Titelbild Chem. Ing. Tech. 10/2016

The so‐called plug & play reactor is a novel reaction device with exchangeable reaction segments as well as modules for heating/cooling and mixing. The applications of the plug & play reactor include gas/solid, liquid/solid as well as gas/liquid/solid reactions. Commercially available HPLC columns, filled with catalyst particles, can be inserted in the reaction segment to act as fixed bed reactors. No external mixing device is needed, which leads to a high compactness of the reactor. The monitoring of the reaction progress and the reaction parameters can be carried out inline and online and an easy scale‐up by increasing the pipe diameter as well as simply numbering‐up by multiplying the modules is possible. Thus, the plug & play reactor is an attractive alternative to existing batch processes and can be easily implemented in existing processes. G. J. Lichtenegger, V. Tursic, H. Kitzler, K. Obermaier, J. G. Khinast, H. Gruber‐Wölfler*, Chem. Ing. Tech. 2016 , 88 (10), 1518 – 1523. DOI: 10.1002/cite.201600013     

Microfluidic dual loops reactor for conducting a multistep reaction

Precise control of each individual reaction that constitutes a multistep reaction must be performed to obtain the desired reaction product efficiently. In this work, we present a microfluidic dual loops reactor that enables multistep reaction by integrating two identical loop reactors. Specifically, reactants A and B are synthesized in the first loop reactor and transferred to the second loop reactor to synthesize with reactant C to form the final product. These individual reactions have nano-liter volumes and are carried out in a stepwise manner in each reactor without any cross-contamination issue. To precisely control the mixing efficiency in each loop reactor, we investigate the operating pressure and the operating frequency on the mixing valves for rotary mixing. This microfluidic dual loops reactor is integrated with several valves to realize the fully automated unit operation of a multistep reaction, such as metering the reactants, rotary mixing, transportation, and collecting the product. For proof of concept, CdSeZn nanoparticles are successfully synthesized in a microfluidic dual loops reactor through a fully automated multistep reaction. Taking all of these features together, this microfluidic dual loops reactor is a general microfluidic screening platform that can synthesize various materials through a multistep reaction.

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The effect of imperfect mixing on an idealized kinetic fermentation model

Mixing patterns in stirred reactors are analyzed in this study from the point of view of the hysteresis behavior of final product rate as a function of the intermediate concentration in the sequential reaction A → B → C. A two-tank system model with an internal recycle stream is studied, in order to simulate the effect of imperfect mixing in a single batch reactor. The extent of mixing between positions at which product and intermediate species concentrations are measured in the reactor is revealed in both the direction of the hysteresis function and the relative magnitude of the inscribed area. Another approach, which apparently simplifies the analysis, is to measure the concentration of B in each of the two tanks and cross-plot the two variables. This latter technique not only avoids the errors resulting from differencing the data, but also leads to correlations between the circumscribed areas and the degree of mixing.     

Non-ideal flow in an annular photocatalytic reactor

This study deals with the modelling of non-ideal flow in a tubular photocatalytic reactor with thin layer of TiO₂ photocatalyst. The objective was to analyse different level of mixing in the photoreactor applying basic principles of chemical reaction engineering. For this purpose photocatalytic oxidation of toluene was used as the model reaction. Photocatalytic reactor was operated in two different flow modes: classic type of an annular reactor with basically ideal (plug) flow with some extent of dispersion and annular flow reactor acted as stirred tank reactor with mixing of reaction mixture accomplished by recirculation. A series of experiments with step input disturbance at the entrance of the reactor with different air flow was performed in order to achieve better understanding of the reactor hydrodynamics. Several reactor models are applied, such as one dimensional model of tubular reactor at the steady state conditions, axial dispersion model at non-stationary conditions and the model of the continuous non-stationary stirred tank reactor. Numerical methods necessary for solving model equations and parameter estimation were described.     

Liquid-Liquid-Gas Triphasic Hydrogenation of Bicarbonate to Formate in a Continuous Flow Tubular Reactor

Multiphasic reaction of bicarbonate hydrogenation to form formate using homogeneous Ru PNP pincer catalyst in a continuous flow tubular reactor is reported. The reaction system consists of three phases. Catalyst is dissolved in toluene while potassium bicarbonate is dissolved in water. The significance of efficient mixing among the organic phase, aqueous phase and gaseous hydrogen to improve hydrogenation reaction by using different inert packing materials is studied by operando visualization and also quantitatively discussed. The bicarbonate conversion of up to 67% is achieved after optimization of important reaction and reactor parameters. The designed reactor setup comprised of effective recycling system that recycles the catalyst with >99% activity.     

泰勒流反应器的流动及反应特性

利用由静态混合器、喷嘴和分气盒组成的新型布气装置在搅拌釜式反应器中诱导生成泰勒流,对反应器流动特性及反应特性进行了实验研究。结果表明,与常规搅拌釜式反应器相比,泰勒流反应器内物料流动更加接近于平推流流型,泰勒流的生成在反应器内构建出局部平推流区域,降低了物料返混程度。反应器反应性能因流动特性改变而得以增强,相同实验条件下,在泰勒流反应器中进行的蔗糖水解反应转化率比在常规搅拌釜式反应器中高出26.7%。在一定操作范围内,局部平推流区域和反应转化率均随搅拌转速或进气量的增加而增大。泰勒流反应器可简化为平推流区和全混流区并联的流动模型,推导出了反应转化率与平推流区域占反应器总体积比率之间的关联关系。     

Characterisation and comparison of the micromixing efficiency in torus and batch stirred reactors

The mixing at a molecular scale (micromixing) plays an important role on selectivity, yield and quality of final products of a large range of competing fast chemical reactions. In this study, we have compared, by the use of iodide–iodate reaction tests, the micromixing in two reactors, the first one is the standard batch stirred reactor and the second is the torus reactor. Various conditions of agitation and feed locations were used for this study. A comparative analysis of the micromixedness ratio (α) in the two reactors was carried out on the basis of the local rate of specific energy dissipation.     

Mass transfer limitations in slurry photocatalytic reactors: Experimental validation

In the present work the existence of mass transfer limitations in slurry, photocatalytic reactors is studied. Experimental validation is made in a flat plate reactor that is part of a recycling system. The reactor is described with a mathematical model previously developed [Ballari et al., 2008a. Chemical Engineering Journal 136, 50], considering a transient, two-dimensional mass balance (TDM). The complete reactor model was developed to show the existence of these effects, which result from the occurrence of concentration gradients in reaction space. They develop when these reactors are operated under some operating conditions whose effects should be always analyzed before assuming the validity of existence of perfect mixing in reaction space. Dichloroacetic acid (DCA) was the adopted model compound. To solve TDM, a kinetic expression for DCA acid was determined before under well mixed conditions [Ballari et al., 2009. Industrial and Engineering Chemistry Research 48(4), 1847]. The studied variables are flow rate, catalyst loading, and irradiation rates. The experimental data agree quite well when they are interpreted in terms of the two-dimensional model (TDM) regardless of the operating mode. The perfect mixing model (PMM), normally employed to describe this and other types of slurry photoreactors, does not have the same level of universal application; i.e. it is restricted to perfect mixing, but in many cases far simpler to use. However, it can be concluded that when the photocatalytic reaction is not fast, employing catalyst loadings below 1 g L^–1, irradiation rates at the reactor wall below 1×10^?6 Einstein cm^?2 s^?1 and good mixing operation (Re>1700) it will be always safe to assume that mass transport limitations in the bulk of the fluid are nonexistent. In a typical batch reactor the above flow conditions are equivalent to very intense mixing. If the catalyst concentration is increased, the mixing conditions should be improved in the same proportion. Within limits, higher solid loadings can be compensated with lower irradiation rates [Ballari et al., 2008a. Chemical Engineering Journal 136, 50]. In addition, with the validated model, additional simulations are shown, operating the reactor under different virtual reactor thicknesses to widen amplitude of the reached conclusions. These findings will be useful in kinetic studies to prevent incursion in certain ranges of experimental conditions that could lead to erroneous interpretation of the obtained kinetic data.     

Kinetic model‐aided reactor design for peroxidase‐catalyzed removal of phenol in the presence of polyethylene glycol

A batch‐mixed reactor and a plug‐flow reactor were shown to have identical reaction kinetics for peroxidase‐catalyzed removal of phenol. Semi‐batch operation and step‐addition operation of the plug‐flow reactor with respect to hydrogen peroxide produced similar output. In the presence of polyethylene glycol, a continuous stirred tank reactor needed a long time to complete the reaction at the minimum enzyme dose. It was not necessary to stir the reaction mixture continuously during the reaction, hence polymerization and settling of products happened simultaneously. Recycling of the precipitates was not beneficial either for the removal efficiency or for the operation of a settling tank. A model previously developed to simulate the reaction in a batch reactor was used to predict the process in different types of reactor system. A plug‐flow reactor system is recommended for the reaction in the presence of polyethylene glycol; the system consisting of a small mixing tank followed by a tank in which reaction and settling occur simultaneously. © 1999 Society of Chemical Industry     

Effect of flow rate and constant operating current on the behaviour of a recirculating electrochemical reactor system

A theoretical analysis is presented for the current-time, voltage-time and power-time relationships and for the energy requirement and process time for the cathodic extraction of copper in a porous flow-through electrode operated as a batch recirculation system under galvanostatic conditions. The anode reaction considered is O₂ evolution and the secondary cathode reaction is H₂ evolution. The copper extraction is under diffusion control, plug flow is assumed in the reactor and perfect mixing in the reservoir. Comparison is made with the case of operation at the limiting current throughout the process duration.     

湍流非预混燃烧数值模拟的代数二阶矩模型

湍流燃烧数值模拟是研究燃烧的一种重要手段,采用的湍流燃烧模型是否恰当直接影响最终结果的准确性。在湍流燃烧中,化学反应速率不仅取决于当地的组分浓度和温度,而且与组分的湍流脉动也有密切关系。通过对湍流燃烧模型进行探讨,发现代数二阶矩模型（ASOM）能综合考虑湍流和反应动力学因素的影响,而且比其他复杂的模型简单。研究将组分混合速率对化学反应速率的影响在一个修正的代数二阶矩模型（RASOM）中进行考虑,更准确地计算出化学反应速率。为了验证模型的准确性,RASOM模型被应用到Sandia实验室测量的甲烷-空气非预混燃烧（Flame-D）的数值模拟中。模拟得到的结果与实验结果以及修正的涡破碎模型（EBU-A）和原ASOM模型的结果进行了对比,发现RASOM模型的效果较好。     

Die Modellierung von Stofftransportprozessen in Mehrphasenströmungen – am Beispiel des Strahlzonen‐Schlaufenreaktors

The efficiency and selectivity of chemical reactions are influenced by the mixing characteristics of the reactor. Existing models often assume homogeneous mixing on micro scale to calculate the reaction yield. However, neglecting the local hydrodynamic phenomena causes a discrepancy between model calculation and experimental data especially considering mass transfer limited reactions. In two‐phase flows different mass transfer phenomena have to be considered: the diffusion in the gas‐liquid boundary layer and diffusion in the Batchelor layer. The aim of the paper is to describe the mass transfer affecting mechanisms in multi‐phase flows and to discuss the first results of the investigation of local mass transfer phenomena in a two phase flow driven jet‐zone loop reactor.     

Effective Ozone Generation in Oxygen Using a Mesh Electrode in an Ozonizer with Variable Linear Velocity

Results of studies on ozone synthesis under discharges proceeding in a metal mesh-ceramic dielectric system have been presented. The experiments were carried out in the reactor with unique reaction space geometry, in which the reacting gas flew with consequently increasing linear velocity. The high voltage electrode was made of a metal mesh, which caused intensification of the gas mixing in the reaction space. Using a simple reactor with one-side cooling of the reaction space, high ozone maximum concentrations (100 g/Nm³) and energy efficiencies (180–200 g/kWh) were obtained at 25 °C.     

T型反应器内流动、混合及界面反应特征

利用平面激光诱导荧光（PLIF）技术和酚酞显色反应,对不同Reynolds数（20<Re<420）下T型反应器内的复杂流动结构及界面反应进行了可视化研究。随着入口Re的增加,反应器内依次出现分离流、稳态吞噬流、非稳态吞噬流及非稳态对称流等流动模式。重点考察了不同流动模式下T型反应器内的界面反应特征,结合流动模式对混合效果、产物浓度分布及时间演化进行了分析,揭示了反应器内复杂流场对混合及界面反应的影响机理。结果表明,吞噬流三维旋涡结构使流体相互卷吸缠绕,通过折叠拉伸形成层状的流体界面,极大增加了反应物的界面接触面积,混合及反应程度显著提升,产物浓度较高且分布均匀。     

New advances in agitation technology for exothermic reactions in very large reactors

Organic reactions, such as polymerisations, often require precisely controlled homogeneous reaction conditions in order to achieve high product quality, minimise waste or rework, and therefore reduce environmental impact. Many such processes were originally manufactured in moderately sized reactors with a height-diameter ratio of around 1. Operators have subsequently driven up their production rates by intensifying the reaction and by increasing the reactor size (and often increased the reactor's height-diameter geometry) in response to the economic requirement for world-scale manufacturing plants.However, as reaction intensities and reactor sizes increase, product quality can be affected due to poorer homogeneity, since reaction rates are faster but bulk mixing is slower. This can lead to a “Limit to Scale” beyond which product quality is unacceptable. Better bulk mixing within very large reactors would increase the Limits to Scale.Laboratory trials were undertaken to compare the homogeneity achieved by different impeller configurations in a model of a typical large reactor. This leads to a new agitator design concept, which achieves rapid mixing in large vessels with high height-diameter ratios using a series of impellers that produce a narrowly confined axial flow, effectively a virtual draft tube, in the centre of the reactor. Fluid returns to the top of the reactor near the walls creating a loop flow pattern within the reactor, effectively an “internal loop reactor”. CFD simulations of the reactor were undertaken to better understand the hydrodynamics, and were validated against experimental results.     

Multi-mode low-dimensional models for non-isothermal homogeneous and catalytic reactors

A systematic procedure based on the Liapunov-Schmidt method of bifurcation theory is used to derive low-dimensional models for different types of non-isothermal homogeneous, catalytic and coupled homogeneous-heterogeneous reactors. These low-dimensional models are described by multiple concentration and temperature modes (variables), each of which is representative of a physical scale of the system. These “multi-mode models” capture mass and thermal micromixing as exchange of material and energy, respectively, between the modes (scales). The multi-mode models retain all the parameters and most of the qualitative features of the full convection-diffusion-reaction equations. While in the limit of vanishingly small local heat and mass diffusion times, they reduce to the classical ideal pseudo-homogeneous reactor models, they are also capable of capturing the mixing or mass (and/or heat) transfer-limited asymptotes for the case of fast reactions. We illustrate the usefulness of the multi-mode models in predicting mixing and selectivity effects on reactor performance and the influence of local transport effects on reactor runaway and bifurcation behavior for the case of non-isothermal homogeneous and catalytic reactors.     

Steady state multiplicity behavior of an isothermal axial dispersion fixed-bed reactor with nonuniformly active catalyst

An isothermal, heterogeneous fixed-bed reactor packed with nonuniformly active catalyst pellets where a biomolecular Langmuir-Hinshelwood reaction occurs, is studied using an axial dispersion model. A catalyst activity distribution given by a Dirac delta function, where the active catalyst is deposited at a specific location within the pellet, is considered. This includes the common case of externally coated pellets with external mass transfer resistance. The steady state multiplicity behavior of this reactor, and its limiting cases: CSTR, PFR and pseudohomogeneous axial dispersion, are examined in detail. The nonlinearity of the reaction kinetics provides two sources of multiplicity, through the heterogeneous nature of the reactor and the presence of axial dispersion in the fluid phase. Their roles in determining reactor multiplicity behavior are fully explored. It is shown that this system can admit at most nine steady state solutions. The limiting behavior of the heterogeneous axial dispersion model as Pe → 0 or ∞ is not represented fully by the CSTR or PFR models because of ignition phenomenon. Finally, the effects of mixing on reactor conversion are discussed.     

Modeling of bubble column slurry reactor for reductive alkylation of p-phenylenediamine

A bubble column slurry reactor (BCSR) model has been developed for the reductive alkylation of p-phenylenediamine (PPDA) with methyl ethyl ketone (MEK) to N,N^′-di-secondary-alkyl-p-phenylenediamine (Di-amine). This particular reaction system is commercially relevant and involves a combination of parallel and consecutive reactions comprising equilibrium non-catalytic (homogeneous) and catalytic (heterogeneous) steps. The proposed model is based on the ‘mixing cell approach’. In this work the mixing cell approach has been extended by including a liquid backflow stream from all but the bottommost mixing cell. The model incorporates the contributions of gas-liquid and liquid-solid mass transfer, heat effects, and complex multistep reaction kinetics. CFD model is used to estimate the extent of backflow among mixing cells and its dependence on operating parameters. The effect of gas and liquid velocities, catalyst loading, inlet PPDA concentration, and temperature on the conversion, selectivity, global rate of hydrogenation, and temperature rise is discussed. The comparison of the current approach with the traditional mixing cell model is discussed. The BCSR model presented here will be useful to provide guidelines for designing and improving overall performance of bubble column reactors.     

New method for the determination of precipitation kinetics using a laminar jet reactor

In this paper a new experimental method for determining the kinetics of fast precipitation reactions is introduced. Use is made of a laminar jet reactor, which is also frequently applied to determine the kinetics of homogeneous gas-liquid reactions. The liquid containing one or more of the precipitating reactants passes a gas-filled reactor as a stagnant jet in which no mixing occurs. The remaining reactant needed for precipitation is supplied in gaseous form and causes the precipitation reaction to occur while it is diffusing into the jet. Hydrodynamics as well as transport phenomena are precisely known for this system, whereas agglomeration can be minimized by adjustment of the concentration of the solute supplied by the gas. The kinetics of the different crystallization steps can be determined by analyzing the size distribution of the produced particles. This new method is experimentally demonstrated for the precipitation of CuS using H₂S gas. The obtained data were successfully used to simulate a packed bed absorber in which H₂S is absorbed by a CuSO₄ solution.     

A shrinking-aggregate two-environment mixing model

A two-environment mixing model is proposed to predict the conversion of a homogeneous reaction in a continuous stirred-tank reactor with premixed feed. The entering fluid is assumed to consist of spherical aggregates segregated from the fluid in the reactor. The aggregates shrink and fluid from the aggregates enter a maximum-mixedness, or nonsegregated, flow region. The aggregate shrinking rate is a function of the aggregate age. The model parameters—the fluid residence time and the entering and exiting flow rates in each environment—are determined a priori using a shrinking-aggregate model and the residence time distribution of the system. Model predictions are compared to experimental results.