External Mass Transfer Limitations Research Articles

Theoretical and experimental evaluation of mass transfer limitation in gas phase dehydration of glycerol to acrolein over supported HSiW catalyst

Experimental and theoretical studies were performed to evaluate the existence of internal and external mass transfer limitations in the gas phase glycerol dehydration to acrolein over supported HSiW catalyst. In the experimental method, the internal and external diffusions were determined by varying the gas mass flow-rate (mg), catalyst pellet size (dp), and catalytic bed volume (Vcat). In the theoretical approach the dimensionless parameters such as effectiveness factor (η), Thiele modules (φ1), and overall effectiveness factor (Ω) were calculated. The experimental results indicated no external mass transfer limitation due to constant glycerol conversion (94–97%) at various conditions. In addition, both the theoretical and experimental approaches confirmed no internal mass transfer limitation in glycerol dehydration reaction with catalyst pellet sizes of dp = 2–4 and 5–7 µm due to effectiveness factor equal to 1 (η = 1). Calculation of the overall effectiveness factor (Ω) also confirmed the absence of external and internal diffusion in presence of catalysts with pellet size of dp < 13–17 µm.

Biosorption of the Reactive Blue 5G Dye in a Fixed Bed Column Packed with Orange Bagasse: Experimental and Mathematical Modelling

The removal of reactive dye in a fixed bed column packed with orange bagasse was modelled using a model that considers the effects of axial dispersion, external and internal mass transfer limitations, and the equilibrium in the liquid-solid interface. The equilibrium and kinetic parameters were obtained through fit the model to the experimental data. In the parameters identification procedure the PSO optimization method was used. The mathematical model showed good performance when describing the dye adsorption process. Furthermore, orange bagasse is an attractive and cost-effective alternative as an adsorbent to remove reactive blue 5G dye from wastewaters.

Excess surface area in bioelectrochemical systems causes ion transport limitations.

We investigated ion transport limitations on 3D graphite felt electrodes by growing Geobacter sulfurreducens biofilms with advection to eliminate external mass transfer limitations. We characterized ion transport limitations by: (i) showing that serially increasing NaCl concentration up to 200 mM increased current linearly up to a total of +273% vs. 0 mM NaCl under advective conditions; (ii) growing the biofilm with a starting concentration of 200 mM NaCl, which led to a maximum current increase of 400% vs. current generation without NaCl, and (iii) showing that un-colonized surface area remained even after steady-state current was reached. After accounting for iR effects, we confirmed that the excess surface area existed despite a non-zero overpotential. The fact that the biofilm was constrained from colonizing and producing further current under these conditions confirmed the biofilms under study here were ion transport-limited. Our work demonstrates that the use of high surface area electrodes may not increase current density when the system design allows ion transport limitations to become dominant.

Evaluation of Presulfided NiMo/γ-Al2O3 for Hydrodeoxygenation of Microalgae Oil To Produce Green Diesel

In the present work, reduced presulfided NiMo/γ-Al2O3, the conventional hydrotreating catalyst, was evaluated for green diesel production via hydrodeoxygenation of unrefined microalgae oil in a microreactor, mimicking the single channel of a monolithic reactor. The effect of reactor inner diameter on space-time yield of hydrocarbon and microalgae oil conversion was studied first to confirm the superiority of the microreactor for three-phase reactions. Based on the external and internal mass transfer limitation analyses, a range of process conditions without mass transfer limitation was determined for catalyst evaluation. The results showed that NiMo/γ-Al2O3 is deactivated due to the accumulation of produced oxygenated intermediates in hydrodeoxygenation reaction, and its selectivity to even-numbered carbon hydrocarbon produced from hydrodehydration correlates with the catalyst activity. The catalyst activity and life can be preserved by increasing hydrogen to oil ratio, residence time, reaction temperatur...

Optimization of ethanol fermentation from discarded carrots using immobilized Saccharomyces cerevisiae

Discarded carrots are a valuable source of biomass amenable for valorization. Their use as raw material for ethanol production by fermentation, using yeasts immobilized in Calcium alginate, was proposed. The biocatalyst immobilization method, the existence of internal and external mass transfer limitations, the effect of the initial pH and the reuse of immobilized yeasts were particularly evaluated. Results indicate that beads made with a 2 % solution of Sodium alginate and a 30 % solution of Saccharomyces cerevisiae were strong enough to allow an efficient nutrient transfer into the matrix and to prevent cell leaking. A stirring rate of 200 rpm was needed to avoid external mass transfer limitations. These beads were used in three successive fermentations. An initial pH of 5.5 reached the best fermentation parameters. Non-enriched, non-sterile carrot must was fermented through immobilized yeasts; and values of ethanol concentration (29.9 g L−1), Yp/s (0.409 g g−1), and productivity (7.45 g L−1 h−1) were obtained. These values were similar to those registered when free cells were used.

Enzymatic Esterification of Oleic Acid and Propanol by Novozym 435

Recently, demand of biolubricants has been continuously increased since it is environmentally friendly and renewable. Therefore, this research focused on the biolubricants production from enzymatic esterification of oleic acid and propanol using Novozym 435 as a biocatalyst. The esterification experiments were conducted under the optimal conditions, as follows: 45°C, oleic acid to propanol molar ratio of 1:2, Novozym 435 loading of 5% based on the weight of oleic acid. It was shown that the optimal rotation speed at 250 rpm could minimize the effect of external mass transfer limitations and maintained the enzyme activity. The conversion of oleic acid from the esterification for 6 h with isopropanol was 76.4%, which was lower than that with n-propanol (88.9%). Novozym 435 could be reused in the production of propyloleate for at least 5 cycles with maintaining FFA conversion of 94% of its initial value. Moreover, the use of molecular sieve to remove water during the reaction could significantly enhance the final FFA conversion from 88.9% to 94.7%.

Negative Impact of High Stirring Speed in Laboratory-Scale Three-Phase Hydrogenations

An increase in stirring speed is generally considered to be an a priori means of reducing external mass-transfer limitations in fast three-phase hydrogenations that are performed in a stirred tank. We provide experimental evidence for a 300-mL stirred reactor that, above a certain impeller speed, the efficiency of gas–liquid mass-transfer decreases, resulting in the decreased reaction rate. The phenomenon is attributed to the high degree of gas recirculation with large cavities behind the blades. The recirculation may decrease hydrogen concentration in the remainder of the tank, thus decreasing the concentration gradient that controls mass transfer. The model reaction in this work was 2-methyl-3-butyn-2-ol semihydrogenation with Lindlar catalyst Pd–Pb/CaCO3. The test impellers were a Rushton turbine, a down-pumping pitched blade turbine, and up-pumping A340 impellers. The kinetic experiments were combined with the measurement of volumetric gas–liquid mass-transfer coefficient, flow pattern analysis and im...

Kinetics of Jojoba oil methanolysis using a waste from fish industry as catalyst

Abstract In this study, the kinetics of the heterogeneous methanolysis of Jojoba oil using calcined shells of Mytilus Galloprovincialis as catalyst has been studied at different temperatures (45–55–65 °C), methanol:oil ratios (6:1–9:1–12:1) and catalyst percents (6–8–10%). The main products obtained are the jojobyl alcohols with pharmaceutical activity and Fatty Acid Methyl Ester (FAME) is produced as co-product. The catalyst was synthesized through calcination at 800 °C during 6 h. The catalyst was characterized by BET method, X-ray diffraction analysis (XRD), inductively coupled plasma atomic emission spectrometry (ICP) and transmission electron microscopy (TEM). The catalyst is mainly CaO which is macroporous, it has low porosity and turns to Ca(OH)2 in the course of the reaction. The reaction was performed immediately after the calcination process in order to avoid the poisoning of the catalyst by H2O and CO2. The poisoning of the catalyst and its effect has also been object of this study. In addition, external and internal mass transfer limitations and the effect of different calcination temperatures on the process have been studied. The proposed kinetic mechanism fits a sigmoidal curve because there are mass transfer limitations which control the process at the beginning of the reaction whereas the chemical reaction is the limiting step when a critical amount of FAME and jojobyl alcohols are formed. The chosen variables in this kinetic study have been the temperature, the initial catalyst percent and the methanol:oil ratio. The kinetic study describes the whole methanolysis process accurately.

Modeling of the SCR reactor for coal-fired power plants: Impact of NH3 inhibition on Hg0 oxidation

A 1D + 1D heterogeneous model of the SCR reactor with honeycomb monolith catalyst is developed; it solves the mass balances of the reacting species along the axial and radial direction, using a lumped approach to describe the gas–solid mass transfer and a differential approach to describe diffusion and reaction within the catalyst wall. The model describes the coexistence of the DeNOX reaction and the highly desired oxidation of Hg0. It is well known from the literature that the conversion of Hg0 is negatively affected by NH3, but the extent of such an inhibition is uncertain because of the inherent complexity of the mercury speciation measurement. We thus apply the model to examine the concentration and reaction rate profiles inside the monolith wall and comprehend the way and extent of the interference of NH3 on Hg0 oxidation. The estimate of the intrinsic kinetic parameters was obtained from lab scale and pilot scale tests over commercial V-based catalysts, which allowed to focus the analysis on realistic conditions. The DeNOX and the Hg oxidation processes are both affected by external and internal mass transfer limitations, but the kinetic control from diffusion is stronger in the case of the DeNOX reaction. Thus along the radial coordinate and along the axial coordinate, the decay of NH3 concentration is more pronounced than the decay of Hg0 concentration. In practice, the main operating parameter which determines the integral impact of NH3 inhibition is the area velocity, that is the sizing of the full scale reactor. Still, a parametric analysis provides useful elements for envisaging the desired catalyst characteristics which can promote Hg0 conversion and minimize NH3 inhibition.

Heterogeneous Kinetic Study for Esterification of Acetic Acid with Ethanol

The esterification of acetic acid with ethanol has been investigated in the presence of a low cost ion exchange resin mainly used in the water treatment industry namely Trilite SCR-B as a heterogeneous catalyst. Internal mass transfer limitations have proven to be negligible using analysis based on Thieles modulus. The results of the experimental runs done by varying stirrer speeds of the reactor showed the absence of external mass transfer limitations. The kinetic experimental results were correlated with the heterogeneous kinetic models Langmuir-Hinshelwood (LH), Eley-Rideal (ER) and pseudo homogeneous kinetic model. The LH model was found to be correlating the experimental results better than the other two models. Reaction and adsorption constants were further evaluated within the temperature range between 335K and 355K, with 1:1 ethanol to acetic moles ratio under a catalyst loading of 50g/L. The study results of adsorption of both water and ethanol on to the cation exchange resin surface were found to be exothermic under the tested conditions. Furthermore, effect of concentration of each reactant on the reaction rate was examined using initial reaction rate measurements. ENGINEER, Vol. 47, No. 01, pp. 9-15, 2014

Influence of the three way catalytic converter substrate cell density on the mass transfer and reaction resistances

The conversion of pollutants in automotive three way catalytic converter (TWC) channels is influenced by fluid mechanics, heat and mass transfer and catalytic reactions. The quantification of the total and relative importance of the physical and chemical phenomena involved is of major importance for the TWC performance optimization. In this paper, we present and implement a method to quantify the magnitudes of the external and internal mass transfer resistances as well as the chemical reaction resistance. The resistance quantification method was applied to analyze the influence of the TWC substrate cell density on the mass transfer and reaction resistances. Two TWCs with different cell density, referred to as TWC 200 cpsi and TWC 400 cpsi, were investigated. The experimental data were obtained under real world TWC operating conditions. For both TWCs and over the range of operating temperatures investigated the results reveal that (i) the magnitude of the reaction resistance is insignificant, (ii) the magnitude of the internal mass transfer resistance is higher than the magnitude of the external mass transfer resistance, and (iii) the washcoat thickness of the TWCs investigated (δC=30μm) is so high that limits the contact between the reactants and the PCM. A comparison between the two TWCs indicates that (i) the magnitude of the external mass transfer resistance for the TWC 200 cpsi is higher than that for the TWC 400 cpsi, and (ii) the magnitude of both the internal mass transfer and the reaction resistance for the TWC 200 cpsi is lower than those for the TWC 400 cpsi. The present study also demonstrates that, keeping constant both the PCM loading and the washcoat mass, a TWC with high cell density along with a thinner washcoat layer allows reducing simultaneously the external and internal mass transfer limitations, which may enhance further the TWC conversions.

Internal and external mass transfer limitations on the activity of immobilised acid urease derivatives differing in enzyme loading

The specific activity of several Eupergit® C 250 L immobilised acid urease derivatives with enzyme loadings (YP/B) of 48–171mg of bovine serum albumin equivalent (BSAE) per gram of dry support (ds) in a model wine solution was satisfactorily reconstructed using the simultaneous film and intraparticle diffusion and Michaelis-Menten kinetic model by assuming that the immobilisation procedure did not alter the intrinsic kinetic parameters of free acid urease. The behaviour of crunched biocatalysts resulted to be practically coincident with that of the free enzyme, while that of the whole beads was affected by diffusional effects. The model prediction capability was also established in long-time reaction tests using two other biocatalysts at different dry bead concentrations (cBd). The effect of YP/B was finally simulated in the pseudo-1st order regime in the presence of a stagnant or highly-stirred medium. At the limiting Sherwood number of 2.0 and YP/B>48mg BSAE g−1 ds, the calculated specific activity was greatly affected by external mass transfer. To help designing stirred bioreactors, the substrate reduction to be achieved in a given time interval was empirically related to cBd and YP/B.

Modelling ammonium-oxidizing population shifts in a biofilm reactor

The dynamic reactor behaviour of a nitrifying inverse turbulent bed reactor, operated at varying loading rate, was described with a one-dimensional two-step nitrification biofilm model. In contrast with conventional biofilm models, this model includes the competition between two genetically different populations of ammonia-oxidizing bacteria (AOB), besides nitrite-oxidizing bacteria (NOB). Previously gathered experimental evidence showed that different loading rates in the reactor resulted in a change in the composition of the AOB community, besides a different nitrifying performance. The dissolved oxygen concentration in the bulk liquid was put forward as the key variable governing the experimentally observed shift from Nitrosomonas europaea (AOB1) to Nitrosomonas sp. (AOB2), which was confirmed by the developed one-dimensional biofilm model. Both steady state and dynamic analysis showed that the influence of microbial growth and endogenous respiration parameters as well as external mass transfer limitation have a clear effect on the competition dynamics. Overall, it was shown that the biomass distribution profiles of the coexisting AOB reflected the ecological niches created by substrate gradients.

Heterogeneous modeling of chemical-looping combustion. Part 1: Reactor model

Heterogeneous modeling is used to investigate the significance of intraparticle diffusion, temperature fluctuations, and dispersion on predicting fixed-bed chemical-looping combustion (CLC) reactors. The model is validated against chemical-looping reduction experiments from the literature that utilize methane as fuel and a Ni-based oxygen carrier. The effect of particle size on CLC reduction reactions is studied by examination of the Weisz–Prater and Mears criteria for internal and external mass transfer limitations. Homogeneous and heterogeneous models are applied for two particle sizes and it is shown that diffusion limitations are significant for the larger particle. Furthermore, non-ideal flow patterns are evaluated, where superior predictions are obtained by considering the effects of dispersion and convection. Radial dispersion in each experiment is analyzed in detail and the predicted temperature fluctuations inside the reactor and particle are presented. Lastly, the heterogeneous model is applied to design a particle that maximizes chemical-looping efficiency, measured as fuel conversion and product selectivity. Enhancements to the overall reaction rate and temperature profile are observed for the optimal particle size as a result of reducing the intraparticle resistance.

Kinetic study of propane dehydrogenation and catalyst deactivation over Pt-Sn/Al2O3 catalyst

The kinetics of propane dehydrogenation and catalyst deactivation over Pt-Sn/Al2O3 catalyst were studied. Performance test runs were carried out in a fixed-bed integral reactor. Using a power-law rate expression for the surface reaction kinetics and independent law for deactivation kinetics, the experimental data were analyzed both by integral and a novel differential method of analysis and the results were compared. To avoid fluctuation of time-derivatives of conversion required for differential analysis, the conversion-time data were first fitted with appropriate functions. While the time-zero and rate constant of reaction were largely insensitive to the function employed, the rate constant of deactivation was much more sensitive to the function form. The advantage of the proposed differential method, however, is that the integration of the rate expression is not necessary which otherwise could be complicated or impossible. It was also found that the reaction is not limited by external and internal mass transfer limitations, implying that the employed kinetics could be considered as intrinsic ones.

Production of ultrapure hydrogen in a Pd–Ag membrane reactor using noble metals supported on La–Si oxides. Heterogeneous modeling for the water gas shift reaction

Two different catalysts, Rh(0.6% wt/wt)/La2O3(27% wt/wt)·SiO2 and Pt(0.6% wt/wt)/La2O3(27%)·SiO2, were tested in the WGS reaction. Their performances were first studied in a conventional fixed-bed reactor. Their activities were similar and they were both very stable. However, as Pt(0.6)/La2O3(27)·SiO2 showed a much higher selectivity to the desired reaction, the performance of a membrane reactor employing this catalyst was studied. The effects of the H2O/CO ratio, space velocity, sweep gas flow rate and size of the catalyst particle on CO conversion and H2 recovery were studied at laboratory scale under isothermal conditions. A 1-D heterogeneous model was developed in order to properly reproduce the experimental results obtaining good agreement between the simulation results and laboratory data. The experimental and theoretical results confirm the existence of significant external mass-transfer limitations in the fluid-particle interface for these very active formulations.

Kinetic model for the catalytic disproportionation of pine oleoresin over Pd/C catalyst

The kinetics of the disproportionation of pine oleoresin (a renewable bioresource) over a carbon-supported palladium catalyst was studied. Kinetic experiments, constructed to eliminate internal and external mass transfer limitations, were performed in the temperature range of 210–250°C. The samples, withdrawn from reaction mixtures at different intervals, were determined by GC–MS and GC. A new reaction scheme together with a lumped kinetic model was proposed to describe the complex reaction system. The kinetic parameters for each involved reaction were estimated using the Levenberg–Marquart method by MATLAB software. The various activation energies of rosin acids in pine oleoresin, for the isomerization, dehydrogenation and hydrogenation reactions, were 83.05–172.75kJmol−1. The various activation energies of monoterpenes in pine oleoresin, for the dehydrogenation and hydrogenation reactions, were 103.97–133.82kJmol−1. The kinetic model well fitted the experimental observations and could be used to predict the concentration distribution of the products at 260°C. The results showed that the disproportionations of rosin acids and monoterpenes followed second order and first order reactions, respectively. In addition, dehydrogenation was the main reaction in the disproportionation of pine oleoresin. Dehydrogenated acid and p-cymene were main components in the final products.

Characteristics of integrated micro packed bed reactor-heat exchanger configurations in the direct synthesis of dimethyl ether

The performance of three integrated micro packed bed reactor-heat exchangers (IMPBRHEs) for direct DME synthesis over physical mixtures of CuO–ZnO–Al2O3 and γ-Al2O3 catalysts was experimentally investigated. Systematic variations in reactor and slit dimensions and configuration were analyzed in terms of thermal behaviour, mass transfer, pressure drop and residence time distribution (RTD). The pressure drop was always small (<0.12bar) relative to the total pressure (50bar), and linear dependence with GHSV confirms the predicted laminar flow for Re=0.1–2. A narrow RTD was estimated by the dispersion analysis. Careful temperature measurements confirmed that the reaction temperature is mainly controlled by the oil heat exchange to give a practically uniform temperature profile for set inlet oil temperatures of 220–320°C. The micro packed beds were found free of the internal as well as external mass transfer limitations, as showed by no significant change in the CO conversion and DME yield for different catalyst particle sizes, no effect of varying the linear gas velocity, and no effect of manipulating reactant diffusion coefficient. Packed bed microstructured reactors hence provide an isobaric and isothermal environment free from transport limitations for the direct DME synthesis, in the kinetic regime as well as at equilibrium conversion.

Comparison of Novozyme 435 and Purolite D5081 as heterogeneous catalysts for the pretreatment of used cooking oil for biodiesel production

The catalytic performance of two types of catalysts, an ion-exchange resin, Purolite D5081 and an immobilised enzyme, Novozyme 435, was compared for the esterification pretreatment of used cooking oil (UCO) for the preparation of biodiesel. The reactions were carried out using a jacketed batch reactor with a reflux condenser. The effect of mass transfer limitations was investigated and it was shown that internal and external mass transfer limitations were negligible. An immobilised enzyme, Novozyme 435, was investigated because it has been shown to give high free fatty acids (FFAs) conversion. This catalyst has been compared to an ion-exchange resin, Purolite D5081, which was developed for the esterification of UCO for the production of biodiesel. It was found that a conversion of 94% was achieved using Purolite D5081 compared to 90% conversion with Novozyme 435. However, the optimum methanol to FFA ratio for Purolite D5081 was 98:1 compared to 6.2:1 for Novozyme 435. In addition, it has been found that with Novozyme 435 there are side reactions which result in the formation of additional fatty acid methyl esters (FAMEs) and FFAs at longer reaction times.

Comparison of Phenomenological and Hybrid Models in the Description of the Ion Exchange Process in a Fixed-Bed Column

In this study, two models (phenomenological and hybrid) were used in the description of the ion exchange process in a fixed-bed column. The external and internal mass transfer limitations and thermodynamic equilibrium at the solid-liquid interface were considered. In the phenomenological model, the mass action law was used to describe the equilibrium, while, in the hybrid model an artificial neural network (ANN) was used. The experimental equilibrium data were used for the validation of the ANN. The performance of the models was compared using experimental breakthrough curves. Both models described well the ion exchange process. The simulation time using the hybrid model was shorter.