Stirred tank reactors dissipation

The approach based on the energy dissipation rate is not limited to a particular type of slurry reactor. Therefore, equations of type 46 and 52 have also been proposed for stirred-tank reactors. For e the total energy dissipation rate originating from both gas and power inputs via the stirrer must then be used. Hence,... 

Chemical engineers intuitively work under the assumption that to improve the mass-transfer characteristics of a gas-liquid reactor, more energy must be dissipated in the fluids to effect a more vigorous contacting of the fluids. For singlephase flow in turbulent systems, this concept has become known as the Chilton-Colburn analogy. Another example of the coupling of hydrodynamics and mass transfer in multiphase systems is the common correlation of the mass transfer and the power input for stirred-tank reactors. 

Fig, 28. Comparison of gas/liquid interfacial area as a function of the energy dissipation rate (necessary power input) in sulphite oxidation systems, [ 1, stirred tank reactor S lReith(118)] 2,bubble column-porous plate G [authors] 3, bubble column-perforated plate S[Reith(118)J 4, ejector nozzle S [Nagel(il8)] 5, nacked bubble column S lNagel(117)) 6, bubble column-injector nozzle G authorsJ. G geometrical area ... 

Equation 6.10 is a definition of the Reynolds number based on power dissipation using a velocity term derived from dimensional arguments. This prompted Middleman (1965) to comment that derivation of Equation 6.11 using Kolmogorov s theory is a sophisticated form of dimensional analysis. Even with this oversimplification. Equation 6.10 still needs to be modified for gas-liquid mass transfer for which has a wide variation, and hence, an average is difficult to define. In view of this, most investigators resorted to correlating the volumetric mass transfer coefficient,. The correlations proposed for stirred tank reactors were therefore of the form (Hickman 1988 Middleton 2000)... 

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. 

The general equations for chemical reaction in a turbulent medium are easy to write if not to solve (2). In addition to those for velocities (U = U + uJ and concentrations (Cj = Cj + Cj), balance equations for q = A u, the segregation ( , and the dissipations e and eg can be written (3). Whatever the shape of the reactor under consideration (usually a tube or a stirred tank), the solution of these equations poses difficult problems of closure, as u S, 5 cj, cj, and also c cj, c Cj in the reaction terms have to be evaluated. The situation is even more complicated when the temperature and the density of the reacting mixture are also fluctuating. Partial solutions to this problem have been proposed. In the case of instantaneous reactions (t << Tg) the "e-quilibrium assumption" applies the mixed reactants are immediately converted and the apparent rate of reaction is simply that of the decrease of segregation, with Corrsin s time constant xs. For instance, with a stoichiometric proportion of reactants, the extent of reaction X is given by 1 - /T ( 2), a simple result which can also be found by application of the IEM model (see (33)). 

Figure 6. Interfacial area vs. energy dissipation rate (23) 1, dual-flow column 2, pipe flow 3, bubble column 4, stirred tank 5, bubble column with 2-phase nozzle 6, co-current packed bed 7, jet tube washer (2-phase nozzle) 8, tube reactor with...

The agitated thin-film reactor was an 0.08 m diameter wiped-film evaporator and was cooled by either convection or evaporation. Because of vacuum requirements for evaporation of the solvent, only convective cooling was utilized. Yields were found to be 10 to 15% higher than in the 250 L stirred tank. The increase in temperature affected the results far less than that which occurred in the stirred tank. Although no exact data on local energy dissipation rates in wiped-film evaporators was available for the unit, higher local energy dissipation rates occurred than in stirred tanks. 

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