US2126420A - Method for heating and fractionating hydrocarbon oils - Google Patents

Description

c. w. STRATFORD METHOD FOR HEATING AND FRACTIONATING HYDROCARBON OILS Filed Dec. 18, 1933 2 Sheets-Sheet 1 INVENTOR GM Sf/afford CRUDE 576/9665 c. w. STRATFORD 2,126,420 METHOD FOR HEATING AND FRACTIONATING HYDROCARBON OILS 2 Sheets-Sheet 2 Filed Dec. 18, 1935 ENVENTOR CW. Srrzvfora ATTOR EY Patented Aug. 9, 1938 UNITED STATES PATENT OFFICE l /IETHOD FOR HEATING AND- FRACTIONAT- ING HYDROC'ARBON OILS Application December 18, 1933, Serial No. 702,883

4 Claims.

My invention relates to a method for heat ing and fractionating hydrocarbon oils.

In the processes and apparatus of the prior art wherein oil is heated and flashed into a fractionating zone in a single flash operation,

difficulty is experienced in obtaining maximum yields of desired products. The charging rate can be varied only through certain limits.

Diiferent types of crudes have difierent boiling ranges which fact makes it necessary to especially design fractionating equipment for the particular type of crude used in processes involving conventional methods. When pipe stills are used as a means for heating the oil, the furnace l5 temperature to which a pipe still must be fired is governed by the charging rate, the type of oil charged and the amount of oil to be vaporized.

In fractionating problems where a number of reboilers are used for supplying heat in the bottom of the fractionating towers, the heat for reboiling cannot readily be supplied by the pipe still which heats the oil for fractionation, since varying quantities of heat must be supplied for reboiling.

It will be seen that the present systems of heating and fractionating oils are not flexible. The

type of crude upon which they operate can be varied only. within certain limits. The charging rate at which the plant is to be operated can be varied within comparatively narrow limits.

In heaters which employ brickwork, the refractory material becomes highly heated and, when it is desired to shut down the furnace, it is necessary to keep on circulating oil even after the fire has been extinguished. Then, too, in furnaces employing brick work, the cost of maintaining the brick work is considerable, due to the high temperatures of firing.

One object of my invention is to provide a method of fractionating hydrocarbon oils which possess flexibility of,

a. Control.

I). Desired products.

0. Charging rate.

(1. Charging stock.

Another object of my invention is to provide a method of heating oil in which the oil flows in a thin film at progressively increasing velocity.

Another object of my invention is to provide a novel method of furnishing the heat of reboiling which will permit a flexibility and accurate control.

Another object of my invention is to provide a heating method having a high eificiency.

Anotherv object of my invention is to provide a method for heating oil independent of,

a. The charging rate.

b. The type of oil.

Another object of my invention is to provide a method of fractionating oils in which the reboiling load may be varied from zero to the maximum permitted by the capacity of the heater.

Another object of my invention is to provide a method of operating a heater to clean it by flowing oil in such a manner as to preclude carbon deposition on the heating surfaces.

Another object of my invention is to provide a fractionating method which will permit the securing of maximum yields of desired products of great uniformity and over a Wide range.

In the accompanying drawings which form part of the instant specification and which are to be read in conjunction therewith and in which like reference numerals are used in the various views to represent like parts:

Figure 1 is a diagrammatic representation of one mode of apparatus capable of carrying out the process of my invention.

Figure 2 is a diagrammatic sectional elevation of the heater used in my process.

Figure 3 is a sectional view taken on a line 3-3 of Figure 2.

Figure 4 is a sectional view taken on a line 4--4 of Figure 2.

Figure 5 is a sectional View taken on a line 5-5 of Figure 2.

Figure 6 is an enlarged fragmentary sectional View of the portion of the heater shown in Figure 2.

In general, my invention contemplates the establishing of a circulating stream of a fairly heavy oil from a reservoir through a heater and back to the reservoir. The circulating oil is that previously separated from the process and may comprise a crude from which the desired lighter products such as gasoline, naphtha, kerosene, and gas oil have been removed. The absence of light products insures that the oil may be heated to desired temperatures without the formation of vapors sothat the heater will always be operated completely full of liquid precluding the formation of vapor in the heater which would tend to insulate the oil to be heated from the heating surfaces. The stream of oil is circulated at high velocity and at a constant rate. Another pump takes suction from the reservoir and discharges to any desired number of heating units through which the oil to be heated passes in heat exchange. From the various heating. units, the oil is returned to the common reservoir. It will be seen that the quantity of oil passing through the heater may remain constant while the reboiling load may vary from zero to the maximum.

The heater comprises structure which will permit a mass of oil to be flowed at high and progressively increasing velocity toward the fired end of the heater. A convection section is fitted to the heater. The crude is charged through a series of heat exchangers through the convection section and into the first fractionating tower in which a desired light product such as gasoline is removed. In the first tower, the bottoms are reboiled by means of heat supplied by a stream of oil withdrawn from the reservoir. It will be appreciated that in this manner close control and great flexibility are achieved. By maintaining the bottom of the tower at a predetermined temperature, maximum yields of the desired product can be obtained. The partially topped crude is then heated by heat exchange with a second stream of oil drawn from the reservoir, and passed into a second tower from which a second desired product is withdrawn overhead. The bottom of the tower is reboiled by a third stream of oil withdrawn from the bottom of the reservoir. The topped crude is then introduced directly into the heated stream of oil which is circulating from the heater. The oil is heated to the desired point in the heater to permit of the vaporization of gas oil and lighter fractions. The quantity of oil flowing through the heater is comparatively great in proportion to the topped crude being admixed with the heated oil. The hot oil will vaporize the gas oil fractions and those lighter, and the admixed vapors and oil are separated in a separating zone which is preferably situated at the top of the reservoir. The unvaporized oil will pass to the body of oil in the reservoir. The vapors are taken overhead and fractionated in a tower in which a desired light product, as for example, a naphtha or kerosene, may be recovered overhead while the heavier products such as gas oil may be withdrawn at any desired point. The unvaporized condensate bottoms of the tower are stripped either in an external or internal stripping zone. The stripped condensate bottoms may be then reheated in a vacuum still for vacuum distillation as is the customary practice, or otherwise processed.

More particularly referring now to the drawings, oil from the lower portion of separator reservoir l is pumped by pump 2 through line 3 into the inlet ring 4 of the radiant section 5 of the heater 6 which will be more fully described hereinafter. The oil is withdrawn from the outlet 1 of the heater and passed through line 8 into the upper portion of the separator reservoir I through tangentially disposed inlet 9, through spiral separator Ill. The oil collects in gutter I l, overflows on to humped plate l2 and rejoins the body of oil [4 in the reservoir portion of tower l. The oil circulation just described is continuous, the same quantity of oil being constantly withdrawn and returned. The crude oil to be fractionated, it being understood of course that any oil may be used, crude being taken merely by way of example, from storage tank I6, is pumped by pump I! through heat exchanger I8, through heat exchanger l9, through heat exchanger 25, through heat exchanger 2|, through heat exchanger 22, into the convection tubes 23 of the heater from which it leaves through line 24 and is discharged into fractionating tower 25. It will be observed that the oil flows through the various heat exchangers in counterflow relation. Vapors formed by the heating of the oil are removed from the fractionating tower 25 through line 26, passed through heat exchanger !8, through condenser 27, into separator 28. In separator 28 water is removed through line 25; gas is vented through line 3:3 and the desired product which may be gasoline, for example, is removed through line 3 l. A portion of the gasoline passes through line 32 to the reflux reservoir 33 formed in the lower portion of the separator, reflux being pumped by pump 36 through line 35, as is well known in the art. The bottom oi tower 25 is maintained at a predetermined temperature by means of heat supplied thereto in reboiler 36 which is substantially a vertical heat exchanger. Pump 37 withdraws oil. from the body of oil I4 within separator reservoir from which it is pumped through line 38, through line 39, controlled by valve 45, through reboiler 35. Valve as may be a temperature controlled valve so that, as the temperature of the bottom of the tower 25 drops, the valve Ml will tend to open, permitting a greater quantity of hot oil to circulate through the reboiler. The tower bottoms pass through line ll, through the reboiler, and return to the tower through line 42 by thermo-siphon flow.

It will be observed that the unvaporized oil withdrawn from the bottom of fractionating tower 25 through line 53 is the crude minus the desired light fraction. The partially topped crude is pumped by pump 44 through heat exchanger 45 in which it is heated by hot oil from the body of oil l4 coming from line 38 through line 4B,

through line 41, in counterflow with the partially topped crude in heat exchanger 45, through line 48, where it joins the oil coming from reboiler 36, through line 49. The oil from reboiler and heat exchanger i5 passes through line 55 into line 5|, which discharges into separator reservoir into the gutter l I thereof. The partially topped crude, heated in heat exchanger passes through line 52 and is discharged into fractionating tower 53. The bottom of fractionating tower 53 is maintained at a. given temperature by means of reboiler 54 which acts similar to reboiler 36 and is supplied with hot oil from the body of oil I4 by oil withdrawn from line 45 through line 55, which is controlled by valve 56 and is returned through the reboiler and line 5'! which communicates with line 5|. Line 5| is the common line for returning all of the oil, with exception of oil to HJI through line 86, that is withdrawn from the body of oil I4 by pump 31, to the body of oil. The bottom of the tower being maintained at a given temperature, a maximum yield of the desired product will be obtained. Vapors of the desired product are taken overhead through line 58, pass through heat exchanger l9 into condenser 59 and thence into separator 65, which is similar to separator 28. Reflux from the reflux accumulator 6| in the bottom of separator 65 is pumped by pump 62 through line 63 controlled by valve 64 into tower 53. The desired product is withdrawn through line 65 and passed to storage. It will be observed that both towers 25 and 53 are provided with lines 66 and 6'5 controlled by valves 68 and 69. These lines are for the purpose of admitting steam into the bottom of towers 25 and 53. In event it is desired to repair, clean or inspect reboilers 36 and 54, these lines may be also used in starting. It is to be understood, of course, that in normal operation the reboilers are used and not steam from lines 66 and 61. The topped crude passes from the bottom of fractionating tower 53 through line 70. Normally, tower 53 will be positioned above separator reservoir I so that oil from to fractionating tower 75.

the bottom of fractionating tower 53 may flow through line 70 by gravity. In the drawings, a pump 'H is shown. The topped crude passes through valve controlled line 12 and is discharged into the circulating stream of hot oil in line 8.

It will be observed that the oil circulating in the primary circulating stream, that is, the oil from the reservoir through the heater back to the reservoir, is highly heated. The oil in the reservoir is usually maintained at about 600 F. The oil leaving the radiant heat section of the heater through line 8 is usually only a few degrees higher in temperature than the oil entering the heater due to the fact that the circulation rate is very high. For example, if the rate of fiow of the topped crude be about 50 gallons per minute, the circulation rate through the heater in the primary circulating stream will be about 3000 gallons per minute. It is obvious therefore that a large quantity of hot oil flowing through the heater will heat the topped crude joining the circulating stream through line 12 to a temperature in the vicinity of 600 F. The hydrocarbons boiling below this temperature will be vaporized. The oil and vapors are discharged tangentially through a spiral passageway in the top of the separator reservoir I. It will be observed further that steam may be discharged into a spiral passageway through valve controlled line 13 to further assist in the removal of light products from the topped crude. The result of this procedure is that the unvaporized oil is separated from the oil vapors. The oil vapors will be withdrawn from the separator reservoir through line M and passed The unvaporized oil will join the main body of oil l4 in the reservoir portion of the separator reservoir. It will be observed further that the oil in the reservoir and forming the body of oil I4 is denuded of all products boiling below the vicinity of 600 F. so that the oil being circulated through the heater is of such a nature that it will not become vaporized when heated to temperatures in the vicinity of 600 F. This results in keeping the heating spaces of the heater liquid full.

:3 are secured to the pumps, the assemblies being supported from tubular member 16. The tubular member 16 passes through the reservoir in the manner illustrated. Within the reservoir, the tubular member is provided with a number of wholes or openings 00 which are covered by a coarse screen 8i which in turn is covered with a fine screen 82. The liquid level of the oil in the reservoir is indicated at 03. This level is maintained by means of level control 84. Just below the liquid level 83 swirl plates 85 are fitted.

The pumps 2 and 31 take suction through the tubular member lfiwhich is supplied from the reservoir through the holes 80, as will readily be appreciated by reference to the drawings. The

' liquid level operates in the following manner.

A bleeder line 86 communicates with the discharge of pump 37. If the level of the liquid drops, air line 0? communicating with level control chamber 0d acts to put pressure on diag phragm valve 80 tending to close the valve and the quantity of oil in reservoir I is substantially constant. The vapors passing into fractionating tower are fractionated, the desired product being tatken overhead through line 90 and passing through heat exchanger and condenser 9| into separator 02, the lower portion of which forms a reflux accumulator 03 similar to separators 28 and 60, reflux being pumped by pump 04 through line 05 to tower 15. The desired product, which may be kerosene, for example, is withdrawn through line 96 and passed to storage. The gas oil fraction may be withdrawn from the tower through side stream drawofi and passes through line 91 through heat exchanger 2|, through cooler 93, through line 99, to storage. The heaviest liquid is withdrawn from fractionating tower 75 through line I00 and is discharged together with the oil being bled from the separator reservoir through line 86 into stripping tower lill in which the heavy fractions are stripped with steam entering tower l0! through line 02. The vapors and stripping steam are passed into fractionating tower 75 through line M3. The stripped bottoms are withdrawn from the stripping tower by pump I00 and passed through line I 05, through heat exchanger 22, through cooler I06, and withdrawn through line I01. Line I01 may pass to the heater of a vacuum distillation system or to other processing as is well known in the art. In a typical example, in which a Roumanian crude was processed, the oil in the primary circulating stream through heater 6 entered the heater at about 320 C. and left the same at about 325 C. The bottom of fractionating tower was maintained at about 180 C. giving a gasoline having an end point of about 140 C. The partially topped crude is heated in heat exchanger and passed into fractionating tower 53 from which a naphtha having an end point of 185 C. was removed, the bottom of the tower 53 being reboiled to a temperature of 250 C. The vapors passing into fracticnating tower 15 through line 74 resulted in an overhead product of a kerosene having an end point of 275 C. The gas oil was withdrawn through line 01.

Referring now to Figure 2, a cylindrical shell 200 is provided with slots 2M and embraced by an annular member 4 provided with a flange connection 202 into which the oil to be heated passes. Disposed within and surrounded by shell 200, I fit a second cylindrical shell 203, disposed in spaced relation to shell 200 adapted to form a manifold into which oil through slots 20! passes. By reference to Figure 6 it will be seen that shell 203 is fitted with nozzles 204. These nozzles are disposed completely around the shell 203 at spaced intervals. It will be appreciated that oil from manifold 205 formed by shells 200 and 203 will pass through the nozzles 204 throughout the entire area. Disposed in spaced relation within shell 203 is a third shell 205 which may be cor rugated in the manner shown. The space between shell 203 and shell 206 forms a thin annular oil passageway. An annular outlet ring I communicates with the oil passage 20'! and is provided with a connection member 208. A burner 200 is disposed to fire the surface of shell 206 which forms in effect a combustion chamber Zlll. A volute 2H supplies air through air register 2l2 of the burner and is adapted to give the products of combustion a whirling motion. A transition fiue 2|3 joins a casing 2H5 to the construction just described. Within member 2M a plurality of tubes 23 are disposed forming a convection section, it being observed that the first portion of the furnace described above forms a radiant heat section.

A second transition flue 2l5 joins an air preheater 2|! of any suitable construction to the convection section. The air preheater 2|! is connected to a stack MB. A blower fan 219 passes air in heat exchange with the hot products of combustion in air preheater 2 l1 and supplies the preheated air through duct 220 to the volute 2| I. The duct 229 is fitted with a suitable damper 22L In operation, oil to be heated flows through connection 2% around annular oil inlet member 4, through slots ZOI into manifold 205 and is jetted by nozzles 204 against the corrugated surface of shell ZUES. Since all of the oil must flow into outlet ring 1, it will be observed that the oil flowing through the fired end in Figure 2 flows in a thin annular sheet to which additional oil is constantly and progressively being added, the oil at the convection section end being supplied by one ring of nozzles. As the oil passes to the left, more and more oil is added to the flowing sheet. Inasmuch as the volume of the passageway is substantially constant, the addition of an increasing quantity of oil will result in an increased velocity in order to take care of the increased mass of oil being supplied. Just before the oil enters the annular outlet member I, the oil passageway 29'! is enlarged by tapering the cylindrical shell 20% at 222 for collecting the oil. The combustion gases have been given a whirling motion as pointed out above. Ihe rate of heat transfer is extremely high due to the fact that the oil is flowing from an extended surface in counterflow to the heating medium and owing to high velocity flow of impinged 011 against the heated shell. There is minimum loss of radiant heat due to the fact that the heating surface completely surrounds the source of radiant heat. The jetting of the oil against the heating surface and the high rate of flow prevent carbon deposition and in a measure act as an automatic cleaning means, carbon deposits being prevented by maintenance of Very low flow temperature. The heater is easily and inexpensively installed, maintained, and repaired, inasmuch as there is a minimum of brick work. Furthermore, when the furnace is shut down, there is a minimum of radiant heated brick work to continue heating the oil. The cost of brick work maintenance is avoided. A heater of my design has a very high efficiency, being between 87 and 90%. I have used an experimental heater of the type shown for a period of one year and a half, without the necessity of cleaning it once.

It will be observed that I have accomplished the objects of my invention. I have provided a method of distillation in which maximum yields of desired products are obtained while providing for flexibility with respect to control and with respect to the types of crudes which can be processed. I am enabled to operate my unit at any charging rate from zero to the maximum. I am enabled to reboil in an efficient and flexible manner. It will be observed that the quantity of oil passing through the primary circulation ring in which heat is supplied is unchanged while the reboiling load may be varied at will. Due to the fact that all light products are removed from the primary circulating stream, the heater operates liquid full. Due to the high velocity of flow in my heater, low film temperatures are obtained so that parasitic cracking is avoided.

It will be understood that certain features and sub-combinations are of utility and may be employed without reference to other features and sub-combinations. This is contemplated by and is within the scope of my claims. It is further obvious that various changes may be made in details within the scope of my claims without departing from the spirit of my invention. It is, therefore, to be understood that my invention is not to be limited to the specific details shown and described.

Having thus described my invention, what I claim is:

1. A method of fractionating hydrocarbon oil including the steps of circulating a stream of oil from and to a body of oil, supplying heat to said stream of oil, heating the oil to be fractionated and fractionating the same in a first fractionating zone, removing vapors from said zone, passing the unvaporized oil in said zone in heat exchange with a second stream of oil circulated from and to said body of oil to reboil the unvaporized oil in said first fractionating zone, removing the unvaporized oil from said zone and passing the same in heat exchange with a third stream of oil circulated from and to said body of oil, fractionating the thus heated oil, and removing oil vapors from the fractionating zone.

2. A method as in claim 1 wherein the unvaporized oil in the second fractionating zone is reboiled by heat exchange with a fourth stream of oil circulated from and to said body of oil, the unvaporized oil is withdrawn from the second fractionating zone and introduced into the first stream of oil whereby vapors are formed, the vapors separated from unvaporized oil, which is returned to said body of oil and the vapors are fractionated.

3. A method of fractionating hydrocarbon oil including the steps of circulating a stream of oil from and to a body of oil, circulating a second stream of oil from and to said body of oil, supplying heat to said first circulating stream of oil, heating oil to be fractionated by heat exchange with said second circulating stream of oil, fractionating the heated oil, withdrawing oil vapors from the fractionating zone, separately removing liquid fractions from the fractionating zone, circulating a third stream of oil from and to said body of oil, and passing the liquid fractions in said fractionating zone in heat exchange with said third circulating stream of oil to reboil the liquid fractions.

4. A method of fractionating hydrocarbon oil including the steps of circulating a stream of oil from and to a body of oil, circulating a second stream of oil from and to said body of oil, supplying heat to said first circulating stream of oil, heating oil to be fractionated by heat exchange with said second circulating stream of oil, fractionating the heated oil, withdrawing oil vapors from the fractionating zone, separately removing liquid fractions from the fractionating zone, introducing said liquid fractions into the heated first circulating stream of oil whereby vapors are formed, separating the vapors from the liquid oil, passing the liquid oil to said body of oil and fractionating the second mentioned vapors in a second fractionating zone.

CHARLES W. STRATFORD.

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