Mass transfer investigation and operational sensitivity analysis of amine-based industrial CO2 capture plant

In this article, the industrial process of CO₂ capture using monoethanolamine as an aqueous solvent was probed carefully from the mass transfer viewpoint. The simulation of this process was done using Rate-Base model, based on two-film theory. The results were validated against real plant data. Compared to the operational unit, the error of calculating absorption percentage and CO₂ loading was estimated around 2%. The liquid temperature profiles calculated by the model agree well with the real temperature along the absorption tower, emphasizing the accuracy of this model. Operational sensitivity analysis of absorption tower was also done with the aim of determining sensitive parameters for the optimized design of absorption tower and optimized operational conditions. Hence, the sensitivity analysis was done for the flow rate of gas, the flow rate of solvent, flue gas temperature, inlet solvent temperature, CO₂ concentration in the flue gas, loading of inlet solvent, and MEA concentration in the solvent. CO₂ absorption percentage, the profile of loading, liquid temperature profile and finally profile of CO₂ mole fraction in gas phase along the absorption tower were studied. To elaborate mass transfer phenomena, enhancement factor, interfacial area, molar flux and liquid hold up were probed. The results show that regarding the CO₂ absorption, the most important parameter was the gas flow rate. Comparing liquid temperature profiles showed that the most important parameter affecting the temperature of the rich solvent was MEA concentration.     

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

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

Investigation of hydrodynamic performance and effective mass transfer area for Sulzer DX structured packing

The hydrodynamic performance in terms of pressure drop (?P) and liquid holdup (h_L), and tshe effective mass transfer area (a_e) of Sulzer DX structured packing were investigated at 293.15 K and 101.3 kPa. In addition, the flooding velocity (u_F) was also calculated based on the experimental results of liquid holdup, and the effective voidage correction factor (?) was obtained by combining the Billet model and the experimental effective fraction. The liquid volume method and pressure difference from just below to above the column packing approach are used to describe the hydrodynamic performance in a structured packing column. Experimental results showed that the operational conditions in terms of gas flow rate, liquid flow rate, viscosity, and liquid systems strongly affect the hydrodynamic performance. The experimental comparison between the pressure drop profiles in air‐water (polyethylene oxide [PEO]) and MEA‐H₂O‐CO₂ systems indicated that both the reacting MEA and CO₂ partial pressure can enhance the pressure drop value. In addition, the Bain‐Haugen correlation model was developed to predict the flooding velocity data with an acceptable AARD of 8.1%, and a model was also successfully proposed to predict the values of liquid holdup with an AARD of 11.8%, which is lower than 14.7% in Billet model. Furthermore, the effective mass transfer area was found to be increased by increasing both the liquid and gas flow rate by using NaOH‐H₂O‐CO₂ system. A model was also proposed to calculate the experimental a_e with an acceptable AARD% of 19.52, and this built model (Eq. 39) can reasonably explain the experimental phenomenon. © 2018 American Institute of Chemical Engineers AIChE J, 64: 3625–3637, 2018     

Effect of ship tilting and motion on amine absorber with structured‐packing for CO2 removal from natural gas

A gas‐liquid Eulerian porous media computational fluid dynamics (CFD) model was developed for an absorber with structured packing to remove CO₂ from natural gas by mono‐ethanol‐amine (MEA). The three‐dimensional geometry of the amine absorber with Mellapak 500.X was constructed to investigate the effect of the tilting and motion experienced on ships and barges for offshore plants. The momentum equation included porous resistance, gas‐liquid momentum exchange, and liquid dispersion to replace structured‐packing by porous media. The mass equation involved mass transfer of CO₂ gas into MEA solution, and one chemical reaction. Parameters of the CFD model were adjusted to fit experimental data measured in the CO₂‐MEA system. As the tilting angle increased, the liquid holdup and effective interfacial area decreased and CO₂ removal efficiency was lowered. The uniformity of liquid holdup deteriorated by 10% for a 3° static tilting, and a rolling motion with 4.5° amplitude and 12 s period, respectively. © 2015 American Institute of Chemical Engineers AIChE J, 61: 4412–4425, 2015     

Mathematical Modeling,Simulation, and Experimental Verification of CO2 Removal in a Turbulent Contact Absorber

A highly efficient technique of contaminant gas reduction, Turbulent Contact Absorber (TCA), is applied to CO₂ removal from a typical flue gas. Aqueous K₂CO₃ sorbent was evaluated as a regenerable sorbent for CO₂ from the flue gas. In order to identify the system, momentum and mass balance equations were written for the TCA tower. A flat plate falling film model was employed to simulate the TCA tower and the effect of turbulence was included in mass and momentum transfer coefficients. To check the accuracy of the model, a pilot scale TCA was built and operated. A Testo type gas analyzer was used to detect gas concentrations at the inlet and outlet of the rig. The model was validated successfully with pilot plant data. The effect of velocity and K₂CO₃ concentration on the TCA performance has also been carried out. It was found that the bed pressure drop increases linearly with gas velocity and then remains constant. An increase in the liquid flow rate increases liquid holdup, which leads to a rise in bed pressure drop. Higher turbulence within the TCA causes a velocity peak to shift from hypothetical gas‐liquid interface towards the falling film plate. An increase of the K₂CO₃ concentration from 1.0 g mol/L to 2.0 g mol/L was found to give an increase in CO₂ removal by about 4 %.     

Absorption of CO2 into aqueous solutions of MEA and AMP in a wetted wall column with film promoter

The process for removing CO₂ from a gas mixture can be achieved by absorption into amines through of different conventional techniques. But, the report of mass transfer parameters that can be used for process design is limited. Thus, a study of CO₂ absorption process into monoethanolamine (MEA), 2-amino-2-methyl-1-propanol (AMP) and their respective blend were developed with the aim of evaluating the absorption performance and of determining a design parameter for a single-amine absorption system. The experimental tests were carried out in a wetted wall column employing a film promoter, a thin stainless steel woven. The experimental device was operated in countercurrent and the different tests were developed at 298 K, atmospheric pressure, a constant gas load (10% CO₂) and three different liquid loads. The results show that the interfacial area is influenced by increasing the liquid load. The K_Ga values of MEA and the blend based on it are higher in relation to AMP. Moreover, the results of the thermodynamical-kinetic parameter show that this absorption process is independent of liquid load, characteristic of rapid chemical reactions. The K_Ga value for the different MEA:AMP mixtures shows that it increases when the MEA proportion is increased in the blend.     

Modeling and Simulation of a Bubbling SO2 Absorber with Granular Limestone Slurry and an Organic Acid Additive

The absorption of SO₂ into limestone slurry containing suspended reactive particles was performed in a bubble reactor with continuous feeding of both gas and liquid phases at a constant pH and high temperature (50 °C). An absorption model with a reaction plane based on the film model was developed. The effect of limestone particle size, concentration, acetic acid additives, and inlet SO₂ concentration on the concentration distribution of chemical species in the liquid film and SO₂ absorption rate were simulated. Increasing the concentration of limestone slurry, adding acetic acid additives into the system or decreasing the limestone particle size or inlet SO₂ concentration caused the reaction plane in the liquid film to shift towards the gas‐liquid interface. Model and experimental results were compared, and it was shown that the model fits the experimental data well.     

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     

Mathematical Modeling and Genetic Algorithm Optimization of Reactive Absorption of H2S

A rate‐based mathematical model was developed for the reactive absorption of H₂S in NaOH, with NaOCl or H₂O₂ as the chemical oxidant solutions in a packed column. A modified mass transfer coefficient in the gas phase was obtained by genetic algorithm and implemented in the model to correct the assumption of instantaneous reactions. The effects of different operating variables including the inlet H₂S concentration, inlet gas mass flux, initial NaOH, concentrations of the chemical oxidants in the scrubbing solutions, and liquid‐to‐gas ratio on the H₂S removal efficiency were studied. A genetic algorithm was employed to optimize the operating variables in order to obtain maximum removal efficiency of H₂S. The model results were in good agreement with the experimental data.     

CFD simulations of bubble column reactors: 1D, 2D and 3D approach

CFD simulations have been carried out for the predictions of flow pattern in bubble column reactors using 1D, 2D and 3D k-ε models. An attempt has been made to develop a complete correspondence between the operation of a real column and the simulation. Attention has been focused on the cylindrical bubble columns because of their widespread applications in the industry. All the models showed good agreement with the experimental data for axial liquid velocity and the fractional gas hold-up profiles. However, as regards to eddy diffusivity, only the 3D model predictions agree closely with the experimental data.The CFD model has been extended for the estimation of an axial dispersion coefficient (D_L) using 1D, 2D and 3D models. Excellent agreement was found only between the experimental values and the 3D predictions. The 1D and 2D simulations, however, yielded D_L values, which were lower by 25-50%. For this, a mechanistic explanation has been provided.     

Measurement of mass transfer coefficient in an airlift reactor with internal loop using coalescent and non‐coalescent liquid media

In this work the sulfite oxidation (SOM), dynamic pressure‐step (DPM) and gassing‐out (GOM) methods were compared for volumetric mass transfer coefficient measurement in an airlift reactor with internal loop. As a liquid phase both, non‐coalescent and coalescent media were used. Among the methods discussed here, the mass transfer coefficient (k_La) values obtained by the DPM appear as the most reliable as they were found to be independent of oxygen concentration in the inlet gas, which confirmed the physical correctness of this method. The difference between data measured using air and oxygen was not higher than 10%, which was comparable to the scatter of experimental data. It has been found that the sulfite oxidation method yielded k_La values only a little higher than those obtained by the DPM and the difference did not exceed 10%. Up to an inlet gas velocity (U_GC) of ?0.03 m s^?1 the GOM using oxygen as a gas medium gave k_La values in fact identical with those obtained by the DPM. At higher flows of the inlet gas, the GOM yielded k_La values as much as 15% lower. The enhancement in oxygen mass transfer rate determined in non‐coalescent media was estimated to be up to +15%, when compared with a coalescent batch. The experimental dependence of k_La vs the overall gas hold‐up was described by an empirical correlation. 1 Copyright © 2004 Society of Chemical Industry     

Absorption of carbon dioxide into aqueous blends of 2-amino-2-methyl-1-propanol and monoethanolamine

This work presents an investigation of CO₂ absorption into aqueous blends of 2-amino-2-methyl-1-propanol (AMP) and monoethanolamine (MEA). The acid gas mass transfer has been modeled using equilibrium-mass transfer-kinetics-based combined model to describe CO₂ absorption into the amine blends according to Higbie's penetration theory. The effect of contact time and relative amine concentration on the rate of absorption and enhancement factor were studied by absorption experiment in a wetted wall column at atmospheric pressure. The model was used to estimate the rate coefficient of the reaction between CO₂ and monoethanolamine at 313 K from experimentally measured absorption rates. A rigorous parametric sensitivity test has been done to identify the key systems’ parameters and quantify their effects on the mass transfer using the mathematical model developed in this work. The model predictions have been found to be in good agreement with the experimental rates of absorption of CO₂ into (AMP+MEA+H₂O).     

Performance of a bubble column reactor for oxidation of ethylene (wacker process)

An experimental investigation of the performance of a bubble column reactor for the liquid-phase catalytic oxidation of ethylene to acetaldehyde (Wacker Process) has been carried out by varying the different operating parameters, such as the inlet gas velocity and catalyst concentration. It has also been shown that, for a given set of conditions, a certain critical ratio of partial pressure of O₂ to that of ethylene in the reactor inlet is required for the overall catalytic cycle to work with maximum efficiency under steady state. The agreement between the experimental results and the predictions of a theoretical reactor model developed herein was found to be excellent.     

CFD modeling of flow,macro-mixing and axial dispersion in a bubble column

The flow pattern in a bubble column depends upon the column diameter, height, sparger design, superficial gas velocity and the nature of gas–liquid system. In this paper, the effect of some of these parameters have been simulated using Computational Fluid Dynamics (CFD). The relationship of these parameters with the interphase force terms has been discussed. A complete energy balance has been established. Using this methodology, the flow patterns reported by Hills (1974), Menzel et al. (1990), Yao et al. (1991) and Yu and Kim (1991) have been simulated. Excellent agreement has been shown between the CFD predictions and the experimental observations. The above model has been extended to homogenization of an inert tracer. In order to confirm this model, mixing experiments were carried out in a 200 mm i.d. bubble column. A radioactive tracer technique was used for the measurement of mixing time. Tc-99m (^99m Tc), in the form of sodium pertechnate salt, was used as the liquid phase tracer. Good agreement has been shown between the predicted and the experimental values of mixing time. The model was further extended for the estimation of axial dispersion coefficient (D_L). Excellent agreement between the simulated and the experimental values of the axial dispersion coefficient confirms the predictive capability of the CFD simulations for the mixing process.     

Simulation of CO<Subscript>2</Subscript> removal in a split-flow gas sweetening process

Split-flow gas sweetening is known to consume less energy than a conventional gas sweetening process when the inlet sour gas contains a high concentration of acid gases. In this work, a computer simulation of a split-flow natural gas sweetening process based on absorption/stripping process with alkanoamine (MEA and DGA) solutions, using Aspen plus, was performed. The input of parameters such as the concentration of sour gases (CO₂, H₂S) in the feed gas has been examined. Simulation results show that the split-flow gas sweetening process can reduce the reboiler duty of a stripping tower better than the conventional gas sweetening process according to the concentration of CO₂ in the feed gas.     

Hybrid neural network for prediction of CO<Subscript>2</Subscript> solubility in monoethanolamine and diethanolamine solutions

The solubility of CO₂ in single monoethanolamine (MEA) and diethanolamine (DEA) solutions was predicted by a model developed based on the Kent-Eisenberg model in combination with a neural network. The combination forms a hybrid neural network (HNN) model. Activation functions used in this work were purelin, logsig and tansig. After training, testing and validation utilizing different numbers of hidden nodes, it was found that a neural network with a 3-15-1 configuration provided the best model to predict the deviation value of the loading input. The accuracy of data predicted by the HNN model was determined over a wide range of temperatures (0 to 120 °C), equilibrium CO₂ partial pressures (0.01 to 6,895 kPa) and solution concentrations (0.5 to 5.0M). The HNN model could be used to accurately predict CO₂ solubility in alkanolamine solutions since the predicted CO₂ loading values from the model were in good agreement with experimental data.     

Reduction of amine mist emissions from a pilot-scale CO2 capture process using charged colloidal gas aphrons

In the CO₂ capture process from coal-derived flue gas where amine solvents are used, the flue gas can entrain small liquid droplets into the gas stream leading to emission of the amine solvent. The entrained drops, or mist, will lead to high solvent losses and cause decreased CO₂ capture performance. In order to reduce the emissions of the fine amine droplets from CO₂ absorber, a novel method using charged colloidal gas aphron (CGA) generated by an anionic surfactant was developed. The CGA absorption process for MEA emission reduction was optimized by investigating the surfactant concentration, stirring speed of the CGA generator, and capture temperature. The results show a significant reduction of MEA emissions of over 50% in the flue gas stream exiting the absorber column of a pilot scale CO₂ capture unit.     

单乙醇胺吸收CO2的热力学模型和过程模拟

采用非随机双流体电解质（ENRTL）热力学模型,通过拟合单乙醇胺（MEA）的饱和蒸气压、热容数据,MEA和水（H₂O）二元体系的汽液平衡、热容、混合热数据,以及二氧化碳（CO₂）在MEA水溶液中的溶解度数据,建立了MEA吸收CO₂的热力学模型,并用核磁共振（NMR）组成数据成功地进行了验证。在此模型基础上,利用平衡级模型建立了MEA吸收/解吸CO₂的过程模拟,利用文献中中试工厂数据验证了过程模拟的准确性。对于质量分数为30%的MEA溶液,固定吸收塔CO₂去除率为90%的条件下,当吸收塔液气质量流率比值为2时,再沸器能耗最小,为3.64 GJ·（t CO₂）^-1。     

Modeling CO2 absorption into concentrated aqueous monoethanolamine and piperazine

Dugas and Rochelle (2011) measured CO₂ mass transfer rates in 7–13 molal aqueous monoethanolamine (MEA) and 2–12 molal piperazine (PZ) from 40 to 100 °C over a large range of CO₂ loading. They observed that the liquid phase mass transfer coefficient (k′_g) was almost independent of amine concentration and temperature. In this paper models are created to explain this behavior.CO₂ reaction rates in MEA and PZ are represented with termolecular (base catalysis) kinetics with activity-based rate expressions. Solving the activity-based rate expressions with a shell balance and implementing diffusion resistance using film theory yielded an expression for the liquid phase mass transfer coefficient, k′_g. Parameters in the k′_g expression were estimated from existing literature data. Two pre-exponential rate constants (k_MEA and k_PZ) are the only parameters that were adjusted to match experimental data.Estimates from independent sources of parameters in the model for k′_g fully account for the observed effects of CO₂ loading, temperature, and amine concentration. The k′_g expressions match the 93 wetted wall column experimental rates measured by Dugas and Rochelle (2011) with average deviations of 13% for MEA and 19% for PZ. The mass transfer expressions also match experimental data obtained by other researchers.This model shows that complex rate behavior in MEA and PZ systems can be fully explained using existing literature data. It also shows that the MEA and PZ systems can be represented by termolecular kinetics on an activity basis.     

Spouted bed hydrodynamics in a 0.91 m diameter vessel

Hydrodynamic measurements were obtained in a flat-based half-cylindrical column of diameter 0.91 m and inlet orifice diameters of 76 to 114 mm. Beds of 3.5 to 6.7 mm diameter particles with static depths of 0.53 to 1.83 m were spouted with air. In agreement with measurements by earlier workers in smaller columns, it was found necessary to operate with inlet orifice diameters less than about 30 times the mean particle diameter in order to be able to achieve stable spouting. Correlations for minimum spouting velocity developed on small vessels generally gave poor predictions for the large diameter vessel employed in this work and failed to predict the observed dependence of U_ms on the static bed height. Substantial dead regions where particles were stagnant were observed in the lower outer portion of the vessel. Other aspects of behaviour studied, including spout diameters and shapes, fountain heights, pressure profiles and gas velocities in the annulus, were qualitatively similar to those in smaller columns, although equations developed for the smaller vessels did not always provide accurate predictions.