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Phase behavior of water and hydrocarbon systems

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The phase behavior of water and hydrocarbon mixtures differs significantly from the vapor/liquid equilibria of normal hydrocarbons in two ways:

  • Aqueous and hydrocarbon components usually separate, with very low mutual solubility
  • Hydrates often form with water and hydrocarbons smaller than n-pentane

The fundamental reason an engineer should consider the equilibria is the possible existence of a water phase, in which hydrates can form, causing:

  • Multiphase flow
  • Flow blockage
  • Other engineering challenges

Flowline blockages can cause losses of millions of dollars of income while blockage remediation is occurring. The most accurate prediction methods allow avoidance of flowline blockages.

Phase behavior of water and hydrocarbons

When hydrocarbon contacts water, the two components separate into two phases in which the mutual component solubility is less than 1.0 mol% at ambient conditions. This splitting of phases affects almost all treatments of mixed water and hydrocarbon systems and is caused by the different molecular attractions within water and hydrocarbons. Hydrocarbon molecules have a weak, noncharged attraction for each other, while water attracts other water molecules through a strong, charged hydrogen bond.

Because hydrogen bonds are significantly stronger than those between hydrocarbon molecules, hydrocarbon solubility in water (and that of water in hydrocarbons) is very small. Hydrogen bonds are responsible for most of the unusual properties water displays:

  • Water’s very high heat of vaporization, which absorbs large amounts of heat and buffers many hydrocarbon reservoir temperatures.
  • Very high normal boiling point water has relative to its molecular weight.

Phase behavior of water and hydrocarbon systems is divided into three main categories:

  • This page covers phase definitions and the Gibbs[1] phase rule, which are used to define the problem. Only the two most common concerns are treated in this page; for a rigorous discussion of water and hydrocarbon phase diagrams, see Harmens and Sloan.[2]
  • Equilibrium of water and hydrocarbon systems without hydrates goes on to cover the simplest case—that of a water and hydrocarbon mixture when all phases are fluid, as vapor and/or liquid, and without hydrate formation. This water and hydrocarbon equilibrium without hydrates exists at high temperature or low pressure or when only large (greater in size than n-pentane) hydrocarbon components are present.
  • Because of the importance of hydrates in water and hydrocarbon equilibria, Equilibrium of water and hydrocarbon systems with hydrates deals with systems containing small hydrocarbon molecules (<9Å) that form hydrates with water. Hydrates are the most common solid-phase problem in flow assurance. They are solid crystalline compounds that typically plug flow channels, valves, drillstrings, blowout preventers, etc.; therefore, hydrate-formation regions normally are avoided. Hydrates provides a more detailed analysis of emerging technologies related to flow assurance and hydrates. For a more complete exposition of the hydrate calculation methods, see Makogon[3] and Sloan.[4][5]

Phase definitions and the Gibbs phase rule

Phases are homogeneous regions of matter—gas, liquid, or solid—that can be analyzed using common tools such as:

  • Pressure gauges
  • Thermocouples
  • Chromatographs

In this discussion, phases are distinct homogeneous regions larger than 100 μm. The shorthand used here for the various phases are:

  • HC = hydrocarbon, typically with a very low (<1%) water concentration.
  • I = pure ice.
  • L = liquid that is either water (LW) or hydrocarbon (LHC).
  • V = vapor that is always a single phase, never splitting.
  • W = liquid water, usually of high (>99%) purity, except where indicated.
  • H = hydrate or solid.

The order of phase listing is by decreasing water concentration. For example, the listing order LW > H > V > LHC means that hydrates (H) contain less water than the liquid water phase (LW), but more water than vapor (V), which in turn contains more water than liquid hydrocarbon (LHC).

The Gibbs[1] phase rule for nonreacting systems provides the most convenient method for determining how many intensive variables are important in phase equilibria. The Gibbs phase rule states:

 ....................(1)

where:

  • F = number of intensive variables (e.g., pressure, temperature, single phase composition) required to define the system (known as the degrees of freedom)
  • C = number of components
  • P = number of phases

For example, when excess gas (excess so that its composition does not change) contacts water to form hydrates, there are three phases (P = 3, namely LW + H + V) and two components (C = 2, namely water and a gas of constant composition), so that F = 1; only one intensive variable (either pressure, temperature, or one phase composition) is needed to define the system. If this system is uniquely specified at a given pressure, for example, the identical temperature, and same individual phase compositions always will occur for the initial components at that pressure, when three phases are present. This system definition assures the engineer of uniqueness, so that the identical equilibrium phase behavior will be achieved. If gas also condenses (P = 4, with the addition of LHC), however, or the gas quantity is small enough to change composition (such that C > 2), then the F = 1 restriction no longer holds.

As a second example, consider again the case of a constant composition hydrocarbon vapor in equilibrium with water, so that C = 2. With two phases (V + LW), by the Gibbs phase rule the degrees of freedom are two (F = 2), so that for this example, both temperature and pressure are needed to determine the water composition in the hydrocarbon at conditions under which the two phases coexist. In contrast to the single variable required for three-phase systems in the above paragraph, with this case of two phases and two components, the saturated water concentration in the hydrocarbon (or water dewpoint) is determined by two variables. The water content in a hydrocarbon vapor is correlated with temperature and pressure in Equilibrium of water and hydrocarbon systems without hydrates. Identical restrictions apply to the hydrocarbon content in the water phase.

The same restriction of F = 2 applies when a constant composition liquid hydrocarbon exists in equilibrium with liquid water. However, because both phases (LW + LHC) are dense, very large pressure changes are required to influence the water content of the hydrocarbon. In general, when condensed phases coexist, temperature and concentrations have a much greater influence than does pressure. With liquid hydrocarbon and water, the temperature controls the mutual solubility (i.e., the concentration of the water in the hydrocarbon liquid, as well as the concentration of the hydrocarbon in the liquid water). The mutual solubility of water and liquid hydrocarbons is correlated with temperature in Equilibrium of water and hydrocarbon systems without hydrates.

The Gibbs phase rule is discussed further in Thermodynamics and phase behavior.

Equilibrium of water and hydrocarbon systems

These are divided into two categories, with and without hydrates, as listed below:

More details on these systems can be found on their respective pages.

Nomenclature

C = in the Gibbs phase rule, the number of components in a nonreacting system
F = in the Gibbs phase rule, the number of intensive variables required to define a nonreacting system (degrees of freedom)
P = in the Gibbs phase rule, the number of phases in a nonreacting system

References

  1. 1.0 1.1 Gibbs, J.W. 1931. The Collected Works of J. Willard Gibbs, Vol. VI. New York: Longmans, Green & Co.
  2. Harmens, A. and Sloan, E.D. 1990. The phase behaviour of the propane-water system: A review. The Canadian Journal of Chemical Engineering 68 (1): 151-158. http://dx.doi.org/10.1002/cjce.5450680118
  3. Makogon, Y.F. 1997. Hydrates of Hydrocarbons, 482 Tulsa, Oklahoma: PennWell Publishing Company.
  4. Sloan, E.D. Jr. 2000. Hydrate Engineering, Vol. 21, 89. Richardson, Texas: Monograph Series, SPE.
  5. Sloan, E.D. Jr. 1998. Clathrate Hydrates of Natural Gases, second edition. Boca Raton, Florida: CRC Press.

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