Michel Cabassud

A thermochemical approach for calcium phosphate precipitation modeling in a pellet reactor

Calcium phosphate precipitation is studied in this article. The P-recovery process is carried out in a fluidized sand bed, the so-called pellet reactor which presents major advantages from the hydrodynamical viewpoint. The associated chemistry is yet relatively complex, due to pH gradient along the column and to the residence time of the various precipitates. The experimental observations showed three different phenomena: first, an agglomeration of fines around the sand grains is observed, second, a stagnation of fines in the bed occurs while a significant amount of fines also leaves the bed with the liquid effluent. The purpose of this work is to validate the thermodynamical model developed in our previous works on a semi-industrial sized pilot. Additional experimental runs carried out for various operating conditions showed the robustness of the model. These results open some interesting perspectives for the determination of optimized operating conditions at industrial scale.

Predictive functional control for the temperature control of a chemical batch reactor

A predictive functional control (PFC) technique is applied to the temperature control of a pilot-plant batch reactor equipped with a mono-fluid heating/cooling system. A cascade control structure has been implemented according to the process sub-units reactor and heating/cooling system. Hereby differences in the sub-units dynamics are taken into consideration. PFC technique is described and its main differences with a standard model predictive control (MPC) technique are discussed. To evaluate its robustness, PFC has been applied to the temperature control of an exothermic chemical reaction. Experimental results show that PFC enables a precise tracking of the set-point temperature and that the PFC performances are mainly determined by its internal dynamic process model. Finally, results show the performance of the cascade control structure to handle different dynamics of the heating/cooling system.

DYNAMIC MODELLING OF CROSSFLOW MICROFILTRATION OF BENTONITE SUSPENSION USING RECURRENT NEURAL NETWORKS

Abstract Using a set of experimental results (permeate flux and deposit thickness) as a function of different operating conditions, obtained during crossflow microfiltration of a bentonite suspension with a laboratory pilot, a dynamic modelisation of this process by means of recurrent neural networks is proposed. The elaborated neural network is able to describe the evolution of permeate flux and deposit thickness from the process variables (transmembrane pressure, crossflow velocity, concentration of the suspension) and the starting point values for permeate flux and deposit thickness. The simulation of the evolution by such a model for experiments limited to a certain timespan, allows us to obtain coherent limit values for both permeate flux and deposit thickness.

Transposition of an Exothermic Reaction From a Batch Reactor to an Intensified Continuous One

The implementation of chemical syntheses in a batch or semi-batch reactor is generally limited by the removal or the supply of heat. A way to enhance thermal performances is to develop multifunctional devices like heat exchanger/reactors. In this work, a novel heat exchanger/reactor is characterized in terms of residence time, pressure drops, and thermal behavior in order to estimate its capacities to perform an exothermic reaction: the oxidation of sodium thiosulfate by hydrogen peroxide. Experimental results highlight the performances of the heat exchanger/reactor in terms of intensification, which allows the implementation of the oxidation reaction at extreme operating conditions. These conditions are finally compared to the ones of a classical batch reactor.

Precise parameter estimation for chemical batch reactions in heterogeneous medium

Abstract A sequential experimental strategy for precise parameter estimation has been used in the case of liquid–liquid dispersions in batch-stirred tank reactors where slow chemical reactions take place. The mathematical model for a batch reaction in a stirred tank reactor is formulated as a system of non-linear differential equations standing for the mass balance of each component. Physical kinetic parameters and chemical kinetic parameters which arise from this model are estimated simultaneously. The estimation problem is posed as a weighted least squares problem and solved by using a standard Levenberg–Marquardt algorithm. In this work, we intend to show how it is possible to develop efficient experimental design strategies that lead to an accurate estimation of the parameters involved in phenomenological models and most particularly in kinetic models. Three design criteria for designing the experiments have been employed in order to increase the precision on the parameter estimates of the model. A standard non-linear sequential quadratic programming method ensures the determination of the operating conditions which define the experimental design. The well-known alkaline hydrolysis of esters in aqueous phase has been treated as a numerical application example.

Dynamic models for start-up operations of batch distillation columns with experimental validation

The simulation of batch distillation columns during start-up operations is a very challenging modelling problem because of the complex dynamic behaviour. Only few rigorous models for distillation columns start-up are available in literature and generally required a lot of parameters related to tray or pack geometry. On an industrial viewpoint, such a complexity penalizes the achievement of a fast and reliable estimate of start-up periods. In this paper, two “simple” mathematical models are proposed for the simulation of the dynamic behaviour during start-up operations from an empty cold state. These mathematical models are based on a rigorous tray-by-tray description of the column described by conservation laws, liquid–vapour equilibrium relationships and equations representative of hydrodynamics. The models calibration and validation are studied through experiments carried out on a batch distillation pilot plant, with perforated trays, supplied by a water methanol mixture. The proposed models are shown by comparison between simulation and experimental studies to provide accurate and reliable representations of the dynamic behaviour of batch distillation column start-ups, in spite of the few parameters entailed.

A General Simulation Model and a Heating/Cooling Strategy to Improve Controllability of Batch Reactors

This paper describes the development of a dynamic simulation model for stirred tank batch or semi-batch chemical reactor and its heating/cooling system. Heat and mass balances are established for the reactor and its jacket. As the general purpose is the thermal control of the reactor, special attention is devoted to the behaviour of the heating/cool-cooling system. The control strategy is based on the use of the thermal flux as manipulated variable. At each sampling time, the controller computes the thermal flux to be exchanged with the fluid flowing inside the jacket to get the desired reactor temperature set-point. This information is then used to select the thermal fluid on the basis of the maximal and minimal thermal flux capacities of each utility. The computer simulation program is flexible, enabling simulation of a batch or semi-batch reactor vessel, ranging from a laboratory pilot plant to full-scale production plant. To demonstrate the good performance of both simulation model and control methodology, experimental results are presented for a pilot plant and simulation studies have been performed for both pilot plant and industrial reactors.

Influence of solvent choice on the optimisation of a reaction–separation operation : application to a Beckmann rearrangement reaction

In pharmaceutical syntheses, the solvent choice generally represents a complex design step. Traditionally, this choice is operated according to criteria connected with the reaction step and without any consideration on the following separation steps. The purpose of this study is to highlight the benefits of a global approach of optimisation for the solvent determination. In this way, an optimisation framework dedicated to global synthesis is applied to a simple reaction–separation operation integrating a Beckmann rearrangement reaction, leading to interesting solvent choices.

A General Framework for Pellet Reactor Modelling: Application to P-Recovery

Emphasis in recent years has been focused on improving processes which lead to enhanced phosphate recovery. This paper studies the precipitation features of calcium phosphate in a fluidized bed reactor in a concentration range between 4 and 50 mg 1−1 and establishes the conditions for optimum phosphate removal efficiency. For this purpose, two models are coupled for predicting the pellet reactor efficiency. First, a thermodynamical model is used for predicting calcium phosphate precipitation vs. initial conditions (pH, [P], [Ca], temperature). The second one is a reactor network model. Its parameters are identified by an optimization procedure based on simulated annealing and quadratic programming. The efficiency is computed by coupling a simple agglomeration model with a combination of elementary systems representing basic ideal flow patterns (perfect mixed flow, plug flow, etc.). More precisely, the superstructure represents the hydrodynamical conditions in the fluidized bed. The observed results show that a simple combination of ideal flow patterns is involved in pellet reactor modelling, which seems interesting for a future control. The experimental prototype used for validation purpose is first described. Then, the thermochemical model is presented for calcium phosphate precipitation. The third part is devoted to the reactor network-oriented model. The approach presented is finally validated with experimental runs.

Semi-batch reactor optimization and control for the epoxidation of furfural

Abstract In attempting to optimize a chemical systhesis, competing side reactions often interfere, which degrades the reactants into other undesired subtances. In those cases, the course of the reaction can be influenced by acting on the manipulated variables temperature and feedrate as the reaction proceeds. The determination of such strategies is usually referred to as optimal control. In the present paper, a methodology of optimal control of batch reactors, the main processing tool in fine chemistry, is presented. The implementation of the methodology involves two phases: (i) an optimization phase where a dynamic mechanistic model is determined and used to optimize the process based on a technical—economic objective function and (ii) a control phase where a dynamic empirical model is used to optimize a performance objective function which penalizes deviations of process variables from their optimum values given by the optimization phase. The first part of the paper presents the global strategy and the necessary theoretical developments. The second part of the paper is an illustration of the strategy using the epoxidation of furfural as a model reaction. The optimization based upon an approximate mechanistic model is compared to a more conventional approach involving the response surface methodology. Then, control studies by simulation and experimentation on a 21 pilot plant reactor for the epoxidation of furfural are performed. The previously determined optimal trajectories are implemented in practical operation.