By Jim Riggs1
In the first article of this series, Jim discussed control relevant issues associated with distillation columns. The second article presented the major disturbances affecting composition control and the importance of properly functioning regulatory controls. The third article discussed the use of product composition measurements in distillation column control and explored single composition control strategies.
This final segment of the series considers advanced issues including dual composition control and constraint control.
Dual Composition Control
The choice of the proper configuration for dual composition control is a more challenging problem than for single composition control because there are more viable approaches and the analysis of performance is more complex.
There is a variety of choices for the manipulated variables, or MVs, including L, D, L/D, V, B, V/B, B/L, and D/V, that can be paired to the four control objectives (x, y, reboiler level and accumulator level). As a result, there are a large number of possible configuration choices although most of them are not practical.
It is assumed that the choice for the control configuration for the column pressure (Figures 3-7) is made separately from the selection of the composition control configuration.
If we limit our choices to L, D and L/D for controlling y and V, B and V/B for controlling x, there are nine possible configurations to consider: (L,V), (D,V), (L/D,V), (L,B), (D,B), (L/D,B), (L,V/B), (D,V/B) and (L/D,V/B). In each configuration, the first term is the MV used to control y and the second term is used to control x.
Figure 10 shows the (L,V) configuration. The set point for the reflux flow controller is set by the overhead composition controller and the set point for the flow controller on the reboiler duty is set by the bottom composition controller. This leaves D to control the accumulator level and B to control the reboiler level.
Figure 11 shows the (D,B) configuration where D is adjusted to control y and B is changed to control x, which leaves L for the accumulator level and V for the reboiler level.
Consider the classifications of these nine control configurations. The five configurations that use either D or B as a MV for composition control are referred to as “material balance configurations” because they use the overall material balance for the column to adjust product compositions. In fact, the (D,B) configuration has been referred to as the super material balance configuration.
The four configurations that do not use D or B as MVs are known as “energy balance configurations” because they directly adjust the vapor/liquid traffic in the column for composition control. The (L/D,V/B) configuration is known as the “double ratio configuration.”
The major factors affecting the composition control performance of a particular configuration are coupling, sensitivity to disturbances and the response time for changes in the MV. The most commonly used configuration is the (L,V) configuration because it provides good dynamic response, is the least sensitive to feed composition disturbances and is the easiest to implement, even though it is highly susceptible to coupling.
On the other hand, the (L/D,V/B) configuration is, in general, the least affected by coupling and has good dynamic response, but is quite sensitive to feed composition disturbances and is more difficult to implement. The (D,B) configuration has advantages for certain high-purity columns if the levels are tuned tightly, but is open-loop unstable, i.e., non-self-regulating.
As a result, there is no clear choice for the best configuration for dual composition control of distillation columns. In fact, there are specific cases for which each of the nine potential configurations listed earlier provides the best control performance.
While it is not possible to a priori choose the optimum configuration, there are some guidelines that can reduce the possibility of choosing a poor configuration for a particular column. In general, for high reflux ratio cases (L/D > 8), configurations that use material balance MVs (D and B) or ratios (L/D and V/B) are preferred while for low reflux ratio cases (L/D < 5), configurations that use energy balance MVs (L and V) or ratios are preferred.
In many cases, the control of one of the two products is more important than control of the other. For these cases, when the overhead product is more important, L is usually the best MV. When the bottoms product is more important, V is the proper MV. If the column is a high reflux ratio column, the MV for the less important product should be a material balance knob (D or B) or a ratio (L/D or V/B).
Because a C3 splitter is a high reflux ratio column and control of the overhead product is more important, the (L,B) or (L,V/B) configuration is preferred, which is consistent with simulation studies that have been performed. If the column is a low reflux ratio column, the less important product should be controlled by an energy balance knob (V or L) or a ratio (L/D or V/B). For example, for a low reflux column for which the bottom product is more important, the (L,V) or (L/D,V) configuration is preferred.
Table 2 (click for large view) summarizes the recommended control configurations for columns in which one product is more important than the other.
Composition Controller Tuning
For most distillation columns, the effective dead-time-to-time constant ratio is relatively small. Therefore, derivative action is not necessary and PI composition controllers are commonly used. When inferential temperature control is used, fast-acting temperature loops with significant sensor lag may require derivative action because of their effective dead time-to-time constant ratios. Because most composition and temperature control loops are relatively slow responding, ATV identification with on-line tuning is recommended.
When one product is more important than the other, it is best to first tune the less important loop loosely (i.e., less aggressively tuned, e.g., critically damped) and then tune the important loop to control to set point tightly (e.g., a 1/6 decay ratio). This dynamically decouples the multivariable control problem by providing relatively fast closed-loop dynamics for the important loop and considerably slower closed-loop dynamics for the less important loop.
In this manner, the coupling effects of the less important loop are slow enough that the important loop can easily absorb them. As a result, the variability in the important loop can be maintained consistently at a relatively low level. In effect, this approach approximates the performance of single composition control without allowing the less important product composition to suffer large offsets from set point.
When the importance of the control of both products is approximately equal, both loops need to be tuned together. In this case, both loops must be detuned equally to the point where the effects of coupling are at an acceptable level.
Some of the most common column constraints include:
Maximum reboiler duty
This constraint can result from (1) an increase in the column pressure that reduces the temperature difference for heat transfer in the reboiler; (2) fouled or plugged heat-exchanger tubes in the reboiler that reduce the maximum heat transfer-rate; (3) an improperly sized steam trap that causes condensate to back up into the reboiler tubes; (4) an improperly sized control valve on the steam to the reboiler that limits the maximum steam flow to the reboiler; or (5) an increase in the column feed rate such that the required reboiler duty exceeds the maximum duty of the reboiler.
Maximum condenser duty
This constraint can be due to (1) an increase in the ambient air temperature that decreases the temperature difference for heat transfer; (2) fouled or plugged tubes in the condenser that reduce its maximum heat duty; (3) an improperly sized coolant flow control valve; (4) an increase in coolant temperature; or (5) an increase in column feed rate such that the required condenser duty exceeds the maximum duty of the condenser.
Kister discusses in detail the three types of flooding, showing that each type results from excessive levels of vapor/liquid traffic in the column.
Weeping results when the vapor flow rate is too low to keep the liquid from draining through a tray onto the tray below.
Maximum reboiler temperature
For certain systems, elevated temperatures in the reboiler can promote polymerization reactions to the level that excessive fouling of the reboiler results. A reboiler duty constraint can be identified when the steam flow control valve remains fully open and the requested steam flow is consistently above the measured flow rate.
Condenser duty constraints are usually identified when the column pressure reaches a certain level or when the reflux temperature rises to a certain value. The onset of flooding or weeping is generally correlated to the pressure drop across the column or across a portion of the column. It should be emphasized that it is the reboiler duty that is adjusted to honor each of these constraints, i.e., prevent the process from violating the constraints.
Three approaches can be used for constraint control of a distillation column:
1. Convert from dual composition control to single composition control.
2. Reduce the column feed rate to maintain the purity of the products.
3. Reduce the product impurity setpoints for both products.
Figure 12 shows how a low select can be used to switch between constraint control and unconstrained control for a maximum reboiler temperature constraint when the overhead is the more important product. When the bottom product is more important, the combined constrained and unconstrained configuration is more complicated, with more potential configuration choices.
Figure 13 shows a control configuration for observing the maximum reboiler temperature constraint when the bottoms product is more important. Note that in this case, the reflux flow rate is used to maintain the bottom product composition while the reboiler duty is used to maintain the reboiler temperature. In this configuration, the reboiler temperature control loop acts relatively quickly while the bottoms composition loop is slower acting because reflux flow is used as the MV.
The advantage of model predictive control (MPC) applied to distillation columns is greatest when MPC is applied to a series of columns, e.g., an entire separation train. This results because the MPC controller can efficiently operate the system of columns against the operative constraints to maximize the throughput for the system.
For a typical industrial separation train, there is a large complex set of constraints that limit the overall process throughput. As a result, there is a large number of combinations of operative constraints that must be controlled if the controller is to maximize the throughput for the full range of operation. Advanced PID constraint controllers applied to this problem require a separate control configuration for each combination of operative constraints, resulting in an excessively large number of separate control configurations.
On the other hand, an MPC controller can directly handle the full range of combinations of operative constraints with a single MPC controller. In addition, the MPC controller is much easier to maintain than a custom-built advanced PID controller for such a complex system. MPC can provide significant control improvements over PID control for a single column in most cases, but these improvements pale in comparison to the economic advantages offered by applying MPC to large-scale processes.
Keys to Effective Distillation Control
For effective distillation control, it is imperative to take care of the basics first.
1. Ensure that the regulatory controls are functioning properly.
2. Evaluate the analyzer dead time, reliability and accuracy.
3. Check that RTDs or thermistors are being used to measure tray temperatures for composition inference and that they are correctly located. Also, ensure that pressure-corrected tray temperatures are used.
4. Use internal reflux controls for changes in reflux temperature.
5. When L, D, V and B are used as MVs for composition control, ratio them to the measured feed rate when column feed rate changes are a common disturbance.
For configuration selection, use the (L,V) configuration for single composition control. For dual composition control, use an energy balance configuration for low reflux ratio cases and use material balance or ratio configurations for high reflux ratio columns. For many dual composition control cases, the control of one product is much more important than the other. For such cases, you should use L as the MV when the important product is produced in the overhead and V when it is produced in the bottoms.
Additionally, the less important product should be controlled using an energy balance knob (L or V ) or a ratio knob (L/D or V/B) for low reflux cases or D, L/D, B or V/B for high reflux columns. For these cases, it is important to tune the important loop tightly and tune the less important loop much less aggressively.
Finally, override and select control should be applied to ensure that all column constraints are satisfied when they become operative.
1. This material is reprinted from Chemical Process Control, 2nd Ed. with the permission of the publisher: Ferret Publishing (806 747 3872).
About the Author
Jim Riggs is a professor of chemical engineering at Texas Tech University, where he has been since 1983. He has served as an industrial consultant and presented a number of industrial short courses on various topics relating to process control. He is the author of several popular chemical engineering textbooks and co-founded the Texas Tech Process Control Consortium in 1992.
Department of Chemical Engineering
Texas Tech University
Lubbock, Texas 79410