Hydrocarbon Processing - June 2022 - 18

Process Control, Instrumentation and Automation
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TC
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FIG. 2. Tray TC used to manipulate reflux.
high, hot vapor bypass is reduced and accumulator
pressure decreases, encouraging
condenser drainage, exposing more
of the condenser area and increasing
condensation to reduce the column pressure.
For this concept to work, column
pressure must be higher than accumulator
pressure, which is accomplished by placing
the condenser below the accumulator.
PROCESS AND
MEMBRANE
TECHNOLOGY
Emission control
(e. g. VRUs) for
terminals, refinieries
and marine loading
www.borsig.de
BORSIG GmbH
EMISSION CONTROL
Phone: +49 (0) 7623 96609-0
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
On what tray should the TC be located?
FIG. 1 does not specify whether
the TC should be placed near or far from
the reboiler. The purpose of distillation
is to separate a light component, called
a light key (LK), from a heavier component,
called a heavy key (HK). Many
columns deal with multiple components,
some lighter than LK and others heavier
than HK. Lighter components would
evaporate to become part of the distillate,
whereas heavier components would sink
to the bottom.
Each column has a composition profile
where vapor and liquid composition
become heavier near the reboiler. While
bottom product specification may call
for 1% or fewer LK contamination, the
tray TC vapor stream would have a much
higher content of LK, ideally 15%-20%
for cut control sensitivity. The tray temperature
is a function of composition, and
upon change, the tray TC would manipulate
the reboiler to correct the tray composition.
Therefore, the tray TC works by
keeping the tray composition constant.
The tray TC location is often determined
before control engineers state a pref18
JUNE 2022 | HydrocarbonProcessing.com
erence. Non-ideal locations would hinder
the tray its usefulness as a cut control tool.
What about a tray TC located in
the rectifying section? Dynamically
manipulating the reboiler to control a
rectifying section tray temperature is
challenging. It is preferred that the TC
manipulates reflux flow rather than reboiler
duty, as shown in FIG. 2. From a heat
balance point-of-view, reboiler manipulation
is like reflux manipulation. Reducing
the reflux would increase distillate yield.
Ideally, that TC would be located on a tray
where the HK content of the tray liquid
would be 15%-20%.
This simple configuration serve as a
default configuration for a distillation
column DCS strategy. It is clean, simple,
non-interactive, easy to operate and keeps
the product's purity near-constant. Upon
receiving lab results indicating that one
product is too clean and the other is too
contaminated, the operator would reset
the TC setpoint to correct the cut. Conversely,
when both products are either too
clean or too contaminated, the operator
would manipulate the reflux flow setpoint
to correct the fractionation.
Y. ZAK FRIEDMAN is a Principal
Consultant with Petrocontrol. He has
practiced APC for most of his career,
specializing in using first-principles
models for inferential process
control and has developed inferential
models to fit many hydrocarbon
processes. Dr. Friedman's experience spans more than
50 yr, working with Exxon Research and Engineering,
KBC Advanced Technology and Petrocontrol. He holds
a PhD degree from Purdue University.
TI
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Hydrocarbon Processing - June 2022

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