Optimal product distribution from laminar flow reactors: Newtonian and other power-law fluids

Abstract

In tubular reactors viscous fluids are in laminar flow. For reactions in series this gives a product distribution different from either plug flow or mixed flow. More importantly, laminar flow depresses the maximum amount of intermediate that can be obtained when compared to plug flow. Here we treat the simple case of an elementary two-step mechanism: (1) [ A→ R→ S] Three special cases of the velocity profile in a tubular reactor are treated: uniform velocity for a plug flow reactor (PFR), parabolic velocity for a Newtonian flow reactor (NFR) and a conical velocity profile for an extreme power-law flow reactor (EPFR). The limit of complete mixing in a mixed flow reactor (MFR) is also included in comparisons. We present design charts which show quantitatively how the rate constant ratio affects the product distribution, and we compare the maximum possible amount of intermediate obtainable from reactors whose flow patterns can be characterized by the four models above. These results are important because they allow an evaluation of the detrimental effects when the ideal plug flow assumption for tubular reactors is inappropriate and a convective model more accurately describes the situation. The entire range of power-law fluid behavior is represented on these plots and we clearly show that even the worst flow pattern which arises is still much preferable to mixed flow.