INVESTIGATION OF THE FORMALDEHYDE METHOD FOR PURIFYING BENZENE FROM THIOPHENE. SECOND COMMUNICATION

Effect of temperature on the efficiency of the process of formaldehyde purification of the BT fraction from thiophene and on the properties of high-molecular-weight condensation products

N. M. SMOLYANINOVA, M. I. ALEKHIN

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

In studying the formaldehyde method for de-thiophenizing benzene, we [1, 2], under laboratory and semi-industrial conditions, demonstrated the possibility and high efficiency of this method when using, as the starting raw material, not commercial grades of benzene but the crude (unwashed) benzene–toluene fraction (BTF). However, the formation of viscous, low-mobility products of the polycondensation of formaldehyde with thiophene, unsaturated and, possibly, benzene hydrocarbons, mixed with the acidic polymers (“acid resin”) of the sulfuric-acid stage of purification, caused considerable difficulties in removing them from the washing apparatus.

Assuming that the presence of “acid resin” adversely affects the mobility of the condensation products, we changed the procedure of the purification process. After treatment of the BT fraction with sulfuric acid, the acid polymers formed were removed from the reactor after settling. A fresh portion of 93% sulfuric acid in an amount of 4% and 1% formalin was added to the fraction purified in this way; that is, it was no longer the crude but the washed BT fraction that was subjected to purification. As a result of this procedure, the mobility of the polycondensation products noticeably improved, but it still remained insufficient for their unobstructed discharge from the mélanger. In addition, with this procedure the consumption of sulfuric acid doubles.

One way of improving the technological suitability of this method may be, in our opinion, to find conditions for carrying out the process that ensure the production of polycondensation products with a lower molecular weight, which could either dissolve completely in the purified product or easily settle out from it, while having sufficiently high fluidity. Unfortunately, the process of polycondensation of thiophene with formaldehyde, especially in the presence of a large excess of benzene hydrocarbons and other chemically active substances, has not been studied theoretically at all.

When purifying the BT fraction with formaldehyde in the presence of sulfuric acid, interaction of formaldehyde is possible not only with thiophene, but also with unsaturated hydrocarbons, as well as with benzene hydrocarbons of the fraction, especially with high-boiling homologues of benzene, which have greater reactivity in comparison with it.

The process of polycondensation of aromatics with formaldehyde has the character of electrophilic substitution of hydrogen atoms of the aromatic

the nucleus by carbonium ions [3, 4]. Thiophene, according to A. Katritsky and D. Lagovskaya [5], enters into the electrophilic substitution reaction almost as readily as mesitylene—approximately \(10^3\) times faster than benzene. Hence follows one more requirement for the process of desulfurizing the BT fraction: the maximum selectivity of the process, ensuring minimal losses of aromatics and excluding their polycondensation with formaldehyde.

We have attempted, on the basis of a study of the literature in the field of the chemistry and physicochemistry of high-molecular compounds, to identify factors that make it possible to regulate the physical properties of polymers. Such factors are temperature [3, 6—10], the amount and concentration of catalyst [7, 11—15, etc.], and the ratio of reagents [8, 14, 15—20, etc.].

In the present work the influence of the process temperature on the properties of the high-molecular products of formaldehyde purification and on the degree of desulfurization of the BT fraction was studied.

The investigation was carried out with a crude BT fraction from coke-chemical production of KMK (its characteristics are given in Table 1) in two variants: in the presence of acid resin and with its preliminary removal before the formaldehyde purification stage.

Table 1

Characteristics of the BT fraction

Experimental part

Into a glass laboratory stirred reactor equipped with a water jacket, reflux condenser, sampling device, thermometer, and stirrer (600 rpm), 500 ml of the fraction is placed. Upon reaching the specified temperature (with the aid of a thermostat), sulfuric acid is fed into the stirred reactor and then, after ten minutes of stirring, formalin. To determine the kinetics of desulfurization of the fraction, samples of the product were periodically taken for analysis 5, 10, and 20 min after the addition of formalin. After 30 min of mixing formalin with the fraction, the benzene hydrocarbons were separated (after settling) from the mixture of acid and polymers, neutralized with 20% alkali, and sent for analysis. To determine the thiophene content in benzene, the benzene cut was distilled off from the BT fraction after its purification (79—90°C).

The yield of still residues was characterized by the content in the neutralized product of the fraction boiling above 180°C; the mobility of the crude polymers was evaluated by the time of outflow from a separating funnel with a stopcock diameter of 2.5 mm and visually.

The yield of solid polycondensation products was determined after neutralization with alkali, boiling with water, and washing to a neutral reaction of the acidic high-molecular residue. The obtained pow-

the resin-like substance was dried in a vacuum drying oven at 50–60°C and weighed.

The yield of solid products and still residues was determined only for the final experiments of each series.

In carrying out experiments with removal of the acidic resin (according to the second variant), the latter was drained from the mixer after ten minutes of stirring the fraction with sulfuric acid and settling; then a mixture of sulfuric acid and formalin was fed into the reactor.

Figure 1 presents graphs characterizing the dependence of the thiophene content in benzene on the temperature of the purification process at different durations of stirring of the fraction with formalin for two variants: in the presence of acidic resin (curves 1, 4, 6, 8)

Fig. 1. Dependence of the thiophene content in benzene on the temperature of the purification process at different stirring times of the BT fraction with formalin: 1 — in the presence of acidic resin, $\tau_{\mathrm{stir}} = 5$ min.; 2 — without acidic resin, $\tau_{\mathrm{stir}} = 5$ min.; 3 — without acidic resin, $\tau_{\mathrm{stir}} = 10$ min.; 4 — in the presence of acidic resin, $\tau_{\mathrm{stir}} = 10$ min.; 5 — without acidic resin, $\tau_{\mathrm{stir}} = 20$ min.; 6 — in the presence of acidic resin, $\tau_{\mathrm{stir}} = 20$ min.; 7 — without acidic resin, $\tau_{\mathrm{stir}} = 30$ min.; 8 — in the presence of acidic resin, $\tau_{\mathrm{stir}} = 30$ min.

and without it (curves 2, 3, 5, 7). These data show that temperature has a great influence on the process of formaldehyde purification of the BT fraction. The thiophene content in benzene passes through a minimum in the temperature range 20–30°C (depending on the stirring time) if purification proceeds in the presence of acidic resin; in its absence, the minimum is observed at 20°C.

It should be noted that, beginning at approximately 30°C and a contact time of more than 10 min, the dethiophenation process proceeds more effectively in the presence of the acid polymers from the sulfuric-acid stage of purification. At lower temperatures, and also with a short contact time (5 min), the acid resin has a negative effect on the process. The latter is apparently associated with a decrease in catalyst activity as a result of the increased viscosity of the polymers that isolate the sulfuric acid from the reaction mixture.

At elevated temperatures, exchange reactions probably begin to accelerate owing to the involvement in the copolycondensation process of activated molecules of aromatic and other compounds of the BT fraction, which leads to poorer purification.

The favorable effect of the acid resin at temperatures above 20°C is possibly the result of the catalytic action of certain sulfonated components of the acid polymers, the probability of whose formation increases with increasing temperature.

Analyzing the data on the kinetics of benzene dethiophenation as a function of temperature, presented in Fig. 1, one may note that over almost the entire temperature range studied the optimum mixing time of the mixture should be considered to be 10 min. Increasing the duration of the process at 15–40°C does not cause a substantial increase in the degree of benzene dethiophenation, while at higher temperatures (50–70°C) the purification even deteriorates (Fig. 1).

Table 2 gives the yields of solid polycondensation products and still residues, as well as the characteristics of the “crude polymers,” as a function of temperature at a duration of the formaldehyde stage of purification of 30 min. These data show that, with increasing temperature, beginning at 25°C, the yield of solid products increases when the process is conducted in the presence of acid resin and is almost independent of temperature in its absence.

If the sum of the yields of solid polymers (still residues and losses of the sulfuric-acid stage of purification) is taken as the total purification losses and this quantity is expressed graphically as a function of temperature, it can be seen (Fig. 2) that in the presence of acid resin the losses are lower than in purification with its removal. When the process is carried out without acid resin, the total losses do not depend on temperature.

Let us consider the effect of temperature on the properties of the high-molecular purification products present in the mixture with sulfuric acid.

As follows from Table 2, when the BT fraction is purified in the presence of acid resin, over the entire temperature range investigated a very viscous mass is formed, which does not flow out of the discharge opening of the funnel.

When the process is carried out with preliminary removal of the acid resin, temperature has a substantial effect on the fluidity of the polycondensation products: at 5°C very mobile substances were formed; at 15–20°C the products were more viscous, but passed satisfactorily through the discharge opening (in 40–60 sec). At a higher temperature the mobility of the polycondensation products drops sharply; their color changes from dark violet to brown and black. The mass formed adheres well to glass and, on discharge, entrains a large amount of acid.

Thus, when the fraction is purified in the absence of acid resin, as the temperature of the process is increased the fluidity of the acid polycondensation products decreases. The cause of this, in all probability, is the presence in the fraction of unsaturated compounds which, in the presence of sulfuric acid, polymerize vigorously with the formation of viscous, resinous black products, insoluble

in benzene [21]. Raising the temperature favors the course of these reactions, leading to the formation of still more viscous substances.

From this standpoint it is clear why, in the presence of acid resin, absolutely nonflowing products are obtained. Preliminary removal

Table

Effect of temperature on the yields of solid polymers and still residues and on the properties of acidic high-molecular-weight products of formaldehyde purification.
(Composition of the reaction mixture: BTF — 500 ml; sulfuric acid — 10 ml (4% by weight), formalin — 5 ml (1% by weight); duration of stirring with sulfuric acid — 70 min, with formalin — 30 min.)

polymers at the sulfuric-acid stage of purification sharply reduces the content of unsaturated compounds in the fraction, but does not completely eliminate their presence. Therefore, at low process temperatures their negative influence is not yet manifested, but with increased heating and in the presence of a fresh portion of catalyst, the residual unsaturated compounds, becoming activated, polymerize vigorously, forming a small amount of very viscous

Fig. 2. Dependence of the total losses in formaldehyde purification of the BT fraction on the process temperature: 1—in the presence of acid resin; 2—without acid resin

resins, which as it were glue together the particles of solid polycondensation products, depriving them of the ability to disperse in sulfuric acid, as occurs at low temperatures.

Of course, this interpretation does not preclude other explanations of this phenomenon. As we have observed, the quality (composition) of the BT fractions has a great influence on the formaldehyde purification process. When working with some samples, even in the presence of acid resin, free-flowing polycondensation products were obtained at room temperature. However, the existing methods for assessing the quality of these fractions are very primitive and do not make it possible to identify differences in their behavior or to make well-founded judgments regarding particular results obtained.

Conclusions

1. The formaldehyde method for purifying the BT fraction from thiophene is highly effective, ensuring the production of benzene of a high degree of purity. The optimum process temperature is 20–30°C.

2. To reduce sulfuric acid consumption, it is advisable to conduct the process in the presence of acid resin; however, its implementation in ordinary apparatus (a melangeur) is impossible because of difficulties in discharging the polymers. Here, apparently, another technological design of the process should be pursued.

3. The process can be carried out in conventional equipment provided that the acid resin is removed beforehand and at temperatures not above 15°C, which is associated with increased acid consumption and the need to use artificial cooling, at least in the summertime.

1. More detailed studies of the composition of the crude and washed fractions of crude benzene must be carried out using modern methods of analysis.

2. The question of the expediency of the formaldehyde purification method for the BT fraction can be resolved only after establishing the dependence of the process on the composition of the fraction and carrying out the corresponding economic calculations.

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