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Failure Analysis of High Temperature Plate Heat Exchangers in Converter Section of Sulfuric Acid Regeneration [SAR] Unit

A Sulfuric Acid Alkylation [SAA] unit in a refinery converts olefins and butane to high octane alkylate using highly concentrated sulfuric acid as a catalyst. The function of this Sulfuric Acid Regeneration [SAR] unit is to regenerate spent sulfuric acid from alkylation process into clean sulfuric acid of 99.2% concentration, which is then recycled back into the SAA unit. The process of SAR can be classified in to following four steps:


• Formation of SO2 by the decomposition of Spent acid and combustion of H2S.
• Cooling and Purification of the SO2
• Conversion of SO2 to SO3
• Absorption of SO2 in H2SO4

Product Number: 51323-19420-SG
Author: Sudharsanan Soundararajan
Publication Date: 2023
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In Sulfuric Acid Regeneration [SAR] Unit, the spent sulfuric acid [H2SO4] from Alkylation Unit is regenerated to produce a clean Sulfuric Acid of 99.2%. As part of regeneration process the spent acid is decomposed to sulfur dioxide (SO2) at 1066oC [1950oF]. After cooling and purification the conversion of SO2 to Sulfur trioxide (SO3) occurs with the presence of Oxygen in the Convertor section of the unit. The conversion process is an exothermic reaction in nature and the converted Hot gas is used to heat the inlet cold gas to the 1st Converter in Hot Gas-Gas Plate type heat exchanger. The metallurgy of the plates is UNS S30400 [SS304] and the operating temperature is around 620oC [1150oF]. During operation, a leak from this exchanger was observed and while carrying out the repair a metal sample was retrieved for analysis. As part of metallurgical analysis, chemical compositional analysis, Optical Microscopy, XRD analysis of the scale, SEM and EDS were conducted. This paper explains the probable reason for the failure of heat exchanger plates and mitigation measure for the same.

In Sulfuric Acid Regeneration [SAR] Unit, the spent sulfuric acid [H2SO4] from Alkylation Unit is regenerated to produce a clean Sulfuric Acid of 99.2%. As part of regeneration process the spent acid is decomposed to sulfur dioxide (SO2) at 1066oC [1950oF]. After cooling and purification the conversion of SO2 to Sulfur trioxide (SO3) occurs with the presence of Oxygen in the Convertor section of the unit. The conversion process is an exothermic reaction in nature and the converted Hot gas is used to heat the inlet cold gas to the 1st Converter in Hot Gas-Gas Plate type heat exchanger. The metallurgy of the plates is UNS S30400 [SS304] and the operating temperature is around 620oC [1150oF]. During operation, a leak from this exchanger was observed and while carrying out the repair a metal sample was retrieved for analysis. As part of metallurgical analysis, chemical compositional analysis, Optical Microscopy, XRD analysis of the scale, SEM and EDS were conducted. This paper explains the probable reason for the failure of heat exchanger plates and mitigation measure for the same.

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