Air separation process and device

Abstract: A process for introducing a multicomponent liquid feed stream (10) at pressure (P 2 ) into a distillation column (C1) operating at lower pressure (P 1 ). The process comprises removing a split stream (12) from the feed stream (10), reducing its pressure (V1) and using the resulting stream (13) to subcool (H1) the feed stream (OV) remainder of the feed stream (10). After being subcooled, the remainder of the feed stream (10) is also reduced in pressure (V2) and both streams (14,16) are fed to different stages of the distillation column (C1). An important embodiment of the present invention is within a conventional double column air separation cycle where the multicomponent liquid stream is the crude liquid oxygen stream from the bottom of the high pressure column which must be reduced in pressure prior to its introduction into the low pressure column.

Description: The present invention relates to an air separation process and apparatus. A common task encountered in air separation processes is where one must introduce a multicomponent liquid feed stream into a distillation column operating at a lower pressure than that of the feed stream. In the state of the art double column air separation cycle, the crude liquid oxygen bottoms from the high pressure column must be reduced in pressure prior to being fed to the low pressure column for further rectification. Typically, as shown in Figure 1, the above noted task is accomplished by simply reducing the pressure of the multicomponent liquid feed stream 10 across valve V1 prior to its introduction into distillation column C1 as stream 11. US-A-3,915,680 discloses a prefractionation process of a multicomponent liquid feed stream at higher pressure than that of the fractionation zone wherein part (or all) of the feed stream is flashed to produce a partially vaporised stream. This is used to cool, by indirect heat exchange, the main feed stream thus increasing the vapour:liquid ratio of the partially vaporised stream. The resultant stream is fed into the fractionation zone at a location below any remaining portion of the liquid feed stream. EP-A-0573176 discloses an improvement to a multicolumn distillation system for distillation, separation and recovery of select components of a multicomponent feed stream such as air. The system comprises a main distillation column and a side column. Thermal integration of the side column with the main column is achieved, in one aspect by withdrawing a liquid fraction from the side column and vaporising it against a vapour fraction wi-thdrawn from the main column. Further aspects of the thermal coupling between parts of the multicolumn distillation system are described. US-A-4,171,964 relates to a process for separating multicomponent gas streams containing hydrocarbons, having an improved ethane recovery rate. It discloses an improvement over known single stage cryogenic expander processes for natural gas separation. The improved process comprises the known steps of cooling and processing an inlet gas, feeding into a high pressure column, expanding/flashing the feed stream from the bottoms of the high pressure column to reduce the pressure for introduction into a demethanizer column, the improvement being further subcooling the feed stream such that the temperature of ethane vapours in the feed stream is reduced thus reducing loss of ethane from the demethanizer column. In one embodiment of the improvement, described in Example 10/Figure 12, a high pressure feed stream is sub-cooled and then divided into two portions; the first portion is expanded and then used to carry out the sub-cooling of the feed stream before being fed to the demethanizer; the second portion is expanded and fed to the demethanizer. Both the streams fed into the demethanizer are vapour/liquid mixtures. It is an object of the present invention to devise an improved scheme for an air separation cycle for introducing a crude liquid oxygen stream from the bottom of a high pressure column into a column operating at a lower pressure whereby the subsequent separation of the crude liquid oxygen stream in the lower pressure distillation column is made more efficient. The present invention is an air separation process wherein a crude liquid oxygen stream is introduced into a distillation column operating at a lower pressure than the pressure of the crude liquid oxygen stream. The process comprises subcooling the feed stream with a split stream removed from the subcooled feed stream and reduced in pressure. The remainder of the subcooled crude liquid oxygen stream is also reduced in pressure and both streams are fed to different stages of the distillation column. According to the present invention, there is provided an air separation process in which a crude liquid oxygen stream is produced from the bottom of a higher pressure column and subsequently introduced into a lower pressure column, wherein the crude liquid oxygen stream is introduced into the lower pressure column by a process comprising the steps of: (a) removing a split stream from the subcooled crude liquid oxygen stream of step (c); (b) reducing the pressure of the split stream; (c) heat exchanging the reduced pressure split stream against the crude liquid oxygen stream, thereby subcooling said crude liquid oxygen stream and warming said reduced pressure split stream to provide a warmed reduced pressure split stream; (d) reducing the pressure of the subcooled crude liquid oxygen stream remaining after removal of the split stream to provide a reduced pressure subcooled crude liquid oxygen stream; and (e) introducing said reduced pressure, subcooled crude liquid oxygen stream of step (d) and said warmed, reduced pressure split stream of step (c) into the lower pressure column as a multicomponent liquid feed stream, the introduction point of said reduced pressure, subcooled crude liquid oxygen feed stream being at least one stage above the introduction point of said warmed, reduced pressure split stream, wherein subsequent to heat exchanging the reduced pressure split stream against the crude liquid oxygen stream and prior to introducing the warmed, reduced pressure split stream into the lower pressure distillation column, the warmed, reduced pressure split stream is used to satisfy condensing duty in a crude argon column. Suitably, the reduction of the pressure of the split stream in step (b) is performed across a first valve; and the reduction of the pressure of the subcooled feed stream in step (d) is performed across a second valve. There may be additional feed streams to the distillation column which are cooled or subcooled by indirectly heat exchange against the reduced pressure split stream. Further, there may be effluent streams removed from the distillation column which are warmed by indirectly heat exchange against the feed stream. The present invention also provides an apparatus for an air separation process of the invention, said apparatus comprising a higher pressure column providing a crude liquid oxygen stream at the bottom thereof, a lower pressure column into which said stream is introduced and a crude argon column, the apparatus further comprising heat exchanging means for subcooling said crude liquid oxygen stream against a reduced pressure split stream thereby warming said reduced pressure split stream; means for removing a split stream from said subcooled crude liquid oxygen stream downstream of said heat exchanging means; means for reducing the pressure of said split stream to provide said reduced pressure split stream; means for reducing the pressure of said subcooled crude liquid oxygen stream remaining after removal of the split stream; means for introducing said warmed, reduced pressure split stream into the lower pressure distillation column at least one stage below the introduction point of said reduced pressure, subcooled crude liquid oxygen stream; and further heat exchanging means in which reduced pressure split stream warmed against the crude liquid oxygen stream provides condensing duty to said argon column. The following is a description by way of example only and with reference to the accompanying drawings of an embodiment of the invention. In the drawings:- Figure 1 is a schematic drawing of the prior art process for introducing a multicomponent liquid feed stream into a distillation column operating at a lower pressure than that of the feed stream; Figure 2 is a schematic drawing depicting the steps involving splitting and heat exchange of the crude liquid oxygen stream in accordance with the process of the present invention, but omitting the crude argon column cooling required by the present invention; Figure 3 is a generalized McCabe Thiele diagram for the prior art distillation process of Figure 1; Figure 4 is a generalized McCabe Thiele diagram for the distillation process of Figure 2 which, when compared to Figure 3, graphically illustrates the improvement over the prior art; and Figure 5 is a schematic diagram depicting an embodiment of the present invention wherein the present invention is incorporated into a state of the art double column air separation cycle. Referring to Figure 2, a split stream 12 is removed from multicomponent liquid feed stream 10. As represented by the dotted lines in Figure 2, the options for removal of the split stream are before the subcooling of the feed stream (stream 12a) and/or during the subcooling of the feed stream (stream 12b) and/or after the subcooling of the feed stream (stream 12c) but in accordance with the present invention, at least part of the split stream is removed after the subcooling. Preferably, the entire split stream is removed from the feed stream after the subcooling of the feed stream. The pressure of the split stream is subsequently reduced across valve V1. The reduced pressure split stream (stream 13) is then indirectly heat exchanged against the feed stream 10 in heat exchanger H1, thereby subcooling the feed stream 10 and warming the reduced pressure split stream 13. After reducing the pressure of the subcooled feed stream across valve V2, both the reduced pressure, subcooled feed stream (stream 14) and the warmed, reduced pressure split stream (stream 16) are introduced into distillation column C1. As shown in Figure 2, the introduction point of the reduced pressure, subcooled feed stream 14 is at least one stage above the introduction point of the warmed, reduced pressure split stream 16. However, Figure 2 does not show use of the latter stream (16) to satisfy condensation duty of the crude argon column as required by the present invention. The skilled practitioner will appreciate the following aspects of the present invention as depicted in the embodiment in Figure 2: 1. Split stream removal location. As noted above, the split stream is preferably entirely removed from the feed stream after the subcooling of the feed stream as represented by stream 12c in Figure 2. This has to do more with practical limitations associated with the heat exchanger as opposed to any thermodynamic driving force. By removing the split stream 12c after it has been subcooled in the heat exchanger H1, there will be less flashing of the split stream into the vapor phase when the split stream is subsequently flashed/reduced in pressure across valve V1 and warmed against the subcooling feed stream in the heat exchanger H1. This, in turn, translates into a simpler heat exchanger design. 2. Valve vs. dense fluid expander for accomplishing pressure reduction. The respective pressure reductions of the split stream and the feed stream in Figure 2 are performed across valves V1 & V2. In the interest of gaining thermodynamic efficiency by performing these pressure reductions largely at constant entropy instead of at constant enthalpy, one could replace one or both of these valves with dense fluid expanders. Such efficiency gain, however, would come at the expense of increased capital and operating complexity. 3. Pressure of streams following their respective pressure reductions. The pressure of the split stream and feed stream following their respective pressure reductions will generally be the operating pressure of the distillation column plus the expected pressure drop between the valve/expander at issue and the column. In the case of the split stream, this expected pressure drop includes the pressure drop across the heat exchanger. Thus the pressure of the split stream following its pressure reduction will generally be slightly higher than the pressure of the subcooled feed stream following its pressure reduction. The benefit of the arrangement shown in Figure 2 is that, as compared to the prior art method as depicted in Figure 1, the subsequent separation of the feed stream 10 in the distillation column C1 is made more efficient. This increased efficiency of separation is graphically illustrated by generalized McCabe Thiele diagrams for the distillation processes in Figures 1 and 2 as are shown, respectively, in Figures 3 and 4. Note how much closer the operating lines are to the equilibrium curve in Figure 4 as compared to Figure 3. (In a McCabe Thiele diagram, the closer the operating lines are to the equilibrium curve, the more efficient the separation becomes). This is not only because there are two distinct feed lines in Figure 4 (as compared to a single feed line in Figure 3), but also because the slope of the feed lines in Figure 4 have been favorably manipulated by the present invention's act of transferring refrigeration between the two streams. By subcooling feed stream 14, the slope of its feed line in Figure 4 is rotated clockwise as compared to the single feed line in Figure 3. (Note that the slope of subcooled feed line 14 is almost vertical in Figure 4 since it is close to its bubble point temperature after being flashed/reduced in pressure in Figure 2; post-flash temperatures colder than feed line 14's bubble point temperature would rotate the slope of feed line 14 even further in the clockwise direction). Similarly, by warming split stream 16, its feed line is rotated counterclockwise as compared to the single feed line in Figure 3. (Note that the slope of warmed feed line 16 is almost horizontal in Figure 4 since it is close to its dew point temperature after being flashed/reduced in pressure in Figure 2; post-flash temperatures warmer than feed line 16's dew point would rotate the slope of feed line 16 even further in the counterclockwise direction). In terms of maximizing the efficiency of the separation, the optimum slopes of the feed lines are those slopes which minimize the area between the operating lines and the equilibrium curve. The optimum slopes of the feed lines are dependent on the specific example, especially with respect to the shape of the equilibrium curve. The improved efficiency of separation resulting from the arrangement shown in Figure 2 as discussed above translates into a better separation (i.e. improved product purities and recoveries) using the same power (i.e. the same boil-up and reflux requirements) and the same number of equilibrium stages in the distillation column. Conversely, the improved ease of separation can translate into an equivalent separation but with a reduction in power and/or the number of stages. An important embodiment of the present invention is within a conventional double column air separation cycle where the multicomponent liquid stream is the crude liquid oxygen stream from the bottom of the high pressure column which must be reduced in pressure prior to its introduction into the low pressure column. This embodiment is depicted in Figure 5. Referring now to Figure 5, an air feed (stream 100) which has been compressed to an elevated pressure, cleaned of impurities which will freeze out at cryogenic temperatures and cooled to near its dew point is fed to a distillation column system comprising high pressure column C1, low pressure column C2 and crude argon column C3. In the interests of simplifying the drawing of Figure 5, the operations relating to the above noted compression, cleaning and cooling of the air feed have been omitted from Figure 5. As is well known to those skilled in the art: (i) the compression of the feed stream is typically performed in multiple stages with interstage cooling against cooling water; (ii) the cleaning of impurities which will freeze out at cryogenic temperatures (such as water and carbon dioxide) is typically performed by a process which incorporates an adsorption mole sieve bed; and (iii) the cooling of the air feed down to its dew point is typically performed by heat exchanging the pressurized air feed in a front end main heat exchanger against the gaseous product streams which are produced from the process at cryogenic temperatures. Continuing the reference to Figure 5, the air feed 10 is specifically fed to high pressure column C1 in which the air feed is rectified into an intermediate gaseous nitrogen overhead (stream 20), a portion of which is removed as a product stream (stream 24), and the crude liquid oxygen bottoms (stream 10). As per the process of the present invention, a split stream (stream 12) is removed from the crude liquid oxygen bottoms 10, reduced in pressure across valve V1 and the resulting reduced pressure split stream (stream 13) is heat exchanged against the crude liquid oxygen bottoms in heat exchanger H1. The pressure of the subcooled crude liquid oxygen bottoms (stream 40) is reduced in pressure across valve V2 and the resulting reduced pressure, subcooled crude liquid oxygen bottoms (stream 14) is fed to low pressure column C2 in which it is distilled into a final gaseous nitrogen overhead (stream 30) and a final liquid oxygen bottoms which collects in the sump of the low pressure column C2. A gaseous oxygen product stream (stream 32) and a waste stream (stream 34) are also removed from the low pressure column C2. The high pressure and low pressure columns are thermally integrated in that a portion (stream 21) of the intermediate gaseous nitrogen overhead 20 from the high pressure column C1 is condensed in reboiler/condenser R/C 1 against a vaporizing portion of the final liquid oxygen bottoms. A first portion (stream 26) of the condensed intermediate gaseous nitrogen overhead is used to provide reflux for the high pressure column C1 while a second portion (stream 28) is used to provide reflux for the low pressure column C2 after being reduced in pressure across valve V3. An argon-containing gaseous side stream (stream 50) is removed from the low pressure column C2 and fed to the crude argon column C3 in which it is rectified into an argon-rich gaseous overhead (stream 60) and an argon-lean bottoms liquid (stream 62). The argon-lean bottoms liquid 62 is returned to a suitable location in the low pressure column C2. The argon-rich gaseous overhead 60 is condensed in reboiler/condenser R/C 2 against the warmed reduced pressure split stream (stream 16). A portion (stream 64) of the condensed argon-rich overhead is returned as reflux to the argon side-arm column C3 while the remaining portion (stream 66) is recovered as a product stream. Both the vapor component (stream 47) of the further warmed split stream and the liquid component (stream 48) are fed to a suitable location in the low pressure column C2. The skilled practitioner will appreciate the following aspects of the present invention when it is incorporated into a more comprehensive distillation operation such as depicted in Figure 5's embodiment thereof: (1) Subcooling of other additional streams by the reduced pressure split stream. When the process of the present invention is integrated into a distillation operation such as shown in Figure 5, there will often be additional streams that can also be advantageously integrated into the present invention's heat exchange step such that these additional streams are also cooled or subcooled by the reduced pressure split stream of the present invention. For example, although not shown in Figure 5, the nitrogen reflux stream 28 to the low pressure column C2 could be subcooled by the reduced pressure split stream 13. Similarly, a turbo expanded portion of the air feed 100 could be cooled by the reduced pressure split stream 13 prior to being fed to the low pressure column C2. (2) Thermal integration of the present invention's heat exchanger with the prior art subcoolers. In the interests of simplifying the drawing of Figure 5, omitted from Figure 5 are the well known prior art subcooling heat exchanger(s) which transfer low temperature refrigeration from various product streams (such as streams 30 and 34 in Figure 5) to various low pressure column feed streams (such as streams 10 and 28 in Figure 5). It should be noted that the present invention's heat exchanger can be integrated with these subcoolers to form a single heat exchanger as can be easily designed by one skilled in the art.

Claims: An air separation process in which a crude liquid oxygen stream-is produced from the bottom of a higher pressure column and subsequently introduced into a lower pressure column, wherein the crude liquid oxygen stream is introduced into the lower pressure column by a process comprising the steps of: (a) removing a split stream from the subcooled crude liquid oxygen stream of step (c); (b) reducing the pressure of the split stream; (c) heat exchanging the reduced pressure split stream against the crude liquid oxygen stream, thereby subcooling said crude liquid oxygen stream and warming said reduced pressure split stream to provide a warmed, reduced pressure split stream; (d) reducing the pressure of the subcooled crude liquid oxygen stream remaining after removal of the split stream to provide a reduced pressure subcooled crude liquid oxygen stream; and (e) introducing said reduced pressure, subcooled crude liquid oxygen stream of step (d) and said warmed, reduced pressure split stream of step (c) into the lower pressure distillation column as a multicomponent liquid feed stream, the introduction point of said reduced pressure, subcooled crude liquid oxygen stream being at least one stage above the introduction point of said warmed, reduced pressure split stream, wherein subsequent to heat exchanging the reduced pressure split stream against the crude liquid oxygen stream and prior to introducing the warmed, reduced pressure split stream into the lower pressure distillation column, the warmed, reduced pressure split stream is used to satisfy condensing duty in a crude argon column.. A process as claimed in Claim 1, wherein: (i) the reduction of the pressure of the split stream in step (b) is performed across a first valve; and (ii) the reduction of the pressure of the subcooled crude liquid oxygen stream in step (d) is performed across a second valve. A process as claimed in Claim 1 or Claim 2, wherein: (i) there are additional feed streams to the distillation column; and (ii) the reduced pressure split stream is also indirectly heat exchanged against one or more of the additional feed streams, thereby cooling or subcooling such additional feed streams. A process as claimed in any one of the preceding claims, wherein: (i) there are effluent streams removed from the distillation column; and (ii) the crude liquid oxygen stream is also indirectly heat exchanged against one or more of the effluent streams, thereby warming such effluent streams. An apparatus for an air separation process as claimed in any one of the previous claims, comprising a higher pressure column (C1) providing a crude liquid oxygen stream (10) at the bottom thereof, a lower pressure column (C2) into which said stream (10) is introduced and a crude argon column (C3), the apparatus further comprising heat exchanging means (H1) for subcooling said crude liquid oxygen stream (10) against a reduced pressure split stream (13) thereby warming said reduced pressure liquid split stream (13); means (12) for removing a split stream from said subcooled crude liquid oxygen stream downstream of said heat exchanging means (H1); means (V1) for reducing the pressure of said split stream to provide said reduced pressure split stream (13); means (V2) for reducing the pressure of said subcooled crude liquid oxygen stream (40) remaining after removal of the split stream; means (16) for introducing said warmed, reduced pressure split stream into the lower pressure distillation column (C2) at least one stage below the introduction point of said reduced pressure, subcooled crude liquid oxygen stream (14) ; and further heat exchanging means (R/C2) in which reduced pressure split stream (16) warmed (H1) against the crude liquid oxygen stream (10) provides condensing duty to said argon column (C3).

Inventors: AGRAWAL RAKESH, WOODWARD DONALD WINSTON, AGRAWAL, RAKESH, WOODWARD, DONALD WINSTON
Applicants: AIR PROD & CHEM, AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1996
Main classification: B01D3
IPC codes: F25J3