Dec-2004

Low temperature SCOT Catalyst 
pays off

Jacobs Nederland B.V. demonstrates how new improvements in low temperature catalysts have boosted the Shell Claus Offgas treating (SCOT) process

Gerrit Bloemendal and Ellen Ticheler
Jacobs Nederland B.V.

Article Summary

In the early 1970s Shell Research Ltd. introduced the Shell Claus Offgas Treating (SCOT) process to reach high sulphur recovery levels in sulphur plants. This process involves the reduction of sulphur species in the Claus tail gas to H2S, followed by cooling of the gas and condensing the bulk of the process water, and subsequent washing of the process gas by an alkanol-amine solvent, such as di-isopropanol amine (DIPA) or methyl-diethanol amine (MDEA). This is similar to gas treating processes.

The standard SCOT process is easily capable of meeting 250 ppmv total sulphur in the SCOT offgas from the absorber, which corresponds to an overall sulphur recovery of 99.8-99.9% (for Claus and SCOT).

In recent years, the demand for higher sulphur recovery efficiencies gave the incentive to improve the SCOT process by lowering the total sulphur content in the offgas from the SCOT absorber to less than 50 ppmv total sulphur, thereby maintaining low operating costs or even decreasing the operating costs. This resulted in a number of improvements and several new versions of the process.

Although the combination of unit operations gives a good performance and recoveries of 99.9% (and higher) are easily met, little effort was spent optimising the SCOT plant. Energy was abundant in the early days of SCOT and the conservation of energy or reduction of CO2 emission were not issues.

The SCOT process does not produce any secondary waste. In additional to its high recovery, this was one of the main targets in its development.

Design improvements and process optimisation
Low Temperature SCOT Catalyst
The conventional SCOT units were provided with a standard hydrogenation catalyst, which required a typical reactor inlet temperature of 280-300°C. Although the catalyst has been increasingly tailored for service in a SCOT unit, until recently this only resulted in a reduction in the amount of catalyst required, meaning it was operated at higher space velocities. The required high inlet temperature was fixed. The fact that this high inlet temperature also called for a substantial amount of utilities to heat the gas was considered one of the drawbacks of the technology. For the purposes of energy conservation, a program was started to develop and commercialise a catalyst with a much lower inlet temperature. This catalyst is now commercially available and proven in refinery application, and as a result the reactor inlet temperature can be lowered drastically.

The catalyst has excellent hydrogenating behaviour. Elemental sulphur is completely hydrogenated, whereas the slip of SO2 from the SCOT reactor is equal to or less than the SO2 slip from conventional catalysts.

For the thermodynamic equilibrium of COS hydrolysis, the low temperature is favourable.
At equilibrium, less COS is found at these temperatures than with conventional catalysts. However, if the temperature is lowered, the reaction velocity also drops. Consequently, the concentration of COS to the SCOT unit must be minimised.

The CS2 in the feed gas to the SCOT reactor will not only be hydrolysed, but a small percentage will be hydrogenated as well, forming traces of methyl mercaptan. This phenomenon was previously found with conventional catalyst at high space velocity, and is found again with the low temperature SCOT catalyst. Therefore, also the CS2 concentration to the SCOT reactor must also be minimised.

Due to these drawbacks in COS and CS2 hydrolysis, the low temperature SCOT catalyst is best applied in combination with a layer of titanium catalyst in the bottom of the first Claus reactor. The titanium catalyst will hydrolyse more than 90% of the COS and CS2 in the feed to the Claus reactor, and will thus minimise the concentration of these components in the Claus tail gas feeding the SCOT.

Waste Heat Boiler
The early SCOT plants were always provided with a waste heat boiler downstream of the reactor. As the gas from the reactor was at such a high temperature (typically 320 to 340°C), use of the heat for the generation of LP steam was an obvious option. However, the equipment for this was not cost effective. Furthermore, the value of low pressure steam is limited in most locations. Thus, the more modern designs were no longer provided with a waste heat boiler without client request.

With the new Low Temperature SCOT catalyst, the outlet temperature from the reactor is even lower, making the economics for waste heat recovery even worse. The lower gas temperature has cut the recoverable heat to approximately 50%. As a result, SCOT units are now designed without waste heat boiler.
 
Reheater
In the conventional SCOT unit, the gas must be heated to 280°C or higher, and is most economic with an inline burner. If steam is used, it must be very high pressure steam (typically 80 bar and superheated at 400°C), and even then a double shell design must be applied using the heat of condensation from the steam and the steam superheat to reach the high temperature. For small units, the combined use of steam and electrical trim heaters has been applied. The use of a feed gas/effluent gas exchanger with an electrical trim heater has also been applied, although these solutions are more expensive than the inline burner.

Although the use of inline burners is most widespread in SCOT units, efficient operation of these burners is often difficult. As indicated, the inline burner is also used for the generation of reducing gas. Since the first SCOT units were put online, experience has shown that the tail gas normally contains sufficient reducing components to convert the sulphur species to H2S. Moreover, it is possible to operate the Claus plant off ratio, thereby suppressing the amount of SO2 in the tail gas and minimising the reducing gas demand of the tail gas.

The inline burner could be operated near a stoichiometric combustion ratio. However, operating the inline burner near its stoichiometric ratio implies the risk that oxygen from the burner is slipped into the tail gas, causing an unwanted temperature excursion in the reactor. If the excess oxygen persists, it will result in catalyst deactivation. To avoid this scenario, the inline burner is normally operated below 95% stoichiometry.

On the other side of the operating window for inline burners is the point at which soot formation occurs. If natural gas is used as the fuel source, the stoichiometric ratio can be as low as 75% without creating soot. However, if a refinery fuel gas is used, which typically contains some C3s and C4+, the substoichiometric ratio cannot be much lower than 85% or soot will form and be deposited on the catalyst bed, causing pluggage of the reactor and deactivation of the catalyst.