Plastics Technology

DEC 2014

Plastics Technology - Dedicated to improving Plastics Processing.

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requisite volume of airfow through the drying hopper, which could be as high as 20 to 30 in. of water column when a heavy loading of regrind and fnes is being dried. DESICCANT (fg. 6): The desiccant is the heart of the dehumidi- fying dryer. It is the substance that removes the moisture from the drying air that is returning from the drying hopper. This moisture needs to be removed from the return air in order to reduce the air's dewpoint back into the range of -30 F to -40 F. The most common type of desiccant used in dehumidifying- air polymer dryers is molecular-sieve desiccant made from a synthetic zeolite material. The molecular sieve will flter out molecules from the drying air that are smaller than the desic- cant's pores. Manufacturers of PET bottles used for soft drinks and water are concerned with the acetaldehyde (AA) levels in the PET preforms they are producing because AA will change the favor of the bottled drink. Virgin PET normally has AA levels in the range of 1 to 3 ppm, while reground PET may have an AA level of 10 to 15 ppm or more. Dryers used for PET bottle production usually employ a 13X desiccant, which adsorbs both AA and water mole- cules from the dryer's return-air circuit. AIR HEATERS (fg. 6): Typical PET drying temperatures are in the range of 300 F to 350 F. Either electrical or natural-gas heaters can be employed in a PET dryer. Electrical heaters are less complex and less costly than gas heaters, but their downside is the skyrock- eting cost of electricity. Depending on your location, the cost of electricity can be several times more than natural gas. If you are considering the purchase of a gas-heated dryer for PET, be careful to specify indirect heat exchangers in the both the drying- air and regeneration-air circuit heaters. An indirect heat exchanger in the drying-air circuit separates the combustion air from the drying air, which will guar- antee no combustion exhaust gases will come into contact with and contaminate the PET being dried. An indirect heat exchanger in the regeneration-air circuit is also important. It separates the combustion air from the regenera- tion air, which will guarantee that no combustion exhaust gases will come into contact with your dryer's desiccant. REGENERATION AIR CIRCUIT (Fig. 6): The regeneration-air circuit of a PET dryer utilizes ambient plant air in a single-pass circuit to purge the adsorbed moisture from the desiccant out to the atmosphere. INLET AIR FILTER: Regeneration air is drawn in from the atmo- sphere and frst fltered of any possible contaminants prior to contacting the desiccant. REGENERATION AIR BLOWER: The regeneration blower moves the air through the single-pass regeneration-air circuit. A sufcient volume of airfow is required to elevate the desiccant's temperature in order to heat the desiccant and purge its adsorbed moisture. REGENERATION AIR HEATERS: Heating the desiccant to a high temperature is necessary for regeneration. These heaters must have enough heating capacity to raise the temperature of the regenera- tion air to the factory-set regeneration temperature in order achieve complete desiccant regeneration. REGENERATION MOISTURE EXHAUST: As the temperature of the desiccant is increased it will begin releasing its adsorbed moisture. The released moisture will be purged to the atmosphere through the moisture exhaust port. DESICCANT COOLING CIRCUIT: The desiccant's temperature on completion of the regeneration cycle is elevated and its residual moisture content is at a very low level. The desiccant must be cooled prior to being moved into the process-air stream (Fig. 7). If hot desiccant is moved into the on-stream position it will not adsorb moisture from the air returning from the drying hopper. The moisture present in the air returning from the hopper will pass right through the hot desiccant, resulting in high-dewpoint air being recirculated into the drying hopper. Typically, a small. controlled volume of air from the drying-air circuit is bled through the desiccant in the cooling position. Some older designs of dryers use ambient air to cool the desic- cant after regenera- tion, which results in preloading the desiccant with some ambient moisture as the desic- cant's temperature is reduced during cooling. The beneft of cooling the hot, freshly regenerated desiccant with a slipstream from the drying-air circuit is that the desiccant is being cooled with the moderately dry air from the dryer's return-air circuit, rather than ambient plant air, which may have a very high moisture content, depending on local weather conditions. ABOUT THE AUTHOR: Pete Stoughton is an independent consultant with over 40 years' experience with dehumidifying dryers and PET crystal- izing systems. He has held various technical positions with the Conair Group, including technical services manager, product manager, and business unit manager. Contact: (814) 516-3838; pstoughton@comcast.net. Temperature is probably the most important consideration in drying any hygroscopic polymer. @plastechmag 55 Plastics Technology D R Y I N G P E T

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