Wet Air Oxidation - Industrial & Engineering Chemistry Research (ACS

2

Znd. Eng. Chem. Res. 1995,34, 2-48

REVIEWS Wet Air Oxidation Vedprakash S. Mishra, Vijaykumar V. Mahajani, and Jyeshtharaj B. Joshi* Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 029,India

Wet air oxidation (WAO), involving oxidation at high temperature (125-320 "C) and pressure (0.5-20 MPa) conditions, is useful for the treatment of hazardous, toxic, and nonbiodegradable waste streams. The process becomes self-sustaining when the feed chemical oxygen demand (COD) is about 20 000 m g L and can be a net energy producer a t sufficiently higher feed COD'S. All the published information on WAO has been analyzed and presented in a coherent manner. Wet air oxidation studies on pure compounds have been critically reviewed. Mechanism, kinetics, and structure-oxidizability correlation for WAO of carboxylic acids, phenols, cyanides, and nitriles have been described. The industrial applications discussed include municipal sewage sludge treatment, distillery waste treatment, black liquor treatment, cyanide and nitrile wastewater treatment, spent carbon regeneration, and energy and resource regeneration. Waste streams from other sources and some miscellaneous applications of WAO have also been included. Special emphasis is given to WAO under supercritical conditions (above the critical temperature of water) and oxydesulfurization of coal. In addition to the industrial applications, some other aspects (like various catalysts and oxidizing agents) of WAO have also been discussed. Recommendations and suggestions for further investigations have been made.

Contents 1.0. Introduction 2.0. Wet Air Oxidation of Pure Compound Solutions 2.1. Wet Air Oxidation of Carboxylic Acids 2.2. Wet Air Oxidation of Phenol and Substituted Phenols 2.3. Wet Air Oxidation of Cyanides and Nitriles 2.4. Wet Air Oxidation of Miscellaneous Compounds 3.0. Industrial Applications of Wet Air Oxidation 3.1. Wet Air Oxidation of Municipal Sewage Sludge 3.2. Wet Air Oxidation of Alcohol Distillery Waste 3.3. Effluent from Pulp and Paper Mill 3.4. Cyanide, Cyanate, and Nitrile Wastewaters 3.5. Regeneration of Spent Carbon and Spent Earth 3.6. Oxydesulfurization of Coal 3.7. Energy and Resource Generation 3.8. Oxidation in Supercritical Water 3.9. Miscellaneous Applications of Wet Air Oxidation 3.9.1. Miscellaneous Applications 3.9.2. Wet Air Oxidation of Other Waste Streams 4.0. Conclusions 5.0. Suggestions for Future Work

2 6

6 9 11

14 15 15 18

19 22 22 25 28

29 32 32 32 35 35

6.0. Nomenclature 7.0. References

38 38

1. Introduction

Fresh and unfrozen water of Earth constitutes only 1%of that in the hydrosphere, the bulk of which (99%) is groundwater and only 1% is surface water in lakes, rivers, soil, and atmosphere. During consumption, the water becomes contaminated with various kinds of substances. We will focus our attention on contamination (pollution) of water due to its use in industrial processing, particularly manufacture of chemicals. The industrial effluents are as varied as the industries themselves, in terms of nature of contaminants, their concentrations, treatment, and disposal methods required. Even the effluent characteristics from a single manufacturing unit varies with time and is quite unpredictable. The contents of effluent may vary from totally inorganic components (as in the case of electroplating industry wastewater) to highly toxic organochemicals (say from a pesticide manufacturing plant). The industrial wastewater can be divided in three categories: cooling water blowdown, boiler blowdown, and process wastewater. It is the process wastewater that is of interest to this work. These wastewaters need to be treated to meet local discharge standards. The choice of the method for the treatment of a particular effluent stream is governed by various factors such as the constituents (organic or inorganic), their concentration, volume to be treated, and toxicity to microbes. The various treatment methods available are chemical treatment, physical treatment (adsorption, reverse osmosis, etc.), biological treatment, wet air oxidation (WAO), incineration, etc. More often combination of the above methods is used to get better results.

* Author t o whom all correspondence should be addressed.

0888-5885/95/2634-0002$09.00/0 0 1995 American Chemical Society

Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 3 Chemical treatment is used for pH adjustment, coagulation of colloidal impurities (using alum, FeS04, polyelectrolytes, etc.), precipitation of dissolved pollutants (metal removal as hydroxides, carbonates, etc.), oxidation using ( 0 3 , ClOz, Cl2, Hz02, and 0 2 ) and reduction, and sludge conditioning. However, chemical treatment is usually prohibitively costly if large volumes are t o be treated particularly if the waste is predominantly organic in nature. In view of this, chemical treatment is usually used as the pretreatment step for pH adjustment, color removal, and removal of toxic compounds so that other treatment methods such as biological method can be used. Biological treatment is a method suitable for nearly all the applications. However it is necessary for the resulting sludge to be disposed of by either landfilling or burning with a corresponding expenditure of energy following elaborate thickening and dewatering procedures. In spite of this, it is the most popular treatment method. It is popular to the extent that the wastewaters that are not suitable for biotreatment due to toxicity or high organic load are treated by other means t o make the final effluent suitable for biotreatment. Biological treatment requires a large area of land, which is costly and may not be always available. Further, biological treatment processes react unfavorably to shock loads. Reverse osmosis is a membrane process used for desalting of brackish water and removing dissolved solids from certain industrial wastewaters. A concentrated and much smaller waste stream is produced, presumably making disposal simpler. However, the technique is not frequently used primarily due to high membrane replacement costs. Activated carbon (and some synthetic resins) is used frequently to treat wastewater by adsorptive process. High molecular weight organics (particularly those having lower solubility in water) are adsorbed preferentially on the carbon surface. Carbon treatment becomes attractive if the spent carbon can be regenerated by biotreatment, solvent extraction, WAO, etc. Wet air oxidation (WAO) is a well-established technique of importance for wastewater treatment particularly toxic and highly organic wastewaters [Zimmermann (1950, 1954a,b, 1958a,b, 19611, Laughlin et al. (19831,Joshi et al. (19851,and Copa and Gitchel(1989)l. This work reviews the available literature on WAO. Wet air oxidation involves the liquid phase oxidation of organics or oxidizable inorganic components at elevated temperatures (125-320 "C) and pressures (0.520 MPa) using a gaseous source of oxygen (usually air). Enhanced solubility of oxygen in aqueous solutions at elevated temperatures and pressures provides a strong driving force for oxidation. The elevated pressures are required to keep water in the liquid state. Water also acts as a moderant by providing a medium for heat transfer and removing excess heat by evaporation. WAO has been demonstrated to oxidize organic compounds to C 0 2 and other innocuous end products. Carbon is oxidized to COz; nitrogen is converted to NH3, NOS, or elemental nitrogen; halogen and sulfur are converted to inorganic halides and sulfates. The higher the temperature the higher is the extent of oxidation achieved, and the effluent contains mainly low molecular weight oxygenated compounds, predominantly carboxylic acids. The degree of oxidation is mainly a function of temperature, oxygen partial pressure, residence time, and the oxidizability of the pollutants under consideration. The oxidation conditions depend on the

COMPRESSOR

WASTE

EXCHANGER PRESSURE

V

N

,

,CO, , S T E A M

O X I O I Z E O Liauie

Figure 1. Basic wet oxidation plant flow sheet.

treatment objective. For instance, in the case of sewage sludges, mild oxidation conditions can be used to achieve 5-15% COD reduction resulting in a sludge which is sterile, is biologically stable, and has very good settling and drainage characteristics. On the other hand, in the case of oxidation of caustic scrubbing liquors, more than 99.9% of the waste components are oxidized. Figure 1 depicts a typical WAO treatment system. The wastewater is brought to the system pressure, using a high pressure pump. Air is added to the reactor using a compressor. Preheating may be necessary to raise the temperature of wastewater. The feed temperature is adjusted such that the exothermic heat of reaction raises the mixture temperature to the operating temperature. Preheating can be done using the treated effluent [Pradt (1972) and Van Kirk (1977)l. The reactor effluent can be cooled by cooling water (to produce steam) or the wastewater-air mixture. Liquid and noncondensible gases are disengaged in a separator. If desired, the offgases can be expanded in a turbine to recover energy (Figure 2). The gases are then treated (using carbon adsorption beds, afterburners, etc.) to reduce the concentration of hydrocarbons and other organic matter present. The liquid phase can be disposed of directly or most often subjected to biological treatment. Wet air oxidation requires much less fuel than other thermal oxidation processes such as incineration. This is because, for WAO, the only energy required is the difference in enthalpy between the incoming and outgoing streams. However, for incineration, not only the sensible enthalpy (combustion products and excess air to be heated to the combustion temperature of about 1000 "C) is to be provided but also it is required to supply heat for the complete evaporation of water. The capital cost of a WAO system is higher and depends on the flow, oxygen demand of the effluent, severity of the oxidation conditions, and the material of construction required. The reactor itself can account for a significant fraction (50%)of the total equipment cost. The capital investment can be reduced if one opts for a WAO system based on oxygen instead of air. However, the higher cost of oxygen has to be compared with the saving in initial capital investment. Prasad and Materi (1990) studied the technological and economic aspects of both systems

4 Ind. Eng.

t Figure 2. Basic flow sheet of wet air oxidation (after Pradt (1972)).

and concluded that oxygen-based WAO systems show greater profitability. The operating costs are almost entirely for power to compress air and high pressure liquid pumping [Wilhelmi and Knopp (1979)l. Further, WAO becomes selfsustaining with no auxiliary fuel requirement when the COD is above 20,000 mg/L [Joshi et al. (198511. In fact, energy recovery (thermal or mechanical) becomes possible when the feed COD is sufficiently high. Chou and Verhoff (1981)studied the effect of reactor temperature, reactor pressure, COD of the feed waste stream, etc. on power generation by WAO. Incineration becomes selfsustaining when the COD is in the range of 300 000400 000 mg/L [Chowdhury and Copa (1986)l. Costs can further be reduced by reducing the severity of the oxidation conditions by use of suitable catalysts (Cu2+, Fez+, CuO/ZnO, Ru, Ce, etc.). In most applications, WAO is not used as a complete treatment method, but only as a pretreatment step where the wastewater is rendered nontoxic and the COD is reduced sufficiently, so that biological treatment becomes applicable for the final treatment. This strategy means that extreme oxidation conditions are no longer necessary. Sampayo and Hollopeter (1979) have discussed the suitability of biological and physicochemical processes for the treatment of WAO effluent. Avezzu et al. (1992) shows that a pretreatment in the form of WAO increased the biodegradability of landfill leachates substantially. Wet air oxidation also makes possible the recovery of inorganic chemicals. For instance, a t one paper mill in Australia, more than 99.9% of the pulping chemicals are recovered for reuse [Teletzke and Pradt (1969)l. The first patent of a WAO system is 83 years old now. In 1911, Strehlenert obtained a patent for the treatment of sulfite liquor (from pulp production) by oxidation with compressed air at 180 "C. Hanglin and Stauf (1927) could purify a solution containing metallic salts and organic impurities using wet air oxidation. The temperature was above 130 "C and the pressures were above 0.2 MPa. However, industrial application of the technique started with the patents granted independently to a Swedish company Stora Kopparbergs BA [Cederquist (195411and an American firm Sterling Drug Inc. [Zimmermann (1954a,b)l. The first known WAO plant was put up in 1958 by Borregaard in Norway for

the treatment of sulfite liquors but was later closed down due to uneconomical operation. A major thrust to the WAO technique, as a commercial process for wastewater treatment, came in the early 1960s with its application for the recovery of pulping chemicals from waste liquors and complete oxidation of sewage sludge. Zimpro built several WAO plants for complete oxidation of sewage sludge. On the basis of the work with sewage sludge, new applications for WAO were discovered such as conditioning of sludge for easy dewatering and improving the settling characteristics besides volume reduction and the spent carbon regeneration. Commercial applications for hazardous wastes started by the early 1970s. Currently more than 200 full scale WAO plants are in operation for the treatment of a wide variety of effluent streams. The major application of WAO is still the treatment of sewage sludge, with more than 50% of the total number of WAO plants built being used for this purpose [Randall et al. (198511. There are several publications which have reviewed the various aspects of WAO. Zimmermann (1958a) discussed the WAO process, process conditions, its application in treatment, and recovery of chemicals from paper mill effluent. They also described the commercial installations available by that time. Most of this information has been updated in recent reviews [Wilhelmi and Knopp (19791,Perkow et al. (1981),Schaeffer (19811, Canney and Schaeffer (1983a,b), Baillod et al. (1985),Joshi et al. (1985),Chowdhury and Copa (19861, Li et al. (1991), and Copa et al. (199111. Wilhelmi and Knopp (1979) described the WAO process, design considerations, and cost factors (compared t o incineration). They also discussed the scheme for treatment of acrylonitrile plant wastewater and coke oven gas scrubbing liquor. Perkow et al. (1981) published an excellent review covering the historical developments; treatment of municipal sludge, pulp and paper mill effluent, and chemical industry effluent by WAO; catalytic WAO; use of oxidizing agents other than oxygen; various available reactor designs and processes; construction materials; and cost comparison with other processes. Schaeffer (1981) discussed the application of WAO in wastewater treatment, resource recovery from waste

Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 5 Table 1. Reviews Published authors

Year

title

Angamennone and Pieroni Baldi et al. Berbenni et al. Beyrich Bowers et al. Canney and Schaeffer

1987 1985 1987 1987 1988 1983a 1983b 1986 1992

wet oxidn of nonbiodegradable waste WAO as wastewater treatment: appl and dev. aspects WAO in waste treatment wet oxidn toxicity redn in ind wastewater discharges detoxification of hazardous wastewater by WAO wet oxidn of toxics: new appl of existing technol WAO of toxic and hazardous org in ind wastewaters treatment in liquid phase of org wastes by molecular oxygen at elevated temp-WAO process use of wet oxidn method in treatment of ind. wastewater trickle bed oxidn reactors catalytic wet oxidn process advanced treatment of wastewaters WAO treatment means for aq hazardous waste streams wet oxidn of wastewater and its catal wet air oxidn LOPROX-flexible way to pretreatment wet oxidn treatment process of wet oxidn eng aspects of treatment of aq waste streams use of wet oxidn in wastewater treatment wastewater treatment according to state of art in big chem plants components for wet oxidn liquid phase oxidn plants for wastewater treatment components for wet oxidn installations industrial wastewater treatment with chem wet oxidn; survey of process advanced wastewater treatment in chem ind catalytic wet oxidn of wastewater appl of wet oxidn technol chem terminology of wastes; simple environ chem; wet oxidn titanium for waste treatment WAO-review waste elimination; thermal treatment of liquid water contg wastes current status and prospect of advanced wastewater treatment wet oxidn: alternative technol for thermal destruction of hazardous wastewater advanced wastewater treatment by catalytic wet oxidn process treatment of industrial wastewater with WAO

Chowdhury and Copa Foussard et al. Gao and Lu Goto et al. Harada Heimbuch and Wilhelmi Higashijima Holzer et al. Imamaura Joshi et al. Kalman et al. Kaltenmeier

Koeppke Malle Mitsui Murukami Murata Oettinger and Hoffman Perkow et al. %her and Grosche Sugahara Schaeffer Tsukuda and Harada xu

1987 1977 1987 1985 1978 1989 1991 1983 1985 1989a 1990 1988 1989a 1989b 1993 1983 1991 1982 1982 1981 1981 1976 1992 1984 1992 1990

streams, design aspects, and cost comparisons in their review. They also discussed energy generation aspects of WAO from waste streams and low-grade fuels. The resource and energy generation aspect has been discussed in detail in section 3.7. Canney and Schaeffer (1983a,b) discussed the application of WAO for dilute and toxic effluents along with the results of some commercial WAO installations in their review. Baillod et al. (1985) presented the prevailing costs of equipment required in a WAO system. They calculated the total capital investment requirements, operating costs, wastewater treatment charges, and possible income (in the form of steam generated). Joshi et al. (1985)have extensively reviewed the WAO (catalytic as well as noncatalytic) of various organic compound solutions, and the oxydesulfurization of coal using WAO. They also discussed the engineering aspects of WAO along with other wastewater treatment methods such as biological and chemical treatment. They have compared the performance of a variety of equipment used for gas-liquid reactions. Chowdhury and Copa (1986) discussed the WAO process and compiled bench, pilot, and full scale performance data for WAO of various toxic and hazardous waste streams in their review. Li et al. (1991) compiled the kinetic parameters for WAO of various organic compounds under subcritical and supercritical conditions. On the basis of these data they proposed a generalized and very useful kinetic model for WAO of organic compounds. The details of the model are given in section 2.1. Copa et al. (1991) published a very good review covering the developmental status, process limitations,

no. of pages 11 14 9 3 5

no of refs 19 33 11 8 24

8 8 28

9

5 51 8 14 7 6 10 16 35 9 4 3 3 3 6 4 7 5 5 24 9 14 5

13 166 8 12 28 12 6 115 187 14 16 0 0 0 14 17 0 10 0 13 53 31 13

8 4

3 3

environmental impact, critical operating parameters, and energy requirements. They also gave an estimate of treatment cost by WAO along with a discussion on the performance of commercial WAO installations in North America. In addition, there are many useful reviews covering various aspects of WAO (Table 1). These reviews are very focused in nature. Though these reviews do not give an overview of all the information available in the literature, they present some aspects of WAO in detail. There is a need for a comprehensive review on WAO covering all the aspects and all the information on WAO of pure compounds as well as industrial effluents published to date. This review is aimed at fulfilling this need. A better understanding of the reactions taking place during WAO (reaction mechanism and kinetics) is important as it leads to reliable design of oxidation reactors and also to cost reduction by optimization of the operating conditions. For this purpose a number of WAO studies have been performed on the aqueous solutions of several pure compounds (important model pollutants) and wastewaters containing toxic and hazardous compounds. Wet air oxidation of aqueous solutions of phenols and carboxylic acids has been studied in great detail with emphasis on the kinetics and mechanism of wet air oxidation. Therefore these two classes of pollutants have been reviewed in separate sections. Also reviewed separately is the information about WAO of cyanide and nitrile aqueous solutions in view of their high toxicity and hence importance in wastewater treatment. Wet air oxidation has been found to be very promising for desulfurization of coal. There is a lot of information on chemistry and kinetics

6 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995

of the process, effect of different parameters on the efficiency of the process, mathematical models of the process, various controlling steps, etc. In view of the continuing importance of coal as a prime energy source, this application of wet air oxidation has been reviewed in detail. The available literature on industrial applications of WAO has also been reviewed. 2.0. Wet Air Oxidation of Pure Compound Solutions 2.1. Wet Air Oxidation of Carboxylic Acids. Carboxylic acids are very valuable commercial products as they find their use in a large number of synthetic organic products. Several dicarboxylic acids are also of commercial importance because of their use in synthetic polymers. Among monocarboxylic acids, formic acid is used as a disinfectant, as a preservative, to make formates and cellulose esters and in the textile and leather industries. Acetic acid is an important solvent in organic processes apart from its major use in cellulose acetates. Other acids also find their use in preparation of pharmaceuticals, dyes, flavoring ingredients, perfumery esters, etc. During manufacture and during their use in synthetic processes, carboxylic acids find their way into the waste streams. Sometimes these acids are formed as byproducts in a process and part invariably find their way in the waste streams. For example, the caprolactam plant waste stream, petrochemical waste stream, and pharmaceutical plant waste stream contain appreciable quantities of carboxylic acids. The understanding of the wet air oxidation of low molecular weight acids is important for yet another reason. During the WAO of a variety of waste streams, the pollutant molecules are broken down mainly to low molecular weight carboxylic acids. These acids may be further oxidized to carbon dioxide and water if oxidation conditions are sufficiently severe. The low molecular weight carboxylic acids in general and acetic acid in particular are quite resistant to oxidation [Imamura et al. (1979, 1980a, 1980b, 19811 and hence accumulate at the latter stages of wet air oxidation [Baillod et al. (1982, 19851, Fisher (1971), Foussard et al. (19891, Friedman et al. (1988),Keen and Baillod (1983, Teletzke et al. (1967), and Wu et al. (1987)l. The slow rate of oxidation of low molecular weight carboxylic acids is a major limitation of the WAO technique. In view of this the understanding of WAO of low molecular weight carboxylic acid achieves great significance. With this realization, the WAO of low molecular weight acids has received fairly good attention in the published literature. Oxidation of various mono- and dicarboxylic acids (c1-c6) has been studied in great detail. The monocarboxylic acids which have been studied are formic, acetic, propionic, butyric, valeric, and caproic acids. Oxalic, adipic, succinic, and glutaric acids are the dicarboxylic acids. These studies were aimed at determining kinetic parameters (orders with respect to substrate and oxygen and the activation energy), percent TOC removal, and the oxidation mechanism in some cases. The temperatures ranged from 112 to 300 "C, although in the majority of cases the temperatures used were in the range of 230-290 "C. Oxygen partial pressures ranged from 0.35 to 12.8 MPa. In view of slow rates of oxidation, the use of homogeneous and heterogeneous catalysts has received a great attention. The homogeneous catalysts (particularly copper salts) are in general more effective oxidation catalysts [Tagashira et al. (19751, Goto et al. (19771,and

Imamura et al. (1982c)l but their use necessitates a precipitation step to recoverhemove the toxic catalyst from the final effluent. In view of this, heterogeneous catalyst systems are preferable. It is important to use catalyst systems which are stable and do not get leached away in the solution by reacting with acids present. This is the case when CuO is used as the catalyst [Njiribeako et al. (1978a,b)]. Catalytic oxidation of waste streams is discussed later in detail [section 3.91. Copper sulfate and copper nitrate have been used as homogeneous catalysts for the oxidation of carboxylic acids. The heterogeneous catalysts used include transition as well as noble metals. The various heterogeneous catalysts that have been used are Cu, Pd, CoO/ZnO (6.5: 82.9, Cu:Mn:La oxides (4:2:1) supported on a spinel support (ZnO and Al203,48.5 and 51.5%, respectively), copper chromite, iron oxide, Co:Bi (51)complex oxides, RdCe, and Mn/Ce. Of the various catalysts studied, the multicomponent catalyst systems like Co:Bi, Cu:Co, Cu:Co:Bi, and Ru/Ce were considerably more active than other catalysts (except Mn/Ce catalyst). Co:Bi (51)was found to be the most active one [Imamura et al. (1982a,b 1988)l. Activity of Co:Bi catalyst was due to the presence of basic sites on the catalyst surface on which acetic acid is adsorbed. This is followed by a redox reaction between catalyst and adsorbed acetic acid to induce its decomposition. Use of MdCe (1:l) catalyst resulted in complete removal of acetic acid at 200 "C. Activity of MdCe catalyst can be further improved by using Ru as a promoter [Imamura et al. (1988)l. The results of all above and other relevant studies have been summarized in Table 2. The order with respect to substrate concentration was in the range of 1-1.5 for catalytic as well as noncatalytic oxidation of carboxylic acids. The order with respect to oxygen for catalytic WAO of formic acid [Baldi et al. (1974)l and acetic acid [Levec and Smith (197613 was found to be 1 and 1.5, respectively. The order with respect to oxygen for catalytic WAO of other acids is not available. The order with respect to oxygen for noncatalytic wet air oxidation of acids was in the narrow range of 0.31-0.46. Merchant (1992) observed the order with respect to oxygen to be close to zero (in the range of 0.1-0.2) for the noncatalytic WAO of acids (CI-cS). As expected, the energies of activation for catalytic WAO (50-134 kJ/mol) were lower than that for catalytic WAO (75142 kJ/mol) of c1-c6 carboxylic acids. During WAO, the longer molecules are oxidized to various intermediate products. Most of the initial intermediates formed (except the low molecular weight carboxylic acids) are unstable and are further oxidized to oxidation end products (COS,etc.) or to low molecular carboxylic acids (mainly acetic acid). The low molecular weight carboxylic acids are resistant to further oxidation as mentioned earlier. Thus, the organics in the effluent from a WAO system can be divided into three groups [as by Li et al. (199111: all initial and relatively unstable intermediates except acetic acid (group A), refractory intermediates like acetic acid (group B) and oxidation end products (group C). A schematic pathway is given below: A

+

ki 0 2

0

*c

+ 02

When expressed in concentration (COD, TOC, or TOD)

Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 7 Table 2. Wet Air Oxidation of Carboxylic Acids

investigators Day et al. (1973)

substrate and catalyst used propionic acid (0.1-0.2 N)

Williams et al. (1973)

butyric acid (8.8-17.6 g/L)

Baldi et al. (1974)

formic acid (12.2 x to 100 x lo-' mol/cm3);CuO/ZnO (16.5%;82.5%) mixture; porosity = 0.67; surface area = 35.9 m2/g

Levec et al. (1976)

acetic acid (8.33 x to 83.33 x lo-' mol/cm3);Cu:Mn:lanthanum oxides in ratio of 4%, 2%, 1%(wt %) on a spinel support (ZnO 48.5% and A1203 51.5%);porosity = 0.55; surface area = 89 m2/g acetic acid ((8.3-70.8) x mol/cm3), Fe oxide as catalyst

Levec and Smith (1976)

Imamura et al. (1982a)

acetic acid (5000 mgh); Co:Bi (5:l) complex oxides

treatment conditions and observations T = 232-287.8 "C, PO,= 1.72-5.17 MPa, m = 0.39, n = 1.43, E = 134.82 kJ/mol, A = 9.32 x 10l2 T = 237-257 "C, TP = 6.8-13.6 MPa, n = 1.4, m = 0.46, E = 124 kJ/mol,A = 1.26 x lo9 T = 200-240 "C, TP = 4 MPa, m = 1,n = 1,E = 146.54 kJ/mol at d, = 0.054 cm (Ef = 0.36 at 212 "C, 0.91 at 224 "C, 1.58 at 232 "C and 2.88 at 240 "C); at d, = 0.291 cm, E = 113 kJ/mol ( E f = 0.19 a t 240 "C); at d, = 0.477 cm, 6 = 1,E = 100.48 kJ/mol (Ef=0.11 at 240 "C) T = 250-280 "C, TP = 6.8 MPa, Ef = 0.78 and 0.59 a t 250 and 280 "C, respectively, E = 71.2 kJ/mol ford, = 0.038 and E = 50.24 kJ/mol ford, = 0.18 cm

T = 250-286 "C, TP = 6.7, 7.3 MPa; m = 1.5 and tends to decrease from 1 to 0 as acetic acid concn decreases; E = 87.92 kJ/mol; 1-5% conv in liquid full and 10-20% conv in trickle bed T = 248 "C, PO,= 1MPa; apparent E = 105.9 kJ/mol; in 60 min 100%TOC removed at catal concn of 20 mM treatment conditions and observations PO,= 1MPa and t = 20 min % TOC removal

investigators Imamura et al. (1982b)

substrate and catalyst used Co:Bi (5:l) complex oxides (2 x 10-2 moVdm3) formic acid acetic acid

propionic acid butyric acid valeric acid hexanoic acid oxalic acid adipic acid succinic acid glutamic acid Foussard (1983)

temp ("C)

PH

catal

no catal

112 180 200 225 248 248 248 248 248 112 140 160 248 248 248

2.8 3.4 3.4 3.4 4.0 4.5 3.5 3.7 3.5 1.4 1.4 2.5 2.8 3.2 8.1

25.9 2.0 7.1 25.1 67.0 79.0 83.4 50.9 80.6 0.0 31.9 86.3 97.4 100.0 82.5

17.3 1.0 2.4 3.2 8.3 6.9 17.5 8.3 12.4 0.0 27.0 90.0 27.3 58.6 72.5

T = 227-288 "C, TP = 2-20 MPa, n = 1,m = 0.31,

oxalic acid (0.1-0.2 g/L)

E = 133.8 kJ/mol, A = 6.83 x lo8 Chowdhury and Copa (1986) formic acid (25 000 mg/L) Imamura et al. (1988) acetic acid (TOC = 2000 m a ) , RdCe; Cu(N0312 and MdCe (12 mM total metal ion concn used) Imamura et al. (1988)

formic acid (TOC = 2000 mg/L); total catal metal concn = 12 mM

Imamura et al. (1986)

acetic acid (5000 mg/L); total catal metal ion concn = 20 mM

Foussard et al. (1989)

acetic acid (about 30 formic acid (24-43 g/L)

T = 300 "C; in 60 min, 98.3% redn obtained without catal T = 200 "C,Po, = 1MPa, t = 1h; with RdCe (5 w t % on Ce) 44.5 and 19.4%TOC removal at pH 2.7 and 6.9, respectively; with Cu(N03)2 at pH 2.5, 32.6% TOC removed; with MdCe (1:l)complete removal of acetic acid achieved T = 150 "C, Po, = 1MPa, t = 1h; reactivity highly dependent on pH; 24.9% TOC redn without catal; at pH 1.9, 64.7% TOC removal using Cu(N03)~;using RdCe (5 wt % on Ce) 100% TOC removed a t pH 1.9 compared to 7% at pH = 5.6 T = 247 "C, PO,= 1MPa, t = 1h; 42% TOC removal without catal, 87.1% TOC removal for Cu(NO& and 99.5% removal for MdCe (70/30)and CoBi (50) both; no elution of Mn or Ce from Mn/Ce catal T = 270-320 "C, TP = 2-20 MPa, n = 1,m = 0.37, E = 167.7 kJ/mol,A = 5.6 x 1 O l o T = 190-313 "C, TP = 2-20 MPa, n = 1.33, m = 0.46, E = 143.5 kJ/mol, A = 3.10 x lo9 % removal

investigators Merchant (1992)

substrate acetic acid propionic acid butyric acid valeric acid caproic acid succinic acid glutaric acid adipic acid

temp ("C) 275 250-275 230-250 240-260 230-250 240-260 240-250 230-260

COD 7 8-32 12-35 11-49 26-70 32-64 24-32 23-70

acid

m

n

E1

E

12-31 12-22 27-65 30-90 20-35 32-54 0-87

0 0 0 0 0 0 0

1 1 1 1 1 1 1

142.4 121.8 208.5 208.1 137.33 76.6 110.9

139.0 105.1 120.6 107.6 128.1 80.8 116.8

8

Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995

terms, [AI = [all initial intermediate organic compounds] - [acetic acid], [Bl = [acetic acid], and [Cl = [end products]. On the basis of the available literature, Li et al. (1991) proposed the reaction to be first order with respect to group A or group B and nth order with respect to oxygen (in most cases, n = 0). The values of kl can be determined from the initial reaction rate, and most of the rate equations in the literature are actually expressions for k ~ .The value of k3 can be determined from the Foussard rate equation for acetic acid [Foussard et al. (198911. The value of k2 can be determined experimentally. It has been observed by the authors that, in addition to acetic acid and other oxidation end products, significant quantities of solid carbonaceous materials are also formed during WAO of several waste streams. This solid carbonaceous material undergoes oxidation in the same manner as the activated carbon during its wet air oxidative regeneration (section 3.5). This aspect of formation and subsequent oxidation of solid carbonaceous material during WAO of waste streams should be incorporated in the model of Li et al. (1991). The ratio of formation of acetic acid to that of oxidation end products [defined as point selectivity, a, by Li et al. (199111 can be used to characterize the strength of waste streams for WAO process. It can be observed from the Table 2 that the oxidation rate increases with an increase in the molecular weight/ carbon number (from acetic acid onward) irrespective of whether a catalyst system is used or not. Formic acid is comparatively easy to oxidize. Further, dicarboxylic acids are more easily oxidized compared to their monobasic counterparts. The rates of oxidation of dicarboxylic (as well as monocarboxylic) acids increases with an increase in the number of methylene groups in the molecule [Danisove et al. (197711. The high molecular weight acids (mono- as well as dicarboxylic)are oxidized to lower molecular weight acids (acetic acid being the ultimate oxidation product in almost all cases). Some of the studies have been directed toward the understanding of the mechanism of oxidation [Fisher (19711, Day et al. (19731, and Williams et al. (1973, 197511. Oxidation of formic acid can be written as

HCOOH

+ 0.50, - H,O + CO,

(2.1)

Formic acid may also undergo thermal decomposition (gas phase) to C 0 2 and H2 (decarboxylation) or CO and H20 (dehydration) [Margolis (19711, Thomas and Thomas (19671, and Ruelle et al. (198611.

4H20

HCOOH

COz CO

+ +

HZ (decarboxylation)

centrations [Baldi et al. (197411. Rate of oxidation is very high (50 times) compared to rate of decarboxylation [Baldi et al. (197411. The addition or presence of acetic acid and acetaldehyde accelerated the decomposition of formic acid. Oxidation of formic acid does not involve formation of any stable intermediate. Oxidation of formate ion is slower than that of formic acid. This is expected in view of higher resonance energy for the formate ion compared to that for formic acid (less effective resonance due to separation of charges). Formate ion is oxidized to bicarbonate, which causes the pH of the solution to rise. No gases are formed.

+ 0.50, - HC0,-

HCOO-

Merchant (1992) observed that the rate of oxidation increases with an increase in the pH of the medium. This is in contrast to the observations reported by Foussard et al. (1989). More work needs to be done to confirm or explain the observed pH effect. Shende (1994) observed that, during the noncatalytic oxidation of formic acid below 200 "C, there is a fast reduction in chemical oxygen demand (COD) of the solution initially and then a stage is reached when no further COD reduction occurs. This behavior was not observed during the catalytic (using copper sulfate) oxidation under similar condition and during the noncatalytic oxidation above 200 "C. For oxidation of formic acid over CuO-ZnO catalyst, participation of lattice oxygen, as in the case of CO oxidation on CuO, has been suggested by Baldi et al. (1974). According to them, formic acid reacts with an active oxidized site on the catalyst and produces C02 and reduced site. The reduced site can be reformed to an oxidized site by oxygen. Similar mechanism has been suggested by Levec and Smith (1976) for the oxidation of acetic acid over FeO catalyst. Oxidation of acetic acid to CO2 has been shown to involve formation of HCOOH and HCHO as intermediates [Margolis (197111. Levec and Smith (1976) assumed the formation of intermediates or free radicals such as HCHO, HCOOH, and CO during the catalytic WAO of acetic acid and proposed the following elementary steps t o explain the mechanism of oxidation of acetic acid. CH,COOH

+ X - CH,COOH*X

(dehydration)

Water molecule acts as a catalyst in the decarboxylation reaction. Hydrogen is formed by taking one hydrogen from HCOOH and the other from H2O [Ruelle et al. (1986)l. The formation of H2 from HCOOH is less probable due to the very high activation energy for the reaction. Decarboxylation is the exclusive route for catalytic decomposition of formic acid. For noncatalytic decomposition as well, decarboxylation is the main route and has been observed even under oxidative conditions [Bjerre and Sorenson (199211. Decarboxylation under oxidation conditions has been observed at very low oxygen concentrations (0.56 x lo-' mol/cm3)and found to be absent at sufficiently high (normal) oxygen con-

(2.3)

+ x - 0.x (2.4) CH3COOH*X+ 0.X - [HCHO] + [CO] + H,O + X 0.50,

+ X - [HCOOH] + X [HCOOH] + 0.X - CO, + H 2 0 + X [COI + 0-x- CO2 + x [HCHO]

H20

(2.2)

(2.5) (2.6) (2.7) (2.8)

X indicates a vacant site and 0.X an oxidized site capable of acting as catalyst for the reaction (2.7). Combination of reactions from (2.5) to (2.7) leads to the overall reaction: CH,COOH

+ 20, - 2C0, + 2H,O

(2.9)

Two parallel paths have been proposed for the oxidation of propionic acid [Day et al. (1973)l. The first path involves direct oxidation of the propionic acid to carbon

Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 9 dioxide, water, and other gaseous oxidation products via several intermediates. The second path oxidizes the remainder of the propionic acid to acetic acid via acetaldehyde as an intermediate. Contribution of both the paths is more or less same; i.e., half of the propionic acid is oxidized by the first path and the remaining half by the second path. The attack a t the a-carbon by oxygen is the key step. However, rupture of a C-C bond at other places (p- or y-positions) has also been considered to be possible [Danisove et al. (197711. It has been established that the -COOH group increases the reactivity of a C-H bond compared to hydrocarbons [Danisove et al. (197711. Butyric acid has been shown to oxidize in a similar manner [Williams (1975)l. Merchant (19921, however, could not detect acetic acid in the samples collected during WAO of propionic acid, indicating that propionic acid is probably directly oxidized to carbon dioxide, water, and other gaseous products. Unfortunately, the investigators did not analyze the COz and other gaseous products formed during the oxidation reaction, which would have proved the issue beyond doubt. Acetone, 2-butanone, and acetaldehyde have been identified to be precursors to acetic acid formation by oxidation of higher molecular weight compounds including carboxylic acids [Day et al. (1973),Fisher (19711,and Taylor and Weygandt (1974)l. Sodium salts of acids are relatively more resistant than the correspondingfree acids [Foussard et al. (1989) and Imamura et al. (198811. Foussard et al. (1989) applied the Bodenstein quasi state principle to the free radicals and hydroperoxides and arrived at the following rate equation for the oxidation of sodium acetate.

r = KO e -E/RT [Al([B]+ P 0 p ) 0 . 5 II

(2.10)

where KO" = 2.8 x 1O1O s-l (m~l/m~)-O.~; E = 167.7 kJ/ mol, and [Bl = 21 mol/m3. They also derived an empirical kinetic model which is given below.

r = Koe-E~RT[Al"[Po2/Hl"

(2.11)

where 270 < T < 320 "C, 36 mol/m3 < iPo.JH) < 136 mourn3,the kinetic parameters KO= 5.6 x 1O'O s-l (mol/ m3)-0.38and E = 167.7 kJ/mol, n = 1, and m = 0.38. The experimental results coincided with both the above equations. They further observed that the oxidation of sodium acetate is slow below 270 "C but quite fast above 320 "C. We tried to correlate the absolute rate constants a t a given temperature [Merchant (1992)l with the number of carbon atoms in the acids and arrived at the following correlations (for 250 "C):

where K1 is the rate constant based on COD reduction for the carboxylic acids having Cn number of carbon atoms and

where K2 is the rate constant based on propionic acid concentration for the carboxylic acid having Cn number of carbon atoms. It appears from the foregoing discussion and the results tabulated in the Table 2 that, except acetic acid

and to some extent propionic acid, acids are relatively easy t o oxidize. Destruction of acetic acid has been achieved at relatively high temperature of 247 "C, 1 MPa oxygen partial pressure using 20 mM of 5:l Co:Bi catalyst and MdCe in 1 h [Imamura et al. (1986)l. Another important aspect of the catalysts is their stability or resistance t o get leached out in the solution from the catalyst. The discrepancies regarding the formation of acetic acid during propionic acid WAO [Day et al. (1973) and Merchant (1992)l and the two-step behavior for noncatalytic oxidation of formic acid [Shende (1994)lshould be explained on the basis of some more detailed investigation. The observation of two paths for the oxidation of propionic acid by Day et al. (1973) gives a hope that by use of suitable temperature, oxygen partial pressure, and catalyst (at an optimum concentration) it may be possible to oxidize the acids exclusively to COZ and water. Results of some preliminary investigations in our laboratory also point to this possibility. This aspect of wet air oxidation should be looked into as it will avoid the acetic acid formation during wet air oxidation of high molecular weight compounds. This will solve one of the major problems of low rate of oxidation during the latter stages of wet air oxidation. Muconic acid, maleic acid, glyoxylic acid, and others such as 3-hydroxypropionic acid are also formed as intermediates in WAO of pollutant molecules, particularly aromatic ones like phenol. Wet air oxidation studies on these acids is desired as these are the very early acid species formed which undergo further oxidation to still low molecular weight acids like acetic, formic, and propionic. It may be worthwhile t o study wet air oxidation of higher carboxylic acids (CS-CZZ) commonly found in nature as such data are lacking in current literature. Although MdCe catalyst has shown the capability of completely destroying acetic acid, by far one of the most difficult molecules to oxidize, more information (kinetic parameters, stability and reusability of the catalyst system, its effectiveness in the destruction of real waste streams by WAO) is highly desirable. In addition to this, the search must be on for a cheaper catalyst system which is at least comparable with MdCe catalyst in terms of efficiency, stability, etc. It may be worthwhile to try to improve the activity of CuO catalysts by using a suitable modifier or promoter. 2.2. Wet Air Oxidation of Phenol and Substituted Phenols. Phenol and substituted phenols are very important chemicals commercially. Phenol, cresylic acids, and cresols are used for making phenolformaldehyde resins and tricresyl phosphates. Phenol, alkylphenol, and polyphenols are important raw materials for the wide variety of organic compounds, dyes, pharmaceuticals, plasticizers, antioxidants, etc. Phenols are mainly of coal tar origin and hence present in the effluent from coke ovens, blast furnaces, and shale oil processing. Phenols are also present in the eMuent from the chemical process industries which are either manufacturing or using them. The importance of phenols in water pollution stems from their extreme toxicity to the aquatic life and resistance to biodegradation. Phenols impart a strong disagreeable odor and taste to water even in very small concentrations. It is generally difficult to biotreat waste stream having phenols above 200 mg/L [Pruden and Le (197611. Pauli and Franke (1971) and Katzer et al. (1976) have put this limit at

10 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995

50 and 70 mgL, respectively. In any case all the above limits are exceeded in most of the waste streams. As mentioned earlier, wet air oxidation is very suitable for such waste streams. There is a comparatively large amount of published literature on the oxidation of phenols in dilute aqueous solutions by molecular oxygen. Most of the studies are for phenol. The earliest studies on oxidation of phenols were performed at comparativelylow temperatures (up fx100 "C) under different conditions [Reich and Strankmuller (1958a-c) in the presence of certain types of ashes and Hamilton et al. (1969) using MnO2 as catalyst]. Under these conditions, the extent of oxidation was very small and mainly oxidative coupling resulted. Borkowski (1967) and Walsh and Katzer (1973) studied vapor phase catalytic oxidation of phenol. Detailed studies on the kinetics and mechanism of oxidation of phenol and substituted phenols in alkaline aqueous solutions (pH 9.5-13) by molecular oxygen in the temperature range of 25-80 "C were made by Kirso et al. (1967, 1968, 1972). Although these are not exactly the conditions employed for WAO, most of the observations made by the investigators regarding the mechanism and the end products of oxidation of phenol are valid under WAO conditions as well. The orders with respect to oxygen and substrate were 0 and 1, respectively for all the phenols and their mixtures studied. The energies of activations were in the range of 5.44-54.01 kJ/mol. They observed that the rate of oxidation increases with an increase in alkalinity of the medium, i.e., with the increasing concentration of phenolate ion. Their results showed regular dependence of the kinetic characteristics on the position of the substituent relative to the hydroxy group. The ortho- and para-substituted phenols are comparatively easy to oxidize due to the conjugation through the aromatic nucleus. In the case of dihydroxynaphthalene series, they observed that the oxidation rate increases with increasing distance between the -OH groups in the molecule. The effect of the position and the type of substituent on the ease of WAO discussed later in the text corroborates the observations of Kirso et al. (1967, 1968, 1972). The end products formed during the oxidation identified by them include polymeric material (tars), pyrocatechol, hydroquinone, and carboxylic acids. These products have been identified during the WAO of phenols as well. The only exception is of Sadana and Katzer (1974a,b), who did not observe the formation of carboxylic acids during the WAO of phenol. However, formation of carboxylic acids (saturated as well as unsaturated) along with other intermediate products during WAO of phenol and substituted phenols has now been established beyond doubt [Ohta et al. (19801,Devlin and Harris (19841, and Mishra et al. (1994a)l. The orders with respect to phenol (0.44) and oxygen (0.55)[Ohta et al. (198011were also in disagreement with the first-order dependencies reported by Sadana and Katzer (1974a). Formation of tars is favored by low temperature of oxidation and high phenol concentration. Involvement of free radicals in WAO of phenols has specificallybeen established [Sadana and Katzer (1974a,b)l. Another important observation in the case of WAO of most of phenols is a pronounced induction period, the length of which depended on the catalyst concentration, oxygen concentration, temperature of WAO, and type of phenol [Shibaeva et al. (1969a,b), Joglekar et al. (1991), and Mishra et al. (1994a)l. The induction period, during which the rate of oxidation is very slow, is followed by a steady state

step during which the oxidation rates are the fastest. Maximum oxidation takes place during this step. Recently Mishra et al. (1994a) indicated that under certain conditions of temperature, the steady state step is followed by a third step characterized by a slow oxidation step. This third step is observed due to the resistance of the low molecular weight fragments (mostly carboxylic acids) formed during the first two steps to further oxidation. Sadana (1979) determined the rate constants for free-radical initiation, uninhibited and inhibited oxidizability, and relative inhibitor efficiency in aqueous phase phenol oxidation by the inhibitor method. The kinetics of free radical inhibitor consumption, phenol hydroperoxide formation, and kinetic chain length growth were examined theoretically along with its implications. They developed a method of estimating the length of induction period and a criterion for the critical catalyst concentration (CCC, infinite length of induction period or no oxidation) expectations in aqueous phase phenol oxidation. Devlin and Harris (1984) studied noncatalytic oxidation of phenol in aqueous solution by molecular oxygen in the temperature range of 150-225 "C and total pressure of 20.69 MPa in great detail and proposed a reaction pathway based on various intermediates isolated during the oxidation reaction. Imamura et al. (1982b,c)developed several catalysts to improve the liquid phase oxidation and found that MdCe composite oxides exhibit much higher activity than even homogeneous copper catalyst [Imamura and Dol (1985) and Imamura et al. (1986,1987)l. Imamura et al. (1988) studied the catalytic effect of the noble metals (Pt, Ru, Rh, Ir, Pd) on WAO of phenol and other important model pollutants. Activities of Ru, Pt, and Rh were higher than homogeneous copper catalyst. Ruthenium showed the highest activity among the three catalysts and indicated a high possibility to be used as promoter for MdCe catalyst. Imamura et al. (1988) achieved 94.8% TOC removal during the oxidation of aqueous solutions of phenol using RdCe catalyst. Pintar and Levec (1992) studied catalytic oxidation of phenol over CuO and ZnO catalysts. Destruction of aqueous solution of phenol by oxidation with sodium sulfite plus oxygen as oxidant and copper sulfate as catalyst has been studied by Kulkarni and Dixit (1991). Cu2+(765 m a ) , and 0.25 MPa By using Na2S03 (3 oxygen partial pressure, they could completely destroy phenol (100 mgL) in 15-18 min a t 110 "C. Other investigators who studied catalytic WAO of phenol in aqueous solution include Imamura and Okuda (1981) (H202, sodium peroxodisulfate, MnO2, Fe203, Cu-FeBi-K, C d C , etc.), Debellefontaine et al. (1991) [H2021 Fe2+l, Higashi et al. (1991) (5% PtJAl203 and other catalysts), Pintar and Levec (1992)(CuO and ZnO), and Eckert et al. (1990). Imamura and Okuda (1981) observed that H202 accelerated the decomposition of phenol but did not reduce the level of TOC. Noncatalytic oxidations of phenols have been reported by Pruden and Le (19761, Helling et al. (19811, Willms et al. (1987a1,Vortsman and Tels (1987),Baillod et al. (197913, 1982), Randall and Knopp (19801, and Joglekar et al. (1991). Li et al. (1991) calculated point selectivity (a)values (ratio of formation rate of acetic acid to C02) for phenol and 2-chlorophenol based on the results of Baillod et al. (1979b)t o be 0.15 and 0.37-0.96, respectively. The a value associated with the oxidation of 2-chlorophenol exhibited some variation with temperature. This according t o Li et al. (1991) was most likely due to the

a),

Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 11 different pathways for the destruction of 2-chlorophenol. The reaction intermediates might be converted by both oxidation and other reactions involving chlorine radicals. The effect of these side reactions is temperature dependent and should be observable in WAO of other chlorinated molecules. Keen and Baillod (1985)studied WAO and the toxicity of the end products of WAO of phenol and substituted phenols. They observed that specific reactants are removed more rapidly at higher pH values. The end products of WAO were 10-20 times less toxic than the starting compounds, but the effluents were generally found to be more toxic than expected, based on the known concentration of starting compound remaining in the effluent. This they proposed may be due t o the combined toxic effect of the end products of WAO and the remaining part of original compound. It may be interesting to study, on the above lines, the toxicity of the effluents obtained by wet air oxidation of other phenols, effluents obtained by oxidation of phenols under more severe conditions of wet air oxidation (high temperature, catalytic oxidation). Joglekar et al. (1991) studied the kinetics of WAO of phenol and nine substituted phenols with chloro, methyl, methoxy, ethyl, and dimethyl as substituent groups. Kinetics was studied in the temperature range of 150190 "C and oxygen partial pressure range of 0.3-1.5 MPa. The kinetics was studied on the basis of reduction in total phenolic content as well as on the basis of reduction in COD. Oxidation of phenol by molecular oxygen has been proposed to be an electrophilic reaction. The reaction between aryloxy radical with oxygen was considered to be the rate-limiting step. They observed the following order of reactivity for the phenols studied: p-methoxyphenol > o-methoxyphenol > o-ethylphenol > 2,6-dimethylphenol > o-methylphenol > m-methylphenol > p-chlorophenol > o-chlorophenol > phenol > m-chlorophenol Phenol and chlorophenols exhibited an induction period, the length of which depended on the oxygen partial pressure, followed by a fast reaction step. In the case of methoxyphenols, the induction period was absent. The methoxyl group favors the formation of aryloxy radical by increasing the electron density on the aryl ring. This results in increased oxidation rate, and thus there is no induction period. Oxidation of alkyl group occurs much more readily compared to that of the ring resulting in the rapid formation of radicals. Due to this, oxidation of alkylphenols is characterized by an initial fast reaction period followed by a slow reaction period. The presence of methoxyl and alkyl group a t ortho/para positions of the phenolic hydroxyl group stabilizes the oxy radical formed by resonance and hyperconjugation, respectively. A substituent in an ortho position causes steric hindrance to the stabilization so that o-methoxyphenol showed a slower rate than p-methoxyphenol. In the case of alkylphenols, m-cresol showed the slowest rate as methyl group in a meta position does not favor stabilization of the oxy radical. Similar observations have been made by Kirso et al. (1967,1968,1972)for the oxidation of phenol in aqueous alkaline solutions (pH 9.5-13.0) in the temperature range of 25-80 "C. In o-alkylphenols, a radical can be easily formed at the alkyl group preferably at the benzylic positions. Though chlorine at ortho/para posi-

tion can stabilize the oxy radical, it has a strong inductive effect. Thus it shows a poor stabilizing effect. It is also seen that, similar to m-cresol, m-chlorophenol also showed low reactivity. In methoxy- and alkylphenols, the radical formation being assisted well, oxidation starts right from the beginning. The orders with respect to phenolic content, COD, and oxygen were 1for all the phenols studied and for both steps (wherever observed). The reaction became mass transfer controlled above 240 "C and at phenol concentration greater than 20 000 mg/ L. The values of mass transfer coefficients were reported. All the information related t o WAO of phenols has been summarized in Table 3. We can see that the order with respect t o oxygen for phenol oxidation is in the range of 0-1.5. Details of the extent of partial pressure variation is not known in the case of Kirso et al. (1967, 1968, 1972). The zero-order dependencies observed by Mishra et al. (1994a) and Yang and Eckert (1988) can be related to the temperature and pressure conditions used by them. Yang and Eckert (1988) used temperatures in the range of 310-340 "C, leading to high rates of oxidation. However, there is a corresponding increase in the oxygen solubility as well. In this case as well as in the case of Mishra et al. (1994a) the range of oxygen partial pressures was very limited. This may be the other possible reason for the zero-order dependence observed by these two investigators. In all other cases, the temperatures are in intermediate range (80-288 "C). It can be seen from above discussion that most of the phenols studied are halophenols, methoxyphenols, and alkylphenols. Wet air oxidation studies should be extended to other commercially important phenols such as bisphenols, polyphenols, dihydroxyphenols such as quinones, etc. which have not yet been studied. Further, as in the case of carboxylic acids, RdCe and Mn/ Ce catalyst systems have been found to be very effective for the destruction of phenolic compounds. It may be worthwhile to study the effectiveness of these catalysts in destruction of real phenolic waste streams. Kulkarni and Dixit (1991) found sodium sulfite to be a very effective auxiliary oxidant (co-oxidant) during wet air oxidation of phenol with CuSO4 as catalysts. It will be interesting to study the effectiveness of other auxiliary oxidizing agents (such as HN03) to improve the efficiency of wet air oxidation of phenol and substituted phenols. 2.3. Wet Air Oxidation of Cyanides and Nitriles. Widespread use of cyanides and nitriles has increased the probability that they will be found in significant concentrations in surface waters and effluents. Alkalimetal cyanides (NaCN, KCN, etc.) are used in extraction of silver and gold from their ores, electroplating, germicidal sprays in agriculture, pharmaceuticals, preparation of organic cyanides, etc. Cyanamide is used to produce calcium cyanide as intermediate for pesticides and is a raw material for dicyandiamide and melamine. Calcium cyanamide is used for steel nitridation and also in agriculture in defoliants, fungicides, and weed killers. Calcium cyanide is used in the preparation of fumigants, rodenticides, and ferrocyanides. Organic cyanides are used in the production of polymers, synthetic rubber (acrylonitrile), textiles (nylon via adiponitrile), plastics, pesticides, dyes, solvents (acetonitrile), etc. Cyanides and nitriles (particularly the unsaturated ones) are highly toxic and nonbiodegradable at the concentrations normally encountered in effluents. Wet

12 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995

Table 3. Wet Air Oxidation of Phenol and Substituted Phenols investigators Shibaeva et al. (1969a)

substrate and catalyst used phenol (1.9-3.8 g/L)

treatment conditions and observations T = 180-210 "C, TP = 3.5 MPa, m = 1,n = 1, E = 107 kJ/mol, A = 1.96 x 109 Sadana and Katzer (1974a) phenol; unsupported CuO T = 180-210 "C, inactive (although as active as supported form, on the basis of unit surface area) 10% CuO on y - A l 2 0 3 ; surface at T = 100 'C, PO, = 1atm, n = 1 for both steps and t 1 =~ 17 h area = 140 mz/g, porosity = 0.42 mz/g at T = 105 "C, PO, = 10 atm, n = 1for both steps, m = 1.5(steady state), and tllz = 28 min at T = 118 "C, PO, = 30 atm, n = 1for both steps, m = 1.5 (steady state), tu2 = 3.5 min at T = 100-128 "C,Poz = 6.17-8.29 atm, E f =0.5, E = 276.3, 184.2 kJ/mol, for initial and steady state regions at T = 140 "C, PO,= 3.5 atm, Ef= 0.05, E = 175.85 kJ/mol for d, < 0.06 mm and low temp, E = 100.48 kJ/mol for d, = 0.4 mm and at high temp 19% MnOz on y - A l 2 0 3 E = 108.9 and 175.9 kJ/mol for initial and steady state region, respectively; rates in steady state region are same for MnOz and cUo/&o3 catalysts; IN ALL THESE CASES NO ORGANIC ACIDS WERE DETECTED at T = 114 and 200 "C and PO, = 3.4-17 atm, n = 1,m = 1.5, Katzer et al. (1976) phenol; 10% CuO on y - A l 2 0 3 E = 57.36 kJ/mol for d, < -150 mesh, Ef= 1 a t 200 "C and 9.5 atm Po,,, greater than 99% conv of phenol to COz and HzO achieved in 9 min Pruden and Le (1976) phenol (1.4-3.0 MPa) T = 200-250 "C, TP = 5.5-15.2 MPa; no hydrolysis a t 300 "C in 1h, n = 1,m = 1, E = 45.22 kJ/mol,A = 4.71 x lo4 a t 250 "C and 15 MPa, about 99% conv obsd % destruction in 1h

investigators Randall and Knopp (1980)

Baillod et al. (1980) Harris et al.

starting concn (mg/L) 10 000 12 410 8 220 10 000 5 000

phenol (0.2-5) x mol/cm3); 10% CuO on y-Al203, surface area = 140 m2/g, porosity = 0.42 cm3/g

Ohta et al. (1980)

investigators Baillod et al. (1982)

substrate and catal used phenol 2-chlorophenol 2,4-dimethylphenol 4-nitrophenol pentachlorophenol

substrate and catal used 2.4-dichioro~henol600m a ) 2;4-dichlorGhenol(23.5 i&L)

Helling et al. (1987) Willms et al. (1987a)

phenol phenol

Yang and Eckert (1988) p-chlorophenol Vortsman and Tels (1987) phenol Kulkarni and Dixit (1991) phenol, CuSO4 (765 mgL) with NazSO3 and oxygen as oxidants, 1L autoclave

substrate and catal used phenol p-chlorophenol

275 "C/Cu2+ 99.88 97.30

T = 121-288 "C,Po, = 5-30 atm, n = 0.44, m = 0.55, order w.r.t. catal = -0.6, K = 0.47 exp(-20400/RT), org acids detected, deactivation of catal below d, = 0.054 cm obsd, at higher d, (0.172 and 0.318) no deactivation obsd

T = 204-260 "C, TP = 3.9-7.1 MPa, m = 1, E = 67.8 kJ/mol,A = 1.84 x lo4 T = 204-260 "C, TP = 3.9-7.1 MPa, n = 1,E = 62.9 kJ/mol,A = 7.4 x lo3 T = 130-250 "C, PO, = 6-11.2 MPa, >95% phenol degradation in 150 "C) in the absence of oxygen. The separation of the solid mass formed results in COD reduction of the waste. However, the COD of the effluent is still quite high and needs to be treated further before discharge. Lele et al. (1990) studied the kinetics of WAO of the effluent obtained after separation of solids formed during thermal preatment. Details of the above studies and other relevant published literature are given in Table 7. From Table 7 it can be seen that the range of temperatures and total pressures are in the range of 150-250 "C and 1-6.6 MPa, respectively. For implementing WAO for the treatment of alcohol distillery wastewater, the major contribution to the operating costs comes from the power for the compressor. Therefore an attempt needs to be made to investigate the catalytic oxidation so that there is a reduction in operating pressure and hence the power costs become manageable. The chloride concentration in distillery waste may be as high as 6.4 kg/m3 [Lele et al. (199211. Further, the effluent contains substantial quantities of salts and these are likely to pose scaling problems on the heat transfer surface. In view of this, there is a need to select a suitable material of construction. Substantial work is still needed for scale-up of the process. Lele et al. (1989, 1990, 1992) have suggested a two-step process in which there is a substantial reduction in the oxygen requirement. This two-step process has promise for implementation, and a vigorous attempt in this direction will be extremely useful. 3.3. Effluent from Pulp and Paper Mill. Effluent from the pulping mill generated after cooking of wood or other suitable raw material is termed as black liquor due to its color. It is highly organic in nature and contains organic matter in the form of suspended solids, colloids, BOD, COD, sulfur compounds, pulping chemicals used, organic acids, chlorinated lignins, resin acids, phenolics, unsaturated fatty acids, terpenes, etc. About

300 m3 of wastewater is generated per ton of pulp manufactured. Thus the liquor has recoverable amounts of chemicals and energy. Recovery of the chemicals is a necessity in order for the manufacturing units to meet local discharge standards. It is an economic necessity as well as in the wake of the rising cost of pulping chemicals and energy and to improve the profitability of a pulping operation. The conventional treatment method involves evaporation followed by incineration in furnaces. Pulping chemicals are recovered in the form of molten salts, and the energy recovery is in the form of steam. However, the chemical and energy recovery efficiency is not satisfactory [Flynn (197611. Wet air oxidation is a very promising treatment method for black liquor. In fact, the first patent specification of a WAO process was for the treatment of sulfite liquor from pulp production in an autoclave at 180 "C with compressed air [Strehlenert (1911)l.The first known WAO plant was put into operation in 1958 for treatment of sulfite liquor, although the plant was later closed due to uneconomical operation [Perkow et al. (198l)l. Since that period, WAO has come a long way by way of highly efficient chemical and energy recovery systems and is economically attractive. The sodium-lignin salt present in the effluent is converted to Na2C03. The soda is recovered in the form of a 10% Na2C03 solution which is free from organics [Schoeffel and Seegert (196611. After caustification, the reusable soda lye is obtained. Sulfur is also recovered in the form of inorganic salt. A plant of this type was put into operation a t Associated Pulp and Paper Mills, Ltd., in Australia in 1966. The WAO plant could recover more than 99.9% of pulping chemicals (soda) and energy in the form of high-pressure steam [Teletzke and Pradt (196911. Several other investigators also studied WAO of black liquor [Teletzke (1964), Zimmermann and Diddams (1960),Morgan and Saul (1968),Jannot (1973), Galassi (19801, Prasad and Joshi (1987), and Foussard et al. (198911. Teletzke (1964) and Zimmermann and Diddams (1960) also discussed a complete plant design with possible recovery of chemicals. Galassi (1980) used ozone as an oxidizing agent to wet air oxidize the black liquor having a COD of 347 000 mgL a t 280-380 "C. Prasad and Joshi (1987) have reported the kinetics of WAO of black liquor in the temperature range of 120180 "C and at 0.3-1.0 MPa oxygen partial pressure. They observed that, unlike in the case of distillery waste, there is no solid formation and no hydrolysis (or thermal degradation) of the black liquor even at 270 "C. More than 96% COD reduction was observed a t 275 "C and 0.3 MPa oxygen partial pressure. They also studied the effectiveness of various metal oxide catalysts (CuO, MnO2, ZnO, SeO2) in WAO of black liquor. Selenium oxide (SeO2) showed a substantial catalytic effect while others were only nominally effective. Foussard et al. (1989) observed formation of low molecular weight organic acid (mainly acetic acid) due to lignin degradation as indicated by Emanuel et al. (1967). The energy of activation obtained for the oxidation of black liquor was found to be close to that of propionic acid [Day et al. (1973)],indicating that the oxidation of low molecular weight organic acids formed is the limiting step for the oxidation reaction. Foussard et al. (1989) developed an empirical model using an exponential rate model to fit their data. The quantity

20 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995

Table 7. WAO of Alcohol Distillery Wastewater investigators

substrate

treatment conditions and observations

T = 150-210 "C, PO,= 0.1-2.5 MPa; init fast oxidn step followed by

Daga et al. (1986) alcohol distillery waste (COD = 35 kg/m3)

Lele et al. (1989) alcohol distillery wastewater (COD = 108 kg/m3)

Lele et al. (1990) thermally pretreated alcohol distillery waste after solid separation

slower step obsd; n = 1for both steps; n = 1.5 if single rate expression fitted for both steps; n = 0 above Po, = 1MPa and at 190 "C; below Po, = 1MPa, m = 0.3 at 150 "C and m = 0.6 at 200 "C; E = 45.38 kJ/mol T = 160-250 "C, PO,= 0, t = 2 h; charred residue formed during treatment; m = 0; COD redn 27.8 and 63% at 160 and 250 "C, respectively; filtrability of slurry (solids formed) increases with increase in temp; at pH 7 and 1.0, COD redn 26 and 59% at 230 "C; filtrate better suited to biological treatment compared to original wastewater T = 230 "C, PO,= 0; charred residue with properties similar to coal in terms of C content and calorific value separated 50%; COD redn in 2 h; on WAO of filtered effluent at 250 "C and at 2 MPa further 75% COD reduction obtained; n = 0 at low temp; n = 0 for both steps observed at higher temp; m = 0.6

T = 204.5 "C, TP = 2.07 MPa, pH = 5, t = 2 h investigators

substrate

Chowdhury and Ross (1975)

effluent from a brewery unit

catalyst none CeOz coo2 MnOz Crz03 Fez03

1 1

1 1 1 1 1 1 (0.8 0.2) 0.25 1

vzo5 Ptoz

+

V Z O+ ~ Fez0 Pt-black Y ?;N -}i mercuric acetate ferric acetate ceric sulfate ceric sulfate and chromium sulfate Na-perborate

investigators Chowdhury and Ross (1975)

substrate effluent from a brewery unit

COD (mg/L) init % redn

catal concn (g)

1 1 1

0.8 0.3

+ 0.2

catal and catal concn (mol of ions/L) HzOz (0.0098 moVL) with (Hg)zS04 (5.19) silver sulfate (9.87) chromic sulfate (2.15) ceric sulfate (2.91) ferrous sulfate (5.53) nickelous sulfate (2.93) manganous sulfate (4.55) cupric sulfate (3.1) ferric sulfate (2.74) cobaltous sulfate (3.56) benzoyl peroxide systems benzoyl peroxide only (0.00636 m o m ) benzoyl peroxide (0.00319 mol/L) with ferrous sulfate (5.553) ceric sulfate (2.91) chromic sulfate (2.15) cobaltous sulfate (3.56) silver sulfate (9.87)

3920 4320 6600 6600 6600 6600 3760 3670 5220 6521 6521

87.70 82.56 86.55 87.50 80.20 83.86 89.94 92.46 77.60 74.87 82.52

4650 6860 4320 3920 5220

66.88 77.22 83.36 80.48 81.10

init

% redn

820 760 4650 4640 7760 6521 6521 6550 6325 8320

83.02 84.02 81.85 88.66 86.30 87.05 86.93 96.0 95.0 83.02

3920

80.04

8320 7760 4650 4650 4650

91.79 80.38 74.96 81.38 83.39

combination effect of metal ions/peroxides investigators Chowdhury and Ross (1975)

substrate effluent from a brewery unit

catal (loadings) (mol/L) Ni2+ + Cu2++ HzOz (0.0047, 0.00123, 0.0098)

+

+

I I I

influent

% redn

8320

88.53

4650

89.98

6120

95.38

Ni2+ Cu2+ benzoyl peroxide (0.0047, 0.00123, 0.00318)

Fe3+ + Cu2++ HZOZ (0.00164,0.00031,0.0196) Fe3+ Cu2++ benzoyl peroxide1

+

(0.00438, 0.00123, 0.00318)

+ (0.00166, 0.00123, 0.0098) MnzC+ Cu2++ HzOz

612

89.17

6325

94.85

(0.00273, 0.0031, 0.0098)

6325

93.7

Fez+ + Cu2+ H2Oz

Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 21 Table 7 (Continued) combination effect of metal ions/peroxides COD (mg/L) investigators Chowdhury and Ross (1975)

substrate effluent from a brewery unit

catal (loadings) (mom) Ce4+ Cr3+ HzOz (0.00146, 0.00215,0.0196) Fez++ Fe3+ HZOZ (0.00166, 0.0011,0.0098) Fe3+ Fez+ Cu2+ H2Oz (0.0011,0.00166,0.00123,0.0098) Fez+ Fe3+ Ni2+ HzOz (0.00166,0.0011,0.00123,0.0098)

+

+ + + +

+ +

+ +

influent

% redn

10096

90.47

6550

94.34

6325

96.72

6325

93.95

I I

Energy of Activation for Different Catalysts energy of activation (kJ/mol) investigators

substrate

Chowdhury and Ross (1975)

emuent from a brewery unit

catal none Fez+ + Fe3+ Cu2+ HZOZ

+

+

fast step 97.43 71.43

Slow Step 84.99 65.48

Relative Catalytic Effects during WAO % COD redn at

investigators

substrate

catal

Chowdhury and Ross (1975)

effluent from a brewery unit

none Fez+ Fe3+ HZOZ Fe3+ HzO2 Fe3+ + Cu2+ + HZOZ Fez+ HzOz Fez+ Fe3+ Cu2+ HZOZ CU'+ HzOz

+ + + + +

+ +

+

15min

30min

2h

52.5 62.0 65.0 78.0 53.0 81.0 86.5

68.5 77.0

84.5 94.5 95.0 94.5 85.0 96.7 96.0

80.0

89.0 70.0 91.0 95.5

Table 8. WAO of Effluent from Pulp and Paper Mills investigators

substrate

Wright (1952) Teletzke and Pradt (1969) Claude et al. (1980) Galassi (1980) Ren et al. (1982) Fenchel(1978) Prasad and Joshi (1987)

black liquor pulping liquor (eucalyptus wood) COD = 130 g/L black liquor black liquor black liquor sewage sludge containing fillers from paper mill black liquor from kraft pulp mill (COD = 24-33 kg/m3)

treatment conditions and observations

T = 40-90 "C, PO,= 0.021 MPa, n = 1 T = 300 "C, TP = 20.7 MPa; COD redn of 98.99%and chemical recovery more than 99.9%achieved energy recovery in form of steam T = 220-320 "C, PO,= 2-13.6 MPa, m = 0.38; E = 13.4 kJ/mol between 220 and 295 OC; between 295 and 320 "C, E = 135.23 kJ/mol T = 280-380 "C, effluent COD < 2000 mg/L T = 180-250 "C, PO,= 1.2 MPa, n = 2, E = 94.1 kJ/mol T = 250-300 "C, TP = 13 MPa; 90%COD redn (init COD = 40-50 g/L) with high level and quality of keoline recovered T = 120-180 "C, PO,= 0.3-1 MPa; no hydrolysis up to 270 "C; WAO proceeds via init fast reaction step followed by slow step; n = 1 in both steps, m = 0.5 for fast step and 0.35 for the slow step; if a single rate equation is fitted for the two steps, n = 1.5 and m = 0.5 E = 126.65 for fast step and 80.81 kJ/mol for the slow step; E = 51.96 kJ/mol on basis of single rate expression; 96%COD redn obtained at 275 "C and 0.3 MPa PO,

of organics that cannot be oxidized (TODE)a t a given temperature was correlated by the following equation: TOD, = TOD,[l - 0.12 exp(3.57 x 10-3T>l

(3.1)

The rate of oxidation of easily oxidizable matter was given by

r = -d(ToD) - K dt

e-E/RT

[TOD - TODEIn@r

(3.2)

where 550 K T 590 K,0.6 kg/m3 PoJH < 9.5 kg/m3,E = 135 kJ/mol, KO= 9.08 x lo8 (kg/m3)-0.38s-l, n = 1, and m = 0.38. Flynn (1979) has discussed some aspects of chemical and energy recovery during wet air oxidation of black liquor. When properly applied, chemical recovery efficiency of nearly 100% and thermal eficiency in excess

of 80% can be achieved. Table 8 gives a summary of published information on WAO of pulping and paper mill effluent. The above studies on WAO of black liquor involve temperatures in the range of 120-317 "C, and the total pressure may be up to 20 MPa. With the aim of reducing the severity of oxidation conditions required, catalytic WAO of black liquor needs to be studied. In view of the formation of carboxylic acids including acetic acid during WAO of the black liquor [Foussard et al. (1989)1,it may be interesting to study the effectiveness of catalysts like RdCe, Co/Bi, CuO, etc. in WAO of black liquor. Oxidation of these low molecular weight acids is a limiting step in WAO of black liquor (as in the case of all WAO systems). The above-mentioned catalysts have shown promise in completely oxidizing the carboxylic acids including acetic acid. The effluent characteristics differ, depending on the type of the process

22 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995

(chemical, semichemical, or mechanical1 and the raw material used (type of wood). Wet air oxidation studies on these effluents should be performed. 3.4. Cyanide, Cyanate, and Nitrile Wastewaters. Cyenides are present in discharges from the electroplating industry, extraction of metals, coke furnaces, petroleum refineries, etc. Sodium cyanide is used in manufacture of pharmaceutical, agrochemical, and dye intermediates. The effluents from these industries contain unreacted cyanides. Among nitriles, effluent from acrylonitrile manufacturing plant has attracted attention because of its high toxicity due to the presence of acrylonitrile, acetonitrile, acrolein, inorganic cyanides, and ammonium sulfate along with large concentration of other organics. For acrylonitrile plant wastewater, chemical treatment alone or in conjunction with physical processes has been reported in the literature [Ennenbach et al. (19771, Manea et al. (19771, and Cruceru et al. (198311. These studies are aimed at reducing the acrylonitrile and the cyanide content. Further, substantially higher quantities of ozone are required to oxidize acrylonitrile to any significant extent. At the same time ozone has an adverse effect on the biodegradability of acrylonitrile [Medley and Stover (198311. Nitriles and cyanides are present in concentrations which are toxic t o microorganisms. In addition to these toxic components, there are other nonbiodegradable components as well in the wastewater. Biotreatment methods reported in the literature for the treatment of acrylonitrile wastewater involve acrylonitrile concentration of less than 200 mgL [Kincannon et al. (1983) and Singh and Desai (198711. Knowles and Wyatt (1988) used a mixed microbial culture to treat actual acrylonitrile effluent and achieved 7 5 9 0 % reduction. Treatment of acrylonitrile-bearing wastewater by WAO has been reported by several investigators [Akitsune (1976), Wilhelmi and Knopp (19791, Inagaki et al. (19801, Sumitomo Chem. Co. (19821, Dietrich et al. (19851, Chowdhury and Copa (19861, and Keckler et al. (199311. Wet air oxidation has been reported for successful treatment of cyanide-containing waste streams from coke oven gas scrubbing [Wilhelmi and Knopp (197911 and other industries such as metal plating and photoreproduction [Chowdhury and Copa (198611 with almost complete (>98-99%) cyanide destruction. For the sake of economic advantage a two-step treatment, WAO followed by biological treatment or biophysical treatment, has been suggested [Wilhelmi and Ely (197611. In the first step acrylonitrile concentration is reduced by more than 99.9% with the corresponding COD removal of 97% by WAO. After WAO the effluent contains a very high concentration of ammonium sulfate which is recovered. The effluent is then subjected to biological or better biophysical treatment involving powdered activated carbon in conjunction with the activated sludge treatment. Mishra (1992) used a similar strategy to treat the wastewater generated from an acrylonitrile manufacturing unit. The wastewater contained 1330 mg/L acrylonitrile and 480 mg/L acetonitrile along with acrolein, inorganic cyanides, and ammonium sulfate. Further, it has a very high COD (12 000 mgL). It should be noted that most of the previous studies reported in the literature pertain to the treatment of acrylonitrile wastewater having much lower COD and nitrile content. The wastewater was subjected to a treatment system involving WAO of the

wastewater followed by aerobic activated sludge treatment in a bench scale sequential batch reactor (SBR). Mishra determined the kinetics of WAO and activated sludge treatment process. At 250 "C and 0.69 MPa oxygen partial pressure, about 60% COD reduction could be achieved in 2 h. More than 96% acrylonitrile destruction could be achieved in 4 h at 225 "C, while more than 77% reduction in acetonitrile could be achieved in 4 h at 240 "C. Comparatively lower temperatures were used (below 250 "C) in the study to ensure that the reaction is in the kinetically controlled regime. The effluent obtained aRer WAO was subjected to activated sludge treatment (in a SBR) after about 4 times dilution with water. Although about 95% COD reductions (from 10 000 to 536 mg/L) were obtained during the biological treatment step, the brown color of the wastewater was not removed t o any significant extent. This necessitated the use of activated carbon to remove the color. The effluent obtained after the combined treatment (WAO activated sludge treatment powdered activated carbon) gave an effluent which was of very good quality (COD reduced to 40 mg/L from about 120 000 mg/L with complete removal of nitriles and color). Table 9 summarizes all the relevant published information about WAO of cyanide, cyanate, and nitrile wastewaters. We can see that cyanide and nitrile wastewater streams from only a few sources (coke oven gas scrubbing, metal plating, and acrylonitrile manufacturing plant) have been studied. There are several other important waste streams containing cyanides and nitriles, e.g., waste streams from petrochemical and steel plants (containing inorganic cyanides), pharmaceutical, pesticide, textile, plastic, and dye manufacture, etc. (containing nitriles). Wet air oxidation studies on these waste streams are needed. 3.5. Regeneration of Spent Carbon and Spent Earth. Activated carbon and activated earth are very widely used in adsorptive separation processes. The regeneration of these materials is very important for making these adsorption processes economically attractive. The various methods available for the purpose are biotreatment, solvent extraction, thermal regeneration, steam regeneration, and acid-base treatment. Loven (1973) has described the various methods for regeneration of activated carbon including the economic aspects. Regeneration of spent carbon (and earth) by biological treatment is not possible when the pollutants are in concentrations toxic to the activated sludge or are nonbiodegradable. Regeneration by solvent extraction [Smisek and Cerny (19701, Hassler (19631, and Sutikno and Himmelstein (1983) is not economical unless the adsorbed components are of high value and product recovery is planned. Further, substrates which are chemisorbed on the carbon surface (like phenols) are difficult to remove by solvent extraction [Suzuki et al. (19781, Goto et al. (19861, and Yonge et al. (198511. Use of supercritical fluids for the regeneration of spent carbon has also evoked some interest recently [Tan and Liou (1988, 1989a,b)l. The method is better than steam regeneration [De Fillippi et al. (1980)l. However, in this case also the components chemisorbed on the carbon surface are difficult to remove [Kander and Paulaitis (1983)l. Thermal, steam, and WAO regeneration techniques take advantage of the fact that adsorption is an exothermic process and, at higher temperature, the adsorptive capacity of the carbon is lowered considerably. Thermal and WAO techniques regenerate the

+

+

Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 23 Table 9. Cyanide, Cyanate, and Nitrile Wastewaters investigators substrate Wilhelmi and Ely (1976) acrylonitrile wastewater (COD = 42 000, BOD = 14 200, CN- = 270, TDS = 57 200 mg/L) Pradt (1972) cyanide-containing acrylonitrile wastewater (COD = 32 000, CN- = 2000 mg/L) Inagaki et al. (1980) acrylonitrile wastewater (COD = 152 000 mg/L) Dietrich et al. (1985)

treatment conditions and observations 97% COD, 93% BOD and 99.9+% cyanide redn by WAO; on subjecting oxidized effluent to biotreatment, further 75% COD, 99% BOD removed; thus overall 99+% COD, 99.9+% BOD, and 99+% cyanide removed by combined treatment COD of effluent = 8000 (75% redn) and cyanide concn in effluent = 6 mg/L (99.7% redn)

coke plant effluent (309 mg/L CN-)

acetonitrile (1040 mg/L) in effluent propionitrile (391 mg/L) in effluent acrylonitrile wastewater Chowdhury and Copa (1986) (COD = 37-46 000 and CN- = 400-900 m a ) Chowdhury et al. (1986) cyanide waste (13.1-2100 mg/L CN-) metal plating waste at Casmalia Resources, CA (COD = 97 g/L, cyanide = 33 g/L) acrylonitrile wastewater

wastewater diluted to COD of 93 000 mg/L with treated wastewater and then subjected to WAO at 245-250 "C in presence of 500 mg/L copper/copper salts; effluent afier oxidn contained 4800 mg/L COD, 0.01 mg/L CN-, and ~ 0 . mg/L 1 Cu2+ T = 279 "C, effluent flow rate = 6.3 gph, t = 69 min, TP = 10.75 MPa, catal used; >99% removal with < 6 mg/L in effluent T = 275 "C, t = 60 min, catal used; 98.4% redn in acetonitrile concn T = 275 "C, t = 60 min, catal used; 98.2% redn in propionitrile concn at 250 "C, total press. of 6.9 MPa and effluent flow rate of 145 gpm, 60-65% COD and more than 99.9% CN- redn obtained in 90 min

T = 275-300 "C, 97.6-99.9% remova; last traces of CNremoved using HzOz 99.99% destruction

T = 254 "C, TP = 7.2 MPa, t = 115 min, Cu2+as catal; 95.7% COD and > 99.9% toxic removal

acrylonitrile wastewater at Asahi Chem Co., Japan (COD = 37-46 000 mg/L and CN- = 400-900 mg/L) coke oven wastewater at Nippon Steel, Japan (33 g/L thiocyanate) at Tokyo Gas, Japan (30 g/L thiocyanate, 26.5 g/L thiosulfate) at Dofasco, Canada (80 g/Lthiocyanate)

T = 250 "C, ef€luent flow rate = 145 gpm, TP = 6.9 MPa, t = 90 min; 60-65% COD and >99.9% CN- redn

T = 270 "C, effluent flow rate = 67 gpm, TP = 7.3 MPa, t = 60 min; 97% SCN- redn

T = 260 "C, effluent flow rate = 17.6 gpm, TP = 7.3 MPa, t = 60 min; 99% SCN- and >99% thiosulfate removal

T = 270 "C, effluent flow rate = 10 gpm, t = 90 min, TP = 8.6 MPa; '99.6% removal

Cyanide Wastewaters, Casmalia Resources, California COD ( m a ) investigators Copa et al. (1989)

substrate caustic cyanide waste

metal plating waste

metal stripper waste photoreproduction waste precious metal recycle waste cyanide waste investigators Copa et al. (1989)

substrate coke oven gas wastewater at Dofasco, Canada

Kalman et al. (1987b)

nitrile plant wastewater

Mishra (1992)

acrylonitrile wastewater (COD = 120 000, acetonitrile = 480, and acrylonitrile = 1330 m a )

T("0 280 280 280 280 280 280 280 270 280 280 270 280 280 257

t (min) 60 60 60 60 60 60 60 60 60 60 60 60 60 80

cyanide (mg/L)

influent

% redn

influent

% redn

47600 1030 13700 12750 29000 14300 4250 39900 24400 34900 42020 38200 15600 37400

86.2 77.7 78.0 79.6 84.1 49.2 78.1 75.9 87.7 82.1 69.7 93.6 69.2 88.8

67900 91.5 2635 31250 19010 19678 449 26285 21100 6527 17220 1880 11340 25390

99.7 99.3 99.0 99.1 99.7 98.6 98.0 99.2 99.9 99.0 98.5 98.8 98.8 99.7

treatment conditions and observations T = 270 "C, waste feed flow = 18.25 gpm, pH = 8.4, TP = 9.05 MPa, COD reduced from 100.1 to 1.9 g/L, NH4SCN reduced from 94.8 to 0.008 g/L,(NH4)2Sz03reduced completely from 24.7 g/L, total S increased from 44.7 to 130.6 g/L T = 160-240 "C, TP = 7-15 MPa, m = 1, E = 52.1 kJimol, A = 2.43 10-5 T = 150-250 "C,Po,= 0.68-1.36 MPa, n = 1,m = 0, E = 39.78 kJ/mol; '95% acrylonitrile removed by WAO at 225 "C in 4 h acetonitrile redn of 77% in 2 h at 250 "C; 65% COD redn in 8 h at 200 "C

spent carbon with simultaneous destruction of the adsorbed pollutants. During thermal regeneration (by hot oxidizing gases a t 650-1000 "C),apart from corrosion problems while handling carbon loaded with s-and C1-containingorganics, the carbon losses may be as high

as 10-60% due to prevailing oxidizing conditions. Regeneration of spent carbon by air ('450 "C1 and steam have been discussed by Hutchinson (19731, Hernandez and Haniot (19761, Chihara et al. (1981), Smith (19751, and Klei et al. (1975).

24 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995

Wet air oxidation is an attractive alternative to the above techniques and is recommended for regeneration of activated carbon [Ledding (1961) and Kalman and Szebenyi (1986,198711. Wet air oxidation regeneration involves thermal as well as oxidative regeneration. For regeneration by WAO, the slurry of carbon (5-10% solids) is subjected to wet air oxidation [Knopp and Gitchel (197013. The advantages with WAO regeneration include direct slurry regeneration without dewatering, absence of particulate emissions, carbon losses less than 7%, and autogenous regeneration, and it is relatively less costly than the thermal regeneration processes [Knopp et al. (19781, Remirez (19771, and Mundale et al. (199111. The combination of carbon adsorption and biological treatment followed by regeneration of spent carbon by WAO can handle the industrial wastes which are too dilute for economic processing by WAO and are nonbiodegradable [Meidl and Burant (197411. The quality of water produced with the use of regenerated carbon equals the best obtained with virgin carbon [Burant and Vollstedt (19731, Burant (19741, and Gitchel et al. (197511. Wet air oxidation restores all the properties and pore structure of parent carbon to a very high degree with the exception of small ( < l o A) pore surface. In addition to this, there is an increase in the surface oxygen content of regenerated carbon. The small pores ( < I O A) are in any case not essential in wastewater treatment as the most likely compounds (CI-C4 compounds) are not adsorbed even by virgin carbon which involve such pores. Further, such compounds are generally quite biodegradable and hence removed by biological treatment [Gitchel et al. (197511. The increase in surface oxygen content of regenerated carbon results in loss of capacity of the powdered activated carbon (PAC) for low molecular weight aromatic compounds [Mahajan et al. (19801, Coughlin (19701, Coughlin and Ezra (19681, Knopp and Gitchel (19701, and Wedeking et al. (1985, 198711. Gitchel et al. (1980) have extensively studied the characteristics of the activated carbon regenerated by WAO and found that the recovered carbon has higher inorganic content, higher organic oxygen, hydrogen and nitrogen content, more surface area and volume in pores of greater than 37 A diameter, smaller particle size, and increased real density compared to virgin carbon. The importance of surface oxygen groups was demonstrated by RecktenWalt (1986). Kipling (19651noted an increase in selectivity of carbon for ethyl alcohol over benzene with the extent of oxidation. However, exposing the oxidized carbon to hydrogen or heating the oxidized carbon to 800-900 "C in vacuo to remove surface oxides recovered part of the adsorptive capacity lost by previous oxidation [Coughlin (19701, Mahajan et al. (19801, Coughlin and Ezra (19681, and Wedeking et al. (198711. The acidic groups formed during or following the activation of carbon limited the capacity of carbon for anionic adsorbate, in direct proportion to the concentration of the groups [Graham (19551, Snoeyink et al. (197411. However, the capacity for methylene blue dye was not affected by the regeneration and was limited by only pore size. The PACT process (activated sludge treatment with powdered carbon added to it) is superior to activated sludge followed by carbon treatment for the removal of pollutants from wastes. Combination of activated carbon-activated sludge treatment followed by regeneration of spent carbon (with simultaneous destruction of excess sludge and nonbiodegradables adsorbed on the

carbon surface) is capable of producing effluent of a quality otherwise obtainable only after tertiary treatment. Richard et al. (19821, Rollins et al. (19821, Randall (19831, and Bryan (1992) have discussed such an application combining PACT and WAO process. Canney et al. (1985) used WAO for pretreatment of a mixed hazardous waste that was then subjected to PACT treatment t o produce water quality suitable for direct discharge. Effect of temperature, time, and biomass on carbon regeneration by WAO has been studied by Ding et al. (1987). The temperature reported for regeneration of spent carbon is in the range of 200-240 "C [Ploss Van Amstel and Rietema (1973) and Charest and Chornet (197611. Above 200 "C, losses of nonloaded virgin carbon were very high. However, in the presence of adsorbed biomass or compounds, oxygen attacks them before oxidizing PAC and the losses were 1-5% in the temperature range of 200-250 "C [Knopp et al. (19781and Gitchel et al. (198011. This indicates that the selectivity is enhanced if there is a competition for available oxygen. Knopp et al. (1978) showed that PAC also catalyzes the oxidation of biomass and organics during WAO. The amount of adsorbate oxidized during WAO regeneration of carbon differs for different sources of carbon due to differences in reactivities of surface oxygen groups. Phenanthrene was found to be readily oxidized when adsorbed on Nuchar SA powdered activated carbon and oxidized by WAO while Hydrodarco carbon was less effective [Richard et al. (198811. Richard et al. (1988) also identified the intermediates (15) ranging from phenol to oxygenated biphenyl derivatives formed during WAO of phenanthrene sorbed on carbon. They hypothesized the involvement of hydroxy radicals and other intermediates on the activated carbon surface for the degradation. Charest and Cornet (1976) studied the kinetics of WAO of activated carbon. The oxidation proceeds rapidly above 200 "C and oxygen partial pressure as low as 3 MPa. The orders with respect to substrate (carbon) and oxygen were 0 and 1, respectively. The activation energy of 33.49 kJ/mol suggests a free-radical process typical of WAO systems. Mundale et al. (1991) provide a detailed study of the steps involved in the regeneration of the spent carbon by WAO and the rate-controlling step. The following steps occur during regeneration by WAO: desorption of adsorbed species from the activated carbon surface, mass transfer from the interior of activated carbon surface to the outer surface of activated carbon particle (intraparticle diffusion), mass transfer from the external surface of the particle to the bulk of liquid (film diffusion), mass transfer of oxygen from bulk gas t o the liquid phase, and the reaction between dissolved oxygen and the sorbate [assuming the reaction to be taking place in the bulk liquid which was confirmed by Mundale et al. (199111. A similar mechanism also holds for regeneration of spent earth. If the reaction proceeds a t another location, the regeneration step would be gas-liquid mass transfer, external mass transfer of oxygen from liquid bulk to the particle surface, internal diffusion of oxygen, and adsorption of oxygen. The rate-controlling step was determined by studying the rates of individual steps. The gas-liquid mass transfer coefficient for oxygen and kinetics of reaction between dissolved oxygen and phenol were investigated in the absence of activated carbon. The rates of internal and external diffusion of phenol was

Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 26 studied in the absence of oxygen. It was observed that, at 195 "C and phenol concentration e200 mg/L, the gas-liquid mass transfer resistance is eliminated above the impeller speed of 15 reds. The kinetics of reaction between dissolved oxygen and phenol was thus studied above the impeller speed of 15 reds, below 195 "C and at phenol concentration 99.9 cresols 38 13 65.8 quinoline investigators Dietrich et al. (1985)

Pilot Scale WAO of Organics in Industrial Wastewater study compound type Tlt ("C/min) C organonitrogen/phosphorus 260f60 260/60 D organonitrogen/sulfur 260/60

feed concn ( m a ) 1887 2585 6.7

% redn

99.6 99.5 99.4

investigators substrate treatment conditions and observations Chowdhury and Copa coke plant effluent T = 279 "C, TP = 10.75 MPa, t = 69 min, Cu2+cat&, 91.3% COD and >99% toxic removal coke oven gas liquor T = 268 "C, TP = 9.36 MPa, t = 83 min, no catal; 99.5% COD and >99.9% toxic removal (1986) T = 174, TP = 1.04 MPa, t = 56 min, no catal: 44.2% COD and >99.9% toxic removal spent caustic scrubbing liquor A T = 320 "C, TP = 20.7 MPa, t = 64 min, no catal; 96.2% COD and >99.9% toxic removal T = 200 "C. TP = 2.97 MPa. t = 47 min, no catal: 94.1% COD and 299.9% toxic removal sDent caustic scrubbing liquor B pesticide wastewater T = 240 "C, TP = 5.85 MPa, t = 120 min, no catal; 30.4% COD and 99.9% toxic removal T = 260 "C, TP = 8.28 MPa, t = 120 min, no catal; 65.2% COD and 99.9% toxic removal herbicide wastewater T = 281 "C, TP = 10.75 MPa, t = 60 min, Cu2+catal; 90.5% COD and 299% toxic removal herbicide wastewater T = 243 "C, TP = 10.92 MPa, t = 130 min, no catal; 70.9% COD and '99% toxic removal Destruction of Specific Pollutants during WAO a t T = 314 "C, TP = 13.4 MPa, t = 128 min investigators assay concn influent (mgL) Chowdhury and Copa (1986) COD 77500 1,a-dichlorobenzene 2213 methylene chloride 60 perchloroethylene 4000 Freon TF 3000 8385 xylene 30 toluene phenols 1556 isopropyl alcohol 1700 methyl ethyl ketone 6000 investigators substrate Matsuno et al. (1989) wastewater with 11.3 and 38 g/LCOD and TOC from diisopropylbenzene Mitsui et al. (19890 wastewater containing 40 g/L COD, 2.5 g L N, 10 g/L suspended solids Lin et al. (1992) caprolactam wastewater dyeing wastewater TNT red wastewater (1:lOO dilution) Hao et al. (1992) TNT red wastewater (1:lOO dilution) Hao et al. (1994) Gao et al. (1988) cotton slurry black liquor coal gasifier unit wastewater containing 92% phenolics

87.9 98.7 99.9+ 99.9+ 99.9+ 99.8+ 98.3 99.9 76.5 99.9+

treatment conditions and observations

T = 200 "C, PO, = 0.6 MPa, t = 4 h; 69% COD and 42% TOC redn T = 100-370 "C, PO, = 0.1-20 MPa, PtTiOz catal; at 240 "C, TP = 5 MPa, t = 60 min, 99% COD, 99.2% N, and 99.99% NH3-N removal

T = 260 "C, TP = 6.9 MPa, t = 60 min; 89% COD removal T = 240 "C,TP = 6.9 MPa, t = 60 min; 95% COD redn T = 340 "C, TP = 14.8 MPa; 99% org carbon and COD removal T = 260 "C, PO,= 0.69 MPa; >84% destruction T = 240-250 "C, TP = 5.4-5.9 MPa, t = 0.7 h; 42-48% COD and 95% color removal; > 90% soda recovery

Green et al. (1983)

% redn

T = 200-225 "C, PO,= 3.55 MPa, supported CuO as catal; 64% COD and 58% TOC removed

38 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 studies have not been performed. These include haloethers, halomethanes, polynuclear aromatic hydrocarbons, etc. In addition to these, there are compounds which although not priority pollutants are resistant t o biodegradation. Wet air oxidation studies on these compounds are needed. 3. Wet air oxidation of various waste streams has been made. However, there is very little published information on the kinetics of WAO of these waste streams. Detailed investigations are needed to get the kinetic data. With a view to reducing the severity of oxidation conditions, a suitable (cheap, stable, and resistant t o poisoning) catalyst system needs to be developed for WAO of waste streams. 4. There is a need t o investigate the mechanism of wet air oxidation. Though the mechanism of oxidation of organic compounds (for organic synthesis) is fairly well studied, there is scanty information on the WAO mechanism. Both catalytic (homogeneous and heterogeneous) and noncatalytic reactions need to be covered. 5 . The WAO is carried out in bubble columns, mechanically agitated reactors, loop reactors, trickle bed reactors, etc. The temperature and pressure range of operation is 125-320 "Cand 0.5-20 MPa, respectively. Though the WAO systems predominantly contain air and water, the organic components can affect the quality of gas-liquid dispersion (i.e., bubble size and rise velocity). For the design of WAO equipment, we need to know the values of the mass transfer coefficient, heat transfer coefficient, extent of mixing in all the three phases, and the pressure drop. For the estimation of these parameters, particularly under the condition of high temperature and pressure, there is no reliable procedure reported in the published literature. A systematic attempt is needed to develop data basehseful correlations which will be useful for the design and scale-up of WAO equipment. 6. Mathematical models need to be developed for the WAO reactions and reactors. 7. As mentioned in section 1.0, the material of construction for the reactor and heat exchanger will significantly affect the cost of the WAO unit. The choice of the material of construction is based on the nature of the wastewater, particularly its chloride content. Perkow et al. (1981)have suggested the use of various materials of construction for a range of chloride concentration under different conditions of temperatures. However, more detailed information is needed to decide the best material of construction for specific waste streams. 6.0. Nomenclature A = preexponential factor d, = diameter of catalyst particles E = energy of activation based on reduction in COD (kJ/ mol) E1 = energy of activation based on reduction in substrate concentration (kJ/mol) Ef= effectiveness factor H = Henry's law constant m = order with respect to oxygen n = order with respect to substrate concentration PO,= oxygen partial pressure (MPa) S, = specific surface area of catalyst (mVg) t = reaction time (min) T = temperature ("C) ATN = decrease in total nitrogen content ATOC = decrease in total organic carbon TP = total pressure (MPa)

GTNIS = decrease in total nitrogen content per unit catalyst

surface area Greek Letters

a = point selectivity 6 = tortuosity factor

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