Understanding Curvature-Driven Enhancement in Performance of Serpentine Tubular Reactors

This work demonstrates the potential of introducing bends in the straight tube as an efficient, simple, and highly scalable tubular reactor for continuous flow synthesis. The effects of bend count and interbend spacing in various configurations of fixed length (1 m) are systematically investigated through extensive experimental and computational studies. The quantitative effect of the number of U-bends per unit length on the hydrodynamics (pressure drop, residence time distribution (RTD), and mass transfer) and the performance for an aromatic nitration reaction have been studied. Increasing the number of U-bends per unit length facilitated Dean vortex interactions, leading to enhanced flow unsteadiness and a broader distribution of energy across frequencies in the spatial energy spectra. The characteristic slope near −5/3 in the temporal energy spectrum indicates the presence of a localized inertial subrange, highlighting the early transition to turbulence due to the periodic and highly dense sequence of U-bends in serpentine geometries. Higher number of U-bends per unit length also improved radial mixing and reduced axial dispersion and reduction in the required mixing length with increasing number of U-bends. The volumetric mass transfer coefficient was seen to have a strong positive dependence on Reynolds number and the number of bends kLa∝ [Re0.71, Nb1.6], which got reflected in the enhancement in conversion for multiphase phenol nitration. For identical tube lengths and at constant residence time of 1 min, phenol conversion increased from 57% to 60.9%, 69%, and 73.1% for a tube with 0 (straight tube), 3, 15, and 35 U-bends, respectively. Overall, the study establishes clear relationships between serpentine geometry and transport behavior, providing useful guidelines for the design of compact continuous multiphase reactors.