Dynamic simulation of a Multi-Effect Distillation (MED) process

• We proposed a numerical methodology to investigate dynamic simulation of MED.
• The governing PDEs are thoroughly formed, discretized and solved for tube-bundle.
• Transient response of start-up and common failures of MED are studied.
• 200 s failure in main steam will delay return to steady conditions for 550 s.
• Feed water decrease will result in overconcentrated brine with high risk of fouling.

In this paper, dynamic simulation of a Multi-Effect Distillation process has been conducted via deriving, discretizing, and solving PDEs governing its tube-bundle behavior. Possessing a complex process and different heat transfer mechanisms has limited recent researches to steady-state simulations. A detailed examination was performed on existing correlations of falling film evaporation on horizontal tubes and condensation inside them. A couple of appropriate correlations were selected and applied to calculate the transferred heat. Results showed that during start-up and feeding the main steam to the process, vapor quality almost drops linearly inside tubes. Moreover, during initial time steps, steam condenses quickly and forms a mass of subcooled liquid in front of the flow stream which results in a slow-down of flow motion. Dynamic study of the process shows that if an unexpected interrupt occurs during steady-state operation such as failure in feeding main steam for 200 s, the process will last almost 550 s to return to its steady normal condition. Another examination showed that by sudden decrease of feed water to less than the half of its nominal value, brine will be concentrated more than designed quantity and the risk of fouling on tubes will be increased.

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