care must be taken to ensure that each bed is operated above the dew point of sulfur. The operating temperatures of the subsequent catalytic stages are typically 240 °C for the second stage and 200 °C for the third stage (bottom bed temperatures). In the sulfur condenser, the process gas coming from the catalytic reactor is cooled to between 150 and 130 °C. The condensation heat is used to generate steam at the shell side of the condenser. Before storage, liquid sulfur streams from the process gas cooler, the sulfur condensers and from the final sulfur separator are routed to the degassing unit, where the gases (primarily H2S) dissolved in the sulfur are removed. The tail gas from the Claus process still containing combustible components and sulfur compounds (H2S, H2 and CO) is either burned in an incineration unit or further desulfurized in a downstream tail gas treatment unit. Sub dew point Claus process The conventional Claus process described above is limited in its conversion due to the reaction equilibrium being reached. Like all exothermic reactions, greater conversion can be achieved at lower temperatures, however as mentioned the Claus reactor must be operated above the sulfur dew point (120–150 °C) to avoid liquid sulfur physically deactivating the catalyst. To overcome this problem, the sub dew point Clauss reactors are oriented in parallel, with one operating and one spare. When one reactor has become saturated with adsorbed sulfur, the process flow is diverted to the standby reactor. The reactor is then regenerated by sending process gas that has been heated to 300–350 °C to vaporize the sulfur. This stream is sent to a condenser to recover the sulfur. Process performance Over 2.6 tons of steam will be generated for each ton of sulfur yield. The physical properties of elemental sulfur obtained in the Claus process can differ from that obtained by other processes. Sulfur is usually transported as a liquid (melting point 115 °C). In elemental sulfur, viscosity increases rapidly at temperatures in excess of 160 °C due to the formation of polymeric sulfur chains. Another anomaly is found in the solubility of residual H2S in liquid sulfur as a function of temperature. Ordinarily, the solubility of a gas increases with increasing temperature but with H2S it is the opposite. This means that toxic and explosive H2S gas can build up in the headspace of any cooling liquid sulfur reservoir. The