GB2041238A - Distillation plate - Google Patents

Abstract

A baffle system for use on a plate for mass transfer operations comprises at least one channel 1, the surface 5 of the channel through which a two-phase mixture of liquid and vapour leaves the channel having a plurality of holes 8. A deflector 9 is immediately adjacent to each hole and the deflectors are aligned to deflect two-phase mixture towards one end of the channel 1. A liquid return zone may be defined by the space 13 between two channels 1 and 2 laterally adjacent to each other. Disengaged liquid from the two-phase mixture collects in the liquid return zone 13 and passes through slots 10 in the channel sides to re-enter the channel. <IMAGE>

Description

SPECIFICATION Distillation plate This invention relates to plates or trays for mass transfer operations, and has particular but not exclusive reference to distillation and absorption proces ses.

The principle of distillation is that when a liquid containing two or more volatile components is boiled, the composition of the vapour is in general different from that of the liquid. It is therefore poss ible to separate components from one another.

Various types of equipment can be used to bring together vapour and liquid for mass transfer operations, in particular plate or packed columns. Plate columns comprise plates superposed to each other.

The plates may be perforated, so that vapour introduced at the column base can pass up the column.

Liquid is introduced at the top of the column, passes down the column, normally flowing in alternate directions across the plates. Each plate has a weir system, comprising an inlet and exit weir, so that the liquid can be retained on the plate and the vapour is caused to bubble through.

Distillation plates depend for their function on the mass transfer which occurs between the rising vapour and the dispersed liquid supported on the plate.

Distillation plates are designed to allowvapourto bubble or otherwise migrate up through a layer of liquid on the plate while the liquid flows across it.

The liquid is held as a dynamic dispersion on the plate by the vigorous action of the vapour emerging with high momentum from orifices in the plate, thereby enabling intimate contact to occur between vapour and liquid for efficient mass transfer.

In the process of dispersing vapour through a liquid on a perforated plate, different flow patterns and dispersion forms arise according to the flow conditions, e.g. the vapour and liquid flow rates, the system properties, the height of the outlet weir, the physical properties of the liquid and vapour, and the plate geometry, namely the form and separation of the plate perforations.

One of the most widely used plates consists of a horizontal uniformly-perforated metal sheet. A percentage free area is usually in the range 3 to 15%.

Liquid enters the plate through one or more inlet downcomers, and is retained on the plate by a weir system. The liquid flows over one or more exit weirs, and flows down outlet downcomers to the next plate below. Downcomers can form part of the plate. The liquid can flow alternatively in opposite directions across the plate, though there can be other liquid flow regimes, such as the liquid flowing to a central downcomer in the plate.

The object of a good plate design is to achieve a high efficiency of mass transfer at high loadings.

There are basically four flow regimes on plates~ bubble flow, cellular flow, froth and spray. These flow regimes are met in the foregoing order as the vapour velocity increases. The transition between regimes is not sharp, and it may not be possible to observe some regimes under certain conditions. It may also be possible to have two or more regimes co-existing.

In practice, it has been observed that as the vapour loading is increased, the behaviour of the liquid dis persion on the plate becomes increasingly erratic so that at any instant vapour is surging through some areas of the plate at a high rate whilst other areas may have no vapour flow. When this happens, liquid sloshes around the plate, which leads to transverse oscillatory waves across the column. The explanation of this erratic behaviour is that the rising vapour bubbles entrain liquid and hence impart an upward flowto the liquid.

At low vapour loadings, the liquid can drain back between the rising vapour bubbles and sustain the circulation. However, as the vapour loading increases, this steady circulation can no longer happen and it is replaced by a transient liquid return mechanism. Liquid accumulates in some regions of the dispersion until it achieves a sufficient density to drain under gravity, whilst other regions become denuded of liquid and vapour rises preferentially in these regions. Large bubbles or"vapourvoids" are therefore formed. The location of these different flow regions is continually changing in a more or less regular oscillatory manner. The effect is reduced mass transfer between the vapour and liquid phases, reduced point efficiency and increased liquid entrainment (such entrainment is detrimental to mass transfer) with increasing vapour flow.It is therefore necessary to reduce the formation of vapour voids. This can be done by ensuring that the liquid from the froth flows back to the plate floor.

A baffle system has been shown in "The Transactions of the Institution of Chemical Engineers" (1973) 51, 188, and "The Chemical Engineer" (1975) July/August, which provides a liquid return zone into the plate. This system uses vertical baffles consisting of thin plates supported on edge at intervals across the plate. The baffles are fixed in a direction which is substantially parallel to the liquid flow across the plate. Liquid drainage, back onto the plate, is assisted by the baffles, since the draining liquid is no longer bounded by rising vapour bubbles on all sides. The baffles also prevent the development of liquid sloshing and transverse oscillatory waves.The baffles are supported so that the gap between them is wider at the top, i.e. the edge furthest from the plate, than at the bottom, and they are also supported clear of the plate by a gap of about the same size as the lower gap separating the baffles.

It has been found that it is possible to achieve an equal reduction of the hydraulic gradient and of liquid back-mixing on the plate whilst achieving a further improvement of mass transfer at close tray spacings for high vapour and liquid loadings.

According to the present invention, a baffle system for use on a plate for mass transfer operations comprises at least one channel, the surface of said channel through which a two-phase mixture of liquid and vapour leaves the channel having a plurality of holes, a deflector immediately adjacent to each hole, and said deflector being aligned to deflect the twophase mixture in a direction towards one end of the channel.

When the baffle channels are in use, a liquid return zone is defined by the space between two channels laterally adjacent to each other.

In a preferred form of the invention, the or each channel is gabled, the surface through which the two-phase mixture of liquid and vapour leaves the channel 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 both along the plate towards one end of the channel and down towards the floor of the plate.

Accordingly, it is possible for the direction of the two-phase mixture to be adjusted depending on the duty, so that with high liquid loadings the angle would favour flow along the plate and at low liquid loadings, the direction could be mostly, or even completely, downwards.

The invention further consists in a baffled plate wherein the baffling is provided by the baffle system comprising channels according to the invention.

By means of the invention, it is possible to have better mass transfer and therefore higher point efficiencies. The flow of vapour is preferentially from the centre region of the channel where the twophases (i.e. vapour and liquid) mix. The vapour flow promotes some internal recirculation of liquid within the two-phase channel, giving more intimate contact between vapour and liquid and therefore better mass transfer.

The dispersion in the two-phase channels is very dense and turbulent because liquid is fed into it rapidly from the drainage channel through the slots provided, and because the gabling at the apex of the channel promotes some internal recirculation of the liquid within the two-phase channel. Also, because the deflectors direct the liquid from the dispersion in a downward direction, a substantial part of the momentum in the rising dispersion is used to prom ote external recirculation via the drainage channels.

Furthermore, the streams of dispersion emerging from the downward inclined holes in adjacent chan nels will impinge on each other and promote further mass transfer.

By means of the invention, the baffle system has provision for higher liquid and vapour loadings without the difficulties of liquid entrainment and hydraulic gradient. Also, the degree of liquid backmixing on the plate will be reduced. This will lead to a higher Peclet Number which combined with the improved point efficiency arising from very rapid mass transfer will generate increased plate efficiency.

It is possible for the invention to have specific mechanical features which cheapen the construction of large columns by permitting large unsupported plate spans with thin gauge material. The mechanical design of the baffle system is very strong and stiff, whether the channels are separately joined to the plate or integral with the plate. The flexing of large distillation plates is a recognised problem which could be cured by this means. It would also probably be desirable to use baffle channels of sufficiently thick material so that a plank placed across them would support the weight of a man.

Because the stiffness imparted by the baffle system allows thin gauge material to be used, it is possible to use smaller diameter perforations in the plate, and these are also beneficial in promoting improved mass transfer.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a plan view of a sieve plate incorporating the invention; Figure 2 shows a perspective view of two baffle channels; Figure 3 shows a plan view of one baffle channel; Figure 4 shows a longitudinal section of one baffle channel; Figure 5 shows an alternative plate layout; Figure 6 shows an end view of a baffle channel; and Figure 7 shows a side elevation of a baffle channel.

Figure 1 shows a plan view of a sieve plate 20 having channels 1 and 2. The liquid flows across the plate in the direction of the arrow A from the inlet downcomer 50 to the exit downcomer 55. It must be appreciated that Figure 1 shows a schematic plan view of a sieve plate and its associated downcomer arrangements.

Two channels 1 and 2 are shown in Figure 2. By way of example, the nearest of the two channels (Channel 1) shown in perspective will be considered.

The channel is defined by two sides 3 and 4, a roof 5 and two flanges 6 and 7. The flanges are used to join the channel to the plate 20, e.g. a sieve plate (when the channel is integral with the sieve plate, the flanges do not exist). The channel can be spot welded or riveted to the sieve plate along the flanges 6and7.

The channel 1 is substantially rectangular in cross-section. When the channel is made independently of the sieve plate, the sides 3 and 4 may be spot welded or riveted to the roof 5 along fold-lines 11 and 12. The shape of the channel may also be obtained by folding or pressing a single sheet of suitable material into the desired shape.

The roof of the channel 5 has holes 8 punched into the material. The shape of the holes are similar to those found on a cheese grater. The flap of material 9 which results from the punching operation is used as a deflector for the two-phase mixture of vapour and liquid. Figures 3 and 4 show a plan and a side view respectively of a channel.

Vapour rises up through the sieve plates 20 and enters the channel 1 (Figure 2). Liquid flows across the plate, shown by the arrow A, through the channel 1. As the vapour enters the channel, it entrains liquid and thereby produces a froth of a two-phase mixture which normally fills the channel.

The two-phase mixture is carried up through the roof of the channel by the rising vapour. The object of the deflector plates 9 by each hole is to deflect the two-phase mixture towards one end of each channel. In practice, the mixture will be deflected towards the outlet weir of each channel. The result is that some of the rising two-phase mixture emerging from each hole is deflected towards the outlet weir.

Additional mass transfer and drainage of the liquid phase back to the plate floor occurs with the impingement of the two-phase mixture on the deflector plate 9.

A disadvantage of the conventional sieve plate and weir system is that a hydraulic gradient develops across the plate. There is more liquid on the plate immediately adjacent to the inlet downcomer than there is near the outlet weir at the other end of the plate. Consequently, the vapour preferentially passes up through the liquid of least depth on the plate, since there is a lower pressure head at that pointthan at the inlet downcomer end of the plate.

Therefore, there is not sufficient contact between the two phases and excessive entrainment. Weeping may also occur.

As the two-phase mixture passes through the holes, there is a slight expansion which promotes disengagement of liquid from vapour. Separation of liquid and vapour occurs above the roof 5 of the channel 1. Liquid then collects in liquid return zones 12. These zones are defined by the space formed between two adjacent channels. The deflector plates 9 have deflected the two-phase mixture towards the outlet weir end of the channel. The disengaged liquid therefore moves towards the outlet weir end of the plate. This therefore helps to reduce the hydraulic gradient across the plate and to equalise the level of liquid across the plate. The vapour proceeds to pass up the column.Thereby the object is achieved of creating a deliberate concentration of vapour flow into certain zones and of liquid flow into certain other zones, enabling the liquid to return to the plate via the liquid return zones where it is protected from the foaming environment.

The disengaged liquid collects in the liquid return zones 12 and accelerates downwards, under gravity between the sides of each adjacent channel before discharging onto the plate. As shown in Figure 2, there are slots 10 cut at intervals along the lower edge of both sides of each channel. The liquid passes through these slots and re-enters the channel, where further mass transfer is promoted in the intimately mixed rising liquid and vapour within the channel.

A further embodiment of the invention is shown in Figure 5, which shows a plan view of the channel arranged in a herring-bone pattern on the plate. The alternative direction of the baffles in Figure 5 largely eliminates the dead spaces near the column wall by initially directing the liquid outwards and then direct- ing it inwards again.

An end view of a further embodiment of a baffle channel bounded on either side by similar channels, is shown in Figure 6. The channel is defined by two sides 203 and 204, a roof portion 205 and two flanges 206 and 207. The flanges are used to join the channel to the plate 220 e.g. a sieve plate (when the channel is integral with the sieve plate, the flanges do not exist). The channel can be spot welded or riveted to the sieve plate along the flanges 206 and 207.

The channel sides 203 and 204 are bent along fold-lines 208 and 209, so that the roof portion 205 is gabled. The roof portion 205 has a small surface 210, flat or curved, which is substantially parallel to the sieve plate when the channel is joined to the plate.

When the channel is made independently of the sieve plate, it may be formed by pressing and folding a single sheet of suitable material into the desired shape, which may be a single channel or an assembly of parallel channels formed side by side.

The sides 211 and 212 of the roof portion 205 have holes 213 punched into the material. The shape of the holes is similar to those found on a cheese grater. The flap of material 214 which results from the punching operation is used as a deflectorforthe two-phase mixture of vapour and liquid.

Vapour rises up through the sieve plate 220 and enters the channel 201 (Figures 6 and 7). Liquid flows across the plate through the channel 215. As the vapour enters the channel, it entrains liquid and thereby produces a two-phase dispersion or froth which normally fills the channel.

The two-phase mixture is carried up through the rows of holes in the roof of the channel by the rising vapour. The object of the deflector plates 214 by each hole is to deflect the two-phase mixture both towards the outlet weir end of each channel and downwards towards the sieve plate floor 220. Deflection along the plate eliminates the problems of hydraulic gradient across the tray. Deflection downwards promotes recirculation with the desired dense froth properties. It also reduces the dispersion height above the baffles and so allows closer tray spacings for a given vapour loading or higher vapour loadings for a given tray spacing.

The angle of deflection is changed by varying the angle a (see Figure 7) which the front of the hole makes with the longitudinal dimensions of the channel, and to a lesser extent by varying the gable angle p (see Figure 6). As the angle a is reduced the liquid is thrown progressively in a downward direction.

Increasing a throws the liquid towards the outlet weir. For very low liquid loadings the angle a could be made zero, in which case pairs of holes, or more numerous assemblies of holes could be punched in rows, all pointing essentially downwards. For a given value of a the effect of decreasing the angle p will be to incline the flow more along the plate and less in a downward direction. The choice of angles and spacings can be optimised for a given duty.

The total flow area through the inclined hole should be made about equal to the flow area up through the two-phase channel 201 so that the plate pressure drop is kept low.

The disengaged liquid collects in the liquid return zones 215 (or 12 as shown in Figure 2) and flows downwards, under gravity between the sides of each adjacent channel before discharging onto the plate.

As with the previous embodiments, there are slots 216 cut at intervals along the lower edge of both sides of each channel. The liquid passes through these slots and re-enters the channel where further mass transfer is promoted in the intimately mixed rising liquid and vapour within the channel.

Any form of construction which achieves the protected return paths for liquid flowing back to the floor of the plate may be employed. The baffle system comprising the channels may reach the full distance from inlet weir to outlet weir, and being a construction which extends the full width of the column, or it may be built up out of modules, each module covering part of the area of the plate.

The baffle system is preferably used with an underflow inlet weir, so that the ends of the baffles reach to the face of the weir. The underflow inlet weir communicates only with the liquid drainage channels 215. The inlet weir may be joined to the baffle system as in my co-pending patent application British Application No.45680/78 (Serial No.2035127) entitled "Weir". The preferred form of outlet is a vertical plate aboutthe same height as the baffles, and more or less touching the ends of the baffle. The outlet weir may have the same shape as the crosssection of the baffles. The outlet weir may be joined to the baffle system as in my co-pending patent application British Application No 45680/78 (Serial No.2035127) entitled "Weir". The baffle system may also be used for the more conventional forms of inlet and outletweirs.

While the above described plate system will be beneficial with any type of downcomer arrangement, the advantage will be greatest when used with downcomers and weirs as described in my copending patent application British Application No.

45680/78 (Serial No.2035127) entitled "Weir". The advantages of this plate system will also apply to gas absorption columns.

Claims (21)

1. A baffle system for use on a plate for mass transfer operations comprising at least one channel, the surface of said channel through which a twophase mixture of liquid and vapour leaves the channel 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,dirnc- tion towards one end of the channel.

2. A baffle system as claimed in Claim 1, wherein the channel is gabled, the surface through which the two-phase mixture of liquid and vapour leaves the channel 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 both along the plate towards one end of the channel and down towards the floor of the plate.

3. A baffle system as claimed in Claim 1 or 2, wherein, in use, a liquid return zone is defined by the space between two channels laterally adjacent to each other.

4. A baffle system as claimed in Claim 3, wherein the liquid return zones are wider at the top than at the bottom near the sieve plate floor, in order to minimise bubble entrainment in externally recirculated liquid.

5. A baffle system as claimed in any of the preceding claims, wherein the two-phase mixture is deflected towards the outlet weir end of the or each channel.

6. A baffle system as claimed in Claim 2, in which the two-phase mixture is deflected towards the outlet weir end of the or each channel and downwards towards the sieve plate floor.

7. A baffle system as claimed in Claim 6, in which the liquid of the two-phase mixture is thrown in a downward direction towards the sieve plate floor by altering the angle each hole makes with the longitudinal dimensions of the or each channel.

8. A baffle system as claimed in any of the preceding claims, in which liquid and vapour disengage of the or each channel.

9. A baffle system as claimed in Claim 8, in which the disengaged liquid collects in the liquid return zone before discharging onto the plate.

10. A baffle system as claimed in any of the preceding claims, wherein slots are cut at intervals along the lower edge of both sides of the or each channel.

11. A baffle system as claimed in Claim 10, wherein the disengaged liquid passes through the slots from the liquid return zone, re-enters the or each channel, and moves towards the outlet weir end of the or each channel.

12. A baffle system as claimed in any of the preceding claims, in which the or each channel is integral with the plate.

13. A baffle system as claimed in any one of Claims 1 to 11, in which the or each channel is attached to the plate by riveting or spot welding.

14. A baffle system as claimed in Claim 10, in which the or each channel is formed by pressing and folding a single sheet of material.

15. A baffle system as claimed in any of the preceding claims, in which the or each channel extends the full width of a column.

16. A baffle system as claimed in any one of Claims 1 to 15, in which the or each channel extends partially across a column.

17. A baffle system as claimed in any of the preceding claims, wherein the or each channel is built up out of modules, each module covering part of the area of the plate.

18. A baffle system as claimed in any of the preceding claims, in which the channels are arranged in a herring-bone pattern on the plate.

19. A baffled plate wherein the baffling is provided by the baffle system as claimed in any of the preceding claims.

20. A baffled plate as claimed in Claim 19, for use with a weir, such that the weir ensures that a flow of inlet liquid is fed to liquid return zones, said zones being defined by the space between two channels laterally adjacent to each other as present in the baffle system.

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