Multistage Evaporation

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Multistage evaporation exploits the latent evaporation heat of the solvent vapors from the first stage as a heating medium for a succeeding stage, where it is condensed. Such systems have a reduced energy consumption compared with the traditional single . stage systems, as only the first stage needs to be heated with steam. Vapors generated in the last stage must be condensed by a cooling medium, thus reducing cooling water usage. Figure 7.20 shows an evaporation unit with four stages and heat integration. As a consequence, the energy consumption decreases with the number of stages as shown in Figure 7.21. In a multistage plant, with n numbers of stages, the steam consumption is approx. only 1/n of the consumption of a single stage evaporation unit neglecting thermodynamic non - idealities.

Preheater

Preheater

Stage 1 Stage 2 stage 3 Stage 4

Stage 1 Stage 2 stage 3 Stage 4

Condensate

Figure 7.20 Evaporation unit with four stages and heat integration; co-current flow of liquid and vapor.

Condensate

Figure 7.20 Evaporation unit with four stages and heat integration; co-current flow of liquid and vapor.

The overall temperature difference, which can be divided among the individual stages, is determined by the highest allowable product temperature of the first stage and the lowest boiling temperature of the final stage [15]. As a result, the temperature difference per stage decreases with an increasing number of stages and the heating surfaces of the individual stages must be made larger accordingly

Figure 7.21 Relationship of energy consumption and heating surface as a function of the number of stages.

01 u iS

Number of stages

Figure 7.21 Relationship of energy consumption and heating surface as a function of the number of stages.

to achieve the required evaporation rate, but with a lower temperature difference. Approximately, the total heating surface of all stages increases proportionally with the number of stages. As a result, the investment costs rise whereas the energy consumption decreases with the number of stages (as shown in Figure 7.21).

For a specified temperature difference between the heating steam in the first stage and the boiling point in the last stage, an optimum number of evaporator stages can be found, subject to given economic and operating constraints.

Pumps can be installed to force liquid to circulate in the evaporator, in cases where the temperature difference in an individual stage is insufficient to provide a natural thermosiphon circulation. Forced circulation leads to an improved heat transfer through increased liquid velocity, and therefore reduced heat transfer area.

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