butane adsorption on activated carbon-fiber composites: modeling and simulation.

1. BUTANE ADSORPTION ON ACTIVATED CARBON-FIBER COMPOSITES: MODELING AND SIMULATION. Rodney Andrews1,2, Marit Jagtoyen2, and Eric Grulke1 1Chemical and Materials Engineering, University of Kentucky 177 Anderson Hall, Lexington, KY 40506-0046, USA 2Center for Applied Energy Research , University of Kentucky 2540 Research Park Drive, Lexington KY 40511-8433, USA

3. Examples of Activated Carbons for VOC removal * flexographic printing: acetates and alcohols * automotive paint booth process equipment * bakeries: ethanol * styrene: emissions ~ 32 million lbs/year in 1990

4. VOC Removal Technologies *Adsorption *Thermal oxidation -VOC ? H2O and CO2 -high energy consumption -dilute air streams *Biofiltration -low concentrations -low operating costs *Combined Processes

5. Advantages of ACF for VOC Adsorption Often very low concentrations of VOCs in large air volumes * short contact times (<0.01 sec) * concentrations down to 10 ppm * requires deep beds of granular activated carbon (GAC) * high pressure drop Rate of adsorption can be greatly increased: *micron vs millimeter dimensions for fibers *Fourier Mass Transfer Time for 90% Change 3 mm granule: 0.032 sec 25 mm fiber: 0.000005 sec

6. Activated Carbon Fibers As-produced Fibers *low bulk density *difficulties in handling and containment Composites * novel monolithic form * design flexibility - produced to any size or shape * rigid, highly permeable, strong * open internal architecture

7. Carbon Fiber Composite

8. Objectives *Adsorption of butane on GAC and carbon fiber composite beds at low butane concentrations. *Apply Quasi-lognormal distribution (Q-LND) approximation to predict breakthrough curves *Compare model with data -applicability and fit to data -calculation intensity and stability -implications

9. Experimental Equal volume beds of activated carbon fiber composites and commercial GAC. Butane: 20 -100 ppm in dry nitrogen. Carbon Properties and Experimental Parameters Sample BET Mass of Density Contact time DP s.a.(m2/g) carbon(g) (g/cc) (s-1) (psi) GAC(F-400) 1014 46 0.48 0.082 1.01 Composite 789 14 0.15 0.081 0.59 Contact time = (bed vol/flow vol/sec)

10. Schematic of Flow System

11. Pressure Drop Requirements

12. Efficiency of Butane Removal Rate of removal of butane at breakthrough per mass (g/hr/g) Concentration(ppm) Composite GAC Ratio 20 0.013 0.0039 3.33 50 0.034 0.0093 3.66 100 0.059 0.015 3.93 Mass Transfer Coefficients from Sherwood correlation Composite: kc = 1.38 m/s GAC: kc = 0.85 m/s

13. Quasi-lognormal Distribution Approximation Developed by Xiu et al AIChE Journal, 43(4), 979, 1997. Modeling fixed-bed adsorbers with: fixed-bed adsorbers axial dispersion external film diffusion intraparticle diffusion Adjustable for varying particle geometry

14. Q-LND Approximation column operates isothermally Fick’s Law of Diffusion axial dispersion intra-particle transport linear adsorption isotherm axial fluid velocity is constant

15. Modeling Experimental Data Step feed input moments of the impulse response quasi-lognormal probability density function Dimensionless forms Peclet number Biot number distribution ratio single, concentration dependent, fitting parameter

16. Breakthrough Profile for Butane on GAC Bed

17. Breakthrough Profile for Butane on Composite Bed

18. Applicability and Fit to Data Fit data well over concentrations studied distribution ratio was single fitting parameter Calculated mass transfer coefficients 0.3 m/s 1.55 m/s good comparison with Sherwood number Assumptions in model appear valid Peclet and Biot numbers within range of those for similar systems found in literature

19. Calculation Intensity and Stability Computationally simple solved in Mathcad: desktop package Converges rapidly to solution Easily tuned for changes in system adsorbent particle shape adsorber bed parameters Solution convergence is stable

20. Implications for Scale-Up Applicability criterion HD is combined particle mass-transfer coefficient ? is bed-length parameter ? is distribution ratio p is particle shape factor May extend range of inequality fit to experimental data outside this range

21. Conclusions Q-LND Approximation predicts breakthrough profiles for butane on GAC and ACFC Model is applicable at low butane concentrations Computation rapid convergence simple (off the shelf solution) easily tunable Allows for rapid simulation of novel systems

22. Future Work Alternative bed configurations candle filters large diameter fibers Heat Transfer and Desorption electrical heating Alternative feed conditions multiple beds ramped feed

23. Acknowledgments This work was sponsored by CAER, University of Kentucky. The authors would like to acknowledge: *Danny Turner and Rodney Johnson for help with experimental work. *Kathie Sauer for help with graphics.