Mathematical modeling of mass transport in partitioning bioreactors

Abstract

Abstract So-called two-phase partitioning bioreactors (TPPBs), like many other reactors, are the heart of production process. Additionally, they are among the most interesting reacting operations, mainly because of their particular characteristics, such as multiphase nature, complex mass transport phenomena, bio-catalytic kinetics, and the intensified design consequence of the incorporation of a second liquid phase. TPPBs are multiphase systems which consist of an aqueous phase, usually containing microorganisms, and an immiscible and biocompatible organic phase that partitions toxic/insoluble substrates/products to the cells. This partition seems to favor metabolic demands, as well as thermodynamic liquid–liquid equilibrium of the system. Nevertheless the most important effect of the partition is improving mass transfer inside the liquid phase; otherwise cells are isolated from nutrients. Particular emphasis has been placed on assessing the nature of organism s interaction with the aqueous-organic interphase. Therefore, proper understanding of these bioreactors needs multidisciplinary participation; however mathematical modeling belongs to the field of reactor engineering. Additionally, TPPBs are quasi-isothermal systems as consequence of their biological nature; therefore their dynamics is governed by characteristic times of mass transport across interphases versus characteristic times of biological reaction. The elucidation of the processes responsible for gaseous substrate (mainly oxygen) mass transport is crucial to achieve optimum design of TPPBs, either in oxygen limited fermentations or in the off-gas treatment of volatile organic contaminants. This chapter, first defines the phases present in this kind of reactors and second introduces the basis of the “method of volume averaging” to propose and evaluate characteristic times in the liquid phase. Later, a macroscopic approach to the gas phase is developed, in order to obtain characteristic times for mass transfer limited systems. Both sets of characteristic parameters are compared finding useful interpretations of characteristic times and probable controlling steps. Finally, brief analyses of some experimental studies in literature are performed; it is found that conclusions obtained by the mathematical models analysis performed in this chapter are in agreement with experimental results reported previously.