Chemical Engineering August 2014 - 50

Engineering Practice
Block column 1: column grand composite curve (stage-H)
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Feed stage 3
Feed stage 7
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Enthalpy deficit MW
Figure 2. An example of using CGCC (S-H) to compare two
design options with different feed-stage locations: stage 3
versus stage 7 (Green is the actual profile, red and blue are the
ideal profiles)
For each stage of the column, the
software simultaneously solves
equations for equilibrium and operating
lines for automatically selected
light and heavy key components
[1-3].
Software generates stage-versusenthalpy
(S-H) and temperatureversus-enthalpy
(T-H) profiles for a
column that represent the theoretical
minimum cooling and heating
requirements in the temperature
range of separation. These curves,
called column grand-composite
curves (CGCC) are used to identify
potential objectives that can be accomplished
by various column modifications
[1-3].
In addition to thermal analysis,
software can be used to perform
exergy analysis. Software calculates
exergy loss at each stage
of the column, accounting for all
material and heat streams entering
and leaving the process. Exergy
loss profiles are generated as
a result, and they can be used to
study how certain factors, such as
momentum loss (pressure driving
force), thermal loss (temperature
driving loss), and chemical potential
loss (mass-transfer driving
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Ideal profile
Actual profile
Block column 1: column grand composite curve (T-H)
Scope for reduction in
condenser and
reboiler duties
~23 MW
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Enthalpy deficit MW
Figure 3. This CGCC (T-H) illustrates the duty reduction potential
for design option 1
force), affect the losses in work potential
due to the irreversibility of
real processes [1-3].
Using thermal analysis results
Engineers use the CGCC and exergy
loss profiles, generated during
thermal analysis, to analyze potential
modifications to columns [1-3].
The CGCC are used to analyze
how energy efficiency can be improved
by a number of column modifications,
including changing feed
location, reflux ratio modifications,
heating or cooling feed conditions,
and adding a side cooler or heater.
Software can produce the CGCC for
temperature versus enthalpy, and
stage versus enthalpy with a click
of a button.
Exergy loss profiles are also easily
generated to analyze exergy loss
at different stages or temperatures,
and these plots can be generated
with a click of the button as well.
Stage-versus-enthalpy plots are
useful in identifying opportunities
for feed-stage location modifications.
Engineers can easily identify
irregularities that resulted from
incorrect feed placement with the
obvious projections at the feed lo50
ChemiCal engineering www.Che.Com august 2014
cation (pinch point) on the stageversus-enthalpy
CGCC (Figure 2).
These projections are a manifestation
of a need for extra local reflux
to make-up for the unsuitable feed
placement. A feed can be introduced
either too high up in the column,
or too low in the column, resulting
in a sharp enthalpy change on
the condenser or reboiler sides on
the S-H CGCC, respectively. This
will guide engineers in whether it
is most appropriate to place the
feed stage down or up the column,
in order to eliminate such distortions
and reduce condenser and
boiler duties. Figure 2 compares
two design options with feed stage
located at stage 3 and stage 7. The
S-H CGCC shows a noticeable projection
on the condenser side at
the pinch point located between
stages 2 and 3. The corrected S-H
CGCC that resulted in moving the
feed stage down the column is also
shown in Figure 2 [2].
Temperatures versus enthalpy
plots are useful in identifying opportunities
for reflux-ratio modifications.
The potential for reductions in
condenser and reboiler heat duties
is characterized by the horizontal
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Stage
Temperature, °C
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Chemical Engineering August 2014

Table of Contents for the Digital Edition of Chemical Engineering August 2014
