Chemical Engineering December 2022 - 43

Neuman & Esser USA
FIGURE 2. Three spiral-tube heat exchangers are shown here (arrows)
within a Neuman & Esser hydrogen diaphragm-compressor package
copper, copper-nickel, titanium,
Hastelloy, Inconel and Incoloy. The
casings are commonly in cast iron,
cast steel, fabricated steel or stainless
steel. Although any material
that can be cold worked (rolled) and
welded may be used for the casing
or shell side.
High-pressure applications
When fluid operating pressure is elevated,
above 750 psig, as an example,
a spiral-tube heat exchanger
is an ideal candidate. New energy applications,
such as hydrogen-fueling
systems or remote natural-gas delivery
systems, create new demand for
this type of heat exchanger. Similarly,
developing markets, such as supercritical
CO2 for botanical extraction
or shelf-stable alternatives to traditionally
frozen foods and also mature
markets for industrial gases, like helium
systems, also require these specialized
heat exchangers.
Hydrogen fueling systems. The
energy transition and search for nonfossil-based
transportation fuels has
brought hydrogen to the forefront as
a fuel for fuel-cell electric vehicles.
The Society of Automotive Engineers
Standard SAE J2601 governs
fuel-station
requirements
for
light
duty vehicles and buses. Dispensing
pressure to the vehicle is either
10,000 psi (70 MPa) or 5,000 psi
(35 MPa). These are extremely high
pressures. Diaphragm compressors
are used to increase hydrogen
pressure to the required storage
pressure. The compressors are
multi-stage, where heat exchangers
remove heat of compression (Figure
2). Spiral-tube heat exchangers are
used for compressor inter- and aftercoolers
to remove heat caused by
compression. At such high operating
pressures and for
system integration,
spiral tube exchangers
are chosen. A
typical heat removal
requirement for a hydrogen
compressor
inter-stage cooler is
100 lb/h of hydrogen
at supercritical pressure
of 2,000 psig
cooled from 300°F
to 100°F. For the final
compression stage
the heat removal requirement
typically
is the same 100 lb/h of supercritical
hydrogen at 10,000 psig cooled
from 250°F to 100°F. Actual mass
flowrate will vary from installation
to installation as will the inter-stage
and final-stage cooling requirement
based upon compressor design.
Another use of spiral-tube heat
exchangers in hydrogen fueling stations
is for precooling the hydrogen
before it is dispensed to a vehicle.
SAE J2601 refers to T40 or T30, for
example, meaning the dispensing
system is to deliver hydrogen to the
vehicle at -40°C or -30°C, respectively.
The temperature is essential for
meeting fueling time requirements.
Hydrogen has a unique thermodynamic
property that is unlike most
other gases, except for helium. Most
gases, when passing through a control
valve, expand adiabatically to a
lower pressure and experience a
reduction in temperature. Due to a
negative Joule-Thomson coefficient
for hydrogen and the operating conditions
of the fueling system, when
hydrogen flows through a flow control
valve and undergoes a pressure
loss, the temperature actually rises.
If the resultant rise in temperature
Advanced Extraction Systems
isn't removed, it affects the vehicle
filling time.
A hydrogen precooler is used to
remove the heat caused by pressure
drop across a flow-control valve in
the supply line to the fuel dispenser
admitting hydrogen into a vehicle.
Here too, pressure is high and in the
range of 10,000 psi for automobiles
or 5,000 psi for mass-transportation
vehicles. The removal requirement is
typically 120°F hydrogen cooled to
-40°F (-40°C) for a J2601 T40 fueling
system.
Developing heat exchanger designs
at pressures of 5,000 psi or
greater with hydrogen in supercritical
state is not ordinary fare.
Botanicals, shelf-stable foods
Supercritical CO2, where pressure is
above 1,075 psia and temperature
in excess of 88°F, serves as an ideal
solvent for separating essential oils
by varying CO2 pressure and temperature.
To effect precise control of
the extraction or separation process,
a heat exchanger is used to heat supercritical
CO2 at pressures in the
range of 4,000 to 5,000 psig from
approximately 32°F to 140°F. Tailoring
the solvating properties of supercritical
CO2 is important for separating
high-purity plant oils. Spiral-tube
heat exchangers handle the highpressure
service economically, provide
reliable outlet temperatures and
integrate into an extraction system
package compactly (Figure 3). Also
to bring CO2 to the 4,000 to 5,000
psig operating pressure diaphragm
or reciprocating compressors are
utilized. Inter-stage and final-stage
compressor coolers apply spiraltube
heat exchangers where heat of
compression is removed.
An emerging market involves the
FIGURE 3. A spiral-tube heat exchanger (blue) is integrated into Advance Extraction Systems' supercritical
CO2 botanical extraction system
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