Abstract:
A cryogenic rectification system wherein feed air is provided into the lower portion of a cryogenic rectification column, and liquid having a defined oxygen concentration is withdrawn from a defined intermediate level of the column above the feed air introduction level, vaporized and optionally diluted with nitrogen for recovery as ultra high purity clean dry air.

Description:
TECHNICAL FIELD 
     This invention relates generally to the cryogenic rectification of air and is particularly useful for the production of ultra high purity clean dry air. 
     BACKGROUND ART 
     Clean dry air is used as a utility fluid in manufacturing processes. For example, clean dry air is used in the manufacture of semiconductors in such applications as pneumatic valve actuation, for driving small motors and for cleaning equipment parts. 
     In the cryogenic separation of air to produce nitrogen and certain other pure gases which may be used in semiconductor manufacturing operations, air is first cleaned of high boiling impurities such as carbon dioxide and water vapor prior to being passed into the column or columns of the cryogenic air separation plant. Clean dry air is taken from this cleaned feed to the cryogenic air separation plant for use in the semiconductor manufacturing operation. 
     Recently several new applications have emerged where clean dry air is used directly in the semiconductor manufacturing equipment. Examples of such new applications include the use of clean dry air as a sweep gas to prevent build up of unwanted contaminants, and the use of clean dry air as a coolant to remove heat from the equipment. Clean dry air for these applications is required at a much higher purity than was previously needed. 
     Accordingly it is an object of this invention to provide an improved system for producing clean dry air. 
     It is another object of this invention to provide a system which can effectively produce clean dry air having an ultra high purity. 
     SUMMARY OF THE INVENTION 
     The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention, one aspect of which is: 
     A method for producing ultra high purity clean dry air comprising: 
     (A) passing feed air at a feed air level into a cryogenic rectification column and producing by cryogenic rectification within the column a nitrogen-rich fluid and an oxygen-enriched fluid; 
     (B) withdrawing nitrogen-rich fluid from the upper portion of the column, and withdrawing oxygen-enriched fluid from the lower portion of the column; 
     (C) withdrawing liquid having an oxygen concentration within the range of from 10 to 50 mole percent from the column at a withdrawal level which is within the range of from 1 to 25 equilibrium stages above the feed air level; and 
     (D) vaporizing the withdrawn liquid and recovering the resulting vapor as ultra high purity clean dry air. 
     Another aspect of the invention is: 
     Apparatus for producing ultra high purity clean dry air comprising: 
     (A) a heat exchanger, a cryogenic rectification column, and means for passing feed air into the cryogenic rectification column at a feed air level; 
     (B) means for withdrawing fluid from the upper portion of the cryogenic rectification column, and means for withdrawing fluid from the lower portion of the cryogenic rectification column; 
     (C) means for passing liquid withdrawn from the cryogenic rectification column at a level within the range of from 1 to 25 equilibrium stages above the feed air level to the heat exchanger; and 
     (D) means for recovering ultra high purity clean dry air from the heat exchanger. 
     As used herein the term “feed air” means a mixture comprising primarily oxygen and nitrogen, such as ambient air. 
     As used herein the term “column” means a distillation or fractionation column or zone, i.e. a contacting column or zone, wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column and/or on packing elements such as structured or random packing. For a further discussion of distillation columns, see the Chemical Engineer&#39;s Handbook, fifth edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13 , The Continuous Distillation Process.    
     Vapor and liquid contacting separation processes depend on the difference in vapor pressures for the components. The high vapor pressure (or more volatile or low boiling) component will tend to concentrate in the vapor phase whereas the low vapor pressure (or less volatile or high boiling) component will tend to concentrate in the liquid phase. Partial condensation is the separation process whereby cooling of a vapor mixture can be used to concentrate the volatile component(s) in the vapor phase and thereby the less volatile component(s) in the liquid phase. Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phases. The countercurrent contacting of the vapor and liquid phases is generally adiabatic and can include integral (stagewise) or differential (continuous) contact between the phases. Separation process arrangements that utilize the principles of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. Cryogenic rectification is a rectification process carried out at least in part at temperatures at or below 150 degrees Kelvin (K). 
     As used herein the term “indirect heat exchange” means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other. 
     As used herein the term “top condenser” means a heat exchange device that generates column downflow liquid from column vapor. 
     As used herein the terms “turboexpansion” and “turboexpander” mean respectively method and apparatus for the flow of high pressure gas through a turbine to reduce the pressure and the temperature of the gas thereby generating refrigeration. 
     As used herein the term “subcooling” means cooling a liquid to be at a temperature lower than the saturation temperature of that liquid for the existing pressure. 
     As used herein the term “top” when referring to a column means that section of the column above the column mass transfer internals, i.e. trays or packing. 
     As used herein the term “bottom” when referring to a column means that section of the column below the column mass transfer internals, i.e. trays or packing. 
     As used herein the term “ultra high purity clean dry air” means a fluid which comprises from 10 to 50 mole percent oxygen with the balance comprised essentially of nitrogen and containing a total of less than 10,000 parts per billion (ppb), more preferably less than 100 ppb, most preferably less than 10 ppb of hydrogen, carbon monoxide, water, carbon dioxide and hydrocarbon impurities. 
     As used herein the term “tray” means a contacting stage, which is not necessarily an equilibrium stage, and may mean other contacting apparatus such as packing having a separation capability equivalent to one tray. 
     As used herein the term “equilibrium stage” means a vapor-liquid contacting stage whereby the vapor and liquid leaving the stage are in mass transfer equilibrium, e.g. a tray having 100 percent efficiency or a packing element height equivalent to one theoretical plate (HETP). 
     As used herein the terms “upper portion” and “lower portion” mean those sections of a column respectively above and below the mid point of the column. 
     As used herein the term “high purity nitrogen” means a fluid having a nitrogen concentration of at least 99 mole percent, preferably at least 99.9 mole percent, most preferably at least 99.999 mole percent. A particularly desirable form of high purity nitrogen is ultra high purity nitrogen which is a fluid having a nitrogen concentration of at least 99.99999 mole percent. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The sole FIGURE is a simplified schematic representation of one preferred embodiment of the cryogenic rectification system of this invention for producing ultra high purity clean dry air which also produces ultra high purity nitrogen. 
    
    
     DETAILED DESCRIPTION 
     In general, the invention is a system for cogenerating ultra high purity clean dry air from a system for the generation of high purity nitrogen, preferably ultra high purity nitrogen. The system for generating high purity nitrogen broadly comprises compression and purification processes, carried out at around ambient temperature or above, and a cryogenic rectification process, carried out at least in part, below 150 K. The cryogenic rectification process comprises at least one rectification column where ultra high purity nitrogen is produced as a top product and where argon and oxygen and other impurities that are heavier than nitrogen, are rejected as a bottom waste stream. The invention takes liquid from a point in the rectification column where the oxygen concentration is within the range of from 10 to 50 mole percent, preferably from 15 to 30 mole percent, most preferably from 19.5 to 23.5 mole percent and where most of the impurities that are heavier than nitrogen and oxygen have been separated. This liquid is then pumped to an elevated pressure determined by the ultimate clean dry air product pressure, vaporized and the resulting vapor warmed. The concentration of oxygen in the extracted stream is measured, and if it is found to be greater than a certain predetermined level, ultra high purity nitrogen product is added to the stream to dilute it. The final stream is recovered as ultra high purity clean dry air product. 
     The invention will be described in greater detail with reference to the Drawing. Referring now to the FIGURE, feed air stream  101  is taken from the atmosphere, filtered and compressed in compressor  110  to a pressure generally within the range of from 150 to 250 pounds per square inch absolute (psia) to form compressed air stream  102 . Compressor  110  commonly comprises a number of compressor stages in series with heat of compression being removed from the air stream in between each stage in inter-coolers, and after the last stage, in an after-cooler. The inter-coolers and the after-cooler also drain any water that might be condensed from the air stream. 
     Stream  102  is then passed through prepurifier unit  120  which removes several atmospheric impurities that are not desired in the final ultra high purity nitrogen product and that might also freeze out at subsequent cryogenic temperatures. The prepurifer unit commonly includes a catalytic reactor that is used to oxidize hydrogen and carbon monoxide gases into moisture and carbon dioxide respectively. Hydrogen has a boiling point significantly above that of nitrogen and without removal, would otherwise pass to the ultra high purity nitrogen product in the subsequent cryogenic rectification process. Carbon monoxide has a boiling point below nitrogen but above oxygen, and without removal, would also appear in part in the ultra high purity nitrogen product. The prepurifier unit also includes an adsorption system to substantially remove moisture and carbon dioxide from the air stream. This moisture and carbon dioxide comprises that already present in the feed air and that which is formed in the catalytic reactor. The adsorption unit removes moisture and carbon dioxide to a level somewhat less than 1 ppm v/v, most typically &lt;0.1 ppm v/v and &lt;0.25 ppm v/v respectively. At this level, these gases will not rapidly build up as frozen deposits in the cryogenic apparatus. Further, since their boiling points are significantly less than that of nitrogen, argon and oxygen, these impurities will not accumulate in the ultra high purity nitrogen product, but rather they will pass out of the process in a waste stream. Other impurities, apart from hydrogen, carbon monoxide, moisture and carbon dioxide, are also present in the air stream, and include hydrocarbon species such as methane and ethane. These hydrocarbons are not appreciably oxidized in the catalytic reactor at the usual operating temperatures of between approximately 300 and 500° F., and are not appreciably adsorbed in the adsorption unit and therefore pass, at least in part, through the prepurifier unit. Methane is the lightest atmospheric hydrocarbon and has a boiling point below nitrogen, argon and oxygen. It and other heavier impurities can therefore easily be separated from the ultra high purity nitrogen product by cryogenic rectification and pass with the argon and oxygen components into a waste stream. 
     Prepurified air stream  103  exits the prepurifier and is split into main air stream  104  and turbine air stream  106 . Stream  104  enters the cryogenic rectification cycle and is first cooled in heat exchanger  160 , from around ambient temperature, to a point close to the dew point of air. Stream  106  is further compressed in compressor  130  to form stream  107 , and is then also passed into heat exchanger  160 . Stream  107  is cooled to a temperature somewhat above it&#39;s liquefying point and then extracted from heat exchanger  160  as stream  108 . Stream  108  is then expanded to a lower pressure in turboexpander  140  to form first low pressure column feed stream  109 . Expansion across turbine  140  causes stream  108  to cool and generate refrigeration for the cryogenic rectification cycle. The amount of refrigeration generated is controlled by the proportion of stream  103  passing to stream  106 , and is commonly in the range of from 5 to 20%. Compressor  130  and turbine  140  are commonly linked by a drive shaft, which removes work from the turbine and supplies that work to the compressor. This arrangement increases the refrigeration that is achieved by the turbine, since it boosts the pressure of stream  106  prior to expansion. It is not necessary however, and indeed compressor  130  may be omitted and the work generated by the turbine may be removed by some other means, for example an oil brake, or other friction coupling. 
     Stream  104  emerges from heat exchanger  160  as stream  111  and is split into streams  112  and  113 . Higher pressure column  100  is operating at a pressure generally within the range of from 130 to 230 psia and contains an upper rectification section  100 A and a lower rectification section  100 B. Stream  112  passes directly into column  100 , below section  100 B. Stream  113  passes into heat exchanger  180 , where it is cooled and at least partially condensed by indirect heat exchange with stream  212 , which is a liquid stream derived from rectification column  200 . Stream  113  exits heat exchanger  180  as stream  114  and is passed into column  100  at a point above section  100 B and below section  100 A. At this point, the composition of the condensed portion of stream  114  approximately matches the concentration of the liquid passing from the bottom of section  100 A into the top of section  100 B. 
     Gaseous stream  112  ascends column  100  and contacts a descending liquid stream that is rich in the heavier components of air. The counter-current contacting that occurs causes light components of air, i.e. nitrogen, to concentrate in the ascending gas stream and heavy components of air such as oxygen and argon to concentrate in the descending liquid stream by cryogenic rectification. Liquid that reaches the bottom of column  100  is removed as high pressure column waste stream  121 . A first portion of the gas that reaches the top of the column passes in stream  21  into the condensing-side passages of higher pressure column top condenser  150  where it is condensed and returned to the top of column  100 , above section  100 A, as reflux liquid in stream  22 . A second portion of the gas stream is taken as ultra high purity gaseous nitrogen stream  122 , and is then warmed back up to close to ambient temperature in heat exchanger  160  to form ultra high purity gaseous nitrogen product stream  125 . 
     Condenser  150  cooling duty is derived from stream  121 , which exits the bottom of column  100 . This stream is sub-cooled by a few degrees in heat exchanger  170  to form stream  126  and then passed into the boiling-side passages of condenser  150 , in indirect heat exchange relation to the condensing vapor in the condensing-side passages. The pressure in the boiling-side passages is significantly lower than the pressure of stream  126  and set such that the boiling point of the boiling-side liquid is sufficiently less than the boiling point of the condensing-side vapor, so as to achieve the necessary indirect heat exchange. Vapor formed in the boiling-side, exits condenser  150  as stream  127 , while any remaining liquid in the boiling-side, exits as stream  131 . In addition to the column reflux generated in the condensing-side of condenser  150 , additional reflux is added in the form of returned high pressure liquid nitrogen stream  208 , which is generated in lower pressure column  200 . 
     Stream  127  exits condenser  150  and is throttled across valve  20  to form second lower pressure column feed stream  128 . Column  200  is operating at a pressure less than that of column  100  and generally within the range of from 50 to 100 psia. Lower pressure column  200  contains upper rectification section  200 A and a lower rectification section  200 B. Stream  128  and stream  109  enter the bottom of lower pressure column  200  below section  200 B. The gaseous stream, ascending column  200  contacts a descending liquid stream that is rich in the heavier components of air. The counter-current contacting that occurs, causes light components of air, i.e. nitrogen, to concentrate in the ascending gas stream and heavy components of air such as oxygen and argon to concentrate in the descending liquid stream by cryogenic rectification. Liquid that reaches the bottom of the column is removed as lower pressure column waste stream  201 . The gas that reaches the top of the column passes in stream  23  into the condensing-side passages of lower pressure top column condenser  250  where it is condensed. A first portion of the liquid is returned to the top of column  200  as reflux liquid in stream  24 . A second portion of the liquid is taken from condenser  250  as liquid nitrogen stream  206 . Stream  206  is pumped in pump  210  to a pressure in excess of the operating pressure of higher pressure column  100 , to form high pressure liquid nitrogen stream  207 . A first portion of stream  207  is taken as returned high pressure liquid nitrogen stream  208  and passed into the top of column  100  as additional reflux. A second portion of stream  207  may be taken as ultra high purity liquid nitrogen product stream  225 . 
     A second liquid stream  211 , is taken from lower pressure column  200  and is used to produce an ultra high purity clean dry air product. Stream  211  is extracted between rectification sections  200 A and  200 B at a level such that stream  211  has an oxygen concentration within the range of from 10 to 50 mole percent. This level is within the range of from 1 to 25 equilibrium stages, preferably from 5 to 20 equilibrium stages, above the feed level where feed stream  109  is passed into the column. The oxygen concentration of the liquid withdrawn in stream  211  is preferably within the range of from 15 to 30 mole percent, most preferably within the range of from 19.5 to 23.5 mole percent. The withdrawal level also corresponds to a position where heavy atmospheric impurities, such as methane and ethane, have been substantially separated. These impurities being heavier than oxygen, concentrate in section  200 B and pass out of the bottom of column  200  in stream  201 . Stream  211  is a liquid air-like stream that may be enriched in oxygen and is substantially depleted in heavy impurities. Stream  211  may then be pumped, if desired, to an elevated pressure by pump  220  to form stream  212 , the pressure of stream  212  being sufficient to provide an ultimate ultra high purity clean dry air product at the required product pressure. Stream  212  is then passed through heat exchanger  180 , where it is at least partially vaporized by indirect heat exchange with stream  113 . Generally the required delivery pressure of the final clean dry air product is sufficiently lower than the required delivery pressure of the final gaseous nitrogen product. This means that stream  113  has a boiling point sufficiently higher than that of stream  212 , and therefore the necessary temperature difference is available to promote indirect heat exchange between the two streams in heat exchanger  180 . Further, the flow of stream  113  can be easily adjusted to match the required heat load necessary to at least partially vaporize stream  212 . Stream  213  exits heat exchanger  180 , at least partially vaporized, and passes into heat exchanger  160  where any remaining liquid is vaporized, and where the vapor is warmed up to around ambient temperature to form ultra high purity clean dry air stream  214  which may be recovered as product. As already mentioned stream  214  has an oxygen concentration generally corresponding to that of air. Accordingly, stream  124 , which comprises ultra high purity nitrogen, may be added to stream  214  in order to dilute stream  214 , and make the oxygen concentration equal to that of atmospheric air, or at least within desired limits. The resulting stream is ultra high purity clean dry air product stream  215 . The pressure of stream  214  must be below that of stream  124 , in order for stream  124  to be added to stream  214 . 
     Top condenser  250  cooling duty is derived from stream  201 , which exits the bottom of column  200 , and stream  131 , which exits the boiling-side of top condenser  150 . Stream  201  is passed into the boiling-side passages of condenser  250 . Stream  131  is subcooled by a few degrees in heat exchanger to  270  to form stream  132 , and then stream  132  is also passed into the same boiling side passages of condenser  250 . The boiling-side liquid formed from streams  201  and  132  is positioned in indirect heat transfer relation to the condensing vapor in the condensing-side passages of condenser  250 . The pressure in the boiling-side passages is significantly lower than the pressure of streams  132  and  201  and set such that the boiling point of the boiling-side liquid is sufficiently less than the boiling point of the condensing-side vapor so as to achieve the necessary indirect heat transfer. Vapor formed in the boiling-side of condenser  250 , exits the condenser as stream  202  and is warmed up in heat exchanger  270  to form stream  203 , which is further warmed up in heat exchanger  170  to form stream  204 , which is still further warmed up to around ambient temperature in heat exchanger  160  to form waste stream  205 . A portion of stream  205  is commonly used as regenerating sweep gas for the adsorption system in prepurifier  120 , while the remaining portion is commonly vented to atmosphere. 
     A number of computer simulations were carried out to demonstrate the invention and the advantages attainable thereby and the results are presented in Table 1. 
     Table 1 lists the expected concentration of several impurities in (A) ultra high purity clean dry air product from the preferred embodiment of the invention, where the product is derived from air-like liquid taken from the lower pressure column of a dual column cycle such as is illustrated in the Figure; (B) clean dry air product from an alternative embodiment of the invention, where the product is derived from air-like liquid taken from the higher pressure column of such dual column cycle; (C) a conventional system, where clean dry air is taken from the prepurifier unit, and (D) atmospheric air. Table 1 shows that all of these heavy impurities are removed to below ppb levels in the preferred embodiment, and that all, except methane are removed to below ppb levels in the alternative embodiment, methane being reduced to approximately 200 ppb. Clean dry air produced by the conventional system however, contains significant amounts of these heavy impurities. Hydrogen and carbon monoxide are removed to below ppb levels, but methane and ethane pass through the prepurifier and remain at their atmospheric concentrations, while moisture and carbon dioxide are still at relatively high levels. In Table 1 all concentrations are reported in ppm mol/mol. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Impurity 
                 A 
                 B 
                 C 
                 D 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Hydrogen (H 2 ) 
                 &lt;0.001 
                 &lt;0.001 
                 &lt;0.001 
                 0.5 
               
               
                 Carbon Monoxide (CO) 
                 &lt;0.001 
                 &lt;0.001 
                 &lt;0.001 
                 0.5 
               
               
                 Methane (CH 4 ) 
                 &lt;0.001 
                 0.2 
                 1 
                 1 
               
               
                 Ethane (C 2 H 6 ) 
                 &lt;0.001 
                 &lt;0.001 
                 0.05 
                 0.05 
               
               
                 Carbon Dioxide (CO 2 ) 
                 &lt;0.001 
                 &lt;0.001 
                 &lt;0.25 
                 350 
               
               
                 Moisture (H 2 O) 
                 &lt;0.001 
                 &lt;0.001 
                 &lt;0.1 
                 1000 to 
               
               
                   
                   
                   
                   
                 28,000 
               
               
                   
               
             
          
         
       
     
     Although the invention has been discussed in detail with reference to certain preferred embodiments, those skilled in the art will recognize that there are other embodiments of the invention within the spirit and the scope of the claims. For example, the invention may be practiced with a single column cryogenic air separation plant and also with a cryogenic air separation plant having more than two columns.