where x var = (x j ) j∈Ivar are the process variables and g i (x var ), i ∈ I MESH are the model equations. In the case of distillation processes they are given by the socalled MESH equations (material-balance, equilibrium-balance, summation-law and heat-balance). Typically there are less equations than variables. The user has to specify a value for each remaining degree of freedom. The simulation software then tries to determine values for all process variables by solving the underlying non-linear system of equations given in Equation (1). If not just any design is to be found, the specifications are modified until a good design is found. This can for example be achieved using an optimization routine.