Ind. Eng. Chem. Res. 1998, 37, 2061-2063
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Inhibition Effect in Supercritical Water Oxidation of Hydroquinone Chommanad Thammanayakatip, Yoshito Oshima, and Seiichiro Koda* Department of Chemical System Engineering, School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan
In the oxidation reactions of hydroquinone under a supercritical condition (temperature of 683 K and pressure of 24.5 MPa), the conversion was found to become saturated despite the very fast initial reaction. This behavior was quite different from that under a subcritical condition (temperature of 633 K and pressure of 24.5 MPa). Under both conditions, p-benzoquinone was found to be an important intermediate. The yield of CO2 was very small, which indicates a strong inhibition effect of hydroquinone and/or its derivatives. These inhibition phenomena should be taken into account very carefully in the application of supercritical water oxidation for treating waste organic materials where a complete decomposition is very important. Introduction The supercritical water oxidation (SCWO) is a promising technology for waste organic materials treatment (Tester et al., 1993). The relevant oxidation kinetics of organic compounds were reviewed recently (Savage et al., 1995). In the treatment of waste organic materials, complete oxidation is a definitive requirement. So far, a variety of organic materials have been tested and found to be oxidized within a reasonable residence time. However, much more detailed kinetic study is necessary for various organic compounds in order to make the SCWO process reliable enough and also to understand the fundamental aspects of reaction progress under the supercritical water conditions (T > 647.3 K, P > 22.1 MPa). In the present paper, we report a case of hydroquinone, which is known as an efficient inhibitor in various radical-chain reaction systems including autoxidations (Ingold, 1961). It is also an important aerobic oxidation intermediate of phenolic compounds (Devlin and Harris, 1984; Yang and Eckert, 1988; Gopalan and Savage, 1994). The objective of the present study of SCWO of hydroquinone is 2-fold. The first is to know whether SCWO can completely decompose even such substances as those possessing strong inhibition ability, and the second is to understand the reaction mechanism of SCWO with this clue. We have found a very specific inhibition behavior under supercritical conditions, and a quite large difference is suggested in the kinetics between supercritical and subcritical conditions. Experimental Section The experiments were performed using a conventional flow reactor. Figure 1 is a schematic drawing of the apparatus used in the present study. An electronically heated fluidized sand bath was used for heating the reactor and the preheat lines. As the reactor, a Hastelloy C276 tubing of 0.108 cm i.d. was adopted. The length was usually 0.75 m. However, two other lengths (1.8 and 6.6 m) were adopted for changing the residence * Corresponding author. Telephone: +81-3-3812-2111, ext. 7327. Fax: +81-3-5684-8402. E-mail:
[email protected].
Figure 1. Reaction apparatus.
time and also for changing the Reynolds number under the same residence time. As the preheat line for the decomposition of H2O2 to O2, a Hastelloy C276 tubing of 0.108 cm i.d. and 9 m length was usually used. A tubing of 5.4 m length was also used in order to check the effect of the decomposition efficiency of H2O2. As the preheat line for the hydroquinone supply, a tubing of 0.108 cm i.d. and 1.8 m length was employed. The temperature of the fluid in the tubing was monitored directly using two thermocouples at the entrance and the exit of the reactor. The difference between the two thermocouples was within (2 K, and their average was adopted as the reaction temperature. The fluid coming out of the reactor was promptly cooled by an external cooling-water flow, depressurized using a backpressure regulating valve, and separated to gaseous and liquid parts in a gas-liquid separator. The gaseous part was analyzed gas chromatographically. CO2 and CO yields were corrected for the dissolution loss into the liquid part in the gas-liquid separator, using the appropriate Henry’s law constants. The liquid part was quantitatively analyzed by HPLC. The species analyzed were hydroquinone and p-benzoquinone, which was found to be the principal intermediate product. Some unidentified product peaks were also observed in HPLC spectra. To avoid the change of hydroquinone and intermediates during the holding period of the sample before the analysis, the liquid samples were always introduced to HPLC within 1.5 min after the sampling. The aqueous solution of hydroquinone and that of H2O2 were pumped up to the desired pressure using high-pressure pumps (Tosoh CCPS) and preheated while flowing in the preheat lines, respectively. The two flows were mixed just before the inlet to the reactor and
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2062 Ind. Eng. Chem. Res., Vol. 37, No. 5, 1998
Figure 2. Comparison of hydroquinone and phenol conversion as a function of residence time. The hydroquinone (0) + O2 and the phenol (b) + O2 reaction were carried out at 693 K and 24.5 MPa, individually. The initial weight fractions (concentrations) are as follows: hydroquinone, 2.2 × 102 ppm (1.2 × 10-4 mol dm-3); phenol, 2.2 × 102 ppm (1.2 × 10-4 mol dm-3); O2, 3.2 × 103 ppm (6.3 × 10-3 mol dm-3).
Figure 3. Hydroquinone conversion vs residence time in hydroquinone + O2 reactions at 24.5 MPa at 633 (b) and 683 (0) K. The initial weight fractions (concentrations) at individual temperatures are as follows: hydroquinone, 2.2 × 102 ppm (5.8 × 10-4 mol dm-3); O2, 3.2 × 103 ppm (2.9 × 10-2 mol dm-3) at 633 K; hydroquinone, 2.2 × 102 ppm (1.3 × 10-4 mol dm-3); O2, 3.2 × 103 ppm (6.9 × 10-3 mol dm-3) at 683 K.
let into the reactor. H2O2 was considered to be completely converged to O2 before the mixing as described below, and thus the oxygen concentration was kept at least 5 times larger than what was necessary for the complete oxidation to yield 6CO2 + 3H2O. The kinetics of H2O2 decomposition
H2O2 f H2O + 1/2O2 in subcritical and supercritical water have been studied in detail up to 723 K (Croiset et al., 1997) in an Inconelmade reactor with a S/V ratio of 8.3 cm-1. In the present experiments, the feed rates of H2O2 were always controlled under the level that more than 99.99% of H2O2 would be decomposed, adopting the decomposition rate constants reported by Croiset et al. in the estimation. The S/V ratio of the present experiments is 37 cm-1, which is larger than 8.3 cm-1 in the work of Croiset et al., and thus the estimation using their rate constants should give the lower bounds in the present experiments, provided that Inconel in the work of Croiset et al. and Hatelloy C276 in the present experiments have similar surface activities for the H2O2 decomposition. We have verified, using a KMnO4 titration method, that the decomposition is always more than 99.7% (0.3% is the uncertainty of the experimental analysis) in the present experiments. Moreover, the fact that the remaining H2O2, if any, was not affecting the obtained results was born out using a shorter preheat line of 5.4 m at 683 K and 24.5 MPa, which gave the same conversion of hydroquinone with that obtained using a 9-m preheat line. Results and Discussion At first we investigated whether any pyrolysis of hydroquinone proceeds. When hydroquinone without oxygen was passed through the reactor at 693 K and 24.5 MPa, less than 1.5% hydroquinone was consumed at a residence time of 25 s, which is much longer than the adopted residence times in the oxidation reactions. Thus, the pyrolysis of hydroquinone in the preheat line as well as that in the reactor could be neglected. Figure 2 shows that hydroquinone is much more easily oxidized than phenol at least in the initial stage. This may be ascribed to the much more rapid radical attack to hydroquinone than phenol (Alfassi and Schuler, 1985).
Figure 4. Time profile of p-benzoquione in hydroquinone + O2 reactions at 24.5 MPa at 633 (b) and 683 (0) K. The initial weight fractions (concentrations) are the same as those of Figure 3.
We have studied the kinetic behavior of SCWO of hydroquinone under a pressure of 24.5 MPa with two different temperatures, 633 K (in the subcritical region) and 683 K (in the supercritical region). A remarkable self-inhibition, that is, a relatively constant conversion of about 0.98 continuing for a long time after the very rapid initial rise, is observed as shown in Figure 3 at 683 K. Any similar time behavior, however, has not been observed at the subcritical temperature, 633 K, where a gradual increase of conversion up to unity is observed. To check the effect of different Reynolds numbers, we have adopted a tubing of different lengths of 1.8 or 6.6 m at the same residence time. The results have shown that the present experiments are not affected by the different Reynolds numbers within the data scatter. The results shown in Figure 2 have been obtained with Reynolds numbers between 1000 and 5800 at 633 K and between 1500 and 10 000 at 683 K. p-Benzoquinone has been found as the principal intermediate product, whose time behaviors at the two reaction temperatures are described in Figure 4. It is noticed that the initial rise of p-benzoquinone corresponds to the consumption of hydroquinone, and then p-benzoquinone gradually decreases again due to some subsequent reactions. The p-benzoquinone yield at 683 K is very high, close to unity at the peak, and its subsequent decrease is relatively slow compared to that at 633 K. It is plausible that hydroquinone is partly recovered from p-benzoquinone, which explains, together with its relatively slow subsequent consumption, why the conversion of hydroquinone appears to remain
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HO-C6H4-OH + O2 f HO-C6H4-O• + HO2• 2HO-C6H4-O• f HO-C6H4-OH + OdC6H4dO HO-C6H4-O• + O2 f OdC6H4dO + HO2•
Figure 5. CO2 yield vs residence time in hydroquinone + O2 reactions at 24.5 MPa at 633 (b) and 683 (0) K. The initial weight fractions (concentrations) are the same as those of Figure 3.
The reason why hydroquinone is an efficient inhibitor is the very easy abstraction of H atoms from the OH groups of hydroquinone to yield the relatively stable p-benzoquinone, together with production of stable radicals during the subsequent radical-chain oxidation processes. The apparently quite different behavior of the kinetics between the subcritical and supercritical temperatures may be understood by analyzing the medium effect on individual oxidation steps under the two different conditions. Concluding Remarks
Figure 6. CO yield vs residence time in hydroquinone + O2 reactions at 24.5 MPa at 633 (b) and 683 (0) K. The initial weight fractions (concentrations) are the same as those of Figure 3.
relatively constant after the very rapid initial consumption. At 633 K, p-benzoquinone is also an important intermediate. However, it seems to react more easily in the subsequent steps. The yields of the atomic carbon base of gaseous products, CO2 and CO, are also plotted against the residence time in Figures 5 and 6, respectively. They are quite small under the supercritical conditions, which indicates that the reaction progress is interrupted at some intermediate stage before the complete oxidation to yield 6CO2 + 3H2O. Although the yields at subcritical temperatures are much higher than those at supercritical temperatures, they are still much smaller compared to the complete oxidation. The above-mentioned phenomenological kinetics are summarized as follows: (1) The reaction kinetics are quite different between the subcritical and supercritical temperatures. (2) In the supercritical region, the initial reaction is very fast, but it is interrupted rather suddenly. The conversion does not reach unity and is kept at about 0.98 for a relatively long period. The detailed reaction progress cannot be described at present; it is known that the aerobic oxidation of phenolic compounds is very complicated (Ingold, 1961; Devlin and Harris, 1984; Yang and Eckert, 1988; Gopalan and Savage, 1994). The following reactions are some of the candidate reactions relevant to the initial consumption and p-benzoquinone formation, consulting the previous aerobic oxidation studies. p-Benzoquinone is one of the common oxidation products from hydroquinone in various oxidation systems (Minisci et al., 1989). When the benzene ring of hydroquinone and/or p-benzoquinone is attacked, ring-opening and dimerization reactions may proceed.
The specific inhibition of SCWO of hydroquinone is important for two reasons. First, the behavior strongly suggests that the SCWO reactions are mostly of radicalchain reactions, though more detailed experiments and analysis are desirable. Second, in SCWO processes of organic substances, compounds similar to hydroquinone and/or the intermediates may be produced. To recognize the existence of such inhibition effects as described in the present paper and to avoid them should be very important for the treatments of various organic wastes using SCWO processes. Acknowledgment The present work is supported by “Research for the Future” Program by the Japan Society for the Promotion of Science (96P00401), which is greatly appreciated. We thank Dr. S. F. Rice for giving us information on the H2O2 decomposition rate. Literature Cited Alfassi, Z. B.; Schuler, R. H. Reaction of azide radicals with aromatic compounds. Azide as a selective oxidant. J. Phys. Chem. 1985, 89, 3359-3363. Croiset, E.; Rice, S. F.; Hanush, R. G. Hydrogen peroxide decomposition in supercritical water. AIChE J. 1997, 43, 2343-2352. Devlin, H. R.; Harris, I. J. Mechanism of the oxidation of aqueous phenol with dissolved oxygen. Ind. Eng. Chem. Fundam. 1984, 23, 387-392. Gopalan, S.; Savage, P. E. Reaction mechanism for oxidation in supercritical water. J. Phys. Chem. 1994, 98, 12646-12652. Ingold, K. U. Inhibition of the autoxidation of organic substances in the liquid phase. Chem. Rev. 1961, 61, 563-589. Minisci, F.; Citterio, A.; Vismara, E.; Fontana, F.; De Bernardinis, S. Facile and convenient synthesis of quinones from phenols. J. Org. Chem. 1989, 54, 728-731. Savage, P. E.; Gopalan, S.; Mizan, T. I.; Martino, C. J.; Brock, E. E. Reactions at supercritical conditions: applications and fundamentals. AIChE J. 1995, 41, 1723-1778. Tester, J. W.; Holgate, H. R.; Armellini, F. J.; Webley, P. A.; Killilea, W. R.; Hong, G. T.; Barner, H. E. Supercritical water oxidation technology. Process development and fundamental research. ACS Symp. Ser. 1993, No. 518, 35-76. Yang H. H.; Eckert, C. A. Homogeneous catalysis in the oxidation of p-chlorophenol in supercritical water. Ind. Eng. Chem. Res. 1988, 27, 2009-2014.
Received for review November 13, 1997 Revised manuscript received February 27, 1998 Accepted March 4, 1998 IE970801J
