US1786315A - Process of cracking mineral oil - Google Patents

Description

Dec. 23, 1930.

A. E. PEW, JR.. ET AL PROCESS OF CRACKING MINERAL OL Filed Feb. 24, 1926 Patented' Dea 23, 1930v UNITED slrA'rE PATENT4 oFFlcE .an'rrrun n raw, JB., or' BRYN mwa., ann HENRY rnoms, or BIDLEY PABX, rumi- A CGRPORATIONOF NEW' f"` EY L ELPHIA, PENNSYLVANIA,

PROCESS F GBACKUIG MINERAL OIL V.Application lecl February 24, 1926.. Serial No. 80,247. i

in the are of treating high hihhg aha high gravity hydrocarbon oils for the purpose of decomposing them to convert them into lower boiling and lower gravity hydrocarbon oils,-

' 5, which art is generally known as cracking,

m useful industrial application is in the production of gasolnefrom such higher boiling and higher gravity petroleumfoils 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 be eHected in vapor phase or in liquid phase or partly in one phase and partly in another. With a suciently high temperature (the minimum of which is about 600 F.) and under a ressure of several atmospheres, 'a considera le percentage of, for example, gas oil, may be decomposed 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) 40 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 oil to be decomposed, which may be initially preheated, through a long tube' or tubes' in immediate contact with heating gases and thereinto 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 chamberv of relatively large cross-section, 'wherein the pressure is maintained, but

wherein the temperature, although lower, is maintained in the cracking zone. 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 procgenerally esses have been carried on in vapor phase, in paixed phase and in liquid phase'. In the liquid phase process, the oil may be allowed to escape through a loaded valve into a va, porizer, wherein there is maintained apressure but little above atmospheric. After passing the valve, the most of the oil immediately vaporizes. The vapors are fractionated and condensed. In the vapor phase process, the cracked, or partially cracked, va.- por 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 boiling point is separated out.

In such processes, the formation of carbon and rcsinous matters gives rise to serious dif- 'Ihe formatlon and' accumulation 'rates the trouble, in that much of the coke that vwould otherwise'form or deposit in the tubes forms or deposits in the reaction chamber orV drum.U The tubes need to be cleaned less frequently, lbut they must be cleaned at intervals and frequent cleaning of the reaction chamber or drum is necessary.

The object of the invention is to provide a practicable process of cracking in vapor and liquid phase in which little or no coke will be formed.

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 appreciated and may not have been heretofore known.

With the usual method of heating the oil to raise it to a cracking temperature, whet-her it is flowing in a long tube or comparatively quiescent in a tank, the temperaraised not only to a decomposition or crackingtemperature 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 been 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 format-ion of the coke starts, its rate of production is continually 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 dierence 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 inthe 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 temperat-ure desired.

However, while, in said experimental and development work, the oil was cracked without substantial formation of coke, asphaltene was formed in substantial amounts. Asphalteneis often mistaken for coke and, like coke, it has insulating qualities and, its formation, when started, proceeds at an accelerating rate. Asphaltene, however, unlike coke, is soluble in different liquids, such as carbon bisulphide, or in petrolene, which latter substance itself may be separated out 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 promotive of the formation of coke would be eliminated. It is also clear that even if the temperature of the heating medium could not be held below a point at which asphaltene will form, and that if the asphalt/ene 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 temperature difference between the heating medium and the oil by means of the indirect application ofthe primary heating agent and the direct transmissions of heat to the oil b means of a secondary heating agent w ich, under practicable absolute pressures, will boil at a temperature to which it is desirable to raise the oil,4 and which, in its vapor form, is flowed into a heat exchange relation with the oil and is condensed thereby and gives up its latent heat to the oil, the condensate returning to a liquidbody of the substance, which is continuously being heated by the primary heating agent and is continuouslyeneratng vapor. In the application o 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 clearly 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 hoil by heat exchan e9e at temperatures within the desirable zone of .oilcracking temperatures. 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 difference 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 temperature difference between the oil and ordinary furnac'e gases or other ordinary heating media. In other words, the4 temperature o 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 form after the process has been in operation for some time; but at an time after .it starts to form, the oil crac ing process may be interruptedfor a few minutes, and the asphaltene may be readily removed by dowin a stream of solvent liquid through the tu es. Other positive advantages are hereinafter enumerated, as they may be more readil appreciated after the process is fully descri d.

ln heating oil by means of furnace gases applied direct to a stream of oil flowing through a tube, a great temperature difterence between the furnace gases and the oil. is not objectionable while the oil is being heated up to a cracking temperature. Below about 600 F., heat can be applied to the oil at a comparatively high rate per square foot of heating surface without injurious local cracking. lt 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.

ln a patent issued to us November 20, 1928, No. 1,692,786, there is set forth a process and apparatus that are particularly adapted to the cracking of oil inits liquid phase. The object of the present invention is to set forth a process that will avoid the objections to prior cracking processes and that is particularly adapted to crack the oil in two phases (liquid and vapor).

While the process is not dependent for its execution on any particular construction and arrangement of apparatus, the layout shown in the accompanying drawing, which is an elevation, in diagram, of a complete cracking plant, `is well adapted to carry out the process in a particularly practicable and advantageous way.

The mercury boiler av is contained in a furnace b which is provided with a down-take c. The furnace gases pass upward around the boiler, thence laterally and down downtake c and thence upward through a chimney d.

Above thel boiler is arranged a cracking unit comprising a shell e functioning as an oil heating and mercury vapor-condensing chamber, and a nest of oil carrying tubes ,extending within the shell.

From the mercury boiler a a mercury vapor lineg extends upward to the-higher end of chamber 3. From the lower end of the charnber is a mercury condensatev return line comprising a trap consisting of a cup h and a goose-neck i, another goose-neck la and a pipe-line m leading back to the, boiler a. The upper part of gloose-neck z' connects with the upper part of t e trap by a vapor pipe j.

drum n adapted to be partly filled with oil, has anv inletpipe o from the upper end header of the oil tubes f andan outlet pipe p leading to a settling chamber r. From chamber r a pipe s extends to the lower end header of the tubes f. In the pipe line s is a pump t. Pipe u aiords an outlet for heavy residue from the settling chamber r.

Vapor from drum n is taken out through pipe lv to a fractionating tower w. The lighter uncondensed products from tower w go to a condenser .The heavier products may go to a condenser y or, by pipe z, may be returned to the drum.

Drum n and the cracking unit e. f are heavily insulatedto reduce the heat losses from exposed surfaces.

At the start of the process drum n may be partly illed with oil from any source of supply. lt desired, the oil may be preheated by circulating it upward, from a supply pipe 8, through a nest of tubes 9 in the down-take c, and thence, through a pipe 10, to drum n, preferably by way of tower wand pipe a.

In a by-pass line 5, connecting the mercury vapor line g with the return mercurycondensa-te line m, isa safety valve or pressure regulating-valve 6 and a condenser 7.

ln the following description of the operation of lthe process, it should be borne in mind that cracking takes place in both. a liquid phase and a vapor phase. and that the amount of cracking that will voccur inl Atemperature of 600 F.). Through line 10 the preheated oil .Hows to tower w and is thence conveyed to the drum. Oil is circulated from drum n through line p, settling chambery', line a, tubes f and line 0 back to the drum. i

, The mercury boiler may be operated at a pressure of about 80-95 pounds gauge pressure, which will give 'a mercurv vapor temperature of about 900 F. The mercury vapor passes through the chamber e of the cracking unit. By heat exchange with the oil in the tubes f, the vapor is condensed and its latent heat is transferred to the oil. The mercury that is condensed in chamber e is returned by gravity to the boiler. In order.

lboiler that (assuming that all the mercury skilled engineer.

- perature in the After the oil enters the cyclic oil heater n f, it is gradually raisedv in temperature.

After the operation has continued long enough to establish normal working conditions, the temperature of the oil in the drum n will be from 7 50o-800 F., while the temperature of the oil leaving the pipes f will be substantially hi her, say 875 F. The temrum, however, is a cracking temperature, but is necessaril lower than the temperature of the oil leaving the tubes: first, because the oil in the tubes receives directly the heat of. the heating medium; second, because of the relatively cold oil that is being fed in through line a; third, because ofthe vaporizationv taking place in the drum; and fourth', because of the cracking reaction which is taking place.

In order to reach the necessary temperature foresubstantial cracking to take place, it is necessary to subject the oil to a ressure of (say) ten atmospheres. The igher the pressure, the greater the pro ortion of the oil that will be cracked in liquid phase. .The higher the temperature, the more rapid the cracking. Undesirably high temperature differences between the mercury and the oil may be avoided by reducing the temperature of the mercury vapor (which is effected by reducing the pressure in the boiler) and effecting the cracking more slowly. However, no part of the oil, even the peripheral film adjacent the tube wall, can be raised above the temperature of the mercury vapor, and if the latter is maintained below the temperature at which material amounts of coke will form, thel tubes cannot become clogged with coke and the oil cannot be more or less insulated, by such coke, from the heating medium, as occurs in ordinary cracking processes, in which the temperature of the heating medium is necessarily v ery much higher than the temperature to which it is desired to heat the oil. p

The heavy unvaporized residual oil will settle in the chamberfr and maybe drawn off, continuously or intermittently, 'through line u.

Vapor from drum n goes to tower w, Where it is fractionated. The lighter products pass in Vapor stage to a condenser either direct or through a purifier (not shown). The heavier condensed products may be returned to drum n -or may be withdrawn from the stem.

The cracking takes place both in tubes f and in the drum n, but, due to the small amount of oil in the tubes in proportion to the total amount of oil in the cyclic system, the larger proportion of oil is cracked in the drum. The main function of the tubesis to transfer heat to the oil in the drum and keep such oil at cracking temperature.

The process may be carried on in batch, but it is preferred to carry it on continuously.

Any foreign matter in the mercuryecollects in the trap h z' and is thus prevented from being carried back into the boiler In the cracking systemdescribed, due to the prevention of localized heating of the oil substantially above the highest predeter mined cracking temperature, no part of the oil reaches the temperature at which coking takes place. There is ultimately some formation of asphaltene, which, at moderately high temperatures, is formed before the production of coke at still higher temperatures, as hereinbefore explained. At any time after asphaltene starts to form and deposit on the tube, 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 ina plicable to other systems of cracking, is oliviously strikingly simple, eX- pedient and economical. 4

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

Little or no heat is wasted in coke formation and no heat is Wasted in peneratingany heat insulating wall of coke. The latent heat released by the mercury on 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 appreciable depreciation of equipment.

To the extent that clogging of the tubes is avoided, the s stem can be operated continuously for a re atively long time, which is another factor that makes for economy.

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

No part of the oil is heated above the maxi mum cracking temperature desired.

In the preferred mode of practicing the process, the primary heatinr medium (i. e., 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.

lltl

While mercury is preferred as the second- 'a i heating agent', it is ossible to substitute oter vaporizable meta ,such as ca and zinc. Y f

We do not herein claim that part oi the described process which involves, in addition to the maintenance, or a tube temperature below the zone of coke-forming temper-vv cracking lzone to decompose higher boiling constituents into lower oiling constituents -i'n the second cracking zone and to continue the decomposition inthe 1| cracg Zone, conveying vapor from the hrst crachng zone to the rractionating zone and therein separating lighter vapors and withdrawing them and condensing heavier vapors, and re- 'turning theicondensate, together with the rst specihed intiowing preheated oil, to the owing circuit ci oil undergoing cracking. In testimony oi which invention, we have hereunto set our hands, at Marcus Hook, Pa, on this 19th day of February, 1926.

v ARTHUR E. PEW, Jn. HENRY THUMAS.

which comprises continuously .llowing the oil from a source of supply into a cracking zone and thence continuously circulating it through a. second cracking zone and back to the first cracking zone, vaporizing a body of liquid mercury and nog the mercury vapor into heating relation with the second cracking zone so as to eiect condensation of mercury vapor and impart, by such condensation, sumcient heat to the oil in the second cracking zone to decoose higher boiling constituents into lower oiling constituentsv in the second'cracking zone and to continue the decomposition in the first cracking zone, continuously removing vapors from the first crack zone and fractionating the vapors outside the specied circuit.

2. The process ot cracking mineral oil which comprises continuously Howing the oil from a. source of supply into a'cracking zone and thence continuously circulatingitthrough a locus of settling, a second cracking zone and back to the first cracmng zone, vaporizing a body of liquid mercury and iiowing the mercury vapor into heating relation with the second cracking zone, so as to effect condensation of mercury vapor and impart, by such condensation, suiiicient heat to the oil in the second cracking zone toA decompose higher boiling constituents into lower boiling constituentsin the second cracking zone and to continue the decomposition in the first cracking zone, continuously removing vapors v from the first cracking zone and fractionating the vapors outside the specified circuit and withdrawing residual oil from. the settling locus.

3. The process of cracking mineral oil which com rises preheating the oil and continuously owing it through a fractionating zone and thence into `a cracking zone and thence continuously circulating the oil through a second cracking zone and back to the first cracking zone, vaporizing a body of liquid mercury and flowing the mercury vapor into heating relation with the second cracking zone so as to effect condensation of mercury vapor and impart, by such condensation, sufhcient heat to the oil in the second

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