Boil-off gas recovery

Natural gas becomes Liquefied Natural Gas (LNG) by cooling it to minus 162 °C . Inside an LNG storage tank LNG wants to become a gas again. It does not remain cold in an vacuum insulated cryogenic tank as there is no cooling system. After a while the LNG starts to “boil”, after which it becomes gaseous at the top which causes a pressure increase in the tank, the so-called boil-off gas.

Cooling back boil-off gas: re-liquefaction

If a LNG service station has little flow through and is not frequently refilled with cold LNG from the major terminals, that boil-off needs to be controlled in order to avoid venting into the atmosphere. Our patented Micro Liquefaction Unit (MLU) automatically and efficiently regulate that entire process. It takes the light fractions from the top of the cryogenic storage tank, re-liquefies the gas and returns it as a liquid to the tank bottom at about minus 135 degrees Celsius.

Boil-off gas Treatment Unit (BTU)


An alternative way of controlling BOG is using liquid nitrogen to cool with spirals, inside the storage tank. However this solution is less effective in terms of TCO, as nitrogen is expensive and needs to be transported to the site by a truck. It furthermore requires active management and logistics of the nitrogen storage that needs to be positioned onsite. 

Cost reduction by innovation

With LIQAL’s Micro Liquefaction Unit (MLU) operational costs are reduced while the quality from the LNG in the tank remains. This installation, which re-liquifies 'reheated' natural gas to LNG, is sometimes delivered in combination with an LNG refuelling station, but also operates as a stand-alone unit and on-board of ships.

Performance Features

Standard Containerized Design (BTU-1)

  • Designed for stationary LNG plants
  • Mobile design with flexible vacuum isolated hosed for process connection
  • Gas treatment capacity 23 kg per hour
  • Storage pressure conditioning between 3.0 and 8.0 barg
  • No redundancy installed, design capacity 1 x 100%
  • Minimized startup time
  • Zero emission solution for LNG filling stations
  • PED 97/23/EC certified
  • System design according EN-1473 for onshore liquefaction processes

A brief recap of the system’s advantages, where reliable and predictable operations are required:

  • Proven technology, excellent performance
  • Unlimited holding time LNG = zero emissions
  • Constant LNG quality (condition ánd composition)
  • No need for active management of storage pressure (OPEX)
  • No LIN consumption and logistics
  • Larger usable volume of LNG storage
  • Possibility to supply cold LNG to clients (2-3 barg p_sat) and effectively serve the new Volvo HPDI.
  • Flexibility on placement dispenser(s) and unloading box
  • Stand alone, modular, CE certified unit (to be placed at other stations in your network if
  • throughput is sufficient / small footprint)
  • Predictable fuelings for clients
  • Low energy use 0.7 kWh/kg feed gas

Modular Skid-Mounted Design (BTU-2 to BTU-5)

  • Designed for LNG bunker storage conditioning
  • Gas treatment capacity up to 1000kg per hour
  • Storage pressure conditioning between 0.1 and 0.5 barg
  • 4 x 33% redundancy installed throughout the complete mechanical design
  • PED 97/23/EC certified

Our liquefaction systems are developed for:

  • Boil-off gas recovery on ships, in fuelling stations or other distribution infrastructure
  • Biogas to Liquefied BioMethane production
  • Natural gas to LNG production
  • Energy efficient integration with and supply of the complementing process equipment for gas upgrading and storage & loading is our specialty.
  • Designed for fully automated and stand-alone operation.



 Use LNG production
capacity (kg/h) 
 Model Dimensions
Power (kW) 
Max absorbed
power (kW) 
BOG feeding
 LNG return
 Stationary  17  BTU-1 10ft high cube container  30  15.3 1x3/4”NPT·M 1x3/4”NPT·M
 Marine  160  BTU-2  11 x 2.6 x 2.7m  290  125  2x DN50 PN16 2x DN50 PN16
   240  BTU-3 13 x 2.9 x 2.7m 433 180 2x DN80 PN16 2x DN80 PN16
   320  BTU-4 15x3.2x2.7m 577 240 2x DN100 PN16 2x DN100 PN16
   500  BTU-5 19x3.2x2.7m 902 370 2x DN150 PN16 2x DN150 PN16


This innovative liquefaction technology, which is suited for efficient control of conditions in on-board and onshore LNG storage tanks, ensures unlimited holding time of LNG in the tank. It also enables supply of both saturated- and cold LNG to effectively fill all available LNG truck types (including the new Volvo EURO 6), without the use of liquid nitrogen.

Call LIQAL now for a free estimate or quotation +31(0)8-54 861 000 or contact us >>

pdf LIQALPlease download our Liquefaction product sheet to find out more!