A rational approach to amine mixture formulation for CO 2 capture applications. Trondheim CCS Conference - 6 June 14 16, 2011 Graeme Puxty

Transcription

1 A rational approach to amine mixture formulation for CO 2 capture applications Trondheim CCS Conference - 6 June 14 16, 2011 Graeme Puxty

2 The ideal solvent for CO 2 post-combustion capture: Process challenges Fast mass transfer and large absorption capacity in the face of low CO 2 partial pressure (5-25 kpa) Minimal energy requirement for solvent regeneration

3 The ideal solvent for CO 2 post-combustion capture: Process challenges Operability (viscosity, density, surface tension, corrosivity) and stability (oxidative and thermal degradation) Atmospheric emissions and toxicity Presence of other acid gases (SO x, NO x ) and O 2

4 The ideal solvent for CO 2 post-combustion capture: Solvent properties Rapid mass transfer Large absorption capacity Gas Solvent p CO2 Concentration p CO2,i [CO 2 ] i [CO 2 ] Solid Surface Low equilibrium partial pressure at low temperature (absorber), high at high temperature (desorber) Gas-liquid interface Enthalpy of absorption, large or small?

5 Solvent properties Solvents are generally aqueous solutions of amines (organic bases containing N) CO 2 predominantly undergoes chemical absorption which is required to achieve rate and capacity requirements at low CO 2 partial pressure There is generally a trade-off between mass transfer, absorption capacity, CO 2 partial pressure and absorption enthalpy Amine mixtures are now being used as a way to selectively achieve this trade-off depending upon priorities

6 Determining speciation and mass transfer

7 Equilibrium model K H + + CO HCO - 3 K 2 H + + HCO 3 - H 2 CO 3 K 3 CO 2 + H 2 O H 2 CO K 3 4 CO 2 + Am AmCOO - + H + H + + Am AmH + H + + AmCOO - H + + OH - K 5 K 6 K 7 AmCOOH H 2 O Requires determination of reactions and equilibrium constants via techniques such as spectroscopy (CSIRO & UoN) Include all reactions required to define complete speciation Solve system of non-linear simultaneous equations defining the equilibrium constants and mass balance

8 Equilibrium model agreement with measurement 4.9 M MEA VLE data PZ speciation data

9 Film model Fick s 2 nd law + chemical reaction Change in concentration with time c t Diffusion coefficient 2 c = D + r 2 x Change in concentration with distance (2 nd derivative) Rate of chemical reaction Requires knowledge of reaction kinetics, equilibria and physical properties (CSIRO & UoN) Kinetics via stopped-flow and physical properties such as viscosity are easily determined Integrated numerically over time and distance for all chemical species using Matlab

10 Film model a software tool in Matlab

11 Film model agreement with measurement PZ flux data PZ + AMP flux data

12 Fast mass transfer A sterically free primary or secondary amine functionality to react with CO 2 Strong interaction with CO 2 To maintain rate the amine functionality needs to remain unprotonated HO NH 2 monoethanolamine HN NH piperazine

13 Large capacity A tertiary / sterically hindered amine that does not form a stable carbamate A good base present in large concentration to maintain a basic solution (ph buffer) K AmCOO CO 2 + Am + B AmCOO - + BH + K HCO3 CO 2 + H 2 O + B HCO 3- + BH + HO NH 2 2-amino-2-methyl-1-propanol K B H + + B BH + K AmCOO [AmCOO ][H ] = KB KHCO3 = [CO 2][A] 2 2 [HCO ][H ] K [CO ][H O] B HO N methyldiethanolamine OH K B + [BH ] = + [H ][B]

14 Low p CO2 in absorber An equilibrium that favours the formation of the reaction products carbamate / bicarbonate / protons Strong interaction with CO 2 at low temperature A good base to accept protons K AmCOO CO 2 + Am + B AmCOO - + BH + K HCO3 CO 2 + H 2 O + B HCO 3- + BH + K B H + + B BH + K AmCOO [AmCOO ][H ] = KB KHCO3 = [CO 2][A] 2 2 [HCO ][H ] K [CO ][H O] B K B + [BH ] = + [H ][B]

15 High p CO2 in stripper Large temperature dependent shift in equilibrium to favour reactants (CO 2 ) Weak interaction with CO 2 at high temperature K AmCOO CO 2 + Am + B AmCOO - + BH + K HCO3 CO 2 + H 2 O + B HCO 3- + BH + dlnk H = 1 d( ) R T K B H + + B BH + K AmCOO [AmCOO ][H ] = KB KHCO3 = [CO 2][A] 2 2 [HCO ][H ] K [CO ][H O] B K B = + [BH ] + [H ][B]

16 Large or small ΔH abs? [CO ] = [CO ] + [AmCOO ] + [HCO ] 2 total 2 3 [HCO ] [AmCOO ] H = H + H + H 3 abs ( ) HCO3 ( ) AmCOO CO2 [CO 2] total [CO 2] total A small ΔH abs reduces the enthalpy contribution to the desorber energy requirement A large ΔH abs increases the temperature dependent shift to favour CO 2 thus increasing p CO2 in the desorber

17 Large or small ΔH abs? It depends if the absorption enthalpy or water condensation makes a larger contribution to the energy requirement q = x m C ( ) p T T heat H2O,lean lean bottom top q = y m H vap H2O,top top vap q abs = H abs qreb = qheat + qvap + qabs

18 Estimating solvent performance To allow a fair comparison solvent performance needs to be evaluated at optimal operating conditions Estimate the optimal energy requirement using the equilibrium model: Rich loading (α rich ) is fixed at 40 C and 5 kpa CO 2 Optimise stripper bottom temperature and lean loading (α lean ) for minimum reboiler energy requirement (assume isobaric) Estimate the log mean mass transfer coefficient for the absorber assuming 40 C and using α rich and α lean values from optimisation

19 Estimating optimal energy requirement Heat limited minimum reboiler energy requirement: qreb,hl = x m Cp( T T ) + H + y m H H2O,lean lean bottom top abs H2O,top top vap Steam limited minimum reboiler energy requirement: mh2o,min qreb,sl = mh2o,min Hvap + Habs y CO2,reb Minimum steam rate from pinch point of operating and equilibrium p CO2 vs. α curves q = max( q, q ) reb reb,hl reb,sl

20 Examples single amines Name Concentration / mol.dm -3 Ln mean k l / m.s -1 (α rich, α lean ) [AmCOO ] [CO ] 2 total [HCO 3] [CO ] 2 total lgk B -ΔH abs / kj.mol -1 AMP (p) 3.0 (27% w/w) 1.52 (0.41, 0.02) (73) Ammonia (NH 3 ) 1.5 (2.5% w/w) 1.57 (0.42, 0.05) Ammonia (NH 3 ) 5.9 (10% w/w) 3.23 (0.35, 0.02) DEA (s) 3.8 (40% w/w) 0.81 (0.35, 0.02) MDEA (t) 3.0 (36% w/w) (0.16, 0.008) n/a (58) MEA (p) 4.9 (30% w/w) 4.8 (0.46, 0.10) (84) PZ (s) 1.0 (9% w/w) 5.7 (0.68, 0.07) All is data at 40 C (60) k l is the liquid side mass transfer determined using the film model with α rich for p CO2 = 5 kpa and α lean (in parenthesis) from optimisation of the energy requirement. ΔH abs was determined over the temperature range C. PZ is a diamine and can form a dicarbamate and diprotonated species, B = PZ + PZCOO - p = primary, s = secondary and t = tertiary amine

21 Examples mixtures Name Concentration / mol.dm -3 Ln mean k l / m.s -1 (α rich, α lean ) [AmCOO ] [CO ] 2 total [HCO 3] [CO ] 2 total lgk B -ΔH abs / kj.mol -1 AMP (p) 3.0 (27% w/w) 1.52 (0.41, 0.02) (73) MDEA (t) 3.0 (36% w/w) (0.16, 0.008) n/a (58) PZ (s) 1.0 (9% w/w) 5.7 (0.68, 0.07) (60) PZ AMP 1.0 (9% w/w) 3.0 (27% w/w) PZ 1.0 (9% w/w) MDEA 3.0 (36% w/w) 6.6 (0.41, 0.02) 5.3 (0.23, 0.02) n/a

22 Energy requirement vs. mass transfer

23 What happens in a mixture? K AmCOO CO 2 + Am + B AmCOO - + BH + K B H + + B BH + For PZ: B = PZ For PZ + AMP: B AMP For PZ + MDEA: B PZ & MDEA

24 Conclusions A detailed and complete chemical model allows accurate prediction of CO 2 -solvent behaviour, including mixtures Chemical equilibria, kinetics and diffusion allows accurate prediction of speciation and mass transfer A combination of log mean mass transfer coefficient, overall equilbrium constants for carbamate and bicarbonate formation, reaction enthalpy and reboiler energy requirement provides a metric for capture performance The approach can be applied to solvents of any complexity and type

25 CSIRO Energy Technology Graeme Puxty Research Scientist Phone: Web: Thank you Contact Us Phone: or enquiries@csiro.au Web: