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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 C tequilibration: 1 h B A 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] 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⁄mX J⁄ ��?@AB 1 � ��?@AB r � − diffusion coef^icient m s J � − mole fractiOn − = J � �� �� � �� �� � − time s �, � − space coORdinate m µ� ≤ � ≤ �� § Mass transport in surrounding fluid: convection [2-4] �̇ − molar lux kmOl⁄s �̇ gA h � − mass transfeR coef^icient X z kmOl⁄m � − suRface of the gel mJ 0 − suRface �̇?@AB,D = �� �?@AB,D − �?@AB,G ∞ − bulk phase ⁄ �̇ gAh − CO2 mass lOw kg s § Finite difference method �̇ gAh + �̇ ?@AB = �����. [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 (�̇ 3 �5)|8 2 in bulk Diffusion fluid � = 0 ∆� EtOH � = � Transport in � boundary layer (�̇ 3 �5)|8:∆8 � = � • �̇ : 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: ��J,•(�, �, �) 1 � J �• ��J,•(�, �, �) = J � ‚ ‚ �?@AB,gAh �J,•(�, �, �) ‚ �• �J,•(�, �, �) ‚ �� � �� �• �� Extracted ethanol from gel particles (acts as source term in bulk fluid): Œ•Ž �† � J ������J,… �J,• �, �, � , �J,… �, � = − ‚ ‹ 4 ‚ � ‚ � ‚ �• ‚ �J,• �, �, � �� �‡ˆ ‚ � ‚ � �� Œ•D Mass transport in bulk fluid/autoclave in axial direction: ��J,…(�, �) �� J � � �, � ‚ � � �, � � �J,…(�, �) J,… J,… = � (�) ‚ − + ������ (� �, �, � , � �, � ) ’ ��J �� J,… J,• J,… �̇ � �J,… �, � = �‡ˆ ‚ � ‚ �…(�J,… �, � ) 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.
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