LNG system with enhanced pre-cooling cycle

Abstract

A natural gas liquefaction system employing a high pressure pre-cooling refrigeration cycle. The natural gas liquefaction system can be advantageously employed in cold weather regions and / or in regions that exhibit large variations in ambient temperature.

Description

BACKGROUND OF THE INVENTION[0001]1. Field of the Invention[0002]This invention relates to a method and apparatus for liquefying natural gas. In another aspect, the invention concerns an improved liquefied natural gas (LNG) facility employing a high pressure pre-cooling cycle.[0003]2. Description of the Prior Art[0004]The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Such liquefaction reduces the volume of the natural gas by about 600-fold and results in a product which can be stored and transported at near atmospheric pressure.[0005]Natural gas is frequently transported by pipeline from the supply source to a distant market. It is desirable to operate the pipeline under a substantially constant and high load factor but often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In or...

AI Technical Summary

Benefits of technology

[0012]A further embodiment of the invention is to provide a high capacity pre-cooling cycle for LNG facilities that does not require significantly larger equipment and / or piping than conventional propane pre-cooling cycles.

[0014]Still another embodiment of the invention is to provide an LNG facility / process that exhibits higher efficiency and operability in cold weather environments.

Problems solved by technology

It is desirable to operate the pipeline under a substantially constant and high load factor but often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline.

The liquefaction of natural gas is of even greater importance when transporting gas from a supply source which is separated by great distances from the candidate market and a pipeline either is not available or is impractical.

Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas.

Such pressurization requires the use of more expensive storage containers.

As the demand for high capacity LNG facilities increases, conventional propane pre-cooling systems are becoming inadequate.

However, a number of disadvantages are associated with increasing the cooling capacity of a propane pre-cooling cycle by simply increasing the flow rate of propane through the system.

For example, increasing the flow rate of the propane refrigerant in the pre-cooling cycle requires larger compressors and larger piping.

Obviously, the use of larger equipment and piping increases the cost of the facility.

Further, larger equipment and piping takes up more space—an undesirable feature when plot space is limited, such as for offshore LNG facilities.

In addition, the capacity of conventional propane refrigerant compressors is already at or near the limits of known technology.

Thus, procuring a propane compressor of significantly higher capacity than currently available propane compressors would be very expensive, if not impossible.

Propane pre-cooling cycles can also present a number of challenges when employed in LNG facilities located in cold weather (e.g., arctic) environments.

For example, the low condensing pressure of propane does not permit the propane compressor to operate at its full capacity in cold weather environments because doing so would require the compressor to berate below its minimum allowable suction pressure.

Further, problems associated with propane pre-cooling cycles can be exacerbated in cold weather regions exhibiting a wide range of extremes in ambient air temperature.

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