GB2035127A - Plate for mass transfer operations - Google Patents

Abstract

A weir and baffle system on a plate for mass transfer operations, e.g. distillation and absorption, comprises a metal weir plate 4 adjacently positioned against the ends of vapour-liquid contact channels 1, 2 and 3 defined by baffles across the plate. The weir is either spot welded or riveted to the channel ends. Liquid return zones 9 and 10 are defined between the channels 1, 2 and 3 and communicate with apertures in the weir plate. The channels are blocked off by the flat surface of the weir plate, and the inlet liquid fed by a down comer to behind the weir plate has therefore to pass initially into the liquid return zones 9 and 10 before crossing the plate in the channels. The outlet ends of the channels and liquid return zones are blocked off by an outlet weir leading to a downcomer. <IMAGE>

Description

SPECIFICATION Weir This invention relates to weirs for use on plates for mass transfer operations, and has particular but not exclusive reference to distillation and absorption processes.

It is known to use two types of weir. The first type is used in small columns, where the weir is formed from the downcomers. The downcomers, which are usually circular, are extended down towards the plate, so that they are at a suitable height to act as a weir. A seal is placed at the bottom of each downcomer, so that vapour is prevented from rising up the downcomer.

The second type of weir is used in larger columns, and is known as a rectangular chord weir. This particular type of weir is a flat sheet which cuts off a part of the plate. It is possible that several downcomers may be used flush with the flat sheet. However, it is also possible that one downcomer, corresponding to the cross-sectional area of the cut-off part of the plate, may be used. A liquid seal at the bottom of the downcomer(s) can be provided by an inlet chord weir. The crosssectional area of the cut-off part of the plate i.e. the part of the plate located behind the weir may be 15 to 20% of the total plate area.

Exit weirs are often placed a short distance from the last row of perforations on the sieve plate. This is to ensure that the liquid can be disentrained from the vapour to allow the downcomers to receive only clear liquid. This means that vapour-free liquid is fed to the inlet weir on the next plate.

Thus, the present invention consists in a weir for use with a baffle system of a plate for mass transfer operations, such that the weir ensures that the flow of inlet liquid is# fed to liquid return zones, said zones being defined by the space between two channels as present in a baffle system.

The weir is particularly useful in conjunction with a baffle system as claimed and disclosed in my co-pending Patent Application British Patents Application No. 45535/78 filed simultaneously and entitled "Distillation Plate".

Therefore, the present invention further consists in a baffle system for use on a plate for mass transfer operations comprising: at least two channels, the surface of said channels through which a two-phase mixture of liquid and vapour leaves the channels having a plurality of holes, a deflector immediately adjacent to each hole and said deflector being aligned to deflect the two-phase mixture in a direction towards one end of the channels; and a weir according to the present invention.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a schematic cross-section of a column having baffled plates; Figure 2 shows an end elevation of a first embodiment of an inlet weir; Figure 3 shows a plan view of an inlet weir; Figure 4 shows a means for securing an inlet weir to a sieve plate; Figure 5 shows an end elevation of a second embodiment of an inlet weir; Figure 6 shows a plan view of a third embodiment of an inlet weir; Figure 7shows a plan view of a fourth embodiment of an inlet weir; Figure 8 shows an end elevation of an outlet weir; and Figure 9 shows a plan view of the outlet weir.

Figure 1 shows a schematic cross-section of a column 1 having baffled sieve plates 2. The baffling is provided by channels 3. Liquid from the plate above passes down an inlet downcomer 4 onto the plate 2. The liquid flows over or through the inlet weir 5, and then crosses the plate in the channel 3. The liquid then flows over the outlet weir 6, and passes down the outlet downcomer 7 to the plate below. The direction of liquid is shown by arrows A.

An embodiment of an inlet weir is shown in Fig. 2. The weir consists of a metal plate 4, which will form part of the downcomer, adjacently positioned against the end of channels 1, 2 and 3. The weir can be spot-welded or rivetted to the channel ends. It must be appreciated that the channels extend across the plate, and that in practice there may be more than three channels on the plate. A liquid return zone is defined by the space between two channels laterally adjacent to each other.

The metal weir plate 4 is slit and bent, as shown in Fig. 4, to form slots 5 and 6 at regular intervals along the plate. The slots match or correspond to the liquid return zones. The teeth 7 and 8 block the two-phase channels 2 and 3, so that the inlet liquid cannot pass into these channels. The inlet liquid is allowed to pass into the liquid return zones between the channels.

Fig. 3 shows a plan view of an inlet weir.

Two liquid return zones 9 and 10 are defined between three channels 1, 2 and 3. The channels are blocked off by the flat surface of the weir plate, and the inlet liquid has therefore to pass into the liquid return zones.

Fig. 4 shows a means for securing an inlet weir to a sieve plate. The metal weir plate 4 can be secured to the sieve plate either by fixing bracket or a tab 12.

A further embodiment of the inlet weir is shown in Fig. 5. The metal weir plate is a continuous sheet 13 drilled with a pattern of holes. The pattern of holes consists of a line of holes 14 adjacent to one of the long edges of the metal weir plate, with lines of similar shape and size holes 15 at regular intervals extending away from the row of the holes 14 towards the base of the weir plate. The lines of holes extending towards the weir plate base 15 and occurring at regular intervals along the weir plate correspond to the liquid return zones. The area of plate between each line of holes 15 blocks off the two-phase channels.

Therefore, inlet liquid is fed to the liquid return zones via the lines of holes 14 and 15.

The weir plate is adjacently position against the ends of the channels, and it may be spot welded or rivetted to the channel ends. The diameter of the holes are less than the width of the liquid return zones. If the liquid load is low the line of holes 14 may be omitted.

Figs. 6 and 7 show further embodiments of an inlet weir. The walls 17 and 18 of the channels 1 and 2 are extended beyond the end of each channel. The extensions 19 and 20 are then bent to meet or overlap each other, so that each channel is blocked off. The extensions 19 and 20 are preferably spot welded or rivetted together 21. This weir design would be used in combination with a flat downcomer which terminated somewhat below the height of the baffles and adjacent to them.

In Fig. 7, the channels 1 and 2 are blocked off by separate filler pieces 22. These can be either made of a resilient material and sprung into the channel openings, or they can be made of a less resilient material and attached to the inside walls of the channels 17 and 18 by spot welding or rivetting.

There is an outlet weir at the opposite end of each channel to the inlet weir. The outlet weir is required to block off the ends of the two-phase channels. The outlet weir should also partly or wholly block off the liquid return zones between the channels. The dispersed liquid phase on each sieve plate is retained on the plate by the inlet and outlet weirs. The outlet weir blocks off the ends of the twophase channels, so that the dispersed liquid phase in each channel is entrained by the rising vapour, and the two-phase mixture is then forced to pass up through holes located in an upper surface of each channel, in order to promote better mass transfer (as claimed and disclosed in my co-pending Patent Application British Application No. 45535/78 simultaneously filed and entitled "Distillation Plate").

Figure 8 shows an end elevation of an outlet weir. The walls 17 and 18 of the channels 1 and 2 are extended away from the ends of the channels. The extensions 19 and 20 are then bent in a direction perpendicular to the channels. so that extensions block off the two-phase channels and liquid return zones. This is shown in plan view in Fig. 9.

Extensions can be spot welded or rivetted together 21.

A similar effect from an outlet weir can be obtained by using a flat metal plate across the ends of the channels. The metal plate can be attached to the ends by either brackets or tabs or by spot welding or rivetting. It may also be beneficial in practice to castellate the upper edges of the outlet weir, so that the liquid return zones have lower weirs than the twophase channels.

While the above described weir will be beneficial with any type of sieve plate, the advantage will be greatest when used with a baffle system for use on a plate for mass transfer operations as described in my-pending Patent Application British Application No.

45535/78 filed simultaneously and entitled "Distillation Plate".

Claims (13)

1. A baffle system for use on a plate for mass transfer operations comprising at least one two-phase channel in which vapour and liquid are intimately mixed, liquid return zones for enabling liquid to flow back onto the plate, the liquid return zones being defined by the space between two channels laterally adjacent to each other, an inlet weir which blocks off one end of the or each two-phase channel, so that inlet liquid is fed to the liquid return zones, and an outlet weir which blocks off the other end of the or each two-phase channel and the liquid return zones.

2. A baffle system as claimed in Claim 1, wherein the inlet weir is a plate adjacently positioned against the ends of the two-phase channels.

3. A baffle system as claimed in Claim 1 or 2, wherein the inlet weir is slit to form slots at regular intervals along the plate, the slots being spaced along the plate to match or correspond with the liquid return zones.

4. A baffle system as claimed in Claim 1 or 2, wherein the inlet weir is a continuous sheet of material drilled with a pattern of holes, the pattern consisting of a first line of holes adjacent to one of the long edges of the weir plate, with lines of holes spaced at regular intervals to match or correspond with the liquid return zones, said lines of holes extending away from the first line of holes towards the base of the weir plate.

5. A baffle system as claimed in any of the preceding claims, wherein the weir plate is spot welded or riveted to the channel ends.

6. A baffle system as claimed in Claim 1, 2, 3 or 4, wherein the inlet weir is integral with the channel ends.

7. A baffle system as claimed in any of Claims 1 to 5, wherein the inlet weir is secured to a sieve plate by a fixing bracket or a tab.

8. A baffle system as claimed in Claim 1, wherein the inlet weir is formed from the longitudinal walls of at least one channel, the walls being extended beyond the end of the or each channel and being bent to meet and overlap each other, so that the or each channel is blocked off to ensure that inlet liquid is fed to the liquid return zones.

9. A baffle system as claimed in Claim 1, wherein the outlet weir is formed from the channel walls being extended away from the ends of the channels, the walls being bent in a direction perpendicular to the channels, so that the extended walls block off the twophase channels and liquid return zones.

10. A baffle system as claimed in Claim 8 or 9, wherein the extended walls are spot welded or riveted together.

11. A baffle system as claimed in Claim 9, wherein the upper edge of the outlet weir is castellated so that the liquid return zones have lower weirs than the two-phase channels.

12. A baffle system as claimed in Claim 1, wherein the inlet weir is formed from filler pieces which block off one end of the or each two-phase channel, the filler pieces being attached to the inside walls of the or each channel by spot welding or riveting.

13. A baffle system substantially as herein described with reference to the accompanying drawings.