Abstract:
A method for the production of air gases with variable liquid production by the cryogenic separation of air can include the steps of sending a purified and compressed air stream to a cold box under conditions effective for cryogenically separating the air stream into an oxygen product and nitrogen using a system of columns, wherein the purified and compressed air stream is at a feed pressure when entering the system of columns; withdrawing the oxygen product at a product pressure; delivering the oxygen product at a delivery pressure to an oxygen pipeline, wherein the oxygen pipeline has a pipeline pressure; wherein during the second mode of operation, the method can include monitoring the pipeline pressure; reducing the difference between the pipeline pressure and the delivery pressure; and adjusting liquid production from the cold box. By operating the method in a dynamic fashion, additional liquid production can be realized in instances in which the pipeline pressure deviates from its highest value.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application Ser. No. 62/356,972 filed on Jun. 30, 2016, which is hereby incorporated by reference in its entirety. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    The present invention generally relates to a method and apparatus for efficiently operating an air separation plant that feeds at least one of its products to a pipeline. 
       BACKGROUND OF THE INVENTION 
       [0003]    Air separation plants separate atmospheric air into its primary constituents: nitrogen and oxygen, and occasionally argon, xenon and krypton. These gases are sometimes referred to as air gases. 
         [0004]    A typical cryogenic air separation process can include the following steps: (1) filtering the air in order to remove large particulates that might damage the main air compressor; (2) compressing the pre-filtered air in the main air compressor and using interstage cooling to condense some of the water out of the compressed air; (3) passing the compressed air stream through a front-end-purification unit to remove residual water and carbon dioxide; (4) cooling the purified air in a heat exchanger by indirect heat exchange against process streams from the cryogenic distillation column; (5) expanding at least a portion of the cold air to provide refrigeration for the system; (6) introducing the cold air into the distillation column for rectification therein; (7) collecting nitrogen from the top of the column (typically as a gas) and collecting oxygen from the bottom of the column as a liquid. 
         [0005]    In certain cases, the air separation unit (“ASU”) can be used to supply one of its air gases to a nearby pipeline (e.g., an oxygen or nitrogen pipeline) in order to supply one or more customers that are not located immediately near the ASU. In a typical ASU supplying a local pipeline, it is common to use a process configuration utilizing an internal compression (pumping) cycle, which in the case of an oxygen pipeline, means that the liquid oxygen produced from the lower pressure column is pumped from low pressure to a higher pressure than that of the pipeline and vaporized within the heat exchanger, most commonly against a high pressure air stream coming from a booster air compressor (“BAC”) or from the main air compressor (“MAC”). As used herein, a booster air compressor is a secondary air compressor that is located downstream of the purification unit that is used to boost a portion of the main air feed for purposes of efficiently vaporizing the product liquid oxygen stream. 
         [0006]    Under normal conditions, the ASU feeding oxygen to the oxygen pipeline is designed to produce oxygen at a constant pressure. This is because ASUs operate most efficiently at steady state conditions. However, pipelines do not operate at constant pressures. For example, it is not uncommon for an oxygen pipeline to operate between 400 and 600 psig (i.e., about a 200 psig pressure variance) during a single day. This can occur due to variable customer demand and/or variable supply to the pipeline. 
         [0007]    In the prior art known heretofore, it is customary to design the ASU to provide the oxygen gas at a constant pressure that is above the highest pressures expected for the pipeline. In order to address the problem associated with pipeline pressure variance, it is customary to let down the pressure of the gaseous oxygen across a control valve to approximately match the pressure of the pipeline just prior to introducing the oxygen gas to the pipeline. However, this method suffers from inefficiencies anytime the pipeline pressure is below that of the design pressure of the ASU. Therefore, it would be advantageous to provide a method and apparatus that operated in a more efficient manner. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention is directed to a method and apparatus that satisfies at least one of these needs. 
         [0009]    In one embodiment, the invention can include a method for adjusting the production pressure(s) of the air gases (e.g., nitrogen and oxygen) to follow the pressure of the pipeline, thereby increasing liquid production when the pipeline pressure decreases. 
         [0010]    In one embodiment, this inefficiency can be minimized by designing the equipments used in the ASU (e.g., main heat exchanger, liquid oxygen (“LOX”) pump, BAC, MAC, etc. . . . ) to have sufficient flexibility for being able to deliver gaseous oxygen (“GOX”) at different pressure levels based on the pipeline pressure. In another embodiment, the method and apparatus can include a process control strategy to automatically and continuously adjust the GOX product pressure coming out of the main heat exchanger to follow the pipeline pressure. 
         [0011]    In another embodiment, as the GOX product pressure can be adjusted to match the oxygen pipeline, the discharge pressure of the BAC can be kept relatively constant, thereby allowing for additional liquid production. Those skilled in the art will also recognize that if the unit does not use a BAC, then the discharge pressure of the MAC can be kept relatively constant in a similar fashion. 
         [0012]    In certain embodiments of the invention, this inefficiency is eliminated by designing the equipments including main exchanger, LOX Pump, MAC, and BAC, etc. with sufficient flexibility of being able to deliver GOX at different pressure levels according to the pipeline pressure and by implementing a process control strategy to automatically and continuously adjust the GOX product pressure to follow the pipeline pressure. In this particular implementation, the automatic pipeline GOX feed valve can be set at 100% open and GOX flow can be controlled by a flow indicator controller (“FIC”) manipulating the LOX pump speed. The lower the GOX pipeline at the delivering point, the lower the GOX pressure from the cold box. 
         [0013]    One efficiency gain that can be realized by lowering the GOX product pressure coming from the cold box is to increase the production of liquid product, either liquid oxygen (“LOX”) and/or liquid nitrogen (“LIN”) without changing the set point of the operating conditions of the MAC or the BAC. Additional liquid production is realized by an overall increase in available refrigeration. For example, by running the LOX pump at a reduced pressure, the LOX pump will produce less excess heat (due to energy losses associated with vibration, friction, etc. . . . ). Additionally, a lowered pressure for the LOX results in less heat of compression. Thirdly, the lower pressure LOX going through the heat exchanger results in a smaller heat loss within the heat exchanger, which results in a gain of additional cold recovery. All three of these factors help to contribute to additional available refrigeration, thereby allowing for an increased liquid production (e.g., liquid nitrogen and/or liquid oxygen). Notably, this increased refrigeration does not require any additional compression or expansion steps, and therefore, the additional liquid production is accomplished without the typical increase in power usage. 
         [0014]    For example, a 1500 st/d O 2  ASU producing GOX at 600 psig can produce about 4150 scfh additional liquid nitrogen when the oxygen product from the liquid oxygen pump is reduced to 450 psig. The overall stability of an ASU process will not be compromised by this pressure variation due to the fact that ASU process usually has faster dynamics than the pipeline and the pipeline often intrinsically contains a large buffer and pressure variation can only occur slowly. 
         [0015]    Although certain embodiments of the invention have only been described for a GOX product sent to an oxygen pipeline, the concept can easily be applied to any product such as high pressure gaseous nitrogen (GAN) that is produced by internal compression process. The control strategy can easily be implemented using any alternative control scheme that can allow GOX and/or GAN pressure to automatically follow the pipeline. For example, the ASU product pressure can be adjusted to follow the pipeline by controlling the pressure differential across the product control valve to the pipeline. For example, instead of directly measuring the pressure of the gaseous product coming from the cold box, the user can measure the pressure drop across the product control valve, and use the control means to derive a desired set point for the pressure drop across the control valve by adjusting the pressure of the gas coming out of the cold box (e.g., if GOX is the product stream, the liquid oxygen pump could be adjusted until the pressure drop across the product control valve is at or below a desired threshold). 
         [0016]    In one embodiment, the pressure differential across the product control valve is less than 5 psi, more preferably less than 3 psi, more preferably less than 1 psi. In another embodiment, the ASU product pressure is within 5 psi of the pipeline pressure, thereby allowing the product control valve to remain fully open, resulting in a minimal pressure loss across the product control valve. In another embodiment, the pressure differential across the product control valve is less than 2%, preferably 1%, more preferably 0.5% of the pipeline pressure. Ideally, the pressure drop across the product control valve approaches zero. 
         [0017]    In one embodiment, a method for the production of air gases with variable liquid production by the cryogenic separation of air can include the steps of: a) compressing air to a pressure suitable for the cryogenic rectification of air to produce a compressed humid air stream, the compressed humid air stream having a first pressure P o ; b) purifying the compressed humid air stream of water and carbon dioxide within a front end purification system to produce a dry air stream having reduced amounts of water and carbon dioxide as compared to the compressed humid air stream; c) compressing a first portion of the dry air stream in a booster compressor to form a boosted air stream, the boosted air stream having a first boosted pressure P B1 ; d) introducing a second portion of the dry air stream and the boosted air stream to a cold box under conditions effective to separate air to form an air gas product, wherein the air gas product is selected from the group consisting of oxygen, nitrogen, and combinations thereof; e) withdrawing the air gas product from the cold box, the air gas product having a first product pressure P P1 ; f) introducing the air gas product to a pipeline, wherein the pipeline is configured to transport the air gas product to a location located downstream of the pipeline, wherein the pipeline operates at a pipeline pressure P PL , wherein the air gas product is introduced to the pipeline at a first delivery pressure P D1 ; g) monitoring the pipeline pressure P PL  within the pipeline; h) adjusting one or more pressure set points within the cold box based on the pipeline pressure P PL ; and i) adjusting liquid production from the cold box based on the one or more pressure set points adjusted in step h). 
         [0018]    In optional embodiments of the method for the production of air gases with variable liquid production by the cryogenic separation of air:
       the one or more pressure set points of step h) is the first product pressure P P1 ;   the first boosted pressure PB 1  is kept substantially constant during steps h) and i);   the cold box comprises a main heat exchanger, a system of columns having a double column composed of a lower pressure column and a higher pressure column, a condenser disposed at a bottom portion of the lower pressure column, and a liquid air gas pump selected from the group consisting of a liquid oxygen pump, a liquid nitrogen pump, and combinations thereof;   the air gas product is oxygen and the pipeline is an oxygen pipeline;   the liquid oxygen pump pressurizes liquid oxygen from the lower pressure column to the first product pressure P P1 ;   the air gas product is nitrogen and the pipeline is a nitrogen pipeline;   the liquid nitrogen pump pressurizes liquid nitrogen from the higher pressure column to the first product pressure P P1 ;   the first product pressure P P1  is adjusted based upon the monitored pipeline pressure P PL .   the liquid production from the cold box is a liquid selected from the group consisting of liquid nitrogen, liquid oxygen, and combinations thereof; and/or   a process controller, in communication with a plurality of flow indicators, pressure indicators, and control valves, is configured to perform steps g) through i);       
 
         [0029]    In another aspect of the invention, a method for the production of air gases with variable liquid production by the cryogenic separation of air can include a first mode of operation and a second mode of operation, wherein during the first mode of operation and the second mode of operation, the method can include the steps of: sending a purified and compressed air stream to a cold box under conditions effective for cryogenically separating the air stream to form a gaseous air gas product using a system of columns, wherein the purified and compressed air stream is at a feed pressure P F  when entering the cold box, wherein the air gas product is selected from the group consisting of oxygen, nitrogen, and combinations thereof; withdrawing the gaseous air gas product at a product pressure P PO ; delivering the gaseous air gas product at a delivery pressure P DO  to an air gas pipeline, wherein the air gas pipeline has a pipeline pressure P PL ; monitoring the pipeline pressure P PL ; wherein during the second mode of operation, the method further can include the steps of: reducing the difference between the pipeline pressure P PL  and the delivery pressure P DO  and adjusting liquid production from the cold box. 
         [0030]    In optional embodiments of the method for the production of air gases with variable liquid production by the cryogenic separation of air:
       the step of reducing difference between the pipeline pressure P PL  and the delivery pressure P DO  further comprises adjusting the product pressure P PO ;   the step of adjusting liquid production from the cold box further comprises the step of maintaining the feed pressure P F  substantially constant;   the product pressure P PO  and the delivery pressure P DO  are substantially the same;   the air gas product is oxygen, wherein the cold box comprises a main heat exchanger, a system of columns having a double column composed of a lower pressure column and a higher pressure column, a condenser disposed at a bottom portion of the lower pressure column, and a liquid oxygen pump;   the cold box further comprises a gaseous oxygen (GOX) feed valve, wherein the GOX feed valve is in fluid communication with an outlet of the liquid oxygen pump and an inlet of the air gas pipeline;   the step of reducing the difference between the pipeline pressure P PL  and the delivery pressure P DO  comprises an absence of adjusting the GOX feed valve;   the step of reducing the difference between the pipeline pressure P PL  and the delivery pressure P DO  includes maintaining the GOX feed valve fully open;   the method may also include the step of providing a main air compressor upstream the cold box, wherein the step of reducing difference between the pipeline pressure P PL  and the delivery pressure P DO  further comprises the step of adjusting the operation of the liquid oxygen pump while maintaining the operation of the main air compressor substantially constant, such that the product pressure P PO  is adjusted while keeping the feed pressure P F  substantially constant;   the method may also include the step of providing a booster compressor downstream a main air compressor and upstream the cold box, wherein the step of reducing difference between the pipeline pressure P PL  and the delivery pressure P DO  further comprises the step of adjusting the operation of the liquid oxygen pump while maintaining the operation of the booster compressor substantially constant, such that the product pressure P PO  is adjusted while keeping the feed pressure P F  substantially constant;   the liquid production from the cold box is a liquid selected from the group consisting of liquid nitrogen, liquid oxygen, and combinations thereof; and/or   a process controller, in communication with a plurality of flow indicators, pressure indicators, and control valves, is configured to perform the steps in the second mode of operation.       
 
         [0042]    In another aspect of the invention, the apparatus for the production of air gases with variable liquid production by the cryogenic separation of air can include 
         [0043]    a) a main air compressor configured to compress air to a pressure suitable for the cryogenic rectification of air to produce a compressed humid air stream, the compressed humid air stream having a first pressure P o ; 
         [0044]    b) a front end purification system configured to purify the compressed humid air stream of water and carbon dioxide to produce a dry air stream having reduced amounts of water and carbon dioxide as compared to the compressed humid air stream; 
         [0045]    c) a booster compressor in fluid communication with the front end purification system, wherein the booster compressor is configured to compress a first portion of the dry air stream to form a boosted air stream, the boosted air stream having a first boosted pressure P B1 ; 
         [0046]    d) a cold box comprising a main heat exchanger, a system of columns having a double column composed of a lower pressure column and a higher pressure column, a condenser disposed at a bottom portion of the lower pressure column, and a liquid oxygen pump, wherein the cold box is configured to receive the boosted air stream and a second portion of the dry air stream under conditions effective to separate air to form an air gas product, wherein the air gas product is selected from the group consisting of oxygen, nitrogen, and combinations thereof; 
         [0047]    e) means for monitoring the pressure of a pipeline, wherein the pipeline is in fluid communication with the cold box, such that the pipeline is configured to receive the air gas product from the cold box, the air gas product having a first product pressure P P1 ; 
         [0048]    f) means for adjusting one or more pressure set points of the apparatus based on the monitored pipeline pressure, wherein the one or more pressure set points of the apparatus comprise a discharge pressure of the liquid oxygen pump; and 
         [0049]    g) means for adjusting liquid production from the cold box. 
         [0050]    In optional embodiments of the apparatus for the production of air gases by the cryogenic separation of air:
       the first product pressure P P1  is adjusted such that the difference between the first product pressure P P1  and the first delivery pressure P D1  is below a given threshold;   the threshold is less than 5 psi;   the threshold is less than 3 psi;   the liquid production from the cold box is a liquid selected from the group consisting of liquid nitrogen, liquid oxygen, and combinations thereof;   means for adjusting liquid production from the cold box comprises a process controller, in communication with a plurality of flow indicators, pressure indicators, and control valves;   the air gas product is oxygen and the pipeline is an oxygen pipeline;   the liquid oxygen pump pressurizes liquid oxygen from the lower pressure column to the first product pressure P P1 ;   the air gas product is nitrogen and the pipeline is a nitrogen pipeline.       
 
         [0059]    In another aspect of the invention, the apparatus for the production of air gases with variable liquid production by the cryogenic separation of air can include: (1) a cold box configured to receive a purified and compressed air stream under conditions effective for cryogenically separating the air stream to form an air gas product using a system of columns, wherein the purified and compressed air stream is at a feed pressure P F  when entering the cold box, wherein the air gas product is selected from the group consisting of oxygen, nitrogen, and combinations thereof, wherein the cold box is configured to produce the air gas product at a product pressure P PO ; (2) means for transferring the air gas product from the cold box to an air gas pipeline; (3) a pressure monitoring device configured to monitor the pipeline pressure P PL ; and (4) a controller configured to adjust the product pressure P PO  of the air gas product coming out of the cold box based upon the pipeline pressure P PL , wherein the controller is also configured to adjust liquid production from the cold box based on the product pressure P PO  of the air gas product coming out of the cold box. 
         [0060]    In optional embodiments of the apparatus for the production of air gases by the cryogenic separation of air:
       the air gas product is oxygen, wherein the cold box comprises a main heat exchanger, a system of columns having a double column composed of a lower pressure column and a higher pressure column, a condenser disposed at a bottom portion of the lower pressure column, and a liquid air gas pump selected from the group consisting of a liquid oxygen pump, a liquid nitrogen pump, and combinations thereof   the controller is configured to communicate with the liquid oxygen pump and adjust a discharge pressure of the liquid oxygen pump.   the product pressure P PO  and the delivery pressure P DO  are substantially the same.   the controller is in communication with the pressure monitoring device.   the apparatus may also include means for withdrawing liquid oxygen from the cold box.   the apparatus may also include means for withdrawing liquid nitrogen from the cold box.   the apparatus may also include a gaseous oxygen (“GOX”) feed valve, wherein the GOX feed valve is in fluid communication with an outlet of the liquid oxygen pump and an inlet of the air gas pipeline, wherein the controller is configured to maintain the GOX feed valve in a fully open position regardless of the pipeline pressure P PL .   the apparatus may also include a main air compressor disposed upstream the cold box, wherein the controller is further configured to adjust the operation of the liquid oxygen pump while maintaining the operation of the main air compressor substantially constant, such that the product pressure P PO  is adjusted while keeping the feed pressure P F  substantially constant.   the apparatus may also include a booster compressor downstream a main air compressor and upstream the cold box, wherein the controller is further configured to adjust the operation of the liquid oxygen pump while maintaining the operation of the booster compressor substantially constant, such that the product pressure P PO  is adjusted while keeping the feed pressure P F  substantially constant.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0070]    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. 
           [0071]      FIG. 1  provides an embodiment of the present invention. 
           [0072]      FIG. 2  provides another embodiment of the present invention. 
           [0073]      FIG. 3  provides a graphical representation of simulation data showing increases in liquid production as a function of gaseous oxygen product pressure. 
       
    
    
     DETAILED DESCRIPTION 
       [0074]    While the invention will be described in connection with several embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all the alternatives, modifications and equivalence as may be included within the spirit and scope of the invention defined by the appended claims. 
         [0075]    Now turning to  FIG. 1 . Air  2  is introduced into main air compressor  10  and compressed, preferably to a pressure of at least 55 psig to 75 psig (or around 5 psig higher than the pressure of the MP column). The resulting compressed humid air stream  12  is then purified of water and CO 2  in front end purification system  20 , thereby producing dry air stream  22 . In one embodiment, all of dry air stream  22  passes via line  26  into cold box  40 . Within cold box  40 , the air is cooled and cryogenically treated in order to separate the air into air gas product  42 . Air gas product  42  is then removed from cold box  40  and passed through product control valve  50  before entering air gas pipeline  60 . 
         [0076]    In a preferred embodiment, the pressure and flow rate of air gas product  42  can be measured by second pressure indicator PI 2  and flow indicator FI 1 , respectively. The pressure of air gas pipeline  60  can be measured by pressure indicator PI 3 . First liquid air gas product  44  and/or second liquid air gas product  48  can also be removed from cold box  40  in certain modes of operation. The flow rate of first liquid air gas product  44  can be measured by flow indicator FI 2 , and the flow rate of second liquid air gas product  48  can be measured by flow indicator FI 3 . In the embodiment shown, control valves  46 ,  47  can be used to control the flow rates of fluids  44 ,  48 . 
         [0077]    In one embodiment, the various pressure and flow indicators/sensors are configured to communicate (e.g., wirelessly or wired communication) with process controller  55 , such that the various flow rates and pressures can be monitored by process controller  55 , which is configured to adjust various settings throughout the process based on the measured flows and pressures. 
         [0078]    Additionally, an embodiment of the present invention may also include booster air compressor  30 . This embodiment is represented by dashed lines, since it is an optional embodiment. In this embodiment, a portion of dry air stream  22  is sent to booster air compressor  30  via line  24  and further compressed to form boosted air stream  32  before being introduced to cold box  40 . While the embodiment of  FIG. 1  shows booster air compressor  30  as a single compressor, those of ordinary skill in the art will recognize that booster air compressor  30  can be more than one physical compressor. Additionally, booster air compressor  30  can also be a multi-stage compressor. 
         [0079]    While the figures show direct lines of communication from the various pressure and flow indicators to the process controller  55 , embodiments of the invention should not be so limited. Rather, those of ordinary skill in the art will recognize that embodiments of the invention may include instances in which certain indicators communicate directly with a related pressure controller. 
         [0080]      FIG. 2  provides a more detailed view of cold box  40  for the optional embodiment that includes booster air compressor  30 . In this embodiment, cold box  40  also includes heat exchanger  80 , turbine  90 , valve  100 , double column  110 , higher pressure column  120 , auxiliary heat exchanger  130 , lower pressure column  140 , condenser/reboiler  150 , and liquid oxygen pump  160 . Turbine  90  can be attached to booster  70  via a common shaft. Just like in  FIG. 1 , air  2  is introduced into main air compressor  10  and compressed, preferably to a pressure of at least 55 psig to 75 psig (or around 5 psig higher than the pressure of the MP column). The resulting compressed humid air stream  12  is then purified of water and CO 2  in front end purification system  20 , thereby producing dry air stream  22 . A first portion of dry air stream  24  is sent to booster air compressor  30 , with the remaining portion of dry air stream  26  entering cold box  40 , wherein it is fully cooled in heat exchanger  80  before being introduced to higher pressure column  120  for separation therein. Following pressurization in booster air compressor  30 , boosted air stream  32  is preferably fully cooled in heat exchanger  80  and then expanded across valve  100 , before being introduced into a bottom portion of higher pressure column  120 . 
         [0081]    Partially boosted air stream  37  is preferably removed from an inner stage of booster air compressor  30  before being further compressed in booster  70  and then cooled in after cooler  75  to form second boosted stream  72 . Second boosted stream  72  undergoes partial cooling in heat exchanger  80 , wherein it is withdrawn from an intermediate section of heat exchanger  80  and then expanded in turbine  90  thereby forming expanded air stream  92 , which can then be combined with second portion of dry air stream  26  before introduction to higher pressure column  120 . 
         [0082]    Higher pressure column  120  is configured to allow for rectification of air within, thereby producing an oxygen-rich liquid at the bottom and a nitrogen-rich gaseous stream at the top. Oxygen-rich liquid  122  is withdrawn from the bottom of higher pressure column  120  before exchanging heat with low pressure waste nitrogen  114  and low pressure nitrogen product  112  in auxiliary heat exchanger  130 , and then expanded across a valve and introduced into lower pressure column  140 . As is well known in the art, higher pressure column  120  and lower pressure column  140  are part of double column  110 , and the two columns are thermally coupled via condenser/reboiler  150 , which condenses rising nitrogen rich gas from higher pressure column  120  and vaporizes liquid oxygen that has collected at the bottom of lower pressure column  140 . In the embodiment shown, two nitrogen-rich liquid streams  126 ,  128  are withdrawn from higher pressure column  120 , exchange heat with low pressure nitrogen product  112  and low pressure waste nitrogen  114 , subsequently expanded across their respective valves, and then introduced into lower pressure column  140 . Medium pressure nitrogen product  129  can also be withdrawn from higher pressure column  120  and then warmed in heat exchanger  80 . 
         [0083]    Liquid oxygen collects at the bottom of lower pressure column  140  and is withdrawn and pressurized to an appropriate pressure by liquid oxygen pump  160  to form liquid oxygen  162 . Liquid oxygen  162  is then vaporized within heat exchanger  80  to form air gas product  42 . The pressure and flow rate of air gas product  42  can be measured via second pressure sensor PI 2  and FI 1 , respectively. As in  FIG. 1 , air gas product  42  flows across product control valve  50  and into air gas pipeline  60 . Liquid oxygen product  44  from liquid oxygen pump  160  is delivered to the storage (not shown). Liquid nitrogen product  48  from top of lower pressure column  140  is delivered to the storage (not shown). Those of ordinary skill in the art will recognize that both product LOX and LIN can be produced as high pressure at the discharge of the pump or low pressure from the columns either in the HP or LP column for LIN or from the bottom of the LP column for LOX. 
         [0084]    As noted previously, the pressure of air gas pipeline  60  tends to drift over time. In methods known heretofore, this problem was solved by adjusting the openness of product control valve  50  to create the appropriate pressure drop. However, there are inefficiencies in doing this. Instead, embodiments of the present invention can adjust the pressure set points within the cold box, for example, the discharge pressure of liquid oxygen pump  160 . By reducing this pressure an appropriate amount, product control valve  50  can be left fully open, thereby resulting in minimal expansion losses across product control valve  50 . In one embodiment, the appropriate amount yields a difference between PI 2  and PI 3  to be less than 5 psi, preferably less than 3 psi. 
         [0085]    By reducing the pressure of liquid oxygen product  162  and keeping the pressure of the incoming air streams at the same pressure set points (e.g., BAC and MAC maintained at constant set points), additional liquid production can be achieved. For example, for an ASU process that is built to produce gaseous oxygen at 610 psig (e.g., stream  42 ), approximately 51 kscfh LOX and 91 kscfh LIN can be produced. However, this same process can produce approximately 57 kscfh more LIN or 54 kscfh more LOX if the discharge pressure of the LOX pump is reduced to produce a gaseous oxygen product stream at approximately 400 psig. 
         [0086]    Tables I-III below show comparative data for various streams with Table I being a base case at 610 psig GOX production, Table II being an embodiment in which LIN production was maximized with GOX production being at 400 psig, and Table III being an embodiment in which LOX production was maximized with GOX production also being at 400 psig. While these examples only show LIN and LOX production being maximized, respectively, those of ordinary skill in the art will recognize that embodiments of the invention are not so limited. Rather, embodiments of the invention can also include instances in which both LOX and LIN production could be both increased at the same time. Those of ordinary skill in the art will recognize that in these embodiments, the increase for each LIN or LOX will not be as much individually as is shown in Table II or Table III. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                 610 psig GOX 
               
             
          
           
               
                   
                   
                 Flow 
                 Pressure 
                 Temp 
               
               
                   
                 Stream # 
                 (kscfh) 
                 (psig) 
                 (° F.) 
               
               
                   
                   
               
             
          
           
               
                   
                 2 
                 8073 
                 0 
                 72 
               
               
                   
                 12 
                 8073 
                 71 
                 87 
               
               
                   
                 24 
                 4010 
                 69 
                 64 
               
               
                   
                 26 
                 3329 
                 69 
                 64 
               
               
                   
                 32 
                 2663 
                 928 
                 87 
               
               
                   
                 37 
                 1347 
                 515 
                 87 
               
               
                   
                 42 
                 1487 
                 610 
                 74 
               
               
                   
                 44 
                 51 
                 30 
                 −297 
               
               
                   
                 48 
                 91 
                 6 
                 −315 
               
               
                   
                 72 
                 1347 
                 790 
                 87 
               
               
                   
                 92 
                 1347 
                 66 
                 −281 
               
               
                   
                 94 
                 4676 
                 66 
                 −248 
               
               
                   
                 162 
                 1487 
                 614 
                 −287 
               
               
                   
                 MP Col 
                 — 
                 66 
                 — 
               
               
                   
                 LP Col 
                 — 
                 6 
                 — 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE II 
               
             
             
               
                   
               
               
                 400 psig GOX -- LIN Production 
               
             
          
           
               
                   
                   
                 Flow 
                 Pressure 
                 Temp 
               
               
                   
                 Stream # 
                 (kscfh) 
                 (psig) 
                 (° F.) 
               
               
                   
                   
               
             
          
           
               
                   
                 2 
                 8073 
                 0 
                 72 
               
               
                   
                 12 
                 8073 
                 71 
                 87 
               
               
                   
                 24 
                 4010 
                 69 
                 64 
               
               
                   
                 26 
                 3329 
                 69 
                 64 
               
               
                   
                 32 
                 2663 
                 928 
                 87 
               
               
                   
                 37 
                 1347 
                 515 
                 87 
               
               
                   
                 42 
                 1487 
                 400 
                 75 
               
               
                   
                 44 
                 51 
                 30 
                 −297 
               
               
                   
                 48 
                 148 
                 6 
                 −315 
               
               
                   
                 72 
                 1347 
                 826 
                 87 
               
               
                   
                 92 
                 1347 
                 66 
                 −281 
               
               
                   
                 94 
                 4676 
                 66 
                 −252 
               
               
                   
                 162 
                 1487 
                 404 
                 −289 
               
               
                   
                 MP Col 
                 — 
                 66 
                 — 
               
               
                   
                 LP Col 
                 — 
                 6 
                 — 
               
               
                   
                   
               
             
          
         
       
     
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE III 
               
             
             
               
                   
               
               
                 400 psig GOX -- GOX 
               
             
          
           
               
                   
                   
                 Flow 
                 Pressure 
                 Temp 
               
               
                   
                 Stream # 
                 (kscfh) 
                 (psig) 
                 (° F.) 
               
               
                   
                   
               
             
          
           
               
                   
                 2 
                 8073 
                 0 
                 72 
               
               
                   
                 12 
                 8073 
                 71 
                 87 
               
               
                   
                 24 
                 4010 
                 69 
                 64 
               
               
                   
                 26 
                 3329 
                 69 
                 64 
               
               
                   
                 32 
                 2663 
                 928 
                 87 
               
               
                   
                 37 
                 1347 
                 515 
                 87 
               
               
                   
                 42 
                 1433 
                 400 
                 75 
               
               
                   
                 44 
                 105 
                 30 
                 −297 
               
               
                   
                 48 
                 91 
                 6 
                 −315 
               
               
                   
                 72 
                 1347 
                 826 
                 87 
               
               
                   
                 92 
                 1347 
                 66 
                 −281 
               
               
                   
                 94 
                 4676 
                 66 
                 −248 
               
               
                   
                 162 
                 1433 
                 404 
                 −289 
               
               
                   
                 MP Col 
                 — 
                 66 
                 — 
               
               
                   
                 LP Col 
                 — 
                 6 
                 — 
               
               
                   
                   
               
             
          
         
       
     
         [0087]    As is shown in the tables above, when the pipeline pressure changes, the pressure of stream  42  is adjusted to match the pipeline pressure and the flow rates of streams  44  or  48  are changed. The remaining streams remain largely unchanged. As will be readily appreciated, being able to produce additional amounts of liquid can be highly beneficial, particularly since liquid streams are at a premium on the market. Furthermore, this is accomplished without any loss of production in terms of flow rate, without any significant upset to the operating conditions of the double column, and with minimal additional capital expenses. 
         [0088]    In an embodiment in which the air gas product is nitrogen, the embodiment may include withdrawing higher pressure nitrogen product  129  as a liquid from higher pressure column  120 , and pressurizing it to an appropriate pressure using a liquid nitrogen pump (not shown) before warming in heat exchanger  80 . The resultant warmed nitrogen gas product would then be introduced to a nitrogen pipeline in similar manner as described with respect to the gaseous oxygen product. Alternatively, a liquid nitrogen stream can be removed from the lower pressure column instead of the higher pressure column. 
         [0089]      FIG. 3  presents a graphical representation of liquid production as a function of pressure of the air gas product (e.g., stream  42 ). As shown in the example, going from a pressure of about 650 psig to 400 psig can yield an almost two fold increase in LIN production (went from about 80 to about 150 kscfh). Similarly, liquid oxygen production was increased from around 40 to about 105 kscfh. While the graphical representation was developed with the assumption that only one of the liquid products was being adjusted at a time, the invention is not intended to be so limited. In fact, it is perfectly acceptable to increase both liquid products at the same time. 
         [0090]    In another embodiment, process controller  55  can be configured to access spot pricing data (or the user can input data into the controller), such that process controller  55  can be configured to optimize/adjust the amount of increased LIN and/or LOX based upon the current spot pricing data. Similarly, process controller  55  can also be configured to keep track of local inventories of LIN and/or LOX, and make adjustments to the production of LIN and/or LOX based on this additional data. 
         [0091]    The terms “nitrogen-rich” and “oxygen-rich” will be understood by those skilled in the art to be in reference to the composition of air. As such, nitrogen-rich encompasses a fluid having a nitrogen content greater than that of air. Similarly, oxygen-rich encompasses a fluid having an oxygen content greater than that of air. 
         [0092]    As used herein, if the pressure of a stream is kept substantially constant, that is intended to mean that the pressure set point for the process equipment that affects said stream pressure remains unchanged. Normal variance due to typical process conditions is meant to be encompassed by this term. 
         [0093]    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. 
         [0094]    The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise. 
         [0095]    “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. 
         [0096]    “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. 
         [0097]    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. 
         [0098]    Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range. 
         [0099]    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.