Method and apparatus for dispersing gas into liquid

Abstract: The invention relates to a method and apparatus for dispersing gas into liquid or slurry, which uses a rotor at least partly submerged in the liquid, and blades connected thereto. According to the invention, the gas to be dispersed is conducted, via a gas conduit (1, 22, 32) to the inside (7, 27, 37) of the rotor (6) and further to the surrounding liquid or slurry through at least one discharge aperture (12, 28, 38) formed in the rotor blade (8, 26, 36) and being adjustable in width.

Description: The present invention relates to a method and apparatus for dispersing gas into liquid, so that the gas used in the dispersion is fed into the liquid through dispersion blades provided in the rotor. The US patent 4,078,026 introduces an apparatus for dispersing gas into liquid, and according to one preferred embodiment of the said apparatus, the gas to be dispersed is conducted via the hollow axis of the rotor and injected through specific gas ducts into liquid slurry. The apparatus of the US patent 4,078,026 is submerged in the liquid or slurry under treatment, so that at least the stator and rotor of the apparatus are located totally underneath the liquid or slurry surface. From the US patent 4,425,232 there is known a rotor-stator pump assembly, where the rotor body includes the hub, blade and top plate members, forming a uniform construction. The gas flow, which is conducted into the gas chamber, is discharged transversally from the gas chamber and flows in gas pockets along the surfaces of the moving blades provided for dispersing the slurry. In both apparatuses according to the above described US patents, the power of the apparatus depends on the supplied amount of gas, and the power increases essentially when the gas supply is cut off. Moreover, after the cut-off, the particles contained in the surrounding slurry may block the gas injection apertures that are important for the dispersion process. Thus, when restarting the apparatus, the dispersion of gas into liquid becomes essentially more difficult or is nearly stopped altogether. DE-A-14 57 186 discloses an impeller for supplying gas into a liquid. This impeller has discharge apertures for the supplied gas which apertures are adjustable in width, in that one wall of the hollow rotor blade on the downstream side is movable along with increasing rotation of the impeller. The width of the aperture is only adjusted as a function of the rotation of the impeller. This has the effect that the discharge opening is closed when the impeller stops. US-2 235 218 discloses a diffusion apparatus where a discharge opening is provided between the circumference of two disks which are brought in tight but not positive engagement. Both disks form a diffuser head for introducing one fluid into another fluid. Normally the two concave disks building a fluid supply chamber in between are pressed by the surrounding liquid together so that no fluid can flow from the liquid supply chamber between the disks into the surrounding liquid. This is only possible if the pressure of the supplied fluid between the disks is raised so as to force the disks in opposite directions as to release the circumferal abutment of both disks. GB-976 560 discloses a mixing device for introducing gas into a liquid. The mixing device has a central rotor surrounded by an arrangement of radially extending hollow blades having an adjustable width to vary the mixing efficency of the gas into the liquid. The object of the present invention is to eliminate some of the drawbacks of the prior art and to create an improved and operationally more secure apparatus for dispersing gas into liquid, in which apparatus the gas discharge apertures are arranged, in order to balance the power taken in by the apparatus, on the dispersion surface of the outer circumference formed by the rotor blades, so that at the beginning of the dispersion treatment, the gas discharge apertures can be cleared of possible particles flown therein. The essential novel features of the invention are apparent from the appended patent claims. According to the invention, the rotor blades are formed to be box-like, so that the liquid pressure created inside the rotor blade essentially extends the distance between the side walls of the rotor blade. Such a liquid pressure is advantageously created while starting the gas supply onto the rotor, so that the amount of liquid flown inside the rotor blades and the gas injection pipework during the stoppage, as well as any harmful components possibly contained therein, can advantageously be removed before starting the dispersion treatment proper. Moreover, according to the invention the inner structure of the rotor is advantageously arranged so that the gas to be dispersed can be conducted in a closed space onto the dispersion surface formed by the outer edges of the side walls of the rotor blades, when seen from the rotor axis. Now for instance in the middle part of the rotor construction there is formed a gas distribution chamber, wherefrom the gas to be dispersed flows into the rotor blades arranged radially with respect to the said chamber. The gas distribution chamber can also be formed inside the rotor, so that above or underneath the rotor blades there is installed a guide member, the inner space thereof being so designed that the gas to be dispersed flows through the guide member either downwardly or upwardly to the rotor blades. Thus the liquid to be aerated during the dispersion treatment falls into contact with the dispersion gas only on the dispersion surface, which is provided with at least one gas discharge aperture per one rotor blade. The rotor blade of the invention is composed of one or several box-like elements arranged on top of each other in an essentially vertical position, the outer edge whereof, when observed from the rotor axis, forms the dispersion surface of the rotor blade in between the dispersion gas and the liquid to be aerated. The box-like element used in forming the rotor blade is further composed of at least two parts, so that the element parts form a closed compartment in cross-section. The element parts thus form the walls of the box. The said parts are manufactured so that at least one of the parts is made of a material which is essentially thinner or more elastic than the rest, or weaker in pressure resistance, in which case the liquid pressure created inside the box makes the walls of the box to be drawn further away from each other. The parts of the box-like element of the rotor blade of the invention are interconnected so that the cross-section is advantageously either rectangular or wedge-shaped, with the peak upwards or downwards. When the dispersion apparatus of the invention should be switched off, the rotation of the rotor is stopped and the supply of the dispersion gas is cut off. Now the surrounding liquid can freely flow into the box-like element through the dispersion gas discharge aperture located on the dispersion surface of the rotor blade. In normal process conditions, the surrounding liquid may contain components that are detrimental for dispersion and may block the dispersion gas discharge aperture; therefore it is possible that through the gas discharge aperture located on the dispersion surface of the rotor blade, into the rotor blade there can also enter such particles that may block the dispersion gas discharge aperture. In the rotor blade of the invention, the discharge apertures are about 1 - 5 mm wide, in which case also the width of the discharge aperture prevents large harmful components or objects from entering the rotor blade. By composing the rotor blade of the invention of at least two interconnected parts, so that in at least one of these parts the resistance to liquid pressure is poorer than in the rest, the components that are detrimental for the discharge of the gas are made to be removed from inside the rotor blade, advantageously at the beginning of the dispersion treatment. The harmful components are advantageously removed from inside the rotor according to the invention, because the parts having different resistance to the pressure of the discharging liquid are drawn apart from each other, and the discharge aperture of the dispersion gas is widened from 2 - 5 times for the duration of the discharge of the liquid pressure; now the detrimental components are advantageously removed from inside the rotor blades prior to the discharge of the dispersion gas proper. When the liquid has flown out of the gas discharge aperture, the drawn-apart wall of the said aperture is returned back to the initial position. While applying the method and apparatus of the invention, the power required by the apparatus is not essentially increased, when the dispersion gas supply to the apparatus is cut off, for instance due to the specific requirements of the process in question. Accordingly, the rotating and actuating members of the apparatus cannot be overloaded. Thus the dispersion apparatus of the invention advantageously achieves an improved oxygen transfer efficiency in between the gas to be dispersed and the surrounding liquid, as well as an improved agitation of the liquid on an advantageous power level. According to an advantageous embodiment of the invention the width of the discharge aperture is adjusted for the duration of the liquid/slurry discharge by means of the pressure of the dispersion gas for pumping the liquid through the discharge aperture. The invention is explained in more detail below, with reference to the appended drawings, where Figure 1 is a side-view illustration of a preferred embodiment of the invention, Figure 2 illustrates the section A-A of the embodiment of figure 1, Figure 3 illustrates the enlarged section B-B of the embodiment of fig. 2, Figure 4 is a side-view illustration of another preferred embodiment of the invention, Figure 5 is a side-view illustration of a third preferred embodiment of the invention, Figure 6 illustrates an advantageous shape of the gas discharge aperture of the rotor blade of the invention, and Figure 7 illustrates another advantageous shape of the gas discharge aperture of the rotor blade of the invention. According to figure 1, during the dispersion treatment, when the rotor shaft 2 is rotated by means of an actuating assembly 13, the dispersion gas, in this case air, is conducted, via an air conduit 1, to an intermediate space 3 formed around the rotor shaft 2. The intermediate space 3 is sealed around the rotor shaft 2 by means of a sealing 4. From the intermediate space 3, air is further conducted into the hollow rotor shaft 2 through an inlet 5. From inside the rotor axis 2, air is discharged into a chamber 7 formed within the rotor 6. From the chamber 7, air is radially discharged into the rotor blades 8 which are made of box-like elements. The box-like element (Figs. 2 and 3) of the rotor blade 8 is formed so that the essentially vertical side walls 9 of the box gradually converge while proceeding outwards from the rotor shaft 2. The box-like element of the rotor blade 8 is further composed of two parts 10 and 11 at a discharge opening 12, forming a closed compartment in cross-section. The part 10 is made of a material somewhat thinner than the part 11, so that the liquid pressure discharged from the rotor blade affects the part 10, and the parts 10 and 11 drawn further apart and the discharge aperture 12 for dispersion gas, located in between the said parts 10 and 11, is widened. Two of the rotor blades in Fig. 2 are illustrated during the operation stage of the apparatus and one of the rotor blades (with section B-B) is illustrated when the apparatus has been stopped. While applying the method of the present invention, the rotor is partly submerged in the liquid or slurry to be dispersed and containing solid particles, and the air serving as dispersion gas can be injected into the rotor shaft 2 via the air conduit 1 and through the intermediate space 3 from above the liquid surface. In connection with the submersion and at other times, when the air supply is cut off, the solid particles contained in the liquid or slurry to be dispersed have free access to flow into the rotor blade 8 via the discharge aperture 12. In that case the blocking of the apparatus also is possible. When the air supply is started, it gives a pressure impact to the liquid or slurry located inside the rotor. According to the invention, this pressure impact advantageously affects the part 10 of the box-like element of the rotor blade 8, which part 10 is, according to Fig. 3, made of a material thinner than that of the part 11 of the rotor blade. Owing to the pressure impact, the part 10 of the rotor blade advantageously yields, so that the width of the discharge aperture 12 increases and the solid particles possibly flown to inside the rotor blade 8 are advantageously removed back into the surrounding liquid or slurry. In figure 4, during the dispersion treatment, an actuating assembly 20 rotates the rotor shaft 21, and the air serving as the dispersion gas is conducted, via an air conduit 22, to an intermediate space 23 located around the rotor shaft 21, which intermediate space 23 is sealed, with respect to the shaft 21, by means of a sealing 24. From the intermediate space 23, air is conducted, via an inlet 25 to inside the rotor shaft 21. From within the rotor shaft 21, air is first discharged to inside a guide member 27 installed underneath the rotor blades 26, and further upwards, to the surrounding liquid through discharge apertures 28 provided on the dispersion surface of the box-like rotor blades 26. The embodiment of figure 5 corresponds to the embodiment of figure 4 in that during the dispersion treatment, the actuating assembly 30 rotates the rotor shaft 31, and the air serving as the dispersion gas is conducted via the air conduit 32 to the intermediate space 33 provided around the rotor shaft 31, which intermediate space 33 is sealed, with respect to the shaft, with a sealing 34. From the intermediate space 33, air is conducted through the inlet 35 to inside the rotor shaft 31. The difference from the embodiment of figure 4 is that from inside the shaft 31, air is first discharged to inside the guide member 37 installed above the rotor blades 36, and further downwards to the surrounding liquid through discharge apertures 38 located on the dispersion surface of the box-like rotor blades 36. Figures 6 and 7 illustrate the shapes of the gas discharge apertures of the rotor blade, so that the aperture 12 of figure 6 is wedge-like with the peak upwards, and that of figure 7 is wedge-like with the peak downwards.

Claims: Method for dispersing gas into liquid or slurry, by using an apparatus comprising a rotor and blades connected thereto, the rotor being at least partly submerged into the liquid or slurry, a rotor blade (8, 26, 36) being composed of at least one box-like element, the parts (10, 11) whereof form the walls of a discharge aperture (12, 28, 38) adjustable in width for the gas to be dispersed, at least one (10) of the parts (10, 11) of the box-like element of the rotor blade (8, 26, 36) consists of a material which is essentially thinner or more elastic than the other parts (11), wherein upon starting of gas supply said one part (10) is drawn apart by pressurized liquid/slurry which has entered the box-like element, e.g. during operation stop, so as to widen the discharge aperture (12) for the duration of the liquid/slurry discharge and when the liquid has flown out of the gas aperture, the drawn-apart part (10) of said aperture is returned to its initial position. A method according to claim 1, characterized in that from a chamber (7) arranged inside the rotor (6), gas is radially fed to the discharge aperture (12) provided in the rotor blade (8). A method according to claim 1, characterized in that from a guide member (27) connected to inside the rotor (6), the gas to be dispersed is fed upwardly to the discharge aperture (28). A method according to claim 1, characterized in that from a guide member (37) connected to inside the rotor (6), the gas to be dispersed is fed downwardly to the discharge aperture (38). A method according to any of the claims 1 - 4, characterized in that the width of the discharge aperture (12, 28, 38) for the duration of the liquid/slurry discharge is adjusted by means of the pressure of the dispersion gas. An apparatus for dispersing gas into liquid or slurry, said apparatus comprising a rotor and blades connected thereto, a rotor blade (8, 26, 36) being composed of at least one box-like element, the parts (10, 11) whereof form the walls of a discharge aperture (12, 28, 38) adjustable in width for the gas to be dispersed, at least one (10) of the parts (10, 11) of the box-like element of the rotor blade (8, 26, 36) consists of a material which is essentially thinner or more elastic than the the other parts (11), such that upon starting of gas supply said one part (10) is to be drawn apart by pressurized liquid/slurry which has entered the box-like element, e.g. during operation stop, so as to widen the discharge aperture (12) for the duration of the liquid/slurry discharge and when the liquid has flown out of the gas aperture, said drawn-apart part (10) is returned to its initial position. A apparatus according to claim 6, characterized in that the discharge aperture (12) of the rotor blade is essentially rectangular in shape. A apparatus according to any of the claims 6 or 7, characterized in that the discharge aperture (12) of the rotor blade is essentially wedge-like, with the peak directed downwards. A apparatus according to any of the claims 6 to 8, characterized in that the discharge aperture (12) of the rotor blade is essentially wedge-like, with the peak directed upwards.

Inventors: LEIPONEN MATTI OLAVI, LEIPONEN, MATTI OLAVI
Applicants: OUTOKUMPU MINTEC OY
Application year: 1993
Main classification: B01F3
IPC codes: B01F7, B01F3