Patent Application
20150168057
The present invention relates to a process for separating air into its components. More specifically, embodiments of the present invention are related to producing merchant or non-merchant grade liquid nitrogen using a pair of turbo-boosters to provide refrigeration and energy for the process.
A process for producing nitrogen through the cryogenic separation of air in a nitrogen production plant is provided. In one embodiment, the nitrogen production plant includes a main air compressor, a heat exchanger, an air separation unit, a recycle compressor, a first turbo-booster having a first booster and a first turbine, a second turbo-booster having a second booster and a second turbine, and a liquid/gas separator. In one embodiment, the process can include the following steps: (a) obtaining a main air feed comprising filtered purified and compressed air at a pressure of at least 5 bar; (b) fully cooling the main air feed in the heat exchanger to form a fully cooled air feed; (c) withdrawing the fully cooled air feed from the heat exchanger and introducing the fully cooled air feed to the air separation unit under conditions effective for rectification of the cooled air feed into gaseous nitrogen and waste gaseous oxygen, wherein the air separation unit comprises a single column; (d) warming the waste gaseous oxygen in the heat exchanger; (e) warming the gaseous nitrogen in the heat exchanger; (f) compressing the gaseous nitrogen in the recycle compressor to form a compressed nitrogen recycle; (g) further compressing the compressed nitrogen recycle in the first booster to form a boosted nitrogen; (h) further compressing the boosted nitrogen in the second booster to form a fully boosted nitrogen; (i) cooling the fully boosted nitrogen in the heat exchanger to form liquefied nitrogen; (j) introducing the liquefied nitrogen to the liquid/gas separator under conditions effective to separate gaseous nitrogen from the liquefied nitrogen, such that gaseous nitrogen is withdrawn from the liquid/gas separator and combined with the gaseous nitrogen from the air separation unit before warming in the heat exchanger; (k) withdrawing recovered liquid nitrogen from the liquid/gas separator as product; (l) withdrawing a fraction of compressed nitrogen recycle from the compressed nitrogen recycle and partially cooling the fraction of compressed nitrogen recycle before expanding the fraction of compressed nitrogen recycle in the first turbine to form a first expanded nitrogen; (m) introducing the first expanded nitrogen into the heat exchanger and combining with the gaseous nitrogen from the air separation unit prior to compression in the recycle compressor; (n) withdrawing a partially cooled boosted nitrogen from the fully boosted nitrogen from an intermediate point of the heat exchanger; (o) expanding the partially cooled boosted nitrogen using a second turbine to form second expanded nitrogen; and (p) introducing the second expanded nitrogen to the liquid/gas separator.
According to other optional aspects of the invention:
According to another embodiment of the invention, a process is provided for producing a liquid nitrogen product through the cryogenic separation of air in a nitrogen production plant, the nitrogen production plant including a main air compressor, a heat exchanger, an air separation unit having a single column, a top condenser, and a bottom reboiler, a recycle compressor, at least one turbo-booster having a booster and a turbine, a liquid/gas separator, and a subcooler. In one embodiment, the process can include the following steps: (a) obtaining a main air feed comprising filtered purified and compressed air at a pressure of at least 5 bar; (b) fully cooling the main air feed in the heat exchanger to form a fully cooled air feed; (c) withdrawing the fully cooled air feed from the heat exchanger and introducing the fully cooled air feed to the air separation unit under conditions effective for rectification of the cooled air feed into gaseous nitrogen and waste gaseous oxygen, wherein the air separation unit comprises a single column; (d) warming the gaseous nitrogen in the heat exchanger; (e) compressing the gaseous nitrogen in the recycle compressor to form a compressed nitrogen recycle; (f) withdrawing a fraction of partially compressed nitrogen recycle from an internal stage of the recycle compressor, cooling said fraction of partially compressed nitrogen recycle in the heat exchanger, condensing said fraction of partially compressed nitrogen recycle in the bottom reboiler, and then flashing said fraction of partially compressed nitrogen recycle into a top portion of the single column as reflux; (g) compressing and expanding the compressed nitrogen recycle using the at least one turbine-booster to provide the refrigeration for the process and produce an expanded nitrogen stream comprising nitrogen-enriched liquid and nitrogen-enriched gas; (h) introducing the expanded nitrogen stream to the liquid/gas separator; (i) subcooling the nitrogen-enriched liquid from the liquid/gas separator in the subcooler to form the liquid nitrogen product; and (j) withdrawing an oxygen-enriched fluid from the single column to provide subcooling for the subcooler.
According to another embodiment of the invention, a process is provided for producing a liquid nitrogen product through the cryogenic separation of air in a nitrogen production plant, the nitrogen production plant comprising a main air compressor; a heat exchanger; an air separation unit having a single column, a top condenser, and a bottom reboiler; a recycle compressor; at least one turbine-booster having a booster and a turbine; a liquid/gas separator; and a subcooler. In one embodiment, the process can include the steps of introducing a main air feed to the single column under conditions effective for separating air to produce oxygen and nitrogen; using gaseous nitrogen withdrawn from the recycle compressor as a heating fluid for the bottom reboiler; and refluxing the single column using a pressurized nitrogen stream cooled in the heat exchanger.
In one embodiment, the invention can also include the step of withdrawing column bottoms from the single column and using a first portion of the column bottoms to provide subcooling for a nitrogen product, and using a second portion of the column bottoms for driving the top condenser.
In one embodiment, the invention can also include the step of withdrawing column bottoms from the single column and using the column bottoms to drive the top condenser, and withdrawing an oxygen-rich liquid column side draw stream from the single column and using the oxygen-rich liquid column side draw stream to provide subcooling for a nitrogen product.
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.
Gaseous nitrogen 28 is also withdrawn from air separation unit 19 and passed through the cold side of heat exchanger 10 to provide additional cooling. However, instead of venting to the atmosphere, gaseous nitrogen 28 is recycled in the process. Nitrogen recycle 38 exits air separation unit 19 and is introduced to recycle compressor 40 and compressed to form compressed nitrogen recycle 46. Compressed nitrogen recycle 46 is then cooled in second aftercooler 43 before being boosted in first booster 50 and cooled in third aftercooler 51 to form boosted nitrogen 52. Boosted nitrogen 52 is then introduced to second booster 53 in order to further compress boosted nitrogen 52 before being cooled in fourth aftercooler 55 to form fully boosted nitrogen 56. In one embodiment fully boosted nitrogen 56 can be at ambient temperature and a pressure of about 45 to about 65 bar prior to entering heat exchanger 10.
Fully boosted nitrogen 56 is then introduced to heat exchanger 10 for cooling. In one embodiment, one portion of fully boosted nitrogen 56 is fully cooled in heat exchanger 10 to form liquefied nitrogen 58, which is subsequently introduced to liquid/gas separator 60 by flashing via valve 59. In another embodiment, another portion of fully boosted nitrogen 56 is only partially cooled in heat exchanger 10 to form partially cooled boosted nitrogen 78. In one embodiment, partially cooled boosted nitrogen 78 is at or above its super critical pressure. Partially cooled boosted nitrogen 78 is then introduced into second turbine 80 in order to expand partially cooled boosted nitrogen 78 to form second expanded nitrogen 82. In one embodiment, second expanded nitrogen 82 can have a temperature that is near or below its dew point and a pressure of about 5 to about 6 bar. In one embodiment, second expanded nitrogen 82 is a two phase fluid consisting of gas and liquid phases. In a preferred embodiment, second expanded nitrogen 82 is introduced to liquid/gas separator 60 in order to separate any gaseous nitrogen from liquid nitrogen. Recovered liquid nitrogen 62 is withdrawn from liquid/gas separator 60 and collected as product. In one embodiment, gaseous nitrogen 68 is withdrawn from a top portion of liquid/gas separator 60 and combined with gaseous nitrogen 28 before introduction to the cold side of heat exchanger 10 and subsequently recycled.
In one embodiment, fraction of compressed nitrogen recycle 48 is withdrawn from compressed nitrogen recycle 46 and fed to the warm end of heat exchanger 10, where fraction of compressed nitrogen recycle 48 is partially cooled before being expanded in first turbine 70 to form first expanded nitrogen 72. In one embodiment, first expanded nitrogen 72 is reintroduced to heat exchanger 10, preferably at an intermediate point, and combined with gaseous nitrogen 28 and subsequently recycled. In one embodiment, first turbine 70 is connected by a common shaft with first booster 50 and helps to provide the energy needed for first booster 50 to compress compressed nitrogen recycle 46. Likewise, second turbine 80 is connected by a common shaft with second booster 53 and helps to provide the energy needed for second booster 53 to compress boosted nitrogen 52. In one embodiment, first turbine 70 and second turbine 80 provide substantially all of the refrigeration needs for the process.
First turbine 70 and second turbine 80 produce refrigeration by work expansion. Their respective boosters, first booster 50 and second booster 53, utilize the produced work to further compress their respective nitrogen streams.
After exiting the cold end of heat exchanger 10, fraction of partially compressed nitrogen recycle 44 is used to provide heat to bottom boiler 21 before being introduced via valve 93 near a top portion of single column 20. Those of ordinary skill in the art will recognize that even though recycle compressor 40 and second recycle compressor 45 are pictured as two different compressors, it is possible to use one compressor and remove fraction of partially compressed nitrogen recycle 44 from an inner stage of that single compressor.
As with all distillation columns, liquids tend to collect near the bottom, while gases rise to the top. In this embodiment, oxygen-rich liquid 24 is withdrawn from the bottom of single column 20 and introduced to subcooler 30 via valve 29. In one embodiment, oxygen-rich condensing fluid 26 is withdrawn from oxygen-rich liquid 24 and introduced via valve 35 near top condenser 23. In one embodiment, top condenser is a bath type condenser.
Gaseous nitrogen near the top of single column 20 travels up tube 27, with a portion being withdrawn as gaseous nitrogen 28 and the rest condensing within top condenser 23 before being reintroduced to single column 20. Oxygen-rich condensing fluid 26 introduced near top condenser 23 provides the needed cooling to condense the nitrogen. Waste gaseous oxygen 22 is withdrawn and used to provide refrigeration to heat exchanger 10. In one embodiment, safety purge 83 can be withdrawn as a safety precaution.
Recovered liquid nitrogen 62 is then introduced to subcooler 30 in order to further cool recovered liquid nitrogen 62 to produce liquid nitrogen product 64. Oxygen-rich liquid 24 is used to provide the necessary cooling. Any gas forming within subcooler 30 is withdrawn as oxygen-rich waste gas 34 and may be combined with waste gaseous oxygen 22 before entering the warm end of heat exchanger 10. In one embodiment not shown, oxygen-rich waste gas 34 may be warmed in heat exchanger 10 separately from waste gaseous oxygen 22 in order to allow for deeper subcooling of liquid nitrogen product 64. Oxygen purge 32 can be withdrawn from the bottom of subcooler 30 as necessary. Recycled liquid nitrogen 66 can be withdrawn from recovered liquid nitrogen 62 and introduced to the top of single column 20 as reflux via valve 36. In an optional embodiment (shown as dotted line 66a), recycled liquid nitrogen 66 can originate from liquefied nitrogen 58 via line 66a.
In certain embodiment, the feed gas to the single column is air, as opposed to a feed gas having a concentration having higher nitrogen content. The single column has both a bottom reboiler and a top condenser, and in certain embodiments, the reboiler is driven by gaseous nitrogen withdrawn from the recycle compressor, preferably at a first stage discharge of the recycle compressor. In another embodiment, the single column can be partly refluxed with liquid nitrogen split-off from a Joule-Thompson stream (e.g., high pressure nitrogen stream exiting the cool end of the heat exchanger such as stream 58). In another embodiment, column bottoms may be split for both product subcooling and for driving the top condenser, or all of column bottoms can used for driving the top condenser with product subcooling being done via an oxygen-rich liquid column sidedraw stream.
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.
