The study on the wet air oxidation of highly concentratedemulsified wastewater and its kinetics

Wenwei Tang a,*, Xinping Zeng a, Jianfu Zhao b, Guowei Gu b, Yiju Li a,Yaming Ni a

a Department of Chemistry, Tongji University, 1239 Siping Road, Shanghai 200092, Chinab State Key Laboratory of Pollution Control and Resource Reuse, Tongji University, 1239 Siping Road, Shanghai 200092, China

Abstract

The highly concentrated emulsified wastewater containing non-ionic surfactant is hardly biodegradable, and few

cost-effective techniques of treatment are available. In this paper a systematic study on its wet air oxidation (WAO) and

the related decrease efficiency of COD, TOC, influencing factors, characteristics of the kinetics was performed in a 2-l

high-pressure batch autoclave. The result indicated that the application of WAO had a good performance in the

treatment of emulsified wastewater and the operation conditions such as temperature, partial pressure of oxygen,

concentration of input wastewater have influence on the oxidation efficiency to different extent. And the temperature is

the key influential factor: COD of the wastewater (initial COD: 48 000 mg/l) was reduced by 86.4% after 2 h oxidation

at 220 8C, with the supply of oxygen 1.25 times more than its theoretical value. A general kinetic model can be used to

explain the WAO process with sufficient oxygen supply, and an exponential model was developed.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Wet air oxidation; Hardly biodegradable organic wastewater; Emulsified wastewater; Kinetics

1. Introduction

Emulsified wastewater usually comes from me-

chanical industry. Its value of CODCr is normally

5000�/100 000 mg/l. It contains all kinds of organic

matters such as surfactants, additives and mineral

oils. The emulsified wastewater belongs to typical

highly concentrated hardly biodegradable organic

wastewater. Normal treating methods concen-

trated on common physical or chemical methods

such as chemical demulsification, electrolytic floa-

tation, separation with membrane, but they have

apparent shortcoming: bringing secondary pollu-

tion and the cost of operation too high.

Wet air oxidation (WAO) is that at upper

temperature (125�/350 8C) and pressure (0.5�/20

MPa) using air or pure oxygen as oxidant to

oxygenolyze organic matters in the fluid phase to

inorganic matters or small molecular organic

matters [1]. Compared with conventional methods,

it has wider adaptive range, high efficiency, lower

secondary pollution, and higher reaction rate. So it

has been received a great attention in the environ-

mental field.* Corresponding author. Tel.: �/86-21-6598-2592.

E-mail address: tangww@mail.tongji.edu.cn (W. Tang).

Separation and Purification Technology 31 (2003) 77�/82

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PII: S 1 3 8 3 - 5 8 6 6 ( 0 2 ) 0 0 1 6 1 - 2

The first patent of WAO was put forward byStrehlenert in 1911 [2]. After the 60s, with the

study and application of WAO to the reclaim of

paper pulp and sludge oxidation in wastewater

factory, it was developed as technology for waste-

water treatment [3]. From the 70s to present, the

research and application range of WAO quickly

spreads from recycle of materials and energy to

treatment of toxic wastewater and materials,especially the treatment of deleterious regents

containing hydroxybenzene, cyanogens, etc. And

the content of study developed deeply into the

reaction mechanism and kinetics [4�/6].

In this paper, WAO technique was used to treat

hardly degradable highly concentrated emulsified

wastewater. The objective is to study the treating

effect, influential factors, and kinetics of WAO inorder to offer theoretical basis and technical

guidance to the application of engineering.

2. Experimental equipment, method and quality of

water for experiment

2.1. Experimental equipment and method

The experiment was carried out in a 2-l high-

pressure batch autoclave and experimental equip-

ment as shown in Fig. 1.

The main equipment is model FYX-2a perma-

nent magnetic stirring high-pressure batch auto-

clave. Making the experiment of batch WAO;

aeration, water inflow and outflow are intermit-

tent. The oxidant is oxygen. First, 400 ml waste-water was added, and then the reaction autoclave

was sealed. After adding sufficient oxygen, it startsto heat. When temperature reaches the set value,

stirrer starts to work and keep a stirring rate of 500

rpm.

2.2. Method of water quality analysis

Conventional indexes: CODCr, using potassiumdichromate method, TOC, using TOC analytical

instrument, pH, using precise digital indicated

acidimeter. Non-ion surfactants: using colorimeter

of complexing extraction.

2.3. Quality of wastewater for experiment

Washing the cut and punched aluminum pro-

ducts on the lathe with non-ion surfactants con-

taining abstergents generated the emulsified

wastewater. The quality of the wastewater is listed

in Tables 1 and 2.

3. Results and discussion

3.1. Influence of temperature

The influence of temperature on the WAO ofemulsified wastewater (CODCr 48 000 mg/l, TOC

14 220 mg/l, pH 9.02) was investigated as shown in

Figs. 2 and 3, and Fig. 4.

The results indicated that: with increasing

temperature, the removal of CODCr and TOC

was evidently increased, especially between 180

and 220 8C. For example: at 160 8C, CODCr and

TOC reduced only by 61.5 and 50.2%, respectively,at 180 8C risen slightly, at 200 8C, 75.4 and

66.4%, at 220 8C, 86.4 and 79.5%, at 240 8C,

90.2 and 85.5%, respectively. But considering cost-

effective, it is better at 220 8C. The removal of

TOC was lower than that of CODCr. Initially the

oxidation rate was high, especially above 200 8C,

Fig. 1. Experimental equipment.

Table 1

Quality of the emulsified wastewater

pH CODCr (mg/l) TOC (mg/l) BOD5/CODCr

9.09�/9.88 53 570�/74 110 15 620�/20 840 0.072�/0.124

W. Tang et al. / Separation and Purification Technology 31 (2003) 77�/8278

indicating the typical characteristic of free radicalreaction.

The pH of output water was decreased at the

beginning of the reaction (except for 240 8C),

after reaching a minimum, it was increased with

increasing reaction time. The lower the tempera-

ture, the longer the time needed to reach the

minimum pH and also the smaller the final pH. It

suggested that during WAO organic acids wereproduced. And the rule of pH change approxi-

mately embodied the rule of eat and flow of

organic acids.

3.2. Influence of oxygen supply

Theoretical amount of oxygen supply was

indicated by corresponding partial pressure of

oxygen (PO2�). The WAO of the wastewater with

different oxygen supply was investigated under

such conditions: 220 8C, CODCr 51 120 mg/l.The result indicated that (Fig. 5): with increas-

ing initial partial pressure of oxygen (PO2), the

reaction rate significantly increased. When PO2was

as 0.5 PO2�, the effect of WAO was remarked

restricted. When PO2was as 0.75 PO2

�, the oxida-

tion rate increased substantially. The difference of

the COD removal when PO2was as 1.0 PO2

� and as

Table 2

The concentration of organic matters in the emulsified wastewater (CODCr 74 110 mg/l)

Mineral oils (mg/l) Polyether (mg/l) Phenolic ether (mg/l) Additive (mg/l)

2198 19 980 7993 3997

Fig. 2. The influence of temperature to COD removal rate.

Fig. 3. The influence of temperature to TOC removal rate.

Fig. 4. The influence of temperature to pH of output water.

Fig. 5. The influence of initial partial oxygen pressure to COD

removal rate.

W. Tang et al. / Separation and Purification Technology 31 (2003) 77�/82 79

1.25 PO2� was about 3%. When PO2

was as 1.5 PO2�

removal of CODCr was 90.7%, but then the total

system pressure increased, and so the cost of

operation must be high. The reaction time should

not be less than 1 h and the amount of oxygen

supply is better between 1.0 and 1.5 PO2�.

3.3. Influence of the concentration of inflow water

The WAO of wastewater with different concen-tration was investigated at 220 8C with 1.2 PO2

�

(Fig. 6). The result indicated that: when the initial

CODCr of wastewater was 8948�/74 110 mg/l, after

2 h of reaction the removal of CODCr was 81.8�/

89.3%. So WAO had good treatment effect in a

wide concentration range. It is a potential techni-

que for the treatment of the emulsified wastewater.

4. Kinetic characteristics of WAO

4.1. General kinetic model

Li (1991) [7] put forward the general kinetic

model of organic matter oxidation shown as

following (Fig. 7 and Eq. (1)). The general kinetic

model used three kinetic parameters (k1, k2 and k3)to associate macroscopically organic oxidation

and change of intermediate product

[A � B]

[A � B]0

�k2

k1 � k2 � k3

e�k3t

�k1 � k3

k1 � k2 � k3

e�(k1�k2)t (1)

A */initial organic matters and unstable inter-

mediate products (using CODCr to express), B */

low-grade organic acids, C */final products of

oxidation, subscript 0 indicated initial value.

The k1, k2 and k3 calculated by using

Levenberg�/Marquardt method with nonlinear

experimental data were shown in Table 3. The

result indicated that: k1 increased with the increase

of temperature and the increase of temperature

was propitious to the oxidation of organic mattersto final products. The increase of k3 according to

the increase of temperature was more apparent,

indicating that the increase of temperature was

more propitious to the oxidation of intermediate

products to final products; divergent point value

k2/k1 token the selectivity of different reaction

approaches. The k2/k1 decreased with the increase

of temperature, indicating that the increase oftemperature was more propitious to the excursion

of reaction to final products and the reduce

amount of organic matters increased substantially.

The forecasted CODCr tallied with experimental

values and the maximum deviation of their re-

moval was 2.89%.

Apparent activation energy and frequency fac-

tor were shown in Table 4. Combining with thecomparison of rate constants we could know that

at lower temperature the oxidation of organic

matters to final products and intermediate pro-

ducts could progress quickly, and the oxidation of

intermediate products to final products was very

slow. After increasing temperature the trend of

reaction to final products directly was accelerated

and more stable intermediate products reachedactivated state, accelerating oxidation.

4.2. Exponential experiential model

The exponential experiential model consisting of

three influential factors of related reaction tem-Fig. 6. The influence of initial COD to COD removal rate.

Fig. 7. General kinetic model of WAO.

W. Tang et al. / Separation and Purification Technology 31 (2003) 77�/8280

perature, concentrations of organic matters and

oxygen supply can be used to further analyze the

kinetics of WAO [8].

�dC

dt�k0 exp

��

Ea

RT

�[C]m[PO2

]n (2)

In Eq. (2), k0 was the factor in front of the

exponent; Ea was activation energy, kJ/mol; R was

gas constant; T was temperature, K ; C was

CODCr, mg/l, PO2was initial partial pressure of

oxygen; m and n were the orders of reaction.

The order of reaction can be obtained by the

plot of Ln(�/DCOD/Dt ) vs. Ln(COD) (Fig. 8)

according to the data of Fig. 6. After the regres-

sion the order, m , was calculated to be 2.015.

Similarly, the order, n�/0.3118, was obtained

from the plot of Ln(�/DCOD/Dt) vs. Ln(PO2)

(Fig. 9) using the data of Fig. 5. The rate constant

k calculated from the original date in Fig. 2 by

minimum quadratic multiplication method is listed

in Table 5. The apparent activation energy was

obtained from the plot of Ln(k ) vs. 1/RT .

According to Eq. (2), the factor in front of the

exponent, k0, was obtained in Table 6.

Thus the exponential model of WAO of emulsi-

fied wastewater is:

Table 3

Parameters of the general kinetic model

Temperature ( 8C) k1 (min�1) k2 (min�1) k3 (min�1) k2/k1 Maximum deviation of CODCr removal (%)

160 0.1664 0.2317 0.0037 1.3927 1.87

180 0.1884 0.2576 0.0041 1.3673 1.50

200 0.2123 0.1509 0.0042 0.7108 2.89

220 0.2003 0.0906 0.0081 0.4526 2.04

240 0.2247 0.0911 0.0102 0.4056 2.35

Table 4

Apparent activation energy and frequency factor of general kinetic model

Ea1

(kJ/mol) A1 (min�1) r2 Ea3

(kJ/mol) A3 (min�1) r2

6.19 0.957 0.8663 24.47 2.864 0.8644

Fig. 8. The plot of Ln(�/DCOD/Dt ) vs. Ln(COD).

Fig. 9. The plot of Ln(�/DCOD/Dt ) vs. Ln(PO2).

W. Tang et al. / Separation and Purification Technology 31 (2003) 77�/82 81

�dCOD

dt�0:09463 exp

��

45:928 � 103

RT

�

� [COD]2:015[PO2]0:3118;

(r2�0:919)

(3)

By using this model, it could be predict the

removal efficiency of COD, the average deviation

between forecasted value and experimental value

after 1 h reaction was small (B/4.3%).

5. Conclusion

Temperature is the key factor that influences the

performance of WAO and 220 8C is a recom-

mended operation temperature. When the COD of

inflow water was 48 000 mg/l, the CODCr and

TOC was reduced up to 86.4 and 79.5%, respec-

tively, after the reaction for 2 h. The change of pH

embodied the eat and flow rule of intermediateorganic acids. The pressure of feeding oxygen is

recommended to be 1.0�/1.25 PO2�. WAO techni-

que could remove significantly COD and TOC in a

wide range of concentrations. By using the general

kinetic model the WAO process with sufficient

oxygen supply can be perfectly predicted. The

WAO exponential model of the emulsified waste-

water was developed.

References

[1] F.J. Zimmermann, Chem. Eng. 65 (1958) 117.

[2] V.S. Mishra, V.V. Mahajani, Ind. Eng. Chem. Res. 34

(1995) 2.

[3] G.H. Teletzke, Chem. Eng. Pro. 60 (1964) 33.

[4] T.L. Randall, P.V. Knopp, J. WPCF 52 (1980) 2117.

[5] M.J. Dietrich, Environ. Prog. 4 (1985) 171.

[6] L. Lei, X. Hu, Water Res. 32 (1998) 2753.

[7] L. Li, J. AICHE 37 (1991) 1687.

[8] S.H. Lin, S.J. Ho, Ind. Eng. Chem. Res. 35 (1996) 307.

Table 5

Kinetic parameters of the exponential model

Temperature (8C) k (mg/l min)�1 r2

160 2.30E�/07 0.8479

180 2.50E�/07 0.8741

200 4.10E�/07 0.8530

220 1.02E�/06 0.9218

240 1.40E�/06 0.9536

Table 6

Apparent activation energy and frequency factor of the

exponential model

Ea (kJ/mol) k0 ((mg/l)�1.015 MPa�0.312 min�1) r2

45.928 0.09463 0.919

W. Tang et al. / Separation and Purification Technology 31 (2003) 77�/8282