Stirred Tanks Research Articles

CFD simulations of the transition between non-aerated and aerated conditions in uncovered unbaffled stirred tanks

The transition between non-aerated and aerated regimes in uncovered unbaffled stirred tanks (UUSTs) was investigated using computational fluid dynamics (CFD). The Volume of Fluid (VOF) method was employed to model the free surface dynamics under various operational conditions. The simulations were able to predict the peculiar experimentally observed behavior of UUSTs and revealed that at velocities below a critical threshold (N_<_Ncrit), the system remains non-aerated, while exceeding this threshold (N_≥_Ncrit) induces bubble ingestion, leading to significant changes in power consumption and flow patterns. The CFD simulations accurately predicted the behaviour of the Power Number (Np) as well as the vortex shape inside the tank both in subcritical and supercritical regimes and showed good agreement with original experimental data and correlations from the literature. Additionally, the modeling of the aerated regime successfully predicted the vortex shape, the bubble dispersion within the tank, and the cavities formed behind the blades.

Application of continuous stirring tank reactor for controllable synthesis of Cu7S4 nanocrystals

As a typical model for a chemical reactor in industry, the continuous stirring tank reactor has been widely used in waste-water treatment, industrial catalysis, and biological fermentation as well as great application prospects in the synthesis of nanomaterials. In this report, a convenient lab-scale continuous stirring tank reactor was assembled and utilized to synthesize Cu7S4 nanocrystals by injecting cupric bromide and sulfur solution in the presence of ascorbic acid and polyethyleneimine at 60 °C. The morphology control of Cu7S4 nanocrystals was accomplished by simply adjusting the agitation speed. Cu7S4 nanofibers with an average diameter of 48 ± 4.42 nm and a length of several micrometers were obtained at 80 rpm. Cu7S4 nanoplates with a thickness of 89 ± 13.56 nm and a plane size of 103 ± 16.47 nm were synthesized at 90 rpm, and the size of the nanoplates was regulated by continuously increasing the agitation speed from 90 rpm to 1000 rpm. Furthermore, the influences of two additional conditions (the mean residence time and the concentration of the feed solution) on the morphology of Cu7S4 nanocrystals, were also investigated. Therefore, these results could facilitate the understanding of the behaviors of CSTR in the synthesis of Cu7S4 nanocrystals and could also provide an important reference for the continuous synthesis of other nanoparticles.

Using CFD and machine learning to explore chaotic mixing in laminar diameter-transformed stirred tanks

Elimination of segregated zones is important to intensify the global chaotic mixing of high-viscosity fluids. Diameter-transformed stirred tank (DTST) reactor as a new process-intensification equipment is proposed. Through the PLIF technique, tracer transport CFD modeling, and Poincaré sections, we investigate the mixing structures inside a DTST reactor, and find that the DTST reactor can trigger global chaotic mixing. This is due to the wall squeezing effect, causing the fluid particle in periodic motion to become disordered. From the perspective of mixing kinetics, an area coverage model A=Amax1-e-Eamt is developed to quantitatively characterize its mixing efficiency, and the maximum area coverage Amax is 98.12 %. In addition, to help practitioners quickly select a suitable DTST reactor under different working conditions, machine learning-based flow pattern prediction models are proposed, among the ANNc model has a highest accuracy of 97.6 %. The decision boundary can be expressed by the Rec-E discriminant lg(Rec)=2.3(E-0.95)0.18-1.

Mitigation of vibration and free-surface instabilities by using anti-vortex baffles in an industrial stirred vessel

Inadequate understanding of mixing dynamics can result in compromised product quality, increased production expenses and revenue loss. This study utilizes vibrometer measurements in both baffled and unbaffled eccentric flat-bottomed stirring vessels, containing a trisodium phosphate–water mixture, to investigate the impact of baffles on reducing vibrations. Through examining the effect of different configurations of baffles, this research effectively eliminates vortices and vibrations generated during agitation, consequently reducing maintenance frequency. Additionally, computational fluid dynamics (CFD) analysis is conducted on baffled and unbaffled stirred tanks, revealing streamlined patterns and mean velocities. By solving the Reynolds-averaged Navier–Stokes (RANS) equations with the k–[Formula: see text] turbulence model, obtained results reveal the importance of baffles in reducing vibrations and vortices while stabilizing the free-surface. However, balancing the number of baffles with impeller blades is crucial to prevent vibration regeneration.

Dynamics of pH Oscillators in Continuous Stirred Tanks in Series.

Complex reaction networks with positive and negative feedback can produce diverse nonlinear phenomena in open reactors, such as multistability and oscillations. pH oscillators driven by hydrogen or hydroxide autocatalytic processes show sustained oscillations in continuously stirred tank reactors (CSTR) but only a sharp pH switch in batch. Here, we present a numerical study on the dynamics of pH oscillators in a series of CSTRs. We show a critical residence time under which bistability and above which oscillations develop. The dynamics of the CSTR cascade show the cross-shaped phase diagram of nonlinear activatory inhibitory systems. In the domain of oscillations, one reactor starts to oscillate autonomously and induces forced complex oscillations in the following tanks with damped amplitudes. These results, with their practical implications, may contribute to understanding the recent experimental observations of nonlinear phenomena in the presence of a residence time ramp and inspire further research in this area.

Discrete Element Method–Volume of Fluid Simulation of Drawdown and Dispersion for Floating Particles in Stirred Tanks: Influences of Impeller Parameters

Discrete Element Method–Volume of Fluid Simulation of Drawdown and Dispersion for Floating Particles in Stirred Tanks: Influences of Impeller Parameters

Sacral neuromodulation device biofilm differs in the absence and presence of infection, harbors antibiotic resistance genes, and is reproducible in vitro.

Sacral neuromodulation (SNM) is effective therapy for overactive bladder refractory to oral therapies, and non-obstructive urinary retention. A subset of SNM devices is associated with infection requiring surgical removal. We sought to compare microbial compositions of explanted devices in the presence and absence of infection, by testing phase, and other clinical factors, and to investigate antibiotic resistance genes present in the biofilms. We analyzed resistance genes to antibiotics used in commercially-available anti-infective device coating/pouch formulations. We further sought to assess biofilm reconstitution by material type and microbial strain in vitro using a continuous-flow stir tank bioreactor, which mimics human tissue with an indwelling device. We hypothesized that SNM device biofilms would differ in composition by infection status, and genes encoding resistance to rifampin and minocycline would be frequently detected. Patients scheduled to undergo removal or revision of SNM devices were consented per IRB-approved protocol (IRB 20-415). Devices were swabbed intraoperatively upon exposure, with controls and precautions to reduce contamination of the surrounding field. Samples and controls were analyzed with next-generation sequencing and RT-PCR, metabolomics, and culture-based approaches. Associations between microbial diversity or microbial abundance, and clinical variables were then analyzed using t-tests and ANOVA. Reconstituted biofilm deposition in vitro using the bioreactor was compared by microbial strain and material type using plate-based assays and scanning electron microscopy. Thirty seven devices were analyzed, all of which harbored detectable microbiota. Proteobacteria, Firmicutes and Actinobacteriota were the most common phyla present overall. Beta-diversity differed in the presence versus absence of infection (p = 0.014). Total abundance, based on normalized microbial counts, differed by testing phase (p < 0.001), indication for placement (p = 0.02), diabetes mellitus (p < 0.001), cardiac disease (p = 0.008) and history of UTI (p = 0.008). Significant microbe-metabolite interaction networks were identified overall and in the absence of infection. 24% of biofilms harbored the tetA tetracycline/minocycline resistance gene and 53% harbored the rpoB rifampin resistance gene. Biofilm was reconstituted across tested strains and material types. Ceramic and titanium did not differ in biofilm deposition for any tested strain. All analyzed SNM devices harbored microbiota. Device biofilm composition differed in the presence and absence of infection and by testing phase. Antibiotic resistance genes including to rifampin and tetracycline/minocycline, which are used in commercially-available anti-infective pouches, were frequently detected. Isolated organisms from SNM devices demonstrated the ability to reconstitute biofilm formation in vitro. Biofilm deposition was similar between ceramic and titanium, materials used in commercially-available SNM device casings. The findings and techniques used in this study together provide the basis for the investigation of the next generation of device materials and coatings, which may employ novel alternatives to traditional antibiotics. Such alternatives might include bacterial competition, quorum-sensing modulation, or antiseptic application, which could reduce infection risk without significantly selecting for antibiotic resistance.

Cultivation of a Novel Strain of Chlorella vulgaris S2 under Phototrophic, Mixotrophic, and Heterotrophic Conditions, and Effects on Biomass Growth and Composition

Microalgal biomass is an excellent platform for producing food, feed, nutraceuticals, pharmaceuticals, and biofuels. This study aimed to investigate the effect of the trophic mode of cultivation (phototrophic, heterotrophic, and mixotrophic) on the growth and biomass composition of Chlorella vulgaris S2. The contents of lipids and carbohydrates, as well as the fatty acid composition of total lipids, were studied. The effects of the carbon-to-nitrogen ratio (C:N) and the organic carbon concentration of the growth media under mixotrophic and heterotrophic conditions were also investigated. The C:N ratio of 30 mol mol−1 favoured lipid synthesis, and the C:N ratio of 10 mol mol−1 favoured carbohydrate synthesis. Maximal lipid and biomass productivities (2.238 and 0.458 g L−1 d−1, respectively) were obtained under mixotrophic conditions at the C:N ratio of 50 mol mol−1 and glucose concentration of 50 g L−1. Fed-batch cultivation conducted in a stirrer tank bioreactor under heterotrophic growth conditions increased biomass (2.385 g L−1 d−1, respectively) and lipid (0.339 L−1 d−1) productivities ~50 and ~60 times compared to the fed-batch phototrophic cultivation, respectively. The trophic mode, growth phase, and growth medium composition significantly influenced the fatty acid composition. Under mixotrophic and heterotrophic growth conditions, lipid accumulation is associated with an increase in oleic acid (C18:1) content. Mixotrophically grown biomass of Chlorella vulgaris S2 under optimised conditions is a suitable source of lipids for biodiesel production.

Feeding frequency efficacy on biogas yield of oily substrate anaerobic digestion in continuous stir tank reactor.

Anaerobic treatment of oily substrate, known as grease trap waste (GTW), was investigated for its practicability via continuous stirred tank reactor (CSTR) at different operating conditions and selected recovery strategies of feeding frequency efficacy. This study determine the performance of feeding frequency efficacy, namely feeding every 24 hours (R24H) and feeding every 12 hours (R12H). Under organic loading rate (OLR) of 2.2 gCOD/L.day, R12H exhibited methane composition of 57%, methane production rate of 0.27 LCH4/L.day, and methane yield of 0.14 LCH4/gCODremoved. At the same OLR, R24H recorded methane composition of 60%, methane production rate of 0.29 LCH4/L.day and similar methane yield as R12H. Findings indicated that R24H showed performance comparable to that of R12H. Given minor variation observed in performance, it is recommended that plant operators may consider scheduling two feedings per day for low loading conditions and switch to one feeding per day for higher loading conditions. This strategy is designed to balance the system and prevent shock loads, which could lead to plant shutdowns. This mechanism will induce their conversion to volatile fatty acids (VFAs); thus, reducing the risk of acid accumulation and pH drops, which could inhibit methanogens to produce methane, especially for oily substrate.

Polymeric surfactants with high acid values for emulsion type pressure-sensitive adhesives

Low molecular weight polymers with hydrophobic-carboxyl groups have been usually used as anionic surfactants in emulsion polymerization for coatings. However, such polymeric surfactants usually have low acid values below 200, because it is difficult to make high acid-value polymeric surfactants using a normal batch reactor. In this work, we polymerized high acid value polymeric surfactant having high acid values successfully up to 380 using CSTR (continuous stirring tank reactor). The synthesized polymeric surfactant was a random copolymer of P(BA-AA) and we applied them as anionic surfactants in making emulsion-type acrylic PSAs (Pressure-Sensitive Adhesives) to investigate the effects of high acid values on the adhesive properties. This CSTR can effectively control the composition and molecular weight of polymeric surfactants through varying monomer ratios, retention time, and initiator concentrations. The surface tension analysis and emulsion stability test indicated that the synthesized polymeric surfactants exhibited a similar levels of critical micelle concentration (CMC) and emulsion stability with other conventional anionic surfactants. Also, the adhesion property analysis showed that the PSA used high acid value polymeric surfactant resulting in superior adhesive properties and heat resistance than the PSAs used low acid value polymeric surfactant or conventional low molecular weight surfactant. It was very notable that polymeric surfactants having unusually high acid values above 300 because of their high contents of carboxylic acid groups could result in superior adhesive properties and heat resistance.

In-situ methane enrichment in anaerobic digestion of food waste slurry by nano zero-valent iron: Long-term performance and microbial community succession

In this work, nano zero-valent iron (nZVI) was added to a lab-scale continuous stirring tank reactor (CSTR) for food waste slurry treatment, and the effect of dosing rate and dosage of nZVI were attempted to be changed. The results showed that anaerobic digestion (AD) efficiency and biomethanation stability were optimum under the daily dosing and dosage of 0.48 g/gTCOD. The average daily methane (CH4) yield reached 495.38 mL/gTCOD, which was 43.65% higher than that at control stage, and the maximum CH4 content reached 95%. However, under single dosing rate conditions, high nZVI concentrations caused microbial cell rupture and loosely bound extracellular polymeric substances (LB-EPS) precipitation degradation. The daily dosing rate promoted the hydrogenotrophic methanogenesis pathway, and the activity of coenzyme F420 increased by 400.29%. The microbial analysis indicated that daily addition of nZVI could promote the growth of acid-producing bacteria (Firmicutes and Bacteroidetes) and methanogens (Methanothrix).

Fluid–Solid Mixing Transfer Mechanism and Flow Patterns of the Double-Layered Impeller Stirring Tank by the CFD-DEM Method

The optimization design of the double-layered material tank is essential to improve the material mixing efficiency and quality in chemical engineering and lithium battery production. The draft tube structure and double-layered impellers affect the flow patterns of the fluid–solid transfer process, and its flow pattern recognition faces significant challenges. This paper presents a fluid–solid mixing transfer modeling method using the CFD-DEM coupling solution method to analyze flow pattern evolution regularities. A porous-based interphase coupling technology solved the interphase force and could be used to acquire accurate particle motion trajectories. The effect mechanism of fluid–solid transfer courses in the double-layered mixing tank with a draft tube can be obtained by analyzing key features, including velocity distribution, circulation flows, power, and particle characteristics. The research results illustrate that the draft tube structure creates two major circulations in the mixing transfer process and changes particle and vortex flow patterns. The circulating motion of the double-layered impellers strengthens the overall fluid circulation, enhances the overall mixing efficiency of the fluid medium, and reduces particle deposition. Numerical results can offer technical guidance for the chemical extraction course and lithium battery slurry mixing.

Insights into the preparation of zein nanoparticles by continuous membrane nanoprecipitation

Nanoparticles (NPs) preparation is limited to an exclusive use in batch processes and small-scale formulations. The use of membranes as high-performance micromixers is expected to open new scenarios to overcome limitations of conventional nanoprecipitation system such as stirred tank (ST) nanoprecipitation. The ability of the porous membrane to add uniformly one phase to another and govern their mixing at the membrane interface seems to be an important parameter for obtaining uniform NPs. Inorganic membranes (pore size of 1 μm) were used to carry out membrane nanoprecipitation (MN) to form Zein nanoparticles (ZNPs) at pores level by non-solvent induced phase separation. A systematic study of the preparation of ZNPs in the ST and MN systems was carried out to establish the Ouzo diagram. The influence of zein concentration and solvent to non-solvent ratio on the size and size distribution of ZNPs was also investigated. A wider stable Ouzo zone was obtained with MN than with the ST process. ZNPs size increased from 100 nm up to 700 nm, while maintaining low polydispersity index (PDI < 0.2). The results demonstrate the suitability of MN for the continuous production of ZNPs and open the possibility of scaling up the nanoprecipitation process.

Data-driven ensemble optimal compensation control for partially known delayed and persistently disturbed nonlinear systems

In almost all industrial processes, the development of an accurate mathematical model that comprehensively characterizes the regulated system poses a substantial obstacle. Diverse factors, such as unidentified system components and time-varying external disturbances, contribute to this phenomenon. This can result in a disparity between the model and the plant and thereby compromising the performance of the designed controller. In this study, a novel data-driven ensemble optimal compensation control framework is proposed for partially known nonlinear systems with state delays and structural disturbances. Initially, the estimated system model is leveraged to construct a robust fuzzy model predictive controller (RFMPC) based on the Lyapunov–Krasovskii functional (LKF), ensuring the input-to-state stability (ISS) of the plant. Then, a reinforcement learning (RL)-based compensator is assembled to gradually minimize the model-plant mismatch across the entire state space. To streamline the training process of the compensator and facilitate its stability, a dynamic network update scheme is devised, incorporating a moving average gradient calculation method and a learning rate tuning strategy. Furthermore, comparative discussions with some prevalent methods are demonstrated. With the established control framework, the performance of the RFMPC can be securely enhanced, and convergence is ensured within a narrow proximity to equilibrium, despite time-varying perturbation dynamics. By integrating this approach with deep learning (DL) methods, the efficacy of numerous industrial processes could be significantly improved. A numerical simulation and a discretized continuous-time stirring tank reactor (CSTR) example validate the effectiveness and benefits of the proposed control strategy.

Enhanced Methanogenesis of Waste-Activated Sludge (WAS) in a Continuous Stirring Tank Reactor with Stealth Electrodes

The integration of a microbial electrolysis cell (MEC) is an effective strategy for enhancing the efficiency and stability of an anaerobic digestion (AD) system for energy recovery from waste-activated sludge (WAS). Typically, electrodes are arranged as separate components, potentially disrupting mixing and complicating the reactor configuration, posing challenges for the scaling up of AD-MEC coupling systems. In this study, electrodes were introduced into a continuous stirring tank reactor (CSTR) in a “stealth” manner by integrating them with the inner wall and stirring paddle. This electrode arrangement approach was validated through a sequential batch digestion experiment, resulting in a remarkable 1.5-fold increase in cumulative methane production and a shortened lag period compared to the traditional CSTR with a nonconductive inner wall and stirring paddle. Both the conductive materials (CMs) employed in the electrodes and the electrochemical processes equally contributed to the observed enhancement effect of the electrodes by regulating the evolution of the microbial community within the electrode biofilms, with a specific emphasis on the enrichment of methanogens (primarily Methanobacterium). This research offers a potential avenue to solve the contradiction between the electrode introduction and the mixing operation in AD-MEC coupling systems and to contribute to its future commercial application.

Effects of Particle Suspension on Surface Vortex in Unbaffled Stirred Tanks through DEM–VOF

Effects of Particle Suspension on Surface Vortex in Unbaffled Stirred Tanks through DEM–VOF

Experimental and computational investigation of chaotic advection mixing in laminar rectangular stirred tanks

The phenomenon of chaotic advection mixing in laminar stirred tanks, particularly when handling high-viscosity fluids, represents a common and challenging issue in various chemical and biochemical industrial processes. The effectiveness of these mixing processes is often constrained by the stretching, folding, and squeezing of fluid particles. Previous research efforts have predominantly concentrated on the design and/or optimization of impellers and operation modes to increase the stretching and folding of fluid particles. However, the mechanism of triggering chaos through the squeezing of fluid particles remains comparatively less explored. This study endeavors to address the intricate problem of laminar mixing by proposing the utilization of unbaffled rectangular stirred tanks. Via a combined approach of experimental and computational investigations, we illustrate that the implementation of rectangular stirred tanks yields significant improvements in chaotic mixing within laminar flow regimes. This type of tank not only enhances the convective transport of fluids but also distorts or completely eliminates the segregation regions by squeezing. Remarkably, as the tank L/W ratio increases from 1 to 2.5, the observed asymptotic area coverage from a frontal perspective escalates from approximately 55 % to about 85 %. Concurrently, the dimensionless mixing time decreased significantly from 1950 to 350. Of particular note, macroscopic homogeneous mixing is observed in stirred tanks with L/W ratios of 2 and 2.5 at Re = 29, with dimensionless mixing times of 550 and 350, respectively. This work highlights the potential of rectangular stirred tanks as a simple and cost-effective system for laminar mixing applications. Furthermore, it also hints at the broader applicability of the approach in various industrial mixing contexts, such as the suspension of floating granules, entrainment of air, and others.

Continuous Synthesis of a Macrocyclic Sulfite of Polyethylene Glycol by Cascaded Continuous Stirred Tank Reactors (CSTRs).

Many macrocyclic compounds are attractive drug-like molecules or intermediates due to their special properties. However, the bulk synthesis of such compounds are hindered by the necessity of using diluted solutions, in order to prevent intermolecular reactions that yields oligomer impurities, thereby resulting in a low production efficiency. Such challenge can be adequately addressed by using continuous reactors, allowing improved efficiency with smaller space footprints. In this work, we proposed a novel continuous process for the synthesis of a macrocyclic sulfite of tetraethylene glycol (PEG4-MCSi), which is a precursor to a very useful building block, PEG4-macrocyclic sulfate (PEG4-MCS). The basic reaction parameters, including stoichiometry and temperature, were first confirmed with small batch reactions, and the effectiveness of coiled reactors and continuous stirred tank reactors (CSTRs) were compared. Cascaded CSTRs were proven to be suitable, and the reaction parameters were subject to further optimization to give a robust continuous process. The process was then tested with 4 parallel runs for up to 64 h. Finally, the merits and demerits of batch and continuous reactions were also compared, demonstrating the suitability of latter in the bulk production of macrocyclic PEG-MCSi compounds.

New insights into relationship between gas cavities with operating conditions and hydrodynamic characteristics in aerated stirred tanks: Integrated in-situ/DIP/ROIMI/BIV quantization and CFD simulation

Current research seeks to understand and optimize the aerated stirred tanks’ (ASTs) intensified transfer process by systematically investigating the relationship between gas cavities with hydrodynamic characteristics and operating conditions. A novel integrated in-situ/DIP (based on TWS Machine Learning)/BIV/ROIMI technique was demonstrated for effectively monitoring, identifying and quantifying hydrodynamic characteristics in complex multiphase mixing systems (1130 < Re < 17192). The CFD-PBM model with a Multiple-Reference-Frame approach was employed to simulate and verify experimental results. Experimental and simulated results showed that under vortex clinging (VC) cavity flow zone, with the area of cavity (Acavity) increasing, the bubble dispersion is better, Sauter diameter (de32) is smaller, holdup (εg) and interfacial area (a) are larger. When the Acavity is more than 150 mm2, the hydrodynamic characteristics are optimal. The significant factors for the formation of cavities were comprehensively analyzed via 180 sets of full-factorial analysis, namely, impeller types, rotation speed, solutions, rotation speed × impeller types, solutions × impeller types, flow rate × impeller types and rotation speed × flow rate. Furthermore, a model (R2 = 0.95) was proposed and successfully predicted Acavity (relative error less than 3 %). This work may provide valuable insights for optimizing the design and operating of ASTs for enhance multiphase mixing processes.

Novel calcium-rich biochar synthesis and application for phosphorus and amoxicillin removal from synthetic and urban wastewater: Batch, columns, and continuous stirring tank reactors investigations

This work examines the synthesis of a novel calcium rich biochar (MB-900) and its application for phosphorus (P) and amoxicillin removal from synthetic and treated urban wastewaters under static and dynamic conditions (laboratory column and continuous stirring tank reactors (CSTRs)). Characterization techniques show that MB-900 has an enhanced structural, textural and surface chemical properties. Besides, batch adsorption tests indicate that MB-900 has important P recovery capacity in comparison with various engineered biochars available in the literature. Furthermore, under dynamic conditions, MB-900 efficiently recovers P from both synthetic solutions and real wastewater effluent. Due to a higher residence time, P recovery in CSTR mode are 94.9 and 82.3 mg g−1 for synthetic and urban wastewater, respectively. These values are 1.4, and 6.1 times higher than those obtained in column tests. Mechanism investigations shows that P recovery occurs through electrostatic interactions, complexation, and especially precipitation as hydroxyapatite. Under column and CSTR modes, AMX behaves like a conservative tracer and was not adsorbed by MB-900. Therefore, the resulting P-loaded biochar can be applied in agriculture as a slow release fertilizer without risks related to AMX adsorption/leaching by plants.