US3480686A

US3480686A - Procedure for the thermal cracking of hydrocarbons

Info

Publication number: US3480686A
Application number: US685286A
Authority: US
Inventors: Guy Martens
Original assignee: Solvay SA
Current assignee: Solvay SA
Priority date: 1966-11-24
Filing date: 1967-11-24
Publication date: 1969-11-25
Anticipated expiration: 1986-11-25
Also published as: BE706661A; DE1618971A1; NL6714561A; ES347181A1; FR1515059A; GB1142559A; CH467736A

Description

United States Patent Int. Cl. (307?: 3/30, 11/24 US. Cl. 260-679 13 Claims ABSTRACT OF THE DISCLOSURE Hydrocarbons are subjected to thermal cracking to produce acetylene, at temperatures of 1000 C. and higher admixed with a diluent of aromatic hydrocarbon.

BACKGROUND OF INVENTION The invention relates to an improved process for the production of acetylene by the cracking of hydrocarbons.

It is well known that acetylene may be obtained from hydrocarbons by providing, in the cracking zone, a temperature in the range of about 1000 to 1600" C., and even higher. However, at these temperatures, the rate of dissociation of acetylene into its elements, carbon and hydrogen is not negligible, and it is recommended that the hydrocarbons remain only for a very short period, within this cracking zone generally between 0.5 and a few tenths of milliseconds.

To prevent the decomposition of acetylene, the reduction of the period during which the cracked products are heated above 1000 C. has been attempted.

This may be made possible by providing a high gas feeding speed by maintaining a low partial pressure of hydrocarbons. The low partial pressure may be obtained by operating under a partial vacuum and/or by adding a sufficient quantity of gaseous diluent to the hydrocarbon. The use of a diluent also has the advantage of promoting the production of acetylene because the equilibrium then has a high tendency to be displaced towards the thus decreasing the quantity of acetylene which may be obtained. The speed of this secondary reaction rapidly increases when the temperature rises from 950 to 1400 C, and in the particular caseof the cracking of ethane, at about 1400 C., H. S. Glick (7th International Symposium on Combustion, 1959, pp. 98-107) has shown that about 5% of the carbon content is converted into CO by the hydrolysis reaction of acetylene, in spite of the very short contact time, which is less than one millisecond.

Even though water vapor is a very useful industrial product, it is obvious that it does not really constitute an inert diluent and it causes serious acetylene losses.

Diluents other than water vapor have been proposed. Particularly, hydrogen, which is more chemically inert, has been suggested but it must be used in a cycle operation, which is difiicult and costly since the gas is not easily condensed.

SUMMARY OF INVENTION It has been found that it is possible to reduce the stay of the cracked gases in the cracking zone, so as to considerably decrease the decomposition of acetylene into its elements, while diluting the hydrocarbon content without causing the above disadvantages.

The invention is specifically directed to an improvement in acetylenic thermo-cracking processes in which the hydrocarbons submitted to the cracking operation are heated to temperatures higher or equal to 1000 C. and are diluted with one or more compounds selected from the aromatic hydrocarbons.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Among the aromatic hydrocarbons, benzene is preferred. However, any aromatic compound may be employed, for example, naphthalene, biphenyl, anthracene or the mixture of condensed products separated by cooling the gases resulting from the cracking process may also be used.

Surprisingly, it has been found that aromatic hydrocarbons are particularly suitable as diluents in thermal cracking. They all have in common excellent heat stability at temperatures of 1000 C. and higher.

Aromatic hydrocarbons are substantially chemically inert with respect to ethylene and acetylene under the residence time conditions required for the acetylenic crackmg.

A fraction of the charge used in the acetylenic cracking produces benzene and other aromatic compounds which generally compensate for the eventual loss of diluent. Normally, the process does not require a benzene diluent supply.

By using an aromatic hydrocarbon diluent, it is possible to completely reduce the losses of acetylene which occur when the cracking process is carried out with water vapor as the diluent.

Furthermore, aromatic hydrocarbons may be separated readily by condensation. By recycling the diluent there is no inconvenient separation problems, such as those which occur when using hydrogen.

Advantageously, the diluent is benzene. Its low heat of vaporization does not require a very important amount of energy to transform it from the liquid state to the vapor state, when overheated under the cracking conditions.

The diluents according to the invention may be used with all acetylenic thermal crackings, since the heat required during the cracking operation may be supplied by means of hot gases, as in partial combustion or pilot flame crackings. Furthermore, when using Wultf type regeneration ovens or the like, the heat may be transmitted by contact with ceramic elements which have been previously heated.

When using regeneration ovens, it has been found that benzene has a favorable effect on thermal transfers. For example, by replacing water with benzene, under identical molar dilutions of the load, it is possible to double the heat transfer coeificients. This is made possible by replacing in the cracking zone, the laminar system with the turbulent system which has been found to have a favorable effect on the thermal transfer.

Generally, the molar ratio of diluent to hydrocarbon may be maintained between about 2/1 and 20/1, preferably between 4/1 and 12/1.

According to the invention, the nature of the hydrocarbon charge to be cracked has no influence on the diluent and the usual range of hydrocarbons from methane to light naphtha may be used.

For example, without limiting the object of the invention to particular cases, many comparative tests were carried out by cracking propane and n-pentane while diluting the hydrocarbon with benzene or with water vapor. The following examples, therefore are intended 3. Process according to claim 1 in which said aromatic hydrocarbon is selected from at least one member of the group consisting of a benzene, naphthalene, biphenyl, anthracene and a mixture of condensed compounds obtained by cooling the gases resulting from the cracking to illustrate the invention but must not be construed as 5 process. limiting its scope in any manner whatsoever. 4. Process according to claim 1 and the additional steps in which said aromatic hydrocarbon used as diluent is EXAMPLE 1 separated by condensation from the products of said crack- Propane diluted with benzene or with water vapor ing operation and is then recycled. in a diluent/C H molar ratio of 5/1 Was f d o a 5. Process according to claim 1, in which the cracking cracking zone maintained at 1200 C., the mixture being operation is carried out by partial combustion. under a total pressure of 0.5 atm. 6. Process according to claim 1, in which the cracking The results obtained as a function of the residence time operation is carried out by means of a pilot flame. and the nature of the diluent appear in the following 7. Process according to claim 1 in which the cracking table. The rates of conversion into C H and C H are of said hydrocarbon is carried out in a regeneration oven. given with respect to the total number of carbon atoms 8. Process according to claim 2, in which the cracking present in the propane charge. operation is carried out by partial combustion.

Diluent Benzene Water Rate of conversion of carbon C2H4 02H: C2H4+C2H2 C2H4 CzHz CZHAi-CzHQ as 3a as as: as sea 8' Residence in 0.20 0.43 0.63 0.20 0.33 0. 53 mmlsmnds- 4o 0. 13 0.41 0. e0 0. 13 0. 33 0. 4a 50 0.08 0. 47 0. 55 0.08 0. 31 0. 39

With equal residence time, it has been found that the 9. Process according to claim 2, in which the cracking rate of conversion of propane into acetylene is always operation is carried out by means of a pilot flame. higher when it is diluted with benzene: the rate of con- 10. Process according to claim 2, in which the cracking version into ethylene is the same in the two cases; it does of said hydrocarbon is carried out in a regeneration oven. not depend on the nature of the diluent. 11. A process according to claim 1 in which said paraf- EXAMPLE 2 tllgdrocarbon is in the range of methane to hght Under the Same temperature and total Pressure C0I1di 12. Process according to claim 1 in which the molar trons as given in Example 1, n-pentane, diluted with benratio of diluent to paraffinic hydrocarbon is from about zene or with water vapor so that the diluent/C H molar 2 1 to 20:1, ratio 1s 11.1/1 was cracked- 13. Process according to claim 1 in which the molar The results also expressed as a carbon fractlon of the 40 ratio of diluent to paraffinic hydrocarbon is from about converted C H appear in the following table: 4:1 to about 12:1.

Diluent Benzene Water Rate of conversion of carbon CzH4 02H: C2H4+OZH2 C2114 02H; C2H4-l-C2H2 $8 8. 3g 8. 2g o. 30 0. 17 0. 57 Resldfimce 30 0: 10 0: 44 01 63 3' 1g g9 59 mllhsecmds- 40 0. 12 0. 4s 0. e0 0112 ii 3 81 0.08 0. 49 0. 57 0.08 0. 30 0. 38

Also in this case, the use of benzene has a marked References Cited increase in the production of acetylene.

What I claim and desire to secure by Letters Patent is: UNITED STATES PATENTS 1. In an acetylenic thermal cracking process in which 3,205,048 9/1965 fi et 260677 a fluid paraffinic hydrocarbon submitted to a cracking 3,392,210 7/1968 Pulstonen 260679 operation is heated to a temperature equal to or higher than 1000" C., the improvement comprising carrying out DELBERT GANTZ Pnmary Exammer said cracking with paraflinic hydrocarbon diluted with at J, M, NELSON, As i t t E amine least one aromatic hydrocarbon in a molar ratio of dilu- 60 cut to said parafiinic hydrocarbon of at least 2:1.

2. Process according to claim 1, in which said aromatic hydrocarbon is benzene.

US. Cl. X.R. 260683