Physical Absorption Of CO2 Research Articles

Demonstrating the key role of kinetics over thermodynamics in the selection of ionic liquids for CO2 physical absorption

A comprehensive study of over 50 ionic liquids (ILs) -integrating molecular simulation, gravimetric experiments and process analysis- has been conducted to evaluate the role of thermodynamics and kinetics on the physical absorption of CO2 by these widely researched solvents. Despite the common view, CO2 molar gas solubility is found as a misleading criterion to select the adequate IL for being used as CO2 absorbent in commercial separation columns, due to the strong kinetic control of the operation. In contrast, low viscosity and molar weight are demonstrated to be key parameters to minimize solvent consumption, energy duty and equipment size. Short-chain imidazolium-based ILs with tetracyanoborate, tricyanomethanide, and dicyanamide anions are proposed as adequate CO2 absorbents with favorable transport and thermodynamic properties. Current results indicate that ILs do not exhibit better absorbent performance in the CO2 capture unit than conventional organic solvents (such as glymes, components of Selexol) already used in the industry. IL regeneration stage need to be considered in future studies in order to state the promising advantages of IL absorbents in terms of thermal stability, energy consume and economy.

Physical absorption of CO2 and H2S from synthetic biogas at elevated pressures using hollow fiber membrane contactors: The effects of Henry’s constants and gas diffusivities

The present study applies the mathematical model to investigate the physical absorption of CO2, H2S and CH4 from synthetic biogas at high pressure using hollow fiber membrane contactors. Henry’s constants are considered pressure and temperature dependent, while the polarity of H2S is included in the calculation of gas diffusivities. The model was validated with the experimental data using two hollow fiber membranes, i.e., polyvinylidenefluoride (PVDF) and polytetrafluoroethylene (PTFE), to analyze membrane wetting. The effects of pressure and temperature on the CO2 and H2S absorption and CH4 loss were investigated. From the model validation, the experimental results using PVDF and PTFE at 1 bar are in good agreement with non-wetting mode. At elevated pressures, non-wetting mode results correspond with the experimental results using PTFE, while the partially wetting mode well predicts the results of PVDF. The modeling results also indicate that increasing pressure and reducing temperature improves the absorption fluxes of CO2 and H2S, while the CH4 losses in this work are lower than 5.5%.

From kinetics to equilibrium control in CO2 capture columns using Encapsulated Ionic Liquids (ENILs)

A novel approach based on Encapsulated Ionic Liquids (ENILs) is proposed for overcoming the mass transfer constraints in CO2 physical absorption by ILs. Absorption process simulation using COSMO-based/Aspen Plus methodology -an a priori approach- was carried to select four ILs with high and similar CO2 absorption capacity but markedly different transport properties: EmimTCM, BmimTCM, BmimDCN, and BmimOcSO4. Simulations using equilibrium- and rate-based column models for these ILs showed that CO2 recovery and the absorbent performance are severely reduced when the viscosity of the IL increases. Experimental gravimetric analyses with the selected ILs confirmed the large differences in solubility and absorption rate, this last also dependent on viscosity. ENILs were prepared by encapsulation of ILs in hollow carbon sub-microcapsules with a porous shell. The experimental gravimetric analysis evidenced that the ILs maintain their CO2 absorption capacity after encapsulation, whereas the absorption rate is ca. 50 times higher for ENILs than neat ILs. ENIL tests in fixed bed operation at different operating conditions yielded bed utilization values dependent on the CO2 solubility in the ENIL, while equivalent mass transfer zone lengths were obtained for all the materials. The results demonstrate the fast CO2 mass transfer rates in ENILs -related to the high contact area provided- allows overcoming the mass transfer limitations controlling the CO2 rate of physical absorption by ILs.

Systems Analysis of Physical Absorption of CO2 in Ionic Liquids for Pre-Combustion Carbon Capture.

This study develops an integrated technical and economic modeling framework to investigate the feasibility of ionic liquids (ILs) for precombustion carbon capture. The IL 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide is modeled as a potential physical solvent for CO2 capture at integrated gasification combined cycle (IGCC) power plants. The analysis reveals that the energy penalty of the IL-based capture system comes mainly from the process and product streams compression and solvent pumping, while the major capital cost components are the compressors and absorbers. On the basis of the plant-level analysis, the cost of CO2 avoided by the IL-based capture and storage system is estimated to be $63 per tonne of CO2. Technical and economic comparisons between IL- and Selexol-based capture systems at the plant level show that an IL-based system could be a feasible option for CO2 capture. Improving the CO2 solubility of ILs can simplify the capture process configuration and lower the process energy and cost penalties to further enhance the viability of this technology.

A modeling study of module arrangement and experimental investigation of single stage module for physical absorption of biogas using hollow fiber membrane contactors

In the present work, the effect of module arrangement on the physical absorption of CO2 and H2S in water at high operating pressure using 2 different types of hollow fiber membrane contactor (HFMC) including polyvinylidenefluoride (PVDF) and polytetrafluoroethylene (PTFE) was studied for biogas upgrading application. The simulation was performed at various liquid velocities (0.12–2.0m/s), pressure differences between liquid and gas phases (0.125–1.0bar for PVDF and 1.0–5.0bar for PTFE), operating pressures (1–30bar) under four different module arrangement scenarios, i.e., (i) single stage module, (ii) multistage module, (iii) multistage module with splitting liquid (α = 0.2 and 0.4) and (iv) multistage module with recycle liquid (φ = 0.5 and 1). The modeling results predicted the significant improvement of CO2 and H2S removal performances when the multistage module was applied for both HFMCs. To obtain the highest removal performance, the suitable liquid velocity, pressure difference between liquid and gas phases and the liquid flow pattern of the module arrangement are the key concerns. For PVDF, the multistage module with splitting liquid (α = 0.2) provides the highest performance at the liquid velocity of 0.12m/s, pressure difference of 0.125bar and the operating pressure of 30bar. For PTFE, at the liquid velocity of 2.0m/s and the pressure difference of 1.0bar, the multistage module with recycle liquid (φ = 1) gives the highest performance at the operating pressure of 1–20bar while the multistage module with splitting liquid (α = 0.4) provides the highest performance at the operating pressure of 30bar.

Feasible ionic liquid-amine hybrid solvents for carbon dioxide capture

Ionic liquid (IL)-amine hybrid solvents have been proved to be an energy-saving system for CO2 capture. Lack of experimental information on the diffusivity and heat of CO2 absorption in IL-amine hybrid solvents hampers the wide application of IL in CO2 capture. In this work, the effects of IL on absorption rate, solubility, diffusivity and heat of CO2 absorption in feasible hybrids of 1-butyl-3-methyl-imidazolium nitrate ([Bmim][NO3]) and monoethanolamine (MEA) have been elaborated using weight method, vapor liquid equilibrium and calorimetric measurement. The results showed that the addition of IL slightly decreases CO2 loading under atmospheric pressure. Based on the measured solubility and calorimeter results, IL is beneficial to improve the physical absorption of CO2 and reduce the heat of absorption in IL-amine hybrids. The diffusivity of CO2 can be maintained at an acceptable level by controlling the concentration of IL. Considering CO2 capacity, kinetics and heat of absorption, the hybrids of 30% MEA+10% [Bmim][NO3]+60% H2O were recommended. Our work gives the meaningful insights on designing novel IL-amine hybrids for CO2 capture.

Analytical study of membrane wetting at high operating pressure for physical absorption of CO2 using hollow fiber membrane contactors

In the present work, an average membrane wetting ratio at high operating pressure of hollow fiber polyvinylidenefluoride (PVDF) and polytetrafluoroethylene (PTFE) membrane contactors was estimated analytically based on log-normal distribution function. The membrane characteristics, liquid velocity, operating pressure were also considered for the calculation. The simulations were validated with experimental data to confirm the accuracy of the calculated average membrane wetting at high pressure.At the operating pressure of 1bar, dry modes are observed for both PVDF and PTFE. While the results of PTFE correspond with the dry mode, the partially wetted mode well predicts the results of PVDF at high operating pressure. The increases of liquid velocity and operating pressure show the higher membrane wetting due to the influences of increasing pressure drop in membrane module and lower surface tension of CO2-dissolved water at high operating pressure. Moreover, PTFE provides lower average membrane wetting compared with PVDF since the average membrane wetting ratio from the calculation occurs at higher trans-membrane pressure at the outlet liquid (>1bar) which can be possible to maintain the dry mode including during the start-up of the process for the highest absorption performance using PTFE.

Removal of carbon dioxide from pressurized landfill gas by physical absorbents using a hollow fiber membrane contactor

This article details the physical absorption of CO2 from synthetic landfill gas (containing 40% CO2 and 60% CH4) using a hollow fiber membrane contactor (HFMC) at elevated pressures in the range of 0.1–1.2Mpa. Polyethylene glycol dimethyl ether (Selexol) was used as absorbent in HFMC for the first time. The effects of operating parameters on the absorption process were investigated. The results show that the use of Selexol as absorbent gives higher CO2 absorption fluxes and removal efficiencies compared to water. The highest absorption flux reached 1.61×10−3mol/m2s at 1.2Mpa. A 2D mathematical model and a resistance-in-series model were used together to analyze the absorption process and membrane wetting at different pressures. Due to the relatively lower surface tension of Selexol, the absorption process suffers more negative effects from membrane wetting at higher pressures. The change of mass transfer resistances in each phase along with the elevated pressures was theoretically calculated. The results reveal that even when slight membrane wetting occurs, resistance in the wetted part of the membrane is nonnegligible.

Ionic liquids for post-combustion CO2 capture by physical absorption: Thermodynamic, kinetic and process analysis

The post-combustion CO2 physical absorption with ionic liquids (ILs) is studied in this work using a COSMO-based methodology that allows including ILs into Aspen Plus. Firstly, the performance of 8 ILs with different nature in the absorption and regeneration individual operations is evaluated in commercial packed columns at different temperature and pressure conditions adding thermodynamic, mass transfer kinetic and technical criteria to the IL selection. Secondly, both the absorption and regeneration are integrated in a complete CO2 capture process simulation. The interdependency of variables and their influence in the total operating cost (OPEX) is estimated. The total energy needed for this capture process is compared to homologue results presented in literature for other CO2 capture technologies. Finally, we provide a preliminary estimation of the capital cost (CAPEX) of the process for a pilot near to industrial scaled plant.

Carbon Dioxide Fixation by Combined Method of Physical Absorption and Carbonation in NaOH-Dissolved Methanol

The present study investigates the potential of a process as one of the carbon capture and storage/utilization (CCSU) technologies. The process involves a technology that combines the CO2 physical absorption and mineral carbonation in NaOH-dissolved methanol and includes CO2 desorption and methanol regeneration. Sodium methyl carbonate (SMC) is precipitated as CO2-fixing material in relatively highly concentrated NaOH solution and the amount of CO2 physically absorbed is estimated to be approximately 4.36 g CO2/500 mL methanol at ambient condition. The CCSU capacity of the process based on the gas phase and cake weight measurement is calculated to be 11.02 g CO2/(6 g NaOH/500 mL methanol) and 10.17 g CO2/(6 g NaOH/500 mL methanol), respectively. The amounts of CO2 desorbed and CO2 fixed by SMC as well as Na2CO3 via the evaporation of solution that remains after carbonation are estimated to be 5.64 g CO2/(6 g NaOH/500 mL methanol) and 4.53 g CO2/(6 g NaOH/500 mL methanol), respectively. Therefore, the CO2 ...

Molecular Dynamics Simulation Study on CO2 Physical Absorption Mechanisms for Ethylene-Glycol-Based Solvents Using Free Energy Calculations

Molecular mechanisms of CO2 physical absorption are significant for the development of higher-performance CO2 absorbents. In this study, the free energy profiles for CO2 approaching several types of ethylene glycol (EG)-based solvent molecules and their derivatives (diols, ethers, and alkanes, etc.) were evaluated using molecular dynamics simulations. A desolvation barrier appeared in the free energy profiles in the pure component solvent, corroborating the negative contribution of a void effect on CO2 affinities. The profiles in vacuum were also estimated to investigate the functional group effects in a constituent solvent molecule. The profiles for the EG-based solvent molecules showed similar minimum values to their derivative diols, while the values for their derivative ethers and alkanes were markedly higher. CO2 solubility is certainly determined by two major factors: the specific CO2 affinities of the constituent hydroxyl groups and a steric void effect near an absorbed CO2 molecule.

High frequency ultrasonic-assisted CO2 absorption in a high pressure water batch system

Physical absorption process is always nullified by the presence of cavitation under low frequency ultrasonic irradiation. In the present study, high frequency ultrasonic of 1.7MHz was used for the physical absorption of CO2 in a water batch system under elevated pressure. The parameters including ultrasonic power and initial feed pressure for the system have been varied from 0 to 18W and 6 to 41bar, respectively. The mass transfer coefficient has been determined via the dynamic pressure-step method. Besides, the actual ultrasonic power that transmitted to the liquid was measured based on calorimetric method prior to the absorption study. Subsequently, desorption study was conducted as a comparison with the absorption process. The mechanism for the ultrasonic assisted absorption has also been discussed. Based on the results, the mass transfer coefficient has increased with the increasing of ultrasonic power. It means that, the presence of streaming effect and the formation of liquid fountain is more favorable under high frequency ultrasonic irradiation for the absorption process. Therefore, high frequency ultrasonic irradiation is suggested to be one of the potential alternatives for the gas separation process with its promising absorption enhancement and compact design.

Technical, Economic and Environmental Viability of Offshore CO<sub>2</sub> Reuse from Natural Gas by Dry Reforming

The recent discoveries of Pre-Salt layers in Brazil require process developments for enhanced sustainability as these reservoirs have oil with associated natural gas exhibiting an expressive amount of CO2 [1,2]. The challenge is thus to expand the supply of this fossil raw material to produce energy and chemicals in a carbon-constrained economy. In this scenario, CO2 capture and reuse techniques are important routes for a moderate transition from the present fossil-based economy to a long-term sustainable future. This work evaluates two process alternatives for a floating unit consisting of physical absorption of CO2 from natural gas, with propylene carbonate, producing a natural gas stream poor in CO2 and a stream consisting of a mixture of CO2 and hydrocarbons. Two alternatives are evaluated for processing the CO2 rich stream to yield methanol. Technical, economic and environmental analyses of the process flowsheets are performed for comparison purposes. Alternative 1 combines dry and steam reforming in one reactor (one pot reactor, Bi-Reforming), while Alternative 2 segregates the two reactions: dry reforming (Dry-Reforming) occurs in one reactor and water gas-shift reaction takes place in a subsequent reactor. The two Alternatives are evaluated with ASPEN HYSYS® (ASPENTECH Inc) simulator for calculation of energy and mass balances necessary to assess economic and environment performances. The economic evaluation employed the software Capital Cost Estimation (CAPCOST) for calculations of CAPEX and OPEX. The software Waste Reduction Algorithm (WAR, EPA) was used to evaluate the potential environmental impacts, by PEI (Potential Environmental Impact). Results indicate that the performance of Alternative 1 was better than Alternative 2: (a) methanol production (17905 kg/h) 4 times higher, (b) CAPEX (U$$75.497.127), 21,7% lower, (c) Sales Revenue (U$$144,890,686) 181% greater, and (d) the index for environment impact (868 PEI/hr) 47,7% lower.

CO2 Solubility in Ether Functionalized Ionic Liquids on Mole Fraction and Molarity Scales

The effect of ether functional group(s) in the cation, anion, and both of the ions in ionic liquids on physical absorption of CO2 are revisited in the present work. The solubilities of CO2 in ether functionalized ammonium and pyrrolidinium salts, [N211MEE][Tf2N], [Pyr1MOM][Tf2N], and [Pyr1MOM][FSA] were previously reported together with the corresponding alkyl analogues. In addition to such cation-modified ionic liquids, we investigate a new family of ether functionalized ionic liquids with alkoxy sulfates; [C2mim][C1(OC2)2SO4], [P444ME][C6SO4], [P444ME][C1OC2SO4], [P444ME][C1(OC2)2SO4], and [N221ME][C1(OC2)2SO4]. The CO2 solubility data on the molarity as well as the mole fraction scales are presented in a series of ether functionalized ILs with a brief overview of the previously reported results. It has become apparent that the introduction of an ether functional group in anions more effectively improves the CO2 solubilities on both the mole fraction and molarity scales than that in cations. The effects...

Physical Absorption Of CO2 in Protic and Aprotic Ionic Liquids: An Interaction Perspective.

The physical absorption of CO2 by protic and aprotic ionic liquids such as 1-ethyl-3-methyl-imidazolium tetrafluoroborate was examined at the molecular level using symmetry adapted perturbation theory (SAPT) and density functional techniques through comparison of interaction energies of noncovalently bound complexes between the CO2 molecule and a series of ionic liquid ions and ion pairs. These energies were contrasted with those for complexes with model amines such as methylamine, dimethylamine, and trimethylamine. Detailed analysis of the five fundamental forces that are responsible for stabilization of the complexes is discussed. It was confirmed that the nature of the anion had a greater effect upon the physical interaction energy in non functionalized ionic liquids, with dispersion forces playing an important role in CO2 solubility. Hydrogen bonding with protic cations was shown to impart additional stability to the noncovalently bound CO2···IL complex through inductive forces. Two solvation models, the conductor-like polarizable continuum model (CPCM) and the universal solvation model (SMD), were used to estimate the impact of solvent effects on the CO2 binding. Both solvent models reduced interaction energies for all types of ions. These interaction energies appeared to favor imidazolium cations and carboxylic and sulfonic groups as well as bulky groups (e.g., NTf2) in anions for the physical absorption of CO2. The structure-reactivity relationships determined in this study may help in the optimization of the physical absorption process by means of ionic liquids.

Dynamic modelling and validation of pre-combustion CO2 absorption based on a pilot plant at the Buggenum IGCC power station

Based on the increasing concern about human-induced global warming, it is expected that constraints on carbon emissions will be much stricter in the future. Integrated gasification combined cycle (IGCC) power plants are an option in the attempt to mitigate the effect of CO2 emissions, because they can be fuelled or co-fuelled with biomass, but also, very importantly, because they can be equipped with pre-combustion CO2 capture units. Moreover, the rapidly growing share of electricity produced by converting renewable energy sources demands for more flexible operation of future IGCC plants in order to compensate for inherently intermittent operation of wind and solar power plants. The CO2 capture unit should therefore be able to follow the dynamic operation of the power plant, and, at the same time, meet environmental targets in terms of CO2 removal. In order to investigate and improve the transient performance of such complex systems, dynamic process simulations performed with accurate physically based models are essential. This paper presents the development and validation of the dynamic model of the absorption and solvent regeneration unit as part of a pre-combustion CO2 capture system, with focus on the absorber column model. The models are implemented into an open source software library following an object-oriented, equation-based modelling approach using the Modelica language. The models are validated by comparison against two sets of transient experimental data generated at the CO2 capture pilot plant realized and operated at the Buggenum IGCC power station in the Netherlands. The model predictions satisfactorily reproduce the dynamic performance of the pilot plant considering the main process variables, such as pressures, flow rates and compositions. One of the results of this work is that, in case of physical absorption of CO2 a well-tuned, equilibrium-based absorber model is able to accurately predict transient operation. The validated process models enable process and control system design aiming at improvement of transient performance of the pre-combustion capture unit.

Development of Porous Asymmetric Polyamide–Imide Torlon® Membranes for Physical CO2 Absorption and Separation

Porous flat-sheet polyamide–imide (PAI) membranes were prepared via a phase inversion method to evaluate CO2 absorption performance in the gas-liquid membrane contactors. Different amounts of polyethylene glycol (PEG-600) were introduced into the polymer solution to investigate the structure and performance of resulted membranes. The membranes were characterized in terms of gas permeation, contact angle measurement and CO2 absorption flux. By introducing 6 wt.% PEG into the polymer dope, N2 permeance of the membrane was significantly improved from 482 to 1320 GPU. Mean while, the effect of PEG on the measured water contact angle was in significant. From CO2 absorption test, the developed membrane presented about 90% higher CO2flux compared to the plain membrane at water flow rate of 70 ml/min. In conclusion, by introducing a polymeric non-solvent additive into the polymer dope, it is possible to enhance surface porosity (permeability) of PAI membranes, which is a key factor for CO2 absorption test.

Effect of general montmorillonite and Cloisite 15A on structural parameters and performance of mixed matrix membranes contactor for CO2 absorption

Abstract Polyvinylideneflouride (PVDF) hollow fiber mixed matrix membranes (MMMs) were fabricated through wet phase inversion method using general MMT and Cloisite 15A as inorganic fillers. The effect of clay filler loadings (1, 3 and 5 wt% of polymer), their particle size and hydrophobicity on the membrane morphology, structure and performance in gas–liquid contacting process were investigated. The fabricated MMMs were characterized by scanning electron microscopic (SEM), gas permeation test, hydrophobicity, wetting resistance and CO2 absorption test. As expected, MMMs showed asymmetrical structure, differing in finger-like portion, porosity and pore size. The fabricated MMMs with higher finger-like area and surface porosity showed higher permeance than plain membrane. Furthermore, the wetting resistance in terms of surface hydrophobicity and liquid entry pressure of water increased with loading. From the point of view of physical CO2 absorption, the membranes incorporated with small particle size filler (general MMT-filled PVDF membranes) exhibited higher absorption fluxes than those embedded with large particle size fillers. The highest absorption flux for membrane containing 1 wt% general MMT is 1.0 × 10−3 mol m−2 s−1 at the flow rate of 3.1 m s−1. That flux was approximately 54% and 82% higher than the flux of MMMs with embedded Cloisite 15A of the same loading and plain PVDF membrane, respectively. The obtained flux of synthesized membrane was superior compared to several in-house made and commercial membranes.

Heat of absorption of carbon dioxide in mixtures of 2-amino-2-methyl-1-propanol and organic solvents

Information is required on the heat of absorption for the correct design and optimization of absorption processes for the removal of CO2 from gas streams. The heat of absorption affects the temperature profile in the absorber, and consequently the capacity of the absorption unit and the achievable purity of the gas from which CO2 is removed. It is also associated with the energy requirement of the stripper. This usually accounts for the greatest operating cost in the absorption process, and if the amine could be regenerated using less energy or with low-grade energy, i.e. waste heat (70–90°C), significant savings could be made.This paper presents experimental data on the heat of absorption of CO2 in solutions of 2-amino-2-methyl-1-propanol (AMP) and an organic solvent, either N-methyl-2-pyrrolidone or triethylene glycol dimethyl ether, at different amine concentrations (15 wt% and 25 wt%) and temperatures (25°C and 50°C). The heat of absorption was greatly affected by the formation of a precipitate, which was observed during some of the experiments, and was significantly lower at CO2 loadings above 0.5mol CO2/mol AMP, as physical absorption of CO2 became the dominating absorption process.

Purification of Biogas for Energy Use

This work determined the absorption efficiency of H2S in the solution of Fe-EDTA in synthetic biogas flows, as well as the efficiency of physical absorption of CO2 in Fe-EDTA. Biogas was bubbled through a pseudo-catalytic solution of Fe-DTA resulting in the formation of sulfur particles. H2S absorbed in the Fe-EDTA suffered a redox reaction. The Fe-EDTA solution used was regenerated with air. It was used two types of catalysts, in order to compare the efficiency of a catalyst of Fe-EDTA synthesized in the laboratory in an inert atmosphere, in relation to a commercial catalyst containing Fe-EDTA. In the batch study, removing H2S from biogas indicated that the concentration of Fe-EDTA applies great influence on the catalytic activity. For a system operating in a continuous regimen, it was possible to completely eliminate the H2S from biogas with Fe-EDTA synthesized in laboratory. The removal of CO2 present in the biogas was also evaluated. It was possible to achieve maximum efficiency of the CO2 absorption in 90 % in steady state, it was obtained an increase of 16 % in the calorific value of biogas, from 8.03 to 9.28 kWh/m3.