Abstract

The sulfuric acid alkylation process chemically combines isobutane with a low octane olefin (usually a mixture of propylene and butylene) in the presence of sulfuric acid catalyst to yield higher octane alkylate product used in blending gasoline pool. Corrosion is primarily driven by sulfuric acid and esters formed in the process. Materials of construction in the plant are generally carbon steel although austenitic stainless steels like UNS S31603 and UNS N08020 are used selectively to resist acid corrosion. Temperature and flow velocity have a direct relationship to corrosion rates, perhaps affecting the passive behavior of this super-austenitic stainless alloy. Operational control of these process variables in combination with mechanical design is important to minimize corrosion rate even in a super-austenitic alloy. This paper describes the frequent failures of super-austenitic stainless alloy UNS N08020 in effluent treatment section of the plant, its active-passive corrosion behavior and recommendations to improve long term reliability, integrity and plant safety.

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