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Nigeria Sugar Manufacturing By Milling Process

Pemas Engineers Ltd specializes in operation and maintenance of cane
sugar industry manufacturing by milling process machinery,mill plant and
sugar processing equipment that are used for sucrose extraction,clarification,
evaporation,sugar boiling,sugar drying and bagging,and ensuring  that these
machinery,plants and equipment are well balanced for better efficiency,and
are suited for efficient optimization of water,steam and power requirements.

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Basically, sugar manufacturing by milling process goes through the process
of sucrose extraction, clarification, evaporation, sugar boiling and sugar
drying and bagging using various machinery,plant and equipment and
chemicals along the production line. The boiler plant is required for steam
production for power plant, mill and knife turbines and steam exhaust for
process use. The sugar crystals produced can be in the form of raw sugar or
refined sugar depending on the extent to which colour in the grains formed
have been removed. In this regard, some factories stopped their sugar
process at what is known as “Brown Sugar” stage without going further to the
next stage referred to as “Refined White Sugar”.

Sugar is sucrose extracted from cane or from Beet. Cane and Beet are
different but sugar can be obtained from them using different technologies.
Sucrose from cane is extracted by milling or diffusion process while sucrose
from Beet is essentially extracted by diffusion process. Sugar production
from Beet is more expensive than sugar production from cane reason being
that while a cane sugar factory can work conveniently for about 5 – 6
months, a Beet factory can only work for about 3 months because Beet is
grown from seed sown in the spring. This means a cane factory could be
only half of a Beet factory in size. From 0.680kg of beet, it is possible to get
0.113kg of sugar. In this write up, we are concerned with cane sugar
manufacture starting from the raw material, which is cane. The basic
agronomic practices involve the following

a. Land Development/Preparation
This process is capital intensive since heavy agricultural machinery are
needed for various operations. Among these operations are tree felling,
clearing, filling and levelling, land surveying, construction of drainages and
canals.

b. Seed Planting
This is done manually or mechanically depending on the farm size,
geological nature of the soil and availability of equipment and labour.

c. Fertiliser Application
Various types of fertilisers are available for use depending on the type of
soil. These include NPK and Urea. Other source of manure like the filtered
mud could also be applied for improved yield.

d. Weeding
This is the process of eliminating competitive weeds e.g. grasses. This could
be done manually or mechanically depending on age of sugarcane.
Chemical weeding is also common in most cases.

e. Irrigation
This is the method by which water is applied at a predetermined cycle. This is
done either by surface or overhead systems. This is a very important aspect
of cane growing; hence, a permanent source of water must be available for
irrigation during the dry period of the year.

f. Harvesting
Canes are harvested at maturity. Maturity in cane is determined by the purity
at the harvesting period, which must be above 80%. Harvesting could be
done manually or mechanically depending on the nature of the cane and
furrow type. However, manual harvesting is common due to availability of
cheap labour during the dry season.

THE MANUFACTURING PROCESS

STAGE 1: Extraction Of Sugar
a. The weighbridge
b. The derricks
c. Conveying and preparation
d. The mills

The first operation is weighing the cane. The cane weighbridge has a
capacity of 30 tonnes.
The cane is brought to the factory in cane carts by tractors.
The unloading derricks system is capable of handling 120 TCH.
The feed tables, which are also suitable for 120 TCH, transport the cane
onto the 1st feeder carrier, which runs under a leveller knife set. The leveller
knife is driven by a 205KW steam turbine. The clearance between the carrier
and the knife is small to allow the cane to be cut to pieces.
The cane on the first feeder carrier is passed to the auxiliary carrier which
runs under a heavy-duty knife set driven by a 205KW turbine. The knives
further cut the cane into further smaller pieces.
The cane leaving the auxiliary carrier goes on to the main carrier and enters
the heavy-duty shredder for fine preparation before entering the mill.
In the mill house, the cut cane will pass successfully through four mills, each
incorporating three rollers. These rollers will be disposed in such a way that
there will be two squeezing of the cane per mills – the first between the top
and feed roller, and the second between the top and back roller. The top
rollers will be free to move vertically to a limited degree in order to provide for
any uneven thickness of the cane blanket, and to ensure that a steady
pressure is at all times maintained. This pressure will approach a total of 450
tonnes on each top roller, and will be imposed by means of hydraulic
pressure cylinders operating on the bearings.
In order to provide for the increased throughput of cane up to 4000 tonnes
per day, which will be required at a later period, provision is made in the
layout of the mill house for the installation when required of two additional
mills.
The mills are driven by steam turbines each developing up to 410KW. The
normal speed of the turbines will be 5700 rpm, which will be reduced to the
mill speed of 4.3 rpm through heavy multiple gearing. In the first instance, -
with the crushing requirement at 2000 tonnes cane per day – the mill was
driven in pairs through gearing by the turbines. This gearing was designed,
however, such that, when the throughput of cane is increased and more
power per mill becomes necessary, a re-arrangement of the gear wheels and
the addition of further turbines could be effected to provide for one turbine to
drive each mill.
The stopping of the turbines – either singly or all together – and individual
adjustment for speed is effected from a central control platform located in a
position from which one attendant will have a free access to the plant in
operation.
The juice squeezed out of the cane by the mills is passed through
mechanically operated strainers and is then pumped across to the process
house where, before being treated, it will be weighed automatically for
purposes of process control.

STAGE 2: Production of Brown Crystal Sugar.
This stage comprises the following successive operations:-
a. The clarification of the juice by the action of chemicals and heat, and the
subsequent precipitation of the impurities, which are thereby separated from
the liquor. The precipitate – normally called “mud” – is subsequently filtered
to enhance the recovery of juice, and hence of sugar.
b. The evaporation of the large quantity of water, which is present in the
juice, followed by, further concentrated to produce sugar crystals.
c. The separation of the crystals from their mother liquor by the application of
centrifugal force.
The first operation is the clarification of the juice expressed by the milling
plant, which has a dark opaque yellow/green colour due to occluded dirt and
other impurities, which in addition to sugar, are extracted from the cane
during milling. For clarification a small quantity of Sulphur dioxide gas is first
bubbled through the juice, after which a controlled amount of milk of lime is
added. The chemically treated juice is then pumped through heaters to bring
the temperature to around 215oF, after which it is passed to the clarifier. In
this the settlement of impurities takes place, and the resultant clear juice is
decanted off from above in to continuous rotary vacuum filters, which
separate the remaining juice, which is returned to process.
The second operation in the process house is to remove the greater part of
the water contained in the juice. This is done in a multiple effect evaporator,
which utilizes the exhaust steam from the various turbines as the heating
medium. The juice passes successively through four vessels and the vapour
provided by the evaporation from each vessel is transferred in turn to the
next. By this means the juice is concentrated to a thick syrup at a relatively
low boiling temperature in the fourth vessel. The vapour from this vessel
passes to a condenser, which maintains the vacuum. The clarified juice
enters the evaporator containing 85% of water by weight, and emerges as a
thick syrup containing about 35%.
Further operation in the process house is to continue to evaporate the water
from the syrup such that, when the liquor becomes saturated, crystals of
sugar will form and can be grown to the size required under carefully
controlled conditions of boiling. This operation is performed in vacuum pans,
which are large vessels, similar to those of the evaporator, but which operate
individually on an intermittent batch system.
This boiling operation produces a composite mass of sugar crystals
embedded in heavy molasses, which is termed massecuite. On discharge
from the vacuum pans the massecuite gravitators into crystallizers located
below, in which it is kept in motion by stirrers until in a condition for passing
to the centrifugals.
The third operation in the process house is the separation of the crystals
from the molasses, this being effected by spinning the massecuite in
centrifugals, which are in the form of an open perforated drum attached on a
vertical spindle driven by an electric motor. When the basket rotates the
sugar crystals are retained within the screen of the basket, and the molasses
passes through the fine perforations. The centrifugals operate at speeds of
1000 rpm and 1500 rpm, creating in the massecuite forces up to 1500 times
that of gravity.
At the conclusion of stage two a brown crystal sugar results. This is the “raw
sugar” of commerce, and contains 97% to 98% chemically pure sucrose. The
brown colour is due to an envelope of mother liquor with which each crystal is
covered, together with a proportion of impurities, which are occluded with the
crystal structure.

STAGE 3: Production of White Sugar.
The removal of the molasses and the other impurities from the brown sugar
produced in stage 2 will be effected by re-melting the sugar and re-
crystallising this operation being done in the white sugar section of the
process house. The brown crystal sugar is dissolved in water under
controlled conditions to produce a liquor of desired density at a particular
temperature. Thereafter this raw melt liquor is subjected to chemical
treatment by the metered addition of a solution of a phosphoric acid followed
by a controlled amount of milk of lime. The liquor is then aerated and fed to a
clarifier in which the precipitated impurities float to the surface to form a scum
and the clarified liquor is drawn off from below. Both the liquor and the scum
are filtered, after which the liquor is passed for boiling in the white sugar
vacuum pans, which are operated in the same manner as those used earlier
in the production of brown sugar.
The separation of the white sugar crystals from the massecuite is carried out
in the white sugar centrifugals. The product from these machines is
thereafter passed through a sugar drier to reduce the moisture content to
less than 0.05%. From the driers the sugar is conveyed out of the process
house in 50kg bags to the sugar store.

Steam Generation and distribution.
The crushed cane – called bagasse – from which the juice has been
extracted by the mills, will be conveyed to the boiler house and will be burned
directly as fuel in specially designed furnaces. The generation of steam for
the total power and heating requirements of the factory will normally be
derived from this fuel source.
Steam will be generated in the boiler at a pressure of 250 p.s.i.g, and will be
sued directly for driving the steam turbines operating the mills and electric
generators. The steam will be exhausted from these turbines at a pressure of
15 p.s.i.g, and will be distributed around the factory as the heating medium
for juice heaters, evaporators and vacuum pans.
A very useful factory by-product is molasses, which the company sells to
Nigerian Yeast and Manufacturing Company Plc as raw material for their
products. Molasses is usually 4% in cane.
In Nigeria, only Nigerian Sugar Company, Bacita and Savannah Sugar
Company, Numan produce refined white sugar. Lafiagi produces raw sugar
otherwise known as “Mill White Sugar” while Sunti Sugar Company is
expected to produce also “Mill white Sugar” when the factory is ready. In
terms of plant and equipment of production, the process of sucrose
extraction, clarification, evaporation, boiling, crystallisation and drying are the
same. The differences being in different manufacturers designs and
economics of operation.
The plant and equipment reviews and operational performances can be
related to the types of plant and equipment installed at respective sugar
companies comparing them to where similar plants and equipment are
installed elsewhere in the world and their operational performances.
The capacity of the mills (factory) in terms of tonnes cane crushed per hour
determines the capacities and input requirements of other plants and
equipment in the factory. It is therefore necessary from the onset to assess
the mill capacities as installed for the factories while from the operational
point of view the cane milling efficiency is equally assessed.
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