Taylor Couette Vortex Flow in Enzymatic Reactors

Roberto Campos Giordano and Raquel de Lima Camargo Giordano

Summary

Taylor vortex flow reactors (VFRs) are especially useful when dealing with fragile biocatalysts, in view of the low-shear stress that is a characteristic of this flow pattern. Hence, they may be an interesting solution for reactions with immobilized enzymes. This chapter presents some basic features of this reactor. It is not intended to provide a review of the extensive literature on Taylor vortex flow reactors; rather it includes some of the available correlations that may be useful to design enzymatic VFRs for specific applications. Some clues concerning reactor operation that may help in everyday laboratory practice are highlighted, too.

Key Words: Enzymatic reactor; fragile supports; shear stress; Taylor vortices; vortex flow reactor.

1. Introduction

The secondary flow pattern that appears above a critical rotation in the gap between an inner (rotating) and an outer (generally, stationary) cylinder is named Taylor vortex flow (1).

One important reason for using vortex flow in biochemical reactors is its ability to promote a gentle, but still efficient, stirring, which is ideal when dealing with fragile cells (e.g., mammalian cells) or with shear-sensitive particles, such as gel matrices for immobilized enzymes (see Note 1). Another advantage of this system is that the rotation of the inner cylinder provides an additional degree of freedom for the operation of the reactor, which facilitates the fluidization of the biocatalyst.

Taylor vortices are probably one of the most widely studied phenomena in the field of fluid dynamics, ever since Taylor, in 1923, solved Navier-Stokes equations for the disturbances of Couette flow (disregarding the nonlinear terms) and predicted the onset of the vortices (2). In the same paper, experimental results supported the theoretical predictions with striking accuracy. Above a critical rotation of the inner cylinder, counter-rotating toroidal vortices appeared, superim

Vortex Flow Reactor

Fig. 1. Continuous vortex flow reactor.

Fig. 1. Continuous vortex flow reactor.

posed to the main Couette flow. Furthermore, the size of the vortices agreed with Taylor's calculations. Chandrasekhar (3) provides an excellent formalization of the linear solution of this problem.

This chapter does not intend to provide a revision of the extensive literature on Taylor vortex flow; for instance, Tagg (4) presents approx 1500 selected references on this topic. Instead, only the main aspects that might be useful from a practical viewpoint, for a possible application of Taylor vortex flow reactors (VFRs) with immobilized enzymes, are addressed here.

Enzymatic VFRs may operate either in continuous or in batch mode. In the first case, an axial Poiseuille flow is forced through the apparatus. Figure 1A schematically depicts a continuous VFR. Figure 1B shows a picture of it after the injection of a tracer.

2. Vortex Flow Bioreactors

Taylor vortices have been used to enhance the performance of bioreactors. Batch cultures of plant cells (5) and the continuous operation of photocatalytic processes (6) are two examples of cultivation processes using VFRs. Taylor flow is also used in downstream operations such as ultrafiltration (7) and adsorption (8).

When working with immobilized enzymes, a possible alternative would be immobilization on the inner cylinder wall (9). This option could overcome a limitation that the fluidization of particles imposes (i.e., a lower limit for rotation rates). Despite this advantage, that configuration has not prevailed, probably because of its lower biocatalytic area. Hence, fluidized bioparticles, inserted in the annular gap, are the usual choice for enzymatic VFRs (see Note 2). Some processes where this configuration was employed are cleavage of heparin with immobilized heparinase in extracorporeal devices (10-12), glucose-fructose isomerization (13,14), casein hydrolysis (15), and cheese whey protein hydrolysis (18,20).

Taylor VFRs Notation

Ar

Archimedes number (dp3gp(pp - p)/v)

d

Reactor gap (Re- Rj), L

dp

Biocatalyst average diameter, L

Dax

Axial dispersion coefficient, L2T-1

D,

Stirred tank impeller diameter, L

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