Condensation of Water Vapor
One method of assuring that hydrates do not form is to assure that the amount of water vapor in the gas is always less than the amount required to fully saturate the gas. Typically, but not always, the gas will be saturated with water in the reservoir. As the gas is cooled from reservoir temperature, the amount of water vapor contained in the gas will decrease. That is, water will condense.
The temperature at which water condenses from natural gas is called its dew point. If the gas is saturated with water vapor, it is by definition at its dew point. The amount of water vapor saturated in the gas is givenby Figure 4-6. The graph shows the water content in pounds of water per MMscf of saturated gas at any pressure and temperature. For example, at 150°F and 3,000 psi, saturated gas will contain approximately 105 Ib of water vapor per MMscf of gas. If there is less water vapor, the gas is not saturated and its temperature can be reduced without water condensing. If the gas is saturated at a higher temperature and then cooled to L50°R water will condense until there are only 105 Ib of water vapor left in the gas. The dotted line crossing the family of curves shows the approximate temperature at which hydrates will probably form at any given pressure. Note the hydrates form more easily at higher pressures.
To keep water from condensing as the gas is processed, it is necessary to dehydrate the gas (that is, remove water vapor) until the amount of water vapor remaining in the gas is less than that required to fully saturate the gas at all conditions of temperature and pressure. Since the dehydrated gas will have a lower dew point, dehydration is sometimes called dew point depression. For example, if the amount of water vapor in the 3,000 psig gas stream referred to earlier were reduced from 105 Ib/MMscf to 50 Ib/MMscf, the dew point would be reduced from 150°F to 127°F. That is, its dew point will be depressed by 23°F.
Figure 4-6 contains an approximate hydrate formation line. This should be used with care, as the position of the line depends on the gas composition. It is better to calculate the hydrate formation temperature or use Figure 4-5 for approximation.