a refining unit treats heavy sweet charge oil with a methyl - 2 - pyrrolidone solvent , hereafter referred to as mp , in a refining tower to yield raffinate and extract mix . the mp is recovered from the raffinate and from the extract mix and returned to the refining tower . a system controlling the refining unit includes a gravity analyzer , a refractometer , a sulfur analyzer and viscosity analyzers ; all sampling the heavy sweet charge oil and providing corresponding signals . sensors sense the flow rates of the charge oil and the mp flowing into the refining tower and the temperature of the extract mix and provide corresponding signals . one of the flow rates of the heavy sweet charge oil and the mp flow rates is controlled in accordance with the signals from all the analyzers , the refractometer and all the sensors , while the other flow rate of the heavy sweet charge oil and the mp flow rates is constant .