Teodoro Espinosa-Solares

Mixing performance induced by coaxial flat blade-helical ribbon impellers rotating at different speeds

The mixing performance (pumping, dispersion capabilities, power consumption) of a new dual impeller mixer composed of a disc turbine and a helical ribbon impeller mounted on the same axis but rotating at different speeds is investigated. The methodology is based on a blend of experimental measurements and 3D numerical simulations in the case of Newtonian and non-Newtonian shear-thinning fluids. It is shown that the dual impeller mixer outperforms the standard helical ribbon in terms of top-to-bottom pumping when the fluid rheology evolves during the process. The power consumption of this new mixer is also studied which allows to derive a generalized power curve.

Mixing Time in Rheologically Evolving Model Fluids by Hybrid Dual Mixing Systems

Mixing time experiments were performed using a hybrid dual mixing system, which included a helical ribbon impeller (HR) and either a Smith (ST) or Rushton turbine (RT). Xanthan gum solutions were used as rheologically evolving fluids to evaluate changes in mixing time under non-aerated and aerated conditions. The helical ribbon agitator and turbine of the hybrid dual mixing system was kept at a constant rotational speed ratio, N T /N HR =10. Experiments showed that performance of the hybrid mixing system was superior to that of the individual impellers. Flow properties and gassing conditions played an important role in mixing time. While mixing time was practically identical under non-gassed conditions for both the ST-HR and RT-HR mixing systems in low-viscosity fluids, differences up to 1000% were observed in high shear-thinning fluids. In these fluids, the RT-HR combination exhibited better performance than the ST-HR. In high-viscosity fluids, gassing enhanced mixing time particularly when the ST-HR hybrid system was used. Both mixing systems showed similar mixing times under the highest gassed condition evaluated in this work.

Power consumption of a dual turbine-helical ribbon impeller mixer in ungassed conditions

Abstract Ungassed power measurements in a dual coaxial mixer composed of a helical ribbon and a Rushton turbine were carried out in laminar mixing conditions for Newtonian and non-Newtonian shear thinning fluids. For the Newtonian case, the power draw constant Kp for the hybrid geometry was not the sum of the individual impellers. This was explained by considering the radial discharge flow in the turbine region as well as the top-to-bottom circulation pattern of the helical ribbon impeller. For the non-Newtonian fluids, the results showed that, at a given Reynolds number, power consumption decreases as the shear thinning behaviour increases. A dimensionless and unique representation of the power draw data was obtained by shifting the non-Newtonian power draw results to the Newtonian curve. This was carried out with a Ks function defined from the Kp(n) data. The predictions for Ks were found to be in good agreement with those obtained using the classical method of A.B. Metzner and R.E. Otto (AIChE J., 3 (1957) 3–10). It was observed that pseudoplasticity tends to shift the upper limit of the laminar region toward Reynolds numbers higher than 10.

Flow patterns in rheologically evolving model fluids produced by hybrid dual mixing systems

The flow patterns produced by two dual mixing systems composed of independently driven impellers were studied. The dual impellers included a turbine rotating at high speed (Rushton or Smith) and a slowly rotating helical ribbon agitator (HR). Visualizations and power input were used to evaluate mixing performance. The influence of the rotational speed ratio on the flow patterns was evaluated. For high shear-thinning fluids, N T /N HR modifies the flow patterns considerably. Three typical behaviors were found with shear thinning fluids: segregation of two principal flow patterns (N T /N HR 10), and a well-distributed flow pattern throughout the tank (N T /N HR =10). For low-viscosity fluids, the motionless HR reduced the vortex length and the T-HR systems eliminated vortex when the impellers rotated in opposite directions at N T /N HR =10. Finally, a relationship between the dimensionless vortex length and the Froude number is proposed for individual turbines as well as for the turbine-motionless HR systems.

Thermophilic anaerobic co-digestion of poultry litter and thin stillage.

The purpose of this study was to test whether the performance of a thermophilic CSTR digester that has been stabilized on poultry litter will be enhanced or diminished by the addition of thin stillage as co-substrate. Replicate laboratory digesters, derived from a stable pilot-scale digester, were operated with increasing ratios (w/w) of thin stillage/poultry litter feedstock. After a period of adaptation to 20% and 40% thin stillage, digester performance showed increases in biogas, percent methane and COD removal, as well as a decrease in volatile acids. Peak performance occurred with 60% thin stillage. However, 80% thin stillage caused significant reduction of performance, including declines of methanogenic activity and COD removal. In conclusion, supplementing the thermophilic digestion of poultry litter with thin stillage improved the bioenergy (methane) output, but thin stillage became inhibitory at high concentrations.

Macroscopic mass and energy balance of a pilot plant anaerobic bioreactor operated under thermophilic conditions

Intensive poultry production generates over 100,000 t of litter annually in West Virginia and 9×106 t nationwide. Current available technological alternatives based on thermophilic anaerobic digestion for residuals treatment are diverse. A modification of the typical continuous stirred tank reactor is a promising process being relatively stable and owing to its capability to manage considerable amounts of residuals at low operational cost. A 40-m3 pilot plant digester was used for performance evaluation considering energy input and methane production. Results suggest some changes to the pilot plant configuration are necessary to reduce power consumption although maximizing biodigester performance.

Thermophilic Anaerobic Digester Performance Under Different Feed-Loading Frequency

The effect of feed-loading frequency on digester performance was studied on a thermophilic anaerobic digester with a working volume of 27.43 m3. The digester was fed 0.93 m3 of chicken-litter slurry/d, containing 50.9 g/L chemical oxygen demand. The treatments were loading frequencies of 1, 2, 6, and 12 times/d. The hourly pH, biogas production, and methane percent of the biogas were less stable at lower feed frequencies. There was no statistical difference among treatments in methanogenic activity. The feed-loading frequency of six times per day treatment provided the greatest biogas production.

Application of Anaerobic Digestion Model No. 1 to describe the syntrophic acetate oxidation of poultry litter in thermophilic anaerobic digestion

A molecular analysis found that poultry litter anaerobic digestion was dominated by hydrogenotrophic methanogens which suggests that bacterial acetate oxidation is the primary pathway in the thermophilic digestion of poultry litter. IWA Anaerobic Digestion Model No. 1 (ADM1) was modified to include the bacterial acetate oxidation process in the thermophilic anaerobic digestion (TAD). Two methods for ADM1 parameter estimation were applied: manual calibration with non-linear least squares (MC-NLLS) and an automatic calibration using differential evolution algorithms (DEA). In terms of kinetic parameters for acetate oxidizing bacteria, estimation by MC-NLLS and DEA were, respectively, km 1.12 and 3.25 ± 0.56 kg COD kg COD(-1)d(-1), KS 0.20 and 0.29 ± 0.018 kg COD m(-3) and Yac-st 0.14 and 0.10 ± 0.016 kg COD kg COD(-1). Experimental and predicted volatile fatty acids and biogas composition were in good agreement. Values of BIAS, MSE or INDEX demonstrate that both methods (MC-NLLS and DEA) increased ADM1 accuracy.

Gas Dispersion in Rheologically‐Evolving Model Fluids by Hybrid Dual Mixing Systems

Gas dispersion experiments (0.18≤Fr≤0.71, 0.02≤Fl≤0.09) were carried out using a hybrid dual mixing system, which included a helical ribbon impeller and either a Smith or a Rushton turbine. Newtonian and non-Newtonian model fluids were used as rheologically-evolving fluids to evaluate changes in gas dispersion performance. A motionless helical ribbon agitator was used as a baffle in low-viscosity Newtonian fluids. Both Smith and Rushton turbines produced a vortex, which was eliminated by the motionless helical ribbon impeller. Gas dispersion in low-viscosity fluids was enhanced when the helical ribbon agitator and turbine of the dual hybrid mixing system was kept at a rotational speed ratio of 10 (N T /N HR =10), which allowed dispersion at a lower Fr than the turbine alone. For moderate-viscosity Newtonian fluids, gas dispersion was achieved at Fr≤0.71 and Fl≤0.05. Flow properties of non-Newtonian fluids played an important role in gas dispersion; transition from dispersing to flooding stages was observed for the fluids that were more shear-thinning (n≤0.38).

Modeling temperature variations in a pilot plant thermophilic anaerobic digester

A model that predicts temperature changes in a pilot plant thermophilic anaerobic digester was developed based on fundamental thermodynamic laws. The methodology utilized two simulation strategies. In the first, model equations were solved through a searching routine based on a minimal square optimization criterion, from which the overall heat transfer coefficient values, for both biodigester and heat exchanger, were determined. In the second, the simulation was performed with variable values of these overall coefficients. The prediction with both strategies allowed reproducing experimental data within 5% of the temperature span permitted in the equipment by the system control, which validated the model. The temperature variation was affected by the heterogeneity of the feeding and extraction processes, by the heterogeneity of the digestate recirculation through the heating system and by the lack of a perfect mixing inside the biodigester tank. The use of variable overall heat transfer coefficients improved the temperature change prediction and reduced the effect of a non-ideal performance of the pilot plant modeled.