Gas Phase Mass Transfer Resistance Research Articles

Overpotential Behavior of Electrolysis and Fuel Cell Modes of Molten Carbonate Cells

Molten carbonate is an high operating temperature electrolyte for the fuel cell (MCFC) and electrolysis cells (MCEC). In particular, the liquid molten carbonates allow the reversible operation between fuel cell and electrolysis cells. The electrode kinetics and mass transfer characteristics of MCFC have been rather extensively investigated compared with those of MCEC. In this work, the mass transfer behaviors between MCFC and MCEC have been focused by experimental way with 100 cm2 class bench cells.The electrodes and matrix in the cell were supplied by KIST in Korea. The cells were operated at 923K under atmospheric pressure. The hydrogen electrode was supplied by a mixture of H2:CO2=1:1, 2:1, 1:2 ratios at 20% or 40% H2O. The oxygen electrode was fed by Air:CO2=0.7:0.3 atm. The steady state polarization (SSP) from 0 to 150 mA cm-2, inert gas step addition (ISA), and reactant gas addition (RA) methods were employed. The ISA adds inert gas to the hydrogen or oxygen electrodes, then the addition varies reactants flow rate and partial pressures. Consequently, the variations are reflected to the cell voltage at open circuit and polarization states. The voltage shift by the flow and partial pressure change represents overpotential behaviors at the electrodes [1]. The RA method uses a reactant gas instead of inert gas in the ISA. Therefore, the RA provides the overpotential information of the added reactant species [2].From the experimental investigations, the total voltage loss at the FC is larger than that of EC mode. In addition, the hydrogen electrode has significant gas phase mass transfer resistance due to the H2, CO2, and H2O species at the FC and EC modes. A similar overpotential was observed at the hydrogen electrode. On the other hand, the oxygen electrode has much less overpotential at the EC mode than the FC mode. Since the oxygen reduction at the FC mode contains O2 dissolution and mass transfer through the carbonate electrolytes, liquid phase mass transfer resistance is dominant. However, the oxidation of carbonate ions to oxygen does not contain liquid phase mass transfer process at the EC mode. Therefore, much less overpotential was observed at the oxygen electrode at the EC mode than the FC mode. This can be a reason of smaller total voltage loss at the MCEC than the MCFC.[1] Samuel Koomson, Choong-Gon Lee, J. Electroanalytical Chemistry, 925, 116896 (2022).[2] C.-G. Lee, H.-C. Lim, J. Electrochem. Soc., 152, A219-A228 (2005).

Effect of temperature on the electrode reactions in a planar solid oxide fuel cell

Understanding the temperature dependence of electrode reactions is attempted with a commercial 100 cm2 class planar solid oxide fuel cell. Experimental investigations are conducted using a specially designed setup, i.e., inert gas step addition (ISA) and reactant gas step addition (RA) methods, to measure the electrode overpotential behaviour at temperatures ranging from 923 to 1073 K. The ISA reveals that the gas-phase mass transfer resistance at the anode is more substantial and becomes immensely larger with increasing temperature due to the effect of pore diffusion resistance (PDR). On the other hand, the gas-phase mass transfer resistance of the cathode shows an unclear dependence on temperature, indicating little resistance in the cathode. The RA method represents that the H2-inducing mass transfer resistance increases with increasing temperature due to the PDR effect, while the O2-inducing mass transfer resistance decreases with increasing temperature because of the reducing solid-phase mass transfer resistance of oxide ions.

A rigorous surface renewal model for transient gas absorption with first-order chemical reaction in a stirred liquid

This work presents a rigorous surface renewal model for unsteady-state gas absorption in a stirred batch cell in which the dissolved gas undergoes a first-order chemical reaction with a liquid-phase reagent for the cases of zero and finite gas-phase mass-transfer resistance, with the concentration of dissolved gas in the bulk liquid being a function of time. For oxygen absorption in water containing a reagent (e.g., sodium sulfite), the model predicts that the rates of absorption and dissolved-gas transfer to the bulk liquid have an inverse behavior during the initial moments of absorption after which they level out as steady state is approached, with the former being higher than the latter due to the consumption of dissolved gas by the reaction occurring in the surface elements. In general, predictions of the dissolved oxygen concentration made with the rigorous model compare quite well with those of a much simpler pseudo steady-state model. However, the rigorous surface renewal model gives a finer grained picture of the phenomenon of absorption, i.e., of the rates of absorption and dissolved-gas transfer, especially during the initial moments of absorption. A hybrid model, which uses both the pseudo steady-state and rigorous surface renewal models, is proposed for performing unsteady-state gas absorption calculations.

Comparison of gas phase transport effects between fuel cell and electrolysis cell modes of a 100 cm2 class molten carbonate cell

• Gas phase transport effects are compared between electrolysis and fuel cell modes with 100 cm 2 class cells. • Symmetric overpotential behaviours are obtained between electrolysis and fuel cell modes. • Severe gas phase mass transfer resistance exists in the hydrogen electrode of electrolysis cell. • Negligible gas phase resistance is observed in the oxygen electrode of electrolysis cell. The gas-phase mass transfer effects of 100 cm 2 class molten carbonate cells (MCCs) were investigated in both the fuel cell and electrolysis modes using steady-state polarization (SSP) and inert-gas step addition (ISA) methods. The SSP results showed symmetric and linear behaviours between fuel cell (FC) and electrolysis cell (EC) modes without showing activation polarization in both modes. The ISA results showed that the hydrogen electrode (HE) has significant gas-phase mass transfer resistance in both modes, and the overpotential shift pattern of HE by the flow rate change is symmetric in both modes. On the contrary, a small gas-phase mass transfer inducing overpotential was observed at the OE in the FC mode compared to the HE, while there was no gas-phase mass transfer-induced overpotential in the EC mode. Therefore, the EC mode of MCCs comprises mainly gas-phase mass transfer controlling processes at HE and negligible at OE. In addition, it can be inferred that reversible operation in both modes is possible, and the reaction mechanisms are symmetrical between FC and EC modes.

Transient physical gas absorption in a stirred liquid using the surface renewal model of mass transfer

This work presents a rigorous surface renewal model for unsteady-state physical gas absorption in a stirred batch cell in which the concentration of dissolved gas in the bulk liquid is a function of time for zero and finite gas-phase mass-transfer resistance. Using experimental parameters from the literature for oxygen and hydrogen absorption in water at 25 °C and atmospheric pressure, predictions of the model reveal that the rates of absorption and dissolved-gas transfer to the bulk liquid exhibit an inverse behavior during the initial moments of absorption after which they merge into a single curve and decline with process time as the water becomes increasingly saturated with the dissolved gas. Predictions for the dissolved oxygen concentration made with the rigorous model compare excellently with those of the standard pseudo steady-state model used in the literature except for short time scales.

Lifetime expectancy of molten carbonate fuel cells: Part II. Cell life simulation using bench and coin-type cells

The lifetime of molten carbonate fuel cells is simulated in terms of electrolyte loss rate, voltage reduction rate, and activation energy using 7 cm2 coin- and 100 cm2 bench-type molten carbonate fuel cells. Arrhenius plots are used to determine the temperature dependence of the anode gas-phase mass transfer resistance, cathode gas and liquid-phase mass transfer resistances and electrolyte loss rate. The gas-phase mass transfer resistance of the anode has positive activation energy, indicating more substantial resistance at higher temperatures. The cathodic gas-phase mass transfer resistance has small and negative activation energy. In contrast, the cathode shows negative and positive activation energies at the mass transfer resistance of superoxide ion (O2−) and CO2 in the liquid electrolytes, respectively. The negative value indicates a lower overpotential at higher temperatures and vice versa. The Arrhenius plot of the electrolyte weight loss rate shows positive activation energy, indicating that an increase in temperature causes a simultaneous increase in electrolyte weight loss. The cell life of a molten carbonate fuel cell is predicted using a factor that relates the voltage reduction and electrolyte loss rates. A lower value of the factor gives a longer cell life.

Simultaneous application of vacuum and sweep gas in a polypropylene membrane contactor for the recovery of dissolved methane from water

Vacuum and nitrogen as sweep gas were used simultaneously (combination mode) to generate the driving force in a microporous polypropylene membrane contactor for the recovery of dissolved methane from water. Experiments were carried out under different liquid flow-rates (5.0–28.0 L h−1), N2 flow-rates (0.5–1.5 L h−1) and vacuum pressures (0–480 mbar). The maximum removal efficiency of methane was >90% at moderate values of gas-to-liquid ratios (G/L) and vacuum of 0.2 and ≤ 200 mbar, respectively. Removal efficiencies obtained in combination mode were usually higher than when using vacuum or sweep gas, separately. To achieve a methane content >35% in the recovered gas, G/L values lower than 0.03 are necessary, with removal efficiency up to 55%. Even at these soft conditions, gas phase mass transfer resistance was demonstrated as being negligible, so the limiting resistance was in the liquid phase. G/L ratios increased until values of 0.3, with a vacuum of 200 mbar, maximised the energy output (>250 kJ m−3 of treated water), in the case of recovered methane mixed with biogas produced in an anaerobic reactor. The results show that combination mode could be used to improve the performance of methane degassing in membrane contactors.

Effect of gas-phase transport in a planar solid oxide fuel cell

Reaction mechanisms and performance improvement in solid oxide fuel cells (SOFCs) have been the major research areas in this field. There are several discrepancies associated with the kinetics of SOFCs due to the contradicting theories proposed by researchers in this area. This work aims at determining the overpotential due to the gas-phase mass transfer resistance at the various electrodes and the possible electrode reaction characteristics using the inert gas step addition (ISA) method proposed by this research group. The ISA method can quantitatively measure the overpotentials due to the mass transfer resistance at each electrode. The ISA method shows an inverse relationship between the overpotential shift and the square root of the reactant gas flow rate according to the boundary layer theory. The overpotential due to the gas-phase mass transfer resistance is expected to be much higher at the anode than at the cathode, indicating the presence of considerable gas-phase mass transfer resistance at the anode.

Pressure effect on the electrode reactions in a molten carbonate fuel cell

The effect of pressure up to 3 atm on electrode reactions was investigated with steady-state polarization, AC impedance and reactant gas addition (RA) methods. Two 100-cm2 class molten carbonate fuel cells (MCFCs) were used in this work. AC impedance revealed that the cathodic mass transfer resistance was reduced by pressurization, whereas the anodic one was slightly enlarged by the increased gas phase mass transfer resistance. The RA method clearly showed that the mass transfer resistance of the H2 gas rose with pressurization, likely due to methanation at the anode. On the other hand, it also revealed that the mass transfer resistance of O2 species in the cathode was decreased by pressurization.

An experiment and model of ceramic (alumina) hollow fiber membrane contactors for chemical absorption of CO2 in aqueous monoethanolamine (MEA) solutions

The chemical absorption of CO2 in a monoethanolamine (MEA) solution by a ceramic hollow fiber membrane contactor (HFMC) was investigated experimentally and numerically to obtain the best compromise between the mass transfer coefficient and structural characteristics such as membrane pore size and porosity. The mathematical model derived is based on the three resistances in the resistance-in-series model. The accuracy of the numerical simulation was verified quantitatively by the experimental data obtained in this study. A good agreement between experimental and computational results was found with an average absolute deviation (AAD) between observed data and predicted values of 2.86%. In addition, the effects of the operating condition (i.e., gas and liquid flow rates) on the mass transfer coefficients for ceramic HFMC systems were also studied, revealing that the membrane and gas-phase mass transfer resistances were dominant factors in the overall mass transfer. In conclusion, the present study suggests that the membrane structure plays a very important role in the optimization of HFMC performance. In fact, the best results were obtained with an intermediate range of the pore size between 102 and 104 nm, corresponding to the best compromise between performance (i.e., overall mass transfer coefficient) and applicability (i.e., breakthrough pressure).

Influence of Key Factors on the Characteristics of Flue Gas Desulfurization of Basic Aluminum Sulfate by Bubbles.

Temperature, reactant concentration, bubble flow field characteristics, and mass-transfer characteristics play an important role in flue gas desulfurization of basic aluminum sulfate (aluminum base). The influence of various factors on desulfurization efficiency (ηabs) was determined from the macroscopic and microscopic levels through experiments and numerical simulations. The temperature of absorption solution had a significant effect on ηabs, and low temperature was advantageous for SO2 absorption performance of aluminum base. The value of ηabs increased with increasing reactant concentration. When the aluminum base concentration was higher than 50%, ηabs remained above 90%. The low aluminum base shortened the beginning time of the rapid decline of ηabs. This outcome was related to the low concentration of active Al2O3. At a low concentration range of 2000–5000 ppm of inlet SO2, the reaction rate determined the ηabs level, and gas-phase mass-transfer resistance was the main factor restricting the increase in ηabs. However, reaction rate and gas-phase mass-transfer resistance determined the ηabs level when the inlet SO2 concentration was as high as 50 000 ppm. The aeration rate affected the bubble size, number, and diffusion state. In addition, a higher aeration rate reduced the gas–liquid contact time and speeded up the consumption of aluminum base. As a result, bubbles along the absorber height showed different desulfurization characteristics, that is, 0–0.1 m was the initial stage of bubble formation and desulfurization (the ηabs was lower than 15%), 0.1–0.34 m was the main stage of bubble diffusion and desulfurization (the ηabs rapidly increased to 86.4%), and 0.34–0.4 m was the stable stage of desulfurization (the ηabs slowly increased to 92.5%).

Temperature effect on the electrode reactions in a molten carbonate fuel cell

A temperature effect on the electrode reactions was investigated in terms of activation energy of overpotential with 100cm2 class molten carbonate fuel cells. The overpotential is attributed to the resistance in the electrochemical reactions on electrodes, thus the activation energy reflects reaction characteristics of the electrodes. The activation energy was measured with electrochemical impedance spectroscopy (EIS) and inert gas step addition (ISA) methods. These methods revealed that the anodic overpotential had positive activation energies showing larger overpotential at higher temperature, whereas the cathodic overpotential had negative activation energies. In particular, the positive activation energy of anodic overpotential is a unique behavior which has not been treated before. The gas phase diffusion resistance in the porous electrode, calling pore diffusion resistance (PDR), would be responsible for the positive activation energy. On the other hand, the temperature dependence of reactant gas dissolution to the carbonate melts and transport through the carbonate film leads to negative activation energy because the cathode is covered by the relatively thick electrolyte film. Insignificant PDR effect is observed at the cathode through the activation energy of gas phase mass transfer resistance at the cathode.

Gas-Phase Mass-Transfer Resistances at Polymeric Electrolyte Membrane Fuel Cells Electrodes: Theoretical Analysis on the Effectiveness of Interdigitated and Serpentine Flow Arrangements

Mass transfer phenomena in polymeric electrolyte membrane fuel cells (PEMFC) electrodes has already been analyzed in terms of the interactions between diffusive and forced flows. It was demonstrated that the whole phenomenon could be summarized by expressing the Sherwood number as a function of the Peclet number. The dependence of Sherwood number on Peclet one Sh(Pe) function, which was initially deduced by determining three different flow regimes, has now been given a more accurate description. A comparison between the approximate and the accurate results for a reference condition of diluted reactant and limit current has shown that the former are useful for rapid, preliminary calculations. However, a more precise and reliable estimation of the Sherwood number is worth attention, as it provides a detailed description of the electrochemical kinetics and allows a reliable comparison of the various geometrical arrangements used for the distribution of the reactants.

Simulation of carbon dioxide absorption by monoethanolamine solution in wetted-wire column

This study is an attempt to investigate the chemical absorption of CO2 in aqueous mono-ethanolamine (MEA) solution in a multi-wire column. A rate-based process model based on the fast reaction for the CO2–MEA system was employed for modelling of mass transfer and chemical reaction inside the column. As the gas phase mass transfer resistance is not negligible comparing to the liquid phase mass transfer resistance, unlike previous models, gas side mass transfer resistance has also been considered in the model. In addition, the heat effects associated with the absorption and chemical reaction are included through energy balances in the gas and liquid phases. A computer program (MATLAB code) was developed to simulate CO2 absorption into aqueous solution of mono-ethanolamine in a multi-wire column. The model is capable of predicting essential information, such as overall mass transfer coefficient, gas absorption rate and efficiency of the column. The modelling results were compared with available experimental data conducted in columns that equipped with both single wire and multiple wires and good agreement was achieved. Thereby it was concluded that the developed model is capable to predict the chemical gas absorption performance of both multi-wire and one-wire gas–liquid contactors.

Gas-surface interfacial and gas-phase resistances to vaporizing cesium hydroxide and chloride in air

The interfacial and the pure gas-phase resistances in air to vaporizing cesium species were evaluated in a comparison of the experimental vaporization flux and the maximum vaporization flux as calculated based on the kinetic theory of gas and the film theory. Gas-phase mass transfer resistances did not show great changes with the temperature, but their values were much greater that those associated with the interfacial resistances. The values of gas-phase resistances were at least a hundred times greater than interfacial resistances for the vaporizing cesium species investigated in this study.

Comparative kinetics of carbon dioxide absorption in unloaded aqueous monoethanolamine solutions using wetted wall and string of discs columns

The kinetics of the reaction between carbon dioxide and unloaded aqueous solutions of monoethanolamine (MEA) were investigated over the temperature range from 298 to 343K, MEA concentration range from 0.5 to 5mol/L in both a wetted wall column reactor (WWC) and a string of discs contactor (SDC). Earlier kinetic studies in the CO2–MEA–H2O system are reviewed and compared first based on the experimental data alone. The comparisons are made for different types of apparatuses to establish whether the recent literature data and the data from this study are consistent with each other. Then the various ways of interpreting the data are reviewed and compared. Finally, the new data are interpreted using the pseudo first order assumption and including gas phase mass transfer resistance. The direct mechanism was used for the interpretation and the reaction order was found to be in the range of 1.36–1.51 with respect to concentrations. A model was developed for the kinetic rate constant explaining the broken order behavior.

Simultaneous removal of CO2 and H2S from pressurized CO2–H2S–CH4 gas mixture using hollow fiber membrane contactors

In the present paper, the simultaneous removal of CO2 and H2S from their pressurized mixture with CH4 (5% CO2–2% H2S–93% CH4) using custom made membrane contactors equipped with micro-porous polymeric hollow fibers is described. Two types of commercial hollow fibers were employed and compared, i.e., expanded poly(tetrafluoroethylene) (ePTFE) and poly(tetrafluoroethylene-co-perfluorinated alkyl vinyl ether) (PFA). The feed gas mixture composition was 2% H2S, 5% CO2 in balance of CH4 to mimic the typical natural gas composition. Distilled water, aqueous sodium hydroxide and amine solutions of different concentrations were tested as absorption liquids to achieve the selective removal of the acid gases from the pressurized gas streams. The effect of pressure on the simultaneous CO2 and H2S absorption rates was investigated by the simultaneous pressurization of the absorption liquid and the feed gas (up to 50bar) and monitoring the residual acid gases in the exit stream. The obtained experimental results indicated that the simultaneous CO2 and H2S fluxes were enhanced by increasing the inlet gas pressure for both physical and chemical absorption solvents. Up to the authors’ best knowledge, this is the first report on (i) the experimental simultaneous removal of CO2 and H2S from pressurized gas streams using HFM contactors and absorption solvents up to 50bar and (ii) the utilization of PFA fibers in custom made high pressure-membrane modules. PFA fibers exhibited impressive higher fluxes (9–10 times) for CO2 and H2S than those obtained with the common ePTFE fibers which could be attributed in part to the associated higher mass transfer coefficient. The anticipated CO2/H2S flux ratio of 2.5 (which matches the ratio of their inlet concentrations in the feed gas) was obtained whenever the co-absorption was unhindered into absorption solvent of sufficient capacity, e.g., 2.0 M NaOH solution. Analysis of the mass transfer coefficients showed that while the overall mass transfer coefficients are determined by the liquid phase mass transfer coefficients at low pressures, gas phase mass transfer resistance contributes considerably to the overall resistance at high pressures.

Significance of gas velocity change during the transport of CO 2 through hollow fiber membrane contactors

A comprehensive 2D mathematical model was developed for the physical and chemical absorption of CO 2 from natural gas containing high percentage of CO 2. Unlike previous mathematical models, the model considers change in the axial gas velocity as CO 2 is being absorbed from the gas mixture. The model was validated with the experimental data and compared with previous model results for a gas mixture containing 10% CO 2, and then was expanded to account for higher percentages of CO 2. For 10% CO 2, the model predictions showed a slight difference between the previous model and the experimental data for the physical absorption of CO 2. However, there was a much improved agreement between the model predictions and the experimental data for the case of chemical absorption using 0.005 M MEA. Although this difference might be small for low content of CO 2 in gas mixtures, the model results showed that the decrease in gas velocity becomes significant for higher content of CO 2, particularly if high absorption rate of CO 2 was achieved and thus, maintaining a high percent of removal of CO 2 due to the increase in residence time. This is a major contrast with the previous model behavior where the percent removal of CO 2 continuously decreased with the introduction of more CO 2 in the gas mixture. Furthermore, the model results showed that the effect of bulk flow contribution for gas mixtures containing high content of CO 2 is insignificant for systems where the gas phase mass transfer resistance is small when compared to that of the liquid phase.

Biofiltration of simulated air pollutants from distillers dried grains with solubles (DDGS) dryer vents at corn‐derived ethanol production facilities

AbstractThe DDGS dryer vents at corn‐derived ethanol production facilities often contain a mixture of odorous and/or potentially hazardous air pollutants including acetaldehyde and formaldehyde. The objective of this study was to investigate the feasibility of utilizing a biofilter system to treat the volatile organic compounds (VOCs) released from the DDGS dryer stack during ethanol production. Single aldehyde elimination capacity experiments indicate that both acetaldehyde and formaldehyde are readily biodegraded in a biofilter at a short contact time of 10 s, although the acetaldehyde removals during the initial start up period were lower than those observed during later operation. A biofilter packed with porous silicate pellets at neutral pH successfully achieved greater than 90% removal of VOC mixtures containing acetaldehyde, formaldehyde, ethanol, and acetic acid. However, extended operation with the four component VOC mixture led to a decline in biofilter pH. At low pH (4.6), acetaldehyde removal declined from 95 to 62% and ethanol removal declined from 90 to 57%. The option of utilizing a water scrubber upstream of the biofilter to remove acetic acid was investigated; it was determined that effective acetic acid removal was possible in a recirculating water scrubber maintained at a pH level above ∼7, where the rate of removal was controlled by gas phase mass transfer resistance. © 2009 American Institute of Chemical Engineers Environ Prog, 2010

Dry-casting: Computer simulation, sensitivity analysis, experimental and phenomenological model studies

A model for the dry-casting process for polymeric membrane formation is generalized to include a broader spectrum of boundary conditions. This model is used to conduct a sensitivity analysis to assess which properties and process parameters are most influential and hence need to be determined most accurately. The model predictions are relatively insensitive to changes in the nonsolvent/polymer and nonsolvent/solvent friction coefficients but moderately sensitive to changes in the solvent/polymer friction coefficient. However, the predictions are very sensitive to changes in the gas-phase mass-transfer coefficient, which needs to be well-characterized and carefully controlled during the dry-casting process. Experimental studies for the water/acetone/cellulose acetate system demonstrate that changes in the boundary conditions can result in dramatically different membrane morphologies. The results of the modeling, sensitivity analysis and experimental studies are used to advance a simple phenomenological model for the dry-casting process that explains all the observed trends. This is used to develop a set of design heuristics for dry-casting based on the effect of changes in the properties and parameters on the casting solution and gas-phase mass-transfer resistances, the solvent concentration at the interface and the mass-transfer rate.