Determination of the effective radial mass diffusivity in tubular reactors under non-Newtonian laminar flow using residence time distribution data

Residence time distribution (RTD) plays an important role in the performance of a laminar flow reactor (LFR). The theoretical E-curve can be derived for the tube flow of a power law non-Newtonian fluid; however, in practice, the streamline flow can be disturbed by the use of corrugated tubes or coils or by the presence of mechanical vibration, high wall relative roughness, pipe fittings or curves. Moreover, stagnation or recirculation zones reduce the active volume of the reactor. The integration of the measured RTD with the process model for flow, heat transfer and reaction can be challenging. The approach proposed in this work is to introduce an effective radial mass diffusivity in the model to take into account the enhanced dispersion and to use the mean residence time as characteristic time. The LFR with radial diffusion was modeled and the simulation results showed the transition from ideal laminar flow to intense radial dispersion according to the modified Peclet number. The measured RTD of a tubular system with high wall relative roughness running with a Newtonian fluid and a pseudo-plastic fluid was compared with the simulated results in order to determine the effective radial diffusivity and the active volume. With these parameters, the LFR model would provide a more reliable prediction of the RTD, hence preventing the representation of complex flow distributions in the reactor.