US3317626A - Olefin pyrolysis initiators - Google Patents

Description

United States Patent Office 3,317,626 Patented May 2, 1967 of Ohio No Drawing. Filed Feb. 19, 1964, Ser. No 345,863

20 Claims. (Cl. 260-680) This invention relates generally to the cracking of olefins. More specifically it relates to a method of improving the etficiency of olefin pyrolysis by use of initiators. Most specifically it relates to methods of improving the efficiency of cracking of certain specific olefins to form specific diolefins employing olefin pyrolysis initiators.

It is known that most olefins may be thermally decomposed or cracked by subjecting them to relatively high temperatures. By the terms cracking, decomposing," cracked," decomposed, pyrolysis and pyrolyzing and the like, as employed throughout this application and the claims appended thereto, is meant that the olefin molecule splits into two fragments from the appilication of heat. (This true thermal pyrolysis process is to be distinguished from dehydrogenation processes which re quire the effect of a surface catalyst as Well as greater temperatures to remove hydrogen efficiently from molecules to form more unsaturated molecules.) These fragments themselves become molecules of other lower molecular weight materials which will contain both carbon and hydrogen atoms. This will be explained later in greater detail. Usually the thermal decomposition or cracking of olefins is conducted in a closed zone or reactor at temperatures usually ranging from about 300 C. to about 1000 C. This pyrolysis is usually conducted in the absence of oxygen. Olefins normally are cracked while they are in a gaseous state and may be cracked either relatively pure or as mixtures with other hydrocarbons, usually in mixture with a saturated hydrocarbon, i.e., a mixed feed stream of pentene and pentane or they may be in mixture with diluents such as nitrogen, steam and the like.

As indicated, the cracking of olefins usually results in the formation of two lower molecular weight materials, each of which contain both carbon and hydrogen atoms. The particular lower molecular weight materials formed when olefins are pyrolyzed depends largely upon the configuration of the olefin subjected to the cracking proc ess. By the term configuration as used throughout this application and claims, is meant the position or location of the double bonds and the position or location and type of the side groups, if any, of the olefin in question. To explain this in more detail, an olefin containing 6 carbon atoms with a side methyl group attached to the second carbon atom of the main or straight chain portion and the double bond in the 2 position, such a material is Z-rnethyl pentene-2, when subjected to cracking, will upon decomposition, produce as the predominant product the diolefin, isoprene or 2-methyl butadiene-l,3. and a lower molecular weight parafiin, methane. On the other hand, a 6 carbon olefin having a methyl group attached to the second carbon atoms of the straight chain and the double bond in the l posiiton, such a compound is Z-methyl pentene-l, when cracked, will produce as the predominant products two other lower molecular weight olefins, isobutylene and ethylene. Therefore, the configuration of the particular olefin employed usually designates the main or predominant products which result from the cracking of the olefin. These differences in product obtained upon cracking of olefinic isomers are due to the fact that olefins crack at the carbon-to-carbon single bond in the position beta to the double bond, that is, the scission of the olefin occurs at the bond that is in a position beta to the double bond or that the split in the olefin occurs between the carbon atom next to the carbon which has the double bond attached to it and the carbon atom adjacent thereto. Thus, to sum up, if olefins are to crack at all by the application of heat, they must contain in their make-up a carbon-to-carbon single bond which is in a position beta to the double bond. Therefore, wherever the terms cracking of olefins," olefins cracked," decomposition or decomposed and the like are employed in this application, is meant olefins which will crack upon the application of heat to form two lower molecular weight olefins or one lower molecular weight diolefin and a saturated hydrocarbon at the bond beta to the double bond.

Employing the most favorable conditions conducive to cracking of olefins, by application of heat alone, to form lower molecular weight materials, it has been found that olefins decompose at a low rate per pass through the cracking zone. The conditions found conducive to the cracking of olefins are the temperature in the cracking zone, the residence time in the zone and the ratio of the olefin to the diluent, if any, employed. It is usually the practice, to effect an increase in the overall yield of such a process, to separate the unreacted or undecomposed olefin from the products resulting from the decomposed portion of the olefin and return or recycle the unreacted olefin to the cracking zone. It has been found, however, regardless of how many recycles are employed, the ultimate yield or the ultimate decomposition of the olefin to the desired products at an appreciable yield per pass is not greater than about 45 mol percent of the olefin being decomposed, the remaining 55 mol percent being converted to undesirable or unwanted products as a result of side reactions caused by the high temperature, the long residence time and the recycling steps employed. Thereby, a fairly high percent of these products are, in a sense, wasted, in that the starting materials are converted to undesired products.

Therefore, this invention has as its main object a method whereby the overall yield of the desired products produced by cracking on olefin may be increased. Another object is to provide a method whereby the yield per pass of desired products obtained when olefins are cracked may be increased. Another object is to increase the ultimate yield or the ultimate decomposition of olefins to the desired products. Still another object is to provide a method whereby the residence time in the cracking zone of olefins may be decreased. Another object is to provide a method whereby olefins may be cracked at lower cracking temperatures. Another object is to provide a method whereby the formation of undesired products produced by side reactions during the cracking of olefins may be decreased. Another object is to provide a method whereby the size of the equipment necessary to crack a given volume of an olefin is reduced. Another object is to reduce the amount of material required to be recycled. Still another object is to provide a method to promote the cracking of olefins to the desired products. Other objects will become apparent as the description proceeds.

In accordance with the present invention, olefins which contain in their molecules a carbon-to-carbon single bond in a position beta to the double bond are subjected to temperatures varying from about 400 C. to about 900 C. for periods of time ranging from about 0.001 to about 3 seconds, while said olefin is in the presence of at least one olefin pyrolysis initiator of the free radical type selected from the group consisting of acetaldehyde, dimethyl ether, ethylene oxide, methylethyl ether, propionaldehyde, acetone, methylethyl ketone, propylene oxide, in order to cleave the carbon-to-carbon single bond in a position beta to the double bond of said olefin.

Generally, the cracking of olefins in accordance with the practice of this invention may be carried out in any conventional manner usually employed in the art of cracking olefins. Generally these conditions employed may be widely varied and are not critical. They usually depend upon the particular olefin to be cracked and the particular products which are desired. For instance, the cracking temperature may be varied widely from about 400 C. to about 900 C. However, it is preferred to practice this invention at temperatures ranging between 500 and 800 C. and it is more preferable to employ temperatures ranging from about 600 C. to about 750 C. The time that the olefins are in the cracking zone during the practice of this invention may range broadly from about 0.001 to about 3 seconds. However, depending upon the particular olefin cracked and the products desired, this time may vary from about 0.05 to about 1 second and it is most preferred that this time range from about 0.1 to about 0.5 second. These times are referred to usually as residence time, that is residence time within the cracking zone and are defined as the time required for 1 molecule of incoming gas, whether it be reactant, diluent or both, to pass through the cracking Zone. The cracking zone may be defined as the zone at which the temperature is elevated to the cracking temperatures as indicated above.

Generally, the olefins which are cracked in accordance with the invention may be in pure form or in mixture with other hydrocarbons. The olefins to be cracked may also be mixed with an inert diluent. It is usually desirable to employ an inert diluent when cracking olefins in accordance with this invention. The term inert diluent is defined as a material which does not appreciably react or interfere with the olefin to be cracked. Likewise, the diluent does not react with the desired products produced by the cracking at the cracking conditions employed or with initiators employed in the cracking process. Furthermore, this diluent likewise should not crack or decompose itself at the conditions employed. Examples of diluents suitable for use in this invention are steam, carbon dioxide, hydrogen, nitrogen, the inert gases such as helium, neon and argon or parafiinic hydrocarbons such as methane, ethane, or various other hydrocarbons which themselves will not crack at the temperatures employed in the cracking conditions of this invention. The ratio of diluent to olefin to be cracked which may be employed in the practice of this invention, if any be employed, may widely vary from about 0.5/1 to about or more mols of diluent per mol of olefin. However, if more than about 15/1 ratio is employed, the improvement gained does not offset the cost accrued and the process could become uneconomical. Therefore, it is usually preferred to use a ratio of diluent to olefin of from about 1/1 to about 3/1 or 4/1 in this invention.

The pressure employed in the cracking zone is not critical and may vary from about 1 millimeter of mercury to about 500 pounds per square inch gauge. However, it is preferred to employ pressures ranging from about 1 atmosphere to about 100 pounds per square inch gauge in the practice of this invention. Generally, it is preferred to employ oxygen-free conditions when practicing this invention.

By the term olefin pyrolysis initiator of the free radical type as used in this specification and claims is meant that the material generates free radicals when subjected to the operating conditions of this invention. In other words,

any material which at the operating conditions will decompose or dissociate to form a free radical will operate as an olefin pyrolysis initiator. The preferred olefin pyrolysis initiators are acetaldehyde, dimethyl ether, ethylene oxide, methylethyl ether, propionaldehyde, acetone, methylethyl ketone, propylene oxide and the like. The amount of olefin pyrolysis initiator which has been found useful in this invention and which shows good improvement over thermal pyrolysis has been found to vary broadly. (Since each mol of initiator generates one mol of free radical, the amounts are best designated as mol percents.) It has been found that as little as 0.4 or 0.5 mol percent of initiator based on the total olefin is sufficient to cause an improvement. While there is no upper lllfllt to the amount of initiator which may be employed it has usually been the practice to employ amounts Iupoto ab-otut mdol percent. However, very little imvemen 1S aine above warrant the eftra Cost. 50 mol percent which would As was stated above, this invention is directed to methods of promoting or increasing the efficiency of cracking olefins which are subject to thermal cracking processes generally. Thus, olefins which, when subjected to the practice of this invention, will split at the carbon'tocarbon single bond which is in the position beta to the double bond. It is most desirable to employ the process of this mvention with those olefins which have a carbonto-carbon single bond in a position beta to the double bond and which have the proper configuration so that when they decompose they result in products which predominantly form diolefins.

Representative among the olefins havin a carbon single bond which is in a positio n be t d tz t lie double bond which will decompose to form as a major product, butadiene-l,3 when cracked in the presence of at least one initiator of this invention. are hexenepentene-Z; cyclohexene; 3-methyl butene-l; Z-heptene' methyl hexene-2; 2-octene; 5methyl heptene-Z' 6-met hyl heptene-Z; and nonene-Z.

Representative among the olefins having a carbontocarbon single bond which is in a position beta to the double bond which will decompse to form, as a major product, Z-methyl butadiene-l,3 or isoprene when cracked in the presence of at least on initiator of this invention are Z-methyl pentene-2; S-methyl pentene-Z; Z-ethyl butene-l; 3,3-dimethyl butene-l; 2,3-dimethyl butene-l' 2 methyl hexene-Z; 3-methyl hexene-2; 3,3-dimethyl pentene-l; Z-methyl heptene-2; 3-methyl heptene-2; 3 3-dimethyl hexene-l; 2,5-dimethyl hexene-Z; 3,5-dimdthylhexene-2; Z-methyl octene-Z; B-methyl octene-Z; 3 3-dimethyl heptene-l; 2,5-dimethyl heptene-2; and 2 6-dimethyl heptene-2.

Representative among the olefins having a carbon-todouble bond, which will decompose to form, as a major product, 2-ethyl butadiene-l,3 when cracked in the presence of at least one initiator of this invention, are 3-ethyl pentene-2; 2-ethyl pentene-Z; and 3-ethyl hexene-Z.

Representative among the olefins having a carbontocarbon single bond which is in a position beta to the double bond, which will decompose to form, as a ma or product, 2.3-dimethyl butadiene-1,3 when cracked in the presence of at least one initiator of this invention, are 2,3-dimethyl pentene-Z; 2,3,3-trimethyl butene-l; 2, 3,3-trimethyl pentene-l; and 2,3-dimethyl heptene-Z.

Representative among the olefins having a carbon-tocarbon single bond which is in a position beta to the double bond, which will decompose to form, as a major product, 3-methyl pentadiene-1,3 when cracked in the presence of at least one initiator of this invention, are 3-rnethyl hexene-3; 3-methyl heptene-3; 3,4-dimethyl hexene-2; and 3,6-dimethyl heptene-3.

Representative among the olefins having a carbon-tocarbon single bond which is in a position beta to the double bond, which will decompose to form, as a major product, 2-methyl pentadiene-1,3 and 4-methyl pentadiene-l,3 when cracked in the presence of at least one initiator of this invention, are 2,4-dimethyl pentene s; Z-methyl heptene-3; 4,4-dimethyl hexene-2; 2-propyl pentene-2', 2-methyl-3-ethyl pentene-l; and 2,6-dimethyl heptene-3.

Representative among the olefins having a carbontocarbon single bond which is in a position beta to the double bond, which will decompose to form, as

major products, the piperylenes, when cracked in the presence of at least one initiator of this invention, are hexene-3; 4-methy1 pentene-Z; heptene-3; 4-methyl hexene-2; octene-3; 4-methyl heptene-Z; 6-methyl heptene-3; 4,5- dimethyl heptene-Z; and 4,5,5-trimethyl hexene-2.

The practice of this invention is illustrated by the following experiments which are intended to be represent ative rather than restrictive of the scope of this invention.

All of the experiments were performed in a cracking assembly consisting of a hairpin" coil prepared from /4 inch OD. 316 stainless steel tubing. This cracking coil was immersed in a bed of fluidized heat transfer powder. This heat transfer powder was a microspheroidal alumina-silica material normally employed as a cracking catalyst. This heat transfer powder was heated both by an electrical resistance heater and by combusting a natural gas flame directly in the fluidized powder bed. The temperature gradient from the top to the bottom of the bed was never more than about 5 to 6 C. and the gradient from the fluidized bed to the cracking zone was never more than about 5 to 6 C. The temperatures were measured within the fluidized bed by means of conventional thermocouple techniques. The cracking coil had conventional thermocopule wells and the temperature within the cracking zone was also measured by conventional thermocouple techniques. The procedure employed was to bring the heat transfer powder up to about 500 C. by employing the electrical resistance heaters, at the same time fluidizing the bed by means of air. Then a direct natural gas/air flame was employed to bring the heat transfer bed up to the desired cracking or operating temperature. The natural gas flame and products of the combustion and additional air was used to fluidize the powdered bed. The promoter or refractory material was mixed with the olefin, which was to be cracked, in the desired mol percentage prior to the olefin being passed through the cracking zone. Water and the olefin containing the promoter, if any, were pumped at the proper rates necessary to produce the desired steam to hydrocarbon ratios and at an overall rate to give the desired residence time of the materials in the cracking zone. When all variables had been adjusted to give the desired operating conditions, the products of the cracking were collected; if liquid, by means of cooled receivers, and, if gas, they were metered at atmospheric pressure and room temperature conditions. The products collected were analyzed for content and yields by conventional analytical methods. Conventional recycle techniques were employed to obtain the utlimate yields and are reported as ultimate reaction efficiencies. pass yields are reported as the yield per pass.

The results of each experiment as well as the operating conditions are reported in the table below wherein column 1 is the run number; column 2 is the residence time in seconds; column 3 is the cracking temperature in C.; column 4 lists the initiator employed, if any, and the amount in mol percent; column 5 is the yield of isoprene reported in mol per cent per mol of olefin charged and IS the single pass yield and column 6 is the ultimate yield and is reported as the mol percent of isoprene obtained per mol of olefin cracked while employing conventional recycle techniques.

In each of these examples products produced are isoprene and methane. All yields and efficiencies are based on the isoprene produced.

The per It can be seen from the results set forth in Table I that more than a 30% greater yield of isoprene per pass is obtained by the practice of this invention as opposed to true thermal decomposition of 3-methyl-2-pentene.

Similar results may be obtained employing other olefins having in their molecule a carbon-to-carbon single bond in a position beta to the double bond and other olefin pyrolysis initiators as well as other reaction conditions, all of which have been discussed elsewhere in this application.

While certain representative embodiments and details have been shown for the purpose of illustrating the invention, it will be apparent to those skilled in this art that various changes and modifications may be made therein without departing from the spirit or scope of the invention.

What is claimed is:

1. A cracking process which comprises providing a mixture of (a) at least one olefin having in its molecule a carbon-to-carbon single bond in a position beta to the double bond and (b) at least one olefin pyrolysis initiator selected from the group consisting of acetaldehyde, dimethyl ether, ethylene oxide, methylethyl ether, and propionaldehyde, said pyrolysis initiator being present in said mixture in an amount of at least 0.4 mol percent based on the olefin, subjecting said mixture to temperatures ranging from about 400 C. to about 900 C. for periods of time ranging from about 0.001 to about 3 seconds. to cleave the carbon-to-carbon single bond which is in a position beta to the double bond of said olefin.

2. A cracking process which comprises providing a mixture of (a) at least one olefin selected from the group consisting of 2-methyl pentene-2; 3-methyl pentene-Z; 2-ethyl butene-l; 3,3-dimethyl butene-l; 2,3-dimethyl-butene-1; 2-methyl hexene-Z; 3-methyl hexene-Z; 3,3-dimethyl pentene-l; Z-methyl heptene-Z; 3-methyl heptene-Z; 3,3-dimethyl hexene-l; 2,5-dimethyl hexene-2; 3,5-dimethyl hexene-2; and (b) at least one olefin pyrolysis initiator selected from the group consisting of acetaldehyde, dimethyl ether, ethylene oxide, methylethylether, and propionaldehyde, said pyrolysis initiator being present in said mixture in an amount of at least 0.4 mol percent based on the olefin, subjecting said mixture to temperatures ranging from about 400 C. to about 900 C. for periods of time ranging from about 0.001 to about 3 seconds, to cleave the carbon-to-carbon single bond which is in a position beta to the double bond of said olefin, and thereby forming isoprene.

3. A cracking process which comprises providing a mixture of (a) at least one olefin selected from the group consisting of 3-ethyl pentene-Z; Z-ethyl pentene-2 and 3- ethyl hexene-2 and (b) at least one olefin pyrolysis initator selected from the group consisting of acetaldehyde. dimethyl ether, ethylene oxide, methylethyl ether, and propionaldehyde, said pyrolysis initator being present in said mixture in an amount of at least 0.4 mol percent based on the olefin, subjecting said mixture to temperatures ranging from about 400 C. to about 900 C. for periods of time ranging from about 0.001 to about 3 seconds, to cleave the carbon-to-carbon single bond which is in a position beta to the double bond of said olefin and thereby forming Z-ethyl butadiene-l,3.

4. A cracking process which comprises providing a mixture of (a) at least one olefin selected from the group consisting of hexene-Z; 3-methyl pentene-l; pentene-Z; cyclohexene; 3-methyl butene-l; Z-heptene and S-methyl TABLE I [Pyrolysis ol' fl-Methyl-Q-Pontene to Isopreue] Exp. No. Residence Temp, C. Initiator and Amount, Isoprcno i Reaction Time, Sec. Mole Percent Yield, Mole Etllc., Percent Percent l l n 0.15 668.5 Nonezusnns." CV... 15.6 61.2 2 0. 15 673. 0 Acetaldchytle, 10% 2t). 4 61. l

hexene-Z and (b) at least one olefin pyrolysis initiator selected from the group consisting of acetaldehyde, dimethyl ether, ethylene oxide, methylethyl ether, and propionaldehyde, said pyrolysis initiator being present in said mixture in an amount of at least 0.4 mol percent based on the olefin. subjecting said mixture to temperatures ranging from about 400 C. to about 900 C. for periods of time ranging from about 0.001 to about 3 seconds, to cleave the carbon-to-carbon single bond which is in a position beta to the double bond of said olefin and thereby forming butadiene-1,3.

5. A cracking process which comprises providing a mixture of (a) at least one olefin selected from the group consisting of hexene-3; 4-methyl pentene-Z; heptene-3; 4- methyl heXene-2; octene-3; 4-methyl heptene-Z and 6- methyl heptene-3 and (b) at least one olefin pyrolysis initiator selected from the group consisting of acetaldehyde, dimethyl ether, ethylene oxide, methylethyl ether, and propionaldehyde, said pyrolysis initiator being present in said mixture in an amount of at least 0.4 mol percent based on the olefin, subjecting said mixture to temperatures ranging from about 400 C. to about 900 C. for periods of time ranging from about 0.001 to about 3 seconds, to cleave the carbon-to-carbon single bond which is in a position beta to the double bond of said olefin and thereby forming piperylene.

6. A process according to claim 1 in which the olefin pyrolysis initiator is acetaldehyde.

7. A process according to claim 1 in which the olefin pyrolysis initiator is ethylene oxide.

8. A process according to claim 2 in which the olefin is Z-methyl pentene-2.

9 A process according to claim 2 in which the olefin is 3-methyl pentene-Z.

10. The method according to claim 3 in which the olefin is 3-ethyl pentene-2.

11. The method according to claim 4 in which the olefin is peritene-2.

12. The method according to claim 5 in which the olefin is heXene-3.

13. The method according to claim 5 in which the olefin is 4-methyl pentene-2.

14. The method according to claim 8 in which the olefin pyrolysis initiator is acetaldehyde.

15. The method according to claim 9 in which the olefin pyrolysis initiator is acetaldehyde.

16. The method according to claim 8 in which the olefin pyrolysis initator is ethylene oxide.

17. The method according to claim 10 in which the olefin pyrolysis initiator is acetaldehyde.

18. The method according to claim 11 in which the olefin pyrolysis initiator is acetaldehyde.

19. The method according to claim 12 in which the olefin pyrolysis initiator is acetaldehyde.

20. The method according to claim 13 in which the olefin pyrolysis initiator is acetaldehyde.

References Cited by the Examiner UNITED STATES PATENTS 2,259,630 10/1941 Frey et al. 260683 3,209,048 9/1965 Burk et a1 260-680 3,211,737 10/1965 Burk et al 260-680 3,238,270 3/1966 Turnquest 260-680 3,254,136 5/1966 Frech 260-680 DELBERT E. GANTZ, Primary Examiner.

G. E. SCHMITKONS, Assistant Examiner.