NEW CORRELATION ACCURATELY CALCULATES NAPHTHENE WATER SOLUBILITIES
Carl L. Yaws, Xiang Pan, Duane G. Piper Jr.
Lamar University
Beaumont, Tex.
A new correlation has been developed that uses the boiling points of liquid naphthenes to accurately calculate their water solubilities down to less than 1 ppm (wt),
The correlation can be used for initial engineering studies, including those involving health, safety, and the environment.
SOLUBILITY ILLUSTRATIONS
The importance of hydrocarbon solubility in water will increase in the future because of health, safety, and environmental issues. The following shows how important this is even at very low concentrations. (See OGJ, Apr. 8, p. 80, for the paraffin water solubility correlation.)
- For human inhalation exposure issues:
The threshold limit value (TLV) for cyclohexane in air is 300 ppm (vol). 1 A concentration of only 0.05 ppm (mol) cyclohexane in water produces about 540 ppm (vol) of cyclohexane in air, at the air-water interface.
This air concentration of 540 ppm (vol) exceeds the TLV of 300 ppm (vol).
- For safety considerations:
The lower explosion limit (LEL) for cyclohexane in air is 1.3 vol %.'A concentration of only 2 ppm (mol) cyclohexane in water produces about 2.1 vol % cyclohexane in air, at the air-water interface.
This air concentration of 2.1 vol % exceeds the lower explosion limit of 1.3 vol %.
- For environmental issues:
Consider a spill of cyclohexane in water. The water will become saturated with cyclohexane. At saturation, the solubility of cyclohexane in water is about 56.1 ppm (wt), or 12 ppm (mol). 3.
This concentration of 12 ppm (mol) at saturation produces about 128,400 ppm (vol), or 12.8 vol % cyclohexane in air, at the air-water interface. The TLV of 300 ppm (vol) and the LEL of 1.3 vol % are both greatly exceeded by this air concentration.
These examples show that very low concentrations (ppm and less) of hydrocarbon in water can produce concentrations in air, at the air-water interface, that exceed both the TLV for human exposure and the LEL for flammability.
NEW CORRELATION
The new correlation for solubility of naphthenes (cycloparaffins) in water is as follows:
log(S) = A + BTb + C(Tb)2 + D(Tb)3
where:
S = Solubility in water @ 25 C., ppm (wt)
Tb = Boiling point, K.
A = -16.7 for cyclohexanes
A = -16.9 for cyclopentanes
B = 177.811 X 10-3
C = - 500.907 x 10-6
D = 411.124 x 10-9
The correlation applies to naphthenes (cyclohexanes and cyclopentanes) with zero, one, two, and three substitutions, which are liquids at ambient conditions (25 C., 1 atm). Liquid naphthenes are C5 and larger molecules containing the cycloparaffin ring.
The boiling point range for the correlation is 301 K. to about 561 K. This represents the range of boiling points of liquid naphthenes for which the correlation is projected to be reliable. The correlation is not applicable to naphthenes that are solids at ambient conditions.
The correlation constants (A, B, C, and D) were determined by regression of the available data.
In preparing the correlation, a literature search was conducted to identify data sources.
The publications were screened and the appropriate data entered into a computer to produce a data base of available experimental solubility values for hydrocarbons.
The data base also served as a means of checking the accuracy of the correlation.
NAPHTHENES AND PARAFFINS
The water solubility curves, as a function of boiling point, for naphthenes and paraffins are shown in Fig. 1. The curves show that much less data are available for naphthenes than for paraffins.
Because more data are available for paraffins, wider solubility and boiling point ranges are encompassed by the paraffin correlation.
Paraffin water solubility decreases with increasing boiling point. This decrease begins at about 300 K. and continues at about the same rate up to about 500 K. At temperatures above 500 K., the rate of decrease in water solubility slows.
The water solubility of naphthenes is higher than that of paraffins with the same boiling point.
Fig. 1 indicates that the data for naphthenes appear to be parallel to the more extensive data for paraffins. This indication suggests that variation of water solubility of naphthenes probably parallels the variation of water solubility of paraffins.
The comparison in Fig. 1 also shows favorable agreement of correlation and experimental data for both naphthenes and paraffins.
In testing the new correlation for C5-C10 liquid naphthenes, it was determined that solubility values are in favorable agreement with experimental data down to very low concentrations. The experimental data covered a solubility range of 0.12-156 ppm (wt).
COMPARISON OF CORRELATIONS
The new correlation is compared to the API correlation in Table 1 and Fig. 2. For the data points in the table covering liquid naphthenes with zero and one substitutions, the average deviation from experimental data is about 9% for the new correlation, and about 20.4% for the API correlation.
For liquid naphthenes with two and three substitutions (trans 1,4-dimethylcyclohexane; cis 1,2-dimethylcyclohexane; 1,1,3-trimethylcyclohexane; and 1,1,3-trimethylcyclopentane), deviations for the API correlation were lower.
A plot of the experimental data and results for the new and API correlations are shown in Fig. 2, for both paraffins and naphthenes.
For paraffins, the figure indicates that the correlations are roughly equivalent up to a boiling point of about 400 K. and a water solubility of about 0.40 ppm (wt).
Because the boiling point and solubility of octane (398.9 K. and 0.431 ppm [wt]) roughly match this condition, the point where the divergence begins roughly corresponds to C8 liquid paraffin. This divergence increases for larger liquid paraffins.
For naphthenes, the figure indicates that the correlations are roughly equivalent up to a boiling point of about 450 K. and a water solubility of about 0.10 ppm (wt). This point where the divergence begins roughly corresponds to a C10 liquid naphthene.
The divergence is projected to increase for larger liquid naphthenes.
EXAMPLES
The following examples show how the correlation is used for calculating the solubility of naphthenes in water.
Example 1: A hydrocarbon spill of cyclohexane occurs into a body of water at ambient conditions (25 C., 1 atm). Substitution of the correlation constants for naphthenes (using the value of A for cyclohexanes) and the boiling point of cyclohexane (353.90 K.) into the correlation equation yields:
log(S) = 1.713885711
S = 51.75 ppm (wt)
The calculated and experimental values compare favorably: 51.75 (calc.) vs. 56.10 (exp.).
Example 2: A hydrocarbon spill of 1-cyclopentylpentane occurs into a body of water at ambient conditions. Substitution of the correlation constants for naphthenes (using the value of A for cyclopentanes) and the boiling point of 1-cyclopentylpentane (453.76 K.) into the correlation equation yields:
log(S) = -0.9470009717
S = 0.11 ppm (wt)
The calculated and experimental values compare favorably: 0.11 (calc.) vs. 0.12 (exp.).
REFERENCES
- Sax, N.I., and Lewis, R.J., Sr., "Hawley's Condensed Chemical Dictionary," 11th ed., Van Nostrand Reinhold Co. Inc., New York, N.Y., 1987.
- Daubert, T.E., and Danner, R.P., "Data Compilation of Properties of Pure Compounds," Parts 1 and 2, Dippr Project, American Institute of Chemical Engineers, New York, N.Y., 1985.
- Sorensen, J.M., and Arit, W., "Liquid-Liquid Equilibrium Data Collection," Vol. 5, part 1, Dechema Chemistry Data Series, 6000 Frankfurt/Main, Germany, 1979.
BIBLIOGRAPHY
McAuliffe, C., J. Phy. Chem., Vol. 70, 1966, p. 1267.
Price, L.C., Bull. Am. Assoc. Petrol. Geologists, Vol. 60(2), 1976, p. 213.
Yaws, C.L., Chen, D., Yang, H.C., Tan, L., and Nico, D., Hydrocarbon Processing, July 1989, p. 61.
"Technical Data Book-Petroleum Refining," American Petroleum Institute, 4th ed., Vol. 2, 1983.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.