Two stage coprocessing of coal with model and commingled waste plastics mixtures

Abstract

Coprocessing of coal with waste plastics in two separate stages was investigated in order to tailor the reaction conditions and catalysts used in each stage for the materials present. In the first stage reaction, waste plastics were liquefied at 440°C (713 K) for 1 h with an initial H 2 pressure of 2.8 MPa and a catalyst, either the zeolite HZSM-5 or one of two cracking catalysts, Low Alumina or Super Nova-D, which was used to break selectively the hydrocarbon chains in the waste polymer. In the second stage, coal was added and the reaction conditions were selected to maximize the coal conversion and hydrotreatment of the dissolving coal molecules. The solvent used in the second stage was the hexane soluble fraction from the first stage reaction. Most of the second stage reactions were performed at 400°C (673 K) for 30 min, with an initial H 2 pressure of 5.6 MPa and a slurry-phase hydrotreating catalyst, either Mo naphthenate, Fe naphthenate, or a combination of the two, plus excess sulfur. Two types of waste plastics, a model mixture composed of 50% high density polyethylene (HDPE), 30% polyethylene terephthalate (PET), and 20% polystyrene (PS), and an actual mixture, composed primarily of HDPE and polypropylene (PP), were used. The catalysts chosen for the first stage reaction were found to have a significant effect on the quality of the hexane soluble liquids produced, and, hence, also affected the second stage processing. The second stage catalyst affected the overall product and boiling point distributions. Separation of the two reactions into two stages afforded the opportunity for selecting reaction conditions and catalysts to obtain a higher conversion and production of materials in the desired boiling point range than that available using single stage processing.