Vortex-type mixing device for a down-flow hydroprocessing reactor

The present invention is directed to a vortex-type mixing device for a down-flow hydroprocessing reactor. In particular, the device improves the effectiveness of an existing mixing volume in mixing the gas phase and liquid phase of two-phase systems. According to the present invention, the mixing device helps create a highly arcuate flow to incoming effluents and a high degree of mixing within a constrained interbed space of a hydroprocessing reactor.

FIELD OF THE INVENTION

This invention is directed to a vortex-type mixing device for a down-flow hydroprocessing reactor. Such down-flow hydroprocessing reactors are used in the petroleum and chemical processing industries for carrying out catalytic reactions of hydrocarbonaceous feedstocks in the presence of hydrogen, at an elevated temperature and pressure. Exemplary reactions including hydrotreating, hydrofinishing, hydrocracking and hydrodewaxing.

BACKGROUND OF THE INVENTION

In fixed-bed hydroprocessing reactors, gas and liquid reactants (e.g. hydrogen and a hydrocarbonaceous feedstock) flow downward through one or more beds of solid catalyst. (See, e.g. U.S. Pat. No. 4,597,854 to Penick).

As the reactants flow downward through the reactor catalyst beds, the reactants contact the catalyst materials and react to produce the desired products. Gas reactants such as hydrogen are consumed, and heat is generated by the catalytic reactions. Controlling the temperature of the feedstock as it travels downward through the reactor is important to ensure the quality and quantity of product yield is maximized toward the target product(s).

Cool hydrogen-rich gas can be introduced between the catalyst beds to quench the temperature rise and replenish the hydrogen consumed by the reactions. In order to maintain overall reactor performance, the temperature of the fluids within the reactor should be as uniform as possible and liquids and gases should be well mixed in order to maximize performance. Poor interbed fluid mixing can limit reactor operation in various ways. When interbed mixing is unable to erase the radial temperature differences, these differences persist or grow as the process fluids move down the reactor. Hot spots in any bed can lead to rapid deactivation of the catalyst in that region which shortens the total reactor cycle length. Product selectivities are typically poorer at high temperatures. For example, hot regions can cause color, viscosity and other product qualities to be off-specification. Also, if the temperature at any point exceeds a certain value (typically 800 to 850° F.), the exothermic reactions may become self-accelerating leading to a runaway event, which can damage the catalyst, the vessel, or downstream equipment.

Due to these hazards, refiners operating with poor reactor internal hardware must sacrifice yield and/or throughput to avoid the deleterious effects of poor interbed fluid mixing. Reactor temperature maldistribution and hot spots can be minimized through mixing and equilibration of reactants between catalyst beds, correcting any temperature and flow maldistributions, and minimizing pressure drops. The mixing of fluids between catalyst beds can be accomplished through the use of distributer assemblies and mixing chambers. With present-day refinery economics dictating that hydroprocessing units operate at feed rates far exceeding design, optimum interbed fluid mixing is a valuable low-cost debottleneck.

Distributor assemblies can be used to collect, mix, and distribute fluids in the interbed region of multi-bed catalyst reactors. Distributor assemblies generally include a trough for collecting and mixing liquid and gas flowing from an overhead catalyst bed, and a mixing device or chamber disposed centrally within the trough for receiving liquid from the trough and further mixing the liquid and gas.

The mixing device is a key component of many distributor assemblies because it provides efficient and thorough mixing of fluids/gases and helps avoid hot spots and poor temperature distribution.

The mixing device has at least one inlet for receiving liquid from the trough and at least one outlet for directing flow toward an underlying catalyst bed. Designs for mixing devices vary, including baffle mixer designs such as ribbon blenders and disk-and-donut type mixers that promote mixing through changing the direction of the fluid and gases.

Another type of mixer is a centrifugal or vortex-type design. This type of mixer collects the liquid and gas streams flowing downward through the reactor, and introduces them into a circular chamber where they make several rotations before being passed downward through a centrally located aperture.

If present, the mixing device is generally located in the interbed space between catalyst beds in a reactor. The interbed space in many reactors is limited due to the presence of support beams, piping, and other obstructions which occupy the interbed region. Due to these space constraints, unique hardware, such as a mixing device scaled to fit the space available, is required to perform efficient two-phase mixing in what amounts to limited volume. In addition, lower height distributor assemblies can increase catalyst loading volume with the same reactor volume, therefore improve utilization of the reactor volume.

Due to the importance of sufficient interbed fluid mixing for good catalyst lifetimes, high throughput, long cycle length, and overall reactor performance, improved mixing devices are needed. In addition, mixing devices that have lower vertical footprint and that can be retrofitted to existing reactors which have limited interbed space are of particular necessity.

SUMMARY OF THE INVENTION

The present invention is directed to a vortex-type mixing device for a down-flow hydroprocessing reactor. The mixing device provides a more effective mixing of fluids in the space between catalyst beds in a multi-bed reactor. In particular, the invention is direct to a mixing device that improves the effectiveness of an existing mixing volume in mixing the gas phase and liquid phase of two-phase systems. The device is well suited for retrofit applications due to its relatively small size and can also be scaled for new reactor designs to achieve efficient fluid mixing in the interbed space of a multi-bed reactor.

The mixing device includes a horizontal cover plate having an inner surface and a base plate extending parallel to the cover plate. The base plate having an inner surface and a base plate aperture.

A plurality of inwardly-curved vanes extend vertically between the inner surfaces of the cover and base plates. A vertical weir ring extends vertically from the base plate inner surface proximal to the circular aperture. The weir ring has a weir ring top edge and a weir ring diameter. A bubble cap extends downwardly from the inner surface of the cover plate into a mixing region. The bubble cap has a bubble cap diameter and a bottom edge, the bubble cap diameter being smaller than the weir ring diameter, and the bubble cap bottom edge extends below the weir ring top edge.

DETAILED DESCRIPTION

It has been found that the vortex-type mixing device of the present invention affords benefits over vortex-type mixing devices known in the art. Such benefits include, a reduced vertical footprint in the reactor (reduction in reactor volume occupied by inter bed distributor assemblies), high throughput, enhanced mixing, lower pressure drop, and enhanced overall reactor performance. Specific embodiments and benefits are apparent from the detailed description provided herein. It should be understood, however, that the detailed description and specific examples, while indicating embodiments among those preferred, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

The present invention is directed to a vortex-type mixing device for a multi-bed hydroprocessing reactor. Referring toFIG. 1, a cross-sectional diagram of a portion of a multi-bed down-flow reactor10is illustrated. The reactor10includes a vessel shell12, upper and lower catalyst beds (14and16, respectively) containing packed catalytic extrudates. Each catalyst bed14,16is supported on a grid screen assembly18(illustrated for catalyst bed14only) composed of a support grid, optional space cloth and screen, all of which are well known in the art. The grid screen assembly is mounted on parallel support beams20that are horizontally mounted to the reactor vessel inner wall22, and extend upwardly into the catalyst bed14.

An interbed distribution assembly24is vertically interposed between the catalyst beds14,16. The interbed distribution assembly24includes a vortex-type mixing device26of the present invention. The mixing device26of the invention is mounted under the catalyst bed14, and in fluid tight communication with a collection plate28adapted to receive and mix liquid and gas flowing down from the overhead. A quench gas inlet tube30distributes quench gas (e.g. hydrogen) into the region above the mixing device26.

FIG. 2is a cross-sectional diagram of the mixing device26,FIG. 3is an isometric view of one half of the mixing device26, andFIG. 4is a top plan view of the mixing device26.

The mixing device includes a base plate32having an inner surface32aand mounted in fluid tight communication with the collection plate28(FIG. 1), and a cover plate34having a cover plate inner surface34awhich extends substantially horizontally relative to the base plate32. The annular collection plate28collects fluids flowing down from the overlying catalyst bed.

A plurality of staggered, inwardly-curved vanes36are fixedly attached to, and extend vertically between, the base and cover plate inner surfaces32aand34a, respectively. In one embodiment, the vanes36are in fluid tight communication with the base and cover plate inner surfaces32aand34a, respectively.

As shown inFIG. 4, each vane has a vane external end38affixed proximal to the outer periphery of the cover plate34, and internal end40located proximal to a mixing region (described herein below) of the mixing device26. The open space between the vanes36define a series of mixing device inlet regions42, each inlet region42being defined as area bound by neighboring vanes36and their respective ends38,40.

Referring again toFIG. 2, a weir ring44extends vertically from the base plate32, proximal to a centrally-positioned circular base plate aperture defined by edge46. In one embodiment, the weir ring44has a vertical height of one-half of the vertical height of the vanes36. A weir ring horizontal plate48extends outwardly from the weir ring upper edge50. The weir ring horizontal plate48is preferably perforated.

A circular bubble cap52affixed to, preferably in fluid tight communication with, the cover plate inner surface34aextends from the cover plate34downwardly into the center of a mixing region of the mixing device26(the mixing region being defined as the area between the plates32,34, excluding the inlet regions42, or, stated differently, the region defined by the area between opposing vane internal ends40). The bubble cap may be keyed as illustrated inFIGS. 2 and 3.

The lower end54of the bubble cap52extends a distance52abelow the weir ring upper edge50. In one embodiment, wherein the bubble cap lower edge54is keyed, the upper edges58defining the keyed openings are positioned below the weir ring upper edge50.

The collection plate28includes a circular aperture defined by edge60, and a riser tube62proximal to the collection plate circular aperture60extends vertically upward from the collection plate28and into the bubble cap52. The top edge64of the riser tube62is situated at or above the bubble cap upper edge58of the keyed openings.

As shown inFIG. 2, the weir ring44, bubble cap52and collection plate aperture60each have a diameter44a,52aand60a, respectively, wherein the measured values of the diameters have the following relationship:60a<52a<44a.

In operation, hydrocarbonaceous liquid feed rains down from the catalyst bed14, through the grid screen assembly18, and onto the annular collection plate28. Typically, the liquid will collect and rise to a liquid level at or above the height of the weir ring horizontal plate48. Gas from the upper catalyst bed14mixed with quench gas (e.g. hydrogen gas) introduced via the quench gas inlet tube30fills the void between the liquid collected on the annular collection plate28and the catalyst bed14.

The liquid and gas enter the mixing device26via the mixing device inlet regions42, wherein the vanes36tangentially direct the liquid and gas to flow in an arcuate or circular flow pattern as the liquid and gas enter the mixing region of the mixing device26. The liquid travels up and over the weir ring44(and through the horizontal weir plate48if it is perforated), and intermixes with the gas as the liquid/gas flow into the bubble cap52under its lower end54and through the keyed openings, over the riser tube top end64and into the riser tube62. The intermixed gas and liquid then travel downward out of the riser tube62, typically to a tray containing a plurality of perforations, downcomers or nozzles, and then on to the lower catalyst bed16. Perforated spiral plates are installed on inner surface of the riser62to further improve gas/liquid mixing while flowing downward in the riser62.

As can be appreciated by one skilled in the art, a mixing device26as described herein is intended for use in a large hydroprocessing reactor designed to process thousands or tens-of-thousands of barrels of feedstock per day (1 barrel=43 gal.; 164 L). Accordingly, the mixing device26described herein may be several feet in diameter and, because of the materials used to construct the device26(e.g. ¼″-½″ plate steel), weigh several hundred pounds (.lbs) when constructed.

The mixing device26of the present invention may be constructed in place by welding or otherwise affixing the individual components together to achieve construction of the finished device26. However, it will be recognized that constructing the device26in place using this method may take several days, delaying operation of the reactor unit. In addition, where the device26is being employed to update or retrofit the design of an existing reactor, it is desirable to reduce the amount of assembly taking place within the reactor vessel (due to safety concerns such as possibly igniting residual hydrocarbon materials remaining in the reactor).

In order to reduce the amount of time needed to construct a new reactor, or retrofit an existing reactor, portions of the mixing device26are preferably pre-assembled to form subassemblies, and the subassemblies are inserted into the reactor and assembled to form the completed mixing device26.

In one embodiment illustrated inFIGS. 2,3and4, the mixing device26consists of two mixing device subassemblies26a,26b, each representing one-half of the mixing device26. Each subassembly26a,26bis provided with one or more lifting lugs66and68, respectively. The lifting lugs66,68are provided for attaching each subassembly66,68to a hoist, crane or other device capable of lowering the subassembly into the reactor and maneuvering the subassembly into place.

Each subassembly26a,26bis provided with a mating flange70and72, respectively, containing a plurality of openings through which a nut/bolt combination (or such other appropriate affixing device) can be inserted to hold the subassemblies26a,26bin place during operation, and further allows the subassemblies26a,26bto be disassembled between operating periods during maintenance to allow access to the areas above and below the mixing device26.

Referring toFIG. 5, the horizontal placement of the vanes36is illustrated. Circle R1, representing the outer diameter of the mixing device26as prescribed by process hydraulic calculations employed by those skilled in the art, along with circle R3, presenting the inner diameter of the mixing device inlet regions42also prescribed by process hydraulic calculations, are illustrated. Circle R2is located half the radial distance between R1and R3.

Angle “A” ofFIG. 5represents the angular offset of each corresponding vane36, which results in a radial overlap of a vane internal end40with a larger portion of a neighboring vein external end38. In one embodiment, A=15° for a 4 vane system, 10° for a 6 vane system, and 8° for a 8 vane system. Angle “B” represents the radial distance the vane36occupies within the region between R1and R3. In one embodiment, B=360°/(the number of vanes). At angle “A”, the inner surface of the vane36intersects with R1. At an angle that is the sum of angles “A” and “B” (A+B) the inner surface of the vane36intersects with R3. At an angle that equals A+B/2, the inner surface of the vane36intersects with R2.

The radial overlap of the vanes36is defined by angle “A”. In operation, the liquid and gas enter the mixing device26via the mixing device inlet regions42, wherein the vanes36tangentially direct the liquid and gas to flow in an arcuate or circular flow pattern as the liquid and gas enter the mixing region of the mixing device26.

The previous description of a preferred embodiment of the present invention is primarily for illustrative purposes, it being recognized that a number of variations might be used which would still incorporate the essence of the invention. Accordingly, reference should be made to the following claims in determining the scope of the invention.