Modeling reactor-focused energy balances for multi-stage CSTR anaerobic digestion systems

2020 
This paper encompasses the development of energy balance models to determine temperature dynamic behavior in single- and two-stage anaerobic digestion systems. Modeling assumptions were made according to the reactor type, its operation, energy exchanges, thermodynamics, and kinetics, as well as involved processes, substances, and phases. Stirring, heating, stream enthalpies, and reaction energies from biochemical processes were taken as the main external inputs. The temperature effect on specific growth constants was determined with a cardinal model. A general energy balance was obtained for a non-adiabatic, non-isothermic, triphasic single stage continuous stirred tank reactor digester, and then it was adapted to the case of a two-stage biohydrogen and biomethane mass balance model to obtain a multi-stage energy balance. The model was implemented and simulated in Modelica, then compared with data from a real-life experiment consisting of the digestion of a 1% glucose solution. Concentration and temperature evolution in both stages were analyzed. Temperature changes due to reaction enthalpies were observed mainly in the first stage, hydrolysis being the predominant process at the startup, followed by sugar acidogenesis. The main mechanics of the proposed model were demonstrated, and a reasonable approximation of the expected results was obtained.
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