Higher Mass Transfer Coefficient Research Articles

In-situ measurement of electrode kinetics in porous electrode for vanadium flow batteries using symmetrical cell design

Electrode kinetics are crucial to electrochemical performance of vanadium flow battery. However, existing studies on electrode kinetics are mostly performed in ex-situ conditions based on planar electrodes, so the obtained rate constant can neither reflect the kinetics in practically used porous electrode nor be used to predict the activation polarizations of a real flow cell. To address this limitation, an in-situ measurement of electrode kinetics in porous electrode is conducted in this study by a symmetrical cell design for vanadium flow battery. For both V2+/V3+ and VO2+/VO2+ couples, the activation polarization as a function of current density is firstly acquired on the symmetrical cell through measurements of polarization curve, high frequency resistance and local mass transfer coefficient, followed by Tafel fitting that yields both rate constant and transfer coefficient. Based on kinetic parameters of both V2+/V3+ and VO2+/VO2+, the simulated full cell activation polarizations coincide well with the full cell experimental result. By further comparing to simulation results using reported kinetic parameters determined from planar electrodes, the reliability of the proposed method is further highlighted. The electrode evaluation is much more involved than Tafel polarization and stack design is not treated in this contribution.

Understanding the effect of transverse vibration on hollow fiber membranes for submerged forward osmosis processes

External concentration polarization (ECP) remains a pervasive challenge for submerged forward osmosis (FO) systems, as it critically reduces the water flux of membranes. In this study, we propose to use transverse vibration on hollow fiber membranes to effectively reduce the ECP and increase the FO water flux performance. The effects of different vibrating amplitude and frequency were investigated. Also, the vibration effect was evaluated under different draw solution concentrations, membrane orientations and draw solute types. Our results show increased water flux with increasing vibrating intensity (i.e., amplitude and frequency) while maintaining a constant specific reverse solute flux (Js/Jw). At the highest amplitude (1.2 cm) and frequency (3 Hz) used, the mass transfer coefficient is increased from 0.7 × 10−5 m s−1 (at no vibration) to 1.8 × 10−5 m s−1, which falls within the optimal flux range as determined from our FO modelling results. The increment in mass transfer coefficient is found to be closely related to the improved hydrodynamic conditions due to vibration, and less dependent on the membrane orientation and draw solution concentration. In addition, both modelling and experimental results show that maintaining a reasonably high mass transfer coefficient is especially important for FO process with severe ECP conditions, such as when dilutive ECP is present (i.e., active-layer-facing-feed-solution orientation) or when a high water flux is applied (usually in the active-layer-facing-draw-solution orientation). However, the severe ECP arising from large draw solutes (e.g., larger sized polymer with low diffusivity and high viscosity) might not gain greater benefits from the vibration or improved hydrodynamic conditions.

Phenomenological modeling and simulation of a textile stenter

Mathematical modeling and simulation of cotton fabric drying in a stenter machine was performed using a finite element method. A standard κ-ɛ turbulence model was coupled with heat/mass transfer in porous media models. A novel approach to simulate all injectors enabled a new perspective of the process and the calculation of local convective coefficients. Three simulations were run: Simulation I reproduced real operational conditions; Simulations II and III estimated the effects of increasing drying air inlet velocity and decreasing translational velocity of the fabric, respectively. The highest drying air velocities occurred at the nozzles on the edges of the injectors, leading to high convective heat and mass transfer coefficients. Results indicated the drying process was not uniform along the fabric’s width. The models were acceptable in realistically predicting the drying of a cotton fabric in a stenter machine and could be useful in optimizing the stenter design considering the final product’s quality and energy consumption.

Evaluating hydrodynamics in a bioreactor with liquid phase dispersion in the gas phase

Abstract Bioreactors are used in many biochemical industries to produce commercial life products such as medicines, enzymes, perfumes, paints and antibiotics. In the presented study, a specially shaped bioreactor has been designed, built and operated to increase the mass transfer coefficient. The constructed bioreactor, according to type of microorganisms, can provide high amounts of oxygen or carbon dioxide. Moreover, the manuscript was aimed at investigating the hydrodynamic properties of the bioreactor. The bioreactor was constructed from three parts including shower in upper part, middle section for mass transfer and bottom section as a reservoir. Liquid flow rate, shower holes diameter, aeration velocity and the middle part height of the bioreactor have been studied as factors influencing the hydrodynamics. The results showed that the highest mass transfer coefficient was 30.1 1/h which was achieved when the liquid flow rate, the shower holes diameter, aeration velocity and middle part height of the bioreactor were 280 mL/min, 2 mm, 0.03 vvm and 60 cm, respectively.

Freeze Granulated Zeolites X and A for Biogas Upgrading.

Biogas is a potential renewable energy resource that can reduce the current energy dependency on fossil fuels. The major limitation of utilizing biogas fully in the various applications is the presence of a significant volume fraction of carbon dioxide in biogas. Here, we used adsorption-driven CO2 separation using the most prominent adsorbents, NaX (faujasite) and CaA (Linde Type A) zeolites. The NaX and CaA zeolites were structured into hierarchically porous granules using a low-cost freeze granulation technique to achieve better mass transfer kinetics. The freeze granulation processing parameters and the rheological properties of suspensions were optimized to obtain homogenous granules of NaX and CaA zeolites 2–3 mm in diameter with macroporosity of 77.9% and 68.6%, respectively. The NaX and CaA granules kept their individual morphologies, crystallinities with a CO2 uptake of 5.8 mmol/g and 4 mmol/g, respectively. The CO2 separation performance and the kinetic behavior were estimated by breakthrough experiments, where the NaX zeolite showed a 16% higher CO2 uptake rate than CaA granules with a high mass transfer coefficient, 1.3 m/s, compared to commercial granules, suggesting that freeze-granulated zeolites could be used to improve adsorption kinetics and reduce cycle time for biogas upgrading in the adsorption swing technology.

Remarkably high Pb2+ binding capacity of a novel, regenerable bioremediator Papiliotrema laurentii RY1: Functional in both alkaline and neutral environments.

The ability of P. laurentii strain RY1 to remediate lead (Pb2+) from water was investigated in batch and column studies. The lead removal ability of non-viable biomass, non-viable biomass immobilised on agar-agar (biobeads) and agar-agar at different pH was compared in batch studies. It was found that among the three, biobeads have maximum ability to remove Pb2+ followed by biomass and agar-agar beads. Maximum and almost equal lead removal by biobeads was observed at both neutral and alkaline pH making it a novel and more applicable bioremediator as all other reported bioremediators have a single pH for optimum activity. Studies were performed to determine the optimum conditions for lead removal from aqueous solutions for biobeads. The physical and chemical characterization of the biobeads before and after Pb2+ biosorption was done by using S.E.M. and F.T.I.R. respectively. The adsorption of Pb2+ on biobeads obeyed the Langmuir adsorption isotherm and pseudo first order kinetics. These mean that the Pb2+ binding sites are identical, located on the surface of the adsorbant and the rate of Pb2+ removal from aqueous solution is directly proportional to the number of Pb2+ binding sites on the biobeads. The thermodynamics of the biosorption process is also investigated. The binding capacity of the biobeads in batch study was found to be 52.91mg/gm which is higher in comparison to other reported yeast bioremediators. The used biobeads can be desorbed using 0.1(M) CaCl2. The desorbed biobeads can be used subsequently for several cycles of lead removal making it cost-effective. Column studies were also performed for biobeads with the help of Thomas model for examining its suitability for industrial application. Maximum specific lead uptake of the biobeads when applied in the column was found to be 58.26mg/gm which being promising makes it suitable for application in industries involved in the treatment of wastewater contaminated with high amounts of lead. The high mass transfer co-efficient indicate that small sized column can be used effectively to remove high amounts of lead which makes the bioremediation process by the biobeads more economical and advantageous for industrial application. Several factors like effectiveness of the biobeads in Pb2+removal at both neutral and alkaline pH, reusability, high mass transfer co-efficient, regenerability and high binding capacity makes it a novel versatile, cost-effective and high utility bioremediator.

Kinetic modeling of microwave osmotic dehydration of mangoes under continuous flow medium spray conditions using sucrose and maltodextrin (10-18 DE) solute mixtures

Microwave osmotic dehydration of mango (Mangifera indica) was carried out under continuous spray mode (MWODS) with four osmotic solute mixtures (sucrose (S), sucrose in combination with maltodextrin (MD) 10 dextrose equivalent (DE), 15DE, and 18DE), at two temperatures (40 °C, 60 °C) and two concentrations (40%, 60%) and four contact times (10, 20, 30, 40 min). The mass transfer kinetics were fitted to a first-order kinetic model, by way of an empirical parameter (k) representing an overall mass transfer coefficient. Azuara model was also evaluated to describe the mass transfer kinetics. The results showed that the overall mass transfer coefficients for moisture and solids contents were influenced by temperature, concentration and, more importantly, the solute composition. The highest mass transfer coefficient for moisture loss (1.56 E−2) and lowest for solids gain (8.60 E−3) observed with sucrose + maltodextrin 10DE (S + MD 10DE) combination at 60%/60 °C and 40%/40 °C, conditions, respectively. In addition, the Azuara model well fitted the experimental data for mass transfer kinetics (R2 > 0.92). The highest moisture loss (ML) along with lower solids gain (SG) was associated with the solute sucrose + maltodextrin 10DE combination. The highest ML/SG ratio of 10 and weight reduction of 55.5% were observed with S + MD 10DE at 40 °C/60%/40 min and 60 °C/60%/40 min, respectively, with S + MD 10DE solute mixture. Overall, it was possible to successfully describe the mass transfer kinetics using empirical first order and Azuara models and MWODS with S + MD 10DE facilitated the maximum moisture loss and low solids uptake, both desirable from osmotic dehydration point of view.

Comparison of structured activated carbon and traditional adsorbents for purification of H2

RPSA technology can offer a more compact, cost-effective and energy-efficient solution for H2 recovery when compared to conventional PSA. The adsorption rate of adsorbents has become a key factor affecting the development of RPSA. Activated carbon, widely used as adsorbent for CO2 in H2 purification is in urgent need for further development to suitable for RPSA system. In this study, a structured activated carbon adsorbent was prepared using the dip-coating method, with nickel foam as a framework. The adsorption characteristics of the material for CO2 adsorption at different adsorption pressures, volume flow rates, and lamellar spacing parameters were investigated, and the kinetic behaviour was studied by the breakthrough curve. The results showed that the activated carbon was bonded to the nickel foam skeleton via an organic-inorganic binder with excellent mechanical strength. Structured adsorbents bed has uniform gas flow distribution, the pressure drop is about 1/3–1/7 of particles packed bed, and the breakthrough curve was sharper than particles. It indicated that the mass-transfer resistance in the structured adsorbents is low and has high effective diffusivity for CO2. The adsorption rate constant was calculated using the Yoon-Nelson model, and the correlation between the model calculation data and experimental data was greater than 0.98. Structured activated carbon adsorbent manufactured by dip-coating method, has a higher mass transfer coefficient and a shorter mass transfer zone length. It is a potential alternative adsorbent for RPSA.

Flow Distribution and Mass Transfer of Gas–Liquid Flow in Parallel Microchannels with Different Tree-Shaped Distributors: Halving-Width versus Constant-Width

The flow distribution and mass transfer for gas-liquid flow in two parallel microchannels with different tree-shaped distributor are studied. Two parallel microchannels, both consist of four parallel branches, operated in the slug flow and with Reynolds number in the range of 0.2-10. The operating range in the halving-width microchannel (Tree-1) is significantly narrower than the constant-width microchannel (Tree-2). As the ratio of gas-liquid flow rate increases, the flow distribution in the Tree-1 gradually deteriorates. While in the Tree-2, there exists a critical ratio of gas-liquid flow rate to attain ideal flow distribution. Meanwhile, the volumetric mass transfer coefficient in the Tree-1 is lower, while in the Tree-2, the opposite is true. However, the high mass transfer coefficient is always accompanied by the high pressure drop. The pressure drop in the Tree-2 is higher than the Tree-1. Furthermore, the performance ratio is adopted to evaluate the mass transfer performance.

Fabric Moisture Uniform Control to Study the Influence of Air Impingement Parameters on Fabric Drying Characteristics.

Impinging dryness is now a widely used and effective way for fabric drying due to its high heat and mass transfer coefficient. Previous studies on fabric drying have neglected the contributions of moisture uniformity and diffusion coefficient to the drying process; though, they have recently been shown to have a significant influence on drying characteristics. This report outlines a step-by-step procedure to investigate the effects of air impingement parameters on a fabric's drying characteristics by controlling the uniformity of its area moisture distribution. A hot air blower unit equipped with an angle adjustable nozzle is used to generate air flow with different velocities and temperatures while the drying process is recorded and analyzed using an infrared thermograph. In addition, a uniform padder is adapted to ensure the fabric's moisture uniformity. Impinging drying is studied under different initial conditions by changing the air flow temperature, velocity, and direction, then the applicability and suitability of the protocol are evaluated.

Modelling of the separation of long-chain normal paraffins from kerosene in a simulated moving bed process: effect of the desorbent

Linear paraffins can be selectively separated from the rest of components of kerosene (branched hydrocarbons, aromatics and naphthenes) by means of liquid phase adsorption on 5A zeolite using the technology of simulated moving bed (SMB). In previous works, the kinetic and equilibrium parameters required for modelling and design of the SMB unit were obtained for pure n-paraffins and n-paraffin mixtures. However, the simulation of the SMB process indicated the presence of n-C5, used as a desorbent, in the separation zone, especially after the feed mixture is introduced. This finding motivated this work, in which n-paraffin mixtures (n-C10, n-C12, n-C14) including n-C5 were studied to address its influence in the process. The kinetic and equilibrium parameters for these mixtures were obtained and included in the model for the simulation of an SMB unit. While mixtures without n-C5 preferentially adsorbed shorter n-paraffins, it was found that including n-C5 in the mixtures reverses the selectivity of the adsorbent. In this case, longer n-paraffins are preferentially adsorbed, matching the trend observed for pure n-paraffins. In addition, n-C5 significantly increases the mobility of n-paraffins, as indicated in their higher mass transfer coefficients. The model was validated by comparing the predicted performance with the reported separation achieved by a commercial SMB unit that separates n-paraffins from hydrotreated kerosene fractions. The predicted separation performance is very similar to that achieved in our previous works, slightly improving the purity (99.6%) of the extract as a trade for a small loss in recovery (95.4%).

EFFECT OF DISC-BLADE INTERCEPTING ANGLE ON MIXING PERFORMANCE IN A MULTIPHASE STIRRED VESSEL

Abstract The impeller geometry is a determining factor for mixing efficiency in a stirred vessel. In this study, the effect of disc-blade intercepting angle on power number, just suspended speed and mass transfer coefficient was investigated in a multiphase system under turbulent conditions. The impellers used in this study were fabricated with 3D printing. The interactions between the experimental parameters were analysed using Response Surface Methodology (RSM). The impeller power number was found to have a linear positive relation with disc-blade intercepting angle from 30° to 90°. Beyond 90°, the impeller power number became lower with increment in the disc-blade intercepting angle. The results confirmed better suspension efficiency in the angle range of 50° to 120° in 5 wt% solid suspensions. The lowest specific power requirement (Ɛjs) at just suspension condition was observed in the angle range of 30° to 80°. The highest mass transfer coefficient was obtained for the impellers with disc-blade intercepting angle in the middle range. Two models were established on power number and mass transfer coefficient for various disc-blade intercepting angles. The study confirmed that the hydrodynamic and mass transfer performance of disc blade impellers could be maximized by changing the disc-blade intercepting angle for a selected system.

Modelling and parameter estimation of breakthrough curves for amine-modified activated carbons under pre-combustion carbon capture conditions

Pressure swing adsorption (PSA) demonstrates high potential for capturing pre-combustion carbon dioxide in Integrated Gasification Combined Cycle (IGCC) power plants, due to a binary mixture of hydrogen and carbon dioxide entering the separation process. In this work, a seven-step PSA model was developed and compared to adsorption experiments under PSA conditions (25 bar and ambient temperatures) performed with tetraethylenepentamine (TEPA) and a novel blend of monoethanolamine-monodiethanolamine (MEA-MDEA) modified activated carbons, using nitrogen and carbon dioxide mixtures. The MEA-MDEA modified activated carbon showed promising results for pre-combustion PSA processes, due to their high carbon dioxide adsorption capacity and delayed break-point of about 200 s compared to the unmodified activated carbons. A sensitivity analysis carried out for the adsorbent parameters in a seven-step PSA process showed that high mass transfer coefficient values yielded to highly purified products, specifically for the light product stream (99.4%). A sensitivity analysis of the process variables showed that purity values of the heavy stream (carbon dioxide) were over 90% when the purge pressure was reduced to 0.5 bar and the carbon dioxide feed fraction increased to 60%.

Numerical study on influencing factors of accelerated erosion experiment based on the consideration of fluid-mass transfer

An accelerated erosion test with improved experimental efficiency is required to study the critical properties of pipe erosion failure in a reactor effluent air cooler used for the hydrocracking process. In this study, a single 90° elbow was simulated numerically with the design of an L16 orthogonal table to discuss the influencing factors of grid refinement at the bend. An analysis of the length–diameter ratio L1/D revealed that when L1/D increases to more than 5, the change in wall shear stress at the inner bend wall is insignificant. The mass transfer coefficient of ferrous ion and the maximum shear stress τm were also utilized to describe the erosion rate, including electrochemical corrosion and hydrodynamics. The factor that influenced mass transfer coefficient most was inlet velocity. The regions with high mass transfer coefficient were located at R1 and R2, with R1 showing a higher risk of electrochemical corrosion than R2. Operating pressure played a critical role in the effect on τm, which presented a low inside/high outside trend. The region with high τm was located at R3, where a high breakage risk of erodent product films was observed.

Prescription de la dose de dialyse

The concept of dose of dialysis is relatively recent. It evaluates the adequacy of extrarenal clearance, in order to provide the best chances of survival to chronic dialysis patients. Although it presents drawbacks, urea Kt/V is recognized as the most clinically relevant indicator. It can be easily calculated online thanks to the equipment of the dialysis monitors with ionic dialysance. The target of balanced Kt/V is greater than 1.2, provided the minimal Kt dialysis dose is received. To reach these targets, it is necessary to use dialysers of large surface, with high urea mass transfer coefficient. The blood pump flow must be high and dialysate flow rate sufficient. With the advent of online hemodiafiltration, a dose of convective dialysis was imposed. To achieve these convective targets, large surface dialysers with high hydraulic permeability must be used, with a high beta 2 microglobulin screening coefficient and low albumin loss. Prescribing the dose of dialysis is essential in an optimal quality-of-care based approach.

Scalability of 3D printed structured porous milli-scale reactors

This study addresses the scalability of in-house designed, and 3D printed structured porous reactors for liquid-liquid reactions. The base structure of these porous reactors consists of cylindrical fibres in defined geometrical arrangements. Their scale-up was realized by increasing the reactor diameter by a factor of 1.5 and 2 respectively while keeping the fibre dimensions constant. Also, the effect of altering the fibre dimensions in proportion to the scale-up factor was assessed. The reactors were characterized in terms of their biphasic heat and mass transfer properties. In stratified flow, the scaled-up structured porous reactors exhibited high interfacial mass transfer coefficients (kLa) at residence times <10 s, whereas in Taylor flow an overall drop in kLa values was observed. Furthermore, the highest biphasic heat transfer coefficients were found for the structured porous reactors with a scale-up factor of 1.5. Moreover, the structured porous reactors were applied to industrially relevant reactions. For the oxidation of nonanol, the scaled-up reactors showed an overall drop in yield, nevertheless with two folds production rate at same pressure drop. For the relatively slow C-N cross-coupling reaction, larger yields were realized by arranging scaled-up reactors in series at same total residence time. Specifically, an arrangement of 8 reactors with a scale-up factor of 2 in series resulted in six times higher production rate than a conventional packed-bed reactor but without any additional pressure drop. For the considered range of residence times, keeping the fibre dimensions constant while increasing the reactor diameter was observed to be advantageous.

Mass Transfer Enhancement for CO2 Absorption in Structured Packed Absorption Column

The acidic gas, Carbon dioxide (CO2) absorption in aqueous ammonia solvent was carried as an example for industrial gaseous treatment. The packed column was provided with a novel structured BX-DX packing material. The overall mass transfer coefficient was calculated from the absorption efficiency of the various runs. Due to the high solubility of CO2, mass transfer was shown to be mainly controlled by gas side transfer rates. The effects of different operating parameters on KGav including CO2 partial pressure, total gas flow rates, volume flow rate of aqueous ammonia solution, aqueous ammonia concentration, and reaction temperature were investigated. For a particular system and operating conditions structured packing provides higher mass transfer coefficient than that of commercial random packing.

Efficient Micromixing for Continuous Biodiesel Production from Jatropha Oil

Introduction:The utilization of biodiesel as an alternative fuel is turning out to be progressively famous these days because of worldwide energy deficiency. The enthusiasm for utilizing Jatropha as a non-edible oil feedstock is quickly developing. The performance of the base catalyzed methanolysis reaction could be improved by a continuous process through a microreactor in view of the high mass transfer coefficient of this technique.Materials &amp; Methods:Nanozirconium tungstovanadate, which was synthetized using sol-gel preparation method, was utilized in a complementary step for biodiesel production process. The prepared material has an average diameter of 0.066 &amp;µm.Results:First, the NaOH catalyzed methanolysis of Jatropha oil was investigated in a continuous microreactor, and the efficient mixing over different mixers and its impact on the biodiesel yield were studied under varied conditions. Second, the effect of adding the nanocatalyst as a second stage was investigated.Conclusion:The maximum percentage of produced methyl esters from Jatropha oil was 98.1% using a methanol/Jatropha oil molar ratio of 11 within 94 s using 1% NaOH at 60 &amp;°C. The same maximum conversion ratio was recorded with the nanocatalyst via only 0.3% NaOH.

Monitoring gradient formation in a jet aerated bioreactor.

Jet aerated loop reactors (JLRs) provide high mass transfer coefficients (kLa) and can be used for the intensification of mass transfer limited reactions. The jet loop reactor achieves higher kLa values than a stirred tank reactor (STR). The improvement relies on significantly higher local power inputs (∼104) than those obtainable with the STR. Operation at high local turnover rates requires efficient macromixing, otherwise reactor inhomogeneities might occur. If sufficient homogenization is not achieved, the selectivity of the reaction and the respective yields are decreased. Therefore, the balance between mixing and mass transfer in jet loop reactors is a critical design aspect. Monitoring the dissolved oxygen levels during the turnover of a steady sodium sulfite feed implied the abundance of gradients in the JLR. Prolonged mixing times at identical power input and aeration rates (∼100%) were identified for the JLR in comparison to the STR. The insertion of a draft tube to the JLR led to a more homogenous dissolved oxygen distribution, but unfortunately a reduction of mixing time was not achieved. In case of increased medium viscosities as they may arise in high cell density cultivations, no gradient formation was detected. However, differences in medium viscosity significantly altered the mass transfer and mixing performance of the JLR.

An investigation into evaporative ink drying process on forced convective heat and mass transfer under impinging air jets

Impinging air jets are widely used in industry for drying, cooling and heating due to high heat and mass transfer coefficients which are developed in the impingement region. High velocity heated air impinging jets have proved to be very effective to enhance evaporative ink drying especially on the rotary presses for the packaging and converting industry. In this paper, to provide data for ink dryer designers, an investigation has been carried out to obtain the heat and mass transfer coefficients which constitute the evaporative drying of thin ink films. Finally, the theoretical results are compared for the constant rate period and falling rate period with some experimental and theoretical results found to be satisfactory particularly for the drying time.