GB2054401A - Counter-current fluid-fluid exchange apparatus - Google Patents

Abstract

A counter-current exchange column is sub-divided by horizontal partitions, a series of identical stages each comprising a mixing zone and a separating zone. In the mixing zone, agitators thoroughly blend the phases yet impart no vertical motion thereto; by reason of their different specific weight, the phases separate in the separating zone. The mixing and separating zone of each stage are connected via ports (9) in the partition between them. The mixing zones of adjacent stages are connected by tubes (10, 11) which pass through the separating zones (3, 8) disposed between them and the partitions separating the adjacent stages. The mixing zone of each stage is connected to the separating zone next to it (i.e. in an adjacent stage) via a connecting element (12, 13) projecting vertically into the separating zone, having a flow path which reverses direction after the fashion of a siphon. In operation, the heavier phase flows through the tubes which connect the mixing zones of adjacent stages, while the lighter phase flows through the connecting elements which connect the adjacently disposed separating and mixing zones (3, 7) of adjacent stages. The structurally simple column arrangement avoids any retro-mixing between stages. <IMAGE>

Description

SPECIFICATION Substance exchange apparatus, particularly for extraction This invention relates to apparatus for the exchange of substances in countercurrent between two specifically differently heaving inter se insoluble or only partially soluble media. In particular, the invention relates to such apparatus, particularly for the extraction of two liquids, having a plurality of stages each having a mixing zone in which the media are mixed and a separating zone in which the mixed media are separated by reason of their different specific weights, the specifically heavier medium being fed to and the lighter medium being discharged from the uppermost stage and the lighter fluid medium being fed to and the heavier medium being discharged from the lowest stage.

Substance exchange apparatus, in particular extraction apparatus, having a plurality of stages each consisting of a mixing and a separating zone, is referred to also as mixer-separators. The mixerseparators having superposed stages, have over mixer separators with adjacently disposed stages, for example the so-called box-type mixerseparators, the advantage that they do not require a large floor area.

One substance exchange apparatus of the type described at the outset is the so-called Lurgi-Tower Extractor, the principle of which is explained in the article by H.W. Brandt et al, "Moderne Flussig/Flussig- Extraktoren-Uebersicht und Auswahlkriterien" (Modern Liquid-Liquid Extractors, a Review and Selection Criteria), Chem.-lng.-Tech. 50 (1978) No. 5, pp. 345-354. In the case of this extractor, all separating zones are disposed one above another in a tower. The mixing zones are located alongside the associated separating zones and have mixer pumps which pass both phases upwards. The bottom of each mixing zone communicates with the lower regions of the separating zones of their and the next higher stage and the upper zone of the separating zone of the next lower stage and, above, with the middle zone of the separating zone of their stage.

Throttle valves at the top outlets of the mixing and separating zones regulate the relevant flow.

In the case of known extractors, the two phases in the mixing zone are not passed in counter-current which is optimum for substance exchange but flow in the same direction, in fact from the bottom upwards. This results in an internal circulation of the heavy phase in each stage which increases the rate of entrainment and so impairs the stage efficiency.

Another disadvantage is the fact that the mixing pumps have to transport the phases because the optimum speed of the pump is, for mixing of the phases, generally different that the speed required for transport. When there are variations or fluctuations in the rate of feed (in the volume flow of phase supply), it is furthermore necessary for the throughflow in the mixing zones to be adapted by means of the thrpttle valves which necessitates considerable control complication.

The invention aims to provide, in at least a preferred embodiment thereof, a structurally simple apparatus of the type described at the outset, which is particularly suitable for extreme phase conditions, and which avoids retro-mixture between the individual stages, guarantees a sufficient dwell time of the phases in the individual stages, operates reliably without regulation even when there are variations in the rate of supply and which avoids the other aforesaid disadvantages of the knownapparatus.

In accordance with the present invention we propose Apparatus for the exchange of substances in countercurrent between two specifically differently heavy inter se insoluble or only partially soluble fluid media, and comprising a column divided into a plurality of stages each having a mixing zone in which the media are thoroughly mixed and a separating zone in which the mixed media are separated by reason of their different specific weights, the specifically heavier fluid medium being fed to and the lighter medium being discharged from the uppermost stage and the lighter fluid medium being fed to and the heavier medium discharged from the lowest stage, one or more ports or the like communicating between the mixing and the separating zone of each stage, a first connecting element communicating between the two mixing zones respectively above and below, each separating zone and a second connecting element communicating between mutually adjacent regions of the mixing and separating zones of adjacent stages wherein the said second connecting element includes a direction-reversing flow path and allows the fluid separated in the said region of the separating zone to flow through into the said region of the mixing zone and forms a hydraulic trap for the other medium.

Other features of this invention are set forth in the appendent claims.

Embodiments of the invention will now be described by way of example with reference to the accompanying diagrammatic drawings, in which: Figure 1 shows a partial axial longitudinal section through a multi-stage column for counter-current extraction of liquids; and Figures 2 to 4 show diagrammatically details of alternative embodiments of a multi-stage column for counter-current extraction of liquids.

The interior of the column shown in Figure 1 comprises a cylindrical housing 1 sub-divided by horizontal partitions into a series of identical stages (one shown) each including a mixing zone 2 and a separating zone. In the Figure, only three partitions are visible, namely the two partitions 4 and 5 which separate the illustrated stage from the adjacent upper and lower stages respectively and the partition 6 which separates from each other the mixing and separating zones 2 and 3 of the stage which is illustrated. Of the mixing zone of the upper stage which follows on from the top of the separating zone 3 and the separating zone of the lower stage which follows on from the bottom of the mixing zone 2, only the areas 7 and 8 immediately adjacent the partitions 4 and 5 can be seen.

The mixing and separating zones 2 and 3 are interconnected via ports 9 in the partition 6, the entire cross sectional area of the ports amounting to about 30% of the interior cross-sectional area of the cylindrical column 1. Atube 10 extending vertically through the separating zone 3 communicates between the mixing zones 2 and the mixing zone 7 of the next stage above. The upper end of the tube fits tightly in an aperture in the partition 4 and is flush with the top surface thereof. At the bottom, the tube 10 ends a little above one of the ports 9 in the partition 6.

The diameter of the tube 10 is selected such that the continuous specifically heavier phase can, even at maximum rate of feed, readilyflowthrough the tube 10. The upper end of a similar tube 11 which extends through the separating zone 8 of the stage below and through which the mixing zone 2 communicates with the mixing zone (not shown) of that stage, fits in an aperture in the partition 5.The mixing and separating zones of adjacent stages, (that is to say the upper part of the separating zone 3 and the lower part of the mixing zone 7 and the lower part of the mixing zone 2 and the upper part of the separating zone 8) communicate in each case via a connecting element (12,13). Each connecting element is in the form of a siphon and has a pot-shaped container 14 within which is a tube 15, the outer wall of which is spaced from the inside wall of the container 14. The upper end portion of the tube 15 passes is fitted tightly in an aperture in the partition 4. The connecting element 12 extends into a middle of the separating zone 3; the annular opening 16 which is defined by the upper end of the container 14 and the tube 15 being spaced below the partition 4 by only a fraction of half the height of the separating zone 3.The annularopening 16 at the upper end of the annular gap 17 between the tube 15 and the container 14 and the upper end of the portion of tube 15 communicates with the mutually adjacent ends of the separating and mixing zones 3 and 7; the flow path extending down through the annular gap 17 and in upwardly through the tube 15.

As with all of the mixing zones, the mixing zone 2 is sub-divided by horizontal partitions 20,21,22 into three mixing chambers 23, 24 and 25, adjacent chambers being in communication via ports 27 having the same cross-sectional area as the ports 9.

Located in each of the mixing chambers 23 to 25 is an impeller agitator 28. The impeller agitators 28 of the mixing zones of all stages are mounted on a common shaft 29 extending vertically through the entire column and are designed so as to impart no motion in the vertical direction. At the partitions 4, 5 which separate the stages from one another, the shaft 29 is mounted in a bearing 30 whereas the shaft 29 passes through clearance holes in the partition 6 and the partitions 20, 21 and 22.

In order to avoid turbulence in the separating zones, 3, 8, the shaft 29 is enclosed by a screening tube 31. Also at the bottom of each separating zone 3, 8 is a supporting grid 32 supporting a coalescence aid, e.g. a filler or a metal mesh not shown in the drawing.

The partition 6 and the partitions 20,21 and 22 are maintained a predetermined distance apart and from the partition 4 by spaces (not shown) and are fixed to the partition 4 by means of (likewise not shown) tie bolts which pass through the psaces and the walls 6, 20,21,22. The partitions 4,5 between the stages and which, in contrast to the partition 6 are disposed inside the stage and the partitions 20, 21, 22 have to be tightly connected to the housing shell 1, are held between flanges 33 of the housing 1 and are sealed by gaskets 34. This tight separation of the stages prevents any retro mixture between stages.

The multi-stage extraction column described above is particularly suitable for counter-current extraction under extreme phase conditions, for example when the volume flow of the phase which is to be dispersed represents a multiple, e.g. 50 times of the volume flow of the continuous phase. The column is intended for dispersing a specifically lighter phase (e.g. toluene) in a specifically heavier phase (e.g. water). For initial commissioning of the column, firstly the continuous heavy phase (water) is poured into the column until the separating and mixing zones of all stages are completely filled.The light phase which, during operation of the column, is fed in at the lowest stage (not shown) is indicated by dotted lines in the drawings and is designated 40, the direction of rising movement being indicated by arrows 41 and the heavier phase which is fed to the highest stage (not shown) moves downwardly, this being indicated by arrows 42.

From the lowest separating zone 8 shown, the lighter phase rises through the connecting element 13 into the lowest chamber 25 of the mixing zone 2. It then passes through the ports 27 one after another into the chambers 24 and 23 of the mixing zone and in doing so gradually becomes more and more intimately blended with the continuous phase by impellers 28. The resultant dispersion then passes through ports 9 into the separating zone 3 where the phases are separated again: the lighter phase 40 separated from the dispersion layer 44 indicated by undulating lines collects in the upper part of the separating zone 3, a (stationary) phase limit 43 between the layer consisting of the lighter phase 40 in the upper part and the dispersion layer 44 in the lower part of the separating zone 3 being automatically created without the need for any control.As a result of further separation of the lighter phase, the lighter phase at the annular opening 16 is forced through the annular gap 17 downwards to the bottom of the tube 15, whence it rises through the tube 15 into the mixing zone 7 of the stage above where - as described above - it is thoroughly mixed with the heavier phase and is again separated from it in the next separating zone.

As mentioned, at initial commissioning, the column is completely filled with the heavier phase. The rate of supply thereof produces a throughflow through the mixing zones but not through the separating zones: in the mixing zone 7, the heavier continuous phase descends into the area adjacent the partition 4. From there, it flows through the tube 10 and the ports 9 directly into the topmost chamber 23 of the mixing zone below, whereupon it flows through the chamber 24 into the lowest chamber 25 and through the tube 11 into the mixing zone of the next lower stage.

If the feed rate of the phases is interrupted, the light and the heavy phases will no longer flow through the connecting elements 12, 13 and the tubes 10 and 11 but remain in the relevant stage, the individual stages then being completely isolated from one another. For this reason, the phase concentration profile in the stages is retained indefinitely.

This has the advantage that even after prolonged periods of shut-down, the column can be brought quickly into operation. With conventional columns, on the other hand, after a period of inactivity, all of the lighter phase collects in the upper part of the column and of the heavier phase collects in the lower part of the column. For renewed operation, therefore, conventional columns require long startup times.

In order to disperse a heavier fluid in a lighter fluid, the column must be so constructed that the connecting elements 12, 13 project not downwardly but upwardly into the separating zones 3,8; and with reference to the drawings, this means column must be inverted.

The above described apparatus can also be used for (chemical) reactions and microbiological processes and operated not only with two liquids but, for example, also with a gas and a liquid.

If it is intended to use the apparatus for countercurrent reaction between gaseous and liquid mediums, it is expedient to arrange the connecting elements 12, 13 coaxially around the shaft 29, i.e. the latter is extended through the tube 15 and the bottom of the container 14, a bearing seal being disposed in the bottom of the container. In this case, the intermediate wall 22 is omitted and the tube of the connecting element 13 is extended to a point near the agitator 28 which guarantees optimum intimate mixing. So that the gas is also passed in the upper chambers 23 and 24 of the mixing zone 2 to the agitators 28, the partitions 20 and 21 may be bell or funnel-shaped. This embodiment is also suitable for substance exchange processes between two liquids.

In another embodiment not shown, the bottom end of the tube 10 is bent over to avoid the dispersion rising through the tube which might otherwise occur with extremely low rates of feed of the heavy continuous phase.

The tube 10 by which the adjacent mixing zones 2 and 7 are connected could also be guided outside the cylindrical housing 1 and along the separating zone 3 and may, for example, discharge into the interior of the housing directly above the partition 4 and 6 (the other tubes 11 would have to be located accordingly).

The housing 1 may also be double walled around the separating zones 3,8 and the partitions 4, 5 which separate the stages from one another may be fixed tightly against the inner wall of the double walled shell of the adjacent separating zone so that the zones of two mixing zones which are adjacent the same separating zone would communicate via the annular gap formed between the two walls of the double walled shell. In the case of this embodiment, the tubes 10,11 become unnecessary.

The connecting elements 12, 13 by which the adjacent mixing and separating zones 7 and 3 or 2 and 8 of adjacent stages communicate could also be constituted by tubes bent into a V-shape or a U-shape. In this case for example the free end of one of the two arms extending obliquely or vertically in the separating zone 3, 8 would pass tightly through the partition 4, 5 into the mixing zone 7,2 and the free end of the other arm would pass through the separating zone 3, 8 to a point close to the partition 4, 5. The for instance U-shaped connecting elements could also be disposed outside of the housing their two ends ending in each case just above and below the relevant partition 4,5, in the housing interior. For the connecting elements 12, 13, various other con- structions are possible, devised after the fashion of a siphon with a direction-reversing flow path.For example, a channel bent into a ring could be disposed coaxially of the shaft 29, close to the housing 1, the partition 4,5 could be provided with an annular slot, the outer side wall of the channel could be connected to the edge of the partition which outwardly defines the annular slot and the edge of the partition inwardly defining the annular slot could be flanged over into the channel and an annular casing could be fixed on it, the bottom edge of the casing being spaced from the bottom of the channel.

Adjacent mixing zones could also communicate by a plurality of tubes 10, 11 instead of by one tube as shown. Likewise, the mutually adjacent mixing and separating zones of adjacent stages could communicate not by one connecting element 12, 13 but also by a plurality of such connecting elements.

So that during filling of the housing with continuous heavy phase (e.g. water- priorto initial commissioning) no air bubbles can form in the upper zones of the separating zones 3,8, venting ducts with a shut-off facility may be provided in the housing 1. For the same purpose, the partitions 4, 5 could, by means of tiny holes, be made permeable to air but not the (liquid) phases (or only negligible quantities of such phases).

The column shown in Figure 1 is made comparatively large. Smaller versions of the column may be constructed in a similar manner is described hereinafter in connection with Figures 2 to 4. The reason for this is as follows: if the column shown in Figure 1 is made very small the cross-section of the connecting element 12 which is formed as a siphon becomes so small that from a specific limit of throughput of the lighter phase, the connecting element 12 no longer produces the desired effect: the riser pipe 15 can then in fact become completely filled with the light phase 40 resulting in extraction of the separated light phase from the separating zone 3. The separating layer 43 between separated light phase 40 and dispersion layer 44 therefore rises to the point of equilibrium or to the opening 16 of the siphon 12 which is filled with ligh phase.The flow thereupon breaks off and the separating layer 43 sinks again sufficiently that light phase 40 again starts to flow through the siphon. This instability is extremely undesirable. The alternatives described hereinafter with reference to Figures 2 to 4 avoid this instability even in the case of the smallest columns.

Common to them is an additional connecting means between the pot-shaped container 14 and the mixing zone 7 disposed above it. Extraction of the light phase is reliably prevented by this additional connecting medium which acts as a relief.

In the case of the embodiment according to Figure 2, there discharges into the bottom part of the pot shaped container 14 an additional portion of tube 35 which prevents vacuum extraction of the light phase, the portion of tube 35 extending through the partition 4 into the mixing zone 7, in which its end is bent over. The upper end of the portion of tube 35 therefore projects beyond the top end of the portion of tube 15 into the mixing zone 7 in which it is bent over, so that the light phase 41 emerging from the portion of tube 15 does not pass back again through the tube 35 into the connecting element 12. With the construction of the end of the portion of tube 35 which is illustrated, this tube virtually picks up only the heavy phase 42 which is dispersed in the mixing zone 7.Furthermore, the bottom edge of the portion of tube 15 has tooth-shaped cut-outs 36 which likewise contribute to avoidance of extraction of the light phase. In the case of the alternative embodimenu shown in Figure 3, the connecting medium is a U-tube 37 having arms of unequal length. The end of the shorter arm is disposed in the bottom of the pot-shaped container 14 while the end of the longer arm is in the partition 4. In the case of the alternative embodiment shown in Figure 4, only a horizontal pipe 38 connects the bottom part of the pot-shaped container 14 to the tube 10 which communicates with the upper mixing zone 7. If the connecting element 12 (or 13) is constructed differently, from that shown in the drawing, e.g. is constructed as a U-shaped or V-shaped tube, then the additional connecting agent 35, 37 or 38 is likewise extended into that part of the connecting element in which the flow path is reversed, in the case of a U-shaped connecting element therefore, in the curved part thereof.

Claims (14)

1. Apparatus for the exchange of substances in counter-current between two specifically differently heavy inter se insoluble or only partially soluble fluid media, and comprising a column divided into a plurality of stages each having a mixing zone in which the media are thoroughly mixed and a separating zone in which the mixed media are separated by reason of their different specific weights, the specifically heavier fluid medium being fed to and the lighter medium being discharged from the uppermost stage and the lighter fluid medium being fed to and the heavier medium discharged from the lowest stage, one or more ports or the like communicating between the mixing and the separat ing zone of each stage, a first connecting element communicating between the two mixing zones respectively above and below each separating zone and a second connecting element communicating between mutually adjacent regions of the mixing and separating zones of adjacent stages wherein the said second connecting element incudes a direction reversing flow path and allows the fluid separated in the said region of the separating zone to flow through into the said region of the mixing zone and forms a hydraulic trap for the other medium.

2. Apparatus according to Claim 1, wherein mixing in the mixing zones is effected by mixing devices so constructed that they impart to the fluid medium no vertical motion.

3. Apparatus according to Claim 1 or 2, wherein, in each stage, the distance from the end of the said flow path of the second connecting element which discharges into the separating zone is spaced from a partition separating the adjacent stages, by a fraction of half the height of the separating zone.

4. Apparatus according to any of Claims 1 to 3, wherein each separating zone includes coalescence aids, such as packages of filler or wire mesh.

5. Apparatus according to any one of Claims 1 to 4, wherein each second connecting element comprises a container suspended in the separating zone with its open end facing a partition separating the adjacent stages, and a tube which, through the partition, communicates with a region of the mixing zone adjacent the partition and which extends into the container so as to define within the container a gap around the tube.

6. Apparatus according to any one of Claims 1 to 4, wherein the second connecting element is Ushaped, the arms of the U-shape extending vertically in the separating zone and the free end of one arm passes tightly through a partition separating the adjacent stages, into a region of the mixing zone adjacent thereto, the free end of the other arm being disposed in a region of the separating adjacent the partition and confronting the partition.

7. Apparatus according to any one of Claims 1 to 6, wherein the mixing devices are located on a common shaft or on a plurality of shafts extending vertically through the column, the shaft or shafts mounted in sealed bearings carried by partitions separating the stages from one another and being enclosed by a screening tube in each separating zone.

8. Apparatus according to any one of Claims 1 to 7, wherein the first connecting element is a tube.

9. Apparatus according to Claim 8, wherein each of the tubes constituting the first connecting element, extends vertically through the separating zone and at one end fits tightly in a partition dividing the separating zone from the mixing zone of an adjacent stage while at the other end is directed toward or passes through the port in another partition between the said separating zone and the mixing zone of the same stage, and!or defined between that partition and the column housing.

10. Apparatus according to claim 9, wherein the other end of the tube is bent over.

11. Apparatus according to one of Claims 1 to 10, wherein each mixing zone is sub-divided by horizontal partitions into a plurality of mixing chambers which communicate with one another via ports in the partitions and or defined between the partitions and the column housing.

12. Apparatus according to Claim 5, wherein the tube projecting into the container has a serrated edge at its end which is disposed in the container.

13. Apparatus according to one of Claims 1 to 12, wherein the part of the second connecting element in which the path of flow is reversed communicates via a third connecting element with the said region of the mixing zone.

14. Apparatus for the exchange of substances in counter-current between two specifically differently heavy inter se insoluble or only partially soluble media, constructed and arranged substantially as hereinbefore defined and as illustrated in the accom panying drawings.