Hydrates

This application calculates the temperature or pressure at which hydrates form in a natural gas system. Hydrates are ice-like solids that can plug pipelines and measurement or regulation equipment — restricting or stopping gas flow — even at temperatures well above 32°F (0°C) due to elevated system pressures. The calculator supports two industry-recognized methods: the S. Ameripour & M. Barrufet (2009) method, which accounts for gas composition and inhibitors, and the M. Motiee (1991) method, which uses a correlation based on specific gravity.

Note: electrolytes and inhibitors are not included in the Motiee method; those fields are disabled when that method is selected.

Background

Hydrates form when water and hydrocarbon gas molecules combine at or below the dew point temperature of the mixture. Because gas pipeline pressures are typically elevated, the hydrate formation temperature is often above 32°F (0°C), meaning hydrates can form even when the gas itself is not frozen. The risk of hydrate formation increases as pressure rises or temperature falls.

Gas composition plays a significant role in hydrate formations. Heavier hydrocarbons — such as propane and isobutane — raise the formation temperature compared to leaner gas mixtures. Hydrogen sulfide also has a notable impact on formation conditions.

One common mitigation strategy to combat hydrates is to add an electrolyte or alcohol inhibitor (such as methanol or sodium chloride) to the gas stream. Inhibitors lower the hydrate formation temperature at a given pressure, or equivalently raising the hydrate formation pressure for a given temperature.

Equations

S. Ameripour & M. Barrufet, 2009

This method uses correlations published in the Journal of Canadian Petroleum Technology (May 2009, Vol. 48, No. 5) for systems with or without inhibitors. It accepts a known pressure or temperature along with the mole fraction composition of the gas and its specific gravity, and optionally accounts for the weight percent of electrolytes and/or inhibitors in the stream.

GASCalc implements the hydrate formation pressure calculation directly from the journal. When temperature is the unknown, the program uses a numerical root-finding algorithm to determine the dew point temperature. The full detail of this approach are available in Reference 2.

M. Motiee, 1991

As implemented in GASCalc, the Motiee method uses the following empirical equations to calculate hydrate formation temperature or pressure from the gas specific gravity. This method does not include the effects of inhibitors or electrolytes.

Hydrate formation temperature:

T = -238.24469 + 78.996674 \times \log P_a - 5.352544 \times (\log P_a)^2 + 349.473877 \times SG - 150.854675 \times SG^2 - 27.604065 \times SG \times \log P_a

T = -238.24469 + 78.996674 \times \log P_a – 5.352544 \times (\log P_a)^2 + 349.473877 \times SG – 150.854675 \times SG^2 – 27.604065 \times SG \times \log P_a

Where:
T − Hydrate formation temperature (°F)
P_a − Absolute pressure (psia)
SG − Specific gravity of the gas (dimensionless)

Hydrate formation pressure:

P = 5.23479 - 0.004243 \times T + 0.000218 \times T^2 - 7.766945 \times SG + 3.50079 \times SG^2 + 0.019557 \times T \times SG - P_{ATM}

P = 5.23479 – 0.004243 \times T + 0.000218 \times T^2 – 7.766945 \times SG + 3.50079 \times SG^2 + 0.019557 \times T \times SG – P_{ATM}

Where:
P − Gauge pressure (psig)
P_a − Absolute pressure (psia)
P_ATM − Atmospheric pressure (psia)
SG − Specific gravity of the gas (dimensionless)
T − Temperature (°F)

Case Guide

Part 1: Create Case

Input Parameters

Part 2: Outputs/Reports

Results

References

• M. Motiee — “Estimate Possibility Of Hydrates,” Hydrocarbon Processing, July 1991.
• S. Ameripour & M. Barrufet — “Improved Correlations Predict Hydrate Formation Pressures Or Temperatures For Systems With Or Without Inhibitors,” Journal of Canadian Petroleum Technology, May 2009, Volume 48, No. 5.

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