  Several studies have reported the importance of optimally operating the
absorption column in a post-combustion CO2 capture (PCC) plant. It has been
demonstrated in our previous work how economic Model Predictive Control (EMPC)
has a great potential to improve the operation of the PCC plant. However, the
use of a general economic objective such as maximizing the absorption
efficiency of the column can cause EMPC to drive the state of the system close
to the constraints. This may often than not lead to solvent overcirculation and
flooding which are undesirable. In this work, we present an EMPC with zone
tracking algorithm as an effective means to address this problem. The proposed
control algorithm incorporates a zone tracking objective and an economic
objective to form a multi-objective optimal control problem. To ensure that the
zone tracking objective is achieved in the presence of model uncertainties and
time-varying flue gas flow rate, we propose a method to modify the original
target zone with a control invariant set. The zone modification method combines
both ellipsoidal control invariant set techniques and a back-off strategy. The
use of ellipsoidal control invariant sets ensure that the method is applicable
to large scale systems such as the absorption column. We present several
simulation case studies that demonstrate the effectiveness and applicability of
the proposed control algorithm to the absorption column in a post-combustion
CO2 capture plant.
