Control Structure Design for Optimal Operation of Thermally Coupled Columns

Control Structure design for optimal operation of thermally coupled columns

Deeptanshu Dwivedi¹, Ivar J. Halvorsen² and Sigurd Skogestad¹

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

² SINTEF ICT, Applied Cybernetics, N-7465 Trondheim, Norway, Email: Ivar.J.Halvorsen@sintef

Keywords: Thermally coupled Distillation Columns, Optimal Operation, Control Structure Design, Divided Wall Column

Abstract:

The dividing wall column (DWC) has gained increased industrial attention due to its energy and capital-saving properties. To obtain the energy-savings in practice, it is required to apply a control strategy that keeps the operation close to the optimal operation in presence of unknown disturbances and model uncertainties. Otherwise, the potential energy savings may easily be lost or lower purity may result, especially in the side stream product. The three-product DWC has been used in more than hundred industrial applications.  As we increase the complexity by going to the 4-product Kaibel column and even the 4-product DWC with two partition walls, and use the column to separate feeds with a large number of components which shall be grouped into suitable product streams, the question of control design and control performance will become even more important. 

Minimum energy operation and tight control are closely related. In a conventional two product column, the most common control strategy is a simple one-point temperature control. With a large the energy input, in event of usual disturbances during operation, this very simple strategy can ensure purity in both products.  This however, usually results in either both or one of the products is over purified.  To obtain minimum energy operation, it is well known that both products should be controlled to their purity specifications. This is actually a more difficult mode of operation.

This work aims to demonstrate the control structure design and performance by operation of a four- product lab scale Kaibel column using the experimental runs followed by modelling and identification. A separation of mixture of four alcohols: methanol, ethanol, propanol and butanol are studied using the lab column.  The lab column is a made of standard vacuum glass sections with 50 mm internal diameter and filled with 6 mm Rachig rings. The height of the set-up is about 8 m. There are 24 temperature sensors inside the column and the column is operated using a Labview interface. The column has a magnetic funnel that divides the liquid between the pre-fractionator and the main column. The vapour split is affected using two motor operated butterfly valves, one each in the pre-fractionator and in the main column. The reboiler is a 2 KW kettle type electrical heater.

A control structure using four decentralised PI temperature controllers is proposed. One temperature in the pre-fractionator and three temperatures in the main column were controlled using four manipulated variables namely liquid split ratio and 3 product flow rates: distillate, side product 1 and side product 2. The column was successfully stabilized and all the four temperature were controlled within ±0.5 C of the set-point. The product samples were collected and analyzed. The preliminary experiments yielded significantly pure top and bottom product while the two side products were not very pure. A suitable temperature profile should be maintained using the 4 PI decentralized loops to improve the purities. This is being further studied using a rigorous model of the same and lab experiments.

An effective vapour split is critical during minimum energy operation of the column. The vapour split valves were examined and an experiment was designed to test their efficacy. The column was run under total reflux conditions using only two components: methanol and ethanol. The vapour split valves are operated under split range control to set a constant temperature difference between a sensor in the pre-fractionator and one in the main column.  This control objective was successful tested for servo and regulatory performance.

The model of the lab setup is a rigorous first principle equilibrium stage based model. There is no vapour hold up and model accounts for non-ideality in liquid phase using Wilson equation. The number ideal stages in the real column is determined experimentally by doing a total reflux experiment and using only two components, methanol & and ethanol. The compositions of the product sample were then analyzed to determine the number of ideal stages using Fenske Equation.  The next step in the identification was to manipulate the inputs in the model to match the experimental conditions and data reconciliation. Here we fit the steady state temperature profiles, and the compositions of the product samples collected during the steady conditions of operations of the lab column. An optimization problem was formulated for the same [2]. A good fit of temperature snap shot from the steady operation condition of the experiment was obtained. The dynamic response of the experiments can be further fit using other parameters of the model like stage hold up and sizing.

This model is then used to test the efficacy of control structure for its regulatory response. The Kaibel column, owing to its highly interactive nature has some unique control problems and demand novel control solutions [3]. The key is to stabilize each internal sub-column profile to ensure that the separation task in each sub-column is carried out properly in an event of disturbances like feed property variations, tray/packing performance variations, measurement uncertainties and uncertainties in setting the internal flow splits. This can be done by use of feedback control, based on knowledge of how the internal sub-columns should behave at and around the minimum energy operating region. 

Here we will show a procedure based on characterizing the optimal operating region.  The V-min diagram [4, 5] is a simple tool that can be used to assess both design and operation issues.  It can be point out the overall minimum energy requirement and flow distribution in fully thermally coupled arrangements. Further, we also get information on allowable variation margins in operation of the internal sections or sub-columns of the arrangement without compromising the overall minimum energy operation. This is very important for practical implementation. Due to inevitable design uncertainties and unknown disturbances an industrial column control system has to be designed to allow for certain slacks and the information about how and where operation can be relaxed is important in order to design the control strategy. For instance, an allowable range for vapour splits, allowable variation in feed properties for a selected control strategy, or if the focus should be on controlling the upper or lower part of the pre-fractionator profile.  By identifying the key operation parameters that need high attention and thereby accurate control, we obtain a system that can give high purity in all products with a simple feedback control strategy.

 References:

1. Kaibel, G. (1987). Distillation Columns with vertical partitions, Chem. Eng, Tech. 10 (1987) 92-98

2. Lid, T. Skogestad, S (2008). Scaled steady state models for effective on-line applications, Comp. & Chem. Engg.Vol. 32, Issues 4-5, April 2008, Pages 990-999

3. Strandberg, J., Skogestad, S., Halvorsen I. (2010), Practical control of dividing wall column, Distillation & Absorption (Eindhoven, Netherlands)

4. Halvorsen, I., Skogestad, S. (2006), Minimum Energy for the four-product Kaibel-column, AICHE Annual meeting, 216d

5. Halvorsen, I., Skogestad, S. (1999), Optimal Operation of Petlyuk Distillation: Steady State behaviour, Journal of Proces. Control, vol 9, 1999, 407-424