Equilibrium of water and hydrocarbon systems without hydrates

This page considers two equilibrium conditions:

1. The point at which, at a given temperature and pressure, water becomes saturated in either hydrocarbon vapors or hydrocarbon liquids and forms a separate fluid phase.
2. The point at which hydrocarbon becomes saturated in liquid water and forms a separate hydrocarbon phase.

Both water and hydrocarbon dewpoints are represented as the maximum solubility of each phase in the other. The discussion in this page assumes that hydrates will not form; prevention of hydrate formation requires:

• High temperature
• Low pressure
• That all hydrocarbons be larger than n-pentane (9Å)

Prediction of hydrate formation is covered in Predicting hydrate formation.

Water solubility (dewpoint) in a hydrocarbon gas

The chart by McKetta and Wehe (Fig. 1) acceptably correlates the water content of hydrocarbon gases as a function of temperature and pressure. Because F = 2, two intensive variables are needed to specify the system. At a given temperature and pressure, the user can determine the saturated water content of gases, the point at which a liquid water phase will precipitate. For this reason Fig. 1 frequently is called the water dewpoint chart.

Before using Fig. 1, however, note that:

• Water content is given in H₂O lbm/MMscf of gas at 60°F and 1 atm.
• Remarkably, the chart can be used for any hydrocarbon gas or gas mixture, regardless of gas composition. However, the water content should be multiplied by a slight correction factor for gas gravity (gas gravity = gas molecular weight divided by air molecular weight). Larger correction factors are required for sour (H₂S + CO₂) gases.
• To construct the chart, data from several investigators were measured at high (>50 lbm/MMscf) water contents and extrapolated (as ln water content vs. temperature) along isobaric lines of constant pressure to lower water content.
• While the semilogarithmic plot adequately correlates data for gases at higher water contents, the correlation cannot be extrapolated to lower water content (<20 lbm/MMscf) because the lines bend sharply downward at the hydrate formation temperature and pressure.
• Dashed lines in Fig. 1 represent metastable equilibrium of water in the vapor, giving a metastable water content that is higher than the equilibrium water content of gas in equilibrium with hydrates, the stable condensed phase at lower temperatures.
• Fig. 1 should not be used for low water concentrations; instead, use a computer program, as discussed in Equilibrium of water and hydrocarbon systems with hydrates.

Despite its limitations, Fig. 1  is very useful and provides a check against high water content values calculated by commercial phase equilibria computer programs.

• 

  Fig. 1 – Water content of hydrocarbon gases (from McKetta and Wehe). C_s = correction factor for salinity, C_G = correction factor for gas gravity.

No similar measurements and charts are available for determining the hydrocarbon content in water vapor, with a separate liquid hydrocarbon phase. To approximate this at low pressures, an engineer may use the rule of thumb that hydrocarbon liquid will condense when the hydrocarbon partial pressure equals its vapor pressure. This calculation rarely is performed, however, because water’s hydrogen bonds cause water vapor pressure to be lower than that of many hydrocarbons. At temperatures below 100°C, only alkanes with carbon numbers above seven (n-C₈H₁₈⁺) have lower vapor pressures than water, because of water’s strong hydrogen bonds.

For this reason, it is much more common for water to precipitate from a hydrocarbon vapor in gas/petroleum operations than it is for hydrocarbon to precipitate from a low-pressure water vapor. Therefore, Fig. 1 is most practical for determining water solubility in hydrocarbon vapor.

Mutual solubility of liquid water and liquid hydrocarbons

Tsonopoulos correlated mutual liquid solubility of liquid water and well-defined liquid hydrocarbons (normal alkanes, 1-alkenes, alkylcyclohexanes, and alkylbenzenes) for molecules that are too large (>9Å) to form hydrates. Solubilities in more general liquids (e.g., petroleum fractions) are not in the open literature and can be approximated using well-defined hydrocarbon fluid solubilities or their mixtures. The correlations for such fluids are given in four parts listed below:

• Solubility of hydrocarbons in liquid water at 298 K
• Solubility of hydrocarbons in liquid water as a function of temperature
• Solubility of water in liquid hydrocarbons at 298 K
• Solubility of water in liquid hydrocarbons as a function of temperature

Pressure does not significantly affect the mutual solubilities of liquids.

Solubility of hydrocarbon in liquid water at 298 K

Because of dissimilarity in bonds between water and hydrocarbon, the mole fraction of well-defined hydrocarbon in water at 298 K always is very small, ranging from a high of 5 × 10^–4 for alkylbenzenes with a carbon number of 6 (N_c = 6), to a low of 2 × 10^–9 for nonane, a normal alkane. Even with these low water concentrations, at the same carbon number, the hydrocarbon in water concentrations decrease in the order of alkylbenzenes > alkylcyclohexanes > linear 1-alkenes > normal alkanes, as shown in Fig. 2. For a given chemical type, larger molecules always are less soluble in water than are smaller molecules.

• 

  Fig. 2 – Solubility of hydrocarbons in water at 298.15 K (from Tsonoupoulos).

The correlation for the mole fraction of hydrocarbons in water (x_HC-W) at 298 K is:

$\ln {x_{{\text{HC}}-{\text{W}}}}=a+bN_{c}+{\frac {c}{N_{c}}},$

(1)

where:

• N_c = the carbon number
• a, b, and c = constants as given in Table 1

For normal alkanes, the correlation does not fit well after N_c = 11.

Table 1—Constants and ranges of application in Eq. 1 for solubility of hydrocarbons in water at 298 K

Type	a	b	c	Nc Range
Normal alkanes	-3.9069	-1.51894	0	C5 to C11
Linear 1-alkenes	-2.24515	-1.54381	0	C5 to C8
Alkylbenzenes	4.25097	-1.59463	-14.89081	C6 to C12
Alkylcyclohexanes	-3.74419	-1.27431	0	C6 to C10

Solubility of hydrocarbon in liquid water as a function of temperature

Liquid hydrocarbon solubility in water generally increases with temperature; however, there appears to be a minimum in solubility that ranges from 291 K for alkylbenzenes to 303 K for alkanes. As the temperature moves below these minimum values, the hydrocarbon concentration in water increases.

These solubilities change enough that the temperature effects for each compound must be treated individually. The hydrocarbon mole fraction for hydrocarbon liquids (x_HC-W) in water, as a function of temperature (T) in K, is well-described by the correlation:

$\ln {x_{{\text{HC}}-{\text{W}}}}=a+{\frac {b}{T}}+c\ln {T},$

(2)

where a, b, and c = constants for normal alkanes, as given in Table 2. For constants for individual 1-alkenes, alkylcyclohexanes, and alkylbenzenes, see Tsonopoulos.

Table 2—Constants and ranges of application in Eq. 2 for temperature effect on solubility of normal alkanes in water

Compound	a	b, K	c	T Range, K
Pentane	-333.59719	14537.472	47.97436	273 to 413
Hexane	-374.90804	16327.128	53.89582	273 to 413
Heptane	-396.93979	17232.298	56.95927	273 to 413
Octane	-415.7563	17975.386	59.55451	273 to 413
Nonane	-433.434	18767.82	61.940	273 to 413

Solubility of water in liquid hydrocarbons at 298 K

The solubility of water in hydrocarbon liquids at 298 K, like that of hydrocarbons in liquid water, is small, ranging from 3.2 × 10 –4 for ethane to 3 × 10 –3 for alkylbenzenes ( N c = 6), as shown in Fig. 3 . The solubility of water in liquid hydrocarbons decreases in the following order for the same N c : alkylbenzenes > linear 1-alkenes > normal alkanes > alkylcyclohexanes.

• 

  Fig. 3 – Solubility of water in hydrocarbons at 298.15 K (from Tsonopoulos).

The correlation for mole fraction of water in liquid hydrocarbons for well-defined fluids is:

$\ln {x_{{\text{W}}-{\text{HC}}}}={\frac {a+bN_{c}}{c+N_{c}}},$

(3)

where a, b, and c = constants as given in Table 3.

Table 3—Constants and ranges of application in Eq. 3 for solubility of water in hydrocarbons at 298 K

Type	a	b	c	Nc Range
Normal alkanes	-79.6677	-6.6547	9.5470	C2 to C16
Linear 1-alkenes	4.6649	-7.3894	-0.3834	C3 to C16
Alkylbenzenes	-78.1518	-7.9107	15.7423	C6 to C16
Alkylcyclohexanes	-102.4415	-6.7228	11.9077	C6 to C16

Solubility of water in liquid hydrocarbons as a function of temperature

Unlike the three above solubility correlations, the solubility of water in hydrocarbons increases dramatically with temperature. At high temperatures (>500 K) the solubility of water in hydrocarbons can exceed 0.1 mole fraction, and may not be negligible, as with some of the above concentrations. These solubilities change so dramatically that the temperature solubility of each compound must be treated individually. The correlation of the mole fraction of water in liquid hydrocarbons as a function of temperature (K) is:

$\ln {x_{{\text{W}}-{\text{HC}}}}=a+{\frac {b}{T}},$

(4)

where a and b = constants as given in Table 4 for normal alkanes, as well as the heat of solution (ΔH₁) defined as the enthalpy of water in the hydrocarbon solution minus that of pure liquid water. Consult Tsonopoulos for constants and ranges for individual 1-alkenes, alkylcyclohexanes, and alkylbenzenes.

Table 4—Constants and ranges of application in Eq. 4 for temperature effect on solubility of water in normal alkanes

Compound	a	b	ΔH1	T Range
 Pentane	6.951930	-4381.365	36.4	273 to 343
 Hexane	6.698073	-4291.186	35.7	273 to 343
 Heptane	6.761260	-4290.700	35.7	273 to 343
 Octane	6.839365	-4290.165	35.7	273 to 539
 Decane	6.476563	-4179.296	34.7	273 to 343
 Hexadecane	6.418156	-4089.393	34.0	273 to 343

Nomenclature

a	=	constant
b	=	constant
c	=	constant
Nc	=	carbon number
T	=	temperature, °F
xW-HC	=	mole fraction for water in liquid hydrocarbon

Noteworthy papers in OnePetro

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External links

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See also

Phase behavior of water and hydrocarbon systems

Phase diagrams

Gas properties

PEH:Phase_Behavior_of_H2O_Hydrocarbon_Systems