3. 190
Natural gas can be cooled and liquefied in
order to allow natural gas to be economically
transported over great distances. In its liquid
form natural gas occupies only 1/600th of its
normal volume and has a temperature of
around -162°C. The Engineering Division of
Linde AG develops tailor made processes for
the liquefaction of natural gas. Linde has proc-
esses for plants ranging in size from 40.000
tons per annum for peak shaving plants and
up to 12 million tons per year for large
baseload plants. Linde Engineering has a
strong history in the LNG industry having
developed, built and started-up over 20 LNG
plants world-wide since 1967.
Contents
1-Thermodynamic Analysis of
LNG Processes
2- LIQUEFACTION CYCLES
3- Closed Cycles
4- Open Cycles
5- CASCADE CYCLES
6- LNG processes with a phase
separator
4. 191
Natural Gas Liquefaction Process
The liquefaction process is the key element of the LNG plant.
Liquefaction is based on a refrigeration cycle, where a refrigerant by
means of successive expansion and compression, transports heat from
the process side to where the natural gas is. LNG plants often consist of
a number of parallel units, called trains, which treat and liquefy natural
gas and then send the LNG to several storage tanks. The capacity of a
liquefaction train is primarily determined by the liquefaction process, the
refrigerant used, the largest available size of the compressor/driver
combination that drives the cycle, and the heat exchangers that cool the
natural gas.
The basic principles for cooling and liquefying the gas using
refrigerants, involve matching as closely as possible the cooling/heating
curves of the process gas and the refrigerant. These principles result in
a more efficient thermodynamic process, requiring less power per unit
of LNG produced, and they apply to all liquefaction processes. Typical
cooling curves are shown in Figure 15. Observing the cooling curve of a
typical gas liquefaction process, three zones can be noted in the
process of the gas being liquefied. A precooling zone, followed by a
liquefaction zone, and completed by a subcooling zone. All of these
zones are characterized by having different curve slopes, or specific
heats, along the process. All of the LNG processes are designed to
closely approach the cooling curve of the gas being liquefied, by using
specially mixed multicomponent refrigerants that will match the cooling
curve at the different zones/stages of the liquefaction process, to
achieve high refrigeration efficiency, and reduce energy consumption.
FIGURE 15 : TYPICAL NATURAL GAS/REFRIGERANT COOLING CURVES
5. 192
The liquefaction process typically accounts for almost 45% of the capital
cost of the overall LNG plant (Knott, 2001), which in turn accounts for
25% to 35% of total project costs, when including the regasification
facility and the dedicated vessels for transport. Key equipment items
include the compressors, used to circulate the refrigerants, the
compressor drivers, and the heat exchangers, used to cool and liquefy
the gas, and exchange heat between refrigerants. For recent baseload
LNG plants, this equipment is among the biggest of its type, and at the
leading edge of technology.
Since LNG liquefaction requires a significant amount of refrigeration, the
refrigeration system represents a large portion of a LNG facility. A
number of liquefaction processes have been developed with the
differences mainly residing on the type of refrigeration cycles employed.
The most commonly utilized LNG technologies are described below,
―Propane Precooled Mixed Refrigerant (PPMR™)/C3 MR Process‖.
There are other processes developed or in development for baseload
LNG applications, which can be, or are being, considered in feasibility
studies or for future projects, but are not discussed here.
As with most process designs, there is a tradeoff between efficiency and
capital cost. In addition, considerations such as ease of start-up, ability to
handle feedstock composition changes, and maintenance costs play a
role. Below the thermodynamic efficiency of LNG processes is explored.
2.1 Thermodynamic Analysis of LNG Processes
In the simplest sense, liquefaction of natural gas could be accomplished
in a single stage cooler/condenser. Since natural gas contains a mixture
of gases, in a real process and as mentioned earlier, the NGL’s are
removed and can be marketed or used separately. Any noncondensable
gases, such as N2 and H2, as well as any CO2, H2S, and water vapor
present are also removed. For the sake of simplicity, in the analysis
below, ―natural gas‖ is assumed to be pure methane. A narrative
example is used here under realistic conditions to demonstrate important
thermodynamic and heat transfer issues. The results can be scaled up or
down depending on the size of the natural gas stream to be liquefied.
Metric units are used because almost all of the published chemical
engineering literature is now in these units.
The raw feed will be taken as 25°C and 40 bar, and the product LNG (liquid
methane) at 4 bar and –150°C. It is important, when comparing
performance indicators, to note particularly the inlet and outlet
specifications. For sizing purposes, one 8-MTPA process in two parallel 4-
MTPA trains is considered.
6. 193
2.1.1 Ideal Cooling Process
For an ideal cooling process, the cooling load can be written as a
basic material and energy balance,
in
in
out
out
cool
h
m
h
m
Q
Since mass in equals mass out, the terms min and mout can be
replaced with m, the heat per unit mass (kJ/kg) can be expressed
as
m
Q
Q cool
cool
.
Heat transfer is given by
T
UA
t
Q
Q
.
Where U is the overall heat transfer coefficient, in W/m2-s-K.
Solving for area
T
t
U
Q
A
The coefficient of performance (COP) for a refrigeration cycle is
equal to Qcooling/Wactual. Classical thermodynamics indicates that the
maximum COP can be calculated in terms of the temperature
differences alone as
1
/
1
0
T
T
W
Q
COP c
2.1.2 Real Cooling Processes
Real processes are less efficient than the ideal reversible processes
described above. The primary sources of inefficiency are friction in the
compressors, finite temperature differences in the heat exchangers,
irreversible flashes across throttling valves, and heat loss to the
surroundings.
2.2 LIQUEFACTION CYCLES
The two most common methods that have been used in engineering
practice to produce low temperatures are Joule-Thomson expansion
and expansion in an engine doing external work.
This section discusses each of these processes in detail and
analyzes them thermodynamically.
7. 194
2.2.1 JOULE-THOMSON CYCLES
The Joule-Thomson coefficient is the change in temperature that results
when a gas is expanded adiabatically from one constant pressure to
another in such a way that no external work is done and no net
conversion of internal energy to kinetic energy of mass motion occurs.
Thermodynamically, it is an irreversible process that wastes the
potential for doing useful work with the pressure drop. However, it is as
simple as a valve or orifice and finds wide use in refrigeration cycles
The thermodynamic definition of the Joule-Thomson coefficient is
h
P
T
One of the more important thermodynamic relations that involves
Joule- Thomson coefficient is
T
p
p
p P
H
C
T
V
T
V
C
)
(
1
)
)
(
(
1
Combination of the above relation with the ideal gas law (PV = RT) gives
m = 0, and thus no temperature change occurs when an ideal gas
undergoes a Joule-Thomson expansion. For a real gas, the Joule-
Thomson coefficient may be positive (the gas cools upon expansion),
negative (the gas warms upon expansion), or zero. The locus of all points
on a pressure−temperature plot where the Joule Thomson coefficient is
zero is known as the inversion curve Figure shows the Joule-Thomson
inversion curve for methane expansions must take place below the curve
to produce refrigeration.
FIGURE 1 : JOULE-THOMSON INVERSION CURVE FOR METHANE AS AFUNCTION OF P AND T
8. 195
The behavior of several gases upon expansion from 101 bar (1,470 Pisa)
to 1 bar (14.5 psia) is shown in Table 2. Two items should be noted. First,
for both methane and nitrogen, the cooling effect upon expansion when
started at ambient temperature (80° F, 27°C) is relatively small. Second,
the cooling effect increases significantly as the initial temperature is
lowered. For helium, the expansion results in heating the gas rather than
cooling. The temperature increase remains constant because the Joule-
Thomson coefficient remains nearly constant over the temperature range
considered
TABLE 1 : THE BEHAVIOR OF SEVERAL GASES UPON EXPANSION FROM 101 BAR (1,470 PISA) TO 1 BAR
(14.5 PSIA)
Because methane, the principal constituent of natural gas, must be cooled
to−258°F (−161°C) before it becomes a liquid at 1 atmosphere pressure, a
liquefier that uses only a Joule-Thomson expansion requires more than a
compressor and an expansion valve if it is to function at reasonable initial
pressures. A counter flow heat exchanger needs to be added to make a
complete system.
The liquefaction cycle begins with natural gas being compressed and sent
through the heat exchanger and expansion valve. Upon expansion, the gas
cools (approximately 84°F [47°C] if the gas is principally methane and the
expansion is from 1,500 to 14.7 psia [101 to 1 bar]), but none liquefies
because a temperaturedrop of approximately 338°F (188°C) is required to
convert the gas to a liquid. Thus, all of the chilled low-pressure gas is
recycled through the heat exchanger for recompression. This cold low-
pressure gas lowers the temperature of the high pressure gas stream ahead
of the expansion valve, which results in a lower temperature upon expansion.
As long as all of the gas being expanded is recycled through the counter
flow heat exchanger to cool the high-pressure gas stream, temperatures will
be progressively lower upon expansion. The process continues until liquid is
formed during the expansion from high to low pressure.
Initial temperature Final temperature
initial
t
t final
o
F (o
C) o
F(o
C) o
F(o
C)
Methane 80 (27) -4 (-20) -44 (-47)
Nitrogen 80 (27) 46 (8) -34 (-19)
Helium 80 (27) 91 (33) 11 (6)
Methane -10 (-23) -125 (-87) -115 (-64)
Nitrogen -10 (-23) -60 (-51) -50 (-28)
Helium -10 (-23) 1 (-17) 11 (6)
Methane -46 (-43) -215 (-137) -169 (-94)
Nitrogen -46 (-43) -107 (-77) -61 (-34)
Helium -46 (-43) -35 (-37) 11 (6)
9. 196
The liquid formed is separated from the low-pressure gas stream in the
liquid receiver and is ultimately withdrawn as the product. The amount of
low-pressure gas recycled to the compressor is now significantly reduced,
which cuts back on the cooling effect in the heat exchanger. With the
addition of makeup gas to the low-pressure side of the compressor to
compensate for the liquid product being withdrawn, a steady-state is
reached in the liquefaction system and no further cooling can be achieved.
The first law of thermodynamics for a steady-state flow system is
v
m
q
m
m
PE
KE
h
.
.
.
)
)
(
0
where PE and KE are, respectively, the potential and kinetic energy per
unit mass. The enthalpy, h, heat term, q, and work term, wS, are on a
mass basis, and m represents the mass flow rate. Application of the
equation to the components inside the thermodynamic boundary gives the
relation
l
q
h
where the overall enthalpy change of the gas, h, on a mass basis equals
the heat leak qL , per unit mass of gas. On a per unit of mass flow of
entering gas, and defining f = m1 / m2 , the fraction of entering gas
withdrawn as a liquid, the equation becomes:
FIGURE 2 : SIMPLE JOULE-THOMSON LIQUEFACTION CYCLE.
10. 197
l
q
h
h
f
fh
1
3
2 )
1
(
2
3
1
3
h
h
q
h
h
f
l
For a given system, h2, h3, and qL are essentially fixed, so the only
way to increase liquefaction is to decrease the inlet gas enthalpy, h1,
which is done by increasing the inlet pressure, assuming that the
compressor outlet gas temperature remains constant. Thus, more
compressor work should lead to more liquid production.
2.2.2 EXPANDER CYCLES
The point was made during the discussion of the Joule-Thomson
expansion that it was a thermodynamically irreversible process.
Expansion of high-pressure gas to the lower pressure in a reversible
or nearly reversible manner provides two distinct improvements over
the Joule-Thomson expansion. First, in the reversible expansion, a
large fraction of the work required to compress the gas can be
recovered and used elsewhere in the cycle. This property provides an
increase in cycle efficiency. Second, the reversible process will result
in a much larger cooling effect.
Several options are available for selection of expanders for LNG use,
both in the type of expander and in the basic cycle itself.
Expanders are basically compressors with the flow reversed and, as
with compressors, positive displacement and dynamic expanders are
available. In 1902, Georges Claude pioneered expander use in air
liquefaction. Claude’s expander was a reciprocating machine, as were
most early machines used in cryogenic processes, such as those
developed by Heylandt in 1912 and later by Collins
Reciprocating expander inefficiencies to four causes:
Inlet and outlet valve losses
Incomplete expansion
Heat transfer
Piston friction
11. 198
Similar to dynamic compressors, dynamic expanders can be centripetal
flow or axial flow. In centripetal machines (i.e., turboexpanders), the gas
enters through nozzles around the periphery of the wheel, expands, and
transmits work to the wheel, which causes it to rotate, and finally exhausts
at low pressure at the axis of the machine. Axial-flow expanders have as
their counterparts steam turbines. Centripetal machines have isentropic
efficiencies on the order of 85 to 90%, whereas axial-flow expanders are
about 80% efficient (Swearingen, 1968). Turboexpanders are high-speed
machines, generally designed to operate from 10,000 to 100,000 rpm,
depending on the throughput .
For design purposes, several techniques may be used to compute the
expected enthalpy change, but the simplest and apparently satisfactory
method is to use the ideal value from a P-H or T-S diagram, and correct
this value with the anticipated turboexpander efficiency (Swearingen,
1968; Williams, 1970). The work generated in the expander must be
removed from the system if the full thermodynamic efficiency of the cycle
is to be realized. The general practice in large-scale operations is to
couple the turboexpander to a gas compressor. Reciprocating expanders
would naturally be coupled with reciprocating compressors, and
turboexpanders coupled with centrifugal compressors. The available
expander work can be very large. Swearingen (1968) states that a
turboexpander handling500 MMcfd (14 Sm3/d) at pipeline pressure would
develop 10,000 hp (7,500 kW). Surprisingly, the turbine rotor would only
be 18 inches in diameter. In small-scale operations, recovery of the
expander work is often not economicallyfeasible. In this case, the
turboexpander is simply coupled to a braking device that dissipates the
work. Expander−compressor combinations require considerable care in
their selection and operation. Swearingen (1970) and the Engineering
Data Book (2005b) discuss what must be considered in the selection,
operation, and maintenance of turboexpanders. As mentioned previously,
several options are available in the type of expander cycle. All expander
cycles fall into two groups: closed cycles and open cycles. Note that most
expander cycles have J-T valves as well as turboexpanders .
2.2.2.1 Closed Cycles
In a closed expander cycle, the fluid being expanded is not the fluid to be
liquefied; the expander simply acts as an external source of refrigeration,
similar to the nitrogen may be used in a closed expander system to liquefy
natural gas. A very nitrogen is expanded, and the cold gas is then used to
cool and liquefy the natural gas stream. Actual cycles for producing LNG
are far more complex. Hathaway and Lofredo (1971) provide a complete
process flow sheet for a plant that has four warm heat exchangers, one
12. 199
large nitrogen compressor, and three turboexpander/ compressor
combinations.
The closed cycle has several advantages over the open cycle, in
which the natural gas itself is expanded. First, if nitrogen is used, safety is
enhanced, because the closed cycle reduces the number of processing
steps in which flammable natural gas is used. Second, the closed nitrogen
cycle has been reported (Anonymous, 1970) to require simpler and less
expensive shutdown procedures than its opencycle counterpart and
appears to be the most economical process under many conditions. Finally,
because the natural gas is not passing through the expander, the process
purification system is not so critical. Gas passing through the highspeed
expander must be free of condensed phases and any components that
solidify at the expander exhaust temperature, because deposition on the
rotor will destroy it.
2.2.2.2 Open Cycles
An open expander cycle uses the gas being liquefied as the expanding
fluid and has the advantage over the closed cycle of being less
complex. A basic expander the expander is simply used as a source of
refrigeration, and the high-pressure gas is liquefied as it expands
through the Joule-Thomson valve. The first law of thermodynamics for a
steady-state flow system applied to the two heat exchangers, the
expander, and the liquid receiver gives the following equation:
2
3
6
4
2
3
1
3 )
(
h
h
h
h
e
h
h
q
h
h
f l
FIGURE 3 : SIMPLE CLOSED YCLE LIQUEFACTION PROCESS
13. 200
where e is the fraction of the gas going to the expander and (h4 − h6) is
the work done by the expander. The quantities m, m , and m represent the
mass flow rate into the liquefier, the mass flow rate of liquefied product,
and the mass flow rate to the expander, respectively. A more detailed
analysis of the overall system is possible (Dodge, 1944). The cycle shown
is only one of a number of possible process arrangements.
2.3 LNG Technology overview
Three main processes:
Cascade cycle:
Separate refrigerant cycles with propane, ethylene and
methane
(Phillips, Atlantic LNG, Trinidad)
FIGURE 4 : OPEN CYCLE EXPANDER PLANT
14. 201
Mixed refrigerant cycle:
Single mixed refrigerant (SMR) (PRICO)
Propane pre-cooled mixed refrigerant (C3/MR) (APCI)
Dual mixed process (DMR) (Shell, Sakhalin)(Liquefin )
Mixed Fluid Cascade Process (MFCP) (Statoil/Linde)
Expander cycle
2.4 LNG Process Systems
Common LNG Process Systems
Phillips Cascade Process
Three Pure Components
• Propane
• Ethylene
• Methane
APCI (Air Products)
Two Components
Propane
Mixed Component Refrigerant
New Emerging LNG Process Systems
Linde Process
Three Mixed Refrigerants
Axens Liquefin Process
Dual Mixed Refrigerant
Shell Process
Dual Mixed Refrigerant
16. 203
2.5 LNG Liquefaction Technology
LNG liquefaction technologies in commercial use:
Air Products & Chemicals Propane Pre-Cooled Mixed
Refrigerant. (APCI), Dual Cycle Refrigeration
Phillips Optimized Cascade (POC), Triple Cycle
Refrigeration
Shell DMR, Similar to APCI, Dual Cycle, both MR
Linde MFCP (Multi Fluid Cycle Process)
Black & Veatch Pritchard, Poly Refrigerant Integral Cycle
Operation II (PRI CO II), Single Cycle Refrigeration
Other LNG Liquefaction Processes
BHP/Linde (Nitrogen, Single Cycle Refrigeration)
TEALARC, Similar to PRICO, Single Cycle MR
TEALARC Conventional Cascade
IFP/CII-1 SMR, Similar to PRICO, Single Cycle MR
IFP/CII-2 DMR, Similar to Shell DMR, Dual Cycles MR
IFP/CII is now Axen Liquefin
2.6 Classification of LNG Liquefaction
Technology
Single Cycle Refrigeration (SCR)
SCR Pure Component
BHP/Linde (Nitrogen Cycle)
SCR Mixed Components
TEAL (Skikda Unit 10, 20, 30, Algeria)
PRICO (Skikda Unit 40, 50, 60, Algeria)
APCI (Marsa El-Brega, Lybia)
CII/BP (proposed concept)
Dual Cycle Refrigeration(DCR)
Propane/MR Cycles (APCI)
Brunei, Algeria Arzew,
Das-Island, Badak,
Arun, Malaysia-1/2/3,
Australia NWS 1/2/3,
QatarGas, RasGas,
Oman
17. 204
Dual MR
Australia NWS-4,
Russia Sakhalin (being built or designed)
Triple Cycle Refrigeration
Propylene/Ethylene/Meth ane
Algeria - CAMEL, Kenai (Alaska), Trinidad
2.6.1 Propane Precooled Mixed Refrigerant
(PPMR™)/C3 MR/APCI Process
The Propane Precooled Mixed Refrigerant process developed by Air
Products & Chemicals Int. started to dominate the industry from the late
1970s on. This liqufaction technology is used in egyptain spanish gas
company (SEGAS).
This process accounts for a very significant proportion of the world's
baseload LNG production capacity. Train capacities of up to 4.7 million
tpy were built or are under construction. There are two main refrigerant
cycles. The precooling cycle uses a pure component, propane. The
liquefaction and sub-cooling cycle uses a mixed refrigerant (MR)
made up of nitrogen, methane, ethane and propane. The precooling
cycle uses propane at three or four pressure levels and can cool the
process gas down to -40 C. It is also used to cool and partially liquefy
the MR. The cooling is achieved in kettle-type exchangers with propane
refrigerant boiling and evaporating in a pool on the shell side, and with
the process streams flowing in immersed tube passes. A centrifugal
compressor with side streams recovers the evaporated C3 streams and
compresses the vapour to 15 - 25 bara to be condensed against water or
air and recycled to the propane kettles.
In the MR cycle, the partially liquefied refrigerant is separated into vapour
and liquid streams that are used to liquefy and sub-cool the process
stream from typically -35 C to between -150 C - -160 C. This is
carried out in a proprietary spiral wound exchanger, the main cryogenic
heat exchanger (MCHE).
The MCHE consists of two or three tube bundles arranged in a
vertical shell, with the process gas and refrigerants entering the
tubes at the bottom which then flow upward under pressure.
The process gas passes through all the bundles to emerge liquefied at
the top. The liquid MR stream is extracted after the warm or middle
bundle and is flashed across a Joule Thomson valve or hydraulic
expander onto the shell side. It flows downwards and evaporates,
18. 205
providing the bulk of cooling for the lower bundles. The vapour MR
stream passes to the top (cold bundle) and is liquefied and sub-cooled,
and is flashed across a JTvalve into the shell side over the top of the cold
bundle.
It flows downwards to provide the cooling duty for the top bundle and,
after mixing with liquid MR, part of the duty for the lower bundles.
The overall vaporised MR stream from the bottom of the MCHE is
recovered and compressed by the MR compressor to 45 - 48 bara. It is
cooled and partially liquefied first by water or air and then by the
propane refrigerant, and recycled to the MCHE. In earlier plants all
stages of the MR compression were normally centrifugal, however, in
some recent plants axial compressors have been used for the LP stage
and centrifugal for the HP stage. Recent plants use Frame 6 and/or
Frame 7 gas turbine drivers. Earlier plants used steam turbine drivers.
FIGURE 5 : C3/MR SIMPLIFIED SCHEME
19. 206
2.6.2 AP-XTM
For very large, single train LNG liquefaction plants, the Air Products AP-X
TM
LNG process offers an attractive means to significantly reduce the specific
capital cost of LNG. The AP-X
TM
LNG process can be configured with
propane or mixed refrigerant pre-cooling as required by ambient conditions
and plant location, and it is expected to provide the lowest unit cost of LNG
on the market today. The economy of scale is superior to splitting equipment
into two process trains or duplicating equipment. The first AP-X
TM
trains to be
built will have a nominal capacity of 7.8 MTA LNG. The AP-X
TM
concept is
depicted in Figure. This process retains the concept of single train equipment
up to the highest capacity and can employ the Split MR
TM
compressor
configuration. The addition of the sub-cooling cycle using nitrogen as working
fluid reduces the propane refrigeration and mixed refrigerant compression
duty per tonne of LNG. Plant capacities can be steadily increased from 7.5 to
10 MTA by increasing the amount of driver power consumed using two or
three frame 9 gas turbines. It has been shown that natural gas liquefaction
capacities of up to 10 MTA and beyond can be achieved with single train
compression equipment for mixed refrigerant and nitrogen. Using AP-X
TM
technology at approximately 7.5 MTA LNG the number of propane
compression casings is increased to two in the same arrangement as for the
C3MR process. Air Products manufactures and supplies the cryogenic
machinery necessary for the nitrogen expansion required in the AP-X
TM
process from its manufacturing facility in the United States.
FIGURE 6: AP-X PROCESS
20. 207
Figure demonstrates the evolution of train sizes over the years by
showing the train capacity for a representative sample of LNG liquefaction
facilities. From the 1960’s until about 2000, train capacities increased from
less than 0.5 MTA to about 3 MTA. Since 2000, train capacity has increased
to about 5 MTA.
The industry is about to take a very substantial step towards increasing
train capacity with the implementation of the AP-XTM process in Qatar in
early 2008. Six trains are currently under construction, each with a
nameplate capacity of 7.8 MTA. The AP-XTM process is an extension of the
C3MR process, maintaining its advantages as well as allowing for a
substantial increase in train capacity.
FIGURE 7: INCREASING IN LNG TRAIN CAPACITY
21. 208
By using SplitMR® technology in which a portion of the mixed refrigerant
compression requirement is driven by the same driver as used for propane
compression (Figure 5b), the power balance becomes evenly split. This
allows for full utilization of gas turbine power and increases train capacity
for the same number of drivers and compressors. At the time of the writing
of this paper, there are four trains in operation with this technology,
RasGas Trains 3, 4, & 5 and Segas. There are also several other trains
currently under construction that will use this technology. In addition, this
concept of using one driver for multiple refrigeration services can be
extended to include other cycles in addition to the C3MR and AP-XTM
processes.
FIGURE 8:(A) SEPARATE DRIVER CONFIGURATION FOR PROPANE AND MR COMPRESSORS (B) SPLITMR®
MACHINERY CONFIGURATION
Propane Casing Arrangement. A four stage single casing propane
compressor can be utilized for C3MR train capacities up to about 5 MTA
and AP-XTM train capacities up to about 8 MTA. For higher train
capacities, two casings may be required due to aerodynamic constraints.
There are several options. One option is to use two 50% units which
permits running at a reduced capacity if one unit must be taken offline.
Another option is to use a 1,4-2,3 split propane compressor casing in
a series arrangement as shown in Figure.
Stages 1 and 4 are in the first casing and stages 2 and 3 in the
second casing. The inlet pressures to the four stages may be
22. 209
different than the single casing compressor design and are
adjusted to maximize efficiency. Note that the discharges from the
third and fourth stages are at the same pressure since they are
connected to a common condenser.
Each stage would typically have multiple impellers. This series arrangement
minimizes the complexity of the suction piping and avoids the potential for
imbalances in compressor duties that can occur with parallel compression.
FIGURE 9:(A) SINGLE CASING PROPANE COMPRESSOR (B) 1,4-2,3 SPLIT CASING PROPANE COMPRESSOR
Compressor Drivers. There are many compressor driver options
available for theC3MR and AP-XTM processes. They include steam
turbines, gas turbines (e.g. Frame 5, 6, 7, 9 and aero-derivatives), and
electric motors. Most of the C3MR projects being executed today use
Frame 7 gas turbines. They have an ISO power of approximately 86
MW at 3600 rpm. The power and efficiency is significantly improved
over the smaller Frame 5 and 6.
Frame 9 gas turbines are being used for the AP-XTM trains in Qatar.
Although widely used in the power generation industry, this will be the
first application for LNGliquefaction. The Frame 9 full load string tests
for driving the propane, mixed refrigerant, and nitrogen compressors
have been completed successfully. Mechanical drive Frame 9 gas
turbines can produce about 50% more power than the Frame 7. They
are about 4% more efficient over their speed range. Because they
operate at a slower speed (3000 rpm), compressors can be designed
for more flow capacity than those for a Frame 7.
23. 210
2.6.3 CASCADE CYCLES
A cascade system consists of two separate single-stage refrigeration
systems: a lower system that can better maintain lower evaporating
temperatures and a higher system that performs better at higher
evaporating temperatures. These two systems are connected by a cascade
condenser in which the condenser of the lower system becomes the
evaporator of the higher system as the higher system’s evaporator takes on
the heat released from the lower system’s condenser.
It is often desirable to have a heat exchanger between the liquid refrigerant
from the cascade condenser and the vapor refrigerant leaving the
evaporator of the lower system. The liquid refrigerant can be sub-cooled to
a lower temperature before entering the evaporator of the lower system, as
shown in the next figure. Because the evaporating temperature is low, there
is no danger of too high a discharge temperature after the compression
process of the lower system.
FIGURE 10 : REFRIGERATION CASCADE SYSTEM LAYOUT. FIGURE 11 : (P-H) DIAGRAM
24. 211
Advantages and Disadvantages
The main advantage of a cascade system is that different refrigerants,
equipment, and oils can be used for the higher and the lower systems. This
is especially helpful when the evaporating temperature required in the lower
system is less than -60°C.
One disadvantage of a cascade system is the overlap of the condensing
temperature of the lower system and the evaporating temperature of the
higher system for heat transfer in the condenser. The overlap results in
higher energy consumption. Also a cascade system is more complicated
than a compound system.
The performance of the cascade system can be measured in terms of 1 kg
of refrigerant in the lower system, for the sake of convenience. If the heat
transfer between the cascade condenser and the surroundings is ignored,
then the heat released by the condenser of the lower system is equal to the
refrigerating load on the evaporator of the higher system.
Applications of cascade systems
1. Liquefaction of natural gas and petroleum vapours
2. Liquefaction of industrial gases
3. Manufacturing of dry ice
4. Deep freezing etc.
5. Medical applications
FIGURE 12: CONOCO PHILLPS OPTIMIZED CASCADE
25. 212
Optimum cascade temperature:
For a two-stage cascade system working on Carnot cycle, the optimum
cascade temperature at which the COP will be maximum, Tcc,opt is given by:
c
e
ccopt T
T
T
Where Te and Tc are the evaporator temperature of low temperature
cascade and condenser temperature of high temperature cascade,
respectively.
Refrigeration effect
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.
In a refrigeration cycle, energy is transferred from lower to higher
temperature levels economically by using water or ambient air as the
ultimate heat sink. If ethane is used as a refrigerant, the warmest
temperature level to condense ethane is its critical temperature of about
90°F. This temperature requires unusually high compression ratios making
an ethane compressor for such service complicated and uneconomical.
Also in order to condense ethane at 90°F, a heat sink at 85°F or lower is
necessary. This condensing temperature is a difficult cooling water
requirement in many locations. Thus a refrigerant such as propane is
26. 213
cascaded with ethane to transfer the energy from the ethane system to
cooling water or air.
2.6.3.1 ConocoPhillips optimized Cascade
One example of cascade cycle is ConocoPhillips currently has at least two
trains in operation: Atlantic LNG, and Egyptian LNG. More trains are being
constructed since this process is expanding to compete with the APCI. It
shares about 5% of the world’s LNG production and it has been in operation
for more than 30 years.
The process uses a three stage pure component refrigerant cascade of
propane, ethylene, and methane .The pretreated natural gas enters the first
cycle or cooling stage which uses propane as a refrigerant. This stage cools
the natural gas to about -35o
C and it also cools the other two refrigerants to
the same temperature. Propane is chosen as the first stage refrigerant
because it is available in large quantities worldwide and it is one of the
cheapest refrigerants. The natural gas then enters the second cooling stage
which uses ethylene as the refrigerant and this stage cools the natural gas
to about -95o
C. At this stage the natural gas is converted to a liquid phase
(LNG) but the natural gas needs to be further sub cooled so the fuel gas
produced would not exceed 5% when the LNG stream is flashed. Ethylene
is used as the second stage refrigerant because it condenses methane at a
pressure above atmospheric and it could be also condensed by propane.
After methane has been condensed by ethylene, it is sent to the third stage
where it sub cools the natural gas to about - 155o
C then it is expanded
through a valve which drops down the LNG temperature to about - 160o
C.
Methane is sent back to the first cooling stage and the LNG stream is
flashed into about 95% LNG (which is sent to storage tanks) and 5% fuel
gas used as the liquefaction process fuel. Methane is used as the sub
cooling stage refrigerant because it could sub cools up to -155 oC and it is
available in the natural gas stream so it is available at all times and at lower
costs
.
27. 214
We will now attempt to perform a simulation of this simple process. We first
recognize that the boiling points of each of the refrigerants will limit the
temperatures at the outlet of each exchanger.
Refrigerant Boiling point (o
c)
Propane -42
Ethylene -103
Methane -161
The duty curve of the simple cascade is shown in the next figure:
FIGURE 13 : SIMPLE CONOCOPHILLIPS CASCADE LNG COOLING CURVE
One source of thermodynamic inefficiency is the finite temperature
difference that must be present for heat transfer. The maximum
thermodynamic efficiency in a liquefaction cycle is realized when the
heating curve of the refrigerant corresponds to the cooling curve of the
natural gas being liquefied. This relation means that both ΔT (and thus
ΔS) are zero for the heat transfer process (Dodge, 1944). Figure 27 is a
schematic of a cooling curve for a natural gas system and heating curves
for a mixed refrigerant system and a three-fluid classical cascade system.
TABLE 2: REFRIGERANTS BOILING POINTS
28. 215
Temperature
o
C
The natural gas and mixed refrigerant curves show marked curvature
because the fluids are mixtures. Thermodynamically, the mixed refrigerant
comes closest to a reversible process because it minimizes the temperature
difference between the two fluids.
The classical cascade attempts to approximate the cooling curve by use
of a series of refrigerants (usually three) in separate loops. Use of more
than three refrigerants allows a closer approximation to the cooling curve
but with the penalty of extra equipment and added cycle complexity. Both
mixed refrigerant cascades and classical cascades will be discussed in
more detail in subsequent sections.
The classical cascade was one of the earliest cycles developed and used
in the liquefaction of the so-called permanent gases, helium and hydrogen.
The classical cascade is attractive because it can be very efficient
thermodynamically. Recently, the classical cascade has been modified
with the introduction of the mixed refrigerant cascade. Because both types
of cycles are in use for LNG production, we discuss the more important
features of both processes.
Heat duty ( 106 K
Kcal/hr )
FIGURE 14 : CASCADE HEATING CURVE
29. 216
2.6.3.2 Hypothetical two - fluid Cascade Cycle Classical Cascade
The classical cascade process starts with a vapor that can be liquefied at
ambient temperature by the application of pressure only. The liquid formed
by pressurization is then expanded to a lower pressure, which results in a
partial vaporization and cooling of the remaining liquid.
This cold liquid bath is then used to cool a second gas so that it may also
be liquefied by the application of moderate pressure and then expanded to
a lower pressure.
The temperature reached in the expansion of the second liquid will be
substantially lower than that achieved by the expansion of the first liquid. In
principle, any number of different fluids may be used and any desired
temperature level can be reached by use of the appropriate number of
expansion stages. In practice, however, three fluid and three levels of
expansion are normal.
The cascade process differs from the previously described liquefaction
techniques, in that the cooling is obtained principally from external circuits,
with one final Joule-Thomson expansion for liquefaction. The Joule-
Thomson expansion and many turboexpander processes use the gas to be
processed as their refrigerating medium
FIGURE 15 : SCHEMATIC OF A HYPOTHETICAL TWO - FLUID CASCADE CYCLE.
30. 217
Following the flow from the compressor discharge (1) for the high-
temperature working fluid, the pure fluid is cooled in heat exchanger E-
1, expanded through a Joule-Thomson valve (2), and goes to liquid-
vapor receiver R-1 (3).
The liquid in the receiver cools the gas feed, and exiting vapor flows
back through heat exchanger E-1 to the second-stage compressor
suction (4).
In addition to cooling the gas feed, part of the liquid from the receiver (3)
flows through heat exchanger E-2, where it is cooled before expansion
through a Joule-Thomson valve (5), which creates a liquid−vapor mixture in
the receiver R-2 (6). Both the temperature and the pressure of the liquid−
vapor mixture in receiver R-2 are lower than in receiver R-1.
The liquid in receiver R-2 further cools the gas feed. The vapor passes
through a liquid separator, two heat exchangers E-1 and E-2, and goes to the
compressor suction (7). Liquid from receiver R-2 goes to heat exchanger E-3
(8) to cool the pure fluid in the second cascade before expansion through a
Joule-Thomson valve (9) to create a cooler liquid−vapor mixture in receiver
R-3 (10). Essentially the same process is repeated for the second working
fluid, which goes through three stages of refrigeration. The feed gas
progressively cools until it leaves receiver R-5. It then passes through the
final Joule-Thomson valve, and becomes a liquid−vapor mixture at the final
desired temperature and pressure. The liquid goes to storage, while the cold
vapor is used for heat exchange before being recycled or used for fuel.
31. 218
For simplicity this loop is not shown. Although the figure and its
discussion are simplified, they do illustrate the fundamental features of a
classical cascade. Industrial cascades for LNG production use three pure
fluids with two or three stages of refrigeration for each fluid. Andress
(1996) discusses an industrial application of the classical cascade.
2.6.3.3 Mixed-Refrigerant Cascade
As mentioned earlier, the maximum thermodynamic efficiency in a
liquefaction cycle is realized when the heating curve of the refrigerant
corresponds to the cooling curve of the natural gas being liquefied. The
classical cascade attempts
To approximate the cooling curve by use of three pure refrigerants in
separate loops. However, replacement of several pure refrigerants in
separate cycles with a single refrigerant composed of many components
condensing at various temperatures in one cycle makes possible a more
closely matched cooling curve of the natural gas. In addition, only a single
compressor is required for the refrigerant. This feature is the principle for
the mixed-refrigerant cascade, the MRC (Linnett and Smith, 1970), the
autorefrigerated cascade, the ARC (Salama and Eyre, 1967), and the one-
flow cascade OFC process
FIGURE 16: QUALITATIVE PLOT OF VAPOR PRESSURE FOR TWO PURE REFRIGERANT FLUIDS THAT SHOWS
THES OVERLAPPING REGION NEEDED FOR CASCADE CYCLES
32. 219
Closed Cycles Figure 34 shows a schematic of a simple closed
cycle(single-flow mixed-refrigerant). The refrigerant would probably be a
mixture of nitrogen, methane, ethane, propane, butane, and perhaps
pentane; the exact composition depends upon the composition of the
natural gas being liquefied. The refrigerant mixture is compressed and then
partially condensed in a water-cooled exchanger. The refrigerant then
undergoes a series of pressure reductions and liquid-vapor separations to
provide the cold fluid needed in the heat exchangers to liquefy the natural
gas. The temperatures attained in the various heat exchangers depend on
the composition of the refrigerant and the pressure to which the gas is
initially compressed. These operating parameters are selected to
approximate the cooling curve of the natural gas being liquefied. In this
cycle, the natural gas passes through all four heat exchangers in series and
is then expanded into a industrial example of this type system.
FIGURE 17 : SCHEMATIC OF A SIMPLE CLOSED CYCLE(SINGLE-FLOW MIXED-REFRIGERANT).
33. 220
Open Cycle—In the open cycle system shown in simplified form in Figure
36 the natural gas stream to be liquefied is physically mixed with the
refrigeration cycle stream. This mixing can take place before, during, or
after the compression process, depending upon the pressure at which the
natural gas feed is available. After compression, the united gas streams
are partially condensed in a water-cooled or air-cooled heat exchanger,
and then separated into liquid and vapor fractions in a separator. From this
point, the process is similar to the closed cycle system. That is, the liquid
fractions are expanded, which results in vaporization and cooling, and
these cold streams are used as the coolant in the heat exchangers. The
vapor from the last separator is condensed in the final heat exchanger,
and then expanded and separated into an LNG product and a flash gas
that would generally be used for plant fuel. To prevent heavy
hydrocarbons from plugging in the low-temperature portion of the cycle a
liquid slipstream may be withdrawn at a relatively high temperature
Summary—The mixed refrigerant cycles discussed above possess several
advantages over the classical cascade system. The principal advantage
is the use of a single-compressor refrigerant system (excluding the
propane precooling) in place of the three refrigerant compressors and
cycles of the standard cascade. This configuration provides simplification
in instrumentation and piping and a better use of compression power.
Further advantages include the ability to readily change the composition of
the refrigerant for cycle optimization, should the composition of the feed
gas change, and the ability to extract cycle refrigerants directly from the
feed gas.
The disadvantage when compared with the standard cascade is the
necessity for having facilities to recover, store, and blend the components
in the refrigerant cycle.
FIGURE 18 : SCHEMATIC OF THE POPULAR OPEN CYCLE MIXED REFRIGERANT SYSTEM
34. 221
The Mixed Fluid Cascade Process (MFCP) developed by Statoil/Linde is
shown in Figure. The purified natural gas is precooled, liquefied, and
subcooled by means of three separate mixed refrigerant cycles. The cold of
the precooling cycle is transferred to the natural gas via two plate fin heat
exchangers, whereas the cold of the liquefaction and Subcooling cycle is
transferred via two spiral wound heat exchangers by the other two
refrigerants (Bach, 2000). The refrigerants are made up of components
selected from methane, ethane, propane, and nitrogen. The three
refrigerant compression systems can have separate drivers or integrated to
have two strings of compression. The process has been designed for large
LNG trains (>4 MTPA). The MFCP is a classic cascade process, with the
important difference that mixed component refrigerant cycles replace single
component refrigerant cycles, thereby improving the thermodynamic
efficiency and operational flexibility.
FIGURE 19:STATOIL/LINDE LIQUEFACTION PROCESS
35. 222
2.6.4Single Mixed Refrigerant Loop Process
The large and expensive LNG projects are often based on
processes which require multiple refrigeration systems. The PPMR
Process requires two sequential refrigeration systems to
accomplish the LNG production task. The best way to reduce the
amount of process equipment is the utilization of a single
refrigeration system. Black & Veatch Pritchard has developed a
mixed refrigerant process, (PRICO®), which has been
successfully used.
The simplest of the mixed refrigerant natural gas liquefaction
processes that do not have any phase separators. This process is
also sometimes called the single refrigerant process. This is a
single mixed refrigerant loop and a single refrigeration
compression system. The mixed refrigerant is made up of
nitrogen, methane, ethane, propane, and iso-pentane. The
component ratio is chosen to closely match its boiling curve with
the cooling curve of the natural gas feed. The closer the curves
match, the more efficient the process is. The mixed refrigerant is
compressed and partially condensed prior to entering the insulated
enclosure for the highly efficient plate fin heat exchangers,
collectively known as the ―cold box.‖ The cold box contains a
number of plate fin heat exchanger cores, which allow multiple
streams to be heated/cooled to extremely close temperature
differences. The MR is then fully condensed before it is flashed
across an expansion valve, which causes a dramatic reduction in
temperature.
FIGURE 20 : SINGLE-STAGE MIXED REFRIGERANT LNG PROCESS WITHOUT PHASE SEPARATORS
36. 223
This vaporizing liquid is used to condense the MR stream, as well as the
natural gas feed stream. The warmed low pressure MR vapor is then sent
to the compressor for Recompression. The natural gas feed stream enters
the cold box and is initially cooled to about –35°C(–31°F) with a propane
chiller.
The gas is then sent to a separator to remove the heavier components,
which are sent to the fractionation plant. The expanded MR then cools the
light components, primarily methane, to the liquefaction temperature.
Use of a single refrigeration system eliminates all the equipment necessary
to link the sequential refrigeration systems in other LNG processes. The
single refrigeration loop greatly simplifies the piping, controls, and
equipment for the liquefaction unit that translates into capital cost savings of
up to 30 percent.
Since the system uses a single mixed refrigerant, there are further
simplification steps which are important to decrease the investment cost.
With a single mixed system, refrigerant makeup can come from storage,
import, or can be made up from the feed gas. Only a small skid mounted
fractionator is required to produce refrigerant makeup streams from the feed
gas. The system is quite small since it is only for occasional makeup, and
high purity streams are not required. This simplification eliminates many
large pieces of equipment. Thus, the simplification resulting from the single
mixed refrigerant makeup philosophy saves capital, versus either the
propane precooled or cascade system (Price et al., 2000). However, the
single cycle process is not as efficient as a multiple cycle process, as it is
very unlikely that it will ever be used in large baseload LNG plants. It is
mainly used for peak shaving applications, due to its lower capital cost
compared to multiple cycle processes.
FIGURE 21: OPTIMIZED SINGLE REFRIGERATION CYCLE OPTIMIZED SMR, PRICO II)
37. 224
z
FIGURE 23: BLACK & VETACH PRICO PROCESS
FIGURE 22 : OPTIMIZED SINGLE REFRIGERATION CYCLE (OPTIMIZED SMR, IFP/CII-1)
38. 225
2.6.5Liquefin™ Process
IFP and Axens have developed the Liquefin™ process with the aim of
producing LNG cheaper than with any other process, at good conditions
of reliability, safety, and friendlier to the environment. With this process
very high capacities can be reached with a simple scheme and
standard compressors (Martin et al., 2003). It is a two mixed refrigerant
process designed for LNG base load projects of train sizes up to 6
MTPA.
The process operates according to the basic flow scheme presented in
Figure 39. All cooling and liquefaction is conducted in Plate Fin Heat
Exchangers (PFHE) arranged in cold boxes. The PFHE arrangement is
at the heart of the liquefaction technology. The refrigerants are made up
of components from methane, ethane, propane, butane, and nitrogen.
The first mixed refrigerant is used at three different pressure levels, to
precool the process gas, and precool and liquefy the second mixed
refrigerant. The second mixed refrigerant is used to liquefy and subcool
the process gas. Using a mixed refrigerant for the precooling stage, the
temperature is decreased down to a range of –50°C to –80°C
depending on refrigerant composition. At these temperatures, the
cryogenic mixed refrigerant can be completely condensed, no phase
separation is necessary, and moreover, the quantity of cryogenic
refrigerant is substantially reduced. The weight ratio between the
cryogenic mixed refrigerant and LNG can be lower than unity.
The overall necessary power is decreased, as the quantity of cryogenic
mixed refrigerant is lower; and a good part of the energy necessary to
condense it is shifted from the cryogenic cycle to the prerefrigeration
cycle. Moreover, this shifting of energy allows a better repartition of the
exchange loads; and the same number of cores in parallel can be used
between the ambient and cryogenic temperature, allowing a very
compact design for the heat exchange line.
A very significant advantage of this new scheme is the possibility to
adjust the power balance between the two cycles, making it possible to
use the full power provided by two identical gas drivers . This process
was initially developed to obtain a 50%–50% sharing of power between
the liquefaction refrigerant cycle and the precooling refrigerant cycle.
The advantages of this process are in the use of a single quality of
liquefaction refrigerant and a simplified PFHE type liquefier .
39. 226
The Liquefin™ process is flexible, and offers more than one possibility
to reach large and highly competitive capacities; either by using very
large gas turbines (combined cycle) to produce electricity, and using
large electrical motors (up to 70 MW) in parallel on each cycle.
Also or by using larger gas turbines. With Liquefin, this would allow
capacities of 7 to 8 MTPA with only two main drivers.
The process represents a real breakthrough, as the plant capacity can be
chosen considering mainly the economics and the marketing
possibilities, without being bothered by technical hindrances. A total
cost reduction per ton LNG is reported to be 20% compared to other
processes. The cost reductions drive from: (1) increasing the plant
capacity, (2) reducing the heat exchanger costs, (3) all over plate fin
heat exchangers, (4) compact plot area, and (5) multi sourcing of all
equipment, including heat exchangers (Mّlnvik, 2003).
The Liquefin™ process uses two mixed refrigerant circuits and PFHE
cold boxes designed to match very accurately the cooling curve of
natural gas. The refrigerant cycle is about 6–7% more efficient than the
other alternatives. If we add to this the effectiveness of the plate fin heat
exchangers, which have a high surface-to-volume ratio, lower pressure
drop than conventional units, and efficient heat transfer, the overall
process is around 15% more efficient than the established competitors
(Knott, 2001).
FIGURE 24: AXENS LIQUEFIN
40. 227
2.6.6 Dual Mixed Refrigerant (DMR) Process
Shell developed a Dual Mixed Refrigerant (DMR) process for
liquefaction, as shown in Figure 36, with two separate mixed
refrigerant cooling cycles, one for precooling of the gas to
approximately –50°C (PMR cycle) and one for final cooling and
liquefaction of the gas (MR cycle). This concept allows the designer
to choose the load on each cycle. It also uses proven equipment, e.g.
spiral wound heat exchangers (SWHEs), throughout the process.
The DMR process is the basis of the Sakhalin LNG plant, with a
capacity of 4.8 MTPA per train (Smaal, 2003).
Process configuration is similar to the Propane Precooled Mixed
Refrigerant (PPMR) process, but with the precooling conducted by a
mixed refrigerant (made up mainly of ethane and propane) rather
than pure propane. PPMR vapor from the precool exchangers is
routed via knockout vessels to a two stage centrifugal PPMR
compressor. Desuperheating, condensation, and subcooling of the
PPMR is achieved by using induced draft air coolers. The PPMR
compressor is driven by a single gas turbine. Another main difference
is that the precooling is carried out in SWHEs rather than kettles. The
cooling duty for liquefaction of the natural gas is provided by a
second mixed refrigerant cooling cycle (MR cycle). The refrigerant of
this cycle consists of a mixture of nitrogen, methane, ethane, and
propane. Mixed refrigerant vapor from the shell side of the main
cryogenic heat exchanger is compressed in an axial compressor
followed by a two stage centrifugal compressor. Intercooling and
initial desuperheating is achieved by air cooling. Further
desuperheating and partial condensation is achieved by the PMR
precooling cycle. The mixed refrigerant vapor and liquid are
separated and further cooled in the main cryogenic heat exchanger,
except for a small slipstream of vapor MR, which is routed to the end
flash exchanger (Dam and Ho, 2001).
FIGURE 25: DUAL MIXED REFRIGERANT (DMR) PROCESS
41. 228
The DMR process has also employed double casing instead of single
casing equipment. This is a reliable method to bring the propane MR
process closer to a capacity of 5 MTPA. With a single precooling cycle and
two parallel mixed refrigerant cycles, the capacity can also be boosted up
to 8 MTPA. The process can either use propane or an MR in precooling.
Proven refrigerant cycles can be used without step changes in technology.
The capacity can be increased further with different (larger) drivers.
Another possibility for the propane-MR process is to transfer power from
the propane cycle to the mixed refrigerant cycle. The closer coupling
between the two cycles, by mechanical interlinking of compressors, is an
operational challenge.
FIGURE 26:THE DMR PROCESS
42. 229
Assumptions
The following assumptions are made in all the examples provided in this
chapter:
The pressure drop in all heat exchangers and phase separators is zero.
The ambient temperature is 300 K.
The minimum temperature approach between the hot and cold
streams is 3 K in all cold heat exchangers.
The adiabatic efficiency of all compressors is 80% and that of
all pumps
is 90%.
The heat in leak from ambient is negligible.
2.6.7 Precooled LNG process without phase
separators
The precooled natural gas liquefaction process shown in Fig. 43 was one of
the first LNG processes to be patented and precedes the single-stage natural
gas liquefaction process by many years. The process can be considered a
precooled PRICO process. In this process, an external refrigerant is used to
cool the process to a temperature of 210 to 245 K. The precooling temperature
is dependent on several factors such as (1) separation of propane and other
high boilers, (2) dew point temperature of the feed, (3) distribution of work
between different compressor drivers, etc. In order to simplify the analysis, the
separation of propane and other high boilers has not been considered in the
present example. A two-stage as well as a three-stage compression process
has been assumed for the precooling refrigerant
FIGURE 27 : PRECOOLED LNG PROCESS WITHOUT PHASE SEPARATORS
43. 230
2.6.8 LNG processes with a phase separator
Most of the problems associated with the possibility of freezing of high
boilers in the refrigerant of the single-stage process can be easily
overcome using phase separators that return high boilers to the
compressor at relatively high temperatures (220 to 260 K), much
above the freezing point of the high boilers. In this process, the
refrigerant mixture composition and operating pressures are so
chosen that partial condensation of the refrigerant occurs in the
condenser. The liquid and vapor phases are separated in a phase
separator before passing through the precooling heat exchanger
(HX-1). The liquid phase, which contains most of the high boilers, is
subcooled and expanded to a lower pressure, to provide the
refrigeration necessary to cool and partially condense the vapor
stream leaving the phase separator to a temperature of 220–260 K.
The natural gas is in a superheated state at the exit of the precooling
heat exchanger (HX-1). The natural gas feed is condensed (liquefied)
and subcooled in the main heat exchanger (HX-2).
FIGURE 28 : SINGLE - STAGE MIXED REFRIGERANT GAS LIQUEFACTION CYCLE WITH A PHASE
SEPARATOR
44. 231
2.6.9 Precooled LNG process with a phase
separator
Consider a precooled natural gas liquefaction process with a single
phase separator.Athree-stage compression process is used for the
main refrigerant, The precooling temperature has been assumed to be
240 K.
The process is similar to the LNG process with a phase separator, except
for the additional precooling heat exchanger (HX-1). Three different
refrigerants
are evaporated in the three heat exchangers to cool the hot fluid streams.
However, the refrigerants evaporated in the second and third heat
exchangers are derived from a single refrigerant using the phase
separator
FIGURE 29 : PRECOOLED LNG PROCESS WITH A PHASE SEPARATOR
45. 232
The overall exergy efficiency of the process is dependent on the
exergy efficiency of the precooling system (ex;pre). Different
arrangements can be used for precooling. Either single-component
(pure fluid) refrigerants or refrigerant mixtures can be used for
precooling. Alternately, the precooling refrigerant can itself be derived
from the main refrigerant passing through the compressors. The
refrigerant can be evaporated at a single pressure or at different
pressures. Different processes result from each of these choices and
are described in the following sections.
2.6.3 LNG processes with turbines
Turbine-based natural gas liquefaction processes have been widely
used in peak shaving plants. They have also been proposed for offshore
plants and for reliquefaction plants on LNG ships . The refrigerant is
compressed to a high-pressure, typically greater than 100 bar. The
refrigerant is precooled in the heat exchanger and expanded to low
temperatures in a turbine. The cold, low-pressure refrigerant is warmed
up in the heat exchanger to cool the natural gas feed and the high-
pressure refrigerant. Both nitrogen and mixtures of nitrogen and methane
have been used as the refrigerant in this process.
FIGURE 30: REVERSE BRYTON PROCESS FOR THE LIQUEFACTION OF NATURAL GAS
46. 233
The exergy loss in the turbine is higher than that in the heat exchanger in
both cases, in spite of the turbine adiabatic efficiency being 80%. Precooling
improves the exergy efficiency of the reverse Brayton liquefiers, as in the
case of Linde–Hampson liquefiers. Any of the different processes discussed
in the previous sections for precooling mixed refrigerant processes can also
be used for precooling the reverse Brayton process. It is also possible to
use a second turbine to provide the necessary precooling. Figure 48 shows
one such patented process, known as the dual expander natural gas
liquefaction process . Methane is used as the refrigerant in the precooling
process, while nitrogen is used in the main process.
The main advantage for introducing the methane refrigerant (stream 6)
directly into the turbine is the reduction in the number of streams in the heat
exchanger from five to four. When the methane refrigerant is replaced by a
mixture of 98 mol% methane and 2 mol% ethane, the exergy efficiency
improves to 53.7% in spite of the mixture entering the turbine at ambient
temperature (300 K). The use of refrigerant mixtures is thus advantageous
even in the dual turbine process. Precooling the dual turbine process will
result in a further improvement in the exergy efficiency. The processwill then
be similar tomanymixed refrigerant processes with three different
refrigerants, one each for precooling, condensation, and subcooling. It is
also possible to use recompression processes or multiple turbines to
provide the necessary refrigeration to liquefy a natural gas feed.
47. 234
2.7 Comparison OF LNG Processes
The Various Processes can be compared on a number of points
The Number Of Equipment Items Required
Compressor Horsepower Requires
Simplicity Of Control and Turndown
Proven Capability
Area Of Heat Exchanger
On the number of equipment items, the cascade type of plant has
approximately 50 % more equipment items than the mixed refrigerant
processes. This in turn leads to a degree of complexity and difficulty in
control and turndown and will result in plant cost of about 20 % greater than
the mixed refrigerant processes.
At one time it was argued that the cascade process was considerably more
efficient than the mixed refrigerant processes, but, with the development of
the propane precooled Tealarc, mixed refrigerant processes have achieved
efficiencies in terms of compressor horsepowers equal or better than the
optimized cascade process.
2.8 Optimization Of Work Required
From an energy point of view refrigeration involves two steps :
Transformation of the liquefaction work into work to be performed on the
refrigerant, which is achieved in the cryogenic heat exchanger.
Compression and cooling of the refrigerant by absorption of the work and
rejection of the heat .
consequently, to optimize efficiency of the cycle both the exchanger and
the compressor efficiency have to be stimulaneously optimized.
To increase the cryogenic exchanger efficiency two things are
important
Reduction of exchanger approach dT . This is especially the case t low
temperatures. This leads to an increase exchanger surface.
Reduction of heat exchanged. This is done by reducing the total weight
flow by using a high Compression ratio .
48. 235
Irreversiblilities in the compression system are mainly due to the fact
that the compression is not isothermal . Note that high compressor
effeciences are obtained with low compression ratio while the opposite
is true for the cryogenic heat exchanger. In practice , the optimum
compression ratio is between 3 and 6 per stage depending on the K
value of the gas.
Of the two types of compressor efficiencies of 85 % while centrifugal
compressor only reach 78 %
Best overall efficiencies are obtained in the processes using two cycles
such as propane precooled MR ( Air Products ) and MR precooled (
Linde and Technp ) . In these processes a single refrigerant is used to
achieve liquefaction. It is a mixture of nitrogen , methane, ethane, or
ethylene , and is a mixture of ethane , propane , and butane.
Total Energy Consumption
For any LNG process eight important factors affect Energy consumption
:
Process Selection Factors
Final Flash or Subcooling
Refrigeration Process.
Mechanical Power System.
Cooling System.
FIGURE 31: CAPITAL COST VS CAPACITY
51. 238
2.9 Summary
A number of mixed refrigerant processes have been discussed in this
chapter. It is evident from the examples provided that processes that use
different refrigerants for precooling (desuperheating), condensation
(liquefaction), and subcooling operate at high exergy efficiency. The
three refrigerants can be derived from a single refrigerant. Alternately,
the condensation and subcooling refrigerants can be derived from a
single refrigerant using a phase separator, and a separate precooling
refrigerant . On the other hand, three separate refrigerants, one each for
precooling, condensation, and subcooling, are used in cascade
refrigerators operating with refrigerant mixtures . There are also attempts
to use a nitrogen expander process for subcooling the natural gas feed,
while two separate refrigerants are used for precooling and
condensation.
The exergy efficiency of most precooled liquefaction processes
described in this chapter is nearly the same. The choice of a process
also depends on other criteria such as the size of heat exchangers, the
cost and availability of equipment, ambient temperatures, etc.
The exergy efficiency of practical large LNG plants is higher than that
shown in the examples due to the use of smaller minimum temperature
approaches in the heat exchangers (typically 1.8 to 2 K), as well as the
use of dense fluid and two-phase expansion turbines instead of
expansion valves.
Most of the processes described in this chapter can be used in small
natural gas liquefiers being proposed for liquefying stranded wells,
biogas from landfills, municipal wastes, etc. Processes with phase
separators such as the Kleemenko process or the PRICO process are
ideal for such applications because of their simplicity. Precooled
liquefaction processes are preferable for large liquefaction systems.
52. 239
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