US1692786A - Process of cracking mineral oil - Google Patents

Description

Nov. 20, 1928.

A. E. PEW, JR., ET AL PROCESS OF CRACKING MINERALOIL Filed Jan. 25, 1926 m 5 m W n Wm m, M, O "M zwfl mfl r 4 w/zwiss:

Patented Nov. 20, 1928.

UNITED STATES PATENT OFFICE.

ARTHUR E. FEW, JR, OF BRYN MAWR, AND HENRY THOMAS, F RIDLEY PARK, PENN- SYLVANIA, ASSIGNORS TO SUN OIL COIPANY, OF PHILADELPHIA, PENNSYL- .VANIA, A CORPORATION OF NEW JERSEY.

PROCESS 0F GRAGKING MINERAL GIL.

Application filed January 25, 1926. Serial No. 38,437.

In the art of treating high boiling and-high gravity hydrocarbon oils for the purpose of decomposing them to convert them into lower boiling and lower gravity hydrocarbon oils,

which art is generally known as cracking, the most common procedure is to subject the oil to be cracked to high temperature and a substantial pressure above atmospheric. The

art is a very old one. In recent years, its most is useful industrial application is in the production of gasoline from such higher boiling and higher gravity petroleum oils as gas oil and fuel oils. The cracking operation may be conducted in batch or it may be carried on in a continuous manner. The cracking may he effected in vapor phase or in liquid phase or partly in one phase and partly in another. With a sulliciently high temperature (the minimum of which is about 600 F.) and under a ressure of several atmospheres, a consideraiile percentage of, for example, gas oil, 1 may be decom osed or cracked into gasoline. At 800 and higher temperatures, under the moderate pressures specified, the conversion will take place largely in vapor phase, or in both phases. If the pressure be greatly increased, a larger proportion of the oil will be cracked in liquid phase. At a pressure of (say) 600 pounds to the square inch or (say) atmospheres, the oil will be cracked almost wholly in liquid phase.

In continuous processes for cracking higher boiling petroleum constituents to gasoline, it is old and well known to convey the 011 to be decomposed, which may be initiallly preheated, through a long tube or tubes in immediate contact with heating gases and therein to raise the oil to a cracking temperature and either complete the cracking therein or perform a part of the cracking therein. In the latter case, the partly cracked oil may be conveyed to a container or reaction chamber of relatively large cross-section, wherein the pressure is maintained, and wherein the temperature of the oil falls somewhat, although not below an effective cracking'temperature. in this chamber the cracking is completed and the oil will be relatively quiescent to allow deposition of the coke which is formed under the high temperature conditions. Such processes have been carried on in. vapor phase, in mixed phase and in liquid phase. n the liquid phase process, the oil may be allowed to escape through a loaded valve into a vaporizer, whereinthere is maintained a pressure but little above atmospheric. After, passing the yalve, the most of the oil immediately vaporizes. The vapors are fractionated and condensed. In the vapor phase process, the cracked, or partially cracked, vapor is taken off from the container and goes either direct to a condenser, or through a fractionating tower in which a product of the desired boilmg point is separated out.

In such processes the formation of carbon and resinous matters gives rise to serious difi'iculties. It is desirable, for various reasons, especially in liquid phase processes, to effect all the cracking in the tubes, but the formation and accumulation of coke in the tubes tends to insulate the oil from the heating gases and clog the tubes, necessitating their frequent cleaning. The process is therefore uneconomical it not commercially impracticable. If only part of the oil is cracked in the tubes and if the partly cracked oil is conveyed from the tubes to a large reaction chamher, in which the cracking is completed, as explained above, the trouble is merely ameliorated. in that some oi the coke that would otherwise form or deposit in the tubes terms or deposits in the large reaction chamber. The tubes nevertheless become lined or choked with carbon, and while they need to be cleaned less frequently, they must be cleaned at intervals, and frequent cleaning of the reaction chamber is necessary, and the process is more economical and practicable only in degree.

The ideal liquid phase process is one wherein the oil is subjected to the cracking operation and is completely cracked while traveling along a long thin stream, from which it continuously escapes into an expansion chamber wherein most of it immediately vaporizes, the gasoline being separated, fractionated and condensed and p-urified by known methods. The object of this invention is to provide a practicable process of this character.

Before describing the process it may be informative to set forth more in detail the cause, nature and effects of coke formation. Certain factors in the problem are well understood, while other factors seem not to be generally appreciated and maynot have been heretofore known. a

With the usual method of heating the oil to raise it to a cracking temperature, whether it is flowing in a long tube or comparatively quiescent in a tank, the temperature diflerence between the furnace gases and the oil is and must be very great. Consequently the film of oil immediately in contact with the metal of the tube is raised to a temperature much above that of the average temperature of the whole body of oil. This causes the highly heated part of the oil to be raised not only to a decomposition or cracking temperature but to a temperature high enough to 'separate out certain constituents and convert them into coke, which is deposited on the tube wall. Substantial deposition of coke does' not occur until after the process has n in operation for some little time. Once, however, the coke starts to form, it forms very rapidly. The coke acts as'an insulator, requiring more heat to penetrate it in order to heat the entire body of oil to a cracking temperature. This raises the temperature of the tubes. The coke that has already formed acts to absorb oil, which, at the increased temperature of the tubes, causes it to form coke rapidly. In short, once the formation of the coke starts, its rate of production is constantly accelerated.

Careful experimental work in connection with the practical development of the present invention, wherein the oil was heated to a cracking temperature under conditions which prevented local heating much above the upper limit of the desirable cracking temperature, resulted in the formation of little or no coke. That is, coke occurs within temperature ranges that are materially above the upper limit of the desirable cracking temperature zone. In the ordinary cracking processes, the oil flowing through the tubes is necessarily subjected to an excessively high temperature because the heat transfer from gases of combustion to the oil is very low and, in a commercial unit, it is essential that the difl'erence in temperature between the heating gases and the oil be very great; otherwise, the large amount of heating surface necessary would involve too high a cost. Further, the insulating effect of the accumulating layer of carbon in the tubes necessitates a still greater temperature difference between the furnace gases and the tubes and the subjection of the peripheral zone of the stream of oil to still higher temperatures in order that the main body of the traveling oil shall be subjected to the lower cracking temperature desire'd.

However, while,in said experimental and development work, the oil was cracked without substantial formation of coke, asphaltene was formed in substantial amounts. Asphaltene is often mistaken for coke and, like coke, it has insulating qualities and, its formation, when started, proceeds at an accelerating rate. As haltcne, however, unlike coke, is soluble in ifi'crent liquids, such as carbon bisulphide, or in petrolene, which latter substance itself may be separated out of asphalt by dissolving the asphaltene in gasoline, precipitating the asphaltene and evaporating the gasoline from the petrolene. Coke that is formed at higher temperatures cannot be dissolved and must be removed from the tubes by mechanical means.

It is clear that if the temperature difference between the heating medium and the oil could be economically reduced, the conditions fpromotive of the formation of coke woul be eliminated. It is also clear that even if the temperature of the heatin medium could not be held below a point at w ich asphaltene will form, and that if the asphaltene remains behind and accumulates in the tubes, the latter could be cleaned expeditiously and economically by simply running through the tubes, at convenient intervals, a solvent of the asphaltene.

We have found it to be practicable to secure the desired low tem erature difference between the heating medium and the oil by means of the indirect a plication of the pnmary heating agent an the direct transmissions of heat to the oil by means of a secondary heating agent which, under practicable absolute pressures, will boil at a temperature to which it is desirable to raise the oil, and which, in its vapor form, is flowed into heat exchange relation with the oil and is condensed thereby and gives up its latent heat to the oil, the condensate returning to a liquid body of the substance, which is con tinuously being heated by the rimary heating agent and is continuous y generating vapor. In the application of Pew & Thomas, No. 13,040, filed March 5, 1925, which is a continuation in part of an application filed March 17, 1924, No. 699,615, the advantage of using a vaporized metal, such as mercury, as a direct heating agent for mineral oil, is clearlv and fully disclosed. Mercury has a high boiling point and high heat conductivity; it will not oxidize or disintegrate when heated or brought into contact with iron; and

it may be condensed and its latent heat transferred to the oil by heat exchange at temperatures within the desirable zone of oil cracking ten'lperatures. The most important and valuable quality of mercury vapor in its application to the cracking of oil, is that, to accomplish the same degree and rapidity of cracking, the difierence between the temperature of the mercury vapor and that to which it is necessary to raise the oil need be very small relatively to the necessary temweaves perature (lifierencc between the oil and ordinary furnace gases or other ordinary heating media. In other words, the temperature of the heating medium from which heat is directly transferred through the tube wall to the oil. may be so relatively low that all the objectionable coke forming conditions hereinbefore recited are minimized or avoided. No hard, insoluble, coky or resinous residue is formed because of the absence of the elevated temperature required to form it; and the conditions promotive of the building up of an insulating i ll of coke between the direct heating medium and the oil are absent.

.However, even'at the much lower temperature of the heating medium, asphaltene starts to form after the process has been in operation for some time; but at any time after it starts to form, the oil cracking process may be interrupted for a few minutes, and the asphaltene may be readily removed by flowin a stream solvent liquid through the tu es. @ther positive advantages are hereinafter enumerated, as they may be more readily appreciated after the process is fully described.

In heating oil by means of furnace gases applied direct to a stream of oil flowing through a tube, a great temperature difference between the furnace gases and the oil is not objectionable while the oil is being heated up to a cracking temperature. Below about 600 :51, heat can be applied to the oil at a comparatively high rate per square foot of heating surface without injurious local cracking. it is therefore found practicable to utilize the waste heat of the primary heating medium by subjecting the relatively cold oil to the heat of the furnace gases after they pass beyond the mercury boiler, thereby quickly preheating the oil to a high temperature, which, however, preferably does not reach, and ought not to exceed, the temperature at which substantial cracking begins.

It is desirable, particularly after the oil has been raised to a cracking temperature, to

raise the temperature gradually through the cracking zone. While the higher the temperature, the more rapid the cracking, and while it is permissible to impart a higher temperature to the oil with mercury vapor as a heating medium than with the usual furnace gases, due to the small temperature difference necessary between the oil and the heating medium, it is objectionable, for reasons hereinbefore fully stated, to crack at an unnecessarily high temperature. While it is usually not necessary to exceed a temperature of about 875 at the exit end of the oil stream, by our method this temperature can readily be raised in cases where the oils being handled demanded this increased temperature. A mercury vapor temperature 25? to 50 above that of the maximum temperature of the oil, is capable of raising the preheated part of the oil stream to the specified maximum temperature and it is practical to subject the entire stream of oil to the temperature of the mercury va )or immediately after the oil leaves the pre eater. it is advantageous, however, to maintain a small temperature difference between the oil entering the stream and the mercury vapor, so as to avoid a sudden rise of temperature of the peripheral part of the stream, or the film adjacent the inner wall of the tube. To maintain thissmall tem erature difierence and to insure a gradual iearting up through the cracking range of the entire body of the oil and to provide a minimum practicable temperature diderence between the mercury and the oil at all points in the latters flow, it is arranged to heat the oil in stages, the term 7 peratnre of the mercury being raised in the successive stages.

ther advantageous features of the orocess will lee-brought out in the following description.

While the process is not dependent for its execution on any particular construction and arrangement of apparatus, the drawings represent the lay-out of a plant in which the process is adapted to be carried out in a particularly practicable and advantageous way.

Fig. 1 is an elevation, largely diagramatic, of a complete cracking plant.

Fig. 2 is a detail view of one of the heat exchange or cracking units.

lhe mercury boiler a is contained in a furnace a which is provided with a down-take c. The furnace gases pass upward around the boiler, thence laterally and thence downward through the down-take c and thence upward through a stack 0).

Above the boiler is arranged a series of cracking units, each comprising a shell e, functioning as an oil heating and nercury vapor-condensing chamber, and a nest of tubes f. The tubes are so arranged that the oil entering from pipe g must flow back and forth repeatedly before leaving at h. It is desirable that the oil should be given suflicient velocity to aid in cleaning the tube walls and the length of travel must be sufficient to allow proper time for the reaction to take place.

From the mercury boiler a a mercury vapor line i extends upward and thence laterally along the cracking units 1, Q, 3and t, and is provided with four feeds 11, 12, 13 and 14: extending to the respective chambers e of the four cracking units 1, 2, 3 and at. Each feed is i rovided with a valve j.

rom the several chambers e extend mercury condensate return lines 21, 22, 28 and 24 to a header p. Each return line comprises a trap consisting of a cup is and a goose-neck m, and another goose-neck n. The upper part of gloose-ncck m connects with the upper part of t 1e cup by a vapor pipe 0. The part lid 11. connects with a header means of apipe 1'.

In the down-take c is a nest of tubes 8 comprising a preheater. Relatively cold oil is pumped into one of the bottom tubes and thence flows back and forth repeatedly and finally leaves one of the top tubes and flows through pipe 9 to cracking unit 1. The oil. leaving the tubes 7 of unit 1 through pipe it enters unit 2, and thence flows through units 2, 3 and 4 in series.

The mercury vaporized in boiler (1 passes through line 2' and thence, through the independent l'ceds 11, 12, 13 and 14, to the several mercury chambers e of the four units 1, 2, 3 and 4.

In each chamber 6 the mercury vapor is condensed and its latent heat is transferred to the oil. Mercury condensate returns from the chambers 6 through the return lines 21, 22, 23 and 24 to the header 7) and thence, through pipe r to the boiler a.

From the last cracking unit 4, the oil passes through a pipe it into a vaporizer 'v. The inlet to the vaporizer may be controlled by a valve u. Beyond the vaporizer is a bubble tower to and beyond that a condenser w.

A more specific description of the operation follows: It should be understood that the more detailed information, such as pressures and temperatures, are illustrative only. The figures iven are to be taken in a relative, rather t an in an absolute, sense. Various ressures, temperatures and temperature dif erences may be desired by the operator, and

' these may be predetermined with a rather close approach to exactitude; I

It should be particularly borne in mind that -'the specific plant above described is adapted particularly to cracking mainly in a liquid phase, as the pressure in the oil system given below indicates.

Oil is pumped through the preheater 8 and thence in series through the tubes f of the cracking units and is maintained, throu hout, at a high pressure, say 600 poumfs per square inch.

The mercury boiler may be operated at a pressure of about 80-85 pounds gauge pres sure, which will give a mercury vapor temperature of about 900 F. The mercury vapor flows through the several cracking units in parallel.

The temperatures of the mercury chambers e bf the successive units should progressively increase.- Thevalve in line 14 controlling the inflow of mercury vapor to unit 4 is left wide open, which will'maintain a tempera-- ture in chamber e of that unit of 900 F. A lower temperature ma be established in any chamber b more or ess throttling the ad mission va ve. I Thus the valve in line 13 is closed to the degree that will give a temperature in the chamber e of unit 3 of 850 F. Chamber e of unit 2 may be held at a temperature of 800- F. by still further throttling the valve in line 12. By still more nearly closing the valve in line 11, chamber of ulnit 1 may be held at a temperature of The oil entering the preheater a may have a temperature of 200 F., and during its flow therethrough is heated to any desired higher temperature, say 600 F. It is desirable that the oil should be heated in the preheater to a temperature below that at which substantial cracking occurs. Indeed it is preferred that there shall be no cracking at all in the preheater. The oil then enters the tubes of unit 1 at a temperature of 600 F. By eat exchange with the mercury vapor in the surrounding chamber e, (which as above stated, is at a temperature of aboutl50 F.) the vapor is condensed and its latent heat is trans ferred to the oil, which leaves unit 1 at a temperature of 670 F. There is thus an average temperature diiference between the heating medium and the heated medium of about 115 F.

The oil enters unit 2 at a temperature of ture of 740 F. There isthus an average temperature difference in this unit between the mercury and the oil of.95 F.

The oil enters unit 3 at a temperature of 740 F. and leaves at a temperature of 810 F., the average temperature difference in this unit between the mercury and oil being 75 F.

The oil enters unit 4 at a temperature of 810 F. and leaves at a temperature of 875 F. The temperature difierence between'the mercury and oil in this unit is 57 F.

The oil leaving pipeh at valve u enters the vaporizer 2:. Here the pressure on the oil is instantly released and nearly all of it immediately vaporizes. A small proportion (say five or ten percent) is thrown down in the vaporizer. The vapors enter the bubble tower en wherein they are fractionated by passing up through descending condensed oil. The uncondensed gasoline vapors leave the .bubble tower and are subsequently condensed in condenser 01. The gasoline vapors may be purified on the way to the condenser or the final condensate may be purified.

The small temperature difierences between I located at such a height abovethc boiler that the column of mercury in the return line may have a head equal to the pressure carried in the boiler.

Any foreign matter in the mercury collects in the traps km, and is thus prevented from being carried back into the boiler. From time to time the traps may be opened and foreign matter removed from the surfaces of the mercury in the cups k. I

A by-pass for excess mercury vapor is provided between the mercury vapor line 11 and the liquid mercury return line 1'. In this by-pass is located a safety valve, or maximum pressure relief valve y and a condenser a.

A longitudinal section of one of the cracking units is shown in Figure 2. The shell e must be made of sufiicient strength to carry the required pressure of mercury vapor. The tubes f must be made of a size and strength suitable for carrying oil atthe high pressure specified. For the liquid phase system, the tubes should have walls about half an inch thick. The tubes may be welded to the heads of the shell 0. By reason of the temperature diiference between the tubes and the shell, provision should be made for expansion and contraction. One way of providing for expansion and contraction is to make the shell in two sections with an expansion joint 0' at the center. These details, however, may be worked out by engineers skilled in the art of heat exchanging.

The process is also adapted to be modified to effect the cracking of the oil in vapor phase or, more conveniently, in a two phase (liquid and vapor phase) process. Such a process is set forth and specifically claimed in another application filed by us February 24, 1926, Serial No. 90,247.

In the crackin system described, due to the prevention 0 localized heating of the oil substantially above the highest predetermined cracking temperature, no part of the oil reaches the temperature at which coking takes place. There is ultimately some formation of asphaltcne, which, at moderately high temperatures, is formed before the production of coke at still higher temperatures, as here inbefore explained. At any time after asphaltene starts to form and deposit on the tubes, a solvent may be pumped through the oil system. which dissolves the asphaltene and thus cleans out the tubes. This method of cleaning out the tubes, wholly unworkable to clean out coke deposits and therefore inapplicable to other. systems of cracking, is obviously strikingly simple, expedient and economical.

The main advantages of the process above described may be enumerated as follows:

Littleor no heat is wasted in coke formation and no heat is wasted in penetrating any heat insulating wall of coke. The latent heat released by the mercury in its condensation is therefore practically all utilized in doing useful work. The cracking operation is therefore conducted with a. high degree of economy and efficiency.

Inasmuch as the tubes are never subjected to high heat, they never burn and there is no depreciation of equipment.

To the extent that clo ging of the tubes is avoided, the system can be operated continu- Ously for a relatively long time, which is another factor that makes for economy.

The rate of heat transfer to the oil may be accurately controlled.

No part of the oil is heated above the maximum cracking temperature desired.-

The oil may beheated through a cracking range, say of 300 F., while the temperature difference between the heating medium and the oil at any stage of the heating, may be very much less than this range.

The primary heating medium (e. g. furnace gases) is utilized to first heat the secondary heating medium (the mercury) that directly furnishes the heat required for cracking, and

to next preheat the oil to about the lower limit of the temperature range for cracking, thereby providing for the most economical utilization of the heat of the primary heating medium.

While mercury is preferred as the secondary heating agent, it is possible to substitute other vaporizable metals, such as cadmium and zinc.

We do not herein claim that part of the described process which involves, inaddition to the maintenance of a tube temperature below the zone of coke-forming temperatures, the discovery that the solid matter that so deposits in the tube is a soluble solid whose removal may be effected by other than mechanical means. Such invention forms the subject matter of a separate application filed February 10, 1926, Serial No. 87 ,24 t, by J. Howard Pew.

What we claim is:

1. The process of cracking mineral oil which con'iprises heating a body of liquid mercury under an absolute pressure required to pro duce mercury vapor at a temperature above that at which substantial cracking of the oil' will occur, but below that required to form IOU any substantial amount of coke, establishing a flowing stream of oil of great length and small thickness at a temperature below that of the mercury vapor and flowing the mercury vapor into a confined space surrounding the flowing elongated stream of oil and into heat exchange relation with the oil to effect condensation of mercury vapor and such transfer of its latent heat to the oil as will decompose higher boiling constituents thereof into lower boiling products, returning the condensed mercury to the body of liquid mercury, and continuously removing the oil from the locus of heat exchange.

of the mercury vapor and flowing the mercury vapor into a confined space surrounding the flowing elon ated stream of oil and into heat exchange re ation to effect condensation of mercury vapor and such transfer of its latent heat to the oil as will decom se higher boiling constituents thereof into ower boiling products, imposing such su eratmospheric pressure on the oil as will maintain it during its decomposition mainly in liquid phase, returning the condensed mercury to the body of liquid mercury, and continuously removing the oil from the locus of heat exchan e.

3. %he process of cracking mineral oil which com rises subjecting the oil to the rocess set orth in claim 2 followed by subecting the partially decomposed oil to another treatment which is the same as that set forth in claim 2 but in which the mercury condensation temperature is higher than that in the first treatment.

4. The process \of cracking mineral oil which comprises flowing a stream of oil successively through a series of cracking zones, flowing mercury vapor from a common source independently into a space in each zone surrounding the oil stream and into heat exchange relation with the oil therein and effecting in each space condensation of mercury vapor and transfer of its latent heat to the oil to the degree required to effect substantial cracking of the oil, maintaining progressively higher mercury condensation temperatures in the successive cracking zones, returning the condensed mercury from each cracking zone to the body of liquid mercury, and continuously removing the oil from the last cracking zone.

5. The process of cracking mineral oil which comprises flowing a stream of oil successively through a series of cracking zones,

heating a body of liquid mercury under the superatmospheric pressure required to vaorize the mercury at a temperature materialy above that required to crack the oil but not more than about 100 F. in excess of the highest temperature to which it is desired to heat the oil, flowing such vapor into the several crackin zones around the oil stream and in heat exc lan e relation with the oil and efi'ecting in eac space condensation of mercury va or and such transfer of its latent heat to t e oil as will decompose higher-boiling constitutents into lower boiling products,

mercur under an absolute pressurerequired to pro uce mercury va or at a tem rature above that at which su stantial cracking of the oil will occur, establishing a..continuously flowing stream of oil at a temperature below I that of the mercury vaporand flowing said stream and the mercury va or into heat exchange relation-to effect con ensation of mercury vapor and such transfer of its latent heat to the oil as will decomposehigherboilin constituents therof into lower boiling pro uctsf maintaining at the locus of heat exchange a predetermined absolute mercury vapor pressure less than that at the locus of mercury vapor eneration by restricting the rate of flow of the mercury vapor to the locus of heat exchange, returning the condensed mercury to the body of liquid mercu and continuousl removing the oil from t e locus of heat exc ange.

7. The process of cracking mineral oil which comprises heating a body ,of liquid mercur under an absolute pressure required to pr uce mercur va or at a temperature above that at whic substantial cracking of the'oil will occur, establishing a continuously flowing streamof oil at a temperature below that of the mercury vapor and flowing said stream and the mercur vapor into heat exchange relation to e ect condensation of mercury vapor and'such transfer of its latent heat to the oil as will decompose higher boiling constituents thereof into lower boiling products, roviding an independent path for the flow ol mercury from the locus of generation of mercury vapor backto the body of liquid mercury, controlling the maximum pressure of the mercur vapor in the flow line by opposing a pre etermined-resistance to flow of mercury throu b said path, continuously removing the oi? from the locus of heat exchange and independently controlling the flow of mercury vapor from the flow line to the locus of heat exchange. e

In testimony of which invention, we have hereunto set our hands, at Philadelphia, Pennsylvania, on this 23rd day of January,

ARTHUR E. PEW, JR. HENRY, THOMAS. 1 v a

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