28
LNG
INDUSTRY
SEPTEMBER
2016
entropy production by using several refrigeration cycles
for each liquefaction stage. By evaporating the liquid
refrigerants, high thermodynamic efficiency can be
achieved. However, the process is complicated and
requires a large number of components, meaning that the
size requirement is large and the capital cost is high. The
high efficiency and high investment cost makes it suitable
for large land-based liquefaction plants.
Mixed refrigerant
MR liquefaction is also based on the Rankine cycle.
However, contrary to cascade cycles, a blend of
refrigerants is used to obtain a close following of the
natural gas cooling curve. By mixing refrigerants, a
temperature glide can be attained, which means that the
temperature at phase change will not be constant. This
is because the components in the mixture evaporate at
different temperatures, causing a change of concentration,
which can be adapted to the process gas cooling curve.
In reality, the MR will cause a curved temperature profile,
which will lower the thermodynamic efficiency, compared
to the cascade cycle. The MR process is suitable for
small scale liquefaction plants where the low equipment
count and simplicity can be a substitute for high efficiency.
Expander cycle
The expander cycle differs from the other liquefaction
cycles by using an expander instead of a Joule Thomson
(JT) valve. The expander is connected to the compressor,
and extracts useful power from the compressed gas.
The refrigerant used is a pure gas, and is only in gaseous
phase, making it insensitive to motion. This also
eliminates issues relating to the distribution of liquid
refrigerants in the heat exchangers, thereby allowing
rapid start-up. A gaseous phase refrigerant, however,
has a limited enthalpy difference, and requires a higher
refrigerant flow than two-phase refrigerants, which limits
the capacity. The process does not follow the cooling
curve of the process gas very well, which results in lower
efficiency than with other technologies. This, on the other
hand, makes the process more forgiving to variations in
the gas composition.
Most expander processes utilise the reversed Brayton
cycle (either closed or open loop) to generate cooling. This
is done either in a single or dual stage or with precooling.
By using an open-loop expander cycle, a fraction of the
process gas is utilised as a refrigerant. This eliminates the
need for excess refrigerants.
The reversed Stirling cycle is another type of expander
process used for liquefaction. The Stirling cycle is a
modified Carnot cycle, where heat from the compression
stage is utilised in the expansion stage, making it a
regenerative cycle.
Offshore reliquefaction
The selection of liquefaction technology for offshore
applications differs from the onshore equivalents. Space
on marine vessels is limited, which increases the need for a
compact solution.
The use of hazardous hydrocarbons has to be limited
for safety reasons.
Small scale offshore reliquefaction is, from a capacity
perspective, quite similar to onshore peak shaving plants.
The expander cycle is a proven technology for these
small scale plants and is a viable choice for offshore
reliquefaction.
Thermal oxidation
Another method for handling BOG is thermal oxidation
(i.e. combustion). This is primarily done by feeding the
excess gas to the consumers (i.e. the ship’s engines). Two
and four-stroke internal combustion engines are normally
used for propulsion and power generation, while two-stroke
engines usually have a high power output and are used for
direct propulsion. Four-stroke engines can be used both as
main and auxiliary engines, the latter being used while in
port, as well as when at sea. Additionally, auxiliary boilers
can be used to produce steam or hot water. If the amount
of BOG does not correspond to the rate of consumption,
the gas can be fed to a gas combustion unit (GCU). The
GCU is a burner which combusts the BOG in a controlled
manner without the risk of releasing unburned natural gas
to the atmosphere. Although a possible solution for BOG
handling, no useful energy can be recovered from a GCU,
which is why it should primarily be recovered by other
means.
Compression
Feeding gas to the engines is one way of handling BOG in
the tanks. Four-stroke engines usually have a suitable fuel
pressure need for Type C tanks and can consume the gas
at tank pressure. Two-stroke engines, however, demand
Figure 3.
The three-stage cooling curve of natural gas
(15 – -161°C) with precooling, liquefaction and subcooling.
