(191h) Simulation of the Release of Non-Condensable Gases in Seawater Evaporation

As potable water is becoming scarcer and scarcer around the world, there is an increasing interest in different desalination processes. A sophisticated sea water model is needed in order to simulate the reactive processes during desalination. The most common modeling approaches of sea water are based on salinity: models describe the properties of sea water as a function of temperature, pressure, and salt fraction. Some efforts have also been done in order to represent sea water based on its true solute composition. However these models are mainly developed for temperatures below 25 ° C.

A multi-phase model of sea water applicable for the simulation of desalination processes in elevated temperatures was developed in this study. The thermodynamic model was based on the published Pitzer parameters. The applicability of the model was evaluated for a multi-stage flash (MSF) desalination process. The multi-phase system of sea water typically includes species of Na, Ca, K, Mg, Cl, and S. In the present work total of four (4) species in the gaseous phase, thirty-seven (37) species in the aqueous phase, and five (5) solid phases were defined. Process temperatures in the multi-stage flash evaporation range from 110 down to 40 ° C and pressures from 130 kPa down to 10 kPa. The model extrapolates the Pitzer parameters to the evaluated temperatures up to 110 ° C.

The results of the study were promising: the multi-phase model of sea water could predict the CO₂ release in evaporation and scale formation due to precipitation. The amount of CO₂ released depends on the carbonate reactions in solution. Such reactive processes can in general be described better using multi-phase models rather than equation of state models. The model also predicted the composition of typical scales in MSF process. The results explain the phenomena usually observed in MSF processes, such as need for venting gases and scale formation. The results encourage continuing the development of more precise sea water models at elevated temperatures in order to fully understand the chemical phenomena in MSF and reverse osmosis desalination plants.