Analysis of a model for a nonisothermal continuous fluidized bed catalytic reactor

A three-phase model, consisting of a dilute or bubble phase, an interstitial gas phase and a solids phase, has been developed for a non-isothermal gas fluidized-bed catalytic reactor with continuous circulation of catalyst particles. The dilute phase is assumed to be in plug flow, the interstitial gas is considered to be either perfectly mixed or in plug flow, and the particles are assumed to be perfectly mixed.It is shown that the conversion in a reactor and steady state temperature and concentration profiles are the same irrespectively of the assumed flow pattern in the interstitial gas.The numerical computations were done for the case of a single irreversible reaction with or without catalyst decay and for the case of two consecutive irreversible reactions. In the case of an adiabatic reactor and ordinary exothermic reactions, or in the case of a cooled reactor with a highly exothermic reaction, multiple steady states may occur.     

Mathematical modelling of fluidized bed reactors—III: Axial dispersion model

An axial dispersion model has been developed for a continuous fluidized bed catalytic reactor with a cocurrent flow of the emulsion phase gas and the catalyst particles. The influence of some parameters on multiplicity of steady states has been reported. Several examples illustrating the transient behavior of the system are presented. In cases where three steady states are possible it appears that the intermediate steady state is unstable, while the lower and the upper steady states are locally stable. It was noted that the initial temperature of the emulsion phase is a predominant factor in determining which steady state will be approached.     

QUALITATIVE OBSERVATIONS OF HEAT AND MASS TRANSFER EFFECTS ON THE BEHAVIOUR OF A TRICKLE BED REACTOR

This paper examines the effect of simultaneous heat and mass transfer on the hydrogenation of cyclohexene in a trickle bed reactor with particular attention given to the problem of liquid phase evaporation and transition to the gas-phase regime of operation. The reaction rates are obtained as a function of temperature and hydrogen flow rate; the concentration of the substrate in the feed displays considerable hysteresis due to an abrupt increase of the reaction rate arising from temperature gradients within the bed and in the gas film surrounding the catalyst pellet, during the transition from the liquid to the gas-phase regime. The transition is accompanied by the change of apparent kinetics of the model reaction as well as by a change of regime and operation of the pellet. In the liquid phase a pellet originally showing inter-phase and intra-particle diffusion resistances changes into the gas-phase regime with a large resistance due to inter-phase diffusion.     

Modelling of fluidized bed reactors—II: Uniform catalyst temperature and concentration

A model based on the simple two phase theory of fluidization including the catalyst particles as a third phase has been developed for a nonisothermal fluidized bed catalytic reactor with continuous circulation of catalyst particles. The dilute phase is assumed to be in plug flow, the emulsion phase gas is considered to be perfectly mixed and the particles are assumed to be perfectly mixed and uniform.Exact criteria for uniqueness and multiplicity of the steady state solutions are presented and some conclusions derived therefrom. Several examples illustrating the influence of some parameters on the steady state multiplicity are reported. The steady states are analyzed for local asymptatic stability using Liapunov's direct method, but the sufficient conditions for stability are found to be rather conservative.Numerical examples illustrating the transient behavior of the system are presented, and it has been found that the initial temperature of the catalyst particles is a predominant factor in determining which steady state will be approached.     

Modeling of transient heterogeneous two-dimensional catalytic packed bed reactor

A two-dimensional transient catalytic packed bed model, incorporating all transport parameters and resistances, along with boundary conditions based on a catalytic single pellet has been developed. Thermal conduction through the solid phase is included in the model. The overall steady state reactor performances of packed bed reactor using a model proposed in this study are compared with those from different models which are often used for a packed bed reactor. The model presented is very useful in the presence of internal temperature and concentration gradients in the catalyst pellets. The dynamic behavior in feed temperature change is examined during ethane hydrogenolysis. A transient thermal runaway is observed by feed temperature decrease. The sensitivities of the computation to each physical parameter and the effects of some simplifying assumptions in the model are also analyzed. The magnitude and position of hot spot in catalytic packed bed reactor are relatively sensitive to thermal parameters and characteristic parameters of a catalyst pellet.     

高效再生催化裂化装置多稳态分析：反应温度开/闭环控制条件对热反馈机制的影响

针对催化裂化反应-再生系统在提升管反应温度开环和闭环控制条件下的输出与输入多稳态问题,分析了烧焦罐式高效再生催化裂化反应-再生系统在两种条件下随着CO助燃剂添加量变化时的多稳态分布。在反应温度开环条件下,因再生温度与反应温度的耦合程度较低,使系统移热曲线呈单调递增,导致了系统出现3个稳态操作点。在反应温度闭环控制条件下,提升管反应器和再生器间热反馈机制发生改变,由于再生剂循环量可以作为额外的自由度对再生温度和反应温度之差进行补偿,再生器和提升管反应器的耦合程度增强,使得系统只会在助燃剂添加量极低时才会出现多个稳态点,而在基准操作条件下只有一个稳态点,规避了系统在提升管反应温度开环时的多个稳态点的问题。     

Investigation of multiphase hydrogenation in a catalyst‐trap microreactor

BACKGROUND: Multiphase hydrogenation plays a critical role in the pharmaceutical industry. A significant portion of the reaction steps in a typical fine chemical synthesis are catalytic hydrogenations, generally limited by resistances to mass and heat transport. To this end, the small‐scale and large surface‐to‐volume ratios of microreactor technology would greatly benefit chemical processing in the pharmaceutical and other industries. A silicon microreactor has been developed to investigate mass transfer in a catalytic hydrogenation reaction. The reactor design is such that solid catalyst is suspended in the reaction channel by an arrangement of catalyst traps. The design supports the use of commercial catalyst and allows control of pressure drop across the bed by engineering the packing density. RESULTS: This paper discusses the design and operation of the reactor in the context of the liquid‐phase hydrogenation of o‐nitroanisole to o‐anisidine. A two‐phase ‘flow map’ is generated across a range of conditions depicting three flow regimes, termed gas‐dominated, liquid‐dominated, and transitional, all with distinctly different mass transfer behavior. Conversion is measured across the flow map and then reconciled against the mass transfer characteristics of the prevailing flow regime. The highest conversion is achieved in the transitional flow regime, where competition between phases induces the most favorable gas–liquid mass transfer. CONCLUSION: The results are used to associate a mass transfer coefficient with each flow regime to quantify differences in performance. This reactor architecture may be useful for catalyst evaluation through rapid screening, or in large numbers as an alternative to macro‐scale production reactors. Copyright © 2008 Society of Chemical Industry     

模拟单颗粒催化剂上多组份反应扩散过程的通用软件的开发

本文在尘气传递模型的基础上,开发了在微机上实现的模拟单粒催化剂多组份多反应体系反应扩散过程的通用软件。它可用于横拟速率方程为任意形式的,扩散系数随温度、浓度变化的,不同形状颗粒、粒外有或无扩散效应时等温或非等温的反应扩散过程。本软件具有管理功能强、模型精度高、算法先进以及适用性广等特点,并配有彩色图形输出,是辅助催化剂设计和分析反应器性能的有效工具。     

The hydrogenation of isophorone to trimethyl cyclohexanone using the downflow single capillary reactor

Despite many innovations in the intensification of catalytic multiphase reactors for the small and medium scale manufacture of chemicals, there have as yet been relatively few commercial successes. One reason for this might be that many of these developments inherently incorporate a fixed catalyst, which may not suit an industry that is based on principles of batch manufacture and multi-product plant. This study evaluates an intensified reactor that encompasses the opportunities demonstrated from structured flows and thin channels, together with a mobile, slurry catalyst, namely a capillary reactor with gas/liquid/suspended catalyst flow. A downflow single capillary reactor (SCR) was designed, built and evaluated for the selective hydrogenation of isophorone to trimethyl cyclohexanone using commercial Pd- and Rh-based catalysts. Using the single capillary arrangement, the reaction was shown to be operating under kinetic control.

Comparison of the rate of hydrogenation with autoclave showed a significant increase of the reaction rate when capillary reactor was used. The temperature of reaction is a crucial factor in tuning the reaction towards different products. The constant relative reaction rate obtained for different catalyst loading as well as the calculated value of the apparent activation energy show that the reaction of hydrogenation of isophorone is not mass transfer limited in the single capillary reactor.     

MASS TRANSFER WITH CHEMICAL REACTION IN PARTIALLY WETTED FLAT PLATE CATALYST PELLETS: RIVULET FLOW ANALYSIS

Mass transfer with chemical reaction is analyzed in a system formed by a flat plate solid catalyst, partially wetted by a flowing rivulet of a liquid in contact with a stagnant pure gas. The paper solves the fluid dynamic problem of the liquid phase first, and afterwards incorporates the mass transfer and the chemical reaction. The system is assumed to be isothermal and at steady state, with a first order kinetics whose limiting reactant is in the gas phase. This work studies the influence of the gas-liquid surface tension, the liquid reactant flow rate, the liquid viscosity and the angle of inclination of the solid, upon the wetting factor. The model proposed also predicts the effect of these parameters and the Thiele modulus on the overall effectiveness factor and the molar flux of the limiting gaseous reactant at the catalytic solid-liquid interface in a direct way. This approach makes the wetting factor a non-manipulated variable.     

THE ROLE OF LATTICE OXYGEN IN THE DYNAMIC BEHAVIOR OF OXIDE CATALYSTS

The lattice of an oxide catalyst used for oxidation reactions can act as a reservoir for oxygen, storing and releasing it for reactions at the catalyst surface under appropriate conditions. The implication of this oxygen storage property of an oxide catalyst on its dynamic response characteristics has been investigated through an experimental study of 2-butene oxidation over vanadium oxide as a model reaction. Isothermal reaction rate measurements in a differential reactor and nonisothermal studies in a single pellet reactor have been carried out. Following a step increase in the feed butene concentration, isothermal reaction rate overshoot and pellet temperature overshoot were observed. These observations could be modelled in a qualitatively correct way by a very simple model accounting for the participation of lattice oxygen in the catalytic reactions under dynamic conditions. It is demonstrated through model simulations that the ignition characteristics of a catalyst pellet are significantly affected by the participation of the lattice oxygen, when steady state multiplicity is present.     

PERFORMANCE OF A PACKED-BED REACTOR

We examined the conversion rates in a packed bed catalytic reactor with a two phase upward flow in a wide range of operating conditions. The oxidation of ethanol to acetic acid in the liquid phase on a Pd-Alumina catalyst was chosen as the test reaction.

Global reaction rates were measured by changing gas velocities, temperature, and feed concentrations of ethanol in the liquid phase. The observed rates were compared with those calculated using two models, assuming a total external wetting of the catalyst. In the first model, a “kinetic” conversion rate was calculated by neglecting any interphase mass transfer resistance. In the second model the interphase mass transfer resistance was considered and expressed by an overall coefficient evaluated from published correlations. The results show that there is an hydrodynamic influence, probably due to the mass transfer and/or to the partial effective wetting of the catalyst. Mass transfer, on the other hand, is better than that observed in other cases. A comparison with the performances of a downflow trickle-bed reactor operating at the same tested conditions showed a much smaller influence of mass transfer and hydrodynamics on the overall conversion rate for the upflow reactor.     

Steady and unsteady co oxidation on platinum with allowance for external diffusion retardation

A mathematical model for catalytic oxidation of CO on a spherical platinum catalyst with allowance for the influence of gas diffusion is developed and studied. The model explains the origin of the effects of bifurcation and oscillation of a steady reaction rate. The model is based on the four-stage catalytic Elley-Ridley mechanism. In contrast to the case where gas diffusion retardation is ignored and only one basic form of catalyst phase plane exists, owing to the consideration of the effect of gas diffusion, we found two additional regimes of catalyst operation. These additional regimes make it possible to explain the origin of the effects of bifurcation and oscillation of the steady reaction rate by which the reaction rate for a steady-state surface is meant. When the surface concentrations of adsorbing reagents or free catalytic sites vary in the course of the reaction, the regime of catalyst operation is called unsteady. The reaction rate when the catalyst reaches a steady state is determined for all possible regimes of catalyst operation. Based on analysis of the results obtained, we give some practical recommendations on an increase in the reaction rate. The hysteresis of the reaction rate is explained.Translated from Fizika Goreniya i Vzryva, Vol. 32, No. 6, pp. 52–61, November–December, 1996.     

微型气液固三相等温微分浆态床反应器的优化

针对气液固三相浆态床催化反应中,传递、反应、催化剂的原位表征均比较复杂的问题,为了有利于气、固相均匀分散于液相和反应温度在反应器中实现等温,通过对气液固三相反应工艺特性和反应器性能要求的分析,对微型气液固三相浆态床反应器进行了优化。根据微型浆态床对气液固三相反应分析的要求,采用图像法研究了分布器为G1、G2、G3,砂板直径为2、2.5、3 cm反应器中的流体力学性能特征,考察了气体流速、温度、反应器直径及气体分布器对气含率、气泡尺寸、气泡上升速率以及气泡分布的影响,并进行流体动力学模拟计算,确定了微型浆态床反应器的直径为2 cm,气体分布器为G3砂板的反应器结构,该反应器可以应用于反应过程中间态及液体产物生成过程的测试。     

Determination of effectiveness factor of a partial internal wetting catalyst from adsorption measurement

The effect of partial internal wetting of catalyst pellets on apparent reaction kinetics at elevated temperatures and pressures is investigated experimentally and by modeling for benzene hydrogenation. A new method that combines adsorption and chemical reaction is introduced to study the kinetics influenced by capillary condensation of reagents at steady-state conditions. It is shown that the extent of liquid filling in the pellet interior has a critical effect on the global kinetics, and the current state of the catalyst depends on the history. Under certain conditions two steady states of the effectiveness factor exist. Moreover, either a decrease in temperature or increase in total pressure can increase the effectiveness factor. The model exhibits good agreement with experimental results.     

Multiplicity of the steady state in fluidized bed reactors—IV. Fluid catalytic cracking (FCC)

A mathematical model for fluid catalytic cracking units is developed. The model takes into account the kinetics of the cracking reactions, as well as the kinetics of coke combustion in the regenerator. A numerical scheme is developed for the solution of the model equations. It is found that multiplicity of the steady states extends over a wide scope of operating variables and parameters. The model investigates the effects of catalyst circulation rate and gas oil flow rate, which have a strong effect on the reactor temperature and hence yield and selectivity. The phenomenon of hysteresis has been investigated. The model can be used for yield optimization and steady state control.     

Effectiveness factor of a catalytic sphere in particle assemblage approached with a cell model

The cell model approach was extended to the catalytic reaction over a spherical catalyst pellet and the effects of several parameters on the effectiveness factor were investigated via numerical simulation. The proposed mathematical model consisted of the partial differential equations for gas reactant flow in the external gas phase around a catalyst sphere, mass transport in the gas phase, and two-dimensional diffusion-reaction in the catalytic sphere. Numerical simulation in the orthogonal boundary fitted reference frame demonstrated the decrease in effectiveness factor of a catalyst sphere due to the external mass transfer resistance and inter-particle hydrodynamic interaction. It was observed that the voidage of the particle assemblage shows more recognized influence than the Peclet and Reynolds numbers of particles and the azimuthal diffusion of reactant inside the particle was negligible. On this basis, a simplified set of cell model equations for catalytic reaction in particle assemblages were proposed. An example of optimizing search indicated the present model may be used to detect the most strong influence on catalytic reaction in particle assemblage over the practical ranges of operating and designing parameters to avoid excessive loss in the effectiveness factor.     

NMR imaging of mass transport processes and catalytic reactions

NMR imaging and spectroscopy techniques are applied to study flow and filtration of liquids, gases and granular solids in various geometries and to the in situ studies of the interplay of mass transport and catalytic reactions in porous media. In particular, quantitative spatially resolved maps of flow velocities of liquids and gases in the channels of monoliths have been obtained. A comparative study of the filtration of water and propane through model porous media has revealed that the dispersion coefficients for water are dominated by the holdup effects even in a bed of nonporous glass beads. Similar experiments performed with the gravity driven flow of liquid-containing fine solid particles through a porous bed have yielded the distributions of particle velocities for various flow rates. The NMR imaging technique was employed to visualize the propagation of autocatalytic waves for the Belousov–Zhabotinsky reaction carried out in a model porous medium. It was demonstrated that the wave propagation velocity decreases as the wave crosses the boundary between the bulk liquid and the flooded bead pack. The images detected during the catalytic hydrogenation of -methylstyrene on a single catalyst pellet at elevated temperatures have revealed that the reaction and the accompanying phase transition alter the distribution of the liquid phase within the pellet.     

The effect of film resistances on the fouling of catalyst pellets—II transient analysis

A transient analysis of the fouling of porous catalyst particles has been made using two models, in one of which an isothermal pellet is assumed while in the other intraparticle gradients of both temperature and concentration are considered. To account for the relative rate of fouling compared with the main reaction and hence to determine whether transient or pseudo-steady analysis is appropriate a new parameter termed the characteristic parameter for fouling is employed. The effect of this parameter and Lewis number are considered and a comparison is made with pseudo-steady state analyses. Finally a comparison is made of catalyst multiple steady states under fouling conditions using both types of analysis.     

连续相变的加氢反应器

引　言加氢反应是一类强放热反应 ,在采用滴流床反应器的情况下 ,经常遇到由于液体分布不均而导致的催化剂不完全润湿的现象[1] 。当液体反应物中含有挥发性组分时 ,在一定条件下 ,液相反应物可能会蒸发 ,伴随着反应速度的突然增加[2 ] ,使催化剂局部过热而失活 .另外 ,由于液体在催化剂床层中的不均匀分布而使床层出现未被润湿的干区 ,在强放热反应中 ,会经常观察到出现热点 ,床层的轴向温度出现振荡[3] .两相并流向上的固定床反应器 ,由于床层内的持液量较高 ,液体分布均匀 ,催化剂能很好地润湿 ,因此有可能避免出现热点[4 ] .这种操作形…