Heat-based thermal energy and work-based mechanical energy are two common forms of energy that appear in process industries. While heat integration has become a matured technology widely used for thermal energy recovery, work integration, however, has not been formally studied. From the thermodynamics point of view, in heat integration, temperature is a state variable and the driving force for heat transfer. In work integration, pressure is a state variable. A system reaches a mechanical equilibrium if at every point within a given system there is no change in pressure with time, and there is no movement of material. Work integration could contribute significantly to mechanical energy recovery through synthesizing work exchange networks (WEN’s), where work exchangers are operated in a batch mode, while compressors and expanders as utility units operated in a continuous mode; these render WEN’s a type of sophisticated hybrid network system.
In this paper, we introduce the concept and fundamentals of work integration. Then we present a thermodynamic analysis method that can be used to quantitatively identify the maximum amount of recoverable mechanical energy of process systems. It is found that the pinch technology widely used for heat integration cannot be adopted directly in WEN synthesis mainly because of the operating mode of work exchangers. We introduce a synthesis method to derive an optimal network for maximum mechanical energy recovery at the lowest cost. The feasibility of work integration through WEN synthesis is demonstrated through a case study in ammonia manufacturing, where significant amount of the wasted mechanical energy can be recovered.
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