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Ind. Eng. Chem. Res. 1998, 37, 336-340
Kinetics of Hydrolysis of PET Powder in Nitric Acid by a Modified Shrinking-Core Model Toshiaki Yoshioka,* Nobuchika Okayama, and Akitsugu Okuwaki Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aoba, Sendai 980-77, Japan
Poly(ethylene terephthalate) (PET) powder from waste bottles was degraded at atmospheric pressure in 7-13 M nitric acid at 70-100 °C for 72 h, to clarify the mechanism of a feed stock recycling process. Terephthalic acid (TPA) and ethylene glycol (EG) were produced by the acidcatalyzed heterogeneous hydrolysis of PET in nitric acid, and the resulting EG was simultaneously oxidized to oxalic acid. The kinetics of the hydrolysis of PET in nitric acid could be explained by a modified shrinking core model of chemical reaction control, in which the effective surface area is proportional to the degree of unreacted PET, (1 - X), affected by the deposition of the product TPA. The apparent rate constant was inversely proportional to particle size and to the concentration of the nitric acid. The activation energy of the reaction was 101.3 kJ/mol. 1. Introduction The hydrolysis of esters in neutral and acid media is well-known. The initial step in the hydrolysis is the addition of a hydrogen ion, followed by addition of H2O, and then, finally, the ester bond is severed. Many researchers have reported on the kinetics of the hydrolysis of polyethylene terephthalate (PET). McMahon et al. (1959), Golike et al. (1960), Davies et al. (1962), and Buxbaum (1968) studied the hydrolysis of PET film between 50 and 150 °C and at 100% relative humidity, with saturated solutions of potassium sulfate, sodium chloride sulfuric acid, sodium bromide, and sodium iodide, and reported that hydrolysis is limited by the diffusion of water molecules into the film. Ravens et al. (1961), on the other hand, investigated the hydrolysis of PET at between 150 and 220 °C, and measured the rate constant and the equilibrium constant. Mandoki (1986), Seo et al. (1991), and Campanelli et al. (1993) studied the hydrolysis of molten PET in an excess of water at 250-300 °C. Campanelli et al. reported that the activation energy in the melting range of PET is 55.7 kJ/mol. It is proposed that the reaction is catalyzed by hydrogen or oxonium ions from the carboxyl end groups and that the activation energy is 112.6 kJ/mol. Some studies of the hydrolysis of PET by acid catalysis, including hydrochloric acid (Ravens, 1960; Davies et al., 1962) and sulfuric acid (Brown and Obrien, 1976; Pusztaseri, 1982; Yoshioka et al., 1994), have been reported. Ravens et al. (1960) investigated the effect of hydrochloric acid concentration on the rate of hydrolysis of PET fiber at 70 °C and explained that the reaction rate is principally determined by the solubility of hydrochloric acid in the amorphous regions of PET. We investigated the effect of sulfuric acid concentrations up to 10 M at 150 °C (Yoshioka et al., 1994). In this case, hydrolysis was greatly accelerated at concentrations above 5-7 M due to an increase in the specific surface area of PET, resulting from the formation and growth of cracks. On the other hand, PET dissolved in concentrated sulfuric acid was quickly hydrolyzed when added to water (Pusztaseri, 1982). Appreciable amounts of terephthalic acid (TPA) and ethylene glycol (EG) were produced by both methods.
The melting point of PET is around 245-265 °C. The hydrolysis of PET is clearly a heterogeneous reaction at temperatures below its melting point and is thought to be a homogeneous reaction at higher temperatures. Since the reaction at temperatures below the melting point proceeds at the solid-liquid interface, the effects of the changing effective surface area of PET on the reaction should be considered. Such considerations for a heterogeneous reaction have not been discussed from the perspective of a shrinking-core model (Yoshioka, 1996) in spite of the importance of these kinetic effects on PET hydrolysis. The authors previously reported on the possibility of a feed stock recycling process for waste PET bottles using a nitric acid solution (JETRO, 1994). In this paper, we investigate the effects of nitric acid concentration (7-13 M), particle size (
