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Ind. Eng. Chem. Res. 1996, 35, 4182-4184
Gel-Based Separation of an o-Toluidine-Water Emulsion† Vivek V. Buwa, Ashish K. Lele, and Manohar V. Badiger* Chemical Engineering Division, National Chemical Laboratory, Pune 411 008, India
The present work describes an attractive and economically viable novel separation technique using superabsorbing hydrophilic polymer gel for dewatering an industrial o-toluidine-water emulsion. It is demonstrated that the initial water content of 30% in the emulsion is reduced to 3% within 10 min by using a semicontinuous separation strategy, which can have significant energy savings over the conventional distillation process. The mechanism of water removal and the effect of the presence of interfacial agents on the rate of water removal are also studied. Introduction Recently, many novel separation strategies based on stimuli-responsive polymeric gels have been demonstrated. Synthetic polymeric gels have been used for size-selective separation of macromolecules from aqueous solutions (Freitas and Cussler, 1987; Cussler et al., 1984; Badiger et al., 1992) and for chemically selective separation of mixtures of organic solvents (Varma et al., 1995; Mukae et al., 1993). In this work, we present a gel-based process for separating an emulsion of an organic liquid and water. Specifically, we demonstrate the dewatering of the o-toluidine (OTD)-water emulsion using a superabsorbing hydrophilic polymer gel. OTD is an important intermediate used extensively for manufacturing dyes, printing textiles in blue-black, and synthesizing various colors that are fast to acids. During its manufacturing process, OTD is obtained as an emulsion containing about 30% by weight water and 1% o-nitrotoluene. Although this emulsion does not contain interfacial agents, it is stable and does not separate easily by gravity or centrifugal action due to a small difference in the specific gravities of OTD and water (1.008 for OTD and 1.0 for water at 25 °C). Distillation is the standard industrial strategy used for separation, since the boiling point of OTD (200 °C) is very different from that of water. However, because of the significant water content, distillation requires highenergy consumption. A gel-based dewatering at ambient temperature would thus provide an alternate costeffective separation strategy. We report here the separation of OTD-water emulsion at room temperature using Jalshakti, which is a commercially available hydrolyzed starch-g-poly(acrylonitrile) superabsorbing polymer with an equilibrium absorption capacity of 150-200 g of water/g of dry polymeric gel. We show that the water content of the emulsion can be reduced from 30% to 3% in 10 min using 1-mm gel particles. We also demonstrate the feasibility of a semicontinuous gel-based separation strategy. The effect of the presence of interfacial agents (surfactant) on the rate of water removal has been studied to investigate the mechanism of dewatering and to check the general feasibility to other kinds of emulsions. Experimental Section OTD was obtained from Hindustan Organic Chemicals Ltd. (Bombay, India), and the Jalshakti gel was obtained from Indian Organic Chemicals Ltd. (Bombay, * Author to whom correspondence should be addressed. † NCL Communication No. 6359.
S0888-5885(96)00240-0 CCC: $12.00
Figure 1. Apparatus for semicontinuous process using stirred tank.
India). Emulsions of OTD and water with and without surfactant were prepared by mixing the desired compositions using a high intensity emulsifier for 30 min. These emulsions were representative of the industrially obtained emulsions based on comparable times required for initial separation by gravity alone. In some of our experiments, the sodium salt of dioctylsulfosuccinate was used as a surfactant, and its concentration was kept well above the critical micellar concentration (cmc). Commercial polymeric gel was attritioned and sieved to obtain particles of 1.1-mm average size. The emulsions were photographed using a camera fitted to a high-resolution optical microscope (Leitz Optical Microscope, Model 12 POL-D). The rapid Brownian motion of tiny water droplets formed in the presence of surfactant was arrested by freezing the emulsion with liquid nitrogen, and then the frozen sample was photographed. Therefore, it is very likely that the measured droplet size is slightly less than the actual droplet size. The rate of uptake of water by the gel when immersed in either pure water or in the emulsion was measured as follows. A weighed amount of predried gel particles was continuously stirred in a known amount of the liquid. After a fixed time, the mixture was quickly filtered by application of slight vacuum and the filtered swollen particles were weighed. The uptake was calculated as
q)
wt of swollen gel - wt of dry gel wt of dry gel
This was repeated for different times to determine the rate of uptake. The water content of the emulsion was measured using the standard Karl-Fischer titration technique. The apparatus shown in Figure 1 was designed to demonstrate the feasibility of semicontinuous gel-based separation of the OTD-water emulsion. The emulsion and dry gel particles were fed to a 1000-mL stirred tank © 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996 4183
Figure 2. Swelling kinetics in pure water and kinetics of water removal from the OTD-water emulsion without surfactant.
through separate ports, and the exit stream was filtered to separate the swollen gel particles. Samples of the exit liquid stream were analyzed for water content by the Karl-Fischer titration. The gel particles were subsequently regenerated. Results and Discussions The swelling kinetics of the polymeric gel in pure water is as shown in Figure 2. The gel swells rapidly, imbibing 100 g of water/g of the gel in the first 6-10 min, after which the swelling rate levels off. The reproducibility of our experiments to measure the uptake of water was found to be satisfactory. Figure 2 also shows the gel-mediated decrease in water content of an OTD-water emulsion which does not contain any surfactant. The water content of the emulsion decreases rapidly from the initial 35% to 3% in 10 min, corresponding to the absorption of water by the gel from the emulsion. It is seen that the kinetics of water removal in the emulsion matches closely with the swelling kinetics of the gel in pure water, indicating that the presence of OTD does not affect the absorption rate. However, the equilibrium uptake by the gel is slightly reduced in the presence of OTD. The swollen gel particles are found to contain less than 1% OTD as confirmed by mass balance on OTD in all our experiments. We believe that this OTD is mostly “adsorbed” on the gel particle surface, since a negligible amount of OTD was imbibed when the gel was immersed in pure OTD. The adsorption of OTD onto the gel surface can be minimized by using highly hydrophilic gels. The results shown in Figure 2 have an important bearing on the design of any continuous process strategy for gel-based separation of the OTD-water emulsion. During our experiments using the semicontinuous apparatus shown in Figure 1, we maintained an average residence time of 2 min in the stirred tank based on the fact that in this time scale, the water content of the emulsion would decrease from 34.8% to 3.2%, corresponding to an uptake (q) of 160 g of water/g of gel. The feed rate of the gel was determined from this calculation. Residence times greater than 10 min are not suitable since only marginally higher quantities of water would be removed (see Figure 2). In a representative run, 1 g/min of gel particles and 500 mL/min of OTDwater emulsion containing 30% water were fed to the stirred tank through separate ports. Once the steady state (usually within 1 min) was reached, samples of
Figure 3. Optical micrographs of the OTD-water emulsions: (i) without surfactant, (ii) with surfactant.
the exit liquid stream were withdrawn and analyzed for water content using Karl-Fischer titration as stated earlier. The average water content from three samples collected between 3 and 10 min was found to be 3.5%, which demonstrates the feasibility of the process. It is seen that separation of water from the emulsion is easily possible with the stirred tank design used here. Similarly, other designs can be employed equally efficiently. For example, a basket reactor or a pipe reactor (Levenspiel, 1972) can be used to effect separation. A choice between these strategies should be based on issues of easy material handling, recycling, and selectivity. In order to further purify OTD, the gel-based process needs to be followed with a distillation process. However, the energy consumption in the distillation process will now be considerably reduced because of the much lower water content in the feed. Thus, a combination of gel-based separation process and distillation can result in a cost-effective and economically viable process for separation of the OTD-water emulsion. Finally, any commercial continuous process would require the gel to be regenerated and recycled. Jalshakti can be regenerated by bringing the particles in contact with an aqueous acidic solution of pH less than 4. The collapsed gel particles can be filtered and dried before recycling. This strategy would generate a small amount of waste acidic stream. Alternately, a thermoreversible gel such as poly(N-isopropylacrylamide) (PNIPA) can
4184 Ind. Eng. Chem. Res., Vol. 35, No. 11, 1996
We believe that the presence of surfactant molecules on the surface of water droplets reduces the relative gel-water interaction and also reduces the coalescence efficiency with the gel particles. Hence, the absorption of water from this emulsion occurs much more slowly than in the absence of surfactant. However, the rate of water removal can be increased by using highly acidic gels like poly(acrylamidopropanesulfonic acid) (PAMPS) and poly(styrenesulfonic acid), wherein the acidic nature of the gel can facilitate the breaking of surfactant layer between gel particles and colliding water droplets. Conclusions
Figure 4. Comparison between the rate of water removal from the OTD-water emulsion with surfactant and without surfactant.
be effectively used in the process. The PNIPA gel can absorb water from the emulsion at ambient temperature, and the swollen particles can be easily regenerated by simply heating them above the lower critical solution temperature (LCST) of 32 °C at which they collapse discontinuously. A study of the mechanism of the gel-based dewatering of emulsions can provide further insights into the design of an industrial process. Figure 3 shows an optical micrograph of the (i) OTD-water emulsion without surfactant and (ii) OTD-water emulsion with surfactant. In the absence of surfactant, water is seen to exist as dispersed droplets of average size 12-50 µm, while in the presence of surfactant, water exists mostly as rapidly moving droplets of about 1-µm average size. It is observed that in the absence of surfactant, the water droplets are absorbed whenever they come in contact with the gel particles, probably due to the strong hydrophilicity of the gel, compared to the much weaker water-OTD interactions. This mechanism of separation is entirely different than the size-selective and chemically selective macromolecular separations mentioned earlier. Thus, the continuous stirring of gel particles in the emulsion is necessary to ensure the contact between water droplets and the gel particles. In the presence of surfactant, however, it is seen that rapidly moving tiny water droplets tend to coalesce faster than being absorbed by gel. Correspondingly, the rate of water absorption by the gel is much slower, as seen from Figure 4.
We have demonstrated an attractive gel-based separation of an industrially relevant o-toluidine-water emulsion. The process strategy enjoys advantages of high selectively, easy materials handling, recycling, and economic viability in terms of energy savings. Specifically, we have shown a reduction in water content of the emulsion from 30% to 3% in about 10 min. We have also demonstrated the feasibility of a semicontinuous process for gel-based separation. Such a gel-based separation strategy would not require high capital costs because of the use of conventional equipments such as a stirred tank. The economics of the strategies would also depend on the choice of gels and recycling options. Literature Cited Badiger, M. V.; Kulkarni, M. G.; Mashelkar, R. A. Concentration of Macromolecules From Aqueous Solutions: A New Swellex Process. Chem. Eng. Sci. 1992, 47, 3. Cussler, E. L.; Stokar, M. R.; Vaarberg, J. E. Gels As Size Selective Extractive Solvents. AIChE J. 1984, 30, 578. Freitas, R. F.; Cussler, E. L. Temperature Sensitive Gels As Size Selective Absorbents. Sep. Sci. Technol. 1987, 22, 911. Levenspiel, O. Chemical Reaction Engineering, 2nd ed.; Wiley Eastern Private Limited: New Delhi, 1972. Mukae, K.; Sakurai, M.; Sawamura, S.; Makino, K.; Kim, S. W.; Ueda, I.; Shirahama, K. Swelling of Poly(N-isopropyl acrylamide) Gels in Water-Alcohol (C1-C4) Mixed Solvents. J. Phys. Chem. 1993, 97, 737-741. Varma, A. J.; Lele, A. K.; Mashelkar, R. A. Separation Based on Chemically Selective Polymer Gels. Chem. Eng. Sci. 1995, 50, 3835-3837.
Received for review April 23, 1996 Revised manuscript received August 5, 1996 Accepted August 11, 1996X IE960240G
X Abstract published in Advance ACS Abstracts, October 1, 1996.
