(151a) Control of Kaibel Distillation Column by Estimated Compositions From Temperature Measurements

Control of Kaibel Distillation
Column by Estimated Compositions from temperature measurements

Maryam Ghadrdan¹, Ivar Halvorsen²,
Sigurd Skogestad¹, Deeptanshu Dwivedi¹

¹ Department of
Chemical Engineering, Norwegian University of Science and Technology,
N-7491 Trondheim, Norway, Email: ghadrdan@nt.ntnu.no,
skoge@nt.ntnu.no, dwivedi@nt.ntnu.no

² SINTEF ICT, Applied
Cybernetics, N-7465 Trondheim, Norway, Email: ivar.j.halvorsen@sintef.no

Reliable
and accurate measurement of product compositions is one of the important issues
in distillation column control. However, direct composition measurements are
expensive, not so reliable and there is also a significant delay.
Temperature measurements are robust, fast and cheap and
represents proven technology in the process industry.

We have used a soft-sensor to estimate the
compositions from temperature measurements. Our estimation method (we call it
Loss method) is a reformulation of the well-developed Self-optimizing method proposed
by Skogestad (2000) to make it adequate for the purpose of estimation (Ghadrdan
et al. 2011). We have formulated the Loss method for open-loop and closed-loop
estimator. With the term ?open-loop? estimator, it is implied that the
predicted variables are not used for control purposes. In
"closed-loop" the estimates are used as feedback in control loops,
and the performance of the closed loop control will depend on the estimator performance.
Since the estimator is based on steady-state information, it is important that
the behavior in a dynamic system does not degrade the control.

In this work, we are demonstrating our
estimation method for a Kaibel distillation arrangement which is a complex
case-study. Kaibel distillation arrangement (Figure 1) separates a
four-component mixture into pure products and thus replaces a sequence of three
conventional distillation columns. There is a potential for significant energy
savings, but this requires that   the column is operated optimally. The extra operational
degrees of freedom in the column are used for the purpose of optimization and
to operate the column as close as possible to the minimum energy. The tight
integration in Kaibel column makes it challenging to control, compared to the
conventional sequence of simple columns. The control performance using the
estimated compositions to be controlled in face of various process disturbances
will be demonstrated by dynamic simulations.

Figure 1.
Schematic of Kaibel Distillation Column

References

Skogestad, S. (2000). ?Plantwide control: the search for the selfoptimizing control structure.? J. Proc. Control 10:487-507

Ghadrdan, M. ,Grimholt,
C., Skogestad, S., Halvorsen, I.J., ?Loss method:
A static estimator applied
for product composition estimation from distillation column temperature profile.? AIChE Annual Meeting, (2011), Minneapolis, USA