SEPARATIONS Supercritical Fluid Fractionation of a Nonionic Surfactant

I n d . Eng. Chem. Res. 1992,31, 1105-1110

1105

SEPARATIONS Supercritical Fluid Fractionation of a Nonionic Surfactant Charles A. Eckert,* Michael P. Ekart, Barbara L. Knutson, Kirk P. Payne, and David L. Tomasko School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100

Charles L. Liotta School of Chemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400

Neil R. Foster School of Chemical Engineering and Industrial Chemistry, University of New South Wales, Kensington, New South Wales, Australia 2033

Supercritical fluid extraction (SFE) is demonstrated for the removal of unreacted dodecanol (lauryl alcohol) from nonionic surfactants (poly(oxyethy1ene) dodecyl ethers, C12E,) and fractionation based on polymer chain length. Carbon dioxide, pure and with 1.0 and 3.5 mol ?& methanol cosolvent, propane, and ammonia were used as supercritical solvents. Experimental solvent loadings and distribution Coefficients were used to develop a batch extraction model and continuous stagewise extraction model. Propane and ammonia exhibited higher loadings while the C02/methanol mixtures showed greater selectivity for removing the dodecanol and C12E1. Conclusions regarding the choice of solvent for specific separations are discussed.

Introduction Although once touted as a miracle medium for separations, supercritical fluids (SCF‘s) are finding their niche as solvent media for highly specific separation processes, for example, in the purification of fatty acids and vitamins and the fractionation and purification of monodisperse polymers. Due to the large throughput, relative invariance of feedstocks, and general ease of separations in hydrocarbon processing, some type of distillation is almost always a suitable technique. However, in specialty chemicals processing and hazardous waste treatment there is neither a large throughput nor an invariance in feedstocks, and many species are much harder to separate. These processes are very much batch-oriented and require continuous adaptation in downstream separations. This is particularly the case for hazardous waste cleanup where even different samples from one site will contain very different distributions of contaminants. In such cases where traditional separation methods are unsuitable or economically prohibitive, SCF processing can be adapted to provide a viable alternative. Several characteristics of SCF’s allow them to be extremely versatile media. Very nonvolatile and thermally labile substances can be dissolved to a substantial degree in SCF’s at temperatures typically less than 150 “C. The addition of a cosolvent can enhance the selectivity for specific components of a mixture. Finally, the solvation power can be manipulated over several orders of magnitude with relatively small perturbations of the temperature or pressure of the fluid, thereby allowing precise phase separation to effect fractionations. We demonstrate here an example of adapting SCF processing to a specific separation. A nonionic surfactant consisting of a distribution of poly(oxyethy1ene) dodecyl ethers (CH3(CH2)110(CH2CH20),,-1CH2CH20H, denoted as C12E,) with an average ethoxylation, n , of 3.7 is extracted with various SCF’s 0888-5885/92/ 2631-1105$03.00/0

to remove the nonethoxylated alcohol. The distribution coefficients obtained from the experiment are then used to model a multistage extraction for continuous processing.

Background Surfactants essentially fall into four chemical types: anionic (negative charge on surface active group), cationic (positive charge), nonionic (no charge), and amphoteric (zwitterionic). Anionic surfactants account for the largest part of U.S.production (66%) while nonionics form the second largest group (24%) (Greek, 1991). Nonionics in turn are comprised of two common types: carboxylic acid esters including mono- and multiglycols with long chains, and ethoxylated long-chain alcohols or alkylphenols. This work focuses on ethoxylated dodecanol (lauryl alcohol) which after ethoxylation suffers from a high “free oil” (unreacted alcohol) content that degrades the surface activity. The ethoxylation of linear alcohols can be acid- or base-catalyzed. Basic catalysts result in few by-products but a broad product distribution. Acid catalysts give narrower distributions, but produce more harmful byproducts. To improve the quality of nonionic surfactants, it is desirable to minimize the amount of unreacted alcohol as well as to narrow the distribution of ethoxylates in the mixture. In the absence of a catalytic scheme to produce these properties, separation of the product mixture must be considered. Previous studies have used gas-liquid chromatography to determine molecular-weight distributions (Stancher and Favretto, 1978) and foaming processes for separating the surfactant from an aqueous solution (Kucharski and Kuciel, 1971; Zwierzykowski and Mgdrzycka, 1973; Zwierzykowski et al., 1975; Kuciel and Makomaski, 1979; Mqlrzycka and Zwierzykowski, 1981). A recent Japanese patent describes a countercurrent fractionation process 0 1992 American Chemical Society

1106 Ind. Eng. Chem. Ras., Vol. 31, No. 4,1992

-

-~

Constant Tempera~reBath

Figure 1. Schematic of supercritical fluid extraction appmatw.

using supercritical COz (Horizo and Yanagi, 1990). The use of supercritical fluids for fractionation on a larger scale than chromatographic separation was f i s t demonstrated by Zosel (1978) for the separation of a mixture of olefis with supercritical ethylene. Many applications have since been demonstrated in the literature ranging from the fractionation of natural products such as fish oils, palm oil, and lemon-peel oil (Peter and Brunner, 1978; Brunner and Peter, 1982; Nilsson et al., 1988,1989;Tilly et al., 1990; Yamauchi and Saito, 1990) to narrowing the molecular weight distribution of polymers (Yilg6r and McGrath, 1984a,b; Scholsky et al., 1986; Elsbemd et al., 1987; Krukonis, 1988; Daneshvar and Gulari, 1989). For general reviews of SCF extraction, there are several excellent sources (Williams, 1981; Paulaitis et al., 1982, 1983; McHugh and Krukonis, 1986; Eggers and Sievers, 1989). Experimental Section The apparatus for the batch fractionation of the surfactant is shown in Figure 1. The saturator consisted of two sample cylinders (Whitey, 304HDF4-75) in series. The bottom sample cylinder was charged with the detergent, and the top cylinder was packed with glass wool to prevent entrainment. The supercritical solvent was dispersed into the sample through a stainless steel frit (Alltech, 050100). The saturator and preheating coil were immersed in a well-stirred water bath (UCON 500 heattransfer fluid, Union Carbide, was used with propane and ammonia) and maintained at these temperatures using a Bayley temperature controller (Model 123). The temperature was maintained within *O.l OC of the set point during a run. For pure solvents, the solvent was pumped into a surge vessel by an air-driven compressor (Haskel AG152). When a solvent mixture was used, a predetermined amount of the m l v e n t was measured carefully into the vessel, which was then filled with the solvent needed to attain the desired mixture compition. The vessel was pressurized well above the pressure desired for the extraction and was thoroughly mixed via magnetic stirring for a period of several hours. As the extraction proceeded, the pressure in the mixing bomb dropped as the mixture was depleted; the extraction was stopped when there was no longer enough pressure in the bomb to maintain flow. Care was taken to maintain the solvent mixtures in the one-phase region at all times. On the other hand, in order to carry out a separation, it was necessary that the solvent/surfactant mixture he in the two-phase region. This was confirmed by visual inspection of the mixtures in a Jerguson gauge; we also confirmed that no entrainment of liquid droplets in the fluid phase was occurring.

The system pressure was regulated using a Tescom 4ooo pressure regulator. The pressure in the saturator was monitored using a calibrated Sensotec TJE/743-06-01 transducer a t the outlet. With this scheme the pressure was maintained within *5 psi during a run. After flowing through the extractor, the fluid went through an Autoclave micrometering valve, releasing the pressure, and into a chilled drop trap where the extract was collected. The drop trap was packed with ultrafine steel wool wrapped with filter paper to eliminate any sample loss. Flow was measured with a GCA/Precision Scientific wet test meter. When working with ammonia, the wet test meter was filled with oil. The extract was weighed and then analyzed by gas chromatcgraphy (Varian 3400) on a 10% SP2100 100/120 mesh packed column (Supelco). Helium was the carrier gas at a flow rate of 25 mL/min. The column temperature was initially a t 100 OC and ramped a t 16 OC/min to 330 "C where it was held for 8 min. Detection was carried out by a flame ionization detector; response factors were measured by Kuo (1991). The surfactants used were Synfac DG 3.7 EO and 4.2 EO obtained from Milliken Chemical Co., and the methanol wm Aldrich HPLC grade 99.98%. Gases were obtained from Matheson (ammonia, anhydrous) or Scott Specialty COZ,SFC grade; propane, instrument grade). Results Extractions were performed on the surfactant using three supercritical solvents and two SCF/cosolvent mixtures. Solvent loadings were determined from gravimetric analysis described above and were accurate within 10%. Solvent-free distrihution coefficients were determined from the GC analyses; the solvent-free distribution coefficient for a given component is the ratio of its mole fraction in the extract (after depressurization and separation of the supercritical solvent) to its mole fraction in the surfactant that was initially charged into the extractor. These distribution coefficientsform the basis for comparing solventa to fractionate the surfactant; a solvent-free distribution coefficient for component i that is greater than 1indicates that the relative amount of solute i with respect to the other soluteg is greater in the fluid phase than in the liquid phase. We have calculated all distribution coefficients from the first cut of the extraction, which usually consisted of 0.3-0.5 g of material out of 25 g of feed. Keeping the first cut small assured that the feed was not depleted of its more volatile components so rapidly that ita composition changed significantlyduring the first cut. This was codirmed by performing further cuts and analyzing the rate of change in the compition of the liquid loaded into the saturator. Uncertainties in the distribution coefficients were determined by replicate experiments. The uncertainties in the distribution coefficients for dodecanol and C,&+ were 10%; for C,,E,,, which were present in much smaller quantities in the extract, the uncertainties were 20%. To check the composition dependence of the distribution coefficients, we extracted feeds with average ethoxylations of 3.7 and 4.2 a t the same conditions; the distribution coefficients were identical within experimental error. First we compare absolute loadings in each SCF as an indicator of the amount of solvent required to achieve a separation. The "loading" of the solvent is defined as weight of extract/weight of solvent and is plotted against reduced density in Figure 2. The CO,/methanol mixture densities were reduced by the density of pure COP As expected, the loading is increased with addition of the cosolvent methanol because of its ability to hydrogen bond

Ind. Eng. Chem. Res., Vol. 31,No. 4,1992 1107

iA

25 A rn

I

.

I

%.

1

o.wo1

I

2 Y

1.5 1

05

.

Lauiyll

2

3 4 component

5

6

7

F m 4. Effect of pmure on distributioncoefficients in pure COS at 40 "C.

Figure 2. Experimental loadingn of surfactant in supermitical solvents at T,= 1.03.

1.6 1

am-

I

I

18

..... -...

16 14 12 X

I

08 06 0 02 Law

1

2

3

4

5

6

1

comlnnent

Camwnenl

Figure 3. Effect of temperatureon distribution wefficients m pure CO, at constant density (0.5 g/cms).

Figure 5. Distribution Coefficients for propane at a reduced density of 1.05.

with the soluteg. The loadings in ammonia and propane are subatantielly higher than in COzeven with a cmlvent (note log d e ) ; however, comparing different SCF's at the same reduced conditions obfuscates the fact that the volatility of the surfactant increases with temperature. Ideally, one would normalize the loading by the vapor pressures, hut these data are not available. The distribution coefficients in pure COz at a density of 0.5 g/cm3 and at 33,40,and 55 "C are shown in Figure 3 where lauryl denotes the free dodecanol and the numbers indicate successive n-mers of ethoxylated alcohol, Cl2E,. There is a clear trend, as expected, of lower distribution coefficients for higher ethoxylates, and since these span from greater than unity to less than unity, the data immediately show the likelihood of selectively removing the more volatile ethoxylates from the solution. Of curious note, however, is the distribution coefficient for ClzEl, which is higher in some cases than the dodecanol even though physical reasoning might suggest that it be less volatile. On the basis of the estimated uncertainties, the distribution coefficients do not appear to be a strong function of temperature. Any temperature dependence at this density in COPis not very significant compared to the pressure (or density) dependence described below. The effect of pressure (density) on the distribution coefficients is seen in Figure 4 for pure COz at 40 OC. Although the low pressure gives very desirable distributions, Le., monotonically decreasing with increasing ethoxylation, the loading of the solvent at these conditions is only 6 X lod g/g solvent, meaning that separation a t these conditions would not he feasible. As the pressure is increased, the mixture critical pressure is approached

and the fluid and liquid phaeea beoome more similar. This means that the loading is increased; however the distribution coefficients approach unity making the separation of the components more difficult. By comparing the trends in Figurea 2 and 4, one can begin to appreciate the trade-off between selectivity and loading which must ultimately be balanced economically. For comparison between solvents, we chose to compare at similar reduced densities in order to m i n i the effect of proximity to the critical point of the pure SCF. The 1300 psi results for CO, correspond to a reduced density of approximately 1.05 which is the same reduced density shown in Figure 5 with pure propane as the solvent at two temperatures. The magnitude of the coefficients is similar although the behavior in propane is slightly different. In propane we do see a significant temperature dependence; for the 99 'C case, the long-chain n-mers have very large coefficients, indicative of nearly complete miscibility. Visual observation of the system at these conditions indicated proximity to the mixture critical point. This hehavior WBS also noted by Horizo and Yanagi (1990), causing them to discard propane as a viable solvent. At this temperature, operating at a lower pressure further from the mixture critical point would provide distribution coefficients more useful for separations, although the loading would be lower. At 107.7 O C and a reduced density of 1.05, the mixture critical pressure is higher; thus the behavior was much more favorable for separations, showing a monotonic decrease with chain length. The results from using ammonia as well as 1 and 3.5 mol 90methanol/C02 mixtures are compared with pure COz and propane in Figure 6. The ammonia was at a slightly

1108 Ind. Eng. Chem. Res., Vol. 31, No. 4,1992

25

2cCO2 wi 3 5% MeOH Y

0 2-

, 2

3 4 Component

5

6

7

Figure 6. Comparison of distribution coefficients for fluids at T,= 1.03 and pr = 1.05.

lower reduced density than the rest due to equipment limitations, and the conditions for the C02-cosolvent mixtures were again reduced by pure C02 critical properties. Ammonia and propane show the desired monotonic decrease with chain length but are not as selective as indicated by the difference in distribution coefficients for the first and last components shown. The C02 mixtures show greater selectivity as well as a higher coefficient for CI2E, than for dodecanol. This may, in fact, be an advantage as the difference in distribution coefficients between C12El and C12E2is much larger in COP and its mixtures than in ammonia or propane. The loadings and distribution coefficients were used to evaluate a separation/fractionation scheme using either a batch or stagewise distillation as discussed in the next section.

Discussion of Modeling In our models of separation processes, we assumed first that the distribution coefficients were independent of the composition. This assumption appears to be valid based upon comparison of the distribution coefficients measured for feeds with different compositions. Second, the supercritical fluid solubility in the liquid phase was ignored, this assumption affects only continuous separation processes where some of the SCF will be removed in the bottoms product. First, we constructed a model analogous to our experiments: a single-stage batch extraction. The assumptions above lead to the relationship (Treybal, 1980): log

FxF,i Fx, = (YiL log RXR,i

RXRJ

where F is the moles loaded into the extractor with composition x F , ~for CI2Eiand xF& for the dodecanol, R is the ~ X R , and a i l , moles of residue with compositions X R and is the relative volatility of C12Eibased upon ckdecanol. The relative volatilities were determined from our experiments and are the ratio of the equilibrium distribution coefficient of the i-mer to the distribution coefficient of the dodecanol. Equation 1 was solved for each of the i-mers simultaneously along with C . x R j = 1 to give the composition for R moles of residue. kesults are shown in Figure 7 for COPat 1300 psi and 40 OC extracting 70 mol % of the feed. This is compared with the composition of the raffhate after an extraction that we conducted under identical conditions. The good agreement between the model and experiment indicate that the model assump-

compo^^

Figure 7. Results from batch distillation model compared to experiment, 1300 psia, 40 O C , 70 w t % of feed extracted.

Yp,i

G

A

Ln+ t

Y ",I G

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Yn-1,i

Ln f

Xn,i

tions are reasonable. Note that even after extracting a large amount of the feed, although the distribution is narrower and shifted toward higher i-mers as desired, there is still about 5 mol ?% lauryl alcohol in the product. In order to obtain an improved product, we examined a continuous stagewise separation. A schematic of the stagewise separation is shown in Figure 8. The supercritical solvent, assumed to be pure, and the feed flow countercurrently into a column with p equilibrium stag-. The fluid and liquid leaving each stage are assumed to be a equilibrium: Yn,i

KiXn,i

(2)

where ynjis the composition of component i above the nth tray in the fluid phase, xnj is the composition of component i below the nth tray in the liquid phase, and Ki is the composition-independent equilibrium distribution coefficient of component i, determined from our experiments. In contrast to the solvent-free distribution coefficients discussed above, these ICs use the actual fluid-phase compositions and are much smaller than the solvent-free coefficients because the fluid phase is composed primarily of the solvent. In addition, mass balances can be written for each component around each tray: Yn-l,iG

+ Xn+l,iLn+l = Y n , i G + x n , i L n

(3)

where G is the fluid molar flow rate and Ln is the liquid molar flow rate below the nth tray. Because the fluidphase solubilities of the liquid-phase components are low, G is assumed to be constant throughout the column.

Ind. Eng. Chem. Res., Vol. 31, No. 4,1992 1109 0.3

0.300

/.,

0.250 0.200

;

0.150

0.100

0.050

--

0.000

Component

Figure 9. Effect of feed rate on continuous stagewise extraction of surfactant a t 40 OC and 1300 psia with five equilibrium stages. 0'350

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0.3W

_____ 1500 psi -2000 pSi 1

7 Wr%RECOVERED

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3

a

6

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Figure 11. Continuous stagewise extraction results using C02with 3.5% methanol as the solvent a t 40 O C and 1300 psia with lo00 mol of solvent/mol of feed and five equilibrium stages.

+

0.250-

ae

0.200-

P

9 0.150 0.100

1

9

4

5

6

f

I

Component

Figure 10. Effect of pressure on continuous stagewise extraction of surfactant at 40 OC with 600 mol of CO,/mol of feed and five equilibrium stages.

Finally, the s u m of the mole fractions in the liquid phase below each tray must be unity:

cx,,i = 1

(4)

With the liquid feed and composition specified, it is necessary to stipulate another variable, such as the fluid flow rate, to solve this system of equations. Figure 9 shows the results for C02 at 1300 psi and 40 "C for two different feed rates of the solvent and five equilibrium stages. Removal of about 60 w t % of the surfactant feed by the SCF results in virtual elimination of the dodecanol and CI2E1;the extract can be removed easily from the SCF stream and undergo further reaction. This separation, however, requires a large amount of COP (up to 2000 mol of C02/mol of surfactant); thus we searched for better conditions or solvents. In Figure 10, separations are carried out with identical amounts of C02 at 40 OC, but with varying pressure. Despite the favorable distribution Coefficients at 1100 psi, virtually no separation is achieved because of the low loading; thus, the resulting distribution is not shown as it is nearly identical to the feed. At 2000 psi, the loading is high, but the distribution coefficient for the lauryl alcohol is not as good (in addition to the increased expense due to operating at higher pressures) leading to a poorer separation. The optimum pressure lies somewhere between these extrema. Figure 11 compares the separation achieved by equal amounts of pure C02 and C02with 3.5 mol % methanol.

0.250-

\I

mob NHWmol feed 19 W h RECOVERED

Component

Figure 13. Continuous stagewise extraction results using ammonia solvent at 2000 psia and 144 OC with five equilibrium stages.

This relatively small amount of methanol dramatically increases the removal of dodecanol and the first three i-mers. Examination of the distribution coefficients shows that all of the solvents are roughly equivalent in terms of selectivity; however the loadings are much different. The results for propane and ammonia are shown in Figures 12 and 13. Note that the degree of separation achieved by 2000 mol of C02can be accomplished with about 500 mol of propane or only 75 mol of ammonia. In addition to

1110 Ind. Eng. Chem. Res., Vol. 31, No. 4, 1992

requiring less solvent, the separation with propane can be attained at lower pressures; however, the somewhat higher temperature and flammability of propane must also be considered. Ammonia is corrosive and requires higher pressures and temperatures.

Conclusions We have demonstrated the fractionation and purification of a nonionic surfactant using a variety of pure and cosolvent-modified supercritical fluids. Distribution coefficients and total solubility were measured in a flow apparatus over the reduced temperature range 1.01 < T, < 1.08 and reduced density range 0.5 < pr < 1.8. The trend in distribution coefficients show the possibility of removing both the unreacted dodecanol and C12E1from the surfactant. The ClZEloften exhibited a higher distribution coefficient than dodecanol. Promising distribution coefficients were noted for lower pressures; however, solubilities (or loadings) were low. As the pressure increased, the loadings also increased but the selectivity decreased. Two models for the removal of dodecanol were developed using the experimental distribution coefficients: a batch extraction model and a continuous stagewise extraction. The batch extraction succeeds in narrowing the distribution of polymer but leaves a product with 5 mol % dodecanol after extracting 70 mol ?& of the feed. The continuous extraction model shows that virtually all of the dodecanol and CI2E1can be removed in five equilibrium stages with 60 mol % of the feed extracted. The continuous extraction model was used to make comparisons between various solvents by determining the amount of solvent required to achieve a given separation. Based strictly on the amount of solvent required per mole of feed, the effectiveness of the SCF solvents increased in the order COz < COz + 3.5% MeOH < propane < ammonia as expected. Other factors must be considered, of course, but this work demonstrates that COz, although convenient, may not always be the most effective solvent for a given separation. In fact, the addition of a casolvent to propane may be worthwhile as long as the higher temperature does not suppress the cosolvent-surfactant interaction. In the absence of economic evaluation, we would recommend a process based on propane or the methanol/carbon dioxide solvent mixture for high selectivity and loading using moderate operating conditions. Acknowledgment We gratefully acknowledge the generosity of Milliken Chemical Co. and the help of Ms. Betsy Kuo. Financial support for this work was provided by E. I. du Pont de Nemours and Co. Registry No. CI2En,9002-92-0.

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Prepr. (Am. Chem. SOC.,Diu. Polym. Chem.) 1987, 28 (2), 399-400. Eggers, R.; Sievers, U. Current State of Extraction of Natural Materials with Supercritical Fluids and Developmental Trends. ACS Symp. Ser. 1989,406,478-498. Greek, B. F. Sales of Detergents Growing Despite Recession. Chem. Eng. News 1991,Jan 28,25. Horizo, H.; Yanagi, Y. Japanese Patent 02160738 A2, 1990. Krukonis, V. J. Supercritical Fluid Processing: Current Research and Operations. Proceedings of International Symposium on Supercritical Fluids; I N P L Nancy, France, 1988;pp 541-560. Kucharski, S.; Kuciel, E. Nonionic Surfactants I11 The Studies on the Foam Fractionation. Tenside 1971,8 (4),191. Kuciel, E.; Makomaski, K. Steady-state Foam Fractionation of Ethoxylated Nonylphenol. Tenside Deterg. 1979,16 (2), 76. Kuo, B. Narrowing the Molecular Weight Distribution of Linear Alcohol Ethoxylates. M.S. Thesis, Georgia Institute of Technology, Atlanta, GA, 1991. McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction: Principles and Practice; Butterworths: Boston, 1986. Mgdrzycka, K. B.; Zwierzykowski,W. Foaming of Ionic and Nonionic Detergents from Their Aqueous Solution Mixtures. Sep. Sci. Technol. 1981,16 (2),185. Nilsson, W. B.; Gauglitz, E. J.; Hudson, J. K.; Stout, V. F.; Spinelli, J. Fractionation of Menhaden Oil Ethyl Esters Using Supercritical Fluid COz. J. Am. Oil Chem. SOC.1988,65 (11,109-117. Nilsson, W. B.; Gauglitz, E. J.; Hudson, J. K. Supercritical Fluid Fractionation of Fish Oil Esters Using Incremental Pressure Programming and a Temperature Gradient. J. Am. Oil Chem. SOC.1989,66(ll), 1596-1600. Paulaitis, M. E.; Krukonis, V. J.; Kurnik, R. T.; Reid, R. C. Supercritical Fluid Extraction. Reu. Chem. Eng. 1982,l (2),179-250. Paulitis, M. E., Penninger, J. M. L., Gray, R. D., Davidson, P., Eds. Chemical Engineering at Supercritical Fluid Conditions; Ann Arbor Science: Ann Arbor, MI, 1983. Peter, S.; Brunner, G. The Separation of Nonvolatile Substances by Means of Compressed Gases in Countercurrent Procesaea. Angew. Chem., Int. Ed. Engl. 1978,17,746-750. Scholsky, K. M.; O’Connor, K. M.; Weiss, C. S.; Krukonis, V. J. Supercritical Fluid Processing of Synthetic Polymers. Polym. Prepr. (Am. Chem. SOC.,Diu. Polym. Chem.) 1986, 27 (2), 140-141. Stancher, B.; Favretto, L. Gas-Liquid Chromatographic Fractionation of Polyoxyethylene Nonionic Surfactants. J. Chromatogr. 1978,150,447. Tilly, K.D.; Chaplin, R. P.; Foster, N. R. Supercritical Fluid Extraction of the Triglycerides Present in Vegetable Oils. Sep. Sci. Technol. 1990,25,357-367. Treybal, R. E. Mass-Transfer Operations, 3rd ed.; McGraw Hill: New York, 1980. Williams, D. F. Extraction with Supercritical Gases. Chem. Eng. Sci. 1981,36(ll),1769-1788. Yamauchi, Y.; Saito, M. Fractionation of Lemon-Peel Oil by SemiPreparative Supercritical Fluid Chromatography. J. Chromatogr. 1990,505,237-246. Yilgor, I.; McGrath, J. E. Novel Supercritical Fluid Techniques for Polymer Fractionation and Purification. 1. Background. Polym. Bull. 1984a,12,491-497. Yilgor, I.; McGrath, J. E. Novel Supercritical Fluid Techniques for Polymer Fractionation and Purification. 2. Fractionation and Characterization of Functional Siloxane Oligomers. Polym. Bull. 1984b,12,499-506. Zosel, K.Separation with Supercritical Gases: Practical Applications. Angew. Chem., Int. Ed. Engl. 1978,17,702-709. Zwierzykowski, W.; Mgdrzycka, K. B. Removal of Detergenta from Aqueous Solution by Foaming. Sep. Sci. 1973,8 (l), 57. Zwierzykowski, W.; Mgdrzycka, K. B.; Chlebus, S. Influence of Molecular Weight of Nonionic Detergents on Their Removal from Aqueous Solutions by Foaming. Sep. Sci. 1975,10 (4),381. Receiued for review October 7, 1991 Accepted December 16, 1991