Absorption - Introduction to Carbon Capture and Sequestration - page 211

500

MEA
PZ

400

300

200

100

0

2000 2002 2004 2006 2008 2010

year

Figure 5.7.2 The parasitic energy of an amine capture process
This fi gure shows how different process designs and different solvents (MEA and PZ)
have reduced the parasitic energy over the years. The blue line represents the thermo-
dynamic minimum. Data from Rochelle et al. [5.13].

process we can also analyze and estimate the minimum energy required
for each step in the overall process. Professor Gary T. Rochelle and
co-workers at the University of Texas at Austin analyzed the amine cap-
ture process shown schematically in Figure 5.7.3. In this fi gure, we
recognize the absorber and the heat exchanger. For the stripper a two
stage process is used, where a fi rst separation is achieved at a higher
pressure than the second one. In the condenser, water is removed from
the CO 2 gas stream before the compression stage. In this process, a
fl ue gas with 12% CO 2 at 40ºC is considered and it is assumed that
90% of the CO 2 is captured and compressed to 150 bar.
Rochelle and co-workers estimated the minimum required energy by
calculating the minimum lost work in each step of the process. The idea
of a lost-work analysis is that any irreversibility in the process creates
additional entropy that is lost for the process. As we recall from fi rst year
thermodynamics, a truly reversible process requires infi nitely small steps
and infi nitely small perturbations from the equilibrium confi guration. In
practice, “infi nitely small” can be approached by building very large
equipment such that the fl ow is so slow that the system is practically at
equilibrium. However, the costs of such equipment will be prohibitively