US2734810A

US2734810A - Method of making oil gas interchangeable with natural gas

Info

Publication number: US2734810A
Authority: US
Publication date: 1956-02-14
Anticipated expiration: 1973-02-14

Description

Feb. 14., 1956 E. s. PETTYJOHN ET AL 2,734,810

METHOD OF MAKING OIL GAS INTERCHANGEABLE WITH NATURAL GAS Filed March 11, 1952 2 Sheets-Sheet 1 14, 1956 E. s. PETTYJOHN ET AL 2,734,810

METHOD OF MAKING OIL GAS INTERCHANGEABLE WITH NATURAL GAS Filed March 11, 1952 2 sh9ebSShe6t 2 (/39 $772202? -\Z4U $227 72? index /230 Z417 142222.262

- 2 6%, wzmm rm United States Patent METHOD OF MAKING 01L GAS INTERCHANGE- ABLE WITH NATURAL GAS Elmore S. Pettyjohn, Evanston, and Henry R. Linden, Franklin Park, 111., and Wiliiarn L. Lee, Atlanta, Ga, assignors to Institute of Gas Technology, Chicago, 111., a corporation of Illinois Application March 11, 1952, Serial No. 275,952 4 Claims. (Cl. 48-211) This invention relates to a method of preparing an oil gas by cracking petroleum oils, particularly under conditions producing carbon deposits in amounts preventing continuous cracking due to the necessity for frequent removal of carbon deposits.

An imporatnt object of the invention is to provide a method of the nature indicated which can be carried out, if desired, in conventional regenerative apparatus for making oil gas with only slight modification of such apparatus, and which is eliective to make oil gas under less severe cracking conditions and at a faster rate and to make an oil gas more interchangeable with natural gas, all as compared with conventional regenerative methods for making oil gas.

Other and further objects and features of the present invention will become apparent from the following description and appended claims as illustrated by the accompanying drawings showing, diagrammatically and by way of examples, apparatus for carrying out the meth- (ads of the present invention. More particularly:

Fig. l is a vertical, longitudinal, sectional view (with parts shown in elevation) of an apparatus according to the present invention for making oil gas;

Fig. 2 is a fragmentary view of the main gas duct of the apparatus of Fig. 1 as modified in accordance with the disclosure in the copending application of Elmore S. Pettyjohn entitled Oil Gas Plant, Serial No. 270,956 filed on February 11, 1952, now abandoned;

Fig. 3 is a vertical, longitudinal, sectional view (with parts shown in elevation) of another apparatus according to the present invention for making oil gas; and

Fig. 4 is a fragmentary view of the main gas duct of the apparatus of Fig. 3 as modified in accordance with the disclosure in the above-noted copending application of Elmore S. Pettyjohn.

Referring now to Fig. 1, the apparatus there shown includes two generators 1t) and 12 disposed adjacently each other and two superheaters 14 and 16, disposed, respectively, adjacently the generator and the generator 12. At their bottoms, the generator 10 and the superheater 14 are interconnected by a duct 18 to form a first generator-superheater pair, while the generator 12 and the superheater 16 are similarly interconnected by a conduit 20 to form a second generator-superheater pair. The generators 10 and 12 and the superheaters 14 and 16 are provided with interior checker brick work 22 made up of layers of closely spaced bricks arranged in closely overlapping position (as between the various layers) so as to provide a maximum number of tortous gas passages whereby highly etficient heat exchange is promoted between the gas and the brick work. Due to the overlapping of the checker bricks, each brick structure forms a unified mass perforated by a large number of gas passages, with the result that the temperature therein will be substantailly uniform. The cracking effected within each brick structure will therefore be uniform throughout each brick work.

The roofs of the generators 10 and 12 are each arched 2,734,810 Patented Feb. 14, 1956 above the brick work therein and are each formed, respectively, with central, vertical apertures 24 and 26 recciving vertical ducts 28 and 30 opening upwardly into a crossover duct 32. The roofs of the superheaters 14 and 16 are likewise arched above the brick work therein and are each formed, respectively, with central,

vertical apertures

34 and 36 receiving vertical ducts 38 and 40 having upper openings closed by valves 42 and 44. Stacks 46 are provided above the upper ends of the ducts 38 and 40 to receive gases vented therefrom when the valves 42 and 44 are opened.

An air duct 48 having a valve 50 discharges into the duct 28 leading into the generator 10 and another air duct 52 having a valve 54 discharges air into the duct 30 leading into the generator 12. An oil spray device 56 for the generator 10 extends through the duct 28 and is supplied with oil to be cracked through a pipe 58 having a valve 60 and with oil to be burned for heating through a pipe 62 having a valve 64. An oil spray device 66 for the generator 12 extends through the duct 30 and is supplied with oil to be burned for heating through a pipe 68 having a valve 70 and with oil to be cracked through a pipe 72 having a valve 74.

A gas duct 76 branches 011 from the duct 38 of the superheater 14 and has connected thereto an air duct 78 valved at 80 as well as a steam pipe 82 valved at 84. Similarly, a gas duct 86 branches off from the duct 70 of the superheater 16 and has connected thereto an air duct 88 valved at 90 aswell as a steam pipe 92 valved at 94. Both ducts 76v and 86 discharge into a main gas duct 96 leading to a wash box 98 containing liquid 1% into which the lower end of the main gas duct 96 dips. Another gas duct 102 conducts the make gas from the wash box 98. A valve 104 in the top of the duct 96 serves alternately to close the orifices of the ducts 76 and 86 into the duct 96.

As so far described, the apparatus of Fig. l is conventional. According to the present invention, a pipe valved at 112 extends through the duct 38 and another pipe 114 valved at 116 extends through the duct 40. The pipes 110 and 114 serve to admit, respectively, into the superheaters 14 and 16 liquid or vaporized propane, butane, gasoline, light naphtha, or a mixture thereof.

In the operation of the apparatus of Fig. 1 through a four-stage cycle, a first blast period is initiated by closing the valve 42, then opening the valve 44, and moving the valve 104 to close the outlet of the duct 76. Primary air is admitted through the conduit 78 by opening the valve 81;. Secondary air is admitted through the conduit 52 by opening its valve 54 and oil for heating is supplied through the pipe 68 by opening its valve 70. ,The primary air passes through the superheater 14 and the generator 10 wherein cracking has been effected in the preceding make stage, and the air then removes carbon deposited in the brick work of this first generator-superheater pair while being preheated before reaching the second generatorsuperheater pair. At the 'end of this blast period, the valves 54, 7t), 80, and 44 are closed. The apparatus may be purged by briefly opening the valve 84 of the steam pipe 82 before the valve 44 is closed.

In the next succeeding make period, oil is cracked in the second generator-superheater pair. For this purpose, the valve 112 of the pipe 110 is opened to supply a light hydrocarbon of the nature indicated and the valve 74 of the pipe 72 is opened to supply oil to the generator 12. The make gas leaves the top of the superheater 16 and passes through the

ducts

86 and 96, through the wash box 98 and into the gas main 102. Duringthis period, carbon is deposited in the brick work of the second generator-superheater pair. At the conclusion of this make period, the valves 74 and 112 are closed and the valve 104 is moved to close the outlet of the duct 86 into the duct 96 and to open the outlet of the duct 76.

In the next succeeding blast period, the valve 42 is opened and the valve 94 of the steam pipe 92 may then be opened briefly to purge the apparatus. Primary air is admitted through the pipe 88 by opening the valve 90 to remove the carbon deposits from the brick work in the generator 12 and superheater 16, the air being preheated while passing therethrough. Secondary air is admitted from the pipe 48 by opening its valve 50 and oil is supplied from the pipe 62 by opening the valve 64. At the end of the blast period, the valves 64, 90, 50, and 42 are closed. Before the valve 90 is closed, the valve 94 of the steam pipe 92 may be opened briefly to purge the apparatus.

In the next succeeding make period, the valve 116 of the pipe 114 is opened to supply a light hydrocarbon of the nature indicated and oil is supplied to the generator by opening the valve 60 in the oil pipe 58. The make gas leaves the top of the superheater 14 and passes through the ducts 76 and 96, through the wash box 98 and into the gas main 102. During this period, carbon is deposited in the brick work in the first generator-superheater pair. At the conclusion of this make period,

valves

60 and 116 are closed and the valve 104 is moved to close the outlet of the duct 76 into the duct 96 and to open the outlet of the duct 86. The valve 94 is then opened, and the valve 84 of the steam pipe 82 may be opened briefly to purge the apparatus. The apparatus is then ready for a repetition of the four-stage cyclic regenerative process above described.

By way of a modified method of operating the apparatus of Figure 1, oil to be cracked while a light hydrocarbon is fed into the apparatus through the pipe 110 may be supplied through either the pipe 58 or the pipe 114 instead of the pipe 72. Or, oil to be cracked while a light hydrocarbon is fed into the apparatus through the pipe 114 may be supplied through either the pipe 72 or the pipe 110 instead of the pipe 58.

In the above-described process, a light hydrocarbon (of the nature previously indicated) is cracked in the superheater of one superheater-generator pair while a heavy oil is cracked in the generator of the other set. The product gas from the light hydrocarbons cracked in the superheater flows into the generator, so that the heavy oil is cracked in an atmosphere of this product gas produced from the above-disclosed light hydrocarbons. As a result, the make gas produced is more completely interchangeable with natural gas. Further, both the generator and the superheater are simultaneously utilized to produce gas, so that the rate of gas production is greatly increased as compared with that possible in conventional regenerative apparatus for making oil gas of high heating value when operated according to conventional methods. Finally, the heat normally used for superheating the I steam conventionally introduced into the apparatus during admission of oil to be cracked is utilized for cracking the indicated light hydrocarbons so that the generation of oil gas is effected with a much smaller fuel consumption per unit of oil gas made than has heretofore been found possible. Only slightly more tar is formed than if the oil had been cracked in the absence of the indicated light hydrocarbons.

A regenerative apparatus of somewhat different design from that shown in Fig. l is illustrated in Fig. 3. The apparatus of the last-mentioned figure is generally U- shaped, having left and right shells indicated generally respectively, at 150 and 152 and interconnected at their bottoms by a conduit 154. The left shell 150 has provided therein spaced lower and upper checker brick works 156 and 158 functioning similarly, respectively, to the generators and superheaters of the apparatus of Fig. l. The right shell 152 is provided with similarly functioning spaced lower and upper checker brick works 160 and 162. An oil spray device 164 projecting into the Cir interspace between the generator 156 and the superheater 158 is supplied with oil to be cracked through a pipe 166 valved at 168 and with oil to be burned for heating through a pipe 170 valved at 172. An air duct 174 valved at 176 also projects into the said interspace. Similarly, an oil spray device 178 projecting into the interspace between the generator 160 and the superheater 162 is supplied with oil to be cracked by a pipe 180 valved at 182 and with oil to be burned for heating through a pipe 184 valved at 186. An air duct 188 valved at 190 also projccts into the interspace between the generator 160 and the superheater 162.

The roof of the shell is arched above the brick work 158 and is formed with a vertical aperture 192 receiving a vertical duct 194 closed upwardly by a valve 196 which when opened permits venting of the shell 150 into a stack 198. Similarly, the shell 152 is arched above the brick 162 and is formed with a vertical aperture 200 receiving a vertical duct 202 closed upwardly by a valve 204 which when opened permits venting of the shell 152 into a stack 206. A gas duct 208 branches off from the duct 194 for the shell 150 and has an air duct 210 valved at 212 as Well as a steam pipe 214 valved at 216 connected thereto. Similarly, a gas duct 220 branches off from the duct 202 for the shell 152 and-has an air duct 222 valved at 224 as well as a steam pipe 226 valved at 228 connected thereto. Both ducts 194 and 202 discharge into a gas duct 230 having a lower end dipping into liquid 232 in a wash box 234 from which make gas is discharged into a gas main 236. A valve 238 at the top of the gas duct 230 is movable between extreme positions closing the outlets into the gas duct 230, respectively, of the

ducts

208 and 220.

As so far described, the apparatus of Fig. 3 is conventional.

According to the present invention, a pipe 250 valved at 252 extends through the duct 194 and another pipe 254 valved at 256 extends through the duct 202. The pipes 250 and 254 serve to admit, respectively, into the

superheaters

158 and 162 liquid or vaporized propane, butane, gasoline, light naphtha, or a mixture thereof.

In the operation of the apparatus of Fig. 3, blasting is first carried out. The valve 196 is closed, the valve 204 is opened, and the valve 238 is moved to close the outlet of the duct 208. Primary air is admitted from the duct 210 by opening the valve 212 to remove anycarbon deposits (formed in a preceding cracking or make period) in the superheater 158 and the generator 156, the primary air then being preheated on passage through these structures. Secondary air is admitted through the duct 188 by opening its valve 190, and oil for burning is supplied by opening the valve 186 of the pipe 184. The generator and the superheater 162 are thus brought to cracking temperatures. At the end of the blast period, the

valves

186, 190, 212, and 204 are closed. Before the valve purged by opening the steam valve 216 briefly.

In the next succeeding make period, the valve 182 of the oil pipe is opened, as is also the valve 252 of the pipe 250. The make gas leaves the shell 152 through the duct 202 and passes through the

ducts

220 and 230 and through the wash box 234 into the gas main 236. Carbon is deposited in the generator 160 and the superheater 162. At the end of the make period, the

valves

182 and 252 are closed and the valve 238 is moved so as to close the outlet of the duct 220.

In the next succeeding blast period, the valve 196 is opened and the apparatus may then be purged by briefly opening the valve 228 of the steam pipe 226. Primary air is admitted from the duct 222 by opening the valve 224, to remove carbon deposited in the generator 160 and the superheater 162, this air being preheated on passing through the brick work of the structures. Secondary air is admitted from the conduit 174 by opening the valve 176 and oil is supplied to the spray device 164 from the 204 is closed, the apparatus maybe i pipe 170 by opening the valve 172. At the end of this blast period, the

valves

172, 224, 176, and 196 are closed. Before the valve 196 is closed, the apparatus may be purged by briefly opening the valve 228 of the steam pipe 226.

In the next succeeding make period, the valve 168 of the oil pipe 166 and the valve 256 of the pipe 254 are opened. The make gas leaves the top of the shell 150 and passes through the

ducts

194, 208, and 230 and through the wash box 234 into the gas main 236. Carbon is deposited in the generator 156 and the superheater 158. At the conclusion of this make period, the

valves

168 and 256 are closed, and the valve 238 is moved so as to close the outlet of the duct 208. The valve 204 is opened, and the apparatus may then be purged by briefly opening the valve 216 of the steam pipe 214, to make the apparatus ready for a repetition of the above-described four-stage cyclic operation.

Apart from the stages hereinabove described, the apparatus of Fig. 3 is operated exactly as the apparatus of Fig. 1 to produce a make gas similar to that formed in the apparatus of Fig. 1.

In the two make periods, the oil to be cracked need not be introduced through the pipe 130 when a light hydrocarbon is admitted through the pipe 250 but can instead be introduced through the pipe 166, or through both

pipes

180 and 166. When a light hydrocarbon is admitted through the pipe 254, oil to be cracked can be introduced through either of the

pipes

166 or 180, or through both of these pipes.

By way of an example, we tabulate hereinbelow the results obtained by operating an apparatus such as that shown in Fig. 3 for cracking Bunker C oil at an average temperature of from 1450 to 1470 F. in an atmosphere provided by admitting propane to the superheaters. The oil was fed into the apparatus at the rate of 85 gallons per run and the propane (in liquid form) at the rate of 20 gallons per run. There were 12 runs per hour producing make gas at the rate of 116,600 s. c. f./hour. The make gas analysis is tabulated as follows:

The above gas is more interchangeable with natural gas than conventional high heating value oil gas. The concentration of diluents (CO2, 02 and N2) can be adjusted to a more desirable mixed heating value and specific gravity by reducing the volume of combustion gas permitted to enter the product gas. The make gas rate when feeding only the 85 gallons of Bunker C fuel oil per run was reduced to 97,000 s. c. f./hour, 75 lbs. per minute of make steam then being used as compared to the 45 lbs. of make steam per minute used when propane was cracked simultaneously with the oil. More than 20 gallons of propane per run can be admitted into the apparatus, with resultant formation of more than 116,600 s. c. f./hour of make gas.

In general, the same type of oil gas can be produced by proceeding as follows. The cracking of the aboveindicated light hydrocarbon in the superheaters is carried out at a temperature range from 1200 to 1650 F. and at a residence time of from to 12 seconds. One part of the indicated light hydrocarbons may be cracked in the superheater for each 2 to 10 parts of oil cracked in the generator. The oil referred to hereinabove as being cracked in the generators includes fuel and gas oils, and the various heavy petroleum oils such as heavy gas oil and Bunker C oil. Ordinarily, heavy petroleum oils are employed. The average cracking temperature ranges from 1350 to 1700 F., and the residence time from 1 to 5 seconds. The cracking temperatures and residence time are maintained in the same manner as in the operation of a conventional regenerative apparatus for making (at atmospheric pressure) an oil gas having a heating value of from 700 to- 1500 and preferably of from 950 to 1200 B. t. u./s. c. f.

it should be noted that in the operation (at atmospheric pressure) of a conventional regenerative apparatus for making oil gas having a heating value of from 700 to 1500 B. t. u./s. c. f. (at a cracking temperature of from 1350 to 1700 F., at a residence time of from i to 5 seconds, and at a partial pressure of the oil gas during cracking ranging from 0.3 to 0.4 atmosphere), the composition of the gas formed is fixed by the diluent (combustion blast or purge gas) free heating value thereof, being independent of the nature of the oil being cracked. The relation between the gas composition and the heating value of the diluent free oil gas is tabulated as follows:

Paralfins Ethylene Higher illuminants Hydrogen i- HMA Gasman In proceeding according to the present invention, the cracking of the. oil in the generators is carried out within the indicated ranges of temperature and residence time. Within these ranges, the conditions are adjusted to give a make gas of the particular heating value desired, due regard being had for the fact that the cracking in the super-heaters of the indicated light hydrocarbons reduces the diluent content of the make gas in proportion to the quantity of light hydrocarbons cracked, correspondinglyreduces the specific gravity of the make gas, raises the paraflin content of the make gas and reduces the make. contents of olefins and other illuminants all as com pared with the make gas of the same heating value obtained in the conventional regenerative apparatus for making oil gas. The net effect is one equivalent to making a final oil gas of reduced diluent-free heating value without increasing the severity of cracking and thereby avoiding increased carbon formation and tar viscosity.

The composition of the oil gas produced according to the present invention is such as to make the oil gas more completely interchangeable with natural gas than the oil gas produced by conventional cracking methods at the same conditions of cracking severity. For a complete discussion of the interchangeability of high heating value oil gas and natural gas, reference is made to the copending application of Elmore S. Pettyjohn and Henry R. Linden entitled Fuel Gas Interchangeable with Natural Gas and Method of Preparing the Same, Serial No. 270,957, filed on February 11, 1952.

It is believed that the higher paraffin to. olefin ratio obtained in the make. gas prepared according to the present invention is in part due to the relatively increased oil gas partial pressure obtained by cracking the oil in an atmosphere of product gas (obtained by cracking the indicated light hydrocarbons in the superheaters) rather than by cracking the oil in a steam atmosphere, as is conventionally done in operating regenerative apparatus for making oil gas.

A still higher paraflin to olefin ratio and a higher paraffin to hydrogen ratio in the make gas (with consequently more complete interchangeability of the make gas with natural gas) can be secured by carrying out the make steps of the above process under superatrnospheric pressure. For this purpose, the apparatus of Fig. 1 may be modified as illustrated in Fig. 2. As there shown, the duct 96 is constricted downstream of the valve 104, as indicated at 106. This constriction is dimensioned so as to dam up the oil gas pressure generated within the apparatus when oil is cracked therein. Further, the conduit 96 is made sufiiciently wide downstream of the constriction 106 to permit expansion of oil gas passing therethrough whereby normal functioning of the wash box 98 is made possible. The modified apparatus of Fig. 2 is operated exactly as the apparatus of Fig. 1, but in the apparatus of Fig. 2 the cracking steps of the cycle are carried out under superatrnospheric pressure (which is not true of the blasting steps), with resultant further irnprovement in the itnerchangeability of the make gas with natural gas, as noted hereinabove. For further details, reference is made to the copending application of Elmore S. Pettyjohn entitled Oil Gas Plant.

By way of a modification of the apparatus of Fig. 3, the duct 230 may be constricted downstream of the valve 238, as shown at 240 in Fig. 4. This constriction is dimensioned so as to dam up the pressure of the oil gas generated in the apparatus when oil is cracked therein. The pressure upstream of the constriction 240 does not interfere with a normal operation of the wash box 234, for the gas issuing from the constriction expands downstream of the constriction before entering the wash box 234. The improved results obtained by cracking under pressure are discussed above in connection with the description of Fig. 2.

Many details of construction and procedure may be variedwithout departing from the principles of this invention, and it is, therefore, not our intention to limit the patent granted on this invention otherwise than necessitated by the scope of the appended claims.

We claim:

1. A cyclic regenerative oil cracking process for making an oil gas for supplementing natural gas which comprises providing a first and a second generator and a first and a second superheater each capable of absorbing heat and of releasing heat on gas flow therethrough, heating said generators and superheaters to a cracking temperature of 1350 to 1700 F., thereafter flowing through said second superheater, second generator, first generator and first superheater in said sequence a. hydrocarbon selected from the group consisting of propane, butane, gasoline, light naphtha and mixtures thereof to crack said hydrocarbon while simultaneously flowing a petroleum oil through said first generator and said first superheater in said sequence at a rate which is equivalent to a residence time of from 1 to 5 seconds, whereby said oil is cracked in the presence of the cracking products of said hydrocarbon and in the absence of steam to form oil gas, cooling said oil gas thus formed, thereafter flowing air through said first superheater, first generator, second generator and second superheater in said sequence to remove carbon deposited in said first generator and said first superheater when said oil is cracked therein, while flowing additional air through said second generator and said second superheater 1n said sequence and flowing oil into said second generator for combustion therein, said flowing of air and oil being continued until said bodies have been raised to the cracking temperature indicated and the hot combustion product gases being vented after flowing out of said second superheater, thereafter flowing said hydrocarbon through said first superheater, first generator, second generator and second superheater in said sequence to crack said hydrocarbon, while simultaneously flowing said oil through said second generator and second superheater in said sequence at a rate which is equivalent to a residence time of 1 to 5 seconds, whereby said oil is cracked in the presence of the cracking products of said hydrocarbon and in the absence of steam to form oil gas, cooling the oil gas thus formed, thereafter flowing air through said second superheater, second generator, first geenrator and first superheater in said sequence to remove carbon deposited in said second superheater and said second generator when said oil is cracked therein, while simultaneously flowing additional air through said first generator and first superheater in said sequence and flowing oil into said first generator for combustion therein, said flow of said air and oil being continued until said generators and superheaters have been raised to a cracking temperature of 1350 to 1700 F. and the hot combustion product gases being vented after flowing out of said second superheater, and thereafter repeating the cycle recited hereinabove beginning with the first recited flowing of said hydrocarbon and said oil.

2. A method according to claim 1 in which said pe-' troleum oil is caused to flow through said second generator, first generator, and first superheater in said sequence while said hydrocarbon is caused to flow through said second superheater, second generator, first generator and first superheater in said sequence, said petroleum oil also being caused to flow through said first generator, second generator and second superheater in said sequence while said hydrocarbon is caused to fiow through said first superheater, first generator, second generator, and second superheater in said sequence. I V

3. A method of making an oil gas interchangeable with natural gas which comprises passing a hydrocarbon selected from the group consisting of propane, butane, gasoline, light naphtha and mixtures thereof through a heated vessel at a temperature of 1200 to 1600 F. and for a residence time of 10 to 12 seconds to crack said hydrocarbon, immediately passing said hot cracking products into a reactor while simultaneously introducing into the reactor a normally liquid petroleum fraction to crack said fraction in the presence of said hot cracking products and in the absence of steam, the cracking of said fraction being carried out at a temperature of 1350 to 1700 degrees Fahrenheit and at a residence time of from 1 to 5 seconds to obtain a gas containing a major proportion of parafiinic hydrocarbons.

4. The method of claim 3 wherein two to ten parts of said liquid petroleum fraction are employed for one part of said hydrocarbon.

References Cited in the file of this patent UNITED STATES PATENTS 543,992 Kirkham Aug. 6, 1895 982,754 Waring Jan. 24, 1911- l,97l,729 Perry Aug. 28, 1934 2,192,815 Johnson et al Mar. 5, 1940 2,580,766 Hall Jan. 1, 1952 2,592,591 Odell Apr. 15, 1952 2,605,176 Pearson July 29, 1952 2,656,307 Findlay Oct. 20, 1953

Claims

3. A METHOD OF MAKING AN OIL GAS INTERCHANGEABLE WITH NATURAL GAS WHICH COMPRISES PASSING A HYDROCARBON SELECTED FROM THE GROUP CONSISTING OF PROPANE, BUTANE, GASOLINE, LIGHT NAPHTHA AND MIXTURES THEREOF. THROUGH A HEATED VESSEL AT A TEMPERATURE OF 1200 TO 1600 * F. AND FOR A RESIDENCE TIME OF 10 TO 12 SECONDS TO CRACK SAID HYDROCARBON, IMMEDIATELY PASSING SAID HOT CRACKING PRODUCTS INTO A REACTOR WHILE SIMULTANEOUSLY INTRODUCING INTO THE REACTOR A NORMALLY LIQUID PETROLEUM FRACTION TO CRACK SAID FRACTION IN THE PRESENCE OF SAID HOT CRACKING PRODUCTS AND IN THE ABSENCE OF STEAM, THE CRACKING OF SAID FRACTION BEING CARRIED OUT AT A TEMPERATURE OF 1350 TO 1700 DEGREES FAHRENHEIT AND AT A RESIDENCE TIME OF FROM 1 TO 5 SECONDS TO OBTAIN A GAS CONTAINING A MAJOR PROPORTION OF PARAFFINIC HYDROCARBONS.