Ind. Eng. Chem. Res. 1997, 36, 4369-4373
4369
Removal of Phenols from Wastewater Using Liquid Membranes in a Microporous Hollow-Fiber-Membrane Extractor A. Nanoti, S. K. Ganguly, A. N. Goswami,* and B. S. Rawat Indian Institute of Petroleum, Dehradun 248005, India
This paper reports experimental data on the removal of phenol from aqueous solutions using emulsion liquid membranes in a microporous hollow-fiber extractor. The hollow-fiber extractor appears to offer significant advantages over conventional liquid-liquid contactors for this separation because emulsion leakage and swell are practically eliminated even when treating high phenolic feeds. The overall mass-transfer coefficients are seen to be more strongly dependent on the phase flow rates among the parameters studied. The experimental mass-transfer coefficients have been predicted by a resistance-in-series model. Introduction Phenols occur as toxic contaminants in effluent waters from industries such as oil refining, coke and coal processing, phenolic resin manufacture, and several other chemical and metallurgical operations. While typical concentrations of phenolics in oily and process waters and caustic wash effluents range from 10 to 200 ppm, in some wastewaters from phenol-producing plants, phenol concentrations may exceed 2000 ppm. Due to its high toxicity, environmental regulations are particularly strict for phenol, and specifications limit its concentration in treated effluents to less than 1 ppm. The removal of phenols from wastewaters is, therefore, an important separation problem, and traditionally, several separation techniques like solvent extraction and carbon adsorption as well as biological treatment procedures have been used in industry to handle this task. Although solvent extraction has several features that favor its applicability for this purpose, one of its disadvantages is the appreciable solubility of the solvent in water which results in high solvent losses and can also be a source of pollution (Mackay and Medir, 1983). Carbon adsorption, though a very effective treatment procedure, is energy-intensive especially with regard to the regeneration of the spent carbon. Biological treatment procedures require large land area, have attendant problems of sludge disposal, and are prone to failure from shock loadings. The novel separation process based on emulsion liquid membranes (ELMs) has emerged as a versatile technique with proven potential in several fields. Several literature reports have appeared concerning the removal of phenols from wastewater (Cahn and Li, 1974; Kataoka et al., 1989) using ELMs. Pilot-plant studies have demonstrated the cost effectiveness of the process, and an industrial plant is reported to be in operation in China (Xiujuan et al., 1988). One of the advantages of the ELM process is the extraction and stripping is carried out in a single stage, thus reducing the extractant inventory and removing system equilibrium constraints. However, ELM extraction of phenols does have some problems with respect to leakage and swelling of the internal phase of the emulsion. These effects may, on occasion, severely limit the efficiency of the process. A new concept of ELM contacting in a microporous hollow-fiber contactor has recently been reported (Raghuraman and Wiencek, 1993) for the separation of * Author for correspondence. S0888-5885(97)00003-1 CCC: $14.00
copper from wastewater . In this method, a water-inoil emulsion containing 0.1 N sulphuric acid as the internal “stripping” phase flows through the lumen of the hollow fiber, while wastewater containing copper ions flows on the shell side of the contactor. This offers the advantage that internal-phase leakage and swell are minimized because of the absence of high shear rates normally encountered in conventional mixers using ELM operations; as in a hollow-fiber contactor, the contacting of phases is nondispersive (Prasad and Sirkar, 1988). Another factor is that the small pore diameters of the micropores in the hollow-fiber wall prevent the presence of internal phase drops in the pores so that direct contact between the internal and external water phase is avoided. Earlier studies (Goswami et al., 1992) on ELM phenol removal using conventional mixers/columns in our laboratory indicated that leakage and swell increased significantly when phenol concentrations exceeded 1000 ppm. The ELM route does not appear to be suitable for treating wastewater with phenol concentrations exceeding 1000 ppm. Such wastewaters are typical effluents from organic chemical industries (e.g. phenolproducing plants) or can result from plant upsets. In the present paper, we report the experimental findings of an investigation on phenol removal from water. Aqueous phenol (3500-5000 ppm) is extracted in a hollow-fiber liquid-membrane extractor consisting of 3600 microporous polypropylene fibers. A caustic emulsion (phase ratio 0.5) of water in oil is pumped through the tube side (lumen) of the fibers, and the oil wets the fibers filling the wall pores. Aqueous phenol passes through the shell side. As phenol is extracted into the oil, it diffuses through the oil-filled pores, transferring to the caustic phase in the emulsion where it reacts to form sodium phenate. Thus, both extraction and stripping are taking place in the shell-and-tube membrane unit. A mass-transfer resistance-in-series model has been used to predict the experimental mass-transfer coefficients. Model Development The concentration profiles of permeating species in an ELM separation using microporous hollow-fiber contactors are depicted in Figure 1. Here the emulsion is in the fiber lumen, while the aqueous feed phase is in the shell side. The mass transfer thus encounters three-diffusional resistances, a film-transfer resistance in the shell fluid, a membrane-transfer resistance during transfer through the oil phase in the pores of © 1997 American Chemical Society
4370 Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 Table 1. Characteristics of the Hollow-Fiber Module fiber wall thickness, µm effective pore size, µm porosity, % effective fiber length, cm fiber internal diameter, µm no. of fibers effective surface area, m2 effective area/vol, cm2/cm3
30 0.05 30 16 240 3600 0.4 65
These same authors show that if unhindered diffusion is assumed in the solvent-filled pores, then the masstransfer coefficient can be written as
km ) θDm/tτ
(8)
Experimental Section
Figure 1. Concentration profile of phenol in a hollow-fiber extractor.
the membrane wall, and a tube-side resistance during transfer through the tube fluid. The mass-transfer rate for each of these transfers is given as
rate of transfer in the shell side ksπd0L(CWB - CWi)
(1)
rate of transfer in the membrane pore kmπdlmL(Cm,1 - Cm,2)
(2)
rate of transfer in the tube side ktπdiL(Cm,2 - CE)
(3)
The partition coefficient of the permeating species between the organic and aqueous phases is defined as
m ) Cm,1/CWi
(4)
From eqs 1-4, the expression for overall mass-transfer coefficient, Kow, can be written as
Kow )
(
de de 1 + + ks dlmkmm dtktm
)
-1
(5)
The overall mass-transfer coefficient can be calculated from the individual transfer coefficients ks, km, and kt. The tube and shell side mass-transfer coefficients are known to depend on the flow conditions in the fiber lumen and shell fluid, respectively, and correlations are available in the literature expressing these dependencies. Thus, Dahuron and Cussler (1988) give the following correlation for the shell-side mass-transfer coefficient:
( )( )( )
ks ) 8.8
Ds de2VsFs ηs de Lηs FsDs
1/3
(6)
Again, for the tube-side mass-transfer coefficient, Prasad and Sirkar (1988) give the following correlation:
( )[( )( )( )]
kt ) 1.64
Dt di
ELM phenol removal experiments have been carried out in a hollow-fiber contactor (Liquicel Model 5-PCM100) supplied by M/s Hoechst Celanese, Charlotte, NC. This contactor consists of 3600 microporous polypropylene hollow fibers arranged in a shell-and-tube configuration. Details of the contactor are given in Table 1, and the experimental setup is depicted in Figure 2. All experiments were run by pumping the water-in-oil caustic emulsion from reservoir R1 under pressure control through needle valve NV2, while the aqueous feed containing phenol is countercurrently pumped on the shell side under pressure control through needle valve NV1. The oil phase in the W/O emulsion will readily wet the pores and directly contact the aqueous phase. By applying a higher pressure on the aqueous phase, the interface between the oil and the aqueous phases is stabilized at the pore entrance on the aqueous side. Emulsion and aqueous phase flow rates were monitored by flowmeters FM attached to respective lines as shown in Figure 2. In all experiments, the fluidphase pressure differential between the emulsion and aqueous phases was maintained at 0.34 kg/cm2. The phenolic content in the treated water was determined by a colorimetric method (API 716-57) based on 4-aminoantipyrine. The method has been found to measure phenol concentrations within precision limits of (4%. The W/O emulsion was prepared by high-speed agitation of an aqueous phase containing NaOH with an oil phase in a baffled mixer shown in Figure 3. The agitation speed was 4000 rpm and was continued for 20 min. The mixer was of i.d. 7.8 cm, and a turbine impeller of diameter 2.6 cm was used for agitation. The oil or membrane phase consists of an oil-soluble nonionic surfactant Polysorgen SMO 80 supplied by M/s Dai-Ichi Karkaria Ltd., Bombay, dissolved in a straight-run kerosene fraction. The leakage of the internal phase of the emulsion into the feed phase was measured by determining the electrical conductivity of the aqueous feed phase, while the emulsion swell was measured by determining the change in the volume of the emulsion after contacting.
di divtFt ηt L ηt FtDt
1/3
(7)
Results and Discussion In Table 2, we report the experimental data of the phenol concentrations in treated water for a typical run, obtained upon operation of the hollow-fiber extractor over an extended period of time of up to 5 h. The data indicate that within the range of experimental observation, the membrane unit shows stable operation. Steady
Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4371
Figure 2. Schematic diagram of a hollow-fiber extractor. Table 2. Experimental Data of the Phenol Concentrations in Treated Water phenol in feed, ppm flow rate of aq feed, mL/min flow rate of emulsion, mL/min emulsion internal phase ratio normality NaOH, N surfactant concn, wt %
3500 10 10 0.5 0.75 5 (in kerosene)
run time, min
treated phenol concn, ppm
run time, min
treated phenol concn, ppm
15 30 60 90 120 150
1700 1050 1100 1125 1125 1100
180 210 240 270 300
1100 1100 1046 1100 1110
tionship (Papedopolous and Sirkar, 1993) out Qf(Cin W - CW ) Kow ) πd0LN∆Clm
∆Clm )
out out in [(Cin W - CE ) - (CW - CE )]
ln
Figure 3. Diagram of the batch mixer.
state is approached in about 30 min, and in all experiments reported in this paper, the unit was operated for 1 h before samples of treated water were collected for analysis. The principal parameters examined in the ELM extraction of phenol in the hollow-fiber contactor are the flow rates of the emulsion and aqueous phases, the phase volume ratio of the internal phase in the emulsion (defined as volume dispersed phase in emulsion/total volume of emulsion), and the concentration of the NaOH in the internal phase. The results have been analyzed in terms of an overall mass-transfer coefficient which has been calculated as per the rela-
[
]
out Cin W - CE in Cout W - CE
(9)
(10)
Here, due to the instantaneous reaction of phenol in the stripping emulsion phase, CE ) 0, so
∆Clm )
( ( )) out Cin W - CW
ln
Hence, we have
(11)
Cout W
( ( )) ln
Kow ) Qf
Cin W
Cin W
Cout W πd0LN
(12)
The model equations (5)-(8) have been used to calculate the overall mass-transfer coefficients under each ex-
4372 Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 Table 3. Physical Conditionsa symbol m Dm Ds ηs ηt Fs Ft
property
value
partition coeff pore liquid diffusivity, m2/s shell fluid diffusivity, m2/s shell fluid viscosity, kg/(ms) tube fluid viscosity, kg/(ms) shell fluid density, kg/m3 tube fluid density, kg/m3
0.98 2.83 × 10-9 1.69 × 10-9 5.80 × 10-4 1.50 × 10-2 995.0 900.0
a Shell fluid: phenolic feed (aqueous). Tube fluid: emulsion (W/O).
Figure 5. Effect of the emulsion flow rate on the overall masstransfer coefficient.
Figure 4. Effect of internal-phase ratio in the emulsion on the overall mass-transfer coefficient.
perimental condition. The physical properties required for these calculations are given in Table 3. While calculating the tube-side mass-transfer coefficient, the diffusivity of phenol in the emulsion phase was calculated by the Jefferson-Witzett-Sibbett model equations (Jefferson et al., 1958). Here the emulsion, which is of composite nature, is modeled as an assembly of spheres of internal aqueous phase in cubic elements of the oil phase. The individual phase diffusivities required were calculated by the Wilke-Chang equation (Wilke and Chang, 1955). In Figure 4, we show our findings on the variation of the mass-transfer coefficient with the phase ratio of the internal phase in the emulsion along with the modelpredicted values. As can be seen, the mass-transfer coefficient remains practically constant, and this is predicted by the model, also. We would expect that the increase in the phase ratio would affect the diffusion coefficient of phenol in the emulsion phase as found in typical stirred tank experiments, but apparently this factor in not critical in the hollow-fiber contactor. A somewhat stronger influence on the mass-transfer coefficient is seen when the emulsion flow rate is increased, as shown in Figure 5. The Kow value increases from 4.33 × 10-7 to 5.97 × 10-7 m/s, suggesting that hydrodynamic conditions in the fiber lumen could be important, though not to a significant extent. This variation is predicted by the model, also. Varying the flow rate of the shell-side fluid (aqueous feed) affects the masstransfer coefficient to a much larger extent, with Kow increasing from 1.9 × 10-7 to 5.02 × 10-7 m/s as shown by the results in Figure 6. This points to the importance of resistances in the shell side in this contactor. The model predicts this trend, too. It is to be noted that due to inhomogeneities in the actual flow patterns in the fiber bundle on the shell side, the equation for ks could be very dependent on the
Figure 6. Variation of the overall mass-transfer coefficient with a flow rate of aqueous feed.
Figure 7. Variation of the overall mass-transfer coefficient with normality of the internal phase.
module, which could be the reason for the small discrepancies noted between model and experiment. A slight decrease in the mass-transfer coefficient is observed when the NaOH concentration is increased beyond 1 N, while the model predicts a constant value as shown in Figure 7. The emulsions prepared with alkali-metal concentrations exceeding 1 N showed creaming in the line leading to the contactor. It is possible that such emulsions entering the contactor are leaner in internal-phase content, leading to poorer extraction. In all experiments, essentially no leakage or emulsion swell was observed. The overall mass-transfer coefficients obtained in this study are in the range of (2-6) × 10-7 m/s. It may be
Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4373
noted that Urtiaga et al. (1992) studied the extraction of phenol from water using a supported liquid membrane with kerosene as the membrane phase. They report mass-transfer coefficients in the range of (4-6) × 10-7 m/s. Furthermore, Sengupta et al. (1988) also studied phenol extraction by hollow-fiber-containing liquid membranes. Their membrane phase was decanol or MIBK. With NaOH as the strip phase, they report high mass-transfer coefficients (around 2 × 10-6 m/s). It is to be pointed out that the distribution coefficient of phenol is higher in these membrane phases compared to kerosene as used in the present study. The combination of ELMs with hollow-fiber membrane extraction for phenol removal as studied in this paper scores a number of points over conventional ELM treatment. Thus, while it retains the advantages of ELMs in providing a large surface area for mass transfer and extraction and stripping in one stage, it circumvents the difficulties associated with ELMs. This method allows treatment of high phenolic feeds and minimizes emulsion leakage and emulsion swell. It may be mentioned that the inability to tackle emulsion leakage and swell accounts in large part for the slow pace of commercialization of ELM technology. It is expected that with the combination of ELMs with hollow-fiber extractors, commercialization prospects for this novel separation technique would become brighter. Conclusions The removal of phenol from wastewater has been studied using emulsion liquid membranes in a microporous hollow-fiber contactor. At the high phenolic concentrations studied (3500-5000 ppm), the typical problems of emulsion leakage and swell encountered in conventional mixers/columns used in ELM treatments have not been observed in our experiments with the hollow-fiber contactor. The experimental mass-transfer coefficients have shown a stronger dependence on hydrodynamic conditions in the fluid phase external to the hollow fiber among the parameters studied, and variations of the mass-transfer coefficients with experimental conditions can be predicted with fair accuracy by a mass-transfer model. The hollow-fiber contactor appears to have a promising potential for applications involving ELMs in environments where leakage/swell effects can become problematic. Nomenclature CWB ) bulk concentration of phenol in water, mol/m3 Cw ) concentration of phenol in water, mol/m3 CWi ) concentration of phenol in water at the water-oil interface, mol/m3 Cm ) concentration of phenol in membrane (pore) fluid, mol/m3 CE ) concentration of phenol in emulsion, mol/m3 ∆Clm ) log-mean concentration gradient, mol/m3 di ) internal hollow-fiber diameter, m d0 ) external hollow-fiber diameter, m de ) shell hydraulic diameter, m dlm ) log-mean diameter of hollow fiber, m Ds ) diffusivity of phenol in shell fluid, m2/s Dt ) diffusivity of phenol in tube fluid, m2/s Dm ) diffusivity of phenol in membrane (pore) fluid, m2/s ks ) shell-side mass-transfer coefficient, m/s kt ) tube-side mass-transfer coefficient, m/s km ) membrane (pore) mass-transfer coefficient, m/s Kow ) overall mass-transfer coefficient, m/s L ) length of hollow fiber, m
m ) distribution coefficient of phenol between organic and aqueous phases N ) number of hollow fibers in contactor module Qf ) flow rate of shell fluid, m3/s t ) thickness of hollow-fiber wall, m Vs ) velocity of shell fluid, m/s Vt ) velocity of tube fluid, m/s Greek Letters ηs ) absolute viscosity of shell fluid, kg/(ms) ηt ) absolute viscosity of tube fluid, kg/(ms) νs ) kinematic viscosity of shell fluid, m2/s Fs ) density of shell fluid, kg/m3 Ft ) density of tube fluid, kg/m3 θ ) porosity of hollow fibers τ ) tortuosity of hollow fibers Superscripts in ) refers to inlet of fluid into contactor out ) refers to exit of fluid from contactor
Literature Cited API Method 716-57. Manual on Disposal of Refinery Wastes. Determination of phenolic materials by 4-amino antipyrine method; American Petroleum Institute: New York. Cahn, R. P.; Li, N. N. Separation of Phenol from wastewater by the liquid membrane technique. Sep. Sci. Technol. 1974, 9 (6), 505. Dahuron, L.; Cussler, E. L. Protein extraction with hollow fibers. AIChE J. 1988, 34, 130. Goswami, A. N.; Sharma, S. K.; Sharma, Anshu; Gupta, T. C. S. M. Removal of phenol from refinery wastewaters using liquid surfactant membranes in a continuous contactor. Ind. J. Chem., 1992, 31A, 361. Jefferson, J. B.; Witzett, O. W.; Sibbitt, W. L. Thermal conductivity of graphite-silicone oil and graphite-water suspensions. Ind. Eng. Chem. 1958, 50, 1589. Kataoka, T.; Nishiki, T.; Kimura, S. Phenol permeation through liquid surfactant membranes. J. Membr. Sci. 1989, 41, 197. Mackay, D.; Medir, M. Industrial effluent treatment. In A Handbook of Solvent Extraction; Lo, T. C., Baird, M. H. I., Hanson, C., Eds.; John Wiley and Sons: New York, 1983; Part III. Papedopolous, T.; Sirkar, K. K. Separation of 2-propanol/n-heptane mixture by perstraction. Ind. Eng. Chem. Res. 1993, 32, 663. Prasad, R.; Sirkar, K. K. Dispersion-free solvent extraction with microporous hollow-fibre modules. AIChE J 1988, 34, 177. Raghuraman, B.; Wiencek, J. Extraction with emulsion liquid membranes in a hollow fiber modules. AIChE J. 1993, 11, 39. Sengupta, A.; Basu, R.; Sirkar, K. K. Separation of solutes from aqueous solutions by contained liquid membranes. AIChE J. 1988, 34 (10), 1698. Urtiaga, A. M.; Ortiz, M. I.; Salazar, E.; Irabion, J. A. Supported liquid membranes for the separation-concentration of phenol. I. Viability and mass transfer evaluation. Ind. Eng. Chem. Res. 1992, 31, 877. Wilke, C. R.; Chang, P. Correlation of diffusion coefficient in dilute solutions. AIChE J., 1955, 1 (2), 264. Xiujuan, Z.; Jianghong, L.; Qiongsia, F.; Tiangsi, L. Industrial applications of liquid membrane separation for phenolic wastewater treatment. Sep. Tech.; N. N., Strathman, H., Eds.; Engineers Foundation: New York, 1988.
Received for review January 2, 1997 Revised manuscript received June 30, 1997 Accepted June 30, 1997X IE970003T
Abstract published in Advance ACS Abstracts, September 1, 1997. X
