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Ind. Eng. Chem. Process Des. Dev. 1904, 23, 410-413
A Novel Method of Recovering Phenolic Substances from Aqueous Alkaline Waste Streams A novel two-phase method has been proposed to remove as well as to recover phenolic materials from aqueous alkaline waste streams. This is based on the reaction of phenolic materials with substances such as benzoyl chloride, p -toluenesulfonyl chloride, etc. dissolved in a water-immiscible solvent such as toluene in the presence of a phase transfer catalyst. In a continuous reactor-settler arrangement it was possible to bring down the phenol content of an aqueous alkaline stream from 2035 ppm to 45 ppm with phenol recovered as phenyl benzoate.
Introduction The removal of phenols from aqueous industrial effluents is a statutory obligation. Due to stringent regulations, the concentration of phenols in industrial wastewater has to be brought down to around 1ppm. Phenols are present in wastewater from petroleum refineries, petrochemical industries, coal conversion processes, and many diverse industries manufacturing phenols and related chemicals (Mancy and Weber, 1971). Methods for removing phenol include adsorption on activated carbon, solvent extraction, chemical oxidation, biological degradation, and membrane processes. In many cases the phenolic constituents are not amenable to biological degradation. It is common to find wastewaters containing a few hundred to a few thousand parts per million of phenolic materials, and recovery of phenolic materials may prove to be an attractive proposition. Among various recovery processes solvent extraction has been widely used. The main criterion for a suitable solvent extraction process is the distribution coefficient (K,) for the solute in the solvent. Polar solvents such as methyl isobutyl ketone (MIBK), diisopropyl ether (DIPE), butyl acetate, etc., are better than nonpolar hydrocarbons. However, because of the higher solubility of polar solvents in water, the wastewater has to be processed further to remove the dissolved solvents before disposal (Earhart et al., 1977). Aim of the Present Work In a number of cases, the phenolic wastewaters are alkaline in nature. For example, it has been reported that the condensate water from coal-conversionprocesses have a pH between 8 and 9.8 and are highly buffered (Greminger et al., 1982). In the alkaline medium, a substantial fraction of the phenolic material exists in the ionized state. Greminger et al. (1982) have also reported KD values for phenol, dihydric phenols, and trihydric phenols in MIBK and DIPE as a function of pH. Their data show that a t high pH, KD values are very low primarily due to the existence of phenolate ions. A large solvent circulation rate ( S ) will be required for a fixed wastewater flow (W), to maintain KDS/ W value between 1.5 and 3.0 suggested as optimum for solvent extraction (Earhart et al., 1977). Hence the removal of phenols via solvent extraction becomes difficult. It would be necessary to bring pH to a neutral/mildly acidic level to allow solvent extraction. Dihydric phenols have been reported to be present in condensate waters from the coal-conversion process (Greminger et al., 1982). Even at a neutral pH, these are difficult to extract by simple solvent extraction because of their high solubility in water and extremely low KD value in nonpolar solvents. Polar solvents give higher KDvalues (Won and Prausnitz, 1975),but their use for extraction has the inherent disadvantage mentioned earlier. We have developed a novel method by which phenols cannot only be efficiently removed from alkaline waste-
waters but are also recovered in the form of industrially useful compounds. In this method, phase transfer catalysis (Starks and Liotta, 1978) has been adopted to transport phenolate ions from alkaline wastewaters to an organic solvent where they react with a suitable organic reagent. For example, with benzoyl chloride as the organic reagent, phenol forms phenyl benzoate; with toluenesulfonyl chloride, phenol forms phenyl tosylate, etc. The products of reaction mentioned above are industrially useful substances. The product is soluble in the organic phase, has practically negligible solubility in water, and can be recovered after removing the solvent. The phase transfer catalyst (PTC) can be recycled. The phase transfer catalyst employed should be completely insoluble in water since a water-soluble catalyst leads to loss of catalyst in the aqueous stream and therefore an additional problem of catalyst recovery from the aqueous stream. Further, some phase transfer catalysts such as quaternary ammonium compounds may be toxic to snails and algae. A variety of phase transfer catalysts are available that are water insoluble, for example, Aliquat 336 (tricaprylmethylammonium chloride), hexadecyltributylphosphonium bromide, etc. (Landini et al., 1977),and in the presence of an organic solvent these highly lipophilic catalysts will be completely partitioned toward the organic phase. The mechanism of phase transfer catalysis for recovery of, say, phenol as phenyl benzoate is depicted in the following scheme (organic phase) (Q+C,H,O-) + C,H,COCl
-
C,H,OOCC,H,
41
Na+C,H,O(aqueous phase)
(interface)
+
(Q'Cl-)
II
J.
c1-
where Q'Cl- represents the phase transfer catalyst and the parentheses denote ion pairs. Since the catalyst is insoluble in the aqueous phase, the ion-transfer step has been shown as a liquid ion-exchange mechanism, wherein Q' resides exclusively in the organic phase and ions are exchanged across the interface. This method can possibly be applied for extraction of dihydric phenols also from aqueous alkaline wastestreams. Once it is established that phase transfer catalysis allows the recovery of phenolic materials, we can then explore the possibilities of using immobilized phase transfer catalysts (Regen, 1982).
Experimental Section Materials and Methods of Analysis. All the materials used were obtained from reputable firms. p-Toluenesulfonyl chloride was recrystallized from hexane before use. Benzoyl chloride was distilled under reduced pressure and used.
0196-4305/84/1123-0410$01.50/00 1984 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 2, 1984
Phenols were analyzed at ppm levels in the aqueous phase by the method reported by Whitlock et al. (1972). Here, the phenol in aqueous phase is coupled with diazotized sulfanilic acid to form a colored azo dye. The absorbance at different concentrations was measured on a Beckman UV/visible spectrometer. The rate of extraction, in the case of stirred cell experiments, was calculated on the basis of chloride ions formed in the aqueous phase. The chloride ions were analyzed by Volhard's method (Vogel, 1962). In order to check the material balance the phenyl benzoate was also analyzed on a gas chromatograph. Experimental Procedures. Experiments were carried out in both batch and continuous modes for recovery of phenol. The mechanism of the phase transfer catalyzed reaction of sodium phenolate with benzoyl chloride was discerned by carrying out batch experiments in constant interfacial area stirred cells. Aqueous streams were prepared by dissolving the appropriate amounts of the phenolic substances in distilled water and adding sodium hydroxide according to stoichiometric requirements. The aqueous streams so prepared had a pH of 11.0 and above. Toluene was chosen as the organic solvent because of its low solubility in water. Aliquat 336 (tricaprylmethylammonium chloride) was used as the PTC for this work because it is insoluble in water and is also one of the least expensive phase transfer catalysts. Stirred Cell Experiments. A 5.5 cm i.d. glass stirred ceIl with two cruciform glass stirrers mounted on a central shaft was used for the measurement of the specific rate of extraction. The two phases were stirred separately without causing dispersion at the speeds of stirring employed, namely, 30 to 60 rpm. Thus, since a plane interface is maintained, the interfacial area is equal to the geometrical area of the stirred cell which is accurately known. Further details of the stirred cell are available elsewhere (Nanda and Sharma, 1966). Aqueous phase (50 mL) containing sodium phenolate of known concentration was inititally taken in the stirred cell. Toluene (50 mL) containing known concentrations of the catalyst and benzoyl chloride was added carefully along the sides of the cell so that toluene is not dispersed into the aqueous phase. Initial and final samples were withdrawn for calculating the specific rate of extraction. The stirred cell was operated at about 30 "C and the batch times were suitably manipulated to maintain all the other parameters at identical conditions when one of them was varied. To study the effect of interfacial area, an experiment was also carried out in a 9.5 cm i.d. stirred cell. The enhancement factor, defined as the ratio of the rates of extraction in the presence of PTC to that without PTC, was obtained by conducting experiments in the presence and absence of the catalyst under otherwise identical conditions in the 5.5 cm i.d. stirred cell. Here also the batch times were suitably manipulated to obtain the same conversion levels ( E 12%) in both experiments. Batch Experiments for Recovery of Phenol. The batch experiments were carried out at room temperature (930 "C) in a 9.5-cm, fully baffled, mechanically agitated glass reactor. The aqueous phase (300 mL) containing about 5000 ppm of the phenolic substance was agitated at 1000 rpm with a stoichiometric amount of benzoyl chloride in toluene (150 mL) containing 0.02 X mol/cm3 Aliquat 336. After a known interval of time (10 min) the aqueous phase was separated and analyzed for phenol. In the case of phenol, in addition to benzoyl chloride, p-toluenesulfonyl chloride was also used to extract phenol as phenyl tosylate. Some experiments were also carried out without the PTC.
411
k'oor b
Figure 1. Reactor-settler unit. Key: (1)Glass reactor; (2) baffles (SAL); (3) stirrer (6-bladed turbine); (4) aqueous phase inlet; (5) organic phase inlet; (6) reactor outlet; (7) settler; (8)wire mesh packing; (9) organic phase outlet; (10) aqueous phase outlet.
Continuous Experiments. Continuous experiments were conducted at room temperature for recovering phenol as phenyl benzoate. The details of the reactor-settler unit used for the continuous experiments are shown in Figure 1. The aqueous and organic phase flows were maintained by constant head reservoirs and monitored by rotameters. The total holdup in the reactor was about 850 mL. It was observed visually that toluene was the dispersed phase at a holdup of 50% toluene. The organic phase contained benzoyl chloride according to stoichiometric requirements. The speed of agitation was maintained at 1700-1800 rpm. A residence time of 15 min was provided in the settler to facilitate good phase separation. Samples of aqueous phase were collected after steady state was attained and analyzed. Results and Discussion The material balance checked well, which indicates that no benzoyl chloride was hydrolyzed for the phase transfer catalyzed reaction of sodium phenolate with benzoyl chloride. Benzoyl chloride has negligible solubility in water and in the two-phase system toluene provides a relatively very high distribution coefficient favoring the organic phase. Besides, in the presence of the phase transfer catalyst, the reaction occurs in the organic phase. Thus the analysis of chloride ions for calculating the specific rate of extraction in the stirred cell experiments should give accurate results. The specific rate of extraction of sodium phenolate with benzoyl chloride catalyzed by PTC was found to be independent of the speed of stirring. The speed of stirring was varied from 30 to 60 rpm in the constant interfacial area stirred cell. Thus, hydrodynamic factors are unimportant in affecting the specific rate of extraction. The specific rate of extraction in the 9.5 cm i.d. stirred cell was 2.27 X lo-' mol/cm2 s, whereas in the 5.5 cm i.d. stirred cell it was 2.257 X lo-' mol/cm2 s, under otherwise similar conditions. Thus the specific rate of extraction was found to be independent of the interfacial area, which shows that the volumetric rate of extraction is proportional to the interfacial area. Figure 2 shows a logarithmic plot of the specific rate of extraction against the average concentration of sodium phenolate, under otherwise uniform conditions. The reaction is first order with respect to sodium phenolate in the range from 0.04 X mol/cm3 to 0.24 X mol/cm3. In any experiment the percentage variation in the con-
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412
-
AVERAGE CONCENTRATION OF SODIUM PHENOLATE I N AQUEOUS P H A S E XlO’, molc/cm’
Table I. Recovery of Phenol, m-(;’resol, and Resorcinol: Batch ExDeriments’ expt phenolic no. material
rt 9
u d .
E IL
1
v)
phenol
2
m-cresol
3
resorcinol
benzoyl chloride p-toluenesulfonyl chloride benzoyl chloride benzoyl chloride
0.8
In the presence of 0.02 X 2 mol/moI of resorcinol. a
0.6
init final % concn, concn, recovP P ~ppm ery 5070 3.8 99.92
5200
2.2
99.96
5140
5.5
99.89
5017
2.4
99.95
mol/cm3 Aliquat 336.
0 The stirred cell experiments indicate that this two-phase reaction conforms to the fast pseudo-first-order reaction regime. In this regime, the volumetric rate of a reaction which is first order in both the reactants is given by
;0.2 0
1
I
4.0
org reagent used
0.1 0 .01
0.02
0.06 0.06
0.2
0.1
0.4
0.6 0.0 1
RAu
AVERAGE CONCENTRATION OF BENZOYL CHLORIDE I N ORGANIC PHASE xio’, m o l t / c m ’
Figure 2. Effect of concentration of sodium phenolate on specific rate of extraction (-A-): [PTC] = 10.66 X lo4 mol/cm3; [benzoyl chloride], = 0.24X low3mol/cm3; speed of stirring = 50 rpm. Effect of concentration of benzoyl chloride on specific rate of extraction (-O-): [PTC] = 10.66 X lo4 mol/cm3; [sodium phenolate], = 0.236 X mol/cm3; speed of stirring = 50 rpm.
*0
# I
21-
17 Y
w a U N
1513-
- 0 UC
y
0
ln
r;
75-
= u[A*](D,~~[B,])’/~
Hence, the dependence of the specific rate of extraction on the square root of benzoyl chloride concentration shows that the reaction is first order in benzoyl chloride as well. The results obtained from the batch experiments in the mechanically agitated contactor for the recovery of phenol, m-cresol, and resorcinol are given in Table I. In the case of phenol, the percentage recovery obtained using p toluenesulfonyl chloride is also given. These results show that the proposed method is attractive and merits attention. In the continuous experiments, by providing a residence time of 8 min in the reactor, it was possible to bring down the phenol content from 10800 ppm to 180 ppm at a PTC mol/cm3. To stimulate a concentration of 0.01 X typical industrial waste stream another continuous experiment was conducted with an aqueous stream consisting of 2035 ppm phenol. By providing a residence time of 12 min in the reactor and with a PTC concentration of 0.01 X mol/cm3 it was possible to bring down the phenol content to 45 ppm. An estimate of the interfacial area, a, in the mechanically agitated reactor was obtained for the continuous extraction experiments from the volumetric rate of extraction and the specific rate of extraction values (calculated from the stirred cell experimental data). The value of a comes to around 720 cm2/cm3 of dispersion volume and is a reasonable estimate at a holdup of 50% toluene and speed of agitation of 1700-1800 rpm (Lele et al., 1982). Since it was established by the stirred cell experiments that the rate of extraction is interfacial area dependent, higher speeds of agitation, addition of emulsifiers to increase a, etc. (Lele et al., 1982),may bring down the phenolic materials to even lower levels in the continuous unit. However, care should be taken to see that no phase separation problems arise. Experiments without the Phase Transfer Catalyst. Some batch experiments were conducted for recovering phenol without the PTC but under otherwise identical conditions. It was observed that a substantial amount of benzoyl chloride underwent hydrolysis. However, when the amount of organic phase was increased (with organic to aqueous phases in equal proportions) it was possible to recover phenolate ions with stoichiometric amounts of benzoyl chloride. From the stirred cell experiments it was observed that a PTC concentration of 0.01 X mol/cm3 in the organic
34 11
2
3
4
5
6
7
B
-
9 1 0 1 1 1 2
CONCENTRATION OF PTC IN ORGANIC PHASE xio‘, moic/cm’
Figure 3. Effect of PTC concentration on specific rate of extraction: [benzoyl chloride],, = 0.24 X mol/cm3; [sodium phenolate],, = 0.236 X mol/cm3; speed of stirring = 50 rpm.
centration of phenol was restricted to around 12%. Figure 2 also shows a log-log plot of the specific rate of extraction against the average concentration of benzoyl chloride, under otherwise uniform conditions. The rate of extraction has an order of 0.5 with respect to benzoyl chloride in the range from 0.04 X mol/cm3 to 0.24 X mol/cm3. The specific rate of extraction was found to be proportional to the PTC concentration when the catalyst concentration was varied from 0.0013 X mol/cm3 to 0.0107 X lC3mol/cm3 (Figure 3). This observation agrees with the theory of phase transfer catalyzed reactions (Herriott and Picker, 1975).
(1)
Ind. Eng. Chem. Process Des. Dev., Vol. 23,No. 2, 1984 413
phase can enhance the rate of extraction by a factor of about 36. Even still higher enhancement factors can be achieved since the rate has been found to be proportional to catalyst concentration. This factor may become relevant in the case of continuous extraction of phenols. Taking into consideration the huge quantities of wastewater to be processed it may prove beneficial to have a low value of the residence time in the reactor from overall economical considerations and this can be accomplished with the PTC, quite apart from handling much lower volumes of the organic phase and lower energy consumption associated with the removal of the solvent.
Scope for Further Work (1) This method can perhaps be extended to recover other phenolic substances such as aminophenols, xylenols, chlorophenols, alkyl phenols, phenol sulfonic acids, naphthols, catechol, and even trihydric phenols from aqueous alkaline waste streams. (2) The use of a water-insoluble organic solvent in which the product is solid and insoluble looks attractive as the product can be recovered by filtration and the organic solvent with the catalyst can be recycled. (3) Triphase catalysis employing immobilized phase transfer catalysts deserves special attention for large-scale adoption (Regen 1975; 1982). Here we may consider two alternatives. In the first case a packed column may be used. In the second alternative, fine particles on magnetized support may be used so that catalyst removal does not pose a problem. Such a strategy has been suggested for demineralization of water and flocculation (Bolto, 1983). (4) The recovery of phenolic substances as their ethers may also be considered. (5) The separation of the catalyst from the product and its recycle needs a more detailed study. Conclusions The recovery of phenolic materials from aqueous alkaline streams can be accomplished successfully with simultaneous conversion to useful compounds in a two-phase system. In a typical case, the phenolic content of an aqueous alkaline stream was brought down from 2035 ppm to 45 ppm with benzoyl chloride in a continuous reactorsettler arrangement.
Acknowledgment One of us (V.K.K.) is thankful to the UGC,New Delhi, for the award of a Research Fellowship.
Nomenclature a = interfacial area, cmz/cm3of dispersion volume [A*] = interfacial concentration of reactant A in the phase B’ where the reaction takes place, mol/cm3 [Bo] = concentration of reactant B in bulk B’ phase, mol/cm3 DA = diffusivity of A in B’phase, cm2/s kz = second-order rate constant, cm3/mol s KD = distribution coefficient RAu = volumetric rate of extraction, mol/cm3 (dispersion) s S = solvent flow rate W = waste water flow rate Registry No. C6H6COC1,98-88-4; C6H500CCBH6, 93-99-2; m-cresol, 10839-4;resorcinol, 108463;p-toluenesulfonylchloride, 98-59-9;phenol, 108-95-2.
Literature Cited Bolto. B. A. Prog. Polym. Scl. 1983, 9 , 89. Earhart, J. P.; Won, K. W.; Wong. H. Y.; Prausnltz, J. M.; King, C. J. Chem. Eng. Prog. 1977, 73(5),67. Greminger, D. C.; Bums, 0. P.; Lynn, S.; Hanson. D. N.; King, C. J. Ind. Eng Chem. Process Des. D e v . 1982, 21, 51. Herrlott, A. W.; Picker, D. J . Am. Chem. SOC. 1975. 9 7 , 2345. Landini, D.; Mala. A.; Montanarl, F. J . Chem. SOC.Chem. Commun. 1977, 112. Lele, S. S.; Bhave, R. R.; Sharma, M. M. Ind. Eng. Chem. Process Des. D e v . 1983, 22, 73. Mancy, K. H.; Weber, W. J. “Analysis of Industrlal Wastewaters”; Wiiey-Intersclence: New York. 1971; p 490. Nanda, A. K.; Sharma, M. M. Chem. Eng. Sci. 1966, 21, 707. Regen, S. L. J . Am. Chem. Soc. 1975. 9 7 , 5956. Regen. S. L. N o w . J . Chlm. 1962, 6(12), 629. Starks, C. M.; Liotta, C. “Phase Transfer Catalysis: Principle and Techniques”, Academlc Press: New York, 1978. Vcgel, A. I . ”A Text-book of Quantitative Inorganic Analysis including Eiementary Instrumental Analysis”, 3rd ed.; Longmn: London, 1982; p 266. Whltiock, L. R.; Slggia, S.; Smola, J. E. Anal. Chem. 1972, 44, 532. Won, K. W.; Prausnitz, J. M. J . Chem. Thermodyn. 1975, 7 , 661.
.
Department of Chemical Technology University of Bombay Matunga, Bombay 400 019, India
V. K. Kriehnakumar Man Mohan Sharma*
Received for review October 13, 1982 Accepted August 19, 1983
