INVESTIGATION OF THE INDIVIDUAL HYDROCARBON COMPOSITION OF THE LIGROIN FRACTIONS OF OILS FROM THE SOVETSKOYE AND SOBOLINOYE FIELDS OF TOMSK OBLAST

N. M. SMOLYANINOVA, S. I. KHOROSHKO, V. M. NENASHEV

(Presented by the scientific-methodological seminar of the CTF)

Analysis of the individual hydrocarbon composition of ligroin fractions is a very complex task, owing to the sharp increase in the number of possible hydrocarbon isomers compared with gasolines. Attempts known from the literature to investigate this fraction have been limited to identifying aromatic hydrocarbons, in some cases also cyclohexane hydrocarbons, and to determining the group composition of the dearomatized portion. This is due primarily to the absence of reference, chromatographically pure hydrocarbons.

To study the individual hydrocarbon composition of ligroins, we propose an analytical scheme requiring a minimal number of reference substances (Fig. 1). In order to facilitate interpretation as much as possible—

                 Initial
              fraction 150–200 °C
                       │
                       ▼
                Separation
                on silica gel
                       │
      ┌────────────────┼──────────────────┐
      ▼                ▼                  ▼
 Intermediate     Aromatic          Paraffin–
   fraction       concentrate       naphthenic part
      │                │                  │
      ▼                ▼                  ▼
Repeated separation  Analysis           Precise
  on silica gel       by GLC           rectification
      │                │                  │
      └────────────────┴──────────────────┘
                       │
                       ▼
                    Analysis
                    by GLC

Fig. 1. Scheme for investigating the individual hydrocarbon composition of the ligroin fractions of oils.

…the scheme provides for a number of simplifications of the composition of the initial mixture, namely: dearomatization and precise rectification of the paraffin–naphthene portion into narrow fractions (on the order of 1°C). A similar scheme was proposed earlier¹ for studying the individual composition of gasolines with an end boiling point of 122°C.

The initial ligroin fraction (150–200°C), obtained by distillation of the oil on a standard ARN-2 apparatus with 20 theoretical plates, was subjected to dearomatization on silica gel. The aromatic concentrate, representing a mixture of hydrocarbons of one class, was studied by the GLC method immediately after separation. The analysis was carried out on a KhL-4 chromatograph with a thermal-conductivity detector. As the liquid stationary phase, cow oil was used, previously thermostated at 250°C and deposited on a prepared INZ-600 support (0.25–0.5 mm) in an amount of 12% by weight. The column length was 3 m.

Chromatograms of the aromatic concentrate were recorded at two temperatures, 413 and 563°K (140 and 190°C, respectively), which were selected as optimal after a series of experiments.

Identification of individual hydrocarbons was carried out by analysis of artificial mixtures and graphically. The results of the analysis are presented in Table 1.

Table 1

Individual hydrocarbon composition of the aromatic concentrate of the ligroin fractions of oils from the Sovetskoye and Sobolinoye fields

It should be noted that cow oil, which had proven itself excellently in the separation of various polar substances with low boiling points, turned out to be the best phase of all those tested (Apiezon, dinonyl phthalate, polyethylene glycol 1000, PEGA, TWEEN-80), but nevertheless in this—

¹ N. I. Lulova et al., Chemistry and Technology of Fuels and Oils, 9, 14, 1962.

conditions is insufficiently effective. This is explained, in our view, by the low thermostability of the oil at elevated temperatures, as well as by the low resolving power of packed columns.

The paraffin–naphthene portion was rectified under a slight vacuum (residual pressure 100 mm Hg) into narrow fractions (about 1°C) on a laboratory column with an efficiency of 40–50 theoretical plates. The narrow fractions of the paraffin–naphthene portion consist of normal and isoalkanes and cyclanes. For more effective separation of this mixture, a nonpolar phase—Apiezon L—was used. The chromatograms were recorded on an KhL-4 instrument. The liquid phase was applied to a prepared support (INZ-600) at a rate of 15% by weight. The column length was 7.2 m; the column operated isothermally at a temperature of 563°K (190°C). Helium served as the carrier gas in both cases.

The data obtained under these conditions served as the basis for identification.

In the qualitative interpretation of the paraffin–naphthene portion, we proceeded from the theoretically justified assumption that the rectilinear dependence of the logarithm of the relative retention time on boiling temperature is preserved in going from hydrocarbons comprising the gasoline fraction to components of the ligroin fraction.

In all, 23 individual hydrocarbons were detected (Table 2). For the most part, both fractions are represented by alkanes, among which isoparaffins predominate. Among the naphthenes, which consist mainly of cyclohexane derivatives, higher rings are encountered.

Table 2

Individual hydrocarbon composition of the paraffin–naphthene portion of ligroins from the oils of the Sovetskoye and Sobolinoye fields

From Tables 1 and 2 it can be seen that both naphthas are similar to one another both in qualitative composition and in the quantitative ratios of hydrocarbons, although the original oils belong to different deposits.

The investigations carried out showed that, using the proposed scheme, it is possible to analyze satisfactorily the individual hydrocarbon composition of naphtha fractions of oils and gas condensates from various fields.

Conclusions

1. A scheme has been proposed for analyzing the individual hydrocarbon composition of naphtha fractions of oils.

2. A study was carried out of two naphtha samples from oils of the Sovietskoye and Sobolinoye fields in Tomsk Oblast. In each fraction, 17 aromatic and 23 naphthenic and paraffinic hydrocarbons were found.