Ind. Eng. Chem. Res. 1992,31, 1709-1717 Marcus, Y.; Kertes, A. S. Extraction by Ion-Pairs Formation. In Zon Exchange and Solvent Extraction of Metal Complexes; Marcus, Y., Kertes, A. S., Eds.; Wiley Interscience: New York, 1969;pp 737-814. Scibona, G.; Danesi, P. R.; Conte, A.; Shuppa, B. Interfacial Equilibria with Quaternary Alkylammonium Salts. J. Colloid Interface Sci. 1971,35,631-635.
1709
Shafer, H. M.; Holland, A. J. US.Patent 2 709 669,Merck and Co., 1955. Vandegrift, G. F.; McCarty Lewey, S.; Dyrkacz, G. R.; Horwitz, E. P.J . Znorg. Nucl. Chem. 1980,42,127-130.
Received f o r review February 18, 1992 Accepted March 4,1992
Membrane Solvent Extraction Removal of Priority Organic Pollutants from Aqueous Waste Streams Chang H. Yun, €&vi Prasad,?and Kamalesh K. Sirkar* Department of Chemistry and Chemical Engineering, Stevens Institute of Technology, Castle Point on Hudson, Hoboken, New Jersey 07030
A recently developed nondispersive microporous membrane-based solvent extraction technique has been used to remove a number of priority organic pollutants simultaneously from a synthetic high strength aqueous waste stream. A microporous hydrophobic hollow fiber based membrane extractor having an order of magnitude higher contact area than conventional extraction devices has been used. The pollutants were phenol, 2-chlorophenol, nitrobenzene, toluene, and acrylonitrile. The extracting solvents were methyl isobutyl ketone (MIBK), isopropyl acetate (PAC),and hexane. The distribution coefficient of each pollutant has been measured for each solvent over a wide concentration range. In the once-through extracting mode, the concentrations of all pollutants in the aqueous phase flowing through the hollow fiber lumen were reduced to less than 20 mg/L using either MIBK or IPAc as solvent flowing countercurrently on the hollow fiber module shell-side. A lumped masstransfer analysis has been made to characterize the observed mass transfer for all pollutants. This technique was shown to be efficient in cleaning high strength wastewaters containing pollutants, which may be polar, nonpolar, high boiling, low boiling, aromatic, aliphatic, etc.
Introduction A common commercial method of recovering phenol from concentrated waste streams is extraction with an immiscible organic solvent (Cusack et al., 1991; Patterson, 1985). The phenol concentration in such treated effluents is low enough for biological treatment to take over without upsets from fluctuations in toxic species loading. A question of interest is could we remove priority organic pollutants, in general, successfully by solvent extraction? Equilibrium distribution studies of priority polar organic pollutants, e.g., 2-chlorophenol, nitrobenzene, isophorone, acrylonitrile, acrolein, and N-nitrosodimethylaniline,between water and various organic solventa indicate that, indeed, there exists considerable potential for solvent extraction (Joshi et al., 1984). Recovery of pollutanta thus extracted and solvent recycle can be achieved in a number of ways including distillation. Conceptual designs and economic analyses of such solvent extraction processes for removal and recovery of priority polar organic pollutants, e.g., acrylonitrile, etc., indicate a cost in the range of $4-10/10oO gal of water (Joshi et al., 1984). Compare this with the orders of magnitude higher cost of incineration: high BTU waste costa $1-2/gal whereas low BTU waste costs $7-8/gal (Magee, 1988). Conventional solvent extraction employs dispersion of one phase as drops in another phase and subsequent coalescence of dispersed phase and phase separation. This mode of operation frequently leads to solvent loss by *To whom correspondence should be addressed. Current address: Department of Chemical Engineering, Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, N J 07102. Current address: Separations Products Division, Hoechst Celanese, Charlotte, NC 28273.
emulsion formation. This is detrimental to the process economics. For example, the loss of costly extractants or chelating agents in heavy metal removal poses a signifcant economic minus. Such solvent loss may also unduly increase the organic loading of the treated stream requiring additional cleanup. Emulsion formation in conventional dispersion-based solvent extraction is thus a major shortcoming. In conventional solvent extraction, a given solvent extraction column can operate efficiently within a small flow range; larger flow variations lead to flooding (Treybal, 1963). Thus, fluctuations in waste stream rate can load or flood the column. Further, there has to be a density difference between the aqueous and the organic stream. Moreover, particulates in an aqueous waste stream are a significant problem in such columns. We have recently developed a dispersion-free solvent extraction technique which eliminates all such problems (Kiani et al., 1984; Frank and Sirkar, 1985; Prasad et al., 1986; Frank and Sirkar,1986, Prasad and Sirkar,1987a,b). In this technique (Figure la) the aqueoumxganic interface is immobilized in the pores of highly open microporous polymeric membranes as the two phases flow on two sides of the membrane. The membrane pores are filled with the phase, preferentially wetting it, while the other immiscible phase is completely excluded. Solvent extraction is easily achieved by transfer of solutes through the aqueous-organic interfaces immobilized at the pore mouths due to a pressure difference between the two phases. There is no coalescence problem since there is no dispersion. Each phase can have any flow rate or any density. Fermentation broths having suspended cells have been easily handled (Frank and Sirkar, 1985,19861, suggesting little problem with fouling streams. However, certain precautions or pretreatments may be necessary with particulates in
0888-588519212631-1709$03.00/00 1992 American Chemical Society
1710 Ind. Eng. Chem. Res., Vol. 31, No. 7, 1992
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determine the solute distribution coefficients in respective solvents to be used for extraction studies. Earlier studies (King et al., 1984) on solvent extraction had recommended methyl isobutyl ketone (MIBK) as a solvent for extraction of phenol and 2-chlorophenol, while the solvent for nitrobenzene was identified as toluene. It was decided that the distribution coefficients of the above solutes between water and the respective solvents would be studied. Data on the variation of the distribution coefficients with concentration of these pollutants in aqueous phase are needed for an accurate scaleup of hollow fiber modular devices for process scale applications. Since toluene was being used as a pollutant in the aqueous stream, it was decided to study some other solvents for the removal of priority pollutants. The solvent to be used for extraction of the organic pollutants should have the following properties: (1)high distribution coefficient, mi,of the organic pollutants in the organic phase; (2) low toxicity of the solvent to downstream biological treatment facilities; (3) easy strippability from the water phase; (4) low solubility in water; (5) low cost; and (6) easy recoverability from the extract containing the pollutants for recycle. Here, the distribution coefficient is defined as m i=
well-packed hollow fiber solvent extraction devices (Prasad and Sirkar, 1989,1992). Hydrophobic, hydrophilic, and composite microporous flat membranes and hydrophobic and hydrophilic hollow fibers have been utilized in different studies. Variations of this technique where a gel is used in the pores of the membrane to eliminate the need of a pressure difference between the two phases have also been explored by Ding and Cussler (1991). The overall solute extraction rate per unit equipment volume in such a technique can be very high when hollow fiber modules are used (Prasad and Sirkar, 1988). Our recent experiments indicate 99% plus recovery of a solute (in the range of 60-200 Da) from a dilute feed (-10000 ppm) in 15-30-cm-length, 1.9-cm-diameter hollow fiber modules (Prasad and Sirkar, 1990). A comparable separation is achieved in industry using a 18.3-m-heightpacked column. It is natural to explore this membrane solvent extraction technique for its capability of simultaneous removal of a number of priority organic pollutants (e.g., 2-chlorophenol, nitrobenzene, phenol, acrylonitrile, toluene) from simulated high strength wastewaters using a hollow fiber module. The objective is to study the simultaneous removal of whole classes of organic pollutants, whether they are polar, nonpolar, high boiling, low boiling, etc. In exchange, the solvent is introduced in the treated water. The general expectation would be that a high degree of toxic solute removal from untreated high strength industrial wastewater will be achieved in an efficient fashion without any solvent loss by dispersion. A further objective is to see whether the mass-transfer coefficients for the solutes can be easily characterized or even predicted. The organic pollutanta selected and their aqueous-phase concentrations are phenol (500 mg/L), 2-chlorophenol (1241 mg/L), nitrobenzene (604 mg/L), toluene (216.5 mg/L), and acrylonitrile (3627 mg/L). The solvents studied were methyl isobutyl ketone (MIBK), isopropyl acetate (IPAc), and hexane. Preliminary Considerations on Solvent Selection Solvent extraction of solutes from aqueous waste streams required initial experiments in batch extraction mode to
concentration of solute i in the organic phase concentration of solute i in the aqueous phase
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A particular solvent will not have all of the desired properties for all the organic pollutants. The solvent to be used should be based on optimization of the above criteria. On the basis of our preliminary studies and literature survey, the list of solvents for this work was narrowed down to methyl isobutyl ketone (MIBK), hexane, and isopropyl acetate (PAC)as the extracting solvents to provide a wide perspective. Experimental Section The priority pollutants were obtained from the following sources: phenol (99+%), Aldrich Chemical Co., Inc., Milwaukee, WI; 2-chlorophenol (99+ %), Aldrich; nitrobenzene (99+ %), Aldrich; toluene (ACS Grade), Fisher Scientific Co., Springfield, NJ; acrylonitrile (99+%), Aldrich. The extracting solvents were obtained from the following sources: MIBK (ACS Grade), Fisher; IPAc (99+%), Aldrich; hexane (95+%, HPLC Grade), Aldrich. The following concentrations of the various organic pollutants, in the aqueous feed stream, were used in the hollow fiber extraction studies: (1)phenol, 500 mg/L; (2) 2-chlorophenol, 1241 mg/L; (3) nitrobenzene, 604 mg/L; (4) toluene, 216.5 mg/L; and (5) acrylonitrile, 3627 mg/L. Since low concentrations ( 2-20 mg/L) of these organic pollutants can be present in the aqueous and solvent phases, it was decided to use a high performance liquid chromatograph (HPLC) for analyses of each phase due to the ease of operation and excellent reproducibility of results. The actual equipment used was a HP-1090 HPLC equipped with a UV filter photometric detector, a C-18 reverse-phase hypersil ODS column (Chrompack Inc., Bridgewater, NJ), an autosampler from Micromeritics Instrument Corp., Norcross, GA (Model 728), and a HP 3390A integrator. It was possible to calibrate the above equipment within accuracies of f4% (relative error) for the water phase when the following conditions were used 210 nm in the W fdter photometric detector, mobilephase
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Ind. Eng. Chem. Res., Vol. 31, No. 7,1992 1711 Table I. Hollow Fiber Module Used SDecification of Fiber hollow fiber type CELGARD X-20 hydrophobic polypropylene material of hollow fiber fiber inner diameter 240 pm fiber outer diameter 290 pm 0.03 rm pore size porosity 40 70 Specification of Module length of module 15.0 cm 1.9 cm diameter of module 1800 no. of fibers area/unit volume 47.5 cm-I packing density 0.43
flow rate for all solutes except toluene, 0.3 mL/min of 24% acetonitrile and 76% water; for toluene the mobile phase, 40% acetonitrile, rest water, the flow rate being 0.4 mL/min or 60% acetonitrile, rest water, the flow rate being 0.5 mL/min. The organic-phase concentration was obtained by a mass balance. Calibrations to measure the aqueous-phase solvent concentration exiting the hollow fiber module were also made for solvents actually used (e.g., MIBK);the conditions were similar to those for four solutes except 280 nm in the UV filter. Prior to hollow fiber extraction studies, the solute distribution coefficients over a wide solute concentration range were determined. The distribution coefficient measurement consisted first of contacting measured and equal volumes (usually 50 mL) of pure organic phase with aqueous solutions of solutes of predetermined strength (for the case of high concentration of solute in aqueous phase after equilibrium, pure water and organic phase containing solute were utilized before contacting and stirring) in a 200-mL Erlenmeyer glass stoppered flask. The flask contents were vigorously stirred on a magnetic stirring table for 12 h, after which the phases were separated in a 125-mL separating funnel. Usually 2 h were sufficient for phase separation, but longer time was needed for high solute concentrations to break the emulsion formed during stirring. After collection of each phase (the emulsion phase was excluded when it was formed), the aqueous phase was analyzed by HPLC for solute concentration. To check the mass balance, the organic phase was sometimes analyzed by HPLC after being diluted with acetonitrile, which was a component of the mobile phase of HPLC. The hollow fiber module used in this study was a 6in.-length test module made by Hoechst Celanese Separations Products Division, Charlotte, NC; it is shown schematically in Figure lb. The module contained hydrophobic microporous polypropylene hollow fibers with epoxy tube sheets in a nylon casing. Further details are given in Table I. The experiments were done with the aqueous feed solution containing organic pollutants flowing on the tube-side while the extracting solvent flowed countercurrently on the shell-side (Figure IC).The experiments were done in the once-through mode. The concentrations of the pollutants in the feed and exit aqueous streams were determined using the methods outlined earlier. The aqueous phase was maintained at a pressure 5 psig higher than the organic-phase pressure (10 psig for aqueous phase, 5 psig for organic phase); the organic phase wetted the membrane pores. In the limit of the low flow rates used here, there was essentially no pressure drop recorded on either the tube- or the shellsides of the extraction module. Figure 2 provides a schematic of the hollow fiber extraction setup. From the extraction experiments in a hollow fiber module, the overall aqueous phase-based mass-transfer
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Experimental Results and Discussion The values of the distribution coefficient of different solutes in different solvents are presented first. This is followed by the results of solvent extraction using the
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microporous hollow fiber module mentioned earlier. Figures 3 and 4 show respectively the distribution coefficients of phenol and 2-chlorophenol between water and the various solvents as a function of the aqueous-phase concentration of the pollutant. It can be seen that, in the low concentration range of the organic pollutant, the distribution coefficient is fairly high except for hexane as a solvent, suggesting that solvent extraction is indeed a viable option for pollutant removal. It should be noted here that, for the purpose of this work, the distribution coefficient is always defined as the ratio of the solute concentration in the organic phase to the solute concentration in the aqueous phase (eq 1). Figure 5 shows the distribution coefficients of nitrobenzene between water and various solvents as a function of the aqueous-phase solute concentration. It is essential to know this behavior since the objective is to reduce the aqueous-phase concentration of the pollutants to around and below the 10 mg/L level, and it is necessary to know the distribution coefficient at low concentration levels for eventual scaleup of the process. The distribution coefficients for acrylonitrile are shown in Figure 6. In the h e m e w a t e r system of Figure 6, the data show that the distribution coefficient of acrylonitrile depends substantially on its aqueous-phase concentration within the required range of aqueous-phase concentrations. So, it is inappropriate to use an average value. Recognize now that the distribution coefficient of toluene in most of the solvents used here is very high. As a result, in batch extraction studies carried out by initially preparing the aqueous phase containing toluene up to 433 mg/L (solubility of toluene for water at 16 OC is 500 ppm;
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Perry and Chilton, 1973) and stirring it with the same volume of pure solvent, the residual concentration of toluene in the aqueous phase (
