(378k) Composition and Temperature Cascade Control with Model Predictive and PI Controllers for Dividing-Wall Distillation Columns

Despite
dividing wall being a very effective technology for process intensification of
distillation, the resultant dividing-wall distillation columns (DWDCs) exhibit
strongly interactive and highly nonlinear behaviors, which may pose great
challenges to process operations. Proportional-integral (PI) controllers can
stabilize the operation of the DWDC, but their performances are frequently far
from satisfactory. Model predictive control (MPC) can potentially improve
performance sharply, but the stringent requirements on the qualities of the
embedded model considerably lower its robustness. To suppress these two
deficiencies, a cascaded control structure by combining the MPC and PI
controllers is proposed for the DWDC in this work. The PI controllers regulate the
stage temperature in the inner layer, serving to quickly stabilize the DWDC,
while the MPC directly controls the product compositions in the outer layer,
addressing the strongly interactive and highly nonlinear behaviors of the DWDC.
Two typical cases i.e., a three-product Petlyuk DWDC
and a four-product Kaibel DWDC, and five control
schemes i.e., two impurity composition controls (PI-ICC and MPC-ICC), two
purity composition controls (PI-PCC and MPC-PCC), and one pure temperature
control PI-TC (which represents the case wherein MPC in the outer layer fails
to work), are studied to evaluate the feasibility and effectiveness of the
proposed control structure. It is found that the cascaded MPC/PI controllers
(i.e., MPC-ICC and MPC-PCC) substantially outperform the pure PI controllers
(i.e., PI-ICC and PI-PCC), with lower maximum and steady-state deviations,
shortened settling times, and suppressed oscillations. In the case of failure
of the MPC, the cascaded MPC/PI controller can still maintain stable operation
of the DWDC. These outcomes highlight that the combination of MPC and PI
controllers can be a potential method for the operation of highly interactive
and complicated DWDCs.

Figure 1 The novel
composition/temperature cascade control scheme

Figure 2
Composition/temperature cascade control of the three-product Petlyuk DWDC using pure PI control

Figure 3
Composition/temperature cascade control of the three-product Petlyuk DWDC using MPC and PI control