Ullmann's Encyclopedia of Industrial Chemistry (2005) Part 2

10779 Pages·151.432 MB·English

Cements,ChemicallyResistant 7
Table3.Resistanceofcementstovariouschemicals
Chemical Silicate Sulfur Hydraulic Bituminous Phenolic Furanresin Epoxyresin Unsaturated Vinylester
cement cement cement material resincement cement cement polyester resincement
mortar cement resincement
Non- + + – + + + O + +
oxidizing
acids
Oxidizing + + – O O O O + +
acids
Silica- – + – + + + + + +
dissolving
acids
Bases – + O + O + + O +
Oxidizing – – O O – O O + +
bases
Salts + +/–a) O + + + + + +
Water – + + + + + + + +
Organicacids+ O – O + + O + +
Aliphatic + O – + + + + +
compounds
Aromatic + O – + + O – +
compounds
Alcohols + + + O + + O + +
Ketones, + – – – + + O – O
esters
Aliphatic + – O – + + – – O
chlorinated
hydrocarbons
Aromatic + – O – + + – – O
chlorinated
hydrocarbons
Aldehydes + – O O + + O O O
Aliphatic + + – + + – O +
amines
Aromatic + + – + + – – O
amines
Phenoles O – – – + + – – O
Fatsandoils + – O – + + O + +
a) resistantwithpH≤7; + resistant; – notresistant; O resistantunderspecialconditions
cangenerallybestoredfor12monthsorlonger 2. W.L.Sheppard:HandbookofChemically
atroomtemperatureandunderdryconditions. ResistantMasonry, C.C.R.M.Inc.,
Specialcaremustbetakeninthetransporta- Havertown,Pennsylvania,1977.
tionofthebindersbecauseoftheirreactivityand 3. F.K.Falcke,G.Lorentz(eds.):Handbookof
toxicity. Acid-ProofConstruction, VCH
Verlagsgesellschaft,Weinheim1985.
5. Toxicology 4. E.Schacht:“Ausmauerungeninchemisch
beanspruchtenBeha¨lternundApparaten,” Z.
The national guidelines for the handling of the Werkstofftech. 5(1974)297–307.
binder must be carefully observed. Indeed, the 5. J.Du¨ck:“Schwund-undQuellverhalten
bindingagentscanbetoxic,irritating,anddetri- reaktionsha¨rtenderKunstharzkittefu¨rden
Sa¨ureschutzbau(ShrinkingandSwelling
mental to health. Some powders can also be
PropertiesofChemicallyCuredResinMortars
physiologically active because of the hardener
forChemicalResistantLinings),” Z.
theycontain.
Werkstofftech. 12(1981)73–83.
6. ASTM-Standards 1977,AnnualBookof
ASTMStandards,vol.04.05.1997.Chemical
6. References
ResistantMaterials;VitrifiedClay;
1. F.K.Falcke:KleinesHandbuchdes Fiber-CementProducts;Mortars;Masonry,
Sa¨ureschutzbaues, VerlagChemie,Weinheim AmericanSocietyforTestingandMaterials,
1966. 1916RaceSt.,Philadelphia,Pa.19103,USA.
8 Cements,ChemicallyResistant
7. DECHEMARichtlinie:Bestimmung verfahrenstechnischenAnlagen;Kombinierte
physikalischer,insbesonderemechanischer Bela¨ge, 1997.
KennwertevonKittenfu¨rdenSa¨ureschutzbau, 10. AGIArbeitsblattS10Part3,Schutzvon
Dechema,Frankfurt. BaukonstruktionenmitPlattenbela¨gengegen
8. DIN28062,ChemischeApparate;Bau-und chemischeAngriffe–Plattenlagen,part3,
Werkstoffefu¨rAusmauerungen; VincentzVerlag,Hannover.
Einteilung–Eigenschaften–Pru¨fung; 1978. 11. W.A.Kuenning:“GuidefortheProtectionof
9. DIN28052–5,ChemischeApparate; ConcreteagainstChemicalAttackbyMeans
Oberfla¨chenschutzmitnichtmetallischen ofCoatingsandotherCorrosion-Resistant
Werkstoffenfu¨rBauteileausBetonin Materials,” Proc.Amer.Concr.Inst. 63
(1966)1305–1391.
Centrifuges, Filtering : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience

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DOI: 10.1002/14356007.c05_c01.pub2 Search All Content
Article Online Posting Date: January 15, 2007
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Abstract
The article contains sections titled:

1. Introduction
2. Fundamentals
2.1. Description of the Pore Liquid
2.2. Cake Formation
2.3. Washing of the Centrifuge Cake
2.4 Dewatering of the Centrifuge Cake
2.4.1.Equilibrium of Centrifugal Dewatering
2.4.2.Kinetics of Dewatering
3. Discontinuous Filtering Centrifuges
3.1. Cycles of the Discontinuous Centrifugation Process
3.2. Filling Strategies of the Centrifuge Drum
3.3. Peeler/Scraper Centrifuge
3.3.1.Horizontal Construction
3.3.2.Vertical Construction: Three Column Centrifuge
3.3.3.Siphon Centrifuge
3.4. Inverting Filter Centrifuge
3.5. Centrifugal Dryer
4. Filtering Centrifuges with Continuous Feed
4.1. Conical Screen Centrifuge
4.2. Vibrating Conical Screen Centrifuge
4.3. Tumbler Centrifuge
4.4. Screen Scroll Centrifuge
4.5. Pusher Centrifuge
4.5.1.Cake conveyance – Transport model
4.5.2.Washing on the Pusher Centrifuge
4.5.3.Feeding – Danger of Basket Flooding
4.5.4.Multistage Pusher Centrifuges
4.5.5.Special Pusher Centrifuges
4.6. Decanting Centrifuge/Solid Bowl Centrifuge
4.6.1.Application for Filtration Dewatering
4.6.2.Dewatering Model
4.7. Screen-Bowl Centrifuge
5. Hybrid Processes
5.1. Hyperbaric Centrifugation/Jet-Stream Centrifugation
5.2. Steam-Enhanced Centrifugation/Steam-Pressure Centrifugation
6. Laboratory-Scale Testing
6.1. Laboratory Centrifuges
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Centrifuges, Filtering : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience
7. Applications
8. References

[Top of Page]
1. Introduction
Centrifuges are machines which are primarily used in solid–liquid separation. The driving force is the centrifugal field,
which acts differently on the particles and the mother liquid. Filtering centrifuges compete with filters ( Filtration), press
filters, and sedimenting centrifuges ( Centrifuges, Sedimenting).
The field of application for filtering centrifuges is quite wide: minerals, fine chemicals, pharmaceutical products, food
products, and bio suspensions. Therefore, different concepts for construction have been developed. Furthermore, most
centrifuges are adapted in detail engineering to their application. Filtering centrifuges can be operated in continuous or
discontinuous mode. The capacity can range from several kilograms up to more than 100 t per hour.
All filtering centrifuges have in common the dewatering process. First, a filter cake is formed on a permeable support,
second, the liquid drains in the direction of the centrifugal field through the filter cake; third, desaturation begins, the liquid
runs off the pores, and air penetrates the pores of the cake. The process comes to an end when the driving force of the
centrifugal field is in equilibrium with the retaining forces in the pores, which primarily are due to wetting effects (Fig. 1).
Figure 1. Process phases during centrifugal filtration: sedimentation, drainage, and desaturation

a) Liquid surface; b) Clear liquid; c) Sedimenting suspension; d) Filtrate; e) Clear liquid; f) Centrifuge cake; g)
Desaturated cake; h) Wet cake
The main component of a filtering centrifuge is the perforated drum or basket. Its interior is covered with a sieve or a filter
medium. The rotation of the basket usually is powered by an electric drive. The performance of a centrifuge can be
estimated by using the centrifugation number C, which is the ratio of the centrifugal acceleration to gravity. It depends on
the angular velocity and the basket diameter R.
(1)

Technical filtering centrifuges operate with a centrifugal number from about 100 up to 2000 or even higher (Fig. 2). The
maximum rotary speed is limited by the mechanical stress and the durability of the steel used for construction.
Figure 2. Operational ranges for filtering centrifuges

a) Inverting filter; b) Peeler centrifuge; c) Screen scroll; d) Pusher centrifuge; e) Tumbler centrifuge; f) Pendulum
centrifuge

The vertical lines perpendicular to the rotor-diameter axis and the rotational speed (n) lines from top right to bottom left
intersect at the operating point. The ordinate of this intersection gives the centrifugal number C. Secondary parameter
lines from top left to bottom right give the circumferential velocity v.
Table 1 gives an overview on the construction size and performance of the common types of filtering centrifuges. The
solids throughput depends mainly on the suspension and particle properties, which must be estimated with laboratory-
scale tests.

Table 1. Types of centrifuges: constructive and operation parameters
Rotor Maximum Minimum Minimum solids
Centrifuge diameter, Throughput, centrifugal particle size, concentration, %
type mm [1], [2] t/h [2], [3] number µm [1], [3] [1], [3]
Peeler 250–2000 0.1–15 1500 (2000) 5–10 10
centrifuge
Inverting filter 400–1300 700–1100 2 5
centrifuge
Centrifugal 400–1300 600–700 2 5
dryer
Conical screen 500–800 1250–2400 50–80 15
centrifuge
Vibrating 300 30–120 300 50
conical screen
centrifuge
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Centrifuges, Filtering : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience
Tumbler 200 50–300 200 50
centrifuge
Screen scroll 200–1000 0.1–100 500–1500 80 20
centrifuge
Pusher 200–1560 2–100 300–1200 50 20
Centrifuge
Decanting > 4000 70 4–5
centrifuge
Screen-bowl 360–1000 0.2–30 1800–3200 20 4–5
centrifuge
[Top of Page]
2. Fundamentals
2.1. Description of the Pore Liquid
The pore liquid in a filter or centrifuge cake consists of differently bound liquid (Fig. 3). The main fraction is the liquid in the
large interparticle pores (a), which are easily accessible. After drainage of the pore liquid, liquid bridges (b) remain within
the porous system at the contact faces of the particles. They are bound by strong surface forces, as is liquid in the surface
roughness of the particles (c). The inner porosity of the solid also contains liquid (d). Any particle surface is covered by a
liquid surface film as well as with adsorbed liquid (e). Centrifugal dewatering is able to remove the liquid from the large
pores. With large particle sizes some of the liquid bridges also can be removed by the centrifugal force. All other liquid
cannot be removed by mechanical means in a centrifugal field or by filtration.
Figure 3. Pore liquid binding mechanisms in the porous matrix

a) Pore liquid; b) Liquid bridges; c) Liquid in surface roughness; d) Liquid in inner porosity; e) Adsorbed liquid film
The centrifuge cake is described by its geometry in the basket and its inner structure. The porosity is the ratio of void
volume V in the cake to the entire cake volume V .
v c
(2)

Dewatering of the porous cake is quantified with different dimensionless numbers: the saturation S, which is the ratio of
liquid volume to pore volume, the liquid load X, which is the ratio of liquid mass to solids mass, and the residual moisture
R , which is the ratio of liquid mass to mass of wet cake.
M
(3)

Due to its dependence on porosity, saturation is a more academic unit. Residual moisture and liquid load are derived from
practical application.
2.2. Cake Formation
Cake formation in the drum must take account of the curvature of the filtering surface. The cake is built up as an annulus
(Fig. 4). Thus, the area through which the liquid drains is a function of cake height. This must be considered when
calculating the pressure drop.
Figure 4. Cake geometry

a) Filter medium; b) Cake; c) Clear liquid pool
The volume flux can be written using the Darcy equation:
(4)

where r is the radius, L the length of the drum, p the pressure, the dynamic liquid viscosity, and r the specific cake
dr L c
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Centrifuges, Filtering : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience
resistance.
Integration within the boundaries of drum radius R and radius of the cake surface r leads to the pressure drop as a
S
function of the cake height (Eq. 5).
(5)

This pressure drop of the volume flux is balanced by the driving centrifugal pressure of the liquid in the basket (Eq. 6). The
liquid in the pores of the cake and the clear liquid in the pool above the cake surface r must be considered:
L
(6)

where is the liquid density.
L
The force balance when the pressure drop is equal to the driving centrifugal pressure allows the volume flux of filtrate
draining off the cake to be determined (Eq. 7).
(7)

In the case of small cake heights the equation can be simplified by replacing the logarithm (Eq. 7) by a linear term (Eq. 8).
(8)

The volume flux can be written with separated groups of parameters (Eq. 9): filtering surface, centrifugal acceleration,
product parameter, and cake geometry.
(9)

The volume flux can be used to calculate the draining time in a centrifuge (Eq. 10). The drum usually is filled with a flow
rate higher than the draining rate. Cake formation is an instantaneous process and the liquid remains above the cake.
Subsequent to draining, a second filling step, a washing step, or desaturation occurs. With the focus on filling andwashing
it is necessary to predict the right moment when the liquid level reaches the cake surface:
(10)

The estimation of the filling time at the optimum feed flux, which is a function of the cake height in the basket, requires an
extended mass balance [1].
2.3. Washing of the Centrifuge Cake
Cake washing is an essential process step to increase product quality. The mechanism and process design differ from
continuous to discontinuous centrifuges. Continuous centrifuges arecharacterized by low specific washing liquid quantities
from 20 to 200 kg/t . Washing mainly is a film washing process. Film washing has its highest efficiency when the wash
solids
liquid is injected directly after cake formation, when the centrifuge cake has its highest saturation. The more liquid has
drained from the pores, the more closed liquid compartments are created within the porous system. These are protected
by the surface tension and therefore they are not directly accessible for the wash liquid. The wash liquid itself is sprayed
onto the cake surface through pressure nozzles. The residence time of the cake in the centrifuge must be high enough to
ensure that both the pore and the wash liquid are able to drain out of the cake. Usually the wash liquid prolongs the film
flow phase of dewatering [4].
Discontinuous centrifuges use a washing mechanism which is similar to that used in filtration. The wash liquidflows in plug
flow through the entirely saturated filter cake. The wash ratio, defined as was liquid per unit pore volume, in most cases is
greater than one.
The washing step can be operated at lower rotation speed than dewatering. The liquid is softly laminated above the cake
surface, which must be disturbed and resuspended as little as possible.
The wash liquid then drains through the cake. The flux can be calculated using the same equation as for the cake
formation process (Eq. 7).
2.4. Dewatering of the Centrifuge Cake
The dewatering process can be split up into different subprocesses. Feeding of the suspension into the drum is directly
coupled to cake formation. Cake formation by sedimentation is characteristic of centrifugal processes in which the solid
density is higher than the liquid density. Formation of the cake is as instantaneous effect.
Subsequent to cake formation the liquid is forced by its own weight to drain through the porous sediment. Desaturation
occurs when the surface forces can be overcome by the mass forces. Finally the dewatering process reaches an
page 4of 19
Centrifuges, Filtering : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience
equilibrium state.
2.4.1. Equilibrium of Centrifugal Dewatering
The driving force of the centrifugation process is the centrifugal pressure p (Eq. 11), which is calculated from the liquid
c
density and the height of the liquid in the drum H:
(11)

Using the approximation that the cake height is small compared to the drum diameter a simple force balance can be
applied (Fig. 5). The filter cake is replaced by a capillary with a representative diameter. This diameter is called the
hydraulic diameter (Eq. 12).
(12)

where r is the specific filter cake resistance, and porosity.
c
Figure 5. Force balance in a characteristic capillary

a) Centrifugal force; b) Surface force; c) Pressure drop (shear force)
The driving force is the centrifugal force and the retaining force is the surface force:
(13)

Transformation of Equation (13) leads to the definition of the Bond number Bd (Eq. 14):
1
(14)

The Bond number Bd also can be seen as a reciprocal relative liquid height in the capillary. The Bond diagram (Fig. 6)
1
shows the equilibrium of dewatering: Saturation is plotted versus Bd .Information on the minimal saturation for a given set
1
of operation parameters can be drawn from the Bond diagram.
Figure 6. Equilibrium of dewatering, characteristic Bond-diagram

a) Saturation S = 1 — no dewatering; b) Bond plateau
The Bond diagram (Fig. 6) contains four characteristic areas:

(cid:122) Bond I — no desaturation occurs, saturation is equal to one, and the surface force equals the centrifugal force.
(cid:122) Bond II — desaturation begins, and with increasing Bd saturation decreasesuntil all liquid is removed from the large
interparticular pores.
(cid:122) Bond III — the so-called Bond plateau; saturation remains constant, and to drain the liquid bridges the centrifugal
force must be increased by at least one order of magnitude.
(cid:122) Bond IV — this area is usually out of range for technical dewatering processes; the liquid bridges start to drain.
Filtering centrifuges are operated in Bond I to III, and continuous filtering centrifuges mainly in Bond III. Discontinuous
filtering centrifuges operated in Bond II create a heterogeneous distribution of liquid within the cake. The cake near the
filter cloth is saturated, and the cake near the surface is desaturated.
The description of the area Bond IV also uses a second Bond number (Eq. 15), which takes into account that the cake
height is no longer an influencing parameter. The dimensionless number Bd is calculated with the square of the hydraulic
2
diameter, which is a characteristic length for the liquid bridges within the pores.
(15)

The Bond diagram can also be plotted versus Bd , and different experimental results the form a master curve in the area
2
Bond IV but spread in the area Bond II, where the cake height has its main influence on desaturation.
2.4.2. Kinetics of Dewatering
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Centrifuges, Filtering : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience
Estimation of dewatering kinetics is relevant to continuous centrifuges, which have alimited residence time from about 2 to
30 s. With the help of the capillary model (Fig. 5) the drainage of the liquid canbe calculated taking into account the friction
force of a laminar flow within the capillary. The force balance (Eq. 16) then includesfriction, surface, and centrifugal forces:
(16)

The solution of the force balance leads to an implicit equation of the dewatering time and the time dependent position x of
the liquid surface within the capillary (Eq. 17). The left side of the equation, which contains the time t, can be merged into
the kinetic parameter. This kinetic parameter can be seen as a reciprocal time.
(17)

In the force balance of the plug flow (Eq. 16) the surface forces also can be expressed by the measured capillary pressure
of the centrifuge cake (Eq. 18). This approach decouples the hydrodynamic properties of the hydraulic diameter from its
effect on the surface force. The decoupling becomes more and more necessary the smaller the hydraulic diameter gets,
because for fine particles the surface force calculated using the hydraulic diameter increases more than in the real cake.
(18)

The calculation of the plug flow determines the moment when the single-phase flow of the pore liquid has reached the filter
media. From then on the remaining pore liquid drains as surface or film flow in the capillaries of the cake.
The regime of film flow can be described by applying the Nusselt film theory of a liquid film draining down a vertical plate.
Finally, the saturation within the pores of the cake can be expressed with the same kinetic parameter introduced for plug
flow (Eq. 17). The film saturation can be written as a power law with two optional parameters a and b (Eq. 19). The
rigorous solution for a smooth and even surface quantifies a to 4/3 and b to 1/2. Usually b is a product parameter and it
depends on the particle size distribution and the roughness of the particles. Therefore the numerical value of b after fitting
to experimental data can vary in the range from 0.27 to 0.6.
(19)

The time-dependent saturation of the centrifuge is calculated by a combination of both approaches. The overall saturation
consists of three contributions: the mechanically nonremovable liquid, the film flow, and the plug flow (Eq. 20, Fig. 7). The
nonremovable liquid consists of the liquid bridges, the adsorbed liquid, and the liquid located in inner porosity. The
nonremovable saturation S amounts to 0.06 to 0.09 for smooth spheres. A typical value of S for mineral products is
non non
about 0.15.
(20)

Figure 7. Characteristic time-dependent desaturation; superposition of film and plug flow

a) Maximum saturation S = 1; b) Plug flow S(t); c) Film flow S(t); d) Superposition; e) Equilibrium saturation
An alternative approach to calculate the time-dependent dewatering of the cake uses the pore size distribution [5], [6],
which is given by a capillary pressure curve. In all classes of capillaries the plug flow iscalculated, without any contribution
of the film flow. But the pore size distribution is not given in most practical problems, and therefore it is difficult to apply this
model.
[Top of Page]
3. Discontinuous Filtering Centrifuges
The process integration of discontinuous centrifuges aims at processes which are operated batchwise. Semicontinuous
operation is possible when several discontinuous centrifuges are combined. The maximum throughput of discontinuous
centrifuges is 5–10 t per hour. The maximal basket diameter of discontinuous centrifuges is about 2000 mm. The
solids
common size of discontinuous filtering centrifuges ranges from 600 to 1500 mm. The maximum centrifugal number is
2000, and the typical operating range C = 600–1500.
3.1. Cycle of the Discontinuous Centrifugation Process
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Centrifuges, Filtering : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience
Discontinuous centrifuges are operated batchwise. The process steps are executed in succession (Fig. 8). First the
centrifuge is accelerated, and then the suspension is fed into the basket. It depends on the filtration properties, the
concentration of the suspension, and the throughput whether more than one filling step is necessary. The rotational speed
during the filling process is lower than the maximum speed, because usually the drive is not designed for the maximum
drive torque, which occurs during the acceleration of the suspension.
After the cake has built up, the centrifuge basket can be accelerated to operational speed. The next steps are one or more
washing steps, if required. After the last washing step desaturation (plug and film flow according to Eq. 19) occurs.
Discontinuous centrifuges do not have technical restrictions on the residence time. Desaturation can be continued until
saturation has reached equilibrium or the Bond saturation.
It depends on the type of discontinuous centrifuge at which rotational speed the basket is unloaded. Modern peeler
centrifuges remove the cake at quite high rotational speed; the inverting filter centrifuge requires a low speed.
Figure 8. Cycle of discontinuous centrifugation

a) Acceleration of the basket; b) Feeding speed (≤ maximum speed); c) Acceleration to maximum speed; d) First washing
step; e) Second washing step; f) Desaturation; g) Discharge at lower speed
3.2. Filling Strategies of the Centrifuge Drum
The cake formation process during filling depends on the ratio f of liquid fed to the basket and liquid consumed by the
r
growing cake, either by being incorporated in the porosity or by passing through as filtrate flux. The filtration ratio can be
approximated for suspensions of lower concentration by the ratio of liquid content of the feed suspension to the filtrate flux
(Eq. 21). Here the liquid which fills the cake porosity is neglected.
(21)

The right filling strategy must be developed to meet the requirements of the centrifuge. If the feed flux is too small, instant
filtration occurs when the suspension hits the filter medium or the surface of the filter cake. In this case there is no
smoothening effect of a liquid pool above the growing cake. The cake is formed where the suspension hits thesurface and
becomes more and more uneven, which leads to an imbalance in the basket rotation. The uneven cake height (Fig. 9)
negatively affects the washing step, because the washing liquid drains through the thinner areas of the cake. The integral
degree of impurity increases.
Figure 9. Regimes of cake formation as a function of feed flux

a) Optimal range of operation with minimal liquid pool; b) Low feed flux and instantaneous cake formation; c) Imbalance
due to irregular cake formation; d) High feed flux and increasing pool depth; e) Imbalance due to liquid waves at the pool
surface
High suspension feed rates lead to a high liquid pool above the cake, which increases the sedimentation distance of the
particles. During sedimentation segregation can occur. Due to this segregation a heterogeneous cake with a higher
resistance and a skin layer is formed, and this also has negative effects on the residence time and the desaturation
properties. The skin layer increases the capillary entry pressure of the cake. Fine products showing this tight skin may
even not be able to be desaturated in the centrifuge.
The optimum feed rate of a discontinuous basket centrifuge must be chosen between these two boundaries. In
conventional processes the feed rate is constant over the filling interval, but the cake permeability and hence the filtrate
flux decreases (Eq. 7). Therefore, the higher the cake grows, the deeper the overlying liquid pool becomes. Modern
approaches try to set up a control strategy based on the filtration and desaturation models [7] to regulate the feed flux.
3.3. Peeler/Scraper Centrifuge
The peeler centrifuge is the most widespread construction of discontinuous centrifuges. The name derives from the
strategy to remove the solids from the basket. A scraper is used which immerges into the cake while the basket still is in
rotation. This automatic discharge replaces the manual labor of the operator which formerly was necessary.
Cake removal with a scraper leaves a thin cake layer in the basket. This layer is characteristic for the centrifuge type. As a
positive effect it prevents any particle transfer through the filter medium in the following filling step. However, the thin layer
ages due to deep-bed filtration effects and therefore its resistance increases from cycle to cycle, which leads tolonger and
longer processing times. When the thin layer has reached a certain resistance it is manually removed.
With the focus on hygienic design and good manufacturing practice the thin layer is a severe problem. It must be removed
after each centrifugation step to prevent, e.g., any cross-contamination of separate batches.
Several concepts to remove the ground layer are available. The layer can be removed with mechanical, pneumatic, or
hydraulic devices. The pneumatic concept either is operated with a strong air blow from the housing through the basket or
page 7of 19
Centrifuges, Filtering : Ullmann's Encyclopedia of Industrial Chemistry : Wiley InterScience
directly with an additional nozzle holder inside. The hydraulic concept feeds a larger quantity of liquid into the basket after
scraping is finished. The ground layer is resuspended and flushed out off the basket. Any further cleaning uses the CIP
(cleaning in place) equipment.
3.3.1. Horizontal Construction
The basket is mounted on a horizontal shaft, supported by fixed bearings. At the opposite side the shaft is powered by an
electric drive via a V-belt. The basket is perforated and equipped withdrainage media and a filtration medium above it. The
front side can be closed with a door. An alternative concept uses a traversable housing, which can be removed to provide
access to the entire basket and the internals.
Figure 10. Peeler centrifuge, in feeding mode (left) and discharge mode (right)

a) Shaft; b) Drive of transport screw; c) Feeding pipe; d) Washing pipe; e) Centrifuge lid; f) CIP nozzle; g) Centrifuge cake
in the basket; h) Housing; i) Scraper during peeling of the cake; j) Transport chute; k) Solids discharge; l) Remaining thin
layer
The feed piping reaches into the basket, and a special distribution device is mounted at its end (Fig. 10). The discharge
scraper and the wash pipes are also located inside the basket. The discharge scraper feeds the solid to a funnel which is
connected either to a chute or a transport screw. During filling, washing, and filtration the discharge unit is turned toward
the inside of the basket.
Cake washing uses wash pipes or nozzles to achieve uniform distribution of the wash liquid over the cake surface. The
flux of the injected wash liquid is high enough to create a liquid pool above the cake, which slowly drains through the cake.
3.3.2. Vertical Construction; Three-Column Centrifuge
The vertical peeler centrifuge consists of a vertically mounted basket in which the drainage and filter media are fixed. All
feed pipes pass through the lid of the centrifuge. Directly after cutting, the solids are transported either upward with a
pneumatic conveyer or fall down through the backplane of the basket into a chute.
To reduce vibration the vertical centrifuge is fixed on a massive frame or plate which lies on rubber dampers. An
alternative concept is the three-column centrifuge. The basket and the drive unit are mounted as a kind of pendulum on
three support points. This concept reduces the vibrations transferred to the ground.
3.3.3. Siphon Centrifuge
The siphon centrifuge is a modified horizontal peeler centrifuge in which a siphon is connected to the filtrate piping. Due to
the hydraulic contact from the pore liquid through the filter medium into the filtrate in the siphon a hydrostatic pressure is
built up. This suction is an additional driving force which accelerates drainage during cake formation, washing, and the
plug-flow phase of desaturation. At the moment when the surface of the plug reaches the filter medium the hydraulic
contact breaks and the suction force becomes zero.
The siphon also allows backflushing of filtrate into the basket, which can be used to regenerate the ground layer. The
efficiency of the siphon can directly be controlled by the position of the skimmer tube (Fig. 11). It can be of advantage to
start the suction force after the feeding of the basket, because a high filtrate flux at the beginning of cake formation favors
formation of an uneven surface.
Figure 11. Principle of the siphon centrifuge

a) Skimmer tube; b) Hydraulic height; c) Siphon; d) Filter media
3.4. Inverting Filter Centrifuge
The inverting filter centrifuge is a specialized horizontal centrifuge (Fig. 12), which is used in integrated processing of high-
quality products. The basket is mounted on the horizontal shaft. The basket can be closed pressure-tight with a special lid.
The lid is connected by six anchors to the backplane of the basket. During the opening of the basket the connected front-
and backplanes are shifted to the discharge side. The filter medium, which is fixed at the backplane and at the front edge
of the basket, is inverted. After the inversion (Fig. 13) the inner side, on which the cake has been processed, has been
turned outside. With a small acceleration the cake detaches from the filter medium into the housing and the discharge
chute.
Due to the pressure-tight construction of the basket the inverting filter centrifuge also can be operated as hyperbaric
centrifuge. The feed pipe, which is inserted from the front side though the lid, can be sealed with special membrane. The
feed pipe is the only pipe which reaches into the basket; it is therefore also used to inject the washing fluid, as well as
pressurized air during hyperbaric processing.
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