Rheological Measurements in the Control of Modern Polymerisation Processes

Alan George Porpoise Rheometers Ltd (http://www.porpoise.co.uk/)

Introduction Today the mainstream Control of Polymerisation focuses on the reactor conditions, through measurements of temperature, pressure, flow rate and gas composition measurements, using well developed sensors. In the constantly developing scene, advanced Computer Control and Catalysts have made important contributions to the control of the polymer structure. All of these activities have greatly improved the products but there remains almost untouched the opportunity to provide the means of controlling the polymer chain structure by direct measurement of the polymer itself. ICI, one of the major pioneers of polymerisation, had this advanced vision, which resulted in the equipping of two reactions, LDPE and PP, with feedback control, governed by rheological measurements. Additionally safety initiatives such as Hazop, Intrinsic Safety, etc. lead the way to the present norm which is to produce in a safe and cost effective manner. Much of this legacy, in particular the wide knowledge of polymerisation projects, process measurement and control, forms roots of the Porpoise capability. Control and rheology Polymer structure can be can be closely defined by measurements that represent molecular properties, namely average molecular weight and molecular weight distribution. When this information is linked back to the process through various control mechanisms, there is the means of adjusting and optimising the process and the resulting product. In turn, this changes the process to one of increased certainty. More so, this mode of control, loosely described as feedback control, avoids the major bottleneck of the polymer industry, which crudely put can only make a selection of products that have already been made. Several major improvements accompany the implementation of feedback control, which include understanding of process modelling, gas analysis (leading to a better understanding and control of impurities), catalyst performance (leading to control of molecular structure and above all the subtle connections between rheology measurements and structure) etc. The products so made, exhibit very repeatable properties and because they can be specifically made to recipe, the processes, in turn, become more efficient. In the past few years some polymerisation processes have been improved by calculation of polymer properties, using sophisticated modelling. This approach has proved to be beneficial, particularly when accompanied by improvements brought about by better instrumentation and computerisation. But this has only reinforced the larger opportunity. The fact remains that there is still no widespread adoption of automated, full-time calibrated rheometers to provide the reliable base data for the computer models, to directly verify reaction product and to certify finished product (in real time). The Porpoise scheme The prime objectives are to enable the polymer producer to improve polymer quality and the increase plant efficiency. To satisfy the objectives in full it is necessary to bring the new philosophy of measurement, primarily for control purposes. For control of all of the reaction stages in each process and for the control of the final product selection.

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This can entail the provision of more than one measurement station, as many processes have multiple reaction stages. For instance the copolymer production of PP can have two main reactors that sequentially feed into one extruder. The extruder is a reactor, when it is being used as a peroxide vis-breaking unit. The minimum complement of rheometers is therefore three. A second example is the LDPE process. This is far simpler than the PP example and can be fully controlled by one rheometer. In this case both Reaction Control and QC can be embraced by the same rheometer, when suitably placed. The schematic of a single reactor PP plant shows two rheometers. These are sited in the laboratory. This scheme more than satisfies the requirements of sampling and measurement delay for precise control of the principal reactor, the main extruder and the finished product batches. The main contention of the scheme is that the in-process measurement functions are to be satisfied by their at-line counterparts, which must sited in the laboratory. In this case the reactor sample, which is in powder form, is extracted from the transfer line immediately after the main reactor. It is degassed and deactivated prior to re-melting in the rheometer. The finished product, in pellet form, is extracted immediately after the de-watering stage, post extruder. The finished product sample serves for both the extruder and the finished product In both cases the samples are transported to the laboratory. In the case of the pellet sample, it is also conveniently supplied via the central distribution equipment that also feeds the film and additive analyser equipment. It should be noted that measurement for Control purposes places significantly higher demands on precision and accuracy than measurement for QC purposes. Except in special circumstances, this effectively rules out any equipment operating outside the closely controlled environment of the laboratory and without full-time calibration. For the purposes of control it is mandatory that both the analyser and the sampler, as an integrated system, are more reliable than the plant. (The integrated system must have provision for redundancy, backup elements and means of repair). The capability of the system The rheological measurements made by the rheometer are Melt Index (and sometimes Polydispersity). A state of full-time calibration is maintained using the Porpoise Transfer Standard ™ system. This system allows the equipment to remain in calibration during plant transitions. Routine calibration cross checks, are done by a combination of off-line comparisons and infrequent cross-checks that are made during steady running. This results in a facility that complements the existing computer controls and provides valuable data for ongoing plant optimisation and product improvement. The system outlined makes the step change in measurement capability, required for this significant process improvement. The rheometer is to be supplied with build verification certification (in association with SGS, who operate globally, through their UK offices). Additionally the all-important ongoing full time calibration is to be certified by SGS. This ensures third party quality assurance to the entire polymer production. These two initiatives are seen as the necessary steps to meet the Best Practice standards now being operated by the leading polymer producers. Porpoise have recently transferred their manufacture of rheometers to a third party in order to focus on the control opportunity and will continue under a new name.

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