Chemical Engineering August 2020 - 40

Hydrogen gas
(High temperature)
(High pressure)
Fill fluid
1. Dissociaation:
2. Protonisation:
H2
HO - ē
2HO
H+
H+
H+
HO
HO
Hydrogen
recombination
2HO
H2
316L/AlloyC
FIGURE 5. Molecular hydrogen can disassociate and form H+ ions , which are
capable of permeating a diaphragm
Fill fluid
Process:
H2 at the high temp.
& high pressure
aqueous solution
& galvanic H2
reduction
H2
H2
as a component
in ammonia
used for fertilizer
production,
a fuel source,
or a reformer
byproduct in hydrotreating
and
hydrocracking
processes. One
lesser known,
but highly important,
use of
hydrogen is in
the production
of ultra-pure
polysilicon.
2H++2ē
H2
Gold
FIGURE 6. A coating of gold on the diaphragm reduces permeability for both
gaseous and aqueous applications. Increasing the coating thickness increases
the effective temperature range
sure measurement setup, the interior
space is vented to atmosphere
through the instrument housing.
As the diaphragm flexes due to
pressure, the distance between the
electrodes changes and the capacitance
value changes with it. This provides
the raw analog signal processed
by the instrument's transmitter. This
configuration is especially well-suited
to hydrogen service because neither
molecular hydrogen nor H+ ions can
diffuse directly through the ceramic
material. Even if hydrogen manages
to penetrate the assembly seal at the
edge, it will have no effect. There is
no fill fluid and the interior space is
vented, so such trace amounts simply
escape through the instrument.
A ceramic diaphragm is significantly
more durable than stainless
steel or high-nickel alloys, even
when they are coated with gold. It
also provides wide temperature and
pressure capabilities, assuring a long
service life with full integrity.
Example: Ultra-pure polysilicon
Hydrogen gas is used in many processes,
both as a component to
facilitate a specific reaction or as
the end product - for example,
38
The polysilicon
wafers used
to form the base
for computer
chips, photovoltaic
cells and
various other
electronic devices
have to be
manufactured
with a degree
of purity that is unimaginable in
most industries. Polysilicon for
photovoltaic cells must approach
99.999999% purity, achieved
through a complex and very hot
process. Polysilicon for computer
chips must be even purer.
The process begins by turning
sand and coal into metallurgical silicon
(99% pure) in an arc furnace.
Crushed metallurgical silicon is then
fed into a fluidized-bed reactor to
mix with hydrogen chloride gas at
300°C (572°F). This creates a mix
of compounds, including trichlorosilane
(HSiCl3) as the primary product.
The mixture can be separated using
multiple distillation steps, isolating
trichlorosilane from the byproducts
and purifying it.
All the byproducts
and
contaminants
must be removed
for the
next step of
the process to
work correctly
and deliver the
necessary final
product. The
purified trichlorosilane is mixed with
purified hydrogen and fed into a
highly specialized reactor.
Inside the reactor, thin filaments
of polysilicon, fabricated
into
squared inverted U shapes, are
mounted on the floor (Figure 8). A
typical reactor supports 15 to 20
of these, often up to four meters
tall, within the vessel. The filaments
are heated electrically to 1,150°C
(2,102°F) while trichlorosilane gas
is pumped in along with molecular
hydrogen to a pressure of 5 bars.
As the reactor interior heats up,
the trichlorosilane decomposes
and deposits pure polysilicon onto
the glowing filaments. Other byproducts
from the reaction are extracted
and captured for recycling.
This process continues for several
days, with a constant flow of
the feed gases, eventually building
up the filaments from a few millimeters
to 200 mm in diameter. Feed
gas flow and pressure must be controlled
carefully to ensure the fastest
material deposition without wasting
unreacted gas.
This is a challenging process, to
say the least. The chemicals involved
are highly flammable, corrosive,
toxic and react violently with
water. The process is energy intensive
and costly, so ensuring careful
control and stability are paramount.
Temperature fluctuations can cause
cracking or splintering, and even the
slightest contamination renders the
product unusable.
For a pressure instrument, this
application is as challenging as
they come. Pure hydrogen at a high
temperature and pressure provides
all the conditions necessary to
drive hydrogen permeation through
a conventional stainless steel or
high-nickel alloy diaphragm. This
Atmospheric pressure
C
Pressure
FIGURE 7. Using a ceramic diaphragm in a dry cell assembly does not allow
any internal pressure to accumulate, thereby protecting the instrument
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Chemical Engineering August 2020

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