GB2148141A - Staged flow distribution grid assembly for ebullated bed reactor - Google Patents

Abstract

In the staged grid assembly, the gas and liquid mixture flows first through a lower secondary flow distribution grid plate (30) into an interim zone (35), and then flows upwardly through an upper primary flow distribution grid plate (12) into the reactor ebullated bed (25). The staged grid assembly contains an upper primary grid plate containing multiple flow tubes (16) covered by bubble caps (18), and a lower secondary grid plate containing multiple flow tubes (32). The assembly enables the primary grid to provide a more uniform distribution of gas and liquid flow upwardly into the ebullated bed across the entire cross- sectional area of the reactor, and thereby provides improved operation of the ebullated catalyst bed reactor. <IMAGE>

Description

SPECIFICATION Staged Flow Distribution Grid Assembly and Method for Ebullated Bed Reactor This invention relates to an improved flow distribution grid plate assembly and method used for providing uniform upward fluid flow distribution in ebullated bed catalytic reactors. It relates particularly to a staged flow distribution method and grid plate assembly having an upper primary grid plate and a lower secondary grid plate located below the primary grid plate, each plate containing multiple vertical flow tubes.

In ebullated catalyst bed reactors operated under elevated temperature and pressure conditions, flow maldistribution problems sometimes exist below the distribution grid plate and in the catalyst bed. Such flow maldistribution is usually due to abnormal operating conditions such as plugging of openings in the grid plate by coke, or to excessive coke deposits on the catalyst particles in the bed. If such plugging of openings in the grid plate occurs, non-uniform flow distribution and bed ebullation results, which is very undesirable. The riser flow tubes and slotted tail pipes as now used in reactor grid plates usually perform adequately in distributing the recycle and feed liquid streams and hydrogen gas into the ebullated catalyst bed.However, the presently used grid plate arrangement has been found to be inadequate for handling severe flow maldistribution in the plenum of the reactor, for it can only moderately improve the flow distribution existing below the plate, but cannot alleviate "spouts" and major operational upset conditions which lead to an uneven depth of hydrogen in the plenum chamber, which can cause a greater length of the tail pipe slot to be exposed with a corresponding increase in hydrogen flow to those particular riser tubes.

Such maldistribution flow conditions in a reactor plenum can be more or less constant depending on the manner in which the feed streams and recycle streams are introduced into the plenum. Also, flow maldistribution could possibly occur as a sloshing effect where the liquid level in the plenum below the distribution grid is constantly tilting from one direction to another.

The use in such ebullated bed catalytic reactors of conventional cylindrical riser flow tubes covered by cylindrical-shaped bubble caps is disclosed by U.S. Patent No. 3,197,286 to Farkas et al; U.S. Patent No.

3,197,288 to Johanson, and U.S. Patent No. 3,475,134 to Weber et al. However, it has been found that inadequate distribution of the gas and liquid flows are usually provided by these reactor designs.

Accordingly, improvements in flow distribution in ebullated bed catalytic reactors have been sought.

An improved staged grid plate configuration has now been developed which more effectively redistributes the gas and liquid flows below the primary grid plate whenever flow maldistribution problems exist below the grid, so as to provide more uniform ebullation of the catalyst bed in the reactor. To aid in maintaining a "smooth" liquid level in the plenum and consequently a reasonably equal length of slot exposure on each riser tube for gas flow into each tube, a secondary grid plate is provided below the primary grid. This secondary grid is similar to the single flow distribution grid presently used in ebullated bed reactors.

However, the secondary grid does not have caps over the riser tubes on the upper side of the plate, but uses only slotted tubes attached to a plate which extends to near the inner walls of the reactor.

This invention provides a staged distribution grid plate assembly and method used for improved flow distribution upwardly into an ebullated catalyst bed of a reactor, in which a lower secondary flow distribution grid feeds gas and liquid flow upwardly to an upper primary flow distribution grid, and thence into the ebullated bed of the reactor. The'flow tubes provided in the lower secondary grid plate are uniformly spaced and are relatively larger in diameter and have greater total cross-sectional area than flow tubes in the upper primary distribution grid, so that the greater and controlling pressure differential occurs across the upper primary grid plate to provide a more uniform flow distribution upwardly into the ebullated bed.This staged grid plate arrangement or assembly enables the upper primary grid to operate more effectively, so that the ebullated catalyst bed will have a more uniform distribution of gas and liquid flowing upwardly therethrough across the entire cross-sectional area of the reactor.

In the staged grid plate assembly of the present invention, the upper primary grid plate can be supported from either the reactor lower head or from the reactor wall, and the lower secondary grid plate is usually supported from the upper primary grid, such as by multiple spacer rods extending between the two grids. Alternatively, the secondary grid plate can be separately supported from the reactor lower head or wall, or it can be structurally integrated with the primary upper grid plate so as to help withstand the total differential pressure across the grid assembly caused by the upward fluid flow th rough the grids.Also, the lower grid plate can be attached integrally to the upper plate by an extension of the flow tubes below the upper plate, so that the catalyst bed weight and total pressure differential across both plates in the grid plate assembly is carried by the assembly.

More specifically, the present invention provides a staged grid plate assembly for providing uniform flow distribution of a gas/liquid mixture upwardly into an ebullated bed of a reactor, said grid plate assembly comprising an upper primary grid plate supported within the reactor near the lower end of the reactor, said primary grid plate containing multiple flow distribution tubes extending substantially vertically through said plate, said primary tubes being uniformly sized and spaced in the plate; a cap covering the upper end of each tube in said primary grid, said cap being rigidly attached to and spaced outwardly from the tube upper end above the grid plate, so as to permit flow of fluid upwardly through the tubes and then outwardly from under the lower edges of the cap into the ebullated bed; and a lower secondary grid plate located below and spaced from said primary grid plate, said secondary grid plate containing multiple flow distribution tubes passing substantially vertically through the secondary plate, said secondary tubes having uniform diameter and spacing, whereby fluid passes upwardly through the flow tubes in the secondary grid, and then upwardly through the flow tubes in the primary grid and outwardly from under the lower edges of the caps to provide uniform fluid flow in the reactor ebullated bed.

In another aspect, the invention provides a method for uniformly distributing gas and liquid flow upwardly into an ebullated bed reactor, which comprises introducing gas and liquid flow streams into a plenum located at the lower end of a reactor below a flow distribution grid; passing said gas and liquid flow upwardly from said plenum through multiple tubular flow passages located in a secondary grid plate into an interim zone located above the secondary grid plate and below a p imary grid plate; remixing and redistributing the gas and liquid flow in said interim zone above said lower grid plate; and passing the remixed gas and liquid upwardly through multiple tubular flow passages located in said upper primary grid plate, then under bubble caps each located over said multiple upper flow passages and uniformly into an ebullated catalyst bed of the reactor.

In liquid phase catalytic reactors for contacting liquids, gases and particulate solids, it is very important for achieving complete and effective catalytic reactions that the upflowing liquid and gas mixture be uniformly distributed across the entire horizontal cross-section of the reactor, so as to maintain the bed of particulate solids or catalyst in a uniformly expanded condition within random motion of the catalyst.For certain reactions, such as the catalytic hydrogenation of heavy oils or coal-oil slurries or the hydrocracking of heavy hydrocarbon feedstreams under elevated temperature and pressure conditions, such as at 50o10000F (260--538"C) temperature and 5005000 psig (34--345 bar gauge) pressure, to produce lower-boiling liquid fractions, flow maldistributions through the reactor flow distributor or grid plate assembly can cause relatively inactive zones in the bed where the catalyst is not in uniform random motion.

This condition leads to the undesired formation of agglomerates of catalyst particles by coking of the hot oil or slurry. The desired uniform flow distribution upwardly through the grid plate into the ebullated catalyst bed can be impaired either by restrictions occurring in the riser tubes due to coking, or by catalyst particles in the tubes. The present invention provides an effective solution to these flow maldistribution problems in the ebullated catalyst bed.

The flow distributor or grid plate assembly must also function to prevent catalyst particles from draining downwardly back through the distributor whenever the reactor is shut down, while most of the liquid contained within the catalyst bed is drained down to below the bed. If catalyst is allowed to drain back through the grid plate flow distributor, it can plug the flow passages therein and interfere with operations so that re-ebullating the catalyst bed becomes very difficult because the flow passages are at least partly restricted. Furthermore, such restrictions can produce undesired flow maldistribution in the catalyst bed. To prevent such backflow of catalyst, a ball check valve is usually provided in each riser tube.

In the present invention, two grid plates are provided in series flow relationship so that a relatively more uniform flow distribution upwardly into the ebullated catalyst bed above the upper primary grid plate is thereby achieved. It is thus a basic feature of the present invention that both grid plates contain multiple flow tubes having uniform size and spacing, with only the tubes in the upper grid plate having caps covering the upper ends of the tubes. The flow tubes used in the secondary grid plate should be uniformly spaced and have relatively larger diameter and total cross-sectional area than the flow tubes in the upper primary grid plate. The secondary flow tubes do not necessarily need to be cylindrical in shape but can be square, rectangular, or triangular in cross-sectional shape or practically any configuration can be employed.

However, the combination of tube effective diameter and number of tubes should provide the desired uniform flow and differential pressure across the secondary grid, which should be between about 0.10 and 0.90 times the differential pressure across the upper primary grid. Also, the length/diameter ratio for the secondary tubes should be at least about 1.0, and usually need not exceed about 5.0.

In operation, the gas liquid mixture which passes through the multiple flow tubes in the lower secondary grid is redistributed in the horizontal space between the two grid plates. Thus, the flow of gas/liquid mixture flowing through the multiple flow tubes in the upper primary grid into the ebullated bed will be more uniform that when only a single grid plate is used.

Reference is now made to the accompanying drawings, in which: Fig. 1 shows a partial vertical sectional view through the lower portion of a reactor vessel containing a staged grid plate assembly having multiple riser tubes therein in accordance with a preferred embodiment of the invention; Fig. 2 shows a portion of the primary upper grid plate containing multiple riser flow tubes each covered by a single bubble cap and containing a ball check; Fig. 3 shows a partial vertical sectional view of an alternative staged grid plate assembly in which both grid plates are supported from the reactor lower head; and Fig. 4 shows a partial sectional view of an alternative grid plate assembly in which the staged grid plates are structurally integrated into a single unit supported from the reactor wall.

As generally shown in Fig. 1, reactor 10 contains a primary upper grid plate 12 which is rigidly supported therein usually at its outer edges by a cylindrical shaped support skirt 13 connected to the reactor lower head 14, and is sealed to the side wall in the lower portion of the reactor, so as to provide a plenum space 15 below a lower grid plate 30. The feedstream to the reactor enters through conduit 11 and the flow is deflected radially outwardly by stationary baffle 11 a. The grid plate 12 serves to support catalyst bed 25 and contains multiple riser flow tubes 16. As shown in greater detail in Fig. 2, each riser tube 16 has at least one opening or slot 17 at its upper end and is covered by a cap 18, which is rigidly attached to the upper end of tube 16 by suitable fastening means 19, such as a threaded bolt and nut.The cap 18 is spaced outwardly from tube 16 to provide for uniform flow of fluid upwardly through the tubes 16 and slot 17 of the grid plate 12 and into the bed 25 of catalyst particles.

As shown in Fig. 2, the lower edge of the cap 18 is preferably provided with notches 1 8a to provide for the localized exit flow of gas and promote the formation of small gas bubbles in bed 25. The notches are intended to let the gas emerge from under the caps as small discrete bubbles instead of large globs of gas, and the notch widths should usually be 510 times the catalyst effective particle diameter. The notches located around the bottom of the caps can be used with individual caps of any shape, such as cylindrical or tapered shape. Also, to prevent backflow lox of catalyst from the bed 25 to plenum 15 below the grid plate following reactor shutdown or loss of recycle liquid flow, a ball check 20 is usually provided and is preferably located in the upper end of each riser tube 16, as shown in Fig. 2.The ball check 20 mates with seat 22 provided within the upper end of the riser tube 16 to prevent any backflow of catalyst from the bed 25 to the plenum 15 below the distributor plate 12. To facilitate the entry of gas such as hydrogen into the lower end of the riser tube 16, openings such as holes 23 or vertical slots 24 are provided in the tube below the grid plate.

Located below the primary grid plate 12 is a secondary grid plate 30, containing multiple parallel flow tubes 32 each having an opening such as holes 33 or vertical slot 34 in the lower end thereof. The secondary grid plate 30 is spaced below and usually supported from the upper grid plate 12, such as by multiple rods 36 with each rods having a spacer tube 37 located around the rod for maintaining the desired space 35 between the upper and lower grid plates, as shown in Fig. 1. The secondary grid 30 can be extended to contact support skirt 13, or preferably can have a small annular space 38 therebetween and be provided with a circumferential skirt 40 extending downwardly from plate 30. The lower end of skirt 40 should extend to substantially the same level as the lower ends of secondary flow tubes 32.Also, the skirt 40 is provided with openings such as holes 39 or slots 41, which are similar to holes 33 or slots 34 in the secondary flow tubes 32. Furthermore, the flow area of the annular space 38 should not exceed about 10 percent of the total flow area for the openings in the secondary grid plate, i.e. that provided both by multiple flow tubes 32 and annular flow space 38.

In operation of the dual grid plate assembly, the gas/liquid mixture fed into plenum 15 forms a gas space 1 5a below lower secondary grid plate 30. The gas and liquid mixture in plenum 15 passes upwardly through multiple flow tubes 32 and annular space 38 into space 35 between the upper and lower grid plates.

In space 35, the gas/liquid mixture is redistributed generally horizontally and the gas portion rises to form gas space 35a above the liquid level 35b. The liquid level 35b is controlled by the vertical location of slots 24 in the lower ends of riser tubes 16 and by the flow rate through the grid plate.

It is thus an advantage of the present invention that the lower secondary grid plate provides for the lateral redistribution of fluid flow below the upper primary grid plate and thereby tends to correct any flow maldistribution below the primary grid plate, which may be caused by flow maldistribution problems on the underside of the grid. Reactor bed ebullation will be generally uniform unless some riser tubes become plugged by coke formation, etc.

In an alternative embodiment of the present invention, as shown in Fig. 3, both the upper primary grid plate 12 and the lower secondary grid plate 30 can be separately supported from the reactor lower head 14 by means of an outer cylindrical support skirt 13 for upper grid 12 and an inner cylindrical support skirt 45 for supporting the lower grid plate 30. For this grid plate configuration, the support rods 36 and spacer tubes 37 used for the Fig. 1 embodiment are not needed. Also, the multiple flow tubes 32 in grid plate 30 are provided with multiple openings 33 or slots 34 to facilitate the entry of gas such as hydrogen into the flow tubes, similarly as for the flow tubes 16 in the upper grid plate 12. If desired, dual nozzles 11 forthe feedstream can be provided into plenum 15.

It is another important feature of the present invention that the two grid plates can be structurally integrated, so that the total pressure differential across the grid plate assembly due to upward fluid flow therethrough and the catalyst bed weight is carried away by the assembly of both plates. As shown in Fig. 4, the riser tubes 42 for the primary upper grid plate 12 are extended downwardly and rigidly attached such as by welding to the lower secondary grid plate 30. Openings such as holes 43 or slots 44 are provided in the lower end of each tube 42 as before, to provide for entry of gas such as hydrogen into the flow tube. Also, the upper primary grid plate 12 can be suitably supported from the reactor inner wall by a continuous ring 26 welded to the wall. The upper grid plate 12 is attached to ring 26 by multiple fastener bolts 28 and nuts 29.In this grid plate assembly, the periphery of lower grid plate 30 can be extended to near the inner wall of reactor 10 so as to provide a small annular space 46 therebetween, and be provided with a peripheral depending skirt 48 and holes 49 similarly as previously described for the Fig. 1 embodiment.

The utility and effectiveness of the staged flow distribution grid assembly is illustrated by the following specific Example, which should not be construed as limiting the scope of the invention.

Example In an ebullated bed catalytic hydrogenation reactor for petroleum feedstock material, a dual grid assembly has the following characteristics and dimensions: Reactor Temperature, "F (OC) 750--850 (399454) Reactor Pressure, psig (bar gauge) 1000-3000 (69-207) Reactor Inside Diameter Ft (m) 12 (3.7) Vertical Spacing Between Primary and Secondary Grid 16(40.6) Plates, in. (cm) Primary Grid Flow Tube Diameter in. (cm) 1.30 (3.3) Bubble Cap Diameter, in. (cm) 3 (7.6) Primary Flow Tube Extension Below Primary Grid 9 (22.9) Plate,.in. (cm) Flow Area of Primary Grid Tubes, in.2 (cm2) 2.16 (13.94) Secondary Grid Flow Tube Diameter, in. (cm) 4(10.2) Secondary Flow Tube Extension Below Grid 5(12.7) Plate, in. (cm) Flow Area of Secondary Grid Tubes, in.2 (cm2) 12.7 (81.94) Pressure Differential Across Upper Primary Grid, 5--8 (0.340.55) psi (bar) Pressure Differential Across Lower Secondary Grid, 1-3 (0.069--0.207) psi (bar) The catalyst ebullation pattern in the reactor is uniform over a wide range of liquid and gas flow rates from the plenum upwardly into the reactor bed.

Claims (17)

1. A staged grid plate assembly for providing uniform flow distribution of a gas/liquid mixture upwardly into an ebullated bed of a reactor, said grid plate assembly comprising: (a) an upper primary grid plate supported within the reactor near the lower end of the reactor, said primary grid plate containing multiple flow distribution tubes extending substantially vertically through said plate, said primary tubes being uniformly sized and spaced in the plate; (b) a cap covering the upper end of each tube in said primary grid, said cap being rigidly attached to and spaced outwardly from the tube upper end above the grid plate, so as to permit flow of fluid upwardly through the tubes and then outwardly from under the lower edges of the cap into the ebullated bed; and (c) a lower secondary grid plate located below and spaced from said primary grid plate, said secondary grid containing multiple flow distribution tubes passing substantially vertically through the secondary plate, said secondary tubes having uniform diameter and spacing, whereby fluid passes upwardly through the flow tubes in the secondary grid, and then upwardly through the flow tubes in the primary grid and outwardly from under the lower edges of the caps to provide uniform fluid flow in the reactor ebullated bed.

2. A grid plate assembly according to claim 1, wherein the total cross-sectional area of secondary flow tubes in the secondary grid plate exceeds that of the primary flow tubes in the primary grid plate.

3. A grid plate assembly according to claim 1 or 2, wherein each flow tube in said secondary grid plate has larger cross-sectional area than each tube in the primary grid plate.

4. A grid plate assembly according to any of claims 1 to 3, wherein said tubes in said lower secondary grid plate have a length/diameter ratio of from 1.0 to 5.0.

5. A grid plate assembly according to any of claims 1 to 4, wherein said secondary grid plate is supported from said primary grid plate by multiple support rods.

6. A grid plate assembly according to claim 5, wherein said secondary grid plate is spaced below said primary grid plate by a spacer means provided on said support rods.

7. A grid plate assembly according to any of claims 1 to 6, wherein said primary grid plate is attached to the reactor head by a skirt means extending below said primary grid plate to an attachment point below said secondary grid plate.

8. A grid plate assembly according to any of claims 1 to 7, wherein both primary and secondary grid plates are structurally integrated so as to withstand the catalyst bed weight and the total differential pressure across the grid plate assembly.

9. A grid plate assembly according to any of claims 1 to 8, wherein the primary tubes contain check valves to prevent backflow of catalyst from above the grid plate to below the grid.

10. A grid plate assembly according to claim 9, wherein said check valve is a ball and a mating seat located in the upper portion of the riser tube.

11. A method for uniformly distributing gas and liquid flow upwardly into an ebullated bed reactor, said method comprising: (a) introducing gas and liquid flow streams into a plenum located at the lower end of a reactor below a flow distribution grid; (b) passing said gas and liquid flow upwardly from said plenum through multiple parallel tubular flow passages located in a lower secondary grid plate into an interim zone located above the secondary grid plate and below a primary grid plate; (c) mixing and redistributing the gas and liquid flow in said interim zone above said lower grid plate; and (d) passing the mixed gas and liquid upwardly through multiple tubular flow passages located in an upper primary grid plate, then under bubble caps each located over said multiple upper flow passages and uniformly into an ebullated catalyst bed of the reactor.

12. A uniform flow distribution method according to claim 11, wherein the fluid differential pressure said upper primary grid plate exceeds that which occurs across the lower secondary grid plate.

13. A flow distribution method according to claim 11 or 12, wherein the inlet gas and liquid flow streams are each introduced through separate conduits into said plenum, and are each passed around a flow distributor baffle associated with each conduit within said plenum.

14. A flow distribution method according to any of claims 11 to 13, wherein the liquid is a hydrocarbon liquid and the gas is hydrogen.

15. A flow distribution method according to any of claims 11 to 14, wherein the liquid temperature is 500--900"F (260--482"C) and the liquid pressure is 5005000 psig (34--345 bar gauge).

16. A staged grid plate assembly substantially as hereinbefore described with reference to the accompanying drawings and/or the Example.

17. A flow distribution method substantially as hereinbefore described with reference to the accompanying drawings and/orthe Example.