The use of syngas, a renewable energy source, has raised increasing interest for the production of chemicals. Syngas, a mixture containing CO, H2, and CO2, can be converted into hydrocarbons, such as ethanol and 2,3-butanediol, via biochemical conversion. In this process, the microorganism Clostridium auto ethanogenum (C. auto ethanogenum) converts syngas into these hydrocarbons under ambient conditions. Still, syngas fermentation has drawbacks which influence the commercialization and scale-up, including low mass transfer rates due to the low solubility of syngas and the slow growth rate of C. auto ethanogenum.

An external loop gas-lift reactor (ELGLR) was considered to potentially overcome the low mass transfer rates. The recirculation of liquid in the reactor induces pneumatic agitation and the recirculation of biomass, which can improve the mass transfer and growth of C. auto ethanogenum and is currently used at an industrial scale by the company LanzaTech.

A 1D model of an external loop gas-lift reactor can provide understanding of the hydrodynamics and syngas fermentation process with C. auto ethanogenum in an ELGLR. A 1D model has the advantage of being less computationally intensive compared to other dimensional models and can be solved dynamically, which is valuable for comprehending the phenomena of syngas fermentation over time in the reactor. The predictions obtained by this model can be used to roughly determine optimal ranges for syngas fermentation, which could be useful for higher dimensional models or experimental setups.

The model considered a continuous inflow and outflow as well as liquid recirculation between the riser and the downcomer. The hydrodynamics of the reactor were based on correlations and equations proposed in the literature, whereas the syngas conversion by C. autoethanogenum was described using a black-box approach.

The hydrodynamic parameters were validated with experimental data proposed in the literature and a CFD model, which was also used for comparison of the syngas fermentation parameters. Additionally, the syngas fermentation in the 1D model was further optimized by adjusting external variable parameters like the dilution rate, inlet biomass concentration, and the gas mass inflow rate.

The syngas fermentation parameters in terms of ethanol productivity and CO conversion yield were compared to the industrial syngas fermentation process operated by the company LanzaTech. The 1D model was found to predict the hydrodynamic parameters in the same order of magnitude as the experimental data. Yet, the hydrodynamic parameters predicted with the CFD and 1D model corresponded less. Additionally, the syngas fermentation parameters varied greatly between the 1D and CFD model, probably as a result of different hydrodynamic parameter estimations in both models.

Using an optimization procedure, a dilution rate of 3 × 10^-5 1/s, gas inflow rate of 2.5 kg/s, and a biomass inlet concentration of 10 g/L were found to be the most advantageous for syngas fermentation in the 1D ELGLR model, resulting in an ethanol productivity of 0.6 g/L/h and 24.9% conversion.

The comparison with the estimated industrial performance deviated by 75.5% and 4.3% in ethanol productivity and CO-to-ethanol conversion yield, respectively. Due to the rough estimations and calculations, the validity of the 1D model to predict the syngas fermentation in an industrial process could not be deduced.

In conclusion, the 1D model was considered to reasonably predict the hydrodynamics of an actual external loop gas-lift reactor and give a rough approximation of the syngas fermentation by C. auto ethanogen. However, more experimental data on the syngas fermentation process is necessary for a complete understanding of the syngas fermentation process within an ELGLR.

The environment in which microscopic organisms live in is dominated by viscous forces because of their small length scales. Inertial forces are of little use to them in their propulsion mechanisms. As a consequence of this, an organism such as the scallop which moves through time-reversible deformations of its body would not propel itself in a regime dominated by visocus forces. Hence, microscopic organisms use appendages like cilia and flagella that are not time reversible to move forward. However, inertial effects become important to microscopic organisms at the relevant time and length scales. For example, inertia is used by a microscopic organism such as Paramecium to escape/attack its predator/prey.
The effects of inertia on the model of a spherically ciliated micro-organism are studied numerically using an Immersed Boundary Method (IBM) in the present work. In this model ,the distortions of the envelope that is generated by connecting all the tips of the cilia together, are prescribed. The unsteady Reynolds number which characterizes the influence of unsteady inertia that is generated by the beat of the organism, is varied from 0.025 to 18. The code which uses a Volume Penalization/Volume of Solid IBM to simulate the distorting sphere is validated for several test cases. The mean swimming velocity of the organism that is obtained numerically from the code is in agreement with the analytical model for two cases of the unsteady Reynolds number. The mean swimming velocity is found to decrease at increasing inertia. The flow pattern that is obtained in the near-field as a result of the distorting sphere is significantly different from those obtained with the existing models available in literature.