CENTRIFUGATION.

The mechanical disruption of a microbial culture will produce a suspension of biological particles in the colloidal size range. The size range and rheology of the suspension will depend on the cell type and morphology, their fermentation history and the homogenisation conditions. Flocculation of the cell debris to aid in its removal will alter the demands being places upon the centrifuge, since the nature of the process stream has been changed. Several conflicting requirements have influenced the design of industrial centrifuges. The capacity of a centrifuge may be described by:

Vo = (i)V (1.10)

18 \L

where Vo is the sedimentation velocity; the first component of the right hand side of the expression describes the terminal velocity of a particle of diameter (d), under the influence of gravity. The second part relates to the performance of the centrifuge. Small deviations from particle non-sphericity will significantly reduce sedimentation efficiency. Another significant factor is hindered settling:

where is the hindered sedimentation velocity and a the geometric factor. The value of a is 4.6 for monosized spherical particles; values for non-rigid, non-spherical particles can be in the range 10-100; this may result in significant deviations from the Stokes law equation. Hence the Stokes law equation can only be used as an approximate guide since biological particles do not have the characteristics of ideal mono-sized spheres. Centrifuge performance will depend upon the density difference between the colloidal solids and the suspending liquor, the size of the particles, the liquid viscosity, the residence time in the unit, and the relative centrifugal force (RCF).

The density difference between the colloidal particles and the surrounding medium tends to be low. Also the disruption of cell with the concomitant release of nucleic acids tends to increase the viscosity; these effects combine to considerably reduce centrifuge performance and the problems are compounded by design restrictions placed upon centrifuges. The radius of rotation is restricted since mechanical stress increases with the square of the radius, hence safe operational limits are soon reached. Biological products are temperature labile, and hence efficient cooling of the bowl is required. Also native process material requires that any aerosols generated by the unit be eliminated, thus the fitting of hermetic seals is required. The advantages of centrifugation include the rapid speed of processing and low sensitivity to variations in the feed stream. ’Soft’ feed systems have been developed to stop air entrainment and hence reduce protein dénaturation at gas-liquid interfaces; they also improve recovery of shear sensitive precipitates because the material is accelerated more slowly.

The particle efficiency recovery of a centrifuge can be expressed by the grade efficiency curve (Mannweiler et a l, 1989). This represents the relationship between the fraction of solids recovered T(d) of size d as a function of the normalised size d/dc. The critical particle diameter dc is the size of particle which will just be recovered in the centrifuge. Mannweiler et a l (1990) has shown that the settling properties of E.coli and yeast cell debris are poor. The settling properties are a

function of particle shape, density difference (between particle and medium) and viscosity. These characteristics combined with the sub-micron size of the debris result in poor centrifugal recovery. From examination of the grade efficiency curve it can be seen that for particles of floes greater than approximately 3 pm almost complete recovery will result. Hence the production of large floes with poor shear resistance is not necessary. Floes in the size range of 5 to 20 pm should produce excellent centrifugal recovery with reasonable resistance to mechanical forces. Grade efficiency curves can be used to evaluate the extent of floe breakup in a centrifuge (Bell & Brunner, 1983). When breakup occurs the particle size distribution for the exit stream will be different from that of the feed. The grade efficiency curve will become negative at low values of d/dc, due to the production of fragments which were not present in the original suspension.

Placek and Teague (1988) have reported on the flocculation of cells by the addition of polyelectrolyte directly into the feed zone of a disc stack centrifuge. The high shear accompanying fluid acceleration provided intensive and rapid mixing. Floes formed under these conditions were reported to be more shear resistant than pre­ flocculated suspensions which were subsequently fed into the unit. This mode of operation was reported to make the requirement for hermetic seals and special bowl geometries unnecessary since the floe were being formed in the region of most intense shear.

Bentham et a/., (1990) have reported the use of a scroll decanter centrifuge for the removal and dewatering of borax flocculated yeast cell debris. Various rheological studies showed that the performance for recovery and dewatering were independent of feed rate and differential scroll rate. Solid material recovery was high (85%) whilst soluble protein loss in the sediment was only 1% due to good dewatering. The advantages of using a scroll decanter centrifuge is the constant removal of solids from the bowl, thus enabling continuous processing of feed streams with a high solids content.