Study Of Polystyrene And Polypropylene Degradation In Near-critical Water

This paper combines the characteristics of supercritical water method and catalytic degradation process and study the pyrolysis method of the mixed plastics polystyrene(PS) and polypropylene(PP) by efficient use of a catalyst in the near-critical water, at lower temperature and pressure, thus search after a new process of the mixed plastics pyrolysis, which achieve the goals such as reducing cost, optimizing reaction condition, recycling resources and so on.Based on the analysis of macroscopic properties (BET surface area and pore size distribution) of different catalyst, we select the ideal plastic waste fluid cracking catalyst——a composite catalyst: Y-SiO2-Al2O3(YSA), which has good BET surface area and pore size distribution . Its BET surface area, volume of mesopore and micropore increase significantly than those of SiO2-Al2O3, and the adsorption characteristic also significantly changed for the better.We can obtain the degradation process conditions of PS by experimental results: the ratio of water/PS is 3/1, reaction temperature is 340℃, reaction time is 1h,catalyst is Y-SiO2-Al2O3 (amount is 3%). In these process conditions, the degradation effect of PS by Y-SiO2-Al2O3 is superior than by SiO2-Al2O3 or Y-zeolite: the liquid yield is nearly 93%, higher 6～10% than others.As a catalyst to Y-SiO2-Al2O3, we study the effect of raw material ratio (water/mixed plastics), PS/PP, amount of catalyst, reaction temperature and time on the pyrolysis reaction of mixed plastics (PS, PP) by orthogonal experiments. From the range analysis and effect curve, we can draw optimization of the reaction conditions: raw material ratio (water/mixed plastics) is 7/2, PS/PP is 7/3, amount of catalyst is 4.5%, reaction temperature is 360℃and reaction time is 3h.We analyze the pyrolysis products by the means of viscosity test, atmospheric and vacuum distillation and GC-MS. The results show that the mixed plastics PS, PP can be fully degradable (liquid yield is 93.34%, gas yield is 4.38%, residue is less than 2.3% and the average carbon number of products is less than 11), when raw material ratio (water/mixed plastics) is 7/2, PS/PP is 7/3, catalyst is Y-SiO2-Al2O3 (amount is4.5%), reaction temperature is 360℃and reaction time is 3h. The liquid degradation products are light oil, which are mainly composed of 12.25% toluene, 46.21% ethylbenzene, 26.63% cumene and a few short chain aliphatic compounds.