Polymer Solvent Drying
Wednesday, February 11th, 2015
Problem: The production of many petroleum and biomass based polymers involves contact with chemical solvents. This contact may occur during the polymerization/reaction step, or to extract impurities from the final product. Typically, the liquid solvent is first removed mechanically by centrifuge or filter. However, after mechanical separation many polymers still contain 30-60 weight percent solvent. Consequently, a solvent extraction drying step is required to produce the desired end product.
One example of this commercial requirement was:
- Polymer (Polyethylene) Solids Rate: 30,000 kg/hr
- Hexane Content: 35% wt
Additional Considerations included:
- Polymer flowability was poor in the 35-25% range
- An existing dryer was to be used as a second stage dryer below 25%
- Polymer softening temperature was 110-120°C
- Hexane was to be recovered so low dryer gas flow rate was desirable to improve solvent extraction efficiency and reduce costs
Centrifuge product entered the Twin TorusDisc at approximately 35% hexane and 65°C. The material was then heated to the normal boiling point of hexane (69°C) and dried to approximately 25%. Heat is supplied to the material by conduction between the material and the hollow rotor discs. The rotor discs were heated with saturated steam at 110°C to avoid softening or melting of the material. The exhaust gas from the Twin TorusDisc was then filtered and cooled to recover liquid hexane. The Twin rotor TorusDisc with intermeshing discs is well suited to effectively mix and convey poor flowing materials.
Benefits and Results:
|Wetcake Values||Final Product|
|Product Consistency||Poor Flowing Wetcake||Wetted Powder with Improved flowability|
- Reduced hexane content to desired level
- Improved flowability of material to the point that could easily be handled by the existing downstream second stage dryer.
- The solvent extraction process allowed for the evaporated solvent to be recovered and recycled back into upstream process.