ASTM D5291 Instrumental Determination of Carbon, Hydrogen, and Nitrogen
ASTM D5291 Standard Test Methods for Instrumental Determination of Carbon, Hydrogen, and Nitrogen in Petroleum Products and Lubricants
3. Summary of Test Methods
3.1 In these test methods, carbon, hydrogen, and nitrogen are determined concurrently in a single instrumental procedure. With some systems, the procedure consists of simply weighing a portion of the sample, placing the portion in the instrument, and initiating the (subsequently automatic) analytical process. In other systems, the analytical process, to some degree, is manually controlled.
3.2 The actual process can vary substantially from instrument to instrument, since a variety of means can be utilized to effect the primary requirements of the test methods. All satisfactory processes provide for the following:
3.2.1 The conversion of the subject materials (in their entirety) to carbon dioxide, water vapor, and elemental nitrogen, respectively, and
3.2.2 The subsequent, quantitative determination of these gases in an appropriate gas stream.
3.3 The conversion of the subject materials to their corresponding gases takes place largely during combustion of the sample at an elevated temperature in an atmosphere of purified oxygen. Here, a variety of gaseous materials are produced, including the following:
3.3.1 Carbon dioxide from the oxidation of organic and elemental carbon,
3.3.2 Hydrogen halides from organic halides (and organic hydrogen, as required),
3.3.3 Water vapor from the oxidation of (the remaining) organic hydrogen and the liberation of moisture,
3.3.4 Nitrogen and nitrogen oxides from the oxidation of organic nitrogen, and
3.3.5 Sulfur oxides from the oxidation of organic sulfur. In some systems, sulfurous and sulfuric acids can also be obtained from a combination of the sulfur oxides and the water vapor.
3.4 There are several accepted ways of isolating the desired gaseous products and quantitatively determining them. These are as follows:
3.4.1 Test Method A - From the combustion product gas stream, oxides of sulfur are removed with calcium oxide in the secondary combustion zone. A portion of the remaining mixed gases is carried by helium gas over a hot copper train to remove oxygen, and reduce NOx to N2, over NaOH to remove CO2, and over magnesium perchlorate to remove H2O. The remaining elemental nitrogen is measured by the thermal conductivity cell. Simultaneously, but separately from the nitrogen measurement, the carbon and hydrogen selective infrared cells measure the CO2 and H2O levels.
3.4.2 Test Method B - From the combustion product gas stream (which is cleaned from sulfur oxides, excess oxygen, etc. as in 3.4.1), the remaining CO2, water vapor, and N2 are flushed into a mixing chamber and are thoroughly homogenized at a precise volume, temperature, and pressure. After homogenization, the chamber is depressurized to allow the gases to pass through a heated column, where the gases separate as a function of selective retention times. The separation occurs in a stepwise steady-state manner for nitrogen, carbon dioxide, and water.
3.4.3 Test Method C - The combustion product gas stream, after full oxidation of component gases, is passed over heated copper to remove excess oxygen and reduce NOx to N2 gas. The gases are then passed through a heated chromatographic column to separate and elute N2, CO2, and H2O in that order. The individual eluted gases are measured by a thermal conductivity detector.
3.5 In all cases, the concentrations of carbon, hydrogen and nitrogen are calculated as functions of the following:
3.5.1 The measured instrumental responses,
3.5.2 The values for response per unit mass for the elements (established via instrument calibration), and
3.5.3 The mass of the sample.
3.6 A capability for performing these computations automatically can be included in the instrumentation utilized for these test methods.
4. Significance and Use
4.1 This is the first ASTM standard covering the simultaneous determination of carbon, hydrogen, and nitrogen in petroleum products and lubricants.
4.2 Carbon, hydrogen, and particularly nitrogen analyses are useful in determining the complex nature of sample types covered by this test method. The CHN results can be used to estimate the processing and refining potentials and yields in the petrochemical industry.
4.3 The concentration of nitrogen is a measure of the presence of nitrogen containing additives. Knowledge of its concentration can be used to predict performance. Some petroleum products also contain naturally occurring nitrogen. Knowledge of hydrogen content in samples is helpful in addressing their performance characteristics. Hydrogen to carbon ratio is useful to assess the performance of upgrading processes.
