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 Introduction Literature review Technological gap Objective
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
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur

3. INTRODUCTION
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
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur

4. Figure 1. Schematic of LNG receiving terminal
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur

6. Figure 1. Schematic of LNG receiving terminal
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur

7. 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
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur

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

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

10. LITERATURE SURVEY Wendong et al., 2014
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.
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
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur

11. LITERATURE SURVEY Xiong et al., 2014
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
kg/s
LNG flowrate
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
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur

12. LITERATURE SURVEY Mehrpooya et al., 2015
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
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur

13. Technological Gap
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.
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur

14. OBJECTIVE
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.
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur

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

16. 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
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur

17. RESULTS AND DISCUSSIONS
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.
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur

18. RESULTS AND DISCUSSIONS
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.
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur

19. RESULTS AND DISCUSSIONS cont..
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.
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.
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur

20. RESULTS AND DISCUSSIONS cont..
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
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
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur

21. RESULTS AND DISCUSSIONS cont..
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, (LNG 1.2 bara)
0.14
283
1.727
0.525
3845
0.382
Xiong et al., (LNG 100 bara)
0.33 -
No LOX
0.597
0.558
Mehrpooya et al., 2015 (LNG 70 bara)
0.72
198
0.864
0.310
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
0.431
0.227
Proposed O2 plant (100 bara LNG)
1.15
237
0.863
4.428
0.446
0.235
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur

22. CONCLUSIONS
Completely avoides compression. Thus capital requirement of LP & HP BOG compressor is eliminated.
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.
ASU products are obtained in liquid form, thus transportation is convenient.
LIN can be stored in dewar vessel to recondense BOG in case NG demand becomes zero due to a unplanned shut own.
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.
Operating engineers can use the presented parametric results to utilize fluctuating flowrate of LNG by having a flexible chain of booster air compressor.
The operation of ASU under fluctuating throughput of LNG is being analysed by the authors as future scope.
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur

23. Thank you!!! Any querries????
Presenter: Prof. Kanchan Chowdhury
Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur