While catalytic converters perform the sulfur conversion portion of the modified Claus unit, sulfur condensers allow for the recovery of that sulfur in a liquid state. After the sulfur is condensed from a vapor to a liquid, it drains through a gravity rundown system into the temporary sulfur storage pit, or collection vessel. At the beginning of the sulfur rundown system, the liquid side must be sealed off from the vapor side to prevent process gas from escaping with the liquid sulfur. The two types of sulfur sealing mechanisms are the traditional underground seal leg, and the increasingly common above-ground sulfur seal. Both types have their strengths and weaknesses.
The traditional underground (in-ground) sulfur seal leg has been used in SRUs for over 50 years. These depend on the head pressure of a liquid sulfur level within the vertical leg to act as a vapor seal and block the process gas. Traditional seal legs function well during normal operation, but in the event of a pressure spike they may cause process gas to blow out and into sulfur storage. While supplemental gas relief can sometimes be desirable, this gas contains hazardous H2S and SO2, and will continue to enter the sulfur pit until the leg is refilled with liquid sulfur. In addition to this, underground seal legs are also cumbersome to remove in the event of plugging off.
Above ground sulfur seals have been around since the 1990s, they utilize a float and orifice to create a vapor seal, similar to a float steam trap. These seals do not allow process gas to enter sulfur storage, because any pressure event will immediately close off the orifice. There is, therefore, no supplemental relief path in these original above-ground designs. While being much easier for maintenance and accessibility, the original design does require periodic cleaning of the filter screen. CSI has, however, developed an advanced version of the above-ground seal which addresses the perceived weaknesses of the original design, called the SxSealTM 2000. This sulfur seal has been very well received and allows for both supplemental pressure relief and simple clean-out without having to open it up and remove a screen. While some companies can be reluctant to deviate from traditional methods, advancing technology will always provide means of improvement, thereby reducing safety hazards and increasing operational effectiveness and ease-of-use.