Pore network model of deactivation of immobilized glucose isomerase in packed-bed reactors II: three-dimensional simulation at the particle level

Immobilized glucose isomerase is widely used for converting glucose to fructose by enzymatic isomerization. The process takes place in a packed-bed reactor consisting of mesoporous particles with distributed pore sizes and interconnectivities. Its efficiency is, however, significantly affected by deactivation of the mesoporous particles. In this paper, we study deactivation of the mesoporous particles using a three-dimensional pore network model of the pore space with distributed pore sizes and interconnectivities, and investigate several plausible mechanisms of deactivation of the porous particles. The results of the present study, which will be used as the input for simulation of the phenomenon at the reactor level, demonstrates the strong effect of the particles’ morphology on the deactivation process.     

Pore network model of deactivation of immobilized glucose isomerase in packed-bed reactors. Part III: Multiscale modelling

A widely used method for converting glucose to fructose is by enzymatic isomerization. This process, which uses immobilized glucose isomerase, takes place in a packed-bed reactor that consists of microporous particles with a range of pore sizes, characterized by a pore size distribution. The micropores are also interconnected, giving rise to a three-dimensional (3D) network of pores with distributed sizes and connectivities. The particles themselves generate a 3D pore network at the reactor level with distributed pore sizes, but with a fixed connectivity. In this paper, Part III of a series, we develop a multiscale modelling approach to this problem, beginning with the relevant phenomena at the scale of the micropores, and integrating them into the particle and reactor length scales. As the efficiency of the process is significantly affected by deactivation of the microporous particles, we take this phenomenon into account at all the relevant length scales. We use a real random packing of particles, originally constructed by Finney (Proc. R. Soc. Lond. A, 319 (1970) 479), and map its pore space onto an equivalent 3D Voronoi network in which the pores are represented by the edges of the Voronoi polyhedra. The flow field in the Voronoi network is determined, and the convection-diffusion-reaction equation is then solved in the Voronoi network, taking into account the gradual deactivation of the microporous particles. Several plausible mechanisms of deactivation of the microporous particles are considered, and their effect on the performance of the reactor is investigated. Good agreement is found between the results of the computer simulations and the relevant experimental data.     

Interconnectivity and permeability of supercritical fluid-foamed scaffolds and the effect of their structural properties on cell distribution

This study aims to investigate interconnectivity and permeability of scCO₂-foamed scaffolds and the influence of structural scaffold properties on cell distribution. Supercritical fluid technology was utilized to fabricated scaffolds from 37 kDa, 53 kDa and 109 kDa PLGA (85:15). Pore size, pore size distribution and porosity were quantified by MicroCT, and window sizes were measured using SEM. A novel interconnectivity algorithm allowed the quantification of scaffold interconnectivity in three space dimensions. To determine the permeability of porous materials direct perfusion experiments were performed, where a known flow rate was applied to measure the pressure differential across the scaffolds. The permeability was calculated using Darcy's law. Largest pore sizes, porosities, interconnectivities and permeabilities were obtained for scaffolds fabricated from 37 kDa PLGA. These scaffolds showed a heterogeneous pore structure and distribution, whereas homogeneous pore structure, smallest pore sizes, porosities, interconnectivities and permeabilities were observed for scaffolds fabricated from 109 kDa PLGA. The distribution of 3T3 fibroblasts through scCO₂-foamed scaffolds was investigated by MicroCT and MTT staining. Cells were further visualized by fluorescent imaging. Uniform cell distribution was observed on scaffolds fabricated from 109 kDa PLGA and an average of 10% of the total scaffold volume was covered with cells that had adhered onto them.     

Effect of dealcoholation of support in MgCl2-supported Ziegler–Natta catalysts on catalyst activity and polypropylene powder morphology

Spherical support was produced using melt quenching method. Scanning electron microscopy (SEM) images showed that support particles have a rough surface morphology with small pore sizes. The size of primary particles determined to be about 50 nm. Produced particles were dealcoholated using hot nitrogen flow. In this study, the dealcoholation apparatus was similar to a fluidized bed reactor. It consists of three main zones: heating zone, bed zone, and expansion zone. This apparatus is capable to dealcoholate and elutriate support particles at the same time. Results showed a significant increase in specific pore volume and surface area of support particles during the dealcoholation. According to the obtained results, two mechanisms were proposed to explain effect of the dealcoholation on the morphology of support particles. Three dealcoholated samples of different alcohol contents were used in the catalyst preparation. Results showed that by decrease in alcohol content of support particles, specific surface area, and pore volume of catalysts increased. Moreover, titanium content of final catalysts increased slightly. Polymerization of propylene was carried out using obtained catalysts in slurry phase. An increase in catalyst activity and enhancement of polypropylene powder morphology were observed as the alcohol content of support particles decreased.     

Numerical calculation with experimental validation of pressure drop in a fixed-bed reactor filled with the porous elements

To study flow dynamics in a fixed-bed reactor, experiment and numerical simulation are used. In this work, the flow dynamics in a fixed-bed reactor filled with porous particles was simulated. For verification, experimental data were used. Porous particles were prepared from grains with sizes from 0.2 to 2.0 mm. Porous particles made by sintering grains in a muffle furnace. The study of pressure drop was performed in the velocity range up to 3 m/s. Numerical calculation was performed for a realistic computational domain obtained by RBD-algorithm (rigid body dynamics). It was found that if the pore size is less than 0.5 mm, then the flow through the porous medium of particles is minimal; if the pore size is larger than 0.5 mm, then there is a flow through the porous elements of the fixed-bed reactor. To take into account the porosity of the particles, γ correlation coefficient was proposed for the Ergun equation.     

Nano‐sized silica supported Me2Si(Ind)2ZrCl2/MAO catalyst for ethylene polymerization

Nano‐sized and micro‐sized silica particles were used to support a zirconocene catalyst [racemic‐dimethylsilbis(1‐indenyl)zirconium dichloride], with methylaluminoxane as a cocatalyst. The resulting catalyst was used to catalyze the polymerization of ethylene in the temperature range of 40–70°C. Polyethylene samples produced were characterized with scanning electron microscopy (SEM), X‐ray diffraction (XRD), differential scanning calorimetry (DSC), and gel permeation chromatography (GPC). Nano‐sized catalyst exhibited better ethylene polymerization activity than micro‐sized catalyst. At the optimum temperature of 60°C, nano‐sized catalyst's activity was two times the micro‐sized catalyst's activity. Polymers obtained with nano‐sized catalyst had higher molecular weight (based on GPC measurements) and higher crystallinity (based on XRD and DSC measurements) than those obtained with micro‐sized catalyst. The better performances of nano‐sized catalyst were attributed to its large external surface area and its absence of internal diffusion resistance. SEM indicated that polymer morphology contained discrete tiny particles with thin long fiberous interlamellar links. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

A pore network model for calculating pressure drop in packed beds of arbitrary-shaped particles

A pore network model is built to predict pressure drop in packed beds of arbitrary-shaped particles, using a method that consists of particle packing by the rigid body technique, pore network construction by the maximal sphere algorithm, and numerical calculation of fluid flow. The pore network model is firstly validated by comparing with experiments, Ergun-type equations, and particle-resolved computational fluid dynamics (CFD). The pore network model is as accurate as the particle-resolved CFD, and is remarkably two to three orders of magnitude less computationally intensive. Then, the pore network model is used to calculate the pressure drops in the beds packed with particles of different shapes and sizes, as well as using different flow media. These calculation results prove the versatility of the pore network model. This work provides an accurate yet efficient pore network model for predicting pressure drop, which should be a powerful tool for designing packed beds.     

基于分形概念的煤焦颗粒气化模型

应用分形概念描述煤焦颗粒内部微观孔洞形态,用Bethe网络模化颗粒孔隙空间拓扑结构,这种方法提供了一种真实自然的框架来描述颗粒的孔洞结构及其伴随着气化过程进行的孔隙开通、增大、汇聚现象.模型还描述了气化过程的逾渗破碎现象,因而可以用来模化各种工况下的实验数据.     

Liquid distribution studies in trickle-bed reactors

Liquid distribution plays an important role in determining the reactor performance in a trickle-bed reactor. Radial liquid distribution was studied in a trickle-bed reactor with five different shapes and sizes of catalyst packing in a uniformly distributed liquid inlet. The liquid tends to flow preferentially along the existing filaments where the porosity is high. The introduction of gas flow into the liquid–solid system smoothens the liquid distribution to some extent due to the competition between liquid and gas phases for the interstitial pore space.     

Experimental investigation on mechanisms of fine particles generation for the Borealis Borstar multistage olefin polymerization process

Simultaneous production of large amount of undesired fine particles is a big trouble in the Borstar multistage olefin polymerization process. Aiming at reducing the fine particles, the formation mechanism and formation location of the fine particles were thus studied. First, the influence of catalyst nature was considered. Second, the ash content, bulk density, morphology, and molecular weight distribution of polyethylene with different particle sizes from the small‐scale loop prepolymerization reactor, supercritical loop reactor, and gas phase fluidized bed reactor were studied, respectively. In combination with particle growth model, scanning electron microscope, gel permeation chromatography, and laser particle size analyzer, the particle morphology and growth kinetics were investigated. The results showed that fine particles were mostly generated in the supercritical loop reactor, and were significantly affected by the particle size distribution, residence time distribution, and particle fragmentation of the catalyst. Furthermore, scanning electron microscope images showed the catalysts with low activity tended to generate more fine particles. Based on these results, several strategies for reducing the amount of fine particles were proposed, which could be applied in the industrial process. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46589.     

Characterization of Palladium Nanoparticles Adsorpt on Polyacrylic Acid Particles as Hydrogenation Catalyst

Palladium particles can be prepared in sizes of a few nanometers from Pd(OAc)₂in the presence of a block copolymer in aromatic unpolar solvents with suitable reducing agents like NaBH₄ or LiAlH₄. After mixing the organic metal solution with a polyacrylic acid (PAA) dispersion of defined concentration and following a crosslinking process with an epoxide, reactive membranes with a well-defined pore diameter were obtained. The catalytic activity of the prepared palladium particles incorporated in the crosslinked PAA network has been proven by the gas phase hydrogenation of propyne in a special membrane reactor.     

Heterogeneously catalyzed oxidative dehydrogenation of isobutyric acid to methacrylic acid: A new application of periodic flow reversal

The heterogeneously catalyzed oxidative dehydrogenation of isobutyric acid in a fixed bed reactor using molybdenum (Mo) heteropoly acids as catalysts shows a loss of Mo into the gas phase due to the formation of volatile Mo-complexes under reaction conditions. To avoid this loss of catalyst and to keep the catalytic material in the fixed bed and thus increase the catalyst's lifetime, the process has been performed under periodic flow reversal within the reactor. In this work, periodic flow reversal is tried in a semi-pilot test reactor as a method to fix the Mo-compounds in the catalyst bed. The influence of this mode of operation on the temperature profile in the reactor, on conversion, selectivity and yield of the product methacrylic acid is investigated in comparison with the process without periodic flow reversal.     

Modeling the reaction of gaseous HCl with CaO in fluidized bed

An integrated mathematical model is developed to evaluate the performance of the reaction of gaseous HCl and CaO in fluidized bed. The model considers initial pore size distribution of solid reactant, pore structure change and attrition caused by particles movement. Bethe network is used to describe the pore space topology, and the percolation theory is used to determine the accessible reaction surface area of the sorbent particles and the effective diffusion coefficient of gaseous HCl. This model prediction accounts for the diffusion of HCl in shrinking pore space as well as in product layer, and clearly demonstrates the increasing diffusion resistance and the isolation of partially reacted pores causing incomplete conversion of solid. The model shows excellent agreement with the experimental data.     

CHAR GASIFICATION: PARTICLE MODEL COMPARISON IN A STIRRED TANK REACTOR

A simple well mixed model is developed to study the effect of single particle models for the gasification of char in a reactor. Two models, a lumped model and a distributed model, are used to describe the processes in the particle. The model consists of the conservation equations along with the residence time distributions and population balance equations. It is found that neglecting the conversion distribution function in the bed causes considerable differences at high temperatures and large particle sizes. The lumped model is rather insensitive to the mean residence time of the particles and the pressure, compared to the dependence observed in the distributed system. The difference in the design parameters predicted by the models is large and further, the particle size influences these parameters to a greater extent in the distributed system, the lumped system being almost independent of the particle size.     

CHAR GASIFICATION: PARTICLE MODEL COMPARISON IN A STIRRED TANK REACTOR

A simple well mixed model is developed to study the effect of single particle models for the gasification of char in a reactor. Two models, a lumped model and a distributed model, are used to describe the processes in the particle. The model consists of the conservation equations along with the residence time distributions and population balance equations. It is found that neglecting the conversion distribution function in the bed causes considerable differences at high temperatures and large particle sizes. The lumped model is rather insensitive to the mean residence time of the particles and the pressure, compared to the dependence observed in the distributed system. The difference in the design parameters predicted by the models is large and further, the particle size influences these parameters to a greater extent in the distributed system, the lumped system being almost independent of the particle size.     

Global Reaction Kinetics of CO and CO2 Methanation for Dynamic Process Modeling

For the design and optimization of methfanation processes detailed modeling and simulation work is advisable. However, only a few kinetics published in literature rely on wide temperature and pressure ranges, which are prevalent at modern methanation applications with dynamic operation. Especially the simulation‐based design of methanation processes with commercial catalysts is difficult due to legal restrictions regarding the publication of kinetic data of those catalysts. In this work, rate equations for the dynamic modeling and simulation of methanation processes operating with commercial Ni/Al₂O₃ catalysts are selected, adapted, and tested in a dynamic reactor model. The results suggest that the catalyst's nickel content is an indicator for the choice of a rate equation. Testing of the equations in a reactor model meets published data for CO and CO₂ methanation and own measurements.     

Controlling nanostructure in thermal plasma processing: Moving from highly aggregated porous structure to spherical silica nanoparticles

The synthesis of SiO₂ nanoparticles in a radio frequency (RF) plasma reactor is studied. Scanning electron microscopy (SEM), nitrogen absorption (BET), and laser diffractometry techniques are used to determine the morphology, product size and aggregation level of the resulting nanopowder. Computational fluid dynamics (CFD) modelling employing Fluent 6.2.16 is used to better understand the flow and temperature fields inside the reactor and their effect on the nanoparticles. The theoretical and experimental results are later combined to describe the effects of the above mentioned parameters on the formation (nucleation and growth) of the nanoparticles by different mechanisms. It is demonstrated that the quench gas configuration and reactor geometry can now be designed to control the morphology and size of nanoparticles in these reactors. Various nanostructured products have been synthesised: i.e., highly aggregated nanostructure, partially sintered nanospheres and spherical nanoparticles with very low levels of aggregation. These nanostructures have their primary particles sized between 10 and 200 nm, while the aggregate sizes can lie in the range of between hundreds of nanometers to several micrometers. The critical parameters that should be considered for the large-scale production process are finally identified.     

Three-dimensional pore network model of biofilter treating toluene: A study of the effect of the pore space morphology

Treatment of polluted air stream with toluene in a biofilter under quasi-steady-state condition is described with a three-dimensional (3-D) pore network model. The efficiency of the process depends on the pore structure of the biofilter media. The previous models for the process, considered transport and reaction equations mostly in the continuum systems. They cannot show the effect of pore spaces’ morphology, different pore size distributions and connectivity types. We used the 3-D pore network model to simulate the pore structure and to study the effects of biofilm growth on pore size distribution and pore connectivity on the biofilter performance. This model predicts the toluene conversion, pressure drop profiles, biofilm thickness and clogging in the biofilter. Our developed model assumes a nonlinear kinetic for the biodegradation reaction of toluene in the biofilm. The differential equations of the model are solved numerically. Model results indicated that biofilter media with smaller mean pore size and higher connectivity have a higher toluene conversion. To investigate the validity of the model, we compared the model results with experimental data and found satisfying similarities.     

Minimum entropy generation for isothermal endothermic/exothermic reactor networks

It was shown in an earlier work by us that entropy generation and energy (hot utility or cold utility) consumption of isothermal, isobaric reactor networks depend only on the network's inlet and outlet stream compositions and flow rates and are not dependent on the reactor network structure, as long as the universe of realizable reactor units and network outlet mixing units are either all endothermic interacting with a single hot reservoir, or all exothermic interacting with a single cold reservoir, respectively. It is shown that when the universe of realizable reactor/mixer units, of isothermal, isobaric, continuous stirred tank reactor networks, consists of both endothermic units interacting with a single hot reservoir and exothermic units interacting with a single cold reservoir, the network's net (hot minus cold) utility consumption depends only on the network's inlet and outlet stream compositions and flow rates (and does not depend on the network's structure). In contrast, the network's entropy generation depends on the network's inlet and outlet stream compositions and flow rates, and the network's hot utility (or cold utility) consumption. The latter, in general, depends on the network structure, thus making entropy generation also, in general, depend on network structure. Thus, the synthesis of isothermal, isobaric reactor networks, with fixed inlet and outlet stream specifications, is equivalent to the synthesis of minimum hot (or cold) utility consuming such networks. The Infinite DimEnsionAl State‐space conceptual framework is used for the problem's mathematical formulation, which is then used to rigorously establish the above equivalence. A case study involving Trambouze kinetics demonstrates the findings. © 2014 American Institute of Chemical Engineers AIChE J, 61: 103–117, 2015     

Continuous Generation of TiO 2 Nanoparticles by an Atmospheric Pressure Plasma-Enhanced Process

A novel method for the continuous generation of titanium dioxide (TiO₂) nanoparticles by dielectric barrier discharge process is presented using titanium tetraisopropoxide (TTIP) and water as precursors. The aerosol generator employs an atmospheric pressure plasma enhanced nanoparticle synthesis (APPENS) process of alternative current (AC). The influences of applied voltage, frequency and precursor molar ratio on the generated particles were described by the SEM, XRD, and SMPS analyses. The results showed that TiO₂ particles appear to be in a broad size range of bi-modal distribution when no voltage is applied. While after applying the AC plasma they become uni-modal distributed with average sizes range from around 30 to 60 nm. The applied electric frequency can be adjusted to either generate nanoparticles after the plasma reactor or develop a thin film in the reactor. An increase in the precursor molar ratio leads larger particles with a broader size distribution.