Liquefied natural gas takes up about 1/600th the volume of natural gas in the gaseous state. It is odorless, colorless, non-toxic and non-corrosive. Hazards include flammability, freezing and asphyxia.
The liquefication process involves removal of certain components, such as dust, acid gases, helium, water, and heavy hydrocarbons, which could cause difficulty downstream. The natural gas is then condensed into a liquid at close to atmospheric pressure (maximum transport pressure set at around 25 kPa/3.6 psi) by cooling it to approximately −162 °C (−260 °F).
The reduction in volume makes it much more cost efficient to transport over long distances where pipelines do not exist. Where moving natural gas by pipelines is not possible or economical, it can be transported by specially designed cryogenic sea vessels (LNG carriers) or cryogenic road tankers.
The process of natural gas liquefaction at the Badak LNG Plant use the multi component refrigeration system from APCI. In general, the LNG processing is as follows:
1. Raw natural gas from the fields is passed through knock out drums to separate the liquid condensate prior to entering the LNG plant.
2. Carbon dioxide is removed by chemical absorption by amine process.
3. Water is removed by molecular sieve.
4. Propane, Butane, and Condensate content is separated from the LNG feed in fractionation column.
5. LNG Feed is Precooling by propane refrigeration.
6. Final cooling and liquefaction of LNG carried out in the Main Cryogenic Heat Exchanger by using a multi component refrigerant as a cooling media.
Quality of LNG
LNG quality is one of the most important issues in the LNG business. Any gas which does not conform to the agreed specifications in the sale and purchase agreement is regarded as “off-specification” (off-spec) or “off-quality” gas or LNG. Quality regulations serve three purposes:
- 1 – to ensure that the gas distributed is non-corrosive and non-toxic, below the upper limits for H2S, total sulphur, CO2 and Hg content;
- 2 – to guard against the formation of liquids or hydrates in the networks, through maximum water and hydrocarbon dewpoints;
- 3 – to allow interchangeability of the gases distributed, via limits on the variation range for parameters affecting combustion: content of inert gases, calorific value, Wobbe index, Soot Index, Incomplete Combustion Factor, Yellow Tip Index, etc.
In the case of off-spec gas or LNG the buyer can refuse to accept the gas or LNG and the seller has to pay liquidated damages for the respective off-spec gas volumes.
The quality of gas or LNG is measured at delivery point by using an instrument such as a gas chromatograph.
The most important gas quality concerns involve the sulphur and mercury content and the calorific value. Due to the sensitivity of liquefaction facilities to sulfur and mercury elements, the gas being sent to the liquefaction process shall be accurately refined and tested in order to assure the minimum possible concentration of these two elements before entering the liquefaction plant, hence there is not much concern about them.
However, the main concern is the heating value of gas. Usually natural gas markets can be divided in three markets in terms of heating value:
- Asia (Japan, Korea, Taiwan) where gas distributed is rich, with an GCV higher than 43 MJ/m3(n), i.e. 1,090 Btu/scf,
- the UK and the US, where distributed gas is lean, with an GCV usually lower than 42 MJ/m3(n), i.e. 1,065 Btu/scf,
- Continental Europe, where the acceptable GCV range is quite wide: approx. 39 to 46 MJ/m3(n), i.e. 990 to 1,160 Btu/scf.
There are some methods to modify the heating value of produced LNG to the desired level. For the purpose of increasing the heating value, injecting propane and butane is a solution. For the purpose of decreasing heating value, nitrogen injecting and extracting butane and methane are proved solutions. Blending with gas or LNG can be a solutions; however all of these solutions while theorically viable can be costly and logistically difficult to manage in large scale.
Currently there are 4 Liquefaction processes available:
- C3MR ( sometimes referred to as APCI): designed by Air Products & Chemicals, Incorporation.
- Cascade: designed by ConocoPhillips.
- Shell DMR
It is expected that by the end of 2012, there will be 100 liquefaction trains on stream with total capacity of 297.2 MMTPA.
The majority of these trains use either APCI or Cascade technology for the liquefaction process. The other processes, used in a small minority of some liquefaction plants, include Shell’s DMR technology and the Linde technology. These processes are less important than the APCI or Cascade processes.
APCI technology is the most used liquefaction process in LNG plants: out of 100 liquefaction trains on-stream or under-construction, 86 trains, with a total capacity of 243 MMTPA have been designed based on the APCI process: the second most used is the Philips Cascade process which is used in 10 trains with a total capacity of 36.16 MMTPA. The Shell DMR process has been used in 3 trains with total capacity of 13.9 MMTPA; and, finally, the Linde/Statoil process is used only in the Snohvit 4.2 MMTPA single train.