Patent Application
20070283719
While the specification concludes with claims distinctly pointing out the subject matter that Applicant regards as his invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which:
With reference to
Air separation system 1 is designed to fractionate, argon, oxygen and nitrogen that is contained within a feed air stream 10. Feed air stream 10 is compressed within a compression unit 12 that may encompass numerous stages of compression with inter-stage cooling. The compression of feed air stream 10 produces compressed stream 14 that is purified within a purification unit 16. Purification unit 16 removes the high boiling contaminants that are present within feed air stream 10 such as carbon dioxide, water, and potentially carbon monoxide. Such a unit can be a temperature swing adsorption unit having beds of alumina and/or molecular sieve adsorbent operating out of phase to adsorb such contaminants present within the feed air stream 10. The purification produces a compressed and purified stream 18.
Compressed and purified stream 18 is divided into first and second subsidiary streams 20 and 22. Typically, first subsidiary stream 20 constitutes between about 65 percent and about 70 percent of compressed and purified stream 18. Second subsidiary stream 22 constitutes between about 30 percent and about 35 percent of compressed and purified stream 18. Second subsidiary stream 22 is then compressed within booster compressor 24 to enable vaporization of the pumped and pressurized liquid oxygen product that will be discussed hereinafter.
Air separation system 1 is provided with a main heat exchanger 26 that typically is one or more units of plate-fin design. First subsidiary stream 20 is cooled within main heat exchanger 26, typically to a temperature in a range of between about 125° K. and about 190° K. Thereafter, first subsidiary stream 20 is expanded within a turboexpander 28 to a temperature at or near the dew point and of a pressure compatible with higher pressure column 30. The expanded second subsidiary stream 20 is then introduced into the base of the higher pressure column 30 as the primary air feed. It is understood that turboexpander 28 expands with a performance of work. Although not shown, such work would typically be applied to a compressor that would compress first subsidiary stream 20.
Higher pressure column 30 is part of a multiple column arrangement 32 that also has a lower pressure column 34 operatively associated with higher pressure column 30 via a condenser reboiler 36 having a core 38 located within a shell thereof. Lower pressure column 34 is so named because it operates at a lower pressure than the higher pressure column 30. As indicated previously, both higher pressure column 30 and higher pressure column 34 could be a series of connected columns. Each of the higher pressure and lower pressure columns 30 and 34 contain mass transfer contact elements 40 and 42 for higher pressure column 30 and 46, 48, 50, 52 and 53 for lower pressure column 34.
Condenser reboiler 36 could be integrated into the columns and the higher and lower pressure columns 30 and 34 as known in the art. Condenser reboiler 36 serves to condense a nitrogen column overhead that collects within the top of higher pressure column 30 against a vaporizing liquid oxygen column bottoms that is produced within the lower pressure column 34 and that collects as liquid oxygen column bottoms 56 within condenser-reboiler 36. A condensed nitrogen stream 58, made up of the nitrogen column overhead is divided into first nitrogen reflux stream 60 that is used to reflux the higher pressure column 30 and a second nitrogen reflux stream 62 that is further cooled within and exchanger 64. Part of second nitrogen reflux stream 62 may be taken thereafter as a nitrogen product stream 66. However, all of second nitrogen reflux stream 62 can be expanded by a Joule-Thompson valve 68 to the pressure of lower pressure column 34 and is then used to reflux the lower pressure column 34.
In higher pressure column 30, first subsidiary stream after having been expanded within turboexpander 28 and introduced into higher pressure column 30 produces an ascending vapor phase that becomes evermore rich in the lower boiling or light components, nitrogen, for example, as it ascends the mass transfer elements 40 and 42 to form the nitrogen column overhead within higher pressure column 30. The vaporized liquid oxygen column bottoms 56 forms an ascending vapor phase within lower pressure column 34 that becomes evermore rich in the lighter component, nitrogen. The descending liquid phase, that is initiated by second nitrogen reflux stream 62, becomes evermore rich in oxygen, the heavier or less volatile component.
As indicated previously, air separation system 1 is designed to produce a high pressure oxygen product. As such, an oxygen stream 70 composed of the liquid oxygen column bottoms 56 produced within lower pressure column 34 is pressurized by being pumped by a pump 72. Pressurized liquid may be extracted in part as a pressurized liquid oxygen stream 74. However, the remaining portion 76, which could be the entire portion of liquid stream 70 if pressurized liquid product stream 74 were not removed, is vaporized within the main heat exchanger 26 against liquefying second subsidiary stream 22.
Second subsidiary stream 22, after having been compressed and cooled, is expanded to the pressure of higher pressure column 30 by way of a Joule-Thompson valve 80 and then divided into first and second portions 82 and 84. Portion 82 is introduced into an intermediate location of higher pressure column 30 as a saturated liquid. Portion 84 is also expanded via a Joule-Thompson valve 86 and is introduced into lower pressure column 34 as a two-phase stream within an intermediate location thereof of appropriate concentration to such stream.
Air separation system 1 and multiple column arrangement 32 thereof also includes an argon column 90 that is provided with mass transfer contact elements 92 to contact an ascending vapor phase with a descending liquid phase formed within argon column 90. An argon and oxygen-containing vapor stream 94 is introduced into argon column 90 to produce an ascending vapor phase to separate the oxygen. Argon column 90 operates at a pressure comparable to lower pressure column 34. Argon and oxygen-containing vapor stream 94 can be rectified within argon column 90 to produce nearly pure argon-rich fraction as an argon-rich column overhead. An overhead stream 96 composed of the argon-rich column overhead is condensed within a condenser 100 having a core 101. The resulting liquid argon-rich stream 110 is divided into a first portion 120 that can be taken as a product and a second reflux portion 122 that is used to reflux argon column 90. An argon depleted oxygen-rich column bottoms 124 is formed within argon column 90 and is pumped by a pump 126 back to the lower pressure column 34 as a stream 128.
Heat transfer duty within the condenser 100 is taken up by part of the crude liquid oxygen column bottoms produced within a higher pressure column 30. However, as indicated previously, the removal of the liquid oxygen product stream 70 and its resultant pressurization to produce the pressurized oxygen product, will result in liquefaction of not inconsiderable part of the incoming air stream. This will result in less nitrogen vapor being introduced into higher pressure column 30 that will in turn result in less nitrogen reflux being introduced into lower pressure column 34 by way of second nitrogen reflux stream 62. At the same time, if a stream composed of all of the crude liquid oxygen were used to condense argon within the argon column, the nitrogen traffic would be increased in the lower pressure column 34 resulting in less argon being washed down to a stage where it could be removed as argon-oxygen containing vapor stream 94 for eventual recovery. Hence, the problem is simply exacerbated when a liquid oxygen product is pressurized and then vaporized within the main heat exchanger.
In order to overcome such problem, in the present invention, a crude liquid oxygen stream 130 is valve expanded within a Joule-Thompson valve 132 to produce a two-phase stream 134. The vapor phase, which is a nitrogen-rich vapor phase, is disengaged from the liquid phase within phase separator 136. A liquid stream 138 composed of the liquid phase is then introduced into condenser 100 to produce streams 140 and 142 composed of the vapor and liquid fractions, respectively, due to the partial vaporization of liquid phase stream 138. However, since the flashed vapor stream 146 has been removed prior to entry into the condenser 100 there will be less nitrogen traffic in the top of lower pressure column 34, thereby increasing the liquid to vapor ratio within lower pressure column 34 in a region above which argon and oxygen-containing vapor stream 94 is removed. It is to be noted here that although one phase separator is shown, there could be successive stages of flash separation in which the liquid produced in an upstream phase separator were subsequently valve expanded and introduced into a downstream phase separator to produce the liquid phase stream from the downstream phase separator.
Liquid stream 138 is typically pumped by a pump 143 back to the condenser 100. It is to be noted that not all of the liquid stream 138 need be sent to the argon condenser. A portion could be sent to the lower pressure column 34 directly. Furthermore, liquid stream 138 could be sent directly to the column with another of other known streams that could be used in connection with condenser 100. In the illustrated embodiment, a piping run serves to lower the pressure of liquid stream 138 to a pressure suitable for introduction of streams 140 and 142 into lower pressure column 134. The pumping is necessary due to the length of the argon column and its design in producing a pure argon product. Hence, there may not be enough pressure within a high pressure column to bring it up to a level of condenser 100. However, the invention is not limited to this specific embodiment and if a crude argon fraction were to be further processed in a shorter column, there might be sufficient pressure to drive liquid stream 138 into condenser 100. In such case, a Joule-Thompson valve would have to be used to lower the pressure and thereby to allow for such introduction of streams 140 and 142 into lower pressure column 34.
A nitrogen-rich stream 146 that is composed of the nitrogen-rich fraction is warmed in the main heat exchanger 26 and then introduced into an appropriate stage of compression unit 12. This is possible where the nitrogen-rich stream 146 has a composition in which the nitrogen component is not more than about fifteen percent of that present within the air, plus or minus. It is to be noted, that it is possible to cold compress nitrogen-rich stream 146, although this would be disadvantageous in that its refrigeration value would thereby be lost. A further possibility is that not all of the nitrogen-rich stream need be recompressed. In fact, the present invention contemplates that only part of such stream or streams, if two or more flash separation stages are used, is recycled back for compression. The remaining portion in an appropriate case could be valve or work expanded and then vented or sent back to the columns.
It is to be further noted, that a nitrogen-rich stream 148 and a waste nitrogen stream 150 having a lower nitrogen concentration of nitrogen-rich stream 148 may be extracted from the top and at a lower location of lower pressure column 34. These streams are warmed in heat exchanger 64 and the main heat exchanger 26 to cool the second nitrogen reflux stream 64 and to also, help cool the incoming streams.
With reference to
While the present invention has been described with reference to a preferred embodiment, as will occur to those skilled in the art, numerous changes, additions and omissions can be made without departing from the spirit and the scope of the present invention.
