New Crystallization of Fatty Acids from Aqueous Ethanol Solution

A new separation process of saturated fatty acids (lauric acid-myristic acid) using crystallization from an aqueous ethanol solution has been examined...
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Ind. Eng. Chem. Res. 1999, 38, 2428-2433

New Crystallization of Fatty Acids from Aqueous Ethanol Solution Combined with Liquid-Liquid Extraction Kouji Maeda,* Yoshihisa Nomura, Kimihiko Tai, Yoshitaka Ueno, Keisuke Fukui, and Syouji Hirota Department of Chemical Engineering, Himeji Institute of Technology, 2167 Shosha, Himeji, Hyogo 671-2201, Japan

A new separation process of saturated fatty acids (lauric acid-myristic acid) using crystallization from an aqueous ethanol solution has been examined. There were two vessels in this separation process: an extraction vessel and a crystallization vessel. The fatty acids in the aqueous phase were first extracted from their organic phase (melt) in the extraction vessel. The fatty acids in the aqueous phase were continuously introduced to the crystallization vessel, and then the fatty acids were crystallized there. The crystals of the fatty acids were collected continuously above the aqueous phase in the crystallization vessel. In this process, the yield and the purity of the crystals over time were measured, and it was found that the purity of lauric acid increased unsteadily up to 0.98 mole fraction of lauric acid with an increase in the yield of the low yield range. The mole fraction of ethanol in the aqueous phase could be significant to control the relationship between the yield and the purity of the crystals. Three different mole fractions of lauric acid in the organic phase were used to be separated in this process. Moreover, we have considered the effective separations of this process, and the maximum yield and purity of the crystals have been estimated by a simple mass balance. Introduction Liquid-liquid extraction is an effective separation process, because both the feed phase and the product phase can be homogeneous fluid phases. Those two liquid phases are partially miscible solutions with respect to each other; therefore, phase separation is easier. For mixtures consisting of similar components, the separation factor of one component between two liquid phases might not be so good according to liquidliquid equilibria (LLE).1 On the other hand, crystallization has been a traditional separation process, and in particular it is used as a high purification method. General crystallization has traditionally been carried out in a suspension system;2 therefore, the crystals must be additionally separated from the solution. When solid-liquid equilibria (SLE) for organic mixtures are simple eutectic systems, the separation factor of the solute between the crystal and the solution will be extremely high.1 If the advantages of both liquid-liquid extraction and crystallization could be combined, an effective separation process would be available. Sun has suggested that water can play an intermediate role for the displacement of adhering mother liquor between organic crystals.3 This method uses one characteristic of immiscible liquid. Davey et al. examined emulsion crystallization, and they found a possibility to overcome the problems of separating eutectic mixtures.4 However, it would be very difficult to consider the obvious mechanism of the emulsion crystallization. The industrial potential of fatty acids is constantly growing. Lauric acid and myristic acid are major components of the coconut resource. Separation and purification of fatty acids is the one channel which * To whom correspondence should be addressed. E-mail: [email protected].

provides the upgrading of fibers, plastics, and other specific chemicals. Separation of fatty acids cannot be accomplished by normal melt crystallization because of the fact that major components have similar carbon chain lengths and insignificant differences in molecular weight. Maeda et al. have previously reported that the fatty acids can be dissolved in the aqueous phase and that the fatty acids can crystallize in the aqueous phase about LLE data and SLE data.5 The distribution of fatty acids between the organic phase and the aqueous phase was shown experimentally. Furthermore, the crystals obtained from the aqueous phase had a high purity of lauric acid.6 The new concept of crystallization combined with liquid-liquid extraction has been imaged recently.7 In this paper, we have experimentally examined a new crystallization combined with liquid-liquid extraction for fatty acids. The relationship between the yield and the purity of the crystals in this process is discussed at different operative conditions. We have estimated the maximum yield and purity in the low yield range using a simple mass balance. The effect on them of mole fraction of ethanol in the aqueous phase was considered. Experimental Section Materials. The GR-grade reagents of lauric acid and myristic acid from Nacarai Tesque Co. without additional purification were used as organic mixtures in our experiments. The purity of the fatty acids was more than 0.99 mole fraction (gas chromatography). The GR grade of ethanol from Nacarai Tesque Co. was used as an organic solvent, and its purity was more than 0.995 mole fraction. Distilled water was used as an antisolvent for fatty acids. Construction of the Apparatus. Two different vessels for liquid-liquid extraction and crystallization should be provided in the proposed process. Figure 1

10.1021/ie980715z CCC: $18.00 © 1999 American Chemical Society Published on Web 05/07/1999

Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 2429 Table 1. Operating Conditions for Liquid-Liquid Extraction and Crystallization temperature Xo

xe

circulating rate/(mL/h)

Te/K

Tc/K

0.11

0.05 0.07 0.10 0.05 0.07 0.07 0.10 0.05 0.07 0.10

540 540 540 540 540 10800 540 540 540 540

333 333 333 333 333 333 333 333 333 333

283 283 283 283 283 283 283 283 283 283

0.49

0.89 Figure 1. Experimental apparatus for the extraction-crystallization method.

shows the experimental apparatus to carry out the new crystallization process. The extraction vessel was connected to the crystallization vessel with two tubes in the lower part of the vessels because the aqueous phase is heavier than the organic phase. The aqueous phase was circulated by a tube pump (EYELA, MP3) between the extraction vessel and the crystallization vessel. The temperatures of both the extraction vessel and the crystallization vessel were controlled by two different water-thermostat baths. The aqueous phase of each vessel was agitated by a magnetic stirrer (Iuchi, M-3). The crystals produced in the crystallization vessel were collected continuously by the upper Nylon monofilament filter (mesh opening ) 80 µm) because the crystals in the crystallization vessel floated in the aqueous phase. Procedures. A total of 0.90 kg of the aqueous ethanol solution having a known mole fraction of ethanol (xe) was put into the extraction vessel and the crystallization vessel. A total of 0.05 kg (Wf) of a fatty acid mixture having a known mole fraction of lauric acid (Xo) was fed into the extraction vessel. The partially miscible solution in the extraction vessel was heated to the extraction temperature (Te ) 333 K). The aqueous and organic phases were stirred so slowly that the aqueous phase and organic phase could not be emulsified. The aqueous solution in the crystallization vessel was cooled to the crystallization temperature (Tc ) 283 K). The aqueous solution was also slowly stirred. After the temperatures of both vessels were kept constant for 3 h, the tube pump began circulating the aqueous phase. The flow rate of the circulation was fixed at 540 mL/h. The fine crystals appeared in the crystallization vessel, and then they accumulated above the aqueous phase. The wet crystals were continuously collected on the Nylon filter. The mass (W) of the crystals was measured after the crystals on the Nylon filter had been dried for 24 h. The purity (Z) of the crystals was analyzed by a gas chromatograph having a FID detector (Shimadzu, GC8A) with a column (Unisole 400, Uniport S 40/80, 3L × 1 m). The purity was defined by the mole fraction of lauric acid in the crystals. The time courses for the mass and purity of the cumulative crystals were experimental data in the process experiments. The mole fraction of ethanol in the aqueous phase (xe) and the mole fraction of lauric acid in the organic phase (Xo) were changed as operative conditions. They are shown in Table 1. The mole fraction of ethanol in the aqueous phase (xe) was defined by excluding fatty acids, and also the mole fraction of lauric acid in the organic phase (Xo) was defined by excluding the aqueous ethanol solution. Further, to estimate the relationship between the yield and the purity of the crystals, one kind of experiment with a high circulating rate (10 800 mL/h) has been

Figure 2. Time courses for the yield of the cumulative crystals at different X° and xe.

done. This experiment is to recognize the limited mass balance in the batch organic phases. Results Time Courses of Crystal Yield at Low Circulating Rate. The fatty acids were extracted from the organic phase (their melt) and dissolved into the aqueous phase in the extraction vessel. The fatty acids were then crystallized in the crystallization vessel. The fatty acid crystals in the aqueous phase were very fine and reflected light like diamond dusts. The floated crystals were accumulated above the aqueous phase. The masses of the crystals were converted into moles (S). Using the total moles of the fed fatty acids (L0), the yield (S/L0) was defined by the mole ratio of the collected crystals to the originally fed fatty acids in the extraction vessel. The yield of the crystals versus the circulating time (t) is shown in Figure 2. The yield of the crystals increased with time linearly. As the mole fraction of ethanol in the aqueous phase (xe) increased, the yield of crystals (S/L0) became considerably large. Ethanol works as an extractant chemical in the aqueous phase in this process. It can be expected that raising the mole fraction of ethanol in the aqueous phase makes the solubility of fatty acid in the aqueous phase increase. The driving force of crystallization of fatty acids should be given by the difference of the solubility of fatty acids between the extraction vessel and the crystallization vessel. That is the supersaturation of fatty acids in the proposed process. The aqueous phase which contained more fatty acids also crystallized more fatty acids because the supersaturation generated by the extraction vessel at Te and the crystallization vessel at Tc became higher. When lauric acid (Xo) increased in the organic phase, the yield of the crystals (S/L0) increased slightly because the solubility of the total fatty acids in the aqueous

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Figure 3. Relationship between the purity and the yield of the cumulative crystals at different ethanol mole fractions in the aqueous phase (Xo ) 0.89).

Figure 5. Relationship between the purity and the yield of the cumulative crystals at different ethanol mole fractions in the aqueous phase (Xo ) 0.11).

Figure 4. Relationship between the purity and the yield of the cumulative crystals at different ethanol mole fractions in the aqueous phase (Xo ) 0.49).

Figure 6. Relationship between the purity and the yield for the cumulative crystals at different circulating rates (Xo ) 0.49, xe ) 0.07).

phase became higher. Of course, the solubility of lauric acid was higher than the solubility of myristic acid in the aqueous phase.5,8 Changes of Crystal Purity with Yield at Low Circulating Rate. The relationship between the yield and the purity of the crystals from the organic phase having 0.89 mole fraction of lauric acid (Xo ) 0.89) is shown in Figure 3. It was surprising that the crystals were very rich in lauric acid (Z > 0.98 mole fraction), as compared with the organic phase (Xo ) 0.89). The purity (Z) increased as the yield (S/L0) increased in the low yield range. Increase of the mole fraction of ethanol in the aqueous phase (xe) caused the purity of the crystals (Z) to be higher. In the case of 0.49 mole fraction of lauric acid in the organic phase (Xo ) 0.49) shown in Figure 4, the purity of the crystals (Z) also increased considerably up to 0.83. Figure 5 shows the case of 0.11 mole fraction of lauric acid in the organic phase (Xo ) 0.11). The aqueous phase containing 0.05 mole fraction of ethanol (xe ) 0.05) did not change the purity of the crystals (Z) considerably. When the ethanol mole fraction in the aqueous phase was 0.07 (xe ) 0.07), the purity of the crystals (Z) increased from 0.11 in the organic phase to 0.3 in the crystals. However, more increases in the ethanol mole fraction in the aqueous phase (xe ) 0.10) made the purity of the crystals (Z) worse. The operations using 0.07 mole fraction of ethanol in the aqueous phase (xe ) 0.07) were efficient to produce the crystals. Change of Crystal Purity with Yield at High Circulating Rate. Figure 6 shows the comparison in the circulating rate on the yield (S/L0) and the purity of the crystals (Z). Higher circulating rate made the

purity of the crystals as a function of their yields lower. The purity of the crystals produced at high circulating rate increased in the low yield range. However, it began to decrease at 0.5% crystal yield (S/L0 ) 0.005). This purity drop is caused by the decrease in mole fraction of lauric acid in the organic phase (Xo) because the organic phase is a batch system without any supplements. We assumed that the decrease in the mole fraction of lauric acid in the organic phase (Xo) could begin the decrease of the purity of the crystals (Z) at low circulating rate, too. Discussions Distribution of Lauric Acid between the Aqueous Phase and the Organic Phase and between the Crystals and the Aqueous Phase. In an earlier experiment,6 we reported the distribution of lauric acid between the aqueous and organic phases. Figure 7 shows that 0.91, 0.52, and 0.20 mole fractions of lauric acid in the aqueous phases (Xa, aqueous ethanol solution free) are distributed from 0.89, 0.49, and 0.11 mole fractions of lauric acid in the organic phases (Xo), respectively. The effective distributions of lauric acid between the crystals (Z) and the aqueous phase (Xa) are also shown in Figure 8. Those distribution data were previously measured at the ethanol mole fraction range: xe ) 0.01-0.05. They would be useful to consider the mechanism of the proposed process even though the process experiments have been done at different ethanol mole fractions in the aqueous phase. It is found that lauric acid was preferentially crystallized in the aqueous phase which contained more than 0.4 mole fraction of lauric acid (aqueous ethanol solution free) and myristic

Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 2431

Figure 7. Distribution of lauric acid between the aqueous phase and the organic phase for the aqueous ethanol solution-lauric acid-myristic acid system at 333 K.

Figure 8. Effective distribution of lauric acid between the crystals and the aqueous phase for the aqueous ethanol solution-lauric acid-myristic acid system at 293 K.

Figure 9. Solid-liquid phase diagram for the lauric acidmyristic acid system.

acid was preferentially crystallized from the aqueous phase which contained less than 0.3 mole fraction of lauric acid (aqueous ethanol solution free).6 The distribution of lauric acid intersects a diagonal in Figure 8. The intersecting points could be recognized as the eutectic compositions for the binary mixture of lauric acid and myristic acid. Figure 9 shows the binary phase diagram for the lauric acid-myristic acid system, which was calculated by the previous report.9 The original eutectic composition is placed in the range 0.6-0.7 mole fraction of lauric acid. The aqueous ethanol solution can move the eutectic composition significantly. Actual results in Figures 3 and 4 follow the distributions of lauric acid in Figure 8. However, in Figure 5, lauric acid rather than the myristic acid was preferentially crystallized from the aqueous phase containing more than 0.07 mole fractions of ethanol. Increasing ethanol in the

aqueous phase causes the distribution of lauric acid between the crystals and the aqueous phase to be higher. It can be thought that the eutectic mole fraction of lauric acid in the aqueous phase will be shifted by less than 0.20, even though the binary eutectic mole fraction of lauric acid is about 0.6. The aqueous ethanol solution has the capacity to shift the eutectic composition of the binary fatty acid significantly. Consequently, 0.89 and 0.49 mole fractions of lauric acid in the organic phase (Xo ) 0.89, 0.49) can be separated to the rich lauric acid crystals in the process, and also 0.11 mole fraction of lauric acid in the organic phase (Xo ) 0.11) could be separated. However, its separation efficiency is not good. Changes of Crystal Purity with Yield in the Low Yield Range. The relationship between the yield and the purity of the crystals is preferable in Figures 3, 4, and 6. The greater the yield of the crystals obtained, the higher was the purity of the crystals in the low yield range. The impurity in the crystals might be caused by adhering of the impurity in the aqueous phase. That effect should be negligible because the impurity is very diluted in the aqueous phase. If the crystals would be produced with a constant effective distribution between the aqueous phase and the crystals (steady state), the purity should decrease, taking the mass balance of the batch organic phase into account. The distribution between the organic phase and the aqueous phase could be assumed to be constant because the operative conditions (temperature, interface area, etc.) of the liquidliquid interface in the process did not change, and also the liquid-liquid extraction process has been used as a equilibrium separation process.1 The crystallization process is a very complex phenomenon which consists of both nucleation and growth in the supersaturated solution;2 therefore, it is difficult to consider the distribution of solute between the crystals and the aqueous phase to be constant. As a result of these experiments in Figure 6, it can be guessed that the effective distribution of lauric acid between the aqueous phase and the crystals might increase gradually in the low yield range, and it would be constant. The turning point observed in Figure 6 is the most important for this process, and it can distinguish the steady process after the turning point from the unsteady process before the turning point. The turning point in this process means the maximum yield and purity of the crystals. The turning point is caused by a decrease in the mole fraction of lauric acid in the batch organic phase. Estimating the turning point is very useful to the design of the proper operation of the proposed process. For high circulating rate, the turning point was placed where the ratio of crystallized lauric acid to the total fed lauric acid is

SZ/L0Xo ) 7.29 × 10-3

(1)

If the ratio is constant for each operation even at low circulating rate, the turning point for each operation can be obtained easily. The relationship between the yield and the purity in the unsteady state (in the low yield range) can be expressed by the following equation.

Z ) a + b log(S/L0)

(2)

The lines in Figures 3, 4, and 6 were correlated with eq 2. Their parameters are listed in Table 2. Role of Ethanol in the Aqueous Phase. Figure 10 shows the effect of mole fraction of ethanol in the

2432 Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 Table 2. Fitting Parameters for Equation 2 Xo

xe

circulating rate/(mL/h)

a

b

0.49

0.05 0.07 0.07 0.10 0.05 0.07 0.10

540 540 10800 540 540 540 540

1.012 5 0.975 84 0.952 98 0.950 44 1.002 7 1.000 4 0.988 11

0.052 758 0.051 289 0.110 65 0.046 964 0.004 712 4 0.0044 53 3 0.001 148 6

0.89

removal of the adhering mother liquor will be easier than the general crystallization processes.2 The hydrophobic effect of organic fatty acid prevents the surrounding impurities from entering into the crystals, and it caused the purity of the crystals (Z) to be considerably high even when it reduced the yield of the crystals (S/ L0). High-purity crystals could be easily obtained. As compared with the extraction process,1 the crystallization process from the aqueous phase recovers the solute more effectively than evaporating the solvent. Our experiments were conducted using fatty acids. The proposed method should be applicable for not only fatty acids but also other organic compounds. A higher purity of organic crystals such as aromatics might also be produced by the proposed method. Conclusions

Figure 10. Effect of mole fraction of ethanol in the aqueous phase on the purity and the yield of the crystals (Xo ) 0.49).

The fatty acid crystals were obtained in the crystallization vessel continuously above the aqueous phase which extracted fatty acids from the organic phase in the extraction vessel. The purity of the crystals increased considerably from the mole fraction of lauric acid in the organic phase or in the aqueous phase. Lauric acid was effectively separated out from a mixture which included the binary mixture of lauric acid and myristic acid. Effects of the mole fraction of ethanol in the aqueous phase on the purity and yield of the crystals were considered in the low yield range. Using a simple mass balance, the maximum purity and yield in the unsteady state were estimated for each operation. The present experiments proved that there are great advantages in using the liquid-liquid extraction-crystallization process. Acknowledgment

Figure 11. Effect of mole fraction of ethanol in the aqueous phase on the purity and the yield of the crystals (Xo ) 0.89).

aqueous phase (xe) on the maximum purity (Z) and yield (S/L0) of the crystals from the organic phase having 0.49 mole fraction of lauric acid. The maximum purity and yield were obtained by simulating eqs 1 and 2. An increase in ethanol in the aqueous phase (xe) made the yield of the crystals (S/L0) better but made the purity of the crystals (Z) worse. Of course, the higher circulating rate caused the purity of the crystals (Z) to be worse. The turning points for the crystals from the organic phase having 0.89 mole fraction of lauric acid are shown in Figure 11. It is found that the purity of the crystals could be more than 0.99 mole fraction of lauric acid easily even though its yield would be very low. The simple estimations for the maximum purity and yield in the process have been done for the fatty acid mixture. However, more detailed observations (nucleation and growth of organic compounds in the aqueous phase) would be necessary to apply the proposed process for other mixtures. If the objective was to obtain highly purified lauric acid, crystallization from an aqueous ethanol solution would be a very effective method. In view of the crystallization process, the advantage of the proposed method can be thought of as the crystals of hydrophobic organic fatty acids float above the aqueous phase. The

We are very grateful to Professor Edward L. Hays in the Department of General Education for proof reading and correcting the English in this paper. Nomenclature a: parameter in eq 2 b: parameter in eq 2 L0: moles of binary fatty acids in the organic phase (aqueous ethanol solution free), mol S: moles of binary fatty acids in the cumulative crystals, mol Tc: crystallization temperature, K Te: extraction temperature, K Xo: mole fraction of lauric acid in the organic phase or fed melt of the binary fatty acids (aqueous ethanol solution free) ) lauric acid/(lauric acid + myristic acid) in the organic phase Xa: mole fraction of lauric acid in the aqueous phase (aqueous ethanol solution free) ) lauric acid/(lauric acid + myristic acid) in the aqueous phase xe: mole fraction of lauric acid in the aqueous phase ) ethanol/(ethanol + water) in the aqueous phase Z: mole fraction of lauric acid in the crystals W: mass of the cumulative crystals collected on the filter, kg Wf: mass of the binary fed fatty acids, kg

Ind. Eng. Chem. Res., Vol. 38, No. 6, 1999 2433

Literature Cited (1) King, C. J. Separation Processes, 2nd ed.; McGraw-Hill: New York, 1980. (2) Myerson. A. S. Handbook of Industrial Crystallization: Butterworth-Heinemann: Woburn, MA, 1993. (3) Sun, Y. C. Water: Key to New Crystallization Process for Purifying Organics. Chem. Eng. 1971, 12, 88. (4) Davey, R. J.; Garside, J.; Hilton, A.; McEwan, D.; Morrison, J. W. Purification of Molecular Mixture below the Eutectic by Emulsion Crystallization. Nature 1995, 375, 664. (5) Maeda, K.; Yamada, S.; Hirota, S. Binodal Curve of Two Liquid Phases and Solid-liquid Equilibrium for Water + Fatty Acid + Ethanol Systems and Water + Fatty acid + Acetone Systems. Fluid Phase Equilib. 1997, 130, 181. (6) Maeda, K.; Nomura, Y.; Fukui, K.; Hirota, S. Extraction and

Crystallization of fatty acids by ethanol aqueous solution. Kagaku Kogaku Ronbunshu 1997, 23, 433. (7) Maeda, K.; Nomura, Y.; Guzman, L. A.; Hirota, S. Crystallization of Fatty acids Using Binodal Region of Two Liquid Phases. Chem. Eng. Sci. 1998, 53, 1103. (8) Hoerr, C. W.; Pool, W. O.; Ralston, A. W. The Effect of Water on the Solidification Points of Fatty Acids. Solubility of Water in Fatty Acids. Oil Soap 1942, 19, 126. (9) Enomoto, H.; Maeda, K.; Fukui, K.; Hirota, S. VaporLiquid-Solid Equilibria for the System Propane or 2-methylpropane-dodecanoic acid plus tetradecanoic acid. J. Chem. Eng. Data 1997, 42, 791.

Received for review November 13, 1998 Revised manuscript received March 15, 1999 Accepted March 25, 1999 IE980715Z