Performance of a packed column for continuous supercritical carbon

a decrease in that of HMT in the extract. Theraffinate has a high concentration of. HMT with trace amounts of LMT. The performance characteristics of ...
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Biotechnoi. Prog. 1993, 9, 70-74


Performance of a Packed Column for Continuous Supercritical Carbon Dioxide Processing of Anhydrous Milk Fat Ajay R. Bhaskar,*pt Syed S. H. Rizvi; and Peter Harriotti Institute of Food Science and School of Chemical Engineering, Cornell University, Ithaca, New York 14853

A continuous system was designed for the processing of anhydrous milk fat (AMF) with supercritical carbon dioxide (SC-CO2). The packed column was operated as a stripping column with SC-CO2 as the continuous phase and AMF as the dispersed phase. The extraction was studied a t 24.1 M P a and 40 "C. T o utilize the functional similarities and make the analysis simpler, the AMF triglycerides were grouped as low-melting (LMT: C24-C34), medium-melting (MMT: C36-C40), and high-melting triglycerides (HMT: C42-C54). There was a n increase in the concentrations of L M T and MMT and a decrease in t h a t of HMT in the extract. T h e raffinate has a high concentration of HMT with trace amounts of LMT. T h e performance characteristics of the packed column were described by the number of transfer units (NoG),the overall volumetric mass transfer coefficient (KoGu),and the height of a transfer unit (HOG).T h e NOGand KOGUvalues decreased and the HOGvalue increased with increasing solvent-to-feed ratio. T h e HOGvalues increased and the NOGand KOGUvalues decreased from L M T t o MMT t o HMT, but the HOGvalues for HMT were greater than expected. T h e values of NOG,HOG,and KOGUfor AMF (as a single component) were intermediate to those of M M T and HMT.

Introduction Milk fat has been traditionally utilized to manufacture butter and other dairy products. Although the pleasing flavor of milk fat is highly desirable in many foods, recognition of its limited dietary value and functional properties in the native form has dramatically reduced its consumption in recent years. Substitutes for milk fat are often used in food formulations. Much interest has been focused on fractionation of milk fat which, in turn, offers the dairy industry new possibilities for milk fat utilization. Application of supercritical fluid extraction (SFE) for fractionation and cholesterol reduction of anhydrous milk fat (AMF) has been shown to offer attractive possibilities (Shishikura et al., 1986;Arul et al., 1987;Rizvi et al., 1989). SFE does not involve the use of additives, surfactants, or organic solvents. The other advantages include separation a t low temperatures, rapid mass transfer, and negligible residual solvent levels. The preferred solvent in the food industry is carbon dioxide because it is nontoxic, inexpensive, nonflammable, and readily available and i t has low critical temperature and pressure compared to other solvents. However, inadequate thermodynamic descriptions of supercritical solvent-solute mixtures and lack of engineering data to facilitate scale-up and design have contributed to the limited acceptance of SFE processes on a commercial scale. Like most applications of SFE, research on milk fat has thus far concentrated only on batch systems. In the case of fluids like milk fat, which can be pumped a t high pressures, it is reasonable to anticipate that the processing time can be minimized and the economics made more favorable by continuous processing (Rizvi, 1991). A packed column is often used for continuous extraction when only a few equilibrium stages are required (Seibert and Fair, 1988). Packed columns were found to be more + Institute of Food Science. t School of Chemical Engineering.


efficient compared to spray columns for the processing of AMF with supercritical carbon dioxide (SC-CO2) (Lim, 1992). The packed column provides a tortuous path for the dispersed phase, increasing the residence time and consequentlyresulting in an increased overall area for mass transfer. Lim (1992) also studied different packing materials and their effect on column efficiency and observed that the column packed with knitted mesh yielded the highest mass transfer efficiency. Scale-up of extraction systems requires reliable data on mass transfer rates. While previous research on SFE has concentrated on phase equilibria, there has been very limited research investigating the hydrodynamicsand mass transfer characteristics of such systems. Only a few studies on both the hydrodynamics and mass transfer efficiency a t high-pressure conditions have been reported. These studies have mostly concentrated on model systems: SCCOdisopropyl alcohol/water (Rathkamp et al., 1987);SCCOdethanoI/water (Seibert and Mooseberg, 1988);toluene/ acetone/water (Lahiere and Fair, 1987). Similarly, there have been few studies on mass transfer efficiencies and hydrodynamics of processing AMF. Yu et al. (1992a) extensively studied the phase equilibria of AMF in SCCO2. They also studied the solubility of AMF and its triglyceride composition(indicated by acyl carbon number) a t different pressures and temperatures (Yu et al., 199213). Raj and Rizvi (1991) and Lim (1992) studied the mass transfer of AMF, as a single component, but did not study the individual triglycerides. Since AMF is a complex mixture of triglycerides (acyl carbon number varying from C24 to C54) with a range of molecular weightsand degreesof unsaturation, its inherent complexity leads to differences in the solubility of the triglycerides (Perry et al., 1949). The diffusivity of the AMF triglycerides in SC-CO2 also varies with molecular weight. AMF or butter oil is very desirable from a process engineering point of view for continuous extraction and fractionation at high pressures. It is virtually water-free,

0 1993 American Chemical Society and American Institute of Chemical Engineers

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T Raffiate filter 4 COzflow 4 AMFflow pressure gauge -b Extractflow -0 thermocouple rupturedisc 4 saftev valve checkvalve 3-way valve a b a c k pressure regulator 9





Figure 1. Simplified schematic of the continuous pilot-scale SFE system used in the present study: (1)feed flow meter; (2) feed pump; (3) liquid COn tank; (4) solvent pump; (5) micrometering valve; (6) solvent flow loop; (7) entrainment vessels; (8)

packed column; (9) view cell; (10) sampling vessel; (11)solvent flowmeter; (12) dry test meter.

homogeneous, and liquid a t around 40 "C (Rizvi, 1991). The specific objectives of this research were to study the mass transfer rates for AMF and its triglycerides, grouped as low-melting triglycerides (LMT), medium-melting triglycerides (MMT), and high-melting triglycerides (HMT). These results should provide a basis for a better understanding of scale-up and design for the processing of milk fat with supercritical carbon dioxide.

Materials and Methods Anhydrous Milk Fat (AMF). Commercial grade butter was converted into AMF by melting it at 60 "C, decanting the top layer, and filtering it through Whatman No. 1filter paper. The oil content in AMF was determined to be at least 98% (AOCS, 1989). Experimental Method and Procedure. Figure 1 shows a simplified schematic of the continous pilot-scale SFE system used in the present study. The basic system consists of a packed column (8), feed and solvent metering controls, and a sampling vessel (10). The packed column is 1.8 m long and 4.9 cm in internal diameter with six inlet/outlet ports. The column was packed with SS 304 Goodloe knitted mesh packing (surface area: 19.2 cm2/ cm3;void fraction: 0.95). The continuous system can be operated in either the cocurrent or countercurrent mode. A Milton Roy Type B (2) reciprocating pump (maximum flow rate: 8.0 L/h) is used for feeding AMF. The feed can be introduced from the top (2= 1.22 m) or from the center (2 = 0.61 m) of the column. The packed column and the sampling vessel are equipped with drain valves at the bottom and heating jackets for temperature control. A positive displacement, reciprocating pump (4) (maximum flow rate: 113 L/h) was used to compress the gas to the desired operating pressure. Pressure in the system was

maintained with the help of back pressure regulators. The AMF and CO2 flow rates were monitored by rotameters (1,11). The packed column was operated as a stripping column with SC-CO2 as the continuous phase and AMF as the dispersed phase. On the basis of the work of LIM (1992), who studied the effect of processing conditions for AMF extraction with SC-C02 in the pressure range of 13.7-24.1 MPa and temperatures of 40 and 60 "C, the optimum conditions of 24.1 MPa and 40 "C were used in this study. The flow through the column was countercurrent with AMF entering a t the center and SC-C02 entering at the bottom. SC-CO2 with the dissolved fat then was passed into the separation vessel through a pressure reduction valve where the soluble triglycerides were precipitated and collected. The volumetric flow rate of CO2 was measured by a dry test meter (12) before it was vented to the atmosphere. Data were collected periodically for feed and solvent flow rates and amount of extract and raffinate. A material balance check was performed a t the end of each run. If the material balance closure was more than 90%, the samples were analyzed and the mass transfer efficiency was determined. The packed column was considered to be in the steady state when the amount of extract and the flow rates (feed and solvent) obtained were constant for three successive time intervals. A typical run required 1h to reach the steady state. Analysis. Triglycerides were analyzed by a modified method of Amer et al. (1985). Complete analysis of triglyceridesin AMF was done for only one run a t a solventto-feed ratio of 62. For the other runs the performance was based on amount extracted. The feed, raffinate, and extract of AMF were directly analyzed for their triglyceride compositions on a gas chromatograph (GC) using a DB-5 capillary glass column (30 m X 0.25 mm) (J and W Scientific, Folsom, CA) fitted with a flame ionization detector (Hewlett-Packard Model 5890, Avondale, PA). The oven temperature was programmed in three stages: first, from 50 to 240 "C a t a rate of 25 OC/min; second, from 240 to 345 "C a t a rate of 3 OC/min and held for 25 min; finally, from 345 to 350 "C at a rate of 0.1 "C/min. The injector and detector temperatures were maintained a t 330 and 345 "C, respectively. The carrier gas used was helium (1.5 mL/min). The variation in the results from analysis was estimated to be 5 % at most on the basis of duplicate analyses. Theory The ability of a given packing to achieve effective mass transfer between a gas phase and a liquid phase is commonly expressed in an empirical form as the height of packing equivalent to one transfer unit (HTU) or the height of packing equivalent to one theoretical plate (HETP) (Peters and Timmerhaus, 1991). HETP varies not only with the type and size of the packing but also very strongly with flow rates of each fluid and for every system with concentrations as well, so that an enormous amount of experimental data would have to be accumulated to permit utilization of the method (Treybal, 1980). The difficulty lies in the failure to account for the fundamentally different actions of tray and packed towers. It does not give an accurate picture of what is happening physically in the column, and hence this method is not preferred. HTU can be expressed on the basis of the number of transfer units calculated from the gas-phase driving force (NTUG) or from the liquid-phase driving force (NTUL)(Peters and Timmerhaus, 1991). The NTU required is a measure of the difficulty of the separation between componentsbeing extracted. Due to the difficulty in measuring solute concentrations a t the gas-liquid

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interface, the resulting mass transfer rates can be expressed as overall coefficients (Treybal, 1980). For this study the overall gas-phase coefficient is based on the driving force Y* - Y, where Y* is the weight fraction corresponding to equilibrium with the liquid-phase composition and Y is the weight fraction in the extract. Thus, if the change in the mass flow rate of the gas phase is small so that V (mass flow rate of gas) and K m (overall volumetricmass transfer coefficient) can be assumed constant (McCabeet al., 1985), the equation for the column height (ZT)can be written as

where S is the cross section. The integral in eq 1is defined as the number of transfer units (NoG),which represents the change in the gas concentration divided by the average driving force. The subscripts show that NOGis based on the overall driving force for the gas phase.

The other part of eq 1is defined as the height of a transfer unit (HOG)and has units of length. (3) Therefore, ZT can be expressed as


(4) Equations 1-3 can be applied to individual components in a multicomponent mixture using the corresponding equilibrium data. For dissolution of a single pure fat component, Y* would be constant throughout the column, and integration of eq 2 would give

NOG = In [(Y*- Yin>/(Y* - Yout)l However, when a mixture is extracted with SC-CO2, the changing fat composition leads to changes in Y* for each component, and exact integration of eq 2 is impossible. However, the values of Y* at each end of the column can be determined. The Y* values in this study were obtained from the data of Yu e t al. (1992131, who studied phase equilibria of AMF with SC-CO2 in a static recirculation system. If Y* is assumed to be a linear function of Y ,then Y* - Y is also a linear function of Y , and eq 2 integrates to

NOG = (Yout-Yin)/(Y*- Y ) b HOGcan be calculated from eqs 4 and 5 as


HOG= ZTINOG From eqs 3 and 6, KocU can be calculated as


KO& = VISHOG (7) Although the values for NOG,HOG,and KooU could be calculated for each component, to utilize the functional similarities and make the analysis simpler, it was convenient to group the triglycerides into three categories defined as low-melting triglycerides (LMT: C24-C34), medium-melting triglycerides (MMT: C36-C40), and high-melting triglycerides (HMT C42-C54). Results and Discussion Optimization of Extraction Conditions. In the design of continuous extraction systems it is important to optimize extraction rates while minimizing process time. To achievethis, the effect of solvent-to-feed ratio on extract loading and extraction yield must be studied. The

L = 1.662 + O.M)1711(S/F) - 0.0001243(S/F)2, R2=0.98


E = 2.011(S/F) - 0.01188(S/F)z, R2=0.98





'40'' " 5

0' " ' 6 0 Solvent-to-feed ratio


' ' 70



$45 0

Figure 2. Extract loading and extraction yield as a function of solvent-to-feed ratio at 24.1 MPa and 40 O C . Table I. Number of Transfer Units (NO& Height of a Transfer Unit (Boa),and Overall Volumetric Mass Transfer Coefficient (K-) at Different Solvent-to-Feed Ratios for Extraction of AMF at 24.1 MPa and 40 O C ~~~

experimentalrun parameters solvent (SC-COz), g/h g/h feed (AMF), solvent-to-feedratio extract (SC-COZAMF),g/h raffmate, g/h NOQ HOG cm K w , g/cm%~ extraction yield, w t %

I 9224 297 31 9370 126 1.34 45 0.0030 49



9022 185 49 9155 49 1.17 52 0.0026 72

9098 124 73 9202 19 0.76 81 0.0017 84

extraction yield (E, wt %) is defined as

E = (m,/mf)lOO The extract loading (L,wt % ) is defined as L = (mJmJ100


(9) where me is the amount of extract, mf is the amount of feed, and m, is the amount of solvent. Figure 2 shows the extract loading and the extraction yield as a function of solvent-to-feed ratio a t 24.1 MPa and 40 OC. A t a low solvent-to-feed ratio the extract loading is high but the extraction yield is low and vice versa for a high solvent-to-feed ratio. Since the AMF dissolved in CO2 is diluted by the excesssolvent, low solute loading is observed a t a high solvent-to-feed ratio. A decrease in the solvent-to-feed ratio results in an increase in the extract loading until the capacity of the supercritical solvent limits further take up to AMF due to phase equilibrium limitations. The maximum extract loading (equilibrium solubility) of AMF a t 24.1 MPa and 40 "C is 2.1 w t % (Yu et al., 1992b). A t high values of solventto-feed ratio more AMF passes intothe C02 phase, thereby increasing the extraction yield. Table I lists the NOG,HOG,and KoGU values for AMF extraction for different solvent-to-feed ratios a t 24.1 MPa and 40 OC. The mixture density of AMF with SC-CO2 for extract and raffinate was 873 and 908 kg/m3,respectively. For this study, mass transfer rates are based on the maximum extract loading (equilibrium solubility). The number of transfer units (Nm) is a measure of the difficulty of separation; i.e., a close approach to saturation of the solvent requires a large number of transfer units. A t a low solvent-to-feed ratio the degree of absorption is high, and hence high NOGand KoGa values are observed. A large mass transfer coefficient and large interfacial area per unit

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Table 11. Trialweride ComDosition (Solvent-Free Basis) of Feed, Raffinate, and Extract as Determined from GC Analysis. ~


AMF fraction LMT MMT HMT overall

mol wt 609.53 663.52 785.49 688.49




mol w t


16.22 50.85 32.93 100.00

666.05 814.43 784.57d

traces 17.07 82.92 99.99

mol wt 609.25 664.10 776.07 678.77d


18.51 54.58 26.91 100.00

extractbwt % 29.63 40.75 29.62 100.00


0.0621 0.0382 0.0216 0.0318

Solvent (SC-COz)= 6787 g/h; feed (AMF)= 109g/h; solvent-to-feedratio = 62. Equilibrium triglyceride compositionof extract in a static recirculation system (Yu et al., 1992b). Equilibrium distribution coefficient in a static recirculation system (Yu et al., 1992b).d Molecular weight determined as weighted average from LMT, MMT, and HMT.

volume result in a low HOGvalue. As the solvent-to-feed ratio increases, there is a decrease in extract loading, the NOG and KoGa values decrease, and the HOG value increases. Similar observations were made by Mizandjian and Massie (1988),who carried out experiments to recover butyric acid from an aqueous solution of acetic acid with SC-CO2. Their results showed that the HTU values increased and KoGa values decreased with increasing solvent-to-feed ratio. At low solvent-to-feed ratios, the composition of the raffinate will shift closer to that of the feed. Also the change in extraction yield after a solventto-feed ratio of 60 is small compared to the change between solvent-to-feed ratios of 30 and 60. Thus, concentration on only high loading leads to an uneconomic choice of process conditions. A solvent-to-feed ratio of 60-65 was chosen with an extraction yield of 75-82 wt % for further analysis. Lim (1992) studied the mass transfer for AMF (as a single component) in SC-CO2 and reported the performance of a packed column. Using a modified Riccatti equation, he expressed the column efficiency as height equivalent to a theoretical stage (HETS). His results showed that there was a slight decrease in HETS with an increase in the solvent-to-feed ratio. This disagrees with the HTU values calculated in this study. Composition Distribution of Triglycerides and Mass Transfer Rates. AMF contains 97-98s triglycerides, 0 . 1 4 4 4 % mono- and diglycerides, 0.3-0.796 free fatty acids, and 0 . 2 4 4 % cholesterol (Arul et al., 1987). It is thus reasonable to assume that the composition of AMF is mostly triglycerides. The feed, raffinate, and extract were analyzed for their triglyceride composition to establish the effectiveness of extraction. Table I1 lists the triglyceride compositions (as determined by GC analysis) and the molecular weights for the feed, raffinate, and extract. The molecular weights for the individual LMT, MMT, and HMT were determined as weighted averages from the 5% composition in AMF, raffinate, and extract as determined by GC analysis. Most of the high molecular weight triglycerides are retained in the column while the low molecular weight triglycerides are extracted with SC-COP. Figure 3 shows the composition distribution of triglycerides in the feed, raffinate, and extract a t 24.1 MPa and 40 OC. The extraction yield was 78 wt % and the raffinate was 21 wt 5%. There is an increase in LMT and MMT and a decrease in HMT in the extract compared to the original AMF. The raffinate is rich in HMT and has trace amounts of LMT. Yu et al. (1992b) calculated the equilibrium distribution coefficient ( m ) for each component of AMF. Their results show that the LMT have a higher m value compared to MMT and HMT, which decreases from LMT to MMT to HMT. The trend of data for triglyceride composition of feed, raffinate, and extract is similar to that obtained by Yu et al. (1992b) using a static recirculation system. The NOG,HOG,and KoGa values were determined for the AMF triglycerides grouped as LMT, MMT, and HMT and are presented in Table 111. The weight % values of


4 4





24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54

Triglyceride (acyl carbon number) Figure 3. Composition distribution of triglycerides in AMF, raffinate, and extract at 24.1 MPa and 40 OC at a solvent-to-feed ratio of 62. Table 111. Mass Transfer Data for AMF and Its Triglycerides at S/F = 62 KOGG

NOG HOG,cm g/cm% LMT 4.51 14 0.0073 MMT 3.62 17 0.0059 HMT 0.39 157 O.OOO6 AMF 0.93 65 0.0015 a Y*bt = Y*bP(X,dXb). Y*bp,see Yu et al. (1992b).


Y*bta O.OOO1 0.287 1.567 2.1

Y*mpb 0.246 0.724 0.305 1.275

YOut 0.622 0.856 0.622 2.1

LMT, MMT, and HMT from Table I1 were used to calculate Y* and Y ,which are also shown in Table 111.The concentration of LMT in the raffinate is very low and cannot be accurately determined due to analytical limitations. This value was, therefore, assigned as 0.01 for calculations to determine the trend in the data. Table I11 shows that the HOGvalues increased and the NOGand KoGa values decreased from LMT to MMT to HMT, but the HOGvalues for HMT were greater than expected. The LMT and MMT are more soluble in SC-C02 compared to HMT. Also, the highest calculated driving force (Y* - Y) is a t the bottom of the column; 1.5 wt % for the HMT compared to 0.0004 wt % for LMT and 0.29 w t % for MMT. The values for NOG,HOG,and KoGa for AMF (as a single component) were intermediate to those for MMT and HMT. Figure 4 shows possible profiles of Y and Y* for LMT, MMT, and HMT. For the HMT, Y* at the bottom increases as the fat passes through the column, since the raffinate is enriched in HMT. The reverse is true for LMT and MMT. But, for the LMT, Y* is almost zero a t the bottom and Y increases slowly as COZrises through the column. Perhaps most of the mass transfer for the LMT occurs in the top half (or less) of the column. According to regular solution theory, the increasing molar volume of the solute dictates that the volatility of triglycerides should decrease as the molecular weight increases. Therefore, triglyceride solubility should decrease with increasing molecular weight (Prausnitz et al., 1986). Thus, it is expected that the mass transfer rates for the triglycerides will vary with their molecular weights.

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information in scale-up and design of extraction systems for a given duty. Acknowledgment Support of this research by the Northeast Dairy Foods Research Center is thankfully acknowledged.

Literature Cited AOCS, Official Methods and Recommended Practices of the American Oil Chemists’ Society, 4th ed.; American Oil Chemists’ Society: Champaign, IL, 1989. Amer,M. A.; Kupranycz,D. B.; Baker,B. E. Physicaland chemical characteristicsof butterfat fractionsobtainedby crystallization 1985, 62, from molten fat. JAOCS, J. Am. Oil Chem. SOC. 1551-1557.

Arul, J.; Bourdeau, A.; Makhlouf, J., Tardif, R.; Sahasrabudhe, M. R. Fractionation of AMF by supercritical carbon dioxide. J. Food Sci. 1987,52,1231-1236. Bamberger,T.; Erickson, J. C.; Cooney, C. L. Measurement and model prediction of solubilities of pure fatty acids, p y e triglycerides, and mixtures of triglycerides in supercritical carbon dioxide. J. Chem. Eng. Data 1988,33, 321-333. Lahiere, L. R.; Fair, J. R. Mass transfer efficiencies of column contactors in supercritical extraction service. Ind. Eng. Chem. 1987,26,2086.

Lim, S. Performance characteristics of a continuous system supercritical carbon dioxide system coupled with adsorption. Ph.D. Dissertation, Cornell University, Ithaca, NY, 1992. McCabe, W. L.; Smith, J. C.; Harriott, P. Unit Operations of Chemical Engineering; McGraw-Hill: New York, 1985;pp 617-659.

Figure 4. Possible profiles of Y and Y* for LMT, MMT, and HMT: Y*, equilibrium solubility; Y, solubility of extract.

Bamberger e t al. (1988) measured the solubilities of pure triglycerides (trilaurin: C36; trimyristin: C42; tripalmitin: C48) in SC-CO2. Their results showed a difference of about 6-fold between the solubilities of trilaurin (0.0439 wt %) and trimyristin (0.0067 w t %) and of about 10-fold between trilaurin and tripalmitin (0.000 42 wt %) a t 24.1 MPa and 40 “C. The HMT are also soluble in the LMT, which helps them keep in solution. Since the LMT concentration decreasesin the raffinate,the characteristics of the HMT shift toward those of pure triglycerides, which are solid a t these operating conditions. This may affect the mass transfer rates of HMT and may be another reason for the greater differences in NOG,HOG,and KoGU values compared to LMT and MMT. The mass transfer rates for LMT and MMT are close to each other. If the true value for LMT in the raffinate is less than 0.01, the NOGand KOGUvalues for LMT would increase. The MMT are about 50 wt % of the triglyceride composition in AMF. The triglycerides close to the MMT show reasonable data for mass transfer rates. Such experiments on mass transfer rates will be essential for economic evaluations, scale-up, and design of extraction systems for processing AMF.

Conclusions The mass transfer rates of AMF and its triglycerides grouped in three categories (LMT, MMT, HMT) were studied for a packed column. The raffinate was rich in HMT and had trace amounts of LMT. The HOGvalues increased and the NOGand KoGa values decreased from LMT to MMT to HMT, but the HOGvalues for HMT were greater than expected. The values for NOG,HOG, and KOGUfor AMF (as a single component) are intermediate to those for MMT and HMT. The mass transfer data for AMF and its triglycerides can give useful

Mizandjian,J. L.;Massie,J. F. Performanceof a packedcontactor in supercritical COZcoutercurrent extraction. In Proceedings of the International Symposium on Supercritical Fluids; Institut National Polytechnique de Lorraine: Vandoeuvre, France, 1988; pp 661-667. Perry, E. S.;Weber, W. H.; Daubert, B. F. Vapor pressures of phlegmatic liquids. I. Simple and mixed triglycerides. J. Am. Chem. SOC. 1949, 71,3720-3726. Peters, M. S.;Timmerhaus, K. D. Plant Design and Economics for Chemical Engineers; McGraw-Hik New York, 1991;pp 649-739.

Prausnitz, J. M.; Litchtenthaler, R. N.; de Azevedo, E. G. Molecular Thermodynamics of Fluid-Phase Equilibria, 2nd ed.; Prentice-Hak Old Tappan, NJ, 1986. Raj, C. B. C.; Rizvi, S. S. H. Processing of milk fat with supercritical carbon dioxide: Mass transfer and economic aspects. Trans. Znst. Chem. Eng., in press. Rathkamp, P. J.; Bravo, J. L.; Fair, J. R. Evaluation of packed columna in supercritical extraction process. Soluent Extr. Zon Exch. 1987,5,367-391. Rizvi, S. S. H. Supercritical fluid processing of milk fat. Newsletter of the Northeast Dairy Foods Research Center; Cornell University: Ithaca, NY, 1991;p 3. Rizvi, S. S. H.; Lim, S.; Nikoopour, H.; Singh, M.; Yu, Z. Supercritical fluid processing of milk fat. Engineering and Food; Spiess,W. E. L., Schubert, H., Ed.; Elsevier: New York, 1989;p 145. Seibert, A. F.; Fair, J. R. Hydrodynamics and mass transfer in spray and packed liquid-liquid extraction column. Znd. Eng. Chem. Res. 1988,27,470-481. Seibert, A. F.; Moosberg, D. G. Performance of spray, sieve tray and packed contactors for high pressure extraction. Sep. Sci. Technol. 1988,23,2049-2063. Shishikura, A.; Fujimoto, K.; Kanedo, T.; Arai, K.; Saito, S. Modificationof butter oil by extraction with supercritical COz. Agric. Biol. Chem. 1986,505,1209-1215. Treybal, R. E. Mass Transfer Operations; McGraw Hik New York, 1980; pp 275-341. Yu, Z. R.; Rizvi, S. S. H.; Zollweg, J. A. Phase equilibria of oleic acid, methyl oleate, and anhydrous milk fat in supercritical carbon dioxide. J. Supercrit. Fluids, in press, 1992a. Yu, Z. R.; Rizvi, S. S. H.; Zollweg, J. A. Fluid-liquid equilibria of anhydrous milk fat with supercritical carbon dioxide. J. Supercrit. Fluids, in press, 1992b. Accepted November 10, 1992.