Pyrolysis of Primary Tar from Brown Coal of the Yarskoye Deposit in Tomsk Oblast
Abstract Generated abstract
This study examines whether aromatic hydrocarbons can be obtained by pyrolyzing fractions of primary tar from brown coal of the Yarskoye deposit in Tomsk Oblast. Debenzolized tar vapors were passed over fireclay packing at 650 to 680°C, with separate treatment of distillates below 300°C and those from 300 to 360°C, followed by analysis of liquid condensates, light gasoline fractions, phenols, bases, gas, and coke. The experiments established product yields and showed that pyrolysis gasoline amounted to about 10% of the original tar, with substantial aromatic and unsaturated content, while the toluene fraction reached 2.47% of the tar and gas yield was about 14%. The results suggest that primary brown-coal tar may be a potential feedstock for producing toluene, phenols, and unsaturated gaseous hydrocarbons under suitable pyrolysis conditions.
Full Text
Pyrolysis of Primary Tar from Brown Coal of the Yarskoye Deposit in Tomsk Oblast
K. K. Stramikovskaya
(Submitted by Prof. I. V. Gebler, Doctor of Technical Sciences)
At the present time, considerable success has been achieved in developing new effective methods of gasification and semicoking, but the question of the utilization and processing of the liquid products obtained in these processes still remains unresolved. Tars, or pitches, obtained as by-products in the low-temperature decomposition of solid fuel are acquiring great importance in the national economy. Along with the study of the chemical composition and properties of low-temperature pitches, no less important for finding rational methods of their utilization is the study of the transformations that pitch and its fractions can undergo during cracking, pyrolysis, and hydrogenation [1].
The aim of the present work was to determine the possibility of obtaining aromatic hydrocarbons by pyrolysis of individual fractions of the primary tar of brown coal from the Yarskoye deposit in Tomsk Oblast. The indicated brown coals occur in Tertiary coal-bearing deposits. The characteristics of the coals, the conditions for obtaining the semicoking tar, and its chemical composition were published by us in works [2, 3].
From the primary tar, a distillate boiling up to 160°C was first isolated in an amount of 5.21% by weight. This light fraction contained 2.4% bases, isolated with 20% sulfuric acid; 6.3% phenols, extracted with an 8% sodium hydroxide solution; 6.7% aromatic compounds, determined by the aniline-point method; and 63.8% unsaturates. The results of rectification of the purified light gasoline with a three-bulb dephlegmator are given in Table 1.
Table 1
Yields of products from rectification of direct gasoline from brown-coal tar
| Name of fraction | Temperature limits of fraction collection, °C | Yield, % of gasoline | Specific gravity |
|---|---|---|---|
| First drop | 40 | ||
| Head fraction | up to 78 | 10.4 | 0.784 |
| Benzene–toluene | 78—108 | 21.6 | 0.785 |
| Toluene | 108—113 | 20.3 | 0.799 |
| Toluene–xylene | 113—137 | 14.7 | 0.804 |
| Residue | above 137 | 33.0 | 0.828 |
The debenzolized primary tar was heated in a metal flask, and the vapors obtained were subjected to pyrolysis at temperatures varying within the range 650—680°C. The vapors leaving the flask were passed through a layer of fireclay grains 3—5 mm in size, placed in a copper tube. Heating of the tube with the fireclay packing was carried out by means of a tube electric furnace with regulation of the temperature in it.
The liquid pyrolysis products were condensed stepwise in two receivers. In the first receiver the temperature varied within the range 50–70°, and in the second within 20–25°C. The uncondensed vapors of the pyrolysis products leaving the second receiver with the gas were absorbed in a column by activated carbon. The gas was collected in a gasometer. The rate at which tar vapors were passed through the reaction tube was regulated by the rate of gas evolution and was maintained at 500–600 cm³/min. The entire pyrolysis process was divided into two parts: 1) pyrolysis of the vapor distillates distilled from the tar up to 300°C (pyrolysis I), and 2) pyrolysis of the vapor distillates distilled from the tar within the temperature range from 300 to 360° (pyrolysis II).
The liquid pyrolysis products condensed in the first receiver are hereinafter called pyrolysis tar, while the liquid products condensed in the second receiver are called light condensate. The residue in the metal flask, the so-called still residue, is rather a pitchy semicoke than pitch. Coke formed as a result of the pyrolysis of hydrocarbons was deposited on the chamotte grains in the copper tube. The yields of the products of distillation and pyrolysis are presented in Table 2 (1296 g of tar was taken).
Table 2
| Product name | Yield, g | Yield, percent of tar |
|---|---|---|
| Products of pyrolysis I | ||
| Tar | 422.87 | 32.63 |
| Light condensate | 141.10 | 10.92 |
| Gas | 136.00 | 10.51 |
| Products of pyrolysis II | ||
| Tar | 198.43 | 15.33 |
| Light condensate | 14.70 | 7.15 |
| Gas | 47.00 | 3.64 |
| Liquid products of pyrolysis I and II absorbed by activated carbon | 25.20 | 1.96 |
| Still residue | 96.00 | 7.43 |
| Coke from cracking and losses | 71.64 | 5.54 |
| Light straight-run gasoline | 67.26 | 5.21 |
| Water | 73.50 | 5.68 |
| Total | 1 296 | 100 |
The liquid pyrolysis products were analyzed for their content of phenols and bases. In addition, fractions boiling up to 150°C, called light pyrolysis gasoline, were distilled from the liquid pyrolysis products. The latter was subjected to rectification with a three-bulb dephlegmator. The results of the analyses are given in Tables 3 and 4.
Table 3
Characteristics of the liquid pyrolysis products
| No. | Parameters | Light condensate of pyrolysis I and II | Pyrolysis tar I |
|---|---|---|---|
| 1. | Specific gravity | 0.904 | 1.006 |
| 2. | Bases, wt. % | 6.47 | 0.23 |
| 3. | Phenols, wt. % | 17.5 | 31.00 |
| 4. | Yield of purified condensate from bases and phenols: a) based on the initial condensate, % based on the initial tar |
76.6 9.25 |
69.8 23.8 |
| 5. | Specific gravity after purification from phenols and bases | 0.876 | 0.965 |
| 6. | Yield of light pyrolysis gasoline | 37.4 | 7.25% |
Yield of pyrolysis gasoline as a percentage of the initial primary tar:
| Source | Yield |
|---|---|
| from light condensate | 4.52 |
| from pyrolysis tar I | 2.37 |
| from pyrolysis tar II | 1.24 |
| absorbed by activated carbon | 1.96 |
| Total | 10.09 |
Table 4
Fractional composition of light pyrolysis gasoline
(first drop at a temperature of 40°C)
| Name of fraction | Temperature limits for collecting the fraction, °C | Specific gravity | Yield in % of total pyrolysis gasoline |
|---|---|---|---|
| Forecut | up to 78 | 0.706 | 17.6 |
| Benzene–toluene | 78–108 | 0.792 | 15.3 |
| Toluene | 108–113 | 0.818 | 14.0 |
| Toluene–xylene | 113–135 | 0.829 | 9.9 |
| Residue and losses | above 135 | 0.854 | 43.2 |
The total content of unsaturated and aromatic compounds was determined with 100-percent sulfuric acid, taken in an amount of three volumes per one volume of fraction. Treatment with sulfuric acid was carried out with cooling and shaking for 20 minutes. The results are given in Table 5.
Table 5
Content of unsaturated and aromatic compounds in the light fractions
| Gasoline | Name of fraction | Temperature limits for collecting the fraction, °C | Amount of aromatic and unsaturated compounds in % by volume |
|---|---|---|---|
| Direct distillation | Gasoline before distillation | up to 150 | 70.5 |
| Direct distillation | Benzene–toluene | 78–108 | 71.3 |
| Direct distillation | Toluene | 108–113 | 71.9 |
| Pyrolysis | Gasoline before distillation | up to 150 | 86.1 |
| Pyrolysis | Benzene–toluene | 78–108 | 92.4 |
| Pyrolysis | Toluene | 108–113 | 92.0 |
| Pyrolysis | Toluene–xylene | 113–135 | 94.2 |
| Pyrolysis | Forecut | up to 78 | 83.0 |
The determination of unsaturated compounds in the indicated fractions was carried out by shaking 2 volumes of 80-percent sulfuric acid with one volume of fraction for 10 minutes with cooling. By this method, 19% unsaturated compounds were found in the pyrolysis gasoline, and 20% in the benzene–toluene fraction. Subtracting the unsaturated compounds from the sum of unsaturated and aromatic compounds gave 67.1% aromatics in the pyrolysis gasoline, and 72% in the benzene–toluene fraction.
Yields of the products, calculated on the primary tar, in weight percent, are given in Table 6.
Table 6
| No. | Product name | From straight-run gasoline | From pyrolysis gasoline | Total |
|---|---|---|---|---|
| 1. | Straight-run gasoline | — | — | 5.21 |
| 2. | Pyrolysis gasoline | — | — | 10.00 |
| 3. | Light ends boiling up to 78° | 0.55 | 1.77 | 2.32 |
| 4. | Benzene–toluene fraction (78–108°) | 1.13 | 1.54 | 2.67 |
| 5. | Toluene fraction (108–113°) | 1.06 | 1.41 | 2.47 |
| 6. | Toluene–xylene fraction (113–135°C) | 0.77 | 1.00 | 1.77 |
| 7. | Phenols | 0.32 | 12.31 | 12.63 |
| 8. | Bases | 0.12 | 0.78 | 0.90 |
| 9. | Gas | — | — | 14.15 |
At present the principal aim of industrial pyrolysis units is directed toward obtaining toluene and unsaturated hydrocarbons. The starting material for pyrolysis plants is kerosene, the demand for which is already very large at present. According to M. A. Dalin [4], in the pyrolysis of thermal-cracking kerosene, the yield of the toluene fraction (108–113°) is 3–3.2% of the feedstock, with a content of aromatic hydrocarbons of about 80%. In the pyrolysis of the primary tar of Yarsk brown coal, under the conditions indicated in the present work, it proved possible to obtain the toluene fraction in an amount of 2.47% of the primary tar (Table 6), with an aromatic-compound content of about 75%. The adopted temperature conditions (650–680°) are not sufficiently severe for obtaining a large gas yield, but the gas obtained contains up to 54.5% unsaturated compounds. With an overall gas yield of 14% of the primary tar, the gaseous unsaturated compounds will amount to 7.7 wt.%, with coke formation of about 5.5%. If one takes into account that in modern pyrolysis units the ethylene yield is 7–8% of the initial kerosene [4], then it is possible that the process of pyrolysis of primary tar for the purpose of obtaining toluene and unsaturated hydrocarbons will prove profitable.
Conclusions
- The yields of the products of pyrolysis of primary brown-coal tar and the fractional composition of the light distillates of the liquid pyrolysis products have been established.
- The content of phenols in the liquid pyrolysis products and their yield as percentages of the primary tar have been determined.
- The yield of pyrolysis gas has been found.
LITERATURE
- Danin V. A. Chemical Processing of Fuels. Proceedings of the Second All-Union Conference on Artificial Liquid Fuel and Technological Gases. Publishing House of the Academy of Sciences of the USSR, 18, 1957.
- Gebler I. V. and Stramkovskii V. F. Izvestiya Tomskogo politekhnicheskogo instituta, 64, 275, 1948.
- Gebler I. V. and Stramkovskaya K. K. Izvestiya Tomskogo politekhnicheskogo instituta, 64, 290, 1948.
- Dalin M. A. et al. Chemical Processing of Petroleum Hydrocarbons. Proceedings of the All-Union Conference on the Integrated Chemical Processing of Petroleum Gases. Publishing House of the Academy of Sciences of the USSR, 97, 1956.