Composition Estimation In DWC Columns Using Temperature Measurements

Composition Estimation in DWC Columns Using Temperature Measurements

Maryam Ghadrdan¹, Ivar J. Halvorsen² and Sigurd Skogestad¹

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

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

Abstract

In this work, we propose a method to estimate the product compositions in a four product dividing wall column based on a combination of a number of temperature measurements.

Keywords: Kaibel column, Thermally coupled column, composition estimation, combination of measurements

 

Introduction

The Kaibel distillation column is considered as a intensified process which can replace three columns and separate a feed to 4 products. This four product divided-wall distillation column (DWC) contains fully thermally coupled sections built into a single shell. This arrangement is so interesting for strongly reduced energy consumption and construction costs. The tight integration makes it challenging to control, compared to the conventional sequence of simple columns.

It is critical to have a good estimate of product compositions. Reliable and accurate measurement of product compositions is one of the important issues in distillation column control. On-line composition measurement devices are expensive and not very reliable to be used directly in closed loop control and there is usually a considerable time delay that may be a limitation to control performance. Temperature measurements are fast, inexpensive and more reliable and have been used for distillation column control in industry instead of composition analyzers. Mejdell and Skogestad [1] have mentioned different reasons of inefficiency of single temperature measurement in distillation columns in their study. Composition changes in feed, the effect of variation of off-key components, noise in measurement devices, temperature variation due to flow pulses and improper mixing on the trays, pressure changes are named as sources of inefficiency. Some of them can be compensated. The only problem which can not be corrected is to get constant composition by fixing a single temperature some trays away from it. They have suggested a method for estimating the product compositions by measuring temperatures of all trays.

In this work, we propose an alternative approach for designing estimator which is to use the self optimizing control strategy and find a combination of temperature measurements in the Kaibel column. This work is a continuation of the work done by Hori et al. [2]. In this work we will include noise. The number of measurements which result in one control variable depends on the number of temperature sensor locations which are put in the column during construction. By keeping the combination of temperatures constant, we can make sure that the process is optimal even after disturbances occure and therefore the compositions will remain constant as the optimal steady state values. This approach will be compared with the Partial Least Square (PLS) approach proposed previously ([3-4]).

The idea behind self-optimising control is to find a variable which characterise operation at the optimum, and the value of this variable at the optimum should be less sensitive to variations in disturbances than the optimal value of the remaining degrees of freedom. Thus if we close a feedback loop with this candidate variable controlled to a setpoint, we should expect that the operation will be kept closer to optimum when a disturbance occur.

 Self-optimizing control is when we can achieve an acceptable loss L with constant setpoint values c, for the controlled variables (Skogestad 2000).

Process Description

As mentioned above, the divided-wall distillation column (DWC) contains fully thermally coupled sections built into a single shell. The DWC is capable of separating three or four products with a single reboiler and condenser. The Kaibel column, which is a 4-product DWC, is shown in Figure 1. The two lightest and the two heaviest products are supposed to be separated in the prefractionator and the products are separated further and drained in the main column.

The temperature at a stage in a distillation column is a good indication of its composition. Skogestad [5] presents some benefits of using temperature loops for controlling the composition:

1. Stabilizes the column composition profile along the column

2. Gives indirect level control: Reduces the need of level control

3. Gives indirect composition control: Strongly reduces disturbance sensitivity

4. Makes the remaining composition problem less interactive and thus makes it possible to have good two-point composition control

5. Makes the column behave more linearly 

 

 The model has six degrees of freedom: boilup rate (V), reflux (L), side stream flows (S1, S2), liquid split (Rl) and vapour split (Rv), from which four will be used to keep the product compositions constant. There will remain two manipulated variables which are used as optimization variables.

Figure 1. Schematic of a 4-product dividing wall column

 

The model used for this study is simulated in UNISIM. The feed stream is an equimolal mixture of Methanol, Ethanol, 1-Propanol, 1-butanol and saturated liquid. The optimal boilup is somewhat higher than the theoretical minimum boilup derived from minimum energy diagram proposed by Halvorsen et al. [6] which is with the assumption of infinite number of stages. This value is used as an initial estimate of the energy needed for a specified separation. All the optimal operating points for different sets of the disturbances are found by applying an optimisation solver in MATLAB with the full non-linear model in UNISIM.

References

1.   Mejdell, T., Estimators for Product Composition in Distillation Columns. 1990, Norwegian University of Science and Technology.

2.   Hori, E.S., S. Skogestad, and V. Alstad, Perfect Steady-State Indirect Control. Ind. Eng. Chem. Res., 2005. 44: p. 863-867.

3.   Mejdell, T. and S. Skogestad, Composition Estimator in a Pilot-Plant Distillation Column Using Multiple Temperatures. Ind. Eng. Chem. Res., 1991. 30: p. 2555-2564.

4.   Weber, R. and C. Brosilow, The use of secondary measurements to improve control. AIChE J. , 1972: p. 614-623.

5.   Skogestad, S. and I. Postlethwaite, Multi-variable Feedback Control, Analysis and Design (2nd Edition). Wiley 2007.

6.   Halvorsen, I.J. and S. Skogestad, Minimum Energy for the four-product Kaibel-column in AIChE Annual meeting 2006. 2006: San Francisco p. 216d