Hydrocarbon Processing - August 2021 - GP-21

SPECIAL FOCUS: SMALL-SCALE LNG
Achieve three efficiencies
in MR compressors for SSLNG
T. PATEL, Atlas Copco Gas and Process, Houston, Texas; M. DREWES, Atlas Copco Gas and Process,
Köln, Germany; and J. ZHAO, EnFlex Group, Houston, Texas
In the equipment they use, investors in small-scale LNG
(SSLNG) face a steep challenge: They must achieve cost efficiency,
operational efficiency and energy efficiency-all
with maximum availability. In this context, the design of single
mixed-refrigerant (SMR) compressors, a critical piece of technology
in SSLNG plants, takes on an important role in both the
plant's efficiency and its return on investment (ROI).
As the benefits of integrally geared compressors (IGCs)
have become better known across hydrocarbon processing industries,
this compressor type has achieved broader acceptance.
For SSLNG, this pertains particularly to compressors requiring
gas or electric drives with a rated power of up to 27 MW
(36,200 hp). Demand for IGCs has been especially strong for
SSLNG plants with varying technical and logistical requirements
compared to large-scale LNG facilities. These include
quick deployment, scalability, redeployment capability and the
ability to respond rapidly to market demand.
Until the early 2000s, inline (i.e., single-shaft) compressors
were the dominant compressor technology in the marketplace.
From then, IGCs have become more popular due to their greater
efficiency and reliability, and their inclusion in the 7th edition
of the API (2002, Ch. 3). Nevertheless, universally applicable
statements on the differences between these two technologies
must be carefully considered. This is because the specific use
case and operating conditions are what ultimately drive the decision
on which compression technology is used.
This article examines the deployment of integrally geared
compressor technology, exploring the drivers behind the rising
demand for SSLNG and the qualities that make IGCs an efficient
compression solution in SSLNG plants. Characteristics discussed
include the IGCs' high flexibility and lower initial investment
and operating costs, which enable operators to realize agile
plant deployment and improve the efficiency of a plant once it is
operational. This article uses the example of a mixed-refrigerant
IGC installationa
operating at an SSLNG plant in China.b
Inside the Zhengtai Yida LNG plant. The Zhengtai Yida
LNG plant (COVER PHOTO) is located in China's Inner Mongolia
region, approximately 700 km (440 mi) from China's capital,
Beijing. The plant has a 0.807-MMtpy capacity (based on
350 days of operation), and it uses an SMR cycle, the most-used
cycle for liquefaction trains with a capacity of 1 MMtpy or less.
The SMR cycle uses various components, such as nitrogen,
ethylene, methane, propane, butanes and/or i-pentane in place of
the pure hydrocarbon refrigerants utilized in the discrete cooling
FIG. 1. The IGC unit's deployment within the SMR cycle.
Gas Processing & LNG | JULY/AUGUST 2021 21
steps of a cascade refrigerant cycle. Basically, SMRs operate by
condensing a hydrocarbon gas mixture to a liquid and then using
cooling through its vaporization to cool natural gas close to its
boiling point of −162°C (−260°F). The final phase change into
LNG is often accomplished with a Joule-Thomson valve.
Many proprietary SMR processes utilize a single-pressure
mixed-refrigerant cycle, which compresses the refrigerant to its
highest pressure. It is then sent to a series of condensers and
separators before it is ultimately delivered to the heat exchanger
to cool LNG. To increase efficiency, other SMR processes vaporize
the mixed refrigerant in two steps: one higher-pressure
and one lower-pressure stage, which can be driven either by two
compressors or, ideally, a single compressor with two separate
stage groupings (FIG. 1).
The relatively narrow range of compression specifications
allows for the standardization of a compressor's aerodynamic
design in three- and four-stage mixed-refrigerant compressors.
Such standardization allows for faster compressor delivery at a
lower initial investment. Given that LNG projects are naturally
CAPEX-intensive (the liquefaction process can account for
more than 50% of total project costs), lower compressor investment
costs help improve a plant's ability to operate profitably.
Lower investment and design priorities. Unlike SSLNG
plants, large-scale LNG liquefaction plants require significant
investment commitments across all project phases. SSLNG
plants have lower installation cost, can be built in much shorter
timeframes, and use natural gas as a cost-efficient feedstock in

Hydrocarbon Processing - August 2021

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