Integrated design of ORC process and working fluid using process flowsheeting software and PC-SAFT

Abstract

Organic Rankine Cycles (ORC) transform low-temperature heat into electrical power. To make best use of a heat source, ORC process and working fluid have to be optimized simultaneously. Thus, integrated design approaches of fluids and processes have been developed. However, integrated design approaches are usually complex and based on specific software tools which prevents fast and easy development of the ORC models. In this work, we have integrated the so-called 1-stage CoMT-CAMD approach into the process flowsheeting software gPROMS ProcessBuilder allowing for integrated design of process and working fluid. In 1-stage CoMT-CAMD, thermodynamic properties are modeled by the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) equation of state, which we use directly from the gSAFT physical property package. To introduce the molecular structure of the working fluid as an additional degree of freedom within the process optimization, we implemented the homosegmented group contribution approach of PC-SAFT and the Computer-aided Molecular Design (CAMD) formulation of 1-stage CoMT-CAMD in ProcessBuilder. Existing model libraries of ProcessBuilder were adapted to employ the Variable Molecular Structure Compound feature (VMSC) of gSAFT during process optimization. The resulting mixed integer nonlinear program (MINLP) optimization problem is solved by the standard MINLP solver integrated in ProcessBuilder. Thereby, the optimal working fluid and the corresponding optimal process are identified in one single optimization. The resulting tool enables the easy definition of integrated design problems based on the drag-and-drop feature of a process flowsheeting software for ORCs and beyond.