(107h) 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