Copyright @ IF AC Management and Control of Production and Logistics, Grenoble, France, 2000 A COMBINED CONTROL AND OPTIMIZATION SYSTEM USED IN SOLVENTS RECOVERING DEVICE Gang Xiong*, Timo R. Nyberg** Paper Machine Automation, Automation and Control Institute, Tampere University o/Technology, p.a. Box 692, FIN-33IOI Tampere. FINLAND * T. +35833652644 F. +35833653950 Email:gang@ad.tut.ji ** T. +35833652/42 F. +35833653950 Email:timo.nyberg@cc.tut.ji Abstract: The solvents recovering process in lubrication oil production needs a lot of energy, and has large stagnancy, strong coupling, time delay and interference etc. To measure the quality of solvents on-line is difficult. How to assure the quality and save energy is discussed. A non-linear predictive control is proposed firstly. Because the mathematical model used for energy optimization can't meet actual need, a combined modeling and optimization strategy is then proposed. Intelligent database, adaptive modeling, combined control and optimization, expert control and intelligent alarming etc. are used to improve the function of the device clearly. Actual production results prove that the system is a great success. Copyright f)2000IFAC Key words: Nonlinear predictive control, control algorithms, computer control systems, optimal system, control system design 1 INTRODUCTION Wax can be separated from lubrication oil by dropping its solidifying point, so that lubrication oil can be used at a lower temperature. Ketone-benzol dewaxing device is a typical device used in separating wax from lubrication oil through distillation. In the production process, two kinds of solvent (ketone and benzol (or toluene)) are added into lubrication oil. The solvents are expensive, one must recover them from oil flow and reuse them. But the recovering process needs a lot of energy, the device has large stagnancy, strong coupling, time delay and interference etc., to measure the quality of solvents on-line is difficult, how to assure the quality of solvents and save energy at the same time is an important challenge (X.Q. Fang, 1994a;.B.zh Guo, 1993; KJ Zhang, 1988). 2 THE TECHNOLOGY AND THE CONTROL AND OPTIMIZATION PROBLEM OF SOLVENTS RECOVERING DEVICE FROM WAX 2.1 The Technology Solvent de-waxing is a continuous physical separating process, which is composed of two similar 1099 units, solvents recovering unit from oil and solvents recovering unit from wax. In the paper, we mainly discuss the latter process; Its main task is to recover solvents from wax with high quality and reduce energy consuming at the same time. Based on different boiling points between wax and solvents, when wax flow that includes solvents is heated up to the boiling point of solvents and is carried to distilling towers, some solvents can evaporate, the blend flow can be divided into gas and liquid. Gas is carried out from the top of the towers, liquid is carried out from the bottom of the towers. After several heating and distilling processes of tower 5, tower 6, and tower 7-1, most solvents can be recovered. The residual solvents can be recovered completely through azeotropic rectification process in tower 7 (Fig. I ). The concrete technology of recovering solvents unit from wax is introduced in G. Xiong (1996). The wax flow includes wax and solvents, after being heated by the exchanging heat that comes from the solvents gas on the top of towerS, tower 6 and tower 7, it is carried into tower 5 where it is divided into gas and liquid firstly, some solvent evaporate. The remaining wax flow is drawn from tower 5 by pump 9, heated by main streams, then is carried into tower 6 where it is divided into gas and fluid secondly, some other solvents evaporate. The remaining wax flow at the