Distillation: Major Disturbances & First-Level Control
By Jim Riggs
I
n the first article of this series, Jim discussed the importance and challenges associated with distillation control and the control relevant issues of distillation operations. Here he continues the discussion by presenting the major disturbances affecting composition control and the importance of properly functioning regulatory controls. Please refer to the previous article for terminology and variable definitions.
Disturbances
The type and magnitude of disturbances affecting a distillation column have a direct effect on the resulting product variability. An analysis of the major types of disturbances encountered in distillation columns follows.
•
Feed composition upsets
Changes in the feed composition represent the most
significant upsets with which a distillation control system must
deal on a continuous basis. A feed composition change shifts the
composition profile through the column resulting in a large upset
in the product compositions. Most industrial columns do not have a
feed composition analyzer; therefore, feed composition upsets
usually appear as unmeasured disturbances. When a feed composition
analyzer is available, a feedforward controller can be applied
using the on-line measurements of the feed composition. Because
feed composition changes represent a major disturbance for
distillation control, the sensitivity of potential control
configurations to feed composition upsets is a major issue for
configuration selection.
• Feed flow rate upsets
The flow rates in a steady-state model of a column with
constant tray efficiencies scale directly with column feed rate.
Therefore, ratio control (using L/F, D/F, V/F or B/F as composition
controller output) is an effective means of handling feed flow rate
upsets. Dynamic compensation is normally required to account for
the dynamic mismatch between the response of the product
compositions to feed flow rate changes and the response to changes
in the MVs. When certain ratios (e.g., L/D, V/B) are used as MVs,
these ratios, combined with the level control, automatically
compensate for feed flow rate changes.
• Feed enthalpy upsets
For columns that use a low reflux ratio, feed enthalpy
changes can significantly alter the vapor/liquid rates inside the
column, causing a major shift in the internal composition profile
and, therefore, a significant upset in the product compositions.
This upset can be difficult to identify because (1) most industrial
columns do not have feed temperature measurements and (2) even if a
feed temperature measurement is available, it does not detect feed
enthalpy changes for a two-phase feed. This disturbance may be
difficult to distinguish from feed composition upsets without a
more detailed analysis. It may be necessary to install a feed
preheater or cooler to maintain a constant feed enthalpy to a
column.
• Subcooled reflux changes
When a thundershower passes over a plant, the reflux
temperatures for the columns can drop sharply. Columns that use
finned-fan coolers as overhead condensers are particularly
susceptible to rapid changes in ambient conditions. If internal
reflux control is not applied, severe upsets in the operation of
the columns result because of major shifts in the composition
profiles of the columns. When internal reflux control is correctly
applied, the impact of a thunderstorm on column operations can be
effectively eliminated.
• Loss of reboiler steam pressure
When a steep drop in steam header pressure occurs, certain
columns (those operating with control valves on the reboiler steam
that are nearly fully open) experience a sharp drop in reboiler
duty. This results in a sharp increase in the impurity levels in
the products. When the steam header pressure returns to its normal
level, the composition control system for the column attempts to
return to the normal product purities. Because of the severity of
this upset, if the composition controllers are not properly tuned,
the upset can be amplified by the composition controllers,
requiring the operators to take these controllers off-line to
stabilize the column, greatly extending the duration of the period
of production of off-specification products. This disturbance is,
in general, the most severe disturbance that a control system on a
distillation column must handle and may require invoking overrides
that gradually bring the operation of the column to its normal
operating window instead of expecting the composition controllers
to handle this severe upset by themselves.
• Column pressure upsets
Column pressure has a direct effect on the relative
volatility of the key components in the column. Thus, changes in
the column pressure can significantly affect product compositions.
A properly implemented pressure control scheme maintains column
pressure close to its set point, with only short-term and
low-amplitude departures. A large class of columns (e.g., refinery
columns) is operated at maximum condenser duty to maximize column
separation, which minimizes steam usage. For these cases, the
column pressure increases during the day, when the cooling water or
ambient air temperature is the greatest, and decreases at night,
but the resulting pressure changes are usually slow enough that the
composition controller can efficiently reject this disturbance.
Regulatory Controls
Improperly functioning flow, level or pressure controllers can undermine the effectiveness of the product composition controllers.
• Flow controllers
Flow controllers are used to control the flow rates of the
products, the reflux and the heating medium used in the reboiler
and their set points are determined by the various level and
composition controllers. To assess the performance of a flow
control loop, you can applying block sine waves and comparing these
results for the dead band and time constant with the expected
performance levels.
• Level controllers
Level controllers are used to maintain the level in the
accumulator, the reboiler and the intermediate accumulator of a
stacked column (i.e., a distillation column composed of two
separate columns when there are too many trays for one column).
Loose level control on the accumulator and reboiler has been shown
to worsen the composition control problem for material balance
control configurations (when either D or B is used as a MV for
composition control). When D or B is adjusted, the internal
vapor/liquid traffic changes only after the corresponding level
controller acts as a result of the change in D or B. On the other
hand, if a level controller is tuned too aggressively, it can
result in oscillations passed back to the column and contribute to
erratic operation. When the reboiler duty is set by the level
controller on the reboiler, a level controller that causes
oscillation in the reboiler can also cause cycling in the column
pressure.
• Column pressure controllers
The column overhead pressure acts as an integrator and is
determined by the net accumulation of material in the vapor phase.
Column pressure is controlled by directly changing the amount of
material in the vapor phase of the overhead or by changing the rate
of condensation of the overhead, which converts low-density vapor
to high-density liquid. A variety of approaches can be used to
control column pressure including:
1. using the maximum cooling water flow rate and allowing the column pressure to float at the minimum pressure level (Figure 3).
2. adjusting the flow rate of a refrigerant to the condenser (Figure 4).
3. adjusting the level of liquid in the condenser to change the effective heat-transfer area (Figure 5).
4. venting vapor from the accumulator (Figure 6).
5. venting vapor from or injecting inert gas into the vapor space in the accumulator (Figure 7).
Note that approaches 1 - 3 directly affect the rate of vapor condensation to control pressure while approaches 4 and 5 directly adjust the amount of vapor in the overhead of the column for pressure control.
The fastest-responding pressure control configurations (i.e., the approaches that should provide the tightest control to set point) are vent flow (Figure 6) and vent flow or inert injection (Figure 7).
The override/select controller in Figure 7 uses vent flow when the measured pressure is above set point and uses the injection of an inert gas when the pressure is below set point.
The speed of the pressure control loops based on manipulating the flow of a refrigerant (Figure 4) and adjusting the effective heat-transfer area (Figure 5) respond considerably more slowly because both of these approaches make changes in the rate of heat transfer to change the column pressure.
Operating at minimum column pressure (Figure 3) allows the column pressure to swing with the maximum pressure normally occurring during the afternoon and the minimum pressure occurring early in the morning.
More on Distillation Control
The application of the best high-level approach to distillation control (e.g., model predictive control) will generally be ineffective if the process is not thoroughly understood and the regulatory controls are not implemented properly.
The next article in this series on distillation control discusses the use of product composition measurements in distillation column control and explores single composition control strategies.
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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.
Jim Riggs
Department of Chemical Engineering
Texas Tech University
Lubbock, Texas 79410
Email: jim.riggs@ttu.edu