Recycling/reuse

Recycle/reuse involves the use of pollutant-laden streams within the process. Typically, separation technologies are key elements in a recycle/reuse system to recover valuable materials such as solvents, metals, inorganic species, and water. 

The various waste-management options namely, source reduction, recycle/reuse, treatment, and disposal. 

Which streams should be recycled/reused To what units ... 

Hamad, A. A., Varma, V., El-Halwagi, M. M., and Krishnagopalan, G. (1995). Systematic-integration of source reduction and recycle reuse for the cost-effective compliance with the cluster rules. AIChE Annu. Meet., Miami. 

Dunn, R. F., and El-Halwagi, M. M. (1993). Optimal recycle/reuse policies for minimizing the wastes of pnilp and papier plants. Environ. Sci. Health A28(l), 217-234. 

Pollution prevention practices have become part of the U.S. National Pollutant Discharge Elimination System (NPDES) program, working in conjunction with best management practices (BMPs) to reduce potential pollutant releases. Pollution prevention methods have been shown to reduce costs as well as pollution risks through source reduction and recycling/reuse techniques.5... 

Primary metals manufacturing operations have experienced source reduction and recycle/reuse benefits similar to those available to metal finishing operations, including conserving waters through countercurrent rinsing techniques, and utilizing electrolytic recovery, customized resins, selective membranes, and adsorbents to separate metal impurities from acid/caustic dips and rinsewaters to thereby allow for recycle and reuse. 

Figures 4.1 and 4.2 depict the superstructures on which the mathematical model is based. Figure 4.1 represents a situation where reusable water storage does not exist. In this situation, water used in each water using operation j can be supplied from the fresh water header, the recycle/reuse water header or a combination of both headers. Water from each operation j can be recycled to the same operation, reused in downstream processes and/or dispensed with as effluent. On the other...

Constraints (4.18) states that the inlet stream into any operation j is made up of recycle/reuse stream, fresh water stream and a stream from reusable water storage. On the other hand, the outlet stream from operation j can be dispensed with as effluent, reused in other processes, recycled to the same operation and/or sent to reusable water storage as shown in constraints (4.19). The inlet concentration into operation j is the ratio of the contaminant amount in the inlet stream and the quantity of the inlet stream as stated in constraints (4.20). The amount of contaminant in the inlet stream to operation j consists of the contaminant in the recycle/reuse stream and the contaminant in the reusable water storage stream. The following storage specific constraints are also imperative for the completeness of the model for scenario 3. 

Constraints (4.27) states that if water is recycled from operation j to operation / at a given time point p, then operation / should commence at time point p. However, the fact that operation / commences at time point p does not necessarily mean that there is a corresponding recycle/reuse stream at time point p. This is due to the fact that operation / could be using freshwater instead of recycle/reuse stream. Constraints (4.28) and (4.29) together ensure that water recycle/reuse from operation j to operation / coincides with the completion of operation j at time point p. Similarly, constraints (4.30) and (4.31) ensure that water recycle/reuse from operation jto operation/ coincides with the start of operation/ at time point p. Constraints (4.32) states that any operation j will start after the previous task in the same operation j is complete at time point p. Constraints (4.33) and (4.34) respectively state that if an operation j starts or ends at two distinct time points, then the later time... 

Constraints (4.38) and (4.39) state that when water stream is transferred from storage to any operation j for reuse, then the time of transfer must coincide with the start of operation j. Constraints (4.40) ensures that whenever a water stream is transferred from storage to operation j at time point p, then operation j must commence at time point p. However, operation j can start at time point p even if there is no reusable water stream transferred from storage, since water could be received from recycle/reuse and fresh water streams. 

Figure 4.9 shows that 1767.84 kg of freshwater is required over the 7.5 h time horizon. This corresponds to 25% reduction in freshwater requirement compared to the situation without water recycle/reuse. Although water from process A is at a relatively lower concentration of 0.1 kg salt/kg water, the time constraints in the absence... 

The formulation for this scenario entails 1411 constraints, 511 continuous and 120 binary variables. The reduction in continuous variables compared to scenario 1 is due to the absence of linearization variables, since no attempt was made to linearize the scenario 2 model as explained in Section 4.3. An average of 1100 nodes were explored in the branch and bound search tree during the three major iterations between the MILP master problem and the NLP subproblem. The problem was solved in 6.54 CPU seconds resulting in an optimal objective of 2052.31 kg, which corresponds to 13% reduction in freshwater requirement. The corresponding water recycle/reuse network is shown in Fig. 4.10. 

The overall model for this scenario involves 5614 constraints, 1132 continuous 280 binary variables. Three major iterations with an average of 1200 nodes in the branch and bound search tree were required in the solution. The objective value of 1560 kg, which corresponds to 33.89% reduction in freshwater requirement, was obtained in 60.24 CPU seconds. An equivalent of this scenario, without reusable water storage, i.e. scenario 2, resulted in 13% reduction in fresh water. Figure 4.12 shows the water recycle/reuse network corresponding to this solution. 

Fig. 4.14 Water recycle/reuse network for the second case study - scenario 1 (Majozi, 2005)...

The objective is then to determine the production schedule that generates the least amount of effluent through the exploitation of wastewater recycle/reuse. It should be emphasised that recycle in this context refers to wastewater being used by the same unit from which it was produced and reuse refers to the usage of wastewater in a different unit to which it was produced. 

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