Spectra Energy Midstream retained the services of Gas Liquids Engineering Ltd. (GLE) to prepare a cost estimate for sulphur plant modifications to be carried out during the 2008 turnaround.
The Nevis Sulphur Plant consists of three Claus converters (Claus plant) followed by a tail gas clean-up unit (TGCU), which is the two-bed Sulfreen unit. The Claus plant uses hot gas bypass reheat lines to set the temperatures for the first two Claus converters and a steam reheater to set the temperature for the third Claus reactor.
The Sulphur Plant was originally designed for 240 LTD of sulphur but the amount of sour feed gas had been steadily declining and the Sulphur Plant was only producing approximately up to 50 LTD of sulphur.
Based on the three-stage Claus plant and the Sulfreen tail gas clean-up unit, the Nevis sulphur facility should have been capable of achieving 99.2% recovery efficiency. The licensed average quarterly sulphur recovery efficiency for the Nevis facility is 98.4%. In 2007 the average monthly/quarterly average efficiency was slightly below the license number. Sulphur Experts Inc. conducted a site visit in June 2007 to review the sulphur plant operation and to determine the causes of lower efficiency.
The 1st Claus converter operating temperature was found to be significantly below the normally recommended value to provide good COS and CS2 conversion. The COS and CS2 not being converted in the 1st Claus converter and COS and CS2 being bypassed to the 2nd Claus converter are responsible for efficiency losses. It was also determined that Reheater X-610 was not sufficient by itself to provide the recommended second stage operating temperature.
The following recommendations were provided and implemented:
- Blanking of the waste heat boiler first pass tubes (66 of the 75 tubes were blanked with ceramic ferrule plugs)
- Relocation of the Steam Reheater X-610
- Bypassing the Third Claus Converter
- Bypassing the Sulphur Separator
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