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CO2 Capture by Adsorption:
General Principles
Douglas Ruthven,
University of Maine,
Orono, ME 04469
Stanford University, May 26 – 27th 2011
Global Warming ??
Orono ME April 1st 2011 (14 inches Snow!)
Outline
1. The challenge - Direct air capture vs point source.
2. Very large scale!
3. Present Technology - Amine Absorption.
4. Other Options – Economic Considerations.
5. Adsorption Systems:
Contactors/pressure drop
Process Schemes
Regeneration of Adsorbent
Adsorbent Choice/New Adsorbents
6. Conclusions
Options for CO2 Capture
Direct Air Capture (DAC) - pCO2 ≈ 3.7 x 10-4 atm.
Advantages: Clean feed, free choice of location
(e.g. to optimize use of solar power).
Disadvantage: Very low feed concentration!
Cproduct/Cfeed ≈ 2800
Point Source Capture (Power Station or Cement Works) pCO2 ≈ 0.13atm.
Advantage: Much higher feed conc.
Disadvantages: Dirty feed, location fixed by site of power plant.
Advantage of higher feed concentration overwhelms other considerations!
See Report of American Physical Society Chemical Capture of CO2 from Ambient Air (2010).
Thermodynamic Considerations
Minimum theoretical Work of Separation = -ΔGmixing
Based on product Wmin increases asymptotically as xf→0.
Energy required for separation from a dilute feed is large.
Selling price as Function of Concentration in
raw feed (log-log plot)
Separation costs are dominant so costs scale with feed concentration
regardless of nature of product. (T.K.Sherwood)
The Challenge: Huge Scale of operation, very
large flow rates
1000 Megawatt Coal Fired Power Station.
Stack Gas Flow Rate = 2.3 x 105 kg.moles /hr.
= 6 x106 m3 /hr.
≈ 1.0 cubic km per week!
CO2 Flow Rate = 3 x 104 kg.moles /hr.
= 1300 Tonne/Hr
≈ One Tractor trailer Load in
Four Mins!
Present Technology - Absorption
Absorption in MEA – Well optimised process (50 yrs)
434 Megawatt → 303 Megawatt net (with CO2 capture!)
COST: $ 60/Tonne CO2 or about 10c/kw hr. More than doubles electricity
cost! [DOE/NETL Report 401/110907 (2007)].
Can we do Better?
Possibilities:
Perm – Selective membrane
Adsorption:
Choice of Contactor
Regeneration Method (PSA/TSA)
Process Scheme/ Cycle Time
Choice of Adsorbent
High throughput requires rapid cycle to keep
adsorber volume within reasonable limits!
Scaling of Costs with Throughput
Membrane Adsorption or Absorption
COST COST
THROUGHPUT THROUGHPUT
At large scales of operation membrane processes are
unlikely to be economic!
Parallel Passage Contactor
Narrow Spacing – How narrow? Advantages:
Uniformity necessary to avoid dispersion Low ΔP
Metal backing to eliminate ΔT Good Mass Transfer
Rapid Response
Isothermal
Practical Monoliths
Optical micrographs of uncoated and washcoated cordierite honeycomb substrates. (a) uncoated honeycomb, (b) 18.2% washcoat (80% silicalite +20% silica binder), and (c) 30% washcoat (60% silicalite + 40% silica binder).
Schematic of Circulating Adsorbent –
Continuous Counter-Current System
Circulating Adsorbent Test System (TDA)
Pressure Swing vs. T Swing
Pressure Swing vs. T Swing
PSA: Requires approx. linear isotherm.
Delta loading is limited.
Rapid cycle is easily achieved (especially with parallel passage contactor).
Cycle times < 1 sec. are possible.
TSA: K = Ko exp(-ΔH/RT)
Small change in T gives large change in K – hence large delta loading.
BUT: Rapid cycle requires very fast heat transfer – difficult to achieve.
Minimum cycle time is minutes (not sec.)
QuestAir RPSA System
Rapid Cycle PSA (RPSA)
Current Status:
RPSA process (air separation) Questair Inc.
Small/Medium scale operation.
Cycle time ~1sec.
What are the limits on size/cycle time??
Possibility of a small system with very high
throughput.
Required scale-up for CO2 capture very
difficult!
Rapid Cycle TSA
PSA (and RPSA) only for weakly adsorbed species.
RTSA would allow use of stronger (higher capacity)
adsorbents! - but fast heat exchange is difficult.
One Possible Approach: Hollow fibre adsorbent
(Lively et al. I and E.C.Res. 2009)
Rapid Cycle TSA (Lively et al. I and E.C.Res. 2009)
.Very low ΔP on gas side.
Heat exchange fluid through central tube.
Rapid response but minutes not seconds.
Possible application for CO2 from stack
gas.
Adsorbents for CO2 CaptureZeolite 5A (Early Mitsubishi Trials 1990s)
400K: KCO2 ≈ 2.5 mmole/g.
(-ΔH) ≈ 42 kj/mole
Capacity at 400K, 0.13 atm ≈ 0.33mmole/g.
High affinity for H2O limits use with humid stack gas.
Adsorbent is unstable to acid conditions (SOx).
TDA Adsorbent (2009) “Alkalized Alumina”
More robust adsorbent! Live steam regeneration.
Similar capacity (0.3 mmole/g. at 400K, 0.13atm.)
Smaller (-ΔH) ≈ 23 kJ/mole – requires larger T swing for regeneration.
Amine Functionalized Silica - Potentially attractive but further testing needed!
Working Capacity (delta loading) of Adsorbent-
The Achilles Heel of Adsorption processes.
Working capacity ≈ 1.3%wt ≈ 0.3 mole/kg adsorbent.
1000 Megawatt power Stn. → 500kg mole CO2 /min.
Adsorbent circulation rate ≈ 1800 tonnes/min.
Assume one minute cycle time (adsorption + regen. time).
Rotary Wheel Contactor diameter 10m, depth 1.0m.
24 Wheels in parallel to provide required
adsorbent circulation rate!!
If one wheel, diameter = 48m - impractical !
Viable process will require a much higher working capacity
and/or shorter cycle time.
Improved Adsorbents?
Amine functionalized mesoporous silicaSayari and Belmabkhout, Adsorption 15, 318 (2009); Ind.Eng.
Chem.Res. 49. 359 (2010); Chem. Eng.J. 158, 513 (2010)
Highly selective for CO2. Capacity ≈ ten times 5A or Functionalized Alumina. High K requires thermal swing regeneration.
Amine functionalized mesoporous silicaEffect of water vapor is minimal! Promising but further
testing under more realistic conditions is needed!
MOFS: Millward and Yaghi JACS (2005)
MOFS: High saturation capacity but relatively low K so
working capacity at low partial pressures appears
modest – comparable with AC.
Conclusions
Parallel Passage Contactor and rapid cycle process to achieve high throughput.
Thermal swing or pressure swing? Both are possible.
P swing is faster but difficult to achieve high working capacity.
Successful demonstration at pilot scale –
Approx. $ 40/Tonne CO2 (c.f. $60/Tonne for current Amine Absorption) – still ≈ 5c/kwh !!
BUT: Scale-Up for 1000 Megawatt Power Stn. Problematic.
Improved adsorbent with working capacity ~10%wt CO2 is needed for economic viability – supported amines?
Wheel system (Inventys) with such an adsorbent: $20/Tonne CO2 (~3 cent/kwh) – estimated.
Thermal Swing – 5min cycle time
Membrane separation
Feed
(A+B)
Retentate (B+trace A)
High P
Permeate (A+ trace B)Low P
High P
Gas or Liquid
Membrane Element
If ph>>pl Sep.Factor(α)→KADA/KBDB=πA/πB=s
Zeolite Monoliths (Crittenden)
Requirements: Channel diameter ~ wall thickness < 0.5 mm. For good mass transfer.
Uniform channels to minimize axial dispersion.
Counter-Current and Simulated
Counter-Current Processes