Membrane Gas-Solvent Contactor Pilot Plant Trials of CO2 Absorption from Flue Gas

Membrane gas-solvent contactors have received much attention for CO₂ absorption, as the approach incorporates advantages from both solvent absorption and membrane gas separation. This study reports on pilot plant trials of three membrane contactors for the separation of CO₂ from flue gas. The contactors were porous polypropylene (PP), porous polytetrafluoroethylene (PTFE), and non-porous polydimethylsiloxane (PDMS), with the solvent PuraTreatTM F^TM. To enable performance comparison, laboratory measurements based on a gas mixture of 10% CO₂ in N₂ were also undertaken on the same contactor–solvent systems. It was found that the PP contactor experienced significant pore wetting in both laboratory and pilot plant studies. In contrast, the PTFE contactor experienced only minor pore wetting in the laboratory. However, in the pilot plant trial of the PTFE contactor extensive pore wetting was observed, and the overall mass transfer coefficient measured was comparable with the PP contactor. The non-porous PDMS contactor had an overall mass transfer coefficient two orders of magnitude less than the PP contactor, due to the greater mass transfer resistance of the polymeric film. However, the non-porous membrane does not experience pore wetting, which resulted in the overall mass transfer coefficient being similar for both laboratory and pilot plant measurements.     

Experimental study on membrane wetting in gas–liquid membrane contacting process for CO2 absorption by single and mixed absorbents

The membrane wetting by the liquid absorbents is an important problem in the operation of gas–liquid membrane contacting process. In order to gain a better understanding on the role of absorbents on membrane wetting, monoethanolamine (MEA, primary amine), diethanolamine (DEA, secondary amine), and 2-amino-2-methyl-1-propanol (AMP, sterically hindred amine) were applied as absorbent solutions. The membrane used for the experiments was the hollow fiber polyvinylidenefluoride (PVDF) membrane. The performance of both single and mixed amine solutions on the CO₂ absorption capacity and membrane wetting potential were investigated. In addition, sodium chloride (NaCl, inorganic salt) and sodium glycinate (SG, organic salt) were added into the MEA aqueous solution to observe CO₂ flux and membrane wetting.The results revealed that the use of MEA solution and SG as absorbents gave highest CO₂ flux. The overall mass transfer coefficients obtained from the experiments also showed the same trend as CO₂ flux, i.e, the values were in the following order: MEA> AMP > DEA. However, the long-term flux was monitored and it was found that MEA also gave lowest flux decline due to the membrane wetting. The use of mixed amine solutions and the addition of NaCl did not help protect the membrane wetting. On the contrary, the addition of SG in to MEA solution can improve flux and resulted in stable CO₂ flux indicating that the membrane wetting was negligible.     

Non-oxidative thermal degradation of amines: GCMS/FTIR spectra analysis and molecular modeling

Aqueous amino solvents, such as monoethanolamine (ETA/MEA), methyl diethanolamine (MDEA) or amine blends, are the most widely used solvents in commercial CO₂ or acid gas separation applications. These commercial solvents have various disadvantages, such as the possibilities of the solvent to be degraded. This research examines the impact of non-oxidative thermal degradations on the performance of the CO₂ absorption and the degradation mechanism of amine solvents. The impact of degradation was conducted by measuring the CO₂ solubility of solvent that had been heated to 120°C for 2 h. Although the performance of CO₂ absorption was not significantly reduced, the degradation of amines was found. Supported by Fourier Transform Infrared (FTIR) and Gas Chromatography/Mass Spectrometer result, the suspected products of non-oxidative thermal degradation of MDEA were MEA and acetone.     

Reduction of CO2 emissions by a membrane contacting process☆

A membrane-based gas–liquid contacting process was evaluated in this work for CO₂ removal from flue gases. The absorption of CO₂ from a CO₂–N₂ mixture was investigated using a commercial hollow fiber membrane contactor and water or diethanolamine as absorbing solvents. Significant CO₂ removal (up to 75%) was achieved even with the use of pure water as absorbent. By using aqueous amine solutions and chemical absorption, mass transfer improved, and CO₂ removal was nearly complete (∼99%). A mathematical model was developed to simulate the process and it was validated with experimental data. Results show that membrane contactors are significantly more efficient and compact than conventional absorption towers for acid gas removal.     

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.     

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

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

复合溶液膜吸收CO2的性能及其对膜孔润湿的影响

利用自行搭建的CO₂膜吸收实验台，采用聚丙烯（PP）膜组件，以质量分数10%的N-甲基二乙醇胺（MDEA）作为主体胺溶液，添加不同配比的哌嗪（PZ）、乙醇胺（MEA）、甘氨酸钾（PG），考察CO₂脱除效率和传质速率的变化，比较不同复配比的复合溶液表面张力以及对PP膜的浸润性，并以10%MDEA+10%PG混合溶液作为吸收液进行长时间实验。结果表明：添加少量的添加剂对MDEA溶液膜吸收CO₂均有显著的促进作用，当配比小于0.2时，促进作用大小为PZ>MEA>PG；当配比大于0.2时，促进作用大小为PZ>PG>MEA；PZ和MEA均随着添加配比的增加，溶液表面张力减小，而PG相反；表面张力小的溶液对膜浸润性较强，容易造成膜润湿；添加剂质量分数均为10%时，对膜溶胀性和疏水性以及膜孔结构影响大小为PZ>MEA>PG；在20天内，PG/MDEA混合溶液作用下的CO₂脱除效率从89.56%下降为83.09%，对PP的疏水性影响较小，膜组件可以稳定运行。吸收液表面张力对膜吸收法脱除CO₂性能的影响显著。所得结果可为膜吸收CO₂吸收剂复配提供依据，并可为揭示膜吸收CO₂过程中膜润湿导致膜失效的机理以及抑制膜润湿提供实验数据。     

Hollow fiber membrane contactor transient experiments for the characterization of gas/liquid thermodynamics and mass transfer properties

We present results from experiments and numerical simulations of contact between a non-reactive gas (N₂O and CO₂) and a physical solvent (H₂O) occurring in a polypropylene (PP) hollow fiber membrane contactor. The closed-loop liquid flow within the experimental setup provides transient curves representing the progressive saturation of the solvent by the gas. We develop an in-house numerical model to fully characterize the gas/liquid mass transfer both in the non-wetted and in the wetted modes, i.e., when the liquid starts partially wetting the pores of the membrane. Using experiments and numerical simulations, we show that the Henry constant (H) and the molecular diffusion coefficient of a non-reactive gas absorbing into a liquid solvent can be extracted by parameter estimation. Both parameters are obtained within a single experiment at a constant temperature and the comparison with temperature-dependant correlations yields excellent agreement over the whole range of temperature studied in this work. Simulations show a partial wetting of the membrane pore by the liquid meniscus during a contact between CO₂ and H₂O, possibly due to the plasticizer effect of CO₂ inside the membrane contactor fibers.     

Removal of carbon dioxide by absorption in mixed amines: modelling of absorption in aqueous MDEA/MEA and AMP/MEA solutions

This work presents an investigation of CO₂ absorption into aqueous blends of methyldiethanolamine (MDEA) and monoethanolamine (MEA), as well as 2-amino-2-methyl-1-propanol (AMP) and monoethanolamine (MEA). The combined mass transfer–reaction kinetics–equilibrium model to describe CO₂ absorption into the amine blends has been developed according to Higbie's penetration theory following the work of Hagewiesche et al. (Chem. Eng. Sci. 50 (1995) 1071). The model predictions have been found to be in good agreement with the experimental rates of absorption of CO₂ into (MDEA+MEA+H₂O) of this work and into (AMP+MEA+H₂O) reported by Xiao et al. (Chem. Eng. Sci. 55 (2000) 161), measured at higher contact times using wetted wall contactor. 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 MDEA/MEA and AMP/MEA.     

Theoretical and Experimental Studies on Acetylene Absorption in a Polytetrafluoroethylene Hollow‐Fiber Membrane Contactor

The separation of acetylene from a gas mixture was investigated using a polytetrafluoroethylene hollow‐fiber membrane contactor and 1‐methyl‐2‐pyrrolidinone as absorbent. The effects of the gas velocity, the liquid velocity, the feed gas concentration, and the module length on the acetylene mass transfer were investigated. The results showed that the acetylene mass transfer flux increased with increasing liquid velocity, gas velocity, and feed gas concentration, but decreased with increasing membrane module length. A mathematical model was used to predict the wetting extent of the membrane and the mass transfer resistance in the acetylene mass transfer process. The wetting extent of the membrane was found to increase with increasing liquid velocity and to be effectively restrained with increasing gas velocity. The liquid phase resistance and the wetted‐membrane phase resistance controlled the acetylene mass transfer in the acetylene absorption process. The acetylene absorption efficiency was maintained at 90 % for 114 h of the C₂H₂ membrane absorption–thermal desorption cycle process.     

Experimental study of CO2 absorption/stripping via PVDF hollow fiber membrane contactor

Gas–liquid hollow fiber membrane contactor can be a promising alternative for the CO₂ absorption/stripping due to the advantages over traditional contacting devices. In this study, the structurally developed hydrophobic polyvinylidene fluoride (PVDF) hollow fiber membranes were prepared via a wet spinning method. The membranes were characterized in terms of morphology, permeability, wetting resistance, overall porosity and mass transfer resistance. From the morphology analysis, the membranes demonstrated a thin outer finger-like layer with ultra thin skin and a thick inner sponge-like layer without skin. The characterization results indicated that the membranes possess a mean pore size of 9.6 nm with high permeability and wetting resistance and low mass transfer resistance (1.2 × 10⁴ s/m). Physical CO₂ absorption/stripping were conducted through the fabricated gas–liquid membrane contactor modules, where distilled water was used as the liquid absorbent. The liquid phase resistance was dominant due to significant change in the absorption/stripping flux with the liquid velocity. The CO₂ absorption flux was approximately 10 times higher than the CO₂ stripping flux at the same operating condition due to high solubility of CO₂ in water as confirmed with the effect of liquid phase pressure and temperature on the absorption/stripping flux.     

CO2 Capture Using Activated Amino Acid Salt Solutions in a Membrane Contactor

An activated solution based on amino acid salt was proposed as a CO₂ absorbent. Piperazine (PZ) was selected as an activating agent and added into the aqueous glycine salt to form the activated solution. A coupling process, which associated the activated solution with a PP hollow fiber membrane contactor, was set up. An experimental and theoretical analysis for CO₂ capture was performed. The performances of CO₂ capture by the coupling process were evaluated using the PZ activated solution and the non-activated glycine salt solution. A numerical model for the simulation of the hollow fiber membrane gas–liquid mass transfer was developed. Typical parameters such as outlet gas phase CO₂ concentration, capture efficiency, and mass transfer coefficient for the activated solution were determined experimentally. The effects of operation temperature and liquid CO₂-loading on mass transfer coefficient and capture efficiency were discussed in this work. Axial and radial concentration profiles of CO₂ in the fiber lumen and mass transfer flux were simulated by the model. Results show that the performances of the PZ activated glycine salt solution are evidently better than that of the non-activated glycine salt solution in the membrane contactor for CO₂ capture. Elevation of the operation temperatures can enhance the overall mass transfer coefficient. The activated solution can maintain higher capture efficiency especially in the case of high CO₂-loadings. The gas phase CO₂ concentration with the activated solution is lower than that with the non-activated solution whether along axial or radial distances in the fiber lumen. The model simulation is validated with experimental data.     

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.     

Sulfur dioxide non‐dispersive absorption in N,N‐dimethylaniline using a ceramic membrane contactor

BACKGROUND: Removal of sulfur dioxide from gas emissions by selective absorption is a common method to separate and concentrate sulfur dioxide and to reduce air pollution and environmental risks. N,N‐dimethylaniline is an organic solvent used in some industrial applications for its sulfur dioxide affinity, leading to a regenerative process. However, the use of scrubbers and equipment in which direct contact between gas and liquid takes place leads to solvent losses due to evaporation and drops dragging. RESULTS: In this work, an innovative procedure based on non‐dispersive absorption in a ceramic hollow fibre membrane contactor was studied in order to avoid drops dragging. The absorption efficiency ranged between 40 and 50%, showing the technical viability of the process. The sulfur dioxide flux through the membrane has a linear relationship with the concentration of SO₂ in the gas stream and an overall mass transfer coefficient K_overall = (1.10 ± 0.11) × 10^?5 m s^?1 has been obtained. CONCLUSIONS: The mass transfer behaviour of a ceramic hollow fibre membrane contactor for sulfur dioxide non‐dispersive absorption in N,N‐dimethylaniline has been studied. The main resistance is found to be the ceramic membrane and the effective diffusivity has been inferred. The mass transfer model and parameters allow the evaluation of equipment design for technical applications. Copyright © 2008 Society of Chemical Industry     

Comparison of solvent performance for CO2 capture from coal-derived flue gas: A pilot scale study

The performance of a proprietary solvent (CAER-B2), an amine-carbonate blend, for the absorption of CO₂ from coal-derived flue gas is evaluated and compared with state-of-the-art 30 wt% monoethanolamine (MEA) under similar experimental conditions in a 0.1 MWth pilot plant. The evaluation was done by comparing the carbon capture efficiency, the overall mass transfer rates, and the energy of regeneration of the solvents. For similar carbon loadings of the solvents in the scrubber, comparable mass transfer rates were obtained. The rich loading obtained for the blend was 0.50 mol CO₂/mol amine compared to 0.44 mol CO₂/mol amine for MEA. The energy of regeneration for the blend was about 10% lower than that of 30 wt% MEA. At optimum conditions, the blend shows promise in reducing the energy penalty associated with using industry standard, MEA, as a solvent for CO₂ capture.     

Mass transfer correlations for membrane gas-solvent contactors undergoing carbon dioxide desorption

Membrane gas-solvent contactors are a hybrid technology combining solvent absorption with membrane gas separation, which demonstrates potential for CO₂ capture through the ability of the membrane to rigidly control the mass transfer area. Membrane contactors have been successfully demonstrated for CO₂ absorption, and there is strong research interest in using membrane contactors for the complimentary CO₂ desorption process to regenerate the solvent. However, understanding and modelling the various stages of mass transfer in the desorption process is less well-known, given the existing mass transfer correlations had been developed from absorption experiments. Hence, mass transfer correlations for membrane contactors are reviewed here, and their appropriateness for desorption analysed. This is achieved through simulating CO₂ desorption through a membrane contactor from loaded 30wt% monoethanolamine solvent to enable comparison of the correlations. It was found that the most cited correlations by Yang and Cussler were valid for shell side parallel flow, while that of Kreith and Black was viable for shell side cross flow. A limitation of all of these correlations is that they assume single phase flow on both sides of the membrane; however, the high temperature of CO₂ desorption can lead to partial solvent vaporisation and hence two phases present on one side of the membrane contactor during desorption. A mass transfer correlation is established here for two phase parallel flow on the shell side of a membrane contactor, based on experimental results for three composite and one asymmetric hollow fibre membrane contactors stripping CO₂ from loaded MEA at 105-108℃. This correlation is comparable to that reported in the literature for mass transfer in other two phase systems, but differs from the standard format for membrane contactors in terms of the exponent on the dimensionless Schmidt and Reynolds numbers.     

Enhancing CO2 absorption efficiency using a novel PTFE hollow fiber membrane contactor at elevated pressure

The internal structure design of membrane module is very important for gas removal performance using membrane contactor via physical absorption. In this study, a novel membrane contactor developed by weaving polytetrafluoroethylene (PTFE) hollow fibers was applied to remove CO₂ from 60% N₂ + 40% CO₂ mixture (with CO₂ concentration similar to that of biogas) at elevated pressure (0.8 MPa) using water as absorbent. Compared with the conventional module with randomly packed straight fibers, the module with woven PTFE fibers exhibited much better CO₂ absorption performance. The weaving configuration facilitated the meandering flow or Dean vortices and renewing speed of water around hollow fibers. Meanwhile, the undesired influences such as channeling and bypassing were also eliminated. Consequently, the mass transfer of liquid phase was greatly improved and the CO₂ removal efficiency was significantly enhanced. The effects of operation pressure, module arrangement, feed gas, and water flow rate on CO₂ removal were systematically investigated as well. The overall mass‐transfer coefficient (K_OV) varied from 1.96 × 10^?5 to 4.39 × 10^?5 m/s (the volumetric mass‐transfer coefficient K_La = 0.034–0.075 s^?1) under the experimental conditions. The CO₂ removal performance of novel woven fiber membrane contactor matched well with the simulation results. © 2017 American Institute of Chemical Engineers AIChE J, 64: 2135–2145, 2018     

Prediction of CO2/O2 absorption selectivity using supported ionic liquid membranes (SILMs) for gas–liquid membrane contactor

Given their unique and tunable properties as solvents, ionic liquids (ILs) have become a favorable solvent option in separation processes, particularly for capturing carbon dioxide (CO₂). In this work, a simple method that can be used to screen the suitable IL candidates was implemented in our modified gas–liquid membrane contactor system. Solubilities, selectivities of CO₂, nitrogen (N₂), and oxygen (O₂) gases in imidazolium-based ILs and its activity coefficients in water and monoethanolamine (MEA) were predicted using conductor-like screening model for real solvent (COSMO-RS) method over a wide range of temperature (298.15–348.15?K). Results from the analysis revealed that [emim] [NTf₂] IL is a good candidate for further absorption process attributed to its good hydrophobicity and CO₂/O₂ selectivity characteristics. While their miscibility with pure MEA was somehow higher, utilizing the aqueous phase of MEA would be beneficial in this stage. Data on absorption performances and selectivity of CO₂/O₂ are scarce especially in gas–liquid membrane contactor system. Therefore, considering [emim] [NTf₂] IL as a supporting material in supported ionic liquid membranes (SILMs), using aqueous phase of MEA as an absorbent would result in a great membrane-solvent combination system in furthering our gas–liquid membrane contactor process. In conclusion, COSMO-RS is a potentially great predictive utility to screen ILs for specified separation applications. In addition, this work provides useful results for the [emim] [NTf₂]-SILMs to be extensively applied in the field of CO₂ capture and selective O₂ removal.     

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     

The effect of membrane pores wettability on CO2 removal from CO2/CH4 gaseous mixture using NaOH,MEA and TEA liquid absorbents in hollow fiber membrane contactor

The present paper renders a modeling and a 2D numerical simulation for the removal of CO₂ from CO₂/CH₄ gaseous stream utilizing sodium hydroxide (NaOH), monoethanolamine (MEA) and triethanolamine (TEA) liquid absorbents inside the hollow fiber membrane contactor. Counter-current arrangement of absorbing agents and CO₂/CH₄ gaseous mixture flows are implemented in the modeling and numerical simulation. Non-wetting and partial wetting modes of operation are considered where in the partial wetting mode, CO₂/CH₄ gaseous mixture and liquid absorbents fill the membrane pores. The deteriorated removal of CO₂ in the partial wetting mode of operation is mainly due to the mass transfer resistance imposed by the liquid in the pores of membrane. The validation of numerical simulation is done based on the comparison of simulation results of CO₂ removal using NaOH and experimental data under non-wetting mode of operation. The comparison illustrates a desirable agreement with an average deviation of less than 5%. According to the results, MEA provides higher efficiency for CO₂ removal in comparison with the other liquid absorbents. The order for CO₂ removal performance is MEA > NaOH > TEA. The influence of non-wetting and partial wetting modes of operation on CO₂ removal are evaluated in this article as one of the novelties. Besides, the percentage of CO₂ sequestration as a function of gas velocity for various percentages of membrane pores wetting ranging from 0 (non-wetting mode of operation) to 100% (complete wetting mode of operation) is studied in this research paper, which can be proposed as the other novelty. The results indicate that increase in some operational parameters such as module length, membrane porosity and absorbents concentration encourage the removal percentage of CO₂ from CO₂/CH₄ gaseous mixture while increasing in membrane tortuosity, gas velocity and initial CO₂ concentration has unfavorable influence on the separation efficiency of CO₂.