Mass transfer limitations in syngas fermentation processes are mostly attributed to poor solubility of CO and H₂ in water. Despite these assumed limitations, a syngas fermentation process has recently been commercialized. Using large-sale external-loop gas-lift reactors (EL-GLR), CO-rich off-gases are converted into ethanol, with high mass transfer performance (7–8.5 g.L^-1.h⁻¹). However, when applying established mass transfer correlations, a much poorer performance is predicted (0.3–2.7 g.L^-1.h⁻¹). We developed a CFD model, validated on pilot-scale data, to provide detailed insights on hydrodynamics and mass transfer in a large-scale EL-GLR. As produced ethanol could increase gas hold-up (+30%) and decrease the bubble diameter (≤2 mm) compared to air–water mixtures, we found with our model that a high volumetric mass transfer coefficient (650–750 h⁻¹) and mass transfer capacity (7.5–8 g.L^-1.h⁻¹) for CO are feasible. Thus, the typical mass transfer limitations encountered in air–water systems can be alleviated in the syngas-to-ethanol fermentation process.

This chapter deals with fermentation processes, converting low cost renewable feedstocks into valuable bio-products, with the help of microorganisms or mammalian cells in bioreactors or fermenters. In industrial vessels, the volumetric conversion rate, i.e. the fermentation intensity, is limited by a transport step: mass transfer, liquid mixing or cooling. In special processes where the growth of the cells is marginal, intensification is possible by active cell retention. A comparison with chemical process intensification reveals that the same four main principles are valid, i.e. (1) maximize the rate at optimal selectivity, (2) minimize the impact of substrate concentration gradients, shear rate gradients and other local differences, (3) relieve the transport limitations and (4) arrange smart integration of operation steps of which cell retention is the most important. In essence, optimized microorganisms in fermentations can be viewed as efficient, smartly integrated cell factories. The main principles are illustrated with four intensification examples, showing that debottlenecking of the oxygen transfer capacity is most important, followed by liquid mixing. The limits of fermentation intensity have been estimated for fed-batch fermentations supplied with air or pure oxygen and point at significant optimization space. In contrast, aerobic continuous fermentation is expected to remain difficult due to fundamental restrictions.