Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Polymers from only ethylenic monomers or processes of...
Reexamination Certificate
2000-04-26
2002-06-11
Wu, David W. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Polymers from only ethylenic monomers or processes of...
C526S065000, C526S067000, C526S348500, C526S348600, C525S053000, C525S054000
Reexamination Certificate
active
06403748
ABSTRACT:
TECHNICAL FIELD
This invention relates to optimizing properties of polymeric resins, particularly polyolefins, made in continuous production runs, including transitions from the manufacture of one polymer product to another. It includes methods and apparatus for utilizing maximum amounts of products having properties which vary within and/or outside a set of specifications.
BACKGROUND OF THE INVENTION
In many polymer manufacturing processes, off-grade resin may be produced when transitioning from one resin grade to another and even during steady state operation. This problem is especially noticeable in a gas phase polymerization because the large fluidized bed polymerization reactor behaves like a continuously stirred tank reactor. Once the reactor conditions are changed for the new grade of resin in a gas phase reactor, it may be necessary to produce as much as three times the bed weight of the resin before the new resin properties achieve an acceptable steady state. Because the product properties change slowly in large volume reactors, and depending on the similarities and differences between the two products, it is not uncommon for up to 15% of the total resin made in a reactor campaign to be off grade (out of specification in at least one respect), mainly due to the product made during the transition from one set of specifications to another. The resin produced during such transitions is usually sold at a significantly lower price than the resin that meets the specifications (sometimes called AIM grade resin herein). It is desirable to reduce the amount of off-grade resin made during transition periods between sets of specifications and at any other time.
In gas phase polymerization operations, it is common to pellet the off grade granular resin and then recycle these off grade pellets back into the pelleter at concentrations which will not cause deviation from specifications in AIM grade production. Repeated pelleting can impart an undesirable thermal history, and the additive content of the product must be continually recalculated and adjusted. In addition, the pelleter capacity is underutilized because a significant portion of the granular product is run through the pelleter more than once. Recycling pellets to the pelleter is undesirable.
Another problem in a gas phase polymerization line is the provision of adequate surge capacity between the reactor and pelleter. Adequate surge capacity is important to allow common pelleter problems to be fixed without having to kill or upset the reactor, which normally produces product more or less continuously. Sudden kills or upsets in the reactor due to the inability of the pelleter to handle the particulate product can result in off grade resin and lengthy restart procedures. In the present state of the art, the purge bin located between the reactor and the pelleter is frequently designed larger than required for simple purging in order to allow extra surge time for the pelleter.
Also in continuous polymerization production, as in a fluidized bed, it is common for one or more properties to vary from a target point but remain within a specification range. While virtually all consumers and users of resin want product within specifications needed for their particular purposes, many also are concerned with the consistency of specified values. Thus the products must not merely be within a specified range of values for one or more properties, but must consistently, from run to run, and from the beginning to the end of a run, be on, or very near to, the target value or target values for one or more properties. Producing final products having reduced variability will increase the process capability limit (CpK) of the process. Persons skilled in the art will realize that a change in a control (set) point to adjust one product property will affect the process in other ways, and that process adjustments of any kind may affect product consistency in more than one manner. Ideally, in spite of the complexity of the process, one will want to produce a product which is consistently on target in all respects.
It would be desirable to alleviate the necessity for excessively large purge bins, to develop a method for minimizing off-grade production, to reduce or eliminate the practice of recycling through the pelleter, and to produce a specification product which is consistently on target for various properties.
Our invention, will be discussed and described with particular reference to the widely used “Unipol” fluidized bed process for olefin polymerization. See, for example, the descriptions in Bernier et al U.S. Pat. No. 5,453,471, Miller U.S. Pat. No. 4,003,712, Aronson U.S. Pat. No. 4,621,952, and other related patents of Union Carbide Corporation. The applicability and value of the invention is not limited to the Unipol process, however; it may be used in any resin producing facility which makes particulate (sometimes herein used interchangeably with “granular”) product more or less continuously whether or not it is to be pelletized.
SUMMARY OF THE INVENTION
This invention provides a method for trimming, or blending, off grade granular material into AIM grade material while keeping the AIM grade material within specifications. The value of the invention is particularly high when trimming granular transition off grade into the leading and trailing products of a transition to reduce the total off grade product. The invention also provides a method for consistently producing resin which is not only within specification ranges but on target or very close to target for desired properties.
The apparatus used in this invention is a set of one or more bins, referred to herein as trim forward bins, located between the reactor and the pelleter, preferably after a purge bin. Resin from the reactor can be transferred to any of the trim forward bins, or be sent to a purge bin, or directly to the pelleter. Our invention, however, concerns the use of the trim bin or bins. Resin can be combined from any of the trim bins before it is sent to the pelleter. The system may comprise any number of bins and the bins may be of any size. An optimum number of bins and the size of the bins can be determined based on the anticipated product cycle of the reactor. Preferably the trim forward bins will be of a design or construction as to provide “plug flow”, meaning that the bin delivers granular material substantially in the sequence in which it was received. The interface between two somewhat different granular materials is substantially conical if the granular material enters at the center of the top of the bin and falls onto a bed of granular material already present. A conical interface between products of somewhat different characteristics, as between on-specification and off-specification polyethylene granules, will remain substantially conical as it descends in the bin. Gravity discharge from the center of the bottom of such a bin assures a substantially clear passage from one type of product to another, although there may be some mixing at the interface when it is first formed and as it emerges from the bottom of the bin. Sometimes “plug flow” in this context is used interchangeably with “mass flow”, and that is our intention in this description. The downward progress of an interface or boundary of two different types of granular products may be calculated and/or projected as discharge from the bin is monitored, and we utilize this fact in our invention. We do not intend to rule out the use of other types of trim bins, however, i.e. ones which may more significantly blur the boundary between one product and another, as there may be instances in the use of our invention where such a blurring will be well defined and/or of little consequence, and the progress of a blurred interface may be followed as well as a sharply delineated one.
Our process includes a method of enhancing the production of a first pelletized resin product having a first set of specifications made as granules in a substantially continuously operating reactor during a campaign including the sche
Jaen Jose Cavetano
Levin Jeff Scott
Morris Carlos Ray
Nelson Mark Louis
Powers Kim Dwight
Cheung William K
Union Carbide Chemicals & Plastics Technology Corporation
Wu David W.
LandOfFree
Enhancing production of resin withing specifications does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Enhancing production of resin withing specifications, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Enhancing production of resin withing specifications will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2908143