Process intensification of particle separation by lift force in arc microchannel with bifurcation

The present study demonstrates a novel hydrodynamic lift force sieving attained in an arc microchannel with a bifurcation at the downstream end. The fluorescent polystyrene particles with diameter of 10 or 20 μm are dispersed in water or NaCl aqueous solutions so that the particles are completely neutrally buoyant, lighter or denser relative to medium. The slurry is fed into the arc microchannel whose radius, width and depth are 20 mm, 200 and 150 μm, respectively. The fluorescent trajectories of flowing particles are recorded at the bifurcation. It is found that the 20-μm particle is sharply focused to an equilibrium position somewhat distant from the outer wall regardless of the given density difference. As a result, all 20-μm particles report to the outer branch of bifurcation. On the other hand, the 10-μm particles dispersed mostly across the channel width are always recovered from the both branches. The results imply that the arc microchannel with a bifurcation will intensify the process of particle separation since the particles completely neutrally buoyant as well as denser and lighter particles can be simultaneously separated or classified without membrane. Finally, a separation process in a series of arc channels is proposed and the process efficiencies are discussed.     

Hydrodynamics and mass transfer characteristics in gas-liquid flow through a rectangular microchannel

Researches on two-phase transfer and reaction processes in microchannnels are important to the design of multiphase microchemical systems. In the present work, hydrodynamics and mass transfer characteristics in cocurrent gas-liquid flow through a horizontal rectangular microchannel with a hydraulic diameter of have been investigated experimentally. Liquid side volumetric mass transfer coefficients were measured by absorbing pure CO₂ into water and a 0.3 M NaHCO₃ / 0.3 M Na₂CO₃ buffer solution. Interfacial areas were determined by absorbing pure CO₂ into a 1 M NaOH solution. Two-phase flow patterns and pressure drop data were also obtained and analyzed. This paper shows that two-phase frictional pressure drop in the microchannel can be well predicted by the Lockhart-Martinelli method if we use a new correlation of C value in the Chisholm's equation. Liquid side volumetric mass transfer coefficient and interfacial area as high as about and , respectively, can be achieved in the microchannel. Generally, liquid side volumetric mass transfer coefficient increases with the increasing superficial liquid or gas velocity, which can be described satisfactorily by the developed empirical correlations. A comparison of mass transfer performance among different gas-liquid contactors reveals that the gas-liquid microchannel contactor of this study can provide at least one or two orders of magnitude higher liquid side volumetric mass transfer coefficients and interfacial areas than the others.     

Robust temperature control for batch chemical reactors

Since batch chemical reactors exhibit an integrating response, temperature control for these systems can be a real problem for conventional PID controllers. Tuning can be extremely difficult due to the reduced stability margins proved for this type of processes. In this work, a simple robust control strategy for temperature regulation in batch and semi-batch chemical reactors is proposed. The feedback controller is composed by an approximate I/O linearizing feedback equipped with a calorimetric balance estimator. Based on standard results from singular perturbations, it is proven that the proposed feedback controller (i) can track a bounded temperature trajectory as close as desired (i.e., practical stability) by adjusting a single estimation parameter, and (ii) after a short transient, the performance of the exact I/O linearizing feedback can be recovered as the calorimetric balance estimation rate is increased.     

Numerical study on the hydrodynamics behavior of a central insert microchannel

In this work, the computational fluid dynamics method is used to study the liquid hydrodynamics behavior in the microchannel without central insert (MC1) and the central insert microchannel (MC2), respectively. The maximum deviation between simulation and experiment is 24%. The formations of flow patterns are explained based on contours and force analysis where the flow pattern maps are established by two-phase flow rate. The effects of aqueous phase viscosity and two-phase flow rate on the characteristic sizes of each flow pattern are also explored. Specifically, four unconventional flow patterns are found in MC2, namely the unique droplet flow, the unique slug flow, the unique coarse annular flow and the unique film annular flow. Though the insert occupies part of the channel, the pressure difference in the channel is significantly reduced compared with MC1. Moreover, the insert significantly changes the formation velocity range of each flow pattern, greatly broadens the formation range of annular flow and also has an important influence on the characteristic size of the flow pattern. The organic-phase dimensionless axial size (L_o/W) and the dimensionless radial size (D_o/W) of the droplet (slug) are negatively related to the aqueous-phase viscosity (μ_a) and flow rate (u_a). The D_o/W of the annular is negatively correlated with μ_a and positively correlated with organic-phase flow rate (u_o). This study provides direct numerical evidence that the insert is key to the formation of bicontinuous phase flow pattern, as well as further strengthens our understanding of the flow characteristics and optimization design of insert microchannels.     

微通道连续流反应器用于传统搅拌釜的工艺改造

采用康宁公司的G1微通道连续流反应器对传统的HATP搅拌釜工艺进行了改造和优化,考察了溶剂使用、反应温度、反应物摩尔当量、流速等对反应的影响,进一步优化了工艺条件。改进后的工艺产品质量分数可达98%,目标产物收率近100%,可实现年产30 t HATP,不再使用任何有机溶剂,实现了整个工艺过程的零排放连续操作。     

Optimal temperature profiles for tubular reactors implemented through a flow reversal strategy

Optimal steady-state reactor temperature profiles in classic jacketed tubular reactors often exhibit a (nearly) trapezoidal shape along the reactor, i.e., in a first part the temperature is raised, then a constant temperature interval follows, and (possibly) the temperature is lowered towards the outlet. The practical realisation of these optimal reactor temperature profiles is, however, complex because of the spatially varying jacket fluid temperature profile that is required for the constant reactor temperature part. Since similar trapezoidal temperature profiles are encountered in reverse flow configurations, this reactor type may be an alternative to implement the optimal profiles in practice. This article conceptually proves the feasibility of this concept and provides an optimisation procedure for its design and operation.     

An investigation of catalytic plate reactors by means of parametric sensitivity analysis

A catalytic plate reactor consisting of closely spaced catalytically coated plates, where endothermic and exothermic reactions take place in alternate channels is studied. The influence of several design parameters on its thermal behaviour and performance is investigated by parametric sensitivity analysis using a two-dimensional reactor model. Parameters considered are: channel size; wall thickness and thermal conductivity; inlet temperature, composition and velocities; and kinetic parameters. It is demonstrated that different catalysts can show similar thermal behaviour and performance but exhibit different sensitivity behaviour. For the system investigated the strongest influence on reactor sensitivity comes from the activation energies of both exothermic and endothermic reactions.     

CHALLENGES AND INNOVATIONS IN REACTION ENGINEERING

The importance of reaction engineering in generating a myriad of products on which developed societies depend is outlined. The challenges of a political, economic, and technical nature that need to be addressed in rendering conversion of raw materials into desired products that are more environmentally friendly and sustainable are briefly discussed. It is shown that multiphase reactors are prevalent in all applications, and improvements in the reactor material and energy efficiencies lead to more environmentally benign processes. This requires, in addition to the selection of green process chemistry, systematic implementation of the multi-scale reaction engineering methodology to accomplish proper reactor type selection and scaleup for commercial applications. It is also illustrated that recent innovations in multiphase reaction engineering basically utilize two key concepts: process intensification (e.g., enhancement in mass and heat transfer rates) and simultaneous reaction and separation. Examples of these are discussed, such as micro-reactors, reactive distillation, etc. It is also shown that commercialization of bench-scale discoveries requires either scaleup in parallel or vertical scaleup. New tools for visualization of opaque multiphase flows and development of appropriate rational phenomenological multiphase reactor models for scaleup and design are also briefly discussed.     

Measurement and control of polymerization reactors

The measurement and control of polymerization reactors is very challenging due to the complexity of the physical mechanisms and polymerization kinetics. In these reactors many important variables, which are related to end-use polymer properties, cannot be measured on-line or can only be measured at low sampling frequencies. Furthermore, end-use polymer properties are related to the entire molecular weight, copolymer composition, sequence length, and branching distributions. This paper surveys the instrumentation technologies, which are of particular interest in polymerization reactors with emphasis on, for example, measurement of viscosity, composition, molecular weight, and particle size. This paper presents a hierarchical approach to the control system design and reviews traditional regulatory techniques as well as advanced control strategies for batch, semibatch, and continuous reactors. These approaches are illustrated by focusing on the control of a commercial multiproduct continuous emulsion polymerization reactor. Finally, the paper captures some of the trends in the polymer industry, which may impact future development in measurement and reactor control.     

Zeolite based films, membranes and membrane reactors: Progress and prospects

The integration of reaction and separation in catalytic membrane reactors has received increasing attention during the past 30 years. The combination promises to deliver more compact and less capital-intensive processes with substantial savings in energy consumption. With the advent of new inorganic materials and processing techniques, there has been renewed interest in exploiting the benefits of membranes in many industrial applications. Zeolite membranes, however, have only recently been considered for catalytic membrane reactor applications. Despite the significant recent interest in these types of membranes there are relatively few reports of the application of such membranes in high-temperature catalytic membrane reactor applications. This can be attributed to a number of limitations that still need to be addressed such as the relatively high price of membrane units, the difficulty of controlling the membrane thickness, permeance, high-temperature sealing, reproducibility and the dilemma of upscaling. A number of research efforts, with some degree of success have been directed to finding solutions to the remaining challenges. This review makes a critical assessment of what has been achieved in the past few years in terms of hurdles that still stand in the way of the successful implementation of zeolite membrane reactors in industry.     

Modelling of packed bed membrane reactors for autothermal production of ultrapure hydrogen

The conceptual feasibility of a packed bed membrane reactor for the autothermal reforming (ATR) of methane for the production of ultrapure hydrogen was investigated. By integrating H₂ permselective Pd-based membranes under autothermal conditions, a high degree of process integration and intensification can be accomplished which is particularly interesting for small scale H₂ production units. A two-dimensional pseudo-homogeneous packed bed membrane reactor model was developed that solves the continuity and momentum equations and the component mass and energy balances. In adiabatic operation, autothermal operation can be achieved; however, large axial temperature excursions were seen at the reactor inlet, which are disadvantageous for membrane life and catalyst performance. Different operation modes, such as cooling the reactor wall with sweep gas or distributive feeding of O₂ along the reactor length to moderate the temperature profile, are evaluated. The concentration polarisation because of the selective hydrogen removal along the membrane length was found to become significant with increasing membrane permeability thereby constraining the reactor design. To decrease the negative effects of mass transfer limitations to the membrane wall, a small membrane tube diameter needs to be selected. For a relatively small ratio of the membrane tube diameter to the particle diameter, the porosity profile needs to be taken into account to prevent overestimation of the H₂ removal rate. It is concluded that autothermal production of H₂ in a PBMR is feasible, provided that the membranes are positioned outside the inlet region with large temperature gradients.     

Development of a microchannel catalytic reactor system

The purpose of this article is to demonstrate the applicability of microreactors for use in catalytic reactions at elevated temperatures. Microchannels were fabricated on both sides of a silicon wafer by wet chemical etching after pattern transfer using a negative photoresist. The walls of the reactor channel were coated with a platinum layer, for use as a sample catalyst, by sputtering. A heating element was installed in the channel on the opposite surface of the reactor channel. The reactor channel was sealed gas-tight with a glass plate by using an anodic bonding technique. A small-scale palladium membrane was also prepared on the surface of a 50-Μm thick copper film. In the membrane preparation, a negative photoresist was spin-coated and solidified to serve as a protective film. A palladium layer was then electrodeposited on the other uncovered surface. After the protective film was removed, the resist was again spin-coated on the copper surface, and a pattern of microslits was transferred by photolithography. After development, the microslits were electrolitically etched away, resulting in the formation of a palladium membrane as an assemblage of thin layers formed in the microslits. The integration of the microreactor and the membrane is currently under way.     

Intensification of cavitational activity in sonochemical reactors using different additives: Efficacy assessment using a model reaction

Sonochemical reactors offer excellent promise for the intensification of different chemical processing applications. The current work deals with intensification of cavitational activity using different additives with an objective of decreasing the processing cost as well as enhancing the applicability of sonochemical reactors for different applications. Potassium iodide oxidation has been used as a model reaction. Experiments have been carried out in a laboratory scale ultrasonic horn reactor. The effects of different additives such as air, solid particles (cupric oxide and titanium dioxide), salts (sodium chloride and sodium nitrite) and radical promoters (hydrogen peroxide, ferrous sulphate, iron metal, carbon tetrachloride and t-butanol) on the degradation of potassium iodide have been investigated. Combination of additives has also been investigated for examining the possible synergistic effects in comparison to the use of individual additions. It has been observed that based on the type of additive, optimum concentration needs to be selected and it may not be desirable always to use different additives in combination. It is desirable to select an additive which can give additional reaction mechanism in the system to aid the desired application under question.     

Catalytic combustion assisted methane steam reforming in a catalytic plate reactor

A theoretical study of methane steam reforming coupled with methane catalytic combustion in a catalytic plate reactor (CPR) based on a two-dimensional model is presented. Plates with coated catalyst layers of order of micrometers at distances of order of millimetres offer a high degree of compactness and minimise heat and mass transport resistances. Choosing similar operating conditions in terms of inlet composition and temperature as in industrial reformer allows a direct comparison of CPRs with the latter. It is shown that short distance between heat source and heat sink increases the efficiency of heat exchange. Transverse temperature gradients do not exceed across the wall and across the gas-phase, in contrast to difference in temperature of outside wall and mean gas phase temperature inside the tube usually observed in conventional reformers. The effectiveness factors for the reforming chemical reactions are about one order of magnitude higher than in conventional processes. Minimisation of heat and mass transfer resistances results in reduction of reactor volume and catalyst weight by two orders of magnitude as compared to industrial reformer. Alteration of distance between plates in the range 1- does not result in significant difference in reactor performance, if made at constant inlet flowrates. However, if such modifications are made at constant inlet velocities, conversion and temperature profiles are considerably affected. Similar effects are observed when catalyst layer thicknesses are increased.     

微通道反应器中精细化学品合成危险工艺研究进展

卤化、氧化、重氮化、硝化以及催化加氢是精细化工生产中的重要反应,通常以间歇方式在釜式反应器中进行,存在安全隐患,并且反应效率低。微通道反应器技术的发展为解决上述问题提供了有效途径,因此,发展基于微通道反应器的安全高效合成工艺成为当前精细化工领域的研究热点之一。该文综述了近年来微通道反应器中涉及精细化工产品合成危险工艺的研究进展,并指出了微通道反应器存在的不足和今后研究的方向。     

Control of periodically operated reactors

Control of periodically operated reactors has in common with control of reactors operating at steady state the objective of minimizing the effect of disturbances on an objective function such as the cost of a product or the deviation of an outlet concentration of a pollutant from a statuary target. Simple feedback control employing feedback PID regulators, however, is not “adequate for most disturbances because of problems with tracking a time-varying output and the necessarily non-linear character of the reactors with respect to controlled as well as uncontrolled inputs. This contribution is a review of the literature and a discussion of research needs. The literature on the control of periodically operated reactors is not voluminous. Nevertheless this literature clearly indicates that model based predictive controllers can be used for this type of reactor”. Further research on the limitations, maintenance and implementation costs of model based controllers in this application would be worthwhile. Experimental studies are wholly absent. Unique regulators for other periodic operations, such as adaptive control or forcing the output toward a reference trajectory using an open loop model based control strategy, certainly warrant study of their application to periodically operated reactors. Additionally, proper design of the reactor may lead to configurations that are simpler to control and that may not require complex control strategies for efficient operation.     

Combustion of methane lean mixtures in reverse flow reactors: Comparison between packed and structured catalyst beds

The scope of this work is to compare systematically the performance of particle beds and monolithic beds in catalytic reverse flow reactors used for combustion of lean methane/air mixtures, using alumina-supported palladium as catalyst. Different values of gas surface velocity (0.1–0.3 m/s), particle diameter (3–6 mm, for particle bed), cell density (200–400 cpsi, for structured bed) and catalyst/inert ratio (0.4–1) were used for the simulation of the combustion of 3500 ppm methane in both kinds of reverse flow reactor. An unsteady one-dimensional heterogeneous model has been developed and solved using a MATLAB code. The model, physical parameters and transport properties used had been experimentally validated in a previous work, operating with a particle bed reverse flow reactor. Results obtained indicate that the reverse flow reactor is more stable when the catalyst particle beds are use, although the difference with the monolith bed decreases as surface velocity increases. In contrast, pressure drops in the bed are higher for the particle bed.     

Averaging theory and low-dimensional models for chemical reactors and reacting flows

A novel approach based on the Liapunov-Schmidt technique of bifurcation theory is presented for the spatial averaging of a class of convection-diffusion-reaction models. It is used to derive low-dimensional averaged models for different types of homogeneous and catalytic reactors, as well as coupled homogeneous-heterogeneous systems. For the homogeneous isothermal case, the averaged models consist of a pair of balance equations for each species A_j in terms of the mixing-cup (C_j,m) and spatially averaged (〈C_j〉) concentrations. The first (global) equation traces the evolution of C_j,m with residence time while the second (local) equation, which is independent of the reactor type, gives the local concentration gradient as a difference between C_j,m and 〈C_j〉 in terms of the local variables (such as species diffusivities, shear and reaction rates). For the wall-catalyzed reaction case, the averaged models are described by a pair of equations for each species in terms of C_j,m and the surface concentration C_j,s and are similar to the classical two-phase models of catalytic reactors. For the coupled homogeneous-heterogeneous case, the averaged models consist of three balance equations for each species in terms of C_j,m, 〈C_j〉 and C_j,s, and contain four mass transfer or exchange coefficients. The accuracy, convergence and the region of validity of the averaged models are examined for some special cases. Finally, the usefulness of the averaged models in predicting the reactor behavior is illustrated with an example for each of the three cases, homogeneous, heterogeneous and coupled homogeneous-heterogeneous case.     

Hydrogen production from propane in Rh-impregnated metallic microchannel reactors and alumina foams

Rh-impregnated alumina foams and metallic microchannel reactors have been studied for production of hydrogen-rich syngas through short contact time catalytic partial oxidation (POX) and oxidative steam reforming (OSR) of propane. Effects of temperature and residence time have been compared for the two catalytic systems. Temperature profiles obtained along the central axis were valuable in understanding the different behaviour of the reactor systems. Gas phase ignition occurs in front of the metallic monolith at furnace temperatures above 700 °C, leading to lower hydrogen selectivity. Lowering the residence time below 10 ms for the microchannel monolith increases the syngas selectivity. This probably due to quenching of the gas phase reactions at high linear gas velocity, and suggests that microchannel reactors have potential for isolating kinetic effects and minimising gas phase contributions. The Rh/Al₂O₃ foam systems show higher initial syngas selectivity than the Rh-impregnated microchannel reactors, but deactivate rapidly upon temperature cycling, especially when steam is added as a reactant.