Methods and apparatus for mixing fluids

A bubble cap (100) has a riser (120) and a cap (130), separated by a divider (140) that extends to a length at least 50% of a distance measured between the top of the riser (122) and the bottom of the cap (134). In another aspect, the riser (120) and cap (130) cooperate to provide a skirt height (160) of no less than 1.5″. More preferred bubble caps (400) have a relatively high skirt height (460) or long slit length (497), or both. In yet another aspect, flow-redirecting vanes (610) and plates (630) cooperate to provide a rough distribution of fluids to subsequent distribution tray (650).

FIELD OF THE INVENTION

The field of the invention relates to fluid mixing and distribution.

BACKGROUND OF THE INVENTION

Many commercial processes involve mixing of fluids, including especially catalytic reactors and large fractionation columns. Such mixing is not always a simple matter, especially where the fluid has multiple phases (such as liquids and gases/vapors), and where large volumes are being rapidly mixed. Numerous mixing apparatus are known, and some of these are described in U.S. Pat. No. 6,098,965 to Jacobs et al. (August 2000), which is incorporated herein by reference in its entirety. Jacobs et al. teach several improvements, some of which involve bubble caps spaced apart on a distribution plate.

Bubble caps generally comprise a riser and a cap, arranged such that a fluid flows upwards in a space between the cap and the riser, reverses direction and then flows downward through a passageway in the riser. In the absence of swirl directors, the fluid flow path is thus generally in the shape of an inverted “U”. Bubble caps are generally affixed to a distribution plate, and the passageway through the riser is confluent with a hole in the distribution plate. Bubble caps often contain a plurality of side slots that provide an entrance for the gas phase into the annular space between the riser and the cap. The gas entrains liquid present in the annular space. See, for example, U.S. Pat. No. 5,158,714 to Shih et al. (October 1992), which is herein incorporated in its entirety by reference.

There must be some mechanism for maintaining the position of the riser with respect to the cap. It is known to use cantilevered arms or other spacers for that purpose. See, for example, U.S. Pat. No. 5,989,502 to Nelson et al. (November 1999) and 4,305,895 to Heath et al. (December 1981), each of which is incorporated herein in its entirety by reference. In the past, such spacers have always been of minimal size to reduce cost and minimize any flow effects. Prior art spacers therefore exclusively serve a positioning function, and do not materially assist in either fluid flow or mixing.

Skirt height has been shown to materially affect the fluid flow and mixing. See, for example, “Optimum Bubble-Cap Tray Design”, Bolles, William L., a four part series inPetroleum Processing, Vol. 11, No.2, pp 65–80; Vol. 11, No.3, pp 82–95; Vol. 11, No.4, pp 72–79, Vol. 11, No.5, pp 109–120, which is incorporated herein in its entirety by reference. In this series of articles, Bolles presents a design methodology for bubble caps of the type commonly used in distillation columns. In such columns, the vapor flow is upward through the bubble cap tray and the liquid flow is transverse, across the bubble cap tray. Such flow is typically described as countercurrent flow. In the Bolles article, at Vol. 11, No.3, p.87, a skirt height of 0.5 inches to 1.5 inches is recommended, and there is a suggestion that greater skirt heights would be disadvantageous. There is certainly no teaching, suggestion, or motivation of which the current applicants are aware, for skirt heights greater than 1.5 inches.

Conversely, Ballard et al. (U.S. Pat. No. 3,218,249) teaches the use of bubble caps as a mixing and distribution means for the concurrent downflow of vapor and liquid. Ballard et al. teaches skirt heights of any distance “. . . above the distribution tray so long as the flow of gas through the downcomers is not sealed off; a reasonable range being from a level corresponding to practically no distance above the tray to a distance of about one foot thereabove.” teaches that “. . . the liquid phase, disengaged from the vapor phase by gravity, fills up on tray 18 to a level below the slot depth in the downcomer caps, such level being determined primarily by the gas flow rate per cap. It is, of course, necessary that some of the slot openings be exposed above the liquid surface to permit passage of vapor therethrough. Where the caps have no slots, the liquid level on the tray will be below the bottom rims of the caps for the same reason. Where unslotted caps are used, clearance between the bottom rim and the tray must be maintained to accommodate the passage of gas and liquid thereunder.” Clearly, the skirt height dimensional range taught by Ballard, et al. applies specifically to an unslotted cap, because vapor flow through a slotted cap can not be blocked off by reducing the skirt height to practically no distance. There is no teaching of a specific dimensional range suitable for slotted bubble caps.

Shih, et al. (U.S. Pat. No. 5,158,714) teaches the use of a dispersion plate to improve the distribution of liquid exiting the riser. Gamborg, et al. (U.S. Pat. No. 5,942,162) teaches the use of a slotted bubble cap, modified such that the cap is non-concentric with the riser, to improve the uniformity of liquid distribution. Gamborg, et al. describe this modified bubble cap as a vapor lift tube, wherein the cap is called an upflow tube and the riser is called a downflow tube. Nonetheless, the fluid flow path is the shape of an inverted “U”, flowing first upward through the upflow tube and then downward through the downflow tube. Jacobs, et al. (U.S. Pat. No. 6,098,965) teaches the use of riser vanes and/or target plates to improve the distribution of liquid exiting the riser. Aside from the patents cited above, the current applicants are not aware of any other information in the public domain that discloses technological advances in the use of bubble caps as a mixing and distribution means for the concurrent downflow of vapor and liquid

Some systems that utilize bubble caps provide for rough distribution of fluids upstream of the bubble caps. A patent granted to Stangeland, et al. (U.S. Pat. No. 5,690,896 November 1997) describes an apparatus for rough distribution comprising a perforated plate located directly above the bubble cap tray. With this approach, the perforations must pass both the gas phase and liquid phase fluids. As a result, the prevailing liquid level on this tray may be quite low, thereby negatively impacting the quality of rough distribution. A patent granted to Grott, et al. (U.S. Pat. No. 5,837,208 November 1998) describes an apparatus for rough distribution consisting of a perforated tray surrounded by cylindrical wall. With this approach, the gas phase fluid can flow through the annular area between the perforated tray and the reactor wall, while the liquid phase fluid flows primarily through the perforations. One drawback of this approach is that the annularly downflowing gas phase fluid can disturb the liquid surface on the bubble cap tray, thereby negatively impacting the performance of the bubble cap tray. Finally, with both of the above approaches, the perforated trays restrict inspection and maintenance access to the bubble cap tray.

Thus, there is still a need for improved methods and apparatus for mixing and distributing fluids, including improvements to bubble cap trays and rough distribution mechanisms.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides devices and methods in which a bubble cap has a riser and a cap, separated by a divider that extends to a length at least 50% of a distance measured between the top of the riser and the bottom of the cap, this distance henceforth being referred to as the “riser/cap span”. In preferred embodiments the divider is preferably at least 70% of the riser/cap span, and more preferably at least 90% of the riser/cap span,. The divider may be attached to either or both the riser and the cap, and there may be two or more such dividers.

In another aspect, the present invention provides devices and methods in which the riser and cap cooperate to provide a skirt height suitable for the liquid volumetric rate passing through the tray. The portion of the riser and cap below the liquid surface acts as a hydraulic resistance to liquid crossflowing the tray. This hydraulic resistance results in a variation in the liquid depth on the tray. Higher liquid depths occur in the areas on the tray where the liquid has been introduced to the tray, while lower liquid depths occur in the areas on the tray where the liquid has arrived by crossflow. These variations in the liquid depth are just as detrimental to the uniformity of liquid distribution as physical variations from levelness of the tray deck itself.

By increasing skirt height, the hydraulic resistance to liquid crossflow is reduced. The preferred skirt height for a specific application is dependent upon, among other things, the liquid volumetric rate passing through the tray. For low liquid rates, bubble caps having a skirt height of no less than 1.5 inches is preferred. At higher liquid rates, bubble caps having a skirt height of at least 2.0 inches is more preferred, and at still higher liquid rates, bubble caps having a skirt height of at least 2.5 inches is more preferred. At very high liquid rates, as may be encountered in very large reactors, bubble caps having a skirt height of 3 inches or higher are contemplated. The unusually high skirt heights are preferably achieved by using an especially long riser rather than using an especially short cap.

In yet another aspect, the present invention provides devices and methods in which chevron-type vanes and plates (e.g., mixing chamber floor and splash deck) cooperate to provide a rough distribution of fluids to subsequent distribution tray(s).

Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing in which like numerals represent like components.

DETAILED DESCRIPTION

InFIG. 1, a prior art bubble cap10generally comprises a riser20and a cap30separated by a spacer40. The bubble cap10is attached to a distribution plate15. The spacer40is very small with respect to the lengths of both riser20and cap30, and the skirt height60is less than 1.5 inches. The fluid flow path70through the bubble cap is generally in the shape of an inverted “U”.

InFIGS. 2A and 2B, a bubble cap100generally comprises a riser120and a cap130separated by a plurality of dividers140. The bubble cap cooperates with a distribution plate115to locally mix the fluids. (As used herein, the term “fluid” means anything that flows, including especially a vapor phase or a liquid phase, or a mixture comprising at least two phases. The term also includes any fluid that is mixed and distributed in a commercial process.)

The riser120has a top122and a riser height125defined by a distance between the top122of the riser120and the top116of the distribution plate115. The riser120also defines an inner passageway190. Contemplated risers can be formed of any suitable material, including carbon steel, stainless steel and other alloys, plastic, and ceramics, depending in large measure upon the temperature and corrosiveness of the fluids being mixed. Such risers can also have virtually any suitable overall dimensions. The overall shapes are also subject to variation. Although tubular risers having circular horizontal cross-sectional areas are preferred, it is also contemplated to provide tubular risers with elliptical, square, rectangular, or other horizontal cross-sectional areas. Risers need not even have uniform passageways along their length. Preferred risers may also have swirl directors150above or within (not shown) the passageways.

The cap130has a top132, a bottom edge134, and a cap length135defined by a distance between the top132of the cap130and the bottom edge134of the cap130. The cap130also has a skirt height160defined as the distance between the bottom edge134of the cap130and the top116of the distribution plate115. Contemplated caps can again be formed of any suitable material, including carbon steel, stainless steel and other alloys, plastic, and ceramics, depending again in large measure upon the temperature and corrosiveness of the materials being mixed. Preferred caps have horizontal cross-sectional areas of similar shape to that of the associated riser, but may also have other shapes. For example, a cylindrical cross-section riser may have a rectangular cross-section cap.

The skirt height160is a function of the riser height125, the cap length135, and the distance between the top122of the riser120and the top132of the cap130. Preferred bubble caps have a riser120and cap130that cooperate to provide a skirt height of no less than 1.5″. More preferred bubble caps have a skirt height of at least 1.75 inches, and even more preferred bubble caps have a skirt height of at least 2.0 inches, at least 2.5 inches, at least 3 inches, and at least 4 inches. The unusually high skirt heights are preferably achieved by using an especially long riser rather than using an especially short cap, although all combinations are contemplated.

Without being limited to any particular theory or contemplated mode of operation, the present inventors contemplate that a skirt height of no less than 1.5 inches is advantageous because it enhances cross-flow of fluids moving on the top116of the distribution plate115. Hydraulic calculations show that skirt heights up to 3 inches or higher may also be advantageous, depending largely upon the quantity of the liquid phase being conveyed across the top116of the distribution plate115, and subsequently through the space180between the riser120and the cap130and the riser passageway190. Although not presently considered to be a preferred embodiment, it is also contemplated that the bubble caps on a distribution plate need not all have the same skirt height. For example, some skirt heights may be less than 2 inches while others are more than 2 inches. Alternatively, all skirt heights may be more than 2 inches, and some may be more than 2.5 inches. It may even be advantageous for the bubble caps having relatively higher skirt heights to be positioned around the periphery of the distribution plate, or in some other manner, depending, at least in part, on where the fluids are introduced to the distribution plate.

Alternatively, the slots can be lengthened. Preferred slots can be at least 2.5 inches long, more preferably at least 3.5 inches long, still more preferably at least 4 inches long, at most preferably at least 5 inches long.

The dividers140inFIGS. 2A and 2Bpreferably span essentially the entire distance from the sidewall of the cap130to the sidewall of the riser120. The dividers are positioned near the top122of riser120. Other embodiments, however, are also contemplated. For example, dividers are currently contemplated to be long enough to have a significant impact on the hydraulics of the fluid flowing in the space180between the riser120and the cap130. Preferred dividers140impact the fluid hydraulics by having a length of at least 50% of the riser/cap span, preferably 70% of that distance, more preferably 90+% of that distance. In an alternative embodiment (not shown), the dividers can extend from the top of the cap al the way to the bottom edge134of the cap. The dividers need not be continuous, in that they may be constructed in several shorter dividers, as long as the sum of the length of the dividers is at least 50% of the riser/cap span. Contemplated dividers (not shown) may also be positioned non-vertically such that they impart a swirl to the fluid rising in the space180between the riser120and the cap130. Still further, any suitable number of dividers are contemplated to be utilized in any given bubble cap, including especially from two, three, four, five, or six dividers.

Dividers140may be attached to the riser, the cap, or both the riser and the cap. Attachment may be direct or indirect. Some of the dividers may assist in maintaining the positioning of the riser to the cap, and some may not assist very much, or at all, in that regard. Preferred methods of attachment include welding, such as tack-welding, stitch-welding, or any other welding means. Dividers may comprise any suitable material or materials. Swirl director150is affixed to the top122of the riser120. The swirl director150directs the fluid170from a space180between the riser120and the cap130to the riser passageway190in a circumferential flow path, which apparently results in a more uniform wetting of the inner wall of the riser120, and a ring-shaped discharge pattern of the fluid170, as the fluid170exits the riser passageway190. The swirl director may be continuous with the riser120, or may be affixed to the riser120by welding or any other suitable method. In operation, fluid170enters the bubble cap100through an opening117between the top116of the distribution plate115and the bottom edge134of the cap130, defined by a skirt height160. If the bubble cap100possesses one or more slots on the side of the cap130, fluid will also enter the bubble cap100therethrough. The fluid170then enters the space180between the riser120, the cap130, and the two dividers140. The fluid170then flows upward through the space180and through the swirl director150where the fluid170is mixed. The fluid then enters the riser120and flows downward through the riser passageway190. The cap length135is shorter than the cap length35ofFIG. 1, allowing the skirt height160to be longer than the skirt height60ofFIG. 1. In the event that two adjacent bubble caps100are at different elevations, due perhaps to a tilted distribution tray115, the two dividers140and the skirt height160allow more uniform splitting of the fluid170between the two adjacent risers than do two adjacent bubble caps10ofFIG. 1.

The distribution plate115is preferably circular, and measures between about 36 inches and about 240 inches in diameter, and between about 0.06 inches and 0.50 inches thick. The size generally depends upon the size of the reactor in which it is utilized. Currently preferred distribution plates are made from stainless steel and other alloys, although any suitable material, including carbon steel, plastics. and ceramics are also contemplated. A typical distribution plate115supports between about 60 and about 1200 bubble caps, although lesser or greater numbers of bubble caps are also contemplated. The risers120are typically rolled into the distribution plate115, such that the riser passageways190coincide with holes118in the distribution plate115.

As depicted in the Jacobs patent referenced above with respect to other bubble caps, the distribution plate115may actually comprise a re-distribution plate because chambered mixing and/or rough distribution may be accomplished upstream. Thus, it should be apparent that distribution plate115may be placed at any appropriate position with respect to other processes and apparatus in any mixing reactor.

InFIG. 3, bubble cap200is similar to the bubble cap100ofFIGS. 2A and 2B, except that the bubble cap200has four dividers240instead of the two dividers140. InFIG. 3, the four dividers240are organized into two sets of two dividers, each set disposed in separate vertical planes within a space280. Within each set, the two dividers are disposed within one vertical plane within the space280, and separated within the space280. As a result, the fluid270may pass through the space280formed between the riser220, the cap230, and past the four dividers240.

InFIG. 4, bubble cap300is again similar to the bubble cap100ofFIGS. 2A and 2B, except that the bubble cap300has a cap length335that is shorter than the cap length135, and a riser height325that is shorter than the riser height125. The result is a skirt height360that is equal to the skirt height160of the bubble cap100, even though the riser heights and cap lengths are different.

InFIG. 5, bubble cap400has a cylindrically curved side433, in which are disposed multiple side slots495. Each of the multiple side slots495extends downward to the bottom434of the cap430, such that the slot length497of any given slot495is the distance from the top496to the bottom434of the cap430. The slot elevation498is defined as the distance between the top496of the slot495and the top416of the distribution plate415. Among other things, such side slots495allow passage into the bubble cap400of a fluid470being mixed and distributed.

The bubble cap400ofFIG. 5has at least eight slots495, four of which are shown. The slot length497is 2.5 inches, and the slot elevation498is 4.5 inches. In alternative embodiments it is contemplated that the slot length497could be anywhere from about 1.5 inches to about 12 inches. Slots495typically have a generally rectangular shape, although they may have any other suitable shape such as a triangular or other tapering shape, a zigzag shape, and so forth. InFIG. 6, a distribution plate516contains a plurality of bubble caps500. The fluid570flows in a zigzag550pattern on the distribution plate516, with the risers520and caps530creating a hydraulic resistance to crossflow. A portion555of the crossflowing fluid570is mixed and distributed by the bubble caps. The plurality of bubble caps500may vary in quantity, depending on a variety of factors. Two of the factors are the cap center-to-center spacing, which influences the number of caps per unit of distribution tray area, and the size of the reactor or any other commercial process being used to mix and distribute fluids. Furthermore, the plurality of bubble caps500may be distributed on the distribution plate516in any manner, preferably in a symmetrical manner to achieve a symmetrical distribution of the fluid. There may or may not be indentations, channels,-baffles, or other paths (not shown) disposed in or on the distribution plate516to modify the cross-flow550.

InFIGS. 7A,7B and7C, a rough distribution apparatus600contains a plurality of chevron-type vanes610. The vanes are disposed between an outlet of a mixing apparatus620and a splash deck630. The presence of the splash deck630forces the fluid exiting the mixing apparatus to flow outward through the passageways612formed by chevron-type vanes610along paths613. The splash deck630is preferably imperforate, but may contain orifices (not shown) to allow a portion of the fluid to pass downward onto the subsequent distribution tray650(which maybe the final distribution tray).

By way of reference,FIG. 7Bdepicts catalyst bed640below subsequent distribution tray(s)650, and reactor wall660.

In a preferred embodiment, the chevron-type vanes610are positioned below the substantially imperforate floor of a mixing chamber (not shown), above a substantially imperforate splash deck630, and surround the outlet orifice(s)620of an upstream mixing chamber (not shown). The vane passageways612thereby formed cause the fluids flowing therethrough to change directions preferably at least two times and provide the sole means of fluid communication between the upstream mixing chamber and the downstream subsequent distribution tray650. The chevron-type vanes610result in a more uniform velocity profile of the fluid exiting the vane passageways612, thereby providing more effective rough distribution of the fluid to the subsequent distribution tray650. When used in conjunction with a mixing chamber that swirls the fluids being mixed therein, the chevron-type610vanes also serve to reduce the tangential component of the fluid velocity. When arranged in circular layout that is concentric with a central outlet orifice of the mixing chamber, the chevron-type vanes610promote a liquid discharge pattern, exiting the vane passageways612, such that the liquid is supplied to the subsequent distribution tray650in an annular ring (not shown). This annular ring supply pattern is an extremely effective method of supplying liquid to the subsequent distribution tray650, provided that the diameter of the ring produced by the liquid is near optimal. The optimal ring diameter is dependent upon the geometry of the final distribution tray650and can be determined by hydraulic calculations. Although chevron-type vanes have been depicted inFIGS. 7A,7B, and7C, other flow redirecting-type vanes have been contemplated. Several examples are depicted inFIGS. 8 and 9.

InFIG. 8, wave plate-type vanes710are spaced apart to form vane passageways712, the passageways providing a flow path713for fluids to pass therethrough.

InFIG. 9, staggered channel-type vanes810are spaced apart to form vane passageways812, the passageways providing a flow path813for fluids to pass therethrough.