Gas-liquid Absorption Research Articles

Modeling and Simulation of Gas Liquid Absorption Column for So2 Removal Process

During coal combustion, sulphur in the coal is converted into sulphur dioxide (SO2).This sulphur dioxide (SO2) is responsible for the formation of acid rain which is one of the widespread forms of pollution all over the world that causes harmful effects to humans and environment. To minimize the adverse impacts of SO2, it must be removed from flue gas. For reduction of SO2, flue gas desulphurization (FGD) is most commonly used. The mathematical viewer is developed and simulated for a gas liquid absorption column to control the flow rate of H2O2.The model equation is developed by considering material balance around the column. The absorption rate is determined by using different concentration of sulphur dioxide with hydrogen peroxide. Hydrogen peroxide, not only absorbs the SO2 but it also produce useful by-product in the form of sulphuric acid (H2SO4).

The Interplay between Charge Transport and CO2 Capturing Mechanism in [EMIM][SCN] Ionic Liquid: A Broadband Dielectric Study.

The hoisted increment in the CO2 emission in the atmosphere is a noteworthy environmental problem. Gas-liquid absorption is a well-known strategy that can be used to control CO2 emissions from an increased rate of fossil fuel industrializations. In this work, a combination of broadband dielectric spectroscopy, Fourier infrared (FTIR) spectroscopy, and quantum chemical calculations were used to study the absorption, desorption and kinetic mechanism of a room temperature imidazolium ionic liquid (IL) with cyanide anion, 1-ethyl-3-methylimidazolium thiocyanate ([EMIM][SCN]) on CO2 exposure. Initially, the charge transport and glassy dynamics of [EMIM][SCN] is investigated in a wide frequency and temperature range using broadband dielectric spectroscopy and differential scanning calorimetry. The conductivity relaxation was well fitted with Havriliak-Negami function in the modulus formalism, while the dc conductivity correlated well with the Barton-Nakajima-Namikawa relation. Then, the conductometric approach was taken to monitor the interplay between the ionic conductivity of [EMIM][SCN] and diffusion of captured CO2 in it. The resistance of the IL increases upon CO2 exposure, indicating a chemical change at the molecular level of [EMIM][SCN]. The possible CO2 capturing mechanisms for [EMIM][SCN] were investigated with density functional theory calculations and FTIR spectroscopy. Thus, this work proposes a new strategy to explain the mechanism underlined in chemisorption of CO2 in the [EMIM][SCN]. This can be extended to more promising CO2 capturing materials including ionic liquids especially imidazolium-based ionic liquids with cyanide anions like dicyanimide, tricyanometanide, tetracyanoborate, etc.

Techno-economic assessment of hydrogen production processes based on various natural gas chemical looping systems with carbon capture

Hydrogen is regarded as a promising energy carrier with several key advantages for future low carbon applications (e.g. no greenhouse gas emission at the point of use, higher energy conversion efficiency). This paper is assessing from a techno-economic point of view, three chemical looping processes suitable for hydrogen production generating high purity hydrogen corresponding to 300 MW thermal output, with a carbon capture rate of at least 90%: i) chemical looping hydrogen production (CLH), ii) sorption enhanced reforming (SER), iii) sorption enhanced chemical-looping reforming (SECLR). Key techno-economic performance indicators were evaluated and compared amongst each other and against a conventional natural gas reforming technology without/with carbon capture by chemical gas-liquid absorption using alkanolamines. The results show that CLH using iron-based (i.e. ilmenite) oxygen carrier seems to be the most promising hydrogen production technology amongst the evaluated systems having the highest energy efficiency at CCR>99%, lower operating and maintenance (O&M) costs, with a hydrogen production cost of 41.84 €/MWh compared to 42.43 €/MWh for no capture conventional reforming and 44.58 €/MWh for amine-based capture with 70% CCR, at a CO2 emissions avoidance cost of 19.46 €/tCO2.

Simulation of carbon dioxide absorption by amino acids in two-phase batch and bubble column reactors

ABSTRACTThe absorption of carbon dioxide (CO2) is an important process in many practical applications. The use of amino acid solutions as absorption solvents has the potential to reduce the amount of energy required by the regeneration process. The goal of this research project is to develop dynamic models to simulate CO2 absorption by using amino acid solutions as absorption solvents. A reaction scheme is proposed to represent the chemical reactions between the amino acid and CO2. Two reaction models, for a two-phase batch reactor and a bubble column, based upon transient mass and energy balances for the chemical species found in CO2 gas-liquid absorption are presented. Computer codes have been written to implement the proposed models. Simulation results are presented and discussed. The proposed models can be used to optimize and control CO2 absorption in practical applications.

Life Cycle Assessment of SEWGS Technology Applied to Integrated Steel Plants

The environmental evaluation of the sorption-enhanced water–gas shift (SEWGS) process to be used for the decarbonization of an integrated steel mill through life cycle assessment (LCA) is the subject of the present paper. This work is carried out within the STEPWISE H2020 project (grant agreement No. 640769). LCA calculations were based on material and energy balances derived from experimental activities, modeling activities, and literature data. Wide system boundaries containing various upstream and downstream processes as well as the main integrated steel mill are drawn for the system under study. The environmental indicators of the SEWGS process are compared to another carbon capture and storage (CCS) technology applied to the iron and steel industry (e.g., gas–liquid absorption using MEA). The reduction of greenhouse gas emissions for SEWGS technology is about 40%. For the other impact indicators, there is an increase in the SEWGS technology (in the range of 7.23% to 72.77%), which is mainly due to the sorbent production and transportation processes. Nevertheless, when compared with the post-combustion capture technology, based on gas–liquid absorption, from an environmental point of view, SEWGS performs significantly better, having impact factor values closer to the no-capture integrated steel mill.

Techno-economic assessment of combined hydrogen & power co-generation with carbon capture: The case of coal gasification

The gasification technology has multiple key advantages for future low carbon power generation scenario e.g. reduced energy and cost penalties for CO2 capture, multi-fuel multi-product operation capability, ability to process lower grade fuels, plant flexibility, etc. This paper is assessing the updated most important techno-economic aspects of IGCC power plant equipped with pre-combustion CO2 capture. Several key design options were assess in view of techno-economic indicators: selection of the gasification reactor, chemical vs. physical gas-liquid absorption used for CO2 capture, F and H-class gas turbines used for the combined cycle power block, flexible hydrogen and power co-generation, etc. Evaluated coal-based gasification concepts generate 400–600 MWe net power with a flexible hydrogen output from zero up to 300 MWth with 90% carbon capture rate. The similar design without carbon capture was also consider as a base case to quantify the technical and economical modifications induced by the CO2 capture step. As the techno-economical results show, the optimised IGCC plant (using dry fed entrained flow gasifier, H-class gas turbine, physical solvents, flexible hydrogen and power co-generation, etc.) provides improved results. In conclusion, the IGCC with pre-combustion capture is one promising technology for the future low-carbon economy.

Carbon Dioxide Absorption Modeling for Off-Gas Treatment in the Nuclear Fuel Cycle

The absorption of carbon dioxide is an important process in many practical applications such as reduction of greenhouse gases, separation and purification processes in the chemical and petroleum industries, and capture of radioactive isotopes in the nuclear fuel cycle The goal of this research is to develop a dynamic model to simulate CO2 absorption by using different alkanolamines as absorption solvents. The model is based upon transient mass and energy balances for the chemical species commonly present in CO2 gas-liquid absorption. A computer code has been written to implement the proposed model. Simulation results are discussed. The reported model simulates well the response to dynamic changes in input conditions. The proposed model can be used to optimize and control the separation of carbon-14 in the form of CO2 in the nuclear industry.

Economic Assessments of Hydrogen Production Processes Based on Natural Gas Reforming with Carbon Capture

Hydrogen is a promising energy carrier for future low carbon economy as fuel or chemical for various industrial applications (e.g. heat and power, petro-chemical, metallurgy etc.) as well as for transport sector. In order to decarbonise the energy sector as well as the transportation sector, the fossil fuels need to be efficiently converted to hydrogen and the resulted CO2 to be captured and then used / stored. The Carbon Capture, Utilisation and Storage (CCUS) technologies are promising options of efficiently combating climate change (by significantly reducing the greenhouse gas emissions) as well as continuing of using the fossil fuels. This paper is assessing the key updated technical and economical performances of hydrogen production based on natural gas reforming with and without carbon capture. As illustrative examples, conventional steam reforming and autothermal reforming (using both oxygen and air) of natural gas were evaluated. The CO2 capture technology was based on pre-combustion capture configuration using gas-liquid absorption. As illustrative cases, Methyl-DiEthanol-Amine (MDEA) as a chemical solvent and SelexolTM as a physical solvent were assessed for their performances. The evaluated hydrogen production concepts have a capacity of 100,000 Nm3/h (corresponding to 300 MWth based on lower heating value) with a purity higher than 99.95 % (vol.). The paper presents in details the evaluated hydrogen production concepts based on natural gas reforming, modelling and simulation aspects, model validation, mass and energy integration issues as well as proposing an integrated methodology for quantification of key economic aspects. As the results show, the conventional steam reforming has higher energy utilisation factor (about 5 net percentage points) than the autothermal reforming cases. The carbon capture rate is about 65 - 70 % for the conventional steam reforming considered as an illustrative case. The economic indicators show better performances for conventional steam reforming in comparison to autothermal reforming in term of specific capital investment cost (about 12 - 24 % lower), operational and maintenance costs (about 7 % lower), hydrogen costs (about 5 - 10 % lower). Physical absorption is more energy and cost effective than chemical absorption as pre-combustion capture method.

Energy efficiency improvements of post-combustion CO2 capture based on reactive gas–liquid absorption applied for super-critical circulating fluidized bed combustion (CFBC) power plants

The environmental impact represents a significant constraint for fossil fuel-intensive industrial processes in transition to a low-carbon scenario. This paper is evaluating the potential energy efficiency improvements in reactive gas–liquid absorption process used for CO2 capture from coal-based super-critical CFBC power plants. Two improved configurations for Methyl diethanolamine (MDEA)-based post-combustion CO2 capture were assessed: absorption intercooling and lean vapour recompression as well as the combination of both. The improved MDEA gas–liquid absorption configurations were compared to conventional MDEA and MEA-based systems for post-combustion CO2 capture as well as the CFBC power plant without carbon capture to assess both the energy and cost penalties for CO2 capture. As the results show, the proposed innovative concepts exhibit better specific thermal energy consumptions for solvent regeneration (2.24–2.58 vs. 2.97 MJ/kg CO2), higher net electrical efficiency (34 vs. 32%) and improved economic indicators (e.g. 2403 vs. 2552 €/kW net power as specific capital investment, 39.2 vs. 41.6 €/MWh as O&M costs, 79 vs. 84 €/MWh as cost of electricity) compared to the conventional MDEA and MEA cases.

Exploiting the hydrophilic role of natural deep eutectic solvents for greening CO2 capture

Abstract CO2 released from fossil fuels has made a great contribution to global warming, and natural deep eutectic solvents (NADES) offer an attractive option for greening CO2 capture. Being economical natural materials, L-arginine (L-Arg) and glycerol (Gly) were used to synthesize a series of novel hydrophilic NADES (L-Arg/Gly, at mole ratios of 1:4, 1:5, 1:6, 1:7, 1:8). The L-Arg/Gly (1:6) DES showed excellent thermal stability (Tmax, 250 °C; Tf, 13.7 °C). The hydrates of L-Arg/Gly DES (at water contents of 0 wt%, 10 wt%, 20 wt%, 30 wt%, 40 wt%, 50 wt%, 60 wt%) were also prepared. The synthesis process of L-Arg/Gly DES and its hydrate were studied with FT-IR and in situ IR spectroscopy. High hydrophilicity of such solvent was demonstrated with fast and efficient capture of CO2 at a high absorption (0.511 mol CO2/mol NADES) under optimized conditions. In the gas-liquid absorption process, the hydrophilic properties of L-Arg/Gly DES and its hydrate greatly reduced the transfer barrier, and accelerated the phase transfer by increasing accessibility of active amine sites on NADES and CO2. And the dynamics for the CO2 absorption was investigated correspondingly. The absorption behaviors of CO2 in L-Arg/Gly DES and its hydrate were studied by in situ IR spectrometer and nuclear magnetic resonance (1H NMR, 13C NMR), and the characteristic infrared peaks and shifts of CO2 absorption were observed. In addition, the L-Arg/Gly DES could be reused for 5 times without obvious inactivation. Indicating the L-Arg/Gly DES would be a potential substitute of the traditional volatile amine solvents for CO2 capture.

Free Radical and Direct Ozone Reaction Competition to Remove Priority and Pharmaceutical Water Contaminants with Single and Hydrogen Peroxide Ozonation Systems

ABSTRACTFrom the application of concepts derived from the gas-liquid absorption film model, the competition between ozone reactions with 72 water emerging or priority contaminants (pharmaceuticals, pesticides, polynuclear aromatic hydrocarbons, etc.) and the initiation steps of the hydroxyl radical decomposition of ozone in ozone alone and combined with hydrogen peroxide oxidation systems has been studied. With this information, the ozone preferential reaction, that is, the ozone direct reaction or the formation of free radicals and the kinetic absorption regime are known. In a second step, the ratio of removal rates of the contaminants studied by reacting with hydroxyl radical and ozone has also been estimated. With this, the way contaminants are preferentially removed (from their reaction with ozone or from the reaction with free radicals) can also be known and, hence, whether or not an ozone advanced oxidation system is convenient to be applied. For instance, most of the contaminants studied in this work at concentrations lower than 50 μgL−1 and hydrogen peroxide at concentrations lower than 50 mgL−1 react with ozone under chemical control regime so that both direct and free radical reactions theoretically compete. However, under chemical control, typical concentration of scavengers present in wastewater or surface water would inhibit the free radical reactions and, at least theoretically, for many contaminants studied here, the direct ozone reaction is the principal removal way. When mass transfer controls the process rate only contaminants with a hydroxyl and ozone rate constant ratio ≥ 1.6x106 M-1s-1 would be preferentially removed through free radical way.

The Study of Carbon Dioxide Removal in Membrane Module by Single and Mixture of Alkanolamine Aqueous Solutions

This work presents an investigation on the performance of CO2absorption into aqueous alkanolamine solution using a hollow fiber membrane gas–liquid contactor. Aqueous solution of ethanolamine (MEA), diethanolamine (DEA), 2–amino–2–methyl–1–propanol (AMP) and piperazine anhydrous (PZ) were chosen as the absorption liquids. A microporous hollow fiber membrane made of polyvinylidene fluoride (PVDF) was used as the medium for gasliquid absorption process. In this study, the operating temperature is fixed at 30°C, while the flowrate of CO2 and alkanolamine were in the range of 1000–5000 ml/min and 50–280 ml/min respectively. The feed gas was introduced directly to the shell of the module at 1–1.5 bar and the liquid flowed through the fiber lumen side. The CO2 transfer through the membrane was found to be reaction controlled and dependent on the type of amine used. The use of piperazine as an accelerator in the mixture of the absorption liquids gives a good impact on increasing the performance for the rate of CO2 transfer.

Carbon capture and utilisation technologies applied to energy conversion systems and other energy-intensive industrial applications

Reducing the greenhouse gas emissions from heat and power sector as well as from other energy-intensive industrial applications is of paramount importance today. Various carbon capture technologies can be applied to reduce the CO2 emissions from the polluting process industries. This paper is presenting, through various illustrative coal-based examples, the carbon capture, utilisation and storage (CCUS) technologies used to reduce the carbon footprint of the overall processes. The evaluations were focused on the conceptual design and the technical & environmental assessment of CCUS technologies with potential applications in power generation, iron & steel, cement and chemicals (including captured CO2 utilisation for production of various chemicals e.g. methanol, substitute natural gas, synthetic fuel).Two reactive gas-liquid and gas-solid methods were evaluated through illustrative examples. The first evaluated CO2 capture option is based on gas-liquid absorption using chemical solvents (alkanolamines). The second capture option is based on calcium looping system. The carbon capture rate is set to 90%. Various coal-based processes were considered as illustrative examples e.g. combustion, gasification, cement production, integrated steel mill, coal to chemicals etc. The proposed conceptual designs were simulated; the mass & energy balances as well as the thermal integration tools were used to quantify the key technical and environmental performance indicators. The assessments show that CCUS technologies have significant advantages in reducing the environmental impact of energy-intensive industrial applications and simultaneously to produce useful products.

Effect of non-solvent additive concentration on CO2 absorption performance of polyvinylidenefluoride hollow fiber membrane contactor

Abstract Membrane contactor for post-combustion carbon dioxide (CO2) capture is a promising alternative to conventional gas-liquid absorption systems. In order to improve the polyvinylidenefluoride (PVDF) hollow fiber membrane structure for CO2 absorption via membrane contactor, the effect of non-solvent additive concentration on the membrane structure and CO2 absorption performance was investigated. Lithium chloride (LiCl), phosphoric acid (PA) and their mixture over a range of concentrations (0–8 wt%) were used as non-solvent additives. The morphological cross sections of the prepared membranes included four segments from the inner to outer surface, which were successively termed the nodule-like zone, transitional zone, porous zone and skin layer. Increasing the concentration of non-solvent additives increased sponge-like structures, decreased finger-like structures in the porous zone, and decreased mean pore size on the membrane outer surface. CO2 membrane contact absorption tests showed a positive correlation between the CO2 absorption flux and the non-solvent additive concentration. The highest CO2 absorption flux of 1.31 × 10−3 mol m−2 s−1 was obtained at an inlet gas (CO2/N2 = 19/81, v/v) flowrate of 130 mL/min and an absorbent liquid (distilled water) flowrate of 70 mL/min by adding 8 wt% of PA in the PVDF dope solution.

Techno-economic evaluations of post-combustion CO2 capture from sub- and super-critical circulated fluidised bed combustion (CFBC) power plants

The heat and power generation sector is facing fundamental changes for transition to a low carbon scenario due to significant environmental constraints. This paper is evaluating from techno-economic point of view the integration of post-combustion CO2 capture technologies into coal-based CFBC power plants operated in sub- and super-critical steam conditions. Two post-combustion CO2 capture technologies were assessed: a chemical gas-liquid absorption using alkanolamines (Methyl-DiEthanol-Amine - MDEA) and a gas-solid adsorption using calcium-based sorbent (Calcium Looping - CaL). The analysis evaluates how chemical gas-liquid absorption/gas-solid adsorption influence the techno-economic performances of CFBC power plants. As benchmark cases used to quantify the energy and cost penalties for CO2 capture, the correspondent sub- and super-critical CFBC plants without CO2 capture were considered. As the results show, the CaL concepts exhibits better net electrical efficiency (35% vs. 32%), higher carbon capture rate (97% vs. 90%) and improved economic indicators (e.g. 1860vs. 2552€/kW net power as specific capital investment, 34vs. 41.6€/MWh as O&M costs, 66vs. 84€/MWh as cost of electricity) compared to the MDEA cases (all these values being reported for the super-critical CFBC designs).

Reducing the carbon footprint of cement industry by post-combustion CO2 capture: Techno-economic and environmental assessment of a CCS project in Romania

Reducing the carbon dioxide emissions from the energy-intensive industrial sectors is of great importance in the fight against climate change. The cement industry is responsible for about 5% of global CO2 emissions. In this article, two reactive absorption and adsorption post-combustion CO2 capture methods are assessed in conjunction with cement production. The gas–liquid absorption method uses alkanolamine (MDEA) as chemical solvent and the gas–solid adsorption method uses calcium looping (CaL) technology. The carbon capture rate is set to 90%. The analysis considers a conventional size of cement plant (1Mt/y) focusing on mass and energy integration aspects of the carbon capture unit as well as quantification of main techno-economic and environmental indicators of the cement plant with carbon capture. The evaluated designs were modelled and simulated, the mass and energy balances being used to assess the overall performances. For comparison reason, a cement plant without carbon capture was also considered to assess the energy and cost penalties for the carbon capture designs. The analysis shows that the CaL system has significant technical and economic advantages compared to the gas–liquid absorption case (e.g. higher energy efficiency, lower capital, operational and maintenance (O&M), cement production and CO2 avoidance costs).

Pilot scale testing and modeling of enzymatic reactive absorption in packed columns for CO2 capture

Efficient processes for carbon dioxide (CO2) capture from post-combustion flue gases are required to combat global climate change. A key stage in post-combustion capture is selective CO2 separation from the flue gas stream. Separation of CO2 from mixed gases using countercurrent gas–liquid absorption in packed columns is a well-established technology for treatment of industrial gas streams. This approach can be adapted to remove CO2 from post-combustion flue gas, however, process improvements are needed to minimize the corresponding capital costs and energy requirements. Special challenges for CO2 recovery from flue gas arise from the very large volumes of gas to be processed, the need to operate the process with an inlet flue gas stream at atmospheric pressure, and the high amount of energy required to regenerate the absorption liquid. Aqueous solutions of the tertiary amine N-methyldiethanolamine (MDEA) are commercially used for high pressure CO2 separation due to high loading capacity for CO2, relatively good chemical and thermal stability and low volatility.Application of MDEA-based solutions to ambient pressure separations, such as CO2 capture from flue gases, is challenging since high reaction rates are required. High reaction rates for the MDEA system are achievable at high temperatures, which is conflicting with the preference of low temperatures to exploit high absorption capacity. This conflict can be overcome with the addition of a rate enhancing catalyst that enables high reaction rates at low temperatures.To put this innovative breakthrough technology closer to industrial application CO2 absorption in 30–50wt.% aqueous solutions of MDEA in absence and presence of the CO2 absorption enhancing enzyme carbonic anhydrase (CA) was evaluated in pilot scale. The pilot scale investigation employed a packed column for parametric testing. Test variables included the liquid phase composition (30–50wt.% MDEA), the column liquid load (8–24m3m−2h−1), the absorber temperature (20–40°C), and the application of CA in a dissolved or immobilized form. The CO2 absorption mass transfer enhancement provided by CA was measured.In the presence of dissolved CA, 30wt.% aqueous MDEA showed superior performance in terms of absorption rates compared to operation using 50wt.% MDEA(aq). No significant change in the CO2 absorption rate was observed for operation at given loads between 20°C and 40°C with dissolved CA present. At 20°C with 30wt.% MDEA the absorption rate with dissolved CA increased by more than 9 times compared to the absorption rate without enzyme. These results broaden the operation window for efficient CO2 absorption using MDEA solutions and allow for the exploitation of new process regimes, wherein high equilibrium loadings are achievable by applying lower absorption temperatures.Based on the experimental results obtained with dissolved CA, a simplified rate-based model of enzymatic reactive absorption (ERA), accounting for enzyme accelerated reaction kinetics, was developed which was capable of accurately predicting CO2 absorption rate when compared with experimental data. Implemented in a process simulator the model allows for the detailed investigation of the process behavior and the complex interactions of ab- and desorption operations in the presence of the CA. The validated model is intended to guide future experimental work as well as further performance optimization.In addition to the work exploiting the catalyst in its free form, the utilization of CA immobilized in a granular form and held in the pockets of Katapak-SP packing was successfully demonstrated.

Gas-liquid absorption in industrial cross-flow membrane contactors: Experimental and numerical investigation of the influence of transmembrane pressure on partial wetting

Membrane wetting is a limitation to mass transfer in industrial Hollow-Fiber Membrane Modules (HFMM). The influence of transmembrane pressure on wetting is investigated through both experimental and numerical works. On the one hand, experiments for CO2 absorption in softened water are carried out at pilot scale with Liqui-Cel® Extra-Flow modules operating in a cross-flow mode. After several hours of operation (50–300min), the membrane partially wetted mode is established at steady state and decreases the absorbed CO2 flowrates up to 10.5% compared to membrane dry mode. Interestingly, CO2 absorption is found to be independent on different transmembrane pressure (0.5 and 2.5bar), both in the dry and partially wetted mode. On the other hand, a 2D predictive tool is developed to help in the design of HFMM gas absorption systems. The model depicts the complex hydrodynamics in commercial cross-flow modules. The liquid in the shellside is flowing through the fiber bundle considered as a porous medium. The liquid mass-transfer coefficient is non-uniform over the membrane module and is estimated at a local scale. The parameters from the empirical Chilton-Colburn correlation are optimized with experimental results in the dry mode through a Particle Swarm Optimization (PSO) algorithm. The simulation results are in good agreement with the experiments (−13/+17%). In the partially wetted mode at steady state, wetting is then described by considering a log-normal pore size distribution and the Laplace-Young equation. The transmembrane pressure decreases the simulated absorbed CO2 flowrate. By comparing to experimental data, the numerical relative error is increasing with transmembrane pressure. Therefore, a suggestion is made to adapt the description of wetting in HFMM gas-liquid models.

Membrane contactors for process intensification of gas absorption into physical solvents: Impact of dean vortices

Membrane contactors are considered as one of the most promising intensification technologies for gas-liquid absorption processes. For chemical absorption systems, such as flue gas CO2 absorption in amine solutions, the mass transfer resistance is significantly located in the membrane due to the accelerating effect exerted by the reaction in the liquid. For physical absorption systems however, such as CO2 absorption in water for biogas purification, most of the resistance is located within the liquid phase. Process intensification requires an increase of the gas-liquid interfacial area (provided by the membrane) together with increased mass transfer performances in the rate limiting phase, namely the liquid. In the liquid phase, a boundary layer is formed, in which the solute concentration is higher than at the bulk, thus decreasing the driving force for the transfer. This study investigates the use of Dean vortices as a tool to increase mass transfer performance in membrane contactors. These vortices, generated by a helical hollow fiber, are applied to CO2 absorption in water using dense polymeric membranes. A straight hollow fiber with water flowing in the lumen is compared to different helical shaped hollow fibers. Firstly, Dean vortices, calculated by a CFD approach, are successfully compared to the velocity maps experimentally obtained by MRI, a non-invasive spectroscopy. Hydrodynamic simulations are thus validated, offering parametric studies and design optimization perspectives. The experimental results for the mass transfer performances of a hollow fiber under different liquid velocity conditions are further compared to simulations. A parametric study, combining CFD and mass transfer simulations, shows that Dean vortices enable an increase of the mass transfer coefficient by a factor of 3, with a moderate increment in pressure drop or energy expenditure, when compared to the straight fiber. Lastly, the best helical geometry, with maximal CO2 mass transfer at the expense of minimal energy requirement is investigated. For a given application, it is shown that a large process intensification effect is attainable; with the same energy requirement, a 2-fold decrease in the surface area is achieved. The potential of the concept for practical hollow fiber modules design and applications are finally discussed.

Kinetics of reaction between CO2 and ionic liquid-carbon dioxide binding organic liquid hybrid systems: Analysis of gas-liquid absorption and stopped flow experiments

It is generally accepted that CO2 emissions from point sources – such as thermal power plants – can be controlled by gas-liquid absorption where the process is intensified significantly by introducing a reactantinto solvent. In this work, blends of 1-ethyl-3-methyl imidazolium bis (trifluoromethylsulfonyl) imide ([emim][Tf2N]) with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (or 1,1,3,3-tetramethylguanidine (TMG)) in 1-hexanol were studied. Gas absorption experiments were carried out in a gas-liquid mini reactor for the hybrid systems containing different weight concentrations of organic bases while keeping the ionic-liquid weight percentage constant. Total amount of absorbed CO2 was measured as function of time and the absorption (or loading) capacities and the initial absorption rates have been obtained at 313K. It was found that increasing the amount of the organic base increased the loading capacity of CO2. Also, reusability and performance loss of hybrid solutions were investigated by using Fourier transform infrared spectrometry under sequential absorption-desorption cycles. The intrinsic reaction rates were measured in a stopped flow equipment for a temperature range of 283–303K. The empirical power law reaction orders with respect to DBU and TMG were found to be between 1.0 and 2.0 at different temperatures and the kinetic data could be interpreted satisfactorily by a modified termolecular reaction mechanism.Finally, the observed reaction rate constants inferred from heterogeneous gas absorption rates – through estimated solubility and diffusivity – were compared with the homogeneous – intrinsic – reaction rate constants obtained by a rapid mixing technique, namely stopped-flow conductimetry. The observed pseudo first order rate constants at 313K were found to be very similar and the deviations remained within 5–15%. Obtaining relevant data by these two very different techniques in the same laboratory provided pros and cons of each method and clarified – often – biased ambiguities.