Hydrate Dissociation P-T Curve — Motiee + Hammerschmidt

Generate a hydrate equilibrium screening curve from gas composition, check whether an operating point sits inside the hydrate-risk region, and estimate MEG or methanol inhibitor rate.

What It Solves

The hydrate curve tool gives a quick equilibrium screen from gas composition and pressure. It helps identify whether a flowing or shut-in condition is inside the hydrate-risk region and how much thermodynamic inhibitor depression is needed to move the condition outside that region.

Motiee 1991 Sweet-Gas Correlation

For sweet natural gas, Petropt uses the Motiee screening correlation with pressure in psia, gas specific gravity relative to air, and hydrate temperature in °F:

T_F = -238.24469 + 78.99667*log10(P) - 5.352544*(log10(P))^2
      + 349.473877*sg - 150.854675*sg^2
      - 27.604065*sg*log10(P)

This equation is cited to Motiee, M., "Estimate Possibility of Hydrates," Hydrocarbon Processing 70(7), 98-99 (1991).

Composition Adjustment

For sour or rich gas, the Petropt screener applies a damped composition shift on top of the Motiee sweet-gas estimate. This adjustment is not from Motiee; it is an engineering judgment damping factor of 0.4 against the standard composition shift so the screen remains conservative without behaving like a full thermodynamic package.

Hammerschmidt Inhibitor Depression

Thermodynamic inhibitor depression is estimated with the Hammerschmidt equation:

Delta T = K*W / (M*(100 - W))

where K = 2335 for methanol, K = 2222 for MEG, M is molecular weight (32.04 methanol, 62.07 MEG), and W is inhibitor wt% in the aqueous phase. Cite: Hammerschmidt, E.G., "Formation of gas hydrates in natural gas transmission lines," Industrial & Engineering Chemistry 26 (1934).

Worked Example

Given: pure CH4, sg = 0.554, pressure = 1500 psia, operating temperature = 70°F.

Motiee hydrate T at 1500 psia ~= 57 deg F
Warm margin = 70 - 57 = 13 deg F

20 wt% MEG:
Delta T = 2222*20 / (62.07*80) = 8.95 deg F
Inhibited hydrate T ~= 57 - 8.95 ~= 48 deg F

Petropt verdict: the 70°F operating point is outside hydrate risk by about 13°F before inhibitor. Adding 20 wt% MEG drops the equilibrium temperature to about 48°F, widening the warm margin.

Limitations

Motiee is a screening correlation, not a vdW-Platteeuw thermodynamic package. The sour-gas H2S/CO2 correction is a damped shift, not a rigorous phase-equilibrium calculation. For final design, use a hydrate package such as Multiflash, PVTsim, or OLGA calibrated to the actual fluid.

References

Motiee, M. (1991). "Estimate Possibility of Hydrates." Hydrocarbon Processing, 70(7), 98-99.
Hammerschmidt, E.G. (1934). "Formation of gas hydrates in natural gas transmission lines." Industrial & Engineering Chemistry, 26.
Sloan, E.D. and Koh, C.A. (2008). Clathrate Hydrates of Natural Gases, 3rd ed. CRC Press.
GPSA Engineering Data Book, Section 20.