a furfural refining unit treats heavy sweet charge oil with a furfural solvent in a refining tower to yield raffinate and extract mix . the furfural 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 flash point temperature analyzer , a sulfur analyzer and viscosity analyzers ; all analyzing the heavy sweet charge oil and providing corresponding signals , sensors sense the flow rates of the charge oil and the solvent 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 furfural flow rates is controlled in accordance with the signals from all the analyzers and all the sensors , while the other flow rate of the heavy sweet charge oil and the furfural flow rates is constant .