The paper describes a conceptual formulation to screen, scope and review design options of complex separation systems without the need for extensive regression experiments. It employs a supertask representation with new synthesis formulation that exploits thermodynamics and engineering insights of primary separation. The thermodynamic and separation efficiency is systematically assessed with the prepositions of general terms, 'conceptual losses'. These terms explicitly account for the competing design drives of the problem and enables trade-offs to become clear in the optimal solution. In all cases, the approach guarantees simple mathematical models that one can solve to global optimality. Furthermore, the approach provides venues to interpret the results and explain the layouts selected by the optimisation. The approach is illustrated with two different examples and results are explained by reviewing important conceptual terms. Indeed, it provides a unique opportunity to understand and build confidence in the selected options.