US1969321A - Treatment of mineral oils by heat - Google Patents

Description

Aug.7, l934. w. J. PERELIS I TREATMENT OF MINERAL OILS BY HEATv Original Filed March 20.

Patented Aug. 7, 1934 UNITED STATES PATENT OFFICE Y mesne assignments, to Universal Oil Products Company, Chicago, Ill., a corporation of South Dakota Application March zo, 1925, lserial No. 16,952 Renewed January 4, 1934 17 Claims.

This invention relates to the treatment of mineral oils by heat and includes a process for converting the material treated into a more volatile fraction and valuable heavier fractions suitable for further use, and includes apparatus suitable for practice of the process.

Numerous procedures have been developed and used for the cracking of petroleum, either in vapor phase or in liquid phase, which lead in each instance to the formation of lighter and more volatile gaseous or liquid fractions, and heavier, relatively non-volatile fractions or residues. A common eiect of the so-called vaporphase processes is the production of large volumes of gaseous substances (and usually carbon) which are not commercially valuable. The liquid-phase and liquid-and-Vapor-phase processes, on the other hand, either fail to yield a suitable proportion of volatile liquid fractions, or if carried out under more extreme conditions for the production of a larger proportion of the volatile liquid fraction, result in causing the remaining fractions to be excessively contaminated with coky material, thus reducing a large part of material passed to a condition substantially useless for fuel or any other of the more exacting, and hence more important, uses.

I am aware of sundry processes for the cracking or conversion of -petroleum by heat, but so far as I am aware none of the several processes and the apparatus used in their practice are capable of advantageous use except upon one particular class of raw material, or capable of optimum production of both the more volatile and the less volatile products of conversion. Processes and apparatus adapted, for example, to a satisfactory or optimum conversion of a light gas-oil fraction or a kerosene fraction are unsuitable for a heavy gas-oil fraction or the residues of topping distillations or for treating crude oil directly. I am not aware of any prior cracking process or apparatus suitable indifferently to the conversion of charging stocks of crude petroleum or of the above-mentioned fractions. Cracking processes of which I am aware seldom attain a good economy of heat.

No process for conversion by heat into a motor-spirit fraction of a relatively heavy oil of which I am aware is capable of the conb'iiitant production of an economically good yield of motor-spirit or cracked gasoline and a commercially satisfactory residual fuel oil.

This invention-therefore has among its objects the provision of a process and apparatus capable of dealing with crude oils, gas-oils, kerosenes or the residues of topping operations indiiferently and with good economy of heat, and capable of satisfactory conversion of the charging stock by economically applied heat into a motor-spirit fraction and a residual fraction of good commercial qualifications and free from detrimental sediment.

Further objects of the invention are to provide a mode of heat treatment for the conversion of mineral oils avoiding in practice the limitations of the prior art, and in particular avoiding any operative disqualification from excessive coking and any danger of forming excessive amounts of degenerated heavy hydrocarbon residues containing tar, carbon or other impurities in suspension, avoiding any necessity for frequentinterruption of the treatment, and nevertheless capable in practice of attaining reliably a comparatively high conversion of the oily material treated into a cracked gasoline or motor 76 spirit and a commercially good fuel oil substantially free from suspended solids. v

The process may comprise among the recommended procedures or steps preheating the oil to be cracked, as by heat exchange with an eiilux 80 of the cracked products; heat treatment adapted n to prevent the excessive separation of cracked vapors from the oil under treatment during flow in a tubular heater, under suicient pressure to control the evolution of vapor and to cause turbulent flow of the liquid and entrained vapor in a tubular heater of relatively great length at a high speed, and during this flow subjecting the oil and entrained vapor stream to successive stages of heat treatment characterized by the attainment of a preliminary optimum temperature, permitting decrease of this temperature within a reactive degree of temperature during the evolution into entrained vapor of the more volatile reaction products, and again raising the temperature of the turbulently flowing stream during flow for a suicient time for economical conversion to the volatile products; subjecting the material to substantial reduction of temperature and releasing the residual pressure upon the oil stream under treatment; and ultimately separating the treated eillux into the desired products, for example crude gasoline, recycled material, gases and fuel oil.

Gaseous orreadily volatile liquid fractions, if any, of the raw-material may be removed before subjecting the material to the cracking process, but these steps are not essential for treatment of a wide class of materials capable of being advantageously treated by this process. It has y been established that, regardless of the source or nature of mineral hydrocarbons, with the exception of solids such as carbon `and coke, fluidity increases with rise of temperature and approaches a common maximum value. Accordingly, whatever the nature of the feed stock employed, this may be reduced to a condition of relativelylow surface tension and ajstate of maximum fluidity by bringing it within the proper temperature range. The charging stock, e'speciallywhen composed of the heavier'hydrocarbons, is warmed or preheated in advance of subhiission to the cracking treatment proper. Such cheating will ordinarily have no eifect upon the cracking operation, other than that of stabil zing the heat balance and minimizing the required further input of heat, and increasing the 'input and output of which the apparatus is capable.

It is recommended that the treatment shall be applied to an oil stream of substantially uniform cross-section throughout. I

A typical example of practice of the process of the invention will now be described with the aid of description of apparatus suitable to practice of the process, one form of which apparatus is indicated in diagram in the accompanying drawing, in which the gure is a diagrammatic elevation of one complete unit of the apparatus.

Referring to the drawing, the charging stock material may be preheated in preliminary heat exchanger 1, from which the oil iiows through pipe a to fractionating apparatus at 2, or other means for separating the gasoline vapors from the insufficiently cracked stock, and topping the feed stock of materials already sulciently volatile. 'Ihence the material iows through pipe 16 to a reservoir 3, and thence iiowsor is pumped through pipe b to a heat exchanger 4, in-'which a further rise of temperature is attained; and thence ows through pipe c to and through a cracking heater 5, from which the treated material returns by pipe d through heat exchanger 4 and passes through a pressure-reducing valve 23 and pipe e into a vaporizer 6, (in which the material may be treated by steam spray, if desired, to aid in separation by vapor and condensate) and from which the vapors pass in counter-currentQ relationship -to a spray of the preheated feed stock in the fractionating apparatus 2, and thence to a condensation apparatus 8 and fractionating apparatus, of which the prel heating heat-exchanger 1 may be a part.

ample through pipe 15.

- the vapors the unconverted The preliminary heater 1 may be associated with other exothermic stages of the flow through apparatus, but as shown is any usual type of conductive heat exchanger, conveniently placed to utilize the heat of the liquid outow at 37 from a still drum 3 4 in which the liquid condensate from a fractionating towerv 33 is reheated,JV

for exampleV to a temperature of the order of 20 F. The oil supply is preferably fed through e determined proportions through each to secure' a temperature of the. mixed stream in pipe a predetermined in accordance with the nature of the charging stock fed, for example 200 F. for some crude oils) by gravity or a pump 12, and thence is delivered into a spray head a1 at the top of a fractionating tower 13, so arranged as to afford counter-current flow of the heated gases and vapors from the vaporizer 6, for ex- 'I'he charging stock spray in tower 13 condenses and separates from but vaporized-fractions for refluxing, and the heat of the vapors preliminary heater 1 or by-pass 11 (in pre'- contributes heat to the charging stock spray already preheated with the effect of vaporizing its more volatile contents. Tower 13 may be provided with any suitable surface-providing filler, or bailles,l if desired, and have a vapor pipe 14 atl or near the top, for outow of the vapors from 6 and the volatiles, if any, from the charging stock spray at al. 'Ihe fluid outlet 16 at the bottom of tower 13 passes the charging stock and the condensed liquid fraction of the vapors from vaporizer 6 to reservoir 3, whence a return vent pipe 17 may be provided to permit the return of anyevolved gases through tower 13. A pump 18 forces the liquid contents from reservoir 3 through pipe b through the heat insulated heat interchanger or preheater 4, and through pipe 'c into and through the convoluted continuous tube ofthe cracking unit 5.

'I'he attained temperature of the oil leaving preheater 4 will ordinarily be maintained at about 740 F., but other temperatures at which the oil is loi' sufllcient uidity and at which the input of cracking heat at the next stage `of the operation is notrequired to be too great for the degree of conversion desired may be adopted within this invention.

Cracking is preferably conducted in at least I realized temperature throughout any cross-section of its path of flow. Preferably the temperature during ilow is permitted to rise, then to fall, and then to rise again to a maximum at exit; for example to a maximum temperature ofthe order of 900 F..with a usual permitted variation from 890 F. to 910 F.

'Ihe cracking unit 5 in one preferred form comprises a relatively great length of uniform diameter tubular heater, arranged for numerous reversals of direction of now, and having capacity for a large volume of iluid contents, but I am not to be understood as excluding the use of constructions in which the diameter of nic ample from 3% inches at entrance to 4%:inches at exit. For example, the cracking stageiof this treatment may be carried out during iiowk in series through three coils 20, 21 andl 22 of iron or steel tubing having respectively capacity to hold about thirty barrels, ltwenty barrels, and sixty barrels of uid, and constructed of 3% inch inside-diameter tubes; each ofthe coils 21 and 22 may, however'be constructed of 4% inch inside-diameter tubes; and thetubes of each coil may be suitably connected at ends of transverse runs by 180 forged steel couplings. Such an installation of'apparatus is suitable for producing about a thousand barrels of rectified gasoline per diem.

Therespective cracking coils 20, 21 and 22 are` preferably mounted in individual lfurnaces F1, F2, F3, in which the entrance coil 20 may be heated by any suitable combustion heater at a rate to advance the temperature of the ow through `about F., for example to 870 F. or somewhat less at exit from that coil. The next coil 21 and its furnace ln `may be heated by the waste heat from the furnaces ffl F1 and F and desirably the rate of input heat here is such as to cause the temperature of the fluid ow to recede through about F.

The next coil 22 is heated at a rate to cause the fluid ow to rise to about 900 F., or somewhat higher at exit. The relative temperatures attained during ow are important to correct practice. In the entrance coil 20 the oil passes to its maximum liquid fluidity or lack of viscosity as a liquid, and attains a cracking temperature during increasing rate and turbulence of flow due to expansion, the impressed pressure and the internal disintegration of its com ponents. The rate of heating to this point, if much higher than that recommended, would cause coking because the rate of input of heat would be too great for a liquid stream of oil not yet in a state of turbulent flow and not yet in that state of maximum fluidity or minimum viscosity which may be developed with the aid of entrained vapor; the oil particles close to the tube wall would be overheated and the center of the stream of oil would remain relatively cool. Coking-would then occur in the oil in contact with the tube walls. But during the time when the temperature is permitted to recede somewhat in the second coil 21, the material has already been subjected to heat during suliciently long a time to permit the development in it of minute vesicles of vapor, which are kept in a state of thorough distribution and individual separation by the rate and turbulence of ow of the stream. The temperature, in coil 21, is still sufficient for continued development by heat of the more volatile products of conversion, and by the time the material passes out of coil 21 its condition is such as to insure inability to overheat its liquid contents by heated pipe-walls which, at an earlier stage, would have resulted in heating the liquid in contact with them to too high a. temperature, so high as to result in too much permanent gas and too much carbon in the products. In other words, the fluid stream has now been vbrought to a state of flow, lack of viscosity, and contents of entrained vapor in which state it is a good vehicle for the convection of heat away from the walls of the tubular heater, as contrasted with greatly less efficiency in this respect in the entrance portions of the rst coil 20.

The fluid stream may therefore be detained during rapid and turbulent ow in coil 22 under higher temperatures for a sufiicient time to complete the conversion of the hydrocarbon material to the degree desired without endangering the formation of too much permanent gas and carbon. Coil 22 may therefore be of such length and maintained at such relatively high temperatures throughout as to eiect the thorough conversion of the material.

It will be understood that the resistance to flow in the tubular heaters 20, 21, 22 is relatively large, and that a relatively large progressive drop in pressure from the initial pressure is a necessary consequence of this. From this cause the pressure in coil 21 is less than in coil 20, and in coil 22 less than in coil 21. From the same cause, the pressures decrease from entrance to exit of each of these coils. This progressive decrease in pressure contributes to the progressive development of fluidity and turbulence of flow caused by the increasing contents of entrained vapors of the volatile substances consequent upon rise of the material to a temperature beyond a vcracking temperature at or before exit from coil 20, and maintenance of temperatures in the cracking range during flow in coil 21 for a suflicient time to make the development of volatiles effective.

'Ihese steps are eifective to cause a higher eiciency in cracking in relation both to the Volume of material treated in a given time and in the use of heat for the conversion. A still greater advantage, however, arises from the effect of the relationship of condition of the flowing stream to the heating Walls. The temperatures to which the material may be subjected without coking are' much increased by the recommended procedure, with corresponding increase in the production of gasoline.

'Ihe pressures at which cracking is carried out may vary as measured at exit from coil 22 from 250 pounds to 450 pounds to the square inch, (for crude oil, residues, heavy gas oil and mixtures of these) to higher pressures for light gas oils, kerosenes and their mixtures.

Elux from cracking as mentioned, now at temperatures of the order of 900 F. passes through pipe d to the heat exchanger 4, which operates not only to heat the incoming flow, but

also to extract from the eiux flow sufficient heat to lower its temperature to`from 750 F. to 550 F. prior to partial release of pressure at valve 23. If the efflux were not cooled at this stagef release of the pressure would be accompanied by a spontaneous evolution of carbon in the liquid residue. The prevention of formation of carbon at this stage is one of the more important features of this invention.

To this end, cooling of the elux stream may be effected after pressure release by valve 43, instead of cooling the eillux stream before the release of pressure, if too-far extended evaporation is checked, as by maintaining turbulent flow. For this reason the position of the pressure regulating valve 23 beyond the preheater 4 is not essential, although recommended if full advantage is to be taken of the heat stored in the eiiluxA stream of oil.

If the heat of the eillux stream is not employed to preheat the incoming oil, the advantage of producing sediment-free residues may still be maintained by resort to any expedient for cooling the elux stream before extensive evaporation; for example by mixing a flow of cold oil l with the efflux from the cracking unit.

When the heat exchanger 4 is not in use, the valves 43 and 23 are closed. The pressure on the unit is then regulated with the help of valve 44 in line 48 which connects with pipe d coming from the heater 22 and connects with the line e leading out of heat exchanger 4, cutting out the valves 43 and 23. The hot cracked oil coming out of the heater 22 flows then through line 48 and valve 44 into the vaporizer 6. Cooling oil can be admixed through the valves 45, 46 or 47. The cooling oil enters through'line 49.

Apparently it has not heretofore been understood that the appearance of carbon as sediment, coke or in colloid suspension in liquid cracked residues is caused by the accumulated heat in the cracked eiiiux, which is sufficient after pressure reduction to vaporize not only the lighter hydrocarbons, but also some of the veryheavy hydrocarbons, which when vaporized have a residue of coke or carbon. It has falsely been assumed that a carboniferous liquid residue was an inevitable consequence of higher temperatures of cracking, and the practice of pors and high-boiling residues will now be ap- A1li preciated.

The vapors are permitted to separate in vaporizer 6 under the reduced pressures and temperatures. Preferably this part of the operation is carried out at temperatures well below F. at pressures approaching or at atmospheric pressure, and ordinarily in the presence of water vapor, the latter especially if the oil treated contains only a small proportion of asphalt.

As shown, low pressure steam may be intro-l duced into the vaporizer 6 through pipe 40 and perforated spray pipe 41, and the liquid hot residue is run off through a cooler 42 to a fuel oil storage tank.

'I'his residue is normally free from more than one per cent of carbon. The residue oil is therefore'quite as valuable commercially as it would have been if no cracking operation had been employed to obtain from the oil the values resulting from the production ofthe more volatile substances.

Upon release of pressure in the vaporizer the oil stream transforms a substantial part of its heat values into the latent heat of vaporization of its more volatile fractions. Accordingly that portion or fraction of the oil stream having a boiling point below 600 F. or thereabouts, is gasied and passes off as distillate through vapor outlet 15, while the heavier portion collects in the bottom of the vaporizer, in liquid form, and may be withdrawn from time to time or continuously through the outlet 24.

'Ihe vapor educt 14 from the fractioning tower 13 leads into condenser 8, which may comprise the heat interchanger 26 and water-cooled cooler 27, a separator 28 having a gas outlet 29 for the gases consequent on the operations, and a storage tank 31 for crude gasoline condensed at this point.. Vapors in the pipe 14 are normally at a temperature of the order of 420 F., and this retained heat may be relied upon to reheat the condensate for rectification. A pump -32 is adapted to return the condensate through the interchanger 26 and thence to the reflux towerl 33 of the still 9, from which the eiiluent vapors are conducted to condensers 34, from which a part of the condensate may be returned to the rctifying column 33 through pipe 35. 'I'he fraction at 36 is rectified gasoline. y

It will be observed that the described preheating and fractionating steps carried out on the material treated are such as to perform a topping separation of the material treated and to reux insufliciently volatile products of the cracking operation as another consequence of the character of exchange of heat between the ingoing material and the eux material.

It' the feed stock, for example, is a heavy material, the entrance temperature in pipe a may be made materially lower (by adjustment of the amount by-passed about the heatl exchanger 1,

necesaror by any other control of 'its temperature), with the result of providing in feed tank 3 enough residual from the topping operation in towerl 13 and enough condensate of the flow from tank 6 for a full-capacity feed to the cracking heater 5. If the feed stock is a lighter material. the entrance temperature in pipe a may be made higher, and the rate of pump 12 increased to provide a full-capacity ow from tank 3 in Aaccordance with the amount of lighter distillates of .the feed stock separating out and passing through pipe 14.

Proper control of the adjustable factors of the apparatus enables these arrangements automatically to care for variation in the material being treated, and theprocess avoids loss by securing treatment only of those portions of the material which will not separate into the Vdesired fractions as a consequence of the preliminary exchanges of heat. Saving of heat is therefore supplemented by an emcient flexibility in operation.

vThe material recovered at 39 is a light gas oil and kerosene fraction which may be in turn subjected to treatment according to the described steps. Preferably this is done in a second group of apparatus in which the pressure in the cracking tube is higher, 4i1.' the initial operation was on a heavy oil; or'during alternate operation of the same unit of apparatus. A very thorough conversion of the original material into rectified gasoline and a superior grade of fuel oil is thus eiected with g'reat economy of heat.

Wherever in this specification the attainment of turbulent ow in the stream `of oil under treatment is mentioned, it will be understood that the purpose of such flow is agitation and thorough admixture of the contents, and it will be apparent that any effective procedure for agitating the contents might be resorted to, although I prefer agitation by causing rapid flow in a tubular container in which the direction of ow is often reversed.

In some cases of practice under this invention it is advantageous to make sure of a sufIicient development of vapor in the flowing stream during the cracking operation by increasing the proportion of volatiles initially in the feed stock.

This may be done by adding lower-boiling pointv oils or distillates or a proportion of the volatile substances produced by the cracking operation grec of occurrence as a bubbly suspension of vapor-in the stages of cracking, including the4 stream of the material under flow in a tubular y heater through a rise in temperature to a 'state of maximum liquid fluidity, causing the flow to recede in temperature within the range of cracking temperatures during continued development of volatile derivatives for a substantial time, and again increasing the temperature during agitating flow to a predetermined maximum temperature.

2. Process for the treatment of mineral oils by heat comprising as a step heating a liquid stream ofthe material under ow in a tubular heater through a rise in temperature to a cracking temperature, causing the flow to recede in temperature within the range of cracking. temitself. These additions serve to increase the deperature during turbulent ow, and again increasing the temperature'during agitation to a predetermined maximum temperature.

3. Process for the treatment of mineral oils by heat, comprising as steps, heating a liquid stream of the material under llow in a tubular heater to a temperaure sufcient for cracking conversion, causing the temperature to recede during turbulent flow during the development of a state of liquid and suspended finely divided entrained vapor while still maintaining a cracking temperature, then increasing the temperature to a predetermined maximum higher than the temperature attained during the rst cracking and thereafter main aining the stream of liquid and entrained vapor during maintained turbulent ilow for a substantial time at said predetermined maximum temperature.

4. Process for the treatment of mineral oils by heat comprising as a step heating a liquid stream of the material under flow in a tubular heater to a temperature suilicient to initiate a maximum liquid fluidity and suicient for cracking conversion, causing the temperature to recede during turbulent flow during the development of a state of liquid and suspended finely-divided entrained vapor, and thereafter increasing the temperature of the stream of liquid and vapor during maintained turbulent flow for a substantial time to a desired maximum temperature Within the cracking range.

5. Process for the treatment of mineral oils by heat comprising maintenance during a predetermined time of ow through a tubular heater of a fluid stream of the material to be treated, said material being for a substantial part of its ilow through said heater in a liquid state with pratcically no vapor and subjected to progressive heating to a cracking temperature, and during a subsequent 'part of its flow being in a state of mixture of liquid and entrained vapor and in a state of turburlent ow at a cracking temperature, while the progressive heating is interrupted, and during a further part of its ow subjecting said stream to a temperature substantially higher than said cracking temperature, while again progressively heating the stream.

6. Process for the treatment of mineral oils by heat comprising maintenance under initial pressure during a predetermined time of flow in a tubular heater of a fluid stream of the material to be treated, said material being for a substantial part of its flow in a state of mixture of liquid and entrained vapor and in a state of turbulent now at a cracking temperature, and comprising as a step reduction of the rate of heat input at a part of the path of flow in which the formation of a finely divided bubbly suspension of the vapors of included volatile substances occurs as a consequence of the attained temperature, and thereafter increasing the temperature of the flowing stream to a maximum during maintenance of said state of bubbly suspension.

7. Process for cracking mineral oils by heat, characterized by subjecting a liquid stream of oil while under turbulent flow under pressure to increase, decrease and increase of temperature in succession, maintaining the temperature at a cracking range during said increase, decrease and increase, the attained temperature of the flowing stream remaining at a reactive degree for cracking, the temperature attained during said second increase being higher than that attained during said first increase.

8. Process for the cracking of hydrocarbon materials by heat comprising as steps heating a flowing stream of the material to a cracking temperature, causing the flow to recede in temperature within the range of cracking temperature, causing agitation in sullicient degree to prevent the formation of coky deposits vwhile the temperature is decreasing within the cracking range, prolonging this treatment for a substantial time and thereafter raising the temperature of the material to cause a maximum degree of cracking at a higher temperature than during said first stage of cracking and during continued agitation, the maximum temperature being reached just before the material leaves the cracking system.

9. Process for the treatment of mineral oils by heat comprising as a step raising the temperature of a stream of liquid oil to a cracking temperature of the order of 850 to 870 Fahrenheit in a tubular heater, permitting the temperature of the material .to decrease during continued flow in a tubular heater, and then increasing the temperature of the stream of the material during continued flow in a tubular heater for a predetermined time to a predetermined maximum of the order of 900 Fahrenheit.

10. Process for the treatment of mineral oils by heat comprising as a step preheating the oil to be treated, raising the temperature of the preheated liquid oil during ilow in a tubular heater to a crackingrange, permitting the ternperature of the material to decrease during continued flow in a tubular heater, and then in-,

creasing the temperature of the stream of the material during continued flow in a tubular heater for a predetermined time to a predetermined maximum of the order of 900 Fahrenheit.

11. A process for the treatment of hydrocarbon materials by heat, comprising as steps heating a flowing stream of substantially liquid oil progressively to a cracking temperature, then interrupting said progressive heating for a period during which generation of vapors in the stream occurs While the oil is at a cracking temperature, then again progressively heating the modified stream to a temperature higher than said first cracking temperature, agitating the material during said heating steps and thereafter ellecting a partial cooling of the stream for the purpose of preventing liberation or separation of generated vapors from liquid while said materials are at a cracking temperature.

12. Process for the treatment of mineral hydrocarbons by heat comprising raising the material to a cracking temperature while flowing in a substantially liquid stream, theninterrupting the progressive heating for an interval during which vapor generation occurs before raising the material to a maximum temperature during maintenance of conditions of turbulent low in a tubular heater preventing the separation from Asuspension of volatile vapors during thev heating, then cooling the material below 700 Fahrenheit while maintaining said conditions of turbulent llow, preventing the separation from suspension of vapors during the cooling and permitting the material to vaporize at said reduced temperature.

13. Process for the treatment of liquid mineral hydrocarbons, comprising causing the liquid to ilow under pressure in a relatively y small stream of uniform cross section in a tubular heater, preheatingthe stream in theearly part of its course to a temperature of maximum liquid uidity, raising the temperature of the oil stream I a cracking temperature, then decreasing the temperature While still maintaining it within a cracking range, and then increasing the temperature to a maximum degree, the attained temperature at the maximum degree being at a reactive degree for cracking.

15. A process for the cracking of mineral oils by heat comprising as steps causing the owof oil under pressure through a series of heaters at a cracking temperature, and keeping the temperature of oil in a plurality of the heatersV higher than the temperature of the oil in an intervening heater of the series, the temperature of the stream during the time it is in said in` tervening heater being at least a cracking temperature and the temperature in the last heater being the maximum temperature of all.

16.y In a" process for cracking hydrocarbon oil, the method of heating the oilr comprising flowing the oil through an elongated heating coil,

heating the oil duringits initial ow and while in the form of a substantially liquid stream to a cracking temperature short of' that at which coke formation will occur and then decreasing the application of heat for a period of flow adequate to permit generation of .such vapor as will form under the pressure employed at theattained oil temperature and thereafter again increasing the application of heat to the modied oil stream to a desired maximum cracking temperature above that to which the liquid oil stream is subjected.

17. In a process for cracking hydrocarbon oil, the method of heating the oil comprising flowing the oil through an elongated heating coil, heating the oil during its initial ow and while in the form of a substantially liquid stream to a cracking temperature short of that at which coke formation will occur and then decreasing the application of heat for a period of ow adequate to permit generation of such vapor as will form under the pressure employed at the attained oil temperature and thereafter again increasing the application of heat to the modied oil stream to a desired maximumv cracking temperature above that to which the liquid oil stream is subjected, an then cooling the jadvancing stream prior to liberation or separation of generated vapors to a temperature below 700 F.

WALTER JAMES PERELIS.

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