LOW CONSISTENCY OZONE BLEACHING REACTOR

Abstract: A reaction vessel (10) which mixes ozone (23a-f) with a low consistency pulp and a dewatering vessel (14g) which removes the liquid (27) from the pulp before it leaves the reactor, regulating this same liquid to the pulp entering the reactor. The inner diameter of the recycle pipe (36) is 7.7 to 45% of the inner diameter of the reaction chamber.

Description: LOW CONSISTENCY OZONE BLEACHING REACTOR BACKGROUND OF THE DISCLOSURE Field of the Invention A device for reacting ozone with wood pulp at very low consistencies and for recycling liquid within the system. Other Disclosures Bentvelzen, et al, U.S. Patent 4,216,054 discloses a process for treating wood pulp with ozone in which the wood pulp is at a consistency under 1%. Meredith et al, U.S. Patent No. 4,310,384 issued January 12, 1982 discloses a system for recycling the water in a low consistency ozone treatment. Oldshue, U.S. Patent 3,966,542 discloses a treatment system in which a number of impellers are attached to the same shaft and mix the material in a number of separate chambers. Kempf, et al, U.S. Patent 4,080,249 discloses a system in which pulp is treated with ozone at a consistency between 1 and 10%. Chupka, et al, U.S. Patent 4,155,841 disclose a high turbulence screen. Verreyne, et al, U.S. Patent 3,660,225 shows an oxygen bleaching apparatus with oxygen recycle. Schnyder, U.S. Patent 3,941,650 shows screen cleaning means for a cellulose digester. Carlsmith, U.S. Patent 3,579,417 and Samuelson, et al, U.S. Patent 3,843,473 show upflow cellulose digesters. Grangaard, et al, U.S. Patent 3,024,158 shows a digester with a side stream. SUMMARY OF THE INVENTION Consistency is the amount of oven dry fiber in a mixture of fiber and liquid. In treating pulp at low consistencies, it is usually necessary to be able to reuse the liquid. At a 1% consistency the liquid weighs 99 times the pulp. At a 1/2% consistency, it weighs 199 times the pulp. For other , processes the pulp should be at a higher consistency. For example, pul should be at 1.2-1.8% consistency for good sheet formation on a vacuum washer. Handling lower consistencies requires a larger filtrate pump of higher power and possibly some modification of the dropleg and takeoff system. Transportation of pulp is usually at 2.5 to 4% consistency to avoid the large pipeline sizes associated with the large flows at lower consis¬ tencies. In the present invention it was decided that the water should be reused at the treatment site and within the treatment system. What is proposed is a reactor which mixes ozone with a low consistency pulp and a dewatering vessel which removes the liquid from the pulp before it leaves the reactor, and adds this same liquid to the pulp entering the reactor. The inner diameter of the return pipe is 7.7 to 45% of the inner diameter of the reactor. Other configurations of return passage may be used. These would have different cross sections or different locations. In each instance the inner cross-sectional area would be equal to a circular cross section having an inner diameter 7.7 to 45% of the inner diameter of the reactor. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a vertical cross section of the reactor showing a bottom dewatering chamber and an outside return line. Figures 2-5 are horizontal cross sections of the reactor taken across the corresponding cross section lines shown in Figure 1. Figure 6 is a vertical cross section of a modified reactor showing a top dewatering chamber and an outside return line. Figure 7 is a vertical cress section of a modified dewatering chamber. It is shown in the reactor of Figure 6 but may also be used as a bottom dewatering chamber. Figure 8 is a horizontal cross section of the dewatering chamber taken along line 8-8 of Figure 7. Figures 9-10 are cross sections of modified • dewatering chambers. They are shown as bottom dewatering chambers but may also be used as top dewatering chambers. Figure 11 is a horizontal cross section of another modified dewatering chamber taken along line 11-11 of Figure 12. Figure 12 is a vertical cross section of the dewatering chamber taken along line 12-12 of Figure 11. Figures 13-14 are isometric views showing details of construc- tion of the basket and screen of Figures 11-12. Figure 13 is with the screen and Figure 14 is without the screen. Figures 15-16 show a modification to the dewatering chamber of Figures 11-13. Figure 15 is a horizontal plan view partially in cross section and Figure 16 is a vertical cross section taken along line 16-16 of Figure 15. Figure 17 is a vertical cross section of a reactor showing an annular return line and a bottom dewatering chamber. Figure 18 is a horizontal cross section taken along line 18-18 of Figure 17. Figure 19 is a vertical cross section of a reactor showing an annular return line and a top dewatering chamber. Figure 20 is a vertical cross section of a reactor showing a central return line and a top dewatering chamber. Figure 21 is a horizontal cross section taken along line 22-22 of Figure 21. DESCRIPTION OF THE PREFERRED EMBODIMENT The reactor 10 has an outer shell 11, a cover 12 and base 13. The reactor is divided into a number of chambers 14a-g by divider plates 15a-f. Each of the divider plates has a central circular aperture 16a-f, each aperture having a diameter that is approximately one-third the diameter of the plate. Material passes from one chamber to the next through these apertures. A cental shaft 17 extends through these apertures. The shaft rotates on bearings, the number depending on the length of the shaft. Three bearings are shown: upper bearing 18, central bearing 19 and lower bearing 20. The central bearing 19 is held in place by struts 21 extending between the outer shell 11 and the bearing. The shaft 17 is turned by motor assembly 22. Each of the chambers 14a-f has an impeller 23a-f attached to the shaft 17. The impeller may be of any appropriate design. A four bladed impeller is shown, but a six bladed impeller could normally be used. It mixes the pulp slurry with the ozone in the chamber. To prevent rotation of the slurry in the chamber, a series of baffles 24a-f are placed in each cell. The number and size of the baffles will depend on the size of the reactor. Four are shown in each cell. The lower chamber of the reactor is a dewatering chamber. The pulp slurry has been at a consistency in the range of 0.01 to about 0.7% during its treatment in the reactor. Liquid is removed from the pulp slurry, before it leaves the reactor so that the slurry will be at a consistency in the range of 0.5-4% as it leaves. The dewatering chamber 14g has a series of baffles 25 which are similar to the baffles 24 in the other chambers except that the baffles 25 have apertures 26 which allow the passage of filtrate around the exterior liquid section of the dewatering chamber. The baffles also act as supports for the screen 27 which is concentric with the shaft 17 and the outer wall 11. The screen 27 divides the dewatering chamber into the outer annular liquid chamber 28 and the inner slurry chamber 29. Impellers 30 are attached to the shaft 17. The outer edges of the impellers 30 have foils 31 which remove the fiber mat from the interior surface of the screen so the liquid may continue to pass through the screen at a high flow rate and continue to prevent pulp from accumulating in the dewatering chamber . The screen 27 acts differently from the usual pulp screen. The usual pulp screen has apertures or slots large enough to allow the pulp fibers to pass through the screen while retaining the larger knots, shives or fiber bundles within or on the screen. The purpose of the screen in the present dewatering chamber, however, is to retain all of the fibers within the dewatering chamber while allowing the water to pass through. The preferred material of construction of the screen is stainless steel. The apertures should not be greater than the length of a pulp fiber being processed to prevent the pulp fibers from passing through them. This can be from 0.8 to 2 mm. The maximum mesh size would be 30. The mesh size normally would be 60. In operation the pulp slurry enters through the upper pulp slurry inlet pipe 32 and passes down through the chambers 14a-f. In each of these chambers the slurry is mixed with ozone by the impellers 23a-f. The ozone is normally admitted beneath the bottom impeller through pipe 38 at the bottom of the reactor and passes upwardly through the chambers and leaves the pulp slurry at the upper surface of the slurry in chamber 14a. In the lower chamber 14g the liquid passes through the screen 27 concentrating the pulp slurry in the slurry chamber 29. The slurry then exits through slurry outlet pipe 34. The excess liquid passes into filtrate outlet 35 and is carried upward through filtrate recirculation pipe 36 and re-enters the reactor and chamber 14a at inlet 37. A pump 38 is used to force the liquid upwardly from chamber 14g to 14a. The action shown in Figure 1 is OMPI countercurrent because the pulp slurry is moving in a direction opposite that of the ozone. The filtrate recirculation pipe 36 has an inner diameter that is 7.7 to 45% of the inner diameter of the reactor. Normally the inner diameter of the recirculation pipe would be no more than 24% of the inner diameter of the reactor. Other recirculation passages may be used in place of this exterior pipe. These will be described later. They may have different cross sections or different locations. In each instance the inner cross-sectional area would be equal to a circular cross section having an inner diameter of 7.7 to 45% of the inner diameter of the reactor. The preferred sizes would also be the same. Figure 6 shows a modification in which the movement of slurry and ozone is co-current. Both the pulp slurry and ozone pass upwardly in the same direction. The only difference between the reactor shown in Figure 6 and that shown in Figure 1 is that the pulp slurry inlet and dewatering chambers are reversed. The pulp slurry inlet is at the bottom of the reactor and the dewatering chamber is at the top of the reactor. This is the preferred location of the inlet and the dewatering chambers. The same reference numerals are used in Figure 1 and Figure 6. In Figure 6 both the pulp slurry and ozone enter at the bottom of the reactor and are carried upwardly through the reactor, again being mixed in each chamber. In the upper chamber the slurry is dewatered in the same manner as in Figure 1 and the excess liquid is carried downwardly to return to the bottom chamber. The relationship on the inner diameter of the recirculation line to that of the reactor is the same as in the system of Figures 1-5. The advantage of having the dewatering chamber at the top is that no pump 38 is required to force the liquid to flow through the filtrate recirculation pipe 36'. The head required comes from the difference in density between the aereated contents of the reactor and the gas free filtrate in the recirculation pipe. A preferred inner diameter of the recirculation pipe when the flow is downwardly from the upper dewatering chamber is 20 to 30% of the inner diameter of the reactor. Figures 7 and 8 show another modification of the dewatering chamber. In this modification the outer annular chamber 39 is the slurry chamber and an inner annular chamber 40 is the filtrate chamber. Because the passage of filtrate is from the outer annular chamber 39 to the inner annular chamber 40, the foils 41 are on the outside of the screen 27". There is less chance of damage to the screen by the impeller. The foils 41 remove the pulp mat from the screen 27". The foils 41 are the inner edges of impeller arms 42 of the dewatering chamber impeller assembly 43. The impeller arms 42 are attached to the central shaft 17" by impeller elements 44. The inner annular filtrate chamber 40 extends upwardly from the divider plate 15a". The annular structure has an inner cylindrical wall 45, an upper wall 46 and the outer cylindrical screen 27" which is concentric with the innner wall 45. The annular space formed by these walls is connected by a pipe 47 to the filtrate outlet 36". There is a tendency for some of the gas to collect within the annular space so the aspirator 48 in the annular space and open at the upper end of the space draws the gas from the space. The rest of the reference numerals are the same. The operation of this dewatering chamber is the same as the other dewatering chambers except that the excess liquid is pulled inwardly through the screen rather than forced outwardly through the screen. The impeller still cleans the fiber from the screen. Figures 9 and 10 show other configurations of screens and impellers in the dewatering chamber. The screen 27'" in Figure 9 is convex toward the center. The screen 27"" in Figure 10 is accordian shaped to provide greater surface area. Figures 11-15 show two preferred constructions for the dewater¬ ing chamber. These constructions maintain the shape of the screen and keep the screen clean. The screen 27'"" is held in a basket 50. The basket 50 has upper and lower rings 51 and 52, and fixed outer bars 53 extending between these rings. The screen 27"'" is placed inside these bars and held against these bars by removable inner bars 54 which are attached to the rings 51 and 52, and the outer bars 53 by screws, bolts or other types of fasteners 55. An apertured backing plate may be used to protect the screen. It is shown in the drawings, but its use will depend upon the pressure being exerted on the screen. The drawings show the apertured backing plates in place. They are on the liquid chamber side of the screen. In Figures 11-15 the apertured backing plates 56 extend between the outer fixed bars 53. In other configurations the backing plates would be omitted. The construction is shown in Figures 12-14. The upper ring 51 has an upper horizontal annular plate 57 and a downwardly extending flange 58. The flange 58 and the plate 57 form inner shoulder 59 and outer shoulder 60. The lower ring has a similar construction - a horizontal annular plate 61 and an upwardly extending flange 62 forming inner and outer shoulders 63 and 64. The outer bars 53 fit into the outer shoulders 60 and 64 and are fixed to the rings 51 and 52. The inner bars 54 fit into the inner shoulders 59 and 63 and are fastened to the rings 51 and 52 and the outer bars 53 by fasteners 55. Screws are shown. The screen 27'"" is held between the inner and outer bars 51 and 52. The pressure of the fiber and slurry against the screen 27"'" may cause the screen to tear along ^ the outer bars 53. Several actions may be taken to prevent this. The corners of the outer bars next to the screen 27'"" may be beveled to remove any cutting edge. An apertured backing plate 56 may also be provided. These plates are on the outflow side of the screen 27'"", support the screen, extend between the outer bars 53, and fit into and are fastened to the outer shoulders 60 and 64 of the rings 51 and 52. Each of the backing plates 56 have apertures 65 through which the liquid passes. The size and number of the apertures will depend upon the pressure of the slurry against the screen. The plate, which is concave to match the contour of the rings and screen, must support the screen and prevent its tearing. If the pressure is not great, then the backing plate may be omitted. The slurry within the cylinder formed by the screen 27'"" is kept in turbulence adjacent the screen by the foils 66 which rotate within the inner circumference of the cylinder formed by the inner faces of the inner bars 54. The turbulence of the slurry created by the foils keeps the fibers from clogging the screen 27'"". The foils 66 are attached to the central shaft 17'"" by upper and lower arms 67 and 68. The number of foils will depend upon the size of the basket, and the types and concentration of the slurry. Four foils are shown. The speed of these foils will also determine the maximum pressure drop across the screen at which the screen can be operated. The OMPI ^ higher the speed, the greater the pressure drop. However, high motor speeds also increase the tendency of the screen to tear. Another action that may be taken to prevent tearing of the screen is the choice of a slower rotor speed. In one experiment, a screen having an exposed area of 1.7 ft. (0.158 m ) and a backing plate successfully thickened the stock and did not tear when the rotor velocity was 6.4 m/sec. and the unit was processing 770 gallons/min. (2914.5 L/min.) of 0.3% consistency pulp. Figures 15 and 16 show the preferred basket screen housing used when the flow of slurry is inward rather than outward. The differences between this construction and that shown in Figures 11-14 are that the outer bars 53' are movable, the inner bars 54' are fixed, the fasteners 55' attach the outer bars to the inner bars, the apertured plates 56' are on the inside of the screen 27""" and the foils 66' are on the outside of the screen and are held by the upper arms 68' only. The other reference numerals, the construction and the operation are the same as in Figure 7. The recirculation line may have a cross section other than circular. For example, it may be square or octagonal. The filtrate recirculation line is an annulus around the outer shell 11. This latter construction is shown in Figures 17-18. The return line is the annulus 69. Except for the annular return line, the reactor is the same as the reactor shown in Figure 1. The only difference is that the filtrate passes from return line 70 into the annular passage 69, is forced upwardly through the annular passage and passes through apertures 71 into upper chamber 14a""'". The other reference numerals are the same as in Figure 1. Figure 19 illustrates the annular return line used in co-current flow. The reactor 10"""" has the same flow pattern as reactor 10' in Figure 6. In reactor 10"""" the filtrate enters the annular passage 69' through aperture 71', travels down the passage 69' and through apertures 72 into lower chamber 14f"""". The rest of the apparatus, the reference numerals and the operation are the same as in Figure 6. The filtrate recirculation line may also be inside rather than outside of the reactor. This is shown in Figures 20-21 in which the recirculation line is the central shaft 73. The upper chamber 14""'"" is the dewatering chamber, and the flow of slurry and filtrate is similar to the flow in Figure 7. The slurry is on the outside of dewatering device 74. The filtrate passes through the screen 75 of dewatering device 74, through the apertures 76 in shaft 73, downwardly through shaft 73, and out through apertures 77 into lower chamber 14f """". The dewatering device 74 is mounted on the shaft 73 by upper and lower plates 78 and 79, and rotates with the shaft. The screen 75 may be mounted on the plates 78 and 79 in any convenient manner. A construction similar to that shown in Figures 15 and 16 is preferred. In that construction, upper wall 46' would be the equivalent of upper plate 78, inner wall 45' the equivalent of shaft 73, and divider plate 15a""" the equivalent of lower plate 79. The foils 80 are on the ends of baffles 24a"""". The circular plate 81 above the apertures 77 in chamber I4f""""' prevents the short circuiting of the filtrate in its upward travel through the reactor. The apertures 77 are between plate 81 and the impeller 23f""" m . The rest of the apparatus, the reference numberals, and the operation is the same as in Figure 19. In all these constructions, the inner cross-sectional size of the return line would be equal to a circular cross section having an inner diameter of 7.7 to 45% of the inner diamter of the reactor. This dimension will be called "equivalent diameter" for all cross sections and types of return lines or pipes. The normal equivalent inner diameter is 7.7 to 24% of the inner diameter of the reactor, and the equivalent inner diameter of the downflow line would be 20 to 30% of the inner diameter of the reactor.

Claims: CLAIMS 1. Apparatus for mixing a wood pulp slurry with liquid to create a low consistency wood pulp slurry, mixing said slurry with a chemical, and thereafter removing said liquid from said slurry and recycling said liquid to new additional slurry, comprising a plurality of chambers, said chambers comprising two end chambers, a passage between each pair of adjacent chambers, one end chamber being a dewatering chamber, the rest of said chambers being mixing chambers, means in said rest of said chambers for mixing said slurry, a screen in said dewatering chamber dividing said chamber into a pulp slurry chamber and a liquid chamber, said screen preventing the passage of pulp fibers from said pulp slurry chamber to said liquid chamber, means for keeping said screen clear for passage of liquid through said screen, an inlet passage in the other end chamber, an outlet passage in said dewatering chamber between said pulp slurry chamber and the exterior of said dewatering chamber, said passage between said dewatering chamber and said adjacent mixing chamber being between said adjacent mixing chamber and said pulp slurry chamber, and a liquid recycle passage between said liquid chamber and said other end chamber, said liquid recycle passage having an equivalent inner diameter which is in the range of 7.7 to 45% of the equivalent inner diameter of said chambers. 2. The apparatus of claim 1 in which said liquid recycle passage is a pipe outside of said chambers. 3. The apparatus of claim 1 further comprising a central shaft, and said mixing means and said screen clearing means being attached to said central shaft. 4. The apparatus of claim 3 in which said liquid recycle passage is in said central shaft. 5. The apparatus of claim 3 further comprising said screen being a cylinder having an axis aligned with the axis of said chambers, a basket holding said screen in position, said basket comprising matched interior and exterior bars aligned with and parallel to said screen axis, said bars defining said cylinder and passages between adjacent pairs of bars, said screen passing between and being held by said pairs of interior and exterior bars. 6. The apparatus of claim 5 further comprising an apertured backing plate in said liquid chamber between said bars and contiguous with said screen. 7. The apparatus of claim 1 further comprising said screen being a cylinder having an axis aligned with the axis of said chambers, a basket holding said screen in position, said basket comprising matched interior and exterior bars being aligned with and parallel to said screen axis, said bars defining said cylinder and passages between adjacent pairs of bars, said screen passing between and being held by said pairs of interior and exterior bars. 8. The apparatus of claim 7 further comprising an apertured backing plate in said liquid chamber between said bars and contiguous with said screen. 9. The apparatus of claim 1 in which said dewatering chamber is the upper chamber of said apparatus. 10. The apparatus of claim 9 in which said recycle passage equivalent inner diameter is 20 to 30% of the equivalent inner diameter of said chambers. 11. The apparatus of claim 9 in which said liquid recycle passage is a pipe outside of said chambers. 12. The apparatus of claim 9 further comprising a central shaft, and said mixing means and said screen clearing means being attached o said central shaft. 13. The apparatus of claim 12 in which said liquid recycle passage is in said central shaft. 14. The apparatus of claim 12 further comprising said screen being a cylinder having an axis aligned with the axis of said chambers, a basket holding said screen in position, said basket comprising matched interior and exterior bars aligned with and parallel to said screen axis, said bars defining said cylinder and passages between adjacent pairs of bars, said screen passing between and being held by said pairs of interior and exterior bars. 15. The apparatus of claim 14 further comprising an apertured backing plate in said liquid chamber between said bars and contiguous with said screen. 16. The apparatus of claim 9 further comprising said screen being a cylinder having an axis aligned with the axis of said chambers, a basket holding said screen in position, said basket comprising matched interior and exterior bars aligned with and parallel to said screen axis, said bars defining said cylinder and passages between adjacent pairs of bars, said screen passing between and being held by said pairs of interior and exterior bars. 17. The apparatus of claim 16 further comprising an apertured backing plate in said liquid chamber between said bars and contiguous with said screen. 18. The apparatus of claim 1 in which said dewatering chamber is the lower chamber of said apparatus. 19. The apparatus of claim 18 in which said recycle passage equivalent inner diameter is 7.7 to 24% of the equivalent inner diameter of said chambers. 20. The apparatus of claim 18 in which said liquid recycle passage is a pipe outside of said chambers. 21. The apparatus of claim 18 further comprising a central shaft, and said mixing means and said screen clearing means being attached to said central shaft. 22. The apparatus of claim 18 further comprising said screen being a cylinder having an axis aligned with the axis of said chambers, a basket holding said screen in position, said basket comprising matched interior and exterior bars aligned with and parallel to said screen axis, said bars defining said cylinder and passages between adjacent pairs of bars, said screen passing between and being held by said pairs of interior and exterior bars. 23. The apparatus of claim 22 further comprising an apertured backing plate in said liquid chamber between said bars and contiguous with said screen.

Inventors: FLECK JOHN ACROYD, MEREDITH MICHAEL DEAN, MOORE STEVEN JAMES, FLECK, JOHN, ACROYD, MEREDITH, MICHAEL, DEAN, MOORE, STEVEN, JAMES
Applicants: WEYERHAEUSER CO, WEYERHAEUSER COMPANY
Application year: 1984
Main classification: D21C7
IPC codes: B01F7, D21C9