Page 305 - Petroleum and Gas Field Processing

12

Gas Dehydration

12.1 INTRODUCTION

Natural gas dehydration is the process of removing water vapor from the gas
stream to lower the dew point of that gas. Water is the most common
contaminant of hydrocarbons. It is always present in the gas–oil mixtures
produced from wells. The dew point is defined as the temperature at which
water vapor condenses from the gas stream. The sale contracts of natural gas
specify either its dew point or the maximum amount of water vapor present.
There are three basic reasons for the dehydration of natural gas streams:
1. To prevent hydrate formation. Hydrates are solids formed by the
physical combination of water and other small molecules of
hydrocarbons. They are icy hydrocarbon compounds of about
10% hydrocarbons and 90% water. Hydrates grow as crystals
and can build up in orifice plates, valves, and other areas not
subjected to full flow. Thus, hydrates can plug lines and retard
the flow of gaseous hydrocarbon streams. The primary
conditions promoting hydration formation are the following:
1. Gas must be at or below its water (dew) point with ‘‘free’’
water present.
2. Low temperature.
3. High pressure.
2. To avoid corrosion problems. Corrosion often occurs when liquid
water is present along with acidic gases, which tend to dissolve
and disassociate in the water phase, forming acidic solutions.
The acidic solutions can be extremely corrosive, especially for
carbon steel, which is typically used in the construction of most
hydrocarbon processing facilities.

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