Chemical Engineering September 2010 - 46

Power-recovery potential
100
150
200
250
300
50
Equation versus simulation deviation
0.05
0.10
0.15
0.20
0.25
-0.05
-0.10
50
60
350
400
450
Fluegas temperature, °F
FIGURE 2. This chart, showing the power-recovery potential
at various luegas temperatures, can be used to approximate
the power-recovery rate for various luegas inlet
temperatures at a given ambient temperature of 70ºF
FIGURE 3. This chart compares the accuracy of the net
power recovery estimated using Equation (1) versus the net
power recovery as predicted by the simulation model. As shown,
the results predicted by the simulation and the results predicted
by the equation differ by no more than -0.1% to +0.23%
TABLE 1. REFINERY WASTE-HEAT-RECOVERY POTENTIAL
Refinery size,
1,000 bbl/d
500 to 600
400 to 499
300 to 399
200 to 299
100 to 199
Net power
recovered, MW
22 to 27
18 to 22
13 to 18
9 to 13
4 to 9
and fluegas composition. The case illustrated
in Figure 1 is based on a
typical fluegas flowrate, temperature,
pressure and composition [1] with an
ambient air-temperature of 70ºF, and
assumes that natural gas is the fuel
used by the fluegas source.
The curve shown in Figure 2 can
be used to approximate the powerrecovery
rate for various fluegas inlet
temperatures at the assumed ambient
temperature. Note that power
recovery varies linearly with fluegas
temperature for a given ambient temperature
and fluegas stream, allowing
for easy interpolation to estimate the
power-recovery rate for a specific application.
Equation
(1) is derived empirically
from simulation data, and can be used
to estimate the potential power recovery
that is possible when using a
simple ORC:
(1)
P = Net recovered power, kW
Qfg = Fluegas flow, million std. ft3/h
Tfg = Fluegas inlet temperature, ºF
Ta
= Ambient temperature, ºF
A = -0.00411 (unitless constant)
B = 0.775 (unitless constant)
Value of recovered
power, $ millions/yr
up to 12.9
up to 10.7
up to 8.6
up to 6.2
up to 4.3
C = 1.122 (unitless constant)
D = -211.63 (constant)
This equation produces estimates of
fluegas temperatures that have an
accuracy of ± 0.23% between 350 and
500ºF with ambient temperatures
varying between 50 and 100ºF. Figure
3 illustrates the percent difference in
the values calculated by the equation
versus those produced by simulation.
Equation (1) also assumes that the
cycle is running in the working fluid's
subcritical region. The subcritical
region is the ranges of temperature
and pressure below the fluid's critical
point - that is, where distinct liquid
and vapor phases exist. Note that this
equation is only provided for quick estimation
purposes. More accurate results
can be easily achieved by simulating
the simple process.
Using Equation (1) to obtain empirical
data on fluegas rates and temperatures
for various refinery units, it is
easy to formulate a table such as that
shown in Table 1. Based on the size of
the refinery, the potential power recovery
and the associated dollar value
of the power recovered can be approximated
with suitable accuracy. For the
case discussed here, the reported dol38
CHEMICAL ENGINEERING WWW.CHE.COM SEPTEMBER 2010
lar values of the power recovered annually
are based on a plant on-stream
factor of 8,000 hours per year, and a
conservative, per kW-h value of $0.06.
Various ORC configurations
Although, the main focus of this article
is on the use of a simple ORC configuration,
specific modifications may be
made to the cycle and to improve the
heat-recovery efficiency and increase
the quantity of power recovered. For
example, as shown in Figure 5, if an
additional heat exchanger (E-102)
were provided to preheat the refrigerant
stream entering the evaporator
(E-100) by heat exchange with the turbine
exhaust stream, then an increase
in power recovery of at least 10% over
the simple cycle may be realized. Depending
on the size of the specific installation,
this could prove to be a very
significant increase.
However, while improved heat-recovery
efficiencies and the potential to increase
the quantity of power recovered
are always gratifying, the addition of
extra equipment requires additional
capital, operations and maintenance
expenditures. These additional costs
must be weighed against the value of
the power recovered. Thus, depending
on the required payback time and
forecasted energy prices, the addition
of a simple ORC system may prove to
be the most cost-effective solution for
a particular application.
Choice of a working fluid. The simulated
properties of the working fluid
used in this study are based on the
physical properties of the refrigerant
R-245fa (1,1,1,3,3 - pentafluoropropane),
which has the trade name Gen500
70
80
90
100
350
3
5 0
400
4 0 0
450
4 5 0
500
Recovery rate, kW / million std. ft3/h fluegas
Deviation from simulated value, %
Ambient temperature, °F
Fluegas
temperature, °F
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Chemical Engineering September 2010

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