MODELING OF THREE PHASE NONCATALYTIC BUBBLE COLUMN REACTORS

Mathematical models have been developed for the design of bubble column slurry reactors wherein the particles with nonuniform distribution of solid reactant take part in the reaction and follow the shrinking core model. The cases of external mass transfer control, ash layer diffusion control and chemical reaction control have been analyzed using solid distribution functions presented by Dudukovic (1984). It was found that the nonuniformity of solid reactant can strongly affect the reactor performance. Correction factors for modifying the performance charts of Joshi el al. (1981) who considered only uniform solid reactant distribution are presented for design and scale-up purposes.     

Reactions of solid particles with nonuinform distribution of solid reactant: The volume reaction model

The effect of non-uniform solid reactant distribution on conversion of solid particles in gas-solid reactions is analyzed based on the volume reaction model. Certain special features of such systems are pointed out. The possibility of ash layer formation in the kinetically controlled regime is discussed. Conditions leading to single or double ash layer formation, both at the center and surface of the particle, in the intermediate regime of diffusion with simultaneous reaction are described. Detailed mathematical equations which are useful for calculation of the conversion-time relationship for particles with non-uniform solid reactant distribution are presented. Comparison is made to reaction of uniform particles and differences in required reaction time for desired conversion are outlined.     

窄分布醇醚合成用催化剂的筛选与催化作用分析

讨论了酸、碱催化剂对窄分布醇醚合成反应的催化性能。从反应物转化率、醇醚产物选择性和醇醚分子量分布窄度几个方面综合考察了酸、碱类催化剂的优缺点,发现酸性催化剂的催化活性普遍较高,均在９５％以上,产物分子量分布较窄,ｎ＜５的产物占醇醚产物总质量的８５％左右,但生成的非醇醚类产物杂质较多,占反应产物质量的１４％以上;碱性催化剂的催化活性不一,其中活性较高的固体碱类镁铝复合金属氧化物（ＬＤＯ）与酸性催化剂活性相当,达９９．９％,且其产物纯净,非醇醚类产物仅占反应产物质量的１．２％,其醇醚产物分子量分布虽然宽于酸性催化剂,ｎ＜５的醇醚占醇醚产物质量的７７．２％,但明显窄于工业上普遍应用的均相催化剂ＫＯＨ（６６．８％）,是较有发展前景的催化剂。     

A zone model for reactions of solid particles with strongly adsorbing species

A new zone model for reaction of solid particles with zeroth order reaction with respect to the fluid reactant and first order with respect to solid reactant has been proposed and analytical solutions for conversion and effectiveness factor obtained. The effect of Biot number and Thiele modulus on model predictions is investigated. Comparison of the model to other models for fluid solid reactions is presented together with the method for model discrimination. The model predicts conversion time behaviors encountered in practice.     

A mathematical model for pyrolysis of a solid particle — effects of the heat of reaction

A transient analysis of the effects of the heat of reaction in a solid pyrolysis system is presented. Two cases, one characterizing the heat-transfer-controlled reaction and the other self-sustaining reaction, are analyzed. To carry out the numerical simulation, utilization is made of the volume reaction model which takes into account the simultaneous heat and mass transfer phenomena, and the method of line which utilizes the second order centered finite difference scheme for the spatial discretization. Effects of the heat of reaction on the solid conversion, solid reactant, and fluid product concentration profiles, temperature distribution, enthalpy, and average fluid product concentration in the particle are examined. The results are graphically presented and interpreted.     

连续多级串联完全混合槽中固体颗粒反应产率计算方法

本文针对级内颗粒完全混合一级间无颗粒返混的多级反应器,探讨了按停留时间分布计算固体颗粒反应率的方法。首先分析了用δ（0）函数示踪法与多元独立随机变量和的分布密度函数法求颗粒停留时间分布的特点;应用后一种方法推得了计算颗粒反应产率的一般公式。从固体颗粒固相反应物浓度分布密度函数也可以推得此公式。在此基础上对一类特殊的反应速度方程,解析求出了直接计算任何一级反应产率的通式;对求解计算通式困难的反应速度方程,论证了两种逐级计算方法:以反应时间作计算参量及以颗粒固相反应物浓度作计算参量。     

Studies in transesterification VI. Ion-exchange catalysis

The preparation of methacrylate esters by transesterification with ion-exchange resins as acidic catalysts and n-butanol as the reactant alcohol is described. Conventional cellular cation exchangers have been used together with a resin having a solid centre and a reactive outer surface. The latter resin is of lower capacity but the reaction rate compares favourably with that of conventional resins while its performance on repeated use is vastly superior.     

Monolithic catalysts as more efficient three-phase reactors

Monolithic reactors are an attractive alternative to conventional multi-phase reactors. Advantages are the low pressure drop, the absence of a need for a catalyst separation, and the large geometrical surface area. The main disadvantage is, however, the lack of practical experience with monolithic reactors. A pilot-scale study has been carried out to evaluate the performance of a monolithic reactor on a larger scale. The results of this study have demonstrated that a higher productivity can be obtained for a monolithic reactor compared to a trickle-bed reactor for a solid catalysed gas–liquid reaction that is mass transfer limited in the gas-phase reactant. A higher selectivity has been demonstrated for the selective hydrogenation of benzaldehyde towards benzylalcohol when a monolithic reactor is used instead of a trickle-bed reactor as a result of the narrower residence-time distribution.     

Microstructured reactor for electroorganic synthesis

In the present work a brief overview of microstructured devices, the advantages and disadvantages as well as the principles of a multiscale design approach are presented. The advantages mainly comprise uniform current density distribution, local control of the process parameters, high single-pass conversion of the reactant and reduced concentration of the supporting electrolyte needed to perform the reaction. The main challenge with this type of cell is gas evolution, a typical reaction taking place at the counter-electrode. The phenomena discussed are documented using the example of anodic methoxylation of 4-methylanisole. An analysis was made of the two-phase flow hydrodynamics inside the microstructured cell. The range of operational conditions suitable for the desired reaction was identified. The results were used together with the information on the electrode reaction kinetics in the form of the Butler-Volmer-type equation for the design of a suitable bipolar microstructured cell. A bipolar cell optimized to perform the desired reaction is reported. The results are compared with the published data. An analysis was performed that proved that the performance of the existing technology is more demanding in terms of energy consumption for the separation of the final product from the reaction mixture. The process intensification was evaluated on the basis of the available data.     

Kinetics of the gas-solid heterogeneous photocatalytic oxidation of trichloroethylene by near UV illuminated titanium dioxide

Kinetics of the gas/solid heterogeneous photocatalytic oxidation of dilute trichloroethylene (TCE) vapors by ultraviolet-illuminated titanium dioxide have been determined using a fixed-bed dynamic photoreactor. Reaction rate dependences on inlet TCE, oxygen and water vapor concentrations were found to consist of both reactant sensitive and insensitive regions. In the reactant sensitive regions, measured limiting apparent reaction rate orders for TCE, oxygen and water vapor are 0.8, 1.7 and — 3, respectively. Water vapor in the reactant stream lowersinitial reaction rates relative to corresponding water free conditions, but is required to sustain photocatalytic activity for extended periods of time.     

Kinetics of acetic acid esterification over ion exchange catalysts

Computer simulation showed that catalytic distillation is an attractive process for the removal of dilute acetic acid from wastewater. Selection of catalysts and kinetic data have been obtained for the design of the catalytic distillation column. Kinetic measurements were conducted in a batch reactor. Methanol was added to the dilute acetic acid solutions and reacted with the acid in water to form methyl acetate and water. The reaction can be catalyzed by solid acid catalysts. It was found that Amberlyst 15 was an effective catalyst for this reaction. The effects of stirrer speed, reaction temperature, reactant concentration and catalyst loading on reaction rate were investigated. A complete kinetic equation for describing the reaction catalyzed by Amberlyst 15 was developed. This equation can be used in the simulation and design of the catalytic distillation column for removing acetic acid from wastewater.     

Concentration fluctuation caused by reaction-diffusion coupling near catalytic active sites

The coupling of reaction and diffusion between neighboring active sites in the catalyst pore leads to the spatiotemporal fluctuation in component concentration, which is very important to catalyst performance and hence its optimal design. Molecular dynamics simulation with hard-sphere and pseudo-particle modeling has previously revealed the non-stochastic concentration fluctuation of the reactant/product near isolated active site due to such coupling, using a simple model reaction of A → B in 2D pores. The topic is further developed in this work by studying the concentration fluctuation due to such coupling between neighboring active sites in 3D pores. Two 3D pore models containing an isolated active site and two adjacent active sites were constructed, respectively. For the isolated site, the concentration fluctuation intensifies for larger pores, but the product yield decreases, and for a given pore size, the product yield reaches a peak at a certain reactant concentration. For two neighboring sites, their distance (d) is found to have little effect on the reaction, but significant to the diffusion. For the same reaction competing at both sites, larger d leads to more efficient diffusion and better overall performance. However, for sequential reactions at the two sites, higher overall performance presents at a smaller d. The results should be helpful to the catalyst design and reaction control in the relevant processes.     

RESIDENCE TIME DISTRIBUTION IN AN OPEN REACTOR:EFFECT OF BACKGROUND REACTANT ON TRACER DISPERSION

A new approach is presented for deriving the residence time distribution(RTD)in open-type reactors with exclusion of residence in the fore section.Inasmuch as failure to predict the con-version of chemical reactions has been evidenced in many occasions,numerical solution of tracerdispersion in the presence of background concentration gradient of reactant is given to demonstratethe strong effect of background reactant on the true RTD a reactant molecule experiences.Withinthe error of computation,the conversion of a first-order chemical reaction under steady state is shownto be equal in both closed and open reactors,despite difference in relevant RTDs     

Early Hydration of Tricalcium Silicate: A Solid Reaction with Induction and Acceleration Periods

The hydration of tricalcium silicate was studied using a sample with a limited particle size distribution. The hydration reaction was analyzed as an example of general solid reactions having induction and acceleration periods. The first product, which may have a structure similar to the reactant, forms on the surfaces of reactant grains. The nuclei of the stable product are produced in the first product layer and act as reaction centers; the main reaction to form a stable product occurs rapidly in the acceleration period. In the hydration of calcium silicates, hydrates produced in this stage are poorly crystalline and metastable and change gradually into a more stable form.     

固状淀粉胶的研制

固状淀粉胶是以淀粉为主要原料，经加热催化分步氧化，同时加入聚乙烯醇合成树脂改善其性能，最后用硬脂酸钠作为赋型剂将其凝胶成固状而制得，它适用于日用纸品的粘合．本文介绍了该粘合剂的反应机理、助剂的选择、配方设计、工艺过程、产品性能及影响因素等几方面的情况．     

Gas-solid reaction model for a shrinking spherical particle with unreacted shrinking core

A heterogeneous reaction model is developed to represent the gas-solid reaction consisting of two-step reactions: formation of a solid intermediate on the core of unreacted solid and consumption of the solid intermediate. The presented model takes into account the shrinkage of both the unreacted core and the solid particle itself. The model successfully represents the fluorination of uranium dioxide where uranium hexafluoride gas is produced through uranyl fluoride as a solid intermediate. The rates of the shrinkage are discussed in detail for general cases by solving an ordinary differential equation for two radii of the particle and the unreacted core. Two parameters, the ratio of the reaction rates between the two steps and the non-dimensional diffusion rate of a reactant gas into the intermediate, control the rates of the shrinkage. Effective range of the model is specified in terms of the two parameters.     

The analysis of an isothermal batch electrochemical reactor with a secondary cathodic reaction

An analysis has been made to examine the effect of a constant applied electrolysing voltage on the performance of an isothermal batch electrochemical reactor in which the rate of the cathodic reaction is limited by mass transfer of a reactant to the electrode surface, a simultaneous secondary reaction occurring at higher cathode potentials.Numerical solutions are presented for a hypothetical process study to show how average production rates for various percentage conversions of reactant are influenced by the magnitude of the electrolysing voltage and the corresponding electrical energy requirement. The most noticeable effect is that the electrical energy requirements for a given production rate are very strongly influenced by both the degree of conversion of reactant and by the extent to which the secondary reaction occurs.     

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.     

Gas‐Lift Reactors for Rapid Reactions with Appreciable Gas Consumption

Gas‐lift reactors offer important advantages for a number of gas/liquid and gas/liquid/solid reactions. However, the design and operation of these reactors can be complex when there is a substantial change in the molar gas flow rate along the length of the reactor, e.g., when a gaseous reactant is converted into a liquid product. In this situation, there is a strong coupling between reactor hydrodynamics and reaction kinetics, which arises from the fact that the rate of liquid circulation through the reactor and the longitudinal profile of gas holdup in the riser are mutually dependent. Several one‐dimensional models have been developed to describe kinetic/hydrodynamic coupling in gas‐lift reactors. These models offer useful insights into the parameters that affect reactor performance. The models can also be used to explore different approaches to scale‐up.     

Modelling of non-catalytic gas—solid reactions—I. Transient analysis of the particle—pellet model

A transient analysis of the problem of non-catalytic gas—solid reaction based on the particle—pellet model, which considers the particulate nature of the pellet and includes the external transport resistances, is presented. The method of solution of the resulting non-linear partial differential equations is based on the orthogonal collocation technique. The transient model has been compared with the pseudo-steady analysis of the problem. The effect of various parameters on the temperature profiles in the pellet and the conversion of the solid reactant has been discussed.