Abstract:
Separation method and apparatus for separating a gaseous mixture, for example, air, in a cryogenic rectification plant in which a compressed stream is divided into subsidiary streams that are extracted from a main heat exchanger of the plant at higher and lower temperatures. The two streams are then combined and expanded in a turboexpander to generate refrigeration for the plant. The flow rates of the two streams are adjusted to control inlet temperature of a turboexpander supplying plant refrigeration and to minimize potential deviation of the turboexpander exhaust from a saturated vapor state. Control of the expansion ratio can advantageously be applied to allow variable liquid production from the rectification plant.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional application of U.S. patent application Ser. No. 11/634,623 filed Dec. 6, 2006, the disclosure of which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method and apparatus for separating a gaseous mixture in a cryogenic rectification plant in which the temperature of a compressed stream of the gaseous mixture fed to a turboexpander and used to supply refrigeration to the plant is controlled by removing two streams of the compressed stream from the plant main heat exchanger, controlling the flow rates of the two streams and then combining the two streams prior to their introduction into the turboexpander. 
       BACKGROUND OF THE INVENTION 
       [0003]    It has long been known to separate a variety of gaseous mixtures by cryogenic rectification, for example pretreated air and natural gas. In such processes, the gaseous mixture to be separated is pressurized, purified and then cooled to a temperature suitable for its rectification. The rectification of the gaseous mixture occurs within one or more distillation columns. Each of the columns has mass transfer elements such as trays or packing, for example, structured packing, which bring liquid and vapor phases of the gaseous mixture into contact with one another and effectuate mass transfer between the vapor and liquid phases. 
         [0004]    The incoming feed is thereby distilled within the distillation columns or columns to form component streams enriched in the components of the gaseous mixture. The component streams can be taken as liquid and gaseous products and are used in the cooling of the gaseous mixture after having been compressed and purified to a temperature suitable for the separation of the gaseous mixture within the distillation column or columns. The cooling takes place through indirect heat exchange that is conducted in a plant main heat exchanger. 
         [0005]    In order to minimize warm end losses in the main heat exchanger and to produce liquid products, refrigeration can be generated by expanding a compressed stream made up of the gaseous mixture and introducing the compressed stream into at least one of the columns in a plant. 
         [0006]    It is also known to mechanically pump a liquid product, for instance in air separation, an oxygen-rich liquid column bottoms stream may be vaporized within the same main heat exchanger against a liquefying compressed air stream provided for such purpose. 
         [0007]    Given that energy supply costs for electric power consumed in compressing the feed can vary with the time of day, there is a growing incentive to be able to manipulate plant product slates and in particular, liquid production rates. For example, high purity oxygen plants are often designed to produce anywhere of up to about 10 percent of the air as a liquefied product. There exists the need to manipulate the flow of products so that at times less than the maximum capability of the plant is utilized, for example, plant operations in which less than 10 percent of the air is taken as the liquid product. In order to change liquid production rates, it is conventional practice to adjust the turbine flow employed in generating refrigeration. An example of this can be found in U.S. Pat. No. 5,412,953. In this patent, a pumped liquid oxygen plant is described in which the liquid product make is adjusted by adjusting flow to the turboexpander. This adjustment of flow is effectuated by recycling air from the bottom of the higher pressure column to a compressor that is used in compressing the air to the turboexpander. Such operation can result in wide swings in air compression requirements that are required for such purposes as vaporizing pressurized column liquids. 
         [0008]    Another possibility in controlling liquid production is to vary the expansion ratio of the turbine expander by increasing or decreasing the pressure of the compressed mixture being introduced into the turboexpander. This also can result in control problems in that as the pressure is increased, the mixture to be expanded may be liquefied at the exhaust of the turbine. In an extreme case where between about 10 and about 15 percent of the compressed process feed is to be liquefied. In such situations, the turbine may suffer from poor efficiency and may incur potential damage. At the other extreme, as pressure is decreased, the temperature of the expanded stream increases when the turbine inlet temperature is relatively fixed by the main heat exchanger design. When such increase is above the saturation temperature of the expanded feed to a column, liquids within the column may vaporize resulting in high local vapor flows, loss of separation performance and potential column flooding. 
         [0009]    In the prior art, it is known to control the turboexpander inlet temperature of an air separation plant in order to prevent liquefaction in the turboexpander exhaust. For example in U.S. Pat. No. 3,355,901, a cascade control system is utilized to ensure that the exhaust of a turboexpander used in supplying refrigeration to an air separation plant is at about saturation temperature or slightly superheated. In this patent, warm vapor is divided into two streams. One stream is cooled within a heat exchanger against a cryogenic gas produced in the air separation process and the other stream by-passes the heat exchanger. The streams are then combined and introduced into the inlet of a turboexpander. The turbine exhaust temperature is sensed and a signal referable to such temperature is fed as an input into the cascade control system to control a valve that in turn controls flow of the stream that is cooled within the heat exchanger. However, it is to be noted that such arrangement is to be used in a plant that does not manipulate expansion ratio and as such the variation of turbine exhaust temperature is limited. It could not be used in a plant where expansion pressure and ratio vary substantially. 
         [0010]    As will be described, the present invention provides a method and apparatus for separating a gaseous mixture in which refrigeration and therefore liquid production is varied by simultaneous manipulation of turbine expansion ratio and inlet temperatures. Simultaneous manipulation of turboexpander inlet temperature allows for greater variability of liquid production than would otherwise exist by manipulation of turbine expansion ratio alone. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention provides a separation method in which a compressed gaseous mixture is separated within a cryogenic rectification plant by purifying the compressed gaseous mixture, cooling the compressed gaseous mixture by indirect heat exchange with mixture component streams after having been purified and then, rectifying the gaseous mixture within a separation unit. The separation unit has at least one distillation column to produce the mixture component streams. 
         [0012]    At least one product liquid stream is discharged from the separation unit that is enriched in one mixture component of the gaseous mixture. At least part of the gaseous mixture after partial cooling thereof during the indirect heat exchange is divided into a first subsidiary stream and a second subsidiary stream. The first subsidiary stream and the second subsidiary stream are withdrawn from the indirect heat exchange at higher and lower temperatures, respectively. The first subsidiary stream and the second subsidiary stream after withdrawal from the indirect heat exchange are then combined to produce a combined stream. At least part of the combined stream is expanded with the performance of work within a turboexpander to supply refrigeration to the cryogenic plant. At least part of an exhaust stream of the turboexpander is introduced into the separation unit. The temperature of the combined stream is controlled such that the exhaust stream is at about its saturation temperature by controlling the flow rates of the first subsidiary stream and the second subsidiary stream. Here it is important to note that as used herein and in the claims, the “control of the flow rate” does not mean that the flow rates of the first subsidiary stream and the second subsidiary stream are necessarily independently controlled. In plant designs in which all of the combined stream is directed to a turboexpander, the active control of the flow rate of one of such streams will control the other of the streams. In plant designs in which not all of the combined stream is routed to the turboexpander the flow rate of such streams could be independently controlled. 
         [0013]    The temperature control of the combined stream is advantageous in any type of cryogenic separation plant and in such plants where a pressurized liquid product is to be vaporized. The present invention, in its most basic aspect has a wider applicability in that such cryogenic separation plants sometimes require fine tuning due to unforeseen operational and environmental impacts. For instance, if the flow to the turboexpander is warmer than expected, the exhaust temperature may be higher than expected so as to cause unforeseen and excessive vaporization of liquids within the distillation columns. This having been said, the present invention has particular applicability where the pressure of the at least part of the compressed gaseous mixture is varied to in turn vary the refrigeration supplied by the turboexpander and the production rate of the liquid streams. In such cases, increasing the turboexpansion inlet pressure by increasing the pressure of the at least part of the compressed gaseous mixture increases liquid production. Decreasing the pressure of the at least part of the compressed gaseous mixture decreases liquid production. During high liquid production, the flow rates of the first subsidiary stream and the second subsidiary stream are controlled such that a flow rate of the first subsidiary stream is greater than that of the second subsidiary stream. During the low liquid mode of production the flow rates of the first subsidiary stream and the second subsidiary stream are controlled such that the flow rate of the first subsidiary stream is less than that of the second subsidiary stream. 
         [0014]    The present invention has particular applicability to the separation of air. In this context, the compressed gaseous mixture can be composed of air. In such application, the mixture component streams are oxygen-rich and nitrogen-rich streams and the separation unit can be an air separation unit having higher and lower pressure distillation columns operatively associated with one another in a heat transfer relationship to produce the oxygen-rich and nitrogen-rich streams. Consequently, the liquid stream is enriched in one of oxygen and nitrogen. 
         [0015]    The liquid stream can be enriched in oxygen and part of the liquid stream is pumped to produce a pressurized liquid stream. The oxygen-rich stream is formed by the pressurized liquid stream and the pressurized liquid stream is vaporized as a result of the indirect heat exchange to produce a pressurized oxygen-rich product. In such case, the compressed gaseous mixture is divided into a first compressed air stream and a second compressed air stream prior to the indirect heat exchange. The at least part of the gaseous mixture is the first compressed air stream. The second air stream, during the indirect heat exchange is condensed by indirect heat exchange with the pressurized liquid stream, thereby forming a liquid air stream. The air contained within the first compressed air stream and the second air stream is rectified within the air separation unit. 
         [0016]    The flow rates of the first subsidiary stream and the second subsidiary stream can be controlled by a first and second pair of valves. Each pair of valves contains a high flow control valve, namely, a valve that is capable of metering high flow rates and a low flow control valve, namely, a valve that is capable of metering very low flow rates. During the high liquid mode of production, the flow rates of the first subsidiary stream and the second subsidiary stream are respectively controlled by the high flow control valve of the first pair of valves and the low flow control valve of the second pair of valves. This is because the flow rate of the first subsidiary stream is greater in such case. As a result, the low flow control valve of the first pair of valves and the high flow control valve of the second pair of valves are set in closed positions. Conversely, during the low liquid mode of production, the flow rates of the first subsidiary stream and the second subsidiary stream are respectively controlled by the low flow control valve of the first pair of valves and the high flow control valve of the second pair of valves. The high flow control valve of the first pair of valves and the low flow control valve of the second pair of valves are set in the closed positions. 
         [0017]    The exhaust stream can be introduced into a bottom region of a higher pressure column. The liquid air stream can be divided into first and second portions and valve expanded into the higher and lower pressure columns, respectively. 
         [0018]    A nitrogen-rich column overhead stream of the higher pressure column can be liquefied against vaporizing oxygen-rich column bottoms of the lower pressure column. This produces first and second nitrogen reflux streams to reflux the higher and lower pressure columns. The second of the nitrogen reflux streams can be subcooled prior to being introduced into the lower pressure column by exchanging heat with a waste nitrogen vapor stream and a product nitrogen vapor stream that is also withdrawn from the lower pressure column. The waste nitrogen and the product nitrogen are the nitrogen-rich streams taking part in the indirect heat exchange, mentioned above. 
         [0019]    A crude liquid oxygen stream formed from the oxygen containing column bottoms of the higher pressure columns can be valve expanded and introduced into the lower pressure column for rectification without being subjected to indirect heat exchange to further cool the crude liquid oxygen stream prior to its being valve expanded. 
         [0020]    In another aspect, the present invention provides a separation apparatus. In accordance with this aspect, at least one compressor is provided to compress a gaseous mixture, thereby to produce a compressed stream. A purification unit is provided to purify the compressed stream. A main heat exchanger is connected to the purification unit and is provided with flow passages for subjecting the compressed stream to indirect heat exchange with mixture component streams. A separation unit is provided consisting of at least one distillation column to rectify the gaseous mixture. The separation unit produces product fractions consisting of the mixture components. The separation unit has at least one liquid product outlet and at least one gaseous product outlet. 
         [0021]    The main heat exchanger is connected to the separation unit such that the mixture component streams flow from the cold to the warm ends thereof. The main heat exchanger is configured to discharge a first subsidiary stream and a second subsidiary stream, respectively; the first subsidiary stream and the second subsidiary stream being made up of the gaseous mixture. The first subsidiary stream and the second subsidiary stream are discharged from the main heat exchanger at higher and lower temperatures, respectively. 
         [0022]    A turboexpander expands at least part of the combined stream with the performance of work to supply refrigeration. The combined stream is formed from the first subsidiary stream and the second subsidiary stream and the turboexpander is connected to the separation unit such that at least part of an exhaust stream of the turboexpander is introduced into the at least one distillation column. 
         [0023]    A flow control network is configured to mix the first subsidiary stream and the second subsidiary stream and thereby to form the combined stream. The flow control network has valves which control flow rates of the first subsidiary stream and the second subsidiary stream and therefore, the temperature of the combined stream to ensure that the exhaust from the turboexpander has an outlet temperature at least at about equal to saturation temperature. 
         [0024]    As indicated above, the gaseous mixture can be air and the compressed stream can therefore be a compressed air stream. The mixture component streams in such an application of the present invention are oxygen-rich and nitrogen-rich streams and the separation unit can be an air separation unit having higher and lower pressure distillation columns operatively associated with one another in a heat transfer relationship, thereby to produce the oxygen-rich and nitrogen-rich streams. The turboexpander is connected to the air separation unit such that at least part of the exhaust from the turboexpander is introduced into the higher or the lower pressure distillation columns. 
         [0025]    A pump can be provided to pressurize part of the liquid stream to produce a pressurized liquid stream. The pump is in flow communication with the separation unit and the main heat exchanger such that the pressurized liquid stream vaporizes as a result of the indirect heat exchange to produce a pressurized gaseous product. The compressed air stream is a first compressed air stream and the at least one compressor is part of a compression system. 
         [0026]    The compression system is provided with a base load compressor. A turbine loaded booster compressor is also provided in flow communication with the base load compressor and operatively associated with the turboexpander to at least be partially driven by the work of the turboexpander. A first compressor is connected to the turbine loaded booster compressor and the first compressed air stream is thereby produced by the turbine loaded booster compressor and the first compressor. Additionally, a second compressor is provided in flow communication with the base load compressor to produce the second compressed air stream. The second compressor is also in flow communication with the main heat exchanger and the main heat exchanger is also in flow communication with the air separation unit such that the second compressed air stream is subjected to the indirect heat exchange causing the vaporization of the pressurized liquid stream and the second compressed air stream to liquefy, thereby to form a liquid air stream and the liquid air stream is introduced into the air separation unit. 
         [0027]    The first compressor can be provided with inlet guide vanes or the compression system can be provided with a by-pass line having a cut-off valve to by-pass the first compressor when the cut-off valve is set in an open position. This allows the pressure of the second air stream to be varied to in turn vary the refrigeration supplied by the turboexpander and therefore, production of the liquid stream. 
         [0028]    The valves of the flow control network can include a first and a second pair of valves connected to the main heat exchanger and each pair containing a high flow control valve and a low flow control valve. During the high liquid mode of production, the flow rates of the first subsidiary stream and the second subsidiary stream are respectively controlled by the high flow control valve of the first pair of valves and the low flow control valve of the second pair of valves. During such time, the low flow control valve of the first pair of valves and the high flow control valve of the second pair of valves are set in closed positions. During the low liquid mode of production, the flow rates of the first subsidiary stream and the second subsidiary stream are controlled by the low flow control valve of the first pair of valves and the high flow control valve of the second pair of valves. At this time, the high flow control valve of the first pair of valves and the low flow control valve of the second pair of valves are set in closed positions. Additionally, the flow control network is provided with a static mixer or similar device interposed between the first and second pair of valves and the turboexpander to mix the first subsidiary stream and the second subsidiary stream. 
         [0029]    In addition, the turboexpander can be connected to a bottom section of the higher pressure column and the main heat exchanger can be connected to the air separation unit so that first and second portions of the liquid air stream are introduced into the higher and lower pressure columns. Expansion valves are positioned between the main heat exchanger and the higher and lower pressure columns so that the first and second portions are valve expanded to the higher and lower pressures of the higher and lower pressure columns. 
         [0030]    Additionally, as also discussed above with respect to the method, a condenser-reboiler can be operatively associated with the higher and lower pressure columns so that a nitrogen-rich column overhead stream of the higher pressure columns can be liquefied against vaporizing an oxygen-rich column bottoms of the lower pressure column to produce first and second nitrogen reflux streams to reflux the higher and lower pressure columns. A subcooler can be provided to subcool the second of the nitrogen reflux streams prior to being introduced into the lower pressure column. The subcooler is configured to subcool the second of the nitrogen vapor stream and a product nitrogen vapor stream withdrawn from the lower pressure column. The subcooler is connected to the main heat exchanger so that the waste and product nitrogen streams are therefore the nitrogen-rich streams taking part in the indirect heat exchange within the main heat exchanger. 
         [0031]    A conduit can connect the bottom region of the higher pressure column to an intermediate location of the lower pressure column to introduce a crude liquid oxygen stream formed from the oxygen containing column bottoms of the higher pressure columns into the lower pressure columns for rectification. A further expansion valve is positioned within the conduit to expand the crude liquid oxygen stream to a compatible pressure of the lower pressure column at its point of introduction. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]    While the specification concludes with claims distinctly pointing out the subject matter that Applicants regard as their invention, it is believed that the invention will be better understood when taken in connection with the accompanying drawings in which: 
           [0033]      FIG. 1  is a schematic view of an air separation plant for carrying out a method in accordance with the present invention; 
           [0034]      FIG. 2  is an elevational view of a main heat exchanger employed in the air separation plant illustrated in  FIG. 1 ; 
           [0035]      FIG. 3  is an alternative embodiment of  FIG. 3 ; 
           [0036]      FIG. 4  is an alternative embodiment of  FIG. 3 ; 
           [0037]      FIG. 5  is an alternative embodiment of  FIG. 3 ; 
           [0038]      FIG. 6  is a sectional view of  FIG. 5  taken along line  6 - 6  thereof; and 
           [0039]      FIG. 7  is a sectional view of  FIG. 5  taken along line  7 - 7  thereof. 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    With reference to  FIG. 1 , an air separation plant  1  is illustrated for exemplary purposes. As indicated above, the present invention in its more broader aspects has equal application to other separation process, for example, those involving natural gas. 
         [0041]    Air separation plant  1  includes a compression system  10  to compress the air to pressures suitable for its rectification within an air separation unit  12  having a higher pressure column  14  and a lower pressure column  16 . Rectification of the air separates the components of the air into oxygen-rich and nitrogen-rich fractions that are extracted as oxygen-rich and nitrogen-rich streams that are introduced into a main heat exchanger  18  to indirectly exchange heat from the compressed air to the oxygen-rich and nitrogen-rich streams and thereby to cool the compressed air to a temperature suitable for the rectification thereof. As would occur to those skilled in the art, in other separation processes, a feed such as natural gas might be obtained at pressure thus obviating the need for compression within the plant itself. 
         [0042]    Having briefly described the air separation plant  1 , a more detailed description begins with compression system  10 . Compression system  10  includes a base load compressor  20  to compress an incoming air stream  22  to a pressure that can be within the range of between about 5 and about 15 bars absolute (“bara”). Compressor  20  may be an inter-cooled integral gear compressor with condensate removal. 
         [0043]    The resultant compressed air stream  24  is then directed to a prepurification unit  26  that may comprise several unit operations, all known in the art, including: direct water cooling; refrigeration based chilling; direct contact with chilled water; phase separation and/or adsorption within adsorbent beds operating out of phase containing, typically an alumina adsorbent. Prepurification unit  26  produces a purified compressed stream  28  that has a very low content of higher boiling contaminants such as water and carbon dioxide that could otherwise freeze within main heat exchanger  18  and hydrocarbons that could collect within air separation unit  12  and present a safety hazard. 
         [0044]    Purified compressed air stream  28  is divided into streams  30  and  32 . Stream  30  is subjected to further compression within a turbine loaded booster compressor  34  that is operatively associated with a turboexpander  36  to recover some of the work of expansion in operation of booster compressor  34 . A stream  38  is produced by the compression that can have a pressure that can be typically between about 15 and about 20 bara. Steam  38  is then further compressed by a compressor  40  to produce a first compressed air stream  42  having a pressure of between about 20 and about 60 bara. 
         [0045]    Stream  32  can constitute between about 25 percent and about 35 percent of purified compressed air stream  28  and is further compressed within a compressor  44  to produce a second compressed air stream  46  having a pressure of between about 25 and about 70 bara. 
         [0046]    As will be discussed, first compressed air stream  42  after having been cooled and subjected to temperature control in accordance with the present invention is introduced into turboexpander  36 . An exhaust of turboexpander  36 , exhaust stream  48 , is introduced into a bottom region  50  of higher pressure column  14 . The second compressed air stream  46 , as will be discussed, condenses within main heat exchanger  18  against the vaporization of a pressurized product to produce a liquid air stream  52  that is valve expanded within an expansion valve  54  to a pressure suitable for its entry into higher pressure column  14  to produce a reduced pressure liquid stream  56 . In this regard, the higher pressure column  14  can operate at a pressure of between about 5 and about 6 bara. A first portion  58  of reduced pressure liquid stream  56  is introduced into higher pressure column  14  and a second portion  60  of reduced pressure liquid stream  52 , after having been expanded in an expansion valve  62  to a pressure suitable for its introduction into lower pressure column  16 , is then introduced into lower pressure column  16  as a stream  63 . In this regard lower pressure column  16  can operate at a pressure of between about 1.1 and 1.4 bara. 
         [0047]    The higher pressure column  14  is provided with mass transfer elements  64  and  68 , schematically illustrated, that can be structured packing. The vapor introduced via exhaust stream  48  initiates an ascending vapor phase that contacts a descending liquid phase that descends within mass transfer elements  64  and  68 . Additionally, first portion  58  of reduced pressure liquid stream  56  descends within packing element  64  and the evolved vapor will ascend through a packing element  68 . As the vapor ascends within higher pressure column  14  it becomes evermore rich in the lighter components of the air, namely, nitrogen and as the liquid descends within the higher pressure distillation column  14 , the liquid becomes evermore rich in the heavier components of the air, namely, oxygen, to produce a crude liquid oxygen column bottoms stream  82  that collects within bottom region  50  of distillation column  14 . 
         [0048]    A nitrogen-rich column overhead stream  70  is introduced into a condenser reboiler  72  located within the bottom of lower pressure column  16  where it vaporizes some of the oxygen-rich liquid column bottoms  74  that collects within lower pressure distillation column  16  by virtue of the distillation occurring within such column. This produces a liquid nitrogen stream  76  that is divided into first and second nitrogen reflux streams  78  and  80  to reflux the higher and lower pressure columns  14  and  16 , respectively. The reflux provided in higher pressure column  14  by virtue of the first nitrogen reflux stream  78  initiates the formation of the descending liquid phase. A crude liquid oxygen stream  82  composed of the crude liquid oxygen column bottoms within higher pressure column  14  is valve expanded within an expansion valve  84  to the pressure of lower pressure column  16  and is introduced into lower pressure column  16  as a stream  85 . The second nitrogen reflux stream  80  is subcooled within a subcooling unit  86  to form a stream  88  to reflux the lower pressure column  16 . All or a portion of stream  88  may be introduced into lower pressure column  16  as a stream  89  after passage through valve  87 . A portion of stream  88  may be taken as a liquid product  102  and directed to suitable storage (not shown). 
         [0049]    The lower pressure column  16  is provided with mass transfer contacting elements  90 ,  92 ,  94  and  96  that contacts liquid and vapor phases within lower pressure columns  16  to produce the oxygen-rich liquid column bottoms  74 , a nitrogen product vapor stream  98  and a waste nitrogen vapor stream  100  that are passed into subcooling unit  86  to subcool second nitrogen reflux stream  80 . 
         [0050]    An oxygen-rich liquid stream  104  composed of the oxygen-rich liquid column bottoms  74  can be pressurized by way of a pump  106  to produce a pressurized liquid oxygen stream  108 . Part of the pressurized liquid oxygen stream  108  is vaporized within main heat exchanger  18 . As illustrated, a pressurized liquid oxygen product stream  109  can be taken as a product. In such case, the remainder, stream  110  is vaporized within main heat exchanger  18  to produce a pressurized oxygen product stream  111  that can be taken as a high pressure oxygen product. Additionally, waste nitrogen stream  100  can also be warmed in the main heat exchanger  18  to form waste stream  112  and product nitrogen vapor stream  98  can be warmed within main heat exchanger  18  to form a nitrogen-enriched product stream  113 . Heat exchange passes  114 ′,  115 ′,  116 ′ and  117 ′ are provided within main heat exchanger  18  for such purposes as have been outlined above and passes  118 , that will be discussed in further detail hereinafter for cooling the first compressed air stream  42 . 
         [0051]    In accordance with the present invention, liquid production of air separation plant  1 , namely pressurized liquid oxygen product stream  109  and liquid nitrogen product stream  102 , are varied by varying the pressure in the first compressed air stream  42 . This variation in pressure can be effectuated by a by-pass line  122  having a valve  124  that can be set in an open and closed position for controlling the by-pass by either allowing flow within by-pass line  122  or cutting off the flow to by-pass line  122 . Alternatively, line  122  may be configured for recirculation of compressor  40 . Additionally, in place of by-pass line  122 , compressor  40  could be provided with variable inlet vanes to vary the pressure of first compressed air stream  42 . 
         [0052]    During a high mode of liquid production, if the pressure of first compressed air stream  42  is increased, there will be more refrigeration produced and more liquid will therefore be produced. Conversely, if the pressure of the first compressed air stream  42  is reduced, there will be less refrigeration produced by turboexpander  36  and therefore a decrease in liquid production. 
         [0053]    However, in high liquid modes of production first compressed air stream  42  can be partly liquefied due to its high pressure and the cooling within main heat exchanger  18 . The control of temperature of the inlet stream to turboexpander  36  is accomplished by configuring the main heat exchanger to discharge the first subsidiary stream  126  and the second subsidiary stream  128  at higher and lower temperature to in turn control the temperature of the stream fed to the inlet of the turboexpander  36 . In order to control the temperature at the inlet of turboexpander  36 , pairs of control valves  130  and  134  are provided. The first pair of control valves  130  has a high flow control valve  136  and a low flow control valve  138 . Similarly the second pair of flow control valves has a high flow control valve  140  and a low flow control valve  142 . These valves are termed “high flow” and “low flow” in a comparative sense. For example, a “high flow” valve is one where the volumetric flow rate is anywhere from about 10 and about 100 times that of a “low flow” valve. However, the sizing of the high flow control valves relative to the low flow control valves would depend on a specific application of the present invention. Physically, the low flow valves are thus much smaller units than the high flow control valves. 
         [0054]    During the high mode of liquid production, high flow control valve  136  is controlling the flow of the predominant part of the flow contained within first subsidiary stream  126 . Low flow control valve  138  will be in a closed position. Additionally, high flow control valve  140  will also be closed and the low flow control valve  142  will be open to control the flow of second subsidiary stream  128  that will be either in a dense phase or a liquid phase. In the low liquid production mode, now most of the flow goes with second subsidiary stream  128 . Thus, high flow control valve  136  is set in the closed position and low flow control valve  138  is set in the open position. Similarly, the high flow control valve  140  now controls the flow of second subsidiary stream  128  and low flow control valve  142  is set in the closed position. 
         [0055]    The flow of first subsidiary stream  126  and second subsidiary stream  128  are then combined within a static mixer  144  to produce a combined stream  146  that can be introduced into the inlet of turboexpander  36  at a controlled temperature. 
         [0056]    As indicated above, the temperature control of combined stream  146  is provided in a manner that ensures that turbine exhaust stream  48  is not substantially liquefied or in other words has a liquid content of no greater than about 5 percent. More preferably, the exhaust stream will remain at or near the saturation vapor temperature. From the standpoint of column operation, variations above saturation temperature may now be effectively limited to less than about 20° C. Hence, the term “about” when used herein and in the claims in connection with the saturation vapor temperature means a temperature that is not lower than a temperature at which more than about 5 percent of liquefaction is in the turboexpander exhaust and not higher than a temperature that will result in a superheating of the exhaust beyond about 20° C. In order to accomplish this, the control of high and low flow control valves  136 ,  138 ,  140  and  142  could be set at pre-specified positions to obtain a controlled temperature of combined stream  146 . More preferably, closed loop control will be employed. In such an approach, the temperature of stream  146  is maintained by sensing the temperature of combined stream  146  and comparing its value to a predetermined value/setpoint and adjusting the positions of valves  136 ,  138 ,  140  and  142  accordingly. Such control is often referred to as PID control (proportional, integral and derivative control) as is well known to the art of process engineering. Alternatively, the temperature difference between exhaust stream  48  and stream  82  could also be monitored. The subject valves would then be manipulated to control the outlet temperature of the turbine in response. In so doing, the turbine superheat is maintained at some predetermined approach to saturation. 
         [0057]    The table below represents a calculated example generated by way of a steady state process simulation that illustrates key operational features of air separation plant during periods of both high and low liquid production. In this example gaseous oxygen stream  111  is produced from the process at a pressure  30  bara. The higher pressure column  14  operates at 5.2 bara. Further, in this example, all of the expansion flow of stream  30  passes through the expander  36  and into column  14 . The temperatures of the first and second subsidiary streams  126  and  128  were obtained by a rigorous solution for a fixed brazed aluminum heat exchanger design such as the one illustrated in  FIG. 2  and described in more detail hereinafter. Upon the initiation of high liquid production mode the exiting second subsidiary stream  128  is in a substantially liquefied state. 
         [0000]                                          TABLE                   Low Liquid   High Liquid       Stream and Operational Conditions   Production   Production                                EXPANSION PRESSURE RATIO of   3.0   8.6       combined stream 146 and turbine exhaust               stream 48               EXPANSION FLOW FRACTION of stream   0.656   0.669       30 relative to purified air stream 28               LIQUID PRODUCT FLOW FRACTION (the   0.034   0.106       sum of flow rates of liquid product streams               102 and 109 divided by the flow rate of the               entire incoming air stream 22)               SECOND SUBSIDIARY FLOW FRACTION   0.989   0.004       of second subsidiary stream 128 to stream 30               TEMPERATURE OF FIRST SUBSIDIARY   −100.6   −93.4       STREAM 126               TEMPERATURE OF SECOND   −133.4   −136.8       SUBSIDIARY STREAM 128               TURBINE EXHAUST STREAM 48   9.5   1.3       SUPERHEAT (in degrees centigrade)                    
A simulation of the subject process in a plant such as air separation plant  1  in which the heat exchanger is designed in the conventional manner (for the low liquid production mode and without temperature control for the turboexpander inlet) results in the turbine exhaust (stream  48 ) exhibiting a liquid fraction of roughly 30 percent. From a thermodynamic standpoint, the turbine work to flow ratio of the conventional approach would be 45 percent lower than that achievable through the application of the disclosed invention. In other words, the refrigeration potential from the same expansion ratio is greatly enhanced through the current invention.
 
         [0058]    It is understood that all of the combined stream  146  need not proceed to expander  36 . If desired, a portion of combined stream  146  can be directed back to the main heat exchanger  18  for further cooling and liquefaction and fed to the air separations unit  12 . Similarly, not all of the exhaust stream  48  need be directed to the air separation unit  12 . For example, a portion of the turbine exhaust  48  could be recirculated to the compressor  20  or the outlet of prepurification unit  26 . Additionally, exhaust stream  48  could be introduced into the lower pressure distillation column  16 . In such case, a portion of the stream could be directed to the waste stream or warmed and then vented. Although not illustrated, the present invention is equally applicable to air separation plants that employ different configurations than that illustrated in  FIG. 1 . For example, the present invention has application to air separation plants in which there is no liquid pumping of a product stream or in which all of the oxygen-enriched liquid is taken as a product and none vaporized. In case of a plant that does not employ liquid pumping, there would be no compressed air stream such as second compressed air stream  46  and the apparatus associated with the production and cooling of such stream. Even where there is vaporization of a product stream within a main heat exchanger, the streams emanating from the base load compression, such as streams  30  and  32 , might be compressed to about the same nominal pressure with the pressure of one of the streams being introduced into a turboexpander varied to vary liquid production together with a temperature control as provided herein. As also indicated above, the present invention has application to other cryogenic separation plants that do not involve the separation of air. 
         [0059]    With reference to  FIG. 2 , heat exchanger  18  is illustrated in more detail. As would be understood by those skilled in the art, heat exchanger  18  is oriented in a vertical position and can be a plate-fin type heat exchanger that has multiple layers of plates defining finned flow passages to define the heat exchange passes  114 ,  115 ,  116  and  117  and thereby to effectuate the heat exchange in a manner known in the art. In this regard, second compressed air stream  46  is introduced into an inlet header  150  and the liquid air stream  52  is discharged from an outlet header  152 . The flow of such streams is throughout the entire length of heat exchanger  18  and between finned flow passages located between plates. Similarly, waste nitrogen stream  100  also flows the entire length of heat exchanger  18  and is introduced though an inlet header  154  and is discharged as waste stream  112  from an outlet header  156 . The nitrogen vapor product stream  98  is introduced into an inlet header  158  and is discharged from an outlet header  160  as nitrogen-enriched product stream  113 . The pumped liquid oxygen-enriched stream  110  is introduced into an inlet header  159  and is discharged as the pressurized oxygen product stream  111  from header  161 . 
         [0060]    First compressed air stream  42  is introduced into heat exchanger  18  through an inlet header  162  and is redirected by distribution fins  163  to flow in a lengthwise direction of heat exchanger  18  and through a finned passage  164 . After partly traversing the length of heat exchanger  18 , the flow is then redirected by distribution fins  165  and is discharged through an outlet header  166  as a stream  167 . Part of such stream  167  is discharged from outlet header  166  as a stream  168  that is then reintroduced into heat exchanger  18  through an inlet header  169  and a remaining part of stream  167  forms first subsidiary stream  126 . Stream  168  is then redirected by distribution fins  170  to flow in the lengthwise direction of heat exchanger  18  through a finned passage  171 . After having been further cooled by partial traverse of heat exchanger  18  through finned passage  171 , stream  168  is then redirected again by way of distribution fins  172  and is discharged through an outlet header  173  as stream  128 . 
         [0061]    It is to be noted that as could well be appreciated by those skilled in the art, the layers of finned passages  164  and  171  thereby form the heat exchange passes, designated in  FIG. 1  by reference numeral  118 , for first compressed air stream  42  that are used in forming first subsidiary stream  126  and second subsidiary stream  128 . 
         [0062]    With reference to  FIG. 3 , in an alternative embodiment of main heat exchanger  18 , a main heat exchanger  18 ′ is provided with an outlet header  166  and inlet header  169  could be placed opposite one another. In such case, distribution fins  165  and  170  are replaced by an arrangement of distribution fins  165 ′ and  170 ′ that are separated by a diagonal partition to divide the flow. 
         [0063]    With reference to  FIG. 4 , in an alternative embodiment of heat exchanger  18 , a heat exchanger  18 ″ is provided with a hard way fin section  165 ′. A hard way fin section is a section of fin arranged to produce a principal flow resistance parallel to the flow direction that is greater than the flow resistance perpendicular to the flow direction. When valve  136  is open, this acts to split the flow so that first subsidiary stream  126  is discharged from outlet header  167 ′ at a higher flow rate than a remaining portion of the stream flowing within finned passage  164 . The remaining portion then flows through finned passage  171  and is then redirected by distribution fins  172  to outlet header  173  as second subsidiary stream  128  that is further cooled due to its continued traverse of heat exchanger  18 ″. 
         [0064]    With reference to  FIG. 5 , a heat exchanger  18 ′″ is presented as an alternative embodiment to heat exchanger  18 . With additional reference to  FIGS. 7 and 8 , a layer of distributor fins  165 ″ is provided to redirect the flow from finned passage  164  to outlet header  166 . The stream  168 , enters inlet header  169  and then flows through distributor fins  170 ′ to be directed to finned passage  171  for discharge from discharge header  173  as second subsidiary stream  128 . Fins  165 ″ and  170 ′ have a height which is approximately half of the main passage height. They are placed on top of one another with a dividing plate in between. In this way the inlet and outlet distribution can be achieved in a smaller volume, although the pressure drop incurred will be higher (as a result of reducing the flow area by half). 
         [0065]    While the invention has been described with reference to a preferred embodiment, as will occur to those skilled in the art, numerous changes and additions can be made without departing from the spirit and the scope of the present invention as recited in the appended claims.