Chemical Engineering December 2022 - 44

Broadwind Heavy Fabrications
FIGURE 4. Spiral-tube heat exchangers within a Broadwind Heavy Fabrications compressed
natural-gas pressure-reduction system
use of supercritical CO2 to produce
quality food products that
do not require freezing, refrigeration
or cold storage. Supercritical
CO2 is used to sterilize foods that
are shelf-stable, thereby eliminating
requirements for cold-storage
infrastructure, such as refrigerated
tractor trailers, cold-storage warehousing
or grocery store refrigerated-food
displays. In this application,
CO2 is compressed to 4,000
to 5,000 psig and spiral-tube heat
exchangers are applied to remove
heat of compression.
Compressed natural gas systems
In
remote locations, where infrastructure
and pipeline-delivery systems
for natural gas do not exist,
compressed natural gas is trailered
to the location. Natural gas with
pressures in the range of 4,000 psig
is delivered in truck trailers. Then
pressure-reducing systems lower
the natural-gas operating pressure
for use by a remote user. Spiral-tube
heat exchangers are applied to heat
the high-pressure natural gas prior
to pressure reduction to maintain
acceptable operating temperatures
fo l l owi ng
adiabatic
expansion
across the
pressurer
educ in g
system. As
described
previously,
this typically
results
in
cooling of
the natural
gas. For
example,
Permian
Basin hydraulic-fracturing
sites can improve
drilling rig economics by using dualfuel
engines that can operate with
diesel or a diesel and natural-gas
mixture. During times of high diesel
fuel prices, diesel/natural-gas
mixture dual-fuel, where natural gas
displaces 60-70% of the diesel,
greatly improves drilling rig operating
cost. Fracking sites often do
not have natural-gas distribution
systems, thus " virtual pipelines "
are employed, where compressed
natural gas at high pressure is
trucked to a remote location where
decompression systems lower the
pressure for use in an industrial application
(Figure 4), such as a dualfuel
engine. Additionally, this type of
" virtual pipeline " may be used as a
fuel source for peak shaving on limited
pipeline capacity and pipeline
outages or for another industrial application
where natural-gas pipeline
infrastructure is not present.
Likewise, spiral-tube heat exchangers
are used on the compression
side of the natural gas value
chain. In the Marcellus Shale Play,
natural gas is co-produced along
with shale oil during the hydraulic
VOC or product recovery
Reducing harmful or valuable emissions
from chemicals processes
is always a top priority for environmental
stewardship and to improve
unit economics. Owing to compactness
and effectiveness of condensation
with minimal pressure loss,
spiral-tube heat exchangers can be
considered. Spiral-tube exchangers
have three common configurations
for vent or process condensing (Figure
5). A low-cost option is where the
coil or bundle fits inside the process
vessel such that process vapors and
gases flow around the outside of the
coil and condensable vapors are
condensed for reflux back into the
vessel. A similar, but more expensive
option is where the condenser
is mounted on a vessel discharge
flange. In this case, process vapors
and gases flow around the outside
of the coil condensing condensable
vapors. In either case, the coolant
flows inside the tubes. A desirable
aspect of these types of condensers
is that they mount directly onto
a process vessel to avoid piping and
associated hydraulic losses.
Figure 6 shows a cryogenically
cooled storage-tank-vent condenser
installed to eliminate 98% of methylene
chloride (CH2Cl2) vapors vented
or released from the storage tank
Graham Corp.
fracturing process. Natural gas can
be compressed to 4,000 to 5,000
psig for use in compressed natural
gas fueling stations, where trailer
trucks are filled with the high-pressure
natural gas. When the gas is
compressed, the heat of compression
must be removed. Spiral-tube
heat exchangers are applied as
compressor inter-stage and aftercoolers
to lower the temperature of
the 4,000-5,000 psig gas subsequent
to compression.
FIGURE 5. Spiral-tube heat exchangers can help lower environmental impact with VOC or product recovery. Three different configurations are shown here
44
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