Читать книгу Plastics Process Analysis, Instrumentation, and Control - Группа авторов - Страница 51

The decomposition of waste plastic has been investigated at a relatively low temperature (95, 96). The process can decompose a mixed stream of waste plastic at a temperature generally less than 375°C in a hot oil medium. The process converts the polymeric structure of the waste plastic or plastics to smaller chemical molecules such as the monomeric units and related chemical structures at a relatively lower temperature. It also serves the market for such products.

The low-molecular weight distillate from the waste plastic may help reduce the demand for imported petroleum products and help decrease our dependence on foreign crude oil.

Before mixed plastic wastes were studied, each individual component plastic was thermally decomposed to have a basis for the difference between mixed and individual resource recovery, and whether the mixed plastic wastes when thermally decomposed had an unexpected interaction.

A laboratory setup was used for preliminary experimentation which consisted of a thermally regulated flask with water condenser. The flask temperature was regulated to within 5°C and the overhead distillate condensed with 16°C water while the amount of uncondensed overhead gas was measured. The SAE 50 oil and the appropriate plastic resin were placed into the flask and the flask was purged with nitrogen before heating began. After the proper time at temperature, the flask was quenched.

The above tests indicated that the apparent optimum temperature for the hot oil decomposition of PE and PP was about 425°C, about 400°C for PS, about 375°C for poly(ethylene terephthalate) (PET), and about 325°C for PVC. Thus a temperature staging process was employed with mixed waste plastics. PET was not employed in this experiment since its solid decomposition product tended to clog the apparatus. A further aspect in omitting PET was that recent trends in recycling of waste plastic have been to separate out the bottles made of PET and recycle them directly to the bottle manufacturer.

Then, a three-stage temperature experiment was performed using 270°C for 20 min, 410°C for 30 min, and 450°C for 45 min.