ENHANCEMENT OF MASS TRANSFER USING ROTATING CYLINDER PROMOTERS

 The aim of present work is to determine experimentally the degree of mass transfer enhancement of dissolved oxygen using turbulent promoters of longitudinal leg extensions constructed on Rotating Cylinder Electrode (RCE) made of brass.The limiting current density (LCD) was evaluated under turbulent flow conditions at three different temperatures 35, 45, and 55 C. The experimental runs were carried out in 0.1 N NaCl salt solution of pH = 6 using a rotational velocity range from 200 – 1000 rpm. The effects of hydrogen evolution on LCD at pH 5, 6, and 7 were also investigated.  Two types of rotating cylinder made of brass, were examined : an enhanced cylinder one, with four leg  rectangular extensions 10 mm long, 10 mm wide, and 1mm thick, and  an enhanced cylinder two with four extensions 30 mm long, 10 mm wide, and 1mm thick. and pH is almost horizontal,showing that the effect of hydrogen evolution in this pH range on i_l'The results showed that the relation between LCD, i_l  values is negligible when compared with velocity and temperature. A better performance was obtained using enhanced cylinder two. The difference between enhanced cylinder one and two is within the accuracy of experimental reproducibility of results. The mass transfer enhancement as compared with a normal rotating cylinder electrode,devoid of promoters, is 53% or 58% higher. The enhancement percentage decreased as rotational velocity increased further, since, seemingly, optimum turbulence has been reached practically by means of rotation in presence of turbulence promoters. Also, enhancement percentage decreased with increasing temperature. As temperature increased mass transfer coefficient increased on both normal and enhanced cylinder electrodes in spite of the fact that solubility of dissolved oxygen decreased. Thus the effect of extensions
acting as turbulent promoters was diminished leading to mass transfer coefficient values to be approximately close to each other and enhancement percentage being reduced. The effect of extensions length on the enhancement was found little, i.e., limited under present conditions. Also, mass transfer due to temperature rise surpassed the effect of reduced oxygen solubility.