a report by
Christian D T Begazo, Erica C Carvalho and José R Simões-Moreira
SISEA – Alternative Energy Systems Laboratory, Mechanical Engineering Department,
Escola Politécnica, Universidade de São Paulo
Natural gas has grown to be an important energy in
the international scenario. The world demand is
steadily increasing and the last figures show that from
2004 to 2005 there was a 2.3% utilisation raise.
1
As
part of the natural gas world market, liquefied natural
gas (LNG) has played an important role. Historically,
LNG came onto the scene when conventional
natural gas gas transport through pipelines was not
possible for reasons such as technical and political
issues, i.e. crossing international and state borders,
forests and seas or even oceans. Within that
framework only large LNG plants have been built
that achieve the remarkable train capacity above
7.5MMtpy.
LNG has been produced in small scale plants lique-
faction (SSL) plants to supply peak shaving demands, as
well as to make available natural gas to regions that
need it but where it is not economically or technically
feasible to build new pipelines. In many countries
natural gas has also been used as fuel for city buses,
trucks, boats, locomotives, or even for automobiles.
Along with the economical advantage comes the
environmental benefit as natural gas emission factors
are usually superior to those from other hydrocarbon
fuels. Today there are many companies manufacturing
SSL turnkey plants in the world market. This paper
succinctly reviews the main technologies available for
natural gas liquefaction in SSL plants.
LNG Process
LNG is the result of cooling natural gas to a cryogenic
condition to condensate methane, the natural gas main
component. A -161.5ºC temperature is required to
produce and keep natural gas in a liquid state at
standard atmospheric pressure. Preceding the
liquefaction process, it is necessary to treat the natural
gas in order to remove humidity, CO
2
, and heavier
hydrocarbon components C3+. Depending on the
natural gas origin it may also be required to remove
acid gases, mercury and sulphur.
A typical LNG plant is built in the following main
stages: natural gas pre-treating, liquefaction, storage and
LNG shipment. Usually, the liquefaction machinery is
the element that demands the most investment,
accounting for 30–40% of the overall capital.
2
Considering that the specific energy consumption is
a non-negligible factor in the LNG industry, new
processes and conventional processes technology
improvements comprise the main goal pursued by
the companies. Overall, thermal efficiency, safety,
and operational costs are some of the other issues one
should also take into consideration in selecting a SSL
plant technology.
Evidently most SSL plant technologies derive from
the large capacity technology that were designed to
produce millions of tons per year (tpy) of LNG. The
first plants used natural gas liquefaction by cooling
the gas using either the refrigerant cascade principle
or a simple mixture of refrigerants. A typical train of
liquefaction capacity was less than 1Mtpy, orders of
magnitude lower than those nowadays. SSL plant
capacity for supplying vehicular stations and peak
shaving systems are in general around 10–500 tons
per day (tpd).
Large LNG plants are long-term capital-intensive
investments, which contrasts with SSL plants. Many
SSL plants are available in containers or modules
ready to be shipped anywhere and for immediate
start-off operation. It is estimated an overall
liquefaction system costs between US$1,500/MMbtu
and US$2,500/MMbtu. According to Cascone,
3
a
considerable amount of the investment cost is spent
on the gas treating system and the main heat
exchanger. Figure 1 gives an idea of the investments
costs distributed according to the several processes in
a SSL plant adapted from GTI’s analysis.
SSL Plant Classification
From a general point of view, the SSL processes can
be grouped into two major groups, namely open-
loop, in which the refrigerant fluid is part of the feed
gas, and closed-loop, where the natural gas cooling
and liquefaction is attained by a auxiliary refrigerant
that flows continuously in a separated circuit. Open-
loop systems are based mainly on a successive
Small-scale LNG Plant Technologies
LNG
28
HYDROCARBON WORLD 2007
Erica C Carvalho is an
undergraduate student in the
Mechanical Engineering Department
at Escola Politécnica of University
of São Paulo.
Christian D T Begazo is a graduate
student in the Mechanical
Engineering Department at Escola
Politécnica of University of São
Paulo, Brazil, where he is
developing a thesis on liquefaction
process simulation. He worked for
four years on lubricating
engineering. He graduated in 2000,
from Universidade Catolica de Sta.
Maria, Arequipa, Peru.
José R Simões-Moreira is Professor
of Mechanical Engineering in the
Mechanical Engineering Department
at Escola Politécnica of University
of São Paulo. He has authored a
book on Psychrometry and several
technical and scientific papers on
flashing mechanisms in phase
change processes as well as on gas
and alternative energy system
studies. He has also undertaken
consulting projects for electrical and
oil and gas companies in Brazil.
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