US2668793A - Apparatus for vapor phase conversion of hydrocarbons at constant temperatures - Google Patents

Description

Feb 9, 1954 w. w. HOLLAND APPARATUS POR VAPOR PHASE CONVERSION OR HYDROCARBONS AT CONSTANT TEMPERATURES Filed Oct. 25. 1948 Patented Feb. 9, 1954 APPARATUS FOR VALOR PHASE CONVER- SION OF HYDROCARBONS AT CONSTANT TEMEERATURES William W. Holland, Baltimore, Md., assignor to The Gyro Process Company, Detroit, Mich., a corporation of Michigan Application October 23, 1948, Serial No. 56,208

(Cl. ISG- 116) Claims.

This invention relates to the conversion of hydrocarbons in the vapor phase, and more particularly to the conversion of liquid hydrocarbons for the production of high anti-knock motor fuel. This invention is directed specincally to the pyrolytic decomposition of hydrocarbons at the higher temperatures employed in vapor phase cracking wherein the temperature is maintained at a substantially constant level during the cracking interval, thereby avoiding the production of an excessive amount oi fixed gas.

When cracking hydrocarbons in the liquid phase temperatures of the order of 750 to 850 are usually employed for the conversion ci' virgin charging stocks, such as reduced crude and gas oil, into lighter hydrocarbons Within the gasoline boiling range; but when cracking the more refractory hydrocarbons, such as kerosene or the socalled cycle stocks, that is, relatively light petroleum fractions which have previously been subjected to cracking conditions, or when reforming virgin gasoline to improve its anti-knock properties, temperatures of 900 to 10D0 F. may be required to effect the desired degree of decomposition. In either case, however, the production of coke and fixed gas increase with the rise in temperature. It is customary therefore in thermal liquid phase cracking to operate at relatively low temperatures in order to avoid an excessive production of coke and gas and to permit continuous operation over extended periods of time.

When cracking in the vapor phase, considerably higher temperatures are required than those employed in liquid phase cracking, but the reaction time is very much shorter. While the pyrolytic decomposition of hydrocarbons in the liquid phase requires several minutes, the vapor phase reaction is completed within a few seconds. 'If the correct time-temperature relationship is used in each case and if the temperature is maintained at a reasonably constant level during the decomposition interval, it is apparent that comparable amounts of gas should be produced. But While thermal liquid phase cracking is conducted on a constant temperature basis, vapor phase cracking is carried out under constantly rising temperature conditions, which promotes gas production.

According to current practice, temperatures of about 1100 to 1350o F. are employed in vapor phase cracking, depending upon the refractory nature of the charging stock. Dried and preheated hydrocarbon vapors en'ter a conversion coil at or about the temperature at which incipient cracking begins and are heated rapidly and progressively thereafter during the course of their passage through the coil, reaching the maximum temperature only at the point of emergence from the coil, after which they are quenched. At no other point in the conversion coil is the temperature arrested and maintained at a constant level for the required time to complete the reaction. Under such conditions, uniform decomposition of the hydrocarbon vapors is impossible, and since the vapors are heated progressively to increasingly higher temperatures throughout the coil, it is obvious that both undercracking and overcracking must occur at certain points within the coil. The overcracked portion of the vapor is responsible for the high gas production, while the undercracked portion detracts from the gasoline yield. While soaking drums are used successfully in liquid phase cracking to provide the requisite reaction time, they have not proven effective in vapor phase cracking because of the short time element and the difliculty of maintaining a uniform temperature throughout a large body of highly heated vaporous hydrocarbons.

Considerably more gas is produced in vapor phase cracking than in the liquid phase conversion of hydrocarbons, which is reflected in a lower gasoline yield unless additional steps are taken to recover liquid fractions from the gas, such as, for example, compression, absorption. polymerization and the like. This practice has been followed heretofore in order to obtain a yield of motor fuel comparable with that from liquid phase cracking, and at the same time derive the advantages afforded by the superior antiknock motor fuel properties of the vapor phase cracked product. There is, without question, a very notable difference in this respect between liquid and vapor phase cracked gasoline, which difference is more conspicuous in actual power delivery in road tests than laboratory block tests seems to indicate. This is due apparently to the presence of certain cyclic and olefinic molecular configurations not found in liquid phase cracked motor fuels.

While the high rate of xed gas production has probably been the major deterrent in the more extensive use of vapor phase cracking for the production of motor fuel, it has never received the attention directed to less important features of the process. Persistent efforts have been directed to various means of recovering liquid fractions from the gas, rather than means of retarding the gas production itself. The opinion that high temperatures and high gas production are in effect synonymous with respect to crackingoperations seems to have been generally accepted by oil reners and technical experts alike, but such is not the case. Therevis, however, some justication for such an opinion in View of the methods heretofore employed in determining the time-temperature relationship in the vapor phase conversion o1" hydrocarbons, wherein the vapors have never for an instant been' maintained under constant temperature conditions at any' stage of the decomposition reaction. This explains the high gas production and the presence of the highly unsaturated compunds, particularly di; olens, in the cracked distillate which render its treatment by conventional methods diiucult.

This invention, therefore, is directed particiularly to the employment of heating equipment and a process of operation wherein preheated hydrocarbon vapors are rapidly heated to the vpredetermined decmpo'sition temperature 'and are thereafter maintained at a constant'temperature for a suitable time interval to effect 'their uniform decomposition without the production of an excessive amount of fixed gas. Under such conditions, the vapors no't only are never exposed t 'the high peakk temperatures productive of gas formation, but the actual cracking temperature is appreciably lower than the temperatures heretofore employed in vapor phase cracking. As a r'iatte'r of fact, according to :current practice, the temperature of the cracked vapors leaving the conversion coil is about 10D degrees or more above that required for the decomposition of the hydrocarbons under constant ltemperature conditions when a slightly longer time element is used. it the inilder operating temperature an appreciably higher yield of liquid products is obtained, the

Agas production is'greatly reduced and the distillate responds 'more 'readily to conventional methods of treatment, while the character of the distillate 'with respect 'to' its anti-knock propertics appears t undergo Il' change whatsoever.

'rho latter is to vhe expected, however, when one considers that the decomposition temperature of vthe hydrocarbon vapors4 is still about 150 to 200 F. above the temperatures vempioyou in liquid phase cracking, *andy/that certain molecular rearrangements talee plats Yatths highsrtompeia- "temperatures throughout the `conversion reaction,

ths 'aylih'g the high peak temperatures heal 'the end 'f th rc'actiri which 'are responsible vfor the high vgas production which always b'h associated with vapor phase cracking.Y

` n Another` object of this ihvehtiohis the piokductionl of` a cracked distillate which may los treated by the Conventional acid-'alkali or ullervs earth cohtaot methods without undue losses in volume or anti-knock properties. y

Other objects of this invention "will A'become apparent in the following description of the process and equipment as shown in the accompanying drawing dforming a part of this specification,

wherein;

Fig. 1 is a diagrammatic View, partly in side elevation and partly in vertical'se'c'tion, of ap-p'aratus suitable 'for Ycarrying into eiiect the ypresent -invention;

Fig. 2 is a vertical transversel sectional View taken through the furnace unit of the apparatus on the plane indicated by the line 2 2 of Fig. l, disclosing the radiant heat sections or zones of the furnace unitn; n K Y Fig. 3 isa similar view on the plane indicated by the line 3 3 of'Fig. 1 and illustrating the convection heat sections or zones of the furnace unit;

Fig. 4 is .a View similar to Fig. 2 but disclosing 'a modied form of furnace unit.

Referringv .more particularly to the drawing, thev numeral ,i `Vrepresents a feed pump which delivers'a liquid hydrocarbon charging stock from a bulk supply tank` (not shown) through a line 2 to a vaporizing coil 3, located preferably in the convection section 4 of the furnace 5, wherein the charge is converted for the most part into the Y vapor state by the waste heat gases from the radiant section E. Leaving the vapolizing coil vthrough line' 1, the mixture of vaporized and unvmixed with steam, are then directed toa second coil Il, preferably located in the higherk telnperature upper zone of the convection section of the furnace, wherein any entrained liquid particles carried along mechanically by the rapidly moving vapor stream are converted into the vaporous state, and wherein the total volume 0f vaporized hydrocarbons are dried andpreheated to a temperature at about which incipient conversion begins. Y

The preheated vapors then pass into the conversion zone proper which rconsists pl'e'ferablyof a bank of roof tubes Vl2 located at least partially in the highest temperature 'Zone 13, Fig'. y2, of 'the radiant section of the fui-nace wherein they 'are 'exposed to intense heating conditions in order to bring them fully up to the 'predetermined conversion temperature in the shortest possible time. Since this requires but 'an instant in 'View of the comparatively small temperature ris necessary Yand because of the high velocity at vthis point,

Ythe vapors how having Vieachei'i their1 maximum velocity, little decomposition takes place at this stage' of th'e operation. Having Vbeen -r'ai'sed to the desired cracking temperature, the vapors then pass through amildei temperature zone It 'wherein they are maintained at a constant temperature for the predetermined time to complete 'the cohver'sionre'action, the respective heating zones 'being cred independently by the burners "it Yfor zone i3 and Yit, lli and ls''or zone it.

The` constant temperature zone may he divided, ifV desired, into a plurality of compartments 19, 2o and 2l, heated independently by 'the burners Y2,2, 23 jahd'zl, respectively, as shown `"ih Fig. '4. This affords more uniform temperature control ofthe vapors passingthi-oiigh the toil 'than caused by lateralconvection 'currents contacting th'e individual tubes of the conversion coil also 'may be reduced to a ihihimum'ih rthis manner. Furthermore, a degreo'of flexibility is added to the heating system in that'the compartment; I9 may be used either as a high temperature or mild heating zone, an alternative which may be desirable under some conditions, such as when processing highly refractory charging stocks.

The dividing Walls 25, 26 and 21, which may be constructed of any suitable refractory material, extend upwardly from the floor of the radiant section of the furnace to Within a few inches of the bottom of the conversion coil tubes. superimposed upon the top of the walls are refractory metal strips 28, 29 and 3U, extending to the roof of the furnace and adapted to fit edgewise between the individual tubes of the coil with sufficient clearance to avoid overheating the tubes. The dividing walls also extend from the front wall of the furnace to the bridge wall 3|, their position in the radiant section being indicated by the dotted line 32, While the refractory metal strips extend from thefront wall to the back wall of the furnace, as shown by the dotted line 33.

This method of controlling the conversion coil temperatures provides effective means of avoiding the present practice of progressively heating the hydrocarbon vapors to increasingly higher temperatures throughout the conversion period, and particularly the excessively high peak temperatures reached at the end of the decomposition reaction, which high temperatures heretofore have been responsible for the excessive gas production when cracking hydrocarbons in the vapor phase.

The decomposed vapors leaving the conversion coil through tube 34 are conveyed by line 35 to a temperature arrester 36 and are thereafter subjected to known refining methods which consist essentially in first quenching the vapors by direct contact with a clean liquid cooling agent produced within the system to a temperature below that at which further cracking can take place, usually within the range of 600 to '700 F. The partially cooled light and heavy fractions, together with some finely divided coke, leave the arrester by way of line 31 and enter a sep- 1 arating column 38 where separation is effected with the aid of steam under partial pressure conditions, the heavy tarry materials and suspended coke particles being withdrawn from the bottom of the column through the valved line f ing range and heavier fractions which may not have been completely decomposed in the conversion operation. The latter, which is known as cycle stock, is withdrawn from the bottom of the fractionator through the valved line 42 and disposed of as the market conditions may dictate. It may be returned to the feed stock for further cracking in order to increase the gasoline yield, although having passed through the system it is more refractory than virgin stock and yields less readily to cracking conditions, or it may be further refined for domestic fuel or other suitable purposes.

In order to secure a clean, carbon-free and entirely satisfactory liquid fraction for the quenching operation in the temperature arrester 36, a side stream is withdrawn from an intermediate point in the fractionating tower through line 43 and cooler 44 which is collected in the working tank 45 from which a portion is delivered by pump 4s through line 47 to the top of the arrester as a quenching medium. Another portion may be withdrawn from the tank by pump 4B and delivered through line 49 to the mid-section of the tower as a cooling agent if desired.

The decomposed hydrocarbon vapors and gas leaving the top of the fractionating tower through line 50 pass through a cooler' 5| and are collected in a gas-liquid separator 52. A portion of the condensate is returned to the top of the tower by pump 53 through line 54 as a temperature control medium, while the remainder of the condensate is withdrawn from the bottom of the separator through line 55, controlled by valve 56, and delivered to the motor fuel treating system, the uncondensed gas leaving the top of the separator through the valved line 5l going to the gas separating and recovery system.

The conversion of hydrocarbons in the vapor phase under constant temperature conditions as above described permits of considerably longer continuous operating periods without the necesi. sity of shutting down for cleaning purposes, due

largely to the limited coke production because of the milder temperature required.

Iclaim:

l. A furnace for converting hydrocarbon oils comprising a furnace setting composed of vertical front, back and side Walls and horziontal floor and roof walls, said walls providing a heatconfining enclosure, a bridge wall extending vertically and transversely of the setting between o said front and back walls and dividing said enclosure internally to form spaced combustion and convection chambers, the bridge wall terminating below and in spaced relation from the roof wall of the setting to provide a space in the upper part of the setting establishing communication between said chambers for the flow of burner gases, a partition wall arranged vertically and longitudinally of said furnace between and in spaced parallel relation with the side walls thereof to form a plurality of independent vertically disposed radiant heat compartments in said combustion chamber, said partition Wall extending from the front wall of said setting to said bridge wall and from the iloor wall to substantially the roof wall of said setting, burner means situated in the lower part of each of said compartments for maintaining independently regulatable temperatures therein, said convection chamber being provided in the bottom thereof with a waste gas outlet, a pipe coil disposed in said convection chamber for vaporizing oil passed therethrough by heat derived from burner gases discharged from said compartments, an oil conversion coil composed of a plurality of parallel tubular elements arranged in the top of said furnace setting adjacent to said roof Wall and in each of said compartments for serial flow of oil vapor therethrough, and means for conducting vaporized oils from the pipe coil in said convection chamber to said conversion coil, the said parallel tubular elements of the conversion coil being independently heated in each compartment to produce a regulatable temperature in the tubes of each compartment independent of that in another compartment.

2. A furnace for converting hydrocarbon oils as defined in claim 1, and wherein the furnace is provided internally with a plurality of said vertically arranged partition walls, the latter extending longitudinally of the furnace between the side walls thereof to form at least three independent vertically disposed compartments in said combustion chamber, and burner means for heating each of said compartments.

3. A furnace for converting hydrocarbon oils as set forth in claim 1, and wherein the partition wall arranged vertically .and longitudinally of the furnacebetween the side walls thereof to form a plurality of independent vertically disposed compartments in the combustion chamber of the furnace is provided at the tcp thereof with a longitudinally extending refractory metal strip, said strip being positioned between the upper edge surface of the partition wall and the adjoining lower surface of the roof wall of the furnace setting and disposed in the horizontal plane of the oil conversion coil arranged in the top of said furnace setting, the said metallic strip extending from the front to the back wall of said furnace setting at the top of the combustion and convection chambers.

4. In apparatus for the conversion of relatively high-boiling hydrocarbons into lower boiling hydrocarbons comprising: a furnace having walls forming a radiant heat zone and a convection carbons from said primary coil are passed, a

converter coil composed of a plurality of longitudinally extending parallel and serially joined tubular elements, the latter being disposed in the top of said furnace above said bridge Wall in registry with the radiant and convectionV heat zones of the furnace, a vertically disposed imperforate longitudinally extending partition wall arranged in said furnace and extending longitudinally of said furnace from a front wall thereof 'to said bridge wall, .said partition wall dividing said radiant zone into a plurality of separate compartments extending from the bottom to the top of the radiant zone, and burner means in each of said compartments for maintaining independently regulable temperatures therein, the tubular elements of said converter coil being disposed in all of said compartments and independently heated in each compartment to produce a regulatable temperature in the tubes of each compartment independent of that inv another compartment.

5. In apparatus for the conversion of relatively higher boiling hydrocarbons into lower boiling hydrocarbons, as dened in claim 4, and further characterized by providing said radiantl zone with a plurality of spaced partition walls defining i11- dependent fuel combustion chambers, each of said chambers containing said burner means, at least one of said combustion chambers possessing a greater width than the remaining combustion chambers.

WILLIAM W. HOLLAND.

References Cited in the file of this patent UNTTED STATES PATENTS

Claims (1)

1. A FURNACE FOR CONVERTING HYDROCARBON OILS COMPRISING A FURNACE SETTING COMPOSED OF VERTICAL FRONT, BACK AND SIDE WALLS AND HORIZONTAL FLOOR AND ROOF WALLS, SAID WALLS PROVIDING A HEATCONFINING ENCLOSURE, A BRIDGE WALL EXTENDING VERTICALLY AND TRANSVERSELY OF THE SETTING BETWEEN SAID FRONT AND BACK WALLS AND DIVIDING SAID ENCLOSURE INTERNALLY TO FORM SPACED COMBUSTION AND CONVECTION CHAMBERS, THE BRIDGE WALL TERMINATING BELOW AND IN SPACED RELATION FROM THE ROOF WALL OF THE SETTING TO PROVIDE A SPACE IN THE UPPER PART OF THE SETTING ESTABLISHING COMMUNICATION BETWEEN SAID CHAMBERS FOR THE FLOW OF BURNER GASES, A PARTITION WALL ARRANGED VERTICALLY AND LONGITUDINALLY OF SAID FURNACE BETWEEN AND IN SPACED PARALLEL RELATION WITH THE SIDE WALLS THEREOF TO FORM A PLURALITY OF INDEPENDENT VERTICALLY DISPOSED RADIANT HEAT COMPARTMENTS IN SAID COMBUSTION CHAMBER, SAID PARTITION WALL EXTENDING FROM THE FRONT WALL OF SAID SETTING TO SAID BRIDGE WALL AND FROM THR FLOOR WALL TO SUBSTANTIALLY THE ROOF WALL OF SAID SETTING, BURNER MEANS SITUATED IN THE LOWER PART OF EACH OF SAID COMPARTMENTS FOR MAINTAINING INDEPENDENTLY REGULATABLE TEMPERATURES THEREIN, SAID CONVECTION CHAMBER BEING PROVIDED IN THE BOTTOM THEREOF WITH A WASTE GAS OULET, A PIPE COIL DISPOSED IN SAID CONVECTION CHAMBER FOR VAPORIZING OIL PASSED THERETHROUGH BY HEAT DERIVED FROM BURNER GASES DISCHARGED FROM SAID COMPARTMENTS, AN OIL CONVERSION COIL COMPOSED OF A PLURALITY OF PARALLEL TUBULAR ELEMENTS ARRANGED IN THE TOP OF SAID FURNACE SETTING ADJACENT TO SAID ROOF WALL AND IN EACH OF SAID COMPARTMENTS FOR SERIAL FLOW OF OIL VAPOR THERETHROUGH, AND MEANS FOR CONDUCTING VAPORIZED OILS FROM THE PIPE COIL IN SAID CONVECTION CHAMBER TO SAID CONVERSION COIL, THE SAID PARALLEL TUBULAR ELEMENTS OF THE CONVERSION COIL BEING INDEPENDENTLY HEATED IN EACH COMPARTMENT TO PRODUCE A REGULATABLE TEMPERATURE IN THE TUBES OF EACH COMPARTMENT INDEPENDENT OF THAT IN ANOTHER COMPARTMENT.

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