Artic LPG Design

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ARCTIC LNG PLANT DESIGN:

TAKING ADVANTAGE OF THE COLD CLIMATE

William P. Schmidt

Technology Manager, LNG Process

Christopher M. Ott

Principal Process Engineer

Dr. Yu Nan Liu

Technical Director, LNG

Joseph G. Wehrman

LNG Machinery Specialist

Air Products and Chemicals, Inc.

Allentown, Pennsylvania, USA

[email protected]

KEYWORDS: LNG liquefaction, LNG processes, refrigeration cycles, dual mixed refrigerant, propaneprecooled mixed refrigerant, DMR, C3MR, arctic, desert, tropical, climate, seawater cooled, air cooled

ABSTRACT

As the LNG industry continues to grow, liquefaction plants are being considered in a wider range of climates,

including arctic environments. This paper compares arctic conditions to the historic tropical and desert

locations, focusing on the key differences of colder year-round ambient and seawater temperatures and

wider seasonal temperature ranges. These differences play a significant role in selecting the optimum

liquefaction cycle, refrigeration driver type, and machinery configuration. The Propane Precooled Mixed

Refrigerant (C3MR) and Dual Mixed Refrigerant (DMR) processes are compared. With the same power

input, these processes produce essentially the same LNG flow with essentially the same specific power,

except in very specific circumstances. A case study is included to provide more detailed information.

Equipment configurations are discussed, including electric motor drive, aeroderivative gas turbines and

refrigerant compressor arrangements.

INTRODUCTION

Historically, almost all baseload LNG facilities have been either in tropical or desert regions of the world, with

only two projects having been completed in arctic climates in the past decade: Snøhvit [1,2] and Sakhalin

Island [3]. The world-wide demand for natural gas is increasing rapidly, and to meet this need, the LNG

industry is expanding into new natural gas sources. Many large natural gas fields are in arctic or sub-arctic1

regions. Developing projects for arctic climates and constructing plants in those regions present many new

challenges. This paper focuses how to optimally design the liquefaction process for arctic climates.

This paper will address only the process implications for the liquefaction area. Outside the paper’s scope are

construction, shipping, and human factors such as extreme cold and very long and short daylight periods.

1 The commonly used Köppen climate classification system defines arctic and subarctic climates as follows:

Arctic Subarctic

Warmest Month Tmean < 10°C T > 10°C (no more than 4 months with Tmean > 10°C)

Coldest month TMean < 0°C TMean < 0°C

This paper will not use these strict definitions, rather we use “arctic climate” to refer to locations with longperiods of time where the ambient temperature is well below 0°C.



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WHAT MAKES THE ARCTIC DIFFERENT?

The obvious answer is that the ambient temperatures are much colder. However, it is worth a closer look

at the specific weather data to understand what the differences are and to quantify them. For this analysis,

three types of climates are considered: desert, tropical, and arctic.

• Desert2 – Qatar represents this climate type. Deserts have very hot summers and mild winters

(temperatures always above freezing). The sea temperature warms significantly in the summertime,

as the shallow Persian Gulf is heated by the intense sun.

• Tropical3 - The island of Borneo is a typical tropical climate. There is virtually no variation in

temperature throughout the year, in either air or seawater temperatures

• Arctic – The Yamal peninsula in northern Russia, on the Kara Sea, has a harsh Arctic climate. The

winters are extremely cold, and the summers are colder than the desert winters. The seasonal air

temperature variation is extremely large. The sea surface freezes in the winter, requiring ice

breakers for shipping.

The figures below quantify these differences. Figure 1 shows the range in the daily average ambient air

temperature for each month [4]. The “error bars” show the typical range of daily average temperatures within

each month. (For details of how these are calculated, see Appendix.) Figure 2 shows the Sea Surface

Temperature (SST) for the three sites, as measured during 2012 [5]. Yamal’s summertime jump in SST

occurred when the icepack melted in June.

2 In the Köppen Climate Classification deserts are defined as any climate with little precipitation. (There are

quantifiable precipitation criteria, not given here). Deserts are either “hot” (e.g. Qatar) or cold (e.g., Gobi).Hot is defined one of two ways, depending on the specific user

• all 12 mean monthly temperatures are greater than 18°C, or

• the coldest month mean temperature is greater than 0°C. For this paper, we are using the simpler, less precise term of “desert” to refer to hot deserts, because there

are few LNG plants in cold desert climates (Peru LNG is one).3 The Köppen Climate Classification would split this classification into tropical monsoon and tropical

rainforest, depending on the seasonal variation in rainfall. A tropical climate has all twelve months withaverage temperatures over 18°C. We do not distinguish between monsoon or rainforest in this paper.

-50

-40

-30

-20

-10

0

10

20

30

40

50

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

T e m p e r a t u r e ( ° C )

Figure 1: Dry Bulb Temperature

Qatar

Borneo

Yamal



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The data is summarized in Table 1:

Table 1: Seasonal Temperatures - Summary

Yamal Borneo Qatar

Air T (°C)(1)

Yearly Avg -10.5 27.2 27.7

Summer High(2)

12.5 33.1 39.7

Winter Low(3)

-42.6 22.1 12.9

Yearly Range

(4)

55.1 11.0 26.8Typical Diurnal

Range(5) 4 - 9 5 - 7 7 - 11

Seawater T(°C)

Yearly Max 5 30.5 34

Yearly Min -2(6,7)

27 21

Yearly Range 7 3.5 13

Notes: (1). Detailed description of these are in Appendix(2) Approximately the 99% summer high temperature(3) Approximately the 99% winter low temperature(4) Difference between summer high and winter low(5) Typical difference between daily high and low.

(6) The salinity of seawater lowers its freezing point toapproximately -2°C.(7) Sea surface freezes in winter; seawater for cooling is taken from

below the surface.

Further observations from this table are:

• The air temperature varies the most in arctic climates; Yamal shows the highest yearly range of

55°C, then Qatar with 27°C, and finally Borneo at 11°C.

• The mean diurnal temperature swings (day-to-night) are fairly similar, all between 5 and 10°C. (Note

that the yearly variation in average daily temperature for the tropical climate is only slightly larger

than its diurnal variation). The challenge that diurnal variations can cause for the liquefaction plant isnot the size of the temperature changes; rather the challenge is that the rate of temperature change

-50

-40

-30

-20

-10

0

10

20

30

40

50

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

T e m p e r a t u r e ( ° C )

Figure 2: 2012 SST

Qatar

Borneo

Yamal

Ice Covered Ice Free

Ice

Covered



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is relatively large—often a few °C/hr. The plant capacity can change by 0.5 to 1%/°C, so the plant

control system and operating procedures must allow for this capacity change.

• For all climates, the seawater temperature varies less than the air temperature. There is virtually no

diurnal variation.

Arctic locations differ from the tropical and desert locations in many other ways. These include

• The number of daylight hours varies much more. Arctic regions have weeks or months where the

sun never rises, with similar periods where the sun never sets.

• The process equipment and piping must be designed for the very cold ambients. This includes

material selection (normal carbon steel is typically only rated to -29°C) and freeze protection for any

lines containing water or other components with relatively high freezing points. This is more of a

concern in the upstream portion of the plant. Much of the equipment in the liquefaction area is rated

for cryogenic temperatures (often down to -40°C in the precooling loop and -160°C in the main

liquefaction area.) However the warm areas of the liquefaction area, including the compressors, gas

turbines and associated cooling water systems, must be designed for these cold ambienttemperatures.

• The winds are extreme which combined with long periods of darkness and the cold temperatures,

make construction, operation and maintenance much more difficult. The annual precipitation is

relatively low, because the very cold air cannot carry much water vapor. However, whatever snow

does fall will stay for the entire winter, never melting, and fog and mist can accumulate and freeze on

buildings and equipment near the sea.

• The sea will contain ice and may freeze over. This complicates shipping tremendously.

The factors above are beyond the scope of this paper, and will not be considered further. Arctic climatesprimarily affect the LNG liquefaction process by changing the cooling medium temperature, and when

cooling with air, the cooling medium heat sink varies much more between seasons.

LIQUEFACTION PROCESS CHOICES

This paper compares how colder temperatures affect two LNG liquefaction processes: Propane Precooled

Mixed Refrigerant (C3MR) and Dual Mixed Refrigerant (DMR). These processes are discussed in detail here

because they are commonly used in baseload LNG plants. They differ mainly in the precooling step. C3MR

uses pure propane as the precooling refrigerant, where DMR uses a blend of low boiling hydrocarbons

(typically C1 through C4). A third process used in baseload plants is the Pure Component Cascade (PCC).

Because both the PCC and C3MR processes use C3 precooling, colder ambients affect the PCC process

similarly to the C3MR process and it is not discussed separately.

C3MR Process - The workhorse of the LNG industry has been the AP-C3MRTM

LNG liquefaction

process [6, 7], as shown in Figure 3.



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This process cools and liquefies gas using two refrigeration loops powered by gas turbine compressors:

propane (C3) for precooling, Mixed Refrigerant (MR) for liquefaction and subcooling.

Natural gas is fed to the propane chilling section where is it cooled from ambient temperatures to

approximately -35°C. Then the pre-cooled feed enters the Main Cryogenic Heat Exchanger (MCHE), which

is a coil wound heat exchanger (CWHE) [8]. There, the natural gas is liquefied with a mixed refrigerant (MR)

which is a combination of nitrogen, methane, ethane, and propane. The composition is selected to maximize

the process efficiency. Finally, the LNG exits the MCHE and goes to the end flash unit (not shown), where itis split into a fuel gas stream and LNG product.

In the MR refrigeration loop, the high pressure MR is cooled against propane to approximately -35°C, and it

partially liquefies. The stream is separated in the HP MR separator into MR liquid (MRL) and MR vapor

(MRV). The MRL enters the MCHE, where it is subcooled. It is removed at an intermediate point of the

MCHE, and sent to the MCHE shell side. The MRV enters the MCHE, where it liquefies and is subcooled. It

exits at the LNG temperature and is returned to the shell side. The MRV and MRL boil on the shell side,

which provides the refrigeration to liquefy and subcool the incoming natural gas and MR. The vapor MR

which leaves the MCHE is compressed in a two or three stage compressor, consisting of a Low Pressure

(LP), Medium Pressure (MP) and High Pressure (HP) MR compressor. In Figure 3, all the MR compressors

are shown as a single body for simplicity; although in practice they will be split and will have one or moreintercoolers. .

DMR – The Dual Mixed Refrigerant (DMR) process has two mixed refrigerant loops:

• Warm Mixed Refrigerant (WMR) pre-cools the Cold Mixed Refrigerant (CMR) and vapor feed

• The CMR liquefies and subcools the feed.

This CMR loop performs very similarly to the MR loop in the C3MR process. The CMR is compressed,

precooled, and split into two streams: MRV and MRL. These are then sent to the MCHE for further cooling.

They are withdrawn at different points, reduce in pressure, and then introduced to the MCHE shell. As they

boil, they provide refrigeration to liquefy and subcool the LNG feed, as well as cool the incoming MRV andMRL.

Figure 3: The C3MR Process

C3 Pre-coolingFeed

MRV

Mixed Refrigerant (MR)

MRL

MR

C3

Precool

Temperature



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The key difference in the DMR process is that the CMR and NG feed are precooled in a CWHE by WMR

before going on to the MCHE. There are many configurations of precooling loops, all giving similar process

efficiencies. The process patented by Air Products and Chemicals [9] is shown in Figure 4 and is used in the

case study below. Here, the WMR is compressed in two stages. It is partially condensed in the intercooler,

with the remaining vapor condensed in the aftercooler. The liquid from the intercooler is pumped and

combined to create the high pressure liquid WMR stream. This is subcooled in the precooler (which is also a

CWHE), reduced in pressure over a Joule-Thompson (J-T) valve, and then vaporized on the precooler shellside to precool the feed and MR.

A key value in the DMR process is the temperature between the precooler and MCHE, known as the

“precool temperature”, which is shown in Figure 4 below.

There are many variations of the DMR process which have been developed over the past 30 years, each

with subtle variations, optimized for various situations [3, 7, 9, 10, 11, 12, 13, 14].

SELECTING THE COOLING MEDIUM

There are four cooling media that have been used in LNG plants:

•

Ambient Air – the process streams flow through tubes, while fans draw ambient air over the outsideof the tubes.

• Direct Seawater – Seawater pumped through heat exchangers at elevated pressure, cooling thevarious process streams, and the seawater is then returned to the ocean.

• Indirect Seawater – Seawater is pumped and used to cool a freshwater stream, and the seawater isreturned to ocean. This freshwater stream is then sent through a closed loop throughout the LNGplant.

• Evaporative Cooling Tower – Treated water is circulated from a basin through the plant cooling loop,absorbing the waste process heat. A fraction of the return water is evaporated in ambient air, coolingthe remaining water.

For this analysis, the evaporative cooling tower and direct seawater are not considered, because these are

not as widely used in recent LNG projects. Table 2 below shows the range of process temperatures that can

be achieved with the different cooling media, for Yamal:

Figure 4: Dual Mixed Refrigerant

LNG

Cold Mixed

Refrigerant (CMR)

MRV

MRL

Natural Gas

Warm Mixed

Refrigerant (WMR)

5

Precool

Temperature



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Table 2 – Arctic Plant

Process Temperatures Produced by Different Cooling Media

Indirect Seawater Ambient Air

Year Round TemperatureVariation

-2 to 5°C -42 to +13°C

Seawater to closed loopapproach T

4°C N/A

Process Approachtemperature

5 to 10°C 10 to 20°C

Process T after cooling

Range 7 to 19°C -32 to 33°C

Min to max <12°C <55°C

Diurnal variations <1°C 4-9°C

Basis: Temperatures for Yamal Peninsula

From the table, one can see that on the Yamal Peninsula, the process temperatures will vary much more

over the year when using air cooling, perhaps as much as a factor of 4 to 5. Even with the higher approach

temperatures of an air cooled system, the process temperatures are colder for much of the year using air

cooling instead of seawater cooling. This is very different from the tropics/desert where the seawater tends to

be similar to or cooler than the ambient air when the approach temperature is taken into account.

Other considerations for selecting the cooling media in arctic operation are given in Table 3 below.

Table 3 – Cooling Media Comparison

Indirect Seawater Ambient Air

Process T after cooling 7 to 19°C -32 to 33°CDiurnal temperature variations <1°C 4-9°C

Freeze Protection of coolingsystem

Use glycol/water mixture Heaters/filters needed on aircoolers to prevent ice

accumulation

Plot space Smaller (area used for indirectcoolers)

Larger (Process coolers arelarge)

Note that air coolers may use louvers or fan speed control to limit the amount of cooling and to prevent ice

accumulation during cold periods. This stabilizes the process operation at the cost of higher efficiency or

production at the lowest temperatures.

CASE STUDY — ARCTIC AIR COOLING WITH TWO FRAME 7 GAS TURBINES

To investigate how these factors affect the liquefaction unit process design, a case study was performed for

an arctic environment, using air cooling. The arctic conditions chosen are extreme, but are not as severe as

Yamal. The air temperature varies over the year from -20 to +22°C, with a yearly average of 4°C. The plant

design basis is to use two GE Frame 7 Gas Turbines. A design point was selected as described below and

used to develop typical compressor curves. Cases were then developed at different air temperatures

maximizing production at each operating temperature by consuming all available power, subject to the

compressors operating on their performance curves. This best simulates real compressors and drivers. This

produces a nominal 6 mtpa at the yearly average ambient of 4°C.



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Details of the design are below:

• Refrigerant Compressor Drivers: Two GE Frame 7 Industrial Gas Turbines, running at 3600 rpm.

The rated ISO power is 86.2 MW. The power output is derated by 9.6% for aging, fouling and

inlet/outlet pressure losses. The available power varies with temperature by 0.65%/°C.

•

Compressor Configuration: This study uses two 50% compressor sets configured to create twoidentical parallel strings, as shown below:

The DMR compressors have a similar configuration, with two 50% strings.

The two x 50% configuration has the following advantages for this study:

• In the range 6 mtpa, a compressor running at 3600 rpm may be constrained by aerodynamic

impeller design limits, such as inlet flow coefficient and inlet Mach number. Using two 50%

compressors makes each compressor smaller (effectively for 3 mtpa), which relaxes the compressor

constraints.

• When the individual compressor power consumption varies, the power shifts automatically between

compressors. There is no need to balance compressor and driver loads.

• Two 50% strings increase availability. In the event that one string is offline, the plant production can

be maintained at 50% or more.

• Compressor design: A design ambient temperature was selected for each case. At the design

point, all compressors (C3, CMR, WMR, and MR) were assumed to have 83% polytropic

efficiencies. Air Products’s proprietary model was then used to develop typical compressor

performance curves, for head and efficiency vs. volumetric flowrate. These typical curves were then

used to rate the plant at the different air temperatures.

The design ambient temperature was chosen at 4°C for the DMR compressors. The C3MR

compressors were designed at 13°C. This choice is explained further in the results section.

STARTER



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• Feed Conditions: The feed is introduced to the liquefaction section at 68 bara. It is relatively lean.

The assumption is that it is either lean from the gas field or it has gone through an upstream NGL

plant. This assumption was made to avoid the complication of integrating the NGL plant or scrub

column with the liquefaction unit. This integration is very much project-specific, so including it would

make it more difficult to generalize any conclusions. The composition assumed for the study is below

in Table 4:

Table 4– Feed Composition for Case Study

N2 1.7%

C1 93.0%

C2 3.3%

C3 1.2%

C4+ Balance

CO2 100 ppm

• Heat Exchangers: The process heat exchangers were set to the following:

• The Main Exchanger is a coil wound heat exchanger (CWHE) for both processes.

• The MR precooler is also a CWHE (DMR only).

• The wound coil heat exchangers are sized to process the feed at the design point, fixing the heatexchanger area. This area is then used to compute the heat transfer performance at otheroperating conditions.

• The C3 evaporators are shell-and-tube kettles, with an assumed 3°C approach temperature(C3MR only).

• Because the cooling medium is cold, only three propane chilling levels are used. In warmerdesert and tropical climates, it is typical to have 4 levels of precooling.

• The C3 condenser has a 20°C approach temperature to ambient air at the cold end (C3MR

only).• The Warm MR condenser has a 10°C approach temperature at the cold end. (DMR only)

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• All compressor intercoolers have an approach temperature of 10°C approach.

• The MR compressor (for C3MR) and CMR compressor (for DMR) have two intercoolers, givingand LP, MP and HP MR/CMR compressors.

• Hydraulic Turbines: The MRL and LNG each have hydraulic turbines to expand the liquid and

recover refrigeration.

• Fuel: The MCHE outlet temperature is adjusted to produce a fixed fuel rate. This fuel rate does not

change with ambient temperature.

• Availability: the plants are assumed to have equal availability of 340 days/year (93.2%).

EXPLOITING THE LOWER TEMPERATURES — THEORY

The key process difference—and potential advantage—of an arctic facility is the cold temperature of the

cooling medium. This is because every process must reject waste heat. In LNG liquefaction processes, this

is primarily the heat removed from natural gas as it is liquefied, along with the heat of compression from the

refrigerant compressors. As that heat is rejected into a colder sink, the process becomes more efficient.

Therefore, all things being equal, the liquefaction process efficiency increases when the cooling medium is

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The C3MR and DMR condenser temperatures differ because the temperature pinch changes location. InC3MR, the pinch occurs at the warm end of the C3 condenser because while the air warms up, the propanetemperature does not change over the exchanger. In the DMR process, the Warm MR cools by over 20°Cfrom inlet to outlet at the design point, and more closely tracks the warming air.



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colder. This efficiency is expressed as specific power, typically in kwh/tonne LNG produced. A lower value of

specific power is a higher efficiency.

The equation below shows the relationship between specific power, power consumption and production:

Ws=kWref

LNG

Where

Ws = Specific Power for liquefier, kwh/tonnekWref = total gas hp for refrigeration compressors, kWLNG = LNG rundown production after any endflash, tonne/hr

Rearranging this equation gives production in terms of power supplied and specific power consumption:

LNG=kWref

Ws

What this equation shows is that LNG production is proportional to the refrigeration power consumed and

inversely proportional to specific power. To increase production, one can increase the power supplied (in our

case study, by the gas turbines), or improve the specific power. Note that the relationship is linear.

Assuming that extra feed is available, a 1% reduction in specific power increases production by 1%,

Likewise, a 1% increase in available gas turbine power also increases production 1%. And if both the

specific and available power increase by 1%, the production increases by 2%.

The question is then how does changing the cooling medium temperature affect the specific power and the

available power?

Cooling Medium Effect on Specific Power

Three specific reasons why colder temperatures improve specific power are listed in order of importance

(most to least important):

• Refrigerant Condenser Pressure – The precooling refrigerant is completely condensed against the

cooling medium with a colder cooling medium lowering the condensing pressure. This lowers the

required refrigerant compressor discharge pressure, which in turn reduces power consumption.

Because the refrigerant condensers reject 80% of the energy removed from natural gas to make

LNG, lowering the condenser pressure is typically the majority of the specific power improvement.

• Compressor inlet temperature – A colder inlet temperature improves the compressor specific

power. For centrifugal and axial compressors, for a given mass flowrate and pressure ratio, the head

developed by the compressor, and thus the power consumption, is proportional to the absolute inlet

temperature.

This is typically 20 to 40% of the specific power improvement.

• Inlet to liquefaction section – With a colder cooling medium, the feed to the liquefaction unit is

colder in the arctic than in the tropics or desert. Reducing the inlet temperature reduces the total

refrigeration needed to convert ambient temperature natural gas into LNG. However, this effect isrelatively small, because it is easier to cool a stream near ambient than when the stream is much

colder. For example, with a 0° ambient, it takes over 6 times more work to remove 1 unit of energy at



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-30°C than at -140°C. Therefore, slightly cooling the liquefier feed stream by itself has a small effect

on specific power, typically less than 20% of the improvement.

Other considerations in cooling the liquefaction unit feed:

1) The raw natural gas entering the plant will contain water, requiring the feed to stay above 20

to 25°C before dehydration to prevent forming hydrates.2) Following dehydration, in tropical and desert plants, the feed is cooled by the precooling

refrigerant after the dehydration unit. In arctic climates, some of this precooling could be with

the cooling medium during most or all of the year (depending on the cooling medium). The

plant design may take advantage of this to lower the precooling refrigerant load by adding

extra heat exchangers.

Cooling Medium Effect on Available Power

The colder air temperature is also very important when the compressors are driven by gas turbines, because

the power available increases with decreasing ambient temperature. For industrial turbines, this is on the

order of 0.7% per °C and for aeroderivative gas turbines, the available power increases approximately 1.1%per °C. However, there is an upper limit for the power increase. This is typically set by mechanical

constraints within the turbine, such as maximum shaft torque or compressor pressure/temperature limits. For

a typical industrial turbine, the GE Frame 7, the available power may increase until the ambient temperature

is below -20°C [16, 17]. Aeroderivative turbines will reach their maximum power output at a higher ambient

temperature, typically between -10°C and +10°C [17]. Further power can be added through helper motors.

EXPLOITING THE LOWER TEMPERATURES — PRACTICE

C3MR Process

The C3MR process was designed for an 11°C ambient temperature, with the parameters given above. At11°C, typical compressor curves were developed using APCI’s proprietary computer modeling and design

tools. The heat exchanger areas fixed. The plant was then rated over the full range of ambient temperatures,

-20 to +22°C. The 11°C design point was chosen to be able to utilize the full gas turbine available power at

the summer high condition of 22°C, while providing maximizing the production at the lower ambient

conditions. The production data is summarized in Table 5 below:

Table 5 – C3MR Production Summary

Ambient T (°C) -20 -10 -3 4 13 22

Production (mtpa) 6.9 6.8 6.6 6.1 5.4 4.8

Relative to 22°C 1.45 1.42 1.38 1.27 1.14 1.00GT Power (MW) 195 184 176 169 158 148

Relative Specific Power 0.91 0.88 0.86 0.89 0.94 1.00

Findings from the study are summarized below:

• The plant produces more LNG as the ambient temperature decreases. Above -3°C, approximately

50% of the increase is due to an increase in specific power and the reminder due to increased

available gas turbine power. With the C3MR process, as stated above, the precooling temperature is

limited by being pure propane (no composition change) and that the propane must be above

atmospheric pressure throughout the loop. Therefore, as the feed and MR are cooled by the ambient

air coolers more and more, there is less need for precooling. For this case study, when the ambient

temperature reaches 3°C, the required precooling duty is low enough that the propane compressor



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goes into recycle. The specific power then begins to increase, relative to -3°C. So below 3°C, the

production increases due to higher available gas turbine power, but the rate of production decrease

slows as the specific power stops improving. This is shown in the Figure 5 below:

• The optimum precooling temperature does not vary significantly with ambient temperature.Therefore, as the cooling medium becomes colder, less of the refrigeration power is needed by the

propane compressor and more by the MR compressor. Below about -3°C, the propane compressor

becomes so unloaded that it begins to recycle to stay within the operating limits. This causes the

fraction of refrigeration power 5 consumed by the propane compressor to level off, even though the

process precooling requirement continues to decrease. This propane compressor inefficiency

causes the C3MR specific power starts to increase, relative to 3°C.

The MR aftercooler approach temperature becomes colder and the MR composition is adjusted so

as to load up the MR compressor as much as possible. This is done by adding more low boiling

point component (N2) and reducing the C3. This lowers the MR dewpoint temperature. These effects

are shown in Table 6 and Figure 6. Note that optimum the MR composition is fairly constant below -

3°C, with only slight changes in N2. This ability to adjust the MR composition is a key feature of any

mixed refrigerant cycle and has been practiced for many years [19].

5 This is the propane compressor power divided by the sum of the propane plus MR compressor powers.

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

-20 -15 -10 -5 0 5 10 15 20 25

R e l a t i v e P r o d u c t i o n

Ambient Temperature (°C)

Figure 5: C3 MR Relative Production

Increase due to

better specific power

Increase due to more

available GT Power



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Table 6 – C3MR Refrigerant Composition DMR Production Summary

Ambient Air (°C) -20 -10 -3 4 13 22

MR Composition (Mole %)

N2 11.5 10.6 9.8 9.0 7.9 6.3

C1 42.7 43.6 45.2 44.7 44.1 42.9

C2 34.7 34.3 33.1 33.2 33.3 33.8

C3 11.2 11.5 11.9 13.1 14.7 17.0

Tdew @ 62 bara (°C) 57.5 58.4 58.7 63.4 69.5 78.1

DMR Process

The DMR process was designed with the parameters given above at the yearly average ambient

temperature of 4°C. This temperature was chosen to allow the compressors to cover the entire coolingmedium temperature range. Typical compressor curves were developed at this point, using APCI’s

proprietary computer tools, and the heat exchanger areas fixed. The plant was then rated over the full

ranges of ambient temperatures, -20 to +22°C. The full gas turbine available power at the each condition

was utilized to maximize production. The production data is summarized in Table 7 below:

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

0

5

10

15

20

25

30

-20 -15 -10 -5 0 5 10 15 20 25

% P

o w e r c o n s u m e d b y C 3 C o m p r e s

s o r

M R D e w T e m p e r a t u r e ( ° C )


Figure 6: C3 MR Refrigeration

MR Dew T

% C3 Power



14

Table 7 – DMR Production Summary

Ambient T (°C) -20 -10 4 13 22

Production (mtpa) 8.5 7.5 6.2 5.5 4.8

Relative to 22°C 1.75 1.55 1.28 1.13 1.00

GT Power (MW) 195 184 169 158 148

Relative Specific Power 0.75 0.80 0.89 0.94 1.00

Findings from the study are summarized below:

• As with C3MR, the DMR cycle produces more LNG as the ambient temperature decreases.

However, unlike C3MR, the production increases linearly over the entire ambient temperature range.

This is because as ambient air temperature decreases and cools the feed and CMR more and more,

it is possible to adjust the WMR and CMR compositions keep the WMR and CMR power split

roughly constant. While C3MR process can make adjustments down to about -3°C, the DMR

process can be adjusted down to -20°C. This ability to fully load the WMR compressor at winter

ambients keeps the specific power rising throughout the ambient temperature range, which in turns

keeps the production rising (Figure 7).

• The production increase shown in Table 6 and Figure 7 is attributed to both the increase in available

power and the decrease in specific power. As with C3MR, each contributes about equally to

increasing the overall production. That is, about 50% of the production rise is due to specific power

improvement and 50% is due to more available GT power.

• To keep improving the specific power, the power split between the WMR and CMR compressors is

maintained essentially constant, with the WMR compressor consuming between 40% and 45% ofthe total refrigeration power over the entire ambient temperature range. This is accomplished by

changing the WMR and CMR compositions, as shown in Table 8 and Figure 8.

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

-20 -15 -10 -5 0 5 10 15 20 25

R e l a t i v e P r o d u c t i o n


Figure 7: DMR Relative Production

Increase due to more

available GT Power

Increase due to

better specific power



15

Table 8: DMR Refrigerant Compositions (mole %)

Ambient T (°C) -20 -10 4 13 22

WMRC1 24.57 21.83 13.73 10.10 6.58

C2 68.75 70.05 70.91 67.92 63.65

C3 6.68 1.74 0.00 0.00 0.00

C4 0.00 2.55 6.14 8.79 11.90

I4 0.00 3.83 9.22 13.18 17.86

Tdew @ 45 bara (°C) 18.4 34.2 55.8 68.2 80.4

CMR

N2 16.9 14.8 12.7 11.4 10.3

C1 51.2 50.1 47.7 46.8 45.6

C2 32.0 34.1 36.4 36.5 37.0C3 0.0 1.0 3.2 5.3 7.0

Tdew @ 62 bara (°C) -21.9 -15.4 -5.5 0.8 5.7

The composition needs to change fairly dramatically over the course of the year. Note that the CMR change

is fairly significant, particularly with the propane and nitrogen. From winter to summer, the nitrogen

decreases by a factor of 0.6, while the propane increases from 0% to 7%. The Warm MR change is even

more significant, with methane reducing by a factor of 4, propane nearly being eliminated, and the butanes

rising from 0% in the winter to nearly 30% in the summer. It is possible to reduce the amount of composition

change to ease operation; however, this will come at the cost of some efficiency. Thus, to maintain optimum

efficiency, the WMR compositions will need to be changed fairly dramatically. To do this efficiently, a

refrigerant reclamation system may be considered, to avoid flaring large quantities of valuable refrigerants.

0%

5%

10%

15%

20%

25%

30%

35%

40%

45%

-40

-20

0

20

40

60

80

100

-20 -15 -10 -5 0 5 10 15 20 25

%

P o w e r c o n s u m e d b y C 3 C o m p r e s s o r

M R D e w T e m p e r a t u r e ( ° C )


Figure 8: DMR Refrigeration

WMR Dew T

CMR Dew T

% WMR Power



16

COMPARING C3MR AND DMR

The C3MR and DMR process are both flexible and both dramatically increase production at lower ambient

temperatures. There is essentially no difference between -3°C to +22°C, which is over 50% of the year. Over

this range, the production is essentially equal between the two processes because the specific powers are

equal, and both processes are able to consume the entire available GT power.

Below -3°C, the DMR process will produce more LNG, provided that sufficient feed is available. While both

processes are able to fully use the available GT power, the C3MR process has a higher specific power,

because the propane loop cannot be fully tuned for the very cold ambient temperatures. The production and

specific power differences are shown in Figure 9 below. To achieve higher production, the WMR and CMR

compositions will need to be continually or periodically adjusted.

The large production increases assume that the necessary feed rate is available at low ambient

temperatures. However, in many situations, the feed flowrate is fixed. For fixed feed flow, as the specific

power decreases, the gas turbines consume less fuel, allowing more feed to be turned into LNG. Thisincreases production, but not by as significantly when more feed is available. As a simple case study for

fixed feed rate, assume that the gas turbines consume 5% of the feed flow as fuel at 22°C. Between -3 and

22°C, the DMR and C3MR processes have the same 14% decrease in specific power, so they will have

identical production increases. From -3°C to -20°C, the DMR specific power improves, so its production

increases to 4.82 mtpa, which is 1.2% above the 22°C production. For C3MR, the specific power declines

below -3°C, so its production falls to 4.78 mtpa, which is 0.5% above the 22°C production. This is shown in

Figure 10 below:

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

0

1

2

3

4

5

6

7

8

9

-20 -10 0 10 20 30

R e l a t i v e S p e c i f i c P o w e r

P r o d u c t i o n ( m t p a )


Figure 9: Comparing C3MR and DMR

DMR Production

C3MR ProductionDMR Specific Power

C3MR Specific Power



17

There are some equipment differences between the DMR and C3MR processes. They both will have 5

compressor bodies, with a two stage WMR compressor replacing the propane compressor with multiple side

feeds. The MR and feed streams are precooled with three or four kettle type exchangers in the C3MR, while

the DMR process has a single large CWHE to precool both Feed and Cold MR.

DOES THE EXTRA POTENTIAL PRODUCTION HAVE VALUE?

From a high level view, the performance improvements due to colder a cooling medium can be taken as

lower power consumption and/or higher production. To decide how to do this, more information is needed:

• What is to be the limiting unit of the LNG facility? It could be the pipeline, slug catcher, Acid Gas

Recover Unit (AGRU), dehydration unit, or the liquefaction unit. Within the liquefaction unit, the

limiting component is usually either the feed rate, the power available to refrigerant compressors (i.e.

the compressor drivers) or the refrigeration compressors themselves.

• How much extra capacity can be used? This answer is mainly commercial. An arctic plant has the

potential to produce more LNG product during the colder months and less during the summer

months. However, this extra product must be commercially useful, in that seasonal customers must

be available. It is of little value to be able to produce more if there is no customer for the extra

product.

• How much will the heating medium change during the year? If air cooling is used, the cooling

medium temperature will vary much more than if seawater is used. In almost all locations, the

amount of variation in the seawater temperature will not significantly increase production. So

significant extra production will potentially be available only when cooling with ambient air.

4.5

4.6

4.7

4.8

4.9

5

-20 -10 0 10 20 30

L N G P r o d u c t i o n ( m t p a )


Figure 10: Production for Fixed Feed

DMR

C3MR



18

Consider a facility where the answers to these questions are simultaneously

• The liquefaction unit (including the refrigerant compressor drivers) limits production

• Extra product can be sold

• The cooling medium varies significantly during the year. (This essentially means that air cooling is

used, and the air temperature varies by more than 30 to 40°C over the course of the year.)

In this special combination of circumstances, the DMR facility can produce more LNG than C3MR during

portions of the year, and this may be considered in the plant economics. However, if the answer to any one

of the above questions is different, then C3MR and DMR will produce the essentially the same amount of

LNG over the course of a year. In these cases, the process selection must be based on factors other than

production capacity.

To illustrate this point, consider how plant location will affect the monthly production. The production is

estimated by assuming that it is linear with temperature, as shown in Figure 96. (This is clearly a

simplification. In actual conditions, the production will not be so linear, but this simplifies the analysis, while

maintaining the essential features of the example.)

= 6.653− 0.0869

Where

T = cooling medium temperature, °C

mtpa = LNG production

Using the seasonal temperatures for ambient air and seawater (see Figures 1 and 2), the year round

production is estimated for desert, tropical and arctic climates. Note that production is maximized using for

two GE Frame 7 gas turbines, without helper motors, for the case study parameters. This is shown in Figure

11 below.

6 This equation is valid only for the case study of this paper. It will not be valid for other combinations of gas

turbines, feed compositions, temperature approaches, etc.



20

• Extra Capacity Not Utilized – In this area, there is insufficient gas turbine power to utilize the total

installed production capacity. In this case, there is no commercial value in investing CAPEX for this

extra production.

• AP-C3MRTM

or AP-DMRTM

– For this combination of temperature variation and desired winter

production increase (relative to the summer production), both the C3MR and DMR processes are

acceptable. They will both be able to meet the facility production needs, and the process selection

can be made for other reasons besides seasonal capacity. Note that this area includes all seawater

cooled sites, tropical air cooled and desert air cooled.

• AP-DMRTM

– When both the temperature range and desired wintertime production increase are

large, the DMR process is the best selection. As shown in the case study, the WMR and CMR

compositions can be adjusted so that the entire power produced by the gas turbines is utilized to

make LNG. Note that boundary between “DMR” and “C3MR or DMR” is not a hard line; is a blurred

transition.

As an example, consider a desert climate with air cooling, having a 30°C seasonal average daily ambient

temperature range. It is desired to be able to product 1 mtpa more LNG in the winter than the summer. This

is located in Figure 12 as Point A, in the region that either C3MR or DMR can meet this requirement. Theprocess selection will be based on other factors besides plant capacity and ability to match seasonal

temperatures.

The trends and general shape of Figure 12 will be true in general. However, the actual breakpoints and

specific values of boundary locations will vary for each case, depending on feed composition and pressure,

driver configuration, site weather conditions.

0

1

2

3

0 10 20 30 40 50 60

I n s t a l l e d C

a p a c i t y a b o v e S u m m e r C a p a

c i t y ( m t p a )

Cooling Medium Temperature Range (°C)

Figure 12: Process Selection Guide

Seawater Cooling

Desert Air Cooling

Tropical Air Cooling

Extra Capacity Not

Utilized

AP-C3MRTM

or

AP-DMRTM

AP-DMRTM

Arctic Air Cooling

Note: This chart is valid only

for the case study of this

paper which fully utilizes the

avaialbel power from 2 GE

Fr7 GT's w/o helper motors A



21

OTHER CONSIDERATIONS FOR ARCTIC LNG LIQUEFACTION UNITS

The discussions above primarily focused on the process design of the liquefaction area; mechanical

considerations were not discussed in detail. These were ignored in the case study, because while a

particular LNG Plant size was selected, the results can easily be scaled to other production rates and gas

turbines. Items that could affect this case study or others are

• The maximum power available from a Gas Turbine

• Alternative driver types: electric motors and aeroderivative turbines

• Speed variation to more efficiently turn down the C3 compressor

• Refrigerant compressor configuration

• Alternative precooling refrigerants

Each of these is now considered in turn.

Maximum GT Power — In the case study here, no limit was placed on the power available from the gas

turbine. It was assumed that as the ambient temperature cooled, more power is available without limit.

However, in practice, turbines will have a maximum power output, due to mechanical limits such as shaft

torque, internal temperatures, etc. The Frame 7 gas turbines continue to be able to deliver more power at

ambients down to -20°C [16, 17], while aeroderivatives reach mechanical limits at warmer temperatures,

typically between -10°C and +10°C [18]. If the maximum GT power is a limit, a helper motor could be added

to provide the extra power at colder temperatures. This could also be used to supplement the gas turbine at

warmer temperatures.

Electric Motor Drivers — Large electric motor can be used to drive the compressors, with the electricity

coming either from the grid or an onsite power plant. To stay within proven motor sizes, three or more

compressor strings would be needed to produce the nominal 5-6 mtpa in this study. However, as stated

above, the results of this study are scalable, and the production can be increased or decreased to match the

driver power.

Aeroderivative Gas Turbines — Aeroderivative gas turbines are becoming more widely used in the

LNG industry to drive the refrigeration compressors. They have been operated in land-based baseload

plants. The first C3MR process with aeroderivative gas turbines is under construction, and is set to come

online in 2014. Their features, relative to industrial turbines, are summarized below [7]:

• Aeroderivatives have higher efficiency, which reduces autoconsumption by up to 25%.

• Aero derivative turbines are dual or triple shaft designs, so they do not need a helper motor tostart

•

They typically have a larger speed range, which can be up to 50 to 105% of nameplate.• Aero derivatives are more sensitive to ambient temperature variations, with their output falling

about 1.2%/°C.

• The maximum power ratings are generally smaller than the industrial turbines, so for baseloadplants, multiple parallel compressor strings have been installed.

• Aeroderivative turbines are designed to be removed from service in a few days, so maintenancecan be performed offline. However, aeroderivatives require more frequent periodic inspections,which reduce their availability.

The most common compressor drivers are compared in Table 9 below:



22

Table 9: Compressor Driver Comparison [7]

Driver Industrial Aero Electric

ISO Thermal Efficiency 29 to 34% 41-43% Gas Turbine less

several percent(2)

Size Discrete Discrete

(smaller max)

Variable

Shaft Type Single and Dual Dual and Triple N/A

Speed range(1)

(% nameplate)

SS: 95-102%

DS: 50 to 105%

50-105% 20-100%

Starters motors

required(1)

SS -Y

DS - N

N N

(VFD req’d)

Availability Good Good Best

Amb T effect Moderate Large Nil (w/ sufficient

electricity supply)

CAPEX Least Least Middle(3)

Notes:1. SS = Single Shaft, DS = Dual Shaft

2. The apparent efficiency depends on the power source. If electricity is generated onsite, then onemust consider the type of turbines (aero derivative or frame), as well as the fractional load. (Lowfractional load gives lower efficiencies.)3. Assumes onsite electrical generation via gas turbine generators

Propane Compressor Control by Varying Speed — As the ambient temperature gets colder, the

propane compressor discharge pressure decreases, as set by the propane condenser. Also, because the

heat load on the propane system decreases, the propane flowrate decreases. As shown on the performance

curve in Figure 13 below, the propane operating point shifts down and to the left. Reducing the compressor

speed will better match the compressor to the desired operating point. The Frame 7 gas turbines used in the

case study have very limited speed adjustment; however, speed control can be used in facilities using aero

derivative turbine or electric motor for compressor drivers. Speed control should be considered if the driver

allows.

Figure 13 – Propane Compressor Operating Points

HEAD

FLOW

Winter Operating

Point

Lower Speed

Higher SpeedSummer

OperatingPoint



23

Compressor Configuration — This study was performed using two 50% parallel compressor strings.

However an alternative is the Air Products SplitMR® configuration. In this configuration, the C3 and HPMR

compressor are driven by one gas turbine, and the LPMR and MPMR compressors are driven by the a

second gas turbine. This reduces the number of compressor bodies on a string from five to four, relative to

the basis of this study.

A key feature of this compressor configuration is that slightly changing the pressure between the MPMR andHPMR compressors can equalize the power consumption of the two strings. This allows the total gas turbine

power to be absorbed and maximizes production for the fixed driver set.

However, as the cooling medium becomes much colder, the propane compressor power consumption will

become too low to be fully balanced with the MR compressors. That is, the driver with the C3 and HPMR

compressors will have available power, and the driver with the LPMR and MPMR will need power, with no

way to move power between the drivers. Therefore, a very wide cooling medium temperature variation will

result in the SplitMR® compressor configuration not consuming all of the available power and producing less

LNG. When the cooling medium range is not so large, such as in a desert or tropical climate, or if arctic

seawater is used as the cooling medium, then this is not an issue.

An additional attribute of the SplitMR® configuration is that it is well proven. It is being used in nine operating

baseload trains, with another eight in construction.

Alternative Precooling Refrigerants — As shown above, one propane limitation is that when the

lowest propane pressure is kept slightly above atmospheric, then the precooling temperature is limited to -

35°C. An alternative pure component precoolant would be a pure component having a normal boiling point

below -40°C, simultaneously with a critical temperature above the cooling medium temperature plus any

temperature approach. The refrigerant should be readily available, preferably from the natural gas feedstock

(although it is possible to import the refrigerant, if needed.)

Three pure component refrigerants potentially meet these basic requirements: propylene, ethane, andethylene. Their properties are summarized below:

Table 11 - Potential Alternative Precooling Refrigerants

Propane Propylene Ethane Ethylene

CriticalTemperature

97 °C 92 °C 32 °C 10 °C

Normal BoilingPoint

-42 °C -48 °C -89 °C -104°C

This table shows that propylene gives a slight improvement over propane; however it is typically not readily

available from the natural gas feed. The slight performance improvement is not typically worth the extra cost

and effort to import. Ethane is a possible choice, provided that the cooling medium is below 10 to 20°C to

ensure that he refrigerant is maintained below its critical temperature. This may be possible for seawater, but

most arctic locations will have short, but significant periods where the ambient air temperature will become

too high. Ethylene has the same issue, only it is even more pronounced; ethylene will need very cold

ambients or seawater which is essentially 0°C. Additionally, ethylene is not readily available from the natural

gas feed and would need to be imported.



24

CONCLUSIONS

This paper shows that C3MR and DMR are both well suited for arctic environments. The two key

differences in arctic climates, when compared to the historic tropical or desert LNG facility locations are

• The yearly average cooling medium (air or seawater) is much colder, typically less than 0 to 5°C.

•

If the cooling medium is air, the summer to winter seasonal variation is much larger; it can be up to60°C or more.

In this paper, we have considered how the C3MR and DMR processes will operate in arctic environments.

This paper shows that both C3MR and DMR have essentially the same production and specific power over a

wide operating range. Both processes are capable of producing significantly more LNG in the winter. In

special circumstances, the most important of which is that air cooling is used and the seasonal variation is

larger than 30 to 40°C, then the DMR process can produce somewhat more LNG in the wintertime. However,

the entire facility and supply chain, including available carriers, must be designed to take this into account.

This will require additional investment to handle the additional seasonal volume. In all other situations, the

production and efficiency are essentially the same. The process selection is then to be made for reasons

other than production.

ACKNOWLEDGEMENT

We would like to thank Wenbin Hu for developing the computer simulations of the DMR and C3MR

processes. His hard work and insights were invaluable for this work.

REFERENCES CITED

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Gas Utilization, AIChE 2012 Spring Meeting, Houston, Texas 2 April 2012.

[2] Vist Sivert, Morten Svenning, Hilde Furuholt Valle, Henrik Ormbostad, Gunder Bure Gabrielsen, Dan

Pedersen, Roy Ivar Nielsen, Jostein Pettersen, Arne Olav Fredheim, “Start-Up Experiences From

Hammerfest LNG - A Frontier Project In The North Of Europe”, Paper PS4-4, LNG 16, 2010.

[3] Verburg, René, Sander Kaart, Bert Benckhuijsen, Padraig Collins, Rob Klein Nagelvoort, "Sakhalin

Energy’s Initial Operating Experience from Simulation to Reality: Making the DMR Process Work”,

Paper PS4-5, LNG 16, 2010.

[4] American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc., ASHRAE ® Handbook:

Fundamentals, Chapter 14, “Climatic Design Information”, 2009 edition, Atlanta, GA,.

[5] “NOAA Optimum Interpolation Sea Surface Temperature Analysis”, Environmental Modeling Center

(EMC), NOAA National Oceanic and Atmospheric Administration (NOAA), SST maps for 2012 at

http://www.emc.ncep.noaa.gov/research/cmb/sst_analysis/

[6] Pillarella, Liu, Petrowski, and Bower, “The C3MR Liquefaction Cycle: Versatility for a Fast Growing,

Ever Changing LNG Industry”, LNG 15, 2007.

[7] Schmidt, W. P., C. M. Ott, Y. N. Liu, W. A. Kennington, "How the Right Technical Choices Lead to

Commercial Success", LNG 16, 2010.

[8] McKeever, Jack, Mark Pillarella, and Ron Bower, “An Ever Evolving Technology”, LNG Industry, SpringIssue 2008.






25

[9] Roberts, Mark Julian, Rakesh Agrawal, “Dual Mixed Refrigerant Cycle for Gas Liquefaction”, US Patent

6,119,479, 19 September 2000.

[10] Liu, Yu-Nan, James W. Pervier, “Dual Mixed Refrigerant Natural Gas Liquefaction”, US Patent

4,545,795, 8 October, 1985.

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[12] Garier, C., Paradowski, H., “Method and Plant for Liquefying a Gas with Low Boiling Temperature”, US

Patent 4,274,849.

[13] Caetani, E., Paradowski, H., “Method of and System for Liquefying a Gas with Low Boiling

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[14] Gauberthier, J., Paradowski, H., “New Trends for Future LNG Units”, Session II, Paper 6, The 9th

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[17] Ekstrom, T.E., P.E. Garrison, “Gas Turbines for Mechanical Drive Applications”, GE Publication GER-

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Propane Precooled Multicomponent Refrigerant MCR® Process”, LNG 5, 1977.



APPENDIX – DRY BULB TEMPEATURES

The dry bulb temperatures in Table 1 are calculated as follows:

• The temperature for each month is the daily average dry bulb temperature as reported in the

ASHRAE handbook [4].

• The “error bars” approximate the range of temperatures covered by 90% of the hours in each month.

The upper end is approximately the 95% high (5% of the hours in that month are warmer than this

temperature) and the 5% low (5% of the hours in the month are colder than this temperature.) This

data is not directly available, so it is estimated. The estimate methods vary, based on the location.

For Qatar and Yamal, which see fairly large seasonal variations, the monthly 5% low and 95% high

are approximately the monthly average, +/- two standard deviations of the monthly average. For

Borneo, which has almost no seasonal variation, the high and low are approximately equal to the

monthly average +/- 0.75 times the monthly mean temperature variation. This is crosschecked by

comparing the predicted yearly high and low, with the independently reported 99% summer high and

1% winter low. These match within a few °C, confirming that these are reasonable estimates.

• The typical diurnal variation is the range of mean daily temperature range over the year.

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