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• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
  • C08F2/00—Processes of polymerisation
  • C08F2/002—Scale prevention in a polymerisation reactor or its auxiliary parts
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J19/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J19/0006—Controlling or regulating processes
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J19/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J19/0006—Controlling or regulating processes
  • B01J19/002—Avoiding undesirable reactions or side-effects, e.g. avoiding explosions, or improving the yield by suppressing side-reactions
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J19/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J19/18—Stationary reactors having moving elements inside
  • B01J19/1812—Tubular reactors
  • B01J19/1837—Loop-type reactors
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
  • C08F10/00—Homopolymers and copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond
  • C08F10/02—Ethene
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
  • C08F2/00—Processes of polymerisation
  • C08F2/12—Polymerisation in non-solvents
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J2208/00—Processes carried out in the presence of solid particles; Reactors therefor
  • B01J2208/00008—Controlling the process
  • B01J2208/00734—Controlling static charge
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J2219/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J2219/00049—Controlling or regulating processes
  • B01J2219/00191—Control algorithm
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J2219/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J2219/00049—Controlling or regulating processes
  • B01J2219/00245—Avoiding undesirable reactions or side-effects
  • B01J2219/00247—Fouling of the reactor or the process equipment
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J2219/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
  • B01J2219/00049—Controlling or regulating processes
  • B01J2219/00245—Avoiding undesirable reactions or side-effects
  • B01J2219/00254—Formation of unwanted polymer, such as "pop-corn"
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
  • C08F10/00—Homopolymers and copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
  • C08F110/00—Homopolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond
  • C08F110/02—Ethene
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
  • C08F2400/00—Characteristics for processes of polymerization
  • C08F2400/02—Control or adjustment of polymerization parameters

Definitions

• the present disclosure relates generally to the production of polymers and, more specifically, to controlling polymer particle size by varying catalyst particle size based on catalyst productivity.
• polyolefin polymers such as polyethylene, polypropylene, and their copolymers
• retail and pharmaceutical packaging such as juice and soda bottles
• household containers such as pails and boxes
• household items such as appliances, furniture, carpeting, and toys
• automobile components such as pipes, conduits, and various other consumer and industrial products.
• polyolefin construction is generally non-reactive with goods or products with which it is in contact as well as with the ambient environment. This property allows polyolefin products to be used in many residential, commercial, and industrial contexts, including food and beverage storage and transportation, consumer electronics, agriculture, shipping, and vehicular construction.
• the wide variety of residential, commercial and industrial uses for polyolefins has translated into a substantial demand for raw polyolefin, which can be extruded, injected, blown or otherwise formed into a final consumable product or component.
• the raw polyolefin is typically produced in bulk by petrochemical facilities, which have ready access to monomers, such as ethylene, that serve as the molecular building blocks of the polyolefins to be produced.
• monomers such as ethylene
• the polymerization reaction itself is exothermic, or heat- generating, and is typically performed in closed systems where temperature and pressure can be regulated to produce polyolefins having the desired properties.
• a polyolefin reactor may foul, such as when the polymerized product is formed on the reactor walls or when the product cannot be maintained as a slurry. Such a foul may result in a loss in heat transfer, such as due to a reduction in circulation or reduced efficiency at a heat exchanger interface, which may impair or completely negate the capacity to maintain the desired temperature within the reactor.
• a reactor foul may also result in a reduction in the circulation of the reactor contents and/or in a variation from the desired percent solids (measured by volume or by weight) of the reactor effluent.
• a reactor foul may result in deviations from the desired reaction conditions, the polymer product produced during such a reactor foul may not meet the desired specifications; that is, the product may be "off-spec."
• control of the reaction may be lost entirely, and the reactor may become plugged with polymer, requiring one to three weeks to clear, during which time the reactor may not be operated.
• a reactor foul may occur due to a variety of different factors, depending on the type of polymerization system and circumstances.
• the external indications that such a foul exists may include deviations from the set reaction temperature or increased demand on the coolant system to maintain the set temperature value.
• an increase in the temperature differential between the coolant inlet temperature and reactor temperature may be indicative of certain types of reactor fouls, such as those which interfere with the transfer of heat through the reactor walls.
• Another external indication of a foul may be an increased motor load as the pump attempts to maintain a velocity within the reactor sufficient to keep the polymer and catalyst particles suspended or attempts to compensate for restriction or obstruction of the flow path.
• a high pressure differential may be observed at the pump and may indicate the presence of some fouls.
• copolymer fouling may occur when the reactor temperature falls above the "fouling curve," describing the suitable reactor temperature ranges for producing polyolefins having a desired density.
• a deviation may result in swelling of the polymer particles and an increased tendency for the particles to agglomerate into larger particles, both which can increase the polymer volume in the reactor.
• the higher volume percent solids may result in a moving bed of polymer rather than a slurry, which decreases the circulation rate.
• the reactor circulating pump must work harder to propel the fluid and particles, resulting in a high motor load and a high pressure differential, i.e., ⁇ .
• a condition known as a "solids foul” may occur in which circulation of the reactants and product in the reactor is interrupted or degraded.
• solids foul a condition known as a "solids foul"
• large polymer particles may be formed which can plug continuous take-off valves or other outlet valves or conduits.
• the large polymer particles may also settle out of the slurry in the reactor, where they may restrict the flow of slurry.
• the large polymer particles increase volume percent solids in the reactor, increasing the flow resistance of the slurry and leading to a corresponding high motor load and a high ⁇ as the reactor circulating pump compensates for the increased resistance.
• fines An increase in fine particles of polymer, i.e., "fines," may also result in a form of fouling.
• an increased number of fines increases the viscosity of the slurry due to the corresponding increase in particulate surface area.
• the reactor circulating pump must work harder, resulting in a higher motor load and ⁇ . In these situations, if the pump is unable to compensate, heat transfer through the reactor walls may be impaired and/or polymer particles may settle out of the slurry.
• Static fouling is typically associated with polymer particles, fines, and/or catalysts being held to the reactor wall by static electricity.
• the catalyst particles and catalyst within the polymer particles and/or fines facilitate polymerization along the reactor wall, resulting in a film or layer of polymer growing on the reactor wall.
• the layer of polymer grows, it decreases the transfer of heat from the reactor to the reactor coolant.
• the loss of heat transfer resulting from the polymer layer may result in a lowering of the coolant temperature at the inlet to maintain the desired production rate.
• the temperature differential i.e., the spread, between the coolant inlet temperature and reactor temperature may increase.
• the layer of polymer restricts the flow of slurry along the reactor wall, resulting in an increased motor load and ⁇ at the circulating pump.
• the polymer particles and fines can become fused together, which may plug the reactor, requiring the reactor to be cleaned.
• a reactor foul may be indicated by some or all of the factors mentioned above. For example, a decreasing heat transfer rate, an increasing temperature differential, an increased motor load, and/or an increased ⁇ may indicate the presence or development of a reactor foul. In response to these indicators, a rapid response is typically required to regain control of the reaction. Depending on the foul, such responses may include adjusting the reactor temperature, increasing the addition rate of diluent (such as isobutane), decreasing the addition rate of monomer, adding anti-static agents, and/or decreasing the addition rate of catalyst. If control of the reaction cannot be regained, it may be necessary to kill or moderate the reaction to prevent the reactor from becoming plugged with polymer.
• diluent such as isobutane
• Fig. 1 depicts a loop slurry reactor in accordance with the present technique
• Fig. 2 is a table depicting fluff size ranges as a function of catalyst size
• Fig. 3 is a table depicting catalyst particle size as a function of catalyst productivity.
• Fig. 4 depicts a cut-away view of a polyolefin reactor segment including a long flow transition portion, in accordance with one aspect of the present technique
• Fig. 5 depicts a cut-away view of a polyolefin reactor segment including a central flow diverter, in accordance with one aspect of the present technique
• Fig. 6 depicts a cut-away view of a polyolefin reactor segment including multiple impellers, in accordance with one aspect of the present technique
• Fig. 7 depicts a loop slurry reactor configured for manual temperature control based on the temperature measurements obtained at local reactor hot spots, in accordance with one aspect of the present technique
• Fig. 8 depicts a loop slurry reactor configured for automatic temperature control based on the temperature measurements obtained at local reactor hot spots, in accordance with one aspect of the present technique
• Fig. 9 is a chart depicting a periodogram with dual traces in accordance with the present technique.
• Fig. 10 depicts a loop slurry reactor configured for manual response to a statistically derived predictive indicator of a foul, in accordance with one aspect of the present technique
• Fig. 11 depicts a loop slurry reactor configured for automatic response to a statistically derived predictive indicator of a foul, in accordance with one aspect of the present technique.
• Fig. 12 is a block diagram illustrating steps in the post-processing of polyolefins produced in a polymerization reactor, in accordance with one aspect of the present technique.
• the present techniques are directed to the detection and reduction and/or prevention of fouls in polyolefin polymerization reactors.
• the prevention of reactor fouls by controlling polymer particle size and/or by controlling the polymerization reaction based upon high temperature readings is initially discussed.
• the detection of impending reactor fouls using statistical methods, such as periodograms, is discussed. Once detected, such impending reactor fouls may be prevented using relatively minor adjustments.
• Statistical methods may also be used to evaluate catalysts for a propensity to foul.
• the present techniques also are directed to controlling polymer particle size.
• the present techniques may be employed to produce relatively small polymer particles.
• at least 70 to 90 percent by weight of the polymer particles may have an individual polymer particle size of 100 to 500 ⁇ .
• the relatively small polymer particles may allow a higher solids level to be achieved within the reactor.
• a catalyst having a relatively small particle size may be employed.
• at least 81 to 100 percent by weight of the catalyst particles may have an individual catalyst particle size of less than 50 ⁇ .
• the catalyst particle size may vary from approximately 1 to 50 ⁇ , depending on the catalyst productivity.
• the catalyst particle size for producing the relatively small polymer particles may be calculated using the catalyst productivity, as well as other variables, such as the catalyst particle density, the polymer particle density, and the target polymer particle size.
• polymer particle size and catalyst particle size refer to the particle diameter, which may be determined by measurement techniques known to those of ordinary skill in the art, such as screen analysis or laser diffraction, among others.
• laser diffraction may be performed in accordance with ISO guideline 13320:2009, entitled “Particle Size Analysis - Laser Diffraction Methods,” which is hereby incorporated by reference in its entirety.
• the laser diffraction analysis assumes a spherical particle shape; however because the polymer and catalyst particles are not perfect spheres, a particle size distribution is determined where the predicted scattering pattern for the volumetric sum of spherical particles matches the measured scattering pattern.
• the particle size distribution shows the amount by volume of particles of each size, or size range, and may be used to determine the percent by weight of particles of a certain size, or within a specific size range, by techniques known to those of ordinary skill in the art. An average particle size may then be determined as the mean average particle size of the particle size distribution.
• a particle size analyzer with dual-wavelength detection may be employed where red light measurements are determined using a helium- neon laser and blue light measurements are determined using a short wavelength blue light source in conjunction with forward and backscatter detection.
• a Mastersizer 2000 or other similar Mastersizer commercially available from Malvern
• the particle size may be determined by analyzing three separate samples of approximately 50 milliliters each to obtain a complete particle size distribution. An average particle size can then be determined as the mean average particle size from the particle size distribution.
• the present techniques may be implemented in conjunction with a variety of polymerization reactions, such as may be carried out in different types of polymerization reactors.
• An exemplary reactor for carrying out polymerization reactions is a loop slurry reactor 10, as depicted in Fig. 1, which may be used to polymerize polyethylene and other polyolefins.
• the loop slurry reactor 10 will be discussed herein, though it is to be understood that the present techniques may be applied to other types of polymerization reactors susceptible to fouling, such as boiling liquid pool and gas phase reactors. Indeed, any type of polymerization reaction or reactor may benefit from the present techniques.
• FIG. 1 an exemplary loop slurry reactor 10 and coolant system 12 is depicted.
• the coolant system 12 removes heat from the loop reactor 10 via reactor jackets 14.
• the loop reactor 10 is generally composed of segments of pipe connected by smooth bends or elbows.
• the reactor 10 may be used to carry out polyolefin polymerization under slurry conditions in which insoluble particles of polyolefin, such as polyethylene, are formed in a fluid medium and are suspended as slurry until removed.
• the fluid medium may include diluent (such as isobutane), ethylene, comonomer (such as hexene), co-catalysts, molecular weight control agents, and any other desired coreactants or additives which are added to the reactor interior prior to or during a polymerization reaction.
• a particulate catalyst may be added to the reactor 10 and suspended in the fluid medium to initiate or maintain the desired polymerization reaction.
• the catalyst can be any suitable catalyst for polymerizing the monomers that are present.
• An example of such a catalyst is a chromium oxide containing a hexavalent chromium (or Cr +6 ) on a silica support, which may be used to polymerize ethylene momomers.
• a motive device such as pump 16, circulates the fluid slurry in the reactor 10.
• the pump 16 may be an in-line axial flow pump with the pump impeller 18 disposed within the interior of the reactor 10 to create a turbulent mixing zone within the fluid medium.
• the impeller may also assist in propelling the fluid medium through the closed loop of the reactor, as depicted by arrows, at sufficient speed to keep solid particulates, such as the catalyst or polyolefin product, suspended within the fluid medium.
• solid particulates such as the catalyst or polyolefin product
• the impeller 18 may be driven by a motor 20 or other motive force.
• the reaction conditions within the reactor 10 may be selected to facilitate the desired degree of polymerization and the desired reaction speed while keeping the temperature below that at which the polymer product would go into solution and/or start to melt in the presence of the fluid medium.
• the cooling jackets 14 may be provided around portions of the closed loop system. A cooling fluid may be circulated within the cooling jackets 14 as needed to remove the generated heat and to maintain the temperature within the desired range, such as between 150° F to 250° F (65° C to 121° C) for polyethylene.
• the monomer (and comonomer if present) polymerizes to form polymers that are substantially insoluble in the fluid medium at the reaction temperature, thereby forming a slurry of solid particulates within the medium.
• the solid polyolefin particulates may then be removed from the reactor 10, such as via a settling leg or continuous take-off 22, and directed to downstream processing. In downstream processing, the polyolefin discharged from the reactor 10 may be extracted from the slurry and eventually formed into parts or products for personal, commercial, and/or industrial use.
• the discharge from the reactor 10 may be directed to one or more additional loop slurry reactors for further polymerization.
• the reactors when operating in series with the first reactor 10, may produce bimodal or multimodal resins where resins having a different average molecular weight and/or density are produced in each reactor.
• a high molecular weight, linear low density polymer could be produced while a low molecular weight, high density polymer may be produced in the one or more additional reactors.
• a low molecular weight, high density polymer may be produced while a high molecular weight, linear low density polymer may be produced in the one or more additional reactors.
• the discharge from the one or more additional reactors may then be directed to downstream processing where the polymer may be extracted from the slurry.
• the polyolefin produced by the reactor 10 may have the desired properties.
• various types of reactor fouls may occur which effectively limit or impair the control of reactor conditions, such as temperature, slurry circulation rate, and/or the percent of solids in the slurry (by weight or by volume). If not prevented or addressed, such reactor fouls may lead to undesirable economic and commercial results, such as off-spec product and/or reactor down-time.
• a solids foul may result from the presence of large polymer particles in the slurry mixture.
• the larger polymer particles require a greater slurry velocity to remain suspended. Failure to maintain sufficient slurry velocity allows the larger polymer particles to settle out of the slurry, leading to a solids foul. Therefore, the tendency of larger particles to be formed limits the solids carrying capacity of the reactor 10, which in turn limits the ultimate production capacity of the reactor 10.
• increasing the solids carrying capacity of the reactor 10 also increases the capability to operate the reactor 10 at higher space-time yield, as measured in pounds of polymer product produced per hour for each gallon of reactor volume or equivalent measures.
• Such an increase in the space-time yield in conjunction with a reduced incidence of reactor fouls may result in increased polyolefin production and throughput at the reactor 10. Therefore, to increase the solids carrying capacity of the reactor 10, it may be desirable to produce polymer particles in a desired size range such that the polymer particles are more likely to remain suspended, thereby allowing a greater weight percentage of solids to be achieved in the reactor 10.
• an Englehard Lynx 100 catalyst which on average produces smaller polymer particles than those produced using a Davidson 969 MS Chrome catalyst, may be used to achieve a higher solids level in a reactor without inducing a foul.
• the polymer particles produced by the Lynx 100 catalyst may be circulated at higher solids levels than comparable polymer particles produced by the 969 MS catalyst.
• the desired polymer particle size range may vary depending on the polymer product and reaction conditions. In one embodiment, to maintain suitable slurry conditions in a loop slurry reactor running under reaction conditions such as those discussed with regard to Fig. 1, less than 1% by weight of the polymer particles are greater than 1,500 ⁇ across. In another embodiment, less than 5% by weight of the polymer particles are greater than 1000 ⁇ across. In yet another embodiment, less than 0.1% by weight of the polymer particles are greater than 1,500 ⁇ across and/or less than 0.5% by weight of the polymer particles are greater than 1000 ⁇ across
• less than 5% by weight of the polymer particles are less than 100 ⁇ across and, in another embodiment, less than 0.5% by weight of the polymer particles are less than 100 ⁇ across.
• more than 70% by weight of the polymer particles are between 300 ⁇ and 500 ⁇ across and, in an additional embodiment, more than 80% by weight of the polymer particles are between 300 ⁇ and 500 ⁇ across.
• more than 90% by weight of the polymer particles are between 300 ⁇ and 500 ⁇ across.
• a catalyst may be employed which, due to the catalyst size, shape, reactive surface area, or other catalyst activity characteristic, produces polymer particles in the desired size range.
• the size of the polymer particles produced by a catalyst generally varies proportionally with the catalyst particle size; that is, smaller catalysts generally produce smaller polymer particles.
• Fig. 2 An example of this is provided in which a table comparing polymer particle size for different sized catalysts is provided. As can be seen in the table of Fig. 2, the weight percentage of different sized polymer particles varies between catalysts and generally corresponds to the catalyst particle size.
• the 25 ⁇ EP30X catalyst does not produce measurable amounts of the polymer particles larger than 1190 ⁇ , unlike the larger catalysts.
• the catalysts smaller than 100 ⁇ produce less than 5% by weight of polymer particles greater than ⁇ , ⁇ across while 100 ⁇ catalysts produce more than 5% by weight of polymer particles greater than ⁇ , ⁇ across.
• catalyst size may be one factor that determines polymer particle size, other factors, such as morphology, active site accessibility, catalyst activity, polymer produced, and so forth, may also contribute to the range of polymer particle sizes produced by a given catalyst.
• polymer particles smaller than a certain size may be desirable to produce polymer particles smaller than a certain size to impede premature setting of the polymer particles, which may cause reactor fouling.
• fines may refer to polymer particles with a particle size less than approximately 150 ⁇ , preferably less than 100 ⁇ .
• the recovery of the formed polyolefin may be improved by reducing problems caused by fine particles in the pneumatic conveying equipment (e.g., plugged filters, low flow rates in feeders due to aeration, and fines flowing through filters).
• the weight percentages of individual polymer particle sizes may be determined from a particle size distribution measured by screen analysis, laser diffraction, or other suitable technique known to those of ordinary skill in the art.
• polymer particles of a smaller particle size may allow a higher solids level within the loop slurry reactor 10 to be achieved.
• smaller polymer particles may allow a higher polymer particle weight percent or volume percent to be achieved, which may increase the solids level without requiring an increased circulation velocity.
• the relationship between the polymer particle size and the polymer particle concentration may be illustrated by the Durand correlation for horizontal pipes. The Durand correlation, as presented by Wasp (See EDWARD J.
• WASP ET AL., SOLID-LIQUID FLOW SLURRY PIPELINE TRANSPORTATION 89 (Trans Tech Publications 1977) ( 1977)) may be expressed as: where VD is the solids deposition velocity, F L is an empirical constant that generally decreases with solids concentration and particle size, g is gravitational acceleration (32.2 ft/sec ), pp is the polymer particle density, pi is the liquid medium density, and D is the reactor internal diameter.
• Solids deposition velocity represents a minimum circulation velocity for a loop reactor because if a particle is deposited out of the main flow of slurry in the loop reactor the particle's heat transfer is lowered. The decrease in heat transfer can cause the particle to overheat from the continuing reaction.
• the particle may start to soften or melt in the presence of the reaction medium, which may cause the particle to fuse together with other particles and/or adhere to the reactor wall. Fusion of particles and/or adherence to the reactor wall may inhibit flow and heat transfer in the loop reactor, which may result in more depositions, increased flow resistance, and flow instabilities.
• the empirical constant F L is a function of polymer particle size and polymer volume concentration, and is significantly lower at smaller particle sizes (e.g., less than 800 microns). Accordingly, as the polymer particle size decreases, the empirical constant F L is also lower, which in turn allows a higher polymer particle density, and thereby a higher polymer concentration, to be achieved at the same solids deposition velocity. In other words, by maintaining a smaller polymer particle size, the polymer concentration may be increased at the same solids deposition velocity.
• the production of polymer particles with a particle size just slightly above the particle size of fines may be achieved by controlling the particle size of the catalyst added to the loop slurry reactor 10 (Fig. 1). As noted above, smaller polymer particles may be produced by employing smaller catalyst particles. However, it is now also recognized that polymer particle size may be a function of the catalyst productivity.
• the catalyst productivity may generally refer to the weight ratio between the amount of polymer produced and the amount of catalyst added. The catalyst productivity may vary based on polymerization conditions, such as the quality of the feedstock, the polymerization temperature, the weight percent of monomer in the flash gas, catalyst activation conditions, and/or the equipment used, among others.
• the expected catalyst productivity for a polymerization run may be determined based on historical data and/or process control models or algorithms, among others.
• the expected catalyst productivity may then be used to calculate the catalyst particle size that should be employed to produce polymer particles of the desired or target size.
• the catalyst particle size may be calculated by dividing the target polymer particle size by the catalyst productivity.
• other properties such as the catalyst particle density and the polymer particle density, may be included in the calculation.
• the relationship between polymer particle size, the productivity, and catalyst particle size may be expressed as: where d p is the polymer particle size, d c is the catalyst particle size, P is the productivity (weight of polymer produced / weight of catalyst added to the reactor), p c is the catalyst particle density, and p is the polymer particle density.
• polymer particles may increase in size by fusing together due to deposition and may decrease in size by breaking apart due to thermal and/or mechanical stresses (e.g., stress from the generation of polymer at multiple catalyst sites on the same catalyst particle or stresses from flow in the reactor, impact with the reactor wall, and/or flow through the reactor pump).
• Equation 2 represents the largest polymer particle size that a catalyst of a certain particle size should produce when the solids deposition velocity is maintained.
• Equation 2 the catalyst particle size is inversely proportional to the productivity. Accordingly, as the productivity increases, smaller size catalyst particles should be employed to achieve the desired smaller polymer particles. Solving Equation 2 for the catalyst particle size yields the following equation, which may be employed to determine the catalyst particle size based on the expected productivity:
• Equation 3 may be employed to calculate the catalyst particle size for the expected productivity based upon a target polymer particle size.
• one of the factors affecting the expected productivity is the type of catalyst employed.
• a silica supported chromium catalyst such as 969 MS commercially available from Davison Chemical, EP30X commercially available from PQ Corporation, or Magnapore® commercially available from Grace Davison
• productivities may vary from approximately 1,000 to 15,000, and all subranges therebetween. More specifically, for chromium silica catalysts, the productivity may range from approximately 2,000 to 10,000, and all subranges therebetween.
• productivities may vary from approximately 1,000 to 15,000, and all subranges therebetween. More specifically, for metallocene catalysts, the productivity may range from approximately 2,000 to 10,000, and all subranges therebetween.
• a Ziegler-Natta catalyst such as a Lynx® polyolefin catalyst commercially available from BASF
• productivities may vary from approximately 10,000 to 150,000, and all subranges therebetween. More specifically, for Ziegler-Natta catalysts, the productivity may range from approximately 20,000 to 100,000, or even more specifically from 50,000 to 80,000. Fig.
• Equation 3 is a table comparing catalyst particle sizes for various productivities and typical polymer particle sizes. Specifically, the table illustrates catalyst particle sizes calculated using Equation 3. Each row in the table represents a desired catalyst productivity, with columns showing the calculated catalyst particle size that should be used to obtain polymer particles that are typically 100 ⁇ , 150 ⁇ , 200 ⁇ , 250 ⁇ , 300 ⁇ , 350 ⁇ , 400 ⁇ , 450 ⁇ , and 500 ⁇ in size. For example, for a desired productivity of 2,000, a catalyst with a particle size of approximately 8 ⁇ may be employed to produce polymer particles with a particle size of approximately 100 ⁇ , while a catalyst with a particle size of approximately 40 ⁇ may be employed to produce polymer particles with a particle size of approximately 500 ⁇ .
• the table also illustrates size ranges for catalyst particles that may be employed to produce polymer particles within a certain size range.
• the corresponding range of catalyst particle sizes may be employed.
• a catalyst with a particle size of approximately 8 ⁇ to 40 ⁇ may be employed to produce polymer particles with a particle size of approximately 100 ⁇ to 500 ⁇ .
• the catalyst particle sizes shown in the table may represent a range of catalyst particle sizes encompassing the catalyst particle sizes included within the maximum and minimum sizes that correspond to the maximum and minimum polymer particle sizes.
• the table also illustrates the maximum catalyst particle size that may be employed to produce polymer particles at or smaller than a specific size. For example, for a desired productivity of 2,000, a catalyst with a particle size of less than or equal to approximately 32 ⁇ may be employed to produce polymer particles of a size less than or equal to 400 ⁇ . In another example, for a desired productivity of 10,000, a catalyst with a particle size of less than or equal to approximately 19 ⁇ may be employed to produce polymer particles of a size less than or equal to 400 ⁇ .
• catalyst particle size may be determined based on the expected productivity and the desired polymer particle size.
• a catalyst employing particles of the determined particle size may then be used to produce the polymer particles as described above with respect to Fig. 1.
• a catalyst of the determined particle size may be purchased from a catalyst supplier and then directed to the reactor 10 (Fig. 1).
• the catalyst purchased from the supplier may have an average particle size of the determined particle size. Further, in certain embodiments, at least approximately 81% to 100% by weight, and all subranges
• the catalyst particles may have an individual particle size less than or equal to the determined catalyst particle size.
• at least approximately 81% to 100% by weight of the catalyst particles may have an individual particle size less than 50 ⁇ , 40 ⁇ , 30 ⁇ , or 20 ⁇ .
• the desired catalyst particle size may represent a range of catalyst particle sizes.
• the catalyst particle size shown in Fig. 3 may represent catalyst particles having an individual particle size that ranges from plus and/or minus approximately 1% to 30% of the shown catalyst particle size, and all subranges therebetween.
• the catalyst of the determined particle size may then be added to the loop slurry reactor 10, as described above with respect to Fig. 1, to produce polymer particles within the target particle size range.
• the catalyst may be added to the loop slurry reactor 10 where the catalyst may be suspended in a fluid medium of diluent, monomer, comonomer, and other coreactants or additives.
• the catalyst and the fluid medium may be circulated within the loop slurry reactor 10 to polymerize the monomer, thereby producing a slurry of the polymer particles.
• the slurry may be removed from the reactor 10 and directed to downstream processing. Further, in certain embodiments, the slurry may be directed to one or more additional loop slurry reactors to carry out further polymerization for producing bimodal or multimodal resins.
• Equation 3 also may be used to calculate catalyst particle size for an expected catalyst productivity within a gas phase polymerization reactor.
• the gas phase polymerization reactor may include a fluidized bed gas phase reactor.
• gas phase polymerization reactors such as horizontal gas phase reactors, among others, may be employed.
• a gaseous stream of monomer may be passed through a reaction zone in the presence of a catalyst to form polymer particles at a velocity sufficient to maintain a bed of formed solid particles in a suspended condition.
• the velocity needed to maintain a suspended condition may be a function of polymer particle size and may be referred to as the minimum velocity.
• larger polymer particles may require a higher velocity to remain suspended and/or to reduce saltation. Accordingly, it may be desirable to produce polymers of a smaller particle size, which have a lower minimum velocity.
• the polymer particles may exit the overhead of the reactor in the unreacted monomer gas stream, which may reduce polymer yield and/or plug downstream equipment, such as filters and pipelines, among others.
• polymer particles of a size slightly larger than the minimum particle size which are still small enough to require a lower minimum velocity, but large enough to refrain from exiting the reactor in the overhead gas stream.
• the gas phase reactor may be operated at a fluidization velocity that is equal to or greater than the minimum velocity.
• the fluidization velocity which may be determined according to techniques known to those skilled in the art, may be greater than the minimum velocity to account for factors such as fluidization nonuniformity, and particle shape among others.
• the fluidization velocity may be approximately 1-20 times greater than the minimum velocity, and all subranges therebetween. More specifically, the fluidization velocity may be approximately 1-10 times greater than the minimum velocity, and all subranges therebetween.
• the gas phase polymerization reactor may be operated under reaction conditions, including fluidization velocity, catalyst particle size, and certain temperatures and/or pressures, among others, designed to produce polymer particles of the target polymer size. According to certain embodiments, it may be desirable to maintain reaction conditions within the gas phase polymerization reactor to produce at least approximately 70% to 100% by weight, and all subranges therebetween, of polymer particles with a particle size of approximately 100 ⁇ to 1,400 ⁇ , and all subranges therebetween.
• reaction conditions within the gas phase reactor may be desirable to maintain reaction conditions within the gas phase reactor to produce at least approximately 70% to 90% by weight, and all subranges therebetween, of polymer particles with a particle size of approximately 100 ⁇ to 1,400 ⁇ , and all subranges therebetween, or even more specifically from approximately 300 ⁇ to ⁇ , ⁇ , and all subranges therebetween.
• Equation 3 may be used to calculate a catalyst particle size for the expected catalyst productivity based upon a target polymer particle size.
• the expected catalyst productivity may vary depending on the catalyst employed. For example, for gas phase reactors, catalysts such as Ziegler-Natta catalysts, silica supported chromium catalyst, and metallocene catalysts may be employed within a range of expected catalyst productivities, as described above.
• the catalyst particle size may then be calculated using Equation 3.
• a catalyst having the calculated particle size may then be purchased from a supplier and directed to the gas phase reactor.
• a gaseous stream of monomer may be passed through a reaction in the presence of the catalyst to polymerize the monomer and form the polymer particles of the target polymer size.
• at least 70% by weight of the polymer particles formed may have an individual particle size less than or equal to the target polymer size.
• approximately 70 to 100% by weight, and all subranges therebetween, of the polymer particles formed may have an individual particle size less than or equal to the target polymer size.
• the catalyst particle size may be determined to be less than approximately 110 ⁇ to 50 ⁇ , and all subranges therebetween. In another example, the catalyst particle size may be determined to be less than approximately 110 ⁇ , 100 ⁇ , or 50 ⁇ .
• the catalyst particle size also is a function of catalyst particle density and polymer particle density.
• the catalyst particle density and polymer particle density were kept constant.
• the catalyst particle density and/or the polymer particle density may vary based on factors, such as the type of catalyst, type of polymer produced, and reactor conditions, among others. Accordingly, in other embodiments, Equation 3 may be employed to calculate catalyst particle size for other catalyst particle densities and/or polymer particle densities.
• a reactor wall 24 having a root mean square (RMS) roughness of approximately 25-50 micro inches, and all subranges therebetween, may be employed. More specifically, a reactor wall 24 having an RMS roughness of 35 micro inches or less may be employed.
• RMS root mean square
• a roughened reactor wall typically measured by the root mean square (RMS) roughness
• RMS root mean square
• the polymer particle size is believed to be related to the reactor friction factor, which is a function of the roughness of the reactor wall 24 (Fig. 1).
• a relatively smooth reactor wall 24 such as a reactor wall having an RMS of 63 micro inches or less, does not break down or attrite polymer particles in a slurry moving along the reactor wall 24 at 30 to 40 feet per second to the same extent that a reactor wall 24 having an RMS between 63 micro inches and 250 micro inches does.
• a reactor wall 24 with an RMS of 250 micro inches or greater may result in an even greater reduction in polymer particle size compared to a reactor wall having a lower RMS.
• the polymer particle size may be smaller, under similar slurry movement conditions, in a reactor 10 having rougher reactor walls 24.
• One possibility, therefore, for reducing average polymer particle size is to use a rough reactor wall, either by machining the reactor wall 24 to increase the RMS or by not smoothing the reactor wall 24.
• the pump 16 and/or its environment may be modified to facilitate the production of smaller polymer particles.
• This may be accomplished in a variety of manners, such as by using an impeller 18 having shorter impeller blades and/or a larger impeller hub, which make it more likely that suspended particles will pass near the faster moving impeller tips rather than the slower moving portions of the impeller blades near the hub.
• an impeller 18 is used that can rotate within the diameter of the main reactor loop, the use of a flared flow transition piece having a greater diameter may be avoided.
• such a flared flow transition piece may be undesirable to the extent that denser polymer particles may not disperse to the periphery of the transition piece, where the fast moving impeller tips are located, but may instead continue through the center of the transition piece, where the slower moving portions of the impeller near the hub are located.
• a flared transition piece is generally about 2 feet in length while the reactor contents are typically circulating at approximately 37 feet/second. Due to the shortness of the transition piece relative to the circulation rate, the denser polymer particles may have approximately one-twentieth of a second to disperse toward the periphery of the transition piece before reaching the end of the transition piece.
• One alternative, depicted in Fig. 4, is to incorporate a long flow transition piece 26 when it is desired to employ an impeller 18 that is larger than the main reactor diameter.
• the depicted long flow transition piece 26 is ten feet in length. Due to the greater length of the long flow transition piece 26, the denser polymer particles are more likely to disperse to the periphery of the long flow transition piece 26, where they may be attrited by the faster moving impeller tips. While a ten foot long flow transition piece 26 is depicted in Fig. 4, the length may vary, with one embodiment including long flow transition pieces 26 between ten feet and twenty feet in length. In general, however, the long flow transition piece 26 is sized to allow the denser polymer particles to disperse around the impeller 18 as opposed to being carried by inertia through the central portion of the impeller 18.
• a transition piece incorporating a flow diverter 28 sized and positioned to correspond to the impeller hub 30 may be employed for directing polymer particles toward the tips of the impeller blades, as depicted in Fig. 5.
• other techniques may also be employed to divert the flow of slurry to the periphery around the impeller 18, where the faster moving impeller tips may attrite the polymer particles to maintain a smaller average polymer particle size.
• impeller designs such as designs that have a more severe effect on the polymer particles, may be employed to increase the attrition of polymer particles.
• the pump 16 may be operated at higher speed to increase attrition of the polymer particles by the impeller 18.
• a multi-stage pump or multiple pumps such as the dual impellers 18 depicted in Fig. 6, may also be employed to increase the overall attrition of the polymer particles provided by the impeller(s) 18.
• One technique that may reduce the likelihood of or prevent such a copolymer foul is to measure reactor temperature at local hot spots 32 in the reactor 10 and to control the reaction based on one of the hot spots 32, such as the hottest point, or on a measure of central tendency of the measured temperatures, such as on the mean, median, or mode of some or all of the measured hot points.
• an integrated function of the temperature measured at the hot spots 32 may be calculated and updated and control maintained based on the integral.
• the local hot spots 32 of the reactor 10 may be determined as a function of the coolant feed locations 34 and monomer feed locations 36. For example, in the depicted eight-leg reactor of Fig. 1, four local hot spots 32 may be present, one for each pair of cooling jackets 14. In particular, if the reactor legs are alternately cooled by cooling jackets 14 receiving fresh (i.e., cold) coolant and used (i.e., warmer) coolant, a local hot spot 32 may be expected between where the slurry exits a reactor leg cooled by used coolant and where it enters the next reactor leg cooled by fresh coolant. In one embodiment, the hot spots 32 are typically at the top of a vertical pair of reactor legs just after the 180 degree bend that is typically not cooled. A local hot spot 32 in the reactor 10 may reach a sufficiently high temperature to cause polymer particles to swell, possibly leading to a copolymer foul, as discussed above.
• the likelihood of a copolymer foul may be reduced by controlling the reactor temperature based on one or more of the temperatures measured at the local hot spots 32 or on a value derived from some or all of the temperatures measured at the local hot spots 32, such as a mean, median, or integral.
• the derived value or measured temperature may be compared to one or more threshold values, such as the set temperature, the upper bound for the fouling curve at a given pressure, or temperatures derived from the fouling curve, which may indicate an undesired temperature.
• the operation of the coolant system 12 may be adjusted to maintain the desired reactor temperature. For example, if the reactor temperature, as measured at the local hot spots 32, is too high, the coolant system 12 may employ cooler coolant or may increase the rate at which coolant is pumped through the cooling jackets 14. These adjustments to the operation of the coolant system 12 may be made manually, such as by an operator adjusting addition of fresh coolant to the system or adjusting one or more valves controlling the flow rate of coolant through the jackets 14. Alternatively, the adjustments to the operation of the coolant system may be made in an automated manner, such as by the automated operation of valves controlling the addition of fresh coolant to the coolant system 12 or controlling the flow rate of coolant through the operation of jackets 14.
• control of the reactor temperature is based on the highest
• the time for the reactor temperature control system to react is reduced.
• the disturbance must travel with the slurry to the temperature control point and be detected. Then the controller acts to send either colder or more coolant to the reactor 10. The changed coolant then must travel from the coolant exchanger to the reactor 10 and into the reactor jacket 14 and to the hot spot to correct the disturbance.
• the time needed for the coolant system 12 to respond is reduced and the temperature disturbance can be controlled sooner.
• temperature may be monitored at each of the local hot points 32, such as by a thermistor 40 or thermocouple.
• the temperatures 42 measured by the thermistors 40 may be displayed visually, such as on a monitor 44 or gauge, for an operator to review.
• a temperature 42 that crosses a temperature control threshold 46 may prompt the operator to adjust one or more reactor conditions, such as the temperature of the coolant entering the cooling jackets 14, the coolant circulation rate, the catalyst addition rate, and/or the monomer addition rate. In this way, the reactor temperature is controlled such that the hottest point in the reactor 10 remains within the fouling curve associated with the desired product, thereby preventing or reducing the incidence of polymer swelling, which might lead to a copolymer foul.
• reactor temperature may be decreased by increasing the percentage of fresh coolant within the coolant system 12 or by increasing the flow of coolant through the cooling jackets 14. Such increases may be accomplished by adjusting a coolant supply valve 48 between a coolant supply 50 and the coolant system 12 or by adjusting a coolant flow valve 52 providing coolant flow to one or more of the cooling jackets 14.
• the actuation of the coolant supply valve 48 and/or the coolant flow valve 52 is accomplished by electrical signals generated by valve control circuitry or a valve control routine, such as may be present on an operator workstation 54, such as a suitably configured general or special purpose computer. While Fig.
• circuitry or routines present on the operator workstation 54 may process the measured temperature data such that only the highest measured temperature is displayed, an average measured temperature is displayed, a median measured temperature is displayed, or some other selected or derived value, such as an integral, is displayed for an operator to monitor.
• the temperature control scheme may be fully or partly automated.
• the temperatures measured by the thermistors 40 located at the local hot spots 32 is provided to a controller 56, such as a suitably configured general or special purpose computer system.
• the controller 56 may be provided with control circuitry or may execute one or more control routines to determine if a measured temperature or a value derived from the measured temperatures, such as the average or median temperature, crosses a temperature control threshold. If a threshold is crossed by a measured temperature or a derived value, the controller 56, via valve control circuitry or routines, generates signals that adjust the flow of one or both of the coolant supply valve 48 and the coolant flow valve 52, as described above.
• Prevention of Fouls Early Detection
• reactor operation data such as temperature, pressure, the addition rate of reactants and/or catalysts, and the power consumption of the pump are examples of reactor operating conditions that may be monitored and measured over time, either constantly or at intervals.
• Such data sets represent a time series for the reactor condition being measured, that is the data set includes data points representing the measured condition at known or fixed times. Time series of data may be analyzed to detect trends or patterns in the data which allow predictions to be made about future conditions, such as future reactor operating conditions.
• the periodogram technique for analyzing time series data of this type may be used to evaluate the randomness of the set of time series data to determine if there are actually non-random, i.e., periodic, components within the data.
• This periodogram technique assumes that the time series data set in question is actually composed of sine and cosine waves of different frequencies. Based on this assumption, the periodogram technique may be used to detect and estimate the amplitude of a sine component buried in the noise of the data or to identify periodic components of unknown frequency within a time series of data.
• periodogram analysis may be used to estimate the amplitude, i.e., the "peak," of a sine component of known frequency, such as identified periodic components.
• periodic components may be identified in the noise of the time series data, such as reactor pump power, and the respective frequency and amplitude of the identified periodic components may be determined and plotted. Trends in the periodic components may then be used to in evaluating the reactor operating conditions for current or future events of interest, such as reactor fouls.
• a periodogram analysis of reactor operation data such as pump power
• a periodogram analysis of reactor operation data may be conducted using the equations set forth below or their computational equivalents.
• the number of measurements N equals 2q+l and the Fourier series model:
• Equations 4-8 if a set of time series data is random, containing no systematic sinusoidal component, each component /(/J) would have the same expected value, with actual values distributed in a chi- square distribution with two degrees of freedom. However, if the set of time series data contains a systematic sine component with a frequency of/ ⁇ , the value of /(/ ⁇ ) would be inflated at or near fi. Statistical deviations from a chi-square distribution may, therefore, be used to discover the presence of periodic components in the time series data and to estimate their frequency and amplitude.
• the mathematical and statistical operations described above may be performed by a statistical analysis computer program, such as StatGraphics, or by a more general application, such as a spreadsheet program, configured to execute suitable algorithms.
• a statistical analysis computer program such as StatGraphics
• a more general application such as a spreadsheet program
• Such computerized implementations may be performed on a general or special-purpose computer system configured to analyze time series data.
• reactor operations data One type of time series data that may be analyzed in this manner is reactor operations data.
• reactor circulating pump power may be of interest since increases in pump power are often an indicator of a reactor foul.
• Trends in the periodic components of pump power may, therefore, be useful in predicting reactor conditions such as fouls.
• pump power in kilowatts is typically monitored during normal operation, providing an existing set of time series data for analysis.
• pump power can be measured at set intervals, such as at five second intervals, and is an independent measure that is relatively free of problems such as data scatter.
• Other reactor data such as reactor density and/or pressure, may also useful in predicting reactor fouls.
• a chart depicting the results of a periodogram analysis of pump power measurements is provided.
• a periodogram analysis of pump power measurement identified two different periodic components of the pump power data prior to the occurrence of a foul.
• the first periodic component was determined, as described above, to have a period of approximately 22.5 seconds while the second periodic component was determined to have a period of 45.4 seconds. Based on these frequencies, the intensities of the periodic components during half -hour time intervals were determined and plotted over time to form the chart of Fig.
• first and second periodic components are represented respectively by the first trace 60 and second trace 62. While it is uncertain what the first and second periodic components represent, one possibility is that they represent pump power oscillations caused by the hydrodynamics of the reactor, a phenomena occasionally referred to as density waves.
• a crossover 64 of the first trace 60 and the second trace 62 preceded a reactor foul, with a foul typically occurring within hours of such a crossover 64.
• action may be taken to avert a foul or minimize its severity, such as lowering reactor solids and/or adding anti-static agents. For example, lowering reactor solids by 0.5% to 1.0% after a crossover event 64 is observed may prevent a foul or reduce its severity.
• crossover events 64 observed in the analysis of pump power data represent one possible predictive indicator of a foul
• other predictive indicators such as divergences, inflection points, maximums, minimums, and so forth
• monitoring, detection, and response to a predictive indicator, such as a crossover event, determined in this manner may be manual or automated.
• a workstation 54 and display 44 are depicted in conjunction with the reactor 10.
• the workstation may include analysis circuitry or may be configured to execute analysis routines for performing time series analysis, such as a periodogram analysis, on one or more reactor operating conditions, such as pump power, temperature, pressure, and so forth.
• the result 66 of the analysis may be displayed on a monitor 44 for review by an operator.
• the operator may adjust the reactor conditions, such as by reducing the solids in the reactor 10 by increasing the flow through a take-off valve 68 or by reducing the flow of catalyst or reactants through an inlet valve 70 of the reactor 10.
• the operator may choose to increase the flow of an additive, such as an anti- static agent, through an additive valve 72 in response to the displayed results 66. While these valve operations may be performed by circuitry or routines accessible to the operator at the workstation 54, the operator may also manually actuate or adjust the valves 68, 70, and 72 in some embodiments.
• the monitoring, detection, and response scheme may be fully or partly automated.
• the pump power or other reactor operation data may be provided to a controller 74, such as a suitably configured general or special purpose computer system.
• the controller 74 may be provided with control circuitry or may execute one or more control routines to analyze and evaluate the results of a time series analysis, such as a periodogram analysis, of the reactor operation data. If a predictive indicator, such as a crossover event, is detected, the controller 74 adjusts the operation of the reactor in accordance with a preconfigured response.
• the controller 74 may perform operations to reduce reactor solids, such as by increasing the takeoff of solids, by reducing the addition of reactants or catalyst, or by increasing the addition of additives, such as anti-static agents, as discussed above.
• valve control circuitry or routines generates signals that adjust the flow solids, reactant, catalyst, and/or additives, as described above.
• the periodogram analysis may also be used to detect the presence of local accumulations or polymer aggregates in the circulating slurry.
• an increase in intensity, i.e., amplitude, of the first trace 60 may indicate the presence of such aggregates while a subsequent decrease in intensity may be indicative of the dissolution of the local accumulation or polymer aggregate.
• the frequencies associated with the first and second periodic components in a periodogram analysis of pump power correspond to the reactor circulation rate. This correspondence may provide an alternate and independent method for estimating the reactor circulation rate.
• the period for the second periodic component was determined to be 45.4 seconds, which generally corresponds to the time in which the reactor contents complete one circuit of the reactor 10.
• the period for the first periodic component was determined to be 22.5 seconds, which generally corresponds to the time taken to complete half of a circuit.
• the periods of the first and second periodic components were observed to have a similar relationship to circulation, with the first periodic component having a period of 11.5 seconds and the second periodic component having a period of 23.5 seconds.
• the technique may also be useful in evaluating catalysts for fouling susceptibility.
• the second periodic component as measured by peak intensity is reduced or absent.
• chrome type catalysts acquired from different suppliers differ in their fouling propensity, with catalysts that are less prone to fouling exhibiting no second periodic component or a second periodic component having a small amplitude.
• the periodogram analysis technique may be used to screen catalysts for fouling susceptibility, such as by determining the presence or scale of a second periodic component in the periodogram analyses of pump power usage for different catalysts.
• periodogram techniques discussed above represent one example of statistical analysis that may be used to derive predictive indicators of fouling from time series data
• other statistical analysis techniques may also be employed.
• analyses based on spectrum and spectral density functions may be employed to derive predictive indicators from suitable time- series data and may offer advantages if the frequency of the underlying periodic components varies over time.
• the spectrum is the Fourier cosine transform of the autocovariance function.
• DCS distributed control system
• Such an implementation may analyze a "sliding window" of time series data, such as the pump power data acquired over the last hour, to generate a statistical analysis, such as the periodogram analyses discussed herein, which may be evaluated for predictive indicators, such as the crossover event 64.
• the analysis may be updated at intervals, such as every ten minutes, based on the sample rate for the time series data.
• polyethylene, polypropylene, or other polyolefins all of which may be formed into respective articles for commercial, residential, or industrial use.
• the present techniques may also be applied to a variety of reactor geometries, such as horizontal, vertical leg, pilot plants, and so forth.
• different periodograms may be derived for different reactors and/or for different catalyst systems, such that the analysis may differ as well.
• events such as crossover event 64 are believed to consistently indicate a pending foul.
• sampling frequency, trace intensity and trace frequency are inter-related so that it may be advisable to periodically re-evaluate assumptions about the plotting of trace points to confirm the calibration of ongoing analyses.
• the dried polyolefin may be processed to remove unreacted reactants and catalyst in a polyolefin recovery system 80, which may include various subsystems such as monomer recovery columns, flash vessels, and cyclones.
• the purified polyolefin is typically provided to an extrusion system 82 where the polyolefin product is typically extruded to produce polymer pellets with the desired mechanical, physical, and melt characteristics.
• Additives, such as UV inhibitors and peroxides may be added to the polyolefin product prior to or during extrusion to impart desired characteristics to the extruded polymer pellets.
• pellets shipped to product manufacturers 86 may include low density polyethylene (LDPE), linear low density polyethylene (LLDPE), medium density polyethylene (MDPE), high density polyethylene (HDPE), and enhanced polyethylene.
• LDPE low density polyethylene
• LLDPE linear low density polyethylene
• MDPE medium density polyethylene
• HDPE high density polyethylene
• the various types and grades of polyethylene pellets may be marketed, for example, under the brand names Marlex® polyethylene or MarFlex® polyethylene of Chevron-Phillips Chemical Company, LP, of The Woodlands, Texas, USA.
• the polyolefin (e.g., polyethylene) pellets may be used in the manufacture of a variety of products, components, household items and other items, including adhesives (e.g., hot-melt adhesive applications), electrical wire and cable, agricultural films, shrink film, stretch film, food packaging films, flexible food packaging, milk containers, frozen-food packaging, trash and can liners, grocery bags, heavy-duty sacks, plastic bottles, safety equipment, coatings, toys and a variety of containers and plastic products.
• adhesives e.g., hot-melt adhesive applications
• electrical wire and cable e.g., electrical wire and cable
• agricultural films shrink film, stretch film, food packaging films, flexible food packaging, milk containers, frozen-food packaging, trash and can liners, grocery bags, heavy-duty sacks, plastic bottles, safety equipment, coatings, toys and a variety of containers and plastic products.
• polyolefins other than polyethylene such as polypropylene, may form such components and products via the processes discussed below.
• polyolefin e.g., polyethylene
• the products and components formed from polyolefin may be further processed and assembled by the manufacturer 86 for distribution and sale to a consumer, such as customer 88.
• a polyethylene milk bottle may be filled with milk for distribution to the consumer, or a polyolefin fuel tank may be assembled into an automobile for distribution and sale to the consumer.
• the pellets are generally subjected to further processing, such as blow molding, injection molding, rotational molding, blown film, cast film, extrusion (e.g., sheet extrusion, pipe and corrugated extrusion, coating/lamination extrusion, etc.), and so on.
• Blow molding is a process used for producing hollow plastic parts, and may employ blow molding equipment, such as reciprocating screw machines, accumulator head machines, and so on.
• the blow molding process may be tailored to meet the customer's needs, and to manufacture products ranging from plastic milk bottles to the automotive fuel tanks mentioned above.
• injection molding products and components may be molded for a wide range of applications, including containers, food and chemical packaging, toys, automotive, crates, caps and closures, to name a few.
• Polyolefin pipe for example polyethylene pipe
• polyethylene pipe may be extruded from polyethylene pellet resins and used in an assortment of applications due to its chemical resistance, relative ease of installation, durability, cost advantages, and the like.
• plastic polyethylene piping has achieved significant use for water mains, gas distribution, storm and sanitary sewers, interior plumbing, electrical conduits, power and communications ducts, chilled water piping, well casing, to name a few applications.
• high-density polyethylene (HDPE) which generally constitutes the largest volume of the polyolefin group of plastics used for pipe, is tough, abrasion-resistant and flexible (even at subfreezing temperatures).
• HDPE pipe may be used in small diameter tubing and in pipe up to more than 8 feet in diameter.
• polyethylene pellets may be supplied for the pressure piping markets, such as in natural gas distribution, and for the non-pressure piping markets, such as for conduit and corrugated piping.
• Rotational molding is a high-temperature, low-pressure process used to form hollow parts through the application of heat to biaxially-rotated molds.
• Polyethylene pellet resins generally applicable in this process are those resins that flow together in the absence of pressure when melted to form a bubble-free part.
• Pellets, such as certain Marlex ® HDPE and MDPE resins offer such flow characteristics and a wide processing window.
• these polyethylene resins suitable for rotational molding may exhibit desirable low- temperature impact strength, good load-bearing properties, and good ultraviolet (UV) stability.
• applications for rotationally-molded Marlex ® resins include agricultural tanks, industrial chemical tanks, potable water storage tanks, industrial waste containers, recreational equipment, marine products, plus many more.
• Sheet extrusion is a technique for making flat plastic sheets from a variety of pellet 38 resins.
• the relatively thinner gauge sheets are generally thermoformed into packaging applications such as drink cups, deli containers, produce trays, baby wipe containers, and margarine tubs.
• Other markets for sheet extrusion of polyolefin include those that utilize relatively thicker sheets for industrial and recreational applications, such as truck bed liners, pallets, automotive dunnage, playground equipment, and boats.
• a third use for extruded sheet, for example, is in geomembranes, where flat-sheet polyethylene material is welded into large containment systems for mining applications and municipal waste disposal.
• the blown film process is a relatively diverse conversion system used for
• blow molding in conjunction with monolayer and/or multilayer coextrusion technologies lay the groundwork for several applications.
• Advantageous properties of the blow molding products may include clarity, strength, tearability, optical properties, and toughness, to name a few.
• Applications may include food and retail packaging, industrial packaging, and non-packaging applications, such as agricultural films, hygiene film, and so forth.
• the cast film process may differ from the blown film process through the fast quench and virtual unidirectional orientation capabilities. These characteristics allow a cast film line, for example, to operate at higher production rates while producing beneficial optics.
• polyolefin pellets may also be supplied for the extrusion coating and lamination industry.

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