US2016304A - Conversion of hydrocarbon oil - Google Patents

Description

at {8, 1935'. R, F. mow 2,016,304

CONVERSION OF HYDROCARBON OIL Filed May 19, 1931 m n e FRESH RICH/78D E TBOW INVE TOR /-//6 ATTORNEY Patented Oct. 8, 1935 UNITED STTES PATENT OFFICE Richard F. Trow, Port Arthur, Tex., assignor to The Texas Company, New York, N. Y., a corporation of Delaware Application May 19, 1931, Serial No. 538,590

1 Claim.

This invention relates to the conversion of higher boiling hydrocarbon oils into lower ones and relates particularly to a method of distilling a clean cracking stock from a relatively dirty charge oil and cracking the clean stock together with a refiux condensate which is formed in the process, a gas oil fraction stripped from a cracked residue which is formed in the process and a residue from the distillation step, in which the maximum practicable ultimate yield of a desirable light product having a high anti-knock value with a minimum yield of gas and carbon is obtained from the charge oil.

The invention broadly contemplates a method of cracking petroleum oils to an ultimate yield of motor fuel and residue where a relatively dirty higher boiling oil such as unclean gas oil or topped crude charge oil is separated by distilla-' tion and dephlegmation into a clean and a less clean cracking stock and is subjected to appropriate cracking conditions and a recycle oil resulting from the cracking operation is resubjected to cracking while an ultimate tarry residue is discharged from the system.

The invention is especially adapted toa conversion process in which substantially no liquid is carried in the reaction vessels of a coil and drum type of apparatus. This process, now becoming popularly known as low liquid level cracking, is one in which both liquid phase and vapor phase cracking is effected simultaneously in the same zone. It is capable of being practiced at a higher temperature than is ordinarily employed in liquid phase cracking, and accordingly yields a product having a much higher antiknock value. It is also capable of being practiced at somewhat lower temperature than, true vapor phase cracking.

It has been found that the class of charge stock which is suitable for cracking in the liquid phase tends to precipitate and deposit coke in the heater coil at the higher temperature em ployed in low liquid level operation. Therefore one of the main objects of the present invention is to develop within the system a stock cleaner than that charged to the ordinary liquid phase cracking process from a relatively dirty charge stock. In other words the invention makes it possible tocharge any petroleum oil containing an appreciable amount of crackable or gas oil constituents to a cracking system and evolve therefrom a clean desirable cracking stock, and crack the stock under such conditions as to produce a maximum yield of superior quality motor fuel and an ultimate residue.

It is not desirable to discharge an appreciable quantity of constituents of the fresh charge oil from the system before it has been subjected to appropriate cracking conditions even though it is quite unclean. Thus a further object of my 5 invention is to subject the residual fraction of the charge oil to suitable cracking conditions in the system so as to "effect such conversion into lighter products as is practicable.

In cracking oil in the conventional coil and drum type of apparatus the continuously developing residue, which is withdrawn from the drums, contains constituents which are capable of being further converted under favorable conditions. T -ese constituents may be autogenously distilled from the residue by the sensible heat of the mixture under reduced pressure.

According to a preferred manner of regulating the conditions in the practice of the process the fraction thus developed may be satisfactorily treated, or rather retreated, in the operation under conditions also favorable to the cracking of the clean somewhat refractory cracking stock developed from the charge oil. Thus it is found advantageous to auto-genously distill the residue developing in the cracking drums to thereby strip out a gas oil fraction and recycle it to the cracking system for further treatment. In other words it is advantageous to separate out and recycle gas oil portions of the products produced in the cracking step to thereby obtain an ultimate yield of only motor fuel and a heavy residue of pitchy consistency from the original charge oil.

These and other objects and advantages of my invention willbe better understood by referring to the following description, taken in connection with the accompanying drawing the single figure of which illustrates in diagrammatical elevation a preferred form of apparatus for practicing the invention.

Element l is a furnace, 2 is a reaction vessel, 3 is a combination reaction vessel and scrubber, 4 is a dephlegmator, 5 is a reflux condenser, 6 is a final condenser, 1 is a trap and 8 is an accumulator drum. Element H] is a combination autogenous distillation and dephlegmating tower, II is a. condenser for the final vapors developed therein, I2 is a trap and I3 is an accumulator.

A line l5 may conduct the fresh charge oil to preheater coils I6 and I1, situated in the reflux condenser 5 and dephlegmator 4, respectively, or by-pass either or both of them by manipulation of the valves 20, 2!, 22, and 23, and lead to the upper portion and/or lower portion of the vessel 55 3. A vapor line 30 connects the upper portion of the vessel 3 with the lower portion of the dephlegmator 4. A pump 32 takes suction thru a line 3| from the bottom of the dephlegmator 4' and discharges into a coil 35 situated in the furnace thru a line 33. A branch line34 connected to the discharge side of the pump serves to furnish a reflux or scrubbing oil to the uppermost portion of the vessel 3.

The oil emerging from the coil 35 is passed thru another coil 36 in the furnace I and emerges therefrom to be conducted to the upper portion of the reaction vessel 2 thru a line 40. A line 4| serves to connect the lower portions of the vessels 2 and 3.

A pump 43 takes suction on the bottom of the vessel 3 thru a line 42 and discharges into the transfer line 40 thru a line 44 while a drain line 45 leads from the bottom of the vessel 2 to the combination still and dephlegmator I0. A vapor line 50 connects the upper portion of the dephlegmator 4 with the reflux condenser 5 while a condensate run-back line 5| is also provided interconnecting the two vessels. A final vapor line 52 conveys vapor from the condenser 5 thru the final condenser 6 to the trap l from which a portion of the liquid may be returned to the upper portion of the dephlegmator 4 thru the lines 53 and 54 by the pump 55. The remainder of the ultimate distillate from the trap 1 is passed thru a line 56 to the accumulator 8, having a liquid outlet 51 and gas outlet 58.

The residue delivered into the combination still and dephlegmator ID from the vessel 2 undergoes autogenous distillation at reduced pressure in the lower portion of the vessel It) and an evolved vapor is conducted to the upper portion thereof thru a line while a remaining residue is released thru a line ll. A pump 15 takes suction on the lower portion of the dephlegmating section of the vessel l0 thru a line 16 and discharges thru a line H into the charge line 5 which leads into the vessel 3. A vapor line 8|) connects the upper portion of the dephlegmating section of ID with a condenser H. A portion of the condensate dropping out into the trap l2 may be returned to the upper portion of the dephlegmating section of |0- thru lines EM and 82 and a line 821) having a valve 82a by a pump 83. The remainder of the condensate is conducted thru a line 84 to the accumulator 3, having a liquid outlet 85 and a gas outlet 85.

When beginning to operate a sufiicient amount of clean oil may be drawn in thru a pipe 29 from a source not shown and pumped into the heater 35. After operating conditions are established a switch may be made to the fresh charge oil which is delivered thru the line I5.

In a preferred mode of operation a fresh charge oil which may be a relatively dirty gas oil or a topped crude, is charged thru the preheaters l6 and I1 and emerging therefrom at a temperature of approximately 700 F. is split between the upper portion and the lower portion of the vessel 3. That portion of the fresh charge oil delivered to the bottom of the vessel is introduced below the surface of the small body of liquid carried therein which is ordinarily only enough to insure a liquid seal so that no vapors are withdrawn from this vessel. 3 a

That portion of the charge which is discharged to the upper portion of the vessel 3 is delivered at about a midpoint of a series of baffle plates which serves to distribute the liquid in a filmlike manner to eifect contact with the hot vapors moving upward towards the outlet pipe 30. Va.- pors leaving the vessel 3 at a temperature of about 810 F. are conducted thru the line 30 into the dephlegmator 4 where dephlegmation is effected to produce overhead vapor and a reflux 5 condensate. The reflux condensate is cooled somewhat in the lower portion of the dephlegmator 4 by the indirect contact of the charge oil which is passed thru the preheater section I! and is withdrawn therefrom at a temperature of 10 7 about 750 F. and. pumped thruthe heater.coils by the pump 43 and are discharged into the transfer line 40 near the outlet from the heater coil.

The thus combined highly heated clean cracking stock and residue from the vessel 3 having an 20 average of about 880 F. is delivered into the up- 7 per portion of the vessel 2. A large portion of the products remaining liquid at the entry to vessel 2 are immediately flashed into vapor. Some atomization of the liquid occurs also due 25 to the vaporization and rapid separation of portions of the oil from the main stream being delivered into the vessel 2 under a preferred pressure of about 400 pounds per square inch which pressure is preferably maintained throughout 30 the remainder of the cracking portion of the system. I

' The vapors and some entrained .or suspended finely divided liquid particles are passed thru the vessel 2 and into 3 by the connecting line 4| 85 While liquid portions being deposited in the bottom of the vessel 2 are preferably drawn off as soon as they are deposited. In some cases it may be desirable to maintain such an amount of liquid in the vessel 2 as to overflow small quantities 40 of it thru the line 4| into the vessel 3.

The hot vapors entering the vessel 3 move upwardly encountering that portion of the combined fresh charge and recycle oil which is dropping down from the bafiies' above. Cooling and con- 5 densation of the vapors is thus effected to a desired extent and the hot vapors in turn serve to 'impart enough heat to the combined fresh charge and recycle oil to vaporize a portion of it. After encountering the charge oil, the up 50 wardly moving vapors encounter the clean scrubbing oil which is delivered to the uppermost portion of the bafiies thru the line 34, as previously described, and are still further cooled and cleansed and finally emerge substantially free of 55 carbonaceous or dirty material thru the line 30. In order to reduce the amount of scrubbing oil refluxed to the top of 5 thru the line 34 it may be found desirable to cool it after it leaves the pump 32 and before it is discharged into the vessel. 0 That portion of the oil which fails to vaporize and thatwhich remains condensed in the vessel 3 is deposited in the bottom thereof in contact with that portion of the combined fresh oil and recycle oil being delivered thereto which serves 5 to further cool the liquid portion being deposited. The amount of cooling supplied here should be at least sufiicient to lower the temperature of the combined oils below Where advanced cracking or cracking to pitch or coke of the least refractory 70 constituents occur. It is in fact preferable to maintain only a temperature at which cracking of the heavier less refractory constituents begins and substantially all of the lighter more refractory constituents of the charge oil and the refluxed oil will have become vaporous at the pressure employed. The resultant vapors from the charge and reflux material are added to those moving upwardly to the top of the vessel while the remaining liquid is withdrawn thru the line 42, as previously described.

The warm vapors conveyed thru the line 30 to the dephlegmator 4 are subjected to any conventional method of fractionation in the first stages of which heat is interchanged between them and the fresh charge which is being passed thru the preheater coil H. The dephlegmated vapors are passed thru the reflux condenser 5, where heavier portions are condensed, and a portion of their heat is imparted to the fresh charge which is being passed thru the coil 56. The final vapor is conducted thru the line 52 to the final condenser E and to storage while a portion condensed in the condenser 5 is returned to the upper portion of the dephlegmator t thru the line 5|, as is also a portioncondensed in the condenser 6 and trapped out in the trap I, as a reflux cooling medium.

The residue resulting from the reaction and being deposited in the vessel 2 is released thru the line 45 into the lower portion or still section of the vessel H] where autogenous distillation is effected at a reduced pressure. The resulting vapors are conducted to the upper or dephlegmating section of the vessel thru the line it while the heavy residual portion is released from the system thru the line H.

The vaporous portion conducted to the dephlegmating section of the vessel is subjected to any conventional method of fractionation therein to produce a relatively small percent of final overhead vapor and a reflux condensate. The vaporous product is comparable to that emerging from the previously described reflux condenser 5 and is condensed in the condenser it and passed thru the accumulator to storage or a portion of it may be returned to the upper portion of the dephlegmating section as a reflux cooling medium. The reflux condensate collects in the bottom of the dephlegmating section of i9 and, as it is capable of being further converted under the cracking conditions established in the cracking zone, is withdrawn by the pump it and delivered into the line i5 to there combine with the fresh charge and be delivered into the vessel 3 as previously described.

In the mode of operation which is described herein the approximate temperatures given are well above the economical and practical operating limits of a liquid phase cracking operation, that is, one where sizable bodies of liquid are maintained in reaction vessels such as 2 and 3. By maintaining temperatures of these higher orders it is possible to produce an ultimate distillate which has a relatively high anti-knock value and because of the shorter times of reaction in the reaction vessels of a combined vapor and finely divided suspended liquid particles, as compared with only a body of liquid, no secondary or advanced cracking in which pitchy or cokey bodies and noncondensable gases are ordinarily formed, has time to occur. On the other hand as the rate of cracking is greater at these higher temperatures a yield of lighter products based on the fresh charge will be obtained which is as good economically speaking as that obtained from average liquid phase conversion.

It is pointed out, however, that in order to maintain these desirable high temperatures it is necessary to provide a cracking stock, that is, the

oil passed thru the heater coils 35 and 36 which is much cleaner than the usual charge oil for a coil heater in a coil and drum type apparatus where cracking in the liquid phase. This is because relatively unclean cracking stock contains constituents which are not capable of being subjected to the higher order of temperature maintained in my coil without readily precipitating cokey particles which deposit in the coils and greatly shorten the time of a single continuous 10 operation.

The formation of these bodies is believed to be due largely to more advanced cracking of the less refractory heavier constituents of the dirtier oils and it may be also due to some substances which may be present in the oil tending to promote the formation of coke due to the accelerated rate of reaction that occurs in the coil at the higher temperature.

In my invention the higher temperature to which the clean cracking stock is heated in the co l immediately quenched or lowered upon its oxiu rem the coil by the addition of a somewhat cooler oil. This quenching medium consists of the previously referred to less refractory or dirty constituents of the fresh charge oil, the heavier portion of recycle oil from the residue stripper residue precipitating from the cracked .products entering thru the pipe ii i. This temperature is only lowered, however, to one at which a desirable further reaction of all of the oil being treated will continue but at a somewhat slower rate. The combined oil thus undergoing treatment is delivered into the reaction vessels for further time of digest as has been described.

The quenching oil, added to the hotter material which is being decomposed at a faster rate, serves not only to lower the temperature and thus slow up the rate of decomposition but also acts as a solvent, it is believed, for incipient pitchy or cokey particles and gas which may be present in the highly heated oil from the coil and which would doubtless evidence itself positively in the reaction vessels if this quenching and solvent oil were not added at this stage before secondary reaction begins.

It is pointed out that among the conditions established in this operation is that of subjecting the vapors to a longer time of digest in the vessels than the liquid is subjected to. The time of digest of the vapors runs from their entrance thru pipe 40 to their emergence thru pipe 30 while the liquid is withdrawn from the bottom of each vessel as soon as it is deposited.

As an alternative mode of operation to the one previously described herein, the charge oil may be delivered directly into the upper portion of the vessel through; pipe 98 having a valve 9|, cutting out coil l1 and perhaps l6 and delivering all of the liquid from the upper section of ID, as before, thru the line i! into the vessel 3, while all of the condensate from the condenser ll may be refluxed through a line S2 having a valve 93 as a coolant to some point in the dephlegrnator 4 to replace the cooling furnished, as shown and described, with the charge oil passing thru coil 11.

A suitable clean cracking stock for passing heating coil 35 may be a gas oil having a straw or light straw color or an oil of 15-20 on the cell, Lovibond scale; however, the use in this stage of other relatively clean cracking stocks is not precluded.

While a preferred mode of operation has been described herein in detail it is to be understood that neither this form nor the approximate tempassing a condensate oil produced in the process through a heating coil in which the oil is subjected to a high cracking temperature, delivering the heated products of cracking into an enlarged cracking zone maintained at cracking temperature under superatmospheric pressure wherein separation of vapors from liquid takes place, directing the separated vapors to a combined cracking and dephlegmating zone, passing vapors at a temperature within the cracking range from said combined cracking and dephlegmating zone to a high pressure fractionating zone wherein fractionation occurs to separate out a condensate, passing said condensate to aforesaid heating coil forcracking, withdrawing liquid from said enlarged cracking zone at a rate adequate to prevent the accumulation of liquid therein, expanding the liquid thus withdrawn into a low pressure auto-distillation zone wherein separation of vapors from liquid takes place, passing the separated vapors into a low pressure fractionating zone and subjecting them to fractionation therein, bringing charging stock intoindirect heat 5 exchange with vapors in said high pressure fractionating zone, introducing charging stock thus heated into said low pressure fractionating zone to dephlegmate the vapors therein, withdrawing Y a liquid fraction from said low pressure fraction- 10 ating zone comprising reflux condensate and unvaporized constituents of the charging stock and introducing the admixed oils into said combined cracking and fractionating zone wherein the oil is subjected to fractionation and cracking, with- 15 drawing a residual fraction from said combined cracking and fractionating zone and passing it to said enlarged cracking zone wherein the time of subjectionof liquid components to cracking 1 conditions is limited by the rapid withdrawal of 0- liquid from the chamber while vaporous constituents are maintained at cracking temperature therein. l

RICHARD F. TROW.

Images