Resolution of Problems With Iron Oxide Slurry Process for Removal of H2S From Natural Gas Case History, Eva O'Byrne Gas Unit, Upshur County, Texas

Abstract

Summary Dramatic process improvements have evolved during operation of a patented iron oxide slurry sweetening plant located on the site of one of Sun Exploration and Production Company's sour gas wells in East Texas. Produced gas containing 580 ppm HS and 3.0 mole percent C02 has been successfully treated to leave less than 1 ppm hydrogen sulfide in the sales stream. Initially the new process was found to be fraught with numerous operational problems and the reactivity of the slurry batch proved to be unreliable, non-repeatable, and seemingly uncontrollable. The most notable problems encountered werecarry-overs of oxide slurry from the contactor tower,inability to raise the gas production rate above 2 MMSCFD (40% of capable rate) during the first 24 hours of operation without carry-over of slurry,occasional failure of a fresh slurry batch to sweeten below the 4 ppm H2S gas sales contract quality specification,inconsistent and short batch life,rapid settling of oxide powder at low production rates,-inability to remove all of spent slurry solids from contactor, leaving plugged tower packing and promoting subsequent channeling of gas,difficulties with slurry fouling of valve seats and plugging small lines, andproblems with sales gas temperatures being objectionably high after lengthy gas well production. These typical faults of the original slurry process have been largely eliminated by employing both mechanical and chemical alterations. A slurry additive has been proven effective in eliminating carry-overs and the need for initial rate restriction with a new slurry batch. The additive, furthermore, has significantly improved the suspension properties of the slurry oxide, and consequently allows for better removal of expended slurry solids during batch change outs. Controlled injection of trace amounts of air into the sour gas stream has resulted in substantially longer slurry batch life and considerably lower outlet H2S content. The process was originally thought to require preheating of the sour gas to prevent condensate collection and associated foaming problems in the slurry contactor. This need for preheating has been proven to be unnecessary and is generally undesirable. Other warranted changes of the process system serve to simplify and minimize equipment needs while increasing process reliability. As a result of the refinements made to the slurry process at Sun's installation, the sweetening cost per MCF has been reduced, plant downtime is now at an acceptable minimum, and future installations of this type can be accomplished with less initial capital cost. Introduction Reasons for Selection of Iron Oxide Slurry Process Several years ago a new non-regenerative batch type chemical process was developed to remove hydrogen sulphide from natural gas streams. The process utilized a proprietary black form of iron oxide powder suspended in an aqueous column. It was known that H2S would react with the iron oxide as the gas stream was bubbled up through the suspension. Advantages of the process included its ability to selectively remove H2S from a natural gas stream containing carbon dioxide. Furthermore, the process offered a scheme of sweetening without the undesirable side effects of S02 emission and spent batch disposal problems.