US 3607730 A
Description (OCR text may contain errors)
United States Patent [561 References Cited UNITED STATES PATENTS  Inventor Robert W. Pieiffer Bronxville,N.Y.
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Primary ExaminerHerbert Levine Attorneys-John C. Quinlan and Margareta Le Maire  APPARATUS AND METHOD FOR CONVERSION OF HYDROCARBONS 25 Clalms, 5 Drawlng Figs.
ABSTRACT: An apparatus and method for effecting fluid catalytic cracking of hydrocarbons to products boiling in the  Field of nvonocmaorq FEED 2a STEAM 22 PATENTEU SEPZI um SHEET 2 UP 2 2 M R. 7 HR 3 .nm, flm 9 EN N E 7 E RM W .0 6 T A 8 TI T B m m 79 FIG.4
21 STEAM 774 FIG.5
HYDROCARBON FEED 73 APPARATUS AND METHOD FOR CONVERSION OF HYDROCARBONS This application is a continuation-in-part of pending application Ser. No. 719,052, filed Apr. 5, 1968 now U.S. Pat. No. 3,492,221.
The present invention relates to an apparatus and a method for conversion of hydrocarbons and more specifically to an apparatus and method for fluid catalytic cracking of relatively heavy hydrocarbons to high quality gasoline product.
In recent years, commercial catalytic cracking catalysts have been developed which are highly active and also exhibit superior selectivity towards the formation of desirable products such as gasoline at the expense of coke and light ends production. Examples of such catalysts are those of the types commonly called high alumina and molecular sieve catalysts. It has been found that maximum benefit is derived from these catalysts by reducing the time the catalyst is in contact with the hydrocarbons undergoing cracking in the reaction zone. For this reason, it is preferred to carry out the catalytic cracking operations employing so-called dilute or disperse phase cracking techniques, i.e., the catalyst is contacted with a hydrocarbon feed stream moving through the reaction zone at sufficiently high superficial velocities that the catalyst is carried along in said stream as a dilute suspension and with a minimum of back-mixing.
In general, catalytic cracking of relatively high boiling hydrocarbons to form substantial quantities of materials boiling in the gasoline range is carried out in the following process sequence: hot regenerated catalyst is contacted with preheated hydrocarbon feed in a reaction zone under conditions suitable for cracking, the cracked hydrocarbon vapors are disengaged from the spent catalyst, which is subsequently fed to a stripping zone where it is contacted with a gasiform stripping agent, whereby volatile hydrocarbon material is stripped from the catalyst. The stripped catalyst is then transferred to a regeneration zone where it is regenerated by burning carbonaceous deposits from the catalyst using an oxygen-containing gas such as air, after which the regenerated catalyst is transferred to the reaction zone for reuse. The hydrocarbon material from the reaction zone and the stripping zone is transferred to a recovery system including suitable fractionation equipment for recovery of gaseous products, gasoline and one or more heavier fractions boiling above the gasoline range. The latter fractions may be withdrawn as products of the process or may at least in part be recycled to the reaction zone for further cracking.
It has been found that substantial economies are realized when the regenerator vessel is superimposed in vertical alignment on a vessel containing the reaction zone, the catalyst disengaging zone and the stripping zone. One important advantage of such an arrangement is that the catalyst may be transferred from the regeneration zone into the reaction zone through a straight line vertical standpipe, and similarly, stripped catalyst can be introduced to the regenerator through a straight line vertical riser, thereby avoiding the erosion and/or fluidization problems heretofore resulting from the lateral transport of dense phase catalyst.
Another important advantage of this arrangement is that the regeneration zone can be operated at a lower pressure than the reaction zone, and consequently the cost of construction of the vessel as well as the cost of compression of the oxygencontaining regeneration gas can be minimized. Moreover, due to the relatively higher pressure of the cracked hydrocarbon vapors, the size and cost of the fractionation equipment as well as the compression cost in handling normally gaseous hydrocarbons in the recovery system also are minimized.
There are, however, many problems connected with the design of a dilute phase reaction zone included in the aforementioned apparatus. It is necessary that the vertical standpipe supplying freshly regenerated catalyst from the upper regeneration vessel to the inlet of the reaction zone, usually located in a lower portion of the lower vessel, be rigidly sup ported by either the upper vessel or by the top section of the lower vessel.
A design rigidly connecting the outlet of said standpipe with the inlet to a dilute phase reaction zone conduit, where the latter is supported by the bottom portion of the lower vessel, cannot be tolerated. This is so, because during startup of such a unit, the considerable downward expansion of the standpipe would result in mechanical failure of the rigid connection between said reaction zone and the standpipe.
In copending application Ser. No. 719,052 an apparatus and a method are disclosed, which solve the aforementioned problems. The method is carried out in an apparatus having an upper regeneration vessel, a lower vessel, and a structure forming an open seal well within the lower vessel. A standpipe communicates with the upper vessel and the open seal for transport of hot freshly regenerated catalyst to the seal well and valve means are provided for controlling flow of catalyst through the standpipe into the seal well. A confined elongated transfer-reaction zone located within the lower vessel depends from said seal well and is in open pressure communication therewith. In the seal well a sealing and pressure-developing fluidized dense bed of regenerated catalyst is maintained and the thus developed pressure is employed for transferring catalyst from said seal well through said transfer zone. Hydrocarbon feed is introduced into the confined elongated transfer zone which is maintained under conditions suitable for cracking of said hydrocarbons. Cracked hydrocarbons and spent catalyst are withdrawn from the transfer zone and separated, the spent catalyst is stripped in a stripping zone and then passed to the upper regeneration vessel where it is regenerated.
Although the above described apparatus and method have enjoyed considerable commercial success, these are instances when said apparatus and method are somewhat restricting as to the desired conversion or hydrocarbon feed throughputs. This is particularly so when an existing commercial installation is to be revamped from a conventional dense bed cracking operation carried out in the lower vessel to short contact time dilute phase operations in transfer lines contained entirely within said lower vessel.
The present invention is an improvement over the apparatus and method of application Ser. No, 719,052, in that it provides for conversion of hydrocarbons under conditions which are largely independent of any limitations imposed by the size of the lower vessel. More specifically, the present invention enables hydrocarbons to be cracked employing any desired contact times and hydrocarbon feed throughputs.
In accordance with the present invention the apparatus for the conversion of hydrocarbons comprises an upper vessel; a lower vessel; a structure forming an open seal well supported by said lower vessel; a standpipe in communication with the upper vessel and the open seal well; a confined elongated transfer zone depending from said seal well and partially located outside said lower vessel and having the outlet portion located within said lower vessel in open pressure communication with said seal well; valve means for controlling flow of solid material through said standpipe into said seal well; means for introducing hydrocarbon feed into the confined elongated transfer zone; a solids stripping zone; means for introducing solids from the transfer zone into said stripping zone, and means for transferring solids from the solids stripping zone to the upper vessel.
In accordance with the present invention there is also provided a method for the conversion of hydrocarbons comprising transferring hot, freshly regenerated catalyst of fluidizable particle size from an upper regeneration zone through a standpipe to an open seal well supported by a lower vessel, maintaining said open seal well in open pressure communication with the outlet of a confined elongated transfer zone depending from said seal well and partially located outside the lower vessel, maintaining a sealing and pressure-developing dense bed of fluidized, regenerated catalyst in said open sea] well; employing the thus developed pressure for transferring catalyst from said open seal well through said depending confined elongated transfer zone, concurrently contacting in said transfer zone said catalyst with hydrocarbon feed under conditions suitable for cracking of said hydrocarbon feed, withdrawing cracked hydrocarbons and spent catalyst from the outlet portion of the transfer zone, stripping spent catalyst of strippable hydrocarbons in a stripping zone, recovering cracked and strippable hydrocarbons in a product recovery zone, and regenerating stripped catalyst in the upper regeneration zone by contact with an oxygen-containing gas at temperatures above those employed in the transfer zone.
The upper vessel may be of any design suitable for the regeneration of spent catalyst employing fluidized dense bed techniques and should include means for introduction of oxygen-containing gas into a bottom portion of said vessel, and preferably cyclones located in an upper portion of said vessel for the recovery of entrained solids from the regenerator flue gases and return of said solids to the bed of catalyst undergoing regeneration. The lower vessel, containing a portion of the confined elongated transfer zone, may be provided with means for maintaining a fluidized dense bed of solid material in the bottom portion thereof, said means including inlets for fluidizing gases such as steam, nitrogen or hydrocarbon feed. The lower vessel should preferably be of a sufficient size to permit the installation of cyclones in an upper portion thereof for the purpose of recovering entrained solids from the vapors exiting the lower vessel. The stripping zone or zones can be provided by suitable partitions located within a lower portion of the vessel or as a lesser diameter downward extension of the lower vessel, or as a separate vessel.
Preferably both the upper and the lower vessels are of an essentially cylindrical shape having spherical or semispherical heads. The upper vessel may be directly superimposed on the lower vessel providing a so-called single-head system, or it can be positioned above the lower vessel such that atmospheric air can circulate between the vessels providing what is commonly known as a two-head system. The single-head structure is generally more economical in regard to systems in which the diameter of the regenerator is not more than about feet. Below such diameters, the amount of metal expansion incurred can be reasonably accommodated by using metal thicknesses in the vessel structure which can be fabricated and handled without excessive cost and without incurring unreliable vessel quality which may render the vessel unsuitable. At greater diameters than the above stated, a two-head system is preferably used.
In the single-head" system the support point of a suspended vertical standpipe communicating with the upper and lower vessels is located in the upper portion of the lower vessel, while in the two-head system a suspended standpipe may be supported in a lower portion of the upper vessel or may be supported in the upper portion of the lower vessel. In the latter system a seal between the standpipe and the lower vessel is provided, e.g., by a bellows-type expansion joint which is required between the upper and lower vessels.
In a single-head" system it is possible to provide the section of the standpipe extending from a lower portion of the upper vessel into an upper portion of the lower vessel with an annular space enveloping said section and means for introducing thereto a cooling gas such as low-temperature steam. This arrangement will serve to protect the nozzle located in the common partition through which the standpipe extends, i.e., said nozzle can then be maintained at temperatures considerably lower than that of the upper vessel and that of the solids flowing through the vertical standpipe. Alternately, by employing a considerably larger nozzle in the head, it is possible to provide sufficient insulation thickness to protect the head from the high temperature solids flowing through the standpipe, and thus obviate the requirement for the nozzle cooling gas.
The vertical standpipe is advantageously provided with means for introduction of aeration gas at points spaced throughout the length of the standpipe to promote a smooth and steady flow of solids therethrough.
The geometric configuration of the open seal well structure enveloping this vertical standpipe is not too important as long as there is sufficient space between the walls of the structure and the standpipe to permit a fluidized dense bed of solid material to be maintained in said space; for instance, a horizontal cross section thereof could be a square, rectangle, ellipse, or of any other suitable shape. However, for ease of construction and maintenance it is preferred that at least the upper portion of said well structure be of a generally cylindrical shape with its longitudinal axis coinciding with that of the standpipe. It is not necessary that the entire seal well structure be within the confines of the lower vessel, and in some cases it might be desirable to extend a lower portion of said structure downwards through the bottom of said lower vessel. However, it is essential that the vapor outlet from said seal well be located within the lower vessel. Fluidizing gas inlet means are located in the lower portion of the seal well.
The first portion of the transfer zone immediately subsequent to the seal well and upstream from the hydrocarbon inlet is generally located mostly outside and below the lower vessel and provides for change of flow of dense phase fluidized material from downward to a substantially upward direction. It is preferably comprised of a section depending from said seal well and extending downwardly therefrom, followed by either a return bend or by a lesser bend and an upwardly sloped lateral section. said lesser bend is preferably a short radius bend, i.e., a bend having a configuration such that the radius of curvature R of the bend is related to the diameter D of the pipe by the relationship R/kll). However, other bends may also be employed, e.g., having R/D=l .5.
It is required that the combined heights of the well structure and the section extending downwardly from said well to the bend be at least sufficient to maintain the above-mentioned fluidized dense bed of solid material in said well such that it will develop sufficient pressure to enable circulation of solids at design rates through the transfer zone depending from said well.
For the purpose of insuring operational stability, it is preferred that the first portion of the transfer zone is provided with a plurality of means for introducing fluidizing gas to the section extending downwardly from the seal well and also to the upwardly sloped lateral section, thereby assuring smooth downward flow of catalyst and providing for a reverse seal consisting of a fluidized relatively dense bed of solids in the upwardly sloped section of the first portion of the transfer zone. The hydrocarbon feed is introduced through one or more inlets located at some distance downstream from said latter fluidizing gas inlets. in case of a temporary pressure upset during the operation of the apparatus, the aforementioned reverse seal will prohibit the passage of vaporizing hydrocarbons into the seal well which otherwise might necessitate the discontinuation of operations to reestablish the correct flow patterns.
At the aforementioned hydrocarbon feed inlets, there should be preferably located means for introducing a dispersing medium such as steam. These will also be in use during startup of the apparatus to aid in establishing catalyst circulation prior to introduction of hydrocarbon feed.
The valve means for control of solid material through the standpipe could be a slide valve in, for instance, the portion of the standpipe located between the upper and the lower vessels in a two-head system. Another and more preferred alternative, which can be used in either the single or two-head system, is a plug valve which seats against the lower end of the standpipe, said valve being vertically reciprocable through a bushing in the bottom of the lower vessel. The total differential movement of the standpipe between its support point and the valve seat at its lower extremity, varies the hot and cold positions of the valve seat. This valve is controlled in such a manner that its opening will give the desired flow rate regardless of the movements of its seat due to expansion resulting from change in temperature.
In two-hea system designs using a slide valve for controlling the flow of solids in the vertical standpipe, the open seal well structure may be supported at an upper portion thereof by an adjacent portion of the standpipe or alternatively at the lower vessel head. The preferred embodiment of the invention, whether a plug valve or slide valve is employed for said purpose, includes the rigid support of the open seal well structure by a bottom portion of the lower vessel.
The invention is not limited to one specific design of the elongated confined transfer zone which is depending from the seal well, the only requirement being that the length and volume be sufi'icient to provide adequate contact between the hydrocarbon feed and the solid catalyst to result in the desired conversion of the feed and that the transfer zone be in open pressure communication with the seal well, i.e., the outlet portion of the transfer zone and the vapor outlet from the seal well must be located in the same vessel. It is preferred however that a second portion of the transfer zone immediately downstream from the hydrocarbon feed inlet be substantially vertical to provide for substantially vertical upflow conditions therein. In order to minimize the actual height of the transfer zone, it is advantageous to provide it with one or more devices for changing direction of flow. Erosion due to the flow of highvelocity eroding suspensions through the transfer zone is preferably minimized by an internal erosion resistant lining, which serves as the primary protection for the metal structure. However, erosion will usually be a maximum in the aforementioned devices, and care should be exercised in their design. It has been found that such erosion can be reduced substantially if the changes of direction of flow are about right-angled, e.g., a vertical straight portion of the transfer zone is closely followed by a horizontal straight portion, and if such portions are connected by appropriately designed devices. These devices function well when the aforementioned angle is between about 75 and 105. One suitable design for such a connection is a so-called side-out straight tee, i.e., a tee used as elbow, entering run. The far end of the run is capped and is thus closed to flow. A more preferred design, however, is comprised of a truncated hollow cone, advantageously an oblique cone, extending from one such portion of the transfer conduit, the far end of the cone being its base of larger diameter, a cap covering and closing said larger diameter base, and another such portion of said transfer conduit extending from an outlet in the conical surface in such a way that the projected axes of the respective portions of the transfer conduit are intersecting at angles in the range from about 75 to about 105 In a preferred embodiment of such a device, there is provided between the base of the cone and the cap an extension such as a hollow cylindrical extension of the same diameter as that of the base. The inlet flow which enters through the smaller diameter area of the cone exits through the side outlet provided in the conical surface, such side outlet preferably having the same cross-sectional area as the inlet. The cone design has the advantage over the tee in that sharp edges at changes of direction, which are subject to erosion, have been removed from the direct line of impingement of a major portion of the solids suspension. In both embodiments, the sudden change in direction of the high-velocity suspension causes solids to collect as a relatively dense suspension within the devices, especially in the respective closed portions thereof (i.e., in the far end of the run of the tee, or in the larger diameter area of the cone or in the extension thereof), and said collected solids serve as a protective cushion on which the suspension impinges. The closed end portions of the aforementioned devices can be periodically opened for inspection and maintenance purposes without necessitating dismantling of any part of the transfer-reaction zone.
In addition, the primary structures forming these devices can advantageously be enclosed within secondary protective structures of suitable designs, e.g., offset cylinders, where the annular space between a device and its corresponding secondary protective structure may be filled with an erosion resistant refractory material. Thus, in case of an actual erosion failure in the primary structure forming the device, the secondary protective structure will continue to maintain the suspension within the transfer zone. Plant operations need not be disrupted due to an erosion failure of the primary device, but can be continued for relatively long periods of time before plant shutdown for maintenance becomes necessary. It is to be understood that any other localized area of the transfer zone found to be subject to severe erosion could also be enclosed within secondary protective structures similar to the ones previously described.
More than one standpipe can be included in the design of the apparatus of this invention, each of these standpipes communicating with a seal well with at least one transfer zone depending therefrom, i.e., the scope of the invention includes designs where two or more transfer zones depend from a common seal well. Such designs are useful in the efficient utilization of the space available in a relatively small lower vessel. I-IOwever, the preferred means for the conversion of dissimilar hydrocarbon feeds to be treated at different operating conditions is in separate transfer-reaction zones depending from separate seal wells.
In a system comprised of one standpipe, one seal well and one transfer-reaction zone, the desired temperature of a transfer-reaction zone is maintained by providing it with a temperature controller which preferably is located near the outlet of the transfer-reactor, and which actuates the valve controlling the flow of hot regenerated catalyst through the corresponding standpipe. As the temperature drops below a desired level, the valve is opened to increase the amount of catalyst to be circulated and vice versa. In a system where one standpipe and one seal well serve more than one transfer-reaction zone, the temperature controller which actuates the catalyst flow valve is preferably located in the common disengaging zone in the lower vessel.
The outlet of the confined transfer zone or zones is located within the lower vessel and consequently is in open pressure communication with its corresponding open seal well. The location of said outlets can either be within an upper portion of a stripping zone, or at points adjacent thereto, in which latter cases the stripping zone is advantageously provided with solids inlet means e.g., in the form of slots in the stripping zone walls in the case of an internal stripper or with a solids control valve in the case of an external stripper. Generally, where the lower vessel contains a fluidized dense bed of solids, the outlet should be positioned above the dense bed to minimize overcracking of the hydrocarbons and to promote the rapid separation of the cracked vapors from the spent catalyst in the disengaging space provided by the vapor space of the lower vessel. It has been found that a further improvement in the separation is obtained by providing the outlet portion of the transfer zone with one or more side-discharge distributing apertures or slots as compared with a simple bottom outlet. Furthermore, it is possible by a suitable arrangement of such slots to guide the flow of separated solids in a desired direction and thereby aid in the distribution of such solids. The use of the discharge slots is not limited to disperse phase catalytic cracking zones, but applies generally to any system for transport of gas-solids suspensions and the subsequent separation, guidance and distribution of such solids.
The stripping zone can be of any suitable design and should include means for introducing stripping gas to a bottom portion thereofand preferably baffles to increase the contact between stripping gas and solids.
The apparatus should also include one or more riser conduits extending from a lower portion of the stripping zone into the upper vessel which serves as a regeneration zone, and said risers should be provided with inlets for lift gas to accomplish the upwards transport of solids through said risers.
The operation of the apparatus will be described with reference to one standpipe, one seal well enclosing said standpipe and one transfer-reaction zone. However, it is understood that there can be plurality of standpipes, each having its corresponding seal well and one or more transfer-reaction zones depending therefrom. Thus, hot freshly regenerated catalyst in amounts controlled by the temperature-actuated valve is transferred from the upper vessel serving as a regeneration zone through a standpipe into a corresponding seal well located within the lower vessel and is subsequently introduced into a confined transfer-reaction zone at least a portion thereof being located outside the lower vessel. The catalyst is preferably aerated to maintain it in a fluidized state while flowing through said standpipe. A dense fluidized bed of such regenerated catalyst is maintained in the seal well extending upwards into the annular space formed by the standpipe and the corresponding seal well structure. The height of the sealing bed is dependent on the pressure drop through the transfer-reaction zone and will fluctuate as the hydrocarbon feed rate, catalyst-to-oil ratio, temperature and pressure are varied. Being in a fluidized state, the sealing bed will act as a pressure-developing column and will provide precisely the necessary pressure for transport of the catalyst through the subsequent reaction zone and any subsequent dense-bed reaction zone which may be superimposed on the transfer-reaction zone.
Hydrocarbon feed, preferably preheated, is introduced to the transfer zone where it is contacted with the hot regenerated catalyst flowing therethrough. The contact causes the formation of a dilute phase suspension of catalyst and oil vapors, which moves through the confined transfer zone at high superficial velocities while cracking of the hydrocarbons is taking place. In a preferred embodiment, a dense fluidized bed of regenerated catalyst is maintained in the first portion of the transfer zone upstream from the hydrocarbon feed inlet, said bed serving as a reverse pressure seal to prevent hydrocarbon vapors from entering the open seal well in the event of a pressure upset in the system. The hydrocarbon feed can be fresh feed alone or a mixture of fresh feed and recycle gas oil, recovered from subsequent product treatment zones. In the cases where more than one transfer-reaction zone is employed, dissimilar feeds which require different operating conditions for optimum results, can be treated separately in what is generally called segregated feed cracking, e.g., the fresh hydrocarbon feed and the recycle stream can be cracked in two separate zones. Examples of suitable feeds for treatment in the apparatus of the invention include gas oils, reduced crudes, waxy feeds, etc.
A suspension of cracked hydrocarbons and catalyst exits from the transfer-reaction zone and is advantageously withdrawn therefrom through one or more apertures or slots to promote rapid disengagement of spent catalyst. The spent catalyst exiting from the reaction zone is discharged above a dense fluidized bed of catalyst which is maintained in a bottom portion of the lower vessel. In one embodiment, said latter bed may serve as a true reaction zone by the additional introduction of a stream of hydrocarbons to said bed, and such an arrangement can be used with advantage for converting a hydrocarbon stream which is relatively hard to crack, e.g., a cycle oil which would require relatively longer times for its conversion into lower boiling material.
In another embodiment, no additional hydrocarbons are fed to the aforementioned dense bed, which in this case will serve as at least a first stripping zone. In one specific aspect, the spent catalyst is contacted in such bed with a sufficient quantity of a gaseous material, such as steam or nitrogen, to maintain the bed in a turbulent dense state of such dimensions that there is provided adequate time to strip off the vaporizable hydrocarbons from the catalyst particles. ln another specific aspect, only such quantity of gaseous material is provided to the bed to assure the fluidized movement of particles therein. In this case, the bed performs very little stripping duty but serves mainly as a support much like a baffle to guide the disengaged and rapidly moving spent catalyst from the transferreaction zone into a separate and confined stripping zone located within said lower vessel.
The effluents from the stripping zone and from the reaction zone are preferably combined and subsequently separated from entrained catalyst particles by means of cyclones located in an upper portion of the lower vessel and are thereafter transferred to conventional product treatment zones including fractionation zones for recovery of normally gaseous hydrocarbon products, gasolines product and higher boiling fractions, which can include light and heavy fuel oil fractions and/or recycle gas oil fractions, the latter to be returned to the reaction zone or zones for further cracking.
The stripped catalyst is transported through one or more riser conduits into the upper vessel, where it is regenerated by burning the carbonaceous deposits on the catalyst in an oxygen-containing atmosphere. The transport and regeneration is carried out in any conventional manner and should preferably include use of at least part of the oxygen-containing regeneration gas for elevating the stripped catalyst to the regeneration zone.
The operating conditions employed to achieve catalytic cracking of hydrocarbons according to the invention include regenerator temperatures between about l,l00 F. and about l,500 F. and regenerator dilute phase pressures from about atmospheric pressure to about 35 p.s.i.g. The density of the sealing bed in the seal well structure, is preferably maintained at values ranging from 20 lb./cu.ft. to about 45 lb.lcu.ft. The same range applies to preferred densities of the bed of regenerated catalyst maintained within the first portion of the transfer zone, which generate the reverse pressure seal. The outlet of the disperse phase reaction zone can be operated in a temperature range above 850 F. and preferably between about 925 F. and about l,000 F. Suitable reaction zone pressures are between about 5 p.s.i.g. and 50 p.s.i.g. The relative weights of catalyst and total hydrocarbons flowing through the elongated confined reaction zone, i.e., the so-called catalyst to oil ratio, is preferably maintained at values ranging between about 2 and about 20. The length and volume of the elongated reaction zone should be sufficient to provide contact times therein from about 0.5 second to about 4 seconds or even higher, while the cross-sectional area of said zone is designed to result in superficial velocities of the suspension ranging between about 15 ft./sec. and about 25 ft./sec. in the vicinity of the hydrocarbon feed inlet and between about 20 ftjsec. and about 60 ft./sec. at the reactor outlet. In those cases where additional hydrocarbon feed is introduced to the dense bed located in a bottom portion of the lower vessel, the conditions for this portion of the reaction include, in addition to the above-cited reaction temperatures and pressures, catalyst/oil ratios ranging between about 2 and about 25, preferably between about 5 and 10, and space velocities, i.e., the hourly weight of hydrocarbons fed to the dense bed divided by the weight of said bed, ranging between about 0.25 and I5 and preferably from about 0.5 to about 5.
In the embodiments where no additional hydrocarbon feed is introduced to the bed maintained in the lower portion of the lower vessel, and where said bed functions mainly as means for directing the flow of spent catalyst into a main stripping zone, the superficial velocity of the fluidizing agent in said bed, such as steam or nitrogen, is preferably maintained in a range from about 0.04 ft./sec. to about 0.2 ft./sec.
The operating conditions employed in the stripping of spent catalyst in a stripping zone includes temperatures above about 825 F and preferably between about 900 F. and about 975 F., and pressures ranging between about 10 p.s.i.g. and about 55 p.s.i.g. The amount of stripping medium relative to the catalyst circulation is advantageously maintained between about 1 and about l0 pounds per 1,000 pounds of circulated catalyst. Superficial velocities of the stripping medium are from about 0.5 ft./sec. to about 2.0 ft./sec.
in order to provide a better understanding of the present invention, reference will be had to the accompanying schematic drawings which form a part of this specification.
In the drawings:
FIG. 1 is an elevated view of a specific example of the apparatus of the invention, which includes two separate standpipe-open seal well-transfer conduit systems, only one of said systems being shown. In this example the lower vessel is of sufficient size to accommodate a major portion of each transfer conduit.
FIG. 2 is a sectional view taken on line AA of the FIG. 1 looking downward.
FIG. 3 is a specific example of a device for changing the direction of flow of an erosive disperse phase suspension.
FIG. 4 is an elevated view of another specific example of the apparatus of the invention wherein a large portion of the transfer zone is located outside the lower vessel.
FIG. 5 is a sectional view taken on line B-B of the FIG. 4 looking downward.
It is to be understood that the drawings are only shown in sufficient detail to fully understand the invention and that some portions of the system such as the fluidized dense bed regenerator as well as reaction effluent outlets and subsequent product recovery zone have not been included since those employed are conventional.
The apparatus shown in FIGS. 1 and 2 is a single-head" system comprising an upper vessel 1 containing a regenerator zone (only partially shown in FIG. 1), which is superimposed on and vertically aligned with a lower vessel 2. Two vertical standpipes 3, which are supported by the partition 4, extend downwardly into open seal well structures 6, which are supported by the bottom of the lower vessel 2. The standpipes and their respective open seal wells are located 180 F. apart in the space provided between the central cylindrical stripping zone 7 and the walls of vessel 2. The steady flow of freshly regenerated catalyst through the standpipe, which is aided by the introduction of aeration gases throughout the heights of the standpipes (not shown on the drawing), is controlled by plug valves 8. The catalyst flows into open seal wells 6, where fluidized dense beds of such material are maintained and which extend upwards in the annular spaces 9. The fluidization of said beds is accomplished by the introduction of aeration steam through distributor rings 11 and 12. The bottom portion 10 of each seal well is partitioned into two sections 13 and 14 by means of vertical baffle 16. These baffles serve to reverse the direction of flow of high-velocity dense phase catalyst issuing downwardly out of the standpipes, resulting in low-velocity upward flowing dense phase suspensions in sections 13. Said suspensions then flow over the respective baffles into the second aerated sections 14 of the seal wells. Subsequently the catalyst flows into the first external sections 17 of the transfer-reaction conduits, said first sections originating within the seal wells and depending in a downward direction from the second aerated sections of the open seal wells. Aeration gases are introduced (not shown) throughout the length of the external section of the transfer zone to aid the flow of catalyst therethrough. A change of direction of the flow of catalyst is obtained by means of bend 18, and the catalyst proceeds in a lateral and upward direction through section 19 equipped with aeration gas nozzles 21 to maintain therein fluidized dense bed reverse pressure seals. Steam nozzles 22 are provided for the injection of emergency steam in case of loss of hydrocarbon feed and for introduction of steam to maintain catalyst flow during startup and shutdown. Hydrocarbon feed is introduced by means of an injection nozzle 23, introducing the hydrocarbon vertically into riser conduit 24 contained within the lower vessel. The resulting dilute suspensions of vaporized hydrocarbons and catalyst flow through the vertical riser portions 24 of the transfer-reaction zones, into the crossover portions 26 and subsequently into vertical downcomer portions 27 provided with discharge slots 28. The respective riser-crossover and crossover-downcomer portions are connected by means of side out tees, which are provided with caps 31 at the far ends of their respective runs. All internal metal surfaces of these devices for changing the direction of flow and all connecting conduits thereto, are lined with an erosion resistant refractory. The devices are shielded by the secondary protective structures 32, the annuli formed thereby being filled with erosion resistance refractory material, if desired. The downcomers are closed below the discharge slots by means of plates 33 and are supported in the example by extensions 34 attached to the bottom of vessel 2. However, other methods for closing and support of the transfer conduit can be employed. A barely fluidized dense bed 36 of catalyst is maintained in the bottom portion of the lower vessel 2 and fluidization is maintained by the introduction of steam through the aeration ring 37. The cracked vapor-catalyst suspensions which exit through the discharge slots are rapidly disengaged in the vapor space above bed 38 and the separated catalyst particles are guided along the surface bed 36 into stripper inlet slots 39, where they are contacted with stripping steam provided through distribution rings 41. The stripped catalyst is transported to regeneration zone 1 by means of riser 42 with the lift gas being provided through hollow plug valve 43. The disengaged cracked vapors emanating from discharge slots 28 are combined with the vaporous stripper effluent from stripping zone 7 and subsequently passed to cyclones (not shown) for recovery of entrained catalyst therein. The vapors then exit from (not shown) the lower vessel and are passed to product recovery zones including fractionation zones (not shown).
With the apparatus of FIGS. 1 and 2 it is possible to achieve at least a 25 per cent increase in either contact time or hydrocarbon feed throughput as compared with the apparatus of application Ser. No. 719,052 FIGS. 1 and 2, wherein the transfer-reaction zone is totally enclosed within the lower vessel. Furthermore, there are few maintenance problems associated with the apparatus of this invention by reason of the external locations of many of the aeration gas nozzles and hydrocarbon feed inlet nozzles thus providing easy access thereto if needed.
FIG. 3 depicts an alternate device to a side out" tee employed for changing the direction of the flow of an erosive solids suspension, such as, for instance, the disperse phase hydrocarbon vapor-catalyst suspension in conduits 24, 26 and 27 of FIG. 1. This device is described hereinafter with respect to its connection to a vertical riser conduit 52 and a horizontal crossover conduit 53; however, it can equally well be employed to connect a horizontal crossover with a subsequent vertical downcomer. The device is comprised of a truncated oblique cone, which is connected at its lesser diameter section to riser conduit 52 and at its oblique surface to the crossover section. A cylindrical extension 54 is provided at the larger diameter area of the cone which is capped by means of plate 56 or alternatively by other suitable closures such as a dished or ellipsoidal head. All internal metal surfaces of the cone device, as well as of all connecting conduits are lined with an erosion-resistant refractory material 57, serving as primary protection therefor. A secondary protection is obtained by the enclosure of the device within a cylindrical structure 58, which is filled with additional erosion-resistant refractory material 59. The high-velocity upflowing solids-suspension from riser 52 causes solids to be collected and held in relatively dense suspension within the portion of the device extending above crossover conduit 53, and said solids act as a further protection against erosion in this area as the high-velocity suspension impinges thereon. The advantage of the device of FIG. 3 over the side out" tees in FIGS. 1 and 2 lies primarily in the removal of sharp projections 61 and 62 from the direct line of travel of the high-velocity eroding suspension and thereby considerably reducing erosion at such projections.
The apparatus shown in FIGS. 4 and 5 is a two-head" system wherein a major portion of the transfer zone, including the vertical riser, the crossover portion and part of the downcomer, is located outside the lower vessel. In other respects the apparatus is substantially identical to the one depicted in FIGS. 1 and 2. For simplicity, the description of the features common to both apparatuses have herein been omitted, the numbering of such details being the same as in FIGS. 1 and 2.
The upwardly sloping lateral sections 19 are connected to extemal vertical risers 71 by means of the vertical sections 72.
Expansion joints (not shown) can be provided if required to prevent mechanical failure of the transfer conduit system due to expansion or contraction during startup and shutdown periods. Hydrocarbon feed is introduced by means of nozzles 73 into external risers 71, which extend to a height above that of the lower vessel 2 but below the upper regenerator vessel (not shown on the drawing). The resulting suspensions of vaporized hydrocarbons and catalyst flow through the risers, into the external crossover portions 74 and subsequently into vertical downcomer portions 76. The first sections of said downcomers are located outside the lower vessel. Steam for startup and emergency use is introduced via nozzle 77. Hollow oblique cones 78 having caps 79 connect the riser, crossover and downcomer sections.
With this apparatus design it is possible to achieve any desired contact times and hydrocarbon feed rates, since the lower vessel is only required to house the outlet portions of the transfer zones and the seal wells. The appurtenant equipment such as strippers and cyclones could, if desired, be located externally to the lower vessel.
It is evident that there are many other possible modifications to the apparatus of the invention. For instance, the transfer line zone could include an external vertical riser portion, a partially external crossover portion and an internal downcomer portion, the crossover portion then entering the side of the lower vessel.
Also, it is not necessary when at least two transfer zones are employed, that they be of the same size and configuration, which is particularly the case when dissimilar hydrocarbon feeds are cracked in the apparatus.
What is claimed is:
1. An apparatus for the conversion of hydrocarbons, which comprises:
an upper vessel;
a lower vessel;
a structure forming an open seal well supported by said lower vessel;
a standpipe in communication with the upper vessel and the open seal well;
a confined elongated transfer zone depending from said seal well and partially located outside said lower vessel and having the outlet portion located within said lower vessel in open pressure communication with said seal well;
valve means for controlling flow of solid material through said standpipe into said seal well;
means for introducing hydrocarbon feed into the confined elongated transfer zone;
a solids stripping zone;
means for introducing solids from the transfer zone into said stripping zone, and
means for transferring solids from the solids stripping zone to the upper vessel.
2. An apparatus according to claim 1 wherein a bottom portion of said well is provided with fluidizing gas inlet means.
3. An apparatus according to claim 1 in which a vertical baftie is provided in the lower portion of the open sea] well structure, dividing said structure into two sections.
4. An apparatus according to claim 1 wherein means are provided to maintain a reverse pressure seal within a first portion of said elongated transfer zone and wherein the hydrocarbon inlet means are located downstream from said reverse pressure seal.
5. An apparatus according to claim 1 wherein a last portion of said transfer zone is an apertured conduit.
6. An apparatus according to claim 1 wherein said valve means is a temperature actuated plug valve located at the outlet from the standpipe and responsive to temperature fluctuations within the transfer zone.
7. An apparatus according to claim 1 wherein fluidizing medium inlet means are provided in a bottom portion of said lower vessel.
8. An apparatus according to claim 4 wherein means are provided for introducing fluidizing gas to said first portion of the transfer zone.
9. An apparatus according to claim 4 in which the first portion of said transfer zone is comprised of a conduit forming about a bend, one leg thereof depending downwardly from said seal well.
10. An apparatus according to claim 4 in which the first portion of said transfer zone is comprised of a first section depending from said seal well and extending downwardly therefrom, a second section forming a bend and a third upwardly sloping lateral section.
11. An apparatus according to claim 4 wherein a second portion of said transfer zone is a substantially vertical riser conduit.
12. An apparatus according to claim 5 wherein the stripping zone is located within the lower vessel adjacent to said last portion of the transfer zone.
13. An apparatus according to claim 10 wherein said second section of the first portion of the transfer zone is a short radius bend.
14. An apparatus according to claim 11 in which the transfer conduit is comprised of the following additional portions:
a first device for changing the direction of flow of material exiting the second substantially vertical riser portion;
a third and substantially horizontal crossover portion;
a second device for changing the direction of flow of material exiting the third portion, and
a fourth and substantially vertical downcomer portion.
15. An apparatus according to claim 14 in which said devices are spatially enclosed within protective structures.
16. An apparatus according to claim 14 in which the vertical riser portion is located outside the lower vessel.
17. An apparatus according to claim 15 wherein the spaces between said devices and their respective protective structures are filled with an erosion-resistant refractory material.
18. An apparatus according to claim 16 in which the horizontal crossover portion is located at least partially outside the lower vessel.
19. An apparatus for the conversion of hydrocarbons which comprises:
an upper vessel;
a lower vessel disposed in vertical alignment with said upper vessel;
a structure forming an open seal well within said lower vessel and rigidly supported by a bottom portion of said lower vessel;
a suspended vertical standpipe in communication with the upper and the lower vessels and extending downwardly into the open seal well;
a plug valve located at the outlet from the vertical standpipe for control of flow therethrough;
first means for introducing a fluidizing medium into a bottom portion of said well;
a confined elongated transfer zone depending from said seal well and partially located outside said lower vessel and having the outlet portion located within said lower vessel in open pressure communication with said seal well;
second means for introducing a fluidizing medium into said first portion of the transfer zone;
at least one hydrocarbon feed inlet to said transfer zone downstream from said second fluidizing medium inlet means;
a solids stripping zone;
means for introducing solids from the transfer zone into the stripping zone, and
at least one riser conduit for the transfer of solids from the stripping zone to the upper vessel.
20. A method for the conversion of hydrocarbons comprising:
transferring hot, freshly regenerated catalyst of fluidizable particle size from an upper regeneration zone through a standpipe to an open seal well supported by a lower vessel;
maintaining said open seal well in open pressure communication with the outlet of a confined elongated transfer zone, said transfer zone depending from said seal well and being located partially outside the lower vessel;
maintaining a sealing and pressure-developing dense bed of peratures above those employed in the transfer zone.
21. A method according to claim 20 wherein a reverse pressure seal comprised of a second dense fluidized bed of regenerated catalyst is maintained in a first portion of the elongated confined transfer zone.
22. A method according to claim 20 wherein two transfer zones are employed and wherein dissimilar hydrocarbon feeds are contacted with catalyst in the respective transfer zones.
23. A method according to claim 20 wherein a dense fluidized bed of catalyst is maintained in a bottom portion of the lower vessel.
24. A method according to claim 20 wherein the cracked hydrocarbons and spent catalyst are withdrawn through discharge slots provided in said outlet portion.
25. A method according to claim 23 wherein a second hydrocarbon feed is contacted with said dense fluidized bed within the bottom portion of the lower vessel under conditions suitable for cracking of said second hydrocarbon feed.