This document proposes two novel processes for separating light hydrocarbons like ethane from liquefied natural gas (LNG) using the cryogenic energy released during LNG regasification. The first process, called the "high pressure process", uses a demethanizer operating at 4.5 MPa to recover over 99.99% methane, then compresses the methane-rich natural gas. The second process, called the "low pressure process", uses a lower pressure demethanizer at 2.4 MPa and re-liquefies the methane-rich stream, pressurizing it with pumps instead of compressors. Both processes produce liquefied ethane and LPG at atmospheric pressure with acceptable power

1. Improved processes of light hydrocarbon separation from LNG
with its cryogenic energy utilized
Ting Gao, Wensheng Lin ⇑
, Anzhong Gu
Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China
a r t i c l e i n f o
Article history:
Available online 5 March 2011
Keywords:
Liquefied natural gas (LNG)
Cryogenic energy utilization
Light hydrocarbon separation
Process
Economic analysis
a b s t r a c t
Liquefied natural gas (LNG) often consists of some kinds of light hydrocarbons other than methane, such
as ethane, propane and butane, which are of high additional value. By efficiently utilization of LNG cryo-
genic energy, these light hydrocarbons (Cþ
2 ) can be separated from LNG with low power consumption and
LNG is gasified meanwhile. Two novel light hydrocarbon separation processes are proposed in this paper.
The first process uses a demethanizer working at higher pressure (about 4.5 MPa). The methane-riched
natural gas from the demethanizer can be compressed to pipeline pressure with low power consumption.
The other one uses a demethanizer working at lower pressure (about 2.4 MPa). By cascade utilization of
LNG cryogenic energy, the methane-riched natural gas from the demethanizer is entirely re-liquefied.
Then the liquid product is pressurized to pipeline pressure by pumps instead of compressors, reducing
the power consumption greatly. By both of the two processes, liquefied ethane and LPG (liquefied petro-
leum gas, i.e. Cþ
3 ) at atmosphere pressure can be obtained directly, and high ethane recovery rate can be
gained. On the basis of one typical feed gas composition, the effects of the ethane content and the ethane
price to the economics of the light hydrocarbon separation plants are studied, and the economics are
compared for these two processes. The results show that recovering light hydrocarbons from LNG can
gain great profits by both of the two processes, and from the view of economics, the low pressure process
is better than the high pressure process.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Natural gas is often liquefied for efficient transportation, and
liquefaction is a high energy consumption process. On the other
side, liquefied natural gas (LNG) should be gasified for normal
use at the receiving site, and great cryogenic energy is released
during the gasification process (about 840 kJ/kg), which can be uti-
lized to recover energy and enhance economic performance [1,2].
The cryogenic energy of LNG can be utilized in several ways,
such as air separation [3], cryogenic power generation [4], seawa-
ter desalination [5], and so on. Recently, a lot of LNG sources
among the international trades are rich gas, which contain more
than 10% of light hydrocarbons other than methane (such as eth-
ane, propane and butane). Ethane is a kind of quality and clean
raw materials for the production of ethylene, thus it has high addi-
tional values. By utilizing the cryogenic energy of LNG during its
gasification, ethane and LPG (liquefied petroleum gas, i.e. Cþ
3 ) can
be produced with low power consumption [6–9].
There have been some patents about separating light hydrocar-
bons (Cþ
2 ) from LNG as early as 1960 in America, and several new
patents have been registered in these years. However, these tech-
niques are usual as a means of heat value control, and the Cþ
2 sep-
arated from LNG are always stay at high pressure, which is
inconvenient for transportation and marketing [10–12]. In recent
years, researches for the production of Cþ
2 form LNG by utilizing
its cryogenic energy have developed in China. Hua et al. [13,14]
proposed several improved processes. Ref. [13] suggested a new
process which integrated the two parts of the heat exchanger net-
works and the light hydrocarbon separation process, and by heat
integration and optimization, the power consumption for separa-
tion was reduced greatly. However, the pressure of the separated
Cþ
2 is still high. Ref. [14] gave a further optimization for the heat ex-
changer networks, and designed a process which got rid of the
compressor. Besides, this process utilized the cold energy of LNG
to sub-cool the separated Cþ
2 , and thus the Cþ
2 remained liquid state
at normal pressure. However, ethane was not further separated
from Cþ
2 in this process, thus the product was not available to be
directly used.
Based on the existing researches, two novel light hydrocarbon
separation processes are proposed in this paper. By both of these
two processes, liquefied ethane and LPG at atmosphere pressure
can be obtained directly with acceptable power level, and high eth-
ane recovery rate can be gained. On the basis of one typical feed
gas composition, the effects of the ethane content and the ethane
0196-8904/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.enconman.2010.12.040
⇑ Corresponding author. Tel.: +86 13764193350.
E-mail addresses: gtwgq@sjtu.edu.cn (T. Gao), linwsh@sjtu.edu.cn (W. Lin).
Energy Conversion and Management 52 (2011) 2401–2404
Contents lists available at ScienceDirect
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journal homepage: www.elsevier.com/locate/enconman

2. price on the economics of the light hydrocarbon separation plants
are studied, and the economics are compared for these two
processes.
2. Process structure
For the two processes proposed in this paper, the first one,
which was called ‘‘high pressure process’’, uses a demethanizer
working at higher pressure (about 4.5 MPa). The methane-riched
natural gas from the demethanizer can be compressed to pipeline
pressure with low power consumption. The second one, which was
called ‘‘low pressure process’’, uses a demethanizer working at
lower pressure (about 2.4 MPa). By cascade utilization of LNG cryo-
genic energy, the methane-riched natural gas from the demethan-
izer is entirely re-liquefied. Then the liquid product is pressurized
to pipeline pressure by pumps instead of compressors, reducing
the power consumption greatly.
HYSYS software (AspenTech) is used for the process simulations
and calculations.
2.1. 1 High pressure process
The high pressure process is shown in Fig. 1.
As shown in Fig. 1, the LNG at atmosphere pressure is firstly
pressurized to 4.5 MPa with a pump, and then it enters into the
demethanizer after being pre-heated by using the heat energy
from the condenser of the deethanizer. The demethanizer works
at 4.3 MPa, by which more than 99.99% of methane is recovered
and released from the top of the demethanizer. The methane-
riched natural gas is then compressed to the pipeline pressure by
a compressor. The Cþ
2 released from the bottom of the demethaniz-
er enters into the deethanizer after being depressurized to 0.2 MPa
by a throttle. The deethanizer works at 0.11 MPa, and liquefied eth-
ane (LC2) at atmosphere pressure is obtained at the top of the
deethanizer, while LPG (LCþ
3 ) at atmosphere pressure is obtained
at the bottom of the deethanizer.
2.2. 2 Low pressure process
The low pressure process is shown in Fig. 2. The LNG at atmo-
sphere pressure is firstly pressurized to 1.5 MPa by pump 1, and
then it is heated twice and becomes gas–liquid two-phase fluid.
For a two-phase fluid, the sensible cooling capacity of the gas phase
Fig. 1. High pressure process.
Fig. 2. Low pressure process.
Table 1
LNG receiving station conditions.
Parameters Value
Composition (mol%)
CH4 90.16
C2H6 5.22
C3H8 3.10
i-C4H10 0.45
n-C4H10 0.82
i-C5H12 0.04
n-C5H12 0.03
N2 0.18
LNG storage pressure (MPa) 0.125
LNG temperature (°C) À158.3
LNG heat value (MJ/Nm3
) 40.2
Pipeline pressure (MPa) 7.65
LNG imported quantity (t/a) 1.10 Â 106
Table 2
Process settings.
Parameters Value
Compressor adiabatic efficiency 85%
Pump adiabatic efficiency 75%
Pressure drop of heat exchanger 10 kPa
Number of stages in the demethanizer 25
Number of stages in the deethanizer 20
Pressure drop of the demethanizer 20 kPa
Pressure drop of the deethanizer 15 kPa
2402 T. Gao et al. / Energy Conversion and Management 52 (2011) 2401–2404

3. and the latent cooling capacity of the liquid phase can be utilized
separately. Afterwards the gas–liquid two-phase fluid enters into
a liquid–vapor separator, and the separated methane-riched gas
is liquefied by using the sensible cooling capacity of the LNG in
heater 1 and is then pressurized to 2.4 MPa with pump 3; mean-
while, the Cþ
2 -riched liquid is pressurized to 2.5 MPa with pump
2 and then enters into the demethanizer. The demethanizer works
at 2.4 MPa. The methane-riched natural gas released from the top
of the demethanizer is liquefied by using the latent cooling capac-
ity of the LNG in heater 2, and then it mixes with the methane-
riched gas out of pump 3. The mixture is firstly pressurized to
5 MPa by pump 4, and then its cold energy is further utilized for
the condenser of the deethanizer. Afterwards it is further pressur-
ized to the pipeline pressure by pump 5. The Cþ
2 released from the
bottom of the demethanizer enters into the deethanizer after being
depressurized to 0.2 MPa by a throttle. The deethanizer works at
0.11 MPa, and liquefied ethane (LC2) at atmosphere pressure is ob-
tained at the top of the deethanizer, while LPG (LCþ
3 ) is obtained at
the bottom of the deethanizer.
For these two processes, the temperature of the reboiler in the
demethanizer is about 20–70 °C, and the heat energy required for
this reboiler can be provided by combusting a fraction of the meth-
ane-riched natural gas; the temperature of the reboiler in the
deethanizer is about À20 to À35 °C, and this reboiler can be di-
rectly heated by air or water.
3. Process performance
Take one of the LNG receiving stations in China as the example,
whose gas source parameters and operating conditions are shown
in Table 1, these two processes are simulated by the software
HYSYS (some of the settings or parameters for the components in
the processes are shown in Table 2), and the performance of these
two processes are calculated, as shown in Table 3. It can be seen
from Table 3 that the performance of the low pressure process is
better than the high pressure process.
However, the high pressure process is more simple and com-
pact, thus it is more suitable for the cases where the space is lim-
ited. Furthermore, the low pressure process requires accurate
temperature matching, and thus its adaptability is worse than
the high pressure process. Therefore, the high pressure process is
also preferable for the cases where the conditions change
frequently.
Table 3
Process performance.
Parameters High pressure process Low pressure process Notes
Natural gas production 98.5 t/h 97.83 t/h
Natural gas purity (methane
content)
98.8% 99.3%
Natural gas heat value 36.19 MJ/Nm3
36.01 MJ/Nm3
Ethane production 9.03 t/h 9.68 t/h Purity: 99.99%; heat value:
47.51 MJ/Nm3
LPG production 13.93 t/h 14.38 t/h Heat value: 46.12 MJ/Nm3
Cþ
2 recovery rate 90.38% 95.20%
Ethane recovery rate 85.67% 91.78%
Power consumption 1534 kW 1016 kW Output pressure of natural gas:
8 MPa
Heat load of the reboiler in the
demethanizer
14.38 MW (natural gas consumption: 1.04 t/
h, about 1%)
8.86 MW (natural gas consumption: 0.64 t/h,
about 0.65%)
Table 4
Economic comparison.
High pressure process Low pressure process
Investment (million CNY)
Equipment 45 60
Others 7.5 7.5
Total 52.5 67.5
Operation cost (million CNY/a)
Electricitya
11.87 7.86
LNG cold energyb
10.54 10.54
Payout for the workersc
0.57 0.57
Others 6.4 6.4
Total 29.38 26.37
Income (million CNY/a)
Natural gasd
À1062.1 À1092.2
Ethanee
568.2 609.05
LPGf
607.76 627.37
Total 113.86 144.22
Net profitg
(million CNY/a) 54.91 74.64
Payback periodh
(year) 1.96 1.90
a
Electricity price: 0.9 CNY/kW h.
b
LNG cold energy price: 10 CNY/t(LNG).
c
Ten workers, salary: 50,000 CNY/a for each worker; welfare factor: 14%.
d
The amount and the heat value of the natural gas decrease after light hydro-
carbon separation, causing a loss of incomes for natural gas. Heat value is used to
measure the value of the natural gas, and the price of it is 0.11 CNY/MJ.
e
Ethane is sold as the raw material of ethylene, the price of it should be higher
than the price measured with its heat value. In order to estimate ethane price by
using its heat value, the heat value price of ethane is assumed as 1.4 times than the
heat value price of natural gas: 0.154 CNY/MJ.
f
LPG is sold as fuel, the price of it is measured with the heat value price of
natural gas: 0.11 CNY/MJ.
g
35% of taxes is eliminated.
h
One year of construction period is considered. Fig. 3. Changes of profit with ethane content and ethane price.
T. Gao et al. / Energy Conversion and Management 52 (2011) 2401–2404 2403

4. 4. Economic analysis
4.1. Economic comparison
Although the low pressure process has better efficiency and
more production than the high pressure process, the high pressure
process is simpler, and therefore requires less equipment invest-
ment and smaller occupied area.
An economic analysis and comparison for these two processes is
studied, and the results are shown in Table 4 (the operation time is
assumed as 8600 h per year).
From Table 4 we can see, recovering light hydrocarbons from
LNG can gain great profits by both of these two processes, and from
the view of economics, the low pressure process is better than the
high pressure process.
4.2. Effects of the ethane content and the ethane price
Considering ethane is the main source of increasing income, the
economics of light hydrocarbon separation plants is very sensitive
to the ethane content and the ethane price. As a result, the effects
of the ethane content and the ethane price on the economics of
these two processes are further investigated.
On the basis of the composition listed in Table 1, the effect of
the ethane content is studied when changing the molar fraction
of methane and ethane while the molar fraction of the other com-
ponents is fixed. Meanwhile, the effect of the ethane price is stud-
ied by altering the times of the heat value price of ethane over that
of the methane (a). The results are shown in Fig. 3.
Fig. 3 indicates that both of these two processes have great po-
tential for economic benefits. As long as the ethane price is higher
than 1.2 times of its heat value price, the light hydrocarbon sepa-
ration plants can make profits by using the high pressure process
when the ethane content of LNG is higher than 5%, while it is 4%
for the low pressure process.
5. Conclusion
Two novel light hydrocarbon separation processes are proposed
in this paper. By both of the processes, liquefied ethane and LPG at
atmosphere pressure can be produced directly with acceptable
power level, and the cryogenic energy of LNG is utilized
reasonable.
On the basis of one typical feed gas composition, the effects of
the ethane content and the ethane price to the economics of the
light hydrocarbon separation plants are studied, and the economics
are compared for these two processes. The results show that,
recovering light hydrocarbons from LNG can gain great profits by
both of the processes, and from the view of economics, the low
pressure process is better than the high pressure process. However,
the high pressure process is more preferable for the cases where
the space is limited as well as the cases where the conditions
change frequently.
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