Agitating – Having specified feed means – Impinging jets
Reexamination Certificate
1999-09-20
2002-07-23
Cooley, Charles E. (Department: 1723)
Agitating
Having specified feed means
Impinging jets
C366S175200
Reexamination Certificate
active
06422735
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to mixing chemicals in a fluid stream and more particularly to hydraulic diffusion flash mixing of chemicals in water treatment and waste water treatment.
BACKGROUND OF THE INVENTION
Chemicals have long been used in the treatment of water and waste water. In instances of treatment practice it is possible to identify the application of chemicals in the form of gases, liquids and solids. In all cases it is advantageous to achieve a uniform dispersal of the chemicals into the water stream as rapidly as possible and to ensure that chemicals that have already been dispersed do not return to the region wherein fresh chemicals are being introduced into the water stream. Meeting this last condition requires that the dispersal and mixing of the chemicals be done in a plug flow regime.
A first class of chemicals applied in the liquid form in water and waste water treatment is coagulants. They are used to induce flocculation of particles suspended in the raw water to be treated. This aggregation of suspended particles allows for more efficient sedimentation and/or filtration downstream. For best results, the initial mixing of the chemical coagulant with the raw water should occur as rapidly as possible to form a homogenized mixture within a second, or less.
The principal objective of this rapid or flash mixing in coagulation practice is to ensure a homogeneous coagulation by completely uniform dispersal of the coagulant throughout the water. In this way, the coagulant can make contact with the maximum number of suspended particles prior to the completion of hydrolysis, enabling intermediate complexes to destabilize the suspended particles initiating aggregation. This chemistry of destabilization sets some of the requirements for efficient rapid mixing.
Chemical coagulants should be dispersed in an unblended stream of raw water. Dispersing chemicals into a blended or partly blended stream (backmixing) can lead to poor destabilization of a fraction of the particles because some might have insufficient surface coverage while others might have too extensive surface coverage by adsorbed chemical species. This wastes chemicals and results in less effective floc formation for a given amount of a coagulant.
Stagnation time, defined as the amount of time that elapses from the addition of a coagulant to the start of mixing, should be reduced for the most effective coagulation. Experts on coagulation suggest that a sufficiently short stagnation time is achieved with a hydraulic jet mixer if the jet plume can be made to cover the entire cross section of the mixing conduit within one conduit diameter downstream of the mixer location when the conduit is flowing at its maximum capacity.
A second class of chemical used in water and waste water treatment includes those associated with disinfection. The primary concern in water and waste water treatment is the elimination of water born disease. Suspended particles are removed from potable water primarily because they can interfere with disinfection by shielding pathogens from contact with biocides. The aesthetics of suspended particle removal is a secondary concern.
The chemicals used in disinfection practice are chlorine, ammonia and ozone. Chlorine and ammonia can be fed as a gas or as a solution in water. Ozone is fed as a gas. In all cases, the concerns about short stagnation time, even dispersion, and avoidance of backmixing that have been enumerated in the discussion of coagulation apply to the disinfection process. When a disinfectant is fed as a gas it is also necessary to ensure efficient transfer of the disinfectant from the gas phase to the liquid phase by minimizing the size of the gas bubbles. Small bubbles are preferred because a disinfectant molecule has a shorter distance to travel from the interior of the bubble to the gas-liquid interface, and because the ratio of surface area to volume is larger for small bubbles. Both of these considerations improve the gas to liquid mass transfer process.
A third class of chemicals is used in water treatment to eliminate unpleasant taste and odor. Most often potassium permanganate and activated carbon are used for this purpose. Potassium permanganate is fed as a solution in water. Powered activated carbon (PAC) is fed as a slurry. When activated carbon is used in the granulated form, the water to be treated is passed through fixed beds of the granulated material. Rapid mixing is used to feed potassium permanganate and PAC and the concerns about short stagnation time, even dispersion, and avoidance of backmixing that have been enumerated in the discussion of coagulation apply to the elimination of taste and odor by the use of these chemicals.
From a mechanical point of view, a rapid mixing device should be simple, practical, and relatively inexpensive and should not create appreciable head loss.
Through the years, in attempting to meet theses chemical and mechanical requirements, many devices have been employed to provide rapid mixing needed for chemical dispersion. These include the weir, the Parshall Flume, and rapid mixing chambers equipped with mechanical rotary mixing devices such as propellers or turbines and in-line blenders. More recently, hydraulic diffusion flash mixing has been used as a method providing rapid mixing without appreciable head losses and lower operating and maintenance costs than mechanical methods. This method also provides more efficient rapid mixing with reductions to 20 to 50 percent in chemical consumption over mechanical methods.
Generally hydraulic diffusion flash mixing operates by drawing off a portion of the water to be treated into a carrying water loop. The chemical to be dispersed is added to this drawn-off portion. The mixture of carrying water and chemical is then injected into the remainder of the water through a diffuser. A pump in the carrying water loop provides the pressure for injection.
Usually the diffuser is a radial diffuser which injects the carrying water and the chemical mixture perpendicular to the flow direction of the remaining water from a deflector plate or from several nozzles equally spaced about the circumference of a tube placed in the center of the conduit carrying the remaining water. Radial injection can also occur by injecting perpendicular to the flow direction from nozzles equally spaced about the pipe periphery. In theory, this alternative reduces head losses, but is more difficult to construct, so central injection is preferred.
In other versions of hydraulic jet diffusion, the jet nozzles are placed on a tube that crosses the major diameter of the conduit carrying the remaining water; or on a grid of tubes that crisscrosses the conduit carrying the remaining water. These jet nozzles can be situated so that they discharge perpendicular to the direction of flow in the conduit, or either upstream or downstream to the direction of flow. These versions can cause objectional head losses in the conduit carrying the remaining water. The multiplicity of nozzles that are required to attain a short stagnation time, even dispersal, and avoid backmixing, require that the chemical be mixed with a relatively large amount of carrying water, That is inefficient in the amount of power required by mixing and requires the use of more chemical because the large dilution by carrying water reduces the efficiency of coagulation and can cause nozzle clogging by increasing the propensity of the coagulant to precipitate. Thus, central injection is preferred.
Sometimes the diffuser is a conical jet diffuser which injects carrying water and chemical mixture parallel to the flow direction of the remaining water through a single nozzle, directed either upstream or downstream with the flow, located in the center of the conduit carrying the remaining water. Both directional options are versions of the central injection scheme. Because flow through a conical nozzle requires more power than the convergent nozzle used in the radial jet versions, and because the water leaving the conical nozzle does not flow entirely perp
Cooley Charles E.
Sorkin David
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