Page 310 - Petroleum and Gas Field Processing

dew point calculation for a multicomponent mixture of hydrocarbon
gases. The basic equation is given by

n
X
y i
¼ 1 ð1Þ
K i
i¼1
where y i is the mole fraction of component i in the gas phase, on a water–
free basis, K i is the vapor–solid equilibrium constant for component i,
defined by K i ¼ y i /x i , and x i is the mole fraction of component i in the
solid phase, on a water-free basis.
Given the gas pressure P and the mole fraction of its components y i ,
the hydrate formation temperature T is obtained through a trial-and-error
procedure according to the following steps:
1. Assume a value for T.
2. Final the values of K for the components of the gas at P and T
using the K charts.
3. Calculate the values of x i , where x i ¼ y i /K i .
4. Find the sum of y i /K i for all components.
5. Convergence to the desired value of T is obtained by trial-and-
error until

n
y i
X
¼ 1
K i
i¼1
A sample chart for the vapor–solid equilibrium constant for isobutane is
given in Figure 3 [3].

12.3 METHODS USED TO INHIBIT HYDRATE FORMATION

Hydrate formation in natural gas is promoted by high-pressure, low-
temperature conditions and the presence of liquid water. Therefore,
hydrates can be prevented by the following:

1. Raising the system temperature and/or lowering the system
presure (temperature/pressure control)
2. Injecting a chemical such as methanol or glycol to depress the
freezing point of liquid water (Chemical injection)
3. Removing water vapor from the gas liquid–water drop out that
is depressing the dew point (dehydration).

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.