The Role of Heat Exchangers In Post-Combustion CO2 Capture

The role of heat exchangers in post-combustion CO2 capture

Warren Ziegler

Motivation:  The overall energy penalty for post-combustion carbon capture by aqueous amines as well as solid sorbent systems can be reduced significantly by recovering heat from the lean/hot solvents coming out of the regenerator and using the energy to pre-heat the rich/cool solvent absorber column. This paper will look at the potential of heat recovery via computer modelling of  the cross exchanger in aqueous amine capture as well as heat recovery in solid from solid CO2 sorbent systems.

For the aqueous amine case Oyenekan shows that lowering the approach temperature from 10C to 5C reduces the reboiler duty from 151 kJ/gmol CO₂ at 10C to 127 kJ/gmol CO₂ at 5C.
 Alfa-Laval modeled one of their plate and frame heat exchangers in an amine post-combustion scenario using a proprietary model similar to the package Xphe from the Heat Transfer Research Inc. with the following assumptions:

● 800MW plant

● No flashing of rich feed

● Hot Side:
257F --> 143F

● Cold Side:
132F --> 228F

Heat Exchangers for Solid CO2 Sorbents

Solex thermal solids heat exchanger

Solex Thermal modeled heat recovery for a solid sorbent CO2 post combustion capture scenario under the following model assumptions:

1) The sorbent will be an immobilized amine sorbent. PEI is deposited on a silica substrate.

2) Bulk density is about 44-lb/cu ft.

3) Average particle size is projected to be 600 micron.

4) Specific heat is 0.45-Btu/lb-^oF as experimentally determined.

5) Moisture content is assumed to be zero (for simplicity at this time)

6) Angle of repose is approximated to be 37^o. 

7) Derived from the MATRIC report where a 500-MW plant was used as a basis.

8) Solid flow (lean sorbent from the regenerator) into the absorber is 568 klb/hr or 4.7 ton per minute. 

9) The solid flow into the regenerator (rich sorbent from the absorber) is 643 klb/hr or 5.36 ton per minute.

Results

1) Multiple coolers/heaters needed

2) Each cooler/heater is 35' h x 6' sq

3) Each heater/cooler has 3000 ft2 plate surface area

4) One cooler processes 35,000 lb/hr lean sorbent

5) Sixteen coolers needed to process 568,000lb/hr for hypothetical 500mw plant

6) Seven (6.5) heaters needed to process 643,000 lb/hr rich sorbent

Could Increase Area and Throughput by 20%

Relative Performance of Liquid Amine and Solid Sorbent Heat Exchangers

1) For liquid/liquid cross heat exchanger for amine scrubbing get a heat transfer coefficient of ~425BTU/ft2/hr/F

2) For solid/liquid heat exchanger for solid sorbents get a heat transfer coefficient of      ~11BTU/ft2/hr/F

References:

Chris Wajciechowski, Alfa-Laval private communication

Stripper Configurations for CO2 Capture By Aqueous Amines? paper presented at Annual AIChe meeting 2010 David H. Van Wagener; Dr. Gary T. Rochelle, The University of Texas at Austin, Austin, TX

Oyenekan, B. A. (2007).  Modeling Strippers for CO2 Capture by Aqueous Amines, Ph.D. Dissertation, The University of Texas at Austin, May 2007

Andy Nix, Solex Thermal private communication