Abstract:
In a process for delivering pressurized gas from an apparatus for separating air by cryogenic distillation, a stream of oxygen-rich liquid or gas having a nominal flow rate is withdrawn from a low-pressure column, an oxygen-rich liquid purge stream is withdrawn as bottoms from the low-pressure column and the oxygen-rich liquid purge stream is sent to a storage tank, in the event of a reduction in the production by the column system or an increase in the demand by a customer, a back-up stream is withdrawn from the storage tank and vaporized in a back-up reboiler and only if the liquid level in the storage tank exceeds a given threshold, a liquid stream is withdrawn from the storage tank constituting at most 2% of the nominal production output of the oxygen-rich stream and is sent to the back-up reboiler.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a §371 of International PCT Application PCT/EP2011/061279, filed Jul. 5, 2011, which claims the benefit of FR1055421 and FR1055423, both filed Jul. 5, 2010, all of which are herein incorporated by reference in their entireties. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to an apparatus and to a process for separating air by cryogenic distillation. 
     BACKGROUND 
     In these apparatuses, it is necessary to take off a purge stream permanently from the external evaporators used to evaporate the liquid oxygen at low pressure and/or from evaporators within the columns, in order to avoid a potentially dangerous accumulation of impurities. These purge streams are withdrawn substantially permanently, for example, from the bottom of a low-pressure column, which produces only gaseous oxygen withdrawn directly from that column, in order to prevent the accumulation of impurities. 
     The purge of the cryogenic liquids from the cold box are generally sent to a specific device for evaporating them:
         gravel pit, for generators of very small size   atmospheric chamber, which collects the liquids, which will subsequently evaporate slowly   evaporator (using heating and ventilation).       

     In EP-A-0605262, a purge liquid is sent into a storage facility, and a liquid portion from the storage facility is sent into a heat exchanger to be mixed with gaseous oxygen. The storage facility is also fed by liquid transported by a truck; the system is therefore one of injection of liquid oxygen with an extra contribution of external liquid, thus allowing the purge to be evaporated with the reheated gaseous oxygen. 
     In EP-A-1202012, a storage facility is fed by a purge stream originating from the bottom of the low-pressure column in the case of a reduction in the production of the column system, a standby stream which is evaporated in a standby evaporator is withdrawn from the storage facility. 
     SUMMARY OF THE INVENTION 
     According to the invention, the purge is able to compensate the evaporation losses in the storage facility, and the excess is evaporated in the standby vaporization system (without recovery of the refrigeration capacities in the cold box). 
     According to the invention, provision is made to send a purge stream to a liquid oxygen storage facility, optionally by means of a pump, depending on its pressure. 
     According to one subject of the invention, a process is provided for separating air by cryogenic distillation, to give a gaseous product, optionally under pressure, wherein
         a) cooled, purified, and compressed air is sent into a column system comprising a medium-pressure column and a low-pressure column   b) i) from the low-pressure column, an oxygen-rich gas stream is withdrawn which has a nominal flow, to form a gaseous product, and an oxygen-rich liquid purge stream is withdrawn at the bottom of the low-pressure column, the purge stream being richer in oxygen and in heavy impurities than the stream having the nominal flow and constituting not more than 2% of the nominal production flow of the oxygen-rich stream, or
           ii) from the low-pressure column, an oxygen-rich liquid stream is withdrawn which has a nominal flow, and is evaporated in a main evaporator, to form a gaseous product, and an oxygen-rich liquid purge stream is withdrawn which constitutes not more than 2% of the nominal production flow of the oxygen-rich stream from the evaporator,   
           c) the oxygen-rich liquid purge stream is sent into a storage facility,   d) if the level of liquid in the storage facility exceeds a threshold corresponding to the maximum capacity of the storage facility, a liquid stream is withdrawn from the storage facility, constituting not more than 2% of the nominal production flow of the oxygen-rich stream, and is sent to the standby evaporator, to make up part of the gaseous product, and   e) if the level of liquid in the storage facility is below the threshold, the liquid purge stream is sent to the storage facility, but the liquid is sent from the storage facility to the standby evaporator only in the case of a reduction in production by the column system or of an increase in demand by a customer for the gaseous product.       

     According to other, optional subjects:
         the oxygen-rich liquid purge stream is sent from the bottom of the low-pressure column or from the evaporator into the storage facility permanently or occasionally, outside of periods of shutdown and startup of the column system.   in the case of shutdown of the column system, the purge stream is not sent to the storage facility.   the stream having a nominal flow is a gas stream withdrawn from the low-pressure column, and the purge stream constitutes the only oxygen-rich stream withdrawn from the column system.   during shutdown of the column system, the storage facility is filled by means of liquid coming from the bottom of at least one column of the column system.   from the low-pressure column, an oxygen-rich gas stream is withdrawn which has a nominal flow, to form a gaseous product, and an oxygen-rich liquid purge stream is withdrawn at the bottom of the low-pressure column, the purge stream being richer in oxygen and in heavy impurities than the stream having the nominal flow and constituting not more than 2% of the nominal production flow of the oxygen-rich stream.   from the low-pressure column, an oxygen-rich liquid stream is withdrawn which has a nominal flow, and is evaporated in a main evaporator to form a gaseous product, and an oxygen-rich liquid purge stream constituting not more than 2% of the nominal production flow of the oxygen-rich stream is withdrawn from the evaporator.   the sending of liquid from the storage facility to the standby evaporator is always initiated if the threshold in the storage facility is exceeded   if the threshold in the storage facility has not been reached, the sending of liquid from the storage facility to the standby evaporator is initiated only in the case of an increase in the customer&#39;s demands or in the case of a reduction in production by the column system.       

     The standby evaporator therefore has a function of evaporating the overflow in the storage facility. 
     According to another subject of the invention, an apparatus is provided for separating air by cryogenic distillation, comprising a column system comprising a medium-pressure column and a low-pressure column, a storage facility, a standby evaporator, and either
         i) means for withdrawing, from the low-pressure column, an oxygen-rich gas stream having a nominal flow, to form a gaseous product, and means for withdrawing an oxygen-rich liquid purge stream at the bottom of the low-pressure column, the purge stream being richer in oxygen and in heavy impurities than the stream having the nominal flow and constituting not more than 2% of the nominal production flow of the oxygen-rich stream, or   ii) a main evaporator, means for withdrawing, from the low-pressure column, an oxygen-rich liquid stream having a nominal flow, and for sending it to the main evaporator, means for withdrawing a gaseous product formed by evaporation of the liquid stream, means for withdrawing an oxygen-rich liquid purge stream constituting not more than 2% of the nominal production flow of the oxygen-rich stream from the evaporator,
           and means for sending the oxygen-rich liquid purge steam to the storage facility, detection and control means for detecting the level of liquid in the storage facility and for initiating, only if the level exceeds a threshold corresponding to the maximum capacity, the withdrawal of a liquid stream from the storage facility, constituting not more than 2% of the nominal production flow of the oxygen-rich stream, and the sending of this stream to the standby evaporator to constitute part of the gaseous product, these detection and control means being able to detect if the level of the liquid in the storage facility is below the threshold and, in that case, for initiating the sending of the liquid purge stream to the storage facility and the prevention of the sending of the liquid from the storage facility to the standby evaporator except in the case of a reduction in production by the column system or of an increase in demand by a customer for the gaseous product.   
               

     According to other, optional subjects:
         the apparatus does not comprise a means for evaporating purge liquid apart from the standby evaporator.   the apparatus comprises means for withdrawing, from the low-pressure column, an oxygen-rich gas stream having a nominal flow, to form a gaseous product, and means for withdrawing an oxygen-rich liquid purge stream at the bottom of the low-pressure column, the purge stream being richer in oxygen and in heavy impurities than the stream having the nominal flow and constituting not more than 2% of the nominal production flow of the oxygen-rich stream.   the apparatus comprises   a main evaporator, means for withdrawing, from the low-pressure column, an oxygen-rich liquid stream having a nominal flow and for sending it to the main evaporator, means for withdrawing a gaseous product formed by evaporating the liquid stream, and means for withdrawing an oxygen-rich liquid purge stream constituting not more than 2% of the nominal production flow of the oxygen-rich stream from the evaporator.   means for supplying the storage facility with a liquid from an external source.   means for sending a bottom liquid from the low-pressure column to the storage facility.   means for sending a bottom liquid from the medium-pressure column to the storage facility.       

     It will be noted that the transfer pump may also be the same one used for the transfer of the production of liquid oxygen, where appropriate. 
     The liquid oxygen storage facility is filled with oxygen of “industrial merchant” grade, which is therefore “pure” (99.5 mol %). In order to supply the customer when the air separation apparatus is not operating, provision is made to evaporate the liquid from this storage facility in a standby evaporator. Sending a purge liquid having an average purity less than that of the liquid from the storage facility will reduce slightly the purity of liquid contained in the storage facility, which will nevertheless be at an average level greater than the customer demand. 
     An oxygen-rich stream—gaseous or liquid—contains at least 75 mol % of oxygen. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention&#39;s scope as it can admit to other equally effective embodiments. 
         FIG. 1  represents an apparatus in accordance with an embodiment of the invention. 
         FIG. 2  represents an apparatus in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The invention will be described in more detail with reference to the figures, which illustrate an apparatus in accordance with various embodiments of the invention. 
     In  FIG. 1 , a stream of compressed and purified air  1  is cooled in a heat exchanger  3  and divided into three. A stream  7  is withdrawn from the heat exchanger at an intermediate level, released into a turbine  9 , and sent to a low-pressure column  15  of a twin column, in gaseous form. Another stream  5 , after having been compressed in the compressor A, is liquefied in the heat exchanger  3 , released into a valve  11 , and sent to the medium-pressure column  13 . A third stream B is sent directly into the medium-pressure column. 
     Streams enriched in nitrogen and in oxygen are sent from the medium-pressure column to the low-pressure column in a conventional manner. At the bottom of the low-pressure column  15 , liquid oxygen accumulates around the reboiler  17 . A liquid oxygen stream  19  containing less than 98% oxygen is withdrawn from the bottom of the low-pressure column  15  and pressurized by a pump  23  to 2 bar; alternatively, the compression may be hydrostatic. A portion of the oxygen at this pressure is sent to an evaporator  24 , where it evaporates by heat exchange with a portion of the feed air ( 5  or B) which is then sent to the column. This evaporated stream constitutes the gas production of the column system in normal operation, and is produced with a nominal flow. 
     A purge stream  121  containing impurities is withdrawn from the evaporator  24  permanently, unless the column system is not operating. This stream is purer in oxygen than the stream  19 . The evaporated oxygen continues its reheating in the heat exchanger  3 , as low-pressure oxygen stream  27 . 
     The remainder of the oxygen is pressurized to a pressure of 10 bar in a pump  25 , and evaporates in the heat exchanger  3  as stream  29 . Alternatively, all of the oxygen may be evaporated to the lower pressure, and the stream  29  will not exist. 
     A first line connects the evaporator  24  to the storage facility  31 , for conveying the purge liquid  121  to said facility permanently or occasionally, unless the column system is not operating. The storage facility  31  supplies a standby evaporator  37  with liquid oxygen  35 . The purge liquid may be sent occasionally, which means that the liquid is withdrawn from the evaporator periodically, according to a cycle, so as to have quantities of liquid that allow proper analysis. 
     Oxygen is evaporated in the standby evaporator  37  in large quantity, when the customer demands are particularly high or when the air separation apparatus is not operating at full capacity, in the case of breakdown, for example. 
     In contrast, during periods in which the customer demands are not particularly high and the air separation apparatus is operating normally, an oxygen-rich liquid stream constituting less than 2% of the nominal flow of gaseous oxygen  19 , and preferably around 1% of the nominal flow, is sent to the storage facility. This liquid stream is sent to the standby evaporator only if the level in the storage facility exceeds a threshold, this threshold typically being that of the maximum capacity of the storage facility  31 . Accordingly, this stream sent to the standby evaporator constitutes the overflow of the storage facility  31 . The evaporated oxygen is sent to the customer. 
     This allows recovery of the molecules of the purge in the liquid  121 , and therefore allows a reduction in the total production of the apparatus (maximum energy gain of around 1% to 2%). 
     If the level in the storage facility  31  is below the threshold, more particularly when the storage facility is not full, the storage facility  31  is filled by sending the liquid  121  to it. 
     The majority of the liquid in the storage facility originates from a tanker truck  61  or from the apparatus itself via the stream  21 , which can be diverted to the storage facility as and when required. The liquid in the storage facility originating from the truck has a purity of 99.5 mol %. The liquid  121  may be purer, as pure, or less pure than the liquid from the tanker truck  61 . 
     Sending this pure liquid  121  to the storage facility  31  via the line  33  carries no risk of affecting the purity of the liquid in the storage facility. 
     On the other hand, consideration may be given to sending, in the case of column shutdown, the liquids accumulated in the bottom of the low-pressure and/or medium-pressure column to the storage facility as well, in the lines  21  and  51 . These liquids will obviously have a purity lower than that of the liquid from the truck  61  or that of the liquid  121 . In certain cases, the customer may tolerate, for the short term, a reduction in purity of the liquid evaporated in the standby evaporator. A valve  41  allows regulation of the flows of these accumulated liquids that are sent to the storage facility  31 . 
     In  FIG. 2 , a stream of compressed and purified air  1  is cooled in a heat exchanger  3  and divided into two. A stream  7  is withdrawn from the heat exchanger at an intermediate level, released into a turbine  9 , and sent to a low-pressure column  13  of a twin column, in gaseous form. Another stream,  5 , is cooled in the heat exchanger  3  and sent to the medium-pressure column  13 . 
     Streams enriched in nitrogen and in oxygen are sent from the medium-pressure column to the low-pressure column in a conventional manner. At the bottom of the low-pressure column  15 , liquid oxygen accumulates around the reboiler  17 . A gaseous oxygen stream  19  containing approximately 98% oxygen is withdrawn from the bottom of the low-pressure column  15  and is reheated in the heat exchanger  3  as low-pressure oxygen stream  27 , before undergoing optional compression (not shown). A purge stream  21  containing impurities is withdrawn from the bottom of the column  15  permanently, to prevent accumulation of impurities in the column bottom. This liquid withdrawal constitutes the only oxygen-rich withdrawal from the column system. A first line connects the column bottom BP to the storage facility  31 , in order to carry the purge liquid  21  to the facility permanently (either continuously or in regular batches), unless the column system is not operating. The storage facility  31  supplies a standby evaporator  37  with the liquid oxygen  35 . 
     The oxygen is evaporated in the standby evaporator  37  in large quantity, when customer demands are particularly high or when the air separation apparatus is not operating at full capacity, in the case of breakdown, for example. 
     In contrast, during periods when the customer demands are not particularly high and the air separation apparatus is operating normally, an oxygen-rich liquid stream constituting less than 2% of the nominal flow of gaseous oxygen  19 , and preferably approximately 1% of the nominal flow, is sent to the storage facility. This liquid stream is sent to the standby evaporator if the level in the storage facility exceeds a threshold, this threshold typically being that of the maximum capacity of the storage facility  31 . Accordingly, this stream sent to the standby evaporator constitutes the overflow of the storage facility  31 . The evaporated oxygen is sent to the customer. 
     This allows recovery of the molecules of the purge in the liquid  21 , and therefore a reduction in the total production of the apparatus (maximum energy gain of around 1% to 2%). 
     If the level in the storage facility  31  is below the threshold, more particularly when the storage facility is not full, the storage facility  31  is filled by sending the liquid  21  to it. In normal operation, therefore, the liquid from the storage facility is not sent to the standby evaporator if the maximum level is not reached. However, initiation of sending from the storage facility to the standby evaporator may be necessary, in the case of a reduction in production by the column system or of an increase in the customer&#39;s demands. 
     The purge stream  21  has a higher molar oxygen purity than that of the stream  19 . 
     The majority of the liquid in the storage facility comes from a tanker truck  61  or from the apparatus itself via a liquid stream which is taken off as and when necessary (not shown), which can be sent to the storage facility. The liquid in the storage facility from the truck has a purity of 99.5 mol %. A pump may be needed in order to send the purge liquid  21  to the storage facility. 
     The purge stream  21  may be purer, as pure, or less pure than the liquid supplying the storage facility from the tanker truck  61 , for example. 
     The liquid in the storage facility  31  may be evaporated in a standby evaporator  37  and sent to a customer in the event of breakdown of the distillation apparatus and/or if the production level of the distillation apparatus is insufficient for supplying the customer. 
     In the possible event of the column system shutting down, consideration may be given to sending the liquids accumulated in the column bottoms during shutdown to the storage facility  31  likewise. In the case of  FIG. 2 , the system comprises a low-pressure column and a medium-pressure column; however, it will be readily appreciated that the invention also applies to the case in which the accumulated liquids sent to the storage facility may come from an argon column, an intermediate-pressure column or a mixing column. This withdrawal of an accumulated liquid during shutdown of an argon column, an intermediate-pressure column, or a mixing column also applies to the case of  FIG. 1 . 
     In the event these impure liquids are sent from the column bottoms to the storage facility during shutdown of a column, it is sometimes necessary to regulate the sending of liquid to the storage facility as a function of the level of liquid in the storage facility. Such regulation is unnecessary when the liquids sent to the storage facility are only high-purity permanent purge liquids. 
     In one embodiment, the essential requirement is to regulate the sending of high-purity purge liquid (or, optionally, of a mixture of high-purity purge liquid and less pure accumulated liquids) such that the liquid sent to the standby evaporator has a purity above a threshold which is acceptable for the customer. 
     While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step. 
     The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise. 
     “Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein. 
     “Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary a range is expressed, it is to be understood that another embodiment is from the one. 
     Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur. 
     Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such particular value and/or to the other particular value, along with all combinations within said range. 
     All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.