US1574547A - Operation of oil stills - Google Patents

Description

Feb. 23 1926. """1';574,5'47

J. E. BELL OPERATION OF OIL STILLS Filed August 10, 1923 3 Sheets-Sheet 1 A'ITORNEYs Feb. 23 1926.

J. E. BELL OPERATION OF OIL STILLS Filed August 10, 1923 3 Sheets-Sheet 2 yr INVENTOR MM IBY 2m ATTORNEYS 7 5 m M w e h M a 5 S O a 9 7 1. m 0\ & vL\4 OJ. 3 H S I L5 L2 |A mm 7 LT L gum B .F E mc I W M 2 B H NF 0 Feb. 23 1926.

INVENTOR- ATTORNEYS Patented Feb. 23, 1926.

- UNITED STATES v 1,574,547 PATENT OFFICE.-

JOHN E. BELL, OF BROOKLYN, NEW YORK, ASSIGNOR TO SINCLAIR REFINING COM- PANY, OF CHICAGO, ILLINOIS, A CORPORATION OF MAINE.

OPERATION OF OIL STILLS.

Ap plicatiomfiled August 10,

To all whom it may concern:

Be it known that I, JOHN E. BELL, a citi- N zen of the United States, residing at Brooklyn, in the county of Kings, State of New York, have invented certain new and useful) Improvements in the Operation of Oil Stills; and I do hereby declare thefollowing to be a full, clear, and exact description of the invention, such as will enable others skilled in the art to which it appertains to make and use the same This invention relates to an improved method of operation of oil stills,-and particularly of oil cracking stills, and involves an improved method of heating of the stills whereby danger of overheating is avoided or minimized and a more uniform heating action. is obtained.

The invention relates particularly to improvements in the heating of pressure stills for cracking hydrocarbons of. high boilingpoints such as gas oil and the like, to form more volatile or lighter hydrocarbons such as those which constitute commercial gasoline.

Among the objects of the invention are the protection of the tubes or heating surfaces of the still from overheating or burning, the lengthening of the time of theiruns between cleaning. with a corresponding in crease in the distillate produced, a more uniforn'i distribution of the heat throughout the heating chamber and other advantages whiclr Pressure stills and other stills for cracking oil areheatedby furnaces in which coal, gas, or oil is burned, and, v for economy in fuel consumption, it is desirable that the fuel be burned in the, furnace with the least amount of excess air and the maximum furnace ten'iperature. Excessive temperatures are, however, objectionable in the heating chambers of oil cracking stills. Carbon is formed during the cracking operation and tends to deposit on the side of the tubes or heating surfaces of the still and'such a layer of carbon or of heavy tar is a poor conductor of heat and insulates the metal surface from the oil on the inside, with the result that the temperature of the metal increases over the temperature of the oil, the amount of this increase being proportional to the thickness of the carbon deposit and to the amount of heat delivered to the external surface heated by the furnace gases.

1923. Serial No. 656,672.

The overheating of the still or of the tubes or heating elements of a cracking still andv the resulting increased tendency of carbon formatlon, tend to give hot spots and involve a considerable element of danger or maintained at pressures approximating 100 pounds per square inch and may be maintained considerably nigher, any break or rupturevis followed by a disastrous fire.

In an oil cracking still the tubes or other heating surfaces most exposed to, the heat of the furnace will be the first to suffer. The danger of overheating of such tubes or surfaces can be decreased by reducing the fuel consumption, but this in turn reduces the capacity of the still; While, if the still is driven hard to increase its capacity by increasing the fuel consumption, the danger of overheating is greatly increased. According to the present invention the heating of the still is modified and regulated to avoid or greatly reduce the danger of overheating.

According to the present invention the products 'of combustion from the firebox are cooled by admixture with hot gases at a lower temperature prior to bringing them into contact with the tubes or other heating surfaces of the still, thereby reducing the will appear from the following description.

temperature to which the first tubes or surfaces are subjected and the amount of heat absorbed by them, and increasing the amount of heat absorbed by the tubes or surfaces over which the gases pass before escaping from the heating chamber. This cooling of the products of combustion is nevertheless efi'ected without sacrificing either fuel economy or capacity of the still, and may even result in a considerable incrgase in still capacity.

lVith the ordinary arrangement of furnace, such as commonly used under stills, a large proportion of the heat in the fuel passes directly from the fuel-bed, where solid fuel is used, or from the flame and hotter brick work, when gaseous or oil fuel is used, direct to the heat absorbing surface of the still by radiation. It has been shown by tests that the amount of heat absorbed by the tubes next to the fire, in the case of a steam boiler, may be and usually is 4:0 or

- amount of heat is principally transferred astimes as great as the average absorption of' heat throughout the whole surface, and this radiant energy. It has also been shown that heat transferred in this way increases veryrapidly with the furnace temperature; the process in general being controlled by Stefans law for radiation which makes the total amount of radiation proportional to the fourth power of the absolute temperature of the radiant. When the furnace is arranged so that the heat absorbing surfaces are shielded from.

direct radiation, suchshields and the gas connecting passages become secondary radiators and the total amount of heat transmitted in this way is greatly reduced; -''but the heat that would otherwise be transmitted by radiation is absorbed by the gases with the result that there is an actual *increase in gas temperature of the gases coming in contact with the heating surfaces of the still, although this increase ing'as temperature does not compensate for the reduction in the radiant heat. I

According to the present invention 7 the heat absorbing surface of the cracking still is protected from the direct radiation of the furnace by the interposition of a high bridge wall and by providing a vertical fine with refactory walls, or by other suitable arrangement of the fire box and heating chamber.

Also, according to the present invention, the products of combustion from the furnace are diluted and cooled with a heated gas before they are passed over the oil heating surfaces. For example, I may introduce a diluting body of air or gas into the connecting flue, between the furnace and still, at a point near the furnace. The addition of ,such a diluting body of air or gas reduces the gas temperature where thegases first contact with the heating surfaces of the still, and also the temperature of the refractory walls. The heat absorbed by the surface or surfaces of the still first in contact with the furnace gases is thus reduced in two ways; namely, by the lower temperature of the gas and second, by a reduction in the temperature of the refractgry secondary radiating surfaces.

If'the dilut'in'g'gas is lint heated, the obvioussresult is a reduction in efliciency such as accompanies the burning of fuel with an excessive amount of cold air. According to the present invention, however, the diluting gas admixed with the products of combusmately the temperature of the waste gases leaving the heating chamber of the still. I have found that if the temperature of the diluting gas is approximately that of the waste gases leaving the still surface, not only is there no reduction in the fuel economy and capacity, but on the other hand, there is a slight increase in both.

Accordingly, in the preferred embodiment of the invention, I recycle a part of the heated gases from near the outlet of the heating chamber and introduce them into the fire box or bring them into admixture with the products of combustion, so that the products of combustion are cooled and diluted thereby before they are passed over the oil heating surfaces. The introduction of the recirculatedflue gasreduces the temperature of the furnace .gases to a point where objectionable overheating can be avoided, while, as'these recirculated gases are at the temperature of the gases leaving the heating chamber, the fuel efiiciency is not decreased.

The present application is in part a continuation of my prior application, Serial No. 532,482, filed January 28, 1922,f,or oil heating furnaces and the improved process of the present invention can advantageously be carried out in apparatus such as described and claimedin such prior application Thus, as. described in said prior application, in one method of operation, the dilutant gas admixed with the products of combustion may be a proportion of the waste gas at the temperature at which it leaves the heating chamber of the still, this waste gas beingdrawn aside and returned to the furnace for admixture with the products ofthe preheated air may be admixed with the products of'combustion. An air heater may be-installed in the flue leading from the still and air forced through this heater and preheated thercin and then utilized as a diluting medium by forcing it into the flue between the furnace and still.

In its broader aspects, accordingly, the invention provides an improved method of operation in which the products of combustion are diluted and cooled with heated gases at a lower temperature, and particularly with gases of approximately the temperature of the flue gases leaving thestill chamber and passing to the flue or stack. This reduction in temperature prevents the high temperatures'of the the box from overheating the first tubes or surfaces of the still and there is a decrease in the quantity of heat absorbed by these initial heating surfaces with an increase in the quantity of heat absorbed by t1on is heated, for example, to approxi-- the remaining surfaces. A more uniform distribution of the heating action is thus accomplished, while the increased volume of the heating gases results in an increase of velocity which increases the rate and ciliciency of heat absorption and has the further desirable result of carrying soot and slagthrough the heating chamber rather The preheating of air by the flue gases and the utilization of the preheatedair for admixing with the products of combustion tends to give an oxidizing atmosphere in the heating chamber of the still; and I consider it more advantageous to recycle a part of the flue gases than to preheat air and utilize the preheated air for admixture with the products of combustion. This recycling of part of the flue gases requires littleadded equipment and little modification of the furnace construction. The recycled gas for example, may be returned through openings or flues in the Walls, floor or lining of the furnace, and, in addition to admixing with and cooling the gases from the fire box may also serve to cool the walls or floors or may be introduced so as to protect the furnace arch or other parts of the furnace from the intense heat of combustion.

l/Vhen the same amount of fuel is burned in the furnace, withrecyc-ling of part of the fine gases, as when the gases are not recycled, it is evident that a measure of the efficiency and capacity of the oil can be made from the temperature and weight of the waste gases. The weight of the gases is not however changed by the recycling of part of the gases, but experience has shown that the temperature of the waste gases escaping to the stack is actually reduced by the recycling of a portion of them. As a result, the efficiency and capacity of the still are somewhat increased.

The increase of the rate of gas flow over the'heating surfaces, while it increases the rate and efficiency of heat absorption, also increases the friction loss and to maintain the same furnace draft it is necessary to have a somewhat increased draft at the still outlet. When this is provided however, not only will the capacity of the still be maintained but the efficiency will be somewhat increased, while, on account of the lower temperature of the gases striking the first tubes or heating surfaces of the still, these surfaces will be protected, in the manner previously described. I

The invention is of more or less general application for the heating of oil stills, and particularly of tubular oil cracking stills where the oil is circulated through tubes passing through the furnace, for example, where the oil is circulated upwardly through vertical tubes or where the oil is circulated through horizontal tubes extending through or arranged in the heating chamber.

In a tubular oil still the circulation of the heating gases is advantageously countercurrent, or in a general countercurrent direction, to that of the oil. Where, for example, the oil is circulated upwardly or in a general upward direction through a tubular still, or through the tubular heater of a still,

the gases are circulated on a general downwardly direction; and the present invention is of particular value for use with such a tubular cracking still. In such' a still, the oil is heated in its upward flow and is hottest in the upper tubes or in the upper portions of the tubes. Where the high temperature products of combustion are introduced into the top of the heating chamber containing such tubes, the tubes containing the hottest oil are. subjected to the greatest heat. According to the, present invention, the excessive temperatures of combustion are moderated and the products of combustion are cooled before they are brought into contact with the tubes or portions of tubes containing the hottest oil. The more uniform distribution of the heating gases gives an increased heating effect in the lower tubes or in the lower portions of the tubes so that in general a more uniform heating of the oil flowing through the tubular heater is accomplished, as well as the avoiding of excessive over-heating of any portions of the tubes.

The invention will be further described in connection with an oil still such as that described and illustrated in Patent No.

1,285,200, granted to the Sinclair Refining Company, November 19, 1918, on the application of Edward IV. Isom; but it will be understood that the invention is of more general application, although it is of particular value with a tubular cracking still of the character referred to in said patent.

In the accompanying drawings I have shown certain forms of apparatus in which the process of the invention can be practiced, and the invention will be further described in connection therewith. It is to be understood however that these specific illustrations and descriptions are for the purpose of exemplification and that the scope of the invention is defined in the following claims,

in Which I have endeavored to distinguish the heater of an oil cracking still of the said general construction illustrated in Patent No. 1,285,200, these three figures showing somewhat modified constructions and arrangements for recycling the flue gas or waste gas. It will be understood that .saidbulk supply tank through the heating tubes and back to the bulk supply. That is, the construction illustrated is one in which the oil is recirculated through heating tubes, as distinguished from a tubular still or heater in which the oil is heated by a single ass therethrough; although the invention 1n its broader aspects is of general application to tubular and other cracking stills of various types and designs including single pass tubular stills.

In the accompanying drawings the corresponding parts of the three figures are indicated by the same reference characters.

Referring to the drawings, the furnace illustrated comprises .a fire box 1, which may be of any usual or preferred form, communicating by an elongated throat 2 with the heating flue or chamber 3 containing the oil heating tubes. The chamber 3 is of considerable vertical length and communicates at its lower end with a stack fine 4;. In Figs. 1 and 3 I have shown the fire box as equipped at 5 with a mechanical stoker and in Figs. 1 and 2 with burners by means of which oil or gas may be used as fuel. The elongated throat 2 isseparated from the heating flue or chamber 3 by a lengthy bridge wall 6 which, in Fig. 1, overhangs at its top as at 30 into the throat of the fire box. The purpose of the elongated throat, and of the overhanging bridge wall in Fig. 1 is to shield the tubes in the heating flue from direct radiation from the fire box.

The battery of heating tubes 7 is conneeted at top and bottom to headers 9, 12, respectively, both of which headers are outside the heating flue. The tube 7, extends through an opening 8 in the top wall of the furnace which is closed by a perforated slab 8 of the refractory material through which the tubes extend. At their lower end the tubes 7 extend through a slab or slabs of refractory material 10 beneath which the header 12 is located. The slabs of refractory material may rest upon collars formed on the tubes as shown in Fig. 1, and the plates or slabs 10 may rest upon angle ironsj The entire battery; of vtubesis sustained from the top header. so, as

13 at their edges.

to be free to' expand and contract. with change of temperature.

The lower header is connected to a manifold 12 which receives oil through the pipe 12 The upper header is connected to a manifold 9 which in turn connects with a pipe 11. The oil received through pipe 12 enters the header 12, flows upward through the bank of tubes to the upper header. and thence through the manifold and pipe 11.

When the heater is employed in a system of the type shown in the Isom patent to which reference has been made, the pipes 11, 12 are connected to the bulk supply tank (not shown) and preferably one or both of them are provided with means for accelerating the circulation as shown in said patent.

The heating flue is provided with baffles 14, 15 which cause the hot gases to pass back and forth among the tubes of the battery a number of times. before reaching the stack flue 4 by which they make their exit. By reason of this thorough circulation among and contact with the oil heating tubes, the heat is efficiently extracted from the prodnets of combustion before they reach the stack flue. The efiiciency is further increased due to the fact that the general direction of the flow of the heating gases and oil is opposed so that the relatively cool oil entering at the bottom of the tubes extracts the heat from the partially cooled gases'and its temperature increases until it is at the maximum in the upper pass of the furnace where it is exposed to the gases fresh from the fire box. The oil heating tubes are however screened from direct radiation from the fire box, even at their upper ends, thus preventing overheating with consequences to which .reference. has heretofore been made.

In order to reduce the temperature of the hot products of combustion coming from the fire box before they strike the oil containing tubes I provide meansfor returning a portion of the partially cooled gases from the exit end of the heating chamber and for mixing it with the fresh products of com bustion from the fire box. For this purpose I. provide a return flue or flues 16 on one or both sides of the furnace which connect with the lower end of the heating chamber at 17 struction illustrated a common Venturi con-' struction is shown with a steam jet indicated at 28, but it will be evident that a positive blower or other positive forcing means may be similarly used. By suitably controlling the forcing means any desired proportion of volume of gases in contact with the tubes and consequently the rapidity of flow is increased, and, therefore, the heating effect is improved and at the same time the heat is more evenly distributed throughout the tubes and throughout the length thereof.

In theconstruction of Fig. 2 the flue gases are in part admitted through a series of apertures or an elongated slot or flue 32 at the base of the bridge wall and in part introduced through a separate flue 30 provid ed in the floor of the furnace in order to cool the same. These gases entering through the flue 30 escape in the form of a protective blanket along the inner front wall of the furnace and over the arch 27, while the gases entering through the fine 32 tend to pass up along the bridge wall and to protect the bridge wall, although iiiterinixture of the products of combustion and of the recycled flue gas will take place such that gases of an approximately uniform compo-- sition will pass over the bridge wall into contact with the tubes.

It will be evident that the amount of flue gases recycled can be varied and the temperature of the combustion gases corre spondin'gly modified before they reach the tubes through which the oil is circulated. The temperature in the fire box may for example, be maintained around 24t00 F. and this temperature may be reduced by admixture of recycled flue gas for example, to a temperature of about 2000 F. before the gases reach the tubes. In their passage through the first pass of the heating cham her the gases may be reduced from about 2000 to about 1300 F. to which temperature they enter the second pass where they may be reduced to about 1000 F. The finally leaving the lower ass may be reduced to about 800 F. at which temperature a part of the flue gas is recycled for admixture with the hot combustion gases and the remainder is discharged at the stack. The regulation of the recycling of the fiue gases is independent of the rate of combustion in the fire box, and, with a sufficient recycling of flue gases, the rate of combustion may be somewhat increased without objectionable overheating of the surfaces of the cracking still by the admixed gases.

In practice there is usually a small leakage of air through the furnace structure and past the bafiies 8 and 10 into the heating chamber which results in the introduction of a certain amount of free oxygen into the furnace gases. This oxygen will be heated by the gases in the furnace chamber and may serve to bring about added com bustion.

It will thus be seen that the present invention provides for the reduction of the furnace temperature of the heating gases at the point where they first come in contact with the surfaces of the cracking still and that this reduction of temperature permits a milder condition of heat absorption in these tubes with resulting less coke formation, while the reduction of temperature in the furnace by re-circulation of gases protects the brick work and prevents slagging of the same. The fast velocity of gases past the tubes tends to prevent the deposit of furnace slag and dust upon the tubes with, resulting impairing of vision of the tubes and heat conductivity.

It will further be seen that these and other advantages are obtained without sacrifice of the efficiency of the furnace but that the efliciency is somewhat increased, as well as giving a more uniform application of heat to the oil heating surfaces. As a re salt, the life of the heating tubes or elements is greatly increased, while the. danger of injury from overheating and burning is greatly reduced, and the apparatus can be operated for longer periods of time without requiring shutting down for cleaning. The furnace linings are also prolonged because of the lower temperature of the gases which are brought in contact there with.

It will further be seen that the recycling of flue gases is advantageously combined with an operation in which direct radiation upon the oil heating tubes is prevented by causing the combustion to take place in a fire box remote from the heatingchamber and interposing a bridge wall or similar obstruction to cut oil direct radiation.

While the invention is of more or less general application to oil stills, and particularly to tubular stills, it is of special value in oil cracking stills operated at a high pressure where there is tendency for carbon deposits to form on the heating surfaces and where the failure of any part of the still is accompanied with great hazard to life and property. In a tubular oil cracking still the invention makes the heating cf the tubes more uniform, increases the efliciency of heat transfer, and decreases or removes altogether the danger of over-heating; while nevertheless taking full advantage of the heat of the furnace gases utilized in heating the still.

I claim:

1. The improvement in the heating of oil cracking stills in which oil is heated to the cracking temperature by hot products of combustion from a furnace passing exter nally over the oil heating surfaces of. the still, which comprises recycling over the oil heating surfaces in admixture with fresh prodnets of combustion from the furnace a portion of the hot waste heating gases which have previously passed over the heating surfaces.

2. The improvement in the heating of oil cracking stills in wli'ich'oil is heated to the cracking temperatufe by heat transferred to the oil through heating surfaces from hot products of combustion passing over the oil heating surfaces from a furnace in which fuel is burned, which comprises diluting the hot products of combustion from the furnace with hot waste heating gases which have passed over the heating surfaces and passing the resulting admixed heating gases over the oil heating surfaces.

8. The improvement in the heating of oil cracking stills in which oil is heated "to the cracking temperature by hot products of combustion from a furnace passing externally over the oil heating surfaces of the still, which comprises diluting the hot products of combustion from the furnace with hot gas at a lower temperature prior to bringing said hot products of combustion into heat interchanging relation with the oil heating surfaces.

4. The improvement in the heating of oil cracking stills in which oil is heated to the cracking temperatureby hot products of combustion from a furnace passing exter nally over the oil heating surfaces of the still, which comprises diluting the hot products of combustion from the furnace with gases at a temperature approximately equal to the temperature of the admixed gases after contact with the heating surfaces of the still.

5. The improvement in the heating of oil cracking stills in which oil is heated to the cracking temperature by hot products of com bustion from a furnace passing externally over the oil heating surfaces of the still, which comprises diluting the hot products of combustion from the furnace with part of the hot waste heating gases from the still and passing the resulting mixture of gases over the oil heating surfaces.

6. The improvement in the heating of oil cracking stills in which oil is heated to the cracking temperature 'by hot products of combustion from a furnace passing externally over the oil heating surfaces of the still, which comprises diluting hot products of combustion from the furnace with hot gases at a lower temperature before passing them over the oil heating surfaces and also protecting the oil heating surfaces from direct radiation from the furnace.

7. The improvement in the heating of oil cracking stills in which oil is heated to the cracking temperature by hot products of combustion from a furnace passing externally over the oil heating surfaces of the still, which comprises recycling a part of the hot waste heating gases which have passed over the heating surfaces, admixing them with hot products of combustion from the furnace and passing the resulting mixture over the oil heating surfaces.

8. The improvement in the heating of tubular oil cracking stills in which oil is heated to the cracking temperature by hot products of combustion from a furnace passing externally over the tubular oil heating surfaces of the still, which comprises recycling a part of the hot Waste heating gases from the still, admixing them with the hot products of combustion from the furnace to give a gaseous mixture of temperature lower than the furnace gases before passing the heating gases over the tubular heating surfaces, and passing the gaseous mixture over the tubes of the tubular still. v

9. The improvement in the heating of tubular oil cracking stills in which oil is heated to the cracking temperature by hot products of combustion from a furnace passing externally over the tubular oil heating surfaces of the still, which comprises circulating the oil and the heating gases in a general countercurrent direction and diluting the hot products of combustion with gas at approximately the ten'iperature of the hot waste heating gases before circulating the gaseous mixture over the heating tubes.

10. The improvement in the heating of tubular oil cracking stills in which oil is heated to the cracking temperature by hot products of combustion from a furnace passing externally over the tubular oil heating surfaces of the still, which comprises passing the heating gases in a generally downward direction over the tubular heating surfaces of the still, and recycling and admixing a part of the hot waste heating gases with the hot products of combustion to reduce their temperature before passing the gaseous mixture in contact with the tubular surfaces of the still.

11. The improvement in the heating of tubular oil cracking stills in which oil is heated to the cracking temperature by hot products of combustion from a furnace passing externally over the tubular oil heating surfaces of the still, which comprises circulating the oil through the tubes in a generally upward direction, circulating the heating gases around the tubes-in a generally downward direction, and recycling a part of the hot waste heating gases and admixing them 7 tecting the heating surfaces from direct radiation from the: furnace and causing the recycled gases to be heated by radiation before the mixture of gases passes over the heating surface. I

13. The improvement in the heating of tubular oil cracking stills in which oil is heated to the cracking temperature by hot products of'combustion from a furnace passing externally oyer the-tubular oil heating surfaces of the still, which comprises reducing the temperature of the heatinggases first coming in'contact with the tubes ,by admixture therewith of recycled hot waste heating gases and thereby obtaining a more uniform heating of the tubular still 'with increased velocity of flow of the gases.

14. The improvement in the heating of tubular oil crackin stills in which oil is heated-tothe crack-ing temperature by hot products of combustion'from a furnace passing externally over thetubular oil heating surfaces of the still, which comprises diluting the heating gases with part of the hot waste heating gases, passing the resulting gaseous mixture over the tubes of the still in a series of passes and returning the hot gases after thelast pass for admixture with the hot furnace gases.

15. The improvement in the heating of 1 oil cracking stills in which oil is heated to the cracking temperature by hot products of combustion from a furnace passing externall over the oil heatin surfaces of the still, which comprises admixing the hot gases from the furnace with returned hot waste heating gases from the still containing free oxygen before passing thev gaseous mixture over the oil heating surfaces.

16. The improvement in the heating of tubular oil cracking stills in which oil is heated to the cracking temperature by hot products of combustion from a furnace passing externally overthe tubular oil heating surfaces of the still, which comprises shielding the cracking tubes from direct radiation from the furnace andintroducing recycled u .y hot waste heating gases from: the still into the hot furnace gases before passing the gaseous mixture over-the tubes of the still. 17 The improvement inthe heating of oil cracking stills in which oil is heated to the cracking temperature by hot products of combustion. from a furnace passing externally over the oil heating surfaces of the still, which comprises recirculating a part of the -hot waste heating gases from the still over and through the furnace structure to fcool the same and introducing the recirculated gases into the hot furnace gases before passing the gaseous mixture over the oil heating surfaces.

18. The improvementin theheating of tubular oil cracking stills in which the oil is circulated from a bulk supply through heating tubes and back to the bulk suply and in which the circulating oil is heated to the cracking temperature by hot products of combustion from a furnace passing externally over the heating tubes, which comprises recirculating a part of the hot waste heating gases from the still and admixing them with the hot products of combustion from the furnace to cool the same prior to passing the gaseous mixture over the heating tubes.

19. The improvement in the heating of circulated from a bulk supply through ventical heating tubes and back to the bulk supply and in which the circulating oil is heated to the cracking temperature by hot products of combustion from a furnace passing externally over the heating tubes, which comprises returning a part of the hot waste heating JOHN E. BELL.

" tubular oil cracking stills in which the oil is

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