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Re: PHREEQ Use in Power Plant Chemistry





> Can PHREEQC be used to predict speciation in power plant cooling towers
at
various levels of evaporation? The towers run to saturation, typically, of
calcium carbonate, but other salts of magnesium, silica, iron, phosphorus,
etc. are possible and would limit the amount of water that could be
evaporated.

PHREEQC is a model, so it may differ from reality, but I think it would be
applicable, provided you stay within a couple limitations. One is
temperature, I would use the llnl.dat database if you are above say 40-50C
or at least compare results between phreeqc.dat/wateq4f.dat results and
results with llnl.dat. Another limitation is ionic strength; the ion
association model of phreeqc starts to be less reliable at higher ionic
strengths, it may be more or less, but I usually use seawater as a cutoff
(.7 molal ionic strength). There is also the question of kinetics,
homogeneous aqueous equilibrium is fairly fast, but uptake and release of
dissolved gasses may be slow and the kinetics of mineral formation may also
be slow.

> I read in the manual that PHREEQC will not give a true "path"
thermodynamic
answer in evaporation questions. But, it would seem that the program should
still be useful in a stepwise calculation of what happens as evaporation
progresses.

You can definitely run evaporation simulations, where you remove water from
the system. Usually, the initial solutions have a kilogram of water or
about 55.5 moles. By removing water (probably with REACTION) you simulate
evaporation.

> If PHREEQC can work in this application, can the effect of scale
inhibitors
(phosphonates, acrylates, etc.) be modeled as well?

Again, it works in theory, but my guess is the inhibitors work by
complexing calcium, silica, or other reactants. To model that you need to
know the binding constants (reaction and log K) for these complexation
reactions. PHREEQC databases have some data for phosphate binding, but you
would probably need to add reactions for the specific inhibitors that are
used.

> In some areas of the country the blowdown from the cooling tower is
processed in evaporators and mixed salt crystalizers to produce cake salt.
Can PHREEQC predict the speciation of the product salts?

I'm not sure what you mean here. PHREEQC can model the equilibrium
precipitation of multiple salts. I don't know how the crystallizers work,
but perhaps that just speed up the reactions by adding lots of
crystallization sites. In this case you should approach the equilibrium
mineral assemblage.

> Are there any references or papers that you know of that discuss using
PHREEQC in this manner?

It's new to me, I'm an ivory tower guy not a cooling tower guy. If you need
help setting up some scoping calculations, I could take a look at your
input files. I don't want you to spend a lot of time if the approach isn't
really workable, but I guess I would like to know whether PHREEQC is useful
for this or not.

David





David Parkhurst (dlpark@xxxxxxxx)
U.S. Geological Survey
Box 25046, MS 413
Denver Federal Center
Denver, CO 80225

Project web page: http://wwwbrr.cr.usgs.gov/projects/GWC_coupled




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