IMPROVEMENTS

IN CONTINUOUS CENTRIFUGATION

TO REDUCE

NON- SUGAR RECIRCULATION

Presented by E.D. Bosse at the 27th General Meeting of the American Society of Sugar Beet Technologists Anaheim, California March 3 to 6, 1993

Author's address: E.D. Bosse c/o BMA AG P.O. Box 3225 3300 Braunschweig Germany

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280

IMPROVEMENTS IN CONTINUOUS CENTRIFUGATION

TO REDUCE NON-SUGAR RECIRCULATION

1. INTRODUCTION

As all of you know. the quality of white sugar made by multi-stage evapo- and

cooling crystallization is subject. in the fi rst place. to the colour of the crystals, the

ash content. the colour in solution and the crystal size distribution.

Let me use the following three- boi ling scheme (Fig. 1) as an example:

~

r

'-­

y UQUOR

'3.S!! 72.00 9.~ 88,57

,. 9.64 ..

HIWI RAW SUQAR

7.22 98.50 .... 7.11 118.110 ~

7.03

AFF. LDW RAW IlASSEc:. I 4,11' 81.Q~4,53 • •51 4.01 .­ ~

AFF. LDW AAW &UGAA 2.72 98.00 2.67 118.00 2.6 ,

22.94 17.20 16.00

THICK JUICE

~ STANOARD uaUOR

36.52 n .aa 26.88 85.02 25.64

... WMm: SUaAR ItIASSEc:.

29.65 9UlO 26.88 95.02 25.64

I T

HIGH-RAW ItIASSEaIITE 15.00 II2.SO 13.117 89.15 12.37

-~

LOW RAW ..... ssecUm: 6.36 85.00 6.04 77.50 • . 68

I

H

m mTS mZ

,4,41

' • . 27 1427

3.40 2.94 1.n

SYMBOLS

WHI'TE SUQAR

MOlASSES

.,TS q

l1li.00 100.00

86.50 59.00

Fig. 1: Simplified 3-boiling scheme

This shows that the lower-valued products from raw and low-raw processing are

separated from their mother syrup, are washed or affinated with a certain medium,

281

and are fed in the form of a solution, i.e. liquor, to recrystallization or recycled as

seed magma for the higher-valued product.

Every sugar technologist, therefore, has to avO'id - to the extent possible - the

inevitable recydi'ng of non-sugars, i.e. non-sugar recirculation and, consequently,

impurities.

Therefore, centrifugation is of great significance to the non-sugar circulation in the

various stages of crystallization , i.e. the effici ency of separation of the mother syrup

from the crystal has a decisive importance.

This separating efficiency in turn depends on several technological factors, such as

- mean crystal size,

- crystal size distrubtion,

- gravity factor,

- thickness of the crystal layer in the centrifugal,

- viscosity of the massecuite, and

- quality and purity of the washing medium used.

The separating efficiency can be understood as the percentage of mother solution

which leaves the respective centrifugal stage as syrup - defined as the ratio of the

dry substance masses concerned.

2. DEVELOPMENT OF A NEW MASSECUITE PREPARATION SYSTEM

FOR CONTINUOUS CENTRIFUGALS

TO REDUCE NON-SUGAR RECIRCULATION

It has been common practice to treat the massecuite, before feeding it into the

basket of a continuous centrifugal employed almost exclusively for low-raw

massecuite, but sometimes also for raw massecuite, in a massecuite preparation

system (Fig. 2).

28 2

Fig. 2: Standard massecuite distributor

In this preparation system, the massecuite is treated with steam and water to reduce

its viscosity and improve its centrifuging properties, homogenized by the rotating pins

shown on the picture, accelerated by the accelerator bell to the final speed of the

machine and then fed into the centrifugal basket.

In recent years a number of investigations were made to improve the afore­

mentioned separating efficiency and, consequently, reduce non-sugar recirculation,

and this led to a novel massecuite preparation system in conjunction with our new

double centrifugal (Fig. 3).

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Fig. 3: Double centri fugal

The double centrifugal is a unit with two baskets arranged one on top of the other.

The sugar leaving the first basket is mixed with affination syrup and immediately

conveyed to the second basket underneath for further processing.

The specific advantage inherent in combining two independent units is that the

stages can be adjusted in an optimum way by simply varying the speeds to the

technologically required G-factor, which cannot be done when the two baskets are

driven by one common shaft.

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To provide for optimum separation of the sugar crystals from the highly viscous

mother syrup, the pre- seperating stage can be operated at up to 2,000 rpm which

means a G-factor of over 2,500 while the affination stage requires a far lower G­

factor.

In developing our new double centrifugal, great importance was attached to

massecuite preparation (Fig. 4) as due to its high viscosity, low- raw massecuite is

very difficult to purge.

I I

-t----,---­-­

Fig. 4: New massecuite distributor

In this respect, central massecuite feed turns out to be of particular advantage. Water

and steam are applied as early as possible - in the massecuite feed pipe. In the

product distributor, acceleration activates the heating and mixing process.

285

The newly developed massecuite preparation system is a three-step distributor bell

which has now been efficiently employed al ready in many centrifugals for 3

campaigns. It consists of a rigid outer section and a rotating inner section with two

shoulders, the so-called accelerator bell which, in addition, is heated with steam.

This additional application of steam in the area of the multi-step accelerator bell

provides for better massecuite mixing and heating, i. e. much more efficient heat

transfer between steam and massecuite, without even the slightest melting of sugar

crystals. The result is an excellent separation of the sugar crystals from the mother

syrup.

This brings about an evenly high massecuite temperature, i.e. it can be heated by up

to 15 ·C as compared with 6 to 8 ·C for the previous model. The three-step

accel'erator bell provides at its shoulders for a certain mixing effect, i.e. for a relative

movement of the crystals, considerably improving the massecuite distribution at the

same time.

Now the new massecuite preparation system can be incorporated not only in the

double centrifugal, but also in most of our other continuous centrifugals.

3. OPERATING RESULTS

Fig. 5 gives the results of a comparison made between a contin uous K 1100 and a

K 1100 Turbo, which is an identical machine but for the new massecuite preparation

system.

286

purity purity low raw difference sugar % molasses

92 I --------------~~

90 I---------~"":;_

++98 1-----------;-----7----t 1.6

1.4

1.2

1.0

0.8

88 I-----------;--~---t 0.6

86 I--__~--------~--~~ 0.4

84 I---------r--~---t 0.2

82 0 1 2 3 4 5

wash water % •

Fig. 5: Comparison of new K 1100 Turbo vs standard K 11 00

At a basket operating speed of 2,200 rpm and the associated gravity factor of 2,900,

as related to the largest inside diameter of the basket, the throughput of the K 1100

Turbo is some 15 - 20% higher than that of a conti nuous centrifugal of the standard

K 1100 type, and the technological data of the sugar and molasses leaving this

machine are identical or even better.

The chart shows the curves of sugar purity and difference in purity between

molasses and cyclone molasses (Nutsch) versus the percentage of wash water

applied at a constant throughput.

287

Figs. 6, 7 and 8 show the results achieved by a double centrifugal processing low­

raw massecuite: They also reveal how close ash, colour and purity, subject to the

quantity of wash water applied, can get to the crystal core figures.

0.5 ASH LOW-RAW UQUOR

0.4

0.3

0.2

0.1

o

I I I I i I \ I I I \ ,

I

\ II t~IJI

CORE-ASH: ci . 05~ I I

I I I I I I I I o 1 234 567 8

WASH WATER % O.m. ___ (BASKET 1+2)

COLO R LOW RAW LIQUOR 300

200

100

o

1\ \

I~

~~ CORE

I I - COlOR: eo IU5eo.... I

I I I I I I 1 2345678

WASH WATeR % a.m. ___ (BASKET 1+2)

Fig.6: Ash of low-raw liquor Fig. 7: Colour of low-raw liquor

t~oo

PURITY 99 LOW-RAW UaUOR 98

97

96

95

I CORE-PURITY: 99,5%

V ,II'

2

I I ,

k--~ ~

•

4 5 6 7 8 WASH WATER % a.m. ___ (BASKET 1+2)

Fig. 8: Purity of low-raw liquor

288

4. NON-SUGAR RECIRCULATION

Now, what do these improved results mean to sugar house work in general in

conjunction with the afore-mentioned boiling scheme?

The non-sugar balances (Fig. 9) show the great variation of non-sugar circulation at

different low- raw purities. The core purity of the raw sugar was assumed to be

constant at qHR = 98.8.

The fo llowing versions relate to extreme low-raw purities of q LR = 92.5 and 98 with

an average of qLR = 95.

Version 1 Version 2 Version 3

Thick Thick Thick Juice Juice Juice

100"_

qLR=92.5%

100%

qLR=95%

100%

qLR= 98%

Fig. 9: Non- sugar balances

289

Version 1:

Assuming that the low-raw product with 92.5% purity is mixed with raw green syrup

and the affination syrup is recycled to the low-raw stage, there is a non- sugar

circulation of 4.8% to the white and of 17.8% to the low- raw stage.

Version 2:

It is evident that better separation in the centrifugal and, consequently, a higher low­

raw sugar purity (95%) reduce the non-sugar circulation to the white sugar station

and, in particular, to the low- raw station from 17.8% to 8.5%.

Version 3:

If it were possible, in an extreme, to produce low-raw sugar with a purity of qLR = 98%, non-sugar circulation would be the lowest, and in addition to that no affinatioll

would be necessary.

5. SUMMARY

These examples very clearly illustrate the influence of the separating efficiency in a

low-raw centrifugal.

The higher the purity of the low-raw sugar, i.e. the better the separation of the

mother syrup by proper massecuite preparation in the centrifugal, the lower the

recirculation and thus the residence time of the non-sugar, which also has a

particularly positive influence on the colour of the products concerned.

Therefore, the first rule to be obeyed is to produce a very uniform low-raw sugar

crystal providing the Ilargest possible mean crystal size so as to achieve the set goal

of optimum separating work.

290

Symbols used:

m = Mass flow

mTS = Mass dry substance

mz = Mass sugar

wTS = Dry substance

q = Purity

qHR = Purity high- raw sugar

qLR = Purity low- raw sugar

References:

- BMA Information

- D. Bosse.

Paper read at Hannover Industrial Fair, 1992

- Klaus Buchholz. Dietrich Schliephake,

Zuckerindustrie 116 (1991 ), 414- 416

291