Numerical Study of the CO2 Absorber Performance Subjected to the Varying Amine Solvent and Flue Gas Loads

The paper is devoted to the amine-based post-combustion carbon dioxide capture technology. The aim of the paper was to analyze the effect of varying flow conditions on the CO₂ capture efficiency of the absorber column. As a research tool, a numerical model of the chemical absorption with aqueous monoethanolamine solution in a packed bed was employed. A complex physio-chemical process including two-phase flow hydrodynamics, heat transfer, and absorption chemistry was simulated by Ansys Fluent commercial software. The parametric study was focused on CO₂ capture efficiency in terms of varying loads of amine solvent (liquid) and flue gas. The corresponding changes of liquid holdup, species concentration, temperature and reaction rate distributions are discussed in detail allowing to better understand the absorption column operation. The simulation results have shown clearly the mutual interactions of partial processes and the sensitivity of the system to varying column loads. They have been found to be useful in defining the optimal ranges of operational parameters.     

Computational fluid dynamics simulation of reactive absorption of CO2 in a structured packed column

In this study, multiphase Eulerian computational fluid dynamics (CFD) modelling is developed to predict the hydrodynamics, mass transfer, and chemical absorption of CO₂ using a monoethanolamine (MEA) solution in a structured packed column. First, the hydrodynamic simulation of liquid dispersion in a structured packed bed using a two-dimensional CFD is performed. The simulation results of the radial distribution of the liquid holdup are compared with the literature experimental data. The model prediction matches the experimental data at the top position of the column, whereas a slight deviation is found at the bottom position of the column. Using a validated CFD model, the reactive mass transfer is modelled to study CO₂ capture in a structured packed column with Mellapak 500.X. The model results are compared to the literature experimental results of CO₂ mole fractions along the height of the column. It is found that the model results match the experimental findings. Furthermore, CFD modelling is extended to investigate the influence of operating conditions such as gas and liquid velocities on CO₂ removal efficiency. The present CFD model demonstrates the porous media approach for reactive absorption of CO₂ in a structural packed bed.     

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.     

Behaviour of absorption/stripping columns for the CO2-MEA system; Modelling and experiments

A model is presented for the steady-state simulation of a CO₂ recovery pilot plant with aqueous monoethanolamine (MEA) solutions. CO₂ absorption is performed in a column packed with 2.54 cm ceramic Pall rings. CO₂ recovery is achieved in a 20 sieve tray steam stripping column. The packed column absorption model was fitted to the experimental data using the specific interfacial area of the irrigated packing as an adjustable parameter. The equivalent average bubble diameter was used as the adjusting parameter in the sieve tray stripping column. Modelling of both towers reproduces within 3% average error concentrations measured in a pilot plant. Measured temperatures were also well correlated.     

Numerical Simulation of Absorbing CO2 with Ionic Liquids

Although separating CO₂ from flue gas with ionic liquids has been regarded as a new and effective method, the mass transfer properties of CO₂ absorption in these solvents have not been researched. In this paper, a coupled computational fluid dynamic (CFD) model and population balance model (PBM) was applied to study the mass transfer properties for capturing CO₂ with ionic liquids solvents. The numerical simulation was performed using the Fluent code. Considering the unique properties of ionic liquids, the Eulerian‐Eulerian two‐flow model with a new drag coefficient correlation was employed for the gas‐liquid fluid dynamic simulation. The gas holdup, interfacial area, and bubble size distribution in the bubble column reactor were predicted. The mass transfer coefficients were estimated with Higbie's penetration model. Furthermore, the velocity field and pressure field in the reactor were also predicted in this paper.     

Gas holdup and energy dissipation in liquid‐gas ejectors

BACKGROUND: Ejectors have excellent mass transfer characteristics with energy efficiency and can be used in place of conventional countercurrent systems, namely, packed bed contactors as well as venturi scrubbers, cyclones and airlift pumps. Although a number of papers have been published in the recent past, none of them provides a theoretical basis for the prediction of gas phase holdup. In this work an attempt has been made to develop a theoretical basis for predicting gas phase holdup based on first principles using Nguyen and Spedding's distribution function (C_o) and initial value parameter (B). RESULTS: In the present work, measurements and correlations are reported for the gas holdup and energy dissipation in a liquid‐gas ejector. The holdup data have been correlated using the theoretical models proposed by Nguyen and Spedding, 26 with an estimated initial value parameter B and the distribution function C_o. The throat and diffuser loss coefficients were found to be constant up to a gas/liquid flow ratio of 1.6 and then it was found to be a function of area ratio, physical properties and gas holdup. CONCLUSIONS: The present proposed correlations for gas phase holdup and energy dissipation, E_mix, should be useful for the efficient design of co‐current ejectors for gas‐liquid contacting, in particular for the removal of CO₂ from natural gas, since the viscosity and surface tension ranges covered in the present study are essentially those encountered in amine–carbon dioxide systems. Copyright © 2008 Society of Chemical Industry     

Tomographic measurement of liquid hold up and effective interfacial area distributions in a column packed with high performance structured packings

In this paper, we report on the use of a high energy and high resolution X-ray tomograph to visualize and quantify the distribution of liquid hold up and of gas–liquid interfacial area in a 0.1 m diameter column filled with MellapakPlus 752.Y packing elements. A standard air–water system at room temperature and atmospheric pressure were used. Tomographic measurements have been carried out in a large number of packing cross sections situated at different heights between the top and the bottom of the packed column, giving access to the evolution of axial profiles of liquid hold up and of gas–liquid interfacial area as a function of the operating conditions. Gas–liquid interfacial area values were also measured by a chemical method (CO₂ absorption from air into a caustic solution). For the first time, a whole set of gas–liquid interfacial area values evaluated from tomographic images are interestingly compared with values measured by a chemical method. A comparison is also presented with literature models.     

Effective Interfacial Area and Liquid Holdup of Nutter Rings at High Liquid Loads

The effective interfacial area of the Nutter Ring NR#1, NR#1.75 and NR#2.5 was measured by absorbing CO₂ from air with NaOH. Liquid holdup was measured using air‐water as test system. The liquid load was varied between 20 and 180 m³/m²h and the influence of the pressure drop was investigated. Correlations are derived from the experimental data which represent the measured data within an accuracy of 10 %. The aim of this work is to present the test results as well as to summarize how the effective interfacial area can be calculated in order to motivate other researchers to publish interfacial area instead of k_OG · a, when using CO₂‐NaOH as test system.     

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     

Control strategies for flexible operation of power plant with CO2 capture plant

About 20% power output penalties will be incurred for implementing CO₂ capture from power plant. This loss can be partially compensated by flexible operation of capture plant. However, daily large variations of liquid and gas flows may cause operation problems to packed columns. Control schemes were proposed to improve the flexibility of power output without causing substantial hydraulic disturbances in capture plant is presented. Simulations were implemented using ASPEN Plus. In varying lean solvent flow strategy, the flow rate of recycling solvent was manipulated to control the CO₂ capture rate. The liquid flow of the absorber and gas flow of the stripper will vary substantially. In an alternative strategy, the lean solvent loading will be varied. Variation of gas throughput in the stripper is avoided by recycling part of CO₂ vapor to stripper. This strategy provided more stable hydraulics condition in both columns and is recommended for flexible operation. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

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.     

Improved gas'liquid contacting in co-current flow

A new method of increasing interfacial area for gas-liquid contacting in co-current flow using screen packings has been evaluated in a 51/2-in. I. D. column using CO₂ chemisortpion in sodium hydroxide solution and CO² physical desorption from water. The study investigated the effect of gas velocity (0.3-2.7-ft/sec.), liquid velocity (0.018-0.1-ft/sec), and of column height (0.67-7.8-ft.) on the interfacial area, a, and the physical desorption liquid phase mass transfer coefficient, K_L. The two types of screen packing tested produced interfacial areas of 2 to 4 times that generated in an unpacked column. K_L showed no effect of gas velocity but increased with liquid velocity, a and K_L decreased with increasing column height. Generally, photographic evaluation gave unrealistically high values of a.     

Mathematical modelling of mass-transfer and hydrodynamics in CO2 absorbers packed with structured packings

This paper presents a mechanistic model that can predict mass-transfer performance and provide an insight into dynamic behavior within structured packings used for CO₂ absorption. The model was built upon the kinetics and thermodynamics of the absorption system, as well as the liquid irrigation features and the geometry of packing elements. A computer program (Fortran 90) was written to simulate CO₂ absorption into aqueous solutions of sodium hydroxide (NaOH) and monoethanolamine (MEA) in a column packed with Gempak 4A, Mellapak 500Y and Mellapak 500X. The simulation gave essential information, including the concentration of CO₂ in gas-phase, concentration of reactive species in the liquid-phase, system temperature, mass-transfer coefficients (k_G and k_L), and effective interfacial area (a_e) for mass-transfer at different axial positions along the absorption column. The simulation also provided liquid distribution plots representing the quality of liquid distribution or maldistribution across the cross-section of the column. Verification of the model was achieved by comparing simulation results with experimental data. Very good agreement was found for wide ranges of operating and design parameters, including liquid load and initial liquid distribution pattern.     

Dynamic modelling and analysis of post-combustion CO2 chemical absorption process for coal-fired power plants

Post-combustion capture by chemical absorption using MEA solvent remains the only commercial technology for large scale CO₂ capture for coal-fired power plants. This paper presents a study of the dynamic responses of a post-combustion CO₂ capture plant by modelling and simulation. Such a plant consists mainly of the absorber (where CO₂ is chemically absorbed) and the regenerator (where the chemical solvent is regenerated). Model development and validation are described followed by dynamic analysis of the absorber and regenerator columns linked together with recycle. The gPROMS (Process Systems Enterprise Ltd.) advanced process modelling environment has been used to implement the proposed work. The study gives insights into the operation of the absorber-regenerator combination with possible disturbances arising from integrated operation with a power generation plant. It is shown that the performance of the absorber is more sensitive to the molar L/G ratio than the actual flow rates of the liquid solvent and flue gas. In addition, the importance of appropriate water balance in the absorber column is shown. A step change of the reboiler duty indicates a slow response. A case involving the combination of two fundamental CO₂ capture technologies (the partial oxyfuel mode in the furnace and the post-combustion solvent scrubbing) is studied. The flue gas composition was altered to mimic that observed with the combination. There was an initial sharp decrease in CO₂ absorption level which may not be observed in steady-state simulations.     

Liquid Phase CO2-Analytics for Standardized Mass Transfer Characterization in Packed Columns

Mass transfer parameters are necessary for the design of absorption and desorption processes in packed columns. To determine the effective interfacial area and liquid side mass transfer parameters, CO₂ absorption and desorption are frequently used. Reliable analytics for concentration determination are essential to obtain correct results. In this work two methods of CO₂ liquid phase analysis are compared: first, the back titration of unreacted NaOH after prior precipitation of the bound CO₂; secondly, the inorganic carbon analysis with a commercial inorganic carbon analyzer.     

Mass transfer with chemical reaction in liquid foam reactors

Mass transfer studies were conducted in a stable liquid foam reactor under various operating conditions to evaluate gas holdup, effective interfacial area, liquid-phase mass transfer coefficient and a modified interfacial mass transfer coefficient to include the surface-active agents employed. Gas holdup and effective interfacial area were evaluated experimentally. The interfacial mass transfer coefficient was evaluated semitheoretically, by considering the interfacial region as a separate phase and using the experimental data developed for mass transfer accompanied by a fast first-order chemical reaction. The liquid-phase mass transfer coefficient was also evaluated semitheoretically, using Danckwert's theory for the liquid phase and the experimental data on mass transfer accompanied by a slow pseudofirst-order chemical reaction. An experimental unit was set up to provide a stable flowing foam column, simulating the foam reactor. Mass transfer rates were studied for superfacial gas velocities in the range from 1.5 × 10⁻² m/s to 5 × 10⁻² m/s, giving gas residence times in the range from 20 to 55 seconds. A cationic and nonionic surface-active agent and three different wire mesh sizes, giving bubble size distributions in the range from 2.2 to 5.4 mm Sauter mean diameters, were employed. It is observed that gas holdup is insensitive to the type of surface-active agent; it is however, dependent on wire mesh size and gas velocity. The bubble diameter and, hence, the interfacial area are found to be insensitive to gas velocity in the range studied; they are, however, strong functions of wire mesh size. The liquid-phase mass transfer coefficient increases with increase in gas velocity. The surface-active agent introduces additional resistance to mass transfer in both reaction cases, this being the controlling one in the case of the fast reaction. A comparison with conventional packed bed contactors indicates the mass transfer rates to be about 8 times lower for the foam reactor, for the fast reaction case; for slow reactions, the foam reactor has mass transfer rates approximately 2-4 times higher than those for conventional packed bed contactors.     

Measurement of local specific interfacial area in bubble columns via a non-isokinetic withdrawal method coupled to electro-optical detector

Precise measurement of gas-liquid interfacial surface area is essential to reactor design and operation. Mass transfer from the gas phase to the liquid phase is often a key feature that controls the overall process. Measurement of gas-liquid interfacial area is often made through a separate measurement of the gas holdup and bubble size with complex and/or sophisticated methods. In this work, an inexpensive method is presented for the simultaneous determination of both local gas holdup and bubble diameter. The method is based on the withdrawal of the air-liquid dispersion under non-isokinetic conditions and on bubble counting via a simple optical device. The method was calibrated in a bubble column with several withdrawal pressures using coalescing and non-coalescing media. During the same calibration experiment, gas holdup was also measured manometrically and individual bubble diameters were measured by a photographic method. With a vacuum pressure of 3 kPa, local interfacial area measured with the withdrawal method produced a relative error below 13%, compared to the manometric/photographic method. The method was then used to characterize local specific interfacial area in a bubble column under several operating conditions with coalescing and non-coalescing media. In coalescing media and with superficial gas velocities (v_g) from 0.25 to 3.5 cm/s, the average interfacial area ranged from 17 to . With non-coalescing media the average interfacial area ranged from 40 to . Under the test condition it was observed that gas holdup is a parameter that has a greater distribution (standard deviation from 30% to 70%) than the volume-mean bubble diameter (standard deviation from 6% to 12%). It is shown that a model previously developed for characterizing gas holdup homogeneity is also suitable for characterizing interfacial area homogeneity.     

Chemisorption of CO2 in a gas–liquid vortex reactor: An interphase mass transfer efficiency assessment

To develop cost-effective CO₂ capture technology process intensification will play a vital role. In this work, the capabilities of a gas–liquid vortex reactor (GLVR) as novel process intensification equipment are evaluated by studying its interphase mass transfer parameters to build up the fundamentals for its future application to for example, CO₂ capture. The NaOH-CO₂ chemisorption system and Danckwerts' model are applied to obtain the effective interfacial area and liquid-side mass transfer coefficient. Results show that the gas–liquid contact in the GLVR is capable of both generating a large interfacial area in a small reactor volume and creating a region with high-energy dissipation to improve mass transfer. A comparison of the volumetric mass transfer coefficients with data reported in literature for conventional and intensified reactor types confirms a superior mass transfer efficiency and, most importantly, a favorable energetic efficiency of the GLVR.     

Ozone absorption in alkaline solutions

Using a packed column, the rate of ozone absorption into KOH aqueous solutions has been measured. The pH range was 8.5–13.5 and the temperature varied from 18 to 27°C.Independent measurements of CO₂ absorption into buffer solutions containing KAsO₂ were used to determine the interfacial area.The results can be interpreted on the basis of a first order reaction in both O₃ and OH^?.     

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