VAPOR/LIQUID CONTACTING CYCLONE WITH DEVICES TO PREVENT BACKMIXING

Abstract: This invention is directed to a cyclonic vapor/liquid contacting device. Liquid feed (1) is introduced near the floor of the cyclone. Vapor (2) enters through sieve holes in the bottom of the cyclonic device. Near the floor are angled tabs or vanes that impart a spin to the vapor rising up through the floor. The tabs or vanes mix the liquid and vapor. The liquid is then thrown toward the cyclone wall (3) where it exits through slots (5) in the wall (3).

Description: VAPOR/LIQUID CONTACTING CYCLONE WITH DEVICES TO PREVENT BACKMIXING This invention is directed to a cyclonic vapor/liquid contacting device and the process of employing it in distillation or related mass transfer or heat transfer applications, such as fluid catalytic cracking. This invention is directed to the use of flaps or baffles (shrouds) near the outside of openings in the barrel of the cyclone which may be used to help direct liquid out through the openings, thereby preventing backmixing of liquid. Cyclone separators are well known devices for separating solids from gases and gases from liquids. Some typical cyclone separator designs are illustrated in Perry's Chemical Engineers' Handbook, published by McGraw-Hill Book Company, New York City. In conventional cyclones, spin is imparted to the vapor and entrained liquid after it enters the cyclone via a downcomer or plenum. The vapor and entrained liquid then moves upward.entering a zone where liquid is removed via slots or other perforations in the barrel or by an annular hat in the top of the barrel. The use of vanes within cyclones is well known. They are used to guide the flow of gases and minimize entrainment of liquids or solids. U.S. Patent No. 3,969.096 to Herbert discloses a cyclone separator having multiple vaned gas inlets. The gas inlet consists of elongated inlet openings (like those of a louver) positioned to deflect incoming gas in a circular path from the gas outlet tube. Some means of preventing backflow from cyclonic devices are known. U.S. Patent No. 4,701,307 (Walters et al.) discloses a ballistic separation device that results from surrounding the downstream end of a progressive flow reactor or riser reactor with a concentric conduit that is in fluid communication with a cyclone separator. The device may also include a bevelled lip or projection at the axial opening of the progressive flow reactor. In Walters, vapor is being separated from solid, as opposed to vapor being separated from liquid as in the instant invention. Shrouds on the inlet of a cyclone, rather than on the outlet, are illustrated in U.S. Patent No. 5,266,187. The concept of the instant invention involves a cyclonic vapor/liquid with asymmetric liquid inlet and outlet, as shown in Figures 1-3. The liquid is preferably brought in from one side of the barrel, and the liquid effluent from near the top of the barrel is always discharged primarily toward the opposite side, using means such as a shroud around the outside of the top portion of the barrel. Means are also used to prevent liquid backflow, such as baffles between the cyclone barrels which extend preferably 4-10 inches up from the tray deck. The net effect is to force the liquid across the tray deck, from one row of cyclones to the next, without appreciable backmixing. This cross-flow pattern is known to give better contacting efficiency than where liquid backmixing can occur on the tray (see Chemical Engineer's Handbook, 5th Edition, R. H. Perry and CH. Chilton, Ed. McGraw-Hill, 1973, pp.15-18). Figure 1 illustrates a cyclone arrangement, located on a distillation tray, with asymmetric discharge. The fluid is released to one side, using means such as a shroud 1 , and vanes to direct fluid flow downward, 2, and baffles between the cyclone barrels, 3. Figure 2 illustrates the top view of the cyclone arrangement of Figure 1 , with baffle 1 and shroud surrounding cyclone, 2. Figure 3 illustrates the details of the cyclone with shroud for asymmetric discharge. There are spin vanes inside the barrel, but they are not shown, in order to enhance clarity. Liquid enters the cyclone at its base, 1. Vapor enters the perforated bottom of the cyclone, at 2, mixes with the liquid and proceeds up the cyclone barrel, 3. The baffle, 4, forces liquid across the tray deck, from one row of cyclones to the next, without backmixing. Liquid, 5 is forced out to the side, due to shroud, 6. Vapor, 7, is forced out the top. Figure 4 illustrates the same cyclone details as Figure 3, with an alternate inlet base, 1 to the cyclone. Liquid is conveyed to the center of the cyclone, then is released into the vapor flow. A circular baffle (not shown) above the central feed inlet to the cyclone base would normally be used here to distribute the incoming liquid out to the sides of the cyclone. Figure 5 illustrates the top view of an alternate cyclone arrangement. Shrouds protrude to discharge liquid on the far side of the baffles. Visible is the inlet downcomer 1 , the baffle 2, and the outlet downcomer 3. In Figures 1-4, the liquid feed to the cyclone must be brought in from one side of the cyclone barrel. In Figure 5, the entire perimeter of the cyclone base is accessible for liquid feeding. The key feature of the cyclone of this invention is that liquid exiting the cyclone is directed primarily to one side. The concept of asymmetric discharge may be employed with most cyclone designs, but it is particularly useful in situations in which a secondary spin zone is employed. Cyclone having shrouds or other devices to prevent backmixing are best suited for situations in which liquid rate is relatively low and gas rate is relatively high. Liquid flow rates, in cyclones having asymmetric discharge, are preferably limited to less than 40 gallons of liquid per square foot of tower area and more preferably limited to less than 20 gallons of liquid per square foot of tower area. In some cyclonic devices of the prior art, the liquid may enter the barrel from a central inlet feed tube. In the instant invention, however, liquid is brought down from one tray level to the next via conventional downcomers, rather than via central inlet feed tubes. Thus, the apparatus of Figure 1 resembles a conventional distillation tray, except that on the tray deck are special cyclonic devices where vapor/liquid contacting and separation take place. The general arrangement of placing cyclonic contacting devices on trays is well known. For instance, U.S. Patent No. 4,838,906 describes a contact and separating elements for a mass-transfer apparatus, with special geometries of ribs and vanes. An annular hat is used to collect the liquid at the top of the cyclone and return it to the tray. The liquid inlets and outlets for this device are pictured as symmetrically distributed around the periphery of the cyclone, so liquid will be drawn in from and discharged to all sides. This can allow some of the liquid that was run through a contacting element to be drawn back into and run through the same element again, which may not be desirable. The major feature that distinguishes this concept from that of U.S. Patent No. 4,838,906 is that in the instant invention the liquid exiting the cyclone is directed to one side, past the inter-barrel baffle, to force the liquid to move across the tray to the next row of cyclones or to a downcomer to the next tray down. In addition, there are numerous other differences in the specific geometry of the cyclone. In its most general form, the concept of this invention is not wedded to any particular details of cyclone geometry. Thus, it can encompass a wide variety of means for introducing the liquid feed from outside the cyclone into the base of the cyclone, for imparting spin to the fluids inside the cyclone and for providing liquid removal from the vapor stream. It is not limited to cyclones with secondary spin zones and barrel exit slots. For instance, it could also apply to cyclones with a single set of spin vanes (either intregral with the floor of the cyclone or mounted some distance above the floor) and to cyclones with an annular hat but no lower exit slots for liquid removal. However, the preferred cyclone barrel configuration is (as described above) a non-spinning zone at the base, surmounted by primary spin vanes, exit slots in the barrel and secondary spin vanes. An annular hat atop the cyclone barrel can conveniently be integrated into the surrounding shroud. The means of introducing liquid into the base of the cyclone could be as simple as holes in the barrel near the bottom of the barrel, as shown in Figure 3, or could involve more elaborate systems of plenums, tubes, troughs, vanes and orifices, that may provide more even spatial distribution of liquid and vapor. Several such feed introduction devices are shown in U.S. Patent Nos. 3,498,028 and 5,683,629. U.S. Patent No. 3,498,028 is directed primarily to the details of a cyclonic air/liquid cyclone contactor. U.S. Patent No. 5,683,629 is directed to the overall hardware configuration that appears in a number of patents assigned to Shell, in which the liquid is brought down from a tray level above to a liquid plenum or supply space, from which liquid is fed to the base via cyclonic contactors. In particular, U.S. Patent No. 5,683,629 describes some liquid feed devices, to be used in connection with their overall configuration. (That configuration differs from that of the instant invention in various significant respects.) The feed devices in U.S. Patent No. 5,683,629 may be useful in our concept.

Claims: CLAIMS: 1. A series of cyclonic devices suitable for use in the contacting of vapor and liquid in a distillation or fractionation process, wherein liquid exiting the cyclonic device is directed primarily to one side, each device comprising: (a) a floor; (b) a continuous side wall having an upper end and a lower end, the lower end terminating into the floor, the upper end defining an upper cyclonic region which may be adjacent to the floor of another cyclonic device, and the side walls defining a contacting volume above the floor and below the upper cyclonic region; (c) at least one vapor opening in the floor through which vapors can flow into the contacting volume; (d) at least one plenum, located near the floor of the cyclonic device, through which liquid may enter the cyclone; (e) at least one liquid outlet located on one side wall, the liquid outlet defining an opening in the side wall through which liquid can flow; (f) at least one set of devices to impart spin to the vapor and entrained liquid. 2. A series of cyclonic devices suitable for use in the contacting of vapor and liquid in a distillation or fractionation process, wherein liquid exiting the cyclonic device is directed primarily to one side, each device comprising: (a) a floor; (b) a continuous side wall having an upper end and a lower end, the lower end terminating into the floor, the upper end defining an upper cyclonic region which may be adjacent to the floor of another cyclonic device, and the side walls defining a contacting volume above the floor and below the upper cyclonic region; (c) at least one vapor opening in the floor through which vapors can flow into the contacting volume; (d) at least one tray located outside the continuous side wall, through which liquid enters the cyclonic device at or near its floor; (e) at least one liquid outlet located on one side wall, the liquid outlet defining an opening in the side wall through which liquid can flow; (f) at least one set of devices to impart spin to the vapor and entrained liquid. 3. The series of cyclonic devices recited in claim 1 or claim 2 wherein, in step (f), there are at least two sets of devices to impart spin to the vapor and entrained liquid and at least one liquid outlet of step (e) is located on the side wall between the first set of spin devices and the second set of spin devices. 4. The series of cyclonic devices recited in claim 1 or claim 2 wherein means to direct liquid flow primarily to one side are selected from the group consisting of a shroud around the top of the cyclone, vanes to direct fluid flow downward and baffles between the cyclone barrels. 5. The series of cyclonic devices recited in claim 4, wherein an annular hat atop the cyclone barrel is integrated into the shroud around the top of the cyclonic device. 6. The series of cyclonic devices recited in claim 1 or claim 2 wherein liquid flow rates are preferably limited to less than 40 gallons of liquid per square foot of tower area. 7. The series of cyclonic devices recited in claim 1 or claim 2 wherein liquid flow rates are preferably limited to less than 20 gallons of liquid per square foot of tower area. 8. The series of cyclonic devices recited in claim 2, wherein the means of introducing liquid into the base of the cyclone is selected from the group consisting of holes in the barrel near the bottom of the barrel. 9. The series of cyclonic devices recited in claim 2, wherein the means of introducing liquid into the base of the cyclone is selected from the group consisting of plenums, tubes, troughs, vanes and orifices. 10. In a process for fractionating, distilling, or the like, the improvement comprising: (a) providing a vertical column having surrounding sidewalls; (b) feeding a first and a second fluid into the column, wherein the first fluid is in its vapor state and the second fluid is in its liquid state within the column; (c) directing the first and second fluids to flow through a series of cyclonic devices located within the column, wherein liquid exiting the cyclonic device is directed primarily to one side, each device comprising: (1) a floor; (2) a continuous side wall having an upper end and a lower end, the lower end terminating into the floor, the upper end defining an upper cyclonic region, which is enclosed in a shroud, wherein the upper cyclonic region is adjacent to the floor of another cyclonic device, and the side walls define a contacting volume above the floor and below the upper cyclonic region; (3) at least one vapor opening in the floor through which vapors can flow into the contacting volume; (4) at least one liquid outlet located on one side wall, the liquid outlet defining an opening in the side wall through which liquid can flow; (5) at least two sets of devices to impart spin to the vapor and entrained liquid, wherein at least one liquid outlet of (4) is located on the side wall between the first set of spin devices and the second set of spin devices; (d) also directing the fluids to flow through a plurality of liquid downcomers comprising: (1) downcomer side walls having an upper portion and a lower portion, the upper portion being located in the upper cyclonic region, and the lower portion extending to a point above the cyclone floor and having a lower portion end; (2) at least one downcomer port located proximate to the lower portion end of the downcomer, the port defining an opening in the downcomer side walls through which liquid can flow; whereby the vapors present within the column flow upward from one cyclonic device to the next through the vapor openings and the liquid present within the column flows downward from one cyclonic device to the next by passing through the liquid outlet and through the downcomers, and whereby the liquid and vapor contact each other in a co- current fashion within the contacting volume of the cyclonic devices.

Inventors: BUCHANAN JOHN SCOTT, STOBER BERNE KIM, BUCHANAN, JOHN, SCOTT, STOBER, BERNE, KIM
Applicants: MOBIL OIL CORP, MOBIL OIL CORPORATION
Application year: 1999
Main classification: B01F3
IPC codes: B01D3, B01F3, B01F5, B04C3