Mixing Characteristics and Bubble Behavior in an Airlift Internal Loop Reactor with Low Aspect Ratio

The present study summarizes the results of macro-and micro-mixing characteristics in an airlift inter-nal loop reactor with low aspect ratio （H/D≤5） using the electrolytic tracer response technique and the method of parallel competing reactions respectively. The micro-mixing has never been investigated in airlift loop reactors. The dual-tip electrical conductivity probe technique is used for measurement of local bubble behavior in the reactor. The effects of several operating parameters and geometric variables are investigated. It is found that the increase in su-perficial gas velocity corresponds to the increase in energy input, liquid circulation velocity and shear rate, decreas-ing the macro-mixing time and segregation index. Moreover, it is shown that top clearance and draft diameter affect flow resistance. However, the bubble redistribution with a screen mesh on the perforated plate distributor for macro-mixing is insignificant. The top region with a high energy dissipation rate is a suitable location for feeding reactants. The analysis of present experimental data provides a valuable insight into the interaction between gas and liquid phases for mixing and improves the understanding of intrinsic roles of hydrodynamics upon the reactor de-sign and operating parameter selection.     

Optimal operation policies in batch reactors

Optimal operation policies in batch reactors are obtained using dynamic optimisation technique. Two different types of optimisation problems, namely, maximum conversion and minimum time problems are formulated and solved and optimal operation policies in terms of reactor temperature or coolant flow rate are obtained. A path constraint on the reactor temperature is imposed for safe reactor operation and an endpoint constraint on undesired waste production (by-product) is imposed to minimise environmental impact.Two different types of models are considered within the optimisation framework. The shortcut model allows determination of optimal reactor temperature profile to be used for detailed design of the reactor. The detailed model allows optimising operating conditions for an already designed batch reactors.     

Dynamic shear in continuous-flow rotating-disk catalytic reactors with stress-sensitive kinetics based on Curie's theorem in non-equilibrium thermodynamics

Stress-sensitive response is simulated in a modified parallel-disk reactor that implements steady and unidirectional dynamic shear in the creeping flow regime. Reactants chemisorb on the surface of the rotating plate, and catalytic sites are replenished from the bulk fluid via radial and tangential flow accompanied by transverse diffusion in the z-direction toward the active surface. Chemical reaction is enhanced by viscous shear at the interface between the bulk fluid and the rotating plate. This heterogeneous catalytic rotational reactor is modeled via radial convection and axial diffusion with angular symmetry in cylindrical coordinates. The reaction/diffusion boundary condition on the surface of the rotating plate accounts for stress-sensitive reactant consumption via the zr- and zΘ-elements of the velocity gradient tensor at the catalytic surface. Linear transport laws in chemically reactive systems that obey Curie's theorem predict the existence of cross-phenomena between scalar reaction rates and the magnitude of the second-rank velocity gradient tensor, selecting only those elements of ?v experienced by reactants that are chemisorbed on the surface of the rotating plate. Stress sensitivity via the formalism of irreversible thermodynamics introduces a zeroth-order contribution to heterogeneous consumption rates that must be quenched when reactants or active vacant sites are not present on the surface of the rotating plate. Rotating-disk reactor simulations are presented for simple 1st-order, simple 2nd-order, and complex heterogeneous stress-free kinetics, where the latter considers Langmuir-type dissociative adsorption of one of the reactants. Accurate design of rotating-disk reactors must consider stress-sensitivity when the shear-rate-based Damköhler number (i.e., ratio of the stress-dependent zeroth-order consumption rate relative to the rate of reactant diffusion toward the active surface) is greater than its threshold value which increases at higher stress-free Damköhler numbers. Modulated rotation of the catalytically active plate demonstrates that these rotating-disk reactors must operate above a threshold for the stress-sensitive Damköhler number, identified under steady shear conditions, before dynamic shear has a distinguishable effect on reactor performance.     

Macro-mixing in a draft-tube airlift bioreactor

Airlift reactors are pneumatically agitated reactors that have been widely used in industries, particularly in bioprocesses. Extensive studies about the flow dynamics in airlift column reactors exist; however, most of these studies have focused on global hydrodynamic parameters using conventional techniques. The local flow characteristics, such as the macro-mixing and the turbulence intensity, are crucial for reliable design and scale-up, and they remain unclear. This work focuses on studying the macro-mixing in a draft-tube airlift bioreactor utilizing an advanced flow dynamic measurement technique, computer automated radioactive particle tracking (CARPT). True residence time distribution analyses for the overall column as well as individual regions, i.e., the riser, the downcomer, the top, and the bottom regions, are conducted for the first time based on CARPT measured particle trajectories. The effects of the superficial gas velocity and the top/bottom clearances on the macro-mixing are also discussed. The results suggest that although the flow structures in the overall draft-tube column reactor, as well as in the riser and in the downcomer, are close to plug flows, bypassing and stagnancy exist in the top and the bottom regions.     

The formation of emulsions during the jet mixing of reagent flows as applied to the sulfuric acid alkylation of <Emphasis Type="Italic">iso</Emphasis>-butane with olefins

The modernization of reactor equipment for the industrial process of the sulfuric acid alkylation of iso-butane with olefins has been considered. Classic alkyl gasoline production technology has been improved by applying small reactors with a divergent-convergent configuration that optimizes the hydrodynamic regime in the reaction space. This emulsion process is subjected to theoretical analysis with the substantiation of the proposed contactor design. Industrial tests that confirm the high efficiency of the applied design compared to the functioning industrial sulfuric acid alkylation processes with regard to the reduction of energy consumption and the optimization of the consumption parameters of feedstock components and a catalyst have been described for this technology.     

淤浆鼓泡反应器的开发研究 Ⅰ.间歇过程

要求实验室间歇鼓泡反应器能重复间歇搅拌釜中多段温控等优化工艺,当反应过程处于反应动力学控制时,才较易实现。两种反应器获得相同工艺条件的关键操作因素分别是表观气速和桨叶转速。这两种操作因素的临界值,包括保证催化剂完全悬浮,排除外扩散阻力等问题作为操作下限,避免鼓泡液泛作为操作上限,均需在开发研究中进行测定。实现优化工艺需要着眼于催化剂表面有足够的氢浓度,这和要求处于反应动力学控制是一致的。按 Hammer 的三维性能图对淤桨鼓泡反应器的开发放大有较大的参考意义。     

Modification and optimization of benzene alkylation process for production of ethylbenzene

In this paper, an industrial ethylbenzene production unit has been simulated and the results are compared against five-day experimental data. According to prevailing unit condition, i.e. recycled ratio of benzene, benzene selectivity, and energy consumption, the unit is not working under its optimum conditions for minimum cost of ethylbenzene production. In the current design, high amount of benzene recycle (6:1) causes to have an additional cost due to fractionation of ethylbenzene from benzene. A new approach is proposed to modify the benzene alkylation process and reduce the unit's energy consumption. In the newly designed scheme, two double-bed alkylation reactors converted into four single-bed reactors. The amount of injected ethylene, alkylation reactors temperature, and recycled stream are regulated as adjustable parameters for the optimization of the process. In the modified process, the reflux ratio reduced to 1.87 and the benzene selectivity increased. The optimized process shows a considerable decrease in the unit's energy consumption in compare to the current process. Also, the mass fraction of ethylbenzene would reach to 99.12% of purity before entering to the transalkylation reactor for further purification. Therefore, if the presented purity is acceptable for the final application, the transalkylation reactor could be eliminated from the new design.     

气升式环流反应器中的流体动力学研究(Ⅱ)——实验及模型结果分析

建立了一直径为0.186m、高3m的内环流气升式反应器。通过电化学方法测得了反应器中的气含率和速度。本文以文(Ⅰ)所得气升式环流反应器的一维两流体模型为基础,推导出了反应器的能量方程。分析了反应中的功耗分配规律,并以此对反应器中的流动规律作了较合理的解释。本文还从理论和实验上较全面地讨论了反应器操作参数和结构参数对气含率和循环液速等的影响。     

Finite element modeling of the laminar and transition flow of the Superblend dual shaft coaxial mixer on parallel computers

The macro-mixing mechanisms of the Superblend coaxial mixer consisting of a Maxblend impeller and a double helical ribbon agitator mounted on two independent coaxial shafts rotating at different speeds are numerically investigated. The simulations are based on the resolution of the Navier-Stokes equations with help of a parallel three-dimensional finite element solver exploiting the capabilities of high performance computers. To model the rotation of agitators a hybrid approach based on a novel finite element sliding mesh and fictitious domain method is used. The power consumption, the flow patterns, the shear rate distribution, the pumping capacity and the mixing time of the Superblend mixer are calculated from the simulated hydrodynamics. The simulations allow observing the flow as it evolves from deep laminar (Re=0.1) to transition (Re=520) regime. As Reynolds number increases, several recirculation zones above and below the middle of the tank are formed. It is found that operating the agitators in co-rotation mode requires less power consumption and exhibits equal or shorter mixing time than counter-rotation mode. The larger power consumption in counter-rotating mode is caused by the presence of high shear vortices generated between the two coaxial agitators. Furthermore it is shown that the shear distribution throughout the Superblend coaxial mixer operating in co-rotation mode is almost homogenous, which is highly desirable for shear sensitive products. In view of the results obtained in this work, the Superblend coaxial mixer is found as a good alternative for tough mixing applications.     

Intensification of biodiesel synthesis using metal foam reactors

This study presents a technology for continuous and high-efficiency alkali-catalyzed biodiesel synthesis using a metal foam reactor combined with a passive mixer. A metal foam reactor with higher pore density produces smaller droplets that result in higher efficiency of biodiesel synthesis. Compared with conventional stirred reactors, the time for high methyl ester conversion can be shortened remarkably by the use of metal foam reactors. Experimental results reveal that a metal foam reactor of 50 pores per inch exhibits an energy consumption per gram biodiesel of 1.01 J g⁻¹, merely 1.69% and 0.77% of energy consumption of the zigzag micro-channel and conventional stirred reactors, respectively. Moreover, biodiesel yield per reactor for the metal foam reactor is approximately 60 times that of the zigzag micro-channel reactor, thus overcoming the problem of numbering up an excessive number of reactors in the application. These results indicate the great potential of metal foam reactors in small-fuel biodiesel processing plants for distributive applications.     

圆盘反应器的混合特性

以工业聚酯终缩聚圆盘反应器开发应用为研究背景,分别考察了介质粘性、弹性、盘径和盘间距对圆盘反应器流型及宏观混合时间的影响,实验结果表明：高粘,弹性,高剪切和小的盘间距有利于混合;釜径增大,混合效率降低。     

化学反应器宏观混合研究展望

宏观混合是过程工业中的一个重要单元操作,习惯上用混合时间来定量描述,半个世纪以来实验和理论研究的积累十分丰富。然而,目前宏观混合时间的定义和测量方法仍有很强的主观性,结果不能科学地反映化学反应器内宏观混合过程的全局。对宏观混合的几个重要问题进行了分析,指出其缺陷,并建议了今后进一步深入研究的方向和提升本领域研究科学性的措施。     

Thermally safe operation of liquid-liquid semibatch reactors Part II: Single diffusion controlled reactions with arbitrary reaction order

The thermally safe operation of an indirectly cooled semibatch reactor in which an exothermic liquid-liquid reaction occurs corresponds to conditions of potentially very high macrokinetic conversion rates compared with the supply rate of the coreactant, which accumulation in the system remains consequently low. This leads to the definition of a target temperature that can be compared with the real temperature-time profile, in order to develop boundary diagrams which summarize all the possible thermal behaviors of the reactor and can be used for safe scale-up purposes. The variable parameters which appear in such diagrams are an exothermicity and a reactivity number derived from the expressions of the conversion rates in the kinetically or diffusion controlled regime, respectively.In this work the influence of the microkinetic rate of reaction on the shape and location of the boundary diagrams for single liquid-liquid diffusion controlled reaction systems is discussed, extending to this regime the results previously obtained for kinetically controlled reactions.Also in the case of diffusion controlled reactions, it is shown that for many practical systems, using boundary diagrams based on (1,1) reaction orders can lead to both unsafe or not necessary low production operating conditions. Consequently, a number of new boundary diagrams for arbitrary reaction orders is presented and some rules-of-thumb useful to their application are discussed.     

Optimal integration for biodiesel production using bioethanol

The production of biodiesel from algae is optimized using bioethanol following four different transesterification paths: alkali, enzymatic, and heterogeneous catalysts and supercritical conditions. The reactors are modeled using response surface methodology based on experimental results from the literature. These reactor models are implemented together with short‐cut methods for the other equipment (distillation columns, gravity separators, etc.) in order to recover the ethanol, separate the polar and nonpolar phases, and purify the glycerol and biodiesel produced to formulate the problem as a superstructure of alternatives. The aim is to simultaneously optimize and heat integrate the production of biodiesel using ethanol in terms of the reaction technology and the operating conditions. The optimal conditions in the reactors differ from the ones traditionally used because these results take the separation stages into account. In terms of the optimal process, the alkali catalyzed process is the most profitable, while the enzymatic one is also promising due to the lower consumption of energy and water, although it requires significant enzyme cost. © 2012 American Institute of Chemical Engineers AIChE J, 59: 834–844, 2013     

环流反应器参数测量和流型识别的研究进展

环流反应器结构简单,传热传质能力高,功率消耗小,对细胞的损害极微,作为化工、环保和生化反应器,有着广泛的应用前景。本文综述了环流反应器中气含率和循环液速的测量方法,归纳了均匀鼓泡区和非均匀鼓泡区两种流型识别的最新方法,提出了今后研究工作的方向。     

A novel reverse flow reactor coupling endothermic and exothermic reactions. Part I: comparison of reactor configurations for irreversible endothermic reactions

A new reactor concept is studied for highly endothermic heterogeneously catalysed gas phase reactions at high temperatures with rapid but reversible catalyst deactivation. The reactor concept aims to achieve an indirect coupling of energy necessary for endothermic reactions and energy released by exothermic reactions, without mixing of the endothermic and exothermic reactants, in closed-loop reverse flow operation. Periodic gas flow reversal incorporates regenerative heat exchange inside the reactor. The reactor concept is studied for the coupling between the non-oxidative propane dehydrogenation and methane combustion over a monolithic catalyst.Two different reactor configurations are considered: the sequential reactor configuration, where the endothermic and exothermic reactants are fed sequentially to the same catalyst bed acting as an energy repository and the simultaneous reactor configuration, where the endothermic and exothermic reactants are fed continuously to two different compartments directly exchanging energy. The dynamic reactor behaviour is studied by detailed simulation for both reactor configurations. Energy constraints, relating the endothermic and exothermic operating conditions, to achieve a cyclic steady state are discussed. Furthermore, it is indicated how the operating conditions should be matched in order to control the maximum temperature. Also, it is shown that for a single first order exothermic reaction the maximum dimensionless temperature in reverse flow reactors depends on a single dimensionless number. Finally, both reactor configurations are compared based on their operating conditions. It is shown that only in the sequential reactor configuration the endothermic inlet concentration can be optimised independently of the gas velocities at high throughput and maximum reaction coupling energy efficiency, by the choice of a proper switching scheme with inherently zero differential creep velocity and using the ratio of the cycle times.In this first part, both the propane dehydrogenation and the methane combustion have been considered as first order irreversible reactions. However, the propane dehydrogenation is an equilibrium reaction and the low exit temperatures resulting from the reverse flow concept entail considerable propane conversion losses. How this ‘back-conversion’ can be counteracted is discussed in part II Chemical Engineering Science, 57, (2002), 855-872.     

Enriched Air or Pure Oxygen as Oxidant for Gas‐to‐Liquid Process with Microchannel Reactors

The syngas production step is one of the most costly steps in a gas‐to‐liquid plant. Commonly, oxygen is used as an oxidant in the reforming step. However, through the introduction of microchannel reactors, the use of enriched air may be justified. The merits of using enriched air versus pure oxygen are analyzed by utilizing an autothermal reformer, with microchannel reactors in the once‐through Fischer‐Tropsch (FT) step. Pure oxygen is provided by a cryogenic air separation unit (ASU) and enriched air by use of air separation membranes. Pure oxygen requires a smaller FT reactor volume, which means lower reactor costs at the expense of having a costly cryogenic ASU to produce pure oxygen. The operating cost of the ASU is lower than that of the air membrane, but the installed cost is higher.     

非热等离子体烃类燃料氧化重整反应器的研究进展

燃料氧化重整(部分氧化)为温和的放热反应,其反应速率快、能耗低,特别适用于在线制取氢气或富氢气体。大气压非热等离子体为燃料氧化重整提供了一种应用前景广泛的新技术,展现了对燃料具有普适性、快速响应和反应器紧凑高效等优点。综述了大气压非热等离子体烃类燃料氧化重整反应器的研究进展,着重阐述了火花和滑动弧放电产生的暖等离子体及其烃类燃料重整反应器。与电晕和介质阻挡放电产生的冷等离子体反应器相比,暖等离子体反应器具有燃料转化率高和能耗低的优点。     

A highway in state space for reactors with minimum entropy production

Thousands of numerical solutions of an optimal control problem for plug flow reactors were found to give, what we call a “highway in the reactors’ state space”. The problem was to find the heat transfer strategy which minimise the entropy production in reactors with fixed chemical conversion. The control variable was always the temperature of the heating/cooling medium along the reactor. The highway represents the most energy efficient way to travel far in state space. Such highways were studied for five reactor systems, endothermic and exothermic ones. Numerical analysis showed that the reactor highway is characterised by approximately constant thermodynamic driving forces/local entropy production for reasonable process intensities. Each solution represents a compromise between the entropy production of reactions, heat transfer and frictional flow (pressure drop). The solutions enter and leave the highway at different positions depending on how far from the highway their initial and final destinations are. Knowledge about the nature of the highway, e.g. when the reactor operates in a reaction mode or a heat transfer mode, may be important for energy efficient reactor design. The theoretical formulation of the optimisation problem is valid for plug flow as well as batch reactors. We showed that important results in literature like the Spirkl-Ries quantity, the theorems of equipartition of entropy production and equipartition of forces are contained in our general formulation. The numerical results showed that the analytical results are good approximations to the optimum also in problems where they do not apply in a strictly mathematical sense.