Control system for an N-methyl-2-pyrrolidone refining unit receiving light sweet charge oil

A control system for a refining unit having an extractor receiving light sweet charge oil and N-methyl-2-pyrrolidone solvent and providing raffinate and extract-mix which are subsequently processed to recover the N-methyl-2-pyrrolidone and to yield refined oil and extract oil, respectively, comprising gravity analyzer means for analyzing the light sweet charge oil and providing a signal API corresponding to the API gravity of the light sweet charge oil, sulfur analyzer means for analyzing the light sweet charge oil and providing a signal S corresponding to the sulfur content of the light sweet charge oil, viscosity analyzer means for analyzing the light sweet charge oil and providing signals KV.sub.150 and KV.sub.210 corresponding to the kinematic viscosities of the light sweet charge oil corrected to 150.degree. F. and 210.degree. F., respectively, flow rate sensing means for sensing the flow rates of the light sweet charge oil and the methyl-2-pyrrolidone and providing signals CHG and SOLV corresponding to the sensed flow rates of the light sweet charge oil and the N-methyl-2-pyrrolidone, respectively, temperature sensing means for sensing the temperature of the extract mix and providing a signal T corresponding thereto, and control means connected to all the analyzer means, to flow rate sensing means and to the temperature sensing means for controlling one of the flow rates of the light sweet charge oil and the N-methyl-2-pyrrolidone flow rates while maintaining the other flow rate constant in accordance with signals API, S, KV.sub.150, CHG, SOLV and T.