The Oxidation of Valeric Acid in Aqueous Solution zy N. NIKOUOU, N. ABATZOGLOU, S. GASSO* and E. CHORNET** Dbpartement de g h i e chimique, Universitt! de Sherbrooke, Sherbrooke, Qubbec Jl K 2Rl The liquid phase oxidation of an aqueous solution of a lower fatty acid, valeric (pentanoic) acid, has been studied at high temperatures (240-325°C) and pressures (10.3-10.4 MPa) in a batch autoclave reactor. The objective of the study was to develop a mechanistic understanding of the wet oxidation of industrial wastewaters contaminated with vegetable oils. Valeric acid was chosen as the prototype molecule. High performance liquid chromatography (HPLC) has been used to identify the liquid products and estimate their concentrations as a function of the extent of oxidation. Moreover the technique of GC-MS has also been used to obtain information on specific intermediate oxidation products that could not be identified by HPLC. The gaseous products of the reaction have been also identified and quantified. zyx A detailed reaction mechanism is proposed based on free radical reactions. L’oxydation en phase liquid d’une solution aqueuse d’acide pentanoique (valtrique) a Ctt Ctudite zyxw B des pressions (10.3-10.4 MPa) et des temp6ratures Clevtes (240-325°C) dans un autoclave. L’objectif de cette ttude est de dtvelopper un mecanisme de l’oxydation humide des eaux ustes industrielles contamintes par des huiles vtgktales. Comme prototype de molCcule, l’acide pentanoique est utilisC pour l’ttude. La chromatographie en phase liquide zyx 2i haute performance (HPLC) est utilisee pour identifier les produits liquides et pour estimer leur concentration en fonction du degrC de l’oxydation. De plus, la technique de GC-MS est aussi utiliste pour obtenir de l’information sur les produits intennuiaires de l’oxydation p i ne peuvent &re identifiks par HPLC. Les produits gazeux de la rtaction ont CtC aussi identifies et quantifits. Un mecanisme de reaction dttaillk est proposC bast sur la formation des radicaux libres et des radicaux peroxydes. Keywords: wet oxidation, he wet oxidation process is an effective method of zyxwvu T destruction of toxic organic molecules present in wastewaters (Foussard et al., 1989). Its application to fatty acids, other than acetic and propionic acids, is, however, not well documented despite the significance of such treat- ment in the destruction of the toxicity in wastewaters derived from food processing plants (oils and fats). Our goal was to investigate the mechanism of oxidation of a fatty acid, valeric acid, in order to understand the mechanistic pathways leading to simple, non-toxic molecules compatible with biological treatments. The literature on oxidation of lower fatty acids is scarce. Day et al. (1973) reported on the wet oxidation of propionic acid and derived an expression for the rate of destruction of propionic acid as well as a mechanistic model leading to the formation of acetic acid and acetaldehyde. In a more recent study, Ito et al. (1989) have proposed that acetic acid is the major intermediate product in the oxidation of carboxylic acids. In a study on phenol oxidation, Devlin et al. (1984) identified, as intermediate oxidation products, mono- and bi-carboxylic acids as well as hydroxy-acids. As well, Foussard et al. (1989) and Li et al. (1991) clearly indicate that upon oxidation of acidic wastes the lower molecular weight fatty acids are the most difficult inter- mediate products to fully oxidize to C02, the ultimate oxidation product. Since we felt that the literature was still incomplete we decided to undertake a detailed study to determine the oxidation pathways. Pentanoic (valeric) acid is the prototype molecule chosen for our studies. The methodology followed was to vary the conversion of valeric acid over the range 0-100% by varying the reaction temperature and time sequence. The intermediary compounds produced during the oxidation were determined and a reaction pathway proposed *On leave from the Instituto de TecnoIogias del Medio Ambiente (ITEMA), Universitat Polittcnica de Catalunya, Barcelona, Spain **Author zyxwvutsrqponm to whom correspondence should be addressed. mechanism, valeric acid. to explain their presence. The mechanistic background is the extensive work of Emanuel et al. (1967) on the liquid-phase oxidation of hydrocarbons. Experimental REACTANTS Pentanoic (valeric) acid, 99+ % pure (Aldrich. Chem. Co.), and standard purity 99.99% oxygen (Liquid Carbonic Inc.) were used. For each series of experiments a pentanoic acid aqueous solution at about 20 g/L was used. No discern- ible impurities were observed during the chromatographic analysis of the initial solution. For each experiment 200 ml of the initial solution were charged to the reactor. APPARATUS AND PROCEDURE The reactor (THP Inc.), having an internal diameter of 50 mm, was a 535 cm3 316 stainless steel stirred autoclave. The autoclave head was secured tightly onto its body with the aid of four symmetrically placed screws. A 316 stain- less steel O-ring was placed in between the head and the autoclave body for effective sealing. The oxygen inlet dip tube and the outlet gas tube were attached to the reactor via cone and thread fittings. The outlet tube was connected through a two way vent valve (Autoclave Engineers Inc.) to either a gas sampling tube or to the atmosphere. After closing the two-way vent valve the oxygen inlet value was opened and the pressure of oxygen was set at 10.3 MPa with the aid of a manometer attached to the inlet tube. At this pressure, the reactor was checked for gas leaks. If no leaks were detected the initial experimental oxygen pressure was then set at 4.1 MPa. It should be noted that this quantity of oxygen is 2.43 times the stoichiometric one needed for the complete oxidation of the pentanoic acid to C02 and H20. The temperature was measured by a thermocouple inserted into a thermowell immersed in the aqueous phase at about three-quarters of the liquid height. The thermocouple was connected to a data acquisition system HP3497A to measure 522 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING, VOLUME 72, JUNE, 1994