US2678956A - Thermal cracking - Google Patents

Description

May 1s, 1954v R. L'. HASCHE THERMAL CRACKING Filed April 8, 1950 E .mw m5 nu MM D M m E L H wbhomk l P S owQ um? Q2u SS a. a U Q uw Il QM, m l bv Wa l/l lo.. .mwmmmwl w l bw d l A w N mv@ @36 w y (Ittomegs Patented May 18, 1954 UNITED STA'l-f' THERMAL CRACKNG of Delaware Application April 8, 1950, Serial No. 154,845

(Cl. 26o-679) d Claims.

llhis invention relates to the manufacture of unsaturated hydrocarbons by the thermal cracking of saturated hydrocarbons. More particularly this invention concerns an improved process or obtaining high yields of acetylene and ethylene by the thermal cracking of mixed satu-J rated hydrocarbons.

As already pointed out in the several Wullic Patents 1,843,965; 1,889,39S; and others, as well as my prior Patents 2,236,534; 2,318,688; and 2,319,679 the older method of manufacturing unsaturated hydrocarbon gases, .such as acetylene from calcium carbide or other methods involving electrical energy are expensive and not adapted to large-volume, low-cost production. The several patents, aforementioned, describe the production of acetylene and ethylene by the thermal cracking of nomacetylenic hydrocarbons under high temperature, short time of contact, conditions.

That is, by subjecting the saturated hydrocarbon to a fraction of a second cracking the saturated hydrocarbon is converted to acetylene without substantial destruction to carbon or formation of carbon monoxide and carbon dioxide. The described methods of thermally cracking saturated hydrocarbons have preferred the use of a feed stock such as butano, ethane, various heavy oils, or the like. The reason for this is that by cracking an even numbered, saturated hydrocarbon, such as a 2 or a-carbon molecule better pyrolysis to the 2 or Ll-carbon unsaturated hydrocarbon acetylene or ethylene is indicated without the liberation or loss oi a single carbon atom. of odd numbers as propane, pentane, oils, and the like may also be cracked and if the conditions are correctly maintained, even though methane radicals are liberated from the odd number of carbon atoms, there may be reformation to acetylene and ethylene as well as aromatics. However, since the desired product is the unsaturated hydrocarbons, such as acetylene or ethylene, any benzene or the like produced even though of value may not be considered as beneiicial to the process in that carbon atoms going into the formation of the aromatics are, of course, lost to the formation of the desired unsaturates.

In many instances today the rich feed stocks comprised largely of butano or the like heavy hydrocarbon are not available. in many parts of the country and the world only mixed stocks are available and while it is possible to separate such mixed stocks into a relatively pure ethane fraction, propane fraction, pentane fraction, or

However, saturated hydrocarbons ,i small.

the like, for the individual cracking of each fraction such separatory processes involve considerable expense, particularly where no one particular fraction largely predominates in the available stock. Also there are some instances where mixed gases are obtained as by-products and wherein the hydrocarbon content of saturated oompounds comprised of 2 or more carbon atoms, even in the aggregate thereof, may be For example, there are available, or may be readily made up, mixed gases which may be comprised of only 2% or 3% ethane, from a few per Cent up to i5% or 20% propane together with minor amounts of heavier saturated hydrocarbons in the nature of oils which presumably are comprised of several carbon atoms in their chain. Such mixed gases may also have present varying amounts of the one carbon atom, saturated hydrocarbon CHt. Therefore, it will be noted that such mixed gases may be considered rather lean feed stocks.

I have found a process whereby such lean stocks of mixed gases may be eiciently pyrolyzed and under reasonable operating conditions, to obtain good yield of unsaturates such as ethylene and acetylene,

This invention has for one object to provide a process for cracking lean, mixed hydrocarbons of the class described to a cracked gas mixture containing a good content of unsaturates. Another object is to provide an improved method for cracking mixed hydrocarbons containing only a few per cent each oi hydrocarbons having 2 or more carbon atoms in the molecule, to ethylene and acetylene. Still another object is to provide a method of cracking which involves the dilution of saturated hydrocarbons having more than 2 carbon atoms in their molecule with a diluent comprised of theYmono-carbon atom compound CB14. A still further object is to provide a thermal cracking process of the class indicated for mixed gases which may be carried out under lower temperature conditions than heretofore thought to be feasible.

Qther objects will appear hereinafter.

In accordance with the present invention I have found that in the cracking of such lean feed stocks having only a small percentage of the heavy saturated hydrocarbons and usually in the presence of some Clit that improved 0peration, yields, and other favorable results may be obtained by rendering the CH4 content of a certain value both in the feed and in the cracked gas. That is, the CH@x in the feed is preferably held within the range of 50% to 75% or 80% by dilution, blending, or otherwise, the balance of the feed stock composition being the heavier hydrocarbons such as ethane, propane, butane, oils, and the like. The pyrolytic cracking of the controlled feed is also so controlled that the content of CHi in the cracked gas (taking into account such items as gas expansion) is of the same general order of magnitude, 110%, as in the feed. Usually as will be apparent from the further description and examples, this permits the use of lower temperatures. Otherwise the higher temperature conditions probably operate to destroy some of the heavier saturated hydrocarbons in causing CH4 formation therefrom or even carbon black formation.

That is, while it is not desired to be bound by any theory, apparently for each specific saturated hydrocarbon there are certain temperature conditions and other factors most favorable to the conversion of that hydrocarbon to the desired unsaturates, acetylene and ethylene. The application of very high temperatures presumably will act upon CHL; to liberate methine radicals which theoretically can combine to form acetylene. Iowever, such temperature conditions may also act upon the heavier saturated hydrocarbons destructively. It is apparent that it would be futile to lose the benefit of the heavier hydrocarbons in having them go to the formation of methane, carbon, or oxides of carbon. From the control of and observation of numerous runs involving the pyrolysis of mixed gases under varying conditions I have discovered that the optimum conditions obtained, in general, as respects the combined yield of the desired unsaturates, ethylene and acetylene, when the process is operated so that the content of CHi in the cracked gas are about the same as that in the feed as will be set forth in further detail.

It will be noted, therefore, that with a given percentage of CH4 dilution within the range of 50% to '75%, aforementioned, that the particular combination of temperature, contact time, steam dilution, as well as individual furnace characteristics which will be involved are those which hold the CH4 content of the product gas approximately the same as in the feed composition and represent the preferred operating conditions of the present invention.

In carrying out the process of the present invention the same general technique and same apparatus described in the several Wulff and I-Iasche patents, aforementioned, may be used. That is, the feed stock herein described may also be diluted with some steam, say for example a 1A to 3 dilution as described in the Wulff and Hasche patents. Likewise in the present process reasonably short contact times are used. However, it will be kept in mind that in the process of the present invention that the combination of a specific contact time and a specific temperature will be such that the percentage of the CHI; is not substantially affected. However, it is to be noted in particular that this does not mean the CII-I4 is inert or does not change during the process. This aspect will be further apparent from a consideration of the several examples hereinafter set forthand the attached curve. In general, therefore, with an increase in temperature the contact time might be shortened slightly to prevent such increase in temperature from acting upon the feed stock in a manner that greatly alters the CH4 content. On the other hand when lowering the temperature the contact time may be varied in another manner.

While several variations in conditions may be accomplished in the process of the present invention the matter of the over-all combination of conditions, just indicated, will be such that a combination is used wherein the content of the mono-saturated hydrocarbon in the feed and in the eiiiuent cracked gases are substantially the same. In preferred operation any variation in feed to cracked gas is only 1% or 2% and in general it is preferred not to have this variation outside of a plus or minus 10%.

It is preferred to use a furnace structure for carrying out the present process of the type shown in Patents 2,432,885 and 2,318,688 of which I am an inventor. Such furnace structures are believed to comprise a particularly efficient unit for carrying out pyrolysis processes of the present type. However, it is not desired that the present process be limited to the regenerative cracking unit just described although best results are obtained therein and as the carrying out of the present process in other apparatus and by other means of heating is not precluded.

For a further understanding of the present invention reference will now be made to several specific examples which examples may be considered as carried out in a regenerative furnace of the aforementioned type wherein eflcient heat transfer was accomplished from properly spaced refractory surfaces.

Example I In accordance with this example the lean feed stock of mixed gas was comprised essentially of 2.4% ethane and 32.6% of propane, these heavier hydrocarbons being diluted by volume with approximately 65% CHi. This feed stock was subjected to thermal cracking in a regenerative furnace of the type already referred to with a steam dilution of 1:1. The temperature of operation was 1225 C. with a contact time for the particu- The expansion of total feed in this particular example was 2.01 and taking this gas expansion into consideration it will be observed that the process was operated in a manner so that the content of CHi in the cracked gases computed on the same basis as in the feed gas was approximately 64.5% as compared with the 65% value in the feed. In other words, in this run in accordance With the preferred operation of the present invention the process was operated so that the amount of CIL; in the eiuents expressed in volume of gas was Substantially equivalent t0 that in the input.

The yield (based on heavy hydrocarbon) was 36% acetylene and 19.8% ethylene (ora combined yield of desired unsaturates of approximately 55%) together with about 8.3% of aromatics. It will be observed that this is a favorable conversion for this particular feed of the heavy hydrocarbon component to the desired products.

Example II In accordance with this example the same type feed stock of Example I was processed in similar equipment. However, in this run a steam dilution of 2:1 was employed together with a temperature of 1210o C. and a Contact time of 0.086 sec. The carbon balance in this example was 82 compared with 80.9 in the preceding example.

The cracked gas analysis Was as follows:

CO2 (2.3 C2H2 9.() C21-I4 3.2 CGI-Ie 0.6 O2 0.2 H2 48.6 CO 3.7 CH4 28.9 N2 5.5

The expansion on total feed was 2.25 and taking this into account (2.25X28-9 CH4) it .will be observed that the content of CI-I4 which was maintained in the cracked gas was substantially identical with that in the input. The yield of acetylene was approximately 40% and of ethylene was approximately 14% (giving a combined production of the desired unsaturates of well over 50% for this particular feed). rIhe yield of aromatics, principally benzene, was in excess of 7%.

Example IIIv In accordance with this example the type of feed and general conditions of operation were the same as in the preceding examples excepting that in the instant run a steam dilution of 3:1 was used. Specifically a temperature of 1200 C. and a contact time of 0.098 sec. were employed.

The gas expansion was 2.20 and taking this into consideration it will be observed that the eliiuent CHi was approximately 66% or 1% higher than that in the feed.

rlhe yields of acetylene were 38.6% and ethylene 12.4%, together with 7.6% of aromatics. Here again the operation was controlled suiiiciently to give a combined production of desired product of a yield greater than 50% for this particular feed.

Example IV In accordance with this example the feed stock was comprised of about 20% in the aggregate of a mixture of the heavier hydrocarbons, ethane, propane, and heavier, diluted with approximately 80% by volume of CH4. The steam dilution of 1/2z1 and a temperature of 1100 C. were employed with a short contact time as already described. The combined yield of ethylene and acetylene, based on the C2 or greater hydrocarbons, was approximately 83%, a very favorable yield, when the operation was conducted so that the content of CH4 in the cracked gas (based on a gas expansion of 1.45) was held at approximately 79 as compared with the 80 value of the feed.

In order to compare the change when the CI-Il. content in the eluent is reduced, similar runs to preceding were carried out but at a 50" C. higher temperature although otherwise similar. In these runs the content of CID. in the effluent dropped to approximately (as compared with the 80% in the feed, aforementioned) based on a gas expansion of approximately 1.5. .Analysis of the cracked gas showed 35.4% acetylene and 39.6 ethylene or a yield of 7 5% which was somewhat lower, about 8%, than in the preceding run where the eilluent CI-I4 had been better controlled.

For further comparison other runs .were carried out in which the temperature was raised about C. over the temperatures in the firstmentioned run. The C2114 in the effluent dropped to about 64 (based on a gas expansion of 1.6). rI'he aggregate yield of acetylene and ethylene was only 73 showing a definite reduction from the first run as increased variation was made in the control of the effluent CH4 as compared with the input CHl.

Example V In this example for further contrast considerable variation in the CH4 content was intentionally accomplished. The feed in this example as in the preceding was comprised of about 20% of C2 or heavier hydrocarbons diluted with approximately 80% of CI-Ir in the feed. The temperature was held substantially in excess of 1200O C. with a contact time of around 0.088 sec. The steam dilution was 1:1. The eliluent CH4 was approximately 64.8 as compared with the 80% input. The combined acetylene and ethylene yield was only about 54% as compared with the greater than 80%, namely the most favorable yields in the preceding example using the same composition feed.

From the foregoing runs it is apparent that when a substantial differential exists between the effluent CHi and the input CI-I4 that there is a marked drop in the aggregate yield of desired unsaturates, ethylene and acetylene from a particular mixed gas feed.

I have found that for any particular feed that more favorable yields and better operation may be accomplished if the pyrolysis is carried out within the temperature range of 1100" C. to 1300o C. and otherwise in a manner so that the eflluent CHI; is of approximately the same magnitude of the input Clie., or at least in preferred operation, does not vary therefrom more than a few per cent and in no case greater than a i10%. As is apparent from the preceding examples, a variation of not more than 2% or 3% is regarded as best operation and can be accomplished when operating at the upper part of the temperature range near 1300 C. with sufliciently short contact time. However, as the examples also show, the optimum results of the present invention can be well accomplished Iwhen using the lower temperatures between 1100 to 1200 C., thereby permitting of more latitude of contact time as will be readily understood by those skilled in the art.

The present invention is to be distinguished from those descriptions in the prior art wherein hydrogen, nitrogen, methane, and the like have been grouped together as inert gases and diluents. Under the conditions of the present invthat the CHl is not inc-rt.

emphasised by the attached drawing forming a vention in an environment of the C2 and heavier hydrocarbons while it may be that methane does function as a diluent it is thought to go beyond this for the reasons set forth below. In considering the following data and curve on tached drawing, it is not desired to limit the present invention to any particular theory of operation but perhaps a clearer understanding will be had of why the present invention may not be considered a simple dilution mechanism. In other words in the present invention the CH4 does more than reduce the partial pressure but also appears to function as an active agent.

'Reference to Examples IV and V above shows that even though a certain predetermined amount of C114 is included in the feed that the amount of the effluent CH4 can be altered by variations in the process conditions. Hence, it is apparent This aspect is further part of the present invention showing a curve based on a fixed feed gas composition subjected to temperature variations over the range 1150" C. to 1400o C.

That is, by reference to the curve at A it will be seen that even though 50 per cent of CII-I4 was initially present that by carrying the process conditions in a certain direction 70% or more CI-Li can appear in the efliuent gas.

On the other hand, as shown by the curve at B the content of CH4 may be caused to decrease. Hence it is apparent that the CH4 is not inert but may be an intermediate which if properly controlled in accordance with the present invention may be caused to exert favorable inuence in the improved production of C2H2 and C2H4 as already set forth in detail.

As indicated above, the present invention has been illustrated by the several examples, but it is not to be unduly limited thereby. rIhe use of steam dilution, preheating, recycling, if desired, and other similar features referred to in connection with the Wulff and Hasche patents outstanding may likewise be employed in the process of the present invention. While 1' prefer regenerative heating it is entirely possible to carry out the present invention in a tube furnace or other type furnace provided the operation is conducted in a manner that the effluent CI-Ii corresponds to the input Cl-Li as has already been described at 5,

length. In operating with such different types of furnaces the characteristics of such furnaces may be somewhat different requiring the utilization of somewhat different conditions of time and temperature, within the ranges given.

In general, choice of temperatures within the range of 1100 C. to 1300 C., in conjunction with an appropriate short contact time will function to give the desired cracking to ethylene and acetylene without material alteration of the CI-Ii content. In my preferred operation I would usually employ temperatures under 1200 C. in conjunction with less than le of a second Contact time. However, as indicated a different type of furnace may indicate slightly different conditions depending on the furnace characteristics. Therefore, the proper combination of temperature, time, and any other factors needing consideration will be chosen that there is a maximum of only Ia few per cent variation of the effluent CIL; as compared to the input CH4 whereby more than 50% yields, and even greater than 80 yields with certain feeds, of ethylene and acetylene in the aggregate will be obtained from the cracking of lean feed stocks.

8 I claim: 1. A process for producing good aggregate yields of unsaturates such as acetylene and ethylene comprising the steps of diluting with methane a lean stock of mixed gases containing only a -few percent each of two or more saturated hydrocarbons having two or more carbon atoms to a final in-gas composition having a methane content in the range of 50 %-80% by volume of the hydrocarbons present, cracking said in-gas by means of heat exchange at a temperature in the range of 1100" C. to 1200 C. for a period of time not exceeding 0.1 second and thereby maintaining an off-gas having a total methane content equal to the total methane content (-53%) of the ingas required to produce said off-gas.

2. A process for producing good aggregate yields of acetylene from a lean mixed hydrocarbon feed stock containing as the only saturated hydrocarbons of more than one carbon atom, a mixture of a plurality of saturated hydrocarbons each of which is present in an amount representing only a few percent by volume, not exceeding about 20%, of the total hydrocarbons in the stock, said method comprising the steps of adjusting the methane content of said feed stock to a value in the range of 50%-80% by volume of the hydrocarbons present, cracking saturated hydrocarbons in said feed stock by heat exchange and maintaining an off-gas having a total methane content equal to the total methane content plus or minus 10% of the in-gas required to produce said off-gas by conducting said cracking at a temperature in the range of about 1100o C. to 1300 C. for a period of time of the order of about, but not exceeding 0.1 second and recovering an off-gas. containing at least 50%, inclusive of a substantial amount of acetylene, of unsaturates based on the hydrocarbons of two or more carbon atoms.

3. A process as defined in claim 2 wherein the step of adjusting the methane content comprises diluting said feed stock by adding methane thereto.

4. A process for producing good aggregate yields of acetylene from. a lean mixed hydrocarbon feed stock containing as the only saturated hydrocarbons of more than one carbon atom a mixture of a plurality of saturated hydrocarbons each of which is present in an amount representing only a few percent by volume, not exceeding about 20%, of the total hydrocarbons in the stock, said method comprising subjecting said stock to a cracking reaction as an in-gas diluted to a methane content having a value in the range of 50 5t-80% by volume of the hydrocarbons present, cracking saturated hydrocarbons in said in-gas at a temperature in the range of about 1100 C. to 1300o C. for a period of time of the order of about, but not exceeding, 0.1 second and controlling the cracking reaction for optimum efciency by maintaining (by approprlate changes of temperature and reaction time) in the olf-gas from said cracking reaction which contains a substantial amount of acetylene, a methane content equal to the total methane content plus or minus 10% of the in-gas required to produce said off-gas.

References Cited in the file 0f this patent UNITED STATES PATENTS

Claims (1)

1. A PROCESS FOR PRODUCING GOOD AGGREGATE YIELDS OF UNSATURATES SUCH AS ACETYLENE AND ETHYLLENE COMPRISING THE STEPS OF DILUTING WITH METHANE A LEAN STOCK OF MIXED GASES CONTAINING ONLY A FEW PERCENT EACH OF TWO OR MORE SATURATED HYDROCARBONS HAVING TWO OR MORE CARBON ATOMS TO A FINAL IN-GAS COMPOSITION HAVING A METHANE CONTENT IN THE RANGE OF 50%-80% BY VOLUME OF THE HYDROCARBONS PRESENT, CRACKING SAID IN-GAS BY MEANS OF HEAT EXCHANGE AT A TEMPERATURE IN THE RANGE IF 1100* C. TO 1200* C. FOR A PERIOD OF TIME NOT EXCEEDING 0.1 SECOND AND THEREBY MAINTAINING

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