THERMOCHEMICAL MODELING OF DIRECT CAUSTICIZATION OF BLACK LIQUOR USING SODIUM TRI-TITANATE

Transcription

1 THERMOCHEMICAL MODELING OF DIRECT CAUSTICIZATION OF BLACK LIQUOR USING SODIUM TRI-TITANATE Risto Pajarre, VTT Processes Adriaan van Heiningen, University of Maine

2 Outline Thermodynamic equilibrium calculations using Gibbs free energy minimization on a spreadsheet (ChemSheet) Inclusion of chemical kinetics in ChemSheet Black liquor gasification and direct causticization application Description of constraining chemical kinetics Computational results with/without recirculation of gasification gas Conclusions

3 3 Chemical Equilibrium and Gibbs Free Energy Minimization The Gibbs free energy G = n i µ i of a closed system reaches its minimum value (for given pressure and temperature) at equilibrium. The equilibrium state can be solved as a non-linear optimization problem where the chemical potentials are typically supplied to the solver as known functions of temperature, pressure and composition: 0 µ i = µ i + where γ = γ i i RT ( x, T, p) j ln x γ i i ( )

4 4 Inclusion of kinetic constraints In many important reaction systems the effect of reaction kinetics can not be ignored Free energy minimization calculation can still be used to calculate the effect of side reactions and various thermodynamic values such as heat production, if the advancement of necessary reactions can be constrained. Necessary kinetic constraints to the system can bet set as new components (= columns in the stoichiometric matrix) t 0 t 1 t Initial inputs Min(G) (with kinetics frozen) State t = t 0 Reaction kinetic calculation Min(G) (with kinetics frozen) State t = t 1 Reaction kinetic calculation Min(G) (with kinetics frozen) State t = t

5 5 Black Liquor Gasification with Titanate Direct Causticization Major equilibrium reactions CO + H O CO + H CO C + CO Na S + CO + H O Na CO 3 + H S CO + 3 H CH 4 + H O Kinetically controlled reactions 5 (Na O 3TiO ) + 7 Na CO 3 3 (4Na O 5TiO ) + 7 CO C + H O CO + H C + CO CO Na SO C Na S + 4 CO

6 6 What Conditions for Kraft BLG with Titanate Direct Causticization Above 675 C to allow reasonable direct causticization kinetics Above 700 C to allow reasonable sulfate reduction Use low pressure steam gasification to keep CO partial pressure low ( 0.1 atm) for direct causticization Medium temperature ( C) atmospheric gasification

7 7 Inputs Elemental Analysis of Black Liquid Solids (Gullichsen and Fogelholm, 000): Element S Na K Cl C H O N Inerts w% Calculated Chemical Species (based on 1 kg bls) Potassium replaced by equimolar amount of sodium 1% of S as elemental sulfur to obtain experimental sulfur pyrolysis loss All chlorine as NaCL Remaining sodium as Na CO 3 Remaining oxygen as CO Remaining carbon as elemental carbon Hydrogen and nitrogen in elemental form Na SO 4 NaCl Na CO 3 CO C H S N SiO Total Mole gram Additions: Steam (1.mol/mol org. C) and Na O*3TiO (0.876kg/kgBLS)

8 8 Simulation of Fast Pyrolysis of Black Liquor Solids Comparison with Pyrolysis Yield Losses Measured by Jian Li (1989) Jian Li used oxidized black liquor No direct causticization, sulfur reduction or carbon gasification Ratio of elemental S/Na SO 4 in bls chosen to obtain agreement with sulfur loss obtained by Li. Yield Loss (%) C O Na H S Overall weight Jian Li This work

9 Reaction Kinetics ( Na O 3TiO ) + 7NaCO3 3( 4NaO 5TiO ) CO 3 X = ( 1 ( 1 k1t) ) X u where X u = 0.9 and *T(K) 1 o k1 = 66560e 1.035h ( at 700 C) Causticization of Na CO 3 9 C + CO / + H [ C] d dt where H O CO CO / 10 7x10 K1[ CO ] e = [ CO ] + K [ CO] 30,070 / T ( K ) 9 3x10 K + [ H ([ Na][, C] ), K = 3.4, 1. 4 K1 = K3 = min K4 = 3 [ H O] e O] + K 4 5,00 / T ( K ) [ H ] Carbon gasification mol 3 m s 4 d C + Na SO4 = Na S + 4CO [ NaSO4] ( kt ) mol = k t[ Na SO ] e dt where k = e T 4 0 m 3 s Sodium sulfate reduction

10 C H O N Ti S Na Cl Si Ar R(C) R(NaCO3) R(NaSO4) Ar(g) 1 CH4(g) 1 4 CO(g) 1 1 CO(g) 1 3 Cl(g) 3 H(g) HO(g) 1 HS(g) 1 HCN(g) HCl(g) 1 1 N(g) NH3(g) 3 1 NO(g) 1 1 NO(g) 1 Na(g) 1 NaCN(g) NaOH(g) O(g) S(g) 1 S(g) SO(g) 1 *4NaO*5TiO C 1 1 NaCO NaO 1 NaO*3TiO 7 3 NaS 1 NaSO NaCl 1 1 NaOH SiO Ti 1 R_C+ 1 R_C- -1 R_NaCO3+ 1 R_NaCO3- -1 R_NaSO4+ 1 R_NaSO4- -1 gas 10

11 11 Batch/Plug Flow Simulation Results Gas Composition versus Time

12 1 Batch/Plug Flow Simulation Results Gas Composition versus Time (Logarithmic scale)

13 13 Batch/Plug Flow Simulation Results Solids Composition versus Time (Logarithmic scale)

14 14 Batch/Plug Flow Simulation Results Reaction Rates versus Time

15 15 Batch/Plug Flow Simulation Results Reaction Rates versus Time

16 16 Batch/Plug Flow Simulation Results Effect of Product Gas Recirculation (10% of total steam flow)

17 17 Batch/Plug Flow Simulation Results Effect of Product Gas Recirculation (10% of total steam flow)

18 18 Batch/Plug Flow Simulation Results Effect of Product Gas Recirculation (10% of total steam flow)

19 19 Conclusions Inclusion of Reaction Kinetics in Gibbs Energy Minimization calculation is a powerful tool for industrial process simulation Complete direct causticization within one hour Final carbon burnout by carbothermic sulfate reduction Carbon gasification limited by high CO concentration Gas recycle decreases carbon consumption by gasification sothat more carbon is available for sulfate reduction