Dec-2008

Capture and convert - handling mercaptans in hydrocarbon streams

With the development of increasingly difficult hydrocarbon streams, mercaptans emerge as a major nuisance. The removal and conversion of mercaptans is becoming a challenge under the ever increasing restraints on emissions. This article gives a summary of available technologies to capture and convert mercaptans.

Gerrit Bloemendal, Sander Kobussen and Frank Scheel
Jacobs Nederland B.V.

Article Summary

Capturing mercaptans in refineries. In refineries, mercaptans can be found in both gaseous streams (fuel gas) and liquid streams (LPG). Raw fuel gas typically contains substantial amounts of H2S, sometimes some CO2, and often organic sulphur compounds such as COS and mercaptans. Usually the fuel gas is treated with amine based solvents (MEA, DEA, DIPA or MDEA) to remove the H2S. If the gas does not contain CO2 (or the CO2 was absorbed by the amine solvent), the gas can be further processed in a caustic scrubber process such as the Merox process. Here, the mercaptans which are by their nature very weak acids, react with the strong base in the caustic, forming a salt according to the reaction:

RSH + Na+ + OH- → RS- + Na+ + H2O

The caustic solution containing the mercaptides can be regenerated by contacting it with air. With the aid of the Merox catalyst in the caustic solution, the mercaptides are oxidised and a disulphide oil (DSO) is formed: 

4 RS- + 4 Na+ + 2 H2O + O2→ 2 RSSR + 4 Na+ + 4 OH-

The DSO is decanted for further processing. Typically, this stream is routed to the inlet of a hydrotreating process in which the disulphides are hydrogenated to form H2S and hydrocarbons. The H2S is subsequently absorbed in the amine system, and further processed in the sulphur plant.

Usually, the removal of CO2 is not targeted, so the gas from the amine section can still contain substantial amounts of CO2. In that case, the Merox process will be less economical since the CO2 reacts with the caustic and results in high chemicals consumption. In such a situation, a caustic prewash system is commonly applied to remove the CO2 before feeding the gas to the Merox system.

For liquid hydrocarbon streams (LPG) a similar strategy is usually applied, with an amine extraction step to remove the H2S followed by a mercaptan extraction system based on the Merox process. In cases where the LPG contains high levels of COS, the amine extraction system can be expanded with one or more mixer/settler combinations in order to reduce the COS to low levels and to off load the Merox unit.
 
Moderate mercaptan removal in natural gas
The process line up that is used to treat gases containing mercaptans depends strongly on the required removal efficiency. If only a moderate degree of removal is required (typically up to 95% removal), a mixed physical/chemical solvent such as Sulfinol or Flexsorb SE can be a good choice. Sulfolane, used as the physical component in these solvents, has a certain affinity for mercaptans and can thus act as an absorbent. The solvent can be tailored by varying the sulfolane content and selecting the amine (Flexsorb for high CO2 selectivity, MDEA for moderate CO2 selectivity, DIPA for deep CO2 removal) to achieve an optimised removal of H2S, COS, CO2 as well as mercaptans.

Deep mercaptan removal in natural gas
Sometimes deep removal of mercaptans is required, e.g. in liquefied natural gas (LNG) production, and additional treating steps are needed. For this purpose, molecular sieves are usually the technology to look at. Preferably, the raw gas is pretreated as described above to minimise the load on the molecular sieve. Molecular sieves achieve a very low concentration of mercaptans in the treated gas, typically as low as 5 ppmv.

The mole sieve unit is a swing type adsorption process in which the organic sulphur is adsorbed at low temperatures. Once the mole sieve bed is (almost) saturated, it is taken offline and stripped with hot gas from a furnace to release the sulphur species. The resulting small gas stream contains the concentrated organic sulphur compounds. After cooling this gas, the bulk of the sulphur species are absorbed in a small contactor applying the same solvent as in the main contactor. The resulting gas is fed back to the front end of the mole sieve to readsorb the remaining mercaptans and will be treated to sales gas specification. The absorbed mercaptans will ultimately be released in the solvent regenerator, from where they are routed along with the acid gas to the sulphur plant. Considering the batchwise nature of the mole sieve process, it is evident that the mercaptans will be released in peaks that can have a very negative effect on the operation of the sulphur plant.

Although mole sieve systems are quite often the technology of choice because of their excellent adsorption characteristics, other regenerative adsorption processes such as the Sorbead process can be used as well, providing an attractive combination of sulphur removal and hydrocarbon/water dewpointing.

An alternative route consists of a physical solvent system followed by a Merox type caustic wash. The solvent (e.g. Selexol) removes the H2S and CO2 from raw natural gas to bring it to treated gas specification. It will also remove the bulk of the mercaptans, thereby reducing the size and chemicals consumption of the downstream Merox system. The disulphide oil formed in the Merox oxidiser has to be handled in the gas plant, so cannot be hydrotreated. Therefore it will be combusted separately and thereafter routed to the thermal stage of the sulphur plant as an additional feed.

Converting mercaptans in a rich acid gas
Once the mercaptans are absorbed and released in the regenerator together with H2S, the resulting acid gas has to be processed further to recover the sulphur. If the acid gas is of a good quality (containing more than 35% H2S), it can be processed directly in a Claus type unit. However, if the gas originates from a mole sieve type unit, the mercaptans are released in peaks that substantially influence the performance of the sulphur plant. Simply relying on the tail gas analyser to handle the peaks is not an advised option, because the disturbances are too large for a feedback system. In those situations, the improved burner control system using a feed forward analyser (known as ABC+) is an excellent option1. With this system the sulphur plant air demand is calculated as a function of the acid gas composition and flow, resulting in more accurate control and improved sulphur recovery.