Case study of water and wastewater minimisation in a citrus plant

This case study describes a water and wastewater minimisation study carried out for a citrus plant located in Argentina (Thevendiraraj et al., 2003) using pinch technology (Wang and Smith, 1994) and its extensions (Kuo and Smith, 1998). Citrus juice processing plants utilise large quantities of fresh water. The objective of this study was primarily based on reducing the overall freshwater consumption and wastewater produced in this plant.

Background of citrus plants

The main products are citrus fruit juice (in concentrated form) with essential oil and dehydrated peels as by-products. The raw material to the plant is fresh fruit and this is subjected to a series of processes. The citrus processing plant is categorised into the following processes (UNIDO, 1969 and Fig. 5.7): (1) selection and cleaning; (2) juice extraction; (3) juice treatment; (4) emulsion treatment; (5) peel treatment.

Water pinch analysis

Water minimisation was achieved by maximising water reuse and by the identification of regeneration opportunities. Water-using operations were characterised by the maximum inlet and outlet contaminant concentrations, which are dictated by equipment corrosion, fouling limitations, minimum

Fresh fruit

Fresh fruit

Essential Concentrated Dehydrated oil lemon juice peel

Fig. 5.7 Schematic flow diagram of a typical citrus plant.

Essential Concentrated Dehydrated oil lemon juice peel

Fig. 5.7 Schematic flow diagram of a typical citrus plant.

mass transfer driving forces and limiting water flowrate through an operation. Targets determined the minimum freshwater requirement using the limiting composite curve and water distribution network. Pinch graphical methods were based on single contaminants and were further extended to multiple contaminants. When dealing with a number of operations, multiple contaminants and multiple water sources, the problem becomes more complex and algorithms using the basic principles have been developed and can be solved using mathematical programming as described by Smith (2005).

Data extraction

The first step is data extraction: each stream is characterised by its contaminant concentration, inlet and outlet concentration levels, and limiting flow-rate through each operation. The data are provided by Lithoral Citrus Plant Authorities (2002) in a schematic flow diagram of the citrus plant (Fig. 5.7), a simplified water distribution network and the mass balance of the water streams in the citrus plant. Eleven freshwater-using operations are identified. The main limiting contaminant for this analysis consists of the COD, because it makes up the largest contaminant in the majority of the water streams and exhibits significantly high values. The overall mass balance is closed with a simplistic assumption of 1 t/h as evaporation losses from the steam system to account for an inconsistency of 1 t/h of water. The data extracted included the current inlet and outlet contaminant concentrations together with the water flowrate through each operation accounting for water gained and/or lost from the process. The total mass load picked up by the fresh water through each operation was then calculated. The 11 water-using operations with the water flowrates entering and exiting each operation are represented in the form of a simplified water network as shown in Fig. 5.8.

The freshwater COD concentration level in the plant is 30 ppm. There is existing reuse of water between processes currently in the plant and these water reuse streams have been left unchanged. The simplified water network presented in Fig. 5.8 shows the freshwater-using operations with existing water reuse streams 'in-built' within each identified operation. The current total fresh water consumed and waste water generated in this citrus plant were 240.3 and 246.1 t/h, respectively.

Water pinch targeting

The existing water network provided a base starting point for the water pinch analysis. The freshwater target was evaluated by the composite curves. The maximum concentration levels were based on the constraints and limitations dictated by process conditions and requirements. The data were represented in the WATER software (2002, 2006) with identified constraints. The WATER software creates a solution to the defined problem using mathematical programming based on water pinch technology

124G.318 I t/h

Fresh water to plant

Packing (1)

Treatment plant (potable)(2) -

Boiler

Loss to t/h environment I 0 |t/h

Selection/cleaning (3)

Blowdown+ condensate losses neglected

Overflow as WW I 47.558~lt/h^~ 1137.558It/h

I 28.093 |t/h Gain from process 1118.093It/h,

APV condenser

Screen 2 and 3 and

and green tank (4)

Vincent press

Vacuum pump (5)

Finisher

I———| Condensate from —15—lt/h distiller stream u

Gain from process

Distiller (8)

Condensate from juice vapour

Distiller condenser and washing spiral 1 (10)

|| 0-476 lt/h Gain from process

Waste water From plant

Total raw water Total waste water Difference

¡240-3181 Total gain from process | 37-391 I

1246-0541 Total losses to process | 31-655 |

5-736 Difference 5-736

Fig. 5.8 Existing water network (simplified).

principles. The program uses a superstructure that includes all feasible structures and linear programming with feedwater minimisation as the objective function. The automated design allows user interaction through specification of additional constraints that are not part of the formulation. The process restrictions on the water type permissible for each operation indicate that operations 2, 4, 5 and 10 require only fresh water as feed (Fig. 5.9). Consequently the minimum fresh water required by the plant operations is 164.4 t/h.

Recycle, 0.83 t/h

4.8 t/h

Operation 7

0.79 t/hi

Operation 6

Fresh water 169.3 t/h

Operation 1

Loss to process, 6.265 t/h

Recycle, 23.7 t/h

Operation 2

2.45 t/h l

K

Operation 3

Loss to process, 4.8 t/h

Loss to process, 4.8 t/h

Operation 4

Recycle, 23.7 t/h

Recycle, 0.83 t/h

Gain from process ^0.48 t/h

Operation 11

Gain from process _^28.09 t/h

Operation 4a

Operation 5

Loss to process, f 10.95 t/h

Operation 9

113.72 t/h

Net gain from process, ^28.09 t/h

Operation 2a/8

Loss to process, f 8.64 t/h

Operation 10

Waste water

175.05 t/h

Fig. 5.9 Water network after pinch analysis as a conventional diagram.

Operation 1 is a batch process and although it allows water reuse at its inlet operation, in this analysis it is assumed that fresh water is continuously available for this operation at any one time. The total minimum freshwater requirement for this plant is 164.4 + 0.5 = 164.9 t/h. The current total freshwater feed to the plant is 240.3 t/h. The maximum theoretical freshwater reduction that is achievable is 31.4%.

The water pinch analysis is then carried out for the existing water network with maximum concentration levels. The overall freshwater target is calculated using the maximum reuse analysis. The water network, represented as a conventional diagram, and the limiting composite curve are presented in Figs 5.9 and 5.10, respectively. The freshwater target is compared with the current freshwater consumption to evaluate the overall water and

0.30E+05

0.25E+05

0.15E+05

0 100 000 200 000 300 000 400 000 500 000 600 000 700 000 800 000 Mass (g/h)

Fig. 5.10 Limiting composite curve generated by WATER software.

wastewater minimisation for this plant. The existing network results in a freshwater demand of 169.3 t/h and a wastewater flowrate of 175.1 t/h. This shows substantial reductions in water and wastewater minimisation, but the design includes reuse of certain streams that require treatment. The analysis is based on COD as the main contaminant in the reuse streams. These streams may contain other contaminants such as solid waste and chemicals in small amounts that may require further treatment prior to their being used for other processes. Further design options need to be developed that deal with process requirements, operating conditions and water reuse suitability.

These reductions may be achieved by introducing further constraints to potential reuse streams, and by utilising the maximum water reuse analysis to obtain optimised designs with improved freshwater targets that meet all process operating conditions and restrictions. The regeneration reuse analysis can also be utilised to explore further design options that allow reuse of regenerated water in some operations. This analysis requires installation of a treatment unit to regenerate waste water, e.g. gravity settling, filtration, membranes, activated carbon system, biological treatment, etc. This further reduces the freshwater target and wastewater generation in the plant compared with the maximum water reuse analysis and was analysed by the WATER software.

Four different design options were generated considering both the maximum reuse analysis and regeneration-reuse analysis. Design options A and B were based on the maximum reuse analysis with both achieving a freshwater consumption of 188 t/h compared with the actual freshwater consumption of 240 t/h, reducing freshwater use/wastewater generation by approximately 22% (Fig. 5.10). There is no further scope for water reuse

x-x Limiting composite s n-n Water supply line /

0 100 000 200 000 300 000 400 000 500 000 600 000 700 000 800 000 Mass (g/h)

Fig. 5.10 Limiting composite curve generated by WATER software.

due to process limitations and restrictions. The finding at the diagnostic stage indicates that the theoretical maximum freshwater/wastewater reduction is 31%. A further reduction in freshwater/wastewater is achievable by regenerating waste water and reusing it in other operations. Design options C and D are generated based on the regeneration reuse analysis, both resulting in total freshwater consumption of 169 t/h, which is equivalent to a 30% reduction compared with the actual plant consumption.

The reduction in fresh water is achieved by re-routing water streams and therefore requires new pipes. Design option A requires the fewest new pipes (five new pipes), followed by design options B and D with seven pipes each and finally design option C with nine new pipes. These reflect the impact on investment costs. The five new pipes identified for design option A are also required in all of the other options for the same function with similar flowrates. This indicates that design option A requires the lowest investment cost and design option C requires the highest investment cost. Design options C and D both require additional costs for investment in a regeneration unit compared with design options A and B. The results are summarised in Fig. 5.11.

Design options A and D (shown as conventional diagrams in Figs 5.12 and 5.13) are the most attractive options for the maximum reuse analysis and the regeneration reuse analysis. Design option A shows lower freshwater reduction with lower investment cost compared with design option D which results in a higher freshwater reduction and requires much higher investment costs. The waste water generated will also reduce correspondingly with freshwater reduction, hence reducing wastewater treatment operating costs in addition to freshwater savings for each of the options. The cost analysis carried out for design option A shows very attractive financial returns for this low-investment option with a payback period of 0.14 years. The outlet water quality of operation 3 requires further analysis and therefore the investment cost for the regeneration process is not

Freshwater savings

Freshwater savings

Design options

Fig. 5.11 Summary of design options.

Design options

Fig. 5.11 Summary of design options.

Operation 1

Loss to process 6.265 t/h t

Operation 2

Operation 7

> Operation 4

PJPE 26.15 t/h

Operation 3

Operation 6

Gain from process 0.48 t/h

Loss to process 10.95 t/h

Gain from process 28.09 t/h 90 t/h

^Operation 4a

Operation 5

118.09 t/h

Operation 2a/8

> Operation 10

Waste water

New pipes

Fig. 5.12 Design option A: simplified water network as a conventional diagram.

available to permit a complete cost evaluation of design option. Further detailed studies need to be carried out to dictate the regeneration process type required and its associated costs in order to fully evaluate this option.

The thermal energy of the reuse water streams proposed in the respective design options are also reviewed to ensure that stream temperatures at the inlet of operations are unchanged. The operating temperatures for the varying operations, with reference to information obtained from the citrus plant authorities, indicate that almost 90% of the operations operate at ambient temperatures with the exception of operation 8 which produces waste water at 90 °C. This stream is highly contaminated which carries some limitations. All the water reuse streams as suggested in the design options are appropriate in terms of temperature requirements and are not expected to affect the operation thermally. The overall hot and cold utility requirements of this plant will not be affected by the changes proposed in the design options.

Fresh water

Loss to process 6.265 t/h

_PIPE 1j

Operation 1

Operation 2

Loss to process

Operation 7

ik26.15 t/h

PIPE 2 1~62t/n

Operation 4

Operation 3

Regeneration unit

Operation 6

Gain from process 0.48 t/h PIPE 3,

Operation 11

Gain from process 28.09 t/h ' 3877t/h

> Operation 4a

Operation 5

Loss to process ^10.95 t/h

Operation 9

118.09 t/h

Gain from process 7.82 t/h

Operation 2a/8

Loss to process 8.54 t/h

Operation 10

Waste water

New pipes

Fig. 5.13 Design option D: simplified water network as a conventional diagram.

Conclusions

The existing freshwater consumption of the plant is 240.3 t/h with a wastewater generation of 246.1 t/h. The design options result in a reduction of freshwater consumption and a corresponding reduction in wastewater generation of 22% for the maximum reuse analysis and 30% for the regeneration reuse analysis. For a practical project the number of modifications is limited. The maximum water reuse analysis requires a minimum of five new pipes, and the regeneration reuse analysis requires seven new pipes. The analysis shows very attractive financial returns with a payback period of 0.14 years. Further detailed studies need to be carried out to specify the regeneration process type required and its associated costs in order to fully evaluate this option.

Water pinch analysis for this citrus plant shows that reductions in freshwater consumption and wastewater generation of up to 30% can be achieved with minimum changes/investments made to the existing plant.

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