Enhancing Efficiency of Anaerobic Digestion by Optimization of Mixing Regimes Using Helical Ribbon Impeller

The appropriate mixing system and approach to effective management can provide favorable conditions for the highly sensitive microbial community, which can ensure process stability and efficiency in an anaerobic digester. In this study, the effect of mixing intensity on biogas production in a lab-scale anaerobic digester has been investigated experimentally and via modeling. Considering high mixing efficiency and unique feature of producing axial flow, helical ribbon (HR) impeller is used for mixing the slurry in this experiment under various conditions. Three parallel digesters were analyzed under identical operating conditions for comparative study and high accuracy. Effects of different mixing speeds (10, 30, and 67 rpm for 5 min h−1) on biogas production rate were determined in 5-L lab-scale digesters. The results demonstrated 15–18% higher biogas production at higher mixing speed (67 rpm) as compared to 10 rpm and 30 rpm and the results proved statistically significant (p < 0.05). Biogas production at 10, 30, and 67 rpm were 45.6, 48.6, and 52.5 L, respectively. Higher VFA concentrations (7.67 g L−1) were recorded at lower mixing intensity but there was no significant difference in pH and ammonia at different speeds whereas the better mixing efficiency at higher speeds was also the main reason for increase in biogas production. Furthermore, model simulation calculations revealed the reduction of dead zones and better homogeneous mixing at higher mixing speeds. Reduction of dead zones from 18% at 10 rpm to 2% at 67 rpm was observed, which can be the major factor in significant difference in biogas production rates at various mixing intensities. Optimization of digester and impeller geometry should be a prime focus to scale-up digesters and to optimize mixing in full-scale digesters.

Caking Phenomena During Pilot-Scale Crystallization of Dextrose Monohydrate

Dextrose Monohydrate (DMH) is a bulk chemical used in the food, beverage, and pharmaceutical industries. The caking often appeared in the crystallization of DMH. Caking is an agglomeration that can affect the product quality of DMH and is dependent on the type of impeller. This study aimed to determine the type of impeller to avoid the caking during the DMH crystallization and identify the DMH caking. The results showed that caking did not occur on the helical ribbon and anchor impeller, while caking appeared on the Rushton turbine impeller. Computational fluid mechanics (CFD) analysis showed that caking occurs due to uneven homogeneity of stirring. Fourier transform infrared (FTIR) and X-ray diffraction (XRD) studies showed that DMH caking and non-caking had the same peak pattern. Meanwhile, optical microscope and scanning electron microscope (SEM) analysis showed that the DMH caking seen agglomerate. Density analysis showed that DMH with caking was 1.257–1.350 kg/L, while the non-caking was 0.504–0.780 kg/L. Caking phenomena during the DMH crystallization can be avoided by using a helical ribbon and anchor impeller. FTIR and XRD analysis cannot be used to identify DMH caking products; meanwhile, optical microscope, SEM, and density analyzes can be used to identify DMH caking products.

Development of a Novel Helical-Ribbon Mixer Dryer for Conversion of Rural Slaughterhouse Wastes to an Organic Fertilizer and Implications in the Rural Circular Economy

Organic wastes of rural slaughterhouses in developing countries comprise of blood and undigested rumen contents harboring infectious microbial pathogens and having impermissible BOD5 and COD values. Previously we demonstrated valorization of blood and rumen contents through drying and conversion to an efficacious organic fertilizer which was free from infectious pathogens and heavy metals. Here we describe fabrication of a novel helical-ribbon mixer dryer for transition from the current small-scale household cooking to equipment-driven sustainable production. Blood and rumen digesta mixed in a 3:1 ratio, having initial moisture of 85%, were dried at 90–110 °C for 3–4 h to attain 15.6% final moisture-containing organic fertilizer. Energy consumption and moisture extraction rate were 49.4 MJ per batch and 18.9 kg h−1 respectively. Using this method, small abattoir owners could emerge as multi-product producers to enhance earnings while farmers could source the fertilizer locally for organic farming. The two activities can be complementary to each other and become a sustainable circular economy model. We applied a spreadsheet-based model for calculation of cash flow, breakeven point and conducted financial cost–benefit analysis on the projected operation of the dryer. Fertilizer production parallel with the meat trade should be profitable for slaughterhouse owners and farmers apart from generating local employment opportunities.

Which impeller should be chosen for efficient solid-liquid mixing in the laminar regime?

The vast majority of solid-liquid mixing studies have focused on high Reynolds number applications with configurations and impeller geometries adapted to this type of regime. However, the mixing of particles in a viscous fluid is an essential element of many contemporary industries. We used the CFD-DEM model previously developed in our group to investigate solid-liquid mixing with close-clearance impellers in the laminar regime of operation. We compared different geometries that is, the double helical ribbon, anchor, Paravisc$^{TM}$, and Maxblend$^{TM}$ impellers. We investigated the impact of fluid viscosity and compared the results with those obtained with the pitched blade turbine, a more commonly used impeller, based on power consumption for equivalent mixing states. This study highlights that the higher the viscosity of the fluid, the more interesting it is to use close-clearance impellers for their ability to generate a strong shear stress and a strong bulk flow in the entire vessel.

Modelling Mechanically Induced Non-Newtonian Flows to Improve the Energy Efficiency of Anaerobic Digesters

In this paper, a finite volume based computational fluid dynamics (CFD) model has been developed for investigating the mixing of non-Newtonian flows and operating conditions of an anaerobic digester. A CFD model using the multiple reference frame has been implemented in order to model the mixing in an anaerobic digester. Two different agitator designs have been implemented: a design currently used in a full-scale anaerobic mixing device, SCABA, and an alternative helical ribbon design. Lab-scale experiments have been conducted with these two mixing device designs using a water-glycerol mixture to replicate a slurry with total solids concentration of 7.5%, which have been used to validate the CFD model. The CFD model has then been scaled up in order to replicate a full-scale anaerobic digester under real operating parameters that is mechanically stirred with the SCABA design. The influence of the non-Newtonian behaviour has been investigated and found to be important for the power demand calculation. Furthermore, the other helical mixing device has been implemented at full scale and a case study comparing the two agitators has been performed; assessing the mixing capabilities and power consumption of the two designs. It was found that, for a total solids concentrations of 7.5%, the helical design could produce similar mixing capabilities as the SCABA design at a lower power consumption. Finally, the potential power savings of the more energy efficient helical design has been estimated if implemented across the whole of the United Kingdom (UK)/Austria.

Study of the Influence of Geometric Parameters in an Innovative Scraped Surface Heat Exchanger With Helical Ribbon

This work aims to characterize the hydrodynamic and thermal behaviors in an innovative scraped surface heat exchanger (SSHE) equipped with helical ribbon by means of the numerical simulation approach. In this study, the conservation equations of continuity, momentum, and energy in the laminar, steady-state and isothermal conditions are resolved using a specific computational fluid dynamics (CFD) code based on the 3D finite volume method. The effects of the gap between the exchanger wall and the tip of the ribbon, the ribbon width, and the number of turns in the ribbon on the hydrodynamic and thermal behaviors are studied. Varying the gap values leads to reveal an optimum value giving the highest heat transfer coefficient. Moreover, numerical results have shown that increasing the ribbon width improves the heat transfer. Furthermore, the influence of the number of turns is carried out for Reynolds number ratios (Rer/Rea) inferior and superior to 1. Results revealed that increasing the number of turns avoids the back-mixing phenomenon and thus improves the heat transfer. In this study, the establishment of correlation is determined with the introduction of dimensionless and geometrical groups to predict the heat transfer coefficient in SSHE.

On the Use of the Coaxial Cylinders Equivalence for the Measurement of Viscosity in Complex Non-Viscometric, Rotational Geometries

The rheology of macroscopic particle suspensions is relevant in many industrial applications, such as cement-based suspensions, synthetic and natural drilling fluids. Rheological measurements for these complex, heterogeneous systems are complicated by a double effect of particle size. On the one hand, the smallest characteristic length of the measuring geometry must be larger than the particle size. On the other hand, large particles are prone to sediment, thus calling for the use of rotational tools that are able to keep the suspension as homogeneous as possible. As a consequence, standard viscometric rotational rheometry cannot be used and complex flow geometries are to be implemented. In this way, however, the flow becomes non-viscometric, thus requiring the development of approximate methods to translate the torque vs. rotation speed raw data, which constitute the rheometer output, into viscosity vs. shear rate curves. In this work the Couette analogy methodology is used to establish the above equivalence in the case of two complex, commercial geometries, namely, a double helical ribbon tool and a square-shaped stirrer, which are recommended for the study of relatively large size suspensions. The methodology is based on the concept of the reduction of the complex geometry to an equivalent coaxial cylinder geometry, thus determining a quantitative correspondence between the non-standard situation and the well-known Couette-like conditions. The Couette analogy has been used first to determine the calibration constants of the non-standard geometry by using a Newtonian oil of known viscosity. The constants have been subsequently used to determine the viscosity curves of two non-Newtonian, shear thinning fluids, namely a homogeneous polymer solution and two heterogeneous concentrated suspensions. The results show that the procedure yields a good agreement between the viscosity curves obtained by the reduction method and those measured by a standard viscometric Couette geometry. The calibration constants obtained in this work from the coaxial cylinder analogy are also compared with those provided by the manufacturer, indicating that the calibration can improve the accuracy of the rheometer output.