Chemical Engineering August 2014 - 51

Block column 1: column grand composite curve (T-H)
Block column 1: column grand composite curve (stage-H)
210
205
200
195
190
185
180
175
170
165
160
155
150
145
5
10
Enthalpy deficit MW
Figure 4. This CGCC (T-H) illustrates the duty reduction potential
for design option 2. Note the much slimmer duty reduction
potential, as compared to Figure 3
gap between the T-H CGCC pinch
point and the ordinate, which represents
the surplus of heat during the
separation process. In order to reduce
condenser and reboiler loads,
reflux ratio can be reduced, while
increasing the number of stages
to sustain an adequate degree of
separation. Comparing Figure 3
and Figure 4 demonstrates how T-H
CGCC are applied in evaluating
different design options to see how
reducing the reflux ratio affects the
condenser and reboiler duties. They
illustrate how changing the reflux
ratio from 7.7 to 1.3, while increasing
the number of stages from 15 to
30, results in a 23.5 MW reduction
in condenser and reboiler duties
[2].
Either S-H or T-H CGCCs can
be applied in finding the appropriate
range of modifications to the
feed quality. T-H CGCC plots will
display sharp enthalpy changes either
on the reboiler side or the condenser
side, depending on whether
the feed is excessively sub-cooled,
or overheated, respectively. It is
also worth noting that changes
in the heat duty of pre-heaters or
pre-coolers will lead to analogous
15
20
Ideal profile
Actual profile
26
21
Sharper enthalpy change
on the reboiler side
16
Ideal profile
Actual profile
31
11
6
2
4
6
8
10
12
14
Enthalpy deficit MW
Figure 5. This CGCC (S-H) shows a sharper enthalpy change
on the reboiler side
changes in the column reboiler or
condenser, based on the same principle.
Figure 5 displays a sharper
enthalpy change on the reboiler
side in the S-H CGCC plot, which
would lead an engineer to a conclusion
that design could benefit
from adding a pre-heater. A table
of the simulation software results
is then used to examine the effects
of adding a pre-heater, resulting in
reduced reboiler duty to the temperature
levels at which the hot
utility (for the reboiler and for the
pre-heating the feed) is required to
be provided [2].
Even though feed conditioning is
a more desirable way to reduce utility
costs, adding a side condenser
or a reboiler can also provide a way
to accomplish this goal. The goal of
placing a side reboiler or a condenser
is to allow heating or heat removal
using a cheaper hot or cold utility,
respectively. Side condensing or
side reboiling provides an external
way to modify column design, and
is typically used when it provides a
more convenient temperature level.
Analyzing the T-H CGCC plots helps
identify the range for side condensing
or reboiling. Engineers look at
the area below or above the pinch
point, that is, the area between the
ideal and enthalpy profiles. A side
condenser can be placed if there is
a significant area below the pinch
point, and a side reboiler can be
used in the opposite case. Figure 6
illustrates a base case. There is a
large area between the actual and
ideal profile above the pinch point,
which leads to a design modification
of adding a side reboiler to the basecase
design. The resulting slimmer
area between the ideal and the actual
profile is shown in Figure 7,
where a side reboiler with a duty of
approximately 6.5 MW was added
at stage 22. The addition of the side
reboiler not only leads to a reduction
in the heat duty of the main
reboiler, it also helps reduce the hot
utility [2]. However, one should keep
in mind the capital cost of adding a
side reboiler or condenser.
Exergy loss analysis is a complementary
tool that is used evaluate
the design modifications mentioned
above [2]. Exergy loss profiles for
different design options can be compared
to determine which design
is optimal. Figure 8 compares two
design options using exergy proChemiCal
engineering www.Che.Com august 2014 51
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Temperature, °C
Stage
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Chemical Engineering August 2014

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