Supercritical Drying

Methods to dry and to shape aerogels: final properties vs. process time

Irina Smirnova Hamburg University of Technology (TUHH) Common steps for all aerogel geometries

• (Solvent exchange pior to drying) to produce suitable organogel • Autoclave loading • Pressurization of the system • Extraction under flow conditions (drying) • Depressurizatuon of the autoclave

2 Gel shaping

3 Aerogels: shapes and forms

Powders Beads Fibers

Lumira Cabot

JIOS SiO2 Monoliths Films Composites

4 Monolithic aerogels: Latest commercial developments - PU aerogels

New polymer-based aerogels @ BASF: Slentite • low thermal conductivity (18 mW/mk) • compression resistance of > 300 kPa • monolithic panels can be handled dust-free

•Production of test samples for industry since 2014

• Large pilot facility in operation since 2015

• More information in „Corpus“ (available online)

www.slentite.com 5 Form of the gel/aerogel

Supercritical extraction Precursors mixing Monolitic Gel Aerogel Gelation monolith + Aging Cross- Particle formation linking (emulsification, prilling , spraying) Supercritical Microspherical extraction aerogel Gel particles

6 Particle generation strategies ? …

Precursor solution Gel (micro)particles Aerogel (micro)particles Ø How to disperse the precursor solution to get particles?

Dispersion in a liquid phase Dispersion in a gaseous phase

7 Dispersion in a liquid phase Emulsion-gelation at lab-scale

Aerogel micro-particles

30 µm

[3] Carbon [1] Alginate [2] Silica (resorcinol- [4] Polyimide [5] Pectin and starch formaldehyde)

d90 = 400 – 1400 µm d90 = 1000 - 1500 µm d90 = 20 µm davg = 10-15 µm d32 = 500 µm

■ Emulsion gelation compatible with a variety of systems ■ Stirred vessel and batch rotor stator machines

Ganesan et al. Materials 11(11) 2144, 2018 8 Emulsion gelation: mobile continuous emulsion-gelation set-up

Trigger emulsion

sol oil

■ Continuous emulsion gelation: ● Two 20 L vessels for biopolymer and oil ● Flow rate controlled with the pump ● Valves to control phase ratio ● 200 L/h emulsion at 20 vol.% → 40 L/h particles

V.Baudron et. al. CIT, 2018, 9 Particle production: Jet Cutting technology

• Production of almost monodisperse beads 200µm- 3mm • Continuous upscalable technique • Well suitable for aerogel production

I.Preibisch et al. Materials, 2018, 11,1287 10 Solvent exchange on a large scale possible: Equipment at TUHH

Ethanol Feed Slurry Tank Pump

Slurry Moving Bed Eductor Column Motive Dynamic Pump Pinch Valve

Working prototype: Technology Readiness Level (TRL): • Prove of concept Transition from Level 6 to Level 7 • Actual maximum outflow: 0.150 L/min • Basic components are integrated together • Ethanol flow rate: 0.250 L/min • Finishing last details to test in an • Single units characteristic curves operational environment identification

www.nanohybrids.eu 11 Infuence of solvent: solvent/gel and solvent/CO2 interaction

Solvent selection framework 1. Optimal aerogel characteristics (e.g. bulk density (low) & surface area (high)) 1. Depend on the precursor system 2. Influences the processing time (shrinkage, final water/solvent concentrations)

3. Compatibility with sc-CO2 drying (shrinkage, rest solvent, process time)

2. “Non aerogel production” aspects of the solvent 1. Price & availability 2. Solvent consumption during the aerogel processing & recyclability 3. Minimization of process risks (fire, health and safety)

→ There is no “ideal solvent” for all aerogels: individual cases should be considered

→ Shrinkage should be considered and modelled (current work)

12 Supercritical drying

13 Supercritical drying evolution: from Kistler’s technique to present day

Supercritical drying: evolution of the process aim upon commercialization of aerogels: • From „how to dry a gel to prevent the 3D structure?“

• To „how to determine an shortest drying time and lowest CO2 consumption?“

Single phase

Two phase

14 Low-temperature supercritical drying

15 15 High pressure phase diagram for EtOH/CO2 system sc-fluid 12 12 °C) K (65 338 9 sc-fluid 9 °C) K (50 323 C) 6 °C) 6 ° 313 K (40 313 K (40 Pressure, MPa Pressure, Pressure, MPa Pressure, 3 L+G 3 L+G

0 0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Mole fraction of CO2 Mole fraction of CO2

vapor phase single phase phase boundary (supercritical) liquid phase

Two phases coexist along the phase Above the critical point only one boundary line phase can exist at any p – T § Analogous to one component mixture: densities of vapor and liquid phases are equal at the critical point 15 Low temperature sc-drying: check list

CO2 above pc – Tc for mixture

aerogel

Check point Literature data/input needed Select solvent(s) suitable for the gel Gelation procedure depending on the precursor preparation Check whether solvents in the gel are Binary CO2/solvent high pressure phase diagram miscible with CO2? If not, perform the solvent exchange Liquid-liquid ambient pressure phase diagram (liquid-liquid extraction) If yes, select operating conditions: Binary CO /solvent high pressure phase diagram temperature and pressure 2

16 Supercritical drying: process realization (typical batch operation)

Ø Important questions for upscale: autoclave loading and product removal

17 Infuence of solvent: solvent/gel interaction – water concentration

Pure Solvent Gel conc. % + Pure solvent consumed mH mH 50 Hydrogel gel/solvent cs 1:1 mass 2 x mH 75 ratio (Final gel conc.) 3 x mH 87.5

4 x mH 93.75

7 x mH 99.2

10 x mH 99.9

14 x mH 99.99

• Each additional step costs time and solvent • Solvent recovery process also important • Ethanol azeotrope 95.5 wt%

→ Each solvent-gel system tolerates different “rest water”

Raman Subrahmanyam, PhD Thesis 2019 18 Addition of solvent to prevent gel shrinkage: always needed?

• Is the solvent evaporation during pressurization significant? 60°C

Gel

60°C

Gel

60°C

Gel

Raman Subrahmanyam, PhD Thesis 2019 19 Autoclave loading: optimal space use vs. supercritical drying time and solvent consumption

�� = ��

�� + �� = ��

�� % = · 100 ��

• At high autoclave loading, no solvent addition generally needed Raman Subrahmanyam, PhD Thesis 2019 20 Solvent spillage: volume expansion during pressurization

conventional sc-drying D

tequilibration: 1 h B

A. 60°C, 0.1 MPa B. 60°C, 5 MPa C. 60°C, 8 MPa D. 60°C, 10 MPa

G G G G

L L1 L1 L1

G L 2 L2 L2

CO2 solubilizes into gel solvent causing liquid volume expansion → Solvent spillage

A. Bueno et al. Ind. Eng. Chem. Res. 2018, 57, 8698-8707 21 Integration of solvent spillage in the drying process

• Exemplary drying profile with account to the solvent spillage

Ca-Alginate 3 wt. %., 308.15 K.

Ø Initial pressurization time is actually a drying (extraction) time

22 Supercritical drying: experimental drying times available in literature

9 8 7 time [h] time 6 5 drying 4 3

Experimental Experimental 2 1 0 0 5 10 15 20 smallest gel thickness [mm]

Ø Very large range: there is no common scheme to choose a proper drying time

23 Supercritical drying: different aerogel geometries/sizes

Main mechanisms influencing the drying time and measures for optimization:

• Diffusion limitation: monoliths - Slow process (hours) - Gel thickness is crucial - Increase the diffusion coefficients: increase temperature, reduce viscosity - Carefull solvent selection: critical point of solvent-CO2 mixture is important

• Transition between diffusion and mass transport limitations: thin films and beads - Intermediate drying time (minutes-hours) - Flow inside the autoclave: geometry of the autoclave is important - Increase diffusion coefficients (as above)

• Mainly dominated by mass transfer: particles in µm range - Much faster process (minutes) - Flow conditions in the autoclave decisive (Re and Bi numbers) - Optimal autoclave geometry - More robust towards solvents selection

24 Understanding an extraction step:

Technische Universität Hamburg-Harburg predictive mass transport model Institut für Thermische Verfahrenstechnik

§ P=const., T=const.

§ Mass transport in porous gel network: diffusion [1-4]

� − concentration kmol⁄m ⁄ �� 1 � �� r � − diffusion coeficient m s � − mole fraction − = � �� − time s �, � − space coordinate m µ� ≤ � ≤ ��

§ Mass transport in surrounding fluid: convection [2-4]

�̇ − molar lux kmol⁄s �̇ � − mass transfer coeficient z kmol⁄m � − surface of the gel m

0 − surface �̇, = �� , − �, ∞ − bulk phase ⁄ �̇ − CO2 mass low kg s § Finite difference method �̇ + �̇ = ��.

[1] Orlovic et al. , J. Serb. Chem. Soc., 2005, 70 (1), p. 125–136. [2] Özbakır and Erkey, J. Supercritical fluids , 2015, 98, p. 153–166. [3] Griffin et al., J. Supercritical fluids, 2014, 94, p. 38–47. [4] Lebedev et al., J. Supercritical fluids, 2015, 106, p. 122–132. 25 •�̇ ��,�� : CO2 inlet flow �̇ , � •�̇ ��,��: CO2 outlet flow • �̇ ��,��: ethanol outlet flow z = 0

CO Transport z (�̇ �)| 2 in bulk Diffusion fluid � = 0 ∆� EtOH

� = � Transport in � boundary layer (�̇ �)|∆ � = � • �̇ : volume flow, •��: fluid density (CO2+ethanol), �̇ , + �̇ , •r: radial coordinate of spherical gel particle

Selmer et al., J. Supercrit. Fluids 140 (2018) 26 Mass transport in gel particles in radial direction:

��,(�, �, �) 1 � � ��,(�, �, �) = � �, �,(�, �, �) � �,(�, �, �) �� Extracted ethanol from gel particles (acts as source term in bulk fluid):

� � ��, �, �, �, � , �, �, � = − 4 � � � �, �, �, � �� Mass transport in bulk fluid/autoclave in axial direction:

��,(�, �) ��

� � �, � � � �, � � �,(�, �) , , = � (�) − + �� (� �, �, � , � �, � ) �� , , , �̇ � �, �, � = � � �(�, �, � )

Selmer et al., J. Supercrit. Fluids 140 (2018) 27 • Theoretical minimal supercritical extraction time t, as function of sphere radius and ratio of gel porosity �� to gel tortuosity �� 1.0E+05 1.0E+04 • T=318 K, 1.0E+03 • P=12 MPa, 1.0E+02 �� • ��,�� = 0.0097 1.0E+01 ε/τ = 1 tse,min [s] 1.0E+00 ε/τ = 0.4 1.0E-01 ε/τ = 0.125 1.0E-02 ε/τ = 0.07 1.0E-03 0.01 0.1 1 10 Radius gel sphere [mm]

Selmer et al., J. Supercrit. Fluids 140 (2018) 28 Supercritical drying: time of the process versus minimal possible drying time

9 Experimental sc. extraction/drying times 8 7 time [h] time 6 5 drying/ 4 3 2 Experimental Experimental 1 Min. theoretical drying time 0 0 5 10 15 20 smallest gel thickness [mm]

Ø In most publications the dyring time is too long: needs optimization at larger scale

29 Supercritical drying: analysis with dimensionless number

•The Biot number Bi relates the inner mass transfer to the mass transfer coefficient β for a single gel body

� �� , � = � � � � �, �

•Keff is an effective diffusion coefficient, calculated via the theoretical minimal supercritical extraction time tse,min of a spherical gel body of the radius R � 1 � �, � = � �, �

•The K1mean number presents the relation between inner mass transfer and outer mass transfer in the bulk fluid (mean value of all K1mean numbers being calculated at each time interval k and space interval s).

� � �1 = � (�, �, �, �) � �, ,, �, �, �, � �

Selmer et al., J. Supercrit. Fluids 140 (2018) 30 Supercritical drying of beads: role of Biot number

• Supercritical extraction time for various particle radii: 15 µm – 2.5 mm relative to the theoretical minimal supercritical extraction time at β→∞ 1.0E+04 limited by internal mass

[%] • T=318 K transfer • P=12 MPa 1.0E+03 tmin limited by external mass transfer

1.0E+02 drying time/

1.0E+01

supercritical supercritical 1.0E+00 1.0E-01 1.0E+00 1.0E+01 1.0E+02 1.0E+03 1.0E+04 Biot-number Bi [-] Ø For very small particles mass transfer is more important than diffusion!

Selmer et al., J. Supercrit. Fluids 140 (2018) 31 Supercritical drying: role of pressure and temperature

• Transition Bi number depends on the process conditions (T,P)

Pressure (MPa) �� (-) 9 12 17

Temperature (K) 313 78.1 66.6 58.9

318 86.5 65.0 61.1

323 - 67.6 60.1

�� • ��,�� (T=313 K)=0.0078, ��,�� (T=318 K)=0.0097, ��,�� (T=323 K)=0.0118)

Ø Transition Bi number reduces at higher pressure and low temperature

Selmer et al., J. Supercrit. Fluids 140 (2018) 32 Supercritical drying: role of pressure and temperature

• Theoretical minimal supercritical extraction time t, at various temperature and pressure for spherical particles with R=250 µm.

Pressure (MPa) ��,�� (s) 9 10 12 13 17 Temperature (K) 313 1.84 1.97 2.13 2.19 2.40 318 1.73 1.77 1.92 1.98 2.18 323 (1.69)* 1.65 1.75 1.80 1.98

�� • Porosity ��=1 and tortuosity ��=1. End points: ��,�� (at T=313 K) = 0.0078, ��,�� (at �� T=318 K) = 0.0097, ��,�� (at T=323 K) = 0.0118).

• Mixture critical pressures: Pcrit (at T=313 K)=7.60 MPa, Pcrit (at T=318 K)=8.37 MPa, Pcrit (at T=323 K)=9.09 MPa

Ø To reach the lowest drying time: higher temperatures and lower pressures

Selmer et al., J. Supercrit. Fluids 140 (2018) 33 Supercritical drying of beads: role of K1 number

• Calculated extraction time relative to the minimal theoretical extraction time and calculated CO2 consumption relative to the initial ethanol mass

20 20 18 420 kg/m^3 18 560 kg/m^3 tse,min 16 16 720 kg/m^3 14 Limitation in 14 12 the bulk fluid 12 ] -

[ 10 10

8 8 [g/g] 6 6 4 4 Diffusion limitation CO2 consumption excl. 2 2 supercritical extraction time/ extraction supercritical 0 0 mass ethanol pressurization/initial 0 2 4 6 8 10 12 14 16 18 K1mean [-] � � �1 = � (�, �, �, �) � �, ,, �, �, �, � � Selmer et al., J. Supercrit. Fluids 140 (2018) 34 Supercritical drying of beads: role of K1 number

20 20 18 420 kg/m^3 18 560 kg/m^3 tse,min 16 16 720 kg/m^3 14 Limitation in 14 12 the bulk fluid 12 ] -

[ 10 10

8 8 [g/g] 6 6 4 4 Diffusion limitation CO2 consumption excl. 2 2 supercritical extraction time/ extraction supercritical 0 0 mass ethanol pressurization/initial 0 2 4 6 8 10 12 14 16 18 K1mean [-]

Ø Universal dependence independent of the gel radius, gel porosity, gel tortuosity,

�/�-ratio, volume and porosity of the packed bed, CO2 density, T-P combination and end condition x, = � � !

Selmer et al., J. Supercrit. Fluids 140 (2018) 35 Supercritical drying of beads: five steps to ideal process (case spherical particles)

1. Determine the gel particle radius R, the gel porosity and gel tortuosity

2. Select a high Keff value and thus determine the supercritical extraction temperature and pressure (preferably high temperature and low pressure)

Pressure (MPa) -9 �� (10 m²/s) 9 10 12 13 17 Temperature (K) 313 33.9 31.8 29.3 28.5 26.0 318 36.1 35.4 32.5 31.5 28.7 323 (36.9) * 37.8 35.8 34.7 31.6

� 1 � �, � = � �, �

3. Determine the corresponding CO2 density

Selmer et al., J. Supercrit. Fluids 140 (2018) 36 Supercritical drying of beads: five steps to ideal process (case spherical particles)

4. Calculate β from using Bi (transition) to stay at the optimal process window where the inner mass transport is as fast as the mass transfer from the gel surface to the bulk fluid

Pressure (MPa) �� (-) 9 12 17 Temperature (K) 313 78.1 66.6 58.9 318 86.5 65.0 61.1 323 - 67.6 60.1

� � = �� , � �(�, �) � �

5. Estimate the mass flow you need to achieve the calculated β using the CO2 density and known mass transfer correlations from literature in accordance with the used geometrical gel/autoclave set-up.

Selmer et al., J. Supercrit. Fluids 140 (2018) 37 Supercritical drying of beads: five steps to ideal process (case cylinders/plate)

• The half of the smallest gel dimension can be approximated as “gel particle radius R” to be used in the first step of the guideline.

• The theoretical minimal supercritical extraction time �, prolongs approximately with the factor 2 in case of a cylinder and with the factor 4 in case of a plate due to geometrical reasons.

• The chosen � value should be multiplied in case of a cylinder with the approximate factor 1/2 and in case of a plate with the approximate factor 1/4 to consider the slower extraction.

• Corresponding smaller mass flows and longer extraction times (compared to the extraction of a gel sphere) are recommended for cylinders and plates.

38 Supercritical drying: different aerogel geometries/sizes

• Plates/monoliths: batch process seems to be the only alternative - Commercially available - One large autoclave or numbering up - Different automatization levels possible

• Beads: both batch or semi-batch process possible - Usage of filling vessels or pumping the particle slurry into the autoclave - Principally autoclave can be opened on the top and on the bottom: faster product removal - If the residence time is not too long: semi-continuos process possible (several autoclaves involved, continuous feed stream to autoclaves)

• Small particles: principally a continuous process possible - Moving bed - Fuidized bed - Extraction column

39 Supercritical drying: scale up

Lab scale-bench scale at TUHH: 50 Liter scale, batch process

• Flexible plant: both monoliths and beads can be dried • Max 50 L solvent: explosion protection

• Solvent separation and recycling realized

Various types of aeerogels dried

40 Supercritical drying of beads: large pilot scale

200 l of Ca-alginate alcogel particles were dried in existing pilot plant

§ Alcogel particles were shipped in standard chemical barrels § Alcogel particles were poured from the barrels § Polymer filter bags for handling & drying in the plant § Supercritical drying using typical conditions § Strong spillage observed during pressurization

Ø Drying of particles works in pilot scale even under not optimized conditions & plant

41 Supercritical drying of microparticles: continuous approach (TUHH)

Particle suspension Principle: CO2 + solvent § CO2 flows through column from bottom to top § Particle suspension is fed at the top of the column

§ Particles sediment against CO2 flow towards the bottom of the column

§ CO2 extracts solvent from particles in countercurrent flow

CO2 § solvent/CO2 flow leaves column at Dried particles the top

CO2 Valveless cont. depressurization

Dried particles 42 Supercritical drying of microparticles: continuous approach (TUHH)

Continuously Batch dried Experimental: dried • Ca-alginate gel particles Pore volume 10.1 cm³/g 10 cm³/g • Column length = 1 m • Suspension mass flow = 13.3 g/min BET surface 600 m²/g 575 m²/g • CO2 mass flow = 20.6 g/min area

dmean 211 µm 157 µm

Ø Countercurrent drying of aerogel particles possible - Comparable properties to batch dried particles - No compaction of particles: free flowing particles

43 Conclusions

Ø Processing from gel to aerogel can be realized for different geometries and different scales at reasonable time

• Understanding of all phenomena of the supercritical drying (solvent exchange, solvent spillage, diffusion, outer mass transfer) to find the optimal process conditions

• Dimensionless analysis (Re, Bi numbers) helps to compare different drying set ups and to analyze the results (No try and error any more!)

• Important considerations for upscaling supercritical drying equipment : - Batch vs. continuous approach - Length-to-diameter ratio of the autoclave - Autoclave diameter (balance between autoclave capacity and cost) - Autoclave orientation (e.g. vertical vs. horizontal) - Flow conditions within the autoclave (balance between energy expenditure and batch length)

44 Welcome to 5th International Seminar on Aerogels

28 – 30. September 2020 in Hamburg