Ind. Eng. Chem. Process Des. Dev. 1982, 21, 331-334
331
Gasohol Production by Extraction of Ethanol from Water Using Gasoline as Solvent Stephen A. Leeper' and Phllllp C. Wankat School of Chemical Engineering and Laboratory of Renewable Resources Engineering, Purdue Universiv, West Lafayette, Indiana 47907
A scheme for producing gasohol by distillation and then extraction of the resulting 90 wt % ethanol, 10 wt % water feed with gasoline is presented. Theoretical calculations for extraction with pure solvents are presented and used as a guide for gasoline extraction. Experimental results from a Scheibel column are presented for extraction with heptane, heptane-toluene blends, a commercial gasoline, and a naphtha. These results agreed qualitatively with the theoretical predictions and showed that a 10% ethanol gasohol mixture can be produced by extraction with
gasoline.
Although it is now a commercial process, the use of fermentation alcohol as a liquid fuel in gasohol is still controversial. The controversy centers around the net energy balance for alcohol production and the overall economics. These questions have been considered in detail by Chambers et al. (1979), Scheller and Mohr (1977), and Scheller (1978). The most energy intensive step in the process is the separation of ethanol from water. This step has recently received considerable attention. Black (1980) modeled improved distillation schemes, Ladisch and Dyck (1979) and Fanta et al. (1980) studied adsorbent drying, Wymore (1962) reported use of cation exchange resins, Gregor and Jeffries (1979) studied membrane methods, and Hartline (1979) reviewed a variety of possible separation methods. In the older literature extraction of ethanol from water was studied by several researchers. Othmer and Trueger (1941) and Othmer and Ratcliff (1943) used n-amyl alcohol and isoamyl alcohol to extract dilute alcohol from water. Schiebel (1950) investigated mixed solvent systems of methyl-n-amyl ketone and glycol, and o-xylene and glycol. Doronin et al. (1963) also studied ethanol extraction. Roddy (1981) investigated the distribution of ethanolwater mixtures in a variety of organic solvents. Recently, the possibility of direct extraction of ethanol with gasoline has invoked a flurry of letters to the editor of Chemical and Engineering News (Meyers, 1980; Arora, 1980; Gelbein, 1980, and Othmer, 1980). Some of these writers were quite critical of the idea. In this paper we present a proposed scheme for separation of ethanol from water by extraction with gasoline, and present experimental data on the extraction step. Separation Method The method proposed for recovery of the ethanol is shown in Figure 1. The predistillation column concentrates the ethanol from a feed concentration of 6 to 12 wt % to approximately 90 wt %. This mixture is then directly extracted with gasoline to form gasohol as a product. The raffinate from the extraction column is sent to a gasoline recovery still and the bottoms from this still is recycled to the predistillation column. It may be possible to send the raffinate directly to the predistillation column and recover the gasoline in this column along with the ethanol. The direct use of gasoline in a fermentation plant is not new since gasoline is used commercially as the entrainer in azeotropic distillations (Hartline, 1979).
'EG&G Idaho, Inc., Idaho Falls, ID 83415. 0196-4305/82/1121-0331$01.25/0
Since gasoline is not a good solvent for ethanol, it is necessary to preconcentrate the ethanol to a high level before extraction. Distillation up to about 90 or 91 wt % is not energy intensive (Ladisch and Dyck, 1979). Because of the unusual shape of the ethanol-water vapor-liquid equilibrium, the energy required per pound of distillate increases slowly as the ethanol concentration in the distillate increases to about 90 or 91 wt %. Above, 90 or 91 wt % the inflection point in the VLE causes a tangent pinch point and forces the use of much higher reflux ratios. Thus the energy requirement increases greatly for distillate concentrations above 91 wt % . Fortunately, 90 or 91 wt % solutions are concentrated enough to allow the direct extraction of ethanol with gasoline. The proposed process has several other advantages. Since gasohol is directly produced, solvent recovery from the product is not required. This process should consume significantly less energy than the current distillation methods. Also, since the ethanol produced is denatured in the process, there should be fewer problems with illegal use of the alcohol. This scheme probably lends itself to retrofitting of existing distillation facilities.
Theory Except for solubility data, we were originally not aware of information on the equilibrium properties of ethanolwater-gasoline mixtures in the literature. Since completing this research, the work of Nakaguchi and Keller (1979), who studied the ethanol-water-gasoline phase diagrams of three gasolines, was brought to our attention. To serve as a guide for the experimental work, we did theoretical calculations for n-heptane-ethanol-water,n-hexaneethanol-water, and toluene-ethanol-water. Equilibrium data for the first two systems are given by Vorob'eva and Karapet'yants (19661, while Washburn et al. (1939) and Mertslin and Nikurashina (1961) studied the toluene system. The extraction calculations were done using the difference point method (Smith, 1963; King 1980). First, graphical calculations were done on triangular diagrams. Then the equilibrium data were fit by interpolation and the calculations were also done on a computer. The two calculation methods agreed with each other. The most marked result of the theoretical calculations is the importance of the feed concentration. A series of computer runs were made to determine the feed concentration necessary to achieve 10 wt % ethanol in the extract. Figure 2 illustrates typical results for n-heptane. To achieve an extract which is 10 wt % ethanol with a solvent to feed ratio, S/F, of 5.5, a feed concentration greater than 0 1982 American Chemical Society
332 Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 2, 1982
Gosohol (-10
wl % Ethanol)
1
n I
'- 1
'r
E
HEPTRNF. I rl
IO R I TOLLCNF
I
lE.5 I TPLUENE _L-
I
I
Roffinate Recycle (65-80 wl % Ethanol)
Figure 1. Proceas for gasohol production by distillation and gasoline extraction.
--
__
_ __. -._ z _i_
co~Poz:-:3\
Figure 2. Theoretical prediction for heptane extraction of ethanol 10.0 ~ wt ~ %; 25 O C , 1 atm. from water. S / F = 5.5; Y N , E= ~
90.6 w t % ethanol is required. The number of stages required decreases rapidly as feed concentration increases to 91%. Other computer runs showed that as S/F decreases the desired extract concentration can be obtained with less concentrated feeds. However, as S/F decreases, the recovery of ethanol in the extract product decreases and a larger recycle stream results. Recoveries in the range 6040% were obtainable for a 10% extract. For n-heptane, increasing the number of stages past 15 had no effect, and most of the desired separation could be done in 5 or less equilibrium stages (see Figure 2). Comparison of the n-heptane, n-hexane, and toluene simulations showed that, as expected, n-heptane was the worst solvent and toluene was the best. Qualitatively, all three systems showed similar results but the critical feed concentrations were lower for n-hexane and toluene. Since gasoline is a mixture of hydrocarbons, we expect qualitatively similar results. With a feed concentration of 90 wt % ethanol, an extractor with 5 equilibrium stages with hexane as the solvent can easily produce a 10 wt % ethanol extract. Since the aromatics and olefins in gasoline are better solvents than hexane, we expect that a gasoline extractor with 5 equilibrium stages could produce a gasohol with 10 w t % ethanol utilizing a feed of 90 w t % ethanol. This prediction was verified by the experiments. Complete details of the theoretical calculations and additional pure component results are presented by Leeper (1980). Experimental Section Experimental extractions were performed using a 1-in. i.d., 36 in. long Scheibel liquid-liquid extractor with five mixing and six settling zones. The extractor was very similar to Scheibel's system (Scheibel, 1948a, 1948b).
I
-.l---
-
1
I --3
1
5OLVCNT-TO-FCEU
R 9 T I O I S/F
f
I
Figure 3. Extract compositions obtained from experimental extractions of ethanol from water with heptane and heptane-toluene blends. Feed concentration 90.5 f 0.9 wt % ethanol.
Toluene, heptane, toluene-heptane blends, and gasoline were used as solvents. Stream compositions were determined using a Carle 8500 Basic Gas Chromatograph with a thermal conductivity detector. An 8% Carbowax 1540 column operated from 85 to 89 "C was used. All composition uncertainties are reported as f 2 standard deviations. Extractions with toluene were not successful. Both toluene and water phases came out of the top and the bottom of the extractor, since the properties of the two phases were too similar. The density of toluene (0.8669) is between the densities of ethanol (0.7893) and water. As a result, there was an inversion within the extractor where the extract phase switched from being the lighter to the heavier phase. Operation of the extractor with n-heptane (Philips, 99+ pure) was successful. The startup characteristics were studied and showed that the raffinate concentration reached steady state in 50 min while other streams took less time. At steady state a low amplitude cycling of compositions and flow rates was observed. Cycling behavior was also observed by Scheibel (1948a, 1948b). A series of experiments were run to determine the HETP for a feed of 90.5 wt %. Unfortunately, this attempt was not successful. Within two standard deviations of the feed composition, f0.9%, the separation obtained experimentally was predicted to be impossible or require less than one stage. Attempts to determine the HETP with lower feed concentrations were also unsuccessful. Comparison of all our experimental runs with computer modeling is consistent with from 3 to 5 stages in the column. This is also consistent with Scheibel's results (Scheibel, 1948a, 1948b). A series of steady-state experimental extractions was done with heptane and two heptanetoluene blends. The results for a feed concentration of 90.5 f 0.9 wt % ethanol are shown in Figures 3 and 4. The blends produce an ethanol extract concentration at least as high as pure heptane. At S/F = 2.3 the 18.5% toluene blend has a significantly higher extract ethanol concentration. The recovery obtained with the blends appears to be somewhat higher than that obtained with pure heptane. These somewhat improved results with the blends are encouraging for gasoline studies since gasoline is a more complex, variable blend. The heptane results are in reasonable agreement with the computer simulations. Ethanol recovery in the extract increases and extract ethanol concentration decreases as S / F increases. Three extraction runs were made with unleaded gasoline as the solvent. The details of these runs are presented in Tables I and 11. The first run used unleaded gasoline
Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 2, 1982 333
x-
Table 11. Extraction with Amoco Naphtha
6o 50
Input composition, wt % H2 0 EtOH solv
solvent
0
o
6
P A
0
2
I
SOLVENT-TO-FEED
100 0
flowrates, g/min
P 1
0 902 1
lo* 1
feed
HEPTRNE. I 10.0
'1
TOLUENE
18.5
).
TOLUENE
solvent, S feed, F
1
1
I
3
Li
5
run 11 5.6 0.6 1.5 f 0.1 3.7 + 0.5 1700 rpm
run I 6.5 f 0.6 1.1f 0.1 5.9 f 0.8 1700 rpm
SIF
stirrer
output
R A I10 I S/F 1
Figure 4. Ethanol recoveries obtained experimentally for extraction of ethanol from water with heptane and heptane-toluene blends. Feed concentration 90.5 f 0.9 w t % ethanol.
flowrates, g/min
H,O
wt % EtOH
solv
run I
Table I. Extraction with Commercial Unleaded Gasoline flowrates, wt % dmin H,O EtOH solv Input solvents, S 9.4 * 0.1 0 0 100 feed, F 3.0 f 0.1 9.9 f 0.8 90.1 t 0.8 0 SIF 3.1 + 0.1 stirrer 1700 rpm output extract, E 12.3 * 0.9 -0 13 f 2 87 f 2 raffinate, R 1.14 f 0.09 23 f 1 62 i- 2 15f 2 recovery
57
i.
7%
purchased at our friendly neighborhood Amoco station. This gasoline worked well and produced an extract which was 13 f 2 wt % ethanol with 57 f 7% recovery of ethanol in the extract. The other two runs used a naphtha provided by Amoco Oil. This naphtha was 43.5% saturates, 22.0% olefins, and 34.5% aromatics. The naphtha's density was 0.778 f 0.002 which is higher than most gasolines and only slightly lower than the density of ethanol. An analysis of ASTM distillation, sulfur and nitrogen levels, and octane numbers was kindly provided by Amoco Oil (see Leeper, 1980). Despite the high density, operation of the extractor was satisfactory. The results shown in Table I1 indicate a 9.5 f 0.4 wt % ethanol extract with 68 f 7% recovery when an S/F of 5.9 was used. The theoretical results indicated that the extract concentration could be increased to above 10% by decreasing the S/F ratio. This was proven in run I1 of Table I1 where an extract with 10.6 f 0.4 wt % ethanol was obtained with 54 f 6% recovery when S/F of 3.7 was used. Note in Tables I and I1 that the solvent concentrations in the raffinate are quite high (14 to 17 wt %). This occurs because the raffinate is highly concentrated in ethanol. This loss of solvent is unacceptable without a solvent recovery step as shown in Figure 1. In these experiments we continually looked for water in the outlet extract (gasohol) streams, but were unable to observe any water with our gas chromatograph. This is an unexpected result which does not agree with the phase diagrams of Nakaguchi and Keller (1979). Calibration of the chromatograph showed we should have been able to detect a water peak down to 0.1 wt % water. Since the samples were exposed to air, it is possible that the volatile water evaporated. Further work to clarify the amount of water in the extract would be advisable. From these runs it is evident that a gasohol with an ethanol concentration greater than 10% can be obtained
extract, E 7.1 f 0.6 -0 raffinat.e, R 0.38 t 0.03 21 19 t 1 E /R
recovery run I1
68
i-
*1
9.5 63.0
t
0.4 90.5 f 0.4 14 i. 1
* 0.4
7%
extract, E 6.9 2 0.6 -0 10.6 t 0.4 89.4 f 0.4 raffinate, R 0.58 f 0.06 19 f 2 63.1 f 0.4 17 * 2 12f 1 E /R recovery
54
i-
6%
by gasoline extraction of a feed containing approximately 90 wt % ethanol. Higher feed concentrations will produce higher extract concentrations and/or higher recoveries. Discussion The uncertainties in the experimental measurements are fairly large. For the heptane and heptane-toluene blends each sample was chromatographed nine times. The gasoline chromatograms took longer and each sample was chromatographed three times. Use of a more sensitive detector with computer analysis of the data would decrease these errors. Despite the relatively large uncertainty, it is clear that a 10% gasohol can be produced by gasoline extraction. We expected gasoline to behave qualitatively like heptane, but it should be a somewhat better solvent than heptane. Comparison of Tables I and I1 with Figures 3 and 4 shows that these expectations were correct. At the same S / F ratios gasoline produced an extract ethanol concentration and recovery that were significantly higher than the results obtained with heptane. Within experimental error the gasoline results were the same as those obtained with the heptane-toluene blends. To compare this extraction process with other separation processes we estimated the energy requirements for the system shown in Figure 1. The energy requirements for the predistillation column were calculated using ethanol-water enthalpy-composition data. The requirements for the gasoline recovery column were estimated from the results reported by Black (1980) for a similar distillation. The extraction was assumed to produce a gasohol with 10 wt % ethanol with 68% recovery of the ethanol in the extract. Feed to the extractor was 90 wt % ethanol and the raffinate was 63 wt % ethanol and 16 wt % gasoline. Heat exchange between the feed to the extractor and the raffinate recycle was assumed as was preheating the feed to the predistillation column. The results of these estimates are shown in Table I11 and compared to the predictions of Scheller (1978) and Black (1980). The extraction process requires significantly less energy than distillation. The extraction results were not
Ind. Eng. Chem. Process Des. Dev. 1982, 21, 334-337
334
Table 111. Estimated Energy Requirements feed, wt % EtOH
distillation process reference Black (1980) Scheller (1978)
6.39
8.0
Btu/lb EtOH 4423 9186
optimized but are close to the results obtained from run I of Table 11. These energy requirements could be reduced by optimization of the extraction, by optimizing heat recovery in the distillations, and by increasing alcohol content in the feed to the predistillation. Note that the results in Table I11 are only estimates. Firmer results would require a detailed simulation of the gasoline recovery distillation column and calculation of the heat recovery obtainable. Conclusions The extraction of ethanol from water with gasoline to form a gasohol with 10 wt % ethanol was achieved experimentally. The feed to the extraction column needs to be about 90 wt % ethanol. Recovery of the gasoline in the raffinate stream and recycle of the raffinate would be required. Behavior of the gasoline extraction can be qualitatively predicted with theoretical calculations done with single component equilibrium data. Acknowledgment Discussions with Dr. George Quinn and Professors Michael Ladisch and George Tsao were most helpful. We wish to thank Amoco Oil for the donation of naphtha and the Laboratory of Renewable Resources Engineering at Purdue University and the State of Indiana for partial support.
Btu/lb gasohol . -
extraction, Btu/lb gasohol
ratio: extrldist
442.3 918.6
300 290
0.68 0.32
.
I
Chambers, R. S.;Heredeen, R. A.; Joyce, J. J.; Penner, P. S. Science 1979, 206, 789. Doronin, V. N.; Zhavoronkov, N. M.; Nikolaev, A. M.; Professy Zhidkosfoni Ekstrakti. Tr. Nauchn - Tekn. Soveshch, Leningrad, 1963, 1961, 32. (Chem. Abstr. 1964,6 0 , 3743e). Fanta, G. F.; Burr, R. C.; Orton, W. L.; Doane, W. M. Science 1980. 210, 646. Gelbeln, A. P. Chem. Eng. News 1980,58(35) 2. Gregor, H. P.; Jeffries, T. W. Government Report Announcement Index (U.S.) 1979, 79(18) 215. Report Order No. NTIS-PB-295645. Hartine, F. F. Science 1979,206, 41. King, C. J., "SeDaration Process", 2nd ed.;McGraw-Hill: New York, 1980; chapter 6. Ladisch, M. R.; Dyck, K. Science 1979,205, 898. Leeper, S.A. M.S. Thesis, Purdue Unhrersky, West Lafayette, IN, 1960. Mertslin, R. V.; Nikurashina, N. I. Russ J. Phys. Chem. 1981, 35, 1293. Mevers. R. T. Chem. €no. News 1980.58117). 4. NakaGchi, G. M.;~Keller,j.L., "Ethanol Fuel'Mddification for Highway Vehicle Use", DOE Final Report, Contract #EY-764-04-3683, Modific., A003, (DOE ALO, EY-764-04-3683-31) July, 1979. Othmer, D. F. Chem. Eng. News 1980,58(36), 4. Othmer, D. F., Ratcliffe, R. L. Ind. Eng. Chem. 1943. 35, 798. Othmer, D. F., Trueger, E. Trans. AIChE 1941,37, 597. Roddy, J. W. Ind. Eng. Chem. Process Des. Dev. 1981,20, 104. Scheibel, E. G. Chem. Eng. Prog. 1948a,44, 681. Scheibel, E. G. Chem. Eng. Prog. 1948b,44, 771. Scheibel. E. G. Ind. Eng. Chem. 1950,42, 1497. Schelier, W. A.; Mohr, 8. J. CHEMTrCH 1977, 7, 816. Scheller, W. A. "Energy Requirements for Grain Alcohol Production", presented at the 176th National Meetlng of the American Chemical Society, Miami Beach, FL, Sept 10-15, 1978. Smith, B. D. "Design of Equiilbrium Stage Processes"; McGraw-Hill: New York, 1963; Chapter 7. Vorob'eva, A. I.; Karapet'yants, N. Kh. Russ. J. Phys. Chem. 1966, 40, 1819. Washburn, E. R.; Beguin. A. E.; Beckord, 0. C. J. Am. Chem. SOC. 1939, 61, 1694. Wymore. C. E. Ind. Eng. Chem. Prod. Res. Dev. 1962, 1, 173. '
Literature Cited Arora, S. Chem. Eng. News 1980,58(29) 4. Black, C. Chem. Eng. Prog. 1980, 76(9) 78.
Received f o r review April 10, 1981 Accepted October 26, 1981
Extraction of Strong Mineral Acids by Organic Acid-Base Couples Aharon Eyal and Avraham Banlel' Casali Institute of Applied Chemistty, The Hebrew University of Jerusalem, Jerusalem, Israel 9 1904
A new method is described for the separation of strong mineral acids from other species present in aqueous solutions, by liquid-liquid extraction. An extracted acid c a n be recovered by backwashing of the extractant phase with water at concentrations approaching those in the original solutions. Even higher concentrations may be obtained
using common ion effects or temperature effects. Extractants consist of a strong organic acid and an amine dissolved in a carrier solvent. Both the organic acid and the amine are water insoluble, in free and in salt form. Extractant characteristics to satisfy the requirements of a given process are obtained through the selection of the acid and of the amine that form the extracting couple and adjustment of their molar ratio. Processes achievable include the separation and recovery of acids from mixtures of acids (e.g., recovery of H2S04,H2SiF,, and H3P04 from "wet process" phosphoric acid) and conversions by reaction in acid media (e.g. conversion of KCI to KNO, by HNO, with the recovery of concentrated HCI).
Introduction Long chain aliphatic amines form stable salta with strong mineral acids. They are therefore useful in the extraction of acids from aqueous solutions. They are also selective, which makes them suitable for the separation of strong mineral acids, present in aqueous solutions, from each other. (Bertocci and Rolandi, 1961; Grinstead and Davis, 1968). The order of selectivity is generally illustrated by the sequence: HCIOI > HN03 > HC1> HBr > H2S04> H3POk However, the extraction is practically irreversible. Reversibility may be obtained in specific systems-for 0196-4305/82/1121-0334$01.25/0
example by hydrolyzing with water at higher temperatures-but the strong acids are recovered only at low concentrations. The only published industrial process based on amine-salt hydrolysis relates to an organic acid-citric acid (Baniel, 1972), which due to its structure must have an unusually high AST contribution to free energy. Reversible extraction at fairly high acidity levels is possible with salts of amines with strong mineral acids as extractants. When only one mineral acid is present in a system the salt of that same acid with an amine is an 0 1982 American Chemical Society
