Page 313 - Petroleum and Gas Field Processing

1. It is non corrosive.
2. It is chemically inert; no reaction with the hydrocarbons.
3. It is soluble in all proportions with water.
4. It is volatile under pipeline conditions, and its vapor pressure is
greater than that of water.
5. It is not expensive.

Methanol is soluble in liquid hydrocarbons (about 0.5% by weight).
Therefore, if the gas stream has high condensate contents, a significant
additional volume of methanol will be required. This makes this method of
hydrate inhibition unattractive economically because methanol is nonrecov-
erable. In such a situation, it will be necessary to first separate the condensate
from the gas. Some methanol would also vaporize and goes into the gas. The
amount of methanol that goes into the gas phase depends on the operating
pressure and temperature.
In many applications, it is recommended to inject methanol
some distance upstream of the point to be protected by inhibition, in
order to allow time for the methanol to vaporize before reaching that
point.

Glycol Injection
Glycol functions in the same way as methanol; however, glycol has a
lower vapor pressure and does not evaporate into the vapor phase as readily
as methanol. It is also less soluble in liquid hydrocarbons than methanol.
This, together with the fact that glycol could be recovered and reused for
the treatment, reduces the operating costs as compared to the methanol
injection.
Three types of glycols can be used: ethylene glycol (EG), diethylene
glycol (DEG), and triethelyne glycol (TEG). The following specific
applications are recommended:
1. For natural gas transmission lines, where hydrate protection is of
importance, EG is the best choice. It provides the highest
hydrate depression, although this will be at the expense of its
recovery because of its high vapor pressure.
2. Again, EG is used to protect vessels or equipment handling
hydrocarbon compounds, because of its low solubility in
multicomponent hydrocarbons.
3. For situations where vaporization losses are appreciable,
DEG or TEG should be used, because of their lower vapor
pressure.

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