This invention relates to the production of polymers such as polyvinyl chloride (PVC) and more particularly to the optimization of the process for producing polymers and improving the quality of the polymer produced by that process.
Description of the Prior Art
PVC is one of the oldest polymers and the second largest thermoplastics in terms of volume manufactured in the world. This widespread use arises from PVC's high degree of chemical resistance and its truly unique ability to be mixed with additives to give a large number of reproducible compounds having a wide range of physical, chemical, and biological properties. This makes PVC a versatile choice over other plastic materials.
More than 75% of the world's PVC resins are produced by the batchwise aqueous suspension precipitation polymerization process. Due to the number of variables involved, such as the amount of monomer charged, monomer impurities, initiator charge and properties, temperature control profile, etc., the process is extremely complex and it is difficult to achieve an optimum operation of the process.
The suspension polymerization process uses a reactor which includes an agitator to facilitate improved monomer/water dispersion. Most current reactors are water-jacketed and lined with glass or stainless steel to minimize polymer buildup on the walls. Typically, a reflux condenser is also used within the process to assist with the removal of heat generated from the highly exothermic polymerization reactions. The process flowsheet of a typical batch suspension PVC reactor 1 is shown in
A vinyl chloride monomer (VCM) which includes recovered vinyl chloride monomer (RVCM) 2 is used in the process. The VCM and included RVCM 2 are first finely dispersed in process water 13 by vigorous agitation using agitator 3. A small amount of primary and/or secondary suspension agents or dispersants 4 such as partially saponified polyvinyl alcohol (PVA) or polyvinyl acetates, are added to control coalescence of the growing grains as a protective coating of polymer is eventually formed. Viscosity changes can be managed with conversion and also injection water, ensuring effective heat transfer to the reactor walls; however, this becomes less important for systems with reflux condensers 5 (since this is where 80% of heat removal occurs).
Polymerization is induced by the addition of oil- or monomer-soluble initiators 6 used either alone or in combination with each other. Materials such as those coming from the diacyl peroxide, peroxydicarbonate, azo initiator or alkyl peroxyester groups are initiators commonly employed in suspension or mass polymerization of VCM. Initiators may also be added by batch or while not so common today at a controlled rate during the polymerization process. The reaction takes place in the coalesced monomer droplets. The reactor's contents are heated to the required temperature by either steam or hot water 7. Once the initiator(s) 6 begin to decompose into free radicals, polymerization commences. The heat of polymerization is transferred from the monomer droplets to the aqueous phase and then to the reactor wall, which is cooled by water 8 flowing through the reactor's jacket.
The reactor design includes a cooling jacket 9 which may or may not provide the means for all heat removal. If the reactor 1 includes a reflux condenser 5, it is typically provided as an upper extension to the reactor for condensing monomer vapor generated in the reactor and refluxing the condensed monomer back into the reactor. The reflux condenser 5 will remove most of the heat. If only a jacket 9 is used, chilled water 8 will normally be used in the jacket 9 unless the cooling jacket 9 is very efficient.
When the free liquid monomer has been consumed, the pressure in the reactor 1 begins to fall as a result of free monomer being consumed in the liquid phase and increased monomer mass transfer from the vapor phase to the polymer phase due to a sub-saturation condition. In industrial PVC production, the reaction is usually stopped when the pressure drops a certain amount. Since PVC is mostly insoluble in its own monomer, once the polymer chains are first generated, they precipitate immediately to form two separate phases in the polymerization droplet (the polymer and an entrapped monomer phase). Reactions continue in both the free liquid monomer phase and the entrapped monomer phase dispersed about the formed polymer. When polymerization is complete, the polymer is in the form of a colloid consisting of spherical particles dispersed in water. If the polymerization conditions are properly chosen through the course of the batch, a polymer having extremely narrow particle-size distributions can be obtained.
Suspension polymerization can be carried to 84% to 88% conversion, under proper pressure and temperature by using oil-soluble initiators. The final conversion determines the finished polymer properties. The reaction temperature is used for molecular weight control. Sometimes, a chain transfer agent may be added to control molecular weight in the free radical polymerization. Polymerization inhibitors may also be used in this system for control of polymerization reactions, kill agents if needed in highly unusual circumstances to immediately stop the reaction and end stop agents at the end of the batch to bring the polymerization reactions to a controlled stop. Typical polymerization times can vary between 3.5 to 6 hours, depending on the molecular weight of the polymer resin being prepared, as well as the heat-removal capacity of the reactor system. After completion of the batch, the mixture (polymer slurry) 11 is transferred to a blow-down vessel (not shown) where unreacted vinyl chloride is recovered. The PVC slurry 11 is then stripped, dried, and stored.
The prior art has dealt mostly with the real-time control of certain parameters within the PVC polymerization process. For example, U.S. Pat. No. 6,106,785 and U.S. Pat. No. 6,440,374 each describe a batch polymerization process controller that uses inferential sensing to determine the integral reaction heat. The integral reaction heat is used to estimate the degree of polymerization which has occurred in the batch reactor. The integral reaction heat can be used in either a feedback mode where it is the direct controlled variable or a feedforward mode where another variable such as reaction temperature is the direct controlled variable. In whatever mode used, the reaction heat tends to be a poor measurement of the degree of polymerization since heats of reaction vary depending upon chain length, the degree of cross-linking and the amount of heat holdup within the reaction vessel which is also affected by heat transfer resistances to the jacket and reflux condenser. Therefore the prior art suffers since it does not provide an ability to backward correlate the degree of polymerization to these other parameters.
Furthermore, the prior art is focused upon maintaining or regulating a particular “desired” value assigned “a priori” to either the integral reaction heat or reaction temperature without focusing upon a better determination of an improved control target of these values based upon a multiple number of other factors. Such factors that can affect the “desired” values include the amount and impurities of monomer charged to the reactor; the amount, time and activity of initiator(s) charged to the reactor; the amount and impurities of water charged to the reactor; the heat exchange coefficients for the jacket and reflux condenser; the remaining time to batch completion; etc. In fact, all parameters will affect the desired temperature target not only for instantaneous control of the reactor but how to best control the reactor over the remaining time of the batch.
In contrast to the polymerization process controller described in the prior art it is desirable to optimally determine all factors affecting the finished polymer prior to initiating the batch, using these optimized parameters in setting up and starting the batch, in an on-line procedure for correcting assumptions made in the optimal determination based upon measurement responses from batch startup, and in an on-line procedure that periodically executes to determine and adapt reactor temperature control profiles across the remaining life of the batch (also estimated by the procedure) to achieve the desired polymer properties and optimal polymer yield. The present invention meets these requirements.
In a polymer plant a method that comprises:
In a polymer plant a method that comprises:
In a polymer plant a method that comprises:
A polymer plant that comprises:
a and 7b show a flowchart for the on-line optimization of the on-line phase of the present invention.
Referring now to
In pre-batch off-line phase 12 the off-line reaction optimizer is executed to determine how to load the reactor 18 shown symbolically in
The reactor 18 is loaded using the recommendations of the off-line phase 12. The reactor 18 is started after it is loaded and controlled at the temperature profile provided by the off-line phase 12.
The technique then enters the on-line phase 14 where on-line dynamic reconciliation and parameter estimation 14a is performed. Up to this point in the technique assumptions on model parameters, efficiencies of initiator(s), VCM and water impurities, etc. have been made in the recipe. On-line dynamic reconciliation and parameter estimation 14a is used to correct for errors in these assumptions. Since the present invention is concerned with dynamic reconciliation and parameter estimation the corrections are performed by 14a only after the process has run for some time collecting measurements from its start, for example, fifteen minutes or one half hour. The technique of the present invention can perform this dynamic correction either only once or on scheduled cycles. Measurements of the PVC reaction process in operation are taken and are used in the on-line dynamic reconciliation and parameter estimation 14a.
The end results of the dynamic reconciliation and parameter estimation is reconciled plant data 14b and a tuned on-line model 14c. That model is used in an on-line optimization 14d of the process as for example to check for and control run away temperatures in reactor 18 and determine the end time of the batch. The on-line optimization may be performed one time or may be periodically scheduled over the course of the batch.
Once the batch is complete the technique 10 enters the post-batch off-line phase 16 where the reactor cleaning optimizer 16a is executed to determine if any of the equipment used with the batch such as reactor 18 should or should not be cleaned. If optimizer 16a determines that the reactor 18 should not be cleaned then the tuned on-line model 14c is transferred to the pre-batch model 12c to become that model for the next batch to be made in reactor 18 and the heat exchange coefficient for reactor 18 calculated during on-line phase 14 is used for the next batch. If optimizer 16a determines that the reactor 18 should be cleaned then the tuned on-line model 14c is transferred to the pre-batch model 12c to become that model for the next batch to be made in reactor 18 and the clean heat exchange coefficient for reactor 18 is used for the next batch. Thus the technique 10 will use the tuned on-line model 14c for the prior batch as the pre-batch model 12c for the next batch as long as that model is available.
Referring now to
The phase then proceeds in 24 to identify the:
Phase 12 then proceeds to 26 where it executes the optimization of the pre-batch model. The optimization results are then in 28 sent to the operator for inspection. If in 30 the optimization results are rejected the existing recipe is used in 32. If in 30 the optimization results are accepted an updated recipe is used in 34. The operator may accept the optimization results if based on experience the results seem reasonable or the results may be automatically rejected in the event of a failure code from the optimizer such as an over-constrained problem.
After the recipe is selected the batch is started in 36 and the technique enters the on-line phase 14.
Referring now to
Referring now to
After the batch is characterized in 44 the technique proceeds to decision 46 where it determines if the operator has the option to either validate the results of the characterization or input different results. If the operator does not have the option the technique proceeds to 48 and then to 50 where the model parameters are updated.
If decision 46 determines that the operator has the option to validate the results or input different results the technique proceeds to 52 where the optimization results are sent to the operator for inspection and then to 54 for operator entry and then to decision 56 to determine if the operator does or does not accept the results. As described above if the operator accepts the optimization results then the model parameters are updated at 50. If the operator does not accept the optimization results then the model parameters are not updated. In either case the technique for the on-line phase proceeds to the on-line optimization 14d.
Referring now to
Referring now to
If the answer to decision 62 is yes, the flowchart 60 proceeds to 68 where the post estimation state variables are identified and then to 70 which represents the function of block 14d of
If the operator does not have the option, the flowchart 60 proceeds to 76 where the control targets are updated. If the operator has the option, the results of the on-line optimization are at 78 sent to the operator for inspection and at 80 the operator makes an entry to either accept or reject the results.
The flowchart then proceeds to decision 74 where it is determined if the operator has or has not accepted the results of the on-line optimization and as described above to 76 where the control targets are updated if the operator has accepted the results of the optimization. If 74 determines that the operator has not accepted the results of the optimization the flowchart 60 proceeds to 82 in
Referring now to
As was described above in connection with
Referring now to
While the present invention has been described in connection with the suspension batch production of PVC it should be appreciated that it can be used in other types of batch production of PVC as well as batch production of other polymers. While the present invention is described above in the context of a single batch it should be appreciated that the off-line optimization 12d, the on-line optimization 14d and the reactor cleaning optimization 16a all of
It is to be understood that the description of the preferred embodiment(s) is (are) intended to be only illustrative, rather than exhaustive, of the present invention. Those of ordinary skill will be able to make certain additions, deletions, and/or modifications to the embodiment(s) of the disclosed subject matter without departing from the spirit of the invention or its scope, as defined by the appended claims.