3B Scientific Critical Point Apparatus User Manual

3B SCIENTIFIC
Critical Point Apparatus 1002670
Instruction sheet
02/13 MH/JS
23
22
19 18 17
®
PHYSICS
1 Vernier scale 2 Fixed scale 3 Grease nipple 4 Threaded bush 5 Handwheel 6 Base 7 Frame 8 Threaded axle with piston 9 Piston cover 10 Outlet for thermal medium 11 Inlet for thermal medium 12 End plate 13 Cylinder 14 Conical seal
1220 21
3 4
15 Measuring cell 16 Valve plate 17 Regulating valve 18 Gas connection fittings 1/8"
(for Minican® gas container) 19 Flush valve 20 Hole for thermocouple
5
21 Heat casing 22 Safety valve 23 Manometer (excess pressure indicator)
876
On delivery, the critical point apparatus is filled with hydraulic fluid. The test gas is not included.
Before filling with the test gas, carry out a volume calibration, as described in chapter 6, using air as an approximation of an ideal gas.
Filling with the test gas is described in chapter 7. Experimental investigations are described in chap-
ter 8. Important notes on storage of the test gas and
equipment (if not in use for a long period) are stated in chapter 9.
1516 14
1213
911
1. Contents of instruction manual
Owing to the inevitable diffusion of the test gas through the conical seal, it is necessary to degas the hydraulic fluid in the equipment, as described in chapter 10. This must be done before the equipment is put away for storage (after removing the test gas) or if it has been in use for a long time.
The threaded bush in the frame must be lubricated regularly and also inspected at lengthier intervals. Refer to section 11 for instructions.
Maintenance work as described in chapter 12 is only required if the rubber components get worn out and their functionality is adversely affected.
1
2. Safety instructions
When used properly, the operation of the critical point apparatus is not dangerous, since both the experimenter and the equipment are protected by a safety valve. However, it is extremely important to observe a few precautionary measures:
Read the instruction sheet thoroughly and follow
the instructions therein.
Do not exceed the maximum permissible values
for pressure and temperature (60 bar and 10­60°C).
Do not operate the equipment without qualified
supervision.
Always wear safety goggles.
Only increase the temperature at low pressure with pure gas phase in the measuring cell.
Before increasing the temperature, wind the
handwheel outwards so that maximum volume is attained in the measuring cell.
When conducting adjustments, make sure that the safety valve does not point in the direction of people who could be injured or objects that could be dam­aged if the valve cover shoots out. When conducting experiments, pay special attention too to the align­ment of the safety valve.
When setting up the apparatus, make sure that
the safety valve does not point in the direction of people who could be injured or objects that could be damaged.
When adjusting the safety valve, wrap your arms
around the apparatus to reach the valve at the back.
If the conical seal is overtaxed, it could get damaged or even destroyed.
Never set a pressure above 5 bar if the regulating
valve or the flush valve is open, i.e. if there is no back pressure from the gas in the measuring cell.
Never create underpressure by turning the hand
wheel inwards when the valves are shut.
In the frame there is a threaded bush, which is to be regarded as a safety-related feature (see section 9).
Lubricate the threaded bush every 100 cycles.
Inspect the threaded bush annually.
To prevent damage by corrosion inside the instru­ment,
use a 2:1 mixture of water and anti-freeze fluid as
the thermal medium.
Only as real gas for SF
and nitrogen as ideal gas.
6
3. Description
The critical point apparatus allows us to investigate the compressibility and liquefaction of a gas. Meas­urements allow determination of the critical point for the gas as well as the recording of isotherms for an adiabatic p-V diagram (Clapeyron diagram). The gas used for testing is sulphur hexafluoride (SF
). SF6 has a
6
critical temperature of 318.6°K (45.5°C) and a critical pressure of 3.76 MPa (37.6 bar) which makes for a simple experiment set-up.
The critical point apparatus consists of a transparent measuring cell of particularly well sealed, pressure­resistant design. The volume of the measuring cell can be modified by turning a fine-adjustment wheel and can be read by means of a fixed scale and a rotat­ing vernier scale to an accuracy of one thousandth of the maximum volume. The pressure is applied via a hydraulic system using castor oil approved for me­dicinal use. The measuring cell and hydraulic system are isolated from one another by a conical seal which rolls up when there is an increase in pressure. This design means that any pressure difference between the measuring cell and the oil reservoir is negligible in practical terms. A manometer measures not the pressure of the actual gas but that of the oil, thus eliminating any need for a space within the measur­ing cell. When observing transitions from gas to liquid or vice versa, the lack of such a dead space means that the development of the very first drop of liquid as well as the disappearance of the last bubble of gas can be observed. The measuring cell is surrounded by a transparent chamber of water. A circulating thermo­stat arrangement (water bath) means that a constant temperature can be maintained during the experi­ment with a high degree of accuracy. The tempera­ture can be read and monitored using a thermome­ter.
The fact that volume, pressure and temperature can all be read with a high degree of accuracy means that accurate p-V diagrams or pV-p diagrams can be re­corded without much difficulty. Pressure and tem­perature-dependent volume correction enable us to achieve accurate quantitative results which are well in agreement with published values.
4. Contents
1 Critical point apparatus, filled with hydraulic fluid
(castor oil). With attached gas connection fittings for MINICAN® gas container and protection for gas supply connections. Test gas (SF
) not included.
6
1 Oil filling device 1 Allen key, 1.3 mm (for grub screw on the vernier
scale) 1 Plastic tubing, 3 mm diameter 1 1/8" tube fitting (wrench width 11 mm) 1 Grease gun
2
J
Δ⋅=
Δ
5. Technical data
Sulphur hexafluoride:
Critical temperature: 318.6 K (45.5°C) Critical pressure: 3.76 MPa (37.6 bar) Critical volume: 197.4 cm
3
/mol
Critical density: 0.74 g/mol
Maximum values:
Temperature range: 10-60°C Maximum pressure: 6.0 MPa (60 bar) Threshold value for
safety valve: 6.3 MPa (63 bar) Theoretical long-term
pressure: 7.0 MPa (70 bar) Theoretical rupture
pressure: >20.0 MPa (200 bar)
Materials:
Test gas: Sulphur hexafluoride (SF
)
6
Hydraulic fluid: Castor oil Measuring cell: Transparent acrylic Temperature coating: Transparent acrylic Recommended
thermal medium: mixture of water and anti­ freeze in the ratio 2:1
Determination of volume:
Piston diameter: 20.0 mm Piston surface: 3.14 cm
Displaced volume: 3.14 cm Maximum volume: 15.7 cm
2
2
× displacement
3
Scale division for displacement: 0.05 mm
Maximum displacement: 50 mm
Determination of pressure:
Manometer: Class 1.0 (max. 1% deviation
from full scale value) Measured quantity: Excess pressure Indicator: 60 bar max. Manometer diameter: 160 mm
Connections:
Hole for temperature sensor: 6 mm dia.
Connections for thermal medium: 7 mm dia.
Connection for regulating valve: 1/8’’ dia.
Gas connection: 1/8’’ (3.17 mm) dia. (as
supplied)
General specifications:
Dimensions: 380 x 200 x 400 mm
3
Weight: 7 kg approx.
6. Volume calibration
6.1 Preliminary notes:
NOP M
Q
L
K
R
S
A
B C E F
Fig. 1: Cross-section of apparatus with measuring cell (A),
conical seal (B), oil chamber (C), piston (D), cylin­der (E), heat casing (F), silicone seal (G), end plate (H), square grommet (I), piston cover (J), threaded axle (K), gasket (L), manometer connec­tion (M), guide tube (N), spring (O), sleeve (P), hole for temperature sensor (Q), circular grommet (R) and valve plate (S)
D
I
H
G
One turn of the handwheel winds the piston into/out of the cylinder by means of a threaded axle. This leads to a change of volume in the oil chamber (see Fig. 1). Since oil is practically incompressible and all the other components other than the conical seal are almost rigid, a change in volume in the oil chamber causes the conical seal to deform, thereby creating an
almost equal change in volume ΔV cell. As a first approximation for ΔV
sAV
G
where
(1)
2
cm143.A= and Δs = displacement of piston.
in the measuring
G
, we can assume:
G
The piston displacement is shown in divisions of 2 mm on the fixed scale. Intermediate values are read on the vernier scale in divisions of 0.05 mm.
The fixed scale can be moved by loosening the two knurled screws. The vernier scale can be repositioned and turned around the threaded axle on loosening the grub screw (between scale positions 0 9 and 1 0).
6.2 Zero point calibration:
The zero point for the volume scale must be deter­mined by conducting a calibration.
For this, we take advantage of the fact that in a pres­sure range of 1-50 bar and in a temperature range of 270-340 K, air acts as a near-ideal gas (the real gas factor has a deviation of less than 1% from 1). There­fore, at a constant temperature (e.g. room tempera­ture) for two piston displacements s
and s1 and for
0
3
the corresponding pressures p0 and p1 of the trapped
T
=
air, we get:
spsp = (2)
1100
Substituting
p
s Δ⋅
1
0
=
pp
s
01
sss Δ+=
10
and rearranging gives:
(3)
Rough calibration of scales:
Open the regulating valve wide.
Loosen the grub screw for the vernier scale by
half a turn (it is now possible to turn the scale easily on the threaded axle without moving the handwheel, although a counterpressure acts against this independent movement).
Wind the handwheel out till you detect a notice-
able resistance.
Without turning the handwheel, turn the vernier
scale on the threaded axle till the 0.0 mark is on the top and the fixed scale shows approx. 48 mm.
Loosen the knurled screws of the fixed scale and
shift the scale to the side till the 48-mm bar is ex­actly above the centre line of the vernier scale (see Fig. 2).
Tighten the knurled screws again. In doing so,
make sure that the fixed scale does not press against the vernier scale.
100 20304050mm
00 19
18
17
Calculate the zero corrected piston position s
1, corr
using Equation 3.
Adjust the vernier scale to the corrected value
and, if necessary, move the scale again.
If required, wind the handwheel out a little and
secure the vernier scale with the grub screw.
Measurement example:
= 1 bar, p1 = 16 bar, p1 – p0 = 15 bar
p
0
s
= 48.0 mm, s1 = 3.5 mm, Δs = 44.5 mm
0
Therefore,
s
= 2.97 mm.
1, corr
The vernier scale must therefore be adjusted so that now only 2.97 mm are shown instead of 3.50 mm.
Note:
After calibrating the zero point, it is possible to obtain qualitatively accurate measured values. With regard to temperature
T and pressure p, it is also possible to
obtain quantitatively accurate measurements of the isotherms in range around to the critical point where the two phases exist simultaneously. However, espe­cially in the liquid phase, the measured isotherms are rather too widely separated.
6.3 Detailed calibration:
The exact relation between the volume VG in the measuring cell and the scale reading
s is dependent
on the volume of oil in the oil chamber. The oil chamber also expands marginally in proportion to the pressure as a result of the spring in the manometer tube. Additionally, when the temperature is in­creased, the castor oil expands to a greater extent than the rest of the equipment. This means that the pressure rises at a slightly greater rate at higher tem­peratures. All of these phenomena can be calculated if appropriate calibration has been effected using air as an ideal gas.
The ideal gas equation would thus be:
Vp
Rn
(4)
=
16
with
J
3148.R =
molK
Fig. 2: Piston position reading at 48.0 mm
Zero correction:
Shut the regulating valve (the pressure in the
measuring cell now corresponds to the ambient pressure
p
= 1 bar. To within the accuracy of the
0
measurement, the manometer should display an excess pressure of 0 bar).
Wind the handwheel in till an excess pressure of
15 bar has been reached (absolute pressure
p
= 16 bar).
1
Read the piston position s
displacement
Δs = s
s1.
0
and calculate the
1
After taking the overpressure reading pressure can be calculated from:
p = p
+ 1 bar (6)
e
The absolute temperature is given by:
T = ϑ + ϑ
where ϑ0 = 273.15°C (7)
0
The volume is given by:
sAV
G
where
(8)
2
cm143,A= and s is the “effective” piston
displacement.
From the measured displacement
p
, the absolute
e
s
, it is possible to
e
calculate the effective piston displacement as follows:
4
(9)
(
)
ϑ⋅++=
CpCsss
ϑ
pe 0
By substituting in equation 4, we get:
ϑββ++
0
pe
ϑ+ϑ
0
Apssp
ϑ
(10)
0
=
Rn
If we take several readings at various temperatures and pressures, we can calculate the term:
n
()
⎛ ⎜
Q
=
=
1i
p
ϑ+ϑ
0
Apssp
ϑββ++
ϑ
ii0ii
2
⎞ ⎟
Rn
(11)
⎟ ⎠
Sample measurements: Table 1: Measured values for calibration
i s
/ mm
e
ϑ
p / bar
1 40.0 20.0°C 6.6
2 20.0 20.0°C 12.4
3 10.0 20.0°C 23.3
4 5.0 20.0°C 41.8
5 3.5 20.0°C 53.9
The free parameters s
, βP, βϑ and n should be appro-
0
priately selected so that the value of Q is reduced to a minimum.
Additionally required (see also chapter 8):
1 Compressor or bicycle pump and valve 1 Bath/circulating thermostat 1008653/1008654 1 Dig. quick-response pocket thermometer 1002803 1 Type K NiCr-Ni immersion sensor, -65°C-550°C
1002804 2 Silicone tubes, 1 m 1002622 1 l Anti-freeze fluid with corrosion-inhibiting additive
for aluminium engines (e.g., Glysantin® G30 ma-
nufactured by BASF)
Conducting the calibration:
Connect the circulation thermostat as described
in chapter 8 and fill it with the water/anti-freeze mixture.
Connect the plastic tube (3-mm internal diameter)
to the 1/8" gas connection fittings.
Open the regulating valve.
Wind the handwheel outwards, making the piston
move till it reaches say the 46.0 mm position.
Use a compressor or a bicycle pump to create an
excess air pressure of approx. 3-8 bar in the measuring cell.
Shut the regulating valve.
To record measurements, vary the volume in the
measuring cell or the temperature of the thermo­stat and wait till a stationary equilibrium has been attained. Then take a pressure reading.
Use appropriate adjustment software to set the s
β
, βϑ and n parameters so that the quadratic
P
,
0
equation for the errors Q is reduced to a mini­mum (see equation 11).
If you like, you can adjust the vernier scale
around s
so that this correction is not necessary.
0
With the set parameters, it is possible to calculate the “effective” piston displacement s from the measured displacement s
using Equation 9 and then to calcu-
e
late the calibrated measuring cell volume using Equa­tion 8.
6 5.0 20.0°C 41.8
7 5.0 10.0°C 38.9
8 5.0 30.0°C 45.3
9 5.0 40.0°C 49.0
10 5.0 50.0°C 53.5
The following parameter values are obtained:
s
= 0.19 mm,
0
P
mm
0230
.=β
bar
n = 0.00288 mol.
7. Filling with test gas
7.1 Handling of sulphur hexafluoride:
Sulphur hexafluoride (SF6) is a non-toxic gas and is absolutely safe for humans. The MAC value for danger of suffocation on account of oxygen deprivation is 1000 ppm. That is equivalent to 6 filled measuring cells per 1 m
However, SF
3
of air.
is extremely harmful to the environment
6
and can give rise to a greenhouse effect 24,000 times stronger than CO
. Therefore, do not allow large quan-
2
tities to be released into the environment.
7.2 Gas connection via fixed pipes:
Additionally required:
1 SF
gas cylinder with manufacturer’s/supplier’s rec-
6
ommended gas fittings/valves, e.g. SH ILB gas cylinder and Y11 L215DLB180 regulating valve from Airgas (www.airgas.com).
1 Pipes with outer diameter of 1/8" and, if necessary, adapters, e.g. from Swagelok (www.swagelok.com).
1 open-end spanner (13 mm), 1 open-end spanner (11 mm)
According to the principles of “good laboratory prac­tice”, it is recommended to utilise a gas supply via fixed pipes, especially if the equipment is regularly in operation.
Filling begins with several flush cycles in which the air is flushed out of the pipe. The number of cycles re­quired to flush out the air depends on the length of
mm
,
0340.=β
ϑ
grd
and
5
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