Multiphase Bioreactor Design

CHAPTER EIGHT

SOLID-TO-SOLID BIOCONVERSIONS:

BATCH OR CONTINUOUS?

M.J.F.MICHIELSEN, RENE H.WIJFFELS, JOHANNES TRAMPER

AND H.H.BEEFTINK

Food and Bioprocess Engineering Group, Wageningen University, P.O.

Box 8129, 6700 EV Wageningen, The Netherlands

ABSTRACT

Studies of solid-to-solid bioconversions in the last decade have mainly

focused on attaining a high conversion at a large scale in batch systems.

Lately, attention has shifted towards batch systems with very high

amounts of solid substrate, thus featuring a small liquid phase. Further

development of these batch systems as well as the development of

continuous systems has been hampered by the lack of mechanistic models,

featuring the kinetics of dissolution, bioconversion, and crystallisation.

Recently, such a model has been developed, giving a new impulse to this

technology.

Based on the current status of biocatalyst immobilisation and of

continuous reaction crystallisation, proposals for continuous systems for

solid-to-solid bioconversions are given. Assuming that such continuous

systems are feasible, a general procedure is developed to determine

whether a batch or continuous mode of operation is most profitable for a

spécifie solid-to-solid bioconversion.

Keywords: Solid-to-solid biocatalysis, batch systems, continuous

systems, costs comparison

INTRODUCTION

In aqueous media, biocatalysts often show low overall volumetric productivities due to
limited substrate solubility and/or inhibition by substrate and/or product (Michielsen et
al., 1999a). Multi-phase systems are generally applied to increase overall substrate
solubility and to reduce substrate and/or product inhibition; a second (or third) phase
serves as a substrate reservoir or product sink. Other advantages of such systems are that
biocatalytic production and downstream processing are integrated, and that high product
concentrations can be gained as the product accumulates (mainly) in one phase.
For extraction of hydrophobic compounds, an organic solvent is often used as the
water-immiscible second phase. However, use of an organic phase may cause loss in
specificity and selectivity of the biocatalyst, product contamination, and it makes the
process less cost efficient and less environmentally friendly.