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