Process intensification is a chemical engineering field which has truly emerged in the past few years and is currently rapidly growing. It consists in looking for safer operating conditions, lower waste in terms of costs and energy and higher productivity; and a way to reach such objectives is to develop multifunctional devices such as heat exchanger/reactors for instance. This review is focused on the latter and makes a point on heat exchanger/reactors. After a brief presentation of requirements due to transposition from batch to continuous apparatuses, heat exchangers/reactors at industrial or pilot scales and their applications are described.

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Heat exchanger/reactors (HEX reactors) : Concepts, technologies: State-of-the-art

Improved safety is one of the main drivers for microreactor application in chemical process development and small-scale production. Typical examples of hazardous chemistry are presented indicating potential risks also in miniaturized equipment. Energy balance and kinetic parameters describe the heat production potential and, together with heat transfer capability, the temperature development in a continuous flow reactor. Besides these calculation procedures, checklists for laboratory safety and risk assessment are the basis for improved laboratory work as well as for equipment-related safety discussions. For complete and larger chemical plants, hazard and operation (HAZOP) studies are the appropriate method of handling hazardous processes and their scale-up.

Safety assessment in development and operation of modular continuous-flow processes

Flow chemistry using milli- and microstructured reactors-from conventional to novel process windows.

Flow processes with microstructured reactors allow paradigm changes in process development and thus can enable a faster development time to the final production plant. They do this by exploiting similarity effects along the development chain (modularity) and intensification. The final result can be a (significantly) reduced number of apparatus in the plant, a (significantly) reduced apparatus size, and a higher predictability in the scale-out of the apparatus. So far, this was mainly achieved via transport intensification given in microstructured reactors – improved mixing and heat transfer which increase productivity and possibly improve selectivity. A more new idea is chemical intensification through deliberate use of harsh chemistries at unusual (high) pressure, temperature, concentration, and reaction environment which again increases productivity. A very new idea is the process design intensification – the reaction-maximized flow processes need less separation expenditure and the small unit size together with the high degree in functionality gives large potential for system integration. Both means change and simplify the process scheme totally which can lead to a reduced number of apparatus and has impact on predictability. The modular nature of the small flow units allow an easy implementation to modern modular plant environments (Future Factories) which enables to perform all the testing cycles (lab, pilot, production) in one plant environment; an example are here container plants. All these measures have large potential for (much) decreased overall development time.

Potential Analysis of Smart Flow Processing and Micro Process Technology for Fastening Process Development: Use of Chemistry and Process Design as Intensification Fields

Flow chemistry is typically used to enable challenging reactions which are difficult to carry out in conventional batch equipment. Consequently, the use of continuous-flow reactors for applications in organometallic and organic chemistry has witnessed a spectacular increase in interest from the chemistry community in the last decade. However, flow chemistry is more than just pumping reagents through a capillary and the engineering behind the observed phenomena can help to exploit the technology’s full potential. Here, we give an overview of the most important engineering aspects associated with flow chemistry. This includes a discussion of mass-, heat-, and photon-transport phenomena which are relevant to carry out chemical reactions in a microreactor. Next, determination of intrinsic kinetics, automation of chemical processes, solids handling, and multistep reaction sequences in flow are discussed. Safety is one of the main drivers to implement continuous-flow microreactor technology in an existing process and a brief overview is given here as well. Finally, the scale-up potential of microreactor technology is reviewed.

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Beyond organometallic flow chemistry : the principles behind the use of continuous-flow reactors for synthesis

The present paper deals with the establishment of a new methodology in order to evaluate the inherently safer characteristics of a continuous intensified reactor in the case of an exothermic reaction The transposition of the propionic anhydride esterification by 2-butanol into a new prototype of “heat-exchanger/reactor”, called open plate reactor (OPR), designed by Alfa Laval Vicarb has been chosen as a case study Previous studies have shown that this exothermic reaction is relatively simple to carry out in a homogeneous liquid phase, and a kinetic model is available A dedicated software model is then used not only to assess the feasibility of the reaction in the “heat-exchanger/reactor” but also to estimate the temperature and concentration profiles during synthesis and to determine optimal operating conditions for safe control Afterwards the reaction was performed in the reactor Good agreement between experimental results and the simulation validates the model to describe the behavior of the process during standard runs A hazard and operability study (HAZOP) was then applied to the intensified process in order to identify the potential hazards and to provide a number of runaway scenarios Three of them are highlighted as the most dangerous: no utility flow, no reactant flows, both stop at the same time The behavior of the process is simulated following the stoppage of both the process and utility fluid The consequence on the evolution of temperature profiles is then estimated for a different hypothesis taking into account the thermal inertia of the OPR This approach reveals an intrinsically safer behavior of the OPR

Evaluation of an intensified continuous heat-exchanger reactor for inherently safer characteristics

The implementation of process intensification by multiscale equipment will have a profound impact on the way chemicals are produced. The shift to higher space-time yields, higher temperatures, and a confined reaction volume comprises new risks, such as runaway reactions, decomposition, and incomplete conversion of reactants. Simplified spreadsheet calculations enable an estimation of the expected temperature profiles, conversion rates, and consequences of potential malfunction based on the reaction kinetics. The analysis illustrates that the range of optimal reaction conditions is almost congruent with the danger of an uncontrolled reaction. The risk of a spontaneous reaction with hot spots can be presumed if strong exothermic reactions are carried out in micro-designed reactors. At worst, decomposition follows the runaway reaction with the release of noncondensable gases. Calculations prove that a microreactor is not at risk in terms of overpressure as long as at least one end of the reactor is not blocked.

Guidance on Safety/Health for Process Intensification including MS Design; Part I: Reaction Hazards

The intensified technologies offer new prospects for the development of hazardous chemical syntheses in safer conditions: the idea is to reduce the reaction volume by increasing the thermal performances and preferring the continuous mode to the batch one. In particular, the Open Plate Reactor (OPR) type “reactor/ exchanger” also including a modular block structure, matches these characteristics perfectly. The aim of this paper is to study the OPR behaviour during a normal operation, that is to say, after a stoppage of the circulation of the cooling fluid. So, an experiment was carried out, taking the oxidation of sodium thiosulfate with hydrogen peroxide as an example. The results obtained, in particular with regard to the evolution of the temperature profiles of the reaction medium as a function of time along the apparatus, are compared with those predicted by a dynamic simulator of the OPR. So, the average heat transfer coefficient regarding the “utility” fluid is evaluated in conductive and natural convection modes, and then integrated in the simulator. The conclusion of this study is that, during a cooling failure, a heat transfer by natural convection would be added to the conduction, which contributes to the intrinsically safer character of the apparatus.

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Dynamic Behaviour of a Continuous Heat Exchanger/Reactor after Flow Failure

The new technology of process intensification by multiscale equipment can significantly contribute to achieve a safer design by going from batch/semi-batch to continuous operation combined with a reduction of inventory of hazardous substances in critical stages. On the other hand, the shift to higher space-time-yields comprises new risks such as runaway reactions with hot spot formation, described in Part I, and handling an explosive atmosphere in the presence of potential permanent ignition sources, described in Part II. A tool was developed for preliminary risk assessments, called HAZOP-LIKE study, to cover the characteristic features of micro-designed equipment that are relatively unimportant when handling conventional equipment. Two generic cases concerning liquid/liquid and gas/gas reactions were studied to demonstrate the method.

Guidance on Safety/Health for Process Intensification Including MS Design. Part III: Risk Analysis

The oxy-combustion process uses CH4/O2/CO2/H2O mixtures at various concentrations, according to the different operation phases. To analyze the risks associated to this process, the safety characteristics of these explosive mixtures have to be taken into account. A literature review showed that some safety features of methane in oxygen or in air were not available. Thus, the flammability ternary diagram of CH4/O2/CO2 mixtures was determined at room temperature and 1 bar pressure. Furthermore, the influence of oxygen content on the explosion severity (Pmax; dP/dt) was investigated. The ternary mixtures were prepared directly in a 20 L spherical test vessel. The concentrations of reactants were adjusted using the relationship between the partial pressure and the molar fraction of gas. The ignition source used was an alumel fusing wire. The flammability limits of methane in oxygen were extrapolated at 5 and 68% vol., by using the established CH4/O2/CO2 mixtures ternary diagram. It also confirmed that when the carbon dioxide concentration increases, the flammability range decreases: no ignition was observed when carbon dioxide content exceeded 73%. A significant influence of the oxygen concentration on the explosion severity has been highlighted for CH4/O2/CO2 mixtures containing respectively 10, 25, 45 and 65% vol. of carbon dioxide. The maximal explosion overpressure and the maximum pressure rise were both measured near the stoechiometry. Maximum values of Pmax and dP/dt measured for a 10% vol. carbon dioxide concentration were 11.2 bar rel. and 5904 bar/s respectively, while they were 3.6 bar rel. and 72 bar/s respectively in the case of a 65% vol. carbon dioxide content in the mixture.

https://hal-ineris.archives-ouvertes.fr/ineris-00976228/document

Wassila Benaissa

Papers

Evaluation of an intensified continuous heat-exchanger reactor for inherently safer characteristics

Guidance on Safety/Health for Process Intensification including MS Design; Part I: Reaction Hazards

Dynamic Behaviour of a Continuous Heat Exchanger/Reactor after Flow Failure

Guidance on Safety/Health for Process Intensification Including MS Design. Part III: Risk Analysis

Experimental study of CH4/O2/CO2 mixtures flammability