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P a r r I n s t r u m e n t C o m p a n y
Gas Measurement Systems
P
arr offers a variety of
accessories for its line
of pressure reaction vessels
to enable the investigator
to accurately determine the
amount of gas consumed in a
reaction conducted at elevated
pressures and temperatures.
There are essentially two
methods used to measure the
amount of gas delivered to a
reaction vessel. These are:
1. The measurement
of the
pressure drop in an auxil-
iary supply vessel of known
volume.
2. The measurement and
integration
of the flow
rates using an electronic
mass flow meter.
Each of these methods has
its advantages and limitations
as discussed below.
Intermediate Supply Tanks
Certainly the simplest
method to measure the
amount of gas consumed in a
reaction is to feed the gas from
a vessel of known volume
and to measure the pressure
drop in this vessel during the
course of the reaction. The
consideration in this method
is to select a supply vessel
with a volume matched to the
amount of gas that will be con-
sumed in the reaction. It needs
to be large enough to contain
enough gas to complete the
reaction and small enough that
the pressure drop will be sig-
nificant and measurable. This
basic technique can be applied
in a number of ways:
1. The supply tank can be
connected directly
to the
reaction vessel. This is the
simplest and least expen-
sive. The principal limitation
of this approach is that the
reaction pressure will fall
as gas is consumed and the
reaction will not be conduct-
ed at a constant pressure.
2. The supply tank can be
fitted with a constant
pressure regulator.
The
regulator must be selected
to match the planned oper-
ating pressure. This regula-
tor will deliver gas to the
reaction vessel at constant
pressure overcoming the
limitation described in (1)
above.
3. Initial and final pressures
in the supply tank
can
be measured with analog
gages, or continuous pres-
sure readings can be made
and recorded using pres-
sure transducers. While the
transducers add cost, they
also add increased resolu-
tion and the opportunity to
follow the rate of the pres-
sure drop and hence the
rate of reaction.
4. Enhanced precision can
be achieved
by measuring
the temperature in the sup-
ply tank and applying cor-
rections as appropriate.
Parr has put together
a series of high pressure
burettes in complete packages
for direct connection to our
reactors. The basic ones are
listed on the following page.
These burettes can also be
equipped with digital pressure
transducers, internal thermo-
couples and data acquisition
and reduction support. Please
contact our customer support
group for information on these
possibilities.
Mass Flow Meters
Contact Parr Technical
Service for help with mass
flow meters or controllers.
Because these meters must
be individually specified and
calibrated to the specified gas
as well as the desired flow rate
and operating pressure, no
attempt has been made here
to identify the possible selec-
tions and specifications.
The main consideration in
selecting mass flow meters
is to specify a pressure and
a measurement range appro-
priate to the reaction. Some
additional considerations are:
• Mass flow meters
tend
to have an accuracy of 1%
of the full-scale flow rate.
Since the meter must be
sized to record the maxi-
mum expected flow rate,
the accuracy is poor when
the reaction is nearly com-
pleted and the flow rate is
lower. Some systems over-
come this by placing two
meters in parallel and shift-
ing over to the lower flow
rate meter once the initial
surge is over.
• Meters are calibrated
for
a specific gas. If the user
will work with only one gas,
e.g. hydrogen, this is not a
significant restriction.
• Electronic flow meters
are relatively fragile and
must be protected with
filters to ensure reliable
service.
Mass Flow Controllers
Mass Flow Controllers add
an automated control valve
to the mass flow meter to
provide gas flows that are
proportional to an electronic
set point. Although normally
used to provide a constant
flow rate to reactors operated
in a continuous-flow mode,
a unique application in batch
reactions is to allow the set
point to be dictated by the
error signal from the reactor
pressure transducer. As gas is
consumed, the pressure drop
signal can be configured to
increase inlet flow. This signal
can be sent to multiple control-
lers, enabling the make-up
gas to be a mixture with an
operator-specified ratio. This
technique is often used in
the study of co-polymers and
ter-polymers.
