Mar-2009

Mercury removal in hydrocarbon streams

The presence of mercury in refinery hydrocarbon streams results in detrimental effects, including catalyst poisoning, corriosion and safety issues

Steve Catchpole
Johnson Matthey Catalysts

Article Summary

Mercury is found in an ever-increasing number of hydro-carbons worldwide. The level of mercury can vary significantly, depending on location. The situation is complex in refineries, as liquid feedstocks can contain elemental, organic and inorganic species. Complete removal of mercury is advisable to avoid catastrophic failures in cryogenic equipment, to prevent poisoning of process catalysts, and for health and safety reasons. Additionally, hydrocarbon products that can be labelled as mercury free are able to command an enhanced product value. For example, mercury-distressed naphtha may be sold at $5—10 less than open-spec naphtha.

There are now over 250 proprietary Puraspec mercury removal units (MRUs) in service on a range of hydrocarbons, with more than 100 units in commercial operation on liquid applications. The new generation of Puraspec absorbents have been shown to give effective removal and long life without impacting negatively on product quality.

Mercury in hydrocarbons
Almost all hydrocarbons contain mercury. In the case of natural gas and natural gas liquids, it is likely to be present as elemental mercury. In the case of crude oil, it may also be present as organo-metallic and ionic mercury. The concentration of mercury in natural gas varies widely from 450—5000 μg/Nm3 in some fields in North Germany to less than 0.01 μg/Nm3 in some parts of the US and Africa.¹ Much higher levels of mercury can be found in crude oil.2 Levels found in some crude oils are shown in Table 1.

Elemental mercury is only sparingly soluble in liquid hydrocarbons3 (the solubility in n-octane at 25°C is 2 ppm), so the high levels recorded show that mercury compounds must also be present.

Mercury has a high boiling point (356.7°C), but has a high vapour pressure at ambient temperature and is surprisingly mobile. Organo-mercury compounds are easily broken down when heated in a reducing atmosphere,  and elemental mercury is released during the hydrotreating and catalytic reforming stages of the refinery. 
As a result, whatever the original form of mercury in the crude oil feed to 
the refinery, much of it ends up as elemental mercury in the C3 to C6 product streams; namely, LPG, gasoline and naphtha.^2,4

Published levels of mercury found in refinery product streams are shown in Table 2a. Figure 1 displays the measured distribution of mercury in the crude feed and product streams of the crude column of a major world-scale refinery. It is also worthwhile to note the prevalence of mercury in the water effluent of the desalting operation.

Data derived from world-scale refineries shed additional light on the distribution of mercury. These data are summarised in Table 2b.
Due to its ability to manifest itself in practically all of the crude column products, mercury will be found in a number of downstream unit operations. Figure 2 shows the measured distribution in a FCC/resid cracker, and Figure 3 shows the measured distribution in a hydrocracker of a typical world-scale refinery.

Although the levels of mercury recorded are low, the tonnages of liquid hydrocarbons handled are enormous, so downstream processing equipment is exposed to a substantial amount of mercury. The main worries are corrosion, poisoning of catalysts, and health and safety. These can involve serious 
financial losses.

Corrosion
Mercury readily wets most surfaces and forms amalgams with a number of metals. A particular concern is aluminium, because it is widely used in cryogenic heat exchangers. This is a potentially reactive metal protected from attack by air and water by an oxide layer. If the protective oxide layer is damaged (say, by a small scratch) and liquid mercury is present, an amalgam is formed, which will allow rapid reaction with air or water:

Hg + Al → Hg(Al)
Hg(Al) + 6H2O → Al2O3.3H2O + H2 + Hg

Aluminium heat exchangers are particularly vulnerable during de-riming and start-up and shutdown. Since the catastrophic failure of an aluminium heat exchanger at Skikda in 19755 there have been many reported incidents.5,6 The most recent was the Santos onshore gas facility in 2004, when there was a massive blast and fire in the liquids recovery section, resulting in the shutting down of gas supplies.

Mercury can also cause liquid metal embrittlement (LME). This can occur in the absence of water and produces rapid brittle fracture. Mercury can diffuse into the grain boundaries to form a liquid amalgam. The cracks produced propagate along the grain boundary. This type of corrosion affects a broad range of materials such as aluminium alloys, copper-based alloys and some types of steel. LME is particularly worrying because it is difficult to detect prior to failure under pressure.

Corrosion is a particular concern for LNG plants, so a mercury limit of less than 0.01 μg/Nm3 is set on the feed. Similarly, ethylene cracker plant operators favour their hydrocarbon feed to contain less than 5.0 ppb of mercury, and the same limit is set for the low temperature storage of LPG.

More recently, following the move to ultra-low sulphur fuels, there are concerns about corrosion of auto fuel meters and pumps.