BOG Spray Recondenser

Technological Chanllenges

Boil-off gas (BOG) is the single most important factor to consider in designing and operating LNG-based systems. The BOG generated during LNG bunkering behaves significantly differently from that of the LNG carriers or onshore terminals where BOG handling is a well-established practice. The fuel LNG storage tank onboard will be pressurized due to the external heat ingress. Before LNG bunkering, the high-pressure BOG in the tank should be removed and transferred to the LNG bunkering facility which may be a bunkering terminal or a bunkering shuttle. This BOG is characterized by being “intermittent” and may represent a considerable peak pressure load. That is, the LNG bunkering operation should consist of two serial sub-operations: one for BOG transfer and the other for LNG transfer.

A conventional type BOG reliquefaction system is based on direct liquefaction of vapor into liquid, and difficult to apply to this case. One solution with the conventional technology is to increase the liquefaction capacity to deal with the peak load. Of course, this is economically discouraging due to the considerable equipment cost. Another solution is to keep the liquefaction capacity for the average load and liquefy the BOG gradually by extending the BOG transfer sub-operation and delaying the LNG entire bunkering operation. Note that both solutions should be on the idling mode for the LNG transfer sub-operation. That means, they should experience frequent load changes which may cause equipment failure and impose operational complexities on the crew.

Solution by Lattice Technology

 The BOG Spray Recondenser of LATTI-CE consists of the BOG recondensing unit and the LNG subcooling unit as shown in the figure. The recondensing unit is a tank half filled with LNG. 

Part of the LNG discharged from the recirculation pump is recycled to the spray nozzles, and the rest is delivered to the LNG subcooling unit and ultimately to the spray nozzles.

Process concept of BOG Spray Recondenser 08

 Operation of the BOG Spray Recondenser is continuous and steady. The incoming BOG during the BOG transfer sub-operation is mixed with the sprayed LNG. This leads to increase in the pressure and temperature of the LNG. Consequently, the pressure of the BOG recondensing unit increases on

over the sub-operation. As the BOG supply is stopped for the LNG transfer sub-operation, the low-temperature LNG from the LNG subcooling unit, which is typically a nitrogen refrigeration system, starts to cool downthe LNG and finally brings thepressure and temperature back close to the initial state. Over the both sub-operations, the rate of the LNG circulation and the load of the LNG subcooling unit are kept constant to attain the steady operation.

Schematic layout of BOG Spray Recondenser