Production of nitrogen of ultra-high purity

Abstract: Air is rectified in a rectification column 24 to produce at its top a gaseous nitrogen fraction relatively to produce a rich in light elements, such as neon, helium and hydrogen. A stream of this gaseous fraction is then partially condensed within a condenser 32 and separated into liquid and vapour phase within a phase separator 48. The liquid phase is lean in the light elements and the vapour phase is rich in the light elements. The liquid phase is removed from the bottom of the phase separator 48 and is introduced into the column 24 as reflux. As the reflux descends from tray to tray it is stripped of light elements. A product stream containing ultra-high purity nitrogen is withdrawn as a liquid stream 62 from the column 24 after suitable stripping of the reflux. The product stream 62 can be further purified by stripping within a stripper column 68.

Description: The present invention relates to a process for producing high purity nitrogen by the low temperature rectification of air. More particularly, the present invention relates to such a process in which light elements such as helium, hydrogen and neon, are removed from a nitrogen fraction to produce a nitrogen product of ultra-high purity. Methods and apparatus for producing high purity nitrogen by the low temperature rectification of air are well known in the art. An example of such a method and apparatus is disclosed in US-A-4 966 002. In this patent, the high purity nitrogen is produced by a single column low temperature rectification process distinguished by its incorporation of a waste recompression cycle. In such a cycle, two partial waste streams of nitrogen are respectively engine expanded and compressed by a compressor coupled to a turboexpander by an energy dissipative brake. The compressed partial waste stream is introduced into the column to enhance nitrogen recovery and the engine expanded partial waste stream is used within the process as a source of refrigeration. Such process and apparatus produces high purity nitrogen at high pressure and at high thermodynamic efficiencies. The product nitrogen is high purity in that it is lean in oxygen. However, the product does contain "light elements" such as helium, hydrogen and neon, which, due to their volatility, tend to concentrate in the nitrogen product stream in an amount that represents a ten fold increase as compared with their concentration in the entering air. For most industrial applications of nitrogen, such concentrations of light elements are unimportant. However, in the electronics industry, ultra high purity nitrogen is required in which the product nitrogen is essentially free of the light elements. The term "ultra high purity nitrogen" is therefore used herein to mean nitrogen whose concentration of light elements is less than the concentration of such elements in air. The term "light elements" as used herein means hydrogen, helium and neon. WO-A-91/19142 discloses a process in which air is cooled and fed into a main fractionating column so as to be rectified; liquid nitrogen is withdrawn from below the uppermost plate in the column and is fed into a secondary rectifier. Nitrogen product is withdrawn from the secondary rectifier at a position below that at which the liquid nitrogen is introduced. US-A-4902321 discloses a process and apparatus for producing nitrogen that is illustrated in connection with a single column apparatus. The process comprises rectifying air within a rectification column to produce a top fraction comprising nitrogen vapour relatively rich in light elements; condensing a stream of the top fraction to form a condensate: returning a stream of the condensate to the top of the rectification column as reflux and withdrawing a liquid nitrogen product stream from the rectification column. In order to perform this process, US-A-4 902 321 discloses an apparatus comprising low temperature rectification means having a rectification column for rectifying air to produce a top fraction comprising nitrogen vapour relatively rich in light elements; condensing means having an inlet connected to the top of the rectification column for condensing a stream of the top fraction and an outlet communicating with the top of the rectification column; and an outlet for withdrawing a liquid nitrogen stream from the rectification column withdrawing a product stream of said ultra high purity liquid nitrogen from the rectification column. The present invention relates to a method that can be used to separate a ultra high purity nitrogen product from air typically using a single rectification column. According to the present invention there is provided a process of producing ultra-high purity nitrogen comprising: the features of claim 1. The product stream can be further purified by using a gas to strip further light elements therefrom. Thus the product stream can be introduced into the top of a stripper column, and the stripper gas introduced into the stripper column below the product stream. This produces further purified ultra high purity nitrogen as liquid at the bottom of the stripper column and a gas at the top of the stripper columns. The further purified liquid is withdrawn from the bottom of the stripper column. Nitrogen production rates can be increased by withdrawing a gas stream from the top of the stripper column, recompressing the gas stream to rectification column pressure, and introducing the compressed gas stream into the rectification column. Alternatively, in order to avoid the expense of recompression, the gas stream can be extracted from the stripper column and partially condensed. The resulting liquid and gaseous phases are lean and rich in the light elements, respectively. The gaseous phase is preferably separated from the liquid phase and returned to the stripper column. Additionally, a process liquid, such as oxygen enriched liquid produced at the bottom of the rectification column, can be withdrawn from the rectification column and heat exchanged with the gas stream withdrawn from the stripper column so as partially to condense the gas stream. The refrigeration potential can then be recovered from the partially condensed stream and used in the rectification to increase production of ultra high purity nitrogen. In accordance with the process of the present invention, a high purity nitrogen process design can readily be modified to produce ultra-high purity nitrogen. The process according to the invention will now be described by way of example with reference to the accompanying drawings in which: FIG. 1 is a schematic view of an air separation process in accordance with the subject invention; FIG. 2 is a schematic view of an alternative embodiment of an air separation process in accordance with the present invention; FIG. 3 is a schematic view of a further alternative embodiment of an air separation process in accordance with the present invention; FIG. 4 is a schematic view of a still further embodiment of an air separation process in accordance with the present invention; and FIG. 5 is yet another embodiment of an air separation process in accordance with the present invention. All of the embodiments illustrated above, represent the process of the present invention applied to an air separation plant illustrated in FIG. 4 of US 4,966,002. For the sake of simplicity of explanation, the same reference numerals will be used in the accompanying drawings for identical components and streams of process fluid passing between the components. Additionally, arrowheads are used to show flow direction of the process fluid between the components. With reference to FIG. 1 of the accompanying drawings, an air separation plant 10 operated in accordance with the present invention is illustrated. In air separation plant 10, air is compressed by a compressor 12 and is then purified in a pre-purification unit 14. Pre-purification unit 14 is a PSA unit having beds of activated alumina and molecular sieve material to adsorb carbon dioxide and water Hydrogen may also be removed. For example, the purification unit may be as described in EP-A-438 282 (which also removes carbon monoxide) An air stream 16 of the now compressed and purified air is then cooled in a main heat exchanger 18 of plate-fin design. Air stream 16 is then split into two portions 20 and 22. Portion 20 of air stream 16 is introduced into a rectification column 24 having, say, 79 trays. The air is rectified within rectification column 24 to produce at the bottom thereof an oxygen rich liquid 26 and at the top thereof gaseous nitrogen. High purity liquid nitrogen is taken from the seventy fifth tray (from the bottom) of the rectification column 24. This tray is spaced 4 trays from the top of column 24 Hence, the gas at the top of the column 24 consists of nitrogen vapour relatively rich in the light elements which tend to concentrate there due to the volatility of the lights elements. A waste stream 30 of oxygen rich liquid is extracted from the bottom of rectification column 24. A back pressure valve 25 is used to maintain column pressure. After passage through back pressure valve 25, waste stream 30 is vaporised and warmed in a condenser 32 and air liquefier 34 of plate-fin design to produce a warm waste stream 36. Warm waste stream 36 is split into two portions 36 and 40. Portion 38 is compressed in a compressor 42 to produce a compressed waste stream 44. Compressed waste stream 44 is cooled in main heat exchanger 18 and is then passed into the bottom of rectification column 24 to enhance the nitrogen recovery rate. A stream 46 of nitrogen is extracted from the top 28 of rectification column 24. In accordance with the present invention, stream 46 is partially condensed in condenser 32 and is then introduced into a phase separator 48. A liquid phase lean in the light elements collects in the bottom of phase separator 48 and a gaseous phase rich in the volatile light elements collects in the top of phase separator 48. Phase separator 48 is connected to the top of rectification column 24 to reintroduce the liquid phase as reflux stream 50, into rectification column 24. Hence, the partial condensation followed by the phase separation of stream 46 acts to purify stream 46 partially by separating the vapour phase from the stream after partial condensation thereof. The vapour fraction is removed as a stream 52 and is subsequently combined with portion 40 of waste stream 36 to form a combined stream 54. A back pressure controller 55 is used to reduce the pressure of stream 52 to that of portion 40 of waste stream 36. The combined stream 54 is heated in main heat exchanger 18, engine expanded in a turboexpander 56 to produce refrigeration in the form of an expanded waste stream 58. It is to be noted that compressor 42 is coupled to turboexpander 56 by a common shaft having an oil brake 60 to dissipate some of the work from the expansion process. Expanded waste stream 58 is warmed in air liquefier 34 and then by passage through main heat exchanger 18 to ambient temperature before leaving the process. In so warming, stream 58 cools incoming air stream 16. As mentioned previously, rectification column 24 has 79 trays, typically 4 more trays than used in the rectification column of the process described in US 4,966,002. The reason for this will become apparent. After reflux stream 50 is reintroduced into the top of rectification column 24, it drops from tray to tray while being stripped of the light elements. Thus, a product stream 62 drawn say 4 trays below the top of rectification column 24 as a liquid is still leaner with respect to the light elements than stream 50 and in fact comprises nitrogen of ultra-high purity. A back pressure valve 64 is used to maintain column pressure in spite of the withdrawal of product stream 62. After passage through back pressure valve 64, product stream 62 is then vaporised and warmed by passing through condenser 32 partially to condense stream 46 and then through air liquefier 34 also to help liquefy portion 22 of cooled air stream 16. The product stream 62 is thus warmed. It is then introduced into main heat exchanger 18 and thereby warmed to ambient temperature. With reference to FIG. 2 of the drawings, an air separation plant 100 is capable of producing a further purified product stream 66 of higher purity than product stream 62 produced by the air separation plant 10 shown in Figure 1. In air separation plant 100, product stream 62 is again withdrawn about 4 trays below the top tray of rectification column 24. Product stream 62 is then introduced into a stripper column 68, a packed column of approximately 4 stages, where product stream 62 is further stripped by a stripper gas having a higher purity than product stream 62. The stripper gas is introduced into stripper column 68 below the point of entry of product stream 62 and is used in forming further purified product stream 66 which collects as a liquid at the bottom of stripper column 68. Further purified product stream 66 is withdrawn from the bottom of stripper column 68 and is then vaporised in condenser 32 and air liquefier 34. Further purified product stream 66, is then split into two partial streams 72 and 74. Partial stream 72 of further purified product stream 66 forms the stripper gas, and, as such, is introduced into the bottom of stripper column 68. The other partial stream 74 of further purified product stream is warmed to ambient temperature in main heat exchanger 18 for delivery to the customer. A gas stream is withdrawn from the top of stripper 68 as stream 78 which is combined with streams 52 and portion 40 of waste stream 36 to produce combined stream 54 which is partially warmed and then expanded in turbo expander 56 to produce expanded waste stream 58. Back pressure controllers 77 and 79 are used to reduce the pressure of streams 52 and 78 to that of portion 40 of waste stream 36. The advantage of this last aspect of plant operation over that of air separation plant 10 is that the the amount of expansion is increased by the increase in flow into turboexpander 56 to allow more nitrogen to be recompressed in compressor 42 for addition to rectification column 24. As a result, the process and apparatus involved in plant 100 allows for the production of ultra-high purity nitrogen product having a greater purity than that produced by the process and apparatus of air separation plant 10 at an equivalent rate of production. FIG. 3 illustrates an air separation plant 200 that is similar in operation to plant 100, illustrated in FIG. 2. The sole difference between plant 200 and 100 is that stream 78, composed of gas from the top of the stripper column 68, is compressed in a recompressor 80 to column pressure and is introduced back into the column 24, at an appropriate concentration level. The additional nitrogen introduced into rectification column 24 enhances the recovery rate of ultra-high purity nitrogen over the plant and process illustrated in Fig. 2. FIG. 4 illustrates an air separation plant 300 capable of producing more ultra-high purity nitrogen than air separation plant 100, illustrated in FIG. 2, without the recompression of gas form the top of the stripper column 68, thereby avoiding the added operational expenses of air separation plant 200 illustrated in FIG. 3. In air separation plant 300, product stream 62 is extracted from rectification column 24 for further purification before delivery. To this end, product stream 62 is introduced into the top of stripper column 68 for further stripping against a stripper gas made up of partial stream 72 of further purified product stream 66. Stream 78 is withdrawn from the top of the stripper column 68 and overhead is partially condensed in a stripper recondenser 82 and is then introduced into a phase separator 84. In phase separator 84, liquid and vapour phases form, lean and rich in light elements, respectively. A stream 86 from the bottom of phase separator 84 is introduced into the top of stripper column 68 along with product stream 62 to enhance the recovery rate of ultra-high purity nitrogen. A side waste stream 30a is extracted from waste stream 30 and is then fully vaporised in stripper recondenser 82. A back pressure valve 31 is provided to maintain the column pressure of rectification column 24. Side waste stream 30a is then introduced into the outlet stream of turboexpander 56 to recover the refrigeration contained therein. The vapour phase is extracted from the top of phase separator 84 as a stream 87 and is then combined with stream 52 from phase separator 48 for expansion with portion 40 of waste stream 36. This produces additional refrigeration and also enhances liquid nitrogen production. Back pressure controllers 89 and 91 are used to reduce the pressures of stream 52 and 87 to that of portion 46 of waste stream 36. FIG. 5 illustrates an air separation plant 400, which contains all of the components of air separation plant 300 with the addition of a phase separation tank 88. The objective of air separation plant 400 is to increase the degree of recompression and expansion over that involved in air separation plant 300 in order efficiently to increase the recovery rate of ultra-high purity nitrogen. Unlike air separation plant 300, side waste stream 30a is only partially vaporised in stripper recondenser 82. The partial vaporisation of side waste stream 30a results in a high enough pressure to recover its refrigeration potential. Such recovery is effected by passing partially condensed waste side stream 30a into phase separation tank 88 for separation into liquid and vapour phases. A stream 90 composed of the liquid phase is extracted from the bottom of phase separator 88. Stream 90 is then added to waste stream 30 to add to the flow to be expanded and increase the amount to be recompressed. In addition, since stream 90 is added to waste stream 30 upstream of its introduction into condenser and air liquefier, more tower overhead can be partially condensed, purified, stripped and recovered. The resultant waste stream 30b is introduced into condenser 32 and air liquefier 34 to produce a warm waste stream 36a. A stream 92 composed of the vapour phase is extracted from the top of phase separator 88. Stream 92 is added to warm waste stream 36a downstream of passage through condenser 32 and air liquefier 34 to form warm waste stream 36 which contains added flow to be expanded and recompressed. The refrigeration potential is recovered by adding streams composed of the liquid phase after vaporisation and warming and the vapour phase into the combined stream 54 to be expanded into turboexpander 56. The process according to the invention is further illustrated by the following examples. EXAMPLE 1 In this example, ultra-high purity nitrogen is recovered though the use of the process and apparatus illustrated in Fig. 1. The nitrogen product obtained from this process is contained within a product stream 62 flowing at a rate of about 1115.0 Nm 3 /hr (Normal cubic metres per hour) and containing approximately 0.5 ppb oxygen, 0 57 ppm neon, and 5 0 ppb helium. It is to be noted that the process and apparatus of Figs. 1-5 also separate hydrogen from high purity nitrogen. Such separation is carried out in the pre-purification unit 14 as well as rectification column 24. Practically, the concentration of hydrogen in the examples will lie between helium and neon. Additionally, in this and succeeding examples, pressures and given in absolute units. Air stream 16 upon entry to main heat exchanger 18 has a temperature of about 278.7°K, a pressure of 11 7 kg/cm 2 , and a flow rate of approximately 2462.0 Nm 3 /hr. Upon leaving mean heat exchanger 18, air stream 16 has a temperature of approximately 109 9°K and a pressure of about 11.00 kg/cm 2 . After division of air stream 16. portion 20 of stream 16 has a flow rate of approximately 2370.0 Nm 3 /hr and portion 22 has a flow rate of about 92.0 Nm 3 /hr. After liquefaction, portion 22 has a temperature of about 107.4°K, and a pressure of about 10.98 kg/cm 2 . Waste stream 30 has a flow rate of approximately 1347.0 Nm 3 /hr, a temperature and pressure of approximately that of the column, namely 109.9°K, and 11.01 kg/cm 2 , respectively. Back pressure valve 25 produces temperature and pressure drops within waste stream 30 to about 101.0°K and about 6.0 kg/cm 2 . After warming, the resultant warm waste stream 36 has a temperature of approximately 106.6°K, and a pressure of approximately 5.87 kg/cm 2 . Portion 38 of warm waste stream 36 has a flow rate of approximately 870.0 Nm 3 /hr, and portion 40 has a flow rate of approximately 1321.0 Nm 3 /hr. After passage through compressor 42, the resultant compressed waste stream 44 has a temperature of about 142.9°K and a pressure of approximately 11.08 kg/cm 2 and after passage through main heat exchanger 18, compressed waste stream 44 has a pressure of approximately 11.01 kg/cm 2 and a temperature of approximately 112.7°K. Stream 52, representing the vapour fraction removed from stream 46 of tower overhead, has a temperature of about 104.5°K, a pressure of about 10.7 kg/cm 2 , and a flow rate of approximately 26.0 Nm 3 /hr. When combined with portion 40 of waste stream 36, combined stream 54 has a flow rate of approximately 1347.0 Nm 3 /hr, After combined stream 54 passes through main heat exchanger 18, it has a temperature of about 142.0°K, a pressure of about 5.77 kg/cm 2 . The resultant expanded waste stream 58 has a temperature of about 106°K and a pressure of about 1.53 kg/cm 2 . Expanded waste stream 58 leaves air liquefier 34 at a temperature of about 106.6°K, and subsequently leaves main heat exchanger 18 with a temperature of about 274.0°K and a pressure of about 1.50 kg/cm 2 . Product stream 62 leaves air liquefier 34 as a vapour at a temperature of about 104.6°K, and a pressure of about 9.67 kg/cm 2 . Back pressure valve 64 produces a pressure and temperature drop within product stream 62 to about 9.79 kg/cm 2 and about 103.2°K. After passing though main heat exchanger 18, product stream 62 has a temperature of about 274.0°K, and a pressure of about 9.55 kg/cm 2 . EXAMPLE 2 In this example, ultra-high purity nitrogen is recovered though use of the process and apparatus shown in Fig. 2. The nitrogen product obtained from this process is contained within partial stream 74 of product stream 66 flowing at a rate of about 1115.0 Nm 3 /hr. and containing approximately 0.5 ppb oxygen, 31 ppb neon, and about 0.03 ppb helium. In this example product stream 74 has a lower concentration of light elements than product stream 66 of the preceding example through the use of stripper column 68. Air stream 16 upon entry to main heat exchanger 18 has a temperature of about 278.7°K, a pressure of 11.17 kg/cm 2 and a flow rate of approximately 2661.0 Nm 3 /hr. Upon leaving mean heat exchanger 18, air stream 16 has a temperature of approximately 109.9°K and a pressure of about 11.00 kg/cm 2 . After division of air stream 16, portion 20 of air stream 16 has a flow rate of approximately 2553.0 Nm 3 /hr and portion 22 has a flow rate of about 108.0 Nm 3 /hr. After liquefaction, portion 22 has a temperature of about 107.4°K, and a pressure of about 10.98 kg/cm 2 . Waste stream 30 has a flow rate of approximately 2405.0 Nm 3 /hr, a temperature of about 109.9° K, and a pressure of about 11.01 kg/cm 2 . Back pressure valve 25 reduces the temperature and pressure of waste stream 30 to 100.9°K and about 6.00 kg/cm 2 . After vaporisation and warming, the resultant warm waste stream 36 has a temperature of approximately 106.6°K and a pressure of approximately 5.87 kg/cm 2 . After division of warm waste stream 36, the resulting portions 38 and 40 flow at about 987.0 Nm 3 /hr and 1418.0 Nm 3 /hr, respectively. Stream 38 is compressed in compressor 42 to form compressed waste stream 44 having a temperature of about 142.9°K and a pressure of approximately 11.08 kg/cm 2 . After passage through main heat exchanger 18, compressed waste stream 44 has a pressure of approximately 11.02 kg/cm 2 and a temperature of approximately 112.7°K. Stream 52, representing the vapour fraction removed from stream 46 of tower overhead, has a temperature of about 104.6°K, a pressure of about 10.71 kg/cm 2 , and a flow rate of approximately 26.0 Nm 3 /hr. Stripper overhead stream 78 has a flow rate of about 102.2 Nm 3 /hr, a temperature of 102.8°K., and a pressure of about 9.53 kg/cm 2 . When stripper overhead stream 78 is added to stream 52 and portion 40 of heated waste stream 36, combined stream 54 has a flow rate of about 1546.0 Nm 3 /hr, a temperature of about 105.7°K., and a pressure of about 5.87 kg/cm 2 . After combined stream 54 passes through main heat exchanger 18 its temperature increases to about 141.0°K. The expanded waste stream 58 has a temperature of about 105.0°K and a pressure of about 1.63 kg/cm 2 . Expanded waste stream 58 leaves air liquefier 34 with a temperature of about 106.6°K, and a pressure of about 1.55 kg/cm 2 and subsequently leaves main heat exchanger 18 with a temperature of about 274.0°K and a pressure of about 1.30 kg/cm 2 . Product stream 62 is introduced into stripper column 68 at a flow rate of about 1217.0 Nm 3 /hr, a temperature of about 103.0°K., and a pressure of about 9.67 kg/cm 2 . Further purified product stream 66 is extracted from the bottom of stripper column 68 at a flow rate of about 1183.0 Nm 3 /hr, a temperature of about 103.0°K, and a pressure of about 9.67 kg/cm 2 . Further purified product stream 66 is vaporised and heated and leaves air liquefier 34 at a temperature of about 106.6°K, and a pressure of about 9.67 kg/cm 2 . Partial stream 72 has a flow rate of about 68.0 Nm 3 /hr and is introduced into stripper column 68 as stripper gas. Partial stream 74 is warmed in main heat exchanger 18 to a temperature of about 274.0°K and a pressure of about 9.55 kg/cm2 and delivered as product. EXAMPLE 3 A nitrogen product of ultra-high purity is recovered having essentially the same purity as the product produced in Example 2. The recovery rate of the nitrogen product is enhanced with respect to that of Example 2 by compressing stripper overhead stream 78 and introducing it into column 24 in the manner and the apparatus shown in Fig. 3. In this regard, partial stream 74 which contains the ultra-high purity nitrogen product flows at about 1115.0 Nm 3 /hr as in the previous example. However, entering air stream 16 in this example flows at about 2467.0 Nm 3 /hr as compared to 2661.0 Nm 3 /hr in Example 2. In the main, the pressures and temperatures of the streams is the same as that in Example 2, except as indicated otherwise in the discussion set forth below. After division of air stream 16, portion 20 of air stream 16 has a flow rate of approximately 2373.0 Nm 3 /hr and portion 22 has a flow rate of about 94.0 Nm 3 /hr. Waste stream 30 has a flow rate of approximately 2199.0 Nm 3 /hr., and after division, the resulting portions 38 and 40 flow at about 873.0 Nm 3 /hr and about 1326.0 Nm 3 /hr, respectively. Stream 52, representing the vapour fraction removed from stream 46 of tower overhead, has a flow rate of approximately 26.0 Nm 3 /hr and is added to portion 40 of heated waste stream 36 to form combined stream 54 having a flow rate of about 1352.0 Nm 3 /hr. After combined stream 54 passes through main heat exchanger 18 its temperature increases to about 142.3°K and after passage through expander 56, the resultant expanded waste stream 58 has a temperature of about 105.9°K. Product stream 62 is introduced into stripper column 68 at a flow rate of about 1212.0 Nm 3 /hr and further purified product stream 66 is extracted from the bottom of stripper column 68 at a flow rate of about 1177.0 Nm 3 /hr. After division of further purified product stream, partial stream 72 has a flow rate of about 62.0 Nm 3 /hr for introduction into stripper column 68 as stripper gas. Stripper tower overhead stream 78 has a flow rate of about 97.0 Nm 3 /hr. After passage through recompressor 80, stripper tower overhead stream 78 has a temperature of about 108.5° K. and a pressure of about 10.73 kg/cm 2 for introduction into rectification column 24. EXAMPLE 4 An ultra-high purity nitrogen product is recovered by the use of the the process and apparatus illustrated in Fig. 4. The purity of the product is essentially that of Example 2 in that it contains approximately 0.5 ppb oxygen, 38.0 ppb neon and 0.03 ppb helium. The recovery rate is greater than that of Example 2 but without the added power consumption arising in Example 3 by recompression of the stripper tower overhead. In this regard, the further purified product flows at about 1115.0 Nm 3 /hr and is produced from air stream 16 entering main heat exchanger 18 at a flow rate of about 2539.0 Nm 3 /hr. Air stream 16 enters main heat exchanger 18 with a temperature of 278.7°K and a pressure of 11.17 kg/cm 2 . Within main heat exchanger 18, the pressure and temperature of air stream 16 drops to about 11.00 kg/cm 2 and about 109.9°K, respectively. After division of air stream 16, portion 20 has a flow rate of approximately 2443.0 Nm 3 /hr and portion 22 has a flow rate of about 96.0 Nm 3 /hr. After liquefaction, portion 22 has a temperature of about 107.4°K, and a pressure of about 10.98 kg/cm 2 . Waste stream 30 as removed from the bottom of rectification column 24 has a flow rate of approximately 2188.0 Nm 3 /hr. and a temperature and pressure of approximately that of the column, namely 109.9°K, and 11.01 kg/cm 2 . Side waste stream 30a is divided from waste stream 30 and flows at about 67 Nm 3 /hr. Waste stream 30 enters condenser 32 at a temperature of about 100.8°K and a pressure of about 6.00 kg/cm 2 and leaves air liquefier 34, as waste stream 36 containing warm vapour, at a temperature of about 106.6° K. and a pressure of about 5.87 kg/cm 2 . Warm waste stream 36 is divided into two portions, portion 38 having a flow rate of approximately 880.0 Nm 3 /hr. and portion 40 having a flow rate of approximately 1308.0 Nm 3 /hr. After passage through compressor 42, the resultant compressed waste stream 44 enters main heat exchanger 18 at a temperature of about 143.0° K and a pressure of approximately 11.09 kg/cm 2 and thereafter, is introduced back into rectification column 24 at a pressure of approximately 11.01 kg/cm 2 and a temperature of approximately 112.7°K. Stream 52, representing the vapour fraction removed from stream 46 of tower overhead, has a temperature of about 104.6° K., a pressure of about 10.70 kg/cm 2 , and a flow rate of approximately 27.0 Nm 3 /hr. When combined with portion 40 of warmed waste stream 36 and stream 86 (having a flow rate of about 23.0 Nm 3 /hr, a temperature of about 102.8°K., and a pressure of about 9.52 kg/cm 2 ) combined stream 54 has a flow rate of approximately 1358.0 Nm 3 /hr, a temperature of about 106.2°K., and a pressure of about 5.87 kg/cm 2 . After combined stream 54 passes through main heat exchanger 18, it has a temperature of about 142.0°K and a pressure of about 5.78 kg/cm 2 . After expansion, side waste stream 30a is added to expanded waste stream 58 having a temperature of about 105.8°K. and a pressure of about 1.61 kg/cm 2 . Expanded waste stream 58 leaves air liquefier 34 with a temperature of about 106.6°K, and and a pressure of about 1.55 kg/cm 2 and then main heat exchanger 18 with a temperature of 274.0°K and a pressure of about 1.3 kg/cm 2 . Product stream 62 is extracted from rectification column 24 at a flow rate of about 1138.0 Nm 3 /hr, a temperature of about 104.6°K, and a pressure of about 10.72 kg/cm 2 . Stripper overhead stream 78 flowing at about 97.0 Nm 3 /hr and having a temperature of about 102.8° K and a pressure of about 9.53 kg/cm 2 is partially condensed against fully vaporised waste stream 30a. Side waste stream 30a enters stripper recondenser 82 at a temperature of about 98.7°K and a pressure of about 5.11 kg/cm 2 . The gas phase is separated from the liquid phase in phase separator 84 and stream 86, comprising the liquid phase, is combined with product stream 62 and introduced into stripper column 68 to increase the recovery rate of the further purified product. The combined stream introduced into stripper column 68 has a flow rate of about 1212 Nm 3 /hr, a temperature of about 102.8° K., and a pressure of about 9.53 kg/cm 2 . Further purified product stream 66 is extracted from the bottom of stripper column 68 at a flow rate of about 1180.0 Nm 3 /hr, a temperature of about 103.0°K., and a pressure of about 9.67 kg/cm 2 . Further purified product stream 66 leaves air liquefier 34 at a temperature of about 106.6°K, and a pressure of about 9.67 kg/cm 2 . Partial stream 72 of further purified product stream 66 having a flow rate of about 65.0 Nm 3 /hr is introduced into stripper column 68 as the stripper gas. Partial stream 74 of further purified product stream 66 is warmed in main heat exchanger 18 for delivery of the product to the customer at a temperature of about 274.0°K and a pressure of about 9.55 kg/cm 2 . EXAMPLE 5 In this example an ultra-high purity nitrogen product is recovered by the process and apparatus illustrated in Fig. 5. The product recovered contains approximately 0.5 ppb oxygen, 1.0 ppb neon and about 0.003 ppb helium. The process consumes air flowing at about 2513.0 Nm 3 /hr and the product flows at a rate of about 1115.0 Nm 3 /hr. Therefore, the process and apparatus of this example are capable of functioning at a greater efficiency than that of Example 4. The reason for this increase in efficiency relates to the fact that a greater degree of compression and expansion are taking place in this example over other examples presented herein. Air stream 16 enters main heat exchanger 18 with a temperature of 278.7°K and a pressure of 11.17 kg/cm 2 . Within main heat exchanger 18, the pressure and temperature of air stream 16 drops to about 11.00 kg/cm 2 and about 109.9°K, respectively. After division of air stream 16, portion 20 has a flow rate of approximately 2415.0 Nm 3 /hr and portion 22 has a flow rate of about 98.0 Nm 3 /hr. After liquefaction, portion 22 has a temperature of about 107.4°K, and a pressure of about 10.98 kg/cm 2 . Waste stream 30 removed from the bottom of rectification column 24 has a flow rate of approximately 2246.0 Nm 3 /hr. and a temperature and pressure of approximately that of the column, namely 109.9°K, and 11.0 kg/cm 2 , respectively. Side waste stream 30a is divided from waste stream 30 and flows at about 366.0 Nm 3 /hr. Stream 90 containing liquid from partially vaporised waste stream 30a is re-added to waste stream 30 to produce waste stream 30b. After such addition, waste stream 30b vaporises in condenser 32 at a temperature of about 100.9°K and a pressure of about 6.00 kg/cm 2 and warms in the air liquefier 34. The resultant warm waste stream 36a has a temperature of about 106.6° K. and a pressure of about 5.87 kg/cm 2 . Stream 36a is combined with stream 92, containing the vapour portion of stream 30a, to produce warm waste stream 36 having a flow rate of about 2246.0 Nm 3 /hr. Warm waste stream 36 is divided into two portions, portion 38 having a flow rate of approximately 897.0 Nm 3 /hr. and portion 40 having a flow rate of approximately 1349.0 Nm 3 /hr. After passage through compressor 42, the resultant compressed waste stream 44 enters main heat exchanger 18 at a temperature of about 143.0°K and a pressure of approximately 11.09 kg/cm 2 . Thereafter, compressed waste stream 44 is cooled in main heat exchanger 18 and introduced into rectification column 24 at a pressure of approximately 11.00 kg/cm 2 and a temperature of approximately 112.7°K. Stream 52, representing the vapour fraction removed from stream 46 of tower overhead, has a temperature of about 104.5°K., a pressure of about 10.7 kg/cm 2 , and a flow rate of approximately 27.0 Nm 3 /hr. After passing through back pressure control valve 89 it is combined with portion 40 of warmed waste stream 36 and stream 87 representing the vapour phase of partially condensed stripper tower overhead (having a flow rate of about 22.0 Nm 3 /hr, a temperature of about 102.8° K., and a pressure of about 9.53 kg/cm 2 ). The resultant combined stream 54 has a flow rate of approximately 1398.0 Nm 3 /hr, a temperature of about 106.0° K., and a pressure of about 5.87 kg/cm 2 . After passage through main heat exchanger 18, combined stream 54 has a temperature of about 141.5°K and a pressure of about 5.78 kg/cm 2 . After expansion, the resultant expanded waste has a temperature of 105.3° K. and a pressure of about 1.63 kg/cm 2 . Expanded waste stream 58 leaves air liquefier 34 with a temperature of about 106.5° K. and and a pressure of about 1.53 kg/cm 2 and then main heat exchanger 18 with a temperature of 274.0°K. and a pressure of about 1.30 kg/cm 2 . Product stream 62 is extracted from rectification column 24 at a flow rate of about 1138.0 Nm 3 /hr, a temperature of about 104.6°K., and a pressure of about 10.72 kg/cm 2 and sent to the stripper 68. Stripper overhead stream 78 flowing at about 125.0 Nm 3 /hr and having a temperature of about 102.8° K. and a pressure of about 9.53 kg/cm 2 is partially condensed against partially vaporising waste stream 30a. Side waste stream 30a enters stripper recondenser 82 at a temperature of about 100.9°K and a pressure of about 6.00 kg/cm 2 . The gas phase is separated from the liquid phase in phase separator 84 and stream 86, comprising the liquid phase, is combined with product stream 62 and introduced into stripper column 68 to increase the recovery rate of the further purified product. The combined stream introduced into stripper column 68 has a flow rate of about 1240.0 Nm 3 /hr, a temperature of about 103.0° K., and a pressure of about 9.67 kg/cm 2 . Partially vaporised side waste stream 30a is then sent into phase separator 88 for separation of the liquid and vapour phases. Stream 90, extracted from the bottom of phase separator 88 and having a flow rate of about 238.0 Nm 3 /hr, a temperature of about 101.5° K. and a pressure of about 6.00 kg/cm 2 , is added to waste stream 30. Stream 92, extracted from the top of phase separator 88 and having a flow rate of about 128.0 Nm 3 /hr, a temperature of about 101.2°K., and a pressure of about 5.87 kg/cm 2 is added to stream 31 after its passage through air liquefier 34 to form warm waste stream 36. The result of such additions is that the refrigeration potential of the partially vaporised side waste stream 30b is being recovered and more material is being added to the amount of waste to be compressed. The foregoing operation is to be compared with that of Example 4 in which the fully condensed side waste stream 30a is at too low a pressure for there to be any meaningful amount of refrigeration to be recovered. Further purified product stream 66 is extracted from the bottom of stripper column 68 at a flow rate of about 1207.0 Nm 3 /hr, a temperature of about 103.0°K., and a pressure of about 9.67 kg/cm 2 . Further purified product stream 70 leaves air liquefier 34 at a temperature of about 106.6°K. and a pressure of about 9.67 kg/cm 2 . Partial stream 72 of further purified product stream 66, having a flow rate of about 92.0 Nm 3 /hr., is introduced into stripper column 68 as stripper gas. Partial stream 74 of further purified product stream 66 is warmed in main heat exchanger 18 for delivery to the customer at a temperature of about 274.0°K and a pressure of about 9.55 kg/cm 2 .

Claims: A process of producing ultra-high purity nitrogen comprising: rectifying ar within a rectification column to produce a top fraction comprising nitrogen vapour relatively rich in light elements; condensing a stream of the top fraction to form a condensate; returning a stream of the condensate to the top of the rectification column as reflux and withdrawing a liquid nitrogen product stream from the rectification column; wherein the stream of the top fraction is only partially condensed whereby the condensate is relatively lean in the light elements and the residual vapour is relatively rich in the light elements; the condensate is separated from the residual vapour, the light elements are stripped from the reflux in a stripping section within the rectification column to form ultra high purity liquid nitrogen below the top of the column; the product liquid nitrogen stream is withdrawn from the ultra high purity liquid nitrogen below the top of the rectification column; the stream of the top fraction is condensed in indirect heat exchange with a stream of oxygen-enriched liquid withdrawn from the rectification column; downstream of its heat exchange with said stream of the top fraction the oxygen-enriched stream is divided into first and second partial streams the first partial oxygen-enriched stream is compressed, is cooled and is introduced into the rectification column, the second oxygen-enriched partial stream is warmed and is expanded with the performance of external work and the resulting expanded stream is used to provide refrigeration for the process; and a stream of said residual vapour from the condensation of the top fraction is combined with the second partial oxygen-enriched stream upstream of the warming of the second partial oxygen-enriched stream. A process according to claim 1 further including purifying the product liquid nitrogen stream by employing in a separate stripper column a stripper gas to strip residual light elements therefrom. A process according to claim 2, wherein the purification of the product stream is carried out by introducing the product stream into the top of the stripper column and the stripper gas into the stripper column below the product liquid nitrogen stream in order to produce a further purified ultra-high purify liquid nitrogen product at the bottom of the stripper column and a gas at the top of the stripper column, and withdrawing a stream of said further purified ultra-high purity liquid nitrogen. A process according to claim 3, in which said stripper gas is itself taken from said further purified ultra-high purity liquid nitrogen. A process according to claim 4, in which said stream of said further purified ultra-high purity liquid nitrogen is vaporised in heat exchange with a stream of condensing air, and the resulting condensed air is introduced into the rectification column as one feed air stream, and in which a second feed air stream is introduced into the rectification column. A process according to any one of claims 3 to 5, further including withdrawing a gaseous stream from the top of the stripper column, recompressing the gaseous stream so withdrawn to rectification column pressure and introducing it into the rectification column. A process according to any one of claims 3 to 6 further including partially condensing a gas stream withdrawn from the top of the stripper column and thereby forming a stripper column condensate relatively lean in light elements and a residual stripping column gas relatively rich in light elements separating the stripper column condensate from the residual stripper column gas, and introducing the stripper column condensate into the stripper column. A process according to claim 7 in which the partial condensation of the gas stream withdrawn from the stripper column is effected by heat exchange with a liquid stream withdrawn from the rectification column. A process according to claim 7 or claim 8, in which said residual stripper column gas is warmed, and is expanded with the performance of external work, and the expanded gas is used to provide refrigeration for the process.

Inventors: STERN SIDNEY S, STERN, SIDNEY S.
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1992
Main classification: F25J3
IPC codes: F25J3