1. A method for optimizing the operation of a lignocellulosic granular matter refining process using a control unit ( 94 ) and at least one refiner stage ( 92 ), said process being characterized by a plurality of input operating parameters, at least one output parameter being controlled by said unit ( 94 ) with reference to a corresponding control target, and at least one uncontrolled output parameter, said method comprising the steps of: i) providing a predictive model ( 84 ) including a simulation model ( 86 ) for said refining process and an adaptor ( 88 ) for said simulation model, said simulation model being based on relations involving a plurality of matter properties characterizing lignocellulosic matter to be fed to said process, said refining process input operating parameters, said controlled output parameter and said uncontrolled output parameter, to generate a predicted value of said uncontrolled output parameter; ii) feeding the simulation model adaptor ( 88 ) with data representing measured values of said matter properties and data representing measured values of said controlled and uncontrolled output parameters, to adapt the relations of said simulation model accordingly; and iii) providing an optimizer ( 214 ) for generating an optimal value of said control target according to a predetermined condition on said predicted value of said uncontrolled output parameter and to one or more predetermined process constraints related to one or more of said matter properties, said refining process input operating parameters and said refining process output parameter.