Mercury in natural gas

M. D. Bingham, Field Detection and Implications of Mercury in Natural Gas, Soc. Petrol Engrs. Production Engrg, 120—124 (May, 1990). 

Mercury in natural gas is also measured by atomic fluorescence spectroscopy (ASTM D6350) and by atomic absorption spectroscopy (ASTM D5954). 

The example above shows the severity of the problem that arises from the presence of mercury in natural gas. Not only is it necessary to determine the levels of mercury present, but also to remove the majority of the mercury prior to any contact with aluminium reactors. The latter of course, further compounds the problem, because if 90% or more of the mercury has been removed, then to determine the remaining mercury is even more difficult. An effective analysis system will he able to measure mercury in its organic and inorganic forms and to do so very quickly. If a mercury removal bed is losing its efficiency then it is imperative to stop the process as soon as possible. In addition, these systems are expensive to operate and it is uneconomic to switch to a new unit if the original still has some life left in it. 

There are many techniques used for the analysis of mercury in natural gas the most commonly used approach is the Jerome Analyzer (Arizona Instrument. Phoenix. Arizona. USA). This collects mercury onto a gold adsorber over a period of time by amalgamation. The 431 -X Mercury Vapour Analyzer is shown in Fig. 3.9. 

The availabihty of a new highly sensitive system for the measurement of mercury in natural gas has allowed a completely new total systems approach to be adopted. This provides an on-hne system to measure the most critical gas stream on a continuous basis, sending detailed results directly to the process computer. It avoids the built-in delays involved in using laboratory measurements and provides better integrity of results. 

We still manage to pollute the atmosphere and environment by the continual use of coal and petrochemicals as our main sources of energy. The impact of mercury in natural gas has been such that there have been at least six experiences of plant failure which have been traced to corrosion due to mercury attacks on aluminium rotors in the plant fabric. The six experiences of plant failure have had significant financial impact on the petrochemical companies. Since very... 

The online measurement of mercury in natural gas presents additional problems operating on a continuous basis put instruments on the design of the instrumentation, and the safety issues involved put an additional requirement on the suppliers. 

The presence of mercury in natural gas has been detected in numerous fields for many years. Mercury found in natural gas is generally in metallic form and its concentration varies from 1 to 75pgNm Natural gas associated condensates are very different First the mercury found in condensates is present in various chemical states elementary, ionic and organometallic second, arsenic is often simultaneously present third, the concentration ranges of mercury and arsenic are generally and, respectively, in the ranges of 10 to 3000 ppb and 10 to 150 ppb. The distribution of mercury and arsenic for an Asian condensate is shown in Fig. 18.30. For this particulate condensate, the large majority of mercury is found in the C3 and C fractions, whereas, arsenic was almost exclusively found in the residue. 

After an initial process equipment assessment confirmed the presence of mercury in natural gas being processed the company industrial hygiene department initiated the mercury exposure risk assessment program. 

The prineiples of Appendix K to Part 75 Quality Assurance and Operating Procedures For Sorbent Traps Monitoring Systems method provides the capability to collect samples over a longer period resulting in an integrated average mercury concentration, lower deteetion limits and a more representative sample. This method also provides a QA/QC protocol with numerical criteria that exceeds that provided by either the ASTM or the ISO methods. The FSTM sorbent traps ein)loyed for the project were especially developed for sampling mercury in natural gas. 

Vickery, J. and Mark Nelson, Measurement and Monitoring of Mercury in Natural Gas Using a Modified Version of EPA Method 308, AIChE, April 2010... 

Natural gas contains both organic and inorganic sulfur compounds that must be removed to protect both the reforming and downstream methanol synthesis catalysts. Hydrodesulfurization across a cobalt or nickel molybdenum—zinc oxide fixed-bed sequence is the basis for an effective purification system. For high levels of sulfur, bulk removal in a Hquid absorption—stripping system followed by fixed-bed residual clean-up is more practical (see Sulfur REMOVAL AND RECOVERY). Chlorides and mercury may also be found in natural gas, particularly from offshore reservoirs. These poisons can be removed by activated alumina or carbon beds. 

In addition, mercury is often present in natural gas and petroleum products, both of which can be the basis of feedstock for industrial reactions. These are generally carried out with aluminium rotors or condensers, and low levels of mercury attack aluminium components, causing stress fractures. Mercury-induced corrosion on aluminium heat exchangers has resulted in at least four long-term industrial complex shutdowns in Algeria, the USA, Indonesia and Thailand. These shutdowns required costly replacements and from several weeks to several months of lost production the plants have now been equipped with mercury removal units. 

Emteborg, H., Baxter, D.C. and Freeh, W. (1993) Speciation of mercury in natural waters by capillary gas chromatography with a microwave emission detector following preconcentration using a dithiocarbamate resin microcolumn installed in a closed flow injection a system. Analyst, 118, 1007-1013. 

Mercury is one of a number of toxic heavy metals that occur in trace amounts in fossil fuels, particularly coal, and are also present in waste materials. During the combustion of fuels or wastes in power plants and utility boilers, these metals can be released to the atmosphere unless remedial action is taken. Emissions from municipal waste incinerators can substantially add to the environmental audit of heavy metals, since domestic and industrial waste often contains many sources of heavy metals. Mercury vapor is particularly difficult to capture from combustion gas streams due to its volatility. Some processes under study for the removal of mercury from flue gas streams are based upon the injection of finely ground activated carbon. The efficiency of mercury sorption depends upon the mercury speciation and the gas temperature. The capture of elemental mercury can be enhanced by impregnating the activated carbon with sulfur, with the formation of less volatile mercuric sulfide [37] this technique has been applied to the removal of mercury from natural gas streams. One of the principal difficulties in removing Hg from flue gas streams is that the extent of adsorption is very low at the temperatures typically encountered, and it is often impractical to consider cooling these large volumes of gas. 

Extending the procedures for the measurement of mercury levels in natural gas and coal fired stack emissions places far more difficult demands on the skills of the analyst. Both present very complex matrices, which although different in many respects, provide similar requirements to overcome the effects of the make up within the matrix. 

The mercury level in natural gas streams can be quite varied and the problems caused by high levels quite catastrophic. As such, online measurement can be essential in spotting potential problems allowing a measured response to increased Hg levels, potentially saving millions of dollars. A schematic block diagram of the online system is shown in Fig. 3. 

Assessment of mercury concentrations in natural gas and other process gas streams. 

ASTM D 6350 98 Standard Test Method for Mercury Sampling and analysis in natural gas by Atomic fluorescence Spectroscopy... 

Although not modified to increased adsorption capacity, the first commercial example was the cobalt chloride-impregnated transition alumina marketed by Alcoa in the 1950s for water adsorption. Calcium, sodium, and potassium salts have also been used commercially for various reasons. The most recent published example is the modified alumina used to separate mercury from natural gas streams in the Arabian oil countries. A related example is the permanganate impregnation of a... 

NcNamara, J.D., and Wagner, N.J., Process effects on activated carbon performance and analytical methods used for low level mercury removal in natural gas applications. Gas Sep. Purif. 10(2), 137-140 (1996). 

Because of differences in ecotoxicity between the different mercury species and as many mercury species are volatile or can easily be transformed into volatile compounds, they can readily be separated by gas chromatography and detected by MIP optical emission spectrometry for speciation. Freeh et al. [520] compared the Bee-nakker microwave-induced plasma (MIP) and a furnace atomization plasma excitation spectrometry (FAPES) source for the determination of derivatized mercury species in natural gas condensate with coupling to high-resolution GC for sample introduction and monitoring the emission of the 253.6 nm mercury hne. The precision of replicate measurements for dimethyl-, methylbutyl-, and dibutyhnercury... 

For mercury species determination in natural gas condensate, isotopically enriched Mc2Hg, MeHgCl and HgCh were prepared and used for species-specific isotope dilution and method assurance tests. ° This method gave excellent detection limits and improved matrix tolerance compared with other speciation approaches for samples in an organic matrix. 

Natural gas may contain helium, hydrogen and mercury, though the latter is rarely a significant contaminant in small quantities. 

An explosion rupturing an ammonia separator (still) in an ammonia production unit, probably because mercury vapour from geological sources entered with hydrogen syngas originating from natural gas and reacted to give explosive nitride deposits. The separator remains crackled when scraped [1]. For a more academic study of the effects of mercury on ammonia plants, including embrittlement and corrosion, as well as explosive deposits [2],... 

Gill and Fitzgerald [481] determined picomolar quantities of mercury in seawater using stannous chloride reduction and two-stage amalgamation with gas-phase detection. The gas flow system used two gold-coated bead columns (the collection and the analytical columns) to transfer mercury into the gas cell of an atomic absorption spectrometer. By careful control and estimation of the blank, a detection limit of 0.21 pM was achieved using 21 of seawater. The accuracy and precision of this method were checked by comparison with aqueous laboratory and National Bureau of Standards (NBS) reference materials spiked into acidified natural water samples at picomolar levels. Further studies showed that at least 88% of mercury in open ocean and coastal seawater consisted of labile species which could be reduced by stannous chloride under acidic conditions. 

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