GRAVITATIONAL MILK OF LIME DOSING SYSTEM. A COST REDUCTION CASE

STUDY NZOIA SUGAR COMPANY.

SOLOMON B. AMWAYI1 (B. Tech. Chemical & Process Engineering)

1Production Department, Nzoia Sugar Co. Ltd, Box 285Bungoma-Kenya

Correspondent author Email: samwayi@yahoo.com

Tel: +254721808663

ABSTRACT.

Liming is a cruel stage in sugar processing. It is taken as the first stage in juice treatment. Mixed

juice from sugarcane extraction is acidic in nature, and thus, provides a fertile condition for

inversion process. Nzoia Sugar Company has been undertaking this crucial stage since its

inception. The main system of dosing has been pumping. High production cost has been the main

challenge facing Kenyan sugar industry; threatening closure from COMESA country

competition. To mitigate this challenge, the company re-engineered gravitational liming dosing

system during the annual maintenance (O.O.C) period in September 2012. This type of system

has been in use and is being improved on in terms of automation. Great cost reductions have

been realized in lime usage, energy consumption and maintenance costs in the tune of two and a

half million shillings in the past one year.

Key word: Liming, Gravitational system, Cost reduction, Baume

2

1.0 INTRODUCTION.

Liming is a process in juice purification/treatment where hydrated lime is introduced into

sugarcane extracted mixed juice in the form of Milk of Lime, (MOL) to neutralize the acids in

the juice and react with phosphoric acid to produce a precipitate. This is a crucial stage in sugar

processing to prevent sugar related losses by inversion. Conventionally, dosing of the prepared

lime is done by pumping system. Previously, Nzoia sugar had been using pumping system,

which despite of being effective had some challenges in terms of high maintenance cost and

frequent pump leakages, which culminated to lime losses. To prevent these losses and reduce

production cost the Company re-engineered gravitational lime dosing system. This system

reduced lime usage from 500kg per shift to the current 350-375kg per shift equivalent to 20%

savings. The energy and maintenance cost have equally dropped by about 20%. The purpose of

this paper is to investigate the effectiveness and efficiency of the gravitational MOL dosing

system as a cost reduction strategy.

1.1 Objective.

The main objective of this paper is to evaluate the gravitational dosing system. Specifically the

paper seeks:

To establish cost reduction made by adopting gravitational MOL dosing system.

To evaluate the reduction in energy consumption at the MOL station.

To develop strategies that can enhance sustainable MOL dosing system.

1.2 Scope of the paper

This paper is limited to the cost reductions made by adopting gravitational MOL dosing system.

The units of operations under consideration are those involving the liming system in the period

2012 and 2013.

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2.0 MATERIAL AND METHOD

2.1 Introduction

The hydrogen ion concentration (pH) of the juice of normal mature sugarcane range from 4.73 to

5.63 but the usual value is between 5.2 – 5.4 (Meade, 1977, p.26) [4]

. MOL is a basic chemical

for clarification processes. The dark-green juice from the mills is acidic and turbid. The

clarification (or defecation) process, designed to remove both soluble and insoluble impurities,

universally employs lime and heat as the clarifying agent (Meade, 1977, p.43) [4]

. Nzoia sugar

has adopted an intermediate liming system. Mixed juice is partially limed to pH of between 6.2 –

6.4, pre-heated to 70 o C, limed to pH between 7.5 – 8.5 and heated to 100 – 105

o C. Heating the

limed juice to boiling or slightly above coagulates the albumin and some of the fats, waxes, and

gums and the precipitate thus formed entraps suspended solids as well as fine particles (Meade,

1977, p.45) [4]

. The limed heated juice is then flashed to remove non condensable gas and finally

sent to the clarifiers (Work Instruction Manual, 2013) [6]

. MOL is a suspension and kept in

constant motion by stirring the content in the preparation and dosing tank.

2.2 Preparation and operating procedures.

MOL, about 1.1 -1.8 lb (0.5 - 0.8kg) CaO per ton of cane, neutralizes the natural acidity of the

juice, forming insoluble lime salts, mostly calcium phosphate (Meade, 1977, p.43) [4]

. MOL is

prepared at a concentration of 6-10o Be

0 i.e. ≈100g CaO per liter density, 1.116g/ml (Meade,

1977, p.114) [4]

. Water is pumped into the preparation tank preferable hot condensate from

evaporators and heaters; untreated water increases risk of incrustation/scaling of evaporators.

When the water gets quarter-way, lime bags are mixed into the water to capacity of the tank. It is

left stirred for about 2 hours before being channeled into the dosing tank. Once empty another

batch is prepared for the next dosage.

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Table 1: Relation between degree Baume and lime content of the milk

Source: Hugot. Handbook of cane sugar engineering ( pg 400) [3]

Key considerations in the process and the variations

Method of adding the lime: as milk, batches or continuously

Regulating quantity of lime: periodic tests; continuous recording of pH; automatic

addition through pH control.

Time of adding lime: before heating; “delay liming” (increase reaction time before

heating); after heating; fractionally before and after heating.

5

Figure 1: Gravitational lime dosing system

S

Source: Author 2014

For calculation on equipment sizing, Eisner (1958) [1]

, made the latest compilation of formula on

capacity of equipment in a booklet.

LIME DOSING SYSTEM FLOW DIAGRAM

LIME PREPARATION

TANK (6m3)

LIME DOSING

TANK (8m3)

PRELIMED JUICE

TANK (8m3)

LIMED JUICE TANK

(12m3)

FM

M

WATER FROM

SERVICE TANK

MIXED JUICE FROM MILLS

PRELIMED JUICE( pH

6.2-6.4) to primary

heating

LIMED

JUICE (pH

7.5-8.5) to

secondary

heating.

DRAIN

DRAIN

PRIMARY HEATED

JUICE FROM HEATERS

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3.0 DATA ANALYSIS AND PRESENTATION.

3.1 Lime consumption.

Table 2: Factory Monthly Production [7]

MONTH

LIME DOSAGE (KGS) CANE GROUND (TONS) CLARIFIED JUICE PH

2012 2013 2012 2013 PH 2012 PH 2013

JAN 31275 32975 82,470.76 76,806.55 6.6 6.8

FEB 29325 30725 67,596.07 68,085.58 6.8 6.8

MAR 34475 31100 79,078.57 57,185.74 6.9 6.8

APR 31975 28925 68,097.19 53,879.44 6.9 6.8

MAY 29425 27950 67,978.49 65,163.21 7.1 6.7

JUNE 26375 19175 65,101.50 61,717.35 7.2 6.7

JULY 29750 28125 60,207.78 65,232.54 7.0 6.7

AUG 27700 22500 58,333.08 66,467.67 7.1 6.8

SEP 18050

63,411.34 6.8

OCT 13725 22225 30,261.50 64,931.25 6.9 6.7

NOV 32825 27425 76,066.84 58,148.44 6.8 6.7

DEC 30900 31325 72,656.69 61,462.49 6.8 6.7

AVERAGE 6.9 6.8

TOTAL 317,750 320,500 727,848.47 762,491.60

Graph 1: Comparison graph

Source: Production shift superintendent log book [8]

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NB: In September 2012, the factory stopped for 6 weeks, scheduled annual maintenance. While

in 2013 it operated without annual maintenance.

3.2 Energy Savings.

This system does not require pumping, thus, the MOL is dosed by gravity eliminating the need

for the pump.

Table 4: Pump & Stirrer power ratings and their respective energy consumption

EQUIP

MENT

TYPE F/R

ATE

SPE

ED

Q

T

Y

MOTOR

POWER

ENERG

Y IN

KWh

per day

ENERGY

CONSUMP

TION PER

YEAR

UNIT

COST

PER

YEAR

(KSH)

M.O.L

Pump 2

Moret 50m3/h

1500

rpm

2 15Hp =

11.03 KW

264.72

KWh

66,180 kWh 1,058,880

Preparati

on tank

stirrer

10

rpm

1 5 HP = 3.68

kW

88.32

KWh

22,080 kwh 353,280

Dosing

tank

stirrer

8

RPM

1 5 Hp = 3.68

KW

88.32

KWh

22,080 kwh 353,280

Source: Nzoia Sugar Company, Electrical Engineering 2014

CONVERSION RATE: 1HP = 0.7355 KWh

1KWh at current market rate costs ksh. 16. (Masinde, 2014) [5]

.

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Assuming Effective crushing days in a year = 250 days

E(kWh) = P(kW) × t(hr) or kWh = kW × hr

(a) Total cost of running the pumped dosing system = Cost of power for running dosing

pump and two stirrers.

(b) Total cost of running the gravitational dosing system = Cost of the two motor stirrers

Savings made is equal to the cost of using the pump. This is equivalent to the cost difference of

running the two systems.

By using the gravitational dosing system the company is able to make a cost reduction of

Ksh. 1, 059,120 per year.

3.3 Maintenance cost related savings.

The dosing pumps (Moret type) were serviced twice per month. The various related maintenance

costs incurred were as follows:-

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A. Pipes.

Table 5: Pipe Description.

ITEM SIZE QUANTITY UNIT COST IN

KSH.PER 6

METER (VAT inc)

TOTAL COST OF

REPLACEMENT

3” Pipe SCH40 7 16820 117740

4” Pipe @SCH40 3 22181 66543

3” Valves

(discharge)

2 33,060 66,120

4” valve (suction) 2 43,848 87,696

Source: Syspro MIS 2014[9]

NB: The pipes are replaced every 2 years due to blockages while the valves are replaced

every 5years

B. Pump service cost.

Table 6: Pump service data

ITEM DESCRIPTION QUANTITY UNIT

COST

IN KSH.

SERVICE

COST IN

KSH

REPLACEMENT

/TOTAL COST

(KSH)

Bearing1 No. 3309 2 12,116 24,232 48,464

Bearings2 No. 6311 2 5,568 11,136 22,272

Man power

hour

Fitters 2 85 per

hour

8,160 8,160

10

Gland

packing

9.5mm 6 12,000

per pack

72,000 72,000

Rubber

coupling

FX70 2 43,326 86,652 86,652

Lubricating

oil

AZORA 68 100 215 per

Litre

21,500 21,500

TOTAL 259,048

Source: Syspro MIS and process Engineering 2014[9]

NB: The bearings are replaced twice per year or when they cease in between. Two mechanical

pump fitters service the pumps after every two months working 8 hour.

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4.0 DISCUSSION AND FINDINGS

4.1 Lime consumption.

It was found that the factory utilize more lime in 2013 than 2012. This was attributed to lack of

O.O.C break in 2013. In 2012, the factory had annual maintenance lasting 6 weeks increasing

amount of lime consumed from 317,750 kg to 320,500kg representing an increased variance of

2,750 Kgs. The cost went up by Ksh. 74,250.

If we factor out the O.O.C stop in both scenario the figure changes as tabled below.

Table 7: Summary of lime consumption

YEAR TOTAL LIME

USED KGS)

AVERAGE

PER MONTH

(KGS)

LESS LIME USED

DURING 6WK

BREAK

EFFECTIVE

TOTAL(KGS)

2012 317,750 28,887 0 317, 750

2013 320,500 26,708 280,437

DIFFERENCE BTW 2012 -2013 37,313

Source: Author 2014

Therefore, effective reduction in lime consumption would be 37,313 kg, while

Effective cost reduction would be

NB: The effective cost reduction is equivalent to the cost reduction made by adopting

gravitational lime dosing system. This is attributed to reduction in leakages of pumps, blockages

in the piping system and other undetermined losses.

4.2 Energy savings.

The collected data revealed that by eliminating the use of pumping system the company reduced

its energy consumption for running the pump by 264.72 KWh translating to Ksh.

(1KWh = ksh 16) per year.

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4.3 Maintenance cost related savings.

The company made a saving by reducing the number of pipes and valves replaced. The

gravitational lime dosing system has minimum piping as the tank is installed adjacent to the

dosing point unlike previous system where the pump was located at the ground floor far from the

dosing point. The pipes were replaced every 5years. This is calculated to be Ksh. 92 141 per year

for pipes and Ksh. 30763 for the valves. Other savings were in the pump maintenance cost. The

summary for the reduction is represented below.

Table 8: Summary of maintenance costs.

ITEM COST REDUCTION (KSH)

Pipes 92, 141

Valves 30, 763

Total maintenance cost 259, 048

TOTAL 381,952

Source: Author 2014

The total cost of maintenance is reduced by adopting the gravitational liming system. This is a

cost reduction to the company. The cost of procuring the pump is assumed to be a sunk cost

(Colin, 2006) [2]

4.4 Overall savings/cost reduction.

Table 9: Summary of overall cost reduction

ITEM COST REDUCTION (KSH)

Lime consumption 1, 007,451

Energy(power) 1, 059,120

Maintenance cost 381,952

TOTAL 2,448,523

Source: Author 2014

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The company makes an overall cost reduction of Ksh. 2,448,523 per year.

5.0 CONCLUSION.

The gravitational MOL dosing system can be concluded to have had an impact on cost reduction.

COMESA is a threat currently facing sugar industries in Kenya. The main issue being high cost

of production which is way high than other eastern and southern countries in the industry. In

order to survive and compete effectively in the global market, existing /operating sugar firms

should adopt a cost reduction strategy. Although, other measures like diversification and

privatization offer practical solution, they are capital intensive. Operating sugar mills should

conduct an audit of its system and optimize its operating capacity, reducing cost on various

sections while maintaining standard quality product.

6.0 RECOMMENDATIONS.

The system should be automated by installing automatic PH meters, transmitters and

controllers.

Handling MOL slurry in pumps, pipes, tanks and valves require frequent cleaning and

constant maintenance. Lime will dissolve in sucrose solution forming calcium saccharate,

a true solution which can be handled without the above problems.

Hot water preferably condensate from evaporator and heaters to be used as a solvent.

Well trained and competent employees to operate the station.

5.0 REFERENCES:

[1] Eisner, Basic calculations for the Cane Sugar Factory, Booker brothers McConnell &

Co. Ltd., London, 1958.

[2] Colin, D. (2006) Cost and Management Accounting: (6th

ed). Thomson learning: London

[3] Hugot, E. (1986). Handbook of Cane Sugar Engineering (Vol7): Elsevier science

Publishers.

[4] Meade-Chen (1977). Cane sugar Handbook: (9th

ed) Canada: John Wiley & Sons.

[5] Masinde, J. (2014, January 14). Geothermal plant to ease pressure from bills. Smart

Company, pg3

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[6] Nzoia Sugar company: Production - Process work instruction manual (Doc no.

NSC/PROD/PROC/WIM-04)

[7] Nzoia Sugar Company: Factory monthly production file (Doc no.

NSC/FACT/LAB/FMPF-01).

[8] Nzoia Sugar Company (2012/13) Production Shift Superintendent Log book (Doc no.

NSC/PROD/PM/SS/LB/12).

[9] SYSPRO Management Information System: Nzoia Sugar Company limited.

6.0 ACKNOWLEDGEMENT.

A special appreciation goes to those who contributed to this paper: Gabriel Situma (Production

Manager) for valuable discussion of the ideas in this paper, Humphrey Silikhani ( Process

Chemist) & William Wakhisi (JT chemist) for guidance in the material and method applied,

Ronald Chembukha (Process Engineer) for providing maintenance schedule for the dosing

system, Kennedy Irungu (Workshop Engineer) for availing spare parts details and Duncan

Odhiambo (Electrical Engineer) for the Energy conversion calculations.