Efficient synthesis of a heteroazeotropic distillation process with shortcut methods

Abstract:
Distillation is an expensive unit operation especially in terms of operating cost. The difficulty in analyzing and deciding how to separate a given mixture is caused by the nonlinear behavior of thermodynamics, namely azeotropic effects and miscibility gaps. Therefore it is desirable to have a decision-making support in the conceptual design phase of a process while the separation synthesis is made. For this purpose a three step approach is described: First different promising flowsheets are generated based on reasonable heuristics. Then each flowsheet is optimized with the objective to minimize the recycle flow rates. The resulting nonlinear programs (NLP) are solved with the sequential quadratic programming algorithm (SNOPT). Finally each flowsheet is evaluated with a suitable shortcut method to determine the minimum energy demand for each separation. For separations outside the miscibility gap the rectification body method (RBM) is applied, the continuous distillation regions-based method (CDRM) is used to deal with occurring phase splits. Based on this evaluation a ranking is created to decide which alternatives should be modeled with more detail e.g. with a rigorous optimization model. In this work the proposed methodology is used to investigate a given set of feed streams each consisting of a highly nonideal mixture of propargyl alcohol (PA), mono-chlorobenzene (MCB) and water. Due to mixing alternatives caused by different feed streams, the number of process variants increases. Further heuristics e.g. the recycle reachability rule help to reduce this additional complexity. All the data, which are relevant to this case study (feed compositions, material properties), are provided by the industrial partner Syngenta Crop Protection.

Keywords:
process synthesis, heteroazeotropic distillation