Determination of Inert Gas in Anhydrous Ammonia

GBH Enterprises, Ltd.

Plant Analytical Techniques
ANHYDROUS AMMONIA: DETERMINATION OF INERT GASES

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ANHYDROUS AMMONIA: DETERMINATION OF INERT GASES
1

SCOPE AND FIELD OF APPLICATION

This packed-column GC method is suitable for the determination of hydrogen,
nitrogen, oxygen, argon and carbon monoxide in anhydrous ammonia. The
determinations of the gases are linear in the range O-100 ppm v/v.

2

PRINCIPLE

The gaseous components of the test sample are separated on the solid phase of
a packed chromatographic column, and are then detected by means of a helium
ionization detector.
Helium carrier gas flows through the detector ionization chamber, where
collisions with primary electrons from a g-source occurs. A titrium
radioactive source is mounted inside the detector cell.

3

REAGENTS AND MATERIALS

3.1

Carrier gas: Helium with 50 ppm v/v neon.

3.2

Service helium, 5 kg/cm2: used to operate the injection valve.

3.3

Water-free methanol.

3.4

Solid phase column packing: Porapak Q-S, SO-80 mesh.

THIS MATERIAL IS HARMFUL BY INHALATION.
3.5

Calibration mixture: Approximately 10 ppm of each component analysed.
The mixture must have a certificate guaranteeing the quality of the
mixture: This mixture can be obtained from BOC or an alternative source.


4

APPARATUS

4.1

Gas-chromatograph, having a helium ionization detector, and an
amplifier. A Carlo-Erba 3700 model, or similar, is suitable.

4.2

Chromatography column: a 4m x 3 mm OD stainless steel tube, suitably
adapted to a suitable size to fit into a 2L vacuum jar.

4.3

Rare gas purifier: a BOC Mk3 model, or similar, is suitable.

4.4

Recorder: 0.1 mv FSD with 1 s response time.

4.5

Flowmeter: O-10 L/hr.

4.6

Vacuum flask: a Dilvac Dewar Flask, model 990/111. or similar, is
suitable.

4.7

Refrigeration unit: a Neslab CryoCool CC-80 model, capable of
maintaining a cooling temperature of -80oC is suitable.

4.8

Sampling 'bomb' as shown in Figure 2.

4.9

Gas sample valve

4.10

Stop-watch.

5

PROCEDURE

5.1

Preparation of Column

Bend the column tubing (4.2) into a suitable size to fit into the vacuum jar;
close one end with a glass wool plug and connect this end via a catch-pot to
a vacuum pump. Connect a small funnel to the other end, start the pump and
fill the column with packing (3.4). vibrating it at the same time. Leave any
pumping for a further 15 minutes and make up any fall in level. Switch off
the pump, remove the funnel and close the open end of the column with a glass
wool plug. Activate the column packing by heating overnight at 230°C in a
stream of helium.


5.2

Operating Conditions

Column temperature:

- 80°C.

Carrier gas

Helium containing 50 f- 10 ppm neon
Inlet pressure 1.5 kg/cm2 (21.4 psig)
Helium service 4 kg/cm2 (57 psig)

Excitation voltage
Attenuation
Helium purifier

380 V
256
Switched on. (Preset by manufacturers
at approximately 730oC.

Recorder chart speed
Injector
Sample volume

2mm/min
Gas loop
3mL

Assemble the components of the chromatograph system as shown
diagrammatically.
in Figure 1, and check that the operating conditions are correctly set.
5.3

Calibration

5.3.1 Connect the primary calibration mixture cylinder (3.5) to the
chromatograph fitted with a suitable exit flow indicator (4.5).
5.3.2 Maintain a purge rate of 3 L/hr through the gas loop, as indicated by
the exit flow indicator.
5.3.3 After a period of approximately one minute, change the "LOAD" setting
on the sampling valve control to "INJECT" and maintain this setting for
exactly 15 seconds, as timed by a stop-watch. (4.10).
5.3.4 Change the setting on the sampling valve control back to "LOAD".
5.3.5 Reduce the flow rate such that it is just showing on the exit flow
indicator. The system should be slowly purged.
5.3.6 Increase the flow rate back up to 3 L/hr and repeat the procedure from
sub-sections 5.3.2 to 5.3.5 inclusive.


5.3.7 Continue this procedure until the standards on the recorder (4.4) show
good repeatability. Carefully check the consistency of the peaks for
oxygen and nitrogen (see Figure 4).
5.4

Sampling

REFER TO LABORATORY SAMPLING PROCEDURES MANUAL DOCUMENT:
SPL-FE, PROCEDURE NUMBER: 52/1/l FOR SAFETY PRECAUTIONS.

5.4.1 The sampling "bomb" is used for this purpose (Figure 2). This consists
of a liquid sample volume V1 and an in-line expansion volume, V2
enclosed by "Hake" ball valves. These volumes are such that the volume,
V1 of liquid ammonia, when allowed to vaporize into a total volume of V1 +
V 2 gives a gas sample at a pressure of three atmospheres.
The test sample is collected as follows:
5.4.2 Connect the vessel to the sample point at the Ermeto coupling
(Figure 3) and purge right through with liquid ammonia sample for 30
seconds. Close valve B and allow the liquid in V2 to evaporate. When the
pressure in V 2 is almost at atmospheric pressure, close valve C.
5.4.3 Close valve A, open, then shut valve B and finally open valve A. The
liquid in V1 will expand into V2. Open valve C again until V2 is almost at
atmospheric pressure, then close valve C.
5.4.4 Repeat the purging procedure (5.4.3) eight times.
5.4.5 Take the final sample in the same manner, but leaving valves C and A
closed. Close the plant sample point and open the purge line, allowing any
ammonia present to rent to atmosphere before disconnecting the sample
"bomb".
5.5

Analysis

5.5.1 Connect the outlet of the sample "bomb" to the column inlet by means of
a micro-valve coupling.
5.5.2 Place the column in the vacuum flask, as shown in figure 1. Switch on
the refrigeration unit and reduce the methanol temperature to - 80°C.
Allow 2 hours for the temperature to stabilize.

5.5.3 Open valve C, with valves A and B still closed, and by means of the
needle valve, regulate the flow of test sample through the flow-meter at 3
L/hr for one minute.
5.5.4 Follow the procedure for the section Calibration from subsection
5.3.3 to 5.3.6 inclusive.
5.5.5 Duplicate tests should be run to check for the absence of air which may
have diffused into the sample lines.
5.5.6 Typical retention times are as follows:
Component
Hydrogen
Nitrogen
Oxygen
Argon
Carbon Monoxide

Retention time
2 mins
5 mins
6 mins
6.5 mins
7.5 mins

6

EXPRESSION OF RESULTS

6.1

Identify each peak by its retention time and measure its height from base
line to tip.

6.2

Calculate the concentration of each component by relating this height to
the peak heights of the standards.

7

NOTES

7.1

Ammonia will be retained on the column at -80°C, and is likely to break
through approximately four hours after injection of the first sample, or
after the analysis of approximately 20 samples.

7.2

The column is reactivated by maintaining it at ambient temperature for
several hours, and preferably overnight. This process can be expedited by
immersing the column in a bath of hot water.

7.3

Small amounts of methane (up to 50 ppm v/v) in the anhydrous ammonia
will show up as a broad interference peak.


Figure 1

SCHEMATIC ARRANGEMENT OF CHROMATOGRAPH FOR
THE DETERMINATION OF INERT GASES IN ANHYDROUS
AMMONIA


Figure 2

ANHYDROUS AMMONIA SAMPLE “BOMB”


Figure 3

ANHYDROUS AMMONIA SAMPLE “POINT”


Figure 4 EXAMPLE STANDARD CHROMATOGRAM FOR CALIBRATION
STANDARDS


Determination of Inert Gas in Anhydrous Ammonia

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