748
Ind. Eng. Chem. Process Des. Dev. 1984, 23, 748-754
Drehmel, D. C.; Martin, G. B.; Abbott, J. H. EPAEPRI Joint Symposium on Flue Gas Desulfurization, Hdlywood, FL. May 1982. Glasson, D. R. J. Appi. Chem. 1958, 8 , 794. Harvey, R. D.; Steinmetz, J. C. Environ. Geol. Notes 1971, 5 0 : Illinols State Geoloaical Survev. IL. ~ , Urbana. . Levy, A."Utlliiition AR 8 TD Contractor's Review Meeting", U S . Department of Energy. PETC, Pittsburgh, PA, March 1982. Ruth. L. A.: Sauires. A. M.:Graff, R. A. Environ. Sci. Technoi. 1972, 6 , 1009. ~
Siegel, S.; Fuchs, L. H.; Hubble, B. R.; Nielsen, E. L. Environ. Sci. Techno/. 1978, 12, 1415. Van Houte, G.; Rodrique, C.; Genet, M.; Dehmon, B. Environ. Sci. Techno/. 1981, 15, 327.
~
Received for revierv April 12, 1983 Revised manuscript received October 24, 1983 Accepted December 8, 1983
Solvent Extraction for Removal of Polar-Organic Pollutants from
Dlllp K. Joshl, John J. Senetar, and C. Judson King" Department of Chemical Engineering, University of California, Berkeley, California 94 720
Factors favoring solvent extraction as a method for removing polar-organic pollutants from water streams are discussed, along with factors influencing solvent choice. Both conventional and chemically complexing extractants are considered. Equilibrium data are presented for extraction of acrolein, acrylonitrile, N-nitrosodimethylamine,
2-chlorophend, isophorone, nitrobenzene, bis(2chloroethyl)ether, and bis(2-~hbroethoxy)methane, using various solvents. These are interpreted in terms of Lewisacid, Lewis-base concepts. Vapor-liquid equilibrium data are reported for mixtures of acrolein or acrylonitrile with several solvents. On the bases of the extraction and VLE results, along with stabilky tests for solute/sohrent mixtures, likely attractive solvents are identified. Conceptual designs and economic analyses have been c a m out for a number of sohrent/solute systems and result in projected costs of $1 to $2.50/m3of water ($4 to $10/1000 gal of water).
Introduction Solvent extraction has been utilized extensively industrially for recovery of certain chemicals (phenol, acetic acid) from aqueous streams, but it has not yet seen widespread use as a method for removing organic pollutants from water. It is worthy of more serious attention for that application. Biological treatment, the most commonly used treatment process for organics-laden water, is a degradation process. Extraction, on the other hand, can recover chemicals in unchanged form. This feature is particularly desirable when the recovered chemical(s) have economic value. Extraction can also be an attractive alternative, or precursor, to biotreatment if some pollutants are toxic to the biological organisms, if some are refractory to biological oxidation, or if fluctuating organics loadings would affect the stability of a biotreatment process. Among recovery processes, extraction competes with stripping and solvent-regenerated adsorption. In comparison with stream stripping, extraction will be most advantageouswhen the steam requirement for regenerating the solvent is substantially less than the steam required for stripping and/or when the solute forms a miscible azeotrope with water. In comparison with solvent-regenerated adsorption, extraction will be most advantageous at higher solute concentrations and when the energy requirement for regenerating the solvent in extraction compares favorably with that for separating the pollutant(s) from the regeneration solvent in adsorption. Solvent extraction may or may not serve as a broadbrush removal process for a wide variety of organics, depending upon the solvent used and the particular classes of organics present. Barbari and King (1982) have shown that extraction with a hydrocarbon solvent, such as kerosene, can be an effective broad-brush process for generic removal of chlorinated hydrocarbons and aromatics from water. 0196-4305/84/1123-0748$01.50/0
In the present work, attention was focused upon removal of more polar organic solutes from water. Extraction has been used for years commercially for recovery of phenols (Wurm, 1968) and acetic acid (King, 1983) from dilute aqueous solutions. In these cases the solvent is selected specifically for its compatibility with the solute(s) of interest, rather than for broad-brush removal purposes. More polar solvents are employed, because of their higher capacities for the solutes of interest. Examples are diisopropyl ether, methyl isobutyl ketone, and isopropyl acetate. These polar solvents themselves have significant solubilities in water, and residual solvent must be removed from the aqueous raffinate for both economic and environmental reasons, unless the water is recycled in an appropriate way. Alternatives for removal of residual solvent from water include atmospheric and vacuum steam stripping (Greminger et al., 19821, inert-gas stripping (Wurm, 1968),and extraction with a nonpolar solvent (Earhart et al., 1977). With higher-molecular-weight solvents, it may not be necessary to include such a step. The particular polar-organic solutes investigated in this study were chosen from the list of Priority Pollutants of the US. Environmental Protection Agency (Keith and Telliard, 1979). Factors considered were high solubility in water, nonbiodegradability, difficulty of stripping, and presence of functional groups which might lead to specific interactions with certain solvents. The solutes selected were acrolein, acrylonitrile, 2-chlorophenol, isophorone, nitrobenzene, N-nitrosodimethylamine, bis(2-chloroethyl) ether, and bis(2-chloroethoxy) ether. The latter three compounds are denoted as " D M A , b2CEE, and bPCEM, respectively. To provide a basis for evaluation of solvents, extensive measurements were made of equilibrium distribution coefficients for different solute/solvent systems. Isolated measurements were also made of vapor-liquid equilibria and of stability of solute/solvent systems when held at high 0 1984 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 4, 1984
749
Table 1. Values of Weight-Fraction-Based Equilibrium Distribution Coefficient (KD) Measured for Extraction of Polar-Organic Solutes from Water into Various Solvents, at High Dilution and Ambient Temperature acrylo2-chloronitrosolvent/solute acrolein nitrile NNDMA phenol isophorone benzene b2CEE b2CEM alkane: 11.0 12.4 0.44 0.15 10.8O n-undecane aromatic: 3.46 0.68 2.1 toluene ketones: 6.5‘ 0.63 600 117 4.9 methyl isobutyl 70 116 1.60 3.52 390 diisobutyl 0.93 1.2 (0.1) 159 50 isobutyl heptyl esters: 5.4 0.6 n-butyl acetate 2.5 2.1 3.5 n-hexyl acetate n-octyl acetate 1.7 1.98 3.5 isobutyl isobutyrate ethers: 2.6 0.27 diisopropyl 1.7 di-n-butyl 1.08 3.2 phosphates: 83 125 tri-n-butyl 1.98 5.3 0.3d 2600 2.7 1200 1.7 tricresyl chlorinated HC’s: 8.6 2.6 methylene chloride 6.6 chloroform 6.5 7.2 4.6 1,1,2,2-tetrachloroethane alcohols: 0.34 175b 1-octyl 0.3 1.25 1.71 2-ethylhexyl acids: 2-ethylhexanoic 1.6 0.42 0.5 neodecanoic 2.5Be 0.43d di-2-ethylhexyl phosphoric (1.3) “Barbari and King (1982). bBanerjee et al. (1980). ‘Possible irreversibility or slow kinetics, or impurity effect. d50 vol % mixture in n-undecane. e 50 vol % in diisobutyl ketone.
temperatures for a protracted period of time. On the basis of these results, promising solvents were identified, and preliminary designs and economic analyses were carried out for a number of different extraction systems. In general, solvents both lower-boiling and higher-boiling than the solutes of interest were investigated. Lowerboiling solvents generally require more energy input, since the bulk of the solvent must be taken overhead in regeneration by distillation. On the other hand, high-boiling solvents may undergo thermal degradation in the reboiler of the regenerator and/or may build up significant concentrations of heavy tars or other nonvolatile pollutants, thereby requiring a purge and/or reprocessing of a sidestream of solvent. These factors are specific to the particular processing situation. Therefore both high- and low-boiling solvents were explored. Equilibrium Distribution Coefficients Procedure. Equilibrium distribution coefficients for particular solute/solvent systems were measured by contacting predetermined amounts of solute-bearing aqueous feed and of solvents in flasks agitated in a mechanical shaker. The phases were then allowed to settle, and the raffinate was centrifuged. Samples of aqueous feed and raffinate were analyzed either by flame-ionization gas chromatography using Porapak columns, or by liquid chromatography using C18p-Bondapak columns (both from Waters Associates). The equilibrium distribution coefficients was calculated from the aqueous feed and raffinate concentrations, using the equation
Here K D is the equilibrium distribution coefficient, expressed as weight fraction solute in the solvent phase divided by weight fraction in the aqueous phase. w F and W R
are the weight fractions of solute in the feed and raffinate phases, respectively, and W and S are the masses of aqueous and solvent phases, respectively. In general, solvent phases were not analyzed; however, in two cases for extraction of acrolein by di-n-butyl ether, solvent-phase analyses were made by gas chromatography. Mass balances showed that 99.87 and 98.38% of the solute in the aqueous feed were accounted for in the product phases. Precautions were taken against solute and solvent losses by vaporization during handling; these tests with a relatively volatile solute confirmed that the procedures used were effective. Solute concentrations in the aqueous feed were chosen to be dilute, but analyzable with precision. Concentrations of 400 to 4000 ppm w/w were typical. The solvent-bwater phase ratio was chosen so that the amount of solute left in the raffinate could be analyzed with sufficient precision and so that the solute would build up to no more than about 1% w/w concentration in the solvent phase. The measured equilibrium distribution coefficients should then be characteristic of conditions of high dilution in both phases. The particular conditions used for measurements with particular solutes are given by King et al. (1984) and by Joshi (1983) for acrolein, acrylonitrile, and NNDMA, and by Senetar (1982) for the other solutes. Acrylonitrile and ” D M A are on the OSHA list of known carcinogens, and handling conditions used for them were appropriate to that fact. Results Table I lists the results of measurements of equilibrium distribution coefficients, presented on a weight-fraction basis as K D . Table I1 gives the same results as K,, the equilibrium distribution coefficient based upon mole fractions. KD may be converted to K , by multiplying KD
750
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 4, 1984
Table 11. Values of Mole-Fraction-BasedEquilibrium Distribution Coefficient (ICz)Measured for Extraction of Polar-Organic Solutes from Water into Various Solvents, at High Dilution and Ambient Temperature (See Key to Footnotes in Table I) acrylo2-chloronitrosolvent/solute acrolein nitrile NNDMA Dhenol isoDhorone benzene b2CEE b2CEM alkane: 3.8 1.3 94O 89 320 95 103 n-undecane aromatic: 450 1800 11.7 3.5 10.7 toluene ketones: 3300 460 650 36' 3.5 methyl isobutyl 27 12.6 3100 490 2200 550 920 27.8 diisobutyl 1630 510 9.5 isobutyl heptyl 12 (1) esters: 16 35 4 n-butyl acetate 17 28 n-hexyl acetate 16 n-octyl acetate 17.3 31 isobutyl isobutyrate ethers: 11 17 1.8 diisopropyl 8.9 26 di-n-butyl phosphates: 39300 710 4300 1230 1850 29.3 78 3d tri-n-butyl 25000 35 55 tricresyl chlorinated HC's: methylene chloride 31 41 12 chloroform 43 43 67 1,1,2,2-tetrachloroethane alcohols: 1-octyl 2.5 9.0 12.4 2 2-ethylhexyl acids: 13 3.4 4 2-ethylhexanoic neodecanoic 29.4e 5.3d di-2-ethylhexyl phosphoric (23)
by the ratio of the solvent molecular weight to that of water. As is noted in the table, a few results from previous studies are included for cases where no measurements were made in the present work. These results are, in general, averages of data for several independent runs. Individual data points are reported by King et al. (1984), Joshi (19831, and Senetar (1982). The number of significant figwes in Tables I and I1 is indicative of the apparent precision of the data. Values in parentheses in Tables I and I1 are averages of runs showing considerable scatter; thus they are more uncertain than the other values listed. In a number of cases, values of KD were measured for back-extraction of solute into water from the extract from a previous run. A higher apparent value of KD for the back-extraction would be indicative of a degradative reaction during extraction, a kinetically limited reaction, or an irreversibility due to an impurity in the solvent. For the systems reported in Tables I and 11, there was no evidence of such behavior. On the other hand, the extraction of acrylonitrile into methyl isobutyl ketone (MIBK) showed apparent values of KD for back extractions averaging about 35% less than those for the forward extraction. In the case of extraction of NNDMA from water into methylene chloride the average KD evidenced by six forward-extraction runs was 6% greater than the average KD from the six back-extraction runs, which is close to the experimental uncertainty. Discussion The mole-fraction-based equilibrium distribution coefficient, K,, is simply related to the activity coefficients of the solute in the two phases YW
K, = Ys where yw and ys are the solute activity coefficients in the
Table 111. Solubilities of Polar-Organic Solutes in Water solubility in water wt mole solute fraction fraction (X,) reference acrolein 0.208 0.088 Smith (1962) acrylonitrile 0.0735 0.0262 American Cyanamid (1959) 2-chloro0.028 0.0040 Callahan (1979) phenol isophorone 0.0120 0.00157 Union Carbide (1978) nitro0.0019 0.00028 Perry and Chilton (1973) benzene b2CEE 0.0102 0.00128 Callahan (1979) b2CEM 0.081 0.0091 Callahan (1979)
aqueous and solvent phases, respectively. Changes in K, can thereby by interpreted in terms of changes in the two activity coefficients. For solutes with low aqueous solubility, the activity coefficients yw may be inferred from the mole-fraction solubility in water, n, as Yw = b w ) - l
(3)
Equation 3 assumes that yw is independent of solute concentration and that the solubility of water in the pure solute is not sufficient to affect the activity of the solute. Values of x , are given in Table 111, along with original references. NNDMA is miscible with water iv all proportions and therefore is not included in Table 111. It should be noted that the solubility cited for b2CEM was calculated from a molecular-structure relationship, rather than measured. The much lower value of lcw for nitrobenzene leads to much higher values of yw than for the other solutes. This is the reason why the values of K, and KD for nitrobenzene are, in general, much higher than those for the other solutes. Similarly, the high solubilities of acrolein and NNDMA-and the corresponding low values of ?,--are
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 4, 1984 751
the reason for values of KD and K , for these compounds being generally lower than those for the other solutes. Changes in K, from solvent to solvent for a given solute are inversely related to changes in ya. These changes may be interpreted in terms of functional-group interactions between solute and solvent molecules, as well as the overall polarity of the solvent medium. Since values of K , for many of the other solvents are much greater than K , for extraction into undecane, sometimes by orders of magnitude, it is apparent that there are very strong functional-group interactions at play, capable of making very large changes in ys. These may be interpreted in terms of concepts of hydrogen bonding and Lewis aciditylbasicity. Among the solvents, the ketones, esters, ethers, and phosphates function as Lewis bases (electron donors) because of the electronegative nature of the carbonyl and phosphoryl groups and the ether linkage
+I I
-?P=O
+I I
-C-O-G-
-
I+
I
The particular chlorinated hydrocarbons used in this work act as Lewis acids (electron acceptors) because they fit into the Class IV hydrogen-donor hydrogen-bonding category of Ewe11 et al. (1944)-namely, molecules having two or three chlorine atoms on the same carbon atom as a hydrogen atom, or one chlorine on the same carbon atom and one or more chlorine atoms on adjacent carbon atoms. The electronegative chlorines serve to increase the electropositive nature of the hydrogen atoms, making them more available as donated hydrogens (electron acceptors). The carboxylic acid and phosphoric acids, of course, serve as strong hydrogen donors, but they can also act as Lewis bases through the electronegative oxygens in the carbonyl and phosphonyl groups. Their solvent power is often diminished by dimerization of acid molecules through hydrogen bonding, making the functional groups less available to interact with solutes. The alcohols can also act as both Lewis bases and Lewis acids. However, they too can self-associate through hydrogen bonding in solution, reducing the availability of the functional group for interaction with solutes. The aromatic, toluene, can interact with acids or bases through T bonding. Lewis aciditylbasicity tendencies for each of the solutes considered can be interpreted as follows. Acrolein. The aldehyde carbonyl group in acrolein can act as a moderatly strong Lewis base. However, because acrolein is an a-/3 unsaturated carbonyl H
the electrons in the olefinic bond are delocalized, giving the molecule moderately strong Lewis-acid properties as well. From Table I1 it can be seen that both Lewis-acid and Lewis-base solvents increase K, well above that for undecane. Acrylonitrile. The nitrogen atom of the cyano group is electronegative, giving the molecule Lewis-base properties. a,/3 unsaturation exists in this molecule as well, delocalizing the olefinic electrons and imparting Lewis-acid properties. Thus both Lewis-acid and Lewis-base solvents are effective with acrylonitrile, giving a behavior rather similar to that for acrolein, with the values of K, in general being somewhat higher than for acrolein, reflecting the lower value of x , (higher y,) for acrylonitrile. NNDMA. There is significant charge separation in the resonance hybrid of NNDMA
(I)
(111)
(11)
The configurational isomers I1 and I11 both exist because of the high barrier to rotation about the N=N bond. The oxygen atom has Lewis-base properties, while the positive nitrogen atom has Lewis-acid properties. Because of steric hindrance about the positive N atom, the molecule should have more ability to act as a Lewis base, thereby rationalizing the substantially higher value of K , found for the Lewis-acid solvent (methylene chloride) than for the various Lewis-base solvents. 2-Chlorophenol. The phenolic hydrogen is acidic, and the presence of the electronegative chlorine atom on the ring makes it more so. Thus the molecule is a very strong hydrogen donor for hydrogen bonding and it complexes strongly with the Lewis-base solvents, giving the greatest factor of increase of KD values over the undecane value for any of the solutes tested. Isophorone. Isophorone is a Lewis base through the carbonyl oxygen and is also a Lewis acid becauses of delocalization of the olefinic electrons in the a,@position. It gives enhanced KD with both Lewis-acid and Lewis-base solvents, in a way similar to acrolein and acrylonitrile, except with generally higher values of K , because yw is higher for isophorone. Nitrobenzene. The nitro- group on the ring serves to withdraw electrons and makes the molecule a strong 7electron acceptor, or a Lewis acid. It has substantially enhanced values of KD with Lewis-base solvents, with less enhancement in the case of the less-basic, more-acid alcohol solvent. b2CEE and b2CEM. The ether linkages in these molecules function as weakly basic sites, but the greater effect appears to come from the influence of the electronegative chlorine atoms in making hydrogens on the same atom more electropositive and available as electron acceptors H
H
H
H
cI-c-c-o-c-c-cI
H
H
H
H
bZCEE
H
H
H
H
cI-c-c-o-c-o-c-c-cI
H H
H
H
H
H
b2CEM
Thus the hydrogens on the two end carbon atoms in both molecules should be active. In addition, the hydrogens on the middle carbon in b2CEM would be activated by the adjacent oxygen atoms, making it a still stronger Lewis acid. Because of this acidity, the Lewis-base solvents give strongly enhanced values of KD for the two chlorinated ethers, with the enhancement being somewhat greater for b2CEM. It can be seen that in some cases conversion of KD to K, removes most of the effect of solvent molecular weight within a homologous series. This can be seen for extraction of acrolein by esters, or, to a lesser extent, for extraction of acrolein and acrylonitrile by ethers. However, among the ketone solvents, the trends in K, vary from one solute to another. For b2CEE K , is relatively constant for the three ketones. For acrylonitrile and 2-chlorophenol K, for extraction into IBHK is substantially less than for extraction into MIBK or DIBK, and for acrolein K , for extraction into DIBK and IBHK is substantially less than for extraction into MIBK. These trends may reflect steric hindrance hampering full access of the solutes to the
752
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 4, 1984
maxima exist, with K, for an intermediate solvent composition being higher than that for either of the two pure solvents. Other mixtures tested did not show this behavior (Senetar, 1982). The maxima may reflect any of the following effects: (1) interaction of the Lewis-acid and Lewis-base components of the solvent phase separately with basic and acidic sites on the solute molecule; (2) enhancement by the diluent of the degree of basicity of the carbonyl oxygen in neodecanoic acid; (3) diminished dimerization of the carboxylic acid brought about by solvation by the diluents, making the acid groups more available for complexation with isophorone.
400 o Toluene Oiluent
D OIBK
300
A
Diluent Undecone Oiluent
3
9.2
0
06
0.4
1.0
08
Mole-Fraction Neodecanoic Acid in Solvent Phase
Figure 1. Equilibrium distribution coefficient (K,) vs. molafraction neodecanoic acid for extraction of isophorone from water with various diluents.
carbonyl oxygen in the presence of the bulky heptyl and isobutyl groups and/or a lesser density of polar functional groups in the higher-molecular-weight ketones. Among the basic solvents, the relative basicities of the functional groups lie in the following order
I
-PP=O
I
phosphoryl
> \c=o > / carbonyl
-c
H0 \oester
I > -c-0-cI
I I
ether
Trends in K, for a particular solute among these solvents reflect this factor, which gives higher K, for stronger bases. When the weight-fraction distribution coefficients, KD,are considered instead, the trends reflect a combination of the strength-of-basicity factor and the effect of higher molecular weight giving a lower value of KD This combination of effects is apparent when K, and KD for tributyl phosphate are compared with values of K, and KD for the lower-molecular-weight ketones and esters. For the strongly acidic 2-chlorophenolthe stronger basic functional group in TBP strongly outweighs the negative effect of the higher molecular weight of TBP. Comparing the two phosphates, TBP generally gives higher values of KD than TCP because of its lower molecular weight and the effect of the K electrons in TCP lowering the basicity of the phosphoryl group. For extraction of acrolein, K bonding apparently makes K, for TCP somewhat greater than that for TBP. TCP, however, is a substance of more environmental concern than TBP. Amines and trioctyl phosphine oxide (TOPO) were also tested as extractants for acrolein and acrylonitrile (Joshi, 1983). Neither showed any advantages in KD for extraction of acrylonitrile, and TOPO showed no advantage for extraction of acrolein. The primary and secondary amines gave very large degrees of extraction for acrolein, but back-extraction experiments and kinetic measurements showed that this was due to a slow, irreversible chemical reaction. Tertiary amines give relatively low values of KD for acrolein (Joshi, 1983). Solvent Mixtures. Several mixtures of conventional solvents were tested for extraction of isophorone. Figure 1 shows values of K, measured for extraction of isophorone into solvent mixtures containing neodecanoic acid in three different diluents. For DIBK and toluene as diluents,
Vapor-Liquid Equilibria To a first approximation, the ease of regenerating the solvent by distillation can be judged from the difference in boiling points between the solute and solvent. Nonidealities in solution can affect the situation, however. Vapor-liquid equilibrium (VLE)data were obtained for binary mixtures of acrolein with methylene chloride, MIBK, butyl acetate, tetrachloroethane, and toluene, as well as for binary mixtures of acrylonitrile with methylene chloride, tetrachloroethane, butyl acetate, and MIBK (King et al., 1984; Joshi, 1983). For this purpose a vapor-recirculating equilibrium still of the sort described by Hipkin and Myers (1954) was employed. Indicated liquid-phase activity coefficients ranged from 0.74 to 1.42, corresponding to only modest departures from ideality. All systems proved to be sufficiently wide-boiling so that regeneration by distillation would not be inordinately expensive, except for the acrolein/methylene chloride system. Here the relative volatility of methylene chloride to acrolein was about 1.30 at mole fractions of acrolein equal to 0.5 and greater in the liquid phase. This would lead to a relatively expensive distillation, and so this system was not considered further, despite the relatively large value of KD for extraction of acrolein into methylene chloride. Stability Tests For mixtures of acrolein or acrylonitrile with various solvents, stability tests were carried out wherein the mixtures were heated under controlled conditions for 72 h. This was usually accomplished by boiling and totally refluxing the mixture at atmospheric pressure, although some tests were also carried out by holding mixtures at 65 O C in a water bath. These mixtures typically contained 15 to 20% w/w of the solute. Observations were made of changes in appearance, changes in boiling temperature, and changes in composition as evidenced by the ratio of the solute and solvent peak areas in a gas-chromatographic analysis of the liquid. Appearance of extra peaks on the chromatogram would be additional evidence of degradation, but no additional peaks were observed. The tests and their results are described in more detail by King et al. (1984) and by Joshi (1983). A mixture of acrolein in isooctane showed no indications of chemical change upon refluxing at 60 " C ; however, mixtures of acrolein with butyl acetate, MIBK, and tetrachloroethane all generated an opaque, white solid upon refluxing at the atmospheric boiling points of the mixtures, ranging from 75 to 90 O C . A t the same time the boiling temperatures increased and the ratio of acrolein to solvent decreased. The white solid possessed properties equivalent to those of acrolein polymer, which could logically form under these conditions. This polymer would have the potential of fouling the regenerator reboiler, as well as other detrimental effects. Two approaches were investigated for discouraging the formation of polymer. Lowering the temperature during
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 4, 1984 753 Table IV. Solvents Recommended for Further Consideration for Extraction of Particular Polar-Organic Solutes from Water low-boiling solvents high-boiling solvents solute bD, OC solvent bD, "C solvent bD, "c -_ acrolein 53 n-butyl acetate 126 117 methyl isobutyl ketone 146 tetrachloroethane 111 toluene a acrylonitrile 78 methylene chloride 40 tributyl phosphate 146 (tetrachloroethane) -N-nitrosodimethylamine 152 methylene chloride 40 111 (toluene) 2-chlorophenol 176 hydrocarbons hydrocarbons 117 218 isobutyl heptyl ketone methyl isobutyl ketone a tributyl phosphate isophorone 215 hydrocarbons hydrocarbons 117 a tributyl phosphate methyl isobutyl ketone diisobutyl ketone 168 111 toluene hydrocarbons b2CEE 178 hydrocarbons methyl isobutyl ketone 117 218 isobutyl heptyl ketone a tributyl phosphate 218 hydrocarbons hydrocarbons b2CEM a tributyl phosphate diisobutyl ketone 168
__
"Tributyl phosphate has a specific gravity of 0.978, and should therefore be used with an appropriate diluent. The boiling point of the diluent will govern the boiling point of the solvent mixture.
Table V. Estimated Costs for Extraction of Polar Organics from Water (King et al., 1984; Joshi. 1982; Senetar, 1982) acrolein acrylonitrile butyl methylene 2CP"/ NB"/ MIBK" acetate toluene TCE" TCE" chloride TCE" TCE" IBHK" DIBK" aqueous feed solute concn, ppm 200 200 200 200 100000 200 200 30000 3000 1000 13.6 13.6 13.6 13.6 56.8 56.8 56.8 11.4 11.4 flow rate, m3/h 13.6 268 1112 896 fixed capital investment, 382 403 793 469 534 148 171 FCI k$ operating costa, $/m3 water labor-related 0.67 0.67 0.67 0.67 0.65 0.15 0.15 0.15 0.44 0.44 capital-relatedb 0.80 0.85 1.68 0.99 0.57 0.57 0.45 0.27 0.37 0.43 solvent loss 0.38 0.38 0.15 0.20 0.18 0.16 0.14 0.14 0.23 0.76 1.22 0.56 0.79 1.45 0.44 0.46 0.16 0.48 utilities8 1.09 0.99 credit for recovered solute (0.14) (0.14) (0.14) (0.14) - (0.10) (0.10) - 0 0 total operating cost without interest, $/m3 water interest on capital, $/m3 watef total operating cost with interest, $/m3 water
NBa/ D1BK"f 1000 11.4 229 0.44 0.58 0.10 0.55 0
2.80
2.75
3.56
2.26
2.19
2.45
1.08
1.02
1.20
2.11
1.67
0.33
0.35
0.70
0.41
0.24
0.24
0.18
0.11
0.15
0.18
0.24
3.13
3.10
4.26
2.67
2.43d
2.69
1.27
1.13O
1.35
2.29
1.91
"Key to solvents and solutes: MIBK = methyl isobutyl ketone; 2CP = 2-chlorophenol; IBHK = isobutyl heptyl ketone; TCE = 1,1,2,2tetrachloroethane; NB = nitrobenzene; DIBK = diisobutyl ketone. 24% of FCE (maintenance, 6%; depreciation, 10%; insurance, 1%; local taxes, 2%; factory expenses, 5%). clO% of FCE. dCost equivalent to 2.46/kg acrolein produced. eCost equivalent to 3.46/kg acrylonitrile produced. !No recovery of dissolved DIBK from raffinate. #Steam at $19.60/1000 kg.
the stability test to 65 "C resulted in clear solutions for MIBK and tetrachloroethane as solvents, and produced only a slight cloudiness with butyl acetate. Losses of acrolein were much reduced or eliminated altogether. Thus vacuum distillation could be a viable alternative for regeneration. The acrolein used in most of the experiments contains 200 ppm of hydroquinone as stabilizer. Increasing the amount of hydroquinone in the solvent mixture with butyl acetate to 9600 ppm w/w resulted in a clear solution after refluxing at 85 "C. With MIBK, increasing the overall hydroquinone concentration to 1100 ppm w/w produced only a slight cloudiness after refluxing at 82 "C. Hydroquinone could be used without problem as a stabilizer in cases where the raffinate water is recycled to a main process, as in acrolein manufacture. However, it would be undesirable for a wastewater process where the effluent is to be released to the environment, since reported values of KDfor extraction of hydroquinone from water into butyl acetate and MIBK are only 7.2 (Luecke, 1980) and 9.9 (Greminger et al., 1982),respectively. It may be
that a less troublesome inhibitor could be found. In the case of mixtures of solvents with acrylonitrile, yellow solids were formed during heating of mixtures with MIBK and butyl acetate at temperatures around 103 "C. This was presumably the result of an irreversible cyanoethylation reaction (see, e.g., Bruson and Riener, 1942). A yellow-brown resin formed for refluxing with toluene at 87 "C, and a milky white solid formed for refluxing with di-n-butyl ether at 89 "C. On the other hand, mixtures with 50% TBP, 50% undecane v/v (100 "C), with methylene chloride (43 "C), and with 1,1,2,2-tetrachloroethane (102"C)remained clear. These results discourage the use of esters, ketones, ethers, and aromatics as solvents for acrylonitrile.
Solvent Selection Table IV summarizes recommendations of solvents which appear to be promising for extraction of the various pollutants considered in the present work. Bases for these conclusions are (1)the experimentally measured values of
754
Ind. Eng. Chem. Process Des. Dev., Vol. 23, No. 4, 1984 Condenser
-oc Solvenl-Extracl Heal Exchanger
Solvenl-Extract Heal Exchanger Distillation
Steam Slripper
I l l
I I
f-Yi--
-
Distillation
Column
Wa$tewaterFeed Ealraclor
liv" Ertractor
Wastewater Feed
Figure 2. Flow diagram for a low-boiling solvent.
K,, (2) differences in boiling point between solute and solvent, and the experimental VLE data in the cases of acrolein and acrylonitrile, and (3) the stability tests for mixtures with acrolein and acrylonitrile, as well as various other criteria. Hydrocarbons (kerosene and various lower- and higher-boiling cuts) are indicated wherever the values of KD into undecane are sufficiently high. Where a recommendation is open to question, the solvent is indicated in parentheses. In the cases of the four high-boiling solvents listed for acrolein, it will be important to use vacuum distillation and/or an inhibitor or some other method for prevention of the formation of acrolein polymer. Preliminary Designs and Economic Analyses Conceptual designs and economic analyses were carried out for a number of different solute/solvent systems. Results of these analyses are shown in Table V. The economic bases postulated are explained in the footnotes and are described further by Joshi (1983) and Senetar (1982). Two of the systems (acrylonitrile/methylene chloride and nitrobenzene/diisobutyl ketone) involve low-boiling solvents, for which the flow sheet is shown in Figure 2. The process includes an extractor, a regeneration column, and a vacuum steam-stripping column for removal of residual solvent from the raffinate, along with appropriate heat exchangers. The other systems involve high-boiling solvents, for which the flow diagram is shown in Figure 3. In all cases except for 2-chlorophenol/IBHK the solvent solubility was sufficient to warrant recovery of dissolved solvent from the raffinate, and a stripping column similar to that in Figure 2 was also included. The two cases with much higher feed concentrations were included to assess the utility of extraction for the primary separation of acrolein or acrylonitrile from water during manufacture of these chemicals. Here extraction appears to be an attractive alternative to distillation. From the results of the two different cases for the nitrobenzeneldiisobutyl ketone system, it can be seen that there is considerable incentive for installing the stripping column for recovery of dissolved DIBK (solubility in water = 500 ppm) from the raffinate water. In the cases of chlorinated-hydrocarbon solvents it was assumed that raffinate stripping would be sufficient to eliminate any environmental hazard from residual solvent. Total costs, including capital-related costs and interest, lie in the range $1.1 to 3.2/m3 of water ($4.20 to $12.201 1000 gal) for all cases except the acrolein/toluene com-
Raft inate
Make-up Solvent
Figure 3. Flow diagram for a high-boiling solvent.
bination. The low-boilingsolvents lead to higher costs than for the comparable high-boiling-solvent cases, because of the steam cost associated with regeneration. Acknowledgment This work was supported through Grant No. R807027 from the U.S. Environmental Protection Agency, administered through the Robert S. Kerr Environmental Research Laboratory, Ada, OK. This paper was presented orally at the annual meeting of the American Institute of Chemical Engineers in Los Angeles, CA, in November 1982. Registry No. NNDMA, 62-75-9; bBCEE, 111-44-4; bBCEM, 111-91-1; MIBK, 108-10-1; DIBK, 108-83-8;IBHK, 19594-40-2; 2CP, 95-57-8; NB,110-83-8;TBP, 126-73-8;TCE, 79-34-5; acrolein, 107-02-8;acrylonitrile, 107-13-1;isophorone, 78-59-1; n-undecane, 1120-21-4;toluene, 108-88-3; n-butyl acetate, 123-86-4; n-hexyl acetate, 142-92-7;n-octyl acetate, 112-14-1; isobutyl isobutyrate, 97-85-8; diisopropyl ether, 108-20-3;di-n-butyl ether, 142-96-1; tricresyl phosphate, 1330-78-5; methylene chloride, 75-09-2; chloroform,67-66-3; 1-odyl alcohol, 111-87-5;2-ethylhexyl alcohol, 104-76-7; 2-ethylhexanoic acid, 149-57-5; neodecanoic acid, 26896-20-8; di-2-ethylhexyl phosphoric acid, 298-07-7.
Literature Cited American Cyanamid Company, "The Chemistry of Acrylonitrile"; New York. 1959. Banerjee, S.; Yalkowsky. S.H.; Valvani, S. C. Envlron. Sci. Techno/. 1980, 14, 1227. Barbari, T. A.; King, C. J. Environ. Sci. Techno/. 1082, 16, 624. Bruson, H. A.; Riener, T. W. J. Am. Chem. Soc. 1042, 6 4 , 2850. Callahan, M. A. "Water-Related Environmental Fate of 129 Priority Pollutants. A Llterature Search"; Office of Water Planning and Standards, U.S. Environmental Protection Agency, 1979. Earhart, J. P.; Won, K. W.; Wong, H. Y.; Prausnitz, J. M.; King, C. J. Chem. Eng. Prog. 1977, 73(5), 67. Ewell, R. H.; Harrison, J. M.; Berg, L. Ind. Eng. Chem. 1044, 3 6 , 871. Greminger, D. C.; Burns, G. P.; Lynn, S.;Hanson, D. N.; King, C. J. Ind. Eng. Process Des. Dev. 1982, 21, 51. Hipkin, H.; Myers, H. S. Ind. Eng. Chem. 1954, 46, 2524. Joshi, D. K. Ph.D. Dissertation, University of California, Berkeley, 1983. Keith, L. A,; Teillard, W. A. Environ. Sc/. Techno/. 1979, 13, 416. King, C. J. "Acetic Acid Extraction" I n Lo, T. C.; Baird, M. H. I.; Hanson, C. "Handbook of Solvent Extraction"; Wiky: New York, 1983. King, C. J.; Joshi, D. K.; Senetar, J. J. "Equilibrium Distribution Coefficients for Extraction of Organic Priority Pollutants from Water", Report No. EPA600/S2-84-060, 2 Vol., U.S. Environmental Protection Agency, April 1984. Luecke, R. H. I n King, C. J.; Lynn, S.; Hanson, D. N.; Mohr, D. H., Jr., Ed.; "Processing Needs and Methodology for Wastewaters from the Conversion of Coal. Oil Shale and Biomass to Synfuels"; Report No. DOE/EV0081, U.S. Dept. of Energy, May 1980. Perry, T. H.; Chiiton, C. H., Ed. "Chemical Engineer's Handbook", 5th ed.; McGraw-Hill: New York, 1973. Senetar. J. J. M.S. Thesis, Unlverslty of Callfornla, Berkeley, 1982. Smith, C. W. "Acrolein"; Wiley: New York, 1962. Union Carbide Corp., Marketing Div., Chemical and Plastics Data Sheet, 1978. Wurm. H.J., Gliickhauf 1988, 104, 517.
Received for review May 2 , 1983 Accepted January 27, 1984
