Enthalpy of absorption of CO 2 in the aqueous solutions of amines Inna Kim and Hallvard F. Svendsen The Norwegian University of Science and Technology (NTNU), Norway
Outline Background Experimental set-up and results Prediction of H abs from K-values Conclusions
Enthalpy(Heat) of absorption, H abs [Lee L.L.,1994] due to chemical reaction (for the n-th reaction): ln K T eq. for n ΔH = RT due to dissolution of the acid gas (CO 2 ) into the liquid (release of the kinetic energies) ln k H H ΔH T RT RT o o H i i( aq) i( gas) dissol = = 2 2 due to non-ideal mixing (preferential solvation and energetic changes with temperature) (for the i-th species) o r 2 ln γ H H i H = = T RT RT o e i( aq) i( aq) Δ i 2 2
Integral and differential H abs Integrated over some loading interval (direct measurement) Calculated using Gibbs- Helmholz equation: ( CO ) dln f 2 ΔH = d( 1/ T) R x CO2 s f CO2 fugacity of CO 2 x CO2 mole fraction of CO 2 Assumption: f CO = P CO 2 2 Disadvantage: differentiation [Lee et al.,1974]
Differential enthalpy of absorption from equilibrium data *: a VLE data: ln(p CO2 ) vs 1/T: a) Fitted with a line, b) Fitted with a 2 nd order polynomial b * [Hoff K.A., Mejdell T., Svendsen H.., 25]
Experimental part Measuring of semi-differential enthalpy of absorption of CO 2 in a reaction calorimeter
Experimental set-up P P 4 W N T Control device P P P-129 F CO2 CO2 to air P CPA122 (ChemiSens AB, Sweden) P (1~4 bar) (-1~15 bar) co2 2a 1 to air P 2b Thermostat P T T CO2 Control device Vacuum (-1~1,5 bar) 3 1 - Calorimeter 2a,2b - CO 2 storage cylinders 3 - Vacuum pump 4 - Feed bottle
Semi-differential H abs (an example of on-line recorded data) 12 14 1 12 Heat Flow, [W]; Reactor temperature, [ o C] 8 1 6 8 4 6 2 4 2 5 1 15 2 25 3 35 4 45 CO 2 flow*1, [L]; Reactor pressure, [bar] REACTOR_TEMP REACTOR_HB_POWER LIN_IN_1A PRESSURE_A -2-4 -2 Time, [sec]
Semi-differential H abs for 3 wt% MEA 18 16 - Hab s, [kj/mol-co2] 14 12 1 8 6 4 2,,2,4,6,8 1, 1,2 1,4 α, [mol-co 2 /mol-amine] 4 C 8 C 12 C [Jou et al., 1994] [Lee et al., 1974]
Integral H abs for 3wt% MEA 12 - Hab s, [kj/mol-co2] 1 8 6 4 2 4ºC 8ºC 12 C,,2,4,6,8 α, [mol-co 2 /mol-mea]
Estimation of the saturation loading point in the example of 3wt% MEA 6 5 H abs =113, kj/mol-co 2 H abs =89,7 kj/mol-co 2 - Habs, [kj/mol-mea] 4 3 2 H abs =8, kj/mol-co 2 4 C 8 C 12 C 1,2,4,6,8 1 α, [mol-co 2 /mol-mea]
Semi-differential H abs for 3 wt% MDEA solution 35 3 -ΔHabs, [kj/mol-co2] 25 2 15 1 4 C 8 C 12ºC [Jou et al., 1994] 5,4,8 1,2 1,6 α, [mol-co 2 /mol-am]
Semi-differential H abs for MDEA 16 3% MDEA, 8º - Habs, [kj/mol-co2] 14 12 1 8 6 4 2-2,5 1 1,5 2 2,5 3 α, [mol-co 2 /mol-amine] 5% MDEA, 75º 35%, 76.7ºC, 6.9 Mpa (.93 Mpa)** 35%, 76.7ºC, 3.45 Mpa (.7 Mpa)** 5%, 76.7ºC, 6.9 Mpa (.86 Mpa)** 5%, 76.7ºC, 3.45 Mpa (.7 Mpa)** 5%, 76.7ºC, 1.38 Mpa (.7 Mpa)** 4%, 6oC [Merkley et al., 1986] **[Oscarson et al.,2]
Integral H abs for 3 and 4wt% MDEA - Habs, [kj/mol-co2] 3 25 2 15 1 5 3% MDEA, 4º 4% MDEA, 4º 5% MDEA, 4º 4% MDEA, 6º 5% MDEA, 75º 3% MDEA, 8º 3% MDEA, 12ºC 3% MDEA 4ºC, 2MPa * 3% MDEA 8ºC, 2MPa * 3% MDEA 12ºC, 2MPa *,5 1 1,5 2 α, [mol-co 2 /mol-amine] 4% MDEA, 6ºC** 35% MDEA, 76.7ºC*** * [Mathonat et al., 1997]; ** [Merkley et al.,1986]; ***[Oscarson et al., 2]
Measured integral H abs for 3wt% MDEA solution at 8oC 18 16 14 -ΔHabs, [kj/mol-co2] 12 1 8 6 4 2 8ºC (int) 8º (int) 8º (int-2) 8ºC (diff),4,8 1,2 1,6 α, [mol-co 2 /mol-am]
Modelling Prediction of the enthalpy of absorption from K-values
The main reactions in the CO 2 /alkanolamine/water system: 2H O= HO + OH 2 3 + K w a a x x = = γ γ + + + HO 3 OH HO 3 OH HO 3 OH 2 2 2 aw xw γw 2H O+ CO = HO + HCO + 2 2 3 3 K CO2 a a x x γ γ = = + + + H3O HCO3 H3O HCO3 H3O HCO3 2 2 2 aco a 2 w xco x 2 w γco γ 2 w HO+ HCO = HO + CO + = 2 3 3 3 K HCO3 a a x x γ γ = = a a x x γ γ + 2 + 2 + 2 HO 3 CO3 HO 3 CO3 HO 3 CO3 w w HCO w 3 HCO3 HCO3 H O+ RRNH ' = RRNH ' + OH + 2 2 K + RR' NH2 a a x x γ γ = = a a x x γ γ + RR' NH + RR' NH + HO RR' NH 3 HO 3 HO 3 + ' w + 2 ' w + RR NH RR NH2 RR' NH w 2 RR ' NH + HCO = RR ' NCOO + H O CO = CO 2( g ) 2( aq) 3 2 K RR' NCOO a a x x γ γ = = a a x x γ γ RR' NH RR' NH HCO RR' NH 3 HCO3 HCO3 ' w RR NCOO RR' NCOO w RR' NCOO w φ y P= H x γ CO2 CO2 CO2 CO2 CO2
Enthalpy of reaction from K-values ln B K = A+ + ClnT + DT T o Δ G = RTln K r o ex j j j Δ H = Δ H +ΔH o j Δ H = ln K 2 j RT T P o ( r ) ΔG / T o H r ln K Δ = = R 2 T T T P P ex j Δ H = RT 2 ln( Πγ ) T i P
Calculation of the overall H abs [Merkley, 1987] Δ H abs = ΔniΔH j 1. HO H + OH 2 2. H O + CO H + HCO + 2 3. HCO3 H + CO3 + + 4. MDEAH MDEA+ H 5. CO + 2 2( aq) 3 2( g ) + Δ n = OH OH 4 1 f i Δ n = HCO HCO CO CO + 2 2 Δ n3 = CO 3 CO f 3 i 2 2 2 3 f 3 i 3 f 3 i [ ] [ EA] Δ n = MDEA MD CO Δ n = [ CO ] [ CO ] 2( aq) 5 2 f 2 i f i
Comparison of equilibrium constants -33-34 -35-36 Dissociation of water, K w Austgen, 1989 Oscarson et al., 1995 Merkley, 1987 Harned,1933-16 -17-18 1st ionization of carbonic acid, K CO2-26 -26.5-27 2nd ionization of carbonic acid, K HCO3 - ln K w -37-38 -39-4 -41-42 -43 2 2.5 3 1/T, [1/K] 3.5 4 x 1-3 ln K CO2-19 -2 Austgen, 1989-21 Oscarson et al., 1995 Merkley, 1987-22 Harned, 1945 Patterson, 1982 (I=.) -23 Patterson, 1982 (I=.5) Read, 1975 (1 bar) Read, 1975 (2 bar) -24 1.5 2 2.5 3 3.5 4 1/T, [1/K] x 1-3 ln K HCO3 - -27.5-28 -28.5 Austgen, 1989-29 Oscarson et al., 1995 Patterson, 1984(I=.) Patterson, 1984(I=.1) -29.5 1.5 2 2.5 3 3.5 4 1/T, [1/K] x 1-3 ln K MDEAH + -17-18 -19-2 -21-22 -23-24 Protonation of MDEA, K MDEAH + Austgen, 1989 Oscarson et al., 1995 Merkley, 1987 Gonzales, 1998 misc. (exp.) Kamps, 1996-25 2 2.5 3 1/T, [1/K] 3.5 4 x 1-3 ln K carb -4-5 -6-7 Carbamate formation, K MEACOO - Austgen, 1989-8 Gonzales, 1998 Chan,1981 Mahajani, 1982-9 Kent-Eisenberg, 1976 Aroua,1999 (exp) -1 Aroua, 1999 (regr) Wang, 21 Barth, 1984-11 2 Poplsteinova, 24 2.5 3 1/T, [1/K] 3.5 4 x 1-3 ln H CO2 9 8.5 8 Henry.s constants, H CO2 7.5 Austgen, 1989 Oscarson et al., 1995 Merkley, 1987 7 Chen, 1979 Ellis, 1963 Carrol et al., 1991 Versteeg et al., 1988 6.5 1.5 2 2.5 3 3.5 4 1/T, [1/K] x 1-3
Comparison of H of reactions from equilibrium constants 65 Δ H w 6 55 5 45 4 35 Dissociation of water, dh w 3 Austgen, 1989 25 Oscarson et al., 1995 Merkley, 1987 2 Harned, 1933 Marshall&Franck, 1981 15 25 3 35 4 45 5 T, [K] Δ H CO2 2-2 -4-6 -8-1 1st ionization of carbonic acid, dh CO2 Austgen, 1989 Oscarson et al., 1995 Merkley, 1987 Zambonin, 1952 Patterson, 1982-12 2 3 4 5 6 T, [K] Δ H HCO3 3 2 1-1 -2-3 -4 2nd ionization of carbonic acid, dh HCO3 Austgen, 1989 Oscarson et al., 1995 Zambonin, 1952 Patterson, 1984(I=.) Patterson, 1984(I=.1) -5 25 3 35 4 45 5 55 T, [K] 55 Protonation of MDEA, dh AmH+ 32 Carbamate formation for MEA, dh AmCOO- 4 Dissolution of CO 2, dh dissco2 Δ H MDEAH + 5 45 4 35 Austgen, 1989 Oscarson et al., 1995 Gonzales, 1998 Oscarson et al., 1989 Merkley, 1987 Δ H AmCOO- 3 28 26 24 22 2 Austgen, 1989 Gonzales, 1998 Δ H dissco2 2-2 -4-6 Austgen, 1989 Oscarson et al., 1995 Merkley, 1987 misc.(exp) Ellis, 1963 3 25 3 35 4 45 5 T, [K] 18 25 3 35 4 45 5 T, [K] -8 2 3 4 5 6 7 T, [K]
Experiment vs model for 3 wt% MEA solution 16 Overall heat of absorption from K-values for 3wt% MEA 7 Overall heat of absorption per mol-amine for 3wt% MEA 14 4 o C (model) 4 o C (exp) 6 o -Δ H abs, [kj/mol-co 2 ] 12 1 8 6 4 8 o C (model) 8 o C (exp) 9 o C (model) 12 o C (exp) o -Δ H abs, [kj/mol-am] 5 4 3 2 4 o C (model) 4 o C (exp) 8 o C (model) 8 o C (exp) 2 1 9 o C (model) 12 o C (exp).1.2.3.4.5.6.7.8.9 1 α, [mol-co 2 /mol-am].1.2.3.4.5.6.7.8.9 1 α, [mol-co 2 /mol-am]
Conclusions Direct calorimetric measurements provide an accurate means of obtaining the enthalpy of absorption for acid gases in solution as function of temperature and loading By keeping the delta in loading between each new equilibrium, rather low (~.5), it is possible to obtain values of H abs semi-differential in loading It seems to be possible to predict the enthalpy of absorption from the equilibrium data (K-values). The model may be used to predict the equilibrium constants from the experimental enthalpy data. The activity coefficient contributions will be taken into account via excess enthalpies.
Acknowledgement This work has been financially supported by the European Commission through the CASTOR Integrated Project (Contract no. SES6-CT-24-52856). Thank you for your attention!