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Liquid Sulfur 83 tion of H2S to SO2 in the Claus oven (or furnace) followed by the catalytic comproportionation of H2S and SO2 to sulfur and water in a second reactor: H2Sþ3=2O2 !SO2þH2O ð1Þ 2H2SþSO2 Ð3=8S8þ2H2O ð2Þ Both the Frasch and the Claus process produce sulfur as a liquid, and in this form, at ca. 140 C, sulfur is usually transported to the customers using either steam-heated pipelines, railroad cars, or ships, the latter with a sulfur capacity of up to 25,000 tonnes. Sometimes, the sulfur is first solidified be- fore shipping. The main users of elemental sulfur are the chemical industry for the pro- duction of chemicals such as sulfuric acid, phosphorus sulfides, etc., as well as the rubber industry for the vulcanization of natural or synthetic rubber. Liquid sulfurhas unique physical properties which have puzzled scientists and engineers for more than 100 years and which have no parallel in any other substance. For example, the color of the liquid changes reversibly from honey-yellow near the melting point (120 C) to dark red-brown at the boil- ing point (445 C) [3]. The dynamic viscosity h of the melt exhibits a mini- mum of 0.007 Pa s (0.07 Poise) at 157 C but increases by more than four orders of magnitude in the small temperature range of 155–190 C; the max- imum viscosity of 93.2 Pa s (932 Poise) is reached at 187 C [4, 5]. Other measurements using commercial and not purified sulfur resulted in a mini- mum viscosity of 0.009 Pa s at 154 C [6]. On heating to temperatures above 190 C the viscosity decreases again to 0.1 Pa s at the boiling point. Obvious- ly, a substantial concentration of polymeric sulfur molecules (S1) is building up at temperatures above 159 C. This so-called “polymerization tempera- ture\X (or “transition temperature”) depends on the external pressure: As higher the pressure as lower the polymerization temperature [7, 8]. At 840 bar (84 MPa) the polymerization takes place at 145.5 C which is identi- cal to the melting temperature of monoclinic b-S8 at this pressure. In other words, under these conditions b-S8 melts directly to a liquid containing a high concentration of S1 [9]. According to DTA measurements by Kuballa and Schneider the curve for the experimental pressure dependence of the polymerization temperature intercepts the melting curve of b-S8 at 155.5 C and 1300 bar (130 MPa) [10]. The density of liquid sulfur (at ambient pressure) decreases with increas- ing temperature from 1.802 g cm3 at 120 C to 1.573 g cm3 at 440 C but the temperature coefficient of the specific volume shows a unique disconti- nuity at 159 C when the density change has a minimal value (density at this temperature: 1.770 g cm3) [11–13]. Similarly, the heat capacity Cp of the melt exhibits a discontinuity at 161 C [14]. The dielectric constant shows a distinct minimum at 159 C [15, 16]. The temperature of 159 C (sometimes given as 159.5 or 160 C) has therefore also been called a transition tempera- ture (“first order l-transition” at “Tl”).PDF Image | Topics in Current Chemistry
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