Gas-liquid Absorption Research Articles

Contribution of Liquid- and Gas-Side Mass Transfer Coefficients to Overall Mass Transfer Coefficient in Taylor Flow in a Microreactor

Abstract Gas-liquid microreactors find an increasing range of applications both in production, and for chemical analysis. The most often employed flow regime in these microreactors is Taylor flow. The rate of absorption of gases in liquids depends on gas-side and liquid-side resistances. There are several publications about liquid-side mass transfer coefficients in Taylor flow, but the data about gas-side mass transfer coefficients are practically non existent. We analysed the problem of gas-side mass transfer resistance in Taylor flow and determined conditions, in which it may influence the overall mass transfer rate. Investigations were performed using numerical simulations. The influence of the gas diffusivity, gas viscosity, channel diameter, bubble length and gas bubble velocity has been determined. It was found that in some case the mass transfer resistances in both phases are comparable and the gas-side resistance may be significant. In such cases, neglecting the gas-side coefficient may lead to errors in the experimental data interpretation.

Renewable hydrogen production concepts from bioethanol reforming with carbon capture

This paper is assessing the hydrogen production from bioethanol at industrial scale (100000 Nm3/h hydrogen equivalent to 300 MW thermal) with carbon capture. Three carbon capture designs were investigated, one based on pre-combustion capture using chemical gas–liquid absorption and two based on chemical looping (one based on syngas and one using direct bioethanol looping). The carbon capture options were compared with the similar designs without carbon capture. The designs were simulated to produce mass and energy balances for quantification of key performance indicators. A particular accent is put on assessment of reforming technologies (steam and oxygen-blown autothermal reforming) and chemical looping units, process integration issues of carbon capture step within the plant, modelling and simulation of whole plant, thermal and power integration of various plant sub-systems by pinch analysis. The results for chemical looping designs (either syngas-based or direct bioethanol) show promising energy efficiency coupled with total carbon capture rate.

Fluid flow and mass transfer characteristics of enhanced CO2 capture in a minichannel reactor

CO2 absorption using amine solvents can be significantly enhanced by the use of microscale reactors having high surface area to volume ratio. The present paper reports an experimental investigation of the fluid flow and mass transfer characteristics during reactive gas–liquid absorption in a minichannel reactor. Absorption of CO2 mixed with N2 into aqueous diethanolamine was studied in a channel having hydraulic diameter of 762μm and a circular cross-sectional geometry. High-speed imaging of the two-phase flow was conducted to visualize the flow regimes. Image-processing analysis of the acquired flow patterns was performed to determine the interfacial area. The performance of the reactor was studied with respect to the absorption efficiency, pressure drop, mass transfer coefficient, interfacial area, enhancement factor, and Sherwood number. Parametric studies investigating the effects of phase superficial velocity, liquid reactant concentration, and CO2 concentration in the gas phase were performed and are discussed. High levels of absorption efficiency, close to 100%, were observed under certain operating conditions. An empirical model for the Sherwood number was developed and compared against experimental data. The mass transfer coefficient was found to be higher at reduced channel lengths, which was attributed to improved utilization of the absorption capacity of the amine solution for a given reactor volume. The presently achieved values of mass transfer coefficient and specific interfacial area are between 1 and 4 orders of magnitude higher than those reported for most conventional absorption systems.

CO2 capture by pyrrolidine: Reaction mechanism and mass transfer

A new carbon dioxide capture process by means of gas–liquid absorption using pyrrolidine aqueous solutions in a bubble column reactor obtaining suitable results in comparison with other commonly used amines is analyzed. The influence of several operation variables such as amine concentration and gas flow rate has been studied. Carbon dioxide mass‐transfer rate data have shown a different behavior than other amine‐based systems because a constant value in absorption rate was observed in the middle of batch experiments. 13C and 1H NMR spectroscopy studies were performed to analyze the species present during the experiments. These data and the carbon dioxide loading allowed to explain the reaction mechanism existed between these reagents. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1098–1106, 2014

Membrane Separation Processes for Post-Combustion Carbon Dioxide Capture: State of the Art and Critical Overview

Membrane processes have been initially seldom considered within a post-combustion carbon dioxide capture framework. More traditional processes, particularly gas-liquid absorption in chemical solvents, are often considered as the most appropriate solution for the first generation of technologies. In this paper, a critical state of the art of gas separation membranes for CO<sub>2<sub/> capture is proposed. In a first step, the key performances (selectivity, permeability) of different membrane materials such as polymers, inorganic membranes, hybrid matrices and liquid membranes, including recently reported results, are reviewed. In a second step, the process design characteristics of a single stage membrane unit are studied. Purity and energy constraints are analysed as a function of operating conditions and membrane materials performances. The interest of multistage and hybrid systems, two domains which have not sufficiently investigated up to now, are finally discussed. The importance of technico-economical analyses is highlighted in order to better estimate the optimal role of membranes for CCS applications.

Techno-economic and environmental evaluations of large scale gasification-based CCS project in Romania

Gasification is a promising technology in terms of reducing carbon capture energy and cost penalties as well as for multi-fuel multi-product operation capability. The paper evaluates two carbon capture options in terms of main techno-economic indicators. The first option involves pre-combustion capture, the syngas being catalytically shifted to convert carbon species into CO2 and H2. Gas–liquid absorption is used for separate H2S and CO2 capture, then clean gas is used for power generation. The second capture option is based on post-combustion capture using chemical absorption. The most promising gasifiers were evaluated in a CCS design.Evaluated coal-based gasification concepts generate 300–500 MW net power with a flexible hydrogen output from zero up to 200 MW with carbon capture rate higher than 90%. The main techno-economic and environmental indicators of evaluated plant concepts with CCS were presented in detail in the context of development of a large scale CCS project in Romania. The evaluations of the IGCC designs with CCS were done in comparison with the conventional IGCC without CCS. The integrated assessments show that IGCC with pre-combustion capture is one promising technology for the future low-carbon economy.

Assessment of Carbon Capture Options for Super-Critical Coal-based Power Plants

Power generation is one of the industrial sectors with major contribution to greenhouse gas emissions. For climate change mitigation, a special attention is given to the reduction of CO2 emissions by applying capture and storage techniques in which CO2 is captured and then stored in suitable safe geologic locations. Carbon capture and storage (CCS) technologies are expected to play a significant role in the coming decades for curbing greenhouse gas emissions and to ensure a sustainable development of power generation and other energy-intensive industrial sectors (e.g. cement, metallurgy, petro-chemical etc.). This paper evaluates super-critical coal-based power plants with and without carbon capture. The analysis is geared toward quantification of main plant performance indicators such as: fuel consumption, gross and net energy efficiency, ancillary energy consumption, carbon capture rate, specific CO2 emissions, capital costs, specific capital investments and operational costs etc. For CCS configurations, two post-combustion CO2 capture options were considered. The first option is based gas-liquid absorption using a chemical solvent (methyl-diethanol-amine – MDEA etc.). The second option is based on calcium looping cycle, in which the carbonation/calcination sequence of CaO/CaCO3 system is used for carbon capture. The power plant case studies investigated in the paper produces around 950 – 1,100 MW net power with at least 90 % carbon capture rate. The mathematical modelling and simulation of the whole power generation schemes will produce the input data for quantitative techno-economic and environmental evaluations of power plants with carbon capture (similar power plant concept without CCS was used as reference for comparison). Mass and energy integration tools were used to assess the integration aspects of evaluated carbon capture options in the whole power plant design, to optimise the overall energy efficiency and to evaluate the main sources of energy penalty for CCS designs.

Mass transfer characteristics of gas–liquid absorption during Taylor flow in mini/microchannel reactors

This paper reports a numerical study of the mass transfer characteristics during Taylor flow in mini/microchannel reactors. A finite-element implementation of the phase field method was used to predict the hydrodynamics of the two-phase flow. The phase distribution thus obtained was used to define the computational domain to model the reactive mass transfer. The reaction system of the absorption of CO2 into aqueous NaOH solution was considered. Channels with characteristic dimensions ranging from 100μm to 750μm were modeled with cross-flow and flow-focusing inlet configurations. The effect of channel length was studied by varying the residence time in the transient simulation. The results indicated that channels having a small characteristic dimension could yield reductions in the residence time, and therefore the reactor size, by as much as 85%. This reduction was further enhanced by higher concentration levels of the liquid reactant and increased temperatures. The inlet mixing region was found to have a significant influence on the total mass transfer. The channel wall wettability was found to affect the mass transfer characteristics negligibly. The predictions from the currently proposed model were compared with available experimental data, as well as with predictions of an earlier unit cell-based model, and a good agreement was obtained.

CO2 absorption in aqueous solutions of N-(2-hydroxyethyl)piperazine: Experimental characterization using interferometry and modeling

This work deals with the experimental characterization of carbon dioxide (CO2) absorption in an aqueous solution of N-(2-hydroxyethyl)piperazine, by using of digital holographic interferometry. This method enables visualizing the phenomena taking place in the vicinity of the gas–liquid interface. Thanks to a calibration, the obtained results yield the time evolution of the CO2 loading field in the aqueous solution and the gas–liquid absorption rate for various operating conditions. The processing method and the obtained results are detailed. A phenomenological model, highlighting the limiting steps of the gas–liquid CO2 absorption, is proposed. A set of experiments is realized and the parameters of this model are estimated for various initial amine concentrations and initial loadings. The results are discussed and correlations are proposed for the model parameters.

Effect of a falling gas–liquid absorption film temperature on entropy generation

In this paper, an analytical study about the effect of a falling gas–liquid absorption film temperature on entropy generation is carried out. Entropy generation formulations due to viscous effects and mass transfer are derived. Results in terms of viscous, mass transfer and total irreversibilities are graphically presented and discussed.

Second Law Analysis of a Gas-Liquid Absorption Film

This paper reports an analytical study of the second law in the case of gas absorption into a laminar falling viscous incompressible liquid film. Velocity, temperature, and concentration profiles are determined and used for the entropy generation calculation. Irreversibilities due to heat transfer, fluid friction, and coupling effects between heat and mass transfer are derived. The obtained results show that entropy generation is mainly due to coupling effects between heat and mass transfer near the gas-liquid interface. Total irreversibility is minimum at the diffusion film thickness. On approaching the liquid film thickness, entropy generation is mainly due to viscous irreversibility.

Modeling strategies of membrane contactors for post-combustion carbon capture: A critical comparative study

Membrane contactors are considered as a promising process for intensification purposes of gas–liquid absorption units. CO2 post-combustion capture is one of the currently intensively investigated applications and experiments are most often performed on lab scale hollow fiber modules with a monoethanolamine (MEA) aqueous solution, usually in a large excess, as chemical solvent. Different mathematical models, showing a broad range of intrinsic complexity, have been already proposed in order to simulate or predict the separation performances of membrane contactors for this application. Unfortunately, no systematic comparison of the different modeling approaches has been performed yet. This study addresses this issue through a series of experiments performed on the two main types of hollow fiber membrane contactors (microporous PTFE and dense PMP skin composite membrane) under different sets of operating conditions. Four different types of models (constant overall mass transfer coefficient, 1D resistance in series, 1D and 2D convection–diffusion models) have been compared with the membrane mass transfer coefficient as the only adjustable parameter. It is shown that the different models lead to comparable predictions of the experimental results, with slightly similar membrane mass transfer coefficient values. This result addresses key questions in terms of strategy for model validation and with regard to the current trend of increasing model complexity. Interestingly, a different situation holds when MEA is significantly converted at the liquid outlet: in that case, a 1D model approach is required (variable mass transfer coefficient), and leads to results which are comparable to 2D models. Guidelines for a relevant model comparison strategy are finally proposed.

Membrane gas separations and post-combustion carbon dioxide capture: Parametric sensitivity and process integration strategies

Reducing the energy requirement is the primary challenge for post-combustion CO2 capture technologies. Growing interest is being shown in membrane processes as an alternative to the reference technologies (such as gas–liquid absorption in amine absorption). However, these membrane processes remain energy intensive when the recommended CO2 purity and capture ratio (both typically at 90%) are imposed. In this study, a systematic parametric analysis of the energy requirement of a single stage membrane gas separation module has been performed. Different inlet CO2 contents and membrane selectivity performances have been compared. In contrast to previous studies, the capture ratio and CO2 purity constraints have been relaxed below the 90% target in order to possibly identify the most appropriate role and place of membrane processes in a Carbon Capture and Storage (CCS) framework. It is shown that diluted CO2 feed streams (such as natural gas turbine flue gases) systematically require excessive energy for CO2 capture. However, single stage membrane units offer interesting possibilities with a very low energy requirement when used as a pre-concentration step for a moderate inlet CO2 content (15–30%) or as a final step for concentrated streams (50% inlet CO2 content or more). Finally, guidelines for improved integration strategies of membrane units in different carbon capture scenarios, with a particular emphasis on hybrid processes, are proposed.

Evaluation of syngas-based chemical looping applications for hydrogen and power co-generation with CCS

This paper evaluates hydrogen and power co-generation based on coal-gasification fitted with an iron-based chemical looping system for carbon capture and storage (CCS). The paper assess in details the whole hydrogen and power co-production chain based on coal gasification. Investigated plant concepts of syngas-based chemical looping generate about 350–450 MW net electricity with a flexible output of 0–200 MWth hydrogen (based on lower heating value) with an almost total decarbonisation rate of the coal used.The paper presents in details the plant concepts and the methodology used to evaluate the performances using critical design factors like: gasifier selection criteria for chemical looping applications, influence of gasifier feeding system (dry fed vs. slurry fed), heat and power integration analysis as potential ways to increase the overall energy efficiency, hydrogen and carbon dioxide quality specifications considering the use of hydrogen in transport (fuel cells) and carbon dioxide storage in geological formation or used for EOR. The results show that syngas-based chemical looping systems ensure lower carbon capture energy penalties and higher overall plant energy efficiencies than more technologically mature capture methods (e.g. pre- and post-combustion capture based on gas–liquid absorption).

Treatment of Exhaust Gas Loaded with Chlorinated VOC by Composite Adsorbent

Three surfactant, sodium dodecyl sulfate (SBS), sodium dodecylbenzene sulfonate(SDBS), Polyoxyethylene Sorbitan Monolaurate(Tween 20), Turkey red oil sodium salt(TROS), were prepared and tested as composite adsorbents to remove chlorinated volatile organic component from exhaust gas. A gas–liquid absorption equipment was used to evaluate the performance of composite adsorbents. It is found that the three composite adsorbents could all effectively remove chlorobenzene but their absorption abilities were rather different. The maximum removal efficiency of chlorobenzene is composite absorbent TROS with low surface tension, reached as high as 85%, much greater than those of Tween 20 and SDBS (38% and 65%, respectively).

Membrane contactors for intensified post combustion carbon dioxide capture by gas–liquid absorption in MEA: A parametric study

Post combustion carbon dioxide capture raises tremendous chemical engineering challenges. For the first generation of industrial installations, gas liquid absorption in chemical solvents is classically considered to be the best available technology. Two major bottlenecks have however to be solved in order to achieve technico-economical targets: decrease the energy requirement of the process (e.g. through novel solvents or heat integration approaches) and decrease the size of the installation (through process intensification).This study intends to explore the possibilities and limitations of membrane contactors, which are considered as one of the most promising strategy for intensified CO2 capture by gas–liquid absorption. A very large number of studies is continuously reported on this topic, including materials, mass transfer or process design issues, but a rigorous evaluation of their effective potential in terms of intensification is still lacking. Moreover, controversial results have been reported such as intensification factors, compared to packed columns, ranging between 10 and 0.8 on a total unit volume basis.This unclear situation results from different factors. First, experimental comparison of membrane contactors vs. packed absorption columns performances is indeed seldom. Second, the evaluation of membrane contactors is systematically performed at laboratory scale, under operating conditions which do not necessarily reflect industrial operation (i.e. fresh amine solutions are used, limited capture ratio are achieved). These simplifying assumptions have obviously to be reconsidered if a realistic comparison for industrial operation is aimed. More importantly, pressure drop levels, which are known to be very small for packed columns (typically 50mBar on the gas side for an industrial packed column), have to be considered in order to minimize the energy impact of the process. An analysis combining intensification and pressure drop aspects for membrane contactors design, with solvent flowing inside the fibers, and the associated trade-off, which, to our knowledge, has not been achieved for CO2 absorption, is presented based on experimental and simulation results. Practical guidelines on the set of conditions for membrane materials (i.e. permeability and thickness), fiber geometry (external diameter, thickness) and module design (length, packing factor) which enable a significant process intensification effect are finally proposed.

Membrane contactors for intensified post-combustion carbon dioxide capture by gas–liquid absorption processes

Abstract The use of membrane contactors for post-combustion CO2 capture by absorption into a chemical solvent is currently one of the most intensively investigated topics in membrane science. The main (if not the only) target of employing membrane contactors for this application is to provide a significant reduction in size of the absorption unit, as compared to the classical gas–liquid absorption technology, based on packed columns. Surprisingly, this key performance characteristic, which is best expressed through a so-called intensification factor, remains essentially unreported in studies dedicated to CO2 absorption in membrane contactors. In this article, we recall the state of the art in terms of operating conditions and process performance for packed columns applied to CO2 capture. Based on this, a series of challenges for membrane material design and critical process engineering issues are discussed in order to set up a methodology for a better comparison between membrane contactors and packed columns. More specifically, an average volumetric CO2 absorption capacity around 1 mol CO2 m−3 s−1 is proposed as a baseline performance of packed columns in order to estimate the intensification factor of membrane contactors.

Dynamic modeling and absorption capacity assessment of CO2 capture process

One of the most promising carbon dioxide capture technology is based on chemical gas–liquid absorption using alkanolamine solutions. In order to optimize the operating conditions of absorption–desorption cycle, the development of a detailed mathematical model is necessary. Several mass transfer and hydraulic correlation models are published in the literature, but study of predictive capacity for absorption process is required. The aims of this work are to check the mass transfer and hydraulic correlation models predictive capacity and to investigate the dynamic behavior and absorption performance of four different type alkanolamine. The study shows that the implemented mathematical model reveals a good quality prediction of the absorption process for all alkanolamines.The analysis of simulation results highlight that the mass transfer correlation model proposed by Wang et al. predicts well the effective mass transfer area and the mass transfer coefficient correlation. Model proposed by Billet and Schultes predicts well in case of MEA and DEA. And correlation model proposed by Rocha et al., predicts well in case of AMP and MDEA. The comparison of three hold up models show that the model proposed by Bravo et al. is capable to predict well this parameter in a wide column loading range.

PENGENDALIAN EMISI GAS BUANG BOILER BATUBARA DENGAN SISTEM ABSORBSI

Absorption study of emission gas (SO2, NO2, CO, and CO2) from coal boiler with NaOH absorber waste of textile industry has been done. Acid gasses neutralized and particulate trapped by NaOH solution. NaOH was chosen for efficiency of waste neutralized and obtained from waste absorber and conventional textile finishing process (NaOH) with the concentration of 100 mol/m3. Emitted disposal gas concentration at flow rate 500, 1000, 2500, 5000, and 10,000 L/hour before and after absorption was measured and analyzed using MATLAB and EXCEL software. There was no significant difference between observation and theoretical calculation data (P&lt;0,05). At flow rate 2500 L/hour concentration of emission gas have shown under threshold values. A liquid-gas absorption model that obtained form this study could be used to predict optimum flow rate required for reducing the emission gas concentration to the quality threshold values.

Pre-combustion carbon dioxide capture by gas–liquid absorption for Integrated Gasification Combined Cycle power plants

Among various configurations of fossil fuel power plants with carbon dioxide capture, this paper focuses on pre-combustion capture technology applied to an Integrated Gasification Combined Cycle power plant using gas–liquid absorption. The paper proposes a detailed study and optimization of plant design (column height and packed dimensions) with CO2 capture process using different solvents as: aqueous solutions of alkanolamine, dimethyl ethers of polyethylene glycol, chilled methanol and N-Methyl-2-pyrolidone. By developing simulations in Aspen Plus, the following performance results of these physical and chemical solvents, mentioned above, are discussed: overall energy consumption (power consumption, heating and cooling agent consumption), CO2 specific emissions, net electric power output and plant efficiency. The paper presents as well, the total investment capital cost of an IGCC coal mixed with biomass (sawdust) power plant generating 425–450MW net electricity with (70% CO2 capture, 80% CO2 capture and 90% CO2 capture) and without pre-combustion CO2 capture.Simulation results show that for evaluated solvents for CO2 capture, the physical solvent, dimethyl ethers of polyethylene glycol, is more energy efficient that the other physical and chemical solvents investigated. Regarding the economic study, implementation of pre-combustion CO2 capture on IGCC plant, using dimethyl ethers of polyethylene glycol, leads to an increase of the capital cost with about 19.55% for 70% CO2 capture, 20.91% for 80% CO2 capture and 22.55% for 90% CO2 capture.