In-line viscosity monitoring smooths batch polymerization

Some polymer producers used to dream of real-time viscosity measuring. Up-to-the-second data could save hours wasted on off-line testing, eliminate guesswork, and prevent missed endpoints.One U.S. company (see users below) polymerizes polyamide grades in 500- to 1,000-lb batches with 12- to 24-hour cycle times depending on grade.

08/01/1998


Some polymer producers used to dream of real-time viscosity measuring. Up-to-the-second data could save hours wasted on off-line testing, eliminate guesswork, and prevent missed endpoints.

One U.S. company (see users below) polymerizes polyamide grades in 500- to 1,000-lb batches with 12- to 24-hour cycle times depending on grade. Samples were previously taken every four hours and more often as each batch's reaction endpoint approached. Endpoint was usually indicated by three unchanged viscosity readings—typically 100 poise at a maximum of 500 °F and reactor pressure as low as 3 mmHg—in a lab using a bench-type Brookfield rotating disk viscometer. This off-line testing typically took two hours, which meant actual endpoints could be missed by two hours. Tests cost $100 to $150 per sample and often amounted to $500 per batch.

Real-time viscosity measurement became a reality for this polyamide producer when the firm adopted Viscoliner, a continuous in-line process control viscometer from Nametre Co. (Metuchen, N.J.). The in-line viscometer's reactor-installed sensor uses a physical torsion-oscillation mechanism to measure viscosity in real time, and gives operators a continuously plotted curve tracking viscosity over time. The sensors's smooth, continuous surface also prevents clogs.

Viscoliner alleviated guessing about endpoints, which improved batch quality, accelerated production, and delivered higher outputs. Viscoliner improved the polyamide producer's cycle time by 10% to 20% and reduced off-grade product.

In-line measuring basics

Viscoliner's torsion-oscillation mechanism measures viscosity with an electronic controller that converts the sensor's torsional motion into a 4-20 mA signal to generate a viscosity charge on a strip chart. This allows operators to view the polymerization cycle from start to reaction propagation to endpoint as indicated by leveling of viscosity readings. This system can even alert operators before the product goes out of specification.

The torsion-oscillation mechanism's spherical bulb vibrates torsionally on its axis at its natural frequency near 650 Hz. The motion is infinitesimal with an east-west, peak-to-peak travel of one micron 10-3mm at the sphere's equator. Viscoliner's electronic controller converts changes in power required to sustain the amplitude of oscillation into fundamental viscosity units. The energy needed to sustain oscillation at one micron amplitude is proportional to viscosity of the batch in the reactor.

Besides eliminating testing delays and reducing cycle times, in-line viscosity analysis has also alleviated mistakes and allowed plant operators to use Viscoliner's reproducible data to anticipate readings and make proactive adjustments. Testing costs have also been cut by more than $500 per batch. Consequently, the polyamide plant recouped its investment in Viscoliner in two to three months from savings on capacity improvement, raw material yields, reduced labor and sampling requirements, and less scrap.





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