In large-scale stirred bioreactors, used for aerobic microbial production processes, the liquid-phase bulk mixing will strongly affect the overall oxygen transfer. This will result in oxygen concentration profiles in the liquid phase, especially in highly viscous fermentation broths.',* However, in a low viscous fermentation broth oxygen concentration gradients can also be observed. These gradients are caused by the relatively high overall mixing time of the liquid phase, compared with the time constant for oxygen consumption and for oxygen t r a n~f e r .~ When a multicompartment model for the liquid flow and the oxygen transfer into the liquid phase is used, it is possible to predict the overall oxygen transfer capacity of the reactor quite a~curately.~ This is contrary to the use of empirical correlations such as those suggested by Van 't Riet.4 The two-compartment model used for such calculations as reported before3 now will be changed into a five-compartment model. This is acceptable only if the extra parameters can be predicted a priori. The number of compartments is determined by the number of impellers, mounted in the vessel, the part of the reactor below the lowest impeller, the reactor part between the impellers (if the distance between the impellers is larger than the impeller diameter) and the upper part of the reactor volume. So, in this case with two impellers, a five-compartment model will be used (Fig. 1). For the model calculations we assume that the gas is introduced into the lowest impeller region, and that the oxygen transfer in the bottom part of the vessel is negligible. The aim of the model is to predict the following reactor operating variables: 1) the overall oxygen transfer capacity of the reactor (OTR); 2) the local liquid dissolved oxygen concentrations, for estimation of bad aerated zones which can introduce negative effects for the microorganisms and as a base for reliable scaledown experiments to estimate those effects5; 3) the gas-phase exhaustion.

Major transitions can be expected within the next few decades aiming at the reduction of pollution and global warming and at energy saving measures. For these purposes, new sustainable biorefinery concepts will be needed that will replace the traditional mineral oil-based synthesis of specialty and bulk chemicals. An important group of these chemicals are those that comprise N-functionalities. Many plant components contained in biomass rest or waste stream fractions contain these N-functionalities in proteins and free amino acids that can be used as starting materials for the synthesis of biopolymers and chemicals. This paper describes the economic and technological feasibility for cyanophycin production by fermentation of the potato waste stream Protamylasse™ or directly in plants and its subsequent conversion to a number of N-containing bulk chemicals.

To simulate production-scale conditions of gluconic acid fermentation by Gluconobacter oxydans, different experimental setups are presented in this study. From the determination of the time constants of a production-scale reactor, it can be concluded that mixing and oxygen transfer are the rate-limiting mechanisms. This results in oxygen concentration gradients which were simulated in a one-compartment reactor in which the oxygen concentration was fluctuated by a fluctuated gassing with air and nitrogen. It could be concluded that only very long periods of absence of oxygen (ca. 180 s) results in lower specific oxygen uptake rates by Gluconobacter oxydans. From scale-down studies carried out in a two-compartment system to simulate a production-scale reactor more accurately, it could be concluded that not only the residence time in the aerated part of the system is important, but the liquid flow in between the different parts of the reactor is also an essential parameter. It could also be concluded that the microorganisms are not influenced negatively by the fluctuated oxygen concentrations with respect to their maximal oxidation capacity. The two-compartment system can also be used for optimization experiments in which the "aerated" compartment was gassed with pure oxygen. From these experiments it was concluded that also a short residence of the cells at high oxygen concentrations diminished the growth and product formation rates. These experiments show the necessity of the scale-down experiments if optimization is carried out. The two-compartment system presented in this study is a very attractive tool for reliable scale-down experiments.

Disposable bioreactors have gained an increasing importance in recent years in the pharmaceutical production. Wave‐mixed reactors were among the first systems which were applied. In contrast to stirred tank reactors, wave‐mixed bioreactors are characterized by low shear forces while the gas exchange is realized by the large gas‐liquid interface. Oxygen transfer rates obtained are often in a range between 50 and 300 h–1. By applying nutrient‐limiting fed‐batch cultivations, in which the oxygen consumption of a culture is controlled, bacteria can be also cultivated in wave‐mixed reactors. This article describes the successful scale‐up of an Escherichia coli fed‐batch cultivation from the 12‐L to the 120‐L scale using a disposable bioreactor, in which a final biomass concentration of 45 g L–1 was obtained.