PURIFICATION OF BENZENE FROM THIOPHENE BY FREEZING

N. M. Smolyaninova, S. I. Smolyaninov, V. K. Potarskii

(Presented by the scientific-methodological seminar
of the Faculty of Chemical Technology)

In addition to chemical methods for purifying benzene from sulfur-containing and other compounds, attempts are being made to use, for the same purposes, differences in the physical properties of benzene and, in particular, thiophene—for example, their large difference in melting points, as well as differences in solubility under the action of various solvents.

Thus, Killing [1] proposed a method for obtaining benzene of a high degree of purity from benzene-containing mixtures whose components boil at a temperature close to the boiling point of benzene, by cooling the mixture below the freezing point of benzene and separating crystalline benzene.

First a solvent is added to the mixture, the freezing point of which is below the freezing point of benzene (for example, high-boiling petroleum products); the solvent must not form azeotropic mixtures with benzene. The resulting mixture is cooled with stirring so that the greater part (50–60%) of the benzene contained in the original mixture passes into the crystalline mass; after separation from the mother liquor, pure benzene is obtained from it by recrystallization. Then a second solvent, forming an azeotropic mixture with benzene (aliphatic alcohols), is added to the mother liquor. This mixture is subjected to azeotropic distillation, and the remaining portion of pure benzene is isolated from the distilled benzene-containing fraction.

E. K. Sierde and M. M. Rauskas [2] also used cooling for the purification of benzene from various impurities by the method of fractional crystallization. Benzene fractions of coke-oven gasolines containing from 83 to 98 mole % benzene were crystallized, and the content of impurities was determined in the separated benzene crystals.

We tested this approach for purifying benzene from thiophene. For this purpose two solvents were used whose freezing points are lower than the freezing point of benzene: coke-chemical xylene (a mixture of three isomers) and methanol.

The choice of the first solvent was determined by its availability under the conditions of coke-chemical production; that of the second, by the greater simplicity of the subsequent purification operations.

The process flow diagram using xylene could have the following form:

pure benzene          xylene \[ \begin{array}{cc} \underline{\text{pure benzene}} & \underline{\text{xylene}}\\ \diagdown & \diagup\\ & \downarrow\\[-0.5em] \multicolumn{2}{c}{\text{mixture (80--90\% benzene and 10--20\% xylene)}}\\ \multicolumn{2}{c}{\downarrow}\\ \multicolumn{2}{c}{\text{cooling to } -24 \div 30^\circ \mathrm{C}}\\ \multicolumn{2}{c}{\downarrow}\\ \multicolumn{2}{c}{\text{separation and washing of crystals (centrifugation, filtration).}} \end{array} \]

The benzene purified in the first stage is sent to the second stage of crystallization, etc.; the xylene is separated from thiophene and part of the benzene by distillation.

The process flow diagram using methanol as solvent is similar in principle, but the separation of thiophene and then of the remaining benzene from the methanol is possible by adding a definite amount of water to the mixture, allowing phase separation, and subsequently regenerating the methanol from the aqueous layer by rectification.

For the investigation, commercial “pure” grade benzene from coke-chemical production of the Kuznetsk Metallurgical Combine was taken; it contained 0.067% thiophene. This product was mixed with a known amount of low-freezing component and cooled to the specified temperature. The crystals that precipitated were separated on a glass filter and washed with a small portion of solvent under vacuum. Directly on the filter, the benzene crystals were pressed with a plug of filter paper.

The experiments carried out showed that the process of purification from thiophene in the presence of methanol proceeds incomparably better than with xylene. Thus, the yield of purified benzene in one crystallization stage, at equal degrees of purification, is 40–50% higher in the case of methanol than with xylene; moreover, purification in the presence of methanol requires considerably lower crystallization temperatures. Thus, at \(-15^\circ\mathrm{C}\) the yield of benzene crystals in one crystallization was about 15% for the mixture with xylene, and almost 60% for the mixture with methanol.

Experiments were carried out on successive stepwise crystallization of a mixture of pure benzene with 20% methanol. The results of these experiments are presented in Figs. 1 and 2.

As can be seen from Fig. 1, at the fifth crystallization stage benzene with a thiophene content of 0.004% can be obtained, i.e., benzene corresponding to the second grade for synthesis according to GOST 8448-61. Its yield, without taking recycle into account, is 10%. Consequently, recirculation of benzene is necessary, with the ratio in the mixture going to crystallization of fresh feed to recycle of approximately 1 : 9. The production of purer benzene will be associated with increasing the number of crystallization stages and increasing the recycle coefficient.

Figure 2 shows the relationship between the yield of purified benzene and the degree of its purification from thiophene. As can be seen from the figure, there is an almost linear dependence of the degree of purification on the yield of purified benzene. Here, too, the advantage of methanol as a solvent in comparison with xylene is clearly evident.

Fig. 1. Dependence of the benzene yield, the thiophene content in it, and the degree of purification on the number of crystallization stages.
1 — benzene yield, 2 — thiophene content, 3 — degree of purification.

Fig. 2. Degree of purification of benzene from thiophene as a function of the benzene yield during crystallization.
1 — in the presence of methanol, 2 — in the presence of xylene.

Thus, there is a fundamental possibility of purifying benzene from thiophene by the freezing method, with the production of one of the grades of benzene for synthesis according to GOST 8448-61. Methanol should apparently be used as the solvent.

Conclusions

1. The possibility of purifying benzene from thiophene by the freezing method has been demonstrated.

2. The yield and degree of purification of benzene depend on the type of low-freezing component.

3. Obtaining a product of a sufficient degree of purity is possible only with multistage purification with a high recirculation coefficient.

References

1. Kling. Method for obtaining benzene of high purity. R. Zh. Khimiya, IV. 1958, No. 1, p. 2508.

2. Sîyrdek E. K., Rauskas M. M. Obtaining pure benzene by the crystallization method. Proceedings of the Tallinn Polytechnic Institute, No. 165, pp. 130–139, 1959.