Hydrodynamics and mass transfer of concentric-tube internal loop airlift reactors: A review

Production of medical-grade biopolymer in air lift bioreactors

Microbes are known to produce biopolymers for societal applications. Economical production of biopolymer (PHB) is desperately required to significantly replace or reduce usage of non-degradable polypropylene produced by disappearing petroleum resources. Besides it is also equally important to ensure abundant availability of low cost medical grade biopolymers which can be used for several medical applications in society. It has been invariably observed that mechanical agitation in the bioreactors features major power consumption in the operation of bioreactors therefore usage of air lift bioreactors are likely to reduce power consumption by mechanical agitation significantly thereby leading to economic biopolymer production. Present investigation evaluates the possible role of pneumatic bioreactors (e.g., Bubble Column, Outer Aeration Inner Settling, Inner Aeration Outer Settling) as alternates to mechanically agitated bioreactors for the economic production of medical grade biopolymers P(3HB) by Bacillus thuringiensis IAM12077 using glycerol and glucose as major substrates. It was observed that Bacillus thuringiensis IAM12077 cultivations featured Biopolymer P(3HB) accumulations of 22.48%, 37.07%, 27.73%, in BC, OAIS, IAOS air lift bioreactors. Relatively higher product yield, volumetric productivity and P(3HB) accumulation was observed in Outer Aeration Inner Settling (OAIS) air lift bioreactor configuration as opposed to other pneumatic bioreactors.

An experimental and numerical investigation of secondary char formation in hydrothermal carbonization: revealing morphological changes via hydrodynamics

Hydrothermal carbonization (HTC) research has mainly focused on primary char production, with limited attention to secondary char, which is formed through polymerization and condensation of dissolved organic compounds in the liquid phase. This research aims to address this gap via an experimental investigation of the impact of stirring on the mass and carbon balance of HTC reaction products, surface functional groups, and surface morphology of secondary char, using fructose as a model compound. A 3D hydrodynamic simulation model was developed for a two-liter HTC stirred reactor. The experimental results indicated that stirring did not significantly influence the pH, mass, carbon balance, and surface functional groups of secondary char produced under the range of experimental conditions (180 °C, 10% biomass to water (B/W) ratio, and a residence time of 0-120 min) studied. Nonetheless, it was observed that a stirring rate of 200 rpm influenced the morphology and shape of the secondary char microspheres, leading to a significant increase in their size i.e., from 1-2 μm in unstirred conditions compared with 70 μm at a stirring rate of 200 rpm. This increase in size was attributed to the aggregation of microspheres into irregular aggregates at stirring rates > 65 rpm and residence times > 1 h. The hydrodynamic model revealed that high turbulence of Re > 104 and velocities > 0.17 m s-1 correlated with regions of secondary char formation, emphasizing their role in particle aggregation. Particle aggregation is significant above a stirring rate of 65 rpm, which corresponds to the onset of turbulent flow in the reactor. Finally, a mechanism is proposed, based on reactor hydrodynamics under stirred conditions, that explains secondary char deposition on the reactor walls and stirrer.

Enhancing random forest model prediction of gas holdup in internal draft airlift loop contactors with genetic algorithms tuning and interpretability

Gas holdup in an Internal Loop Airlift Contactor (ILAC) is one of the primary hydrodynamic features that governs the performance of the contactor. Prediction of gas holdup assumes importance in the design of airlift contactor. The key challenge in prediction of gas holdup is due to the difficulty in obtaining reliable phenomenological models. The improved prediction accuracies can provide better contactor design and equipment performance. The current work focuses on the use of data driven models for prediction of gas holdup, as the existing empirical correlations were found to be inadequate. In this work, 324 data points from the reported works on internal draft airlift contactor are consolidated. The input part of the examples in the data set consists of ten features which are related to geometry, fluid properties and operating conditions. This study improves the predictive accuracy of gas holdup, the output variable, in ILACs using a Random Forest (RF) model optimized via Genetic Algorithms (GA). SHapley Additive exPlanations (SHAP) based interpretability reveals the contribution of key features. The tuned model achieved a Coefficient of Determination (R²) score of 0.9542 and Mean Absolute Error (MAE) 0.0059, surpassing traditional parameter sets and providing insights into hydrodynamic control.

A critical review of ozone-based electrochemical advanced oxidation processes for water treatment: Fundamentals, stability evaluation, and application

In recent years, electrochemical advanced oxidation processes (EAOPs) combined with ozonation have been widely utilized in water/wastewater treatment due to their excellent synergistic effect, high treatment efficiency, and low energy consumption. A comprehensive summary of these ozone-based EAOPs is still insufficient, though some reviews have covered these topics but either focused on a specific integrated process or provided synopses of EAOPs or ozone-based AOPs. This review presents an overview of the fundamentals of several ozone-based EAOPs, focusing on process optimization, electrode selection, and typical reactor designs. Additionally, the service life of electrodes and improvement strategies for the stability of ozone-based EAOPs that are ignored by previous reviews are discussed. Furthermore, four main application fields are summarized, including disinfection, emerging contaminants treatment, industrial wastewater treatment, and resource recovery. Finally, the summary and perspective on ozone-based EAOPs are proposed. This review provides an overall summary that would help to gain insight into the ozone-based EAOPs to improve their environmental applications.

Performance and mechanism of simultaneous nitrification and denitrification in zeolite spheres internal loop airlift reactor

An internal loop airlift reactor was constructed with zeolite spheres as biofilm carriers (ZS-ALR), and the performance and mechanism of nitrogen removal were investigated. The results indicated that the TN, NH₄⁺-N and TOC removal efficiencies of ZS-ALR reached 96.12%, 100% and 94.54% under appropriate conditions (HRT of 6-8 h, aeration rates of 80-120 mL/min, C/N ratios of 4-6), and the highest TN removal rate constant was 0.01156 min^-1. Further investigating the influence of ammonia-N concentrations on nitrogen removal and biofilm stability revealed that catabolism was important in TN removal, and the prominent genera for nitrogen removal included Sphaerotilus (42.20%), Flavobacterium (17.47%) and Fusibacter (6.14%). Meanwhile, the abundance of amoA, napA, narG and nosZ genes was markedly influenced by ammonia-N concentrations. The nitrogen removal of ZS-ALR was mainly through ammonia-N adsorption by zeolite spheres and simultaneous nitrification and denitrification by biofilm.

Use of Pressurized and Airlift Bioreactors for Citric Acid Production by Yarrowia lipolytica from Crude Glycerol

Citric acid production is generally carried out in an aqueous medium in stirred tank reactors (STR), where the solubility of oxygen is low and the oxygen demand of microbial cultures is high. Thus, for this bioprocess, providing adequate oxygen mass transfer rate (OTR) from the gas phase into the aqueous culture medium is the main challenge of bioreactor selection and operation. In this study, citric acid production by Yarrowia lipolytica W29 from crude glycerol, in batch cultures, was performed in two non-conventional bioreactors normally associated with high mass transfer efficiency: a pressurized STR and an airlift bioreactor. Increased OTR was obtained by raising the total air pressure in the pressurized STR and by increasing the aeration rate in the airlift bioreactor. An improvement of 40% in maximum citric acid titer was obtained by raising the air pressure from 1 bar to 2 bar, whereas, in the airlift bioreactor, a 30% improvement was attained by increasing the aeration rate from 1 vvm to 1.5 vvm. Both bioreactor types can be successfully applied for the citric acid production process using alternative ways of improving OTR than increasing mechanical stirring power input, thus leading to important operating saving costs.

Recent progress on converting CO2 into microalgal biomass using suspended photobioreactors

Inhomogeneous light distribution and poor CO₂ transfer capacity are two critical concerns impeding microalgal photosynthesis in practical suspended photobioreactors (PBRs). To provide valuable guidance on designing high-performance PBRs, recent progress on enhancing light and CO₂ availabilities is systematically summarized in this review. Particularly, for the first time, the strategies on elevating light availability are classified and discussed from the perspectives of increasing incident light intensity, introducing internal illumination, optimizing flow field, regulating biomass concentrations, and enlarging illumination surface areas. Meanwhile, the strategies on enhancing CO₂ light availability are outlined from the aspects of generating smaller bubbles, extending bubbles residence time, and facilitating CO₂ dissolution using extra additives. Given the microalgal biomass production using current PBRs are still suffering from low productivity and economic feasibility, the possible future directions for PBRs implementation and development are presented. Altogether, this review is beneficial to furthering development of PBRs as a practical technology.