Process synthesis considering sustainability for both normal and non-normal operations: P-graph approach

Process synthesis usually determines the normal operation of the process. In addition to the cost, recently, sustainability has also become essential in selecting the process. In case of failures of some redundant operating units of the process, it may still be operational as a non-normal operation. This work shows that the sustainability performance of a process in non-normal operation can be much worse than that of the normal case.Consequently, during process synthesis, it is essential to consider the sustainability of a process for both normal and non-normal operations. The current work is the first in offering synthesis of a process network taking into account a pre-specified limit of the process sustainability indicator for both normal and non-normal modes. This synthesis procedure is formulated in a general way, to interface with any sustainability indicator, expressed as a scalar, here Sustainable Process Index has been used in the case studies. The presented method is illustrated on two case studies to showcase the capability of synthesizing processes for multiple operation. The obtained results indicate that it is possible to simultaneously reduce environmental impact and process cost by more than 20%, by appropriate modification of the operating modes.