Measurement and enhancement of gas-liquid mass transfer in milliliter-scale slurry reactors

Due to the limited availability of chemical reactants in the early process development of pharmaceuticals and fine chemicals, and sometimes the high-cost of catalyst, it is increasingly popular to use milliliter-scale slurry reactors with reaction volumes of 20 ml or less to screen catalyst candidates for three-phase reactions. To ensure the success of catalyst screening, it is advantageous to run reactions under kinetically controlled conditions so that the activities of different catalysts can be compared. Because catalysts with small particle sizes are used in slurry reactors, the reactions are susceptible to gas-liquid mass transfer limitations. This work presents an efficient way of enhancing gas-liquid mass transfer in milliliter-scale reactors through the use of magnetically driven agitation with complex motion. In the reactor described here, gas-liquid mass transfer coefficients can be doubled over those obtained with the agitation technique used in commercial milliliter-scale units. In addition, the reactor can achieve the top range of mass transfer coefficients obtained in a full-scale reactor. This work also presents the first measurements of gas-liquid mass transfer coefficients in milliliter-scale reactors, which are two orders-of-magnitude smaller than systems for which mass transfer coefficients have been reported earlier. Both physical and chemical absorption techniques are used.     

Spatial distribution of functionalities in an adsorptive reactor at the particle level

The concept of adsorptive reactors has attracted considerable attention as a hybrid process to enhance reaction selectivity and conversion for heterogeneously catalysed gas phase reactions. Transport resistances affect the performance of adsorptive reactors adversely and the integration of functionalities within the same particle circumvents this limitation. Instead of a simple uniform integration of functionalities within the particle, one can also non-uniformly distribute the functionalities over the particle to exploit the concentration profiles arising from transport limitations for process enhancement. A detailed numerical investigation has been carried out to identify the optimal distribution of catalyst and adsorbent functionalities at a particle and reactor level using Aspen Custom Modeler. Though process enhancements are possible by non-uniform distribution within particle, the benefits are marginal. Nevertheless, the integration of functionalities within a particle offers significant improvements in adsorptive process performance.     

下行床反应器内催化裂化过程的CFD模拟

耦合湍流气粒多相流模型和催化裂化集总动力学模型,建立了描述下行床内多相流动和催化裂化过程的反应器数学模型,并利用计算流体力学单元模拟软件CFX4.3对下行床内的催化裂化过程进行了数值模拟及分析.模型能预测出在工业应用中反应器内最受关注的诸多参数,如固含率、相间滑移速度、压降、气固相的加速区以及各组分浓度的分布情况.预测结果表明,气相反应的进行将导致反应器内的气粒流动行为发生较大变化,充分考虑反应与流动行为的耦合十分重要;而反应器床径的增大将导致转化率和各产物收率的下降.     

Two‐phase production of cyclohexene and cyclooctene oxides in water as an approach to pollution prevention

Liquid–liquid two‐phase epoxidation from cyclohexene and cyclooctene in aqueous potassium peroxymonosulfate (commercially available as Oxone^®) solution was studied as an application in pollution prevention. To avoid potential emissions of volatile organic compounds an aqueous solution was employed to replace the usual chlorinated solvents used in epoxide production. A droplet column reactor and stirred tank reactor were used to investigate two‐phase synthesis of epoxide. An aqueous Oxone^® solution was used to oxidize a dispersion of alkene droplets and form epoxide. The study of aqueous epoxidation in both reactors showed that the epoxidation of alkenes can be represented as a first order reaction with respect to alkene. The salting out effect of Oxone^® concentration was studied in both reactors and found to be very similar at optimal conditions. In comparing the two reactors, it was found that the droplet column reactor produces larger quantities of product per unit reactor volume for the same reaction time. The objective of this study is to provide an alternative reactor design and synthesis route that can meet pollution prevention goals. Copyright © 2004 Society of Chemical Industry     

The Behavior of Constant Rate Aerosol Reactors

An aerosol reactor is a gaseous system in which fine particles are formed by chemical reaction in either a batch or flow process. The particle sizes of interest range from less than 10 Å (molecular clusters) to 10μm. Such reactors may be operated to study the aerosol formation process, as in a smog reactor, or to generate a product such as a pigment or a catalytic aerosol. Aerosol reactors can be characterized by three temporal or spatial zones or regions of operation for batch and flow reactors, respectively. In zone I, chemical reaction results in the formation of condensable molecular products which nucleate and form very high concentrations of small particles. The number density depends on the concentration of preexisting aerosol. Zone II is a transition region in which the aerosol number concentration levels off as a result of hetergeneous condensation by the stable aerosol. In zone III coagulation becomes sufficiently rapid to reduce the particle number concentration. There may be a zone IV in which agglomerates form. Chemical reaction may continue to generate condensable material throughout the various zones. This paper deals with reactors in which aerosol material is generated at a constant rate. Design parameters of interest are the particle size distribution, number density, surface area, and mass loadings. For ideal systems composed of spherical coalescing particles, these can be predicted theoretically for certain limiting cases. However, the irregular agglomerates which may form in zone IV are more difficult to characterize theoretically.     

Computational fluid dynamics in the development of catalytic reactors

Computational fluid dynamics is becoming an important tool in the study of chemical engineering processes and apparatuses (in particular, the share of works with the application of this method is nearly 6% of the total number of all chemical engineering works issued by Elsevier Science Publishers in 2010). The possibilities of computational fluid dynamics are demonstrated using examples from three different chemical engineering fields: developing a method for loading a tubular reactor for the steam conversion of natural gas, studying heat transfer in a reactor for the hydrogenation of vegetable oils upon the replacement of a catalyst, and investigating the transitional processes in an automobile neutralizer. The results from computational fluid dynamics are verified by comparing them with experimental data in developing a method for loading a tubular reactor, using the problem of decelerating a catalyst particle with a flow of air as an example. The obtained data are compared with classical measurement data on the aerodynamic drag of a ball and a cylinder and represent the further development of works on the flow around particles of complex shape. In this work, the results from inspecting a reactor for the hydrogenation of oils with allowance for the possible heating and uniform distribution of a flow before its entering the catalyst bed are presented. It is shown that the construction of the reactor does not ensure homogeneity of the reaction flow at the desired level and requires modification of heating elements. The efficiency of computational fluid dynamics for investigating fast processes with a chemical reaction is exemplified by studying the transitional processes in an catalytic automobile neutralizer (the effect of flow dynamics and heat transfer on the thermal regime in a honeycomb catalyst particle is very difficult to study by experimental methods). The application of computational fluid dynamics allows us to reduce considerably the time and cost of developing and optimizing the designs of efficient catalytic fixed-, fluidized-, or moving-bed reactors (particularly multiphase stirred (slurry) reactors), along with mixers, adsorbers, bubblers, and other chemical engineering apparatuses with moving media.     

The bifurcation behavior of continuous free-radical solution loop polymerization reactors

The bifurcation behavior of continuous free-radical solution loop polymerization reactors is analyzed in this work. A mathematical model is developed in order to describe the impact of the recycling pump and other external reactor parts upon the process dynamics and stability. Stability analysis is performed using bifurcation theory and continuation methods. It is shown that under certain operational conditions as many as seven steady states are predicted for the loop polymerization reactor. Oscillatory behavior is observed for a wide range of process parameters and onset of oscillations is observed during the transition from operation without material recycling to operation with partial recirculation of the polymer solution. Besides, at certain constrained range of operation conditions, complex dynamics can be observed, including the onset of chaotic behavior. It is also shown that the thermal parameters of the reactor and recycling pump exert a profound effect upon the process stability. For this reason it is shown that oscillatory behavior is very unlikely to occur in actual industrial reactors.     

Transport processes and conversion in isothermal slurry reactors

Transient mass transfer and chemical reaction in isothermal continuous-flow and batch agitated reactors is studied mathematically. Analytical solutions are derived in the form of infinite integrals. The solutions include the effects of fluid-to-particle diffusion, intraparticle diffusion and reversible first-order adsorption for a first-order reaction on the particle surface. The conversion at steady-state is explored in some detail. An efficient numerical scheme for evaluating the infinite integral in the transient solution is given. A large number of calculations are presented, for both the continuous feed and the batch reactor, for the case with negligible adsorption rate resistance. The influences of the different mass transfer resistances on the conversion are clearly demonstrated.     

Investigation of the liquid recycle in the reactor cascade of an industrial scale ebullated bed hydrocracking unit

One of the commercial means to convert heavy oil residue is hydrocracking in an ebullated bed. The ebullated bed reactor includes a complex gas–liquid–solid backmixed system which attracts the attention of many scientists and research groups. This work is aimed at the calculation of the internal recycle flow rate and understanding its effect on other parameters of the ebullated bed. Measured data were collected from an industrial scale residual hydrocracking unit consisting of a cascade of three ebullated bed reactors. A simplified block model of the ebullated bed reactors was created in Aspen Plus and fed with measured data. For reaction yield calculation, a lumped kinetic model was used. The model was verified by comparing experimental and calculated distillation curves as well as the calculated and measured reactor inlet temperature. Influence of the feed rate on the recycle ratio(recycle to feed flow rate) was estimated. A relation between the recycle flow rate, pump pressure difference and catalyst inventory has been identified. The recycle ratio also affects the temperature gradient along the reactor cascade. Influence of the recycle ratio on the temperature gradient decreased with the cascade member order.     

Performance characteristics of a cstr containing immobilised enzyme particles

The effects of internal and external substrate diffusion resistances on the performance of a continuous stirred tank reactor are analysed in this work. Both immobilised enzymatic reactions with and without substrate inhibitions are considered. The substrate conversion for the reaction without substrate inhibition is dependent on four dimensionless parameters: the Thiele modulus, the dimensionless Michaelis constant, the mass transfer Nusselt number and β, which represents a combination of particle hold-up, maximum reaction rate, input substrate concentration and substrate residence time in the continuous stirred tank reactor. For the corresponding reaction with substrate inhibition, the effect of the additional dimensionless inhibition constant on the substrate conversion is also very significant. The substrate conversion generally decreases with decrease in dimensionless parameter β, increase in Thiele modulus and decrease in mass transfer Nusselt number. For the reaction with small Thiele modulus, β and strong substrate diffusion resistances, a multireactor system may be needed if a certain desired substrate conversion is required. The single CSTR model can be extended to describe the multireactor system and the effect of the number of reactors on the substrate conversion for a two- or more reactor system is also examined.     

A new technique to derive particle size distributions and aerosol number concentrations directly from thermophoretic data

The manufacture of many high value-added powders takes place by the decomposition of gaseous precursors in aerosol tube reactors. Historically, process improvements were achieved by making changes on the outside of the reactor and observing what comes out at the end of the pipe. The development of increasingly accurate aerosol dynamics models based on engineering first principles has been limited because models were typically validated on integral properties of ex situ product, instead of particle properties measured at multiple positions inside the reactor. In this study, a model reactor was equipped to capture samples thermophoretically from 15 internal positions. Additional in-line measurements were achieved with a multi-stage inertial impactor and by traditional analysis of ex situ product. Calculations were performed to verify that thermophoresis was the dominant mechanism of particle capture. The thermophoretic samples were analyzed by electron beam microscopy and image analysis to develop particle size distributions at each of the internal positions inside the reactor. An approximation of Talbot's Equation for thermophoretic velocity allowed experimental measurements to be combined with thermophoretic sample data to give predictions of particle number concentration corresponding to the precise sampling locations. The combinations of particle size distributions and number concentrations provide powerful insights on particle nucleation and growth dynamics.     

Economic feasibility of silica and palladium composite membranes for industrial dehydrogenation reactions

This article addresses the economic feasibility of silica and palladium composite membranes for gaseous dehydrogenation reaction schemes. Unlike other methodologies addressed so far, this work presents the economic assessment of dehydrogenation reaction schemes using a conceptual design based simulation methodology for the comparative economic assessment of membrane reactors with conventional reactors. The suggested methodology is applied to two industrially prominent reaction schemes namely styrene (from ethylbenzene) and propylene (from propane) production using silica and palladium composite membrane reactors. Various sub-cases studied in this work include the influence of membrane area per reaction zone volume, reaction zone temperature, reaction and permeation zone pressure, membrane thickness and sweep gas flow rate on process economics. Based on this work, the propylene production scheme is evaluated to provide 60–70% excess profits using membrane reactors when compared with the conventional reactor based technology. However, the gross profit profiles for both conventional reactor and membrane reactor configurations have been found to be similar for styrene production case. For all cases, the cost contribution of membranes and other auxiliary equipment is estimated not to exceed 20% of the total costs. In addition, similar economic performance has been observed for both silica and palladium membranes. Based on these studies, it has been concluded that the industrial applicability of membrane reactors is economically suitable for those dehydrogenation reactions that enable significant conversion enhancement with respect to the conventional reactor technologies.     

Free‐radical solution polymerization of styrene in a tubular reactor—effects of recycling

The styrene free‐radical solution polymerization reaction in a tubular loop reactor is studied here both experimentally and through simulation. An attempt is made to compare the performances of tubular loop reactors when the recycle ratio is varied, based on steady‐state and dynamic responses and on the quality of the polymer produced at different conditions. It is shown here that steady‐state responses of loop reactors and traditional tubular reactors are very similar as far as the quality of the polymer obtained is concerned. Therefore, the recycle ratio cannot be used as a fundamental operation parameter for grade transitions at plant site. However, it is also shown that the recycling of polymer material is very important to accelerate the attainment of the final steady‐state in tubular reactor configurations, because recirculation of material homogenizes the distorted radial profiles of the axial flow velocities.     

Validation of pressure drop prediction and bed generation of fixed-beds with complex particle shapes using discrete element method and computational fluid dynamics

Catalytic fixed-bed reactors with a low tube-to-particle diameter ratio are widely used in industrial applications. The heterogeneous packing morphology in this reactor type causes local flow phenomena that significantly affect the reactor performance. Particle-resolved computational fluid dynamics has become a predictive numerical method to analyze the flow, temperature, and species field, as well as local reaction rates spatially and may, therefore, be used as a design tool to develop new improved catalyst shapes. Most validation studies which have been presented in the past were limited to simple particle shapes. More complex catalyst shapes are supposed to increase the reactor performance. A workflow for the simulation of fixed-bed reactors filled with various industrially relevant complex particle shapes is presented and validated against experimental data in terms of bed voidage and pressure drop. Industrially relevant loading strategies are numerically replicated and their impact on particle orientation and bed voidage is investigated.     

MASS TRANSFER AND HYDRODYNAMICS IN AN AIRLIFT REACTOR WITH VISCOUS NON-NEWTONIAN FLUID

In an internal loop airlift reactor of 55L working volume,the gas-liquid volumetric oxygenmass transfer coefficient k_Lα,gas holdup ε_G and liquid circulation time t_c were measured with the sol-ution of carboxymethyl cellulose(CMC)to simulate the performance of a reactor with highly viscousbroth.Electric conductivity and oxygen probes were used to measure the local gas holdup,liquidcirculation time and oxygen mass transfer coefficient in the individual sections of the reactor(riser,downcomer and the gas-liquid separating section at the top of the reactor)and the total reactor,respectively.The values of k_Lα for the riser,downcomer and separation sections of the reactor were alsoestimated and compared with that for the total reactor.The results show that,both k_Lα and ε_G in-crease but t_c decreases with increasing gas velocity.Correlations and comparisons with works reportedin the literature are also presented.Data show that the methods developed for k_Lα measurements inthe individual section and     

IDEAS based synthesis of minimum volume reactor networks featuring residence time density/distribution models

This work addresses for the first time, the synthesis of globally minimum volume reactor networks, featuring segregated flow reactors (SFR) and/or maximum mixedness reactors (MMR), with the same normalized residence time density (NRTd) function. Global optimality is ascertained by demonstrating that the input–output information maps of SFR and MMR with general RTd/RTD models satisfy all properties required for the application of the infinite dimensional state-space (IDEAS) approach to the RTd/RTD reactor network synthesis problem. The resulting IDEAS formulation is shown to possess a number of novel properties, which can be used to facilitate its solution. The power of the proposed methodology is demonstrated on three case studies featuring segregated laminar flow reactors (SLFR) in which the Trambouze reaction scheme is carried out. In one of the case studies, the identified reactor network is shown to have volume that is as low as half the volume of a single reactor.     

Recuperative coupling of exothermic and endothermic reactions

Coupling energy intensive endothermic reaction systems with suitable exothermic reactions improves the thermal efficiency of processes and reduces the size of the reactors. One type of reactor suitable for such a type of coupling is the heat exchanger reactor. In this work, a one-dimensional pseudo-homogeneous plug flow model is used to analyze and compare the performance of co-current and counter-current heat exchanger reactors. A parametric analysis is carried out to address the vital issues, such as the exit conversion of the endothermic reaction, the temperature peak (hot spot) of the exothermic reaction and the reactor volumetric productivity. The measures to reduce the hot spot by different catalyst profiling techniques are also addressed. Some features of the dynamic behavior exhibited by these reactors, which are important from design, operational and control point of view, are presented.     

Numerical simulation of flow field and residence time of nanoparticles in a 1000-ton industrial multi-jet combustion reactor

In this work, by establishing a three-dimensional physical model of a 1000-ton industrial multi-jet combustion reactor, a hexahedral structured grid was used to discretize the model. Combined with realizable k–ε model, eddy-dissipation-concept, discrete-ordinate radiation model, hydrogen 19-step detailed reaction mechanism, air age user-defined-function, velocity field, temperature field, concentration field and gas arrival time in the reactor were numerically simulated. The Euler–Lagrange metho...     

Reactors for hydrogenation of edible oils

Due to the characteristics of the hydrogenation of edible oil, by far the most common type of reactor has been the batch-type shurry hardener. Although continuous reactors offer several advantages compared to batch reactors, they are seldom used in the industry. This review paper describes the most commonly used full-scale reactors, both batch and continuous. Several different laboratory reactors are also described. The experimental results obtained from those reactors indicate that it is possible to achieve selectivites and reaction rates in a continuous reactor as high as in a slurry batch reactor.