Squaring the circle in drying high-humidity air by a novel ...

Squaring the circle in drying high-humidity air by a novel composite sorbent with high uptake and low pressure-drop

Meltem Erdogan1, Claire McCague2, Stefan Graf1, Majid Bahrami2 and André Bardow1

1 Institute of Technical Thermodynamics, RWTH Aachen University, Germany 2 Laboratory for Alternative Energy Conversion (LAEC), School of Mechatronic Systems Engineering,

Simon Fraser University, Canada V3T 0A3

HPC 2018, 16-19th September

2

Adsorber

Adsorptive Drying

Squaring the circle in drying high-humidity air by a novel composite sorbent

05.02.2020

Humid Air Dry Air

Key: Sorbent

Silicagel Composite Sorbent

• Uptake

• Pressure drop

• Dehumidification Rate

• Hydrothermal Stability

3

Novel Composite Sorbent for Drying


05.02.2020

Preparation

Characterization

• Isotherms

• Uptake

• Pressure drop



4

Preparation of Composite Sorbent I/II


05.02.2020

• Prepared in batches of 400 g

• 55 wt.% mesoporous silica gel, 30 wt.% CaCl2 and

15 wt.% polyvinyl alcohol

Irregular grains:

0.2-0.5 mm

Mean pore diameter:

16 nm

Surface area:

276 m2/g

Pore volume:

1.1 cm3/g

B150 SiliaFlash

No.

Silicycle

SiliaFlas

h

Type

Mean pore

diameter

(nm)

Pore

vol.

(cm3 g-1)

No.

Mean

pore

diameter

(nm)

Pore

vol.

(cm3 g-

1)

S4 B40 4 0.58 CaCl2-

S4 7 0.27

S6 B60 6 0.75 CaCl2-

S6 9 0.37

S9 B90 8 0.83 CaCl2-

S9 10 0.58

S15 B150 16 1.10 CaCl2-

S15 18 0.60

• PVA (85,000-124,000 MW) combined with an-

hydrous calcium chloride and dissolved in 600 mL

distilled water

• Solution was combined with a mesoporous silicagel

• Oven dried at 80 °C and cured at 150 °C

Porosimetry: SBET = 104 m2/g, 0.44 cm3/g

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CaCl2/silica gel composite sent to LTT: Batch 1 & Batch 2


05.02.2020

Composite

batch 1

0

20

40

60

80

100

120

140

0.00 0.20 0.40 0.60 0.80 1.00

H2O

Up

take

(%M

ass)

H2O P/P0

Water sorption isotherm for A3 batch 1 and batch 2 at 25°C(55% Silica gel, 30% CaCl2 , 15% PVA)

A3 batch 1

A3 batch 2

A loose gain composite of CaCl2 in silica gel

B150 would have too high a pressure drop

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Preparation of Composite Sorbent II/II


05.02.2020

• The sorbent was further baked at 250 °C for 24 h, during which it darkened

PVA Binder: Tg = 85°C

mp = 180-190°C (partially hydrolyzed), 230°C (fully hydrolyzed)

Decomposes above 200°C; undergoes pyrolysis at high temperature

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Pyrolysis of PVA Binder


05.02.2020

PVA

Intermediates

Char

Volatiles

Volatiles

Gilman et al, Thermal decomposition chemistry of poly(vinyl alcohol), ACS 1995

0°C 200°C 400°C 600°C

Temperature

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Silica Gel “Grace Sylobead B127” isotherms


05.02.2020

0.0

0.1

0.2

0.3

0.4

0.5

0 5 10 15 20 25 30

w(g

/g)

P (mbar)

14.9C Ads 14.9C Des 19.8C Ads 19.9C Des 24.6C Ads 24.7C Des


15 °C 20 °C

35 °C

25 °C

40 °C

78 °C

• Silica gel provided by

RWTH Aachen team

• Isotherms measured at

with a thermogravimetric

vapor sorption analyser

(Hiden Isochema IGA-

002) at LAEC, SFU

• Adsorption solid symbols

• Desorption open symbols

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Silica gel isotherm set plotted as a function of P/P0


05.02.2020

0.0

0.1

0.2

0.3

0.4

0.5

0.0 0.2 0.4 0.6 0.8 1.0

w(g

/g)

P/P0



• P/P0 plots are easy to

read

• Equilibrium uptake

capacity for 60, 70, and

80 RH%

70%

RH

80%

RH 60%

RH

• The data can also be

plotted as a function of

adsorption potential

ΔF =-RTln(P/P0)

10

Silica gel uptake characteristic curve


05.02.2020

I don’t have this much data or quality of data for the composites (yet). For the

composites, collecting isotherm data at high P/P0 is slow (slow adsorption once the

porous matrix is flooded, and slow uptake by the binder).

The “hook”

Curve section for 55 to 85% RH

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05.02.2020

Preparation

Characterization

• Isotherms

• Uptake

• Pressure drop



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Isotherms of Composite Sorbent and Silicagel


05.02.2020

• The composite sorbent takes up more than

twice the water as silicagel

• For high humidity air (> 80 % RH), the

composite sorbent reaches a maximum

uptake of more than 1 kg/kg

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.0 0.3 0.6 0.9

w (

g/g

)

P/P0

Sylobead B127silica gel

Composite "A3"

Z02

13

Fixed Packed Bed


05.02.2020

Diameter: 100 mm

Length: 50 mm

Volume: 0.393 L

Composite Sorbent Silica gel

Open porous sponge

structure formed inside

adsorber

108 g dry weight inside

adsorber

Spherical beads of

~2 mm in diameter

332 g dry weight inside

adsorber

Adsorber

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Process Parameters & Experimental Setup


05.02.2020

• Adsorption temperature of 40 °C

• 60, 70 and 80 % RH

• Regeneration temperatures of 100 °C and 150 °C

• Air flow of 40 m³/h

Adsorber

15

Water Removal


05.02.2020

• Water removal of composite sorbent

• 37 % lower than silica gel (> 60 % RH)

• 20 % lower than silica gel (> 80 % RH)

• Water removal of silica gel higher

because bulk density is about 3 times

higher compared to composite sorbent

Adsorption temperature of 40 °C

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Specific Uptake


05.02.2020

• For high humidity air (> 80 %

RH), the composite sorbent

had a maximum uptake of

about 0.78 kg/kg and absorbs

2.6 times more than silica gel

• Uptake of composite sorbent

more than twice as high as for

the silica gel

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Pressure Drop across the Bed


05.02.2020

Composite sorbent has a

8-10 times lower pressure

drop than silica gel

Due to sponge

structure inside the

adsorber instead of

packed bed of 2 mm

beads

18

Dehumidification Rate (DR)


05.02.2020

• DR lower for higher regeneration temperature, due to longer cooling period

• Highest DR for both adsorbents at 100 °C and 70 % RH

Composite ≈ 83 gWater/h Silica gel ≈ 103 gWater/h

• DR of silica gel about 24 % higher for the best case

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Specific Dehumidification Rate (SDR)


05.02.2020

• Range of SDR

Composite ≈ 0.5 - 0.77 gWater/gSorbenth Silica gel ≈ 0.2 - 0.3 gWater/gSorbenth

• SDR of silica gel about up to 2-3 times lower compared to SGR of composite

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Hydrothermal Stability


05.02.2020

Thermogravimetric vapor sorption

analysis:

• 0 to 1.2 kPa swings in water

vapor pressure

• 40 minute cycles at 35 ˚C

• Average change in water content

per cycle was 0.1662 ± 0.0006

0.160

0.162

0.164

0.166

0.168

0.170

0 50 100 150

Δw

(g/g

)

Cycle

no measurable loss in uptake

capacity across 150 cycles

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05.02.2020

Preparation

Characterization

• Isotherms

• Uptake

• Pressure drop



Suitable

&

highly promising

for drying high humidity air

Composite

Sorbent

22

Thank you for your attention!


05.02.2020

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23

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Water - Silica Siogel” working pair for adsorption chillers: Adsorption equilibrium and dynamics, A. Sapienza, Renew. Energy 2016


05.02.2020

Siogel

Bulk density: 0.62-0.8 g/cm3

Internal surface area: 800 m2/g

Pore volume: 0.40 cm3/g

Specific heat: 0.6-0.8 J/(g∙K)

Pore average diameter: 2.0 nm

By comparison, the Dubinin-Astakhov approximation, w [kg/kg]= w0∙exp[-(F/E)n], has three fitting parameters w0 = 0.38 kg/kg, E = 220 kJ/kg, and n=1.1.

Both approximations have almost the same standard deviations. Therefore, they provide a good analytical description of the whole set of experimental data and can be used for modeling.

Only w0 has a physical meaning (maximal mass of water that can be adsorbed).

Temperature-invariant curve of adsorption

Equilibrium uptake, w [kg/kg], vs the Dubinin-Polanyi potential

F = -RT ln(PH2O/Ps)

Equilibrium loading w can be precisely described by the simple

exponential expression w [kg/kg] = w0∙exp(-b∙F) where

w0 = 0.4031 kg/kg and b = 0.0051 kg/kJ are fitting parameters

determined from the experimental equilibrium data.

Siogel Surface area: 800 m2/g

Pore volume: 0.40 cm3/g

Pore average diameter: 2.0 nm

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CaCl2 in silica gel porosimetry


05.02.2020

No.

Silicycle

SiliaFlash

Type

Mean pore

diameter

(nm)

Pore vol.

(cm3 g-1) No.

Mean pore

diameter

(nm)

Pore vol.

(cm3 g-1)

S4 B40 4 0.58 CaCl2-S4 7 0.27

S6 B60 6 0.75 CaCl2-S6 9 0.37

S9 B90 8 0.83 CaCl2-S9 10 0.58

S15 B150 16 1.10 CaCl2-S15 18 0.60

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Isotherms of Composite Sorbent and Silicagel


05.02.2020

• The composite sorbent takes up more than twice the water as silicagel

• For high humidity air (> 80 % RH), the composite sorbent reaches a

maximum uptake of more than 1 kg/kg

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LTT open system test bed

Conditions: TDesorption=150°C

TAdsorption=40°C, RH ~60%

Volume flow: 40 m3/h

Sorbent sample: 34.5 wt% CaCl2

in Sylobead B 127

~170 g, diameter

100 mm

Issue: Obvious salt wash out by

30% RH

Application: Dishwasher drying

cycle, open system

Objective: Compare potential of

CaCl2 composites to the zeolite

currently used in commercial

systems (favorable theoretical

comparison was presented by

Meltem at IMPRES)

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30 wt% CaCl2, 55 wt% SiliaFlash B150

silica gel (~15 nm pores, 0.2-0.5 mm

irregular grains, Silicycle Inc.), 15 wt%

polyvinyl alcohol (PVA) binder (85,000-

124,000 MW)

Open performance tests by Meltem

250°C baking at Aachen (partial

pyrollization of binder)

Aachen ‘A3’ preparation

Squaring the circle in drying high-humidity air by a novel ...

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