CFD Approach for Non-Ideally Mixed Bioreactor Modeling

In this paper, we address the progress, challenges and prospect of modeling mixing in bioreactor by the utilization of Computational Fluid Dynamics (CFD). Efficient operation of bioreactor is essential in the biotechnological process, not only to ensure good yield but also to maintain consistent product quality. Process modeling technique has frequently been adopted in the bioreactor design, optimization and control as well as in scale-up process. One of the critical issues in bioreactor modeling is the need to overcome the mass and heat transfer limitations imposed on the bioreactor performance, particularly the close interaction between fluid flow and the biological reactions. CFD can be applied to overcome this obstacles by integrating the flow rates between adjacent zones, and fluid mechanical quantities, as the integration of mixing phenomena into the bioreactor modeling is considered a vital aspect in the efficient design and scale-up of bioreactor system

Experimental and CFD–PBM investigation of an agitated bioreactor using a dual helical ribbon impeller

Throughout past decades, the management of solid waste by producing methane gas, as a renewable source of energy, has featured as an important research objective. Anaerobic digesters are widely used in countries with environmental initiatives and green approaches, where biogas produced from a bioreactor is a carbon neutral source of energy. Biogas contains 70% methane, 30% CO2 and some other gases. The by-product of an anaerobic digester is solid sludge that can be used as either fertilizer or compost. Anaerobic digestion biogas plants can benefit industries by adding value to solid organic waste, reducing fossil fuel usage, eliminating solid waste disposal costs, in addition to generating power. Setting up an anaerobic digestion biogas plant is a green investment for industries interested in environmentally friendly biological processes. A variety of organic solid waste including municipal, industrial, livestock, poultry, meat, and food waste can be digested in an anaerobic system. To treat the large volume of waste generated by industries and urban sewerage systems, more efficient digesters and a continuous improvement of digestion processes are required. To accomplish these objectives, crucial factors including the size, design, and shape of a bioreactor, its working temperature, pH and the hydrodynamics of a system need to be studied. A considerable amount of literature has been published regarding the hydrodynamics of anaerobic digesters. Further, several studies have explored the factors thought to influence the hydrodynamics of anaerobic digesters. These studies have identified that the hydrodynamics of a system could be influenced by the rheological characteristics of sludge, as well as mixer type and shape. Inadequate and poor mixing in a digester can cause the failure of a reactor, non-uniform distribution of mass and heat, imbalanced microbial activity, as well as formation of sediment and scum. Although studies have successfully demonstrated that close-clearance mixers (screw, helical, anchor impellers) increase biogas production, the information about hydrodynamic characteristics and flow field generated by these types of agitators is inadequate. Although hydrodynamics and the rheology of sludge have been studied in the past, more research is required to address these gaps. The application of visual and measuring instruments could facilitate further research on sludge behaviour in an agitated anaerobic digester, but this type of study is not possible due to the opaque nature of real sludge. The main objectives of this project are (i) to find a safe, cheap, clear and stable material that can emulate digested sludge rheological characteristics in a laboratory; (ii) to study and optimize the mixing performance of a dual helical ribbon as an efficient impeller to create an ideal mixing pattern (iii) to investigate the flow pattern and hydrodynamics of a shear thinning fluid in a batch gas-liquid reactor using a combination of a computational fluid dynamics (CFD) simulation and a population balance model (PBM). Study 1 has analysed and compared the Zeta potential, pH resistance, flow curve, viscoelasticity, and thixotropy of four popular model fluids reported previously as ideal simulant of primary, activated, and digested sludge. The results of the correlational analysis indicate that xanthan gum is the best simulant to mimic the rheological characteristics of activated sludge that is sheared less than 100 S-1. There are similarities between the viscosity and flow curve of activated sludge and xanthan gum which can be described by its internal network and molecular structure. This study also compares rheological properties of 2% NaCMC solution and digested sludge containing 3.23% solid sheared between 10-300 S-1, concluding that they behave in an essentially identical manner. The findings from this study provide several contributions towards selecting and applying a clear and safe polymer that emulates the rheological behaviour of sludge. Study 2 has evaluated the performance of a dual helical ribbon impeller in agitating shear thinning fluid. The effects of impeller rotational speed, gas flow rate, clearance to the bottom, and viscosity on power uptake and mixing time have been studied. This study suggests that determining optimum operating conditions can minimize power consumption and time required to achieve the maximum volume of uniformity in reactor. Although the study successfully reports a significant positive correlation between the rotational speed of the impeller and the performance of mixing, there is still a threshold limit for rotational speed. Experimental data shows that power consumption would increase with rotational speed however increasing the rotational speed beyond the certain level does not affect the mixing time significantly. This study suggests two practical equations to estimate power consumption and mixing time under specific operating conditions by applying an ANOVA method. To cover some of the limitations related to the experimental study of hydrodynamics of gasliquid systems, a combination of computational fluid dynamics (CFD) simulation and population balance model (PBM) has been used in the third study. The main purpose of this work is to evaluate the impacts of using a dual helical ribbon on the hydrodynamics of a multiphase reactor. The governing equations and turbulent model of agitated bubbly flow have been solved through a standard k-e model and Eulerian-Eulerian (E-E) multiphase approach. Following grid sensitivity analyses, findings through simulation have been verified by PIV measuring tests. Further, the PBM model has been discretized into five bubble size groups. The results show a positive relationship between rotational speed and bubble breakage. The comparative study indicates an increase in the likelihood of bubble channeling when the rotational speed is insufficient to break the gel-like structure of the liquid. By increasing rotational speed, the bubble hits the blades, breaks, and disperses, leading to improved interfacial area between phases. Further, rotating mechanical blades induce shear stress to bulk of liquid, resulting in a significant drop in viscosity and diminishing the stagnant regions

On the design and simulation of an airlift loop bioreactor with microbubble generation by fluidic oscillation

Microbubble generation by a novel fluidic oscillator driven approach is analyzed, with a view to identifying the key design elements and their differences from standard approaches to airlift loop bioreactor design. The microbubble generation mechanism has been shown to achieve high mass transfer rates by the decrease of the bubble diameter, by hydrodynamic stabilization that avoids coalescence increasing the bubble diameter, and by longer residence times offsetting slower convection. The fluidic oscillator approach also decreases the friction losses in pipe networks and in nozzles/diffusers due to boundary layer disruption, so there is actually an energetic consumption savings in using this approach over steady flow. These dual advantages make the microbubble generation approach a promising component of a novel airlift loop bioreactor whose design is presented here. The equipment, control system for flow and temperature, and the optimization of the nozzle bank for the gas distribution system are presented. (C) 2009 The Institution of Chemical Engineers. Published by Elsevier B.V All rights reserved

Hydrodynamic Characterization of Physicochemical Process in Stirred Tanks and Agglomeration Reactors

A short review of the state of the art in experimental and computational fluid dynamics (CFD) characterization of micro-hydrodynamics and physicochemical processes in stirred tanks and agglomeration reactors is presented. Results of experimental and computational studies focusing on classical mixing tanks as well as other innovative reactors with various industrial applications are briefly reviewed. The hydrodynamic characterization techniques as well as the influence of the fluid dynamics on the efficiency of the physicochemical processes have been highlighted including some of the limitations of the reported modeling approach and solution strategy. Finally, the need for specialized CFD codes tailored to the specific needs of fluid-particle reactor design and optimization is advocated to advance research in this field

Improving the rheometry of rubberized bitumen: experimental and computation fluid dynamics studies

Multi-phase materials are common in several fields of engineering and rheological measurements are intensively adopted for their development and quality control. Unfortunately, due to the complexity of these materials, accurate measurements can be challenging. This is the case of bitumen-rubber blends used in civil engineering as binders for several applications such as asphalt concrete for road pavements but recently also for roofing membranes. These materials can be considered as heterogeneous blends of fluid and particles with different densities. Due to this nature the two components tends to separate and this phenomenon can be enhanced with inappropriate design and mixing. This is the reason behind the need of efficient dispersion and distribution during their manufacturing and it also explains while realtime viscosity measurements could provide misleading results. To overcome this problem, in a previous research effort, a Dual Helical Impeller (DHI) for a Brookfield viscometer was specifically designed, calibrated and manufactured. The DHI showed to provide a more stable trend of measurements and these were identified as being ‘‘more realistic” when compared with those obtained with standard concentric cylinder testing geometries, over a wide range of viscosities. However, a fundamental understanding of the reasons behind this improvement is lacking and this paper aims at filling these gaps. Hence, in this study a tailored experimental programme resembling the bitumen-rubber system together with a bespoke Computational Fluid Dynamics (CFD) model are used to provide insights into DHI applicability to perform viscosity measurements with multiphase fluids as well as to validate its empirical calibration procedure. A qualitative comparison between the laboratory results and CFD simulations proved encouraging and this was enhanced with quantitative estimations of the mixing efficiency of both systems. The results proved that CFD model is capable of simulating these systems and the obtained simulations gave insights into the flow fields created by the DHI. It is now clear that DHI uses its inner screw to create a vertical dragging of particles within a fluid of lower density, while the outer screw transports the suspended particles down. This induced flow helps keeping the test sample less heterogeneous and this in turns allows recording more stable viscosity measurements

Overview on the hydrodynamic conditions found in industrial systems and its impact in (bio)fouling formation

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cej.2021.129348.Biofouling is the unwanted accumulation of deposits on surfaces, composed by organic and inorganic particles and (micro)organisms. Its occurrence in industrial equipment is responsible for several drawbacks related to operation and maintenance costs, reduction of process safety and product quality, and putative outbreaks of pathogens. The understanding on the role of operating conditions in biofouling development highlights the hydrodynamic conditions as key parameter. In general, (bio)fouling occurs in a higher extension when laminar flow conditions are used. However, the characteristics and resilience of biofouling are highly dependent on the hydrodynamic conditions under which it is developed, with turbulent conditions being associated to recalcitrant biodeposits. In industrial settings like heat exchangers, fluid distribution networks and stirred tanks, hydrodynamics play a dual function, affecting the process effectiveness while favouring biofouling formation. This review summarizes the hydrodynamics played in conventional industrial settings and provides an overview on the relevance of hydrodynamic conditions in biofouling development as well as in the effectiveness of industrial processes.This work was financially supported by: Base Funding - UIDB/00511/2020 of LEPABE and UIDB/00081/2020 of CIQUP funded by national funds through the FCT/MCTES (PIDDAC); Project Bio cide_for_Biofilm - PTDC/BII-BTI/30219/2017 - POCI-01-0145-FEDER 030219, ABFISH – PTDC/ASP-PES/28397/2017 - POCI-01-0145- FEDER-028397 and ALGAVALOR - POCI-01-0247-FEDER-035234, fun ded by FEDER funds through COMPETE2020 – Programa Operacional Competitividade e Internacionalizaçao ˜ (POCI) and by national funds (PIDDAC) through FCT/MCTES; Portuguese Foundation for Science and Technology (FCT) under the scope of the strategic funding of UIDB/04469/2020 unit and BioTecNorte operation (NORTE-01-0145-FEDER 000004) funded by the European Regional Development Fund under the scope of Norte2020 - Programa Operacional Regional do Norte; FCT/ SFRH/BD/147276/2019 (Susana Fernandes) and SFRH/BSAB/150379/2019 (Manuel Simoes).info:eu-repo/semantics/publishedVersio

Computational Fluid Dynamics Analysis of Two-Phase Chemical and Biochemical Reactors

In this work, the numerical analysis of turbulent two-phase processes in stirred tanks and bioreactors is performed with a computational fluid dynamics (CFD) approach. The modelling of the turbulent two-phase phenomena is achieved in the context of the Reynolds Averaged Navier-Stokes (RANS) equations and the Two-Fluid Model (TFM). Different modelling strategies are studied, tested and developed to improve the prediction of mixing phenomena, interphase interactions and bio-chemical reactions in chemical and process equipment. The systems studied in this work are a dilute immiscible liquid-liquid dispersion and dense solid-liquid suspensions, both in stirred tanks of standard geometry, a gas-liquid system consisting of a dual impeller vortex ingesting fermenter for the production of biohydrogen, analyzed in two different configurations of the supports for the attached growth of biomass, and two different bioreactors, of different scale and configuration, subject to substrate concentration segregation. Purposely collected experimental data and data from the literature were extensively used to validate the numerical results and either confirmed the goodness of the models and the modelling techniques, helped the definition of the limits and the uncertainties of the model formulations or guided the development of new models. In all cases, particular attention was devoted to the precision of the numerical solution, and to the validation with experimental data to quantify the appropriateness of the models and the accuracy of the CFD predictions

Analysis of a mechanical mixer performance in anoxic reactor

Denitrification is a key process in wastewater treatment since it is responsible for the effective nutrient removal. It requires anoxic conditions, where only chemically bound nitrogen is used as an oxygen source, and no aeration is applied. In suspended biomass systems the growth and homogenization of biomass is essential, high degree of mixing is required, which is achieved only by using mechanical mixers. Mechanical mixing performance relies on the mixing power determined by the equipment dimensions and rotational speed. In this paper the effect of three different rotational speed (rpm: 100, 400, 900 min-1) on flow field and mixing conditions are evaluated. As a result of the simulations, the acceptable flow field was achieved at 400 rpm. The outcome of this research is that the high degree of energy transfer from mixers to fluid flow deteriorated mixing efficiency

Dual mechanism model for fluid particle breakup in the entire turbulent spectrum

This work provides an in-depth understanding of different breakup mechanisms for fluid particles in turbulent flows. All the disruptive and cohesive stresses are considered for the entire turbulent energy spectrum and their contributions to the breakup are evaluated. A new modeling framework is presented that bridges across turbulent subranges. The model entails different mechanisms for breakup by abandoning the classical limitation of inertial models. The predictions are validated with experiments encompassing both breakup regimes for droplets stabilized by internal viscosity and interfacial tension down to the micrometer length scale, which covers both the inertial and dissipation subranges. The model performance ensures the reliability of the framework, which involves different mechanisms. It retains the breakup rate for inertial models, improves the predictions for the transition region from inertia to dissipation, and bridges seamlessly to Kolmogorov-sized droplets