GB2059800A - Gas-liquid separating column - Google Patents

Abstract

Various disadvantages which arise in providing multiple channel overflows 3 on the trays 2 of a co- current separating column are obviated or mitigated by enclosing the co-current elements on each tray in separator housings 7 upstanding on the tray. Preferably the bottom edges of the separator housings are fixed to the trays either directly or through solid or perforated vertical baffles, or through baffles combined with vertical separators. Vertical sections or sloping-roof sections of the separator housings are made with either V or U section corrugations or channels. The active surfaces are defined by the system of channel overflows. <IMAGE>

Description

SPECIFICATION Separating column This invention relates to the separating column used for diffusion and washing processes, working on the principle of co-current contact between the liquid and the gas phases. Diffusion processes denote here processes of distillation and rectification, and also processes of absorption and desorption. By washing processes is meant, above all, the washing of solid or liquid particles from gases.

The separating column according to the invention can also be used for direct heating or direct cooling of two-phase liquid and gas systems.

In addition to conventional trays with one, two, three, or four flow passages there are at present in existence trays which are referred to as trays with multiple overflows, used especially for large liquid loads. These trays use either conventional siphon-type or dynamic overflows. Trays with dynamic multiple overflows were first described in the Federal Republic of Germany in 1956, in patent No.

1,051.605. The object of said patent is a tray with multiple channel overflows arranged on the tray with relatively small spacings, parallel to one another. The openings for the passage of liquid are with this tray made in the lateral walls of the channels. Later, a similar design of the tray found wide application, making also use of parallel channel overflows; in this version, however, the orifices for the liquid were made in the bottom of the overflow channels. This latter design was the object of US patent No 3410,540.

The two arrangements described above led to an increase in the active surface of the tray since also the area below the overflow was made use of here. The long overflow crest of this arrangement, together with the application of sieve trays, made it also possible to maintain a small dynamic height of the liquid, and consequently of the foam, which permitted a reduction in the distance between the individual trays. Another advantage of this arrangement was the fact that due to the channel overflows the tray itself was very rigid so that there was no need to use a supporting structure since the overflow channels served simultaneously as supports of the tray.

Another substantial advantage of these trays consisted in that with increasing diameter of the column the specific liquid load of the overflow weir did not fall, which with conventional tray overflows always resulted in limiting the increase in the column diameter.

The tray arrangement described, however, has also some drawbacks, the principal of which is the reduction of the working range of the tray, i.e. the ratio of the maximum and the minimum gas load of the tray, which ranges from 60 to 100%. Furthermore, the flow path was reduced in the described arrangement, bringing with it a reduction in dividing efficiency so that to obtain the same dividing effect, when compared with conventional trays, a greater number of trays had to be used. Another disadvantage consisted in that at locations where the liquid flowed onto the tray, weeping appeared. In spite of these disadvantages the considerable economic benefits of these types of tray led to an extensive application of trays with multiple channel overflows.

In recent years, new problems appeared in connection with the application of co-current apparatus which, compared with conventional tray types, are three to five times as efficient with respect to gas load. This entails a corresponding reduction in the active surface of the tray and a relative increase of overflows which amount to 25 to 40% of the total crosssectional area of the column. These problems exclude the possibility of applying conventional trays with conventional overflows for cocurrent apparatus. The only alternative is the application of trays with multiple overflows, either of the siphon or the dynamic types.

Owing to the considerable extent of the overflows, which in the case of applying multiple channel overflows can amount up to 40% of the tray surface, the liquid flow path must of necessity be made shorter, or possibly the distance between adjacent overflows, which will result in reducing the dividing efficiency of the tray, in impairing the conditions of the liquid flowing onto the trays, and also in deteriorating the conditions for locating co-current elements on the narrow active portion of the tray. Moreover, in the case of the existing multiple channel overflows applied to a co-current tray there is a danger that with large specific liquid loads the admissable load limits will be exceeded, which will result in jets of liquid falling over the overflow weirs of one overflow coming together and thus increasing the tendency of the tray to flood the overflows.

The separating column according to the invention makes it possible to apply multiple channel overflows in co-current installations while suppressing as much as possible the disadvantages given above. The virtue of the separating column according to the invention which is formed by co-current elements above which separators are located arranged between mutually parallel multiple channel overflows, consists in that co-current elements are situated on active surfaces, above which separators are mounted. The bottom edges of the separators are fixed to the trays, either directly or through solid or perforated vertical baffles, or through baffles combined with vertical separators. The vertical or sloping sections of separators are made either broken or corrugated. The active surfaces are defined by the system of channel overflows.Another pos sible version of the arrangement of active surfaces are mutually perpendicular systems of channel overflows, where the number of channel overflows in one system is by one greater than the number of overflows in the other system.

The tray arrangement with parallel channel overflows is simpler in design, but the application of two mutually perpendicular systems of channel overflows results in that, in comparison with the above trays with multiple cocurrent channel overflows, the liquid flow path and the active area of the tray will be increased, or with the same liquid flow path a much greater length of overfow weirs will be obtained and the length of the path of the liquid jet will be reduced.

This results in the possibility of narrowing the width of the overflows in favour of the active surface of the tray so that co-current elements are easy to mount on the individual square or rectangle sectors of the active surface of the tray.

The arrangement of the column according to the invention thus enables a larger loading of the whole column at increased liquid flow path, and in connection with this a higher degree of dividing efficiency. It also reduces the risk of flooding the overflow channels and provides for extreme specific liquid loads, which could not be coped with while applying the multiple overflows described above.

Of special advantage is the arrangement where the number of channel overflows in one system is by one greater than the number of channel overflows in the other system. This arrangement makes it possible to manufacture all the trays of the same type and by turning the adjacent tray by 90 have the overflows of one tray open into the centre of the quadrangular active surface of the nearest lower tray.

With large liquid and gas loads, in cases when the foam from one part of the tray could reach over the overflow into the other part of the tray and when there would be a danger of flooding the overflows, it is of advantage to use baffle walls made of solid or of perforated material or of layers of metal knitwork or netting. These baffle walls are placed in the longitudinal axis of the channel overflows and their lower edge reaches below the level of the upper edge of the overflow weir while their upper edge terminates below the level of the nearest higher tray.

When treating large flows of liquid it is of advantage if on either side of the channel overflows the upper ridges of overflow weirs are provided with breaking means, which alternate with lowered crest of the overflow weir. The length of the breaking means is chosen to be smaller than the length of lowered crests, and the breaking means on one side of the channel overflow are arranged in such a way that they are displaced with respect to the breaking means on the opposite side of the overflow.

Generally, the principle must be observed that the total length of breaking means does not exceed one third of the length of the crest.

The breaking means are used to break up the current of liquid above the overflow weir and to enable the gas which is above the tray to penetrate into the space of air or gas formed below the jet of liquid flowing over the overflow weir. From this space, which in the lower part is defined by the liquid level in the overflow, the gas is drawn off by the jet of flowing liquid and as long as this space is not connected with the gas space above the tray, a premature flooding of the overflows may occur. In cases when the flow path results small, this arrangement is unnecessary because something similar happens spontaneously in each corner of the quadrangular active surface at the point where the channel overflows of the two systems intersect.

Examples of embodiments of the separating column according to the invention are schematically illustrated on attached drawings, where Figure 1 shows a vertical section through a portion of the column with three trays, provided with multiple dynamic channel overflows forming two mutually perpendicular systems, where adjacent trays are always turned by 90 and co-current elements are for the sake of clarity not shown on the tray, Figure 2 shows the corresponding horizontal section through the plane A-A of the column according to Fig. 1, Figure 3 is an axonometric view of the tray in the embodiment as shown in Figs. 1 and 2, Figure 4 shows a vertical section through the plane A-A of a portion of the tray according to Fig. 5, illustrating in detail the embodiment of the channel overflow whose crests are provided alternately with breaking means and lowered crests, Figure 5 shows an axonometric view of a portion of the tray with one channel overflow in whose overflow weirs breaking means have been made, Figure 6 shows a vertical section through a portion of tray which one channel overflow made as a siphon overflow, Figure 7 shows a vertical section led through the column axis of the slot-type tray with a simple overflow and two rows of separators in the form of saddle roofs, Figures 7a and 7b show two alternatives of the sloping portions of the separators, Figure 8 shows in a vertical section a detail of the embodiment of two types of separators in the form of a lean-to roof, Figure 9 shows in a vertical section a detail of the asymmetric embodiment of the separator in the form of a saddle roof supported with vertical baffles.

Figure 10 is an axonometric view of the tray of the slot-type, where for the sake of clarity one row of slots have been left uncovered while the other rows are overlaid on the one hand by saddle-type separators located perpendicularly to the longitudinal axis of the overflow, on the other hand by separators located parallel to this axis; in the left part of the tray an embodiment is illustrated of separate shortened one-sided separators overlapping circular or quadrangular openings, Figure 11 shows a vertical section through a portion of the column provided with oneflow trays with conventional overflow and round openings overlaid with separators in the form of saddle roof, Figure 12 shows a vertical section through a portion of the column provided with trays with a multiple overflow and round openings overlaid with separators in the form of saddle roof, Figure 13 shows a section through the horizontal plane C-C of the tray according to Fig. 11, Figure 14 shows a section through the horizontal plane D-D of the tray according to Fig. 12.

In a jacket 1 of the column according to Figs. 1 and 2 located at regular spacings above one another in horizontal planes are trays 2, which are made e.g. as sieve trays or perforated sheet trays, provided with two mutually perpendicular systems of channel overflows 3. In the embodiment illustrated the two systems of channel overflows 3 are interconnected but from the viewpoint of rigidity it is of advantage to have the channel overflows of one system made uninterrupted and the overflows of the other system be connected to them. The channel overflows 3 of the two systems thus form squares of active surfaces 5 in which the co-current elements are situated (not shown here). In the bottoms of the channel overflows 3, or in their lateral walls, and below the level of the tray 2, openings 4 are made.The lay-out of the openings 4 is such as to maintain maximum possible liquid flow paths on the individual trays. The openings 4 of the overflow channels 3 are thus concentrated in places close to the centre of the square active surface 5 of the lowerlocated tray 2.

The spatial lay-out of the two mutually perpendicular systems of the channel overflows 3 can best be understood from the axonmetric view shown in Fig. 3. Overflow weirs 8 of the individual channel overflows 3 are interrupted at points of intersection; from the viewpoint of strength and design rigidity, however, it is possible to have in the lower or lateral portions of the overflows 3 only openings which will allow the liquid to flow from one system to the overflows 3 into the other system.

Fig. 4 illustrates in a partial vertical section of a portion of the tray according to Fig. 5 the flow of liquid in the channel overflow 3. The jet of liquid flowing over the overflow weir 8 together with the liquid surface defines a space 21 filled with the gas phase. From this space the gas or the vapours of the gas phase are drawn off by the liquid jet and absorbed so that as long as the space 21 is not connected with the gas space above the tray 2 flooding of the overflow 3 may occur. To prevent flooding, breaking means 9 are used, as shown in Fig. 5. These breaking means 9 break up the current of liquid above the overflow weir 8 and allow the gas which is above the tray 2 to penetrate into the space 21.

Shown in Fig. 6 is another embodiment of the tray according to the invention, in which a conventional siphon closure 10 is used. This closure 10 is located in the bottom part of the channel overflow 3 at such an elevation that the overflow 3 and the siphon closure 10 do not extend to the upper surface of the nearest lower tray 2 and are sufficiently distant from it. The liquid flows from the channel overflow 3 via a slot 12 in the lateral wall of the overflow 3. Fig. 7 shows a separating column in whose jacket 1 the trays 2 with conventional overflows are situated spaced regularly above one another. The trays 2 are provided with openings 6 in the form of elongated slots.Seated by their lower edges on the nonperforated portion of the tray 2 are separators 7 in the form of saddle roof, whose sloping walls form with the tray plane an angle of 60". The separators 7 are situated on the tray in such a way that their longitudinal axis is parallel to the longitudinal axis of the crest line. Each of the separators 7 overlies a row of the openings 6. In the embodiment according to Fig. 7 the openings 6 are shown running parallel to the longitudinal axis of the separator 7 along the whole diameter of the tray 2.

Also possible, however, is an embodiment in which the openings 6 are orientated perpendi cular to the longitudinal axis of the separator 7. In the embodiment illustrated, the separators bear with their edges down to the inner wall of the jacket 1 of the column. Figs. 7a and 7b show two versions of the sloping portion of the roof of any type of separator.

The corrugated or broken shape of the sloping walls of the separators increases their overall separating surface, the entrainment velocity of the penetrating gas is proportionately increased and thus the dividing effect of the separator increases.

Fig. 8 shows two alternatives of the embodiment of the one-sided separator 14 in the form of a lean-to roof. In the embodiment shown in the left part of the Figure a vertical separator 17 links up with the upper edge of the one-sided separator 14 and continues with the vertical baffle 15 fixed to the upper surface of the tray 2. The application of the vertical separator 17 also increases the separ ating surface of the separator 14 and reduces accordingly the entrainment velocity of the gas. The combination of the vertical baffles 15 with the vertical separator 17 can be used in the case of large liquid loading of the tray 2, i.e. in the case of an unfavourable entrained liquid to vapour ratio. In the embodiment illustrated in the right part of Fig. 8 the vertical separator 17 has been omitted, and the vertical baffle 15 is extended up to the upper wall of the one-sided separator 14.The lateral walls of the separators 14 are in both embodiments closed with lateral baffles 16, provided in the lower part with openings 18.

Fig. 9 shows the embodiment of an asymmetric separator 13, which is formed by two sloping parts or walls 19 and 20 differing in size. Linked up with the lower edges of these two walls are the vertical baffles 15 fixed with their lower edges to the upper surface of the tray 2. The side walls of the separators 13 are again closed by the lateral baffles 16 with the openings 18.

In Fig. 10 various versions of the embodiment of separators and tray perforations are shown. The tray 2 is provided with the multiple channel overflows 3 between which, in the major part of the tray 2, the openings 6 are made, perpendicular to the longitudinal axis of the channel overflows 3. The openings 6 in the left portion of the tray 2 are for the sake of clarity not covered with the separators, the openings 6 in the right portion of the tray 2 are partially overlaid with the saddle separators 7 orientated perpendicularly to the axis of the channel overflows 3, and partially with the separators 7 orientated parallel to the longitudinal axis of the overflows 3. For simplicity, the saddle separators 7 in this section of the tray are not closed with the lateral baffle 16.In the extreme left of the tray 2 the shortened separate one-sided separators 14 are illustrated combined with the vertical separators 17 and the vertical baffles 15. In this case the individual separate separators 14 are closed with lateral baffles 16. Both the lateral baffles 16 and the vertical baffles 15 of the separate one-sided separators 14 are in their lower portion provided with the openings 18.

Similar openings 22 are provided in the lower edges of the channel overflows 3 below the level of the tray 2.

The trays in the embodiment according to Figs. 11 and 13 are arranged in such a way that the openings 6 in the tray 2 are circular and always arranged in two rows below the saddle separators 7. The trays are of the single-flow type with conventional overflow.

Illustrated in Figs. 12 and 14 are similar embodiments of a multiflow tray provided with a multiple channel overflow 3.

The tray embodiments shown by way of example in attached drawings do not, of course, cover all the possible modifications and variations in the tray design. Besides longitudinal slots and circular openings also rectangular openings can be provided in the tray. The free surface of the tray, i.e. the ratio of the area of openings in the tray to the active surface of the tray, must exceed 25% of the total tray surface. The angle formed by the sloping walls of separators with the horizontal surface of the tray can generally range from 30 to 75 . The separators can be made in the form of pyramidal or conical roofs or roofs in the form of truncated cone or pyramid, or roofs in the form of a part of torus.

Channel overflows on multiflow trays can be replaced by multiple tubular overflows, etc. In general, the principal must be observed that the built-in structure must be as simple as possible and have as few components as possible.

The column in the embodiment according to the invention works in principle as follows: the liquid which is supplied to the tray via the overflow of the nearest higher tray flows on the tray in horizontal direction around the openings in the tray towards the overflow or overflows of this tray. The gas ascending through the openings in the tray entrains by virtue of its own kinetic energy the liquid towards the separators where the liquid is separated and due to gravitation flows down the sloping surfaces of the separators back to the same tray, where it gets mixed with the newly coming liquid supplied from the overflow of the higher-located tray and is recirculated with it. The remaining part of the liquid moves towards the overflow and flows through this overflow onto the lower-located tray.The gas, from which most of the entrained liquid has been separated, ascends to the higher-located tray, where the whole process is repeated.

The volume of the liquid supplied through the openings 18 in the vertical baffles 15 or in the lateral baffles 16, or of the liquid entering through the bottom part of the separators is precisely defined and, consequently, it is possible to determine accurately the volume of recirculated liquid to maintain the pressure drop of the tray, and from these values derive also the efficiency of the apparatus. As a result, the respective diffusion, washing or other process can be controlled within predetermined limits. The designer of the apparatus is thus given the opportunity of complying exactly with separation conditions at given hydraulic conditions.

List of reference characters column jacket 2~column tray 3 channel overflow 1 openings in the bottom of channel overflow 3 5 active surface 6~opening in column tray 2 7~saddle separator 8~lowered crest; overflow weir 9~breaking means 10~siphon closure 11~baffle plate 12~lateral slots of channel overflow 3 1 3 asymmetric separator 14~one-sided separator 1 5~vertical baffle 16~lateral baffle 17~vertical separator 18~openings in baffles 15and 16 19~shorter (vertical) part of asymmetric sep arator 13 20~longer (sloping) part of asymmetric sep arator 13 21~space of air 22~openings in lower edges of channel overflows 3

Claims (9)

1. A separating column comprising a series of vertically spaced trays, overflow means on each tray providing for flow of a layer of liquid over the active surface of the tray, and the downward flow of liquid from the tray to the next lower tray, co-current means on the active surface of each tray providing for upward flow of gas through the liquid layer to the next higher tray, and separating means upstanding on the tray and enclosing the cocurrent means to separate entrained liquid from the gas flow and return the separated liquid to the liquid layer.

2. A separating column according to claim 1, wherein the separating means include housings having gas-permeable roof portions sloped downwards towards the tray to provide for gravity return flow of separated liquid downwards into the liquid layer.

3. A separating column according to claim 2, wherein the roof portions are channelled and slope downwards towards the liquid layer to carry the separated liquid.

4. A separating column according to any one of claims 1 to 3, wherein the overflow means include a plurality of channel overflows defining the active surface of the tray, and the separating means are disposed between the channel overflows.

5. A separating column according to claim 4, wherein the active surface is defined by two mutually perpendicular systems of parallel channel overflows, the number of channel overflows in one system being greater by one overflow than the number of overflows in the other system.

6. A separating column according to claim 5, wherein adjacent trays in the column are located in such a way that they are mutually at right angles to each other.

7. A separating column according to any one of claims 4 to 6, wherein in the longitudinal axis of the channel overflows baffle plates are located which are made of solid or perforated material or layers of metal openwork or netting, with the lower edge of the baffle plate disposed below the level of the upper edge of an overflow weir and its upper edge terminating below the level of the next higher tray.

8. A separating column according to any one of claims 4 to 7, wherein (a) the channel overflows are provided with breaking means formed in the upper edges of the overflow weirs on either side of the channel overflows, (b) the breaking means alternate with the lowered crest of the overflow weir, the crest length exceeding the length of the breaking means, and (c) the breaking means at one side of the channel overflow are staggered in relation to the breaking means at the opposite side of the channel overflow.

9. A separating column substantially as hereinbefore described with reference to the accompanying drawings.