Agitated reactors mass transfer coefficient

This research used mechanically agitated tank reactor system shown in Fig. 1. The reactor, 102 mm in diameter and 165 mm in height, was made of transparant pyrex glass and was equipped with four baffles, 120 mm in length and 8 mm in width, and six blades disc turbine impeller 45 mm in diameter and 12 mm in width. The impeller was rotated by electric motor with digital impeller rotation speed indicator. Waterbath thermostatic, equipped with temperature controller was used to stabilize reactor temperature. Gas-liquid mass transfer coefficient kia was determined using dynamic oxygenation method as has been used by Suprapto et al. [11]. 

In addition to the parameters involved in a bubble-column reactor, that is, g, a, or a-, and mass transfer coefficients, the required power input, Pj, is a design parameter for an agitated tank. 

Fig. 45.5 Dependence of the volumetric mass transfer coefficient with the speed of agitation for correlations of Eqs. (42) and (43) and comparison with experimental data for (a) a 25 cm3 tank reactor equipped with a four-blade impeller and (b) a 300-cm3 tank reactor equipped with a four-blade gas-inducing turbine.

If the process is continuous and in the complete mixed-flow mode, for both the gas and slurry phases, the equations derived for agitated sluny reactors are valid (see Section 3.5.1) (Ramachandran and Chaudhari, 1980) by simply applying the appropriate mass transfer coefficients. Note that in sluiiy-agitated reactors, the material balances are based on the volume of the bubble-free liquid. Furthermore, in reactions of the form aA(g) + B(l) — products, if gas phase concentration of A is constant, the same treatment holds for the plug flow of the gas phase. 

The fu st step in the solution of the example is the design of the agitated tank, which will determine its hydraulic parameters and thus the mass transfer coefficients, which are an input in the model of the reactor. 

For example, Beltran and Alvarez (1996) successfully applied a semi-batch agitated cell for the determination of kL k,a, and the rate constants of synthetic dyes, which react very fast with molecular ozone (direct reaction, kD = 5 105 to 1 108 L mol-1 s l). In conventional stirred tank reactors operated in the semi-batch mode the mass transfer coefficient for ozone kLa(03) was determined from an instantaneous reaction of ozone and 4-nitrophenol (Beltran et al., 1992 a) as well as ozone and resorchinol (l,3-c//hydroxybenzene) or phloroglucinol... 

M. Bouaifi, M. Roustan, Bubble size and mass transfer coefficients in dual-impeller agitated reactors, Can. J. Chem. Eng. 76 (1998) 390-397. 

Ridgway D, Sharma RN, Eanlay TR. Determination of mass transfer coefficients in agitated gas liquid reactors by instantaneous reactions. Chem Eng Sci 1989 44 2935-2942. 

The liquid-phase mixing in a multistage mechanically agitated reactor is best correlated by Eq. (2.31) in the absence of gas flow and by Eq. (2.32) in the presence of gas flow. The mixing time can be estimated from the study of Paca et al. (1976). Experimental work is needed to estimate gas-phase back-mixing. The use of Eq. (2.36) for the calculation of the gas-liquid volumetric mass transfer coefficient in a multistage mechanically agitated column is recommended. 

In some cases, a slurry reactor with multiple agitation is used. For example, Bern et al. (1976) used the reactor shown in Fig. 15 for the hydrogenation of oils. In this reactor type, horizontal partitions are also introduced at various stages to reduce the extent of backmixing. These authors proposed the following correlation for the gas-liquid mass transfer coefficient, kLaL, in this type of reactor based on pilot-plant data (30 and 500 L capacity) ... 

For a conventional mechanically agitated biological reactor, the information provided for aqueous gas-liquid and gas-liquid-solid systems in Sections II, III, and VII is applicable here. For power consumption, the most noteworthy works are those by Hughmark (1980) (see Eqs. (6.15) and (6.16)) and Schiigerl (1981). For gas-liquid mass transfer, the relationship kLaL = (P/V, ug) is applicable for biological systems. The relationships (6.19) and (6.20) are also valuable, and their use is recommended. The most generalized relation for kLaL is provided by Eq. (6.18). The intrinsic gas-liquid mass transfer coefficient is best estimated by Eq. (6.23). For liquid-solid mass transfer, the use of the study by Calderbank and Moo-Young (1961) (Eqs. (6.24)-(6.26)) is recommended. For viscous fluids, Eq. (6.27) should be used. 

An interesting study of the gas liquid mass transfer in a three-phase agitated slurry reactor was recently reported by Joosten et al.51 They showed that in the absence of solids, the volumetric mass-transfer coefficient can be well correlated to total power (power dissipated by stirrer + gas) per unit volume, but poorly correlated to the power dissipated by the stirrer only, as done in Fig. 9-14. Their data were well correlated by the correlation of Van Dierendock.23... 

Any form of convection, of course, increases the value of Ks. In slurry operation with no liquid flow, gas flow induces convection. In an agitated slurry reactor, stirring causes convection. In a pulsating slurry reactor, pulsation of the slurry induces convection and in a three-phase fluidized bed, the movements of both gas and liquid phases cause convection. Any one or more modes of convection will increase the value of the solid-liquid mass-transfer coefficient. In broad terms, the convective liquid-solid mass-transfer coefficient is correlated by-two steady state theories. Here we briefly review and compare them. 

In this section we consider the rate of absorption of gases into liquids that are agitated so that dissolved gas is transported from the interfacial surface to the interior by convective motion. The next section, based on this one, treats chemical methods for determining interfacial areas and mass-transfer coefficients in agitated gas-liquid reactors. 

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