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Problem with kinetic of precipitation/dissolution


Marek Svab
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Dear GWB team,

 

I am still learning how to effectively work with the GWB system. Now I was trying to deal with some kinetic effect. I started with simplest possible task (only for my understanding of the program). I wanted to calculate precipitation/dissolution of zincite. When I placed (in react) zincite as kinetic reactant, set surface area and kinetic constant, it was ok - I obtained desired curve in time (how zincite precipitated during the time).

However, I prefer possibility of inclusion of kinetic effect onto species presented in the basis (or created by some reactions later in X1t). For this purpose I prepared script enclosed to this message (you have to use the thermodataset "thermo.com.v8.r6+.dat"). The task is simple - we have 2000 mg/kg of zinc in solution with pH 8. It is clear zincite is supposed to precipitate under such conditions, only zincite is allowed as mineral. Without kinetic, zincite is precipitated. But when kinetic was included (the same parameters which were working correctly in my previous task, only nucleus density was added because there was no mineral in the system at the beginning), the results were absolutely non-understandable. Please look at the results of the enslosed script. React correctly predicts zincite as mineral for precipitation (0.00193 moles - ok), includes it as kinetic reactant, but it also immediatelly writes that the same amount of zincite already reacted (at the first kinetic result). This situation remain the same within entire time period. This is why that no meaningful results were obtained. I was trying to find what is the problem for several hours, but I really do not know what to do with this. I was trying also various changes in the script, but without success. Since I want to work with the kinetic effects in the future, I must understand such simple tasks first. Could you help me with this?

Thank you!

Best regards,

Marek Svab

precipitation_of_zincite.rea

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Dear GWB team,

 

I am still learning how to effectively work with the GWB system. Now I was trying to deal with some kinetic effect. I started with simplest possible task (only for my understanding of the program). I wanted to calculate precipitation/dissolution of zincite. When I placed (in react) zincite as kinetic reactant, set surface area and kinetic constant, it was ok - I obtained desired curve in time (how zincite precipitated during the time).

However, I prefer possibility of inclusion of kinetic effect onto species presented in the basis (or created by some reactions later in X1t). For this purpose I prepared script enclosed to this message (you have to use the thermodataset "thermo.com.v8.r6+.dat"). The task is simple - we have 2000 mg/kg of zinc in solution with pH 8. It is clear zincite is supposed to precipitate under such conditions, only zincite is allowed as mineral. Without kinetic, zincite is precipitated. But when kinetic was included (the same parameters which were working correctly in my previous task, only nucleus density was added because there was no mineral in the system at the beginning), the results were absolutely non-understandable. Please look at the results of the enslosed script. React correctly predicts zincite as mineral for precipitation (0.00193 moles - ok), includes it as kinetic reactant, but it also immediatelly writes that the same amount of zincite already reacted (at the first kinetic result). This situation remain the same within entire time period. This is why that no meaningful results were obtained. I was trying to find what is the problem for several hours, but I really do not know what to do with this. I was trying also various changes in the script, but without success. Since I want to work with the kinetic effects in the future, I must understand such simple tasks first. Could you help me with this?

Thank you!

Best regards,

Marek Svab

 

Hi Marek:

 

Your script is fine, you've just set a rate constant that is too fast or a time length that is too long. Here is your input:

 

  • Nucleus density = 1 cm2/cm3. The nucleus density prescribes a minimum surface area value for a supersaturated mineral. You've prescribed that for 1 cm3 of fluid, 1 cm2 Zincite surface area is available for precipitation. Given a 1 kg (~1L) solution, the minimum surface area (As) throughout the calculation will be 1000 cm2.
  • rate constant (k+) = 1e-6 mol/cm2 sec
  • initial Zincite equilibria, very supersaturated; log Q/K ~ 2.85

 

In GWB, the kinetic rate (r, mol/sec) for mineral precipitation/dissolution is given as: r = k+ * As * (1-Q/K)

 

Thus, in your script: r = 1e-6 * 1000 * (1-700) ~ -.7 mol/sec (a negative kinetic rate indicates mineral precipitation).

 

From the equilibrium speciation, we know that this solution will precipitate .0019 moles of Zincite at equilibrium. Given the rate we've calculated, this all happens within the first second (or first reaction path step).

 

If you set a rate constant of 1e-14, you'll see a precipitation curve distributed over your chosen simulation time- I have attached an example of this. You might also consider dropping your nucleus density to lower your minimum available surface area.

 

I hope that helps,

 

Tom Meuzelaar

RockWare, Inc.

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Hi Tom,

 

thanks for fast response.

Yes, it is clear... and simple reason.... :unsure:

But I would like to ask you ... even the curve is ok now...:

Look at the text output (I studied only text output before, this is also why I missed the reaction rate...).

I enclose one part (after 60 days):

 

moles moles grams cm3

Reactants remaining reacted reacted reacted

----------------------------------------------------------------------------

Zincite 0.001873 -0.001873 -0.1524

 

Kinetic rate const. sp. surface net surface rxn rate

mineral rxns (mol/cm2 s)* area (cm2/g) area (cm2) (mol/kg sec)

----------------------------------------------------------------------------

Zincite 1.000e-014 80.00 12.19 -2.814e-011

*Units in absence of promoting or inhibiting species.

 

No minerals in system.

-----------------------------------------------

What does it mean, that "moles remaining" is the same like "- moles reacted". Perhaps it only expresses some simple fact that zincite is precepitating toward equilibrium.

However, It is not absolutely clear to me... (onoly from about 70 % :rolleyes: ). Is it so that "moles remaining" means in our case "zincite already in the system", while "moles reacted" means "zincite precipotated till this time" - this is why the numbers are equal (-). Also - why there is mark "no minerals in system" - when zincite is already preciptated (even not to equilibrium amount). May be it has some principal/software reasons, but now it is, in my opinion, a bit confusing, isn't it?

Many thanks!

Marek.

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Hi Marek:

 

What does it mean, that "moles remaining" is the same like "- moles reacted". Perhaps it only expresses some simple fact that zincite is precepitating toward equilibrium.

However, It is not absolutely clear to me... (onoly from about 70 % :rolleyes: ). Is it so that "moles remaining" means in our case "zincite already in the system", while "moles reacted" means "zincite precipotated till this time" - this is why the numbers are equal (-).

 

The Reactants section in your output file keeps track of incremental mass changes that describe the reaction path you define in the Reactants tab of the React module. Kinetic mineral dissolution or precipitation is one type of reaction path model - in this case, the Reactants section tracks the evolution of kinetic mineral mass along the reaction path. Kinetic mineral dissolution mimics titrating a normal mineral mass into the Basis, since you start with a positive mineral mass in the Reactants tab, and it decreases along the reaction path. Hence it is given a positive rate in the moles reacted column of the Reactants section of the output file. In your case (mineral precipitation), however, you start with zero mineral mass in the Reactants tab- the kinetic mineral mass increases along the reaction path. This is represented by a negative rate in the moles reacted column of the Reactants section of the output file. The moles remaining and moles reacted columns will have the same value throughout your reaction path model. The moles remaining section, in this case, simply reflects the increasing mass of precipitated Zincite.

 

 

 

Also - why there is mark "no minerals in system" - when zincite is already preciptated (even not to equilibrium amount). May be it has some principal/software reasons, but now it is, in my opinion, a bit confusing, isn't it?

 

Agreed- it is a bit confusing. If you look at Rock Mass in the general fluid parameters section for each step of the reaction path, you'll note that there is mineral mass present in the system. The no minerals in system refers to non-kinetic mineral mass.

 

I hope that helps,

 

Tom

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