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Mercury determination in natural gas

INTRODUCTION

The natural gas is a significant source of ingres s of mercury in the environment. The typical mercury concentration in natural gas may vary from 0.01 to 200 μ g/m3 . Mercury, being present in hydrocarbon gas, initiates corrosion of pipelines and catalyst poiso ning during gas transportation and processing, and these effects have serious implications for the gas-processing industry.

Mercury extraction and purification of gases, as well as processing the products of gas deposits, which involve mercury separating as a fairly toxic element, is very important for the env ironment protection. It is important ev en in cases when mercury is not an appreciable component in the natural gas: the huge amounts of world annual gas consumption should be taken into account.

The use of a mercury analyzer RA-915M/RA-915+ with Zeeman background correction of nonselective absorption provides direct real-time merc ury determination in natural gas at a ng/m3 level.

MEASURING METHOD

The method of atomic mercury concentration measuring in natural gas is based on the use of atomic absorption spectrometry with Zeeman background correction employed in a mercury analyzer RA- 915M/RA-915 +. The use of the Zeeman background correction and a multipath analytical cell provides high selectivity and sensitivity of measurements, essentially reducing the effect of interfering compounds. Thus, even with 25% hydrogen sulfide content in the gas (whose non-selective absorption amounts to 93%) the detection limit doesn’t exceed 0.5 μ g/m 3.

As a result, the analyzer provides direct determination of mercury in a hydrocarbon gas due to the elimination of preliminary precipitation and collection of mercury in absorption traps. Therefore, analyses can be carried out with the ultra low detection limit (0.01 μ g/m 3) in real time. To perform a measurement, the analyzer is placed near a gas well or other sampling point (gas pipeline, string, separator, etc). The gas flows continuously through Teflon hoses into the RP-91NG attachment and then arrives to the analytical cell.

The gas flow rate is controlled by a valve. The blank signal is regularly checked by passing the gas through a special filter with the Hg absorption efficiency of 98–99%. If necessary, a simple device is mounted upstream of the instrument to separate the gas from a liquid phase (water, condensate or oil). The mercury concentration is measured once per second and is processed by a computer with a simultaneous data display.

If it is impossible to carry out direct analysis near a sampling point, the gas can be analyzed in a laboratory. In this case, it is recommended to deliver the gas samples to the laboratory in special bags made of materials that don’t absorb mercury (e.g., Tedlar® gas sampling bags).

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