EVALUATING PROCESS PERFORMANCE OF DAC SORBENT

- ADSORPTION ISOTHERMS AND WORKING CAPACITY

Researcher’s Seminar 15.-16. February 2016, Lappeenranta

Jere ElfvingVTT

Thesis results summary

Goal Measurements Result

Identify matrix FTIR Polystyrene

Identify functionalization FTIR Primary amine

Particle size & morphologyLaser diffractometrySEM

Median size 0.6 mmShape spherical, mesoporous

Surface area BET 26-32m2/g

Porosity BET, pycnometer Porosity 0.1 (low)

Study thermal stability TGA/DSC Sintering at 180CC

Adsorption capacity TGA ~0.75-0.90 mmol/g at 25CC

Thesis results summary

• Important qualities of a DAC sorbent:– High working capacity– Regenerability– High CO2 purity (selectivity)– Low regeneration temperature– Good overall performance in different conditions

Need for adsorption isotherm tests to produce models for estimating performance

Bench-scale experimental

Bench-scale experimental

• Start with dry gas mixtures– Only N2/CO2

• Total gas flow constant at 1l/min• Tests conducted at 25oC, 35oC, 50oC• Concentration levels 100-10000 ppm(=1%) CO2

• Points allocated to 100-1000 ppm to increase accuracy of the models for atmospheric conditions (400 ppm CO2)

Estimating the adsorption capacity

• Adsorption started

• 2 h total adsorption time

• Saturation achieved

• N2 purge started

• 30 min purge

• Desorption started by start of heating the column temperature to 80-90oC

• 1 h desorption time

Estimating the adsorption capacity

• The adsorption capacity calculated by integrating the concentration data

• Can be estimated from adsorption or desorption trends– True saturation point may be hard to evaluate from adsorption

trend (FTIR noise & difference in signals) too high or too low capacity

– Desorption capacities fairly close to adsorption capacities, but variate less from the fits

Desorption data used

Different isotherm models

partial pressure of CO2 (bar) 10-30 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

0

0.5

1

1.525oC fits

Desorption dataLangmuirFreundlichSips

• Which isotherm is the best?– Langmuir clearly fits poorly– Freundlich & Sips are very close to each other– Sips is a combination of Langmuir & Freundlich

Determining the best isotherm for adsorption capacity modelling

0

0.05

0.1

0.15

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Langmuir Freundlich Sips

Norm

aliz

ed s

tand

ard

devi

atio

n of

q (%

)

Averages of errors of fit in 3 temperatures

Physisorption

Total desorption

Chemisorption

% =/

1• measured value

• fitted value

Adsorption isotherms using Sips

partial pressure of CO2 (bar) 10-30 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

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0.5

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1.5Sips isotherm fits

25oC data25oC Sips fit35oC data35oC Sips fit50oC data50oC Sips fit

• Significant difference between temperatures especially in the low-concentration region

Toth isotherm

• Sips gives a good fit and the possibility to estimate the adsorption capacity at 25oC, 35oC or 50oC

• What about 30oC, for example?Temperature-dependent model neededToth isotherm

=1 +

• adsorption capacity (mmol/g)• maximum adsorption capacity (mmol/g)• Toth constant (1/bar)• Partial pressure of CO2 (bar)• Toth constant

Toth isotherm

partial pressure of CO2 (bar) 10-30 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

0

0.5

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1.5Toth model fits

25oC points25oC Toth fit35oC35oC Toth fit50oC50oC Toth fit

• Fits are reasonable• However, plateau ( ) increases very high

(>60mmol/g) at 25oC

The contributions of chemi- and physisorption

0.000

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0.200

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0.400

0.500

0.600

0.700

0.800

0.900

0 1000 2000 3000 4000 5000

q phy

s/qto

t

Concentation of CO2 (ppm)

Fraction of physisorption

25 C

35 C

50 C

• Physisorption lowest at 25oC (~30% at 400ppm)• At 50oC, physisorption dominant at already

200ppm

The contributions of chemi- and physisorption

• Usually physisorption considered negligible or not studied separately

• Both contribute to the capacity significantly in our caseIf only total capacity modelled, the contribution of these mechanisms is neglected

Also causes difficulty to estimating the parameters b,qm,t as shown

Use Toth model with both mechanisms recognized:

= + [1]

[1] Serna-Guerrero R, Belmabkhout Y, Sayari A, Modeling CO2 adsorption on amine-functionalized mesoporous silica: 1.A semi-empirical equilibrium model, 2010, Chem. Eng. J. 161, 173-181

The contributions of chemi- and physisorption

For chemisorption:• = 2.60 10 1/• = 0.718 /• = 0.300

For physisorption:• = 196 1/• = 15.1 /• = 0.226

partial pressure of CO2 (bar) 10-30 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

0

0.5

1

1.5Chemisorption and physisorption at 25oC

Physisorption pointsPhysisorption Toth fitChemisorption pointsChemisorption Toth fit

Working capacity

• Toth enables the estimation of working capacity: =– How much CO2 gained in different adsorption & desorption conditions

• Toth contains the parameters:

– = , where isosteric heat of adsorption (J/mol)

– = + (1 ) , where parameter of the temperature-dependent model

– = ( ) , where parameter of the temperature-dependent model

• These parameters determined in each temperature from the isotherms

Use to solve for T-dependent parameters , &

Gain T-dependent model

Working capacity

• Example results:– Adsorption conditions:

• T =25oC• CO2 concentration 400 ppm • Total pressure 1atm qads 0.509 mmol/g (from isotherm)

– Desorption conditions:• TSA: 80oC, ptot = 1atm • PSA: 25oC, ptot = 0.1atm (100 mbar)• TVA: 80oC, ptot = 0.1atm (100 mbar)

Working capacities:TSA: 0.2966 mmol/g ( 58% desorbed)PSA: 0.2863 mmol/g ( 56% desorbed)TVA: 0.4625 mmol/g ( 91% desorbed)

What next?

• Extending CO2 pressure range to 20-100%– To gain more consistent results

• plateau for physisorption still tends to increase too muchaffects the estimation of working capacity

• Adding humidity to the gas mixture– Expected improvement in q at least 20-30%

• Using N2/O2/CO2– Lower adsorption capacity due to oxygen?

• The effect of extreme conditions– Sub-zero temperature?