US3006693A - Disengagment of solids from lift gas in pneumatic lift - Google Patents

Description

INVENToRs. 'rl K 'lazma ffl:

Ma/Vex L9 Y RS 1 B Jwm Hmmm msmn.

F www@ LNmm @wsu .MAF KAG Oct. 31, 1961 United States Patent Oiice 3,006,693 DISENGAGEMENT F SOLIDS FROM LIFT GAS IN PNEUMATIC LIFT Charles K. Clalmch, Jr., Clayrnont, Del., and Numer lvl.

Kapp, Swarthmore, Pa., assigner-s to Hendry Process Corporation, Wilmington, Del., a corporation of Bela- Ware Filed Jan. 31, 1958, Ser. No. 712,428 4 Claims. (Cl. 302--S9) This invention relates to the pneumatic elevation of granular material, particularly as applied to a pneumatic lift employed in a hydrocarbon conversion system to maintain continuous circulation of frangible granular contact material, such as catalyst, through the various treating zones of the system. The invention has particular application to a multiple lift wherein a plurality of lift pipes discharge at a common level within a common disengaging zone.

More specically, the invention relates to improvements in the art of disengaging the rfrangible solids from the lift gas so as to collect and separately withdraw the solids with a minimum of attrition or breakage and to separately discharge the lift gas with a minimum carryover of the solids or attrited portions thereof in the outgoing gas stream.

Multiple lifts employed in hydrocarbon conversion systems commonly have a plurality of rlift pipes arranged in a circular group either surrounding the treating vessel er vessels or to one side thereof, with means for introducing solids and lift gas at the lower end of each lift pipe. A common practice is to provide a single large disengaging vessel to receive the upper end portions of all the lift pipes, the diameter of the vessel being such that the velocity of the expanding streams of discharged lift gas will be decreased suiciently to permit the solids to become disengaged from the lift gas and fall to the bottom of the' disengager. The solids are then discharged from the bottom of the disengager, while the total lift gas is discharged as an overhead stream.

While a single disengager for the plurality of lift pipes has been found to be most practicable, such arrangement may create a serious problem in connection with attrition of the frangible solid particles. Because of the high replacement cost of the granular contact material, such as catalyst, comprising the circulation medium and the difficulties attendant the continuous removal of the socalled fines resulting from such attrition, primary consideration must be given to the matter of disengaging the solid particles from the rising streams of lift gas in such manner as to minimize severe particle-to-particle contact as well as excessive impact between the rising or the falling particles and the hard metallic surfaces of the disengager vessel and its internal equipment. ln addition to the problem of attrition there is a constant problem of solids carryover by reason of the solid particles being deflected or ricocheting into the region immediately surrounding the lift gas outlet of the disengager vessel and, where the gas discharge velocity is above supporting velocity for the particular particle, being carried out of the vessel by entrainment in the discharging gas stream. This latter problem is especially acute when the Alift gas outlet is not shielded by suitable bat-lie arrangements, and when the velocity of the discharging gas stream is near or well above supporting velocity of -any substantial portion of the solid particles.

In order to minimize such attrition, it is desirable to deect the discharging stream of solids so that the particles will have a 4trajectory which carries them away from the zone vertically above the discharge end of the t l lift pipe, so that the substantial major portion of the free- `by the location of the gas outlet conduit.

falling disengaged solid particles will descend out of contact with any substantial portion of the rising stream of solids discharging from the lift pipes. ln this way, severe particle-to-particle contact between rising and falling solids is greatly minimized.

In accordance with the present invention solids are disengaged from a plurality of confined lift streams by discharging the streams at a common level and at spaced locations about the lower peripheral region of a common disengaging zone. The direction of discharge of each of the separate streams may be vertical or may be slightly inclined toward the vertical axis of the disengaging zone. ln either case, the particles have a trajectory which carries them inwardly toward the axial or central region of the disengaging zone. Within the central region, the particles are channeled so as to cause the falling streams of solids to be distributed substantially uniformly around the axial region. Such channeling is effective also to prevent any substantial portion of the solids in one discharging stream from maintaining a trajectory which will carry it across the central or axial region of the disengaging zone into the rising stream or streams of solids at the opposite side thereof.

The lift gas discharging from the plurality of lift paths also reverses its direction of iiow and is withdrawn from the disengaging zone through an outlet located on the vertical axis of the disengaging Zone and below the inner envelope formed by Ithe converging streams of solids. Preferably, the lift gas is Withdrawn in an upward direction from the highest practical level within such region, especially when the disengaged solids are caught in their downward fall at a level not greatly below the lift discharge level.

The velocity of the withdrawn lift gas is preferably below supporting velocity for the average size granular fmaterial, such as a velocity of about 60 to 80% of supporting velocity, thus requiring that the cross-sectional area of the gas discharge outlet be relatively large. Under such low velocity discharge conditions there is little likelihood that any of the larger particles of the granular material descending in the region immediately surrounding the lower end of the gas outlet or any particles rebounding frorn the bottom of the disengaging zone to the region adjacent to the gas outlet, will become entrained in the outgoing stream of lift gas and be carried overhead out of the disengaging zone.

In a preferred form of the invention a plurality of lift pipes, such as about ten to twelve, are arranged with their discharge ends in a horizontal circle concentrically located in the lower peripheral region of the disengaging zone. The bottom of the disengaging zone is formed by a tube-sheet which may be the actual bottom of the disengager vessel or may be an intermediate horizontal partition therein. Solids draw-off conduits are provided in the tube-sheet, the level of solids withdrawal being suiciently below the discharge level of the lift pipes as to preclude any accumulation of solids at the bottom of the disengager from rising over and spilling into the discharge ends of the lift pipes. A relatively-large lift gas outlet conduit extends axially upward through the top or" the disengager from a level therein located a substantial distance above the discharge level of the lift pipes, but well within the envelope of the merging streams of solids rising from the plurality of lift pipes. Preferably the mouth of the gas outlet conduit is at the highest practicable position within the disengaging zone, but is yso located that substantially none or" the solid particles rising from the lifts will pass directly or by deflection into the mouth of the gas conduit. The rising or falling solids moving laterally toward the axial region of the Vessel are prevented from crossing to the opposite side of the vessel Lf desired,

Patented Oct. 3l, 1961- longitudinally extending radial fins may be provided along the surface f the gas conduit so as to further intercept or channel the laterally moving solids.

For a fuller understanding of the invention reference may be had to the accompanying drawings forming a part of this application in which:

FIG. 1 is a diagrammatic illustration, in sectional elevation of one form of disengager adapted to carry out the method of the invention;

FIG. 2 is a plan view taken along the line 2 2 of FIG. 1, and

FIGS. 3 and 4 are fragmentary elevation views, showing modified forms of the gas withdrawal conduit.

In FIG. l of the drawings the disengager vessel 5 is shown as comprising an elongated upright cylindrical portion 6 closed at its ends by upper and lower dished heads 7 and 8, respectively. The lift pipes 9, of which there are ten in the illustrated embodiment, extend upwardly through the lower dished head and terminate at a common level in the lower region of the disengaging Zone 10.

The lift pipes 9 may be assumed to be disposed in a circular row, either around a treating unit or to one side thereof. In the former case, if the treating unit is wider than the lift disengager, the upper end portions of the lift pipes may be inclined slightly toward the axis of the disengager so that the ends may be received into the lower end of disengager 5.

The lower portion of the pneumatic lift and the hydrocarbon conversion apparatus with which it is associated have not been illustrated, for the reason that such illustration is not believed essential to an understanding of the invention. It is not necessary in all cases that the upper ends of the lift pipes be inclined to the vertical. Where the relative sizes of the vessels permit, or where the lift is located to one side of the treating vessel, the lift pipes may be vertical throughout their length.

Extending axially through the upper dished head 7 of the disengager there is a relatively-wide lift gas discharge conduit 11 having its lower or receiving end at an intermediate level within the disengaging zone 10. The portion of conduit 11 projecting within vessel 5 is provided With longitudinal ns 12 extending from the lower end of the conduit 11 to the underside of the upper dished head 7. The number of fins 12 corresponds to the number of lift pipes 9, each tin extending radially outward and lying in a vertical plane passing midway between the projected axes of adjacent lift pipes.

Each stream discharging from the upper end of a lift pipe 9 comprises a solids stream in the form of a gradually widening cone of dispersed solid particles extending upwardly from the lift pipe. At some intermediate level within the disengager all the rising cones of solids tend to merge, so that the entire available cross-sectional area of the disengager is eventually occupied by dispersed particles of rising or falling solids. Since the lowermost end of the `gas discharge conduit 11 is located within the central region of vessel 5, and below the merging streams of upwardly moving solids, all of the discharged solids rise into the annular space 13 formed between the walls of the vessel 5 and the outer surface of conduit 11. FIG. 1 shows that solids in the peripheral regions of the discharging solids streams, indicated by the broken lines 14, strike the inside wall of the vessel as well as the outer surface of conduit 11 and the exposed surfaces of ns 12. 1It is to be noted, however, that the angle of impact is relatively small, so that the particles are caused to ricochet or be deliected rather than to rebound. It is necessary that the lowermost 'edges of fins 12 do not project outwardly into the path of the streams of rising solids, because of the danger of 'erosion and increased solids attrition resulting from the direct impact of solids against the bottom edge ofthe tins. Thus, the lower end portion of the tins may be ch'amfered, as shown at v15 in FIG. 1, so lthat 'the solids colliding with the chamfered edge of the ns will be deflected, thereby reducing the force of impact, or the outer edge of the tins 12 may extend below the lower end of conduit 11, with the fin being chamfered on the inside, as shown at 15 in FIG. 4.

The solid particles disengaged from the streams of lift gas within the annular region 13 descend by force of gravity and by the downwardly flowing lift gas to the bottom of the disengaging zone 10. The disengaged particles pass countercurrently through the rising cones of solids and fall in major part alongside the rising streams of solids, that is, in the areas between adjacent lift pipes and in the central region below conduit 11.

Disengaged solids collected on the surface of dished head 8 at the bottom of vessel 5 are continuously and freely drained through a plurality of discharge conduits 16. The lower region of disengaging zone 10, in the illustrated embodiment, is not used to provide storage or surge capacity for the circulating mass of solids. The solids drained through discharge conduits 16 are received in a storage chamber below the disengager, not shown, from which they may eventually gravitate into the treating section of a conversion unit.

Since it is a primary object of the invention to effect substantial reduction in overall height of the unit as a whole, as well as of the disengager vessel, it may in some cases be desirable to eliminate the separate supply or surge chamber below the disengager and to provide for surge capacity at the bottom of disengager. In such case, a substantially greater vertical distance will need to be provided between the discharge level of the plurality of lift pipes and the bottom of the vessel 5, so that a compact moving bed of solids having substantial depth may be maintained at the bottom of the vessel. In such arrangement, the lower dished head may serve as an intermediate tray to provide stagewise descent of the solids. In any case, suitable level control means is provided to limit the maximum height of rise of the surface of the storage bed within the disengager, so that solids cannot overow the discharge ends of the lift pipes.

Experimental investigation with respect to velocity distribution has revealed the fact that the highest local velocity in the region of lift gas withdrawal from the disengaging zone occurs at the mouth of the gas outlet conduit, adjacent and approximately parallel to the inner Wall surface. In order that solids descending around the outside of the gas outlet conduit 11 may be prevented from being drawn into the conduit as a result of the high velocity gas flow around the lower perimeter thereof, the conduit should be of such size that the highest local velocity will be less than the catalyst supporting velocity. Allowing for a reasonable safety factor, it has been found that the most practical average velocity across the mouth of the withdrawal conduit is about 70% of the catalyst supporting velocity. The range for satisfactory operation, however, is not narrow and, in certain cases, it may be desirable to operate at discharge velocities above or below such figure. To illustrate, in a multiple lift ofnine lift pipes, each 13.25" inside diameter, wherein the total lift gas rate is 1033 cu. ft./sec., the solids velocity in each lift pipe is 59 ft./sec., and the solids supporting velocity is 60 ft./sec., the preferred gas rate through the outlet conduit would be 70% of such supporting velocity, or 42 ft./sec.

The fact that the present disengager provides a reversal of flow for both lift gas and solids in the upper region of the disengaging zone, followed by disengagement and separate withdrawal of the lift gas and solids in the lower region thereof, achieves certain operational advantages. With disengagers of present commercial design, having the gas outlet at the top of the disengager, catalyst carryover begins when maximum velocities within the lift approach about 45 ft./sec. We have found that a catalyst disengaging unit of the type disclosed permits the use of higher gas rates and at the same time produces lower attrition rates at the same rate of catalyst circulation. In the system of the present invention the lift is fully stable when the separate lift pipes operate at maximum velocities more nearly in the range of 55-60 or more ft./sec., there are definite advantages to be obtained in withdrawing the lift gas in such manner as to be relatively independent ofthe maximum lift velocity. Where the total gas is withdrawn from the lower region, of a disengaging zone, as in the present invention, the carryover of solids is not a function of the velocity in the lift, but rather a function of the velocity in the gas outlet pipe. Thus, by operating with a gas outlet velocity of about 70% of the solids supporting velocity, there is substantially no carryover of solids other than nes, regardless of the maximum lift pipe velocity.

The present invention practically eliminates any danger of solids ricocheting into the exiting gas stream and being carried therewith out of the disengager. While it is true that disengaged solids descending to the bottom of the disengager bounce upon the surface of dished head 8 or upon a surface of solids already supported thereon, such bouncing of the particles, which observation'has shown to be sometimes as high as 90% of the distance of fall, is not a serious problem with respect to the matter of solids carryover. Even though the bouncing particles enter the mouth of the gas outlet conduit 11, they will return again by free fall to the bottom of the disengaging zone so long as the velocity of gas flow within the outlet conduit is substantially below the supporting velocity of the solids.

While U.S. Patent 2,643,161 of R. M. Shirk, discloses the withdrawal of lift gas from the lower region of the disengaging zone in connection with a single lift, such method has possible disadvantages. Where savings in the vertical height of supporting equipment and vessels is a primary and important consideration, the withdrawal 'of the total lift gas at a level substantially below the lift pipe discharge level requires the provision of a substantially greater vertical distance between the lift pipe discharge level and the bottom of the disengaging zone. Furthermore, where the total lift gas is withdrawn through an outlet conduit extending downwardly through the bottom of the disengager rather than upwardly through the top, as in the present invention, i-t is necessary to guard against admission of the solids into the open mouth of the gas outlet conduit. While a cap placed above the mouth of the gas outlet conduit prevents the free-falling disengaged solids from dropping directly into the mouth of such gas outlet conduit, it does not prevent bouncing solids from gaining admittance to the mouth of the outlet conduit. If the width of the annular space formed between the outer perimeter of the gas outlet conduit and the inner perimeter of the cap is relatively great, the gas velocity through such annular space will be relatively low but there will be a relatively wide opening through which the bouncing solids may pass. On the other hand, if the width of the annular space is relatively small, in order to minimize the size of the opening available to bouncing solids, the gas velocity will be increased, possibly to the point where solids approaching the mouth of the annular space may be carried therein by entrainment in the high velocity gas stream. In order to keep the gas discharge velocity within acceptable limits there is necessarily formed an open narrow passageway of sufficient size to receive a substantial portion of bouncing particles.

'in those cases where it is nevertheless desired to operate with a relatively-high gas outlet velocity, there is the danger that the solids which are descending immediately about the lower end portion of the gas withdrawal conduit will be sucked into the outgoing gas stream by reason of the normally higher gas velocity along the inner lower perimeter of the outlet conduit. A relatively narrow ring ange may therefore be provided about the lower end portion of the conduit 11, preferably slightly above its lower end. The ledge formed by such ange serves to deflect falling solids away from the lower perimeter of the conduit 11, so that solids dropping from or passing by the outer perimeter of the ange are not seriously eiected by the high velocity conditions existingimmediately adjacent to the mouth of the outlet conduit. A suitable flange for effecting such purpose is shown in the fragmentary view of the gas outlet conduit shown in FIG. 3, the ring ange being indicated by the numeral 17. Where such ange is employed, it is essential that the ange also be located outside the perimeters of the rising streams of solids in order to avoid severe impingement of the solids against the bottom edge surface of the flange.

Whereas solids carryover in the overhead gas discharge type of disengager may result primarily from high maximum solids velocities in the lift, solids carryover from a lift disengager improved in accordance with the present invention is independent of the maximum solids velocity in the lift, because the disengagement of the solids from the lift gas occurs after the direction of gas and solids ilow is reversed. In the present disengager, solids carryover is a function of the gas outlet velocity. Thus, the important variable in the matter of eliminating solids carryover is the height of the disengaging zone with the standard type of disengager, and is the diameter of the gas outlet pipe with the disengager of the present invention. Where an existing lift disengager of the standard type is functioning with excessive carryover of solids, it is necessary to change the lift operation by reduction of the lift velocity in order to correct the adverse condition. With the present type of disengager, the operation of the lift need not be changed. Excessive carryover of solids can be eliminated merely by changing the diameter lof the gas withdrawal pipe so as to provide a lower gas outlet velocity. While this would, of course, involve temporary shutdown and dismantling of a portion of the lift apparatus, the modification may be effected with less inconvenience than would be required to overcome the same problem in the standard type disengager. When it is desired to modify the operation of a multiple lift having the standard type of disengager so as to increase circulation rate through increased lift velocity, it may be necessary to increase the height of the disengager vessel in 'order to avoid excessive carryover. With the present type of disengager, an existing lift may readily be modified to accommodate a substantial increase in lift velocity merely by increasing the size of the gas outlet pipe. Or, in a case where the gas outlet pipe has been designed to provide a normal outlet gas velocity considerably below the supporting velocity of the solids, there may be a certain amount of flexibility in operation without any structural modification of the disengager.

While the disengager of the present invent-ion has been disclosed in but one form, it will lbe obvious to those skilled in the art that it is susceptible to various modifications and improvements within the spirit of the invention, and it is desired therefore that only such limitations shall be placed thereon as are specifically set forth in the appended claims.

It is contemplated, for example, that normally the lift pipes Will be placed so as to enter the disengager vessel as close to the inner wall thereof as may be practicable, and that the diameter of the vessel, as compared to the diameter of the central gas outlet conduit, will be great enough to permit the rising streams of solids to assume an inward trajectory which will enable the substantial major portion of the solids to become disengaged from the gas and to descend in the annular region between the vertical projections of the lift pipes and the gas outlet conduit. With such arrangement, only a relatively minor portion constituting the innermost particles of the rising solids streams (relative to the vertical axis of the disengager vessel) will during their upward travel impinge against the outer surface of the gas outlet conduit.

What is claimed is:

l. A method for disengaging granular solids from a combined stream of lift gas and solids discharging from the upper end of a conned lift path, which comprises the steps of: discharging said combined stream at a low level Within the peripheral region of a broad conned disengaging zone having its upper boundary at a distance above the discharge end of the lift path sutlicient to per mit complete gravitational deceleration of a substantial major portion of the discharged solids, the axis of said discharge being inclined slightly away from the adjacent side of said zone so that a major portion of the decelerated solids descend outside the divergent envelope of the rapidly-rising lower portion of the discharging stream of solids; collecting and withdrawing the disengaged solids at the bottom of said Zone, below the discharge level of said lift path; and withdrawing said lift gas from said zone in an upward direction and at below solids-supporting velocity from a location horizontally spaced from said envelope and well'above the maximum level of rebound for ydisengaged solids rebounding from the surface of the layer of solids collected at the bottom of said Zone, whereby the solids rising from the upper end of the conned lift path and the solids rebounding from the bottom of said zone remain sufficiently remote from the lift gas withdrawal outlet, and the disengaged solids falling adjacent to said outlet have sufficient gravitational acceleration, to minimize their reentrainment in the outgoing gas stream.

2. The method as in claim 1 including a plurality of said conned -lift paths discharging into said disengaging zone at spaced locations along said peripheral region, said lift gas being withdrawn Vfrom said zone iat a central location wholly within the annular envelope formed by the merging cones of solids rising from said plurality of lift paths, said location being immediately surrounded substantially entirely by descending solids disengaged from said lift gas.

3. A method as in claim 2 in which said plurality of lift paths discharge into said disengaging zone at equispaced points along an inner peripheral circle concentric to said zone; in which said location of lift gas withdrawal is on the vertical axis of said disengaging zone; and in which the withdrawn lift gas is conveyed as a conned stream upwardly out of said zone along the vertical aXis thereof.

4. The method as in claim 1, in which said lift gas is withdrawn from said vessel at a velocity in the order of about of said solids supporting velocity.

References Cited in the iile of this patent UNITED STATES PATENTS 2,818,307 Elkin Dec. 31, 1957 2,873,146 Cross et al. Feb. l0, 1959 2,887,341 Cross May 19, 1959

Images