Data processing: generic control systems or specific application – Specific application – apparatus or process – Chemical process control or monitoring system
Patent
1997-12-09
2000-11-07
Grant, William
Data processing: generic control systems or specific application
Specific application, apparatus or process
Chemical process control or monitoring system
700266, 700268, 481275, 20415715, 585700, G05B 2100
Patent
active
061448974
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a control method for processes of synthesis of chemical products. It also relates to a control device for making use of this method and to a process of synthesis, in particular of polymer, controlled by this method.
2. Description of the Background
In a process of synthesis of chemical products which is conducted conventionally, controllers of PID (proportional-integral-differential) type are used for controlling individually a larger or smaller number of variables (temperatures, flow rates, pressures, etc.) which affect the course of the synthesis. In other words, for of each temperature, flow rate or pressure to be controlled, its actual value is measured continuously (or intermittently), and a PID controller compares this actual value with a set point and acts on the variable to be controlled so as to reduce, if appropriate, the difference between the set point and the measured value.
In view of the complexity of most industrial processes of chemical synthesis, the set points of the various controllers must today still be adjusted empirically in order to obtain finally the desired properties for the synthesized product. Recipes are used for this purpose, which provide combinations of parameters determined empirically to obtain, in a steady state, the desired properties for the synthesized product.
Empirical relationships between the controlled variables and the properties of the synthesized product could be deduced from these recipes with the help of more or less sophisticated statistical tools. It is obvious, however, that these empirical relationships can hardly take into account the many interdependencies which exist between the various variables which are controlled separately, or the unknown perturbations such as the impurity contents of the raw materials.
It is also obvious that a traditional closed-loop control, employing measurements of essential properties for the synthesized product as feedback corrections, is difficult to apply to most processes of synthesis. This is because the dead times involved either in the process or in the measurements or analyses used as feedback corrections are too long and the independences between the various variables ruling the process are too complex.
International Application WO 93/24533 describes a control method for a process of gas phase polymerization of an alpha-olefin in a horizontal reactor, in which control parameters make it possible to act on the course of the process in order to make the melt index (MFR) of the polymer equal to the corresponding set point, the method including the following steps: the reactor and a first series of parameters, to a predetermined value.
It has been known for a long time that processes of synthesis, especially processes of continuous polymer synthesis (polymerization processes), equipped with controllers with empirically adjusted set points have major disadvantages which can be summarized as follows: quantities of off-specification product; quantities of transitional off-specification products; synthesized per time unit is difficult to chance without impairing the properties of this or these products; is often poor, even in a steady state.
In order to avoid empirical adjusting of the set points, it has been proposed in the specialist literature to use control methods for processes of synthesis which make use of characteristic equations modelling the process of synthesis in order to relate some properties of the synthesized product(s) to the running conditions of the reactor(s) during the synthesis. However, in order to limit the complexity of these characteristic equations, it was hitherto considered that in practice it was necessary either to consider solely the static case (steady state), or to limit one-self to a highly simplified empirical modelling of the process dynamics. The use of a static model is limited to the control of a fairly steady state production.
In the case of empirical modelling the characteristic equations are v
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Grant William
Marc McDiennel
Solvay Polyolefins Europe--Belgium (Societe Anonyme)
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