The chemical engineering approach to some electrochemical processes

In recent years much research has been carried out in combining electrochemical processes with chemical engineering techniques, and in general, to apply principles of analysis and design of this discipline in search for new electrochemical technologies. This paper reviews seven major areas of activity in modern applied electrochemistry where the chemical engineering approach has been successful: Fluidized bed electrochemical reactors; Bipolar particulate reactors; Pulsed electrochemical reactors; Gas phase electrochemical reactors; Electrocrystallization and electrodissolution; Enhancement of heat and mass transfer in electric fields; dynamics, modelling and optimization of electrochemical reactors.     

Reaktorstabilität und sichere Reaktionsführung

Reactor stability and safe reaction engineering . Stable operating behaviour of chemical reactors and stability in the sense of stability theory of chemical processes are by no means synonymous concepts. Thus, appropriate control can permit reliable operation at a unstable operating point, while even global stability will not necessarily rule out a runaway reaction. The latter is the case when strongly exothermal reactions are characterized by a pronounced parametric sensitivity. In order to delineate the concepts, ?stability”? and ?parametric sensitivity”? are explained first for stirred tank and tubular reactors with strongly exothermal reaction. Both the conventional view of reaction engineering and the theory of heat explosion commonly used in reaction engineering are considered in detail. Practical problems of safe reaction engineering are subsequently discussed for the control of batch reactors, the control of semibatch reactors, and the behaviour of bundled-tube reactors.     

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.     

Keramische Membranen: Materialien – Bauteile – potenzielle Anwendungen

Ceramic membranes can be divided into dense and porous membranes. The materials, microstructure, and manufacturing methods are described and insights into current research topics are given. By adapting the material properties and tailoring microstructures, membranes and components suitable for a variety of processes can be developed. In applications, a distinction can be made between pure gas separation and membrane reactors. In the latter ones, in addition to gas separation, a chemical reaction takes place on one or both sides of the membrane. Membrane reactors can be used to produce basic chemicals or synthetic fuels. The supply of gases can be of interest for power plants, cement, steel or glassworks as well as for the medical sector or for mobile applications.     

Simulation of hydrodynamics and inhibitor consumption in hydrometallurgical plants

Mathematical models are presented for the consumption of the inhibitors of electrocrystallization in hydrometallurgical plants involved with the copper electrorefining and zinc electrowinning. Continuously-stirred tank reactors (CSTR) and plug flow reactors (PFR) in which first order chemical and electrochemical reactions take place are used in these models. The time dependent behaviours of the industrial plants are predicted. Tests with metallic tracers show the validity of the models. Possible uses in electrocrystallization studies are described.     

Recent Advances in MRI Studies of Chemical Reactors: Ultrafast Imaging of Multiphase Flows

NMR has long been established as an in situ technique for studying the solid-state structure of catalysts and the chemical processes occurring during catalytic reactions. Increasingly, pulsed field gradient (PFG) NMR and magnetic resonance imaging (MRI) are being exploited in chemical reaction engineering to measure molecular diffusion, dispersion and flow hydrodynamics within reactors. By bringing together NMR spectroscopy, PFG NMR and MRI, we are now able to probe catalysts and catalytic processes from the angstrom-to-centimeter scale. This article briefly reviews current activities in the field of MRI studies applied to catalysts and catalytic reactors. State-of-the-art measurements, which can already be used in real reactor design studies, are illustrated with examples of single-phase flow with and without chemical reaction in a fixed-bed reactor. The ability to obtain high spatial resolution (< 200μm) in images of the internal structure and flow field within reactors is demonstrated, and the potential uses of these data in reactor design and understanding bed fouling phenomena are discussed. In particular, MRI has produced the first detailed measurements of the extent of heterogeneity in the flow field within fixed-bed reactors. The example of a fixed-bed esterification process is used to show how NMR spectroscopy and MRI techniques can be combined to provide spatially resolved information on both hydrodynamics and chemical conversion within a process unit. The emerging area of ultrafast MRI is then highlighted as an area of particular interest. Recent advances have demonstrated that it is possible to record 2D images over timescales of ～100ms in the magnetically heterogeneous environments typical of heterogeneous chemical reactors. These advances open up opportunities to image many unsteady state processes for the first time. Examples are given of real-time visualization of bubble-train flow in a ceramic monolith and exploring the stability of the gas–liquid distribution as a function of liquid flow rate in a trickle-bed reactor.     

Modular Simulation of Fluidized Bed Reactors

Simulation of chemical processes involving nonideal reactors is essential for process design, optimization, control and scale‐up. Various industrial process simulation programs are available for chemical process simulation. Most of these programs are being developed based on the sequential modular approach. They contain only standard ideal reactors but provide no module for nonideal reactors, e.g., fluidized bed reactors. In this study, a new model is developed for the simulation of fluidized bed reactors by sequential modular approach. In the proposed model the bed is divided into several serial sections and the flow of the gas is considered as plug flow through the bubbles and perfectly mixed through the emulsion phase. In order to simulate the performance of these reactors, the hydrodynamic and reaction submodels should be integrated together in the medium and facilities provided by industrial simulators to obtain a simulation model. The performance of the proposed simulation model is tested against the experimental data reported in the literature for various gas‐solid systems and a wide range of superficial gas velocities. It is shown that this model provides acceptable results in predicting the performance of the fluidized bed reactors. The results of this study can easily be used by industrial simulators to enhance their abilities to simulate the fluidized bed reactor properly.     

Electrochemical microstructured reactors: design and application in organic synthesis

The use of micro structured reactors is an accepted technology in fundamental chemical research as well as in industrial applications. The application of electrochemical microreactors (ECMR) has not attracted as much attention as continuously performed reactions in a confined space. Nevertheless ECMRs are in use to perform electro-organic reactions. In this review, different aspects of ECMRs with structured electrodes and interelectrode distances of mainly ≤100 μm are investigated and discussed, together with various manufacturing techniques and prototypes described therein. Based on representative examples described in various publications for electrolysis (for direct and indirect electrolysis) advantages and disadvantages of electrochemical microreactors are presented and compared with those of conventional electrochemical reactors.     

Wirbelschicht-Prozesse für die Chemieund Hütten-Industrie,die Energieumwandlung und den Umweltschutz

Fluidized bed processes for the chemical and metallurgical industries, energy conversion, and pollution control . This article presents a review of, and selection criteria for, gas/solids reactors with the aid of examples of industrially operating fluidized bed processes. The choice of optimum reactor design with regard to flow and reaction conditions, heat and mass transfer, grain size, and retention time of solids and gas is considered. In conclusion, various processes are described in terms of several process flowsheets.     

Application of catalysts to coal gasification processes. Incentives and perspectives

There are several processes used for coal gasification, but these have three characteristic types of reactors — moving-bed, fluidized-bed and entrained phase. Design and comparison of different processes and reactors is possible if kinetic data relevant to technical performance of processes are available. The state of reaction kinetics of the non-catalysed and catalysed steam-carbon reaction as the basic coal gasification reaction is discussed. Catalysts only have effects on gasification rates in the temperature range of chemical reaction or pore diffusion control. Furthermore the increase in reaction rates by catalysts can also be reached without using catalysts. Therefore the application of catalysts seems not to be attractive in conventional gasification processes because the advantages (less coal and oxygen consumption and lower heat losses) are compensated by their disadvantages (additional costs, side effects). Only such processes in which a temperature increase is limited for different reasons does the application of catalysts have significant advantages. Such processes are the allothermal gasification using nuclear heat and processes leading to synthetic natural gas (SNG).     

多相反应体系的微界面强化简述

加氢、氧化等气液慢反应过程广泛存在于现代过程工业之中,这些反应过程一般受传质速率控制。因此,对这类多相反应体系的传质强化一直是研究热点之一。但总体而言,除外场和微通道（微流控）强化等一类强化反应器外,以往的研究大多集中于界面尺度为毫-厘米级的传统反应器的搅拌与混合方式、气泡分布状态、流体流型、构效关系等方面,而鲜有将研究视角投放到传统以米为直径计量单位的反应器平台上如何构建尺度为微米级的界面体系及其特殊效应方面。探讨了多相反应体系的微界面反应强化理念,并简述了微界面的涵义、微界面反应强化与构效调控方法、微界面反应器的结构与形成原理、微界面体系的微颗粒测试与相界面表征技术、微界面反应强化面临的问题与挑战等,以与本领域同行共同研讨。     

Population balance model for mixing in continuous flow systems

A population balance model is developed for describing macromixing-micromixing hierarchy of continuous flow systems based on the concept of interactive populations of fluid elements. Macromixing is represented as motion of fluid elements in the physical space described by convection-dispersion models while micromixing is described as motion in the composition space induced by randomised mass exchange interactions of fluid elements. A generalised coalescence/redispersion model is formulated, characterised by the intensity of coalescence/re-dispersion interactions and a set of random variables describing the transfer rates of species between the fluid elements. The model allows defining different micromixing rates for different species.The axial dispersion-coalescence/redispersion (ADCR) model, formulated as a special case of the general population balance model describes macromixing-micromixing hierarchy for multicomponent processes in chemical reactors. An infinite system of moment equations is derived for the joint moments of concentrations of species which is closed by applying a cumulant-neglect closure for higher order chemical reactions. Verification of the ADCR model carried out by comparing the computational results of the second-order moment equation reduction with Vassilatos’ experimental data in the case of a tubular reactor with bimolecular quasilinear chemical reaction shows good correspondence. The properties of macromixing-micromixing interactions are examined by numerical experiments using the second-order moment equation reduction. Predictions of the model in maximally micromixed states provide excellent qualitative and quantitative agreement with the results of the continuum balance equation model, and appears to be a good tool for direct modelling of macromixing-micromixing hierarchy in chemical reactors.     

Vinyl chloride suspension polymerization with constant rate. A numerical study of batch reactors

Based on the analysis of a simple mathematical model, different temperature profiles are generated in order to provide vinyl chloride (VCM) suspension polymerization with constant reaction rates in batch reactors. In order to reproduce these temperature profiles in industrial-scale reactors, some process variables, such as coolant temperature, initiator concentration, and rate of water and monomer condensation, have to be manipulated. It is shown that those temperature practices can almost never be applied to large-scale reactors if the jacket temperature is the only variable that can be manipulated. It is also shown that developing an initiator feed procedure or using a reflux condenser may be advantageous.     

OPTIMIZATION OF HETEROGENEOUS CATALYTIC PROCESSES IN FLUIDIZED BED

A number of large-scale chemical processes is carried out in fluidized-bed reactors (FBR). Optimization of processes in FBR is thus of great importance both for the increase of efficiency of the existing plants and for the design of new ones. Selection of optimal conditions for exothermic processes should recognize conditions for heat removal and multiplicity and stability of stationary states rather than just the reaction kinetics and fluidized bed hydrodynamics. Mathematically this problem is rather cumbersome. A process in FUR is described by the boundary problem for differential equations while statement of an optimization problem requires some additional limitations to be imposed on concentrations at the input and output of the reactor, on temperature, design parameters etc. One should take special notice of the limitation on the process stability which (owing to great reverse thermal agitation) is much more important in FBRs than in packed-bed reactors.     

OPTIMIZATION OF HETEROGENEOUS CATALYTIC PROCESSES IN FLUIDIZED BED

A number of large-scale chemical processes is carried out in fluidized-bed reactors (FBR). Optimization of processes in FBR is thus of great importance both for the increase of efficiency of the existing plants and for the design of new ones. Selection of optimal conditions for exothermic processes should recognize conditions for heat removal and multiplicity and stability of stationary states rather than just the reaction kinetics and fluidized bed hydrodynamics. Mathematically this problem is rather cumbersome. A process in FUR is described by the boundary problem for differential equations while statement of an optimization problem requires some additional limitations to be imposed on concentrations at the input and output of the reactor, on temperature, design parameters etc. One should take special notice of the limitation on the process stability which (owing to great reverse thermal agitation) is much more important in FBRs than in packed-bed reactors.     

Flow visualization and modelling of a filter-press type electrochemical reactor

A study of flow visualization and residence time distribution is provided in order to model the flow between two electrodes in a commercial filter-press reactor, the ElectroSynCell® from Electrocell AB. Flow visualization indicates that both axial and lateral dispersion phenomena occur and a global plug flow behaviour is observed. The flow distribution is asymmetric due to the design of the inlet system in the active zone. The flow throughout the cell is described by a dispersed plug flow model for which the mean residence time and the Pe´clet number are determined. The reaction area and the inlet system are separately analysed by locating conductimetric probes inside the electrochemical cell. The reaction area is also well described by a dispersed plug flow model, and characterized by high dispersion. The inlet system is, respectively, described by a dispersed plug flow model and by a cascade of continuous stirred tank reactors. The high number of reactors in the cascade denotes a quasi plug flow behaviour. The results are confirmed by two cascades of continuously stirred tank reactors in series. The dispersion coefficients obtained throughout the reaction area of the cell are not constant. This shows that the flow is not well established at the entrance of the reaction zone and depends on the entrance conditions.     

Mixing models with varying stage size

Two progression models which can be employed to represent flow systems with varying degree and mode of mixing along the direction of flow such as a shallow fluidized bed, radial flow reactors, and short tubular reactors, etc. are proposed. Residence time distributions are derived from the standpoint of stochastic processes. The use of progression models is illustrated with a second order, isothermal reaction.     

工业废水电化学处理技术的进展及其发展方向

综述了工业废水电化学处理技术的进展和应用.介绍了电化学氧化、电化学还原、电絮凝、电气浮、电渗析等方法.阐述了电化学氧化技术目前没有工业化的主要原因是低的电流效率、高能耗和大的操作费用,如何提高传质特性、电流效率、开发用于废水处理的高效电解槽是亟待解决的问题.特别指出了未来工业废水电化学处理技术的发展方向是生物难降解废水处理用的阳极材料、电化学反应器、电化学组合技术、生物膜电化学反应器工艺.     

Moderne Methoden der Meß- und Regelungstechnik und ihre Bedeutung für die Automatisierung verfahrenstechnischer Prozesse

Modern methods of measurement and control engineering and their significance for the automation of chemical engineering processes . This article demonstrates that modern system-theoretical methods open up new approaches in the solution of difficult measurement and control problems. These methods are frequently based on the assumption that a certain a priori knowledge of the process behaviour is available, which can be formulated as a mathematical process model. Model-supported measuring procedures are of particular significance and are discussed at length. These methods greatly enhance the information value of measured values, with regard to both extent and reliability. It is shown how the problem of early recognition of dangerous reaction states in reactors in the case of exothermic reaction can be overcome. In order to minimize the computational effort, methods of model reduction are considered.