US2103719A - Method of and apparatus for heating oil - Google Patents

Description

8, 1937- A. E. HARNSBERGER METHQD OF AND APPARATUS FOR HEATING OIL Filed Dec. 7, 1954 INVENTOR fludgsy E H urns-buyer /v i7 ATTORNEY Patented Dec.28,1937

METHOD OF AND APIBARATUS FOR HEATING V .Audley E. Harnsberger, Winnetka, Ill., assignor to The Pure Oil Company, Chicago, 111., a corpo ration of Ohio Application December 7, 1934, Serial No. 756,385

' 3 Claims. (Cl. 196-47) This invention relates to method of and apparatus for heating oil, and more particularly to method and apparatus for converting mineral oils into motor fuel. i v

Tube stills have almost entirely replaced drum stills for heating and cracking mineral-oils due to the more rapid and eflicient transfer of heat from the heating medium to the oil. Tube stills, however, possess certain disadvantages. At the high temperatures to which the 'oilis heated for cracking, carbon deposits form in the tubes,

clogging them and necessitating shut-downs for the purpose of cleaning. Furthermore, tube failure occurs frequently, particularly when carbon deposits form in the tube thereby preventing efiicienttransfer of heat to the oil pass ing through the tubes and resulting in local overheating.

When oil is passed through a tubular heater at low velocity, turbulent flow is not obtained. As a'result, the film of oil adjacent the inner surfaceof the tubing is heated to a temperature considerably higher than themain body'of oil. The high temperature of the film is conducive to decomposition with resultant carbon formation and deposition. Since carbon is a relatively poor conductor of heat, the rate of heat transfer'from the tubin'gto the oil decreases and as a result the tubing attains a temperature in excess of its physical limit of endurance. The

. overheating of the tubes causes "creep to take place with consequent bulging of the tubes. If such tubing is not replaced itwill burst, frequently' causing both loss of life" and property.

In order tomitigate carbon deposition it has been proposed to pass oil through tubular heaters at" high velocities. 'As a resultgthe oil assumes turbulent flow'so that itfis more uniformly heated and lessens the likelihood of'formation 'and'deposition of carbon by reasonof local overheating adjacent.theinnersuriace of the tubes. 7

However, since the velocity at which oil can be pumped is limitedby friction losses, local overheating cannot-be entirely eliminatedwhen high- 1y heated oil .is subjected to heating with high temperature furnace gasesin order to crack it. The tubing acts as a heattransfer medium between the gasand oil and assumes a temperature intermediate the two. Consequently, when both the. gases and the oil are at a high enough temperature, the tubing attains a temperature in excess ofitsphysical limit, resulting in tube failllleb In order to avoid this difficulty in cracking, it 'has heretofore been customary -to heat-the oil to or near cracking temperature in a tubular heater'and then pass the oil to a cracking drum wherein the cracking reaction takes place. By minimizing cracking in the tubes, the combination of high temperaturejheating gases and high temperature oil is avoided to'a large extent and the deposition of carbon inthe tubes is likewise greatly diminished. I

One of the primary objects oimy invention is to avoid high metal temperatures in tubular heaters.

attendant upon the use of'tubular heaters in the cracking'of oil. Since the tubing is maintained'at a low temperature, local overheating of'the film of oil-adjacent the inner wall of the tubing is minimized thereby greatly decreasing coke deposition. Moreover, by maintainingthe metal tubing at arelatively low temperature, creep; isreduced to a minimum and the life of the tubing is greatly extended. Another object of my invention is to provide an oil heaterin whichoil can berapidly heated to cracking temperature'without forming largedeposits of coke in the tubesor heating coils.

Another object of my invention is to provide By avoiding high tube temperatures I am able to overcome the drawbacks heretofore a heater in which mineral oil can be cracked while passing through thetubes.

A further object of my invention is to provide a tubular heater which provides flexibility of operation so that various types of oils can be heated and cracked therein without resort to a cracking chamber.

Still a further object of my invention is to H provide a method of cracking oil or reforming naphtha in a tubular still.

In order to accomplish the foregoing objects of my invention, oil is passed through a series bank of 'tubes located in the convection zone of g a furnace in which combustion gases, which have been cooled by previous radiant heat transfer to other banks of tubes, are passed over the bank. The preheated oil then passes through a bank of tubes which are heated substantially only by radiant heat of high temperature combustion gases. The oil leaves this bank of tubes substantially at the cracking temperature and passes through a second radiant heat tube bank where the oil at or near the cracking temperature is heated to or' maintained at the cracking temperature for a period of time sufficient to crack the oil. The temperature of the gases in the second radiant heat chamber is consider! ably below the temperature in the first'ra'diant heat chamber. As a result of the particular arrangement of the tubes in the furnace and the distribution of heat, whereby the oil. at lower temperature is subjected to the higher radiant heat and the oil at higher temperature is subjected to the lower radiant heat, the cracking of the oil in the tubes can be accomplished with unusually low amount of carbon deposition and an astonishingly low number of tube failures.

In order to more fully understand the foregoing and other objects of my invention, reference should be had to the following description and drawing of which the single figure is a vertical cross section of the heater which forms a part of my invention.

Referring to the drawing, l designates generally the furnace built with suitable refractory material and set on a suitable foundation. The furnace is divided into three chambers designated by numerals 2, 3, and 4. A bank of horizontally disposed tubes 5 which are serially connected from'bottom to top are located in chamber 2. Bailles, 6 are provided on the inside of the walls I and 8 in order to prevent the gases from channeling along the sides of the walls. A hollow bridge wall designated generally by the numeral I0 separates chambers 2 and 3. Beneath the roof I I of chamber 3 and extending over the top I2 of bridge wall I0 is located a horizontal row of horizontally disposed tubes I4 which are serially connected from one end to the other. A thermocouple I6 is inserted in a suitable well in the roof of the furnace. A vertical row of horizontally disposed tubes I8 may be placed along the inside of the wall 20 which forms part of the hollow bridge wall [3. The top I2 of the bridge wall I0 is spaced from the'roof I I of the furnace in order to form a passageway for combustion gases from chamber 3 to chamber 2. A line 24 connects the end tube 25 of the top row of tubes of the bank 5 to the initial tube 26 of the bank I4. Suitable burners 21, which may be combination gas and oil, are placed in the lower portion of the side walls of chamber 3. A second hollow bridge wall designated generally by the numeral 28 separates the chamber 3 from the chamber 4. The lower end 32 of the bridge wall 28 is spaced from the bottom of the furnace in order to form a gas passageway from the chamber 4 to the chamber 3. A suitable burner 34, which may be a combination oil and gas burner, is placed in the lower end of the front wall 36 of the chamber 4. A double row of horizontally disposed tubes 38 and 40, serially connected, are placed along the side wall 42 of the bridge wall 28, immediately beneath the roof 44 of the chamber 4 and alongside the inside of the front wall 36. The end tube 45 of the bank I4 is connected by line 46 to the farthermost tube 48 of the inside row of tubes 48. The oil flows through the bank of tubes 40 from the tube 48 to the tube 50 and thence to the outside bank of tubes 38 entering the tube 52 and leaving the row of tubes 38 at tube 54 through outlet line 56. It will be understood that the lines connecting the various tubes as well as the inlet and outlet lines are located outside the furnace walls and are heavily insulated to prevent heat loss. The tubes are all preferably placed transversely to the length of the furnace. The inside row of tubes 40 is staggered in relation to the outer row 38 so as to form an obstruction to radiation between the heating chamber and the roof and side walls thereof. The bank of tubes I8 may be omitted if desired. This bank may be used to superheat steam or to heat light oil fractions for rerunning. A thermocouple 60 may be placed in a suitable well in roof 44, and a thermocouple 6| may be placed in a well extending through the side wall of chamber 4 in order to regulate the temperature therein.

The tubes .are connected in series by return bends which are located outside of heat resistance alloy plates which form the side Walls of the furnace adjacent the tubes, and are protected against heat loss by heat insulated doors. The Walls and roof of the furnace are built up of suitable refractory material except at those portions of the side walls of the furnace opposite the tubes. The tubes are supported within the furnace by suitable supporting plates which are in turn supported by hangers suspended from overhead horizontal beams or affixed to vertical beams. A passageway 62 is provided at the lower end of chamber 2 to'exhaust combustion gases therefrom to the flue.

In the operation of the heater, the burner 34 is supplied with an excess amount of air by means of which the temperature of the combustion gases may be regulated. Instead of excess air, flue gases from the stack may be recirculated to chamher 4 in order to lower the temperature therein. A temperature of from 1200 to 1250 F. is maintained in the chamber 4 as determined by the thermocouples 60 and GI. The gases then pass into chamber 3 where additional heat is supplied by means of the oil and gas burners 21. If excess air is supplied in chamber 4 it is utilized for completing combustion in chamber 3. The temperature in this chamber is maintained at approximately 1500 to 1600 F. From the chamber 3 the gases then pass to the convection heating chamher 2 and are exhausted through the passage 62. The oil enters the bank of tubes 5 through inlet line 64 and may be at a temperature of approximately 80 to 300 F., and leaves the top of the bank at a temperature of approximately 550 to 700 F. The preheated oil is then passed through the row of tubes I4 where the temperature may be raised to approximately 900 F. The oil leaves the row I4, passes through the line 46, and enters the row of tubes 40 at 48, and the temperature may be raised to approximately 960 F. during passage through this bank. The oil then passes to the outer bank of tubes 38 in which the temperature is maintained at this reacting temperature with only a slight increase in temperature. The oil leaving through the outlet line 56 may have a final temperature of 970 F. The oil is preferably maintained under super-atmospheric pressure during its passage through the tubes and this pressure may be up to 2000 or more pounds per square inch.

By regulating the heat of the gases in the chamber 3, the length of time during which the oil is subjected to the cracking reaction in the banks 38 and 40 may be controlled. Thus by lowering the temperature of the heating gases in chamber 3 the temperature of the oil passing from bank l4 to bank 40 will be low thereby necessitating a period of time during travel through the bank 40 to bring the oil to cracking temperature, and as a result, shortening the period of time during which the oil is subjected to cracking conditions. Conversely, if the temperature of the gases in chamber 3 is raised, the cracking period may be lengthened. In this manner it is possible to operate upon various types of charging stock. The heater has been used successfully for reforming naphtha, cracking gas oil and topped crude oil.

If desired, more than one bank of tubes can be placed under the roof II, and likewise more than two banks of tubes may be placed in the chamber 4. However, with one bank of tubes under the roof I l and two banks in the chamber 4, efiicient working conditions are established. It has been determined that operating with the temperatures above given, theamount of heat transferred to the row of tubes I4 is of the order of 20,000 B. t. u. per square foot of tubing, the heat transferred to the bank 40 is of the'order of 5,000 B. t. u. per square foot, and the amount transferred to the outer bank 38 is of the order of 2,000 B. t. u. per square foot.

By employing a heater in accordance with my invention, various types of oil can be cracked without resort to a cracking chamber. However, it is to be understood that the heater may be used in conjunction with a cracking chamber, and this may be desirable when cracking low grade crudes.

What I claim is:

1. An oil heater comprising three chambers, a bank of tubes in one chamber disposed in the direct path of combustion gases; a bank of tubes disposed adjacent the roof of an intermediate chamber, and a plurality of rows of tubes disposed adjacent the roof and side walls of the third chamber, means for passing oil from said first mentioned bank of tubes to said second mentioned bank, means for passing oil from said second mentioned'bank to said rows of tubes, an upwardly extending bridge wall between said first and intermediate chambers, a passageway for combustion gases from said intermediate to said first chambers above said bridge wall, a downwardly extending bridge wall between said intermediate and said third chambers, a passageway for combustion gases below said last mentioned bridge wall, heating means located in said third chamber, separate heating means in said intermediate chamber, said heating means in said chambers being so disposed as to impart mainly radiant heat to the tubes in said third and said intermediate chambers.

2. The method of cracking oil in a restricted stream under conditions to prevent excessive tube temperatures which comprises preheating the oil, passing the preheated oil in a restricted stream through a zone wherein the temperature is maintained at approximately 1500 to 1600 F.

and whereinthe oil is rapidly heated chiefly by means of radiant heat to a temperature of approximately 900" F., then passing the heated. oil

in a restricted stream through a second zone maintained at a temperature of approximately 1200 to 1250" F. wherein the oil is heated chiefly by means of radiant heat to a maximum of approximately 970 F., and maintaining the oil in said second zone for a period of time sufficient to crack a substantial portion of the oil to hydrocarbons boiling within the gasoline range without attaining tube temperatures beyond the physical limit of endurance of the tubes, and heating the oil in said first mentioned zone by means of a mixture of combustion gases from said second zone and fresh combustion gases.

3. An oil heating furnace consisting of three heating chambers, a substantially vertical bridge wall extending upwardly from the fioor of the furnace between the first and second chambers, a passageway for combustion gases between the roof of said furnace and said bridge wall, a second bridge wall extending downwardly from the roof of said furnace between the second and third chambers, a passageway for combustion gases between the floor of said furnace and the bottom of said second bridge wall, heating coils in said first chamber located directly in the path of combustion. gases, heating coils directly beneath the roof of said second chamber, connected to the coils in said first chamber, coils adjacent the side walls and roof of said third chamber, said last mentioned coils'being connected to the coils in said second chamber, means for passing oil serially through said coils from first to third chamber, heating means in said second and third chambers whereby to heat the coils therein chief 1y by radiant heat, means for passing combustion gases from said third chamber through said second chamber, and means for passing combustion gases from said second chamber through said first chamber.

AUDLEY E. HARNSBERGER.

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