Solids Suspension Vessel

Adding a second impeller typically has a very small effect on the just-suspended speed. In multiple-impeller systems, zoning occurs when the impeller separation is too large. The most efficient solids mixing occurs just before the 

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Reeently, Corpstein et al. [43] found that high-effieieney impellers provide the same levels of solids suspension at redueed eapital and operating eosts. They introdueed the term just-suspended for the most eommonly eneountered level of liquid solid agitation. This oeeurs when none of the solid partieles remains stationary on the bottom of the vessel for longer than 1-2 see. They developed a eorrelation of the speed required to aehieve just-suspended eonditions as ... 

In order to achieve uniform solid suspension or pickup of solid particles off the bottom, the upward velocities of the fluid streams in all portions of the vessel must exceed the terminal setding velocity of the particular particles. This can be determined by small scale tests. 

Agitation levels 1 and 2 are characteristic of applications requiring minimal solids-suspension levels to achieve the process result. Agitators capable of scale levels of 1 will Produce motion of all of the solids of the design settling velocity in the vessel Permit moving fillets of solids on the tank bottom, which are periodically suspended. 

Agitation levels 3 and 5 characterize most chemical process industries solids-suspension applications and are typically used for dissolving solids. Agitators capable of scale levels of 3 will Suspend all the solids of design settling velocity completely off the vessel bottom Provide slurry uniformity to at least 1 /3 of fluid-batch height Be suitable for slurry draw-off at low exit-nozzle elevations. 

Birch, D J., Ahmed, N., Solids suspension in aerated agitated vessels Role of sparger design, 9th European Conference on Mixing Mixing 97, Vol. 11, 177 (1997)... 

Suspension of solids is maintained by upward movement of the liquid. In principle, use of a draft tube and an axial flow impeller will accomplish this flow pattern most readily. It turns out, however, that such arrangements are suitable only for low solids contents and moderate power levels. In order to be effective, the cross section of the draft tube must be appreciably smaller than that of the vessel, so that the solids concentration in the draft tube may become unpractically high. The usually practical arrangement for solids suspension employs a pitched blade turbine which gives both axial and radial flow. 

The study of solids suspension in quantitative terms normally involves a method of sampling. Typically, samples are withdrawn from the side of the mixing tank through openings or tubes inserted into the vessel wall. It may also be done by submerging a container and quickly removing... 

Just as the critical gas velocity is required to suspend the particles in a three-phase fluidization in an agitated reactor, some minimum agitation intensity is required to keep the particles in suspension. Calderbank11 has described the methods of estimating this minimum agitation intensity. For an agitated liquid-solid slurry vessel, the correlation of Zweitering150 for the calculation of the minimum impeller speed which completely suspends the particles, namely... 

P.M. Armenante and E. Nagamine, Solids Suspension in Agitated Vessels with Impellers Having Small Off-Bottom Clearances, paper presented at the 1996 AIChE Annual Meeting, Chicago, 1996. 

N.H. Chowdhury, Improved Predictive Methods for Solids Suspension in Agitated Vessels at High Solids Loadings, PhD Dissertation, University of Arkansas, 1997. 

Solids suspension is usually carried out in mechanically agitated vessels with or without draft tubes, as shown in Figs. 5 and Dished heads (ASME dished, elliptical, or torispherical heads) are preferred. A single impeller is usually sufficient for off-bottom solids suspension in vessels with dished heads and 7 < 1.3. Dual impellers are recommended for vessels with 13 < H/T < 2.5, which are used for uniform suspension of fast-settling solids. Three or more impellers... 

Typical values for the impeller clearance off the vessel bottom are 7/4 for hydrofoil impellers and 7/3 for pitched-blade turbines. Hydrofoil impellers may be a poor choice when solid suspension is accompanied by other mixing duties, such as liquid-liquid dispersion or gas dispersion. For such cases, a multiple impeller system consisting of a high-efficiency impeller in combination with a 45"" pitched-blade impeller should be evaluated. Small pitched blade impellers with a... 

In agitated vessels, the degree of solid suspension, or solid suspension regime, is generally classified into the following three levels (Fig. 8). ... 

The use of multiple impellers usually has a negligible impact on Vjs, as solid suspension is dominated by the hydrodynamics at the bottom of the vessel, which is primarily affected by the lowest impeller. 

Table 2 Value of the S parameter for solid suspension in dished-bottomed vessels...

In solid suspensions, there is a distinct level to which most of the solids are lifted within the fluid even at speeds above Njs- The distance from the bottom of the vessel to this level is called the cloud height (//doud)-The liquid below this height is solid rich, while above it there is only an occasional visit by a few small solids. 

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