BUTANE ADSORPTION ON ACTIVATED CARBONFIBER COMPOSITES: MODELING AND SIMULATION'

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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

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Introduction

• Air Quality Issues
• Volatile organic compounds (VOCs) are precursors
  to the formation of ozone and smog detrimental to
  the environment and health
• acute respiratory problems
• decrease lung capacity
• impair immune system
• Amendments to the clean air act of 1990
• reduction in emissions of 149 (VOCs)

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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

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VOC Removal Technologies

• Adsorption
• Thermal oxidation
• -VOC ? H2O and CO2
• -high energy consumption
• -dilute air streams
• Biofiltration
• -low concentrations
• -low operating costs
• Combined Processes

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Advantages of ACF for VOC Adsorption

• Often very low concentrations of VOCs in large
  air volumes
• short contact times (lt0.01 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

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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

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Carbon Fiber Composite
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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

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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)

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Schematic of Flow System
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Pressure Drop Requirements
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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

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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

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Q-LND Approximation

• column operates isothermally
• Ficks Law of Diffusion
• axial dispersion
• intra-particle transport
• linear adsorption isotherm
• axial fluid velocity is constant

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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

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Breakthrough Profile for Butane on GAC Bed
Data ? 100 ppm, 50 ppm, x 20 ppm. Model
Continuous curves.
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Breakthrough Profile for Butane on Composite Bed
Data ? 100 ppm, 50 ppm, x 20 ppm. Model
Continuous curves.
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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

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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

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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

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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

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Future Work

• Alternative bed configurations
• candle filters
• large diameter fibers
• Heat Transfer and Desorption
• electrical heating
• Alternative feed conditions
• multiple beds
• ramped feed

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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.