Minimization of Fresh Water Use 13221 Description of the Problem

A number of different processes must be supplied with water. This water may be used

• for heat transfer processes (cooling, evaporation, condensation, etc.),

• for mass transfer processes (washing, absorption/desorption, distillation/condensation, reaction, etc.).

The rising costs of energy, fresh water and wastewater treatment make it necessary to save both energy and water (Pauli 1997). The following considerations are to be made for water management.

The problem is presented by Fig. 13.6, showing four different water-using processes in parallel, each of them using water for mass transfer processes.

Fig. 13.6 Fresh water use in each of the four processes leading to a fresh water flow rate of 112.5 m3 h-1.

Following the discussion of Wang and Smith (1995), Smith et al. (1994) and Baetens (2002), we will at first assume that 112.5 t h-1 fresh water are used for all four processes. The mass transfer data are given in Table 13.2.

Table 13.2 Example for process data of the process of Fig. 13.7.

Process No. Mass load of contaminant Concentrations Water flow rate j Q Cout Cin Cout Q

30 4

E 41

E 112.S

For process 1 with a given contaminant mass load of Q1c1out = 2 kg h-1 impurities and a given concentration of c1out = 0.1 kg m-3, a flow rate of Q1 = 20 m3 h-1 results. Correspondingly for the mass loads of processes 2, 3 and 4 (Table 13.2), given as 5, 30 and 4 kg h-1, flow rates of 50, 37.5 and 5 m3 h-1 can be calculated, which yields a sum of:

How much water do we need if we do not operate the four processes in parallel and look for a more economical water use?

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