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(11) | EP 0 816 784 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Apparatus for fractionating air, or a mixture of oxygen and nitrogen, for the simultaneous production of nitrogen and oxygen or for the production of nitrogen |
| (57) An apparatus for fractionating air, or a mixture of oxygen and nitrogen, for the
simultaneous production of nitrogen and oxygen or for the production of nitrogen,
the apparatus comprising a fractionating column (1; 101) having an intake duct (2;
102) for air or a mixture of oxygen and nitrogen under pressure, a nitrogen discharge
duct (4; 104) located at the top of the fractionating column, and a discharge duct
(3, 3a; 103) for the oxygen or the ternary mixture constituted by oxygen, nitrogen,
and argon located at the base of the fractionating column (1; 101). The apparatus
also comprises a closed circuit (6; 106) which provides a gas-based refrigeration
cycle, the gas being moved by a compressor (7; 107) and flowing through: a heat exchanger
(8; 108), which acts as a condenser for the gas and causes the evaporation of the
oxygen or of the ternary mixture at the base of the column (1; 101); a subcooling
unit (9; 109); an expansion element (10; 110); and another heat exchanger (11; 111)
which acts as an evaporator for the gas and condenses the nitrogen at the top of the
column (1; 101), producing the rectification reflux. |
[0001] The present invention relates to an apparatus for fractionating air, or a mixture of oxygen and nitrogen, for the simultaneous production of nitrogen and oxygen or for the production of nitrogen.
[0002] Apparatuses for fractionating air to produce nitrogen and oxygen simultaneously , with purities above 95% for oxygen and above 98% for nitrogen, are known. These apparatuses operate by means of two fractionating columns which operate at mutually different pressures. In the first column, or medium-pressure column, the compressed air, optionally partially liquefied, is fractionated so as to obtain, at the top of the column, nitrogen mainly in the liquid state with a purity of more than 98% and, at the base of the column, a mixture of nitrogen and oxygen in the liquid state. The reflux liquid nitrogen for rectification is obtained by condensing the gaseous nitrogen rising in the first column in a heat exchanger, which acts as a condenser-evaporator, causing the evaporation of the liquid oxygen produced in the second column. The second column, or low-pressure column, is supplied with the mixture of nitrogen and oxygen in the liquid state that originates from the first column, obtaining, at the top of the second column, gaseous nitrogen with a purity of more than 98% and, at the base, oxygen with a purity of more than 95%.
[0003] The reflux liquid nitrogen for rectification originates from the first column, or medium-pressure column, after being condensed by evaporating, as mentioned, the liquid oxygen produced in the second column or low-pressure column.
[0004] These apparatuses entail the drawback that they require high investment and running costs due to the use of two fractionating columns.
[0005] There are also air fractionating apparatuses for producing only compressed nitrogen with a purity of more than 98%, which operate with a single fractionating column. In these apparatuses, the fractionating column is supplied with partially liquefied compressed air, obtaining, at the top of the column, compressed nitrogen with a purity of more than 98% and, at the base of the column, a liquid ternary mixture constituted by oxygen, nitrogen, and argon. The reflux liquid nitrogen for rectification is obtained by condensing, in a heat exchanger which acts as a condenser-evaporator, part of the gaseous nitrogen rising in the column, by evaporating, after subcooling and adiabatic expansion, the liquid ternary mixture obtained at the base of the column.
[0006] At the output of the exchanger, which acts as a condenser-evaporator, the ternary mixture, after heating, undergoes isentropic expansion, supplying the units of refrigeration required to compensate for the heat losses of the apparatus and to obtain part of the produced nitrogen in the liquid state.
[0007] The nitrogen produced with this apparatus is only a part of the nitrogen contained in the air treated in the fractionating column, since part of the nitrogen is in the liquid ternary mixture that forms at the base of the fractionating column.
[0008] With apparatuses of this kind, a production of nitrogen is generally obtained which is generally equal to 40-50% of the air fed to the fractionating column.
[0009] A principal aim of the present invention is to provide an apparatus for fractionating air, or a mixture of oxygen and nitrogen, which allows to simultaneously produce oxygen with a purity of more than 95% and nitrogen with a purity of more than 98%, with reduced investment and running costs with respect to conventional apparatuses, or which allows to produce compressed nitrogen with a purity of more than 98% with a higher extraction efficiency than allowed by conventional apparatuses.
[0010] Within the scope of this aim, an object of the invention is to provide an apparatus which, in the combined production of oxygen and nitrogen, allows to obtain from the fractionating column all the oxygen and nitrogen contained in the treated air or mixture of oxygen and nitrogen, practically at the same pressure at which the air or mixture is fed to the column.
[0012] This aim, these objects, and others which will become apparent hereinafter are achieved by an apparatus for fractionating air, or a mixture of oxygen and nitrogen, for the simultaneous production of nitrogen and oxygen or for the production of nitrogen, characterized in that it comprises:
-- a fractionating column having an intake duct for air or a mixture of oxygen and nitrogen under pressure, a nitrogen discharge duct located at the top of the fractionating column, and a discharge duct for the oxygen or the ternary mixture constituted by oxygen, nitrogen, and argon located at the base of the fractionating column;
-- a closed circuit which provides a gas-based refrigeration cycle and comprises, in sequence:
-- a compressor;
-- an exchange line to bring the refrigerating gas to a temperature that is close to liquefaction;
-- a first heat exchanger acting as a condenser for said cycle and producing the evaporation of the oxygen or of the ternary mixture at the base of said column;
-- a second heat exchanger acting as a subcooling unit for the gas that leaves said first heat exchanger;
-- an expansion element;
-- a third heat exchanger acting as an evaporator for said cycle and condensing the nitrogen at the top of said column for rectification reflux.
[0013] Further characteristics and advantages of the invention will become apparent from the following detailed description of two preferred but not exclusive embodiments of the fractionating apparatus according to the invention, illustrated only by way of non-limitative example in the accompanying drawings, wherein:
figure 1 is a schematic view of a first embodiment of the apparatus according to the invention for the simultaneous production of oxygen and nitrogen;
figure 2 is a schematic view of a second embodiment of the apparatus according to the invention for the production of nitrogen.
[0014] With reference to the above figures, the fractionating apparatus according to the invention, in the two illustrated embodiments, comprises a fractionating column 1, 101 which can be constituted by a conventional fractionating column, for example a plate column, a filling column, or another type of column which has been tested and is normally used in air fractionation systems.
[0015] The fractionating column 1, 101 is provided with a duct 2, 102 wherethrough the compressed air, or more generally a mixture of nitrogen and oxygen, is fed into the fractionating column 1, 101.
[0016] Proximate to the base of the column 1 of the first embodiment of the apparatus there are provided a discharge duct 3 for the liquid oxygen and a discharge duct 3a for the gaseous oxygen; at the top of the column 1 a discharge duct 4 is provided for the gaseous nitrogen rising in the fractionating column 1.
[0017] In the second embodiment of the apparatus, proximate to the base of the column 101 there is provided a discharge duct 103 for the ternary mixture constituted by oxygen, nitrogen, and argon which descends towards the base of the column 101, and at the top of the column 101 there is provided a discharge duct 104 for the gaseous nitrogen rising in the fractionating column 101.
[0018] The fractionating apparatus also comprises a closed circuit, generally designated by the reference numerals 6 and 106 in the two embodiments and clearly shown in the figures, which performs a gas-based refrigeration cycle.
[0019] More particularly, the circuit 6, 106 comprises a compressor 7, 107 for compressing the gas which is circulated by said compressor 7, 107 along the circuit 6, 106.
[0020] The gas circulated inside the circuit 6, 106 is preferably constituted by argon or by a gas mixture containing at least 40% argon.
[0021] As an alternative, the gas circulated inside the circuit 6, 106 can also be constituted by a conventional refrigerant gas.
[0022] Along the circuit 6, 106, starting from the compressor 7, 107, there are provided: an exchange line for bringing the refrigerant gas to a temperature which is close to liquefaction; a first heat exchanger 8, 108, acting as a condenser for the gas circulated in said circuit; a second heat exchanger 9, 109, acting as a subcooling unit; an expansion element 10, 110; and a third heat exchanger 11, 111 acting as an evaporator.
[0023] The exchange line for bringing the refrigerant gas to a temperature which is close to liquefaction comprises a fourth heat exchanger 12, 112, which is interposed between the compressor 7, 107 and the first heat exchanger 8, 108.
[0024] With particular reference to figure 1, the first heat exchanger 8, wherein the gas circulating in the circuit 6 is condensed, uses the oxygen at the base of the column 1 as a refrigerant fluid.
[0025] The second heat exchanger 9, wherein the gas circulating inside the circuit 6 is subcooled, uses part of the nitrogen that leaves the top of the column 1 as a refrigerant fluid.
[0026] The expansion element 10 can be constituted, in a per se known manner, by an isenthalpic laminar-flow valve or by an isentropic expansion machine.
[0027] The third heat exchanger 11, wherein the gas circulating in the circuit 6 is evaporated, removes heat from the nitrogen at the top of the column 1, causing its condensation; the resulting downward reflux is used for rectification.
[0028] The fourth heat exchanger 12, which acts as a cooling device for the gas that circulates in the circuit 6, uses the following as coolants: the gaseous oxygen leaving the column through the duct 3a proximate to the base of the column 1; the nitrogen leaving the top of the column through the duct 4; and the gas leaving the third heat exchanger 11 before it is fed into the compressor 7. The air is also cooled in the fourth heat exchanger 12 before it is fed into the column 1.
[0029] For the sake of completeness in describing figure 1, it should be noted that along the line for supplying the air or mixture of nitrogen and oxygen there are provided, in a per se known manner: a filter 13; at least one pair of series-connected compression stages 14 and 15, at least one whereof can be operated by means of a turbine 16 which isentropically expands part of the nitrogen leaving the column 1 and is heated in the exchanger 12, a prepurifier 17, and a phase separator 18.
[0030] It should be noted that the second heat exchanger 9 can use, as a coolant, in replacement of the nitrogen leaving the top of the column 1, or as an addition thereto, one or more of the following fluids: the nitrogen that leaves the expansion turbine 16, the argon or other refrigerant gas that leaves the exchanger 11; an external refrigerant fluid.
[0031] Operation of the apparatus shown in figure 1, for the simultaneous production of nitrogen and oxygen, is as follows.
[0032] The compressed air, or the compressed mixture of oxygen and nitrogen, preferably at a pressure substantially between 2 and 20 bar, is fed through the duct 2 and the phase separator 18 into the fractionating column 1.
[0033] The liquid oxygen settles at the base of the fractionating column 1 and exits therefrom through the duct 3. Part of the oxygen, in the gaseous state, is removed through the duct 3a.
[0034] The nitrogen in the gaseous state rises along the fractionating column 1 and exits therefrom through the duct 4.
[0035] As a consequence of the condensation of the nitrogen produced by the heat exchanger 11, part of the nitrogen flows back downwards, performing rectification.
[0036] The gas that circulates in the circuit 6 preferably has, at the output of the compressor 7, a pressure between 3 and 30 bar and is conveyed through the fourth exchanger 12, where it undergoes a first cooling. The gas leaving the fourth exchanger 12 is sent through the first heat exchanger 8, where said gas undergoes condensation, transferring heat to the liquid oxygen and causing its evaporation.
[0037] The gas of the circuit 6 that leaves the first exchanger 8 is conveyed to the second heat exchanger 9, where it is subcooled.
[0038] After subcooling, the gas of the circuit 6 is subjected to isentropic or isenthalpic expansion and then conveyed into the third exchanger 11, where it evaporates, removing heat from the nitrogen and causing it to condense. In practice, the third exchanger 11 acts as an evaporator for the circuit 6 and as a condenser for the nitrogen at the top of the fractionating column 1.
[0039] The first heat exchanger 8 acts as a condenser for the circuit 6 and at the same time acts as a reboiler for the oxygen at the base of the fractionating column 1.
[0040] The gas of the circuit 6, which leaves the third heat exchanger 11, returns to the compressor 7 after being returned to ambient temperature by means of the exchanger 12.
[0041] Assuming a pressure of substantially 6 bar for the supply of air or mixture of oxygen and nitrogen, through the duct 2, the nitrogen is delivered, along the duct 4, at a pressure around 5.8 bar, with a purity of more than 98%, whilst the oxygen is delivered through the duct 3, or through the duct 3a, at a pressure of around 5.9 bar and with a purity of more than 95%.
[0042] With particular reference to figure 2, the first heat exchanger 108, wherein the gas circulating in the circuit 106 is condensed, uses the ternary mixture at the base of the column 101 as a coolant.
[0043] The second heat exchanger 109, wherein the gas circulating in the circuit 106 is subcooled, uses as a coolant the nitrogen that exits at the top of the column 101 as well as the gas that leaves the third heat exchanger 111. The second heat exchanger 109 can also use an external coolant in addition to, or in replacement of, the nitrogen or gas leaving the third heat exchanger 111.
[0044] The expansion element 110 can be constituted, in a per se known manner, by an isenthalpic laminar-flow valve or by an isentropic expansion machine.
[0045] The third heat exchanger 111, wherein the gas circulating in the circuit 106 is evaporated, removes heat from the nitrogen at the top of the column 101, causing its condensation, and the resulting downward reflux is used for rectification.
[0046] The fourth heat exchanger 112, which acts as a cooling unit for the gas circulating in the circuit 106, uses the following as coolants: the ternary mixture leaving the column through the duct 103; the nitrogen leaving the top of the column through the duct 104; and the gas of the circuit 106 leaving the third heat exchanger 111 before being fed into the compressor 107. In the fourth heat exchanger 112, the air is also cooled before being fed into the column 112 and the ternary mixture is further heated after an expansion step performed in a turbine 113.
[0047] More particularly, part of the ternary gas mixture, removed through the duct 103, is heated in the heat exchanger 112 and then subjected to isentropic expansion in the turbine 113. The composition of the gaseous ternary mixture is different from the liquid ternary composition that forms at the base of the fractionating column 101. The isentropic expansion of the gaseous ternary mix provides the units of refrigeration required to compensate for the heat losses of the apparatus and to obtain part of the produced nitrogen in the liquid state.
[0048] For the sake of completeness in describing figure 2, it should be noted that on the line for supplying the air or the mixture of oxygen and nitrogen there are provided, in a per se known manner: a filter 114; a compressor 115 which can be actuated by means of the turbine 113 that expands the preheated ternary mixture; a prepurifier 116; and a phase separator 117.
[0049] Proximate to the top of the column 101 it is also possible to provide a duct 104a for removing nitrogen in the liquid state.
[0050] The operation of the apparatus according to the invention, shown in figure 2, for the production of nitrogen is as follows.
[0051] The compressed air or mixture of compressed nitrogen and oxygen, preferably at a pressure substantially between 2 and 20 bar, after the drying and decarbonatation performed in the prepurifier 117, is introduced, partly in the liquefied state, through the duct 102 and the phase separator 117 into the fractionating column 101.
[0052] The ternary mixture composed of oxygen, nitrogen, and argon sinks towards the base of the fractionating column 101 and exits therefrom through the duct 103.
[0053] The nitrogen in the gaseous state rises along the fractionating column 101 and exits therefrom through the duct 104.
[0054] As a consequence of the condensation of the nitrogen performed by the heat exchanger 111, part of the nitrogen flows back downwards, performing rectification.
[0055] The gas circulating inside the circuit 106, at the output of the compressor 107, has a pressure preferably between 0 and 30 bar and is fed through the fourth heat exchanger 112, where it is subjected to a first cooling. The gas leaving the fourth heat exchanger 112 is sent through the first heat exchanger 108, where said gas condenses, transferring heat to the ternary mixture and causing its evaporation.
[0056] The gas of the circuit 106 that leaves the first heat exchanger 108 is conveyed to the second heat exchanger 109, where it is subcooled.
[0057] After subcooling, the gas of the circuit 106 is subjected to isentropic or isenthalpic expansion and then conveyed into the third heat exchanger 111, where it evaporates, removing heat from the nitrogen and causing its condensation. In practice, the third heat exchanger 111 acts as an evaporator for the circuit 106 and as a condenser for the nitrogen at the top of the fractionating column 101.
[0058] The first heat exchanger 108 constitutes a condenser for the circuit 106 and at the same time acts as a reboiler for the ternary mixture at the base of the fractionating column 101.
[0059] The gas of the circuit 106, at the output of the third heat exchanger 111, returns to the compressor 107 after being returned to ambient temperature by means of the heat exchanger 112.
[0060] The nitrogen leaving the fractionating column 101 through the duct 104 has a purity of more than 98%.
[0061] The amount of liquid nitrogen being produced depends on the pressure whereat the air or mixture of oxygen and nitrogen is fed to the fractionating column 101 and on the composition of the gaseous ternary mixture leaving the fractionating column 101, since the lower the percentage of oxygen in said ternary mixture, the higher the amount of mixture that is sent to the isentropic expansion performed by the turbine 113.
[0062] Tests have shown that the amount of nitrogen obtained with the apparatus according to the present invention is higher than 50% and can reach 80% of the amount of air fed into the fractionating column 101.
[0063] In practice it has been observed that the apparatus according to the invention fully achieves the intended aim, since it allows to produce, with a single fractionating column and therefore with modest investment and operating costs, compressed oxygen with a purity of more than 95% and compressed nitrogen with a purity of more than 98%, using all the oxygen and nitrogen contained in the air or in the mixture introduced in the fractionating column, or nitrogen with a purity of more than 98% with a considerably higher extraction efficiency than conventional nitrogen production systems.
[0064] A particularly advantageous aspect of the apparatus according to the invention in the simultaneous production of oxygen and nitrogen is the considerable energy saving which arises from the lower pressure of the processed air; in conventional apparatuses with two fractionating columns, the units of refrigeration required for the operation of the apparatus are produced by isentropically expanding part of the air upstream of the medium-pressure column, which operates at 4.5-5.5 bar, whilst in the apparatus according to the present invention the units of refrigeration required for its operation are obtained by isentropically expanding all or part of the nitrogen leaving the column and therefore downstream of the fractionating column.
[0065] The apparatus thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the inventive concept; all the details may furthermore be replaced with other technically equivalent elements.
[0066] In practice, the materials employed, as well as the dimensions, may be any according to requirements and to the state of the art.
[0067] Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.
1. An apparatus for fractionating air, or a mixture of oxygen and nitrogen, for the simultaneous
production of nitrogen and oxygen or for the production of nitrogen, characterized
in that it comprises:
-- a fractionating column having an intake duct for air or a mixture of oxygen and nitrogen under pressure, a nitrogen discharge duct located at the top of the fractionating column, and a discharge duct for the oxygen or the ternary mixture constituted by oxygen, nitrogen, and argon located at the base of the fractionating column;
-- a closed circuit which provides a gas-based refrigeration cycle and comprises, in sequence:
-- a compressor;
-- an exchange line to bring the refrigerating gas to a temperature which is close to liquefaction;
-- a first heat exchanger acting as a condenser for said cycle and producing the evaporation of the oxygen or of the ternary mixture at the base of said column;
-- a second heat exchanger acting as a subcooling unit for the gas that leaves said first heat exchanger;
-- an expansion element;
-- a third heat exchanger which acts as an evaporator for said cycle and condenses the nitrogen at the top of said column for rectification reflux.
2. An apparatus according to claim 1, characterized in that said gas of the circuit providing
a refrigeration cycle is constituted by argon.
3. An apparatus according to claim 1, characterized in that said gas of the circuit providing
a refrigeration cycle is constituted by a gas mixture containing at least 40% argon.
4. An apparatus according to claim 1, characterized in that said gas of the circuit providing
a refrigeration cycle is constituted by a refrigerant gas.
5. An apparatus according to claim 1, characterized in that said exchange line comprises
a fourth heat exchanger which acts as a cooling unit for said gas, said fourth heat
exchanger being arranged between said compressor and said first heat exchanger and
using, as refrigerants, the gas leaving said third heat exchanger, the nitrogen leaving
the top of said column, and the oxygen or ternary mixture leaving the base of said
column.
6. An apparatus according to claim 5, characterized in that it comprises an isentropic
expansion turbine the intake whereof is connected to the duct for the nitrogen that
leaves said fourth heat exchanger.
7. An apparatus according to claim 5, characterized in that it comprises an isentropic
expansion turbine the input whereof is connected to the duct of the ternary mixture
that leaves said fourth heat exchanger.
8. An apparatus according to claim 7, characterized in that the duct leaving said isentropic
expansion turbine passes through said fourth heat exchanger again and then through
a prepurifier for the air, or mixture of oxygen and nitrogen, which is fed to said
fractionating column.
9. A method for simultaneously producing oxygen and nitrogen or for producing nitrogen
by fractionating air, or a mixture of oxygen and nitrogen, in a fractionating column,
which consists in introducing in the fractionating column compressed air, or a compressed
mixture of oxygen and nitrogen, characterized in that part of the nitrogen at the
top of the fractionating column is liquefied by passing through a heat exchanger which
constitutes the evaporator of a closed circuit, which provides a gas-based refrigeration
cycle, for rectification; and in that the liquid oxygen or the liquid ternary mixture
at the base of the column is used as a refrigerant in a heat exchanger that constitutes
the condenser of said closed circuit.
10. A method according to claim 9, characterized in that the gas of said circuit, in output
from said heat exchanger that constitutes the condenser of the circuit, is subcooled
in an additional heat exchanger the refrigerant whereof is one or more of the following
fluids: the nitrogen leaving the top of the fractionating column; the nitrogen leaving
the top of the fractionating column after cooling and expansion; the gas leaving the
heat exchanger that constitutes said evaporator; an external refrigerant.
11. A method according to claims 9 and 10, characterized in that the gas of the circuit
is constituted by argon.
12. A method according to claims 9 and 10, characterized in that the gas of said circuit
is constituted by a gas mixture containing at least 40% argon.
13. A method according to claims 9 and 10, characterized in that said gas is constituted
by a refrigerant gas.
14. A method according to claim 9, characterized in that the air or mixture of oxygen
and nitrogen is fed to said fractionating column at a pressure which is substantially
between 2 and 20 bar.
15. A method according to claim 10, characterized in that the gas of said circuit, after
compression, is cooled in an exchange line before being fed into the heat exchanger
constituting the condenser of said circuit.
16. A method according to claim 15, characterized in that said exchange line comprises
a fourth heat exchanger, which acts as a cooling unit for said gas and uses, as refrigerants,
the gas of said circuit that leaves said additional heat exchanger, the nitrogen that
leaves the top of said column, and the oxygen that leaves the base of said column.
17. A method according to claim 10, characterized in that said exchange line comprises
a fourth heat exchanger the refrigerants whereof are: the nitrogen that leaves the
top of said column, the ternary mixture that exits proximate to the base of said column,
and the gas of said circuit that leaves said additional heat exchanger.
18. A method according to claim 17, characterized in that the ternary mixture leaving
the base of said fractionating column is subjected to isentropic expansion after heating
in the heat exchanger of said exchange line and is then sent again through said heat
exchanger of the exchange line in order to provide the units of refrigeration which
are required to compensate for the heat losses of the apparatus and to obtain part
of the produced nitrogen in the liquid state.