INVESTIGATION OF THE PROCESS OF FORMALDEHYDE PURIFICATION OF THE BENZENE–TOLUENE FRACTION FROM THIOPHENE

THIRD COMMUNICATION

Balance of the purification process

N. M. SMOLYANINOVA, M. I. ALEKHIN

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

In previous communications [1] it was shown that the formaldehyde method for purifying the crude benzene–toluene fraction ensures the production of commercial benzenes of the highest grades: the grades “especially pure” and “pure for synthesis” of the 1st grade; the optimum conditions for carrying out the process were determined.

In the present communication the results are presented of balance experiments on formaldehyde purification of the BT fraction. The experiments were carried out at the Kuznetsk Metallurgical Combine in a five-liter washing apparatus of VUKhIN according to two variants: in the presence of acid resin and with its preliminary removal.

Three BT fractions, taken during operation of the dicyclopentadiene unit, were subjected to purification; their characteristics are given in Table 1. Operation of the indicated unit, owing to features of the technology,

Table 1

Characteristics of BT fractions of the KMC

as studies of the KMC coke-chemical production laboratory have shown, has an unfavorable effect on the composition of the BT fraction, which considerably complicates its dethiophenation both in ordinary sulfuric-acid purification and in formaldehyde purification. On this basis, in carrying out the balance experiments the consumption of sulfuric acid was increased by 1.5–1.7% in comparison with that established earlier, and the working procedure was somewhat changed.

Procedure

In experiments carried out in the presence of acid resin, the washing apparatus was charged with 3 l of the fraction, and, with the stirrer switched on, concentrated sulfuric acid was fed in an amount of about 4 percent (by weight). After 30 min of stirring, formalin (1.0%) was introduced into the reaction mixture, and after 10 min an additional 1.5–1.7% of acid was added, after which stirring was continued for another 20 min. The reaction mixture was then discharged, with the stirrer switched off, into a previously weighed vessel, where the benzene hydrocarbons were allowed to settle from the acid and the “crude” polycondensation products for one hour. After weighing the vessel with the reaction products, the fraction separated from the acid and high-molecular-weight products was again placed in the melanger for neutralization with 20% alkali, while the “crude polymers” were subjected to further treatment. After settling, the spent alkali was drained from the apparatus, after which benzene hydrocarbons were distilled off from it and combined with the alkaline BT fraction (raffinate).

In experiments with preliminary removal of acid resin, after 30 min of stirring of the fraction with sulfuric acid, the reaction mixture was allowed to settle for 20 min from the acid and acid resin, which were drained from the apparatus, after which benzene hydrocarbons were distilled off from the resin. Then 4% (by weight) sulfuric acid and 1% formalin were fed, with stirring, into the “washed” BT fraction. After 20 min, another 4% of acid was introduced into the reaction mixture and the contents of the melanger were stirred for another 10 min. In all other respects the procedure was analogous to the preceding one.

After settling from sulfuric acid and its separation, the crude polycondensation products were sent for distillation of benzene hydrocarbons; they were then separated by filtration from the acid solution, washed with distilled water, dried, and weighed.

The concentration of the spent sulfuric acid was determined by titration. The distilled-off benzene hydrocarbons were not mixed with the raffinate, since they had increased sulfur content, and were analyzed separately for thiophene content and degree of coloration (GOSTs 8448-61 and 2706-54). In drawing up the purification balance, it was conventionally assumed that the distilled-off BT fraction corresponded in composition to the bulk of the purified product (raffinate).

The content of pure components in the raffinate and in the fraction previously subjected to sulfuric-acid purification was determined by rectification on the so-called “American column” [2]. Since it was not always possible to separate xylenes clearly from solvents, only their combined yield was taken into account.

Results

Table 2 gives data on the yields and quality of the raffinate, sulfur-free benzene, crude polycondensation products, spent acid, and distilled-off fraction for various purification regimes.

The balance of the purification process is presented in Table 3.

On the basis of these data it may be stated with certainty that, from almost all points of view, except for the possibility of draining the polymers by the usual method, the most effective process is the one carried out in the presence of acid resin, i.e., combining the stages of sulfuric-acid and formaldehyde purification of the BT fraction. The losses of pure products here are two times lower, and those of acid 1.5 times lower, than in purification with preliminary removal of acid resin; the quality of the puri-

Table 2

Yield and quality of products of formaldehyde purification of BT under various conditions.
Formalin consumption — 1.0% (wt.)

^x^ In experiments 8 and 9, crude benzene of the “pure” grade was used for nitration.

^xx^ Losses consist of reaction water, which was not taken into account, as well as polymers partially removed from the spent apparatus.

Continuation of Table 2

Table 3

Balance of the process of formaldehyde purification of the BT fraction from thiophene

* Substances removed with the acidic resin.

** By polymers are meant impurities of unsaturated and sulfur-containing compounds of the BTF, which are removed with the acidic resin at the sulfuric-acid purification stage and with the acidic polymers at the formaldehyde stage; they also count as purification losses.

…purified benzene both with respect to thiophene content and to crystallization temperature. The yield of polymers, unfortunately, is not indicative in these experiments, since, owing to the characteristics of the apparatus used, it was difficult to achieve complete discharge of the polymers, which in some experiments led to a large discrepancy in determining their composition (Table 2, exps. 5–9).

This variant has the same advantages for the purification of “pure for nitration” benzene (Table 2, 3, exps. 8, 9).

An increase in the consumption of sulfuric acid had a very unfavorable effect on the indices of the purification process: the quality of the benzene did not improve, but the yields of pure products—benzene, toluene, and especially xylenes and solvents—decreased considerably; the amount of still residues increased, and washing losses, especially of pure products, rose sharply (Table 3).

True, for the fraction used in experiment 6 (in the absence of acid resin), this method made it possible to obtain good discharge of the polymers. However, when working with a fraction of another composition, and even with nitration benzene, discharge, although possible, did not remove the polymers completely.

It should also be noted that the crude polymers carry with them a considerable amount (up to 11%, calculated on BT fraction) of benzene hydrocarbons, especially when purification is carried out in the presence of acid resin on a fraction with a high benzene content (exps. 1, 2, Table 2). This makes their steaming necessary.

It turned out that the steamed hydrocarbons have a thiophene content 10–50 times higher than that of the raffinate (apparently as a result of exchange reactions occurring during steaming) and a high degree of coloration with sulfuric acid. Consequently, this part of the product cannot be mixed with the purified BT fraction and must either be processed separately to obtain benzene of lower grades, for example “pure for nitration,” or, after being mixed with crude BT, again sent for purification. True, when purifying a fraction containing 79–80% benzene, 12–14% toluene, and 2–3% xylene (experiments 5–7, Table 2), i.e., of a composition typical for plants in the East [3], this difficulty is in fact eliminated or, at any rate, substantially eased, since the amount of BT fraction entrained by the polymers decreases to 1.2–3.7%.

As a disadvantage of formaldehyde purification, it should also be pointed out that it is necessary to use regenerated sulfuric acid, which has a low concentration (23–53%, Table 2), making its use difficult.

In concluding the discussion of the results of the balance experiments, it should be emphasized that all the data obtained are by no means optimal, since the experiments were carried out in a washing apparatus with poor mixing of the reagents and using a BT fraction of poorer-than-usual composition (owing to the operation of the dicyclopentadiene unit). When this method is used to purify crude BT fractions typical of most plants in the East, better results may be expected.

REFERENCES

1. N. M. Smolyaninova, M. I. Alekhin, I. A. Mykolnikov, V. N. Sitarkina. Investigation of the formaldehyde method for purifying benzene from thiophene. Communication 1. This volume. Izv. TPI.

2. L. D. Gluzman, I. M. Edelman. Laboratory Control of Coke-Chemical Production. Kharkov, Metallurgizdat, 1957.

3. L. Ya. Kolyandr. Processing of Crude Benzene. Metallurgizdat, Kharkov, 1962.