A numerical investigation of the hydrodynamics and mass transfer in a three-phase gas-liquid-liquid stirred tank reactor

Towards industrial biological hydrogen production: a review

Increased production of renewable energy sources is becoming increasingly needed. Amidst other strategies, one promising technology that could help achieve this goal is biological hydrogen production. This technology uses micro-organisms to convert organic matter into hydrogen gas, a clean and versatile fuel that can be used in a wide range of applications. While biohydrogen production is in its early stages, several challenges must be addressed for biological hydrogen production to become a viable commercial solution. From an experimental perspective, the need to improve the efficiency of hydrogen production, the optimization strategy of the microbial consortia, and the reduction in costs associated with the process is still required. From a scale-up perspective, novel strategies (such as modelling and experimental validation) need to be discussed to facilitate this hydrogen production process. Hence, this review considers hydrogen production, not within the framework of a particular production method or technique, but rather outlines the work (bioreactor modes and configurations, modelling, and techno-economic and life cycle assessment) that has been done in the field as a whole. This type of analysis allows for the abstraction of the biohydrogen production technology industrially, giving insights into novel applications, cross-pollination of separate lines of inquiry, and giving a reference point for researchers and industrial developers in the field of biohydrogen production.

The effect of Alcanivorax borkumensis SK2, a hydrocarbon-metabolising organism, on gas holdup in a 4-phase bubble column bioprocess

To design bioprocesses utilising hydrocarbon-metabolising organisms (HMO) as biocatalysts, the effect of the organism on the hydrodynamics of bubble column reactor (BCR), such as gas holdup, needs to be investigated. Therefore, this study investigates the first use of an HMO, Alcanivorax borkumensis SK2, as a solid phase in the operation and hydrodynamics of a BCR. The study investigated the gas holdup in 3-phase and 4-phase systems in a BCR under ranges of superficial gas velocities (U_G) from 1 to 3 cm/s, hydrocarbon (chain length C₁₃-₂₁) concentrations (H_C) of 0, 5, and 10% v/v and microbial concentrations (M_C) of 0, 0.35, 0.6 g/l. The results indicated that U_G was the most significant parameter, as gas holdup increases linearly with increasing U_G from 1 to 3 cm/s. Furthermore, the addition of hydrocarbons into the air-deionized water -SK2 system showed the highest increase in the gas holdup, particularly at high U_G (above 2 cm/s). The solids (yeast, cornflour, and SK2) phases had differing effects on gas holdup, potentially due to the difference in surface activity. In this work, SK2 addition caused a reduction in the fluid surface tension in the bioprocess which therefore resulted in an increase in the gas holdup in BCR. This work builds upon previous investigations in optimising the hydrodynamics for bubble column hydrocarbon bioprocesses for the application of alkane bioactivation.

CFD-Based and Experimental Hydrodynamic Characterization of the Single-Use Bioreactor XcellerexTM XDR-10

Understanding the hydrodynamic conditions in bioreactors is of utmost importance for the selection of operating conditions during cell culture process development. In the present study, the two-phase flow in the lab-scale single-use bioreactor XcellerexTM XDR-10 is characterized for working volumes from 4.5 L to 10 L, impeller speeds from 40 rpm to 360 rpm, and sparging with two different microporous spargers at rates from 0.02 L min-1 to 0.5 L min-1. The numerical simulations are performed with the one-way coupled Euler-Lagrange and the Euler-Euler models. The results of the agitated liquid height, the mixing time, and the volumetric oxygen mass transfer coefficient are compared to experiments. For the unbaffled XDR-10, strong surface vortex formation is found for the maximum impeller speed. To support the selection of suitable impeller speeds for cell cultivation, the surface vortex formation, the average turbulence energy dissipation rate, the hydrodynamic stress, and the mixing time are analyzed and discussed. Surface vortex formation is observed for the maximum impeller speed. Mixing times are below 30 s across all conditions, and volumetric oxygen mass transfer coefficients of up to 22.1 h-1 are found. The XDR-10 provides hydrodynamic conditions which are well suited for the cultivation of animal cells, despite the unusual design of a single bottom-mounted impeller and an unbaffled cultivation bioreactor.

Evaluation of the absorption performance of new compound absorbents for toluene under extremely high load

Absorption is an effective way to control volatile organic compound (VOC) industrial air pollution, and the key variable in this process is the selection of suitable liquid absorbents to absorb as many organic pollutants as possible. The objective of this study was to prepare a series of high-efficiency absorbents with different proportions of vegetal oil, mineral oil, and waste engine oil, which can be used for toluene absorption. The absorption efficiency (AE), saturated absorption (SA), and effective absorption time (EAT) of various absorbents were systematically analyzed. The results showed that when the inlet concentration of toluene was 8000 mg/m³ and the inlet flow was 1 L/min, the SA capability of vegetal oil, mineral oil, and waste engine oil was 7.15, 12.43, and 18.16 mg/g, respectively. With the 4000 mg/m³ inlet concentration, the SA of the absorber which was made in the ratio of 2:3:1 was increased to 50.93 mg/g. According to the thermodynamic equilibrium and absorption results, it is proved that the influence of the composition of the absorbent on absorption is greater than viscosity. It is also to be noted that the AE of the composite absorbent can still reach more than 80% after three times of heating and air purification.

Towards enhancement of gas-liquid mass transfer in bioelectrochemical systems: Validation of a robust CFD model

Mass transfer has been identified as a major bottleneck in gas fermentation and microbial conversion of carbon dioxide to chemicals. We present a pragmatic and validated Computational Fluid Dynamics (CFD) model for mass transfer in bioelectrochemical systems. Experiments were conducted to measure mixing times and mass transfer in a Duran bottle and an H-cell. An Eulerian-Eulerian framework with a simplified model for the bubble size distribution (BSD) was developed that utilized only one additional equation for the bubble number density while including the breakup and coalescence. Validations of the CFD model for mixing times showed that the predictions were within the confidence intervals of the measurements, verifying the model's capability in simulating the hydrodynamics. Further validations were performed using constant and varying bubble diameters for the mass transfer. The results showed the benefits of a simplified BSD model, as it yielded improvements of seven and four times in accuracy when assessed against the experimental data for the Duran bottle and H-cell, respectively. Modeling of the H-cell predicted that a lower stirring rate improves mass transfer compared with higher stirring rates, which is of great importance when designing microbial cultivation processes. The model offers a feasible framework for advanced modeling of gas fermentation and microbial electrosynthesis.

A General Review of the Current Development of Mechanically Agitated Vessels

The mixing process in a mechanically agitated vessel is a widespread phenomenon which plays an important role among industrial processes. In that process, one of the crucial parameters, the mixing efficiency, depends on a large number of geometrical factors, as well as process parameters and complex interactions between the phases which are still not well understood. In the last decade, large progress has been made in optimisation, construction and numerical and experimental analysis of mechanically agitated vessels. In this review, the current state in this field has been presented. It shows that advanced computational fluid dynamic techniques for multiphase flow analysis with reactions and modern experimental techniques can be used with success to analyse in detail mixing features in liquid-liquid, gas-liquid, solid-liquid and in more than two-phase flows. The objective is to show the most important research recently carried out.

Scaling-up of Haematococcus pluvialis production in stirred tank photobioreactor

The objective of this study was to evaluate three most common scale-up criteria for Haematococcus pluvialis production from cultivation bottles to 2 and 10 L of stirred tank PBRs. Constant volumetric power input (P/V) was found to be the most suitable criterion for H. pluvialis production. Total carotenoid amount per biomass concentration in 2 L and 10 L stirred tank PBRs were determined to be 4.57 mg/g and 4.77 mg/g, respectively. Antioxidant activity of total carotenoids extracted from H. pluvialis was also higher at constant P/V criterion where 46.91% inhibition rate with a total phenolic content of 11.76 mg gallic acid/L was achieved. Obtained results could be used to expand the bioproduction of H. pluvialis and its extracts in commercial scale.

Insights from Mathematical Modelling into Process Control of Oxygen Transfer in Batch Stirred Tank Bioreactors for Reducing Energy Requirement

Significant energy savings can be made in aerobic stirred tank batch bioreactors by the manipulation of agitator power (Pag) and air flowrate per unit working volume (vvm). Control is often implemented to maintain the oxygen concentration in the bioreaction liquid (COL) at a constant value. This work used model simulations to show that controlling the Pag and vvm continuously over time, such that it is operated at or near the impeller flooding constraint results in the minimum energy requirement for oxygen transfer (strategy Cmin); however, this might prove impractical to control and operate in practice. As an alternative, the work shows that dividing the bioreaction time into a small number of constant Pag time segments (5–10), where a PID controller is used to control vvm to maintain COL constant in each segment, can achieve much of the energy saving that is associated with Cmin. During each time segment, vvm is increased and a sudden decrease in COL is used to detect the onset of flooding, after which there is a step increase in Pag. This sequence of Pag step increases continues until the bioreaction is completed. This practical control approach was shown to save most of the energy that is associated with Cmin.