BACKGROUND OF INVENTION
1. Field of the Invention
This invention relates to a liquid-liquid extraction tower and a method of operating the same. As used herein, the words “tower” and “column” can be used interchangeably.
2. Description of Related Art
Liquid-liquid extraction involves the transfer of mass from one liquid phase into a second immiscible liquid phase. As defined herein, a “feed” is a liquid supplied to the tower having components that are to be separated, and can have as a major component a feed solvent and as minor components, one or more solutes. As defined herein, “extraction solvent” is the immiscible liquid added to the tower for the purpose of extracting one or more solutes from the feed. As defined herein, the “extract” is the liquid removed from the tower containing the extraction solvent plus the one or more solutes. As defined herein, the “raffinate” is the liquid removed from the tower that contains primarily the feed solvent; it is the remaining feed liquid after the one or more solutes have been removed.
FIG. 1 is an illustration of a type of liquid-liquid extraction tower based on the design of Scheibel. Various versions of this general type of design are shown in, for example, U.S. Pat. Nos. 2,493,265; 2,850,362; & 3,389,970 and pages 15-40 to 15-42 of Perry's Chemical Engineers' Handbook, Seventh Ed. (Copyright 1997).
The introduction of the feed into the side of the tower sets up a large-scale circulation that can be deceptively significant and can provide mixing and entrainment of the feed in the assumed single phase extract leaving the tower. This impacts the quality of the extract being removed from the tower. What is needed, therefore, is an improved method of introducing the feed to the tower such that large scale circulation in the area of collection is reduced.
BRIEF SUMMARY OF INVENTION
In one embodiment, this invention relates to a liquid-liquid extraction tower comprising at least one feed inlet, at least one extraction solvent inlet, at least one extract outlet, and at least one raffinate outlet; wherein the tower further has a plurality of stages and an area for collection of the extract; at least one stage comprising a mixing zone having a driven center horizontal impeller positioned between a pair of horizontal annular shroud baffles having open centers, the impeller having a center impeller plate; the stage further having openings between the pair of horizontal annular shroud baffles and the side of the tower; wherein the feed inlet is positioned adjacent the center impeller plate.
In another embodiment, this invention relates to an improvement in the process for liquid-liquid extraction of an extract from a feed using an extraction solvent, using a liquid-liquid extraction tower having at least one feed inlet, at least one extraction solvent inlet, at least one extract outlet, and at least one raffinate outlet; and wherein the tower further has a plurality of stages and an area for collection of the extract; and at least one stage having a mixing zone having a driven center horizontal impeller positioned between a pair of horizontal annular shroud baffles having open centers, the stage further having openings between the pair of horizontal annular shroud baffles and the side of the tower;
the improvement comprising symmetrically introducing the feed adjacent the center impeller plate.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an illustration of one type of liquid-liquid extraction tower based on the design of Scheibel.
FIG. 2 is an illustration of the flows in the area of collection of extract in the tower.
FIG. 3 is an illustration of one type of device for symmetrically introducing the feed using a ring-type distributor.
FIG. 4 is a perspective detail of the device for symmetrically introducing the feed as shown in FIG. 3.
FIGS. 5A & 5 B are illustrations of alternative types of devices for symmetrically introducing the feed utilizing a plurality of open pipes angled to direct the feed either toward or away from the centerline of the tower.
FIGS. 6 & 7 are illustrations of other alternative types of device for symmetrically introducing the feed utilizing either a single pipe or a plurality of pipes that direct the feed vertically around the center drive shaft of the tower.
FIG. 8 is an illustration of the liquid-liquid extraction tower having device for symmetrically introducing the feed adjacent the center impeller plate wherein the extract is taken from the bottom of the tower.
FIG. 9 is an illustration of the liquid-liquid extraction tower having a device for symmetrically introducing the feed adjacent the center impeller plate wherein the extract is taken from the top of the tower.
FIG. 10 is a detail from FIG. 9 showing an alternative type of device for symmetrically introducing the feed utilizing a plurality of pipes that direct the feed vertically around the center drive shaft of the tower.
DETAILED DESCRIPTION OF INVENTION
This invention relates to a liquid-liquid extraction tower comprising at least one feed inlet, at least one extraction solvent inlet, at least one extract outlet, and at least one raffinate outlet, and has a plurality of stages and an area for collection of the extract. The tower has at least one stage comprising a mixing zone having a driven center horizontal impeller having a center impeller plate with the impeller positioned between a pair of horizontal annular shroud baffles having open centers. The stage further has openings between the pair of horizontal annular shroud baffles and the side of the tower. The feed inlet is positioned adjacent the center impeller plate. In a preferred embodiment, the feed inlet is positioned in the area of collection such that the feed is substantially symmetrically introduced adjacent the center impeller plate such that large scale circulation in the area of collection due to the introduction of the feed into the tower is reduced.
FIG. 1 is a representation of the prior art liquid-liquid extraction tower based on the design of Scheibel. The tower 10 has an inlet for the feed 11, an inlet for the extraction solvent 12, an outlet for the extract 14, and an outlet for the raffinate 15. The tower also has a plurality of stages designated by the zones 20 and a plurality of mixing zones designated by 22. At either end of the tower there are collection areas 21. Each mixing zone 22 has a horizontal impeller 23a having a number of impeller blades attached to impeller plate 23b and positioned between a pair of horizontal annular shroud baffles 24 supported by suitable supports 24a, many of which are not shown for clarity. The horizontal impeller is driven by a center shaft 25. Each of the horizontal shroud baffles has an open center 26 and another opening 27 between the baffles and the side of the tower. The tower is also provided with a plurality of annular flow baffles 28 that direct flow from the side of the tower. An optional element that can be employed is the use of packing or wire mesh 29 (not shown for clarity) in the areas of A, B, or C.
FIG. 2 is a representation of a typical prior art arrangement of a bottom collection area 21 of the tower of FIG. 1, with an inlet for feed 11 located on the side of the tower and an outlet for extract 14 located at the bottom of the tower. It is typical for the inlet of the feed 11 to continuously discharge into the bottom collection area 21 from the side of the tower, and the extract in the form of one phase being continuously removed from the bottom of the tower via extract outlet 14. Discharging the feed into the side of the tower sets up an undesirable large-scale circulation in the collection area. The lower density feed phase 11 immediately starts to rise and travel up the side of the tower as shown by flow arrows 30 and 31. It has been found that the energy imparted into the fluid in the collection area by the feed is proportional to the distance the low density liquid in the feed travels through the collection area. Also, if the volumetric rate of feed to the tower is significant, the feed inlet can have a horizontal flow component 32 that flows to the center of the collection area and the open center 26 of the horizontal annular shroud baffles 24. The combination of the upward and horizontal flows set up a large-scale circulating flow in the collection area as shown by flow arrows 33, 34, & 35. This circulating flow can be deceptively significant and can provide mixing and entrainment of undesirable low density material from the feed as small bubbles in the second phase, which is the extract. This impacts the quality of the extract being removed from the tower.
FIG. 2 illustrates an embodiment wherein the collection area is at the bottom of the tower and the extract is the high density phase. In other embodiments, the same principles would apply to the mirror image of FIG. 2, wherein the collection is at the top of the tower with the extract being the lower density phase. In either case, undesirable circulating flow can provide mixing in the collection area and entrainment of the undesired phase in the desired phase of the extract.
In some embodiments, the feed inlet is a sparger having vertical and/or angled orifices. FIG. 3 illustrates one embodiment of a sparger 40 providing a feed inlet for the feed 11. The sparger 40 is positioned in the area of collection such that the feed is substantially symmetrically introduced (as shown by flow arrows 41) into the open center 26 of one of the pair of horizontal annular shroud baffles 24, beneath and adjacent the center impeller plate (not shown). Large scale circulation in the area of collection due to the introduction of the feed into the tower is reduced. This effectively introduces the feed substantially in mixing zone 22 rather than in the collection zone 21, and provides much less circulation and entrainment of the feed in the extract 14.
In some embodiments, the sparger 40 or other device for providing the feed adjacent the center impeller plate must be positioned around or compensate for the center shaft 25 that drives the horizontal impeller 23, that is positioned between the pair of horizontal annular shroud baffles 24. If required, the sparger or other device can be positioned around and/or attached to suitable supports 24a for the horizontal annular shroud baffles.
FIG. 4 illustrates one embodiment of a sparger 40. In this embodiment, the sparger is a circular ring having a plurality of slots 41 for distributing the inlet flow in substantially a symmetric manner. While a plurality of slots is shown, many different various orifices could be used as long as they are practical. For example one continuous circular slot could be used, or the slots could be replaced with simple holes of either circular or other shape. The orifices can be machined or positioned to cause the flow to be essentially vertical, or could be angled towards the centerline of the tower. Alternatively, in one preferred embodiment, the orifices can be machined or positioned to cause the flow to be angled away from the centerline of the tower and directed towards the blades of the horizontal impeller, with the feed inlet being either above or below the open center of the horizontal annular baffle. In this case, in a preferred embodiment, the flow is angled no more than about 80 degrees from the vertical, while still being symmetrically distributed adjacent the center impeller plate. In other embodiments, the flow is angled at least 40 degrees from the vertical, while still being symmetrically distributed adjacent the center impeller plate.
Also, while the sparger in FIG. 4 is shown directing the flow substantially vertically upward, as would be the case wherein the collection area is at the bottom of the tower; a similar sparger that directs the flow substantially vertically downward could be used if the collection area is at the top of the tower. If the collection area is at the bottom of the tower, “adjacent the center impeller plate” means the feed inlet is below the nearest center impeller plate; while if the collection area is at the top of the tower “adjacent the center impeller plate” means the feed inlet is above the nearest center impeller plate.
In some other embodiments, the feed inlet is two or more tubes symmetrically distributed around the centerline of the extraction tower. In some preferred embodiments, the two or more vertically-oriented tubes are angled or bent toward either the centerline of the tower or toward the horizontal impeller blades of the mixing zone. FIGS. 5A and 5B are possible illustrations of such devices made from a plurality of angled or bent tubes for symmetrically providing the feed. Device 50 has at least two tubes that direct the flow towards the center of the open center of the horizontal annular baffle. Device 55 has at least two bent tubes that direct the flow away the center of the open center of the horizontal annular baffle but still into that open center. For clarity, these are shown below the horizontal annular baffle, however, it is understood these could and preferably would extend through the open center of the horizontal annular baffle and into the mixing zone.
In some embodiments, the feed inlet is a substantially vertically-oriented tube located on the centerline of the extraction tower. FIG. 6 illustrates this embodiment for symmetrically providing the feed, wherein a single pipe 60 is used that is specially adapted or sized to surround or compensate for the center shaft 25 that drives the plurality of horizontal impellers in the tower. In this embodiment, the center shaft is actually inserted into the tube and serves the additional function of helping to distribute the feed in an annularly symmetric manner adjacent the center impeller plate. FIG. 7 illustrates still another embodiment for symmetrically providing the feed, wherein a plurality of vertical tubes 70 are used that are specially adapted or positioned to compensate for the center shaft 25 and distribute the feed in a symmetric manner adjacent the center impeller plate. This illustrates the tubes extending through the open center of the horizontal annular baffle.
As used herein, the term feed inlet means either the end of the feed supply tubing or the surface of any sparger or other device having holes or slots or other orifices where the liquid feed flow is actually exiting the feed piping. In some preferred embodiments the vertical distance between the feed inlet and the bottom edge of the center impeller plate is one inch or less. In some other preferred embodiments, the vertical distance between the feed inlet and the open center of one of the horizontal annular shroud baffles is less than one feed inlet tube diameter. In either case, the vertical distance is measured from the closest point of the end of feed tubing or the surface of any sparger or other device to the plane of the horizontal annular shroud. It is believed the feed inlet can be preferably be positioned as close as 0.25 inches (0.64 cm) vertically from the bottom edge of the center impeller plate; any closer it thought to have the potential to cause operability problems without added benefit.
By the term “symmetrically distributed” it is meant that if there is a plurality of feed inlets, the inlets are uniformly distributed in relation to the centerline of tower and are symmetrically located; that is, at least one vertical plane can be drawn through the centerline of the tower that shows symmetry of the feed inlets. If there is a single feed inlet, at least one vertical plane showing symmetry can be drawn through the centerline of the tower and that inlet.
FIG. 8 illustrates one embodiment of a tower 80 including a sparger or other device 81 for providing the feed adjacent the center impeller plate, wherein the feed inlet and collection area for the extract is in the bottom of the tower. For clarity, the feed inlet is shown below the horizontal annular baffle, however, it is understood this could and preferably would extend through the open center of the horizontal annular baffle and into the mixing zone. The tower has all the features as previously described, including an inlet for the feed 11, an inlet for the extraction solvent 12, an outlet for the extract 14, and an outlet for the raffinate 15. The tower also has a plurality of stages and a plurality of mixing zones as previously identified. Each mixing zone has a horizontal impeller positioned between a pair of horizontal annular shroud baffles as previously identified.
FIG. 9 illustrated another embodiment of a tower 90 including a sparger or other device 91 for providing the feed adjacent the center impeller plate, wherein the feed inlet and collection area for the extract is in the top of the tower. As before, the feed inlet is shown above the horizontal annular baffle, however, it is understood this could and preferably would extend downward through the open center of the horizontal annular baffle and into the mixing zone. The tower has all the features as previously described, including an inlet for the feed 11′, an inlet for the extraction solvent 12′, an outlet for the extract 14′, and an outlet for the raffinate 15′. The tower also has a plurality of stages and a plurality of mixing zones as previously identified. Each mixing zone has a horizontal impeller positioned between a pair of horizontal annular shroud baffles as previously identified. FIG. 10 is a detail of view 10-10 from FIG. 9. showing one possible arrangement of the device 91, showing one possible arrangement for positioning around or compensating for the center shaft 25 that drives the pluarality of horizontal impellers in the tower.
In another embodiment, this invention relates to an improvement in the process for liquid-liquid extraction of an extract from a feed using an extraction solvent, using a liquid-liquid extraction tower having at least one feed inlet, at least one extraction solvent inlet, at least one extract outlet, and at least one raffinate outlet; and wherein the tower further has a plurality of stages and an area for collection of the extract; at least one stage having a mixing zone having a driven center horizontal impeller positioned between a pair of horizontal annular shroud baffles having open centers, the impeller having a center impeller plate; the stage further having an openings between the pair of horizontal annular shroud baffles and the side of the tower; the improvement comprising symmetrically introducing the feed adjacent the center impeller plate.
In some preferred embodiments, the feed is introduced within one inch or less of the center impeller plate. In some other embodiments, the feed is introduced within one feed inlet tube diameter of the center impeller plate. The feed can be introduced directly into the mixing zone adjacent the center impeller plate by use of a sparger having vertical and/or angled orifices; a vertical tube; or a plurality of vertically-oriented, angled, or bent tubes.
In some other embodiments, the feed is directed through the open center of one of the horizontal annular shroud baffles and into the annular inlet of the mixing zone. In some other embodiments, the feed is symmetrically directed through the open center by use of a sparger having vertical and/or angled orifices; a vertical tube; or a plurality of vertically-oriented, angled, or bent tubes.
Patent Application
20110147305
