Tier 3 Processing Options for Natural Gasoline from NGL Fractionation

Natural Gasoline is the bottoms product from the debutanizer or last distillation column in a Natural Gas Liquids, NGL, Fractionation facility.  Typically a copper strip or doctor test is used to determine corrosiveness of Natural Gasoline.  To pass the test, the Natural Gasoline is “sweetened”.  Reactive sulfur molecules, i.e. mercaptans (R-SH) are converted into disulfides (R-S-S-R) which are more stable.  The disulfides are left in the Natural Gasoline stream so the sulfur content remains unchanged.  The sulfur content of Natural Gasoline varies but can typically be 100 to 200 parts per million by weight, ppmw. 

Natural Gasoline can be sold and shipped to petroleum refiners who process it further and ultimately blend it into finished gasoline for sale to consumers.  Currently, regulations limit the sulfur content of finished gasoline to an average of 30 ppmw with a maximum content of 80 ppmw.  A recently finalized EPA rule called Tier 3, becomes effective in 2018, and limits sulfur in finished gasoline to 10 ppmw annual average and leaves the maximum allowable sulfur per batch of finished gasoline unchanged.  The new rule has created interest in processing options for NGL fractionation facilities producing and selling Natural Gasoline to produce a “neutral” blend stock containing a maximum of 10 ppmw sulfur.  Since traditional sweetening will not reduce sulfur content, a different processing scheme will be needed to produce a 10 ppmw Natural Gasoline stream. 

This paper analyzes process options including Fractionation, Extraction, and Hydrotreating of Natural Gasoline from NGL Fractionation facilities for Tier 3 compliance.