(82g) Experimental and Modeling Studies of Residence Time Distribution in Partially Filled Laminar Flow Reactors

Authors: 
Ramji, S., Indian Institute of Technology Madras
Vir, A., Indian Institute of Technology, Madras
Pushpavanam, S., Indian Institute of Technology, Madras
The concept of Residence Time Distribution (RTD) is useful in understanding the amount of time spent by fluid elements in a reactor i.e., the degree of macro-mixing. Residence time has a significant effect on the reaction conversion and hence is an important parameter to be considered while designing reactors. The RTD for a fully filled Laminar Flow Reactor (LFR) is available in the literature. If the flowrate of the reactant in the LFR is not sufficiently high, only a part of the reactor is occupied by the reactant and the rest is filled by air. The RTD for partially filled tubular reactors with different liquid hold up is not reported so far to the best knowledge of the authors. In this work, we study the residence time distribution of partially filled circular millichannel for different liquid hold up both experimentally and theoretically. Towards this, a pulse of NaCl tracer was introduced at the reactor inlet and the exit tracer concentration was measured using a conductivity meter as a function of time. This is carried out for different liquid holdup in the reactor. The residence time distribution is then analyzed in terms of E and F curves for different phase hold up.

The major challenge in modeling partially filled LFR is that neither the Cartesian nor the cylindrical coordinate system can describe the gas-liquid interface and channel wall simultaneously. To overcome this challenge, we employ a special coordinate system known as the bipolar cylindrical coordinate system for the study. The governing equation for fluid flow was derived in the bipolar cylindrical coordinate system and solved analytically using the method of separation of variables [3]. The velocity profiles were analyzed for different liquid hold up. Then, the diffusion free species transport equation considering a unit step tracer injection is solved numerically using two approaches: a) the finite difference method using a first order upwind scheme for the convection term and b) the Lax Wendroff method. The results from the two methods were compared with each other and validated with experiments. Finally, we study how RTD for different liquid holdup influences the conversion for a first order reaction.