David Van Wagener The University of Texas at Austin Research Review Meeting January 11, 2008

Overview Background Introduction to stripper modeling Minimizing stripper energy requirement Solvent and configuration options Recent pilot plant results/rate based stripper model Equilibrium stripper model in Aspen Plus ® System model results Solar powered stripping Stripping by flashing Conclusions Future Work

Absorption/Stripping Steam 30 psia AbsorberStripper Gas in Sweet Gas Out Makeup Water Lean SolventRich Solvent CO2

The Need For Stripper Modeling Stripper energy requirement accounts for a large % of total operating cost Reboiler and compressor operation consumes generated steam from power plant Stripper design is critical to minimize energy requirement and reduce operating cost for CO 2 removal

Contributions to Energy Requirement Sensible heat Influenced by heat capacity and liquid flow rate Latent heat Controlled by heat of desorption: Stripped steam Described by H 2 O/CO 2 ratio in product vapor Related to energy requirement through heat of vaporization of water

Solvent Choices Prior work by Oyenekan concluded performance is enhanced by using solvents with: High ΔH desorption High capacity High reaction rate with CO 2 MEA Industry standard: 7m MEA Great ΔH des and reasonable capacity Reaction rate with CO 2 hinders performance Degradation at high T is an issue

Solvent Choices K+/PZ 5m/2.5m has a high ΔH des and fast rates, but only marginal capacity 4m/4m improves the capacity and maintains other qualities MDEA/PZ Also has a high ΔH des and fast rates Additionally, it has exceptional capacity

Stripper Configurations Various configurations of stripping columns can improve performance by: Reducing reboiler duties Reducing stripped steam Decreasing the lean loading and solvent rate Significant work was done by Oyenekan to identify potentially beneficial configurations Internal exchange stripper Multipressure stripper Double matrix stripper  Determined to be most beneficial

Double Matrix Stripper Rich Solvent Lean Solvent Semi-lean Solvent Product CO 2 High P Low P Water Knockout

Flashing Stripper using Solar Energy Rich Solvent Lean Solvent Product CO 2 Water Knockout Solar energy via heated medium T 1, P 1 T 2, P 2 T 3, P 3 1.Initial heating of solvent to high temperature using solar energy 2.3 sequential adiabatic flashes

Levels of Aspen Calculation Rigorousness Equilibrium Mass Transfer Equilibrium stages Equilibrium Radfrac and ACM Thermal equilibrium in stage Specify CO 2 efficiency and number of stages Equilibrium reaction stages Ratefrac (Freguia) and Fortran (Tobieson, NTNU) Access built-in models for a k l a, k g a Specify packing type and height Kinetics with Mass Transfer Rate approximation Rate-based Radfrac and ACM Simple boundary layer Specify k g ’=f(ldg, T, k l ) Rigorous rate calculation RateSep Multiple boundary layer segments Specify rate constants Current absorber modeling approach

Modeling Software Equilibrium Stages Equilibrium Reactions Rate-Based Reactions ACM√√√ RadFrac√ RateSep√√ ACM is very functional, but requires extensive programming RateSep is an add-in function of RadFrac which uses discretized segments and rigorous calculations to approximate mass transfer and reaction rates

Optimization of Lean Loading 1. Independent stripper section (constant rich loading) Trade-off of sensible heat with stripped steam Optimum lean loading occurs with lowest equivalent work: 2. Stripper coupled with absorber (constant absorber specs) Predicts the rich ldg to accompany a specified lean loading Higher lean loadings result in higher rich loadings and/or solvent flows

Task: Analyze Recent Pilot Plant Run 35%wt MEA was used to remove CO 2 Analysis of stripper section: Loadings: 0.48  0.36 P reboiler : psia Max temperature: 216.9°F (≈102°C) Removal: 63% Equivalent work: 41.2 kJ/mol CO 2 (no compression or pumping) Currently the results are being evaluated using: Hilliard VLE Equilibrium reactions in RateSep Simulation flowsheet reflecting pilot plant operation Regressions will be used to reconcile differences

Stripper feed Stripped vapor Pilot Plant Reboiler Design Reboiler is separate from stripping column A fraction of the sump drawoff goes to the reboiler The reboiler only vaporizes a portion of the incoming liquid Stream temperatures vary depending on flow split Sump drawoff Reboiler Reboiler bypassLean solvent Reboiler vapor Remaining liquid solvent

Model predictions 212.5°F (216.9°F) Inlet specified 198.2°F (189.4°F) 122.3°F (112.8°F) 159 lb/min (155 lb/min) ldg: 0.36 (0.36) 198°F (190°F) 194°F (190°F) 193°F (196°F) Uses measured reboiler duty of MMBtu/hr and 75% of sump directed to reboiler Flows and loadings are closely predicted, but temperatures are off 207.8°F (208.8°F) Aspen (Measured)

Adjusting Reboiler Section 216.9°F (216.9°F) Inlet specified 193.2°F (189.4°F) 130.5°F (112.8°F) 159 lb/min (155 lb/min) ldg: 0.41 (0.36) 193°F (190°F) 191°F (190°F) 190°F (196°F) Reboiler duty changed to MMBtu/hr, 15% of sump directed to reboiler Column temperature estimates are still inaccurate, and lean loading is also off 208.4°F (208.8°F)

4m K + /4m PZ System Modeling Gas in Semilean return Lean return H 2 O return CO2 product Absorber Section Cross Exchange Section Stripper Section Compression Section Lean in 5.5 kmol/s 40°C 12.7% CO 2 90% removal of CO kPa base pressure Equilibrium reactions in strippers Hilliard K+/PZ model 500 MW plant specifications

Design Specifications  90% removal in the 15 m packed absorber  Equal CO 2 flow in stripper and absorber lean streams  Equal reboiler temperatures  Cold side 5° approach in lean exchanger  Cold side 5° approach in semi-lean exchanger  Lean amine flow rate into absorber  Low-pressure stripper reboiler duty  High-pressure stripper reboiler duty  High-pressure stripper feed temperature  Low-pressure stripper feed temperature SpecificationVary

Optimized Double Matrix/Intercooling vs. Simple Stripper/No Intercooling Optimum loading is slightly different for two cases Double matrix configuration yields energy savings, but not overwhelmingly The magnitude of savings do not agree with Oyenekan data, but could be attributed to difference in loadings MatrixSimple Rich Loading (mol CO 2 /mol alk) Lean Loading (mol CO 2 /mol alk) Pressure (kPa)265- Split Equivalent Work (kJ/gmol CO 2, to 1MPa) Equivalent Work (kJ/gmol CO 2, to 10MPa)

ToTo dT Solar stripping analysis MEA absorber model was used to determine a rich loading with a given lean loading (initial value of 0.4) Goals: Change the temperature step to attain original loading Optimize the lean loading to minimize equivalent work

Solar Stripping Analysis

Conclusions The data from a pilot plant run was evaluated The thermal efficiency is low An integrated system model was designed in AspenPlus This model demonstrated the double matrix stripper configuration is advantageous over the simple stripper Only 40% of the savings compared to previous isolated stripper models Equivalent work is most sensitive to changes in loading “Flashing strippers” are being investigated as an option for using several heat levels of solar generated steam in stripping

Future Work Implement new models for MEA, K + /PZ, and ROC-16 Upgrade to rate-based simulations Explore mass transfer mechanisms in stripper Flashing in top stage Rate-based reboiler as opposed to equilibrium Complete Aspen tasks with MEA Regress and reconcile differences for pilot plant run Verify accuracy of new Hilliard MEA model Quantify the feasibility of solar stripping