Abstract

Large scale fermentations face challenges in mixing and mass transfer as well as in the design and construction of the equipment. Scale-up from laboratory and pilot scale experiments is difficult because different phenomena — such as mixing times and mass transfer conditions — scale in a different way.

We study the effect of mass transfer, reactor type and scale on the growth of Pichia pastoris yeast. Batch cultivation experiments monitoring the cell growth and ethanol formation are conducted in laboratory scale in two reactor types — stirred tank and an Outotec OKTOP^®9000 draft tube reactor. Model for the yeast growth — including respirative and fermentative metabolism and the effect of dissolved oxygen — is formed based on literature. For scale-up studies, the growth model is used along with one dimensional reactor model that accounts for liquid mixing, gas phase dynamics and local gas hold-up and mass transfer coefficient.

By using a realistic growth model along with the reactor model, the simulated effects of scale-up are presented in terms of cell yield. A decrease in yield is noticed due to oxygen depletion in gas and insufficient liquid mixing. Potential improvements are related to the gas handling capacity and liquid mixing of the reactor.