This gravity-flow, upright dryer saves on floor space and is particularly economical for removing volatiles to low or even non-detectable levels. By introducing gas near the discharge, the Purge Vessel obtains a nearly perfect counter flow of gas to material flow. This drying system requires the lowest possible quantity of stripping gas. Residence time variability is minimal, but length of residence can vary from a few minutes up to 30 hours. The Purge Vessel can be operated in batch mode to heat, dry and cool in a single unit.
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This horizontal design minimizes height requirements and provides continuous plug flow holding for up to four hours. Product material is conveyed in plug flow condition by a rotating screw shaft.
Gas purge can be introduced at any point either through the bed or in the freeboard space above for partial pressure control. |
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A special metering mechanical discharge device is available when operating with materials which have a high tendency to bridge or sinter, or when residence time distribution is most critical. This discharge also allows metering of reactor discharge. |
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APPLICATIONS SECOND STAGE DRYING (<0.05% TOTAL VOLATILES)
- Polyethylene, Polypropylene, PET, Polyoximethylene, PVA, STEAM STRIPPING, Polycarbonate
SYSTEM #1 -
Crystallization-Drying
 Heat treatment of polyester polymer is required to change its original amorphous state to a crystalline form. This process is call crystallization. The change is structure is exothermic and the rate of crystallization will decrease as the polymer becomes increasingly crystalline.
As the polymer changes from amorphous to crystalline form, it passes thorough a sticky stage until the product is surface crystallized. It is necessary to agitate it mechanically or pneumatically to prevent the crystals from sintering. The polymer is crystallized to 30% or more so further processing can be achieved without fusing the particles.
Drying polyester involves diffusion of moisture from the interior of the solid to the surface and then through the boundary layer surrounding the polymer particle to the gas phase. The rate of diffusion is dependent on the temperature of the polymer and the dew point or partial pressure of the gas surrounding it. Therefore, if the polymer can be held for a period of time at a high temperature in a gas having a low partial pressure, diffusion will take place. A Hopper Dryer is used to do this.
A drying gas with low dew point is purged counter current to the product flow. This carries off moisture to the top of the Hopper. The rate of discharge from the Hopper Dryer is held the same as the feed rate. The volume of the Hopper is determined by the residence time required by the polymer to dry. A 4:1 length/diameter ratio and an internal baffle are used to assure a close approximation to plug flow.
SYSTEM #2 - Copolymer Processing
 Polymer producers are constantly developing new grades of copolymers for use in a variety of fiber film and packaging applications. Polyester copolymers are difficult to crystallize because they have reduced rates of crystallization. These copolymers put increasing emphasis on material handling methods which will be reliable and produce the high quality polymer required. Bepex International has responded to these needs with a number of products.
- Crystallizers with residence time capability of more than one hour and multiple heating stages.
- Holding vessels to provide optimum agitation/plug flow conditions.
- Discharge devices which overcome occasional sintering or bridging problems.
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