Catalytic wet-air oxidation of high strength industrial wastewater

& ENVIRCNMENTAL Applied Catalysis B: Environmental 9 (1996) 133- 147

EUEVIER

Catalytic wet-air oxidation of high strength industrial wastewater S.H. Lin *, S.J. Ho Departmenf

of Chemical Engineering, Yuan Ze Institute ofTechnology, Neili, Taoyuan 320, Taiwan, AOC Received 10 July 1995; revised 1 December 1995; accepted 24 January 1996

Abstract Treatment of desizing wastewater, a typical high strength industrial wastewater, by catalytic wet-air oxidation (WA01 is studied. The desizing wastewater sample was obtained from a large textile dyeing and finishing plant. Experiments were conducted to investigate the effects of temperature and catalyst dosage (CuSO, and Cu(NO,),) on the pollutant (chemical oxygen demand or COD) removal. It is observed that over 80% of the COD removal can be realized in an hour of the catalytic WA0 process. A kinetic model was also developed and a two-stage, first-order kinetic expression was found to represent well the treatment reaction. The correlations between the reaction rate coefficients and the temperature and catalyst dosage were also determined. Keywords: Biodegradibility; Catalysts; High-strength wastewater: Reaction kinetics; Wet-air oxidation

1. Introduction Organic compounds are involved in the manufacturing of a wide variety of commercial chemical products. Utilization of these organic compounds in a manufacturing process invariably results in generation of different types of wastewater that contain significant amounts of the waste organic compounds. This is especially true in many chemical and petrochemical industries. Discharge of these wastewaters without treatment into a natural water body is definitely unacceptable because the wastewater discharge can upset the water quality of the receiving water body. In addition to the potential toxicity of the organic compounds, very often the dissolved oxygen (DO) concentration of the receiv-

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9 (19961 133-147

ing water body polluted by the untreated chemical wastewater can fall below the level deemed necessary for normal aquatic life. Hence increasingly stringent restrictions have been imposed by government on the concentration of these organic compounds in the wastewater for safe discharge. Treatment of these wastewaters has thus become an integral part of the operations of chemical and petrochemical plants. Traditionally, activated sludge treatment is the most widely used method to deal with various kinds of chemical wastewater because of its simplicity and relatively low cost [l]. However, the microorganisms in an activated sludge system, even well acclimatized, can only handle chemical wastewaters containing relatively low concentrations of organic compounds primarily due to low biodegradibility and inhibitory effects of those organic compounds [2,3]. The chemical wastewaters containing high concentrations of organic compounds, in excess of 10000 mg/l for instance, as those occurring in many heavy chemical and petrochemical industries need to be treated by other methods. The wet-air oxidation (WAO) treatment process represents such method. The WA0 process has been subjected to numerous investigations by researchers in past decades as a potential alternative to incineration because of its capability to oxidize high-concentration organic compounds in the chemical wastewaters [4,5]. The WA0 treatment of organic compounds in aqueous phase at elevated temperatures and pressures followed a rather complex reaction pathway [6]. However, the oxidation process has been shown to be rapid and its efficiency high indeed

[4,51. Typical operating pressures and temperatures of a WA0 treatment process can easily exceed 100 MPa and 300°C respectively [4,5]. Due to its severe operating conditions, the WA0 treatment process does have several inherent disadvantages. Maintaining the WA0 treatment process at these operating conditions is energy consuming, implying that it is a relatively costly process in comparison with other conventional wastewater treatment methods. Furthermore, presence of corrosive chemical compounds in the wastewaters along with high temperatures and pressures renders the reaction environment in a WA0 reactor highly corrosive [7,8]. Such a severe reaction environment makes material selection for the WA0 reactor a difficult task. Therefore, from a practical standpoint, it is highly desirable to operate the WA0 treatment process at a sufficiently lower temperature and pressure which still enables retention of good oxidation efficiency of the WA0 process. The present investigation is to consider catalytic WA0 treatment of desizing wastewater of the textile industries. The desizing wastewater is generated in the sizing/desizing operation of a dyeing and finishing plant. A typical desizing wastewater contains significant amounts of various organic compounds, including primarily polyvinyl alcohol (PVA), corn starch, carboxymethyl cellulose (CMC) and several surfactants. Beside invariably having a high COD concentration of tens of thousand mg/l, the main organic components of the desizing

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wastewater (i.e. PVA, CMC and various types of surfactant) are highly refractory. Furthermore, what makes the desizing wastewater treatment particularly difficult to handle is its high temperature, usually in the neighborhood of 95°C. Hence the hot desizing wastewater is usually mixed with other wastewater streams from various sources of a dyeing and finishing plant and goes to the wastewater treatment plant [9- 181. Due to the particular characteristics associated with the desizing wastewater, it may be more practical and desirable to separate it from the rest of textile wastewater streams and treat it separately by an efficient chemical method. By doing so, treatment of the remaining textile wastewater could be much simplified and the treatment efficiency considerably enhanced. The present study attempts to experimentally investigate the treatment of the desizing wastewater by catalytic wet-air oxidation (WAO). The WA0 treatment, either in non-catalytic or catalytic form, has been successfully employed by many investigators [4,5,19,20] in treating a wide variety of industrial wastewaters. Extensive review of literature of the WA0 process for various wastewater treatment applications has been given by Mishra et al. [5]. Many investigators have utilized catalytic WA0 in treating different types of wastewaters [20-341. Scrutiny of the literature reveals that the majority of past investigations of catalytic WA0 treatment were concerned with simulated wastewaters with single organic compound [20-341. Very little attention has been paid to the real high strength industrial wastewaters. This is presumably due to the highly complicated physical and chemical properties of those wastewaters. The present work will focus on the catalytic WA0 treatment of desizing wastewater, a typical high strength industrial wastewater. Specific attention will focus on examination of the effects of various catalysts and WA0 operating conditions on the overall treatment efficiency and thus optimization of the WA0 treatment process. Also considered in the present work is the examination of the oxidation kinetics of the catalytic WA0 process.

2. Experimental A schematic of the experimental WA0 treatment process is shown in Fig. 1. The reaction vessel is a high pressure Parr reactor (Model 4563, Parr Instrument, USA). It is made of SS-316 stainless steel and has an effective volume of 600 ml. The reactor is equipped with a 6-bladed turbine type impeller mixer. A thermal sensor, cooling coil and external heating element are also provided in the reactor for temperature control with an accuracy of + 1°C. The operating pressure of the oxidation reaction was controlled by a regulator in the exit air line. The gas exiting the reactor passed through water-cooled cold trap to condense out the possible volatile organic components carried by the exit gas.

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136

Catalysis B: Environmental 9 (19961 133-147 Pressure

Gauge Cooling Water

Gas

-

Liquid Sample out

Thermocouple

He&g Element Fig.

1. Schematic of the experimental set-up

Both bottled air and oxygen were employed as the oxygen supply for the oxidation reaction. The desizing wastewater employed for the experiment was obtained from a large dyeing and finishing plant in northern Taiwan. During the experimental period, the desizing wastewater was invariably rather hot, being in the range 90 to 95°C and had a light brown color. The chemical oxygen demand (COD) concentration was high and in the range of 15 000 to 30000 mg/l. Variation of the COD concentration of the desizing wastewater was attributed to the fact that the cloths processed in this dyeing and finishing plant came from different weaving companies which employed sizing agents of various proportions of PVA, CMC, starch, surfactants and other chemicals. The biological oxygen demand (BOD) concentration varied between 5000 and 7000 mg/l with a BOD/COD ratio around 0.2, indicating that the desizing wastewater is not suitable for activated sludge treatment. In the experiments, 300 ml of the desizing wastewater were placed in the reactor. After it was sealed, the reactor was rapidly pressurized and heated to the desired pressure and temperature. The operating temperature was chosen to be between 150 and 250°C. The air flow rate was held constant at 1 l/min which was found sufficient to maintain an excess dissolved oxygen concentration in the liquid phase for oxidation [20]. The operating pressure was kept at 7 MPa which was sufficiently high to keep the WA0 reaction in the liquid phase at the chosen operating temperatures. The mixer was set at 300 rpm to minimize the interfacial mass resistance between the gas and liquid phases and to keep the reactor content well mixed. The catalysts utilized in the present study included copper sulfate (CuSO,) and copper nitrate (Cu(NO,),). The two catalysts have also

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been employed by Imamura et al. [30] and Kulkami and Dixit [32]. After a test run started, samples were taken periodically during and at the end of the treatment period for analysis. The COD concentration of the samples was determined according to the standard method [35].

3. Results and discussion Fig. 2 demonstrates the effect of CuSO, dosage on the COD removal at 220°C 7 MPa and an air flow rate of 1 l/min. There is a considerable difference in the COD removal between the catalytic and non-catalytic WA0 treatment processes. The COD removal is seen to increase with an increase in the CuSO, dosage. At 30 min and with a dosage of 50 mg/l CuSO,, the COD removal has exceeded that of non-catalytic WA0 treatment for 120 min. It is further noted that addition of 50 mg/l CuSO, improves the COD removal by nearly 20% at 120 min. However, as the CuSO, dosage is raised from 50 to 1000 mg/l, the COD removal is improved by only about 5%. Due to the marginal improvement in the COD removal, a CuSO, dosage above 50 mg/l is not warranted. Similar effects of Cu(NO,), dosages on the COD removal are shown in Fig. 3. The basic characteristics of the catalytic effect as demonstrated in Fig. 2, are largely retained in Fig. 3. A low dosage of catalyst is sufficient to affect a significant increase in the COD removal. The single most important operating variable of the catalytic WA0 treatment process is the temperature. The effect of this variable on the COD removal is demonstrated in Fig. 4 for CuSO, for a 7 MPa pressure while the temperature is

CuSO4 Dosage, a ??

0

mg/l

0 500 1000

Fig. 2. Effect of CuSO, dosage on the COD removal at 220°C.

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Catalysis B: Environmental 9 (1996) 133-147

IO00

_ u 200

0.0

0

0

I

0

20

50

250 500 1000

I

40

I

60

80

I

100

1

Time, min Fig. 3. Effect of CU(NO~)~ dosage on the COD removal at 220°C.

increased from 175 to 260°C. The COD removal is seen to improve markedly with an increase in temperature from 175 to 260°C. However, the largest improvement in the COD removal occurs when the temperature is elevated from 175 to 200°C. Above 200°C the improvement in the COD removal is diminished considerably. For example, when the temperature is raised from 200 to 26O”C, the COD -removal is improved by about- 6% only. Hence for practical purpose, a temperature of 200°C would be deemed sufficient for the present

Temperature, ?? 175 0 200 = 220 0 240 A 260

“C

Time, min Fig. 4. Effect of temperature

on the COD removal with 500 mg/l

&SO,.

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80 0

n

-ii

g

60 0

E E

40.0

8 20 0

0

20

Fig. 5. Effect of temperature

40

Tim::

mm*’

LOO

on the COD removal with 500 mg/l

120

CdNO,),.

catalytic WA0 process because of the higher energy consumption associated with a higher operating temperature. Fig. 5 shows the temperature effect on the COD removal for Cu(NO,),. Basically, similar characteristics, as those shown in Fig. 4, are observed here except that minor improvement in the COD removal becomes apparent at 240°C and above. The pH changes of the wastewater in the reactor with temperature are demonstrated in Figs. 6 and 7, respectively, for CuSO, and Cu(NO,),. The pH is seen here to decrease rapidly with an increase in the oxidation time up to

I

t 2.01

0

20

40

80

Time: Fig. 6. Effect of temperature

100

120

min

on the pH change of the catalytic WA0 process with 500 mg/l

CuSO,.

140

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Catalysis B: Environmental 9 (19961 133-147

800

6.00

4.00 01_1

120

Time, Fig. 7. Effect of temperature

min

on the pH change of the catalytic WA0 process with 500 mg/l

CU(NO~)~.

about an hour, depending on the oxidation temperature. The pH trend then reverses and increases rapidly. The initial pH decrease was presumably caused by decomposition of organic pollutants to low molecule organic acids (mainly acetic acid). The ensuing pH increase was due to subsequent conversion of those organic acids to CO, and water. In the present WA0 investigation, the oxygen content in the liquid phase was maintained at an excess level for complete oxidation of all organic compounds [20]. Hence the oxygen dependence of the WA0 reaction kinetics was avoided. The oxidation reaction kinetics of the WA0 process can be represented by dC = -kc” dt

(1)

where C is the COD concentration, k the reaction rate coefficient, t the time and n the order of reaction. For a first-order reaction (i.e. II = l>, Eq. (1) is integrated to yield

in which Co is the initial COD concentration. According the above equation, a plot of In(C/C,> vs. t will yield the reaction rate coefficient. Figs. 8 and 9 display such plots for CuSO, and Cu(NO,),, respectively. Apparently, two-stage, first-order reaction kinetics describe reasonably well the present catalytic WA0 treatment of the high strength desizing wastewater. Similar oxidation reaction kinetics have also been observed by Joglekar et al. [19] and Lin and Wu [20] for

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Catalysis B: Enuironmentul9 (1996) 133-147

141

260 240

0

20

40

60

80

Time, Fig. 8. Effect of temperature 500 mg/l CuSO,.

on the two-stage,

first-order

100

120

140

160

min reaction kinetics of the catalytic

WA0 process with

WA0 treatment of simulated wastewaters. The reason behind adopting the two-stage, first-order reaction kinetics is that the primary pollutants (PVA, CMC, starch etc. in the present study) are decomposed in the first stage into small molecule organic compounds including acetic acid [6]. Those small molecule organic compounds are consequently further converted into carbon dioxide and water. The intermediate acetic acid had been experimentally ob-

3w

TernpeF$re, 2 50

‘C

?? 200

0 . 200

0

20

220 240 260

40

60

Time, Fig. 9. Effect of temperature 500 mg/l Cu(NO,),.

on the two-stage,

first-order

80

LOO

120

min reaction kinetics of the catalytic

WA0 process with

142

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Catalysis B: Environmental 9 (1996) 133-147

-1.00

-2.00 -

-3.00 ti 9

-4.00 -

-500

m

-

-6.00 m 1.80

1.90

200

210

2.20

2.30

UT, 10-3K-’ Fig. 10. The Arrhenius

plot of the catalytic WA0 process with 500 mg/l

CuSO,.

served by Wu et al. [36] and Foussard et al. [37]. Joglekar et al. [19], Lin and Wu [20] and the present work confirmed the reaction mechanism. Figs. 8 and 9 show that the transition time from the first-stage reaction to the second-stage tends to decreases with an increase in the operating temperature. This is due to the fact that as the temperature increases, the organic compounds in the liquid phase become easier to decompose. It should be noted that the WA0 reaction kinetics could also be described by a generalized model [4]. Since this model is significantly more complicated to use than the two-stage, first-order one, hence it is not considered in the present work. The reaction rate coefficients observed in Figs. 8 and 9 clearly reflect their dependence on the treatment temperature. Such a dependence can be represented by the Arrhenius equation

(3) in which A E is the activation energy, k, the frequency factor, R the gas constant and T the treatment temperature. A semi-logarithmic plot of k vs. l/T permits determinations of k, and DE, as demonstrated in Figs. 10 and 11, respectively, for CuSO, and Cu(NO,),. The frequency factors and the activation energies obtained for the first- and second-stage reactions are listed in Table 1. It is apparent in this table that the frequency factor and the activation energy for each catalyst are of the same order of magnitude. However, comparison of the two figures reveals that the second-stage reaction, there is a significantly smaller slope for the CuSO,-catalyzed WA0 reaction than that of Cu(NO,),. This is

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Catalysis B: Environmental 9 (19961 133-147

143

-loo > -3.00 id ,d -400 I

-5 00

I-

-.50q

;

c I 90

UT, Fig. 1 I. The Arrhenius

2.10

2 00

220

230

10-3K-’

plot of the catalytic WA0 process with 500 mg/l

Cu(NO,),

also effected by a smaller frequency factor and activation energy of the former reaction in Table 1. As revealed in Figs. 2 and 3, the reaction rates of the present catalytic WA0 process were strongly influenced by the catalyst dosage. Hence it would be of much interest to develop the correlations between the reaction rate coefficient and the catalyst dosage for both catalysts. Plots of ln(C/C,) vs. t for various CuSO, and Cu(NO,), dosages are, respectively, shown in Figs. 12 and 13. The two-stage, first-order kinetics apparently yields a reasonable representation of the catalytic reactions for both cases. The catalyst dosage dependent rate coefficient of the reaction kinetics is assumed to be described by the following mth order equation k=k,W”

(4) where W is the catalyst dosage (mg/l) and m the order of catalyst dosage dependence. The logarithmic plots of k vs. W are displayed in Figs. 14 and 15 which permits easy determination of the order of dependence (m). Table 2 lists the catalyst dosage dependent orders for CuSO, and Cu(NO,),. The order of catalyst dosage dependence for the first-stage kinetics is practically the same for

Table 1 The frequency

factors and activation

energies of the catalytic

WA0 process Second stage

Catalyst

First stage k,, min-’

A E, kJ/mol

k,. min-

c&So, Cu(NO,),

3,498 4,778

46.2 48.3

0.033 7.198

’

A E, kJ/mol 6.3 26.9

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Catalysis B: Enuironmental9 (1996) 133-147

250,

CuS04 Dosage,

5

I

I

I

0

20

mg/l

I

I

40

I

I

60

Time, Fig. 12. Effect of CuSO, dosage on the two-stage, 220°C.

I

80

I

I

I

100

120

140

I

160

min

first-order

reaction kinetics of the catalytic WA0 process at

both CuSO, and Cu(NO,),. But that for the second-stage reaction differs quite significantly. Based on this and the previous development, the overall rate kinetic model for the present catalytic WA0 treatment process can be rewritten as ln[g]

=k,W”exp(

-g)

(5)

3 00

CU(NO~)~ Dosage, ?? 0

2.50

0 ?? 200

0

mg/l

s: 1000

4

G

Y

u,'

50

c _; loo

0.50

"CC -'--6

20

60

40

Time, Fig. 13. Effect of Cu(NO,), process at 220°C.

dosage

on the two-stage,

80

100

120

min first-order

reaction

kinetics

of the catalytic

WA0

S.H. Lin, S.J. Ho/Applied

Catalysis B; Environmental 9 (1996) 133-147

1 -3 00

ti

-4 00

,"

2nd -5.00

1

-6 00 -I

I

I

-8.00

I

-100

-600

-: 00

In (CuSOd) Fig. 14. Plots of the reaction rate coefficient

vs. the CuSO, dosage at 220°C.

-200

-3 00

!A

-400

m

2nd -5 00

2 -600

2

L

-900

/

’

’ -8 00

’ In

’ -700

(cu(No3)2

Fig. 15. Plots of the reaction rate coefficient

Table 2 Order of catalyst dosage dependence Catalyst

cuso, Cu(NO,),

of treatment

’

’ -6.00

’

’ -5.00

’

I

-4 00

>

vs. the Cu(NO,),

dosage at 220°C.

kinetics

Order of catalyst dosage dependence

(m)

First stage

Second stage

0.132 0.138

0.023 0.119

145

146

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Catalysis B: Environmental 9 (1996) 133-147

4. Conclusions Catalytic wet-air oxidation of desizing wastewater containing high concentration of various refractory organic compounds was investigated. Experimental results indicated that over 80% reduction of chemical oxygen demand (COD) concentration of the wastewater can be achieved in 60 min of WA0 treatment operating at 200°C 7 MPa and 1 l/min air flow rate. The mixer speed at 300 r-pm was found to provide sufficient mixing for the present WA0 oxidation system. Temperature was observed to be an important control variable of the catalytic WA0 treatment process. But at a temperature above 200°C the COD removal enhancement of the present catalytic WA0 process became less important. The catalytic WA0 treatment was found to be represented reasonably well by two-stage, first-order kinetics. The reaction rate coefficients for both stages of reaction were observed to be highly temperature and catalyst dosage dependent. The temperature dependent rate coefficients follow the Arrhenius equation. A power law in catalyst dosage is found to describe well the catalyst dosage dependence.

Acknowledgements The authors sincerely thank the Yuan Ze Memorial Foundation for the financial support (under grant DRA 83001) of this project.

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