(717f) Global Solution for Mercury Removal in Saturated Wet Gas Streams : From Rational Material Design to Innovative Process Issues

Elemental mercury is a natural contaminant of natural gas. In particular, mercury is a safety issue in LNG plants, as mercury forms amalgams with aluminum-based alloys, leading to corrosion issues on cryogenic exchangers required to liquefy natural gas.

Conventionally Mercury Removal Units (MRU) were located downstream Acid Gases Removal Units (AGRU) and dryers. However, in this configuration, mercury tends to contaminate all the processes streams and equipment located upstream of the MRU. Therefore, in recent gas plant design, MRU are always located at the most upstream possible location, i.e. generally just after condensate and water removal units. In such cases, natural gas is saturated with water and hydrocarbon condensate vapors. Moreover, in such a location, liquid carry-over in MRU beds is likely to occur. Liquid water has a very detrimental influence on conventional mercury guard bed performances: mercury capacity loss for sulfur-impregnated carbons and mechanical strength loss due to binder swelling induced by liquid water for bulk adsorbents. The use of water-resistant mercury adsorbent becomes thus an absolute necessity for MRU in gas plant after water and condensate removal units.

The way IFPEN has optimized MRU solutions dedicated to the total elimination of elemental mercury from saturated wet gas streams is based on a dual complementary approach : on one hand the development of water-resistant mercury adsorbent by deposition of CuS on optimized alumina and, on the other hand, the design of innovative cost-effective process solutions aiming at the reduction of relative humidity in MRUs.

The optimization approach of water-resistant mercury adsorbents will be presented. In particular, the effects of pore size and surface chemistry of dedicated alumina supports on water vapor capillary condensation mechanism will be discussed.

Concerning the process aspects, conventional solutions proposed to prevent any liquid presence in MRU beds rely on natural gas heating or dew pointing, which constitute energy-intensive operation units. Innovative energy-efficient process solutions aiming at the reduction of relative humidity of natural gas in MRU have been recently patented by IFPEN. For similar performances, heat consumptions for such operations have been reduced by an order of magnitude compared to existing solutions. Process modeling and economics calculation on the innovative solutions proposed by IFPEN will be presented.