A case study sugar processing

Sugar manufacturing is a large industrial sector that has an important economic impact in more than 30 European countries. The total sugar output of Europe is about 30 million tonnes per year. Nearly two-thirds of this amount originates from EU countries, among which Belgium, the Czech Republic, France, Germany, Hungary, Italy, the Netherlands, Poland, Spain and the UK are the largest producers. The remaining one-third of the European sugar output is predominantly supplied by the New Independent States (NIS): Belarus, Ukraine and Russia.

The total energy consumption of the European sugar industries is of the order of 300 000 TJ/year. The energy efficiency differs widely throughout the region. When compared with the 'old' fifteen EU countries, specific energy consumption (per unit mass of sugar produced) in the new EC countries, and especially in the NIS, is more than twice as high.

Beet-sugar production is one of the oldest and most intensively explored branches in the food processing industry with high energy consumption. The heat systems are characterised by a high degree of efficient heat recovery. However, despite this, there is often the opportunity for further improvements (Vaccari et al, 2002, 2004). For example, sugar plants are designed to operate with a minimum temperature difference (ATmin) of 815 °C in the utility pinches due to vapour bleeding from multiple-stage evaporation. Contemporary economic conditions state that the optimal ATmin should be in the region of 4-6 °C. This reduction of minimum temperature difference can therefore lead to an 8-10% reduction in the hot utility consumption and with under an economically justified increase in heat exchange surfaces. The main problem remains the inappropriate placement of the evaporation system as a whole, and particularly the inappropriate placement of vacuum pans due to the low temperatures of the vapours contained within them.

The plant undergoing analysis was producing white refined sugar and confectioneries. The raw material was sugar beet and the plant had medium production levels. The diffusion system was a Decline double-screw extractor DC, equipped with hot juice purification, quadruple evaporation effect and a double-products crystallisation scheme. The plant production flowsheet is given in Fig. 4.12.

The hot utility was dry saturated steam at 136 °C, supplied by a public utility power plant. The designed hot utility consumption was 42.2 kg per 100 kg beet with an actual hot utility consumption of 48-52 kg per 100 kg beet. The required cold utility was provided by an internal spring. The cooling water, however, had a range of temperature, fluctuating within the range of 15-40 °C. Heat transfer was secured by a five-level evaporator in quadruple effect and 15 heat exchangers. The existing heat exchangers are shown in Fig. 4.13, along with the network and the overall process layout. The stream data extracted from the process flowsheet and used for the construction of the hot and cold composite curves are given in Table 4.3.

The composite curves were generated using the SPRINT software and are shown in Fig. 4.14. The related grand composite curves, also generated by SPRINT, are shown in Fig. 4.15.

The initial analysis of the data, using these heat integration techniques, has shown a number of areas for potential improvement. Firstly, there is a considerable excess of hot utility consumption, up to 35%, above the designed value, due to the fact that the plant is operating at a capacity lower than the designed one. Secondly, there is unsteady loading of the vacuum pans, with a low degree of process control resulting in 33 t/h of actual steam consumption compared with 26.4 t/h of designed steam consumption. There is also a considerable surplus of heat exchange area and a low level of heat exchange loading; 5601 m2 are available but only 4317 m2 are employed.

The potential for improvement in energy use in the process was centred on the following: increasing the number of evaporation system effects; increasing the use of available heat exchange through the reduction in the temperature driving forces; better heat integration and overall shifting of the vapours bleeding system towards the last evaporation effects. In

PRESSING

Exhausted slices 22.0 t/h 55 °C

Thick juice 16.9 t/h f

Vapour 61.9 t/h

FIRST COOKING

DIFFUSION

HEATING

PRE-LIMING

HEATING

MAIN LIMING

FIRST SATURATION

HEATING

FILTRATION

HEATING

EVAPORATION

CRYSTALLISATION 116.6 t/h '

CENTRIFUGATION C

White syrup 1.4 t/h

DRYING

HEATING

HEATING

HEATING

72.0 t/h

62 °C

PRE-LIMING

145.1 t/h

145.1 t/h

86 °C

MAIN LIMING

152.3 t/h

' 86 °C

FIRST SATURATION

HEATING

Vapour 61.9 t/h

78.8 t/h

Green syrup 7.2 t/h

SECOND COOKING 17.2 t/h

CRYSTALLISATION I 7.2 t/h

CENTRIFUGATION

I Green crystal 5.0 t/h

AFFINATION

Molasses 2.5 t/h

CENTRIFUGATION

Affin. syrup 2.2 t/h

Fig. 4.12 Sugar plant production flowsheet.

addition, there was the possibility of improving the utilisation of condensate heat and modifying the condensate gathering system.

The suggested retrofit modifications included the transformation of the quadruple-effect evaporation system into a quintuple-effect system through the inclusion of the existing reserve evaporation body. Also included were internal changes in the network structure comprising the rearrangement of the existing heat exchangers (Fig. 4.16). Further, it was also suggested that the condensate gathering system should be changed to a pressure sequenced system. The modified heat exchanger network is shown in Fig. 4.17, and the simplified flowsheet diagram representing the heat exchanger network is

Thin juice

Satur. gas

V

Water F2

Sludge

□ c

tJ

4

jhí ;

tH

5

JHt JM

PR Exh

Exhausted HX15 air Primary

L

condensate

a

n

DR

Sugar

Green crystal

Molasses

White syrup t

Second. condensate

Thin juice

Lime

PR Exh

Second. condensate

Exhausted slices

Water Beet slices

Green crystal

Molasses

Exhausted slices

Water Beet slices o o a

Fig. 4.13 Existing heat exchanger network and overall process.

Table 4.3 The stream data extracted for construction of hot and cold composite curves

Streams

Heat capacity flowrate (kW/°C)

Inlet temperature (°C)

Outlet temperature

(°C)

Heat duty (kW)

Heat exchangers

Cold

1.

Fresh water

58.2

40

65

1 455

HX1

2.

Raw juice

75.9

30

43

1 150

HX2

2.

Raw juice

75.9

43

62

1 271

HX3

3.

Juice liming

153.1

70

82

1 562

HX4, HX5

3.

Juice liming

153.1

82

86

870

HX6

4.

Juice filtrate

86.4

86

88

173

HX7, HX8

5.

Juice satur.

83.8

88

91

251

HX9

5.

Juice satur.

83.8

91

94

251

HX10

6.

Thin juice

83.1

93

100

582

HX11

6.

Thin juice

83.1

100

108

665

HX12

6.

Thin juice

83.1

108

123

1 247

HX13, HX14

7.

Diffusion

-

-

-

1 362

DF

8.

Cooking

LH

75

75

7 716

VP

9.

Air

9.0

15

60

405

HX15

10.

Boil. I eff.

LH

128

128

16 747

EV1

11.

Boil. II eff.

LH

119

119

15 024

EV2

12.

Boil. III eff.

LH

107

107

7 047

EV3

13.

Boil. I eff.

LH

95

95

4 734

EV4

Hot

1.

Primary

27.4

90

75

405

HX15

cond.

2.

Secondary

65.3

90

72

1 150

HX2

cond.

3.

Vap. cook.

LH

65

65

7 017

CO

4.

Vap. I eff.

LH

126

126

15 915

EV

5.

Vap. II eff.

LH

117

117

15 323

EV2

6.

Vap. III eff.

LH

103

103

7 526

EV3

7.

Vap. IV eff.

LH

88

88

4 228

EV4

Abbreviations are: satur., saturation; eff., effect; cond., condensate; boil., boiler; vap., vaporation; cook., cooking.

Abbreviations are: satur., saturation; eff., effect; cond., condensate; boil., boiler; vap., vaporation; cook., cooking.

presented in Fig. 4.18. The updated composite and grand composite curves, obtained using SPRINT, are given in Figs 4.19 and 4.20.

The main achievement of the heat integration retrofit methodology was the reduction of designed steam consumption from 26.4 t/h to 24.0 t/h (9%) without any additional capital costs (beside the cost of re-piping). This was achieved by making use of some low-usage heat transfer area and the re-allocation of some heat exchanger area to more appropriate usage. Moreover, the solution advocated was especially beneficial for a sugar plant that is run for less than five months per annum. The retrofit potential modifications and the benefits of these suggestions are given in Table 4.4.

Enthalpy (kW)

Fig. 4.14 Sugar plant composite curves.

Enthalpy (kW)

Fig. 4.14 Sugar plant composite curves.

140.00 r

140.00 r

0.00 2000.00 4000.00 6000.00 8000.00 10000.00 0.12E+05 0.14E+05 0.16E+05 0.18E+05 0.20E+05

Enthalpy (kW)

Fig. 4.15 Sugar plant grand composite curves.

0.00 2000.00 4000.00 6000.00 8000.00 10000.00 0.12E+05 0.14E+05 0.16E+05 0.18E+05 0.20E+05

Enthalpy (kW)

Fig. 4.15 Sugar plant grand composite curves.

In addition to the retrofit suggestions already considered for the sugar processing plant, further improvements in steam consumption could be achieved through the application of a heat exchanger retrofit analysis and improved process control. If these modifications were adopted they would provide a further 9.0 t/h reduction in the steam consumption (27% compared with the actual consumption). This measure would require a more detailed economic analysis (for some guidelines see Donnelly et al., 2005).

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