Fine Chemical Processes: Diagnosis and Performance Optimisation

Abstract

Fine chemical processes involve very different types of reactions. They are run in a continuous or batch way, in reactors which volume can range from hundreds of liters to several dozens of m3. Technological choices are also linked to corrosion constraints. In such a context, the various fields of competence necessary to diagnose how to optimise operating conditions need the help of specific methods and tools. A classification of these processes of problem resolution are proposed. They are presented below. Influence of gas/liquid mass transfer in a process of slurry catalytic gas-liquid-solid oxidationDuring the development of the process, it appeared that it was necessary to minimise the dissolved oxygen in order to reach good performance in terms of selectivity, avoiding the damage of the catalytic sites. On the contrary, an important value of dissolved oxygen concentration promotes the productivity. The optimum compromise between productivity and catalyst consumption could be reached with fine modelling between the reaction kinetics (taking deactivation into account) and mass transfer. The designed process consists of an association of several reactors in series, with a gas/liquid transfer coefficient decreasing from low conversions (with high reaction rates) towards high conversions (with low conversion rates). Coupling between hydrodynamics and heat transferAn industrial gas-liquid process of fermentationA detailed analysis of the industrial process showed that heat and mass transfer coefficients were insufficient, leading to an improvable productivity. These parameters were optimised on a mock-up, a modification of the agitation system and conditions could be recommended in accordance with the process constraints. The choice was based on gas/liquid mass transfer coefficients (so-called gassing-outmethod), dissipated power, mixing times measurements. The technology was validated on a typical fermentation. That is how an improvement of the productivity could be obtained, for an identical quality of product. An industrial process of emulsion polymerisationThe plant was bottlenecked by the heat transfer performance during the final step of monomer feeding. In order to propose industrial improvements of the technology, the approach of diagnosing and optimisation was followed:- hydrodynamic approach of mixing in the reactors, taking into account the rheological properties of the reacting medium and its evolution with time; design of agitators; - numerical (CFD) approach in order to validate the previous conclusions and optimise the configuration of the agitation;- experimental validation at the laboratory and industrial scales. The heat transfer coefficient could be improved by 30%, leading to a benefit in productivity. Processes characterised by specific reaction kineticsOptimisation of the temperature and feeding profiles of a reactant (for a semibatch process), depending on whether we know or not the reaction kinetics When a kinetic study could be achieved in a laboratory and/or a calorimetric reactor during the development of the process, the optimisation of the operating conditions can be reached by a traditional approach. A specific software, allowing the coupling between hydrodynamics, kinetics, heat and mass transfers, has to be used for a performing optimisation. On the opposite, when the reactional scheme is complex, and when the development of the process does not allow any complete parametric study, the use of a grey boxmodel is particularly dedicated. With a limited set of data, the software generates a hypothetical reactional scheme which is used afterwards to determine another experiment that has to be done to validate the model and so the optimal operating conditions. This type of approach is of much interest for fine chemical processes development, where times to market get more and more reduced. Optimisation of the technology of the reactor when the chemistry is sensitive to local overconcentrations (micromixing)For certain types of chemical reactions, for instance competitive-consecutivereactions, the feeding mode of the sensitive reactant has to be chosen with precautions. An example is given, where the modification of bromine alimentation in a bromation reactor led to an improvement of the selectivity by several points. The aim of this presentation is to show how we adapt our approach and tools facing a diagnosis, and to present the indispensable tools for solving the specific questions related to fine chemical processes reactors: complex reaction schemes and kinetics; control of the coupling of mixing and transfers.