New considerations for modelling a MED-TVC plant under dynamic conditions

The multiple-effect distillation (MED) technology is nowadays the most promising desalination process to be coupled with variable heat sources, thus leading to a more sustainable way to produce water. In order to prove the potential of this, it is of major interest to develop powerful modelling tools to predict the performance of this coupling. Only a few models have been presented so far. They show promising results but were based on some simplifying assumptions and non-physical constraints that could limit the analysis of the dynamic behaviour of a MED plant. This paper presents new considerations for the dynamic modelling of a MED plant associated with a thermal vapour compression unit, starting from a previous work “A dynamic model for MED-TVC transient operation”. After several improvements, this model is now more representative of the real operating modes of a MED-TVC plant by considering real process inputs. This paper also highlights the importance of accurately modelling the interconnection between effects, the evaporation and condensation processes and the other components, such as the pre-heaters. Here is also presented a control strategy for operating a MED plant under dynamic conditions. Indeed, when a perturbation occurs in the motive steam pressure, it is possible to stabilize the whole plant by a simultaneous variation in the intake seawater mass flow rate at the final condenser. The model has been validated in steady-state conditions with experimental data from a MED-TVC plant operated in Trapani (Sicily) and was used to perform dynamic simulation to prove the feasibility of operating a MED-TVC plant under dynamic conditions, which is a major step toward proving the possibility of a coupling with renewable energies.