Local Mass Transfer Coefficients Research Articles

Experimental study of a leakage location method based on plug flow mass transfer characteristics

Leaks in water supply pipes pose a threat to water security. In order to save labor investment in pipeline leak location and to find a method to obtain the leak location by calculating upstream and downstream related data, an experimental study was conducted to investigate the plug flow mass transfer characteristics in horizontal pipelines. The results show that the addition of carbon dioxide gas can better detect the occurrence of small area ruptures in the pipeline. The translational velocity equation for elongated bubbles has better accuracy for both the insoluble gas air and the slightly soluble gas carbon dioxide. The local volumetric mass transfer coefficient shows a better linear relationship with the residual gas volume. The iterative equation derived from such a law can effectively predict the distribution of carbon dioxide concentrations in the pipeline. The study shows that the iterative equation, combined with the carbon dioxide concentration at a point downstream and the bubble translation velocity, can make a good localization judgment of the pipeline rupture point.

Local mass transfer and flow visualization around a cylinder in a liquid-solid fluidized bed

Adsorption method has been used for flow visualisation and determination of the local and average mass transfer coefficients around a horizontal cylinder in a liquid-solid fluidized bed. The obtained concentration fields on the adsorbent (silica-foils) represent a clear qualitative flow pattern around the cylinder. By comparison of the concentration fields around the cylinder in the single-phase flow and in the fluidized bed, a significant disturbance of the boundary layer by fluidized particles can be observed. Local mass transfer coefficients were shown to be dependent on the angular position around the cylinder with the maximum determined at the angle 140? (measured from the stagnant point 0?). Selected correlations were applied to predict the average mass transfer coefficients and the two best have shown deviations from the experimental data up to 5 %.

Phenomenological‐based model of direct blue 2 adsorption on corncob in a fixed‐bed

AbstractThis work presents a phenomenologically based semi‐physical model (PBSM) for the liquid–solid adsorption process, which has been experimentally validated. Two modes of operation of adsorption of direct blue 2 dye (DB2) on corn cobs in a fixed bed are discussed by applying the PBSM. The first mode of operation is the typical continuous liquid flow, whereas the second is a semicontinuous mode with the recirculation of liquid leaving the column. These modes of operation were used to illustrate different ways in which the system reaches equilibrium conditions. The equilibrium distribution curve (EDC) for low DB2 concentration was reached with the best fit using the Langmuir–Freundlich model (). The local mass transfer coefficient for the solid phase was evaluated by the PBSM deduction, and subsequently, the diffusivity of the surface solid was experimentally evaluated through comparison in terms of the PBSM. Regardless of the mode of operation, surface diffusion is associated with only the nature of the adsorbent material rather than the concentration of the liquid phase. In addition, the PBSM exhibited liquid and solid concentration changes at different points in the column. Moreover, different operating curves of the process were constructed and validated, exhibiting similar results when comparing the predictions of the model and the experimental data. These findings support the strength of the constructed model, enabling its extension to other adsorbent–dye systems.

Numerical investigation of SO2 absorption into a droplet: deformation and internal flow facilitating mass transfer

The SO2 absorption into a droplet falling freely is investigated numerically based on a detailed three-dimensional mode in this work. Gas-liquid multiphase flow simulations coupled with interphase mass transfer are performed using the coupled Level-set and Volume of Fluid method (CLSVOF) to trace the gas-droplet interface. The results of simulation reveal that droplet would deform and deviate from the standard sphere during falling, with an approximately 5.6 % increase in surface area under the discussed working conditions. The evolution mechanism of the internal circulation in the droplet is clarified: the increase in the relative velocity of gas-droplet facilitates the formation and intensification of the internal circulation in the droplet. As the internal circulation develops into the stable stage, the convective mass transport would dominate the mass transfer process and promote the mass transfer. The distribution laws of the local mass transfer rate and coefficient over the droplet surface are found analogous to the typical case of heat transfer around a sphere. Finally, considering the effects of both droplet deformation and internal circulation on mass transfer concurrently, the variation of Sh as a function of Reynolds number is obtained, which appears the same trend but yields slightly higher values compared to the results of empirical correlations.

A Methodical Approach to Scaling Up an Aerated Coaxial Mixer Containing a Shear-Thinning Fluid: Effect of the Fluid Rheology

The evolution of the rheological behavior of the fluid is one of the primary challenges encountered in the scale-up of the most aerated reactors. In fact, mixing efficiency diminishes because of the changes in the fluid rheological properties. Under these circumstances, it is challenging to keep the mass transfer constant upon scale-up. A thorough literature review revealed that no scale-up study of aerated coaxial mixers has considered the effect of varying fluid rheological properties. Therefore, to analyze the scale-up of these aerated mixing systems, the effects of the impeller speed and type, fluid rheology, and aeration rate on the gas hold-up, local mass transfer coefficient, bubble size distribution, hydrodynamics, and specific power consumption were explored. Electrical resistant tomography, dynamic gassing-in, and computational fluid dynamics methods were employed. For the first time, a systematic approach based on a constant volumetric mass transfer has been proposed for the scale-up of aerated coaxial reactors at different fluid rheological properties.

A methodology for the analysis of distillation processes within rotating packed beds based on local temperature measurements

A temperature-based measurement methodology for the investigation of the local separation efficiency within the packing of Rotating Packed Beds (RPB) has been developed, which enables the derivation of design criteria and optimal operating conditions. Key therein is the acquisition of experimental data providing a local resolution of the temperature profile within the packing. For this purpose, independent data loggers were adapted and installed to capture radial temperature profiles for the distillation of the binary mixture ethanol and 1-propanol. Based on the positions of the sensors, local separation efficiencies and mass transfer coefficients were determined experimentally. Effects on the analysis due to maldistribution were analysed by deploying variations in packing arrangement. By dividing the packing into different sections and averaging the performance indices, the impact of maldistributions is then compensated. This yields in consistent and reproducible results, showing a decrease in mass transfer rates with increasing radial position and a general enhancement of mass transfer by rotational speed. To our knowledge, this is the first experimental study providing radial temperature profiles for RPBs for varying rotational speeds and a methodology for its analysis.

Local volumetric mass transfer coefficients in sections of multiple-impeller stirred tank reactors: Data analysis

The analysis of local values of volumetric mass transfer coefficient, kLa, was performed to identify the effects of local hydrodynamics on the mass transfer rate in multiple-impeller stirred tank reactors. The database of kLa values measured by Dynamic Pressure Method was analysed. The database provides data for two vessel sizes (laboratory and pilot-scale reactor), a wide range of operational conditions (gas flow rate, impeller speed), various impeller geometries and sizes and four different batches including viscous media. The local kLa values in multiple impeller configurations were measured in several positions corresponding to the individual impeller regions. The overall kLa value averaged through the whole volume is considered as the best representation of the correct kLa value. Different hydrodynamic effects on mass transfer rate were obtained in the two end regions. In the bottom region, the local kLa values increase with impeller speed faster than the overall kLa. The opposite behaviour was obtained in the uppermost region corresponding to the decrease of power consumption and the mass transfer rate with increasing surface aeration under high impeller speeds. The best representation of the overall kLa value was obtained in the middle position of the vessel, where the exchange flows bring the liquid saturated to different measure from the end regions. Even if the local kLa data in individual impellers' regions are shifted to the average values due to the inter-impeller exchange flows, the differences between them are still significant. We show the local kLa values by the combination of the data measured at different liquid level heights. The results can be used in the fermenter design with various impeller combinations.

Modeling multiphase fluid flow, mass transfer, and chemical reactions in bioreactors using large-eddy simulation.

We present a transient large eddy simulation (LES) modeling approach for simulating the interlinked physics describing free surface hydrodynamics, multiphase mixing, reaction kinetics, and mass transport in bioreactor systems. Presented case-studies include non-reacting and reacting bioreactor systems, modeled through the inclusion of uniform reaction rates and more complex biochemical reactions described using Contois type kinetics. It is shown that the presence of reactions can result in a non-uniform spatially varying species concentration field, the magnitude and extent of which is directly related to the reaction rates and the underlying variations in the local volumetric mass transfer coefficient.

Experimental and numerical investigations of aerosol transportation phenomena from single bubbles

Enhanced aerosol transportation is required in numerous industrial applications. Typically, bubble columns are used for achieving mass transfer. However, limited studies have been conducted on the aerosol mass retention capability of single bubbles in bubble columns. This study focused on aerosol transportation resulting from the flow field inside single bubbles. First, the aerosol mass transfer efficiency of a single bubble was measured to be used as benchmark data. The results revealed that inertial force was the dominant factor for aerosol transportation within the present measurement conditions. Next, a Eulerian gas–liquid–solid three-phase model was developed to understand the mechanism of aerosol transportation and was subsequently experimentally validated. Finally, a parametric analysis of aerosols and bubble diameters was conducted to investigate the mass transportation phenomena of aerosols of various diameters. The effect of various bubble diameters on aerosol retention was explained by the effect of the flow field on the local mass transfer coefficient. The results of the study provide guidelines for the development models to predict the emission rate of hazardous materials in bubble columns.

Investigating the Effects of Operation Variables on All-Vanadium Redox Flow Batteries Through an Advanced Unit-Cell Model

Next-generation redox flow batteries will benefit from the progress of macroscopic continuum models that enable the optimization of new architectures without the need of expensive fabrication and experimentation. Despite previous attempts, there is still need for robust and thoroughly validated models. Here, a steady-state two-dimensional unit-cell model of an all-vanadium redox flow battery is presented. The model integrates state-of-the-art descriptions of the fundamental physical phenomena, along with new features such as local mass transfer coefficients for each active species, precise sulfuric acid dissociation kinetics, and experimental data of the electrochemical parameters and electrolyte properties. The model is validated at different states of charge and flow rates using polarization, conductivity and open circuit voltage measurements. Then, the contribution of operating conditions on battery performance is studied by analyzing its separate effect on the various phenomena that affect cell performance, such as local pore mass transfer limitations, parasitic hydrogen evolution reactions, crossover and self-discharge fluxes. The resulting model is a reliable tool that can be used to assess the relevance of these coupled phenomena that take place simultaneously within the reaction cell. This important information is critical to optimize cell components, reactor design and to select optimal operating conditions.

Regulating flow field design on carbon felt electrode towards high power density operation of vanadium flow batteries

Flow batteries promise a great practice to integrate with renewable energy sources in electric grid applications. However, high power density operation of flow batteries remains a challenge due to mass transport limitation and flow resistance in porous carbon felt electrode, which urges the need of advanced flow design to synergistically lower concentration polarization and reduce pressure drop. Herein, we realize a remarkably enhanced power density operation for vanadium flow batteries by regulating flow field design on carbon felt electrodes. Finite element analyses firstly reveal significantly reduced pressure drop, well-distributed reactant and promoted flow velocity on carbon felts with parallel and interdigitated flow designs. On the basis of measured local mass transfer coefficients, both interdigitated and parallel based felts exhibit a notable reduction in simulated concentration polarization at 200 mA cm−2 with parallel flow outperforming interdigitated design. Experimental validations further confirm a superior voltage efficiency of 78 % and significantly enhanced discharge capacity at 200 mA cm−2 for the flow cell adopting parallel based felts. Finally, dynamic modelling and simulation of an industrial-scale 32 kW stack highlight a desirable system efficiency of ca. 70 % for the parallel flow felt design at 200 mA cm−2, signifying a great potential of regulating flow field on carbon felts for design and scale-up of practical flow battery stacks.

Multi-mode reduced order models for real time simulations of monolith reactors with micro-kinetics

We present a novel procedure for computing washcoat diffusional effects in monolith reactors with detailed micro-kinetic models. First, we show that the short time scales associated with the adsorption/desorption steps of micro-kinetic models requires using a significantly larger number of mesh points within the washcoat to obtain a solution that is independent of the mesh size. Next, we present a multi-mode reduced order model that eliminates the degrees of freedom associated with species diffusion in the washcoat using the local property dependent internal mass transfer coefficient matrix. This matrix is shown to be diagonal for most micro-kinetic models of practical interest and can be calculated accurately. We illustrate the new approach with an example of H2/CO/C3H6 oxidation over a Pt/Al2O3 catalyst with a 20 step micro-kinetic model. Even for this simple single site model, the small time (and length) scales associated with the species adsorption steps make the computation of the mesh size independent solution of the 1＋1D model time consuming. However, the reduced order model leads to an accurate solution while speeding-up calculations by about three orders of magnitude.

Study on the Effective Mass Transfer Area and the Local Gas-Side Mass Transfer Coefficient in a Rotor–Stator Reactor

Study on the Effective Mass Transfer Area and the Local Gas-Side Mass Transfer Coefficient in a Rotor–Stator Reactor

Performance analysis of aqueous LiCl and CaCl2 based falling film dehumidifier with surface modification

• Analysis is carried out on a falling film based liquid desiccant dehumidifier. • The surface is modified with sinusoidal profile to increase the dehumidification. • The sinusoidal wavy profile increases the desiccant surface area by 7%. • Aqueous LiCl solution enhances the dehumidification by 29.15% compared to CaCl 2 . • With aqueous LiCl, the modified surface increases the dehumidification by 33.18%. In summer air-conditioning, conventional vapor compression systems cool the process air to its dew point temperature for dehumidification and further heat it to required supply temperature. This involves extra cooling and subsequent heating. Liquid desiccant systems avert this issue by separately handling latent and sensible loads, while running primarily on low grade thermal energy. Falling film type liquid desiccant dehumidifiers are advantageous over other types, mainly, as they reduce desiccant carryover into supply air stream. The current work aims towards the analysis and the performance improvement of a falling film type dehumidifier with the help of wavy profile of the working surface. Detailed comparison of the two commonly used desiccants, i.e., LiCl and CaCl 2 are carried out to analyse the performance of the dehumidifier. The present work also includes the characterisation of the wave profile and the analysis in terms of various important parameters, such as, the dehumidification effectiveness, the moisture removal rate and the change in specific humidity, missing in earlier studies. In order to study the performance, a simplified 2–D transient finite volume model is taken into consideration to simulate a multiphase, multi-component conjugate heat and mass transfer system using ANSYS Fluent 19.2. The volume of fluid method is used to trace out the liquid–gas interface. With back-end modification of the solver, the thermophysical properties of the liquid film are evaluated using empirical relationships, as a function of temperature and concentration. The penetration model of mass transfer provides the local mass transfer coefficients. The moisture removing the potential of the aqueous solutions of LiCl and CaCl 2 are evaluated at quasi steady-state conditions. The numerical solver is validated with the already published experimental results with a maximum deviation of 10.31%. A sinusoidal wavy profile of the wall introduced in the system enhances the turbulence intensity by 3.8% and the dehumidification performance by 33.18% and 18% for LiCl and CaCl 2 , respectively. It is due to the enhanced liquid–gas interface area. The mean film thickness is found to improve by 15.25% and 13.38% for LiCl and CaCl 2 , respectively. The effect of process parameters on the performance of the dehumidifier is also studied. It has been observed that the dehumidification effectiveness decreases from 86.14% to 31.1% with increase in inlet air velocity from 0.2 m/s to 2.0 m/s. It has also been found that a lower concentration of the desiccant solution and lower velocity of air yield higher dehumidification effectiveness. However, the moisture removal rate of the system will be compromised. Compared to the previously studied cases, the current configuration of the sinusoidal profile with amplitude to wavelength ratio of 1:15, yields superior dehumidification performance.

CFD modelling of mass transfer in liquid–liquid core-annular flow in a microchannel

This work reports Computational Fluid Dynamics (CFD) modelling of mass transfer in liquid–liquid Core-Annular Flow (CAF) in a microchannel. The model solves Navier-Stokes and convection–diffusion equations after incorporating interfacial boundary conditions of velocity, shear stress, flux and concentration jump. The model has been validated with the analytical and previously reported results. The validated model has been used to understand and quantify the dependence of concentrations boundary layer thicknesses on the core side and annulus side on Reynolds number, Schmidt number, dimensionless length and distribution coefficient. Furthermore, the dependenceof local mass transfer coefficients on the core and annulus side on diffusivity and concentration boundary layer thicknesses is quantified and compared with the predictions of film theory and penetration theory. Finally, the results obtained from the CFD model have been used to obtain the correlations of Sherwood number on the core side and the annulus side.

Determination of mass transfer coefficients in high-pressure CO2-H2O flows in microcapillaries using a colorimetric method

This study presents a method for the experimental determination of the local volumetric mass transfer coefficient kLaL in a high-pressure two-phase flow of water (H2O) and carbon dioxide (CO2) in a micro-capillary using a colorimetric method. H2O and CO2 are fed continuously and co-axially injected at high-pressure (10 MPa) and moderate temperature (303 K) into a microcapillary. Under the flow conditions studied, a segmented flow of CO2 in H2O is formed. The CO2 dissolves into the H2O-rich phase, thereby reducing the pH to about 3.3, depending on the pressure and temperature. The pH of the H2O-rich phase is determined over the entire length of the capillary using a pH sensitive indicator coupled with high-speed imaging and analysis. The concentration of CO2 in the water-rich phase is deduced from this pH value using literature experimental data. The CO2 concentration data and the unit-cell model, which has been modified to account for high pressure conditions, have then been used to determine the volumetric mass transfer coefficient, kLaL, of CO2 into the liquid phase along the entire length of the microreactor. The experimentally derived kLaL ​​ranges between 1 and 13 s−1.

Low-dimensional models for real time simulations of monolith reactors with dual washcoat layers

We present a novel and accurate procedure with local property dependent internal and external mass transfer coefficient matrices for real time simulations of monolith reactors with dual washcoat layers. Our approach extends the prior work based on the Thiele matrix method for accurately describing the washcoat diffusional effects in single layered systems. It combines the vector form of the multi-scale averaged model with calculation of the local property dependent internal, external and cross-Sherwood number matrices for multi-component systems. We illustrate the usefulness of the reduced order model using the example of a dual-layered model system with first washcoat layer of selective catalytic reduction (SCR) catalyst and second washcoat layer of lean NOx trap (LNT) catalyst. We determine the mesh size independent solutions of the full 1+1D detailed model and compare the same with that obtained with the reduced order model. We show that the proposed method has the same accuracy as the mesh size independent detailed solution while speeding up calculations by about three orders of magnitude for the example studied, and possibly by several more orders of magnitude for real systems with complex and detailed micro-kinetic models to describe the storage and reactions in the catalytic layers.

Local Distribution of Oxygen Mass Transfer Coefficient in CMC Solutions in Bioreactors Furnished with Different Types of Coaxial Mixers

Aerated stirred tank bioreactors are one of the most commonly used systems in various processes such as biofermentation where ensuring adequate mass transfer rate is the sole purpose. In this study, the crucial role of the central impeller type (including Rushton turbine, pitched blade turbine, elephant ear impeller and their combinations) on the performance of an aerated bioreactor equipped with two central impellers and an anchor on the local and overall volumetric mass transfer coefficient was investigated at a variety of operating conditions such as non-Newtonian solution concentrations, and central and anchor impeller speeds. Carboxymethyl cellulose (CMC) solution was used as a power-law fluid. Through the use of electrical resistance tomography, simplified dynamic pressure method, and computational fluid dynamics, several aspects of the aerated coaxial mixer namely local and overall gas holdup and mass transfer were studied. The local mass transfer coefficients were measured using three dissolved oxygen sensors installed at three various heights of the bioreactor. The local gas holdup was also measured using tomography. The results showed the positive effect of the anchor impeller on the mass transfer coefficient up to 10 rpm for different coaxial mixing configurations with various impeller type combinations and CMC concentrations. Among different impeller types, the system with two Rushton turbines resulted in the highest volumetric mass transfer coefficient, which was attributed to the higher gas holdup provided by this configuration as well as the flow pattern generated by the coaxial mixer. Moreover, the local distribution of gas holdup and mass transfer coefficient demonstrated a reduction in both parameters from the bottom to the top of the bioreactor.

Parametric numerical study and optimization of mass transfer and bubble size distribution in a gas-liquid stirred tank bioreactor equipped with Rushton turbine using computational fluid dynamics

Abstract Designing and optimizing a bioreactor can be an especially challenging process. Computational modelling is an effective tool to investigate the effects of various operating parameters on bioreactor performance and identify the optimum ones. In this work, a computational fluid dynamics-population balance model (CFD-PBM) was developed to elucidate the effect of different geometrical and operating parameters on the hydrodynamics and mass transfer coefficient of a batch stirred tank bioreactor. The validated model was projected to predict the effect of different parameters including the gas flow rate, the impeller off-bottom clearance, the number of agitator blades, and rotational speed of the impeller on the velocity profiles, air volume fraction, bubble size distribution, and the local gas mass transfer coefficient (K l a) in the bioreactor. Air bubble breakup and coalescence phenomena were considered in all simulations. Factorial experimental design approach was employed to statistically investigate the impacts of the aforementioned operating and geometrical parameters on K l a and bubble size distribution in the bioreactor in order to determine the most significant parameters. This can give an essential insight into the most impactful factors when it comes to designing and scaling up a bioreactor.

The Economic Infeasibility of Salinity Gradient Energy via Pressure Retarded Osmosis

Pressure retarded osmosis (PRO) harvests the chemical potential difference between water sources of two different salinities and has been widely researched as a source of low carbon energy. This study identifies the cost-optimal design and operation using a nonlinear programming model to minimize the levelized cost of electricity (LCOE) of a PRO system for specified process and financial parameters. Our model includes a detailed finite difference module-scale model that considers the pressure drop along the module, nonconstant solution properties, local mass transfer coefficients, and equipment inefficiencies that previous analyses lacked. Under realistic case specifications and present-day costs, the expected median LCOE ranges from $2.37/kWh (seawater draw) to $0.11/kWh (Dead Sea draw). The optimistic case specifications, which assume significant improvements in process performance with no additional increase in cost, lower these median costs to $1.00/kWh (seawater draw) and $0.05/kWh (Dead Sea draw). These estimates exclude variable pretreatment costs and are likely to underestimate the actual LCOE. Membrane costs and maintenance are the primary cost components for all cases. Comparing the LCOE and capacity factor of PRO to other low carbon energy sources suggests that reductions in PRO component costs are unlikely to make PRO cost competitive with renewable energy technologies.