> 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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