Modeling Energy Performance of Aqueous MDEA/PZ for CO 2 Capture

Transcription

1 Modeling Energy Performance of Aqueous MDEA/PZ for CO 2 Capture Peter Frailie Gary T. Rochelle The University of Texas at Austin Luminant Carbon Management Program TCCS-6 June 16, 2011

2 Overview Why MDEA/PZ? MDEA/PZ Aspen Plus Framework Thermodynamics Hydraulics Kinetics Process Modeling Absorber Intercooling Stripper Simple stripper vs. 2-Stage Flash Conclusions

3 Why MDEA/PZ? High capacity 7m MDEA/2m PZ 0.83 mol CO 2 /kg solvent 7m MEA (0.60) 8m PZ (0.76) High CO 2 Absorption Rate k g comparable to 8m PZ at 40 o C Does not exhibit solubility limitations of conc. PZ Commercially used for H 2 and CH 4 treating MDEA is less expensive than PZ

4 Amine Modeling

5 Aspen Plus Modeling - Thermo Overall goal: construct 1 model that represents MDEA, PZ and MDEA/PZ using Aspen Plus enrtl method Over wide temperature, loading, and amine concentration ranges Sequential regression: amine amine/h 2 O amine/h 2 O/CO 2 Minimizes the number of regressed parameters Process models more likely to converge Improves confidence in parameter values Thermodynamically consistent methodology Speciation and thermodynamic properties calculated using same set of thermodynamic parameters

6 Aspen Plus Modeling - Thermo Incorporated all available experimental data C P, VLE, amine volatility, speciation, H ABS, pka,γ CO2 Improves thermodynamic consistency Final model utilized 54 independently adjusted parameters MDEA (17), PZ (33), MDEA/PZ (4) Focused on operationally significant conditions Loading 0.5 and 5 kpa CO 2 Temperature 40 o C to 150 o C Amine concentration wt%

7 Aspen Plus Modeling - Hydraulics FORTRAN subroutines used to fit data Functions of amine concentration, loading, and temperature Density Dugas (2009) Viscosity Weiland (1998) Diffusivity Dugas (2009) Fit over same temperature, loading, and amine concentration ranges as thermodynamic data

8 Aspen Plus Modeling - Kinetics Fit using WWC simulation in Aspen Plus RateSep TM Adjusted k 0 and E A for select kinetic reactions Reactions selected based on predicted speciation k = k exp Final model uses 7 independently adjusted parameters 3 k 0, 3 E A, and D 0 0 E R A 1 T K Amine System Temperature ( o C) CO 2 Loading (mol/mol alk) 8m PZ m MDEA/2m PZ m MDEA/5m PZ

9 Flux pred /Flux exp 1,3 1,2 1,1 1,0 0,9 0,8 0,7 40 o C 60 o C 80 o C 100 o C Lean = 8m PZ = 7m MDEA/2m PZ = 5m MDEA/5m PZ Rich Error avg = 6.7% 0,01 0, P CO2 (kpa)

10 Process Modeling - Absorber

11 Absorber ~1.2 kpa CO 2 (90% removal) Intercooling to 40 o C 12 kpa CO 2 40 o C 100 kpa L/L min = kpa Mellapak 250X Lean 40 o C Column diameter set to 80% flood in bottom stage. Rich o C

12 L/G (mol basis) m MDEA/5m PZ 0.24 mol CO 2 /mol alk Not Intercooled Absorber Height (m)

13 L/G (mol basis) m MDEA/5m PZ 0.24 mol CO 2 /mol alk Not Intercooled Absorber Height (m)

14 m MDEA/5m PZ 0.24 mol CO 2 /mol alk Not Intercooled L/G (mol basis) m Absorber Height (m)

15 Capacity (mol CO 2 /kg H 2 O + Amine) 1 0,9 0,8 0,7 0,6 0,5 0,4 Isothermal 7m MDEA/2m PZ 5m MDEA/5m PZ 8m PZ 0, P CO2 at 40 o C (kpa)

16 Capacity (mol CO 2 /kg H 2 O + Amine) 1 0,9 0,8 0,7 0,6 0,5 0,4 Not Intercooled 7m MDEA/2m PZ 5m MDEA/5m PZ 8m PZ 0, P CO2 at 40 o C (kpa)

17 Capacity (mol CO 2 /kg H 2 O + Amine) 1 0,9 0,8 0,7 0,6 0,5 0,4 Intercooled Not Intercooled 7m MDEA/2m PZ 5m MDEA/5m PZ 8m PZ 0, P CO2 at 40 o C (kpa)

18 Process Modeling - Stripper

19 Simple Stripper Rich Pump 40 o C 150 bar 99.9% CO 2 Rich conditions set by absorber results HeatX Cold T = 5 o C o C 4-14 bar Mellapak 250X Trim Cooler Lean Pump

20 2 Stage Flash Rich Pump Rich conditions set by absorber results HeatX Cold T = 5 o C Trim Cooler HP Flash HP and LP flashes at same temperature Equal vapor flow rates Lean Pump 40 o C 150 bar 99.9% CO 2 LP Flash

21 Equivalent Work Analysis (0.5 kpa Lean Loading) Amine Stripper T ( o C) 7m MDEA/2m PZ 120 5m MDEA/5m PZ 120 8m PZ 120 8m PZ 150 IC? W EQ, SS (kj/mol CO 2 ) W EQ, 2SF (kj/mol CO 2 ) Abs Ht (m) No Yes No Yes No Yes No Yes W eq n reboilers = i=1 Ti 0.75 Qi + 5K T T + 5K i sin k + W pumps + W comps

22 Conclusions Thermodynamic, hydraulic, and kinetic data can be simultaneously fit for MDEA, PZ, and MDEA/PZ using enrtl model and RateSep TM in Aspen Plus Intercooling significantly improved the capacity of each solvent tested Also improved associated W EQ Increased absorber height W EQ for 2SF systematically higher (~1.5 kj/mol CO 2 ) than that of SS. Higher stripper temperature did not necessarily improve energy performance Best W EQ observed for 5m MDEA/5m PZ with an intercooled absorber

23 Questions?

24 Reaction Rate Constants Am 2 + B + CO AmCOO + BH + (kf1) (kf2) (kf3) (kf4) (kf5) (kf6) PZCOO MDEA PZ H 2O CO PZH HCO PZ 2 PZ CO PZH + PZCOO + + MDEA + CO MDEAH + PZCOO + + H 2O + CO2 MDEAH + HCO3 + MDEA + CO Nine possible amine/base combinations for MDEA/PZ 2 ( ) + 2 MDEAH PZ COO Cut down to 6 reactions by analyzing predicted speciation 12 total parameters (6 k 0 and 6 E A ) further reduced to ( ) + 2PZCOO + CO2 H PZCOO + PZ COO 2 2