CHEOPE (CHEmical OPErations) is an ENICHEM proprietary package, developed to solve problems on chemical process operation and design, by an equation-oriented approach (Pagani and d`Arminio Monforte 1987). CHEOPE solves three problem classes: simulation, design and optimization. The package is organized in a modular way to make it easier for the user to input. CHEOPE uses the “modules” to generate suitable equation blocks for each process unit. All the equations are then collected in a large system whose, Jacobian matrix has a quasi-tridiagonal block structure. A Newton-Raphson method is then used for solving the equations. The roots of the linearized system are found by an upper-triangulation method applied to the block-matrix (Pagani and d`Arminio Monforte 1985), otherwise, in optimization and inexact simulation problems, CHEOPE uses a Successive Quadratic Programming algorithm (Pagani et al. 1989). In particular inexact simulation is a very useful technique in distillation design, in flowsheeting and, of course, in plant operation and in process identification. In fact, in these cases, we often have to solve problems where some constraints are unfeasible, conflicting, in numer greater than the degrees of freedom and not equally “important”. We call these constraints “inexact design constraints” and this problem “inexact simulation” (Pagani and d`Arminio Monforte 1991).