Chemical Engineering August 2014 - 52

Engineering Practice
Block column 1: column grand composite curve (stage-H)
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Scope for side
reboiling
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Enthalpy deficit MW
Figure 6. This CGCC (T-H) displays a large area between
the actual and ideal profile above the pinch point, which helps
identify an opportunity for design modification of adding a side
reboiler with heating duty of 6.5 MW to the base-case design
files: one with sub-cooled feed, and
one with pre-heated feed. Note the
higher exergy loss at the feed stage
of Design 1, where the feed is subcooled.
Design 2 shows a significantly
reduced exergy loss by preheating
the feed.
Hydraulic analysis can be used to
together with the thermal analysis
to remove possible bottlenecks in
distillation processes and increase
energy efficiency of the column [2],
but that subject is not covered in
this article.
Pinch analysis
Thermal analysis of column operation
is an important tool in guiding
decisions in column modifications;
however, to achieve best results,
it is important to use this tool in
conjunction with and concurrently
with other tools, in an " integrated
context " [2]. When the processsimulation
model is applied simultaneously,
and is closely integrated
with software that looks at the heat
exchanger network (HEN) and performs
pinch analysis of the entire
process, taking into account other
process heat sources and sinks,
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Enthalpy deficit MW
Figure 7. The CGCC (T-H) after an addition of a side reboiler
that displays a much slimmer area between the ideal and actual
enthalpy profiles
faster and more effective process
modeling results can be achieved
[2]. Software capable of performing
pinch analysis can also be applied
when analyzing the best ways to integrate
column modifications with
the plant utility system [2].
When used separately and iteratively,
it is a painstaking task to
rebuild the process heat network
based on the process model. However,
when used in an integrated
mode, process modifications can
be made and the HEN can be rapidly
regenerated to look at the energy
impact of various alternatives
quickly. Other important factors to
keep in mind when performing energy
optimization of columns are
the greenhouse gas (GHG) emissions.
For example, installing a side
reboiler might lead to increased
GHG emissions, due to increases in
utility consumption [1]. There are
tools available to the industry that
perform pinch analysis of the entire
process, looking for ways to improve
heat integration by comparing
ideal design to the actual design
and suggesting improvements. At
least one commercial software tool
52 ChemiCal engineering www.Che.Com august 2014
(which incorporates an optimization
algorithm that evaluates many
heat integration opportunities and
selects the best ones) can deliver
suggestions for overall energy optimization
with a click of a button,
while computing utility cost savings
potential and greenhouse gas emissions
savings potential.
Such a tool automatically determines
what types of utilities can
be used for various plant processes
and compares them to what is actually
being used, displaying potential
savings percentages. For example, a
cooler might be set to use a refrigerant
in the base case, but cooling
water, a cheaper utility, can be used
instead. Each utility is associated
with a certain GHG-emission contribution,
and that is calculated together
with utility consumption as
well. The user would just need to
specify the appropriate utility costs
and net carbon tax.
Evaluate design/retrofit options
Some of the design modifications
resulting from the thermal column
analysis are actually not beneficial
to the process. For example, as
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Ideal profile
Actual profile
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Ideal profile
Actual profile
Block column 1: column grand composite curve (T-H)
Temperature, °C
Temperature, °C
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Chemical Engineering August 2014

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