Abstract
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.
Highlights
Using elemental sulfur is the most viable route to the production of soluble phosphates, which are necessary for feeding a large portion of the world’s population
Marriott et al used the same technique to further characterize the physically dissolved H2S from the chemically bonded H2S (H2Sx) in the liquid sulfur matrix [15]. This technique was employed in this study to elucidate the possible effects of H2S on the rheology of molten sulfur; more quantitative results will be included in a future manuscript
There was a nontime dependant thermal hysteresis noted when the liquid was heated to above 210 C and recooled. This hysteresis was attributed to 50 ppm native/dissolved hydrocarbons, which react with Sx at T > 210 C
Summary
Using elemental sulfur is the most viable route to the production of soluble phosphates, which are necessary for feeding a large portion of the world’s population. Though highly regarded in the literature, the comprehensive experimental viscosity data of molten sulfur by Bacon and Fanelli were reported over 70 years ago [4] These and other investigators have studied the low-shear viscosity of sulfur and have assumed that molten sulfur is Newtonian in its entire liquid temperature range or that shear effects on sulfur’s viscosity are insignificant [4,5,6]. The temperature regime of interest includes that encountered during many liquid sulfur handling processes and possible injection of molten sulfur into depleted underground reservoirs (120 < T < 280 C) These measurements were carried out using an Anton-Paar high-pressure rheometer with a custom built gravimetric charging rig. 0.4 semiempirical model, based on the reptation model of Cates [8], has been optimized and reported with the measurements from this study
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