Hydrocarbon Processing - June 2022 - 17

Process Control, Instrumentation
and Automation
Y. ZAK FRIEDMAN
Petrocontrol
Distillation column DCS control configuration
Is there a consensus about a distillation
column distributed control system (DCS)
control strategy? Much has been written
about this topic, covering configuration
possibilities, constraints and interacting
control considerations. This article proposes
a default DCS configuration to serve
as a basis. Deviations from the basis are
permitted, though reasons for these deviations
should be discussed and understood.
Can one DCS configuration be valid
for most cases? Distillation columns
typically fractionate a mixture of components
into a light boiling fraction (distillate)
and a heavy boiling one (bottom).
The control strategy must first set distillate
yield (or cut) correctly to draw all the light
components in the feed. This is important
because failure to set the cut correctly
would contaminate one of the products.
Once the cut is set, the column vapor
and liquid traffic (or fractionation) must
be set. Separation is not absolute, and
cross-product contamination is a function
of the internal liquid-to-vapor ratio. It is
necessary to define the reflux or reboil ratio
to achieve the desired product purities.
Therefore, distillation control is about manipulating
two handles: cut and fractionation.
Precise cut control is more important
than fractionation. A 1% cut error may
cause significant contamination; whereas,
for fractionation, even a 5% shift in the reflux
ratio is of minor consequences.
A proposed default configuration that
would work for many cases is illustrated
in FIG. 1. This DCS structure is called a
heat balance-type configuration, augmented
by a tray temperature controller
(tray TC). The operator sets the reflux
flow, whereas the distillate draw is on the
accumulator level control. Distillate yield
is determined by the heat balance. If reboiler
heat duty increases, more vapors
will reach the condenser, liquify and be
extracted as distillate.
The operator has no precise knowledge
of the feed composition and no clue what
distillate yield is desirable. Beyond that,
feed composition and throughput may
vary. Tray TC is a rudimentary inferential
controller of the cut. Should the feed become
heavier, the tray would warm, and
the TC would then cut down reboiler heat
duty, reducing distillate yield.
Is a tray TC the perfect inference?
No, but it responds in the right direction
upon feed composition or enthalpy changes.
Without a tray TC, heat balance configurations
are difficult to operate. A more
elaborate inferential model is feasible,
considering more column measurements,
internal vapor and liquid traffic. That
model could be implemented as a part of
the advanced process control (APC) application,
and it would reset the tray TC
setpoint. However, an elaborate inferential
model is not recommended as part of a
default DCS strategy.
What about reflux control? Shouldn't
the default DCS control strategy include
some form of fractionation control? Remember
that automatic reflux control
is not as critical as cut control because
TI
PC
LC
LC
TI
FC
TC
FC
LC
FC
TI
FIG. 1. Heat balance control configuration with stripping section tray TC.
Hydrocarbon Processing | JUNE 2022 17
FC
FC
the operator knows what reflux ratio is
needed for the separation. Slow ratio controller
of reflux to feed or distillate is not
out of the question, although dynamic
considerations suggest that fractionation
control logic should be left for APC. To
illustrate the dynamic issues, consider an
event when feed flow is ramping up. The
tray TC becomes cold, and the controller
works hard ramping the reboiler to correct
the cut. The last thing needed at that
moment is an increase in reflux flow that
would further cool the column. The APC
would permit increasing the reflux only
when the TC is at or above the setpoint.
What about pressure control? The
pressure control method should not affect
the separation control strategy discussed
above, but for completeness, FIG. 1 shows
one popular configuration used for total
condensers where the condenser is located
below the accumulator. This slows
down condenser drainage, which builds
up liquid levels in the condenser, covering
condenser tubes, reducing condenser
effectiveness and increasing the column
pressure. When column pressure is too
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