LNG technology and equipment

A review of improvements to the Shan Shan LNG processing facilities, ranging from gas treatment and liquefaction with a single mixed refrigerant cycle in spiral-wound heat exchangers through storage to unloading and distribution in China

Xiang Dong Xin Jiang, Guanghui Liquefied Natural Gas Development Co Ltd Eginhard Berger, Linde AG
Albert Meffert, Tractebel Gas Engineering
Li Wei Bin, Sinopec Shanghai Engineering Co Ltd

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Article Summary

In 2004, Xin Jiang Guanghui Liquefied Natural Gas Development Co Ltd established the operation of a unique LNG chain. Natural gas, which until recently was flared at the Tuha oilfields some 300km southwest of Urumqi, can now be used as a clean primary energy source. This new LNG scheme is a feasible and workable alternative to existing peak-shaving and conventional base-load plants. With a LNG production capacity of 0.4mtpa (million tons per annum), it represents a new category of LNG plant types.

The gas is treated and liquefied in an LNG plant near Shan Shan in the Xin Jiang Province of China. The plant is operated in a base-load mode and employs intermediate storage of the LNG product in an insulated tank before it is loaded into tankers. A large number of these trucks then carry the LNG over long distances to satellite and vehicle fuelling stations in various cities of China. After revapourisation of the LNG at these stations, the natural gas is finally distributed partly via pipeline to a variety of industrial and private consumers. Since it is considered to be the most environmentally friendly hydrocarbon fuel, it is expected that this domestic natural gas initiative through LNG will create new gas markets and greatly improve the tight energy-supply situation in China.

Design basis
The plant consists of natural gas treatment and gas liquefaction technology, LNG tank storage tanks and a distribution system. The LNG production capacity of the plant is 1 500 000 m3(n)/d, with an expected on-stream time of 330 days per year. Design hourly liquefaction capacity is 54t/h and storage capacity is 30 000m3 in liquid form of LNG, which is equivalent to 12 days’ production. The send-out and distribution system is able to load 100 tankers or movable containers within 16 hours, usually 30% as tankers and 70% as containers. The product LNG specification is: Composition (mole %): nitrogen 0.8 (max 1.0), methane 82.4, ethane 11.1, propane 4.6, others 1.1. Pressure and temperature at LNG tank:  0.01MPaG, -163°C. The design LNG has a density of about 490kg/m3 in the LNG tank.

Feed gas conditions at plant battery limits are: Composition (mole %): nitrogen 3.81, methane 81.02, ethane 9.99, propane 4.10, butanes 0.93, i&n-pentane 0.05, C6+ < 0.0021. In addition, CO2 and traces of H2S and sulphur are present in the feed gas. The feed gas operating pressure can range from 0.6–1.1MPaG. The design pressure is 0.7MPaG. The feed gas operating temperature can range from -15–40°C. The design temperature is 28°C.

Process features
The main process and utility units are illustrated in Figure 1. The liquefaction process is based on a single closed mixed refrigerant cycle, which requires nitrogen, methane, ethylene, propane and pentane, and all but methane and nitrogen have to be purchased from external sources. Refrigerant nitrogen and purge nitrogen are identical and generated in a nitrogen package.

Make-up water for the closed cooling water cycle and demineralised water as make-up water for the MEA in the CO2 wash unit are also sourced from outside the plant. A mixture of compressed LNG tank return gas and feed gas is used as normal fuel gas, while feed gas is used as start-up fuel gas. A closed hot oil cycle is used as a heating medium, and an MEA (monoethanolamine) water solution is used as solvent for the CO2 wash unit.

The average ambient temperatures range from 37.1°C in the warmest month to -15.6°C in the coldest. Design temperature for the gas turbine air inlet and for air cooling is 30°C, while the average ground temperature in the hottest month is about 37°C, and the extreme maximum ground temperature is around 75°C. The plant elevation above sea level is about 790m.

Process and utility description
The Shan Shan LNG plant has a medium-sized production capacity. Peak-shaving or backup plants with intermittent operation and production have capacities of up to 500 000Nm3/day, while base-load plants with continuous operation and production have capacities of 5 000 000–17 000 000m3(n)/day. At 1 500 000m3 (n)/day, Shan Shan is about three times the size of the largest existing peak-shaving plant, but three times smaller than a small base-load plant.

As previously mentioned, the liquefaction process is based on a highly efficient single closed mixed refrigerant cycle. This provides cold temperatures by Joule-Thomson expansion at three pressure levels and is recompressed in a three-stage turbo-compressor, which is driven by a gas turbine. In order to enhance plant efficiency, the waste heat from the gas turbine is recovered by heating a hot oil cycle, which covers the heating needs of the process plant.

Natural gas (feed gas) has a low pressure at the battery limit, so solid and liquid particles are removed by the feed gas filter separator before the gas is compressed. During the first stage, the gas is cooled in an intercooler against ambient air to about 40°C and condensed water is separated in the inter-stage drum and then fed to the wash unit. The feed gas is further compressed in the next stages with inter- and after-cooling in air coolers. It is then routed to the CO2 wash unit for the removal of CO2, and the sweet feed gas leaving the CO2 wash column is routed to the drier station.

Natural gas liquefaction
After CO2 and H2O removal, the natural gas is routed to the cold part of the process, which contains three spiral-wound heat exchangers designed and built by Linde and integrated in one shell, together with several separation vessels. The natural gas is first cooled in the precooler (E1), then potential off-spec heavy hydrocarbons are separated in the heavy hydrocarbon separator (D3), where only marginal liquids during design feed gas operation are expected. The gas is then condensed in the liquefier (E2) and subcooled in the subcooler (E3). The subcooling temperature is controlled by the amount of tank return gas required as fuel gas for the operation of the gas turbine. The cooling is provided by the closed multi-component mixed refrigerant cycle.

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