Abstract:
A cryogenic process for the production of oxygen by cryogenic distillation using an air separation unit comprising a double column, said double column comprising a high pressure column and a low pressure column comprising the steps of: sending compressed, cooled and purified air to the high pressure column in gaseous form, sending oxygen enriched fluid from the bottom of the high pressure column to the low pressure column, removing nitrogen enriched gas from the top of the high pressure column, sending a first portion of nitrogen enriched gas to a lower reboiler in the low pressure column following compression in a cold compressor having a cryogenic inlet temperature, sending a second portion of nitrogen enriched gas to an upper reboiler in the low pressure column, sending nitrogen enriched liquid from at least one of the first and second reboilers to at least one of the high pressure column and the low pressure column, expanding a stream of nitrogen enriched gas from the high pressure column in an expander following a warming step, driving the cold compressor using a motor and producing oxygen rich gas from the low pressure column by at least one of the steps of removing a gaseous stream and by vaporizing a liquid stream.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a cryogenic air separation process and apparatus. 
         [0002]    All pressures listed in the document are absolute pressures. 
         [0003]    Because of the global warming effect caused by the increased release of CO 2  (carbon dioxide) generated by combustion processes, efforts have been made by utility companies and governments worldwide to reduce and minimize the CO 2  emission. One major source of CO 2  emission is the power generation plant&#39;s combustion process. There are mainly two types of power plants based on combustion processes: coal combustion and natural gas combustion. Both of these processes produce CO 2  when generating power. The most efficient approach to reduce or minimize the CO 2  emission is to capture most of the CO 2  emitted by the power plants. For this effort to be efficient, it must also target the existing coal combustion plants that represent a large portion of the power generation plants worldwide. The oxy-combustion technique is quite advantageous since it can be adapted to existing facilities as well. 
         [0004]    A traditional power plant uses air as the source of oxidant (oxygen) to combust the coal. Steam is generated by heating boiler feed water with the hot combustion products (steam, CO 2 , N 2 , excess air). The steam is then expanded in steam turbines to produce power. The combustion produces CO 2  as a by-product, which, along with other gases such as residual nitrogen of combustion air, moisture, etc., constitutes the flue gas of the combustion. Due to the high content of nitrogen in air (78 mol %), the CO 2  is very much diluted in the flue gas. To insure full combustion, the power plants must also run with an excess air ratio, which further dilutes the CO 2 . The concentration of CO 2  in the flue gas of an air combustion plant is only about 10-20 mol %. The diluted composition of CO 2  increases the size and the power consumption of the CO 2  recovery unit. Because of this dilution, it becomes very costly and difficult to recover and capture the CO 2  especially with the low pressure of the flue gas. Therefore, it is desirable to produce more concentrated CO 2  in flue gas, about 95 mol % of CO 2  purity is preferred, to minimize the abatement cost. An alternative technology for CO 2  recovery from flue gas utilizes an amine contact tower to scrub out the CO 2 . However, significant amount of heat is needed to regenerate the amine and to extract the CO 2  such that the amine process is not cost effective. 
         [0005]    In order to avoid the dilution of CO 2  in N 2 , the power generation industry can switch to an oxy-combustion process: instead of utilizing air as oxidant, pure oxygen of 95% purity or better is used in the combustion process. The combustion heat is dissipated in a recycled flue gas concentrated in CO 2 . By doing so, since there is very small amount or almost no nitrogen in the system, it becomes possible to achieve a flue gas containing about 75-95 mol % CO 2  which is a significant improvement over the previous 20 mol % of air combustion. The purity of CO 2  in oxy-combustion&#39;s flue gas depends on the amount of air leakage into the system and the purity of oxygen being utilized. An air separation unit normally supplies the pure oxygen for combustion. The flue gas rich in CO 2  exiting the boiler is cooled and treated to recover the CO 2  for subsequent disposal. 
         [0006]    The potential main users of oxycombustion technology are existing pulverized coal power plants since an oxygen plant, a CO 2  recycle blower and a CO 2  recovery from flue gas can be added to the existing plant to retrofit it so that the converted plant can comply with new CO 2  emission standard. New grass root plants are likely to base on cleaner IGCC technology. 
         [0007]    For retrofitted oxycombustion coal plants, it is clear that the effort to capture CO 2  is hindered by the cost of the oxygen plant. Furthermore, the power consumption of the oxygen plant, which can be about 10% of the power plant output, also introduces additional cost issues: part of the power generated by the power plant must be diverted to supply the oxygen plant. Therefore less power will be available to supply the grid, especially during peak demand when power is scarce and power costs are premium, resulting in reduction of power plant&#39;s revenue. In this situation, the economics of CO 2  capture and disposal by oxycombustion technique depend strongly on the cost and power consumption of the oxygen plant. Without an efficient setup for the oxygen generation, the cost penalty would be such that it would become uneconomical to operate the clean and CO 2 -free oxycombustion power plants. 
         [0008]    In order to optimize the oxygen supply for oxycombustion scheme, several studies conducted by the power industry with the co-operation of various oxygen suppliers have concluded that a low purity oxygen (about 95 mol %) is sufficient for oxycombustion and provides a low cost, low power consumption oxygen plant. This takes into account the impact of the purity of CO 2  in flue gas caused by the oxygen purity on the subsequent CO 2  concentrating and purification equipment. However further cost reduction of the cost of oxygen is needed to improve the economics of the CO 2  capture. 
         [0009]    The Integrated Gasification Combined Cycle (IGCC) is a new highly efficient power plant wherein, instead of performing the direct combustion of coal to generate hot flue gas for steam generation, the coal is subjected to a partial oxidation process in which it is gasified to yield a mixture containing mostly of H 2  and CO called fuel gas. This fuel gas, after being treated to remove various pollutants or corrosive chemicals, is sent to a gas turbine where it is combusted to heat the compressed feed air prior to the expansion. Pure oxygen of about 95% supplied by an air separation unit is used in the partial oxidation reaction of the coal gasifier. Since the CO of the fuel gas is combusted to yield CO 2  in the burner of the gas turbine, the resulting CO 2  is also mixed with the nitrogen of the gas turbine feed air such that the CO 2  recovery is also a costly and difficult task. To avoid difficult CO 2  recovery due to a high flow, low pressure and much diluted CO 2  of the gas turbine, the fuel gas is subjected to a shift conversion wherein the CO reacts with steam to produce H 2  and CO 2 . The CO 2  can then be recovered economically by scrubbing with a solvent like in the Rectisol process. The fuel gas free of CO and CO 2  and containing mostly H 2  and steam is then burned in the gas turbine to yield an almost CO 2 -free exhaust gas. In IGCC facilities, the oxygen plant is usually needed for the partial oxidation portion, regardless of the need of the CO 2  capture. 
         [0010]    It can be seen from the above simple process description that pure oxygen gas, supplied by an air separation plant, is used in either the direct combustion of an oxycombustion or the partial oxidation of an IGCC process. The production of oxygen requires additional capital investment and consumes significant power to drive the compression equipment. It is obvious that the power consumption of the oxygen plant and the oxygen plant cost must be optimized to reduce the impact of CO 2  capture on the final cost of electricity. In addition to the oxygen plant, the CO 2  recovery from flue gas of an oxycombustion or from the fuel gas of an IGCC also consumes power and requires significant investment since the CO 2  must be further concentrated to about 95 mol % and then compressed to about 100 bar or higher for disposal. 
         [0011]    This invention addresses the potential savings in power and cost of an oxygen plant integrated with an oxycombustion power plant for CO 2  capture or an IGCC plant. 
         [0012]    Power plants supply electricity to the grid. It is well known that power demand varies during the day, there are “peak” periods of high demand, hence high power cost, and there are “off-peak” periods of low demand, low power cost. Peaks usually occur in the day time of the week days, for example, from 9 AM to 5 PM. Off-peaks usually take place at night time for example from 9 PM to 5 AM and week ends. There are also some intermediate demands and costs. The duration of peaks and off-peaks also depends on the seasons, the variations of local temperature, the weather changes etc. 
         [0013]    Because of the variable demand, power plants usually run at or near its design capacity during peaks, but must idle at very low output during off-peaks. Off-peak demand can be as low as 15-20% of the rated capacity. Power cost is high during peaks and sometimes the utility companies must purchase additional power from other suppliers to satisfy demand. The situation is reversed for off-peaks: power supply is abundant but demand drops sharply such that the power generating equipment must be turned down to minimum and sometimes shut down. Utility companies encourage users to consume more power during off-peaks to avoid costly equipment shutdown by lowering the power cost sharply for off-peaks. The power cost for peak or high demand periods can be 3 to 5 times higher than the power cost of the off-peak or low demand periods. 
         [0014]    The addition of an oxygen plant to the power generation plant worsens the power cost structure especially for peak periods. Indeed, the oxygen plant must generate maximum oxygen flow to match with the maximum power output; its power consumption is therefore at the highest during peaks. This additional power consumption is quite costly because it deprives the utility companies from having the available kW to sell on the grid at the premium value. As an indication, an oxygen plant consumes as much as 10% of the power output of a power plant. During off-peaks, the power output is at the minimum level; the consumption of the oxygen plant is also at its lowest level and cannot take advantage of the lower power cost. 
         [0015]    Therefore, there exists a need for an oxygen production process capable of tracking economically the demand curve of a power generation plant such that: 
         [0016]    The oxygen plant can minimize its power consumption during peaks while maintaining its supply of oxygen to the power plants at the rated level. This reduction in power consumption will free up more kW for the grid. 
         [0017]    The oxygen plant can maximize its power consumption during off-peaks to take advantage of the lower power cost while remaining capable of supplying oxygen at reduced level. This high power consumption creates a power demand and keeps the power generating equipment running above it minimum rate, thus potentially avoid costly equipment shutdown. 
       BRIEF SUMMARY OF THE INVENTION 
       [0018]    According to this invention, there is provided a cryogenic process for the production ofof supplying oxygen by cryogenic distillation using an air separation unit comprising a double column, said double column comprising a high pressure column and a low pressure column comprising the steps of:
       a) sending compressed, cooled and purified air to the high pressure column in gaseous form;   b) sending oxygen enriched fluid from the bottom of the high pressure column to the low pressure column;   c) removing nitrogen enriched gas from the top of the high pressure column;   d) sending a first portion of nitrogen enriched gas to a lower reboiler in the low pressure column following compression in a cold compressor having a cryogenic inlet temperature;   e) sending a second portion of nitrogen enriched gas to an upper reboiler in the low pressure column;   f) sending nitrogen enriched liquid form at least one of the first and second reboilers to at least one of the high pressure column and the low pressure column;   g) expanding a stream of nitrogen enriched gas from the high pressure column in an expander following a warming step;   h) driving the cold compressor using a motor; and   i) producing oxygen rich gas from the low pressure column by at least one of the steps of removing a gaseous stream and by vaporizing a liquid stream.       
 
         [0028]    Optionally the process comprises:
       a. During a first period:
           i) feeding a first air stream to the air separation unit at a first flow rate;   ii) feeding liquid oxygen from athe liquid oxygen tank to at least one of a column and a heat exchanger of the air separation unit;   iii) recovering a gaseous oxygen stream with a higher flow than the liquid oxygen stream from the air separation unit; and   iv) sending at least one air derived component liquid to at least one air derived component liquid tank.   
           b. During a second period:
           i) feeding the at least one air derived component liquid stream from the at least one air component liquid tank to the air separation unit;   ii) extracting a liquid oxygen stream from a column of the air separation unit to the liquid oxygen tank;   iii) recovering a gaseous oxygen stream from the air separation unit; and   iv) increasing the flowrate of the first air stream, feeding the air separation unit to a value greater than the first flowrate.   
               
 
         [0039]    Optionally 
         [0040]    At least one air derived component liquid is liquid nitrogen and wherein step iv) of period a) of claim  1  comprises removing liquid nitrogen from a column of the air separation unit. 
         [0041]    At least one air derived component liquid contains  80  mol % nitrogen or greater. 
         [0042]    At least one air derived component liquid is liquid air. 
         [0043]    At least one air derived component liquid contains 35 mol % oxygen or greater wherein step iv) of period a) of claim  21  comprises removing liquid nitrogen from a column of the air separation unit. 
         [0044]    The air flow to the air separation unit varies between the first and second period by at most 10%. 
         [0045]    According to a further aspect of the invention, there is provided a cryogenic apparatus for the production of oxygen by cryogenic distillation using an air separation unit comprising at least a double column, said double column comprising a high pressure column and a low pressure column, said low pressure column having a lower and an upper reboiler, said apparatus comprising a cold compressor, a conduit for sending compressed, cooled and purified air to the high pressure column in gaseous form, a conduit for sending oxygen enriched fluid from the bottom of the high pressure column to the low pressure column, a conduit for removing nitrogen enriched gas from the top of the high pressure column, a conduit for sending a first portion of nitrogen enriched gas to the lower reboiler in the low pressure column following compression in the cold compressor having a cryogenic inlet temperature, a conduit for sending a second portion of nitrogen enriched gas to the upper reboiler in the low pressure column, a conduit for sending nitrogen enriched liquid from at least one of the upper and lower reboilers to at least one of the high pressure column and the low pressure column, an expander for expanding a stream of nitrogen enriched gas from the high pressure column following a warming step, said cold compressor being coupled to a motor, a conduit for removing at least one of a gaseous and a liquid stream from the low pressure column to produce an oxygen product. 
         [0046]    A heat exchanger may vaporize liquid oxygen removed from the low pressure column. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0047]      FIG. 1  is a view of one embodiment according to the invention; 
           [0048]      FIG. 2  is another view of one embodiment according to the invention; 
           [0049]      FIG. 3  is yet another view of an embodiment according to the invention; 
           [0050]      FIG. 4  is also a view of one embodiment according to the invention; 
           [0051]      FIG. 5  is another embodiment according to the invention; 
           [0052]      FIG. 6  illustrates another view of one embodiment according to the invention; 
           [0053]      FIG. 7  is also an illustration of one embodiment according to the invention; 
           [0054]      FIG. 8  is a view of one embodiment according to the invention; and 
           [0055]      FIG. 9  is a final view of one embodiment according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0056]    The invention will be described in greater detail with reference to the figures, wherein  FIG. 1  represents the Prior art approach in which the air separation unit simply supplies the oxygen to the power plant.  FIGS. 2 and 3  represent an air separation unit operating according to the invention at different periods,  FIGS. 4 and 5  represent different phases of operation of plant according to the prior art and  FIGS. 6 ,  7  and  8  represent an air separation units capable of operating according to the invention. 
         [0057]    As shown on  FIG. 1  for the Prior art, during peaks in power demand, 1000 Nm3/h of feed air  6  is treated to yield 200 Nm3/h of oxygen  12  required for peak demand by power generation plant  10 . If the demand is reduced, less air is sent to the oxygen plant  13  to yield less oxygen. The air flow is essentially proportional to the oxygen demand. 
         [0058]    As shown on  FIG. 2  for the bascule approach, during peaks in power demand, 110 Nm3/h of liquid oxygen  53  from a liquid oxygen tank  15  is fed to the oxygen plant  13 , vaporized and combined with the oxygen produced from the feed air to yield 200 Nm3/h of a high oxygen stream  12  required for peak demand by the power generation plant  10 . The recovered refrigeration from the vaporization of liquid oxygen is used to liquefy liquid nitrogen  49  and to store it in a liquid nitrogen tank  17 . Since a portion of the oxygen is provided by vaporizing the liquid oxygen, the air flow  6  to the oxygen plant can be reduced by about the same proportion to 450 Nm3/h resulting in significant power reduction while maintaining the total rated flow of oxygen to satisfy the peak demand. 
         [0059]    As shown in  FIG. 3 , during average periods, 900 Nm3/h of feed air  6  are sent from compressor  1  to the oxygen plant  13  to produce 150 Nm3/h of oxygen  45 . 30 Nm3/h of liquid oxygen are sent from the oxygen plant to the oxygen tank  15  whilst liquid nitrogen  49  is sent from the nitrogen tank  17  to the oxygen plant. 
         [0060]    As shown in  FIG. 3 , during off-peaks, instead of being reduced, the air flow  6  is at 900 Nm3/h to produce more oxygen than the demand. 80 Nm3/h of the excess gaseous oxygen is liquefied by feeding and vaporizing the liquid nitrogen  49  produced during peaks. The produced liquid oxygen  53  is then stored in the liquid oxygen tank  15  to restore the oxygen inventory which will be needed in the subsequent peak periods. 
         [0061]    In order to illustrate this concept, a simple model of power demand (or generation rate) can be used: 
         [0062]    Power generation: 
         [0000]    
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                   
                 Cents 
                 Oxygen 
               
               
                   
                   
                 (US$) per 
                 requirement/ 
               
               
                   
                 Power Demand 
                 kWh 
                 Nm 3 /h 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Peak (⅓ of the time) 
                 100% 
                 4.5 
                 200 
               
               
                 Average (⅓ of the time) 
                 75% 
                 3 
                 150 
               
               
                 Off-peak (⅓ of the time) 
                 50% 
                 1.5 
                 100 
               
               
                   
               
             
          
         
       
     
         [0063]    The performance of an Air Separation Unit (ASU) can be approximated by a simple oxygen recovery ratio of about 20%: for 1000 Nm3/h feed air to the ASU, the corresponding recovery rate of oxygen is 200 Nm3/h. For low purity, low pressure oxygen for oxycombustion application, the power consumption of the oxygen plant is mainly the power consumption of the air feed compressor, hence the air feed flow. 
         [0064]    The prior art process consists of a basic Air Separation Unit, its oxygen production output is adjusted simply by adjusting the air feed flow to the unit. As an approximation, the power consumption is assumed to be proportional to the feed air flow. 
         [0065]    Let us compare the air feed rate of this new process with the prior art process: 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                 Case 1 
               
             
          
           
               
                   
                 New 
                 Prior 
               
               
                   
                 Process 
                 Art 
               
             
          
           
               
                   
                   
                 Air 
                 Power 
                 Air 
                 Power 
               
               
                 Daily basis 
                 Cents/kWh 
                 Flow 
                 Cost 
                 Flow 
                 Cost 
               
               
                   
               
             
          
           
               
                 Peak 100% Power - 8 
                 4.5 
                 450 
                 16200 
                 1000 
                 36000 
               
               
                 hours 
               
               
                 Average 75% Power - 
                 3 
                 900 
                 21600 
                 750 
                 18000 
               
               
                 8 hours 
               
               
                 Off-peak 50% Power - 
                 1.5 
                 900 
                 10800 
                 500 
                  6000 
               
               
                 8 hours 
                   
                   
                   
                   
                   
               
               
                 Total 
                   
                   
                 48600 
                   
                 60000 
               
               
                   
                   
                   
                 (−19%) 
               
               
                   
               
             
          
         
       
     
         [0066]    Note: Assuming the air compressor can be turned down to 50% (for example by using 2 compressors in parallel). 
         [0067]    It can be seen from above table that:
       a) The maximum air flow of the new ASU is 90% of the maximum air flow of the prior art. This represents smaller equipment and a reduction of plant cost.   b) The cost of power to operate the new process is reduced by about 19% based on the above model. This is a significant cost reduction. The economics of oxycombustion and CO 2  capture in particular can therefore be improved.       
 
         [0070]    c) During peak periods, the power consumption of the oxygen plant is sharply reduced by 55%; this represents an important availability of power to supply the demand of the grid. If demand cannot be satisfied, utility companies usually have to purchase additional power from another network at a very high cost. This reduction of consumption of the oxygen during peaks can alleviate the situation and will result in major savings for utility companies. 
         [0071]    The concept can also be applicable to situations where the demand remains constant throughout the high demand periods (highest power cost) or low demand periods (lowest power cost). In this situation, during high demand, the air flow to the ASU is reduced to the limit of machinery&#39;s turndown to minimize its power consumption. During low demand, the air flow is increased not only to satisfy the demand but also to produce liquid oxygen to be vaporized during the periods when power cost is high. 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                 Case 2 
               
             
          
           
               
                   
                 New 
                 Prior 
               
               
                   
                 Process 
                 Art 
               
             
          
           
               
                   
                   
                 Air 
                 Power 
                 Air 
                 Power 
               
               
                 Daily basis 
                 Cents/kWh 
                 Flow 
                 Cost 
                 Flow 
                 Cost 
               
               
                   
               
             
          
           
               
                 Peak 100% Power - 8 
                 4.5 
                 600 
                 21600 
                 1000 
                 36000 
               
               
                 hours 
               
               
                 Average 100% Power - 
                 3 
                 1200 
                 28800 
                 1000 
                 24000 
               
               
                 8 hours 
               
               
                 Off-peak 100% Power - 
                 1.5 
                 1200 
                 14400 
                 1000 
                 12000 
               
               
                 8 hours 
                   
                   
                   
                   
                   
               
               
                 Total 
                   
                   
                 64800 
                   
                 72000 
               
               
                   
                   
                   
                 (−10%) 
               
               
                   
               
             
          
         
       
     
         [0072]    Since the oxygen plant must supply a constant oxygen flow and the additional oxygen liquid for the high demand periods, the plant size for the off-peaks in this example must be increased by 20%. However, the saving achieved is still significant at 10% and the ASU power can be cut back by 40% during the periods of high demand. 
         [0073]    This new process can be used to minimize the plant cost and still provides significant power cost saving. Indeed, in the above example of Case 1 we can arrange to have the following configuration: 
         [0000]    
       
         
               
             
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                 Case 3 
               
             
          
           
               
                   
                 New  
                 Prior 
               
               
                   
                 Process 
                 Art 
               
             
          
           
               
                   
                   
                 Air 
                 Power 
                 Air 
                 Power 
               
               
                 Daily basis 
                 Cents/kWh 
                 Flow 
                 Cost 
                 Flow 
                 Cost 
               
               
                   
               
             
          
           
               
                 Peak 100% Power - 8 
                 4.5 
                 750 
                 27000 
                 1000 
                 36000 
               
               
                 hours 
               
               
                 Average 75% Power - 
                 3 
                 750 
                 18000 
                 750 
                 18000 
               
               
                 8 hours 
               
               
                 Off-peak 50% Power - 
                 1.5 
                 750 
                  9000 
                 500 
                  6000 
               
               
                 8 hours 
                   
                   
                   
                   
                   
               
               
                 Total 
                   
                   
                 54000 
                   
                 60000 
               
               
                   
                   
                   
                 (−10%) 
               
               
                   
               
             
          
         
       
     
         [0074]    As can be seen, the air flow can be kept constant and the oxygen demand can vary during the peaks and off-peaks. This strategy results in 25% reduction in plant size while preserving a good 10% reduction in power cost. 
         [0075]    A similar approach can be used to estimate the savings in some other models and the concept appears to be advantageous in most situations. 
         [0076]    It is useful to note that by liquefying liquid nitrogen when liquid oxygen is vaporized, or vice versa by liquefying liquid oxygen when liquid nitrogen is vaporized, we can recover and store the refrigeration under the liquid form such that there is no major power expenditure to liquefy these important amounts of liquid involved in the transfers. 
         [0077]    The above example use liquid nitrogen as a means to transfer and store the refrigeration during periods of peaks and off-peaks. The process can be applicable to a liquid of another composition derived from air such as liquid air, a liquid rich in oxygen (greater than 35% O 2 ) or a liquid rich in nitrogen (greater than 80% N 2 ). Two or more liquid streams can also be used if needed, for example, during peaks, liquid oxygen is fed and vaporized in the ASU, a stream of liquid N2 and a stream of liquid air can be extracted from the ASU to compensate for the refrigeration. 
         [0078]    The term “bascule” is used to describe the cryogenic air separation process in which, in one phase, a first liquid stream is used to liquefy an oxygen stream. In a next phase, liquid oxygen produced is then fed to the process to allow extraction and restoration of the first liquid stream. Since the process simply exchanges refrigeration between liquid oxygen and the first liquid stream, it does not require power intensive equipment to liquefy a gaseous stream like in traditional liquefaction equipment. 
         [0079]    In the new invention, during low demand periods, the ASU can increase the air feed to restore the liquid oxygen inventory by re-feeding the liquid nitrogen produced in the high demand periods back into the system. The higher feed air coupled with low power cost can provide an added advantage: some small amount of liquid can be extracted from the cryogenic cold box of the oxygen plant with almost no power or cost penalty, for example by simply increasing the flow of a cold box&#39;s expander. This additional liquid can be fed back to the cold box during peaks, reducing the need to operate the cold box&#39;s expander(s) during peaks thus increasing the efficiency and ability of the system to better track the demand. The cryogenic oxygen plant may be equipped with cold compression equipment, which consumes refrigeration. Such small amount of additional liquid generated inexpensively during off-peaks, coupled with the liquid resulting from refrigeration exchange of the bascule, can improve the cold requirement of the system during peaks. Therefore, the cold box&#39;s expander(s) can be throttled or even shut down to further cut back the air flow and still be able to maintain good efficiency of the distillation columns and satisfy the refrigeration need of cold compression equipment, thus increasing the saving of the bascule. 
         [0080]    Oxygen plants equipped with “bascule” features have been utilized in the industry for some time. However, this usage has been limited to the tracking of the usage demand of the clients of the oxygen plant, independently of the power demand of the utility companies like the object of this new invention. In another word, the bascule has been applied previously to the client side of electricity business, however this new invention addresses the integration of the bascule oxygen plant to the supply side, and in particular to the power generation aspect created by the need of oxycombustion or the partial oxidation requirement of IGCC plants. 
         [0081]    In prior art, the economics of an air separation plant can also be improved by liquefying a first liquid stream during the off-peak periods when power cost is low. When power cost is high, the liquid is then vaporized in the air separation unit (ASU) allowing reducing the air flow to minimize the power. The basic difference between this technique and the present invention is that the excess of refrigeration produce by vaporizing the liquid is mainly used to compress a cold gas stream of the ASU at cryogenic temperature to higher pressure, and not to recover an equivalent liquid flow for subsequent use. Power intensive liquefaction equipment such as high pressure compressors and additional gas expanders must be provided to run the liquefaction unit during off hours. The liquefaction equipment can be integrated with the oxygen plant. This prior art is illustrated schematically in  FIGS. 4 and 5 . 
         [0082]      FIG. 4  shows the high power demand phase wherein nitrogen compressed in a cold compressor  16  is sent to the power plant&#39;s gas turbine, a portion of this cold compressed nitrogen can be optionally recycled to the air separation unit  13  to improve the distillation or to vaporize liquid oxygen, and liquid air from storage tank  18  is sent to the air separation unit. 
         [0083]      FIG. 5  shows the low power demand phase wherein air is sent via a warm compressor to the power generation plant  10 . Liquid air formed in liquefier  14  is sent to liquid air storage tank  18 . 
         [0084]    It is clear the concept of this invention can use a combination of both techniques: a bascule feature and some ability to generate additional liquid during low demand and low power cost. The added liquid can, for example, be fed back to the system during peaks to enable an economical cold compression of gaseous nitrogen to higher pressure for the IGCC&#39;s gas turbine injection to lower the compression power requirement in the peak demand period. 
         [0085]    In summary, all power plants are subjected to daily usage variations and this variable characteristic can be utilized advantageously by the bascule approach of the new invention such that the cost of oxygen supply for oxycombustion power plants can be minimized. The concept is directly applicable to IGCC plants. 
         [0086]      FIG. 6  shows an air separation unit capable of operating according to the invention. The plant uses a double column with a medium pressure column  9  operating at around 3.6 bar to 4.0 bar and a low pressure column in a dual reboiler configuration operating at around 1.32 bar. 
         [0087]    For a normal run, air is compressed in compressor  1  and purified in purification unit  5 . The air is then cooled in exchanger  7  as stream  6  and sent in essentially gaseous form to column  9 . 
         [0088]    Oxygen enriched liquid  19  is sent from the medium pressure column  9  to column  11 . Medium pressure nitrogen is used to reboil condenser  21  at an intermediate location between oxygen enriched feed and the bottom of column  11 . Part of the medium pressure nitrogen  25  is compressed by motor-driven compressor  27  and used to reboil condenser  29  at the bottom of column  11 . The liquid formed is expanded in valve  22  and sent back to the top of the column  9 . The condensed medium pressure nitrogen is used as reflux  35  for column  9 , reflux  51  for column  11  and feed  49  for nitrogen tank  17 . A stream of nitrogen  37  at the pressure of column  9  is sent to the exchanger  7  where it warms and is then sent to turbine expander  39  where it is expanded and then fully warmed in the exchanger  7  to form waste stream  43 . Product oxygen  45  is withdrawn as a gas from a section between the two reboilers  21 ,  29 . Low pressure nitrogen is warmed in exchanger  7  and exits as stream  47 . 
         [0089]    During high power demand, the expander  39  does not function or sees its flow sharply reduced. Liquid nitrogen is sent to the tank  17  as stream  49  and liquid oxygen  53  is sent from tank  15  to the bottom of column  11  wherein it vaporizes. The airflow is reduced by reducing the flow of compressor  1 . 
         [0090]    Because of the wide flow fluctuations of the nitrogen expander in various modes, it is not practical to use the power generated by the nitrogen expander to drive the cold compressor  27 . Indeed, in peak mode, the duty required by the cold compressor is very high to vaporize maximum flow of oxygen, meanwhile the flow of the expander is sharply reduced or even zero such that there is not sufficient power of the expander to drive any equipment. Therefore an electric motor is a proper choice to drive the cold compressor. 
         [0091]    During low power demand, the expander  39  functions at or near its peak. Liquid nitrogen is sent from the tank  17  to section  11  as stream  49  and liquid oxygen  53  is sent to tank  15  from section  31 . 
         [0092]    The process according to the invention could of course be operated using other types of apparatus for example that of  FIG. 7  wherein the oxygen from section  31  is optionally pumped by pump  32  and then vaporized in an external exchanger  50 . Part of the cold compressed medium pressure nitrogen  55  condenses in exchanger  50  to provide the necessary heat for vaporization of oxygen. In this arrangement, the cold compressor  27  provides the pressurized nitrogen needed for condensation in both exchangers  50  and  29 . One can also opt to further compress stream  55  by another cold compressor (not shown) should the required pressure of oxygen stream  45  be higher. 
         [0093]    Another embodiment is shown in  FIG. 8 : air to vaporize the oxygen is produced by booster  8 , which compresses about a quarter of the feed air flow. This air condenses in exchanger  7  against the vaporizing liquid oxygen of stream  45  withdrawn from the bottom of the column. 
         [0094]    Liquid oxygen from storage  15  can be fed to the column  11  or in an external vaporizer without passing via the column. It can be vaporized in the exchanger  7 ,  50 , or another exchanger and the resulting gaseous oxygen is mixed with gaseous oxygen produced by the column. 
         [0095]      FIG. 9  shows a variant of  FIG. 7  in which all the air is compressed to a single pressure in compressor  1 , purified in a purification unit  5  and sent to the column  9  as gaseous stream  6 . The rest of the figure is as in  FIG. 7  except that there is no longer a reflux air stream sent from column  9  to column  11 . 
         [0096]    For all the figures, the apparatus uses a single turbine, that turbine being a high pressure nitrogen turbine.