Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-11-02
2002-11-26
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S646000
Reexamination Certificate
active
06484533
ABSTRACT:
INTRODUCTION TO THE INVENTION
The invention relates to the liquefaction of gas to form a cryogen. In particular, the invention relates to an improved method and apparatus for the production of a liquid cryogen from gas by liquefaction.
An important method for the production of liquid cryogen, such as, for example, liquid nitrogen, involves compressing a stream of gas comprising feed gas and recycled gas using a multistage intercooled recycle compressor, cooling the compressed gas, liquefying part of the cooled gas, and work expanding other parts of the gas in one or more expansion turbines to provide heat exchange refrigeration cooling and condensation duty for the process.
It is well known in the art that the use of two expansion turbines operating over different temperature spans, each running at a speed to maximize its performance, is significantly more efficient than the use of a single expansion turbine, due to the closer temperature approaches achieved in the main liquefier heat exchanger. To maximize the overall efficiency of the liquefier, the substantial mechanical power developed by the expansion turbines in producing the refrigeration should be recovered efficiently. For low overall cost, the capital associated with the recovery of the turbine power must be low.
Broadly speaking, there are two methods of recovering the mechanical power generated by expansion turbines. First, it is well known to load expansion turbines with electrical generators thereby recovering the power generated in the form of electricity. However, whilst this method is efficient, such electrical generators are expensive and therefore increase the overall capital cost of the liquefaction process. In addition, an electric generator load expansion turbine usually requires a speed reduction gearbox and mechanical energy is lost in the gearing between the expansion turbine and the electrical generator.
The second method is to recover the power mechanically. For air separation processes, it is known for the mechanical power generated by liquefier expansion turbines to be used to drive a compressor compressing an air separation unit process stream. Such compressor/expander combinations can be a cost effective and efficient means of recovering power developed by a single expander. However, these combinations have the disadvantage of requiring both the expander and the compressor to run at the same speed which is unlikely to be the optimum speed for either component. Further, this combination requires the liquefier expander be associated with the air separation unit feed compressor that is disadvantageous if one wishes to operate the liquefier separately from the air separation unit.
It is also known in the liquefier art to use an expansion turbine to drive a stage of compression, typically to further compress at least part of the gas compressed by the recycle compressor. Such expansion turbine/compressor combinations are sometimes referred to as “companders” and provide about 10-20% of the total compression power. For liquefiers, there is a particular advantage in using an expansion turbine to further compress at least a portion of the gas from the recycle compressor. However, while companders are efficient, they are relatively expensive, require aftercoolers and require instrumentation or controls to prevent then overspeeding. This extra equipment increases the overall cost of the liquefier.
As far as the inventors are aware, there is no prior public disclosure of recovering the turbine power by mounting the turbines on the liquefier recycle compressor or of the advantages derivable therefrom.
An efficient, low cost liquefaction method and apparatus has now been developed in which the power generated by the expansion turbine(s) is recovered in an efficient and cost-effective manner. The improvement results, in particular, in a reduction of the capital cost and ease of construction of liquefiers without sacrificing efficiency.
SUMMARY OF THE INVENTION
According to the first aspect of the present invention, there is provided in a method of liquefying a gas to produce a liquid cryogen comprising:
compressing a gas stream comprising a recycle gas stream in a compressor to provide at least one compressed gas stream;
cooling at least a portion of said compressed gas and work expanding the resultant cooled compressed gas in at least one expansion turbine to provide an expanded gas stream and generate mechanical power;
cooling and at least partially condensing the gas to be liquefied by heat exchange with said expanded gas stream providing refrigeration duty for said cooling and condensation; and
recycling without compressing said heat-exchanged expanded gas stream to the compressor to provide said recycle gas stream,
the improvement consisting in that the expansion turbine is mechanically linked to the compressor so that the mechanical power generated by said expansion turbine provides a portion of the mechanical power required to drive the compressor.
A feed gas stream may be introduced into the process cycle and introduction can occur at numerous different locations. For example, the feed stream may be combined with the recycle stream prior to compression in the compressor. If the pressure of the feed stream were high enough, it could join newly compressed gas stream downstream from the compressor. If the pressure of the feed stream were at a suitable intermediate pressure then the feed stream could join the cycle as an interstage feed stream to the compressor or be combined with an interstage outlet stream from the compressor. The feed stream may be available as a cryogenic gas and join the circulating fluid at a suitable point inside a cold enclosure. Part of the feed stream may be available as a cryogenic gas and part as a warm gas, each part joining the cycle at an appropriate point.
In certain embodiments of the present invention, the feed stream may never join the circulating stream because it has a different composition. In these embodiments, the feed stream is cooled and condensed to form a product liquid against returning expanded gas streams of the recycling fluid. None of the circulating fluid is condensed. An example would be if the circulating fluid were air and the feed and product streams were nitrogen or if the circulating fluid were standard nitrogen and the feed and the product streams were ultrapure nitrogen.
The feed gas may be any suitable gas that is capable of producing a liquid cryogen. Particular examples of suitable gases include any of the common atmospheric gases such as nitrogen, oxygen and argon, many hydrocarbon gases such as methane and ethane, and mixtures of these gases such as air and natural gas.
The improved method is particularly suitable for liquefaction processes using at least one multistage intercooled integrally geared centrifugal compressor for feed and recycle compression duty assembled with one or more expansion turbines. In particularly preferred embodiments of the invention, expansion turbines drive the compressor via a gear drive.
Preferably, the gas to be liquefied comprises a portion of the compressed gas and the compressed gas comprises make-up and recycle gas. Alternatively, the gas to be liquefied consists of a portion of the compressed gas and said compressed gas comprises make-up gas and recycle gas. In other embodiments, the gas to be liquefied does not comprise recycle gas.
In preferred embodiments of the first aspect of the present invention, there is a single expansion turbine to provide a portion of the mechanical power required to drive the compressor. In this embodiment, the expansion turbine may be mounted opposite the compressor on a single pinion. The expansion turbine may drive the compressor by a dedicated pinion. In a different arrangement, the expansion may be mounted on its own pinion and drive the compressor via a gear drive.
In particularly preferred embodiments, the cooled compressed gas is expanded at different temperatures in at least two expansion turbines, each expansion turbine providing a portion of the mechanical power requir
Allam Rodney J.
Cotton Rebecca J.
Dillon, IV John Lloyd
O'Connor Declan P.
Air Products and Chemicals Inc.
Capossela Ronald
Jones II Willard
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