FR2853405A1 - Cryogenic distillation air separation procedure and plant uses lightening gas formed at least partly from purging gas drawn from vaporizer-condenser - Google Patents

Abstract

The air separation procedure consists of transferring at least one liquid via a rising duct with a pressure reducing valve from a first stage operating at one pressure to a second stage operating at a lower pressure. The air separation procedure consists of transferring at least one liquid via a rising duct (6, 9, 12) with a pressure reducing valve (7, 10, 13) from a first stage operating at one pressure to a second stage operating at a lower pressure. In advance of the reducing valve a lightening gas is injected, delivered at a pressure greater than the sum of the second pressure and the pressure created by a column of liquid between the point of the injection and the point at which the liquid is introduced into the second stage. A flow of air (5) is sent to one column (2) of a double separating column, and a fluid rich in nitrogen and another rich in oxygen are drawn from the low-pressure column of the double separation column. At least part of the aforementioned lightening gas consists of a purging gas drawn from the vaporizer-condenser, the gas containing at least one component that does not condense at the lowest operating temperature of the vaporizer-condenser.

Description

The present invention relates to methods and installations for

  production of cryogenic air gases.

  In this type of installation, there are circuits starting from equipment in which a fluid is withdrawn to arrive in another equipment through a regulating member in which it undergoes an expansion (for example, a valve). .

  In a number of cases, the type of flow in the piping downstream of the valve may require the installation of a gas injection at a time to ensure a steady and stable flow but also sometimes to make possible the flow fluid in all the different cases of steps envisaged for the production unit (for example steps reduced in air flow, starting or restarting).

  An injection of a gas at a predetermined level downstream of the valve helps the flow of the fluid expanded in the valve, as described in EP-A0567360.

  In order to minimize the irreversibilities due to the introduction of this gas into the fluid, the sample gas is generally chosen to have a composition and a temperature close to the fluid in which it is injected. This gas must of course have sufficient pressure. This injection flow rate is generally low (in mass) relative to the flow of the main fluid, which sometimes makes it possible anyway the injection of fluid of very different nature.

  In EP-A-0567360, several examples are proposed.

  The rich liquid is drawn off in a medium pressure column (having a conventional pressure around 5 bars a) and then expanded in a valve (after undercooling) before being introduced at an intermediate level into the low pressure column (having a conventional pressure around 1.5 bars a). Since this 3.5 bar pressure difference is sometimes insufficient to bring the rich liquid up to the low pressure column, a flow of air in the air entering the medium pressure column is injected downstream of the pressure column. relaxation valve.

  Likewise, a nitrogen-rich fluid is withdrawn from the head of the medium pressure column and then expanded in a valve (after subcooling) before being introduced at a high level into the low pressure column. To facilitate the rise of this liquid, nitrogen gas drawn at the top of the medium pressure column is injected downstream of the expansion valve.

  The disadvantage of this gas injection system downstream of the expansion valve lies in the fact that it takes from the medium pressure column a gas which does not participate in the distillation in the medium pressure column thus generating a loss of power of rectification.

  In all the double column air separation apparatus, there is a medium pressure column and a low pressure column, thermally connected to each other by a vaporizer-condenser which serves to condense a nitrogen enriched gas from the medium pressure column. against an oxygen enriched liquid of the low pressure column. The nitrogen enriched gas is partially condensed and the condensed fluid is returned to the medium pressure column and / or the low pressure column as reflux. Since the nitrogen-enriched gas contains gases which do not condense at column temperatures, a vaporizer containing, for example, helium, neon, hydrogen and nitrogen as described above must be withdrawn from the vaporizer. in "The Separation of Gases" by Ruhemann, 1945, pp.220-221, otherwise these gases would accumulate in the vaporizer. In general, this purge rate is expanded and then mixed with a residual nitrogen stream withdrawn from the low pressure column.

  Other non-condensed gas purge rates are withdrawn from the other vaporizers-condensers of the apparatus.

  The basic idea of this invention is to use the gas purge rate that does not condense in a vaporizer-condenser of the apparatus as a flow rate of gas to be injected into the riser pipes.

  By using, as injection gas, a fluid which does not participate anyway in the rectification, no loss of yield is generated.

  The present invention relates to methods and facilities for the production of cryogenic air gases.

  In this type of installation, there are circuits starting from equipment in which a fluid is withdrawn to arrive in another equipment through a regulating member in which it undergoes a relaxation (for example, a valve).

  In a number of cases, the type of flow in the piping downstream of the valve may require the installation of a gas injection at a time to ensure a steady and stable flow but also sometimes to make possible the flow of the fluid in all the different cases of steps envisaged for the production unit (for example steps reduced in air flow or starts).

  An injection of a gas at a predetermined level downstream of the valve helps the flow of the fluid expanded in the valve, as described in EP-A0567360.

  In order to minimize the irreversibilities due to the introduction of this gas into the fluid, the sample gas is generally chosen to have a similar composition and temperature of the fluid in which it is injected. This gas must of course have sufficient pressure. This injection flow rate is generally low (in mass) with respect to the flow rate of the main fluid, which makes it possible anyway to inject fluid of very different nature.

  In EP-A-0567360, several examples are proposed.

  The rich liquid is drawn off in a medium-pressure column (having a conventional pressure around 5 bars a) and then expanded in a valve (after under cooling) before being introduced at an intermediate level into the low pressure column (having a pressure classic around 1.5 bars a). Since this pressure difference of 3.5 bar is sometimes insufficient to make the rich liquid rise to the low pressure column, an air flow taken in the air entering the medium pressure column is injected downstream of the valve. of relaxation.

  Similarly, a nitrogen-rich fluid is withdrawn from the head of the medium pressure column and then expanded in a valve (after subcooling) before being introduced at a high level into the low pressure column. To facilitate the rise of this liquid, nitrogen gas withdrawn at the top of the MP is injected downstream of the expansion valve.

  The disadvantage of this gas injection system downstream of the expansion valve lies in the fact that it takes from the medium pressure column a gas which does not participate in the distillation in the medium pressure column thus generating a loss of power of rectification.

  In all the double column air separation apparatus, there is a medium pressure column and a low pressure column, thermally connected to each other by a vaporizer-condenser which serves to condense a nitrogen enriched gas from the medium pressure column. against an oxygen enriched liquid of the low pressure column. The nitrogen-enriched gas is partially condensed and the condensate is returned to the medium pressure column and / or the low pressure column as reflux. Since the nitrogen-enriched gas contains gases which do not condense at the column temperatures, a vaporizer containing helium, neon, hydrogen and nitrogen as described in US Pat. The Separation of Gases "by Ruhemann, 1945, pp.220-221, otherwise this gas would accumulate in the vaporizer. In general, this purge flow rate is expanded and then mixed with a flow of residual nitrogen withdrawn from the low pressure column.

  Other non-condensed gas purge rates are withdrawn from the other vaporizers-condensers of the apparatus.

  The basic idea of this invention is to use the gas purge flow rate which does not condense in a vaporizer-condenser of the apparatus as a flow rate of gas to be injected into the riser pipes.

  By using, as injection gas, a fluid which does not participate anyway in the rectification, no loss of yield is generated.

  Examples of implementation of the invention will now be described with reference to the accompanying drawing, in which: - Figure 1 shows schematically an air distillation plant according to the invention.

  The air distillation plant shown in FIG. 1 essentially comprises a double distillation column 1. This comprises a medium pressure column 2 surmounted by a low pressure column 3. A vaporizercondenser 4 puts in exchange relation thermally the overhead vapor of column 2, consisting of substantially pure nitrogen, and the bottom liquid of column 3, consisting of oxygen at a purity determined.

  A pipe 19 withdraws a purge gas containing nitrogen, helium, neon and hydrogen from the head of the vaporizer-condenser 4, helium, neon and hydrogen being gases which do not do not condense at the coldest operating temperature of the vaporizer-condenser 4.

  In operation, air at a pressure typically of 5 x 105 to 6 x 105 25 Pa is introduced into the vat of column 2 via a supply line 5. "Rich liquid" (oxygen-enriched air) is drawn off in the tank of this column 2 via a pipe 6 equipped with an expansion valve 7, subcooled in a subcooler 8 upstream of this expansion valve, expanded in the latter at a pressure slightly greater than the atmospheric pressure, and introduced at an intermediate point of the low pressure column 3. Between the subcooler 8 and the expansion valve 7 is stitched a riser 9 equipped with an expansion valve 6 and leading to the condenser head of a separation column oxygen / argon (not shown) coupled to the low pressure column 3 in a conventional manner.

  "Lean liquid" (impure nitrogen) is withdrawn at an intermediate point of column 2 via a pipe 11 equipped with an expansion valve (not shown) and, after subcooling and expansion, is introduced at an intermediate point of column 3. Substantially pure liquid nitrogen is withdrawn at the top of the column 3 via a line 12 equipped with an expansion valve 13, subcooled in a subcooler 14 upstream of this expansion valve, expanded in the latter and introduced at the top of column 3.

  FIG. 1 also shows a pipe 15 for producing gaseous oxygen leaving the tank of the column 3, a pipe 16 producing pure nitrogen, starting from the top of this column 3, a pipe 17 of blowing air at an intermediate point of the column 3, and a conduit for discharging waste gas (impure nitrogen) from the upper part of this column.

  It is seen that three different liquids must be raised from the lower column 2 to the upper column 3 and that a liquid must be raised to the top of the oxygen / argon separation column. If these columns are of the packed type, in particular structured, and / or have many theoretical plates, and / or if the vaporizer-condenser 4 is of a type with a small difference in temperature, it may happen that the pressure difference between the two columns 2 and 3 is barely sufficient to ensure these rising liquid.

  In order to guarantee a good rise of the liquids, in a regular and controlled manner, in all the operating modes of the installation, a purge gas pipe 19 is stitched on the vaporizer-condenser 4 and divides into three branches 20, 21 and 24. The branches 20, 21 are each equipped with an expansion valve 22, 23 and respectively join the lines 6 and 9 just downstream of their expansion valves 7 and 10. Likewise, the branch 24 of gas equipped with an expansion valve 25 joins the pipe 12 just downstream of the expansion valve 13.

  Another gas line 26, equipped with an expansion valve (not shown), starts from a location in the column 2 near the lean liquid withdrawal point (line 11) and joins this line 11 just downstream of the expansion valve of this one.

  In operation, a small flow of purge gas conveyed at the feed pressure of the column 2, through the pipe 19, 20, is expanded in the expansion valve 22 and injected into the rich liquid which has just been expanded in the expansion valve 7. The gas bubbles alleviate the rich liquid and reduce the pressure necessary to raise it up to the column 2.

  For the same purpose, a small purge gas flow conveyed by the pipe 19, 21 is expanded in the expansion valve 23 and injected into the rich liquid which has just been expanded in the expansion valve 10. purge gas through line 19 is low, typically less than 1% of the air flow entering the installation.

  Likewise purge gas conveyed by the pipe 24 is expanded in the expansion valve 25 and injected into the liquid nitrogen which has just been expanded in the expansion valve 13, and impure nitrogen conveyed by the pipe 26 is, after relaxation, injected into the poor liquid conveyed by the pipe 11 and relaxed.

  In practice, the main expansion valves 7, 10 and 13 are placed as low as possible to guarantee their supply by free-flowing liquid, and gas bubbles are introduced just downstream of these expansion valves to assist the propulsion. up the liquids in question. More precisely, the pressure of the lightening gas must be sufficient to overcome the sum of the height of liquid which overcomes the injection point of the gas and the pressure of the low pressure column and this pressure is obtained, in the example shown, by virtue of the fact that each gas, which is available at the pressure of the column 3, is injected above the point of withdrawal of the associated liquid. The lightening gas can also be injected into a riser pipe of liquid air. In pump units, there is a flow of air that is liquefied by heat exchange with a pumped liquid from the column. This air flow is liquefied at a level near the ground and then all or a portion of the liquid is sent to an intermediate level of the low pressure column after expansion in a valve. The lighter gas can be injected over this valve.

  Similarly, line 19 may be supplied wholly or partially from one or more of the following purge gas sources: the head vaporizer at the top of a conventional argon mixture column as described in Figure 4.34 of Hausen and Linde's Tieftemperaturtechnik, 1985, the head vaporizer-condenser and / or the vessel vaporizer-condenser of a conventional denitrogenization column as described in Figure 4.35 of Tieftemperaturtechnik of Hausen and Linde, 1985, a vaporizer-condenser exterior as described in Figure 4.26 of Tieftemperaturtechnik Hausen and Linde, 1985, the top vaporizer-condenser and / or the vaporizer-condenser vessel of a column operating at intermediate pressure as described in EP-A-538118 or in "The Very Low Temperatures" of Gomonet, 1952, a backup vaporizer and condenser as described in US-A-5566556 or a condenser vaporizer of the low pressure column head of a azotonne.

  Of course purge gases from different vaporizer-condensers can be used as lightening gas in the lines of the separation apparatus, without having been mixed.

Claims (16)

  A method of separating air by cryogenic distillation in an air separation apparatus having a column system in which at least one liquid is transferred by means of a riser provided with an expansion valve of a first equipment operating at a first pressure to a second equipment operating at a second pressure, less than the first pressure, into which is injected into the riser pipe, downstream of the expansion valve, a lighter gas available at a higher pressure to the sum of the second pressure and the pressure created by a column of liquid between the injection point of the gas and the point of introduction of the liquid into the second equipment, wherein a flow of air is sent to a column of a double air separation column and a nitrogen-enriched fluid and an oxygen-enriched fluid are withdrawn from the low pressure column of the double separation column characterized by in that at least part of the lightening gas is constituted by a purge gas withdrawn from a vaporizer-condenser of the apparatus, this gas containing at least one component which does not condense at the most cold operation of the vaporizercondenser.   2. The process as claimed in claim 1, wherein the average pressure column and the low pressure column are thermally bonded by a condenser vaporizer which partially condenses a nitrogen enriched gas originating from the medium pressure column and in which at least a portion of the feed gas. lightening is constituted by a gas withdrawn from this vaporizer-condenser.   3. The method of claim 2 wherein the vaporizer-condenser 25 is used to vaporize an oxygen-enriched liquid of the low pressure column.   4. Method according to one of claims 1 to 3 wherein a liquid transferred by means of a riser is an oxygen-enriched liquid from the medium pressure column and for the low pressure column or the condenser of a column argon fed from the low pressure column.   5. Method according to one of claims 1 to 4 wherein a liquid transferred by means of a riser is a nitrogen-enriched liquid from the medium pressure column and for the low pressure column.   6. Method according to one of claims 1 to 5 wherein a liquid transferred by means of a riser is liquefied air available at a level of the medium pressure column and for the low pressure column.   7. Method according to one of claims 1 to 6 wherein a portion of the lightening gas is constituted by supply air from or not coming from the medium pressure column and / or gas enriched in medium pressure nitrogen and / or gas enriched with medium pressure oxygen.   8. A method according to one of claims 1 to 7 wherein the air separation apparatus comprises an argon column fed from the low pressure column and having a head vaporizer-condenser and at least a portion Lightening gas comes from the top vaporizer-condenser of the argon column.   9. Method according to one of claims 1 to 8 wherein the air separation apparatus comprising an argon column fed from the low pressure column and a denitrogenation column supplied from the column 15 of argon, the denitrogenation column having a head vaporizer and / or a tank vaporizer-condenser and at least a part of the lightening gas comes from a vaporizer-condenser of the denitrogenation column.   10. Method according to one of claims 1 to 9 wherein the separation apparatus is constituted by a medium pressure column, a low pressure column and a column operating at an intermediate pressure between the medium and low pressures, the column operating at an intermediate pressure being supplied by an oxygen enriched fluid from the medium pressure column and containing a head vaporizer-condenser and / or a vaporizer-tank condenser and at least a part of the lightening gas comes from a ( a vaporizer-condenser of the column operating at intermediate pressure.   11. Method according to one of claims 1 to 10 wherein vaporizing a liquid from a column of the column system in an external vaporizercondenser outside any column by heat exchange with a heat sink fluid and at least one Part of the lighter gas comes from the external condenser vaporizer.   12. Apparatus for separating air by cryogenic distillation in an air separation apparatus having a column system comprising first equipment operating at a first pressure, second equipment operating at a second pressure, less than the first pressure, a second riser pipe equipped with an expansion valve and connecting a liquid withdrawal point in the first equipment and a liquid introduction point in the second equipment, an injection means in the riser, downstream of the valve expansion, a lighter gas available at a pressure greater than the sum of the second pressure and the pressure created by a column of liquid between the injection point of the gas and the point of introduction of the liquid into the equipment, means for sending an air flow to a column of a double air separation column, means for withdrawing a nitrogen-enriched fluid and an enriched fluid in oxygen of the low pressure column of the double separation column 10 and means for withdrawing a purge gas from a vaporizer-condenser of the apparatus characterized in that at least a portion of the lightening gas is constituted by the purge gas, which gas contains at least one component that does not condense at the coldest operating temperature of the vaporizer condenser.   13. Installation according to claim 12 wherein the vaporizercondenser is a vaporizer-condenser of the low pressure column.   14. Installation according to claim 12 or 13 comprising an argon column and means for sending an argon-enriched gas therefrom from the low pressure column in which the vaporizer-condenser is a vaporizer-condenser of a column head. 'argon.   15. Installation according to one of claims 12 to 14 comprising an argon column and means for sending an argon enriched gas from the low pressure column and a denitrogenation column supplied from the argon column, the denitrogenation column having an overhead vaporizer-condenser and / or a vaporizer-tank condenser and means for withdrawing at least a portion of the lightening gas from a vaporizer-condenser of the denitrogenation column.   16. Installation according to one of claims 12 to 15 comprising a column operating at an intermediate pressure between the medium and low pressure 30 having an overhead vaporizer-condenser and / or a tank vaporizer condenser and means for withdrawing at least a portion lightening gas of a vaporizer-condenser of the column operating at an intermediate pressure.