Mill Material Balance

Introduction I-1

CHAPTER I

INTRODUCTION

Mill material balance is a tool for the milling control normally used to evaluate the performance

of sugar factory operations, mainly the extraction done by the mills for juice, the Brix and the

Pol (sugar) as well.

There were several methods of calculation for mill material balance, such as the Brix Method

and the Dry Method.

The weighing works for cane, mixed juice and imbibition water are required for the purpose of

mill material balance calculation. We also need the analyses of cane, the Pol and Brix degrees of

the juice extracted by each mill and the analyses of bagasse exhausted by the last mill.

Empirical test reference and some unlovable assumptions were used by those methods; such as

for instance the Brix Method uses 2 (two) major assumptions, i.e.:

The Brix degree of cane juice has the same value with the Brix degree of the first extracted

juice.

This will make a significant error in the calculation, in which a residual material will be ob-

tained, whichs normally called the Brix free cane water. Thus the calculation is not

exactly balance.

The Brix degree of first mill juice is carried along for the calculations of juice extracted by

each mill after the second mill.

The juice extracted by mill after the second mill is not relevantly related with the first mill

juice.

While the Dry Method needs additional analyses for bagasse exhausted by each mill, which

considered time consuming when it should be done as routine work every day.

Another method uses assumption for the value of re-absorption factor (defined from the

empirical tests), assumption or projection for the total sugar (Pol) extraction and assumptions for

the other figures as well.

The calculation system created here has only 2 (two) assumptions, i.e.:

1. The density of juice in the incoming material equals to the density of juice extracted by the

respective mill (no significant error).

2. The density of bagasse when being milled is equal to the density of fiber (in terms of

atmospheric humidity), which is of course acceptable.

The same analysis applies for Pol and Brix degrees as normally done in every sugar factory, they

are the analyses for cane and juice extracted by each mill, the mixed juice and for the last mill

bagasse.

Mill Material Balance

Introduction I-2

The result of calculation presents a complete mill material balance. All the figures normally used

as assumptions for evaluation such as, re-absorption ability factor, fiber index, fiber loading,

sugar extraction, imbibition rate, mill settings, etc. will be determined and obtained by this

complete mill material balance calculation system. And further, enable us to directly evaluate

and define the followings:

The extraction of juice, Pol and Brix due in each mill.

Is the mill settings coincidentally agreeable to the cane quality / the local condition?

Is the fore worker (knives, shredder, etc.) working properly, or needs additional /

adjustment?

Is there any juice extracted by the feed opening?

How much is the suitable quantity of imbibition water and what is the best application?

Obtaining the correct ratio of the feed and delivery work openings.

Is there any mill in danger for crack during the operation?

1.1 The extractions of juice, Pol and Brix due in each mill.

Besides the extractions by weight for juice, Pol and Brix individually in each mill, the

calculation also defined the weights and volumes for the juice content together with the

Pol and Brix percentages in the incoming as well as the out-coming portions of the

respective mill. So that the performance of each mill can be evaluated, at least knowing

which mill of the tandem has not extracting juice normally, and their constraints. Hence,

enable us to define applicable remedies to promote the extraction (especially during the

campaign).

By knowing the extraction individually, consequently we will also know the overall

extraction for juice; Pol and Brix made by the whole train.

1.2 Is the mill settings coincidentally agreeable to the cane quality / the local condition?

If the calculation of mill material balance made daily or weekly as routine work (with the

program), especially within the period after four to six weeks from the beginning of the

crushing season, it will be detected easily whether the mill settings agreeable to the

average cane quality recently crushed.

The mill material balance program includes the calculation of fiber index as well as the

fibers loading occurred in each mill.

The fiber index indicates the capability of the respective mill to press the fiber for every

escribed volume unit of the work opening. The bigger the value means good capability.

While the fiber loading indicates the capability of the respective mill, i.e. the loading of

fiber when pressed for every square unit of the roller escribed surface. For a better

Mill Material Balance

Introduction I-3

extraction each roller has a limited value of fiber loading, and it depends on the size of

its diameter.

Usually, assumption for the value of fiber index is taken for the calculation of mill

settings. Frequently different actual value found when calculation made during the

evaluation. In this regard, the mill setting is not agreeable to the recent cane crushed.

Nevertheless with the use of the program, the value for each mill will be defined during

the run of the evaluation program, and the value is the maximum fiber index affordable

to the mills. Hence, the mill settings should be made with the defined values instead of

the assumptions previously made.

1.3 Is the fore worker working properly, or needs additional / adjustment?

We have the philosophy that, the first mill has to extract juice as much as possible. The

capability of extraction of the first mill depends on how the ultimate equipment (the fore

worker) does the preparation work. This could be detected by the value of fiber index

and the compression ratio due in the first mill (compression ratio = the ratio of volume of

the incoming material to the volume of bagasse when pressed under the delivery opening

in the same time unit).

For the first mill, if both the values are lower than the projection or the assumption, the

other conclusion is that the fore worker does not work properly. The Index of Preparation

of the cut cane is lower than 86%. It may needs adjustment, or replacement for the

knives rotor. Or, if it would not help, then additional knives or shredder has to be

installed.

Both the values of fiber index and the compression ratio due for the first mill can be

detected directly by the program and for the other mills as well.

1.4 Is there any juice extracted by the feed opening?

At least well know the extraction of a mill is good, when sufficient amount of juice is

extracted by the feed opening. But if the extracted juice by the feed opening is too much,

the extraction process could be hindered by the slip occurred in the respective mill.

While observing the mills operation, sometimes well found some of the mills were not

or merely not extracting any juice from its feed openings. This is because incorrect ratio

between the feed and delivery work openings was applied during the setting of the

respective mill. The ratio definitely was too high.

The evaluation program of material balance includes the calculation to detect such, while

the projection program determine the best setting of the subject ratio.

Mill Material Balance

Introduction I-4

1.5 How much is the quantity of imbibition water and what is the best application?

The projection program for the material balance includes the facility for the simulation of

imbibition water quantity and how the application should be made. This is to resolve the

frequent trouble faced by most of mill engineers, how to eliminate choking of the last

mill operation and obtaining sufficiently dry bagasse.

1.6 Obtaining the correct ratio of the feed and delivery work openings.

Usually, the ratio between the feed and delivery work openings was determined by the

value experienced from the previous seasons practiced. They could have the opinion to

set the mills with decreasing values from the first to the ensuing mills. On the other hand,

increasing values were also used by most mill engineers, but it is to be noted that, too

narrow ratio especially for the first mill, caused the crushing capacity will not be

achieved easily.

Since the program includes the calculation for the subject ratio proportional to the

compression ratio occurred in each mill, then it is easy for the mill engineer to set the

mills with the subject ratio defined by the program.

1.7 Is there any mill in danger for crack during the operation?

The program of mill material balance also includes the calculation, whether any mill in

the tandem has potential for crack during its operation. Especially when the mill settings

are made on mill with headstocks provided without hydraulic for the top roller, or when

the mills with hydraulic but are set with fixed gap only for its lifting.

The calculation is simple. The different between the height made by the bagasse when

pressed at the delivery opening with the actual work opening when set, equals to the

potential deflection made by the roller.

The theory and the formulas defined for all the above-mentioned items will be thoroughly

enlighten in this book.