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Mass Transfer Rate (mass + transfer_rate)
Selected AbstractsHydrodynamic Modulation Voltammetry with a Dual Disk Chopped Flow-Microjet Electrode (CF-MJE)ELECTROANALYSIS, Issue 18 2003Nafeesa Simjee Abstract A novel form of hydrodynamic modulation voltammetry (HMV) is described, based on the periodic variation of mass transport in a microjet electrode (MJE) system, in combination with phase-sensitive detection techniques. In the configuration developed, a jet of solution is fired from a nozzle that is aligned directly over the surface of a dual disk Pt-Pt ultramicroelectrode (UME). The potential at each electrode is controlled separately. A rotating blade, positioned between the nozzle and the UME probe, is used to periodically interrupt flow to the electrode surface, resulting in modulation of the overall mass transfer rate between two defined extremes. The use of a dual disk UME enables two transport-limited current signals to be recorded simultaneously, one for the analyte of interest, and the other for a ,reference species' (oxygen for the studies described herein). The latter current response corresponds to the variation in mass transport rate in the chopped flow (CF) arrangement and is used as the signal for phase sensitive detection of the analyte current. Studies of potassium hexachloroiridate (III) [IrCl] oxidation in aqueous solution are used to demonstrate the capabilities of the technique. HMV in the CF-MJE arrangement allows quantitative concentration measurements, down to at least 5×10,7,M. [source] Measurement of mass transfer coefficient in an airlift reactor with internal loop using coalescent and non-coalescent liquid mediaJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 12 2004M Bla Abstract In this work the sulfite oxidation (SOM), dynamic pressure-step (DPM) and gassing-out (GOM) methods were compared for volumetric mass transfer coefficient measurement in an airlift reactor with internal loop. As a liquid phase both, non-coalescent and coalescent media were used. Among the methods discussed here, the mass transfer coefficient (kLa) values obtained by the DPM appear as the most reliable as they were found to be independent of oxygen concentration in the inlet gas, which confirmed the physical correctness of this method. The difference between data measured using air and oxygen was not higher than 10%, which was comparable to the scatter of experimental data. It has been found that the sulfite oxidation method yielded kLa values only a little higher than those obtained by the DPM and the difference did not exceed 10%. Up to an inlet gas velocity (UGC) of ,0.03 m s,1 the GOM using oxygen as a gas medium gave kLa values in fact identical with those obtained by the DPM. At higher flows of the inlet gas, the GOM yielded kLa values as much as 15% lower. The enhancement in oxygen mass transfer rate determined in non-coalescent media was estimated to be up to +15%, when compared with a coalescent batch. The experimental dependence of kLa vs the overall gas hold-up was described by an empirical correlation.1 Copyright © 2004 Society of Chemical Industry [source] Influence of acid treatment on the surface activity and mass transfer inhibition of a splittable surfactantJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 7 2004Yuh-Lang Lee Abstract A splittable surfactant, Triton SP-190, was used to evaluate the effects of acid treatment on the mass transfer rate of an extraction process and on the interfacial tension-lowering activity of a system containing this surfactant. Equilibrium and dynamic interfacial tensions at the interface of CCl4 and the aqueous phase containing surfactant were measured by using pendent drop tensiometry enhanced by video digitization. A single-drop extraction apparatus was used to obtain the extraction percentage of acetic acid from the dispersed CCl4 droplets to the aqueous phase. The results indicate that the inorganic acid treatment can inhibit the dynamic and equilibrium interfacial tension-lowering activity of Triton SP-190. The mass transfer resistance induced by the addition of Triton SP-190 can also be reduced by the pre-treatment of acid. The effectiveness of acid treatment on both properties was greater at low pH values, lower surfactant concentrations, and longer treatment times. With HCl treatment, the equilibrium interfacial tension was not able to increase to the value of a surfactant-free system, but approached a maximum value which was independent of the pH value, but dependent on surfactant concentration. On the contrary, the extraction percentage, which has decreased due to the presence of surfactant, can be recovered completely to that of a surfactant-free system by acid treatment. The acid-treatment time required to achieve a significant recovery of mass transfer rate was much longer than that required to recover the interfacial tension. The present results also demonstrate that the constituents contained in an acid-treatment system had different effectiveness in affecting the interfacial tension and mass transfer rate due to the different mechanisms involved. Copyright © 2004 Society of Chemical Industry [source] Kinetics of Solids Leaching During Rehydration of Particulate Dry VegetablesJOURNAL OF FOOD SCIENCE, Issue 3 2004A. MARABI ABSTRACT: Air-dried and freeze-dried carrots were rehydrated in a computerized mixing system, and the medium was analyzed for sugar content with high-performance liquid chromatography (HPLC) and for total organic carbon (TOC). Leaching of solids was significant at very short rehydration time. Understanding the mechanism of the leaching process could provide information required for simulation. TOC values were significantly higher than those derived by HPLC, indicating that other organic components were extracted. The difference between TOC and HPLC values varied with time, indicating the existence of a different mass transfer rate. Sugars and TOC values followed an exponential behavior. Quantifying solids leaching is important for modeling and simulation of the rehydration process and for product optimization. [source] Drying-induced birefringence of polyimide optical filmsAICHE JOURNAL, Issue 3 2010Po-Ju Chen Abstract Drying-induced birefringence of polyimide film was investigated. The polyimide solutions were coated and dried on two different types of substrates and then removed for optical measurements. The operating window, which was a region for stable and uniform film formation, was determined experimentally. The out-of-plane birefringence (OPBR) was found to increase with decreasing dry film thickness, and the increase became more significant for films less than 10 ,m thick. The experimental OPBR results were compared with the predictions of two theoretical models. The results agreed reasonably well with the one-dimensional model of Lei et al. (J Appl Polym Sci. 2001;81:1000-1013). On the other hand, a simple viscoelastic model, which is an extension of the elastic model of Croll (J Appl Polym Sci. 1979;23:847-858), could provide clear physical insight, but its applicability was somewhat limited. The effects of several variables such as elasticity number, solidification concentration, yield stress, and mass transfer rate on OPBR were examined. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] Explaining the enhanced performance of pulsed bioreactors by mechanistic modelingAICHE JOURNAL, Issue 5 2008Amaya Franco Abstract In this work, steady-state mass balance based models were applied to two UASB reactors and three UAF for a better understanding of the role of pulsation on the efficacy improvement. Models were defined taking into account the hydraulic behavior of each digester and the limiting mechanism of the overall process kinetics (mass transfer or biochemical reaction rate). The application of the model allows to identify that mass transfer was the controlling step in all the reactors, except for the nonpulsed UASB, where methanogenic activity controlled the reactor performance in the last operation steady states. Mass transfer coefficients were higher for pulsed reactors and, in general, a good agreement between those estimated by an empirical correlation and from the model was obtained. Damköhler number values supported that the external mass transfer resistance was not negligible with respect to the process kinetic and in addition, in most cases, it controls the overall process in the reactors. The relative importance of external and internal mass transfer rate was calculated through the Biot number. The values of this dimensionless module indicated that external transport was the main contributor to overall mass transfer resistance. © 2008 American Institute of Chemical Engineers AIChE J, 2008 [source] Effect of hydrodynamic multiplicity on trickle bed reactor performanceAICHE JOURNAL, Issue 1 2008Werner van der Merwe Abstract Multiple hydrodynamic states in trickle bed reactors have been the subject of numerous hydrodynamic investigations. The extent of variation in the hydrodynamic parameters (like holdup and pressure drop) is large and this variation can be expected to have a significant impact on the conversion in a reaction system. This study presents reaction data for ,-methyl styrene hydrogenation in a trickle bed reactor over a range of conditions that include gas and liquid limitations. It is seen that liquid flow rate variation induced hysteresis has a large impact on the conversion. For gas-limited reactions, the upper branch of the pressure drop hysteresis loop has a higher conversion than the lower branch at the same linear fluid velocities and catalyst weight, while for liquid-limited reactions the lower branch has a higher conversion than the upper branch (the difference in productivity being up to 20%). These trends cannot be explained by differences in wetting efficiency. Instead, it is proposed that for this system the gas,liquid mass transfer rate is the limiting step in gas-limited reactions, while the liquid,solid mass transfer rate is the limiting step in liquid-limited reactions. © 2007 American Institute of Chemical Engineers AIChE J, 2008 [source] Modeling of Semibatch Esterification Process for Poly(ethylene terephthalate) SynthesisMACROMOLECULAR REACTION ENGINEERING, Issue 4 2007Himanshu Patel Abstract The esterification kinetics of terephthalic acid (TPA) and ethylene glycol (EG) in poly(ethylene terephthalate) (PET) synthesis were studied using a semibatch reactor. Rate constants were optimized by data fitting with the oligomeric chain length, the fraction of carboxyl groups in the terminal groups (,) and the water generation curve for different EG/TPA feed ratios. The influence of the TPA particle size distribution on the solid-liquid mass transfer rate and on acid conversion (,) was investigated. It was observed that conversion became more sensitive towards TPA particle size as the EG/TPA feed ratio was lowered. It is advantageous to use the model based on TPA particle size for mass transfer limited esterification reactors. The effect of the monomer feed ratio on conversion, chain length and system heterogeneity can be predicted with this model. [source] On the evolution of the nova-like variable AE AquariiMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2002P. J. Meintjes Abstract A possible evolution for the enigmatic cataclysmic variable AE Aquarii is considered that may put into context the long orbital period and short white dwarf rotation period compared with other DQ Her systems. It has been shown that mass transfer could have been initiated when the secondary KIV,V star was already somewhat evolved when it established Roche lobe contact. In this initial phase the orbital period of the system was probably Porb,i, 8.5 h, and the white dwarf rotation period P*,i > 1 h. Mass transfer in the form of diamagnetic gas blobs will result in an initial discless accretion process, resulting in an efficient drain of the binary orbital angular momentum. Since the initial mass ratio of the binary was probably qi, 0.8, a high mass transfer rate and a slow expansion of the Roche lobe of the secondary star followed, accompanied by a fast expanding secondary following the mass loss. This could have resulted in the KIV,V secondary flooding its Roche surface, causing a run-away mass transfer of that lasted for approximately , during which time the binary expanded to an orbital period of approximately Porb, 11 h. During this phase the mass accretion rate on to the surface of the white dwarf most probably exceeded the critical value for stable nuclear burning , which could have resulted in AE Aqr turning into an ultrasoft X-ray source. The high mass transfer terminated when a critical mass ratio of qcrit= 0.73 was reached. Disc torques spun-up the white dwarf to a period close to 33 s within the time-scale before the high mass transfer shut down when qcrit was reached. The decrease in the mass loss of the secondary allowed it to re-establish hydrostatic equilibrium on the dynamical time-scale (fraction of a day). From this point when qcrit is reached the mass transfer and binary evolution proceed at a slower rate since mass transfer from the secondary star is driven by magnetic braking of the secondary on a time-scale , which is the same as the thermal time-scale tth, 6.3 × 107 yr, i.e. the time-scale on which the secondary shrinks to restore its perturbed thermal equilibrium after the high mass loss. The significantly lower mass transfer in this phase will result in mass ejection from the system. This propeller,ejector action erodes the rotational kinetic energy of the white dwarf, channelling it into mass ejection and non-thermal activity, which explains the non-thermal outbursts that are observed at radio wavelengths and occasionally also at TeV energies. [source] Bubble shape, gas flow and gas,liquid mass transfer in pulp fibre suspensionsTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 3 2010L. K. Ishkintana Abstract Gas,liquid mass transfer in pulp fibre suspensions in a batch-operated bubble column is explained by observations of bubble size and shape made in a 2D column. Two pulp fibre suspensions (hardwood and softwood kraft) were studied over a range of suspension mass concentrations and gas flow rates. For a given gas flow rate, bubble size was found to increase as suspension concentration increased, moving from smaller spherical/elliptical bubbles to larger spherical-capped/dimpled-elliptical bubbles. At relatively low mass concentrations (Cm,=,2,3% for the softwood and Cm,,,7% for the hardwood pulp) distinct bubbles were no longer observed in the suspension. Instead, a network of channels formed through which gas flowed. In the bubble column, the volumetric gas,liquid mass transfer rate, kLa, decreased with increasing suspension concentration. From the 2D studies, this occurred as bubble size and rise velocity increased, which would decrease overall bubble surface area and gas holdup in the column. A minimum in kLa occurred between Cm,=,2% and 4% which depended on pulp type and was reached near the mass concentration where the flow channels first formed. le transfert de masse gaz-liquide dans des suspensions de fibres de pulpe, dans une colonne à bulles de traitement en lot, est expliqué par des observations faites dans une colonne 2D de la taille et de la forme des bulles. Deux suspensions de fibres de pulpe (pulpes kraft de bois dur et de bois tendre) ont été étudiées sur un intervalle de concentrations en masse des suspensions et sur un intervalle de débits de gaz. Pour un débit de gaz donné, on a observé que la taille des bulles augmentait avec l'augmentation de la concentration de la suspension, passant de petites bulles sphériques/elliptiques à des bulles plus grosses de forme quasi-sphérique avec capuchon à elliptique avec dépression. À des concentrations en masse relativement basses (Cm,=,2,3% pour le bois tendre et Cm,=,,7% pour la pulpe de bois dur), des bulles distinctes n'étaient plus observées dans la suspension. Au lieu de cela, un réseau de canaux se formait, au travers duquel le gaz s'écoulait. Dans la colonne à bulles, le taux de transfert de masse volumétrique gaz-liquide, kLa, diminuait avec l'augmentation de la concentration de la suspension. À partir des études 2D, cela se produisait lorsque la taille des bulles et la vélocité ascendante augmentaient, ce qui devrait faire diminuer la surface d'ensemble des bulles et la retenue de gaz dans la colonne. Un minimum de kLa a été observé avec Cm,=,2% et 4% (selon le type de pulpe) et était atteint à proximité de la concentration en masse pour laquelle les canaux d'écoulement commençaient à se former. [source] A Short Note on Steady State Behaviour of a Petlyuk Distillation Column by Using a Non-Equilibrium Stage ModelTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 3 2006Erika Fabiola Abad-Zarate Abstract A Petlyuk distillation column, considering equilibrium and non-equilibrium stage models, was studied. Rigorous simulations were conducted using Aspen PlusÔ RATEFRAC Module for the separation of ternary mixtures. According to the equilibrium model, the energy-efficient design of the Petlyuk column requires that the intermediate component be extracted from the maximum point in the composition profile in the main column. It was found that, for the intermediate component, mass transfer occurs from the vapour to the liquid phase from the top of the column to the stage where the side stream is extracted, from this point mass transfer occurs in the opposite direction. This point, considering the non-equilibrium model, corresponds to the stage in which the net mass transfer rate is zero. For the case of two segments per stage, it was found that the heat duties predicted by the equilibrium model are significantly lower than those obtained by using the non-equilibrium model, which is consistent with previous reported results. However, it is important to say that despite the higher energy duty predicted by the non-equilibrium model; both models predict significant energy savings. On a étudié une colonne de distillation de Petlyuk en considérant des modèles d'étage en équilibre et hors équilibre. Des simulations rigoureuses ont été menées au moyen du module RATEFRAC d'Aspen PlusÔ pour la séparation de mélanges ternaires. Selon le modèle en équilibre, une conception énergétiquement efficace de la colonne Petlyuk nécessite que le composant intermédiaire soit extrait du point maximum du profil de composition dans la colonne principale. On a trouvé que, pour le composant intermédiaire, le transfert de masse se produisait de la vapeur vers le liquide de la tête de la colonne jusqu'à l'étage où le courant secondaire est extrait; à partir de ce point le transfert de masse se produit dans la direction opposée. Ce point, en considérant le modèle hors équilibre, correspond à l'étage où le taux de transfert de masse net est nul. Dans le cas où il y a deux segments par étage, on a trouvé que les rendements thermiques prédits par le modèle en équilibre étaient significativement plus faibles que ceux obtenus à l'aide du modèle hors équilibre, ce qui est cohérent avec les résultats existants. Cependant, il est important de préciser que malgré le rendement énergétique plus élevé prédit par le modèle hors équilibre, les deux modèles fournissent des économies d'énergie significatives. [source] Some Issues on Core-Annulus and Cluster Models of Circulating Fluidized Bed ReactorsTHE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2002Hsiaotao T. BiArticle first published online: 19 MAY 200 Abstract The one-dimensional cluster model and the core-annulus model are examined based on existing correlations. The core-annulus model is found to give reasonable agreement with ozone decomposition data when the effective interphase mass transfer rate constant is equal to about 0.1 1/s, which is one order smaller than the reported values based on gas tracer tests. The prediction from the core-annulus model that the reactor performance decreases with increasing the riser diameter is found to be inconsistent with limited experimental data The one-dimensional cluster model predicts that a riser reactor performs very close to the pseudo-homogeneous plug flow reactor because of the high mass transfer rate between the cluster and the dilute phase. The improvement of model predictions lies in the better characterization of the cluster shape, size and the mass transfer rate between the cluster and the dilute phases. Le modèle de grappes unidimensionnel et le modèle c,ur-espace annulaire sont examinés d'après des corrélations existantes. On a trouvé que le modèle c,ur-espace annulaire décrivait raisonnablement bien les données de décomposition de l'ozone lorsque la constante de taux de transfert de matière entre phases réelle est égale à environ 0,1 s-1, ce qui est d'un ordre de grandeur plus petit que les valeurs obtenues d'après des tests par gaz traceurs. La prédiction du modèle c,ur-espace annulaire selon laquelle la performance du réacteur diminue avec l'augmentation du diamètre de la colonne montante s'avère non cohérente avec les quelques données expérimentales. Le modèle de grappes unidimensionnel prédit que la performance d'un réacteur à colonne montante est très proche de celle d'un réacteur à écoulement piston pseudo-homogène du fait du taux de transfert de matière élevé entre la grappe et la phase diluée. L'amélioration des prédictions du modèle repose sur une meilleure caractérisation de la forme des grappes, de leur taille et du taux de transfert de matière entre les grappes et les phases diluées. [source] Efficiency of non-reactive isothermal bubble column based on mass transferASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 4 2008Subrata Kumar Majumder Assistant Professor Abstract Bubble column reactor as a gas,liquid contactor is extensively used in the chemical and biochemical industries. Mass transfer coefficients governing the transport processes in bubble contactors are a growing concern in chemical and biotechnological processes whose rates are often limited by the mass transfer rate. The influences of different physical, dynamic and geometric variables affect the efficiency characterization of the equipment which are involved in gas/liquid mass transfer processes. This characterization has great importance to optimize the process plant design. In this article, efficiency of two-phase mass transfer in bubble column reactors has been analyzed based on dynamic, geometric and physical variables of the system. An empirical correlation for mass transfer efficiency has also been developed in terms of those variables. The present analysis on the gas,liquid mass transfer efficiency of bubble column may give insight into a further understanding and modeling of multiphase reactors in industrial applications. Copyright © 2008 Curtin University of Technology and John Wiley & Sons, Ltd. [source] Effect of particle,particle shearing on the bioleaching of sulfide mineralsBIOTECHNOLOGY & BIOENGINEERING, Issue 3 2002N. Chong Abstract The biological leaching of sulfide minerals, used for the production of gold, copper, zinc, cobalt, and other metals, is very often carried out in slurry bioreactors, where the shearing between sulfide particles is intensive. In order to be able to improve the efficiency of the bioleaching, it is of significant importance to know the effect of particle shearing on the rate of leaching. The recently proposed concept of ore immobilization allowed us to study the effect of particle shearing on the rate of sulfide (pyrite) leaching by Thiobacillus ferrooxidans. Using this concept, we designed two very similar bioreactors, the main difference between which was the presence and absence of particle,particle shearing. It was shown that when the oxygen mass transfer was not the rate-limiting step, the rate of bioleaching in the frictionless bioreactor was 2.5 times higher than that in a bioreactor with particle friction (shearing). The concentration of free suspended cells in the frictionless bioreactor was by orders of magnitude lower than that in the frictional bioreactor, which showed that particle friction strongly reduces the microbial attachment to sulfide surface, which, in turn, reduces the rate of bioleaching. Surprisingly, it was found that formation of a layer of insoluble iron salts on the surface of sulfide particles is much slower under shearless conditions than in the presence of particle,particle shearing. This was explained by the effect of particle friction on liquid,solid mass transfer rate. The results of this study show that reduction of the particle friction during bioleaching of sulfide minerals can bring important advantages not only by increasing significantly the bioleaching rate, but also by increasing the rate of gas,liquid oxygen mass transfer, reducing the formation of iron precipitates and reducing the energy consumption. One of the efficient methods for reduction of particle friction is ore immobilization in a porous matrix. © 2002 Wiley Periodicals, Inc. Biotechnol Bioeng80: 349,357, 2002. [source] Simultaneous extraction and concentration of penicillin G by hollow fiber renewal liquid membraneBIOTECHNOLOGY PROGRESS, Issue 2 2009Zhongqi Ren Abstract In this article, hollow fiber renewal liquid membrane (HFRLM) technique was used for recovery of penicillin G from aqueous solution. The organic solution of 7 vol % di-n-octylamine (DOA) + 30 vol % iso-octanol + kerosene was used as liquid membrane phase, and Na2CO3 aqueous solution was used as stripping phase. Experiments were performed as a function of carrier concentration in the organic phase, organic/aqueous volume ratio, pH, and initial penicillin G concentration in the feed phase, pH in the stripping phase, flow rates, etc. The results showed that the HFRLM process was stable and could carry out simultaneous extraction and concentration of penicillin G from aqueous solutions. As a carrier facilitated transport process, the addition of DOA in organic phase could greatly enhance the mass transfer rate; and there was a favorable organic/aqueous volume ratio of 1:20 to 1:30 for this system. The mass transfer flux and overall mass transfer coefficient increased with decreasing pH in the feed phase and increasing pH in the stripping phase, because of variation of the mass transfer driving force caused by pH gradient and distribution equilibrium. The flow rate of the shell side had significant influence on the mass transfer performance, whereas the effect of flow rate of lumen side on the mass transfer performance was slight because of the mass transfer intensification of renewal effect in the lumen side. The results indicated that the HFRLM process was a promising method for the recovery of penicillin G from aqueous solutions. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009 [source] Continuous bioremediation of phenol-polluted air in an external loop airlift bioreactor with a packed bed,JOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 6 2006Hossein Nikakhtari Abstract An external loop airlift bioreactor with a small amount (99% porosity) of stainless steel mesh packing inserted in the riser section was used for bioremediation of a phenol-polluted air stream. The packing enhanced volatile organic chemical and oxygen mass transfer rates and provided a large surface area for cell immobilization. Using a pure strain of Pseudomonas putida, fed-batch and continuous runs at three different dilution rates were completed with phenol in the polluted air as the only source of growth substrate. 100% phenol removal was achieved at phenol loading rates up to 33 120 mg h,1 m,3 using only one-third of the column, superior to any previously reported biodegradation rates of phenol-polluted air with 100% efficiency. A mathematical model has been developed and is shown to accurately predict the transient and steady-state data. Copyright © 2006 Society of Chemical Industry [source] Terminal and transient drop rise velocity of single toluene droplets in waterAICHE JOURNAL, Issue 1 2010Mirco Wegener Abstract The knowledge of the drop rise velocity in dispersed systems is of fundamental importance. Especially, the residence time is needed for calculation of mass transfer rates in extraction columns. This work deals with fluid dynamic measurements of toluene droplets rising in water ranging from 1.0 to 7.0 mm, with the premise of high purity of the used chemicals. The toluene/water-system is widely used as a test system with high interfacial tension. A semiempirical correlation for pure systems to predict the terminal velocity of single rising/falling droplets based on experimental data is presented. Results show that a distinction between maximum and characteristic mean values of the drop rise velocity is necessary, especially in the diameter range 2.4,3.0 mm where unexpected velocity fluctuations occur. Two distinct terminal rise velocities were observed for 3 mm droplets. Furthermore, comparisons of the Weber-Reynolds-correlation and the drag coefficient with correlations from literature show good agreement. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Polymer hydrogenation in pulsed flow systems with extrusionAICHE JOURNAL, Issue 4 2008Alan Bussard Abstract The hydrogenation of poly(styrene) over a Pd/Al2O3 catalyst was studied in reactors where pulsed flows are present due to both extrusion and forced pulsing. The reaction was investigated over a range of flow rates, polymer concentrations, and pulsing conditions. Observed activities were highly related to gas-to-liquid mass transfer rates predicted by a correlation for slug flow in catalyst monoliths. A reactor fed by a liquid-starved extruder is an attractive choice for hydrogenation at low polymer concentrations, where intrinsic reaction rates are approached. Higher polymer concentrations (10 wt %) lead to higher mass transfer resistances and a decrease in observed activity. But in this case forced pulsing has a greater effect on productivity; an optimum pulsing frequency was observed. Selectivity was higher and power input lower than in a stirred tank at comparable conditions. The optimal frequency is higher than those found in previous work on hydrogenations. © 2008 American Institute of Chemical Engineers AIChE J, 2008 [source] Prediction of gas sorption kinetics for porous media using MRIAICHE JOURNAL, Issue 9 2006Matthew J. Watt-Smith Abstract Diffusion and reaction within porous media involving condensable vapors are important processes in catalysis, fuel cells, and membrane separations. In this work, 3-D maps of the spatial variation of porosity, pore size and network tortuosity within a porous solid, derived from magnetic resonance images, have been used to construct a structural model for a mesoporous catalyst pellet. Simulations of the kinetic uptake, adsorption and capillary condensation of butane vapor within the porous solid, conducted on the structural model, have successfully predicted experimental measurements of the effects of the onset of capillary condensation on mass transfer rates without the need of the various adjustable parameters prevalent in other models. These findings suggest that accurate mathematical models for both the complex void space of the porous medium, and the mass transport processes taking place within it, have been successfully developed. © 2006 American Institute of Chemical Engineers AIChE J, 2006 [source] Gas dynamics of the central few parsec region of NGC 1068 fuelled by the evolving nuclear star clusterMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2010M. Schartmann ABSTRACT Recently, high-resolution observations with the help of the near-infrared adaptive optics integral field spectrograph Spectrograph for INtegral Field Observations in the Near Infrared (SINFONI) at the Very Large Telescope proved the existence of massive and young nuclear star clusters in the centres of a sample of Seyfert galaxies. With the help of three-dimensional high-resolution hydrodynamical simulations with the Pluto code, we follow the evolution of such clusters, especially focusing on stellar mass loss feeding gas into the ambient interstellar medium and driving turbulence. This leads to a vertically wide distributed clumpy or filamentary inflow of gas on large scales (tens of parsec), whereas a turbulent and very dense disc builds up on the parsec scale. In order to capture the relevant physics in the inner region, we treat this disc separately by viscously evolving the radial surface density distribution. This enables us to link the tens of parsec-scale region (accessible via SINFONI observations) to the (sub-)parsec-scale region (observable with the mid-infrared interferometer instrument and via water maser emission). Thereby, this procedure provides us with an ideal testbed for data comparison. In this work, we concentrate on the effects of a parametrized turbulent viscosity to generate angular momentum and mass transfer in the disc and additionally take star formation into account. Most of the input parameters are constrained by available observations of the nearby Seyfert 2 galaxy NGC 1068, and we discuss parameter studies for the free parameters. At the current age of its nuclear starburst of 250 Myr, our simulations yield disc sizes of the order of 0.8,0.9 pc, gas masses of 106 M, and mass transfer rates of 0.025 M, yr,1 through the inner rim of the disc. This shows that our large-scale torus model is able to approximately account for the disc size as inferred from interferometric observations in the mid-infrared and compares well to the extent and mass of a rotating disc structure as inferred from water maser observations. Several other observational constraints are discussed as well. [source] Basic equations of mass transfer through biocatalytic membrane layerASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 3 2009Endre Nagy Abstract The diffusive and convective mass transport through biocatalytic membrane layer (membrane bioreactor), without and with biochemical reactions, have been investigated. Similarly to the two-film theory for gas-liquid system with diffusive mass transport, only, mass transfer rates have been developed for the biocatalytic membrane layer and for the concentration boundary layer on the feed side of the membrane. Overall mass transfer coefficient has also been defined involving the two-layer, simultaneous mass transfer, namely the mass transfer through the concentration boundary layer and biocatalytic membrane layer. The effect of the convective velocity (Pe number) and the biochemical reaction rate, namely first-order and zero-order reactions as limiting cases of the Monod kinetics, are demonstrated on the mass transfer coefficients accompanied by chemical reaction and on the concentration profiles. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source] Effects of ionic strength on lysozyme uptake rates in cation exchangers.BIOTECHNOLOGY & BIOENGINEERING, Issue 2 2005I: Uptake in SP Sepharose FF Fluorescence scanning confocal microscopy was used in parallel with batch uptake and breakthrough measurements of transport rates to study the effect of ionic strength on the uptake of lysozyme into SP Sepharose FF. In all cases the adsorption isotherms were near-rectangular. As described previously, the intraparticle profiles changed from slow-moving self-sharpening fronts at low salt concentration, to fast-moving diffuse profiles at high salt concentration, and batch uptake rates correspondingly increased with increasing salt concentration. Shrinking core and homogeneous diffusion frameworks were used successfully to obtain effective diffusivities for the low salt and high salt conditions, respectively. The prediction of column breakthrough was generally good using these frameworks, except for low-salt uptake results. In those cases, the compressibility of the stationary phase coupled with the shrinking core behavior appears to reduce the mass transfer rates at particle-particle contacts, leading to shallower breakthrough curves. In contrast, the fast uptake rates at high ionic strength appear to reduce the importance of mass transfer limitations at the particle contacts, but the confocal results do show a flow rate dependence on the uptake profiles, suggesting that external mass transfer becomes more limiting at high ionic strength. These results show that the complexity of behavior observable at the microscopic scale is directly manifested at the column scale and provides a phenomenological basis to interpret and predict column breakthrough. In addition, the results provide heuristics for the optimization of chromatographic conditions. © 2005 Wiley Periodicals, Inc. [source] Carbon Monoxide Mass Transfer for Syngas Fermentation in a Stirred Tank Reactor with Dual Impeller ConfigurationsBIOTECHNOLOGY PROGRESS, Issue 3 2007Andrew J. Ungerman This study compares the power demand and gas-liquid volumetric mass transfer coefficient, kLa, in a stirred tank reactor (STR) (T = 0.211 m) using different impeller designs and schemes in a carbon monoxide-water system, which is applicable to synthesis gas (syngas) fermentation. Eleven different impeller schemes were tested over a range of operating conditions typically associated with the "after large cavity" region (ALC) of a Rushton-type turbine (D/T = 0.35). It is found that the dual Rushton-type impeller scheme exhibits the highest volumetric mass transfer rates for all operating conditions; however, it also displays the lowest mass transfer performance (defined as the volumetric mass transfer coefficient per unit power input) for all conditions due to its high power consumption. Dual impeller schemes with an axial flow impeller as the top impeller show improved mass transfer rates without dramatic increases in power draw. At high gas flow rates, dual impeller schemes with a lower concave impeller have kLa values similar to those of the Rushton-type dual impeller schemes but show improved mass transfer performance. It is believed that the mass transfer performance can be further enhanced for the bottom concave impeller schemes by operating at conditions beyond the ALC region defined for Rushton-type impellers because the concave impeller can handle higher gas flow rates prior to flooding. [source] | |||||||||||||