Hindered settling and scale up

The above discussion only applies to dilute particle suspensions. There are further deviations from ideal settling due to operation at high solids concentrations.

For solids concentrations > 2% (v/v), the free settling velocity of a particle under gravity is hindered by interactions with other particles and the suspending liquid. The reduction in settling velocity increases with solids concentration (Muschelknautz 1987). According to Richardson and Zaki (1954):

Wg - Ug ( l - C v f 7

hindered settling velocity (ms'^)

where

V =

Cy = volume concentration of solids in suspension (m^m*^) a = particle shape factor

For rigid, spherical particles a = 4.6, but it can increase to 10-100 for non-rigid or irregular particles. This can lead to large errors where Stoke's law is assumed (Datar and Rosen 1987). For flocculated suspensions of yeast, a = 12-20 (Brohan and M^Loughlin 1984).

Bamea and Mizrahi (1973) made further reductions to Ug* by taking into account wall effects where the vessel diameter is only 1 - 2 orders of magnitude

larger than the particle size. However, centrifugal sedimentation at high solids concentrations is not hindered as much as in gravity settling due to the continuous dilution effect of the feed throughput (Svarovsky 1990).

Figure 1.3.2: The role o f feed channels in a disc stack centrifiige.

Figure 1.3.3: Fluidflow within the gap o f a disc stack centrifuge.

ou t e r wall i n n e r wall s p a c e r s w a k e s p a c e s p a c e r s t r a c e r h o l e

Figure 1.3.4: Critical Rotation number Ro as a function o f the hydrodynamic

parameter X in a disc stack centrifuge.

ae r ” __ . i Ro • \ . . , r 10 12 U 16 18

where ° critical values in the free gap (point spacers)

With S theory scale up, corrections for hindered settling can be produced simply by substituting the reduced settling velocity expression into the derivation for critical particle diameter, d^ (see Section 3.2) to obtain:

= 8

' i A p A e g ( i - c , r

where m = dynamic viscosity (kg m'^ s'^)

Ap = solid-liquid density difference (kg m'^)

Ag = equivalent settling area (m^)

g = acceleration due to gravity (m s'^)

Grade efficiency curves (Section 3.2) for hindered settling can in theory be superimposed onto those for unhindered settling using this method. One of the difficulties to overcome in the application of hindered settling theory to centrifugal separation of biological solids is the often poorly defined density difference between suspending liquid and solid particles, as in the case of cell debris.

1.3.4 Scale down of the disc stack centrifuge

Work was carried out by Obeng (1983) on a small, scaled down disc stack centrifuge (Westfalia SAOOH-205) to investigate if it could be used to predict the under-run capabilities of a larger disc stack by reducing its separation area. The positions of the active discs were found to have a profound impact on the separation.

Obeng used two sets of active disc arrangements: 6 active discs at the top

of the stack, and 6 active discs at the bottom. 53 blanking discs, ie. vrith no

separator caulks, were used to replace the equivalent of 32 missing active discs. Breakthrough curves were plotted using turbidity measurements for a 6.8% soya

protein precipitate at flow rates 9 - 30.6 Lh’^ for both arrangements.

With the active discs placed at the top of the stack, initial clarification was high at all flow rates. Particle breakthrough into the supernatant was observed after the equivalent of approximately 2 total bowl volume changes at

of the high flow rate and occurred less suddenly. Active discs placed at the bottom of the stack gave poor initial clarification, which declined further during 2.5 total bowl volume changes. The same result was observed at all flow rates.

An explanation was offered relating to fluid flow patterns inside the centrifuge. Improved separation observed in the active top stack arrangement was thought to be due to a large contribution to separation in the solids hold up region of the centrifuge bowl.

The solids dewatering in the top stack arrangement was 51-52 % dwt/wwt, compared to 43-44 % in the bottom stack arrangement. It was thought that this corresponded to a significant amount of recovery occurring in the sediment holding space. However, the total run time and hence solids residence time was nearly twice as long in the experiments with the top stack arrangement and is therefore more likely to have been the real cause for the increased dewatering.

This work preceded that of Mannweiler (1990), who used a larger disc stack centrifuge, and found that active discs placed near the bottom of the stack gave comparable performance to a separator with a complete set of active discs.

Mannweiler (1990) based his scale down method on observations of particle separation efficiency of the clarified effluent. He used a disc stack centrifuge with a bowl volume of 3L normally fitted with 72 active discs. Using an assortment of "blanks", ie. solid disc blocks which obstruct fluid flow, he reduced the active settling area by up to 90%.

Comparisons of the grade efficiency curves obtained in the scaled down and fully active stacks showed that separation was reproducible at all flow rates if the active discs were placed at a height equivalent to 7 active discs from the bottom of the stack. This corresponds to a position approximately 1/10 th of the way up the full stack. Active discs placed below this position in close proximity to the inlet stream were subject to turbulent flow, which disturbed the flow of solids down the discs. Hence solids re-suspension occurred, causing uncharacteristically poor separation, especially at high flow rates. Active discs placed too high gave rise to better separation than that observed in the fully

active stack, due to pre-settling in the sediment holding region, i.e. the centrifuge behaved as a low capacity tubular bowl.

Dye tracer studies revealed that the fluid volume required for nearly all the tracer material to be removed, V9 9, was 8.2 L, irrespective of the active discs’

position. However, experiments were carried out using 15 L feed. This was equivalent to 5 bowl volume changes required to reach steady state according to a rule o f thumb for continuous stirred tank reactors (CSTR). However, 15 L was more than 1 0 bowl volume changes if the volume occupied by the blank discs is

taken into account.

Since scale down of the process stream was based on bowl volume, then there was no reduction in the amount of test material actually required for each experiment. Any saving of process material was therefore made simply by the use of lower flow rates in the scaled down stack contributing to the ease of operation.

Comparisons of performance were made in terms of separation efficiency between pilot and scaled down stacks on the basis of the particle size distribution of the clarified supernatant relative to that in the feed. The full separation effect, including the extent of dewatering and the ability to discharge solids cannot be predicted using this method. For example, Milbum et al (1990) found that yeast cell homogenate flocculated with PEI gave good initial clarification in a disc stack centrifuge, but that blockage of the discharge nozzle due to the rheological nature of the dewatered solids hindered continuous operation.