Natural Gas Liquefication Process: LNG

Natural Gas Liquefication Process:

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 separations.

(1) In the pretreatment process, undesired substances are removed from the gas taken from a gas field. Then the gas is separated using a separator/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).