Ind. Eng. Chem. Process Des. Dev. 1084, 23, 109-115
x, = liquid mole fraction of component j xlr = interfacial liquid mole fraction of component j y, = gas mole fraction of component j y,, = interfacial gas mole fraction of component j z = packing height measured from bottom of column X = latent heat of vaporization at To
109
Kelly, R. M.; Rousseau, R. W.; Ferrell, J. K. €PA Symposium on the Environmental Aspects of Fuel Conversion Technology, V, St. Louis, MO, Sept 1980. Kelly, R. M.; Rousseau, R. W.; Ferrell, J. K. Sep. Sci. Techno/. 1981, 16, 1389. Landolt-Bornstein "Zahienwarte und Funktlonen" 6. Auflage IV. Band Technlk, 4. Teil Warmetechnik Bestandteil Ci, "Absorption in Flussigkeiten mit nledrigem Dampfruck"; Springer-Veriag: Berlin-Heidelberg-New York. 1976. McDaniei, R. Ph.D. Dissertation, Texas A and M University, College Station, TX, 1970. McDanlel, R. A.; Bassyoni. A.; Holland, C. D. Chem. Eng. Sci. 1970, 25, 633. Mlzushima, T.; Olshi, J.; Hashimoto, N. Chem. Eng. Sci. 1959, 10, 31. Olander, D. R. A I C M J . 1960, 6 , 346. Olander, D. R. Ind. Eng. Chem. 1981, 53, 121. Onda, K.; Takeuchi, H.; Okumoto, Y. J . Chem. Eng. Jpn. 1966, 1 , 56. Puranlk, S. S.: Vogelpohl, A. Chem. Eng. Sci. 1974, 29, 501. Raai, J. D.; Khurana, M. K. Can. J . Chem. Eng. 1973, 57, 162. Rousseau, R. W.; Kelly, R. M.; Ferrell, J. K. "Proceedings Gas Conditioning Conference"; The University of Oklahoma, Norman, OK. 1981; p H-1. Rubac, R. E.; McDaniel, R.; Holland, C. D. AIChe J . 1989, 15, 568. Sherwood, T. K.; Pigford. R. L. "Absorption and Extraction", 2nd ed.; McGraw-Hill: New York, 1952; Chapter 6. Sherwood, T. K.; Pigford, R. L.; Wilke, C. R. "Mass Transfer"; McGraw-Hill: New York, 1975; Chapter 9. Shulman, H. L.; Ullrlch, C. F.; Prouix, A. A.; Zlmmerman, J. 0. AIChE J . 1955, 7 , 253. Stephenson, M. J. "Analysis of a Fractional Gas Stripper"; Report No. K1895, Oak Ridge National Laboratory, Oak Ridge, TN, 1978. Stuke, B., Chem. Eng. Techno/. 1958, 25, 677. Treybal, R. E. Ind. Eng. Chem. 1969, 67. 36. von Stockar, U.; Wilke, C. R. Ind. Eng. Chem. Fundam. 19778, 76. 88. von Stockar, U.; Wilke, C. R. Ind. Eng. Chem. Fundam. 1977b, 76, 94.
Registry No. COz, 124-38-9;H,S, 7783-06-4; COS, 463-58-1; MeOH, 67-56-1. L i t e r a t u r e Cited Bolles, W L.; Fair, J. R. Spring Meeting of the American Institute of Chemlcal Engineers, Houston, TX, April 1981. Bourne, J. R.; von Stockar. U.; Coggan, G. C. Ind. Eng. Chem. Process Des. Dev. 1974, 13, 115. Bravo, J. L.;Fair, J. R. Ind. Eng. Chem. Process Des. Dev. 1982, 27, 162. Brittan, M. I.; Woodburn, E. T. AICh€ J . 1966, 72, 541. Cornell, D. W.; Knapp, G.; Fair, J. R. Chem. Eng. f r o g r . 19608, 56(8),68. Cornell, D. W.; Knapp. G.; Close, H. J.; Fair, J. R. Chem. Eng. Progr. 198Ob, 56(8),48. Denbigh, K. "The Principles of Chemical Equilibrium", 3rd ed.; Cambridge University Press: London, England, 1971; Chapter 8. Dodge, B. F.; Dwyer, D. E. Ind. Eng. Chem. 1941. 33, 485. Feintuch, H. M. Ph.D. Thesis, New York University, New York, 1973. Feintuch, H. M.; Treybai, R. E. Ind. Eng. Chem. Process Des. D e v . 1978, 17. 505. Felder, R. M.; Ferrell, J. K.; Kelly, R. M.; Rousseau, R. W. Env. S d . Techno/. 1961, 74, 658. Fellinger, L. D.Sc. Thesis, Massachusetts Institute of Technology, Boston, MA, 1941. Ferrell, J. K.; Felder, R. M.; Rousseau, R. W.; McCue, J. C.; Kelly, R. M.; Willis, W. E. "Coal Gaslfication/Gas Cleanup Test Facility: Volume 1. Description and Operation"; EPA-600/7-80-046a (March 1980). Hlral, E.; Hayashi, Y.; Oshlma, T. Kanazawa DaigakuKogakuba 1968, 5 , 67. Kelly, R. M. Ph.D. Thesis, North Carolina State University, Raleigh, NC, 1981.
Receiued for reuiew October 14, 1982 Accepted May 2, 1983
Factors Influencing Solvent Selection for Extraction of Ethanol from Aqueous Solutions Curtls L. Munson and C. Judson Klng' Department of Chemical Engineering, University of California, Berkeley, California 94720
Measurements of equilibrium distribution coefficients (capacity) and separation factors (selectivity with respect to water) are presented for extraction of ethanol from dilute aqueous solution by a number of different solvents and solvent mixtures. The results are interpreted in terms of the trade-off between capacity and selectivity. Among classes of solvents, Lewis acids have much more favorable combinations of capacity and selectivity than do Lewis bases. Branching is also favorable, a resutt which is rationalized through steric (cone angle) factors. Requirements for solvent selectivity can be ameliorated by the use of an extractivedistillationdewatering step, receiving the extract a s feed. Other important solvent properties are its volatility in a distlllative solvent-recovery step and the ease with which residual soivent may be removed from water by stripping or other means.
weight fraction of ethanol in the solvent phase to that in the aqueous phase, a t equilibrium, and, in most cases, a t high dilution. The greater the value of the distribution coefficient, the less the solvent-to-water ratio that will be required for effective extraction. Lower solvent-circulation rates, in turn, lead to less energy consumption for solvent regeneration. The ability of the solvent to remove ethanol selectively over water may be described by the separation factor, CY, which is defined as the ratio of the distribution coefficient of ethanol to that of water. The value of CY is independent of the units used for the equilibrium distribution coefficients. To the extent that the value of a is greater than 1,the solvent preferentially extracts ethanol rather than water. As has been discussed, for example by King (1980), in the absence of specific molecular interactions there is a natural trade-off between solvent capacity and selectivity.
Introduction
There has been renewed strong interest in technology for separating ethanol from dilute solutions in water. This has been brought about by the energy-intensive nature of conventional distillation and axeotropic-distillation processes. When applied to recovery from dilute solutions, these processes require energy costs which are substantial fractions of the value of the product ethanol as a fuel constituent or chemical intermediate. The separation problem is particularly important in the production of ethanol from biomass by fermentation processes. The choice of solvent is a first and basic consideration for an extraction process. Two desirable characteristics for a solvent are a high capacity for ethanol and a high selectivity for ethanol over water. The capacity to extract ethanol is described by the equilibrium distribution coefficient, KD, which is here defined as the ratio of the 0196-4305/84/1123-0109$01.50/0
0
1983 American Chemical Society
110
Ind. Eng. Chem. Process Des. Dev., Vol. 23,No. 1, 1984
Since both factors are important, it is desirable to seek a solvent or extractant which offers specific functional-group interactions with ethanol. The principal purpose of this paper is to examine what classes of solvents offer such favorable interactions. In addition, some systems of mixed solvents are examined, and other factors important in solvent selection are discussed. Experimental data were obtained for solvents covering a range of functional groups, degrees of Lewis acidity/ basicity, molecular weights, and isomeric configuration. Put together with other available data and guidance from various theoretical approaches, these offer bases for inferring desirable solvent properties from a molecular viewpoint. One obviously important factor influencing solvent selection in fermentation-based processes can be solvent toxicity. This can severely limit the types of solvents which can be considered for direct contact with recycled fermentation broths and can lead to ways other than conventional extraction for implementing solvents- for example, solvent entrapment in polymeric membranes. Since toxicity considerations are quite specific to the type of overall process that is contemplated, they are not considered further in this paper. Experimental Section Procedure. Extractions were performed by placing 10-mL of a solution containing 0.78% w/w ethanol in water into a sealed 125-mL Erlenmeyer flask, along with 50 g of organic solvent. The feed concentration and phase ratio were selected to provide concentrations of ethanol in both phases that were dilute and yet high enough to be analyzed readily. The flasks were vigorously shaken in a temperature-controlled water bath for 24 to 48 h and were permitted to settle for 1-2 h. Samples were then removed from each phase and centrifuged before analysis. Both aqueous and organic phases were analyzed by gas chromatography with a Varian Model 3700 chromatograph with a thermal-conductivity detector. A column, 0.63 cm inside diameter and 152 cm long filled with Poropak Q packing, was used a t 160 "C with temperature programming where necessary. Solvents. Solvents were obtained from a number of different sources, as indicated in Table I. In some cases practical-grade or commercial solvents were used, as shown. Results The results obtained are tabulated in Table I. For cases where repeated measurements were made, the number of measurements and the standard deviation (a) of the data are given in parentheses. Comparison with data from two other comprehensive studies is presented in Table 11. The degree of agreement with the results of these other studies is for the most part reasonable, considering the differences in temperature and ethanol concentrations among the different investigations. The values for the separation factors for TBP and DIBK in this investigation are, however, somewhat higher. Extraction with TBP was repeated with material from two sources with identical results. Differences should not be due to entrainment in the present work, since this phenomenon results in a lowering of the separation factor toward unity. Differences in separation factors obtained from different studies can also be assessed by reference to Table 111, where the indicated distribution coefficients (solubilities) are compared with reported of water, obtained as KD/a, values of the solubility of water in the solvent in the absence of ethanol. The majority of the solubilities presented in Table I11 are the recommended values of Sorensen and Arlt (1979). For many systems there is a wide scatter in
0
Diluent MIBK
DIBK 0 IEHK
0
08
20
00
40
neodeconoic ocid ---2-ethyl hexonoic ocid 60 00 100
Wt % Carboxylic Acid in Solvent Mixture
Figure 1. Distribution coefficients for extraction of ethanol from water by solvent mixtures.
0
loo'
MIBK
A Toluene
DIBK 0 IEHK 0
I
-8 80-
'\ \
L
0
"
c
.
2
L0 L E
0
60-
L
0 L
/-
----a'
neodeconoic ocid -- 2-ethyl hexonoic ocid
0
1
0
20
I
I
40
60
,
80
1
IO0
W t % Carboxylic Acid
Figure 2. Separation factors for extraction of ethanol from water by solvent mixtures.
the reported values of water solubility. It is not surprising, therefore, that agreement is not always close. One would expect that the presence of ethanol in the solvent phase would probably increase the solubility of water, if anything, for the solvents considered here. The values presented are in good agreement with the result of this work for 2ethylbutanol and TBP. For 1-hexanol the solubility falls midway between that calculated from the present data and that reported by Souissi and Thyrion (1981). The value reported here, however, does fall properly above that for the binary system. Deviations for DIBK are not large enough to account for the differences in reported values of a. Figures 1and 2 show values of KD and a measured for extraction of ethanol with solvent mixtures composed of various proportions of carboxylic acids (2-ethylhexanoic acid or neodecanoic acid) in MIBK, DIBK, IBHK, and toluene as diluents. Discussion Lewis Acidity/Basicity. For extraction systems of ethanol, water, and many of the solvents considered here it is useful to interpret solvent capacity and selectivity through donor-acceptor concepts, or Lewis acidity and Lewis basicity, The donor and acceptor numbers of Gutman (1978) are useful for this purpose. The donor
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 1, 1984 I
neodecaAoic a c i d
.
2.ethyl - 4 - m e t h y l pentonoic acid 2-ethyl hexonoic acid
A
I
'A
-
o 2,6-dimethyl - 4-heptanol
i
A
2-ethyl hexanol
x
Roddy n-dcdecanol
'
n-actono1>*\ acid
n-tridecono%, \
0
Acids 0 Alcohols
02.3.4-trimethyl -3-pentanol
\
..
\
\
\
'. '\
Ketones Phosphoryls Amines 0 Carboxylic Acids Chlorinated Hydmcorbons Alcohols Esters Aromatics Other
I ' 0.001
111
n-decanol o\ n-octanolO\\,
\.,n-hexanaic
acid
2 - m e t h y l cyclahexanol o nmnanol 0 3-methyl - 2 - b u t a n o l \gn-hexanol \
21 -2
~
I
0.0 I
-I
In
e
0
I
1
1
1
0 .I
I
KD
0
I
I
Figure 4. Equilibrium distribution coefficients and separation factors for extraction of ethanol from water into alcohols and carboxylic acids.
KD
Figure 3. Equilibrium distribution coefficient vs. separation factor for extraction of ethanol from water. Data of Roddy (1981),Souissi and Thyrion (1981),and the present work for temperatures between 20 and 35 "C.
number is a measure of Lewis basicity and is defined as the molar enthalpy for the reaction of the donor molecule with SbCl,, an electron acceptor, in a M solution in 1,2-dichloroethane. The acceptor number is a measure of Lewis acidity and is a dimensionless number related to the NMR chemical shift of 31Pin Et3P0 in the particular solvent. For this purpose hexane and 1,2-dichloroethane are reference solvents which are assigned values of 0 and 100, respectively. A number of values of donor and acceptor numbers are presented in Table IV. Extraction equilibrium data from the present work, Roddy (1981),and Souissi and Thyrion (1981) are plotted in Figure 3, in the form of separation factor (a)vs. equilibrium distribution coefficient (KD). The data were taken a t different temperatures and ethanol concentrations, both of which affect a and KD. Despite the variety of experimental conditions and solvents, the natural trade-off between a and KD is clearly apparent. At any given K,, however, a large range of values of a may be seen. Ethanol and water have both electron donating and electron accepting capabilities. Ethanol, however, has a slightly larger donor number and a lower acceptor number than water. It is therefore reasonable to expect Lewis acids to associate preferentially with ethanol in the organic phase and provide higher separation factors. This trend can be seen in Figure 3. The data for the more basic extractants, such as amines, phosphoryls, and ketones, fall below those for the more acidic extractants, such as chorinated hydrocarbons and carboxylic acids. Thus, for a given equilibrium distribution coefficient, greater selectivity for ethanol over water will be obtained by use of solvents which are stronger Lewis acids than Lewis bases. Isomeric Configuration of Solvent. Figure 4 is a plot of a vs. KD, limited to carboxylic acids and alcohols as solvents. The normal acids and alcohols fall roughly onto two parallel lines with that for the normal carboxylic acids lying above that for the alcohols. It can be seen that branching of both the carboxylic acids and the alcohols is favorable, giving substantially higher separation factors. Roddy (1981) also noted that branching appeared to be favorable, on the basis of more limited data.
Figure 5. Equivalent cone angles of a hydrogen atom and of a methyl group bonded to a carbon atom.
Steric effects due to branching could reasonably be expected to limit access to the basic electrons of the hydroxyl oxygen of ethanol more than to that of the acidic proton. In support of this argument it is useful to consider a steric parameter, or cone angle, similar to that sometimes used in the inorganic chemistry literature (Tolman, 1977; Ferguson et al., 1978). The calculation method used here for this purpose is illustrated in Figure 5. Each molecular group is considered as an equivalent cone of apex angle, y, and height, d. The conical apex is taken to be a t the center of the atom to which the group is bonded. For a hydrogen atom bonded to a carbon atom, the cone angle has a height equal to the bond length and a base radius equal to the van der Waals radius of hydrogen. For a carbon atom, a side is determined by that line of minimum length which touches the base of the equivalent cone of the substituent group. The cone angle and height thereby determined are then used to define the equivalent cone. Figure 6 uses the characteristic angles thus calculated to illustrate the influence of steric hindrance on selectivity. A family of curves is obtained for the alcohols of various molecular weights, with selectivity increasing with increasing steric hindrance of the hydroxyl group. This trend is not obeyed by the smaller alcohols. The presence of solvent alcohol dissolved in the aqueous phase can be expected to reduce the activity coefficient of ethanol in that phase. The solubilities of the alcohols in water increase with decreasing molecular weight. For the C6alcohols this effect is apparently enough to offset the effect of increased solvent branching in the organic phase. Solvent Mixtures. Mixed solvents in associating systems can give equilibrium distribution coefficients and separation factors substantially different from what would be predicted by simple interpolation between the values measured for the individual solvent constituents as pure solvents. Cases have been reported (e.g., Ricker et al., 1979) where much higher values of KDare realized with solvent mixtures than with either solvent constituent alone. In other cases mixed solvents may give lower extraction capacity than with either constituent alone. The behavior
112
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 1, 1984
Table I. Experimental Measurements of Equilibrium Distribution Coefficients and Separation Factors, from the Present Work solvent/water solvent (repetitions) KDb (O) 01 ( u ) T,"C volratio source of solvent (purity) .-, Ketones 0.50 15 30 4.0 M.C.B.C (reagent) MIBK' 0.19 (0.01) 35 (2.0) 30 4.15 Eastman (practical) DIBK' ( 3 ) 42 (2.8) 30 4.75 Union Carbide (unknown) IBHKa ( 4 ) 0.13 (0.01) Aldrich (98%) 15 30 3.6 isophorone 0.79
________I____________________I_
_ _ _ _ _ _ I I _ ~ _ _ _ _ _ _ _ _ _ _ _ _ _ _ - _ _ - - - _ _ _ I _ _ _
n-hexanoic acid ( 2 ) 2-ethylhexanoic acid ( 3 ) n-octanoic acid ( 2 ) 2-ethyl-4-methyl-n-pentanoic acid neodecanoic acid
1.0 (0.1) 0.52 (0.01) 0.60 (0.05) 0.49 0.23
Carboxylic Acids 15(0.5) 30 44(2.1) 30 30 23 (1.1) 51 30 13 30
4.2-4.6 4.0-4.5 4.4-5 5.6 2.52
M.C .B. (practical) Aldrich (99%) Aldrich (spectrophotometric) Pfaltz and Bauer (unknown) Exxon (technical)
TBPa ( 4 )
0.79 (0.03)
Phosphoryl 12(1.0) 30
2.4-4.9
M.C.B. (unknown) and Mobil (unknown)
benzene toluene nitrobenzene
0.058 0.040 0.091
Aromatics 115 25 100 25 36 30
4.4 4.3 4.6
Aldrich (spectrophotometric) Mallinckrodt (spectrophotometric) Aldrich (99%)
Adogen 368
0.040
4.5
30
3.6
Rohm and Haas (unknown)
methylene chloride carbon tetrachloride 1,2-dichloroethane tetrachloroethane
0.10 0.021 0.074 0.12
Halocarbons 79 140 49 150
30 30 30 30
5.82 7.9 3.1 5.0
Mallinckrodt (unknown) Aldrich (spectrophotometric) Aldrich (unknown) Aldrich (98%)
1-hexanol 1-hexano 1 3-methyl- 3-pentanol 4-methyl-2-pentanol aethylbutanol ( 2 ) 3-ethyl-3-pentanol 3-methylcyclohexanol 1-octanol 2-ethylhexanol 2,3,4-trimethyl-3-pentanol 1-nonanol(2) 2,6-dimethyl-4-heptanol 1-decanol
1.0 1.2 1.3 1.1 0.97 0.06) 1.1 0.93 0.64 0.66 0.82 0.72 0.01) 0.53 0.57
Alcohols 9.5 1.2 13 17 19.6 (0.3) 18 15 11 24 23 13(0.7) 34 13
25 30 30 30 30 30 30 25 30 30 30 30 30
4.13 4.13 4.14 4.04 4.16 4.20 4.65 4.13 4.2 4.95 4.10 4.10 4.15
Kodak (practical) Kodak (practical) Aldrich (unknown) Eastman (unknown) Aldrich (98%) Eastman (unknown) Eastman Kodak (practical) Aldrich (99%) M.C.B. (practical) Pfaltz and Bauer (unknown) Kodak (practical) M.C.B. (technical) Calbiochem (unknown)
Amine
Mixed Solvents _
_
_
_
_
I
~
_
_
___I
solvent 1
solvent 2
% solvent 1(w/w)
KD
a
T, "C
2-ethylhexanoic acid 2-ethylhexanoic acid 2-ethylhexanoic acid 2-ethylhexanoic acid neodecanoic acid neodecanoic acid Adogen 368 Adogen 368 Adogen 368
MIBK MIBK toluene DIBK IBHK toluene DIBK tetrachloroethane tetrachloroethane
62.8 42.9 48 50 50 50 50 50 25
0.81 0.76 0.27 0.41 0.30 0.16 0.13 0.30 0.21
36 21 67 33 41 81 4.3 33 39
30 30 30 30 30 30 30 30 30
MIBK, methyl isobutyl ketone; IBHK, isobutyl heptyl ketone; DIBK, diisobutyl ketone; TBP, tri-n-butyl phosphate. K D expressed as weight fraction ethanol in solvent phaselweight fraction ethanol in aqueous phase, a t equilibrium. M.C.B. = Matheson, Coleman and Bell. a
encountered in any particular case seems to be a complex and difficult-to-predictcombination of a number of factors, including the influence of various constituents on the degrees of acidity or basicity of the other constituents, preferential association between the components of a solvent mixture, the degree of solvation of a complexed form of the solute by the solvent mixture, and/or the ability of multiple solvent constituents to interact separately or synergistically with multiple functional groups on the solute molecule. For the case of ethanol extraction from water, Souissi
and Thyrion (1981) reported measurements of equilibrium distribution coefficients with solvent mixtures of 2ethylhexanol and isodecanol, and of either of these components with ethyl hexyl acetate, DIBK, dibutyl phthalate, and xylene. No substantial departures from simple blending of the pure solvent properties were seen, except that an equivolume mixture of the two alcohols may have given a slightly higher value of KD than either alcohol alone. Figures 1 and 2 show the variation of KDand CY with solvent composition for carboxylic acid extractants with
Ind. Eng. Cham. Process Des. Dev., Vol. 23, No. 1, 1984 113 Table 11. Comparison of Results from the Present Study with Some Previous Measurements
_ - _ _ _ _ - _ - _ _ _ _ _ 1 _ _ 1 _ ~ _ _ I _ _ _ _ _ _ _ _ _ _ _ _ _ _
this work
Souissi and Thyrion (1981)
Roddy (1981)
_ _ _ _ _ _ _ _ l l l l _ _ _ _
solvent
a
KD
T, "C
CY
C E ~
MIBK DIBK TBP benzene
0.5 0.19 0.8 0.058
15 35 12 115
30 30 30 25
0.78 0.78 0.78 0.78
toluene
0.040
100
25
0.78
1-hexanol 2-ethylbutanol 1-octanol
0.97 0.64
25 30 25
0.78 0.78 0.78
1.0
9.5 19.6 11
____I_-___
KD
CY
T,"C
CEO
0.11 0.56 0.053 0.045 0.039 0.037
19 8.3 110 85 100 110
.25' 25 25 20 25 20
4.6 4.6 4.6 0.46 4.6 0.46
0.83 0.61
30 12
25 25
4.6 4.6
KD
CY
T , "C
0.52 0.43 0.79
11.8 11.8 8.7
35 35 35
0.63
15
35
CE = ethanol concentration in aqueous feed, wt %.
Table 111. Comparison of Reported Solubilities of Water in Different Solvents with Calculated Eauilibrium Distribution Coefficients for Water equilibrium distribution coefficients Roddy (1981)
Souissi and Thyrion (1981)
this work
solubility (ref)
TBP
0.0675125 "C
0.0963135 "C
0.0666130 "C
DIBK
0.0058/25 "C
0.0662125 "C (Hardy, 1964) 0.0624140 "C 0.0055/25 "C (Sorensen and Ark, 1979) 0.022130 "C (Sorensen and Arlt, 1979) 0.076130 "C (Sorensen and Arlt, 1979) 0.0349125 "C (Sorensen and Arlt, 1979) 0.046120 "C (Mellan, 1970) 0.00046125 "C (Sorensen and Arlt, 1979) 0.00069125 "C (Sorensen and Arlt, 1979)
solvent
0.0054130 "C 0.0333130 "C 0.105130 "C 0.0508/30 "C 0.0487130 "C 0.00035/25 "C 0.00048125 "C
0.0441122 "C 0.042135 "C
1-hexan ol 1-octanol 0.058125 "C 2-ethylbutanol 0.0277125 "C toluene 0.00039125 "C benzene 0.00048125 "C Table IV. Donor and Acceptor Numbers for Some Compounds (Gutman, 1978)
solvent
DN
1,2-dichloroethane benzene nitro benzene water diethyl ether ethanol tri-n-butyl phosphate hexane cc1, CH ;Cl, CHCI,
0 (ref) 0.10 4.4 18 19.2 20 23.7
AN 8.2 14.8 54.8 3.9 37.1
0 (ref) 8.6 20.4 23.1
o This work 40
-
0
Roddy (1981)
2,2 dimethyl- 3-octonol
5 - 30L
0
e
3-ethylI-heptonol
u
I? C
0 .-c
:2 0 a
2-ethyl hexonol 2-ethyl
butanol
01
2,3,4-trimethyl- 3-pentanol
0 3.ethyl -3-pentonol
ffl
I-decanol
I-nonanol I -octanol
I
3-methyl - 3-pentonol
I I I I20 I30 140 Characteristic Angle, 2 y ( d e g )
J
Figure 6. Separation factors for alcohols a a function of steric cone angle.
various diluents, as measured in the present work. For most mixtures there is a slight positive deviation for the distribution coefficient and a corresponding negative deviation for the separation factor as the relative proportions of the solvent components are varied. More interesting is the case of 2-ethylhexanoic acid and MIBK as the solvent mixture. Here there is a large increase in the distribution coefficient with only a slight decrease in separation factor. This mixed solvent will therefore allow a substantial increase in capacity with only a slight loss in selectivity. Other favorable combinations of extractants are possible. Other Important Solvent Properties. In discussions of solvent extraction for recovery of ethanol from dilute aqueous solution it has sometimes been implied that a very high separation factor is needed, which can be achieved only with a solvent giving an equilibrium distribution coefficient that is so low as to make the process uneconomical. For example, a separation factor of 610 would be needed to enable production of 97% w/w ethanol (above the atmospheric ethanol-water azeotropic composition) directly from an extract which has been equilibrated with a feed containing 5% w/w ethanol in water, without further dewatering. However, in analyzing the separation factors required for the extraction, it is important to recognize that the solvent itself can provide favorable vapor-liquid equilibrium properties for dewatering the extract by azeotropic or extractive distillation. Such an approach has been used effectively on a large commercial scale for many years for the manufacture of glacial acetic acid, obtained by extraction of acetic acid from aqueous solutions. The extraction processes used for acetic acid commonly employ solvents such as isopropyl acetate or mixtures of ethyl acetate and benzene and utilize the solvent as the entrainer in a subsequent azeoptropic distillation to dewater the extract (Eaglesfield et al., 1953; Brown, 1963; King, 1983). These solvents do not provide particularly high values of KDfor acetic acid-around 0.7 for isopropyl acetate (Ea-
114
Ind. Eng. Chem. Process Des. Dev., Vol. 23,No. 1, 1984
r
Extractor/
1
L--4
i Recycle f o r _Extractive _ _ _ _Distillat*on _
Solvent water
i-- , 1 I
Out
Solvent Recycle
t
Figure 7. Use of solvent to volatilize water in a subsequent extractive distillation, thereby lessening requirements for solvent selectivity in extraction. Heat exchangers are not shown.
glesfield et al., 1953)-but they are chosen because of their carrying capacity for water in the azeotrope which passes overhead in the azeotropic-distillation dewatering column. Isopropyl acetate provides separation factors in the range of 12 to 17 for aqueous acetic acid concentrations of 3 to 10% w/w (Eaglesfield et al., 1953), but the isopropyl acetate-water azeotrope is able to carry off that much water. The favorable azeotrope water-carrying-capacity property more than outweighs the rather low equilibrium distribution coefficient and thereby makes isopropyl acetate an effective solvent. Ethanol has a much lower boiling point (78.5 OC) than does acetic acid (118 "C). Hence it is difficult to find a lower-boiling solvent for ethanol which is sufficiently immiscible with water. Chloroform and methylene chloride are two possibilities which give high separation factors and probably also provide good azeotroping capacity for water removal, but the values of KD (0.12 and 0.10) would make for very high solvent-circulation rates, which would probably not be economical. High-boiling solvents are more plentiful. Here again, the solvent properties can be used to advantage for dewatering, this time in an extractive-distillationdehydration step immediately following the extraction and receiving the extract as feed (Figure 7). Similar processes for recovery of acetic acid by extraction with high-boiling solvents have been described by Othmer (1958) and Brown (1963). Such a process requires one more distillation column than the combined extraction and azeotropicdistillation processes for acetic acid, since with a lowboiling solvent the aceotropic-distillation dewatering column can be made to serve two purposes: an overhead decanter can create both recycle solvent and an aqueous phase to be sent to the solvent stripping column (Brown, 1963; Eaglesfield et al., 1953). The capabilities of the solvent for preferentially volatilizing water in the extractive-distillation column relate to the activity coefficient of water in the solvent, which in turn relates directly to the solubility of water in the solvent and thereby to the separation factor in the extraction, albeit at a different temperature. Solvents giving higher activity coefficients for water, and hence having lower equilibrium distribution coefficients for water and higher extraction separation factors, will be more effective for dewatering by extraction distillation. The volatility of the solvent is also important; it must propagate high enough in the distillation column to keep water more volatile than ethanol on the upper stages. Alternatively,
it may be necessary to recycle a portion of the recovered solvent to the upper rectifying section of the extractive distillation column, as shown in Figure 7. The solvent volatility must be sufficiently less than that of ethanol to facilitate separation in the solvent-recovery distillation column, yet it should not be so low as to create a large preheat demand and/or an excessively high reboiler temperature for that distillation. If necessary, a diluent can be incorporated into the solvent so as to provide volatility in this reboiler. The solubility of the extraction solvent in water usually is great enough to make it necessary to remove and recover residual solvent from the aqueous raffinate leaving the extractor. In the process of Figure 7 this is accomplished by steam stripping, often carried out a t atmospheric pressure. Atmospheric steam stripping is not the only alternative, however. Other possibilities are (1)inert-gas stripping, as in the Phensolvan process for extraction of phenols (Wurm, 1968), (2) extraction into a highly immiscible, nonpolar sovlent (Earhart et al., 1977), and (3) vacuum steam stripping (Greminger et al., 1982), which reduces or eliminates the need for preheating the stripper feed. In the cases of vacuum and atmospheric steam stripping, both the solvent volatility and its activity coeficient in water (related to the solubility of the solvent in water) are important design parameters. Selection of the solvent for extraction can therefore be strongly influenced by these parameters.
Conclusions Selection of solvents for extraction of ethanol from dilute aqueous solution should be guided by considerations of selectivity with respect to water (separation factor), as well as equilibrium distribution coefficient for ethanol. A natural trade-off exists between equilibrium distribution coefficients and selectivity; however, solvents that are Lewis acids show distinctly more favorable combinations of these properties than do solvents that are Lewis bases. For alcohols and carboxylic acids as solvents, branching of the solvent molecule has been found to be favorable. This phenomenon is rationalized through steric concepts, using cone-angle calculations. Solvent mixtures of 2ethylhexanoic acid and MIBK give more favorable equilibrium distribution coefficients than either solvent alone, with little loss in selectivity for ethanol over water. The requirements for a high separation factor in extraction of ethanol from water can be relieved somewhat by the use of the volatilizing properties of the solvent for water in an extractive-distillationdewatering step receiving the extract as feed, as shown in Figure 7. Other important solvent properties relate to its volatility in a solvent-recovery distillation column and the ease with which residual amounts of solvent can be removed and recovered from the raffinate water by stripping or other means. Registry No. MIBK, 108-10-1; DIBK, 108-83-8; IBHK, 19594-40-2;ethanol, 64-17-5; isophorone, 78-59-1; n-hexanoic acid, 142-62-1;2-ethylhexanoic acid, 149-57-5;n-octanoic acid, 124-07-2; 2-ethyl-4-methyl-n-pentanoicacid, 108-81-6; neodecanoic, 26896-20-8; benzene, 71-43-2; toluene, 108-88-3; nitrobenzene, 98-95-3; methylene chloride, 75-09-2; carbon tetrchloride, 56-23-5; 1,2-dichloroethane, 107-06-2; tetrachloroethane, 25322-20-7; 1hexanol, 111-27-3; 3-methyl-3-pentano1, 77-74-7; 4-methyl-2pentanol, 108-11-2; 2-ethylbutanol, 97-95-0; 3-ethyl-3-pentano1, 597-49-9; 3-methylcyclohexanol, 591-23-1; 1-octanol, 111-87-5; 2-ethylhexanol, 104-76-7; 2,3,4-trimethyl-3-pentanol, 3054-92-0; 1-nonanol, 143-08-8;2,6-dimethyl-4-heptanol, 108-82-7;1-decanol, 112-30-1.
Literature Cited Brown, W. V. Chem. f n g . P m g . 1963, 59(10), 65. EaglesfleM P.; Kelly, B. K.: Short, J. F. Ind. Chem. 1953, 29, 147, 243.
Ind. Eng. Chem. Process Des. Dev. 1984, 23, 115-121 Earhart, J.; Won, K. W.; Wong, H. Y.; Prausnitz, J. M.; King, C. J. Chem. Eng. frog. 1977, 73(5), 67. Ferguson, G.; Roberts, P. J.; Alyea, E. C.; Khan, M. Inorg. Chem. 1978, 77(10), 2965. Greminger, D. C.; Burns, G. P.: Lynn S.;Hanson, D. N.; King, C. J. Ind. Eng. Chem . Process Des. Dev. 1982, 2 7 , 5 1. Gutman. V. "The Donor-Acceptor Approach to Molecule Interactions"; Plenum Press: New York, 1978; Chapters 1-3. Hardy, C. J.; Fairhurst, D.; McKay, H. A.; Wilson, A. M. Trans. Faraday SOC. 1964, 60, 1626. King, C. J. "Separation Processes", 2nd ed.; McGraw Hill Book Co.: New YOrk, 1980; pp 757-763. King, C. J. "Acetic Acid Extraction", chapter in Lo, T. C.; Baird, M. H. I.; Hanson, C., Ed., "Handbook of Solvent Extraction"; Wiley-Interscience: New York, 1983. Mellan, I."Industrial Solvents Handbook"; Noyes Data Corporation: Park Ridge, NJ, 1970. Othmer, D. F. Ind. Eng. Chem. 1958, 50(3), 60A.
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Ricker, N. L.; Michaels, J. N.; King, C. J. J. Sep. Proc. Techno/. 1979, 7(1), 36. Roddy, J. W. Ind. Eng. Chem. Process Des. Dev. 1981, 20, 104. Sorensen, J. M.; Ark, W. "Liquid-Liquid Equilibrium Data Collection, Binary Systems", Chemical Data Series, Vol. V., Part 1, DECHEMA, Schon & Wetzel GmbH: Frankfurt, W. Germany, 1979. Souissi, A.; Thyrion, F. C. Proc. 2nd World Congr. Chem. Eng. 1981. 4, 443. Tolman, C. A. Chem. Rev. 1977, 77, 313. Wurm, H. J. Gfickauf 1988, 704, 517.
Received for review October 12, 1982 Revised manuscript received April 11, 1983 Accepted April 18, 1983
This paper was presented at the International Solvent Extraction Conference (ISEC'83),Denver CO, Sept 1 1983.
Rheological and Conductometric Properties of Two Different Crude Oils and of Their Fractions Jos6 A. Fernhnder-Lorano' and Yldrls M. Roddguer Department of Chemical Engineering, Postgraduate Studies, Universidad de Oriente, Nircleo de Anzdtegui, Puerto la Cruz, Venezuela
The resistivity and viscosity of two crudes, one asphaltic (Boscan) and the other waxy, and of their fractions were simultaneously determined for different test conditions in a modified Fann viscosimeter. The relationship between resistivity and viscosity is correlated by an equation of the general form R = K X IO"'", where R is the resistivity in ohms, v is the viscosity in centipoises, and K and m are experimental constants. Waxes are responsible for anomalous flow behavior, time dependency, and deposits. The results suggest the presence of a micellar structure of which waxes are the main constituents in an oil base. From the contribution of waxes, asphaltenes, and resins to the electrical resistivity it can be concluded that the structures formed are of very different nature. The application of voltage to a waxy crude greatly reduces its viscosity, and paraffin deposition can be efficiently inhibited by hard asphalt and polyethylene.
Introduction The flow and static properties of many crude oils, in particular those of the waxy type, show pronounced changes in the rheological properties with temperature, age, and previous history. These changes make pumping difficult or may even stop the flow of these crudes if gelling occurs. The importance of determining the effects of age, temperature, and shearing on the static and flow properties of crude oils has been recognized in recent years, and several studies have been done in this field (FernindezLozano, 1969; Ackroyd et al., 1960; Verschuur et al., 1971; Knegtel and Zellinga, 1971; Barry, 1971; Ferntndez-Lozano and Gago, 1982). Some investigators (Ackroyd and Cawley, 1953; Ruse1 and Chapman, 1971; Seillers and Wyllie, 1960) have shown that asphaltenes, resins, and waxes under certain conditions can form an internal structure in crude oils, and that this structure may be broken down as a result of shearing. It has also been shown that the anomalous rheological behavior of crude oils is due to this structure (Lagoutte and De Saint-Palais, 1963). Crude oils may be envisaged as colloidal systems formed by asphaltenes and resins (solutes) in a paraffin base (solvent). The colloidal picture of crude oils was rather confusing because the polar compounds were postulated as macromolecular, micellar, and association colloids (Winniford, 1963). Although experimental findings (Ray et al., 1957; Witherspoon and Munir, 1960) supported the view that petroleum and asphalt colloids possessed micellar 0196-4305/84/ 1123-01 15$01.50/0
structure, a new definition of a micelle structure was not established. Therefore, references were made in the later studies to the early micelle model of Pfeiffer and Saal (1940), who defined the micelle as a colloidal particle composed of central asphaltenes which are stabilized by resins and the resins in turn are surrounded by aromatic and naphthenic hydrocarbons. This situation prevailed until Neuman (1965) published the results of his ultrafiltration study. In this study it was postulated that the central core of the micelle was formed by polar compounds (asphaltenes and resins) stabilized by a protective envelope of naphthenic hydrocarbons. However, it is not necessarily true that the stabilizing envelope has to be of naphthenic hydrocarbons. Studies performed by the author reveal that polar compounds might be solvated with any nonpolar solvent which does not destroy the micelle core association. In the past, conductometric studies of crude oils made in static systems were done to establish the colloidal state of asphaltenes in oils (Eldib, 1962; Katz and Ben, 1944; Preckshot et al., 1943). No prior studies of the conduct ometric properties of crude oils in flowing systems have been published from which the conductometric behavior of a waxy crude oil could be developed. It will be shown later in this work that the conductometric behavior of oils may be explained, in part, on the basis of the general concepts, properties and behavior of molecules and colloidal systems under an electrical field (Atkins, 1978; Davies, 1967). From previous considerations, marked 0 1983
American Chemical Society
