Device and method for liquefying a natural gas and ship comprising such a device

Abstract

The device (400) for liquefying a natural gas comprises: a compressor (105) of a first vaporized coolant chemical mixture, a means (110) for fractionating the compressed mixture into a heavy fraction and a light fraction, —a first body (115) for exchanging heat between the heavy fraction of the first mixture and the natural gas to cool at least the natural gas, a second body (120) for exchanging heat between the light fraction of the first mixture and the natural gas cooled in the first exchange body to liquefy the natural gas, —a conduit (125) for returning the first vaporized coolant mixture in the heat exchange bodies to the compressor, —a regulator (405) for the liquefied natural gas, a collector (410) for the evaporation gas produced during the expansion of the gas in the regulator, —a conduit (415) for injecting the evaporation gas at the inlet of the second exchange body, upstream of an inlet (116) for the natural gas in the first exchange body (115), a third body (420) for exchanging heat between the natural gas and a second chemical coolant compound, —a means (425) for compressing the second vaporized compound, a means (430) for cooling the second compressed compound and a conduit (435) for transferring the second cooled compound towards the third exchange body.

Description

TECHNICAL FIELD OF THE INVENTION[0001]The present invention relates to a device for liquefying a natural gas, a method for liquefying a natural gas, and a ship comprising such a device. It applies, in particular, to the offshore or onshore liquefaction of natural gas.STATE OF THE ART[0002]Liquefying the gas allows natural gas to be transported in a smaller volume compared to transporting non-liquefied natural gas.[0003]Over the last few decades, liquefaction technologies have focused on large gas capacities for reasons of economy of scale.[0004]Implementation of the technologies thus used requires very substantial investments and has very high transportation costs (marine liquefaction and reception facilities). As a result, firstly the trend for liquefaction capacities has been to increase the volume of natural gas transported in order to obtain economies of scale and to make these projects economically more attractive. Secondly, the investments made to implement these technologies ...

AI Technical Summary

Benefits of technology

The patent describes a method for improving the flexibility of a device used for liquefying natural gas. By using certain types of evaporation gas and optimizing the flow rate of natural gas, the method reduces product loss and increases efficiency. The method also avoids the use of heavy components, minimizes the formation of heavy residue, and reduces the size of the exchange surfaces between the natural gas and the coolant mixture. This results in a more simple and compact system. Overall, the method improves the overall performance of the liquefaction device.

Problems solved by technology

Implementation of the technologies thus used requires very substantial investments and has very high transportation costs (marine liquefaction and reception facilities).

However, these opportunities are too small to justify the use of technologies intended for large-scale production (the transposition of conventional technologies is not appropriate, as they are too complex and cannot be used to support the economic viability of these technologies), hence the need to propose new technologies that can meet the two main challenges relating to liquefaction on a small scale:reducing investment costs as much as possible while keeping efficiency as high as possible in order to minimize operating costs; andincreasing the efficiency of the method so as to minimize product loss: the gas volumes to be upcycled are small, which makes every molecule important.

This system has several drawbacks:plate heat exchangers are very sensitive to the distribution of fluids, which poses a problem of marine adaptation for the marine applications;the coolant mixture has a significant number of components, especially heavy compounds, and these compounds crystallize in the heat exchangers under particular pressure and temperature conditions whose arrival is difficult to forecast; andthe method has limited flexibility, especially in terms of operating flow rate, and a limited production capacity per compression means.

For the CII system the main drawbacks are that:the CII method had been developed for large-scale LNG production on land;the method is not very flexible: efficiency drops significantly with any deviation from the operating / design point;the coolant mixture contains too many constituents (hydrocarbons), making the logistics and operational aspect more complex;storage increases the weight of the facilities, critical for offshore facilities;the method has difficulties concerning the ethane supply, which poses significant difficulties for offshore facilities;the method presents risks of alteration to the equipment (exchangers) due to the risk of crystallization of the pentane (iC5 and nC5) contained in the coolant mixture;the efficiency of the method is limited by the dimensions of the exchanger and by manufacturing constraints; andthe method presents problems concerning the installation of equipment at sea (significant drop in the performance of the plate exchanger if the distribution is not right).

A casing can contain from eight to a maximum of ten wheels; the greater the number, the more the compressor is likely to present stability problems.

Conversely, reducing the investment leads to solutions that are less efficient and / or significantly less flexible.

Conversely, a reduction in the number of casings results in a loss of operational flexibility, sometimes accompanied by a loss of efficiency.

However, with or without a multiplication mechanism, the rotation speeds must be proportional or identical, depending on the case, which makes the compression train inflexible when the flow rates entering each section are not identical or proportional.

This can pose problems of mechanical stability.

In addition, a large drop in efficiency is observed in the high-pressure section, even more significant when one wishes to include both sections in a single casing.

Lastly, this configuration is not very flexible, which limits the field of opportunities that can be addressed: a drop in efficiency, which can be significant, can be observed if one deviates from the operating point for which the equipment was sized (natural gas, and precise production conditions) and mechanical stability problems can occur leading to more frequent maintenance.

The current CII systems present the following drawbacks:a variation in the flow rate between the low-pressure and high-pressure sections of the compression train leading to an imbalance between these two sections, which can lead to mechanical instability problems during the shutdown and start-up phases;a significant drop in efficiency is observed between the low-pressure section and high-pressure section; andlimited flexibility for the compression train in terms of flow-rate range and composition.

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