RATIONAL

UTILIZATION

OF

PETROLEUM

RAW

MATERIALS

AND

RESIDUES

OBTAINING NEW PRODUCTS ON THE BASIS OF GAS OIL FRACTIONS AND HEAVY RESIDUES

A. P. Zinov'ev, P. L. Ol'kov, Sh. T. Aznabaev, and Z. I. Syunyaev

UDC 665.775.625.7.038

Petroleum raw materials entering processing contain considerable quantities of surface active materials (SAMs). As a rule, surface active properties are possessed by compounds whose molecules consist of hydrocarbon radicals and one or several active (functional) groups. In petroleum fractions the role of the latter is played by oxygen-containing (ether, carboxyl, carbonyl, hydroxyl), nitrogen-containing (nitro-, amino-, amido-, imido-groups), sulfur-containing (sulfide, thiol, sulfonate), and phosphorus- and sulfur and phosphoruscontaining groups [i]. In the course of processing the petroleum the natural SAMs undergo considerable changes: They break down, are removed from or are redistributed among the petroleum fractions. Thus, water-soluble SAMs are almost completely removed (with aqueous phases), while the oil-~oluble SAMs are concentrated in their original form or in the form of conversion products in the heavy intermediate products (asphalts, cracking residues). In the processes for purifying the petroleum distillates (sulfuric acid, contact-adsorption, catalytic, selective, etc., processes) there is a complex removal of the SAMs from the product composition without any systematic utilization. The heavy petroleum residues containing tarry asphaltene materials (TAMs) are mainly directed towards bitumen production and for the manufacture of protective structural coatings. The SAMs contained in petroleum products determine to a considerable extent their thermal stability, corrosion aggressiveness, and anti-friction, low-temperature, and other properties. Thus, the introduction of heavy petroleum residues with considerable contents of TAMs (Table i) which have well-expressed surface-active properties into such highly aromaticized products as gas oils from coking and catalytic cracking (Table 2) or extracts from the purification of lubricating oil fractions makes it possible to regulate the low-temperature, lubricating, protective, and binding properties of the mixtures which are obtained. The physicochemical properties of the lubricating oil extracts (fraction 300-400~ are as follows: density at 20~ 938 kg/m3; relative viscosity at 50~ 1.91 ~ congealing temperature: -2~ flashing temperature (in open crucible): 168~ sulfur content: 2.1 wt. %; group contents of hydrocarbons (wt. %): paraffinic-naphthenic, 31.4; aromatic, 65.4, including monocyclic, 29.6, bicyclic, 28.6, polycyclic 7.2; tars desorbed by benzene, 2~ tars desorbed by alcohol-benzene mixtures, 0.8. The petroleum products produced on the basis of the gas oil fractions and heavy petroleum residues can be used in various areas of the national economy: in mining and metallurgical plants, in industry, and in construction (for preventative measures, as dust-binding agents, and for fixing soils). The development and introduction into use of new, highly effective petroleum products based on unpurified distillates with heavy residues is one route to the rational utilization of the petroleum SAMs, since by introducing a certain quantity of heavy petroleum residues into highly aromaticized products it is possible to control the usage properties which are required in these petroleum products which are under development. For given mixtures it is usual to find changes of the physical-mechanical properties which show maxima and minima depending on the concentration of cracking residue (Fig. I) since various structural units are formed as a result of the interaction of the TAMs with the solid hydrocarbons in the disperse petroleum systems (DPSs). By varying the ratio of heavy petroleum residue in the mixture it is possible to control the process of structure forma-

I. M. Gubkin Institute of Oil and Gas, Moscow. Ufa Petroleum Institute. from Khimiya i Tekhnologiya Topliv i Masel, No. 12, pp. 5-9, December, 1988.

0009-3092/88/1112-0525512.50

Translated

9 1989 Plenum Publishing Corporation

525

TABLE i

Parameter Density at 20~ kg/m= Molecular weight Relative viscosity at lO0~ ~ [conventional viscosity] Temperature, ~ of congealing of flashing (in open crucible) Sulfur content, wt. % Group content of hydrocarbons, wt.% paraffinic-naphthenics light aromatics middle aromatics heavy aromatics tars desorbed by benzene tars desorbed by alcoholbenzene mixtures asphaltenes carbenes, carboids Concentration of PMTs x i018 spin/g Q u a l i t y of asphaltenes

molecular weight content of C atoms, wt. % content of H atomsp wt. % concentration of PMTS• sDin/g

I Cracking residue Asphalts resid- distil- from Arlansk Petroleum ual late feedfeedstocks stocks 1006 580 4.2

1095 532 12.3

992 560 17.4

12 176 2.47

30 208 3.2

26 258 3.76

18.7 7.6 12.3 34.4 7.7

6.9 3.1 I0.2 44.2 9.5

16.2 8.1 9.3 30.2 ii.2

i0.5 8.6 0.2

12.6 12.6 0.9

17.3 7.5 0.2

6. ~1

9.23

4.26

1020 84.38 5.58

980 88.83 6.58

2280 83.24 6.54

7.52

8.~0

~.77

tion. The TAMs contained in petroleum residues influence the structural-mechanical strength, viscosity, and temperature properties of the DPSs. They possess good depressor properties, particularly in highly aromaticized distillates with small contents of solid paraffinic hydrocarbons. In addition, TAMs improve the binding properties of disperse petroleum systems. The need for combining in a given product good low temperature, lubricating and dustbinding properties required a complex investigation of the heavy petroleum residues as monofunctional additives (depressor, antifriction, and adhesion additives). The low temperature properties are one of the most important usage properties of the new products. When they congeal, structural coagulation of the disperse phase occurs; as a result of this, a three-dimensional network is formed in the system of paraffins and lowcongealing hydrocarbons which prevents its movement by immobilizing the liquid phase [2]. As a result of adding the TAMs the structure formation begins at lower temperatures. The depressor properties of the TAMs has long been known [3]. However, until recently the TAMs or their concentrates (tars, cracking residues) have not been used for formulating new products with good low-temperature properties. The TAMs were first proposed as depressor additives in the development of gas turbine fuels [4]. In investigating the effect of surface-active materials containing TAMs on the process of crystallization of paraffinic hydrocarbons it was found that depending on their chemical composition, they showed a bulk or adsorption (surface) mechanism of action. Resins act'as depressors of the bulk type. Their molecules are characterized by the presence of long aliphatic side chains which penetrate into the crystal structure of the paraffinic hydrocarbons, showing a bulk effect expressed by a change in the crystal form. Asphaltenes act as additives of the surface type; they adsorb onto the crystals separating out and prevent them from coming together. To a certain SAMs of the two types can show synergistic or antagonistic effects. The fact that the dependence of the congealing temperature of mixtures of the gas oil fractions with cracking residues on their fractional compositions can show maxima or minima (see Fig. I) is caused by the features of structure formation in DPS in question. Visual observations under the microscope have shown that the paraffinic hydrocarbons in gas oil fractions crystallize in the form of thin plate-like crystals (lamellae) of the orchorhombic system. Bulk electric charges are formed on these crystals as a result of the diffusional transfer of heat transfer medium through the interface between the liquid and solid phases [5]. In the absence of steric hindrance the crystals orient themselves perpendicular to each 526

TABLE 2 Gas oil from catalytic cracking

Gas oil from cokin6 .......................

Parameter

[

Density at 20~ kglm3 Temperature, ~ of congealing - 9f..flashing ( i n open crucible) Viscosity at 20~ m 2 / s e c Group content of hydrocarbons, paraffinic 9 naphthenic

aromatic. monocyclic noncondensed polycyclic Condensed polycyclic alkylbenzenes indanes, tetralins

190-- { 190--350aC [ 190--400~ 320~

190--300~

I 350 '9~~

[ 400 '9~~

190-300 ~

882

884

902

916

881

883

892

898

--30 80

--26

5,2

--4 92 22,8

--36

4,1

--20 86 I0~,

80 2,1

--22 82 3,7

--8 84 6,2

2 86 12.2

16,4 23,0

12,2 18,9

17,7 = 17,5

18,8 19,0

21.4 19,8

23,2 22,1

12,6 10,4

10,2 8,7

10,3 7,2

12,1 6.9

13,4 6,4

16,8 5,3

60,6

68,9

64,8

62,2

58,3

-- 54,7-

5,l 8,7 12,6 14,5 16,6

5~ 6,1 9,5 16,2 28,_9

I0,I 14,3 22,4 lO~ _3,2

8,2 lO,l 18,4 15,2 5,4

6,3 6,1 15,9 16,4 9,8

5,1 3,8 12,0 17,6 [2,4

30 36 38

12 22 24

24 36 42

32 40 44

28 34 36

12 18 22

82

19,3

18~

24,2

23,3

13,0

12,8 10,5 58,5 62 9,8 14,9

11,2

56,5 6.5 12,8

naphtlmlenes acenaphthenes antht-acenes, phenanthrenes

_

190-320oc

18,3 8,2

4,1

12,6

10,6

Maximum depression of the con' gealing temperature,"C for residual cracklingresidues - - 26 28 for distillate cracking residues 40 32 for a mixture of~distillate and 132 34 residual cracking residues (60:40) ............

a~

b

;0

f

4J

-70

0

~0

60 0

~0

8O

Fig. I. The congealing temperatures tr of gas oils from coking (a) and catalytic cracking (b) as functions of the content c of distillate cracking residue in them: I) 190-300~ 2) 180-320~ 3) 190-350~ 4) 190-400~C.

other and are distributed uniformly over the volume, which makes it possible for even a small quantity (less than i wt.%) of the disperse phase to immobilize the entire volume of liquid. Upon introducing up to 5 wt.% of cracking residue the congealing temperature of the gas oil mixture falls sharply, and a minimum occurs on the curve for the dependence of the congealing temperature of the gas oil fraction on the cracking residue content. During the cooling of these petroleum products adsorption occurs of the asphaltenes as the most active components of the TAMs from molecular solution onto the rapidly growing faces of the crystals of the solid hydrocarbons, which retards their further growth. In the presence of TAMs the rate of growth of the individual faces is equalized and structural units of isometric form are produced. At cracking residue contents of greater than 5 wt.% the equilibrium forms of the structural units become nonuniform again as a result of the adhesion of separate monocrystals to give so-called skeletal and dendritic forms. The congealing temperature of the mixture increases as a result. At cracking residue contents of more than 25 wt.% blocking of the surfaces of the crystals of solid hydrocarbons occurs in the earlier stages of their growth. As a result, a fine crystalline structure is formed and the form of the structural units becomes close to spheri-

527

20

a

-20

20

/

10

-10

25 .... b

/

/

l

,...J U

0

5 ZO

#0

0

60

/ ..........

20

~0

6O

Ct, vr.. ~

Fig. 2 . Dependence of the congealing temperature t= (a) and the viscosity E (b) "of the extract from a 300-400~ lubricating oil fraction on the tar content C c in it.

TABLE 3 ,,,

Parameter L

IUniversin V (hi~. ] Univers~ (s=~r) L

Universia S (.orU~e~)

jing tO TU Tech. ~( co" TU [T~cn.

TU [T_ech.

I v i s c o s i t y ) a c c o ~ - t accora~q~ t o

Spec.] 38.1[0~1776-7

Relative viscosity at 50~ ~ Temperature, ~C, of flashing (in open crucible), not less than coCOngeal~ing, not Ereater than ntent~ ~, not Er~ater than~of water mechanical impuriti6S

15--25

i 15o

15

0,5 0,3

c.

S

"1

,,,,,,,,,,,,,

accora'xp~ t o

L

S ec.

,

3,5--5

15o

--5 . . . . . . . . . . .

0,3

,,,,

.

t,

,,,,

,

Severin-2

according to~J Tech. S ec. ]

18.10186~-81 .

,,,,,,,

,,

1,3--3

9

8o . . . .

...... . ~

. . . . . . .

80

~40 ....................... - - ~ Traces

0,3

~

_

1,1--1,5 . . . . .

......

0,2

cal. Some of the reasons for the transition to crystallization in the spherolytic form are: an increase in the rate of secondary nucleation, and a reduction in the diffusion coefficient of the TAMs as the concentration of the heavy aromatic hydrocarbons of the cracking residues increase in the dispersion medium. Two types of spherolites are formed in the crystallization process: Those of radial structure, which consist of thin crystalline fibers (fibrils) radiating from a center (nucleus), and those in the form of bulbs, where the main structural units are not fibrils but lamellae. The spherolytic structural units are more compact than dendrites and have a smaller tendency to form structural frameworks, which leads to a reduction in the congealing temperature of the mixture. As a result of this a second minimum occurs on the curves giving the dependence of the congealing temperature of the gas oil fraction on the concentration of cracking residue. The mechanism noted above takes into account only the phase changes of the solid hydrocarbons and the change in shape of the structural units which are formed from the crystals. There are also TAM components of the heavy residues in real conversion systems. The molecules consisting of high-molecular-weight compounds interact among themselves and form supermolecular structures, physical associations (Van der Waals interactions) and chemical complexes (chemical interactions) [6]. Dissimilar physical associations are formed at different ratios of the high molecular weight compounds (paraffinic, asphaltenic) in the petroleum system. In the gas oil fractions the structurization of the liquid begins long before reaching the temperature at which crystallization begins as a result of the formation of the supermolecular structures of TAMs. However, the degree of structurization increases noticeably only at temperatures below the cloud point. When up to 5 wt.% of cracking residue is introduced into the gas oil fraction a first minimum appears on the curves for the congealing temperature. For all the fractions this minimum is observed at the critical concentration for micelle formation (CCM). At cracking residue concentrations greater than 5% all of the excess quantity of TAMs goes towards the formation of mixed super-molecular structures consisting of molecules of

528

tars and asphaltenes. The process of the formation of elementary associations predominates over the process of their adsorption on the surfaces of the crystals of the solid hydrocarbons. As a result, the concentration of the elementary associations increases, which leads to an increase in the congealing temperature of the mixture. At cracking residue concentrations of more than 25% (by weight) there is a decrease in the concentration of the crystalline phase, and a coagulated structure is formed from the super-molecular formations. In addition to the adsorption action of the TAMs, a bulk mechanisms of action also begins to appear: The high molecular weight paraffinic hydrocarbons are trapped by the structural units of the TAMs, which is one of the forms of solubilization. The presence of a second minimum on the curve for the congealing temperature of the mixture is mainly determined by the quantity of the crystalline phase. At cracking residue concentrations above the threshold value the structure formation of the TAMs increases the congealing temperature of the gas oil fractions, and the latter subsequently approaches the congealing temperature of the cracking residue. The presence of two extrema on the curve for the congealing temperature is a prerequisite for the development of low-congealing products in two groups. The compositions of the first group have very low congealing temperatures (-60~ low viscosities, high lubricating properties with respect to metallic surfaces. They can be used for preventative lubrications, preventing the adhesion by freezing of moist free-flowing materials to mine transportation equipment. Compositions of the second group are characterized by congealing temperatures of -40~ and good binding, lubricating, and adhesion properties. They can be used as winter dust-binding materials in open-pit mines and roads. Gas oil fractions of different origins having the same boiling temperature range have different responses to petroleum residues as depressor additives. The response of gas oils from catalytic cracking to heavy residues is somewhat higher than that of coking gas oils, except for the 190-400~ fraction. The concentration of paraffinic hydrocarbons and the viscosity of the gas oil fraction play important parts. As the viscosity decreases, the diffusion of the TAMS (acting as depressors) to the surfaces of the crystals of solid hydrocarbons is facilitated, and this changes the conditions for the formation of the solid phase at low temperatures. The fractional composition of the gas oil fractions should satisfy the following conditions: A low boiling limit will cause a fire-hazard problem with the product being developed, i.e., a low flash point. For products which are to be used in the winter period this temperature should be not lower than 80~ while for the summer period, it should be not lower than 150~ which correspond to temperatures for the onset of boiling of the fractions of 190 and 270~ respectively. The temperature of the end of boiling of the fraction is determined by the response to the heavy petroleum residue and by the requirements with respect to the congealing temperature of the product. The data on the effect of the temperature of the end of boiling of the gas oil fraction on the response to the cracking residue show that the maximum depression of the congealing temperature occurs for the coking gas oil with a temperature of the end of boiling of 350~ and for the catalytic cracking gas oil with a corresponding temperature of 320~ On the basis of these data it is proposed that the base components for the low-temperature products should have the following fractional composition: coking gas oils, 190-350~ catalytic cracking gas oils, 190-320~ The way in which the congealing temperature of the gas oil fraction changes as a function of the nature of the petroleum residue is determined by the properties of the TAMs contained in the latter. The components of the cracking residues of distillate origin contain molecules which are condensed mainly in two-dimensional planes; this leads tO their increased mobility even at high degrees of condensation. For the components of cracking residues of residual origin it is usual to find less condensed systems, and their threedimensional molecular structure causes a lower mobility of the components. This explains the sharper increase of the congealing temperature of the gas oil fraction at TAM concentrations above the optimum. When cracking residues of residual and distillate origins are mixed, a synergistic phenomenon is observed with respect to the depressor effect [7]. In this case the tars contained in the distillates cracking residue increase the solubility of the asphaltenes in the residual cracking residue. The low-temperature properties of the gas oil fractions are improved when they are mixed. A mixture of distillate and residual cracking residues in the 529

ratio 60:40 gives the greatest depressor effect on the gas oil fraction. This dependence of the change in the congealing temperature of the mixture obtained here is also characteristic for other highly aromaticized products: extracts from the purification of lubricating oil fractions when heavy petroleum residues such as tars from high-sulfur oils are added to them. The depressor properties of tars from high-sulfur oils (Arlansk petroleum) are less marked than those of cracking residues (Fig. 2a). This is explained by the relatively low asphaltene content in them, the degree of their condensation, the quantity of high molecular weight hydrocarbons in the TAMs, and their chemical composition. The occurrence of side chains on the asphaltene molecules is a steric obstacle to the formation of dense layers on the crystal surfaces. As the length of the side chains in the TAM molecules of the tars increase the adsorbed layer is converted into an usual membrane through which the molecules of the paraffinic hydrocarbons are able to diffuse freely and take part in the growth of the crystal. The requirements with respect to the viscosity properties of the products being developed are determined by the technology of their utilization to make it possible to pump and atomize them through spraying devices. For preventative agents and dust-binding materials which are used in the winter season of the year the relative viscosity should fall in the ranges 1.1-1.5 and 1.3-3 at 50~ respectively; for summer use, dust-binding materials and fixing agents for soils and friable surfaces should have values in the ranges 3.5-5 and1525, respectively. This makes it possible to pump and atomize them without preheating. The relationship for the viscosity of mixtures of extracts from lubricating oil fractions with the tars from Arlansk oil (Fig. 2b) shows that for summer dust-binding materials the limiting content of tar is 30%, while for fixing agents for soils and friable surfaces it is 70%. Depending on the loading of the highly aromaticized product with heavy petroleum residues, the resulting petroleum products will fall in the following sequence: -preventative agents against adhesion by freezing and sticking of moist friable materials to mine transport equipment, for which the low-temperature and viscosity-temperature properties are the main requirements, as well as the lubricating properties. The quantity of filler (heavy petroleum residue) in them is therefore 5-10 wt. %; -dust-binding materials for combating dust formation in open-pit mines and roads, for which the main usage requirements are the binding, dust absorption, lubricating and lowtemperature properties. The quantity of filler in the mixture therefore consists of 20-50 wt. %; -fixers (stabilizers) for solid and friable surfaces, for which the main usage property is the binding ability; the quantity of heavy residues in the mixture amounts to 60-70 wt. %. The new petroleum products have been developed on the basis of a single technology by filling gas oil fractions with petroleum residues. Thus, by introducing into gas oils from coking or catalytic cracking 5-10% of cracking resiudes, the preventative agents Niogrin and Severin-2 are obtained [8, 9], which are used for preventing the freezing together and adhesion by freezing of coals and other moist friable materials in railway and mine transport equipment. When up to 30 wt. % of a mixture of residual and distillate cracking residues are introduced into these gas oils, the low-congealing dust-binding material Universin S (northern) is obtained [I0], which is used for laying dust on open-pit mine roads in the winter period of the year for producing ores by the open-cast method in the Far North and East Siberia regions. When the extracts from the selective purification of the 300-400~ lubricating oil fractions are filled with the tars from the high-sulfur Arlansk oils to the extent of 30 wt. %, Universin L (s-mmer) is obtained, which is used for dust suppression and for preventing the sticking of rock in the spring and fall periods. When 70 wt.% of tar is added, Universin V (high-viscosity) is obtained, which is used for dust laying on open-pit mine roads in the summer period and for stabilizing soils and the friable embankments of mine roads [ii, 12]. The petroleum products which have been developed are low toxicity compounds (class IV according to COST [USSR State Standard] 12.1.007.76), have a moderate locally irritating effect on the skin and mucous membranes of the eyes, and have little effect on higher plants and the zymotic activity of soils [13]. These materials (Table 3) have been tested in more than 40 plants in the mining industry under various climatic and mining-geological conditions in the Urals, Kazakhstan, Central Asia, Eastern Siberia, and the Far North [14, 15]. The application of Universin grades L and 530

S is carried out by means of a modified PM-130 sprinkling machine or a special universal machine based on a BelAZ-540 truck. The specific consumption of the product is 0.8-1.2 kg/m z, and the effective period of dust suppression is 15-20 days. The dust content of the air at the shoulder of the road was reduced from 90-160 to 3-12 mg/m 3. Upon prolonged application of the Universin the covering of the mine roads becomes elastic and better resists the mechanical (abrasive) wear of the truck tires. A tacky quality is guaranteed two hours after treating a road, which facilitates safety of motion; after i0 hours the coefficient of friction becomes close to its value on the dry ground. The preventative agent Severin-2 is deposited on the surfaces of mine transport equipment when it is moving unloaded; it is atomized through nozzles at the preventative treatment points. Its usage is as follows: 6-8 kg per VS-IO0 dump truck; 8-10 kg per gondola car; 1218 kg per P-90 gondola wagon. The application of the soil stabilization agent for combating thermoerosion and also for consolidating the bed and shoulders of mine roads is carried out by the road grader after mixing the Universin V with sand; the consumption of Universin is 239 by weight of the sand. The technology for producing Severin-2 and Universin S has been adopted in the NovoUfimsk NPZ [Petroleum Processing Plant], and that for the production of Universin types L and V at the XXII Congress of the KPSS NPZ [Petroleum Processing Plant] at Ufimsk. Together with an increase of 13-159 in the capacity of mine transport equipment and an improvement in working conditions, the development and introduction into use of the new products based on heavy petroleum residues made it possible to achieve an economic gain of more than one million rubles.

LITERATURE CITED I. 2. 3. 4. 5. 6. 7o

8. 9. I0. ii. 12. 13.

14. 15.

Yu. N. Shekhter, S. E. Krein, and L. N. Teterina, Oil-Soluble Surface-Active Materials [in Russian], Khimiya, Moscow (1978). L. C. Gurvich, The Scientific Fundamentals of Petroleum Refining [in Russian], Gostoptekhizdat, Moscow-Leningrad (1940). V. A. Kalichevskii, Modern Methods of Lubricating Oil Production [Russian translation], Gostoptekhizdat, Moscow-Leningrad (1947). Z. I. Syunyaev, O. I. Rogacheva, and R. Ro Khabibullin, Khim. Tekhnol. Top. Masel, No. I, 21 (1965). V. A. Kuprin, P. L. Ol'kov, and R. N. Gimaev, Khim. Tekhnol. Top. Masel, No. 3, 38-40 (1982). Z. I. Syunyaev, Khim. Tekhnol. Top. Masel, No. 6, 2-5 (1985). V. A. Maksyutov, P. L. Ol'kov, Z. I. Syunyaev, et al., in book: The Chemical Technology of Petroleum and Gas Refining [in Russian], S. M. Kirov Chemico-Technological Institute, Kazan', No. 6 (1978), pp. 18-21. Z. I. Syumyaev, P. L. Ol'kov, O. I. Rogacheva, et al., Niogrin - a New Petroleum Product against Freezing and Adhesion by Freezing [in Russian], Bashknigoizdat, Ufa (1977). V. Ya. Medvedeva, G. A. Subbotina, N. I. Shal'nova, et al., Promyshlennyi Transport, No. 2, 12-13 (1979). A. N. Kunin, R. R. Zagidullin, A. P. Zinov'ev, et al., in book: Combating Silicosis [in Russian], Nauka, Moscow, Vol. 12 (1986), pp. 141-147. USSR Patent No. 507,702. USSR Patent No. 1,337,526. G. C. Maksimov, T. N. Burenko, P. L. Ol'kov, et al., in book: Industrial Hygiene, Preventative Pathology, and Toxicology [in Russian], MNllgigieny im. F. F. Erismana, Moscow (1984), pp. 83-86. P. L. Ol'kov, Z. I. Zyunyaev, O. I. Rogacheva, et al., Cornyi Zh. No. 2, 48-49 (1976). P. L. Ol'kov, A. P. Zinov'ev, O. I. Rogacheva, et al., Promyshl. Transport, No. 2, 11-12 (1977).

531