Marine Loading
1. Design of the VRU
Q(i): is given by the maximum loading capacity of the terminal (max. N° loading arms connected x capacities) + vapour growth factor. The vapour growth factor is given by the pressure drop of the vapour collector and the vaporization of the vapours above the liquid.
Q(c): as loading is continuous it is directly based on Q(i)
Q(4): again as the loading is continuous it is based on Q(i) – vaporization effect which stabilized during loading.
Q(d): Not considered as loading is continuous over a long period and no adjustment can be done.
Concentration: is a uctuating value as maximum concentration is reached only at the end of the loading operation and is almost nil at the beginning.
Vapour composition is based on the same principle than for trucks, composed of light ends HC.
2. Vapour connection
A: The distance is generally long between jetties and VRU and a blower may be necessary to drag the vapours through the system. The blower is located at the outlet of the unit as it is the safest place (clean air in normal operation).
B: The dock Safety Unit design depends on Terminal philosophy but due to modern vessels design a shutdown valve and a DA is sufficient (see our HAZOP)
3. Product loading
Hydrocarbon vapours are heavier than air and form a layer on top of the product. This layer prevents further evaporation
After 80% of the loading time the new vapours start leaving the vessel.
At the end of the loading operation the relative vapour saturation will reach approximately 80%
During unloading of the tanker inert gas or exhaust vapour is injected into the tanker. The Oxygen content is less than 5% by volume; Overall vapour concentration after unloading is relatively low.
Crude Oil
Relatively high content of Methane
- Methane has a very low loading factor on activated Carbon
- The fraction retained will not re-absorbed in an Absorbent
- Finally all Methane goes to Atmosphere
Mercaptan / Hydrogen Sulphide
- H2S concentrations are usually low
- H2S decomposes to elementary Sulphur in the Pores of the Activated Carbon,
which cannot be removed by Vacuum - Treatment is possible by additional pre- lters using Limonite (Iron Ore) or special
impregnated Carbon
4. Absorbent Circulation
- Circulation on one of the product tanks of the terminal.
- Circulation on the product loading line.
- Circulation on a dedicated or intermediary tank.
- Possibility of Liquefaction of the desorbed Vapour by Compression and Cooling.