This article describes features of an automated, closed sampling system that cuts waste and improves safety at a plastics factory. In order to monitor product composition and quality, tests must be performed on material entering and leaving the fluid-bed vessels. This means that samples must be drawn at eight locations, two in each processing stream. Morris, Illinois, complex of Equistar Chemicals LP, considered alternatives that could operate without an employee on the spot. Although sampling normally would be initiated by the computers that run the production process, a pushbutton override was installed at each location to allow on-demand sampling, which is helpful in problem-solving or gauging the progress of product changeover. Samples are collected and delivered by hand to the laboratory. At one time, hydrocarbons, or volatile organic compounds, which are by-products of the manufacturing process, would be present when samples were collected. That potential problem is avoided because the Bristol sampler body provides a built-in purging port intended for introducing an inert gas or fluid to flush clinging materials out of the spool. Automating has given the company more confidence that its samples and their data are representative. Because the samplers are completely enclosed, the finer particles remain as part of the sample.
Drawing quality control samples of dry powders as they are processed under pressure can be tricky, especially when the samples are accompanied by hydrocarbon gases. Opening a sampling valve can release gas and dust into the air, or let product spill onto the ground. In short, opening the process stream creates the potential for a multitude of problems involving safety, cleanup, and waste in one swoop.
At the Morris, 111., complex of Equistar Chemicals LP, a major supplier of polyethylene and polypropylene compounds, such concerns became history with a switch to automated, closed-loop sampling. Now, on command of process control computers, pneumatic devices reach into pressurized piping and pull out measured samples without creating an open hole that would allow anything to escape.
One of 13 Equistar manufacturing plants in the United States, only two of which produce polypropylene, the Morris plant is located about 60 miles southwest of downtown Chicago, near Joliet. Employing approximately 500 people, the plant occupies a 1,000-acre site, including about 70 acres maintained as a wildlife habitat.
Sampling is particularly important for polypropylene production at Morris, where almost all of the plant’s 280- million-pound annual output consists of copolymers tailored to buyers’ specifications. There is relatively little production of commodity homopolymers at the plant.
The Morris plant turns out more than 100 variations of random and impact copolymers to fill customers’ orders, typically in railcar- or hopper truck-size shipments. Each variation can have a different melt flow range, additive package, or ethylene content, as required for such distinct uses as carpet fibers, auto parts, food containers, battery cases, and bottle caps.
According to the plant’s polypropylene operations super intendent, Loren G. Meisinger, satisfying such diverse and specialized needs starts with the olefins unit, which controls the quality of the essential polymer-grade propylene gas feedstock by producing it on site. Feedstock is made from liquefied petroleum gases or from refinery-grade propylene piped in from a nearby oil refinery. The plant’s polypropylene unit combines this feedstock with ethylene, hydrogen, and several proprietary catalysts in a reactor, to precipitate polypropylene as a white powder.
The powder must pass through a catalyst deactivation unit, essentially a 30-foot-long fluid-bed vessel using nitrogen as the fluidizing medium. After deactivation, the powder is piped to nearby silos, which continuously feed extruders that melt the powder to form a material that is chopped into pellets. Finally, nearly all of the pelletized output is bulk-shipped to customers on a just-in-time schedule by rail or truck.
Four polypropylene production lines, each complete with reactor, fluid-bed deactivator, and extruder, allow simultaneous runs of different formulations. A run of any one formulation can last from one-half to several days. While each line stops for scheduled maintenance several times a year, the overall operation runs steadily around the clock, all year long.
Tested Coming and Going
In order to monitor product composition and quality, Meisinger said, tests must be performed on material entering and leaving the fluid-bed vessels. This means that samples must be drawn at eight locations, two in each processing stream.
Incoming samples, drawn every four hours, are given a melt-flow index test. By determining how much polymer flows through a particular orifice at a given time, the melt flow is a measure of the processability of the plastic and is usually specified by the customer. The lab uses a nuclear magnetic resonance spectrometer to check ethylene and rubber content, which imparts impact strength.
Outgoing samples, usually drawn only twice a day unless special circumstances such as product changeovers merit more frequent monitoring, are tested for catalyst residue levels by an X-ray fluorescence spectrometer. All tests are conducted in a lab maintained on site and certified by the American Association for Laboratory Accreditation.
“As soon as results are available, usually in little over an hour, the lab faxes the data back to the production control room,” Meisinger said. “If anything is drifting out of spec, we make the needed adjustments there.”
If the process is proceeding normally, the product will be a coarse powder, with particle sizes ranging from 80 to 30 mesh, or 200 to 800 microns. If it comes out like talcum powder, in the range of 20 to 50 microns, operators know right away that something is wrong, and corrections are made without waiting for test results.
The lab staff records test data in a computerized sample manager program and charts the data for statistical process control purposes. These records help Equistar’s technical service group, located in Cincinnati, respond to customers’ questions, develop new compounds, and resolve processing problems.
In earlier days at Morris, polypropylene line samples were drawn manually, through ball valves fitted into the 14-inch- diameter piping adjoining the fluid-bed vessels. “Someone would have to go to each valve and open it long enough to blow about a pint of powder into a bag,” Meisinger explained. “To make sure it was a current sample, the collector would let the flow run out for a few seconds to clear out the valve fine before putting the bag in place.”
A few years ago, according to senior project engineer Dean A. Karstensen, engineers started looking for a cleaner sampling method and explored several alternatives.
“One approach was to divert material right out of the reactor into a small separation vessel,” he said. “That didn’t work out because some of the samples are best drawn farther downstream where the reaction is more complete. We also tried a lock-hopper arrangement in which an upper valve could be opened to collect a sample and then closed again before a lower valve released the collected material. But with no separation of powder and gas, it all came out together anyway, and someone still had to stand there and turn the handle."
According to Karstensen, the company considered alternatives that could operate without an employee on the spot. “The automated concepts we looked at still allowed dust and gas to escape, or couldn’t meet our system pressure specs,” he continued. “Although the normal pressure at the sampling points is fairly low, about 5 psi, everything in the system must be able to withstand a surge well above the operating pressure, without damage.”
A search ultimately brought engineers to the neighboring town of Yorkville, 111., barely 20 miles from Morris, where Bristol Equipment Co. makes an automated dry powder sampling system known as Isolok. The Bristol design mounts onto an access port installed in the side of a tank, pipeline, or process vessel. The sampler body encloses a plunger that can be activated by a pneumatic cylinder to extend through the port and into the product stream.
Equistar mounted its samplers at points where the powder is in vertical free fall. Each sampler has an extended nose, or scoop, that extends into the drop chute and remains permanently in the product flow. The same design also can be used in a diagonal chute.
The plunger has a narrow section, a hollow ring, near its tip that leaves a space called the sample spool, which captures a measured volume of product. The spool has seals at either side, and when it retracts, it retains a sample of the process material flowing past it at that time, and drops the sample into a closed container attached beneath the sampler body.
Several features were especially important for the Morris application, Karstensen pointed out. Heavy elastomer seals around the plunger at both ends of the spool keep the port closed regardless of plunger position, so there’s no outpouring of process gas or dust.
When a sample is taken, the emergence of the plunger clears the extended nose of collected material. As a result, the sample that the spool takes consists only of the powder in the falling process stream at that moment. This eliminates any need to clear old material out of the sampling line, and so eliminates the extra cleanup workload that would create.
One Becomes Eight
After experimenting with a small Isolok sampler for more than a year, Karstensen had a larger trial unit installed in one of the production lines. “These things aren’t cheap,” he pointed out. “We wanted to be very sure they did what we wanted before making a full commitment.” He eventually converted all eight manual sampling points to Isoloks.
According to Con Phalen, Bristol’s assistant sales manager, a single Isolok sampling instrument costs about $5,300, exclusive of installation equipment and controls.
Although sampling normally would be initiated by the computers that run the production process, a pushbutton override was installed at each location to allow on-demand sampling, which is helpful in problem-solving or gauging the progress of product changeovers.
All units at Morris were fitted with seals of food-grade EPDM elastomer, because any line may be called upon to produce material that will go into FDA-approved containers, bottle caps, or closures.
Samples are collected and delivered by hand to the laboratory. At one time, hydrocarbons, or volatile organic compounds, which are by-products of the manufacturing process, would be present when samples were collected. That potential problem is avoided because the Bristol sampler body provides a built-in purging port intended for introducing an inert gas or fluid to flush clinging materials out of the spool.
“That port allowed us to connect the samplers to a nitrogen-purging gas collection system that runs throughout the plant, gathering up unreacted hydrocarbons from various point sources and piping them back to the olefins unit for recycling into new feedstock,” Karstensen said. “What little process VOCs come into the sampler spools along with the powder bleeds off into the gas collection system before the operator empties the bottles.”
“Automating has given us more confidence that our samples and their data are representative,” Karstensen said.
Samplers are fitted with spools designed to capture 100cc at a time. Bristol also markets a smaller, 18cc version. It has a flush nose design recommended for use in hoppers or in screw conveyors.
To get a pint-sized sample of its polyethylene powder, Equistar programs its 100cc samplers to cycle five times over a 10-minute period, which spreads the total sample over a much bigger volume of product. “That’s something we wouldn’t send a person to do,” Karstensen pointed out.
Because the samplers are completely enclosed, the finer particles remain as part of the sample. According to Karstensen, “The best benefit is that we no longer have people out there at all while the samples are being drawn. Keeping them out of harm’s way is just good proactive safety practice.”