KIN_DELTA sign

[MichaelZ20]

Hi David!
I supposed that KIN_DELTA = KIN(t+1) - KIN(t). Therefore, at precipitation KIN_DELTA should be positive.
I tried to model calcite precipitation by using Thermoddem kinetic database and your simplified approach for considering nucleation.
Unexpectedly I got negative KIN_DELTAs. Where am I wrong?

Code: [Select]

SOLUTION 1
pH 7 charge
Na 100
Cl 100

EQUILIBRIUM_PHASES
Calcite 0 10
CO2(g) -2
SAVE SOLUTION 1
END

USE SOLUTION 1

RATES
Calcite
# Confidence level: 3
-start
1 dpmet = ((1 / TK) - (1 / 298.15)) / 8.3144
2 SRmin = SR("Calcite")
10 moles = 0
20 If (m <= 0) and (SRmin < 1) Then GoTo 220
30 S = 0.7 # average BET; suggested value in m2/g
40 Mm = 100.1 # molar mass in g/mol
50 If (SRmin > 1) Then GoTo 173  ## nucleation
########## start precipitation bloc ##########
130 knu = 0.000000148 * exp((-63500) * dpmet)
140 k1 = 0.00658 * exp((-56100) * dpmet) * (ACT("HCO3-") ^ 1.982)
150 kpre = (-1) * (knu + k1)
160 theta = 0.5
170 eta = 2
########## start nucleation       ##########
173 rate = 1E-8                   ##### a small amount as a seed
175 GOTO 210
180 If (m <= 0) then GoTo 220
190 rate = S * m * Mm * kpre * (ABS(1 - SRmin ^ theta) ^ eta)
200 GoTo 210
########## end precipitation bloc ##########
210 moles = rate * Time
220 Save moles
-end

KINETICS
Calcite
-formula CaCO3 1
-m0 1e-4
-steps 86400 sec in 24

INCREMENTAL_REACTIONS true

USER_GRAPH
-headings Time KIN_DELTA_Calcite SI_Calcite
-initial_solutions true
-axis_titles "Time, hours" KIN_DELTA SI
-start
10 GRAPH_X SIM_TIME/3600
20 GRAPH_Y KIN_DELTA("Calcite")
30 GRAPH_SY SI("Calcite")
-end
END

[dlparkhurst]

I think your rate is always 1e-8. The positive rate represents dissolution of calcite and a negative KIN_DELTA. Ultimately, the moles of calcite goes to zero.

The solution for the kinetic reaction is already in equilibrium with calcite, so there should actually be no kinetic reaction at all.

[MichaelZ20]

Thank you, David!