Ind. Eng. Chem. Process Des. Dev. 1985, 2 4 , 339-343
339
A Novel Coupling-Dephenolization Scheme for Full-Strength Coal-Conversion Wastewaters: Results of Wastewater Study and a Cost Analysis Kal C. Chln, John A. Cha, and Phool K. Llm' Department of Chemical Engineering, North Carolina State University. Raleigh, North Carolina 27695-7905
Results of a feasibility study using an actual coal-gasification wastewater confirm the promising nature of a coupling-dephenolization scheme which we have proposed previously. Kinetic results indicate that the aerobic coupling of phenols, which is the heart of the proposed scheme, occurs at a sufficiently high rate in the actual wastewater to make the scheme practical. A cost anal sis of the scheme indicates that for a commercial-scale of synthetic natural gas and 3 X lo6 gal of wastewater, coal gasification plant with a daily output of 200 X 10' the treatment cost for a 99% removal of phenols from a wastewater containing 4 g of phenol/L is $8.6/103gal (1980 dollar); this compares favorably with the estimated costs of $12.911 O3 gal for a biooxidation scheme and $12.81lo3 gal for a solvent extraction scheme. Additional advantages of the coupling scheme are its simplicity and fast treatment under mild conditions, ability to withstand process interruptions, easy disposal of organic precipitates by incineration, and the ability to remove other pollutants such as sulfides and aromatic amines.
J
Introduction
A recent kinetic study from our laboratory (Lim et al., 1983) indicates that aqueous phenols, in the presence of oxygen and a catalytic amount of cuprous chloride, undergo a facile oxidative coupling reaction to form insoluble coupling products. The reaction involves primarily the oxidative oligomerization of phenols, although it is also accompanied by a limited amount of hydroxylation reaction. The coupling products share the characteristics of humic acids which are known to be formed in the permanganate oxidation of phenol (Ficek and Boll, 1980);they have been tentatively identified as low molecular weight humic acids. The results have led us to propose a novel dephenolization scheme for treating full-strength coal-conversion wastewaters which are not easily handled by such conventional wastewater treatment schemes as solvent extraction (Beychok, 1974; Cavanaugh et al., 1977; Greminger et al., 1982) and biooxidation (Benefield et al., 1977; Cavanaugh et al., 1977; Holladay et al., 1978; Luthy and Tallon, 1980). Phenol extractants are inherently polar in nature; they are sufficiently soluble in water to warrant an expensive solvent recovery step in an extraction scheme. Biooxidation, on the other hand, is limited to dilute phenolic wastewaters (typically less than 1000 mg/L of phenols); it is slow and highly sensitive to process interruptions and fluctuations in temperature, solution pH, nutrients, and loadings (Holladay et al., 1978; Luthy and Tallon, 1980; Vela and Ralston, 1978). We have tested the feasibility of our proposed coupling-dephenolization scheme using an actual full-strength wastewater that was generated in a fixed-bed coal gasifier (run no. 99) at the Morgantown Energy Technology Center (METC), Morgantown, WV (Fernald, 1983). In this paper we present the results of the feasibility study along with the results of a cost analysis of the proposed scheme. The results show that the coupling scheme is simpler and more cost-effective than the more conventional dephenolization schemes based on biooxidation and solvent extraction. Experimental Section
The aerobic coupling of phenol, cresols, and xylenols that were present in the METC wastewater was studied batchwise in a glass Morton reactor. The as-received 0196-4305/85/1124-0339$01.50/0
full-strength wastewater was treated with powdered lime to precipitate out the dissolved carbonate and to adjust the wastewater pH to the desired mark. The lime-treated wastewater was filtered, and 250 mL of the wastewater was used for each run. Except for the kinetic runs which were designed to establish the reaction order with respect to phenol, the lime-treated and filtered wastewater was used without further treatment. For the kinetic runs designed to establish the reaction order with respect to phenol, reagent-grade phenol was added as needed to the wastewater to extend its concentration range. Cuprous chloride was used as the coupling catalyst as supplied, i.e., in the powder form. Control of the reaction temperature was achieved in a thermostated water bath. The oxygen pressure was monitored manometrically and was maintained constant during each run by an oxygen regulator furnished by Air Products Co. The reaction solution was stirred vigorously by a 38-mm magnetic stirring bar to avoid oxygen mass-transfer limitation. The reaction was initiated by adding a predetermined amount of cuprous chloride to the wastewater which had been thermostated to the desired temperature. The progress of the reaction was followed by monitoring the concentrations of phenols in the reactor solution as a function of reaction time. Solution samples were withdrawn from the reactor at timed intervals, quenched in dilute acetic acid solutions to arrest the reaction, and then analyzed for phenols. A Perkin-Elmer Series-4 high performance liquid chromatograph was used for the analysis of phenols and fractionation of the coupling products. The column used was a Perkin-Elmer HS-5 CI8 column; the eluant was a mixture of 40% methanol, 59% water, and 1%acetic acid; its flow rate was 2.0 mL/min. The reverse-phase HPLC technique provided a convenient and reliable quantitative analysis of mixtures of various phenols and their soluble oxidation products, which are formed in a minor amount. In this study, "phenol concentration" refers specifically to that of simple phenol, whereas "phenols concentration" refers to the total concentration of various phenols expressed in terms of equivalent phenol. Results and Discussion (A) General Observations. The characteristics of the
METC wastewater are given in Table I and Figure 1. Representative profiles of concentration vs. reaction time 0 1985 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 2, 1985
340
Log R a t e (Mlhr)
Table I. Characteristics of METC Coal-Gasification Wastewater (Fernald, 1983) Darameter measurement pH (20 "C) 8.8 total phenols (phenol equiv) 3800 mg/L carbonate 5000 mg/L bicarbonate 16000 mg/L ammonia 3400 mg/L total nitrogen (Kjeldahl) 8400 mg/L chloride 1800 mg/L thiocyanate 840 mg/L BOD 14000 mg/L COD 19000 mg/L 5200 mg/L total organic carbon 280 mg/L oil and grease ,
13
,
l
l
'
l
l
'
l
'
l
'
l
I
- 1 3
'
t
1
'
I
'
1
'
1
'
1
'
A
T=6l0C
l
12 -
Basis
11
-
10
-
PH
2 0 0 ml
-
9
-2 0
-1 8
-1.6 LOO
-1.4 [PhOH](M)
-1 2
-1 0
Figure 3. The dependence of the coupling reaction rate on the phenol concentration in METC wastewater.
Amount at Calcium Hydroxide Added, 9
Figure 1. Variation of the pH of METC wastewater with the amount of calcium hydroxide powder added to the wastewater.
Rate, M/hr
0.014 0
0028
10
5 '
I
'
'
~
15
I
Reacton Tme, HOUIS 20 25
35
30
40
" 0.012-
0 . 001100T= 8 l°C
p n = 10.2
[P hOH] = ,, 0.033M
0.008 -
P Po o =36 2
i n Hg
0.006
1
0 008C
t
0.004
0 0041
0.002
0 0
1
1
/
/
2
1
~
3
l
4
/
l
Reaclm Tme Hours
5
1
l
8
/
l
7
l
1
Figure 2. The copper-catalyzed aerobic coupling of phenol in METC coal-conversion wastewater.
for the aerobic coupling of phenol are shown in Figure 2. It is evident that lime pretreatment has a profound effect on the rate of the coupling reaction. As was the case in the simulated wastewater study (Lim et al., 1983),very little soluble products (
