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BondChemBond (Civil/Environmental) (OP)
5 Dec 06 16:56
We have an outdoor storage tank for 40% ferric chloride solution. Looking at various MSDS, I have seen freeze point of ferric chloride solution listed from -10C to -50C. If -10C, we would need some heat tracing, but probably don't have to worry about it if it is lower. So which number is right and are heat tracing generally required for Ferric Chloride storage tank? The design outdoor temperature is 0F.
BigInch (Petroleum)
7 Dec 06 13:27
How is it you have a design temperature of 0ºFarenheit, but you say you need heat tracing at -10ºC?  If your system is supposed to operate at -10º, don't you need a design temperature that goes to -10, or are you shutting down at 0F and only trying to keep the tank from freezing up at lower temperatures?  What do you mean?

I think FeCl freezing temperature can vary with concentration between -50 to -10 and the actual freezing temperature you will have is dependent on the actual concentration of the FeCl solution you have in YOUR tank.

If you don't want to freeze it, you'll need heating.

BigInchworm-born in the trenches.
http://virtualpipeline.spaces.msn.com

kenvlach (Materials)
25 Dec 06 6:03
The literature values vary for the freezing point of 40 wt% FeCl3 solution.  From the more reliable sources, I give the value as -14+2oC.

To understand the variation in freezing point with composition, look at the solubility table & H2O(ice)-FeCl3.6HO phase diagram given by Solvay:  http://www.solvaychlorinatedinorganics.com/docroot/chlo_inorg/static_files/attachments/pch_1610_0007_w_en_ww.pdf

H2O(ice) and FeCl3.6HO form a eutectic, with eutectic point 33-34 wt% FeCl3, -55oC.  So, starting with pure ice, increasing FeCl3 lowers the freezing point from 0 oC to a minimum of -55oC at the eutectic, whereupon further increasing FeCl3 raises the freezing point.  From data in Perry's, 7th Edn., the solubility limit is 42.7 wt% FeCl3 at oC. Interpolation gives a freezing point of -15oC at  40 wt% FeCl3.  Pretty good agreement with the -13.5oC given in the Solvay table.

There are 2 points worth considering:
1)  The slope of the FeCl3.6HO-rich liquidus at the 40% FeCl3 composition is about 5.5oC per wt.%.  
So rather than heating, you can lower the freezing point to -25oC by increasing the water content by 2 wt%.

2) The liquid only partially freezes as long as the temperature doesn't drop all the way to -55oC.  The fraction that solidifies can be calculated from the lever rule for phase diagrams.   Note that FeCl3.6HO is 60 wt% FeCl3.
Example:  40 % FeCl3 solution that begins freezing at -14oC is cooled to -24oC.  The liquidus composition at this temperature is 37.5 wt%.  The weight fraction of solid FeCl3.6HO will be (40-37.5)/(60-37.5) = 0.111 or 11%.
It is unlikely that this slush would damage your system, although it may be difficult to pump.  If the piping is buried in soil, it will be warmer, and thus less problematic.
bimr (Civil/Environmental)
26 Dec 06 9:57
Try contacting your supplier and get the freeze properties directly from him.

Different suppliers have have conflicting freeze properties becasuse they are making their products with different mixtures, perhaps even using byproducts instead of pure chemical. Some may have higher salt concentrations which would affect the freezing point.

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