boil off gas in LNG terminal may re-condensate by liquid nitrogen in cold box

1. Recondensation of excess boil-off gas by
liquid nitrogen produced using LNG
cold in a regasification terminal
Prof. Kanchan Chowdhury
Rohit Singla
Jubil Joy
Cryogenic Engineering Centre
IIT Kharagpur

2. CONTENTS
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur
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 Introduction
 LNG receiving terminal
 Dahej LNG terminal
 Applications of LNG cold
 Air separation unit
 Existing system of LNG regasification in ASU
 Literature review
 Technological gap
 Objective
 Integration of ASU with LNG terminal
 Results and discussions
 References

3. INTRODUCTION
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur
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 Lower physical volume (600 times less than
NG’s) of liquefied natural gas (LNG) compared to
NG favors its transportation through cargo ships
 LNG receiving terminal supplies CNG to
residential or industrial customers after
unloading it from LNG carrier.
 Unloaded LNG stored as liquid in insulated tanks
suffers from heat inleak, that generates boil-off

4. Figure 1. Schematic of LNG receiving terminal
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur
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Delivery
pipeline
LNG Carrier
LNG
LNG Storage
tank
Boil-off
gas
(BOG)
Unloading
arm
Vapor return
line
Blower
BOG Compressor
Recond-
enser LNG
Vaporizer
HP send out
pump
LP in tank
pump
Fluid
removing cold
HP
compressor
A
B
C
D E
F
LNG Receiving terminal

6. Figure 1. Schematic of LNG receiving terminal
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur
6
Delivery
pipeline
LNG Carrier
LNG
LNG Storage
tank
Boil-off
gas
(BOG)
Unloading
arm
Vapor return
line
Blower
BOG Compressor
Recond-
enser LNG
Vaporizer
HP send out
pump
LP in tank
pump
Fluid
removing cold
HP
compressor
A
B
C
D E
F
LNG Receiving terminal

7. Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur
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Applications of LNG cold
Application of
LNG cold energy
Use of cold energy Purpose of cold energy
utilization
Deep frozen or
cold storage
Food preservation and
maintenance of hygiene
Saving of energy as it replaces
conventional refrigeration
Seawater
desalination
Obtain pure water by
crystallization process
Saving energy that is required
for thermal desalination
Liquefaction and
solidification of
carbon dioxide
For the liquefaction and
solidification of carbon
dioxide
Reduces the power consumption
of the system
Liquefaction
and separation
of air
To obtain oxygen,
nitrogen and argon in
liquid and gaseous form
Reduction in compressor size
and power: save energy

8. Figure 2. Block diagram of ASU
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur
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Air separation unit

9. Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur
9
Existing system of LNG regasification in ASU

10. LITERATURE SURVEY
Wendong et al., 2014
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur
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Air intake 5.556 kg/s
LNG mass 0.772kg/s
LNG pressure 1.2 bara
NG Toutlet 283 K
SPC 0.383 kWh/Nm3 LOX
•Requirementof LNG is only 13.9 % of
the net air feed into the plant due high
outlet temperature of LNG. High
temperature was possible due to low
pressure of LNG.
•At 1.2 bara saturation temperature of
LNG is 111.5K due to which its cold can
be utilized by low pressure air and
recycled nitrogen. However, recycling
nitrogen adds irreversibilities two
times into ASU.
•Another constraint of the cycle is that
NG is obtained at 1.2 bara only, due to
which LNG is required to be
compressed before supplying to the
end users.

11. LITERATURE SURVEY
Xiong et al., 2014
Cryogenic Engineering Centre, Indian Institute of Technology,
Kharagpur
11
 To gasify 100 bara LNG, nitrogen
is atleast required at 70 bara
pressure. Eventhough cold
compression is done to expend less
power, but adding heat of
compression at cryogenic
temperature leads to high addition
of irreversibilities in the cycle.
 Plant Produces LIN and 43 bara
GOX. To gasify pumped GOX ,
pressurised air has to fed through
main HX. This also consumes high
power.
 Heat of compression of MAC is
utilized to warm NG to higher
temperature, this leads to lowering
of temperatures at the inlet of
MAC stages, which saves power.
Air flowrate 104.167 kg/s
LNG flowrate 34.444 kg/s
LNG pressure 100 bara
NG outlet temperature -
SPC w.r.t LIN 0.596 kWh/ Nm3 LIN
SPC w.r.t 43 bara
GOX
0.272 kWh/ Nm3 O2

12. LITERATURE SURVEY
Mehrpooya et al., 2015
Cryogenic Engineering Centre, Indian Institute of Technology,
Kharagpur
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 Eventhough air in this
configuration is fed at 3 bara,
but nitrogen, which is ¼th of
the air feed to plant is
recycled nitrogen and cold
compressed to 66 bara to
gasify LNG.
 Recycling nitrogen after each
cold compression stage into
main HX adds,
irreversibilities several times
into the cold box of ASU.
 Oxygen recovery is only 50%
due to less boil-off.
Air feed 40302 kg/s
LNG utilized 28966 kg/s
LNG pressure 70 bara
NG outlet temperature 198 K
SPC w.r.t LIN 0.314 kWh/ Nm3 LIN
SPC w.r.t LOX 0.864 kWh/ Nm3 LOX

13. Technological Gap
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur
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 BOG has to be compressed to about 8 bara in LP
compressor so that it gets recondensed by
pumped LNG.
 Also with reduced flow of LNG, a part of BOG
remains unabsorbed in the recondenser and the
same has to be compressed in high pressure (HP)
compressor.
 The HP and LP compressor are the main power
consuming equipment in LNG regasification
system.

14. OBJECTIVE
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur
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 We propose to recondense BOG by an air
separation unit (ASU) which utilizes cold of LNG to
produce complete liquid products: LOX and LIN.
 The analysis of main heat exchanger is also carried
out to understand the pressure requirement of air
with a varying pressure of LNG.

15. Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur
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Integration of ASU with LNG terminal

16. Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur
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Proposed ASU utilizing cold of LNG to
recondense BOG
 LNG cold is absorbed
by air stream in the
Main HX.
 Less vapors rise in
HPC leading to lower
condenser-reboiler
duty.
 This reduces recovery
of liquid products.
 BOG recondensation
by boiling liquid at the
bottom of HPC
nullifies the
disadvantage of high
liquid feed to the

17. RESULTS AND DISCUSSIONS
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur
17
 Minimum pressure of LNG
as used by Wendong et al.,
2014 was fed to the cold side
of main HX
 Least possible pressure of air
to warm waste nitrogen
(WN2) and LNG was found
for a δT pinch of 0.5 K.
 LNG go through a phase
change as both fluids are not
above super critical pressure.
 So to avoid pinch in the main
HX air has to be above 37
bara, the critical pressure of
air.

18. RESULTS AND DISCUSSIONS
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur
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 At atleast 100 bara
pressure of air is required
to gasify LNG upto 198 K
the phase change
temperature of super
critical 70 bara LNG.
 A decision has to be taken
by compromising between
higher cold utilization
from least possible mass of
LNG or power
consumption in ASU to
pressurise air.
Figure 10. T-H plot of Main HX for a fixed
70 bara LNG inlet when super critical LNG
just changes phase at 198 K to cool air at
least 100 bara.

19. RESULTS AND DISCUSSIONS cont..
Cryogenic Engineering Centre, Indian Institute of Technology,
Kharagpur
19
 Shaded area shows the least
pressure of air is required to
gasify 70 bara LNG after avoiding
pinch.
 A pressure of 44 bara of air was
chosen to gasify LNG as LNG is
gasified to 160K and mass about
1/10th of 400kg/s throughput of
Dahej terminal.
 The shaded area can also become
a warning for design engineers
that the plant cannot function if
pressure of air is lies in that
region.
Region in which HX
will fail to warm LNG
Figure 8. Requirement of mass of LNG for
a fixed 70 bara inlet design pressure of
LNG with variation in pressure of air fed to
main HX.

20. RESULTS AND DISCUSSIONS cont..
Cryogenic Engineering Centre, Indian Institute of Technology,
Kharagpur
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Figure 11. Variation of UA of main HX when
70 bara LNG is warmed up in main HX with
pressure of pressure of air fed to main HX
Figure 11. Variation of minimum temperature
difference between air and cold fluids with
pressure of LNG fixed at 70 bara with
pressure of air fed to main HX
44 bara
44 bara

21. RESULTS AND DISCUSSIONS cont..
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur
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LNG feed
per unit air
fed to plant
Outlet
temperature of
LNG from
ASU
Recondensed BOG
Equivalent LOX product
SPC (kWh/
Nm3 LOX)
SPC (kWh/
Nm3 LIN)
Flowrate
(kg/s)
SPC (kWh/
Nm3 BOG)
Flowrate
(Nm3/hr)
SPC (kWh/
Nm3 *LOX)
Wendong et.al,
2014 (LNG 1.2
bara)
0.14 283 1.727 0.525 0 0 3845 0.382
Xiong et al.,
2014 (LNG 100
bara)
0.33 - No LOX 0.597 0 0 - 0.558
Mehrpooya et
al., 2015 (LNG
70 bara)
0.72 198 0.864 0.310 0 0.000 3.84E+07 0.217
Proposed plant
(1.2 bara LNG) 0.26 300 0.692 3.551 7.689 0.358 73585 0.191
Proposed O2
plant (70 bara
LNG)
1.21 214 0.834 4.279
7.689
0.431 73585 0.227
Proposed O2
plant (100 bara
LNG)
1.15 237 0.863 4.428
7.689
0.446 73585 0.235

22. CONCLUSIONS
Cryogenic Engineering Centre, Indian Institute of Technology,
Kharagpur
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1. Completely avoides compression. Thus capital requirement of LP & HP
BOG compressor is eliminated.
2. HP compressor has to function for a fluctuating flow of BOG coming from
recondenser due to fluctuating throughput of LNG, which was also a major
handicap of BOG recondensation cycle is also negated.
3. ASU products are obtained in liquid form, thus transportation is
convenient.
4. LIN can be stored in dewar vessel to recondense BOG in case NG demand
becomes zero due to a unplanned shut own.
5. Presented parametric analysis of main heat exchanger in the ASU can
become a guide for the design engineers to modify ASU which utilizes
different pressures of LNG.
6. Operating engineers can use the presented parametric results to utilize
fluctuating flowrate of LNG by having a flexible chain of booster air
compressor.
7. The operation of ASU under fluctuating throughput of LNG is being
analysed by the authors as future scope.

23. Cryogenic Engineering Centre, Indian Institute of Technology,
Kharagpur
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Presenter:
Prof. Kanchan Chowdhury
Chowdhury.kanchan @gmail.com