Mass Transfer Limited Process Research Articles

Numerical and Experimental Investigation of Pressure Drop in Periodic Open Cellular Structures for Intensification of Catalytic Processes

Periodic Open Cellular Structures are envisioned as potential enhanced catalyst substrates for heat and mass transfer-limited processes. To enable their rational design, in this study, we propose a combined numerical and experimental approach to assess the pressure drop in the tetrakaidekahedral and diamond lattices since generalized correlations are not present in the literature. Deviations are observed between the model predictions available in the literature, which are possibly due to the different methods of investigation, that is, numerical on ideal geometries and experimental on reproductions. To reconcile the two approaches and prevent discrepancies, careful attention is paid here to the quality of 3D-printed replicas for experimental investigation to obtain results representative of ideal lattices. Computational Fluid Dynamics simulations and experiments are then employed together to cross-validate the results and then to perform a parametric analysis of the effect of the morphological properties on the pressure drop. The effect of the cell size and the porosity are discussed, enabling the derivation of engineering correlations for the prediction of the pressure drop across the lattices within the range of 1 < Reds < 300. Finally, the performances of lattice materials are compared with those of conventional structured supports by evaluating the trade-off between the fluid–solid mass transfer rate and the pressure drop, which is crucial for several catalytic processes. Results show that the diamond lattice outperforms other cellular materials and can outperform ceramic honeycomb monoliths at low Reynolds numbers.

Model-guided design of high-performance membrane distillation modules for water desalination

The significance of geometrical and physical parameters of hollow fiber membrane modules in the membrane distillation process has not been fully evaluated. In this study, we develop a three-dimensional multi-physics model of a hollow fiber membrane module in order to investigate the effect of operating and design parameters on the module performance. The permeate flux and thermal efficiency of the system are considered as the characteristic parameters of the module, operated in direct contact membrane distillation mode (DCMD). The simulation results indicate that the permeate flux for the module can be enhanced by 54% when 1) the hollow fibers are in close-packed configuration, and 2) the interspacing parameter, the ratio of a fiber radius to the center-to-center distance between neighboring fibers, is adjusted properly. We identify the fiber interspacing parameter as a critical parameter for the module design. The permeate flux significantly drops when the interspacing parameter is equal to a value of 0.5, implying that the fibers are adjacent to each other. Moreover, the results indicate that, in our system, the time constant for the mass transfer process through the membrane is higher than that of heat transfer, meaning that the DCMD process for a hollow fiber membrane module under parallel flow condition is a mass transfer limited process.

A multifluid-PBE model for simulation of mass transfer limited processes operated in bubble columns

Modeling of reactive dispersed flows with interfacial mass transfer limitations require an accurate description of the interfacial area, mass transfer coefficient and the driving force. The driving force is given by the difference in species composition between the continuous and dispersed phases and thus depends on bubble size. This paper shows the extension of the multifluid-PBE model to reactive and non-isothermal flows with novel transport equations for species mass and temperature which are continuous functions of bubble size. The model is demonstrated by simulating the Fischer-Tropsch synthesis operated in a slurry bubble column at industrial conditions. The simulation results show different composition and velocity for the smallest and largest bubbles. The temperature profile was independent of bubble size due to efficient heat exchange. The proposed model is particularly useful in investigating the effects of bubble size on strongly mass transfer limited processes operated in the heterogeneous flow regime.

A study on the absorption of ammonia into water in a rotor‐stator reactor

This article presents experimental and modelling studies on the absorption of NH3 into water in a rotor‐stator reactor (RSR). The influences of operating parameters such as the rotation speed of rotor, liquid flow rate, and gas flow rate on the overall volumetric mass transfer coefficient (Kya) of NH3 were investigated. It was found that Kya increased with an increasing rotation speed, liquid flow rate, and gas flow rate. A correlation to predict the Kya of NH3 in the RSR was established and found to be in agreement with the experimental data with deviations within 10 %. A comparison was made between the Kya of NH3 in the RSR and RPB, and the result showed that Kya of NH3 in the RSR was 13 % higher than that in the RPB, thus signifying that the RSR has better intensification effect for mass transfer limited processes than the RPB.

Assessing dissolution kinetics of powders by a single particle approach

A novel methodology for studying the dissolution of powders based on a “single particle” approach is presented. A single particle is the basic unit composing the bulk of a powder, and vast information could be gained from its dissolution kinetics. The dissolution of a single particle was measured by means of a microscopy-based experimental method with custom-developed image analysis algorithms. The effects of various liquids and their physical properties on the dissolution kinetics were studied. A mathematical model based on a shrinking sphere was utilized to describe the dissolution process. The rate constant was derived from the experimental data for each tested condition and correlated with the viscosity of the dissolving medium. A significant effect, mostly at low viscosity values was found. The derived values are in accordance with the diffusive behavior as predicted from the Einstein–Stokes equation. Isothermal calorimetry was used to measure the enthalpy of dissolution, and correlated with the rate constant derived from the single particle dissolution measurements. Faster dissolution rate corresponded with the lowest endothermic dissolution enthalpy. Consequently, it is proposed that the dissolution of particles is not simply a mass transfer limited process and it also includes heat transfer related mechanism(s) that should be further studied. © 2007 Elsevier B.V. All rights reserved.

Theoretical and Experimental Study of the Absorption rate of H 2S in CuSO 4 Solutions : The Effect of Enhancement of Mass Transfer by a Precipitation Reaction

In this paper the desulphurization of gas streams using aqueous copper sulphate (CuSO 4) solutions as washing liquor is studied theoretically and experimentally. The desulphurization is accomplished by a precipitation reaction that occurs when sulphide ions and metal ions are brought into contact with each other. Absorption experiments of H 2S in aqueous CuSO 4 solutions were carried out in a Mechanically Agitated Gas Liquid Reactor. The experiments were conducted at a temperature of 293 K and CuSO 4 concentrations between 0.01 and 0.1 M. These experiments showed that the process efficiently removes H 2S. Furthermore, the experiments indicate that the absorption of H 2S in a CuSO 4 solution may typically be considered a mass transfer limited process at, for this type of industrial process, relevant conditions. The extended model developed by Al-Tarazi et al. (2004) has been used to predict the rate of H 2S absorption. This model describes the absorption and accompanying precipitation process in terms of, among others, elementary reaction steps, particle nucleation and growth. The results from this extended model and results obtained with a much simpler model, regarding the absorption of H 2S in CuSO 4 containing aqueous solutions as absorption of a gas accompanied by an instantaneous irreversible reaction were compared with experimental results. From this comparison it appeared that the absorption rate of H 2S in a CuSO 4 solution can, under certain conditions, be considered as a mass transfer rate controlled process. Under a much wider range of conditions the error that is made by assuming that the absorption process is a mass transfer controlled process, is still within engineering accuracy. Application of the simple model allows for a considerable reduction of the theoretical effort needed for the design of a gas–liquid contacting device, thereby still assuring that the desired gas specification can be met under a wide range of operating conditions. A comparison of the experimental results and the simulated results showed that the extended model gives an under prediction of the H 2S absorption rate for the experimental conditions applied.

Bioconversion of synthesis gas to hydrogen using a light-dependent photosynthetic bacterium, Rhodospirillum rubrum

Biological hydrogen production from synthesis gas was carried out in batch culture. The phototrophic anaerobic bacterium, Rhodospirillum rubrum was used to oxidize CO and water to CO2 and hydrogen. The bacteria were grown under anaerobic conditions in liquid medium; also acetate was used as carbon source in presence of synthesis gas. Biological hydrogen production was catalysed by R. rubrum via the water–gas shift reaction. A light-dependent cell growth modelled with a desired rate of hydrogen production and CO uptake was determined. The effect of light intensity on microbial cell growth was also studied at 500, 1,000 and 1,500 m.cd. A complete conversion of CO to hydrogen and maximum light efficiency were obtained with an acetate concentration of 1 g/l and light intensity of 500 m.cd. Utilization of the carbon monoxide from the gas phase was often considered as a mass transfer limited process, which needed to diffuse through the gas–liquid interface and then further diffuse into liquid medium prior to reaction. The results from this study showed that maximum cell propagation and hydrogen production were achieved with a limited light intensity of 1,000 m.cd. It was also found that high-light intensity may interfere with cell metabolism. In low-light intensity and substrate concentration, no inhibition was observed, however at extreme conditions, non-competitive inhibition was identified. The adverse effect of high-light intensity was shown at 5,000 m.cd, where the CO conversion drastically dropped to as low as 21%. Maximum CO conversion of 98% and maximum yield of 86% with an acetate concentration of 1.5 g/l and a light intensity of 1,000 m.cd were achieved.

Optimisation of the Cathode Composition for the Intermediate Temperature SOFC

Electrochemical characteristics of the half-cells Ce 0.8 Gd 0.2 O 1.9 La 0.6 Sr 0.4 CoO 3-δ (Sys 1), Ce 0.8 Gd 0.2 O 1.9 | Pr 0.6 Sr 0.4 CoO 3-δ (Sys 2) and Ce 0.8 Gd 0.2 O 1.9 | Gd 0.6 Sr 0.4 CoO 3-δ (Sys 3) have been studied by electrochemical impedance, cyclic voltammetry and chronoamperometry methods at various electrode potentials ΔE and temperatures T. The analysis of Z,Z' -plots shows that at lower temperatures, the kinetically mixed process is probable, characterised by the slow electron transfer to an adsorbed and thereafter dissociated oxygen atom O ads as well as by slow mass transfer (i.e. diffusion-like process) of electroactive species inside the cathode or O ads at the internal surface of the porous cathode. The total polarisation resistance increases in the order Sys 1 Sys2> Sys 1. The transfer coefficient for the total oxygen reduction reaction a c > 0.5 dependent on the half-cell studied has been obtained from the Tafel-like evervoltage versus current density plots, indicating the deviation of the mainly charge transfer limited process toward the mass transfer limited process (Sys 2 and Sys 3) in the porous cathode with decreasing temperature. The electrochemical behaviour of half-cells Sys 2 and Sys 1 has been tested during long operation times, t ≤ 1200 hours.

The removal of hydrogen sulfide from gas streams using an aqueous metal sulfate absorbent: Part I. The absorption of hydrogen sulfide in metal sulfate solutions

The desulfurization of gas streams using aqueous iron(II)sulfate (Fe(II)SO 4), zinc sulfate (ZnSO 4) and copper sulfate (CuSO 4) solutions as washing liquor is studied theoretically and experimentally. The desulfurization is accomplished by a precipitation reaction that occurs when sulfide ions and metal ions are brought into contact with each other. A thermodynamic study has been used to determine a theoretical operating window, with respect to the pH of the scrubbing solution, in which the metal sulfate solution can react with hydrogen sulfide (H 2S), but not with carbon dioxide (CO 2) from the gas or hydroxide ions from the scrubbing solution. When the absorption is carried out in this window the proposed process should be capable of removing H 2S from the gas stream without uptake of CO 2 or the formation of metal hydroxides. The pH operating window increases in the order of iron, zinc to copper. Experimental verification showed that the proposed process indeed efficiently removes H 2S when an aqueous Fe(II)SO 4, ZnSO 4 or CuSO 4 solution is used as absorbent. However, for an efficient desulfurization the lower pH of the experimental pH operating window using the Fe(II)SO 4 or ZnSO 4 solution was higher than indicated by thermodynamics. The reason for this must probably be attributed to a reduced precipitation rate at decreasing pH. When a CuSO 4 solution is used as washing liquor the solution can efficiently remove H 2S over the entire pH range studied (as low as pH = 1.4). In this case only the upper pH boundary of the operating window (that indicates the possible formation of copper hydroxide or copper carbonates) seems to be a relevant limit in practice. The laboratory experiments indicate that the absorption of H 2S in a CuSO 4 solution, at the experimental conditions tested, is a gas phase mass transfer limited process. This allows a high degree of H 2S removal in a relatively compact contactor. In addition to the lab scale experiments the potential of the new desulfurization process has also been successfully demonstrated for an industrial biogas using a pilot scale packed bed reactor operated with a fresh and regenerated CuSO 4 solution. This study indicates that the precipitation reaction of metal sulfates with H 2S can be used successfully in a (selective) desulfurization process, and that it can be an attractive alternative to the desulfurization methods currently used.

Electrochemical Impedance Characteristics of Some Medium Temperature Semicells for SOFC

Electrochemical characteristics of the semicells Ce 0.83 Gd 0.17 O 1.9 La 0.6 Sr 0.4 CoO 3-δ ; Ce 0.8 Sm 0.2 O 1.9 | La 0.6 Sr 0.4 CoO 3-δ and Ce 0.83 Gd 0.17 O 1.9 La 0.6 Sr 0.4 CoO 3-δ -Ag clusters have been studied by electrochemical impedance and chronoamperometric methods at various electrode polarisations ΔE and temperatures T. The polarisation resistance values, depending on ΔE and T, have been established from the electrochemical impedance data. The activation energy has been found to decrease slightly with the increase of |-ΔE|. The cathode reaction charge transfer coefficient α c ≃ 1.0 has been obtained from the Tafel-like overvoltage η c , logi-plots at T>823K, indicating the mass transfer limited process in the cathode material. The Z,Z'-plots have a complicated shape, and at least two time-constant values (two limiting stages) can be obtained. Thus, the kinetically mixed process, characterised by the slow transfer of electron to O ads and the diffusion-like step of O ads in the cathode material, is possible.

Evolution of hydraulic conductivity by precipitation and dissolution in carbonate rock

The evolution of hydraulic conductivity and flow patterns, controlled by simultaneous precipitation and dissolution in porous rocks, was examined in a series of laboratory experiments. Linear flow experiments were performed in columns of crushed calcareous sandstone by injecting different concentrations of HCl/H2SO4 mixtures at various flow rates. The effect of simultaneous calcium carbonate dissolution and gypsum precipitation was analyzed. Changes in head gradient, recorded at specific time intervals during the experiments, were used to calculate overall hydraulic conductivity of each column. The effluent acid was analyzed for Ca2+ and SO42− concentrations in order to calculate porosity changes during the experiments. After each experiment, the rock sample was retrieved and sectioned in order to study the pore space geometry, micromorphology, and mineral concentrations. A range of injected H+/SO42− ratios and flow rates was identified which leads to oscillations in the effective hydraulic conductivity of the evolving carbonate rock samples. Because the dissolution of calcium carbonate is a mass transfer limited process, higher flow rates cause a more rapid dissolution of the porous medium; in such cases, with dissolution dominating, highly conductive flow wormholes were observed to develop. At slower flow rates, no wormhole formation was observed, but the porosity varied in different parts of the columns. Analysis of the sectioned parts of the column, after each experiment, showed that total porosity increased significantly by dissolution of carbonate mineral near the inlet of the column and decreased along the interior length of the column by gypsum precipitation. These findings are in qualitative accordance with conceptual understanding of such phenomena.

Comparative TEM of Freeze-fractured Colloidal Liquid Aphrons and Conventional Emulsions

Abstract Colloidal Liquid Aphrons (CLAs) are composed of surfactant doped non-aqueous inner cores surrounded by thin surfactant films. These are created by intense stirring of the non-aqueous phase in an aqueous surfactant solution. Large interfacial areas, due to small size, and high stability (much greater than conventional emulsions) make aphron dispersions attractive for a variety of mass transfer limited processes. The unusual stability of CLA dispersions has been attributed to the multi-layered structure that includes an entrapped water shell around the non-aqueous core as postulated by Sebba. However, no direct evidence for the structure of the surfactant-stabilized interface surrounding the CLAs has been previously published. CLAs were prepared as previously reported. Comparative conventional emulsions were made similarly, with the lipid phase free of the Tergitol®(15-S-3) surfactant used in the CLAs. Aliquots (≈30 μl) of the dispersions were spread on 0.15 mm thick, 10 mm diameter copper supports These supports were ultra-rapidly frozen via plunging in liquid propane(−190°C). The frozen samples were inserted onto a pre-cooled rotary stage of a freeze-etch unit (modified Balzers BA-510) and fractured by knife at 150°C with a residual pressure of approx. 6.5 x 10−5 Pa. The replicas were shadowed with ≈2nm of Pt at 30° followed by 10-15 nm of carbon by arc evaporation. The resulting replica was washed in an 10% graded isopropanol series, rehydrated, washed in chromic acid, rinsed in de-ionized H2O, and then mounted on 300 mesh grids. TEM was performed on a JEOL 100CX operated at 120keV.

FORCED CONVECTION MASS TRANSFER INSIDE LARGE HEMISPHERICAL CAVITIES UNDER LAMINAR FLOW CONDITIONS

Rates of mass transfer at hemispherical cavities machined in the wall of a vertical rectangular duct were measured by an electrochemical technique which involved measuring the limiting current of the cathodic reduction of K3Fe(CN)6 Variables studied were solution flow rate, physical properties of the solution and cavity diameter. The mass transfer coefficient inside the cavity was found to be less than the flat surface value (the duct wall), and decreased with an increase in the cavity diameter. Mass transfer data inside the cavity were correlated by the equation: where Sh and Sc and Re are Sherwood, Schmidt and Reynolds numbers, respectively; and de, and d are the duct equivalent diameter and cavity diameter, respectively. Practical implications of the present results in mass transfer limited processes such as diffusion controlled corrosion, electrochemical machining, electroplating and etching have been highlighted

Study of gaseous substrate fermentations: Carbon monoxide conversion to acetate. 1. Batch culture

Biological processes may be used to convert gas phase substrates, such as H(2)S, CH(4), CO, H(2), and CO(2), to useful products. Utilization of these substrates is often a mass transfer limited process, first requiring absorption across the gas-liquid interface and diffusion through the culture medium to the cell surface, prior to reaction. This article presents a method for determining fermentation parameters of a gaseous substrate in convenient batch vessels using a modified Monod model. The procedure is illustrated with experimental data for the conversion of carbon monoxide to acetate by the strict anaerobe Peptostreptococcus productus.

Transport of particulate suspensions in porous media: Model formulation

AbstractA mathematical model is formulated for the general class of problems that involve the transport of stable particulate suspensions in porous media. The porous medium is represented by a network of pore bodies (sites) and pore throats (bonds). Population balances for the species responsible for particle retention and permeability reduction are written in terms of the various mechanisms of particle capture and reentrainment. Rates of capture and release are evaluated using appropriate physical models. We specifically concentrate on mass transfer limited processes. The effective‐medium theory is suitably formulated to determine the fluid flow distribution in the network and to calculate the permeability. The network representation of the porous medium together with the population balances and the rates of deposition and release provide a consistent model that finds application in filtration and fines migration processes.