US2216471A - Process for converting mineral oils - Google Patents

Description

Oct. 1. 1940- A. P. FRAME ET AL ,4

PROCESS FOR CONVERTING MINERAL OILS Filed Dec. 29, 1936 Q. IN.

PRESSURE Pouuos I0203040506070809OIOOIIOI20|3OI4OI5O Z5 STLL TUBES AIR gs-Am 34 on. m :2 X k 3, 46 2 I 36 INVENTORS 1, u'xnac'xnynnxx, ,]///////;/,6963261114 ALFRED R FRAME I WILHAM H. PRICE 4 2g 35 ATTbRNEY Patented Oct. 1, 1940 PATENT OFFICE rnoonss FUR CONVERTING MINERAL OILS Alfred Putnam Frame, Haddonfield, and William Harold Price, Camden, N. J assignors to Powerv Patents Company, Hillside, N. J a corporation of Maine Application December 29, 1936, Serial'No. 118,040

10 Claims.

This invention relates to an improved process and apparatus for cracking mineral oils and more particularly to a controlled method of effecting such crackingin pipe still heaters.

The present practice employed in the cracking of mineral oils, regardless of the type of process, almost invariably involves the use ofa pipe still heater or furnace in which-the oil to be cracked is passed through two ormore banks of tubes mounted in the furnace.- Some of these banks are subjected to radiant heat from the combustion taking place in the furnace fire box, while most furnaces include at least one convection bank of tubes shielded from direct radiation. It is quite usual for certain of the tubes of this pipe still furnace to become coated inside with a thin layer of carbon or coke, particularly where the furnaces are used for cracking oils at high inlet pressures of from400 to 800 pounds per square inch, and at transfer line temperatures of from 890 to 950 F. Experiments-have shown that this coating of carbon'or cokeon the inside walls of certain tubes occurs in a particular part of the furnace and that after a certain period of time the coatingbecomes so important that the cracking unit must be shut downand the tubes cleaned. y

The investigation of this coke" formation disclosed the fact that for example, in the cracking of topped crude oil at furnace'outlet temperatures of about 915'-F.-the coking in the furnace tubes occurred at -a point where theoilite'mperature was between about 860 and 890 F., when operating the furnace at an inlet'pressure of about 575 lbs. per square inch and an outlet pressure of approximately 250 lbs. per'square inch; Having in mind the-foregoing difficulties and disadvantages resulting from the usual pipe still furnace practice, the primary object of the present invention is to provide a process which will overcome these difliculties and at the same time secure as good or better results from the heating and cracking operation. I

A further object of the invention is the provision of a process for cracking mineral oils in which the pressure, temperature and heating conditions to which the oil is subjected in the pipe still furnace duringthe cracking operation are correlated-in such a way as to overcome the disadvantages referred to. Another object of the invention is the provision of the improved apparatus for carrying out the process of the present invention.

According to the improved process, the present invention comprises the steps, of passing theoil to be cracked through two or more tube banks mounted in a pipe still heater in which the temperature and pressure conditions imposed upon the oil are correlated in such a way that coke is not deposited-as in prior operations. The process also includes the step of controlling the temperature and pressure at various points in the stream of oil through the pipe still heater in relation to the temperature to which the oil is heated as it advances through the heater.- For example, the operation is controlled to maintain apressure below approximately 400 lbs. per square inch on the oil after it-reaches a temperature of approximately 860 F. The process ofthe present invention is particularly adaptedfor the cracking of stocks containing residual oil.

The improved apparatus for carrying'out the process of the present invention comprises a'pipe still heater provided with a combustion cham ber and a convection heating chamber separated by a bafiie wall around one side of'which products of combustion flow from the combustion chamber into the convection chamber. A plurality of similar tubes for heating mineral oil are mounted in the pipe still heater in a position to receive radiant heat from the combustion chamher, and a bank of tubes similar'to those receiving radiant heat is mounted in the convection chamber and heated by combustion gases from the combustion chamber. The tubes'of the bank in the convectionchamber are connected to form at least two separate coils of similar length for the" passage of oil therethrough. Means are provided for introducing oilinto, -and for passing it in series, through the tubes receiving radiant heat from the combustion chamber. The apparatus includes means for passing the oil products leaving said tubes, and for uniformly distributing the products to the coils of the tube bank mounted in the convection chamber. Means are also provided for conducting the heated oil products from the coils of the convection bank into a transfer line for the pipe still heater.

Other objects and advantages of the process of the present invention will be more readily understood from the following detailed description taken in connection with the accompanying drawing, in which:

. Fig. 1 is a chart which shows by means of I curves the pressures at various points in the pipe still furnace under the prior practice and under the new practice employed in accordance with the features of the present invention.

Fig. 2 is a diagrammatic vertical sectional view of .a pipe still furnace showing the relationship of tube banks and circuit suitable for carrying out the features of the present invention.

Referring to Fig. 2 of the drawing, the improved apparatus employed for carrying out the process of the present invention may be briefly described as follows: The oil to be cracked which may be fresh charging stock or a mixture of preheated charging stock and reflux condensate at a temperature of approximately 650 F. is passed under high pressure through a furnace inlet line 2 and then through an economizer tube bank 4 mounted in the flue gas outlet from the furnace. The oil then passes through a connecting line 6 and through a bank of radiant heating tubes 8 mounted vertically along the wall of the combustion chamber of the furnace. From the tube bank 8 the heated oil passes through a connecting line l0 and a radiant roof bank l2. A connecting line l4 from the bank l2 conducts the heated oil to the lower portion of the furnace where it is divided into two streams and passed through a convection bank of tubes IS in parallel streams. The products are discharged from this bank of tubes through lines 20 which lead to a transfer line 22 provided with a valve 24.

The banks of tubes in the pipe still furnace are heated by hot products of combustion supplied fr'om a plurality of burners 26 through parallel tunnels 28 in the bottom of the furnace as shown. The products from the tunnels 28 discharge into a combustion or fire box chamber 30 to the left of which is mounted the radiant tube bank 8. The tube bank l2 mounted above the combustion chamber also receives radiant heat. The products of combustion from the furnace pass upwardly through the combustion chamber and over a thin heat conducting bridge wall 32, and then downwardly over the tubes of the convection bank [8 and the economizer bank 4. The spent products of combustion are discharged from the furnace through a conduit 34 and a portion of these hot products are recirculated back to the combustion chamber by means of a connecting branch conduit 36, a blower 38 and a conduit 40. The recycled products of combustion are discharged into the combustion chamber at points opposite the outlet of tunnels 28.

; The flue gas which is not recirculated through the furnace passes to an air preheater 42 from the pipe 34 and finally to a stack from an outlet conduit 44. The air for use in the furnace is passed through an inlet pipe 46 and in heat exchange with the flue gas in preheater 42. The preheated air is conducted to the burners 26 from a conduit 48.

. The process of the present invention relates primarily to the treatment of the oil in the pipe still furnace, but it is to be understood that the cracked products discharged throughthe transfer line 22 may be subjected to a time reaction by introducing them, for example, into the upper part of an enlarged vertical reaction chamber (shown mostly broken away). The materials from the reaction chamber are fractionated to remove the residual constituents and separate the distillate products into gasoline and higher boiling fractions.

In a typical prior operation in which a furnace of the type shown in Fig. 2 was employed, but where the convection tube bank l8 was a single series tube bank, the oil was charged through the line 2 at a pressure of approximately 575 pounds per square inch in order to obtain an outletlpressure in the transfer line 22 of approximately'250 lbs. per square inch. In this operation, the charging stock was a mixture of topped crude oil and gas oil recycle stock produced in the cracking operation. ,In this process in which the oil temperature in the transfer line 22 was about 917 F. the operation was continued for approximately 100 hours when it was observed that although the outlet pressure in the furnace was maintained substantially constant, the inlet pressure had increased to such an extent that it was necessary to shut down the cracking unit and clean the tubes. After the unit had been shut down and cooled and the tubes opened, it was found that there was coking only in about 25 or tubes in the lower part of the convection tube bank. The coking in this part of the furnace could not be readily accounted for because it was practically in the zone of lowest flue gas and furnace temperatures. Furthermore it was in the zone in which there was no radiant heating of the tubes.

In a subsequent similar run, the temperatures at various points in the tube banks were determined (after the unit was on stream and stabilized) and it was found that the tubes which had previously been coked were in the zone in which the oil temperatures ranged from approximately 860 F. to 890 F. In these runs the pressure conditions prevailing in the coil at various points were measured and when plotted, gave the upper curve shown in Fig. 1 of the drawing.

In carrying out the improved process involving the use of the same number of tubes in the convection bank 18, but divided into two separate streams of equal length, it was found that a very much lower pump pressure was required to maintain an outlet pressure of approximately 250 lbs. per square inch. The pressure conditions for the new operation are shown in the lower curve in Fig. l, in which the gauge pressure in pounds per square inch is plotted against the number of tubes in the pipe still furnace,

A typical run in accordance with the process of the present invention on a charging stock substantially the same as the mixture referred to above continued for a period of approximately 400 hours when the unit was shut down for inspection. The tubes in which the coating of coke formerly occurred were free of cokeand the remainder of the furnace tubes were also found to be free from coke. In this operation the furnace inlet pressure was approximately 425 lbs. per square inch or approximately 150 lbs. per square inch less than the pressure required in the old type of operation as shown by the curves in Fig. 1. At this inlet pressure it was found'possible to increase the feed rate over the old operation by approximately 30%, and at the same time secure a higher yield of gasoline which had an equivalent octane number to the, gasoline produced in the old operation.

It appears that the relationships of the particular temperatures and pressures employed in the heating operation contribute to the important improvements obtained by the process of the present invention. The velocity of the products passing through the various portions of the coil comprising the tube banks may also affect the success of the operation. In any case these temperatures have been correlated in such a way as to provide an extremely efficient process for the cracking of hydrocarbon oils in pipe still heaters.

The temperature of 860 to 890 appears to be a critical temperature for a mixture of topped crude oil. and gas oil reflux condensate. The new conditions provided for this temperature range have avoided the formationof coke incrustations in the coil. The critical temperature for other stocks containing more, or less residue, may be somewhat higher or lower than this range, but it is possible to control the temperature at various parts of the furnace so that any cracking stock may be effectively treated without the formation of coke in the coil.

From the foregoing description it will be readily apparent to those skilled in the art that various other modifications may be made in the process and apparatus without departing from the spirit and scope of the present invention and such changes are contemplated within the scope of the appended claims.

Having thus described the invention in its preferred form, what is claimed as new is:

1. In the cracking of mineral oils in which the oil to be .cracked is passed through a plurality of tube banks in a pipe still heater, the improvement which comprises passing an oil mixture including gas oil recycle stock at a high initial pressure in excess of 400 lbs. per square inch in a single stream through the tube banks except the last, and then in a plurality of streams through the last of said tube banks, heating said single stream of oil to a temperature slightly below that at which the oil reaches a critical state at which coke would be deposited on the walls of the tubes, heating the plurality of streams of oil through a temperature range from slightly below said critical stage temperature to a discharge temperature of from 890 F. to about 950 F., maintaining a pressure below 400 lbs.

per square inch on the oil of said streams to prevent coke formation in the tubes through which said streams pass, and maintaining a substantial superatmoshperic pressure not exceeding 400 lbs. per square inch on the oil products of said streams as discharged from the last of said tube banks.

2. The process of cracking mineral oils as defined by claim 1 in which the oil feed rate to the pipe still heater is controlled in relation to the discharge pressure from the heater to maintain a relatively low rate of pressure drop in the plurality of streams of oil flowing through the last tube bank.

3. In the cracking of mineral oils containing recycle stock in which the oil to be cracked is passed through a plurality of tube banks in a pipe still heater, the improvement which comprises passing the oil to be cracked at a high initial pressure substantially in excess of 400 lbs. per square inch in a single stream through the tube banks except the last and then in a plurality of streams of similar diameter through the last of said tube banks, heating the stream of oil in the tube banks except the last to a temperature slightly below that at which the oil would deposit coke on the walls of the tubes, heating the plurality of oil streams in the last of said tube banks through a temperature range from slightly below said coke deposition temperature to a discharge temperature above 890 F. and not substantially above 950 F., controlling the operation to maintain a pressure at below approximately 400 lbs. per square inch on the oil after it reaches a temperature of approximately 860 F., and discharging the resulting oil products from said heater at a substantial superatmospheric pressure.

4. The process of cracking oil as defined by claim 3 in which said plurality of streams of oil are heated while passing through a convection through a series of heating zones in a pipe still furnace, and in which under usual operating conditions .coke is deposited in the tubes of the pipe still furnace at a point therein at which the oil passing therethrough reaches a critical stage at a relatively high pressure, the improvement in such cracking operations for the prevention of such coke formation, which comprises passing an oil mixture includingcrude oil residual constituents in a flowing stream into said pipe still furnace at a relatively high inlet pres-' sure substantially in excess of 400 lbs. per square inch, passing the oil of said stream through the heating zones of said pipe still furnace and heating the oil as it passes therethrough to a discharge temperature of from 890 F. to a temperature not substantiallyl exceeding 950 F., controlling the heating operation in said pipe still furnace to heat the oil of said stream up to a temperature below but approaching that at which the oil would deposit coke on the walls of the pipe still tubes, dividing the oil products flowing through said heating zones into a plurality of streams at a point therein immediately preceding that at which the oil would reach a critical stage and deposit coke on the walls of the pipe stilltubes, whereby coke deposition in the tubes of said pipe still furnace is prevented, and discharging the cracked oil products from said furnace at a substantial superatmospheric pressure."

7. In the cracking of mineral oils containing recycle stock in which the oil to be cracked is passed under pressure through zones in a pipe still heater, the improvement which comprises passing the oil to be cracked in a flowing stream into said pipe still heater at a high inlet pressure substantially in excess of 400 lbs. per square inch, passing the oil of said stream through said pipe still heater and heating the oil constituents passing therethrough to a discharge temperature of from 890 F. to a temperature not substantially above 950 F., heating the oil of said stream in said heater to a temperature below but approaching that at which coke would normally be deposited on the walls of the tubes of the pipe still and then dividing the oil of said stream into a plurality of streams at a point preceding that at which said cokedeposition would occur thereby preventing coke deposition in the pipe still tubes, controlling the pressure maintained on the oil flowing through said heater so that the pressure on the oil is below 400 lbs. per square inch after the oil of said stream has been divided into a plurality of streams, and. discharging the oil products of said heating from said pipe still heater at a substantial superatmospheric pressure but below a pressure of 400 lbs. per square inch.

8. In the process of converting and cracking mineral oils containing recycle stock in which the oil to be converted is passed through a series of heating zones in streams of restricted cross section, the improvement which comprises passing the oil to be cracked and converted at an initial high pressure substantially in excess of 400 lbs. per square inch in a. flowing stream of restricted cross section into the first of the series of heating zones, passing the oil of said stream through the series of heating zones and heating the oil products passing therethrough to successively raise the temperature thereof and heating them to a discharge temperature of from 890 F. to a temperature not substantially above 950 F., heating the oil products of said stream introduced into said series of zones to a temperature below but approaching that at which coke would be deposited from the oil products being heated and then dividing the Oil products of said stream into a plurality of streams of restricted cross section at a point preceding that at which coke deposition would occur thereby preventing the deposit of coke from the oil streams in said heating zones, and discharging the resulting cracked oil products from the series of heating zones at a substantial superatmospheric pressure.

9. In the cracking of mineral oils in which the oil to be cracked is passed through a plurality of tube banks in a pipe still furnace and in which the last bank of tubes is a convection tube bank heated by hot products of combustion, the improvement which comprises passing the oil to be cracked at a high initial pressure substantially in excess of 400 lbs. per square inch in a single stream through the tube banks of said furnace except the last, and then in a plurality of streams through the last of said tube banks, heating the stream of oil in said tube banks except the last to a temperature below but approaching that at which the oil would deposit coke on the walls of the tubes, heating the plurality of oil streams in the last of said tube banks by passing products of combustion in a generally countercurrent direction to the flow of said streams through said last bank, the oil passing through said last bank being heated to a discharge temperature above 890 F., and not substantially above 950 F., controlling the opera-.

tion to maintain a pressure below approximately 400 lbs. per square inch on the oil after it reaches a temperature of approximately 860 F., and discharging the resulting heated oil products from said pipe still furnace at a substantial superatmospheric pressure.

10. In the cracking of mineral oils in which the oil to be cracked is passed under pressure through a series of heating zones in a pipe still furnace and in which under usual operating conditions coke is deposited in the tubes of the pipe still furnace when the oil passing therethrough reaches a critical stage temperature at a relatively high pressure, the improvement in such cracking operations for the preventing of such coke formation, which comprises passing the oil to be cracked in a flowing stream into said pipe still furnace at a high inlet pressure substantially in excess of 400 lbs. per square inch, passing the oil of said stream through the heating zones of said pipe still furnace and heating the oil constituents passing therethrough to a discharge temperature of from 890 F. to a temperature not substantially above 950 F., controlling the heating operation in said pipe still furnace so that the pressure on the oil products is below 400 lbs. per square inch after the oil products flowing therethrough reach a temperature approaching about 860 F., providing the oil products flowing through said heating zones into a plurality of streams at a point in said furnace immediately preceding that at which the oil would reach a critical stage temperature, thereby preventing coke formation in said pipe still furnace which would normally occur if the oil products flowing through the furnace were not divided into a plurality of streams.

ALFRED PUTNAM FRAME. WILLIAM HAROLD PRICE.

Images