Chapter 7: Acid Gas Treating

7.1 INTRODUCTION

Natural gas, while ostensibly being hydrocarbon in nature, contains large amounts of acid gases, such as hydrogen sulfide and carbon dioxide. Natural gas containing hydrogen sulfide or carbon dioxide is referred to as sour, and natural gas free from hydrogen sulfide is referred to as sweet. The corrosiveness nature of hydrogen sulfide and carbon dioxide in the presence of water (giving rise to an acidic aqueous solution) and because of the toxicity of hydrogen sulfide and the lack of heating value of carbon dioxide, natural gas being prepared for sales is required to contain no more than 5 ppm hydrogen sulfide and to have a heating value of no less than 920 to 980 Btu/scf. The actual specifications depend on the use, the country where the gas is used, and the contract. However, because natural gas has a wide range of composition, including the concentration of the two acid gases, processes for the removal of acid gases vary and are subject to choice based on the desired end product.

There are many variables in treating natural gas. The precise area of application of a given process is difficult to define. Several factors must be considered: (1) types and concentrations of contaminants in the gas,1(2) the degree of contaminant removal desired, (3) the selectivity of acid gas removal required, (4) the temperature, pressure, volume, and composition of the gas to be processed, (5) the carbon dioxide-hydrogen sulfide ratio in the gas, and (6) the desirability of sulfur...