Page 271 - Petroleum and Gas Field Processing

11

Sour Gas Treating

11.1 INTRODUCTION

Natural gas usually contains some impurities such as hydrogen sulfide
(H 2 S), carbon dioxide (CO 2 ), water vapor (H 2 O), and heavy hydrocarbons
such as mercaptans. These compounds are known as ‘‘acid gases.’’ Natural
gas with H 2 S or other sulfur compounds (such as COS, CS 2 and
mercaptans) is called ‘‘sour gas,’’ whereas gas with only CO 2 is called
‘‘sweet gas.’’ It is usually desirable to remove both H 2 SandCO 2 to
prevent corrosion problems and to increase heating value of the gas.
Sweetening of natural gas is one of the most important steps in gas
processing for the following reasons:

1. Health hazards. At 0.13 ppm, H 2 S can be sensed by smell. At
4.6 ppm, the smell is quite noticeable. As the concentration
increases beyond 200 ppm, the sense of smell fatigues, and the
gas can no longer be detected by odor. At 500 ppm, breathing
problems are observed and death can be expected in minutes. At
1000 ppm, death occurs immediately.
2. Sales contracts. Three of the most important natural gas pipeline
specification are related to sulfur content, as shown in Table 1.
Such contracts depend on negotiations, but they are quite strict
about H 2 S content.
3. Corrosion problems. If the partial pressure of CO 2 exceeds
15 psia, inhibitors usually can only be used to prevent corrosion.
The partial pressure of CO 2 depends on the mole fraction of CO 2
in the gas and the natural gas pressure. Corrosion rates will also
depend on temperature. Special metallurgy should be used if
CO 2 partial pressure exceeds 15 psia. The presence of H 2 S will
cause metal embrittlement due to the stresses formed around
metal sulfides formed.

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