The first part of this paper explains the concept of exergy and gives calculation procedures of exergy by equation of state (PR) and some technical details.It also analyzes the obscure concept of “negative exergy” caused by low pressure

especially when the condition “vacuum” is taken into consideration

showing that like the exergy at a low temperature

“negative exergy” is another valuable kind of exergy.The last part of the paper contains exergy analysis reports about two important chemical processes

rectification and heat exchange with the support of the “three layers” theory.The research indicates that energy-saving in the process of rectification is a specific case study instead of adjusting reflux ratio only.In the process of heat exchange

the pressure exergy loss should be specially chosen as well as heat exergy loss to save expensive high level exergy—electricity.