Chemical & Process Technology

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Following are some typical results :

• 1 x 10⁶ tonnes LNG = 52 x 10¹² BTU 
• 1 x 10⁶ tonnes LNG = 8.68 x 10⁶ barrels oil eq
• 1 x 10⁶ tonnes LNG = 1.23 x 10⁶ tonnes oil eq
• 1 x 10⁶ tonnes LNG = 2255.822 x 10³ m3 LNG 
• 1 x 10⁶ tonnes LNG = 1.38 x 10⁹ m3 NG 
• 1 x 10⁶ tonnes LNG = 48.7 x 10⁹ ft3 NG

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Comments :

1. Exact conversion subjects to molecular weight. Above is only approximation.
2. Finding shows above was based on "BP Statistical Review of US, Energy" as source.
3. Present version is 1.1. Only cover mass-volume conversion. Wish develop can expand it to cover flowrate to ease users.
4. There is no clear definition of volumetric flow at condition i.e. at Standard or Normal condition, etc.

Related Topics

• Contaminants and Impurities Limit in LNG
• Design of Compact Plate Fin Heat Exchanger
• Do not Under-estimate The Impact of Altitude Change
• Quick Way to Estimate Insulation for Cold Services
• Quick Way to Estimate BOG
• How Boil-Off-Gas (BOG) is Generated
   

Corrosion & Cracking

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Stratification & Rollover

Toxicity
Present of H2S, COS and merceptants in LNG would results high toxicity LNG. The following list out typical level in LNG :

Component	Practical Acceptable Concentration (ppmV)	Remark
H2S	4
COS	0.5
Total Sulfur	10 - 200	Subject to country
Merceptants Sulfur	6 - 15	Subject to country

Above are typical value for quick reference. Figures may change according to LNG composition, liquefaction temperature, etc and will varies from case to case. Shall be used as reference only.



Related Topics

• Design of Compact Plate Fin Heat Exchanger
• Do not Under-estimate The Impact of Altitude Change
• Quick Way to Estimate Insulation for Cold Services
• Quick Way to Estimate BOG
• How Boil-Off-Gas (BOG) is Generated
• Techniques to Achieve Cryogenic Temperature
• PFHE & CWHE Comparison in LNG Plant
• LNG and Supply Chain

Operating pressure is commonly written as gauge pressure e.g. kPag, barg, etc in engineering whilst absolute pressure e.g kPaa, bara, etc. Absolute pressure equal to gauge pressure plus atmospheric pressure.

Example :
5 bar abs = 5 barg + 1.01325 bar = 6.01325 bara

The standard atmosphere pressure is pressure defined as being equal to 101,325 Pa or 101.325 kPa and normally refer to mean sea level (h=0 m). Atmospheric pressure is decreased with altitude (elevation from mean sea level or earth surface) with following relation.

where
P_b = Static pressure (Pa)
T_b = Standard temperature (K)
L_b = Standard temperature lapse rate -0.0065 (K/m) in ISA
h = Height above sea level (meters)
h_b = Height at bottom of layer b (meters; e.g., h1 = 11,000 meters)
R = Universal gas constant for air: 8.31432 N.m /(mol.K)
g₀ = Gravitational acceleration (9.80665 m/s2)
M = Molar mass of Earth's air (0.0289644 kg/mol)

Altitude = 0 – 11000m, T_b = 288.15 K, L_b = -0.0065 K/m, P_b = 101325 Pa
Altitude =  11000 – 20000m, T_b = 216.65 K, L_b = -1 x 10E-30 K/m, P_b = 22632.1 Pa
Altitude =  20000 – 32000m, T_b = 216.65 K, L_b = 0.001 K/m, P_b = 5474.89 Pa

By inclusion of specific parameters, for altitude = 0 – 11000m, atmospheric pressure is

Example
At altitude of 500m above mean sea level, atmospheric pressure is approximately 95460.84 Pa.
At altitude of 1000m above mean sea level, atmospheric pressure is approximately 89874.57 Pa.

How altitude impacting LNG production rate ?
Let compare LNG production rate change with a plant built at seaside (h = 0) and another LNG plant built at a site with altitude of 1000m. For both plant, LNG store at same gauge pressure (e.g. 50 mbarg), same LNG rundown temperature (e.g. -163 degC), same LNG tank dimension with same in-leak heat (normally higher altitude is with lower ambient temperature, lower in-leak heat is expected. However, the impact is negligible).

LNG from Main Cryogenic Heat Exchanger (MCHE) outlet is set at 50 barg and negative 164.8 degC. A JT valve is letting down pressure to LNG storage tank operating pressure of 50 mbarg. MCHE outlet mass flow set at 50000 kg/h. LNG is pure Methane (C1). Assumed same in-leak heat of 300 kW.

Case : Seaside
Altitude, h = 0m
Atmospheric pressure = 101.325 kPa abs
LNG operating pressure = 50 mbarg = 5 + 101.325 kPa abs = 106.325 kPa abs
From simulation (see below image), BOG flow = 1374 kg/h,
LNG production rate = 48626 kg/h

Case : Inland
Altitude, h = 1000m
Atmospheric pressure = 101325 (1 - 2.25577E-05 x 1000)^5.25588 = 89.8746 kPa abs
LNG operating pressure = 50 mbarg = 5 + 89.8746 kPa abs = 94.8746 kPa abs
From simulation (see below image), BOG flow = 1863 kg/h,
LNG production rate = 48137 kg/h

Same facilities and operating condition, the LNG production in Inland (at 1000m) reduced by 1%.

How altitude impacting Air Compressor / Blower power requirement ?


Concluding Remark
Atmospheric pressure change with altitude and this will have impacts to facilities design. Do not under-estimate this impact.

Earlier post "How Boil-Off-Gas (BOG) is Generated" and "Quick Way to Estimate BOG" discussed several ways result Boil-Off-Gas (BOG) generation and simple way to estimate BOG. Heat leak into piping wrapped with Cold insulation is one of the way possibly result significant BOG generation, in particular in large base load plant where product is loaded into ship. Long rundown and loading line from production plant and from storage tank to ship can generate significant BOG. Proper selection and determination of insulation thickness can minimize BOG generation economically.




Rule of thumb for economic heat flux for heat in-leaks range from 25 to 35 W/m². This is the key rule to derive an economic insulation for cold service whilst minimizing BOG generation.

Common insulation material used in cold services (sometime called cryogenic service in extremely low temperature case e.g. LNG with operating temperature of approximately -162 degC) are typically Polyurethane and Polyisocyanurate Foam (PIR). These insulation material have very low conductivity minimizing heat from ambient entering cold fluid. Typical thermal conductivity for these material is approximately 0.023 W/mK @ 20 degC.
Estimation of piping in-leak heat from ambient starts with estimation of heat transfer coefficient using Nusselt number which is a function of Prandtle and Reynolds number.

Overall heat transfer cooefficient estimate from Nusselt number :

Piping in-leak heat can be estimated with following equation

with heat flux

Piping nominal size = 6"
Piping external diameter = 152.4 mm
Piping length = 100 m
Insulation material = Polyisocyanurate
Insulation thermal conductivity = 0.023 W/mK
Assumed Insulation thickness = 60 mm
Piping external diameter (including insulation) = 152.4 + 2x60 =  272.4mm

Air :
Air Velocity = 5 m/s
Air Thermal Conductivity = 0.029 W/mK
Air specific heat = 1009 J/kgK
Air density = 1.038 kg/m3
Air viscosity = 0.02 cP


Calculation :
Air Prandtle Number = 0.6959

Air Reynolds Number =70687.8
Overall Heat transfer Coefficienct = 18.096 W/m2K
Piping In-leak heat = 2125.72 W
Piping surface area = 85.58 m2
Average heat flux = 24.8 W/mK < 25 W/mK...OK

Similar calculation applied to 1" to 28"
1" - 50mm insulation thickness
2" - 50mm insulation thickness
3" - 60mm insulation thickness
4" - 60mm insulation thickness
6" - 60mm insulation thickness
8" - 70mm insulation thickness
10" - 70mm insulation thickness
12" - 70mm insulation thickness
16" - 70mm insulation thickness
20" - 70mm insulation thickness
24" - 70mm insulation thickness 
28" - 70mm insulation thickness

Earlier post "How Boil-Off-Gas (BOG) is Generated" has discussed several ways can result Boil-Off-Gas generation. They are listed below :

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Atmospheric pressure at sea level is 101.325 kPa abs. Atmospheric pressure is reduced with increase in altitude. For example, at elevation of 1,000 meter, the atmospheric pressure can be as low as 89.81 kPa abs. Cryogenic storage may be designed to operate between 50-70 mbar gauge. If the cryogenic storage tank is at beach (sea level), the operating pressure in the tank is approximately 106.325 - 108.325 kPa abs. If this cryogenic storage is at 1,000 meter, the operating pressure in the tank is approximately 94.81 - 96.81 kPa abs. Lower operating pressure in tank can results higher vaporization and more BOG is generated. Therefore, it is always a good practice to use absolute pressure whenever dealing with cryogenic storage tank. Correct pressure modeling in process simulator is extremely important in finding quantity of BOG generated.

Heat leaks into cryogenic fluid can be via rundown / circulation piping, loading arm & storage tank. Proper selection, installation and maintenance of insulation is one of the key factor in minimizing heat leaks into cryogenic system, hence BOG generation. Besides insulation, other external factors such as wind speed, solar radiation, ambient temperature, sand conductivity and etc, affect heat leak. However, these factors are hard to be managed. Heat leaks into system can be calculated by considering heat conduction, convection and radiation. However, this type calculation involve a lot of uncertainties, assumption and rather complicated. Based on past experiences, an approximate method using vaporization coefficient in determining BOG generation due to heat leaks via storage tank, may be considered during conceptual phase.

Vaporization coefficient (k)  may range from 0.04% to 0.06% for LNG whilst 0.06% to 0.1% for Propane, Butane and LPG. One may take note that above are typical for large storage tank e.g. 160,000m3. Higher k factor should be used for smaller storage. For example, 60,000m3, k of 0.08 - 0.1% may be considered.



Above equation is applicable to storage tank which is low surface area-to-volume ratio. However, piping with very low volume and high surface area may experience higher heat input comparatively. Following equation may be used to estimate BOG generated due to piping.
Average heat flux subject to piping diameter. In general, k_p of 25 -35 W/m² may be considered.

Energy is transferred to pump to move quantity of liquid. Part of the energy will loss due to deficiency. and results BOG generation. Following equation may be considered to estimate BOG generated due to pump deficiency.

Pump efficiency can be range from 55% - 75% for common centrifugal pump.

Cryogenic liquid produced from main plant and transfer to cryogenic liquid storage tank. Inflow liquid will displaced vapor and add-on to BOG generation. Following equation may be used.

Other factors result generation of flashed vapor or BOG generation such as present of non-condensable gasses, negative Joule-Thompson effect and "hot" rundown cryogenic liquid into "cold" cryogenic liquid, will possibly be modeled in process simulator.


Cooling of loading arm and tank in ship / truck may generate substantial amount of vapor initially and reduce as loading arm and tank is cooled. This BOG generation may required dynamic simulation which will not be presented in this post.

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Liquefied Petroleum Gas (LPG) contains mainly Propane (C3) and Butane (i-C4 & n-C4), Liquid Ethylene (C2=) and Liquefied Natural Gas (LNG) contains mainly Methane will evaporate at ambient condition e.g. 20 degC @ 101.325 kPag.

LPG, Liquid Ethylene and LNG can be stored in refrigerated vessel at its bubble point and atmospheric pressure. Their bubble point can be as low as -40, -104 and -162 degC are commonly known as cryogenic temperature and fluid as cryogenic fluid.

Heat leaks into the cryogenic fluid will results vaporization and lead to Boil-Off-Gas (BOG) generation. Other than heat leak, there are other scenarios can lead to BOG generation :

Environment pressure and temperature change affects BOG generation. Maximum BOG generation during summer, noon and high elevation (with low barometric pressure). On the other hand, minimum BOG generation during winter, mid-night and near sea side (high barometric pressure). 

Ambient heat leaks cryogenic fluid will be limited by insulation layer. Heat  leaks into subjects to insulation thickness, thermal conductivity, installation quality, etc. Higher insulation thickness, lower thermal conductivity, high installation quality and etc maintain good heat insulation and reduce BOG generation.

Pumps is required for transferring cryogenic liquid from production plant to storage tank and from storage tank to ship/truck. Pump will absorb power to move cryogenic fluid and any deficiency will generate heat and it will transfer into cryogenic fluid. Pump heat leaks subject to pump capacity, develop head and efficiency. Larger pump, higher head and lower efficiency lead to excess heat generation and leaks into cryogenic fluid. 

Present of inert / non-condensable gasses such as nitrogen, carbon monoxide in cryogenic fluid may flash in the storage tank.

Another phenomenon is negative Joule Thompson (negative JT) where pressure decrease in rundown line lead to higher temperature. Typical gas is Hydrogen. 

"Hot" rundown into "cold" cryogenic fluid 
Hot cryogenic fluid from one train with hotter temperature which carries heat and rundown into cryogenic  tank with colder temperature can results vaporization.

Related Topics

• Techniques to Achieve Cryogenic Temperature
• PFHE & CWHE Comparison in LNG Plant
• LNG and Supply Chain
• Gas Processing, NGL Extraction & LPG Fractionation
• Typical Gas Processing Flow Scheme
• Typical Refinery Flow Scheme
• Floating Gas Refinery Unit
• Use Wobbe Index to Manage Fuel Quality to Gas Burner

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The process is based on a suitable modified Claude cycle which minimizes the umber of heat exchangers and also takes care to accommodate the in house developed turbo xpander. The process design is carried out using the standard calculation procedure and is validated by using process simulation software, Aspen Hysys. parametric analysis is carried out to access the role of different component efficiencies in predicting overall system efficiency at the design and off design conditions. In this analysis, the available turbo expander efficiency is considered to evaluate the feasible heat exchanger efficiency in order to optimize the plant efficiency. The thermodynamic parameters (temperature, pressure, pinch point temperature) are evaluated to obtain the optimum mass fraction through turbo expander for maximum liquid yield. This investigation not only gives the analysis of nitrogen liquefier, but also it will act as a basic frame work for any liquefier and helium liquefier in particular as a future mission.

Related Topics

• PFHE & CWHE Comparison in LNG Plant
• Techniques to Achieve Cryogenic Temperature
• LNG and Supply Chain
• Gas Processing, NGL Extraction & LPG Fractionation
• Typical Gas Processing Flow Scheme
• Typical Refinery Flow Scheme
• Floating Gas Refinery Unit
• Use Wobbe Index to Manage Fuel Quality to Gas Burner

Main Cryogenic heat exchanger (MCHE) is one of key equipment in natural gas liquefaction (LNG) plant. MCHE used in liquefaction of natural gas having some special characteristic :

Two type of compact MCHE widely used in LNG plant. There are Plate-Fin Heat Exchanger and Coil (Spiral) Wound Heat Exchanger. Below images are typical CWHE and PFHE.

CWHE

PFHE