Mercury removal from liquid hydrocarbons
Various methodologies for the measurement of mercury levels in gaseous and liquid hydrocarbons are available, while inorganic routes for mercury removal have a number of advantages over the organic approach
Peter J H Carnell and Steve Willis
Johnson Matthey Catalysts
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Mercury is widely distributed in the earth’s crust, but only rarely does it occur in more than trace amounts. Understandably then, there was little interest in removing mercury, until two events caused worries over levels of mercury. The first was in the early 1950s when fishermen and their families around Mina Mata Bay in Japan were found to be suffering from mercury poisoning caused by trace levels of mercury in a factory effluent being converted into an organo-mercury that accumulated in fish.
The second incident was in the early 1970s when there was a catastrophic failure of a process heat exchanger at Skikda in Algeria. Investigations concluded that this was caused by trace levels of elemental mercury collecting during the cooling stages of LNG production and then attacking an aluminium heat exchanger.
Most naturally occurring hydrocarbons contain low levels of mercury. In the case of natural gas, it is usually present as elemental mercury and levels as high as 4400Âµg/m3 have been reported. Natural gas liquids and crude oil can contain complicated mixtures of all forms of mercury too, and this content can be several ppm. Mercury is present in almost all coal measures as well; reported levels in US coal, for example, range from 70—33000ppb.
Mercury is unusual in that it can exist as a liquid metal, inorganic salt, organo-metallic compound or organo-mercury salt. Furthermore, certain bacteria can convert elemental and inorganic mercury into organo-mercury, favouring the particularly toxic dimethyl mercury. Mercury and organo-mercury compounds are high boiling liquids (Table 1) and all mercury compounds release mercury on heating so that it can be transferred throughout the plant during the processing of hydrocarbons.
The main problem in analysing for mercury arises from the ease with which the metal is lost on sample lines and container walls. Scrupulous purging procedures have to be followed to ensure accurate and reproducible results. Mercury in gases is now measured using atomic fluorescence spectrometry. A known volume of gas is passed over a gold trapping tube in order to concentrate the mercury. Gas at high pressure may need to be let down through an electrical heated regulator to avoid condensation of liquids during depressurisation. Mercury is desorbed from the collector tube by heating to 800°C with a carrier gas for passage into the analyser. This technique can be modified to measure mercury in liquid streams by using an attachment to vapourise a measured volume of liquid over a collector.
Recent work using Neutron Activation Analysis (NAA) has produced encouraging results in determining mercury in naphtha down to 1ppb. In this method, a small sample is irradiated in a nuclear reactor to create radioactive isotopes and their decay is then monitored. This method is attractive because the analysis can be made on the sample tube, thus avoiding handling losses.
There have been many studies on the distribution of mercury on gas-processing plants as part of the measures to avoid corrosion in cryogenic plants. Mercury is split between the gas and NLGs, but significant amounts are released from the vents of the acid gas removal and drier units.
Refineries have long relied on the hydrotreating stage to remove sulphur, nitrogen and metal contaminants from middle distillates, catalytic cracker feeds, and so on. This works well for non-volatile metals such as lead, arsenic and vanadium. One to five per cent of these metals can be retained on the cobalt or nickel molybdate catalysts without any loss in overall performance. However, mercury is not retained on the hydrotreating catalysts under the conditions used and may end up in the liquid streams.
Analysis of shipments of naphtha supplied to a major petrochemical complex over a five-year period showed a wide variation in the levels of mercury present. Of the 547 batches received, the mercury content varied from < 2—16ppb. The concern with mercury in naphtha is that it is a poison for the precious metal C3 hydrogenation catalysts. Analyses showed mercury levels of 100—600ppm on a 0.25% Pd-based catalyst. There have also been incidents of metal embrittlement on the cold train. These problems have resulted in there being a premium for mercury-free naphtha.
The traditional method for the removal of mercury relies on its reaction with elemental sulphur. The sulphur is deposited on a support, typically carbon, and the resulting captive mass is used in a fixed-bed reactor. The reaction is rapid and high levels of mercury can be absorbed onto the bed (10—15%w/w). There are numerous units in service on gas-processing plants around the world. However, there are several drawbacks to this method of removal:
- There is no use for the spent material
- The only environmentally acceptable route for disposal is by combustion followed by condensation of the mercury evolved
- Sulphur is soluble in hydrocarbons, particularly in aromatics. In fact, benzene has been used to remove sulphur released from gas-processing equipment.
Modern thinking has moved towards inorganic routes for the removal of mercury from gaseous and liquid hydrocarbons. These rely on the high reactivity of mercury with the metal sulphides of certain variable-valency metal sulphides.
Hg + MxSy = MxSa + HgS
The reactive metal is incorporated in an inorganic support and the absorbent is supplied with the reactive sulphide present, or this is formed in situ by reaction with H2S in the hydrocarbon to be treated. The inorganic approach has a number of advantages over the organic approach.
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