Search

• Marangoni convection gradually transforms to Rayleigh convection without gas flow. • The drag force-induced circulation inhibits Marangoni and Rayleigh convections. • The suppressed Marangoni convection cells recur at high ammonia concentrations. • A calculation model for ammonia absorption into a moving droplet was derived. The mass transfer characteristics during ammonia absorption into a sessile water droplet in a small controlled-atmosphere wind tunnel were investigated using particle tracking velocimetry (PTV) with polystyrene microspheres. The results showed that the flow field inside the droplet transformed from Marangoni convection to Rayleigh convection in the absence of gas flow. As the gas velocity increased, the drag force-induced circulation inhibited the Marangoni convection in the initial stage of the mass transfer process and Rayleigh convection in the advanced stage. However, the suppressed Marangoni convection cells recurred at high initial ammonia concentrations. Compared with the initial ammonia concentration in the wind tunnel and droplet size, the gas velocity significantly increased the gas-phase mass transfer coefficient. Furthermore, a calculation model for ammonia absorption by a moving droplet was derived based on the Colburn analogy and the two-film theory. The gas-phase mass transfer coefficients calculated using the model coincided with the experimental results. These results will help the design of water curtains. [ABSTRACT FROM AUTHOR]

• Experimental data for SO 2 removal process via spray tower were produced. • A large set of data was collected to evaluate several process conditions. • Optimization presented higher removal efficiency and lower solution consumption. • New ANN model for SO 2 removal and k g a ¯ in spray tower was proposed. • ANNs are more precise than the usual correlations found in the literature. Spray towers to can be used to absorb pollutants gases. Due to the complexity of the process, to propose mathematical models for prediction of removal efficiency and volumetric mass transfer coefficient, is a difficult task. This work aimed to evaluate the influence of variables on the SO 2 removal process in a spray tower and to obtain an artificial neural network (ANN) to predict the removal efficiency and the volumetric mass transfer coefficient for columns with different heights, gas and liquid flow rates. The tower built in this study reached a maximum efficiency of 99.47% and showed that it was possible to achieve high levels of removal without the need to build high towers. The best ANN obtained presented error of 2.76% for the removal efficiency of the SO 2 and 6.79% for the volumetric mass transfer coefficient. The obtained results were very promising in the prediction of important parameters in the process of removing atmospheric pollutants via spray tower. [ABSTRACT FROM AUTHOR]

• The overall catalytic combustion kinetics of H 2 in air is studied. • Experimental data in low Peclet number region is generated (Pe 1.4 - 14). • The mass transfer coefficients were estimated for different Re p (7–77). • A correlation is developed in terms of Sh , Re p and S c for H 2 -air system. • Contribution of individual resistances was estimated at different Re p (7–77). The overall kinetics of catalytic combustion of hydrogen in air is decided by the contribution of the different resistances offered in individual steps such as external diffusion, internal diffusion and surface reaction. To estimate the contribution of external mass transfer resistance, empirical correlations in terms of dimensionless numbers viz. , Sherwood number, particle Reynold's number, and Schmidt number are used in general. The Schmidt number for hydrogen in air is approximately 0.2 and most of the mass transfer correlations in packed bed reactors are valid for Schmidt number higher than 0.6, which corresponds to that of water vapour in the air. The external mass transfer correlations applicable for hydrogen-air system are scarce. In the present study, the external mass transfer effects were decoupled from internal diffusion and intrinsic reaction by conducting experiments with different particle Reynolds numbers in a packed bed batch reactor with complete recycle. Further, the system is theoretically modeled and the external film mass transfer coefficients were estimated at different particle Reynold's numbers using experimental data. Based on the experimental data, a new mass transfer correlation has been proposed to determine the gas film mass transfer coefficient of hydrogen in the air in packed bed reactors, in terms of conventional dimensionless numbers i.e., Sherwood number, Schmidt number, and particle Reynolds number. Further, the contribution of external mass transfer resistance on the overall reaction kinetics was also estimated. [ABSTRACT FROM AUTHOR]

Highlights • An inverse relationship between the mass transfer and permeability was found. • Mass transport processes are strongly influenced by microstructural anisotropy. • Mass transfer correlations were presented for media with varying anisotropy. Abstract A numerical method for calculating the mass transfer coefficient in fibrous media is presented. First, pressure driven flow was modelled using the Lattice Boltzmann Method. The advection-diffusion equation was solved for convective-reacting porous media flow, and the method is contrasted with experimental methods such as the limiting current diffusion technique, for its ability to determine and simulate mass transfer systems that are operating at low Reynolds number flows. A series of simulations were performed on three materials; specifically, commercially available carbon felts, electrospun carbon fibers and electrospun carbon fibers with anisotropy introduced to the microstructure. Simulations were performed in each principal direction (x,y,z) for each material in order to determine the effects of anisotropy on the mass transfer coefficient. In addition, the simulations spanned multiple Reynolds and Péclet numbers, to fully represent highly advective and highly diffusive systems. The resulting mass transfer coefficients were compared with values predicted by common correlations and a good agreement was found at high Reynolds numbers, but less so at lower Reynolds number typical of cell operation, reinforcing the utility of the numerical approach. Dimensionless mass transfer correlations were determined for each material and each direction in terms of the Sherwood number. These correlations were analyzed with respect to each materials' permeability tensor. It was found that as the permeability of the system increases, the expected mass transfer coefficient decreases. Two general mass transfer correlations are presented, one correlation for isotropic fibrous media and the other for through-plane flow in planar fibrous materials such as electrospun media and carbon paper. The correlations are Sh = 0.879 Re0.402 Sc0.390 and Sh = 0.906 Re0.432 Sc0.432 respectively. [ABSTRACT FROM AUTHOR]

Numerical simulations of Taylor flow are carried out using the unit cell approach. The hydrodynamics of two-phase flows are computed using COMSOL Multiphysics software and a moving mesh approach to track the interface. Gas-liquid mass transfer is then solved, by varying the diffusion coefficient. Surprisingly, this work shows that the global Sherwood number S h ∞ plateaus with the Péclet number Pe when it is larger than 1000, which was not previously reported. Local Sherwood numbers for transfer through the film (S h b f) and the caps (S h b s) allow to distinguish their contributions. S h b f evolves as given by the falling film theory, whereas S h b s follows the penetration theory only at moderate Péclet number. A dimensionless correlation to predict S h ∞ gathers results from contrasted configurations: S h ∞ is mainly sensitive to both Pe (before saturation) and the gas hold-up ϵ G. The benefit of working with unit cells of small ϵ G in milli-reactors is therefore highlighted. • Mass transfer coefficient in Taylor flow is evaluated from CFD simulations. • Sherwood number increases then saturates at large Péclet number. • Sherwood number decreases with the gas hold-up in the unit cell. • The film contribution over that from the caps directly depends on the ratio of surfaces. • A scaling law for the global Sherwood number is proposed. [ABSTRACT FROM AUTHOR]

Highlights • Gas-liquid mass transfer for the Fischer-Tropsch synthesis was studied. • Conversion varied in the range 45–90% depending on choice of mass transfer model. • Conversion decreased linearly with increased inlet Sauter-mean diameter. • Bubble size dependency of the mass transfer coefficient showed effect on conversion. Abstract At high catalyst volume fractions the Fischer-Tropsch synthesis (FTS) operated in a slurry bubble column (SBC) is driven into the mass transfer limited regime. This study utilized literature models for the gas-liquid mass transfer coefficients in a multifluid-population balance model in which the gas-phase composition was a function of bubble size. The results confirmed that mass transfer limitations occur and that the choice of mass transfer coefficient model is crucial, yielding final conversion results ranging from 45% to 92% depending on the choice of k L models. At smaller k L values the composition is highly dependent on bubble size, whilst for the largest k L values the composition is not a function of bubble size at all. The population balance modeling (PBM) allowed for explicitly keeping track of the bubble size distribution. Varying the inlet Sauter-mean diameter (SMD) resulted in a linear decrease in conversion as the inlet SMD was increased from 5 mm to 20 mm. Illustrative models for the bubble size dependency of k L were implemented, which provided additional information compared to traditional models which use (bubble size) averaged values for the liquid-phase mass transfer coefficient k L and/or the gas-liquid interfacial area a and composition. [ABSTRACT FROM AUTHOR]

The knowledge of the influence of swarm effects on gas-liquid mass transfer is still limited. In this article, an optical measurement method for the identification of local mixing properties, mass flow rates, and mass transfer coefficients in the wake of rising bubbles with and without swarm turbulence is presented. Two-dimensional concentration data is temporally rearranged to reconstruct a quasi tri-dimensional concentration wake. By this means, the path of the bubble-wake-complex can now be visualized. The analysis shows that mixing time decreases strongly with superimposed swarm turbulence and becomes independent of bubble size. This is expected to have a major influence on chemical reactions. Especially slow parallel and consecutive reaction steps are promoted by poor mixing. Additionally based on locally measured data, mass transfer coefficients of bubbles rising in counter-current flow and with superimposed swarm turbulence are calculated. With the presented method, concentration distributions are now accessible in bubble flows with high spatiotemporal resolution. This paves the way for validating numerical simulations and developing physically determined models. [ABSTRACT FROM AUTHOR]

By combining the prominent advantage of directional fluid flow in an internal airlift loop reactor with the cheap cost of liquid-solid separation in a hydrocyclone, a new slurry reactor for process intensification integrating solid catalytic reactions with liquid-solid separation for clear liquid products is proposed and designed to cut down the capital and operating costs. The reactor can be operated properly when the superficial gas velocity is above 0.0108 m/s, and the solid particles of aluminum oxide are all retained in the slurry reactor if the particle diameter is larger than 57.9 μm. The influences of superficial gas velocity and top clearance on the performance of hydrodynamics and mass transfer in the gas-liquid two-phase and the gas-liquid-solid three-phase systems are measured and discussed systematically, and some data base for the rational design, optimization and scaling-up of this type of promising reactor is provided. [ABSTRACT FROM AUTHOR]

A series of Pd/zeolite/cordierite (zeolite = MCM-41, SBA-15, Beta, or MCM-22; cordierite = COR) monolithic catalysts, as well as Pd/ZIF-8/COR, Pd/Al 2 O 3 /COR, and Pd/SiO 2 /COR, were prepared, characterized, and evaluated for the hydrogenation of 2-ethylanthraquinone (eAQ) to 2-ethyl-9,10-anthrahydroquinone (eAQH 2 ). We found that a 0.8% Pd/MCM-41/COR catalyst exhibited the highest H 2 O 2 yield (7.54 g L −1 ) and selectivity (85.3%) toward active quinones for hydrogen peroxide (H 2 O 2 ) production using the anthraquinone method. Notably, the Pd efficiency (1573 gH 2 O 2 g −1 Pd h −1 ) for the 0.8% Pd/MCM-41/COR monolithic catalyst was much higher than that of a commercial pellet catalyst (500 gH 2 O 2 g −1 Pd h −1 ). The intrinsic kinetics of hydrogenation of eAQ over 0.8% Pd/MCM-41/COR was measured, and the kinetic equation parameters were incorporated into a computational fluid dynamics (CFD) model. The mass transfer coefficients for the monolithic catalysts are 5–20 times higher than the pelleted catalyst. In addition, we found that the gas-liquid mass transfer is the controlling step, showing the unique advantages of monolithic catalysts for process intensification. [ABSTRACT FROM AUTHOR]

In the industrial fermentation processes, most liquids are non-coalescent and often exhibit increased viscosity. However, due to the limitations of most measurement methods, there is a lack of reliable data for predicting volumetric mass transfer coefficients ( k L a ) for viscous batches, especially under high dissipated energies, as the accurate determination of oxygen concentration profile in viscous liquids is not as easy as in low viscosity ones. Our goal is to develop reliable technique for k L a determination in viscous liquids and to establish suitable correlation shapes to describe k L a data. We used the dynamic pressure method (DPM), the experimental set-up of which has been modified for the measurement in viscous liquids. Dissolved oxygen (DO) probes were placed in bypass measuring cells. This set up brings well defined transient characteristics of DO probes, which is crucial for correct k L a evaluation. Measurements were conducted in two phase multiple-impeller fermenters with a non-coalescent viscous Newtonian batch under a wide range of experimental conditions and in the apparatuses of two scales. Using pure oxygen as gas phase, it was confirmed that DPM yields k L a ’s independent of the driving force of absorption even in viscous batch. The improved set up of DPM enabled to use also optical DO probes as well as polarographic ones. It was confirmed that optical DO probe can be used for k L a values up to 0.4 s −1 . Based on the experimental data, correlations were developed to predict k L a in industrial fermenters. Standard correlation k L a = 2.99 ⋅ 10 −3 ⋅( P g / V L ) 0.891 v s 0.556 with standard deviation, SD, 30%, based on gassed power input P g and superficial gas velocity v s , has low standard deviation but it is scale specific. On the other hand, when the term of impeller tip speed ( ND ) is used instead of P g , predicted data exhibit neither over- nor under-estimation of k L a for particular apparatus scale; so the effect of the vessel scale is properly described using this term. In addition, the impeller power number P o was found to be a reliable predictor of k L a in a common correlation for various impeller types, when used together with the impeller tip speed term. The correlation k L a = 0.295⋅( ND ) 2.083 v s 0.461 P o 0.737 with SD 28% suggested in this work can be used for fairly accurate design of industrial fermenters. Both the experimental technique and the correlation shape are ready to be used to obtain the design tool for other batches with various viscosities. [ABSTRACT FROM AUTHOR]

The influence of gas/liquid mass transfer on the valorisation reaction of CO 2 with epoxides using epichlorohydrin in a stirred batch reactor was investigated in this study. The rate of absorption of CO 2 into the liquid was studied in a dynamic way between 50 and 90 °C and at different stirring speeds (100–500 rpm). Two different systems were observed: (i) CO 2 /epichlorohydrin and (ii) CO 2 /epichlorohydrin/catalyst, in order to compare and determine the mass transfer coefficient and the enhancement of the absorption when the latter takes place in the presence of a catalyst. The results suggest that the valorisation reaction followed a slow reaction regime in which the reaction did not occur in the mass transfer film but instead took place within the liquid bulk. Therefore, the reaction rate seemed to control the overall process, but according to the specific reaction regime found, both the liquid holdup and the interfacial area should be enhanced to increase the rate of the reaction and the efficiency of the process. Under the conditions studied in this work, the mass transfer coefficient of the liquid phase (k L a) was found to be in the range of 1.0 × 10 −3 to 1.0 × 10 −2 s −1 and the enhancement factor (E) was between 0.4 and 0.9. [ABSTRACT FROM AUTHOR]

Mechanical agitation or pulsation is usually introduced to a solvent extraction column to improve the efficiency. Recently a number of industrial scale pulsed disc and doughnut (PDD) solvent extraction columns have been found to have higher extraction efficiency while running with non-pulsing. In this study, the mass transfer performance of a PDD column was measured under pulsing and non-pulsing conditions using a 72.5 mm diameter disc and doughnut column. The effects of pulsation intensity and dispersed and continuous phase velocity on the mass transfer performance have been investigated. The results show that, under non-pulsing conditions, the overall mass transfer coefficient increased with increasing continuous phase flow rate and slightly decreased with increasing dispersed phase flow rate. With increasing pulsation intensity, the overall mass transfer coefficient decreased at first and then increased. Two correlations are proposed to predict the height of mass transfer unit and overall mass transfer coefficient in a PDD column including pulsing and non-pulsing conditions. [ABSTRACT FROM AUTHOR]

• Averaged macroscale models of mass transfer in gas–liquid aerated cultures were deduced. • Sets of closure problems defining closure parameters in the macroscale models were proposed. • Dispersion tensors of different phenomenological nature were obtained in the averaged models. • Closure problems for coefficients of axial liquid dispersion and mass transfer were discussed and solved. The upscaling of gas–liquid mass transfer and bioreaction in aerated cultures was performed within the volume averaging theory. Effective transport equations, including their associated closure problems, were derived for 1) the main substrates and biomass (non-transferable components), and 2) the gaseous substrate (transferable component). For the transferable component, two approaches were applied: 2a) the local mass equilibrium (one equation), and 2b) the two-fluid model (two equations). The nature of the dispersion tensors in the three averaged models is discussed. Numerical computations of the axial dispersion coefficient and the volumetric mass transfer coefficient were performed by solving the associated closure problems in unit cells (0.1 ≤ Re ≤ 300; 1 ≤ Sc ≤ 550; 0.1 ≤ Pe ≤ 300; Mo = 2.5 × 1011; 0.001 ≤ ε G ≤ 0.05). The results of both parameters agree with trends reported in literature; some differences in orders of magnitude for D Lzz are attributable to the mismatch between the experimental conditions and the occurrence of mesoscale interactions undetected in the computational cells. [ABSTRACT FROM AUTHOR]

Industrial catalyst development is benefiting from the use of high throughput research. This is typically accomplished by running large numbers of experiments in parallel, using common reactant feed manifolds and analytical systems. In order to fit within a reasonable laboratory footprint, high throughput reactors must be small, yet retain mass and heat transfer rates that allow gradientless operation. For heterogeneous catalytic reactions there are a number of high throughput systems available for testing powdered, or granular catalysts in banks of plug flow reactors, but no reports of CSTR systems capable of testing formed catalyst particles used in commercial fixed bed reactors. We describe the development and engineering characterization of a small prototype CSTR for testing whole catalyst particles. The reactor has inside dimensions of 6.35 cm diameter by 2.8 cm deep and shows well-mixed behavior, based on methane tracer studies. Sublimation of non-porous cylindrical naphthalene cylinders (7.5 mm diameter by 7 mm long) was used to characterize the solid–gas mass transfer coefficient. It was found that the prototype reactor exhibits mass transfer rates similar or higher than reported in the literature for the much larger traditional Carberry and Berty-style reactors. This makes the new device suitable for further development of high throughput arrays of CSTRs for testing commercial catalyst pills. [ABSTRACT FROM AUTHOR]

The removal and enrichment of aliphatic amines with low molecular weights from their individual aqueous solutions was investigated theoretically and experimentally via a hollow-fiber supported-gas-membrane (SGM) process. Aqueous solutions containing 200–5000 mg L −1 amine were tested as feed, and an aqueous solution of 10 wt% sulfuric acid was used as an absorbing solution. Amines such as methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine and triethylamine, as well as ammonia, were tested in the SGM process. The experimental data demonstrated that the overall mass transfer coefficient was a strong function of the polarity of the amine or amine/water volatility and was not a simple function of the molecular weight or boiling point of amines; the order was determined to be trimethylamine>ammonia>triethylamine>diethylamine>ethylamine>dimethylamine>methylamine. The influences on mass transfer coefficients of the feed-in temperature, feed-in concentration, feed flow rate, and the concentration of NaOH pre-added to the feed were also investigated. Among these operating factors, the feed-in temperature and NaOH concentration were crucial; increasing the feed-in temperature and the NaOH concentration led to a significant increase in the mass transfer coefficient, especially when the amine concentration in the feed was low. Mathematical models incorporating laminar flow, ion and molecular diffusion, dissociation equilibrium and vapor–liquid equilibrium were established and solved numerically. When the surface tension of the feed solution was >45 mN m −1 , the SGM process demonstrated good stability during a test period of at least 30 days. Greater than 95% of the amine was recovered, and the amine could be enriched by >20 times in the absorbing solution. Thus, this SGM-based separation process is suitable to remove, recover, and concentrate amines from their aqueous solutions. [ABSTRACT FROM AUTHOR]

This paper presents a model extension to dual-layer monolithic catalysts from a one-dimensional (1D) ( Balakotaiah, 2008; Joshi et al., 2009 ) mathematical model used for single-layer monolithic catalysts. The novel transient non-isothermal model computes average concentrations inside the bulk flow and in each layer of washcoat at every computational grid point along the monolith channel. The model has the capability that physical (e.g. diffusivity, pore radius, and thickness) and chemical (reaction kinetics) properties of the two layers of the washcoat can be different from each other. The relatively low computational cost offers real-time simulations of multi-layer monolithic catalysts. Analytically derived expressions for transverse mass transfer coefficients between the bulk flow and washcoat layers are presented for single-layer and dual-layer catalysts with circular cross-section, which can be easily extended for multi-layer catalysts. In addition, three expressions are presented for calculation of three internal Sherwood numbers, which are used in the dual-layer model, based on the values of Thiele moduli of the washcoat layers. These expressions are especially useful for calculation of the mass transfer coefficients for dual-layer catalysts with non-circular cross-sections. Application limitations of the 1D multi-layer model with respect to physical and chemical operating conditions are discussed. The derived model is applied for an automotive aftertreatment catalyst and the results of this model are compared with the results of a detailed 2D model (selective catalytic reduction/platinum group metals dual-layer catalysts). [ABSTRACT FROM AUTHOR]

Results for the rate of absorption of carbon dioxide gas into a pneumatic (i.e. continuously rising) foam formed from a sodium carbonate–bicarbonate buffer solution stabilised by a polyglycol frother are presented. Previous studies upon the use of gas–liquid foam for gas absorption operations have demonstrated limited potential for the technique, mainly because the liquid films rapidly become saturated with the absorbing gas due to the relatively low liquid fraction within the foam. However, by adding washwater to the free surface of the foam we created a wetter and more stable foam that (1) exhibits a high liquid fraction and therefore avoids film saturation, and (2) creates greater liquid advection past the gas-liquid surfaces, thereby enhancing the mass transfer coefficient. The interfacial area achieved within the foam was in the range 2100–3200 m 2 m −3 (i.e. much higher than in conventional gas-liquid contactors and the liquid-side mass transfer coefficient was the same magnitude as that achieved in packed beds (i.e. 4–8×10 –5 m s −1 ). Indeed, by modelling the foam as a packed-bed of solid spheres (with no adjustable constants), the mass-transfer coefficient in a bed of closely packed solid spheres was found to be in satisfactory agreement with the mass-transfer measured in the wet gas-liquid foam. The hydrodynamic state and the slip velocity between gas and liquid phases is required for the design of wet foam absorption columns, and this can be estimated using a theory of pneumatic foam ( Stevenson, 2007 ). [ABSTRACT FROM AUTHOR]

• Study of oxygen desorption in model wine and wine. • Desorption k L a and k L measured for a variety of model wines and wines. • Presence of protein in model wines reduced k L a and k L. • Reducing the protein concentration in wine increased k L a and k L. Removal of dissolved oxygen through desorption is commonly performed in winemaking, however, industry reports that this process varies for different wines, adding complexity to standard operating procedures. In order to better understand the factors inducing these differences, and therefore allow for better operation industrially, this article examines the mass transfer of six wines and three model wine solutions by evaluating the oxygen desorption volumetric mass transfer coefficient (k L a) , the Sauter mean bubble diameter (D 32), the interfacial area, and the oxygen mass transfer coefficient (k L). The k L for different wines was found to vary significantly (between 0.015 and 0.05 mm/s), while there was no significant variation in the D 32 and the interfacial area. Results after adding proteins (yeast extract) to model wine, or reducing protein from real wine (using bentonite) suggest that proteins reduce k L , and that their presence and concentration contribute to some of the variation in desorption of oxygen. [ABSTRACT FROM AUTHOR]

Abstract: The present paper reports an original, predominantly experimental study of the mass transfer efficiency in biofilters. Hydrodynamic and mass transfer parameters were investigated at two different scales. These parameters have been first considered (i) at a global scale in a three-phase fixed bed reactor with investigating the influence of the physic chemical properties of liquid phase and then (ii) from a local point of view by focusing at the bubble/packing contact. Experiments at global scale have been conducted in a semi-industrial scaled reactor (4.5m height, 0.15m in diameter) operating in batch mode, in co-current gas–liquid upflow. Air was injected at the bottom of the reactor through porous disc diffusers. Two kinds of packing, Meteor and Biolite, which have been thoroughly studied by Garcia-Maldonado et al. (2008), have been tested and compared. The impact of the liquid phase was investigated for different solutions containing clear water and some additives (salts, sugar, suspended solids and varied pH) that can be encountered in industry. For each liquid phase tested, gas holdup, pressure drop, slip velocities and bubble sizes were estimated, as well as volumetric mass transfer coefficients under different superficial gas velocities (ranging from 2.3×10–3 ms−1 to 2.9×10−2 ms−1). For all the tested cases, variations in the hydrodynamic behavior were observed with increasing superficial gas velocity and with all the compounds added to the liquid phase. Mass transfer coefficients decreased with all the tested compounds except for low concentrations of salts, acid and basic solution. Local-scale experiments were performed in a 2D cell made of PMMA, with a height of 200mm, width of 100mm and thickness of 2mm, to investigate the visualization of mass transfer and hydrodynamics in the axial profile of bubbles rising through a fixed bed. A high speed camera was used with an oxygen sensitive dye to visualize oxygen transfer and Kalliroscope particles to visualize bubble hydrodynamics. A specific approach was proposed for estimating the mass transfer coefficient in such a configuration. It was found that the mass transfer coefficient k L depended on bubble behavior through the packing. Low porosity of packing, bubble size and velocity were the principal parameters influencing the hydrodynamics and mass transfer coefficients at this scale. This study considered a new approach to obtain precise data on biofilter systems, investigate the hydrodynamics and gas–liquid mass transfer at two scales, enriching the database on biofilters and providing new insights that could improve this system in industry. [Copyright &y& Elsevier]

Abstract: Heat-exchanger reactors are an important part of process intensification technology. For plate geometries, one solution for intensifying transfer and increasing residence times is to construct two-dimensional meandering channels. Supported by this scientific context, the present work aims at characterising gas–liquid mass transfer in the same square millimetric meandering channel, as in , this constituted the preliminary step required for performing exothermic gas–liquid reactions. Firstly, the gas–liquid hydrodynamics were characterised for a water/air system. When compared to a straight channel of identical compactness and sectional-area (2×2mm2), the meandering channel induced (i) a delay in the transition from Taylor to annular-slug regimes, (ii) a rise of 10–20% in bubble lengths while conserving almost identical slug lengths, (iii) higher deformations of bubble nose and rear due to centrifugal forces (bends). Secondly, an original method for verifying the relevancy of the plug flow model and accurately determining k l a was used (measurements of concentrations in dissolved oxygen along the channel length). For the Taylor flow regime, k l a increased coherently when increasing j g , and the meandering geometry had a small influence. On the contrary, this effect was found no more negligible for the slug-annular flow regime. Whatever the channels, the NTU l remained low, thus showing that, even if millimetric channels allowed to intensify k l a, a special attention should be paid for generating sufficient residence times. At identical compactness, the meandering channel was found to be the most competitive. Finally, results on gas–liquid interfacial areas and mass transfer coefficients were confronted and discussed with respect to the predictions issued from the model developed by . [Copyright &y& Elsevier]

Abstract: One of the operational challenges associated with membrane bioreactors (MBRs) is the fouling of the membranes. In tubular side-stream MBRs, fouling reduction can be achieved through controlling the hydrodynamics of the two-phase slug flow near the membrane surface. The two-phase slug flow induces higher shear stresses near the membrane surface, which generate high mass transfer coefficients from the surface to the bulk region. However, measuring the mass transfer coefficient is difficult in complex heterogeneous mixtures like activated sludge and existing techniques (e.g. electrochemical methods) cannot be applied directly. As an alternative, in this work, a multidisciplinary approach was selected, by exploiting dimensionless analysis using the Sherwood number. Mass transfer coefficients were measured at various superficial velocities of gas and liquid flow in a tubular system. Due to the variability of the mass transfer coefficient obtained for each experimental condition, the results were compiled into, mass transfer coefficient histograms (MTH) for analysis. A bimodal MTH was observed, with one peak corresponding to the mass transfer induced by the liquid flow, and the other peak induced by the gas flow. It was noted that coalescence of bubbles affects the MTH. Coalescence increased the “width” of the peaks (i.e. the estimate of the variability of the mass transfer coefficient) and the height of the peak (i.e. amount of time that a mass transfer coefficient of a given value is maintained). A semi-empirical relationship based on the Lévêque relationship for the Sherwood number (mass transfer coefficient) was formulated for the laminar regime. A test case comparison between water and activated sludge was performed based on full-scale airlift MBR operational conditions. It was found that the Sherwood number in the non-Newtonian case is 8% higher than that in the Newtonian case. [Copyright &y& Elsevier]

Abstract: Steady state simulation of the hydrodynamics and mass transfer in bubbly flow in an axisymmetric internal airlift loop reactor (IALR) with the special treatment of interphase decoupling and outlet boundary conditions is performed, which is based on a Favre averaging two-fluid model for multiphase flow. Different models of mass transfer coefficient are compared and validated with the literature data. A model for describing the mass transfer for a wide range of superficial gas velocity is chosen and the predicted mass transfer coefficient agrees well with the experimental data in the literature. The numerical procedure can be used as a tool for the design and scale-up of IALRs. [Copyright &y& Elsevier]

Abstract: The impeller power and volumetric mass transfer coefficient were measured in a pilot-plant single-, double- and triple-impeller vessels of inner diameter 0.6m. The experimental conditions corresponded with those used earlier in geometrically similar laboratory scale vessel of inner diameter 0.29m [Fujasová et al., 2007, Chem. Eng. Sci. 62, 1650–1669]. The same impeller types and their combinations were used as well as the experimental techniques and forms of the data treatment/correlations, which distinguish bottom and upper section behaviour. Concretely, 23 combinations of the following impeller types were used: Rushton turbine (RT), six-pitched-blade impeller pumping upwards (PBU) and downwards (PBD), Lightnin A315 (LTN) impeller, and Techmix 335 pumping upwards (TXU) and downwards (TXD). Distilled water, representing a low-viscosity coalescent batch, was used as the liquid phase. It was found that the correlations established on the basis of the laboratory scale data might be used to describe the transport characteristics in the pilot-plant vessel. The more precise correlations, based on the data from both the laboratory and the pilot-plant scale vessels have also been established. The specific powers dissipated by impellers under gassed conditions (Pg ) were within the interval from 10 to 8500Wm−3 in the experiments. General correlations of the relative power down under aeration (Pg /P 0) are presented separately for the bottom and upper sections of the vessel. kLa were measured by dynamic pressure method in the individual vessel sections simultaneously. Their values moved within the interval from 0.002 to 0.21s−1. The best fit provided correlating the single- and the multi-impeller (double and triple) vessels data separately. Correlation of the kLa data measured in the middle height of the triple-impeller vessel, the method often used in literature, is also included. Of the triple-impeller configurations, 3RT gave the best mass transfer performance. The configurations utilizing the same impeller type have shown that the radial flow impellers provide higher (20 up to 50%) mass transfer coefficients than the axial flow impellers. The combined configurations (i.e., those with an RT impeller in the bottom section) do not achieve the mass transfer performance of 3RT. The kLa values produced by RT+2PBD and RT+2PBU were only 15–20% lower than those achieved using 3RT at the same power input. The 3LTN and RT+2LTN configurations provided the poorest mass transfer coefficients at the same power input, both being up to 40% lower than those of 3RT. [Copyright &y& Elsevier]