US1885716A - Oil converter - Google Patents

Description

Nov. l, 1932. A. E. HARNSBERGER OIL CONVERTER Filed Jan; 25, 1930 Patented Nov. 1, @1932 UNITED STA rss PATENT oFFl'cE AUDLEY E'. HARNSBERGER, OF CHICAGO, ILLINOIS,4 ASSIGNOB T0 G-YRO PROCESS COMPANY, 0F DETROIT, MICHIGAN, A CORPORATION 0F MICHIGAN on. CONVERTER.

Application led January 25, 1930. Serial No. 423,521).

This invention relates to improvements in converters of the character employed in connection with oil cracking processes, and an outstanding object thereof resides in the provision of a converter wherein is provided a plurality of advantageously arranged tube banks, through which the oil to be heated and converted is passed in a vaporous state, and wherein the arrangement of the tube banks is such as to provide for the eiiicient heating of theoil vapor to cracking or conf version temperatures and the maintenance of the vapors at such temperatures for desired periods of time, ,whereby there is secured from the oil vapor passed through the tube bank a maximum yield of the desired end product and minimum quantities of undesired fractions such as fixed gas and recycle stock.

In vapor phase crackin-gef hydrocarbon oils, as the temperature of the vapors increases they expand in laccordance with known laws and as the pressure of the vapors decreases, due, for example, to, frictional drop, the volume also increases. It is desirable to heat the vapors whenfirst introduced into the converter rapidly until the vapors attain a cracking temperature and to then decrease the quantity of heat suppliedv to the vapors to maintain the latter at a substantially constant temperature duringthe cracking period. To secure satisfactory tube life at positions of` highest heat input it is necessary to employ high vapor velocities in order to conduct away the heat from the metal walls of the tubes. If the velocity is not high, the tube reaches a very high ternperature, approaching that of the actual furnace temperature, which results in short life of the tubes. Therefore, high vapor velocity is necessary at the points of highest heat input, but as the cooler zonesI of the furnace are reached the-.vapor velocity can be safely reduced, or if not reduced at least held constant, and this factor permits of the use of larger tubes in the cooler cracking zone to compensate for the increase in volume dueto increased temperatures and reduction in pressure. Again, at the point in the converter Where cracking takes place there is liberated acertain amount of so-called fixed gases, which also greatly lincrease the volurne of material passing through the cracking coils, and in these portions of the coil if increased flow or tube area is not provided, undesirably high frictional resistance will be encountered, which the present invention avoids.v

Itis, therefore, another object of the present invention to provide a converter for cracking oil in the vapor phase, and wherein use is made of a cracking coil provided with increased cross-sectional or tube area at various .points throughout the unit or coil to provide, first, for high velocity of oil vapor flow at the entrance portion of the coil and, sec- 0nd, for gradually increasing vapor crosssectional area toward the discharge or outlet end of the coil.

For a further understanding of the invention reference is to be had to the following description and the accompanying drawing,

wheren:

Figure 1 is a vertical sectional'view taken through a converter constructed in accordance with the present/invention, and

Figure 2 is a transverse cross-sectional view on the line 2-2 of Figure 1.

Referring more particularly to the drawing, the numeral 1 designates a settin of the furnace or converter. The interior o the latter is provided with a transversely extending bridge wall 2 which divides, the setting* internally into combustion and tube chambers 3 and respectively. The combustion chamber is provided with burners 5 for developing highly heated furnace gases, the latter passing generally upwardly through the combustion chamber and then sweep horizontally along the roof of the furnace and turn substantially downwardly for movement through the tube chamber 4. 'The lower portion of the tube chamber is provided with a horizontally'directed duct V6 whichcommunicates with a vertically disposed stack 7 tllrough which the gases escape to the atmosere. A p Arranged in the duct 6 is a bank 8 of oil vpreheating or vaporizing tubes. This bank of tubes is preferably heated by the waste gases discharged from the chamber 4, a feature which permits of the eiicient utilization of the heat developed by the furnace so that the gases discharged from the stack 7 will be of a minimum temperature. The charging stock enters the bank of tubes 8 by wa of a line 9, and is forced serially through the tubes of said bank in order that the oil may be heated to a vaporizing temperature of approximately 700 to 750 F. Ordinarily little orv no crackingpof the oil takes place in the tube bank 8, and preferably, the charging stocks delivered to the vaporizing tubes comprise a clean fraction of as oil boiling range from which carbon pro ucing high boiling oils have been removed.

The oil discharged from the bank 8 passes by way of the pipe line 10 to an evaporator 1l, wherein the hi h boiling point oils, which remain in the liquid state at the temperatures specified, are dropped to the bottom of the evaporator and withdrawn by way of the valved outlet 12. These high boiling oils may be disposed of as fuel oils. The lighter oils which are in a vaporized condition in the evaporator 11, pass overhead from the latter by way of the outlet line 13 and are delivered to a series of drying tubes 14 arranged in the low temperature side of the tube chamber 4. Superheated steam may be introduced'into the outlet line 13 by way of the steam line 15 in desired quantities. p The drying tubes 14 are of relatively large cross-sectional area, for example, eight inches in diameter, and the oil vapors fiow serially through the several tubes comprising the row 14, whereby the vapors are heated to a dry substantially gaseous condition and liquid oils effectually removed therefrom. Some carbon deposit may take place in the tubes 14, but owing to the large cross-sectional area of these tubes such deposits will have little or no practical effect on the transmission of heat to the vapors or in the matter of seriously obstructing oil vapor How through the tubes.

Afterthe vapors have been dried by passage through the tubes 14 they are continuously transmitted to a bank of cracking or converting tubes 16, consisting of a plurality of serially connected pipes or tubes, which are arranged in the radiant heat zone of the combustion chamber 3 and are disposed preferabl around the sides and top walls of the cham er 3. In the operation of systems of this kind it is desirable to raise the temperature of the oil vapors which enter the conversion zone rapidly from a vaporizing or drying temperature of approximately 750 F. to an active cracking 'temperature in excess of 1000 F. A very practical method of accomplishing this consists in placing the first bank of tubes comprising the cracking section in the radiant heat zone of the furnace, as indicated at 16. It will be understood that therate of heat transfer by radi ation-is proportional to the fourth power of the difference' 'between the absolute temperature of radiating and adsorbing structures, whereas the rate of heat transfer by convection is proportional tothe Iirst power and the diderence between the absolute temperature of the hot gases and the heat absorbingstructure. Accordingly, where heat transfer is effected simultaneously by radiation and convection, the temperature of the heat absorbing structure may become excessive. Therefore, to prevent injury to the tubes 16 it is important that these tubes be kept clear of carbon deposit and the' velocities of oil travel therethrough maintained at a relatively high point in order that the heat applied to said tubes may be quickly absorbed by the rapidly moving oil vapors and conducted away. The drying tubes serve to eliminate carbon deposit in the cracking tubes 16, and since the latter are of relatively small cross-sectional area the velocity of oil travel therethrough is high, in excess of 2500 feet per minute. This results in keeping the Walls of the tubes 16 at a temperature where they will not be deleteriously affected by the high heat of the furnace.

After the oil has been discharged from the bank of tubes 16 it possesses a temperature in eXcess of 1000o F. and less than 1200o F. Since cracking is a product of both time and temperature, it is necessary to-maintain the vapors for a desired period of time at aparticular cracking temperature to secure a de- Vsired rate of conversion. Careful regularelatively placed to secure a uniform appli-I cation of heat thereto at given temperatures to overcome unequal heating of the oil vapors which'lpass through the tubes.

In the attainment of these ends the chamber 4 is provided with a horizontally placed row of tubes 17, which are located in the upper portion of the chamber 4 and are subjected to a high degree of furnaceheat, which is of such a character that the oil vaporsdischarged from the tubes 16 and then passed serially through the tubes 17 are being constantly and uniformly heated. This rate of heat input is such as to more than overcome loss of heat due to the endothermic reactions taking place in the oil vapors while the latter are undergoing cracking. From the tubes 17 the oil vapors flow serially pors are passed serially through a third row of tubes 19 which are .disposed between the tube rows 17 and 18. The tubes compris-- ing the rows 17, 18 and 19 are progressively increased in cross-sectionall a,`rea. This is done in order to accommodate for the eX- pansion of vapors due to the temperature rise and also to the drop ,in pressure by reason of decreased frictional resistance between the moving oil vapors and the walls of the tubes. The largest part of this increase in volume is undoubtedly due to the drop in pressure rather than the increase in tempera? ture. Thus the oil vapors passing through the tubes 16, 17 18 and 19 are constantly heated to secure a continuation of the desired cracking reactions, but in the tubes 17, 18 and 19 preferably lower vapor velocities obtain over the velocities prevailingin the tubes 16 to secure the desired time factor in the conversion of the oil. The oils traveling through the tubes 18 and 19 are heated sulficiently to maintain the same at a cracking temperature in excess of 1000 F. and it follows that' these tubes are not to be considered as mere soaking chambers but active heat .applying elements.

rIhe cracked oil vapors discharged from the tubes 19 are introduced into an arrester 20 where the vapors are brought into direct contact with a large quantity of cold oil which is sprayed into the arrester. Rapid heat exchange takes place andthevapors are dropped in temperature from 1000 F. to a fractionating temperature not in excess of 600 F. /This sudden drop in temperature results in minimizing the liberation of free carbon in the act of cooling the vapors. 'The vapors released from the arrester may be fractionated in any suitable Way and s'ubsequently condensed for various purposes. The liquids obtained from the bottomJ of the arrester may be employed as recycle stock. ln recapitulation itwill be seen that the vapors introduced into the converter are first A 17 will be not greater, but possib y less, than passed through the drying tubes 14 to insure that no liquids, with its coke-forming tenden-V cies, will be introduced into the high radiantheat zone in which the tubes 16 are located. In the tubes 16 high vapor velocities'are maintained in order to successfully conduct away the heat fromV the metal walls of the tubes and to quickly heat'the vapors to a desired crackinga temperature in excess of 1000o F. The vapors are then conducted to the tubes 17, which Vpossess greater crosssectional area than the tubes 16 in order that thevelocity of vapor flow throu' h the tubes the velocities of flow in the tubes 16, this reduction in velocity serving to decrease the rlctionalback pressure of the cracking coils.

The vapors'are then conducted to a still cooler heating zone of the furnace, namely the tubes 18 and 19, where the vapors having. reached a cracking temperature by passage through the bank 17 will be held at substantially alconstant Itemperature by supplying only' the heat .necessary to overcome endothermic heatI losses. The tubes and 19 are of greater cross-sectional area*V` than the tubes 16 and 17 since in this section a considerable volume of light products, such as motor fuel and permanent gas are generated, which cause an increase in volume and it is, therefore, desirable to provide for this in crease in volume by an increase in crosssectional area, -so that high frictional back pressure will not be set up in this portion of the converter where high velocities on account of heat transfer are not required. The conversion reactions having been completed in the tubes 18 and 19 are suddenly and substantially' instantaneously arrested by the operation of the unit 20.

4 In order to demonstrate the necessity for providing for increase in cross-sectional area in various heating portions of thev converter on account of the change that takes place in the vapors as they travel through the converter, the following table is given:

From the above it will be noted that in a representative converter the drying section 11 will be made of tubes having an internal diameter of eight inches, permitting low velocity and 'very slight friction loss. At.

thispoint volume is desirable in order to provide space for the formation of such coke that is produced during the drying operation. It is desirable to have suflicient volume in the tubes 14 to provide for extended converter runs Without shutdowns for cleaning purposes. However, as soon as the tubes 16 are reached the tube size is reduced to four inches in diameter in order to provide high vapor. velocities which will insure heating the vapors in the minimum of time and, also, that the tube Walls will remain at a safe operating temperature under furnace temperatures of i2000o 4to 2400" F. In .the tube bank 17 a siii inch diameter tube is em'- ployed in order that there will not be a large increase in velocity of the vapors and consequently back pressure on the evaporator 11. The vapors, beinglighter, will not have the same heat absorbing capacity in the section 17 as in the section 16.' However, at. this point the furnace temperature is reduced to approximately 180091?. and, therefore, the tube walls will not reach an excessive teniperature. In the tube rows 1S and 19 the size ofthe tubes will be again increased in order to provide a sufficient time factor and at the same time prevent back pressure by preventing excessive vapor velocities.v The vapor velocity in this latter section will be considerably decreased since the tubes at this point receive a very low rate of heat transfer by convection from the flue gases, which will be in the neighborhood of 1500o F. The lue gases are then transferred to the outlet duct 6, where they are 'used to heat or vaporize the charging stock passing through the tubes 8, after which the gases may escape to the atmosphere by the o utlet stack 7.

What is claimed is:

1. In a converter for cracking hydrocarbons in the vapor phase, a setting formed with an internal bridge wall which divides the setting into combustion and tube chambers, the latter being in relative open communication over the top of the bridge Wall, burner mechanism arranged in said combustion chamber, an outlet duct in communica.

tion with the lower part of the tube chamber, a bank of drying tubes arranged in the lower part of the tube chamber, means for supplying vaporized oils to said drying tubes, a bank of cracking tubes arranged in said combustion chamber, said cracking tubes possessing a cross-sectional area materially less" than that of the drying tubes, means for passing oil vapor from'the drying tubes to the cracking tubes, and a second bank of cracking tubes arranged in said tube chamber and through which the vaporized oils are passed following their passage through said first named cracking tubes, the cracking tubes in said tube chamber being individually of greater cross-sectional area than the individual cracking tubes in the combustion chamber but `of less cross-sectional area than the individual drying tubes.l

2. In a converter for cracking hydrocarbons in the vapor phase, a settingformed with an internal bridge Wall which divides the setting into combustion and tube chambers, the latter being in relative open communication over the top of the bridge wall, burner mechanism arranged in said combustion chamber, an outlet duct in communication with the lower partiof 'the tube chamber, a bank of drying tubes arranged in the lower ing vaporized oils to said drying tubes, a bank of cracking tubes arranged in saidcombustion chamber, said cracking tubes possessing a cross-sectional area materially less than that of the dryingtubes, means, for passing oil vapor from the drying tubes to the cracking tubes, a second bank of cracking tubes arranged in said tube chamber and through part of the tube chamber, means for supply-V

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