Chemical Absorption Of Carbon Dioxide Research Articles

Enabling the direct solution of challenging computer-aided molecular and process design problems: Chemical absorption of carbon dioxide

The search for improved CO2 capture solvents can be accelerated by deploying computer-aided molecular and process design (CAMPD) techniques to explore large molecular and process domains systematically. However, the direct solution of the integrated molecular-process design problem is very challenging as nonlinear interactions between physical properties and process performance render a large proportion of the search space infeasible. We develop a methodology that enables the direct and reliable solution of CAMPD for absorption–desorption processes, using the state-of-the-art SAFT-γ Mie group contribution approach to predict phase and chemical equilibria. We develop new feasibility tests and show them to be highly efficient at reducing the search space, integrating them in an outer-approximation algorithm. The framework is applied to design an aqueous solvent and CO2 chemical absorption–desorption process, with 150 CAMPD instances across three case studies solved successfully. The optimal solvents are more promising than those obtained with sequential molecular design approaches.

CO2 absorption in aqueous NH3 solutions: Novel dynamic modeling of experimental outcomes

It is well known that CO2 capture and re-use is one of the main challenges to be pursued in order to tackle global warming issues. Aqueous ammonia solutions are among the most promising sorbents for post-combustion CO2 capture and could represent a valid and potentially economical alternative to the use of conventional alkanolamines, due to their higher absorption capacity, lower energy requirements for sorbent regeneration and greater resistance to oxidative and thermal degradation. Despite its apparent simplicity and convenience, the dynamic evolution of CO2 − NH3 system needs to be further investigated through proper mathematical models that permit to design, optimize, and control the capture process.In this work, the chemical absorption of carbon dioxide contained in a simulated flue gas (N2+CO2; CO215%v/v) by means of aqueous NH3 solutions was investigated both experimentally and theoretically. In particular, a rigorous mathematical model, capable to quantify the CO2 capture efficiency dynamics and the sorbent chemical composition during the process, is proposed for the first time. The model is validated by comparing modeling results with experimental data obtained under different operating conditions. The effect of both operating temperature and sorbent concentration are investigated. The good agreement between model results and experimental data confirms the effectiveness and the reliability of the developed tool that turns out to be able to quantify the dynamics of capture efficiency during the variation of the operating conditions. Therefore, it may be exploited to properly design, optimize and control the capture process and the absorbent regeneration section whose energy requirements also depend on the species concentration into the absorbent solution.

Process Modeling of CO2 Absorption with Monoethanolamine Aqueous Solutions Using Rotating Packed Beds

A first-principle process simulation model is presented for the chemical absorption of carbon dioxide (CO2) with monoethanolamine (MEA) aqueous solutions using rotating packed beds (RPB). Built on a proven rate-based packed bed absorber model, the RPB model rigorously simulates the phase and chemical equilibria at the vapor–liquid interface, the heat and mass transfer across the gas and liquid films, the fast reactions between MEA and CO2 in the liquid film, and the RPB hydraulics. Estimation of the rate of transfer of mass across the liquid film is central to accurate simulation of the CO2 absorption process with MEA aqueous solutions. We show that the literature lab-scale RPB data for CO2 removal efficiency can be satisfactorily correlated by introducing a correction factor for the effective packing surface area predicted by the Onda correlation. Given the validated RPB model, we further show that, among the gas-phase mass transfer coefficient, the liquid-phase mass transfer coefficient, and the reaction rate constant for the reaction between amine and CO2, the reaction rate constant is the controlling step with the greatest potential to enhance the CO2 absorption performance in RPB.

Flexible Operation of the Potassium Taurate Solvent Absorption Section for CO2 Removal in a Coal-Fired Power Plant

To reduce global warming effects due to the increase of the carbon dioxide concentration in the atmosphere, CO2 can be removed from large emitting industrial sources as power production stations with several available technologies. Among these, the most common one is chemical absorption with aqueous amines. The benchmark solvent is MonoEthanolAmine (MEA), though presenting a few disadvantages such as being corrosive and toxic and requiring large amounts of energy for its regeneration. An aqueous solution of potassium taurate has the potential to replace MEA because of degradation resistance, low toxicity and low energy requirements. In addition, at specific conditions, it can enhance the chemical absorption of carbon dioxide by precipitating and forming a slurry with presence of solid taurine. The considered process scheme is similar to the one for the MEA system (basically composed of absorption column and stripping or distillation column), with the addition of a heat exchanger for the dissolution of any solid particle present in the system before being fed to the regeneration section. When applied to a power plant, CO2 removal decreases the revenues from the sale of electricity. Indeed, the requirement of thermal energy for running the reboiler of the regeneration section and of electrical energy for compressing the separated CO2 are some of the main drawbacks of the application of Carbon Capture and Storage (CCS) and may result in a significant decrease in the power output and profit. Operating the CO2 removal section in flexible mode significantly reduces effects on the profit and CO2 emissions. The research reported here focuses on determining the best operating conditions for the potassium taurate solvent absorption section with the aim of limiting the impact of the CO2 capture operation and maintaining a substantial reduction of CO2 emissions in a coal-fired power plant. The process of post-combustion scrubbing is well-suited for operation in flexible mode because it is located downstream of the power production system. For simulating the process, a tool previously developed and based on the commercial software ASPEN Plus® has been employed. The process simulator had been user customized for the representation of the chemical reacting system of carbon dioxide with potassium taurate, which is not present by default in the database. The Vapor-Liquid-Solid Equilibrium (VLSE) is considered in the thermodynamic model and rate-based simulations, also taking into account the kinetics, have been performed. An in-house tool, taking as input the data for electricity demand and price and the result of simulations in ASPEN Plus®, has been then used for analyzing the flexible operation of the plant. The study of a flexible solution for carbon dioxide removal has been carried out in this work, considering the amount of electricity sold during the day for which data are available from the Italian national service institution.

Chemical absorption of carbon dioxide in non-aqueous systems using the amine 2-amino-2-methyl-1-propanol in dimethyl sulfoxide and N-methyl-2-pyrrolidone

Non-aqueous amine systems have been suggested as energy-efficient alternatives to conventional aqueous amine systems in post-combustion carbon capture, as low regeneration temperatures can be achieved. The solubility of CO2 and heat of absorption in non-aqueous systems were studied using the sterically hindered amine 2-amino-2-methyl-1-propnaol (AMP) in the organic solvent dimethyl sulfoxide (DMSO). 13C NMR was used to study the product species in solution as CO2 reacts with AMP in either DMSO or N-methyl-2-pyrrolidone (NMP). The solubility of CO2 in AMP/DMSO showed that low loadings could be achieved at 80–88 °C, indicating that regeneration can be carried out at lower temperatures than in conventional aqueous systems. Precipitation occurred at 25 wt% AMP in DMSO, increasing the overall capacity of the system. The heat of absorption decreased with increasing temperature, and was explained by physical absorption dominating the absorption mechanism at higher temperatures. This was also confirmed by the results of NMR, as less chemically absorbed species were observed at higher temperatures. The reaction products observed in AMP/DMSO and AMP/NMP were identified as the AMP carbamate, bicarbonate from water impurities, and the AMP carbonate from CO2 reacting with the hydroxyl group of AMP.

Theoretical investigations on the effect of absorbent type on carbon dioxide capture in hollow-fiber membrane contactors.

Chemical absorption of carbon dioxide from flue or natural gas in hollow-fiber membrane contactors (HFMCs) has been one of the most beneficial techniques to alleviate its emission into the environment. A theoretical research study was done to investigate the change in membrane specifications and operating conditions on CO2 absorption using different alkanolamine solvents. The mathematical model was developed for a parallel counter-current fluid flow through a HFMC. The developed model’s equations were solved based on finite element method. The simulations revealed that the increase in membrane porosity, length and the number of fibers has a positive impact on CO2 removal, while the gas flow rate and tortuosity enhancement resulted in the reduction of CO2 absorption. Furthermore, it was found that 4-diethylamino-2-butanol (DEAB) with approximately 100% CO2 absorption is suggested as the best solvent in this system, but ethyl-ethanolamine (EEA) with only 46% CO2 absorption had the lowest capacity for CO2 absorption (DEAB>MEA>EDA>MDEA>TEA>EEA). It is worth pointing out that the CO2 absorption can be improved using EEA solvent via change in membrane specifications such as increase in membrane porosity, length and the number of fibres.

Bubble dynamics and mass transfer characteristics from an immersed orifice plate

AbstractBACKGROUNDBubble dynamics plays an important role in performance enhancement of multiphase reactor. The formation of the bubbles at the orifice is critical for the dynamics of bubbles and the mass transfer process during the rising stage. However, few studies have focused on the detailed changing characteristic of bubble formation and its relationship with mass transfer.RESULTSThe evolution of the bubble dynamics from millimeter to micrometer‐sized immersed orifice plates is in situ visualized. Bubble inrush is observed from the orifices with diameters below 421 μm, resulting in significant fluctuation on the gas–liquid interface. The base of the leading bubble varies with the orifice diameter. Smaller orifice is preferred to produce more trailing bubbles with smaller size. The inrush time of the trailing bubble is prolonged with decreasing orifice diameter and increasing gas velocity. The inrush time and diameter of the latter trailing bubble are higher than the former at a certain gas velocity. The volumetric mass transfer coefficient (KLa) is obtained by measuring the bubble volume during the rising process using chemical absorption of carbon dioxide (CO2) with sodium hydroxide (NaOH). Increasing gas velocity, decreasing orifice diameter and bubble inrush make an apparent positive impact on the KLa.CONCLUSIONSThe bubble inrush happens at micrometer orifice. Decreasing orifice diameter and increasing gas velocity are important to intensify oscillation of the leading bubble and increase the frequency of bubbles with high specific interfacial area, which are conducive to the enhancement of the volumetric mass transfer coefficient. © 2020 Society of Chemical Industry

Kinetics of chemical absorption of carbon dioxide into aqueous calcium acetate solution

AbstractIncreasing energy demand in the world leads to more electricity generation mainly at fossil fuel power plants. Greenhouse gases are thus produced and mostly emitted to the atmosphere directly, resulting in global warming and climate change. Carbon dioxide is believed to be a main pollutant among greenhouse gases responsible from global warming. Conventional systems using mostly amine solutions to capture carbon dioxide at the source have some disadvantages, and alternatives are constantly being searched. In this work, a benign system of aqueous calcium acetate solution was investigated for this purpose. Calcium acetate is easy to produce, relatively cheap, environmentally friendly, nonhazardous, and noncorrosive. These properties make it a great alternative for use in capturing carbon dioxide. This absorption process is accompanied by chemical reaction. Therefore, the reaction kinetics needs to be investigated before its use in absorbers. A stirred cell reactor was used in the experiments using aqueous calcium acetate solution of different concentrations (2‐20% w/w) and different carbon dioxide concentrations in gas mixtures (4.5‐100% v/v dry carbon dioxide) at temperatures ranging from 286 to 352 K. The Gibbs free energy change for the overall reaction between carbon dioxide and aqueous calcium acetate solution was found to be –2.75 kJ/mol that shows the reaction is exergonic and occurs spontaneously. It was also found out that the reaction is pseudo–first order with respect to carbon dioxide which was also proven by calculating the Hatta number. Activation energy and Arrhenius (frequency) constant were also determined experimentally.

Chemically reactive membrane crystallisation reactor for CO2–NH3 absorption and ammonium bicarbonate crystallisation: Kinetics of heterogeneous crystal growth

The feasibility of gas-liquid hollow fibre membrane contactors for the chemical absorption of carbon dioxide (CO2) into ammonia (NH3), coupled with the crystallisation of ammonium bicarbonate has been demonstrated. In this study, the mechanism of chemically facilitated heterogeneous membrane crystallisation is described, and the solution chemistry required to initiate nucleation elucidated. Induction time for nucleation was dependent on the rate of CO2 absorption, as this governed solution bicarbonate concentration. However, for low NH3 solution concentrations, a reduction in pH was observed with progressive CO2 absorption which shifted equilibria toward ammonium and carbonic acid, inhibiting both absorption and nucleation. An excess of free NH3 buffered pH suitably to balance equilibria to the onset of supersaturation, which ensured sufficient bicarbonate availability to initiate nucleation. Following induction at a supersaturation level of 1.7 (3.3 M NH3), an increase in crystal population density and crystal size was observed at progressive levels of supersaturation which contradicts the trend ordinarily observed for homogeneous nucleation in classical crystallisation technology, and demonstrates the role of the membrane as a physical substrate for heterogeneous nucleation during chemically reactive crystallisation. Both nucleation rate and crystal growth rate increased with increasing levels of supersaturation. This can be ascribed to the relatively low chemical driving force imposed by the shift in equilibrium toward ammonium which suppressed solution reactivity, together with the role of the membrane in promoting counter-current diffusion of CO2 and NH3 into the concentration boundary layer developed at the membrane wall, which permitted replenishment of reactants at the site of nucleation, and is a unique facet specific to this method of membrane facilitated crystallisation. Free ammonia concentration was shown to govern nucleation rate where a limiting NH3 concentration was identified above which crystallisation induced membrane scaling was observed. Provided the chemically reactive membrane crystallisation reactor was operated below this threshold, a consistent (size and number) and reproducible crystallised reaction product was collected downstream of the membrane, which evidenced that sustained membrane operation should be achievable with minimum reactive maintenance intervention.

Experimental study of carbon dioxide absorption by mixed aqueous solutions of methyl diethanolamine (MDEA) and piperazine (PZ) in a microreactor

In this study, chemical absorption of carbon dioxide (CO2) was performed by using MDEA+PZ (aMDEA) solvent. A T-shaped microreactor mixer was used as the mass transfer device to improve the absorption rate. Operating conditions used in this study were operating temperature (15–55°C), input solvent flow rate (0–0.4lmin−1), inlet gas flow rate (1-9lmin−1), and amine concentration in the solvent (0–40 weight percent). All the experiments were carried out at atmospheric pressure and CO2 concentration of 10% in the feed gas. The central composite design method was used to set an experimental design. The results of optimization showed that the use of a microreactor for CO2 adsorption using aMDEA solvent significantly increased the total coefficient of mass transfer in the gas phase, as compared with other mass transfer devices. The maximum value obtained through the model within the range of operating variables for the response (total mass transfer coefficient in the gas phase) was 1769kmolm−3h−1kPa−1.

Rate-based simulation and techno-economic analysis of coal-fired power plants with aqueous ammonia carbon capture

Ammonia is a promising solvent for the chemical absorption of carbon dioxide owing to its characteristics of low toxicity, low degradation, low cost, and low heat of desorption. The present work evaluates the energy and economic performances of the aqueous ammonia technology applied to a coal-fired power plant. First, the kinetics of absorption are characterized in the Aspen Plus software and are calibrated against the experimental data from the Munmorah pilot plant. Then, the validated model is used for sizing the full-scale plant columns and for simulating the overall capture plant. After determining the sizes of the plant components and their energy consumptions, the capture plant is integrated with the coal-fired power plant, and the economic analysis is assessed with both retrofitted and green-field approaches as described in reports from the European Benchmarking Task Force and the United States National Energy Technology Laboratory, respectively. The novelties presented here are the new approach in the absorption reactions implemented in the rate-based model, which changes the dependency of the ammonia concentration in the absorption process, as well as the full economic analysis with both retrofitted and green-field assumptions, which enable a sound comparison of the outcomes. The results with the retrofitted approach compared with the monoethanolamine case of the European Benchmarking Task Force report returned higher electric efficiency (36.3% versus 33.50%), lower levelized cost of electricity (87.66 €/MWhel versus 92.27 €/MWhel), and lower cost of carbon dioxide avoided (47.03 €/tonCO2 versus 51.62 €/tonCO2). The results using the green-field approach compared against the Cansolv case of the National Energy Technology Laboratory report returned higher electric efficiency (33.06% versus 32.50%), lower cost of electricity (124.3 $/MWhel versus 133.2 $/MWhel) and lower cost of carbon dioxide avoided (66.5 $/tonCO2 versus 75.25 $/tonCO2). In both comparisons, aqueous ammonia had appreciably better performance than the ammine reference cases.

Mass transfer enhancement factor for chemical absorption of carbon dioxide into sodium metaborate solution

Hydrogen is getting increasing attention as a medium for energy storage, and sodium borohydride is accepted as a suitable carrier for hydrogen. The main product of the process by means of which hydrogen is produced from sodium borohydride is sodium metaborate. Our aim was to find an alternative use for sodium metaborate and specifically investigating the feasibility to use it for carbon dioxide capture from flue gases. The products of this chemical absorption are sodium carbonate, sodium bicarbonate and boric acid, all of which are industrially important chemicals. A bubble column was used in the experiments. Oxygen desorption technique was employed to determine the liquid side physical mass transfer coefficient. Chemical mass transfer coefficient was determined by absorption of carbon dioxide from its mixture with nitrogen into sodium metaborate solution. Enhancement factor was then calculated and a correlation was developed for it.

Theoretical investigation of carbon dioxide capture by aqueous boric acid solution: A termolecular reaction mechanism

Hitherto, boric is suggested and used as a promoter or catalyst for carbon dioxide capture in various chemical absorption reactions, such as, absorption by aqueous potassium carbonate solution to increase mass transfer rate. But in this study, a single step termolecular reaction mechanism is suggested for the chemical absorption of carbon dioxide directly by boric acid and water. The reaction thermochemistry and reaction kinetics for termolecular mechanism are investigated by using density functional theory calculations at the B3LYP/6-31G(d) level of theory by taking into account of the implicit solvent effects of water through the polarizable continuum model and dispersion corrections. The findings obtained from theoretical calculations indicate that it is possible to capture carbon dioxide with boric acid in the form of B(OH) 2 OCOOH.

Tuning the Carbon Dioxide Absorption in Amino Acid Ionic Liquids.

One of the possible solutions to prevent global climate change is the reduction of CO2 emissions, which is highly desired for the sustainable development of our society. In this work, the chemical absorption of carbon dioxide in amino acid ionic liquids was studied through first-principles methods. The use of readily accessible and biodegradable amino acids as building blocks for ionic liquids makes them highly promising replacements for the widely applied hazardous aqueous solutions of amines. A detailed insight into the reaction mechanism of the CO2 absorption was obtained through state-of-the-art theoretical methods. This allowed us to determine the reason for the specific CO2 capacities found experimentally. Moreover, we have also conducted a theoretical design of ionic liquids to provide valuable insights into the precise tuning of the energetic and kinetic parameters of the CO2 absorption.

Rigorous modelling of adiabatic multicomponent CO2 post-combustion capture using hollow fibre membrane contactors

The modelling of adiabatic multicomponent chemical absorption of carbon dioxide by means of hollow fibre membrane contactors is presented. One-dimensional and two-dimensional approaches are compared. A relatively wide operating domain was covered, ranging from fresh solvent and partial MEA conversions which correspond to experimental data, to partially loaded solvent and almost complete MEA conversion which represent industrial conditions. Both models predicted solvent flux reversal through the membrane and significant temperature peaks, up to 75°C. Moreover, important solvent losses are expected for the absorption unit. For considerably reduced calculation times, the one-dimensional model provided average results that compared well with those of the two-dimensional model. A relative difference of only 2.88% was found in the average absorbed specific flux of CO2. In the considered operating domain, the diffusion of the complex reaction product, protonated-amine-carbamate, could be considered non-limiting. Furthermore, the use of an average equivalent membrane mass-transfer coefficient was found to be adequate. Finally comparison with the packed bed technology evidenced similar trends and significant process intensification.

Chemical absorption and CO2 biofixation via the cultivation of Spirulina in semicontinuous mode with nutrient recycle

The chemical absorption of carbon dioxide (CO2) is a technique used for the mitigation of the greenhouse effect. However, this process consumes high amounts of energy to regenerate the absorbent and to separate the CO2. CO2 removal by microalgae can be obtained via the photosynthesis process. The objective of this study was to investigate the cultivation and the macromolecules production by Spirulina sp. LEB 18 with the addition of monoethanolamine (MEA) and CO2. In the cultivation with MEA, were obtained higher results of specific growth rate, biomass productivity, CO2 biofixation, CO2 use efficiency, and lower generation time. Besides this, the carbohydrate concentration obtained at the end of this assay was approximately 96.0% higher than the control assay. Therefore, Spirulina can be produced using medium recycle and the addition of MEA, thereby promoting the reduction of CO2 emissions and showing potential for areas that require higher concentrations of carbohydrates, such as in bioethanol production.

Hydrodynamic analogy approach for modelling reactive absorption

Abstract Design of separation units for reactive gas–liquid systems is usually based on conventional modelling methods employing the stage concept, either equilibrium or rate-based. The parameters of such models (e.g., mass transfer coefficients) are determined experimentally, and, most often, significant experimental effort is necessary when new processes are explored or new types of column internals are developed. In the context of a novel approach based on hydrodynamic analogies, rigorous partial differential transport equations are applied for modelling of non-reactive and reactive separation processes in columns filled with structured packings. The approach has already been verified for distillation and reactive desorption processes. In this work, its application to reactive absorption – also known as chemical absorption – is examined. Different types of reactions appearing in a wide range of absorption systems are considered. To account for absorption processes operated at high gas and liquid loads, an adjustment of the turbulent gas flow description is performed. Therefore, different test systems are used, namely physical absorption of ammonia into water, chemical absorption of sulphur dioxide and chemical absorption of carbon dioxide into sodium hydroxide solutions. The absorption of ammonia and sulphur dioxide took place under turbulent gas flow conditions, while in the absorption of carbon dioxide, the gas flow was laminar. Based on the carbon dioxide system, different types of corrugated sheet packings were compared. The hydrodynamic analogy model was successfully validated using available experimental data.

Application of 15N-NMR Spectroscopy to Analysis of Amine Based CO2 Capture Solvents

Abstract 15 N NMR spectroscopy is a useful tool for amine reactivity characterization since it can provide information about the availability of the lone pair of electrons on nitrogen through the measured chemical shift values, which depend on molecular structure and medium effects. Although the amino nitrogen is the focal nucleus of the carbon dioxide-amine reaction, 15 N NMR measurements have so far received little attention in the field of amine-based chemical absorption of carbon dioxide. In this study, from one hand, through 15 N NMR chemical shifts measurements, the effect of solvent on the electron density of the nitrogen of 2-methyl-2-amino-1-propanol (AMP) in solvent blends was investigated; from the other hand, 15 N NMR chemical shifts of aqueous primary non-hindered and hindered alkyl amines were related to the corresponding carbamate forming and carbamate stability equilibrium constants as well as carbamate forming kinetic constants, showing linear relationships. Such correlations could be useful for predictive estimation of carbamate-related equilibrium and kinetic constants.

Comprehensive modeling and CFD simulation of absorption of CO2 and H2S by MEA solution in hollow fiber membrane reactors

A comprehensive mathematical model has been developed for the simulation of simultaneous chemical absorption of carbon dioxide and hydrogen sulfide by means of Monoethanolamine (MEA) aqueous solution in hollow fiber membrane reactors is described. In this regard, a perfect model considering the entrance regions of momentum, energy, and mass transfers was developed. Computational Fluid Dynamics (CFD) techniques were applied to solve governing equations, and the model predictions were validated against experimental data reported in the literature and excellent agreement was found. Effects of different disturbances on the dynamic behavior of the reactor were investigated. Moreover, effects of various parameters such as wetting fraction, gas and liquid inlet velocities, inlet temperature of the solvent, MEA concentration, and CO2 and H2S compositions were carefully studied. It was found that for large values of gas velocity or small values of liquid velocity, the thermal energy equation can play an important role in the model predictions. © 2013 American Institute of Chemical Engineers AIChE J 60: 657–672, 2014

Combined physical and chemical absorption of carbon dioxide in a mixture of ionic liquids

Ionic liquids have attracted great interest recently as the basis of a potential alternative technology for the capture of carbon dioxide. Beyond the inherent tunability of properties of individual ionic liquids, a further strategy in optimising the ionic liquid sorbent for this application is the use of mixtures of ‘pure’ ionic liquids. Some ionic liquids absorb CO2 physically, whereas others do so chemically. Both mechanisms of absorption present advantages and disadvantages for a CO2 capture process operating in a continuous regime. In this work, a mixture of 1-ethyl-3-methylimidazolium acetate (an ionic liquid that reacts chemically with CO2) and 1-ethyl-3-methylimidazolium ethylsulfate (an ionic liquid that absorbs CO2 only through a physical mechanism) was investigated for the absorption of CO2 as a function of temperature and at pressures up to 17bar. The absorption/desorption studies were complemented by the characterisation of thermal and physical properties of the mixture of ionic liquids, which provide extra information on the interactions at a molecular level, and are also critical for the assessment of its suitability for a proposed process and for the subsequent process design.