Nitrogen Removal on LNG Plant

Sylvain Vovard's picture
Sylvain Vovard, LNG Process Engineer, Process and Technology Division, Technip
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Technip is one of the world’s largest engineering and construction companies operating in all sectors of the hydrocarbon industry including LNG.

Sylvain Vovard is an LNG Process Engineer working in Technip’s technology centre inParis. He is the author of several patents. He is the young flag carrier of a team that has 50 years of continuous LNG experience that goes back to CAMEL, the first ever baseload plant, Technip is today a top tier EPC contractor that together with its JV partners has delivered the world’s 6 largest LNG trains in Qatar and the first plant in Yemen. At the other end of the scale, the Hanas small-scale LNG plant is now operating inChina. The company’s unique experience in LNG, floating production platforms and subsea systems, explains why it is the leading engineering contractor in Floating LNG through contracts between Technip-led consortia and Shell and Petronas.

While focused on project execution, Technip continuously develops new technologies to meet the  changing needs of the industry and the processes for nitrogen rejection from LNG described in the Gastech 2012CoTEsession on cryogenics are a prime example.

Nitrogen is often found in non negligible quantities in natural gas, with content varying according to the location and the type of the field. Certain non-associated gases and associated gas with oil recovered after nitrogen injection EOR giving the highest levels. When present in large volumes, it leads to poor combustion and needlessly increases the duty of compression stations. Regulations usually limit the nitrogen content of pipeline gas to below 4 to 5% mol, which sometimes requires N2 removal.

For LNG, the nitrogen specification is even more restrictive (<1 %mol) to avoid roll over in the

LNG carrier during transportation. A typical process to achieve this specification is to expand the LNG downstream the MCHE to the lowest possible pressure. The flash gas, still rich in methane, is re compressed and disposed of as fuel gas. However in the case of high nitrogen feed gas, or low fuel gas consumption, this solution may result in lean fuel gas that does not meet the lower limit of Heating Value or Wobbe Index specified by the gas turbine manufacturers. Furthermore, gas turbine trips can result from the rapid variation in Wobbe Index when a sudden switch is made between lean and rich fuel gas sources.

Nitrogen is most efficiently separated from methane at cryogenic temperatures. From all the schemes commonly used in the natural gas processing, the single column process is the one having the greatest potential to achieve the future standard of less than 0.1%mol C1 in the nitrogen vent gas.

Derived from this well known scheme, a new Nitrogen Removal process developed and patented by Technip allows the production of valuable by-products from an N2 rich LNG stream:

  • Helium, even present at trace quantities in the feed gas can be easily extracted before being further processed,
  • High purity nitrogen can be produced as a gas or as a liquid, removing the need for an air separation unit otherwise needed to produce the nitrogen required for liquefaction refrigerant make-up. This is particularly interesting on FLNG’s using nitrogen cycles.

Sylvain’s presentation will review the processes currently available in the industry. Then the solutions developed by Technip to deal with nitrogen in the LNG industry will be presented. Finally the presentation will focus on a successful process that has become a standard in the LNG industry with its application in the six Qatar LNG Mega Trains.

Sylvian Vovard will be presenting Nitrogen Removal from LNG at the Cryogenics stream at Gastech 2012 Exhibition’s Centres of Technical Excellence (CoTEs). To learn more, register here for this free seminar on 10 October 2012 at the ExCeL centre in London.