The rheology of liquid elemental sulfur across the λ-transition

Seventy million tons of sulfur were produced worldwide in 2016. Much of the transportation and handling of molten sulfur require sulfur pumps which are challenging to operate due to the anomalous behavior of sulfur's liquid viscosity at temperatures near its λ-transition region. Sulfur's viscosity decreases from ca. 30 × 10−2 to ca. 7 × 10−2 Pa s when heating from the melt at T = 115 °C and then increases dramatically at T > 160 °C to a maximum of 93 000 × 10−2 Pa s at T = 187 °C. While the temperature of this viscosity transition is well known, no shear related information can be found in the literature and previous low-shear data do not address thermal hysteresis behavior (viscosity differences for cooling or heating) when sulfur is heated beyond 210 °C. This work is focused on reinvestigating the low-shear viscosity of molten sulfur which was initially studied by Bacon and Fanelli [J. Am. Chem. Soc. 65, 639–648 (1943)] and the effects of high-shear. The rheology of molten sulfur was studied using an Anton-Paar MCR 302 rheometer equipped with a 1000 bars pressure cell. Viscosity profiles, including attenuation during heating or cooling are discussed, as well as the rheological behavior of molten sulfur encountered in pipe flow and pumping shear regions. Data were used to fit a semiempirical equation based on a previous reptation model. In addition, critical shear rates were measured and reported, where liquid sulfur becomes non-Newtonian at high-shear and in regions where polymerization is significant.