(628c) Multi-Loop Control Structure for Dividing-Wall Distillation Columns

Dividing-Wall Distillation columns represent a major advance in the distillation area because they can result in significant reductions of energy consumption and capital investment. Some estimates for the reductions are as high as 30% over conventional separation sequences (Christiansen et al., 1997; Kiss & Bildea, 2011; Massimiliano et al., 2009). As reduction of energy requirements is one of the key reasons for sustainable process technology, DWCs can play an important role as a separation technology for the future (Dejanović et al., 2010). In fact, the benefits of DWCs has already resulted in the installation of over 100 DWCs in industry over the last couple of decades (Dejanović et al., 2010).  Nevertheless, the integration of two columns into a single shell leads also to changes in the control and operating mode due to modifications in the number of degrees of freedom. Therefore, the benefits of DWC technology can only be fully exploited with proper control structures that provide a stable and robust operation of the separation process.

Compared to a conventional distillation system, the control of a DWC is more difficult due to increased interactions among the controlled and manipulated variables. Nevertheless, it has been reported that thermally coupled sequences have good controllability properties(Hernández & Jiménez, 1999; Jiménez et al., 2001), provided that an appropriate control structure is selected (Kiss & Bildea, 2011). An important aspect for evaluating performance of control systems applied to DWCs is the rigor of the model that the controllers are developed on and applied to. The most commonly used dynamic models incorporate several simplifying assumptions, which may not be the most realistic scenario for evaluating control structures.

In this study, we  address this last point in that a rigorous, first-principles-based dynamic model of a DWC used for separation of a ternary mixture is developed. This model is implemented and simulated in gPROMS and consists of 217 ODEs and 589 algebraic equations. Step responses are simulated using this model and a transfer function model is derived around the nominal operating point. RGA analysis is performed and a multi-loop control scheme based upon PI controllers, tuned using IMC, is subsequently used to develop a control scheme. The developed control scheme is then evaluated by application to the detailed dynamic model. The results show that the presented control configuration is able to reject commonly occurring disturbances effectively and also achieves good set point tracking performance. 

References

Christiansen, A. C., Skogestad, S., & Lien, K. (1997). Complex distillation arrangements: Extending the petlyuk ideas. Computers & Chemical Engineering, 21, Supplement, S237-S242.

Dejanović, I., Matijašević, L., & Olujić, ?. (2010). Dividing wall column—A breakthrough towards sustainable distilling. Chemical Engineering and Processing: Process Intensification, 49, 559-580.

Hernández, S., & Jiménez, A. (1999). Controllability Analysis of Thermally Coupled Distillation Systems. Industrial & Engineering Chemistry Research, 38, 3957-3963.

Jiménez, A., Hernández, S., Montoy, F. A., & Zavala-García, M. (2001). Analysis of Control Properties of Conventional and Nonconventional Distillation Sequences. Industrial & Engineering Chemistry Research, 40, 3757-3761.

Kiss, A. A., & Bildea, C. S. (2011). A control perspective on process intensification in dividing-wall columns. Chemical Engineering and Processing: Process Intensification, 50, 281-292.

Massimiliano, E., Giuseppe, T., Ben-Guang, R., Daniele, D., & Ilkka, T. (2009). Energy saving and capital cost evaluation in distillation column sequences with a divided wall column. Chemical Engineering Research and Design, 87, 1649-1657.