Hydrocarbon Processing - April 2021 - GP-42

BACK TO BASICS

LNG
Cold zone
Propane
refrigeration
closed loop

Spiral-wound
heat exchanger

Scrubber
Warm zone

Raw gas

C2+
Compander
Multi-stream
heat exchanger

MR loop
MR compressor

FIG. 4. Propane-precooled SMR liquefaction technology.

The refrigerant vapor from the knockout drum is cooled in the warm zone and
passes through the tube circuit in the cold
zone, wherein it is liquefied and possibly
subcooled. After pressure reduction, it
flows downward on the outer side of the
spool bundle and evaporates, thereby
providing the refrigeration duty to both
the feed stream and the refrigerant vapor coming from the knockout drum.
The refrigerant flowing downward in the
SWHE becomes totally vaporized upon
reaching the bottom.
The exchange configuration previously described is known as " top cold. "
The opposite arrangement is " bottom
cold. " In the latter configuration, LNG is
withdrawn from the bottom rather than
from the top.
Note: The vaporization of the MR fluid gradually flowing downward increases,
and the heat transfer mechanism changes
from two-phase boiling heat transfer at
the top of the warm zone to single-phase
vapor heat transfer at the bottom. However, the geometric data (e.g., coils diameter,
tubes outside diameter, radial tubes spacing, tubes pitch and winding angle) are often kept constant throughout the bundle,
meaning that the thermal design of these
systems is the result of a tradeoff among
the various heat transfer mechanisms.
Aluminum is the material of construction used for the SWHE and BAHE;
therefore, a mercury removal unitb must
be installed in the conditioning section
of the processing facilities.
42

MARCH/APRIL 2021 | GasProcessingNews.com

In a more recent enhancement, the
C3MR has been integrated with an N2
recycle refrigeration cycle in the rear
end of the process. In doing so, the LNG
subcooling duty is shifted from the cold
zone of the SWHE to the N2 recycle
cycle, and the capacity of a single LNG
train can be as much as 8 metric MMtpy-10 metric MMtpy, if the dimensions
are left unchanged. This plant configuration licenses AP-X process technology.
In arctic regions, where the temperature can vary from -40°C to 30°C, the
propane cycle becomes a bottleneck in
the process because it is not possible to
fully utilize the power from the compressor over the wide temperature range. In
replacing the propane refrigeration cycle
with an MR, the maximum utilization
of power available from the compressor
drivers can be attained while maintaining efficient refrigerant compressor operation over the wide temperature range.
In these cases, the MR is constituted by
a blend of ethane and propane. Increasing the proportion of propane creates
a heavier mix suitable for summer operation, while increasing ethane yields a
lighter mix for winter usage.
Dual mixed refrigerant (DMR). The
process where the precooling duty is supplied by an MR heavier than that used
for liquefaction and subcooling is known
as dual MR (DMR). The DMR process
provides the highest thermal efficiency
in severe environmental conditions, but

also the greatest equipment count, complexity and multiple refrigerant handling.
The cascade process is an alternative
technology. In this process, natural gas
precooling is carried out in an evaporator/
condenser that is common to the highand low-temperature refrigeration cycles.
Since each refrigerant circuit is controlled
separately, this technology does not need
to adapt the refrigerant composition to
natural gas. The most well-known commercial cascade process is ConocoPhillips' Optimized Cascade LNG process. It
cascades three pure refrigerants: propane,
ethylene and methane.
It should be noted that plant components in cryogenic service are installed
inside cold boxes. These boxes are created of a structural framework closed with
steel plates, generally painted white, and
filled with insulating material like perlite
to avoid heat exchange with the surrounding environment.
Finally, the modularization concept is
the emerging approach to economies of
scale for baseload plants. Modularization
entails multiple parallel, standardized, independent, small-scale LNG plants. In this
way, new, identical production lines (modules) can be added as the market expands.
This enables cost savings and capital budgeting over a longer span of time. GP
NOTES
By increasing the size of the plant, the CAPEX
increases less than proportionally with the plant
capacity (Q ), according to the generally used estimate of 0.67. That is, CAPEX1 = CAPEX0 × (Q1/
Q0) × 0.67, where CAPEX0 is the investment cost of
a plant of capacity Q0 < Q1.
b
This process was discussed in the Back to Basics
article, " Natural gas phase separation and mercury
removal, " published in the July/August 2020 issue of
Gas Processing & LNG.
a

LORENZO MICUCCI is a Senior
Director at Siirtec Nigi SpA.
He has more than 30 yr of
experience in the engineering
and contracting industry, most
of which have been spent in the
natural gas sector. In 2001, he
joined Siirtec Nigi in Milan, where he directed the
process design and operations department and the
research and development department. During his
time as R&D head, three patents have been granted
to Siirtec Nigi, two of which have been implemented
on an industrial scale. At present, he is the Senior
Director of the technology and marketing
departments. Mr. Micucci also worked for Saipem
(Snamprogetti) as a Plant Designer for integrated
gasification combined cycle and gas-to-liquids
plants. He holds an MS degree in chemical
engineering from the University of Bologna in
Italy and is enrolled as a Qualified Engineer
in the Register of Milan Order of Engineers.

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Hydrocarbon Processing - April 2021

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