Internal Mass Transfer Research Articles

Enhancement in synthesis of citronellyl laurate flavour by combined effect of ultrasound and immobilized lipase as heterogeneous biocatalyst

This study aims to improve the performance of heterogeneous biocatalysts by using the combination of immobilized and ultrasonic lipases in the esterification reaction for the synthesis of citronellyl lauric flavors. Synthesis was conducted out through the reaction of citronellol with lauric acid by using immobilized lipase and ultrasonic. The results showed that immobilized lipase provided stability for up to 9 weeks and could be reused for 5 cycles of reactions. Immobilized lipase 473 U / gram was applied to the synthesis of citronellyl laurate with a reaction time of 90 minutes, 450C, with and without ultrasonic. The result of citronellol conversion with ultrasonic is 3.6 x higher than conventional methode. It shows that the immobilization method provided stability to the biocatalyst, but because it had a limited internal mass transfer so the rate of reaction of ester formation reduced. On the other hand, ultrasonic contributed to increase the dispersion and collision of substrate molecules, reducing reaction time, and intensifying catalytic processes. Thus, the combination of immobilized enzyme and ultrasonic methods can be applied to biochemical processes widely.

Electroless copper deposition and interface characteristics of ionic electroactive polymer

In this study, the cheap copper electrode ionic polymer metal composite (IPMC) was prepared by chemical deposition. The formation mechanism of the copper electrode layer was analyzed firstly, then the surface and cross section morphology, roughness and porosity of IPMC were measured, and the interface profile was simulated and predicted based on Weierstrass-Mandelbrot (WM) function, at last, the adhesion force of the interface was calculated. The results show that the coarsening of Nafion and the adsorption of Ag+ ions provide a good catalytic activation site for the nucleation and rapid growth of copper particles. The copper layer on the surface of IPMC is arranged in granular, distributed compactly and uniformly, with the porosity of 52.3% and a strong (111) preferred orientation, which are beneficial to the performance of IPMC. Based on the WM function, the fractal dimension, characteristic scale and characteristic roughness are used to reflect the flatness and complexity of surface profile. The adhesive strength between the substrate and the coating is mainly due to the surface force field caused by the coarsening treatment. The theoretical and simulation studies of the interface characteristics are helpful to understand the internal mass transfer mechanism of IPMC and characterize the complex interface morphology, which also lays a foundation for the later research and application of IPMC.

Simulation of Reactors under Different Thermal Regimes and Study of the Internal Diffusional Limitation in a Fixed-Bed Reactor for the Direct Synthesis of Dimethyl Ether from a CO2-Rich Input Mixture and H2

This paper presents the simulation of reactors under different thermal regimes using the most used kinetic models for the simulation of the direct synthesis of DME with the objective of optimizing the coupling between heat and mass transfer. A pseudo-homogeneous and a heterogeneous plug-flow model are developed to simulate isothermal, adiabatic, and isoperibolic fixed-bed (shell-and-tube) reactors for the direct synthesis of DME from CO2 and H2, enabling the analysis of the limitations by the internal mass transfer. Concentrations, reaction rates, and temperature gradients within the catalyst are assessed. Catalytic efficiency factors are calculated and represented along the reactor tube. The results show that the choice of the kinetic model is crucial for optimal coupling of these functions since it strongly influences the design and sizing of the reactor for synthesis of DME from CO2-rich feedstocks. Therefore, this analysis demonstrates that the direct synthesis of DME can be limited by internal mass transfer, which promotes the DME synthesis at low conversions, therefore requiring particular attention for future reactor optimization.

Kinetics Study of the Hydrodeoxygenation of Xylitol over a ReOx-Pd/CeO2 Catalyst

In this study, we elucidate the reaction kinetics for the simultaneous hydrodeoxygenation of xylitol to 1,2-dideoxypentitol and 1,2,5-pentanetriol over a ReOx-Pd/CeO2 (2.0 weight% Re, 0.30 weight% Pd) catalyst. The reaction was determined to be a zero-order reaction with respect to xylitol. The activation energy was elucidated through an Arrhenius relationship as well as non-Arrhenius kinetics. The Arrhenius relationship was investigated at 150–170 °C and a constant H2 pressure of 10 bar resulting in an activation energy of 48.7 ± 10.5 kJ/mol. The investigation of non-Arrhenius kinetics was conducted at 120–170 °C and a sub-Arrhenius relation was elucidated with activation energy being dependent on temperature, and ranging from 10.2–51.8 kJ/mol in the temperature range investigated. Internal and external mass transfer were investigated through evaluating the Weisz–Prater criterion and the effect of varying stirring rate on the reaction rate, respectively. There were no internal or external mass transfer limitations present in the reaction.

An assessment on kinetic modeling of esterification reaction from oleic acid and methyl acetate over USY zeolite

In this work, commercial USY Zeolite was employed in oleic acid with methyl acetate esterification. Kinetic studies of esterification were carried out at methyl acetate to the oleic acid molar ratio of 10:1, with 5 wt% and 10 wt% of catalyst content and at 453, 473, 493 and 513 K. The Pseudo-homogeneous (PH) and Heterogeneous kinetic models, according to the Eley-Rideal (ER) and the Langmuir-Hinshelwood (LH) mechanisms, were investigated to correlate experimental data. The Thiele modulus analysis confirmed that the effect of the pore sizes on the internal mass transfer limitations could be neglected, and the study of the internal and external mass transfer limitations showed that the chemical reaction is the rate-controlling step. The coefficient of determination (R 2 ), the Akaike's information criterion (AIC), the relative Akaike weight (λ i ), and Spearman's rank correlation coefficient ( ρ i , j ) were utilized to evaluate the kinetic model's adequacy. The PH model provided the best results according to Akaike's information criterion (AIC) and relative Akaike weight (λ i ). Spearman's rank correlation coefficient ( ρ i , j ) showed that some parameters of ER and LH models presented high correlation and confidence intervals in their parameters, not satisfactorily representing the observed kinetic data. The thermodynamic study through PH model shows that oleic acid with methyl acetate esterification is an endothermic (ΔH = 59.83 kJ mol −1 ) and endergonic (ΔG = 118.66–126.45 kJ mol −1 ) reaction. The activation energy ( E a ) was 41.77 kJ mol −1 , and the negative value of entropy variation (ΔS = −129.8 J mol −1 K −1 ) might be associated with its associative mechanism. The decrease of catalyzed activation energy compared to the uncatalyzed reaction indicates that the USY zeolite was active to the esterification of oleic acid with methyl acetate. • USY zeolite was active in the esterification reaction with methyl acetate. • Three kinetic models were assessed and fitted well to experimental data. • Effect of pore sizes on the internal mass transfer limitations could be neglected. • Statistical criterion discriminated overparametrized models and unreliable parameters. • Pseudo-Homogeneous model was the most suitable.

To the justification of the calculation of the heat conduction process in the dough piece

The main process that ensures the production of bread as a marketable product is the external thermal energy supply during baking. This process is accompanied by a complex physical phenomenon - moisture exchange between the dough-bread with the external environment and the steam-air environment of the baking chamber, as well as internal heat and mass transfer in the heated dough piece. At the same time, biochemical processes associated with the transformation of starch and proteins. Taking into account that the main factor realizing the efficiency and quality indicators of baked bread is heat exchange in the dough piece, in this article, taking into account the peculiarities of the dynamics of the heat transfer phenomenon, a quantitative analysis of the course of this process is carried out. In this work, in contrast to the results obtained by other researchers in the field of calculating the processes of heat treatment of food products, a quantitative analysis of the process is proposed based on a new methodological approach to the problem of physical and mathematical substantiation of the heat transfer process in a dough piece. That can be effectively used in the simulation of innovative samples of baking equipment.

Interaction of Intrinsic Kinetics, Catalyst Durability and Internal Mass Transfer in the Oxidation of Sugar Mixtures on Gold Nanoparticle Extrudates

Sugar monomers originating from well-controlled hydrolysis of hemicelluloses appearing in biomass are important platform molecules and they can be further valorized by catalytic hydrogenation, oxid...

Analysis of herbicide biosorption by means of a phenomenological mathematical distributed parameter model

ABSTRACT A distributed parameter model and two lumped parameter models were used in order to find the rate-limiting step in the adsorption process of a herbicide (Diuron) by Moringa oleifera husks, a possible low-cost adsorbent. For that, four kinetics assays, differentiated by the initial Diuron concentration, were performed. Langmuir isotherm well represented the equilibrium data and through this evaluation, Moringa husks proved to be a potential adsorbent for Diuron removal from water. The internal mass transfer resistance, analysed as a distributed parameter model, was found to better represent the experimental data. This fact enabled the simulation of the process according to the variation of time and space, which contributed to the better understanding of the adsorption process.

COMPUTER PREDICTION OF TECHNOLOGICAL REGIMES OF RAPID CONE-SHAPED ADSORPTION FILTERS WITH CHEMICAL REGENERATION OF HOMOGENEOUS POROUS LOADS

Mathematical models for predicting technological regimes of filtration (water purification from the present impurities), backwashing, chemical regeneration and direct washing of rapid cone-shaped adsorption filters, taking into account the influence of temperature effects on the internal mass transfer kinetics at constant rates of the appropriate regimes, are formulated. Algorithms for numerical-asymptotic approximations of solutions of the corresponding nonlinear singularly perturbed boundary value problems for a model cone-shaped domain bounded by two equipotential surfaces and a flow surface are obtained. The proposed models in the complex allow computer experiments to be conducted to investigate the change of impurity concentrations in the filtration flow and on the surface of the load adsorbent, temperature of the filtration flow, filtration coefficient and active porosity along the filter height due to adsorption and desorption processes, and on their basis, to predict a good use of adsorbents and increase the protective time of rapid cone-shaped adsorption filters with chemical regeneration of homogeneous porous loads.

Decolorization kinetics and mass transfer mechanisms of Remazol Brilliant Blue R dye mediated by different fungi

The release of synthetic dye into the environment causing abnormal growth of phytoplankton may lead to a decline in the photosynthetic performance of aquatic ecosystem. Scientific knowledge of Remazol Brilliant Blue R (RBBR) decolorization is essential for designing the engineered bioremediation systems of employing fungal mycelium. The biodegradation of RBBR dye mediated by an appropriate fungus was analyzed using the modified mass transfer factor models to get better understanding on the decolorization kinetics and mechanisms of external and internal mass transfer. The results showed that the limited capacities of the kinetic and isotherm models are still not able to comprehensively explain many important phenomena of RBBR decolorization mediated by the T. citrinoviride, T. koningiopsis and Pestalotiopsis sp. strains. The rate-limiting step of RBBR decolorization depends on the EMT resistance and the vegetative growth rates of T. citrinoviride, T. koningiopsis and Pestalotiopsis sp. strains can be described by second-order polynomial equation. The analysis of decolorization performance may provide a new insight on the role of fungus in the degradation of RBBR dye.

Liquid–Solid Mass Transfer in Adsorption Systems—An Overlooked Resistance?

In liquid-solid adsorption, fluid film diffusion is typically faster than intraparticle diffusion, especially for microporous adsorbents. However, fluid film diffusion might play a significant role in the overall rate of the process for mesoporous-macroporous and non-porous solids. In most adsorption modeling studies, the fluid film diffusion step is typically ignored, which is not always justified. This article critically discusses the theory behind the liquid-solid mass-transfer coefficient in stirred vessels and presents the dissolution and adsorption methods adopted for estimating its value. Then, starting from the definition of the Biot number, an original analysis is developed with reference to selected studies. Surface versus pore diffusion of the adsorbate in the adsorbent is taken into account, and external versus internal mass-transfer resistance is considered to put the fluid film resistance back in the picture when needed. Iso-Biot charts where the operating points can be visualized are presented as well.

Kinetic study for styrene carbonate synthesis via CO2 cycloaddition to styrene oxide using silica-supported pyrrolidinopyridinium iodide catalyst

Reaction kinetics for heterogeneous catalysis of styrene carbonate (SC) synthesis via CO2 cycloaddition to styrene oxide (SO) using silica-supported pyrrolidinopyridinium iodide (SiO2-PPI) catalyst has been investigated. The results obtained from the mixing study and theoretical analysis based on Thiele modulus (< 0.4) and effectiveness factor (∼1.0) revealed no external and internal mass transfer limitations. The experimental results were compared with pseudo-homogeneous (PH) and Langmuir-Hinshelwood (LH), Eley-Rideal (ER) heterogeneous kinetic model. The experimental data was found to be the best fit with the ER model among the three kinetic models. Moreover, the Arrhenius and Eyring equations were used to determining the activation energy (i.e. 64.9 kJ mol–1) and thermodynamic activation parameters such as enthalpy of activation (ΔH‡= 60.6 kJ mol–1), the entropy of activation (ΔS‡ = –148.3 J mol–1 K–1) and Gibb’s free energy (ΔG‡ = 115.9 kJ mol–1 at 373 K). The SiO2-PPI heterogeneous catalyst exhibited excellent reusability: up to five cycles without any substantial decrease in activity and selectivity towards SC formation.

Thermal Management of Edge-Cooled 1 kW Portable Proton Exchange Membrane Fuel Cell Stack

Air-cooled proton exchange membrane (PEM) fuel cell stacks are gaining momentum as power sources for portable applications such as electric bikes, unmanned aerial vehicles (UAVs), forklifts, etc. Usually the power of portable stacks (PS) is up to several kW [1,2]. Most of the heat generated during PS operation is dissipated into the surrounding atmosphere via forced air convection along the cooling channels or edge-cooling fins, while the higher power stacks require liquid cooling. The emphasis during the design phase of PS is primarily directed towards determining the most suitable material for bipolar plates and the accompanying thermal management. Some of the thermal management objectives are: (i) ensuring the appropriate heat transfer from the stack during startup, i.e. accurately determining the required convection flow rate, to achieve sufficiently high operating temperature from startup at ambient temperature, thus preventing flooding (occurring if the flow rate is high and temperature of the stack is low) or membrane dehydration (occurring if the flow rate is low and temperature inside the stack is high); (ii) ensuring sufficiently high heat removal rates via forced air convection during the operation characterized by stable temperature of the stack, i.e. loosely termed steady-state operation; (iii) ensuring as low as possible temperature difference between the inside and outside portions of the stack by choosing the materials with favorable thermal properties; (iv) properly design the stack and edge-cooling fins to ensure sufficiently high heat removal rates; (v) design the frame for the stack which will ensure that the cooling air is directed to uniformly flow along the edge-cooling fins. In previous studies [3,4] it was shown that the operating temperature inside PEM fuel cell is highly non-uniform and it is not accurate to represent the operating temperature with just one parameter, while at the same time if the temperature profile is controlled it can be exploited to result in high performance of the cell by manipulating the water vapor saturation profile, i.e. relative humidity profile inside the cell. The main contributions of this work to the research field are evident in outlining that the commonly used lumped models can be misleading for dynamic analysis of PEM fuel cell performance due to inability to accurately calculate the maximal temperature inside the stack and this work also outlines under which circumstances the lumped models can be used. This work also presents a novel transient CFD model for analysis of thermal management of PS which gives insight in mass and heat transfer both inside and outside of the cell (internal heat and mass transfer and forced air convection edge-cooling), while other works focus on only one aspect and simplify the other to a significant extent.In order to study thermal management influence on the performance od 1kW edge-cooled proton exchange membrane fuel cell stack without external humidification comprehensive numerical analyses are conducted. Two numerical approaches are considered and compared for a prescribed load profile: (i) lumped model and novel (ii) real-time transient computational fluid dynamics model incorporating realistic modeling of forced air convection on the edge-cooling of the stack. The developed computational fluid dynamics model is used to study the influence of (i) bipolar plate materials (ii) operating delta pressure along the flow field and (iii) different cooling fin configurations on the water and heat balance inside the stack. The results indicate that (i) maximal and average temperatures of the stack are almost linearly correlated to the thermal conductivity of bipolar plate materials and maximal temperatures can be significantly higher (ii) the operating delta pressure can be manipulated to increase the performance of the stack and (iii) the cooling fin redesign has major influence on the overall temperature uniformity across the stack. Additionally, the heat transfer between the stack and metal hydride tank is studied.

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.

Industrial scale 3D printed catalytic converter for emissions control in a dual-fuel heavy-duty engine

A full size ceramic substrate was successfully prepared using a robocasting 3D printer and tested as a methane oxidation catalyst in the after treatment system (ATS) of a heavy-duty diesel engine, converted to co-combust (dual fuel) with natural gas (NG). The 3D printed substrate performance exceeded that of a commercially sourced straight-channelled DOC over most working conditions, despite the 3D printed structure having a lower precious group metal (PGM) loading and channels per square inch (CPSI) density. At moderate and high inlet temperatures, where the reaction rate is limited by internal and external mass transfer, the enhanced catalytic activity of the 3D printed substrate is attributed to the generation of internal turbulence, which increases oxidation rates of methane (CH4) and non-methane hydrocarbons (NMHC). In contrast, there is relatively little difference between the catalytic activity of the 3D and straight-channelled substrates at low temperatures (e.g. cold start up), where the reaction is kinetically controlled and the additional turbulence/mass transfer of the 3D printed complex structure did not measurably alter the catalytic converter performance. Computational fluid dynamics (CFD) confirmed the increased turbulence within the channels of the 3D printed structure. We also report the effects of NG substitution on the fuel combustion efficiency under different engine load settings. The findings provide proof of concept evidence that 3D printing is a suitable means of designing a catalytic converter prototype with higher reaction activity than a conventionally extruded structure. This has significant implications for the design and potential mass production of new catalytic converters with enhanced efficiencies.

Mathematical Modeling of the Process of Convective Drying of Materials Taking into Account their Shrinkage

The authors have discussed the physical regularities of internal mass transfer in drying colloidal capillary-porous materials, which is characterized by the fact that in addition to the phenomenon of mass conductivity (diffusion of moisture) in a fixed coordinate system, the transfer of moisture by the matrix of the material due to its shrinkage is observed. As applied to this case, for a body in the shape of an unbounded plate dried symmetrically from both surfaces, a mathematical model has been written that describes mass conductivity and convective transfer of moisture in the body. The convective transfer of moisture depends on the normal coordinate to the plate’s surface: it is maximum at the surface of the plate and is equal to zero in its central plane. The authors have formulated the problem describing the process of symmetric drying of an unbounded plate on the basis of the convective-diffusion equation with a uniform initial distribution of the concentration and a boundary condition of mass transfer of the third kind. It has been noted that the formulated problem can be solved by numerical methods. For analysis of the degree of influence of the convective component on internal mass transfer, an analytical solution to the problem at constant parameters of the process has been obtained. According to the obtained solution, calculations of the change in the volume mean relative concentration as a function of the Fom number have been performed for the internal problem (Fom = 100) and the mixed diffusion problem (Fom = 10). It has been shown that the maximum effect of the influence of the convective component is observed for the intradiffusion problem.

Highly active and efficient Cu-based hydrotalcite-like structured materials as reusable heterogeneous catalysts used for transcarbonation reaction

A series of robust and efficient Cu-Al hydrotalcite-like compounds (HTLc) as catalysts were prepared by the simple precipitation method with different Cu/Al molar ratios and investigated for the transcarbonation of glycerol with dimethyl carbonate (DMC) for glycerol carbonate (GC) synthesis in a batch reactor. The structural and textural properties of the Cu-Al (HTLc) catalysts were analyzed by several methods like N2-sorption, SEM-EDX/TEM, XRD, FTIR, CO2-TPD, TGA/DTA and ICP-OES. It was found that the transcarbonation of glycerol is directly dependent on the strong basic sites of the catalysts. The Cu/Al molar ratio has easily tuned the glycerol conversion and the GC yield. Among all synthesized catalysts, the Copper-Aluminum (3Cu-Al@500) catalyst showed excellent catalytic activity for a glycerol conversion (96%) and a GC yield (86%) with reaction rate (irrespective to glycerol) of approximately 0.106 mol L−1 h−1. Furthermore, the optimization of the reaction conditions (i.e. molar ratio of the reactants, catalyst mass, reaction time and temperature) and the reusability of the 3Cu-Al@500 catalyst for glycerol conversion and GC yield with TOF value were studied. In addition, the effect of stirring speed and particle size on the minimization of external and internal mass transfer resistance, respectively, was investigated.

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.

Pyrolysis of single large biomass particle: Simulation and experiments

Pyrolysis and heat transfer characteristics of single large biomass particle were investigated using three-dimensional unsteady heat transfer model coupled with chemical reactions. The consumption of biomass and the production of products were simulated. Some experiments were designed to provide model parameters for simulation calculations. The simulation was verified by pyrolysis experiments of large biomass particle in a vertical tube furnace. The simulation results show the internal heat and mass transfer law during the pyrolysis of large biomass particle. When the biomass particle diameter is between 10 and 30 mm, for every 5 mm increase in particle diameter, the time required for complete pyrolysis will increase on average by about 50 s. When the pyrolysis temperature is between 673 K and 873 K, a slight decrease in the pyrolysis temperature will cause the time required for the biomass to fully pyrolyze to rise significantly. And the phenomenon is more obvious in the low temperature range. The results indicate that the numerical simulation agrees well with the experimental results.

Testing Metal–Organic Framework Catalysts in a Microreactor for Ethyl Paraoxon Hydrolysis

We explored the practical advantages and limitations of applying a UiO-66-based metal–organic framework (MOF) catalyst in a flow microreactor demonstrated by the catalytic hydrolysis of ethyl paraoxon, an organophosphorus chemical agent. The influences of the following factors on the reaction yield were investigated: a) catalyst properties such as crystal size (14, 200, and 540 nm), functionality (NH2 group), and particle size, and b) process conditions: temperature (20, 40, and 60 °C), space times, and concentration of the substrate. In addition, long-term catalyst stability was tested with an 18 h continuous run. We found that tableting and sieving is a viable method to obtain MOF particles of a suitable size to be successfully screened under flow conditions in a microreactor. This method was used successfully to study the effects of crystal size, functionality, temperature, reagent concentration, and residence time. Catalyst particles with a sieved fraction between 125 and 250 µm were found to be optimal. A smaller sieved fraction size showed a major limitation due to the very high pressure drop. The low apparent activation energy indicated that internal mass transfer may exist. A dedicated separate study is required to assess the impact of pore diffusion and site accessibility.