Solid suspension in stirred tank

K.J. Bitterof and S.M. Kresta, Prediction of cloud height for solids suspension in stirred tanks, CHISA Proceedings, Prague, August 2002. 

Bittorf KJ, Kresta SM. (2003) Prediction of cloud height for solid suspensions in stirred tanks. Trans IChemE., 81(Part A) 568-577. 

Sharma RN, Shaikh AA. (2003) Solid suspension in stirred tanks with pitched blade turbines. Chem. Eng. Sci., 58 2123-2140. 

Derksen (2006a) continued along this line of approach and—by means of a clever strategy—mimicked the long-time behavior of solids suspension in an unbaffled tall stirred tank equipped with four hydrofoil impellers (Lightnin A310). The time span covered by his LES amounted to some 20,000 impeller revolutions (some 20 min). Running a LES for a Reynolds number of 1.6 x 10 over the entire time span is not an option, and for that reason a particular flow... 

Kraume, M., and P. Zehner, "Suspendieren in Ruhrbehalter—Vergleich Unter-schiedlicher Berechnungsgleichungen" ("Suspension of Solids in Stirred Tanks— Comparison of Different Calculation Methods"), Chem. Ing. Tech., 60,822 (1988). 

Stirred tanks are typically used for the thermochemical pretreatment. To simulate flow of corn stover slurries in stirred tanks, the rheologic properties of these suspensions must be known. The corn stover slurries in stirred tank reactors typically range from 10 to 40% solids (3). 

Armenante, P.M. Uehara Nagamine, E. Effect of low off-bottom impeller clearance on the minimum agitation speed for complete suspension of solids in stirred tanks. Chem. Eng. Sci. 1998, 53, 1757-1775. 

The solids are kept in suspension if the pumping capacity of the impeller causes strong enough circulation of the liquid. In most processes, complete suspension of the particles is not required. Often, so-called off-bottom suspension is sufficient, which means that all particles are moving above the bottom of the tank with some vertical velocity. Radial flow impellers are usually not very effective in suspending solid particles. Actually, about three times more power is required for a radial turbine to provide the same degree of uniformity compared to an axial turbine. This is because the radial turbines pick up particles from the bottom of the tank by the suction side of the impeller, which is only half of the total flow from the impeller. Due to the appearance of an upper and a lower circulation zone, the contents of the two zones are not sufficiently mixed. Axial impellers are therefore most frequently used for the suspension of solids in stirred tanks [65]. 

Solid particle size in stirred tank reactos, loop reactors and bubble columns is usually in the range of 100 - 200 and solid concentration is rather low whereas in multiphase fluidized beds the solid concentration as well as solid particle size is larger. The density difference of liquid and solid usually is large, except in bioreactors, where one is faced also with agglomeration problems and nonnewtonian behavior of the suspension. 

Similar to there is absolutely no information in the literature on the critical speed for suspension of solids, in stirred tank reactors fitted with cooling coils. Nikhade et al. (2005) have provided Equations 7A.32,7A.33, and 7A.34 for AA, for the three impellers referred to earlier. is defined as the difference between... 

Ayranci I, Machado MB, Madej AM, Derksen JJ, Nobes DS, Kresta SM. (2012) Effect of geometry on the mechanisms for off-bottom solids suspension in a stirred tank. Chem. Eng. Sci., 79 16T-176. 

Bao Y, Hao Z, Gao Z, Huang X, Shi L, Smith JM, Thorpe RB. (2006) Gas dispersion and solid suspension in a three-phase stirred tank with multiple impellers. Chem. Eng. Commun., 193 801-825. 

Bao Y, Zhang X, Gao Z, Chen L, Chen J, Smith JM, Kirkby NF. (2008a) Gas dispersion and solid suspension in a hot sparged multi-impeUer stirred tank. Ind. Eng. Chem. Res., 47 2049-2055. 

Congjing R, Xiaojing J, Jingdai W, Yongrong Y, Xiaohuan Z. (2008) Determination of critical speed for complete solid suspension using acoustic emission method based on multiscale analysis in stirred tank Ind. Eng. Chem. Res., 47 5323-5327. 

Processes for precipitation of solid products from dissolved reactants are almost always carried out in stirred tanks. The reactors are operated either semi-batchwise or continuously. In both operation modes, one reactant is added to a stirred solution containing an excess of the other reactant. The functions of the stirrer are mixing of the reactants, suspension of the formed solid particles, and promotion of heat transfer to the wall. 

Guichardon etal. (1994) studied the energy dissipation in liquid-solid suspensions and did not observe any effect of the particles on micromixing for solids concentrations up to 5 per cent. Precipitation experiments in research are often carried out at solids concentrations in the range from 0.1 to 5 per cent. Therefore, the stirred tank can then be modelled as a single-phase isothermal system, i.e. only the hydrodynamics of the reactor are simulated. At higher slurry densities, however, the interaction of the solids with the flow must be taken into account. 

Various reactor combinations are used. For example, the product from a relatively low solids batch-mass reactor may be transferred to a suspension reactor (for HIPS), press (for PS), or unagitated batch tower (for PS) for finishing. In a similar fashion, the effluent from a continuous stirred tank reactor (CSTR) may be transferred to a tubular reactor or an unagitated or agitated tower for further polymerization before devolatilization. 

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