Hydrocarbon Processing - April 2021 - GP-39

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Process technologies for LNG production
L. MICUCCI, Siirtec Nigi, Milan, Italy

Process technologies for LNG. Once
natural gas has been pretreated to reduce
the impurities to trace levels-i.e., water to 0.1 ppm, CO2 to 50 ppm and Hg
to less than 10 ng/Nm3-LNG is liquefied and subcooled by cooling the gas to
-162°C (at 1 bar abs).
Natural gas liquefaction requires a
significant amount of refrigeration. Since
the adopted refrigeration cycles differentiate the commercial liquefaction
processes, the most widely used refrigeration systems in the LNG industry are
outlined in the subsequent sections, followed by the commonly used LNG production technologies.
Refrigeration cycles. The refrigerant

fluid can be a single-component fluid
(N2, for instance) or a mixture of light
hydrocarbons, generally termed as mixed
refrigerant (MR).
In the latter case, the MR speciation
can be varied to accommodate changes in
the operating conditions. For example, in
locations characterized by strong seasonal
variations in ambient temperature, the
MR composition is varied to accommodate these changes.
The selection of the refrigeration cycle is driven by the LNG production pattern. When the LNG is to be produced

for peakshaving purposes, for example,
the liquefaction plant does not operate
on a continuous basis and the capacity is
small. In these cases, the focus is on CAPEX rather than high efficiency, and the
refrigeration cycle may be based on an inverted Brayton refrigeration (BR) cycle.
In the BR cycle, refrigeration is supplied
by expanding a single-component fluid
(N2) without fluid phase change. For
medium to large production plants where
high efficiency is important, a compression refrigeration cycle (CRC) with refrigerant phase change is used.
Inverted Brayton cycle. In the basic

BR cycle, the cooling duty is provided
by expanding nitrogen through a Joule-
Thomson valve, or through an expander,
without causing a change in the fluid
state. The pressure of N2 is raised from
point (1) to point (2) and cooled at constant pressure to point (3) (FIG. 1A). In
the following isentropic expansion to the
pressure P4 < P3, the temperature ideally
drops to T4 , as per the formula shown
in Eq. 1, providing the cooling stream
against which natural gas can be cooled
and liquefied:
T4 = T3 × (P4 ÷ P3)^[(k - 1) ÷ k] < T3

(1)

N2
N2

(2)
(2)
Temperature

LNG is purified natural gas converted
into liquid form. In this physical state,
NGL takes up 1/600th the volume of gas
in its natural form, making it economical to
be stored in large-volume containers and
transported over long distances via tanker.
The English scientist Michael Faraday
first developed a way to liquefy methane
in 1820. However, LNG first made its way
to commercial application in 1959 when
the first LNG tanker, the Methane Pioneer,
sailed from Lake Charles, Louisiana, U.S.
to Canvey Island, UK, where the government wanted to convert coal to natural
gas following the Great Smog of 1952.
LNG international trade followed with
LNG export from Algeria to the UK in
1964. Soon after, France imported LNG
from Algeria, and Spain and Italy imported LNG from Libya. Today, the LNG
industry has become the catalyst for the
transformation of the global gas sector.
The LNG industry consists of three
main processes: liquefaction and storage,
ocean transportation via tankers, and
storage and regasification. As an alternative to land-based projects, the liquefaction, storage and regasification steps can
also take place on vessels, which include:
* FSRU: Floating storage and
regasification units
* FLNG: Floating LNG vessels
* FPSO: Floating production,
storage and offloading systems
* FPS: Floating production systems
* FSO: Floating storage and
offloading systems
* FSU: Floating storage units.
In addition to the traditional uses of
natural gas, and for its reduced environmental impact relative to coal and other
fossil fuels, LNG has become popular
as a clean energy vector in the maritime
and transport sectors. This trend has also
stirred interest in small-scale LNG plants.

(3)

Compressor

Expander
(4)

(1)

Compressor

Expander

(3)

(4)
NG
Enthalpy
(A)

LNG
BAHE
(B)

LNG
BAHE
(C)

FIG. 1. N2 expansion refrigeration cycle.
Gas Processing & LNG | MARCH/APRIL 2021

Hydrocarbon Processing - April 2021

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