(70g) 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

Energy Cost $/MMBTU	$5.00
Approach Temperature F	LMDT	Duty MMBTU/Hr	HT Area FT2	Additional Energy Recovered MMBTU/hr	**Yearly Energy Benefit $	Cumulative yearly Energy Benefit $
11	18.6	760.8	96120
10	17.6	767.3	104460	6.5	$334,941	$334,941
9	16.6	773.9	111720	6.6	$340,094	$675,035
8	15.5	780.4	119580	6.5	$334,941	$1,009,976
7	14.4	786.9	127380	6.5	$334,941	$1,344,918
6	13.3	793.5	134260	6.6	$340,094	$1,685,012
5	12.2	800	146300	6.5	$334,941	$2,019,953
4	11	806.6	195480	6.6	$289,080	$2,309,033
					**Assumes 85% burner efficiency

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