Numerical study on microstructured reactor with chaotic heat and mass transfer and its potential application for exothermic process

Design of microstructured reactors with thermal control function is investigated through numerical simulation. It consists of one middle channel for handling chemicals and two other channels attached to its top and bottom for cooling purpose. Three designs are examined. Reactor A uses simple straight channels. In reactor B, chaotic flow is applied to the middle channel, and in reactor C chaotic flow is applied to all the three channels. Results show that in comparison with the straight channel, the Nusselt number in current design is greatly improved through chaotic flow. Rapid mixing is also achieved. Potential application of the design for continuous exothermic process is analyzed. For reactor A, it is not workable as the temperature of the chemical solution continuously increases over the channel. In comparison, for both reactors B and C the temperature can be well controlled within the required range. As the coolant flow in reactor C is also chaotic, it provides a higher heat removal capacity.     

微结构反应器气固相催化过程强化的研究与工业化进展

轻烃的气固相催化转化与合成是重要的基础能源化工过程,其苛刻的反应条件与显著的热效应严重影响了反应器生产效率、过程能耗与排放。微结构催化反应器传递性能优越,可兼顾紧凑性与低压降,能在高反应通量下精确调控轻烃气固相催化转化与合成过程,适应日益增长的分布式生产需求。介绍了微结构催化反应器的制造并重点讨论了催化剂-反应器集成方式,以强吸热的甲烷蒸汽重整过程和强放热的甲烷化、乙烷氧化脱氢过程为例,综述了微结构反应器气固催化过程强化的研究与工业化进展,展望了新技术的未来发展方向。     

Single-phase fluid flow distribution and heat transfer in microstructured reactors

Single-phase microreactors and micro-heat-exchangers have been widely used in industrial and scientific applications over the last decade. In several cases, operation of microreactors has shown that their expected efficiency cannot be reached either due to non-uniform distribution of reactants between different channels or due to flow maldistribution between individual microreactors working in parallel. The latter problem can result in substantial temperature deviations between different microreactors resulting in thermal runaway which could arise from an exothermic reaction. Thus advances in the understanding of heat transfer and fluid flow distribution continue to be crucial in achieving improved performance, efficiency and safety in microstructured reactors used for different applications. This paper presents a review of the experimental and numerical results on fluid flow distribution, heat transfer and combination thereof, available in the open literature. Heat transfer in microchannels can be suitably described by standard theory and correlations, but scaling effects (entrance effects, conjugate heat transfer, viscous heating, and temperature-dependent properties) have often to be accounted for in microsystems. Experiments with single channels are in good agreement with predictions from the published correlations. The accuracy of multichannel experiments is lower due to flow maldistribution. Special attention is devoted to theoretical and experimental studies on the effect of a flow maldistribution on the thermal and conversion response of catalytic microreactors. The review concludes with a set of design recommendations aimed at improving the reactor performance.     

Unsteady state treatment of very lean waste gases in a network of catalytic burners

Catalytic reactors in forced non-stationary operation enable autothermal VOC oxidation even at extremely low adiabatic temperature rise. A network of three reactors in series with large inert sections is presented as an alternative to the reverse-flow reactor.

A one-dimensional model for simulation of VOC oxidation in forced non-steady state packed bed catalytic reactors has been developed and implemented into a program that uses standard mathematical software. Numerical simulation reveals that the network of three catalytic reactors in series with large inert sections is a suitable design for VOC oxidation at low concentration. The system can be controlled by a simple set of two switches acting according to temperature setpoints.

Maximum temperature is very sensitive to heat transfer, which cannot be considered as infinitely fast despite small temperature gradients between gas and solid. These dependencies are somewhat less pronounced at higher load.     

Design of a microstructured reactor with integrated heat-exchanger for optimum performance of a highly exothermic reaction

The activity and the heat transfer characteristics of several microstructured reactors have been compared in the ammonia oxidation on Pt catalyst. The main parameters which influence reactor performance are catalyst loading, temperature, and the intrinsic conductivity of the reactor material. In case of aluminum as a reactor material, hot spot temperatures were within 5°C at full conversion of 6 vol.% NH₃. Temperature gradients were considerably larger when the microreactor was made from pure platinum due to the smaller intrinsic material conductivity. As a result, the maximum N₂O selectivity was by 20% lower than in the case of the aluminum-based reactor due to considerable differences in the selectivities between the central and wall channels. Experimental data obtained on the above microreactors were used to design an externally cooled cross flow microreactor/heat-exchanger operating at almost isothermal conditions even with a reaction mixture corresponding to an adiabatic temperature rise of about 1400°C. Such system can provide new opportunities for improvement of existing gas/solid catalytic processes with strongly exothermic reactions.     

Microstructured reactors and supports for ionic liquids

Different types of microstructures and their applications with respect to the synthesis and the use of ionic liquids are presented. Microstructured reactors are suitable for reactions with fast intrinsic kinetics, requiring high mass and heat transfer performances. Chemical synthesis can be performed safely under operating condition (e.g. high temperature, pressure, etc.) difficult to obtain in traditional reactors. The examples presented clearly indicate that microstructured reactors offer superior performance for the synthesis of ionic liquids in comparison to conventional equipment. For the use of ionic liquids as reaction media, existing ionic liquids show some limitations due to their higher viscosity compared to conventional solvents. Therefore, future research should be focused on the development of low viscosity ionic liquids.The approaches to use ionic liquids in microstructured reactors and in combination with microstructured supports for catalytic reactions show many advantages in view of high product selectivity and yield. The use of supported ionic liquids on microstructured materials seems to be particularly promising for gas phase as well as for gas/liquid reactions.     

Characterization of a Modular,Scalable Millistructured Plate Reactor

Micro‐ and millistructured reactors offer significant advantages compared to conventional reactors, e.g., in terms of heat and mass transfer as well as safety. For the substitution of larger batch reactors by continuously operated millireactors a comprehensive reactor characterization is required. The heat transfer and hydrodynamics of the millistructured plate reactor PR37 are studied. The meandering of the process channel and its periodically changing cross‐sectional area increase the heat transfer significantly and lead to a dependency of the heat transfer coefficient on the Reynolds and Prandtl number that is not found in straight channels at Reynolds numbers below 2300.     

Heat Exchangeability of a Catalytic Plate Reactor and Analysis of the Reactivity of Steam and CO2 in Methane Reforming

An assemble‐type plate reactor was developed and its intensified heat transfer compared to that of a conventional tubular reactor in methane reforming was confirmed. This characteristic enables accurate reaction kinetic analysis because of quasi‐isothermal operation with mild pressure loss. Reduced experiment cost is one of the features of the assemble‐type reactor. Simple thermal design equations applicable to plate reactors were also assessed. From experiments and accurate reaction analysis using the plate reactor it is suggested that H₂O and CO₂ have similar reactivity for a commercial Ni/α‐Al₂O₃ catalyst. The partial pressure of the oxidizing agent had much more influence on the reactivity of methane reforming than the species of this agent.     

Hydrogen production from steam methane reforming coupled with in situ CO2 capture: Conceptual parametric study

The work deals with fundamental analysis of the sorption-enhanced steam methane reforming (SE-SMR) process in which the simultaneous removal of carbon dioxide by hydrotalcite-based chemisorbent is coupled. A two-section reactor model is developed to describe the SE-SMR reactor, decoupling the complexity in process analysis. The model defines two subsequent sections in the reactor: an equilibrium conversion section (upstream) and an adsorption reforming section (downstream). The material balance relationship in the equilibrium conversion section is directly determined by thermodynamic equilibrium calculation, providing an equilibrated atmosphere to the next section. The adsorption reforming section is described using an isothermal multi-component dynamic model into which the SMR reactions and the high-temperature CO₂ adsorption are embedded. The multiple requirements (including H₂ purity, H₂ productivity, CH₄ conversion enhancement, and carbon oxides concentrations) are taken into account simultaneously so as to analyze and define feasible operation window for producing high-purity hydrogen with ppm-level CO impurity. The performances of the reactors with different dimensions (laboratory-scale and pilot-scale) are explored, highlighting the importance of operation parameter control to the process feasibility.     

Scale-up concept for modular microstructured reactors based on mixing, heat transfer, and reactor safety

Microstructured reactors are characterized by rapid mixing processes and excellent temperature control of chemical reactions. These properties allow the safe operation of hazardous chemistry in intensified processes. Problems occur during scale-up of these processes, where heat transfer becomes the limiting effect. With high flow rates and transitional or even turbulent flow regimes in small channels, rapid mixing and excellent heat transfer can be maintained up to high production rates. For exothermic reactions, limits for parametric sensitivity and safe operation are shown from literature and combined with convective heat transfer for consistent scale-up. Good knowledge of reaction kinetics, thermodynamics and heat transfer is essential to determine runaway regions for exothermic reactions. From these correlations, consistent channel design and continuous-flow reactor setup is shown.     

径向移动床反应器流场特性及其数学模拟

本文票据主流道变质量流及颗粒床层气固体力学理论,建立了完整的径向移动床反应器流体力学数学模型,开发了模拟计算床层气相二维流场的一种新的数学方法及相应的计算程序。黛此可以模拟计算床层气相压力和轴径向速度的二维分布、内外主流道压力和流速分布以及布气孔道的过孔气速和压降。根据模拟计算结果,提出首先优化设计两主流道截面积分配,然后采用变开孔率设计消除开孔区端效应的两步设计方法。借此实现径向移动床反应器的优     

Small/medium nuclear reactors for potential desalination applications : Mini review

Small/medium nuclear reactors (SMRs) are a promising alternative for powering large scale desalination plants. The modern generations of these systems manifest cost effectiveness and built-in safety features. The compatibility with geological and topological challenges is an added advantage. Moreover, funding opportunities and packages could be easily arranged for small/medium nuclear reactors (SMR). This mini review article provides the latest technical features of SMR nuclear plants with emphasis on pressurized light water reactors (PWR), boiling water reactors (BWR), heavy water reactors (HWR), gas cooled reactors (GCR), and liquid metal fast breeder reactors (LMFBR). Preliminary cost indicators for typical units were investigated as a part of joint effort to develop a cost database for these types of reactors. Security and safety features of small/medium reactors are identified and reviewed. This paper identifies and briefly discusses the various types of small/medium nuclear reactors to provide a preliminary evaluation and consideration of using this type of reactor in potential seawater desalination applications.     

Organische Synthese mit mikrostrukturierten Reaktoren

Organic Synthesis with Microstructured Reactors This article describes the chances microstructured reactors offer for chemical plant engineering. This suitability for chemical production is commonly regarded to be the key to the market penetration. Seen in the long term, there is potential that new plants can be equipped with microstructured reactors. Only economic balances, however, which draw up profitability, will open the door to the usage of chemical micro process engineering for plant construction. Main arguments for using microstructured reactors are thus enhanced conversion and selectivity, increased space‐time yields, waste reduction and more safety via small reactor volumes. Credit‐card sized reaction systems allow one to perform the screening of multi‐phase reactions. More prominent, similar screening is carried out for single‐step reactions. Moreover, safe processing with microstructured reactors in the explosive regime enlarges the traditional range of processing. The reaction guidance by microstructured reactors can further influence subsequent processing steps such as product purification and, in this way, can lower the energy costs of processes.     

化学链反应器研究进展

化学链技术是目前能源技术研究的热点之一,其关键技术包括载体材料的制备和反应器的设计。综述了化学链技术的应用前景,总结了化学链反应器设计原理,回顾了目前世界上公开报道的设计完成、在建或已经运行的化学链反应器,归纳讨论了不同反应器设计细节的共同点及目的。开展以微小颗粒、纳米颗粒作为载体材料时,颗粒聚团在宏观反应器尺度下的流动传递规律、循环反应机理和系统运行控制特性的研究;开展反应器内颗粒流动-传递-反应耦合机制研究,建立多尺度统一模型;在全尺寸化学链反应器上进行系统自热实验研究;利用数值模拟方法研究和开发用于固体燃料转化过程的高效炭/灰分离器是未来化学链反应器发展需要关注的几个方面。     

Hollow fiber enzymic reactors for a two substrate process: analytical modeling and numerical simulations

The use of immobilized enzyme reactors in biotechnology and biomedicine is rapidly expanding. This study concentrates on hollow-fiber (HF) enzymic reactors for continuous, single-pass operation. The enzyme, in a soluble form, is physically confined within the shell section of the reactor and the substrate solution flows through the lumen section of it. We consider here a two-substrate reaction proceeding via the Ping-Pong mechanism, with substrates and reaction products diffusing through the fiber wall. The developed analytical model enables to calculate the expected conversion as a function of the volumetric flow rate, kinetic constants, diffusion coefficients, geometric dimensions of the reactor, the flow regimen in the apparatus and substrates concentrations. The model equations are solved by a numerical procedure and the system performance is simulated. Depending on the operation conditions employed, the reactor is controlled by kinetic processes, diffusion processes, or both.     

Heat transfer and temperature profiles in flow-through catalytic membrane reactors

In flow-through membrane reactors, a porous membrane is used as a microstructured catalyst support, which provides for an intensive contact between reactants and catalyst. When performing exothermal gas phase reactions, large temperature differences between feed and permeate side are observed. This work systematically derives an axial temperature profile inside the inaccessible membrane pores by combining a one-dimensional reactor model of mass and energy balances with experimental measurements of reactor temperatures and conversion, applying ethene hydrogenation as a model reaction. It is shown, that the anodized membrane reactor can be regarded as isothermal under any operating conditions and the heat transfer mechanisms inside the membrane prove to be irrelevant for the resulting membrane temperature. By applying the derived heat transfer model to the performed ethene hydrogenation experiments, the reactor temperature can be predicted satisfactorily in the whole range of performed experiments.     

Effects of process and design parameters on heat management in fixed bed Fischer-Tropsch synthesis reactor

A two-dimensional pseudo-homogeneous model of wall-cooled fixed bed Fischer-Tropsch synthesis (FTS) reactor with Co/Re/γ-Al₂O₃ catalyst was developed to study the effect of process and design parameters on heat generation and removal characteristics. The influence of liquid-phase formation on heat transport was accounted for by using two-phase correlations. The effect of intraparticle diffusion on heat generation was considered. Detailed numerical simulations were performed to analyze the effect of process and design parameters on the reactor performance in terms of heat management. Results show that thermal behavior of FTS fixed bed reactors is very sensitive and any large disturbances can lead to temperature runaway. Large tube diameters are shown to be particularly unfavorable, with d _t >5 cm resulting in axial and radial gradients greater than 20 K and 13 K, respectively. The importance of detailed reactor modeling when designing and optimizing FTS fixed bed reactors is highlighted.     

Model‐Based Optimization of a Photocatalytic Reactor with Light‐Emitting Diodes

The complicated interplay between mass and photon transfer within a photocatalytic reactor calls for an integrated design approach. A model‐based optimization approach for LED‐based photocatalytic reactors is presented. First, a model that describes the distribution of reactants and photons within a photocatalytic reactor is developed. Then, several design variables related to the reactor dimensions and light sources are optimized simultaneously using the photocatalytic degradation of toluene as a model system. The results demonstrate how different formulations of the problem can be used to either minimize the reactor cost or to obtain a specified concentration profile within the reactor.     

TiO2平板微反应器设计优化及光催化性能研究

光流体学可以将光催化反应与微流控技术结合,大幅提高光利用率和反应速率,实现对光催化水处理的高效强化,其中微反应器结构的设计与优化是研究重点之一。首先利用流体力学模拟分析优化通道级数,设计了5级树状通道平板反应器。进而,通过调控通道高度,研究不同高度的微通道对其光催化性能的影响。研究发现50 μm微反应器的降解性能及连续操作性能均优于100 μm。同时以亚甲基蓝为模拟废水对微反应器及釜式反应器的反应动力学、微反应器污染问题、连续操作性能等进行研究。研究表明,微反应器在不同流速下对亚甲基蓝均能实现连续高效降解,降解率远高于釜式反应器。当流速为55 μl/min时对5×10^-5 mol/L亚甲基蓝可达到95%的降解率并保持较好的连续操作性能及重复使用性能。     

Design and optimisation of batch and semi-batch reactors

This paper addresses a systematic methodology for batch and semi-batch reactor design and optimisation for both ideal and non-ideal mixing. It can be applied to non-isothermal and multiphase systems. The method starts from a general representation in the form of a temporal superstructure based on the similarity of between plug flow reactors and ideal batch reactors. The temporal superstructure of a batch reactor exists in both the space and time dimensions. For non-ideal mixing, this paper addresses a mixing compartment network model to represent mixing inside reactors. The mixing compartment network is then included into the temporal superstructure to model non-ideally mixed batch reactors and the mixing pattern optimised with the other variables. Besides the operation variables for batch reactors, this method can also suggest the optimum mixing pattern and promising reactor configurations for mechanical design. A profile-based approach is proposed to make a search of the profiles for temperature, pressure and feed addition. This approach starts from a set of initial profiles of temperature, pressure and feed addition. Then the performance of the batch reactor is evaluated against the objective function under different profiles. An optimal set of profiles is then found by this profile searching process. A stochastic optimisation technique based on simulated annealing is employed to obtain optimal solutions. This method is also extended to multiphase reaction systems based on the concept of shadow reactor compartments. A number of case studies are presented to illustrate the use of the proposed methodology.