The exergy of a system is the maximum useful work that can be extracted from
that system until it reaches equilibrium with its environment. Exergy can
be destroyed by irreversibilities of a process. An exergy analysis (or 2nd Law
analysis) is a very powerful way of optimizing complex thermodynamic
systems. In the context of PSA, an exergy analysis can be used to identify
which components of the system are responsible for irreversibility, or lost work
and developments can then be directed at those components. The
diagram below (from R.Banaerjee, et al."Exergy
analysis of pressure swing adsorption processes for air separation",
Chemical Engineering Science 45(2), pg467,1990) shows an example of Grassman diagram which is a
graphical depiction of the exergy flows during operation of a PSA system.
Work
enters through the compressors (100% of the input exergy) and exits the system
at various points along the way. Note that the exergy destroyed in the PSA
beds amounts to 40% of the input exergy, a significant fraction. It is
this component that can be addressed by intelligent cycle design. Typical
PSA plants contain adsorbent beds, compressors, valves, and tanks. Common
irreversibilities are pressure drop during flow through valves, inefficiencies
of the compressors, heat transfer from/to the adsorbent beds, mixing of gases
within the adsorbent beds and tanks, and mixing of gas streams with the
environment. While exergy calculations for many of these components can
easily be done (and has been done in earlier studies), it is the exergy
destroyed within the adsorbent beds that is of most interest to us since this
feature will help guide cycle design. To calculate exergy associated with
adsorption/desorption, it is necessary to develop equations based on Adsorption
Thermodynamics.
Our research program
is directed at developing detailed models for PSA systems including exergy
analysis which, together with overall capital and operating cost methods will
allow us to optimize the design and operation of PSA systems. For more
information on this project, contact Paul Webley.