ANALYSIS OF CRACKING GAS BY GAS–LIQUID CHROMATOGRAPHY

S. I. Smolyaninov, A. V. Kravtsov, I. I. Vishnevetsky,
L. F. Kotlova, L. F. Danilova

(Presented by the scientific-methodological seminar of the organic departments)

When a high-voltage pulsed discharge acts on liquid hydrocarbons, a gas, condensation products (soot), and a liquid medium altered in its physicochemical properties are formed [1]. Up to 70% of the cracked paraffinic and petroleum feedstock and about 30% of aromatic feedstock is converted into gas. The gas obtained during the cracking of aromatic hydrocarbons contains up to 70 vol.% hydrogen and up to 20 vol.% acetylene. During the cracking of paraffinic and cyclic hydrocarbons, a gas is formed containing up to 60 vol.% hydrogen, 25 vol.% acetylene, and 15 vol.% ethylene. In addition to these principal components, it contains 4–12 vol.% methane, up to 2 vol.% ethane, about 0.2 vol.% propane, up to 2 vol.% propylene, methylacetylene, propadiene, vinylacetylene, divinyl, and diacetylene.

The analysis of such a mixture involves considerable difficulties. At the Saratov Chemical Combine and at the Institute of Petrochemical Synthesis of the Academy of Sciences of the USSR, methods have been developed for gas analysis using four chromatographs: the first for determining hydrogen, the second for air and methane, the third for determining the content of all the remaining components, and the fourth for a more accurate determination of the content of acetylene homologues, with a column packed with INZ-600 support with dimethyl phthalate or with a complex phase: 10% triethylene glycol adipate and 15% dimethyl phthalate. When these phases are used, ethane and ethylene, and propane and propylene, leave the column unresolved. On vaseline oil and dinonyl phthalate these components, as we established, also are not separated.

Therefore, in our studies, solid supports—silica gel and alumina—were selected for identifying the hydrocarbon portion of electrocracking gas. The best separation quality was obtained when alumina was used. The order of elution of the principal components is as follows: hydrogen, methane, oxygen, nitrogen, ethane, ethylene, propane, propylene, acetylene, dienes, and acetylene homologues. However, it proved impossible to achieve good separation and detection using only a single column. When nitrogen or argon is used as carrier gases, small amounts of components cannot be determined because of their low thermal conductivity. When helium is used, the hydrogen concentration cannot be determined accurately, since the sensitivity coefficient for hydrogen on helium is 56. Hydrogen is the principal component; therefore its determination must be carried out with sufficient accuracy. Determination of hydrogen, methane, and air was performed by us on a column with NaX molecular sieves.

The analysis conditions are as follows:

1. A 3.5 m column packed with NaX molecular sieves, frac-

fraction 0.5–0.25 mm. Carrier gas—argon, detector current 90 mA, column temperature 50°C, volume of the introduced sample exactly 1 ml.

1. A 4.5 m column packed with alumogel modified with NaOH alkali, fraction 0.5–0.25 mm. Carrier gas—helium, detector current 100 mA, column temperature 80°C, sample volume 5 ml.

Chromatograms of the gas separation are shown in Fig. 1.

Fig. 1. Chromatogram of the separation of the hydrocarbon part of the gas (carrier gas He)

Fig. 2. Chromatogram of gas separation on molecular sieves (carrier gas Ar)

The quantitative calculation of the gas is as follows. A calibration plot is constructed for a mixture of hydrogen with air in the ratios: 1 : 0; 0.9 : 0.1; 0.8 : 0.2; 0.7 : 0.3; 0.6 : 0.4; 0.5 : 0.5; 0.4 : 0.6; 0.3 : 0.7; 0.2 : 0.8; 0.1 : 0.9. The plot is a straight line. Therefore, in daily analyses, for control it is sufficient to check one or two points. To determine the sensitivity coefficients of air and methane, calibration mixtures of these components with air were prepared. The coefficient was calculated by the formula:

\[ k_i=\frac{S_{H_2}\cdot c_i}{S_i\cdot c_{H_2}}, \]

where \(k_i\) is the sensitivity coefficient of the component;

\(S_{H_2}, S_i\) are the peak areas of hydrogen and of the component on the chromatogram, obtained by multiplying the peak height by the half-width;

\(c_{H_2}, c_i\) are the concentrations of hydrogen and of the component in the calibration mixture.

For air and methane the values of \(k\) are 10.9 and 4.7, respectively.

From the chromatogram of the separation of hydrogen, air, and methane, the amounts of these components in the gas are calculated using the cali-

calibration graph for hydrogen, or from the following formulas:

\[ c_{H_2}=\frac{S_{H_2}}{S_{1\text{ ml }H_2}};\qquad c_{\text{air}}=\frac{S_{\text{air}}}{S_{1\text{ ml }H_2}}\cdot 10.9;\qquad c_{CH_4}=\frac{S_{CH_4}}{S_{1\text{ ml }H_2}}\cdot 4.7, \]

where

\(c_{H_2}, c_{\text{air}}, c_{CH_4}\) are the concentrations of hydrogen, air, and methane in the gas;
\(S_{H_2}, S_{\text{air}}, S_{CH_4}\) are the areas of these components, obtained by multiplying the peak height by its half-width;
\(S_{1\text{ ml }H_2}\) is the area obtained upon introducing 1 ml of hydrogen. The sum of the concentrations of all the other components separated on the column with alumogel is determined from the difference \(100-(c_{H_2}+c_{\text{air}}+c_{CH_4})\). The concentrations of the hydrocarbon components are calculated by the method of internal normalization [2].

Analysis of the gas mixture takes 20 minutes; about 20 minutes are required for measurements and calculation. Thus, about 40 minutes are required for the analysis and calculation of one gas sample, taking into account the performance of parallel analyses.

Conclusions

1. A procedure has been developed for analyzing electrocracking gas from individual hydrocarbons and natural mixtures using gas chromatography.

2. Separation of hydrogen, oxygen, nitrogen, and methane is carried out on a column with NaX molecular sieves as the adsorbent.

3. Separation of a mixture of paraffinic, olefinic, and acetylenic hydrocarbons is carried out on a column with alumogel as the adsorbent.

References

1. Chemical Reactions of Organic Products in Electrical Discharges. Edited by Prof. N. S. Pechuro. Moscow, “Nauka,” 1966.

2. K. A. Golbert, M. S. Vigdergauz. A Course in Gas Chromatography. Moscow, “Khimiya,” 1967.