WET AIR OXIDATION OF HAZARDOUS WASTE

Professor I. M. Mishra Department of Chemical Engineering,

Indian Institute of Technology, Roorkee, Roorkee-247 667.

1. Wet Air Oxidation

Wet oxidation is a hydrothermal treatment of aqueous solutions of biologically

recalcitrant and hazardous chemicals/wastes. It is the oxidation of dissolved or suspended

matter in water using an oxidant such as ozone, oxygen, hydrogen peroxide, air etc. It is

referred to as Wet Air Oxidation (WAO) when air is used as an oxidant. The oxidation

reactions generally occur at temperatures above the normal boiling point of water (100

°C) but below its critical point (374 °C). The system must also be maintained under

pressure i) to maintain the solution in liquid form; ii) to avoid excessive evaporation of

water and also iii) to conserve energy, as the evaporation needs latent heat of

vaporization. Under wet conditions, many compounds get oxidized which would

otherwise not oxidize under dry (not wet) conditions, even at the same temperature and

pressure.

When the hydrogen peroxide is used as the oxidant, the oxidation is referred to as

the Wet Peroxide Oxidation (WPO). Due to high cost of peroxides, it is used only to

create free initiation radicals as a pretreatment step followed by either air oxidation or

oxygen oxidation.

Wet air oxidation (WAO) is a well-established technique of importance for

wastewater treatment particularly toxic and highly organic or inorganic wastewaters

[Zimmermann (1950, 1954a, b, 1958a, b, 1961), Laughlin et al. (1983), Joshi et al.

(1985), Copa and Gitchel (1989), and Joshi and Mishra (2006)]. Wet air oxidation

involves the liquid phase oxidation of organics or oxidizable inorganic components at

elevated temperatures (125-320 °C) and pressures (0.5-20 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 C02 and their

innocuous end products.

Carbon is oxidized to CO2; nitrogen is converted to NH3, NOx 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 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. Fig. 1 depicts a typical WAO treatment system.

STORAGETANK

RE

AC

TO

R

SEPARATOR

GAS

HEATEXCHANGER

OXIDIZEDLIQUID

AIR

AIRCOMPRESSOR

PUMP

WASTE

Fig. 1: Basic Wet Oxidation flow Sheet

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.

2. Chemistry of Wet Oxidation

Before discussing the various aspects of CWO in detail, it is necessary to discuss

the aspects of noncatalyzed WO. In most systems, both WO and CWO occur

simultaneously and reaction pathways (and, hence, the reactions that occur) for both

processes are usually very similar, because the main partial oxidation product of both

WO and CWO processes is usually acetic acid. Although the main aim of WO (the

removal of organic compound(s) by conversion to carbon dioxide) and methods for

evaluating WO (percentage of TOC and/or COD removed) are usually explained in very

simple terms, the chemistry that occurs during WO of single organic compounds and

mixtures of organic compounds is quite complex. This complexity is partially due to the

different types of chemical reactions (both oxidative and non-oxidative) that can occur

for various organic compounds under typical WO conditions and the high total number of

reactions that can occur during the WO of even a single organic compound. Different

types of chemical reactions that can cause/lead to oxidation of organic compounds under

typical WO conditions include auto-oxidation (free radical reactions involving oxygen),

heterolytic/homolytic cleavage (oxidative or non-oxidative thermal degradation),

hydrolysis, decarboxylation, alkoxide formation followed by subsequent oxidation

(alkaline solution), and carbanion formation followed by subsequent oxidation (alkaline

solution). The total number of reactions that can occur during the WO of a single organic

compound can be extremely high, even for a simple low molecular-weight organic

compound such as propionic acid. For example, Day et al. (1973) proposed a 16-step

free-radical reaction mechanism for the WO of propionic acid.

3. The Two Main Stages of Wet Oxidation (WO).

WO of an organic compound involves two main stages: (i) a physical stage, which

involves transfer of oxygen from the gas phase to the liquid phase, and (ii) a chemical

stage, involving the reaction between the transferred oxygen (or an active species formed

from oxygen) and the organic compound. Although various other phenomena can

influence and/or cause the WO of an organic compound, such as co-oxidation (oxidation

of an organic compound via a free-radical intermediate produced during oxidation of

another compound), the two main stages either directly or indirectly (as in the case of

cooxidation) determine the rate at which an organic compound undergoes WO.

3.1 Physical Stage.

The physical stage of WO, which involves the transfer of oxygen from the gas

phase to the liquid phase, has been described in detail by Debellefontaine and Foussard,

2000. According to their study, the only significant resistance to oxygen transfer is

located at the gas/liquid interface (film model), with the three limiting cases being (i)

oxygen reacts within the film because of a rapid chemical reaction (in this way, the

oxygen transfer rate is enhanced); (ii) oxygen reacts rapidly within the bulk liquid, where

its concentration is close to zero (the overall rate is equal to the rate at which oxygen is

transferred); and (iii) the oxygen concentration within the bulk liquid is equal to the

interface (or equilibrium) concentration (the overall rate is the chemical step rate, and it is

usually low). Fig. 2 shows the path of gaseous reactant to catalyst surface in slurry

reactor.

Fig 2. Path of gaseous reactant to catalyst surface in slurry reactor

According to Debellefontaine and Foussard (2000) the effect that the rate of

oxygen transfer has on the overall rate can often be eliminated through high mixing

efficiency, which then enables unencumbered chemical kinetic rates to be determined.

4. Wet Oxidation Kinetics.

The effect of reaction temperature, oxygen partial pressure, and organic(s)

concentration on WO reaction rates have been described in detail in recent reviews.

Briefly, the effect of these parameters on the reaction rate of non-catalytic wet oxidation

in simple solutions can be described by the following kinetic model (Kolaczkowski et al.,

1999):

where, rr is the reaction rate, A the pre-exponential factor, E the activation Energy, R the

universal gas constant, T the reaction temperature, Corg the organic compound

concentration in the bulk liquid, and CO2, L the oxygen concentration in the bulk liquid.

The superscripts m and n are the orders of reaction. The partial order, with respect to the

organic compound, is usually 1, whereas the order with respect to dissolved oxygen is

~0.4. For complex wastestreams that contain a mixture of organic compounds, more-

detailed kinetic models are required to explain the effects of the main reaction parameters

on the rate of WO.

These models are usually based on the existence of two general types of

compounds/intermediate compounds present in complex solutions:

(i) compounds and intermediate compounds that undergo relatively fast oxidation to

carbon dioxide and water, and

(ii) compounds and intermediate compounds that are difficult to oxidize (mostly acetic

acid).

Most researchers refer to these classes of compounds as classes “A” and “B”.

Generalized kinetic models for various complex solutions have been developed by

several researchers, using the aforementioned class descriptions. Kinetic models are

important for determination of the effects of various reaction parameters on rates and for

the design of reactors.

5. Chemical Reaction Aspects.

To improve the rates of the chemical reactions that occur during WO, a better

understanding of the types of reactions that are occurring is required. The general

consensus among researchers in the field is that the chemical reaction stage of WO occurs

mostly via free-radical chemical reactions.

Numerous free-radical chemical reactions, from each of the three main types of

free-radical chemical reactions (initiation, propagation, and termination) have been

proposed to occur during WO of various individual organic compounds and mixtures of

organic compounds.

5.1. Initiation.

Bimolecular reactions:

Unimolecular reactions:

Trimolecular reaction:

Alkaline solution only:

where X carboxylate group,

5.2. Propagation.

Alkaline solution only:

where X represents a carboxylate group.

5.3. Termination.

Alkaline solution only:

where X represents a carboxylate group,

The general material balance for the WAO process is as follows (Debellfontaine

and Foussard, 2000):

2zyxwknm O))zy(2k5.0))x3n(25.0m(PSNClOHC ++−−++

→ HeatzPOySOwClxNHOH)x3n(5.0mCO 34

24322 +++++−+ −−−

The heat evolved in the above reaction is found to be around 435 kJ (mole O2 reacted)-1.

6. Mechanism of the Reaction

The oxidation of the organic compounds takes place according to a chain reaction

mechanism. The following reaction steps are involved in the WAO process:

Organic compounds + O2 → Hydroperoxides

Hydroperoxides → Alcohol

Alcohols + O2 → Ketones (or aldehydes)

Ketones (or aldehydes) + O2 → Acids

Acids + O2 → CO2 + H2O

Actually, an organic radical •R is coupled with molecular oxygen to propagate

the reaction in the WAO reaction. •R radical is originated as a result of the reaction

between weakest C-H bonds and oxygen, which forms •2HO , this •

2HO then combines

with RH forming hydrogen peroxide. The hydrogen peroxide obtained decomposes

readily to hydroxyl radicals due to temperature. The last reaction is a propagating step

leading to oxidized species. The mechanism of WAO can be understood better according

to the following reactions:

•• →+− ROOROO

•• +→+ 22 HORORH

222 OHRHORH +→+ ••

MHO2MOH 22 +→+ •

ROOHRRHROO +→+ ••

As for most of the molecules, reaction, the initiation step is a limiting step too,

which depends on the temperature with an activation energy, which can exceed 100 or

200 kJ mol-1. That is why the WAO does not take place at room temperature, but requires

high temperatures (> 250 or 300 oC). As this mechanism shows the importance of free

radicals, so the use of catalysts and promoters can reduce the severity of the operating

conditions required for the reaction.

The overall WAO mechanism includes two steps. One step has been discussed

above, i.e., chemical reaction between the organic matter and the dissolved oxygen. The

second step involves the transfer of oxygen from the gas phase to the liquid one and the

transfer of CO2 from liquid to gas phase. During designing of a wet air oxidation

reaction, it is considered that gases must be diffused rapidly within the gas phase.

Li et al. (1991) presented a generalized kinetic model based on a simplified

reaction scheme with acetic acid as the rate-limiting intermediate product as shown in

Fig. 3.

Organic Compound + O2 → 1k CO2

k1 k2

CH3COOH + O2

t)kk(

321

31tk

321

2

o3

3 213 e)kkk(

)kk(e

)kkk(

k

]COOHCH.C.O[

]COOHCH.C.O[ +−−

−+−+

−+=

++

where, O.C. = Organic compound

Fig. 3: Simplified kinetic model for wet air oxidation process

Table 1. Gives an idea of the basic reaction mechanisms involved in the WAO

process.

Table 1: Wet air oxidation process reaction mechanisms (Liu and Bela, 1995)

Reaction Mechanism Typical Effects Strongest Influences

Hydrolysis Dissolved solids splits long-

chain hydrocarbons

pH, temperature

Mass Transfer Dissolves, absorbs oxygen Pressure, Presence of

liquid gas interface

Chemical Kinetics Oxidizes organic chemicals Temperature, Catalysts,

Oxygen activity

7. Commercial Catalytic Wet Air Oxidation (CWAO) Pr ocesses

The operating conditions of the WAO can be much reduced by the use of

catalysts, which allow substantial gains on temperature, pressure and residence time. The

same amount of destruction of compound/ COD can be achieved at low temperatures by

the use of catalysts. CWAO can be divided in two types:

(a) Heterogeneous CWO process

(b) Homogeneous CWO process

7.1 Heterogeneous CWAO Process

Since the mid-eighties, three CWAO technologies have been developed using

heterogeneous catalysts containing precious metals supported on titania or titania-

zirconia. In comparison to standard WAO process, these processes were able to oxidize

two refractory compounds, i.e., acetic acid and ammonia. The technologies were

developed after studying the important aspects of heterogeneous catalysts, such as,

chemical and physical stability of the heterogeneous catalysts during WAO, which

include leaching and sintering of the active phase and reduction in surface area of the

support. Leaching of the metal can be controlled by pH adjustment and proper choice of

catalyst. The running commercial processes employ catalysts comprising of precious

metals, such as Pt and Pd or a mixture of precious and base metals. The support used is a

mixture of titania and zirconia.

7.2 Homogeneous CWAO Process

In the last decade, wet air oxidation of the toxic waste streams has been carried

out in the presence of homogeneous transition metal catalysts. The problem encountered

with such catalysts is to separate before disposing. So there is a need to develop such

methods, which can separate the active metal ions, so that the catalyst can be recycled

and reused.

8. Applications of Wet Air Oxidation

Non-catalytic and catalytic WAO is an attractive alternative process to biological

oxidation. This process is particularly useful for toxic and refractory pollutants, and can

also be used as an alternative to incineration. This method is useful for the following

situations (Joshi et al., 1985):

(i) For the treatment of pulp and paper mill effluent, where excess energy in

the form of steam can be recovered due to high chemical oxygen demand

(COD).

(ii) Activated carbon can be regenerated. Using this treatment method, the

charcoal loss is much lower than by the thermal regeneration process.

(iii) Filterability and dewaterability properties of municipal sewage can be

improved and the pathogens present in sludge can be destroyed.

(iv) Several non-biodegradable wastewater streams can be treated. Recovery

of chemicals is also possible.

8.1 Wet Air Oxidation of Pure Compound Solutions

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.

8.2 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

effluent 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.

8.3 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. Alkali metal

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.

8.4 Catalytic wet air oxidation of olive mill wastewater

Catalytic wet air oxidation (CWAO) for the treatment of olive mill wastewater

(OMW). Experiments were performed in a high pressure reactor at 100 and 200 8C under

an oxygen partial pressure of 6.9 bar, using carbon supported platinum (1 wt.% Pt) and

iridium (5 wt.% Ir) catalysts prepared by incipient wetness impregnation. At 100 8C,

refractory organic compounds persisted even after prolonged reaction time (8 h). At 200

8C, complete total organic carbon and colour removal was obtained with the Pt/C catalyst

after 8 h of reaction. A kinetic model was developed taking into account catalytic and

non-catalytic reactions, formation of refractory compounds and catalyst deactivation.

Very good agreement between the proposed model and CWAO experimental data at 200 oC was found.

8.5 Biological treatment of Industrial Wastewater containing Sodium Dodecylbenzene Sulfonate (DBS)

Wet air oxidation (WAO) and catalytic wet air oxidation (CWAO) were

investigated as suitable precursors for the biological treatment of industrial wastewater

containing sodium dodecylbenzene sulfonate (DBS). Two hours WAO semi-batch

experiments were conducted at 15 bar of oxygen partial pressure (PO2 ) and at 180, 200

and 220 ◦C. It was found that the highest temperature provides appreciable total organic

carbon (TOC) and chemical oxygen demand (COD) abatement of about 42 and 47%,

correspondingly. Based on the main identified intermediates (acetic acid and

sulfobenzoic acid) a reaction pathway for DBS and a kinetic model inWAO were

proposed. In the case of CWAO experiments, seventy-two hours tests were done in a

fixed bed reactor in continuous trickle flow regime, using a commercial activated carbon

(AC) as catalyst. The temperature and PO2 were 140–160 ◦C and 2–9 bar, respectively.

The influence of the operating conditions on the DBS oxidation, the occurrence of

oxidative coupling reactions over the AC, and the catalytic activity (in terms of substrate

removal) were established. The results show that the AC without any supported active

metal behaves bi-functional as adsorbent and catalyst, giving TOC conversions up to

52% at 160 ◦C and 2 bar of PO2 , which were comparable to those obtained in WAO

experiments. Respirometric tests were completed before and after CWAO and to the

main intermediates identified through the WAO and CWAO oxidation route. Then, the

readily biodegradable COD (CODRB) of the CWAO and WAO effluents were found.

Taking into account these results it was possible to compare whether or not the CWAO

or WAO effluents were suitable for a conventional activated sludge plant inoculated with

non adapted culture.

9. Industrial Applications of Wet Air Oxidation

9.1 Wet Air Oxidation of Alcohol Distillery Waste.

The molasses generated in sugar manufacture (from sugar cane) is a prime raw

material for ethyl alcohol production all over the world. This process is of particular

importance to India and Brazil, major sugar-producing countries. The molasses is

fermented by yeast after suitable dilution. The fermented solution contains about 6-12%

ethyl alcohol which is recovered by distillation. The effluent remaining after alcohol

recovery (spent wash or stillage) is dark brown in color and about 6-15 times by volume

of the alcohol produced. The spent wash has a very high organic content (COD = 60-200

kg/m3) and is very complex in nature. Treatment of this spent wash is a major pollution

problem faced by distilleries. The spent wash also has high sulfate content due to SO2

used for bleaching of sugar, making it difficult to biotreat without dilution. The effluents

generated by distilleries using beet molasses are less concentrated and comparatively

easily treatable.

10. Catalytic wet oxidation of the pretreated synthetic pulp and paper mill effluent under moderate conditions

The black liquor originating from the chemical cooking of wood and other raw

materials in the pulping process contains lignin, organic acids, unsaturated fatty acids,

resin acids, phenolics, terpenes, sulfur compounds, like sulphides, thiosulphate, etc.

Besides dissolved substances, it also contains high suspended solids, colloids and has

high biochemical oxygen demand (BOD) and chemical oxygen demand (COD). Lignin

and other dissolved compounds impart intense black colour to the black liquor. The

amount of wastewater generated from an integrated pulp and paper mill having chemical

recovery units varies from 20 to 250 m3 ton-1 of pulp produced, depending upon the raw

materials and the process conditions employed (World Bank Group, 1998; Garg et al,

2005). In India, most of the paper mills discharge an effluent having a COD value of

600–6250 mg l-1 and have a COD/BOD ratio in the range of 4.0–7.0. Several authors

have performed the treatment of the pulp and paper mill effluents having a COD value of

around 7000 mg l-1 (Stephenson and Duff, 1996; Laari et al., 1999; Garg et al., 2005).

Diluted black liquor from kraft pulp and paper mill can be used as substrate for bench

scale studies to simulate the final pulp mill effluent (Peralta-Zamora et al., 1998; Buzzini

et al., 2005). The pulp and paper mill wastewater cannot be treated directly by biological

methods, as it contains non-biodegradable, refractory organics and toxic inorganics. Wet

air oxidation (WAO) can be an alternative method to treat such wastewaters. It is carried

out at elevated temperature (398–593 K) and pressure (0.5–20 MPa) in the presence of

oxygen (viz. air, oxygen, ozone, hydrogen peroxide) (Mishra et al., 1995). To reduce the

severity of oxidation conditions, various heterogeneous catalysts like metal oxides (ZnO,

CuO, MnO2, SeO2, TiO2, ZrO2, etc.), noble metals on alumina support (Pt, Pd, Ce singly

or in mixed form) and metal impregnated on activated carbon (Cu, Co, Bi, Fe, Mn) have

been used in wet oxidation (WO) studies (Prasad and Joshi, 1987; Pintar et al., 2001).

Apart from heterogeneous catalysts, homogeneous copper sulfate catalyst has also been

used in the oxidation of pure toxic compounds such as glyoxalic and oxalic acids and

sulfide bearing spent caustic from petroleum refineries and concentrated waters from

paper mills at moderate operating conditions (Shende and Mahajani, 1994; Jagushte and

Mahajani, 1999). The catalyst was found quite effective exhibiting almost complete

conversion of thiosulfate into sulfate within 8 min at a temperature and oxygen partial

pressure of 393 K and 0.69 MPa, respectively. Verenich et al. (2000) have used copper

sulfate for the WO of concentrated wastewaters and obtained around 75% COD reduction

at 473 K and 1 MPa oxygen partial pressure.

10.1 Effect of Different Catalysts on CWO of the Filtered Effluent at 423 K

Temperature

The effectiveness of different catalysts on COD reduction by destructive

oxidation of the pretreated paper mill effluent (pH0 = 8) at 423 K and 0.85 MPa total

pressure is shown in Fig. 4. The oxygen partial pressure was kept at 0.4 MPa. The

catalysts used in WO were CuSO4, 5%CuO/95% activated carbon, 60%CuO/40% MnO2

and 60%CuO/40% CeO2. The catalyst concentration used for the reaction was 8 g l-1. The

results show that among all catalysts, C and E show the best activity under the operating

Figure 4. COD reduction of the catalytic thermolysis treated paper mill effluent (i.e. filtrate) due to wet oxidation (COD0 = 2700 mg l-1, T = 423 K, P = 0.85 MPa, pO2 = 0.4 MPa, Cw0 = 8 g l-1, pH0 = 8.0).

conditions, exhibiting a COD reduction of around 78%. The COD reduction without any

additional catalyst (A) not exhibit significant improvement in COD reduction. It can also

be seen from the figure that an induction period exists in the CWO reaction with 5%

CuO/95% activated carbon, showing a very small COD reduction in first 2 h (<10%).

However, it was difficult to identify any induction period in all other cases.

10.2 WO with additional copper sulfate

10.2.1. Effect of catalyst loading.

The effect of fresh addition of copper sulfate on wet oxidation of the filtrate solution is

shown in Fig. 5a. The results are also compared with the COD reduction obtained in the

absence of the fresh catalyst during wet oxidation. It is found that at Cw = 8 and 10 g l-1,

the COD reduction is higher (63% and 67% respectively), but the reduction is not

significant when these are compared to 59% COD reduction without any fresh catalyst.

The COD reductions at Cw = 3 g l-1 and 5 g l-1 are lower (i.e. about 32% and 57%,

respectively) than that without any fresh catalyst. These values of copper sulfate

correspond to an addition of 0.76 g l-1 and 1.27 g l-1 fresh copper ions, respectively. From

the results, it appears that the concentration of the active Cu2+ (0.876 g l-1), already

present in the effluent, is sufficient to remove about 60% of the COD. The addition of

fresh copper sulfate upto 5 g l-1 does not enhance the COD It can also be seen from the

figure that an induction period exists in the CWO reaction with 5% CuO/95% activated

carbon, showing a very small COD reduction in first 2 h (<10%). However, it was

difficult to identify any induction period in all other cases.

10.2.2 Effect of temperature

Fig. 5b shows the effect of temperature on the COD reduction as a function of treatment

time, tR at a fresh copper sulfate mass loading of 8 g l-1. The temperature of reaction was

varied from 383 to 443 K at 0.85 MPa pressure. The COD of the pre-treated wastewater

is reduced by 78% from COD0 = 2700 mg l-1 at 443 K than that of 63% at 423 K in 4 h

treatment time. At 383 K, the COD reduction is quite low (~33%) in comparison to those

at higher temperatures. A portion of the total COD reduction (~22%), however, is

obtained during the pre-heating period of the filtrate from room temperature to 383 K and

the WO treatment for 4 h at 383 K results in only 11% COD reduction from the zero

time.

Figure 5. Effect of (a) catalyst loading (T = 423 K), and (b) temperature (Cw0 = 8 g l-1) on the COD reduction of the treated pulp and paper mill effluent using copper sulfate as catalyst during wet oxidation (COD0 =2700 mg l-1, P = 0.85 MPa, pO2 = 0.4 MPa, pH0 = 8.0, tR = 4 h).

10.3. CWO with 5% CuO/95% activated carbon as the catalyst

10.3.1. Effect of catalyst loading.

Fig. 6a show the effect of Cw on the COD reduction of the wastewater was found

to be shown as a function of the reaction time, tR. CWO was performed at 423 K, 0.85

MPa pressure for tR = 4 h. The ordinate shows the COD of the effluent at zero time for

different Cw. The effect of the pre-heating period is clearly visible, as the COD at zero

time is much lower than the initial COD of the filtrate (2700 mg l-1). About 50% COD

reduction is obtained at zero time with catalyst loadings higher than 5 g l-1 due to

preheating of the filtrate to 423 K. The COD values at zero time were 2000, 1900, 1500,

1300, 1200 mg l-1 at Cw = 0, 3, 5, 8 and 10 g l-1, respectively. The increase in the catalyst

loading results in greater COD reduction at zero time. Pre-heating caused a reduction in

COD from 2700 to 2000 mg l-1 and further COD reduction was due to the adsorption on

activated carbon catalyst. COD reduction increases with the increase in catalyst loading

due to the presence of more adsorption sites for organic components. From the figure,

this is clear that no significant decrease in COD can be observed during the first hour of

WO reaction (especially for higher catalyst concentration runs). This can be interpreted

that any further reduction in COD after 1 h is only due to the oxidation. Catalyst loading

above 8 g l-1 does not show any appreciable increase in the COD reduction (78% at 8 g l-1

to 82% at 10 g l-1). Therefore, further CWO experiments were carried out at a catalyst

loading of 8 g l-1.

10.3.2 Effect of temperature.

The reaction temperature for WO was varied from 403 to 443 K. Fig. 6b shows

the effect of temperature on the COD reduction as a function of reaction time, tR. At 443

K and 0.85 MPa total pressure the COD reduced from 2700 to 300 mg l-1 in 4 h reaction

time. It can be seen that at ‘zero time’, the COD reduction decreases as the reaction

temperature increases. The corresponding COD values at zero time were found to be

1100, 1200, 1300 and 1400 mg l-1 at 403, 413, 423 and 443 K, respectively. Again, pre-

heating caused a COD reduction of around 26% and the rest of the reduction was due to

adsorption. Also, the increase in temperature reduces the amount of adsorption of

reaction species, which is evident from the result. However, total COD reduction

increases with increase in the reaction temperature at tR = 4 h. At 403 K, the COD

reduction was very low at tR = 4 h (from 1100 mg l-1 at tR = 0 to 1000 mg l-1 at tR = 4 h).

Figure 6. COD reduction of catalytic thermolysis treated paper mill effluent using 5% CuO/95% activated carbon as catalyst during wet oxidation as a function of (a) catalyst loading (T = 423 K), and (b) temperature (Cw0 = 8 g l l-1) (COD0 = 2700 mg l-1, T = 423 K, P = 0.85 MPa, pO2 = 0.4 MPa, pH0 = 8.0, tR = 4 h).

10.3.3 Without Additional Catalyst

WO of the filtrate solution was also performed at 423 K and 443 K without adding any

fresh catalyst. The COD reductions were 59% and 78%, respectively, after tR = 4 h. Thus,

the COD of the effluent reduces to 1100 and 600 mg l-1, respectively from an initial value

of 2700 mg l-1.

10.3.4 Effect of different catalysts on CWO of the pretreated effluent filtrate at 443

K temperature

The COD reduction of the effluent with 5% CuO/95% activated carbon and

copper sulfate at Cw = 8 g l-1were compared with that obtained without any fresh

catalyst. The experiments were carried out at 443 K and a total pressure of 0.85 MPa with

oxygen partial pressure being 0.1 MPa. The COD of the effluent is reduced by about 89%

from 2700 to 300 mg l-1 with 5% CuO/95% activated carbon, whereas with fresh copper

sulfate (Cw = 8 g l-1) and without any fresh catalyst COD reduction was equal (78%) in 4

h treatment time. Since the effluent from the catalytic thermal pretreatment of the

wastewater has sufficient amount of Cu ions, further addition of copper sulfate to the

effluent does not show enhanced COD removal.

10.3.5 Change in the pH value with time

Fig. 7 shows the change in final pH of the solution after treatment with different

catalysts, as a function of time. The pH of the solution after zero time first reduces and

then increases. The same trend has also been reported by Zhang and Chuang (1999). At

zero time, the pH of the solution with copper sulfate goes down to about 5.0, which drops

further to about 2.5 in 1 h at 443 K and thereafter increases with time. The decrease in the

pH of the filtrate on heating to 443 K with 5% CuO/95% activated carbon and without

addition of any other catalyst is only slight (from pH 8.0 to pH 7.4). Further heating at

443 K from zero time reduces pH slightly. pH variations of Fig. 7 indicate that the

heating of the filtrate solution with or without addition of fresh catalyst in the presence of

oxygen results in breakdown of organics into lower molecular weight species and

possibly carboxylic acids. With the passage of treatment time, the carboxylic acids are

oxidized to form CO2 and water with the resultant rise in pH. Lowering of the pH after

WO of softwood kraft pulp mill effluent has also been reported by others (Zhang and

Chuang, 1998a, b).

Figure 7. Variation in final pH with time using different ca talysts during wet oxidation (pH0 = 8.0, P = 0.85 MPa, Cw0 = 8 g l-1, COD0 = 2700 mg l-1). 11. Sulphidic Refinery Spent Caustic

Using typical conditions of 23O °C, 4 MPa and one hour retention time, 88%

destruction of chemical oxygen demand was achieved after wet oxidation of a sulphidic

refinery spent caustic. The reduced sulphur compounds in the waste were oxidized

completely to sulphate, thus eliminating the odour associated with this waste product.

The odour was not present in samples taken as early as 5 minutes after the start of

reaction. This waste is illustrative of the fact that, in the wet oxidation system, carbon-

sulphur or sulphm-hydrogen bonds appear to be broken preferentially, and sulphur more

readily oxidized than either carbon or hydrogen. Advantage of this fact is taken in

applying wet oxidation techniques to the problem of chemical desulphurization of coal.

11.1 Chlorinated hydrocarbon pesticide residues

In this test a 90% COD reduction was achieved after one hour’s residence time.

One particular component of concern in this residue was DDT. A destruction efficiency

of greater than 98% was measured for DDT.

11.2 Cyanide wastes from electroplating operations

A number of different plating wastes containing up to 50,000 mg/litre of cyanide

were oxidized. Oxidation occurs quite readily for all but the most resistant ferricyanides.

Residual cyanide levels of less than 1 mg l-1 or destruction efficiencies of greater than

99.998% were achieved. Since the nitrogen in the cyanide appears in the effluent as

ammonia, much of the cyanide destruction must, in fact, be a hydrolysis reaction. In

subsequent experiments, it was shown that the reaction could be achieved with no

addition of oxygen or air to the system, but just by heating the reactor up to about 230°C

and maintaining that temperature for l-2 hours. This hydrolysis process is being

developed and is presently the subject of a commercial demonstration being supported by

the American Electroplaters’ Society. Wood preservative liquor Many wood preservative

liquors contain pentachlorophenol which is a toxic compound of some concern to

environmental regulatory agencies around the world. Earlier work undertaken by ORF

showed a reduction of 99.99% in pentachlorophenol with an equivalent overall COD

reduction in the stream of 76%. In a recent study we have examined both a wood

preservative waste containing pentachlorophenol (PCP) and pure PCP. In this study, the

effluent streams from the oxidation were examined to try to determine the nature of any

breakdown product. PCP destructions of up to 99.96% were observed. Of this, 99.4% of

the chlorine was accounted for as inorganic chloride ion. Trace quantities of other

chlorinated organic materials were observed in GClnass spectrometry analysis of these

effluents. PCP destruction via wet air oxidation has also been investigated in a Zimpro

high-temperature, high-pressure system. At 320” C and 275” C, PCP removal efficiencies

of 99.88% and 81.96% respectively were achieved. At 275”C, and employing a copper

catalyst, PCP removal efficiency was increased to 97.30%. Other sulphur and nitrogen

containing chemicals In addition to the work on PCP described above, work was recently

undertaken on two other pure compounds as examples of organic compounds containing

nitrogen and sulphur. The compounds chosen were mercaptobenzothiazole (MBT) and

diphenylamine (DPA). With mercaptobenzothiazole, a destruction efficiency of >99.99%

was observed. The major by-product observed in the oxidized effluent of batch testing of

MBT was nitrobenzene at a concentration of 0.03% of the original MBT. Traces of

azobenzene, aniline and azoxybenzene were also observed. In tests run in the continuous-

flow pilot plant, rather high concentrations of aniline and nitrobenzene were observed in

the vapour phase effluent from the reactor. These compounds are steam strippable, and it

would suggest that, since only trace quantities of aniline were observed in the batch

reactor effluent, had the aniline not been stripped from the reactor, it would have further

oxidized to nitrobenzene and other products. The wet oxidation of pure diphenylamine

indicated a >99.99% reduction in DPA. Traces of aniline, nitrobenzene, azobenzene,

dimethylazobenzene and chloro-aniline were detected by GC/MS in the effluent from the

batch oxidation tests. In the continuous flow tests carried out on DPA, aniline and

nitrobenzene were again detected in the vapour phase effluent from the reactor in

quantities equivalent to -20% of the original DPA in the influent to the reactor. In a

programme to be undertaken later, Ontario Research will be examining vapour phase

catalysis as a means of “polishing” the destruction of steam strippable components in the

vapour phase effluent from a continuous flow Wetox reactor.

11.3 Spent Caustic Oxidation CATOx2

Environment division of EIL (Mishra and Joshi, 2006) in association with

Department of Chemical Engineering, IIT Roorkee, has developed a technology for the

treatment of spent caustic waste. Process is based on catalytic oxidation of spent caustic

waste at pressure & temperature levels that are consistent with the levels of

pressure/temperature of plant air and steam that are usually available in the plant. The

initial trial run on lab-scale was conducted at IIT, Roorkee and the results were quite

satisfactory in terms of sulphide reduction. The Process is found to be economical and

environment friendly when compared with other technologies using chemical oxidation

with H2O2 or wet air oxidation using patented processes.

Lab scale studies were conducted in order to critically examine the effect of

various process parameters. One of the highlights of lab scale tests was that actual spent

caustic samples from Panipat refinery were used to carry out tests in addition to synthetic

samples, which were prepared to examine the effect of following listed parameters:

1. Effect of phenol in spent caustics

2. Kinetics determination

3. Catalytic oxidation

4. Reaction order with respect to Air

5. Kinetic regimes for a first order gaseous reaction

6. Reaction order with respect to sodium sulfide

7. Reaction order with respect to Copper based Catalyst

8. Temperature effect on reaction rate constant

9. Comparison of air and oxidants

The information thus generated was used to develop a flow scheme for the process,

after which EIL developed a skid mounted unit at their R&D center. This was designed

for a flow of 100-400 lph on a continuous basis. This skid was moved to Panipat refinery

for demonstration purposes and to test the process performanc while handling actual

samples of spent caustic generated in the refinery with the following objectives:

1. To check the efficacy of the process at actual site conditions and variable loads.

2. To minimize H2O2 consumption for the treatment of spent caustic streams.

3. Develop Wet Air Oxidation technology indigenously as patented technologies are

very expensive and are unable to treat spent caustic if sulphide levels are more

than 35000 ppm and COD more than 100000 ppm.

4. Start with a hybrid approach consisting of a combination of H2O2 oxidation, wet

air oxidation and biological treatment.

5. Stagewise development of technology so that users are benefited right at the

beginning.

Other aspects, which are related to test the process at site conditions, are as

follows:

1. To establish efficacy of catalyst based Wet Air Oxidation process at pilot scale

Plant.

2. To re-establish kinetics for the medium pressure Wet Air Oxidation process as

developed in lab scale plant.

3. To find out effect of different process variables on sulphide removal efficiency.

11.3.1 Distinct Features of EIL’s Process:

1. Approximately 50% sulfide reduction is achieved before spent caustic is oxidized

in the oxidation tower.

2. Approximately 65% conversion of sulfide is achieved after the addition of

catalyst before oxidation tower.

3. Addition of water helps in enhancing the reaction rate by lowering the sulfide

concentration.

4. It can treat high concentration of sulfide. During the pilot plant test run, sulfide

level as high as 80000 ppm was treated successfully.

5. Approx. 99.8% of conversion of sulfides is achieved at the outlet of oxidation

tower.

11.3.2 EIL’s process(CATOx2)

Sulphuric spent caustic is stored in a tank and subjected to oil removal. After intial free

oil removal, it is subjected to further removal of aoil thoriugh a media coalescer so that

oil left in the spent caustic is less than 10 ppm. This oil free spent caustic is aerated for 2

hours in aeration tank. The air flow rates are adjusted in such a way tha tagitation and

mixing are minimized that helps in lowering the foam formation if small quatities of

nephthenates are present. After aeration, spent caustic is transferred to another tank i.e.

the oxidation tank where copper based catalyst is added and again aeration is done for

another 30 minutes. The aeration is required for providing intimate mixing of spent

caustic with the catalyst and air for oxidation. This spent caustic is heated to 50° C with

the help of condensate recovered in the process. First stage of oxidation is achieved at

this stage and 65% of conversion of S is achieved.

The spent caustic is pumped to a guard filter and is passed through a pre-

heater where oxidized spent caustic exchanges the heat with the incoming feed of spent

caustic. This preheated spent caustic is transferred to start uo heater where desired

temperature 150- 170° C for the oxidation reaction is achieved. This spent caustic is now

oxidized in the oxidation tower where air is also mixed and released through sparger at

the bottom of the reactor. The top of the reactor contains packing and provides intimate

mixing and space for the breaking of bubbles generated if any during the reaction in the

process. The reactions in the oxidation tower are exothermic and self sustaining and the

need for start-up heater isonly at the beginning. The temperature and pressure maintained

in the reactor are 150-170° C and pressure up to 10 bars.

The oxidized spent caustic after transferring its heat to the feed is cooled down in

a cooler to 50° C and depressurized in a separator. Off gases released from the process

are passed through an activated carbon column and oxidized spent caustic is released to

WWTP for further treatment/disposal. The following table provides a comparison of

various technologies. Table 2 below shows the potential benefits through replacement of

existing process with EIL’s CATOx process.

Table: 2 Potential benefits through replacement of existing process with EIL’s CATOx process

Conclusions:

Most of the compounds are amenable to WAO except low molecular weight

carboxylic acids (particularly acetic and propionic acid) and polychlorinated biphenyls

(PCB’s). However, during WAO, pollutant molecules are broken down to low molecular

weight carboxylic acids. The slow rate of oxidation of the low molecular weight

carboxylic acids is a major limitation of the WAO technique. Some catalyst systems have

shown promise for WAO of these acids under less severe conditions.

Wet air oxidation is a very attractive technique for the treatment of waste streams

which are toxic and dilute. When the feed COD is higher than 20 000 mg/L, WAO

becomes energetically self-sustaining with no auxiliary fuel requirement and may in fact

produce energy in the form of high-pressure steam at sufficiently higher feed COD.

Wet air oxidation system is capital intensive, although the operating costs are

almost entirely for the power requirement of the air compressor and the high-pressure

liquid pumping. The capital cost of a WAO system depends on the flow, oxygen demand

of the effluent, severity of oxidation conditions, and the material of construction required.

The severity of oxidation conditions can be reduced by use of a suitable catalyst system.

Wet air oxidation has been tested to treat the waste streams generated by various

industries such as distilleries, pulp and paper manufacturing units, cyanidelnitrile bearing

wastes, and a host of other waste streams. Wet air oxidation regenerates the spent carbon

with relatively less carbon loss (14 %) and at the same time destroys the adsorbed

organics thus avoiding the need for the separate treatment step. Oxydesulfurization of

coal is another promising application of WAO for removal of pyritic as well as organic

sulfur present in coal. Wet air oxidation has been suggested for energy and resource

generation from low-grade fuels and waste biomass such as peat, forestry, and municipal

residues.

The potential benefits of the CWO process over other conventional water

treatment processes, such as low reaction temperatures and residence times and the

formation of harmless products, will be a key driver for more research in the field. The

main challenge faced in the development of successful industrial-scale CWO processes

for treating specific wastewaters seems to be the development/discovery of suitable

catalysts, i.e., a catalyst that is highly active, economical and environmentally friendly.

The total saving of Rs. 78.34 crores can be made in cost if EIL’s CATOx process

is used in all 17 refineries in India.

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