Gas-Liquid Chromatographic Determination of Solution Properties of Oxygenated Compounds in Water Daniel L. Shaffer and Thomas E. Daubert Department of Chemical Engineering, The Pennsylvania State University, University Park, Pa. 16802
Equilibrium distribution properties of volatile solutes at dilute concentrations in water and associated heat effects are critical in design of absorption and stripping operations in the chemical industries. This work is a study of the application of gas-liquid chromatographic methods to determine activity coefficients and differential heats of solution of volatile oxygenated organic solutes at dilute concentrations in water. Solute-water equilibrium was assured at all points in test columns by high levels of dispersion of water on the inert solid support. Activity coefficients at infinite dilution calculated using the equilibrium absorption vessel model of the chromatographic column were within =k 10% of static values while differential heats of solution at infinite dilution were within =t 10% of calorimetric values. Finite solute concentrations up to 5.0 mole per cent were achieved by injecting large solute samples.
IN 1956, A. J. P. Martin ( I ) speculated that gas-liquid chromatography (GLC) might be useful for studying the thermodynamics of solutions. Kobayashi et al. ( 2 ) reviewed the work of numerous investigators who determined partition coefficients, activity coefficients, and differential heats of solution at infinite dilution using GLC methods. These experimental values obtained from GLC have been shown to be essentially identical with those obtained from static equilibrium measurements. All cited work involved use of volatile organic solutes in conventional nonvolatile liquid substrates such as squalane, diisodecyl phthalate and polyethylene glycol. The present study sought to extend GLC techniques to include systems with water as the solvent phase. This extension permits the measurement of thermodynamic equilibrium parameters of more common binary solutions than those previously studied. The first use of water as the solvent (liquid) phase was reported by Pollard and Hardy (3) in seeking to separate the chloromethanes. These authors also considered dilute solution thermodynamics of methanol in water and found that the major problem hindering this work was the nonstationary nature of the liquid phase as the column’s water was lost by evaporation. Kwantes and Rijnders (4, in studying binary solutions of adjacent homologs of normal paraffins, sought to alleviate this problem by using a forecolumn or presaturator to saturate the carrier gas with solvent at the system’s temperature and pressure upstream of the GLC column. This reduced solvent “bleeding” significantly, but expansion of the gas phase as it passed through the column continued to cause small evaporation losses. Pecsar and Martin reported the first thorough study of solution thermodynamics from GLC with water as the solvent phase (5). These authors used a broad range of organic so(1) A. J. P. Martin, Analyst, 81, 52 (1956). (2) R. Kobayashi, P. W. Chappelear, and H. A. Deans, Ind. Eng. Chem., 59,(10) 63 (1967). (3) F. H. Pollard and C. J. Hardy, “Vapor Phase Chromatography,” D. H. Desty, Ed., Butterworth’s, London, 1957. (4) A. Kwantes and G. W. A. Rijnders, “Gas Chromatography
1958,” Butterworth’s, London, 1958, p. 125. 38,1661 (1966). (5) R. E. Pecsar and J. J. Martin, ANAL.CHEM.,
lutes including alkanes, chlorornethanes, esters, aldehydes, alcohols, and amines to determine activity coefficients at infinite dilution in water. To extend this work into the finite solute concentration region, Pecsar and Martin assumed that the basic equations derived for infinite dilution could be applied without modification to determination of finite solute concentration activity coefficients. Activity coefficients at 30 “C. were obtained for ethanol in water up to a liquid phase ethanol mole fraction of 0.022. Reasonable agreement with static equilibrium measurements was obtained. Heats of solution for the lower alcohols in water were also calculated although necessary experimental data were minimal. THEORY
The “theoretical plate” model of GLC (6)yields the basic relation between the partition coefficient K and solute retention volume VRo: VEO = VM
+ KVL
(1)
The retention volume as expressed in Equation 1 is dependent upon chromatographic parameters of the test column. For an invariant property at a given temperature, Littlewood et al. (7) defined the specific retention volume V gas :
vg= V‘Q- v, rn
The quantity V g is independent of column size or packing density. Combining Equations 1 and 2 and substituting for K ( K E XNL/YN,):
(3) This expression can be combined with the ideal gas law and the definition of the activity coefficient (y = yP/xpo)to yield:
(4) This is the general expression relating the activity coefficient to specific retention volume and liquid phase column pararneters. If solute is present at infinite dilution, Equation 4 reduces to (5)
Equations 4 and 5 are derived for a system operating at constant pressure. For an actual column to operate with gas flow, a finite pressure gradient must exist within the column. To correct for the effect of the pressure gradient, the specific retention volume must be multiplied by a factor j defined as (6) A. J. P. Martin and R. L. M. Synge, Biochem. J., 35, 1358 (1941). (7) A. B. Littlewood, C. S. G. Phillips, and D. T. Price, J . Chem. SOC., 1955,p. 1480. VOL. 41, NO. 12, OCTOBER 1969
1585
Figure 1. Schematic diagram of equipment Helium tank Pressure regulator C. Needle valve D. Rotameter
A. B.
Saturator F. Injection port G. Pressure gauge H. GC column E.
Detector Electrometer K. Recorder L. Hydrogentank M . Oxygen tank
I. J.
The expression for j is nearly unity for short columns and experimental pressures near atmospheric. The differential or partial heat of solution is defined as that quantity of heat (per mole of solute) evolved or required when a differential amount of solute is added to enough solution such that the original solute concentration remains unchanged. At infinite dilution the differential heat of solution becomes equal to the total or integral heat of solution, and therefore in dealing with very dilute solutions it is customary to disregard the distinction between them. The differential heat of solution AH, is related to the temperature variance of the activity coefficient by an equation derived in numerous standard thermodynamic texts :
EXPERIMENTAL Apparatus. The schematic diagram of Figure 1 shows the general layout of the experimental apparatus. The system centered around a Model 810 F & M Research Chromatograph. The gas chromatograph was equipped with a 1.0mV Minneapolis-Honeywell recorder and an F & M Model 50 Automatic attenuator. The inert carrier gas helium, supplied at 99.9 purity, was available at 50 psig from a central source tank (A). The helium stream was throttled down to 10 psig by a pressure regulator ( B ) before passing through a needle control valve (C). Helium flow rates were monitored by a Brooks rotameter (D)which was accurately calibrated with a soap-film flow meter. The carrier gas stream next passed through a 500-ml bubbler filled with distilled water. This bubbler (E> was fitted with a sintered glass disperser head to ensure small gas bubbles for intimate gas-liquid contacting. Before the humidified gas stream entered the system's solute injection port (F), its pressure was measured (G). This pressure, taken as the chromatographic column's inlet pressure, was taken upstream from the injection port to avoid any solute holdup in the neck of the gauge. All columns ( H ) employed were 14-15 inches in length and constructed of '/4-inch 0.d. stainless steel tubing. The column packing was 45/60 mesh Chromosorb G , a Johns-Manville acid washed diatomaceous earth. This material was chosen for its inertness and porous structure which affords large contact area with relatively coarse particles as well as the ability to 1586
ANALYTICAL CHEMISTRY
minimize peak tailing for polar compounds. Columns were packed with a rubber vibrating hammer to ensure a reasonably constant void fraction throughout the entire length. The detector used in this chromatograph was of the flame ionization type. Hydrogen and oxygen were supplied to support the flame in approximate proportions of 1 :4, respectively, with no discernible change in detector sensitivity accompanying small changes in these proportions. Procedures. The first step in preparation for an experiment was the accurate determination of the bone-dry weight of the column. The packed column was placed in the chromatograph, and the oven temperature raised to 200 "C. Dly helium was passed through the column at the rate of 50 ml per minute for 4 hours. To check for true dryness, a 2 . 0 4 sample of an acetone-pentane mixture was injected into the column. Because acetone has a vastly greater solution afinity for water than pentane, the two solute bands elute from the column at the same time only if no water is present. The observation of a single sharp peak on the recorder established the column's complete dryness. This check also reaffirmed the inert nature of the Chromosorb G packing. Next the solid support was coated with the liquid substrate, water. Distilled water was drawn through the packed column with vacuum. Excess water was blown out of the column with humidified air until the desired amount remained. Humidified air was used to keep any evaporative drying at the column entrance minimal. The column was weighed to determine the amount of water added. The column's gas volume V,I was determined before each experiment. Normal pentane, which has very little solution affinity for water, was injected into the helium carrier stream and was carried through the column at essentially the same rate as the helium. Very sharp n-pentane peaks indicated negligible hydrocarbon-water interfacial adsorption. The retention time for the n-pentane multiplied by the helium volumetric flow rate yielded the column's gas volume. System temperatures were controlled by the chromatograph's oven assembly enclosing columns. Internal column temperatures were measured at the conclusion of experiments by substituting an identically packed column fitted with two copper-constantan thermocouples. The average value of these thermocouple temperatures (normal difference of less than 0.1 "C) was taken as the system temperature. When the recorder chart showed complete elution of a solute band, retention time was recorded and the column removed from the oven chamber. The column was weighed to determine the water lost during the experiment so that an average mass of water could be used in subsequent calculations. Columns were used without rewetting of the packing until the amount of water on the packing of columns dropped below 1.0 gram. RESULTS AND DISCUSSION Infinite Dilution Studies, Activity coefficients at infinite dilution in water were calculated for methanol, ethanol, npropanol, acetaldehyde, propionaldehyde, ethyl acetate, acetone, and methylal using Equation 5 with the pressure gradient correction factors. Results obtained in the present work and values reported by other investigators are given in Table I. Table I reflects the considerable scatter in reported values of activity coefficients at infinite dilution for oxygenated organic compounds in water. In static equilibrium measurements this scatter is primarily the result of extrapolations of data from finite solute concentrations into the infinitely dilute region. Extrapolations are necessitated by difficulties encountered in accurately and precisely measuring very low equilibrium concentrations of solute in the vapor and liquid phases. The rather sharp curvatures in the infinitely dilute region of plots of activity coefficient versus solute concentra-
tion cannot be taken into account by methods which normally extrapolate to infinite dilution from a finite solute concentration of the order of 0.02 mole fraction. The activity coefficients calculated in the present study correspond to solute mole fractions of less than 2.0 X Reproducibility of the present study’s activity coefficients presented in Table I varied from =k4 for methanol in water to f10 for propionaldehyde in water. As solute retention time decreased either because of high solute activity coefficient or low level of water in the column, the ability to reproduce a result declined. Finite Concentration Studies. Activity coefficients in water for methanol, ethanol, acetone, and propionaldehyde were determined at solute concentrations up to 5.0 mole per cent. These activity coefficients were determined to support the values obtained in the infinite dilution study and to test the applicability of the equations derived for infinite dilution to systems at finite solute concentrations. First attempts at determining activity coefficients at finite solute concentrations were conducted using the experimental technique proposed by Chueh (8). This technique depends upon establishment of solute vapor-liquid equilibrium in the column before a small solute sample is injected. Equilibrium can be obtained by using a presaturator (bubbler) filled with a solute-water mixture. However, Chueh’s technique could not be used in this study because of experimental difficulties with the detector. A flame ionization detector was needed to eliminate the “noisy” background signal given by the column’s water bleed. However, the flame unit exhibited erratic response behavior when monitoring continuous passage of solute. To overcome this limitation, Barker and Hilmi (9) suggested dilution of the column’s effluent stream with nitrogen before entering the detector. Nitrogen was mixed with the column’s effluent gas over a broad range of proportions, with no appreciable improvement in detector response. The failure of Chueh’s method for this work limited the present study to use of the same technique employed for infinite dilution with the exception that relatively large solute samples were used to obtain finite solute concentrations in the columns. Experimental variation of solute concentrations was achieved by varying the quantity of water in the columns and the size of the injected solute sample. Concentrations were based upon the total amount of solute and water in the column. Solute injection sizes ranged from 1.0 p1 to 60 pl while the weight of water in the 14-15 inch columns was varied from 0.3 to 1.3 grams. In this manner solute mole fractions to 0.05 were obtained. from 1.0 X Calculation of activity coefficients at finite concentrations differs from the infinite dilution calculation in that the more general relation of Equation 4 is necessary because mass of liquid divided by moles of liquid no longer equals the solvent molecular weight. Activity coefficients calculated using Equation 4 for methanol, ethanol, acetone, and propionaldehyde in water at 24 =t0.1 “C are plotted cs. solute mole per cent in Figures 2-5 together with values reported by other researchers. The most significant feature of Figures 2-5 is the rather sharp curvature shown for all four systems in the very dilute regions. This curvature cannot be accounted for by simple predicting equations or by nonlinear extrapolations from higher solute concentrations.
(8) C. F. Chueh, Ph.D. Thesis, Georgia Institute of Technology, Atlanta, Georgia, 1962. (9) P. E. Barker and A. K. Hilmi, J . Gas Chromatog., 5,119 (1967).
Table I. Solute Activitv Coefficients at Infinite Dilution in Water Temperature, 7” “C Method Reference Solute 2.12 24.3 GLC Present work Methanol 0 2.3 Static (11) 1.51 25 Static (12) 25 2.4 Static (13) 2.2 27 GLC (3) 2.53 30 GLC (5) Ethanol 4.74 24.3 GLC Present work 3.48 25 Static (11) 20 4.5 Static (14) 25 4.0 Static (15) 6.15 30 GLC (5) 24.1 GLC Present work n-Propanol 17.2 12.5 25 Static (11) 18 25 Static (16) 30 26 GLC (5) 4.36 24.3 GLC Present work Acetaldehyde 25 4.9 Static (17) 4.4 30 GLC (5) 24.2 GLC Present work Propionaldehyde 24.6 21.8 25 Static (14) 17.6 30 GLC ( 5 ) 24.2 GLC Present work Ethyl Acetate 142.8 130 25 Static (14) 7.96 24.1 GLC Present work Acetone 6.6 25 Static (18) 25 9.4 Static (13) 25 9.6 Static (12) 18.5 24.1 GLC Present work Methylal
Figure 5 shows that the activity coefficient of propionaldehyde in water increases as the concentration of the propionaldehyde increases. Thus, the activity coefficient reaches a maximum value at some finite concentration before falling to 1.0 at a propionaldehyde mole fraction of 1.0. This behavior is in contrast to that of the other systems investigated, all of which yielded maximum values of activity coefficient at infinite dilution. The behavior exhibited by the propionaldehyde-water system is uncommon, but it is not unique, Chloroform in ethanol shows similar behavior (ZO). Although the finite concentration activity coefficientscalculated in the present study show good agreement with statically determined literature values, one must keep in mind that Equation 4 is theoretically valid only for infinite dilution or for systems with linear partition isotherms (activity coefficient constant). Data presented here indicate that this relation can be extended to include the dilute region for water-oxygenated organic systems having activity coefficients which vary with concentration. (10) H. S. Scatchard and B. E. Raymond, Jr., Znd. Eng. Chem., 26, 412 (1934). (11) J. A. V. Butler, D. W. Thomson, and W. H. Maclennan, J . Chem. SOC.,1933,p. 674. (12) D. F. Othmer, Ind. Eng. Chem., 20, 743 (1928). (13) J. A. Griswold and C. B. Buford, Ind. Eng. Chem., 41, 2347 (1949). (14) E. W. Washburn, Ed., “International Critical Tables,” Vol. 111, McGraw-Hill, New York, 1928. (15) R. S. Hanson and F. A. Miller, J . Phys. Chem., 58,193 (1954). (16) R. Gadwa, Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1936. (17) R. N. Furnas and B. S. Leighton, Ind. Eng. Chem., 29, 709 (1937). (18) S. D. York and N. E. Holmes, ibid., 34, 345 (1942). VOL. 41, NO. 12, OCTOBER 1969
1587
2 2 1 '
'
I
I
'
I
'
I
' ' 1
i
1
5.4
3.8 1 0
5
I 2 3 4 METHANOL MOLE PERCENT IN LIQUID PHASE
0
Present work Data of Othmer (12)
Differential Heats of Solution. The differential heat of solution is related to the temperature variance of the activity coefficient by Equation 7. This equation shows that for constant pressure and constant solute composition the tangent to a plot of logarithm of activity coefficient versus reciprocal temperature at any value of temperature gives the differential heat of solution. Activity coefficients for binary solutions of methanol, ethanol, acetone, and propionaldehyde in water at infinite dilution were determined in the column chamber at temperatures ranging from 24-60 "C. Several experimental problems were encountered in measuring the GLC parameters at elevated temperatures. The first problem was the accurate measurement of the column's true internal temperature. The chromatograph's oven chamber was constructed to maintain reasonably constant temperatures up to 500 "C, making establishment of truly steady temperatures in the 20-60 "C range difficult. To check the accuracy of the oven's pyrometer reading, a column shaped identically to the test column and fitted with two internal thermocouples was placed in the oven (with carrier gas Aow) and allowed to equilibrate at the previous oven setting. This check showed that the internal column temperature at a given oven temperature varied over a = t 3 "Crange. The second experimental problem associated with higher system temperatures was the accelerated rate of column water loss. Because the averaging method used to
0
I 2 3 4 ACETONE MOLE PERCENT I N LIQUID PHASE
0 Present work
1588
Data of Griswold and Buford (13)
ANALYTICAL CHEMISTRY
: \
-
A A
0 l
,
I
I
l
,
l
.
l
,
~
,
Y ~
,
~
,
0 I 2 3 4 5 E T H A N O L PERCENT IN LIQUID PHASE Figure 3. Ethanol activity coefficient in water us. ethanol concentration at 24 "C 0 Present work a Data of Pecsar and Martin (5) d Data of Hansen (15)
calculate the mass of water present during the solute band's passage weakened as the water loss increased, any time holdup due to unsteady detector response or carrier gas flow fluctuationsjeopardized the success of the experiment. The temperature variances of activity coefficients for methanol, ethanol, acetone, and propionaldehyde at infinite dilution in water are given in Figure 6. Plots are given as log y us. reciprocal temperature to conform to Equation 7. The data show that for each system activity coefficients increase with increasing temperature, indicating exothermic mixing effects. Scatter observed in Figure 6 is caused by previously mentioned experimental difficulties. Most reliable values are those at lowest temperatures, approximately 24 "C in each case. Because the true curved nature of these log y us. 1/T plots is unattainable from the experimental data, the best approximations of the tangents to these unknown curves at 24 "C are straight lines through the 24 "C end points as shown. Slopes of the straight lines so drawn give the values of AH, in Equation 7. These differential heats of solution are presented in Table I1 along with calorimetrically determined values reported in the literature. Values found in the present
5
Figure 4. Acetone activity coefficient in water us. acetone concentration at 24 "C I
Lo
3.0 -
Figure 2. Methanol activity coefficient in water us. methanol concentration at 24 "C A
3.4
I 3
20 0.0
0.5
1.5 2.0 1.0 PROPIONALDEHYDE M O L E PERCENT IN LIQUID PHASE
2.5
Flgure 5. Propionaldehyde activity coefficient in water us. propionaldehyde concentration at 24 "C
~
,
5
9
I1
'
"
3.0
'
I
I
'
'
3.2
3.1
'
3.3
1
3.4
IO ~ / T(OK)
t
'I
Table 11. Differential Heats of Solution of Methanol, Ethanol, Acetone, and Propionaldehyde at Infinite Dilution in Water Differential Tempera- Heat of ture, solution, Solute "C Btu/lb mole Reference Methanol 24 - 3600 Present work 25.0 -3608 (19) 25 -3200 (20) 25.0 -3160 (21) Ethanol 24 - 4800 Present work 25.0 -4584 (19) 25.0 -4282 (21) 25.0 - 4300 (22) Acetone 24 -4500 Present work 25 -4300 (20) 25.0 -4520 (19) Propionaldehyde 24 - 7200 Present work 25.0 -7320 (19)
2
L
NOMENCLATURE
W
differential heat of solution, Btu/lb mole pressure correction factor = partition coefficient = solvent molecular weight = solvent mass, gram = molar density of vapor phase, moles/cc = molar density of liquid phase, moles/cc = pressure, atm = column inlet pressure, atm = column outlet pressure, atm = solute vapor pressure, atm = gas constant = temperature, O K = gas phase volume, cc = liquid phase volume, cc = retention volume, cc = specific retention volume, cc/gram = solute gas mole fraction = activity coefficient = activity coefficient at infinite dilution = solute liquid mole fraction
=
I
3.1
3.0
I
32
33
3.4
~
3.1
3.0
32
3.3
3.4
I 03/T (" K 1
=
1 , -
70 6o 60
50
i
I
l
/
I
I
,
r
i
1
40 40: 30 - \&
201
io
'
3.1
i.2 ' 103/T("K)
'
313
'
;,4
'
Figure 6. Methanol, ethanol, acetone, and propionaldehyde activity coefficients at infinite dilution in water us. reciprocal temperature in degrees kelvin
study show good agreement with literature values, especially considering the linear approximations used to obtain the results.
Received for review January 29, 1969. Accepted July 14, 1969. Presented at the 4th Middle Atlantic Regional Meeting of the American Chemical Society, Washington, D. C., February 14,1969. (19) E. C. Williams, Ed., "International Critical Tables," Vol. V, McGraw-Hill, New York, 1929. (20) E. R. Bose, Z.Physik. Chem., 58, 585 (1907). (21) A. N. Bertrand, R. S. Neale, J. R. Rose, and G. S. Phillips, J . Phys. Chem., 70,699 (1966). (22) National Bureau of Standards Circular 500, U. S . Gov't. Printing Office, Washington, D. C., 1952.
VOL. 41, NO. 12, OCTOBER 1969
1589
