Process and apparatus for mixing a gas and a liquid

Abstract

In a process for mixing a gas and a liquid in an apparatus having an axial flow down-pumping impeller located in a draft tube and preceded by a first baffle wherein a vortex and turbulence are created and gas is drawn into the vortex, the improvement comprising providing liquid velocity of at least one foot per second, increasing the turbulence, providing a second baffle below the impeller, and a high shear zone between impeller and the second baffle.

Description

, of the illustrated embodiment of the apparatus of the invention, a vortex is formed downward from the surface of the liquid in the vicinity of baffles 4 and conical inlet 5 such that the gas is drawn into and down draft tube 2. The amount of gas ingested into the liquid is a function of the depth to the liquid above conical inlet 5, and the pitch of impeller 6 and its rotational speed. The amount of gas ingested will vary, however, in different processes having different steps and conditions and using different materials.

The parts of the apparatus are made of conventional materials. Various metal alloys, plastics, and glass may be used depending on the composition of the liquids and gases to be mixed. Examples of materials used in the construction of the apparatus are stainless steel, rubber coated steel, and titanium.

For a two foot diameter impeller, the liquid is maintained at a level in the range of about 1 inch to about 15 inches above the uppermost part of draft tube 2, which, as noted, includes conical inlet 5 . in the illustrated embodiment. The preferred level for a two foot diameter impeller is in the range of about 2 inches to about 10 inches above the upper end of the draft tube, e.g. above said conical inlet 5. For a two inch diameter impeller, it is about one half inch. There will be a different liquid level for each draft tube of different diameter and for each rotational speed. Optimum liquid levels are determined by trial and error. As noted, the liquid level must be such that it will permit the vortex to form and be maintained throughout the process. It will be understood that when the liquid level is below the upper edge of the draft tube, liquid will not be circulated downward through the tube and gas will not be aspirated, and where the liquid level is too high for the draft tube diameter and/or rotational speed, the vortex, if one forms at all, will be too high above said upper edge, e.g. the upper edge of conical inlet 5, to feed gas into draft tube 2.

The rotational speed for a two foot diameter impeller is in the range of about 50 revolutions per minute (rpm) to about 1000 rpm and is preferably in the range of about 200 rpm to about 400 rpm.

The helical impeller typically has a single or double helix configuration. Helical impeller 6 is, in fact, two helical impellers connected to shaft 3 such that the leading edges are at an angle of 180 degrees. While helical impellers are preferred, other axial flow impellers such as marine impellers can be used. Radial impeller 8 can be a flat blade turbine with 2 to 8 blades or another type of radial impeller such as a pitched blade impeller. Baffles 9 are similar to vertical inlet baffles 4 in construction. They are also positioned vertically and usually symmetrically, and 2 to 8 baffles, or more, equilaterally spaced, are used, preferably 4 to 8. The plate or sheet which forms each of baffles 4 or 9 are placed vertically so that one vertical edge is at the outer perimeter of draft tube 2 and the other vertical edge is proximate to shaft 3 or its axis. Thus, a top or bottom view would show a pie-shaped structure divided into wedges. In the case at hand, a top or bottom view of baffles 9 would show a pie-shaped structure with four wedges or a cross. The distance from the bottom of radial impeller 8 to the top of baffles 9 is generally less than about one half (0.5) of the diameter of radial impeller 8 and is preferably no greater than about two tenths (0.2) of the diameter of radial impeller 8.

Increased turbulence of the shaft proximate to the helical impellers is caused by bars 7 affixed to shaft 3. These protuberances or projections can take other forms and can be located on the helical impeller as well as the shaft. The turbulence can also be created by providing indentation on the shaft or helical impeller at the position described above for protuberances 7. The indentations can take the form of slots, holes, or dimples.

It is found that the process and apparatus described above are capable of appreciably increasing the flow of gas through the axial flow impeller when compared with similar prior art processes and apparatus and reducing the size of the bubbles exiting baffles 9 to diameters in the range of 0.0625 to 0.125 inch or less. This reduction in bubble size increases the area of contact between the gas and liquid, thus increasing the efficiency of subject process and apparatus.

Claims

1. In a process for mixing a gas and a liquid in an apparatus comprising, in combination:

(a) a vessel;

(b) a cylindrical hollow draft member open at both ends and having a theoretical axis running from end to end, said axis being in a vertical position; and the upper end of the draft member being conically flared;

(c) an axial flow down-pumping first impeller fixedly connected to a rotatable shaft, (i) the first impeller being positioned within the draft tube; (ii) the shaft corresponding in position to the axis; and (iii) the diameter of the first impeller being less than, but proximate to, the diameter of the draft tube;

(d) first vertical baffling means disposed above the impeller;

(e) means for rotating the shaft; and

(f) means for introducing the gas and the liquid into the vessel and for removing gas and liquid from the vessel, the process comprising:

(A) energizing the shaft to provide the first impeller with a rotational speed sufficient to cause (i) vortex formation downward from the surface of the liquid in the vicinity of the first vertical baffling means and the flare of the upper end of said draft member such that the gas is drawn into and down the draft tube and (ii) turbulence in the draft tube;

(B) introducing a sufficient amount of liquid into the vessel to provide, during operation, a liquid level above the upper end of the draft tube; and

(C) recovering liquid from the vessel, the improvement comprising

(1) providing a rotational speed to the first impeller sufficient to impart a liquid velocity down the interior of the draft tube of at least one foot per second;

(2) increasing the turbulence of the liquid at the shaft proximate to the first impeller;

(3) in the area in the draft tube below the first impeller or in the area below, and immediately exterior to, the lower end of the draft tube, providing second vertical baffling means; and

(4) in the area in the draft tube between the first impeller and the second vertical baffling means providing a radial flow impeller fixedly connected to the shaft whereby a high shear zone is created in the area heretofore mentioned in this paragraph (4).

2. In an apparatus for mixing a gas and a liquid comprising, in combination:

(a) a vessel,

(b) a cylindrical hollow draft member open at both ends and having a theoretical axis running from end to end, said axis being in a vertical position; and the upper end of the draft member being conically flared.

(c) an axial flow down-pumping first impeller fixedly connected to a rotatable shaft, (i) the first impeller being positioned within the draft tube; (ii) the shaft corresponding in position to the axis; and (iii) the diameter of the first impeller being less than, but proximate to, the diameter of the draft tube;

(d) first vertical baffling means disposed above the impeller;

(e) means for rotating the shaft; and

(f) means for introducing the gas and the liquid into the vessel and for removing liquid from the vessel, the improvement comprising:

(1) protuberances or indentations located on the shaft or first impeller of sufficient size, and positioned, to increase the turbulence at the shaft proximate to the first impeller;

(2) second vertical baffling means located in the area in the draft tube below the first impeller or in the area below, and immediately exterior to, the lower end of the draft tube; and

(3) a radial flow impeller fixedly connected to the shaft located in the area in the draft tube between the first impeller and the second vertical baffling means whereby a high shear zone is created in said area. 3. The process of claim 1 in which the upper end of said draft member is conically flared. 4. The apparatus of claim 2 in which the upper end of said draft member is conically flared.