Solid–liquid mass transfer in relation to diffusion controlled corrosion at the outer surface of helical coils immersed in agitated vessels

The diffusion controlled dissolution of copper in acidified dichromate solution technique was used to study solid–liquid mass transfer at the outer surface of helical coils immersed in unbaffled agitated vessel. Variables studied were impeller rotation speed, impeller geometry (four blade 90° turbine and 45° pitched blade turbine), distance between coil rings (coil pitch) and physical properties of the solution. The data were correlated in the form of dimensionless equations, for four blade 90° turbine impeller, the data fit the equation:Sh=0.023 Sc0.33 Re0.59dcS0.49while for four blade 45° pitched turbine impeller, the data were correlated by the equation:Sh=0.13 Sc0.33 Re0.51The above equations were obtained for unbaffled vertical cylindrical vessel of flat bottom in the range: 4705 < Re < 52,750, 1104 < Sc < 1506 and 8.75 < (dc/S) < 17.5. The possibility of using the present correlations to predict the rate of diffusion controlled processes that may occur at the outer surface of such coils was highlighted.

► Solid–liquid mass transfer process at helical coil outer surface was investigated. ► The effect of coil pitch, impeller geometry and rotation speed were studied. ► Radial flow type impeller gave a higher rate of mass transfer than axial flow type. ► The coil pitch was found to have an inverse effect on the rate of corrosion. ► All variables were correlated by dimensionless equations that can be used in design.