Chemical Engineering August 2014 - 49

Feature Report
Engineering Practice
Distillation Column Thermal Optimization:
Employing Simulation Software
Applying process simulation
software in distillationcolumn
design and operational
analysis can lead to significant
reductions in operational
and maintenance costs and
improved column performance
Irina Rumyantseva and Ron Beck
Aspen Technology
S
everal market conditions, including
availability of light
crude oilds in the U.S., low
natural-gas prices and improving
macroeconomics, are driving
debottlenecking projects related to
columns in both petroleum refining
and chemical settings. Fortunately,
the process-simulation tools that
can be used to perform the front-end
of these projects have advanced significantly
in the past several years.
Distillation columns present
one of the most challenging design
and operational challenges in most
chemical and petroleum-refining
processes. According to the U.S.
Dept. of Energy, over 40,000 distillation
columns are involved in
plant operations in the chemical
and petrochemical industries in
North America, and they consume
approximately 40% of the total energy
used [1].
The hydraulics and phase behavior
of chemicals within a column
are complex and are dependent on
a number of geometrical and maintenance
factors, making it difficult
to optimize column performance by
hand calculations. Thus, industry is
employing process-simulation software
in order to optimize energy
use in columns and pinpoint potenFigure
1. Thermal and hydraulic column analysis in advanced processsimulation
software can be initiated with a mere check mark selection
tial column modifications to implement
in column design and retrofitting.
Modern process-simulation
software has all the tools necessary
to maximize column energy efficiency,
reduce utilities cost, improve
thermodynamic driving forces to reduce
capital investment, and aid in
column debottlenecking. Engineers
routinely use process-simulation
software to perform thermal and
hydraulic analyses (Figure 1) of columns
[1-3].
This article focuses on applying
software to perform thermal and
exergy analyses of columns in conjunction
with pinch analysis of the
entire process for optimized conceptual
designs and improved plant
operations. One of the key values
of using process-simulation software
in column design and troubleshooting
is the ability to screen and
evaluate a variety of process-configuration
options and operating conditions
rapidly. As is also described
in this article, some simulators can
provide integrated economic evaluation
so the costs of different alternatives
can be contrasted. Further,
steady state simulation can be augmented
by modeling in a dynamic
mode to look in more detail at the
column behavior.
Thermal and exergy analyses
Process simulation can be applied
when performing process design
or retrofit analysis to identify potential
design modifications to improve
energy efficiency and reduce
energy consumption.
The simulation compares column
performance based on user inputs
to the column performance in
thermodynamically ideal columnoperating
conditions, which are
determined theoretically, based on
the practical near-minimum thermodynamic
condition (PNMTC)
approximation [3].
PNMTC assumes thermodynamically
reversible distillationcolumn
operation, where reboiling
and condensing loads are evenly
distributed over the operating
temperature range of the column.
This ideal column is assumed to
operate at minimum reflux, has an
infinite number of stages, and each
stage has heaters and coolers with
proper heat loads, so that the column-operating
line coincides with
the equilibrium line at any given
stage. This approximation also
accounts for losses and inefficiencies
caused by column-design parameters,
such as pressure drops,
multiple side-products and so on.
ChemiCal engineering www.Che.Com august 2014 49
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Chemical Engineering August 2014

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