Modeling and Optimization of CO2 removal in power plants by uzz16657

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									    TWMCC Conference, Spring 2007




Modeling and Optimization of CO2 removal in power plants

                             Sepideh Ziaii Fashami
                              Dr. Thomas F.Edgar
                              Dr. Garry T.Rochelle


                         Chemical Engineering Department
                    University Of Texas at Austin ,Austin ,TX 78712
 Modeling and Optimization of CO2 removal in power plants


Summary

    The public interest in CO2 removal from flue gases has recently
increased, due to more restrictive regulations on emissions of
greenhouse gases. The use of a classic absorption/stripping process
downstream of power plants is the only option so far.

   Aqueous MEA is the most common solvent used to absorb CO2. Its
problem is that the energy required for solvent regeneration is high.
Modeling and Optimization of CO2 removal in power plants


 Process description

      The absorption/stripping process consists of an absorber, a
stripper, a cross heat exchanger, rich and lean solvent pumps and
lean solvent cooler. In this base case, flue gas contacts the
aqueous solvent in a countercurrent, packed absorber.
     The source of flue gas is a coal-fired plant giving 13 mol%
CO2.The rich solvent is stripped by steam in a countercurrent
reboiled column to produce pure CO2. Heat is recovered from the
hot lean solvent by cross heat exchanger with cold rich solvent.
The lean solvent is typically cooled before entering the absorber.
Modeling and Optimization of CO2 removal in power plants


  Process flow sheet
Modeling and Optimization of CO2 removal in power plants

Reactions
•Equilibrium reactions
    HCO3- + H2O ↔ CO3-- + H3O+
    MEAH+ + H2O ↔ MEA + H3O+
    2 H2O ↔ H3O+ + OH-
•Non-equilibrium reactions             +        −
                                                    +
                            ⎯
      CO 2 + MEA + H 2O ⎯ ⎯ ⎯ → MEA + HCO 3
                        K 1, MEA



       CO2 + MEA + H 2O ⎯⎯,MEA→ MEACOO − + H 3O +
                         K2
                            ⎯
                            ⎯
                                                        +
       MEACOO − + H 3O + ⎯⎯−⎯ → CO2 + MEA + H 2O
                                 ⎯
                          K 2 , MEA


                     −
       MEA + + HCO 3 ⎯ ⎯−⎯ → CO 2 + MEA + H 2O
                              ⎯
                       K 1 , MEA
 Modeling and Optimization of CO2 removal in power plants

   Objective
        • Obtaining valid kinetic expressions for non-equilibrium
        reactions of CO2 with (MEA).
        •Modeling Absorption /stripping process to remove 90%
        CO2 from a coal-fired flue gas with MEA in Aspen plus
        software.
        • Optimizing required energy of CO2 capture plant
Model Development
   Absorber : Radfrac Packed column, rate based calculation, Equilibrium
   and non-equilibrium reactions
   Stripper : Rebiled column with one equilibrium stage excluding Reboiler
   Thermodynamic model : Electrolyte-NRTL
   Kinetic Model: (Concentration based)
                                                                          2152
                  Hikita (1977)              Log 10 k 2 , MEA = 10 . 99 −
                                                                   5771 T
                  Little (1971)                ln k1,MEA = 21 −
                                                                      T
Modeling and Optimization of CO2 removal in power plants

 Model Matching
                       The concentration range of Hikita model is 0.015-0.018, much lower than the 5 M used
                       industrially and studied in this work. Therefore, Hikita model is corrected to match
                       kinetic experimental data provided by Aboudheir (2002) and Little model is modified to
                       get consistent bicarbonate composition in the absorber.
                                -7
                       2.2x10


                                -7
                       2.0x10


                                -7
                       1.8x10
      kPa cm )
  2




                                                                                0.30 loading
  .




                                -7
                       1.6x10
  .
      k ' (mol/s




                                -7
                       1.4x10                                5m MEA               7m MEA                            9m MEA                 11m MEA
                   g




                                               o                                               0.40 loading
                       1.2x10
                                -7        60       C


                                                                                                    o
                                -7                                                             50       C
                       1.0x10

                                                                                                                                                          o
                                                                                                                                                     40       C
                                -8
                       8.0x10
                                0.00300                               0.00305                       0.00310                      0.00315                  0.00320
                                                                                                              - 1
                                                                                                    Temp

                                                       Figure 2.Calculated mass transfer coefficients of MEA solutions at 0.30 and 0.40
                                                       loading by Ross(2006) using Aboudheir kinetic data.

      Result: Activity based kinetic model, calculated and used in this work,
                                                                                                 2628                                                5771
                                               Log     10   k 2 , MEA = 26 . 7 −                                             ln k 1 , MEA = 29 −
                                                                                                   T                                                   T
       Modeling and Optimization of CO2 removal in power plants


        Sensitivity analysis
  Sensitivity analysis is done to minimize the equivalent work of modeled plant.
  As a result, Absorber with 22.5 M packing height and MEA with 0.4 lean
  loading is selected as an optimum condition.

   Equivalent Work :

               ⎛T    + 10 − 40 ⎞
W = 0 .75Q reb ⎜ reb
               ⎜ T
                               ⎟+W
                               ⎟            +W
                   reb + 10 ⎠
                                   richpump    leanpump
               ⎝


 W = Equivalent work
 Qreb= Reboiler Duty
 Treb= Reboiler Temperature (◦C)

                                                          Figure 3. Sensitivity analysis
Modeling and Optimization of CO2 removal in power plants

Conclusion and future work

In this work, lean loading is the only one variable used for
optimization. Basically, in addition to lean loading, other
parameters such as stripper pressure, packing height and cross
heat exchanger temperature approach are effective and can be
accounted for optimization.
     In next work, another process configuration (Double matrix)
will be modeled and optimized. We are determined to implement
some proper optimization techniques to consider all the variables
that are effective in optimization and also implement economic
issues to have more complete objective function.

								
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