Why Re-liquefy?

The gas cargo carried by liquefied gas carriers is usually gaseous under Normal temperature and atmosphere conditions. Large-scale bulk transportation is possible by liquefaction of the gas substance. There are three ways to liquefy a gas:

• compression at normal room temperature (fully pressurised method)
• cooling and compression (semi-refrigerated, pressurised method) and
• cooling at normal atmospheric pressure (fully refrigerated method).

In a fully refrigerated type liquefied gas carrier, cargo is liquefied in accordance with the vapour pressure and temperature curve of the particular substance, by cooling the substances to less than the saturated vapour temperature of that substance in normal atmospheric pressure.

While the saturated vapour temperature of propane, a typical cargo of a fully refrigerated type LPG carrier is -42.1ºC, that of methane, the typical cargo of a LNG carrier, is -161.5ºC. Fully refrigerated type liquefied gas carriers load and carry the cargo at such low temperatures. However, during the voyage, despite the insulating material surrounding the cargo tank, there is a small rise in temperature of the cargo liquid due to natural heating from the atmosphere and "boil-off gas" occurs. As a result of this boil-off gas, the pressure inside the cargo tank rises. The designed pressure of the cargo tank of the fully refrigerated type liquefied gas carrier is usually about 0.025Mpa. The "Rules and Guidance for the Survey and Construction of Steel Ships" and the IGC code require that means of temperature/pressure control is provided to prevent the pressure inside the tank from exceeding the design pressure. One such means, which cools down the boil-off gas mechanically to control the pressure inside the cargo tank, is called a re-liquefaction plant.

Methodology

Each substance has a temperature (the critical temperature) at which it cannot be liquefied even if it is further pressurised. Since this temperature is 96.7 degrees for propane, it can be liquefied even at normal temperature by being pressurised to just more than the saturated vapour pressure. However, since that of methane (the usual LNG carrier cargo) is much lower, -82.6 degrees, it cannot be liquefied even if being pressurised, unless cooled down to less than that temperature. Therefore a re-liquefaction plant is usually used as a means of temperature/pressure control device on fully refrigerated type LPG carriers. It is comparatively simple, since seawater can be used as a cool-ant, because the cargo can be liquefied at normal temperature.

The principle is fundamentally as follows:

• Compressing the boil-off gas with a compressor forms a high-temperature and high-pressure gas.
• The seawater, which acts as a coolant in the condenser, absorbs the latent heat of the gas, and a high-pressure liquid is formed.
• It is returned into the cargo tank as low-temperature, low-pressure liquid by effecting an adiabatic expansion via an expansion valve.

On the other hand, to re-liquefy methane, for example on a LNG carrier, a device for refrigeration is necessary because a cryogenic substance must be used as the refrigerant to liquefy the methane. Generally, the economics of actual re-liquefaction plants for LNG carriers have not been favourable. Hence the means of "temperature/pressure control" for the cargo of LNG carriers has been a process whereby the boil-off gas is burned in the main boiler as a fuel for the steam turbine propulsion system, with obvious economic advantages.

In adopting this method, the loss of the cargo cannot be avoided due to the use of the boil-off gas. But, at times when the price of LNG rises for example, the adoption of the LNG re-liquefaction plant can be re-considered as a more economical method. The first LNG re-liquefaction plant "ZERO-LOSS " announced in 1973, adopted the "closed Brayton cycle" in which nitrogen was used as the refrigerant. This system had the advantage of being simple, with the least number of components, thus offering the highest reliability and ease of operation. Another advantage of the system was that control of the plant capability could be achieved by changing the volume flow of the nitrogen in the closed refrigeration cycle. Thanks to the advantages, effective unmanned or automatic operations were possible, even if the amount of boil-off gas varied widely during the operational mode of the ship at the time of fully loaded voyages and ballast voyages.

• Main machinery and equipment being used is as follows:
• N2 compressor: 2-stage radial type
• Turbo expander/compressor: Single stage radial type, connected to each other
• BOG compressor: Single stage radial type
• Cryogenic heat exchanger: Brazed plate fin type
• N2 buffer tank: Cylindrical type

All of this machinery and equipment has been technically proven in cryogenic industries on shore. And the designs have been established based on further actual service. How-ever, the reason why it has not been adopted in ships until now is said to be the fact that it has been uneconomical in relation to the amount of fuel consumption for operation along with the initial cost of these devices. Currently, the first LNG carrier to install a LNG re-liquefaction plant is being constructed at Mitsubishi Heavy Industries, Ltd. Nagasaki and will enter service from this year. The plant has been developed jointly by Osaka Gas Co. Ltd., Nippon Yusen Kabushiki Kaisha, Mitsubishi Heavy Industries, Ltd. and Chiyoda Corporation. Confirmation of the reliability and improved economics over time may also see the complete replacement of the conventional steam turbine with more efficient modern diesel engines on LNG carriers.

The author is with Fluidflow Consultation Inc.