Hydrocarbon Processing - April 2021 - GP-41

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levels with the condensation curve of
process streams.
On the contrary, the boiling curve of
a designed mixture of refrigeration fluid
can better approach the natural gas cooling curve. In doing so, less external work
is required for the liquefaction. MR used
for liquefying natural gas generally contains methane (40%), ethane (48%), propane (9%) and nitrogen (3%). It should
be emphasized that the actual MR speciation used in a specific plant depends on
the natural gas composition and other
project constrains, including environmental conditions.
Production plants. The selection of a
specific LNG production plant is driven
by the feed composition, plant capacity, location, ambient conditions, safety
and energy cost. As a short reference,
TABLE 1 provides a list of the main commercial processes segmented according
to plant capacity.
Although comparatively less efficient,
the N2 recycle process and its modification account for a large share of production plants in operation today. The
simplest system comprises a brazed aluminum plate-fin heat exchanger, a compressor and an expander, as shown in FIG. 1B.
The circulating N2 is compressed with
a reciprocating, multistage compressor
(for micro- and small-scale plants). The
warm, compressed gas is cooled to 150K-
175K by means of an external utility and
auto-refrigeration (i.e., by heat transfer
to the cold, low-pressure N2 stream) and
expanded in a Joule-Thomson throttling
valve or through a gas expander.
The thermodynamic efficiency of this
scheme is very low; the specific power
required to liquefy natural gas can be
greater than 1.5 kW/kg of produced
LNG. The efficiency of this cycle can be
improved by allowing the circulating gas
to condense so that the latent heat of N2
can provide part of the refrigeration duty.
In this case, the basic N2 recycle evolves
into the process diagram shown in FIG. 1C.
Other modifications have been introduced to the basic scheme to improve the
overall efficiency of the N2 refrigeration
system. One modification consists of expanding N2 in three expanders, with each
serving the cooling duty of an individual
step (precooling, liquefaction and subcooling) of natural gas liquefaction, thereby achieving a split-pressure arrangement.

The selection of a specific LNG production plant
is driven by the feed composition, plant capacity,
location, ambient conditions, safety and energy
cost ... Although comparatively less efficient, the N2
recycle process and its modification account for a
large share of production plants in operation today.
Moreover, the multiple expansion allows
for an increase in LNG capacity from 0.5
metric MMtpy to 1 metric MMtpy.
In a recent development, the efficiency is improved by combining an N2 cycle
with a CH4 cycle. The former cycle is for
supplying precooling and liquefaction
duty, while the latter is for subcooling the
produced LNG.
Single mixed refrigerant (SMR). Further improvement of efficiency is attainable by replacing N2 with an MR that has
an adjustable composition to " simulate "
the cooling of natural gas from ambient
to cryogenic temperature. The SMR combines the simplicity of the plant configuration with operational flexibility while
enhancing the overall plant efficiency by
10%-15% relative to the N2 recycle plants.
In the baseload LNG industry, the
most commonly used process configuration is a combination of propane precooled and mixed refrigeration (C3MR)
processes. Generally, the propane cycle
includes a three-stage refrigeration system where propane is boiled at three distinct temperature levels and the boiling
curve forms three distinct steps.
Large production plants are arranged
in multiple trains with parallel compressors and relevant drivers. The size of each
train is increased to the maximum possible to pursue economies of scalea so that
the unit cost-i.e., the CAPEX/t of produced LNG-is as low as possible.
Since a large flowrate of gas must be
cooled from nearly ambient temperature to yield LNG at -162°C, the high
heat transfer required entails a large heat

transfer area. The heart of a baseload
plant is the main heat exchanger (e.g., a
spiral-wound heat exchanger, or SWHE).
The SWHE consists of pressure vessels
containing a number of tubing bundles
fabricated with a large number of long,
aluminum tubes helically wound around
a mandrel or a central core. Numerous
tube layers are formed in the radial direction. Each layer is separated from adjacent layers by spacers.
The SWHE comprises a warm exchange zone and a cold exchange zone.
Together, the tubes clustered in the
warm/cold zone constitute a single, coilwound bundle. In the multi-tubes shown
at the left of the warm zone in FIG. 4,
the feed gas is cooled and partially condensed against a vaporizing refrigerant
on the shell side of the bundle. The resulting two-phase flow is directed in the
bundles of the cold zone, where it is further cooled and extracted as LNG.
The refrigerant in the shell side is
a mixture of light hydrocarbons. After
being cooled and partially condensed
in the MR refrigeration loop, the twophase flow is separated in a knockout
drum. The liquid from the knockout
drum is subcooled in the tubes circuit,
shown at the right of the warm bundle,
and then throttled and mixed with the
refrigerant flowing downward from the
cold area. The MR flows downward over
the outside of the spool bundle. By vaporizing and warming while flowing
downward, the MR provides the refrigeration for cooling the feed gas and subcooling the liquid phase extracted from
the knockout drum.

TABLE 1. Commercial liquefaction plant capacity selection
Scale

Capacity, metric MMtpy

Micro

0.03-0.1

N2 expander

Process technology

Small

0.1-0.5

N2 expander, SMR

Medium

0.5-2.5

Baseload

> 2.5

SMR, DMR, C3MR, AP-LNG
C3MR, DMR, AP-X
Gas Processing & LNG | MARCH/APRIL 2021

Hydrocarbon Processing - April 2021

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