Prof. Joe Wood Dr Yu Rong, Dr Jiawei Wang*

School of Chemical Engineering University of Birmingham, UK

*School of Chemical Engineering and Applied Chemistry, University of Aston, Birmingham, UK

April 2014

Studies of Hydrotalcite Clays for CO2 Adsorption

STEPCAP Project: Step Change Adsorbents for Post-Combustion Carbon Capture

Aim: To develop advanced adsorbents for post-combustion CO2 capture

Adsorbents should have desirable kinetics, capture capacity, stability and ability to be regenerated

Performance Parameter Target Operating Temperature

: adsorption 40 – 80 °C : desorption 85 – 160 °C

Cyclic capacity > 3 mmol g-1 Operating pressure ~1015 mbar CO2 product purity > 95 % CO2 capture > 80 %

Proposed operating conditions for capture plant (T. Drage et al, J. Materials Chem 2011)

Post – combustion technology

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Adsorbents Manufacturing method and Characterization

Temperature swing adsorption Cyclic operation using fixed bed Process modelling Sensitivity analysis

Evaluation of NiMgAl N2 in post-combustion capture Optimization

Conclusions

Overview

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Layered double hydroxides (LDH) Hydrotalcite-like compounds composed of positively

charged layers with charge balancing anions located in the interlayer region

Structure: Advantages

High capacity and stability of CO2 adsorption at elevated temperature

They are inexpensive to prepare and are environmentally friendly.

Layered double hydroxides (LDH) Hydrotalcite-like compounds composed of positively

charged layers with charge balancing anions located in the interlayer region

Structure: Advantages

High capacity and stability of CO2 adsorption at elevated temperature

They are inexpensive to prepare and are environmentally friendly.

Layered double hydroxides

O. Aschenbrenner et al., 2011, Chemical Engineering Research & Design, 89, 1711 -1721

Layered double hydroxides (LDH) Hydrotalcite-like compounds composed of positively charged

layers with charge balancing anions located in the interlayer region

Structure:

Advantages Stability of CO2 adsorption at elevated temperature Reasonable adsorption rate They are inexpensive to prepare and are ‘environmentally

friendly’.

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Amine modified layered double hydroxides

Amine adsorbent

Si

O

OO

NH2

Si

O

OO

NH

NH2

Si

O

OO

NH

NHNH2

3-Aminopropyl triethoxysilane (N1)

N-(2-Aminoethyl-3-aminopropyl) trimethoxysilane(N2)

N-3-(Trimethoxysilyl)propyl) diethylenetriamine (N3)

Different Types of Aminosilanes

JW. Wang et al., 2012, Chemical Engineering Science, 68, 424-431

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Manufacturing method

P. Harlick, and A. Sayari, Ind. Eng. Chem. Res. 2007, 46, 446-458.

Step 1: Dodecyl sulfate (DS) anion intercalated LDH was synthesized by co-precipitation (NiMgAl DS)

Step 2: NiMgAl DS was exfoliated in toluene

Step 3: Single-layer suspension was reacted with amino group (NiMgAl Nx)

Water-aided exfoliation method

Step 1 Step 2

Step 3

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CO2 adsorption capacity of N1, N2 and N3 amine modified HTLCs vs temperature

J. Wang et al, Chem. Eng. Sci. 68 (2012) 424-431

Adding water enhances N2 and N3 amines grafted upon the LDH. CO2 uptake is peaked with 0.2-0.4 ml/g water added for NiMgAl N2.

Characterization

Water added (ml/g) C

O2 u

ptak

e (m

mol

/g)

Netzsch TG 209 F1 thermogravimetric analyzer

(CO2 uptake)

Flash EA 1112 elemental analyzer

(Amine loading)

Water added (ml/g)

Am

ine

load

ing

(m

mol

N/g

)

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Characterization

Model Parameter 25 oC 50 oC 80 oC

Avrami nA 1.09 0.8 0.78 kA (min-1) 0.029 0.07 0.142

Err (%) 1.18 4.28 4.36

1st order models

kF (min-1) 0.027 0.053 0.11 Err (%) 2.62 8.01 9.17

Kinetics

Adsorbents: NiMgAl N2 Kinetic models: Avrami and Lagergen’s pseudo-fist order models; Parameters in the kinetic model were calculated from experimental data through linear regression.

( )teFt qqk

tq

−=∂∂

( )te1nn

At qqtk

tq AA −=∂∂ −

1st order:

Avrami:

Time

CO

2 upt

ake

(mm

ol/g

)

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Temperature swing adsorption using NiMgAl N2 Performance Parameter Target NiMgAl N2 Operating Temperature

: adsorption 40 – 80 °C 65-85 °C : desorption 85 – 160 °C ~140 °C

Cyclic capacity > 3 mmol g-1 ~ 2.7 mmol g-1 CO2 product purity > 95 % 97-98 % CO2 capture > 80 % 90-95 %

Properties of the optimised

adsorbent (NiMgAl N2)

TSA Cyclic Operation

Step 1: Adsorption is operated at ~80 oC; pressure close to ambient.

Step 2: Operating temperature is raised to ~ 140 oC.

Step 3: Desorption continues until meet the recovery target.

Step 4: Cooling returns back to Tad.

1 3 Tde = ~140 oC

Tad = 65-85 oC

2

4

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Cyclic operating conditions

Fixed bed reactor (L/D = 5~9) 100~200 ml/min CO2/N2 mixture 10~15% CO2 in feed gas 80 °C for adsorption 140 °C for desorption ~1 bar pressure

Experimental Procedure

Tem

p (o

C)

CA/

C0

Adsorbents- NiMgAl N2

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Fixed bed model Gas concentration

C = concentration of component (mol/m3) DL = axial dispersion coefficient (m2/s) H = heat of adsorption (J/mol) P = pressure (Pa) T = temperature (K) us = superficial velocity (m/s) µ = viscosity (Pa·s) ρ = density (kg/m3)

( )t

qερε

zC

εu

zC

Dt

C isisiL

i

∂∂−

−∂∂

−∂∂

=∂∂ 1

2

2

• Temperature of gas phase

• Temperature of solid phase

( ) ∑

∂∂

−+−=∂∂

tq

HρTTdh

tT

Cρ iissg

p

fsss Δ

6

( )t

TCρε

zT

uCCzT

ελt

TCCε s

ssg

sg,fg

Lg

v,f ∂∂

−−∂

∂−

∂

∂=

∂

∂12

2

( ) ( )wgint

wiis TT

dh

tq

Hρε −−

∂∂

−−+ ∑ 41 Δ

Energy balance

( ) ( )3

2

32

2 11εd

uερB

εduεμ

AzP

p

sg

p

s −+

−=

∂∂

−

Pressure drop

T.L.P. Dantas et al., 2011, Chemical Engineering Journal 169, 11-19

R. Serna-Guerrero, 2010, Chemical Engineering Journal 161, 173-181

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Assumptions: The gas phase follows the ideal gas law; Constant gas flow rate and uniform void fraction along the column; The mass and temperature gradients in the bed radial direction are negligible.

Model validation A dynamic fixed bed model has

been developed for

Gas separation simulation

Process evaluation

Optimisation

Feed gas: F0 = 105 ml min-1 CO2 = 14.3 %

Validated by experimental-simulation fit

Feed gas: F0 = 150 ml min-1 CO2 = 15 %

Feed gas: F0 = 200 ml min-1 CO2 = 10 %

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• Base case: No CO2 capture • Downstream CO2 capture using NiMgAl N2 adsorption

Overall: Process Modelling

Constraints: • Downstream flue gas properties • CO2 capture and recovery target • Operating condition (temperature, pressure and residence time)

Cycle design: • Dimension of the column(s) • Operating conditions

Performance: • Power for steam/gas stream fed into column(s) • Steam for desorption processes • Cost of fuel (and CO2 emissions) for supplementary energy • Operating cost per unit of CO2 avoided

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Adsorption-desorption cycle

Target Adsorption:

Capture: 90% of feed CO2

Desorption: Recovery: 85% of adsorbed CO2

Flue gas Pressure (bar) 1.4 Temperature (oC) 93.1 Gas flow (mol/s) 200 Composition (mol%, dry) CO2 14.3 N2 80.7 O2 5

J. Zhang et al., 2008, Energy Conversion and Management, 49, 346 -356

SEQUESTRATION

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Adsorption step

Constraints (Retention time, pressure) Gas-adsorbent interaction Breakthrough curve

Cyclic operating

Fixed bed column - Internal diameter : 3.1 m - Length : 6.34 m

16.3 ton NiAlMg N2 do=2.5 mm

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Steam temperature 120~270 oC

Steam flow rate 100~300 mol/s Pressure 1.1~1.4 bar Desorption time < 60 mins

Desorption step

To recover 80% of adsorbed CO2

Initial point:

Saturated NiMgAl N2 q = 0.82 mol kg-1

Column temperature T = ~95 oC

Bounds set based on industrial practices and material limitations

Fixed bed

Steam

Steam +CO2 Flue gas

Emission

Separation & compression

Cyclic operating

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Mohammad R. M. Abu-Zahra, Carbon Dioxide Capture from Flue Gas, 2009

• Base case: CO2 emissions 100 ton per day • Desorption operating • Flue for supplementary energy: coal

Effect of steam temperature

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Effect of steam flowrate • Base case: CO2 emissions 100 ton per day • Desorption operating • Constraints: operating time; pressure

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Optimize cyclic operating • compare performance to base case (no CO2 emissions avoided)

Objective function: Minimizing energy penalty per unit of CO2 emissions avoided

Design variables • operating pressure • steam flow rate • steam temperature

Constraints • outlet pressure • operating time • variable bounds

Optimization

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Optimum variables Optimised case

Bounds Lower/Upper

Pressure (bar) 1.22 1.1/1.4 Steam Temperature (oC) 180 120/270 Steam Flow rate (mol/s) 157.07 100/300

Objective function

Minimize energy penalty per unit of CO2 emissions avoided

Process optimisation - Results

Optimum results Base case Post-capture CO2 emissions from process

coal fired power plant (t/d) 100 0 downstream capture (t/d) - 10

Power demand supplement for blower (MW) - 0.16 Heat demand for desorption (MW) - 1.66

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Extra energy demand

Base case

Post-combustion capture

Absorption (29 % MEA)

NiMgAl N2 adsorption (opt.)

Energy for process (GJ/t CO2) - 2.35 1.5

CO2 emissions

CO2 emissions from process (t/d) 100 10 10

CO2 from utility system (t/d) - 18.7 11.8

Net reduction in CO2 emissions (t/d) - 71.3 78.2

Operating cost

Extra utility cost ($ based on 100t/d feed)

- 589 375

Cost of CO2 emissions reduction ($/t) - 8.25 4.79

Comparisons

L.M. Romeo, 2008, Applied Thermal Engineering. 28, 1039–1046

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Conclusions

Amine modified LDHs were synthesized via water-aided exfoliation and grafting route and studied as adsorbents for CO2 at the elevated temperature.

The highest adsorption capacity for CO2 was achieved by NiMgAl N2 when the amount of water added was 0.2-0.4 ml/g.

Avrami’s kinetic expressions was selected to describe the adsorbent and adsorbate interactions;

Non-isothermal model was built to predict the CO2 adsorption process in the fixed bed;

The fixed bed model was successfully reproduced all of the breakthrough curves.

Objective function is minimizing the energy penalty; Variables focus on the desorption step (i.e. pressure, inject gas flowrate and temperature)

Minimal energy penalty for the adsorption using NiMgAl N2 is 1.5 GJ/t CO2 avoid.

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Acknowledgements

EPSRC and E.ON for funding

Professor Trevor Drage (University of Nottingham, UK)

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Thank you. Questions?