Centrifugal Sifter Dewaters Resin to 10% Surface Moisture
Reprinted from: Chemical Processing Alan E. Model, Senior Technical Editor Saves more than $87,000; handles slurry flow surges
Sybron Chemicals Inc., Birmingham, NJ, has successfully applied a centrifugal sifter to convert a batch dewatering process to a closed-loop, continuous process. Since the conversion, the product quality has been enhanced, spillage reduced and yields have been higher . The potential for product contamination has been lessened.
Spillage/safety demand attention
After polymer spheres are formed in a reactor, they require washing to remove a suspension. In the old lines, the slurry of polymer beads and wash water flowed into a dewatering tank from which employees shoveled the wet polymer into a hopper. The hopper emptied onto a conveyor that took the polymer to a heated dryer. A parallel processing line required hoisting and moving a hopper of polymer beads on a ceiling-mounted track. The hopper was centered over an open dryer feed bin. Sometimes, spillage was inevitable. The line posed safety risks, a potentially messy processing area and yield losses.
Even heating and drying of the polymer was difficult to ensure without curtailing production. The shoveling created an uneven flow to the dryer. Excessively wet or over-dry polymer beads would jeopardize quality. The polymer moisture content must be within 0.3% limits to meet quality standards. Sybron met the tight specifications using a resistance temperature sensor, an electronic controller and a screw feeder to adjust the rate at which the moist polymer is conveyed into the dryer. Productivity was reduced because the dryer operation had to be limited to 50% of capacity. When Production Supervisor John Lazevnick and Process Engineer Robert Blantz joined Sybron Chemicals, they recognized an opportunity to make improvements. A centrifuge purchased by a previous engineer was too small for the processing line. A larger one would have cost Sybron over $100,000 .Rather than propose such an expensive device, Lazevnick and Blantz sent polymer samples to screener manufacturers' facilities for testing to dewater the polymer beads from a 65% water slurry to about 10% surface moisture content.
Circular screen separators, which are often used for dewatering, did not provide enough force to remove the free water. A centrifugal sifter was tried and proved successful. Blantz explains, "There is a certain surface adherence of the water to the bead. Unless the water is driven off with some centrifugal force, the bead won't dewater far enough."
Sybron rented a centrifugal sifter for in-plant testing. Problems were encountered with standard screens so Sybron modified the centrifugal sifter screen basket design. Using a design developed at Sybron, the manufacturer built a basket that extends the support strips farther out from the center and includes a diagonal molded plastic strip that joins the fabric ends of the nylon screen. The extensions that position the support strips prevent the screen from contacting the struts as the screen bulges repeatedly from the load being pushed by the rotating paddles. The diagonal molded plastic strip, which was designed by the manufacturer, prevents screen failure by allowing screen flexing caused by the incoming slurry.
Customized sifter yields benefits
The customized centrifugal sifter, installed in the summer of 1992 converted resin dewatering from a batch process to a closed-loop, continuous process. Among the benefits realized are:
Contamination potential is lessened;
Product quality is improved and yields have increased since spillage has been virtually eliminated;
Labor requirements for dewatering the beads decreased by about 25%, permitting plant personnel to accomplish more tasks on other processing,~ lines;
The overhead hopper no longer showers the workers and floor with moist polymer or presents a safety hazard;
Fluctuation of the moisture content in the polymer resin being fed to the dryer is very rare so that productivity is higher;
The new processing line configuration has freed the second toner polymer dryer for other operations;
Over $87,000 was saved by selecting a centrifugal sifter over a centrifuge.
Customized centrifugal sifter converted resin dewatering from a batch process to a closed-loop, continuous process at Sybron Chemicals Inc.