than the boiling point of water the T T U retort achieves thermal efficiency without external heat exchangers. The maximum processing rate of the retort has not been determined for solids which do not shrink during processing. The processing rate would be expected to exceed or equal that observed for conventional gas-combustion retorts.
L i t e r a t u r e Cited AI-Hal-Ali, N. S., Albus, C. J., Parker, H. W., Paper 33c, 68th Annual Meeting of the American Institute of Chemical Engineers, Los Angeles, Calif., Nov 16-20, 1975. Alired, V. D., Nielson, G. I., Chern. Eng. Symp. Ser., 61 (54), 60-67 (1965). Berry, V. J., Jr., Parrish, D. R., Pet. Trans. AM€, 219, 124 (1960). Burger, J. G., Sahuquet. B. C.. SOC.Pet. Eng. d., 410-422 (Oct 1972). Carman, E. P., Graf, E. G., Corley, R. C., U.S. Bur Mines Bull., No. 563 (1957).
Massie, J. R., Parker, H. W., Paper 43a, 74th National AlChE Meeting, New Orleans, La., Mar 12, 1973. Matzick, A., Dannerberg, F. O., Ruark, J. R., Phillips, J. E., Landford. J. D., Gruthrie, G.. U.S. Bur. Mines Bull., No. 635 (1966). Parker, H. W., Marx, J. W., Trantham, J. C., U.S. Patent 3 035 638 (May 22, 1962). Ruark, J. R., Sohns, H W., Carpenter, H. C., U.S. Bur. Mines, Rep. Invest., No. 7540 (1971). Ruark, J. R.. Sohns, H. W., Carpenter, H. C., U S .Bur. Mines, Rep. Invest., No. 7303 (1969). Smith, G. L., Albus. C. J., Jr., Parker, H. W., Paper 35e, 76th National AlChE Meeting, Tulsa, Okla., Mar 10-13, 1974. Warren, J. E., Reed, R. L., Price, H. S.. Pet. Trans. AM€, 219, 109 (1960).
Rweived f o r rPr>iewM a r c h 4, 1976 Accppted August 2, 1976 T h i s work was p r i m a r i l y supported by Texas State Organized Research Funds via the Institute for Environmental Technology a t Texas Tech University.
Evaluation of Isobaric Liquid-Vapor Equilibrium Data for Alcohol-Water Systems Saturated with Salt Derek Jaques Department of Applied Chemistry, Royal Melbourne Institute of Technology, Melbourne, Victoria, 3000, Australia
The evaluation procedure relies upon the visual examination of raw and derived data for the detection of spurious points, undue bias, and internal inconsistencies. It is shown that the result of the modified area test has to be interpreted with caution for systems in which the alcohol is methanol, ethanol, or 1-propanol. Four systems are examined in detail and their relative quality is evaluated. The first three data points for the 1-propanol-water system saturated with sodium nitrate are shown to be in error.
Introduction The need to have a method for the evaluation of liquidvapor equilibrium data has been recognized for a long time, and since the first test for thermodynamic consistency (Herington, 1947) many data have been found to be of doubtful validity. However, of late the value of the area test has been questioned by, among others, Ulrichson and Stevenson (1972), Van Ness et al. (1973), and Christiansen and Fredenslund (1975), who suggested that it should be replaced by a test which is based upon the difference between the calculated and observed vapor compositions being equal to or less than the experimental error. So far this work has been applied to isothermal data only because the heat of mixing term complicates the procedure for isobaric data. The first attempt to apply a consistency test to salt data was made by Kogan (1960). He used the Gibbs-Duhem equation directly on smoothed activity coefficient data because Herington’s area test was not applicable to his un-normalized data. The present author has applied a modified Herington test for isobaric data to alcohol-water systems saturated with salt (Jaques and Furter, 1972a; Jaques, 1974). The method uses normalized activity coefficients and basically the ternary is treated as a binary composed of each solvent saturated with salt; hence
The approach was dictated by a complete absence of heat of mixing data for salt-saturated mixed solvent systems.
The purpose of this work is to illustrate some of the techniques which can be used to assess the quality of experimental data. In particular, we show that the result of the area test for consistency of salt-saturated data has to be interpreted with caution, and it is advisable to make use of more than one testing procedure in the evaluation of experimental data. The need for some evaluation procedure is very apparent even today when grossly inconsistent U-T-x-y data continue to appear in the literature. Comparison of t h e Results of t h e Area Test and Vapor Composition Prediction Details of the modified Herington method for thermodynamic consistency (Jaques and Furter, 1972a) and Barker’s method for the prediction of vapor composition using the twoor three-constant Wilson equation (Jaques, 1976a,b) are given elsewhere. In both cases the ternary system of two solvents and a salt at saturation is treated as a binary in which each solvent is assumed to be saturated with salt, and at each temperature the solvent vapor pressure can be replaced by the vapor pressure of the solvent saturated with salt. This procedure further assumes that a graph of soluhility vs. liquid composition is a straight line, which it rarely is, but invariably it is convex and hence the addition of two salt-saturated solvents gives a salt-saturated mixture with the precipitation of excess salt. For each alcohol the values of ( K - ID I ) and cr, for the salts are arranged in decreasing and increasing order, respectively, to allow a qualitative comparison. (Although the actual values Ind. Eng. Chern., Process Des. Dev., Vol. 16, No. 1, 1977
129
0.4 1
- ..-
- -
- - - - - - - .- .
- - .- -
.
0
0
0.5
x
0
1.0
.*
0.5
x
Figure 1. System 1-propanol-water-sodium nitrate. Vapor phase deviations for (A) T-x fit and (B) GE/RT-n fit. Broken line shows arithmetic mean of bias.
Figure 3. System 1-propanol-water-cobalt(I1) chloride. Temperature vs. liquid and vapor composition.
O
L
1
o 0.2 0.4 0.6 a8 1.0 Figure 2. System 1-propanol-water-cobalt(I1) chloride. Vapor vs. liquid composition. 800
of the Herington test were not intended to have significance except for sign, they serve here as a rough guide to order. Only grossly different positions are noteworthy.) Methanol-Water.
Area test NH4Cl> NaCl > KC1 > NaN03 > salt-free > HgClz > KI > Pb(N03)Z > NaI > KBr > NaBr Vapor composition prediction salt-free > NaCL > KI > NH4C1 > NaN03 > HgClZ > Pb(NO: KCl > KBr > NaBr > NaI Although the orders are not identical, there is some similarity between the groups and no gross abnormalities occur. Ethanol-Water.
Area test Ca(NO:3)2 > KCl > KN03 > (NH&S04 > HgIz > NH4C1> salt-free > NaCl > NaN03 > Ba(N0& > KBr > KzSO4 > CuCl > KI > NaBr > HgBrz > HgC12 > NaI Vapor composition prediction salt-free > Ca(N03)Z > CuCl > KzSO4 > NH4C1> NaCl > HgI2 > HgClz > B a ( N 0 3 ) ~> KBr > KN03 > KI > NaN03 > KCl > I-IgBrz > NaI > NaBr > (NH4)2S04 Although 10 out of 18 systems are within two places, ammonium sulfate is 14 places out of agreement, potassium chloride is 12, and copper(1) chloride is 10. 1-Propanol-Water.
Area test NaNO:{ > NaCl > salt-free > HgC12 > Pb(NO3)2 > KCl NH4CI Vapor composition prediction HgClz > P b ( N 0 3 ) ~> salt-free > NH4C1 > KCl > NaCl NaNO:3
> >
Now we see very little correlation and the ultimate in miscorrelation has occurred. The sodium nitrate system certainly appears to be thermodynamically consistent by the area test 130
Ind. Eng. Chem., Process Des. Dev., Vol. 16,No. 1, 1977
02
W
Q6
08
3
Figure 4. System ethanol-water-barium acetate. Temperature vs. liquid composition. and yet it is the poorest in vapor composition prediction (see Figure I). There is further discussion of this system in a later section.
Suggested Procedure for Evaluation of Data I. Graphical Examination of Raw Data. A graph of ( y - x ) vs. x will indicate any spurious values but little else. However, T-x and T-y graphs should be continuous and consistent with known phase behavior. Unfortunately, some recent data are not consistent among themselves. For example, Alvarez and Galan (1974) have reported data for the 1-propanol-water system saturated with cobalt(I1) chloride under isobaric conditions. A graph of y vs. x (Figure 2) shows no evidence of azeotropic behavior but T vs. x and y (Figure 3) indicate the presence of an azeotrope, and also the diagram cannot be reconciled with known phase equilibrium behavior. The most likely source of error is in composition. The temperature error is comprised of a random and a systematic contribution. The random error can be roughly assessed by plotting temperatures against composition and calculating the mean deviation from the hand-drawn curve. This should be close to the precision claimed by the authors. Figure 4 shows data for the ethanol-water system saturated with barium acetate for which Meranda and Furter (1971) claim that the composition and temperature data were determined at f0.005 mole fraction and f O . l "C, respectively. The present author estimates the mean random error as f 1 . 3 "C with a maximum error of 3.5 "C. Incidently, the minimum boiling azeotrope suggested by the data is not confirmed by the y vs. x plot. The systematic error is more difficult to assess, but the observed boiling points of salt-free solvents should be close to the literature values. Comparison of the reported boiling points for pure solvents with the best available values in the
004
001 O
.. . .
-- ---- - - -
:* 0
Figure
05
x
0
10
05
?O
x
0
System methanol-water-sodium chloride. Vapor phase deviations for (A) T-x fit and (B) GE/RT-x fit. Broken line shows arithmetic mean of bias. 5.
4
.
0
1 05
x
10
Figure 6. System methanol-water-sodium nitrate. Vapor phase deviations for (A) T-x fit and (B) GE/RT-x fit. Broken line shows arithmetic mean of bias.
Table I. Boiling Point Data of Methanol (Meranda and Furter, 1974)
Pressure, Torr
Measd temp, "C
Lit. (Ambrose et al., 1975), "C
767 750
67.3 67.9
64.8 64.2
Difference 2.5
3.7
literature is given in Table I. A smaller error should be obtainable with the pure solvent even when an Othmer still is employed as in salt-saturated work. The absence of a pump in the Othmer still means that the true boiling point is unlikely to be obtained. However, a comparison of literature values with the measured pure solvent boiling points of Johnson and Furter (1960) suggest a maximum error of f0.3 "C. This would appear to be a more reasonable error with the Othmer still. With respect the present author would hope that the future design of experiments in this field will include a realistic estimation of experimental errors. 11. Examination of Derived Data. Recently Jaques (1976a) has shown that it is better to use a T-x fit rather than a GEIRT-x fit where the measured vapor compositions are used in the correlation. The area test uses n-T-x-y data and this also mitigates against its use. Most reliance should be placed upon the results of Barker's method, but the data with better internal consistency should do well in the excess free energy f i t and not suffer from rounding error. If the vapor pressures of the salt-saturated solvents over the required temperature range are available, the calculation can be carried out with more confidence than if the temperature-independent vapor pressure ratio of the solvent saturated with salt to pure solvent has to be employed. For small elevations of boiling point there is little difference between these two approaches.
0 5 x
-002
-
1 K)
-.
- - . - - .- - - .
05
x
10
Figure 7. System ethanol-water-potassium nitrate. Vapor phase deviations for (A) T-x fit and (B) GE/RT-x fit. Broken line shows arithmetic mean of bias.
I t is now proposed to examine representative systems and attempt to draw conclusions about the relative quality of the data. All systems had passed a visual examination of the raw data. For each system the difference between observed and calculated vapor composition for the T-x fit and the GEIRT-x fit are plotted against liquid composition. The arithmetic mean of the bias from the zero axis is shown by a broken line. The data for the methanol-water system saturated with sodium chloride appears to be good as shown by little or no average bias (Figure 5 ) for the calculated vapor compositions of both fits. The datum for the lowest alcohol concentration is the only poor value and this can be qualitatively accounted for by the largest experimental error occurring in this region of liquid composition. This system also passes the area test quite easily and shows satisfactory values of sample deviations (Table 11). The 1-propanol-water system saturated with sodium nitrate, on the other hand, appears to be poor in comparison (Figure 1).The average bias is very high and that of the first three data points is excessive. Although the visual examination revealed no obvious experimental values in error, little reliance can be placed upon the data. This conclusion is borne out by the large sample deviations but not by the area test (see Table 11). We have considered three possible explanations for the poor performance of this system. Firstly, it is a result of the high solubility with its concurrent elevation of boiling point of water and the presence of the miscibility gap. The methanol-water system saturated with sodium nitrate which shows the same elevation of boiling point of water appears consistent and does not exhibit undue bias (Figure 6). The ethanol-water system saturated with potassium nitrate where there is a similar solubility in water and partial miscibility shows no excessive bias on the T-x fit (Figure 7 ) .Hence it appears that high solubility and partial miscibility are not the cause of the discrepancy. Secondly, it is possible that the values of m and c for this system are incorrect. However, this is unlikely as the values are based upon independent data (International Critical Tables, 1929), and also the same values for the methanol and ethanol cases gave log y vs. x curves which approached zero in a satisfactory manner. The corresponding graph for the 1-propanol system (Figure 8) shows that the curves approach zero correctly, but the first three points appear to be in error. By ignoring these data the sample deviation for the T-x fit is reduced from 0.138 to 0.037 and the bias is reduced from +0.049 to -0.011. These values are still larger than those for the full range data of similar nitrate systems. Hence we conclude that the third and most likely explanation for the poor performance of this system lies mainly in the excessive errors of one or more of the experimental measurements for the first three data points. Ind. Eng. Chem., Process Des. Dev., Vol. 16,No. 1, 1977
131
Table 11. Comparison of Derived Data for Alcohol-Water Systems Saturated with Salt Alcohol
Salt
MeOH" MeOHa EtOH b , r l-PrOHa a Johnson and Furter (197%) are incorrect.
on
I
w
Figure 8. System 1-propanol-water-sodium nitrate. Logarithm of activity coefficients vs. liquid composition. The order of quality of data is as given in Table I1 and somewhat fortuitously it is exactly mirrored by all four sample deviations but not by the area test. It had been hoped to make use of the empirical criterion of consistency of Christiansen and Fredenslund (1975), who proposed that if the difference between observed and calculated vapor composition is less than or equal to the sum of experimental uncertainties in liquid and vapor compositions, the data point is said to be thermodynamically consistent. However, with the present data there are problems. One must assume that the error in pressure or temperature is sufficiently small to be negligible in comparison with the composition errors, and also the method requires the original authors to include reliable estimates of their experimental errors. The absence of such data and the uncertainty of the temperature determination prevent a quantitative answer on consistency. Conclusion Evaluation of isobaric data for salt-saturated systems is hampered by errors associated with the use of the Othmer still and by the complete absence of heat of mixing data. However, within these limitations it is possible to order the sets of data. The best data will perform well in the visual examination of both raw and derived data. For those schools which do not have access to a computer it is recommended that all raw data are subjected to a visual examination, and a graph of logy vs. x should approach zero in a satisfactory manner. The area test can be used with caution to indicate some of the poorer data as it appears that good data will satisfy the area test. This makes its use very limited but the author believes that even this is preferable to no evaluation at all.
132
fJCE/RT
UY
'
( K - 101)
0.015 7.4 9.0 0.014 0.014 NaCl 0.018 0.022 6.9 16.9 0.016 NaN03 0.053 5.8 29.1 0.020 0.055 KN03 0.202 0.156 6.6 41.0 0.138 NaN03 (1960). Rieder and Thompson (1950). The sample deviations previously quoted by Jaques and Furter
i
o
fJY
Ind. Eng. Chem., Process Des. Dev., Vol. 16, No. 1, 1977
Nomenclature G E = excess free energy of mixing ( K - ID I ) = result of modified Herington area test; a positive value indicates thermodynamic consistency m,c = empirical constants in the equation: log pz' = m log PZ0 - c pi' = vapor pressure of solvent i saturated with salt pi0 = saturation vapor pressure of solvent i R = gasconstant T = temperature x = mole fraction of alcohol in the liquid phase, calculated on a salt-free basis y = mole fraction of alcohol in the vapor phase
Greek Letters yL = activity coefficient of component i Py = difference between observed and calculated vapor
composition using the T-x fit Py' = difference between observed and calculated vapor composition using the GE/RT-x fit n = total pressure fJC;/RT = sample deviation of the excess energy uY = sample deviation of the vapor composition corresponding to the GEIRT-x fit uI1 = sample deviation of the total pressure uY = sample deviation of the vapor composition corresponding to the T-x fit 4i = correction term for nonideality of component i in an ideal gaseous solution
Subscripts 1 = alcohol 2 = water Literature Cited Alvarez, J. R.. Galan, M. A,, An. Quim., 70, 271 (1974). Ambrose, D., Sprake. C. H. S., Townsend, R., J. Chem. Thermcdyn., 7, 185 (1975). Christiansen, L. J., Fredenslund, A., A./.Ch.€. J., 21, 49 (1975). Herington, E. F. G., Nature, 180, 610 (1947). "International Critical Tables", Vol. 111, pp 362-374, McGraw-Hill, New York, N.Y., 1929. Jaques, D., Furter, W. F., A.i.Ch.E. J., 18, 343 (1972a). Jaques, D., Furter, W. F., Adv. Chem. Ser., 115, 159 (1972b). Jaques. D., A.I.Ch.€. J., 20, 189 (1974). Jaques. D., Ind. Eng. Chem., Process Des. Dev., 15, 236 (1976a). Jaques. D., 170th National Meeting, American Chemical Society Chicago, Ill., Aug 24-29, 1975; Adv. Chem. Ser., in press, 1976b. Johnson, A. I., Furter, W. F., Can. J. Chem. Eng., 38, 78 (1960). Kogan, V. B.. Russ. J. Phys. Chem., 34, 1331 (1960). Meranda, D.. Furter. W. F., A.i.Ch.€. J., 17, 38 (1971). Meranda, D., Furter. W. F., A./.Ch.E. J., 20, 103 (1974). Rieder, R . M., Thompson, A. R., Ind. Eng. Chem., 42, 379 (1950). Ulrichson, D. L., Stevenson, F. D., Ind. Eng. Chem., Fundam., 11, 287 (1972). Van Ness, H. C., Byer, S.M., Gibbs, R . E., A.I.Ch.€. J., 19, 238 (1973).
Received for reuiew March 18, 1976 Accepted July 1,1976
