What is Boil off Gas and Estimating of BOG rate

What is boil-off-gas?

LPG, Liquid Ethylene, LNG tankers are designed to carry natural gas in liquid form at a low-temperature of – 40°C, – 104°C, – 163°C, close to the vaporization temperature. Despite tank insulation designed to limit the admission of external heat, even a small amount of it will cause slight evaporation of the cargo. This natural evaporation, known as boil-off-gas (BOG) is unavoidable and has to be removed from the tanks in order to maintain the cargo tank pressure or structure integrity.

Other than heat leak, there are other scenarios can lead to BOG generation:

Vaporized vapor due to barometric pressure decrease & ambient temperature increase. 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). 
Heat leaks into Cryogenic fluid rundown piping, circulation / loading line, ship / truck loading arm & storage tank. 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.
Heat generated by rundown pump & in-tank pump and leaks into Cryogenic fluid. 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. 
Flashing of non-condensable gasses. Present of inert / non-condensable gasses such as nitrogen, carbon monoxide in cryogenic fluid may flash in the storage tank.
Negative Joule-Thompson effect. 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.
Cooling of loading arm & tank in ship / truck. Loading arm is heated to ambient temperature when it is unrest for long time. Cryogenic tank in ship / truck is heated by ambient when it is returned with empty tank. All loading arm and tank in ship/truck will needs cooling prior to storage. Large amount of BOG is generated during cooling time.

How to Handling the pressure?

To relieve the pressure in LNG tanks, BOG can be re-liquefied, or used as fuel, or burned in a gasification unit, BOG header-flare.

Re-liquefaction occurs when evaporated LNG is cooled and reverted back to its liquid state. However this presents its own problems. Re-liquefaction requires a lot of equipment, meaning tanks become very large. The process is more practical for large land based liquefaction plants, where space is not as much of an issue.

At sea, BOG can also be managed through combustion. Excess gas is fed to the ship’s engines which have a suitable fuel pressure to consume it. The LNG gas is used as fuel, with some tankers relying on BOG to perform certain maneuvers. Another alternative is to burn the unwanted gas in a gasification unit, but this results in wastage of materials and valuable energy.

Quick way to estimate BOG flow!

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 pipingloading 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, kp 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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