Abstract:
The present invention relates to a process and apparatus for the separation of air by cryogenic distillation. In particular, it relates to a process for separation of air using three cryogenic distillation columns for the production of gaseous oxygen. Certain embodiments of the invention are particularly efficient for the production of gaseous oxygen at pressures between 30 and 45 bars abs, in which the oxygen is produced by removing liquid oxygen from a distillation column, pressurizing the oxygen and vaporizing the pressurized liquid by heat exchange with air.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. §119(e) to European application No. 12305244.1, filed Feb. 29, 2012, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to a process and apparatus for the separation of air by cryogenic distillation. In particular, it relates to a process for separation of air using three cryogenic distillation columns for the production of gaseous oxygen. 
     The process is particularly efficient for the production of gaseous oxygen at pressures between 30 and 45 bars abs, in which the oxygen is produced by removing liquid oxygen from a distillation column, pressurizing the oxygen and vaporizing the pressurized liquid by heat exchange with air. 
     SUMMARY OF THE INVENTION 
     According to an object of the invention, there is provided a process for the separation of air by cryogenic distillation in which air is purified, cooled and sent to a first distillation column of a column system wherein it is separated into an oxygen enriched liquid and a nitrogen enriched gas, oxygen enriched liquid or a liquid derived therefrom is sent from the first column to a top condenser of a second column operating at a lower pressure than the first column and is partially vaporized therein, the bottom of the second column is warmed via a bottom reboiler, liquid from the bottom of the second column is sent to an intermediate point of a third column operating at a lower pressure than the second column, nitrogen enriched liquid from the top of the second column is sent to the top of the third column, oxygen rich liquid is removed from the bottom of the third column, pressurized and vaporized by heat exchange with air, characterized in that oxygen enriched liquid from the top condenser of the second column is sent to an intermediate point of the second column to be separated therein. 
     According to other optional features:
         all the fluid sent to be separated in the second column comes from the top condenser or from the top condenser and the third column.   all the oxygen enriched fluid removed from the bottom of the first column is sent to the top condenser.   the oxygen enriched liquid or the liquid derived therefrom is pressurized after being removed from the top condenser and before being sent to the second column.   the liquid is pressurized by a pump and/or by hydrostatic pressure.   the liquid sent to be separated is derived from the oxygen enriched liquid by cryogenic separation in a fourth column operating at a pressure lower than the pressure of the second column to enrich the oxygen rich liquid still further in oxygen.   the fourth column is fed at the top by nitrogen enriched liquid from the first column.   the fourth column is fed at the bottom by feed air.   the process comprises expanding purified and cooled air and sending it to the fourth column.   the oxygen rich liquid is pressurized to a pressure between 30 and 45 bars abs.   no gaseous nitrogen stream is removed as a gaseous product from the first column.   the air is cooled in a heat exchanger from a temperature above 0° C. to a temperature below −150° C., at least part of the air being removed from an intermediate point of the heat exchanger, compressed in a cold compressor, sent back to the heat exchanger and separated in the column system.   at least 35%, preferably at least 40%, or even at least 50% of the air sent to the column system is expanded in a first turbine to the pressure of the third or a fourth column.   the inlet temperature of the first turbine is lower than the inlet temperature of the cold compressor.       

     According to another object of the invention, there is provided an apparatus for the separation of air by cryogenic distillation comprising a column system having a first column, a second column and a third column, a heat exchanger, means for sending purified, cooled air from the heat exchanger to the first distillation column wherein it is separated into an oxygen enriched liquid and a nitrogen enriched gas, a conduit for sending oxygen enriched liquid or a liquid derived therefrom from the first column to a top condenser of the second column operating at a lower pressure than the first column, the second column having a bottom reboiler, a conduit for sending liquid from the bottom of the second column to an intermediate point of a third column operating at a lower pressure than the second column, a conduit for sending nitrogen enriched liquid from the top of the second column to the top of the third column, a conduit for removing oxygen rich liquid from the bottom of the third column, a pump for pressurizing the oxygen rich liquid, a conduit for sending pressurized oxygen rich liquid to the heat exchanger to be vaporized by heat exchange with air, characterized in that it comprises a conduit for sending oxygen enriched liquid from the top condenser of the second column to an intermediate point of the second column to be separated therein. 
     The apparatus may also comprise
         pressurization means, which may be a pump and/or hydrostatic pressure, to pressurize the liquid from the top condenser upstream of the intermediate point of the second column.   a turbine and a conduit for sending air from the heat exchanger to the turbine and a conduit for sending expanded air from the turbine to the third column and/or a fourth column.   a fourth column adapted to send oxygen enriched liquid from the fourth column to the top condenser.   the fourth column is positioned above the third column or above the second column.       

     One advantage of the present invention is that by sending a large amount of expanded air to the second or (where present) fourth column, the amount of liquid reflux sent to the second column is reduced. Thus, since the amount of gaseous nitrogen produced is constant, it will be understood that the feed and reflux streams to the low pressure column will be subcooled to a greater degree than is usually the case, so that there is less flash. 
     Another advantage linked to the high turbine flow of air sent to the second or (where present) fourth column is that the turbine temperature can be cooler and consequently liquid may formed at the turbine outlet. Approximately 4.5% of the expanded air is liquefied in the turbine, in this case. This means that more of the feed air can be sent to the distillation in gaseous form. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention&#39;s scope as it can admit to other equally effective embodiments. 
         FIG. 1  represents a column system to be used in accordance with an embodiment of the invention. 
         FIG. 2  represents a heat exchange system to be used in accordance with an embodiment of the invention. 
         FIG. 3  represents a heat exchange system to be used in accordance with an embodiment of the invention. 
         FIG. 4  represents a column system to be used in accordance with an embodiment of the invention. 
         FIG. 5  represents a column system to be used in accordance with an embodiment of the invention. 
         FIG. 6  represents a heat exchange system to be used in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The invention will be described in greater detail with respect to the figures. 
     In the process of  FIG. 1 , a column system is used including a first column  100  operating at a high pressure, a second column  102  operating at an intermediate pressure, lower than the high pressure and a third column, thermally integrated with the first column via a bottom reboiler, operating at a low pressure, lower than the intermediate pressure. 
     Gaseous air  2  is the principal feed to first column  100  which is also fed by a stream of liquid air  4  at a higher introduction point than that of stream  2 . Liquid air stream  4  is shown as a single stream but can be composed of multiple liquid air streams (not shown) resulting from the thermal optimization of the main heat exchanger. A stream of air  6  is expanded in a turbine  8  and sent to an intermediate point of third column  103 . No air is sent directly to second column  102 , though this could be envisaged. Oxygen enriched liquid  10  is removed from the bottom of column  100 , expanded in a valve and sent to the top condenser  107  of the second column  102 . In the top condenser, the oxygen enriched liquid is partially vaporized by heat exchanger with the top gas of the second column  102 , thereby condensing the top gas which returns to the second column  102  as reflux. This option gives the optimal temperature for the top condenser; however it is also possible to send only a part of the oxygen enriched liquid  10  to the top condenser and to send the rest to the third column  103 , for example. 
     The non-vaporized liquid  26  from the condenser is divided in two. One part  25  is sent to the third column  103  and the rest  24  is pressurized in a pump  110  and sent to a lower region of the second column  102  as feed. The reboil of the second column  102  is ensured by a stream of gaseous nitrogen enriched fluid from the top of the first column. The fluid is liquefied in bottom reboiler  106  of the second column  102  and sent back to the top of the first column as stream  53 . A stream of the same gas is also condensed in the bottom reboiler of the third column. Gaseous nitrogen may be removed at the top of the first column as a product stream. 
     Liquid  60  containing between 65 and 75% mol. oxygen is removed from the bottom of the second column, expanded and sent to the third column  103 . Vaporized oxygen enriched liquid  123  from the top condenser is also fed to column  103 . Nitrogen enriched liquid from the top of the second column  102  is expanded and sent to the top of the third column  103  as stream  23 . 
     A liquid stream  62  having a composition similar to air is removed from the first column, expanded and sent to the third column. A liquid nitrogen stream from the top of the first column is sent to the top of the third column as stream  41 . 
     Nitrogen enriched gas  59  is removed from the top of the third column  103 . Oxygen enriched liquid  30  is removed from the bottom of the third column  103 , and pressurized in pump  120  to between 30 and 45 bars to form high pressure stream  31 . 
       FIG. 2  shows a heat exchange system to be used to cool the feed streams and warm the product streams of  FIG. 1 . Thus the air  1  is compressed in compressor  3  to form compressed stream  3 . After cooling and purification for moisture and carbon dioxide removal (not shown), the compressed air is divided into three portions. One portion  72  is cooled completely in heat exchanger  10  and sent to the bottom of the first column as stream  2 , the column system being designated as ASU. Another portion  70  is boosted in a warm booster compressor  11 , partially cooled in heat exchanger  10  and expanded in a turbine  8  to form stream  6  to be sent to the third column  103 . 
     A final portion  71  is compressed in a further warm booster  9 , cooled partially in heat exchanger  10 , further compressed in cold booster  13 , cooled in the heat exchanger  10 , liquefied and sent to the column system as liquid stream  4 . 
     The high pressure liquid oxygen  31  at between 30 and 45 bars is vaporized in the heat exchanger  10  to form gaseous pressurized oxygen. The nitrogen enriched gas  59  is also warmed in the heat exchanger  10 . Boosters  9  and  13  can be driven by electric motor(s). 
       FIG. 3  shows that it is also possible to modify  FIG. 2  to avoid using the booster  11 . Two streams  70 ,  72  enter the heat exchanger at the outlet pressure of compressor  1 . In this case, it is possible to send stream  72  to another turbine  18  after partial cooling in the heat exchanger. In this case, part of stream  70  as part of the air  8 A is fully cooled in the heat exchanger  10 , liquefied and sent to the column system ASU. The rest of stream  70  is partially cooled in exchanger  10 , expanded in turbine  8  and sent to the column system ASU as stream  8 . 
     In this case, two cold boosters  13 , 13 A are arranged in series to compress air  4 C to be liquefied. The efficiency can be improved by cooling and liquefying a fraction of stream  73  to form liquid stream  4 B. Similarly, liquid stream  4 A can be extracted after compression of booster  13 A. All liquid air streams  4 A,  4 B,  4 C and  8 A are sent as feeds to the column  100 . For illustration purposes, these streams can be combined and shown as a single stream  4 . 
     The high pressure liquid oxygen  31  at between 30 and 45 bars is vaporized in the heat exchanger  10  to form gaseous pressurized oxygen. The nitrogen enriched gas  59  is also warmed in the heat exchanger  10 . Booster  9  can be driven by electric motor(s). Stream  71  is compressed in warm booster  9  to form stream  73 . Part of stream  73  is completely cooled in the heat exchanger to form stream  4 B. The rest is partially cooled, compressed in cold booster  13 A, warmed in exchanger from one intermediate temperature to another and divided in two. One part  41  is cooled to the cold end of the exchanger and expanded as stream  4 A. 
     The rest  4 C is compressed in cold compressor  13 , having an inlet temperature colder than that compressor  13 A, sent back to the exchanger at an intermediate temperature and cooled to the cold end of the exchanger before being expanded into the column system. 
     Both of the cold boosters  13  and  13 A are driven by turbine  8 . 
     In  FIG. 4 , a fourth column  104  is placed above the top of the third column  103  and operates at a pressure just slightly below that of the third column This column  104  is fed at the top by part  42  of the nitrogen enriched liquid  40 , the rest  43  being sent as before to the top of the third column  103 . A gas  52  and a gas  51  are removed from the tops of the third and fourth columns respectively, both being nitrogen enriched. The liquid  21  from the bottom of the fourth column is sent via a pump  210 , or by hydrostatic head if the layout permits, to the top condenser  107  to be vaporized therein, to ensure that there is sufficient cooling for the top condenser. 
     The fourth column is also fed at the bottom by the air stream  6 , no longer sent to the column  103 , via turbine  8 . 
     In other respects, the column system is as in  FIG. 1 . 
     In  FIG. 5 , the fourth column  104  is placed above the second column, such that the top condenser  107  becomes the bottom reboiler of the fourth column. The fourth column can operate at a pressure slightly lower than that of the third column. The second column operates at 2.3 bars. The oxygen enriched liquid  10  is expanded and fed to the bottom of the fourth column  104  and is separated in the column. Air from the turbine  8  is also sent to the bottom of the fourth column  104  via stream  6 . A nitrogen enriched gaseous stream  51  is removed from the top of the fourth column. The liquid stream  26  leaving the top condenser  107  is divided in two and the liquid  24  is as before used to feed the second column  102 . 
       FIG. 6  shows the heat exchanger system wherein the air compressed in compressor  3  to 7.7 bars is divided in two. One part  71  is boosted to 9.6 bars and divided to form stream  73 ,  74 . The stream  73  is cooled partially in heat exchanger  10  and expanded in turbine  18 , before being again cooled in the heat exchanger to the cold end and sent to the column system as stream  2 . Stream  70  at the outlet pressure of compressor  3  is cooled to an intermediate point in the heat exchanger  10 , expanded in turbine  8  and sent to the column system to the third column  103  or the fourth column  104  of  FIG. 3 or 4  as stream  6 . The remainder  74  is boosted in booster  9  to 12 bars, partially cooled in the heat exchanger and divided in two. One part is compressed in cold compressor  13  to 53 bars, thus having a compression ratio of 4.5, further cooled in exchanger  10  and then expanded into the column system. The rest of the air boosted in booster  9  is cooled to the cold end, expanded and sent to the column system. 
     The oxygen stream  30  at 95% mol oxygen is pressurized and vaporized at 40 bars a. 
     The advantage of this particular set-up is that since the second column  102  is at a lower pressure of 2.3 bars, as opposed to 2.5 bars for  FIG. 3 , the oxygen content in the bottom of the second column can be increased. 
     In all of the figures, the stream  6  expanded in turbine  8  can be partially liquefied. Preferably between 2 and 5% of the expanded air is liquefied. 
     In all of the figures, the air stream  70  represents at least 35%, preferably at least 40% or even at least 50% of the total feed air to be separated. Because of the large amount of air sent directly to the second or fourth column, the first column can have a much smaller diameter than usual, for example twice as small as usual. In the case where the turbine expanded air is sent to the fourth column  104 , the third column can also have a much reduced diameter. 
     Another advantage of the process is that the majority of the waste gas  59  is not sent to the regeneration of the adsorption system for purifying the air. It is this feature which allows the fourth column or minaret to operate at a lower pressure than the third column. 
     The turbine expansion of a large quantity of air down to a particularly low temperature produces a great deal of refrigeration and the use of the cold booster can dissipate efficiently this refrigeration such that the energy consumption can be reduced considerably. 
     Preferably for all the figures, reboiler  106  is a falling film vaporizer. The minimum temperature difference is 0.5° C. and the average temperature difference is between 0.9 and 1.1° C. The expected vaporization rate is less than 33%. Preferably for all the figures, condenser  107  is a falling film vaporizer. The minimum temperature difference is 0.5° C. and the average temperature difference is between 0.9 and 1.1° C. Again, the expected vaporization rate is less than 33%. 
     Although not shown in the figures, it is possible to send feed air to the second column in gaseous or liquid form. In all of the figures, the process produces no or a small amount of liquid product (about 3% of oxygen product) as a final product. 
     In all of the figures, pump  110  may be replaced or supplemented by hydrostatic pressure. 
     While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step. 
     The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise. 
     “Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein. 
     “Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary a range is expressed, it is to be understood that another embodiment is from the one. 
     Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur. 
     Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such particular value and/or to the other particular value, along with all combinations within said range. 
     All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.