Internal Mass Transfer Effects Research Articles

Application of response surface methodology on photocatalytic degradation of Astrazon Orange G dye by ZnO photocatalyst: Internal mass transfer effects

The present study involves the removal of Astrazon Orange G dye from aqueous solutions by ZnO in a heterogeneous system of using UV-C light as a source of energy. Crystalline structure and BET surface area of the photocatalyst were characterized. The optimum conditions were determined as 200 mg dm-3 of dye concentration, 4 g dm-3 of catalyst amount, pH of 6, the temperature of 40 °C, and 0.2 dm3 min-1 of the rate of airflow. Under these conditions, the dye and TOC removal efficiencies at 75 min were determined as 97.9 % and 54.7 %, respectively. Response surface methodology was also used to investigate the interactive effects of the selected parameters and the optimum conditions for maximum degradation of AOG dye. The kinetics of the Astrazon Orange G dye photodegradation by using ZnO represented by the first-order equation. The effect of the temperature on the AOG dye photodegradation was shown by using of the Arrhenius equation. The activation energy was found to be 4.28 kJ mol-1. In addition, internal mass transfer effects on photodegradation of AOG dye were investigated with different sizes ZnO and effectiveness factors, Thiele modulus, and average effective diffusion coefficient were calculated. These results showed that the intraparticle mass transfer did not have much effect on the reaction rate and the chemical step largely controlled the rate. The potential performance of these photocatalytic processes on the synthetic wastewater was also tested.

Co-immobilization of lipase and laccase on agarose-based supports via layer-by-layer strategy: Effect of diffusional limitations

Co-immobilized enzymes present desirable advantages to performing cascade reactions when employed in one-pot processes, mainly because of the shorter lag times owing to better reagent diffusion across the active sites of the different enzymes. But high product yields require high enzyme loads, usually achieved with high surface area and porous supports, where diffusional limitations may be introduced or increased. Heterogeneous biocatalysts were prepared by co-immobilizing Pseudomonas fluorescens lipase (PFL) and Aspergillus sp. Laccase (Novozymes 51003) on two agarose-based supports, DEAE-Sepharose and Octyl-agarose. A layer-by-layer strategy was used by adding polyethyleneimine (PEI) to allow the attachment of the upper enzyme layer onto the lower adjacent layer. A final step, crosslinking with glutaraldehyde, was performed to prevent enzyme leaching under harsh environmental conditions. The heterogeneous biocatalysts showed lower temperature dependence and a significant increase in thermal stability at 50 ºC. No internal mass transfer effects were observed when a low protein load was used during co-immobilization (2 mgprotein/gsupport of each enzyme). However, increasing the enzyme load (more than 10 mgprotein/gsupport) promoted internal diffusion limitations. The layer-by-layer strategy defined in this work enabled the preparation of a multi-enzymatic heterogeneous biocatalyst with high activity, thermal stability, and no diffusional limitations, depending on enzyme concentration.

Single catalyst particle growth modeling in thermocatalytic decomposition of methane

ThermoCatalytic Decomposition of methane (TCD) is studied as a method to convert natural gas into hydrogen and functional carbon. In these processes the carbon typically formed on top of a catalyst phase leading to particle growth. Therefore, the development of a particle growth model is necessary to understand the limitations of thermocatalytic decomposition of methane and to assess optimal parameters and process conditions. In this paper, a particle growth model is presented to describe the growth of functional carbon on the catalyst particle. This coupled model requires kinetic equations and information on deactivation rates which have been studied from literature. The morphology of the particle changes due to carbon formation, which leads to eventual deactivation. Therefore, these kinetic expressions are coupled to a particle growth model based on the analogy with the growth of particles in polyolefin production. To combine the effects of particle growth, kinetics, and internal heat and mass transfer, the Multi-Grain Model (MGM) was used. Results confirm that with the currently available catalysts the carbon yield is not affected by heat and mass transfer limitations, however, with the availability of more active catalysts these limitations will become important. Temperature, however, has a significant role in that it regulates the kinetic rate and thus growth rate, which in turn influences the catalyst deactivation. The optimum temperature for the production of nano-carbon, within a reasonable process time, therefore sensitively depends on the choice of catalyst. • Development of a particle growth model for Thermocatalytic decomposition of methane. • Combined mass/heat transfer, kinetics and particle growth using a multi-grain model. • Assessment of transfer limitations and temperature using available kinetic models.

Kinetic optimization of multilayered photocatalytic reactors

Translucent structured reactors have proven to be an effective design to scale up microreactors. By generating surface area, this flexible reactor design allows to increase the catalyst loading without increasing the catalyst layer thickness, which is beneficial in tackling diffusion limitations in single-channel reactors. However, adding more depth to such a structure by replicating the channels increases the number of scattering boundaries which leads to energy losses. As a result, there is a design problem which seeks to define the optimal catalyst layer thickness and optimal number of repeating boundaries on a light path. Most of the models are numerically solved and very specific to the reactor type being modeled. In this work, a catalyst layer mass balance model is used to construct a model of a translucent structured reactor which takes into account internal mass transfer effects and which can be used to design an optimal structure. The model is simplified to obtain a graphical tool and an analytical model which is validated to estimate the overall reactor kinetics as a function of dimensionless groups. For a conventional range of parameters, the optimal catalyst layer thickness and optimal number of structural layers was equal to 2 µm and 4, respectively. The presented tools in this work are a step forward in the fabrication of design methods for photocatalytic reactor structures. This way, the designer can easily estimate the design outcome without any complex calculations.

Kinetic Modelling and Simulation Studies for the Esterification Process with Amberlyst 16 Resin

The reaction of methanol with acetic acid in a simple isothermal batch reactor was carried out to produce methyl acetate and water with and without catalyst. Amberlyst 16 used as the solid resin catalyst. The reaction mixture temperature was adjusted for 313.15 K - 353.15 K with and without Amberlyst 16. The concentration for catalyst changed from 0.048 mol H + /L - 0.24 mol H + /L based on volume of mixture. The reactant mole ratio for acid to alcohol was varied from 1:1 - 1:4. The influence of catalyst loading, reaction mixture temperature, initial reactant mole ratio, catalyst size, agitation speed on acetic acid conversion has been investigated. An experimental results show that reaction is kinetically controlled when compared to internal and external mass transfer effects. Kinetic rate equation of second order is used to fit experimental data. The reaction rate constants and activation energies were calculated from Arrhenius plot with and without catalyst. This equation can be used in the modelling and simulation of reactive distillation process.

Internal mass transfer considerations in biofilms of succinic acid producing Actinobacillus succinogenes

The rumen bacterium Actinobacillus succinogenes is reputable for its high productivity, -yield and -titre fermentative production of succinic acid under biofilm conditions. The paper presents an analysis of internal mass transfer effects in biofilm fermentations of A. succinogenes. Internal mass transfer effects were investigated by batch fermentations using attached- and resuspended biofilms as biocatalysts. In the latter, the biofilms were resuspended after initial development to simulate mass transfer free (free cell) fermentations. Intrinsic kinetics for succinic production obtained from resuspended free cell fermentations predicted faster production rates than for the attached biofilms runs (biofilm thicknesses in the range of 120–200 µm), indicating internal mass transfer restrictions. A developed biofilm reaction diffusion model gave good predictions of attached biofilm batch results by accounting for internal mass transfer in the biofilm. Biofilm effectiveness factors ranged from 75% to 97% for all batches at the inception of batch conditions but increased with progression of batch operation due to increased succinic acid titres which inhibited production rates. Biofilm thickness and succinic acid concentrations were shown to have a significant effect on internal mass transfer. A simplified algorithm was developed to estimate the pseudo-steady state glucose penetration and biofilm effectiveness of A. succinogenes biofilms without the requirement to solve the overall mass transfer model. The results clearly showed that internal mass transfer need to be considered in biofilm fermentations involving A succinogenes as high biomass concentrations may not always equate to increased productivities if mass transfer effects dominate.

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...

Production of isoamyl acetate by enzymatic reactions in batch and packed bed reactors with supercritical CO2

Batch reactors (BRs) are probably the most widely used reactor designs for biocatalysis in supercritical fluids, but for industrial scale applications packed bed reactors (PBRs) are frequently preferred. The objective of this work was to investigate and compare BR and PBR in the synthesis of isoamyl acetate through enzymatic reactions in SCCO2 media. Experiments were performed to determine the kinetic parameters and to evaluate the internal and external mass transfer effects. The results indicate that the reaction is kinetically controlled, independently of particle diameter and agitation, therefore intra-particle diffusion and external mass transfer resistance are unimportant. The kinetic Ping-Pong Bi–Bi model described well the esterification of isoamyl alcohol with acetic anhydride at the evaluated conditions, indicating that the affinity of the enzyme to acetic anhydride is larger than to isoamyl alcohol. The PBR achieved the highest productivity of isoamyl acetate, although its conversion was lower than that obtained by the BR. In summary, PBR is the best option to synthesize isoamyl acetate in SC-CO2 media.

Kinetics of hydrogenation and hydrogenolysis of 2,5-dimethylfuran over noble metals catalysts under mild conditions

Ketones, derived via the selective ring-opening hydrogenation of biomass-derived furanic compounds, are useful synthons for the production of fuels and lubricant. In this study, 2,5-dimethylfuran (DMF) was used as a model to investigate the liquid-phase hydrogenation of substituted furans over noble metals. The activity and selectivity of carbon-supported Pt, Pd, Rh, and Ru were compared under identical conditions. Pt/C exhibited the highest activity and selectivity for CO bond hydrogenolysis, whereas all other catalysts exhibited a high selectivity for ring hydrogenation to form 2,5-dimethyltetrahydrofuran. The reaction kinetics were measured in detail for liquid-phase hydrogenation of DMF over Pt/C, under conditions that excluded the effects of internal or external mass transfer (333K–363K, pH2=0.41–0.69MPa). A reaction mechanism is proposed to explain the parallel formation of 2,5-dimethyltetrahydrofuran, 2-hexanone, and 2-hexanol. This mechanism is consistent with what is known about the adsorption of DMF on Pt, and explains why ring opening occurs directly from adsorbed DMF and not via secondary reaction of 2,5-dimethyltetrahydrofuran. The kinetics of DMF hydrogenation are well represented by the mechanism and the assumption of non-competitive adsorption of organic species and hydrogen atoms. The step involving CO bond cleavage of the aromatic ring had the highest activation energy, 45.4kJ/mol, and is favored over the steps involving CC bond hydrogenation of the furan ring at elevated temperatures and low pH2. The reaction kinetics reveal that DMF hydrogenation can be tuned to produce 2-hexanone with 92% selectivity at 100% conversion under relatively mild liquid phase conditions (0.41MPa, 393K).

Kinetic modeling of the simultaneous etherification of ethanol with C4 and C5 olefins over Amberlyst™ 35 using model averaging

A kinetic study on the simultaneous liquid-phase etherification of ethanol with isobutene (IB), 2-methyl-1-butene (2M1B) and 2-methyl-2-butene (2M2B) catalyzed by Amberlyst™ 35 to form ethyl tert-butyl ether (ETBE) and tert-amyl ethyl ether (TAEE) is presented. Isothermal experimental runs were carried out in a stirred tank batch reactor in the temperature range 323–353K at 2.0MPa, starting from different initial concentrations. Obtained reaction rates were free of catalyst load, internal, and external mass transfer effects. Mathematical fitting of a series of systematically originated models, model selection, and model averaging procedures were applied to find the best model and to draw conclusions about the reaction mechanism. The selected model involves a saturated catalytic surface with the participation of two active sites in etherification reactions and one active site in isoamylenes isomerization. Apparent activation energies for ETBE formation from IB and EtOH, TAEE formation from 2M1B and EtOH, TAEE formation from 2M2B and EtOH, and double bond isomerization between 2M1B and 2M2B were 72.8±1.4, 74.9±2.8, 81.2±2.2 and, 76.5±7.2kJ/mol, respectively. The alkenes with the double bond in terminal position were more reactive towards EtOH than 2M2B, with the double bond in internal position.

Effects of mass transfer on Fischer-Tropsch kinetics over mesoporous silica-supported Co[sbnd]Mn[sbnd]Ce nano catalysts in a fixed-bed reactor

An impregnated ternary mixed oxide (CoMnCe) on nano silica support was applied, as a novel Fischer-Tropsch catalyst, in a fixed-bed reactor. Using catalysts of various pellet sizes, the Thiele modulus was determined to quantify regions of pore diffusion resistance and surface reaction. Mechanistic studies were conducted via intrinsic kinetic experiments under no pore diffusion conditions (i.e. Thiele modulus<0.4) as follows: T = 483.15–583.15 k, P = 2–6 bar, H2/CO = 1–2, and GHSV = 4500 hr−1. Based on Langmuir–Hinshelwood–Hougen–Watson (LHHW) approach, the optimum mechanism was found to be associative adsorption of CO and H2 with the irreversible associative hydrogen, with the carbon monoxide adsorption determined as the rate-controlling step. For the optimum model, intrinsic activation energy, R-squared value, and mean absolute relative residual were obtained as 62 kj/mol, 0.97 and 10.41%, respectively. Finally, a promising graphical approach was proposed to predict effective reaction rate, considering the effects of internal mass transfer in catalyst beads.

Hydrogen production from steam reforming of methanol over CuO/ZnO/Al2O3 catalysts: Catalytic performance and kinetic modeling

A series of CuO/ZnO/Al2O3, CuO/ZnO/ZrO2/Al2O3 and CuO/ZnO/CeO2/Al2O3 catalysts were prepared by coprecipitation and characterized by N2 adsorption, XRD, TPR, N2O titration and HRTEM. The catalytic performances of these catalysts for the steam reforming of methanol were evaluated in a laboratory-scale fixed-bed reactor at 0.1MPa and temperatures between 473 and 543K. The results showed that the catalytic activity depended greatly on the catalyst reducibility and the specific surface area of Cu. An approximate linear correlation between the catalytic activity and the Cu surface area was found for all catalysts investigated in this study. Compared to CuO/ZnO/Al2O3, the ZrO2-doped CuO/ZnO/Al2O3 exhibited higher activity and selectivity to CO, while the CeO2-doped catalyst displayed lower activity and selectivity. Finally, an intrinsic kinetic study was carried out over a screened CuO/ZnO/CeO2/Al2O3 catalyst in the absence of internal and external mass transfer effects. A good agreement was observed between the model-derived effluent concentrations of CO (CO2) and the experimental data. The activation energies for the reactions of methanol-steam reforming, water-gas shift and methanol decomposition over CuO/ZnO/CeO2/Al2O3 were 93.1, 85.1 and 116.5kJ·mol−1, respectively.

Mass Transfer Effects in Stagnation Flows on a Porous Catalyst: Water-Gas-Shift Reaction Over Rh/Al2O3

Abstract Water-gas-shift (WGS) and reverse water-gas-shift (RWGS) reactions are numerically investigated in a stagnation-flow on a porous Rh/Al2O3 catalyst. External and internal mass transfer effects are studied using three different models for the mass transport and chemical conversion inside the porous catalyst: the dusty-gas model, a set of reaction-diffusion equations, and the effectiveness factor approach. All three models are coupled with the boundary layer equations to describe the potential flow on the stagnation disc, and a multi-step surface reaction mechanism is implemented. The numerically predicted species profiles in the external boundary layer are compared with recently measured profiles. Internal mass transfer limitations are more significant than external ones in case of the 100 μm thick catalyst layer. The effects of catalyst structure (thickness, mean pore diameter, porosity, tortuosity) as well as flow rate and pressure on chemical conversion are discussed.

Mass transfer characterization of gamma-aminobutyric acid production by Enterococcus faecium CFR 3003: encapsulation improves its survival under simulated gastro-intestinal conditions.

Gamma-aminobutyric acid (GABA) production by free and Ca-alginate encapsulated cells of Enterococcus faecium CFR 3003 was investigated. Mass transfer rates characterizing the GABA production process using encapsulated cells were investigated. Experiments were performed to investigate external film and internal pore diffusion mass transfer rates. The Damkohler and Thiele analysis provides a good description of external film and internal pore diffusion resistances, respectively. The experiments revealed that the external film effects could be neglected but the process is affected to the greater extent by internal mass transfer effects and was found to be the principal rate-controlling step. Protective effect of encapsulation on cell survivability was tested under digestive environment, when challenged to salivary α-amylase, simulated gastric fluid and intestinal fluid. Viability of encapsulated cells was significantly higher under simulated gastro-intestinal conditions and could produce higher GABA than those observed with free cells. The results indicate that the Ca-alginate encapsulated probiotics could effectively be delivered to the colonic site for effective inhibitory action.

Numerical modeling of stagnation-flows on porous catalytic surfaces: CO oxidation on Rh/Al2O3

A stagnation-flow on a catalytic porous plate is modeled one-dimensionally coupled with multi-step surface reaction mechanisms and molecular transport (diffusion and conduction) in the flow field and the porous catalyst. Internal mass transport inside the porous catalyst is studied with three different models: instantaneous diffusion (infinitely fast mass transport), effectiveness factor, and one-dimensional reaction-diffusion equations. A new computer code, DETCHEMSTAG, is presented to execute the numerical model. The oxidation of CO over a porous Rh/Al2O3 surface is studied exemplarily. Experimental measurements are carried out to apply the developed model and the computer code. External and the internal mass transfer effects in front of and inside the porous catalyst are discussed. Internal mass transfer limitations become important in case of a thick catalyst layer for accurately predicting the experimental results.

Sorption and diffusion parameters from vacuum-TPD of ammonia on H-ZSM-5

This work aims at filling the gap in vacuum-TPD modeling methodology for microporous samples. The specific objective was to assess and distinguish external and internal mass transfer effects from the intrinsic sorption dynamics during temperature-programmed desorption, as illustrated by ammonia on H-ZSM-5. The external mass transfer pattern was confirmed to be free of bed-depth effects, the intraparticle mass transfer resistance proved to be significant in the ammonia-TPD system, and equipment-related artefacts showed to be negligible based on preliminary experiments. Thus a consistent set of 10 TPD curves was collected, including two adsorption temperatures, three heating rates and two separate particle fractions. The experimental data was successfully modeled with a system including intraparticle mass transfer phenomena and intrinsic sorption kinetics. By combining a transient kinetic approach to a well-designed set of high-quality experiments vacuum-TPD can provide decoupled information on mass transfer and sorption for porous materials as we demonstrate in this work.

Drying in porous media with gravity-stabilized fronts: Experimental results

In a recent paper [Yiotis et al., Phys. Rev. E 85, 046308 (2012)] we developed a model for the drying of porous media in the presence of gravity. It incorporated effects of corner film flow, internal and external mass transfer, and the effect of gravity. Analytical results were derived when gravity opposes drying and hence leads to a stable percolation drying front. In this paper, we test the theory using laboratory experiments. A series of isothermal drying experiments in glass bead packings saturated with volatile hydrocarbons is conducted. The transparent glass cells containing the packing allow for the visual monitoring of the phase distribution patterns below the surface, including the formation of liquid films, as the gaseous phase invades the pore space, and for the control of the thickness of the diffusive mass boundary layer over the packing. The experimental results agree very well with theory, provided that the latter is generalized to account for the effects of corner roundness in the film region (which was neglected in the theoretical part). We demonstrate the existence of an early constant rate period (CRP), which lasts as long as the films saturate the surface of the packing, and of a subsequent falling rate period (FRP), which begins practically after the detachment of the film tips from the external surface. During the CRP, the process is controlled by diffusion within the stagnant gaseous phase in the upper part of the cells, yielding a Stefan tube problem solution. During the FRP, the process is controlled by diffusion within the packing, with a drying rate inversely proportional to the observed position of the film tips in the cell. Theoretical and experimental results compare favorably for a specific value of the roundness of the films, which is found to be constant and equal to 0.2 for various conditions, and verify the theoretical dependence on the capillary Ca(f), Bond Bo, and Sherwood Sh numbers.

Kinetic and diffusion study of acid-catalyzed liquid-phase alkyl formates hydrolysis

Alkyl formates are typically hydrolyzed with the aid of homogeneous catalysts, but heterogeneous catalysts provide a promising pathway for the process. The hydrolysis of alkyl formates by a solid acid catalyst, ion-exchange resin was accomplished in a batch reactor, a stirred autoclave operating isothermally at 60 °C and 90 °C with a constant initial water-to-ester molar ratio. A mathematical model, which incorporates the particle size distribution of the solid catalyst, was developed to study the kinetics and internal mass transfer effects in the porous particles and it was able to predict the concentrations in the bulk phase and inside the catalyst particles. A combined reaction–diffusion model is necessary to describe the behavior of the system. The model was able to predict well the experimental results.

Efficient simulation of an ammonia oxidation reactor using a solution mapping approach

A highly efficient single channel monolith reactor model for the oxidation of ammonia is implemented. A mechanistic model for ammonia oxidation on platinum is used, and all internal and external mass transfer effects are included. The model efficiency is derived from the use of pre-computed solutions of the mass balance equations. Spline interpolation functions are constructed from pre-computed solutions of the mass balances of one volume element of the reactor model. It is shown that these spline functions reproduce the exact outlet concentrations of the volume element with a relative error of less than 3%. This solution mapping approach allows computation of concentration profiles in the reactor by simple successive calls of the interpolation function without any numerical solution. Outlet concentrations of the complete reactor computed in this way show a relative error of less than 2%. Application of the solution mapping approach speeds up the solution of the concentration profiles by a factor of more than 3200, compared to the numerical solution of the mass balances. In this way one steady state concentration profile in an ammonia oxidation catalyst can be computed in less than 0.003 s, despite the fact that the model uses mechanistic surface kinetics and includes the radial diffusion in the washcoat.

Solid-liquid reaction kinetics – experimental aspects and model development

Abstract Solid-liquid reactions are commonly encountered in industry as well as every day life. Vast amounts of the applied bulk and specialty chemicals are produced employing solid-liquid reac-tions. The knowledge of the kinetics is crucial for the design and development of these processes. Quantitative modeling of reactive solids behavior in liquids is a big challenge. The reaction mechanism can be a very complex one, often com-prising several unknown elementary steps. The structure and structural changes of the solid material are also often diffi cult to determine. Kinetics and mass transfer effects are coupled; to determine the intrinsic kinetics, the experiments should be free from mass transfer limitations. External mass transfer resistance can be suppressed by creating strong enough turbu-lent effects around the solid particles, but internal mass trans-fer effects can be present for porous particles. Due to these facts, modeling of dissolution reactions is often simplifi ed. Even though such simplifi cations are necessary, a more in-depth investigation of the related phenomena combined with quantitative studies and more thorough modeling brings new understanding and precision to process development and opti-mization. Several example cases of solid-liquid reactions were chosen for this work, based on their industrial relevance and varying nature. Moreover, different experimental cases were studied to illustrate a basis for the theoretical development. Two of the cases are organic reactions, while the rest are inor-ganic ones. The objective of the current review is to exemplify challenges related to studying solid-liquid reactions and how these challenges can be overcome. This is done through the nine example cases, which serve as good examples of the vari-ous diffi culties often encountered. The published articles serve as a convenient route for fi nding more detailed information on the topic. Based on the literature evaluation and the experi-ence gained in the present work, theoretical development was made based on physico-chemical fundamentals. The main advances of the current work are in the quantitative modeling of a solid phase during its reaction with a liquid. This includes the dynamic and fl exible implementation of the surface mor-phology and particle size distribution, as well as the reaction mechanism into an overall kinetic model, in which the internal and external mass transfer phenomena are taken into account. This helps in model development and discrimination between rival models, as well as in the interpretation of curiosities in kinetic models which can be found in literature.