1 Kinetics of carbonate systems Hanna Knuutila Trondheim, 15 June 009
Outline Activity and concentration based kinetic constant Kinetic constant of infinite dilution Promotion Conclusions
3 String of discs
4 Experimental work Concentration of carbonate (wt-%) Temp. ( o C) Na CO 3 5-30 7-70 K CO 3 5-50 7-70 10 wt-% MEA 7-65 10 wt-% MAPA 7-60 10 wt-% MEA+KCO3 0 7-67 10 wt-% MEA+ NaCO3 10, 0 7-67 10 wt-% MAPA+NaCO3 15, 0 7-60 10 wt-% MAPA+KCO3 0 7-60
5 Absorption of CO into carbonate solution The reactions (1) CO + - HCO 3 - () HCO 3- + - CO 3 - + H O Overall reaction CO + H O+ CO 3 - HCO 3 - Reaction is a proton transfer reaction and much faster than reaction 1 reaction 1 is rate determining
6 Kinetic constants CO + - HCO 3 - Concentration based Activity based Second order kinetic constant r k [ ][ CO ] r k [ ] [ CO ] CO Second order kinetic constant at infinite dilution ( - 0) r k [ ][ CO ] r k [ ][ CO ] k k
7 Activity based kinetic constant Equals the concentration based second order kinetic constant at infinite dilution Independent of the concentrations of ions The VLE models used to model the system needs to be able to predict also the solubility of CO into the liquid r k [ ] [ CO ] CO Experimental VLE models N O solubility measurements H p i app* CO * CO C i CO xco
8 r k [ ] [ CO ] CO Second order kinetic constant at infinite dilution 1000 Data from this study+ Pinsent et al. + Pinsent and Roughton Data from this study Kucka et al., 00 1 - activity of potassium carbonate solutions 5 w t-% 10 w t-% 0 w t-% 30 w t-% 40 w t-% 50 w t-% koh- infinite [m 3 /mols] 100 10 Pohorecki and Moniuk, 1988 Pinsent and Roughton,, 1950 Pinsent et al. 1956 Pinsent et al., 1956 [-]*γοη [mol/l] 0.1 0.01 1 0 10 0 30 40 50 60 70 Temperature ( o C) 0.001.9 3 3.1 3. 3.3 3.4 1/T*1000 [1/K]
9 Defining the kinetic constant at infinite dilution Calculate second order kinetic constant based on concentrations r k [ ][ CO ] Plot the result for each concentration Fit an exponential line to the data ln k [m 3 /kmols] 18 16 14 1 10 8 6 4 0 y = -443.5x + 3.709 R = 0.983 y = -4741.8x + 30.75 R = 0.9935 y = -485.9x + 3.788 R = 0.99 y = -485.7x + 8.456 R = 0.9699 y = -4707x + 7.041 R = 0.9487 y = -4065.6x + 3.983 R = 0.9669 0.009 0.003 0.0031 0.003 0.0033 0.0034 Temperature [1/K]
10 Defining the kinetic constant at infinite dilution Plot for different temperatures the kinetic constant as a function of concentration. Extrapolate the line to zero concentration ln k 0 16 1 8 the kinetic constant at specific temperature is now found 4 0 0 5 10 15 0 K CO 3 concentration [mol/m3]
11 Defining the kinetic constant at infinite dilution Fit a straight line though the points 14 1 y = -6701.3x + 31.511 R = 0.9985 ln (k inf) 10 8 6 4 This study (40-70 oc) Roughton and Pinsent, 1950 Pinsent et al., 1956 Linear all data 0 0.008 0.003 0.003 0.0034 0.0036 0.0038 1/T
1 Pohorecki and Moniuk, 1988
13 Pohorecki and Moniuk, 1988
14 Second order kinetic constant at infinite dilution 1000 Data from this study+ Pinsent et al. + Pinsent and Roughton Data from this study koh- infinite [m 3 /mols] 100 10 Kucka et al., 00 Pohorecki and Moniuk, 1988 Pinsent and Roughton,, 1950 Pinsent et al. 1956 Pinsent et al., 1956 1 0 10 0 30 40 50 60 70 Temperature ( o C)
15 Promotion of carbonate solutions with amines 10 wt-% MEA/MAPA was added to sodium and potassium carbonate solutions Modeling Zwitterion mechanism R k [ ] k RR NH CO CO 1 Termolecular mechanism R { k [ ] k RR NH CO T amine 1 3 k H O RR NH k [ ] RR NH k [ CO ] RR NH } CO T T T H O 1 1 CO 3 1
16 Results 10000000 1000000 kobs (1/s) 100000 10000 1000 10 w t-% MEA 10 w t-% MEA + 10 w t-% NaCO3 10 w t-% MEA + 0 w t-% NaCO3 10 w t-% MEA + 0 w t-% KCO3 Zw itterion mechanism Termolecular mechanism 0.009 0.003 0.0031 0.003 0.0033 0.0034 1/T (K)
17 Results 9 8 7 10 wt-% MEA 10 wt-% MAPA unpromoted, 50 oc Kg ov * 1000 [m/s ] 6 5 4 3 1 0 10 wt-% NaCO3 0 wt-% NaCO3 0 wt-% KCO3 Temperature 40±1 oc
18 Conclusions An expression for the second order kinetic constant for the CO reaction CO + - HCO 3- at infinite dilution was obtained up to 70 o C Second order kinetic constant based on activities is a real constant and independent of the solvent concentrations and equals the concentration based second order kinetic constant at infinite dilution. This should be confirmed with measurement with K, Na and Li
19 Conclusions Aqueous K CO 3 absorbs CO faster than Na CO 3 Promoting the carbonate solutions with amines gives 10 times higher absorption rates Promoting potassium carbonate gives higher absorption rates than promoting sodium carbonate Both zwitterion and termolecular reaction mechanisms are able to predict the kinetic constants
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1 Calculations Concetration based K ov, G 1 = 1 H + k Ek G app CO 0 L Activity based K ov, G 1 = 1 H + k Ek G app CO 0 L Ha = kd 1 CO k o L Ha k D 1 CO CO o kl E = Ha E = Ha k 1 ov, G H app CO 1 1 K k ( ) g D CO k 1 HCO 1 1 K k ov, G CO ( ) g D CO k k1 [ ] k k1 [ ]