Thermodynamic Inhibitors #1

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1. Chemicals can be injected into the production stream to depress the likelihood of significant hydrate formation.

2. A therrnodynamic inhibitor alters the chemical potential of the hydrate phase such that the hydrate formation point is displaced to a lower temperature and/or a higher pressure.

3. Generally, an alcohol or one of the glycols—usually methanol, ethyiene glycol (EG), or diethylene glycol (DEG)—is injected as an inhibitor. All may be recovered and recirculated, but the economics
of methanol recovery will not be favorable in most cases.

The most common inhibitor in field gas situations, where the inhibitor will not be recovered and reused, is methanol. It is a relatively inexpensive inhibitor. Methanol is soluble in liquid hydrocarbons, about 0.5% by weight. If there is condensate in the stream, additional methanol is required because some of that methanol will dissolve in the condensate, Also, some of the methanol vaporizes and goes into the gas state.

Ethylene glycol is the most common recoverable inhibitor. It is less soluble in hydrocarbons and has less vaporization loss than methanol. This is common on the inlet to gas processing plants.

The Hammerschmidt equation may be used in calculating the amount of inhibitor required in the water phase to lower the hydrate temperature:

cal-4-3

The amount of inhibitor required to treat the free water, as given by Equation 4-3, plus the amount of inhibitor lost to the vapor phase and the amount that is soluble in the hydrocarbon liquid will be the total amount required. Figure 4-9 is a chart for determining the amount of methanol that will be lost to the vapor phase. Approximately 0.5% will be soluble in the hydrocarbon liquid.

Ratio of methanol vapor to liquid composition vs. pressure at various temperatures.

The procedure for calculating methanol usage can best be explained by an example. Given a flowing temperature for one well of our example field of 65 °F (as could occur with a remote well and subsea flow line), calculate the methanol required to prevent hydrates from forming. Assume that at the high flowing pressure there is no free water, but the gas is saturated.

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