Importance Of Mass Transfer Research Articles

Machine-learning-based heat transfer and pressure drop model for internal flow condensation of binary mixtures

Machine learning regression models were developed to predict the heat transfer coefficient and frictional pressure gradient during condensation of three zeotropic mixtures (ethane/propane, R245fa/pentane, and R410A) in micro- and macro-channels (0.76 mm < D < 14.45 mm). 1032 data points were used to train, test, and validate four machine learning models: support vector regression (SVR), random forest regression (RFR), gradient boost (GB), and artificial neural networks (ANN). For data not used for training, SVR predicted the apparent Nusselt number the best, with a mean absolute percent error (MAPE) of 5.0%, while GB predicted the two-phase friction factor the best with a MAPE of 5.5%. To determine the impact of the additional heat and mass transfer resistances in zeotropic condensation, Shapley analysis was performed on the models to weigh the importance of all dimensionless parameters used to predict the heat transfer and pressure drop. Overall, the three most important parameters to predict heat transfer were the Reynolds number of both phases and a dimensionless temperature glide; while the most important parameters to predict pressure drop were the Bond number, Weber number, and vapor-phase Reynolds number. The importance of mass transfer is discussed along with the applicability of other simple mixture models.

A mass transfer study of the wet oxidation of cellulose

The wet oxidation (WO) process is currently receiving attention as a promising solution to increasing levels of waste production. Although this process has been investigated from different perspectives, the importance of mass transfer as one the main influencing parameters in the design and development of the process has largely been ignored. This study employed a classical Shrinking Core Model to study WO of cellulose (as a model compound). The relative contributions of mass transfer and reaction kinetics to the overall reaction rate were studied in experiments performed at different temperatures and mixing rates, ranging from 220 to 260 °C and 350–700 rpm respectively. The dimensionless Hatta number was used as the criterion for determining the contribution of reaction kinetics and mass transfer to the overall reaction rate. Contrary to the assumption that mass transfer is insignificant, mass transfer was found to contribute up to 37% of the overall rate of the WO process, with higher contribution seen at higher temperatures and lower mixing rates. The Hatta number of >0.02 at higher temperatures (≥250 °C) confirmed this finding, indicating an undeniable role for mass transfer. The model presented in this paper provides an opportunity for predicting limiting steps of the WO process and can be applied in the design and optimisation of the process.

The Effect of Flow Field Property in Toluene Direct Electro-Hydrogenation Electrolyzer

Introduction Recently, the utilization of renewable energy has been expected to decrease the carbon dioxide emission. For effective utilization of the renewable energy that fluctuates and uneven distributes, the large-scale storage and transportation of hydrogen as secondary energy is important technology. Toluene (TL)/ methylcyclohexane (MCH) organic hydride system is the candidates of energy carrier, and have the advantages of efficiency, safety and handling. We have studied the high-efficient electrolytic direct-hydrogenation of toluene with water decomposition for the organic hydride system as an efficient energy carrier process.1 In these studies, the importance of mass transfer in porous carbon flow field for TL/MCH has been clarified. In this study, we have investigated the effect for electrochemical property on the thickness of the porous carbon flow field to improve mass transfer for the cathode reaction in the toluene direct electro-hydrogenation electrolyzer. Experimental Figure 1 shows the schematic drawing of toluene direct electro hydrogenation electrolyzer. A DSE® electrode (De Nora Permelec ltd) for the oxygen evolution reaction, a carbon paper (29BC, thickness=235 μm or 10BC, thickness=385 μm, SGL carbon ltd.) applied 0.5 mg-PtRu cm-2 of PtRu/C (TEC61E54, TKK) catalyst with ionomer, and Nafion 117® (Du Pont) were used for the anode, the cathode, and the proton exchange membrane (PEM), respectively. The carbon paper as the flow field was loaded 0.02 mg-Pt cm-2 of Pt particles by the impregnation method for the chemical-hydrogenation catalyst.2 The cathode was hot-pressed on the PEM at 120oC and 0.1 MPa for 3 min to fabricate a membrane cathode assembly. Geometrical electrode area was 11.6 cm-2. Operation temperature of the electrolyzer conducted at 60oC. The anode and cathode compartments were circulated 1 mol dm-3 H2SO4 and 1, 5 and 10% toluene diluted by methylcyclohexane, respectively. The tip of the Luggin capillary of a reversible hydrogen electrode (RHE) was placed near the anode. The electrochemical measurements were conducted for the linear sweep voltammetry (LSV), the electrochemical impedance spectroscopy (EIS), the chronoamperometry (CA) and the current efficiency measurement. Results and discussion Figure 2 shows the cell voltage and the internal resistance of R­1 (see inset of Fig. 2) as a function of the current density curves for two types thickness of flow field with 1, 5 and 10% TL feed at 60oC. Internal resistance of R­1 was obtained from the high frequency intercept of the Nyquist plot of the EIS mesurement. The cell voltage and R­1 of 10BC flow field was lower than that of 29BC at all TL concentration. Especially, the cell voltage of 10BC exhibited high performance about 1.83 V at 400 mA cm-2. Figure 3 shows the iR corrected anode and cathode polarization curves of with 1, 5 and 10% TL feed at 60 oC. The anode polarization curves were not affected for the change of the TL concentration and the type of flow field. On the other hand, the cathode overpotential of 10BC was lower than that of 29BC at all TL concentration. This difference was remarkable at high current density region, and it would be affected by the improvement of TL mass transfer in the flow field. Figure 4 shows the current efficiency with 1, 5 and 10% TL feed at 60oC. At 1% TL feed, obvious difference for current efficiency between 10BC and 29BC were not observed. In the initial stage of current efficiency drop at 5 and 10 % TL feed, the current efficiency of 29BC was higher than that of 10BC. However, in higher current region, 10BC showed higher current efficiency than 29BC. In this experiment, the flow rate for two types of flow field was same. At this condition, the thinner 29BC has higher linear velocity than 10BC, and is easy to supply with toluene to the catalyst layer. Hence, 29BC might exhibit the high current efficiency in lower current density region. Under 90-95% current efficiency, which the hydrogen generation increased, the thicker 10BC exhibited the higher current efficiency than the 29BC. It would be considered that the thinner 29BC is easy to occupy the hydrogen gas in the flow field, and the toluene supply to the catalyst layer was insufficient. Acknowledgements This work was partially supported by the Toyota Mobility Foundation and Core Research for Evolutional Science and Technology (CREST) “innovative reaction” (Funding agency: JST). Anode was supplied by De Nora Permelec Ltd. Reference Mitsushima, Y. Takakuwa, K. Nagasawa, Y. Sawaguchi, Y. Kohno, K. Matsuzawa, Z. Awaludin, A. Kato, and Y. Nishiki, Electrocatalysis, 7, 127 (2016). Nagasawa, Y. Sawaguchi, A. Kato, Y. Nishiki, and S. Mitsushima, Electrochemistry 86, 339 (2018). Figure 1

Liquid-liquid Slug Flow in a Microchannel Reactor and its Mass Transfer Properties - A Review

Mass transfer is a basic phenomenon behind many processes like reaction, absorption, extraction etc. Mass transfer plays a significant role in microfluidic systems where the chemical / biological process systems are shrinkened down to a micro scale. Micro reactor system, with its high compatibility and performance, gains a wide interest among the researchers in the recent years. Micro structured reac-tors holds advantages over the conventional types in chemical processes. The significance of micro re-actor not limited to its scalability but to energy efficiency, on-site / on-demand production, reliability, safety, highly controlled outputs, etc. Liquid-liquid two phase reaction in a microreactor system is highly demandable when both reactants are liquids or when air medium cannot be suitable. This arti-cle overviews various liquid-liquid flow regimes in a microchannel. Discussions on the hydrodynamics of flow in micro scale are made. Considering the importance of mass transfer in liquid-liquid systems and the advantage of slug regime over other regimes, the article focuses especially on the mass trans-fer between two liquid phases in slug flow and the details of experimental studies carried out in this area. The advantages of slug flow over other flow regimes in micro structured reactor applications are showcased. © 2014 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0)

Flow-Dependent Mass Transfer May Trigger Endothelial Signaling Cascades

It is well known that fluid mechanical forces directly impact endothelial signaling pathways. But while this general observation is clear, less apparent are the underlying mechanisms that initiate these critical signaling processes. This is because fluid mechanical forces can offer a direct mechanical input to possible mechanotransducers as well as alter critical mass transport characteristics (i.e., concentration gradients) of a host of chemical stimuli present in the blood stream. However, it has recently been accepted that mechanotransduction (direct mechanical force input), and not mass transfer, is the fundamental mechanism for many hemodynamic force-modulated endothelial signaling pathways and their downstream gene products. This conclusion has been largely based, indirectly, on accepted criteria that correlate signaling behavior and shear rate and shear stress, relative to changes in viscosity. However, in this work, we investigate the negative control for these criteria. Here we computationally and experimentally subject mass-transfer limited systems, independent of mechanotransduction, to the purported criteria. The results showed that the negative control (mass-transfer limited system) produced the same trends that have been used to identify mechanotransduction-dominant systems. Thus, the widely used viscosity-related shear stress and shear rate criteria are insufficient in determining mechanotransduction-dominant systems. Thus, research should continue to consider the importance of mass transfer in triggering signaling cascades.

Investigating the importance of mass transfer on the formation of HCCI engine emissions using a multi-zone model

The focus of the present study is to investigate the importance of mass transfer on the formation of the most important HCCI engine emissions, i.e. unburned HC and CO emissions. A multi-zone model is used for this purpose. The multi-zone model includes sub-models for the heat transfer between zones and to the cylinder wall and for the mass transfer between the hotter and colder regions of the combustion chamber. The combustion mechanism is modeled using a reduced set of chemical reactions coupled with a chemical kinetics solver. The results indicate that mass transfer during combustion and expansion plays a significant role on the formation of both the unburned HC and the CO emissions and therefore must be taken into account for the closed part of the engine cycle, i.e. compression, combustion and expansion. According to the model results, the formation of these emissions is located mainly at the crevice and the near-the-wall regions and is determined by the temperature field and mass transfer. It is shown that neglecting mass transfer in these regions during combustion and expansion would result to a significant deviation from the values predicted with the inclusion of mass transfer.

Opposite effects of gas flow rate on the rate of formation of carbon during the pyrolysis of ethane and acetylene on a nickel mesh catalyst

The importance of mass transfer in the formation of carbon on a nickel catalyst during the pyrolysis of ethane and acetylene was investigated. This included the formation of a composite of carbon fibre and amorphous carbon, using a non-porous nickel mesh catalyst in a tubular reactor at 750 °C. Increases in the gas flow rate through the Ni mesh decreased the formation of carbon during the pyrolysis of ethane. On the contrary, a similar increase in gas flow rate increased the formation of carbon during the pyrolysis of acetylene. Characterisation of the carbon products by SEM, TEM, TGA and FT-Raman spectroscopy also showed the opposite effects of gas flow rate on the type and structure of carbon formed on the nickel catalyst. These results can be explained by considering the relative ease with which various C-containing radicals and H radicals desorb from the nickel surface as the mass transfer resistance for radicals is decreased with increasing gas flow rate.

Scale‐up of enzymatic production of lactobionic acid using the rotary jet head system

Enzymatic oxidation of lactose to lactobionic acid (LBA) by a carbohydrate oxidase from Microdochium nivale was studied in a pilot-scale batch reactor of 600 L working volume using a rotary jet head (RJH) for mixing and mass transfer (Nordkvist et al., 2003, Chem Eng Sci 58:3877-3890). Both lactose and whey permeate were used as substrate, air was used as oxygen source, and catalase was added to eliminate the byproduct hydrogen peroxide. More than 98% conversion to LBA was achieved. Neither enzyme deactivation nor enzyme inhibition was observed under the experimental conditions. The dissolved oxygen tension (DOT) was constant throughout the tank for a given set of operating conditions, indicating that liquid mixing was sufficiently good to avoid oxygen gradients in the tank. However, at a given oxygen tension measured in the tank, the specific rate of reaction found in the RJH system was somewhat higher than previously obtained in a 1 L mechanically stirred tank reactor (Nordkvist et al., 2007, in this issue, pp. 694-707). This can be ascribed to a higher pressure in the recirculation loop which is part of the RJH system. Compared to mechanically stirred systems, high values of the volumetric mass transfer coefficient, k(L)a, were obtained when lactose was used as substrate, especially at low values of the specific power input and the superficial gas velocity. k(L)a was lower for experiments with whey permeate than with lactose due to addition of antifoam. The importance of mass transfer and of the saturation concentration of oxygen on the volumetric rate of reaction was demonstrated by simulations.

Effect of Internal Diffusion in Supported Ionic Liquid Catalysts: Interaction with Kinetics

Kinetics and mass transfer for supported ionic liquid catalysts (SILCAs) were studied for complex catalytic hydrogenations of unsaturated aldehydes. The system can be characterized as a gas−liquid−liquid−solid system, where gaseous hydrogen and an unsaturated aldehyde, citral, diffuse from molecular solvent to a thin layer of ionic liquid containing the catalytically active species, metallic palladium nanoparticles. The intrinsic kinetics was determined at 353−423 K (80−150 °C) and pressures ranging from 10 to 50 bar. A kinetic model was proposed that was able to predict the experimental concentration data successfully. In order to obtain more understanding about the importance of mass-transfer and diffusion limitations for these kinds of processes, a number of models describing the concentration profiles of the reactants in the catalysts were developed. The developed reaction-diffusion models revealed important insight into the potential limiting phenomena for this type of reaction systems.

The Reproducible Preparation of Alkanethiolate Monolayers on Gold: The Importance of Mass Transfer During Self-Assembly

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Removal of selected ozonation by-products in pilot scale drinking water biofilters: compound interactions and mass transfer considerations

Biofilters are finding increasing use in drinking water treatment. Among the substances they remove are ozonation by-products such as low molecular weight aldehydes and carboxylic acids. Because a number of these compounds also participate in or are produced by major bacterial metabolic pathways, their behaviour in biofilters may be complex. This pilot scale study investigated biodegradation relationships between methylglyoxal and pyruvate. A second objective was to determine the impact of mass transfer on removals. The formation of pyruvate was observed when methylglyoxal was the sole easily biodegradable compound fed in the filter influent. The removal rates of formaldehyde and methylglyoxal were found to be comparable when each was the sole easily biodegradable compound fed. When fed alone, complete removals of methylglyoxal were obtained under steady-state conditions. When fed alone, the removal rate of pyruvate was more than twice than that of either formaldehyde or methylglyoxal. The removal rate of pyruvate was reduced by more than 50% when it was fed together with methylglyoxal. The reduced rate was probably due both to competition and to the formation of pyruvate from methylglyoxal. Backwashing was shown to have essentially no effect on removals of the compounds investigated. The mass transfer investigations showed that neither external nor internal mass transfer was limiting. Therefore removals were governed by biodegradation rates. The lack of importance of mass transfer is significant because it simplifies modelling for the removal of easily biodegradable substances in drinking water biofilters.

Effect of Morphology of Sulfurized Materials in the Retention of Mercury from Gas Streams

The retention of mercury vapor by sulfur supported on sepiolite has been studied, and the utility of sepiolite as a dispersant for the active phase, sulfur, has been thoroughly ascertained. Samples with 10% S supported on sepiolite of varying size and shape have been prepared from powders sulfurized by reaction/deposit, and their efficiency in depurating air streams with 95 ppm mercury has been tested in a dynamic system using a fixed-bed glass reactor and fluid velocities ranging from 3.1 to 18.9 cm s-1. From breakthrough curves under various sets of conditions, the importance of mass transfer under the process conditions has been proven. The progress of the reaction is limited by the resistance to reactant diffusion inside the solid through the layer of product formed. Sulfur reaction to HgS is reduced to an external zone of the solid, giving rise to an egg-shell deposit whose extension is related to sulfur dispersion and porosity of the adsorbent. Then, conversion and capacity of the samples are related to their porosity and S/V ratio. The use of SEM helps to confirm those statements. The 10% S samples compare well with the more conventional S/activated carbon, with their use being advantageous for the low price and abundance of the substrate.

Optimization of tungsten-based catalysts for the hydrotreatment of coal-derived liquids

The preparation and testing of tungsten-based catalysts for the hydrotreating of coal-derived liquids has been carried out. Catalyst performance was markedly affected by coke deposition, particularly in the top portion of the bed, and means of minimizing deactivation had to be developed. The importance of mass transfer in the systems was confirmed by the finding that catalyst compositions in the outer third of a catalyst pellet have most effect on performance. Tungsten-based catalysts were found to perform better than a commercial nickel-molybdenum-based catalyst. Surprisingly, cobalt-tungsten-based catalysts containing only small amounts of cobalt performed at least as well as some nickel-tungsten-based catalysts. Further testing is required to explore the potential of this type of catalyst.

Modelling of growth rate uniformity of CdTe epilayer by MOCVD

A 2-dimensional mathematical model was established to simulate the growth rate uniformity of CdTe epilayers which were grown in a horizontal MOCVD reactor. Both transport phenomena and the surface decomposition kinetics of diethyl telluride were considered. The calculated effects of reactor pressure and substrate temperature on growth rates along the flow axis were compared with experimental results. They agreed well with each other. These effects were then explained by the relative importance of mass transfer to reaction kinetics. In general, higher growth temperature and higher reactor pressure both favor a mass transfer controlled CVD growth situation. However, to obtain uniformly grown epilayers, one would wish to operate under reaction kinetics controlled region,i.e. low substrate temperature and low reactor pressure.

An Analytical Model for One-Dimensional, Three-Component Condensing and Vaporizing Gas Drives

Abstract An analytical model based on the method of characteristics is presented for the calculation of onedimensional (1D), three-component condensing and vaporizing gas drives. The model describes (1) mass transfer between oil and gas, (2) swelling and shrinkage, (3) viscosity and density changes, (4) gravity stabilization, and (5) rock/fluid interaction. The main assumptions of the model are local thermodynamic equilibrium and the absence of dispersion, diffusion, and capillarity. Example calculations are presented that bring out the main features of both condensing and vaporizing gas drives and also indicate the importance of mass transfer between the phases. In the special case of "developed miscibility," the model predicts a piston-like displacement having a complete recovery at gas breakthrough. The main applications of the model are in (1) conceptual studies of gas drives in which mass transfer plays an important role and (2) the calibration and checking of numerical reservoir simulators for multicomponent, multiphase flow.

Importance of mass transfer and intraparticle diffusion in polymer-supported phase-transfer catalysis

Importance of mass transfer and intraparticle diffusion in polymer-supported phase-transfer catalysis