Optimization-based design of waste heat recovery systems

Today, a large portion of the products that are vital to sustainability of our society are chemical products. These products have a wide range of applications within healthcare, medicine, agriculture, food, plastics and industrial processes. Therefore, chemical product development is regarded as one of the most important areas of chemical engineering today.

The process efficiency and sustainability, in a considerable number of applications, depends on an array of chemical products. To ensure efficient, safe and environmental friendly processes, new chemical products need to be designed and/or selected. This dissertation focuses on the chemical product and process systems used for waste heat recovery.

Here, chemical products are working fluids, which are under continuous development and screening to fulfill regulatory environmental protection and safe operation requirements. Furthermore, for the recovery of low-grade waste heat, new fluids and processes are needed to make the recovery technically and economically feasible.

As the chemical product is influential in the design of the process system, the design of novel chemical products must be considered with the process system. Currently, state-of-the-art computer-aided design methods can only inadequately design novel chemical products and processes as they are considered separately and independently.

Heuristics and know-how can provide feasible alternatives, but requires much user interruption, many resources and can only consider few candidates. Other than working fluids, the thesis presents other product types and applications of relevance, including solvent design. In this thesis, a holistic framework is presented for the design of novel chemical products as a means of process systems design.

The framework ensures optimal design of the chemical product and process system in terms of efficiency and sustainability. Today, some of the most important chemical product design problems are solvents and working fluids. Solvents are a vital part in the recovery of valuable resources in separation processes or waste water treatment.

Working fluids are needed for the recovery of industrial waste heat and in refrigeration, air conditioning and engines, where many fluids today are phasing out due to regulative measures. The developed framework can design new chemical products, as demonstrated in cases of working fluid and solvent design, with the optimal design of the process system it is applied in.

The framework requires the input of the chemical product and process needs, which through a set of systematic steps and algorithms can be formulated into a mathematical program. The program is then solved through a selection of solution strategies and mathematical optimization solvers. The designed framework was implemented in new programs for the application to seven case studies.

Three of which are highlighted in the following. Two industrial case studies have been solved. The first addresses recovery of waste heat from a marine diesel engine. An organic Rankine cycle and a novel pure working fluid was designed for the recovery of the exhaust gas waste heat from the 37 MW marine diesel engine.

The new process system can generate 1.2 MW of power with 2,2,3,3,4,4,5,5-octafluorohexane, which has been shown to outperform other conventional fluids, in terms of performance and sustainability. The fluid was novel and generated through the framework. In the second case study, waste heat recovery from a milk powder production spray dryer was addressed.

A heat pump was designed with a mixed working fluid for the optimal heat recovery and transfer for the low-grade waste heat from effluent spray dryer air. 25% isobutene and 75% 1,3-difluoropropane and a process with a coefficient of performance of 3.22 was designed. The design provided new binary mixture and optimized cycle process that was an improvement compared to conventional systems.

Furthermore, the fluids were not before used as refrigerants and are readily available in the market. In a third case study, a new solvent was designed for the recovery of acetic acid from water. The new design with a liquid-liquid extraction process could get 98.3% recovery of acetic acid using novel solvent butane-2,3-diyl diformate.

The case study served to illustrate the framework application in other chemical product design areas. For the design of a new generation of working fluids, a new property prediction method has been developed that can calculate the properties of the new generation of working fluids, hydrofluoroolefins, that today have one of the best environmental properties.

The developed framework, models and tools in this thesis have been shown to successfully design new chemical products and process systems for waste heat recovery and additional application areas. Furthermore, the impact of this work was emphasized through the comparison with conventional methods and designs where the framework application has been an improvement.

The implications of these new results are shown and discussed through several published work that are presented here, which are supported by a synopsis summarizing the results.