Patent Application
20080202158
The invention relates to the field of re-liquefaction of boil-off gases from liquid natural gas (LNG). More specifically, the invention relates to a method and system for cooling a boil-off gas stream, as set out in the introduction to the independent claim 1.
A common technique for transporting natural gas from its extraction site, is to liquefy the natural gas at or near this site, and transport the LNG to the market in specially designed storage tanks, often placed aboard a sea-going vessel.
The process of liquefying the natural gas involves compression and cooling of the gas to cryogenic temperatures (e.g. −160° C.). The LNG carrier may thus transport a significant amount of liquefied gas to its destination. At this destination, the LNG is offloaded to special tanks onshore, before it is either transported by road or rail on LNG carrying vehicles or re-vaporized and transported by e.g. pipelines.
LNG boils at slightly above −163° C. at atmospheric pressure, and is usually loaded, transported and offloaded at this temperature. This requires special materials, insulation and handling equipment in order to deal with the low temperature and the boil-off vapor. Due to heat leakage, the cargo (LNG) surface is constantly boiling, generating vaporized natural gas (“boil-off”)—primarily methane—from the LNG.
Plants for the continuous re-liquefaction of this boil-off gas are well known. The re-liquefaction of boil-off gases on LNG carriers results in increased cargo deliveries and allows the operator to choose the most optimal carrier propulsion system. LNG carriers have traditionally been driven by steam turbines, and the boil-off gases from the LNG cargo have been used as fuel. This has been considered a costly solution.
One such alternative to using the boil-off gas as fuel is the Moss RS™ Concept, wherein the boil-off gas is liquefied and the resulting LNG is pumped back to the cargo tanks. The Moss RS™ Concept, described in Norwegian Patent No. 305525 B1, is based on a closed nitrogen expansion cycle, extracting heat from the boil-off gas. Boil-off gas (BOG) is removed from the cargo tanks by two conventional LD compressors operating in series. The BOG is cooled and condensed to LNG in a cryogenic heat exchanger (“cold box”), to a temperature between the saturation temperature for compressed CH4 and N2 before being fed into a separator vessel where certain non-condensibles (mainly N2) is removed. The LNG coming out of the separator is pumped back to the cargo tanks, while the non-condensibles (i.e. gases) are sent to a flare or vent stack.
The patented Moss RS™ concept has so far been designed for implementation onboard LNGC vessels ranging up to gross volumes of 216 000 m3. However, as newer and larger vessels are designed, the onboard power supply systems are not enlarged proportionally with the ship's physical dimensions. This provokes changes in process design in order to present more energy efficient solutions for the re-liquefaction of LNG boil-off gas.
The present Moss RS™ concept is based on a nitrogen Brayton cycle with three-stage compression and one-stage expansion. Using only one expander reduces the complexity of the compander-unit (compressors and expander) to a minimum, but the internal temperature approach between hot and cold streams is inadequately large in the middle sections of the cold-box. This is shown in
The exergy losses can be reduced through the introduction of an additional expander. A recognized method for implementing such a unit is to split the refrigerant stream at a given temperature level and hence let two expanders work in parallel as described in Norwegian patent application 2004 0306.
It is, however, an object of the invention to reduce the exergy losses, without splitting streams. It is a need for a more efficient system that will improve performance and reduce the power demand.
The present invention meets that need, in that it provides a method for cooling a boil-off gas (BOG) stream prior to compression in a boil-off reliquefaction plant where the BOG stream following compression is reliquefied in heat exchange with a closed-loop refrigeration system comprising a coolant being compressed, before said reliquefied BOG being returned to a storage vessel, characterized by the following steps:
The invention also provides a system for cooling a boil-off gas (BOG) stream prior to compression in a boil-off reliquefaction plant, comprising a line for feeding BOG into a compressor prior to heat exchange with a closed-loop refrigeration system, said refrigeration system comprising compressors and expanders and a number of heat exchangers for heat exchange with the BOG stream, characterized in that the expanders are arranged in series.
An embodiment of the invention will now be described in more detail, with reference to the accompanying drawings, where like parts have been given like reference numbers.
Implementing two expanders 8, 9 in series instead of in parallel, the exergy losses are reduced without splitting streams. The closed coolant (nitrogen) loop will then be similar to that of
Choosing two expanders in series, the exergy loss area is reduced by the introduction of a new local temperature pinch. This becomes clear when investigating the composite curve of
The BOG is in most cases precooled before compression (by the compressor 11) prior to the cold-box. This is done in order to ensure a reasonable temperature profile in the cold-box and to achieve a more efficient compression in a reasonably sized LD-compressor-unit 11. The issue of precooling has been described in several other patents, involving methods such as
To be able to liquefy most BOG compositions with a minimum of exergy losses, it is crucial to bring as much of the nitrogen refrigerant as possible down to the liquefier section of the cold-box. However, taking out nitrogen before (i.e. upstream of) the expander, will work against this principle. The same goes for the methods involving recycled liquefied BOG to be used in the precooling process. This will imply higher BOG flow rates at the low temperatures, and hence more circulated nitrogen will be necessary to cope with the increase in cooling demand at this temperature level.
In
Turning now to the refrigeration system,
As shown in
Thus, by using a fraction of the low-pressure nitrogen to precool the BOG stream, as in the invention, several favorable effects are seen:
As a direct consequence of these measures the power demand of the invented process as illustrated in
Another effect of splitting the low-pressure refrigerant stream is that it can be used, not only to precool the boil-off gas to the LD-compressor 30, but also to intercool the BOG between the two LD-compressor stages. This could potentially reduce the LD-compressor 11 work with around 50 kW (depending on amongst others the compressor efficiencies), but a slight increase in power demand to the nitrogen compander will equalize much of the power gained when considering the overall system. However, choosing such a solution offers more flexibility to adjust the temperature of the BOG entering the cold-box. This will, for different operational modes, reduce thermal stresses in the plate-fin heat exchanger, and open the possibility for reducing power under various operating conditions such as ballast voyages and voyages with nitrogen-rich LNG cargos.
Thus, a preferred embodiment and a flexible solution for integrating both precoolers and intercoolers is shown in
It is also possible to choose only one split, as shown in
An alternative solution is to feed the cold nitrogen stream to the precooler from the intermediate-pressure nitrogen stream between the two expansion stages. This can in principle be done at any point between the two expanders, shown as points A, B, and C in
| Number | Date | Country | Kind |
|---|---|---|---|
| 20051315 | Mar 2005 | NO | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/NO2006/000090 | 3/8/2006 | WO | 00 | 9/5/2007 |
