Desulphurisation and Mercury Removal from Natural Gases
Figure 2: Gas Sweetening with Combined Amine and Fixed Bed Absorber
aMDEA adsorption
TEG dehydration
Table 1: Distribution of Mercury in a 50 mmscfd Gas Plant Process stream
Raw gas
Acid gas removal vent Dryer vent Condensate Sales gas
maintenance operations; • Sweet
Dry/Sour gas from gas/NGL separation
Amine regeneration Figure 3: Integration of Membranes and Fixed Bed Polishing Raw gas Lead/Lag system
Membrane guard bed
Fixed bed absorbent reactors
CO2
rich gas to flare
Gas polishing
PURASPECJM reactor
sales gas difficulty in disposal of mercury-contaminated equipment;
• emissions to the environment; and • potential liabilities resulting from mercury-contaminated product streams.
Two major types of mercury-corrosion can be observed. These are amalgam corrosion and liquid metal embrittlement (LME). Amalgam-induced corrosion is shown by any metal capable of forming an amalgam with mercury. Most metals owe protection from corrosion to the presence of an oxide layer. If this protective layer is damaged in the presence of liquid mercury, then the metal can show its full reactivity and attack by air or water is rapid.
LME involves the diffusion of mercury into the grain boundaries and results in cracks developing along the grain boundary. This type of attack does not involve air or water and once initiated progresses rapidly, often resulting in a catastrophic failure.
Membrane unit Sales gas
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 to 5,000 μg/Nm3 in some fields in north Germany to less than 0.015 μg/Nm3 in some parts of the USA and Africa.
Although the levels of mercury recorded are low, the tonnages of hydrocarbons handled are enormous so downstream processing equipment is exposed to a substantial amount of mercury. Thus a
During the early stages of gas processing projects it is sometimes difficult to get a detailed composition of the gas stream and its associated impurities.
typical 10,000 tes/day liquefied natural gas (LNG) plant would use
600 mmscfd of natural gas and if this contained 100 µg/m3 mercury the plant would receive 582 kg mercury per year. The main concerns associated with mercury are:
• corrosion of process equipment; •
34 exposure of workers to high levels of mercury during Removal Processes
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 to 15 % w/w). There are numerous units in service on gas processing plants around
HYDROCARBON WORLD – VOLUME 6 ISSUE 2
Little data has been published on the distribution of mercury on gas processing plants and its monitoring has tended to be only from specific feed and product streams. Over the past few years, a number of surveys have been carried out by Johnson Matthey Catalysts on gas processing plants located in the UK, North Africa, the Far East and South America. Measurements were made using the Sir Gallahad II mercury analyser.
The nature of the plants and the difficulty in carrying out the measurements meant that it was not possible to carry out a mass balance on the distribution of mercury through the various process streams. Instead, a number of readings were obtained showing the steady state concentrations of mercury at the various process stages. Not all of the plants had all of these processing stages and only two of the seven plants quoted had mercury removal beds.
Figure 4 clearly shows that mercury is distributed right through the plant and is adsorbed on all the metal surfaces, which makes maintenance and decommissioning of redundant equipment highly hazardous.
A mass balance was also carried out for a gas processing plant in the Far East. The gas flowrate was 50 mmscfd and the mercury balance was carried out across the main processing units. The results are shown in Table 1.
Mercury (kg/year) 220 22 3
45 150
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