US2878076A - Transportation of fluent solid particles - Google Patents

Description

March 17, 1959 T. H. MlLLiKEN, JR

TRANSPORTATION OF FLUENT SOLID PARTICLES Original Filed April 28, .1950

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AGENT I TRANSPORTATION or FLUENT SOLID PARTICLES ThomasH. Milliken,'Jr., Moylan, Pa., assignor to Houdry Process Corporation, Wilmington, Del., a corporation of Delaware Original application April 28, 1950, Serial No. 158,658. Divided and this application October 25, 1954, Serial v No. 464,315

- Claims.. (Cl. 302--53) The present invention relates to systems in which fluent solid particles or granules of contact material are continuously circulated through a downflow path and an upflow path, the contact material being contacted in at least one ,zone in the downflow path with a process gas and thereafter elevated, conveyed or transported through theupflow path for recirculation through the downflow path. Such systems are particularly adapted to the requirements of processes commonly employed in the chemical processing and oil refining industries. Two types of suchsystems are used in the latter industry and are characterized, among other differences, by the type of flow of solids through a process zone, better known as fluid or as moving bed processes. The granular contact material may be a porous or a fused solid and may comprise or consist of a catalyst or catalytically inert material, the latter being used for heat exchange, absorption or fractionation of gases.

7 The present invention involves the moving bed type of process in which relatively large particles or granules, such as sized particles, pellets, formed spheres and the like ranging in size from about 0.05 to 0.5 inch in diameter, flow principally or solely by the influence of gravity through a downflow path or paths of a system in which there are one or more process zones in such downflow paths. The gravitational flow of solids through such process zones is principally or solely as compact downwardly moving non-turbulent beds; a method which is discussed generally in The T. C. C. Catalytic Cracking Process'for Motor Gasoline Production by R. H. Newton, G. S. Dunhamand T. P. Simpson, Transactions of the American Institute of Chemical Engineers, volume 4 l'," p'age 215, April 25, 1945, and the articles there cited. '1 'In such'a moving bed type of system, it has been found advantageous to transport the fluent solid particles (i. e., contactmatcrial) by one or more pneumatic or gas lifts which serve as an upflow path or paths. The system may comprise .a single downflow path in which there are several'proc'e'ss zones at diiferent heights through which zones the contact material flows consecutively so that the solid particles 'need be transported by the gas lift only once in a complete cycle of operation or the system may comprise a plurality of gas lifts and/or downflow paths each containing a single process zone. An exemplary system of the former type has been described in an article entitled,

Houdriflow: New Design in Catalytic Cracking, appearing in: the Oil and Gas Journal, page 78, January 13, 1949. I

i .lnf such systems, the use of a gas lift, instead of the mechanical elevators formely employed, produces many advantages both as to control of the processing variables.

(operating conditions) and as to efiiciency of operation (including cost and maintenance) but, at the same time, creates ne'wproblems. One of the chief problems in the operation of gas lifts ofthe type'referred to above is that ofja'chi'eving simultaneously a high rateof circulation of solid particles and easy and rapid return of solid particles;

from the upflow path to the downflow path, while mainp I well. as operating with about 20 to 25 percent less elevat 2,878,076 Patented Mar. 17, 195? taining a low' rate of attrition or breakage of the solid:

top of the lift, in which vessel the particles reverse flowi and are collected for flow through the downflow path, must be tall. Since such a vessel can be, and frequently is, used for disengaging elevating gas from the solid particles, it is commonly referred to as the disengaging vessel,

hopper or zone. Because the disengaging vessel is at the top of the gas lift, which may be from to 200v or more feet high, the support and bracing of such a vessel presents a considerable problem.

In order to determine the necessary height of such vessels, the height of rise of particles from a gas lift was investigated. The investigation showed that a streamof'; particles and elevating gas emerged from the top of the,

lift as a jet which continued for about 5 pipe diameters (of the lift) before the elevating gas spread out into the,

surrounding space. However, even after the gas has dissipated, the particles still have momentum and hence continue upward until slowed down by the force of gravity and the drag of the surrounding gas. In a specific system;

in which particles of approximately 0.1 inch in average diameter and air at atmospheric temperature and pressure emerged from a lift pipe 12 inches in diameter, the particles traveling at a rate of 40 feet per second (herein abbreviated to ft./sec.), it was shown that the maximum height of rise is about 20 feet. In order to reduce the height of rise to a more practical value, for example 10 feet, it is necessary to reduce the emergent velocity of the particles (i. e., to about 20 ft./sec.). (When velocities in the lift are referred to herein, it is on the basis that such velocities are the same whether the particle is near the middle or the periphery of the lift pipe, high speed movies having proved that this basis is substantially correct.) However, such a reduction in particle velocity would reduce the rate of particle elevation or circulation by approximately 50 percent (other conditions, including particle concentration, remaining the same) and thus re-f sult in a considerable decrease in the efficiency of opera tion of the gas lift. Obviously, the momentum of the particle cannot be reduced by impact because of the resultant breakage or attrition of the particles.

In accordance with the present invention, gas lifts of' the type herein described are constructed and operated so that the particles travel along the lower (and major) portion of the upflow path or gas lift at substantial velocities, smoothly decelerate in the upper (and minor) por- I tion of the upflow path and thereafter continuously dischargefrom the top of said path into the disengaging zone surrounding said top at low velocity.

When gas lifts are constructed and operated in accordance with the present invention, as described in detail below, there is less particle attrition for the same rate of particle circulation through the lift and the disengaging vessel or zone at the top of the lift is smaller,

more efficient and less costly. Moreover, lifts constructed and operated according to thepresentinvention have several processing advantages over lifts not so constructed in that the former type operate at a lower pres- Sure P, uch as as much as ten percent, than the lat-"' ter the Same Weight fate of Particular'transfer or circulation. and same emergent velocity from the lift) ing gas for the same weight rate of particle circulation.

Additional advantages are set forth below in connection with the discussion of thedrawings.

The invention is especially applicable to systems involving the catalytic conversion of petroleum, and for the purpose of illustrating the invention, it will be described hereinafter in connection with a catalytic cracking system for the conversion. of high boiling hydrocarbons to motor gasoline. Such a system is set forth in detail below in connection with the description of the drawings in which:

Figure 1 is a schematic generalized representation of a typical system in which the present invention may be used.

Figure 2 is an enlarged view of the gas lift employed in Figure 1.

Figure 3 is a cross sectional view of the gas lift in Figure 2 taken along the line 3-3.

In Figure l is shown a typical embodiment of the type of system with which the present invention is concerned. Since the invention is directed primarily to the upflow portion of the system and since the operation and construction of the reactor and regenerator or kiln of the typical system shown in Figure l are adequately described in the aforementioned article appearing in the Oil and Gas Journal, detailed illustration and description of the latter have been omitted for the sake of brevity. It is to be understood that various arrangements of kiln and reactor may be employed in connection with the present invention, as for example arrangements shown in the Houdry Pioneer, vol. 5, No. 1, February 1950.

As indicated by the labelled parts in Figure 1, relatively large particles of solid cracking catalyst, such as particles of between about 1 to and preferably about 2 to 8 millimeters in diameter, flow downwardly through a reactor or cracking zone as a downwardly moving compact non-turbulent bed and contact hydrocarbons with the resultant formation of cracked products (synthetic crude); and are transferred through a conduit or seal leg to a regencrator, kiln or regenerating zone in which the coke deposited on the particles of catalyst in the cracking zone is removed. Compositions effective as hydrocarbon cracking catalysts (typically natural or synthetic aluminosilicates) and the conditions in the reactor and kiln are Well known to the art and need not be repeated here.

Catalyst particles are withdrawn from the regenerator and flow downwardly in a conduit as a compact nonturbulent column to a gas lift inlet chamber, supply hopper or particle supply zone indicated generally at 10 surrounding the bottom of a gas lift, and are transported, lifted" or elevated vertically upward as a continuous stream of solid particles by a transporting, elevating or lifting gas introduced at a substantial pressure to inlet chamber 10 by conduits 11 and/or 12, the particles of contact material passing upwardly through an elongated vertical cylindrical passageway or conduit 13 to a closed housing, vessel, hopper or separator surrounding the top of lift 13 and indicated generally at 14, which vessel comprises a disengaging zone. The disengaged lifting gas is removed from vessel 14, as from the top thereof by conduit 15. If desirable, disengaged cases may then pass to a cyclone separator 16, in which entrained fine particles of catalyst are separated from the lifting gas.

Gas, freed of fine particles, is removed from the top of cyclone 16 by conduit 17 while the separated fine particles are removed from the bottom of cyclone 16 through conduit 18, thereafter passing to a bin (not shown).

Since the solid particles are subjected to grinding and abrasion during circulation through the system, the fine particles so formed are preferably continuously removed, as by entraining fine particles in the gas sent to the cyclone shown and/or. by use of an elutriator such as that disclosed in U. S. Patent No. 2,423,813 issued on July 8,

4 1947 to C. H. Lechthaler et a1. At any event, it is desirable to. process the mass of. particles flowing in the system so as to leave a range of particle sizes such that a gas can be passed countercurrently through a downwardly moving compact mass of particles at a pressure drop up to about 4 to 8 inchesof Water per foot of mass depth. A fairly narrow range of particle size, is preferably maintained both for the purpose of insuring flow of gas through compact beds of the particles at practical pressure drops and for operation of the gas lift at low attrition rates, a particularly desirable size relationship being such that the ratio of the average size of largest 5 percent of the total particles to the average size of the smallest 5 percent of the total particles is less than about 20 to l and preferably between 5 to 1 and. 10 t'o1l. Particles of catalyst disengaged from the transporting or lifting gas settle on the surface of bed 19 in vessel 14, from which bed the particles flow through a conduit as a relatively compact non-turbulent column of particles to the reactor. It is to be understood that a par-- ticular type of separator, such as vessel 14', is not a part of this invention and that separators other than the one illustrated, which perform thefunction of separating the elevating gas and particles of catalyst by various specific methods, may be employed. Indeed, the gas, after disengagement from the particles of catalyst at the top of the lift may, if desired, be passed downwardly through the bed of catalyst in the separator (as, for example; where hydrocarbons are employed as the lifting medium and cracking, in addition to that occurring in the left, is desired), the lifting gas being finally disengaged from this TLparticles of catalyst at the bottom of the bed in vesse In a system, as described above, wherein fluent solid particles are elevated by gas through a gas lift or confined upfiow path, which upflow path extends between zones or vessels (10 and 14) located at different elevations and preferably in the same vertical line, the zone at the top of the upflow path is necessarily at a lower gaseous pressure than the zone at the bottom thereof so that there is a gaseous pressure drop through the upfiow path. This pressure drop may be considered. as resulting principally from the expenditure of energy by lifting gas in supporting and elevating the particles; the magnitude of the pressure drop being directly related to the work expended in lifting the particles.

Control of the rate of addition of the lifting gas and of the particles to the upflow path provides a means of controlling the operation of the gas lift. In order to obtain the desired high rates of particle circulation, lift inggas is introduced into the lift in an amount and at a pressure such that its velocity is sutiicient'to accelerate the particles to a substantial velocity. Experience has shown that aparticularly advantageous range of particle velocity is about 30 to ft./sec; (as measured when the particle is substantially completely accelerated) although lower velocities, such as down to about 10" ft./sec. may be employed generally in relatively short lifts, such as those of 50 feet or less.

The lift operation may also be controlled by controlling the concentration of particles in the lift simultaneously with or independently of a control of the amount and pressure of the lifting gas, as by controlling the relative amounts of lift gas introduced by conduits 11 and 12 so as to produce a range of particle concentration of from about 0.5 to 20 percent of the apparent bulk density of the particles (i. e., the density of the packed mass of particles). For present commercial catalysts having apparent bulk densities of from about 40 to 55 pounds per cubic foot, average concen trations of about 1 to 7 pounds per cubic foot have proved particularly advantageous.

In accordance with the embodiment of the present invention shown in detail in Figures 2 and 3, the lower and major portion of the gas lift such as from 60 to 95 percent thereof (indicated by the dimension L in Figure 2), has a constant circular horizontal cross sectional area and the upper and minor portion such as from 5 to 40 percent of the gas lift (indicated by the dimension U in Figure 2) is tapered outwardly and upwardly at a small angle (indicated as a in Figure 2) such that the velocity of the catalyst particles in this section is gradually and smoothly reduced (i. e., without slugging or flowing intermittently) to between about to 40 percent of the velocity at which the particles enter the tapered section. Investigation has shown that the angle of taper measured as indicated in Figure 2 must be less than 7 degrees and should preferably be less than 5 degrees, such as between about 0.15 to 3 degrees, in order to prevent a sudden expansion of the lifting gas since angles of 7 degrees or greater develop localized high concentrations of particles at the point of expansion or thereafter with resultant slugging or intermittent flow of the particles. At any event the velocity of slip between the elevating gas and the particles should be less than the velocity of slip for a single particle. When the upper section is so tapered the diameters of the bottom and top of the tapered section (indicated as d and d in Figure 3) are related so that the ratio of the square of the latter to the square of the former is between about 1.3 to 5.0, this ratio being equal to the ratio of the horizontal cross sectional flow areas at the designated locations in the upflow path. In most commercial installations (i. e., for lifts of from about 100 to 300 feet total height) the length of the tapered section will be between about 20 to 40 feet for the velocity range referred to above. Thus, from the ranges of operating conditions and dimensions of the lift given above, one may select a particular lift and taper for a given set of operating conditions.

As stated above, the velocity of the particles emerging from the tapered section is a fraction of the velocity at which the particles enter this section and disengagement of the particles is thereby considerably simplified. Preferably, the emergent velocities are reduced to less than 25 ft./sec. such as between about 5 to 20 ft./sec. In general it is preferred to reduce velocity at a rate such that the particle concentration in the tapered section does not exceed about 50 percent of the apparent bulk density of the particles so as to avoid slugging. The above reduction in velocity is equally effective where the major portion of the lift is slightly tapered at an angle less than that of the tapered section, as for example, at an angle such that the velocity of the particles is substantially constant along most of the lower and major portion of the upfiow path. When a tapered lower portion of the lift is employed, the same method of reducing the velocity as is stated above can be employed (i. e., the ranges of dimensions, such as included angle and ratio of diameters, given above for a constant diameter lower section are used).

As an example of an operation utilizing the present invention in a catalytic cracking system in which catalyst particles having average diameters of 0.12 inch and an apparent bulk density of 45 pounds per cubic foot continuously circulate, a lift conduit having a total height of 268 feet is employed, the lower 248 feet of this lift pipe being a circular pipe having a constant diameter of 19 inches and the upper 20 feet of the lift being tapered constantly outwardly at an angle of about 43 minutes to the vertical, so that the circular top of the lift pipe is 25 inches in diameter. The temperature of the particles in hopper 10 is approximately 1000 F. Air at a pressure of 5.8 pounds per square inch gauge and a temperature of 1000 F. is introduced into the supply hopper and passes upwardly through the lift pipe at the rate of 3560 standard cubic feet per minute carrying with it catalyst particles at the rate of 200 tons per hour to the disengaging hopper, where the gaseous pressure is about 0.3 pound per square inch gauge. The velocity of the particles of catalyst is calculated under these conditions to be 30 ft./sec. when the particles enter the bottom of the tapered section and 12.6 ft./sec. as the particles leave the top of the tapered section, thus effecting a reduction in velocity of 58 percent.

This application is a division of my application Serial No. 158,658 filed April 28, 1950, and now abandoned.

Obviously many modifications and variations of the invention as hereinbefore set forth may be made without departing from the spirit and scope thereof and therefore only such limitations should be imposed as are indicated in the appended claims.

What is claimed is:

1. In a process for elevating fluent solid particles through a confined lift path by means of lift gas, wherein said particles are conveyed upwardly through said path at substantially less than bulk density and are discharged into an enlarged disengaging zone for separation of said solids from said lift gas solely by complete gravitational deceleration and free fall thereof to the bottom of said enlarged zone, the steps which comprise: continuously accelerating the upward movement of said particles along the lower 60 to 95 percent of said lift path to a desired maximum velocity, and decelerating said upward movement of particles in the upper 5 to 40 percent of said lift path by a gradual increase in the cross-sectional flow area of said upper portion, the deceleration of said particles being such as to provide within said upper portion a particle concentration up to about percent of the apparent bulk density, whereby said particles are discharged into said disengaging zone at a velocity in the range of about 10 to 40 percent of said maximum velocity.

2. A process as in claim 1 in which said maximum velocity is in the range of 30 to 100 feet per second and said discharge velocity is in the range of about 5 to 20 feet per second.

3. A process as in claim 2 in which said lower portion comprising to percent of said lift path is of uniform cross-sectional area, and in which said gradual increase in cross-sectional flow area is such that the ratio of the flow area at the discharge end of said lift path to said uniform area is in the range of about 1.3 to 5.0.

4. A process as in claim 1 wherein said solid particles are at elevated temperature and said lift gas comprises hydrocarbon vapors formed within said lift path by contacting said particles with liquid hydrocarbons.

5. In a pneumatic lift system for elevating granular contact material from a lift inlet chamber to a disengaging vessel by means of a gaseous lift medium, the improvement which comprises an upright elongated lift pipe extending from said inlet chamber to an intermediate level within said disengaging vessel, the lower 60-95% of said lift pipe being of uniform cross-sectional area and the upper 540% thereof being of gradually expanding cross-sectional area such that the upper expanding portion of said lift pipe is tapered at an included angle in the range of about 0.15 to 3.0 and such that the flow areas at the top and at the bottom of said tapered portion are in the ratio of 1.3 to 5.0.

References Cited in the file of this patent UNITED STATES PATENTS 2,398,759 Angell Apr. 23, 1946 2,666,731 Bergstrom Jan. 19, 1954 2,684,872 Berg July 27, 1954 2,684,873 Berg July 27, 1954 2,684,932 Berg July 27, 1954

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