i

DEDICATION

It is to my dear family that I dedicate this report for their priceless support throughout my life

and to the staff at Nzoia Sugar Company Limited for their cooperation and hospitality during my

attachment at the company.

ii

ACKNOWLEDGEMENT

I take this privilege to thank the Almighty God for His love, care and protection during my

attachment at Nzoia Sugar Company Limited.

Secondly I would like to appreciate the Nzoia Sugar Company management, the training centre

department for the chance they gave me to undertake my industrial attachment in the company. I

would also appreciate the production department under which I worked for the period of twelve

weeks (Three months). I personally thank Mr. Elly Owiti whom I worked under as my

supervisor and industrial trainer.

I wont forget to register my heartfelt gratitude to Moi University School of Engineering,

department of Chemical and Process Engineering. I also acknowledge the sacrifice made by my

university supervisor Ms Ajiambo for her support and advice.

I sincerely acknowledge my family for their material support during the attachment period.

iii

DECLARATION

I, Michael Mvita, do hereby declare that this report is my original work. To the best of my

knowledge and understanding, it has not been presented for any award in any other university or

institution for the purpose of learning or examination.

Signature Date:

iv

Table of Contents

DEDICATION ................................................................................................................................. i

ACKNOWLEDGEMENT .............................................................................................................. ii

Declaration ..................................................................................................................................... iii

PREFACE ...................................................................................................................................... ix

CHAPTER ONE ............................................................................................................................. 1

NZOIA SUGAR COMPANY LIMITED ....................................................................................... 1

1.0 Introduction ........................................................................................................................... 1

1.1 Occupational Health and Safety ............................................................................................ 2

1.2 Raw Material Base................................................................................................................. 2

1.3 Company Vision and Mission ............................................................................................... 3

1.3.1 Vision Statement ............................................................................................................. 3

1.3.2 Mission Statement ........................................................................................................... 3

1.4 Core Values, Objectives and Goals ....................................................................................... 3

1.4.1 Core Values ..................................................................................................................... 3

1.4.2 Objectives ....................................................................................................................... 3

1.4.3. Goals .............................................................................................................................. 4

1.5 Organization Structure of Nzoia Sugar Company ................................................................. 4

CHAPTER TWO............................................................................................................................. 6

SUGARCANE ................................................................................................................................ 6

2.0 Introduction ........................................................................................................................... 6

CHAPTER THREE ......................................................................................................................... 7

COMPOSITION OF CANE AND JUICE ...................................................................................... 7

3.0 Introduction ........................................................................................................................... 7

CHAPTER FOUR ........................................................................................................................... 9

RAW SUGAR MANUFACTURE AND REFINING .................................................................... 9

4.0 Cane harvesting ..................................................................................................................... 9

4.1 Cane transport ........................................................................................................................ 9

4.2 Cane Weighing .................................................................................................................... 10

4.3 Cane Yard ............................................................................................................................ 11

4.3.1. Post harvest deterioration ............................................................................................. 12

v

4.4. Cane Handling. ................................................................................................................... 13

4.5 Cane Preparation (Pre-Milling) ........................................................................................... 16

4.5.1 Swing Hammer fibrizor ................................................................................................ 16

4.5.2. Fibre leveler ................................................................................................................. 18

4.6. Juice Extraction (Milling) ................................................................................................... 19

4.6.1. General Mill Description ............................................................................................. 19

4.6.2 Imbibition ...................................................................................................................... 20

4.7 Juice purification: Clarification. .......................................................................................... 20

4.7.1. Juice heating................................................................................................................. 21

4.7.2 Clarifier ......................................................................................................................... 24

4.8 Evaporation .......................................................................................................................... 25

4.8.1 Multi-Effect evaporator ................................................................................................ 25

4.9 Crystallization/Sugar boiling (A- Pan Boiling)/High grade pan boiling ............................. 27

4.9.1 Condenser ..................................................................................................................... 29

4.10. Crystal separation (A Massecuite separation)/ Centrifugaling and purging..................... 31

4.10.1 Batch centrifugal ......................................................................................................... 32

4.11. Sugar Recovery/ Molasses Re-boiling ............................................................................. 33

4.11.1 Continuous centrifugals .............................................................................................. 35

4.11.2 Molasses ...................................................................................................................... 40

4.12. Sugar Drying, Packaging and Bagging............................................................................. 40

CHAPTER FIVE ........................................................................................................................... 44

STEAM GENERATION AND DISTRIBUTION (BOILERS).................................................... 44

5.0. Introduction ........................................................................................................................ 44

5.1.Fire tube boilers ................................................................................................................... 44

5.2. Water tube boilers. .............................................................................................................. 45

5.3. Design Specifications FCB Boilers (Boiler 1 &2) ......................................................... 53

5.4. Design Specifications Alpha Boiler (Boiler 3) ............................................................... 54

5.5. Cleaning during operation .............................................................................................. 54

5.6. Boiler Corrosion ............................................................................................................ 54

5.6.1. Other Causes of Corrosion ........................................................................................... 55

5.7. Boiler Embrittlement ...................................................................................................... 57

vi

5.8. Boiler fittings and accessories ........................................................................................ 57

CHAPTER SIX ............................................................................................................................. 59

POWER GENERATION AND DISTRIBUTION (POWERHOUSE) ........................................ 59

6.0. Introduction ........................................................................................................................ 59

6.1. The Energy Conversion Processes ..................................................................................... 59

6.2. Working principle of the Steam Turbine (Prime Mover) ................................................... 59

6.2.1. Impulse Turbines ......................................................................................................... 60

6.2.2. Reaction Turbines ........................................................................................................ 60

6.3. Power generation principle ................................................................................................. 62

6.4. Turbine major components and systems ............................................................................ 64

CHAPTER SEVEN ....................................................................................................................... 68

COMPRESSED AIR SUPPLY (AIR COMPRESSORS) ............................................................. 68

7.0. Introduction ........................................................................................................................ 68

7.1 Types of Compressors ......................................................................................................... 68

7.1.1. Centrifugal compressors .............................................................................................. 68

7.1.2. Reciprocating compressors .......................................................................................... 69

7.1.3. Rotary vane compressors ............................................................................................. 71

7.1.4. Rotary screw compressors ........................................................................................... 71

CHAPTER EIGHT ........................................................................................................................ 74

LABORATORY OPERATIONS (Quality Control) ..................................................................... 74

8.0. Introduction .................................................................................................................... 74

8.1. Juice analyses ................................................................................................................. 74

8.2. Chemical Dozing ............................................................................................................ 75

8.3. Bagasse analysis ............................................................................................................. 75

8.4. Massecuite analyses ........................................................................................................ 75

8.5. Dry Sugar Analysis ......................................................................................................... 75

8.6. Boiler Water Analysis .................................................................................................... 76

CHAPTER NINE .......................................................................................................................... 77

WATER TREATMENT ............................................................................................................... 77

9.0 Introduction ......................................................................................................................... 77

9.1 Raw Water Tank .................................................................................................................. 77

vii

9.2 Chemical treatment .............................................................................................................. 77

9.2.1 Coagulation ................................................................................................................... 77

9.2.2 Flocculation................................................................................................................... 78

9.3 Sedimentation ...................................................................................................................... 78

9.4 Filtration .............................................................................................................................. 78

9.5 Chlorination ......................................................................................................................... 79

9.6 Water softening.................................................................................................................... 79

CHAPTER TEN ............................................................................................................................ 80

WASTE WATER /EFFLUENT TREATMENT........................................................................... 80

10.0 Introduction ....................................................................................................................... 80

10.1 Aerobic Lagoon System .................................................................................................... 80

10.2 Cooling towers ................................................................................................................... 83

CHAPTER ELEVEN .................................................................................................................... 85

INSTRUMENTATION AND CONTROL WORKSHOP ............................................................ 85

11.0 Introduction ....................................................................................................................... 85

11.1 Level control ...................................................................................................................... 86

11.2 Pressure control ................................................................................................................. 86

11.3 Flow control ....................................................................................................................... 86

11.4 Temperature control .......................................................................................................... 88

11.5 Alarms and Safety Trips .................................................................................................... 88

11.6 Interlocks ........................................................................................................................... 88

CHAPTER TWELVE ................................................................................................................... 89

MECHANICAL WORKSHOP ..................................................................................................... 89

12.0 Introduction ....................................................................................................................... 89

12.1 Pumps ................................................................................................................................ 89

12.2. Types of Pumps ................................................................................................................ 90

12.2.1. Positive displacement Pumps ..................................................................................... 90

12.2.2 Impulse Pumps ............................................................................................................ 93

12.2.3. Velocity pumps .......................................................................................................... 94

12.2.4. Gravity pumps ............................................................................................................ 95

12.2.5. Steam pumps .............................................................................................................. 95

viii

12.2.6. Valveless pumps ........................................................................................................ 96

12.3 Bearings ............................................................................................................................. 96

12.3.1 Classification of bearings. ........................................................................................... 96

12.4. Welding ............................................................................................................................ 97

CHAPTER THIRTEEN ................................................................................................................ 98

SAFETY ........................................................................................................................................ 98

CHAPTER FOURTEEN ............................................................................................................... 99

PLANT LOCATION AND SITE SELECTION ........................................................................... 99

CHAPTER FIFTEEN .................................................................................................................. 106

ENVIRONMENTAL IMPACT ASSESSMENT (EIA) ............................................................. 106

15.0. Introduction .................................................................................................................... 106

15.1. Purpose of EIA ............................................................................................................... 107

15.2. Steps involved EIA ......................................................................................................... 108

15.3. Potential Health Effects .................................................................................................. 108

15.4. First Aid Measures ......................................................................................................... 109

15.5. Exposure Controls/Personal Protection .......................................................................... 109

15.6. Personal Protection Equipment ...................................................................................... 110

NZOIA SUGAR COMPANY FLOWSHEET ............................................................................ 111

PROBLEMS ENCOUNTERED AT THE FACTORY. ............................................................. 112

SOLUTIONS TO THE ABOVE PROBLEMS ........................................................................... 112

RECOMMENDATION .............................................................................................................. 112

CONCLUSION ........................................................................................................................... 113

REFERENCES ............................................................................................................................ 114

ix

PREFACE

Industrial attachment is among the undergraduate programmes at Moi University School of

Engineering. The program normally takes a period of eight weeks. Students in the school of

engineering have to attend three industrial attachments, one at the end of third year of study, the

second at the end of fourth year of study and the last one at the end of fifth year.

It helps students to attain practical knowledge.

Objectives of industrial attachment

i. Make the student familiar with what happens in industry and be able to bridge theory and

practice.

ii. Comprehend the entire production process in specific industry.

iii. Familiarize with different departments at the firm level e.g. laboratory, workshop,

production and processing equipment maintenance, stores and procurement, safety

procedures etc.

iv. Participate in a practical design and understand production management and techniques.

v. Identify and solve engineering problem

1

CHAPTER ONE

NZOIA SUGAR COMPANY LIMITED

1.0 Introduction

Nzoia Sugar Company is located in Bungoma County, Bungoma South District, and 5 kilometers

from Bukembe, off the Webuye-Bungoma highway. The company serves over 67,000 farmers in

the larger Bungoma, Kakamega, Lugari and Malava Districts. It was established in 1975 under

Companys Act Cap. 486 of the Laws of Kenya and commissioned in 1978.

The objectives of the company when it was established were:

i. To increase the countrys GDP through exports.

ii. Open rural industrialization development.

iii. Curb rural-urban migration.

iv. Create employment.

v. Create-social economic enhancement.

vi. Improve rural infrastructure and communications systems.

vii. Attain self-sufficiency in sugar production.

The Kenyan Government is the majority shareholder owning 98% shares while Fives Cail

Babcock (FCB) and Industrial Development Bank owning the remaining shares. The company

produces sugar and supports cane production through the provision of extension services to

farmers with an extensive company nucleus estate covering 3,600 ha and an out-grower zone

spanning more than 23,500 ha of cane.

The company is endowed with water resources from Chalicha Springs and River Kuywa which

traverses its nucleus estate. The permanent water source is not only critical for the factory

production and domestic use but also for the community surrounding the company.

2

1.1 Occupational Health and Safety

Nzoia Sugar Company puts a lot of emphasis on issues of the environment and safety. The

company has put in place policies, set objectives and targets based on the knowledge about its

environmental, health and safety impacts associated with its activities, products and services.

This ensures that the significant environmental, health and safety impacts associated with these

aspects are taken into account in setting the environmental objectives.

To achieve these objectives, the company carries out annual environmental, health and safety

audits in compliance with the environmental management and co-ordination act 2007. The audits

carried out includes: environment audit, safety and health audit, noise level, thermal, air quality,

waste water, hazardous substance, fire safety, risk assessments, plant inspections and medical

examination among others. The environmental, health and safety aspects identified forms the

basis for the development of environmental, health and safety management plan for

implementation and continual review. It also forms part of the company strategic plan on

addressing potential impact to the organization.

To this end, the company has successfully implemented and reserved substantial number of

aspects and significant impacts as identified in the audits. These include: installation of a wet

scrubber to reduce fly ash emission, replacement of asbestos roofing materials, establishment of

additional oxidation ponds to improve on waste water quality, safe handling of used oil,

reduction in accidents and incidents, procurement of new and modern fire engine, creating

awareness on environment, health and safety, construction of adequate and modern ablution

blocks and provision of wholesome water.

1.2 Raw Material Base

The company has a nucleus estate spanning 3,600 ha and an out-grower zone spanning 23,500

ha. The out-grower zone encompasses 67,000 farmers in total.

Varieties of sugarcane grown include CO421, CO945, N14, EAK70-97, EAK70-76, KEN82-

472, D84-84, KEN82-808 and KEN83-737. The current sugarcane yields an average 80 tones per

hectare (TCH), which translates to 90-120 TCH per Plant crop and 60-80 TCH per ratoon.

3

To support the cane production programmes, the company provides extension services to farmers

on proper crop husbandry through a network of outreach offices where farmers get information

and advice on fertilizer, herbicide and seed usage. Weed control is integrated and includes

mechanical, manual and chemical interventions.

1.3 Company Vision and Mission

1.3.1 Vision Statement

To be globally competitive in production of sugar and other products

1.3.2 Mission Statement

To efficiently and innovatively produce and market sugar and other products in a clean and safe

environment to the satisfaction of all stakeholders.

1.4 Core Values, Objectives and Goals

1.4.1 Core Values

In pursuit of its vision and mission, the company is guided with the values below;

i. Teamwork and mutual respect

ii. Strong customer service orientation

iii. Respect for the environment and the surrounding community

iv. Equal opportunity for all the stakeholders

v. Dedication and hardwork

vi. Transparency and accountability

1.4.2 Objectives

Its main objective was to establish sugar cane plantation and produce sugar and molasses as its

by-product. Other key objectives are;

i. Achieve sales growth

ii. Increase profitability

iii. Reduce production costs

4

1.4.3. Goals

The company is determined to achieve the following goals;

a. To facilitate the economic growth of the country

b. To create employment

c. Attain self-sufficiency in sugar products

d. Improve living standards of Kenyans

e. To open up rural industrialization

1.5 Organization Structure of Nzoia Sugar Company

The company is headed by a board of directors below which there is the managing director and

managers to departments respectively. The composition of the Board of Directors is as follows:

1. Chairman presidential appointee

2. Managing director presidential appointee

3. P.S/Appointee ministry of agriculture

4. PS/Appointee ministry of finance

5. Managing director- IDB

6. General manager/alternative- FCB

7. Company secretary- NSC

8. Bungoma district constituency directors-5 directors

5

6

CHAPTER TWO

SUGARCANE

2.0 Introduction

Sugarcane is a tropical grass belonging to the same tribe (andropogonae) as sorghum,

johnsongrass and corn (maize). More specifically, modern sugarcane is a complex hybrid of two

or more of the six species of the genus saccharum: s. barberi Jeswiet, s. officinarum L., s.

robustum brandes abd jesw. Ex Grassl, S. sanguinarum Grassl, s. sinese Roxb, and s.

spontaneum L. many forms of thses species interbreed making highly diverse genus.

The goal of the sugarcane harvest is to produce sugarcane stalks of high quality. Quality is

reduced by damaging cane, increasing trash in delivered cane and delaying cane delivery.

Removal of cane tops is of prime importance in any harvesting operation. Cane tops have little

sucrose but are high in starch and reducing sugars. Starch and reducing sugars lower sugar yield

in the boiling house and the residue from the tops absorbs sucrose and emerges from the mills

with more sucrose than when it entered. Cane leaves also have a high silica content and

contributes to mill wear.

The quality of sugarcane tends to improve with age, reaching maximum and gradually declining.

Rapid deterioration begins from the moment of harvest. Deterioration may begin before harvest

in pest-ridden cane or in fields affected by fire, freezes or wind storms.

After cutting, sugarcane loses water (1 to 2 % daily for the first week). This loss gives an

apparent but false increase in sugar content. The enzyme invertase, already present, converts

sucrose to reducing sugars thus lowering purity. Sucrose inversion varies with temperature and

moisture and is most rapid in hot, dry periods. Stale cane is anathema to the industry; growers

lose tonnage and processors lose sugars.

Unlike sugar beets, sugarcane cannot be stored for processing without excessive inversion; thus

harvesting and processing to raw sugar are concurrent.

7

CHAPTER THREE

COMPOSITION OF CANE AND JUICE

3.0 Introduction

When cane is cut and cleaned by hand, and delivered fresh, processors receive the best possible

starting material for sugar production. Cane that is cut and loaded by machine invariably

contains tops, leaves stubble and roots, as well as soil, water and other extraneous matter.

Sucrose in the juice and cellulose in the fibre are the two main constituents of sugarcane and

both are made of simple sugars. The simple sugars glucose and fructose occur free in sugarcane,

usually in lesser amounts than sucrose. The production of sugar from sugarcane juice is based on

the ability of sucrose to crystallize from thick syrup while glucose and fructose remain dissolved.

Other sugars occur in cane but not in the free state; these are constituents of gums or cell walls.

Sugar, in the ordinary sense, is sucrose. It is the sugar of household and industry and is the most

common sugar in the plant kingdom. Sucrose occurs in all parts of the sugarcane plant and is

most abundant in the stalk, where it is found in the watery vacuoles of storage cells. The sucrose

content is lowest in the actively growing regions, especially the soft portions of the stem tip and

the leaf roll.

The monosaccharide sugars, glucose and fructose condense to form sucrose and water.

Glucose content exceeds that of sucrose only in the actively growing portion of the cane plant.

The glucose content of cane juice is high early in the harvest season, decreasing with maturation.

Also called fruit sugar, fructose is sweeter than sucrose and glucose but of the three, it is the least

abundant in cane. Like glucose, it is most abundant in the growing parts of the plant and least

abundant in the lower stalk and rots. Fructose decrease with maturity and may be undetectable in

some high purity varieties at maturity. Fructose is usually present in lesser amounts than glucose.

Fructose molecules condense to form inulin, a storage product of some plants.

In chemical sense, ` inversion means the changing of dextrorotatory optical activity to

levorotatory, or the converse. Usage in sugar technology has evolved a new meaning: the acidic

or enzymatic hydrolysis of sucrose to invert sugars. `inversion is wrongly but widely used to

8

refer to deterioration following severe burning or freezing when the sucrose is metabolized by

bacteria.

9

CHAPTER FOUR

RAW SUGAR MANUFACTURE AND REFINING

4.0 Cane harvesting

Sugarcane takes 12-16 months to mature. When it is ready for harvesting it stands two to four

meters tall.

Cane harvesting involves cutting the cane at the base, de-trashing and then topping. This process

is labour intensive. Base cutting is important and if it is higher, millable cane stalk is lost and if it

is below, ground roots and soil adheres to the stalk. Soil is very abrasive in the milling process

and increases milling costs.

At Nzoia Sugar Company (NSC), Cane cutting is done by contracted cane cutters. Cut cane is

then stack together awaiting transportation.

Figure 1: Cut Sugarcane

4.1 Cane transport

Cane transport is done by contracted transporters who use tractors for transportation. The tractors

have baskets provided with wire ropes attached to a fixed bar on the right hand side and a

removable bar on the left hand side. The removable bar has some gloves which play a key role

during offloading. The hooks of the hydro-un-loader hooks on the gloves and when lifted the

10

cane in the basket is offloaded into the cane yard or the feed tables. The wire ropes loosely lie on

the basket and during loading they hold the cane for the ease of offloading.

Cane loading into tractors is done by three wheeled grabs. Loaded tractors then transport the

cane to the company.

4.2 Cane Weighing

Cane weighing is done using weighbridges. The weighbridge is made up of load cells which act

as the weighing devices. A load cell is transducer used to convert a force into an electrical signal.

The conversion is indirect and happens in two stages; through a mechanical arrangement where

the force being sensed deforms a strain gauge. The strain gauge then measures the deformation

(strain) as an electrical signal since the strain changes the effective electrical resistance of the

wire. The output of the transducer is scaled to calculate the force applied to the transducer and

the measurement displayed in master load cell readout (computer or any other display unit).

At NSC, there are two weighbridges (western and eastern bridge) lying parallel to each other

used to weigh the mass of cane delivered into the factory either from outgrowers or the

companys nucleus estate. The loaded trucks with sugarcane arrive at the factory and passes over

the western weighbridge which weighs the tonnage of the truck together with the sugarcane. The

truck then offloads the cane in the cane yard and passes over the eastern weighbridge which

measures the weigh of the empty truck as it leaves the factory.

The two weighbridges are computerized and the net weight of the cane is obtained by simple

arithmetic difference of the weight of the loaded truck and the empty truck.

11

Figure 2: Weighbridge showing electrical connection of load cells

Cane is weighed at the cane weighbridge for the following purposes:

i. For purpose of paying farmers based on tons delivered.

ii. For contractors payment purposes which include transporters and cane cutters.

iii. For effective control of factory operations and planning.

iv. For purposes of planning in agriculture department.

4.3 Cane Yard

This refers to a specially designated area where cane is temporarily stored as a bank for use when

the supply of cane stops during the night. It ensures continued feeding of cane to the factory

without unplanned stoppages for twenty-four hours. Cane storage is very critical since cane

deteriorates very fast after harvesting

The weighed cane is offloaded and stored in a cane yard. The offloading is done in such a way

that there is minimal spillage, trampling and crushing of cane.

12

The factory has a cane yard with two storage zones where the cane received is stored. The

storage zones include zone A and B. In zone A, the cane is manually offloaded from the trucks

whereas in zone B the cane is mechanically offloaded by use of hillow cranes.

Figure 3: Sugarcane stored in cane yard awaiting milling

4.3.1. Post harvest deterioration

Cane starts deteriorating immediately after it has been cut. This deterioration is caused by:

i. Inversion of sucrose by enzymes naturally present in the plant.

ii. Infection of the cane by microorganisms which secrete sucrose inverting enzymes.

Microorganisms are always present in cane and re-infestation by insect, rain, wind or direct soil

contamination occurs rapidly after harvesting.

13

The rate at which harvested cane deteriorates is influenced by temperature at the time of

harvesting, the state of the stalk (whole/ chopped, burnt/ trashed), the humidity and the variety of

cane.

For a given cane variety and agricultural practices, temperature is recognized as being a major

factor.

Cane looses mass as it deteriorates mostly through dehydration since the mass of fibre remains

constant. Since the cane mass decrease as cane deteriorates, the mass of pol in the cane decreases

with deterioration time. This is direct loss to the industry and it is however not the only loss. As

the microorganisms consume sucrose, they secrete a number of impurities which cause

processing problems in the factory and these inevitably result in more sucrose being lost, mostly

through lower molasses exhaustion. It has been well established that the concentration of

impurities such as gums and dextrin increase exponentially with cane deterioration time. These

cause severe viscosity problems, slow crystallization rate of sucrose and cause crystal

deformation.

It is for this reason that cane feeding into the feed table is done in the first in first out procedure

to prevent cane staying in the yard too long which would lead to cane deterioration. (Inversion of

sucrose to glucose by microbial activity)

4.4. Cane Handling.

Several systems are used in the cane yard to store and transfer cane.

Hillow cranes are widely used to offload cane from cane trucks. The cane lies on the cradle of

chains that line the width of the truck, one end of each chain being attached to a single beam.

The cane is offloaded by lifting this beam using the hillow crane, spilling the cane into the feed

table or into storage area.

This is an overhead crane consisting of a bridge and a trolley. There are two cranes each having a

capacity of 12 tonnes. They have a span of 82ft (distance from one side of the rail to the other).

They travel on rails of 60m long. The trolley travels on 65 long rails that are mounted on the

bridge. The cranes are 49 6 high.

14

The bridge is fitted with bumpers made from polyurethane rubber which act as stoppers to the

bridge at the end to the travel.

The gantry cranes cane and stackers (a caterpillar) are used to stack together offloaded cane in

the cane yard. Apart from cane stacking, the cane stacker is also used to clean the yard and

loading mud filters (filter cake) to trucks for transportation to farms. Mud filters are used as farm

manure.

It is also from this section where cane is fed into the cane feed table and then further fed into the

cane carrier enroute to pre-milling.

There are two types of cane feeding into the cane feed table: direct and indirect feeding. In direct

feeding cane is directly fed onto the cane feed table from trucks by help of a hillow crane while

in indirect feeding, cane is indirectly fed into the feed table from cane stacks stored in the cane

yard by help of overhead gantry crane.

Figure 4: Cane feed table with spiky chain conveyors

15

The chain conveyors at the feed table then convey the cane into the cane carriers. The cane

carrier is made of metallic slats and conveys the cane into the pre-milling section.

Figure 5: Cane carrier showing metallic slats

16

4.5 Cane Preparation (Pre-Milling)

The pre-milling process is for the preparation of the cane by breaking down the hard structure

and rupturing the cells. This process is done through the first and second knife and a fibrizor.

The purpose of pre-milling is for size reduction of cane into fine fibres in order to increase the

bulk density of cane thus increasing the capacity of the mills, to break down the hard cell

structure (rind) of the cane and to expose cells for easy juice extraction and increase the

imbibitions dilution.

The offloaded cane is fed onto a feed table that has a revolving chain which takes the cane to a

cane carrier. The cane carrier moves the cane to the first knife which has 32 revolving blades

powered by a 120hp motor which chops off the cane into small pieces.

From the first knife, cane is carried to the second knife by the cane carrier for further size

reduction. The second knife has 64 revolving blades and is also powered by a 120hp motor.

The cane carrier further carries the chopped cane from the second knife to the swing hammer

fibrizor which is powered by a steam turbine with an output power of 11.25hp. The fibrizor is

basically a hammer mill consisting of 90 hammers which undertakes finer crushing of cane into

cane fibres to expose the sucrose cells.

4.5.1 Swing Hammer fibrizor

The Fibrizor prepares cane for juice extraction. The basic function of cane preparation with the

Fibrizor is to rupture the maximum possible number of sucrose containing cells while still

maintaining the fibre of sufficient length for better mill feeding and improved permeability of

prepared cane material.

The cells are torn, open, ruptured and disintegrated conveniently and feeds prepared cane to mill

for improved extraction with reduced load on the mills.

Proper cane preparation increases throughput and reduce load at the mills. Preparation index is

expressed as the percentage ratio of brix in ruptured cells to the total brix in the cane. For

optimum/best results of juice extraction and manageable loads on the preparation equipment

preparation index (PI) should be 80%.

17

The Fibrizor hammer design is swing type which is an improved version over fix type hammer

design having the following advantages:

i. Accommodation of load fluctuations

ii. Minimization of breakdowns when foreign material/iron pieces enters

Figure 6: Rotor of the swing hammer fibrizor

Figure 7: The swing hammers dimensions

4.5.2. Fibre leveler

It is used to restrict or allow the required amount of cane fibre to

via the magnet to mill 1, in order to avoid chokes at the magnet.

The cane fibres are then taken to the milling system for juice extraction. As they are taken to the

milling system, they are passed through an electromagnet whi

18

: The swing hammers dimensions

It is used to restrict or allow the required amount of cane fibre to be conveyed from the Fibrizor

via the magnet to mill 1, in order to avoid chokes at the magnet.

The cane fibres are then taken to the milling system for juice extraction. As they are taken to the

milling system, they are passed through an electromagnet which removes any metallic materials

be conveyed from the Fibrizor

The cane fibres are then taken to the milling system for juice extraction. As they are taken to the

ch removes any metallic materials

19

which might be present in the crushed cane. There is also eye observation of the fibres for

removal of any foreign non magnetic materials from the crushed cane.

4.6. Juice Extraction (Milling)

The milling process of sugarcane is done through a series of five mills. In each of the five mills,

there are four rollers: the feed roller, the top roller, the bottom roller and the discharging roller.

The rollers contain grooves and studs that crush the cane fibres producing a juice and in the

process exposing sucrose molecules. Crushing is done from mill one up to mill five until the

fibres are exhausted with sucrose juice. Imbibition water is added to the cane fibre for maximum

extraction of sucrose from the fibres. The exhausted cane fibres are referred to as bagasse and are

released at 2.5% pol (Pol-Is the percentage of sucrose content in juice) while the mixed juice has

a purity of 85% (purity is the sucrose content as a percentage of the dry substance or dissolved

solid content) The juice from mill 1 and 2 is known as express juice and the imbibitions water is

added to the second mill and last (integrated Imbibition).

4.6.1. General Mill Description

All the 4 rollers are provided with a circumferential grooving having a 3 pitch with the angle of

grooving being 45o and the depth of 3.The top roller meshes with the feed and discharge rollers.

During milling, cane fibre tends to pack at the bottom of the grooves thus reducing the rupturing

efficiency. This fibre is removed by the help of scrappers.

The feed and discharge rollers rotate in the opposite direction to that of the top roller via the

pinions whose profile of teeth are of double involute tooth design that permits operating centres

from a maximum of 38.5 to a minimum of 35.

The exhausted cane fibres (bagasse) and are released at 2.5% pol.

The bagasse is then taken to bagasse storage by use conveyor belts and dried for later usage as

fuel to fire the factory boilers. The factory has 3 boilers which are all fired by bagasse thus

making process economical and environment friendly. The bagasse heated boilers generate steam

which is used to turn turbines in the powerhouse thus generating electricity for the company and

the excess steam is directed to the sugar processing plant for use in various reactions that require

heat.

20

4.6.2 Imbibition

This is the addition of water into bagasse to enhance juice extraction. There are two types of

imbibitions as described below.

i. Simple Imbibition

This refers to addition of fresh water. It can be single simple, double simple imbibition and so on

as described below.

Single Simple Imbibition-this is addition of water between the last mill and its previous mill

only.

Double simple imbibition is the addition of water between the last mill and the second last mill,

and between the 2nd

last mill and 3rd

last mill.

ii. Compound Imbibition

This refers to the addition of the juice extracted from the succeeding mill back before the

preceding mill. If the juice obtained from the second last mill is pumped to the fibre before the

previous mill, this becomes triple compound imbibition and so on.

Biocide is also added to the mixed juice from the mills to prevent microbial growth which would

otherwise lead to sucrose inversion to glucose and then screened to remove cush cush (stream of

wet bagasse/bagacillo separated from raw juice by juice screens). The cush cush from the screen

are then recycled and mixed with fibrized cane from the fibrizor enroute to milling. The mixed

juice is then pumped to juice treatment and evaporation section for purification.

4.7 Juice purification: Clarification.

The dark-green juice from the mills is acidic and turbid. The clarification (defecation) process,

designed to remove both soluble and insoluble impurities, universally employs lime and heat as

the clarifying agents. The purpose of this process is to produce the right quality juice and achieve

the optimum sugar recovery. Milk of lime neutralizes the natural acidity of the juice, forming

21

insoluble lime salts, mostly calcium phosphate. Heating the limed juice to boiling or slightly

above coagulates the albumin and some of the fats, waxes, gums and the precipitate thus formed

entraps suspended solids as well as finer particles.

In the juice purification, the mixed juice is first weighed using a digital flow meter. This is done

to determine the number of tonnes crushed per hour which helps to determine the efficiency of

the process, estimate boiling house efficiency and estimate the sucrose content in the juice before

treatment. The mixed juice is taken to a pre-liming process where milk of lime is added to raise

the PH from between 5.2 to 5.8 to between 6.2 to 6.4.The pre-limed juice is then taken to a

primary heating process in a shell and tube heat exchanger where its temperature is raised to

about 75-80C so as to catalyze the reaction between milk of lime and juice.

The juice is then limed by adding more milk of lime which raises the PH of the juice to between

7.5 and 8.0 The limed juice is further heated in a secondary heating process to raise its

temperature to about 103-105 C. The juice is now at optimum conditions of PH and

temperature.

4.7.1. Juice heating

The heating process is done in a series of 5 heaters (heat exchangers) whereby primary heating is

done in heater 1 while secondary heating is done from heater 2 up to 5. Heating is done to

destroy micro-organism that would cause sucrose inversion to glucose and fructose subsequently

causing loss of sugar and formation of a gummy product, provide optimum conditions for liming

and enhance chemical reaction.

4.7.1.1 Principle of operation of heat exchangers

A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to

another. The media may be separated by a solid wall to prevent mixing or they may be in direct

contact. They are widely used in space heating, refrigeration, air conditioning, power plants,

chemical plants, petrochemical plants, petroleum refineries, natural gas processing, and sewage

treatment.

At NSC, shell and tube heat exchangers are used for heating of the juice to aid in clarification.

22

4.7.1.2 Shell and tube heat exchangers

A shell and tube heat exchanger is a class of heat exchanger designs. It is the most common type

of heat exchanger in oil refineries and other large chemical processes, and is suited for higher-

pressure applications. As its name implies, this type of heat exchanger consists of a shell (a large

pressure vessel) with a bundle of tubes inside it. One fluid runs through the tubes, and another

fluid flows over the tubes (through the shell) to transfer heat between the two fluids. The set of

tubes is called a tube bundle, and may be composed of several types of tubes: plain,

longitudinally finned, etc.

4.7.1.2.1 Theory and Application

Two fluids, of different starting temperatures, flow through the heat exchanger. One flows

through the tubes (the tube side) and the other flows outside the tubes but inside the shell (the

shell side). Heat is transferred from one fluid to the other through the tube walls, either from tube

side to shell side or vice versa. The fluids can be either liquids or gases on either the shell or the

tube side. In order to transfer heat efficiently, a large heat transfer area should be used, leading to

the use of many tubes. In this way, waste heat can be put to use. This is an efficient way to

conserve energy.

Heat exchangers with only one phase (liquid or gas) on each side can be called one-phase or

single-phase heat exchangers. Two-phase heat exchangers can be used to heat a liquid to boil it

into a gas (vapor), sometimes called boilers, or cool a vapor to condense it into a liquid (called

condensers), with the phase change usually occurring on the shell side. Boilers in steam engine

locomotives are typically large, usually cylindrically-shaped shell-and-tube heat exchangers. In

large power plants with steam-driven turbines, shell-and-tube surface condensers are used to

condense the exhaust steam exiting the turbine into condensate water which is recycled back to

be turned into steam in the steam generator.

23

Figure 8: Two pass shell and tube heat exchanger

The heated juice is then taken to flash tank where it is flushed to remove gases/vapour and also

lower the temperature to about 95-98 C.

Flushed juice is then taken for flocculation as it enters a clarifier. In this process, a flocculant

(clarifying agent) is added to assist the colloids suspended come out in form of flake or floc. The

flocculant is a high molecular carbon compound that assists in settling down of suspended solid

to attain a proper clarity.

Flocculated juice is allowed to settle in the clarifier for about 30 minutes to allow the suspended

solids to settle through gravitational sedimentation.

From the clarifier, a clear juice is obtained on top while mud is obtained at the bottom of the

clarifier.

The mud is conditioned by addition of milk of lime, water, bagacillo (fine fraction of bagasse

obtained by screening or pneumatic separation generally used as a filter aid in filtration) and a

flocculant to obtain a slurry. The slurry is then filtered in rotary drum filter operated under

24

vacuum to obtain a filtered juice and a mud filter (filter cake). The filtered juice is recycled into

the pre-liming process while the mud filter is manually scrapped from the rotary drum and used

as farm manure because of its phosphate content.

4.7.2 Clarifier

A lamella clarifier (inclined-plate clarifier) is designed to remove particulates from liquids. They

are often employed in primary water treatment in place of conventional settling tanks. They are

used in industrial water treatment. Unlike conventional clarifiers they use a series of inclined

plates. These inclined plates provide a large effective settling area for a small footprint. The inlet

stream is stilled upon entry into the clarifier. Solid particles begin to settle onto the plates and

begin to accumulate in collection hoppers at the bottom of the clarifier unit. The sludge is drawn

off at the bottom of these hoppers and the clarified liquid exits the unit at the top by weir.

Figure 9: The juice clarifier

Clear juice from the clarifier is taken to a clear juice tank from which is then pumped to

evaporators for concentration through evaporation in a multi-effect evaporator.

25

4.8 Evaporation

The clarified juice, having much the same composition as the raw extracted juice except for the

precipitated impurities removed by lime treatment, contains about 85% water. Two-thirds of this

water is evaporated in vacuum multiple effects evaporator arranged in series so that each

succeeding body has a higher vacuum, therefore boils at a lower temperature.

At the evaporation station, the clear juice is concentrated in a multi-effect evaporator consisting

of 5 evaporator units to a thick syrup of brix of 60-68 brix in the last evaporator body. The

evaporator is powered with exhaust steam from mills and vapour from each unit is fed to the

chest of the next evaporator thus making the process economical and efficient.

For maximum evaporation, juice level in the evaporator bodies is monitored and maintained as

set by automatic controls i.e. body 1 should boil at bottom sight glass i.e. a third of the calandria

height while body 2,3,4 and 5 should boil between first and second sight glass.

Physical conditions of the evaporator such as temperature, steam/vapour pressure and vacuum in

the last body are monitored and maintained as follows: temperature in body 1 -125-130C, body

2 -100C, body 3 -90C, body 4 -80C, body 5 -60-65C while exhaust steam from mills to the

first body should be between 15-22psi (1-1.5bars), vacuum in the last body of 18-25inHg, raw

syrup brix from last body of 60-68brix and condensate to be free of sugar trace.

Condensate from body 1 is used as boiler feed water as it is free from sugar drains while that

from second body is used as imbibitions water in the milling section to ensure maximum juice

extraction and sugar curing in centrifugals.

The concentrated raw syrup from the last body is then pumped to a raw syrup receiver at the

mother liquor tank (liquor 1 tank).

4.8.1 Multi-Effect evaporator

A multiple-effect evaporator, as defined in chemical engineering, is an apparatus for efficiently

using the heat from steam to evaporate water. In a multiple-effect evaporator, water is boiled in

a sequence of vessels, each held at a lower pressure than the last. Because the boiling

temperature of water decreases as pressure decreases, the vapor boiled off in one vessel can be

26

used to heat the next, and only the first vessel (at the highest pressure) requires an external

source of heat.

Multiple effect evaporation commonly uses sensible heat in the condensate to preheat liquor to

be flashed. In practice the design liquid flow paths can be somewhat complicated in order to

extract the most recoverable heat and to obtain the highest evaporation rates from the equipment.

Multiple-effect evaporation plants in sugar beet factories have up to eight effects. Six effect

evaporators are common in the recovery of black liquor in the Kraft process for making wood

pulp.

At NSC, a five effect multi-effect evaporator is used to concentrate the juice into thick syrup.

Figure 10: An example of multi-effect evaporator with six effects

The juice purification, evaporation and mud filtration target parameters are as follows:

Juice PH Temperature

i. Pre-limed juice 6.0-6.6 70-85 C

ii. Limed juice 7.5-8.5 100-105 C

iii. Clear juice 6.8-7.2 95-98 C

iv. The clear juice colour should be yellowish green and sparkling clear and free of suspended

solids

v. Filter cake pol. 3.0%

vi. Filter cake moisture 70-72%

vii. Filtered juice pH 7.5-8.0

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4.9 Crystallization/Sugar boiling (A- Pan Boiling)/High grade pan boiling

Crystallization is (natural or artificial) process of formation of solid crystals precipitating from a

solution, melt or more rarely deposited directly from a gas and it occurs in a crystallizer.

Crystallization is therefore is therefore an aspect of precipitation, obtained through variation of

the solubility conditions of the solute in the solvent. Crystallization process consists of two major

steps: nucleation and crystal growth. Nucleation is the step where the solute molecules dispersed

in the solvent start to gather into clusters thus elevating the solute concentration and become

stable under the operating conditions. Crystal growth on the other hand is the subsequent growth

of the nuclei that succeed in achieving the critical cluster size. Nucleation and growth continue to

occur simultaneously while the super-saturation exists.

In NSC, crystallization is done in single-effect vacuum pans, where the syrup is evaporated until

saturated with sugar. At this point, seed grain is added to serve as nuclei for the sugar crystals

and more syrup is added as the water evaporates. The growth of the crystals continues until the

pan is full and the discharged.

The is process is started by preparation of the pan through admitting hot water and steaming the

pan to dissolve out any sugar crystals present in the pan after which footing material is

administered.

There are two methods for formation of footing material and either of which can be used

depending on the prevailing conditions.

The first method is by using fresh syrup. In this method, syrup is admitted into the pan to just

cover the calandria top plate. Steam/vapour to the steam chest is opened to start boiling and

concentrating the syrup. After concentrating sufficiently and attaining the critical point, slurry

which is acts as the seed material is admitted.

The second method which is the most commonly used method in NSC is that of using seed

magma. B-magma is used as the footing material for A-strikes. B-magma is drawn into the pan

to cover the calandria (150HL) and temperature raised upto 70-75C by allowing steam/vapour to

the calandria to facilitate sugar boiling. Sugar boiling in the vacuum pan boilers is done under

vacuum so as to facilitate boiling of sugar at low temperatures, boiling sugar in suspension and

facilitate the movement of vapour and condensate out of the pan. Hot water is gradually let in to

28

wash out the smaller crystals leaving a uniform crop of crystals of the required size (Thinning).

Once the magma has been washed, steam/vapour is increased to the calandria and feeding of

syrup is started. In case of having grained from slurry, for a time, immediately following the

introduction of the slurry, the level of super-saturation should be maintained to allow crystals to

grow. During this time, the rate of crystallization and evaporation should be balanced out. A

small stream of water is run into the pan to assist crystals in hardening. This water maintains the

state of super-saturation which acts as the driving force for crystallization. Care is taken not add

a lot of water because it would dissolve the crystals.

Once grains/crystals have sufficiently developed, the strike is gradually filled upto 450HL by

letting in controlled quantity of syrup.

For good boiling and crystallization, the following conditions should prevail:

i. A vacuum of 20-22inHg

ii. Steam of 8-10 psi (0.5-0.7bar).

iii. Good material circulation in the pan

iv. Brix of the material in the pan should increase as the pan fills

The massecuite is then brixed for exhaustion of sucrose in the mother liquor. The brix of the pan

contents increases as it fills up. This is done by regulating the pan feed.

When the massecuite is properly brixed, steam supply to the calandria is stopped, vacuum to the

pan is broken and the massecuite discharged into a massecuite receiver in a process termed as

striking. Once massecuite has been discharged in to the receiver, the pan is thoroughly cleaned

by steaming out to ensure all crystals have been washed out. Any sugar left adhering to the

calandria poses two dangers:

i. Presence of oversized crystals in the next strike.

ii. Caramerilization of sugar left over in contact with the hot metal leading to appearance of

brown crystals in the next strike.

Condensate from the calandria is continuously drained as the pan boils to avoid hammering and

heat loss.

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4.9.1 Condenser

A condenser is a device or unit used to condense a substance from its gaseous to its liquid state,

typically by cooling it. In so doing, the latent heat is given up by the substance, and will transfer

to the condenser coolant. Condensers are typically heat exchangers which have various designs

and come in many sizes ranging from rather small (hand-held) to very large industrial-scale units

used in plant processes. For example, a refrigerator uses a condenser to get rid of heat extracted

from the interior of the unit to the outside air. Condensers are used in air conditioning, industrial

chemical processes such as distillation, steam power plants and other heat-exchange systems.

At NSC, condensers are used to create vacuum in boiling equipment to help boil the juices and

liquors at low temperatures than their boiling points. The types of condensers used include

barometric condensers and multi-jet spray condensers.

4.9.1.1. Barometric leg condensers

Barometric condensers are utilized to de-superheat and condense the incoming vapors plus cool

the exiting gases, while developing the lowest possible pressure.

4.9.1.1.1 Design Description

There are two principal types of condensers, namely counter flow and parallel flow. The counter

flow condensers are constructed so that the vapors and the condensing fluid flow in opposite

directions while the parallel flow condensers flow in the same direction.

The condensing fluid and condensed vapors are removed by either the use of a tailpipe or a

pump. When a tailpipe is utilized, the unit is elevated to a sufficient height to permit drainage by

gravity. When a pump is used, the system is called a low level barometric condenser. It should be

noted that other items such as level control devices and valves are required in addition to the

pump on the low level design

Barometric condensers are constructed of cast iron, carbon steel, fiberglass reinforced plastic,

Haveg, graphite and all weldable alloys.

There are two (2) basic parts of a condenser: (1) the shell body, and (2) the spray device(s). The

nozzle connections on units constructed of carbon steel and other weldable alloys may have weld

30

ends rather than flanged and/or screwed. A bolted removable cover is supplied on the top of all

units through size 24. Shell internals and spray device(s) can be inspected by removing the top

cover. A manhole is supplied on sizes 26 and larger. The spray devices are fastened internally

by either a flanged connection or threaded ends.

Figure 11:Barometric leg condenser

4.9.1.2 Multi-jet spray condensers

Multi-Jet Barometric Condensers are generally employed where low cost water is available in

ample quantity. It is the simplest design of all barometric condensers, and requires no auxiliary

air pump or pre-cooler. It is probably the ideal type where load conditions are constant and there

is little air leakage. The Multi-Jet Barometric Condenser is also used where the vacuum handled

is not high and a moderately large terminal difference is permissible.

31

Figure 12: Multi-jet spray condenser

4.10. Crystal separation (A Massecuite separation)/ Centrifugaling and purging

Crystal separation is carried out in centrifuge. The centrifuge works using the sedimentation

principle, where the centripetal acceleration causes denser substances to separate out along the

radial direction (the bottom of the tube). By the same token, lighter objects will tend to move to

the top (of the tube; in the rotating picture, move to the centre).

The A massecuite from A massecuite receiver is separated in a centrifuge purposely to separate

A sugar crystals and A molasses from the A massecuite.

In the process of centrifugation, the massecuite from the massecuite receiver is drawn into the

centrifugals. The centrifugals consists of cylindrical basket suspended on a spindle perforated on

its sides and lined with a wire mesh inside of which is metal sheets which acts as screen. The

basket revolves at speeds from 1000-1800 rpm. The perforated lining retains the sugar crystals

which are washed with wash water to rinse mother liquor retained on the surface of the crystals.

The mother liquor (A molasses) passes through the lining because of the centrifugal force

exerted and after the sugar is purged, it is cut down leaving the centrifugal ready for another

charge of massecuite. The A centrifuge used herein are batch centrifugals.

32

The wet A sugar separated from A molasses is discharged into a vibrating conveyor which feeds

a rubber conveyor which conveys the wet sugar for drying.

The A molasses is collected and pumped into A molasses storage tank (liquor II tank) while A

wash (the water used for washing the A sugar crystals) is stored in one of the two chambers in

liquor III storage tank.

4.10.1 Batch centrifugal

Batch centrifugals have perforated basket furnished with a backing screen which is a coarse

woven stainless steel screen (8 mesh). This screen is covered by another stainless steel screen

(5mesh) and this is so called sandwich screen is in turn covered by a perforated sheet of stainless

screen designed to retain crystals. To avoid clogging of the screens by small crystals getting

stuck in the slots, the slots are made diverging which means that the slots get wider from the

insider to the outside.

The mother liquor finds its way through the perforations of the screen, the mesh permits it to

flow to the wall of the basket where it escapes through the basket perforations into the housing

where it is collected and drained off. Massecuite is charged onto the basket from the top and

builds up against the screen until a wall of massecuite of a certain thickness is formed. The

centrifugal rotates rather slowly at this stage as it is very important that the massecuite is spread

evenly over the surface of the screen to prevent imbalance.

When the basket has been loaded with massecuite, the machine accelerates to high speed

(1000rpm) and the molasses will be separated, water is sprayed on the inside of the sugar wall

and it washes away the residue of molasses from the crystals. After spraying, spinning goes on

for a short while to remove as much of the wash water as possible. Next, a break is put into

action which slows down the basket, the discharge valve opens and the sugar is removed by

means of a plough which is lowered into the basket.

When the sugar has been removed, the discharge valve is closed again and the basket can take

the next load.

33

Molasses removal is more efficient when;

i. The time of spinning at a high speed is longer.

ii. The size of crystals is uniform.

iii. The crystals are bigger.

iv. The viscosity of molasses is low.

v. The thickness of the massecuite walls is smaller.

4.11. Sugar Recovery/ Molasses Re-boiling

The A molasses still contain sucrose and it need to be recovered to increase efficiency of the

process and prevent loss of sugars in molasses.

The process of sugar recovery starts with boiling A molasses in B vacuum pans in a process

termed as B-boiling to form B massecuite. The footing material for B-boiling is A-molasses, C-

magma (which acts as seed crystals), C-light molasses from C affination (C double curing) and

raw syrup.

After preparation of the pan as earlier explained, the footing material is admitted into the pan in

the required proportion and granulation done sufficiently, the strike is gradually filled upto

380HL by letting in controlled quantity of syrup. The massecuite is then brixed for exhaustion of

sucrose in the mother liquor and the B massecuite strike discharged into B massecuite receivers.

The B massecuite is then separated into B sugars and B molasses in B centrifugals. B

centrifugals are continuous centrifugals unlike A centrifugals which are operated batchwise.

The B sugars are added with hot water and mixed to form B-magma which is then pumped into

the B-magma tank for usage as seed for A footing in A-boiling. On the other hand, B molasses is

pumped into B molasses storage (liquor IV tank) which is later used as footing material for C

boiling.

The B molasses from the storage is then drawn and admitted into the C vacuum pan for C

boiling. The footing material for C boiling is the B molasses and raw syrup. Upon calculation the

amount of syrup and B molasses required to be mixed in order to end up with a given purity of

34

the final C massecuite, the given proportions of both materials are admitted into the C vacuum

pan for granulation which is further followed with boiling and gradually admitting syrup for the

crystals to grow. Boiling is carried under vacuum to allow the boiling of the massecuite to occur

at low temperatures. When the massecuite is properly brixed, steam supply to the calandria is

stopped, vacuum to the pan is broken and the C massecuite discharged into C massecuite

receiver.

The massecuite is the taken to C centrifugals for C sugar separation. The C centrifugals operate

continuously separating the C massecuite into C sugars and C molasses. C molasses forms the

final molasses and is pumped into the final molasses tank ready for sale. The C sugar is added

with hot water forming C magma, part of which is pumped to C magma receivers which is later

used as seed in B boiling. The remaining C magma is taken for double curing in another

centrifuge. In the process of double curing, the C magma is separated into C double cured sugar

and C-light molasses. The C light molasses is pumped into the second chamber of liquor III tank

and later used as part of the footing material for B boiling. The C doubled cured sugar is screw

conveyed into C melt tank for re-melting using raw syrup forming C melt. Apart from re-melting

doubled cured sugar, the C melt tank is used for re-melting of sugar from the sugar warehouse

which is not properly dried and that which recovered from spills. The C melt is then pumped into

the raw syrup tank (liquor I tank) for use in A boiling.

The following is a summary of the target parameters for boiling house materials:

MASSECUITE BRIX PURITY

A 92-93 86-88

B 94-95 74-76

C 96-98 58-62

MOLASSES BRIX PURITY

A 78-80 72-74

B 80-82 54-56

C 84-86 35-37

35

MAGMA BRIX PURITY

B 92-94 88-92

C - 85-86

C-Melt 60-65 85-86

Syrup 60-68 86-88

Generally B and C centrifugals are continuous centrifugals while A centrifugals are batch

centrifugals.

4.11.1 Continuous centrifugals

These centrifugals have a cone shaped basket; belt driven from underneath by a motor; mounted

upside down alongside the machine. The machine rotates at a fixed speed of 1,500 rpm.

The continuous centrifuge is designed to separate sugar crystal from the mother liquor in low

grade massecuite. The whole process is carried out within a single free standard unit.

4.11.1.1 Principles of operation

Massecuite is fed at a carefully controlled rate into a central feed inlet cone on the top of the

centrifuge. This feed cone facilities to add steam to reheat and improve the fluidity of the

massecuite and to add water in order to lubricate the flow. The massecuite then flows into a

perforated inverted cone which accelerates and distributes the flow, ready for uniform feeding

into the base of the basket.

Centrifugal forces cause the massecuite to flow up the conical basket and also separate the

mother liquor from the crystals. The mother liquor passes through the filtering screens and

perforations and is collected in the inner chamber of the casing and discharged via a pipe

beneath. A spray bar allows the crystals to be washed with water as they move up the basket. The

crystals are eventually expelled off the top rim of the basket and fall down the outer annulus of

the casing into the discharge chute leading to a conveyor running beneath the centrifuge.

36

i. Water and Steam Systems

Lubrication water is sprayed onto the massecuite rope as it enters the centrifuge from a ring

around the inside of the centrifuge feed cone. Wash water is applied onto the sugar crystals as

they move up the basket, from the spraying bar inside the centrifuge

Reheat steam is also added to the massecuite

ii. Discharge Systems

Sugar is discharged from the outer annulus of the monitor casing. A sugar discharge chute

attached around the bottom edge of the casing is required to direct the sugar onto a converter

running side by side beneath the centrifuge on its anti-vibration mounts. The bottom of the

discharge chute must not be rigidly attached to the conveyor. Molasses is discharged from the

large bore tube at the back of the monitor casing.

iii. Principles of Centrifugal Separation

The separation of the massecuite into its constituent solid crystals and mother liquor is performed

in the conical basket assembly. It is basically a filtration process assisted by centrifugal force due

to rotation.

Massecuite is introduced evenly into the bottom of the basket from the accelerator cone. The

angle of the basket causes the massecuite to flow up the basket and at the same time the liquor

starts to purge through the finely slatted filtering screen. At this stage, the process is one of the

liquor draining through the crystals.

As the product moves up the basket, the layer thickness decreases to about a single crystal

thickness. This is partly because the circumference of the basket is increasing and partly because

the liquor is purging out. As most of the liquor purges away, the color of the product on the

screen changes from dark brown to light brown or white. This is clearly visible and is called

color-line. Above the color line, the product mainly comprises crystals with a thin layer of liquor

adhering to the surface. At this stage the process is drying where the centrifugal force tries to pull

these last remnants of liquor of the crystals.

37

Low grade massecuite have small crystals and very viscous liquor and are therefore difficult to

purge in batch type centrifuges. Product layer thickness and conical basket continuous centrifuge

are particularly suitable for separating lower grade massecuite. One disadvantage is that the

crystals tend to be damaged by the action of sliding along the screens and the impact with the

casing after filing off the basket rim. However, the sugar from the lower grade massecuite is

usually re-dissolved for recycling so crystal breakage is not then a process problem.

The process objectives for a centrifuge are:

i. Optimize the massecuite throughput time to suite the overall process. Usually this involves

maximizing the rate that massecuite can be separated so as to require the minimum number

of centrifuges

ii. Minimize the amount of impurities in or adhering to the sugar crystals. This can be

expressed as purging efficiently which is defined as the percentage of impurities in the

massecuite that end up in the molasses outlet stream and should be as high as possible.

iii. Minimize the loss of sugar through the filtering screens into the molasses. Sugar can pass

through the filtering screen either as crystal fragments or as a solution in water. This can be

expressed as molasses purity rise and should be as low as possible.

A high colour line means that there is a lot of mother liquor (high throughput) and this is

therefore not being purged from the sugar until close to the top of the basket (low purging

efficiency). With high throughput, the layer of the crystals on the screen will be thicker and a

small portion will be lost through the screens (low purity rise). The opposite applies for a low

color line.

Generally, the best overall performance will be obtained with the color line 25-50% up from the

base of the basket.

iv. Filtering screens

The crystals are continually sliding over the screens, so the configuration of the perforations is

more important than in batch centrifuge. Continuous centrifuge screens are usually perforated

with fine slots roughly aligned with the direction of motion of the crystals. The slots should be

narrow to reduce the amount of crystals falling through (to avoid a high molasses purity rise) but

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have a larger open area (to increase purging efficiency). Unfortunately, the limitations of

manufacturing process mean that it is only possible to have both these screens which are either

very thin or hence shorter life or made by special process which increase the cost. For example a

screen with slot width of 0.06mm and 10% open area is generally a reasonable compromise for

B massecuite (crystal size 0.3-0.5mm) and C massecuite (crystallize 0.2-0.4mm).

The abrasive action of the crystal eventually causes deterioration of the screens; the slot wears

them to sag into the gasps between the baking mesh wires, which open out the slots to an overall

shape. Both these effects allow more crystals to pass through. A progressive molasses purity rise

increase of 1-2 % over a short time indicates a worn out screen to reduce the sagging effect,

centrifuges incorporate additional finer intermediate mesh between the filtering screen and the

coarse baking mesh. This provides a better support and thus extends filtering screen life.

v. Water addition

Water can be added either as lubrication water to the massecuite as it enters the feed cone or

sprayed directly as wash water onto the product layer as it moves up the screens. In both cases,

the water dilutes the mother liquor and reduces its viscosity and surface tension thus improving

purging efficiently and paradoxically increasing the final dryness. On the other hand too much

water dissolves sugar which increases the molasses purity rise. Generally, adding about 3% by

weight of water relative to massecuite is reasonable initial setting.

Water addition is confined to adding lubrication water and spray washing below the color line as

washing above the color line is wasteful, because the crystals are widely dispersed and the wash

does not come into contact with the crystals. There is also a risk of wash liquor being carried

over the basket lip.

For low grade C massecuite with high viscosity mother liquor, water addition should generally

be based more to lubrication rather than spray washing. For intermediate grade B massecuite,

spray lubrication water addition.

vi. Steam addition

The viscosity of the mother liquor in the massecuite roughly halves for every 10 degrees rise in

temperature. A higher massecuite temperature therefore improves purging efficiency, by

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allowing the liquor to flow out more easily through the gaps between the crystals. Adding feed

reheat steam to the massecuite as it passes through the centrifuge feed cone can therefore be

effective since it tends to heat the mother liquor but do not cause significant dissolving of the

crystals.

Factors that affect crystal separation

i. Viscosity of the massecuite

ii. Grain size: false grain formation

iii. Amount of wash water applied at the centrifugals (steam)

Figure 13: Continuous centrifuge.

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4.11.2 Molasses

Molasses is a syrup, by-product from the processing of sugarcane into sugar. The thick viscous

syrup used in jaggeries, production of ethanol and also one that provides the robust bittersweet

flavor to baked beans and gingerbread has been available throughout the years since the

companys inception

4.12. Sugar Drying, Packaging and Bagging

Although the objective of crystallization is to produce a pre-determined number of crystals of a

pre-defined size, the reality is that the crystals produced in vacuum pans have a crystal size

range, some narrower than others, a crystal regularity, a quantity of fine and coarse crystals,

agglomerates and conglomerates. Moreover because of the open molecular structure of the sugar

crystal, moisture, or H2O molecules will be present in the crystal structure and can be divided

into three types of which only two can be analytically measured:

i. Surface water content; the water adhering to the surfaces of the sugar crystal and

removed by evaporative drying.

ii. Included water content; not able to be measured because it is not possible to extract it

from the crystal structure

iii. Total water content; by measuring this value, using the Karl Fischer method with a

solvent, such as Formamide, as a solvent for the sugar, the inclusion sugar content can be

determined by subtraction.

In freshly dried sugar, the Total water content should be < 0.1% with the Surface water content

being 0.03 0.05%, leaving 0.07 0.05% of Inclusion water content.

The problem with the inclusion water content is that it is very difficult to extract, but will migrate

slowly to the outer surface of the crystal over time in storage. The quantity of water involved in

Inclusion water is a significant percentage of the Total water content of dried sugar and caking,

lumping together of sugar crystals can occur readily in storage, depending on the temperature

relative to ambient and relative humidity in the storage facility

Plants which bag sugar direct from production will tend to have a subsequent problem with

increased moisture. The post-bagging storage conditions become more important, because:

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i. The sugar will often be wa