Abstract

Effective removal of hydrogen sulfide (H2S) is one of the challenging problems associated with the production of natural gas. Since H2S is often present in natural gas along with varying levels of carbon dioxide (CO2), selective removal of H2S from sour gas in the presence of CO2 is very important for cost effectiveness of a scavenging operation. For over ten years, triazine based scavengers have been successfully applied in the sweetening of natural gas. However, observations of the effect of CO2 on the scavenging capacity of triazine based scavengers have not been very consistent. Initial documentation and case histories on performance of triazine based scavengers by Dillon [1,2] reported that CO2 did not have any effect on the H2S scavenging capacity of triazine. However, recent studies by GRI [3] indicate that CO2 significantly reduces the H2S removing capacity of triazine. To address this discrepancy, this paper critically evaluates the effect of CO2 on the scavenging performance of a triazine based H2S scavenger. Results from controlled laboratory experiments, using a wide range of CO2 partial pressure and H2S to CO2 ratios indicate that CO2 has very little effect on the H2S scavenging performance of a triazine based H2S scavenger. Field data supports the observations made in the laboratory experiments.

Introduction

H2S and CO2 are two contaminants that naturally exist in sub-quality natural gas. Removal of H2S from the gas is often required because of safety, environmental or corrosion considerations. However, these factors do not dictate the removal of CO2. Chemicals that react with H2S, very often also have an affinity for CO2. The concentration of CO2 relative to H2S in natural gas is often very large (percent levels CO2 compared to a few parts per million (ppm) of H2S), which further makes removal of CO2 less attractive and more uneconomical. The need for selective removal of H2S therefore emerged as a key criterion for cost effective sweetening of natural gas.

Triazines form an important class of non-regenerable H2S scavengers. They are produced from the reaction between formaldehyde and an alkanolamine comprising 1 to 6 carbon atoms [1]. Various sour gas-sweetening processes have been used for a number of years.

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