It is the intention of this article to provide a brief overview of the

extremely interesting, but highly complex process, surrounding the

production of sugar. Special attention is given to the main

separation stage of the process, which involves the use of centrifuges.

The author wishes to make it clear from the onset that this is a

very general overview. To quote a director of a previous employer:

“He who knows just enough about sugar manufacture to

appreciate the advisability of keeping quite and listening when

those who really know are talking, is on a much better wicket

when the conversation turns to centrifuges”.

Essentially there are two main sources from which sugar can be

commercially produced, namely sugar cane and sugar beet. Beet is

perhaps more expensive to cultivate, but it generally contains a

higher proportion of sugar, typically 15-17%. Furthermore, it is

easy to extract the sugar from the beet . Cane, on the other hand, is

cheaper to cultivate, but it takes a great deal more power and heavy

machinery to cut, crush and squeeze the sugar from within. Juice

from the cane contains troublesome waxes and gums, and its yield is

slightly less at around 10-13% of the weight of cane. However, there

are some countries where they are fortunate to have cane that can

yield close to 20%. Beet is fairly stable in its formation, while cane

structure varies in different parts of the world because of soil type,

climate, cane-eating pests and the skills of the growers.

Process differencesActual sugar processing varies in several ways:

• To produce high quality white sugar from beet can be

successfully achieved by passing the material through the

factory just the once. Cane on the other hand requires

a second pass or refining stage, usually at a separate location.

• Sucrose laden juice is extracted from cane by a combination

of large shredders (knives) and rollers. Beet is sliced into thin

slices to allow the sugar juices to diffuse.

• Climate dictates what can be grown when and the length of

the growing season.

• Residues from the cane is used as fuel in the factory boilers,

where as all the residues and pulp from the beet process are

mixed together and typically turned into animal feed.

Sugar processingSugar processing incorporates several specific stages (Figure 1). For

simplicity the following outline of the process is primarily followed

when producing sugar from sugar beet, but several stages are

common to both processes.

(i) DiffusionBeet enters the combined beet hopper and slicer, having first

had the soil, weeds and other debris from the growing fields

removed. The beet is sliced thinly into slices. It then enters

the diffuser, where it is stirred with water. Due to the cell structure

formation of the flesh, given enough water it will, of its own

accord, assist to expel the sugar and absorb water in its place.

All the fleshings or pulp, now virtually sugar free, are

transported to a pulp press and dryer, where it is mixed with other

waste material to form food pellets for cattle. Sugar laden liquor

from the diffuser, i.e. raw juice, is sent forward to the next stage of

the process.

(ii) Carbonation and filtrationRaw juice must be first cleaned before entering the next stage of the

process. The cleaning of raw juice is achieved by passing the juice

through a CO2 carbonation system, with the addition of lime.

Having passed the first carbonation system it is sent to a large

clarification tank. The clarified liquor then passes to the second

carbonation system. On exiting the second carbonation system the

‘clear’ juice moves forward.

The sediment that accumulates in the clarifier is pumped away

for treatment, usually by rotary vacuum filters or plate and frame

presses.

28 October 2004 ISSN 0015-1882/04 © 2004 Elsevier Ltd. All rights reserved

Why centrifuges play an importantrole in the production of sugar

The sugar production process is extremely complex, involving many differentstages. One of the most important of these is the separation of the sugar. In the following article, Nigel Day, freelance writer for Filtration+Separation,

discusses in detail the sugar production process and explains why industrial centrifuges, such as the parallel batch basket filtration centrifuge

and the conical continuous basket filtration centrifuge, play such animportant role in producing a high-quality product.

featurearticle

Thick Juice Syrup or

Raw Sugar

Water Vapour

White or Raw Sugar

Steam

Molasses

Evaporation Crystallisation Centrifugation

Figure 1: The many proceses that are involvedin the production of sugar.

(iii) Evaporation

Evaporation involves a series of large evaporators. Their purpose

is to evaporate water from the process juice.

It is here that the process engineers do wonderful things with

evaporators by using a multi-stage process with three or four

evaporators connected in tandem. Vapour from the first evaporator

is utilized to heat the juice passed to the second, while vapour from

the second heats the juice as it passes through the third, and so on.

The evaporators are configured so that the last evaporator has

the highest vacuum, and therefore the lowest boiling temperature.

The liquor is progressively concentrated as it passes through the

evaporators. This thickened liquor leaves the last evaporator and

moves forward to the ‘boiling pans’.

(iv) Boiling pansIt is in the pans that sugar crystals are allowed to grow. Pan boiling

is a crucial stage of the process despite up-to-date control and

monitoring systems available this area is both a science and an art,

with the experience of the pan boiler having a great deal of input.

When certain conditions are achieved sugar crystals will begin

to grow. Once crystals are fully-grown the heat to the pan is

turned off to stop unwanted further evaporation. With the

vacuum released the slurry falls freely into the awaiting

crystallizer or receiver below. This viscous slurry is referred to as

‘massecuite’.

(v) Crystallisers or strike receiver This is simply a long cylindrical tank laid on its side. Within it is a

large paddle agitator that rotates slowly, which prevents the sugar

crystals from settling. The massecuite leaving the boiling pans is

super saturated and hot, but given a few hours in a crystalliser it

will cool, the crystals will grow even larger and the degree of super

saturation will be reduced. From here the massecuite flows into a

buffer tank or centrifuge feed tank, where it is fed to the awaiting

battery of centrifuges via several exit ports.

Sugar separation processThe majority of the worlds sugar is produced by what is com-

monly referred to as a ‘Three Pan Boiling System’, although it is

rare to find the system being used that is without some kind of

modification. Each of the three systems comprises a boiling pan, a

receiver/crystalliser and a battery of centrifuges. Each set is referred

to as the ‘A’, ‘B’ and ‘C’ set (Figure 2).

Boiling pan ‘A’ produces the ‘A’ massecuite that is fed to the ‘A’

battery of centrifuges. These centrifuges are batch basket types, as

shown in the photograph enclosed. The sugar crystals from the ‘A’

massecuite are retained in the basket, washed, dried and then

discharged as final product, or table white sugar.

The mother liquor (molasses) extracted from the batch basket

centrifuge is then sent forward to the ‘B’ pan and processed in a

similar way to the ‘A’ stream. As the crystals form, what was ‘A’

molasses becomes ‘B’ massecuite.

This is much thicker and contains poorer quality crystals than

the ‘A’ massecuite. This change in sugar quality and viscosity

means that extraction and washing of the crystals is harder.

It is partly for this reason that a centrifuge of different

configuration is required. Here the straight walled batch basket

centrifuge of the ‘A’ stream is replaced with a battery of

continuous conical basket centrifuges.

The extracted molasses from the ‘B’ centrifugals is still laden

with sucrose, as well as impurities, but the volume is reduced by all

the water evaporated off in the ‘A’ and ‘B’ pans and all the sugar

removed by the ‘A’ and ‘B’ centrifuges. Sugar from the ‘B’ centrifuges

is normally returned to the front of the process for further refining.

The molasses becomes more viscous and is of an even poorer

quality, but it still has some sugar that can be recovered, so it is sent

to the next stage of the process or ‘C’ stream.

Filtration+Separation October 2004 29

featurearticle

A B C

Melter

Bagging

Pelleting

Evaporators

Figure 2: ‘Three Pan Boiling System’ process.

Figure 3: A battery of parallel batch basket centrifuges.

The ‘C’ stream centrifuges are the continuous conical basket

type similar to the ‘B’ units. ‘C’ sugar quality is not very good and

is not normally a saleable product. It is usually sent back to the

front of the separation process, re-melted and added to the initial

juice along with ‘B’ sugar for further refining.

The ‘C’ molasses leaving the ‘C’ centrifuges is regarded as the

end point in the sugar extraction process. In most beet sugar

factories ‘C’ molasses is sent back to the front of the factory and

mixed with the beet fleshings and pressed together to produce

cattle food pellets.

Sugar centrifugesCurrent centrifuges employed in the sugar processing industry

have two very distinct configurations:

• Parallel batch basket filtration centrifuges (Figure 3)

• Conical continuous basket filtration centrifuges (Figure 4)

Parallel batch basket centrifugesThese are primarily used when processing ‘A’ massecuite, although

there are some installations around the world that employ them on

‘B’ massecuite.

With the basket rotating at a feed speed of around 350 rpm ‘A’

massecuite flows quickly into the basket, filling it in approxi-

mately 15 seconds. As soon as a full basket is detected it quickly

accelerates to spin speed. During the time it takes to accelerate

from feed speed to spin speed the captured sugar crystals receive a

product wash.

It only takes just over one minute to fully de-liquor the solids

retained in the basket. It then quickly decelerates to a solids

discharge speed of 75-100 rpm, where the solids are ploughed from

the basket, which exit through the bottom of the basket onto a

waiting conveyor system.

As soon as the solids discharge stage is complete the basket

accelerates back to feed speed and the cycle starts all over again.

Modern day batch basket centrifuges can discharge close to 1000 kg

per cycle with around 22-24 cycles/h.

Conical continuous basket centrifugesConical continuous basket centrifuges complete the remaining

sugar separation stages of the process, the ‘B’ and ‘C’ massecuites.

As the description suggests these are continuous separation

centrifuges operating at a single speed of around 2000 rpm and

generating approximately 2000 x g.

The ‘B’ and ‘C’ massecuite enter the base of the conical basket

via a feed delivery pipe that passes through the centre of the

centrifuge from above. As can be seen in the photograph below.

Surrounding the feed delivery pipe is a steam jacket, which

increases the temperature, and therefore reduces the viscosity to

help assist purging.

Massecuite flows up the conical basket with molasses being

continuously extracted as it travels. Approximately two-thirds up

the basket the sugar receives a product wash before final drying in

the remaining one-third of the basket.

Solids discharge themselves over the lip of the basket into a

solids discharge chamber then fall down into the solids processing

system below. A series of internal baffles keeps the separated solids

and liquors apart.

Current conical continuous centrifuges designed specifically

for the sugar industry are capable of processing approximately 30

tonnes/h.

ConclusionThe sugar industry, which technology and processing is now well

established, offers many opportunities for those involved in the

field of liquid solid separation, none more so than the

manufacturer of industrial ‘sugar’ type centrifuges.

Such machines are continuously evolving, in some ways far

more than other types of centrifuges in other processing sectors.

Major developments are being made all the time, some of which

the author has had pleasure in being involved with.

Challengers for the centrifuge design engineers and machine

drive systems, as well as complying with more and more stringent

specifications, is ensuring that the sugar producing organizations

have the most comprehensive centrifuge system at their disposal.

Sugar producing organization that fail to take full advantage of

these highly sophisticated centrifuges available in the market place

may well ultimately regret their decision.

AcknowledgementsThe author would like to thank Thomas Broadbent & Sons Ltd and

P D Thompson for their help in putting the above article together

ReferenceWilliam Hallatt, 1959. Sugar & Sugar Centrifugals.

30 October 2004 www.filtsep.com

featurearticle

Figure 4: Conical continuous basket centrifugesare fed the massecuite via a feed delivery pipe.

About the authorNigel Day owns NCD Separation Solutions Ltd, a unique company

that offers a complete range of centrifuge consultancy and

centrifuge engineering services, products and technical support.

The company benefits from experience, expertise and knowledge

that has been built up over 21 years of working in the process

separation industry. Nigel can be contacted on info@ncdsep-

solutions.co.uk, or visit www.ncdsep-solutions.co.uk for more

company information.