Znd. Eng. Chem. Fundam. 1984,23, 120-123
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COMMUNICATIONS Phase Separation Technique for Liquid Disperslons A new technique is developed to obtain the constituent liquids of a two-phase liquid dispersion from a continuous flow stirred tank extractor. The phase separation required for solute mass transfer estimation is achieved by in situ filtration of the stirred tank dispersion. The in situ filtration through an extremely thin membrane renders the separation almost instantaneous and eliminates the possibility of additional mass transfer during the separation. The technique is also well suited for continuous on-line monltoring of solute concentration and Is superior to previous methods reported in the literature.
Introduction
The contact of two immiscible liquids involving mass transfer with or without chemical reaction in stirred tanks occurs frequently in chemical process industries. In spite of this operational familiarity, very little is known about the fundamentals of liquid-liquid contacting. The highly interactive and complex hydrodynamic environment of a drop in a turbulent swarm of a mechanically agitated dispersion has precluded any realistic theoretical formulation of interphase mass transfer. Experimental estimation of interphase mass transfer also encounters major difficulties and is reflected by the relative scarcity of mass transfer data in liquid-liquid turbulent contactors such as a continuous flow stirred tank reactor (CFSTR). This paper reports the development of a new experimental technique which facilitates the estimation of mass transfer efficiencies in liquid extractors. Experimental Problems
The solute balance over the continuous phase in a CFSTR gives the continuous phase mass transfer efficiency as
E, =
c, - CC$ cc,a
- C,f
Similarly, the dispersed phase mass transfer efficiency is given by
The soluteequilibrium concentration in the continuous phase (C,,J is given by (3)
while the equilibrium concentration for the dispersed phase is cd,e
= KC,,,
(4)
where K is the solute equilibrium distribution constant. For a given set of operating conditions, the feed concentrations C , f and Cd,f are known. As seen from eq 1and 2, the estimation of mass transfer efficiencies requires the measurement of either C, or Cd. The difficulty arises since the dispersion must be separated into constituent phases before the solute concentration
measurement can be made. The separation is necessary because the solute detection or monitoring devices cannot handle liquid dispersions. Some instruments such as optical densitometers or refractometers require absolutely clear liquids. In addition, it is very important to complete the phase separation in a minimum amount of time to prevent additional interphase mass transfer outside the extractor. Also, the mass transfer efficiencies in stirred vessels are generally high and this puts an extra premium on accuracy and precision of the mass transfer measurements so that the true extractor performance variation with the change in the reactor operating conditions can be discerned. Previous Experimental Methods
The stability of a dispersion depends upon the physiochemical properties of the two liquid phases. The ease with which the phase separation can be effected for a given contactor also depends upon the flow patterns of the liquids. In a liquid-liquid extraction column operation, entrainment notwithstanding, continuous and dispersed phases exit the column at the oppostie ends enabling mass transfer efficiency of the entire column to be determined with ease (Allen et al., 1966; Cruz-Pinto and Korchinsky, 1980). This countercurrent flow pattern also allowed Smoot and Babb (1962) to obtain continuous and dispersed phase concentration profiles in a pulsed extraction column. The downward moving continuous phase was obtained by hypodermic stainless steel needles inserted through polyethylene gaskets of the column. The upward moving dispersed organic phase was sampled by needles with small flared polyethylene sleeves slipped over the end of the needles. The needle sleeves were pointed downward to trap the rising droplets of the lighter-dispersed phase. The sample purity ranged from 50 to 100%. In the absence of an effective linear velocity of the liquids, the problem of phase separation in stirred vessels becomes more acute. Table I gives the summary of previous attempts to determine the mass transfer efficiencies in stirred vessels. In addition to the earlier simple but crude gravity settling technique, a fine grit made of a material preferentially wetted by the required liquid has been used as a sampling device. In the present investigation, it was observed that the condition of the fritted glass filter affected the concentration of the solute in the continuous phase aqueous filtrate. Furthermore, the sintered glass filter became loaded with the dispersed organic phase and on prolonged filtration produced an aqueous filtrate contaminated with the organic phase. The deployment of an ultrahigh speed
0196-4313/84/1023-0120$01.50/0@ 1984 American Chemical Society
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Table I. Previous Phase Separation Techniques Employed for Liquid-Liquid Dispersions in Stirred Vessels investigators system method remarks aqueous iodine solution The removed aqueous solution contained batch; carbon Hixon and Smith removed by pipet few d r o p of organic phase. It was tetrachloride-iodine(1949) believed that they had n o significant water effect on the subsequent analysis. Relatively long times elapsed before gravity settling flow; toluene/ Flynn and Treybal benzoic acid concentration could be kerosene-benzoic (1955) determined by titration. aci d-water The effluent two-phase flow; keroseneAdditional mass transfer takes place Overcashier et al. mixture was led to high n-butylamine-water during phase separation. Material (1956) void volume stainless balance closed to within 4.4%. steel matting in a small pipe cross. The coalescence resulted in separation of phases. Batch: gravity settling; flow: Extraction continued during the gravity batch and flow; Ryon et al. (1959) organic phase by fritted settling. Present work has indicated uranium extraction Teflon and aqueous phase by Dapex process that solute concentration depends on by fritted glass filtration the filter condition. measurements affected by drop In situ conductivity Nagata and Yamaguchi batch; multiple measurements interference liquid-liquid system (1960) In situ conductivity Calibration of the cell is difficult, Rushton et al. (1964) batch; xylenemeasurements octanoic acid-water Drops interfere with the measurement. filtration through fritted The present work shows that the Schindler and Treybal flow; ethyl acetate-water filtrate solute concentration depends glass (1968) on the filter condition. Keey and Glen gravity settling flow; isooctaneThe errors are large; at 85% mass transfer o-nitrophenol-water efficiency, the estimated error is +12%. (1969) Mansoori and Madden batch; oxidationfiltration through sintered Considerable time is lost in separation. glass sparger tube reduction system Present work shows that solute (1969) for tetravalent cerium concentration in the filtrate depends on thhe filter condition. filtration through fritted Mok and Treybal flow; paraldehydeThe present work shows that solute water glass concentration in the filtrate depends (1971) on the filter condition. ultrahigh-speed flow; cyclohexane/ Bapat (1982) additional mass transfer occurs in the centrifugation carbon tetrachloridecentrifuge and phases emerge iodine-water equilibrated. ~~
centrifuge downstream of the extractor was equally unsuccessful. In spite of its small residence time (0.3 s compared to about 100 s of CFSTR), the intense shearing experienced by the entering dispersion accelerated the interphase mass transfer and the effluent phase emerged nearly equilibrated with each other. Nagata and Yamaguchi (1960) and Rushton et al. (1964) utilized the fact that some techniques such as emf measurement, conductometry, and radiometry are not so sensitive to the presence of a foreign phase as the optical methods. They used in situ conductivity measurements to monitor interphase mass transfer in the batch systems. The method does not require phase separation but the conductivity cell calibration is very troublesome. The dispersed phase droplets cause substantial interference in solute concentration measurement, especially at high dispersed phase fraction.
Present Experimental Technique In view of the above deficiencies of the existing methods, a new technique of dispersion phase separation was developed to investigate the performance of a CFSTR as an extractor. A 1-L CFSTR system was used to study the interphase mass transfer of iodine between the dispersed organic phase mixture of cyclohexane and carbontetrachloride (density 970 kg/m3) and continuous aqueous phase consisting of singly distilled water. The stirred reactor was made of a 100-mm internal diameter Pyrex glass cylinder bounded at the two ends 102 mm apart by 303 austenitic stainless steel plates. The stirring was done by a centrally located six-flat-bladed Rushton turbine of 51 mm diameter.
For separating the continuous phase, the separation device uses a nuclear microfilter obtained by irradiating a polymer film by nuclear particles. The narrow trails of damage produced in the polymer matrix are etched out to give circular capillary holes. This filter membrane with an extremely small thickness of 5-10 pm and randomly located straight through circular capillary pores approaches the concept of a two-dimensional screen with surface filtration. The cut-off is sharply defined. Cellulosic membranes or other conventional filter media, on the other hand, consist of a labyrinth of interconnecting isotropic tortuous pores and rely to a great extent on the random entrapment within the matrix for particle retention above the rated pore size. As a result, the cut-off of these filters is not as sharp as the nuclear membranes and this results in contamination of the filtrate by the unwanted phase on prolonged filtration. In addition to the true sieve-like performance and precise cut-off, the nuclear membranes have very good mechanical and chemical properties. Except for very strong acids and alkalies, the chemical resistance of polyester and polycarbonate nuclear membranes is as good fluorocarbon membranes. Figure 1shows the separator assembly for obtaining the continuous aqueous phase for iodine analysis by spectrophotometry. A hydrophilic polycarbonate Nuclepore (registered trademark of Nuclepore Corporation) membrane (nominal size 1pm) is placed on the top of a supporting Teflon (registered trademark of E. I. du Pont de Nemours & Co.) disk (C). The supporting disk has seven holes in a regular hexagonal geometry to allow the aqueous phase from the reactor (A) to pass through the filter. The
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Ind. Eng. Chem. Fundam., Vol. 23, No. 1, 1984 100
-
90
-
50
-
,\"
>-
V
z
w :o -
kw LT
60-
50
-
LL W u-l z
RESIDENCE TIME 60s MASS TRANSFER FROM DISPERSED TO CONTINUOUS PHASE Cd,f ~IXIO-3M;C,f Z O
'/' r
0
4c-
a
Figure 1. Phase separator assembly. Table 11. Dispersion Properties a t 298.15 K a
liquid water cyclohexane1 carbon tetrachloride mix tu re
density, kg/m3 997 970
viscosity, Pa,s 0.00090 0.00082
interfacial tension, N/m
2000
0.0471
5000
5000
10000
1
1
I
I
I
250
300
350
400
450
STIRRER, R PM
Figure 2. Mass transfer efficiency vs. specific power dissipation for cyclohexane/carbon tetrachloride-iodine-water system.
a Iodine equilibrium distribution constant, (Cd,e/Cr.e)= 62.17.
sealing is achieved by axial Viton (registered trademark of E. I. du Pont de Nemours & Co.) O-rings (H)held in compression by a Teflon plug (E) screwed in the bottom plate (B) of the reactor. The step (G) in the reactor bottom was kept as thin as was structurally possible for maximum exposure of the filter membrane to turbulent convection in the reactor. This was done to minimize the permanent deposition of dispersed phase droplets on the filter. A Teflon tube (F,3 mm 0.d.) press fitted into the plug, carries the aqueous filtrate from the collecting cavity (D) beneath the supporting disk to the spectrophotometer cell a t the sampling station. The lighter dispersed organic phase is obtained by a similar assembly in the upper plate of the reactor. Due to the organic nature of the phase to be separated, the filter employed should be highly hydrophobic. A film of organic phase covers the filter in the separator assembly. The rate of phase withdrawal for solute concentration analysis should not exceed the rate a t which the organic film is replenished by coalescing droplets. A specially treated hydrophobic nuclear membrane can be used for this purpose. However, unlike the continuous phase separation which requires the nuclear membrane's idealized pore structure for exclusion of dispersed phase droplets, the hydrophobicity rather than the pore structure is of primary importance for organic dispersed phase separation. A silicone-treated hydrophobic filter paper performed satisfactorily for this purpose. However, Teflon filter membranes have been found to be superior to the filter paper because of their greater mechanical strength and wider range of porosity. The filtrate flow through the separation device is sustained by the back pressure in the reactor (up to 70 kPa gauge) generated by the feed pump. Increasing the reactor pressure increases the filtration rate. The filtration rate can also be enhanced by using the membrane of larger pore size. Judicious combination of reactor pressure and membrane pore size (0.015 to 12.0 pm for nuclear and 2 to 30 Mm for Teflon membranes) can be made to obtain optically clean phases at the required rate for spectrophotometric analysis. Figures 2 and 3 show the results obtained for cyclohexane/carbon tetrachloride-iodinewater system with the new phase separator technique developed. The operating conditions are indicated in the
4000
POWER DISSIPATION FER UNIT MASS, cm2/s3
/p=-*PHASE FRACTION
i
i
OF DISPERSED PHASE 010 RESIDENCE T I M 90s
1
2000
4000
6000
8000
10000
POWER DISSIPATION FER UNITMASS. cm2/s3 I
1
I
1
I
250
300
350
400
4 50
STIRRER, R P M
Figure 3. Mass transfer efficiency variation with direction of mass transfer for cyclohexane/carbon tetrachloride-iodine-water system.
figures while the dispersion properties are given in Table 11. Figure 2 depicts the variation of the extraction efficiency against the power dissipation with dispersed phase fraction as the parameter. Clear trends on the variation of the efficiency with power dissipation and phase fraction are observed. The increase in mass transfer efficiency with higher power dissipation is caused by the increase in the interfacial area accompanied by the decrease in the continuous phase mass transfer resistance. The increase in the extraction efficiency with increase in the dispersed phase fraction is attriubuted to increased drop interaction. Figure 3 gives the effect of mass transfer direction on the extraction efficiency. The difference in the extraction efficiency with mass transfer direction is caused by the interfacial tension gradients along the drop boundary as the solute is exchanged between the two immiscible phases. The change in the interfacial area available for mass transfer due to change in the average drop size resulting from the change in the dispersed phase coalescence frequency is a measure of the surface activity of the transferring solute. In spite of the low surface activity of iodine, the present technique differentiates between the two modes of solute transfer and thus provides a proof of its efficacy in mass transfer experimentation for liquid-liquid dispersions.
Ind. Eng. Chem. Fundam. 1984, 23, 123-126
Unlike gravity settling, centrifugation, or accelerated coalescence over a matting, the separation by the present technique occurs at the reactor boundary and thus eliminates the possibility of additional mass transfer in the separation process outside the reactor. The extremely small thickness of the filter also renders the separation almost instantaneous and thus the response time of the solute monitoring system is governed by the transport lag and the response time of the instrumentation itself. In the present investigation, the system response time did not have significant relevance because only the steady-state operation was studied. However, for the unsteady-state investigations, the response time can be considerably reduced by incorporating micrometering pumps in the sampling system. In addition to reducing the transport lag, the same pump can be used to send a reverse flow pulse to the nuclear membrane to clear the clogged pores to assure a continuous supply of clean phase at the required flow rates during the entire experimental run. It should be noted that the developed technique relies mainly on the preferential wetting characteristics of the filter media and may fail for systems in which such preference is not significant. This may occur for a dispersion with low interfacial tension and a solute with high surface activity or mutual solubility. The excellent performance for the system used in this work is attributed to the low surface activity of iodine and high interfacial tension between the cyclohexane/carbontetrachloride mixture and water. The technique is presently being used satisfactorily in our laboratories in a hydrometallurgical solvent extraction investigation in which iron [Fe(III)] in aqueous sulfate medium is extracted by Kelex 100 in xylene. Conclusions A new experimental technique for obtaining optically clear liquids from the two-phase liquid-liquid CFSTR for solute concentration measurements required for mass transfer efficiency estimation has been developed. Its superior performance as compared to earlier techniques has been demonstrated by the steady-state CFSTR performance evaluation. This in situ instantaneous phase separation technique has the potential to be used in the transient analysis of two-phase liquid extractors employed
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for reactor control and stability investigations. Acknowledgment The financial support by the National Science Foundation, Grant CPE 80-21039, and the general financial support by the Department of Chemical Engineering of Illinois Institute of Technology are gratefully acknowledged. Nomenclature C = solute concentration, g-mol/L E = mass transfer efficiency F = volumetric flow rate to the stirred tank, L/s Greek Letters = solute equilibrium distribution constant 4 = dispersed phase fraction
K
Subscripts c = continuous phase d = dispersed phase
e = equilibrium f = feed Literature Cited Allen, P.; Kropholler, H. W.; Spikins, D. J. The Chem. Eng. 1966, 4 4 , 182. Bapat, P. M. Ph.D. Thesis, Illinois Institute of Technology, Chicago, IL, 1982. Cruz-Plnto, J. J. C.; Korchinsky, W. J. Chem. Eng. Sci. 1960, 35, 2213. Flynn, A. W.; Treybal, R. E. AIChE J . 1955, 1 , 324. Hixon, A. W.; Smith, M. I. Ind. Eng. Chem. 1949, 4 1 , 973. Keey, R. B.; Glen, J. B. AIChE J . 1969. 15, 942. Mansoori, G. A.; Madden, A. J. AIChE J . 1969, 15, 245. Mok, Y. I.; Treybal, R. E. AIChE J . 1971, 1 7 , 916. Nagata, S.; Yamaguchl, I. Mem. Fac. Eng. Kyoto Univ. 1980, 22, 249. Overcashier, R. H.; Kingsley, H.A., Jr.; Olney, R. E. AIChEJ. 1958, 2 , 529. Rushton, J. H.; Nagata, S.; Rooney, T. B. AIChE J . 1964, 10, 298. Ryon, A. D.; Daley, F. L.; Lowrie, R. S. Chem. Eng. Prog. 1959, 10, 70. Schindler, H. D.; Treybal, R. E. AIChE J . 1966, 14, 790. Smoot, L. D.;Babb, A. L. Ind. Eng. Chem. Fundam. 1982, 1 , 93.
Department of Chemical Engineering Pradeep M. Bapatt Illinois Institute of Lawrence L. Tavlarides*t Technology Chicago, Illinois 60616 Received for review July 12, 1982 Accepted September 26, 1983 Department of Chemical Engineering and Materials Science, Syracuse University, Syracuse, NY 13210.
Determination of Hydrate Thermodynamic Reference Properties from Experimental Hydrate Composition Data A rigorous method for using the experlmental cyclopropane hydrate data of Dharmawardhana et al. (1980, 1981) to evaluate the reference chemical potential and enthalpy dlfferences, A&,a and At/$, of structure I hydrate gives values of 1299.4 and 1861 J/mol, respectively, for these two properties. The reference chemical potential is in good agreement with that obtained in the Dharmawardhana study (I297 J/mol), but the reference enthalpy obtained in the previous study is considerably lower (I389 J/mol). The values obtained by the present analysis are probably the best that can be obtained from the experimental cyclopropane hydrate composition data alone and the efficiency of other values of the reference properties in generalized h drate phase equilibrium predictions should be attributable to the insensitivity of the calculations to values of A&, to the limitations of the general model, and to the fact that the model Kihara parameters are adjustable depending upon the value of the reference properties. Slight adjustments in the Kihara parameters can partially compensate for errors in the reference properties.
Introduction Experimental determination of thermodynamic reference properties for gas hydrates has been an elusive goal for over 20 years since van der Waals and Platteeuw's (1959) original work on the subject. The most important
of these properties in terms of predicting hydrate equilibria are the reference chemical potential difference, Ap@, which is the difference in chemical potential between the hypothetical gas free hydrate lattice (@)and ice (a)phases at (273.15 K and 0 kPa), and the reference enthalpy dif-
0196-4313/84/1023-0123$01.50/00 1984 American Chemical Society
