Sub-dew point Claus processes are a modification of the traditional Claus process used to recover sulfur from hydrogen sulfide gas. In the traditional Claus process, the hydrogen sulfide gas is burned in a furnace to produce sulfur dioxide, which is then reacted with more hydrogen sulfide in a series of catalytic reactors to produce elemental sulfur. In a sub-dew point Claus process, the gas is cooled below the dew point of sulfur, causing the sulfur to condense out of the gas stream. The remaining gas is then processed in a modified Claus unit to recover additional sulfur. The sub-dew point process can increase the overall sulfur recovery efficiency of the Claus process, as more sulfur can be recovered from the gas stream.There are several variations of the sub-dew point Claus process, including the sub-dew point Claus process with oxygen enrichment (SDP-OE), the sub-dew point Claus process with tail gas recycle (SDP-TGR), and the sub-dew point Claus process with oxygen enrichment and tail gas recycle (SDP-OE-TGR). These variations involve different modifications to the process to improve sulfur recovery efficiency and reduce emissions.