PRO/II Process Simulation Natural Gas Liquefaction by Multiple Turbo Expander Cycle by Greenwood

Floating natural gas liquefaction plant will be used more for remote small gas field or for off shore gas develoment. In such case, safety becomes very important. Normally, mixed refrigerant is used for onshore plant for its economy. Turbo expander cycle using nitrogen gas as refrigerant is inferior in terms of thermal efficiency, but it is safer when leakage occurs.

Land based plant use brazed aluminum heat exchangers (BAHE) but it is more preferable to use BAHE only for nitrogen circuit. But for natural gas circuit, spiral wound heat exchangers would be prefarable for natural gas cooling by nitrogen. In this case, bottom cool design would be used for spiral wound heat exchangers.

The Purpose of  simulation

The purpose of simulation is to find out thermal efficiency of this system using PRO/II.

Feed Gas Condition

Total Feed gas 6,000kgmol/h (roughly 100ton/h)

Composition:

Feed pressure: 60 Bar G

Feed temperature: 40^oC

Simulation Model

Basically, the process repeats same scheme 3 times for desuper heatibng zone (40 to -30C), condensing zone (-30 to -100C) and sub cooling zone (below -100C) of the natural gas as illustrated below.

PRO/II model

Pressure drop of each heat exchanger was taken as 0.2Bar.

Heat loss of exchanger was taken as 3%.

Nitrogen Compressor Adiabatic efficiency: 80%

Expander adiabatic efficiency: 90%

Outlet temperature of all water coolers:40C

Nitrogen suction temperature to C11, C21, C31  compressor temperature: 30C.

Nitrogen compander suction temperature to C1, C2, C3, C22, C32compressor temperature 40C.

Inter cooler are required for C21-C22 and C31-C32.

Each heat exchanger were divided into 10 zones to check minimum temperature When zone analysis is requested for each LNG exchanger, and data review window in output pull down menu is opened, internal temperature difference such as zone MITA and mean temperature difference could be viewed after each trial run.

Calculation Results

C11 compressor: suction volume: 23,098m³/h, power consumption: 5,227kW, discharge/suction pressure: 17.9/9.9BarG.

C21 compressor: suction volume: 216,291m³/h (1.4 time of 1 st.stage MR compressor), power consumption: 13,862kW, discharge/suction pressure: 4.2/1.9BarG.

C31 compressor: suction volume: 130,206m³/h, power consumption: 11,128kW,discharge/suction pressure: 5.0/1.6BarG.

C1 compressor: suction volume: 13,938m³/h, power consumption: 6,249kW

EX1 expander discharge volume: 17,315m³/h, power generation: 6,275kW

C2 compressor: suction volume 80,773m³/h, power consumption: 15,238kW

EX2 expander discharge volume 104,894m³/h, power generation: 15,232kW

C3 compressor: suction volume 29,046m³/h, power consumption: 7,311kW

EX3 expander discharge volume 40,461m³/h, power generation: 7,300kW

Total power consumption: 52,472kW

For reference, total power consumption of MR cycle is 39,747kW. Therefore Expander/MR power ration is 1.32.

Total liquid mol fraction after flash: 0.9344

6.56mol % of flash gas is compressed and used as dryer regeneration gas and then plant fuel to drive compressors.

LNG products quantity: 98.6ton/h (790,000ton/y)

Power consumption per ton of LNG: 532kWh/ton LNG or 22.2W/(tonLNG/day). 

For reference, MR cycle power consumption per ton of LNG is 403kWh/ton LNG or 16.8kW/(tonLNG/day). 

This means that further optimization, is required.

Plant fuel for power generation: 158kg (When power cycle efficiency: 30%, heat of combustion of methane:890kJ/mol, MW:16.04, J=Ws )

For reference, plant fuel for power generation: 120kg (When power cycle efficiency: 30%, heat of combustion of methane: 890kJ/mol, MW: 16.04, J=Ws )

Overall efficiency: 88.5%.

For reference overall efficiency of MR cycle is 91.3%

Following table is an output of PRO/II.

Composite Cooling Curve of top cold aluminum heat exchanger with no liquid slippage

I have made my own model for this purpose.

Compsit curve

Comparison with Carnot Efficiency

Carnot efficiency is defined as

h=W/Q_c=(Q_h-Q_c)/Q_c=Q_h/Q_c-1=T_h/T_c-1

When condensing curve is assumed straight line from ambient temperature of 40°C to LNG temperature of -160°C and heat sink temperature is 40°C,  integral of Carnot efficiency with incremental temperature of 10°C\ is 0.525.

Actual liquefaction enthalpy change Q_c=21,136,000kcal/h, and actual Actual work required W=52,472kWh (45,147000kcal/h).

W/Qc=2.14

This means that actual COP is 4.07 times of Carnot efficiency.

Acknowledgment

Author is grateful to Invensys Systems Japan, Inc. for letting author use PRO/II v.9.

August 24, 2011

Rev. September 10, 2012