Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Effecting a change in a polymerization process in response...
Reexamination Certificate
2002-08-02
2004-04-20
Teskin, Fred (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Effecting a change in a polymerization process in response...
C526S059000, C526S135000, C526S235000, C526S901000, C526S905000
Reexamination Certificate
active
06723805
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the controlled production of high density polyethylene (HDPE) or of linear low density polyethylene (LLDPE) in one or more gas phase reactors, in the presence of either chromium or Ziegler-Natta catalysts, said process possessing an on-line control of certain process variables as well as the control of certain physical properties of the final resin product. More specifically, such process comprises: i) the use of models for inferring the physical properties as well as the process variables which are not measured continuously and ii) models which are relevant for the control of said properties and operating conditions of the process under study. The control of the process variables provides further the maximization of the production rate as well as of the catalyst yield in the polymerization reaction.
BACKGROUND INFORMATION
The control of the process variables in petrochemical plants is normally manually effected by operators who periodically sample the product to be tested and act to maintain or to correct the operating conditions as to obtain a product having the desired characteristics. This involves delays related to required corrections, since the sampling and laboratory tests normally lag the on-line process, besides possible human errors. Further, the dynamics of petrochemical processes is generally slow. Thus, long periods of time may be required so that the effect of the adjustments performed on the process input variables become effective. Therefore, the slow dynamics is a feature which renders difficult the control of the industrial unit since the worker may not know if there has been a sufficient time for the effects of the performed adjustments to be apparent or either if more time is required so that said effects are completely perceived.
On the other hand, techniques for the empirical as well as rigorous modeling are known, which may be used for obtaining process models. Such mathematical models are able to infer the value of certain process variables that are periodically measured from other process variables that are continuously measured. Besides, the mathematical models may also be used to predict the future behavior of process variables caused by modifications in the operating conditions of an industrial plant.
The techniques of rigorous modeling are based on the physical principles, which describe the basic interactions between the process variables. As compared to the empirical techniques, the rigorous models are more difficult to construct and require a deep knowledge of the process. Besides, the complexity of the equations, which make up the rigorous model, may render it, at least in some cases, unsuitable for on-line implementation, this difficulty arising from the long time to resolve such equations, even making use of computers.
On the contrary, the techniques of empirical modeling do not require such a deep knowledge of the process being modeled and originate simpler mathematical models which may be quickly executed, being therefore suitable for being executed in real time. A disadvantage of the empirical models is that they cannot be used under operating conditions different from those used in their identification. Models obtained from linear and non-linear regressions as well as neural networks are among the relevant empirical modeling techniques described in the literature.
Neural networks are networks of either neurons or elements which are interconnected in a unique way. Typically, the networks are made up of input neurons which receive signals or information from outside the network, output neurons which transmit signals or information to outside the network and at least one intermediate layer of neurons which receive and transmit the information to other neurons.
Besides the fact of a model being rigorous or empirical, it may also be characterized as static or dynamic. A model is said static when it yields as a result the steady state values of the process variables in view of the values assumed by the input variables of the system. On the other hand, the dynamic model presents, besides the information on the steady state of the system, information on the way by which the output variables move between two steady states.
A large number of references describe processes provided with control, for the production of polyolefins.
U.S. Pat. No. 3,636,326 teaches how to adjust the production rate of a polymerization reactor based on the catalyst yield calculated in real time. Thus, possible modifications in the catalyst yield may be automatically compensated by means of a feedback control loop. This kind of control may be practiced when the variables which affect the catalyst yield are not known or not measured. In this case the best to do is to automatically adjust the catalyst rate as soon as a variation in the catalyst yield is perceived. However, by using process models it is possible to preview that catalyst yield will undergo modifications due to certain changes in the operating conditions. Thus, it is possible to previously adjust the catalyst rate so that the change in catalyst yield does not significantly affect the production rate of the reactor.
This and other advantages to be mentioned hereinbelow are features of the controlled process to be described and claimed in the present application.
U.S. Pat. No. 3,998,995 teaches a process in which the production rate and monomer concentration are controlled in a polymerization reactor able to restrict the concentration of the main monomer and the solids concentration at maximum values. In said process, the production rate is controlled by the flow rate of olefinic monomer while the concentration of monomer in the reaction zone is controlled through the adjustment of a diluent feed fed to the process. If the maximum limiting value for the monomer concentration is reached, monomer flow rate is then adjusted to reduce said concentration and no longer to control production. If the maximum limiting value for the solids concentration is reached, the flow rate of diluent is adjusted to reduce said concentration and no longer to control the monomer concentration. Whenever the constraints are violated the goal of controlling production rate and monomer concentration is abandoned in favor of the continuity of operation. The control system which is unable to deal with constraints will not be able to guarantee the safety and the operational continuity required for an industrial unit to accomplish its goals. In spite of the fact that the said U.S. Patent presents a control procedure able to deal with the plant constraints, the system works only from the moment when the constraint has been effectively violated. A controller which could predict the future behavior of the process variables would be able to promote the required adjustments so that the constraint would not be violated or would be just slightly violated. A process whose constraints are so controlled, would allow that the main control objectives would not be abandoned.
A method for the control of the intrinsic viscosity of a polymer which is being produced has been taught in U.S. Pat. No. 3,878,379. The technology unveiled in said U.S. patent, besides being specifically directed to the production of polyethylene terephthalate, relates to one single variable and does not contemplate the control of different polymer properties.
U.S. Pat. No. 4,469,853 provides a process for the production of polyolefins in the liquid phase having a few well defined properties. Such process describes the use of chromatography for the measurement of the concentrations of olefin monomers and hydrogen in the gaseous phase, which is formed on the top of the reactor. The control of the flow rates of ethylene and comonomers is adjusted so as to keep constant the ratio between the concentrations of said reagents so as to lead to a polyolefin having predetermined density. Further, said U.S. patent achieves the control of the ratios between the concentrations of hydrogen and main monomer by adjusting the flo
Braganca Antonio Luiz Duarte
De Oliveira Artur Toledo Martins
Demoro Esdras Piraguacy
Birch & Stewart Kolasch & Birch, LLP
Braskem S.A.
Teskin Fred
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