3 replies to this topic

[mieko]

Please help, I'm having confusions about water distillation process..I just want to ask guidance about the calculations, I've found out that distilled water can be produced via flash distillation, however, I do not have equilibrium data needed for the calculation. The main objective of the process is to remove the minerals from water, so do I regard this as a binary system of water and impurities? Do I need vapor pressures of the water components? Can I design flash distillation unit without plotting the equilibrium line? Thank you and please help

[ankur2061]

mieko,

Refer the link provided below. It discusses isothermal flash using a flash vessel with a further link provided for flash evaporation inMilton Beychok's response. The equilibrium K-values for many compounds are provided in the chart in the spreadsheet attached with this post:

http://www.cheresour...3896#entry33896

Hope this helps.

Regards,
Ankur.

[kkala]

1. A useful description of water desalination by flash distillation can be found in http://en.wikipedia....sh_distillation. Following points are also understood:
α. Approx 15% of sea water feed is desalinated, rest is rejected as brine.
β. Due to scale formation, sea water is preheated below 120 oC.
γ. Heat consumption is about 90 MJ/ton of desalinated water.
2. Attached "dessal.xls" is a try to indicate some way of calculation through a specific example of 5 successive flash stages and a steam heater, treating 14.4 ton/h of sea water. Basis is the web reference referred above. The calculation is subject to criticism by others, since it has been made for first time, probably with some misunderstandings. It reminds multistage evaporation calculation, without need for equilibrium data, only steam properties and flash evaporation of water is used.
3. Way of calculation is: Assume operating pressures, linearly decreasing from stage to stage , which also specifies temperature of each stage. Assume heat transfer area per stage, estimate heat transfer coefficient (same for all stages); estimate temperature of sea water (through heat transfer), passing from the coils of each stage, then heat duty of the exchanger. Hot SW returns to the system, flashing at each stage. Flashing steam is condensed to desalinated water. Relevant latent heat is given to to sea water as above, - correct heat transfer area accordingly.
Finally change of parameters can be tried to approach operating parameters of para 1, which is not realized in the example. Taking example from evaporation, heating surface per stage had better be equal.
4 Above calculation scheme has weak points. Assumption of pressure linearly decreasing is under question. Sea water is considered same as pure water (no boiling point elevation). Calculations could be more precise, there may be errors, etc, etc, yet it may be more useful than a generic explanation, being very difficult in this case. Hopefully it is useful.

Note: "dessal.xls" uses water97_v13.xla of IAPWS - IF97, http://www.cheresour...water-and-steam. In case this is not installed and some cell values are not legible, see the sheet "values" of 2nd page of spreadsheet, but then steam properties will not be automatically calculated.

Attached Files

•  dessal.xls   60.5KB   27 downloads

Edited by kkala, 23 December 2011 - 05:01 PM.

[mieko]

Thank you so much for the help Sir ankur2061 and Sir kkala! I would study all the references you gave me. Thank you!