US2037379A - Cracking of oil - Google Patents

Description

April 14, 1936. E. c. HERTHEL 2,037,379

CRACKING OF OIL Filed Sept. 24, 1923 INVENTOR ATTORNEY;

Patented Apr. 14, 1936 STATES CRACKING OF OIL Eugene@ Herthel, Chicago, Ill., assigner toSinclair Refining Company, Chicago, Ill., a corporation of Maine Application September 24, 1923, Serial No. 664,502

2 Claims.

This invention relates to an improved method and apparatus for cracking heavier hydrocarbon oils for the production of lighter hydrocarbon oils therefrom.

In the cracking of heavier hydrocarbon oils in externally heated stills, the heavier oil is heated to a high temperature under pressure and the heat necessary for raising the oil to the cracking temperature is applied directly to the walls of the pressure still, in the case of a shell still, and to the heating tubes in the case of a tube still. As the cracking operation progresses, the formation of carbon and tar constituents takes place and the extent to: which the cracking operation can be carried in a single run is limited by the accumulation of such constituents which tend to settle or separate out on the heating surfaces and to form a layer of so-called carbon thereon which interferes with the heat transfer and, unless periodically removed by cleaning, causes overheating of the walls or tubes of the still. To avoid or minimize the danger of such carbon separation on the heating surfaces, the run is commonly stopped before the accumulation of heavy tar constituents becomes too great, or the tar is continuously withdrawn and fresh` feed supplied to maintain the charge in a suiciently fluid condition to permit continued operation for a longer period of time. The danger of such carbon separation and resulting failure of the still walls or tubes is greatest where the intense heat of the furnace gases rst comes in contact with the still walls or tubes and where the greatest amount of heatingtakes place.

In carrying out the n present invention, the pressure still itself, whether of a shell or tube type, advantageously is protected from the intense furnace heat, and this heat is applied in such a way as to increase theeiiiciency andcapacity of the still. v

In carrying out the present invention, I find it advantageous to provide an auxiliary heating coil arranged between therebox andthe main heating surfaces of the still so that the auxiliary coil will be subjected to the hottest heating gases and the intense furnace heat thus utilized and reduced before the heating gases pass over the heating surfaces of the still proper. By arranging a preliminary cracking coil intermediate the lirebox and the still p-roper I utilize the highest temperature of the heating gases to heat the oil flowing through the auxiliary'coil and I then use the furnace gases at a lower temperature for heating the main charge of oil in the still itself.

The auxiliary cracking coil is-used for carrying Vout a separate cracking operation which may b e independent of the cracking operation carried out l in the kpressure still itself; but which is advantageously combined with the pressure still so that the oil subjected to the cracking operation in the preliminary cracking coil is discharged into the or the reflux or heavier ends of the pressure distillate, may advantageously'be utilized as charging stock for the auxiliary cracking coil, so that this reflux will be returned through the auxiliary cracking coil to the main body of oilY in the pressure still. Thisreflux oil returned to the still itself may advantageously be admixed with fresh feed which may be utilized for assisting the refluxing action in the reflux tower and then supplied, freed from its lighter constituents and admixed with the reflux, to the auxiliary cracking coil, and subjected to a partial cracking operation therein before it is introduced into the pressure still itself.

By utilizing in the auxiliary cracking coil a fresh charging stock or a mixed reflux and feed, which is free from carbon and tarconstituents, the danger of objectionable carbon deposit on the walls of the auxiliary cracking coil is avoided or minimized, even though this cracking coil is subjected to the intense furnace heat; while the furnace gases are sufficiently cooled by their contact with the auxiliary cracking coil`so that in# tense heating of the walls or tubes of the pressure still itself is avoided, and a milder heating of these walls or tubes obtained such that the danger of objectionable carbon deposit is reduced and the cracking operation canrbe carried to a further extent than would otherwise be feasible.

In the application of the invention in connection with a pressure still of the shell type, theauxiliary coil may be arranged in the heating flue or in the path of the hottest gases at an intermediate point between the firebox and the still proper, so that the furnace gases which come in contact with the heating surfaces of the still will have been cooled considerably below their initial temperature, while the intense heat ofthe high temperature products of combustion from the firebox will be absorbed by the 'oil in the auxiliary cracking coil. Where such a shell still is provided with a reflux tower, or where the distillate therefrom is subjected to reuxing and separation of the heavier constituents therefrom at a lower pressure than that ofthe still itself, the redux,l

either with or without additional feed, can be used as the charging stock of the auxiliary cracking coil and the cracked oil from such coil may then be introduced into the still proper.

Where the auxiliary cracking coil is combined with a tubular cracking still, the coil is similarly arranged between the firebox and the heating chamber or chambers containing the tubes through which the oil is circulated and in which it is heated. This arrangement is of particular value in connection with such tubular cracking stills for the reason that it protects the tubes from the intense furnace heat and enables al milder heating of the tubes to be obtained. With such a tubular still, the auxiliary cracking coil may likewise be combined to receive the reflux from the still itself or the heavier part of the distillate therefrom and may similarly discharge the partially cracked oil back into the main body of oil undergoing cracking in the tubular still.

The invention is of special value and application in conjunction with pressure stills provided with reflux towers into which the fresh feed is introduced and from which the reflux and admixed feed can becirculated through the auxiliary cracking coil before their introduction into the still proper. The reflux towers may be operated at the same pressure maintained in the pressure still and the reiluxed oil, or refluxed oil with admixedV feed, circulated through the auxiliary cracking coil, either by gravity or con- Vection circulation or by means of a pump; or the vapors from the pressure still may be reduced to a lower pressure and subjected to a reiluxing operation and the heavier relluxed distil late with or without admixed feed, pumped through the auxiliary cracking coil and thence back to the main body of oil in the pressure still.

Where the reflux tower is operated at still pressure, the tower and auxiliary coil may be so arranged relative to each other, and the coil may be so constructed that flow of the oil through the coil and into the still will be obtained by gravity and convection; although this circulation may be effected or increased by means of a circulating pump or other means for increasing the circulation.

The invention will be further described in connection withl the accompanying drawing illustrating an embodiment of the apparatus of the invention adapted for the practice of the process of the invention, but it will be understood that the invention is illustrated thereby but not limited thereto.

In the accompanying drawing,

Fig. 1 shows a tubular pressure still embodying the invention, the showing being largely conventional and diagrammatic, and partly in section and partly in elevation;

Fig. 2 is a sectional view of one end of the auxiliary cracking coil taken on the line 2-2 of Fig. 3;

Fig. 3 is a top view of the auxiliary cracking coil taken on the line 3-3 of Fig. 2.

While the invention is applicable in connection with cracking stills of different types, yet it is of particular advantage in connection with a tubu` lar cracking still of the character illustrated. This cracking still is of the general construction illustrated in U. S. Letters Patent No. 1,285,200, granted to the Sinclair Refining Company, November 19, 1918 on the application of Edward W. Isom.

1 The pressure still illustrated in Fig. 1 is made up of the bulk supply tank I located away froml the heating furnace, the vertical heating tubes 2 arranged in the heating flue of the furnace, and circulating pipes 3, 4 and 5 connecting the lower and upper ends of the vertical tubes, respectively, with the bulk supply tank and a circulating pump 6 for circulating the oil from the bulk supply tank through the vertical tubes and back to the bulk supply tank. Arranged above the bulk supply tank is the reflux tower 1 having a series of baffles therein and connected with the vapor dome of the bulk supply tank through the vapor line 8. An overflow reflux line 9 leads back from the reflux tower to the bulk supply tank, and prevents the liquid from rising to a level which will choke up the vapor line 8. The vapors from the top of the reflux tower escape .through the vapor line I0 to the condenser 2 where the vapors are condensed and received in the receiver I3 from which the uncondensed vapors and gas are drawn olf through the pipe I4 and the liquid condensate through Ithe pipe I5. The pressure may be regulated and reduced by means of a regulating Valve I I between the reux tower and condenser or by the valve I4 located beyond the receiver.

The furnace construction for heating the tubes of the still comprises a firebox I6 suitably heated as by gas or oil burners Il, a vertical flue I8 leading upwardly from the firebox to the upper end of the heating flue containing the vertical tubes, a heatingv flue having baflles 29 and 2| therein for causing the gases to pass over the vertical tubes in three successive passes and an outlet flue 22 leading to the stack.

Located above the bridge wall I9 and between the Vertical flue I8 and the flue containing the vertical tubes 2 is the auxiliary cracking coil 23 having an inlet connecting pipe 24 and outlet pipe 25 leading to the manifold at the upper ends of the vertical tubes. A pipe 26 having a valve 21 therein connects the inlet of the coil to the discharge side of the circulating pump 6. rllhis pipe may be used in starting the still in operation, as well as in draining the auxiliary coil and co-nnecting piping when the still is pumped out.

An auxiliary tank or reservoir 28 is arranged to receive the reflux and admixed feed from the reflux tower 'l through connections 29. This tank or reservoir has a vent pipe 3D leading back to the reflux tower. The tank or reservoir 28 is also connected through the pipes 3| and 32 to the feed line 24 of the auxiliary coil. A pump 33 is arranged to pump the oil through the auxiliary coil, while the connecting pipe 34 permits circulation by gravity and by convection when the pump 33 is not used. A branch pipe 36 leads from connection 32 to the bottom of the bulk supply tank. The pipes 3| and 36 have control valves therein by which one or the other or both of these pipes can be shut off when desired. A feed line 35 enters the inlet pipe 24 of the auxiliary coil and permits the direct introduction of fresh feed as'it is desired. A tar draw-off connection for use during the operation 0I" the still is shown at 3'! and a tar draw-off for discharging the still is shown at 38.

The auxiliary reservoir or tank 28 is located Within the bulk supply tank although it is not in open communication therewith except through the pipe connections. It will be evident that valves such asthose in the pipes 3| and 36 should have operating handles which extend out through suitable stufling boxes in the walls of the bulk supply tank, VTest cocks (not shown) may be provided for ascertaining the' level of the oil in this auxiliaryl tank or reservoir. y

A feed line 39 is provided for feeding fresh oil to the top of the reflux tower lf. An additional feed line 49 is provided for feeding oil to the bearings of the pump 6. Additional .feed can also be introduced through the auxiliary coil through the feed line 3'5 as above noted.

The auxiliary coil- 23 is made up of a series of pipes connected at their ends as indicated in Figs. 2 and 3 and arranged so that the oil flows therethrough in a single pass in a generally upward direction, the flow being thus assisted by convection.

When the still is initially charged, oil may be circulated from the main charge of oil through the auxiliary cracking coil, for example, by closing the valve in the pipe 3| and opening the valve in the pipe 36 when the circulating pump 33 will draw oil from the bulk supply tank I and circulate it through the auxiliary coil; or the valve 21 may be opened and oil from the discharge side of the pump 6 may be circulated through the auxiliary coil; or fresh feed may be introduced through the feed line 35.

After the pressure still has been raised to cracking conditions of temperature and pressure, the cracked vapors from the still will rise in the reflux tower 1 and will come in contact with the fresh feed introduced through the line 39 to the top of the tower. By providing a sufficient number of bales practically all of the feed for the still can be introduced in this way and a pressure distillate approximating a gasoline distillate directly obtained with return of the heavier constituents in the liquid state lto the bottom of the reflux tower. This reflux and admixed feed collects in the reservoir or tank 28 which, in a still of the type illustrated, may have a capacity for example of about one thousand gallons. From this reservoir the oil is circulated either by gravity or convection or by pump, through the auxiliary cracking coil 23 where it is subjected to the intense heat of the hottest gases from the furnace. The cracked oil is discharged into the top circulating line of the still and is returned to the bulk supply tank. The unvaporized portion of the oil so returned forms a part of the still charge and is circulated therewith through the vertical tubes. It will be evident that the rate of circulation of the oil should be regulated so that a constant supply is provided and so as to maintain a level of oil in the reservoir 28. By introducing all or practically all of the fresh feed into the top of the reflux tower and bringing it into intimate contact with the vapors therein, a constant supply of reflux and admixed feed is provided. Should the reservoir 28 become full the additional reflux and feed will collect in the bottom of the reflux tower and will overflow through the pipe 9 to the bulk supply tank I.

In a still of the construction illustrated, when the auxiliary coil is not provided, the greater part of the heat is absorbed in the front rows of tubes in the upper pass of the heating chamber where the hot furnace gases first come in Contact with the tubes. In the commercial operation of such stills, around six million to eight million B. t. u. are absorbed per hour by the oil. By interposing the auxiliary coil where it will be subjected to the hottest gases from the furnace, the temperature of these gases will be reduced, for example, from around 2400D F. to around 1600 F. by their passage through the auxiliary coil and a large proportionof' the heat will be absorbed in this way at Ythe maximum temperature of the heating gases. Where, however, the oil fed through 'this auxiliary coil is ad-'mix'ed feed and` reflux which is free from tar, the coil can be subjected' to such high temperatures and a large amount of heat can be absorbed thereby without objectionable carbon deposit on the walls of the tubes of the coil.

After the high temperature furnace gases have been reduced in temperature by passage through the auxiliary coil, they then pass over the vertical tubes of the pressure stilrat a lower tem'- perature and serve to heat these tubes without danger of objectionable overheating'. A milder heating and cracking action is thus obtained so that the still charge can be cracked to a further extent without danger of objection-able carbon deposit on the tubes and with the obtaining of heavier residuum from the cracking operation.

In a commercial still ofthe character illusfrated, the auxiliary cracking coil may be made up of four inch pipe and the oil may be circulated iherethrough at a regulated rate so that objectionable overcracking will be avoided while effective cracking will nevertheless be obtained, and the resulting cracked oil will then be introduced into the pressure still to admix with the oil and vapors therein.

The operation of the pressure still in this way, with circulation ofthe feed and reflux through the auxiliary coil, and with circulation of the main charge of the still through tubes heated to lower temperatures, gives an increased furnace efciency with longer running time before it is necessary to shut down for cleaning, and with increased capacity of operationof the pressure still, while also permitting a reduction of pressure tar while under pressure to a lower gravity withoutrdetriment to the operation,

The higher temperature to which the auxiliary cracking coil is subjected also permits the crackingv therein of" kerosene fractions and lighter oils which are more difllcult to crack in ordinary pressure stills. v- Such kerosene fractions may be those produced by the pressure still itself or supplied from another source.

The addition of an auxiliary cracking coil in conjunction with the operation of the pressure still also permits the introduction of an increased amount of feed through the top of the reflux tower and a fractionation of the vapors from the still to much nearer a ilnishedgasoline than is commonly obtained, and this without any difficulty in regular operation.

The pressure still may be further protected from the intense furnace heat by recirculating a part of the heating gases which have passed over the heating surfaces of the still and admixing this recirculated cooler gas with the fresh heating gases from the furnace before they are passed over the heating surfaces of the still proper, so that the heating gases used for heating the main charge of oil in the still itself are tempered by the combined effect of the recirculated gases and the action of the auxiliary cracking coil in absorbing the initial intense heat of the heating gases. 'I'he recirculation of waste heating gases in, this way increases the velocity of flow of the heating gases improving heat transfer and since the waste heating gases are returned at substantially the temperature at which they would otherwise be discharged, both the eiliciency and capacity of the pressure still may be further increased. The Waste heating gases may be recirculated by means of a blower, as illustrated, or by other suitable means.

It will thus be seen that by this improved construction and method of operation of pressure stills, the intense furnace heat of the heating furnace is utilized in an auxiliary cracking operation where such intense heat is advantageous, and that the cracking operation of the main pressure still is carried out with the furnace gases which have been cooled by their utilization in the preliminary cracking operation.

It will further be seen that the preliminary cracking operation which utilizes the intense heat of the highest temperatures of the furnace may advantageously be combined with the operation of the pressure still utilizing the furnace lgases subsequently at a lower temperature such that a milder cracking action is obtained; and that, where these operations are combined, the feed for the auxiliary cracking coil may advantageously be supplied in the form of reflux, or of reflux and admixed feed from the reux tower of the pressure still. Y

'Ihe temperature and pressure employed in the carrying out of the process and in the operation of the apparatus can be varied with different characters of charging stock. `Where ordinary gas oil Y is employed in the main pressure still pressures of around 90-125 lbs. may be employed; while with lighter oils, higher pressures may be used, for example of around 300-325 lbs. The pressure maintained in the auxiliary cracking coil may be the same as that in the pressure still or only enough higher to cause thedesired circulation; or the auxiliary cracking coil may be maintained at a considerably higher pressure and the pressure reduced -in Acase the cracked oil is to be discharged into the pressure still at a lower pressure.

I claim:

1. A hydrocarbon oil cracking process which comprises passing the oil through a heating zone and heating the oil therein to a cracking temperature, separating vevolved vapors from unvaporized oil in a vapor separating zone and subjecting saidv vapors to dephlegmation thereby forming reflux condensate, collecting said reflux condensate in a body out of contact with vapors evolved in said vapor separating zone, withdrawing a portion of the reflux condensate from said body, separately heating said portion to a temperature higher than that to which the oil is heated in said first mentioned heating zone and thereby vaporizing a portion of said condensate, passing vapors evolved by said separate heating in indirect vheat exchange relation with said body of relux condensate, combining with the vapors being dephlegmated'vapors evolved by said separate heating of the reflux condensate and vapors evolved from said collected body of reux, and condensing the dephlegmated vapors.`

2. An apparatus for cracking hydrocarbon oils comprising a furnace structure having a fire box and a heating chamber, a tubular heater arranged in the heating chamber of said furnace, a vapor separating chamber, means for passing oil to be cracked through said heater and thence into said vapor separating chamber, a dephlegmator and a connection for conveying vapors from said vapor separating chamber to said dephlegmator, a reux collecting receptacle arranged within said vapor separating chamber above the normal liquid level therein, a reflux return connection leading from the lower end of said dephlegmator tosaid reflux collecting receptacle, a vent leading from the upper portion of said reilux collecting receptacle to a point in said dephlegmator above the juncture of said reflux return connection, a second tubular heater arranged in said furnace between the re box and the first mentioned tubular heater, means for withdrawing liquid from the lower portion of said reux collecting chamber and supplying it to one end of said second tubular heater, a connection leading from the other end of said second tubular heater to said vapor separating chamber, and means for condensing vapors discharged from said dephlegmator.

EUGENE C. HERTHEL.

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