lng example
DESCRIPTION
Lecture Notes - LNG exampleTRANSCRIPT
ENSC3019 Unit Operations S2 2015
LNG / Gas Process Engineering Example for Process Modules
Acknowledgments: Dr. John Boxall (2012) Various Slides adapted from Terry Edwards, Process Modules lectures Wesfarmers process slides for Process Modelling course Figures GPSA Handbook and Campbell, Gas Conditioning and Process Vol2
Liquefied Natural Gas (LNG) Background
http://energy.gov/sites/prod/files/2013/04/f0/LNG_primerupd.pdf
Liquefied Natural Gas (LNG) Background
http://energy.gov/sites/prod/files/2013/04/f0/LNG_primerupd.pdf
Liquefied Natural Gas (LNG) Background
https://www.spe-qld.org/useruploads/files/aug_2011_final_lng_presentation_rev3_%5Bcompatibility_mode%5D.pdf
Not necessary on NWS
LNG Processing
LNG Storage and Sales
LPG Extraction LPG
Sales
Domestic Pipeline Sales
Ethane Extraction
Petrochemicals & Gas to Liquids
Separation and Processing of Well Fluids
LNG Production – Cascade Refrigeration
Inlet Gas: Methane (and some N2) • Always requires some front end treatment
– acid gas removal – dehydration – mercury removal
• Liquid hydrocarbon removal – Distillation and absorption
• Liquefaction through a refrigerant cycle
– Refrigeration – Heat Exchangers
6 Phillips Optimized Cascade LNG Process
-161 °C
GAS TREATING (SWEETENING)
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Gas sweetening (CO2 removal, also H2S)
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Gas Sweetening Absorption Processes
• Separate CO2 from NG – Gas supply at high pressure (>35 bar) – Typically also removes hydrogen sulphide (H2S) as well
• Removal specifications – < 2 % (pipeline) – < 50 ppm (LNG plant feed)
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Covered in Gas-liquid absorption columns
Gas sweetening: Example
• CO2/H2S Removal using amine based solvent
• Information: – 1,000 Sm3/day gas @ 6000 kPa – 0.4% H2S, 3.0% CO2 – 20% solution of DEA
• What circulation rate is required?
• Estimate the plant requirements
10 Simplified Design Calcs: GPSA Handbook
Dr, Dc in mm, Q in MSm3/day, P in kPa
Gas sweetening: Example • QDEA = 360(Qy/x) = 360(1.0*3.4/20) = 61.2 m3/h
– Add both acid gas concentrations – 1 MSm3 = 1000 Sm3
• Dc = 10750*sqrt(1.0/√6000) = 1221 = 1200 mm (1.2 m)
– Based on gas flowrate and density (pressure) – Quite reasonable diameter for a column
• Dr = 160*sqrt(61.2) = 1251 = 1300 mm (1.3 m) – Based on amine flowrate – Above feed point regen diameter ~67% or 900 mm
• Reboiler – H = 93*61.2 = 5690 kW – A = 4.63*61.2 = 286 m2
– etc. for other process equipment
Simulation tools providing greater design flexibility/accuracy – Example VMGSim
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DEHYDRATION
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Dehydration to get Natural Gas & Condensate to Shore: 1. To prevent hydrate formation which would block subsea pipeline
2. To prevent internal corrosion of subsea pipeline
Dew-point Control or
Glycol Dehydration
Glycol and Molecular Sieve
Dehydration
Dehydration onshore: 1. As above – but may need even tighter dew-point control
2. Meet a sales gas water content spec.
Export Gas Pipeline
Dehydration of a gas stream, TEG absorption and regeneration
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What is the saturated water content for a sweet gas that is at 50 °C and 15000 kPa if the gas has a molecular weight of 45 g/mol?
If the first gas processing stage will decrease the temperature to 10 °C without significantly changing the pressure, is dehydration critical prior to this initial processing stage, and if so why?.
Gas Dehydration: Water Content
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Information: Temp 50 °C Pressure 15000 kPa Molecular weight of 45 g/mol
Gas Dehydration
Uncorrected Water Content: 1050 mg water/sm3 wet gas
Corrected Water Content: = 1050 * 0.93 = 977 mg water/sm3 wet gas
Correction for molecular weight of gas / gas density: 0.93
50 °C 0.93
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If the first gas processing stage will decrease the temperature to 10 °C without significantly changing the pressure, is dehydration critical prior to this initial processing stage, and if so why?.
Gas Dehydration
19 (Petrobras Hydrate Plug Removed from Pig-Catcher)
TEG Dehydration example
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A Natural gas stream saturated with water enters a triethylene glycol (TEG) contactor at 50 °C. The gas leaving the contactor must have a water dew point below 0°C. What is the minimum concentration of TEG solution coming back from the regeneration required to dehydrate the gas if a 10 °C approach to equilibrium in the column is assumed? Equilibrium data for TEG contactors is provided in the Figure.
TEG Dehydration example
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Information: Temperature 50 °C Leaving water dew point 0°C 10 °C approach to equilibrium Temperature
~ 99.4 wt% TEG
Physical properties of glycols
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Hysys Example of a TEG Stripper and Regeneration – Process Modelling
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Why are Glycols not enough for LNG?
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Other Dehydration
• Gas adsorption – Molecule Sieves common in gas processing – As pointed out adsorption topics not considered in process
modules
• Gas Permeation – Membrane dehydration
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Packed in a tower
Wet Feed Gas in at top
Dry Feed Gas out at bottom
Between 5000kg & 10000 kg in each tower
Mole Sieve gas dehydration
Adsorption and Regeneration Cycling
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Dehydration at Apache’s plant at Varanus Island, NWS
Wet gas
Dry gas
GAS PROCESSING
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Successive distillation/absorption to remove heavier components – mostly just methane for LNG
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Absorption De-ethanizer De-propanizer De-butanizer
To LNG
Westfarmers Straddle NGL plant: Distillation fractions heavier components to LPGs
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Simulation (Hysys etc.) would typically be used for multi-component distillation (MCD) But, McCabe-Thiele Approach for MCD can give initial estimate.
• Example: de-propanizer
– 0.01 ethane, 0.64 propane, 0.3 butanes, 0.05 pentanes – top product with < 0.01 butane, bottom <0.02 propane
• Light Key – Propane
• Heavy Key – n-Butane
• Relative Volatility, α = 3.0
• Non key components: ethane, n-pentane
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Equilibrium line - relative volatility of key components
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Propane Liquid Composition
Propane Vapour Composition
Also need: Operating lines, Reflux Ratio, Feed
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Example Feed: Saturated Vapour – leaving an expansion and heat removed through heat exchanger Assume 65% propane
Lecture 2
Example Reflux Ratio: 1.3
Goal: top product with < 0.01 butane, bottom <0.02 propane
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Rectifying section operating line using reflux ratio
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Y
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intercept 0.99/(1.3+1) = 0.43
0.99,0.99
(< 0.01 butane)
Feed line for the saturated vapour
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Saturated Vapour – Horizontal Line
(Feed = 65% propane)
0.65 feed (0.64 + 0.01)
Operating line for stripping section
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Bottom composition: 0.02, 0.02
Feed and operating line intersection
(< 0.02 propane)
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McCabe-Thiele MCD w/ binary of Key components
Y
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Draw in Stages
McCabe-Thiele MCD w/ binary of Key components • Theoretical stages - 29
– In reality would require more stages – Efficiency of each stage not 100% - not at equilibrium – And of course this is only an estimate
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Example Hysys simulation for the production of natural gas liquids (NGL)
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REFRIGERATION
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LNG Production: Refrigeration required to −160 °C
-161 °C
Even NGL significant cooling required LNG - MCHE
Wesfarmers example: Turbo-expander
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Example using the information provided: Expansion from 5900 kPag to 2800 kPag. Inlet Temperature -45 °C, Exit -75 °C (~230, 200 K). Draw Process on P-h diagram (methane) and Estimate Isentropic Efficiency
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out in isentropic isentropich h h ε= +Δ
Expansion in a turbo expander
Need: hout hin hisentropic
Δhisentropic
650 750
605 645 670
605
645
670
-65
( ) /isentropic out in isentropich h hε = − Δ
40 %
Turbo-expansion process
40% Isentropic efficiency reasonable?
• Short answer, no – Typically greater than 80%
• Why?
• Methane used as the refrigerant for the P-h diagram, example from Wesfarmers is on their NGL train and would contain other components
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Interested in process design? http://www.arrowenergy.com.au/__data/assets/pdf_file/0003/1938/0620-20Project20Description20-20LNG20Plant.pdf
Interested in learning more? http://www.arrowenergy.com.au/__data/assets/pdf_file/0003/1938/0620-20Project20Description20-20LNG20Plant.pdf