Model Predictive Control of a Kaibel Distillation Column

Abstract

A Kaibel distillation column separates a feed into four products with significant lower energy consumption than a conventional sequence of binary columns. Optimal operation and control of such systems is an important task in order to obtain the potential energy savings.A laboratory column has been built at NTNU, Department of Chemical Engineering. At the time of the diploma work the laboratory column has unfortunately not been available for MPC experiments.In practical operation a control structure based on temperature measurements is chosen for the given case. This structure gives a four-by-four multivariable system. The candidate shall base his work on a model developed by Jens Strandberg. The motivation of using a Kaibel distillation column is primarily its energy saving potential. There is no reason for using such an energy saving column if the product purities are below some acceptable values. These purities can be kept above these acceptable values by sufficient control of the column.The simulation model of the Kaibel column was extended to include an efficiency parameter which describes an insufficient vapor mixing effect that occurs in distillation columns. This insufficient mixing leads to increased impurity flows in open-loop, but this is counteracted by increased reflux flow in closed-loop. The column can obtain sufficient purities for increasing insufficient mixing until the reflux flow reaches its maximum value.A single layer MPC and a supervisory MPC approach have been described, implemented and tested on the simulation model of the column. These MPCs show improved dynamic responses compared to the existing decentralized control approach. These MPCs have also been compared in a sensitivity analysis part. The sensitivity analysis shows a clear improvement in the robustness properties for a supervisory MPC compared to decentralized control in terms of input uncertainty. The supervisory MPC has also shown to be more robust than the single layer MPC.A brief qualitative discussion regarding alternative MPC approaches has been done, where inferential control is suggested as an alternative MPC approach for the Kaibel column.An MPC implementation has been done for use at the existing laboratory column at Department of Chemical Engineering. Also here, based on implementation issues, the supervisory MPC is preferable compared to a single layer MPC. A parameter adjusted version of the developed simulation model is recommended as the MPC's internal prediction model.

Optimal operation mode switching of dividing wall distillation based on model-predictive control

“…General industrial automatic control system was shown in Figure 3, one of which was operating function block which executed orders from plan/scheduling function layers, control function blocks were composed of feedback and feedforward algorithms, the algorithm can be classic PID or MPC or other advanced control strategies [16], the reference set points and associated logic blocks were from the operation function blocks. The movement time series of operation and control were on different time scales, the current way was control function automatically fast scale and manual low-scale operation function.…”

Numerical and Experimental Research on a Kaibel Divided-Wall Column: Design and Steady-State and Dynamic Operation

“…The Kaibel divided-wall column (KDWC) that separates a four-component mixture with a single wall was first proposed in 1987 and attracted increasing academic interest until Skogestad et al numerically investigated the control of a Kaibel column through model-predictive control (MPC) in 2010 due to its high complexity. Obviously, larger potential benefits, including capital and operation costs, can be gained with a KDWC than with conventional direct and indirect distillation sequences. , Furthermore, KDWC is the simplest physical structure in four-component mixture separation DWCs.…”

“…Additionally, a pressure-compensated temperature control strategy was proposed to solve the large offset of the lower sidestream product when facing large feed disturbances in a KDWC . In addition to the traditional single-loop PID controller used in the aforementioned literature, model-predictive control (MPC) was also applied to KDWC control . Furthermore, a two-layer control structure that combines PID and MPC and fast model-predictive control (FMPC) were utilized for KDWC control .…”

Modeling and model predictive control of dividing wall column for separation of Benzene–Toluene-o-Xylene

“…Their study proves the real-life practicability of MPC; they did not provide an analysis of the transient behavior of DWC under desired disturbances. Kvernland, Halvorsen, and Skogestad (2010) applied MPC on the Kaibel column that separates a feed stream into four product streams using only a single column. The main objective for optimal operation was to minimize the total impurity flow.…”