US2763703A - Cracking of isobutylene with steam to produce substituted acetylenes and diolefins - Google Patents

Description

W W56 J. HAPPEL ET AL 2,763,770

CRACKING OF ISOBUTYLENE WITH STEAM TO PRODUCE SUBSTITUTED ACETYLENES AND DIOLEFINS Filed Sept. 29, 1952 Moles (3 H formed/I00 moles (3 charged IO 20 30 40 5O CONVERSION (moles C H decomposed/I00 moies C H charged) United States Patent fiice 2,763,703 Patented Sept. 18, 1956 CRACKING F ISOBUTYLENE WITH STEAM TO PRODUCE SUBSTITUTED ACETYLENES AND DIOLEFINS John Happel, Yonkers, and Charles J. Marsel, New York, N. Y.

Application September 29, 1952, Serial No. 312,157

11 Claims. (Cl. 260-678) This invention relates to novel processes for the production of acetylenic hydrocarbons, particularly methyl acetylene, and certain diolefins, chiefly allene, by the thermal cracking of branched chain monoolefins, and more particularly it relates to a new and superior method for making methyl acetylene by the thermal cracking of isobutylene using a combination of critical conditions.

It has been known to pyrolyze olefins to give diolefins and other products. The pyrolysis of isobutylene has also been known to the art. Previously, in order to obtain a feasible and operative process, it has been necessary to conduct the thermal cracking of the branched chain olefins, such as isobutylene, under very special conditions of controlled temperature and under vacuum in order to obtain appreciable amounts of the more valuable products such as methyl acetylene. In fact, studies conducted on the chemical mechanism of the thermal decomposition of isobutylene during pyrolysis have indicated that this cracking operation is extremely sensitive to operating conditions with regard to both yield and identity of products as well as to quantity of worthless tars and fixed gases obtained. Accordingly, the previously employed methods used for obtaining acetylene from branched chain monoolefins required the use of greatly reduced pressures as well as carefully controlled temperatures and contact times and was accompanied by the usual problems of operation at reduced pressures and high temperatures. These include expensive equipment, difliculties of control, poor heat transfer and uncertainty of products and yields. These difficulties, of course, are intensified in commercial scale operations.

It has now been discovered that low molecular weight substituted acetylenes and especially the valuable methyl acetylene, can be made in good yield and purity by the thermal cracking of iso-monoolefins in the presence of steam and under conditions of high temperatures and low contact time of the feed. By this new method, the diliiculties of operating under vacuum are completely avoided, since atmospheric or even slightly superatmospheric pressures are quite satisfactory. There is no coking of the equipment, and consequently no loss of olefin feed stock in such useless materials. There is obtained a product having maximum yields of both methyl acetylene and allene, both of which are highly useful products.

It has been discovered that steam has a unique and hitherto unknown ability to aid the particular desired reaction by cutting down tar formation and greatly facilitating the formation of the desired mixture of methyl acetylene and allene.

To summarize, it has been found that, by maintaining proper temperature and contact time relationships, and controlling certain other secondary variables, maximum yields of methyl acetylene and allene from thermal cracking of isobutylene are obtained which are higher than those obtained by more inconvenient and less practical methods.

This relationship is shown graphically in the accompanying figure which is a plot of the relation of total production of C3H hydrocarbons (i. e., allene and methyl acetylene), per unit of isobutylene throughput through a pyrolysis coil to the degree of decomposition of the isobutylene, expressed as a percentage, which takes place in the same pyrolysis coil. It will be noted that a maximum yield of CsH4 hydrocarbons, the desired product, is reached at about 30% conversion. At conversions lower than the optimum, although the desired reaction occurs to a considerable extent, the amount of unreacted isobutylene remaining is undesirably high. At conversions higher than the optimum, however, a considerable portion of the C3H4 hydrocarbons produced are decomposed by secondary reactions, resulting in a loss of the desired product.

Therefore, these data and the graph clearly indicate that the pyrolysis reaction should be carried out at conversions in the range of about 20% to 40% based on the isobutylene used. For the temperature range of 800 to 900 C., the corresponding contact times for 20-40% conversion have been determined. These data are shown below in Table I.

TABLE I Contact Time, Seconds Temp, 0.

20% Con- 30% Con- 40% Conversion version version Substantially atmospheric pressures are employed within the cracking zone for best results. Steam should be employed admixed with the reactive olefinic feed in mole percent concentration of more than 50% and preferably in the range of to mole percent. It is preferred to mix the iso-olefin and the steam prior to its entrance into the reaction zone. A preheating of the steam is also desirable.

The feed can be the pure isobutylene or a mixture containing substantial amounts of this olefinic feed can also be used, particularly when it is initially obtained as a commercial fraction as by distillation or extraction in petroleum operations or the like.

The operation may be carried out in a pyrolysis tube or a series of tubes or coils made of stainless steel, quartz or the like. Also, brick checkerwork or stoves of the type used for pyrolysis may be used. Such stoves commonly use the principle of regenerative cooling for economy of operation. Any such methods providing hot surfaces are quite satisfactorily employed.

This novel process is particularly valuable for producing maximum yields of methyl acetylene from feeds of isobutylene or mixtures of olefins containing relatively large amounts of isobutylene. In addition, maximum yields of allene are obtained simultaneously with the methyl acetylene. Conditions which are especially adaptable for making methyl acetylene together with allene are SOD-900 C. and .O110 seconds contact time and 8090 mole percent steam. Unconverted isobutylene may be recycled through the reaction zone to insure that all the isobutylene has reacted.

In order to achieve controlled low time of contact, it is necessary to cool the cracked gases very quickly to at least 500 C. after they leave the thermal cracking zone. Shock or quick cooling of the cracked gases serves at least two purposes. Firstly it serves to bring the temperature of the reacted gases quickly below the pyrolytic temperature in order to keep decomposition of the products and secondary reactions to a minimum and secondly,

it'reduc-es the temperature of the methyl acetylene as well as the allene to the lower temperatures at which polymerization reactions of these materials are at a minimum.

These two objectives may be accomplished by shock cooling of the cracked gases. A direct water quench may be. placed immediately after the heating Zone. The gases may be passed directly into a stream or spray of cold water or oil. Cool gases may be mixed with the exit gases immediately after they leave the heating zone. The substituted acetylene is separated by condensation of the steam, further compression to knock out additional "water, and subsequent pressure distillation to separate methyl acetylene and allene from the reaction products. Recovered isobutylene is desirably recycled back to the cracking coil.

The following example is a typical illustration of the process of the invention but it is not intended that the invention be limited in any way thereto.

Example A gas mixture containing mole percent isobutylene and 90 mole percent steam is passed through a stainless steel cracking tube. A thermocouple probe in the cracking tube gave a temperature profile over the length of the tube of about 850-900 C., which included preliminary heating of the feed prior to its entrance into the reaction zone. The contact time in the furnace was between 0.05 and 0.5 second. The product was then quenched with additional steam. Upon analysis of the reaction mixture, it was found that a minimum of 5.6 mole percent of the isobutylene was converted to methyl acetylene in a single pass, and approximately an equal amount of allene was simultaneously formed. The 'remaining reaction product consisted of a mixture of isobutjylene, methane, hydrogen and small amounts of other hydrocarbons.

The production of methyl acetylene is of especial impenance, since this chemical compound has unique propcities which make it useful as a combustible fuel for welding operations and jet engines, and as a chemical intermediate to yield new and valuable synthetic chemical products.

What is claimed is:

1. A process for the production of methyl acetylene which comprises subjecting a feed containing substantial amounts of isobutylene and more than 50 mole percent of "steam to temperatures of from 800 to 900 C. at contact times ranging from .01 to 10 seconds.

2. A process for the production of methyl acetylene and other unsaturated products which comprises subjecting a hydrocarbon feed containing isobutylene and in the presence of more than 50 mole percent of steam, to temperatur'es of from 800 to 900 C. at a contact time of less than 10 seconds.

3. A process for the production of CsHa unsaturated hydrocarbons which comprises subjecting a hydrocarbon feed containing isobutylene and in the presence of more than SO mole percent of steam, to temperatures of from 800 to 900 C. at a contact time of less than 10 seconds.

4. A process for the production of methyl acetylene which comprises-subjecting an olefinic hydrocarbon feed containing isobutylene and more than 50 mole percent of 4 steam, to temperatures of from 800 to 900 C. at a contact timeof less'than l second.

5. A process for the production of a mixture of methyl acetylene and allene which comprises subjecting a feed mixture containing isobutylene and more than mole percent of steam to a thermal cracking at a temperature within the limits of 800 to 900 C. at a contact time of .01 up to 10 seconds, and a conversion of isobutylene of from 20 to-40%.

6. A process which comprises subjecting a mixture of isobutylene and from to mole percent of steam, to a thermal cracking at a temperature within'the limits of 800 to 900 C. at a contact time of less than 1 second, and isolating methyl acetylene from the resultant cracked gases.

7. A process'which comprises subjecting a mixture containing from 10 to 20 mole percent isobutylene and from 90 to 80 mole percent steam to a cracking step at a temperature in the range of 800 to 900 C. at a contact time between .01 and 1 second and isolating methyl acetylene from the resultant cracked gases.

8. A process for making a mixture containing substantial quantities of methylacetylene and allene which comprises subjecting a mixture of isobutylene and from '80 to 90 mole percent of steam to a thermal cracking at a temperature in the range of 800 to 900 C. and a contact time of less than 1 second.

9. A process for making a cracked mixture containing methyl acetylene which comprises the steps of preheating a mixture containing about 10 to 20 mole percent of isobutylene-and about 90 to 80-mole percent steam, immediately passing said preheated mixture to a thermal cracking at a temperature of 800 to 900 C. and contact time of -.O1 to 10 seconds, separating themethyl acetylene from the resulting c-rackedgases, and again subjecting the unrea'cted isobutylene to the preheating and cracking steps.

10. Apr'ocess for-making a cracked mixture containing substantial amounts of methyl acetylene and allene which comprises'prehea'tinga mixture containing 10 mole percent isobutylene and 90 mole percent steam, passing said'mixtureto a cracking Zone in which it is subjected to a temperature of' 850-900 C. for a period of about .05 to 0.5 second, quenching the hot mixture with steam, and isolating methyl acetylene from the cooled gases.

11. A process which comprises subjecting a mixture containing from '10 to 20 mole percent isobutylene and from 90 to 80 mole percent steam to a cracking step at a temperature in the range of 800 to 900 C., a contact time between .01 and 10 seconds, and a conversion of isobutylene of from 20 to 40%, and isolating methyl acetylene from the resultant cracked gases.

References Cited in the file of this patent "UNITED STATES PATENTS OTHER REFERENCES Groggins: Unit Proc'e'sses in Organic Synthetic, third ed., page 704 (1947'), McGraw-Hill Book Co., Inc, New York.

Claims (1)

1. A PROCESS FOR THE PRODUCTION OF METHYL ACETYLENE WHICH COMPRISES SUBJECTING A FEED CONTAINING SUBSTANTIAL AMOUNTS OF ISOBUTYLENE AND MORE THAN 50 MOLE PERCENT

Images