Activated carbon beds heating

Chen et al. [70] suggested that temperature gradients may have been responsible for the more than 90 % selectivity of the formation of acetylene from methane in a microwave heated activated carbon bed. The authors believed that the highly nonisothermal nature of the packed bed might allow reaction intermediates formed on the surface to desorb into a relatively cool gas stream where they are transformed via a different reaction pathway than in a conventional isothermal reactor. The results indicated that temperature gradients were approximately 20 K. The nonisothermal nature of this packed bed resulted in an apparent rate enhancement and altered the activation energy and pre-exponential factor [94]. Formation of hot spots was modeled by calculation and, in the case of solid materials, studied by several authors [105-108],... 

An initially clean activated carbon bed at 320 K is fed a vapor of benzene in nitrogen at a total pressure of 1 MPa. The concentration of benzene in the feed is 6 mol/m3. After the bed is uniformly saturated with feed, it is regenerated using benzene-free nitrogen at 400 K and 1 MPa. Solve for both steps. For simplicity, neglect fluid-phase accumulation terms and assume constant mean heat capacities for stationary and fluid phases and a constant velocity. The system is described by... 

In the second process, the caffeine-loaded CO2 flows through an activated-carbon bed and the caffeine is adsorbed. Usually the activated carbon is recycled by heat treatment up to 600°C and the caffeine is destroyed. 

The filter is considered to be adiabatic, i.e. there is no heat exchange between the activated carbon bed and the environment. This is certainly not true, and some models do take the heat exchange (loss) with the environment into account However, this is not a feature of the activated carbon bed, but is directly related to the filter housing or canister. 

The counter-current pattern of adsorption and desorption favors high removal efficiencies. Desorption of the adsorbed solvents starts after the delay required to heat the activated carbon bed. The specific steam consumption increases as the residual load of the activated carbon decreases (Figure 22.1.6). For cost reasons, desorption is not run to completion. The desorption time is optimized to obtain the acceptable residual load with a minimum specific steam consumption. The amount of steam required depends on the interaction forces between the solvent and the activated carbon. The mixture of steam and solvent vapor from the adsorber is condensed in a condenser. If the solvent is immiscible with water the condensate is led to a gravity separator (making use of the density differential) where it is separated into a aqueous and solvent fraction. 

The first term represents accumulation of energy in the solid, the second term is the heat transfer rate from the fluid to the solid, and the last term is the heat generated by adsorption. This last term can be quite large. Increases in gas temperature of over 100°C can occur in gas adsorption systems, and if oxygen is present activated carbon beds can catch on fire. 

If the heat production extends the heat transport in any place in the bed of activated carbon, self-heating will take place and as a result a temperature peak, a so-called hot-spot , will develop. The consequence of this phenomenon may be the auto-ignition of the bed. 

Critoph, R.E., and Turner, L., Heat transfer in granular activated carbon beds in the presence of adsorbable gases, Int. J. Heat Mass Transfer, 38(9), 1577-1586 (1995). 

Stea.ming Retjuirements. The steaming of fixed beds of activated carbon is a combination of thermal swing and displacement purge swing. The exothermic heat released when the water adsorbs from the vapor phase is much higher than is possible with heated gas purging. This cycle has been successhiUy modeled by equiUbrium theory (128). 

Critoph, R.E. and Thorpe, R.N., Momentum and heat transfer by forced convection in fixed beds of granular active carbon. Applied Thermal Engineering, 1996, 16,419 427. 

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