Johannes Schilling

From molecules to dollars: integrating molecular design into thermo-economic process design using consistent thermodynamic modeling

The right molecules are often the key to overall process performance and economics of many energy and chemical conversion processes, such as, e.g., solvents for CO2 capture or working fluids for organic Rankine cycles. However, the process settings also impact the choices at the molecular level. Thus, ultimately, the process and the molecules have to be optimized simultaneously to obtain a thermo-economically optimal process. For a detailed design of the process and also the equipment, a thermodynamic model is required for both equilibrium and transport properties. We present an approach for the integrated thermo-economic design of the process, equipment and molecule on the basis of a comprehensive, thermodynamically consistent model of the molecule. For this purpose, we developed models for transport properties based on entropy-scaling of the perturbed-chain statistical associating fluid theory (PC-SAFT) equation of state. Thereby, a single model predicts both equilibrium and transport properties in our optimization-based approach for the integrated design of the process, equipment and molecule, the so-called 1-stage CoMT–CAMD approach. The predicted transport properties allow for the design and sizing of unit operations as degrees of freedom during the optimization. Computer-aided molecular design allows the design of novel molecules tailored to the specific process while considering safety and environmental issues. The presented approach is exemplified for the design of an organic Rankine cycle showing the merits of detailed sizing of heat exchangers with different heat transfer types and the rotating equipment as part of the optimization. Single-objective optimization is used to obtain a ranking of potential working fluids. The detailed trade-off between the total capital investment and the net power output of the ORC is studied using multi-objective optimization. Thus, the 1-stage CoMT–CAMD approach allows for efficient and holistic designs linking the molecular scale to economics.

One-stage approach for the integrated design of ORC processes and working fluid using PC-SAFT

Abstract Organic Rankine Cycles (ORC) can transform low-temperature heat into electrical power. To ensure optimal use of a heat source, process and working fluid need to be tailored to the specific application. We present a one-stage approach for the integrated design of ORC process and working fluid, which identifies the optimal working fluid and the corresponding optimal process in a single optimization problem. For this purpose, a process model is combined with a modern thermodynamic model of the working fluid. The process model is based on equilibrium thermodynamics. The perturbed-chain statistical associating fluid theory (PC-SAFT) is used as physically-based thermodynamic model of the working fluid. The fluid model is extended by a group-contribution method based on PC-SAFT to enable Computer-aided molecular design (CAMD) of novel working fluids within the optimization. The full model enables the integrated design of process and working fluid. The optimization is an MINLP problem depending on two kinds of design variables: continuous process variables and integer variables representing the molecular structure of the working fluid. The one-stage approach is exemplified in a case study for a subcritical ORC process. The approach is shown to efficiently identify the optimal working fluid and the corresponding optimal process parameters. Integer cuts are employed to generate a ranked list of candidates.