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mixingtank

Conditions for suspension of solids in agitated vessels

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An Agitation Equipment is a device or mechanism to put something into motion by shaking or stirring. There are several types of agitation machines, including washing machine agitators (which rotate back and forth) and magnetic agitators (which contain a magnetic bar rotating in a magnetic field).[citation needed] Agitators can come in many sizes and varieties, depending on the application. In general, agitators usually consist of an impeller and a shaft. An impeller is a rotor located within a tube or conduit attached to the shaft. It helps enhance the pressure in order for the flow of a fluid be done.[1] Modern industrial agitators incorporate process control to maintain better control over the mixing process.

Though much has been done in the examination of the critical impeller speed, NJS, just necessary for suspension of solid particles, any theory available does not fit well the data for a wide range of variables. An attempt is made to develop a theoretical model of the process based on the comparison of the terminal velocity of a particle and the characteristic velocity of the agitated liquid round the particle at the bottom. The velocity field near the bottom is deduced from the values of local shear rates as estimated before by an electrodiffusion method. Literature data on NJS indicate that the model is acceptable even under extreme conditions. BJS is a constant which may depend on the particle shape. The model even explains effects of other variables such as impeller and tank diameters, liquid viscosity, and densities. Some of these effects cannot be interpreted by a single-power function valid for a wide range of covering industrially important problems of the solid suspension.

We recently showed that the underlying hydrodynamics involved in the suspension of non-dilute suspensions are different in the transitional regime than in the turbulent regime. We took measurements using the pressure gauge technique (PGT) in an unbaffled stirred tank equipped with a pitched blade turbine and found that the dependence on the just-suspended speed of fluid viscosity µ and particle diameter d p were the opposite of what was predicted by the Zwietering correlation and other correlations, including those proposed by Nienow, Baldi, Rao, Takashi, Armenante, and Grenville et al. This is not surprising given that these correlations were derived for the turbulent regime.

Fluid dynamics simulations were applied for evaluating the suspension of particles in stirred vessels. The spatial distribution of particles throughout the vessel was characterized by a single parameter, here called the suspension quality (s). Based on simulation results, a semi-empirical correlation was developed that correlated the suspension quality with the vessel geometry and solid and fluid properties, including a large variety of conditions, such as vessel and impeller diameters, impeller clearances, rotational speeds, particle densities and sizes. Comparison of the model with experimental data from the literature (Bohnet and Niesmak, 1980) suggests that the model can be extended to systems with different impeller geometries by adjustment of one single empirical parameter. The model can be used in the design of stirred vessels for the identification of the rotation speeds necessary to promote a specified suspension quality.

Thickening Equipment is used for the continuous gravity settling (sedimentation) of solids in suspensions. Suspension is fed into one or more basins or chambers and, whilst it is passing through, the solids settle out. The thickened solids are removed together with a portion of the liquid as thickened "underflow". The liquid, ideally containing no solids, forms the "overflow" from the thickener. Thickeners vary widely in size and configuration, but they all comprise: a. a vessel to provide volume and area needed for thickening, with the area being large enough to allow the solids to settle at a velocity faster than the upward velocity of the liquid; b. a system for introducing the feed and directing it into the flow paths that best utilize the vessel volume and area; c. an overflow system for collecting clarified liquid; d. a mechanism to convey settled solids to a discharge point.

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