Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Effecting a change in a polymerization process in response...
Reexamination Certificate
2000-11-03
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, C526S064000, C526S065000, C526S905000
Reexamination Certificate
active
06723804
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
FIELD OF THE INVENTION
This invention relates to the use of Raman spectrometry in processes for polymerizing olefins and in methods of monitoring and controlling olefin polymerization processes, reactants and other components. More particularly, a Raman fiber optic probe may be placed in an olefin polymerization reactor, or before or after such a reactor, for Raman spectrometry analysis. The present processes and apparatus may employ low resolution Raman spectrometry and measurement of liquid phase and/or gas phase components of an olefin polymerization process. The present processes and apparatus allow for quantitatively monitoring the olefin polymerization process in situ and constitute an improvement over gas chromatographic analysis conventionally employed in monitoring olefin polymerization reactions.
BACKGROUND OF THE INVENTION
Olefin monomers, such as ethylene and propylene, can be polymerized to form polyolefins. For example, ethylene or propylene may be homopolymerized to form polyethylene and polypropylene, respectively, or they may be copolymerized together or with higher 1-olefins such as 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene and others. Processes for manufacturing polyolefins typically employ catalyst systems comprising one or more catalytic metal compounds, typically transition metals, perhaps together with a co-catalyst and/or a support such as alumina or silica. Different types of olefin polymerization processes are available. For example, olefins may be polymerized in homogeneous processes or in heterogeneous processes.
One type of polymerization process employs a slurry as the reaction mixture. In slurry polymerization processes, solid olefin polymers such as polypropylene, polyethylene and copolymers, are formed under polymerization conditions that include a slurry as the reaction mixture. The slurry comprises the solid olefin polymer particles suspended in a liquid diluent that is inert in the polymerization reaction and in which the polymer is insoluble under polymerization conditions. Typically, slurry polymerization processes are conducted in a relatively high-pressure continuous reactor, such as a loop reactor. Such reactors may be operated at pressures of about 600 psi, for example, and at temperatures of about 60 degrees C. to about 100 degrees C. In many situations, slurry polymerization processes are relatively more commercially desirable than other polymerization processes.
In the slurry polymerization process, components such as one or more monomers, a diluent, and a catalyst system and possibly other reactants (such as, for example, comonomers or hydrogen) are introduced to the polymerization reactor to form a reaction mixture. The reaction mixture is maintained under polymerization conditions for formation of polyolefin. After a suitable period, the slurry or a portion thereof is discharged from the reactor through a product take-off line into a settling leg. The solid polyolefin settles out from the slurry, leaving a clear liquid comprising diluent and reactants such as the monomer. The clear liquid and solid polyolefin then may then re-mix as they are transferred to one or more separation chambers or flash tanks where, for example, they are flashed to a low pressure such as about 15 or 20 psi. Some slurry loop polymerization equipment includes both a high pressure flash tank and a low pressure flash tank. Further information and details of slurry polymerization processes and loop reactors, including examples of suitable reaction conditions as well as control schemes for other important variables, such as solids concentration and production rate, can be found in U.S. Pat. No. 3,998,995 and U.S. Pat. No. 3,257,363, which are incorporated herein by reference.
It is desired to monitor and control the polymerization reaction so that one may obtain polyolefins having particular properties. Obtaining particular properties in a polyolefin may be done by control of the component concentrations or ratios during the polymerization process. Small changes in components can affect the properties of the final polyolefin product. Control of the concentration of olefin monomer, and if present, comonomer and hydrogen, is required to ensure reliable finished polyolefin product properties. Other important control parameters may include the degree of polymerization, molecular weight, or size of the polymer chain. Therefore, it is desirable to monitor the olefin polymerization process by determining monomer content and, when one or more co-monomers are present, by determining co-monomer content(s). It may also be desirable to determine diluent content and product content.
Current methods of monitoring slurry polymerization processes and the components (reactants, products, and diluent) in such processes are less than optimal for several reasons. In loop polymerization processes, monomer and co-monomer content have typically been determined by gas chromatography (“GC”) analysis of the flash gas, that is, the gas released at one of the flash tanks where pressure is released. For example, U.S. Pat. No. 3,257,363 discloses methods of controlling the composition of the reaction mixture in a loop polymerization reactor wherein a gas chromatographic analyzer may be used to determine the amounts of ethylene and 1-butene reactants from a polymer-free off-gas line or with a sample stream from anywhere in the reaction system.
However, monitoring of the olefin polymerization process by analysis of the flash gas is less than optimal for several reasons. One reason is the amount of time for such analysis. If an analysis takes too much time, it generally has less value for monitoring, controlling or adjusting the olefin polymerization process. Also, another concern arises when the polymerization equipment includes more than one flash chamber, such as a high pressure flash chamber and a low pressure flash chamber. In such instances, gas chromatographic analysis of flash gas takes more time and is potentially less accurate when both high pressure and low pressure flash tanks are in operation.
While the contents of the olefin polymerization reactor may be determined by removing a small sample for analytical testing in a remote laboratory, this is less favorable than monitoring in situ. It may be dangerous and difficult to remove a sample from a hot process stream, and there are risks that the sample may not be representative of the overall reactor contents or that removing the sample may alter the sample. Sampling is time-consuming, and delay may cause the sample not to be representative of the reactor contents. A significant amount of material may be produced in the time required to remove, prepare, and analyze a sample. The analytical data obtained from the delayed sample is therefore of limited value for proactive process control. Furthermore, additional processing of the extracted sample may be required yet is undesirable.
A preferred method of monitoring the polymerization process would monitor the process as it happens, or as soon thereafter as practical. It is also preferable that an analysis method be performed in situ, as opposed to being performed on samples removed from the polymerization equipment. An in situ method would reduce the need to remove samples from the production environment, improve safety, and yield faster measurements. However, there are obstacles to providing in situ on-line chemical information in a process environment. The analytical method must be sufficiently accurate and precise under hostile physical and chemical conditions. The analytical method must be capable of remote detection and analysis.
Analyses of slurry olefin polymerization processes in situ, that is, within a slurry loop polymerization reactor or associated equipment, have been difficult if not impossible to do, since such olefin polymerization reactions are carried out at high pressures. However, spectrophotometric apparatus such as a spec
Chevron Phillips Chemical Company LP
Teskin Fred
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