Two-phase modeling of heat and mass transfer in packed bed absorbers: Implications for process design

Removal of micropollutants in packed bed adsorbers constitutes an environmentally benign water purification process. Rather than adopting engineering correlations, the present work focuses on process design starting from the simulation of a single unit operation. Water contaminated with p-nitrophenol flows through a cylindrical tube, which is packed with spherical activated carbon particles acting as the adsorbent. The process is simulated numerically by modeling transient heat and mass transfer in both phases (solid and liquid) that comprise the packed bed. Breakthrough curves are computed and the overall efficiency of the process is evaluated. It is shown that in order to lengthen the time during which the effluent water is decontaminated, the convective transport of the contaminant in the interstitial space must equilibrate quickly with diffusion of the contaminant inside the particles. Expressions for the characteristic length and time scales to reach this equilibrium are derived. It is demonstrated that the design of the adsorption device can be optimized by decreasing the bed length and/or by increasing the diameter of the particles, when keeping the pressure drop and the volume of the bed fixed. Regarding the heat released by adsorption, it is shown that the temperature ahead of the mass transfer zone rises uniformly due to convective transport.