Mercury can be present in natural gas from zero to quite high concentrations and may be found in different forms. Having the right mercury analyzer is vital to accurate measurement.

Why measure Mercury in natural gas?

Knowledge of the total mercury content of natural gas is extremely important for the following reasons: firstly mercury is highly toxic and is of environmental concerns and secondly it can cause damage to petrochemical and natural gas refineries in addition to loading/unloading terminal plants which can be financially crippling especially when unscheduled shutdowns are forced.

The particular areas which can be affected are:
a) Contamination of environment, particularly potable water sources and fish stocks
b) Poisoning of expensive palladium based catalysts used for the hydrogenation of acetylenic species in steam cracking plants
c) Corrosion of aluminium based heat exchangers at natural gas plants leading in extreme cases to explosion risks.

To avoid potential damage many companies install mercury removal systems to ensure the important parts of the plant are protected. This means periodic measurements of the outlets of these removal systems need to be made to ensure they are working correctly. The inlet concentrations also need to be checked to help calculate the expected life of the removal bed. Other parts of the plant need to be checked to help provide a fuller understanding of the whole process.

As these measurements are so critical, the frequency of measurement needs to be high therefore the preferred method of obtaining this data is by using online instruments.

What is involved in analysing natural gas?

As natural gas is present at elevated pressures and the analyser operates at ambient pressure then the sampling is integral to the analysing process as a whole. Therefore a typical online natural gas analyser consists of the following interrelated components

1 Sample probe
2 Sample line from probe to sampling system
3 Sampling system
4 Purged enclosure for the analyser
5 Analyser system

Photo of Online System for Mercury in Natural Gas

Note Systems can be built to NEC, IECEx or ATEX safety standards with either metric or imperial sizing.

All of the components are configured to provide the utmost reliability, sensitivity and the least possible contamination.

1 Sample Probe
In the majority of cases customers connect to existing sample points, however, if required PSA can provide either traditional type probes or membrane type probes

2 Sample Line
In order to prevent losses from the sample point to the sampling system, Teflon sample lines are preferred: however as the sample is normally at plant pressure at this point it is important to have sample lines which can stand the high pressures. For this purpose PSA utilize the stainless steel braided Teflon lines.

3 Sampling system
Sampling of natural gas can be difficult because of the high pressures normally involved, the inflammable nature of the matrix and the fact that mercury can amalgamate with metallic surfaces.

When the pressure of natural gas is decreased, heavy hydrocarbons condense into a liquid from the vapour phase. When the heavy hydrocarbons condense on the mercury collection medium it reduces the collection efficiency which gives lower mercury results.

In order to obtain a representative sample of gas from a high pressure gas stream PSA use heated regulators during the pressure let down stage. In this way the sample gas at ambient pressure will have the same composition as that at the higher operating pressure.

Losses of mercury by surface adsorption on stainless steel components can greatly affect results. To overcome this, a high flow rate primary bypass for the sample is normally used.

4 Purged enclosure for the analyser
As the analyser is not explosion proof and the area in which the analyser will be located is hazardous, the analyser system needs to be housed in a purged enclosure. The purge can be either air or nitrogen. The purged enclosure has a controller which only allows power to be applied to the instrument when it is safe to do so.

The system can be constructed to meet NEC, IECEx or ATEX safety  standards.

5 Analyser system
The purged enclosure contains the online analyser which consists of the following modules

1 PSA 10.525 Sir Galahad mercury analyser operating in dual amalgamation mode

2 a stream selector unit

3 an automatic calibration system

4 an industrial computer with monitor running the PSA online software

The preconditioned sample is delivered from the sampling system to the stream selector unit. This unit, controlled by the online software, determines the in which the sample streams are analysed plus the frequency and timing of calibration and check samples. On selection of the stream, the valve is actuated so the gas is delivered to the Amasil filled sample trap in the Sir Galahad mercury analyser. The sample volume collected and flow rate over the trap is controlled using a digital mass flow controller. During the collection period the sample tube containing the amasil is heated to ensure that the amasil is not contaminated by any heavy hydrocarbons condensing in the tube. After the collection period the sample trap is heated to 900 degrees and purged with a stream of argon for 30 seconds. This thermally desorbs the mercury collected from the sample and delivers the mercury onto the second trap which is maintained at ambient temperature. The second trap is them heated to release the mercury which passes to atomic fluorescence detector (AFS) where it is measured.

Optimisation of flow rates and sampling times allows a wide concentration range to be measured with the same calibration.

A schematic diagram of the system is shown below.

Diagram of Online dual amalgamation system

What do I do if I have a customer interested in Mercury in Natural gas?

1 Complete the PSA gas questionnaire form Qgas questionnaire with the customer and send to the PSA sales department.

2 Literature: As above plus

Application Note APP060 Determination of Mercury in Natural Gas

Application Note APP082Determination of Mercury in gases containing High Levels of H₂S using Dual Amalgamation AFS

Application Note APP087 Determinationof Total mercury in Mixed LPG using

ISO-6978 and ASTM 6350

Poster Online Determination and Removal of Mercury from Natural Gas Streams – ICMGP 2004

There are also PowerPoint presentations available from PSA Sales department

3 References

We have sold several hundred Sir Galahad systems for the determination of Mercury in Natural gas to all the oil and gas majors, the majority of the field testing companies and many of the mercury removal system suppliers. The Sir Galahad complies to the latest ISO and ASTM methods for mercury in natural gas ISO 6978:2 and ASTM 6350.

We have sold many online mercury in Natural gas /LNG systems worldwide, including BP Amoco, Chevron, Texaco, Conoco, Phillips, Oman LNG, British gas, PTT, Petrobras and Saudi Aramco. Many of these companies have multiple systems with most of these systems handling multiple streams and have been in operation for many years.

4 Support

PS Analytical has been involved in the determination of Mercury since its inception in 1983 and has been heavily involved in the oil and gas industry for over 15 years. Please contact the sales department if you have any questions at all on this application. We often have these systems which are built to order and specification which are available to be shown to visitors to PSA.

What types of companies are interested in the online measurement of mercury in natural gas?

Oil and Gas production companies

Land and sea gas terminal contractors

Oil field testing and support companies

Mercury removal system producers