Values Of Mass Transfer Coefficient Research Articles

Influence of drying air parameters on mass transfer characteristics of apple slices

To efficiently design both new drying process and equipment and/or to improve the existing systems, accurate values of mass transfer characteristics are necessary. The present study aimed to investigate the influence of drying air parameters (i.e. temperature, velocity and relative humidity) on effective diffusivity and convective mass transfer coefficient of apple slices. The Dincer and Dost model was used to determine the mass transfer characteristics. The obtained Biot number indicated that the moisture transfer in the apple slices was controlled by both internal and external resistance. The effective diffusivity and mass transfer coefficient values obtained to be in the ranges of 7.13 × 10−11–7.66 × 10−10 and 1.46 × 10−7–3.39 × 10−7 m s−1, respectively and the both of them increased with increasing drying air temperature and velocity, and decreasing relative humidity. The validation of the model showed that the model predicted the experimental drying curves of the samples with a good accuracy.

Experiments and a three‐phase computational fluid dynamics (CFD) simulation coupled with population balance equations of a stirred tank bioreactor for high cell density cultivation

AbstractThis research describes a three dimensional (3‐D) computational fluid dynamics (CFD) simulation coupled with population balance equations (PBE) to study hydrodynamics and biomass production in a laboratory‐scale stirred‐tank bioreactor. The gas‐liquid‐solid flow was modelled using a Eulerian multiphase and turbulence (RNG) model. The energy dissipation rates, gas holdup, flow patterns, Sauter mean bubble diameter, and volumetric mass transfer coefficient were investigated for three different types of impeller using a multiple reference frame (MRF) model within the whole multiphase bioreactor. The effects of aeration rate and impeller speed on gas holdup and volumetric mass transfer coefficient were investigated owing to oxygen limitation in high cell density cultivation (HCDC). As high viscosity puts a limit on the efficiency of the bioreactor, the influences of viscosity on Sauter mean diameter, gas holdup, and volumetric mass transfer coefficient were also assessed. To determine growth kinetics as well as gas holdup, a set of experiments was performed. The numerical results of gas holdup and kLa were compared with the experimental data. Obtained results suggest that the Scaba impeller results in higher values of volumetric mass transfer coefficient, and subsequently higher biomass concentrations. One of the greatest problems in HCDC is feed accumulation in particular places. As depletion of substrate occurs near the impeller, the best spot for feeding purposes is in the vicinity of the impeller. Current research gives insight into the determination of the optimal operating conditions of HCDC in stirred bioreactors.

Process intensification characteristics of a microreactor absorber for enhanced CO2 capture

Gas separation processes, including post-combustion carbon capture (PCCC) by chemical absorption using liquid solvents can be substantially enhanced using high performance micro-structured surfaces to enhance the surface area available for reaction. The present paper studies the hydrodynamics and mass transfer performance of gas–liquid absorption of CO2 into aqueous diethanolamine in a micro-structured reactor. The system was designed to comprise 15 straight parallel channels in a cross flow inlet configuration. The hydraulic diameter of each channel was 456μm. The performance of the reactor was studied with respect to the absorption efficiency, mass transfer coefficient, acid gas loading ratio, and pressure drop. A flow pattern map was developed using available regime transition criteria. Parametric studies varying the gas and liquid flow rates, as well as their respective concentrations at the reactor inlet, were conducted. The two-phase pressure drop was compared against the predictions of a piecewise model and a reasonably good agreement was obtained. Absorption efficiencies close to 100% were observed under certain operating conditions. The presently achieved values of liquid-side volumetric mass transfer coefficients were between 1–3 orders of magnitude higher than those reported for most conventional gas–liquid absorption systems, which can be attributed to the inherent high specific interfacial area provided through micro-structured surfaces. The results reported here indicate the substantial levels of process intensification that can be achieved using microreactors.

Experimental performance evaluation of a combustion heat-assisted catalytic flat plate fuel reformer for fuel cell grade hydrogen

Combining exothermic catalytic combustion heat on one side of a very high heat convective thin flat plate and endothermic catalytic reforming reaction on the other side, catalytic flat plate provides excellent heat integration into a compact fuel reformer system. Based on a two-dimensional computational fluid dynamics (CFD) model simulation results, a real world compact catalytic flat plate fuel reformer was built and experimentally evaluated its performance, for generating fuel cell grade hydrogen, in an actual high temperature polymer electrolyte membrane (PEM) fuel cell system. The two-dimensional CFD model helps to optimize various design parameters necessary to build the actual compact reformer and eliminated the uncertainties of heat and mass transfer coefficient values arise in one-dimensional CFD model. The performance of the compact catalytic flat plate fuel reformer was evaluated using a 5-cell high temperature PEM fuel cell stack at 160 °C with reformate fed directly from the reformer to the anode stream of fuel cell stack. For performance comparison, a 5-cell high temperature PEM fuel cell stack was first evaluated with industry-standard pure hydrogen (0% CO) as well as with various percentage of CO mixed hydrogen (2% CO and 5% CO). Experimental results showed that if the average cell voltage drop within 0.03–0.05 mV of a 5-cell high temperature PEM fuel cell stack would be acceptable then the reformate can be used at 160 °C instead of pure hydrogen. The experimental results also indicated that the reformer will be able to provide actual fuel cell grade hydrogen that could be directly pumped from the compact catalytic flat plate fuel reformer into the high temperature PEM fuel cell stack.

Liquid–liquid extraction in microchannels and conventional stage-wise extractors: A comparative study

The objective of the study is to compare the performance of microchannels and conventional stage-wise extractors for liquid–liquid extraction by using a standard phase system. Three different microchannels – a T-junction microchannel, a serpentine microchannel and a split-and-recombine microchannel – have been used in the experiments. Conventional extractors are represented by a mixer-settler and an annular centrifugal extractor. The phase system used in the experiments is water (succinic acid) n-butanol system which is one of the standard phase systems recommended by the European Federation of Chemical Engineering. The extractors have been compared on the basis of percentage extraction, overall volumetric mass transfer coefficient and specific extraction rate. When compared on the basis of percentage extraction, performance of the microchannels and the conventional stage-wise extractors is found to be almost similar. Maximum values of overall volumetric mass transfer coefficient and specific extraction rates are found to be more in the microchannels than in the conventional stage-wise extractors. The ratio of maximum overall volumetric mass transfer coefficient in microchannels and conventional stage-wise extractors is found to range from 1 to 8.1. The ratio of maximum specific extraction rate in microchannels and conventional stage-wise extractors is found to range between 2.3 and 9.7.

Mass Transfer from Single Taylor Bubbles in Minichannels

AbstractMass transfer from single CO2 Taylor bubbles in vertical minichannels was measured for various channel hydraulic diameters Dh. The effects of channel geometries on the mass transfer were also investigated by using square ducts and circular pipes. Bubble rising velocities, vB, in the ducts were much faster than those in the pipes due to large liquid flow areas in the corners of the ducts. The values of mass transfer coefficients in the pipes were almost the same as those in the ducts, in spite of a large difference in vB. Sherwood numbers, ShD, using Dh as a characteristic length, are well‐correlated in terms of the Eötvös number. The proposed ShD correlation can well predict a long‐term dissolution process of a Taylor bubble.

English

Performance of chemical deacidification of crude palm oil (CPO) using aqueous NaOH solution in a polysulfone hollow fiber ultrafiltration membrane was investigated. The effects of operating temperature, NaOH concentration and flow rates on percentage of free fatty acids (FFA) removal, oil loss, soap entrainment and overall mass transfer coefficient were evaluated. Overall mass transfer coefficients, soap content in oil and neutral oil loss all increased when the temperature was increased from 60 to 70°C due to an increase of the FFA distribution value. A minimum 0.25 N of NaOH or a NaOH to FFA molar ratio of about 7.62 was required to facilitate the expected extraction efficiency. The increased oil flowrate slightly enhanced the solute transport kinetics, while the aqueous phase flowrate did not significantly influence deacidification efficiency or mass transfer coefficient. About 97% of FFA removal was achieved within 4 hours. The maximum oil loss observed was 11% and the highest soap content in the oil without separation step was 3150 ppm. The values of the overall mass transfer coefficient varied from 2.97×10 -7 to 7.71×10 -7 m/s. These results show the potential of using the non dispersive membrane contacting process for chemical deacidification of CPO as well as other vegetable oils.

Influence of hydrodynamic cavitation on biological objects

Cavitation, which yet has not found a wide application in industrial technologies, is an effective method of hydro-mechanical intensification of chemical-engineering, food, pharmaceutical, biological and other processes. The effectiveness of cavitation process is enabled by cavitation crashing of the solid phase, turbulent diffusion of boundary layer of the fluid by cavitation bubbles, emergence mass transfer aspects, which are characterized by high values of mass transfer coefficients. Processing of biological objects in a cavitation field can be used as an independent process and ensure an inactivation and extinction of microorganisms or as cyanobacteria pre-treatment stage with the aim of increasing the mass transfer surface for the following extraction or biochemical processes. The results of microscopic analysis showed that for certain modes of processing of liquid substances in the cavitation field the decrease of contrast cells is observed as well as damage to the cell membrane, reducing the clarity of contours, cell shape change, aggregation and mechanical damage. Experimental studies have established that in the case of hydrodynamic cavitation to increase the effectiveness of the process of obtaining energy from cyanobacterial biomass, treated in a hydrodynamic cavitation field cyanobacterial biomass, a degree of lipid extraction is increased by 54,3 % and the number of synthesized biogas increases by 28,3 %.

Modified Dincer and Dost Method for Predicting the Mass Transfer Coefficients in Solids

Abstract The analytical model developed by Dincer and Dost [3] for the estimation of diffusivities and mass transfer coefficients has been reviewed, and the equations used for the evaluation of the mass transfer coefficients have been corrected. The corrected equations have been used for the calculation of mass transfer coefficients from literature data. It was observed that the corrected diffusivities showed significant differences from the previously calculated values. The use of the modified equations in future drying calculations will lead to more realistic mass transfer coefficient values.

Study on the heat and mass transfer in AGMD module with latent heat recovery

A novel air gap membrane distillation (AGMD) module for desalination with latent heat recovery which consisted of parallel hollow fiber membranes and heat exchange hollow fibers was successfully developed. The temperature profiles of AGMD module designed with latent heat recovery along the radial direction and mass transfer coefficient across membrane (Cm), Knudsen diffusion coefficient across membrane (CKm), molecular diffusion coefficient across membrane (CMm), and molecular diffusion coefficient in air gap (CMa) were obtained by establishing a heat and mass transfer model. The permeate flux (JD) was highly sensitive to the hot feed inlet temperature, Thi. The maximum value of JD could reach 15.5 kg/m2 h for module 1 and 20.2 kg/m2 h for module 2 at Thi = 90°C and Tci = 30°C. The value of actual mass transfer coefficient across the membrane was in close to the value of Knudsen diffusion coefficient across the membrane, especially at high hot feed brine temperature. The membrane wall was the main resistance for mass transfer and contributed 60–70% to the total mass transfer resistance. The heat transfer across the heat exchange hollow fiber was the main heat transfer resistance and the heat transfer across the heat exchange hollow fibers was only around 3,238.1 W/m2°C. The temperature polarization coefficient was introduced to measure the actual driving force.

Effect of cation transport of SPEEK – Rutile TiO2 electrolyte on microbial fuel cell performance

Rutile TiO2 was synthesized using the sol gel method and used for the preparation of polymer composite electrolyte membrane of TiO2+SPEEK. The composite membranes prepared by solvent casting method showed improved oxygen mass transfer coefficient values and the transport of cations other than protons. The antifouling property of the composite membranes was observed in SEM images which were additionally supported by hemocytometer results. The improved ion exchange and water absorption capacity of composite membranes showed enhanced proton conductivity and electrochemical properties for the composite membranes. The prepared composite membranes were evaluated in single chamber microbial fuel cell and the results were compared with that of the Nafion 117® under similar conditions. The results revealed that all the prepared composite membranes showed higher maximum power density and current density than that of the commercially available Nafion117®. Among the prepared membranes, SPEEK+7.5% TiO2 composite membrane showed the highest power density (98.1mW/m2) and current density (300mA/m2) along with high proton conductivity, antifouling property.

Features of respiration and formation of steroidal glycosides in Dioscorea deltoidea cell suspension culture grown in flasks and bioreactors

An important task is to monitor the physiological state of the cell population when scaling apparatus cultivation of higher plants’ cell cultures to industrial volumes. Investigations were carried out on the culture of Dioscorea deltoidea Wall—strain producer of steroidal glycosides (protodioscin and deltoside)—for which growth characteristics and content of furostanol glycosides were determined during the semirunning growth in bubble bioreactors (20 and 630 L); total rate of oxygen uptake and the portion of participation of alternative oxidase (AO) in the process were measured in the exponential growth phase. The measurements were performed for 3–4 subculture cycles for each apparatus, starting from the second cycle from the moment of inoculation. The values of oxygen mass transfer coefficients (KLa, h−1) for bioreactors were determined at modes of mixing and aeration optimal for growing strain. The maximal portion of cyanide resistant respiration was determined in 20-liter bubble bioreactor with a point aerating device (up to 60–65% of total respiration rate). The minimal values of (KLa (2.0–2.4 h−1) were obtained for the same apparatus, as well as lower levels of furostanol glycosides (4.2–8.0% of dry cell weight). Apparently, this effect was due to insufficiently uniform distribution of oxygen in the nutrient medium. Minimal activity of AO (within 15–25%) was noted for the system with more intensive mass transfer of oxygen-in 630-liter bubble bioreactor with the annular aerator (KLa 7.1–8.0 h−1). Higher biosynthetic rates (7.7–13.9% of dry cell weight) were found in the same bioreactor.

A Kinetic Study for the Noncatalytic Esterification of Palm Fatty Acid Distillate

ABSTRACTIn this work, the reaction scheme for the esterification of palm fatty acid distillate performed under the noncatalytic and high‐temperature condition (230–290°C) was investigated with a rigorous mathematical modeling. The esterification reaction was assumed to be the pseudo–homogeneous second‐order reversible reaction, and the mass transfer effectiveness factor (η) was introduced in the modeling framework to systematically and collectively consider both evaporation and reaction, which are simultaneously and competitively occurred in the liquid phase. The nonlinear programming problem was constructed with the objective function consisting of the errors between experimental data and the estimated values from the reaction model. The problem was solved by using the Nelder–Mead simplex algorithm to identify kinetic parameters, reaction rate constants, and mass transfer coefficients. The values of mass transfer coefficients were found to follow the Hertz–Knudsen relation and expressed as a function of reaction temperature. From the reaction rate constants obtained from the proposed kinetic models, the apparent activation energy was estimated to be 43.98 kJ/mol, which is lower than the value obtained from the reaction using heterogeneous catalysts. This low value indicates that reactants and products behave as an acid catalyst at relatively high operating temperature and constant pressure.

Optimum solids concentration for solids suspension and solid–liquid mass transfer in agitated vessels

The effect of solids concentration on specific impeller power consumption and solid–liquid mass transfer coefficient was investigated in a 0.2 m diameter baffled agitated vessel with standard six-bladed Rushton turbine for solids concentration up to 0.40 (v/v). It was found that the increase of solids concentration significantly increases the mass transfer coefficient up to an optimum solids concentration and decreases thereafter when the system is operated at a just-suspended condition. The increase in mass transfer coefficient with an increase of solid concentrations is mainly due to the increase in Njs (critical impeller speed) with increasing solids concentration, thereby leading to an upsurge of turbulence around the particles. The solids concentration at which the highest mass transfer coefficient is obtained is designated as the effective solids concentration. In a geometrically similar 0.3 m diameter tank, the trends in impeller energy efficiency and solid–liquid mass transfer coefficient values with increasing solids concentration are similar. The experimental data are satisfactorily correlated using the concept of the Kolmogoroff's theory of isotropic turbulence to develop an equation to estimate the solid–liquid mass transfer coefficient in agitated vessels.

Dehydration characteristics and mathematical modelling of lemon slices drying undergoing oven treatment

In this study, the effect of drying temperature on drying behaviour and mass transfer parameters of lemon slices was investigated. The drying experiments were conducted in a laboratory air ventilated oven dryer at temperatures of 50, 60 and 75 °C. It was observed that the drying temperature affected the drying time and drying rate significantly. Drying rate curves revealed that the process at the temperature levels taken place in the falling rate period entirely. The usefulness of eight thin layer models to simulate the drying kinetics was evaluated and the Midilli and Kucuk model showed the best fit to experimental drying curves. The effective moisture diffusivity was determined on the basis of Fick’s second law and obtained to be 1.62 × 10−11, 3.25 × 10−11 and 8.11 × 10−11 m2 s−1 for the temperatures of 50, 60 and 75 °C, respectively. The activation energy and Arrhenius constant were calculated to be 60.08 kJ mol−1 and 0.08511 m2 s−1, respectively. The average value of convective mass transfer coefficient for the drying temperatures of 50, 60 and 75 °C was calculated to be 5.71 × 10−7, 1.62 × 10−6 and 2.53 × 10−6 m s−1, respectively.

Solute transport through porous media with scale-dependent dispersion and variable mass transfer coefficient

In this paper, we investigate the behavior of breakthrough curves in mixed heterogeneous soil column experiments. Advective dispersive transport equations are used for solute transport through mobile–immobile porous medium. A hybrid finite volume method is used to solve the governing equations for solute concentration in mobile region. In first part of the study, constant dispersion, linear, and asymptotic distance-dependent dispersion functions are used to describe the scale effect and to simulate experimental breakthrough curves observed in long soil column experiment. Also, a comparative study has been done among distance-dependent and constant dispersion models, while simulating the experimental data of solute transport through soil column with constant mass transfer coefficient. In second part of the study, variable mass transfer coefficient as function of pore velocity and travel distance is considered and an empirical relation is derived from observed data from experiments. It is shown that asymptotic dispersion model with variable mass transfer coefficient simulates the observed breakthrough curves better as compare to constant and linear distance-dependent dispersion models. Finally, present study also shows that the value of mass transfer coefficient is dependent on pore velocity and movement of solute through porous media.

Studies on Mass and Momentum Transfer at the Outer Wall of a Circular Conduit with Entry Region Spiral Tape Wound on Rod as Turbulence Promoter in Homogeneous Flow

In this study, an experimental investigation is carried out for augmentation of mass transfer coefficients in homogeneous flow by inserting spiral tape wound on rod in a circular conduit. Mass transfer coefficients are evaluated from the limiting current densities and the concentrations of the reacting ions. The variables studied are the geometric parameters of the promoter width, height, pitch of the promoter and the flow rate of the electrolyte, each parameter effect on mass transfer and momentum transfer are investigated. Experiment is conducted for 48 promoters with varying geometric parameters. Reynolds number range covered in the experiment is 4853–14533. Improvement in mass transfer is assessed by comparing the obtained mass transfer coefficient (kL) values with the smooth tube data. It is found that kL increased with increased flow rate. The increase in tape height and width of the promoter resulted in slightly increased mass transfer coefficients. Pitch of the promoter showed marginal effect on kL. Keywords: Turbulence promoter, spiral tape wound, limiting current densities, turbulence promoter, homogeneous flow

English

Published correlations for estimating the liquid phase mass transfer coefficients of structured packings are compared using experimental evidence on the efficiency of Montz-Pak B1–250MN and B1–500MN structured packings as measured in total reflux distillation tests using the chlorobenzene/ethylbenzene system at two operating pressures. Large differences are found between different correlations with respect to both the absolute values of mass transfer coefficients and the fraction of liquid phase based resistance and their trends with respect to increasing vapor and liquid loads. A new Delft Model liquid side mass transfer coefficient correlation that incorporates a more appropriate definition of the liquid film exposure length is presented which now generates lower values. The revised liquid film model, combined with an enhanced turbulent vapor phase mass transfer coefficient, leads to doubling the fractional liquid phase resistance with respect to that based on penetration theory assuming equal contact times. This effect results in predicting efficiencies which are slightly more conservative and agree reasonably well with experimental HETP data presented in this paper.

PH Control during the Struvite Precipitation Process of Wastewaters

The high concentration of phosphorus and nitrogen in wastewater and sludge could be lowered to a certain level by struvite (MgNH4PO4·6H2O) crystallization. One of the main factors for struvite formation is the solution pH. It can be adjusted by non-reagent carbon (CO2) dioxide stripping through the process of aeration. The intensity of the mass transfer between the air and the supernatant of dewatering sludge obtained from wastewater treatment plant is characterized by the volumetric liquid-side mass transfer coefficient, which can be estimated theoretically. It is found that the rate of pH increase depends strongly on the sparging area of the air distribution system while the air flow rate does not influence considerably the Dissolved Oxygen (DO) level which governs the CO2 stripping process. The theoretical calculated values of the volumetric mass transfer coefficient have been compared with those obtained experimentally. Based on the data obtained, relationships of pH/kLa (mass transfer coefficient) were developed. These correlations serve as a tool for prediction of pH during the struvite precipitation process.

Determination of the ozone volumetric mass transfer coefficient in bubble column with two-fluid nozzle gas distributor

Experimental investigation of gas-liquid mass transfer of ozone in water, in bubble column with two-fluid nozzle gas distributor (BKDM), under different operating conditions, are presented in this work. The main objective was to determine the ozone volumetric mass transfer coefficient, kL a, in calm uniform section of the column, under different values of gas and liquid flow rates. Obtained values of these coefficients were compared with the values in countercurrent bubble column. The critical liquid flowrate, when gas hold up reaches its maximum, was experimentally determined. It was shown that the maximum value of the ozone volumetric mass transfer coefficient is obtained just when liquid flowrate is at its critical value.