Conceptual Models > Kinetics and rate controlling factors

Methane kinetic

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Hamed:
Dear all, I am working on a model that simulates methane generation kinetically. At the same time, I want to include the rate of methane oxidation by sulfate so that a portion of CH4 is converted to CO2. However, the issue I am facing is that including the oxidation rate (R2) does not affect the results (the outcomes remain the same even when R2 is removed). So, something is wrong with the implementation of the oxidation rate (llnl database)
Any help would be appreciated.

Below is my code:

--- Code: ---SOLUTION_MASTER_SPECIES

Mtg Mtg      0 Mtg 16.032

SOLUTION_SPECIES

Mtg =  Mtg
-llnl_gamma 2.5000
log_k 0
-delta_H 0 kJ/mol
END

PHASES
Mtg(g)
Mtg  = Mtg
log_k           -2.8502
-delta_H -13.0959 kJ/mol # Calculated enthalpy of reaction CH4(g)
# Enthalpy of formation: -17.88 kcal/mol
-analytic -2.4027e+001 4.7146e-003 3.7227e+002 6.4264e+000 2.3362e+005
#       -Range:  0-300
END

RATES

R1
    -start
10 mu_max=4.1/(3600*24)
20 K_acc=230e-6
30 K_donor=2e-6
40 rate=-mu_max*MOL("Acetate-")/(MOL("Acetate-")+K_acc)*MOL("Acetate-")/(MOL("Acetate-")+K_donor)*M
50 moles=rate*TIME
60 SAVE moles
    -end

R2
    -start
10 mu_max1=2.05/(3600*24) 
20 K_acc1=115e-6     
30 K_donor1=1e-6     
40 rate1 = -mu_max1*M*(MOL("SO4-2")/(MOL("SO4-2")+K_acc1))*(MOL("Mtg")/(MOL("Mtg")+K_donor1))
50 moles = rate1*TIME
60 PUT(rate1, 1)
70 SAVE moles
    -end


SOLUTION 1
units   mol/kgw
pH      7
temp    20
pressure   1

Ca              0.007
Na              0.3
K               0.005
Sr              0.00005
Cl              4.5e-01 charge
S               0.02         
Acetate         0.01
N               0.001
       
GAS_PHASE 1
-fixed_pressure
-pressure 1
-temperature 25

CO2(g) 0.0
N2(g)  1.0
Mtg(g) 0.0


KINETICS

R1
    -m0 5e-7
    -formula    Acetate    12.685   N   0.2  H   12.385  O   -24.87  C   -12.21  Mtg   -12.16 
    -tol 1e-8

R1
    -m0 0
    -formula    Mtg  1  H  -4  C -1  S 0  O  0
    -tol 1e-8 


    -steps 24200 in 200 steps
-runge_kutta    6
INCREMENTAL_REACTIONS true


USER_GRAPH 1
    -headings               time Mtg CO2
    -axis_titles            "Day" "Mtg" "CO2"
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 GRAPH_X TOTAL_TIME/86400
20 GRAPH_Y MOL("Mtg")
30 GRAPH_SY MOL("CO2")
  -end
    -active                 true

--- End code ---

dlparkhurst:
Your kinetics definition has R1 twice; shouldn't one of them be R2?

I can't sort out your reaction network. If you want methanogenesis from acetate, I would write the formula as follows with a positive number for SAVE:

-formula HAcetate -1 Mtg 1 CO2 1

I'm not sure what you are doing with nitrogen.

Your R2 reaction multiplies by M to get the rate, but M is specified to be zero. So, the reaction cannot proceed.

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