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March 10, 2004

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Technology Notebook: Desiccants and drying, optimized






Editor?s note: Kaihan Tavakoli is product manager at ACS Group.

Desiccant dehumidifying is a key tool in the plastics processing industry?s arsenal of resin-drying options. It is a preferred method for handling the high moisture loads that result from the need to dry hygroscopic polymers in large quantities for applications such as PET preform molding, high-volume extrusion of sheet and film, or large-part injection molding. This article addresses the basics of desiccant drying, characteristics of desiccant materials, and recent technical developments in desiccant-dryer design.

Drying with Desiccant

A desiccant dryer works by moving humid air through a bed of desiccant beads. These beads adsorb moisture out of the airflow (see box, lower right). The resulting dried air re- adsorbs moisture when directed through the resin hopper. After a set time, or when the dewpoint of the air reaches a setpoint, the dryer switches airflow from one desiccant bed to another?dryers typically have two or more?while the first bed is regenerated. (The dewpoint is the temperature at which the water vapor content of the air exceeds saturation and falls out as condensation, or dew.)

The bed being regenerated is now heated with air that is very hot, perhaps 500F. This hot air makes the desiccant give up the moisture adsorbed previously. The desiccant is then actively cooled or simply allowed to cool to ambient temperature to begin repeating the cycle. Many types of desiccant are available. Those used in plastics processing include silica gel, lithium chloride salt, and most prominently, a molecular sieve composed of a crystalline alumino-silicate material.

  • Silica gel has a high capacity for moisture. While it can adsorb up to 35% of its weight by moisture, it is not widely used in resin drying because of performance limitations. As dewpoints get lower, the capacity of silica gel to adsorb moisture rapidly falls off. Under typical PET drying conditions, as much as four times more silica is required, by weight, as compared to molecular sieve adsorbent. Silica gel is, however, commonly used for mold dehumidification purposes.

  • Lithium chloride salt also has a high affinity for moisture, but it is not easily regenerated. Also, its low crush strength results in poor mechanical stability, which adversely affects its service life?another reason that makes it unsuitable for dryer applications. However, lithium chloride is used in dewpoint sensor technologies.

  •  A molecular sieve is often used in plastics resin drying because it has a high affinity for moisture at very low dewpoints. It is typically regenerated at 400 to 500F. When regenerated at 450F, molecular sieves are left with 3% to 4% moisture content depending upon the dewpoint of the air used in regeneration. When a molecular sieve is cooled to 120 to 130F, it exhibits approximately 7% to 8% moisture adsorption capabilities with a dewpoint below -40F. This capacity is reduced when the cooling cycle employs ambient air instead of air recirculated through a closed loop.

Different shapes and sizes of molecular sieves are commonly used in resin drying. 4A corresponds to a pore size of 4 Angstroms, 5A to 5 Angstroms, and 13X to 10 Angstroms. Because of its large pore size, 13X can adsorb more moisture and regenerate faster. It has a lower mechanical crush strength, slightly reducing its service life. Molecular sieve crush strength is reduced to 20% when saturated (27% moisture content) vs. dry (3% moisture content).

Molecular sieves are available in different particle shapes including spherical beads (4 by 8 ? 1/8-inch diameter, 8 by 12 ? 1/16-inch diameter), cylindrical beads, and also in chip form. The spherical shape is most used in plastics because air flows over it with much less pressure drop. Also, the smaller beads are more suited to desiccant dryers because of their greater ratio of surface area per density that allows them to heat up and cool down faster. The 8 by 12 bead size is optimum.

In large-capacity dehumidifying dryers used specifically for PET applications, where a low dewpoint is critical to achieving a final moisture content of 30 to 50 parts-per-million (.003% to .005%), 13X molecular sieve, with 8 by 12 bead size, is recommended. Because of its large pore size, 13X can go beyond just dehumidification to also remove some of the contaminates that can out-gas from resin during the drying process and could potentially cause problems when processing PET bottles and extruding sheet and fiber.

Solid Desiccant Canister Design

Typical large dryer designs use solid, noninsulated desiccant canisters that require as much as four to eight hours for desiccant regeneration. Insulation is not used because, for part of the cycle, desiccant has to be cooled and the presence of insulation would delay this process. Because of the lack of insulation, uniform heating of desiccant is impossible and more desiccant and airflow is required to transfer heat.

For purging of moisture from the desiccant, a blower pushes air through a heater until the air is heated to a high temperature. The heated air is then forced through the face surface of the desiccant and travels its length, then purges to the atmosphere. After the moisture is purged, the desiccant must be cooled to 130F or 120F to obtain maximum efficiency. When these lower temperatures are reached, desiccant is ready to adsorb moisture again.

Solid desiccant canister designs require high-horsepower blowers to overcome pressure losses. So, if a dryer is placed in process while saturated (such as at startup), material is exposed to heat for as much as 10 to 12 hours and to humid air for more than 4 to 8 hours. Typical recommended drying time for PET is 4 to 6 hours. PET degrades when it is exposed to heat and humidity for longer periods.

Hollow-core Desiccant Canister Design

In an effort to advance the efficiency and reliability of drying PET and other large loads (2000 to 3000+ lb/hr), ACS Group (Wood Dale, IL) is introducing a new line of large-capacity molecular sieve dryers. The dryers incorporate a significant advance in the way molecular sieve desiccant is presented to the airflows of heating and cooling cycles, along with several other improvements.

The new desiccant drying development places electric heaters or gas-fired heat exchangers directly within a hollow-core desiccant canister. Designated HiCore (Heater-in-Core), this desiccant canister technology provides large dryers with rapid and efficient regeneration and cooling, improved energy efficiency, and enhanced safety compared to solid-canister designs. After leaving the process blower, air comes into contact with the outer wall of the desiccant canister, further cooling it prior to entering the outer cylindrical screen section of the canister. Air entering the outer perimeter of the desiccant cylinder encounters the large surface area (over large volume), which produces a lower velocity and lower pressure for longer contact time between air and desiccant. As the air travels across the desiccant cylinder to get closer to the inner wall, desiccant volume decreases while air pressure and velocity increase for increased moisture adsorption.

Performance is improved because the hollow-core construction induces airflows across the desiccant in such a way that pressure and velocity changes significantly improve adsorption and regeneration efficiency. The hollow desiccant canister helps reduce regeneration time to less than 1 hour. The net result is a reduction in blower and heater energy consumption, translating to energy cost savings of about 10% to 15%.

In the hollow-core design, heaters located within the desiccant canister heat the incoming air. The heated air pressurizes the inner hollow core of the desiccant canister, which is uniformly distributed through the wall of the canister. As the heated air moves through the desiccant, air pressure and velocity are reduced causing the air to expand?allowing longer contact time between air and desiccant?and resulting in efficient desiccant heating. Cooling is achieved by de-energizing the heaters while continuing the airflow.

In addition to the HiCore canister technology, the large-capacity molecular sieve desiccant dryers recently developed by the ACS Group include the following features:

  • Double-wall constructed stainless steel heater boxes, and stainless construction throughout,

  • A simple closed loop cooling valve for the regeneration cycle,

  • A cooler for closed loop regeneration cooling,

  • One valve vs. the typical two-large- valve design,

  • An electric motor-driven valve vs. conventional pneumatic power, and,

  • NEMA 12 control boxes.

Contact information
ACS Group, Wood Dale, IL
Kaihan Tavakoli
(630) 595-1060
[email protected]

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