Interfacial Area For Mass Transfer Research Articles

Influence and Optimization of Gas Diffusion Layer Porosity Distribution along the Flow Direction on the Performance of Proton Exchange Membrane Fuel Cells

Influence and Optimization of Gas Diffusion Layer Porosity Distribution along the Flow Direction on the Performance of Proton Exchange Membrane Fuel Cells

Modification of liquid film thickness by the action of centrifugal force in a falling film microreactor: Effect on the overall process performance of a catalytic cyclohexene hydrogenation

This work presents a process intensification strategy for mass transfer enhancement by developing a system that provides centrifugal acceleration to a falling film microreactor (FFMR). The effective interfacial area for mass transfer was increased by modifying the liquid film at varying flow rates due to the applied centrifugal force's influence. The Nusselt and Kapitza correlations often utilized in predicting liquid film thickness were modified to account for an externally induced force acting normal to the flow direction. Consequently, the effect on film thickness was quantified, enabling the assessment of the overall process performance at different rotational speeds of the reactor unit.For this purpose, hydrogenation of cyclohexene conducted over a Pt/Al2O3 catalyst coated on the micro channel's surface was used as a model reaction. Overall process performance was significantly improved, and mass transfer rate was enhanced by a factor of more than 4.

Electrical Capacitance Volume Tomography (ECVT) for Characterization of Additively Manufactured Lattice Structures (AMLS) in Gas-Liquid Systems

Against the background of current and future global challenges, such as climate change, process engineering requires increasingly specific solutions adapted to the respective problem or application, especially in gas–liquid contact apparatuses. One possibility to adjust the conditions in this kind of apparatuses is an intelligent and customized structuring, which leads to consistent fluid properties and flow characteristics within the reactor. In the course of this, the interfacial area for mass transfer, as well as residence times, have to be adjusted and optimized specifically for the respective application. In order to better understand and advance the research on intelligent customized additively manufactured lattice structures (AMLS), the phase distributions and local gas holdups that are essential for mass transfer are investigated for different structures and flow conditions. For the first time a tomographic measurement technique is used, the Electrical Capacitance Volume Tomography (ECVT), and validated with the volume expansion method and a fiber optical needle probe (A2PS-B-POP) for an air-water system for different modes of operation (with or without co-current liquid flow in empty or packed state). The ECVT proved to be particularly useful for both in the empty tube and the packed state and provided new insights into the phase distributions occurring within structured packings, which would have led to significantly underestimated results based on the visual reference measurements, especially for a densely packed additively manufactured lattice structure (5 mm cubic on the tip). Particularly for the modified structures, which were supposed to show local targeted differences, the ECVT was able to resolve the changes locally. The additional use of a pump for co-current flow operation resulted in slightly higher fluctuations within the ECVT data, although local events could still be resolved sufficiently. The final comparison of the empty tube at rest data with a fiber optical needle probe showed that the results were in good agreement and that the local deviations were due to general differences in the respective measurement techniques.

Mass transfer process intensification for SO2 absorption in a commercial-scale wet flue gas desulfurization scrubber

To intensify SO2 absorption process in wet flue gas desulfurization (WFGD) spray scrubber, a comprehensive CFD model coupled transfer process with chemical reactions is established by Eulerian‒Lagrangian method and verified in a commercial-scale device of 330 MW coal-fired power unit. The desulfurization performance is significantly affected by both the property of each droplet and the dispersion of droplet swarm, which are determined by the spray behaviors. Three dominant factors, including droplet diameter, injection direction and spray morphology, are explored to optimize the spray conditions for intensifying SO2 absorption process. Differing from the ideal plug flow, the optimal diameter of droplet in non-ideal flow is obtained by considering the synergistic effect of the uniformity of gas–slurry distribution and the dynamics of SO2 mass transfer. Compared with the single-direction spray with downward or upward injection, the dual-direction spray reaches a great balance between the competitive effects of the uniformity of gas‒slurry flow and the interfacial area for mass transfer. The influence of spray morphology on mass transfer process is essentially the difference of dispersion characteristics of droplets in the middle part of scrubber. A design scheme combining full-cone and hollow-cone nozzles is suggested for improving the overall SO2 removal efficiency.

Aerobic oxidation of benzyl alcohol in a slug flow microreactor: Influence of liquid film wetting on mass transfer

AbstractHomogeneous Co/Mn/Br catalyzed aerobic oxidation of benzyl alcohol in acetic acid to benzaldehyde was performed in polytetrafluoroethylene microreactors operated under slug flow at temperatures up to 150°C and pressures up to 5 bar. Depending on the bubble velocity and length, a wetted or dewetted slug flow was observed, characterized typically by a complete or partially wetting liquid film around the bubble body. The latter flow suffered from a limited interfacial area for mass transfer. Experiments at temperatures up to ca. 90°C were under kinetic control given no product yield difference under wetted and dewetted slug flows and were used to establish a simplified kinetic expression (first order in benzyl alcohol and zero order in oxygen). This allows to develop a mass transfer model combined with an instantaneous reaction regime that well described the experimental results at higher temperatures where mass transfer was limiting in the dewetted slug flow.

Point efficiency of sieve trays at elevated liquid viscosities

Liquid viscosity plays a major role in determining the mass transfer efficiency in distillation through its contribution to the liquid-phase mass transfer coefficient, interfacial area of mass transfer, and vapor and liquid phase residence times. Most of the distillation efficiency correlations in the literature were developed or validated based on efficiency data at liquid viscosities less than 1.5mPas. However, distillation at liquid viscosities greater than 1.5mPas is encountered in practice in the chemical process industry. Therefore, distillation efficiency data at liquid viscosities greater than 1.5mPas are required to accurately quantify the effect of liquid viscosity on distillation efficiency and to develop improved efficiency correlations. Point efficiency defines the most fundamental mass transfer efficiency for distillation trays upon which tray and column efficiencies are calculated. In this paper, the effect of liquid viscosity on point efficiency of sieve trays is reported with new data for four different test systems at liquid viscosities ranging from 0.2 to 4.2mPas. These new data are critical for evaluating the validity of the existing tray and point efficiency correlations and for extending the existing correlations or developing new correlations for distillation column design at elevated liquid viscosities.

Evaluation of the validity of tray and point efficiency correlations at elevated liquid viscosities and proposal of an improved point efficiency correlation

The accuracy of the distillation efficiency correlations plays a critical role in cost-effectively designing distillation columns. Liquid viscosity is considered to be a major factor that influences distillation efficiency through its contribution to the liquid-phase mass transfer coefficient, interfacial area of mass transfer, and vapor and liquid phase residence times. Nearly all of the distillation efficiency correlations in the literature were developed or validated on the basis of efficiency data at liquid viscosities less than 1.5mPas. However, distillation at liquid viscosities greater than 1.5mPas is encountered in several cases in the chemical process industry. In this paper, the evaluation of three different tray and point efficiency correlations using Oldershaw column data at liquid viscosities from 0.2 to 4.2mPas is presented. In addition, we propose an improved point efficiency correlation, based on the Duss and Taylor correlation, with a modified liquid viscosity exponent for the liquid-phase mass transfer coefficient. The proposed correlation was validated with commercial sieve tray efficiency data and Oldershaw column data. This proposed correlation will aid in the cost-effective design of distillation columns, especially at elevated liquid viscosities.

Batch reactor vs. flow column – Mechanistic investigation and modeling of Au(III) ions adsorption from aqueous solutions containing Ni2+, Na+, Cl− and ClO4− as impurities

In this paper, a mechanistic analysis of the adsorption and reduction of gold(III) chloride complex ions on the activated carbon surface were described. All experiments were performed in the presence of nickel ions. Obtained results confirm that there is no influence of light and heavy cations on the adsorption process. From the Arrhenius equation, activation parameters such as activation energy (17.53 ± 0.98 kJ/mol) and pre-exponential (28.7 ± 8.91 min−1) factor were determined. SEM and XRD, as well as XPS analysis, have confirmed the presence of metallic gold on the surface of activated carbon. The concentration distribution of gold inside activated carbon after adsorption process both for continuous stirred tank reactor and continuous flow reactor was determined. New estimator for interfacial area of mass transfer was defined.

Synergistic effect between spraying layers on the performance of the WFGD spray column

AbstractIn a WFGD spray column, spraying droplets have dual duties of optimizing flow uniformity and providing interfacial area for mass transfer but have contradiction in selection of diameter. In this paper, a synergistic effect between smaller and bigger droplets is proposed to improve the performance of the column, which is to enlarge the interfacial area and optimize the flow uniformity in different layers, and it is used to create a novel design scheme. Through the study of the flow uniformity in the spray column, the detail of this design scheme is proposed as followed: The minimum flow rate and proper droplet diameter should be selected in the bottom layer to achieve the flow uniformity, and smaller droplets can be used in the rest layer to enlarge the interfacial area as much as possible. Compared with the traditional scheme, the liquid hold‐up has increased 16%–30%, and the interfacial area has increased 32%–56% in novel scheme. In practice, this novel design scheme can provide reference and guidance for large scale spray columns.

Reaction Kinetics of CO2 with Monoethanolamine in n-Propanol. 1. Reaction Kinetic Data and Comparison with Existing Rate Law Expressions

The reaction kinetics of CO2 with monoethanolamine is of industrial significance with respect to both CO2 sequestration applications and characterizing the effective interfacial mass transfer area of packed separation columns. Reaction kinetic data were previously, by necessity, only measured under pseudo-first-order conditions with respect to CO2. Furthermore, mass-transfer limitations encountered by the heterogeneous techniques restricted the validity range of the reaction kinetic models developed from the data. New reaction kinetic data, independent of mass-transfer limitations and outside pseudo-first-order conditions, are presented. The data, collected via a previously developed novel, in situ FTIR technique, were subsequently compared with the predictions of two widely accepted rate expressions, the power rate law and pseudo-steady-state hypothesis (PSSH) rate law. The expressions were modeled on the data using a novel multiobjective goal attainment algorithm also developed in this study. The PSSH r...

Mass transfer studies on catalyst-aided CO2 desorption from CO2-loaded amine solution in a post-combustion CO2 capture plant

Two solid acid catalysts of contrasting characteristics were studied on their effects on the liquid-side mass transfer performance for the desorption of CO2 from CO2-loaded 5M monoethanolamine (MEA) solvent. The characteristics of the catalysts were both physical (surface area of catalyst which increases the interfacial area for mass transfer) and chemical (the catalyst acid sites) to catalyze CO2 desorption as illustrated by the use of protonated zeolite socony mobile-5 (HZSM-5) representing a higher Brφnsted/Lewis acid site ratio in a catalyst and γ-Al2O3 representing lower Brφnsted/Lewis acid site ratio. The experiments were conducted in a 1.07m×0.047m2 packed column in full cycle operation. The packing structure consisted of a 0.51m bed height of 6mm inert marble randomly mixed with a specific mass of catalyst and a 50.8mm Sulzer LDX structured packing acting as top and bottom support beds. The effects of amine flow rate and catalyst bed temperature on desorption performance were also analyzed. The catalyst physical activity in terms of the added interfacial area showed higher count for HZSM-5 as compared to γ-Al2O3. The mass transfer coefficient increased by an average of 16% and 22% using γ-Al2O3 and HZSM-5 catalysts, respectively at a CO2 desorption temperature of 85°C. These results are consistent with the greater ability of HZSM-5 to act as a proton donor to facilitate MEACOO- breakdown throughout the loading range of desorption as compared with γ-Al2O3, an amphoteric oxide, which plays the role of HCO3- to release CO2 with lower energy penalties at leaner CO2 loadings. The high percentage increase implies a corresponding reduction in the size of the desorber column which further translates to lower capital costs.

Mass Transfer and Liquid‐Film Formation in a Spray Tower for SO2 Removal in Sodium Hydroxide Solution

AbstractSpray towers allow for controlling air pollution in which a liquid is sprayed in small droplets to produce a large interfacial area for mass transfer between a gas and a liquid phase. An experimental study of a spray tower for removing SO2 is described. The experiments were carried out under different operating conditions by varying the gas velocity, liquid flow rate, and SO2 concentration. SO2 removal efficiency, volumetric mass transfer coefficient, and liquid‐film formation as a result of the collision of droplets against the tower wall are investigated. Removal efficiency and volumetric mass transfer coefficient are analyzed as a function of gas velocity, liquid flow rate, and SO2 concentration, while liquid‐film formation is evaluated as a function of tower height. The results indicate high removal efficiency. Correlations to predict the volumetric mass transfer coefficient are also proposed.

Mass transfer and hydrodynamic studies in a 50 mm diameter centrifugal extractor

Centrifugal extractor is a preferred device for process intensification in solvent extraction. The complexities of high radiation damage to solvent and resultant degradation of solvent performance also necessitate the need for fast contactors like centrifugal extractors in the nuclear fuel recycle operations. In this study, mass transfer performance as well as hydrodynamic characterization was reported for a 50mm diameter annular centrifugal extractor. Mass transfer studies included extraction and stripping experiments with solvent-aqueous pair of 30% Tri-n-butyl phosphate/n-dodecane and aqueous nitric acid solutions for different operating conditions like rotor speed, combined throughput and phase flow ratios. Flooding zone was characterized. Dispersed-phase holdups in annular as well as rotor regions and interfacial area for mass transfer were measured at different operating conditions. Non-ideal effects were studied by conducting residence time distribution analysis using aqueous phase as well as organic phase pulse tracer experiments. Various models like dispersion and tanks-in-series models were applied to experimental data. The contactor in study, was approximated as N number of tanks in series with a finite dead volume.

Correlation between DNAPL distribution area and dissolved concentration in surfactant enhanced aquifer remediation effluent: A two-dimensional flow cell study.

During the process of surfactant enhanced aquifer remediation (SEAR), free phase dense non-aqueous phase liquid (DNAPL) may be mobilized and spread. The understanding of the impact of DNAPL spreading on the SEAR remediation is not sufficient with its positive effect infrequently mentioned. To evaluate the correlation between DNAPL spreading and remediation efficiency, a two-dimensional sandbox apparatus was used to simulate the migration and dissolution process of 1,2-DCA (1,2-dichloroethane) DNAPL in SEAR. Distribution area of DNAPL in the sandbox was determined by digital image analysis and correlated with effluent DNAPL concentration. The results showed that the effluent DNAPL concentration has significant positive linear correlation with the DNAPL distribution area, indicating the mobilization of DNAPL could improve remediation efficiency by enlarging total NAPL-water interfacial area for mass transfer. Meanwhile, the vertical migration of 1,2-DCA was limited within the boundary of aquifer in all experiments, implying that by manipulating injection parameters in SEAR, optimal remediation efficiency can be reached while the risk of DNAPL vertical migration is minimized. This study provides a convenient visible and quantitative method for the optimization of parameters for SEAR project, and an approach of rapid predicting the extent of DNAPL contaminant distribution based on the dissolved DNAPL concentration in the extraction well.

Airlift bioreactor for biological applications with microbubble mediated transport processes

Airlift bioreactors can provide an attractive alternative to stirred tanks, particularly for bioprocesses with gaseous reactants or products. Frequently, however, they are susceptible to being limited by gas–liquid mass transfer and by poor mixing of the liquid phase, particularly when they are operating at high cell densities. In this work we use CFD modelling to show that microbubbles generated by fluidic oscillation can provide an effective, low energy means of achieving high interfacial area for mass transfer and improved liquid circulation for mixing.The results show that when the diameter of the microbubbles exceeded 200µm, the “downcomer” region, which is equivalent to about 60% of overall volume of the reactor, is free from gas bubbles. The results also demonstrate that the use of microbubbles not only increases surface area to volume ratio, but also increases mixing efficiency through increasing the liquid velocity circulation around the draft tube. In addition, the depth of downward penetration of the microbubbles into the downcomer increases with decreasing bubbles size due to a greater downward drag force compared to the buoyancy force. The simulated results indicate that the volume of dead zone increases as the height of diffuser location is increased. We therefore hypothesise that poor gas bubble distribution due to the improper location of the diffuser may have a markedly deleterious effect on the performance of the bioreactor used in this work.

Impact of Surfactant Addition on Oxygen Mass Transfer in a Bubble Column

AbstractThe impact of sparger design and surfactant addition on the oxygen transfer rate in a bubble column was examined. Additionally, measurements were also made of the holdup and bubble size distribution, allowing both the interfacial area for mass transfer and the liquid film mass transfer coefficient to be determined for a range of industrially relevant superficial velocities. It was found that for the velocity range examined changes in the superficial velocity had a minimal impact on the observed value of liquid film mass transfer coefficient. In contrast, addition of both hydrophilic and hydrophobic surface‐active compounds led to an approximately threefold reduction in liquid film mass transfer coefficient.

Mass transfer studies in shallow bubble column reactors

Mass transfer studies are carried out in a bubble column with an internal diameter of 14cm and various static liquid heights. The mass transfer coefficient is evaluated by using an oxygen sorption method. A model considering the gas holdup flushing and the sensor response is used. The interfacial mass transfer area is determined according to the measured bubble size distribution. The liquid-side mass transfer coefficient is also estimated from the volumetric mass transfer coefficient and the interfacial mass transfer area found. Results show that the effect of static liquid height on gas–liquid mass transfer is primarily on the interfacial mass transfer area. The mass transfer process is also governed by the type of gas distributor used. A single nozzle distributor is not suitable for shallow bubble column operations due to the large initial bubbles and the large volume of dead zone generated. It is also found that the different dependence of the liquid-side mass transfer coefficient on the superficial gas velocity observed in the literatures is due to the different bubble rising regimes.

New cost-effective method for conversion of vegetable oil to biodiesel

A low cost method to convert triglycerides to biodiesel by transesterification reaction has been investigated. The new approach is based on controlled feeding of oil into a pool of alcohol allowing easy dispersion of oil creating large interfacial area for mass transfer. It also enabled equilibrium constants to shift in the forward direction due to excess of alcohol in the dispersion. As a result product yield higher than 95% could be achieved at lower energy input compared to conventional method. Comparisons made with conventional batch and co-solvent enabled method show differences and highlight improvements. Reactions were conducted in one liter reactor at various residence time, temperature and molar ratio of canola oil with methanol in presence of potassium hydroxide as the catalyst. Samples were collected at regular intervals and analyzed by gas chromatography (GC) to determine the reaction progress.

Mass Transfer Performance of CO2 Absorption into Aqueous Solutions of 4-Diethylamino-2-butanol, Monoethanolamine, and N-Methyldiethanolamine

The mass transfer performance of the absorption of CO2 in an aqueous solution of monoethanolamine was evaluated experimentally in a lab-scale absorber packed with high efficiency DX structured packing and compared with that of methyldiethanolamine (MDEA) as well as that of a newly developed tertiary amino alcohol, 4-diethylamino-2-butanol (DEAB). The absorption experiments were conducted at atmospheric pressure, using a feed gas mixture containing 14.9% CO2 and 85.1% nitrogen in an absorption column containing DX structured packing. The absorption performance was presented in terms of the CO2 removal efficiency, absorber height requirement, effective interfacial area for mass transfer, and overall mass-transfer coefficient (KGav). In particular, the effects of parameters such as inert gas flow rate and liquid flow rate were compared for both DEAB and MDEA. The results show that the DEAB has a much higher removal efficiency for CO2 along the height of the column than MDEA. Also, the KGav of DEAB was much higher than that for MDEA. For all the solvents, the KGav increased as the liquid flow rate was increased. An empirical correlation for the mass transfer coefficient for the CO2-DEAB system has been developed as a function of the process parameters. In terms of comparison, the results show that the DEAB system provided an excellent overall mass transfer coefficient, which is higher than that of the MDEA system but less than that of MEA.

Experimental and Modelling Studies of Mass Transfer in Centrifugal Contactors

Experimental trials have been undertaken to determine interfacial area for mass transfer in 5cm and 25cm diameter annular centrifugal contactors. All trials used 30% TBP in Exxsol D80 diluent, with extraction of HNO3 being studied at varying flowrates and volumetric flow ratios. Droplet size was determined using a laser reflectance probe.Results from the 5cm rig showed two different regions of operation and the value of the dimensionless constant (K) in the Kolmogoroff expression was estimated to be 0.28 for O/A ratios below 2.5 and 0.30 above 2.5. From the 25cm rig trials K was estimated to be 0.26.