Centrifugation

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE SPG PEG 304

CENTRIFUGATION

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Process Engineering Guide:

Centrifugation

CONTENTS
Synopsis

1

INTRODUCTION

1.1
1.2
1.3
1.4

1.5
1.6
1.7
1.8

Scope of the Section
Centrifuge Types
What is a Disc Centrifuge and How Does It Work?
Operating Regimes – What Limits the Machine’s Output?
1.4.1 The Mechanics of separation
1.4.2 Nozzle Characteristics
1.4.2.1
Effect of Solids Throughput
1.4.2.2
Effect of Nozzle Diameter
1.4.2.3
Theory of Separation
Thickening capability (solids-limitation)
Clarification capability (Hydraulic limitation)
Hindered settling (Flux limitation)
Comments

2

USEFUL LABORATORY TESTS

2.1

Background
2.1.1 Basic Suspension Properties
2.1.2 Settling Kinetics
2.1.3 Network Strength Measurements
2.1.4 Floc Stability Assessment
Test Techniques

2.2


3

Scale-up Theories

3.1

Scale-Up When Clarification is the Limiting Process
3.1.1 Qualitative
3.1.2 Quantitative
3.1.3 Validity
3.1.4 Application
3.2
Scale-Up When Thickening is the Limiting Process
3.2.1 Background
3.2.2 Scale-Up Factor
3.2.3 Three Methods of Application
3.2.3.1
Semi-Empirical Approach I
3.2.3.2
Semi-Empirical Approach II
3.2.3.3
Ab Initio Approach
3.2.4 Estimation of Necessary Parameters and the Problems
Encountered
3.2.4.1
Relationship between ṫ and P
3.2.4.2
Volume of Slurry in centrifuge
3.2.4.3
Pressure (or “field”) Factor
3.2.4.3
Concept of Ultimate Thickness
3.3
Scale-Up When Hindered is the Limiting Process
3.3.1 Background
3.3.2 Gravity Thickeners – Behavior
3.3.3 Batch Thickeners
3.3.3.1
Continuous Thickeners
3.3.4 Gravity Thickeners – Interpretation of Batch Settling Test
3.3.5 Application to Centrifuges
3.3.5.1
Batch Flux Curve
3.3.6 Discussion


4

A SYSTEMATIC APPROACH TOCENTRIFUGE SCALE-UP

4.1

Validity of Different Methods

Table 1:
Table 2:
Table 3:
Table 4:
Table 5:
Table 6:
Table 7:

Steps in Ab Initio Scale-Up for Clarification Limited Systems
Steps in Semi-Empirical Scale-Up for Clarification
Limited Systems
Prediction of Ultimate Thickness for Solids Limited Systems
Steps in Semi-Empirical Scale-Up for Solids
Limited Systems
Steps in Ab Initio Scale-Up for Solids Limited Systems
Steps in Semi-Empirical Scale-Up for System Limited
by Hindered Settling Systems
Steps in Ab Initio Guide Calculations for Systems Limited by
Hindered Settling Systems

5

Worked Examples

5.1
5.2

Solids-Limited Scale-Up: Background to Examples
Solids-Limited Examples: Semi-Empirical and Ab Initio Methods
5.2.1 Semi-Empirical Approach to Solids Limited Operation
(simplified method)
5.2.2 Standard Semi-Empirical Approach
5.2.3 Ab Initio Approach to Solids Limitation

5.3

Calculation of Thickening Ultimate Limit
Example: “Structural” limit for Thickening in “Pruteen” Centrifugation
Scales-Up for Clarification Limited Systems
5.4.1 Ab Initio Clarification Calculation for P2 Type Single Cell Protein
5.4.2 (Ab Initio) Clarification Limit for Unflocculated Bacterial Cells
5.4.3 Semi-Empirical Scale-Up for Clarification
Limited Systems
Ab Initio Calculation for Hindered Settling Limitation
Comments

5.4

5.5
5.6

REFERNCES


SYNOPSIS
The principles underlying centrifugal separation of particulate species are briefly
considered, and the main types of separator available are noted. The procedures
available for scale-up from laboratory or semi-technical data are then discussed
in detail with particular reference to perhaps the most important class of machine
for fine particle processing: the disc-nozzle centrifuge.
Starting with the basic concepts behind their design, discussion follows to explain
the factors which may limit centrifuge performance. It is shown how a few simple,
laboratory scale tests can give a valuable insight into the design and operation of
full-scale industrial machines. Various scale-up theories are presented and
worked samples given, using data drawn from GBHE Agricultural Division
experience in biological separations.

1 INTRODUCTION
1.1

Scope of the Section

Centrifugal separation of solids is governed by the action of a centrifugal field in
either enhancing settling of particles or promoting deliquoring by filtration. The
former effect is the more important for fine particle processing (species < 10 µm
in size > and thus we have chosen to focus attention on such operations in this
section.
Filtration in general is considered in GBHE SPG PEG 300 Filtration.
The discussion is centered upon the scale-up and operation of (continuous)
disc-nozzle machines: a significant amount of data is available for this very
important class of centrifuges to demonstrate the strength and limitations of
scale-up procedures. It is considered that many of the concepts described can be
readily extended to batch or intermittently discharging machines, and also to
Centrifuges of radically different design, such as decanters. However at the
current time there are insufficient working data to warrant detailed discussion of
the use of scale-up techniques in these other circumstances.
To exemplify the various test and calculation methods we have selected a
number of “case histories*' from bioseparation experience at GBHE. This is one
of the most demanding duties conceivable for a centrifuge due to the small
particle size and the cohesive nature of many of the suspensions concerned.


1.2

Centrifuge Types

In centrifugal separators the rate and degree of disengagement of solids from a
liquid medium is enhanced by a centrifugal field due to rotation of portions of the
machine. Equipment of this type falls into two main classes: “centrifuge filters”
and settling centrifuges”.
In “centrifugal filters” the phenomenon which gives concentration of the solids is
a filtration mechanism. The particles are trapped by a filter medium of some
description whilst passage of the liquid is accelerated by the field. Estimates of
the performance of such operations are made by appropriate modification of
standard filtration theory, based on Darcy’s Law (see Sections 3.2 and 3.5)) to
allow for the “g” force. Procedures of this kind are described in references such
as [1], [7] and [10]. Though far from satisfactory, these predictive techniques are
the best that are currently available. As a rule centrifugal filters are not used for
separation of very fine particles: about 10 µm is the usual lower limit on particle
size whilst in most applications for which filtering centrifuges are employed the
species are =: 100 µm or greater. Accordingly, such processes are on the margin
of the scope of this manual and will not be further considered.
In centrifuges based upon a settling principle, it is migration of the solid particles
through the liquid phase that is enhanced by the centrifugal field. As will be seen
later, this may involve either sedimentation of single particles in a Stoke’s Law
type of “free fall” or consolidation of a structured (thickened> sludge. As noted
above, settling rather than filtering centrifuges are appropriate for fine species;
particles down to the order of 0.01 µm can be dealt with in appropriate
circumstances. Machines come in very many designs – disc nozzles, decanters
and so on - all of which have their particular operating regimes. Details of the
principal kinds of settling centrifuge are given in reference [10]. It should also be
noted that centrifuges may have intermittent or continuous solids discharge or
may be operated in batch mode.
Figure 1 represents schematically the continuous disc-nozzle centrifuge which Is
employed extensively in fine particle separations. Feed Is introduced into the
many separation channels via distribution holes In the stack of discs. The whole
assembly is rotating at high speed and the strong centrifugal field produced flings
solids to the outside of the bowl. Here they are discharged through nozzles.
Hydrostatic pressure forces the less-dense liquid phase to flow against the
gravitational field and towards the centre of the machine where it overflows a
level-controlling weir.

Figure 1
In this section we will focus our discussion on this kind of process. We will
consider the way that the output of a given centrifuge is limited by one of three
main mechanisms, viz:
(i)

Failure to capture the fine particles contained in the otherwise clear
centrate.

(ii)

Failure to give sufficient time for the concentrated stream to adequately
thicken.

(iii)

Flux limitation caused by particle:particle interaction.

The first of these is likely to be the limiting mechanism when centrifuging weak
suspensions of discrete, compact particles of small diameter. The second occurs
when attempting to obtain a thick concentrate from an open, flocculated system
having a tendency to form a cohesive structure at low concentrations. The third
concerns a transition region between particle capture and thicks consolidation
and is most likely to occur with a dilute feed of slowly sedimenting particles.


It will be demonstrated how a knowledge of the free fall velocity under a known
gravitational pull, the resistance to compression of the thickened matrix and
some typical centrifuge parameters (dimensions, speed, etc) allow a ready
assessment of the likely limiting mechanism. A strobe-centrifuge test, a
measurement of elastic modulus and some machine specifications are
(theoretically) all that are required.
Armed with these data it is then possible to predict the performance which would
be obtained on any commercial centrifuge. An insight into the limiting mechanism
will also allow a quantitative assessment of the benefit of modifying process
conditions (e.g. feed concentration > or machine parameters (e.g. number of
discs) without the need for time consuming, expensive trials. An informed
judgment on the suitability of centrifugation for a given duty can also be readily
obtained.
Currently, the battery of scale-up techniques available for disc-nozzle machines
have not been properly extended to, for example, decanters. It is probable that
the concepts of Section 3.4.3 are applicable in such circumstances. Further work
In this area will be desirable In the future, especially as existing procedures,
based exclusively on Ʃ theory (see Section 3.4.3 and reference [16] ), are
obviously inadequate. In particular, decanters are perhaps at best advantage for
thickening to high solids (when their method of solids discharge is more
appropriate than that of the disc-nozzle design). In such a regime it will be
probable that the process Is solids rather than clarification limited as is assumed
in the Ʃ approach.
1.3

What is a Disc Centrifuge and How Does It Work?

First, consider a simple clarifying tank in which a single, dense particle is settling
out under the force of gravity: Figure 2


The particle will have a downwards velocity of ug m/s (terminal velocity for free
fall under gravity). It will also be carried along horizontally by the bulk liquid at
velocity v = L/bh m/s.
In order to reach the solids discharge point, the particle must have fallen distance
h before traversing distance 1

Therefore the maximum feed rate that the tank can clarify is given by

i.e.

Max Liquid Rate = Tank Surface Area x Settling Velocity under gravity

The performance of the tank can be improved in one of two ways:
(i)

By increasing the gravitational field. An n-fold increase will give an n-fold
Increase in settling velocity and hence allow an n-fold increase In
throughput.

(ii)

B y reducing t h e height the particle has to fall, e.g. by partitioning the
tank as shown below:
Figure 3


Drawn apart for clarity, this arrangement effectively gives twice the tank area for
the same physical volume, ie the tank separation capacity has been doubled.
Taking the above comments to their logical conclusion would give a conceptual
centrifuge design as below, In which a series of concentric cylinders (5 tanks) are
placed In an enhanced (rotational) gravitational field.

Figure 4
This is not a practical arrangement because there is no convenient means of
separating solid and liquid flows. In practice, discs are Inclined to the
gravitational field with the feed entering through holes in the discs. The solids
migrate along the underside of the discs to the bowl periphery whilst the clarified
supernatant flows inwards.


Figure 5
Liquid inventory in the centrifuge is kept constant by a weir arrangement as
shown below, the liquid finding Its own level with respect to the applied
gravitational field. The thickened sol ids discharge through a number of nozzles
at the periphery of the bowl, the discharge rate being controlled (for some
machines) by varying the nozzle diameter; less sophisticated machines may not
have on-line control but different sizes of nozzles will generally be available.
It will also be noted that the feed enters the machine axially, typically from above.
Ribbed rotating surfaces in the feed zone help accelerate the Incoming liquid to
bowl velocity. Any necessary further acceleration will be imparted by the rotating
disc-stack.
As a further variation, the solids may be transferred through tubes from the bowl
apex back towards the centre of the machine before reaching a nozzle and being
discharged. The reduced hydrostatic pressure here (smaller radius of rotation
and lower liquid depth > mean that larger nozzles can be used for the same
discharge rate, thus reducing the risk of blockage by thickened material.


Figure 6

1.4

Operating Regimes – What Limits the Machine’s Output?
1.4.1 The Mechanics of separation

Despite engineering complexities (see Figure 6) the mechanics of separation in a
disc-nozzle centrifuge are straightforward. Feed Introduced to the disc stack Is
conveyed to the periphery by the strong centrifugal field consequent on the
rotation of the disc stack and bowl. The thickened solids are discharged through
nozzles whilst the less dense liquid is forced out at the weir by hydrostatic
pressure. The specific capacity of a centrifuge to concentrate a particular
suspension is however determined by details of the machine's geometry and
other characteristics: the properties of the feed, and the degree of thickening
needed for a given duty.


1.4.2 Nozzle Characteristics
1.4.2.1

Effect of Solids Throughput

It is important to realize that, for a given rotational speed and nozzle diameter,
the volumetric discharge rate of thicks through each nozzle is fixed. This Is
because the pressure drop across the nozzle is determined solely by the liquid
depth and centrifugal field In the machine.
Increasing either the feed rate or feed concentration will increase the quantity of
solids to be discharged in this constant volume, and hence increase the thicks
concentration. This represents one of those rare occasions when driving a piece
of equipment harder will improve its performance!
Eventually, however, the machine's separation capacity will be exceeded (for one
of a number of reasons to be discussed later) and solids will be lost into the
centrate. The point at which this occurs is called the breakthrough point.

1.4.2.2

Effect of Nozzle Diameter

Increasing the nozzle diameter (or the number of nozzles) will allow a greater
volumetric rate of thicks to pass. It is now possible to reach a new breakthrough
point where the solids throughput is higher. Generally, however, the solids
concentration will have been reduced (see later).
The locus of breakthrough points which can be achieved by changing the nozzle
diameter Is termed the breakthrough curve. By using a variable diameter nozzle
it is possible to operate with clear centrate at any point on or below this curve.
The above points are sketched below, where thicks concentration is plotted
against solids throughput. The latter represents a measure of both feed rate and
concentration.


Figure 7

1.4.2.3

Theory of Separation

The detailed hydraulics of high-speed centrifuges are horrendous. Even
“simplified” treatments are often mathematically and conceptually complex
despite the gross assumptions that are usually made. Only concepts directly
relevant to separation power will therefore be discussed.
For the purposes of many applications the separating power of a centrifuge may
be defined as the rate and concentration of thicks attainable at a given solids
throughput while maintaining a nominally clear centrate.
This immediately suggests two separate mechanisms which could limit centrifuge
performance, i.e. the ability to thicken concentrate and the ability to clarify
centrate.


1.5

Thickening capability (solids-limitation)

When solids are flung to the bowl periphery they will rapidly form a structured
mass from which supernatant has to be squeezed. The rate and extent to which
this compression can occur will be roughly proportional to the applied force - a
greater gravitational field will speed up the thickening process and also Increase
the maximum concentration that can be achieved at low rate (the ultimate
thickening or “structural” limit of Section 3.2).

Figure 8
The gravitational field increases with the square of the rotational speed and is
proportional to the radial distance from the axis of the machine. The time
available for compaction will be roughly proportional to the volume of the
peripheral space and is inversely proportional to throughput.
Two points then follow:(1)

A larger machine will generally pull a greater gravitational field and so
improve the degree of thickening potentially available.

(2)

Increasing the throughput will decrease the residence time and reduce the
actual thickening achieved. This is one possible cause of the trade-off
between rate and concentration referred to earlier.


Relatively quick laboratory-scale stroboscopic centrifuge and pulse shearometer
Tests allow the compaction characteristics of the solids to be readily quantified
(see Section 3.4.2). A knowledge of a machine's dimensions then allows a
reasonable prediction of its thickening potential.
Wlth true solids-limited operation the feed concentration entering the centrifuge is
relatively unimportant. Compression times tend to be dominated by the residence
time available at the higher concentrations (where resistance to further
compaction is greatest).
The disc stack is also comparatively unimportant for solids-limited applications;
the discs provide surface area for clarification rather than residence times for
solids compaction. If a machine is thought to be solids limited, there Is therefore
little point In Installing a pre-thickening stage or modifying the discs.

1.6

Clarification capability (Hydraulic limitation)

The following sketch illustrates the paths followed by particles in the disc stack.

Figure 9


The gravitational field throws the particles onto the upper disc where they are
captured and can subsequently migrate to the bowl periphery. As shown, particle
E just fails to reach the disc before being swept out with the centrate; all other
particles are captured. Increasing the centrate flow will increasingly sweep more
particles out In the centrate stream, le the breakthrough point has just been
passed.
This clarification limit depends on centrate flowrate and therefore the machine's
solids handling capacity will be strongly dependent on feed concentration. Disc
stack configuration is similarly important: doubling the number of discs will
roughly halve the distance each particle has to travel before being captured. This
will theoretically double machine capacity, although other effects, e.g. disc
thickness, flow maldistribution, etc, may be significant.
Clarification limitation can be readily identified If different feed concentrations are
fed to a machine because a whole family of breakthrough curves will be
obtained, one for each feed. This follows from a simple mass balance If It Is
recognized that breakthrough will always occur at a constant centrate rate.
A comparison between solids and clarification limitation Is shown below:

Figure 10


A knowledge of the terminal velocity of particles falling through supernatant
under a known g-force should allow a quick estimate of the clarification power of
a given centrifuge. Such knowledge can be obtained from a strobe centrifuge test
(see later).

1.7

Hindered settling (Flux limitation)

This limitation is less obvious than the previous two but may be equally as
important. It is a well known effect in gravitational thickeners (see Section
3.3.4(c)).
Settling of solids through supernatant requires particles to move relative to liquid.
The relative velocity is generally proportional to the applied gravitational field but
decreases as the solids concentration builds up. This latter phenomenon is called
“hindered settling”. It results from increasingly significant effects of hydrodynamic
interaction between settling species as the average distance between them is
reduced by rising solids content.
For a constant volumetric flowrate of fluid the solids flux must, in the absence of
particle-particle Interactions, be proportional to solids concentration. If hindered
settling is taken into account, however, It can be shown that there is an absolute
maximum to the solids flux that can be obtained.
The effect is typified in gravity settlers by a critical concentration which fills the
settler; solids In excess of the maximum flux are lost to the clarified liquor. The
critical concentration reached depends on the feed concentration but will typically
be before compression effects come into play.
There are two possible places where a maximum solids flux may limit centrifuge
performance:
(1)

In the disc stack where solids are settling out of the clarifying centrate.

(2)

In the bowl periphery where solids are migrating towards the nozzles.


Analysis is complicated by the geometry of centrifuges and the lack of knowledge
of flow regimes which pertain. Considerable work on point (1) was carried out in
1979 C [18] which showed that the net effect is not dissimilar from clarification
limitation. A full analysis of point (2) has yet to be completed.
Further work in this area is required. However, there is no fundamental reason
why the maximum solids handling capacity of a given machine should not be
predicted from knowledge of particle terminal velocity as a function of
concentration. Such information should be yielded by a strobe centrifuge test.
1.8

Comments
(1) The underlying principles of centrifuge operation are relatively easy to
comprehend even If the mathematics are frightening.
(2) A few strobe centrifuge and pulse shearometer tests of material on a
laboratory scale can theoretically yield all the information required to
predict centrifuge performance. Extra rheological measurements aid
interpretation and provide a useful check on the validity of the models
used.
(3) Reality is actually much more complicated than the previous discussion
suggests (maldistribution, vortex formation, counter-current shear, etc). A
truly reliable theoretical model is therefore a remote possibility. However,
the general principles discussed should hold to some extent and an
estimate of likely centrifuge limitations should be possible.

In subsequent sections we detail useful laboratory tests for prediction of
centrifuge performance; develop scale-up theories; and describe and exemplify
the procedures necessary for their application.


2
2.1

USEFUL LABORATORY TESTS
Background

The centrifugation characteristics of a particular suspension may, of course, be
assessed empirically by carrying out trial runs in a centrifuge of similar design to
that proposed for use in the separation process. Alternatively, and generally
more desirably, various procedures can be employed to characterize the material
in the laboratory and estimate behavior over a range of regimes. The second
route Is, as a rule, less expensive and time-consuming and it gives a predictive
capacity unattainable by the empirical approach. Also, when novel solids
separation problems appear, it provides the only method of obtaining order-ofmagnitude values for centrifuge requirements, other than a completely "hit or
miss" series of tests with a variety of machines. We would caution, however, that
complete dependence on laboratory characterization is unwise: because of the
complexity of the effects involved in centrifugation It is prudent to back up smallscale assessment of suspension behavior with at least a few semi-technical runs
to test predictions.
The laboratory tests used to characterize the centrifugation of a suspension are
relatively few in number, and simple in concept. They are best understood by
reference to the basic phenomena underlying all separations dependent upon
solids settling, whether accelerated or not. Figures 11 and 12 illustrate these
effects. The first of the diagrams shows typical settling curves under normal
gravity and under enhanced "g". In the Initial stages the sedimentation graph
tends to be linear, then settling slows as particle interaction or "crowding"
occurs. Eventually, solids content reaches a limiting value as the forces
promoting consolidation are balanced by the internal strength of the sediment.


Figure 11
Typical settling curves for a suspension sedimenting under "lg" and "ng”

Figure 12

Modulus (i.e. network strength > versus solids content for a typical cohesive
suspension.
In many cases there are attractive particle-particle interactions in the suspension
being centrifuged, either due to deliberate flocculation or to a naturally occurring
aggregated state. Here the ultimate solids content achievable (“the structural
limit") is a consequence of the buildup of a cohesive structure between the
interacting floes (see Section 3.4.3(b)). In some cases the suspension being
centrifuged is colloidally stable, i.e. there are repulsive interactions between the
particles. Here the ultimate solids content achievable is when the particles form a
close packed cake.
The effect of increasing acceleration to "ng" is two-fold: settling rates are
enhanced (usually linearly with n In the early stages) because o f the greater
driving force for separation, and the "structural limit" is pushed up as greater
sediment strength (i.e. solids content> is required to resist the increased
compression. The settling curves are dependent upon suspension characteristics
(e.g. degree/strength of flocculation) and the physical set-up of the experiment
used to derive the data < e.g. the number of "g" employed; height of column of
sediment used). In principle, a family of such graphs contain u the information on
a particular suspension needed for prediction of its large-scale centrifuge
behavior. Initial settling rates allow calculation of throughput in the clarification
regime, whilst those for later stages of consolidation are needed for estimation of
centrifuge requirements under solids-limitation conditions. The behavior of the
"structural limit", as a function of *ng” also enables calculation of the maximum
solids that could ever be attained from a particular size/speed of machine,
irrespective of the manipulation performed to increase residence time available
for consolidation.
In practice, it Is useful to have a bit more data than simply the raw settling curves
for a sequence of accelerating conditions. There are a number of reasons for
this, perhaps the most important being the difficulty in identifying transitions from,
say, " free fall " of particles to a hindered settling or compression regime (Section
3.3.1). However, this problem is readily circumvented by measuring the shear
modulus (explained In 3.2 and which Is a measure of strength of any cohering
structure> as a function of solids. Figure 12 shows the general form of
modulus/solids content curve observed for almost all suspensions. In the
absence of structure the system is almost certainly going to be clarification
limited, though conceivably, if the zone extended to relatively high solids contents
(say > 10-15% solids by volume) hindered settling phenomena could begin to
matter. Equally, where moduli are large 0 104 dynes cm-2) the rate determining
effect in centrifugation will probably be sediment consolidation and the material
will display solids limitation in the machine.

Two final points are apposite before a description of actual test methods:
(i)

There is some evidence [2] that at extreme values for sediment cohesion
thickening becomes limited by the material becoming too viscous to pump
(a “rheological limit”). The position of the limit is difficult to define precisely
as it is dependent upon machine configuration, and as It is really a
function of viscosity at high shear rates, the latter quantity being only
indirectly related to modulus [3,4]. However, it is reasonable to expect
potential problems once the modulus rises to ~ 106 dynes/cm2 and the
material starts to exhibit substantial (stiff) paste-like qualities. It should be
noted that the “rheological limit” for , say, a scroll discharge decanter
machine will be greater than that for a disc-nozzle centrifuge due to the
difference in method of sol ids discharge.

(ii)

Because of the asymptotic manner in which the “structural limit” Is
approached, it appears not to be feasible to operate a centrifuge to thicken
anywhere close to this value: required residence times become
impractically long. Thus if calculations suggest that a proposed centrifuge
is just capable of thickening to the required degree, without any
substantial “margin”, it is likely that it will not prove satisfactory in service.
In Figure 11 we have indicated the kind of “practical” thickening limit often
seen in operation.
2.1.1 Basic Suspension Properties
Before carrying out any other measurements, it is essential to obtain
certain basic data concerning the feed, viz the particle size, the density of
the solids, the state of aggregation of the species. Simple procedures,
such as optical microscopy, are often entirely appropriate to the purpose.
Further details on methods which can be applied are given In reference
[19]. For biological particles such as bacterial cells the density may often
be accurately measured via a silica suspension density gradient [2].

2.1.2 Settling Kinetics
The standard, and so far best, approach to measurement of settling
kinetics involves use of a stroboscopic centrifuge. Essentially this is a
laboratory (batch) centrifuge with synchronized stroboscope which allows
observation of the rate of settling of the material over time. Suitable
machines can either be bought from a commercial manufacturer, or can
be readily developed by simple adoption of standard laboratory
centrifuges.

Although the strobe centrifuge allows measurement of required rate data
for calculation of centrifugation behavior in both thickening and clarification
limited regimes, some precautions are necessary for extraction of the
appropriate data In the latter case. What is required is the "free fall"
velocity of a particle or individual floe. Thus, for example, settling velocity
must be measured at several dilutions to ensure that weak (concentrationdependent) flocculation between the particles, is not perturbing the
Stokesian single-particle sedimentation. Alternatively the Stokesian
sedimentation rate can be estimated from first principles (Section 3.3.1).
If the suspension has a wide particle size distribution It is also important to
ensure that the appropriate velocity is measured. With a dilute suspension
of fairly uniform (non-cohering) particles a sharp descending interface
forms In the initial stages of the centrifugation experiment whereas for a
hetero-disperse system the larger particles will often have formed cakes at
the bottom of the tube before the smaller ones have settled appreciably.
These systems require the appropriate sedimentation velocity of a given
clarification criterion: e.g. if the separation criterion Is 100% clarity of
centrate then the sedimentation velocity of smallest particles is the most
appropriate quantity for calculation and so on (see Example (d) Section
3.4.5).
Further details on the precautions to be observed in the measurement
and interpretation of settling kinetics are given in reference [19].
Though strobe centrifuge measurements provide the best approach to
characterizing settling kinetics, we would note that information obtained
from even a basic bench centrifuge is better than nothing. Indeed this
appears to be the procedure followed by centrifuge manufacturers under
such names as "spin test" [6].

2.1.3 Network Strength Measurements
These can be made in two ways: from slow-speed centrifuge experiments
or by employment of the so-called pulse shearometer. Details of these
techniques are provided in Section 3.2 of this manual.


2.1.4 Floc Stability Assessment
Fine particle, particularly biological, suspensions are usually flocculated
prior to centrifugation to achieve acceptably rapid separation. It is
necessary, of course that the Flocs are sufficiently robust to stand up to
the disruptive forces present in the centrifuge otherwise the benefits of
size enlargement are dissipated. Unfortunately, current tests for floe
stability are rather limited in diagnostic capacity and can only act as a
rough guide to actual behavior In a large scale machine. The main reason
for this state of affairs Is the difficulty in characterizing the complex shear
and elongational stresses prevalent in a disc-nozzle centrifuge. Test
apparatus cannot be designed because of lack of information on the
situation likely to prevail In practice. However, some assessment can be
made of the stability of a flocculated suspension towards breakup, albeit of
a rather empirical nature. The standard approach involves measuring a
quantity dependent on Floc size, subjecting the suspension to a
prescribed amount of agitation from some kind of mixer, and then remeasuring the property. The degree to which the two values differ is an
Index of Floc (in) stabllity.
Settling rate (as determined by, say, the strobe centrifuge) Is a good Flocsize-dependent characteristic to observe, though filtration times are also
employed [8,9]. (Filtration is generally faster the larger the Flocs. If
significant floe breakup occurs, filtration rates slow dramatically due both
to reduced cake permeability and to blinding effects. ) The problems arise
with respect to prescription of the appropriate degree of agitation for the
reasons mentioned above. Commercial apparatus (ex Triton Electronics
Limited) Is available for the task (see reference [81, p179] but a simple
agitator in a beaker probably serves as well. Clearly the method cannot
work on an absolute basis but satisfactory results can be obtained by
comparison.
In addition a relatively new instrument produced by Rank Brothers, the
Photometric Dispersion Analyzer, allows the correlation of Floc size and
strength with flow-, and hence, shear rate. For example, suspensions
obtained by use of different flocculants can be assessed in order to
determine the most effective agent In terms of Floc robustness. Similarly,
the stability of new systems can be compared with that of a material
known to retain integrity in the large-scale centrifuge under consideration.


Finally we would note that a number of other laboratory techniques can
play a useful, but lesser role in providing data for prediction of
centrifugation behavior. Rheological methods help to identify when the
flow behavior of the material may be limiting [2] whilst optical microscopy
can be of great value in diagnosing variations in aggregate size which can
lead to erratic performance [2, 4]. The latter is probably also the only way
that the presence of attached gas bubbles - a great hindrance in
centrifuging biological suspensions due to their buoyancy effect - can be
proved. Simple sedimentation tests also have their place - settling usually
stops under “lg” just where network strength becomes significant. Thus the
suspension concentration process up to this point will probably be
clarification limited.

2.2

Test Techniques

Laboratory test methods resolve themselves into four main classes:(i)

Basic suspension properties;

(ii)

Procedures for measuring settling kinetics;

(iii)

Network strength characterization methods;

(iv)

Techniques for assessing the likelihood of aggregate breakup
in the machine.


3

SCALE-UP THEORIES

The appropriate scale-up theory depends on the controlling mechanism likely to
pertain, e.g. clarification, compression or flux limitation (see Section 3.4.1(d) for
details. These will be considered In turn.
3.1

Scale-Up When Clarification is the Limiting Process

3.1.1 Qualitative
Referring back to Section 3.4.1(c), if the solid particle is unable to reach
the base of the gravity settling tank before leaving, then it will pass out in
the supernatant stream, ie full clarification of the liquid overflow will not be
achieved. It was shown that:
Max Liquid Rate = Tank Surface Area x Terminal Velocity under one
Gravity
The clarification capacity of a disc centrifuge can similarly be quantified
and is given the symbol Ʃ
.
The Ʃ value of a centrifuge represents the area of a simple gravity settling
tank (m2) which would be required to give the same separation capacity.
3.1.2 Quantitative
Calculation of the Ʃ value of a known centrifuge is reasonably
straightforward and the standard derivation is outlined below:
For a single disc


Figure 13
Area of disc between r and r+ δ r is given by


Gravitational field resolved perpendicular to disc

3.1.3 Validity
Although Westfalia Separator use the above equation for Ʃ, Alfa Laval use
an empirically “corrected” version of Ʃ, called KQ, where

Centrifuge capacities may be compared by means of their relative KQ
values. However, unlike Ʃ, KQ has no absolute physical interpretation.
The use of either Ʃ or KQ is widespread and indeed often considered to
be the only scale-up parameter of importance. It is, however, only
meaningful when clarification is the limiting process. It has no relevance to
solids-limited applications.
Other mechanical factors may also reduce the applicability of the above
formula, e.g. too narrow disc spacing or Inadequate feed acceleration.
Other factors that should ideally be considered include the significant
Coriolis forces present, and the inevitable spread in actual terminal
velocities.
3.1.4 Application
The Initial settling rate of particles under a known gravitational field can be
determined using a strobe centrifuge test. If the terminal velocity is u (m/s)
under n gravities, then the gravitational terminal velocity


It has previously been shown that a gravity settling tank of area Ʃ (m2) can
cope with a liquid throughput of

Similarly, a centrifuge of equivalent settling area Ʃ (m2) could cope with a
liquid throughput (i.e. centrate) of

A more empirical approach may be preferable if data from e.g. semitechnical centrifuges are available. If the performance of a small centrifuge
is known the likely throughput on a large machine may be predicted from
the ratio of the respective Ʃ values for the two:

It must be emphasized that this empirical approach should be used whenever
possible. To scale from the behavior of a single particle in a spinning test-tube to
a commercial scale machine may be a useful scouting technique but Is less than
accurate.


3.2 Scale-Up When Thickening is the Limiting Process
3.2.1 Background
A rather different scale-up method is required when the centrifuge may be
in a solids-limitation regime, i.e. the key kinetic process is the (slow)
consolidation of the network of Flocs in the partly thickened sludge.
In this case the principal quantity of importance is ṫ, the residence time
essential to give thickening to the required solids content, under the given
centrifugal field, This is solely a property of the material and thus the
suspension properties require characterization before accurate scale-up
estimates can be made.

To reach a given thickness, a minimum residence time ṫ, must be allowed.
For a given radius, this then limits the volumetric rate of the concentrate
stream to be

where V is the volume of solids held in the machine. The calculation
of ṫ and V will be discussed shortly.
3.2.2 Scale-Up Factor
Ideally, a simple scale-up factor analogous to the Ʃ value in clarification
theory is desirable. Such a factor, F, is described below.
The concentrating pressure, P, developed in a centrifuge can be defined
as


The time required for a given degree of consolidation will clearly depend
on the concentrating pressure available. It is reasonable to assume that

The effective capacities of two centrifuges can now be directly compared if
their volume and concentrating pressures are known, viz

The R.H.S. of the equation is thus the required scaling factor, F.

3.2.3 Three Methods of Application
3.2.3.1 Semi-Empirical Approach I
If data on a given material is available on the semi-technical scale,
then the performance of a full-scale centrifuge can be predicted
directly from equation (16) provided that the machine's geometry
and speed are known, and provided that a value for y can be
estimated (see below).


3.2.3.2 Semi-Empirical Approach II
A check on the above method is available if the capacities of the
two centrifuges are known on another similar type of material. In
these circumstances the ratio of concentrating pressures will be
dominated by differences in machine configuration rather than
differences in feed material; the volume term is clearly material
independent. For centrifuges 1 and 2, and feed materials A and B,
It then follows that

I.e. use scale-up data obtained on material A to predict how
material B will behave in the larger centrifuge.

3.2.3.3 Ab Initio Approach
Insight into the compaction characteristics of a material can be
gained quickly from small quantities of feed by using a strobe
centrifuge, where the time required to achieve a given compaction
under a known concentrating pressure can be found. Knowing the
concentrating pressure and volume of a larger centrifuge, the
maximum allowable nozzle discharge rate can be readily
calculated.

3.2.4 Estimation of Necessary Parameters and the Problems
Encountered
3.2.4.1 Relationship between ṫ and P
The value of y in equation (15) is difficult to establish a priori.
Fortunately, the concentrating pressures developed by both semitechnical and full-scale centrifuges (e.g. 'Westfalia NA7, Alfa Laval
FEUX 320) are not dissimilar, since they are limited by materials of
construction, and putting y = 1 is probably sufficiently accurate for
the above semi-empirical approaches. Note that this choice (y = 1)
is essentially the basis of the scaling rule presented earlier in
Section 3.2.7.

At present, however, strobe centrifuges give lower concentrating
pressures and extrapolation to full-scale is thus liable to major
inaccuracies. The Ab initio approach, therefore, although useful as
an initial scouting technique, should not be relied on for detailed
work.
3.2.4.2 Volume of Slurry in centrifuge
The total volume which might be filled by thickened slurry
(including, for example, the volume occluded by the spinning discs)
can be evaluated by simple geometry from the known dimensions
of the centrifuge. As a first approximation it appears reasonable to
take the maximum allowable inventory of thickened slurry as, say,
half this volume. (In the semi-empirical approach errors in this
assumption will largely cancel but the Ab initio approach is
potentially more sensitive to the basis at this part of the calculation.)
3.2.4.3 Pressure (or “field”) Factor
Estimates of this factor are based upon equation (14). For the
strobe centrifuge the quantities to be used are self evident and are
detailed in the worked examples (see later) but for the continuous
machine some explanation is needed.
(1)

Ho, the depth of the bed of consolidating sediment, is taken as the
distance between the disc stack periphery and the nozzle.

(2)

“ng" is some mean of the centrifugal field at the disc periphery and
at the machine wall. For the present example we employed an
arithmetic average of the two values in question.

(3)

Ø.... n is taken as the solids content where the system starts to
display a perceptible cohesive structure, eg ~0.1 weight fraction
solids in the case of flocculated "Pruteen" suspensions. This is
because thickening to the so-called "gel point" will generally be a
much faster process than the subsequent consolidation of the
cohesive sediment. Note that, strictly, volume rather than weight
fractions should be used for i, but as the same conversion factor
will be used in all cases, and we are only interested in ratios of
pressures, it is satisfactory to approximate f by a weight fraction.


3.2.4.3 Concept of Ultimate Thickness
It is often useful to establish the maximum solids content which
could be attained with a particular centrifuge given an infinitely long
residence time in the machine (NB this is the structural limit, rather
than the kinetic limits discussed hereto). This can readily be
predicted from laboratory-scale tests using a pulse shearometer or
a slow-speed centrifuge (see Section 3.2 of this manual or
Appendix A, reference [19]).
The calculated ultimate thickness allows one to rapidly decide
whether a desired concentration can sensibly be achieved by
centrifugation.

3.3 Scale-Up When Hindered is the Limiting Process
3.3.1 Background
The terminal velocity of a swarm of particles is generally less than that of a
single particle falling in an infinitely wide pond; the more crowded the
particles, the slower they fall. This is called "hindered settling".
The Ʃ theory (Section 3.4.3(a)) implicitly deals with the initial capture of
isolated particles from virtually clear supernatant. For capture to be
completed, however, these particles need to be consolidated into a film on
the underside of the discs so that they can subsequently migrate towards
the bowl periphery. If, as a result of hindered settling, a "traffic-jam"
prevents this consolidation, the disc-space will fill up with solids and lead
to carry-over in the centrate.
As with the derivation of Ʃ, the problem of hindered settling will first be
analyzed in terms of a simple gravity thickener (cf Section 3.3.4). This will
then be related to centrifuge performance.


3.3.2 Gravity Thickeners – Behavior
The theory of even "simple" gravity thickeners is complex. An outline of
the key points is given in reference [14]. What follows is a simplified,
overall summary. (Further discussion may be found under Sedimentation,
Section 3.3.4(c) as well as many relevant references.)

3.3.3 Batch Thickeners
The flux-rate of settling particles is proportional to their settling velocity
and their concentration. However, because their velocity will fall and their
concentration increases, the following relationship results:

Figure 14


3.3.3.1 Continuous Thickeners
In a continuous thickener there is a net bulk downflow caused by
the continuous removal of sludge at the base of the thickener which
gives an extra downward flux (proportional to the solids
concentration) .This modifies the above plot to give:

Figure 15

There will often be a minimum in this curve, shown occurring at
concentration G* in the above plot. If, as is frequently the case, the
feed concentration is lower than G* and the sludge concentration
higher, then G* represents the maximum solids flux that the
thickener can handle.
If a greater solids loading is imposed, then the thickener can no
longer cope. It will fill up with material of concentration G* and
excess solids will be lost to the previously clear overflow.

3.3.4 Gravity Thickeners – Interpretation of Batch Settling Test
In a batch gravity test, the sediment Interface height above the vessel
base is measured as a function of time:


Figure 16
(subscript o refers to the initial conditions)
The Yoshioka Method can now be used to determine the limiting flux rate.
This method is detailed in reference [10] and only the mechanistic steps
are shown below:
From the plot of Interface height against time

A more time consuming, but potentially more accurate, method would be
to make up feeds of varying concentration and measure the initial settling
rate. This may well provide a useful check on the quicker, less reliable
construction used above.
From the required discharge concentration, co, construct a tangent to the
batch settling curve and read offs the intercept:


Figure 17

3.3.5 Application to Centrifuges
3.3.5.1 Batch Flux Curve
The batch settling test will typically be performed in a strobe
centrifuge. The equivalent gravity test can be obtained by
expanding the time scale proportionately to the mean g-force
prevailing.
The preceding analysis will then give the minimum allowable flow
area in a gravity settler to permit the desired concentration to be
obtained.
The equivalent settling area of a centrifuge is given by Ʃ. The solids
flux calculated above may therefore be multiplied by Ʃ to test
whether a given centrifuge can cope with the required duty.

3.3.6 Discussion
(1) The gravity settling theory is fraught with assumptions and by no means
universally valid. To further extend this to the complex flow regimes of a
centrifuge must be questionable.
(2) It is not clear what concentration to choose for the .underflow" In the
above graphical construction, It is probably ML the nozzle discharge
concentration but this should give a conservative result.
(3) The identification of points a and b is generally far from straightforward
given the quality of data experienced in the real world. Point b is the most
important to determine as the analysis breaks down beyond this. Possibly
it may be most appropriately determined experimentally as the
concentration to which particles will separate under gravity in the absence
of applied or hydrostatic pressure. An alternative approach to the problem
utilizes a log-log plot based on Figure 16 to identify the compression point
or the so-called Roberts plot (see reference [3] of Section 3.3, pp112-114).
(4) Scale-up from strobe to production centrifuges is not to be recommended.
However, the Ʃ ratio between two disc centrifuges probably reflects their
relative capacities for hindered settling limitation. This comes as a great
relief it is a simple result from a complex situation.

4

A SYSTEMATIC APPROACH TOCENTRIFUGE SCALE-UP

The following steps are recommended:
(1) Unless there is strong supporting evidence one way or the other, It is
dangerous to jump to a conclusion regarding the limiting mechanism In
centrifuge operation.
(2) Carry out a strobe test and modulus measurements on the material in
question. This will provide useful information on the likely mode of
limitation. Follow the three "ab initio" calculation methods given later ,
and estimate ultimate thickening limits, to establish what is likely to
limit centrifuge performance.


(3) Whenever possible, carry out semi - technical trials. By varying
parameters such as nozzle diameter, feed concentration and number
of discs, the limiting mechanism can be checked and quantified.

(4) If possible, compare results with those for any similar materials where
large-scale operating data may exist. Use the semi-technical machine
on the other materials as well so that the 'semi-empirical" calculation
methods can be employed.

(5) Use the scale-up methods to predict how many centrifuges of a given
type will be needed to just cope with the envisaged duty.
(6) It must be remembered that operational centrifuges generally run away
from their theoretical maximum to ensure centrate remains clear
despite minor process fluctuations. Allow, say, 20% installed spare
capacity.
(7) Further allowances for longer term deviations from ideal plant
conditions may be appropriate.
(8) CIP sequences (e.g. up to 1 hour per shift) need to be accommodated.
(9) Finally, some spare capacity to cope with the inevitable mechanical
breakdowns should be considered.

See next section for some worked examples.

4.1

Validity of Different Methods
Certain of the procedures - e.g. the Ab lnitio approach to calculation of
centrifuge performance in a clarification limited regime - give a definite
upper band to performance. However, this kind of useful relationship does
not hold for the semi-empirical technique based on Ʃ theory unless one is
working rigorously within a clarification regime. Outside of the latter,
thickening becomes relatively independent of number of discs [11-13]
whereas Ʃ and KQ methods assume a linear dependence on this factor.
Thus scaling from semi-technical data, derived from a centrifuge with
closely-spaced discs, to predict performance of a machine with more


widely-separated elements can give misleading results. . An example of
this phenomenon is given later in the examples.

There are no particularly obvious ways round this problem but the best
elementary precaution is to vary the disc-spacing on the semi-technical
machine. If this has an effect, then the separation is likely to be either
clarification or hindered settling limited. In either case, the Ʃ or KQ
theories should be valid.
Manipulation of acquired data and close observation of the trials may
themselves provide valuable insight. We suggest that all scale-up
limitations be tested and quantified as far as possible, but that
commonsense and intuition are still important when working out the
implications. The need for a representative number of trials before
committing large sums of capital cannot be overstressed.
The overall scale-up procedure is displayed diagrammatically in Figures
18 and 19 whilst details on how to perform individual scale-up methods
are provided in Tables l-7.


Figure 18

Figure 19

Table 1
Steps in Ab Initio Scale-Up for Clarification Limited Systems
(see Section 3.4.3(a)
I

Determine free fall velocity of a single particle under a known gravitational
field from a strobe centrifuge test (Section 3.4.2 (b)).

II

Calculate the free fall velocity under a single gravity (equation (8)).

III

Calculate the C theory for the centrifuge under consideration (equation
(6)) using manufacturer’s specifications.

IV

Calculate the maximum centrate rate per machine (equation (10)).

V

Calculate the maximum operational feed rate by simple mass balance,
given the desired thicks concentration.

VI

Compare with other scale-up methods.

Table 2
Steps in Semi-Empirical Scale-Up for Clarification Limited Systems
(see Section 3.4.3(a)
I

Calculate I values for the semi-tech and production centrifuges (equation
(6)).

II

Scale centrate rates achievable on semi-tech centrifuge using equation
(11) to give maximum centrate rate possible on the production centrifuge.

III

Check semi-tech behavior as a function of number of discs or feed
concentration, le establishes whether clarification is the limiting factor.

IV

Calculate possible speed of maximum feed rates.

V

Compare results with those from other methods.


Table 3
Prediction of Ultimate Thickness for Solids Limited Systems
(see Section 3.4.3(b))
I

Use pulse shearometer to measure shear modulus of thickened solids as
a function of concentration (Section 3.4.2).

II

Calculate the internal pressure required to give the desired solids
concentration (reference [19]),

III

Calculate the maximum internal pressure that can be generated by the
centrifuge being considered (equation (14)).

IV

Check whether the available pressure is sufficient for the required duty. If
not, choose a more powerful centrifuge or modify the feed material. If it is,
check the clarification and compaction kinetics.

Table 4
Steps in Semi-Empirical Scale-Up for Solids Limited Systems
(see Section 3.4.3(b)
I

Determine breakthrough solids content as a function of nozzle rate for a
small-scale centrifuge, of comparable design to the subject (possible
production) machine.

II

Estimate relative maximum allowable Inventories of thickened slurry for (a)
small -scale machine; (b) subject centrifuge (see text for details).

III

Evaluate relative values of P for (a) small-scale machine; (b) subject
centrifuge, from their known speed, dimensions and solids loading.

IV

From II and III estimate the scale-up factor between the centrifuges as
discussed In Section 3.4.3(b).

V

Estimate maximum allowable nozzle rate, to give required thickening in
subject centrifuge, from equation (16).

VI

Calculate the maximum feed - rate, given the known feed concentration.

VII



Alternatively, where scale-up data for the two machines is known for a similar
material, the simplified procedure described in Section 3.4.3(b) (Iii) can be
substituted for steps I-V.
Table 5
Steps in Ab Initio Scale-Up for Solids Limited Systems
(see Section 3.4.3(b)
I

Measure settling curve(s) in strobe centrifuge.

II

From I evaluate ṫ, i.e. the residence time, to give required solids content In
strobe centrifuge.

III

Evaluate P (the consolidating pressure) in strobe centrifuge from its known
speed, dimensions, and solids loading (reference [19]).

IV

Evaluate P in subject centrifuge from its known speed, dimensions and
solids loading.

V

Using equation (15), evaluate required residence time for subject
centrifuge (assuming y = l),

VI

From centrifuge dimensions evaluate an approximation to maximum
achievable Inventory at thickened slurry in subject machine (see text for
details).

VII

Estimate maximum allowable nozzle rate, to give required thickening in
subject centrifuge, from equation (13).

VIII

Calculate the maximum feed-rate from simple mass balance.

IX



Table 6
Steps in Semi-Empirical Scale-Up for System Limited
by Hindered Settling Systems#
(see Section 3.4.3(c)
I

Calculate I values for the semi-technical and production centrifuges
(equation (6)).

II

Scale “thicks” rate achievable on the semi-technical centrifuge (for desired
degree of concentration) to give maximum probable rate on production
machine by multiplying flux by Ʃ production I semi-tech.

III

Calculate the corresponding maximum feed rates.

IV

Compare results with those determined by other techniques.

# NB Ab Initio scale-up from strobe centrifuge data should be treated with caution
but if such a calculation is needed as a guide it should follow the lines of Table 7.

Table 7
Steps in Ab Initio Guide Calculations for Systems Limited
by Hindered Settling Systems#
(see Section 3.4.3(c)
I

Determine batch settling flux for a known centrifugal field using strobe
centrifuge and the calculation procedure detailed in Section 3.4.3(c).

II

Convert above results to their equivalent for a single gravity settler.

III

Calculate the E value for the centrifuge under consideration (equation (6))
using manufacturer’s specifications.

IV

Calculate maximum thicks flux from product of I, and the single gravity flux
for the considered machine.

V

Calculate maximum allowable feed rate corresponding to IV.

VI

Compare results with those from other methods.

# See Table 6 re caveats concerning the results of this technique.

5 WORKED EXAMPLES

The following examples are taken from recent GBH Enterprises experience with
biochemical separations which are currently amongst the most demanding
centrifugation duties.
5.1

Solids-Limited Scale-Up: Background to Examples
(1) A European Pruteen plant separates biomass from fermenter liquor (3 wt
%) by flocculating the cells, concentrating the cells by flotation to 10X, and
subsequently centrifuging the cells to 17 wt %.
The preferred process now uses a different flocculation technique, giving
Flocs which are more amenable to centrifugation. The flotation stage
would be omitted.
It was necessary to estimate how many centrifuges would be required for
a full-scale plant using the new process, and to quantify the possible
benefits of using a larger but untested centrifuge.
(2) The existing plant has three Alfa-Lava1 FEUX 320 disc-nozzle centrifuges,
each capable of producing up to 2.9 te/h (dry basis> of 17 wt % material
(see Figure 20).
Semi-technical trials on a Westfalia NA7 machine (see Table 8) were
carried out on both the existing and the proposed new flocculation routes,
called Pl and P2 respectively.
A possible alternative to the FEUX 320 machines was the new Westfalia
HDA 300 disc-nozzle centrifuge, a more expensive but larger machine.
(3) Experience has shown that increasing the number of discs in the FEUX
320 machines had no effect on instantaneous capacity, i.e. the process is
probably solids limited. (Indeed, narrow disc spacing was detrimental due
to blockages.)
The semi-technical results showed that roughly similar throughputs on the
NA7 were possible for the two process routes, despite the great difference
in feed concentration. Given the broadly similar nature of the smaller
biomass debris, this also suggests that clarification is not a problem.


Centrifugation

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