ABSTRACT
I. INTRODUCTION Improving performances in industrial processes is a constant concern
the problem comes up at the plant design stage, again if an extension or a modification of an existing process is being envisaged; finally when choosing the operating conditions and the control of the plant in view of the production required : the economic conditions at a given time poses the question in constantly changing terms. Over the last few years, process control has evolved : it started with single analogic loops, the set points of which were modified manually, and is now evolving towards direct digital control where the process computer can decouple the variables and adjust the independent parameters to the operating conditions.
