Polymerization process controller

Details Polymerization process controller Polymerization process controller

1999-11-08

2002-08-27

Tran, Hien (Department: 1764)

Chemical apparatus and process disinfecting, deodorizing, preser

Control element responsive to a sensed operating condition

Control element responds proportionally to a variable signal...

C422S108000, C422S110000, C422S131000, C422S138000

Reexamination Certificate

active

06440374

ABSTRACT:

BACKGROUND
The invention pertains to polymerization reactions, particularly to monitor and control the rate and the amount of conversion in such reactions. More particularly, the invention pertains to accurately determining the rate and amount of conversion at a particular moment in a polymerization reaction so as to control the rate of conversion and to optimize cooling resources.
Most polymerization reactions today are run open loop with respect to the product quality (end-use) properties. Also operations involved in the manufacturing process are scheduled by a simple timer without attention to the actual progress of reaction.
In the last decade, the affordability of powerful computers finally made it possible to exploit the advanced control concepts control theorists have been developing since the 1960's. As a result, control of continuous processes like refinery distillation columns or power generation units has seen a rapid evolution from single loop proportional, integral and differential (PID) controllers to multivariable predictive controllers with built-in constraint optimization whose performance cannot be matched by the old PID solutions.
For a number of reasons, this progress so far has avoided batch processes. Control wise, most batches are still run the way they were thirty or more years ago. If there was a change, it affected control hardware, but not control algorithms. A batch recipe still prescribes time profiles of temperatures or pressures to be followed by a batch reactor in order to make the product. Feedback controllers, usually PID's, are routinely used to make the batch track the recipe in the presence of variations in feedstock concentration and purity, catalyst activity, reactor fouling and so on.
Maintaining batch recipe temperatures and pressures is important but it should not be the control objective. After all, the process owner does not sell batch temperatures or pressures. They are mere process parameters and, by themselves, are not even sufficient ones. It is well known and exemplified below for the case of polymerization processes, that two batches with perfectly identical temperature and pressure profiles can still have different rates at which monomer is converted into polymer, and thus yield products with inconsistent quality. When comes to the end-use parameters of the real product, which are determining its marketable quality, most batch processes are still run open loop, with all the negative consequences that an open loop recipe execution entails.
With the present invention, that approach is replaced with a feedback controller for polymerization processes that closes the loop using a measurement directly tied to the product's marketable quality, and thus employs feedback to eliminate quality variations and inconsistencies due to the fluctuations of process inputs and operating conditions.
The invention is a polymerization control that allows the user to specify independently the reaction mixture temperature and the degree of monomer conversion profiles as a function of time, and execute them under feedback control. This both improves the run-to-run consistency of the product and reduces the uncertainty of the reaction time and coolant consumption at any given instant. Because the coolant availability often is the limiting factor of production capacity, the improved predictability of individual batch runs offers an opportunity to improve batch planning and scheduling and thus increase the plant yield without expensive retrofits.
SUMMARY OF THE INVENTION
This invention enables the controller to employ feedback for the control of product properties without the need for specialty sensors to measure the properties and run the polymerization process on the basis of its inner time reflecting its actual progress. As a result, the invention makes it possible, first, to manufacture polymers with consistent quality and, second, to improve process yield by allowing for better utilization of the available cooling capacity without sacrificing process safety.
The invention includes an inferential sensor, whose concept is based on the observation that for polymerization processes, in which heat is released by a single reaction, the amount of heat released is proportional, albeit in a nonlinear way, to the degree of the monomer conversion. Hence, by carefully calculating the reactor's thermal balance on-line one can continuously infer the degree of conversion and use it for control. Once the actual degree of conversion can be determined and ultimately controlled, one can also control the cooling duty of the reactor and thus make it conform with the cooling capacity allotted to it by the plant scheduler.
Superficially, an advanced batch control system utilizing the inferential sensor looks very much the same as a conventional one. In both cases, measurements of temperatures and flows of the reactor coolant as it enters and leaves the reactor jacket or cooling coil will constitute the bulk of input data. In addition to that data, the inferential sensor may require additional data reporting temperatures at some other reactor spots and on the amounts and temperatures of feedstocks and catalysts. If some data on their composition are available, they can also be used with advantage for a more accurate inference.
The significant difference is in what the controllers do internally with the data. In a conventional batch controller, the data are used directly to control the reactor mixture temperature by manipulating the incoming coolant flow and temperature. In an advanced controller, the data are fed into the inferential sensor instead, where they are used to infer the current value of the degree of monomer conversion. This quantity is then passed to the controller part of the advanced batch controller.
Even though the inferential sensor could be implemented as a stand-alone device and thus resemble physical sensors, this option is unlikely. The reason is that the sensor involves a nonlinear dynamic model of both the process and the reactor, whose state must be kept in sync or coordinated with reality using a state estimation algorithm driven by the measured temperatures and flows. Once the model is available, it is shared with the advantage of a model-based (nonlinear) controller.
Polymerization reactions are exothermic (i.e., a chemical change in which there is a liberation of heat, such as combustion). The overall amount of heat released by a reaction from its start up to a given instant depends on how much of the monomer(s) has been converted into polymer. This measure of released heat indicating the degree of monomer conversion is a more reliable indicator of reaction progress than physical time because the same reaction can be running slower or faster depending on the initiator (i.e., catalyst) activity, reactant purity and other effects that may be difficult to measure directly. Moreover, for many polymerization reactions the degree of conversion is linked to the product quality and thus can be used for closing the loop for the product quality feedback control in place of specialty sensors.
The degree of conversion is not measured directly, but the invention involves inferring its running value by dynamically evaluating the reactor heat balance. This invention involves four concepts. First, there is the way of inferring the degree of conversion from the dynamic evaluation of the reactor heat balance. Second, the use of the degree of conversion replaces specialty sensors for feedback control with respect to the product quality (end-use) properties. Third, the use of the degree of conversion replaces physical time for the timing of process related operations like valve opening and closing, controlling the heat supply/removal, dosing the reactants, and so forth. Fourth, the sensor allows an accurate prediction of the batch evolution and thus makes it possible to accurately predict the cooling need profile from the current instant. out to the batch termination.
In this invention, the reaction mixture temperature and the integral heat ra

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