The approach exhibits a large flexibility in the choice of the optimization variables and the choice of the MESH equations that are presolved. Indeed, both approaches described above can be modeled. Either by presolving all MESH equations and adding no equation to the optimization problem, or by using all process variables as optimization variables and adding all MESH equations to the optimization problem. But also other choices are possible. For example Kravanja and Grossmann 1996 solved complete operating units by an external function. Biegler and Hughes 1982 added only equations introduced by socalled tear streams to the optimization problem. It is also possible to calculate thermodynamic properties such as the vapor liquid equilibrium externally. This has for example be done by Despite this flexibility, the goal is to choose a small subset of optimization variables. For this reason, not only the equations describing the thermodynamic properties but a majority of the model equations are presolved. This way only a small subset of the model equations and of the original variables need to be incorporated in the optimization problem which makes the problem small. In addition, the mapping that determines all process variables from the optimization variables must be evaluated multiple times. Thus, it must be possible to do this quickly and robustly. For this reason, most, but not all, of the equations describing a unit are solved by an external routine. The right choice depends on the considered operating unit. presented a routine for a conventional distillation column. By this was extended to the interconnection of multiple distillation columns. The goal of this paper is to develop a method for simple and multiple dividing wall columns.