US1965945A - Process for heating and cracking oil - Google Patents

Description

July 10, 1934. A. G. PAGE 1,965,945

PROCESS FOR HEATING AND CRACKING OIL Filed Sept. 17, 1927 3 Sheets-Sheet ALEXANDER ERIE/771' H4615 99 L my K ATTORNEY-S.

PROCESS FOR HEATING AND CRACKING OIL Filed Sept. 17, 1927 3-Sheets-Sh'eet '2 0 000000000 OOOOOOOOOO OOOOOOOOOOQOOOOOOOOOO INVENTOR. ALEXANDER GRIFFITH PAGE Fig. 2. BY A? A TTORN E YS.

July 10, 1934. A G PA E I 1,965,945

PROCESS FOR HEATING AND CRACKING OIL Filed Sept. 17, 1927 S SheetS-Sheet 3 IN V EN TOR.

. ALEXANDER ammm PAGE I 'I 4 Fig.5 M-

ATTORNEYS.

Patented July 10, 1934 UNITED STATES PATENT OFFICE PROCESS FOR, HEATING AND CRACKING OIL Alexander Griffith Page, Los Angeles, Calif., as-

,signor to Union Oil Company of California, Los Angeles, Calif., a corporation of California Application September 17, 1927, Serial No. 220,136

1 Claim.

This invention relates to a process for heating a decomposable fluid and particularly petroleum oil. It is more particularly directed to a process and apparatus for heating oil under conditions to induce cracking of the oil.

The process has for one of its objects, a method ofheating oil to its cracking temperature, said heating being controlled to prevent local overheating of the oil beyond a desirable temperature. It is more particularly directed to process for heating the oil to a cracking temperature and regulating the heating of the oil near or at said temperature in such manner as to prevent local overheating and excessive cracking with resulting undesirable carbon deposition and carbon formation.

It has for another of its objects, a process wherein the oil is heated to a cracking temperature and maintained at the chosen temperature whereby the heating of the oil at approximately the cracking temperature is regulated so as to maintain the oil at, at least, said cracking temperature for a prolonged period of time without overheating the oil beyond a safe, desirable cracking temperature.

It is well known that oil heated to a cracking temperature in ordinary furnace construction is in danger of being superheated beyond a critical point to cause excessive carbon and gas formation. This is due to the fact that in ordinary furnace construction, the heating is not capable of such fine control as to prevent this overheating. The fire box temperature and amount of fuel burned is determined entirely by the amount of heating to be accomplished and the temperature to which the material being heated must be raised. In heating oil in tubular stills, the additional criterion of preventing excessive carbonization and decomposition must be observed. In order to avoid these difiiculties, various means have been devised; such as controlling the course of the 'oil in a peculiar manner or by limiting the initial temperature of the furnacegases so .as to prevent superheating of the oil at or about its critical cracking temperature.

I have found that the desirable operation of a tube-still in which part of the coil is used to raise the oil to a cracking temperature is such that the initial part of the heating should occur between cold oil and the hottest gases so that the temperature gradient will be the great est and when the oil has reached the region of its cracking temperature, it should be subjected to a'zone' of heating wherein the oil is not overheated beyond a predetermined level. In furnaces now designed, whereby furnace gases are recirculated to the combustion chamber, the stream of gases is modified to a predetermined temperature before impingement on surfaces over which oil flows to the end that the gas, when it reaches the region where the heated oil cracks, shall be at a temperature to prevent overheating of the oil. However, this reduces the efficiency of the heat transfer to the cold oil zone requiring a larger number of tubes for the preheating zone and reducing the capacity of the furnace for cracking operations.

I have found that, if the tube-still is used to cause cracking of the oil wherein the oil is heated for a period of time while at its cracking temperature, it is desirable to maintain the oil in this zone at uniform temperature and at a temperature below that at which excessive carbonization occurs but yet at sufliciently high level so as to obtain a high reaction velocity i. e. at an optimum reaction temperature and velocity.

If recirculation of furnace gases is employed to prevent overheating of the oil in the heating and cracking zones, the gases passing by the oil at the cracking temperature must be at a sufficiently mild temperature, and the recirculation of the furnace gases must be controlled with this end in View. Oil in cracking absorbs heat and therefore the temperature of the gases in contact with the outgoing oil must be at a sufliciently high temperature to impart heat to the cracking oil at its outgoing temperature. The control of the heating operation in the cracking zone must have this end in view. It will be seen that the recirculation of furnace gases to the combustion chamber controlled to prevent superheating in the cracking zone must be at the expense of efliciency in the preheating zone, where a rapid heating is desirable for maximum efficiency.

It is an object of this invention to control the heating of these two stagesin a furnace independently to cause the preheating of the oil under its optimum conditions and to cause the cracking to occur at its optimum condition.

Oil as it is progressively cracked increases in liability for carbon formation. This is apparproportioned to the rate at which the oil can absorb heat in cracking. If the rate of heating is greater than the rate of absorption of the heat due to cracking, the temperature of the oil will rise beyond a safe cracking temperature.

In cracking furnaces, constituted of banks of tubes, there must be of necessity a temperature gradient across the furnace in the direction of gas flow. Therefore, the initial temperature must be sufficiently high so that the temperature of the gases in contact with the outgoing oil shall be at the correct temperature level.

At the beginning of .the cracking zone, the rate of heat transfer may be much larger than at the end of the cracking period. This is be-- cause the cracking reaction is much more rapid at the beginning of the cracking reaction. The temperature difference between the heating gases and the oil in this first stage may be greater than in the last stage, yet less than in the preheating stage. As the cracking proceeds the temperature difference must be reduced or the rate at which heat is transferred to the oil will be greater than the rate at which it is absorbed in cracking at the desired temperature and the temperature will rise beyond the desired level. However, the control of the temperature for the beginning or end of the heating stage cannot result in temperature conditions or heat transfer which are optimum for the heating stage other than the one under control.

It is, therefore, another object of this invention to control the heating of the oil during its crack ing process in its various stages to maintain the cracking at an optimum rate and at an optimum temperature throughout the course of the heat-- ing and cracking of the oil, so that the cracking rate may be at a maximum with a minimum of carbonization, i. e., without overheating at any stage. It is a further object to control the preheating and cracking of the oil so as to ob-- tain the optimum conditions of heat transfer in the preheating stage and the optimum conditions of heating and cracking in the cracking stage.

I have found that the above conditions can be obtained in a highly satisfactory manner by recirculating the outgoing furnace gases to various parts of the furnace and coils under controlled conditions.

In one of the preferred embodiments, furnace gases are recirculated to the combustion chamber and to the zone above the preheating coils independently. In another of the preferred embodiments, no recirculation is directed to the combustion zone. The preheating occurs at as high a rate as is possible. In operating with some oils such as fuel oils and residuums, the preheating stage must be modified as well, for under such circumstances these easily cracked oils may carbonize in the preheating stage. In operating with oils such as kerosene or gas oil, it has been found unnecessary to modify the temperature of the furnace gases in the preheating zone. The preheating can occur at as high a rate as possible. However, when the oil reaches approximately the cracking temperature, care must be taken to continue the heating to the desirable cracking temperature, or for a prolonged period at the desirable cracking temperature under rigorously controlled conditions. It is desirable at this point to modify furnace gases. If the continued heating is to occur for any length of time requiring a large number of tubes it has been found desirable to break the banks into smaller banks and to control the circulation of furnace gases to the various levels at those banks, independently, in order that the temperature gradient across any bank be limited and the rate of heating of the bank be controlled to that at which the heat is absorbed in cracking at the desired temperature, i. e. Without an undue increase in the temperature of the oil. In this fashion, the coils in which the various stages of cracking occur are each submerged in a practically uniform temperature bath where they are heated at such rate as will induce the maximum rate of cracking with no superheating to the point of excessive carbon and gas formation.

Thus, in the preheating stage, the heating of the cold oil is by high temperature gases. The relatively cold oil is rapidly brought to approximately the desired cracking temperature. At this stage there is little or no danger of excessive cracking or carbon formation. When the oil approaches the cracking range, the temperature must be controlled.

The high temperature of the gases will mean excessive heating both due to the radiation therefrom and due to convection. By introducing recirculated gases to reduce the temperature of the furnace gases, the temperature difference between the oil and the gases is diminished and the velocity of the gases increased. The efficiency of the heating is thus not diminished and this initial cracking zone is not necessarily increased in size. When the oil has been cracked to a desired amount, the gases are again modified in temperature by admission of more recirculated gas to further diminish the temperature differential between the oil and the gases and further increase their volume so that the rate of heat transfer shall be proportioned to the rate at which the oil can absorb heat and the heating conducted in the most efficient manner, the increased velocity volume of the gases here counteracting the decrease in the temperature of the gases to re-establish the heating efiiciency. Thus the amount of heat transferred is held at the efficient rate but the temperature at which it is transferred is such that the oil is not heated to an excessive cracking temperature.

The invention, therefore, provides a process whereby the oil may be preheated at as high a rate as convenient and the cracking occurs under modified and rigorously controlled temperature conditions at a maximum cracking velocity and with a minimum amount of carbon formation.

Further objects will be understood from the following description of the drawings.

Fig. 1 is a vertical section through a cracking furnace;

Fig. 2 is a vertical section through the cracking furnace taken on line BB of Figs. 1 and 3. Fig. 1 is taken on line AA of Fig. 2;

Fig. 3 is a horizontal section of the furnace taken on line CC of Figs. 1 and 2. Fig. 1 is taken on line AA of Fig. 3.

In these drawings 1 is the wall of the furnace. 2 is a fire brick lining thereof. 3 is the roof of the furnace formed by a suspended brick arch as is usual. 4 is the Dutch oven arch. 5 is the floor of the furnace. The space above the floor 5 may be considered to be the combustion space 6 is the front of the Dutch oven. 7 are demountable doors. 8 is the supporting steel frame work for the furnace. 9 is the concrete foundation. 10 are peep holes. 11 is a fuel feed pipe connected to a series of burners 12, situated in the burner arch 13. 14 is a preheated air flue confleeting to a preheater (not shown) situated in flues 24. 15 is a preheated airadmission to burners 12. l6, l7 and 18 are-banks of coils connected in any desired manner by crossovers, as is usual in such cases. These coils are supported by tube sheets 19. 20 are waste gas exit flues of which there are two, one disposed on each side of the furnace. These flues connect via flue 21 to a stack flue 24 connecting to stack 34. Connected into stack flue 24 is a fan 25 driven by a motor 26. Fan 25 is connected to recirculating gas conduit 27 controlled by dampers 32. Connected to conduits 2'? is a plurality of manifolds shown as three in number in the drawings, respectively 28, 29 and 30, there being a corresponding number on both sides of the furnace. Each manifold is controlled by a damper 33. These manifolds are connected into the furnace by a plurality of ducts 23 situated at various heights in the furnace shown here as beneath each bank of tubes. Every alternate duct 23 has a distributing nozzle 22 made of heat resisting metal. The manifolds at each level are cross-connected by a horizontal duct 31 (see Fig. 3). The operation of the furnace will be understood from the above.

Preheated air passing through a preheater (not shown) in flue 24 passes through duct 14 and opening 15 and fuel passes through 11 to be burned by nozzles 12. The combustion gases developed in Dutch oven pass up through the furnace to heat the tubes, finally exiting through 20. passing downward through 21 into flue 24 and out stack 34. The recirculating fan 25 recirculates a portion of these combustion gases into the furnace. The amount of gases circulated is controlled by the operation of the fans and by the dampers 32 and dampers in the connections of the fans to the flue 24 and the various dampers 33 at the various levels. By means of distributing nozzles 22, the gases are introduced to mix thoroughly across the furnace and because of the cross-connection of the flues 24 and manifolds 28, 29 and 30, via the connections 31, a uniform pressure is maintained at the various levels in the furnace.

As specific applications of my invention and not as limiting thereof the following specimen examples are furnished by Way of illustration:

In operating on residuum i. e. heavy fuel oil fractions, it has been found advisable to modify the furnace gases before heating of the oil as well as to modify the gases passing over the cracking section. Thus the furnace gases are modified by recirculation of furnace gases to produce a temperature of approximately 1200 F. The gases pass counter-current to the oil in the preheating zone. The oil is heated to 730 F. It then enters into the cracking section. The gases passing to the cracking section are further modified by introduction of a second stream of flue gases to produce a furnace gas temperature of approximately 784 F. It then passes in heat exchange with the heated oil, cracking it and coincidentally slightly raising its temperature to the exit temperature of 769. The furnace gases issue to the flue at about 784 F. They may pass in heat exchange with incoming air and issue to the stack at approximately 500 F. The gas may be recirculated to the furnace, either at 784 F. or at 500 F. In the example given gas at 758 F. Was recirculated. The difference in temperature is accounted for by heat losses from the recirculating system. In the specific system here described the oil was held at 500 pounds pressure before passing to a reaction chamber, also under pressure. 4

When gas oil is being cracked, relatively cold oil, in this case at about 400 F. from a heat exchanger is introduced into bank 16 to run countercurrent to hot gases generated say at 2000 F. The recirculation manifold 28 is shut off by means of its damper 33. If cracking is to be conducted in the coils, the heated oil at 850 F. is introduced into the bank 17 to run, for in stance, concurrent with the heating gases, recirculation gases are returned via pumps 25 and conduit 2'7 and introduced via 29 by proper regulation of the damper. The amount of recirculated gases introduced at this pointshould be such as to drop the temperature of the gases issuing from bank 16 to a desirable temperature, i. e., say 950 F. This increase in gases circulating over the bank of coils 1'? increases the velocity and volume of gases in this part of the furnace, causing a uniform distribution of the gases over the coils. If the purpose is merely to heat the oil to a cracking temperature and to maintain it but for a short time before passing it to a digestion chamber or to a pressure still, it will be found that this initial recirculation is sufficient. The oil may be passed through coils 18 with no recirculation through manifold 30, coils 17 and 18 functioning here as one bank. However, it is preferable either to remove coils 18 or to design the furnace for this use or to use coils 18 as an economizer coil on cold oil.

However, if a more prolonged heating is desired so as to cause practically all of the desired cracking in the coils, the oil is further heated 110 and cracked. The oil is heated in bank 17 to 860 F. At this stage the oil is passed into bank 18. The gases issuing from bank 17 may be at 930. More gas is recirculated via manifold 30 by proper control of the damper, to produce a temperature of 910 F. at the bottom of bank 18. The temperature of the oil exiting from the furnace may be at 870 while the temperature of the gases issuing from the furnace are at 900. The recirculated gases are split so that the temperature difference and the gas flow across each bank gives the maximum of cracking in each zone without that superheating that results in excessive gas formation and carbonization. It will be observed that the temperature differential diminishes as the cracking proceeds and that there is a practically constant temperature bath of furnace gases in the cracking zone. By regulating the volume of recirculated gases the rate of heating is proportioned to the rate of cracking.

This process thus results in a preheating at the maximum rate and a cracking at the optimum conditions. Each stage of cracking occurs at the desired temperature and at the desired rate. 135 The depth of the banks are so designed so that the temperature gradient is minimized, thus approaching a constant temperature bath of hot gases for each cracking stage with its attendant increase of control for that stage. In the above 14 the oil was maintained at 2000 pounds pressure.

In the above examples the gases were recirculated before passing over the air preheaters. The gases in passing over this are cooled to approximately 500 F. It is obvious that by recirculating 145 these gases, either alone or combined with the recirculation of furnace gases before heat exchange with the air, a further control of the temperature of the furnace gas stream is available. The additional conduits, dampers and pumps, if 150 desired, are easily added to the disclosed embodiment, and anyone skilled in the art will readily understand these modifications The temperatures given in the above examples are of course merely illustrative of the applications of the process to specific oils and to specific operations. Further refinements and improvements as well as the adjustment of the temperatures and volumes of the furnace gases, the amount of recirculation and the temperatureof the recirculating gases, the adjustment of the rate of heating, the temperature differentials and the temperature of the oil will. all vary with different stocks. Anyone skilled in this art will readily understand this and will be able to adjust the conditions by trial to obtain the optimum conditions by employing the above process.

The description is not limiting but merely illus-' trative of the best mode of utilizing my invention, which is,

I claim: a

. A method of cracking oil comprising passing oil in a continuous confined stream through a series of coils disposed in a furnace, burning a fuel in said furnace to generate combustion gases, passing said combustion. gases ever said series of coils, heating the oil in said coils to a cracking temperature above 730 F., continuing the passage of the heated oil through the series of coils to maintain the oil at a cracking temperature, recirculating combustion gases which have passed in heat exchange with said entire series of coils to a point intermediate the ends of the series of coils to heat exchange substantially all of the oil at a cracking temperature with combustion gases diluted by recirculated combustion gases and to heat substantially all of the oil not at a cracking temperature with undiluted combustion gases.

ALEXANDER GRIFFITH PAGE.

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