Abstract:
Device for separating air comprises a pipe for withdrawing liquid oxygen from the low-pressure column and for sending it to a first pump, a pipe for sending liquid oxygen that has been pressurized to a pressure of below 9 bar as from the first pump to a vaporizer, a pipe for sending gaseous oxygen from the vaporizer to a main exchanger Where it is heated up, a pipe for sending liquid oxygen for purging from the vaporizer to a second pump to pressurize it and a pipe for sending the pressurized oxygen from the second pump to an exchanger where it vaporizes to form gaseous oxygen.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a §371 of International PCT Application PCT/FR2012/050500, filed Mar. 9, 2012, which claims the benefit of FR 1152272, filed Mar. 18, 2011, both of which are herein incorporated by reference in their entireties. 
     
    
     TECHNICAL FIELD OF THE INVENTION 
       [0002]    This invention relates to an apparatus and method for separating air by air distillation. 
         [0003]    In particular, it relates to the production of gaseous oxygen at a pressure below 9 bars abs, even below 5 bars abs. The gaseous oxygen can possibly contain less than 98% mol. oxygen. 
       BACKGROUND 
       [0004]    It is required to produce large quantities of oxygen having these characteristics in order to supply oxy-combustion apparatuses, among others. 
         [0005]    It is known in WO-A-10/109149 to vaporise a flow of liquid oxygen at low pressure in an exterior vaporiser in order to produce gaseous oxygen which is then heated in a main exchanger. 
         [0006]    It is known to vaporise the purge of a distillation column in order to recover its refrigeration, for example in U.S. Pat. No. 5,408,831. 
         [0007]    U.S. Pat. No. 5,76,5396 and U.S. Pat. No. 5,251,451 describe installations according to the preamble of claim  1 . 
       SUMMARY OF THE INVENTION 
       [0008]    However this invention proposes to vaporise the deconcentration purge of a vaporiser in an exchanger in order to recover the refrigeration, this vaporiser being the exchanger making it possible to vaporise a liquid of the apparatus under pressure in order to produce a pressurised gaseous product. 
         [0009]    It is desirable to send the purge to a storage of pressurised gases making it possible to maintain stable production in terms of flow as well as in terms of pressure. 
         [0010]    According to an embodiment of the invention, an apparatus for separating air is provided comprising a double column comprising a medium-pressure column and a low-pressure column, a main exchanger, a vaporiser, a main compressor, means for sending all of the air to be treated in the double column to the main compressor in order to produce air substantially at the pressure P 1  of the medium-pressure column, means for sending a portion of the air substantially at a high pressure P 2  to the main exchanger and then to the vaporiser, a pipe for sending air at least partially condensed in the vaporiser to at least one of the columns, a pipe for sending air at the pressure P 1  to the medium-pressure column, means of pressurising, a pump, a pipe for withdrawing the liquid oxygen from the low-pressure column and for sending it to the means of pressurising, a pipe for sending liquid oxygen that has been pressurised at a pressure below 9 bar abs from the means of pressurising to the vaporiser, a pipe for sending gaseous oxygen from the vaporiser to the main exchanger to be heated in order to form a first flow of gaseous oxygen, a purge pipe for sending liquid purge oxygen from the vaporiser to the pump in order to pressurise it characterised in that the purge pipe is not connected to a purge liquid storage and in that it comprises a pipe for sending pressurised oxygen from the pump to an exchanger in order to be vaporised, with the exchanger being connected to a pipe for compressed air connected to the main compressor and to a pipe connected to the double column, in order to form a second flow of gaseous oxygen. 
         [0011]    Optionally:
       the exchanger connected to the oxygen purge pipe is the main exchanger.   the exchanger connected to the oxygen purge pipe is an exchanger separate from the main exchanger.   the exchanger comprises passages connected to a supply air inlet pipe and passages connected to a liquid refrigerant inlet pipe, possibly coming from the double column.   the apparatus comprises a storage of pressurised gases connected to the purge oxygen vaporisation exchanger in order to collect the gaseous oxygen.       
 
         [0016]    According to another embodiment of the invention, a method is provided for separating air in an apparatus comprising a double column comprising a medium-pressure column and a low-pressure column, a main exchanger, a vaporiser, a main compressor, means of pressurising, a pump, wherein all the air to he treated is sent in the double column to the main compressor in order to produce air substantially at the pressure P 1  of the medium-pressure column, a portion of the air substantially at a high pressure P 2  is sent to the main exchanger and then to the vaporiser, air at least partially condensed in the vaporiser is sent to at least one of the columns, air at the pressure P 1  is sent to the medium-pressure column, liquid oxygen is withdrawn from the low-pressure column and it is pressurised, the liquid oxygen that has been pressurised is sent at a pressure below 5 bar abs to the vaporiser, a first flow of gaseous oxygen is sent from the vaporiser to the main exchanger to be heated and liquid purge oxygen of the vaporiser is pressurised in the pump characterised in that the liquid purge oxygen is pressurised without having been stored and is then vaporised in an exchanger through exchange of heat with the air, compressed in the main compressor and intended for the double column, in order to form a second flow of gaseous oxygen. 
         [0017]    Optionally:
       the purge oxygen is pressurised at a pressure of at, least 10 bars abs, more preferably at least 15 bars abs, even at least 20 bars abs in the second pump.   the purge oxygen is vaporised in the main exchanger.   the purge oxygen is vaporised in an exchanger other than the main exchanger.   the second flow of gaseous oxygen is sent to a storage of pressurised gases and is used for backup production.   a variable quantity of the second flow of gaseous oxygen is mixed with the first flow in order to produce a substantially constant mixed flow.   the liquid oxygen withdrawn from the low-pressure column contains at least 80% mol. oxygen   the liquid oxygen withdrawn from the low-pressure column constitutes the only flow containing at least 80% mol. oxygen withdrawn from the low-pressure column.   the liquid oxygen withdrawn from the low-pressure column contains at most 98% mol. oxygen.       
 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    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. 
           [0027]      FIG. 1  shows an embodiment of the present invention. 
           [0028]      FIG. 2  shows another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    The invention shall be described in more detail by referring to the figures which show apparatuses for separating air according to the invention. 
         [0030]    In  FIG. 1 , the apparatus comprises an exchange line  21  and a double column constituted of a medium-pressure column  27  and a low-pressure column  29 . 
         [0031]    All of the air I is compressed in the main compressor  2  in order to produce air at the pressure P 1  substantially equal to the pressure of the medium-pressure column  27 . The air at the pressure P 1  is cooled in a cooler  7 , purified in a purification unit  9  and divided into three fractions. The first fraction  11  is supercharged in a supercharger, able to be constituted of the last stage of the main compressor, with this last stage being part of the second portion of the compressor. The pressure P 1  is below 5 bars abs, even 4.5 bar abs, preferably below 4 bar, and even below 3.5 bar abs. 
         [0032]    The first fraction  11  is brought to a pressure P 2  by the booster  5  or an independent compressor  5  and cooled at this pressure in a cooler (not shown) before being sent to the exchange line  21 . The exchange line is comprised of a brazed-plate aluminium indirect heat exchanger. The fraction  11  is then sent in gaseous form to a vaporiser  41  where it is condensed at least partially before being expanded and sent to the medium-pressure column  27 . The pressure P 2  is below 15 bar abs, preferably below 10 bar, and even below 6 bar abs. The fraction  11 . is less than half of the flow  1 , and preferably less than a third of the flow  1 . 
         [0033]    The second fraction 1.3 at the pressure P 1  is cooled entirely in the exchange line  21  and is divided into two flows. The first flow  23  is sent to a tank reboiler  33  from the low-pressure column  29  where it is condensed at least partially and is sent to the medium-pressure column, mixed with the flow  11 . The second flow  25  is sent in gaseous form to the median-pressure column  27 . 
         [0034]    The third fraction  15  is pressurized in a boostercharger  17 , partially cooled in the exchange line  21 , withdrawn from the exchange line at an intermediary level of the latter and expanded in a turbine  19  coupled to the supercharger  17  before being sent to the low-pressure column  29 . 
         [0035]    A flow of oxygen-rich liquid  55 , an intermediate flow  53  and a flow of nitrogen-rich liquid  51  are withdrawn from the medium-pressure column  27 , cooled in the exchanger  31 , expanded and sent to different levels of the low-pressure column  29 . 
         [0036]    Medium-pressure gaseous nitrogen  49  is condensed in an intermediate vaporiser  35  of the low-pressure column  29  and sent as a reflux at the head of the medium pressure column  27 . Another flow of medium-pressure gaseous nitrogen  47  is heated in the exchange line. 
         [0037]    Liquid oxygen  37 , containing at least 80% mol. oxygen and possibly at most 98% mol. oxygen, is withdrawn from the tank of the low-pressure column  29 , pressurised by a pump  39  at a pressure below 9 bars abs, even below 5 bars abs and sent to the vaporiser  41 . Other than a liquid purge  43 , the oxygen is vaporised in the vaporiser  41  by exchange of heat with the fraction of air  11  at the pressure P 2 . This oxygen then forms the first flow of pressurised gaseous oxygen  45  which is heated in the exchange line  21 . The fraction of air  11  is partially condensed and is sent to the double column. 
         [0038]    The purge liquid  43  is pressurised to a pressure of at least 10 bars abs, or at least 15 bar abs, even at least 20 bars abs in a pump  63  then is vaporised in the exchange line  21 . The second gaseous flow produced as such  59  is sent to a storage of pressurised gases  3  and expanded in order to be mixed with the flow  45  via the pipe  61 . 
         [0039]    Here the vaporisation of purge liquid is carried by using primarily substantial heat, in such a way that no airflow exiting the exchanger  21  is fully condensed, or even is condensed. 
         [0040]    Alternatively as shown in  FIG. 2 , the pressurised purge liquid  43  can be vaporised in an auxiliary exchanger  21 A, separate from the exchange line, against an airflow  25 A and with a liquid refrigerant, for example a flow of nitrogen  57 A heated with the method of separating. 
         [0041]    The flow  25 A cooled in the exchanger  21 A is mixed with the cooled flow  25  and the flow of nitrogen  57 A heated in the exchanger  21 A is mixed with the heated flow  57 . 
         [0042]    The second flow of gaseous oxygen  59 ,  61  formed by the vaporisation can be used as a backup gas during an interruption of the production of gaseous oxygen  45 . 
         [0043]    As such the only airflow which is used to vaporise the purge oxygen  43  remains in gaseous form in the exchanger  21 A and the vaporisation is carried out by substantial heat exchange. 
         [0044]    The pressurisation by the pump  39  and/or  63  can be replaced with a hydrostatic pressurisation in all of the eases described. 
         [0045]    For all of the figures, a variable quantity of the second flow of gaseous oxygen is mixed with the first flow in order to produce a substantially constant mixed flow. 
         [0046]    This variable quantity of the vaporised purge liquid can be mixed with the first flow  45  in order to smooth. out the variations of flow, due, for example to variations of the pressure of the oxygen network. 
         [0047]    By detecting a reduction in pressure in the line  45 , due, for example to an increased oxygen demand, oxygen can be expanded and sent from storage  3  to the line  45  via the pipe  61 . 
         [0048]    In the event of a breakdown of the apparatus for separating air, the flow of oxygen  45  will be reduced or will be nonexistent. In this case, the flow of oxygen  63  from storage  3  can supply a client, the time for a backup vaporiser to be operational in order to prevent any stoppage of production. 
         [0049]    The flow  37  is the only flow containing more than 60% mol. oxygen withdrawn from the low-pressure column. 
         [0050]    The storage  3  operates at a pressure that is higher than the flow  45 . 
         [0051]    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. 
         [0052]    The singular forms “a”, an and “the” include plural referents, unless the context clearly dictates otherwise. 
         [0053]    “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. 
         [0054]    “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. 
         [0055]    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. 
         [0056]    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. 
         [0057]    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.