Selective desorption of carbon dioxide from sewage sludge for in situ methane enrichment - Part II: Modelling and evaluation of experiments

In situ methane enrichment in anaerobic digestion of sewage sludge has been investigated by experiments and by modelling. Sludge from a full scale digester was fed to a pilot scale bubble column having 0.3 m diameter for preferential desorption of carbon dioxide. In this second part, a model describing the steady-state performance of the bubble column for desorption of carbon dioxide and methane has been developed. The column is assumed to operate in the homogeneous flow regime, and with respect to carbon dioxide and methane both phases are described by the axial dispersion model. The model treats the chemical reaction equilibrium between carbon dioxide and bicarbonate as being slow and the bicarbonate concentration as being constant. The model is correlated to previously reported experimental results to determine the liquid side mass transfer coefficient in each experiment. A simple power law model is used to correlate the mass transfer coefficient to the gas flow rate. In general, the model allows for a reasonable qualitative description of the behaviour of the bubble column performance but the quantitative agreement with experimental results is not satisfactory. It is believed, though that the main problem is actually not in the model but is related to experimental uncertainties concerning inlet concentrations of liquid phase methane and bicarbonate.