Work Exchange Network Synthesis for Mechanical Energy Recovery
Aida Amini Rankouhi and Yinlun Huang
Department of Chemical Engineering and Materials Science
Wayne State University, Detroit, MI 48202
Abstract
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.