(6cq) Study of the Performance Characteristics of a Stirred Tank Reactor Suitable for Diffusion Controlled Liquid-Solid Catalytic Reactions

The aim of the present work is to develop high space-time liquid-solid batch stirred tank reactor suitable for conducting diffusion controlled liquid-solid catalytic and electrochemical reactions. To this end single and multi-layers of wire screens lining the wall of a square stirred tank were used as a reaction surface in view of their high specific area and turbulence promoting ability. The performance of the reactor is evaluated by measuring the rate of liquid-solid mass transfer at the screen surface using the electrochemical technique. The proposed reactor was used for removing heavy metals from wastewater solutions. Variable studied were:

1)      Impeller rotational speed

2)      Impeller geometry [ 45^o pitched blade turbine (axial flow impeller) and 90^o flat blade turbine (radial flow impeller)]

3)      Geometrical parameters of the screen such as mesh number and wire diameter

4)      Physical properties of the solution ( ρ, µ and Ɗ)

5)      Number of closely packed screens forming the reactions surface

The mass transfer data at a single screen of different mesh numbers were correlated by the eqauations, for the axial flow impeller the data for the conditions 1735 < Sc < 4032 ; 1510 < Re < 12133 ; 0.147 < (r_h/d_w) < 0.784 fit the equation:

Sh = 0.0022 Sc^0.5 Re^0.51 (r_h/d_w)^0.72

with an average deviation of ± 25%.

For the radial flow impeller the data for the conditions 1735 < Sc < 4032, 1510 <Re< 12133; 0.147 < (r_h/d_w) < 0.784 fit the equation:

Sh = 0.00779 Sc^0.5 Re^0.4 (r_h/d_w)^0.75

with an average deviation of ±21%.

The study revealed that the mass transfer coefficient increases with increasing impeller rotational speed. On the other hand, increasing screens mesh number, number of screens per stack reduces the mass transfer coefficient obtained. For a given set of conditions it was found that using radial flow impeller was found to produce higher rates of mass transfer than axial flow impellers. Also, the data showed that the rate of mass transfer at screens is much higher than that at the flat plate by a factor ranging from 1.09 to 16.25 depending on Re and screen geometry. The volumetric mass transfer coefficient obtained using screen electrode under optimum conditions (4 screens per array, 5 mesh number and Re =10763) could reach 46.14 times the value obtained using flat plate electrode. 

The proposed reactor is suitable for wastewater treatment, electroorganic synthesis, catalytic reactions. In this study, it was use to remove copper ions form acidified copper sulphate solution and the results showed that 54.71% removal of copper ions could be reached under certain conditions (time of electrolysis = 1 hour, initial concentration of 0.05M CuSO₄ and the operating limiting current value was obtained from the overall dimensionless correlations obtained using the standard ferricyanide system).