Study of the Distribution of Sulfur Compounds in Crude Benzene Rectification Products
Abstract Generated abstract
This study examines the distribution of carbon disulfide and thiophene during sharp rectification of crude-benzene rectification products, including a washed benzene toluene fraction, pure benzene, and benzene for nitration. Using a laboratory column with an efficiency of 28 theoretical plates, the authors collected successive distillate fractions and determined carbon disulfide and thiophene contents by standard titrimetric and photocolorimetric methods. The results indicate that carbon disulfide is concentrated in the early distillate fractions and can be almost completely removed with limited benzene loss, while thiophene shows only gradual enrichment in later fractions and cannot be effectively separated at this rectification efficiency. The paper concludes that industrial improvements in rectification sharpness may be suitable for carbon disulfide removal, whereas thiophene purification should rely on other, especially chemical, methods.
Full Text
STUDY OF THE DISTRIBUTION OF SULFUR COMPOUNDS IN CRUDE BENZENE RECTIFICATION PRODUCTS
N. M. Smolyaninova, A. P. Anokhin
(Presented by the scientific-methodological seminar of KhTF)
The purification of benzene hydrocarbons from sulfur is reduced mainly to the removal of carbon disulfide and thiophene, since other sulfur compounds (mercaptans, sulfides, elemental sulfur, etc.) are readily removed in the processes of preliminary rectification of crude benzene and ordinary sulfuric-acid purification of its fractions.
We have carried out a study of the possibility of sharp rectification as a method for purifying benzene from carbon disulfide and thiophene. The rectification was applied to the washed (i.e., after sulfuric-acid purification) BT fraction (benzene–toluene), pure benzene, and benzene for nitration from the Kuznetsk Metallurgical Combine, whose characteristics with respect to the content of sulfur compounds are given in Table 1.
For sharp rectification a laboratory column with an efficiency of 28 theoretical plates was used. Distillate was withdrawn every 30 ml; the distillation of each product was carried out continuously
Table 1
| Crude-benzene rectification products | Quantitative content, % carbon disulfide | Quantitative content, % thiophene |
|---|---|---|
| Washed BT fraction | 0.024 | 0.072 |
| Pure benzene | 0.040 | 0.069 |
| Benzene for nitration | 0.014 | 0.046 |
for 28–30 hours. The fractions obtained were examined for the content of carbon disulfide and thiophene. The content of carbon disulfide was determined by the action of an alcoholic solution of caustic potash followed by titration with iodine, according to GOST 8448-61; thiophene was determined by a photocolorimetric method using isatin-sulfuric acid (the same GOST). The distillation curves of the indicated products and the results of the analyses are given in Figs. 1–3.
The experiments showed that, at the given sharpness (28 t.p.), a characteristic distribution of carbon disulfide and thiophene among the fractions is observed. Thus,
in the case of the washed BT fraction (Fig. 1), it is evident that carbon disulfide is concentrated mainly in the head fraction and is completely concentrated in the first 20% of the distillate.
The preliminary-rectification column of the KMK coke-chemical production plant does not provide this degree of sharpness. This is explained by the insufficient
Fig. 1. Distribution of carbon disulfide (curve 3) and thiophene (curve 2) in the washed BT fraction and its distillation curve (1).
efficiency of the carbon-disulfide column, which has 28 bubble-cap plates (about 14 theoretical plates).
As can be seen from Figs. 2 and 3, the carbon disulfide contained in pure benzene can be withdrawn (at a rectification sharpness of 28 theoretical plates) in the first 15% of the distillate, and in benzene for nitration—in the first 12%.
It is interesting to note that, judging from the distillation curves, these 15–12% are not pure benzene but, apparently, a mixture of benzene with other compounds, of which, besides carbon disulfide and thiophene, the most probable are saturated hydrocarbons, which are usually present in amounts of 0.2–0.8%. They are represented chiefly by cyclohexane, which forms with benzene an azeotropic mixture boiling at 78.5°C, and by normal heptane, which also forms an azeotropic mixture with benzene. Consequently, withdrawal of the head fraction will be associated with insignificant losses of benzene and simultaneous purification from other impurities.
As regards the separation of thiophene from benzene, this compound has a slight difference in boiling points compared with
Fig. 2. Distribution of carbon disulfide (curve 3) and thiophene (curve 2) in pure benzene, and its distillation curve (1).
Fig. 3. Distribution of carbon disulfide (curve 3) and thiophene (curve 2) in benzene for nitration, and its distillation curve (1).
with benzene and is much more difficult to separate. As can be seen from the graphs, under our distillation conditions, in almost all products a gradual increase in the thiophene content was observed as the percentage of the fraction being taken off increased.
Of particular interest are the thiophene distribution curves in pure benzene and in benzene for nitration (Figs. 2 and 3). As is known, both of these products are obtained in the rectification shops of coke-chemical enterprises on one and the same unit, but under different operating conditions of the final rectification column. Usually, benzene for nitration is taken off at a higher reflux ratio. As can be seen from Fig. 2, in pure benzene the first fractions contain 0.011% less thiophene than the subsequent ones, while in benzene for nitration they contain 0.022% less. However, it is quite obvious that it is impossible to obtain benzene containing no thiophene with a rectification sharpness of 28 theoretical plates.
From the data obtained, it follows first of all that carbon disulfide can be completely taken off without large losses of benzene. Consequently, for production it is possible to recommend increasing the sharpness of rectification by reconstructing the existing (carbon-disulfide) columns or, if there is a sufficient number of plates, by changing the technological regime.
As for removal of thiophene by means of rectification, one thing is clear: an efficiency of 28 theoretical plates is insufficient for this purpose. Judging from the literature data [1, 2, 3], in order to separate liquids with a boiling-point difference of 4° by rectification, it is necessary to have a column with an efficiency of 50 theoretical plates (100 real bubble-cap plates).
Thus, apparently, thiophene should be removed by other methods, in particular chemical ones. This purification route may prove promising, if only because thiophene belongs to the class of fairly reactive compounds [4].
Conclusions
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It has been shown that by rectification it is possible to obtain practically complete separation of carbon disulfide from benzene with a comparatively low efficiency of the rectification apparatus.
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Removal of thiophene by the modern rectification means of coke-chemical enterprises is not advisable; chemical methods should be considered more effective.
Literature
- E. Krell. Handbook of Laboratory Rectification. IL, 1960.
- Physical Chemistry of Separation of Mixtures. Collection No. 1. Distillation and Rectification. IL, 1949.
- M. I. Rozengart. Technique of Laboratory Distillation and Rectification. Goskhimizdat, 1951.
- Heterocyclic Compounds, Vol. 1, ed. R. Elderfield, IL, Moscow, 1953.