Ind. Eng. Chem. Res. 1992,31, 2727-2731 Hrudkova, H.; Svec, F.; Kalal, J. Reactive Polymers. XIV. Hydrolysis of the Epoxide Groups of Copolymer Glycidyl Methacrylate-Ethylene Dimethacrylate. Br. Polym. J. 1977,9,238-340. Iwata, H.; Saito, K.; Furusaki, S.; Sugo, T.; Okamoto, J. Adsorption Characteristics of an Immobilized Metal Affinity Membrane. Biotechnol. B o g . 1991, 7, 412-418. Kim, M.; Saito,K.; Furusaki, S.; Sato, T.; Sugo, T.; Ishigaki, I. Adsorption and Elution of Bovine Gamma-Globulin Using an Affmity Membrane Containing Hydrophobic Amino Acids as Ligands. J. Chromatogr. 1991,585, 45-51. Mitsubishi Kasei Manual Zon-ExchangeResin DZAION; Mitsubishi Kasei Co.: Tokyo, 1975. Saito, K.; Ito, M.; Yamagishi, H.; Furusaki, S.; Sugo, T.; Okamato, J. Novel Hollow Fiber Membrane for the Removal of Metal Ion during Permeation: Preparation by Radiation-Induced Cografting
2727
of a Cross-Linking Agent with Reactive Monomer. Znd. Eng. Chem. Res. 1989a, 28, 1908-1812. Saito, K.; Kaga, T.; Yamagishi,H.; Furueaki, S.; Sugo, T.; Okamoto, J. Phosphorylated Hollow Fibers Synthesized by Radiation Grafting and Cross-Linking. J. Membr. Sci. 1989b, 43,131-141. Tsuneda, S.; Saito,K.; Furusaki, S.; Sugo, T.; Okamoto, J. Metal Collection Using Chelating Hollow Fiber Membrane. J. Membr. Sci. 1991,58, 221-234. Yamagishi, H.; Saito,K.; Furusaki, S.; Sugo, T.; Ishigaki, I. Introduction of a High-Density Chelating Group into a Porous Membrane without Lowering the FLUX. Znd. Eng. Chem. Res. 1991, 30,2234-2237.
Received for review April 15, 1992 Accepted August 17,1992
Triphase Catalysis for Recovery of Phenol from an Aqueous Alkaline Stream Narendra N. Dutta,* Somiran Borthakur, and Gajanan S. Patil Chemical Engineering Division, Regional Research Laboratory, Jorhat 785 006,Assam, India
Use of a polymer-supported phase-transfer catalyst for the removal/recovery of phenol from an aqueous alkaline stream has been demonstrated. Phenol was extracted into the organic phase as phenyl benzoate via reaction with benzoyl chloride dissolved in toluene using two types of catalysts. Catalyst with a tributylphosphonium ion as the active centre was found to be more effective than Amberlite IRA 401(C1) resin catalyst. The apparent reaction rates were found to obey pseudofirst-order kinetics under suitable conditions.
Introduction In a previous communication (Dutta et al., 1992), we presented the results of the esterification reaction between benzoyl chloride and phenols in a two-phase system using hexadecyltributylphosphonium bromide (HTPB) as the phase-transfer catalyst (PTC). Conversion of phenols as high as 99.85% could be achieved within 10-15 min using HTPB and Aliquat 336 (Krishnakumarand Sharma, 1984) as well. Phase-transfer-catalyzed esterification of phenol with various aliphatic acid chlorides gives good quantitative yields of products (Direktor and Effenburger, 1985). Various other classes of reactions under phase-transfer conditions are feasible, and a comprehensive review on the same has been published (Dutta, 1990). Certain products such as benzoates which have commercial value are insoluble in water, and they can be recovered by simple phase separation and evaporation of organic phase. Such an extractive reaction scheme was shown to be quite promising for a phenolic wastewater treatment process. However, for industrial applications, a heterogeneous catalyst in the so-called phenomenon of “triphase catalysis” (Regen, 1975) would be desirable in order to simplify catalyst separation and reuse. Though activity of immobilized PTC is generally low, considerable efforts have been made toward its improvement. For instance, activity can be dramatically improved by using a long spacer chain between the polymer matrix (Tomoi et al., 1986) and the active ion. The most commonly used polymer matrix tested as support for P T C is polystyrene cross-linked with divinylbenzene. A number of commercially available ion-exchange resins such as IRA 401; Dowex 1 x 8 and 1x2; and AGMP-1 contain quaternary ammonium groups. Such resins have been studied as heterogeneous PTCs for al-
* T o whom correspondence should be addressed. 0888-5885/92/2631-2727$03.00/0
kylation (Ragaini et al., 1986,1988) and oxidation (Ido et al., 1986) reactions. In this paper, we report a comprehensive study on the use of a typical ion-exchange resin and a freshly prepared polymer-supported PTC containing phosphonium ion for reaction of phenol with benzoyl chloride.
Experimental Section Materials. Phenol (ArOH), benzoyl chloride (RC1 or RX), and all other reagents were of analytical grade and were procured from reputed firms. Two catalyst types were used. One is a gel-type Amberlite IRA 401(C1) manufactured by BDH Chemical Co. This resin contains quaternary ammonium radical in the matrix consisting of styrene-divinylbenzene copolymer with 8% degree of cross-linking. Pretreatment of the resin was made by NaOH and HC1 to obtain in the final chloride form. The major size fraction, 150-210 pm, obtained by seiving was used for reaction. The other catalyst was prepared as described below. Procedure. A. Preparation of Immobilized Phase-Transfer Catalyst: Immobilizing Tri-n -butylphosphine on Polymer Support. A mixture of 25 g of microporous chloromethylpolystyrene (3.5 mequiv of Cl-/g and 2% divinylbenzene cross-linked), 40 g of tri-nbutylphosphine, and 400 mL of 1,2-dichloropropane were heated under refluxing condition in Nz atmosphere for about 8 h. After thorough washing in sequence with methanol, acetone, and methanol and drying under vacuum a t 60 OC, the chloride content was determined by a Volhard titration. It has a titer value of 1.15 mequiv of c1-/g. B. Triphase Reaction. Reaction was carried out in a 300-mL fully baffled cylindrical vessel provided with a four-bladed turbine impeller of 1-cm width, sampling port, and feed inlet. The impeller to vessel diameter ratio was 1992 American Chemical Society
2728 Ind. Eng. Chem. Res., Vol. 31, No. 12, 1992
maintained at 1:3, and the impeller was located at the interface of the two phases. The reactor was immersed in a constant-temperature water bath to maintain the temperature at 30 f 0.5 OC. Known quantities of sodium hydroxide and ArOH were dissolved in water and introduced into the reactor. Measured quantities of distilled RCl dissolved in toluene and catalyst were then added. The stirring was immediately started and the speed was maintained above 800 rpm (in all experiments), ensuring vigorous mixing. During the reaction, an aliquat sample (1mL) was withdrawn from the reaction mixture at an interval of about 10 min. The organic and aqueous phases were separated after a few seconds. The organic phase waa analyzed by a gas chromatograph (CCI; Baroda, India). At the end of each experiment, the whole amount of reaction mixture was subjected to phase separation for analysis of the respective phases. The aqueous phase was analyzed for ArOH by a spectrophotometer (Whitlock et al., 1972) and for chloride (Vogel, 1978) content by the Volhard method, respectively. A final cross-check on ArOH, RC1 converted, and phenyl benzoate formed was made from a material balance calculation based on analysis of both phases. No phenyl benzoate was found to partition into the aqueous phase.
Results and Discussion The overall reaction rate in a triphase system depends primarily on factors such as mixing of the triphase (i.e., mass-transfer effects), catalyst particle size, degree of cross-linking of the polymer support, intrinsic activity of the active sites, and the solvent. The four fundamental steps in polymer-supported phase-transfer catalysis are (i) mass transfer of reactant from bulk solution to catalyst surface, (ii) diffusion of reactant through the polymer matrix to the active site, (iii) reaction at the active site, and (iv) diffusion of the product to the surface of the catalyst and mass transfer of the product to bulk solution (Tomoi and Ford, 1981). In order to simplify the kinetics, triphase reactions are, in general, carried out with an excesa of the ionic reagents in the aqueous phase. Thus, in most of the reactions reported so far, pseudo-fit-order kinetics have been observed. In our study, stoichiometrically equivalent amounts of ArOH and RC1 have been deliberately taken, because in reactions of practical relevance it is imperative to use a high ArOH to RCl ratio if maximum utilization of the aqueous substrate and reduction of the organic inventory have to be ensured. The stirring speed was maintained well above 800 rpm since, based on the literature review, it can be expected that the film diffusion effect on the conversion will be negligible under otherwise identical conditions. Effect of Catalyst Type. The effect of the two catalyst types on the rate of conversion is shown in Figure 1. The conversion profile for the uncatalyzed reaction is also shown in Figure 1, which clearly indicates the advantage of the catalyzed reaction. The observation is quite reasonable as a substantial quantity of benzoyl chloride is hydrolyzed in the uncatalyzed reaction (Dutta et al., 1992; Krishnakumar and Sharma, 1984). The particle sizes of the ion-exchange resin and supported phosphonium ion (P+)catalyst were marginally different owing to the limitation in the maximum size of the copolymer bead used. Yet we feel that the results presented in Figure 1are quite adequate for meaningful interpretation under otherwise identical conditions. Catalyst with P+ ion as the active center is more effective than the resin IRA 401 (Cl). Amberlite resin is more hydrated and not well suited for association with alkyl halide type of compounds (Tomoi and Ford, 1981). The low activity of the resin catalyst may also
100
5.0
0
0 ~
20
-
20 ~~
30 +
40
70 60
50
80
90
N o cafalyst
1W
1tG 120
'30 140 150
mln
Time
Figure 1. Effect of catalyst type on rate of conversion. Reaction conditions: temperature 30 f 0.5 O C ; [ArOH] 4.25 X lo-* mol L-l; [RCl] 4.25 X lo-* mol L-l; phase volume ratio (VJV,) 1:l; catalyst loading 0.5 molar equivalent; Dashed line represents data for NaOH/ArOH of 1.5 (molar). represents data for uncatalyzed reaction.
be attributed to a higher percent of cross-linkingand lower interionic distances imposed by the methyl chain. Even though both catalysts are lipophilic, the catalyst with P+ ion in a location close to the polymer backbone is more active than the catalyst with N+ ion (Ford and Tomoi, 1984). The structures of the catalysts are shown below in order of their activities: @
G
C
H
,
-
P*(n-Bu)3CI-
(2, 1.15)
The values in the parentheses indicate percent crosslinking and milliequivalent of C1- per gram of catalyst, respectively. A more highly cross-linked polymer matrix leads to a more torturous diffusion path and smaller diffusivity of reactants as the dimension available for diffusion is smaller (Tomoi and Ford, 1981). Further, the presence of an n-butyl chain is likely to render the P+ion catalyst more lipophilic in nature, thus ensuring better solvation of the aqueous anion and high selectivity. The conversion vs time profiles (Figure 1)were tested for validity of pseudo-fmt-order reaction kinetics that can be expressed by the equation ln(1-x) = -kappt
(1)
where k, is the apparent rate constant and x is the fractiona! conversion. A satisfactory fit of the data was not obtained, and the correlation coefficients of the linear plot were found to range between 0.80 and 0.85. In a series of experiments with P+ ion catalyst and the mole ratio of NaOH to ArOH raised to 1.5, keeping other conditions same, the conversion vs time profile was found to represent well a case of pseudo-fiborder kinetics. This may be due to the fact that in order to keep phenol in the aqueous phase as phenolate anion, an excess of NaOH or high pH is necessary as incomplete ionization of phenol will lead to reduction in reaction rate. Similar observations were made in the case of the reaction of 2,4,64ribromophenol with allyl bromide using N+ ion catalyst (Wang and Yang, 1991) wherein an optimal value of 1.9 of the said ratio was found to be necessary to realize a high value of the pseudo-fiborder rate constant at the same time maintaining a 1.567 mole ratio of 2,4,6-tribromophenolto allyl bromide. In view of this, all subsequent experiments were carried out a a NaOH to ArOH mole ratio of 1.5.
Ind. Eng. Chem. Res., Vol. 31, No. 12, 1992 2729
2o!t
.^
0
2 : 1
A 0
3 :1 4 : 1
’
0:
1’0
1’0
o;
6b
lo
50
c
T i m e , min
;o
do
9’0
IO0 1;,
40
130
li0
Figure 2. Effect of V J Voon rate of conversion (P+ion catalyst). Reaction conditions: temperature 30 0.5 O C ; [ArOH] 4.25 X lo-* mol L-l; [RCl] stoichiometric to [ArOH]; catalyst loading 0.5 molar equivalent; particle size 150-200 pm and NaOH/ArOH of 1.5 (molar).
Figure 3. Effect of particle size on rate of conversion (P+ion catalyst). Reaction conditions: temperature 30 0.5 O C ; [ArOH] 4.25 X mol L-l; [RCl] stoichiometric to [ArOH]; catalyst loading 0.5 molar equivalent V J V , 2:l and NaOH/ArOH of 1.5 (molar).
Effect of Aqueous to Organic Phase Volume Ratio Val V, is useful for a commercial process application, it was felt necessary to study the conversion at different phase volume ratios. The results presented in Figure 2 pertain to the P+ion catalyst only as the same was found to be superior for the reaction under study. For the same aqueous-phase concentration, an increase in ValV, (while maintaining the level of organic substrate stoichiometrically equivalent to the phenolate ion) will increase the concentration of organic-phase reactant. Thus,since the reaction rate is nearly fmborder with respect to the concentration of organic substrate, the rate should increase for an increase of Val V, as the concentration of the active catalyst ion in all cases was maintained at 1 molar equilvalent with respect to the concentration of organic substrate. The conversion profile obtained at Val V, of 1:l and 2:l cannot be distinguished quite clearly, though at a ratio of 31, the rate of conversion above 65% appears to be reduced significantly and such an observation cannot be easily explained. A phase volume ratio of 4:l gives distinctly poor conversion. In triphase catalysis, an environment under which the catalyst can shuttle between the two liquid phases can perhaps be maintained at a threshold level of dispersion which appears to depend on both stirring speed and phase volume ratio between the two phases. It is essential that the catalyst particle should behave as the dispersed phase with respect to both liquid phases. Even though benzoyl chloride has negligible solubility in water and in the two-phase system, toluene provides a relatively high distribution coefficient favoring the organic phase. At a Va/V, ratio of 1:1, some benzoyl chloride was found to be lost in the aqueous alkaline phase as sodium benzoate. This loss could be suppressed (Pahari and Sharma,1991) only by decreasing the Val V, ratio at least to a value of 1:2. We observed that, even by prolonging the reaction for 24 h, conversion more than 90% was not achieved. This may be due to irreversible loss of benzoyl chloride as a result of hydrolysis. At higher stirring speed, when Val V, is increased, there will be a subsequent increase in the liquid-liquid interfacial area, which will favor hydrolysis rather than triphase reaction. This will result in the reduced rate of reaction by reducing the effective concentration of benzoyl chloride in the organic phase. Similar observations on lowering of overall rate at higher Va/V, ratio were made in the case of homogeneously catalyzed reaction also (Dutta et al., 1992). At higher ratio of ValV,, increase in stirring speed will reduce the organic
droplet size (Tavlarides, 1981) below the catalyst particle size used,rendering poor dispersion of the catalyst particle in the organic phase. While studying reactions of 1bromooctane with aqueous sodium cyanide at a Val V, of 1.5:1, Tomoi and Ford (1981) found that a mechanically agitated reactor a t 600 rpm stirring speed gave a faster reaction rate than a turbulent vibromixer even though the later device generated an organic-in-water dispersion of droplet size as small as 0.05 nm. Further, they found that the dependence of reaction rate on stirring speed decreased as the particle size decreased. On the basis of the above argument and as a special case of reaction that has been studied here, decrease in reaction rate at higher ratios of Val V, seems quite reasonable. It may be appropriate to select a ratio of 2:l for a practical processing scheme. Eff&t of Particle Size. The copolymer beads did not exhibit a wide size distribution, and it was possible to collect only two size fractions (by sieving) in appreciable proportions. A set of data were collected for finely ground particles of size below 45 pm. Figure 3 shows the effect of particle size on conversion profile. It is to be believed that, under otherwise identical reaction and hydrodynamic conditions of the reactor, intraparticle diffusional limitation is lowered for smaller size particles. Therefore, observation of marginally high conversion in the case of 65100-pm particle size is quite reasonable but statistically not very different. For nucleophilic substitution reactions conducted with particle size below 100 pm, the masstransfer coefficient in the external boundary layer becomes insensitive to increase in stirring speed (Marconi and Ford, 1983). The role of increased stirring speed in enhancing mass transfer from the dispersed phase to the particle surface is more pronounced in the case of particle size above 100 Nm. Thus, in our study, it is likely that external mass-transfer resistance in the case of reaction with 601Wpm particle size will play a role in lowering the reaction rate even though the stirring speed was reasonably high. Finely ground particles (C45 pm) give markedly low conversion. Such small particles suspend in the organic medium whereas particles above 65 pm settle to the interface (Tomoi and Ford, 1981). Observed low conversion may be solely attributed to insufficient contact of the particle with the aqueous phase and loss of benzoyl chloride to the aqueous phase. In this case also, mass transfer from the bulk phase to the particle surface becomes drastically low and reaction rate is reduced, yet the rate is better than that achieved in uncatalyzed reaction as evident from Figure 1.
( V,/V,,).Since a high ratio of
*
2730 Ind. Eng. Chem. Res., Vol. 31, No. 12, 1992
On the basis of the interpretation presented in the preceding sections, the mechanism of the reaction in the triphase system under ideal condition may be considered akin to that in a two phase system. The reaction occurs in the organic solvation shell rather than in aqueous phase. The mechanism proposed for triphase reaction is
- 4'5 - 1.0 -3'5
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;
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-
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Time, m,n
Effect of Catalyst Loading. Effect of catalyst loading
has also been studied in order to get some b i g h t into the reaction mechansim. Catalyst loading was based on molar equivalent phosphonium ion with respect to RX. ABshown in Figure 4, catalyst loading has a profound effect on conversion through an enhancement of the observed rate constant varying almost linearly. The rate of reaction is first-order with respect to the catalyst concentration, too. It is expected that an increase in catalyst loading will maximize the amount of aqueous-phase anion bonded to the catalyst cation. A few experiments carried out using a 2-fold increase in ArOH concentration in the aqueous phase under otherwise identical conditions did not show an enhanced conversion profile (data shown in Figure 4 by crossed circles and triangles for 0.3 and 0.5 molar equivalent catalyst loading, respectively), but the results ensure a pseudo-first-order kinetics, dependent only on overall RX concentration. In view of this, it may be inferred that transport of organic substrate may be the rate-limiting step rather than ion exchange. This step may be improved by increasing the RX concentration (but not increasing the ValV, ratio in view of the reasons discussed with reference to the ratio effect) and amount of the catalyst, the effect being realized through improved intrinsic activity as well as elimination of film diffusion resistance. The rate is also likely to be affected by intraparticle diffusion of either RX or the ArO- ion as the particle size used was not the optimized one. The diffusivity of ArOion is expected to be higher than that of RC1. Thus unleas the catalyst amount is increased and the value of Val V, ratio maintained at 521,poor contact of the catalyst with the organic phase leads to the smaller diffusion rate of RCl in the dispersed organic phase than that of ArO-,, in the continuous aqueous phase within the particles. Reaction Mechanism. In a two-phase system using a soluble PTC, the mechanism (Krishnakumar and Sharma, 1984; Dutta et al., 1992) proposed is (o*c6H50-)+ C&&OCI
I C6H50-
+ Na'
+ Na'
CI-
aqueous phase
Figure 4. Effect of catalyst loading on rate of conversion. Reaction mol L-l; conditions: temperature 30 f 0.5 OC; [ArOH] 4.25 X [RCl] stoichiometricto [ArOH]; VJ V, 21; particle size 150-200 pm and NaOH/ArOH of 1.5 (molar). Crossed symbols represent data mol L-'. for [ArOH] 9.5 X
organic phase
C6H50-
-
C~H&OOC6H5
+ o+cI-
I
t ci-
aqueousphase
and the reaction conforms to fast pseudo-first-order regime.
where 0- represents the polystyrene backbone. In fact, the mechanism may be realized through two steps: aqueous phase C6H50Na+ @-P+ c1organic phase C&COC1+
C6H50-P+@-
-
+ NaCl,,
,k
@"""PfC6H50@W??+cl-
+ C&,COOC6H5
where @-P+Cl- and @-P+C6H50- represent the active sites before and after the reaction of ArO-,, with the active center of of the catalyst and k, and k, are the rate constants based on aueous and organic phase reactions, respectively. The overall reaction is C6H50Na+ C6H&OC1
5 C6H5COOC6H5+ NaCl
It may be assumed that the reactants ArON4, and R&undergo substitution reaction with the active sites within the catalyst particle. Such a mechanism was also proposed for the alkylation of 2,4,&tribromophenol (Wang and Yang, 1991). Some Aspects of Catalyst Recycle. Recovered catalyst after reaction was found to contain increased Cl- ion as compared to that present in the fresh catalyst. A rather slow increase of conversion of RC1 after a certain time may be attributed to retention of C1- ion and depletion of the ArO-anion exchange site of the catalyst or to decomposition of phosphonium ion which is likely to occur in the reaction environment particularly when excess ArOH and NaOH are used. By simple washing of the catalyst (water-methanol-water-methanol-watersequence) and repeated use for three times, the activity was found to be retained in the sense that the conversion obtained after 1h of reaction under otherwise identical conditions was almost the same. Thus catalyst recycle appears quite feasible. Conclusion This study shows that phenol can be recovered as phenyl benzoate from dilute alkaline stream via reaction with benzoyl chloride under triphase conditions at 30 O C . A study in a fixed bed reactor that will be desirable for industrial application is in progress. Acknowledgment The authors acknowledge the financial support of the Ministry of Environment and Forest, Government of In& (Grant No. 119/86/89/RE).
I n d . Eng. Chem. Res. 1992,31,2731-2740 Registry NO.PhOH, 108-95-2; PhCOCl, 98-88-43 PhCOzPh,
2731
Solid Reacton. Znd. Eng. Chem. Process Des. Dev. 1986, 25, 878-885.
93-99-2.
Literature Cited Direktor, D.; Wenberger, R. Phase Transfer Catalyaed Eeterification of Phenols with Aliphatic Acid Chlorides. J. Chem. Tech. Biotechnol. 1985, %A, 281-284. Dutta, N. N. P h Transfer Catalysis for Phenolic Waste Water Treatment. Chem. Eng. World 1990,25,79-81. Dutta, N. N.; Borthakur, S.; Patil, G. S. Phase Transfer Catalyzed Extraction of Phenolic Substances from Aqueous Alkaline Stream. Sep. Sci. Technol. 1992,27,1435-1448. Ford, W . T.; Tomoi, M. Polymer Supported Phase Transfer Catal y s t ~Reaction Mechanism. In Advances in Polymer Science: Springer Verlag: Berlin, 1984; Vol. 55, pp 49-104. Ido, T.; Tariki, H.; Sakurai, K.;Goto, S. Intraparticle Diffusion in a Solid Phase-Transfer Catalyst for the Oxidation of Benzyl Alcohol. Znt. Chem. Eng. 1986,26, 105-113. Krishnakumar, V. K.;Sharma,M. M. A Novel Method of Recovering Phenolic Substances from Alkaline Waste Streams. Znd. Eng. Chem. Process Des. Deu. 1984,23,410-413. Marconi, P. F.; Ford, W.T. Catalytic Effectiveness Due to Mass Transfer Limitations in Triphaee Catalysis by Polymer-Supported Quaternary Onium Salta. J. Catal. 1983,83,160-167. Pahari, P. K.;Sharma,M. M. Recovery of Morpholine via Reactive Extraction. Znd. Eng. Chem. Res. 1991,30, 2015-2017. Ragaini, V.; Venella, G.; Ghignone, A.; Colombo, G. Fixed-Bed Reactors for Phase-Transfer Catalysis. A study of a Liquid-Liquid-
Ragaini, V.; Chiellini, E.; DAntone, S.; Colombo, G.;Barzaghi, P. Phenylacetonitrile Alkylation with Different Phase-Transfer Catalysts in Continuous Flow and Batch Rsactors. Znd. Eng. Chem. Res. 1988,27,1382-1387. Regen, S. L. Triphase Catalysis. J. Am. Chem. SOC. 1976, 97, 5956-5957.
Tavlarides, L. L. Modeling and Scale-up of Dispersed Phase Liquid-Liquid Reactors. Chem. Eng. Commun. 1981, 8, 133-164. Tomoi, M.; Ford, W.T. Mechanism of Polymer-Supported Catalyais. 1. Reaction of l-Bromooctane with Aqueous Sodium Cyanide Catalyzed by Polystyrene-Bound Benzyltri-n-butylphoephonium ion. J. Am. Chem. SOC. 1981,103,3821-3828. Tomoi, M.; Kori, N.; Kakiuchi, H. Phase-Transfer Catalytic Activity of Phosphonium Salts Bound to Microporous Polystyrene Reains by Long Spacer Chaina. Makramol. Chem. 1986,187,2753-2761. Vogel, A. I. A Textbook of Quantitatiue Inorganic Analysis Zncluding Elementary Instrumental Analysis, 4th ed.; Longman: London, 1978, pp 34Ck-342. Wang, M. L.; Yang, H. M. A Pseudo-Steady-State Approach for Triphase catalysis in a Batch Reactor. Znd. Ena. - Chem. Res. 1991,30, 2384-2390.
Whitlock, L. R; Siggia, S.; Smola, J. E. Spectrophotometric Analpis of Phenols and of Sulfonates by Formation of an Azo Dve. Anal. Chem. 1972,44,532-535.
Received for review March 30, 1992 Revised manwcript received August 24, 1992 Accepted September 14, 1992
Effects of Low-Amplitude Forced Oscillation on the Rate of Mass Transfer to Circulating Liquid Droplets Timothy C . Scott Chemical Technology Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6224
Aqueous droplets suspended in upflowing 2-othyl-l-hexanol have been forced to undergo low-amplitude shape oscillations by imposition of a pulsed dc electric field. The effects of the imposed oscillation on mass transfer from the continuous phase have been experimentally investigated by measuring the rate of accumulation of a fluorescent dye in droplets. The rates of transport to droplets in each of three oscillation conditions (O%, 5 % and 10% amplitude) were observed at four continuous-phase concentrations in the range of 50-500 ppb of the fluorescent dye. The effects appear to be more pronounced a t the lower concentrations, with the 10% and 5% oscillations producing 44% and 9.5% increases in mass flux, respectively, at a continuous-phase concentration of 50 ppb. All three curves are virtually parallel until the upper limit of the concentration range is approached, a t which point the oscillating drop rates appear to be upwardly diverging from the base-case data.
Introduction Solvent extraction is one of the basic separation schemes employed in the chemical process industries. The driving force for separations is the preference of chemical species to interact with one liquid phase over another. Therefore, separationsare accomplished through selective partitioning of components into immiscible liquids. In order to obtain satisfactory mass-transfer rates, energy must be put into the system to disperse one phase in the other, thereby forming interfacial contact area and promoting interfacial and intrafacial convection. Thus, the system can be characterized by understanding the interactions that occur between droplets of a dispersed phase with the surrounding continuous liquid medium. In order to discern potential effects of various treatments on solvent extraction operations, it is convenient to examine a simple, well-defined model system that is characteristic of the conditions present in the real process. For solvent extraction, a logical approach is to study the be-
havior of a single droplet surrounded by a continuous liquid phase. In this simple system, mass-transfer rates depend upon both the physicochemical properties of the transferring species and the solvents, as well as on the hydrodynamic state of the system. One could attempt to improve the performance of such systems by two approaches: (1) increasing the chemical driving force for separation or (2) altering or enhancing the mode of energy input. The first approach involves identification or synthesis of new solvent systems or additives that will increase the relative affinity of desired species for one of the liquid phases. In the second, one would focus on the effective use of energy inputs into the system to create surface area and promote convection. In this paper, the second type of approach is investigated. Transient electric fields are used to alter velocity profiles within and around liquid droplets with the intent of increasing the rate of mass transfer through enhanced convection.
~ s ~ a - ~ s a ~ ~ ~ ~ ~ ~ s0~1992 i - American ~ ~ ~ 1Chemical $ 0 ~ . Society 00/0
