Mass transfer coefficient stirrer speed effect

Internal recycle reactors are designed so that the relative velocity between the catalyst and the fluid phase is increased without increasing the overall feed and outlet flow rates. This facilitates the interphase heat and mass transfer rates. A typical internal flow recycle stirred reactor design proposed by Berty (1974, 1979) is shown in Fig. 18. This type of reactor is ideally suited for laboratory kinetic studies. The reactor, however, works better at higher pressure than at lower pressure. The other types of internal recycle reactors that can be effectively used for gas-liquid-solid reactions are those with a fixed bed of catalyst in a basket placed at the wall or at the center. Brown (1969) showed that imperfect mixing and heat and mass transfer effects are absent above a stirrer speed of about 2,000 rpm. Some important features of internal recycle reactors are listed in Table XII. The information on gas-liquid and liquid-solid mass transfer coefficients in these reactors is rather limited, and more work in this area is necessary. 

Complete suspension or complete oiF-bottom suspension. All the particles are suspended off the tank bottom or do not stay on the bottom more than 1 or 2 s. When this condition is just reached, there will generally be concentration gradients in the suspension and there may be a region of clear liquid near the top of the tank. The gradient in solid concentration will have little effect on the performance of the unit as a dissolver or a chemical reactor, and the mass-transfer coefficient will not increase very much with further increases in stirrer speed. 

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