DESSICANT DECISION

07/11/2017
Canadian Plastics

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Life is full of choices. Some are bad either way, like having to give up a bought and paid for seat with United Airlines or getting dragged off the plane. When it comes to resin drying, plastics processors have much better options. Molders have a range of dryers to choose from, including hot air, compressed air, infrared, and vacuum drying styles. And then there are desiccant dryers. Desiccant is the magical material that filters the water molecules out of the drying air. Desiccant dryers pass hot air through a hopper full of resin to heat it and carry away moisture from the pellets to the desiccant bed, where moisture is adsorbed. The desiccant must be regenerated after each resin batch has dried, first by heating to around 350° to 400°F — or 175° to 200°C — to drive off moisture, then cooling to recover full adsorption capacity. Desiccant dryers can achieve the -40˚F dewpoint considered adequate for drying any hygroscopic resin — from mild to moderately hygroscopic materials such as ABS, acetal, acrylic, PC, and some TPOs, TPEs, and TPUs to strongly hygroscopic and difficult-to-dry materials like bottle-grade PET and nylons — which is why they account for about 80 per cent of the dryers in use, period, with typical drying residence times of between one to four hours, depending on the resin. They come in two basic styles: dual bed or twin tower designs with two or more desiccant beds, or models with rotating “honeycomb” wheel desiccants. So if you’re running moisture-absorbing resins, choosing a desiccant dryer is a textbook no-brainer. Less obvious, however, is which of the two styles to go with. We asked some of the experts to give us the pros and cons of each. 

New from Una-Dyn, the U.S. arm of Piovan SpA, the Vantage dual bed desiccant dryer is said to virtually eliminate temperature spikes, for example. “The off-line bed is cooled so that process temperatures are not affected during bed changeovers,” Una-Dyn said. And when it comes to dewpoint spikes, the problem might be exaggerated in the first place. A low dewpoint reading is good, but this doesn’t mean your resin is dry — just that the air is dry and has the ability to dry your resin. [...] Dual bed dryers employ two desiccant beds to limit the batch drying process to the roughly four hours required to extract moisture. Wheel dryers, by contrast, use molecular sieve desiccant formed into a continuously rotating wheel that constantly brings fresh desiccant on-line while the rest of the wheel is being regenerated and cooled. This continuous process means there is less variation throughout the drying cycle because there is internal cooling after desiccant regeneration and bed changeover is eliminated — which is why desiccant wheel dryers have become popular picks for many applications. 

 

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