Ind. Eng. Chem. Process Des. Dev. 1985, 2 4 , 556-560
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A Novel Extraction Process for Separating Ethanol and Water Gurmukh D. Mehta’ and Malcolm D. Fraser Science Applications International Corporation, McLean, Virginia 22 102
The feasibility of a novel extraction process for separating ethanol and water is presented. A solvent that exhibits complete misclblllty with ethanol above a certain temperature and only partial miscibility below this temperature is used as the extractant. Hot solvent is contacted with the ethanol-water feed solution to extract ethanol. The solvent-ethanol mixture is then simply cooled to separate the product ethanol and the solvent, which is recycled. White light paraffin oil was found to show the most promise as a suitable solvent. Phase compositlon data for the ethanol-water-paraffin oil ternary system were obtained both at 115 O C , the temperature of the extraction, and at 30 O C , the temperatue of the sohent/ethanol separation. The experimental data indicated that the proposed scheme can produce ethanol with less than 1% water concentration. A conceptual design of a bassline system
is presented.
Introduction One important consideration in the production of ethanol for use as fuel via fermentation is the energy required to separate and purify the ethanol from the fermentation mixture. Extraction is one possible means of accomplishing this separation. However, the problem of separating ethanol and water is then traded for the problem of separating the ethanol from the solvent used in the extraction process, although this latter separation may be easier, depending upon the choice of solvent. In the proposed scheme, a solvent that is completely miscible with ethanol above a certain temperature but only partially miscible or immiscible below this temperature is used as the extractant. Thus such a solvent would extract the ethanol from the fermentation mixture at a temperature in the complete-miscibility range, above the upper critical solution temperature (UCST). The temperature of the solvent-ethanol mixture could then be lowered below the critical solution temperature, and the solution would divide into two solutions, one rich in solvent and the other rich in ethanol, with equilibrium compositions fixed by the temperature. Such coupled solutions are called conjugate solutions. A number of organic liquids including higher hydrocarbons and vegetable oils from conjugate solution pairs with ethanol and may be considered as possible solvents for the proposed extraction process. In separating and purifying alcohol by the proposed process, there is no change of phase, which is generally associated with a significant input of thermal energy as latent heat; only sensible heat input is required. In addition, a significant fraction of the required sensible heat input can be recycled through heat exchange with the mixture as it is cooled down to separate the conjugate solutions. To investigate the feasibility of the proposed low-energy extraction process, work was done to identify the most appropriate solvent and to evaluate the energy requirements of the proposed scheme. Experimental work was conducted on promising solvents to obtain the necessary phase-equilibrium data. The solvent showing the maximum potential was further tested to develop the necessary data for a flow sheet for a base-line system. Material and
* Current address: Science Applications International Corp., Dahlgren, VA 22448. 0196-4305/85/1124-0556$01.50/0
energy balance equations were used to analyze the overall performance of the complete process. Candidate Solvents The project started with the search for a suitable solvent, based on criteria developed for the identification of promising candidates. The most important of these criteria was that the solvent-ethanol binary should exhibit partially miscible behavior with a UCST in the range of 60 to 100 OC. The UCST should be in this temperature range because the extraction process will take place about 15 OC higher than the UCST. The higher the UCST, the higher the source temperature must be, and therefore the cost of energy required by the proposed scheme will be higher. But this temperature must not be too low or else insufficient separation between the conjugate solutions will occur at the heat sink temperature. At present, to our knowledge, there is no theory or even correlation to predict the existence or properties of conjugate binary liquid systems. The search for the extractant, therefore, was based on those systems for which at least UCST data existed in the literature. Literature sources containing data on UCSTs for binary mixtures were surveyed to determine candidate solvents (e.g., Francis, 1961, 1963; Sorenson and Arlt 1979). Other criteria on the list were typical criteria to be met by the solvent in any extraction process: insoluble in the solution to be extracted, relatively inexpensive, noncorrosive, etc. In addition, however, the solvent was not to interfere with the use of the alcohol as a fuel as it was anticipated that the alcohol product would contain a small amount of solvent. Ethanol forms conjugating binary solutions with UCSTs with a wide variety of organic compounds. Most of these compounds are not suitable, however. Ethanol tends to form conjugating binary solutions with high-molecularweight aliphatic compounds. Vegetable oils also form conjugating binaries with ethanol, and one of these compounds may well be an attractive candidate, depending upon availability. From the preliminary list of candidates, the three most promising solvents chosen for further screening included hexadecane, cottonseed oil as the representative vegetable oil, and white light paraffin oil. The white light paraffin oil was purchased from Fisher Chemical Co. and had the viscosity value of 125-135 Saybolt. For brevity it is referred to as “paraffin oil”. The UCST for a binary mixture 0 1985 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985 557
--
Table I. Coniunate-Phase Data for Ethanol-Water-Paraffin Oil Ternary Solution at 30 O C " ethanol-rich phase oil-rich phase E 0 W E 0 W E 50.0 96.66 0.026 0.16 3.32 5.82 94.02 94.51 94.12 90.33
3.61 2.22 0.75
1.88 3.66 8.93
3.58 2.56 1.83
96.38 97.40 98.16
0.036 0.034 0.020
49.0 48.0
45.0
bulk solution 0
W
50.0
0.0
50.0 50.0 50.0
2.0
1.0
5.0
Weight percentage of E (ethanol), 0 (oil), and W (water); due to the round-off error, the percentages may not add to exactly 100.00. The alcohol ;sed in the& testa was 99.9% ethanol.
of each solvent with ethanol was experimentally determined, as shown: solvent (composition)
UCST*,"C
hexadecane (50%mol) cottonseed oil (50% wt) white light paraffin oil (50% wt)
49.1
63.5 102.5
(UCST* is the critical solution temperature at the given composition, but it is close to the UCST, which is the maximum critical solution temperature in the phase diagram.) Thus, on the basis of the UCST, hexadecane appeared to be superior because the thermal energy source temperature required for this solvent for the extraction process would be the lowest of the three. However, to make the process efficient, it is necessary to select that solvent which has a more complete separation of alcohol and solvent in the two conjugate phases at the temperature of separation. Consequently, experimental tests were conducted to estimate the conjugate compositions of the three binaries at room temperature by using these solvents and ethanol. Subsequently, ternary-phase data for solvent-ethanolwater at the extraction and the separation temperatures were generated for the most promising solvent for use in the process analysis.
Experimental Procedures The experimental setup used to measure the UCST and to determine the compositions of conjugate phases involved immersing a closed test tube containing the sample in a heated constant-temperature bath. The bath contained a thermometer and a magnetic stirring bar. The sample in the test tube was also stirred in the same way. For temperatures up to 90 "C, a water bath was used, while for higher temperatures, a cooking oil bath was used. Each test tube containing a binary or ternary mixture was closed from the top by means of two septums. To ensure that there was no vapor loss and that the septums did not come off (due to the high vapor pressures of ethanol and water at temperatures above 30 "C), the septums were held tightly by means of ring clamps. T o measure the UCST, the bath temperature was gradually increased, and the temperature at which the binary solution became a single phase (one clear solution) was noted as the UCST. Then the bath temperature was allowed to drop, and the temperature at which the solution became cloudy, manifesting the start of the formation of two phases, was also noted. Typically each UCST measurement was repeated several times, and to eliminate any procedural error, each mixture composition was repeated three times or more. For determination of the tie line data, a binary or a ternary solution with a known bulk composition was prepared in a test tube, which was then sealed from the top by means of the two septums. Typically the same bulk composition was prepared in three separate test tubes. The test tubes were placed in a constant-temperature water bath and gently mixed once every 15 min for a total
of about 8 h. Then the test tubes were left undisturbed overnight in the bath for about 16 h. During this period, the bulk solution separated into two distinct conjugate phases. Then by means of a syringe with a long needle, samples were gently withdrawn first from the top phase and then, about 0.5 h later, from the bottom while the test tubes were still in the bath. These samples were then analyzed by using the procedure described below to determine the percentages of oil, water, and ethanol in each sample. Typically each sample had to be at least 2.0 to 2.5 g for satisfactory analysis. A procedure found in the literature (Magne and Skau, 1953) for analyzing ethanol-water mixtures containing cottonseed oil was used for determining the composition of mixtures containing paraffin oil. In this procedure, the sample to be analyzed was split into two parts of known weights. One was heated in a water bath at about 85 "C to boil off most of the ethanol in the sample and then placed in an oven at about 105 "C until the sample reached a steady weight. By this procedure, all the ethanol and water in the sample were evaporated, thereby yielding the residual fraction of oil. Typically, it took about 4 to 6 h each for a sample to reach a steady weight both in the water bath and in the oven. The other part of the sample was analyzed with a Karl Fischer titrimeter to determine the percentage of water in the given sample. The percentage of ethanol was then determined by subtracting the percentage of oil and water from 100. The Karl Fischer titrimeter was also used to determine the moisture content in the ethanol used in the experimental work. Phase Equilibrium Data For screening the three candidate solvents, experiments were conducted to determine the potential separation of solvent and ethanol by finding the compositions of the conjugate binary solutions at room temperature. The results obtained for the cottonseed oil-ethanol system were similar to literature data (Magne and Skau, 1953) for a binary of cottonseed oil and 99.3% ethanol, showing that the experimental procedure gave satisfactory results. On the basis of the binary phase composition data at room temperature, which showed the equilibrium solutions near the separation temperature, paraffin oil was found to offer the best overall separation characteristics. In addition, paraffin oil offered the following attractive features: 1. Paraffin oil is in the fuel oil range, and therefore a small percentage of this substance in the final product should not hurt the combustion properties of the ethanol. 2. The presence of a small percentage of paraffin oil in ethanol should not pose any regulatory problem because it is presently required to denature pure alcohol anyway with the addition of 3-5% of specified compounds such as methanol or gasoline. 3. A small residual amount of paraffin oil in the weak solution or stillage recycled to the cooker would perhaps not hurt the animals consuming the stillage as part of their feed. However, this issue needs to be further investigated.
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Table 11. Conjugate-PhaseData for Ethanol-Water-Paraffin Oil Ternary Solution at 115 O C water/ethanol-rich phase oil-rich phase no. 0 W E 0 W E 0 1 2
3 4 5 6 7 8 9 10 11
12 13 14 15 16 17 18 19 20
1.68 1.62 0.40 1.19 0.61 1.64 2.18 0.18 0.20 0.33 2.34 0.59 0.44 2.42 1.72 1.94 3.82 3.67 8.16 11.77
95.41 92.65 92.26 87.04 78.13 86.98 81.33 77.60 71.36 59.22 53.14 62.55 58.46 45.96 18.26 22.33 14.42 7.51 3.57 2.01
2.91 5.72 7.34 11.77 21.26 11.38 16.50 22.22 28.44 40.45 44.52 36.86 41.10 51.62 80.02 75.73 81.76 88.82 88.27 86.23
98.32 98.20 95.78 94.81 97.52 96.41 94.18 97.29 96.88 97.49 92.36 96.40 96.53
0.13 0.12 0.41 0.13 0.21 0.28 0.40 0.11 0.12 0.20 0.47 0.08 0.07
1.55 1.68 3.81 5.06 2.36 3.31 5.42 2.60 3.00 2.31 7.16 3.51 3.40
88.60 81.27 79.15 80.57 81.11
0.70 0.70 0.45 0.22 0.13
10.70 12.03 20.40 19.21 18.75
50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0 50.0
bulk solution W
E
48.0 46.0 45.0 42.0 40.0 40.0 38.0 35.0 30.0 30.0 25.0 25.0 20.0 20.0 10.0 10.0 6.0 4.0 2.0 1.0
2.0 4.0 5.0 8.0 10.0 10.0 12.0 15.0 20.0 20.0 25.0 25.0 30.0 30.0 40.0 40.0 44.0 46.0 48.0 49.0
Table 111. Design of ExtractorGontactor and Material Balances as Function of Reflux Ratio
reflux ratio = reflux/ extract product
lower limit feed concn, w t fracn EtOH
EtOH concn in feed wt fracn
0.05
0.175
0.20 0.30 0.40 0.25 0.30 0.40 0.30 0.40 0.45 0.36 0.45 0.42 0.45
0.04
0.23
0.03
0.28
0.02
0.34
0.01
0.40
no. Of stages enrich strip 2 1 1
2 1 1 2 1 1
4 1
3 1
material balance around extractor-contactor, extract product = 100 extractant raffinateC into reflux feed product extractor
3 3 3 6 5 4 9 5 4 14 8 14 11
5 5 5 4 4 4 3 3 3 2 2 1 1
9.27 2.38 1.36 13.06 8.23 4.72 14.10 8.08 6.64 13.81 9.41 13.65 12.18
8.38 1.45 0.43 11.23 6.38 2.85 11.32 5.27 3.82 10.07 5.65 8.95 7.41
94.11 94.07 94.07 94.17 94.15 94.13 94.22 94.19 94.18 94.26 94.24 94.30 94.29
material balance" around separator, extract product = 100 makeupb EtOH extracproduct tant 0.53 0.53 0.53 1.53 1.53 1.53 2.53 2.53 2.53 3.53 3.53 4.53 4.53
0.07 0.03 0.03 0.12 0.10 0.08 0.17 0.14 0.13 0.21 0.19 0.25 0.24
"Totalethanol product from separator = 5.53 and extractant stream recycled = 94.47 for a l l cases. *Makeuu extractant was calculated bv means of a balance for paraffin bil taken over the entire system CRaffinateproduct is assumed to contain 16.5% ethanol by weight.
Complete ternary-phase composition data were then obtained with paraffin oil, ethanol, and water. Experiments were performed to generate these data for the ethanol-water-paraffin oil system a t room temperature, 30 "C, and 115 "C. The results of these experiments at 30 "C are summarized in Table I. When the data were plotted on a triangular phase diagram, all the tie lines passed through the bulk solution composition points, thereby confirming the accuracy of the results. On the basis of these results, it appears that the separator, if operated a t 30 "C, can produce an ethanol stream containing 94.5% ethanol, 3.6% oil, and 1.9% water as the product and an oil stream containing 96.4% oil, 3.6% ethanol, and 0.04% water as the solvent for recycling to the extraction subsystem, if the bulk solution contains 1% water. Comparison of these data a t 30 "C with the corresponding data at room temperature shows that the separation of oil and ethanol could be improved if the separator operates at a lower temperature. The ethanol stream would contain 95.3% ethanol, 2.7% oil, and 2.1% water as the product, and the recycled solvent stream would contain 97.4% oil, 2.6% ethanol, and 0.05% water. Because the UCST of the paraffin oil-ethanol binary system is about 102 "C, the extraction subsystem in the proposed scheme should operate at a temperature some-
what higher than 102 "C;it was decided to generate the ternary-phase data at 115 %, shown in Table 11. The accuracy of these data is somewhat poorer than that of the ternary-phase data a t 30 "C. Unfortunately, it was not possible to generate more accurate data for the oil-rich phase by using the present procedure. However, the present data were deemed to be sufficiently accurate for screening purposes and this initial feasibility study. A correlation was developed for smoothing out these phase composition data and the tie lines, and this correlation is discussed in the next section. Based upon these data, the following general comments can be made. 1. Except at high ethanol concentrations, the paraffin oil and water have very low miscibility even at 115 "C. 2. As the ethanol concentration in the bulk solution is increased, most of it goes to the water-rich phase rather than the oil-rich phase, indicating that ethanol has much higher affinity for water than oil even at 115 "C. 3. For tie lines 1to 12, Table 111, the oil-rich phase was on the top and the water-rich phase was at the bottom because the density of water is higher than that of oil. However, for tie lines 15 to 20, the ethanol-rich phase was on top of the oil-rich phase. In experiments in the intermediate region, the two conjugate phases had almost equal density, and consequently the two phases were intermingled and did not form a distinct single interface,
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985 559
9
moouci REFLUX
ENRICHING SECTION
5
HEAT
WC
[El
STRIPPING SECTION
115pc
HEAT EXCHANGER
51T
EXTRACTANT RECYCLED
c
Figure 1. Conceptual flow sheet for the base-line system.
thereby making the withdrawal of the samples from the two phases almost impossible. For this reason, no reliable data could be generated for this zone. 4. Depending upon the feed and product reflux rates, the maximum concentration of ethanol in the solventphase product will be in the range 7-10%. Description and Analysis of Proposed System The phase data were used to prepare and analyze a conceptual flow sheet for the proposed system, which is shown in Figure 1. The heated ethanol solution is fed to the extraction column along with heated solvent, solution 1. The output extract from the extraction column, solution 2, after being cooled goes to a separator from which the ethanol product 5 and the solvent recycle 4 emanate. Heat exchange is used extensively to conserve energy in heating up the ethanol solution and the solvent for extraction and cooling the extract product for separation. However, no change of phase with a latent heat effect is involved anywhere within the process. The extraction column is visualized as a multistage unit with countercurrent flows of solvent and ethanol solution. To obtain the richest possible extract product, the extraction column includes both a stripping section and an enriching section with stages above the feed point with a portion of the product ethanol 8 introduced as reflux. Although the phase-equilibrium data a t 115 "C were plotted on a ternary-phase equilibrium diagram, it was found to be more convenient for design calculations to replot the data as an x-y diagram ( x , concentration of ethanol in the water phase; y, concentration of ethanol in the oil phase). To smooth the data, particularly in the region of low ethanol content, and develop a more consistent equilibrium line from the tie line data, the data on x and y were first plotted vs. the independent variable controlling the compositions-the weight fraction of ethanol in the bulk solution, which was determined accurately in making up the solution (the oil fraction was kept constant). Figures 2 and 3 show these correlations of conjugate phase composition data with concentration of ethanol in the bulk solution. As Figure 2 shows, the data on concentration of ethanol in the water phase were fairly consistent. However, the data on concentration of ethanol in the oil phase showed considerable scatter at low concentrations, and this correlation was very useful in smoothing the data. An equilibrium x-y diagram was developed by selecting and plotting pairs of x and y pointa from the correlations. This smoothed equilibrium curve is shown in Figure 4. This curve was used for determining the design of the extractor-contactor. As a practical limit, the maximum concentration of ethanol in the extract product was chosen to be 8.5%. The corresponding equilibrium concentration of ethanol in the
0
0.1
0.2
0.3
0.5
0.4
WEIGHT FRACTION ETHANOL IN BULK SOLUTION
Figure 2. Correlation of ethanol-water-paraffin oil ternary-phase data at 115 OC.
0.12
/
>w
8
0.10
d
Ez
0.08
6
f
eY
0.08
E
5
0.04.
0
0.02.
0
0.1
0.2
0.3
0.4
WEIOHT FRACTION ETHANOL IN BULK SOLUTION
Figure 3. Correlation of ethanol-water-paraffin oil ternary-phase data of 115 OC.
water phase is 47.5%. This pair of equilibrium concentrations is found a t a bulk concentration of 30% ethanol (in the experiments to determine the phase equilibrium diagram with 50% paraffin oil), as may be seen in Figures 2 and 3. From the ternary-phase equilibrium data, this
500
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 3, 1985
WEIGHT FRACTION ETHANOL IN THE WATER PHASE XE
Figure 4. Correlation of ethanol-water-paraffin oil ternary-phase data at 115 "C.
extract product was found to contain about 91% paraffin oil and 0.5% water. The extract product is, of course, cooled to about 30 "C to separate an ethanol-rich product from an oil-rich solvent phase that is then recycled. To determine the compositions of the two separated phases by interpolation, the equilibrium-phase data obtained at 30 "C (Table I) were plotted vs. the independent variable controlling the compositions-the weight fraction of water in the bulk solution [the oil fraction was kept constant at 50%, (Mehta and Fraser, 1982)]. With a bulk solution concentration of water of 0.5%, it was estimated that the ethanol-rich phase contains about 4.5% oil and 0.8% water, the ethanol concentration by difference being then 94.7%. The solvent phase contains 96.5% oil and essentially no water, the concentration of ethanol then being 3.5%. It was estimated that the extract product upon being cooled would split into the ethanol-rich product and the solvent to be recycled in the ratio of 1:17. Because the solvent to be recycled contains 3.5% ethanol, the feed to the extraction column must have an ethanol content higher than the concentration in equilibrium with this amount. Otherwise, ethanol will not be transferred from the water to the oil phase. The lower limit on feed ethanol content for this process in its present state of development is therefore 15% ethanol, as may be seen from Figure 4. Furthermore, 15% ethanol is the lower limit on the raffinate product as well. The extraction column was designed by means of standard procedures (e.g., Treybal, 1955),and the results of the preliminary analysis are in the literature (Fraser and Mehta, 1983). A design of the extraction column was determined for each of a number of combinations of reflux ratio and concentration of ethanol in the feed. As a practical limit, the ethanol content of the raffinate product
was taken to be 16.5% for all cases. For each design of the extraction column, ma^ balance equations were solved for relative flow rates. The results of the extractor design calculations and the material balance calculations are shown in Table 111. This table shows that with an increase in reflux ratio, the ethanol product rate and the limit on the feed ethanol concentration that can be treated by the system decrease. The energy balance calculations (Fraser and Mehta, 1983) showed that the ratio of heat input to product fuel value is also a strong function of reflux ratio. For a reflux ratio of less than 0.03, it was estimated that less than 7.3% of the product fuel value is used as the thermal energy input for the proposed separation system. As the ethanol product from this process does contain 4.5% of paraffin oil, 7.1% of the product fuel value is contributed by the residual oil. However, the product contains almost no water (less than 1%) and is suitable for use as a fuel as it is produced from the process. Conclusions On the basis of the preliminary analysis of this proposed process, the following conclusions can be drawn: 1. The proposed ethanol-water separation scheme using a partially miscible conjugating liquid system should be feasible, although further development and process improvements are needed. 2. The proposed scheme using white light paraffin oil between 115 "C and 30 "C should produce a final product with 94.7% ethanol, 4.5% oil, and 0.8% water by weight. This product can be directly used as motor-grade fuel. 3. The thermal energy required by the proposed system will vary between 4% and 34% of the fuel value of the final product. Literature Cited Francls, A. W. "Critical Solution Temperatures'', Advances in Chemistry Series, Number 31; America1 Chemical Society: Washington, D.C., 1981. Francls, A. W. "Liquid-Liquid Equilibriums"; Intersclence Publishers: New York, 1963. Fraser, M. D.; Mehta, G. D. Presented at the lSth Intersoclety Energy Conversion Engineering Conference, Orlando, FL, Aug 1983; American Institute of Chemical Englneers: New York, 1983; p 592. Magne, F. C.; Skau, E. L. J . Am. OUChem. Soc. 1953. 27, 289. Mehta, G. D.; Fraser. M. D. "FeasJbiHty of a Novel Low-Energy Extraction Process for Sepratlng Ethanol and Water Using Conjugating Solutions", Draft Report from Science Applications; InterTechndogylSoler Corporation: Mclean. VA, 1982. Sorenson, J. M.; Arb, W. "LiquidUquid Equllbrlum Data Collection: Binary System", Chemlstry Data Series, Volume 5, Part 1; DECHEMA: Frankfurt, 1979. Treybai, R. E. "Mass-Transfer Operations", McGraw-Hill: New York, 1955.
Received for review October 20, 1982 Revised manuscript received August 29, 1984 Accepted October 24, 1984 This work was funded by the Solar Energy Research Institute under Contract No. XB-1-9189-1to InterTechnology/Solar Corporation. The work waa carried out by Science Applications, Inc., under a subcontract from InterTechnology/Solar.
