David Van WagenerThe University of Texas at Austin

Research Review MeetingJanuary 11, 2008

OverviewBackground

Introduction to stripper modelingMinimizing stripper energy requirementSolvent and configuration options

Recent pilot plant results/rate based stripper model

Equilibrium stripper model in Aspen Plus®

System model resultsSolar powered stripping

Stripping by flashingConclusionsFuture Work

Absorption/Stripping

Steam 30 psia

Absorber Stripper

Gas in

Sweet Gas Out

Makeup Water

Lean SolventRich Solvent

CO2

The Need For Stripper ModelingStripper 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 CO2 removal

Contributions to Energy RequirementSensible heat

Influenced by heat capacity and liquid flow rate

Latent heatControlled by heat of desorption:

Stripped steamDescribed by H2O/CO2 ratio in product vaporRelated to energy requirement through heat of

vaporization of water

2,COdesH

TCOmol

LCH

capacity

solvP

1

2,

OHvapHCOmol

OHmolH

2,2

2 *

Solvent ChoicesPrior work by Oyenekan concluded

performance is enhanced by using solvents with:High ΔHdesorption

High capacityHigh reaction rate with CO2

MEAIndustry standard: 7m MEAGreat ΔHdes and reasonable capacityReaction rate with CO2 hinders performanceDegradation at high T is an issue

Solvent ChoicesK+/PZ

5m/2.5m has a high ΔHdes and fast rates, but only marginal capacity

4m/4m improves the capacity and maintains other qualities

MDEA/PZAlso has a high ΔHdes and fast ratesAdditionally, it has exceptional capacity

Stripper ConfigurationsVarious configurations of stripping columns

can improve performance by:Reducing reboiler dutiesReducing stripped steamDecreasing the lean loading and solvent rate

Significant work was done by Oyenekan to identify potentially beneficial configurationsInternal exchange stripperMultipressure stripperDouble matrix stripper

Determined to be most beneficial

Double Matrix Stripper

Rich Solvent

Lean Solvent

Semi-lean Solvent

Product CO2

High P Low P

Water Knockout

Flashing Stripper using Solar Energy

Rich Solvent

Lean Solvent

Product CO2

Water Knockout

Solar energy via heated medium

T1, P1

T2, P2

T3, P3

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 CO2 efficiency and

number of stages

• Equilibrium reaction stages• Ratefrac (Freguia) and Fortran

(Tobieson, NTNU)• Access built-in models for a

kla, kga• Specify packing type and

height

Kinetics with Mass Transfer• Rate approximation

• Rate-based Radfrac and ACM• Simple boundary layer• Specify kg’=f(ldg, T, kl)

• Rigorous rate calculation• RateSep• Multiple boundary layer

segments• Specify rate constants• Current absorber modeling

approach

Modeling SoftwareEquilibrium 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 Loading1. 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

KTKT

TKTQW

reboilersn

i i

iieq 313,

10

10*75.0 sink

1

sink

Task: Analyze Recent Pilot Plant Run35%wt MEA was used to remove CO2

Analysis of stripper section:Loadings: 0.48 0.36Preboiler: 15.23 psiaMax temperature: 216.9°F (≈102°C)Removal: 63%Equivalent work: 41.2 kJ/mol CO2 (no compression or

pumping)

Currently the results are being evaluated using:Hilliard VLEEquilibrium reactions in RateSepSimulation flowsheet reflecting pilot plant operation

Regressions will be used to reconcile differences

Stripper feed

Stripped vapor

Pilot Plant Reboiler DesignReboiler 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 0.488 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 0.387 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

H2O return

CO2 product

Absorber Section

Cross Exchange Section

Stripper Section

Compression Section

Lean in

5.5 kmol/s

40°C

12.7% CO2

90% removal of CO2

160 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 CO2 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

Specification Vary

Optimized Double Matrix/Intercooling vs. Simple Stripper/No Intercooling

Optimum loading is slightly different for two casesDouble matrix configuration yields energy savings,

but not overwhelminglyThe magnitude of savings do not agree with Oyenekan

data, but could be attributed to difference in loadings

Matrix Simple

Rich Loading (mol CO2/mol alk) 0.500 0.485

Lean Loading (mol CO2/mol alk) 0.385 0.397

Pressure (kPa) 265 -

Split 0.305 -

Equivalent Work (kJ/gmol CO2, to 1MPa) 31.48 33.71

Equivalent Work (kJ/gmol CO2, to 10MPa) 39.73 41.96

To

dT

Solar stripping analysisMEA 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 loadingOptimize the lean loading to minimize equivalent work

Solar Stripping Analysis

ConclusionsThe data from a pilot plant run was evaluated

The thermal efficiency is lowAn integrated system model was designed in

AspenPlusThis 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 WorkImplement new models for MEA, K+/PZ, and

ROC-16Upgrade to rate-based simulations

Explore mass transfer mechanisms in stripperFlashing in top stageRate-based reboiler as opposed to equilibrium

Complete Aspen tasks with MEARegress and reconcile differences for pilot plant

run Verify accuracy of new Hilliard MEA model

Quantify the feasibility of solar stripping