Chemical Engineering September 2010 - 47

Refinery waste-heat-recovery breakdown
Engineering Practice
etron and is a product of Honeywell
[2,3,4]. The characteristics of R-245fa
make it particularly suitable for fluegas
heat-recovery applications using
ORC technology. This refrigerant has
the required thermodynamic properties
- namely, relatively high critical
and condensing temperatures -
which are necessary for operating in
the temperature range between typical
fluegas temperatures and the ambient
temperature of the heat sink.
This particular refrigerant also has
a relatively high decomposition temperature,
which is desirable for heat
exchange with hot fluegas streams.
Furthermore, R-245fa has desirable
environmental traits that make it a
refrigerant of choice for many industrial
refrigeration systems, in light
of environmental concerns related
to the ozone layer and fugitive emissions.
For instance, the fluid has a
zero ozone-depletion potential (ODP),
a relatively low global-warming potential
(GWP), and it is not considered
a volatile organic compound (VOC) in
the U.S. [4]
The economics and heat-recovery potential
of an ORC are a function of the
thermodynamic properties of the selected
refrigerant. Thus, the selection
of the best working fluid for a given application
is important to the successful
implementation of any waste-heat-recovery
system. Refrigerant blends may
also be considered, and the choice of
refrigerant or refrigerant blend must
be optimized on a case-by-case basis.
Designers and marketers of ORC systems
and their component machinery
have process simulations and empirical
data that are needed to accurately
model and optimize ORC systems specific
to any given fluegas stream.
It is tempting to choose a working
fluid based solely on the desire to maximize
the power-recovery efficiency,
but there are other factors to consider,
as well. For instance, depending on
the specific application and location of
the installation, it may be necessary to
select a working fluid that is environmentally
friendly, safe, and economical
in addition to being inexpensive
and readily available.
Air-cooled heat exchangers vs. cooling
water. The results of this study
are based on the assumption that in
Utility boilers - 9.7% Atm distillation - 13.6% Vacuum - 3.7%
Alkyation - 11.3%
Hydrodealkyation - 4.6%
Naptha HDS - 1.8%
Diesel HDS - 1.4%
CCR - 12.6%
Jet/kero HDS - 0.7%
Delayed
coker - 3.7%
CCR = Continuous catalytic
reforming
FCC - 37.0%
FCC = Fluid catalytic cracking
HDS = Hydrodesulfurization
FIGURE 4. In this breakdown of waste-heat recovery by reinery unit, one can see
that particularly high rates of potential waste-heat recovery are possible for luid catalytic
cracking, atmospheric distillation and catalytic reforming units and utility boilers
the particular application, air-cooled
heat exchange is more economical
than systems based on cooling water.
Air coolers, depending on the cooling
duty required, are generally not
high energy consumers but they may
require a significant amount of plot
space. The decision of which type of
heat exchange methodology, in terms
of capital cost, operating cost, operating
logistics and potential impact on
the overall heat-recovery potential,
must be evaluated on a case-by-case
basis for each application.
Acid-gas condensation issues.
When establishing the temperature at
which the fluegas will exit the wasteheat-recovery
unit, consideration must
be given to the potential for acid condensation.
This occurs when the water
vapor in a fluegas stream is allowed
to condense, carrying with it any compounds
that may form an acid when
combined with water.
In most cases, industrial fluegas
streams are produced by the combustion
of a fossil fuel. In many cases,
this fuel contains some form of sulfur,
which will likely be converted to some
form of SOx during combustion. If
cooled below the acid dewpoint, these
combustion byproducts, along with
carbon dioxide, can combine with the
water vapor present in the fluegas
stream and form corrosive acids [5].
To prevent equipment corrosion,
any item that comes in contact with
the cooled fluegas stream must be constructed
of special materials that can
withstand damage from such acids,
and these special materials can add
considerably to equipment costs. Such
fluegas systems are also commonly designed
to remain above the acid dewpoint
to avoid corrosion.
While these design and operating
considerations can add considerably
to the overall system cost, the condensation
of fluegas provides additional
opportunity for energy capture,
because as the water vapor in the
fluegas stream condenses, the latent
heat of vaporization of the water will
be transferred to the working fluid,
thereby increasing the potential power
recovered via the ORC system.
An economic analysis must be performed
to balance the tradeoffs and
determine whether cooling the fluegas
stream below its acid dewpoint and
capturing the extra energy is worth
the additional capital necessary to
implement the metallurgical requirements
to withstand the potential for
acid attack.
Plant economics and air permit
benefits of ORC technology. If
properly designed, the use of an ORC
system can provide a facility with an
opportunity to produce additional electric
power without an increase in air
emissions. As a general rule of thumb,
any combustion-related facility can
benefit from ORC installations on furnace
stacks if electrical power from external
sources is limited, expensive, or
unreliable by providing the plant with
internal power-generation capabilities
and an additional source of electricity
for captive use at the site.
It is also worth noting that facilities
in certain regions face restrictions on
air emissions that make it costly or impossible
to amend air permits to add
furnaces, cogeneration units or other
means of increasing utility power, a
situation that can limit expansion and
modernization opportunities. However,
the installation of an ORC system
on existing furnaces allows for the
generation of electrical power without
increasing air emissions by capturing
waste heat from furnaces already in
operation.
CHEMICAL ENGINEERING WWW.CHE.COM SEPTEMBER 2010 39

http://WWW.CHE.COM

Chemical Engineering September 2010

Table of Contents for the Digital Edition of Chemical Engineering September 2010