Improved interzone mixing device

Abstract: An improved fluid mixing device in a multi-bed catalytic reactor comprises an arrangement of successive spaced-apart, horizontal plates (16, 21, 25, 28) provided with means for efficiently pre-mixing a process stream of fluid flowing downwardly from catalyst bed (12) and a quench fluid introduced into vessel (10) by sparger (32s). The pre-mixing means comprises a cylindrical cap (18) covering a central opening in first plate (16) and having angular openings (20) in its sidewall (19) to direct the fluid mixture in a swirling motion through the central opening; a horizontal table (37) positioned on second plate (21) in axial alignment with the central opening in the first plate (16); an annular ring of openings (22) around the periphery of the second plate, these openings (22) communicating with conduits (23) having slots (24) therein orientated to create another swirl pattern below plate (21), the fluids then flowing downardly through a central opening in the third plate (25); and a plurality of openings (29) in the last plate (28) for distributing the pre-mixed fluids.

Description: The present invention relates to an interzone mixing device suitable for efficiently premixing fluids of different temperatures or composition. In particular, the present invention is suitable for thoroughly mixing a minor amount of fluid with a major amount of fluid in a relatively short mixing zone. Many catalytic processes are carried out in reactors which contain a series of separated catalytic beds. In such processes frequently mixing means, such as quench boxes, are disposed between beds. The purpose of the quench boxes is to provide rapid and efficient mixing of the fluid steams being processed in the reactor with a cooler fluid stream supplied from an external source thereby controlling the temperature of the process stream coming into contact with the catalyst in the next succeeding catalyst bed. As will be readily appreciated, the better the mixing, the better the temperature and reaction control and, hence, the better the overall reactor performance. Examples of interzone mixing devices can be found in U.S. Patent 3,143,581, U.S. Patent 3,480,407, U.S. Patent 3,502,445, U.S. Patent 3,705,016, U.S. Patent 3,723,072, U.S. Patent 3,880,961, U.S. Patent 3,895,919, U.S. Patent 4,235,847 and U.S. Patent 4,836,989. Some of these devices are complicated. Some are prone to plugging. Some really need a relatively large space to provide the desirable degree of mixing. In U.S. Patent 4,960,571, there is described a quench assembly that provides a significant improvement over known quench boxes. Notwithstanding, the improvements achieved by that device, in today's competitive processing industry there is a need to constantly seek further improvement. Therefore, the present invention addresses the need to provide a device for thoroughly mixing in the space between two reaction zones, a stream of fluid supplied from an external source with the main process stream fluid. The present invention also addresses the problem of providing a device for efficiently mixing a relatively minor amount of fluid with a relatively major amount of fluid, especially in an interzone space. The present invention, in common with US 4,960,511 relates to an interzone mixing device for mixing a secondary fluid with a main process stream fluid in a vertical vessel, comprising: a horizontally disposed first plate having a first surface, a second surface and a central opening therein for main process stream fluid to enter the interzone mixing device; a horizontally disposed second plate having a first surface facing the second surface of the first plate and a second surface, the second plate being spaced apart from the second surface of the first plate, the second plate having a plurality of openings in an annular ring around the periphery thereof; a plurality of conduits extending vertically from the second surface of the second plate and communicating with the peripheral openings therein, the conduits having slots therein oriented in the inward radial direction or at a predetermined angle thereto; a horizontally disposed third plate having a first surface facing the second surface of the second plate and a second surface, the third plate being spaced apart from the second surface of the second plate, the third plate having a central opening therein; a horizontally disposed fluid distribution plate spaced apart from the second surface of the third plate and having a plurality of openings therein; and means for introducing a secondary fluid on the first surface side of the first plate. The invention is characterised by a cylindrical cap having a sidewall extending vertically from the first surface of the first plate, the cap being sized and positioned to cover the central opening in the first plate, the cap having a plurality of angular openings in the sidewall which are orientated at an angle to the inward radial direction to direct the flow of fluid in a swirling direction toward the central opening in the first plate, and further characterised by a horizontally disposed table positioned on the first surface of the second plate and axially aligned with the central opening in the first plate. The present invention will be more readily understood by referring, by way of example, to the accompanying drawings, wherein:-. Figure 1 is a vertical sectional view of a downflow vertical vessel containing at least two catalyst beds and having one form of interzone mixing device in accordance with the present invention. Figure 2 is an enlarged sectional view, taken along lines 2-2 of Figure 1 showing slotted openings in a cylindrical cap of the interzone mixing device. Figure 3 is a detailed view of a slotted conduit opening into a jet stirred mixing zone of the mixing device. Figure 4 is a schematic illustration of an alternate cylindrical cap which constitutes part of the interzone mixing device. Figure 5 is a top plan view of the arrangement of vanes of the cylindrical cap of Figure 4. Figure 6 is a top plan view of an alternative slotted conduit opening into the jet stirred mixing zone. Figure 7 is a vertical sectional view of an upflow vessel containing at least two catalyst beds and an interzone mixing device. The invention will now be described with reference to its use in a multi-bed, catalytic reactor in which the mixing device is located in the zone between two catalytic beds. The mixing device, of course, can be used in non-catalytic vessels or reactors if so desired. Referring now to Figure 1 of the drawings, reference number 10 generally indicates a fluid downflow, multi-bed catalytic reactor or vessel having a vertical cylindrical sidewall 11 enclosing a plurality of beds of catalyst. The vessel 10, therefore, has an upper fluid inlet (not shown) and a lower fluid outlet (also not shown) for introducing a fluid stream for downward flow through the vessel and ultimate removal of the stream after its passage through the vessel. Shown in Figure 1 are catalyst beds 12 and 13. The bed 12 of the catalyst is supported by a catalyst bed support 14 such as a perforated plate, a grid covered with screening or the like, which permits fluid to exit the bottom of the bed. As is known in the art, a plurality of ceramic balls, Raschig rings or the like are disposed on top of the catalyst beds to aid in the distribution of gases as they enter the bed and also to prevent catalyst particles from being disturbed from the position in which they were placed when the vessel was loaded with catalyst. In the Figure 1 embodiment, ceramic balls 15 are shown on the top of catalyst bed 13. Thus, each of the multiple catalyst beds within the vessel comprise catalyst particles, a lower support layer and a covering layer. Interposed between successive catalyst beds 12 and 13 is a mixing device for introducing and mixing a secondary fluid, from an external source, with the partially reacted process fluid exiting from the catalyst bed 12 above the mixing device. The secondary fluid typically is a quench fluid that has a lower temperature than the process stream and is introduced to control the temperature of the process fluid as it enters the next catalyst bed. The secondary fluid may also be added for adjusting the composition of the process stream prior to its entry into the next catalyst bed. As shown in Figure 1 the mixing device includes a horizontally disposed first plate 16 that extends across the vessel 10 and is secured by any convenient manner. First plate 16, as shown, has a central opening through which the fluids may flow as they progress through the vessel 10. Also, plate 16 is located below and spaced apart from the catalyst support 14. The size of the central opening in plate 16 will depend upon the allowable pressure drop across plate 16. As a general guide, the open area in plate 16 will be in the range of from about 1 percent to about 15 percent of the cross-sectional area of the vessel 10. A cylindrical cap 18 with sidewalls 19 extends upwardly from first plate 16 and is positioned to cover the central opening in plate 16. Thus, cap 18 has a diameter greater than the diameter of the central opening in plate 16. Cap 18 also has a height less than the distance between the catalyst support 14 and plate 16 and preferably from about 1/2 to about 1/3 that distance. Cap 18 includes a plurality of angular slots 20 to impart a swirling motion to fluid passing through the cap in its downward progress though vessel 10. Preferably, slots 20 are tangentially oriented. Also, as can be seen in Figure 2, there are ten slots 20 equally spaced around the circumference of the cap 18. The number of slots 20, of course, can vary widely. Basically, the number and size of the slots will be selected to provide a predetermined pressure drop through the cap 18. In an alternative embodiment, shown in Figures 4 and 5, the cylindrical cap 18 has a plurality of vertical vanes 19 which constitute the sidewalls extending upwardly from plate 16. As is seen in Figure 5, these vanes 19 are oriented at an angle to the radial line, preferably from about 15° to about 75° and, most preferably, at about 45°. In a preferred embodiment, a cylindrical pipe 17 having the same diameter as the central opening in plate 16 extends upwardly from plate 16 for a distance less than the height of cap 18 and generally from about 1/2 to about 3/4 the height of cap 18. Referring again to Figure 1, as shown, slots 20 in cap 18 have a height that is about 1/3 the height of the cap; however, this dimension is not critical. Preferably, however, the slots 20 in cap 18 will have a height no greater than the height of cylindrical pipe 17. Located below plate 16 but spaced apart from it is a horizontally disposed plate 21 which has a plurality of openings 22 arranged in an annular ring around the periphery of plate 21. Vertically downwardly from plate 21 and communicating with openings 22 are a plurality of conduits 23. In other words, for each of the openings 22, there is a conduit 23 that extends downwardly from plate 21 and communicates with the opening 22. Conduits 23 have openings or slots 24 through which fluids flow. These slots 24 may be opened inwardly so that the fluids will be directed radially into the vessel 10; however, it is particularly preferred in the practice of the present invention that the slots 24 be oriented at an angle with respect to the inward radial lines so that the fluids emanating from conduits 23 are projected in a swirling direction. Slots 24 may be oriented at an angle of from 0° to 90° and preferably from about 15° to about 75° to the radial line. Indeed, a 45° orientation is most preferred. Conduits 23, as shown in Figures 1 and 3, are generally cylindrical pipe sections with slots 24 cut out. Alternatively, conduits 23 may be a generally U-shaped member such as is shown in Figure 6. In this form, the U-shaped member extends from plate 21 to plate 25 and has the opening 24 of the U angularly oriented in the same manner as slot 24 of the conduit 23 of Figures 1 and 2. As can be seen in Figure 1, located below horizontal plate 21 is a horizontally oriented plate 25 which has a vertically extended annular ring or arm portion 26 which spaces plate 25 from horizontal plate 21. Also, plate 25 has a large central opening in it. This opening, in general, will be in the range of from about 25% to about 75% and preferably from about 50% of the total cross-sectional area of the reactor. Centrally located on and extending upwardly from plate 21 is a cylindrical table 37. Cylindrical table 37 has a diameter at least equal to the diameter of the central opening in plate 16. Thus, table 37 is located on the plate 21 so that it is below the central opening in horizontal plate 16. Table 37 extends upwardly for a distance of about 1/2 to 1/3 the spacing of plate 21 and plate 16. In the present preferred embodiment, the improved interzone mixing device includes a fluid distribution plate 28 which is horizontally disposed in vessel 10 and spaced apart from horizontal plate 25. As can be seen in Figure 1, plate 28 contains a plurality of openings 29 from which there are upwardly extending conduits 30 provided with standard bubble caps 31 for controlling the flow of fluid through the distributor plate 28. A pipe or conduit 32 for introducing a secondary fluid from a source (not shown) external the vessel 10 is located between the cap 18 and the catalyst support 14. Pipe 32 terminates in a centrally positioned ring sparger 32S that has a plurality of openings 33 through which secondary fluid is fed transverse to the generally downward flow of the process stream fluid. As will be readily appreciated, the mixing device is equally suitable for use in upflow reactor vessels. This arrangement is shown schematically in Figure 7. In this embodiment, the various members of the interzone mixing device share substantially the same function and the same relationship to each other as in the downflow reactor vessel and, hence, the various components will be described briefly. Complete details concerning these members can be determined from a review of the description of the downflow reactor. Turning now to Figure 7, reference number 70 generally indicates a fluid upflow, multi-bed catalytic reactor or vessel having a vertical cylindrical sidewall 71 enclosing a plurality of beds of catalyst. The vessel 70, therefore, has an upper fluid outlet (not shown) and a lower fluid inlet (also not shown) for introducing a fluid stream for upflow through the vessel and ultimate removal of the stream after its passage through the vessel. Shown in Figure 7 are catalyst beds 12 and 13. The bed 12 of the catalyst is supported by a catalyst bed support 14. Interposed between successive catalyst beds 12 and 13 is a mixing device for introducing and mixing a secondary fluid, such as a quench fluid. As shown in Figure 7, the mixing device includes a horizontally disposed first plate 76 that has a central opening through which the fluids may flow as they progress through the vessel 70. Also, plate 76 is located above and spaced apart from the catalyst bed 13. A cylindrical cap 78 with sidewalls 79 extends vertically downwardly from first plate 76 and is positioned to cover the central opening in plate 76. Cap 78 includes a plurality of angular slots 80 to impart a swirling motion to fluid passing through the cap in its upward progress through vessel 70. In a preferred embodiment, a cylindrical pipe 77 having the same diameter as the central opening in plate 76 extends downwardly from plate 76. Located above plate 76 but spaced apart from it is a horizontally disposed plate 81 which has a plurality of openings 82 arranged in an annular ring around the periphery of plate 81. Vertically upwardly from plate 81 and communicating with openings 82 are a plurality of conduits 83. Conduits 83 have openings or slots 84 through which fluids flow. Located above horizontal plate 81 is a horizontally oriented plate 85 which has a vertically extended annular ring or arm portion 86 which spaces plate 85 from horizontal plate 81. Also, plate 85 has a large central opening in it. Centrally located on and extending downwardly from plate 81 is a cylindrical table 87. In the present preferred embodiment, the improved interzone mixing device includes a fluid distribution plate 88 which is horizontally disposed in vessel 70 and spaced apart from horizontal plate 85. As can be seen in Figure 7, plate 88 contains a plurality of openings 89 from which there are upwardly extending conduits 90 provided with standard bubble caps 91 for controlling the flow of fluid through the distributor plate 88. A pipe or conduit 92 for introducing a secondary fluid from a source (not shown) external the vessel 70 is located between the cap 78 and the catalyst bed 13. Pipe 92 terminates in a centrally positioned sparger 92S that has a plurality of openings 93 through which secondary fluid is fed transverse to the generally downward flow of the process stream fluid. Obviously, the precise dimensions of the various zones, openings, conduits and the like in the interzone mixing device will depend upon various factors such as permissible pressure drop, the space between the beds available for the mixing device and the like. The operation of the interzone mixing device will now be described in connection with a process stream of fluid exiting from catalyst bed 12 and flowing downwardly through vessel 10. This process stream is premixed with quench fluid introduced into the vessel via sparger 32S. The quench fluid and the process fluid then enter into the mixer through openings 20 and are given a swirling motion as they begin their downward flow through the central opening in plate 16. This swirling motion, of course, results in mixing of the process stream and the secondary or quench fluid. The combined quench fluid and process fluid, after passing through the opening in plate 16, then are deflected horizontally by table 27 which imparts a new torodial flowing pattern as the fluids move outwardly to the outer openings 22. The mixing action is then regenerated as the streams are sprayed out angularly through the openings 24 of pipes 23 thereby creating another swirl pattern below the horizontal deck 21. Finally, fluids flow downwardly through the center area of the plate 25 and are distributed by the bubble cap tray distributor disposed below the horizontal plate 25. The following examples demonstrate the significant mixing improvements achieved by the mixing device. In the examples, the following test procedure was employed. Test Procedure A For this mixing test, a stream of air was passed downwardly through the mixer and a flow of helium amounting to 1 to 5 volume percent of the total air flow passing through the mixing device in the examples which follow was injected into the outer edge of the air flow at a location just ahead of the mixing device. The mixing test device had 28 equally spaced peripheral outlets. Samples out of 8 nearly equally spaced peripheral outlets from among the 28 total outlets were analyzed for helium content using a standard calibrated thermal conductivity detector. This detector exhibits a linear response to helium content and can therefore be used to determine the helium content of the air stream reliably. The helium content was calculated from the thermal conductivity signal. Using the individual helium content measurements, an arithmetic deviation of the helium content from the mean was calculated by averaging the absolute normalized deviation of the helium concentration from the mean. The mean deviation multiplied by 100 is taken as the percent mean arithmetic deviation (ARD). The ARD value is a measure of the mixing occurring in the device. Perfect mixing would be denoted by an ARD of 0%, i.e. all samples have the same helium content. Thus, high ARD values correspond to low helium content uniformity exiting the mixing device and poor mixing performance and low ARD values correspond to good mixing performance. Example 1 A device was built as disclosed herein with a top plate measuring 4 inches (10.1 cm) in diameter, an orifice of 3 inches (7.6 cm) in diameter with 26 vanes angled at 150 to the radial lines, the vanes measuring .5 inches high (1.3 cm) and .5 (1.3 cm) inches deep. The box below the orifice was three inches deep and it contained a cylindrical table 8 inches in diameter and 1.5 inches (3.8 cm) high, centrally located below the orifice. The box diameter was 26 inches (66.0 cm) and at the outer periphery were located 28 equally spaced outlets, 0.375 inches (9.5mm) in diameter. The air flow rate was 181 SCFM (5.1 m³/min) and the helium flow rate 1.83 SCFM (0.052 m³/min). Using the test procedure outline above, the ARD for a 15° vane mixer was 42%. Example 2 The vane mixing device of Example 1 was modified to have the vanes at an angle of 30° to the radial line with all other dimensions kept the same. Also, the gas flow rates were the same. The ARD was determined to be 22% showing an improvement due to the steeper angle of the vanes. Example 3 The vane mixing device of Example 2 was modified to have the vanes at an angle of 45° to the radial line with all dimensions kept the same. The air flow rate in this instance was 180 SCFM (5.1 m³/min); the helium flow rate was 1.83 SCFM (0.052 m³/min). The ARD was determined to be 9% showing an improvement due to the steeper angle of the vanes in comparison to the device of Examples 1 and 2. Example 4 The device of Example 1 in which the top plate is 6 inches (15.2 cm) in diameter, the central hole 5 inches (12.7 cm) in diameter, 35 vanes at 150 to the radial 0.5 inches (1.3 cm) high by 0.5 inches (1.3 cm) deep with the spacer 0.75 inches (1.9 cm) high and 5 inches (12.7 cm) in diameter and the mixing box 1.5 inches (3.8 cm) high by 26 inches (66.0 cm) in diameter. The helium tracer was introduced through a straight pipe sparger placed above the mixing device. The sparger had 24 holes 0.16 inches (4.1 mm) in diameter. The test consisted of mixing 660 cubic feet (18.7 m³) per minute air with 6.8 cubic feet (0.19 m3) per minute of helium. The ARD was determined to be 23%. It should be readily appreciated that the present invention is, of course, suitable for use in various catalytic processes in which reactants or partially reacted reactants and quench fluids are to be homogeneously mixed so as to control the temperature profile of such materials, their composition or both. Indeed, the fluids may be all gases or gases or liquids, depending upon the nature of the process being carried out in the vessel.

Claims: An interzone mixing device for mixing a secondary fluid with a main process stream fluid in a vertical vessel, comprising: a horizontally disposed first plate (16) having a first surface, a second surface and a central opening therein for main process stream fluid to enter the interzone mixing device; a horizontally disposed second plate (21) having a first surface facing the second surface of the first plate (16) and a second surface, the second plate being spaced apart from the second surface of the first plate, the second plate (21) having a plurality of openings (22) in an annular ring around the periphery thereof; a plurality of conduits (23) extending vertically from the second surface of the second plate (21) and communicating with the peripheral openings (22) therein, the conduits having slots (24) therein oriented in the inward radial direction or at a predetermined angle thereto; a horizontally disposed third plate (25) having a first surface facing the second surface of the second plate (21) and a second surface, the third plate being spaced apart from the second surface of the second plate, the third plate (25) having a central opening therein; a horizontally disposed fluid distribution plate (28) spaced apart from the second surface of the third plate (25) and having a plurality of openings (29) therein; and means (32) for introducing a secondary fluid on the first surface side of the first plate (16), characterised by: a cylindrical cap (18) having a sidewall (19) extending vertically from the first surface of the first plate (16), the cap (18) being sized and positioned to cover the central opening in the first plate, the cap having a plurality of inward openings (20) in the sidewall (19) which are orientated at an angle to the radial direction to direct the flow of fluid in a swirling direction toward the central opening in the first plate; and a horizontally disposed table (37) positioned on the first surface of the second plate (21) and axially aligned with the central opening in the first plate (16). A device as claimed in claim 1, wherein the table (37) on the second plate (21) is cylindrical and has a diameter at least equal to the diameter of the central opening in the first plate (16). A device as claimed in claim 1 or 2 wherein the plurality of angular openings (20) in the sidewall (19) of the cap (18) are equally spaced and tangentially oriented. A device as claimed in claim 1 or 2, wherein the sidewall (19) of the cylindrical cap (18) consists of a plurality of vertical vanes (19) tangentially oriented. A device as claimed in any preceding claim, including a cylindrical pipe (17) aligned with, and having a diameter the same as, the central opening in the first plate (16) and extending vertically from the first plate (16) for a distance less than the height of the cap (18). A device as claimed of claim 5 wherein the slots (24) in the conduits (23) are oriented at an angle of 450 with respect to the radius of the vessel (10). A device as claimed in any preceding claim, wherein the means for introducing secondary fluid is a conduit (32) terminating in a centrally positioned cylindrical sparger pipe (32s) having a plurality of openings (33) for introducing secondary fluid radially outwardly.

Inventors: KOROS ROBERT MARTIN, ROSSETTI SALVATORE JOSEPH, WYATT JOHN THEODORE, KOROS, ROBERT MARTIN, ROSSETTI, SALVATORE JOSEPH, WYATT, JOHN THEODORE
Applicants: EXXON RESEARCH ENGINEERING CO, EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1991
Main classification: B01J8
IPC codes: B01F5, B01F3, B01J8, C10G49