Patent Application
20140157824
The present invention relates to a method and device for improving the thermal performance of a nitrogen refrigeration cycle.
Gas liquefaction is a known process used to liquefy gases and commonly employs a nitrogen refrigeration cycle to provide the necessary refrigeration. The nitrogen refrigeration cycle, in its most basic terms, includes the following steps: nitrogen gas is compressed, cooled, and then expanded in a turbo booster. During this expansion, the temperature of the gas drops and the expanding gas moves the turbine, which in turn performs work on the compressor of the turbo booster. The cold gas is then used to provide the necessary cooling to liquefy the target stream.
One problem with refrigeration cycles is that there are refrigerant losses. The mechanical equipment used in the process contains seals, which have gas losses to the atmosphere that must be made up. Although mechanical designs are available which recover a portion of these seal gas losses back to a low pressure stream within the refrigeration cycle, this does not completely eliminate the losses and can incur additional capital cost. One solution to this problem is shown in
However, there is a need for an improved refrigeration cycle that requires less power and/or makeup fluid.
The present invention is directed to a device and a method that satisfies at least one of these needs. Certain embodiments of the present invention relate to the use of an improved refrigeration cycle as part of a liquefaction process, in which makeup refrigerant is introduced on the cool side of the heat exchanger as opposed to the warm side of the heat exchanger.
In one embodiment, the method for producing a liquefied gas stream from an inlet gas feed stream can include the steps of:
cooling at least a portion of the inlet gas feed stream by heat exchange contact with an expanded refrigerant to produce the liquefied gas stream, wherein the expanded refrigerant is circulated in a refrigeration cycle; and
introducing a liquid refrigerant makeup stream into the refrigeration cycle by adding the liquid refrigerant makeup stream to the expanded refrigerant prior to heat exchange contact with the inlet gas feed stream to provide for makeup losses in the refrigeration cycle and to provide additional refrigeration to the refrigeration cycle.
In another embodiment, the inlet gas feed stream to be liquefied is natural gas. In another embodiment, the refrigeration cycle is a nitrogen refrigeration cycle and the liquid refrigerant makeup stream originates from a liquid nitrogen tank containing liquid nitrogen. In another embodiment, the expanded refrigerant is nitrogen. In another embodiment, the expanded refrigerant is expanded in a device selected from the group consisting of an expansion valve, a turbo-expander, and a liquid (or dense fluid) expander. In another embodiment, the method can further includes the steps of withdrawing a utility nitrogen stream from the nitrogen vapor stream subsequent to heat exchange contact with the inlet gas feed stream, and introducing the utility nitrogen stream to a second user for use as utility nitrogen.
In another embodiment, the method producing a liquefied gas stream from an inlet gas feed stream can include the steps of:
cooling at least a portion of the inlet gas feed stream by heat exchange contact with a nitrogen refrigeration cycle to produce the liquefied gas stream; and
adding a liquid refrigerant makeup stream into the expanded refrigerant prior to heat exchange contact with the inlet gas feed stream to provide for makeup losses in the nitrogen refrigeration cycle and to provide additional refrigeration to the nitrogen refrigeration cycle. In one embodiment, the nitrogen refrigeration cycle can include the steps of: expanding a nitrogen stream to a cold nitrogen vapor stream, cooling at least a portion of the inlet feed gas stream by heat exchange contact with the cold nitrogen vapor stream, compressing the cold nitrogen vapor stream to form a compressed nitrogen vapor stream, and cooling at least a portion of the compressed nitrogen vapor stream by heat exchange contact with the cold nitrogen vapor stream.
In another aspect of the invention, an apparatus for producing a liquefied gas stream from an inlet gas feed stream is provided. In one embodiment, the apparatus can include a heat exchanger, a refrigeration cycle, and a makeup liquid refrigerant tank. In one embodiment, the heat exchanger has a cold side and a warm side and is configured to receive the inlet gas feed stream and to produce the liquefied gas stream. In one embodiment, the refrigeration cycle is configured to provide refrigeration to the heat exchanger. Preferably, the refrigeration cycle further includes a compressor and an expander, wherein the compressor is configured to compress a refrigerant to a compression pressure, wherein the expander is configured to receive the compressed refrigerant and expand the compressed refrigerant to an expanded pressure to produce a cold refrigerant at an outlet of the expander, and wherein the outlet of the expander is in fluid communication with the cold side of the heat exchanger. In one embodiment, the makeup liquid refrigerant tank is configured to introduce a makeup liquid refrigerant with the cold refrigerant prior to heat exchange with the inlet gas feed stream.
In another embodiment, the makeup liquid refrigerant tank is in fluid communication with the refrigeration cycle at a point downstream the outlet of the expander and upstream the cold side of the heat exchanger. In another embodiment, the expander is a device selected from the group consisting of an expansion valve, a turbo-expander, and a liquid/dense fluid expander. In an additional embodiment, the apparatus can further include a utility nitrogen withdrawal line configured to withdraw a nitrogen stream from the refrigeration cycle at a point downstream the warm side of the heat exchanger and introduce the nitrogen stream to a second user for use as utility nitrogen.
Certain embodiments of the present invention advantageously do not require supplemental energy for the vaporization of the make up fluid.
These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.
While the invention will be described in connection with several embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all the alternatives, modifications and equivalence as may be included within the spirit and scope of the invention defined by the appended claims.
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As is clearly shown in Table I, embodiments of the present invention can achieve approximately an 8% improvement over methods and apparatus of the prior art. This is extremely beneficial as embodiments of the present invention can provide a cost savings on both the capital cost because of the reduced size of the refrigeration cycle equipment and on the energy required to run the high pressure compressor.
While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, language referring to order, such as first and second, should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps or devices can be combined into a single step/device.
The singular forms “a”, “an”, and “the” include plural referents, unless the context clearly dictates otherwise.
Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.
Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.
