EAGER: Work Integration through Work Exchange Network Synthesis

Wayne State University, Detroit MI

Investigators

Abstract

Over the past decade, accelerated natural resource depletion, greenhouse gas (GHG) emissions, the consequences of climate change, and economic globalization, among other factors, had industries reviewing their practices with a greater focus on sustainability. The results have been improvements in their operations evidenced by energy savings, waste reduction, and gains in productivity. But, according to the DOE Industrial Technology Program (ITP), the energy loss in the manufacturing sectors is still high (approximately 57% of the total energy consumed), and total combustion emissions have reached approximately 1,260 million metric tons of CO2e (carbon dioxide equivalents). The chemical process industries (CPI) account for approximately 20% of the total energy consumed in the U.S. This research project is aimed at improving energy conservation in chemical plants. Intellectual Merit: Heat and work are two common forms of energy in chemical plants. Heat integration, using heat exchanger networks (HENs), is a technique used for thermal energy recovery in the CPI. While process work is more expensive than process heat, using and reusing process work effectively has not been seriously studied. The research goal here is to develop work exchange networks (WENs) that are similar to HENs. From a thermodynamics point of view, heat flow, with temperature as the state variable, is directly related to thermal energy efficiency, while work flow, with pressure as the state variable, can be used to evaluate mechanical energy efficiency. The key mechanism of work integration is work exchange among coupled process streams due to pressure differences. This study is an investigation of using work integration in process systems engineering. The proposed thermodynamic analysis on work exchange under isothermal, non-isothermal, and adiabatic conditions will be used in the work exchange regions for thermodynamically feasible and economically justifiable mechanical energy recovery. Since a WEN is operated in a hybrid mode, the WEN synthesis methodology will be different from those for heat and mass exchanger networks, which are operated in a continuous mode. The methodological novelty will be also demonstrated by the approaches for pinch point identification, energy-cost target setting, and optimal process stream matching. Broader Impacts : In a variety of chemical manufacturing systems, process streams need to be pressurized, which requires work for compression, while other processes can produce work through expansion. Ammonia synthesis, reverse osmosis, and freezing purification are among well-known examples. Another example is the gas processing industry, where high-pressure natural gas needs to be cooled with liquid CO2 and then expanded to a lower pressure to exchange heat with liquid N2, and it is then further depressurized in a turbine to reach its storage pressure. If the available mechanical energy in the high-pressure streams is utilized to pressurize the lower pressure streams, the mechanical energy recovery can greatly reduce operating cost. This is of not only economic significance but also has environmental implications, as energy efficiency improvement could help reduce CO2 emission.