Residence time, mass transfer and back-mixing of the liquid in trickle flow reactors containing porous particles

A residence time distribution (RTD) model to describe the liquid trickle fl ow in a trickle-bed reactor packed with porous particles and operated both under partially and fully wetted conditions was proposed based on a simple representation of the liquid flow structure. The model views the external l iquid stream as divided into a dynamic zone where an axially dispersed plug flow pattern prevails, and an external stagnant (or static) zone contiguou s to both the dynamic zone and the partially wetted porous particles. The m odel incorporates mass transfer between (i) external dynamic and stagnant z ones, (ii) dynamic tone and nearby partially wetted porous particles, (iii) stagnant zone and adjacent partially wetted particles, and (iv) finally in traparticle diffusion. The model parameters were derived from liquid reside nce time distribution tests with various air/Newtonian and air/non-Newtonia n systems. Analysis of the dynamic tracer impulse-response data of the liqu id revealed the significance of the mass transfer resistance between static liquid and adjacent wetted particles, intraparticle diffusion resistance, as well as partial wetting. By properly accounting for intraparticle diffus ion, peculiarly high liquid axial dispersion coefficients were obtained for low liquid velocities and high carboxymethyl-cellulose (CMC) concentration s. Finally, the deficiency of lumping static liquid-solid mass transfer, in ternal diffusion, and partial wetting in the Peclet number and the number o f transfer units was discussed. (C) 1999 Elsevier Science Ltd. All rights r eserved.