CARBON MATERIALS FOR ENVIRONMENTAL PROTECTION Professor Barry Crittenden Department of Chemical Engineering Faculty of Engineering and Design University of Bath, Bath, UK CESEP09, Torremolinos, Spain, October 2009 AN ENGINEERS PERSPECTIVE OF THE ROLE OF ACTIVATED CARBON MONOLITHS

Granular activated carbons (GAC) have enjoyed a long and successful record in removing pollutants from aqueous and gaseous environments. A VERY GOOD MATERIAL…

Granular activated carbons (GAC) have enjoyed a long and successful record in removing pollutants from aqueous and gaseous environments. With pressures now to reduce energy demands and CO 2 emissions in all forms of processing, focus is shifting towards ways of operating AC systems at very much reduced pressure drops. NOW FOCUS ON ENERGY…

Granular activated carbons (GAC) have enjoyed a long and successful record in removing pollutants from aqueous and gaseous environments. With pressures now to reduce energy demands and CO 2 emissions in all forms of processing, focus is shifting towards ways of operating AC systems at very much reduced pressure drops. As an example, activated carbon monoliths (ACMs) can be used to remove volatile organic compounds (VOCs) from air streams with substantially reduced energy demands because (i) they have intrinsically low pressure drops, and (ii) they can be thermally cycled much faster than GAC and hence their carbon inventories can be reduced considerably. TO REDUCE ENERGY DEMANDS…

Granular activated carbons (GAC) have enjoyed a long and successful record in removing pollutants from aqueous and gaseous environments. With pressures now to reduce energy demands and CO 2 emissions in all forms of processing, focus is shifting towards ways of operating AC systems at very much reduced pressure drops. As an example, activated carbon monoliths (ACMs) can be used to remove volatile organic compounds (VOCs) from air streams with substantially reduced energy demands because (i) they have intrinsically low pressure drops, and (ii) they can be thermally cycled much faster than GAC and hence their carbon inventories can be reduced considerably. Replace the large GAC bed with a much smaller volume of structured ACM BY REDUCING THE INVENTORY OF CARBON…

Granular activated carbon needs to be heated with steam or hot gas for its regeneration. This is a slow process, causing thermal swing adsorption (TSA) cycle times to be high, eg 8 hours. Recovery of the solvents is more difficult if steam is used although this is a good heating medium. COMPARE AND CONTRAST

Granular activated carbon needs to be heated with steam or hot gas for its regeneration. This is a slow process, causing thermal swing adsorption (TSA) cycle times to be high, eg 8 hours. Recovery of the solvents is more difficult if steam is used although this is a good heating medium. Activated carbon monoliths can be heated electrically at low potential difference. This is a fast process, thereby allowing operation at much shorter cycle times, eg 60 minutes. The consequence is that much less adsorbent is required and so the equipment is much smaller. COMPARE AND CONTRAST

COMPARE THE PRESSURE DROPS… Skin friction only (laminar flow) Patton et al (2004) Skin friction and form drag with the latter as a function of u 2 predominating Ergun (1952)

A WORLD-WIDE PROBLEM UK GDP = 4.6% of global GDP (Scaruffi, 2004) UK VOC emissions reduced to 1.5 million tonnes in 2002 (Environment Agency) Worldwide VOC emissions therefore are approximately 32.6 million tonnes/year Adsorption is used for 25% of total VOC control market (Frost & Sullivan, 2000) Therefore about 8 million tonnes/year might be controlled by adsorption worldwide A typical VOC concentration might be 2 g/m 3 Volume of air to be cleaned therefore could be 130,000 m 3 /s

Assume an average 20% loading (w/w) on both the granular and monolithic adsorbents (it has been shown that the kinetic performances of the two systems are quite similar). Assume an 8.0 hour cycle time for granular (spherical) adsorbent. Assume that the Ergun equation is applicable for the pressure drop through a bed of granular activated carbon (GAC). Assume a 1.0 hour cycle time for monolith adsorbent. Assume that the Poiseuille equation is applicable for the pressure drop for laminar flow through the monolith channels. What is the potential benefit from switching from GAC to ACM for VOC control? BASIS OF PRESSURE DROP COMPARISON

GLOBAL POTENTIAL Potential power saving is substantial. Potential for CO 2 reduction depends on energy sources.

EFFECT OF SUPERFICIAL VELOCITY The effect of velocity squared in the packed bed is very significant.

MANUFACTURE OF ACMs GadkareeCarbon from high carbon yield phenolic resin impregnated on ceramic honeycomb support 1998 Yates et al.Activated carbon mixed with silicate clay before extrusion 2000 Tennison et al.Binder-less activated carbon made from extruded phenolic Novolak resin 2001 Fuertes et al.Carbon from phenolic Novolak resin mixed with Nomex fibres 2003 Valdés-Solis et al.Carbon from phenolic Novolak resin dip-coated on ceramic support % linear & 50% volumetric shrinkage on carbonisation

1. Phenolic resin 2. Controlled cure 3. Mill & classify 4. Extrude 5. Carbonise 6. Activate MAST CARBON MANUFACTURING STEPS

EXTRUDED ACMs IN VOC RECOVERY UNIT Place R N, Blackburn A J, Tennison S R, Rawlinson A P and Crittenden B D Method and equipment for removing volatile compounds from air US Patent (2005), European Patent EP (2005) Winner – IChemE Severn Trent Water Safety Award 2002

ISOTHERMS FROM HIDEN ANALYTICAL INTELLIGENT GRAVIMETRIC ANALYSER Tóth model: The adsorption capacity is deemed to be fixed but the affinity parameter b is allowed to vary with temperature. DCM

EFFECTIVE DIFFUSION COEFFICIENT FROM THE IGA DCM on ACM Sqr-21

FITTING LDF EQUATION TO IGA KINETIC DATA LDF is described by the following (Chagger et al, 1995): q(t) is the uptake at time t, Δq is the total change in uptake for a given pressure increment and k is the adsorption rate constant. q o is the equilibrium uptake at t o (the initial time of the pressure increment) and q 1 is the equilibrium uptake at the equilibrium time. ORIGINLAB software was used to fit the experimental data to the LDF model equation.

FUNDAMENTAL DESIGN Decisions: 3D diffusion & convection in the channel gas phase Various flow regimes in the channels (eg plug, axially dispersed plug, fully developed, developing) 3D diffusion in the solid phase Adsorption at the gas-solid interface (eg Langmuir, Tóth) Isotropic, anisotropic solid phase Isothermal, non-isothermal Uniform, non-uniform channels

FLOW AND CONVECTION Mass balance gas phase: Mass balance solid phase: Tóth model: Energy balance gas phase: Energy balance solid phase: Energy balance interface: Gas channel heat transfer coefficient:

BREAKTHROUGH OF DICHLOROMETHANE 7 litres/min (STP) air flow (Re = 43; u ave = 0.98 m/s) with 2000 ppmv DCM 103 mm long by 18.6 mm diameter ACM with nominal channel size of 0.7 mm, nominal wall thickness of 0.35 mm fractional free cross sectional area of 0.44.

MODEL TAKES CHANNEL NON-UNIFORMITY INTO ACCOUNT

FINALLY – A COMPARISON WITH GRANULES

CONCLUSION Once the channel shape has been selected, two independent design variables (the channel dimension and the wall thickness) can be altered to provide internal and external mass transfer coefficients similar to those for an equivalent bed of granules. GAC has one less independent design variable. Reducing the channel dimension improves the monolith external mass transfer performance but increases its pressure gradient. Reducing the wall thickness improves its internal mass transfer performance. Channels just less than 1 mm and walls just less than 0.5 mm provide the same free space and mass transfer performance equivalent to a bed of 1-2 mm granules but with a considerably reduced pressure drop. This does not take into account the even greater reduction in pressure drop achievable because the ACM is electrically regenerable. Form drag and roughness are the principal weaknesses of granular materials. In laminar flow, the roughness of monoliths does not affect the pressure drop, and hence energy requirements and carbon dioxide emissions.

QUESTIONS?