Systems (e.g., H2O + C2H6, C3H8, or i-C4H10) with Upper Quadruple Points

Figure 4.2b shows the equivalent of Figure 4.2a to be slightly more complex for systems such as ethane+water, propane+water, isobutane+water, or water with the two common noncombustibles, carbon dioxide or hydrogen sulfide. These systems have a three-phase (LW–V–LHC) line at the upper right in the diagram.

This line is very similar to the vapor pressure (V–LHC) line of the pure hydrocarbon,

because the presence of the almost pure water phase adds a very low vapor pressure

(a few mmHg at ambient conditions) to the system.

Figure 4.2b shows that at the intersection of the LW–V–LHC line with the

LW–H–V line, a second quadruple point (Q2 = LW–H–V–LHC) is formed. Measured

upper quadruple points for simple natural gas components are shown in

Table 4.2. Point Q2 is the origin for two additional three-phase lines: (1) a LW–

H–LHC line that is almost vertical due to the three incompressible phases and

(2) a H–V–LHC line, of less concern, because it exists within the LW–H–LHC and

the LW–H–V boundaries.

For systems with two quadruple points, the hydrate region is bounded by line

I–H–V at conditions below Q1, line LW–H–V between Q1 and Q2, as well as line

LW–H–LHC at conditions above Q2. Hydrates can form at lower temperatures and

higher pressures to the left of the region enclosed by the three lines in Figure 4.2b; to

the right, no hydrates are possible. Upper quadruple pointQ2 is often approximated

as the maximum temperature of hydrate formation, because line LW–H–LHC is

almost vertical; however see data in Chapter 6 for exceptions.

In Figure 4.2b, the areas between the three-phase lines represent two-phase

regions held in common with the three-phase lines. The area bound by three

three-phase lines (I–LW–H, LW–H–V, and LW–H–LHC) is the LW–H region in

which hydrates are in equilibrium only with liquid water. Similarly, the H–V

region is between the three three-phase lines (H–V–LHC, LW–H–V, and I–H–V).

Finally, the H–LHC two-phase region exists between LW–H–LHC and H–V–LHC

lines and the I–H two-phase region exists between the I–LW–H and I–H–V lines.

Estimation Techniques for Phase Equilibria 201

See Section 4.1.5 for a T – x diagram with another perspective of these two-phase

regions.

Note that the last paragraph contains two-phase regions(H–V,H–LHC, and I–H)

for hydrate equilibrium with a phase that is not liquid water. There is a common

misconception that hydrates cannot form without a liquid water phase, a condition

clearly possible in these diagrams. Professor Kobayashi’s laboratory measured

hydrate conditions without a free water phase from vapor or liquid systems from

1973 to 2000. Such equilibria are of interest for gas and gas condensate pipelines

without a free water phase.

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