution

The simultaneous absorption of hydrogen sulfide and carbon dioxide into di-isopropanolamine (DIPA) solution was investigated in a 183 cm long, 2.72 cm OD wetted-wall column at atmospheric pressure. The influence of liquid flow rate, gas flow rate, temperature and liquid concentration on the absorption rate, overall gas-phase mass transfer coefficient and selectivity factor were studied at a constant gas feed ratio. The results show that the absorption rate of CO₂ increases rapidly with increasing liquid flow rate (the Reynolds number of the turbulent liquid film ranges from 2600 to 4350) but increases moderately with increasing gas flow rate (G = 18-91 L/min), indicating that it is liquid-phase mass transfer controlled. In contrast, the absorption rate of H₂S increases very slowly with increasing liquid flow rate but increases rapidly with increasing gas flow rate, indicating that it is gas-phase mass transfer controlled. The absorption rate of CO₂ also increases with increasing temperature (26-80°C) but H₂S absorption rate decreases with increasing temperature. When the concentration of DIPA solution increases from 0.2 to 2.6 mol/L, the absorption rate of both CO₂ and H₂S increases but with a larger rate of increase for CO₂ For selective H₂S removal, it is preferable to operate at low liquid and high gas flow rates, low temperatures and low DIPA concentrations.     

Mass Transfer Flux of CO2 into Methyldiethanolamine Solution in a Reactive-Absorption Process

The mass transfer parameters of both gas and liquid phases affect the mass transfer flux of CO₂ in absorption processes. In this study, an accurate correlation is proposed to calculate the CO₂ mass transfer flux in an absorption-reactive process by methyldiethanolamine (MDEA) solution using the Buckingham π theorem. The various parameters include film parameter, CO₂ loading, concentration ratio, partial-to-total pressure ratio, film thickness ratio, and diffusion ratio which are incorporated in the model. An average absolute relative error of 4.4 % for the calculation of mass transfer flux was stated.     

Surfactant adsorption onto activated carbon and its effect on absorption with chemical reaction

The effects of surfactant contaminations and activated carbon addition on physical gas absorption, and absorption with fast and instantaneous reaction (sulphite oxidation, carbon dioxide absorption into sodium hydroxide and monoethanol amine (MEA) solutions) have been studied in a stirred cell with a flat gas/liquid interface. Surfactants significantly decrease the liquid-side mass transfer coefficient k_L even at very small concentrations. The surfactants can be removed by adsorption onto activated carbon (“surfactant grazing”).In absorption with fast chemical reaction of the gas (sulphite oxidation), the liquid side mass transfer coefficient k_L has no effect on the absorption rate and, consequently, there are no effects of surfactant and activated carbon. CO₂ absorption into sodium hydroxide solution may occur in the instantaneous absorption regime; then, any change in k_L causes a proportional change in the absorption rate. In CO₂ absorption into MEA solution, however, in the instantaneous regime, much stronger effects of surfactant and of its removal by activated carbon are observed. It is suggested that in the absence of surfactants surface convection (Marangoni instability) may occur in MEA solutions.     

离子液体-水复配吸收剂捕获CO2性能

基于绿色合成方法制备出亲水性离子液体(ILs)[NH₂-C₃mim][Br],从有效降低CO₂吸收-解吸操作成本出发,采用ILs-H₂O复配吸收剂,开展了常温加压CO₂吸收及吸收剂常温减压解吸再生实验。结果表明,比CO₂吸收量(基于复配吸收剂或离子液体组分)随复配吸收剂中ILs组分浓度而变;吸收初期,CO₂吸收速率随吸收剂配比变化显著;以CO₂高吸收率和吸收剂低成本为目标,优选出新型水基复配吸收剂(离子液体与水质量比为1.38:1)。分别以水基离子溶液、改良热钾碱液和活化复配醇胺液为吸收剂,在自行搭建的超重力场强化吸收-连续逆流接触(加热或减压)解吸再生台架实验装置上进行了CO₂捕获与吸收剂再生连续化实验。结果表明,在超重力场作用下,改良热钾碱液和活化复配醇胺液对CO₂有较好的捕获,吸收率分别在98%、96%和90%以上,3种吸收剂经加热或减压解吸再生后均可循环回用,水基离子溶液吸收剂在常温减压下解吸更具有实际可操作性。     

Numerical simulation of the mass transfer process of CO2 absorption by different solutions in a microchannel

The flow and mass transfer characteristics of CO₂ absorption in different liquid phases in a microchannel were studied by numerical simulation. The mixture gas phase contained 5 vol% CO₂ and 95 vol% N₂ , and the different liquid phases were water, ethanol solution, 0.2 M monoethanolamine solution, and 0.2 M NaOH solution, respectively. Based on the permeation theory, the distribution of velocity and concentration in the slug flow was obtained by local simulation of flow and mass transfer coupling and was described in depth. The influence of contact time and bubble velocity on the mass transfer of the whole bubble was highlighted. The volumetric mass transfer coefficient on the bubble cap and liquid film, CO₂ absorption rate, and enhancement factor were calculated and analyzed. The results showed that the volumetric mass transfer coefficients of chemical absorption were ~3 to 10 times that of physical absorption and the CO₂ was absorbed more completely in chemical absorption. The new empirical correlations for predicting the mass transfer coefficient of the liquid phase were proposed respectively in physical absorption and chemical absorption, which were compared with the empirical formulas in the literature. The volumetric mass transfer coefficients obtained by predictive correlations are in good agreement with those obtained by simulation in this paper. This work made a basic prediction for CO₂ absorption in microchannel and provides a foundation for later experimental research.     

Gas-liquid mass transfer in trickle-bed reactors: Gas-side mass transfer

New data of gas-liquid mass transfer for cocurrent downflow through packed beds of non-porous particles are presented. Mass transfer parameters for air/carbon dioxide/water and air/carbon dioxide/sodium hydroxide systems were evaluated by least squares fit of the calculated CO₂ concentration profiles in the gas phase to the experimental values. The dependence of k_Ga on gas and liquid flow rates is caused by the dependence of gas-liquid interfacial area, not by the gas-side mass transfer coefficient k_G. In the case of the absorption of dilute carbon dioxide the gas-side resistance is considerably smaller than the liquid-side resistance. In the pulse flow regime, gas-liquid interfacial area calculated from k_La and k_L values obtained by physical, respectively, chemical absorption are lower than the gas-liquid interfacial area evaluated from the measurements under reaction conditions.     

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.     

Influence of polyethylene oxide on absorption of carbon dioxide into aqueous <Emphasis Type="Italic">N</Emphasis>-methyldiethanolamine solution

Carbon dioxide was absorbed into aqueous polyethylene oxide (PEO) solution containing N-methyldiethanolamine (MDEA) in a flat-stirred vessel to investigate the effect of non-Newtonian rheological behavior of PEO on the chemical absorption rate of CO₂, where the reaction between CO₂ and MDEA was assumed to be a first-order reaction with respect to the concentration of CO₂ and MDEA, respectively. A unified correlation equation containing the Deborah number, which reflects the viscoelastic properties of a non-Newtonian liquid, was used to obtain the volumetric liquid-side mass transfer coefficient of carbon dioxide in aqueous PEO solution. The elastic properties of PEO accelerated the absorption rate of CO₂ compared with that of a Newtonian liquid based on the same values of viscosity.     

Carbon Dioxide Mass Transfer into Solvents in a Zeolite Hybrid Device

Carbon dioxide (CO₂) mass transfer processes are analyzed in hybrid equipment which involves a zeolitic membrane and a physical or chemical solvent. This separation device was chosen because the membrane can be used to produce a stream of higher CO₂ concentration to be treated by gas‐liquid absorption. The analysis of the mass transfer behavior of this gas through the solid phase is an important step before more complicated gas streams are applied. The combined use of both techniques can improve the global separation process because they allow performing a previous separation with a positive effect on the cost of the later separation operations. The influence of the liquid phase nature used in one chamber of the membrane contactor upon CO₂ global mass transfer is analyzed. Also the effect caused by the absorption regime, liquid and gas flow rates, and the pressure corresponding to the gas chamber on CO₂ mass transfer is studied to evaluate the importance of each variable.     

Modelling reactive absorption of CO2 in packed columns for post-combustion carbon capture applications

A rate-based process model for the reactive absorption of carbon dioxide (CO₂) from a gas mixture into an aqueous monoethanolamine (MEA) solution in a packed column is developed. The model is based on the fast second-order kinetics for the CO₂-MEA reactions and takes into account the mass transfer resistances. The heat effects associated with the absorption and chemical reaction are included through energy balances in the gas and liquid phases. Appropriate correlations for the key thermodynamic and transport properties and for the gas-liquid mass transfer are incorporated into the model to ensure reliable predictions. The model predictions are validated by simulating a series of experiments conducted in pilot and industrial scale absorption columns with random and structured packings reported in the literature. Comparisons between the simulation results and the experimental data reveal good quality predictions of the gas phase CO₂ and MEA concentrations and the liquid temperature along the column height. The sensitivity studies reveal that the correlations for the gas- and liquid-film mass transfer coefficients given by Onda et al. (1968) provide better predictions than the penetration theory of Higbie (1935) and the correlation of Bravo et al. (1985).     

Dynamic modeling and simulation of absorption of carbon dioxide into seawater

In order to elucidate the dynamic performance of the CO₂ ocean disposal process, effects of operating parameters, such as gas flow rate, salinity and temperature, on the absorption of CO₂ into seawater were examined. The rate-based model consisting of the rates of chemical reaction and gas-liquid mass transfer was developed for simulating dynamic process of CO₂ ocean disposal. In modeling, non-ideal mixing characteristics in the gas and liquid phases are described using a tanks-in-series model with backflow. Experiments were performed to verify dynamic CO₂ absorption prediction capability of the proposed model in a cylindrical bubble column. The operation was batch and continuous with respect to liquid phase and gas phase, respectively. Experimental results indicate that the CO₂ gas injection rate increased the absorption rate but the increase in salinity concentration caused inhibition of the absorption of CO₂. The proposed model could describe the present experimental results for the dynamic changes and the steady-state values of dissolved CO₂ concentration and hydrogen ion concentration. The proposed model might effectively handle the prediction of the absorption of CO₂ into seawater in the CO₂ ocean disposal.