Abstract

When sour natural gases are sweetened with a regenerative solvent, the extracted acid gases H2S and CO2 have to be further handled to dispose of the sulfur in the H2S. The sulfur can be recovered by various processes, which are generally quite expensive. For disposal of small rates of sulfur in the form of H2S, compression of the acid gas stream and injection into an underground zone may be a suitable option. This paper discusses the unique properties of acid gases upon compression and cooling, and the problems that arise in the handling of high pressure acid gas mixtures saturated with water. Methods of overcoming the problems are also reviewed. Process and metallurgical choices are discussed and opportunities for research to minimize costs in acid gas compression and injection are presented.

Introduction

Sour natural gas contains hydrogen sulfide (H2S), which has to be removed to meet specifications for sales gas. Sour natural gas also contains carbon dioxide (CO2). The removal of H2S and CO2, usually called acid gases, from sour natural gas is accomplished by means of a regenerative solvent. There are several amine solvents used for this purpose. Upon regeneration of the solvent, the acid gases are liberated, and are usually sent to a modified Claus plant, where most of the H2S is converted to elemental sulfur. The acid gas stream to the modified Claus plant consists of H2S, CO2, water vapor and minor amounts of hydrocarbon gas. When the concentration of CO2 is considerably greater than the concentration of H2S in the acid gas mixture, the Claus plant has difficulty in achieving a high sulfur recovery. If the total sulfur rate is small, say less than 5 tonnes/day (t/d), it may be more economical to recover the sulfur by some other process. Such other processes, however, have many drawbacks of their own.

An alternative to recovering sulfur on a small scale is to compress and inject the acid gases into a suitable underground zone, in a manner similar to the disposal of produced water. This is environmentally desirable as it eliminates the emission of sulfur compounds and CO2 to the atmosphere.

This paper discusses the technical considerations for acid gas compression and injection into an underground zone.

Properties of H2S and CO2

Upon removal of the acid gases H2S and CO2 from the sour gas, an acid gas mixture is obtained at low pressure that may also contain about 1 % to 3 % hydrocarbon gases, and which is saturated with water vapor. This is the mixture that is compressed through 4 stages of compression, from about 100 kPa (ga) to around 8 to 10 MPa for underground disposal. In this process water condenses, creating the potential for corrosion and hydrate formation. In addition, at such final compressor discharge pressures, the acid gas becomes a liquid or a dense phase when cooled to ambient temperatures.

While experimental results of studies of the physical properties of acid gas mixtures without hydrocarbon components have not been published in the technical literature, the properties of pure CO2 and H2S have been examined and reported. Additionally, the properties of each of the acid gases have also been studied in the presence of water at elevated pressures and temperatures. These results can be used as a guide to indicate how the mixed acid gas streams would behave under the conditions of pressure and temperature when compressed to the injection pressure level. A brief review of the properties of the pure acid gases and the CO2-water and H2S-water binaries is therefore appropriate.

Vapor/Liquid Properties of Pure Compounds. In their pure state, CO2 and H2S exhibit the normal vapor/liquid behavior with pressure and temperature, as indicated in Figure 1. P. 193

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