US3335198A - Ethylene pyrolysis, with acetylene, to butadiene - Google Patents

Description

United States Fatent O i 3,335,198 ETHYLENE PYROLYSIS, WITH ACETYLENE, T BUTADIENE Edwin L. Anderson, Beaumont, Tex., assignor to Mobil Oil Corporation, a corporation of New York No Drawing. Filed Sept. 26, 1963, Ser. No. 311,628 5 Claims. (Cl. 260680) The present invention relates to the thermal conversion of hydrocarbons into unsaturated acyclic hydrocarbons containing four carbon atoms per molecule; more particularly, it is concerned with the pyrolysis of an ethylene-rich gas containing acetylene to produce 1,3-butadiene.

The thermal conversion of hydrocarbons in the presence of steam into olefins and diolefins has been extensively studied with especial emphasis on the manufacture of ethylene and butadiene. While there is usually a greater demand for ethylene than 4-carbon unsaturates, it is desirable under certain commercial conditions to pyrolyze ethylene to produce unsaturated C, straight chain hydrocarbons, especially 1,3-butadiene.

In the prior art, laboratory experiments have been reported on heating mixtures of various proportions of acetylene and ethylene with carbon monoxide and also sometimes steam at temperatures of the order of 1130 F. and atmospheric pressure for reaction times of about 12 to 25 seconds. However, these experiments produced poor yields of butadiene coupled with considerably greater production of hydrocarbon liquids and in some instances more propylene than butadiene. Also, carbon formation was observed, apparently in the glass reactor, in some runs of less than 12 hours duration.

In the conventional production of ethylene by the thermal cracking of various petroleum distillates ranging from refinery fuel gases of substantial C;;-}- content up to naphthas, etc. in the presence of steam, small quantities of acetylene are also formed. Most of this acetylene is recovered in admixture with ethylene in separating an ethylene-rich stream from the lighter gases and heavier hydrocarbons in the crude pyrolysis product by fractional distillation. Heretofore, it has been considered that acetylene had poor thermal stability and contributed to fouling of pyrolysis equipment. Accordingly, acetylene has been removed from olefinic streams prior to subjecting such streams to pyrolysis or certain other processing operations. This acetylene removal has been accomplished by various techniques including selective catalytic hydrogenation and selective absorption in suitable solvents as indicated by the article entitled Ethylene-What You Should Know, in pages 125-144 of the Petroleum Refiner, issue of March 1960. Such removal procedures of course increase the production cost and complicate the processing by requiring additional operations and equipment.

It has now been discovered that under certain pyrolysis conditions described hereinafter for the conversion of ethylene into unsaturated acyclic hydrocarbons having four carbon atoms per molecule, the acetylene may be left in the pyrolysis charge. Coking does occur in the quench zone only, but the rate of carbon accumulation is so slow despite the high temperatures employed that commercial operations are feasible with occasional shut downs for cleaning. It was even more surprising to find that a commercially significant improvement in the yield of butadiene is realized with the new process. It will also be appreciated that this substantial improvement in yield is not only unexpected but also quite unusual in view of 3,335,198 Patented Aug. 8, 1967 ice the very small and seemingly insignificant amount of acetylene present in the pyrolysis charge.

The present invention is a thermal conversion process which includes heating a charge containing a hydrocarbon mixture of minor proportion of acetylene and a major proportion of ethylene at a maximum temperature be tween about 1460 and 1545 F. (preferably between about 1475 and 1535 in forming a conversion effluent containing a higher concentration of unsaturated acyclic hydrocarbons having 4 carbon atoms per molecule than said charge.

Other aspects of the invention relate to adding steam to the reaction, charging a hydrocarbon mixture containing a molar ratio of acetylene to ethylene between about 0.2: and 2.01100, respectively, (which are typical proportions in ethylene produced by the cracking of petroleum fractions), restricting the effective residence time of the reaction mixture above 1000 F. by quenching the conversion reaction eflluent, and particularly in controlling the reaction severity to convert between about 6 and 25 mol percent (preferably about 6 to 20 mol percent) of the hydrocarbons charged. This invention encompasses the combination of the aforesaid ethylene pyrolysis reaction with preceding steps of thermally cracking a hydrocarbon fraction (preferably a normally liquid fraction) followed by fractionating the cracked material to produce the ethylene-rich stream utilized in the ethylene pyrolysis reaction.

Unless otherwise indicated herein, all temperatures are set forth in degrees Fahrenheit, all proportions in terms of weight and all boiling points or ranges according to the ASTM procedure at atmospheric pressure.

The ethylene pyrolysis process conditions necessary for obtaining the benefits of the instant invention may be delineated herein in terms of conversion or reaction severity, preferably in combination with selected ranges of certain operating conditions of which the maximum reaction temperature and reaction time are especially significant. Total conversion may be expressed for the present purposes as the number of mols of C unsaturates reacted in a single pass per 100 mols of hydrocarbons charged to the ethylene pyrolysis reaction. Reaction severity is increased by increasing the reaction temperature or by increasing the residence time of the reacting mixture at temperatures above the pyrolysis threshold or by decreasing the weight ratio of steam to hydrocarbons in the charge, and the severity is decreased when these operating variables are reversed. Other factors such as lowering the reaction pressure and decreasing the quantities of other hydrocarbon diluents present in the charge also tend to increase the severity but to a much less pronounced degree. Accordingly, in view of the number of process variables, their varying degrees of effectiveness and the fact that one may counterbalance another, the most practical way of correlating and defining suitable reaction conditions is the aforesaid conversion range of about 6 to 25 mol percent. It has been found that secondary reactions tend to appear at about 20% conversion and to become significant and generally undesirable when the conversion exceeds about 25%. On the other hand, conversions below about 6 mol percent are of little or no interest from a commercial standpoint.

Conventional pyrolysis furnaces may be used for converting the ethylene. For all practical purposes, the reaction temperature is desirably expressed in terms of the maximum temperature reached by the reaction mixture in this endothermic reaction, namely the temperature of the mixture at the charge outlet of the furnace. In general, the temperature at the charge outlet may be at any level within the range of about 1460 to 1545 F. and the range of about 1475 to 1535 is preferred. The maximum reaction temperature may be easily set and controlled by suitable adjustment of the rate of firing the furnace, especially the final section of a multicellular furnace, or of the reactant charge rate.

The residence time of the reaction mixture at temperatures above the ethylene pyrolysis reaction threshold which is about 1 000 F. is quite brief and generally does not exceed about 2.5 seconds before the reaction products are quenched by contact with a spray or curtain of water or a liquid hydrocarbon fraction of suitable boiling, range and/or by indirect cooling in heat exchangers. Typical residence times are about 0.9 to 2.2 seconds and may be altered at will by adjusting the rate of charging the hydrocarbon mixture.

Operating pressures have not been found to be critical in the ethylene conversion step except that relatively high pressures are not particularly desirable since they tend to retard conversion. In general, pressures of about to 50 pounds per square inch gage (p.s.i.g.) are suitable for the present purposes.

For suppressing coke formation in the ethylene pyrolysis, steam is desirable charged along with the ethylenic steam. In general, a steamzhydrocarbon charging ratio of at least 0221 by weight is recommended. More steam tends to improve the yields of 1,3-butadiene, but this also increases utility costs. In commercial practice, the use of a steam ratio higher than about 1:1 is not desirable from a standpoint of economics.

Substantial quantities of impurities other than acetylene may be present in the ethylenic charge stream such hydrogen, nitrogen, methane, cyclopentane, benzene, etc. for they appear to merely function as diluents Without any apparent deleterious effect on the reaction. However, diluents do reduce the productive capacity of commercial equipment; therefore, it is desirable that the ethylene content of the hydrocarbon materials charged contain at least a major molar proportion, that is at least 50% of ethylene; and it is preferable that the ethylene concentration amounts to at least 83 mol percent. For optimum results a total ethylene and acetylene content of at least 90% is desirable.

In the overall combination process with which the present invention is concerned, a hydrocarbon charge is thermally cracked, desirably in the presence of steam under conventional conditions, and then quicklyquenched in the manner described earlier for the other pyrolysis reaction to produce olefins, particularly ethylene and usually some diolefins for subsequent conversion into C unsaturates. The charge here may range from a relatively light ethane-rich gas recycled from other operations to a preferred liquid petroleum fraction such as a kerosine, gas oil or light naphtha. This charge material should be vaporized before it reaches the highly heated section of the cracking furnace in order to minimize or avoid coking. Cracking conditions are selected in known manner for the particular feed employed and may range from about 1300 to 1600 F. and about 5 to 50 p.s.i.g. pressures for reaction times of about 0.5 to 4 seconds with steam: oil weight ratios of about 0.1 :1 to 3 1. The cracked product stream is fractionated in a conventional recovery system to separate the C stream of the type described herein, which is relatively rich in ethylene and contains a minor amount of acetylene, such as 0.20 to 2.0 mol percent based on the ethylene. No acetylene removal step is necessary prior to charging the ethylene-rich stream along with steam to an ethylene pyrolysis reaction under conditions described elsewhere herein, and the effluent from this conversion reaction is quickly quenched to a temperature below 1000 F. The quenched effluentis passed into a recovery system of a type known in the art where the 1,3-butadiene fraction is separated as well as a stream containing butenes. Most of the lighter gases separated here may be recycled to either the first thermal cracking furnace or the ethylene pyrolysis reactor.

Numerous advantages over the prior art derived from the present invention in addition to those mentioned earlier of simplifying the process by eliminating the acetylene removal step and obtaining an increased yield of butadiene per pass through the ethylene conversion reactor. The rate of coke deposition is so slow and confined to a relatively small part ofthe processing equipment as to be readily acceptable in commercial operations. The process improves versatility in petrochemical processing by employing the same feed for butadiene production that is employed in manufacturing high purity ethylene, so that changing market demands can be met. Thus a special feed prepared in additional processing equipment is not required for the instant process. Also, in contrast with prior acetylene-ethylene reactions, higher production rates are obtained with much shorter residence times and much higher temperatures without excessive coking. And this is accomplished with an insignificant conversion ofvaluable ethylene into less valuable liquid hydrocarbons, therefore, most of the unconverted ethylene may be recycled to the ethylene pyrolysis reaction to further improve the overall yield of butadiene.

For a better understanding of the nature and objects of this invention, reference should be had to the following detailed examples.

EXAMPLES Various batches of light deisopentanized naphtha of the following average characteristics are employed for the preparation of ethylene streams for subsequent conversion into 1,3-butadiene.

Boiling points-- F.:

Initial 109 10% 120 177 Endpoint 266 Gravity API 78.4

Mol percent Ethylene 94.3 Acetylene 0.4 Ethane 3.6 Methane 1.7 1

Similar cracked C fractions are subjected to selective catalytic hydrogenation to substantially eliminate the acetylene therein in preparing feeds for comparative Examples A, B andC.

The following analysis is typical of the hydrogenated product.

Mol percent Ethylene 91.7 Ethane 7.5 Methane a- 0.8 Acetylene parts per million l0 The acetylenefree and normal ethylenic streams in ad mixture with steam are subjected to pyrolysis under the conditions listed in the table hereinafter and then quenched, first by a water injection that reduces the reaction elfiuent temperature to approximately 1200 F. and

then by further chilling in an indirect heat exchanger to about 750 F.

the residence time of the reaction mixture at temperatures above about 1000 F. to less than about 2.5 seconds.

EXAMPLE Acetylene; Ethylene, rnol ratio 0. 32:100 0. 43:100 0. 431100 SteamzHydrocarbon, wt. ratio 0.53 0. 53 0.53 0. 53 0.53 0.53 Furnace outlet temperature, F l, 495 1, 510 1, 515 1,495 1,507 1, 515 Residence time above 1,000 F., sec 1. 4 1. 4 1. 4 1. 4 1. 4 1. 4 Reaction pressure, p.s.i.g .1 19 30 24 20 22 22 Total conversion, mol percent of feed 8. 2 12. 4 19. 5 8. 7 12. 4 17. 4 Conversion to 1,3,butadieue, mol perc t of feed 2. 92 3. 94 5. 42 4. 04 5. 04 6. 34 Butadiene yield, mol percent of total conversion 35.6 31.8 27. 8 46. 5 40. 7 36. 4

The two conversions set forth in the table are expressed respectively as total mols of C unsaturates reacting per 100 mols of hydrocarbons charged and mols of C unsaturates reacting to form 1,3-butadiene per 100 mols of hydrocarbons charged.

Upon comparing vapor phase chromatographic analyses of the products of illustrative Examples 1, 2 and 3 with those of comparative Examples A, B and C, respectively, it is apparent that the simpler process of the present invention provides substantial relative increases in yields of the desired 1,3-butadiene ranging up to as much as 38% under the particular conditions specified.

Fractionation of the pyrolysis effluents of Examples 1, 2 and 3 yields in addition to the butadiene product, smaller amounts of butenes, a substantial light fraction containing principally unconverted ethylene and some propylene along with lesser amounts of ethane, methane and hydrogen. Most of this light fraction may be recycled to the ethylene pyrolysis reactor for further conversion with a portion diverted to other uses to prevent the build up of diluents in the ethylene pyrolysis charge. The production of liquids is insignificant as it amounts to less than 1% of the weight of the charge to the ethylene pyrolysis.

Coking in commercial apparatus operating under the conditions of Examples 1, 2 and 3 is not apparent during the first week of operation. Thereafter an increasing pres sure drop through the quench apparatus is noticed and a shut down is usually desirable after a run of about two weeks to clean coke deposits from the quench zone.

While the present invention is described in much detail in the illustrative embodiments set forth above, it will be appreciated by those skilled in the art that there are many other embodiments included within the scope and spirit of this invention. Accordingly, the instant invention should not be construed as limited to any particular features disclosed except as set forth in the appended claims or as may be required by the prior art.

I claim:

1. A thermal conversion process which comprises heating a hydrocarbon charge mixture containing at least 83 mol percent of ethylene and between about 0.2 and 2 mols of acetylene per 100 mols of ethylene to a maximum temperature between about 1460-and 1545 F. for a period of time sufficient to convert between about 6 and 25 mol percent of said hydrocarbon charge mixture in forming a conversion effiuent containing a substantially higher concentration of unsaturated acycllc hydrocarbons having 4 carbon atoms per molecule than said hydrocarbon charge mixture.

2. A thermal process for the conversion of ethylene which comprises heating a charge of a mixture of hydrocarbons containing acetylene and ethylene in a molar ratio between about 0.2:100 and 2.0: 100, respectively, and steam to a maximum temperature between about 1460 and 1545 F. for a period of time suflicient to convert between about 6 and 25 mol percent of said hydrocarbon mixture in forming a conversion effluent having a substantially higher concentration of unsaturated acylic hydrocarbons having 4 carbon atoms per molecule than said charge and quenching the conversion effluent to restrict 3. A thermal conversion process for the production of 1,3-butadiene which comprises heating a charge of be tween about 0.2 and 1.0 part of steam per part by Weight of a mixture of hydrocarbons containing a total of at least mol percent of acetylene and ethylene in a molar ratio between about 0.2: and 2.0:100, respectively, to a maximum temperature between about 1475 and 1535 F. for a period of time sufficien-t to convert between about 6 and 20 mol percent of said hydrocarbon mixture in forming a conversion efiiuent having a substantially higher concentration of 1,3-butadiene than said charge and quench- .ing the conversion efiluent to restrict the residence time of the reaction mixture at temperatures above about 1000 F. to less than about 2.5 seconds.

4. A thermal process for the conversion of hydrocarbons which comprises thermally cracking a hydrocarbon fraction to form ethylene, quenching the reaction effluent, separating from the reaction eifiuent an ethylenic stream containing acetylene and ethylene in a molar ratio between about 0.20:100 and 2.0:100, respectively, heating a charge comprising said ethylenic stream and steam to a maximum temperature between about 1460 and 1545 F. for a period of time sufiicient to convert between about 6 and 25 mol percent of the hydrocarbon content thereof in forming a conversion effluent having a substantially higher content of unsaturated acyclic hydrocarbons containing 4 atoms per molecule than said charge and quenching the conversion efiluent to restrict the residence time of the reaction mixture at temperatures above about 1000 F. to less than about 2.5 seconds.

5. A thermal conversion process for the production of 1,3-butadiene which comprises thermally cracking a normally liquid hydrocarbon fraction in the presence of steam to form ethylene, quenching the cracking reaction effluent, separating from the quenched efiluent a stream containing a total of at least 90 mol percent of acetylene and ethylene in a molar ratio between about 0.20: 100 and 2.01100, respectively, charging between about 0.2 and 1.0 part of steam per part by weight of said ethylenic stream to a heated reaction zone, pyrolyzing the charge by heating to a maximum temperature between about 1475 and 1535 F. for a period of time sufiicient to convert between about 6 and 20 mol percent of the hydrocarbon content thereof in forming a conversion efiluent having a substantially higher 1,3-butadient content than said charge and quenching the conversion eflluent to restrict the residence time of the pyrolysis reaction mixture at temperatures above about 1000 F. to less than about 2.5 seconds.

References Cited UNITED STATES PATENTS 2,411,256 11/1946 Frey 260-680 OTHER REFERENCES Naragon et al: Thermal Reaction of Ethylene With Acetylene, Ind. Eng. Chem. 34(3), pp. 355-358, March 1942.

DELBERT E. GANTZ, Primary Examiner. G. E. SCHMITKONS, Assistant Examiner.

Claims (1)

1. A THERMAL CONVERSION PROCESS WHICH COMPRISES HEATING A HYDROCARBON CHARGE MIXTURE CONTAINING AT LEAST 83 MOL PERCENT OF ETHYLENE AND BETWEEN ABOUT 0.2 AND 2 MOLS OF ACETYLENE PER 100 MOLS OF ETHYLENE TO A MAXIMUM TEMPERATURE BETWEEN ABOUT 1460 AND 1545*F. FOR A PERIOD OF TIME SUFFICIENT TO CONVERT BETWEEN ABOUT 6 AND 25 MOL PERCENT OF SAID HYDROCARBON CHARGE MIXTURE IN FORMING A CONVERSION EFFLUENT CONTAINING A SUBSTANTIALLY HIGHER CONCENTRATION OF UNSATURATED ACYCLIC HYDROCARBONS HAVING 4 CARBON ATOMS PER MOLECULE THAN SAID HYDROCARBON CHARGE MIXTURE.