Gas-lift digester configuration effects on mixing effectiveness

Computational fluid dynamics simulations were used to study the effect of bottom configuration and a hanging baffle on the mixing inside a gas-lift digester filled with non-Newtonian sludge. The Navier–Stokes and continuity equations were solved numerically using commercially available finite element method-based solver. The results from this simulation were found to be in good agreement with previously reported experimental findings. At a gas recirculation rate of 84.96 l/h, the poorly mixed zones inside a flat bottom digester were about 33.6% of the digester volume, while in the case of digesters with 25° and 45° conical bottoms poorly mixed zones were about 31.9% and 29.6%, respectively. The power law viscosity index, n, did not have a significant effect on the mixing pattern under the conditions studied. Introduction of a hanging baffle in combination with a 45° hopper bottom resulted in reduction of the poorly mixed zone by a factor of three compared to a flat bottom without baffle configuration. Although the introduction of a hanging baffle was able to significantly reduce the size of the poorly mixed zones inside a gas-lift digester, further optimization of the digester geometry may lead to additional improvements.