Determination of Hydrocarbons in Anhydrous Ammonia By Gas Chromatography

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Plant Analytical Techniques ANHYDROUS AMMONIA: DETERMINATION OF

HYDROCARBONS BY GAS CHROMATOGRAPHY

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ANHYDROUS AMMONIA: DETERMINATION OF

HYDROCARBONS BY GAS CHROMATOGRAPHY

WARNING AND SAFETY PRECAUTIONS: VAPOR: CONCENTRATED VAPOR CAN CAUSE SKIN IRRITATION AND HIGH CONCENTRATIONS MAY GIVE SERIOUS BURNS TO EYES, RESPIRATORY PASSAGES AND LUNGS. CAN FORM EXPLOSIVE COMPOUNDS WITH CHLORINE, HYPOCHLORITE OR MERCURY. 1 SCOPE AND FIELD OF APPLICATION The method is suitable for the determination of hydrocarbons from C1 to C4 (see 6.4.2) in gaseous ammonia, or in mixtures of ammonia and air. It is valid for concentrations in the range 10-10000 ppm. The method may be used for the analysis of the atmosphere from a ships hold After purging with ammonia and for the analysis of gasified liquid anhydrous ammonia during or after loading. In these cases, hydrocarbon contamination may arise from the previous cargo of the vessel, the nature of which should be ascertained prior to carrying out the analysis. 2 PRINCIPLE A portion of the gaseous sample is analyzed by gas chromatography, each hydrocarbon being identified by retention time and quantified by comparison of peak height with that of an external standard. The gas chromatograph is fitted with a column maintained at a temperature of 75oC, packed with ethylene glycol dimethacrylate beads, and a flame Ionization detector.



3 MATERIALS REQUIRED 3.1 Carrier gas and auxiliary gases 3.1.1 Argon at not less than 250 KPa pressure 3.1.2 Hydrogen at not less than 400 KPa pressure. WARNING - This material is potentially explosive (see Clause 8). 3.1.3 Air at not less than 400 KPa pressure 3.2 Materials for the preparation of calibration samples.

Each of the hydrocarbons to be determined (see 6.4.2) should be available at a purity of 96% or greater.

3.3 Materials for the preparation of columns 3.3.1 Ethylene glycol dimethacrylate beads, particle size approximately 120 µm

to 150 µm. Porapak T (100 - 120 mesh) manufactured by Waters Associates Inc. is suitable.

4 APPARATUS 4.1 General Description 4.1.1 Gas chromatographic system

The system comprises a gas chromatograph with a flame ionisation detector and ambient 6-port gas sample valve.

4.1.2 Characteristics of the assembly 4.1.2.1 Column temperature

The column oven should be maintained at a temperature between 70 and 80°C, with a tolerance of +/- 0.5oC or less. Visible indication should be given if these criteria cannot be met due to malfunction.



4.1.2.2 Detector temperature

The detector should be heated to a temperature between 160 and 180°C, with a tolerance of +/- 2oC or less. Visible indication should be given if these criteria cannot be met due to malfunction.

4.1.3 Gas Controls and Flow Measurement

Pressure regulators capable of controlling gas flows as follows:

(a) Argon - flow rate of approximately 25 ml/min at a pressure of 55 KPa.

(b) Hydrogen - flow rate of 35ml/min at a pressure of 140 KPa.

(c) Air - flow rate of 200 ml/min at a pressure of 200 KPa. Pressure regulators supplied by manufacturers of gas chromatographic equipment will be adequate providing they offer discrimination of 5 KPa or better.

4.2 Injection equipment 4.2.1 Injection device. Polythene bag fitted with a manually operated isolation

valve, of approximately 10 litres capacity. 4.2.2 Injection system. A six-port gas sample valve fitted with sample loop of

5ml capacity, at ambient temperature. 4.3 Columns 4.3.1 Construction. The column shall be constructed of stainless steel tubing

(washed with acetone and dried prior to packing) of internal diameter 4mm and packed length 3m. When packed with stationary phase (see 4.3.2.1) it should have a lifetime of at least 1 year under the specified operating conditions.

4.3.2 Packing 4.3.2.1 Stationary phase



Ethylene glycol dimethacrylate beads (3.3.1) are used without modification.

4.3.2.2 Conditioning. Purge for 5 hours with argon (3.1.1) at a temperature

of 175oC using a flow rate of between 40 ml/min and 50ml/min and with the column outlet disconnected from. the detector.

4.3.3.1 Efficiency and resolution

The performance criteria specified in 4.3.3.1, 4.3.3.2, and 6.3.3 shall apply to both the calibration mixture and the samples.

4.3.3.2 Resolution. The value of the Ratio shall not exceed 0.05.

The valley is measured from the baseline as shown on the typical chromatogram (see figure 1).

4.4 Flame ionization detector

The detector should be capable of satisfactory performance (see 4.3.3 and 6.3.3) with a carrier gas flow rate of 25 ml/min.

The detector/amplifier measuring system shall be capable of detecting 5ppm of propane in 5ml of the sample under the conditions of the method. An amplifier fitted with a binary attenuator or similar, and with a time constant of 0.5s or less is recommended.

4.5 Potentiometric recorder, having the following characteristics:

(a) 0.4s (nominal) time for 0-95X full scale deflection. (b) Chart width 200mm or greater.

(c) Suitable for a nominal chart speed of 2mm/min.



(d) Dead zone within 0.2X full scale deflection. (e) Nonlinearity less than 0.3% full scale deflection.

(f) Noise less than 1% of full scale. The measurement of peak heights requires a scale with discrimination of lmm.

4.6 Integrator. If an integrator is used, at least 100 counts should be

obtained for each peak of interest. Most systems available for gas chromatographic use are suitable.

4.7 Gas-tight syringe. 1 litre capacity syringe with a body of glass or inert

plastic, and graduated at 50ml intervals. This is used for the preparation of calibration samples.

5 Sample The sample is taken into a polythene bag fitted with an isolation valve

(see 4.2.1). The sample may be stored prior to analysis for up to 1 hour without deterioration.

WARNING: Ammonia is both toxic and explosive (see 8.1 and 8.2) 6 PROCEDURE 6.1 Setting up the apparatus 6.1.1 Injection system. The gas sample valve should be maintained in the “load”

position except for the 15 seconds immediately following sample introduction.



6.1.2 Oven and column 6.1.2.1 Column temperature. Maintain the temperature at 75oC (nominal). 6.1.2.2 Rate of flow of carrier gas

(a) Column. Adjust to obtain a time in the range 25s to 35s for the ‘air’ peak, calculated from the elution time of methane. This corresponds to an inlet pressure of approximately 55 KPa.

(b) Auxiliary gas flow rates. Follow the apparatus manufacturers

recommendations. 6.1.3 Detector temperature. The detector zone temperature should be

maintained at 170°C (nominal). 6.2 Calibration. Calibrate by external calibration. 6.2.1 Standard mixtures 6.2.1.1 Purity of components. Each of the hydrocarbons to be determined

should be available at a purity of 96% or greater. The argon diluent should be of the same purity as that used as carrier gas.

6.2.1.2 Operating frequency. Carry out the calibration prior to analysis, or

at least once per week for routine operation, and recalibrate whenever the apparatus has been shut down and restarted.

The calibration is satisfactory if the variation from the previous calibration is within +/- 10% (relative).

6.2.1.3 Preparation of standard mixtures. Prepare calibration standards by

sequential dilution of the purge gaseous hydrocarbon (see 3.2) using a gas-tight syringe (see 4.7) as follows. Ensure that the syringe is clean and dry; expel air as fully as possible by depressing the plunger. Connect the hydrocarbon source using the shortest possible length of PVC tubing.



WARNING: Hydrocarbon gases are explosive (see 8.2); this operation should be carried out in a fume cupboard With the syringe tap in the purge position, introduce a steady flow of gas to remove air from the line. Operate the tap and allow gas to flow into the syringe. Fill to just below the maximum calibration mark, then isolate the hydrocarbon source and close the syringe tap. Disconnect the line at the syringe, open the tap, completely expel the contents of the syringe, then close the tap. Reconnect the line and repeat the procedure described in the preceding two paragraphs twice further, then fill the syringe with hydrocarbon gas, isolate the source, close the tap, and disconnect the line at the hydrocarbon source. Expel gas to the required volume (minimum lOOm1, dependent upon the desired dilution ratio), allow a few seconds for equilibration on atmospheric pressure, then close the tap. Connect the line to a source of argon. With the tap in the purge position, introduce a steady flow of gas to remove air and residual hydrocarbon from the line. Operate the tap to allow gas to flow into the syringe. Fill to the maximum calibration mark, then isolate the argon supply and close the tap. Effect further dilution to the desired concentration by successive use of the procedure outlined in the preceding three paragraphs. 6.2.1.4 Conditions specific to the use of the standard mixtures

Use the procedure described under 6.3 for chromatographing the standard mixtures (see 6.2.1.3) and measuring the results, except that the standard mixture may be fed into the gas sample valve directly from the syringe.

6.2.2 Presentation of calibration data



6.2.2.1 Calibration graph

(a) Prepare a graph for each hydrocarbon in which the product of peak height (mm) and amplifier attenuation is plotted against concentration, expressed as ppm, of the standard mixture.

(b) The calibration graph should take the form of a smooth line such that no point lies more than 5% (relative) from the mean line. If such a point is found, check the whole apparatus for faults and repeat the entire calibration procedure.

6.3 Test 6.3.1 Introduction of the test portion, operation of equipment and time

between injections.

With the gas sample valve in the purge position, connect the sample bag by the shortest practicable length of tubing and open the isolation valve. Purge at least 200 ml of sample through the valve, then switch the valve to the inject position and close the isolation valve. Leave the gas sample valve in the inject position for 15s. In operating the equipment, make and record attenuation changes in order that peaks remain between 10% fsd and 90% fsd. Allow 2 hours between successive injections unless it is known that elution of all impurities will be complete in a shorter interval.

6.3.2 Recording

(a) Base line drift. The baseline drift throughout the entire analysis, as shown on the typical chromatogram, shall not be greater than 2% of full scale deflection at the lowest attenuation used in the analysis.

(b) Base line noise and wander. The baseline noise and wander shall

not be greater than 2% of full duration of the chromatogram.

(c) Retention time. The retention times shall fall between the following limits:



(d) Peak height of sample components. The peak height to be

measured for each component shall lie in the range 10-90% fsd. (e) Integrator counts. If digital integration is used, not less than

100 counts shall be obtained for the smallest peak of interest. 6.4 Examination of the chromatogram 6.4.1 Typical chromatogram. A typical chromatogram for the analysis is shown

in Figure 1. 6.4.2 Identification. A list of possible contaminants, together with their

typical absolute retention times, is given below (see also figure 1). The retention times measured for the calibration mixtures should be used for identifying the sample peaks.



6.4.3 Determination of hydrocarbon content

Identify and disregard the ammonia peak as shown on the typical chromatogram (figure 1). Identify each peak by comparison of its retention time with that obtained for the calibration mixtures (see 6.4.2). draw in the peak base, and measure the height, in millimeters, to the nearest millimeter. For each peak, calculate peak height x attenuation and read from the appropriate calibration graph the concentration of the hydrocarbon.

REPRESSION OF RESULTS The considerations given in 7.1 shall apply in the interpretation of results obtained for the determination of hydrocarbon content.

7.1 Repeatability

Statistical data unavailable at present.



8 HAZARDS 8.1 Toxicity

a) Ammonia is a toxic irritant: occupational exposure limit 25 ppm

(see 10.1 ~135, 10.2 and 10.3).

8.2 Explosively a) Ammonia is explosive when mixed with air at concentrations

ranging approximately from 16% to 26% (v/v) (see 10.1 ~135, 10.2 and 10.3).

b) Hydrocarbon gases and vapors are explosive when mixed with air

(see 10.1). (c) Hydrogen is explosive when mixed with air at concentrations

between 4% and 75% (v/v) (see 10.1 ~280). TEST REPORT The test report shall include the following particulars: a) the concentration and identity of each hydrocarbon present,

expressed as ppm by volume to two significant figures; b) any unusual features noted during the determination;

10 BIBLIOGRAPHICAL RFFERENCES

10.1 Muir G D, ed. Hazards in the Chemical Laboratory (second edition). The

Chemical Society, 1977. 10.2 Laboratory Services Permanent Instruction LSG/201. 10.3 Anhydrous Ammonia Hazard Sheet



Figure 1 : Example Chromatograph

