Cap8_Raw juice handling

Prévia do material em texto

8 RAW .JUICE HANDLING
189
8.1 Juice screening 8.1.1 Types 01' screen
The juice from the cxtraction plant contains a
signiticant quantity 01' small picccs 01' bagas se. This
can cause problcms in subsequent processing, lead­
ing to blockages in equipment like heaters. A large
proportion 01' this can be easily removed by screen­
ing the juice. The bagas se separated is commonly
referred to as cush cush, and is returned to the ex­
traction plant.
The juice from a milling train is often referred
to as mixed juice, being a mixture 01' primary juice
from the tirst mill and secondary juice from the rest
of the mills. These two streams are still sometimes
screencd separately. The juice from a diffuser is 01'­
ten called draft juice, but raw juice is a better term
to represent both cases.
The juice coming from a milling tandem can
contain over 3 kg insoluble solids/lOO kg raw juice,
bUI is generally aboul half that value. The screen­
ing process will normally reduce this to about 0.5
kg solids, most 01' which will be small particles
which do not cause further trouble and are removed
in claritication. Raw juice from diffusers has much
less bagasse in it, due to the tiltering action of lhe
diffuser. 11is common practice in a number 01' dif­
fuser mills to dispense with screens altogether. This
is a common practice in Soulh Africa where sus­
pendcd solids are less than 0.1 kg/IOO kg raw dif­
fuscr juice (sce Section 6.7.4).
Cush cush screen. Before the advent of choke­
less pumps on milling tandems, it was common to
have a single cush cush screen, sometimes referred
to as a cush cush conveyor, consisting of a long drag
slat carrier on top of a horizontal perforated screen
deck running alongside the mills. The juice from
each mil I passed through the screen, and the drag
carrier conveyed ali the bagasse strained out to a
cush cush return system. It is now general practice
to use chokeless pumps and this arrangement is not
often seen in practice. Details of this system and
also 01' vibrating screens, which also are no longe r
often used, can be found in Hugot (1986).
The remnants of the cush cush conveyor screen
system can still be seen in some mills that have in­
stalled chokeless pumps on the mills, where it has
been shortened and only strains out the bagasse
from primary and secondary mill juice on the drag
carrier scraped screen. The carrier also does the job
of elevating the cush cush to a return conveyor. The
chain operates in a very corrosive environment and
needs to be made of corrosion resistant steel; the
slats are generally wooden. Wedge wire screen is
the best material for the screen, because it is less
likely to clog, a common problem with these sys­
tems. Otherwise stainless steel screen is used, re­
placing copper and brass which was used in the
past.
ReJerellces fi. 200
190 8 Raw juice handling
DSM sereens. These are sometimes referred to
as "ski jump" screens because 01' their shape. Juice
is carefully fed from a weir to the top surface 01'
the concave screen and the juice passes through
the curved static screen while the bagasse particles
are retained on the screen and slide down to be dis­
charged at the bottom by gravity. The screens come
in sections between 0.6 and 1.6 m wide and the arc
01' the screen on the frame generally encloses an
angle 01' 45°. A sketeh is shown in Figure 8.1. The
screen itself is made 01' hard stainless steel wedge
wires arranged at right angles to the juice ftow.
The wedge wire design results in an opening in the
screen which is diverging so that it does not block
with pieces of bagasse. This concept is successful
in giving a non-clogging screen. Different profiles
01' wedge wires are shown in Figure 8.2.
The gap size between wires may be between 0.5
and 1.6 mm. When new, the leading edge 01' each
wire presents a sharp edge to the ftowing stream,
but after prolonged operation the edge wears. It is
possible then to remove the screens and replace
them aI'ter turning them through 180° so that a new
edge is leading. The worn edge in the trailing posi­
tion is sharpened again in service.
The capacity 01' the screens is affected by the
size 01' the opening between screens. Brolherto/l
and Nob/e (1982) established maximum juice load­
ings; relating liquid ftow rates to screen arca, these
ranged from 34 t/(h . m") with the widest opening
01' 1.6 mm down to 21 t/(h . m") with an opening of
0.65 mm. Ma/lso/l and Ames (1982) presented simi­
lar maximum tlow rates, varying from 42 t/(h· m2)
with the widest opening 01' 1.6 mm down to 24 t per
(h· m") with an opening 01'0.35 mm. [n South Af­
rica, equivalent capacity based on installed equip­
ment on milling tandems is much lower at about
12 t/(h . m2). with aperture sizes 01'0.5 and 0.75 mm.
Figure 8.3 shows a collection ofthe data reported by
Brolherto/l and Noble (1982) and Ma/lso/l and Ames
(1982). [nstalled capacity is likely to incorporate
juice loadings below the maximum values shown
here, to allow for tlow surges and periodic cleaning
01'the screens. Brolherlo/l et a!. (1981) recommend
that the aperture size should not be more than 0.8
mm. particularly when the cane is well prepared. A
designjuice tlow 01' 15 t/(h· m") with aperture sizes
01'0.5 mm to 0.75 mm seems reasonable.
Chen (1993) quotes a "conservative" design fig­
ure 01' 89 t/h 01'juice per m 01'screen width. Since
most screen sections are about 1.6 m long. this cor­
responds to 55 t/(h . m") - a rather high figure for an
unspeci fied screen aperture.
The amount 01' bagacillo in the screened juice
depends on the size 01' the aperture in relation to the
size 01' particles in the feed. Somewhere between
75 to 90 % 01' the bagacillo is removed on screen­
ing, although Engel (1966) reported lower remov-
!Feed
. "1:;'.
Proflle bars .f~;
-/,"i:,
Undersize / /j,: Oversize
Raw
juice
feed
1.4mm
3+'\1_t_ 90
T
_1-
2.2 mm
~- --I
1.8mm
'FU_1- 7
Figure 8.1: DSM 4SO wedge wirejuice screen Figure 8.2: Wedgc wirc profiles
8.1.1 Types of screen
80
E 70t •
..<:: 60
$' 50.S
.~ 40u
hl .
u ••
~ 20. ~. )
.x 10 •
O 3
O 0.5 1 1.5 2
Screen aperture in mm
191
Fi~urc 8.3: Maximlll11 DSM screen liqllid loadings as a
fllnclion of screen apertllre. showing data from firo/her­
/(lI! and Noble (1982) and MlIIlSOI! and Ames (1982)
ais around 40 %. Il seems that nearly ali the sand
and soil goes through the screen with the juice. so
the percentage removal of total suspended solids is
somewhat lower. The juice is expected to have about
0.2 kg bagacillo/lOO kg juice. but the suspended
solids content will be higher. closer to 0.5 kg/lOO
kg juice on average.
The way in which the feed is applied over the
screen encourages the fiber to be retained. and the
bagacillo which passes through the screen with the
juice is finer than may be expected from the aper­
ture size. Ma/lso/l and Ames (1982) showed that the
cutoff particle sil.e in the screened juice is hal f the
aperture si I.e.
The screens need little attention in operation
and are cheap and simple devices. The cush cush
usually has some time to drain on the screen before
dropping off the bottom of the screen and so is dry­
er than that from a cush cush screen. The overflow
weirs at the top of the screens must be carefully lev­
eled to get an even loading ofjuice across the width
01' the screen. Routine checks should be made of the
juice feed arrangement to ensure that the liquid is
fed uniformly across and smoothly onto the screen.
If noto carryover ofjuice with the cush cush back to
the extraction plant occurs. with a serious effect on
the efficiency of extraction.
Corrosion is rapid unless corrosion resistant
materiais are used for the supporting frame ar ad­
equate surface protection is employed.
Fi~urc 8.4: Conlra-Shear rotating juice screen
1 Screen diverters; 2 Solids discharge; 3 Idler trunnions
(driven on the other side), 4 0.5 mm apertllre wedge wire;
5 Feed trollgh; 6 Juice entry; 7 Feed weir; 8 Screenedjuice
discharge
Rotating scrcens. These inclinedrotating cy­
lindrical screens are becoming more common in
new installations. They differ from ali the old de­
signs described by Tromp (1936) in that wedge wire
screen is used and the juice is introduced inside the
rotating screen. with the separated bagasse falling
out of the end of the sloping screen. A sketch of a
typical arrangement made popular by Contra-Shear
is shown in Figure 8.4. Juice is introduced from
a head box. falling almost tangentially onto the
screen. but in a direction opposite to the direction
01' rotation 01' the screen. This arrangement seems to
result in minimal blocking of the screen and gives
high throughputs and a high screening efficiency.
Bro/herto/l et aI. (1981) found that the perfor­
mance is improved if the juice is added in the same
direction as the rotation of the screen. A design
figure of 40 t/(h· m2) is recommended. signifi­
cantly higher than that achieved on DSM screens.
This loading was confirmed by Bickle and Webster
(1982). with feed addition in a direction opposite to
the direction of rotation. Screens were reported to
be free of blockage with fiber. Suppliers data tends
10 be more conservative. recommending values
closer to 20 t/(h . m2).
These screen systems typically have a diam­
eter of 1.6 m and a length up to 4.5 m and are sup­
plied with a 0.5 mm screen aperture. They are large
enough for a single screen to be used for high juice
rates. with lhe largest units having a dia meter of
Referel/ces p. 200
192 8 Raw juice handling 1
Linear belt filter. An installation was reported
by Gierke (1989) which worked reasonably well. but
the coarse monofi lament cloth needed to be replaced
every few weeks because of gradual blocking due
presumably to slime build up. 11was scrapped when
a diffuser replaced the milling tandem. for which no
juice screening was required.
3 m and a length of over 5 m. They are easier to
clean since a single set of steam nozzles can clean
the whole drum. but they will require more main­
tenance than DSM screens. The drum rotates at a
speed between 5 and 15 min-' and a 7.5 kW motor is
commonly used on the drive. The same comments
with regard to corrosion applied to the DSM screens
apply here as well.
Usually the cush cush after screening is still
very moist. with a moisture content greater than 90
gflOO g: however Bickle and Webstcr (1982) report
figures of about 80 g moisture /100 g screen returns
from a rotary screen. [n any event the returns to the
mill are still very wet. so if a gravity return is not
possible. a screw conveyor is generally used to con­
vey the cush cush back to the mills.
11is common for the cush cush from the primary
juice and the secondary juice to be combined and
returned to the intercarrier between the tirst and
second mills. [n some cases it is returned to the cane
before the tirst mill. but this is less common. From
an extraction efticiency point of view. the material
should be returned to the cane or bagasse stream
that is closest to the cush cush in terms of the juice
Brix. in which case returning cush cush after the
first mill is probably the better option.
The optimum situation involves screening pri­
mary and secondary juice separately. returning the
primary juice cush cush to the cane before the first
mill and the secondary juice cush cush between the
tirst and second mills.
When this change was made at Darnall in South
Africa. an increase in extraction 01' 0.3 % ensued
as a result. This was at a mill achieving 97 % ex­
traction: the beneli! is likely to be greater when the
extraction levei is lower.
Diffuser juice is often not screened at alI. When
screening is done. the screens are normally located
on top of the diffuser so that the cush cush can drop
straight down into the diffuser close to the feed
end.
Sercen c1eaning
Serccning c1arified juice
8.1.3
One of the problems with juice screening is the
fact that juice screens provide an ideal environment
for fermentation 01' sugar. The microorganisms at­
tach themselves to the screens 01' supporting steel­
work and readily consume sugar. since the tem­
perature 01'the juice is often very suitable for their
growth. The evidence is normally seen in the form
01' si ime bui Id up. a product of Lellcu//ustuc //Iese//­
teroides. The losses caused by other organisms are
not as obvious butjust as serious. since the by-prod­
ucts 01' their activity lead to impurities that increase
the loss in molasses. due both to an increase in the
quantity 01' moiasses and to higher viscosities.
The design 01' the screening system should have
free draining areas with clean steelwork and mini­
mal nooks and crannies to provide the organisms
purchase. The areas under cush cush screens with
poor access are particularly bad areas. Regular
cleaning 01' screens and surrounding steelwork is
necessary. Steam cleaning is the most effective but
labor consuming. Biocides when properly used can
also be effective.
Rotary screens can be designed to be most hy­
gienic. Steam sprays mounted close to the drum are
effective in cleaning the drum and require no labor
when operated on a timer.
8.1.4
Screening 01' clarified juice is sometimes prac­
ticed. particularly in cases where direct consump­
tion sugar is produced. to catch any carryover of
fine bagacillo. Alternatively it is sometimes recom­
mended when problems are experienced with the
blocking 01' backing screens in centrifugals with
tine bagacillo. E//gel (1966) proposed tha! screen­
ing of claritied juice could be undertaken on DSM
screens. using a spacing between wedge wires 01'
0.35 mm. f)uyle and AIf{IrII (1998) tried a vibrat­
ing screen and achieved a screening rate of up to
45 m.1f(h . m~) with a screen aperture in the range
01' 0.3 to 0.5 mm.
Cush eush return8.1.2
8.2.1 Batch scales 193
Screening clear juice sometimes leads to vapor
generation, heat loss and spillage, unless well-de­
signed. Some in-line strainers are available which
are more compact and totally contained. I-ligher
rates per unit screen arca can be achieved with the
larger pressure difference available in pumped flow,
but generally a periodic purge of bagacillo back to
raw juice is required.
Raw juice
from extraction
plant
Quick acting
butterfly
valve
Load cell
Supply tank
Weigh tank
Overflow to
extraction plant
Load cell
1_
8.2 Juice mass flow measure­
ment
Filtrate
return
Raw juice
receiving tank
Quick acting
butterfly
valve
A batch scale system consists of a supply tank
abovc a weigh hopper, which accepts batches of
juice and records the mass before discharging.
The weigh hopper is tared before accepting a new
batch. Older systems incorporated mechanical/hy­
draulic mechanisms for weighing and controlling
juice flows. Typical of this type are the Maxwell­
Boulogne and Servo-Balans scales, which were
the standard for many years. Servo-Balans scales
incorporated test weights in the installation, so that
the weighing system could be checked routinely and
accurately.
Subsequently load cell systems have replaced
them. The weigh hopper is supported on three or
four load cells to record the mass. The electronic
For the purposes of factory performance con­
trol, it is necessary to measure the quantity of su­
crose entering the system, thus enabling an estimate
of recovery and losses to be made. Si nce even losses
of a fraction of a per cent have vast financial con­
sequences for a sugar mill, this measurement also
needs to be very accurate. Thus it is common prac­
tice to employ batch scales to measure the mass of
raw juice produced, because most other metering
devices do not approach tbe same accuracy. In in­
dustries like that in South Africa, the mass of su­
crose in raw juice is used to calculate thc mass of
sucrose in cane for the cane payment system, and
batch scales have to be routinely assized. Whatever
system is used, it should be frequently calibrated
and checked to maintain accurate figures.
8.2.1 Batch scales
I~
to pump
Figure 8.5: Schematic of juice batch weighing system
systemsare far more reliable and accurate than the
mechanical scales and require less maintenance and
attention. Quick acting tight shutoff butterfly valves
have been adopted, which has speeded up the op­
eration of the scales. The weigh hopper may have a
capacity between 2 t and 7 t and the supply tank is
generally about the same size. A sketch of a typical
system is shown in Figure 8.5.
The scales will generally read to about 0.05 %
and the accuracy in operation is within 0.1 %. This is
quite suitable for factory control purposes. Routine
checks with the test weights should be carried out,
preferably once per week. The scale tanks should
also be thoroughly cleaned on mill shut downs.
There are a number of practical issues to be
considered in ensuring ongoing acceptable accu­
racy from batch scales:
The supply tank must not be allowed to overflow
and bypass the weigh hopper into the receiving
tank. The overflow should be routed back to the
tank from which it was supplied and if neces­
sary a high levei alarm should trip the supply
pumps.
The juice should be admitted to the weigh hop­
per somewhat gradually to prevent toa much of
a shock to the load cells. This is important only
when large weigh hoppers are used and is eas­
ily achieved by adjusting the speed at which the
valve admitting juice operates.
ReferellceII'. 200
194 8 Raw juice handling
8.3 Juice sampling and analysis
Just as important as the measurement of the
mass ofjuice entering the process is the analysis of
the juice. Equal importance must be given to ensur­
ing that the sample of juice taken is a representative
sample anel that the analysis is accurate.
The outlet pipe from either the supply tank 01'
the weigh hopper should not protrude into the
liquid in the tank below. This imroduces a
weighing error.
The butter!1y valves should be visible, so that
they can be readily checked for leaks.
The structure supporting the weigh hopper
should be rigid enough to ensure accurate mass
determinations and preferably be free from vi­
bration.
Magnetic flow meters are sometimes used in an
effort to measure juice !1ow. However they cannot
be expected to give an accurate enough measure­
menl. The installed accuracy is generally about I
% and since it measures volume and not mass ftow.
it is affected by juice density and by entrained air.
Even incorporating an online density meter imo the
system does not lead to acceptable accuracy.
The only ftow meter which has the required char­
acteristics is a Coriolis meter. 11can measure ftows
with an installed accuracy of 0.15 % and measures
mass not volume flow. Most meters can also give
a simultaneous output of density anel temperature.
This type of meter can handle suspended solids in
the liquid. is insensitive to variations in temperature
anel pressure. has a high turndown and is virtually
maintenance free. However the cost of these meters
rises exponentially as the size increases. The 80 mm
meters cost of the oreler of $10000, and more than
one meter would be required fÓr 110wsabove about
150 t/h. Larger size meters are becoming available
but at a cosI. Nevertheless installation is cheap anel
easy anel no lengths of straight piping are requireel
before 01' after the meter. 11is expected that prices
will reduce in future as further improvements are
maele to these meters.
Sampling systems
Raw juice
~
: .- - - - '------'
... --------
i Batch scale
Sample i discharge
mllK'" s.~ pipc
tube
8.3.1
11 is common practice to collect a composite
raw juice sample over the period of an hour. If batch
scales are inuse, a representative sample can be ob­
tained by taking a small sample from each tip of
the scale. A sample tube projecting into the outlel
stream of the weigh hopper is adequate. The sample
tube neeels to be carefully designed to ensure that
it does not clog with bagacillo. A sketch of a com­
monly used system is shown in Figure 8.6. 11con­
sists of a 19 mm dia meter pipe with 2.5 mm holes
elrilled in it. located in the outlet from the batch
sc~tles. The perforated end must cover the full width
of the !1ow and lhe angle of the holes is aeljusted
so that the sample container is about three-quarters
full at the end of the sampling period. The container
shoulel be titted with a lid. and ifthe raw juice is hot.
both the sampling tube and the container should be
water cooleel. The sample tube should be replaceel
periodically (typically once/shift) with another
tube that has been cleaned and drieel.
It is important that a representative sample is
obtained, proportional to the juice !1ow. Thus tak­
ing a small amount from the discharge of each scale
tip achieves this. The holes in the sampler must not
be allowed to block or clog.
If batch scales are not in use, representative
sampling is somewhat more difticult. Various meth­
ods of sampling juice automatically are describeel
by Chell (1993).
Sampling raw juice can be hazardous anel can
easily give unreliable results if certain basic pre­
cautions are not taken. The most serious problem
is the potential for the sample to deteriorate. be­
cause conelitions of temperature and concentration
are ideal for fermentation. This requires that ali the
Fi!(urc 8.6: Raw juicc sampler installcd at olltlet ofjuicc
sc~tlcs (SASTA 2(05)
Other metering systems8.2.2
8.3.2 Suspended solids sampling 195
sampling lubes and conlainers be regularly cleaned,
preferably by sleaming, and that some preservalive
is added imo the sample conlainer before collecting
the sample. The second problem is the potential for
evaporalion. Thus the sample container should have
a secure lid and lhe sample should be kept cool.
Mereurie ehloride or lead subacelale was com­
monly used as a preservative. These are generally
no longer acceplahle and a carhamale approved hy
the US FDA is now lhe hesl option. SASTA (2005)
recommend lhe use of 0.2 mL in one liter of sam­
pie. Mat/sen and Day (2005) recommend lhe use
of a mixed dithiocarbamate at levels greater than 5
mg/kg ju ice.
Those industries which rely on sampling firsl
expressed juice for cane payment purposes gener­
ally arrange for juice samples to be lransferred au­
tomatically to the lahoratory for analysis. Again a
numher 01' precaulions need lo he laken lo ensure a
representative sample is received. The sample needs
to be laken across the rull width of lhe mil I.
lhis needs 10 be borne in mind in using the numbers
generaled in calculations. The double polarization
method can be used to get around the major effect
of monosaccharides, but does not account for the
other polarizing material (Section 25.1.1).
As noted in Section 2.4.4, HPLC or GC can be
used to get an accurate estimate of sucrose in juice.
The ralio of pol/sucrose has been found to vary be­
tween 0.97 and 0.99 in South African raw juice but
is somewhat lower in Louisiana in the range 0.95 to
0.97. What this means is lhat using pol underesti­
mates lhe amounl of sucrose entering lhe factory by
between 2 and 4 g/lOO g sucrose. The magnitude of
the error is such that accurate estimates of undeter­
mined loss can never be achieved without measur­
ing true sucrose.
The Brix of juice measured by a refractometer
has been shown to give an accurate estimate of dry
substance in the raw juice, so that no correction for
dissolved subslance is necessary.
8.3.2 Suspended solids sampling
8.4 Juice pumping
Ali routine mill laboratories use polarization,
commonly called pol, to estimale the sucrose C(Jt1­
tent of juice. Although this is an excellenlmeasure
for pure sucrose solutions, it becomes progressively
less reliable as lhe purity reduces. This is due 10 the
presence 01' olher carbohydrates, particularly mono­
saccharides, which rol ale plane polarized light and
The analysis of juice samples is carried out on
filtered liljuid and the presence 01' solid suspended
matter in lhe juice stream needs 10 he laken inlo
accounl. Since the suspended matter can conslitule
hetween 0.1 and I g/IOO g juice, ilS effect can he
very significanl. The mass 01'suspended solids has
to be sublracled from the lolal mass recorded.
Sampling ofjuice to get a representative sample
that is not biased in terms of particle size is difficull.
The swing sampler used in South African faclories
has given adeljuate service (SASTA 2005). A cul
of lhe tola I juice slream is taken by swinging the
sampler ljuickly through the juice slream. Inlhis in­
stance a catch sample once per hour is adequale.
Liquid line sizing. Theoretically the problem
of line sizin'g should be based upon economic con­
siderations. However, in the average sugar mill, op­
timum economic line size is seldom realized owing
to unknown factors such as future flow rate allow-
Raw juice contains some sand and fibrous ba­
gasse particles and is abrasive and corrosive. The
pump discharge pressure is fairly high, because
the juice is pumped lhrough heaters and to a flash
tank at an elevated height and so pump wear can be
problematic. In some cases an intermediate pump is
used between primary and secondary heating, par­
ticularly if intermediate liming is practiced. Care­
fui design of the system is necessary to minimize
costs.
To specify the pump duty, it is necessary to know
the average and maximum flow rates to be pumped,
including filtrate relurn, as well as the head on the
pump under both conditions. This requires an eSli­
mate of the pressure drop in the pipes and the heat­
ers, as well as the difference in elevation between
the suction and discharge liquid elevations.
Pump duties8.4.1
Pol vs. sucrose analysis8.3.3
Neferel/ces p. 200
196
10.0
~
E
'=
ç
g 1.0
ai>"s
o-
'.:J
0.1
1
Pipe diameter in mm
10
S Raw juice handling
100
Flow rate in m3/h
1000
00
500
10000
Figure S.7: Discharge and suction piping diameler sizes as a fUllclioll of liquid t10w rale
(S.I)
ances or the actual pressure drop characteristics of
certain process equipment (e.g. heat exchangers.
control valves).
Where no special process considerations apply.
the chart shown in Figure S.7 may be used for the
tentative sizing of liquid process lines. The areas for
suction and discharge lines on this graph are based
upon the following economic pressure drop ranges
for water at 20 DC:
Suction piping-
3.5 to S kPa per 100 equivalent meters of pipe:
Discharge piping -
15 to 60 kPa per 100 equivalent meters of pipe.
This normally results in a liquid velocity of I to 3
m/s in the discharge line. Suction line sizes are gen­
erally larger than discharge sizes by about one pipe
size. to assist with NPSH. This leads to velocities in
the suction piping of about I m/s. Valve throttling to
adjust Ilow is always done on the discharge side.
FrictioJ1 pressnrc drop iJ1 pipcs. The general
equation for friction pressure drop is given by Dar­
ey's equation and can bc represcnted by:
f·'·//H=--
2·g·d
whcrc:
H friction loss cxpressed in meters of liquid in m.
, equivalent length of piping in m.
li liquid mean velocity in m/s.
g gravitational accelcration constant (9.S1 m/s2).
d internal pipe diametcr in m.
f dimensionless friction factor.
The Darey cquation is valid for laminar or turbu­
lent flow 01' any liquid in a pipe. The most useful
and widely accepted data on friction factors for use
with the Darey formula are those attributed to L.F.
Moody (1944). shown in Figure 9.5 in Section 9.2.5.
This equation may also be used for the calculation
8.4.1 Pump uuties 197
Pump curve
Figure lU!: Effcct of control valve on pump system operat­
ing poinl
of entrance/exit losses anu pressure urop through
valves anu fittings by converting these into
equivalem meters of pipe length or equivalem
number of velocity heaus. The preferreu system
for sugar mills is the two-K methou of Hoofler
(1981). since it is easy to use anu more accurate
at low flows. particularly with more viscous nu­
ius obtained when the liquid Brix is high.
Note that the FWlIlillg friction factor useu in
some texts is one quarter times the Moody fric­
lion factor. Priction pressure urop through ancil­
lary equipment such as heat exchangers is best
obtaineu by actual fielu measurement if it is an
existing unit. or by specifying apressure urop
consistem with the process requirements in a
new installation.
E
.S
--o
'"
wI
---'- f
Static head
-.-L
System curve
Vm
Volumetric flow in m3/s
(8.2)
•••
I'ump selection. The total pump heau requireu
is maue up of the fÓllowing:
The difference between liquiu levels on suction
and discharge siues.
Any uifTerence between vacuum anu/or pressure
on the inlet anu outlel siue liquiu free surfaces.
Priction. i.e. the heau requireu to overcome re­
sistance to ftow through pipes anu fittings.
Pressure urop across the control valve.
J·leau loss through equipment such as healers in
the syslem.
The velocity heau at final uischarge.
As a general guiue safety factors of 20-30 % of the
friction factor will aecolllmouate the change in sur­
face roughness for steel pipe with average service of
5-10 years: beyonu this periou the situation usually
relllains static and will not ueteriorate furlher. This
still uoes not account for increaseu pressure urop
due to increased f10w rates or for reuuceu efflcieney
as a result of pUlllp wear. In most juice pumping ap­
plications. the maximum flow rate is about 25 %
higher than lhe average.
With centrifugal pumps there is a strong rela­
tion between capacity anu heau. which facilitates
steauy pIam control. There are. however. certain
limiting factors in connection with the selection of
centrifugal pumps. Pump curves anu characterislics
neeu to be taken into account. A typical example is
shown in Figure 8.8. The system curve. giving the
pressure urop as a function of flow. is plotteu on the
pump characteristic curve graph. The heau at zero
f10w is the static heau anu the heau is a function
01' lhe flow rate squareu. At the mean flow ~" the
systelll heau is less than that shown by the pump
characteristic. This head difference is absorbed by
the control valve regulating the f1ow.The maximum
ftow obtains with the valve fully open where the
system curve intersects the pump curve. This is the
maximum power poinl and the motor musl be sizeu
to ueliver lhe power requireu at this point.
In raw juice pumping applications. the liquiu
contains abrasive soliu particles. anu has a lower
than neutral pH. This requires the use of harueneu
materiais of construclion. If lhe quanlity of sus­
penueu solius is high. an open impeller is used 10
proviue a "chokeless" pump. Some pump efficiency
is sacrificed in Ihis uesign.
If the discharge head is greater than abOUI 100
m of water. a multistage pump is requireu insteau of
a single stage pump. This is nol usually the case and
a single slage pump is usual in Ihis dUly. A lower
speed may also be specified. since wear due to ero­
sion is proporlionallo speeu to the fourth power.
Powcr rcquircmcnts. Power input to the pump
is given by the relationship:
\i·p·g·Hp=---
1000'11
where:
P power inpul in kW.
V pump capacity in m3/s.
H pump heau in m.
p liquid density at pumping lemperature in kg/m3•
11 hydraulic pump efficiency expressed as a frac-
lion.
Rl1erences p. 200
198 8 Raw juice handling
1.0
This simplifies 10:
NPSH requiremcnts. The pressure ai lhe ínlel
10 lhe pump musl be high enough 10 preveni cavila­
líon. This occurs when part of lhe liquid vaporizes
because lhe pressure has dropped below lhe vapor
pressure of lhe liquid. Nel Posilive Suclion Head
(NPSH) is lhe pressure ai lhe pump suclion above
lhe vapor pressure of lhe liquid, expressed as a head
of liquid. The NPSH available in lhe suclion sys­
tem musl exceed lhe NPSH required by lhe pump.
whích is a funclion of lhe design of lhe pump and
will be specified by lhe pump manufaclurer. This is
nol normally an issue in pumping raw juice because
the temperature is relatively low. However il is ex­
tremely important in pumping clarified juice, which
is close to its boiling point.
MateriaIs of construction
The available NPSH is calculaled as the pres­
surein the feed lank (generally atmospheric), plus
the difference in head belween the minimum liquid
levei in the feed lank and lhe centerline of the pump
(negalive if lhe levei is below lhe pump cenlerline).
less lhe pressure drop in the suclion piping to lhe
pump, less the vapor pressure of the liquid at that
temperature.
Some guidelines regarding NPSH and system
design are:
Actual NPSH requirements rÓr a pump should
be furnished by the manufaclurer by shop tesl
wherever possible.
The NPSH available musl exceed NPSH re­
quired for lhe entire pump capacily range.
When system NPSH is limited, pump NPSH
requiremenls can be reduced by specifying a
lower shaft speed. a double suclion impellcr or
a differenl Iype of pump: a combinalion ofthese
measures may be required in extreme cases.
The suclion pipe must be large enollgh, gener­
ally one size larger than lhe pllmp nozzle, wilh
a velocilY less Ihan 1.5 m/s.
Long sllclion lines should be avoided, wilh a
minimllm of bends and fittings.
8.4.2
Raw juice generally has a pH belween 5 and
5.5. but may be closer to 6 for raw juice from a dif­
fuser in which liming is pracliced. [n either case.
the juice is corrosive. and wilh the effect of sand
in juice. the nel effecI of corrosion combined wilh
abrasion is rapid deteríoration of mi[d steel. Leaks
ofjuice around raw juice tanks are not uncommon.
11is generally worlhwhile to use materiais resislanl
to these condilions. The most cost-effective mate­
rial for lanks and pipes is 3CR 12. a high chrome
steel wilh properties dose to slainless sleel.
PlImps and valves on raw juice dllly are also
generally chosen to incorporale abrasion resistanl
materiais.
(8.3)
........
....
0.2 , ....
0.8
V·H·pp=--
102'11
Higher efficiencies are possible wilh higher shafl
speeds, albeil ai lhe expense of NPSH requíre­
menls. The efficiency of cenlri fugal pumps depends
on Iheir size and lhe values given in Figure 8.9 can
be used 10 eSlimate power requiremenls for prelimi­
nary design purposes. Efficiency data is supplied by
the manufaclurers, bUI values are Iypically 0.5 for
small pumps and 0.75 for large pumps. The efficien­
cy of reciprocaling pumps is usually aboul 0.9.
The designer should always ensure Ihat lhe pro­
posed drive power is sufficienl 10 meel lhe pumping
power requirements over lhe enlire pumping range
under consideralion. This is covered by lhe range of
flow rales and heads given in Figure 8.8.
>­uc
.'!' 0.6
~
<li
a. 0.4
E
:o
"--
Figure 8.9: Efficiencies 01' centrifugal pumps
11is desirable 10 keep the holdllp of raw jllice to
a minimllm. because il provides the opportunily for
microbiologicalloss of sugar to occur. However. in
o
10° 10' 102
Capacity in m3/h
103
8.4.3 Raw juice tank sizing
8.4.4 Juice flow control 199
Generally the flow needs to bc controlled at
a steady rate, so lhal sleady temperature and pI-!
control can be achieved, and to get the besl perfor­
mance from the clarifiers. The first requiremcnt is
adequate tank capacity so that lhe short-term Iluc­
tualions can be smoothed ou!.
order to achieve steady enough tlow to the clarifiers,
a minimum volume 01'surge capacity in the form 01'
a raw juice tank is required. The tank also serves
to smooth out ftow surges that occur with lips from
batch scales, if installed, into the system.
The magnitude 01' the raw juice Ilow in m3/h
is in most cases similar in magnitude to the cane
erushing rate in le/h. This is lhe case for instance
with cane 01' 12 % fiber content and 25 % imbibition
on cane, or with 15 % fiber in cane and an imbibi­
tion levei 01'30 % on cane. Total juice ftow includes
in addition lhe fillrale relurn 01'about 15 t per 100
t raw juice. In order 10 get an idea 01' the size 01'
surge tank required, consider lhal the tank has 10
cope with a juice ftow surge 01'30 % over 10 min­
utes. This will require surge capacity 01'5 minules
at average ftow rate. Assuming a tank half full, a to­
lal juice retention time 01' 10 minutes seems reason­
able. This represents roughly 20 m3 tank capacity
for a mill crushing 100 te/h.
The raw juice tank should have a sloping bot­
tom, so that ali sol id malerial can be pumped out
with lhe juice. 11is good practice to run lhe tank
empty periodically, to remove alI solid material,
which olherwise constitutes a haven róI' microor­
ganisms lhal degrade sucrose. For the same reason,
il is good practice to use the scale discharge and
filtrale return to stir up solids that Iloat on the 10p
surface and get them enlrained inlo the juice being
pumped ou!. Each pump should have its own suc­
tion line, preferably sloping to lhe pump, with the
suction isolating valve Ilush with the tank outle!.
Butterf1y valves ralheI' than gate valves should be
used, so that sealing occurs even in the presence 01'
sand.
Where holdup ofjuice for degradation 01'slarch
by nalural enzymes is provided in thejuice Ilow sys­
tem, the additional surge capacily available means
that lhe surge capacity in the raw juice tank can be
reduced correspondingly.
8.4.4 Juice flow control
The control system shou Id aim to keep the ftow
as steady as possible, allowing the leveI to ftuctuate
without the tank running cmply 01'overftowing. The
simplest arrangement is to run the controller with a
widc proportional bando A better solution is a gap
action control, with lhe flow kept constanl belween
an upper and lower levei (a dead band), with adjusl­
menl to the flow set point occurring only oUlside
these limits. The steadiest flow involves an element
01' predictive control, taking into account the cane
crushing rate and lhe levels in downstream tanks.
The implementation 01' a predictive control systcm
is described by Me/mole (2003).
Flow control is achicved through the use 01' a
variable speed pump or a control valve on the dis­
charge line from the pump. Sizing 01' the control
valve can be done through the use 01'manufacturer's
equations.
Butterfty valves are often lhe valve 01'choice in
large juice lines because 01'their low cosI and high
throughput capacity. Because 01' sand in the juice,
a stainless sleel disc is necessary to protect against
abrasive wear.
11 was believed in the past that the required
pressure drop across the control valve under design
conditions should be at leasl 50 % 01' the dynamic
head 01' 25 to 30 % 01' the total pressure drop in
the whole system in order to achieve good control.
BishofJ et aI. (2002) suggest that the pressure drop
across the valve should be 33 % ofthe dynamic losses
in the system 01' 103 kPa, whichever is greater. This is
often considered to be unnecessary and wasteful
01' energy. The recommended pressure drop across
the conlrol valve is the higher 01' 5 % 01' the total
system pressure (i.e. static head plus pump head at
maximum ftow), 01'35 kPa for rotary control valves,
or 70 kPa for globe valves (Balll11a/1/1 1998).
Variable speed AC motors are now becoming
viable options for flow control instead 01' using a
control valve. The main advantage is the facl lhal
the pUlllp on average runs at a lower speed and
pumps against a lower discharge pressure, so that
wear in the pump due to sand is Illinimized. This
also means thal considerably less power is used,
roughly proportional to the reduction in discharge
pressure. The saving is evident from Figure 8.8,
since the pump will operate along the system curve
instead ofthe pUlllp curve. Thus the saving in power
is represented by the difference in pump head and
systelll curve head at any flow rate.
l<eJerellce" 1'. 200
200 8 Raw'JUicehárÍdlírig,},
Referenees
Ral/mann HO. (1998): Contrai Valve Primer. 3rd Ed. Inslrumenl
Saciety aI' Ameriea, Research Triangle Park, NC. 27-28.
lIick/e R.E.; lVe/wer M. \V (1982): Ralary juice screening aI Pia­
neer mill. Prae. Ausl. Sac. Sugar Cane Technol. 4,249-253.
Ris/wl' T.; Chal'eal/x M.; laller L.; Nair K.; Pale/ S. (2002): Ease
conlral valve seleelian. Chem. Eng. Prag. 98, 11. 52-56.
IIrolherton C.A.; Noble A.C.; Swindel/s R.l. (1981):Juice screen­
ing. Prac. Ausl. Saco Sugar Cane Teehnal. 3, 117-124.
IIrotherton C.A.; Noble A.C. (1982): Perfarmance and capac­
ity af juice screening systems. Proc. Aust. Soe. Sugar Canc
Technol. 4, 243-248.
Chen l.C.p. (1993) in: Chen J.c.P.; Chol/ c.c. (Eds): Cane Sugar
Handbook. 12lb Ed. John Wiley, New Yark.
Oo)'/e C.D.; Auard R.C. (1998): Screening 01' faclory liquor us­
ing a vibratory unil. Prac. Ausl. Sac. Sugar Cane Technol.
20,470-476.
Enlie/ L. (1966): Screeningjuice wilh the DSM. Sugar y Azucar.
61. I. 35-37.
Cierke \VM.U. (1989): The applicalian 01' a linear helt filter for
clIsh-cush remova) from a mill mixed juice at Maidstone.
Prae. S. Afr. Sugar Cane Technol. 63, 33-35.
flool'er \VB. (1981): The two-K method predicls head loss in pipe
fittings. Chem. Engng. 88, 17,96-IOD.
fll/gol E. (1986). lIandhoak ofCane Sugar Engineering. EIsevier,
Amsterdam. 3rd Ed. 353-358.
Madsen L.R.; Da)' O.F. (2D05): Mixed dilhiocarbamates for lhe pres­
ervalion 01'sugar cane juice. Inl. Sugar J. 107, 1282, 576-580.
MallI(}// P.C.; Ames R. V. (1982): Fibre remaval fram juice. Prae.
Ausl. Soco Sugar Cane Technol. 4, 255-259.
Me/roIe L.l. (20D3): Practical management ofjuice ftow and lev­
eis using prcdictivc lTlodcl control. Proc. S. Aff. Sugar Canc
Technal. 77, 423-451.
Mood)' LY (1944): Friclion faclOrs for pipe Ilow. Trans. Amer.
Soco Mech. Eng. 66, 671-678.
SASTA (2005): SASTA Lahoratory Manual. 4lh Ed. S. Afr. Sug­
ar Tech. Ass. CD-ROM
Troml' L.A. (1936): Machinery and Equipmenl oflhe Cane Sugar
Faclory. Norman Rodger, London. 644 p.
..
• No spare gaskets - No flow inverter required
• Clear free flow channels with generolls spacing for the l11ixcd, limcd and elcar juiecs
• Very easy and fast access for cleaning
• Compact and easy to insta 11
• Robust and maintenance free construction
• Multi-strcams juiec hcatcrs are available
For ali rclcvant information plcasc contact lIS.
g~~'[,'~J!,9~<~n~'~
11-&, rue C. I)cmhlon - -&6JO Sf)um~lJ:nc / nd~ium
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