A perforated tray column and a method for improving the same

Abstract

A perforated tray column, wherein each tray comprises conductor tubes (6) for conveying a liquid (L) flowing downwards to the next tray, wherein the conductor tube comprises: a first portion (7) extending above the perforated tray, a second portion (8) extending below the perforated tray towards the next perforated tray, and an end shield (9) for preventing the gas phase from entering the conductor tube.

Description

A perforated tray column and a method for improving it Explanation of the invention Invention technical knowledge The invention relates to a perforated tray column. Prior knowledge of the invention The perforated tray column is a well-known device found in many chemical plants. The space inside the column is divided by a perforated tray assembly to facilitate the interaction between two process streams, typically a downward flowing liquid and an ascending gas or vapor. In particular, perforated trays enhance heat exchange and volume transfer between the process streams. The liquid flowing over a tray before entering the conductor tube reacts with the vapor flowing upward and passing through the valves provided in the selected section of the tray. The following is a list of definitions commonly used in describing perforated tray columns. Active zone: The active zone is the portion of the total area of ​​the cross-sectional column that can be equipped with the aforementioned valves and where liquid-vapor contact occurs. Open area: The open area is the part of the active area that is equipped with valves (the open area indicates the total area of ​​the valves). Conductor Tube Zone: The conductor tube zone is the zone occupied by conductor tubes to allow liquid to flow from a higher tray to a lower tray and where there is no effective contact between the liquid and the vapor. An effective contact is achieved in the active zone by means of valves. Total column surface area: The total surface area of ​​a perforated tray column represents all cross-sectional areas covered by the tray, including the active area and the conductor tube area. Perforated tray columns are used, among other things, to create adsorbents or regenerators in the CO2 removal section of some power plants for ammonia synthesis. The technical problem in designing such columns is that the downward moving liquid stream has a certain residence time above each tray and is transferred to the next tray below. Typically, a perforated tray has a sealing ring around its perimeter to prevent liquid leakage and may include one or more baffles to define the liquid path or to allow a liquid surface to form above the tray. A guide tube is provided to collect liquid from a selected position in the tray and convey the liquid to the next tray below. A typical example of the prior art has only one guide tube per tray. The guide tube is usually placed on one side of the perforated tray; thus each tray has one side where liquid is received from above, and an opposite side where liquid is collected and sent to the next tray. Accordingly, each tray has a guide tube inlet area, both of which reduce the available active area. For this reason, the prior art suffers from the drawback that a large portion of the tray area is occupied by the guide tube inlet area and the guide tube outlet area, and is therefore not available as active area. Generally, the size of the conduit is determined by the maximum fluid velocity through the conduit itself, and therefore relatively large conduits are required. Large conduit sizes not only reduce the active area but also affect the fluid dynamics of the system. Wavebreakers can also reduce the active area. For example, a prior art example of a single-pass tray has a circular passage section with only a central strip available as an active region. The remaining left / right parts are occupied by conductor tubes and belong to the conductor tube inlet and outlet regions. Another drawback is that each tray provides a completely horizontal main flow direction and a uniform liquid flow is directed from the liquid inlet where the liquid is received to the liquid outlet where the liquid enters the conducting tube to the next tray. This liquid flow is in a cross-flow arrangement with the gas phase flowing upwards; however, it has been found that this flow regime is not desirable for the required reaction with the gas phase. The flow regime is also developed in the case of multi-pass trays or equivalent designs in two main directions, usually the two main directions being 180º apart. US 2016 / 0271516 discloses a flow tray for a volume transfer column. Summary of the invention The object of the present invention is to overcome the above drawbacks of the perforated tray columns in the prior art. In particular, one object of the present invention is to increase the active area in the perforated tray column. Another object is to provide a more efficient flow regime. The object is achieved by a perforated tray column according to claim 1. Preferred features are set out in the dependent claims. In this invention, liquid is transferred from one tray to another through a number of conduits. Each tray has a set of conduits and each conduit has an upper inlet section, which extends above the tray, and a lower outlet section, which extends towards the next tray. The lower outlet section terminates with an end shield to prevent rising gas from entering the conduit. A perforated tray column according to the invention includes a duct with a vertical axis, and the perforated tray assembly includes at least a first perforated tray and a second perforated tray located next to and below the first perforated tray, and the first perforated tray includes a plurality of guide tubes, wherein each guide tube includes: a first portion extending above the first perforated tray, a second portion extending below the first perforated tray in the direction of the second perforated tray, wherein the first section of the conduit terminates with a conduit inlet section located above the first perforated tray, wherein the second section of the conduit terminates above the second perforated tray with an outlet section and an end shield. The end shield is configured to retain said outlet portion submerged in the fluid flowing through the conduit, wherein each guide tube associated with the first tray is axially aligned with a corresponding guide tube associated with the second tray. Typically, the column comprises multiple (more than two) perforated trays. In this case, for each pair of adjacent perforated trays comprising a first tray (the upper tray) and a second tray (the lower tray, which is located next to and below the first tray), each guide tube associated with said first tray pair is axially aligned with the corresponding guide tube associated with the second tray pair. For each pair of adjacent perforated trays, the conductor tubes of the first perforated tray of the pair are essentially stacked with the conductor tubes of the second perforated tray of the pair. This is applicable to all adjacent trays, i.e. the second tray of the first pair mentioned above can be considered as the first tray of the other pair. Some preferred embodiments are described by dependent claims. In preferred embodiments, the guide tubes are vertical tubes and are vertically arranged, i.e., each guide tube of the first perforated tray has a common vertical axis with the corresponding guide tube of the second perforated tray. Preferably, the second portions of the conductor tubes of the first tray terminate above the second perforated tray at a distance from the second perforated tray that is greater than the height of the first portions of the conductor tubes of the second tray. The second sections of the conductor tubes are essentially suspended from the first perforated tray. The lower ends of the conductor tubes are above the second perforated tray and preferably above the inlet of the next set of conductor tubes. This arrangement of conductor tubes does not require a conductor tube inlet region because a perforated tray receives liquid flowing down from the upper tray without requiring a conductor tube inlet region (liquid receiving region) in the receiving tray. Accordingly, the first advantage of the invention is the active area, which can be increased compared to the previous invention. The second advantage of the invention is that the liquid is conveyed through a number of conductors instead of one large conductor. Accordingly, each tray receives the liquid at several points and the liquid is distributed more uniformly over the tray. This is also due to the large active area when the conductors can be distributed. Each conductor can be considered as a means of conveying the liquid between the trays and therefore, the provision of several conductors distributed over a larger area provides a more uniform liquid transfer compared to the previous invention. A third advantage is that the distribution of the liquid through several conducting pipes facilitates turbulent flow of the liquid over the surface of the perforated trays, which improves contact, heat exchange and volume transfer with the gas phase. This effect is further improved by arranging the conducting pipes at regular intervals, for example with a square slope or a triangular slope. The need for wave-breakers is reduced and more active surface area can be obtained on the trays. The inventor has also found that several relatively small conducting tubes can transport the liquid more efficiently while increasing the active area of ​​the trays. Another advantage of the invention is that the inlet section of the conductor tubes can be at a significant height above the surface of the perforated tray, as a result of which the liquid level on the trays (seal height) can be greater than in the prior art, which provides for longer contact times and higher tray efficiency. Preferably, a perforated tray column according to the invention is fed with a liquid stream (liquid phase) and a gas stream (gas phase), wherein the liquid stream flows downwards through the column and the gas stream flows countercurrently to the liquid stream in the perforated region upwards. Preferably, the perforated trays are perpendicular to the vertical axis of the reactor. Preferably, each conductor pipe is a straight vertical pipe. More preferably, each conductor pipe is a straight vertical pipe with a circular passage section. The inlet section of the conduit is preferably 300 mm to 2.5 m above the upper surface of the first perforated tray; thus, the corresponding volume above the first perforated tray can be filled with liquid, thus forming a significant volume of liquid which is carried by the upwardly flowing gas phase. The liquid height thus obtained above the trays is substantially greater than the liquid height which could be obtained previously. A related advantage is the increased efficiency of the perforated tray assembly in terms of the interaction between the liquid phase and the gas phase. In one aspect of the invention, the guide tubes are arranged in a pattern such that the liquid on the tray flows around each guide tube in a radial flow that is uniformly distributed in all radial directions around the guide tube, i.e. without a preferred direction of liquid flow. The above pattern can be applied to the guide tubes of all the trays of the column. Accordingly, cross-flow defects are avoided and liquid-gas interactions are improved. In a preferred embodiment, the guide tubes are distributed regularly with a square or triangular pattern over at least a portion of the surface of the trays. This arrangement, in particular, creates the radial flow mentioned above around each guide tube of the tray, without a preferred direction of liquid flow. The total cross-sectional area of ​​said conductor tube is preferably in the range of 4% to 30% of the total surface area of ​​the column. The cross-sectional area of ​​each of the conductor tubes is preferably in the range of 0.4% to 10% of the total surface area of ​​the column (as defined above). The end shield preferably has a bottom surface below the outlet section and a peripheral wall extending around the outlet section from the bottom surface to an upper edge above said outlet section. Said peripheral wall is preferably a cylindrical wall. A column according to the invention is, in a preferred application, an absorber or generator of the CO2 removal section of a power plant for ammonia synthesis. The CO2 removal can be carried out, for example, on a blend gas for ammonia synthesis obtained by hydrocarbon reforming. The CO2 is produced in the carbon monoxide CO shift reaction and must be separated before conversion, since the CO2 damages the catalyst for ammonia synthesis. A known technique for removing CO2 from the blend gas involves absorbing the CO2 in a suitable medium and subsequently regenerating it. These steps are carried out in an absorber and generator which can be embodied in the form of perforated tray channels. The number of perforated trays in a perforated tray assembly is typically between 3 and 9. Preferably, the trays are evenly spaced in a vertical direction. Accordingly, the invention includes a perforated tray column for reacting a downwardly flowing liquid with an upwardly moving gas stream, the column comprising a vertical-axis conduit and a perforated tray assembly comprising a plurality of perforated trays, wherein each said plurality of perforated trays comprises a plurality of conduits, wherein each conduit comprises: a first portion extending above the perforated tray, a second portion extending below the first perforated tray towards the second perforated tray, the second portion being dependent on the upper tray without the need for a conduit entry area for the lower tray, wherein the first section of the conduit terminates with a conduit inlet section located above the first perforated tray, The second section of the guide tube above the second perforated tray ends with an outlet section and an end guard, The end shield is then shaped to keep the outlet section submerged in the fluid flowing through the conduit. wherein, for each pair of adjacent perforated trays comprising a first tray and a second tray, each guide tube associated with the first tray of said pair of adjacent trays is axially aligned with the guide tube associated with the second tray of said pair. The end shield of the upper tray, in preferred embodiments, also acts as a cap for the axially aligned conduit located in the next lower tray, preventing any direct downflow of liquid from the upper conduit to the lower conduit. Each perforated tray (lower tray) that receives the liquid flowing downward from the upper perforated tray (upper tray) preferably has no conductor tube inlet area, the liquid being received as a trickle from conductor tubes suspended from the upper perforated tray. The fluid then flows in a preferred radial direction around each installed tube without a preferred flow direction. An aspect of the invention is also the improvement of a column. The method includes improving the original assembly and installing a perforated tray assembly, wherein the new assembly has an arrangement of perforated trays and conductive tubes in accordance with at least one of the preferred embodiments disclosed above. The advantages of the invention will become more apparent with the help of the following more detailed description in relation to the preferred embodiment. Explanation of maps Figure 1 is a diagram of a perforated tray column according to the invention. Figure 2 is a cross-sectional view of the lower end of the guide tube of the perforated tray assembly of the column of Figure 1, in the preferred embodiment. Figure 3 is a cross-section of the plate. Figure 4 is a comparison between the new configuration based on multiple tray tubes and the standard configuration based on a single pass tray. Detailed explanation A perforated tray column has a pressure channel 1 with a vertical axis AA and includes a perforated tray assembly 2 for contacting a downwardly flowing liquid L with an ascending gas phase G. The perforated tray assembly 2 comprises a number of perforated trays, which are arranged in a regular manner within a channel 1 at a distance. Figure 1 shows a pair of trays comprising a first perforated tray 3 and a second perforated tray 4. The second tray 4 is positioned next to and below the first perforated tray 3. Both trays 3, 4 have a number of holes 5 (visible in Figure 3). Preferably, all trays are of the same shape and size. The first perforated tray 3 includes a number of conducting pipes 6 to transfer the liquid L to the lower tray 4. Each guide tube 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 lower protective end 9. The upper part ends with an inlet section 10 of the guide tube 6. Said inlet section is located at a height h above the first perforated tray 3. Due to the elongated shape of the conductor tubes 6, the height h can be significantly greater than in prior art columns, for example greater than 300 mm and preferably in the range of 300 mm to 2.5 m. Preferably, the inlet sections 10 of all tubes 6 are at the same height above the tray 3. The lower part 8 ends with an outlet section 11 which is above the second perforated tray 4. The outlet section 11 is at the end of the guard 9 (Figure 2). This lower section 8 does not require any part of the lower perforated tray area to provide an inlet piping area since the liquid is fed to the lower tray which only flows from the upper tray. The lower part 8 ends at a height h2 from the lower tray, where h2 is greater than h. Accordingly, the lower part 8 remains above the conductor tubes of the next tray. The end of the guard 9 is also above the second perforated tray 4. Said end of the guard 9 has a lower surface 12 below the outlet portion 11 and a peripheral wall 13 extending around and above the outlet portion 11, from the lower surface 12 to an upper edge 14. Said upper edge 14 is the peripheral wall 13 above the outlet portion 11. Preferably, the wall 13 is cylindrical. The end of the guard 9 is then shaped into a chamber around the outlet section 11, which is designed to keep the outlet section 11 immersed in the liquid L. When the liquid L reaches the upper edge 14, it overflows towards the second perforated tray 4. It is understood that the end of the guard 9 acts as a hydraulic guard to prevent the rising gas G from entering the tube 8. In contrast, the gas G bubbles through the holes 5 and mixes with the liquid L above the tray 3. The first tray 3 has a peripheral seal 15 so that the liquid can only flow downwards through the conductor tubes 6. Each conductor tube 6 has no other inlet than the upper inlet section 10. Figure 2 shows an example of an end protector 9 fixed to the bottom of a tube 6. The figure shows an example in which the end of the tube 8 is conical and the end protector 9 is hooked with metal plates 16, to hold the bottom part 12 below the outlet part 11. In use, the liquid L overflows from the level indicated by the edge line 14 (the upper edge of the wall 13). Figure 3 shows an example of an arrangement of several conductor tubes 6. The conductor tubes are preferably distributed over the entire surface of the tray, or at least part thereof. The second perforated tray 4 has a corresponding set of conducting pipes 6' to convey the liquid L to the other perforated tray below the tray 4. The conducting pipes 6' can be realized in accordance with the conducting pipes 6 of the first tray 3 described above. In particular, the second tray 4 has the same height h as the pipes 6' above the first tray 3. Preferably, the upper inlet portion of the conductor tubes 6' of the second tray 4 is below the bottom of the end guards 9 of the conductor tubes 6 of the first tray 3, which is shown in Figure 1 in a stacked configuration. Accordingly, and more generally, there is a free space between the lower ends of the conductor tubes of each tray and the inlets of the conductor ends of the next tray. Figure 1 shows an example of stacked conductor tubes. Each conductor tube 6 of the first tray 3 is vertically aligned with the corresponding conductor tube 6' of the second tray. The pairs of aligned conductor tubes 6 and 6' have the same axis. It is understood that the figures show only two trays 3,4 but in most cases the assembly 2 will comprise several trays, each tray having a set of conductor tubes, described above as tubes 6, to convey the liquid L to the next tray. Preferably, all the conductor tubes are axially aligned as disclosed above. In operation, the liquid L covers the surface of the first tray 3 and accumulates on the top of the tray 3 until it reaches the inlet sections 10 of the conductor tubes 6. From here, the liquid L flows through the tubes 6, fills the lower end shields 9 and overflows over the edges 14 onto the lower tray 4. At the same time, the gas bubbles G come into contact with the liquid passing through the holes 5; thus, effective heat and mass transfer is achieved. Figure 4 shows a comparison between a) a tray with a passage in the prior art and b) a tray according to an example of the present invention. In the prior art of Fig. 4(a), only a central band 20 of the perforated tray is available as the active region, the remaining sections 21,22 are occupied by conductors. Specifically, one section (e.g. section 21) is the conductor output region and the other section is the conductor input region. Both reduce the active region. The invention, as shown in Figure 4(b), does not require a conductor entry area and thus increases the active area. Also, the conduction is better distributed due to the arrangement of several conductor tubes on the tray. Example In the example below (see Figure 4) taken from an industry application, the conductor area must be designed to provide a liquid velocity of 0.18 m / s (liquid flow rate m3 / h), while the total cross-sectional area is 11.3 m2 (cylinder diameter 3.8 m). A comparison has been made between a standard configuration with a tray passage and a configuration according to an example of this invention based on 7 circular conductor tubes with a diameter of 400 mm. Standard configuration (prior knowledge) New configuration (invention) General cross-sectional area [m2] 11.3 11.3 Conductor inlet area [m2] 0.88 - Conductor outlet area [m2] 0.88 0.88 Active area [m2] 9.7 10.6 It is clear from the table above that the new configuration provides a larger active area for the steam, which reduces the possibility of flooding problems in the column. On the other hand, in the case of the new tower, the use of a new configuration based on multiple stacked tubes suspended from the upper trays allows the tower to be designed with a smaller radius than the column radius with trays based on the standard configuration.

Claims

Claims 1. A perforated tray column for interacting a downwardly flowing liquid (L) with an upwardly ascending gas flow (G), the column comprising a conduit (1) with a vertical axis and a perforated tray assembly (2) comprising at least a first perforated tray (3) and a second perforated tray (4) positioned next to and below the first perforated tray, the first perforated tray comprising a plurality of guide tubes (6), each guide tube comprising: a first portion (7) extending above the first perforated tray, a second portion (8) extending below the first perforated tray towards the second perforated tray, the first portion of the guide tube terminating in an inlet portion (10) of the guide tube located above the first perforated tray, the second portion of the guide tube terminating above the second perforated tray with an outlet portion (11) and an end shield (9). It is found that said end guard (9) is shaped to hold said outlet portion (11) immersed in the liquid flowing through the conductor tube, and wherein each conductor tube (6) associated with said perforated tray (3) is axially connected to a corresponding conductor tube (6') associated withThe second perforated tray (4) is aligned.

2. A column according to claim 1, wherein the second portion (8) of the conductor tubes (6) of the first tray (3) terminates above the second perforated tray (4) at a distance (h2) from the second perforated tray which is greater than the height (h) of the first portions of the conductor tubes (6') of the second tray (4).

3. A column according to claim 1, wherein the inlet section (10) of each conductor tube (6) is at a height of 300 mm to 2.5 m above the upper surface of the first perforated tray (3).

4. A column according to any preceding claim, wherein the first perforated tray and the second perforated tray are perpendicular to the vertical axis of the duct.

5. A column according to any preceding claim wherein each conductor tube is a vertical straight tube.

6. A column according to any preceding claim, wherein the conductor tubes are arranged in a pattern such that the fluid flow around each conductor tube is a radial flow that is uniformly distributed in all radial directions around the conductor tubes.

7. A column according to any preceding claim, wherein the conductor tubes are distributed in a regular manner over a portion of the surface of the first tray with a square or triangular gradient.

8. A column according to any preceding claim, wherein the cross-sectional area of ​​said conductor tubes is in the range of 4% to 30% of the surface area of ​​the first perforated tray.

9. A column according to any preceding claim, wherein the end shield (9) comprises a chamber around the outer section (11), said chamber, which is filled with liquid, acting as a hydraulic shield preventing the upwardly directed gas flow from entering the conductor tube through the outlet section (11).

10. A column according to claim 9, wherein the end shield (9) has a bottom surface (12) below the outlet section (11) and a peripheral wall (13) extending around the outlet section from said bottom surface (12) to an upper edge (14) above the outlet section (11) to define said chamber.

11. A column according to any preceding claim, wherein the conductor tube has no inlet or outlet other than said inlet section and said outlet section.

12. A column according to any preceding claim, wherein the perforated tray assembly (2) comprises a plurality of perforated trays and each of said plurality of perforated trays has a plurality of said conductor tubes (6, 6') for conveying liquid to the next tray, and wherein, for each pair of adjacent perforated trays, each conductor tube associated with the upper tray of said pair of adjacent trays is axially aligned with a corresponding conductor tube associated with the lower tray of said pair.

13. A tray according to claim 12, wherein each perforated tray receiving the downwardly flowing liquid has no conductor inlet area and the liquid is received from conductor tubes suspended from the upper perforated tray.

14. A column according to the preceding claims, wherein the column is an adsorbent of a generator in the CO2 removal section of a power plant for ammonia synthesis.

15. A method for upgrading a perforated tray column, particularly an adsorber or a generator of the CO2 removal section of an ammonia power plant, wherein the method comprises removing a perforated tray assembly from the column and installing a perforated tray assembly in the column, wherein: the new perforated tray assembly includes at least a first perforated tray and a second perforated tray adjacent and below the first perforated tray, and the first perforated tray includes a plurality of guide tubes, wherein each guide tube includes: a first portion extending above the first perforated tray, a second portion extending below the first perforated tray toward the second perforated tray, and an end shield, the first portion of the guide tube terminating with a guide tube inlet portion above the first perforated tray, the second portion of the guide tube terminating above the second perforated tray with an outlet portion and an end shield, the end shield being shaped so that said outlet portion is submerged The fluid flowing through the conductor tube is retained, and in the new perforated tray assembly, for each pair of adjacent perforated trays, each conductor tube associated with the upper tray of the pair isSaid adjacent trays are axially aligned with the corresponding conductor tube associated with the lower tray of said pair.

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