Multicomponent Natural Gas Systems

In Figure 4.2c for natural gases without a liquid hydrocarbon (or when liquid

hydrocarbons exist below 273 K), the lower portion of the pressure–temperature

phase diagram is very similar to that shown in Figure 4.2a. Two changes are

(1) the LW–H–V line would be for a fixed composition mixture of hydrocarbons

rather than for pure methane (predictions methods for mixtures are given in

Section 4.2 and in Chapter 5) and (2) quadruple point Q1 would be at the

intersection of the LW–H–V line and 273 K, at a pressure lower than that for

methane. The other three-phase lines of Figure 4.2a (for I–LW–H and I–H–V)

have almost the same slope at Q1. Otherwise, the same points in Section 4.1.1

apply.

However, for the case in which natural gases contain heavier components, the

upper portion of the diagram is more like that shown in Figure 4.2b. A straight

line labeled LW–H–V represents the hydrate formation region equivalent to the

region between quadruple point Q1 (I–LW–H–V) and the upper quadruple point

Q2 (LW–H–V–LHC) in Figure 4.2b. One significant change in Figure 4.2c is that

quadruple point Q2 becomes a line, as indicated in the next paragraph.

When a liquid hydrocarbon mixture is present, the LW–V–LHC line in

Figure 4.2b broadens to become an area, such as that labeled CFK in Figure 4.2c.

This area is caused by the fact that a single hydrocarbon is no longer present, so

a combination of hydrocarbon (and water) vapor pressures creates a broader phase

equilibrium envelope. Consequently, the upper quadruple point (Q2) evolves into

a line (KC) for the multicomponent hydrocarbon system.

Line KC may not be straight in the four-phase region but is drawn that way

for illustration. The location of the lower point K is determined by the intersection

point of the phase envelope ECFKL with the LW–H–V line, determined

by the methods of Section 4.2 or Chapter 5. To determine the upper point C,

first a vapor–liquid equilibrium calculation is performed, assuming the liquid

phase (exiting the envelope at point C) equals the vapor composition at point K.

That liquid is used to calculate a vapor composition which is used in a vapor–

liquid water–hydrate calculation to determine the upper intersection with the phase

envelope ECFKL.Amore thorough treatment of the calculation of multicomponent

equilibrium with a condensed hydrocarbon phase is given in Sections 4.3.2.

Гидраты смесей углеводородных и неуглеводородных компонентов.

Гидраты природных углеводородных газов.