|Title||Recommended Practice for the Design, Fabrication, and Inspection of Tanks for the Storage of Petroleum Refining Alkylation Unit Spent Sulfuric Acid at Ambient Temperatures|
Sulfuric acid (H2SO4) is the largest-volume commodity chemical in use today. It plays some part in the production of nearly all manufactured goods. One large use of concentrated sulfuric acid is as a catalyst for refinery alkylation units. In these units, C3-C5 olefins such as propylene or butylene are reacted with isobutane to form gasoline-blending components such as isoheptane and isooctane. These products are used to boost octane for automobile and aviation fuels.
Most refineries have an alkylation unit that uses either hydrofluoric acid (HF) or sulfuric acid as the alkylation catalyst. This standard deals with spent sulfuric acid associated with the sulfuric acid alkylation process only.
Refineries using sulfuric acid alkylation typically require tanks for the storage of fresh (not yet used in the alkylation process) and spent (used in the alkylation process and in need of regeneration) acid. Design, fabrication, and inspection of fresh sulfuric acid tanks are covered in NACE Standard RP0294.1 This standard covers additions and deviations from RP0294 that apply to spent sulfuric acid storage tanks.
Large vertical sulfuric acid storage tanks are usually built in accordance with API(1) Standard 6502 or API Standard 620,3 and horizontal cylindrical tanks are built in accordance with the ASME(2) Boiler and Pressure Vessel Code, Section VIII, Division 1.4 While these codes and standards are sufficient for design strength and toughness considerations, they do not address the peculiarities of corrosion by alkylation unit spent sulfuric acid service. In addition, alkylation unit spent acid can contain dissolved hydrocarbons and hydrogen that release into the vapor space of these tanks and potentially produce an explosive environment. Thus, special care must be taken to deal with vapor leakage from the vapor space of these tanks and air intrusion into the vapor space of these tanks.
Carbon steel corrodes moderately when in contact with alkylation unit spent sulfuric acid. If tanks are properly designed and adequately maintained, use of carbon steel is an economical option for the storage of these acids at ambient temperatures. However, accelerated corrosion can occur in various forms, and a catastrophic failure of a spent acid tank in Delaware City, Delaware, U.S.A., has focused attention on the hazards associated with corrosion, vapor space leakage, and hot work on or around alkylation spent sulfuric acid tanks. The Chemical Safety Board(3) report5 can be reviewed for more details regarding this failure.
This standard is to be used in conjunction with NACE Standard RP0294. It is intended for use by owners/operators and fabricators of alkylation unit spent sulfuric acid storage tanks. This standard was prepared by Task Group (TG) 300 on Petroleum Refining Spent Sulfuric Acid Storage Tank Requirements: Standard. TG 300 is administered by Specific Technology Group (STG) 34 on Petroleum Refining and Gas Processing, and sponsored by STG 36 on Process Industry—Materials Performance in Chemicals. It is issued by NACE International under the auspices of STG 34. These committees include representatives of companies involved in the production, transportation, and use of large quantities of spent sulfuric acid.
Section 1: General
1.1 Spent sulfuric acid, generated by refinery alkylation units (spent acid), may differ from fresh sulfuric acid (fresh acid) in several ways. The most notable differences between alkylation unit fresh acid and spent acid are the acid concentration, water content, possible contaminants from the alkylation process, temperature of the spent acid entering the tank from the alkylation unit, and the presence of dissolved hydrocarbons and hydrogen. These differences must be taken into consideration when developing inspection strategies, setting inspection intervals, conducting an external inspection, and/or internally inspecting a tank that is in spent sulfuric acid service. This standard was developed to address these differences between fresh acid and spent sulfuric acid as they may have an impact on the integrity of spent sulfuric acid storage tanks.
1.2 NACE Standard RP0294 shall be followed for all aspects of spent sulfuric acid storage not covered by this standard.
1.3 This standard presents additions to and deviations from NACE Standard RP0294 that apply to alkylation unit spent sulfuric acid storage.
1.4 HF alkylation unit spent acid is not within the scope of this standard.
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