Process and apparatus for air separation by cryogenic distillation

Abstract

Title: Process and apparatus for air separation by cryogenic distillation In a process for air separation by cryogenic distillation, at least part of the cold required for the process is provided by expansion in at least one turbine (92) of a gas (5) containing at least 95 mol% of oxygen at a pressure of at least 5 bars which comes from an external source. Abstract figure: FIG.1

Description

Title of the invention: Method and apparatus for air separation by cryogenic distillation

[0001] The present invention relates to a method and apparatus for separating air by cryogenic distillation.

[0002] A process for separating air by cryogenic distillation requires a source of cold, which may be the expansion of air or nitrogen produced by the separation or otherwise the addition of a cryogenic liquid from an external source. It is known from US to expand oxygen from a low pressure column of a double air separation column in an expansion turbine to provide cold.

[0003] It has also been known for decades to produce hydrogen by electrolysis of water using electrical energy. This electrolysis also generates pressurized oxygen gas.

[0004] A flow of the present invention is to valorize this by-product of electrolysis by producing cold for an air separation apparatus by cryogenic distillation. Since the purpose of electrolysis is to produce hydrogen, the oxygen may not be sufficient to meet the demand or may be in surplus. If the oxygen produced is insufficient, the presence of an air separation apparatus can make it possible to compensate for this lack of oxygen produced by electrolysis.

[0005] It is therefore interesting to find a solution integrating the production of oxygen by electrolysis and the production of oxygen by cryogenic distillation.

[0006] The present invention relates to a method of using oxygen from an external source by sending it into the main exchanger to cool against the fluids produced by the cryogenic separation of air. This oxygen is then expanded in a turbine and returned to the main heat exchanger. Thus the cold usually obtained by expansion of air or nitrogen is provided by this means and in certain cases, no air or nitrogen turbine may be required. In addition, the energy efficiency of the process is improved.

[0007] The term "main heat exchanger" herein refers to an apparatus in which the supply air is cooled approximately to the dew point.

[0008] The main heat exchanger may consist of several blocks connected in parallel and / or in series.

[0009] The electrolysis of water forms a flow of hydrogen and a flow of oxygen.

[0010] The first is supplied as a hydrogen product.

[0011] The latter contains water due to the production process and is therefore freed from water and other components in a dryer.

[0012] The dryer consists, for example, of at least two beds of adsorbents, which operate alternately.

[0013] According to an object of the invention, there is provided a process for separating air by cryogenic distillation in which: i. An air stream is dried, stripped of CO2, cooled in a heat exchanger and separated in a column system comprising at least one column operating at a cryogenic temperature to form at least one fluid enriched in oxygen relative to air and at least one fluid enriched in nitrogen relative to air. ii. The at least one fluid enriched with oxygen or nitrogen is heated in the heat exchanger. iii. At least part of the cold required for the process is provided by expansion in a turbine of a gas containing at least 95 mol% oxygen at a pressure of at least 5 bar abs which comes from an external source, is cooled to an intermediate temperature of the heat exchanger, is expanded in the turbine and is reheated in the heat exchanger.

[0014] According to other optional aspects: • The external source is a water electrolyzer. • The gas containing at least 95% mol of oxygen has been expanded for at least 10 ab bars. • Gas containing at least 95% mol of oxygen is not compressed between the external source and the turbine. • The column system comprises a first column operating at a first pressure and a second column operating at a second pressure lower than the first pressure, the head of the first column being thermally connected to the vessel of the second column. • The turbine also expands a flow containing at least 93% oxygen from the second column. • The gas expanded in the turbine is mixed with a flow containing at least 93% oxygen from the second column forming a mixture and the mixture is heated in the heat exchanger. • The gas expanded in the turbine is mixed with a flow containing at least 93% oxygen from the second column forming a mixture and the mixture is expanded in a second turbine forming an expanded mixture which is heated in the heat exchanger. • No air or nitrogen gas flow is expanded in a turbine to provide cooling to the process. • No cryogenic liquid flow is sent to the process from an ex- source exterior. • The heat exchanger has one end operating at a first temperature and one end operating at a second temperature lower than the first temperature, the intermediate temperature being between the first and second temperatures. • The expanded gas in the turbine heats up from the operating end to the second temperature. • The gas expanded in the turbine is expanded upstream of the turbine in a valve. • The gas comprising at least 95 mol% oxygen is the only gas expanded in the turbine. • The column system comprises a column having an overhead condenser and a nitrogen-enriched gas is withdrawn at the top of the column. • The column system comprises a column having a bottom reboiler, fed at the top with a bottom liquid from the column having an overhead condenser. • The gas containing at least 95 mol% oxygen is sent to a plurality of first passages of the heat exchanger to cool, at least some of the first passages being adjacent to at least one passage where air is cooled and / or at least one passage where nitrogen gas from the column system is heated, the air and / or the nitrogen gas being at a pressure higher than that of the gas containing at least 95 mol% oxygen which is cooled. • The expanded gas containing at least 95 mol% oxygen is sent to a plurality of second passages of the heat exchanger to be heated, at least some of the second passages being adjacent to at least one passage where air is cooled and / or to at least one passage where nitrogen gas from the column system is heated, the air and / or the nitrogen gas being at a pressure higher than that of the gas containing at least 95 mol% oxygen which is heated. • the gas containing at least 95% mol of oxygen is purified of water upstream of the heat exchanger.

[0015] The invention will be described in more detail with reference to the figures where:

[0016] [Fig-1] represents a method according to the invention.

[0017] [Fig.2] represents a method according to the invention.

[0018] [Fig.3] represents a method according to the invention.

[0019] [Fig.l] represents a method according to the invention in the cold of an oxygen flow 5 containing at least 95 mol% oxygen from an external source, for example an elect- water lyser also producing a hydrogen flow. The oxygen flow 5 is at least 5 bar abs, or even at least 10 bar abs and preferably has not been compressed downstream of the external source. On the other hand, if it contains water, it must be dried upstream of the exchanger 12.

[0020] Dried and CO2-purified air 1 is sent to a hot end of a main heat exchanger at a temperature of 20.0°C, a pressure of 9.9 barA, and a flow rate of 962 Nm3 / h, cooled in the main exchanger 12 to -163.3°C and / or its dew point and sent via line L1 into a separation column 2 operating at a first pressure.

[0021] The column system of the air separation apparatus comprises column 2 and a column 4 operating at a second pressure lower than the first pressure. Column 2 has an overhead condenser 3 heated by nitrogen from column 2 to vaporize liquid oxygen from the bottom of column 4. Air 1 separates in column 2 forming an oxygen-enriched liquid in the bottom and a nitrogen-enriched liquid in the top. These two liquids are sent to column 4.

[0022] 52 Nm3 / h of nitrogen-enriched liquid from the head of column 2 is cooled to - 179°C in a subcooler 8 and then sent to the top of column 4. The nitrogen gas 19 accumulated at the top of column 2 is withdrawn as a product with a flow rate of 19,300 Nm3 / h, heated to 19.0°C in the heat exchanger 12 and produced at a pressure of 9.6 barA, which is that of column 2. In the bottom of column 2, an oxygen-enriched liquid containing approximately 35.8 mol% is withdrawn with a flow rate of 564 Nm3 / h, cooled to -169°C in the subcooler 8 and then sent to the middle of column 4.

[0023] Column 4 comprises a head condenser 23 which condenses the nitrogen gas at the top of the column and returns it to the top of column 4. The head gas is withdrawn as a second nitrogen-rich product 17 with a flow rate of 349 Nm3 / h. After reheating in the subcooler, it is heated to 19.0°C in the heat exchanger 12 and is produced at a pressure of 4.1 barA corresponding to the pressure of column 4. In the bottom of column 4, an oxygen-enriched liquid 13 containing 75.5 mol% oxygen is withdrawn with a flow rate of 267 Nm3 / h, cooled to -179°C in the subcooler 8 and then sent to the head condenser of column 4 to cool it. The liquid is vaporized in the condenser to produce an oxygen-enriched gas which is reheated in the subcooler 8, heated to 19°C in the exchanger and produced at a pressure of 1.2 barA.

[0024] The oxygen-rich gas 5 containing at least 95 mol% oxygen is introduced into the hot end of the heat exchanger 12 at a temperature of 35°C, a pressure of 10 barA, and a flow rate of 46 Nm3 / h, cooled to -75°C which is an intermediate temperature of the exchanger 12, leaves the exchanger 12 and is expanded to 1.28 barA in an expansion turbine 92, cooled to -152°C by the expansion and is reintroduced into the heat exchanger 12 at the cold end. This expanded oxygen can be mixed with a waste gas.

[0025] It will be noted that the process does not include any air turbine or nitrogen turbine. In addition, there is no delivery of cryogenic liquid from an external source.

[0026] An improvement in energy efficiency of approximately 3% is observed.

[0027] In the variant of [Fig. 2], the column system comprises a first column 2 operating at a first pressure and a second column 4 operating at a second pressure lower than the first pressure. The columns 2, 4 are thermally connected by a condenser 3 which condenses nitrogen from the top of the column 2 against liquid oxygen from the bottom of the column 4.

[0028] The column system of the air separation apparatus comprises column 2 and a column 4 operating at a second pressure lower than the first pressure. Column 2 has a head condenser 3 heated by nitrogen from column 2 to vaporize liquid oxygen from the bottom of column 4. Air 1 separates in column 2 forming an oxygen-enriched liquid in the bottom and a nitrogen-enriched liquid at the top. These two liquids are at least partly sent to column 4.

[0029] The process produces liquid oxygen LOX in the bottom of column 4 and liquid nitrogen LIN at the top of column 4 as products to be exported. No flow of gaseous nitrogen is withdrawn at the top of column 2 and column 4 does not include a head condenser.

[0030] According to this variant, a flow of gaseous oxygen 15 is drawn off above the liquid accumulated in the tank of the column 4 and is mixed with the oxygen expanded in the turbine 92, to heat the mixture in the heat exchanger 12 from the cold end.

[0031] In the variant of [Fig.3], based on [Fig.2], the oxygen 5 is expanded in two stages. In the turbine 92, it is the only expanded flow. It is expanded to the pressure of the column 4 which is higher than atmospheric pressure by at least 3 bars. Then the oxygen expanded in the turbine 92 is mixed with the oxygen 15 and the mixture 16 is expanded to a pressure slightly above atmospheric pressure in the turbine 102.

[0032] In this case, the oxygen 5 must be at a minimum pressure of 10 bars.

[0033] The apparatus comprises an argon column 6 which produces gaseous argon Ar, the The head condenser is fed by a fraction of the oxygen-enriched liquid.

[0034] To reduce security issues: • The gas 5 containing at least 95 mol% of oxygen is sent to a plurality of first passages of the heat exchanger 12 to cool, at least some of the first passages being adjacent to at least one passage where cools air 1 and / or at least one passage where gaseous nitrogen 17, 19 from the column system 2, 4 is heated, the air and / or the gaseous nitrogen being at a pressure higher than that of the gas containing at least 95 mol% of oxygen which is cooled.

[0035] If the expanded gas is not mixed with another gas, the expanded gas 5 containing at least 95 mol% oxygen is sent to a plurality of second passages of the heat exchanger 12 to heat up, at least some of the second passages being adjacent to at least one passage where air 1 is cooled and / or to at least one passage where nitrogen gas 17, 19 from the column system is heated, the air and / or the nitrogen gas being at a pressure higher than that of the gas containing at least 95 mol% oxygen which is heated up.

[0036] If the expanded gas is mixed with another gas 15, the formed gas mixture is sent to a plurality of second passages of the heat exchanger 12 to be heated, at least some of the second passages being adjacent to at least one passage where air 1 is cooled and / or to at least one passage where nitrogen gas 17, 19 coming from the column system is heated, the air and / or the nitrogen gas being at a pressure higher than that of the mixture which is heated.

[0037] It will be understood that [Fig.2] may also include an argon column.

Claims

[Claim 1] Claims Process for air separation by cryogenic distillation in which: i. An air flow (1) is dried, purified of CO2, cooled in a heat exchanger (2) and separated in a column system (2, 4) comprising at least one column operating at a cryogenic temperature to form at least one fluid enriched in oxygen (15) relative to the air and at least one fluid enriched in nitrogen (19) relative to the air, ii. The at least one fluid enriched with oxygen or nitrogen is heated in the heat exchanger, and iii. At least part of the cold required for the process is provided by expansion in at least one turbine (92) of a gas (5) containing at least 95 mol% oxygen at a pressure of at least 5 bars which comes from an external source, is cooled to an intermediate temperature of the heat exchanger, is expanded in the at least one turbine and is reheated in the heat exchanger.

Images