# Is thermodynamic data of fresh water a good approximation for saline water?

I am currently working on desalination project for my final year. The design is based on flashing of hot liquid in low pressure vessel and then condensing the extracted the steam to produce pure water.

The problem I am facing is that there doesn't seem to be any thermodynamic data (under saturation dome) available for saline water. The need for saturation data is because I need to calculate the required mass flow rate based on daily desalination goal.

For example, if the goal is to produce 100 L/day with average sunlight of 8 hrs, then desalination rate must be 0.0035 kg/s. If I know the quality (percentage of steam produced) of the liquid gas mixture, I can easily calculate the mass flow rate of saline water corresponding to desalination goal.

The only data I could find related to this problem was from a MIT resource Thermophysical properties of seawater, but unfortunately it only provides a single value for enthalpy at any given saturation temperature or pressure.

Right now, I can see only three options:

1. Use steam table, and consider the values an approximation and incorporate the error produced in my design by slightly overdesigning
2. Consider the enthalpy of seawater (from MIT resource) as enthalpy of saturated liquid (i.e. $$h_f$$), and enthalpy of vapor (from steam table) as $$h_g$$ and then perform my calculations
3. Thirdly, I could take the enthalpy of seawater (from MIT resource) as the value that is halfway between $$h_f$$ and $$h_g$$ (from steam table), and then calculate the approximated value of $$h_f$$.

$$h_f=$$ Enthalpy at saturated liquid line; $$h_g=$$ Enthalpy at saturated vapor line

Any help/guidance in this regard will be highly appreciated

• Check out things like miami.pure.elsevier.com/en/publications/… – Solar Mike Feb 16 '19 at 11:36
• This isn't easy and I'm not the guy to answer it. But there are tables of heats of solution (also called enthalpy of dissolution). Basically, there is a nonlinear factor relating to the increasing concentration of the brine, which is highly variable on temperature and concentration, and there is the heat of vaporization at the current brine condition. The trick is to figure out what degree of concentration and decantment optimizes costs. Some free waste heat from other processes is usually needed to make this work. Efficient heat recovery from the waste brine is crucial If you are buying heat. – Phil Sweet Feb 16 '19 at 13:12
• For lookup terms, See Multistage Flash Desalination (MDF), Multi-effect Desalination (MED), and see this document - researchgate.net/publication/… – Phil Sweet Feb 16 '19 at 13:23
• The MIT page you linked to has 2 papers with correlations (and matlab files), from dkimming the abstracts those cold help you to arrive at the data you need? Did you check them, wha'ts missing? – mart Feb 16 '19 at 20:37
• I usually use EES software, and there was some error with the file provided and the library was not recognized by the program. Following your advice, I downloaded the MATLAB files and it seems that all the data is there. – Uzair Feb 16 '19 at 21:20

$$h_f$$ can be approximated from enthalpy of sub-cooled liquid near saturation temperature using SW_Enthalpy function and the value of $$h_{fg}$$ can be easily obtained by using the function SW_LatentHeat.