Cap6_Cane Diffusion

Prévia do material em texto

Extraction 01' sucrose from sugar beet was always
carried out by means 01' diffusion. This is a process
in which dissolved molecules in solutions 01' differ­
ent concentration will diffuse due to a concentration
gradient until equilibrium is reached. In the case 01'
beet immersed in water, sucrose that is contained in
the cells will ditTuse across the cell walls, denatured
by heat, into the extracting liquido To speed up the
process, beet is cut into thin slices a few mm thick
and mixed with water or with a juice whose sucrose
concentration is lower than that 01' the juice in the
beet cells.
When equilibrium is reached, the slices are sep­
arated and the treatment repeated until most 01' the
sucrose is extracted. The cumbersome batch process
has been replaced by a continuous countercurrent
extraction process, which requires relatively simple
equipment and has a low power requirement. As ex­
traction proceeds, the concentration gradient dimin­
ishes and the concentration of nonsugar substances
in the extract increases. The beet diffusion juice has
a purity higher than that contained originally in the
beet cells.
Differences between cane and beet extraction
hinge around the considerable differences in raw
material to be extracted. Because 01' the nature 01'
sugar beet, it can be conveniently cut into long thin
slices, but it was only once adequate preparation 01'
cane for diffusion was achieved that cane diffuser
installations were successfu!' Adequate preparation
implies that most 01' the juice-bearing cells are rup­
tured, thus making the juice in the cane available to
the extracting liquid.
6 CANE DIFFUSION
There are a number 01' reports 01' cane diffusers
having been installcd since before 1900 in different
countries. Early diffusion involved batch ditTusers,
however, as Prillsell Geerligs (1909) pointed out,
diffusion "as yet has not met with general approval,
and has even turned out a complete failure in most
cases."
A batch cane diffusion system operated in Egypt
for over 50 years (7{I/1/mvi 1(65), but since the first
successful continuous cane diffusers installed in the
early 1960s, diffusers have been operated as contin­
uous countercurrent solid/liquid systems. Early de­
signs evolved from beet diffusers, and even the name
diffuser came from the beet industry, although the
tenninology is not really appropriate in the context
of extraction ofjuice from cane.
Since the InOs its adoption has been rapid in
some regions, particularly in Southern Africa, where
more than 90 % 01' ali cane is processed in diffusers.
I-Iowever, milling is still the predominant extraction
process in a large number 01' cane sugar producing
areas.
The early diffuser installations were not without
their difficulties, and the type 01' problems encoun­
tered, and the steps taken to solve them are com­
prehensively reported by vali der Riet and Relltoll
(1971). The tirst installations were bagasse diffus­
ers, i.e. they were preceded by a mil!. Cane diffu­
sion, i.e. extracting sugar from prepared cane in
the absence 01' a first mill, was pioneered in Hawaii
(PaYlle 1968).
Cane diffusers have been consistently capable
01' achieving higher extraction than milling tandems.
The gradual introduction 01' diffusion in South Afri­
ca has been responsible for the progressive increase
in extraction (Reill 1(99), where the industry aver­
age extraction achieved is dose to 98 %.
6.1
6.1.1
Theory
Mechanism of extraction
149
Referell,.es p. /73
150 6 Cane diffusion
Laboratory and pilot plant work showed very
elearly that more intensive preparation 01' cane
makes more of the sucrose containing juice readily
accessible to the extracting liquid, minimizing the
amount of sucrose which has to be extracted by the
much slower dif1'usional mechanism (Rein 1971). It
is not surprising that cane preparation is the most
important variable affecting extraction. The extrac­
tion plant should be designed in the first instance to
handle well-prepared cane.
The size of the diffuser in relation to the cane
lhroughput will have a big effect on extraction. The
longer the cane is in the diffuser, the greater is the
extraction achieved. The size is measured in terms 01'
its screen area for a given bed height, ar by the total
volume occupied by the cane bed. Relating this to
the cane throughput gives the cane residence time in
the diffuser, and this is the fundamental variable 01'
importance. The volumetric throughput Vc is related
to the throughput in lerms of fiber using the fiber
bulk density in the bed Pb.F as:
The extraction process from cane can be mod­
eled as two mass transfer processes in parallel (Rein
1971). One is fast and represents the extraction 01'
sugar in broken cells from the surfaces of the parti­
eles. The rate 01' transfer is influenced mainly by the
velocity 01' liquid flow past the particles. The second
mass transfer process is much slower, representing
the rate of transfer 01' sugar from unbroken cells or
from broken cells in the interior of the partieles. The
rate 01' transfer is dependent on temperature which
affects the rate of diffusion.
It was found that in a fully mixed environment,
lhe ratio 01' rate coefticients for the two processes is
of the order 01' 100. In a packed bed environment,
the degree of solid-liquid contact is not as good and
the ratio is somewhat lower and closer to 50 (Rein
1972). In a packed bed, a high juice velocity pro­
motes the rate of transfer and also improves contact
with the particles, thus reducing the proportion 01'
juice which has to be extracted by the slower diffu­
sion processo Since the ratio is so large, increasing
the amount of sugar which can be extracted by the
faster process is very significant.
Vc = IilF I Ph.F
From these two equations, the relalionship between
throughput and residence time t can be seen:
(6.2)
(6.3)
Fiber packing density
t = (A . /1. Ph.F)1 li/I'
It is more convenient to consider throughput in
terms of the flber rate li/I" which is independent 01'
the amount 01' juice which may be associated with
it. The volumetric throughput is also related to the
screen area A and bed height /1:
6.1.3
The quantity 01' imbibition water added to the sys­
tem in relation to the quantity 01' cane is also im­
portant. Because the mass transfer process involves
concentration gradients as the driving force for mass
transfer, the use of a larger quantity of water will
result in generally larger concentration differences,
with a beneflcial elTect on the rate 01' extraction. The
velocity of liquid flowing past the cane particles
also affects the rate 01' mass transfer. It is necessary
therefore to attempt to improve the degree 01' con­
tact between liquid and solid, determined largely
by the percolation rate 01' liquid through lhe bed 01'
cane. The maximum percolation rate which can be
achieved is therefore also an important variable.
Temperature has two effects on extraction.
Firstly it promotes the rate of mass transfer through
reducing the viscosity and increasing the rate 01' mo­
lecular difTusion. Secondly it denatures the protein
lining 01' the cell walls and therefore improves the
perrneability 01' unbroken cells, making the extrac­
tion 01' sugar from these cells possible. Both these
effects however are somewhat less important than
the effect 01' the other variables involved.
The density 01' the bed is measured in terms 01'
the amount 01' fiber per unit volume, otherwise re­
ferred to as the flber packing density or flber bulk
density. The amount 01' juice associated with the fI­
ber varies in the system and the flber packing density
is the best representation of the degree 01' compac­
tion 01' the bed.
This quantity is important since it dictates the
volume required for a given cane residence time (see
equation (6.3)) and also affects the percolation rate
Vc=A·/1/t
(6.1)
Variables affecting extraction6.1.2
6.1.4 Juice holuup 151
Under high ftow rate conditions,the static holdup
represents about 3 kg juice per kg fiber.
11was founu too that the beu uensity is uepenuent on
partide size, with a higher uensity being obtaineu
with a fineI' bagasse. If the mean particle size in mm
obtained by screen analysis d" is incluued, the 1'01­
lowing equation is obtained:
through the beu. lt is uependent on the fiber con­
tent 01' the cane H'"c anu the height 01' the cane beu
11 in meters (L(JI'e anu Rein 1980). In practice fiber
packing uensities have rangeu arounu 70 to 80 kg
fiber/mJ. The following equation was ueriveu for the
fiber bulk uensity:
26.5 . H',.c ( )Por =---·0.5811+4.3
. d,,+21.2
lt was also found that the packing density is greater
with a larger size uistribution variance (Rein 1972).
Thus a wiue variety of sizes enables the particles to
pack together more closely. With cane preparation
carried out by a heavy duty shredder, equation (6.4)
has been found to be quite reliable in predicting beu
density.
(6.7)
colation velocity tiL is the uownward velocity of the
liquiu as it moves between cane particles, expressed
in m/mino The percolation rate can be thought 01'as
the superficial liquid velocity tlO•L Le. the rate 01'ap­
plication 01' liquid applied to the top surface 01' the
bed expressed as mJ/min per m2 01' bed area. In gen­
eraltlO.L is numerically smaller than tiLanu the ratio
seems to be consistently dose to 0.7 (Lave anu Rein
1980). This is an indication of the reduced open area
for ftow in the bed due to the presence 01' the solid
material.
Measurements 01' percolation rates in full­
scale diffusers have been found to cover a range 01'
0.1-0.2 mJ/(m2 • min). The corresponding perco­
lation velocities in the bed are 0.14 to 0.3 m/mino
(Love and Rein 1980). This is somewhat lower than
the values measured in a pilot plant investigation,
which yieldeu the following correlation:
4980
ti =---
O.L S . P~.r
where S is the specific surface or fineness 01'the par­
ticles in m2/kg. This shows that the percolation rate
is lower with a fineI' preparation and a more compact
bed (higher fiber packing density). Experience with
full-scale diffusers confirms the strong effect 01' fiber
packing density.
The percolation rate is an important variable be­
cause it determines where the interstage juice sprays
should be located in order for the juice to appear in
the correcl tray at lhe bottom of the diffuser. Details
of how the interstage juice sprays should be located
in oruer to achieve the desireu percolation condi­
tions in a ditTuser are outlined by Rein and Inghalll
(1992). This involves changing the amount 01'juice
being recycled.
The optimum positioning 01'juice spray pipes
or weirs can be calculated if the percolation veloc­
ity is known. Referring to Figure 6.1, if the rate at
which the liquid percolates vertically downward
is tiL' the time it spends in the bed is h/tiL' During
this time, the cane bed is moving at a velocity lIc
The horizontal distance moved during this time is
(tlc . h)/tlL. Thus for liquid pumped from the
(i + I)th stage to exit in the ith stage, the spray 01'
weir should be positioned a distance IA from the
middle 01' the ith stage, where the spray advance IA
is given by:
(6.5)
(6.4)
(6.6)
Juice holdup
Juice percolation rates
Pol = H'r.c . (0.5811+ 4.3)
6.1.4
qUI' = 25.2 - 0.18· Po.r
6.1.5
The total juice holuup in the bed can be uivided
into two components, a dynamic anu a static holuup.
The static holdup represents the juice in and between
cane particles which is stagnanl and does not partici­
pate in the ftow 01' liquid through the bed. The higher
the ftow rate, the greater is the dynamic holdup anu
the smaller the static holdup. In beds 01' cane, total
juice holdup quI' has a value under high ftow con­
ditions 01' about 12 kg juice per kg liber. 11is also
dependent on the bed density (Love and Rein 1980)
and can be estimated from the following equation:
11is necessary to differentiate between the per­
colation rate and the percolation velocity. The per- (6.8)
ReJerel/ces f!. /7J
152 6 Cane diffusion
:_ Spray advance _~,
: IA :
~~J~~
- - _• I ~_ - -' - - , Bed
""Bed- -_ ,~-: -\-_ --\-_ '--; height
speed I 1 I i -+' 1 h
~ :
A __ Sprayadvance
, IA
Ju;ce flow
t Bed
'-jheight
h
Figure 6.1: Schematic oiagram of stages in a oiffuser
B __ Sprayadvance
, IA
where b is the breadth or width 01' the diffuser. 11'
uO•L is the maximum percolation rate, then VI. has its
maximum value. This is the optimum condition i.e.
the maximum flow rate possible.
where li is the length of a single stage. It has been
shown that if the liquid is applied as a curtain, as
from a weir, the effective point of addition is closer
to the feed end 01'the diffuser. This is due to the fact
that the overloading of the bed directly below the
weir causes liquid to move horizontally toward the
feed end where the bed is not saturated with juice
(Lave and Rein 1980). The effective displacement
01' the center 01' the juice application is 01' the or­
der 01' one meter. This effect is not apparent with
sprays which apply liquid uniformly over the area
01'a stage.
Calculation 01'interstage juice tlow rates. 11'in the
ideal case ali the juice from the ith stage appears
in the (i - 1)th stage, the theoretical flow rate VL,o
through a particular stage can be calculated and is
given by (Rein and Ingha//1 1992):
, lil [lil (100-1.25'WFB)]
VL,o = -----E.. ~ + qUF,St:llic - .
~ //1F w~
(6.9)
Here qlJF.Sta,ic is the static juice holdup and generally
has a value 01'about 3 kg juice/kg fiber.
11'this f10w rate VL.O is too large in relation to the
percolation rate, f100ding will occur. The amount 01'
liquid flowing from the bottom 01'the bed into a tray
is given by VL in m3/min as:
----JBed
, height
'h
Juice flow
Calculation 01' spray advance. In practice the
ideal case on which equation (6.9) is based does not
occur and some juice will either recycle or bypass
the stage. 11'the bed 01' cane is moving too slowly,
part of the liquid which should have come out in
stage i actually find its way into stage i - I. This is
known as "bypassing" and the effect is to reduce the
quantity ofjuice coming out in stage i; therefore the
quantity 01' liquid pumped on to stage i - I is like­
wise reduced. This can result in the situation where
too little liquid is applied to stage i, with the result
that inefticienl conlacting 01'solid by lhe liquid oc­
curso This is illustrated in Figure 6.2 A.
11'on lhe other hand the horizontal velocity 01'
the cane is increased or the percolation rale reduces,
some ofthe liquid which should have percolated into
slage i is carried past that lray and finds its way into
lray i + I. This is kllown as "recirculatioll", because
juice is reappearillg in the tray from which il was
pumped. This can lead to substantially increased
flow rates, to the poinl where the f100ding flow rale
may be exceeded. This is shown in Figure 6.2 B.
The degree 01'recirculation or bypassing call be
intluenced by lhe factors which alTecl lhe maximum
percolalion rate and by lhe posilion 01' the spray or
weir. The flow rate relative 10 lhe f10w rale ill the
Figure 6.2: Schematic representalion of
A Uypassing ano B Recirculation ofjuice in a oiffuser
-
Bed
speed
Uc
(6.10)Vc = li/I' I Pb,F
6.1.6 Mass and cncrgy balances 153
As is also necessary for a milling tandem, suf­
ficient measurements should be taken to enable ex­
traction to be calculated. This rcquires cane, bagasse
and raw juice analyses and two of the three mass
flow rates. Extraction is calculatcd from eqations
(5.1) or (5.2) in Scction 5.1.1.
An overall total mass balance cannot normally
be carried out becausc of direct injection steam into
the trays and because of evaporation of water from
the hot bagasse, both of which are not mcasured. In
practice they are both of the order of 5 % of cane rate
and almost compensate. In most cases the imbibition
rate is calculated from a mass balance and represents
the net effect of water addition, direct steam injec­tion and loss by evaporation.
The quantity of press water can be estimated by
undertaking moisture, fiber and DS (dissolved sol­
ids) balances around the dewatering mill(s). This
yields the following equation for the press water
quantity:
. Ii/" . [( lI'"s." + 11'1'.,,). II'w.I1" / (100 - II'w.I1") - II'W.8 ]
II/pw = [ () ( )](100 - lI'I1s.Pw - II'so.!'w) - lI'os.Pw + II'so.!'w . II'w.I1" / 100 - II'W.D"
absence of recirculation or bypassing indicates the
dcgree of recirculation or bypassing occurring. Frac­
tional rccirculation x can bc calculated as:
If bypassing occurs, then the value of x is negative.
For no bypassing or recirculation to occur, lhe
sprays should be located a distance 'A as given by
equation (6.8). If rccirculation is requircd, the liquid
application point should be closer to the tray from
which it originated. Conversely if bypassing is nec­
essary. the liquid application should move further
away from the tray from which it originaled. i.e. to­
wards the feed end 01' the diffuser. Then the required
spray advance can be calculated (Rein and Ingllall/
1992) from:
(6.13)
Mass and energy balances6.1.6
(6.11 )
(6.12)'A = 'i .(I - x) + li" . li
111.
Mass balance
Figure 6.3: Typicalmass and energy halances for a diffuser processing 100 l/h cane
Energy balance
This takes into account
the solids in the press
water wso.pw and the
DS of the press water
w"s.pw' With moisture
contents of 80 % ex
the diffuser and 50 %
in final bagasse, the
press water quantity is
generally of the order
of half the quantity of
cane being processed.
A typical mass
and energy balance for
a diffuser processing
100 t cane/h is shown
in Figure 6.3.
The major heat
losses occur from the
body 01' the diffuser
and from cooling of
the press water during
the dewatering opera-
Imbibition 40 t/h
Imbibition 3.9 MW 85
Mill
00,,''''"30 t/h
Mill
0°0 ~Bagasse 63 O(
1.5 MW
'" Heat loss'\. 1.5 MW
Press water
60°(,3.4 MW
Wet
bagasse
80 t/h
Wet
bagasse
800(
6.4 MW
Press wate
50 t/h
Diffuser
Diffuser
Heat loss !3MW
Raw juice
600(7.1MW
Raw juice
110 t/h
(ane
100 t/h
Vapor heating
6.8MW
(ane 25 O(
24MW
Rejáel/ce" fi. /73
154 6 Cane diffusion
This involves specifying the screen area required
to achieve the required residence time of the solid
material and hence the desired extraction leveI. This
area requirement will be affected by ali the variables
discussed above.
For comparison purposes, the area of a diffuser
related lo the fiber throughput in lonnes fiber/hour
i.e. Alii!F is often used. From equation (6.3) it can be
seen that this is given by:
tion. The diffuser is normally installed in the open,
and is generally insulated on the roof and sides. If
the heat losses from the diffuser could be eliminated
entirely, by heavily insulating the whole diffuser and
the piping (saving the 3 MW loss in Figure 6.3), the
heating vapor requirement would be almost halved.
This is neither normally practical nor necessary.
The temperature of the press water returned to
the diffuser is generally about 60 De. The raw juice
from a diffuser is at a higher temperature (60 to
65 0c) than that from a milling tandem and this heat
is not losl. However the bagasse too is at an elevated
temperature and the heat in this is lost (although it
probably helps to reduce the moisture content of the
bagasse before reaching the boilers).
Measurements on vapor usage in a diffuser have
yielded a figure of about 1I ti I00 l cane.
A T- --
li/ , 11 . PhY
In the South African industry, extraction levels 01'
around 97.5 % are oblained with a screen area of 11
m2/unit throughput of fiber/hour, i.e. 11 (m2. h)/tF
Assuming average values of bed height 01' IA m and
a fiber packing density of 80 kg/mJ, from this equa­
tion a screen area of 11 (m2 . h)/tr is equivalent to a
residence time of 1.23 hours or 74 minutes.
In this Chapter, the screen area is taken to be the
plan area of the diffuser occupied by the bed ofjuice
and fiber. It does not include the screen arca between
the tailshaft and the feed plate; this may be includ­
ed in quoted gross screen areas. In some instances,
an "effective" screen area is quoted, excluding the
feed stage area and in some cases also excluding the
drainage area after the press roller. Although there
may be some justification in excluding the feed stage
area, because extraction conditions are not fully es-
6.2 Plant and equiprnent
Types of diffuser6.2.1
A descriplion 01' the different types 01' diffuser
which have been used over the years is given else­
where (Rcill 1995). The types 01' cane dilTuser which
have been used can be categorized as follows:
True countercurrent dilTusers (e.g. DOS,
Saturne).
Moving bed diffusers (e.g. BMA, De Smet, Si 1­
ver Ring, Tongaat-Hulett).
Other types (e.g. FS/van Hengel, Rolocel).
Experience with the dilTerent types 01' diffusers
over many years has resulted in the situation where
only moving bed dilTusers are still used, with very
few exceptions. The DOS diffuser (Brullicllc-O!sclI
1966) was adopted from the bcet industry but proved
unsuitable ror large cane throughputs. Saturne dir­
fusers were based on the Miag wheel, and the long
liquid path length was impractical (D 'Espaigllcl
and Rival/mil! 1974). The FS diffuser conceived
There are essentially two variants ofthe process,
termed bagasse and cane dilTusion. The former has
a single mill ahead of the diffuser and the latter ac­
cepts prepared cane directly into the diffuser. Eariy
installations favored bagasse diffusers, since they
represented a smaller step-change from milling, and
are still required in countries where payment for
cane is based on an analysis 01' first expressed juice.
Cane dilTusers have shown themselves to be
considerably more cost-ctlcctive and are almost ex­
clusively ravored in preference lo bagasse diffusers
in new installations. Therefore, what follows is 1'0­
cused more on cane rather thall bagasse diffusers,
although the principies involved are largely the same
in both cases.
tablished, the contact time ofjuice and fiber beyond
the press roller (usually 4 to 5 minutes) stillmay re­
sult in a reduction in concentration differences and
hence increase extraction. There can be no justifica­
tion in excluding the chain runner arca from the total
area; the maximum reslriction to percolation is not
the screen but occurs in the most compacted part 01'
the fiber mat above lhe screen.
(6.14)
Sizing of diffusers6.1.7
6.2.2 Moving beel elitfusers 155
Figure 6.4: Schel11atic diagral11 01' a 1110vingbed diffllser
I Direct injection vapor: 2 Heating vapor: 3 Heaters: 4 Prepared cane: 5 Raw jllice; 6 Dif­
fllser; 7 Press water: R Lifting screws: 9 Il11bibition water: 10 Final bagasse; 11 Dewatering
l11ills: 12 Stage trays: 13 Press roller: 14 Diffllser bagas se
hy VWl Hengel (FitZl?erald et aI. 1978) was a gooel
concept anel achieved some good extraction values
but proveel to be mechanically unreliable and expen­
sive. Moving hed diffusers have proven to be low
in capital and operating eost and are the only type
considered here.
These eliffusers are countercurrent extraetion ele­
vices, but operate on a staged hasis. Juiee is pumped
onto a moving beel 01' prepareel cane or bagasse, about
50-60 m long, in anything from 10 to 18 stages. A
sehematic diagram 01' a moving bed cane eliffuser is
shown in Figure 6.4.
9
The De Smet cane diffuser is essentially the
same as the De Smet beet diffuser. The cane or ba­
gasse bed forms on a horizontal slow moving screen.
The Silver Ring diffuser is similar in concept, but the
screens move in a circle insteael of along a straight
line (Payne 1968).
The BMA and Tongaat-Hulett eliffusers ditfer
from the De Smet in having a fixeel screen, with a
series of chains that transport the cane bed along the
screen. This generally results in a cheaper diffuser
for the same screen area. A representation of a BMA
diffuser is shown in Figure 6.5.
A comparison of moving screen andfixed screen
eliffusers leaels to the following consielerations:
Beeause of reduced friction, the elrive power re­
quireel on a moving screen eliffuser is generally
lower, typically
30 kW for a 300 tlh
diffuser compared
to 75 kW for a fixed
screen diffuser.
Discharge of cane
from moving screen
ditfusers is by lift­
ing screws (Silver
Ring) or lifting
elrum to smooth out
the flow (De Smet).
Diseharge from
fixeel screen dif­
fusers is by gravity
with a simple kick­
er to smooth out the
flow.
2
Moving bed diffusers
5
6.2.2
Figure 6.5: BMA l110ving bed cane diffllser (collrtesy 01' BMA)
Referelices p. /73
L
156 6 Cane diffusion
Early diffusers had a maceration carrier that wet
the cane before depositing it in the diffuser, which
was not satisfactory (va/l der Riel and Ren/o/l 1971).
Nowadays cane is fed into the diffuser by a slat con­
veyor running at right angles to the diffuser, gen­
erally over a miter plate, shaped so that cane falls
uniformly across the width of the diffuser. The miter
plate has adjustable sections to modify the distribu­
tion across the diffuser. Alternatively the distribu­
tion is changed by cutting 01' welding pieces onto
the miter plate. Once a uniform distribution has been
achieved, it is generally not necessary to modify it
again. A more sensitive adjustment can be achieved
by baffles at the point where the cane is transferred
onto the slat conveyor, which effect a changed dis­
tribution in the slat conveyor and thereby changes
the distribution across the diffuser. It is important to
have a uniform height bed across the width of the
di ffuser.
In some diffusers juice is sprayed onto the cane
as it enters the diffuser to compact lhe cane bed
below the feed conveyor. This is sometimes neces­
sary if insufticient head has been allowed for the
compaction of the cane below the feed conveyor.
It can lead to localized flooding and classification
of the cane particles at lhe feed end. It requires an
up-turned screen area outside the feed end of the dif­
fuser, so that liquid tlowing oul under lhe feed plate
does nol overflow onlo lhe ground. This is shown in
Figure 6.6 A.
Chains dragging cane across the lixed screen
generally result in the formation of a more com­
pact cane layer at the screen, which could affect
percolation.
The moving screen diffuser requires double the
screen area, because half the screen is inopera­
tive on the return strand.
The moving screen elements are somewhat frag­
ile, and the installation is therefore not as ro­
bust.
Fixed screen diffusers have a heavy press roller
riding on the cane, which leads to a lower mois­
ture content of the discharge bagasse. This is
precluded in a moving screen diffuser because
of damage to the screen.
Fixed screen diffusers seem to be better able to
handle excessive soil in cane, for reasons which
are not clear.
The suspended solids content of the raw juice
from a fixed screen diffuser is generally low­
er than from a moving screen diffuser (Rei/l
1995).
Solids in the press water from the dewatering mills
have been found to plug the cane bed where the press
water is returned to the diffuser. Early installations
required a press water clarifier similar to a raw juice
clarifier to remove solids. Some press water clarifi­
ers are still in use, but it is more common now to use
rotating lifting screws in the cane bed at the point
at which the press water is returned, to mix the fine
solids into the cane bed, and obviate the formation
of an impermeable layer of fines.
6.2.3 Cane feed arrangements
Feed
mi"~~eBj._ tt="D~ W=
~ De""" ~"
~plate ~Iate~ ~ ~ ..... ~ ''''';J .,~,'"---.J + + (+) Í+
Figure 6,6: Various schemes for feeding cane into a diffuser. Shadcd arca reprcscms arca cut out of botlom deck of fced
conveyor.
6.2.4 Diffuser drive requirements 157
where quI' is the juice holdup calculated from equa­
tion (6.6). The amount of fiber in the diffuser is cal­
culated from the volume occupied by the cane mass
V in m\ and the fiber bulk density. Thus:
To obtain a rough estimate of the chain pull, ne­
glecting the mass of the chain, equations (6.15) and
(6.17) can be used, assuming a fiber bulk density of
75 kg/m3, a juice holdup of 12 kg juice/kg fiber and
a coefficient 01' friction of 0.3. This leads to the 1'01­
lowing chain pull in kN:
In practice, a value of the constanl of 5 in equation
(6.18) is adequate to allow for maximum load con­
ditions in a fixed screen diffuser. A lower value ap­
plies to a moving screen diffuser. The load has to be
shared among the strands of chain in use. The type
of chain and the number of strands should be chosen
so that the working load does not exceed one-fifth of
the ultimate strength 01' the chain.
The torque, M, is given by:
(6.17)
(6.18)F=2.9·V
A preferred arrangement employs dry feeding
of cane into the difTuser, with liquid applied to the
cane only after the bed has been formed. Experi­
ence with moving bed diffusers has shown lhat more
stable liquid percolation conditions are achieved if
the prepared cane is fed to the diffuser dry, with liq­
uid applied only after the cane bed has been formed.
This requires a distance between the bottom of the
feed conveyor and the screen deck about three times
the final consolidated bed height. This approach
prevents classification 01' fines and fibers which can
happen with a wet feed. [n lhis case a vertical feed
plate is adopted and a nat screen behind lhe feed
plate can be used without any egress 01' liquid. The
cane bed formed provides an adequate seal. Different
conveyor arrangements have been used, as shown in
Figure 6.6 B to E. A slightly slewed conveyor such
as shown in Figure 6.6 [) enables a good distribution
to be achieved without any def1ecting bafftes.
It becomes more difficult to get a uniform distri­
bution with a wide diffuser. The 12 m wide diffusers
at Fe[ixton are choke-fed across their widths, with a
surplus being continuously circulated. This achieves
the purpose of a uniform bed but is considered to
be unnecessarily complicated and would not be re­
peated.
M=F·d/2 (6.19)
where 111 is the mass of fiber and juice in the diffuser
bed, IIIchai" the mass 01' chain (plus mass of slats) in
kg and fl is the coefficient 01' friction. The mass 01'
fiber and juice in the diffuser is calculaled from:
The drive power required depends on the length
of the diffuser and the mass 01' cane in the diffuser.
[t is affected by the friction 01' the conveyor and the
efficiency of the drive. Return run friclion is not sig­
niticant if return id[ers are used. Chain pull F in kN
is calculated from the mass 01' chain and material
being conveyed and from the frictional force and is
approximately given by:
Diffuser drive requirements
8ased on a typical bed velocity of 0.6 m/min, the
installed power required in the example given above
is 1.5 . 3712 . (0.6/60) = 56 kW.
(6.20)P = 1.5 . F . lIc
where d represents the drive sprocket diameter in m.
A smaller diameter sprocket reduces the torque, but
this is usually determined by the pitch 01' the chain.
To give an idea of the torque levels involved, consid­
er a 9 m wide, 55 m long diffuser, with a bed height
of 1.5 m. The total volume is 742.5 m3 and the chain
pull is 5 times this i.e. 3712 kN. For a 1.4 m diam­
eter sprocket, the torque from equation (6.19) is
2600 kN . m.
The motor power required, P, can be calculated
from the speed 01' the chain lIc in m/s and the chain
pull at the headshaft, and a multiplier 01' 1.5 is ap­
propriate for sizing the motor required to allow for
changes in operating conditions:
(6.15)
(6.16)
6.2.4
F = g. /1' (111 + IIIch"'")
1II=1II~·(I+q[jF)
Referem·e.,· 1'. /73
158 6 Cane diffusion
Hcadshaft and drivc. Apart rrom the chain,
the headshaft and drive on a dilTuser are major cost
Scrccns. [n a fixed screen diffuser, the screen
is made 01' perforated plate, either stainless steel or
3CR 12, to resist the effects 01' corrosion and abra­
sion. Hole diametersvary from 6 to 12 mm with an
open area of 25 to 40 %. Although some difrusers
have used different hole sizes in different parts 01'
the diffuser, it has been established that the screen is
not the major resistance to flow and the hole size and
open area is not important. Moving screen diffusers
usually employ specially fabricated sections bolted
to the chains which transport them.
Chains. Depending on the size 01' the diffuser,
a fixed screen di fI'user wi 11have between 8 and 16
strands 01' chain, with slats between each pair of
chains, usually on each alternate chain link, The
chain is a major cost component 01' a diffuser. The
cheapest chain incorporates alloy steel components
(pins and bushes) and generally lasts from 5 to 10
years before requiring replacement. [t is important
that correction 01' pH with lime is used in the dif­
fuser, or else rapid corrosion of the pins may occur.
This situation is aggravated by the juice film trapped
between chain components in which much lower pH
values are possible as the trapped juice degrades.
Stainless steel pins and bushes in the chain can
prolong the chain life to 25 years or more. Great care
needs to be taken in the heat treatment 01' the com­
ponents to get the hardness required without making
the pins brittle. A Rockwell C hardness 01' 46 to 48 is
generally specified. [n this instance, the opportunity
to dispense with lime addition is a feasible option.
Generally, the type of material chosen involves a
trade-off between higher initial first cost and lower
maintenance/replacement costs.
Chain pitch varies between 250 0101 and 500
mm and the chains in use have a breaking load in
the range 01' about 150 to 300 tonnes. Larger pin di­
ameters spread the bearing pressure and longer pitch
chain reduces the number 01' pins required. The opti­
mum chain design varies with the size and geometry
ofthe diffuser and affects not only chain, but also the
headshaft speed and therefore drive costs. Generally
the longe r pitch chains are more cost-effective op­
tions in larger diffusers.
Thus a longer diffllser reqllires a faster bed speed for
the same residence time.
(6.21 )
(6.22)
items. Drive power requirements are low, usually
less than 100 kW, hut drive torque values are very
high as illustrated in Section 6.2.4. A variahle speed
drive is required, and AC variable speed motors are
usually used. Flange mounted motors with planetary
gearboxes have proved to he a neat and elTective
drive system. Total diffuser electrical power require­
ments are low, with power absorbed 01' the order 01'
2 (kW . h)/tc The major part of this is for the inter­
stage juice pumps.
For relatively narrow diffusers, a solid shaft
with shrunk-on sprockets can be considered. For
wide ditTusers, the required diameter 01' the shaft
becomes too great and a hollow shaft with cast-in
sprockets may be used. The drive must he equipped
with a torque limiting system to prevent damage in
the event 01' problems.
This shows tha! the required residence time depends
on the bed height and the screen area A. The total
area is important and the ratio of length to breadth
does not affect the residence time. Generally a
length 01' 65 m is no! exceeded because the chain
pull hecomes too large. A longer and narrower dif­
fuser reduces the cost 01' the drive and headshaft dlle
to a raster bed speed. Since the screen area A is the
product 01' length I and hreadth h and the volumetric
flow rate is also given hy the prodllct 11 . h . IIc eqlla­
tion (6.21) can be written as:
Aspcct ratio. Cane diffuser widths vary between
4 01 and 12 01, and have lengths 01' between 50 and
65 m. The choice 01' the length/breadth ratio is made
purely on a capital cost basis; it has no effect on the
efticiency 01' extraction. Rearranging equation (6.2)
gives the rollowing equation:
1: = (A .11)/V"
Bcd scrcws. Press water clarification was prac­
ticed in early installations to remove fine solids
which otherwise plllgged the top surrace 01' the cane
bed when returned to the dilTuser. \'(//1 der Riet and
Re/lto/l (1971) showed that the clarifier could he dis­
pensed with if the bed were disturhed at the point at
1: = I1 IIc
Mechanical details6.2.5
6.2.6 Juice heating 159
At the feed end 01' the diffuser, juice is heated up
to about 90°C to heat the incoming cane to the re­
quired operating temperature as quickly as possible.
Sealding juice is circulated through the heaters at
the feed end of the diffuser at a rate 01' about 300 %
on cane. Flow meters on the lines to the heaters are
sometimes installed to monitor the amount 01' juice
rccirculated.
Tray volumes. The tray volumes below the
screen should be large enough to hold ali the juice
that will drain from a stage when a mill stop occurs.
This depends on the dimensions of the stage, and
can be calcula!ed assuming roughly 6 kg juice will
drain from each kg of tiber (roughly half the juice in
the bed). This ensures that there will be no mixing
of juice from different stages if a mill stop occurs,
which would otherwise destroy the Brix gradient
and prolong the time before steady state is achieved
after restarting.
Press J'()lIer. Diffusers generally have a large
press roller loca!ed before the discharge 01' the dif­
fuser, riding on the cane bed, wilh a diameter of
abou! 3 m. Its purpose is to prevent flooding ofjuice
and cane out 01' the diffuser and to reduce the mois­
!ure conten! 01' the bagasse leaving the ditfuser by
lhe weight of the roller squeezing the bed. The roller
has to be free to move up and down with the bed as
the bed height changes. In a moving screen diffuser,
generally two lighter press rolls are used to reduce
the pressure on the screen.
(6.23)
Interstage juice application
Juice pumped from each of the juice trays has
!o be applied uniformly across the width 01' the dif­
fuser. This is achieved using an overftow weir or a
horizontal spray pipe. The spray system is able to
apply the juice more uniformly over the stage in
question, whereas the weir provides a curtain 01' liq­
uid, which results in localized liquid overloading 01'
the bed at that point.
A weir system has to be carefully leveled to
ensure that uniform distribution across the diffuser
is achieved. There is generally little bagasse in the
stage juice to interfere with the distribution. None­
theless most weirs have drain plugs in the bottom 01'
the weir to purge any suspended solids periodically.
Spray pipes need to be designed to achieve equal
flows through each nozzle. This requires that the
pressure drop down the pipe be small in relation to
the pressure drop through each nozzle. By the cri­
terion propounded by Knaebel (1981), the pipe di­
ameter dpiPc must be larger than the minimum value
calculated from:
where do represents the diameter 01' each 01' the N
outlet nozzles. The nozzles should be manufactured
as inserts 01' a hardened material to resist erosion and
corrosion. Drain holes are required at the ends of the
pipes to ensure that the pipe drains on shut-down.
In order to vary the point of addition 01' the juice
to controllhe stage percolation rate, as described in
Section 6.1.5, various options are available. With a
spray pipe with downward pointing nozzles, plates
or baftles can be positioned below the sprays and
adjusted to deflect the juice forward or backward to
change the juice applicalion area (Rein and Inghalll
1992). A recent BMA proposal rotates the whole
spray pipe to achieve the same ends. If a weir is
used, a variable position baffte below the liquid cur­
tain can be used to move the juice addition point.
The temperature along the diffuser is maintained
by direct injection of vapor into some of the stage
!rays. Direct contacl healing 01' press water or stage
juice is possible using sub-atmospheric vapor (vapor
3 or 4) to achieve better steam economy (Singh and
Allwright 2000). Further details are given in Section
9.4.
6.2.7
Juice heating
which the press water was returned. This was done
with a number of helical screws installedvertically
in the cane bed, which rotate and lift the cane at the
point of press water return.
This is now common practice. The screws proj­
ect down to about 100 mm from the screen and are
spaced about one me ler apart, with a flight pitch be­
tween 300 and 450 mm. They are commonly driven
by 7.5 kW molors. 11'the screws trip and stop rotat­
ing, the screws will be broken by the force of the
moving bagasse. Thus the installation generally in­
cludes a shear pin which will break in this eventual­
ity so that the screws can pivot upwards out 01' the
way of the moving cane bed.
6.2.6
R~fáences fi. /73
160 6 Cane diffusion
1
Typical control loops found on a ditTuser are
shown in Figure 6.7. Ali control systems are single
loops. The diffuser speed is set to correspond to
the particular set point on the cane throughput rate
controller. The speed is chosen to give the required
bed height. Levei indicators at the feed end 01' the
diffuser indicate whether the desired levei is being
achieved. If nol. a minor correction to the diffuser
speed set poinl is required.
Controls on a 12-stage diffuser shown in Figure
6.7 are as follows:
Temperature controllers on the scalding juice
heaters regulate vapor flow 10 the heaters.
Two pH control loops measure the pH in stages
2 and 7 and control the speeds of the peristaltic
lime dosing pumps.
10 levei control loops regulate liquid levels in
the bed by adjusting the interstage juice sprays.
Percolation rate control may otherwise be car­
ried out manually.
The imbibition water conlroller admits water 10
lhe diffuser at a set rate.
Two temperature controllers regulate vapor to
intermediate diffuser trays to maintain tempera-
6.2.8 Instrumentation and control tures along the diffuser.
High levei alarms on the stage trays give an indi­
cation of any trays full of juice.
Control 01'juice recirculation through lhe scalding
juice heaters is done manually. to achieve the de­
sired heat input aI lhe feed end.
Electrical interlocks that will prevent the dif-
fuser from operating are typically:
Bed screw motor trip.
Diffuser drive bearing lube pumps off.
Bagasse conveyors downstream 01' diffuser
tripped.
Di ffuser kicker trip.
Chain tension detector activated (if fitted).
6.3 Recycle of clarifier mud
A full evaluation of l11udrecycling at Maidstone
l11ill (lemel/ 200 I) del110nstraled the feasibility of
recycling clarifjer underflow to the diffuser, dispens­
ing with the filter station altogether. The l11udshould
be returned at a point where lhe Brix 01'the 11111dis
close to the jllice Brix in the ditTlIser, so as not to
interfere with extraction eff1ciency. To ensure that
Q<0
Raw juice
Figure 6.7: Diffuser instrull1cntation and control systcll1s
~
~
Vapor
Ill1bibition
water
"'-.....
~ Speed
contrai
lime
6.4 Factors affecting diffuser work 161
station. Most 01' the diffuser mills in SOllthern Africa
have been changed to incorporate mud recycle.
6.4 Factors af'fecting diffuser
work
Cane preparation for diffusion is no different
from that reqllired for milling. In both cases, good
preparation helps to achieve high extraction. This is
the most important variable affecting extraction in
difTusers. 11'high extractions are to be achieved, it
is essential that the cane is prepared in a heavy-duty
shredder so that most 01' the sugar-containing cells
01' the cane stalk are rllptured. A PI 01'> 90 is nor­
mally specified.
The way in which the cane is prepared is also im­
portanto Ideally the type of preparation should result
in material where ali the cells are ruptured but with
long fibers still evident, providing a cane bed that
is stable and open enough to allow high percolation
rates to be achievecl. In practice it has been found
that this is best achievecl in heavy-cluty shreclders
with a minimum 01' knifing, since intensive knifing
reclllces the average fiber length.
PaYl1e (1968) provides some data to show that
increasing the clisplaceability index from 92 to 94
increases extraction by about 0.4 %; increasing the
displaceability inclex from 88 to 92 increases extrac­
tion by abollt I %.
thc return of clarifier muds does not interfere with
percolation in the diffuser hy plugging the bed with
fines, the mud is returned dose to the first set 01'bed
lifting screws. Details 01'the arrangement are shown
in Figure 6.8. 11 was found that mud Oow rate aver­
aged only 4 t! I00 t raw juice, largely because 01'the
low solids content in juice from diffusers. Thus the
amount 01'sllgar recycled is in fact quite smal!.
Operation 01' mud recycling at a number 01'dif­
fuser mills has established that extraction and perco­
lation conditions are not afTected adversely. A num­
ber 01' significant advantages have been identified:
The operational and maintenance costs associ­
ated with running a filter station are eliminated.
Loss 01'sugar in cake is eliminated.
Chemical and bacteriological losses associated
with filler station operation are eliminated.
The cost 01'disposing 01'the cake is saved.
Water washing 01'the cake is obviated, reducing
evaporation requirements.
Equipment for bagacillo and cake conveying is
not required.
The bagasse supply to the boilers is increased.
However, the amount 01'ash in bagasse is increased
hyabollt 10 %. This cOllld have implications for in­
creased boi ler tuhe wear, depending on the boiler
design and in particular gas velocities through the
boi ler tube hanks. While reducing the ash (sand)
content 01'cane delivered to the mills is the best so­
lution, this is often not under the control 01'the mill
operator.
This approach enhances the attractiveness 01'dif­
fusion relative to milling, hy eliminating the filter
Lime
Heaters
6.4.1 Cane preparation
Cane
Lime
Clarifier I Clarified
JUlce
Figure ti.!,: Schcmatic diagram of clarificr mlld recycle to a diffllser
ReJerellce" p. /73
162 6 Cane diffusion
6.4.2 Cane residence time Table 6.1: Effect of cane residence time/screen area on
extraction
Tablc 6.2: Effect of changes in imbibition levei on ex­
traclion in a diffuser
Extraction Cane residenceScreen area
in %)
time inminin (m2 . h)/tF
98
8713
97
6710
96
548
As with any solid liguid extraction process, the
more extracting liquor that is added, the easier is the
extraction. So it is with cane diffusion. where higher
imbibition rates will invariably result in higher ex­
tractions. The amount 01' imbibition water added is
generally related to the guanlity 01' fiber bcing pro­
cessed. since it is the fiber which effectively forms
the cane bed and it is the fiber which removes with it
juice in final bagasse.
There is no maximum or minimum imbibition
rate for diffusion. Since high imbibition rates will
enable a smaller diffuser to be utilized to achieve
a given extraction. the reduction in the cost of the
diffuser would have to be balanced against the cost
of additional evaporator capacity and cost 01' steam.
The optimum imbibition rate for any extraction
plant therefore is dependel1t on the local factors al
that mil!.
The effecl 01' imbibition 011extraction is demon­
strated in Table 6.2. based on the resulls 01' a simula­
tion using a malhematical model and confirmed by
results from milling operations. lt is evidenl that the
effect 01' imbibition levei is greater in the case where
extraction is lower.
Experience in Soulh Africa has shown that very
high imbibition rates 01' over 400 % on fiber can be
handled. with conseguent eXlraction benefits. pro-
Imbibition % fiber
98.3
95.4
350300
98.0
95.0
250
97.6
94.3
Imbibition rate6.4.3
Case 1: high extraction
Case 2: low extraction
The longer the time the prepared cane spends
in the diffuser the higher will be the extraction
achieved. Provision 01' adeguate residence time is
probably one 01' the most important design specifi­
cations.
In practice the effect 01' residence time is not al­
ways evident in operating results. This is due to the
fact that as the cane throughput through the ditfuser
is changed, it causeschanges in the percolation rates
in the ditfuser, which may obscure the effect 01' fiber
residence time. These changes relate to the optimum
positioning 01' the interstage juice flows as a function
01' cane throughput rate (Section 6.6.3).
The relationship between fiber residence time 1:
and screen area 01' the diffuser A is given by egua­
tion (6.21). For a given volumetric throughput Vc at
a fixed bed height 11, the residence time is directly
related to the screen area 01' the diffuser. In practice,
bed heights in the diffuser range between 1.1 m and
1.8 m. 11 is clearly 01' benefit from a residence time
point 01' view to operate with as high a bed height
as possible. In practice however percolation condi­
tions become less stable at the greater bed heights
and unless a system is installed to cope with changes
in percolation conditions, diffuser operators have
found it generally more satisfactory to operate with
lower bed heights.
As a rough guide. the values in Table 6.1 apply in
South Africa but can vary substantially depending on
imbibition rate, bed height and degree 01' preparation.
For a bagasse ditfuser, Lamu.I'se and Fitz­
gerald 1974) reported thal a screen area of 5.5
(m2• h)ltp would enable an extraction 01' over 96 %
to be achieved. In fact in its last season 01' operation
in 1982. the bagasse diffuser at Empangeni operat­
ing at a fiber loading 01' 6.0 (m2 . h)/tp achieved an
extraction 01' 97.8 %.
The amount 01' juice held up in the saturated
cane bed 01' a diffuser is roughly 12 times the mass
01' fiber (Lave and Rein 1980). 11' the average fiber
content is 12 to 15 % in cane, the amount of juice
held up in the diffuser relative to incoming cane is
12 x 0.12 = 1.44 to 12 x 0.15 = 1.8 kg juice/kg cane.
Since the raw juice offtake from the diffuser is rough­
Iy egual to lhe tonnes 01' cane entering the diffuser,
average juice residence time in the diffuser is 1.5 to
2 times the residence time 01' fiber, higher if the juice
trays below the diffuser are not kept empty.
!:\\.~:~'
1"_:.-',
6.4.4 Number of stages 163
viding the interstage juice system is properly set up.
It should be emphasized however that it is not neees­
sary to employ a higher imbibition rate on a diffuser
than on a milling tandem. Some difTusers operate
with an imbibition rate below 200 % on fiber.
cases, but in any evcnt destroys concentration gra­
dients along the cane diffuser with a severely detri­
mental efTect on extraction.
6.4.6 Temperature
Although preparation is the most important
variable alTecting extraction in a cane diffuser, the
percolation rate is probably the next most important
variable. This is the rate at which liquid percolates
down through the bed of prepared cane. Laboratory
and pilot plant studies (Rein 1972) have shown that
an increase in percolation rate promotes the rate of
mass transfer and increases the proportion of the
juice in open cells that is accessible to the extract­
ing liquid.
The upper limit to the percolation rate is known
as the Ilooding rate. Flooding of the cane bed oc­
curs when more liquid than can actually percolate
downward between the cane particles is applied to
the top of the bed surface. This causes a number of
operational problems, including washing of cane out
of the feed or discharge end of the diffuser in severe
The use of a number of stages rather than a
single big mixed tank enables higher concentration
differences between sucrose in cane and sucrose in
percolating liquid to be achieved. As the number of
stages is increased, the case 01' true countercurrent
f10w is approached more closely. However the ben­
elit drops off as the number 01'stages increases and
the marginal improvement becomes very small.
The earliest cane dilTusers were installed with
aboul 18 stages. The trend has been in more recent
years to reduce the number of stages resulting in a
more cost-effective designo High extraction cane dif­
fusers need have no more than 12 stages.
From a mechanical structure point of view, a
lower limit on the number 01'stages may be imposed
by the length of an individual stage. If the length 01'
a stage is more than about 4 m, an additional bay per
stage is required as a consequence of the mechanical
design of the structure. The resulting cost increase is
larger than the saving in stage pumps and piping.
6.4.4
6.4.5
Number of stages
Percolation rate and ftooding
Higher temperatures are advantageous in that
they increase the rate of extraction through higher
molecular diffusivity and rcduced liquid viscosity.
However this effect is not as important as the effect
of preparation and liquid f10w rate. Nonetheless it
was estimated that an increase in temperature of
5 °C from 75°C to 80 °C would lead to an increase
in extraction of about 0.2 % (Rein 1974).
The most important reason for keeping the tem­
peraturc above 75°C is to control microbial activity.
This is discussed in Section 6.6.7. Generally diffus­
ers are operated at round about 85°C, allowing low
quality steam bled from evaporators to be used for
heating purposes.
6.5 Dewatering of bagasse
In some early installations a French screw press
was used as a dewatering device (Pa)'ne 1968). bet­
ter suited to this duty because the high moisture con­
tent and high temperature reduce friction. Howevcr
this device proved to be troublesome and subject to
considerable wear duc to sand.
Because of the large quantity of juice to be re­
moved and the high temperature of the juice in dif­
fuser bagassc, the dewatering of diffuser bagasse in
conventional mills was initially found to be a very
diflicult operation. Slipping of bagasse in the mill
was a serious problem, but this has now largely been
overcome by the development of suitable techniques
for roughening mill rolls. It is essential to ensure that
the dewatering mill rolls are kept rough if good ba­
gasse moistures are to be achieved. In addition good
juice drainage and tall feed chutes are important in
achieving good results.
Because of the quantity of juice, the dewatering
generally has to be done in two stages. If it is at­
tempted with a single four roll mill, it is often found
that there is considerable overf1ow of expressed juice
over the top roll onto the bagasse, and two dewater­
ing mills in series are requircd. the first largely to re-
-
AÇUCARBIRA ZIW> lDRBNZmI S/A
Referellees". /73
164 6 Cane diffusion
Table 6.3: Effecl 01' a reuuction 01' 1unil in bagasse moisturc content from 51 to 50 % on
extraction. Sucrose and tiber conlent 01' cane are assumed to bc 13.0 anu 15.0 g per 100
g respectively anu residual juice purity is 75.0.
Moisture in bagasse '"W.A
Sucrose extraction E
Sucrose in bagasse '"S.B
Fibcr in bagasse '"r.B
Sucrose in juicc in bagasse '"5.!
New moisture in bagasse '"W.B
Sucrose in juice in bagasse '"s.J
(unchangeu. as above)
Fiber in bagasse '"r.A
Sucrose in bagasse '"S.B
Sucrosc extraclion E
Change in extraction tJ.E
95
2.01
46.32
4.77
4.77
47.41
1.94
95.27
0.27
51
96
97
1.62
1.23
46.84
47.36
3.92
3.02
50
--
3.92
3.02
47.91
48.41
1.57
1.19
96.22
97.16
0.22
0.16
98
0.83
47.89
2.07
2.07
48.93
0.8
98.11
0.11
move a good proportion 01' the juice. An alternative
arrangement is to attach apressure feeder to the mil!.
The pressure feeder is able to remove sufficientjuice
from the diffuser bagasse so that adequate bagas se
moisture can be obtained in a single milling unit.
It has also been found in South Africa that low
mill roller surface speeds are necessary if moisture
contents below 50 % are to be obtained. In addition
larger grooving and slightly higher feed/discharge
work opening ratios than in conventional milling are
required. Analysis of diffuser dewatering installa­
tions in South Africa in the 1990s showed a depen­
dence ofbagasse moisture content on dewatering mill
capacity. In order to achieve 50 % moisture. a total
roller volume 01' about 0.4 m3 per tonne throughput
01' fiber per h was required;since that time progress
in achieving lower moistures has been evident.
Some dewatering mill installations operale with
two dewatering mills run in parallel rather than in
series. This leads to much lower mill speeds which
help to achieve low bagasse moistures. The optimum
arrangement depends on the size 01' the mills and the
fiber throughput. Operation 01' two four-roll mills in
series is less difficult to set up and operate than two
pressure fed mills in paralle!. Bagasse conveyors too
are less complicated in a series operation.
Various lightweight dewatering devices ahead
01' dewatering mills have been installed at various
times on different installations. One 01' these was
the Sucatlan hydraulically driven dewatering de­
vice fitted to Saturne diffusers. While attractive in
concept. many attempts to use a light-weight de­
watering device have failed. I-Iowever more recent
installations in Brazil are claimed to be effective.
Figure 6.9 shows the type of light-weight dewater-
Bagasse díscharge
to intermediate
carrier
Figure 6.9: Light two-roll bagasse uewatering uevice
6.6.1 Monitoring of the efficiency of extraction 165
with the different values of sucrose and flber in ba-
it is possible to calculate new values for the sucrose
and fiber content. Assuming ali the fiber in cane
ends up in the bagas se, extraction can be calculated
as:
If the moisture content of the bagasse is reduced,
the sucrose and fiber contents will also change. Us­
ing equation (6.24) and the following mass balance
relalion:
gasse.
Table 6.3 gives calculated values of extraction
improvement due to a reduction in bagasse mois­
ture using this procedure. The change in extrac­
tion is not sensitive to lhe values of sUCl'ose and
fiber in cane assumed. Various measurements have
substantiated the theoretical values in practice in
spite of the assumptions made and these are be­
lieved to be reliable estimates. It can be seen that
at low extraction levels, the effect of improving
bagas se moisture has a more signiticant effect on
extraction.
Monitoring of the efficiency of
extraction
Control and operation of
diffusers
Analysis of cane and bagasse is necessary in
order that extraction can be monitored on a routine
basis. Measurement of the moisture content of the
bagasse from each dewatering mill is often also ad­
vantageous. Extraction in a diffuser is less depen­
dent on cane quality than in a milling tandem. Thus
the various forms of reduced extraction which have
been proposed are not really applicable to diffusers
(Rei/l 1975).
It is not possible to monitor the extraction
achieved in a ditfuser on its own because of the large
amount of juice associated with the wet bagas se
leaving the ditTuser. An attempt has been made to
overcome this problem by washing off the attached
surplus juice and measuring the amount of sucrose
!eft unextracted and not available by simple wash­
ing. This led to the concept of "difficult Brix" ex­
traction and the measurement of what was termed
specific extraction (Fergllso/l et aI. 1972). This ap­
proach does allow the extraction achieved in the
diffuser only to be assessed, but the measurement
is technique-dependent and is not used on a routine
basis.
Because of the important effect of cane prepara­
tion on eXlraction, it is important that Pior POC be
measured on a routine basis. Very often a drop in ex­
traction is associated with a deterioration in prepara­
tion, which can be identified from routine analyses.
It is common practice to take juice samples from
each stage in a diffuser on a regular basis, every hour
or once a shift. Dissolved solids (Brix), temperature
and pH of each sample are measured and a profile of
these variables through the diffuser is plotted. An ex­
ample is shown in Figure 6.10. These show whether
adequate temperature control is being achieved and
whether the juice flow system is operating satisfac­
torily. Ideally, the Brix profile along the diffuser
should show a steeply descending slope at the feed
end of the diffuser, tailing off as the discharge end is
approached. Any unevenness in this Brix curve is an
indication of problems with the liquid fiow system.
It is of vital importance that the bed height in the
diffuser be uniform across the widlh of the diffuser.
6.6.1
6.6
(6.25)
(6.26)
(6.24)
II'W.U+ 11'1'.11+ lI's.1I . 10011' = 100
E = I00 . [I - ( I\' S.I! 11\' F.I! ) I ( I\' s.e 11\' re )]
IVS.J = lI's.u 1(1-1.251\'1'.11 11(0)
ing device in use. Typically it has a grooved top
squeezing rol I of about 0.9 m diameter, while the
bottom roll is a larger perforated drum with 10 mm
diameter holes to facilitate drainage of juice. The
device is fed by a conventional [)o/l/lel/y chute and
the rolls may be on fixed cenlers or the top roll may
be lightly loaded. Power requiremenls are low, typi­
cally 1.5 to 2 (kW . h)/tr-
While it is imporlant to reduce bagas se mois­
tures to improve lhe value of bagasse as a fuel, the
additionaljuice thal is recovered also promotes ex­
traction.
It is possible to calculate the magnitude of this
effecl, assuming that the residual juice in lhe ba­
gasse is ali at the same concentration and that its
purity P does not change with a small change in
moisture content.
If the analysis of bagasse is given, the concen­
tration of sugar in the juice in the bagasse, assuming
25 % Brix-free water, is:
Referel/c"" p. J 73
~
]66 6 Cane diffllsion
11'there is a low point in the distriblltion across the
diffllser, it reslllts in a glllly rllnning the length of the
diffllser. This allows jllice to ftow IIp and down the
diffllser on top of the bed, destroying the concentra­
tion gradient in the diffllser and callsing perco]ation
and ftooding problems in some instances. lt is far
less important if the bed height varies a]ong the dif­
fllser, callsed perhaps by an lInsteady feed rate.
The temperature should be up to 75°C within
one stage and at the required operating temperature
within two stages, and then remain steady ali the
way through the diffllser. A temperature p]ot sllch
as that in Figure 6.] O shows if the control set points
or points of addition of heating steam need to be
changed.
Press water shollld be returned to a point where
its Brix (normally about 1-2) corresponds to the
interstage juice Brix. This is generally one ar two
stages from the discharge end 01'the ditTuser.
oR 20 96
o 18 92
c
;;; 16
88 ~
-o
~ 14 84 .~-o 12 80 ~
'" -'" 10~
76 1§
8
'"
'6 272 ~
-o
6c 68 ~'" '"
4
64::J 3ro
2>
60
I
O
"-
Raw 1 2 3 4 5 6 7 8 910111213141516
juice
No. of circulation juice stage
Fi/:urc 6.10: Typical mcasurcd valucs 01'tcmperaturc, pH
valuc and dissolvcd solids contcnl from diffcrcnt stagcs
ofa diffuscr
I Tempcraturc: 2 pH valuc: 3 Dissolvcd solids content
mF
b . h . Pb•F
Best performance is obtained from a diffuser running
lInder totally steady conditions. Attempts have been
The cane feed to a diffllser involves the contro]
of the volllmetric or mass feed rate into the diffuser.
This is generally done in one 01'three ways:
Control 01'the speed of a belt conveyor feeding
the diffuser, measuring the height 01'cane on the
belt and keeping constant the product of convey­
or speed and height.
Using the speed of feed rolls beneath a choked
feed chute feeding into a shredder ahead 01'the
diffuser, as the means 01' controlling the feed
rate.
Control of the mass ftow rate of cane with a belt
weigher on a variab]e speed belt conveyor ahead
01'the di ffuser.
Once the volumetric ftow rate 01' cane is approxi­
mate]y constant, the diffuser bed speed is selected to
give the required bed height. It is important to keep
the diffuser running at a constant speed if good re­
sults are to be achieved. The bed speed can be cal­
culated from a combination 01' equations (6.3) and
(6.22) which leads to:
made to vary lhe bed speed to keep a constant bed
height. Becallse 01'time lags involved, this has never
proved successflll. Even if some short term varia­
tions in cane throughput rate occur, it ispreferable to
run the bed at constant speed and the bed screws will
assist in leveling the bed. This policy greatly assists
the dewatering mills, which otherwise have to cope
with a ftllctllating feed. The bed speed shollld only
be changed if a change in crushing rate is desired, or
if the bed height is to be changed.
Flooding or perco]ation problems are often a
feature 01'diffllser operation if the dilTuser is not set
IIp correct]y. Allderso/l and Smith (1981) give a de­
scription 01'problems that can be experienced.
Ideal ftow conditions in a diffllser as depicted in
Figure 6.\ are not lIsually achieved. There are two
sources 01'non-ideality:
I. Sideways dispersion leads to jllice entering trays
on either side of the preceding tray, as tracer tests
have shown (Love and Reill 1980). As Brix dir­
ferences between adjacent stages are not large,
other than in the first few stages, sllch sideways
mixing does not have a significant adverse effect
on extraction.
Control of percolation in
diffusers
6.6.3
(6.27)
Control of feed of cane and bed
speed
IIc
6.6.2
6.6.4 pl-! Control 167
The side walls and the screen 01' diffusers are
made 01' stainless or corrosion resistant steel to over­
come the problems of combined corrosion/erosion.
Likewise the pins and bushes 01'the chain are some­
times made 01'stainless steel. as the pl-! 01'lhe juice
film between the chain components can easily drop
to a low levei due to microbial action and is there­
fore very corrosive.
The pl-! 01' the juice in the ditTuser is usually
controlled by adding lime into the diffuser. The pl-!
does not affect extraction and lime is added only as
a means 01' conlrolling corrosion. The temperature is
low enough that even at the normal juice pl-!between
5 and 5.5. inversion 01'sucrose is not significam.
Adequate pl-! control can be achieved by add­
ing lime imo two or three stages 01' the diffuser. 11
is generally dosed into the trays. gets mixed in the
stage pump and the pl-! is Illeasurcd on the discharge
pipe after the pump. Care must be taken to ensure
that over-liming does not occur in any one stage
(pl-! > 7). This has been shown to affect percolation
adversely (Lo!'/' and Reill 1980) and its effect is not
reversible if the pl-! is subscquenlly reduced. Regular
inspection 01' data plolted as in Figure 6.10 helps to
assess whether the controls are working adequately
and whether the set points chosen are appropriate.
In addition. over-liming 01' juice in diffusers can
lead to the hydrolysis 01' acetyl groups from the hemi­
cellulose 01'the cane fiber leading to the production
01' acetie acid. The acetic aeid may then be volatilized
in the evaporators causing serious corrosion 01' pan
calandrias and vapor and condensate piping (Beckell
and Gral1ol11 1989). In general. acetic acid levels in
cane from direct analysis. average around 200 mgl
kg DS. In a difTuser undcr good control. a value 01'
around 300 mg/kg DS can be expected. Values mea­
sured at Felixton have. at times. been as high as 1000
mg/kg DS under poor conditions. ln order to obvi­
ate these problems. lime is preferably added in more
than I stage. and control is sometimes easier with a
dilute slurry 01' about 4 °Baumé. ldeally however the
diffuser should be made 01'stainless or non-corrosive
steel or have anticorrosion coatings applied. so that
the need for lime addition is obviated.
2. Juice pUlllped from a slage lray does not ali find
its way to lhe preceding tray. in a true stage-wise
system. There is always some degree 01'recircu­
lation or bypassing occurring. depending on the
operating conditions and the spray positioning.
This is illustraled in Figure 6.2. The results 01'
inappropriate conditions are eilher flooding or
percolation rates Ihat are too low.
Relalively small changes in throughput rate Ihrough
the diffuser. or cane variety. or preparation. can have
a significant elTect on the oplimulll spray posilions.
For this reason a system has been devised where the
point 01'application 01' inlerslage juice can be auto­
matically controlled. This is done by measuring the
liquid levei in the cane bed and adjusting lhe posi­
tion aI which the liquid is applied to the top 01' the
bed 10 Illainlain the liquid levei at the optimum levei
(Reill and 111;;1101111992). This gives belter results
than merely selting interstage sprays 10 handle aver­
age conditions.
The optilllum spray advance can be calculated
from equations (6.10) to (6.12). This requires an
estimate 01' the percolation velocity in a dilTuser.
which can be obtained from a fairly simple set 01'
Illeasurements. This involves establishing steady
conditions in the diffuser with a liquid levei in the
bed. but without any Ilooding. and then switching
olT ali lhe spray pumps. The rate at which lhe liquid
levei drops. observed through the windows in the
sides 01' the diffuser. gives the percolation velocity.
This may be repeated a number 01' times to get an
estimate 01' the average and range 01' percolation ve­
locities.
Ideally lhe point 01'application 01'the juice frolll
each stage should be done automatically. Alterna­
tively manual adjustment is possible. advancing the
spray position if nooding occurs and retarding the
spray position if the bed is too dry i.e. a liquid levei
is not visible in the bed.
11 is common practice to back 01'1'lhe degrec 01'
cane preparation when flooding occurs. This is an
inappropriatc response and has a seriously dctrimen­
tal effect on extraction. A belter approach makes use
01' variable throw sprays to enable the point 01' appli­
cation ofjuice to bc adjusted instead. and extraction
is maximized.
In the absence of adjustable sprays. the practice
01' turning 01'1'one or more stage pumps where the
flooding oceurs is less damaging to eXlraction effi­
ciency.
6.6.4
6.6.5
pH control
Corrosion control in dif'fusers
-
R,jerellces fi. /73
Low Brix raw juices degrade readily as a result
of microbial activity. At room temperatures a large
range of organisms termed mesophiles will ferment
sugar juices. Perhaps the most evident microbial ac­
tivity is shown by Leltcol1ostoc sp, which are slime­
forming bacteria. Such slime is commonly observed
in milling tandems where insufticient attention is
given to cIeanliness of the mills. Actions required in
a milling tandem are outlined in Section 5.12.3.
The natural pH of cane juice is such that seri­
ous corrosion of the mild steel parts of a diffuser
can occur. For this reason, lime is generally (but not
always) added into one or more of the juice trays,
under automatic pH control. The objective is to keep
the pH at about 5.8 to 6.0 in ali stages of the dif­
fuser.
The internal roof of the ditTuser and the stage
trays need to be protected against corrosion, either
by regular painting or other protective coating. or by
using a corrosion resistant material such as 3CR 12.
One of the major advantages of diffusion com­
pared with milling is the greatly reduced mainte­
nance cosI. Very little routine weekly maintenance
is required, and only the following items need atten­
tion on an annual basis:
Chain runners wear and need to be replaced ev­
ery few years.
Lifting screw t1ights wear in spite of hard-facing
on the edges of the t1ights. They need periodic
attention.
Routine pump maintenance is required.
A check on corrosion of trays and structural
steelwork is necessary, particularly at the feed
end of the diffuser where pH values are lower.
Chain pins and bushes need to be replaced. They
have a life of between 5 and 25 or more years.
depending on the materiais and the type and de­
sign of chain.
The diffuser drive (variable speed motor and re­
duction gears) needs routine maintenance.
Headshaft bearings need routine inspection.
168
6.6.6
6.6.7
Maintenance of'dif'f'users
Microbiology of'extraction
6 Cane diffusion
In diffusers where temperatures are considerably
higher. mesophilic organisms are rendered inactive,
but hyperthermophiles may be active. They are gen­
erally lactic acid producingbacteria, anel are active
at temperatures up to 70 0e. The pH range from 5
to 6.5 found in mills and diffusers does not have a
signitlcant etTect on microbial activity.
Control 01'microbial activity in diffusers simply
requires operation aI an average of about 85 0e. This
ensures thal the temperature at no stage drops below
75°C, which is considereel to be the minimum op­
erating temperature. Sufticient heater capacity must
be installeel on scalding juice eluty at the feed end 01'
the dilTuser. in order to achieve a bed temperature of
at leasl 75°C within I stage. Uneler these conditions,
raw juice leaving the diffuser is at a tell1perature of
about 60 to 65 0e. No biocides are necessary if the
temperatures are kept above the minimull1 required.
In eliffusers, losses 01' sugar can be very high
if temperatures are not kept well above 70 0e. It is
not considered feasible to operate ditTusers aI lower
temperatures, as losses under these conditions can
be severe.
The use of biocides in cane extraction plants is
expensive. In diffusers. the attainll1enl of satisfactory
levels 01'temperature is a much sill1pler, cheaper anel
more effeclive means of microbiological control.
The major degradation product of hyperthermo­
philes is laclic acid, which can be routinely mea­
sured. In addition, severe cases of microbiological
losses will be evielent in an accompanying drop in
juice purity. The routine measurell1ent 01'lactic acid
in juice is recomll1ended as a control measure. Aver­
age values of 300 mg/kg lactic aciel on RDS repre­
sent a realistic target for both ll1ills and ditTusers.
In the beet sugar industry. pre-scalders are used.
in which the raw juice is cooled down by heating up
the beet. The rationale is to produce a cooled raw
juice which can use last vapor or even condenser
water to transfer heat anel improve steam economy.
This is likely to be very dangerous with cane in terms
of ll1icrobiological losses. because the temperatures
are ideal for the activity of a number 01'dilTerent or­
ganisms. Biocides would have to be used heavily.
It is less appropriate for cane than beet because the
ambient and cane temperatures tencl to be much
higher and microbiological 110raare often well es­
tablished on cane, particularly if it is chopper har­
vestecl.
,
6.7.1 Capital costs 169
Lalllllsse (1984) has also estimated that the
maintenance costs of a milling tandem are 70 to 80
% higher than those on an equivalent diffusion ex­
traction system. This comparison takes into account
the fact that a dilTuser chain either has to be rcplaced
or have new pins and bushes installed periodically.
Alldersoll and SlIIith (1981) provide similar numbers
to confirm this.
Analysis of stores costs over five seasons. for the
diffuscr and mill eXlraction lines at Maidstone mill
in South Africa, including cane preparation, showed
in-scason, off-crop and total stores costs for the dif­
fuscr line to be 71 %.48 % and 64 % rcspectively of
those for the milling line. If the cane handling and
preparation costs were to be rcmovcd. the estimates
01' Lalllllsse and Alldersoll and SlIIith above would
The extraclion plant is generally a high cost item
in the total sugar mill complex. In this respect the
diffuser offers considerable advantage over conven­
tional mills.
Comparisons should be made between a diffuser
together with its dewatering mills compared with a
fullmilling tandem. Ll/IIII1SSe (1984) has suggested
that the capital costs 01' the diffusion plant would be
about 66 % of the cost of a milling tandem and per­
haps even as low as 55 % if a single mill only is used
for dewatering afler lhe diffuser. In house figures de­
rived by Tongaat-Hulett Sugar confirm the figure 01'
66 % where two dewalering mills are required and
where an extraction of 96 % is desired (Reill 1999).
However once a higher extraction of 98 % is re­
quired, the ratio of diffusion to milling plant capital
cost decreases to 60 %. Thus lhe capital cost advan­
tage of diffusion increases as higher extractions are
sought. These values are higher if pressure feeders
are required on the dewatering mills in order to
achieve acceptable bagasse moistures. but are re­
duced if pressure feeders enable a single dewatering
unit to be used.
Comparison with milling probably be substantiated (Reill 1995). Klllllar and
Rao (2000) report that the operation and mainte­
nance costs of a diffuser, including the dewatering
milL are 44 % of the costs for a four-mill tandem.
Fewer people are required to operate and main­
tain a dilTuser. In general it can be assumed that both
maintenance and operating costs will be about half
the costs of a milling tandem for an equivalent sized
diffuser.
Eft'ect on steam balance and
power requirements
6.7.3
Additional heat is required in the diffusion sys­
tem, generally obtained from vapor I, 2 or 3 bled
from evaporators at an amount of about 1I % on
cane. The raw juice leaving the diffuser is at a higher
temperature and so roughly half of this heat is re­
covered, but the rest of the energy is lost in the final
bagasse. If vapor I is used for heating, the net effect
afler evaporation is to increase the total amount of
steam required in a conventional sugar mill by about
3 % on cane. If vapor 2 or vapor 3 is used. a diffuser
mill will not require more steam than a conventional
mil!.
A disadvantage of diffusion is the fact that more
of the sand coming in with the cane ends up in the
final bagasse and less in lhe mixed juice. Typically
for cane containing 2 % ash, ash % bagasse from a
milling tandem would be 3.3 by comparison with a
figure of 4.0 from a cane diffuser (Lalllllsse 1984).
By contrast Allders(J/l and SlIlith (1981) report very
little increase in ash in bagas se from a bagasse dif­
fuser in Australia relative to milling. The effect of
increased ash is to reduce the calorific value of the
bagasse marginally (see equation (27.1) in Section
27.2.1), but a more severe disadvantage is the fact
that additional sand in bagas se may lead to more
wear in boilers. The effect of this can be minimized
in the design of the boi ler generating tube banks.
Ensuring that gas velocities through the boi ler tubes
are less than 12 m/s will generally eliminate serious
erosion (Section 27.5.8).
vali Hellgel (1990) has shown thal a diffuser
factory requires far less prime mover steam. so
that lower pressure boilers can be used. Alterna­
tively, diffusion is of considerable advantage to
a factory that exports power. Typically installed
operatingand
Capital costs
Maintenance
costs
6.7.1
6.7.2
6.7
ReJaellces 1'. /73
170 6 Cane diffusion
Talde 6.4: Comparison 01' bagasse and steam figures for
a mill and a diffuser processing 300 t/h at an imbibition
rate 01' 300 % on fiber, factory producing steam at 3.2
MPa. using vapor I on the pan IJoor and diffuser heating
duties
Table 6.5: Comparison 01' the quality 01' raw juice from
diffusion and milling (avcrage 01' 10 years data from
Maidstone mill). from Reill (1995)
Diffusion Milling
The two most marked effects 01' diffusion on
juice quality are a higher color in juice and a much
clearer juice, i.e. with lower suspended solids. Juice
color depends on the cleanliness 01' the cane and the
diffuser temperature, but can be an importam issue
when attempts are made to produce a low color raw
sugar or a mill white sugar.
power values (excluding cane preparation) are
90-100 (kW· h)ItFfor a milling tandem and 45-50
(kW· h)ltr for a diffusion piam including dewater­
ing mills, i.e. roughly half 01' the power required in
milling. More comprehensive details are given else­
where (Rein and Hoekstra 1994). Further compara­
tive data is given by Kumar and Rao (2000), which
confirm these nllmbers.
The results 01' a simulation 01' energy balances
for a diffuser and a milling tandem, each crushing
300 t/h, are reported by Reill and Hoekstra (1994)
and are shown in Table 6.4.
Other results 01' this simulation exercise showthat the diffuser factory steam usage can be reduced
below that 01' the mill if vapor 2 is used for process
heating, and that 4.5 MW more power can be ex­
ported from the diffuser factory.
lI' mud recycle from the clarifiers is pracliced,
the bagasse to the filter station is saved and is avail­
able for fueluse.
Effect 00 raw juice quality
Milling Diffusion
84.9
0.64
0.67
0.985
560
97.2
84.]
0.16
0.12
0.988
270
97.7
Season MillingCaneIndustry
diffuscrs
average
1986/87
0.800.090.37
1987/88
0.870.080.39
GC sucrose purity in %
Suspended solids in g/IOO gjuicc
L'l Raw juice purity - cane purity
Pol/sucrose ratio
Lactic acid in mg/kg DS
Sucrosc cxtraction in %
Table 6.6: Average suspended solids Icvcls in g/lOO g
raw juicc in South African sugar mills
Comparison 01' juice quality from the milling
and diffuser tandems is shown in Table 6.5. This
shows clearly that the suspended solids contem 01'
raw juice is considerably lower from a diffuser, and
that lactic acid, a good indicator 01' microbiological
activity, is significantly lower in difTuser juice.
Juice from the ditTuser has a slightly lower pu­
rity, bUl this is largely due to the higher extraction.
Suspcndcd solids. In South Africa the suspend­
ed solids in raw juice is accurately measured at ali
mills, because it affects the payment for sucrose in
raw juice. Thus it is easy to make valid comparisons
between juice from milling tandems and diffusers.
Table 6.6 shows average suspended solids levels
in raw juice for South African milling tandems and
cane diffusers for two high production years at a pe­
riod during the 1980s when roughly half the cane in
the industry was processed in ditTusers. lt is appar­
ent that suspended solids in mill juice are an order 01'
magnitude higher than in diffuser juice. These mea­
surements are taken after the jllice screens. In most
cases,jllice screens are not installed in diffllser mills,
as the solids levels in dilTlIserjuice are so low.
Data reported on South African mills indicates
that sllspended solids vallles in jllice from fixed
screen diffusers are lower than from moving screen
152.3
50.8
53.5
91.5
1.5
90.0
7200
180
75.7
14.3
143.8
47.9
50.7
87.5
3.0
84.5
7497
176
68.7
15.8
High pressure steam in t/h
Steam % cane - instantaneous
Steam % cane - total
Bagasse produced in t/h
Bagasse to filter station in t/h
Bagasse available in t/h
Bagasse NCV in kJ/kg
Fuel value 01' bagasse in MW
Bagasse for steam generation in t/h
Bagasse surplus in t/h
6.7.4
6.7.5 Juice screening and filtration 171
Because of the lower suspended solids content
in the raw juice from a diffuser, juice screens are
often dispensed with entirely. In addition, the filter
area required is very much reduced because 01' the
reduced rÍlud quantities. In Southern Africa the filter
screen area required has been found to be roughly
halved with a diffuser. 11'mud recycling is practiced,
the filter station is eliminated entirely.
ever, showed that a 10 °e change in temperature
changes the color 01' juice by 12 %, but this ap­
plies to hand-cleaned cane.
From these observations, it is dear that, although
din·usion gives a higher color juice than milling,
other factors such as cane variety, cleanliness 01' the
cane, climatic conditions and time 01' season can
have a significantly greater effect on color of juice
and sugar.
The effects 01' differences in juice quality on re­
covery are both positive and negative. The amount
01' filter cake produced is roughly half that in a dif­
fuser mill compared to a milling tandem and so the
loss in cake is halved. 11'clarifier muds are recycled
to the diffuser, cake loss is eliminated altogether.
Monosaccharide/ash ratios in juice from diffus­
ers 01' mills appear to be the same, so that identical
final moi asses purities can be expected. No effect on
final moi asses viscosities is evident. Speculation that
a diffuser leads to increased losses in moI asses has
Othcr constitucnts. The starch content of dif­
fuser raw juice is much lower than juice from a mill­
ing tandem. The higher temperatures in a ditfuser
cause gelatinization 01' starch granules, which render
the starch available to natural enzymes that eliminate
starch in the cane (Buyes 1960). This has meant that
the use 01' amylase to reduce starch in raw sugar has
generally been discontinued in diffuser factories.
Starch levels in sugar from diffuser factories are
found to be 25 % lower and gums levels 12 % lower
than those in sugar from conventional mills (Kuster
1995).
Effect on overall sucrose
recovery
Juice screcning and filtration
6.7.6
6.7.5
Juicc and sugar color, On average, the color of
juice from a diffuser is about 10 to 20 % higher than
mill juice, but is affected by a number 01' factors.
Rei/l (1999) showed how sugar color has varied in
South Africa for a variety 01' reasons. The following
issues are of interest:
Processing the poor cane obtained in a drought­
afTected season leads to an increase in color of
40-50 %, for both milling and difTusion.
When the change from processing half of its
cane supply by dilTusion to full difTusion at
Maidstone mill took place in 1995, no signifi­
cant color change was evident as a result 01' the
second difTuser.
By contrast, the change from milling to diffu­
sion at Umzimkulu in 1991 resulted in sugar
color moving from below industry average to
above industry average.
The juice from different varieties of cane in
South Africa can show very large differences
in color. The varieties giving the highest color
have juice colors more than twice the color 01'
the lowest color variety (Lio/l/let 19S5).
There is a very large time-of-season effect on
cane and sugar color. The sugar color in the first
and last month 01' the season can be 50-100 %
higher than the color in the middle part of the
season.
The presence 01' tops and leaves has a signifi­
cant effect on lhe color of diffuser juice. Liu/I­
/let (1988) found lhat average levels of tops and
leaves found in South African sugar mills would
increase color ofjuice by about 20 %. According
to PaY/le (1968), leaves either green or dry, are
the major cause 01' color development.
Higher temperatures lead to higher juice colors.
Measurements at Amatikulu mill showed that a
reduction in temperature 01' about 10 °e resulted
in a drop 01' 25 % in color. Liu/I/let (1988), how-
ditTusers. Typically, these values are 0.1 and 0.5
g/l 00 g juice respectively.
PaY/le (I96S) reports that juice clarification can
be undertaken in the diffuser itself because of the
filtering action of the cane bed. This involves heat­
ing the juice to about 90 De and increasing the pH
to somewhere in the range 7 to 7.3. Thejuice is lhen
sent straight to the evaporators. However this is the
only reported case of clarifying in the diffuser itself
and the practice is not used elsewhere.
Referel/ces p. J 73
172 6 Cane diffusion
Because 01' the long residence time 01' cane in
the diffuser, start-up anel liquidation operations are
somewhat more prolongeel with a eliffuser. 11is com­
mon practice to till ali the stages 01' the diffuser with
water before starting up so an adequate supply 01'
water has to be available during the maintenance
shutelown. Then there is a period 01' about an hom
before bagasse gets through to the boilers. This
means that an aelequate bagas se store and system 01'
reclaiming bagas se to the boilers is necessary.
Likewise on shutting elown, liquielation 01' the
eliffuser takes a much longer time anel the clarifiers
been demolished by careful study 01' data covering
many years (Kuster 1995). He showed conclusively
that there is no preferential extraction 01' nonsucrose
in a diffuser, and the lower purity is likely to be elue
to the higher extraction. In fact, Koster believes that
a higher recovery is possible in a dilTuser factory,
due to the lower cake losses.
Microbiological losses in milling tanelems have
not receiveel much attention, particularlyas they
are very difficult to measure. Even when extensive
losses occur in the milling tanelem, no significant
reeluction in apparent juice purity is evident. This is
partly due to the fact that elextran proeluced in large
quantities in mills by mesophiles is strongly elextro­
rotatory. artificially inflating the pol measurement.
In general the extent 01' losses in milling tanelems is
unknown. since a means 01' measuring such losses
routinely is not available.
Experiments in the laboratory have established
an approximate conversion equivalence between
lactic aciel formeel and sucrose lost (Mackrory et aI.
1984). Each part 01' lactic acid formed in a diffuser
corresponds to 2 parts 01' sucrose loss. A elifferent
equivalence was founel at temperatures correspond­
ing to milling tanelem operation; in this case, each
part 01' lactic aciel formed means a loss 01' about 8
parts 01' sucrose. However this relationship is con­
sidereel to be less reliable.
Data from Maielstone mill in Table 6.5 show that
the lactic acid content in mill raw juice can easily be
twice that in diffuser juice. Baseei on the equivalence
ratios between lactic acid and sucrose established by
Mackrory et aI. (1984), the loss in mills is probably
far more significant than in diffusers.
generally have to hanelle a redllcing Brix jllice dllr­
ing liquidation.
In operation, diffllsers are more Ilexible than
mills in coping with a wider range 01' throllghpllt
rates. The diffuser speed can be run as slow as the
elrive will allow. i.e. the turndown is very good, and
may be extendeel even fmther by changes in bed
height. The maximlllll eliffllser beel speed will be set
by the ability 01' the elewatering mills to handle the
quantity 01' eliffuser bagasse.
11' long stops are encountered dlle to mechani­
cal breakdowns. it is generally advisable to empty
the dilTuser if the stop is to last more than about six
hours. 11'this is not done. significant deterioration 01'
the sugar in the eliffuser can occur.
Most eliffusers have been installed as a reslllt 01'
a requireel increase in crushing capacity. Small in­
creases in capacity can be obtained from a Illilling
tandem by fitting pressme feeders or by replacing
critical milling units, say the first and the last Illill
with larger units, but the increase obtainable by this
means is lilllited. A cost-elTective method 01' expan­
sion involves installing a diffuser anel utilizing the
best 01' the existing milling units as dewatering mills
for a eliffuser. Rivallalld ( 1984) has confi rmed the cf­
fectiveness 01' this approach from a capital cost point
01' view.
Once the diffuser is installed and further expan­
sion is requireel, the installation 01' a complete ad­
elitional diffuser can be expensive. Alternatively. a
reduction in extraction as a result 01' reduceel cane
residence time is an option. However if it is envis­
ageel that an expansion will be required at the time
that a new eliffuser is being installed, it is probably
wise to pre-invest in incorporating headshaft anel
chain designs that can operate with an expanded dif­
fuser. 11is relatively cheap to increase the length 01' a
diffuser to obtain aelelitional capacity in this way. by
extending the diffuser at the tailshaft (feed) end.
In Zilllbabwe the capacity 01' a eliffuser was in­
creaseel frolll 220 to 300 tonne cane/h by increas­
ing the bed height frolll 1.3 to 1.8 m (Reill 1999).
This involveel raising the top section 01' the eliffuser.
incllleling the press roller. and replacing and rear­
ranging the stage juice sprays to accolllmodate the
Expansion of mill and diffuser
capacity
6.7.8
Effect on operations6.7.7
6.7.9 Maximllm capacity 01' a single extraction line / References 173
Refcrcnccs
higher throughput and the more compact bed reslllt­
ing from the increased bed height. It was necessary
first to check that the headshaft and drive were ca­
pable 01' handling the greater load. The modification
was sllccessful, with the dilTuser handling the higher
throllghput without a drop in extraction.
Ander.wll N. W; Smirh f).f~ (1981): Pcrformancc and opcration 01'
Inkcrrnan's diffuscr/milling train. Proc. Âust. Soe. Sugar Canc
Teehno!. 3. 255-25'1.
Becketl J.: GrllhllUl WS. (1989): Acctatc cxlractio!1 in a cane dir­
fuser Proc. S. Afr. Sugar Teehno!. Ass. 63. 28-32.
80yes P.N. (1960): Starch in lhe rnanufacllIrc 01' raw sugar. Prac. S.
Afr. Sugar Teehno!. Ass. 34.91-97.
Bnmic!ie-Olsell 11. (1966): Recenl experience wilh lhe DOS c:mc
tliffuser. Sugar Azuear 61. 8. 27-29.
D'Es!,ai!(1l1'1J.T: Ri\'(/I/alld J.f:R. (1'174): Operalion of a Saturne
diffuser in Maurilius. Proc. (11t. Soe. Sugar C,me Tcchnol. 15.
14'1'1-1511.
Fergu.wll A; Jel11ling\' R.?; Reilll~ \\1:; Scllfl11lllllll C:J:; \'em lIellgel
li. (1972): Diffuser performanee appraisal - a new approaeh.
Proc. S. Afr. Sugar Teehno!. Ass. 46. 54-63.
Filzgerald .I.R.: SI/lI G.E.: 1'1/11 Hel/gel A. (1978): The f'S diffuser
(van lIengel syslem). 1111.Sugar 1. 80. 3-9.
Jellst'11 C.H.C. (2001): The climinatiol1 01' IIltercakc in a cane sugar
factory by rccycling dcfccation Illuds to the cxtractiol1 plant.
Prol'. Inl. Soe. Sugar Canc Tcchnol. 24. 231-236.
Klll/l'hel K.S. (1981): Simplilietl sparger tlesign. Chem. Eng. 88.
3.116-117.
There is a maximum size 01' dilTuser gener­
ally dictated by mechanical considerations. Gener­
ally the length is limited by lhe requirements of the
chain and the size of the headshaft becomes imprac­
tically large as the width 01' lhe diffuser is increased.
At present the largest dilTuser has a width of 12 m,
which depending on cane quality would generally be
adequate for 450 te/h (65 t/h) with 98 % extraction
or 700 tJh (100 t!h) at 9f. % extraction (at average
imbibition rate). 11' a bagasse diffuser were chosen,
this size diffuser would handle rollghly 650 and
1000 te/h at these extractions respectively.
The largest milling units can process higher
crushing rates if heavy duty pressure feeder lInits are
used, IIp to well over 1000 IJh for 2.5 m wide units.
However once these very large sizes are considered,
two diffuser lines could be a better option. Only a
comprehensive economic analysis can show which
is preferred.
KI/Sla K.C (I '1'15): Some tlownstream effeelS resulting frum dif­
fusioll eomparetl wilh milling as publishctl by lhe Soulh Af­
riean Sugar Induslry. Proe. S. Afr. Sugar Teehllo!. Ass. 6'1,
201-204.
KWI/I/r R.N.; RI/O G. VSY (2000): Cane tliffusion - an energy ef­
tlcicnt juice extraction processo Conf. Sugar Proc. Res. Inst.
162-175.
IÁll1lll.l".Ie.I.P.: Fillgerald JR. (I '174): Diffusion in Soulh Afriea.
Proe. Inl. Soe. Sugar Cane Techno!. 15. 1486-14'18.
1-lll1lll.1".I1'.I.P. (1984): The choice belween tliffusion antl milling.
Rev. Agr. Suer. IIc Mauriee 63, 35-45.
UOlll/l'I G.RE (1988): Kinelics and cquilibrium in cane pulp/water
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1-01'1' /)../.: lIeill I~IV (1'180): The pereolation hehaviour of a eane
tlifTuser. Proe. 11ll.Soe. Sugar Cane Teehno!. 17, 1'100-1924.
Mackrorv L.M.: CI/ZI/lel J.S.: Sl1lilh 1.11.(1984): A comparison of
lhe microbiological activity associatcd with milling anu cane
diffusion. Proe. S. Afr. Sugar Teehnn!. Ass. 58, 86-89.
1'1/\"111'J.H. (I '168): Cane Diffusion - lhe displacemenl proeess in
principie antl praelice. Proc. Inl. Soe. Sugar Cane Teehllo!. 13,
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I'ril/.\·I'I/ Geerligs H.C (I '10'1): Cane Sugar antl ilS Manufaeture.
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Reil/ I~IV (1972): A Sludy of lhe Cane Sugar Diffusion Proeess.
PhD Thesis. Ulliv. of Nala!. 330 p.
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Reill l~IV (1975): A slatislieal analysis of lhe effect of eane qualily
on cxtraction pcrformance. Proc. S.Afr. Sugar Tcchnol. Ass.
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Rl'il/ P.IV; Illghl/l1l P..l.S. (1992): Diffuser performanec optimiza­
tion Ihrough eontrol of liquid Ilow pallerns. Prue. Inl. Soc.
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cncrgy cconomy in a sugar mil!. lnt. Soe. Sugar Cane Tcchnol.
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Reill P.IV (1995): A comparison of eane tlilTusion and milling.
Proe. S. Afr. Sugar Tcehno!. Ass. 69.196-200.
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mills. 1111.Sugar J. 101. 1204E. 192-196 antl232-234.
Ril'al/mlll.l.FR. (1984): Milling antl ditTusion in Mauritius. Rev.
Agr. Sucr. lIe Mauricc 63. 22-30.
Sil1~h I.; Alhvrixhr./. (2000): Press water heating in a direct contact
heater llsing sub-atmosphcric prcssure vapour. Proc. S. Arr.
Sugar Teehno!. Ass. 74. 280-284.
7llll1mri M.H. (1965): Egyplian sugar cane diffusion processo Proc.
In!. Soe. Sugar Cane Teehno!. 12, 14'16-1504.
1'1111 da Ril'l Cli.; Rmloll R.H. (1'171): The Empangeni tliffuser
inslallalion: 1967-1970. Proc. S. Afr. Sugar Teehno!. Ass. 45.
49-60.
wm Henp,e1 A. (1990): Diffusion as steam saver. Zuckcrindustric
115.7.551-554.
Maximum capacity 01' a single
cxtraction linc
6.7.9
R~ramces!,. /73