'No-hands' resin quality control.

A new unmanned resin analysis lab being set up by Houston-based Chevron Chemical Co. at its Orange, Texas, polyethylene plant is believed to be a first in comprehensive, on-line testing of resin direct from a reactor. The totally automated, "lights-out" laboratory, from Rheometrics Inc., Piscataway, N.J., is set up in a standard shipping container. This "lab-in-a-box" will receive samples of reactor flake that are automatically extracted and transported pneumatically from the reactor.

Polymer samples, taken as often as 25 times an hour, will be monitored for particle-size distribution, density, melt-flow rate, and molecular-weight distribution. A small extruder will even convert some of the reactor sample to film or sheet for measuring gel count and other qualities. And a computer screen will display what's happening at each step.

This unusual q-c setup is a dramatic indication of how seriously resin producers are taking issues of quality control. It will provide Chevron with much faster feedback on process variations that could lead to off-spec production. "Our lab monitors polymer reactions while they're in progress so that problems in the process can be corrected before off-grade material is made," says Robert Callahan, director of process technology for Rheometrics, which began developing the automated lab concept in 1988, after introducing its on-line Melt Flow Monitor. In addition to the Melt Flow Monitor, Rheometrics' lab integrates a complex assembly of instruments and equipment from several other suppliers.

SHORTER REACTION TIMES

Typically, on-line rheology testing is performed on melt samples taken downstream at the compounding extruder--perhaps several hours after the resin is actually produced--and not on resin directly sampled from the reaction process itself. Custom installations in Canada and Italy do test reactor-sampled material, but not for so many properties, and certainly not for film properties as in this case, Rheometrics says.

"Resin companies don't like to admit it, but we estimate they lose 6-10% of their production to off-spec because of mechanical/chemical problems," says Rheometrics' manager of mechanical engineering Paul Mode. Off-spec material must either be sold off at a discount or be downblended with prime material, raising issues of quality and consistency.

Rheometrics' lab-in-a-box was delivered in January; it will initially monitor a pilot scale low-pressure PE reactor. "Our plan is to have it on a commercial-sized reactor this year," says Chevron project coordinator Russell Dillow. The lab will channel resin data to plant operators for faster process monitoring. "Normally, samples are taken at periodic intervals, say once per hour, and sent to the lab for analysis. It may take up to 30 minutes or more to get results back," Dillow says. "By the time operators see a change in the data, the reaction process may have already changed again. Or operators may choose to wait until a second sample is tested to see if the change is real. Now we'll have up to 25 data points per hour, providing operators with more statistically sound information, and we expect off-spec production to go down by at least half."

Later on, Chevron hopes to use the lab for automatic feedback controls, so that the reactor process becomes self-correcting and the only off-spec production would be from problems in the feedstock or catalyst.

Rheometrics is currently negotiating with several other resin producers also interested in reactor sampling and testing.

THREE STAGES OF TESTING

Rheometrics' lab, called a Process Monitoring Module, provides three physical forms of polymer for analysis: powder, melt and film/sheet. Powder will be tested for density and particle-size distribution; melt for melt-flow rate, molecular-weight distribution and comonomer content; and film for gels. Modules range in price from $250,000 to over $800,000 "fully loaded," and can include cyclones, hoppers, pumps, an extruder, tape die, rheological and other instruments, chill rolls, sensors, nip rolls and a grinder. Chevron's module is packed in a standard shipping container, with one cyclone extending 4 ft out the roof. Once the module has been proven at Orange, Chevron may order more units to monitor all of its commercial HDPE production systems, Dillow says.

Chevron will initially measure MFR and MWD using Rheometrics' on-line Melt Flow Monitor (MFM) and later add equipment before the extruder to analyze particle size and density. At the downstream end of the lab line, Chevron will add photo-optic analyzers for gels in the film. Photo-optic analysis could also measure clarity and comonomer content. (Chevron also recently installed in the plant's normal q-c lab a camera-based system, from Flow Vision Inc., Wilmington, Mass., which is used to test samples taken manually from the pelletizing stage.)

Rheometrics' lab module is designed to be entirely self-tending. It starts automatically when a sample arrives, even grinds its own waste automatically, and should need only one half-hour inspection a day, Rheometrics says. For maintenance, it has a built-in boom with block and tackle, so one operator can access and handle all heavy components. Safety systems in the module include a safety door for rapid exit; oxygen level sensors; a video monitor and C|O.sub.2~ fire extinguisher system.

The reactor sampling device is a scaled-down version of existing flash-chamber technology, which is used for normal product removal from the continuous reactor process. Automatic sampling hasn't yet been developed for high-pressure reactors, though work is progressing for slurry and solution polymerization processes.

Samples of reactor flake are collected and flashed to remove residual hydrocarbons, then mixed with nitrogen and shot through a pressurized venturi system to the test module. One venturi pump can move a sample 300 ft in seconds. Sensors make sure the sampling rate doesn't get ahead of the test-instrument sequence. If sampling is halted or interrupted for longer periods, like a reactor shutdown, the lab module can switch to feed from an optional storage reservoir to keep the extruder running. Reservoir material is also used to purge the system and to calibrate instruments periodically.

A quarter-pound sample is transferred in nitrogen every 2 min, for about 7.5 lb/hr of powdered material. Powder is separated from nitrogen by a cyclone. Samples are measured volumetrically by level in the throat of the feed hopper. Additives and stabilizers are then added, and low nitrogen pressure is maintained on the feed throat of the extruder to keep material moving. A blast of nitrogen can also be used to break up any powder bridging that may occur at the extruder feed throat.

The extruder is a standard 1.25-in., vented lab model with a two-stage, 38:1 L/D screw with a mixing section. A small continuous screen changer is optional. A side stream of melt can be run through Rheometrics' Melt Flow Monitor (see PT, Sept. '88, p. 81) as often as every 3 min. The rest of the melt is fed through a slit die and extruded vertically into 6-in.-wide tape of 0.5-mil to 40-mil thickness. After observing the tape for gels, the extrudate is ground into flake in a small grinder. Rheometrics had to come up with its own grinder design modifications after it couldn't find a commercial model that could process both thin film and sheet efficiently.

An Allen-Bradley PLC controls the reactor sampling device and the whole lab module and distributes data from the test instruments back to the plant control engineers. Among other things, the PLC directly controls temperature zones on the extruder with cooling blowers and heater bands. "If you tell the PLC what material you're running, it knows what parameters to set. It's also smart enough to know when to shut itself down," notes Rheometrics' Callahan. Different PLCs can be used, depending on the computer environment at a plant. In fact, one of Rheometrics' next modules will use a Modicon PLC. (CIRCLE 8)