A PERFORATED-TRAY COLUMN AND A METHOD OF REVAMPING THE SAME

Abstract

A perforated-tray column wherein each tray comprises downcomer pipes for conveying a downwardly flowing liquid to a next tray, wherein the downcomer pipe includes: a first portion extending above the perforated tray, a second portion extending below the perforated tray towards the next perforated tray, and an end guard to prevent a gaseous phase from entering the downward pipe.

Claims

1-15. (canceled)

16. A perforated-tray column for the interaction of a downwardly flowing liquid with an ascending gaseous stream, the perforated-tray column comprising: a vessel with a vertical axis; and a perforated tray assembly including at least a first perforated tray and a second perforated tray which is next to and below the first perforated tray, the first perforated tray includes a plurality of downcomer pipes, wherein each of the plurality of downcomer pipes includes: a first portion extending above the first perforated tray; and a second portion extending below the first perforated tray towards the second perforated tray; wherein the first portion terminates with an inlet section of the downcomer pipe located above the first perforated tray; wherein the second portion terminates above the second perforated tray with an outlet section and an end guard; wherein the end guard is configured to maintain the outlet section submerged by the liquid flowing through the downcomer pipe; and wherein each of the plurality of downcomer pipes associated to the first perforated tray is axially aligned with a corresponding one of the plurality of downcomer pipes associated to the second perforated tray.

17. The perforated-tray column according to claim 16, wherein the second portion of downcomer pipes of the first perforated tray terminates above the second perforated tray at a distance from the second perforated tray that is greater than a height of first portions of downcomer pipes of the second tray.

18. The perforated-tray column according to claim 16, wherein the inlet section of each of the plurality of downcomer pipes is at a height of 300 mm to 2.5 m above the upper surface of the first perforated tray.

19. The perforated-tray column according to claim 16, wherein the first perforated tray and second perforated tray are perpendicular to the vertical axis of the vessel.

20. The perforated-tray column according to claim 16, wherein each of the plurality of downcomer pipes is a vertical straight pipe.

21. The perforated-tray column according to claim 16, wherein the plurality of downcomer pipes are arranged in a pattern such that the liquid flow around each downcomer is a radial flow that is evenly distributed in all radial directions around the downcomer.

22. The perforated-tray column according to claim 16, wherein the plurality of downcomer pipes are regularly distributed with a square pitch or a triangular pitch over at least a portion of the surface of the first perforated tray.

23. The perforated-tray column according to claim 16, wherein the plurality of downcomer pipes have a cross-sectional area in a range from 4% to 30% of a surface area of the first perforated tray.

24. The perforated-tray column according to claim 16, wherein the end guard includes a chamber around the outlet section and the chamber, filled of liquid, acts as a hydraulic guard preventing a gaseous flow directed upward from entering the downcomer pipe through the outlet section.

25. The perforated-tray column according to claim 24, wherein the end guard has a bottom surface below the outlet section and a perimetral wall extending around the outlet section, from the bottom surface to an upper edge above the outlet section, to define the chamber.

26. The perforated-tray column according to claim 16, wherein the downcomer pipe has no liquid inlet or outlet other than the inlet section and the outlet section.

27. The perforated-tray column according to claim 16, wherein: the perforated tray assembly includes a plurality of perforated trays, and each of the plurality of perforated trays has a plurality of the downcomer pipes for conveying liquid to a next tray; and for each pair of adjacent perforated trays, each downcomer pipe associated to the upper tray of the pair of adjacent trays is axially aligned with a corresponding downcomer pipe associated to the lower tray of the pair.

28. The perforated-tray column according to claim 27, wherein each of the plurality of perforated trays that receives a downwards flowing liquid from an above perforated tray, has no downcomer inlet area and the liquid is received from downcomer pipes hanging from the above perforated tray.

29. The perforated-tray column according to claim 16, wherein the perforated-tray column is an absorber of a regenerator in a CO.sub.2 removal section of a plant for the synthesis of ammonia.

30. A method for revamping a perforated tray column, particularly an absorber or a regenerator of a CO.sub.2 removal section of an ammonia plant, the method comprising: removing a perforated tray assembly from the perforated tray column; and installing a new perforated tray assembly in the perforated tray column, wherein: the new perforated tray assembly includes at least a first perforated tray and a second perforated tray that is next to and below the first perforated tray, and the first perforated tray includes a plurality of downcomer pipes, wherein each of the plurality of downcomer pipe includes: a first portion extending above the first perforated tray; and a second portion extending below the first perforated tray towards the second perforated tray; wherein the first portion of downcomer pipe terminates with an inlet section of the downcomer pipe above the first perforated tray; wherein the second portion of downcomer pipe terminates above the second perforated tray with an outlet section and an end guard; wherein the end guard being is configured to maintain the outlet section submerged by the liquid flowing through the downcomer pipe, and wherein in the new perforated tray assembly, for each pair of adjacent perforated trays, each of the plurality of downcomer pipes associated to the upper tray of the pair of adjacent trays is axially aligned with a corresponding downcomer pipe associated to the lower tray of the pair.

Description

DESCRIPTION OF THE FIGURES

[0059] FIG. 1 is a scheme of a perforated tray column according to an embodiment of the invention.

[0060] FIG. 2 is a cross section of the bottom end of a downcomer pipe of the perforated tray assembly of the column of FIG. 1, in a preferred embodiment.

[0061] FIG. 3 is a cross section of a plate.

[0062] FIG. 4 is a comparison between the new configuration based on multi tray pipe and a standard configuration based on single pass tray.

DETAILED DESCRIPTION

[0063] A perforated tray column has a pressure vessel 1 with a vertical axis A-A and includes a perforated tray assembly 2 for contacting a downwardly flowing liquid L with an ascending gas phase G.

[0064] The perforated tray assembly 2 comprises a plurality of perforated trays, regularly spaced inside the vessel 1. FIG. 1 illustrates a pair of trays including a first perforated tray 3 and a second perforated tray 4. The second tray 4 is located next to and below the first perforated tray 3. Both trays 3, 4 have a plurality of holes 5 (visible in FIG. 3). Preferably all trays are identical in shape and size.

[0065] The first perforated tray 3 comprises a plurality of downcomer pipes 6 to convey the liquid L to the below tray 4.

[0066] Each downcomer pipe 6 includes a first portion (upper portion) 7 extending above the first perforated tray 3, a second portion (lower portion) 8 extending below the first perforated tray 3 towards the second perforated tray 4, and a bottom guard end 9.

[0067] The upper portion 7 terminates with an inlet section 10 of the downcomer pipe 6. Said inlet section is located at a height h above the first perforated tray 3.

[0068] Thanks to the elongate shape of the downcomer pipes 6, the height h can be significantly greater than prior art columns, for example more than 300 mm and preferably in the range of 300 mm to 2.5 meters. Preferably the inlet sections 10 of all pipes 6 are at the same height above the tray 3.

[0069] The lower portion 8 terminates with an outlet section 11 which is above the second perforated tray 4. The outlet section 11 is within a guard end 9 (FIG. 2).

[0070] This lower portion 8 is not requiring any part of the lower perforated tray area to provide an inlet downcomer area because the liquid is fed to the below tray just falling from the above tray.

[0071] The lower portion 8 ends at a distance height h2 from the lower tray, where h2 is greater than h. Accordingly, the lower portion 8 remains above the downcomers of the next tray.

[0072] The guard end 9 is also above the second perforated tray 4. Said guard end 9 has a bottom surface 12 below the outlet section 11 and a perimetral wall 13 extending around and above the outlet section 11, from the bottom surface 12 to an upper edge 14. Said upper edge 14 of the perimetral wall 13 is above the outlet section 11. Preferably the wall 13 is cylindrical.

[0073] The guard end 9 is then configured as a container around the outlet section 11, which is adapted to maintain the outlet section 11 submerged by the liquid L. When the liquid L reaches the upper edge 14, it overflows towards the second perforated tray 4. It can be understood that the guard end 9 acts as hydraulic guard to prevent the ascending gas G from entering the pipe 8. The gas G, on the contrary, bubbles through the holes 5 and mixes with the liquid L above the tray 3.

[0074] The first tray 3 has a peripheral seal 15 so that the liquid can only flow downward through the downcomer pipes 6. Each downcomer pipe 6 has no inlet other than the upper inlet section 10.

[0075] FIG. 2 shows an embodiment of the end guard 9 fixed to the bottom of the pipe 6. The figure shows an embodiment where the terminal part of the tube 8 is conical and the end guard 9 is anchored by metal sheets 16, to keep the bottom 12 below the outlet section 11. In use, the liquid L overflows from the level indicated by the edge line 14 (upper edge of the wall 13).

[0076] FIG. 3 illustrates an example of the arrangement of several downcomer pipes 6. The downcomer pipes are preferably distributed over the entire surface of the tray, or at least over a portion thereof. FIG. 3 refers to the plate 3; the plate 4 is similar.

[0077] The second perforated tray 4 has a respective set of downcomer pipes 6′ to convey the liquid L to another perforated tray below the tray 4. The downcomer pipes 6′ can be realized in accordance with the above described downcomer pipes 6 of the first tray 3. Particularly, the second tray 4 has the same above-tray height h of pipes 6′ as the first tray 3.

[0078] Preferably the upper inlet section of the downcomer pipes 6′ of the second tray 4 is below the bottom of the end guards 9 of the downcomer pipes 6 of the first tray 3, as illustrated in FIG. 1 with a piled configuration. Accordingly, and in more general terms, a free space exists between the lower ends of downcomer pipes of one tray and the inlets of the downcomer ends of the next tray.

[0079] FIG. 1 illustrates an embodiment of stacked downcomer pipes. Each downcomer pipe 6 of the first tray 3 is vertically aligned with a corresponding downcomer pipe 6′ of the second tray 4. Pairs of aligned downcomer pipes 6 and 6′ have the same axis.

[0080] It can be understood that the figures show only two trays 3, 4 but in most cases the assembly 2 will include several trays, each tray having a set of downcomer pipes, embodied as the pipes 6 above described, to convey the liquid L to the next tray. Preferably, all downcomer pipes are axially aligned as disclosed above.

[0081] In operation, the liquid L covers the surface of the first tray 3 and accumulates above the tray 3 until it reaches the inlet sections 10 of the downcomer pipes 6.

[0082] From here, the liquid L flows through the pipes 6, fills the bottom end guards 9 and overflows from the edges 14 over the underlying tray 4. In the same time, the bubbles of gas G contacts the liquid passing through the holes 5. An efficient heat and mass transfer is therefore achieved.

[0083] FIG. 4 illustrates a comparison between a) a prior art single-pass tray and b) a tray according to an embodiment of the invention.

[0084] In the prior art of FIG. 4 (a), only a central band 20 of the perforated tray is available as active area, the remaining sectors 21, 22 being taken by the downcomers. Particularly, one sector (e.g. sector 21) is the downcomer outlet area and the other sector is the downcomer inlet area. Both reduce the active area.

[0085] The invention, as illustrated in FIG. 4 (b), does not require a downcomer inlet area and therefore increases the active area. Also, the liquid transfer is better distributed over the tray due to the arrangement of the multiple downcomer pipes.

Example

[0086] In the following example (see FIG. 4) taken from an industrial application, the downcomer area must be designed in order to allow a liquid speed of 0.18 m/s (liquid flow rate 570 m.sup.3/h), while the overall sectional area is 11.3 m2 (cylindrical column diameter 3.8 m).

[0087] A comparison is developed between a standard configuration with a single tray pass and a configuration according to an embodiment of the present invention based on 7 downcomer circular pipes having 400 mm diameter.

TABLE-US-00001 Standard New configuration configuration (prior art) (invention) Overal sectional area [m.sup.2] 11.3 11.3 Downcomer inlet area [m.sup.2] 0.88 — Downcomer outlet area [m.sup.2] 0.88  0.88 Active area [m.sup.2] 9.7 10.6

[0088] From the above table is evident that the new configuration makes available a greater active area for the vapour decreasing the possibility of flooding issue in the column.

[0089] In another perspective, in case of a new tower, the application of the new configuration based on multi stacked pipe hanged to the above trays, allows the design of a tower having a diameter of the column smaller than that of a column with trays based on a standard configuration.