GB2066228A - Method for production of acetylene black - Google Patents

Abstract

Acetylene gas is fed to a zone of thermal decomposition together with hydrogen, nitrogen, steam, carbon monoxide, carbon dioxide and/or a exothermally decomposable hydrocarbon at a linear feed rate of from 10 to 45 m/sec and the temperature of the zone of thermal decomposition can be thus kept within the range of 1700 DEG C to 2400 DEG C to produce acetylene black. The acetylene black is excellent in hydrochloric acid absorption and electric resistance.

Description

SPECIFICATION Method for production of acetylene black This invention relates to a method for the production of acetylene black by the thermal decomposition of acetylene at a constant temperature within a fixed range.

As an excellent carbon black exhibiting high electro-conductivity and good adsorbability, acetylene black has a high commercial value. To obtain acetylene black of desired quality, however, the temperature at which the thermal decomposition of acetylene is performed must be kept within a fixed range.

Incidentally, the theoretical temperature of the flame which rises from the thermal decomposition of acetylene reaches more than about 2500"C.

Since the carbon structure which constitutes one of the important characteristics of acetylene black is seriously impaired at temperatures exceeding 2400"C, it is necessary to cool to a level below 2400"C. On the other hand, acetylene begins to decompose generally at temperatures exceeding 800"C. When the decomposition temperature falls below 1700 C, the carbon structure of the produced acetylene black becomes so weak as to be readily broken by mechanical forces and the amount of tarry substance in the black analytically rated as a volatile component is increased. This is an undesirable product.The production of acetylene black, therefore, must be performed at a temperature within the range of from 1700 C to 2400"C, preferably from 1900 C to 2300"C and befitting the quality the produced acetylene black is desired to acquire, with the temperature maintained constantly throughout the decomposition.

Conventionally in the production of acetylene black by use of acetylene as a sole raw material, for the purpose of maintaining the temperature of the zone of acetylene black formation within a desired range, it has been a usual practice to fix the heat radiating property due to the external cooling of the thermal decomposition furnace operated for thermally decomposing acetylene in accordance with the quantity of acetylene black intended to be produced. In this type of the furnace used for the production of acetylene black, however, the produced acetylene black adheres to the inner wall surface of the furnace and the hydrocarbons formed during the thermal decomposition of acetylene decompose on the inner wall surface and produce carbon.Consequently, the thickness of the carbon layer formed on the furnace's inner wall and the thermal conductivity are changed with lapse of time, and the heat radiating property of the furnace and the inner temperature of the furnace are proportionally changed. The quality of the produced acetylene black, therefore, lacks constancy.

Further, since the conventional method involves the external cooling of the decomposition furnace, it entails a disadvantage that the temperature difference between the central portion and the peripheral portion in the zone of thermal decomposition is so large as to impair the uniformity of the quality of black product For the production of acetylene black by the combined use of acetylene and other hydrocarbons, several methods have been proposed. The method disclosed in Japanese Patent Publication No. 30314/1968, for example, aims to improve the structure of carbon black by first subjecting other hydrocarbons to partial combustion and thermal decomposition and secondarily incorporating into the produced carbon black aerosol a supplementary amount of acetylene.Japanese Patent No. 162,029 discloses a method for producing carbon black by treating a mixture of acetylene with other various hydrocarbons in the same externally-cooled thermal decomposition furnace as used in the aforementioned thermal decomposition of acetylene alone. The production of carbon black by use of this particular furnace, however, entails the disadvantage mentioned above. Moreover, the publications mentioned above fail to recognize significance in the difference between endothermally decomposing hydrocarbons and exothermally decomposing hydrocarbons as the components for the mixtures involved.

This invention aims to eliminate the various disadvantages suffered by the conventional method and, to that end, provide a method for producing acetylene black excelling in hydrochloric acid absorption, electric resistance and other properties, by feeding to the zone of thermal decomposition, acetylene gas in conjunction with at least one member selected from hydrogen, nitrogen, steam, carbon monoxide, carbon dioxide and exothermally decomposable hydrocarbon, with the linear feed rate of the mixed gas fixed within the range of from 10 to 45 m/sec and the temperature of the zone of thermal decomposition kept within the range of from 1700"C to 2400"C and the selected temperature maintained constantly thereby enabling the acetylene to be decomposed at the constant temperature.

This invention will be described in further detail below.

In the production of carbon black by the thermal decomposition of acetylene, varying methods are available for the purpose of maintaining the zone of thermal decomposition at any temperature within the range of from 1700"C ta 2400"C and allowing acetylene to be decomposed at a constant temperature. Among these methods is counted a method which comprises feeding acetylene downwardly from the top of a vertical furnace to the zone of thermal decomposition in conjunction with at least one member selected from hydrogen, nitrogen, steam, carbon monoxide and carbon dioxide thereby maintaining the zone of thermal decomposition constantly at a temperature within the range of from 1700 C to 2400 C.

In the production of acetylene black, there is generally adopted a vertical thermal decomposition furnace for ensuring smooth discharge of the acetylene decomposition products including the formed acetylene black. As the acetylene is fed downwardly from the top of the vertical furnace, it is thermally decomposed to produce acetylene black and hydrogen gas. The thermal decomposition of the acetylene to be subsequently supplied is continued owing to the large amount of heat of decomposition generated during the thermal decomposition.Acetylene decomposes at temperatures exceeding the level of about 800"C. To obtain acetylene black of good quality which possesses stable carbon structure and good electric resistance and other properties, however, the decomposition temperature must be maintained constant within the range of from 1700 C to 2400"C, preferably from 1900 C to 2300"C. Generally, the temperature for the production of acetylene black is determined by the amount of heat evolved by the decomposition of acetylene, the amount of heat radiated from the zone of reaction to the zone outside it and the amount and specific heat of substances present within the zone.If, in this zone of reaction, any reaction other than the thermal decomposition of acetylene takes place, then the decomposition temperature is additionally affected by the amount of heat generated or absorbed by such other reaction.

This invention contemplates feeding, in addition to acetylene as the raw material, at least one member selected from hydrogen, nitrogen, carbon monoxide, carbon dioxide and steam in an amount regulated proportionately to the change in the amount of the discharged heat due to the change in the thickness of the carbon layerformed on the inner wall surface ofthefurnace and, by virtue of the ensuing endothermal reactions such as water gas reaction and producer gas reaction which occur between such feed gas and carbon, maintaining constant the internal temperature of the furnace, particularlythetemperature of the zone of the acetylene black formation and precluding otherwise possible variation in the quality of the produced acetylene black.

Although at least one member selected from hydrogen, nitrogen, carbon monoxide, carbon dioxide and steam may be fed as mixed with acetylene into the furnace or separately of acetylene via an inlet formed on the furnace wall, it is used more effectively when it is fed as mixed with acetylene.

Alternatively, the desired maintenance of the zone of the acetylene black formation constant at a fixed temperature can be effected by a method which comprises feeding an exothermally decomposable hydrocarbon together with acetylene.

This method, on being applied to the production of carbon black by the thermal decomposition of acetylene which has a feed temperature of 25"C and which, on thermal decomposition, gives a decomposition product consisting of hydrogen gas and carbon and possessing a theoretical maximum temperature of about 2500"C, enables the decomposition of acetylene to proceed constantly at a fixed temperature within the range of from 1700"C to 24000C by admixing an exothermally decomposable hydrocarbon having a lower theoretical maximum temperature than acetylene, in a gaseous state, with acetylene thereby lowering the overall apparent theoretical maximum temperature to a level within the range of from 1700"C to 2400"C and, proportionately to the change in the amount of heat spontaneously discharged from the furnace, allowing the mixed gas either in a pre-heated state or in a state having the acetylene content suitably increased, to undergo exothermal decomposition within the furnace.

Specifically, as a measure for enabling the decomposition temperature of the hydrocarbon to be maintained constant at a fixed level during the production of acetylene black by the exothermal decomposition of acetylene, this method involves determining the composition of the mixed gas on the basis of the amount of the energy which is released during the fission of the interatomic bonds within the molecule of the hydrocarbon as the raw material, so that the apparent theoretical maximum decomposition temperature will be within the range of from 1700"C to 24009C.

Although the furnace to be used in this invention is not externally cooled on purpose, it nevertheless radiates heat. To make up for that particular loss of heat, therefore, the raw materials are preheated.

Optionally, this compensation may be effected by increasing the production of acetylene.

The other gas which is used as mixed with acetylene gas in the method of this invention is required to have no significant effect upon the thermal decomposition of acetylene, to react very little with the acetylene black and the hydrogen produced by the decomposition and to remain substantially inactive in the production of acetylene black. Examples of gases satisfying this requirement include hydrogen, nitrogen, carbon monoxide, and carbon dioxide and steam which are inductive of the water gas reaction and the producer gas reaction.

The feed amount of this addition gas relative to that of acetylene cannot be easily fixed specifically, because it is variable with the structure of the decomposition furnace and the linear feed rate of acetylene.

Generally, however, it is within the range of from 2 to 50% by volume, preferably from 5 to 25% by volume.

The reason for this range is that the purpose of this invention cannot be attained when the feed amount falls outside this range.

It apparently seems that for the hydrocarbon used in the present invention to fulfil the role of adjusting the apparent maximum temperature of the resultant mixture, it could be endothermal in nature. In actuality, however, it must be exothermal. This is because, as shown by a study made on the decomposition reaction from the microscopic point of view, a temperature difference, if any, between the decomposition point of the acetylene molecule and that of the addition hydrocarbon brings about an undesirable effect upon the structure of the produced carbon black. It follows that the exothermal addition hydrocarbon gives better results when the amount of heat generated by its decomposition is large to some degree. Furthermore, the exothermally decomposable hydrocarbon is preferred from the standpoint of permitting the decomposition to continue smoothly.

Examples of hydrocarbons which meet the requirement described above include ethylenic unsaturated hydrocarbons, aromatic unsaturated hydrocarbons, monocyclic unsaturated hydrocarbons and polycyclic unsaturated hydrocarbons.

When the quality of the produced carbon black, the amount of heat produced by the decomposition, the availability, the economy, and the like are taken into consideration, commercially advantageous hydrocarbons turn out to be benzene, toluene, ethylene, butadiene, etc.

Table 1 shows the theoretical maximum temperatures and heats of formation involved in the decomposition of some of hydrocarbons.

An unsaturated hydrocarbon must be converted in advance to a gaseous phase so as to be uniformly mixed with acetylene. Although the gaseous hydrocarbon may be fed to the zone of decomposition via an inlet different from the inlet used for acetylene, it is desired to be fed as mixed with acetylene.

The addition of the unsaturated hydrocarbon not merely ensures stable supply of acetylene black of high quality but also offers a significant advantage that the carbon present in the unsaturated hydrocarbon being used for the adjustment of the product quality can be recovered as part of the carbon black.

TABLE 1 Maximum temperature Heat of formation reached Kcal/kg. MOL ("K) ("C) (1 atm, 250C) C2H2 (acetylene) 2,770 2,500 54.19x103 (G) C3H4 (methyl acetylene) 1,730 1,460 44.32x103 (G) C4H6 (ethyl acetylene) 1,280 1,010 39.48x103 (G) C4H6 (1,2-butadi ene) 1,260 990 38.77x103(G) C4H6 (dimethyl acetylene) 1,180 910 34.97x103(G) C4H6 (1,3-butadi ene) 990 720 26.33x103 (G) C5H8 (1,2-penta diene) 990 720 34.80x103(G) C8Hs (styrene) 870 600 35.22x103 (G) C2H4 (ethylene) 870 600 12.50x103 (G) C6H6(benzene) 750 480 19.82x103 (G) C7H8(toluene) 550 280 11.95x103(G) C3H6(propylene) 450 180 4.88x103(G) C8H1o (ethyl- benzene) 400 130 7.12x103 (G) C8H1o (p-xylene) 350 80 4.29x103 (G) The feed amount of the addition hydrocarbon relative to that of acetylene is such that the apparent theoretical maximum temperature of the mixed gas will reach a level within the range of from 1700"C to 2400"C. Specifically, it is 5 to 40 mol%. It is within the range of from 10 to 40 mol% in the case of benzene (2250C) and from 5 to 30 mol% in the case of toluene (2250C).

The linear feed rate of acetylene to the thermal decomposition furnace must be within the range of from 10 to 45 m!sec. When acetylene is used as mixed with at least one member selected from hydrogen, nitrogen, carbon monoxide, carbon dioxide and steam and an exothermally decomposable unsaturated hydrocarbon, the linear feed rate of the mixed gas must be within the range mentioned above. When the linear feed rate is less than 10 m/sec, there ensues a disadvantage that the possibility of the flame of decomposition escaping from the interior of the furnace into the inlet device and inlet duct for acetylene gas or the mixed gas. When the linear feed rate exceeds 45 m/sec, however, there ensues a disadvantage that the hydrochloric acid absorbing property, a characteristic of the produced acetylene black, is seriously impaired.

The structure of the acetylene black can be generally estimated from the electron photomicroscopes or by the magnitude of hydrochloric acid absorbing property. When the acetylene black is found to have a large capacity for hydrochloric acid absorption, it can be safely concluded to possess a high carbon structure.

The method of this invention can be applied to the furnaces of the type generally adopted heretofore for the production of acetylene black. It proves particularly effective, however, when it is carried out in furnaces of a thermally insulated construction such as those built of refractory bricks and insulating bricks.

The following are characteristics of the present invention: (1) It permits easy adjustment of the temperature of formation of acetylene black abnd easy adjustment of the quality of the produced acetylene black and, therefore, warrants production of acetylene black of stable structure.

(2) It ensures production of acetylene black of consistent quality.

(3) It obviates the conventionally inevitable suspension of operation due to decline of the quality of acetylene black and promises a generous addition to the operating time.

Now, the present invention will be described more specifically by the following examples, which are purely illustrative of and not limitative in any sense of the invention, with reference to the accompanying drawings, in which: Figure 1 shows the relation between the molar fraction of benzene and the decomposition temperature, and Figure 2 shows the relation between the molar fraction of toluene and the decomposition temperature.

The values indicated along the curves of the graphs represent preheating temperatures.

Example 1: Acetylene was fed at a rate of 40 Nm3/hr into a vertical thermal decomposition furnace 0.4 m in inside diameter and 2.4 m in length via an acetylene inlet device disposed on the top of the furnace, and subjected to thermal decomposition. To control the quality of the produced acetylene black, the produced black was sampled and assayed at fixed intervals of 6 hours. Depending on the product quality so determined, the feed volume of steam added was adjusted within the range of from 0.5 to 1.2 kg/hr.

The linear feed rate was 27.0 to 27.6 m/sec. The inner temperature of the furnace was measured by inserting a carbonaceous protective tube through a temperature measuring port formed art a position of the furnace 0.6 m from the furnace top, placing the leading tip of this tube at the center of the furnace interior, and measuring the temperature of the tip of the tube by means of an optical pyrometer. Thus, the temperature was found to be within the range of from 2050"C to 21 50"C.

This operation was continued for seven days without a stop. The quality of the acetylene black so produced is shown in Table 2. It is noted from this table that the product had high quality, showing a high hydrochloric acid absorbing without any appreciable variation and also exhibiting a small variation in the electric resistance.

Example 2: Acetylene black was produced by feeding a mixture of 40 Nm3/hr of acetylene and 0.5 to 1.6 Nm3/hr of carbon dioxide gas to the same apparatus as used in Example 1.

The amount of carbon dioxide mixed with acetylene was adjusted within the range mentioned above, in accordance with the quality of acetylene black determined at fixed intervals of 6 hours. The linear feed rate of the mixture of acetylene and carbon dioxide gas was 26.5 to 27.2 m/sec.

The interior temperature of the furnace measured by the same method and at the same position as involved in Example 1 was within the range of from 2070"C to 21 800C.

The properties of the acetylene black produced by the operation performed continuously for seven days without a stop are shown in Table 2.

Comparative Example 1: Acetylene alone was fed to a furnace having the same inside diameter and length as the furnace of Example 1 and cooled externally with water by means of a jacket.

To maintain the quality of the produced acetylene black as uniformly as possible, the feed volume of acetylene was adjusted within the range of from 34 to 40 Nm3/hr. The results of the operation continued for seven days without a stop are shown in Table 2. In this case, the temperature of the zone of thermal decomposition was 21 20"C to 22909C.

TABLE 2 n=28 Example 1 Example 2 Comparative Example 1 X R X R Amount of hydrochloric acid absorbed 16.1 0.4 16.2 0.4 15.5 1.0 (m1/5g ) Electric resistance 0.208 0.014 0.210 0.011 0.215 0.034 (Q-cm) (Note 1) The amount of hydrochloric acid absorbed and the electric resistance were determined in accordance with JIS K-1469.

(Note 2) Xdenotes the average value and R the range.

Example 3: The procedure of Example 1 was repeated, except half of the steam supplied was substituted with carbon dioxide. The results were similar to those obtained in Example 1.

Example 4: Acetylene was continuously fed at a rate of 40 Nm3/hr into a vertical decomposition furnace 0.4 m in inside diameter and 2.4 m in length via an acetylene gas inlet device provided at the top of the furnace, and subjected to thermal decomposition. Hydrogen gas was admixed continuously at a rate of 4 Nm3/hr with acetylene. At the same time, 4 to 10 Nm3/hr of hydrogen was fed through four inert gas inlets disposed on the furnace at a distance of 0.4 m from the top of the furnace. The amount of hydrogen fed through the inert gas inlets was adjusted within the range mentioned above, according to the quality of acetylene black sampled at fixed intervals of 6 hours.

The linear feed rate of the mixed gas of acetylene and hydrogen to the furnace interior was 28.3 m/sec. The interior temperature of the furnace was determined by inserting a carbonaceous protective tube through a temperature measuring port disposed at a distance of 0.6 m from the top of the furnace, placing the leading tip of the tube at the center of the furnace interior and measuring the temperature of the tip of the tube by means of an optical pyrometer. The temperature was in the range of from 2080"C to 21700C.

The operation was continued under these conditions for 7 days without a stop. The properties of the acetylene black so obtained are shown in Table 3. It is noted from the table that the product had excellent quality, showing a high capacity for hydrochloric acid absorption without any significant variation and displaying a small variation in the electric resistance.

The results indicate that the amount of hydrochloric acid absorbed is correlated with the carbon structure of the acetylene black, that this amount increases with the degree of stability of the structure and that, therefore, when the acetylene black shows a high capacity for hydrochloric acid absorption, it can safely be concluded to possess a high degree of structure. The values of electric resistance given in Table 3 were those of crushed powders of acetylene black measured under the pressure of 49 MPa. Since the electric resistance is heavily varied by the temperature of the zone of decomposition, it has been difficult to decrease this variation.

The amount of hydrochloric acid absorbed and the electric resistance were determined in accordance with JIS K-1469.

Example 5: In the same apparatus as used in Example 4,40 Nm3/hr of acetylene was fed at a linear feed rate of 25.8 m/sec to the furnace. Nitrogen gas was used as an inert gas, and the whole amount of the inert gas was fed into the furnace interior through an inert gas inlet. The amount of the inert gas used was adjusted within the range of from 4 to 10 Nm3/hr, depending on the quality of acetylene black sampled at fixed intervals of 6 hours. The inner temperature of the furnace determined by the same method as adopted in Example 4 was in the range of from 2030"Cto 21500C.

The acetylene black obtained by continuing the operation for 7 days without a stop, similarly to that of Example 4, had excellent quality, showing a high capacity for hydrochloric acid absorption without any significant variation and exhibiting a small variation in the electric resistance. The results are shown in Table 3.

Comparative Example 2: In a furnace having the same inside diameter and length as the furnace used in Example 4 and cooled externally with water by means of a jacket, the procedure of Example 4 was repeated, except that the flow volume of acetylene was adjusted within the range of from 34 to 40 Nm3/hr to maintain the quality of the produced acetylene black as uniformly as possible and use of the inert gas was omitted.

TABLE 3 Example 3 Example 4 Comparative Example 2 X R X R X R X R Amount of hydrochloric 16.2 0.3 16.2 0.4 15.5 1.0 acid absorbed (ml/5g) Electric resistance 0.210 0.016 0.212 0.013 0.215 0.034 (Q-cm) Example 6: A mixture of 40 Nm3/hr of acetylene gas and 4.5 kg/hr of vapor-like benzene preheated to 120"C was fed to a vertical decomposition furnace 0.4 m in inside diameter and 2.4 m in length, via a nozzle provided at the top of the furnace.

The produced acetylene black was sampled at fixed intervals of 6 hours and assayed for hydrochloric acid absorption and electric resistance. The amount of benzene mixed with acetylene was adjusted within the range of from 4.5 to 12.4 kg/hr, depending on the quality of the produced black. The linear feed rate of this mixed gas was in the range of from 26.6 to 28.0 m/sec.

The inner temperature of the furnace was determined by inserting a protective tube via a temperature measuring port provided at a distance of 0.6 m from the top of the furnace, placing the leading tip of the tube at the center of the furnace interior and measuring the temperature of the tip of the tube by means of an optical pyrometer. The temperature was found to be 2040"C to 2130"C.

The operation was continued under these conditions for 7 days without a stop. The produced acetylene black showed a larger capacity for hydrochloric acid absorption with a smaller variation than the conventional countertype product. The results are shown in Table 4.

Example 7: The procedure of Example 6 was repeated, except that ethylene of 2.6 to 6.8 Nm3/hr was used in the place of benzene. The linear feed rate of the mixed gas of acetylene and ethylene was 27.4 to 30.1 m/sec. The inner temperature of the furnace was from 2060"C to 21 40"C.

Comparative Example 3: In a furnace similar to the furnace of Example 6 except that the furnace was externally cooled with water by means of a jacket, the procedure of Example 6 was repeated, except that the feed volume of acetylene gas was adjusted within the range of from 34to 40 Nm3/hr and the addition of the unsaturated hydrocarbon was omitted.

TABLE 4 Example 5 Example 6 Comparative Example 3 R R X R Amount of hydrochloric 16.2 0.4 16.2 0.3 15.5 1.0 acid absorbed (ml/5g) Electric resistance 0.210 0.008 0.213 0.05 0.215 0.034 (Q-cm) (Note) The amount of hydrochloric acid absorbed and the electric resistance were determined in accordance with JIS K-1469.

Claims (8)

1. A method for the production of carbon black by the thermal decomposition of acetylene, which method is characterised by feeding acetylene to the zone of thermal decomposition in conjunction with at least one member selected from hydrogen, nitrogen, carbon monoxide, carbon dioxide, steam and exothermally decomposable hydrocarbons, with the linear feed rate of the mixed gas kept within the range of from 10 to 45 m/sec, thereby allowing the thermal decomposition to proceed at a fixed temperature constantly within the range of from 1700"Cto 2400"C.

2. The method according to Claim 1, wherein the mixed gas is preheated proportionately to the amount of heat spontaneously generated in the zone of thermal decomposition and this preheated mixed gas is allowed to undergo exothermal decomposition within the zone of thermal decomposition for the purpose of enabling the thermal decomposition to proceed constantly at a fixed temperature falling within the range of from 1 700"C to 2400"C.

3. The method according to Claim 1, wherein the amount of the addition gas to be used as mixed with acetylene is adjusted within the range of from 5 to 20% by volume where the addition gas is one member selected from hydrogen, nitrogen and carbon monoxide.

4. The method according to Claim 1, wherein the amount of the addition gas to be used as mixed with acetylene is adjusted within the range of from 2 to 50% by volume where the addition gas is one member selected from steam and carbon dioxide.

5. The method according to Claim 1, wherein the hydrocarbon is an exothermally decomposable hydrocarbon whose decomposition product has a theoretical maximum temperature lower than acetylene, and this hydrocarbon is one member selected from ethylenic unsaturated hydrocarbons, aromatic unsaturated hydrocarbons, monocyclic unsaturated hydrocarbons and polycyclic unsaturated hydrocarbons.

6. The method according to Claim 1, wherein the amount of said hydrocarbon to be mixed with acetylene is such that the apparent theoretical maximum temperature of the mixed gas will fall within the range of from 1700"Cto 2400"C.

7. The method according to Claim 1, wherein the amount of said hydrocarbon to be mixed with acetylene is within the range of from 5 to 40 mol%.

8. The method according to Claim 1 substantially as described in any one of the Examples.