Jan-2014

Mercury treatment options for natural gas plants

Removing mercury as close as is practicable to the front end of a natural gas processing system is the best choice

SATYAM MISHRA
UOP, a Honeywell Company

Article Summary

Mercury in natural gas and natural gas liquids is most likely to be in the elemental state. Although mercury has a high boiling point (357°C), it also has a high vapour pressure, which makes it very mobile. Mobility presents a challenge for today’s gas processors as mercury can disperse throughout gas plant assets, making it difficult to determine how and where it should be removed. Left unchecked, mercury will deposit on surfaces including those common to pipelines and plant assets. Mercury can then desorb back into gas streams, passing through contaminated pipelines.  Because of this, long periods of time can elapse between the installation of an upstream mercury removal unit (MRU) and the complete purging of a pipeline. How best to remove mercury has consumed a lot of thought on behalf of processors and treatment companies alike. Industry experts suggests that removing mercury as close to the front end of a natural gas processing or gas transmission system as is practicable is the best choice.

Mercury removal process options
The market has a number of approaches to treatment for mercury removal. These options can be categorised as regenerative adsorbent and non-regenerative adsorbent solutions for mercury contaminant removal.

Non-regenerative adsorbent for mercury removal
A non-regenerative MRU is sulphur impregnated on carbon or metal sulphide beds. The common and traditional approach to mercury removal has been through the use of sulphur impregnated carbon beds. Existing sulphur impregnated activated carbon options tend to be less effective at positions upstream of molecular sieve drying systems or glycol injection due to the risk of capillary condensation of water and heavier hydrocarbons in the micropores of the carbon sub-structure. Sulphur impregnated carbon products are thus not a product of choice, particularly where MRU locations have been in the ‘up-front’ position, where raw gas is often at or close to its dew point and entrained liquids are common. This is where fixed bed, metallic based MRU products find greatest success in their ability to treat ‘wet’ gas streams in ‘up-front’ positions.

UOP’s non-regenerative metal sulphides (UOP GB series of products) are a set of versatile non-regenerative mercury removal adsorbents. They are operationally flexible and can be used to process gas that is at or close to its dew point in a variety of process locations.

Gas and liquid streams containing thousands of micrograms or parts per billion levels of mercury can be treated to extremely low effluent levels using GB adsorbents. These high capacity mercury adsorbents are engineered using a copper based active component finely dispersed across an alumina substrate. This high capacity leads to infrequent change-outs and a longer lifespan, reducing the cost of mercury removal over time. GB adsorbents can be supplied in their oxidised form or in their sulphided form, which offers flexibility, and they can be pre-sulphided or sulphided in situ.

Mercury is reactively adsorbed from the gas/liquid hydrocarbon stream by reaction with cupric sulphide. As Figure 1 shows, cupric sulphide is the product of choice for mercury removal over cuprous sulphide.

Careful selection of the carrier substrate is essential for maximising mercury adsorption as these beds are designed to operate effectively at short contact times and are exposed to hydrocarbons close to their dew point. Therefore it is critical to specifically engineer the pore distribution of the adsorbent. As Figure 2 shows, an engineered mix of micro- and mesoporous structures ensures rapid mercury adsorption with the ability to withstand liquid condensation and minimise mass transfer limitations.

Regenerative adsorbent for mercury removal
UOP HgSIV adsorbents are regenerative molecular sieve products that contain silver on the outside surface of the molecular sieve pellet or bead. Mercury from the process fluid (either gas or liquid) amalgamates with the silver and a mercury-free dry process fluid is obtained at the bed outlet. An additional layer of HgSIV adsorbent, to an existing dryer configuration, results in removal of design water load and the mercury from feed gas. Mercury and water are both regenerated from the HgSIV adsorbents using conventional gas dryer techniques where the mercury- silver amalgam on the molecular sieve is regenerated thermally. The mercury desorption profile from the HgSIV adsorbent is similar to a typical water regeneration profile, except that mercury is completely removed from the HgSIV adsorbent well before the full regeneration temperature is reached (for water removal).

Mercury exits the bed during the regeneration step along with the spent regeneration gas. Plants have the option to consider installation of a smaller, non-regenerative guard bed to treat the mercury laden, spent regeneration gas. This ensures removal of mercury from the gas processing section of the plant.

Physically, UOP HgSIV adsorbents have a similar appearance to conventional molecular sieves. These HgSIV adsorbents are loaded into an adsorption vessel in the same way as are conventional molecular sieves. There is no need for special care such as the use of nitrogen blanketing during the installation. For unloading, only the same precautions need to be taken as when unloading conventional molecular sieves.

Regenerative adsorbents for mercury removal, such as HgSIV adsorbents, can be used to treat mercury laden gas in plants which were not originally designed for mercury from feed gas. These adsorbents can be installed in the existing dehydration beds without capital expenditure of installing non-regenerative MRUs. This also helps users avoid additional pressure drop upstream of cryogenic separation.

HgSIV adsorbents also help users treat their gas in situations where their non-regenerative guard beds are either out of service or are under performing. 

Mercury removal schemes
Non-regenerative bed for upstream mercury removal
UOP’s GB range of non-regenerative metal sulphide adsorbents can remove mercury from the raw gas, upstream of the amine unit and the dehydration vessels (see Figure 3). Using larger MRU vessels protects the brazed aluminum heat exchanger and significantly reduces mercury contamination in and around the process plant. This option has become increasingly popular since it minimises the total mercury present before there is any opportunity for mercury to migrate to various locations within a gas processing plant and avoid the risk of subsequent partitioning into processed natural gas and condensate streams. This option also avoids subsequent adsorption onto any pipeline asset or piece of equipment downstream.

While the recommendation to position the MRU upstream of the acid gas removal units remains the ideal solution for natural gas plants, there are many operating facilities with MRU vessels downstream of the dryers (see Figure 4).