Cryogenic rectification method and apparatus

Abstract: In a cryogenic rectification method for producing a product stream from a gaseous mixture having higher and lower volatility components and heavy impurities, the mixture is separated by cryogenic rectification employing one or more columns 28 and 30 having plates, trays or packing for intimately contacting ascending vapour and descending liquid streams within the column. The mixture to be separated is compressed in compressor 14 and is combined with a recycle stream 18 to produce a combined stream which is purified in a prepurification unit 20 that is designed to remove the heavy contaminants. The combined stream 20 is divided into major and minor streams 22a and 22b respectively. The major stream 22a is cooled and separated into liquid and vapour phases. Residual heavy impurities concentrate in the liquid phase. The liquid phase is taken as the recycle stream 18, and is pumped to a high enough pressure for vaporization of the impurities to occur with the rest of the liquid in the heat exchanger 25. The resulting vapour is reduced in pressure by passage through a valve 40 and combined with the incoming gaseous mixture. The concentration of heavy impurities in the vapour phase is reduced to a sufficient extent that a product stream 62 comprising the lower volatility component will have a reduction in its heavy impurity concentration.

Description: The present invention relates to a cryogenic rectification method and apparatus for separating a gaseous mixture into lower and higher volatility components with a reduced concentration of impurities in the lower volatility component. Gas mixtures are separated into their higher and lower volatility components by cryogenic rectification which is generally carried out in rectification columns having trays or packings. The separation is characterized by a countercurrent vapour-liquid contact of a descending liquid phase with an ascending vapour phase on the trays or within the packing. The liquid phase becomes ever more concentrated in the lower volatility components as it descends within the rectification column and the vapour phase becomes ever more concentrated with the higher volatility components as it ascends within the rectification column. Heavy impurities concentrate within the descending liquid phase. In the example of cryogenic air separations, heavy impurities such as carbon dioxide can present problems in carrying out the separation in the first instance. As an example, in cryogenic air separation plants that produce gaseous oxygen at a delivery pressure by vaporizing pumped liquid oxygen in a main heat exchanger, heavy impurities such as carbon dioxide and hydrocarbons can exceed their solubility limit in the liquid oxygen as it vaporizes. As a result, carbon dioxide contained within the liquid oxygen can solidify to thereby plug up the heat exchange passageways within the main heat exchanger and hydrocarbons such as acetylene can come out of solution to present a safety hazard. Generally speaking in case of liquid oxygen production, the heavy impurities are removed from the incoming air by an adsorptive prepurification unit. Some impurities, however, remain, and as a result, heavy impurities will concentrate within the lower volatility component of air, namely oxygen. As will be discussed, the present invention provides a method to increase heavy impurity removal so that the product has a reduced concentration of the heavy impurities. According to the present invention there is provided a method of separating a gaseous mixture comprising higher and lower volatility components and heavy impurities to obtain a product stream predominantly containing said lower volatility component(s) of said gaseous mixture, said method comprising: compressing said gaseous mixture; pumping a recycle liquid stream containing in said heavy impurities to a sufficient pressure such that on vaporisation said heavy impurities vaporise with said liquid; vaporising at said pressure and then reducing the pressure of said recycle stream to a pressure about equal to that of said gaseous mixture; combining said recycle stream with the compressed gaseous mixture to form a combined stream, prepurifying the combined stream by removal of heavy impurities therefrom, cooling to a cryogenic temperature at least a major part of the prepurified stream, separating a liquid relatively concentrated in residual heavy impurities from the cooled stream and thereby forming a vapour relatively lean in residual heavy impurities; subjecting said vapour in a distillation stage to rectification to produce said product stream; and forming said recycle stream from the liquid relatively concentrated in residual heavy impurities. The invention also provides an apparatus for separating gaseous mixture comprising higher and lower volatility components and heavy impurities to obtain a product stream predominantly comprising said lower volatility component(s) of said gaseous mixture, said apparatus comprising: a compression stage for compressing a gaseous mixture; a pump for pumping a recycle liquid stream containing heavy impurities to a sufficient pressure that on vaporisation the heavy impurities vaporise with the liquid; heat exchange passages which communicate with an outlet of the pump and in which, in use, said recycle stream vaporises at said pressure; a valve for reducing the pressure of the recycle stream having an inlet in communication with an outlet from said heat exchange passages and having an outlet in communication with a conduit for the flow of the gaseous mixture such that, in use, said recycle stream combines with said gaseous mixture to be separated to form a combined stream; a prepurification stage for purifying the combined stream by removal of heavy impurities therefrom; further heat exchange passages communicating the prepurification stage such that, in use, at least a major part of the combined stream flows through the further heat exchange passages and is cooled to a cryogenic temperature; a separation vessel for separating a liquid relatively concentrated in heavy impurities from the cooled combined stream and thereby forming a vapour relatively lean in heavy impurities having an inlet communicating with an outlet from the further heat exchange passages and having an outlet communicating with an inlet to the said pump; a distillation stage for separating said vapour by rectification in order to obtain said product stream. The present invention has application to any process and any plant configuration in which a product stream predominantly containing lower volatility components of the gaseous mixture to be separated is to be obtained. In case of air separation, the present invention is applicable to any plant that has an oxygen product, for example a single column oxygen generator. The present invention is particularly suitable for use in a double column plant in which liquid oxygen is produced at the bottom of a lower pressure column. The present invention is particularly intended for use in pumped liquid oxygen plants in which the liquid oxygen is pumped to a high pressure and then vaporized within a main heat exchanger by indirect heat exchange with a minor part of the incoming air stream that is boosted in pressure typically to a pressure above that at which the higher pressure rectification plant operates. In such a plant, depending on the delivery pressures of the oxygen product, the heavy impurities will tend to remain after the liquid oxygen vaporizes. For example, as mentioned above, heavy impurities as carbon dioxide can freeze to obstruct heat exchange passages within the main heat exchanger and the hydrocarbons can present an explosion hazard. By substantially reducing the level of the heavy impurities, these foregoing problems can be alleviated in the operation of an air separation plant designed to produce a high pressure oxygen product. The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of an apparatus for carrying out a method in accordance with the present invention; and Figure 2 is a fragmentary view of alternative embodiment to Figure 1. With reference to Figure 1, apparatus 10 is specifically designed to produce a high pressure oxygen product. However, the present invention is not limited to producing high pressure oxygen products nor is it limited to the rectification of air. The present invention does concern cryogenic rectification in which compression and cooling stages are used to compress and cool a gaseous mixture so that the gaseous mixture can be separated in a distillation stage into higher and lower volatility components of the gaseous mixture. Heavy impurities are substantially removed from the gaseous mixture in a prepurification stage, but, as mentioned above, some heavy impurity content remains in the gaseous mixture. In apparatus 10 an air stream 12 after having been filtered to remove dust particles and the like, is subjected to a compression stage including a compressor 14 and an aftercooler 16 to remove the heat of compression. Air stream 12 is then combined with a recycle stream 18 and is purified in a prepurification stage consisting of a prepurification unit 20 of the type designed to remove water and carbon dioxide from air stream 12. Prepurification unit 20 can, for example, consist of adsorbent beds operating out of phase from one another for regeneration purposes. Recycle stream 18 and air stream 12 make up a combined stream 22 to be purified within prepurification unit 20. The combined stream 22 is divided downstream of the prepurification unit 20 into major and minor portions, designated as major and minor subsidiary streams 22a and 22b. The minor subsidiary stream 22b is further compressed by a booster compressor 23. Heat of compression is removed in an aftercooler 24. The use of a booster compressor in such a manner, well known in the art, facilitates efficient vaporization of the high pressure product stream by countercurrent heat exchange with the minor subsidiary stream within a cooling stage formed by a main heat exchanger 25. The minor subsidiary stream 22b is cooled within main heat exchanger 25 to a temperature suitable for its rectification. This temperature is normally at or near the bubble point temperature of air. As is well known in the art, the pressure of minor subsidiary stream 22b must be sufficiently boosted to serve efficiently in the requisite vaporization duty. The major subsidiary stream 22a is also cooled to about its air dew point temperature by passage through the main heat exchanger 25. A double column air separation unit 26, which serves as the distillation stage of apparatus 10, has a higher pressure rectification column 28 and a lower pressure rectification column 30. Columns 28 and 30 are in a heat exchange relationship with one another by provision of a condenser-reboiler 32 which will be discussed hereinafter. Columns 28 and 30 have liquid/vapour contact elements such as random or structured packing, sieve plates, or bubble cap trays. These contact elements are used to bring descending liquid and ascending vapour phases of the mixture into intimate contact with one another. As the vapour rises within each column, from tray to tray or through packing, it becomes ever more concentrated in the more volatile components of air, e.g. nitrogen. As the liquid descends within the column, it becomes more concentrated in the less volatile components of the mixture to be separated, in case of air, oxygen. The descending liquid also becomes more concentrated in the heavy components. Therefore, the heavy components of air, namely carbon dioxide and hydrocarbons, will tend to concentrate in the oxygen product. Higher pressure column 28 has an extended bottom portion 34 containing liquid-vapour contact elements illustrated by reference numeral 36 (e.g. trays or packing). Incoming major subsidiary stream 22a is introduced into extended bottom portion 34 as a vapour, and is scrubbed of heavy impurities by descending liquid to concentrate the heavy impurities in the liquid at the very bottom of extended bottom portion 34. All of the liquid concentrated in the heavy impurities is removed as recycle stream 18. Recycle stream 18 is pumped by a pump 38 to a pressure sufficiently high that on vaporisation of recycle stream within main heat exchanger 25, the heavy impurities vaporize with the other components of the liquid collected in extended bottom portion 34. Recycle stream 18 is reduced in pressure by a pressure reduction valve 40 to the pressure of air stream 12 (downstream of compression by compressor 14) to facilitate the introduction of recycle stream 18 into the air stream 12. As can be appreciated, prepurification unit 20 is thereby continually removing not only most of the heavy impurities of incoming air stream 12 but also those impurities concentrated within the liquid phase collected in extended bottom portion 34 of higher pressure column 28. At the same time, since the heavy impurities concentrate within the liquid phase, the resultant vapour phase has a concentration of the heavy impurities that is far lower than the heavy impurity concentration of the compressed air exiting the aftercooler 16. It is the vapour phase which is taken for rectification (in the lower pressure rectification column 30) thus enabling an oxygen product low in heavy impurities to be produced. Minor subsidiary stream 22b is also introduced into higher pressure column 28 through a pressure reduction valve 41 at an intermediate location thereof. Although minor subsidiary stream 22b is not subjected to scrubbing, it has a lower flow rate than major subsidiary stream 22a. Hence, the reduction of heavy impurity concentration levels of major subsidiary stream 22a predominate(s) to lower overall heavy contaminant concentrations within the liquid oxygen. In operation of the air separation unit 26, the vapour phase rising in column 28 becomes ever more concentrated in nitrogen. A nitrogen stream 42 (comprising a nitrogen-rich fraction of the air) is removed from the top of higher pressure column 28 and is condensed by condenser-reboiler 32. A first reflux stream 44 is returned to higher pressure column 28 so that a descending liquid phase is thereby created which becomes ever more concentrated in liquid oxygen to form an oxygen-rich fraction of the air. The descending liquid is collected and removed from higher pressure column 28 as a rich liquid stream 46. As illustrated in Figure 1, rich liquid stream 46 is removed at a level of higher pressure column 28 located above liquid vapour contacting elements 36. As such, rich liquid stream 46 is not allowed to co-mingle with the liquid phase collected within extended bottom portion 34 of higher pressure column 28. A second reflux stream 48 is also extracted from the condensed nitrogen stream 42. Rich liquid stream 46 and second reflux stream 48 are subcooled in a subcooler 50, reduced in pressure to that of the lower pressure column 30 by pressure reduction valves 52 and 54 respectively, and are introduced into lower pressure column 30. In order to obtain a mass balance in the higher pressure column 28, reflux stream 55 is removed, subcooled within subcooler 50, reduced in pressure by passage through pressure reduction valve 56, and introduced into the lower pressure column 30. In the lower pressure column 30, the liquid phase descends and becomes ever more concentrated in oxygen so as to collect as a liquid oxygen fraction of the air in the bottom of lower pressure column 30. Liquid oxygen, so collected, is vaporized by the nitrogen stream 42 passing through condenser-reboiler 32. A waste nitrogen stream 57 formed from the nitrogen fraction of the air passes through subcooler 50 to subcool second reflux stream 48, rich liquid stream 46 and reflux stream 55. This provides some warming of waste nitrogen stream 57. Waste nitrogen stream 57 is warmed to approximately ambient temperature within main heat exchanger 25 where it serves to help reduce the temperature of major and minor subsidiary streams 22a and 22b. At least a part of waste nitrogen stream 57 also serves to regenerate prepurification unit 20. In order to balance cold box heat leakage and warm-end heat content differences of apparatus 10, a refrigerant stream 58 is extracted from the higher pressure column 28 and warmed within main heat exchanger 25 to a chosen temperature between the cold end and warm end temperatures of the heat exchanger 25. After turboexpansion within turboexpander 60, the refrigerant stream 58 is introduced into lower pressure column 30. An oxygen product stream 62 is removed from the bottom of lower pressure column 30 where it is pumped by pump 64 to the requisite high pressure. Product oxygen stream 62 is then vaporized within main heat exchanger 25. Residual heavy impurities that have concentrated within oxygen product stream 62 vaporize with the oxygen. With reference to Figure 2, extended bottom portion 34 can be devoid of liquid-vapour contact elements 36 and instead serves as a phase separator of the major portion of combined stream 22 (major subsidiary stream 22a). In such case, major subsidiary stream 22a is partially condensed within main heat exchanger 25 so that it has liquid and vapour phases which can be separated within extended bottom portion 34. The heavy impurities concentrate in the liquid phase and collect at the very bottom of extended bottom portion 34 of higher pressure column 28. The vapour phase, lean in heavy impurities, is subjected to the distillation. In the illustration, rich liquid stream 46 is removed from the lowermost tray 66 to prevent co-mingling of the liquid phase to be further refined within lower pressure column 30 and the liquid collected within extended bottom portion 34. As can be appreciated, although higher pressure column 30 is provided with a trayless bottom portion, a separate phase separator could be used and attached to a conventional high pressure column. The following is a calculated example showing the operation of apparatus of Figure 1. In the example carbon dioxide is the contaminant. However, it is applicable to any contaminant which may separate from the boiling product oxygen as a pure contaminant phase or contaminant-rich phase. Prepurification unit 20 in air separation plant 10 removes 99.98% of the carbon dioxide that enters the unit. Incoming air stream 12 contains 350 vpm (parts per million on a molar basis) of carbon dioxide. The partially purified air contains 0.07 vpm of carbon dioxide, which normally collects in the liquid oxygen withdrawn from the lower pressure column 30 where it will normally contain about 0.32 vpm of carbon dioxide. The product oxygen is required at 3.0 bara, which requires a boosted air pressure of about 8.6 bara. However, with about 0.32 vpm carbon dioxide content, the liquid oxygen would need to be pressurized to about 4.5 bara to avoid phase separation of carbon dioxide during vaporization in the main heat exchanger. Such a vaporization pressure would require a boosted air pressure of about 11 bara, an extra and unnecessary expenditure of energy. In accordance with the invention, air which enters the higher pressure column 28 as vapour is scrubbed with a small amount of liquid within liquid-vapour contacting elements 36 (Figure 1) to clean it of carbon dioxide or alternatively is partially condensed forming a liquid containing most of the carbon dioxide (Figure 2). The liquid oxygen withdrawn from the low pressure column now contains 0.093 vpm of carbon dioxide, suitable for a 3.1 bara vaporization pressure. Recycle stream 18 containing about 2.5 vpm of carbon dioxide, is pumped to a suitable high pressure of about 17 bara to prevent precipitation of carbon dioxide. A boosted air pressure of about 8.6 bara is adequate to effect the heat transfer for the vaporization of the liquid oxygen and the scrubber bottoms. Refrigeration of recycle stream 18 is recovered in main heat exchanger 25 and its pressure is partially recovered with its oxygen content by adding the recycle stream to the air stream 12 upstream of prepurification unit 20. In more detail, air stream 12 at a flow rate of 1000 Nm 3 /hr is compressed to about 5.5 bara, cooled and passed to prepurification unit 20. About 322 Nm 3 /hr of air leaving prepurification unit 20 as minor subsidiary stream 22b is further compressed to about 8.6 bara. Both major and minor subsidiary streams 22a and 22b are cooled in the main heat exchanger where the major subsidiary stream 22a exits close to its dew point and the minor subsidiary stream 22b exits mostly liquefied. Major subsidiary stream 22a is scrubbed of its carbon dioxide content by about 20 Nm 3 /hr of liquid in lower section 34 of the higher pressure column. The resulting liquid containing about 40% oxygen is extracted as recycle stream 18 which is pumped to about 17 bara and passed through main heat exchanger 25. When recycle stream 18 emerges, it is throttled into air stream 12 upstream of the prepurification unit 20 to form combined stream 22. The process is a normal double column process with turboexpansion of a stream to produce refrigeration. About 220 Nm 3 /hr of a liquid oxygen product containing 95% oxygen is withdrawn and pumped to 3.1 bara, passed to the main heat exchanger 25 where it is vaporized and heated and delivered as a product at approximately 3 bara. Waste nitrogen stream 57 from the top of lower pressure column 30 passes through a subcooler 50 and then is warmed in the main heat exchanger. Part of this gas may be heated and used for regeneration of the prepurification unit. The flow rate of the recycle stream is approximately equal to that of the liquid which scrubs the cooled, combined stream (i.e. 20 Nm 3 /hr). Accordingly, its flow rate is an order of magnitude less than that (220 Nm 3 /hr) of the product oxygen stream. Therefore, the recycle stream has relatively little effect on the operation of the main heat exchanger.

Claims: A method of separating a gaseous mixture comprising higher and lower volatility components and heavy impurities to obtain a product stream predominantly containing said lower volatility component(s) of said gaseous mixture, said method comprising: compressing said gaseous mixture; pumping a recycle liquid stream containing heavy impurities to a sufficient pressure such that on vaporisation said heavy impurities vaporise with said liquid; vaporising at said pressure and then reducing the pressure of said recycle stream to a pressure about equal to that of the compressed gaseous mixture; combining said recycle stream with the compressed gaseous mixture to form a combined stream, prepurifying the combined stream by removal of heavy impurities therefrom, cooling to a cryogenic temperature at least a major part of the prepurified stream, separating a liquid relatively concentrated in residual heavy impurities from the cooled stream and thereby forming a vapour relatively lean in residual heavy impurities; subjecting said vapour in a distillation stage to rectification to produce said product stream; and forming said recycle stream from the liquid relatively concentrated in residual heavy impurities. A method according to claim 1, wherein said liquid relatively concentrated in residual heavy impurities is formed as condensate during the cooling of the prepurified stream, and is disengaged from residual vapour. A method according to Claim 2, wherein the disengagement is performed within the distillation stage. A method according to Claim 1, wherein said residual heavy impurities are concentrated within said liquid by scrubbing the cooled stream with a descending liquid within said distillation stage. A method as Claimed in any one of the preceding claims, wherein said product stream is withdrawn from distillation stage in liquid state and said method further comprises: pumping said liquid product stream to an elevated pressure; vaporizing said liquid product stream within said cooling stage to produce said product stream at said elevated pressure; taking as a minor stream a part of the compressed, prepurified stream, raising its pressure further in a booster-compressor, cooling the said part of the compressed prepurified stream by countercurrent heat exchange with said liquid product stream; and introducing said cooled minor stream into said distillation stage. A method according to Claim 5, wherein: said gaseous mixture comprises air and said heavy impurities include carbon dioxides and hydrocarbons; said distillation stage comprises a double rectification column having a higher pressure column connected to a lower pressure column in a heat exchange relationship; the air is separated within said higher pressure column into oxygen-rich and nitrogen-rich fractions, and an oxygen-rich stream composed of said oxygen-rich fraction is introduced into said lower pressure column for further separation thereby to produce a liquid oxygen fraction and a nitrogen fraction; and said liquid product stream is removed from said lower pressure column and comprises said liquid oxygen fraction. A method according to Claim 5 or Claim 6, wherein the flow rate of the recycle stream is an order of magnitude less than that of the liquid product stream. An apparatus for separating gaseous mixture comprising higher and lower volatility components and heavy impurities to obtain a product stream predominantly comprising said lower volatility component(s) of said gaseous mixture, said apparatus comprising: a compression stage for compressing a gaseous mixture; a pump for pumping a recycle liquid stream containing heavy impurities to a sufficient pressure that on vaporisation the heavy impurities vaporise with the liquid; heat exchange passages which communicate with an outlet of the pump and in which, in use, said recycle stream vaporises at said pressure; a valve for reducing the pressure of the vaporised recycle stream having an inlet in communication with an outlet from said heat exchange passages and having an outlet in communication with a conduit for the flow of the gaseous mixture such that, in use, said recycle stream combines with said gaseous mixture to be separated to form a combined stream; a prepurification stage for purifying the combined stream by removal of heavy impurities therefrom; further heat exchange passages communicating the prepurification stage such that, in use, at least a major part of the combined stream flows through the further heat exchange passages and is cooled to a cryogenic temperature; a separation vessel for separating a liquid relatively concentrated in heavy impurities from the cooled combined stream and thereby forming a vapour relatively lean in heavy impurities having an inlet communicating with an outlet from the further heat exchange passages and having an outlet communicating with an inlet to the said pump; a distillation stage for separating said vapour by rectification in order to obtain said product stream. Apparatus according to Claim 8, wherein said separation vessel is a phase separator. Apparatus according to Claim 8, wherein said distillation stage comprises a distillation column having a section for separating the liquid relatively concentrated in heavy impurities from the combined stream, whereby the distillation column is also able to function as the separator vessel. Apparatus according to Claim 9 or Claim 10, wherein said distillation stage has an outlet for the withdrawal of the product stream in liquid state and the apparatus additionally includes a further pump, communicating with an outlet from the distillation stage for raising the pressure of the product stream, and yet further heat exchange passages for vaporising the pressurised product liquid stream by indirect heat exchange with a further part of said combined stream; and a booster-compressor for raising the pressure of said further part of the combined stream upstream of its heat exchange with the liquid product stream. Apparatus as claimed in Claim 11, wherein: said gaseous mixture comprises air and said heavy impurities include carbon dioxide and hydrocarbons; said distillation stage comprises a double rectification column having a higher pressure column connected to a lower pressure column in heat transfer relationship; and said separator vessel comprises an extended bottom portion of said higher pressure column.

Inventors: MOSTELLO ROBERT A, STERN SIDNEY S, MOSTELLO, ROBERT A., STERN, SIDNEY S.
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1995
Main classification: F25J3
IPC codes: F25J3