TOKYO GAS TOP > Technical development > For the stable production of city gas > LPG BOG Suppression Technology Utilizing Cold Energy of LNG

LPG BOG Suppression Technology Utilizing Cold Energy of LNG

Objective


The LPG used to adjust the calorific value of city gas is stored virtually under atmospheric pressure. The entry of heat into tanks causes some to evaporate as “boil-off gas” (BOG). Conventionally, BOG is pressurized by means of a compressor and then re-liquefied using cooling water. As this method uses a motor-powered compressor, however, it consumes considerable electricity.

In order to cut the cost of processing BOG and reduce energy consumption, therefore, Tokyo Gas has introduced a new process that uses the cold heat of LNG. This new method cools and condenses LPG-BOG by feeding super-cooled LPG that has been cooled by LNG in a heat exchanger into a tank’s gas phase. As this does away with the need for a high-pressure compressor, this method is potentially much less expensive overall than the conventional one.
 
Comparison of stages in conventional and new systems
 
  
 

How it works


A part of LPG pumped out of a tank is cooled with LNG in a heat exchanger. This super-cooled LPG is sprayed into the gas phase in a tank to condense the BOG. Tank pressure can be kept within a certain range by controlling the flow rate or temperature of the super-cooled LPG.
Figure(Example of results of continuous pressure control test) shows some typical results of a continuous pressure control test. In this test, tank pressure was controlled by changing the flow rate of LPG.
 
Example of results of continuous pressure control test
 
  
 

Features


The main features are as follows:
 
  • Facilities cost is expected to be much less than that of conventional methods.
  • Maintenance cost is also reduced as no high-pressure compressor is needed.
  • In the BOG processing system, separate consideration needs to be given to dealing with non-condensing gas that may be in the tank.
  
 

Prospects


This system has been installed at the Ohgishima LNG Terminal, and it is under a stable operation since its start in autumn 1998.

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