External Mass Transfer Limitations Research Articles

Mass transfer effects on biodegradation of methylene blue by immobilized cell in a packed bed bioreactor

AbstractAerobic degradation of methylene blue by a pure culture of Ralstonia eutropha was studied by both freely suspended cells and kissiris‐immobilized packed bed reactors. Kissiris as a cell support matrix favored the retention of the cells in the packed bed reactor during of the methylene blue biodegradation process. The biodegradation experiments showed that the kissiris‐packed bed reactor had higher performance in removal of the dyestuff for initial concentrations equal or higher than 150 mg L−1 when compared with the freely suspended cells. To evaluate the effect of mass transfer phenomena in the immobilized system, mass balance of methylene blue for a single kissiris piece was applied to derive expressions correlating the methylene blue concentration in the bulk liquid medium and inside of the bioparticle. Kinetics studies revealed a self‐inhibitory effect of methylene blue on its biodegradation at concentrations higher than 85.76 mg L−1 in the free cell and 190.1 mg L−1 for immobilized system. The mass transfer modeling indicated the protective role of the support from cells against the inhibitory role of methylene blue is related to both external and internal mass transfer limitations in the immobilized system. In the kissiris packed bed reactor, the effectiveness value was higher than unity for initial methylene blue concentration of 150 mg L−1.

Periodic reactor operation for parameter estimation in catalytic heterogeneous kinetics. Case study for ethylene adsorption on Ni/Al2O3

Periodic operation of chemical reactors has usually been analyzed for a potential gain in performance. However, at the laboratory scale, periodic operation of a catalytic reactor can be used for kinetic studies with better parameter estimation, which is inherent to transient kinetic studies.Different mathematical methods are developed and compared in this study to simulate and model concentration cycling in a fixed bed catalytic reactor. Different cases including dissociative and molecular adsorption are considered with or without external and internal mass transfer limitations. The analysis of these cases gives information on the contribution of the different parameters on the phase lag and gain loss. This information can be used to better plan experimental kinetic studies.Ethylene adsorption over a Ni/Al2O3 catalyst has been studied experimentally in a dedicated set-up with a high-speed Infrared analyzer capable of analyzing the outlet gas mixture with a data acquisition frequency up to 30 Hz. The data are modeled with one of the analytical expressions developed in this work to estimate the equilibrium adsorption constant.

Development of a Polyaniline-coated Monolith Reactor for the Synthesis of Cephalexin Using Penicillin G Acylase Aggregates

A monolith reactor for the synthesis of cephalexin was developed using capillary columns. The micro channel in the monolith reactor was coated with polyaniline (PANI), and penicillin G acylase was aggregated with PANI using 0.5% of glutaraldehyde as a cross-linker. The developed monolith reactor exhibited many advantages over other enzyme reactors such as batch and continuous reactors. It showed fast enzyme reaction rates owing to the decrease in external mass transfer and internal diffusion limitations. The reactor can easily be scaled up by bundling together multiple monolith reactors, enabling a corresponding increase in feed rate. Furthermore, the monolith reactor showed good operational stability, with 95% of its original activity maintained after 48 h of continuous operation. The PANI coating on the surface of the capillary column increased the enzyme immobilization capacity and conversion was increased from 15.4% to 70.6% after PANI coating. The conversion ratio increased to approximately 70.6% with an increase in residence time and reactor length.

Biofilms in 3D porous media: Delineating the influence of the pore network geometry, flow and mass transfer on biofilm development

This study investigates the functional correspondence between porescale hydrodynamics, mass transfer, pore structure and biofilm morphology during progressive biofilm colonization of a porous medium. Hydrodynamics and the structure of both the porous medium and the biofilm are experimentally measured with 3D particle tracking velocimetry and micro X-ray Computed Tomography, respectively. The analysis focuses on data obtained in a clean porous medium after 36 h of biofilm growth. Registration of the particle tracking and X-ray data sets allows to delineate the interplay between porous medium geometry, hydrodynamic and mass transfer processes on the morphology of the developing biofilm. A local analysis revealed wide distributions of wall shear stresses and concentration boundary layer thicknesses. The spatial distribution of the biofilm patches uncovered that the wall shear stresses controlled the biofilm development. Neither external nor internal mass transfer limitations were noticeable in the considered system, consistent with the excess supply of nutrient and electron acceptors. The wall shear stress remained constant in the vicinity of the biofilm but increased substantially elsewhere.

Modeling Mass Transfer in a Three-Phase Minireactor Filled with Trilobe Catalytic Extrudates in Series

The internal and external gas-liquid mass transfer limitations in a three phase spiral string bed mini-reactor are estimated, using as a model reaction the benzene hydrogenation over a commercial tri-lobe Ni/Al2O3 catalyst at elevated pressures and temperatures. A detailed two phase flow model incorporating liquid volatility, intrinsic catalytic reaction rates, gas-liquid mass transfer effects, liquid-solid mass transfer effects, internal mass transfer effects and wetting of the catalytic particles, was used for the simulation of the spiral string bed reactor performance. The benzene intrinsic reaction kinetics was estimated conducting benzene hydrogenation experiments over crushed catalyst particles. The gas-liquid mass transfer coefficient was estimated from the data of this study and the liquid-solid mass transfer effects were estimated from data of a previous communication. The proposed correlation for the prediction of gas-liquid mass transfer coefficients can describe the gas-liquid mass transfer li...

Decoupling the Role of External Mass Transfer and Intracrystalline Pore Diffusion on the Selectivity of HZSM-5 for the Catalytic Fast Pyrolysis of Biomass

Catalytic fast pyrolysis (CFP) using HZSM-5 zeolite catalysts serves as a promising route for the production of renewable chemicals from lignocellulosic biomass. However, this reaction is limited by the formation of coke, which can exceed 40% of carbon atoms present in the raw biomass feedstock. The role of structural parameters on coking remains unclear with both internal micropore diffusion and external mass transfer limitations hypothesized to actively contribute to carbon deposition. Here, we decouple the role of these parameters by comparing conventional in situ pyrolysis tests using model compounds to experiments performed with the reactant preadsorbed onto the zeolite catalyst. Experimental results supported by calculation of the mass transfer Biot number point to micropore diffusion as the dominant cause of coke formation. Specifically, the presence of defects such as internal crystal grain boundaries and extraframework species in the zeolite’s micropores actively contribute to this undesired side...

Byproduct Formation Mechanisms and Effect of Mass Transfer in Plant Sterol Hydrogenation

Plant sterols, a mixture of several des-4-methyl sterols, were hydrogenated over a Pd/C catalyst by varying catalyst amount and stirring rate. Reactions were carried out under kinetic regime and under the influence of external mass transfer limitations. All reaction were done in the absence of internal diffusion limitations. Under external mass transfer limitations of hydrogen more byproducts due to hydrogenolysis and double bond migration were formed. Under mass transfer limited reactions higher catalyst amounts led to a more extensive byproduct formation. Because of double bond migration two very similar byproducts from sitosterol were formed having trans- and cis-fused rings. Interestingly hydrogenolysis also resulted in two similar stereoisomers of sitostane, in the same way as sitostanone, indicating that the hydrogenolysis occurred during the double bond migration when the double bond is in the Δ3 position. An intermediate product of stigmasterol was observed, in which the ring structure is hydrogen...

Levulinic acid hydrodeoxygenation, decarboxylation and oligmerization over NiMo/Al2O3 catalyst to bio-based value-added chemicals: Modelling of mass transfer, thermodynamics and micro-kinetics

Levulinic acid (LA) hydrotreatment at solvent-free conditions was studied in a slurry reactor over the sulphide form of NiMo/Al2O3 catalyst. The influence of process parameters on the H2 equilibrium solubility in the liquid (calculated by Soave–Redlich–Kwong, EOS), on the external and internal mass transfer limitations, adsorption and desorption rates and the rates of surface and homognenous reactions were determined experimentally and computationally by utilizing a newly-developed micro-kinetic model. Main catalytic hydrodeoxygenation (HDO) product γ-valerolactone (GVL) was formed on NiMoSx phase (surface sites concentration of 0.33μmolm−2 was determined) exclusively by LA hydrogenation to hydroxypentanoic acid, and its subsequent intramolecular esterification, while cyclisation of LA to angelica lactones and their saturation was negligible. Activation energies determined by the regression analysis for the rate-determining step of catalytic LA HDO (Ea 39kJmol−1) and competitive LA decarboxylation in bulk liquid (Ea 134kJmol−1) show, that the temperature increase from 225°C to 275°C accelerated the decarboxylation rates much faster than HDO, which reflected in the drop of selectivity towards GVL formation from 90 to 42%, while its yield was not significantly affected (2.5±3molL−1 within 60min at final temperature). LA oligomerization by aldol addition and subsequent dehydrative cyclisation is reported for the first time and its rate was independent of the catalyst loading or H2/N2 pressure. Internal mass transfer had no influence on the global reaction rate, while the external mass transfer was eliminated at high agitation rate.

Biosorption of chromium (Cr) onto algae (Ulva.lactuca): application of isotherm and kinetic models

The biosorption of chromium (VI) ions to Ulva.lactuca studied in a batch system with respect to the concentration, initial pH and initial metal ion concentration. The algal biomass exhibited the highest chromium (VI) adsorption capacity at 20°C, at the initial pH value of 3.0. Biosorption capacity increased from 0.70 to 1.08 mg g -1 with an increase in initial chromium (VI) concentration from 29.14; 38.28; 58.47 mgL -1 . Freundlich and Langmuir isotherm models were tried to represent the equilibrium data of chromium (VI) biosorption depending on concentration. Equilibrium data fitted very well to the Langmuir model in the studied concentration range of chromium (VI) ions at all the concentrations studied. The pseudo second-order kinetic model was also applied to experimental data assuming that the external mass transfer limitations in the system can be neglected and biosorption is sorption controlled.

Gas-solids kinetics of CuO/Al2O3 as an oxygen carrier for high-pressure chemical looping processes: The influence of the total pressure

Copper oxide on alumina is often used as oxygen carrier for chemical looping combustion owing to its very high reduction rates at lower temperatures and its very good mechanical and chemical stability at not too high temperatures. In this work, the redox kinetics of CuO/Al2O3 have been studied at elevated pressures and temperatures. All the experiments have been started under the same initial conditions to assure the same starting point. While other studies reported a negative effect of the total pressure on the redox kinetics, this study shows that this negative effect of the pressure is most probably caused by external mass transfer limitations in previous studies. Additionally, as long as external mass transfer limitations are prevented, the total pressure at which the reduction is performed does not affect the redox kinetics nor the morphological and chemical structure of the oxygen carrier. The sudden decrease in the reduction rate at higher particle conversions was not influenced by the operating pressure and was attributed to limitations in the spinel reduction kinetics.

Process optimization and kinetic modeling for esterification of propionic acid with benzyl alcohol on ion-exchange resin catalyst

Benzyl propionate, an ester with floral and fruity odor, has significant applications in perfumery and flavor industries. This paper describes the optimization of the synthesis of benzyl propionate catalyzed by Amberlyst-15. The effects of various process parameters such as catalyst loading, alcohol-to-acid molar ratio and reaction temperature on propionic acid conversion and yield of ester were assessed by response surface methodology (RSM). The external and internal mass transfer limitations were found to be absent. Analysis of variance (ANOVA) showed that the acquired quadratic model successfully interpreted the experimental data with the coefficient of determination values, (R2>0.98) and adjusted R2 values, (>0.97). The RSM model was validated by good agreement between the model predicted and experimental values for responses. Pseudohomogeneous (PH) kinetic model was used and validated (R2>0.95) with the experimental data. The activation energy and frequency factor were evaluated as 42.07 kJ mol−1 and 19,874.64 L mol−1 min−1, respectively.

Effect of dilution rate on dynamic and steady-state biofilm characteristics during phenol biodegradation by immobilized Pseudomonas desmolyticum cells in a pulsed plate bioreactor

Pulsed plate bioreactor (PPBR) is a biofilm reactor which has been proven to be very efficient in phenol biodegradation. The present paper reports the studies on the effect of dilution rate on the physical, chemical and morphological characteristics of biofilms formed by the cells of Pseudomonas desmolyticum on granular activated carbon (GAC) in PPBR during biodegradation of phenol. The percentage degradation of phenol decreased from 99% to 73% with an increase in dilution rate from 0.33 h–1 to 0.99 h–1 showing that residence time in the reactor governs the phenol removal efficiency rather than the external mass transfer limitations. Lower dilution rates favor higher production of biomass, extracellular polymeric substances (EPS) as well as the protein, carbohydrate and humic substances content of EPS. Increase in dilution rate leads to decrease in biofilm thickness, biofilm dry density, and attached dry biomass, transforming the biofilm from dense, smooth compact structure to a rough and patchy structure. Thus, the performance of PPBR in terms of dynamic and steady-state biofilm characteristics associated with phenol biodegradation is a strong function of dilution rate. Operation of PPBR at lower dilution rates is recommended for continuous biological treatment of wastewaters for phenol removal.

3-Chloro-1,2-propanediol biodegradation by Ca-alginate immobilized Pseudomonas putida DSM 437 cells applying different processes: mass transfer effects.

3-Chloro-1,2-propanediol (3-CPD) biodegradation by Ca-alginate immobilized Pseudomonas putida cells was performed in batch system, continuous stirred tank reactor (CSTR), and packed-bed reactor (PBR). Batch system exhibited higher biodegradation rates and 3-CPD uptakes compared to CSTR and PBR. The two continuous systems (CSTR and PBR) when compared at 200mg/L 3-CPD in the inlet exhibited the same removal of 3-CPD at steady state. External mass-transfer limitations are found negligible at all systems examined, since the observable modulus for external mass transfer Ω≪1 and the Biot number Bi>1. Intra-particle diffusion resistance had a significant effect on 3-CPD biodegradation in all systems studied, but to a different extent. Thiele modulus was in the range of 2.5 in batch system, but it was increased at 11 when increasing cell loading in the beads, thus lowering significantly the respective effectiveness factor. Comparing the systems at the same cell loading in the beads PBR was less affected by internal diffusional limitations compared to CSTR and batch system, and, as a result, exhibited the highest overall effectiveness factor.

Nobel metal free, oxidant free, solvent free catalytic transformation of alcohol to aldehyde over ZnO-CeO2 mixed oxide catalyst

The catalytic transformation of alcohols to aldehydes under oxidant-free condition has drawn significant attention from the perspective of green chemistry. In this work, we designed noble metal free ZnO-CeO2 mixed oxide catalyst in four different ratios and tested for vapor phase benzyl alcohol dehydrogenation reaction as a model reaction under oxidant free condition. The ZnO-CeO2 mixed oxide catalyst having ratio Zn/Ce=30/70 composition showed highest selectivity towards formation of benzaldehyde. Interestingly in addition to benzaldehyde, toluene was formed in the reaction due to hydrogenolysis of benzyl alcohol. The lowest Ce3+/Ce4+ ratio was observed from the XPS analysis of Ce(3d) core electron for the catalyst having Zn/Ce=30/70 composition compared to others. CO2-TPD results proved that mostly the medium strength basic sites were responsible for hydrogen abstraction from benzyl alcohol producing benzaldehyde. H2-TPR results showed that ZnO-CeO2 catalyst (Zn/Ce=30:70) had lowest reduction temperature which is in the 673K to 573K temperature range. The amount of toluene was higher for the ZnO-CeO2 catalyst having Zn/Ce=40:60 ratio which had less basic sites and higher fraction of Ce3+ ion. The ZnO-CeO2 catalyst (Zn/Ce=30:70) did not deactivate for a reaction time up to 2h. While successive regenerations of the catalyst, toluene selectivity were increased. This may be due to the reduction of Ce4+ to Ce3+ by adsorbed hydrogen species. Also the ZnO-CeO2 catalyst (Zn/Ce=30/70) showed activity for the formation of ethanal, propanal, butanal and octanal along with corresponding alkanes from ethanol, 1-propanol, 1-butanol and 1-octanol in oxidant free condition demonstrating the in-situ generation of hydrogen. The micro kinetic analysis showed that there is no external and internal mass transfer limitation in the present case.

Optimal Catalyst and Cocatalyst Precontacting in Industrial Ethylene Copolymerization Processes

In industrial-scale catalytic olefin copolymerization processes, catalyst and cocatalyst precontacting before being introduced in the polymerization reactor is of profound significance in terms of catalyst kinetics and morphology control. The precontacting process takes place under either well-mixing (e.g., static mixers) or plug-flow (e.g., pipes) conditions. The scope of this work is to study the influence of mixing on catalyst/cocatalyst precontacting for a heterogeneous Ziegler-Natta catalyst system under different polymerization conditions. Slurry ethylene homopolymerization and ethylene copolymerization experiments with 1-butene are performed in a 0.5 L reactor. In addition, the effect of several key parameters (e.g., precontacting time, and ethylene/hydrogen concentration) on catalyst activity is analyzed. Moreover, a comprehensive mass transfer model is employed to provide insight on the mass transfer process and support the experimental findings. The model is capable of assessing the external and internal mass transfer limitations during catalyst/cocatalyst precontacting process. It is shown that catalyst/cocatalyst precontacting is very important for the catalyst activation as well as for the overall catalyst kinetic behavior. The study reveals that there is an optimum precontacting time before and after which the catalyst activity decreases, while this optimum time depends on the precontacting mixing conditions.

Effectiveness Factors and Conversion in a Biocatalytic Membrane Reactor.

Analytical expressions of the effectiveness factor of a biocatalytic membrane reactor, and its asymptote as the Thiele modulus becomes large, are presented. The evaluation of the effectiveness factor is based on the solution of the governing equations for solute transport in the two regions of the reactor, i.e. the lumen and the matrix (with the biofilm immobilized in the matrix). The lumen solution accounts for both axial diffusion and radial convective flow, while the matrix solution is based on Robin-type boundary conditions. The effectiveness factor is shown to be a function of the Thiele modulus, the partition coefficient, the Sherwood number, the Peclet number, and membrane thickness. Three regions of Thiele moduli are defined in the effectiveness factor graphs. These correspond with reaction rate limited, internal-diffusion limited, and external mass transfer limited solute transport. Radial convective flows were shown to only improve the effectiveness factor in the region of internal diffusion limitation. The assumption of first order kinetics is shown to be applicable only in the Thiele modulus regions of internal and external mass transfer limitation. An iteration scheme is also presented for estimating the effectiveness factor when the solute fractional conversion is known. The model is validated with experimental data from a membrane gradostat reactor immobilised with Phanerochaete chrysosporium for the production of lignin and manganese peroxidases. The developed model and experimental data allow for the determination of the Thiele modulus at which the effectiveness factor and fractional conversion are optimal.

Performance of Ni/Si-pillared clay catalytic extrudates for benzene hydrogenation reaction

Ni supported on Si-pillared montmorillonite systems have been prepared and studied as catalysts for the test reaction of benzene hydrogenation. The Ni/Si-pillared clay catalytic systems were characterized as to their crystallographic structure, specific surface area, specific volume and apparent density. Their reactivity was evaluated over a temperature range of 70–150°C under atmospheric pressure, in a differential bench scale reactor, and compared with other Ni catalysts supported on conventional carriers, such as γ-Al2O3 and Vycor. Apart from the intrinsic catalytic rates, the effect of both internal and external mass transfer limitations was also studied for particle sizes commonly used in industrial praxis. The change of the overall reaction rates with particle size was investigated. The mass transfer effects were examined for diluted catalytic beds of three extrudate sizes and diffusional limitation within the particles were decoupled from external mass transfer resistances. Particle diffusional limitations and their dependence on process temperature and particle porosity were determined and discussed.

External liquid solid mass transfer for solid particles transported in a milli-channel within a gas–liquid segmented flow

This article focuses on the measurement of external liquid–solid mass transfer coefficient for micro-particles (dP=40–200μm) transported in a gas–liquid Taylor flow. The particles are kept in motion due to the secondary vortices present in the liquid slugs of these segmented flows. Acidic ion exchange particles and dilute sodium hydroxide solutions are used to perform ion exchange and neutralisation under external mass transfer limitations. Concentration profiles are monitored along the reactor with 4 conductivity cells which provides accurate mass transfer coefficients for each experiment. Two phase velocity (2.5–28cm/s), mean particle size (100–160μm), solid loading (5–18g/L), reactor orientation (vertical down flow-horizontal) and liquid phase nature (Sc=790–2300) were investigated. In that experimental window, it was found that the solid charge and the flow direction have a small influence on the L–S mass transfer even though the direction of the flow influences particle distribution in the liquid slug. Similar mass transfer coefficients and dependencies on the two phase velocities and liquid properties were found. Increasing the two-phase velocity leads first to increase the mass transfer coefficient. Then at highest velocities (uTP>20cm/s), a much lower impact is observed and the mass transfer coefficient tends to stabilize. A first correlation form for the L–S Sherwood number in “slurry Taylor” flow is proposed for both flow orientations. These findings are used in a second part to illustrate the high potentialities of this advanced structured flow in comparison with other conventional reactors using suspended particles like stirred tank and bubble column reactors. The G–L–S “slurry Taylor” offers compelling advantages combining excellent overall external mass transfer (G–L and L–S) and almost ideal plug flow behaviour for the three phases and very good heat transfer capacities.

Reaction kinetics with catalyst deactivation in simultaneous esterification and transesterification of acid oils to biodiesel (FAME) over a mesoporous sulphonated carbon catalyst

In this work, a careful analysis of the reaction kinetics upon simultaneous esterification and transesterification of acidic oils over a mesoporous sulphonated carbon catalyst is discussed. A batch reactor system was used and the synthesized carbon catalyst were characterized by N2-physisorption, transmission electron microscopy, elemental analysis and NH3-TPD. A second order pseudo-homogeneous kinetic model was proposed which explained the experimental results obtained for three different feedstock oils with ⩾98% accuracy. The rate constants (k), activation energies (Ea) and equilibrium constants (Keq) of the individual reactions were determined by regression analysis which confirmed that the reaction steps were kinetically controlled and not limited by inter-particle diffusion or external mass transfer limitations (Ea>25kJmol−1). Furthermore, the composition feedstock was found to have a distinct effect on the solubility of methanol and oil phase which influenced k, Keq and Ea values, eventually determining the final biodiesel (FAME) yield. To account for the loss of activity upon catalyst reuse, a deactivation model was also proposed which explained our results with ∼94% accuracy. In fact, the loss of activity was accounted for by incorporating a concentration-independent ‘deactivation constant’ kd in the reaction rate equations. Moreover, under optimized reaction conditions (120°C and 20:1 methanol-to-FFA molar ratio), FAME yields up to 79–91wt% could be obtained in one step process from oils containing 21–41wt% FFA.

Intrinsic kinetics of clofibric acid photocatalytic degradation in a fixed-film reactor

This study presents a kinetic model for the photocatalytic degradation of an organic pollutant in a fixed-film reactor for water treatment. The pharmaceutical drug clofibric acid (CA) was chosen as the model compound. Experiments were carried out employing TiO2 Aeroxide P25 immobilized on the glass window of a laboratory scale reactor under UV radiation. A kinetic model to represent the degradation of CA was proposed, which explicitly takes into account the absorption of radiation by the catalytic film. Experimental results for different irradiation conditions and different number of TiO2 coatings were used to estimate intrinsic kinetic parameters. External and internal mass transfer limitations were found to be negligible under our operating conditions. Good agreement was obtained between experimental data and model predictions, with a root mean square error of 9.9%.