Conceptual Models > Equilibrium assumptions

Batch reactor equilibria liquid/gas

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gdavilao:
I am conducting batch experiments using 3grs of sandstone sample with a liquid (low salinity, 150mL) to head space (150mL) ratio. The head space is hydrogen (10% by volume) blended in nitrogen (90%). The initial conditions are 1000 psi and 80C. I am periodically collected liquid and gas samples for analysis. The gas bottle is always feeding the pressure vessel in which the head space is always constant at 1000 psi.
I am measuring my liquid samples concentrations using ICP-MS and IC and the pH was measure after one day at 25C. The gas sample was measured using GC and give me the ppm by volume that can be converted to a partial pressure.
Then my question here is, if I am selecting fixed_volume option, as:

--- Code: ---SOLUTION 1 SHASTA Exp1-1 step 4 Calculate ppH2
pH  7.233
pe 5.2875     
density  0.9924
redox N(-3)/N(+5)
units  mol/kgw
Na     4.46e-2
N(-3)  4.52e-4 as NH4
Cl     4.560e-02
N(+5)  1.07e-4 as NO3
S(6)   4.19e-4 as SO4
Mg     2.46e-4
Al     4.82e-7
Si     1.00e-7
K      1.46e-3
Ca     5.81e-4
Fe     1.00e-7
C(4)   1.67e-3
O(0)   1.00e-7
N(0)   0.1 N2(g)  1.7871 !logppH(68.05*0.9)
H(0)   0.1 H2(g)  0.8187 !logppH(68.05*0.0968)
GAS_PHASE 1 Fixed-volume gas phase
-fixed_volume
-pressure 68.05
-volume 0.150
-temperature 80.0
H2(g)   6.59
N2(g)  61.46
CH4(g) 0.0
H2S(g) 0.0
CO2(g) 0.0
NH3(g) 0.0
SO2(g) 0.0
O2(g)  0.0
-equilibrate 1
end
--- End code ---

I am not understanding why the results do not report 68.05 of total pressure? it is reporting 67.84 atm. Also why once start to do the batch equilibrium, the total pressure increase and the hydrogen is not 10% anymore. Is the base line conceptual model not right? if I a setting my program a fixed-pressure is even worse and the calculated volume is 0.1 instead of 0.15 L. Does anyone knows if I am setting the liquid/headspace in a bad way?
In addition, if I collecting the liquid samples, this liquid/headspace ratio is changing, even that I have a constant total and partial pressures, right? the volume must be changing?


--- Code: -----------------------------------------------------------
Beginning of initial gas_phase-composition calculations.
--------------------------------------------------------

Gas_Phase 1. Fixed-volume gas phase

WARNING: While initializing gas phase composition by equilibrating:
         Gas phase pressure (67.84 atm) is not equal to solution-pressure (1.00 atm).
         Pressure effects on solubility may be incorrect.

-----------------------------------Gas phase-----------------------------------

Total pressure: 67.84      atmospheres
    Gas volume:   1.50e-01 liters
  Molar volume:   3.61e-01 liters/mole

                                                        Moles in gas
                                            ----------------------------------
Component                log P           P     Initial       Final       Delta

CH4(g)                  -99.99   0.000e+00   0.000e+00   0.000e+00   0.000e+00
CO2(g)                   -2.33   4.672e-03   2.864e-05   2.864e-05   0.000e+00
H2(g)                     0.82   6.587e+00   4.039e-02   4.039e-02   0.000e+00
H2S(g)                  -99.99   0.000e+00   0.000e+00   0.000e+00   0.000e+00
N2(g)                     1.79   6.125e+01   3.755e-01   3.755e-01   0.000e+00
NH3(g)                   -7.25   5.686e-08   3.486e-10   3.486e-10   0.000e+00
O2(g)                    -4.40   3.949e-05   2.421e-07   2.421e-07   0.000e+00
SO2(g)                  -99.99   0.000e+00   0.000e+00   0.000e+00   0.000e+00

-----------------------------------------
Beginning of batch-reaction calculations.
-----------------------------------------

Reaction step 1.

Using solution 1. SHASTA Exp1-1 step 4 Calculate ppH2
Using gas phase 1. Gas phase after simulation 1.

-----------------------------------Gas phase-----------------------------------

Total pressure: 59.13      atmospheres
    Gas volume:   1.50e-01 liters
  Molar volume:   4.14e-01 liters/mole

                                                        Moles in gas
                                            ----------------------------------
Component                log P           P     Initial       Final       Delta

CH4(g)                   -2.31   4.935e-03   0.000e+00   3.026e-05   3.026e-05
CO2(g)                   -6.23   5.876e-07   2.864e-05   3.602e-09  -2.864e-05
H2(g)                    -4.75   1.790e-05   4.039e-02   1.097e-07  -4.039e-02
H2S(g)                   -6.01   9.810e-07   0.000e+00   6.014e-09   6.014e-09
N2(g)                     1.77   5.912e+01   3.755e-01   3.625e-01  -1.302e-02
NH3(g)                   -3.35   4.421e-04   3.486e-10   2.711e-06   2.710e-06
O2(g)                   -73.61   2.457e-74   2.421e-07   0.000e+00  -2.421e-07
SO2(g)                  -28.15   7.078e-29   0.000e+00   0.000e+00   0.000e+00

-----------------------------Solution composition------------------------------
--- End code ---

dlparkhurst:
Let's start by considering redox equilibrium. In the SOLUTION definition, you can define redox disequilibrium. In particular, you are defining H(0) by equilibrium with H2(g) and N(0) by equilibrium with N2(g). They happily coexist in the SOLUTION calculation.

However, as soon as that solution reacts--with GAS_PHASE, or EQUILIBRIUM_PHASES, or RUN_CELLS, for example--redox equilibrium will be calculated. In this case, H2 will reduce N2 to NH3. Here, I use RUN_CELLS, and you can see that H2 is nearly completely consumed, there is less N2, and NH3(aq) is produced.

The first question is whether you want this reaction to occur? phreeqc.dat, Amm.dat, and pitzer.dat have inert versions of N2, Ntg, and H2, Hdg, so that the redox reaction will not occur. My guess is that will mimic your experiment better than redox equilibrium, but I don't know. So, first decide the first question, then we can go on to gas-solution interactions.


--- Code: ---SOLUTION 1 SHASTA Exp1-1 step 4 Calculate ppH2
pH  7.233
pe 5.2875     
density  0.9924
units  mol/kgw
-pressure 68.05
-temp 80
N(0)     0.1 N2(g) 1.7871 !logppH(68.05*0.9) log10(61.245)
H(0)     0.1 H2(g) 0.8187 !logppH(68.05*0.0968) log10(6.58724)
END
RUN_CELLS
-cell 1
END

--- End code ---

gdavilao:
Hi pdlparkhurst,
Thank you for your fast response. No, I don't want to let thee solution in equilibrium mode, since as you mentioned H2 will completely consumed and the N2 diminish to produce NH3(aq).  This is not happening in my experiments and in the ideal case will be hard to achived in the lab.
If I would like to mimic my batch experiment from the bigining, I will incorporate my initial water composition and i will set my initial partial pressure (10%H2, which is 6.8 atm, and 90%N2) and fix the volume. However, this will again go to equilibrium if I don't set a time step. if I set this reaction inert as you suggested, they are also not mimicking what I am seeing at the lab, which is less H2 (6.4atm). So, i think this will require more work.
However, this is not the case that I am trying to represent here. The solution composition that I am imposing is for t = t1 (lest say 1hr). I impose my pH at 25C(that is the one I measure), and all concentration of species that i measured. Also I se the partial pressure of hydrogen that I have from my GC. I follow this method to calculate the pH at 80C: Step 1: calculate the charge balance of the solution as it is, Step 2: set the Cl concentration as Charge and obtain a better charge balance and
Step 3: With that Cl, I calculate the pH at 80C. Then I am trying to have a model that check that my calculation are good. For this reason, I set up a new input with all the information as before but with my Cl and pH at 80 C, the partial pressures and with the fixed volume option I can check that the pressure remains the same as the impose one, is this a good way to check?
This simulations will give me an idea of the saturation state of the solution at that time and see which minerals can dissolve or precipitates. Does this make sense for you?

dlparkhurst:
Why don't you go ahead and try the simulation. If you are satisfied, then good. If you still have questions, a script will give me a better idea of what you have in mind.

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