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COMPARISON OF DIRECT EXPANSION AND COMBINED DIRECT EXPANSION

AND ORGANIC RANKINE CYCLE FOR POWER GENERATION BY UTILIZING LNG
COLD ENERGY

Conference Paper · May 2017

DOI: 10.18462/iir.cryo.2017.087

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 PAPER ID: 0033
DOI: 10.18462/iir.cryo.2019.0033

Utilization of cold energy of liquefied natural gas in air separation

plant in a safer and more energy efficient way
Jubil JOY(a), Rohit SINGLA(a), Kanchan CHOWDHURY(a)
(a)
Cryogenic Engineering Centre, Indian Institute of Technology Kharagpur
Kharagpur,721302, India, jubiljoy25@gmail.com

ABSTRACT
LNG cold can be utilized to reduce specific energy consumption (SPC) in an air separation unit (ASU). LNG
cold can be used to produce pressurized gaseous oxygen (PGOX), pressurized gaseous nitrogen (PGAN),
liquid oxygen (LOX) and liquid nitrogen (LIN). The proposed configuration is safer than that proposed in a
published paper by Tesch et al. (2017) where PGOX is produced by pumping LOX and vapourizing it in main
heat exchanger (MHX) instead of getting pressurized in a compressor. Further, we avoid bringing flammable
fuel LNG to the ASU premises. Nitrogen from ASU to the regasification terminal gets converted to LIN in a
heat exchanger and the same returns to the ASU. Power consumption, the purity of the products, exergy
destruction and exergy efficiency are determined by simulating the plant on Aspen Hysys 8.6TM process
simulator. The results show that apart from improved safety, the exergy efficiency of the proposed system is
higher than that of the configuration compared.
Keywords: LNG, Regasification, Cold Energy, Cryogenic Air Separation, Exergy, Safety.
1. INTRODUCTION
The cold energy of LNG, which is usually rejected to sea water or air during the regasification process (Jin et
al., 2014), can be utilized for the power generation and other purposes such as liquefaction and separation of
air, cold storage, seawater desalination and liquefaction and solidification of CO2 (Hirakawa and Kosugi,
1981). One of the options for the utilization of LNG cold energy is to reduce power consumption of an air
separation unit (ASU) which produces nitrogen, oxygen and argon in liquid and gaseous forms.
Integration of LNG cold energy and a liquid oxygen (LOX) plant was established in 2010 in Putian, China
(Sharrat, 2012). Cold energy of LNG is used in a recycled nitrogen compressor and in main air compressor
(MAC) by using ethylene glycol as an intermediate fluid (Air Products Co., Ltd., 2006). The cold of LNG is
utilized in a separate heat exchanger for the liquefaction of nitrogen (Yong-qiang and Ben, 2007). Wendong
et al. (2014) suggested a configuration where a double column ASU produces liquid and gaseous nitrogen
(LIN and GAN) apart from LOX and gaseous oxygen (GOX). Another configuration produced only LIN and
LOX. The cold energy of LNG is utilized in the main heat exchanger (MHX) for the liquefaction of nitrogen.
Absence of pure oxygen in MHX makes the process safer for LNG cold to be extracted. Cold energy of LNG
is used in the MHX by Mehrpooya et al. 2015. Ebrahimi and Ziabasharhagh (2017) have proposed plants to
produce pressurized gases in compressors. Tesch et al. (2016 and 2017) have proposed ASUs for utilization of
LNG cold energy in a double column ASU producing nitrogen and oxygen in liquid and gaseous forms. The
GOX and gaseous nitrogen (GAN) are separated and both pressurized to 20 bara in external gas compressors.
Cold energy of LNG is used to cool other streams in the MHX and liquefy recycled nitrogen in a separate heat
exchanger. Part of the cold energy of LNG, which is available at relatively higher temperature, is used for
inter-cooling and after-cooling of MAC.
2. ISSUES INVOVED AND OBJECTIVE
The study of literature involving utilization of LNG cold through air separation process brings a few issues to
the fore:
1) Presence of the high-potency oxidizer (near-pure oxygen) and a fuel gas (natural gas) at the same premises
or in the same equipment poses a great fire safety risk. It may not be safe to vaporize LNG in MHX where
LOX or GOX is also present. It may not be also a good idea to bring LNG to the ASU premises.
2) In modern ASUs, gaseous oxygen is no longer compressed in an oxygen compressor. It is because
handling oxygen in gaseous form, particularly at high pressure, poses a risk of fire. Heat of compression and
presence of unwanted solid particles caused many fire incidents in GOX compressors around the world. The

The 15th Cryogenics 2019, IIR Conference, Prague, Czech Republic, April 8-11, 2019
process of oxygen compression in a compressor has now been replaced by vaporization of pumped LOX and
warming up to ambient temperature in MHX. ASU proposed by Tesch et al. (2017) have been termed as safe
by the authors because oxygen and LNG are not present in the same heat exchanger. However, compression
of GOX still remains as a potential safety issue in their proposed system.
3) In an attempt to capture LNG cold beyond 200 K, the authors in Tesch et al. (2017) have proposed inter-
cooling and after-cooling of MAC to sub-atmospheric temperature. They have not considered the fact that
freezing of moisture in the process air may choke the air passages. Thus the utilization of LNG cold beyond
200 K may not be practically implementable in the present form.
The present work proposes a new configuration of double column air separation plant with LOX pumping for
the utilization of LNG cold energy.
3. METHOD OF ANALYSIS

3.1. Description of LNG cold energy utilization systems by Tesch et al. (2017)
Schematic diagram of double column cryogenic ASU with the utilization of LNG cold energy, which is
published by Tesch et al. (2017), is shown in Fig. 1. Air from atmosphere is compressed to 5.6 bara. The
ethylene glycol-water is cooled by gasified natural gas and the same is used for inter-cooling and after-cooling
of the compressed air. Air, which is cooled in the main heat exchanger (MHX) to nearly 100 K, enters the high
pressure column (HPC). The low pressure column (LPC) and HPC are thermally connected by
condenser/reboiler (CR). HPC produces LIN and GAN from the top. Nitrogen from MHX is compressed to a
supercritical pressure of 46 bara and is cooled by incoming nitrogen stream and LNG cold energy (in HX2).
The pressurized and cooled high pressure nitrogen is divided into two parts and expanded in valves to LIN at
pressures of 5.6 bara and 1.2 bara separately. Low pressure LIN is fed into the top of the LPC as reflux, while
the higher pressure (5.6 bara) LIN is mixed with the condensed nitrogen from CR. A part of high pressure LIN
is taken as product and the remaining is fed into the top of the HPC as a reflux. Bottom product of HPC (highly
impure LOX) is throttled and fed into LPC as feed. Pure LOX is taken from the bottom of LPC. The gaseous
products of LPC such as oxygen and nitrogen are gasified in the MHX and compressed to a pressure of 20
bara. The pressurized gaseous products are cooled by waste nitrogen (WN2) from LPC and ethylene glycol-
water solution. This plant is modified in the present work as described below.

3.2. Modified ASU utilizing LNG cold energy which is safer and more efficient
Schematic of cryogenic ASU with LOX pumping and utilization of LNG cold energy is shown in Fig. 2. Water
is used for the inter-cooling and after-cooling of MAC and booster air compressor (BAC). LOX is pumped
and vaporized in the proposed system. To achieve vaporization of cryogenic fluids in the heat exchanger with
a reasonable temperature approach, air has to be compressed to 33 bara before entering the MHX. Cooled air
outlet at 126 K is expanded in a JT valve and enters the HPC as feed. Impure liquid (IL) nitrogen from HPC
(stream 20) is sub-cooled, throttled and fed into the LPC as reflux. This improves the purity of PL from HPC
and GAN from LPC. The impure product from the bottom of HPC is sub-cooled in the sub-cooler 2, expanded
in JT valve and fed into the LPC.
GAN and WN2 from the sub-cooler 2 are warmed in the MHX. GAN is compressed to 20 bara using 5 stage
nitrogen compressor. The air inlet temperature to the MHX is 305 K and the temperature approach at the warm
side of MHX is 3 K. A part of LOX from the bottom of LPC is taken out as product and the remaining is
pumped to a pressure of 20 bara. The pressurized LOX is vaporized and warmed in MHX. A part of the GAN
(stream 39) from HPC is warmed in MHX and is compressed in a compressor, while the other part of GAN
(stream 40) is cold-compressed, both at 40 bara. These nitrogen streams are used to accept LNG cold from the
regasification terminal and use the same in the air separation plant.

3.3. Thermodynamic analysis of ASU with LNG cold energy utilization

3.3.1. Exergy analysis of the system
General equation for exergy balance for steady state, steady flow for a fixed boundary is,
∑ (1 − (𝑇0 /𝑇𝑗 ))𝑄̇ j - 𝑊̇ + ∑ 𝐸̇ x total in -∑ 𝐸̇ x total out - 𝐸̇ XD = 0 Eq. (1)

The 15th Cryogenics 2019, IIR Conference, Prague, Czech Republic, April 8-11, 2019
∑ (1 − (𝑇0 /𝑇𝑗 ))𝑄̇ j is exergy due to heat interaction, where 𝑄̇j (𝑘𝐽⁄𝑘𝑔) is the rate of heat transfer at the
boundary, 𝑇𝑗 (K) represents instantaneous temperature and 𝑇0 (K) is reference temperature. The heat in-leak
from the system is considered as the loss from the system boundary.

GOX GAN WN2 22

Air NG

21
N2 Control
45 Comprs
volume
HX3
Ethylene glycol -
46 Water pump 37
51
49
Air filter
HX4 47 HX6 33
HX5
Air
GOX 20
1 compr
2 3 4 5 6 44
PPU 7 8
40 41 HX1
HX2
48 23
50 34 38
GAN
32 39 compr 19 24

25 35
36 26
11

MHX

LPC
14
15

16 13
12 CR

30
18
17
31
HPC 27
9
29 28
9

10

43
42
LNG PUMP

LIN LOX

Figure 1: Double column air separation plant with compression of oxygen and utilization of LNG cold energy
(external compression plant) (Tesch et al., 2017)
Air
WN2
P GOX CNG

P GAN 2 51

Filter 39
GAN HX 1
42 44
1 45 43
3-stage 5 stage GAN
C unit 46
MAC unit 3 stage N2
30 50
38 34 C unit
2 2 stage
Air LNG
35 LNG pump
PPU 41 N2 C unit
19

3 4 18 21 22 31
MHX
3-stage Sub-cooler 1
BAC unit 40 47 16
39 17 11
Control
5
32 volume
14 13 15 36
12 C/R

48
49
20

25
HPC

6 23
24
Sub-cooler 2 26
9 10
33

37
28
29
LOX pump 27
LIN
LOX

LNG regasification
Air separation section section

Figure 2: Double column air separation plant with LOX pumping and utilization of LNG cold energy (internal
compression plant)

The 15th Cryogenics 2019, IIR Conference, Prague, Czech Republic, April 8-11, 2019
𝑊̇ (𝑘𝐽⁄𝑘𝑔) is exergy transfer due to work (electrical or mechanical) interaction. EX total in and EX total out are the
combined total physical and chemical exergy transfer associated with in and out of the system;
𝐸̇ XD is the exergy destruction in the process and it is the sum of all exergy destructions in the components.
The physical exergy transfer , EX PH = 𝑚̇ ×exPH Eq. (2)
𝑚̇ (kg/s) is the mass flow rate and ex is the physical exergy per unit mass flow rate (kg/s)
exPH = [(h-h0) - T0(s-s0)] (𝑘𝐽⁄𝑘𝑔) Eq. (3)
‘h’ and ‘s’ are the specific enthalpy and specific entropy of working fluid. h 0 and s0 are the corresponding
values at reference environment temperature (T0) and pressure (P0) at 288.15 K and 1.013 bara respectively.
The chemical exergy transfer , EX CH = 𝑚̇ × exCH Eq. (4)
𝑚̇ (mole/s) is the molar flow rate across the system boundary and exCH is the chemical exergy per unit molar
𝑘
flow rate (mole/s). The chemical exergy of each stream is calculated by the equation, exch = ∑ 𝑥𝑘 𝑒𝑥𝑐ℎ + RT0
𝑘
∑ 𝑥𝑘 ln(𝑥𝑘 ), where 𝑒𝑥𝑐ℎ is the standard chemical exergy value of gas and 𝑥𝑘 is the mole fraction of gas.
The rational exergy efficiency of the system is defined as the ratio of desired exergy output to the exergy used.
Used exergy includes the exergy input from regasification of LNG. Applying Eq. (1) to control volume shown
in Fig.1, the desired exergy output,
𝐸̇ x desired output = (𝐸̇ x total GOX + 𝐸̇ x total GAN + 𝐸̇ x total LOX + 𝐸̇ x total LIN) – 𝐸̇ x total air Eq. (5)
̇ ̇ ̇ ̇
The used exergy 𝐸 x used = (𝐸 x total LNG - 𝐸 x total NG) + 𝑊 MAC + 𝑊 GOX + 𝑊 GAN + 𝑊 NC ̇ ̇ ̇ Eq. (6)
𝐸̇x desired output
From Eq. (5) and (6), rational exergy efficiency of the system, ψex = ×100% Eq. (7)
𝐸̇x used

3.3.2. Assumptions
a. Operations are in a steady state condition.
b. Adiabatic efficiencies of turbine and pumps are not varying with mass, pressure and temperature.
c. Pressure drop and heat loss in pipes and heat exchangers are neglected.
d. UA (UA is the thermal size of the heat exchanger in (W/K) which is the product of overall heat transfer
coefficient U (W/m2 K) and heat transfer area A (m2) of a heat exchanger) includes deterioration factors
(F) arising from irreversibility due to configuration, axial heat conduction, flow maldistribution, heat-in-
leak etc.

3.3.3. Solution procedure

Aspen HYSYS 8.6TM is used to evaluate and compare the ASUs which utilizes the LNG cold energy.
Thermo-physical properties of working fluids have been calculated by using the Peng-Robinson equation
of state. The operating parameters which are used for the analysis of the systems are listed in Table 1.

Table 1. Operating conditions of LNG cold energy utilization systems

Parameter Value
Inlet temperature (K) and pressure (bara) of LNG 111.2 and 1
CH4 – 86.98, C2H6 – 9.35, C3H8 – 2.33,
Composition of LNG (mol %)
C4H10 – 0.63, N2 – 0.71
Composition of air (mol %) N2 – 78.12, O2 – 20.95, Ar – 0.93
Final delivery pressure of CNG 70 bara
Adiabatic efficiency of compressors and turbine (%) 84%
Adiabatic efficiency of pump 70%
4. RESULTS AND DISCUSSIONS
Table 2 compares the performances of the system proposed by Tesch et. al (2017) and the present work in
terms of air flow, products, utilization of LNG cold energy, total power consumptions, and purity of the
products of the plant. The increase in the power consumption of our work lies in BAC where the air is
compressed to 33 bara before entering to the MHX. The compression of air is required to avoid the temperature
cross in the MHX where high pressure pumped LOX is vaporized. The higher power consumption of MAC in
the present work is due to the use of seawater in the inter-stage cooling instead of chilled water from LNG cold

The 15th Cryogenics 2019, IIR Conference, Prague, Czech Republic, April 8-11, 2019
energy. The use of LNG cold energy in inter-stage cooling is avoided by us because it will lead to formation
of ice in air flow.

Exergy analysis of the plants

Fig. 3 and Fig. 4 shows the exergy input and exergy output of all the streams which consists of physical and
chemical exergy. The higher the mass flow rate of LNG leads to higher the cold exergy input to the plant. The
compressor work is the major input of exergy for both the plants. The total exergy output of products such as
PGOX, PGAN are increased in the present work as the chemical exergy is increased by increasing the purity.
The total exergy of LIN and LOX are approximately same. The recovery of exergy in the present work is
higher by 7.90%.

Figure 3: Comparison in exergy input to the internal Figure 4: Comparison in exergy output from the
compression and external compression plants internal compression and external compression plants

4.1.1. Calculation of exergy efficiency of the plants

The detailed exergy efficiency calculation of internal and external compression plants are given below.
a) System which is proposed by Tesch et al. (2017) (external compression plant)
Applying Eq. (5) to the control volume of system shown in Fig. 1, the desired exergy output,
𝐸̇ x desired output = (𝐸̇ x total PGOX + 𝐸̇ x total PGAN + 𝐸̇ x total LOX + 𝐸̇ x total LIN) – 𝐸̇ x total air = (𝐸̇ x total products – 𝐸̇ x total air)
= (978.35 kW + 1907.16 kW + 373.68 kW + 2076.59 kW) – 49.35 kW
= 5335.78 kW – 49.35 kW = 5286.43 kW
Applying Eq. (6) to the control volume of system shown in Fig. 1, the used exergy,
𝐸̇ x used = (𝐸̇ x total LNG - 𝐸̇ x total CNG) + ( 𝑊̇ MAC + 𝑊̇ GOX C + 𝑊̇ GAN C + 𝑊̇ N2C) = (𝐸̇ x total LNG - 𝐸̇ x total CNG) + 𝑊̇ input
= (521894.10 kW – 516196.20 kW) + 2979.67 kW + 876.21+2197.51 kW + 1575.50 kW
= 5697.90 kW + 7628.89 kW = 13326.79 kW
Applying Eq. (7), rational exergy efficiency of the system,
(𝐸̇x total products – 𝐸̇x total air ) 5335.78 kW – 49.35 kW
ψex = ×100% = ×100%
(𝐸̇x total LNG input − 𝐸̇x total CNG 𝑜𝑢𝑡 ) + 𝑊̇total input 5697.90 kW + 7628.89 kW
5286.43 kW
= ×100% = 39.69 %
13317.75 kW

b) Modified system (internal compression)

Applying Eq. (5) to the control volume of system shown in Fig. 2, the desired exergy output,
𝐸̇ x desired output = (𝐸̇ x total PGOX + 𝐸̇ x total PGAN + 𝐸̇ x total LOX + 𝐸̇ x total LIN) – 𝐸̇ x total air = (𝐸̇ x total products – 𝐸̇ x total air)
= (1059.44 kW + 1939.69 kW + 386.81 kW + 2079.90 kW) – 41.14 kW
= 5464.94 kW – 41.14 kW = 5423.80 kW
Applying Eq. (6) to the control volume of the system shown in Fig. 2, the used exergy,
𝐸̇ x used = (𝐸̇ x total LNG - 𝐸̇ x total CNG) + ( 𝑊̇ MAC +𝑊̇ BAC + 𝑊̇ LOX pump + 𝑊̇ GAN C + 𝑊̇ warm N2C + 𝑊̇ cold N2C)
= (𝐸̇ x total LNG - 𝐸̇ x total CNG) + 𝑊̇ input
= (323596.86 kW – 321819.87 kW) + 3356.55 kW + 3422.62 kW + 8.64 kW + 2349.11 kW + 1110.79 kW +
639.61 kW = 1776.99 kW + 10887.20 kW = 12664.31 kW
Applying Eq. (7), rational exergy efficiency of the system,

The 15th Cryogenics 2019, IIR Conference, Prague, Czech Republic, April 8-11, 2019
 (𝐸̇x total products – 𝐸̇x total air ) 5464.80 kW – 41.14 kW
ψex = ×100% = ×100%
(𝐸̇x total LNG input − 𝐸̇x total CNG 𝑜𝑢𝑡 ) + 𝑊̇total input 1776.99 kW + 10887.32 kW
5423.80 kW
= ×100% = 42.83 %
12664.31 kW
The higher exergy efficiency of our system in spite of higher power input is due to use of less exergy from
LNG cold. This is not a welcome situation however and our system needs to be improved by reducing power
consumption and increasing LNG cold for the same size of air separation plant represented by input air flow.
Table 2. Comparison in cold utilization and overall exergy analysis of the plants
Systems
Tesch et al. Tesch et al. Present work
Parameter
(2017) (2017) (our
(Claimed) simulation)
MAC air flow in Nm3/h 45420 45420 45420
Flow Rate of PGAN in Nm3/h (purity in mole%) 18970 (0.9910) 18970 (0.9824) 18970 (0.9945)
Flow Rate of PGOX in Nm3 /h (purity in mole%) 7510 (0.9657) 6900 (0.9616) 7510 (0.9713)
Flow Rate of LIN in Nm3 /h (purity in mole%) 8350 (1.000) 8350 (1.000) 8350 (0.9991)
Flow Rate of LOX in Nm3 /h (purity in mole%) 1280 (0.9758) 1280 (0.9749) 1280 (0.9713)
Mass flow Rate of LNG (kg/s) 10.0 10.0 6.2
Total cold input to the system by LNG (kW) 8490 8490 4873
Cold utilized in the recycled nitrogen (%) 40.6 40.6 40.0
Total power consumption (kW) 8350.00 7619.95 10887.20
Total exergy used in the plant (kW) --- 13317.76 12664.19
Total exergy destruction in the plant (kW) --- 8440.07 7239.5
Rational exergy efficiency of the plant (%) --- 39.69 42.83
5. CONCLUSIONS
A modification of double column air separation has been done with the LOX pumping instead of GOX
compression. Both the systems are safe as far as avoiding LOX or GOX co-existing with LNG in the MHX is
concerned. An additional safety feature of the system in the present work is the use of LOX pumping instead
of GOX compression. The suggestion of input air being intercooled at sub-zero level is not found to be
acceptable and hence done away with in the present system. Power consumption in present work is higher by
42%. The exit temperature of LNG from the present work is 203 K after utilization of LNG cold energy in
recycled nitrogen which is acceptable. Exergy efficiency is 7.90 % higher in the present work even if the
exergy of LNG cold energy is lower. However, power consumption of our system should be improved.

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The 15th Cryogenics 2019, IIR Conference, Prague, Czech Republic, April 8-11, 2019
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