Influence of activation processes on the activated carbon felts microstructure and impact on the. acoustic performances

Transcription

1 Influence of activation processes on the activated carbon felts microstructure and impact on the H. Karpinski 1,2, O. Umnova 1, R. Venegas 3, J. A. Hargreaves 1, M. Nahil 4 and S. Lehmann 2 acoustic performances 1 University of Salford, UK, 2 Carbon Air Ltd, UK, 3 ENTPE, University of Lyon, France, 4 University of Leeds, UK

2 Contents Background Model and parameters required Parameter determination Measurement technique Results Conclusions and future work

3 Multi scale porous materials in acoustics First described in Boutin et al Int.J.Solid.Struct double porosity materials, two porosity scales, microscale porosities and macroscale porosity, with total porosity Modelling using two scale asymptotic homogenization procedure for periodic media Auriault et al, John Wiley&Sons 2009 expansions in powers of l ε = <<1 λ φ m High permeability contrast between the scales effective density determined by the macroscale structure, microscale affects effective bulk modulus 1 1 (1 φ p ) = + Fd ( ω) K ω K ω K ω eff φ total = φ + 1 ( ) ( ) ( ) p p ( φ ) p φ m lm ε 0 = <1 l Lower bulk modulus leads to lower wave speed and high low frequency attenuation p m φ p

4 Granular activated carbon Packing with double porosity particles Triple porous material inter granular voids and two inner porosity scales Model assumes spherical particles and cylindrical inner pores Model parameters: Particle radius (mm) macroscale porosity (around close packing porosity) 2 inner porosities and 2 inner pore radii (nm)

5 Absorption coefficient of granular activated carbon compared with conventional absorber Sound absorption coefficient foam granular activated carbon d=2cm Frequency [Hz]

6 Microstructure modelling granular activated carbons Macroscopic scale Mesoscopic scale REVp Γ sp Ω p Packing of spheres with two inner porosity scales L Microporous domain Ω sp Ω fp l p Viscous and thermal losses in the inter granular voids l n Nanoscopic scale Ω REVn n Γ sn Ω fn Nanoporous domain Ω sm Solid Γ Pores fp Microscopic scale Ω REVm m t 1 n Ω fm Γ sm l m Boutin&Geindreau, JASA 2010 Losses caused by pressure diffusion from the intergranular voids to larger inner pores (high-permeability contrast) Losses induced by sorption processes in nanoscopic inner pores Γ Nanopores Micropores fn Γ fm Model in Venegas &Umnova J.Acoust.Soc.2016 Fitted parameter values used for two inner pore radii and Langmuir constant

7 Activated carbon felts Lightweight - up to 6 times lighter than granular activated carbons Do not need containers Can bend good for silencer applications High surface area promise for good low frequency sound absorption Double porosity material inter fibre pores and a single inner porosity scale (nm in size) Possibility of direct measurement of all parameters for the model

8 Absorption coefficient of activated carbon felt Sound absorption coefficient foam granular activated carbon activated carbon felt d=2cm Frequency [Hz]

9 Previous work on activated carbon felts Acoustical properties of activated carbon felts strongly depend on activation conditions Yue Shen & Gaomin Jiang Journ.Text.Inst. 2015

10 Microstructure modelling of activated carbon felts L Micropore scale (nm) Macropore scale (µm) l l m Regular array of identical cylindrical porous fibres Two porosity scales inter fibre voids (µm) and inner pores (nm) Viscous and thermal losses in inter fibre voids (Umnova et al, J.Acoust.Soc.Am 2009) Losses due to sorption and diffusion in inner nm size pores (Venegas&Boutin WaveMotion 2016) Full model in Venegas&Boutin WaveMotion 2016

11 Local model for adsorption and diffusion in nm size pores Langmuir isotherm, equilibrium state: ρ p a = H m ρ 0 P 0 l m Fickian diffusion equation in microporous (nm in size) domain: D e jωh e ρm = De ρm, Micropore scale (nm) Apparent diffusion coefficient accounts for surface and bulk diffusion in micropores Condition at the boundary microporous domain (fibres): Continuity of mass flux: Continuity of pressure: ρ Venegas&Boutin WaveMotion 2016 ρ 0un = D e nρ p p p a m = ρ 0 m = P0 ω 1 1 φ Can be <1! φ 1+ H e φ Full model predictions for normalised static bulk modulus: K ( ) =. eff

12 Parameters of the model and their measurement methods Fibre radius and orientation image analysis Microporosity (fraction of nm pores relative to unit material volume) Φ m from measured specific micropore volume V n and material density ρ: Φ m = V n ρ Macroporosity φ p -from total porosity φ t and Φ m : φ p = φ t Φ m Langmuir constant H=Δ ρ a P 0 /ΔP ρ 0 - from the isotherm slope Inner pore radius r m isotherm measurements Heat of adsorption isotherm measurements (needed for diffusion coefficients estimation)

13 Materials studied 4 felts with different levels of activation Raw felt, S1, S2, S3 Type of felt Thickness d, mm Raw (Novoloid fibres) S S S Total porosity, φ t Activation level

14 SEM images of S3 felt Collection of fibres Single porous fibre Fibre surface

15 Fibre size measurements Raw S1 S2 S3

16 Determination of nanoscale parameters Felt Micropore volume V n, cc/g DR DH Nm pore radius r m, nm DR DH Adsorption energy. kj/ mol S S / S / Parameter values are different for different measurement techniques Porosity of nm size pores Φ m around 4-5% (porosity of fibres φ m = Φ m /1 φ p is 46-67

17 Measured adsorption isotherms 620 S3 Isotherm, ACN S1 ACN Volume on N, cm 3 /g Volume on N, cm 3 /g adsorption isotherm desorption isotherm adsorption isotherm desorption isotherm p/p p/p 0 Langmuir constant is measured from the isotherm slope around p/ P 0 1 Type 1 isotherm for S3 and S2 inner pores less than 2nm Type 2 isotherm for S1 inner pores 2-50nm (inconsistent with measured pore size!)

18 Impedance tube measurements with three microphones Modified B&K impedance tube: 100 mm diameter: Low-frequency limit: 35Hz High-frequency limit: 1.9kHz R= e j k 0 x H 12 / H 12 e j k 0 x H 0d = 1+R/ e j k 0 L +R e j k 0 L H 23 k= 1/d cos 1 ( H d0 ) Z c =j Z s tan (kd) ρ eff = Z c k/ω effective density K eff = ωz c /k effective bulk modulus O.Doutres et al, Appl.Acoust. 2010

19 Results absorption coefficient of raw felt d=0.011m Absorption coefficient Frequency [Hz] Confirms that regular array of fibres is a good approximation for the macroporosity scale

20 Results effective bulk modulus 2 S2 ACN-15 2 S3 ACN Normalised bulk modulus Re(K(ω)/ P 0 ) Im(K(ω)/ P 0 ) Normalised bulk modulus Re(K(ω)/ P 0 ) Im(K(ω)/ P 0 ) Frequency [Hz] Frequency [Hz] Model assuming no microscale pores Full model

21 Results - absorption coefficient S2 0.5 S2, ACN-15, d=1.1cm d=1cm (4 layers) 0.4 Absorption coefficient Frequency [Hz] Model assuming no microscale pores Full model with DR parameters Full model with DH parameters

22 Results - absorption coefficient S3 0.4 S3, ACN-20, d=0.8 cm d=8mm (4 layers) Absorption coefficient Frequency, [Hz] Model assuming no microscale pores Full model with DR parameters Full model with DH parameters

23 Results - absorption coefficient S1 0.8 S1, ACN d=1.1cm -10, d=1.1cm (4 layers) Absorption coefficient Model assuming no nanoscale pores Frequency [Hz] Full model, DR parameters (adsorption slope) Full model, DH parameters (adsorption slope) Full model, DR parameters (desorption slope) Full model, DH parameters (desorption slope)

24 Comparisons of different felts Absorption coefficient Raw felt, 1.1 cm ACN-15, S2, d= cm cm ACN-20, S3, d=0.8 8 mm cm Frequency [Hz] S3 outperforms raw felt with 20% in thickness and 40% in weight reduction

25 Conclusions and future work Done: Experimental procedure developed to characterise nanoporous fibrous materials Satisfactory agreement with the microstructure based model with independently measured parameters Plans: Develop a rig to measure isotherm at normal conditions Reduce the uncertainties of the measurements

26 New test rig Bulk modulus rig to measure isotherms at normal conditions q Based on Frequency response method q 3 sensors: Displacement Temperature Pressure q Measurement of sorption properties at normal pressure and temperature conditions 26

27 Sorption in nanopores Adsorp(on is a physical or chemical process in which the fluid molecules are adhered on to a surface. The adherence in physical adsorp(on is caused by weak van der Walls forces while by chemical bonding in chemical adsorp(on Desorp(on is the opposite phenomenon whereby the fluid molecules are released from the surface. The release of the molecules is caused by either an increase of temperature or a decrease in pressure. 27