WO1998051390A1

WO1998051390A1 - Vapor liquid contact tray with relief weir sumps

Info

Publication number: WO1998051390A1
Application number: PCT/US1998/009808
Authority: WO
Inventors: Chang-Li Hsieh
Original assignee: Koch-Glitsch, Inc.
Priority date: 1997-05-12
Filing date: 1998-05-12
Publication date: 1998-11-19
Also published as: AU7571698A

Abstract

A vapor-liquid contact tray (16) is provided with a tray deck (40) containing vapor flow apertures (48) and a downcomer (50) positioned at an opening (46) in the tray deck (40). Spaced apart sumps (60) are positioned in cutouts (61a and 61b) formed in the tray deck (40) and inlet wall (52) of the downcomer (50) to receive liquid from the tray deck (40) and convey it into the downcomer (50). As a vapor-liquid mixture or froth descends into the sumps (60), vapor is able to disengage from the liquid, thereby increasing the liquid handling capacity of the downcomer (50). A weir (70) positioned along the outlet end of the tray deck (40) causes liquid to accumulate to a preselected level on the tray deck (40) before it overflows the weir (70) and enters the sumps (60) and downcomer (50). End portions (74) of the weir (70) are displaced in the upstream direction around the cutouts (61a) in the tray deck (40), thereby increasing the overall length of the weir (70).

Description

VAPOR LIQUID CONTACT TRAY WITH RELIEF WEIR SUMPS

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 60/046,230, filed May 12, 1997.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

BACKGROUND OF THE INVENTION

This invention relates in general to mass transfer columns and, more particularly, to vapor-liquid contact trays placed within the columns. The invention also relates to methods for using the trays to effect mass transfer between vapor and liquid streams flowing within the column.

Vapor-liquid contact trays are used in mass transfer columns to facilitate interaction and mass transfer between vapor and liquid streams flowing through the column. The trays typically have a tray deck with liquid inlet and outlet ends and an opening formed at the outlet end of the tray deck. A downcomer is positioned at the opening in the tray deck and provides a passage for removing liquid from the outlet end of the tray deck and directing it downwardly to a liquid receiving area at the inlet end of the underlying tray deck. A weir is typically positioned at the outlet end of the tray deck to force liquid to accumulate to a preselected level on the tray deck before it spills over the weir and enters the downcomer inlet. The liquid exiting the downcomer discharge outlet then flows across the underlying tray and enters the downcomer at the outlet end of that tray deck for passage to the next underlying tray. This pattern is then repeated on each underlying tray.

As liquid is flowing across the tray deck on these vapor-liquid contact trays, vapor passes upwardly through apertures provided in the "active area" of the tray deck and interacts with the liquid to form a frothy two-phase mixture. Most of the vapor then disengages from the mixture and passes upwardly through the apertures in the overlying tray deck. A portion of the vapor, however, remains entrained with the liquid entering the downcomer and passes downwardly to the underlying tray. The presence of vapor in the liquid traveling downward through the downcomer takes up a portion of the fixed volume of the downcomer, thereby reducing the amount of liquid that can flow through the downcomer. The return of the vapor to the underlying tray is also generally undesirable in that it limits the mass transfer efficiency of the tray.

One approach to reducing the volume of gas entering the downcomer with the liquid involves moving the end portions of the weir in the upstream direction on the tray deck to form tray deck calming sections between the weir end portions, also known as "relief weirs," and the downcomer inlet opening. These calming sections do not contain vapor flow apertures and the liquid-vapor froth entering these sections is not further agitated by the upward flow of vapor through the tray deck. As a result, the liquid entering the downcomer from the calming sections may contain less vapor than the liquid that passes over the intermediate portion of the weir and enters the downcomer without first passing through the calming sections. It is also known to extend the downcomer into those areas of the tray deck lying downstream from the displaced end portions of the weir. These extended downcomers, however, discharge liquid vertically downward onto the underlying tray and vapor apertures are normally omitted from the liquid receiving areas of the tray to prevent undesirable weeping of the liquid through the vapor apertures.

Another advantage to forming relief weirs in the manner described above is it increases the length of the weir, thereby reducing the height of the liquid crest spilling over the weir. This slight reduction in the height of the liquid crest is believed to increase the effectiveness of the tray by increasing the space between the liquid crest and the bottom surface of the overlying tray which is available for vapor-liquid interaction. The length of the weir can also be lengthened by simply increasing the size of the downcomer inlet opening. Although a larger downcomer inlet can increase the liquid handling capacity of the downcomer, the effectiveness of the tray can be reduced because of the resulting decrease in the active area of the tray.

While the tray modifications described above can increase the liquid handling of the downcomer, further advances would be desirable.

BRIEF SUMMARY OF THE INVENTION

It is an object of this invention to provide a vapor-liquid contact tray with a downcomer having increased liquid handling capacity by providing relief weirs and sumps into which the vapor-liquid froth flows prior to entry into the downcomer so that vapor disengagement is facilitated by gravitational effects as the froth flows over the relief weirs and into the sumps positioned below the level of the tray deck.

It is also an object of this invention to provide the vapor-liquid contact tray with relief weir sumps that increase the vertical distance through which the froth flows prior to entry into the downcomer to provide additional opportunity for vapor disengagement.

It is another object of this invention to provide the vapor-liquid contact tray as described with sumps that feed liquid into the downcomer rather than discharging the liquid directly onto the underlying tray deck so that vapor apertures can be placed in the tray deck underlying the sumps to increase the active area and effectiveness of the tray.

To accomplish these and other related objects of the invention, in one aspect the invention is directed to a vapor-liquid contact tray, a plurality of which can be positioned in vertically spaced apart relationship within a mass transfer column. The trays have a tray deck containing an opening for removing liquid from an upper surface of the tray deck and a plurality of apertures for allowing vapor to flow upwardly through the tray deck to interact with the liquid and form a two-phase mixture. A downcomer positioned at the opening in the tray deck extends downwardly toward the underlying tray deck and has an upper inlet to receive the liquid entering the opening from the tray deck and a lower discharge outlet through which liquid exits the downcomer and is directed onto an underlying vapor-liquid contact tray. The downcomer is formed by an inlet wall connected to the edge of the tray and other side walls that are connected to the inlet wall to form a liquid confining structure. The other side walls may be formed by the column shell. In accordance with the invention, two spaced apart sumps are positioned at the outlet end of the tray deck along the column shell. The sumps extend below the plane of the tray deck and cutouts are formed in the tray deck and downcomer inlet wall at the location of the sumps so that liquid may flow into the sump from the tray deck and then from the sump into the downcomer. The sumps are formed by side walls and a bottom plate that are joined together to form a structure open at the top and at the downstream side. The side walls and bottom plate preferably do not contain vapor flow apertures so that froth entering these sumps is not further agitated by vapor flowing through the side walls or bottom plate. A weir extends along the tray deck at the outlet end thereof to cause liquid to accumulate to a preselected height on the tray deck before entering the downcomer and sumps. A center portion of the weir extends along and separates the outlet end of the tray deck from the opening that forms the inlet for the downcomer. End portions of the weir are connected to the center portion but are displaced toward the upstream end of the tray deck and extend around the cutouts for the sumps. Liquid which overflows the center portion of the weir flows directly into the downcomer inlet while liquid overflowing the end portions or relief weirs flows first into the sumps and then into the downcomer. The sumps provide calming areas to help in the disengagement of vapor from liquid prior to entry of the liquid into the downcomer. Because the sumps are positioned below the plane of the tray deck, vapor disengagement is further facilitated by gravitational forces acting on the froth as it descends into the sump.

In another aspect, the invention is directed to a method of utilizing the vapor-liquid contact trays to facilitate vapor-liquid interaction within the column. As described above, the sumps help to remove gas from liquid prior to entry of the liquid into the downcomer, thereby increasing the liquid handling capacity of the downcomer. Notably, because the sumps feed into the downcomer rather than directly onto the tray below, vapor flow apertures can be placed in the underlying portions of the tray to increase the active area and tray effectiveness.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the accompanying drawings which form a part of the specification and are to be read in conjunction therewith and in which like reference numerals are used to indicate like parts in the various views:

Fig. 1 is a fragmentary perspective view of a mass transfer column with portions broken away to illustrate the internal placement of a plurality of vapor-liquid contact trays constructed according to the present invention; and

Fig. 2 is a fragmentary top perspective view of the column illustrating the vapor-liquid contact trays; and

Fig. 3 is a fragmentary side elevation view of the vapor-liquid contact trays taken in vertical section along line 3-3 in Fig. 1. DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings in greater detail and initially to Fig. 1 , a mass transfer or heat exchange column is designated generally by the numeral 10 and includes an upright cylindrical shell 12 which defines an open interior region 14 in which a plurality of vapor-liquid contact trays 16 are contained. Column 10 is of a type used for processing liquid and vapor streams, including to obtain fractionation products. Although column 10 is shown in a cylindrical configuration, other shapes, including polygonal, may be used. The column 10 can be of any suitable diameter and height and can be constructed from suitable rigid materials. One or more liquid streams can be directed to the column 10 in a conventional manner through side stream feed line 18 and overhead reflux return line 20. Likewise, one or more vapor streams can be charged to the column through side stream feed line 22 or can be generated within the column. In addition to trays 16, internals such as beds of packing 24 and 26 and liquid distributors 28, 29 and 30 and collector 31 can be located within the column to carry out the desired processing of the liquid and vapor streams as they flow in countercurrent relationship through the column 10. Vapor product is removed from the top of column 10 through overhead removal line 32 and liquid product is removed as a bottoms through removal line 34. Liquid can also be removed at an intermediate portion of column through side stream drawn off line 36. A plurality of manways 38 extend through the column shell 12 to facilitate installation and removal of internal column components. Other appropriate system components such as reboilers, condensers and the like are not illustrated because of their conventional nature.

Turning additionally to Figs. 2 and 3, the vapor-liquid contact trays 16 of the present invention are positioned in the interior region 14 of column 10 and are placed in vertically spaced apart relationship. The trays 16 are generally horizontally disposed and are supported on rings 39 mounted on the inner surface of the column shell 12. Each tray 16 comprises a tray deck 40 having an inlet end 42 where liquid is introduced onto the tray deck 40 and an outlet end 44 where liquid is removed from the tray deck through an opening 46 in the tray deck 40. A plurality of apertures 48 are uniformly distributed across the portion of the tray deck known as the "active area." The apertures 48 permit vapor to pass through the tray deck 40 and interact with liquid flowing across the upper surface of the deck. The apertures 48 have a size, shape and distribution selected for the particular operating conditions in which the tray 16 will be utilized.

Each tray 16 includes a downcomer 50 positioned in the opening 46 at the outlet end 44 of the tray deck 40 and extending downwardly below the tray deck a preselected distance toward the underlying tray. The downcomer 50 is formed in part by an inlet wall 52 which can be planar and vertically positioned as illustrated, or can be sloped, curved, multi-segmented chordal, or other desired configuration. Some or all of the remaining walls of the downcomer 50 can be formed by the column shell 12. The downcomer 50 operates to remove liquid from the outlet end 44 of the tray deck and direct it downwardly to the inlet end 42 of the underlying tray deck. The downcomer 50 has an upper inlet 54 through which liquid enters the open top of the downcomer for downward passage therethrough and a discharge outlet 56 through which liquid is discharged onto the inlet end 42 of the underlying tray deck 40. The portion of the tray deck 40 onto which the liquid is discharged is referred to as a liquid receiving area 58 and typically does not contain apertures 48 because the downward force of the liquid could cause the liquid to weep through the apertures. If desired, however, louvres or other devices designed to impede liquid weep can be placed in the liquid receiving area. In addition, a perforated bottom plate (not shown) can be used at the downcomer discharge outlet 56 to cause liquid to accumulate within the downcomer to impede vapor entry through the discharge outlet 56.

In accordance with the present invention, downwardly extending sumps 60 are formed in the tray deck 40 at the outlet end 42 thereof to receive a portion of the liquid exiting the outlet end of the tray deck 40 and feed it into the downcomer 50. The sumps 60 are preferably positioned in spaced apart relationship in cutouts 61a and 61b, respectively, formed in the portions of the tray deck 40 and downcomer inlet wall 52 adjacent the column shell 12. The cutout 61a in the tray deck 40 is preferably contiguous with the opening 46 in the tray deck that forms the inlet 54 entry into the downcomer. Each sump 60 comprises an inclined, planar bottom plate 62 connected at an inlet end 64 to the edge of the tray deck 40 and sloping downwardly to an outlet end 66 connected to the top edge of the cutout 61b in the downcomer inlet wall 52. Rather than being formed as a planar piece, the bottom plate 62 can be curved, can extend parallel to the tray deck, or can be formed from two or more planar or curved panels connected together. Side walls 68, one of which is formed by a portion of the column shell 12, are joined to the bottom plate and close the sides of the sump 60 so that it is open only at its top and outlet end. Preferably, the bottom plate 62 and side walls 68 do not contain vapor apertures 48. The sumps 60 thus form liquid confining structures that receive liquid from the tray deck, allow it to flow downwardly without being agitated by vapor flowing upwardly through apertures 48, and then discharge the liquid into the downcomer 50 through the cutout in the downcomer inlet wall 52.

The spacing between the sumps 60 and the depth of each sump can be varied to suit the requirements of particular applications. Although two spaced apart sumps 60 have been illustrated, a single sump or more than two sumps may be utilized if desired.

Also in accordance with the invention, a weir 70 is positioned along the edge of the outlet end 44 of the tray deck 40. A center portion 72 of the weir 70 extends between the cutouts in the tray deck 40 and end portions 72 of the weir extend along the cutouts from center portion 72 and abut the column shell 12 at their free ends. These end portions 72 of weir 70 form what are known as "relief weirs" because they are displaced toward the inlet end 42 of the tray deck 40, thereby increasing the overall length of the weir 70. The weir 70 causes liquid to accumulate to a preselected level on the tray deck 40 before it overflows the weir. That portion of the liquid that overflows the center portion 72 of the weir 70 flows directly into the inlet 54 of the downcomer, while the portion of the liquid that spills over the end portions 74 of the weir flows first into the sumps 60 and then into the downcomer 50.

The vapor-liquid contact trays 16 can be used in processes to facilitate vapor and liquid interaction and to help cause vapor to disengage from a portion of the liquid before that portion of the liquid enters the downcomer. In use, vapor and liquid streams are charged to the area of the column 10 containing the trays. The liquid flows downwardly onto the liquid receiving area 58 of the tray and then flows across the tray where it interacts with vapor ascending through apertures 48 to form a two-phase mixture or froth. A portion of the vapor then disengages from the froth and ascends to the next overlying tray where it passes through apertures 48 for interaction with liquid flowing across that tray deck 40. Another portion of the vapor remains engaged with the liquid spilling over the weir 70. That part of the liquid that overflows the end portions 74 of weir 70 flows downwardly into the sumps 60. As the liquid is descending into the sump, gravitational affects facilitate disengagement of the vapor from the liquid. As a result, the liquid discharged from the sumps 60 into the downcomer 50 is degassed to a greater extent than liquid entering the downcomer 50 from the center portion 72 of the weir 74. Because the liquid has reduced vapor content, increases in the liquid handling capacity of the downcomer can be obtained. In addition, because the sumps 60 feed into the downcomer rather than directly onto the underlying tray deck, the underlying portion of the tray deck can contain vapor apertures 48 without the disadvantage of liquid being forced to weep therethrough as a result of the liquid's downward momentum. The placement of vapor apertures in this underlying portion of the tray deck increases the active area and tray effectiveness. Liquid that descends through the downcomer 50 exits through discharge outlet 56 onto the underlying liquid receiving area 58 and then flows across the tray deck 40 for further vapor interaction.

Although the invention has been described with respect to a tray of a type known as a "single-pass" tray because the liquid flows as a single stream across the tray deck, the invention also applies to multiple-pass trays on which two or more streams flow into one or more downcomers. For example, in a two-pass tray a downcomer is positioned in the center of one tray and two downcomers are placed at opposite ends of the underlying tray. The liquid stream exiting the center downcomer is split into two streams that flow in opposite directions to the end downcomers on the underlying tray.

The two streams are then conveyed downwardly to the next underlying tray and flow toward the center downcomer. Other modifications can also be made to the tray, such as having a raised liquid receiving area 58, and remain within the scope of the invention.

From the foregoing, it will be seen that this invention is one well adapted to attain all the ends and objects hereinabove set forth together with other advantages that are inherent to the structure.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.

Claims

CLAIMSHaving thus described the invention, what is claimed is:

1. A vapor-liquid contact tray comprising: a tray deck having an opening at an outlet end for removing liquid from an upper surface of the tray deck and a plurality of apertures for allowing vapor to flow upwardly through the tray deck; a downcomer defined in part by an inlet wall extending downwardly at said opening in the tray deck, said downcomer having an upper inlet to receive liquid entering said opening from the tray deck and a lower discharge outlet through which liquid exits the downcomer; a sump extending below the tray deck and positioned in a cutout formed in the tray deck and a cutout formed in the downcomer inlet wall to receive liquid from the tray deck and feed it into the downcomer; and a weir positioned on said tray deck at the outlet end to cause liquid to accumulate to a preselected height on the tray deck before overflowing the weir into the sump and the downcomer.

2. The vapor-liquid contact tray as in claim 1 , including another sump spaced from said first mentioned sump and positioned in another cutout in the tray deck and another cutout in the downcomer inlet wall.

3. The vapor-liquid contact tray as in claim 2, wherein the weir has a center portion separating the tray deck from the downcomer upper inlet and end portions positioned along said cutouts in the tray deck.

4. The vapor-liquid contact tray as in claim 3, wherein the cutouts in the tray deck are contiguous with the opening in the tray deck.

5. The vapor-liquid contact tray as in claim 2, wherein the sump comprises a bottom plate and side walls defining a liquid confining structure open at the top to receive said liquid from the tray deck and at an outlet side to feed said liquid to the downcomer.

6. The vapor-liquid contact tray as in claim 5, wherein the bottom plate is substantially vapor impermeable.

7. A mass transfer column comprising an external shell defining an open internal region and a plurality of vapor-liquid contact trays supported in the open internal region, at least one of the vapor-liquid contact trays comprising: a tray deck having an opening at an outlet end for removing liquid from an upper surface of the tray deck and a plurality of apertures for allowing vapor to flow upwardly through the tray deck; a downcomer defined in part by an inlet wall extending downwardly at said opening in the tray deck, said downcomer having an upper inlet to receive liquid entering said opening from the tray deck and a lower discharge outlet through which liquid exits the downcomer onto an underlying one of said trays; a sump extending below the tray deck and positioned in a cutout formed in the tray deck and a cutout formed in the downcomer inlet wall to receive liquid from the tray deck and feed it into the downcomer; and a weir positioned on said tray deck at the outlet end to cause liquid to accumulate to a preselected height on the tray deck before overflowing the weir into the sump and the downcomer.

8. The mass transfer column as in claim 7, including another sump spaced from said first mentioned sump and positioned in another cutout in the tray deck and another cutout in the downcomer inlet wall.

9. The vapor-liquid contact tray as in claim 8, wherein the weir has a center portion separating the tray deck from the downcomer upper inlet and end portions positioned along said cutouts in the tray deck.

10. The vapor-liquid contact tray as in claim 9, wherein the cutouts in the tray deck are contiguous with the opening in the tray deck.

11. The vapor-liquid contact tray as in claim 8, wherein the sump comprises a bottom plate and side walls defining a liquid confining structure open at the top to receive said liquid from the tray deck and at an outlet side to feed said liquid to the downcomer.

12. The vapor-liquid contact tray as in claim 11 , wherein the bottom plate is substantially vapor impermeable.

13. A method of intermixing vapor and liquid streams in a mass transfer column containing a plurality of vertically spaced vapor-liquid contact trays, each tray having a tray deck containing apertures, a downcomer positioned at an opening in the tray deck, and spaced apart sumps extending below the tray deck and positioned in fluid flow communication with said tray deck and the downcomer, said method comprising the steps of: (a) flowing a liquid stream across the tray decks of the trays toward said opening; (b) passing a vapor stream upwardly through said apertures in the tray decks and interacting the vapor stream with the liquid stream on said tray decks to form a vapor-liquid mixture;

(c) directing part of the vapor-liquid mixture from said tray decks into said sumps and another part of said mixture into the downcomer;

(d) passing said part of the vapor-liquid mixture downwardly through the sumps to allow vapor to disengage from the liquid in said mixture and flow upwardly to the tray above;

(e) feeding the remaining part of the vapor-liquid mixture in the sump into the downcomer;

(f) passing said parts of the vapor-liquid mixture in the downcomer downwardly therethrough and disengaging remaining portions of said vapor from the liquid in the mixture; and

(g) discharging the resulting liquid from said downcomer onto an underlying tray.

14. The method as set forth in claim 13, including causing said liquid stream to accumulate on said tray deck by placing a weir on said tray deck to separate the tray deck from the downcomer and sumps.