The SDA unit removes asphaltenes from vacuum resid, producing an asphalt product and a resid that can be used for baseoil manufacturing or cracker feed.

Extraction is done using a light hydrocarbon, such as propane or butane, and is done in stages to remove asphaltenes and resins separately.  By varying temperature, extraction of can be done selectively, separating asphaltenes and resins from the resid and from each other by taking advantage of their different supercritical temperatures (temperature at which they precipitate out of solution).

There are two common processes licensed by UOP (DEMEX) and Kerr McGee (ROSE).