Mass transfer phenomena during potassium carbonate catalysed carbon steam gasification reactions in a microbalance setup

Abstract

A description is given of the external mass transfer properties in a Cahn microbalance setup by means of naphthalene sublimation experiments. The functional relationship found can be expressed by: Sh = 1.2 + 1.2 Re 0.25 Sc 0.6 From studies of the potassium carbonate catalysed reaction of activated carbon with low pressure steam this relationship appears to be applicable up to temperatures of 1200 K. It is shown that both the reactor geometry and free convection have little influence on measured mass transfer coefficients at Reynolds numbers commonly applied in a microbalance. “Classical” flow rate variation tests in order to reveal external mass transfer limitations can lead to an incorrect interpretation of the mass transfer behaviour in a microbalance. It appears that carrier gas variations provide more reliable information. Experimentally determined effective diffusion coefficients within the carbon bed agree satisfactorily with theoretical values. It is finally shown that for carbon gasification reactions studied in a microbalance, the region of inter particle and external mass transfer limitations can be entered simultaneously.