Continuous, Online Viscosity Measurement Raises Polymer Productivity
At Leeuwarden, raw methyl methacrylate is pre-polymerized in 5 m3 reactor vessels before being cast into acrylic sheet. As the process advances, the polymer chains grow, increasing the viscosity of the solution. After some 2.5 hours, the process becomes exothermic and viscosity increases rapidly, offering just a 20-second window in which the chain length is optimal for strong, translucent acrylic sheet. At the critical point, the reactor jacket is flooded with coolant to quench the polymerization; flooding too early seriously affects yield, flooding too late incurs 20 minutes of rework to re-establish acceptable quality. Atoglas decided to automate this stage of the process by installing on-line process viscometers.
To meet the Atoglas specification, the viscometer had to respond to changes in viscosity within a few seconds, be EEx d rated (flameproof), and offer high reliability with minimal maintenance, particularly since the polymer solution is extremely corrosive and liable to create coating buildup on any instrumentation.
Following discussions with Solartron systems integrator ThIS (Thermo Instrument Systems), of Breda, Holland, Atoglas selected the 7827 on-line process viscometer. The Solartron 7827 is designed to withstand vigorous mechanical cleaning in the event of coating buildup, with no moving parts and no seals, thus minimizing maintenance needs. Viscosity is measured by electronically maintaining a tuning fork in resonance when its tines are immersed in the fluid.
The 7827 calculates dynamic viscosity using the two 1/2 power point frequencies and not an amplitude measurement, ensuring accuracy is unaffected by temperature. Uniquely, the Solartron 7827 also measures line density, enabling it to compute kinematic viscosity under quite extreme process conditions and so determine base viscosity accurately using ASTM D341-based methods relating viscosity change to temperature. Atoglas uses the relationship between kinematic viscosity and molecular weight—defined in ASTM D 2502-92—to determine the correct moment to quench the polymerization process.
"Throughout the trial, the 7827 achieved a 100% success rate, quenching the polymerization process within two seconds of the optimum quality window opening," notes Peter Postma, project leader at Elf Atoglas. "By installing 7827 viscometers on all our polymerization reactor vessels we expect to eliminate substantial costly rework and increase plant output by some 10%."
Viscosity is one of the most important control parameters in a wide diversity of hydrocarbon processes but control engineers are frequently limited by the need to measure viscosity in the laboratory or analyzer house. Although such measurements are accurate, they are at least 20 minutes old, making it impossible to meet today's tight quality and materials management targets. The real-time process viscosity measurement provided by the Solartron 7827 offers closed-loop control even in highly dynamic processes, allowing the engineer to develop his control philosophy unencumbered with the restrictions enforced by laboratory measurement.
Benefits accrued include improved product quality and greater flexibility in plant management, allowing a wider diversity of feedstocks to be used with better return on capital.
Edited by Nick Basta
For more information, please contact:
<%=company%> USA
Tony Wright, Solartron, Inc.
19408 Park Row, Suite 320
Houston, TX 77084
Tel: (281) 398-7890
Fax: (281) 398-7891
email: 101346.647@compuserve.com
Rest of World
Chris Bishop, Communications Manager
Solartron Ltd.
Victoria Road
Farnborough, Hampshire GU14 7PW, UK
Tel: +44 (0) 1252 376666
Fax: +44 (0) 1252 543854
email: cbishop@solartron.com
Web site: www.solartron.com
ThIS
Reinier van der Wal
ThIS Analytical BV
Heerbaan 220, 4817 NL Breda, Holland
Tel: +31 76 571 3717
Fax: +31 76 581 2028