US1949029A - Method of cracking hydrocarbon oil - Google Patents

Description

Feb. 27, 1934. H. A, SMITH METHOD OF' CRACKING HYDROCARBON OIL Filed June 27 BY 'l Wim/f hTTTORNEY QL SS m Q operation to produce gasoline.

Patented Feb. 27, 1934 PATENT OFFICE 1,949,029 METHOD F CRACNG HYDROCARBON Harris A. Smith, Port Arthur, Tex., assigner to The Texas Company, New York, N. Y., a corporation of Delaware Application June 27, 1930. Serial No. 464,130

7 Claims.

This invention relates to an improved method of cracking relatively heavy hydrocarbon oil and relates more particularly to an improved method of cracking hydrocarbon `oil in which -a fresh charge oil is cracked in an initial pressure cracking operation to produce gasoline, a residue from the cracking operation is autogenously distilled at reduced pressure and a resultant vaporous product is cracked in a vapor phase cracking Additionally the condensatel resulting from the autogenous distillation may be returned to the initial cracking operation for further cracking or may be vaporized and delivered to the vapor phase cracking operation for cracking in that phase and a pitchy residue developed in the autogenous distillation step, which is not capable of being further cracked, is Withdrawn from the system.

` The invention in its broadest aspect contemplates cracking or decomposing relatively heavy hydrocarbon oil in two or more appropriate steps so as to `producea maximum percent of valuable gasoline and a minimum percent of less valuable residue from the charge oil under the most economical operating conditions.`

In the average pressure cracking operation Where a coil and drum type of apparatus is employed, oil to be cracked is charged to the heater coil Where it is heated to a cracking temperature under suitable pressure and is then delivered into one or more stills wherein a body of the heated oil may be maintained from which a continuously forming vapor fraction is conducted to al fractionator where a final gasoline fraction is developed, While a liquid fraction which is developed in the fractionator may be returned to the heater coil for further treatment. As reaction of the oil and vaporization of the products of the reaction goes on in the stills the heavier xremaining residue or tar settles out of the liquid body to the bottom of the stills and is drawn off and conducted to' autogenous distillation means, or as it is more commonly called, tar stripping means. Means of this type may usually com- 1 prise a surge chamber through which all of the tar from the cracking operation is passed, a ash distillation chamber, one or more dephlegmators and condensing means.

Ordinarily, when autogenously distilling or stripping this tar a heavy residue or pitchy substance, one o-r more fractions of gas oil and a comparatively small 'naphtha or gasoline fraction is developed. This gas oil, which-is more refractory than the original charge oil, may be thereafter delivered to suitable cracking Vmeans alone or blended off gradually in a fresh charge oil and returned to the same cracking process while the gasoline fraction from the tar stripping operation is mixed with other gasoline stocks and may be ultimately treated to develop a commercial gasoline and the pitchy tar is discharged from the system.

It is to be understood that gas oil developed in the tar stripping step is of necessity cooled down somewhat and condensed and may even be stored in tankage before it is ultimately returned to further cracking means. Consequently a substantial loss of heat is suffered thereby, besides additional storage facilities and pumping facilities are required in addition to the possibility that valuable time may be lost While this material lies idle in storage. As previously mentioned, however, this is a relatively refractory material and it does not respond well to the same cracking conditions on subsequent passes there-through and it is believed that upon repassing several times a gas oil fraction will develop which will not be at all responsive to the initial cracking conditions. However, the gas oil product from the tar stripper has capabilities of yielding a high grade anti-knock motor fuel when it is treated or cracked under appropriate conditions.

Therefore, one of the objects of the present invention is to conserve and utilize the bulk of the heat imparted to the oil in the heater coil of an initial cracking operation by conducting an evolved refractory gas oil fraction separated from the cracking still tar in a tar stripper to suitable further cracking means While it is hot.

Another object of the present invention is to remove the relatively refractory gas oil fraction from the tar stripper while it is in vapor form and appropriately crack it so as lto develop a maximum percent of gasoline therefrom having high anti-knock properties.

A still further object of the invention is to convert substantially all of the convertible constituents of the original charged oil into a maximum percent of motor fuel and a minimum percent of pitchy residue of no further value as cracking stock in a combined process of several stages.

These and other objects and advantages of the invention will be better understood from the following description and the accompanying drawing which illustrates diagrammatically a preferred form of combination apparatus for practicing the process of the invention.

Referring now to the drawing, fresh charge oil is delivered from a source not shown through a pipe 1, under Aseveral atmospheres of pressure, into `7a. The stills 10 and 11 have communicating pipes 12 and 13 situated so as to maintain a substantial body of oil in each still, although it is pointed out that the situation of the communieating pipes 12 and 13 may be altered so as to maintain any lesser amount of liquid desirable in the stills.

Drawoi pipes 14 and 15 having valves 14a and 15al are provided for withdrawing precipitating tar from the stills 10 and 11 while an overhead vapor pipe 16 is provided for conducting vapor from the upper portion of the still 10 into a fractionator 20.

K The fractionator 20 may be of any suitable type and preferably has in connection with it a re-` flux condenser 21 into which vapors from the fractionator pass through a pipe 22 and from` which a reflux condensate is returned to the fractionator through a pipe 23. A drawoff pipe 24 having a valve 24a is provided for withdrawing a liquid fraction which collects in the lower portion of the'fractionator while a'drain pipe 25 having a valve 25a is provided for draining the fractionator. A vapor pipe 26 conducts a final vapor from the upper portion of the reflux condenser to a condenser 27 from which a resulting having a valve 46 into a surge or cushioning chamber 47 that cushions any shocks or pressure irregularities which might be caused by intermittent withdrawal of tar from the stills 10 and 11.

A float indicator 50 is situated in the chamber 47 to aid in keeping the liquid body in the chamber 47 drawn down toa desirable minimum. The pressure in the surge chamber 47 is preferably only a few pounds below that existing in stills 16 and 1l, that is to say, just enough less to insure prompt discharge of tar from the stills and its passage into the surge chamber. Tar collecting in the chamber 47, the level of which is indicated by float indicator 50, is usually withdrawn in a continuous stream through a pipe 51 having a valve 51a and a relatively small by-passv 52 having a valve 52a, which latter is used for finely regulating the rate of withdrawal of liquid from the chamber 47. An emergency relief pipe 53 having a relief valve 53a is provided for relieving any undue or unexpected pressure which may occur in the surge chamber 47 while a drain pipe 54 having a valve 54a is provided for draining all of the liquid from the chamber 47.

The hot tar is conducted from the chamber 47 through the pipe 51 into a flash or distillation chamber 55 which is preferably operated under a substantially constant pressure reduced greatly from that existing in the surge chamber. An inlet pipe having a valve 60a is provided in the lower portion of the chamber for admitting steam which is to aid in the vaporization of the lighter more liquid portion of the hot tar therein.

Substantially all of the lighter constituents of the tar are distilled or flashed into vapor in this chamber and there remains only a heavy pitchy residue or tar which gathers in the lower portion of the flash chamber and is regulatably drawn off through a pipe 62 having a valve 62a which is regulated by a float regulated mechanism 63 that is responsive to the level of liquid in the chamber 55. A pipe 6l is provided for conducting away the heavy residue as it is withdrawn and a valve 61a is provided for draining the chamber.

Vapor developed in the flash chamber is conducted through a pipe 56 into a dephlegmator 65 where it is subjected to dephlegmation by which a smaller portion of it is precipitated and gathered in the lower portion of the dephlegmator. Steam may be admitted through an inlet 66 having a valve 66a to the lower portion of the dephlegmator to aid in the dephlegmation of the vapor and to aid in revaporizing any lighter fraction which might have become entrained by the precipitating particles of heavier liquid. The liquid gathered in the lower portion of the dephlegmatoris regulatably withdrawn by meansl of a float regulated valve 67a in the by-pass 67 which is regulated by the float regulating mechanism 68. A pipe 69 is provided for conducting away the condensate withdrawn and a valve 69a is provided for draining the dephlegmator when that desirable.

In the operation of tar stripping means of this general type heretofore, without the present improvements, one or more additional dephlegmators have been supplied together with condensing means and other incidental apparatus and communicating pipes as indicated by the dotted lines on the drawing. It is to be understood, however, that these are shown on the accompanying drawing merely for purposes of comparison and that their inclusion in the present setup is entirely unnecessary to the success of my invention.

Vapor emerging from the dephlegmator is drawn through a pipe having valves 75a and 75h in indirect heat interchange with a hotter vapor emerging from a vapor phase converter 86, which is situated in a furnace 82, by a suitable compressor 77 and is forced into the converter 80 wherein conversion of the vapor takes place at relatively high temperature but preferably at relatively low pressure.

The highly heated converted products from the converter 80 passing in heat interchange with vapor` from the dephlegmator 65, as previously mentioned, pass through a pipe 84 into a separator 85 from which a vapor portion is conducted overhead through a pipe 86 .into a dephlegmator 88 while a condensate, which is of the nature of a tar similar to or somewhat heavier than the tar developed in the stills of the initial or higher pressure cracking unit, is Withdrawn from the lower portion of the separator 85 through a pipe 87 having a valve 87a by a suitable pump 87b and is preferably returned to the surge chamber 47 through the pipe 45.

The vapor entering dephlegmator 88 is subjected to dephlegmation therein and the dephlegmated vapor is passed into a reflux condenser 90 through a pipe 91 fre-m which a reflux condensate is returned to the dephlegmator through a pipe 92. A condensatewhich is developed in the dephlegmator is gathered in the 'lower portion thereof and is eventually regulatably withdrawn through a pipe 89 by a regulating valve 89a, which is regulated by the fioat regulating mechanism 89h, and is conducted through the pipe 89 to commingle with the condensate withdrawn from the dephlegmator 65. A check valve 69h is provided in the pipe 69 to prevent the condensate from the dephlegmator 88 entering the dephlegmator 65. A valve 89e and a branch pipe having the valve 89d is provided for draining the condensate from dephlegmator 88 from the system.

Anal vapor is withdrawn overhead from the reflux condenser 90 through a pipe 93 to a condenser 94 from which the resulting condensate is delivered into a vessel 95 which is provided with a gas relief pipe 96 having a valve 96a while the liquid is withdrawn through a pipe 9'7 having a valve 97a..

It is to be understood that the vapor phase cracking portion of the apparatus may preferably be of such size as to operate solely on the amount of vapor furnished from the dephlegmator 65 of the tar stripping means, or it may Z5,

pipe 4S having a valve 48a is so connected to the 35l pipe 40 and the discharge of a pump 49 that the condensate from the dephlegmator 65 might be delivered into the pipe 40 through which the condensate from the fractionator 20 may be rey turned to the heater coil 2 for further cracking in the pressure cracking zone, it is usually preferable to revaporize the bulk of this condensate While it is still relatively hot and deliver it to the converter 80. Consequently a vaporizer 7S and a communicating separator 79 having a drain pipe '79a and a vapor outlet pipe 8l is provided for revaporizing the condensate, separating a developing tarry residue from the vapor and conducting the vapor to the converter 8() wherein it may be commingled with the vapor from the dephlegmator 65 and be converted and further treated as previously described.

When practicing the present invention in one of the preferred manners a fresh or virgin charge oil may be delivered into the heater coil 1 Where it is raised to a cracking temperature under say 300 pounds pressure per square inch. The temperature of the oil leaving the preheater may be of the order of 850 F. and the temperature of the oil body maintained in stills l() and 1l only slightly less than that leaving the heater coil. Such difference as exists will normally be caused by heat losses due to radiation in the transfer pipes and in the stills. .The pressure existing in the stills is preferably substantially the same as that at the outlet of the coil heater. The vapor leaving the top of the still 11 through the pipe 16 is passed into the fractionator 20, as previously described, and in that portion of the system including the reflux condenser 2l and the condenser 27 the pressure is maintained at approximately 300 pounds per square inch, or slightly less, due to normal pressure drop, and the temperatures are graded down so as to deliver a final .vapor from the top of the reflux condenser 2l which has an end boiling point of approximately 420 F.

The hot tar which is withdrawn from the bottoms of stills 10 and l1 through the pipes 14 and l5 is usually about 750 F. when it enters the chamber 47. The pressure maintained in the chamber 4'7 is about 250 pounds per square inch.

The bot tar is released from the surge chamber into the flash chamber 55 in a regulated stream and is distributed therein so as to promote the flash distillation of its lighter constituents. The distillation in the tar stripping zone is autogenous, that is, it is accomplished solely by the sensible heat of the oil as the pressure on the oil is reduced as it leaves the surge chamber and from that point on the pressure in the tar stripper is maintained at about 25 pounds per square inch. Steam is admitted to the lower portion of the flash chamber to aid in the flash distillation of the more liquid portion of the tar. In this phase of the operation most of the liquid constituents of the tar are flashed into vapor and the remaining pitchy tar is preferably discharged from the system.

As has been previously described in connection with the accompanying drawing, the vapor developed in this chamber is preferably conducted to a dephlegmator 65 wherein the vapor undergoes such dephlegrnation as will precipitate the heaviest portion of the vapor which may ordinarily contain solid cokey particles in entrainment. Steam is also admitted to the lower portion of the dephlegmator, not only to aid in the dephlegmation of the vapor but also to aid in revaporizing such portion of lighter constituent as may have become entrained and precipitated with the heavier liquid portion. The remaining vapor portion is withdrawn by the compressor '77 and is delivered into the converter 89 where cracking is carried on in the vapor phase. The pressure existing in the dephlegmator 65 is about the same as that existing in the flash chamber 55, that is, about-25 pounds per square inch. Accordingly, as good cracking conditions in the vapor phase are obtained where the vapors are converted under about 109 pounds pressure at a temperature or about 1000 F. the compressor 77 is required to boost the pressure to whatever is desirable in the converter. As the heavier constituents of this vapor fraction withdrawn from the upper portion of the dephlegmator would be condensed under such an elevated pressure, it is necessary to add enough heat to the vapor in transit from the dephlegmator to the compressor, in the heat interchange step, to prevent any condensation of the vapor which might tend to occur before its delivery into the converter.

Although a dephlegmator and its function has been described in conjunction with the tar stripping means, it is to be understood that in some modes of operation a connection may be had between the vapor pipe 56 and the vapor pipe 75 through which all of the vapor emerging from the flash chamber 55 may be passed directly through the heat interchange step and forced into the converter thus ley-passing the dephlegmater 65. Usually, however, the dephlegmator performs a desirable function and will be employed.

The condensate which is withdrawn from the lower porton or" the dephlegmator 55 is of a less refractory nature than the vapor portion which is delivered to the vapor phase converter, but it is more refractory than the original charge stock. Thus, in most cases, it is preferable to revaporize this comparatively small liquid fraction and, after separating from it the entrained solid matter or residue which fails to vaporize in the revaporing step, deliver the vapor to the vapor phase converter and-crack it therein, together with the previously mentioned Vapor portion withdrawn from the dephlegmator 65 through the vapor pipe 75.

The hot products of conversion emerging from the converter 80 are passed in heat interchange kwith the Vapor from the dephlegmator 65 and are delivered into the separator wherein the developing tarry matter is precipitated and the remaining vapor is passed to a dephlegmator, where it is dephlegmated and a vapor is conducted therefrom to form a iinal distillate having high anti-knock properties and preferably of about the same end point as that developed in the initial cracking operation. This distillate may be commingled with the distillate formed in the pressure cracking unit and the mixture may be further treated to form a high-grade commercial motor fuel.

The tarry substance which is precipitated in the separator 85 is normallycommingled with the hot tar from the stills l0 and l1 in the pressure cracking and is deliveredA into the surge chamber 47, as it will contain some liquid constituents which may not have been fully acted upon in the vapor phase cracking step and which are consequently capable or" yielding motor fuel when again subjected to cracking in the vapor phase.

The condensate developed in the dephlegmator 88 isV withdrawn therefrom as previously described and is preferably returned to the vaporizer 'i8 for recycling through the vapor phase cracking step. Normally, the amount of this condensate is relatively small and eventually it may become so refractory that itis preferable to draw portions of it from the system through the valve 89d.

It is obvious that the process is capable of being practiced in a number of ways which might depart materially from the foregoing description and the accompanying drawing and yet not depart from the spirit and the scope of the invention, and while the description and illustration of the invention is directed to a preferred embodiment of apparatus and mode of operation it is desired that the invention be limited only by the appended claims.

What I claim is:

1. The method of cracking hydrocarbon oil which comprises cracking and vaporizing the oil at superatniospherio pressure to produce a gasoline fraction and a liquid residue, separately distilling the residue at reduced pressure in a flash distillation Zone, subjecting the vapor developed in the dash distillation zone to dephlegmation to produce a reflux condensate and a vapor` iraction, revaporizing the reiiux condensate from the dephlegmation step, separating an unvaporized tarry portion from the revaporized condensate, conducting the revaporized condensate free from tar to a converter in a vapor phase cracking zone, passing the uncondensed vapor fraction from the dephlegrnaticn step into said converter, ccmrningling the two said vapors therein and subjecting them to Vvapor phase cracking.

- 2. The method of cracking hydrocarbon oil which comprises subjecting it to cracking conditions in a pressure cracking zone, withdrawing a vapor fraction and a liquid residue, conducting the residue to a zone oi reduced pressure to flash distill the lighter components thereof, dephlegmating the resulting vapor, returning a reflux condensate from the dephlegmating step to the pressure cracking zone, continuously withdrawing the `dephlegmated vapor and cracking it in the vapor phase, separating out and returning to the zone of reduced pressure a tarr'y portion of the cracked vapor, subjecting the cracked vapor free from tar to dephlegmation, withdrawing a reflux condensate from the last mentioned dephlegmation operation and commingling it with the first mentioned condensate and returning the mixture to the pressure cracking zone while condensing the remaining vapor to form a final gasoline fraction.

3. The method of cracking hydrocarbon oil which comprises subjecting the oil to pressure cracking conditons, withdrawing a vapor fraction and a liquid residue, conducting the residue to a Zone of reduced pressure to flash distill the lighter components thereof, dephlegmating the resulting vapor, subjecting the dephlegmated vapor to heat interchange with the hot products from a vapor phase cracking operation, subjecting a reflux condensate from the dephlegmating step to revaporization to separate a vapor fraction from an unvaporized residue, commingling the last mentioned vapor fraction with the dephlegrnated vapor from the heat exchange step and subjecting the mixture of vapors to cracking in said vapor phase cracking operation.

Il. The method of cracking hydrocarbon oil which comprises subjecting it to pressure cracking conditons, withdrawing a vapor fraction and a liquid residue, conducting the residue to a Zone of reduced pressure to rlash distill the lighter components thereof, dephlegmating the resulting vapor, subjecting a reflux condensate from the dephlegmating step to revaporization to separate a vapor fraction from an unvaporized residue, commingling the last mentioned vapor fraction with the vapor from the dephlegmating step, subjecting the mixture of vapor to cracking in the vapor phase, separating out and returning to the Zone of reduced pressure a tarry portion of the cracked vapor, subjecting the cracked vapor free from tar to dephlegmation, withdrawing a reflux condensate from the last mentioned dephlegmation step and commingling it with the nrst mentioned reflux condensate undergoing revaporization while condensing the remaining vapor to form a gasoline fraction.

5. The method of cracking hydrocarbon oil which comprises 'subjecting the oil to pressure cracking conditions, withdrawing a vapor frac-` tion and a liquid residue, conducting the residue to a zone of reduced pressure to flash distill the lighter components thereof, dephlegmating the resulting vapor to remove a condensate therefrom, passing the dephlegmated vapor in heat interchange with the hot products from a vapor phase cracking operation to heat the dephlegmated vapor, subjecting the condensate from the dephlegmating step to revaporization to separate a vapor fraction from an unvaporized residue, commingling the last mentioned vapor fraction with the dephlegmated vapor' from the heat exchange step, subjecting the commingled vapors to said vapor phase cracking operation, separating out and returning to the zone of reduced pressure a tarry portion of the cracked vapor from the vapor phase cracking operation,` subjecting said cracked vapor free from tar to` dephlegmation, withdrawing a condensate fromthe last mentioned dephlegmating step, commingling it with the rst mentioned condensate to undergo revaporization While condensing the remaining Vapor to form a gasoline fraction.

6. The process of treating hydrocarbon oil which comprises heating said oil under superatmospheric pressure, reducing the pressure thereon to produce vaporization of a portion thereof, dephlegmating the resulting vapors to separate a condensate therefrom, cracking the dephlegmated vapors in the vapor phase, dephlegmating the Vapor phase cracked products to separate therefrom a condensate, combining said

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