The Kryopak EXP®
liquefied natural gas (LNG) Process
Other major commercial LNG processes usually design for base load
(large scale) LNG plant by employing complicated processes and
equipment to reduce operation energy consumption.
For small scale LNG plant, like remote gas liquefaction, it is neither
economic nor practical to use major commercial processes. This is due
to their complicated processes, limited flexibility for feed gas flow
rate, and process gas composition fluctuating.
Please observe the Q/T diagram of a typical gas liquefaction process,
especially three zones noted in the process of the gas being liquefied
(See Below).
Q/T Diagram of Typical Gas Liquefaction
A pre-cooling zone is followed by a
liquefaction zone and completed by
a sub-cooling zone.
All of these zones are characterized by having different curve slopes
(specific heats) along the process.
The Q/T curve shape is different for different gas compositions; in
other words, the cooling, liquefaction and sub-cooling zones are in
different temperature and cooling duty zones for different gas
compositions.
Mixed Refrigerant cycles designs try to closely approach the cooling
curve of the gas being liquefied. They use special mixes of
multi-component refrigerants trying to match cooling curves at
different zones/stages of the liquefaction process to achieve high
refrigeration efficiency and reduce energy consumption.
When gas composition is away from the design gas, the Q/T curve shifts,
but refrigeration does not because it is fixed by premixed
refrigerants.
Temperature approaches are not achieved as designed and therefore
refrigeration efficiency deviates.
Kryopak’s EXP® process employs refrigeration generated by a single
semi-closed isentropic expansion of gases.
Composition of refrigerant gas is the same as the vapor generated from
final product flash (See Below). By this means no mechanical
refrigeration is used in the system as seen in the mixed refrigerant
processes.
Kryopak EXP®
simplifies the gas liquefaction process accordingly. By
recycling the gas, the slope of the Q/T curves are accordingly changed
per the cooling input at the different zones by the recycling
refrigerant gas.
The Kryopak EXP® gas liquefaction process obtains profiles close to the
gas refrigerant used, thus minimizing deviations due to gas composition
fluctuations.
The amount of flow used in the recycles, feed gas, and refrigerant
pressure levels alter the shape of the Q/T curves, which are part of
the design parameters used in achieving an optimum design. The control
system recognizes these fluctuations and automatically changes as
required for optimum system performance.
Simplified Kryopak EXP® Process Diagram
Simplified Kryopak EXP® Q/T Composition Curves
In the Kryopak EXP® process, work and refrigeration are extracted from
the expansion process. The refrigeration is used to aid the
liquefaction process and work extracted is utilized to partially
recompress the refrigerant gas.
Thermodynamically speaking, this expansion cycle is as efficient as the
most advanced mixed refrigerant cycles. Today, compressor/expander
efficiencies are reaching more than 85%. This represents an emerging
opportunity to assist gas liquefiers using this technology in the LNG
business.
Using this design philosophy, the Kryopak EXP® process is able to
achieve a mid level specific refrigeration energy consumption of 0.20 -
0.23 hp/lb LNG (13.5 – 15.5 kW/ton-day LNG) compared to the reported
design energy consumption of 0.18-0.25 hp/lb LNG (12.2 – 16.8
kW/ton-day LNG) for existing MR LNG processes.
One of our typical designs with 150,000 Nm3/day (125 tons /day LNG) has
a total refrigeration energy consumption of only 2,550 kW.
Advantages and
benefits of Kryopak’s EXP® process:
Extremely simple compared with the MR processes
Adaptable to different quality of feed gas without changing plant
efficiency, as the refrigeration cycle works with the purpose of
producing refrigeration to satisfy the refrigeration demand imposed by
the LNG requirements
The refrigerant is always in gas phase, which eliminates the
requirement of refrigerant inventory and separators for their handling,
with the correspondent impact in the safety on the facility, and
therefore simplifies the construction of the cold box exchanger also.
There is no need for premixing of refrigerant
All of the equipment employed by the Kryopak EXP® process are standard
conventional oil and gas field processing equipment using no new or
unproved technology.
As evaluated by Project Technical Liaison & Associates (PTL), one
of the world's leading LNG facility evaluation firms:
“The Kryopak EXP® process has the advantage of using a simple cooling
system with a relatively small number of pieces of equipment operating
at cryogenic temperatures and a fast approach to equilibrium upon warm
start-up. The simplicity of the design and the self equilibrating
nature of the process provide straightforward operation. The process is
not very sensitive to reasonable changes in feed gas composition (as
evidenced by the three cases examined)… The equipment required in the
plant is standard and is readily available from various vendors…”
Kryopak EXP ® LNG
Process Applications…
Remote gas recovery, peakshaving, small scale, base load.
1.0 MMscfd to 11 MMscfd (2x104 Nm3/day to 30x104
Nm3/day) reciprocating
compressor without heat recovery, specific energy consumption of
0.22-0.23 hp/lb LNG depending on design air temperature, full skid
plant.
15 MMscfd to 200 MMscfd (40x104 Nm3/day to 5x106
Nm3/day) centrifugal
compressor with heat recovery, specific energy consumption of 0.20
hp/lb
LNG, partially skid plant.
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