GAS-LIQUID CONTACTING TRAY

Abstract: Gas-liquid contacting column provided with a plurality of axially spaced trays, each tray having two tray sections separated along a diametrical line on the tray, each tray section comprising a plurality of parallel rectangular downcomers, a plurality of liquid receiving areas and bubble area between the downcomers and the liquid receiving areas, which liquid receiving area is positioned just below the downcomer of a consecutive tray just above and which downcomers are open at the upper end and closed at the bottom end, which bottom end is provided with a plurality of liquid discharge openings and which downcomers extend from the diametrical line to the circumferential of the tray in such a manner that the ends of the downcomers of the two tray sections are positioned along the diametrical line in an alternating manner, wherein the area of the liquid discharge openings per downcomer length as viewed from below and close to the diametrical line is less than the average area of the liquid discharge openings per downcomer length as viewed from below.

Description: GAS-LIQUID CONTACTING TRAY The invention is related to a gas-liquid contacting column provided with a plurality of axially spaced trays, each tray having two tray sections separated along a diametrical line on the tray, each tray section comprising a plurality of parallel rectangular downcomers, a plurality of liquid receiving areas and bubble area between the downcomers and the liquid receiving areas, which liquid receiving area is positioned just below the downcomer of a consecutive tray just above and which downcomers are open at the upper end and closed at the bottom end, which bottom end is provided with a plurality of liquid discharge openings and which downcomers extend from the diametrical line to the circumferential of the tray in such a manner that the ends of the downcomers of the two tray sections are positioned along the diametrical line in an alternating manner . Such a gas-liquid contacting column is described in GB-A-1422131. This publication from 1976 describes such a column being provided with a plurality of gas-liquid contacting trays, wherein the rectangular downcomers have vertical walls. The tray is divided by a supporting beam along a diametrical line in two tray sections . Each tray section has a row of parallel rectangular downcomers arranged perpendicular to the supporting beam. The row of downcomers in one section are staggered relative to the row of downcomers in the opposite tray section. According to this publication the staggering tray layout promotes an even distribution of the liquid over the entire tray. Although the tray as described in GB-A-1422131 has proven to be a good gas-liquid contacting tray in many applications there is room for improvement. A problem often encountered with this design is a reduced contacting efficiency of the upwardly moving gas with the downwardly moving liquid. The object of the present invention is to provide a gas-liquid tray which incorporates the advantages of the tray as described in GB-A-1422131 with a higher efficiency. This object is achieved with the following gas-liquid contacting column. Gas-liquid contacting column provided with a plurality of axially spaced trays, each tray having two tray sections separated along a diametrical line on the tray, each tray section comprising a plurality of parallel rectangular downcomers, a plurality of liquid receiving areas and bubble area between the downcomers and the liquid receiving areas, which liquid receiving area is positioned just below the downcomer of a consecutive tray just above and which downcomers are open at the upper end and closed at the bottom end, which bottom end is provided with a plurality of liquid discharge openings and which downcomers extend from the diametrical line to the circumferential of the tray in such a manner that the ends of the downcomers of the two tray sections are positioned along the diametrical line in an alternating manner, wherein the area of the liquid discharge openings per downcomer length as viewed from below and close to the diametrical line is less than the average area of the liquid discharge openings per downcomer length as viewed from below. Applicants have found that by varying the liquid discharge openings along the downcomer lower end according to the invention a higher gas-liquid contacting efficiency is achieved. With close to the diametrical line is preferably meant a distance of 0.2 m. The invention shall be further illustrated by making use of figures 1-4. Figure 1 is a top view of a tray according to the invention as placed in a column. Figure 2 is a cross-sectional view AA' of Figure 1. Figure 3 is a cross-sectional view BB ' of Figure 1. Figure 1 shows a horizontal cross-sectional view of a gas-liquid contacting column. In this figure a top view of a tray (1) and the column wall (2) is illustrated. Tray (1) is provided with a supporting beam (3) along the diametrical line dividing the tray in two tray sections (4,5) . Such a supporting beam is preferably present in trays having a large diameter, preferably having a diameter of above 2 m. Smaller diameter trays can be used without a supporting beam. Each tray section (4,5) is provided with a plurality of parallel rectangular downcomers (6,7) . The downcomers (6,7) have two elongated sides along its length and two smaller sides along its width. The downcomers (6,7) are open at the upper end (8) and closed at the bottom end (9), which bottom end (9) is provided with a plurality of liquid discharge openings (10) . The shape of these openings may for example be circular or have the form of slits parallel or perpendicular to the widths of the downcomer. Each tray section (4,5) is further provided with a segmental downcomer (11,12) also provided with a bottom end which bottom end is provided with liquid discharge openings (13) . Each tray section (4,5) is provided with liquid receiving areas (14,15). The liquid receiving area (14,15) is the area just below the rectangular and the optional segmental downcomers of the tray just above the illustrated tray (1) in Figure 1. Between the liquid receiving area (14,15) and the downcomer (6,7) bubble area (16) is present. Bubble area (16) is provided with openings (17) for passage of upwardly moving gas. This liquid receiving area (14,15) may also be provided with openings for gas passage. Because the liquid being discharged from the tray above will fall on this liquid receiving area with some force, liquid may pass these gas openings. Because this is not desirable the liquid receiving area is either provided with no openings or with special gas openings, for example floating or fixed valves. Another example of such special openings are described in US-A-5702647. If the liquid discharge openings (10) are grouped, the part of the liquid receiving area not directly below these grouped liquid discharge openings may be provided with additional gas openings, for example the same openings as used in the bubble area (16) . In a preferred embodiment of the invention additional gas openings are present in the liquid receiving area (14,15) close to the supporting beam (3) in section (18) as illustrated in Figure 1. These openings can be the same as used in the bubble area (16) . Because almost no liquid is discharged onto this area (18) from the consecutive upper tray in a column according to the invention, it is possible to provide additional gas-liquid contact area in areas (18) on the tray. This is advantageous because liquid flowing from a downcomer in one tray section to the downcomer opening in the opposite tray section in a lower tray will thus be contacted with upwardly moving gas, thereby increasing the gas-liquid contacting efficiency. The downcomers (6,7) extend perpendicular from the supporting beam (3) to the circumferential of the tray (1). Downcomer ends (19,20) of both tray sections (4,5) meet the supporting beam (3) in an alternating manner. The opposite ends (21,22) nearest the wall can be parallel to ends (11,12) or follow the circumferential of the tray, in for example a straight or curved line, in order to optimise downcomer opening area. As can be seen in Figure 1 less liquid discharge openings (10) are present near the downcomer ends (19,20) resulting in that the area of the liquid discharge openings per downcomer length close to the supporting beam is smaller than the average area of the liquid discharge openings per downcomer length. Preferably no liquid discharge openings are present close to the supporting beam (3) . Figure 1 shows so-called sloped downcomers, meaning that the downward directed downcomer walls along its length are sloped, such that these elongated walls (23) are inclined towards each other in a downward direction. This is advantageous because the width of the liquid discharge area of a downcomer will be reduced, resulting in that the associated width of the liquid receiving area (14,15) on the tray just below will also be reduced. In addition to the illustrated trays having sloped downcomer walls downcomers having vertical walls may also be used in combination with the tray according to the invention. According to textbooks like "Distillation Operation" by Henry Z. Kister, McGraw-Hill Inc, 1990, pages 173-175, the ratio of the cross sectional area at tray level to the cross sectional area at the bottom of a sloped downcomer is between 1.5 and 2.0. Figure 2 shows a cross-sectional view AA' of the column of Figure 1 illustrating two trays (1,101) directly positioned above each other. The trays (1,101) are mirrored along the supporting beam (3,103). This results in that the liquid discharge openings (10) of tray (1) are vertically placed above liquid receiving area (115) of tray (101) just below. Figure 2 shows a cross-section of downcomers (7) placed in tray section (5) and a side view of downcomers (6) placed in tray section (4) . Downcomer (7) has two sloped walls (23) and at its end (20) a side wall (24) at the supporting beam (3) . As shown the side wall (24) is vertically positioned. Figure 3 also shows another preferred embodiment which may be used in combination with the column according to the invention. Figure 3 is a cross-sectional view along part of line BB' of Figure 1 illustrating two consecutive trays (1,101) in column (2) (not shown) near to the supporting beams (3,103) . Side walls (24) are sloped such that the lower part of side wall (24) points away from the supporting beam. Due to the sloping of this side wall (24) no liquid discharge openings will be present in the part of the downcomer close to the supporting beam and a somewhat directional flow is created in the first liquid discharge openings (10,110) nearest the supporting beam (3,103). Because the direction of the discharged liquid will point away from the supporting beam (3,103) and because of the increased distance to the supporting beam (3) less by-pass (26) of liquid flowing from downcomer (6) into downcomer (107) of the tray (101) just below results. The angle which side wall (24) makes with an imaginary vertical plane through supporting beam (3) is preferably between 10 and 35 degrees. Figure 3 also shows liquid receiving areas (15,114) having an area (18,118) close to the supporting beam (3,103) which is provided with gas openings (27,127) in order to increase the contacting of liquid flow (26) and the upwardly moving gas (28) through these openings (27,127) . Figure 3 also shows a preferred embodiment wherein a weir (25) is present on the downcomer which extends above the tray level. The weir (25) is a device positioned on or about on the boundary of the bubble area (16) and the downcomer (6,7) which ensures that a certain pre-selected amount of liquid is present on the bubble area (16) . In an even more preferred embodiment (not shown) part of weir (25) at the downcomer ends (19,20) extends more above the tray (1) than the rest of the weir (25) . This even further reduces by-pass (26) . In a further preferred embodiment (not shown) the liquid discharge openings (10) nearest the supporting beam (3) are provided with a flow directing lip, directing the liquid flow away from the imaginary vertical plane through the supporting beam (3) . Because of the flow directing lip, which may be easily be fabricated by stamping from the metal plate of which the downcomer is made, less area from below will be observed nearest the supporting beam (3). This will further result in that less by-pass (26) will be observed. The diameter of the tray may be between 1.5 and 10 m. The number of rectangular downcomers per tray section may be between 1 and 10. Additionally segmented downcomers are suitably present at the intersection of the supporting beam and the circumferential of the tray. A downcomer preferably extends to between 50 and 90% of the tray spacing below the tray. Tray spacing is defined as the distance- between two consecutive contact trays in the column. Suitably the tray spacing is between 0.2 and 1 m. For the present invention it is not critical which kind of openings are used in the bubble area of the tray. Examples of possible openings are bubble cap openings, sieve tray openings, valve tray openings and fixed valve openings. Examples of these openings can be found in general text books such as Distillation Design, Henry Z. Kister, McGraw-Hill Inc, 1992, page 260-267 and in US-RE-27908, US-A-5120474 , WO-A-9828056, WO-A-9737741, US-A-5911922, US-A-3463464 and US-A-5454989. The downcomer is preferably provided with a rectangular shaped baffle vertically positioned along the longitudinal centre in the opening of the downcomer. This anti-jump baffle plate preferably extends to between 30 and 70% of the tray spacing above the tray level. The lower end may extend from tray level to the downcomer lower end. The anti-jump baffle is suitably supported from both sides by supports which are fixed to the inside of the downcomer walls. It has further been found advantageous to provide the downcomer with so-called anti-sloshing baffles. Applicants have found that when vertical plates spaced apart from each other and arranged parallel to the width of the downcomer are present in the lower half of the downcomer less sloshing will occur. Sloshing occurs especially in elongated downcomers, especially in downcomers longer than 0.5 m as measured at tray level, where liquid moves from one end to the other and back. These plates may extend a certain distance from the inner side of the downcomer wall or may optionally extend all the way to the opposite downcomer wall. In the last embodiment the anti-sloshing baffles will also add to the mechanical strength of the downcomer itself. In an even more preferred embodiment the anti-sloshing baffles are at the same time the supports for the above described anti-jump baffle. The column according to the invention is preferably used as a gas-liquid contacting or separation column. Contacting can be absorption processes wherein a downwardly moving liquid is contacted with an upwardly moving gas. Separation is typically a distillation process to separate one or more components from a feed. Typically the feed of a distillation process is supplied to an intermediate position in the column, wherein trays are present above and below said inlet position. Such a column is further provided with reboiler, condensation and reflux means. The invention shall be illustrated by the following non-limiting Example. Example 1 The efficiency was calculated of a tray according to Figure 1 with a tray diameter of 2.44 m and equipped with 8 rectangular downcomers having no liquid discharge openings in the first 0.2 m of the downcomer as seen from the diametrical line. The calculated efficiency is 90%. The calculations were performed using the tray efficiency model as published in the Chemical Engineering Journal, Volume 63, pages 167-180 (1996) "The effect of downcomer layout pattern on tray efficiency" in combination with experimental observations. Comparative Example A Example 1 was repeated except that additional liquid discharge openings were present in the first 0.2 m of the downcomer as seen from the diametrical line. An efficiency was calculated of 85%.

Claims: C L A I M S 1. Gas-liquid contacting column provided with a plurality of axially spaced trays, each tray having two tray sections separated along a diametrical line on the tray, each tray section comprising a plurality of parallel rectangular downcomers, a plurality of liquid receiving areas and bubble area between the downcomers and the liquid receiving areas, which liquid receiving area is positioned just below the downcomer of a consecutive tray just above and which downcomers are open at the upper end and closed at the bottom end, which bottom end is provided with a plurality of liquid discharge openings and which downcomers extend from the diametrical line to the circumferential of the tray in such a manner that the ends of the downcomers of the two tray sections are positioned along the diametrical line in an alternating manner, wherein the area of the liquid discharge openings per downcomer length as viewed from below and close to the diametrical line is less than the average area of the liquid discharge openings per downcomer length as viewed from below. 2. Column according to claim 1, wherein area with openings is present on the tray close to the diametrical line and just below the downcomer of a consecutive tray just above. 3. Column according to any one of claims 1-2, wherein close to the diametrical line is meant a distance of 0.2 m. 4. Column according to any one of claims 1-3, wherein the two elongated walls of the downcomer and the wall nearest the diametrical line are sloped such that they are inclined towards each other in a downward direction. 5. Column according to any one of claims 1-4, wherein along the boundary of the downcomer opening and the bubble area a weir is present, which weir has a part at the downcomer ends nearest the diametrical line extending more above the tray than the rest of the weir. 6. Column according to any one of claims 1-5, wherein the liquid discharge openings close to the diametrical line are provided with a flow directing lip, directing the liquid flow away from the imaginary vertical plane through the diametrical line. 7. Column according to any one of claims 1-6, wherein the lower half of the downcomer is provided with vertical plates spaced apart from each other and arranged parallel to the width of the downcomer. 8. Column according to claim 7, wherein the length of the downcomer as measured at tray level is more than 0.5 m. 9. Column according to any one of claims 1-8, wherein downcomer is provided with a rectangular shaped baffle vertically positioned along the longitudinal centre in the opening of the downcomer, extending to between 30 and 70% of the tray spacing above the tray level. 10. Column according to claim 9, wherein the support of the baffle are vertical plates spaced apart from each other and arranged parallel to the width of the downcomer and positioned in the lower half of the downcomer. 11. Column according to any one of claims 1-10, wherein the tray diameter is more than 2 meter and along the diametrical line a supporting beam is present. 12. Use of a gas-liquid contacting column according to any one of claims 1-11 as a distillation column or absorption column.

Inventors: BOSMANS BERNARDINUS HENRICUS, KONIJN GERRIT, KUSTERS KAREL ANTONIUS, BOSMANS, BERNARDINUS, HENRICUS, KONIJN, GERRIT, KUSTERS, KAREL, ANTONIUS
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 2001
Main classification: B01D3
IPC codes: B01D3, B01J10, B01J19