Ind. Eng. Chem. Res. 2008, 47, 4325–4331
4325
Wet Air Oxidation of Benzene Basim A. Abussaud, Nilgun Ulkem, Dimitrios Berk,* and George J. Kubes Department of Chemical Engineering, McGill UniVersity, Montreal, Que´bec H3A-2B2, Canada
The wet air oxidation of benzene has been studied in a 1.24 L stainless steel autoclave at a temperature in the range of 190-260 °C. The oxygen partial pressure was varied from 0.69 to 1.72 MPa. The initial C6H6 concentration was 5.63 mmol/L. The influence of the pH was studied, and the main reaction intermediates were determined. It was found that the temperature has a great influence on the wet air oxidation of benzene. The effect of the oxygen partial pressure was significant only at the beginning of the reaction. Decrease of the initial pH of the reaction results in a considerable increase in the benzene degradation rate, especially at the lower temperature. The main intermediates were found to be acetic acid and formic acid. Introduction Processing of organic compounds produces a large amount of wastewater that contains toxic (hazardous) materials that cannot be discharged to the environment without treatment. As restrictive environmental constraints increase, new technologies are needed to treat those toxic materials before discharging them to the environment. Wet air oxidation (WAO) is one of these methods. It is defined as the oxidation of soluble or suspended oxidizable components in an aqueous environment using either pure oxygen or air as an oxidizing agent at elevated temperature and pressure. It is a very attractive method for wastewater treatment, especially when the effluent is too dilute for incineration and either too toxic or concentrated for biological treatment. Typically, WAO takes place at a temperature of 125-320 °C and at a pressure of 0.5-30 MPa. The residence time ranges between 15 and 120 min. The need for the high pressure is to maintain the liquid phase at the elevated temperatures and to accelerate the oxygen diffusion/dissolution rate in the aqueous phase. The final products in this method are water, carbon dioxide, and low-molecular-weight organics. The overall reaction for organic materials can be expressed as organics + O2 f CO2 + H2O + LMWO
(1)
Petrochemical Wastewater Effluent. The waste effluents from the petrochemical industries contain a large amount of various organic compounds, which cannot be discharged to the environment without treatment. Among many methods, available an activated sludge treatment is the most widely used method because of its simplicity and low cost.1 However, it cannot be used for highly concentrated wastes because of their low biodegradability and inhibitory effects of the organic compounds.2 For the wastewaters containing phenol and phenolic compounds, which are very common in petrochemical industries, solvent extraction is an economical method if the concentration of these compounds is high (i.e., >1%).3,4 Incineration is an economically attractive alternative, although it is limited only for concentrated wastes and it contributes to the air pollution.1,5,6 Finally, another treatment method that needs to be investigated is wet air oxidation, which is used to convert most of the organic compounds that exist in the waste into carbon dioxide and water. Literature Review. During the last few decades, WAO has been the subject of intensive studies in both chemical and * To whom correspondence should be addressed. Tel.: (514) 3984271. Fax: (514) 398-6678. E-mail:
[email protected].
environmental engineering literature. The most important applications are municipal sewage sludge treatment, industrial wastewater treatment, carbon regeneration, and pulping spent liquors. Many of the studies that have been reported in the literature dealt with both phenol and carboxylic acids.5,6 WAO of Phenol. Phenol is considered to be one of the most common and important pollutants that can be found in the effluent streams of most chemical plants because of its high toxicity even at low concentration. Because of that, wastewater that contains phenol should be treated before it can be discharged to the environment. WAO is found to be an effective method for treatment of effluent that contains phenol. Table 1 shows some of the studies that dealt with the WAO of phenol. WAO of Formic Acid and Acetic Acid. Low-molecularweight carboxylic acids, especially acetic acid, are resistant to oxidation.14–17 They are either originally found in the waste or generated as an intermediate during the WAO of various waste streams and accumulated at the latter stages of WAO, hence becoming a major limitation of the process.18–24 Many studies that deal with the oxidation of the mono- and dicarboxylic acids were done. The aim was to determine the kinetic parameters together with total organic carbon (TOC) and chemical oxygen demand (COD) removal. Most of the studies were done using a high temperature that is in the range of 230-290 °C and a pressure of 0.35-12.8 MPa. Table 2 summarizes some of the studies that have been conducted on the WAO for both acetic acid and formic acid. Effect of Initial pH on the WAO. Initial pH has a significant effect on both the WAO reaction rate and pathway.9 However, the operating pH has a complex effect on reaction rate.9,13 Relatively few studies have been reported about the effect of pH on the wet oxidation of organic compounds.13 The pH of the solution has an effect on both the type of free radical reactions that occur during the WAO and the stability of the free radical intermediates formed. The oxidizing strength of free radical intermediates decreases at alkaline conditions in most cases.6 Kolaczkowski et al.9 studied the WAO of phenol at 200 °C and P of 3.0 MPa with different initial pH’s. It was found that the initial pH has a significant effect on the WAO reaction rate. For pH e 2 and between 7 and 10, no conversion was noticed. However, when no acid or base was added, i.e., pH is approximately 4, significant phenol decomposition was achieved. Also, when pH was adjusted to be above the pKa of phenol, rapid oxidation was achieved with no induction period. One reason for this can be the effect that pH has on the chemical structure of the phenol, i.e., the phenolate ion is highly
10.1021/ie800162j CCC: $40.75 2008 American Chemical Society Published on Web 05/29/2008
4326 Ind. Eng. Chem. Res., Vol. 47, No. 13, 2008 Table 1. Summary of the Results for WAO of Phenol reaction conditions T (°C)
PO2 (MPa)
150-180 300 200 300 220 220 230
results (removal) >90% COD 100% COD 95% phenol in 14 within 2 h at 165 °C.32,33 Imamura et al.28 showed in their study of the catalytic WAO of formic acid that the formic acid reactivity depends remarkably on pH. 100% TOC removal was achieved at 150 °C and PO2 ) 1.0 MPa at pH 1.9, while only 7% degradation was achieved at pH 5.6, which suggests that the reactivity of the formic acid is more than that of the formate ion since the pKa of formic acid is 3.75. They showed also that even the oxidation of the acetic acid was affected by the pH. While 44.5% TOC degradation was achieved at 200 °C in 1 h at pH 2.7, in the oxidation catalyzed by Ru/Ce, only 19.4% was achieved at pH 6.9. To our knowledge and by searching the literature, no single paper that deals with the wet air oxidation of benzene has been published. Benzene is known to be a carcinogenic compound that is usually found in the wastewater effluent of the petrochemical industries. This study deals with the WAO of benzene at different temperatures and pressures. The influence of the pH on the reaction has been investigated. Finally, some of the reaction intermediates have been determined. Experimental Section Materials. Benzene ACS grade is obtained from Fisher Scientific with a 99.9% purity and was used as received from the supplier. Oxygen from a cylinder with a minimum purity of 99.6 was used as oxidant. Helium, which is the inert gas used in the experiment, is also used from a cylinder with a purity
Figure 2. Schematic diagram of the experimental setup.
of 99%. Both oxygen and helium were obtained from BOC Gases in Montreal, Canada. Setup. A 1.24 L, stainless steel laboratory rocking autoclave was used to perform the experiments (Figure 1). A schematic diagram of the experimental setup is shown in Figure 2. For heating purposes, the bomb is fitted inside an autoclave shell that contains a 1600 W heating coil and is rocked by a small motor through approximately 30° to achieve a good mixing. The temperature is controlled to an accuracy of (2 °C by connecting the thermocouple directly to a temperature process controller. As a bomb modification, the capillary tubing is attached to the sampling line and the sample line is wellimmersed in the reaction mixture. The unit is also equipped with a pressure safety valve. Procedure. Benzene (5.63 mmol) was added to 1000 mL of water and stirred for about 1 h. Since the benzene is a very dangerous material, this was done in the fume hood and gloves were worn. The solution was then transferred to the reactor, which was then sealed at once. After that, all auxiliary components were connected and the reactor was purged with helium to get rid of any oxygen that existed in the reactor. Helium was added to the system before heating in order to keep the effluent in the liquid phase. Later, the reactor was placed inside the autoclave that has been heated for about 75 min to minimize the time needed to reach the target temperature. Then, the system was heated to the target temperature. The operating temperature was in the range of 190-300 °C. When the desired
Ind. Eng. Chem. Res., Vol. 47, No. 13, 2008 4327
Figure 3. Degradation of benzene with time at different temperatures: PO2 ) 1.38 MPa; 2, 220 °C; ×, 230 °C; 9, 240 °C; +, 250 °C; [, 260 °C.
temperature was reached, the reactor was pressurized with O2, and this was considered as time zero for the reaction. A liquid sample was withdrawn from the reactor periodically during and after the treatment. Later, the sample was analyzed by a gas chromatograph (GC) to measure the amount of benzene, and ion chromatography (IC) was used to determine the amount of acids produced. The samples were also tested for the total organic carbon (TOC). When measuring the TOC, both total carbon (TC) and total inorganic carbon (TIC) will be determined. The TOC will be the difference between both the TC and the TIC. This has been done in order to minimize any error that may occur because of the interference of the inorganic content. The above-mentioned procedure was repeated at different temperatures and pressures.
Figure 4. Degradation of benzene at T ) 220 °C and PO2 ) 1.38 MPa: [, pH ) 6; 9, pH ) 5; 2, pH ) 4.
Figure 5. Degradation of benzene at T ) 240 °C and PO2 ) 1.38 MPa: 2, pH ) 6; [, pH ) 5; 9, pH ) 4.
Results and Discussion The degradation of benzene has been studied at different temperatures, pressures, and pH’s to show their effects on the degradation. At the beginning of the study, two experiments were carried out without oxygen (using only inert gas) to see whether any pyrolysis of the benzene takes place. These two experiments were done with 11.3 mmol/L of benzene solution at 20 and 250 °C, respectively, under 1.136 MPa nitrogen pressure. In the absence of oxygen, no degradation was observed. These experiments also showed that the sampling procedure did not have any effect on the liquid concentration and all of the benzene stayed in the liquid phase throughout the experiment. Effect of Temperature on the Degradation of Benzene. The oxidation of benzene was studied at different temperatures ranging from 190-260 °C. In those experiments, the oxygen pressure was kept constant at 1.38 MPa at a given temperature. Figure 3 shows the degradation of benzene with time at 220, 230, 240, 250, and 260 °C, respectively, at pH 6. It can be noted from the figure that, as the temperature increased, the degradation of benzene became faster. Also, it can be seen that the reaction consisted of three steps, namely, induction time, fast reaction, and termination step. These steps have been reported by many researchers.7,10,34,35 During the induction time, there was an accumulation of the hydroxyl radicals, and as soon as there was enough hydroxyl radical, the reaction preceded quickly until most of the reactant was oxidized.36 After that, the reaction became slower until it was terminated. As the temperature increases, the induction time decreases until it disappears at 250 °C. The experiments were also performed at 190 °C in which no degradation of benzene was noticed. Influence of pH. The initial pH of the reaction medium has a significant effect on the benzene oxidation, especially at low oxidation temperatures. Figure 4 shows the effect of the initial
Figure 6. Degradation of benzene at T ) 190 °C and PO2 ) 1.38 MPa: [, pH ) 4; 9, pH ) 6.
pH on the benzene degradation at T) 220 °C and PO2 ) 1.38 MPa. It is clear that, when the initial pH was reduced from 6 to 5, the reaction became faster. However, when the initial pH was reduced from 5 to 4, the change in the reaction was only noticed at the first 5 min and after that the results were comparable. During the course of the reaction and because of the intermediates that were produced, the pH of the media kept changing; this complicated the understanding of the effect of the pH. Reducing the pH from 6 to 5 has a large effect on the degradation of benzene at 240 °C and PO2 of 1.38 MPa, while this effect disappeared when the pH was reduced from 5 to 4, which can be seen clearly in Figure 5. The effect of the pH was also studied at a temperature at which no benzene degradation was noticed, i.e., 190 °C, in order to determine whether or not the pH has a significant effect on the benzene degradation. Figure 6 shows that, when the initial pH was 6, almost no degradation was achieved even after 5 h of oxidation time. When the initial pH was lowered to 4, about 98% degradation was achieved in 1 h, which shows a significant improvement on the degradation. This proves the significant effects that pH has on the wet air oxidation as reported in the literature, since the pH of the solution has an effect on both the
4328 Ind. Eng. Chem. Res., Vol. 47, No. 13, 2008
Figure 7. Degradation of benzene at T ) 220 °C and pH ) 4: [, 0.69 MPa; 9, 0.86 MPa; 2, 1.03 MPa; -, 1.38 MPa; ×. 1.72 MPa.
Figure 10. Influence of PO2 on acetic acid formation, T ) 220 °C, pH ) 4: 9, 0.86 MPa; 2, 1.03 MPa; ×, 1.38 MPa; [, 1.72 MPa.
Figure 8. Degradation of benzene at T ) 240 °C and pH ) 4: 2, 1.03 MPa; 9, 1.38 MPa; ×, 1.72 MPa. Figure 11. Influence of PO2 on formic acid formation, T ) 220 °C, pH ) 4: [, 0.69 MPa; 9, 0.86 MPa; 2, 1.03 MPa; ×, 1.38 MPa; -, 1.72 MPa.
Figure 9. Degradation of benzene at T ) 260 °C and pH ) 4: 2, 1.03 MPa; 9, 1.38 MPa; ×, 1.72 MPa.
type of free radicals reactions that occur and the stability of the free radical intermediates formed.6 Effect of Oxygen Pressure on Benzene Degradation. The effect of oxygen partial pressure on the benzene degradation was studied at 220, 240, and 260 °C, respectively, at pH 4. At 220 °C, the degradation was studied in the range 0.69-1.72 MPa. It is apparent from Figure 7 that the degradation of benzene is faster with the increase of the partial pressure, especially in the first 5 min in which the reaction is fast. Figures 8 and 9 show the degradation of benzene at 240 and 260 °C with the partial pressure within the range of 1.03-1.72 MPa. Also here it was noted that, during the first 5 min, the reaction became faster with the increase of the oxygen partial pressure, and after that, the degradation became almost comparable. The main reason is that, at high oxygen pressure, more oxygen was dissolved in the water and, hence, faster reaction was achieved. Reaction Intermediates. Ion chromatography (IC) was used to determine some of the reaction intermediates. The main intermediates were acetic acid and formic acid. Negligible amounts of propanoic acid and glycolic acid were also detected. In addition, by using gas chromatography-mass spectrometry (GCMS), a negligible amount of benzoquinone was identified.
Figure 12. Influence of PO2 on acetic acid formation, T ) 240 °C, pH ) 4: 2, 1.03 MPa; 9, 1.38 MPa; ×, 1.72 MPa.
Most of the intermediates that were formed were degraded to CO2 and H2O as the reaction proceeds; however, only a small amount of acetic acid produced during the reaction was degraded, and most of it remained as a degradation product. Figures 10, 12, and 14 show the influence of PO2 on the acetic acid formation at 220, 240, and 260 °C, respectively, at pH ) 4. At all temperatures, the same trend was observed, which suggests that, at the beginning of the experiment, the acetic acid was produced and, as the reaction proceeded, some acetic acid was degraded while the remaining amount remained as a reaction product. However, as the temperature increased, a lower amount of acetic acid was produced, which can also be seen clearly in Figure 16. Also, it can be noted from the figures that there was no significant effect of the oxygen pressure on the production and degradation of the acetic acid. In Figures 11, 13, and 15, the influence of the PO2 on formic acid formation is shown again at 220, 240, and 260 °C and pH 4. It can be noticed that the higher the PO2 pressure, the higher was the amount of formic acid produced, especially at lower temperatures. Also, as the temperature increased, better degrada-
Ind. Eng. Chem. Res., Vol. 47, No. 13, 2008 4329
Figure 13. Influence of PO2 on formic acid formation, T ) 240 °C, pH ) 4: 2, 1.03 MPa; 9, 1.38 MPa; ×, 1.72 MPa.
Figure 14. Influence of PO2 on acetic acid formation, T ) 260 °C, pH ) 4: [, 1.03 MPa; 9, 1.38 MPa; 2, 1.72 MPa.
Figure 15. Influence of PO2 on formic acid formation, T ) 260 °C, pH ) 4: [, 1.03 MPa; 9, 1.38 MPa; 2, 1.72 MPa.
Figure 16. Influence of temperature on acetic acid formation, PO2 ) 1.38 MPa, pH ) 4: [, 220 °C; 9, 240 °C; 2, 260 °C.
tion was achieved (Figure 17). At 240 °C, all of the formic acid produced during the reaction was degraded after 30 min, while at 260 °C, no formic acid was noticed in the reaction product after 15 min. From these results, it can be concluded that, as the temperature increased, a lower amount of acids was produced, which suggested that, at higher temperatures, most
Figure 17. Influence of temperature on formic acid formation, PO2 ) 1.38 MPa, pH ) 4: [, 220 °C; 9, 240 °C; 2, 260 °C.
Figure 18. TOC vs time at different temperatures, PO2 ) 1.38 MPa, and pH ) 6.
of the benzene went to complete oxidation. On the other hand, at lower temperatures when larger amount of acids were produced, which lowers the pH of the reaction medium, slightly more benzene oxidation was achieved after 30 min of oxidation. In addition, total organic carbon (TOC) tests were performed for two reasons. The first one is to investigate whether most of the benzene degraded to organic compounds, or whether total oxidation to CO2 and H2O was achieved. The second reason is to make sure that the main reaction products were the acetic acid and the formic acid. The TOC had been done for different experiments that have been performed at different temperatures to study the effect of the temperature on the benzene oxidation. Figure 18 shows the TOC results obtained at 220, 240, and 260 °C. As the temperature increases, the TOC reduction increases only at the beginning. However, as the reaction proceeds, the reduction at these temperatures is comparable. This shows that, when the temperature was increased, the reaction proceeded fast to the end products, i.e., CO2 and H2O. Also, most of the product, which was mainly the acetic acid, stayed as acetic acid and would not degrade at the temperatures studied. On the other hand, the amount of TOC was compared to the total amount of carbon in the produced acetic acid and formic acid and in the unreacted benzene during the experiment to check the carbon balance. From Figure 19, it is obvious that no other products were found with a noticeable amount except acetic acid, formic acid, and benzene. In addition, another approach was used to double-check the reaction product in which the amount of CO2 produced during the oxidation was measured in order to close the reaction balance and, hence, to prove that there was no other product formed other than those mentioned earlier. The amount of CO2 in the gas phase was collected by passing the gas through a solution of a 2 M NaOH and then analyzed in the TOC analyzer. The results were then compared with the theoretical amount that should be present if only benzene, acetic acid, and formic acid were found in the
4330 Ind. Eng. Chem. Res., Vol. 47, No. 13, 2008
extended to National Sciences and Engineering Research Council of Canada (NSERC) for their financial support. Literature Cited
Figure 19. Change in carbon concentration with time, pH ) 6: [, total carbon from benzene, acetic acid, and formic acid at T ) 260 °C; 9, TOC at 260 °C; b, total carbon from benzene, acetic acid, and formic acid at T ) 220 °C; 2, TOC at 220 °C.
Figure 20. CO2 measurement, T ) 260 °C, and pH ) 6.
product. From the results, it was found that there was
