The most important scale-up parameters of aerated bioreactors are investigated in 42 and 340 L vessels, with water and various xanthan gum and carboxymethyl cellulose solutions. The study focuses mainly on mass transfer (k L a) measurements under various operating conditions. The relevance of existing correlations is discussed. The traditional viscosity-contribution approach appears unable to predict the changes in k L a during scale-up and an alternative formulation is proposed. The effect of rheology on power consumption and mixing time is in fair agreement with works published on this topic.

The present work investigates the impact of the physical properties and loading strategies of wheat straw and miscanthus on enzymatic hydrolysis at high DM concentration. Three parameters have been chosen to evaluate the enzymatic hydrolysis performance: (i) the mixing time, (ii) the energetic mixing consumption and (iii) the glucose concentration. It was demonstrated that the hydrolysis of miscanthus is easy to perform and has low viscosity. On the contrary, the higher porosity grade of wheat straw than miscanthus (73% against 52%) contributed to have a very high viscosity at 20% w/w DM. The development of a fed-batch strategy allowed the reduction of viscosity inducing the energetic consumption lowering from 30 kJ to 10 kJ. It has been also proven that the miscanthus addition in wheat straw achieved to decrease mixing energy consumption at 5-8 kJ, when it represented more than 30% of the total mass of the reaction medium.

Working at high values of lignocellulosic Dry Matter (DM), as wheat straw, increases the reaction medium viscosity, making the mixing inefficient with the traditional agitators. Batch and fed-batch tests were conducted using different impellers: i) inclined blades, ii) marine impeller, iii) anchor, iv) paravisc and v) double helical impeller. Inclined blades appeared an inadequate device for batch and fed-batch tests. On contrary, double helical impellers and anchor gave optimal performances. An alternative to improve the reactor's rheology is the modification of the feeding strategy. A particular fed-batch strategy allowed keeping low the reaction medium viscosity by a gradual increasing of the DM content in the reactor. In this way, three main benefits were achieved: i) a very good performances in terms of glucose concentration (85 g/L), ii) a strong reduction of the energetic consumption compared to batch test and iii) the adoption of a simple mixing devise.

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