Chemical Engineering December 2013 - 54

Feature Report
Engineering Practice
Optimization of a Steam
Network
HP steam
generators
(process)
HP steam header
Proper configuration of
steam-turbine drivers
affects power generation
in petroleum refineries
C.C.S. Reddy and S.V. Naidu
Andhra University
G.P. Rangaiah
National University of Singapore
P
etroleum refineries generate
power from a number of
sources, including cogeneration
plants and steam networks.
Generally, steam networks use turbines
to improve operational reliability,
specifically as drivers for rotating
equipment. Due to increasing
energy costs, it is essential to maximize
the power-generation capabilities
of steam networks. This article
presents a spreadsheet-based optimization
model for a steam network.
Figure 1 shows a typical steam network
in a petroleum refinery.
For maximizing the total power
generation, a mixed-integer linear
programming (MILP) model
is formulated by linearization of
equipment models and use of a
spreadsheet for estimating steam
properties. The solution of this model
determines the optimum operational
configuration of steam-turbine drivers
for maximum power generation,
while satisfying constraints related
to material balances, equipment selection
and capacity limitations. One
case study is presented to demonstrate
the potential and benefits of
the spreadsheet-based approach for
optimizing steam networks.
Due to the large volume of steam
required for power generation, and
the various turbine equipment
To electrical
substation
Vent
HP steam
consumers
(process)
MP steam header
Vent
MP steam
generators
(process)
Generator
MP steam consumers
(process)
LP steam generators
(process)
LP steam header
LP condenaste header
Cooling
water
LP steam consumers
(process)
Cold condensate header
Vent
Condensate flash drum
Deaerator
DM water header
Figure 1. A typical petroleum refinery steam network features equipment such as
turbines, generators, boilers and supply headers
HP steam
mHL, HHP
W
mHL, HHP
LP steam
Pump or
compressor
Back pressure steam
turbine driver (HP to LP)
mHM, HMP
MP steam
HP steam
mHM, HHP
W
Pump or
compressor
Back pressure steam
turbine driver (HP to MP)
mML, HLP
LP steam
MP steam
mML, HMP
W
Pump or
compressor
Back pressure steam
turbine driver (MP to LP)
Figure 2. There are three different types of back-pressure steam-turbine drivers,
based on their configuration relative to steam supply headers
types, steam-system considerations
have been the subject of many studies
over the years [6,7,10]. However,
almost all of these studies focused
on steam systems with a few very
large steam turbines, with greater
than 1.2 megawatt (MW) output,
for which models are developed by
data regression. Significant errors
will result if these models are applied
to smaller steam-turbine drivers
(below 1.2 MW output). Also,
previous studies did not consider
the effects of condensate recovery
on steam-system optimization. An
54 ChemiCal engineering www.Che.Com DeCember 2013
optimization model must also accurately
represent very small steamturbine
drivers, since, for reliability
purposes, petroleum refineries may
drive pumps or compressors with
smaller turbines, some of which may
generate power as low as 5 kW.
This article's spreadsheet-based
approach provides the flexibility to
realistically evaluate a steam network,
without the constraints and
licensing fees incurred by using
commercial process-simulation
software. With accurate models for
both smaller and larger steam turBoiler
Boiler
Boiler
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Chemical Engineering December 2013

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