Solid-Solid Reactions in Fluidized Bed
Advancing the chemical engineering fundamentals
Particulate Systems (T2-3P)
Keywords: Solid-Solid Reactions, Fluidization, Sintering, Factorial design
Solid-Solid Reactions in Fluidized Bed
J.S.N.Murthy*, V.Surendar Reddy, T.Sankarshana
University College of Technology, Osmania University, Hyderabad-500007, India.
Although solid- solid reactions are known to play an important role in industries, such as those associated with the manufacture of ferrites, semiconductors, ceramics, dry cells, cement production, catalyst preparation, fertilizers, drugs, metallurgy etc. have not received significant attention in chemical engineering literature. The usual manufacturing process involves mixing of the component solid reactants in fine powdered form and firing at high temperature to obtain the desired products.
The factors that affect the solid-solid reactions are rate of heat transfer, rate of diffusion and reaction kinetics. In conventional methods, the rate of heat transfer and rate of diffusion are very slow since there is no mixing. In order to overcome these problems and to reduce the time of reaction to obtain a desired conversion, an attempt is made to investigate these reactions employing fluidized bed technology.
The addition reaction of Phthalic anhydride and p-Nitro aniline system forming phthaloyl derivative of p-Nitro aniline taking place between 80-120oC is selected for investigations. To conduct the reaction, the reactants are ground to fine powders and mixed. To overcome the difficulty in fluidization of these cohesive types of powders, they are mixed with sand (an inert material) in 1:3 proportion and stirring is also provided for proper mixing and smooth fluidization and also to improve rate of heat transfer.
The hydrodynamic studies of fluidization of the mixed powder with stirring are first carried out in a glass column of 0.1m diameter and 1m height at atmospheric conditions. The reaction studies are conducted in a stainless steel column of the similar dimensions. Heating of the reactants is carried by sending hot air at the required temperature. The effect of variables viz. reaction temperature, stirrer speed and flow rate of fluidizing medium on maximum conversion, time for maximum conversion, rate constant and activation energy is studied.
The conventional single factor approach requires a large number of experiments to analyze the experimental data. Hence, the experiments are performed based on statistically designed factorial experiments to analyze the data and formulate the required mathematical equations for the desired response variables.
Development of model equations is done in two steps: 1. Modeling of the solid-solid reaction between phthalic anhydride and p-nitro aniline. 2. Development of relationship between kinetic parameters and the operating parameters.
Among the models proposed in the literature for solid-solid reactions for various mechanisms, Jander’s model under diffusion controlled mechanism given by the equation: [1-(1-X) 1/3]2 = k t, where X is the conversion, t is time and k is rate constant is found to be suitable.
It was observed that the time of reaction is reduced considerably and activation energy is reduced by about 14 times compared to that reported in literature
The equations are then formulated as per the factorial design analysis for the prediction of rate constant, activation energy, maximum conversion and time for maximum conversion in terms of operating variables.
The results predicted by factorial design analysis are compared with results obtained from experiments under different conditions other than those used for the development of equations and found to agree within + 10%. It is found that considerably less time for reaction and activation energy are required in a fluidized bed compared to that of conventional approach to obtain desired conversion for the reaction. These are attributed to good mixing and rate of heat transfer in addition to prevention of sintering in fluidization operation.
Presented Monday 17, 13:30 to 15:00, in session Particulate Systems (T2-3P).
