Oxidation of Industrial Dyeing Wastewater By Supercritical Water Oxidation in Transpiring-Wall Reactor

By Gong, Wei-Jin Li, Fang; Xi, Dan-Li

ABSTRACT: Industrial dyeing wastewater was oxidized in supercritical water in a transpiring-wall reactor, using hydrogen peroxide as an oxidant. Experiments were performed at 595 to 704 K and 18 to 30 MPa, with an oxidant dosage ratio ranging from 0.6 to 2.0. A chemical oxygen demand (COD) removal of more than 98.4% was achieved at 704 K and 28 MPa, with a retention time less than 35 seconds, which increased with the temperature, pressure, and oxidant. A modified first-order rate expression was regressed from experimental data, taking into account the influence of induction time. The resulting pre-exponential factor, A, and activation energy, Ea, were 1.07 seconds^sup -1^ and 12.12 kJ * mol^sup -1^, respectively, while the reaction order for feed wastewater (based on COD) and oxidant were assumed to be 1 and 0, respectively. Gas chromatography/mass spectrometry analysis for effluents indicated that carbon dioxide, carbon monoxide, and nitrogen were the main reaction products, and phenol; benzenecarboxylic acid; 1, 2- benzenedicarboxylic acid; and isoquinoline were detected as intermediates. Water Environ. Res., 80, 186 (2008).

KEYWORDS: supercritical water oxidation, industrial dyeing wastewater, reactive dye, transpiring-wall reactor, kinetic.

doi:10.2175/106143007X221067

Introduction

Supercritical water oxidation (SCWO), defined as oxidation in water at a temperature and pressure above its critical point (647 K and 22.1 MPa), is a very powerful technology for the destruction of aqueous waste streams (Svishchev and Plugatyr, 2006) and hazardous (Veriansyah et al., 2005) or high-risk wastes (Kronholm et al., 2003; Vostrikov and Psarov, 2002). Supercritical water is completely miscible, with organic material and the oxidant (oxygen, air, and hydrogen peroxide) creating a homogeneous reaction media, without interface mass-transfer limitations. The resulting effluent complies with the most stringent environmental regulations and can be disposed of without further treatment. Several papers (i.e., Kritzer and Dinjus, 2001; Savage, 1999) have reviewed the main characteristics of SCWO.

The SCWO has been studied since the beginning of the 1990s. Many papers were published between 1995 and 2000 with model compounds and simulated wastewater as main objects (i.e., Biancheria et al., 1999; Dagaut et al., 1996; Mizuno et al., 2000). Currently, SCWO has been developed at the pilot and industrial scale to treat industrial wastewater. Some researchers (Baura et al., 2005; Cocero et al., 2000; Jin et al., 2001; Portela, Lopez, and Nebot, 2001) have studied oxidation of industrial wastes using SCWO. Most results showed high destruction efficiency for chemical oxygen demand (COD) or total organic carbon removal of more than 99%, and carbon dioxide, water, molecular nitrogen, and small molecular organic compounds as main products.

However, corrosion and salt deposition are still the most severe problems that SCWO process faces currently. Different reactors designed during SCWO to handle salt deposition and corrosion have been proposed; one of them was a transpiring-wall reactor (TWR) (Bermejo, Fernando, and Cocero, 2006), which consists of a reaction chamber limited by a porous wall, through which clean water flows continuously. This cold water creates a thin layer that protects the wall against corrosion and salt deposition. A number of reports (Bermejo and Cocero, 2006; Lee et al., 2005; Prikopsky et al., 2006) stated that TWR reactors show high decomposition efficiency of reactant and great performance against corrosion in treating artificial or industrial wastewater.

Because of the presence of dyes, textile industry wastewater is difficult to treat by traditional wastewater treatment technology. As a result of their complex structure and synthetic nature, degradation of dyes, especially azo dyes, is difficult. Azo dyes are resistant to biodegradation because of biotoxicity and the possible mutagenic and carcinogenic effect (Chang et al., 2001). Although the degradation of dyes by physical (Goloba et al., 2005), chemical (Rajkumar and Kim, 2006), and even biological (Zheng and Liu, 2006) processes is widely reported, these techniques suffer disadvantages of incomplete degradation, sludge generation, adsorbent regeneration, and formation of more toxic byproducts.

Although many studies have been conducted regarding SCWO of artificial and industrial wastewater, there is a lack of available data concerning industrial dyeing wastewater.

The aim of this work is to present the efficiency of industrial dyeing wastewater treated in supercritical water using a TWR reactor with hydrogen peroxide as an oxidant, to study the effects of oxidant dosage, temperature, and pressure on COD removal and to analyze the reaction kinetic and oxidation pathways.

Methodology

Materials and Analytical Methods. Wastewater from the effluent of a fabric dyeing plant was used as a feed solution and hydrogen peroxide with 30% purity was used as an oxidant. Diluted oxidant solutions were prepared using deionized water. Porous ceramic tubes with an inner diameter of 9 mm and length of 500 mm were used for the transpiring wall.

Liquid samples collected from an effluent storage tank were stored at 277 K until the time of analysis. The operational temperature and pressure values in the experiment were within the range 595 to 704 K and 18 to 30 MPa, respectively. Gas samples were analyzed by gas Chromatograph (HP 5890series II, Hewlett-Packard, Houston, Texas). Reaction intermediate was extracted from the liquid phase using aether and analyzed by gas chromatography/ mass spectrometry analysis (HP-6890, HP-5973; Hewlett-Packard).

Procedure and Operating Conditions. Figure 1 shows a schematic diagram of the SCWO system using the TWR. The major components of the system were three feed tanks for wastewater, oxidant, and pure water, respectively; high-pressure plunger pumps; preheaters; TWR reactor; manometers; thermocouples; temperature controllers; cooler, gas-liquid separator, and back-pressure regulator. The reactor body was made of Hastelloy C-276 (Fengqu Special Alloy Corporation, Shanghai, China). The porous ceramic tube was located inside the reactor vessel as an internal reaction zone with a volume of 31.8 mL.

Three feed streams (i.e., wastewater, oxidant, and pure water) were preheated and pressurized into the system by separate plunger pumps (ZJ-X series 2/50, Ligao Industrial Equipment Corporation, Zhejiang, China). Wastewater and oxidant streams mixed at the reactor entrance and entered the reaction zone. Simultaneously, pure water was supplied to the space between the pressure vessel and porous ceramic tube. The water penetrated the porous ceramic tube through numerous micro-pores. As a result, the water was completely mixed with wastewater and oxidant within the internal reactor. The reactor vessel was protected from corrosion by a water layer between the vessel and ceramic tube. The effluent was cooled rapidly in a tube heat exchanger, and then the product stream was separated into liquid and vapor phases via a liquid-gas separator. Tests were operated at the desired temperature and pressure, and samples were collected for further analysis.

All hot sections of the system were insulated with asbestos. The temperature inside the reactor was controlled by a temperature controller and monitored by thermocouples (K-type). The reactor pressure was adjusted by a back-pressure regulator and monitored by a pressure gauge. The residence time was controlled by adjusting the flowrate with a high-pressure plunger pump, which was calibrated frequently with water at the test pressure.

The TWR system used in this work has been presented in a previous study (Lee et al., 2005). However, the main difference from that paper is that a cooling chamber was designed. The temperature in the cooling chamber is at a water subcritical point. It is possible for salt precipitated in the reactor to dissolve at this section. Another difference is that the micro-pore diameter of the ceramic tube is 2 [mu]m bigger than the 0.1 to 0.2 [mu]m reported in the literature (Bermejo, Fernando, and Cocero, 2006), which can slow the rate of pore plugging and prolong the life of the ceramic tube.

The experiments were conducted in the TWR reactor, and the reaction retention time was calculated using following equation:

Results and Discussion

Effect of Operating Condition on Supercritical Water Oxidation Industrial Dyeing Wastewater. Effect of Oxidant Dosage on Chemical Oxygen Demand Removal. Because of the complexity of dyeing wastewater, the amount of hydrogen peroxide needed to fully oxidize organic compounds in wastewater could not be calculated using established chemical reaction equations. However, we could estimate stoichiometric amounts of hydrogen peroxide according to the amount of COD in the feed wastewater. In the tests, the dosage ratio actually added to stoichiometric amounts of hydrogen peroxide was defined as r, which represented the level of oxidant added.

The effect of oxidant dosage on COD removal was investigated at T = 693 K, P = 28 MPa, and t =34.3 seconds. As shown in Figure 2, without the addition of oxidant (r = 0), the value of X was 42.8%. A possible reason for this is the hydrolysis of reactive dyes, which increased with the reaction temperature. In the presence of hydrogen peroxide, X increased rapidly, from 42.8 to 98.4%, as r increased, from 0.6 to 2.0. At r = 1.2, X increased rapidly; over that point, X hardly changed. A possible reason for this was that, when r < 1.2, the oxidant dosage was not enough for the reaction, and the decomposition rate of dye molecules was accelerated by the addition of oxidant, resulting in a rapid increase in X. However, beyond the point 1.2, most organic compounds existing in wastewater have been oxidized to stable products. Beyond the point 1.2, the addition of oxidant hardly favored a reaction. In summary, according to the experiments, the hydrogen peroxide dosage had a positive effect on COD removal. Even though X increased continuously with the increase in r, from 1.2 to 2.0, we investigated the following experiments at r = 1.2, taking into account economic cost. Effect of Temperature and Pressure on Chemical Oxygen Demand Removal. It is well-known that pressure and temperature are important parameters in the SCWO process, because they produce changes in the phase behavior and thermodynamic properties of the system (Bermejo, Bielsa, and Cocero, 2006). However, the influence of pressure on the oxidation rate is still not clear. Many researchers have studied the effect of pressure and temperature on feed wastewater oxidation efficiency, but different influences have been discussed. Some researchers (Anitescu and Tavlarides, 2000; Martino and Savage, 1997; Oshima et al., 1998) concluded that oxidation efficiency increased with pressure. Nevertheless, others (Krajnc and Levee, 1996; Li et al., 1991) disagreed with such conclusions.

In this work, experiments have been performed to investigate the effect of temperature and pressure on X at t = 34.3 seconds and r = 1.2, with temperature and pressure changing from 595 to 704 K and 18 to 30 MPa. The results are shown in Figures 3 and 4, respectively.

As shown in Figure 3, X increased with the reaction pressure, but the trend was slow. The value of X was higher under a supercritical condition than that at a subcritical state, which indicated that supercritical water oxidation was more efficient than water-air oxidation for this kind of wastewater. Figure 4 shows that X was influenced by the reaction temperature. After increasing the temperature from 595 to 704 K, X increased from 81.3 to 98.4%. This was partially because of the change of thermophysical properties of the reaction mixture-especially the heat capacity and density of the mixture. Thus, as the heat of reaction was almost constant with pressure, the temperature increased in the reaction mixture, favoring the reaction, resulting in an increase of COD removal (Bermejo, Bielsa, and Cocero, 2006). Additionally, an increase of reaction temperature with pressure was always observed in experiments. During the experiments, temperatures at the entrance, middle and outlet of the TWR reaction chamber were monitored. According to the changes in temperature at these three positions, the supercritical area in the reaction chamber could be estimated. The temperature at the outlet was always down to the critical point in our work, which indicated that the foil reactor was not completely under a supercritical state. An increase in temperature with pressure indicated that the effective SCWO zone, defined as the actual reaction zone under a supercritical condition, was prolonged. Thus, the retention time of the mixture increased relatively, resulting in the increase of X. In addition, the density of supercritical water increased with pressure, leading to an increase of reactant concentration and X. In our work, a positive influence of temperature and pressure on COD removal was observed, but the effect of pressure was minor compared with temperature.

Effect of Retention Time on Chemical Oxygen Demand Removal. The influence of retention time on the COD removal (X) was determined by conducting experiments at P = 28 MPa and r = 1.2. The COD concentration at the inlet of the reactor was fixed at 7102.88 mg/ L. In these experiments, the retention time was varied from 13.6 to 34.3 seconds; the temperature was increased from 595 to 704 K. The results are shown in Figure 5, which shows that X increased remarkably with the reaction retention time. When P = 28 MPa and T = 704 K, the value of X increased from 83.8 to 98.4%, as t increased from 17.2 to 34.3 seconds. Figure 5 also confirmed a significant effect of temperature on X.

Oxidation Pathway and Kinetic of Supercritical Water Oxidation Industrial Dyeing Wastewater. Oxidation Pathway of Supercritical Water Oxidation Industrial Dyeing Wastewater. Li et al. (1991) pointed out that oxidation of organic compounds in SCWO can be explained by a series of radical reactions, including the following five group reactions: radical initiation, propagation, radical transfer, branching, and termination. In this work, hydrogen peroxide, used as an oxidant, was decomposed to molecular oxygen and hydroxyl radicals by preheating, and molecular oxygen reacted with organic compounds (RH) and water, as shown in eqs 3 to 6.

The HO^sub 2^ and OH radicals were unstable and reacted rapidly with dye molecules and other organic compounds in wastewater. The decomposition rate of organic substance was increased, resulting in the increase of X. Intermediate products were formed through the reaction among R* radicals. In our experiments, several organic compounds (i.e., phenol, benzenecarboxyhc acid, isoquinoline, and 1, 2-benzenedicarboxylic acid) were detected in the effluent liquid samples. Carbon dioxide, carbon monoxide, and nitrogen were the main reaction products in the gas samples.

Kinetic Analysis of Supercritical Water Oxidation Industrial Dyeing Wastewater. For engineering purposes, it is often sufficient to develop a global-power rate model, described as eq 7, to express decomposition of wastes in SCWO.

Jeffrey et al. (2003) researched the effect of water concentration on the decomposition of phenol. The result indicated an inhibiting and accelerating influence of water at different temperature and pressure values. Some researchers (Anikeev et al., 2004; Goto et al., 1999; Portela, Nebot, and Martinez de la Ossa, 2001) pointed out that water concentration had no explicit effect on the decomposition of reactants, and a zero reaction order was assumed in their articles. It can be concluded that the dependence of water concentration on the SCWO reaction was not yet understood.

Our experiments were not designed to evaluate the effect of water concentration. Thus, it was assumed that reaction order of water in the reaction was zero, c = 0. Additionally, as mentioned above, the dosage of oxidant always was in excess (r = 1.2) during the SCWO process. The reaction rate became independent of the oxygen concentration, so that a reaction order of zero for oxygen, b = 0, was assumed in our study. To investigate the effect of initial COD concentration on COD removal, X, a series of experiments was performed at T = 680 K, P = 28 MPa, t = 34 seconds, and r = 1.2. As shown in Figure 6, X increased linearly with initial COD concentration, which indicated that the global reaction order for COD concentration was approximately a first-order reaction. Thus, for this kind of wastewater, the reaction orders for water, oxidant, and feed wastewater were assumed to be zero, zero, and first, respectively. Consequently, substituting [COD] for [C] and rearranging eq 7, with respect to the X defined by eq 2, the global power-law reaction rate could be expressed as eq 9.

Vogela et al. (2005) reviewed kinetic data of SCWO methanol and pointed out that the first-order rate law was not accurate because of the existence of induction time (t^sub ind^) at the early time of oxidation. Holgate and Tester (1993, 1994) and Meyer et al. (1995) reported the presence of an induction period before the onset of oxidation of hydrogen, carbon monoxide, and acetic acid in supercritical water. These induction periods were estimated to be approximately 1 to 3 seconds in duration, by assuming a first-order dependence of the reaction rate on the feed concentration. Induction time was influenced by initial feed concentration, mixing of feed wastewater and oxidant flow, reaction temperature, and so on, and estimated by linearly extrapolating data plotted as In(1 – X) versus t (as shown in Figure 7) back to the point of zero conversion. The retention time, corresponding to the extrapolated zero-conversion point, was interpreted as a purely kinetic induction time (Phenix et al., 2002). Thus, the first-order rate law had to be modified (as shown in eq 10) to describe the organic compound conversion considering induction time.

According to eqs 7 to 10, linear fit analysis for experimental data (as shown in Table 1) was used to obtain the value of the rate constant k, induction time t^sub ind^, active energy Ea, and pre- exponential factor A (as shown in Table 2). The kinetic equation for the oxidation of industrial dye wastewater was expressed as eq 11.

Conclusions

Oxidation of industrial dyeing wastewater in supercritical water was examined at a temperature from 595 to 704 K, pressure from 18 to 30 MPa, and retention time from 13 to 34 seconds; the oxidant dosage ratio ranged from 0.6 to 2.0. Experimental data showed that COD removal efficiency greater than 98.4% can be obtained at T = 704 K and P = 28 MPa with a retention time less than 35 seconds. The reaction conditions (T, P, r, and t) had a significant affect on the COD removal. Phenol; benzenecarboxylic acid; isoquinoline; 1, 2- benzenedicarboxylic acid; carbon dioxide; carbon monoxide; and nitrogen were detected in effluent samples. Kinetic equations of SCWO dyeing wastewater were regressed from experimental data using a modified first-order rate expression. The reaction orders for feed wastewater (based on COD) and oxidant were assumed to be 1 and 0, respectively. The values of the Arrhenius parameters, A and Ea, were 1.07 seconds^sup -1^ and 12.12 kJ * mol^sup -1^, respectively. The induction time t^sub ind^ changed from 1.37 to 2.93 seconds.

Credits

The authors thank Fang Li, Donghua University, Shanghai, China, for help in the experiment and revision of this manuscript.

Submitted for publication January 23, 2007; revised manuscript submitted June 25, 2007; accepted for publication September 7, 2007.

The deadline to submit Discussions of this paper is May 15, 2008.

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Wei-Jin Gong*1,2, Fang Li1, Dan-Li Xi1

1 School of Environmental Science & Engineering, Donghua University, Shanghai, China.

2 School of Energy & Environment Engineering, Zhongyuan University of Technology, Zhengzhou, China.

* School of Environmental Science & Engineering, Donghua University, Shanghai, China; e-mail: hggwj@yahoo.com.cn.

Copyright Water Environment Federation Feb 2008

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