Cryogenic liquids, such as liquefied natural gas (“LNG”), liquid argon (“LAR”), liquid nitrogen (“LIN”), liquid oxygen (“LOX”) or liquid hydrogen, are stored in insulated storage vessels to minimise the loss of liquid by vaporization. However, despite the insulation, some of the stored cryogenic liquid will inevitably vaporize due to heat leaking into the vessel (and the associated piping), pump work into re-circulating liquid streams and flash vaporization from liquid feeds, etc. Steps must therefore be taken to avoid a dangerous build-up of pressure in the storage vessel.
It is often preferable economically to use very large, low pressure storage vessels to store cryogenic liquid. For example, currently, the largest storage vessels for LOX or LIN have a capacity of about 5000 m3 and the largest storage vessels for LNG (usually found on ships in groups of up to 5 storage vessels) each have a capacity of about 40,000 m3. The scale of the problem of pressure build-up due to boil-off is relative to the size of the storage vessel.
It would be possible to simply vent the boil-off vapor to the atmosphere. However, such a solution is not desirable as it would be very costly in terms of wasted cryogenic liquid (which is expensive to produce) and, if the vapor is inflammable (e.g. natural gas or hydrogen), might result in a potentially dangerous build-up of vapor, particularly where very large storage vessels are used.
Another solution to combat boil-off from the storage vessel is to sub-cool sufficiently the liquid feed to the storage vessel to re-condense the boil-off vapor. However, unless the sub-cooled feed is brought into good mass and energy contact with generated vapor, equilibrium will not be achieved. Without equilibrium, either much of the vapor would still have to be vented or the amount of sub-cooling would have to be increased to increase the driving force for the vapor condensation.
To sub-cool the liquid feed by more than the minimum amount required to re-condense the boil-off vapor would result in additional energy consumption. The “excess” sub-cooling will generally result in the sub-cooled (and therefore denser) feed liquid sinking to the bottom of the storage and forming a lower stratified cold liquid zone.
To provide the required close thermal contact between the sub-cooled feed and the boil-off vapor, it is known to spray the sub-cooled liquid feed in the vapor space of the storage vessel. Such a spray technique is carried out to collapse the pressure of returning cryogenic liquid road trailers when being re-filled. However, spray techniques such as these are not very practical for very large storage vessels and are less effective when the storage level is high due the shorter contact time between the liquid and the boil-off vapor. In addition, spray (or “sparge”) devices generally require an increase in the liquid feed pressure due to the pressure drop of the device and this increased supply pressure might not be available.
There are other techniques for condensing boil-off vapor known in the art. For example, U.S. Pat. No. 3,894,856 (Lofredo et al; published 15 Jul. 1975) discloses a process for purifying and liquefying natural gas. One of the objectives of the process is to maintain a constant composition of liquefied natural gas (“LNG”) in a storage tank by liquefying vapors which are generated in the tank. In the exemplified embodiment, LNG boil-off vapor from the storage tank is condensed outside the tank by indirect heat exchange against LIN. The condensed vapor is then returned to the storage tank thereby maintaining the composition of the LNG in the tank.
U.S. Pat. No. 6,470,706 (Engdahl; published 29 Oct. 2002) discloses a boil-off vapor condenser in which boil-off vapor is condensed by direct heat exchange against a liquefied gas. It is disclosed that the condenser has particular application in storage and distribution systems for LNG. In these systems, the LNG is stored in a storage tank. Boil-off LNG vapor is fed to a vapor condenser provided outside the storage tank where it is condensed by direct heat and mass transfer with LNG pumped from the storage tank. Heat and mass transfer may be provided using random packing (such as 2 inch (5 cm) Pall rings), structured packing, tray columns or spray elements. The condensed LNG vapor is then fed to high pressure pumps, from which it is then routed to a distribution pipeline.
It is desirable to have a method of utilizing the refrigeration of a sub-cooled liquid feed to reduce boil-off in a compact device that will not significantly increase the required supply pressure of the liquid feed. It is particularly desirable that the method be suitable for use in very large low pressure storage vessels.
It is an object of preferred embodiments of the present invention to provide a storage vessel for cryogenic liquid in which at least some vapor piping is eliminated thereby reducing the cost and complexity of the storage vessel.
It is a further object of preferred embodiments of the present invention to provide a storage vessel for cryogenic liquid in which a boil-off vapor condenser does not require a leak tight enclosure.
It is a still further object of preferred embodiments of the present invention to provide a storage vessel for cryogenic liquid that does not require the use of an external refrigerant to provide refrigeration duty to condense boil-off vapor.