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REALTIME MANUFACTURE PROCESS CONTROL FOR CRYSTALLINE MICROPARTICLES - COMIPA

Objective

Fine organic chemicals are generally obtained by precipitation or crystallization. There is a real need for a better control of reactive precipitation, because the main physico-chemical properties (purity, size, shape) are determined during this key step of the process.

For instance, narrow crystal size distribution is required for high technology applications (e.g. micro electronics, ceramics, reinforcement) so that currently cheap product (like calcium carbonate) could reach another class of high market value materials.

The project is aimed at developing a new crystallization process for controlling the production of tailored particles.
After kinetic data determination and complete modelling of the precipitation of calcium carbonate, a model-based control strategy has ben proposed in order to control the Crystal Size Distribution. This strategy, little demanding on computer power, is based on a process model and can make up for the lack of sensors for supersaturation and solid growth rate, the 2 driving forces of a precipitation reactor. Simulation results show that this technique is efficient for the control of the reactor through the control of its driving forces, and robust.

During the accomplishment of this study, the following relevant conclusions and facts have been put in evidence:

- kinetics studies on calcium carbonate have shown that the great majority of the published results did not take into account the problem of polymorphism correctly. The identification of the industrial operating conditions for pure calcite production enables the development of a continuous process of pure calcite production resulting in a reduced Crystal Size Distribution standard deviation:

- industrial production of pure calcite : It has been demonstrated that, by properly choosing the reacting solutions, the inlet calcium/carbonate ratio, the initial pH and the working temperature, pure calcite can be produced at an industrial scale,
- obtention of monodispersed particles : This problem was tackled by using initial homogeneous solutions formed by mixing solutions of one of the component, Ca, previously complexed by organic ligands, such as acetate or citrate anions, with solutions of potassium bicarbonate. This initial solution has long induction period for nucleation, which is used to produce homogeneous nucleation and growth leading to monodispersed systems in batch experiments,
- coating technologies : The calcite coating assay has defined a selection of products and technical procedures to quench the growth process and functionalyze the materials for their further use as charges,

- the technologies developed in the framework of COMIPA are extremely useful to understand the precipitation of salts from multiprotonized anions, and substances presenting polymorphism or precipitating multiple phased from a common initial solution (Ostwald stages law),

- modelling studies have also proved that the rule stating that a stationary state can be obtained after 7 residence time, for a continuous precipitation MSMPR, is false in the case of CaCO3 precipitation. However, this rule is generally considered in chemical engineering handbooks as true for common chemical reactors. Oscillating states, with large variations, could of course appear, with a period of around 10 residence times and large fluctuations on the supersaturation, indications of an alternance of intense nucleation and pure growth.

These oscillating states fully justify the necessity of a model-based control system, capable of inducing the disappearance of these oscillations (by using estimators),

- MEDUSE constitutes a high performance general software package, especially useful and efficient for the optimization of continuous precipitation reactors. This efficiency is due to the choice of high performance numerical recipes and algorithms (intensive use of the notion of 'equivalent', Jacobi matrix calculation, fast calculation of the efficiency factor for the evaluation of diffusional limitations, use of a Newton method coupled with an original initialisation subroutine),

- a simplified model, derived from MEDUSE, has been established for a selected operating point. This physical model, based on the use of simplified momentum balances, has a high degree of generality and reproduces strong coupling between solid and liquid phases, birth of the oscillation phenomena,

- from this model, an original control policy has been conceived, based on the real-time estimation of the two process driving-forces, and able to solve the problem of these oscillating states. This policy is also very general and can be applied to numerous cases of continuous precipitation,

- SIMULINK revealed to be an extremely efficient tool for the generation of a reduced model, for the subsequent phases of linearization and reduction and for the fast visualisation of the model capabilities. it also constitutes a very economical testing bench for the simulation and evaluation of different control policies.
Fine organic chemicals are generally obtained by precipitation or crystallization. There is a real need for a better control of reactive precipitation, because the main physico-chemical properties (purity, size, shape) are determined during this key step of the process.

For instance, narrow crystal size distribution is required for high technology applications (e.g. micro electronics, ceramics, reinforcement) so that currently cheap product (like calcium carbonate) could reach another class of high market value materials.

The project is aimed at developing a new crystallization process for controlling the production of tailored particles.

Coordinator

Bertin et Cie
Address
Avenue Du 1Er Mai - Zone Industrielle
40220 Tarnos
France

Participants (3)

Consejo Superior de Investigaciones Cientificas
Spain
Address
Marti Y Framques
08028 Barcelona
Rhône-Poulenc Chimie SA
France
Address
52 Rue De La Haie-coq
93308 Aubervilliers
TRION PRAEZISIONSELEKTRONIK GMBH & CO KG
Germany
Address
Voltastraße 5
1000 Berlin