Patent Publication Number: US-2005124770-A1

Title: Process for the production of polymers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a continuation-in-part of U.S. patent application Ser. No. 10/731,729 filed Dec. 9, 2003. 
    
    
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates a process for preparing polymers. The present invention particularly relates to a process for preparing polymers employing on-line analyzers.  
      2. Background of the Art  
      Laboratory analysis of production samples during the production of chemical products, and especially when those products are polymers, may be a necessary part of the production process. The purposes for these analyses may include process control and to ensure product quality, especially during transitions between product grades.  
      Such chemical analyses may be off-line or on-line. An off-line analysis may be accomplished by taking a sample of a process stream and then subjecting it to a laboratory analysis. An on-line analysis may be accomplished by conducting a portion of the process stream directly to a chemical process analyzer or other device that may be inserted directly into the process stream. For example, a pH probe can be inserted directly into a process stream to do real time on-line pH determinations in contrast to taking process stream samples and doing off-line acid/base titrations or pH paper determinations. On-line chemical process analyzers may offer significant advantages in reducing sample analysis time, which may in turn improve product quality and reduce costs by means of reducing waste in the form of off specification product or prevent premature maintenance which has both direct and indirect costs to chemical manufacturers.  
     SUMMARY OF THE INVENTION  
      In one aspect, the present invention is a process for preparing a polymer. The process includes preparing a polymer using a process including at least one process stream. The process stream is a dispersed phase fluid including polymer particles, and the process further includes contacting at least a portion of the process stream with a sensor probe connected to a turbidity meter and passing light from a light source through the sensor probe and into the turbidity meter. The interaction of the process stream and the light passing through the senor probe is measured and used to define a value for the turbidity of the process stream. The value for the turbidity of the process stream is a component of an algorithm used to determine in real time the polymer particle size or the concentration of polymer particles in the process stream to monitor, control, or monitor and control the process for preparing a polymer.  
      In another aspect, the present invention is a process for preparing a polystyrene polymer. The process includes preparing a polystyrene polymer using a process including at least one process stream. The process stream is a dispersed phase fluid including polystyrene polymer particles, and the process further includes contacting at least a portion of the process stream with a sensor probe connected to a turbidity meter and passing light from a light source through the sensor probe and into the turbidity meter. The interaction of the process stream and the light passing through the senor probe is measured and used to define a value for the turbidity of the process stream. The value for the turbidity of the process stream is a component of an algorithm used to determine in real time the polystyrene polymer particle size or the concentration of polystyrene polymer particles in the process stream to monitor, control, or monitor and control the process for preparing a polymer.  
      In still another aspect, the present invention is a process for preparing a polyethylene polymer. The process includes preparing a polyethylene polymer using a process including at least one process stream. The process stream is a dispersed phase fluid including polyethylene polymer particles, and the process further includes contacting at least a portion of the process stream with a sensor probe connected to a turbidity meter and passing light from a light source through the sensor probe and into the turbidity meter. The interaction of the process stream and the light passing through the senor probe is measured and used to define a value for the turbidity of the process stream. The value for the turbidity of the process stream is a component of an algorithm used to determine in real time the polyethylene polymer particle size or the concentration of polyethylene polymer particles in the process stream to monitor, control, or monitor and control the process for preparing a polymer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a detailed understanding and better appreciation of the present invention, reference should be made to the following detailed description of the invention and the preferred embodiments, taken in conjunction with the accompanying drawings, wherein:  
       FIG. 1  is a graph showing the predicted and actual particle size in a process stream from an impact modified polystyrene unit for Example 1;  
       FIG. 2  is a graph showing the variance between calculated and observed particle size for Example 1; and  
       FIG. 3  is a graph showing the predicted particle size plotted against actual particle size for polyethylene for Hypothetical Example 2. 
    
    
     DETAILED DESCRIPTION OF INVENTION  
      In one embodiment, the invention is a process for preparing a polymer. The polymer may be any that may be prepared using a process that includes a process stream that is a dispersed phase stream including particles of the polymer. Such polymers include, but are not limited to, polyethylene, polypropylene, and polystyrene. Copolymers of polyethylene, polypropylene, and polystyrene may also be examples of such polymers.  
      An embodiment of the invention is a process for preparing a polymer. The process includes preparing a polymer using a process including at least one process stream where the process stream, or at least a portion of the process stream, is contacted with a sensor probe connected to a turbidity meter and passing light from a light source through the sensor probe and into the turbidity meter. Turbidity meters may be used to determine the quantity of small particles suspended in a fluid by measuring the lateral scattering of light produced by suspended particles when a train of light is interacted with a fluid sample. As a beam of light interacts with the fluid sample, the light can be dispersed upon striking particles within the fluid. The intensity of the beam diminishes as the beam interacts with the fluid. Under these conditions, the fluid turbidity is proportional to the ratio of the scattered light to the transmitted light. As the turbidity increases, more light will be scattered and the ratio will increase.  
      A turbidity meter is used to monitor, control, or monitor and control the production of a polymer by contacting a process stream with a sensor probe connected to turbidity meter. The sensor probes useful with the process include any that would be useful for monitoring the turbidity in the process stream. For example, the probe may be selected from the group consisting of transmittance probes, reflectance probes, attenuated reflectance probes, and the like. In one embodiment, the probe used with the process of the present invention is a reflectance probe. In another embodiment, the probe is a double bounce reflectance probe.  
      The sensor probe is contacted with at least a portion of the process stream. In one embodiment, the sensor probe may be installed directly into the conduit through which the process stream is moving. In another embodiment, a sample stream from the process stream, sometimes referred to in the art as a side stream, may be contacted with the sensor probe. In yet another embodiment, a continuous sampling and dilution system such as that disclosed in U.S. Pat. No. 5,907,108 to Garcia-Rubio, et al., which is incorporated herein in its entirety by reference, may be used with the process.  
      In one embodiment, a turbidity meter may be connected to a probe using a fiber optic cable. The probe may be also optically connected to a light source. In one embodiment, the turbidity meter may be local and in another embodiment, the turbidity meter may be a remote turbidity meter. The fiber optic cable may actually be a pair of fiber optic cables wherein a first cable may be used to supply light to the subject probe and the second cable may be used to bring the light passing through the probe to the turbidity meter. In one embodiment, the optical cables are a bundle of multimode fibers allowing the transmission of light in both directions on a single cable.  
      The process includes a light source capable of producing light useful in measuring turbidity. Any light source capable of producing such light may be used. Acousto-optic tunable filters may be used to allow the source to be tuned repeatedly to precise wavelengths reliably over long periods of time. Conventionally tuned sources may also be used so long as they have the capability of working in conjunction with the meter to make fast, accurate measurements. The source may be stand alone or it may be a part of an integrated combination of meter and source. The turbidity meters useful with present invention may be automated and have computers integrated therewith to perform some or all of the processes of interpreting results and making calculation therewith.  
      Any turbidity meter that may make measurements with sufficient precision and reliability to be useful in monitoring, controlling, or monitoring and controlling a polymer production process may be used with the process. For example, METTLER TOLEDO sells several types of such turbidity meters. These meters may be of the back-scattering type or of the forward scattering type.  
      The process may also be used with more than one probe in the same or different process streams having at least one characteristic of interest. One advantage of using a remote meter is that it makes it possible to have multiple probes from different parts of a polymer production facility, but only one meter to maintain. Another advantage of having a remote meter is that it allows such maintenance to be performed in a remote and possibly less hostile environment. In one embodiment, there are two probes in the same process stream connected to the spectrophotometer. In another embodiment, there are two probes used with the process, but the probes are in different process streams. Use of fiber optic cables allows for the separation of probes and the turbidity meter optics to a range of 150 meters or more.  
      A process stream can be any part of a polymer production process wherein a liquid stream including polymer particles is passing through a conduit. For example, the process stream can be in a pipe, a reactor, or be an overhead stream from a distillation column. In one embodiment of the process, a sensor probe is contacted with at least a portion of a process stream. As the process stream move past the sensor probe, the polymer particles in the process stream interact with the probe scatter light. This scattering of light results in an intensity differential between the light entering the probe and the light exiting the probe and allows for the determination of the turbidity of the process stream.  
      In one embodiment, the process for producing polymers is used to prepare high density polyethylene produced in an industrial high density polyethylene slurry loop reactor. This embodiment of the process includes using the turbidity value of a process stream to control or monitor or control and monitor a production unit. This value may be included in an algorithm that is in a manual calculation process or, in another embodiment, in a manual spreadsheet, or in still another embodiment, it may be incorporated into the logic circuits of a controller. In one embodiment, the controller is a neural net or other artificial intelligence (AI) controller.  
      The theoretical relationship between turbidity, particle size, and particle concentration may be used to formulate the algorithm. When two variables are known, the third can be calculated. Thus, when the concentration of polymer particles and turbidity is known, then the polymer particle size can be determined and used to control, monitor, or control and monitor a polymer process. Similarly, if the polymer particle size and turbidity is known, then the polymer particle concentration can be determined and used to control a process.  
      Rather than depending upon the theoretical relationship between turbidity, particle size, and particle concentration, one embodiment of the process refines the algorithm by correlation using historical data or theoretical versus observed data. This can be useful when there are process variables that may influence turbidity.  
      Many operations in a polymer chemical process may be routinely controlled using a Proportional Integral Derivative (PID) controller. The algorithm including the value for turbidity may be incorporated or programmed into such controllers. In another embodiment, many PID controllers are used in conjunction with a second controller that may receive data from the PID controller and then reprogram the PID controller based upon the algorithm including the turbidity value. An AI controller capable of accepting multiple inputs and sending multiple outputs may also be used with process. For example, one such controller is a controller using Process Perfecter® software developed by Pavilion Technologies.  
      The use of automated controllers with the process may be useful for controlling the polymer production process, but the use of the present invention manually should not be discounted. When incorporated into a spreadsheet, the process may be very useful, particularly when changing polymer grades or production rates. In either case, the process may be used, for example, to optimize and control a loop reactor to produce polyethylene or polystyrene, polypropylene in real time.  
      The term “real time” means immediate and without substantial delay. Real time, in embodiments of the process also means a period of time sufficiently brief to allow for meaningful changes to operating parameters during the production of a polymer. For example, in a conventional process, a test sample from a process stream could be collected and then taken to a laboratory. The sample would then be tested for turbidity and the results then reported to the production unit. The delay due to sample collection, transportation and conventional testing can be minutes but is often hours. Since many polymer units produce polymers at a very fast rate, an hour of production of off specification material could represent tons or even tens of tons of off specification material that may be unusable or, at the very least less valuable.  
     EXAMPLES  
      The following examples are provided to illustrate the present invention. The examples are not intended to limit the scope of the present invention and they should not be so interpreted. Amounts are in weight parts or weight percentages unless otherwise indicated.  
     Example 1  
      An impact modified polystyrene process is monitored using an embodiment of the process of the present invention.  
      A reflectance probe is installed into a process line in a pilot scale production unit at a point in the process where the process stream includes homopolystrene and a dispersed phase including rubber with grafted polystyrene particles (Impact Modified Polystyrene). A fiber optic cable runs from reflectance probe to a METTLER TOLEDO turbidity meter. Output from the turbidity meter is sent to a Distributive Control System from Foxboro. The concentration of the Impact Modified Polystyrene in the process stream is known. The process is controlled using the calculated particle size of the Impact Modified Polystyrene.  
      Samples of the Impact Modified Polystyrene corresponding to specific calculations are taken and tested in the laboratory for actual particle size. The method used for this includes the steps of dissolving the sample in methyl ethyl ketone and instrumentally determining particle size using an optical bench. The results from calculating particle size using the turbidity meter and the observed particle size from the laboratory results are displayed below in  FIG. 1 . The Variance between calculated and observed particle size in displayed below.  FIG. 2  as is the standard deviation of 6.6 percent.  
     Hypothetical Example 2  
      Known quantities of polyethylene fluff having known diameters are suspended in mineral oil. The samples of polyethylene are then tested for turbidity and the results used to calculate particle diameters. These results are then compared to the laboratory determined values for particle size. These results are shown below in the table and also displayed in  FIG. 3 .  
                               TABLE                       Particle   Measured       Predicted   Percent       Concentration   Diameter   Turbidity   Diameter   Deviation                                                    5   202   64   207   2.66       10   202   122   211   4.65       15   202   180   215   6.63       20   202   240   204   1.23       5   404   37   408   0.96       10   404   92   434   7.48       15   404   161   356   11.80       20   404   215   390   3.44       5   785   1.2   674   14.06       10   785   35   859   9.40       15   785   104.2   779   0.71       20   785   169   733   6.65                   Avg   5.81                  
 
 Comments Regarding the Examples 
 
      Example 1 shows that a polymer process may be controlled using particle size as determined using a turbidity meter. Hypothetical Example 2 is hypothetical in that no process was controlled, but the results showed that a polyethlene process can be controlled using a turbidity meter to monitor particle size and that variance between predicted particle size and laboratory measured particle may increase with particle size.