RESEARCH ARTICLE

Assessment of Standby Evaporator as an Energy ConservationMeasure: Case of Sudanese Sugar Industry

Mahdi Bashir • Bashir M. Elhassan •

Ali A. Rabah

Received: 24 May 2011 / Accepted: 2 August 2011 / Published online: 25 August 2011

� Society for Sugar Research & Promotion 2011

Abstract This work presents analysis of historical data of

four Sudanese sugar factories namely Assalaya, Sennar,

Gunned and New Halfa. The historical data covers more

than 40 years (1966–2010). In about 38 years (1966–2004)

of this period the sugar industry operates with one set of

evaporator. In 2005 a standby set of evaporator is installed

in each factory. The installed set of evaporator is similar in

configuration (Robert type) to existing set however, with a

relatively large heat transfer area and made of stainless

steel rather than brass. The result of the analysis revealed

that before 2004 the total downtime is 26.6% (53 days) of

the operation period on average; 10.4 days (19.6%)

of which is for evaporator scale removal. The provision of

standby evaporator has completely eliminated the evapo-

rator downtime. The improvement in total downtime has

made significant saving on bagasse. The bagasse saved in

10 days is 14379 ton ($215691 based on bagasse price $15/

ton). The shortening of crop season made by standby

evaporator has saved the sugar cane from over maturation

and improved sugar recovery by 0.2% (from 9.3 to 9.5%).

The savings due to bagasse and sugar recovery only made

the evaporator payback period of 2.8 years. The standby

evaporator has also improved the overall steam economy

and evaporation coefficient. The steam economy of the

standby and existing sets of evaporators is about 3.1 and

2.6 respectively.

Keywords Sudan sugar industry � Downtime �Standby evaporator

Introduction

In the past energy was not of concern in the sugar industry.

In fact the design of the process equipment gave very little

consideration for efficient use of energy and therefore

consumption of more energy—more bagasse—helped

reduce cost of safe storage bagasse. With more utilization

opportunity for bagasse coming up for examples bulb and

paper, fiber board and electricity generation, the energy

conservation in sugar industry is receiving increasing

interest.

The reduction of energy consumption in sugar produc-

tion usually includes improvement in those energy systems

comprising power plants, millings, multiple-effect evapo-

rator and process heating equipment. Over years many

energy conservation systems have been developed in sugar

industry. These include retrofit of existing systems,

cogeneration, bagasse drying and total electrification to

mention a few.

In the sugar factory Hohenau, Austria, a four-effect

evaporator station, originally comprising four natural-

circulation units and a tubular falling film unit, was

reconstructed. The natural-circulation units were removed

and replaced by four plate-type falling film units. Another

new unit of the same type was installed on a new support

structure. The reconstructed station is arranged in six effect

including two pre-evaporation ones. The retrofit of the

evaporator station and heat exchanger network resulted in a

reduction of the specific energy consumption by 17%

(Urbaniec 2004). Another example of evaporator retrofit is

the case for sugar factory Krasnystaw, Poland. The retrofit

resulted in the reduction of energy consumption by 29%

(Urbaniec et al. 1997). A third example of evaporator ret-

rofit is the case of sugar factory Sajkaska located in Zabalj

in the Republic of Serbia. The multi-effect evaporator

M. Bashir � B. M. Elhassan � A. A. Rabah (&)

Department of Chemical Engineering, University of Khartoum,

P.O. Box 321, Khartoum, Sudan

e-mail: rabahss@hotmail.com

123

Sugar Tech (July-Sept 2011) 13(3):179–185

DOI 10.1007/s12355-011-0092-2

station was modernized by replacing old Robert evapora-

tors with falling film plate evaporators (GEA-Ecoflex). In

this way, the energy consumption in the Sajkaska sugar

factory was reduced by about 20% in comparison with the

previous consumption (Prodanic et al. 2008).

Cogeneration is the combined production of heat and

power (CHP) from bagasse. International Sugar Organi-

zation (2009) provided a survey on the status of cogene-

ration on sugar cane processing sectors. It shows that

bagasse-based production of electricity for export to the

national grid is fast becoming a major activity of sugar

mills. It indicated an increasing number of mills in a

growing number of countries are already involved or are

planning to start in the near future electricity production in

excess of captive consumption. A review of the current

situation and prospects for cogeneration in 13 countries in

Africa (South Africa, Kenya), Asia (India), Latin America

(Brazil and Guatemala,) and Oceania (Mauritius) shows

that the scope for efficient, competitive and environmen-

tally friendly electricity production could be sizeable.

Sudan has installed a 15 MW cogeneration plant in Assa-

laya sugar factory in White Nile state. Other congregation

plants are planned for sugar factories at Sennar, Gunned

and New Halfa (personal communication).

In sugar industry mill rolls, centrifugals, air pumps and

compressors, pumps, conveyors and elevators, etc are

steam turbine prime movers. A change from steam turbines

to electric motors as prime movers is often regarded as an

important step in sugar industry. Ramjatun et al. (1999) has

listed the advantage of electric motors as

1. More steam is directed to the new power house where

it is used more efficiently,

2. The long and cumbersome steam pipelines are

removed and replaced by the very much smaller

electric cables,

3. The reliability of the system is increased,

4. Less floor space is occupied,

5. Maintenance cost is reduced,

6. Individual electric motors allow automatic control,

7. Improves reliability, cleanliness, and flexibility of

plant.

Two examples of electrification are available in the

literature. Natal sugar, South Africa has electrified the

auxiliary motors and Cuba has electrified 5 plants out of 15

plants. Sennar sugar factory, Sudan has installed 3 MW

cane preparation electric driven mill. The energy saving

was estimated as 10%.

Sugar Industry in Sudan

Sudan has five sugar factories namely Gunied founded in

1963, New Halfa in 1966, Assalaya in 1980, Sennar in

1977 and Kenana in 1975. The former four factories are

owned by Sudanese Sugar Company (SSC) and the later is

owned by Kenana Sugar Company. There are other three

factories under construction. These are White Nile, Blue

Nile and Deuem sugar factories. The former is expected to

be commissioned in 2012. Figure 1 shows the sugar pro-

duction in the period 1963–2010. The total production

jumps form 500000 ton in year 2000 to more than 750000

in the year 2010. Besides sugar production the production

of ethanol is now introduced in Kenana and under con-

struction in other factories.

Case Study

The study investigates the Sudanese Sugar factories with

special emphasis on Sennar sugar factory. Sennar factory

is located along the Blue Nile about 400 km south of

Khartoum. Both the existing and new (standby) sets of

evaporators are forward quadruple effect of Robert type.

The material of construction of the existing set and

standby are brass and stainless steel respectively. The

standby set is commissioned in 2004. The total cost of

standby evaporator including installation cost is

$1502059.00. Table 1 shows the heat transfer area of the

two sets of evaporators.

0

200

400

600

800

1960 1970 1980 1990 2000 2010

Season

Gunied

N.Halfa

Sennar

Assalaya

Kenana

Total

To

n (

000)

Fig. 1 Sudan sugar production (1963–2010)

180 Sugar Tech (July-Sept 2011) 13(3):179–185

123

Results and Discussion

Downtime Before the Standby Evaporator (1976–2004)

Figure 2 shows the total downtime of the four Sudanese

sugar factories. There is no significant difference between

the downtime of the four factories. It can also be observed

that the downtime assumes a decreasing trend in general.

Hence the downtime of Sennar sugar factory will be con-

sidered for further discussion. Table 2 shows the operation

time and downtime in Sennar sugar factory for the period

from 1976 to 2004. This covers the period before the

installation of the standby set of evaporator. It can be seen

that, in the period 1976–2004, the operation time which

usually extend from November to May, varies between 155

and 264 days with an average of 199 days. It can also be

seen that the total downtime in the period from 1976 to

2004 varies between 65.03% in 1976/77 season to 24.12%

in the 2000/2001 season. The average downtime is 39.37%,

which is equivalent to 78.3 days.

The downtime is generally attributed to

1. Field (referred to as no cane downtime): This may be

due to machinery failure of cane or transportation

systems or inadequate cane cutting laborers. The

no-cane-downtime varies from less than one percent

(in 1977) to more than 20% (in 1996).

2. Factory (referred to as technical downtime): This is

divided further into two types of downtimes as planed

downtime and preventive maintenance.

a. Planned downed: This is the time required for

scale removal of evaporators. Table 3 shows the

downtime for scale removal. It can be seen that the

scale cleaning downtime is 226 h (9.4 days) or 12

% of the total downtime on average.

b. Preventive maintenance downtime: This is due to

equipment breakdown such as cane mills, pumps,

air compressors and conveyors among other. By

subtraction of planned downtime, the preventive

downtime in the year 1999–2000 is about 18% of

the total downtime.

The downtime has significant impact on overall perfor-

mance of both factory and field. The main impacts of

downtime include:

1. Drop in sugar recovery: The compensation of down-

time will prolong the crop season and bush it to hot

summer days. Long crop season means sugar cane over

maturation or in other words loss of sucrose. Figure 3

shows the sugar recovery of the four sugar factories. It

can also be seen that there is no significant variation in

sugar recovery among the four sugar factories. For

Sennar the sugar recovery before the installation of the

standby evaporator is about 9.3% on average and after

the installation of the standby evaporator is about

9.5%; an enhancement of 0.2%. The poor sugar

recovery is attributed to over maturation of sugar cane.

2. Consumption of bagasse: During downtime the plant is

normally not completely down; in particular the power

house is running to provide electricity for irrigation,

residential area and administrative building and steam

for service. The shortage of bagasse normally forces

the plant either to run the boiler on furnace oil or buy

electricity from national grid.

3. Evaporator efficiency: In hot summer days the effi-

ciency of the evaporator will go down due to the

increase in the temperature of cooling water in the

vacuum condenser of the last effect.

There are a number of measures that are recommended to

reduce the downtime. These include

1. Strict implementation of proper maintenance programs

for machinery and harvesters as well as for process

equipment and ancillaries.

2. Supply of adequate number of field machinery e.g.

harvesters, laborers and tractors

Table 1 Heat transfer area of evaporators

Evaporators Heat transfer area (m2)

I II III IV

Standby 2600 2000 2000 2000

Existing set 2400 1900 1700 1700

0

20

40

60

80

1960 1970 1980 1990 2000 2010

Year

Gunied

Halfa

Sennar

Assalaya

Dow

ntim

e %

Fig. 2 Total downtime in sugar factories

Sugar Tech (July-Sept 2011) 13(3):179–185 181

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3. Training of machinery operators

4. Scale or fouling control.

5. Installation of a second set of evaporator which

completely eliminate factory stoppage for evaporator

cleaning.

The first three measures are in fact implemented by the factory

since crop season of 2000–2001. As a result, it can be seen that

there is a substantial reduction in the total downtime in the

period 2001–2004; the total down is 26% on average.

Table 4 shows the analysis of specimen of scale removed

from the evaporator. The result indicates that Ca2? constitutes

the main element of the scale; its concentration is 550 ppm.

Besides Ca2?, the Mg2? and Na1? are presented with signif-

icant amount. The Ca2? and Mg2? can be attributed mainly to

Table 2 Operation and downtime in Sennar sugar factory (1976–2004)

Season Cane Sugar Down time (%) Furnace oil

Year Days Crushed Production Tech No cane Total Consumption

1976 1977 212.00 376957.00 26465.00 64.39 0.64 65.03 6242.00

1977 1978 183.00 408240.00 31675.00 28.00 9.25 37.25 5920.00

1978 1979 100.00 232879.00 18177.00 20.20 0.91 21.11 926.00

1979 1980 171.00 401770.00 30001.20 23.34 14.31 37.65 2138.50

1980 1981 184.00 338156.00 26115.00 32.23 13.15 45.38 1875.00

1981 1982 185.00 313790.00 22326.40 27.04 16.87 43.91 2229.50

1982 1983 182.00 463758.00 40601.00 28.77 4.13 32.90 1907.00

1983 1984 221.00 702830.00 57710.00 33.54 2.28 35.82 3360.50

1984 1985 220.00 760924.00 68884.90 29.70 1.94 31.64 1514.50

1985 1986 237.00 498047.00 37623.00 35.24 20.98 56.22 1602.00

1986 1987 264.00 646372.00 47199.90 42.53 11.82 54.35 3461.00

1987 1988 229.00 573415.00 44157.00 45.68 4.19 49.87 3649.00

1988 1989 228.00 536004.00 44115.00 33.58 15.79 49.37 3741.50

1989 1990 156.00 553396.00 50660.00 30.75 1.79 32.54 909.00

1990 1991 168.00 626503.00 58788.80 32.00 1.49 33.49 804.50

1991 1992 211.00 631881.00 55565.00 30.75 7.94 38.69 648.50

1992 1993 231.00 594920.00 52539.10 31.40 14.13 45.53 3217.50

1993 1994 226.00 528766.30 41835.10 39.89 9.12 49.01 1802.00

1994 1995 232.00 519176.47 42244.40 41.47 7.40 48.87 2238.50

1995 1996 182.00 545889.87 44650.00 27.97 10.05 38.02 0

1996 1997 177.00 47161.27 36000.00 30.99 13.35 44.34 0

1997 1998 160.00 508033.93 43009.20 24.44 13.00 37.44 0

1998 1999 230.00 715580.74 55063.80 35.90 1.42 37.32 0

1999 2000 190.00 734374.45 64522.80 24.05 6.28 30.33 0

2000 2001 155.50 663482.49 62205.90 12.70 11.42 24.12 0

2001 2002 193.50 802022.68 78186.50 18.02 7.52 25.54 0

2002 2003 217.00 928325.54 85021.00 23.18 2.16 25.34 0

2003 2004 238.70 893696.09 78692.00 27.30 3.90 31.20 0

Average 199.42 31.25 8.12 39.37

Table 3 Operation period between two successive cleaning and

downtime

No. Date of scale removal Downtime (h) Operation time (day)

1 13 December 1999 18

2 3 January 2000 26 21

3 22 January 2000 26 19

4 7 February 2000 26 18

5 25 February 2000 24 18

6 13 March 2000 23 18

7 31 March 2000 26 16

8 16 April 2000 31 16

9 7 May 2000 26 21

Total 226 147

182 Sugar Tech (July-Sept 2011) 13(3):179–185

123

1. The soil and mud associated with sugar cane

2. The imbibition water is untreated water (raw water)

3. Chemical used in juice clarifications and pH control.

Lime is used in this particular factory

The Na1? is mainly stem from the fertilizer used in the

sugar cane field.

In other industry scale is commonly controlled using

chemicals, however, this option is not likely in food

industry. The other options include the reduction of Ca,

Mg, Na and other salts in imbibition water and minimi-

zation of mud in cane and enhancement of the juice clar-

ification efficiency. The reduction of these salts in

imbibition water may be made by treating the imbibition

water before utilization. Alternative option is to use con-

densate driven off the evaporators. Minimizing of mud may

require harvesting and cane preparation systems. Reduction

of salt by increasing the clarifier efficiency may represent

the most viable and economical option. To enhance the

clarifier efficiency the most important parameters to control

are temperature and pH.

Downtime After the Standby Evaporator (2005–2010)

Table 5 shows the downtime of Sennar sugar factory for

the period 2005–2010 when the standby evaporator was

installed and fully commissioned. It is clear that the

downtime has been reduced significantly from the year

2005 on; it varies from 20.2 to 10.6% with an average of

14.6%. The downtime to evaporator cleaning is completely

eliminated. The remaining downtime is due to no cane and

preventive maintenance. It can also be seen that the no cane

downtime is also significantly reduced. This is due to the

supply of adequate number of field machineries such as

harvesters, transportation and loading systems.

Potential Savings

Table 6 shows the production data averaged for each

decade of operation before and after the installation of the

standby evaporator. The comparison will be made between

the last period before the installation (2000–2004) and the

period after the installation (2005–2010) of the standby

evaporator. It can be seen that even before the installation

of the standby evaporator, the no cane downtime has

already improved significantly. This is due to the imple-

mented measures indicated early such as provision of

harvesters and maintenance programs. After the installation

it can be seen that the improvement is significant in the

technical downtime. The no cane down remained almost

constant. The technical downtime is reduced from 20.3%

(2000–2004) to 9.5%. The standby evaporator has sub-

stantial impacts on the crop period; it is cut by 15 %

(30 days). Accordingly the sugar recovery is improved by

0.2% (from 9.3% to 9.5%) or enhancement by 2.4%.

Table 7 shows the bagasse and sugar saved due to

improvement in the technical downtime. The bagasse saved

in 10 days (downtime used to clean the evaporator) is

14379.4 ton. The sugar savings due to recovery improve-

ment is 6361.2 ton/season. The lowest price of $15/ton

0

5

10

15

1960 1970 1980 1990 2000 2010

Year

GuniedHalfaSennarAssalaya

Su

gar

Rec

ove

ry %

Fig. 3 Sugar recovery

Table 4 Properties and composition of evaporator scale

Property Sample number Unit

1 2 3 4 5

pH 9.52 9.38 11.95 12.08 9.52

EC 1.13 1.29 4.67 6.38 8.16 S/cm

TOC 3.04 1.44 2.56 2.48 1.62 %

OM 5.38 2.55 3.41 4.39 2.87 %

N 0.05 0.05 0.04 0.03 0.03 %

P 0.84 0.78 0.82 3.28 0.96 %

K 1.57 1.55 1.98 1.08 1.19 ppm

Ca 546.2 541.4 614.2 605.7 599.1 ppm

Mg 9.34 9.15 7.49 3.69 2.47 ppm

Na 21.44 12.49 12.03 11.66 19.53 ppm

Mn 1.72 0.94 \DL \DL \DL ppm

Fe \DL \DL \DL \DL \DL ppm

Cu 9.56 5.39 3.89 \DL \DL ppm

Zn 4.64 9.31 2.54 0.66 0.67 ppm

Ni 0.16 0.02 \DL \DL \DL ppm

Pb \DL \DL \DL \DL \DL ppm

pH is measured in 1:5 ratio, EC in mg/cm

TOC total organic carbon, OM organic matter, \DL below detection

limit

Sugar Tech (July-Sept 2011) 13(3):179–185 183

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bagasse is considered. The profit in sugar is taken as

$50/ton (direct communication with SSC). The cost of the

evaporator including capital and installation cost for the

base year 2003 is $1502059 (Personnel communication

with SSC). The total saving (bagasse ? sugar recovery)

yields a payback period of 2.8 year. As a simple indicator

this payback period is economically feasible.

Steam Consumption

The operation period between successive cleaning of

evaporator is already shown in Table 3. It can be seen that

before the installation of the standby evaporator the oper-

ation period between two successive cleaning times extend

from 16 to 21 days. Figure 4 shows the operation period

after the installation of the standby set of evaporators. First

the cleaning downtime is completely eliminated; as one set

in operation the other set is in scale cleaning. Secondly the

operation time before cleaning is reduced to 5–12 days by

each set instead of the old practice of 16–21 days. The

Table 5 Operation and downtime after the installation of new set of evaporator

Season Cane Sugar Down time (%) Furnace oil (ton)

Year Day Crushed (9103 ton) Production (9103 ton) Tech No cane Total

2004 2005 182.00 835.4 72.4 19.24 0.93 20.17 14.00

2005 2006 184.00 894.2 80.6 13.50 3.70 17.20 256.00

2006 2007 175.33 941.3 92.0 6.80 4.87 11.67 160.00

2007 2008 164.67 871.4 85.5 7.26 5.55 12.81 0

2008 2009 168.50 894.6 87.1 5.91 4.71 10.62 0

2009 2010 151.25 768.7 76.6 4.52 10.75 15.27 0

Average 170.96 9.54 5.09 14.62

Table 6 Production data in Sudanese Sugar Factory (1976–2010)

Seasons Cane Bagasse

(ton/day)

Sugar Down time (%)

Production

(9103 ton)

Recovery

(%)

Tech

(%)

No cane

(%)

Total

(%)

Year Days Crushed

(9103 ton/year)

1976 1980 170.0 351.6 736.8 26.5 7.6 33.6 7.7 41.3

1981 1990 209.0 567.5 964.8 47.2 8.3 33.9 8.1 42.0

1991 2000 204.3 536.2 916.4 48.4 8.3 31.9 9.2 41.1

2001 2004 201.2 821.9 1437.9 76.0 9.3 20.3 6.3 26.6

2005 2010 171.0 867.6 1779.3 82.4 9.5 9.5 5.1 14.6

Enhancement 30.2 457.1 341.3 0.3 0.2 -10.8 -1.2 -11.9

% 15.0 5.6 23.7 23.7 2.4 -53.0 -18.6 -44.9

Table 7 Bagasse and sugar savings

Item Ton per year Price ($/ton) Dollar savings

Bagasse 14379.4 15 215691

Sugar 6361.2 50 318059

Total dollar savings 533750

Capital cost of evaporator ($) 1502059

Payback period (year) 2.81

0

20

40

60

80

0 5 10 15 20 25 30

Operation (Day)

Brix in

Brix out

Old

Set

Old

Set

Sta

nd

by

Sta

nd

by

Bri

x

Fig. 4 Operation time after the installation of standby evaporator

(15 January to 14 February 2011)

184 Sugar Tech (July-Sept 2011) 13(3):179–185

123

apparent advantage of short operation time is the main-

taining of steady syrup brix (referred to as brix out in

Fig. 4). The prolonged operation time has adverse effect on

steam consumption. As the time progresses the scale

buildups in the evaporator. The scale increases the thermal

resistance to heat transfer hence high steam consumption is

needed to maintain the syrup brix level.

Table 8 shows sample of operation data of two succes-

sive runs of each set. The data is averaged over the test run.

Table 9 shows the evaporator total vapor, steam con-

sumption, economy and evaporation capacity. The evapo-

ration capacity is defined as amount of vapor withdrawn

per surface area of evaporator. These parameters are cal-

culated using material and energy balances.

The steam economy of the standby set is better than the

old set. This is may be attributed to aging problems. The

recommended level varies between 3.0 and 3.8. Evapora-

tion capacity ranges between 7.0 and 8.0 kg/h m2.

Conclusion

The work provided historical data on Sudanese sugar

industry. The data covered the crushed cane, downtime,

sugar recovery, fuel consumption for the period from 1963

to 2010. The work also reports the experience gained by

using a standby set of evaporator. The analysis concluded

that the standby evaporator is an effective energy conser-

vation measures as it significantly reduce the downtime

with direct consequences on bagasse savings, sugar

recovery and steam economy.

Acknowledgement The authors acknowledge the assistance pro-

vided by the Sudanese Sugar Company (SSC) in term of data and

facilitation of visits to Sennar Sugar factory.

References

International Sugar Organization. 2009. Cogeneration—Opportunities

in the world sugar industries. In MECAS 05.

Prodanic, B.B., A.I. Jokic, J.Ð. Markovic, and Z.Z. Zavargo. 2008.

Improving the economic performance of the beet-sugar industry.

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and power system in the Mauritian cane sugar factories. In

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Table 8 Operation data of quadruple effect evaporator (runs 1 and of standby and 3 and 4 of existing evaporator)

Run no. Feed I IV

Calandria Body Bx Body Bx

F (t/h) T (�C) Brix P (kg/cm2) T (�C) P (kg/cm2) T (�C) Out VAC (cm Hg) T (�C) Out

1 130 120 20.3 0.57 113 0.27 106 24.8 56.8 65.2 57.2

2 170 120 20.7 0.75 116 0.33 108 27.8 58.3 63.4 62.7

3 131 120 17.9 0.74 116 0.30 107 27.8 57.0 65.0 62.8

4 125 120 19.3 0.60 113 0.23 106 25.1 54.0 68.3 60.8

Table 9 Evaporator performance parameters

Evap. Run no. V1 (ton/h) Vt (ton/h) ks (kJ/kg) k1 (kJ/kg) FCp(Tf - T1) (kJ/h) Ms (ton/h) E E. coeff. (kg/h m2)

Standby 1 23.7 84.2 2223.3 2240.6 7245.2 27.1 3.1 7.38

2 43.3 113.7 2214.1 2236.9 8485.2 47.6 2.4 9.97

Existing 3 46.7 93.5 2214.6 2238.7 6893.7 50.4 1.9 8.91

4 29.0 85.6 2221.8 2243.1 7443.0 32.6 2.6 8.15

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