Control system for an MP refining unit receiving medium sweet charge oil

A control system for an MP refining unit receiving medium sweet charge oil and N-methyl-2-pyrrolidone, one of which is maintained at a fixed flow rate while the flow rate of the other is controlled by the control system, wherein said refining unit treats the received charge oil with the received methyl-2-pyrrolidone to yield extract mix and raffinate, comprising gravity analyzer means for sampling the medium sweet charge oil and providing a signal API corresponding to the API gravity of the medium sweet charge oil, refractometer means for sampling the medium sweet charge oil and providing a signal RI corresponding to the refractive index of the charge oil, viscosity analyzer means for sampling the medium sweet charge oil and providing signals KV.sub.150 and KV.sub.210 corresponding to the kinematic viscosities, corrected to 150.degree. F. and 210.degree. F., respectively, sulfur analyzer means for sampling the medium sweet charge oil and providing a signal S corresponding to the sulfur content of the medium sweet charge oil, flow rate sensing means for sensing the flow rates of the medium sweet charge oil and of the N-methyl-2-pyrrolidone and providing signals CHG and SOLV, corresponding to the medium sweet charge oil flow rate and the N-methyl-2-pyrrolidone flow rate, respectively, means for sensing the temperature of the extract mix and providing a corresponding signal T, and control means connected to all of the analyzer means, to the refractometer means, and to all the sensing means for controlling the other flow rate of the charge oil and the N-methyl-2-pyrrolidone flow rates in accordance with signals API, KV.sub.210 , KV.sub.150, S, RI, CHG, T and SOLV.