Agitating – Having interrelated feed and discharge means – Recirculating from and to mixing chamber
Reexamination Certificate
2002-10-11
2004-11-23
Cooley, Charles E. (Department: 1723)
Agitating
Having interrelated feed and discharge means
Recirculating from and to mixing chamber
C366S173200
Reexamination Certificate
active
06821011
ABSTRACT:
FIELD
The apparatus and methods described herein relate generally to tank mixing systems and, in particular, to tank mixing systems for sludge storage tanks and digester tanks requiring surface mixing.
BACKGROUND
Storage tanks are often used for municipal and industrial sludge and other applications, such as storing sludge from municipal and industrial waste treatment facilities. The sludge generally comprises both solid and liquid components. The storage tanks may be used for storing the sludge when received from a waste treatment facility prior to processing and after processing. In addition, storage tanks may be used for treatment processes, such as aerobic and anaerobic digestion. The storage tanks are typically large, ranging from about 10 feet in diameter up to and beyond 150 feet in diameter. The depths of such tanks likewise have a broad range, varying between about 10 feet to about 40 feet and above.
Due to the mixture of liquid and solid components forming the sludge, and the large volumes of sludge frequently present in the tanks, settling of the solid components relative to the liquid components often occurs. The solid components of the sludge tend to settle in a layer toward the bottom of the tank over time, while the liquid contents remain above the accumulated solid layer on the bottom floor of the tank. In order to facilitate removal and/or further processing of the sludge in the tank, including both liquid and solid components, it is desirable to break up the solid layer on the bottom floor of the tank and resuspend the solid components into the liquid components. Such resuspension involves mixing of the tank contents to move the solid components from the floor in order to create a generally homogenous liquid and solid slurry within the tank. A variety of mixing systems aimed at suspending the solid components back into the liquid components of the sludge have been developed. In some instances, flow patterns are developed within the tanks in order to mix the solid and liquid components of the tank contents together in an efficient and effective manner. One such system is disclosed in U.S. Pat. No. 5,458,414.
During the mixing process, gas entrapped in the solid components often causes large chunks of solid debris to rise toward the surface of the tank and even float on the surface of the tank contents, particularly as the solid layer on the tank floor is broken up. Solid debris floating on the surface of the tank in large chunks is undesirable because mixing processes can occur more efficiently beneath the surface of the liquid tank contents. Solid debris on the surface can be difficult to break up and resuspend into the liquid. When flow patterns are developed in the tank contents, it is desirable to have the solid debris submerged for entrapment in the flow pattern to break up the solid debris. Floating solid chunks can reduce digestive capacity and performance, may result in plugged pipes and pumps, and generally inhibit mixing of the tank contents.
Scum layers may also form on the surface of tank contents during the mixing process. Scum layers might appear on the liquid surface of anaerobic digesters and contain grease, vegetables and mineral oils, and other floating materials such as hair, rubber goods, animal fats, bits of cellulose material, pre-fatty acids, and calcium and magnesium soaps. Scum accumulations can have a specific gravity less than the specific gravity of the sludge, causing the scum to rise toward the surface of the tank contents and even float on the surface.
When the scum accumulations are floating on the surface of the tank contents, it is very difficult to break up or entrap them in the flow pattern beneath the surface of the tank. The scum layers can vary in size from a few inches to several feet in depth. The depth of the scum layer and degree of solidification depends on a variety of factors, such as the volumes of grease and oil in the sludge in the tank, whether sedimentation in the tank is treated separately, the temperature of digester contents, the degree and type of tank mixing, the frequency of cleaning, and whether a tank has a fixed or floating cover. The scum, similar to solid debris floating on the surface, is undesirable because it is difficult for typically submerged tank mixing systems and flow patterns to adequately mix the scum layers and suspend the solid components thereof into the liquid for facilitating removal from the tank or further processing.
In addition to scum, foam can also develop on the liquid surface in anaerobic digesters. Foam can be caused by high grease content, inadequate mixing, a high percentage of activated sludge in food, sludge thickening by dissolved air floatation, several temperature fluctuations, high CO
2
content, high alkalinity, low total solids, excessive mixing rates, and high organic content in the food sludge. Foaming is similar to scum except foam typically has entrapped gases that causes the foam, and the contents thereof, to rise to the surface of the tank. Foam, similar to solid chunks and scum, presents a problem for tank storage systems because it is difficult to break up the foam layer and resuspend the solid contents thereof into the liquid solution for facilitating removal from the tank or further processing. A variety of approaches have been developed for attempting to address foam and scum control. For example, when foam and scum is developed due to excessive grease, grease can be removed from the process train using primary clarifiers. However, the use of primary clarifiers in order to remove the grease complicates the tank storage system and increases the cost.
Another solution developed in an attempt to address foam and scum accumulation problems is to continuously mix the contents of the tank to reduce settling of the solid components. However, mixing continuously can be inefficient and can result in even more scum and foam production when excessive mixing rates are used. Rapid mixing can lead to an increase in entrapment of gasses associated with foaming in solid components, resulting in an increase in foam and scum production.
Other complicated methods of attempting to reduce scum and foam involve minimizing temperature fluctuations. However, temperature variations of just two to three degrees Fahrenheit can cause foam problems. Therefore, controlling foaming by reducing temperature variations can be impractical. Scrubbing digester gases to remove CO
2
has been done in the past but requires expensive and complicated scrubbing mechanisms. The use of actinomycetes have also be used, but requires time intensive and trial and error experimentation and may not be reproducible due to the large variations in the characteristics in the tank contents frequently present.
In some instances, the use of nozzles positioned above the surface of the tank can be used to break up scum and foam layers present on the surface thereof. Such nozzles require manual operation, such as an operator positioned above the tank on a platform and aiming and directing a fluid stream from the nozzle at the foam and scum deposits on the surface of the tank in a random manner. Typically, the nozzles are rotatably and pivotably mounted allowing an operator to aim the fluid stream as needed at the solid components present on the surface of the tank to break them up and urge them back under water where they can be effectively mixed by the tank mixing system. The nozzles can be problematic due to the requirement of an operator to selectively aim the fluid stream at solid deposits, scum and foam. Not only are the nozzles inefficient due to the increased time and operator effort that must be expended in order to break up the sludge deposits, which can take several hours, but the pumping energy required to pump fluid and discharge fluid through the nozzle can add to the increased cost of operating the tank storage system by substantially disrupting the fluid flow patterns within the tank. Moreover, such nozzles are impractical for use with covered storage tanks, where operator access is often impossibl
Cooley Charles E.
Fitch Even Tabin & Flannery
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