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Technology

Liquefaction Technology

Chiyoda Corporation has designed and constructed a number of liquefied natural gas plants over the more than 35 years since 1976 when Chiyoda Corporation built a one-million-ton liquefied natural gas plant in Abu Dhabi in the Middle East in 1976. In 2009, we completed two liquefied natural gas trains each with the world’s largest capacity of 7.8 million tons per year in Qatar. Liquefaction technology has steadily progressed since 1976, and the liquefied natural gas plants that are built now are much more efficient and reliable than the liquefied natural gas plants of a few decades ago. Chiyoda Corporation has been the leader in this industry.

Outline of a liquefied natural gas plant

A liquefied natural gas plant (LNG plant) is roughly divided into five processes: (1) pretreatment, (2) acid gas removal, (3) dehydration, (4) liquefaction and (5) heavy oil separation.

(1) In the pretreatment process, undesired substances are removed from the gas taken from a gas field. Then the gas is separated using a slug catcher into oil and water which are then weighed.

(2)Natural gas taken from a gas field contains environmental pollutants like hydrogen sulfide (H2S) and carbon dioxide (CO2). These impure substances are absorbed and removed from natural gas with an amine absorber (acid gas removal or AGR). With the use of a sulfur removal unit (SRU), sulfur is extracted from the hydrogen sulfide in the removed pollutant.

(3)An adsorbent is used to remove water from the natural gas from which impure substances have been removed so that ice will not form during the subsequent liquefaction process.

(4)Traces of harmful mercury are removed before liquefaction.

(5)The heavy compounds separation process is the core of an LNG plant in which natural gas is cooled and liquefied to –160°C or less using the principle of refrigeration. Because gas is cooled and liquefied to an extremely-low temperature during the process, an enormous amount of energy is consumed. How much this energy can be reduced is important, so various ingenious processes have been proposed and commercialized.

Major liquefaction processes are as follows:

1)C3-MR method: The C3-MR method is currently the main method. Propane and mixed coolants (nitrogen, methane, ethane and propane) are used as the coolant (APCI), and an improvement on this method called the AP-X method is also used for large LNG plants.

2)AP-X method: As liquefaction trains get larger, they approach a limit on the size of heat exchanger that can be produced and transported. This process can increase LNG production capacity by adding LNG sub-coolers with nitrogen coolant used according to the C3-MR method, without increasing the size of the main heat exchanger (APCI).

3)Cascade method: This method sequentially uses propane, ethylene and methane as the coolant (Phillips).

4)DMR method: This method uses two kinds of mixed coolants (an ethane and propane mix and a nitrogen-methane, ethane and propane mix) (Shell).

5) SMR method: This method is called the PRICO process and uses only one kind of mixed coolant (Black & Veatch).

All of these methods require enormous refrigeration compressors. Gas turbines used for giant power plants are used to drive them, so elaborate engineering based on experience and high-level knowledge is required to design, produce and assemble the compressors and gas turbines. We will continue to be the leader of LNG plants because of our ability to deal with LNG and advanced technology based on our abundant experience.

Example of an LNG process flow