US2848383A - Method and apparatus for heating hydrocarbon fluids - Google Patents

Description

Aug. 19, 1958 B. E. BAILEY 8,

METHOD AND APPARATUS FOR HEATING HYDRQCARBQN FLUIDS Filed 001;. 20, 1954 BRADFORD E. BAILEY INVENTOR.

ATTORNEY United tates Patent METHOD AND APPARATUS FOR HEATING HYDROCARBON FLUIDS Bradford E. Bailey, Elizabeth, N. 1., assignor to Esso Research and Engineering Company, a corporation of Delaware Application October 20, 1954, Serial No. 463,480 6 Claims. (Cl. 196-110) This invention relates to a method and apparatus adapted for heating fluids and more particularly relates to heating hydrocarbon fluids to conversion or cracking temperature.

Various furnace designs are known for heating hydrocarbon fluids and various arrangements of oil tubes are known to obtain the desired heating of hydrocarbon fluids. In the conversion of hydrocarbon fluids for the production of motor fuel it is desired to quickly raise the temperature of the hydrocarbon fluid as it passes through the heating apparatus to raise the temperature of the hydrocarbon fluid to the conversion or cracking temperature. During this heating the danger of coke or carbon deposition in the heater tubes or coil is relatively small and large amounts of heat may be added per unit of heating surface. After the hydrocarbon fluid has reached a conversion or cracking temperature the rate of heating of hydrocarbon fluid must be decreased and controlled to minimize carbon deposition and coke formation in the heater tubes but suflicient heat must be added per unit of heating surface to effect the desired extent of conver- S1011.

According to the present invention roof and wall tubes are provided in the radiant heating section and floor tubes are submerged in a bed of fluidized solids in the radiant section. Radiant heat adsorbed by the surface of the fluidized bed of solids is transferred to the tubes by the turbulent mixing of the solids in the fluidized bed. By using a fluidized bed the heating of the floor tubes is maintained substantially uniform. By recycling a portion of the hot stack gases higher floor tube heat densities can be obtained in the fluidized bed.

Further according to this invention the aerating means for the fluidized bed may be subdivided into sections so that the fluidity of the different sections of the fluidized bed may be varied and the heat densities of the tubes in the different floor sections can be maintained at different values and in this way the oil heating curves can be controlled.

Further, low quality fuel oil can be sprayed on the top of the fluidized bed and additional air admitted to the recycle gas for burning the oil in the fluidized bed of solids to produce additional heat in the fluidized bed to be transferred to the oil tubes submerged in the fluidized bed.

In the drawing the figure represents a vertical trans verse cross section of one form of apparatus adapted to carry out the present invention.

Referring now to the drawing the reference character 10 designates a furnace or heating apparatus including a radiant heating section 12 and a convection heating section 16. The radiant heating section is fired by means of burners 18 which burn fuel such as gas to supply heat to the radiant heating section. The burners are preferably arranged in the lower portion of the radiant heating section. The convection heating section 16 is separated from the radiant heating section 12 by vertical partition 22 which extends from the roof of the furnaceto a level 2 above the floor 24 of the furnace so as to leave a space 25 for the combustion gases to pass from the radiant section into the bottom of the convection heating section.

The furnace shown in the drawing is a box type rectangular furnace with the heater tubes horizontally arranged adjacent the wall surfaces of the furnace. Convection tubes 26 extend horizontally through the convection heating section. The ends of the heating tubes both in the convection section and the radiant heating section are connected by suitable headers (not shown) so as to provide a heating coil or a continuousconfined passage for the hydrocarbon fluid being heated.

Wall tubes 28 are arranged adjacent the partition 22 in the radiant heating section and heater tubes 32 are arranged along the opposite wall 33 of the radiant section. Roof heating tubes 34 are provided adjacent the roof of the radiant heating section 12 and floor tubes 36 are provided adjacent the floor of the radiant heating section. These floor tubes are submerged in a fluidized bed of finely divided inert refractory solids 37 and will be described in greater detail hereinafter. The tubes are connected to form a coil or continuous passageway as shown diagrammatically in the drawing.

The combustion gases from the radiant heating section pass down through space 25 under the bottom of the partition 22 and up through the convection heating section 16 to the'stack 38 which is provided with a stationary spiral vane diagrammatically shown at 42 in the stack 38 to separate out solids from the flue gases leaving the stack by centrifugal action so that solid particles are thrown toward the inner wall of the stack 38.

The stack I58 is provided with an annular solids and gas drawofl 44 arranged adjacent the top of the rotary vane to permit withdrawal of a portion of hot stack gases for recycling to the radiant heating section and for use as a portion of the fluidizing gas for the fluidized bed of solids. The annular drawofl 44 also acts to receive the solids centrifugally separated by vane 42. Separating means well known in the art such as cyclone separators or the like may be used instead of the vane 42 and annular drawoff 44. The separated solids and hot stack gases are withdrawn from the annular space 44 through line 46 and passed through the line by blower 48 and through lines 52, 54 and 56 as a fluidizing medium for the inert solids in the fluidized bed arranged on the floor of the radiant heating section and for return of separated solids to the bed. If desired, separated lines for return of solids and gases from annular space 44 may be provided.

The fluidized bed of inert refractory solids is supported on a perforated grid 58 which is spaced from the floor of the radiant heating section and extends from the wall adjacent the burner 13 to the vertical partition 62 which extends upwardly from the floor 24 of the furnace for a short distance. The top of the partition 62 is spaced from the convection heater section partition 22 and also terminates at a lower level than the bottom portion of the partition 22 so as to provide a passageway 25 for the combustion gases leaving the radiant heating section and passing to the convection heating section.

The space under the grid member 5;; is subdivided into three sections by vertical partitions 64 and 66 which extend longitudinally of the furnace and extend from the floor 24 of the furnace to the grid member 58. Line 56 for introducing fluidizing gas and recycling flue gas communicates with space 68 below the grid memher, the space 68 being located between the partition 66 and the lower portion of end partition 62. Line 54 communicates with space 72 below the grid member 58 between partitions 64 and 66. Line 52 communicates with space 74 below the grid member 58 and between partition 64 and the end wall of the furnace adjacent burner 18. Lines 52, 54 and 56 are provided with 3 valves so that the amount of fluidizing gas passing to the diiferent sections of the fluidized bed 37 may be varied as desired to vary the heat input into the oil passing through the heater tubes.

The inert refractory solids comprise finely divided solids having an average particle size between about 1 and 100 microns and as inert solids silica, alumina, kieselguhr, sand, fullers earth, quartz, spent silica alumina cracking catalyst and the like may be used. For fluidizing the inert solids in the fluidized bed 37 the superficial velocity of the fluidizing gas is between about /2 and 2 /2 feet per second. In passing the fluidizing gas through the fluidized solids the particles are maintained in an agitated turbulent condition and extremely good heat transfer is obtained between the solid particles and the heater tubes.

A by-pass line 78 is provided leading from the floor of the convection heating section for recycling a portion of the hot combustion gases to the fluidized bed instead of permitting the combustion gases to pass through the convection heating section before being recycled. In this way the fluidized bed of solids can be maintained at a higher temperature.

Further, oil sprays diagrammatically shown at 82 may be provided above the surface of the fluidized bed for spraying low quality fuel oil on the top of the fluidized bed and in this case additional air from line 84 is introduced into line 46 for burning the fuel oil to provide additional heat for the fluidized bed 37. The combustion or flue gas is used as the fluidizing medium for the fluidized bed 37. The flue gas is formed from the fuel and air introduced into burner 18. If more fluidizing gas is desired, it may be introduced via line 84 or by a line (not shown) ahead of blower 48 for feeding into line 46.

Oil, such as gas oil, to be heated is introduced through line 86 by pump 87 into the oil heater tubes in the upper portion of the convection heating section and the preheated oil is then passed to the radiant heating section and through wall tubes 28, roof tubes 34, wall tubes 32 and floor tubes 36 and the heated oil or cracked products are withdrawn through line 88 and further treated to separate motor fuel from gas and constituents boiling higher than motor fuel.

By using a fluidized turbulent bed of inert solids, the floor tubes 36 are protected from the direct radiant heat which is absorbed by the solids and transferred to the heater tubes. In this way more uniform transfer of heat to the heater tubes is obtained. Because of the extremely high rate of heat transfer of a turbulent fluidized bed, the floor tubes 36 have a more uniform heat density than the radiant roof tubes. With the coil outlet leading from the floor tubes, coking of the hydrocarbon cracked products near the coil outlet will be minimized.

As above pointed out after the hydrocarbon oil has reached a conversion of cracking temperature it is important to control the amount of heat being supplied to the oil to effect the desired extent of conversion without coke deposition in the heated tubes. By providing a plurality of spaces below the grid member for the fluidized bed the different sections of the fluidized bed can be fluidized at different velocities and the heat densities of the floor tubes in the different floor sections can be maintained at desired values.

A specific example of heating hydrocarbon oil will now be given.

All the heater tubes are about 3 inches in outside diameter and about 2.25 inches in inside diameter. In the convection heating section there are '92 tubes, each of which is about 32 ft. long. There are 46 wall and roof tubes 28, 34 and 32 and 30 floor tubes 36 all of the same length as the convection heating tubes.

About 250 barrels per hour of a virgin gas oil under pressure of about 250 p. s. i. g. and at a temperature of about 600 F. are passed through line 876 and through 4 the u u salon heat n s ct n 1 t pr he the by: drocarbon oil to about 700 F. The burners 18 are operated to supply a heat density or a rate of heat transfer about 12,000 E. t. u./hr./sq.. ft. for the roof tubes 34 and wall tubes 28 and 32.

The fire box temperature or the wall temperature of the radiant heating section will be about 1400" F. and

'at this temperature the fluidized bed of solids 37 will be at a temperature of about 1200 F. The quantity of radiant heat transmitted from the fire box or heated walls to the solids in the fluidized bed will then equal the quantity of heat transferred from the solids in the fluidized bed 37 to the heater tubes submerged in the fluidized bed. The corresponding heat density of the tubes will be about 10,000 E. t. u./hr./sq. ft. with the oil at a temperature of 1000 F. and an overall heat transfer coeflicient between the solids and the heater tubes of 50 B. t. u./hr./F./sq. ft.

The inert refractory solids are silica alumina cracking catalyst having a particle size between about 1 and microns. The depth of the fluidized bed 37 is 2 feet and the width of the fluidized bed 37 is about 25 feet and the length about 32 ft.

The hydrocarbon oil passing through the radiant wall tubes 28, roof tubes 34 and Wall tubes 32 has its temperature raised from about 700 F. to 900 F. during passage through these tubes. The heated oil is then passed through the floor tubes 36 and as shown in the drawing is introduced into the heater tube adjacent the vertical partition 62. The floor heater tubes are submerged in the fluidized bed of solids and as pointed out above, means are provided for separately fluidizing difierent sections of the fluidized solids bed. As shown in the drawing means are provided for separately fluidizing three sections of the fluidized bed but more or less of such sections may be used.

In the first section fluidized by line 56, the amount of flue gas passing to the first section of the fluidized bed from space 68 'is controlled to provide a superficial velocity of the gas passing up through the fluidized bed of about 2 feet per second to produce a highly turbulent bed for supplying a large amount of heat to the oil passing through the heater tubes. During passage of the oil through this first section the oil has its temperature raised to about 950 F.

The heated oil then passes through the next set of floor tubes arranged in the section of the fluidized bed 37 above space 72 into which fluidizing gas is introduced through line 54. Here the amount of fluidizing gas is reduced so that the superficial velocity of the gas passing through the middle section of the fluidized bed is about 1% feet per second to reduce the turbulence of the solids in the fluidized bed and to reduce the amount of 'heat being supplied to the oil passing through these heated tubes. In passing through this middle section of the floor tubes the oil is heated to a temperature of about 980 F.

The oil then passes through the last section of the fluidized bed above the space 74 into which fluidizing gas is introduced through line 52. The amount of fluidizing gas introduced into this section of the fluidized bed is reduced so that the superficial velocity is about 1 ft. per second and the fluidized bed has less turbulence than in the other portions of the fluidized bed. This last section of the heater tubes provides a soaking section where the heated oil is maintained at a temperature of about 1000 F. to effect the desired extent of cracking. The cracked products are then withdrawn from line 82 and further treated to separate desired products.

While in the preferred form the different sections of the fluidized bed are maintained under different degrees of turbulence, it is within the contemplation of the invention to maintain the turbulence substantially constant throughout the entire fluidized bed when heating oils fcr other purposes.

The cracking temperature may vary between about 900 and 1100 F. The invention may be used for heating fluids generally without being restricted to cracking temperatures.

.As above pointed out, oil may be introduced through nozzles 82 to supply additional heat to the fluidized bed 37. Instead of this, coke particles from any source but particularly coke particles made in the fluid coking process may be added to the inert solids bed 37 or the entire bed 37 may be made up of such finely divided coke particles which may have particles of a size from a few microns up to about 300 to 500 microns. When using coke particles as all or part of the fluidized bed 37, air is supplied to line 46 and lines 52, 54 and 56 through line 84 or an additional line (not shown) as a fiuidizing and combustion supporting gas.

What is claimed is:

1. A furnace for heating fluids comprising an outer shell, a series of exposed interconnected tubes mounted adjacent the roof and vertical side walls of said shell, a horizontal tray having a perforated bottom mounted in the lower portion of said shell, a layer of fluidizable solids positioned in said tray, means for passing a fluidizing gas upwardly through the perforations in the bottom of said tray at a rate controlled to maintain said solids in a fluidized condition, a series of interconnected tubes submerged in said fluidized solids in said tray, the said tubes being in communication with the tubes mounted adjacent the roof and side walls of said shell, burners extending through the walls of said shell adapted to discharge fuel and oxidizing gas to effect flamed combustion in the central section of said shell and above said layer of fluidizable solids to thereby provide a source for radiant heating the surface of said fluidized solids and the tubes mounted adjacent the roof and walls of 36 such shell, and means for removing combustion gases from said shell at a point removed from said burners and positioned to prevent the direct impingement of the flame on said tubes.

2. In the apparatus defined in claim 1 the further improvement which comprises means for passing a portion of the combustion gases removed from said shell upwardly through the perforations in said tray to fluidize said solids.

3. In the apparatus defined in claim 1 the further impassed to the different sections of said fluidized solid for controlling the heat input to diiferent sections of said tubes submerged in said fluidized solids.

4. A method for heating hydrocarbon fluid which comprises passing a confined stream of hydrocarbon oil through a convection heating section to heat the oil to a predetermined temperature, then passing the heated oil as a confined stream through the upper portion of a radiant heating section in a heating zone to raise the temperature of the oil rapidly, then passing the heated oil as a confined stream through the bottom portion of said radiant heating section while the confined stream is submerged in a dense fluidized turbulent bed of heated solids to supply radiant heat to the confined stream of oil,

controlling the heating of the oil as it passes through the fluidized bed of solids to control the amount of heat being supplied to the oil as it passes therethrough.

5. A method according to claim 4 wherein the surface of said dense turbulent bed is sprayed with fuel oil and oxygen-containing gas is used as a fiuidizing gas for said bed.

6.-A method according to claim 4 wherein hot combustion gas from the radiant heating section is used as a fiuidizing gas for said bed.

References Cited in the file of this patent UNITED STATES PATENTS 2,108,687 Mekler Feb. 15, 1938 2,129,900 Barnes Sept. 13, 1938 2,169,086 Auer Aug. 8, 1939 2,224,917 Mekler Dec. 17, 1940 2,229,262 Stiner Ian. 21, 1941 2,362,279 Hemminger Nov. 7, 1944 2,373,059 Smith Apr. 3, 1945 2,493,498 Peery Jan. 3, 1950 2,608,474 Gilliam Aug. 26, 1952 2,729,428 Milmore Jan. 3, 1956 OTHER REFERENCES Mickley et al.: Ind. Eng. Chem., vol. 41, No. 6, May 26, 1949 (pp. 1135 to 1143).

Claims (1)

1. A FURNACE FOR HEATING FLUIDS COMPRISINGS AN OUTER SHELL, A SERIES OF EXPOSED INTERCONNECTED TUBES MOUNTED ADJACENT THE ROOF AND VERTICAL SIDE WALLS OF SAID SHELL, A HORIZONTAL TRAY HAVING A PERFORATED BOTTOM MOUNTED IN THE LOWER PORTION OF SAID SHELL, A LAYER OF FLUIDIZABLE IN THE LOWER PORTION OF SAID SHELL, A LAYER A FLUIDIZING GAS UPWARDLY THROUGH THE PERFORATIONS IN THE BOTTOM OF SAID TRAY AT AT RATE CONTROLLED TO MAINTAIN SAID SOLIDS IN A FLUIDIZED CONDITION, A SERIES OF INTERCONNECTED TUBES SUBMERGED IN SAID FLUIDIZED SOLIDS IN SAID TRAY, THE SAID TUBES BEING IN COMMUNICATION WITH THE TUBES MOUNTED ADJACENT THE ROOF AND SIZE WALLS OF SAID SHELL, BURNERS EXTENDING THROUGH THE WALLS OF SAID SHELL ADAPTED TO DISCHARGE FUEL AND OXIDIZING GAS TO EFFECT FLAMED COMBUSTION IN THE CENTRAL SECTION OF SAID SHELL FLAMED COMBUSLAYER OF FLUIDIZABLE SOLIDS TO THEREBY PROVIDE A SOURCE FOR RADIANT HEATING THE SURFACE OF SAID FLUIDIXED SOLIDS AND THE TUBES MOUNTED ADJACENT THE ROOF AND WALLS OF SUCH SHELL, AND MEANS FOR REMOVING COMBUSTION GASES FROM SAID SHELL AT A POINT REMOVED FROM SAID BURNERS AND POSITIONED TO PREVENT THE DIRECT IMPINGEMENT OF THE FLAME ON SAID TUBES.

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