Method and apparatus for producing argon by cryogenic air separation

Abstract

A method for producing an argon-rich fraction by cryogenic air separation comprises:-introducing feed air into a distillation system for oxygen-nitrogen separation comprising a low pressure column; -withdrawing an argon-containing fraction from the low pressure column and introducing it into a crude argon column having a crude argon overhead condenser indirectly cooled by the cryogenic working fluid; -drawing an argon-rich fraction from the upper section of the crude argon column; -measuring the oxygen content in the argon-containing fraction, characterized in that-measuring the nitrogen content in the argon-containing fraction; -predicting the nitrogen content in the crude argon condenser on the basis of the measured nitrogen content; and controlling the operation of the crude argon column as a function of such corrected estimated argon concentration dependent on such predicted nitrogen content in the crude argon condenser. Corresponding devices are also claimed.

Description

[0001] The present invention relates to a method for controlling a process for producing argon by air separation and a corresponding apparatus as defined in the preamble of the independent claims.

[0002] The "distillation system for oxygen-nitrogen separation" of the present invention may be a classical Linde double column. Other systems may be used, such as a system having two or more columns side by side or a system having three or more columns.

[0003] The main products of a cryogenic air separation plant are nitrogen and / or oxygen. If the plant is large enough, argon is usually produced as an additional product. A common configuration of such a plant has a crude argon column and a double column as the "distillation system for oxygen-nitrogen separation", as shown in Figure 2.3A on page 22 of the 2008 edition of "Industrial Gas Processes". In order to achieve a high argon production, the crude argon column must be controlled. For this purpose, it is known to measure specifically the oxygen content in the argon-containing fraction fed to the crude argon column by means of a standard oxygen analyzer.

[0004] The object of the present invention is to increase the argon production of such a system. This object is achieved by the features defined in the second part of claim 1.

[0005] The present invention does not use the classical ways that affect the product yield of the distillation product, i.e., changing the number of theoretical trays or the type of mass exchange elements, or smoothly changing the reflux, etc. During the present invention, it has surprisingly been found that the classical control methods lead to instabilities, and these instabilities can be greatly reduced by nitrogen measurement, and such features indirectly increase the argon production.

[0006] Since the argon-containing fraction (also known as the argon transition fraction) does not contain much nitrogen, it is not reasonable to use the nitrogen content for control. The typical composition of such a fraction is 90 mol% argon, 10 mol% oxygen and 150 mol-ppm nitrogen. On the other hand, even a fast nitrogen analyzer can measure at a frequency of several minutes. It is not expected that they can help with control with a shorter response time.

[0007] However, in the present invention, it is shown that using the nitrogen value to correct the known oxygen-based control can greatly increase the argon production. The benefit of argon recovery can be up to 1%, and depending on the product state (liquid or gaseous), the ASU configuration is based on the use of design margins. The recovery rate can be increased by implementing a control concept based on the measurement of nitrogen in the vapor fed to the crude argon column. The advantage of the nitrogen control concept over the existing concepts is that the source of inert gas that may block the crude argon condenser is monitored at the inlet of the system. The existing measurements for the same purpose are located further upstream in the low-pressure column or downstream of the crude argon condenser. These measurements are indirect measurements with a delay time, and there may be other reasons for the values to change. In contrast, the present invention provides a direct measurement with almost no delay.

[0008] Preferably, a gas chromatograph, specifically a fast gas chromatograph, is used to measure the nitrogen content. A normal gas chromatograph gives the measurement result after a delay time of 10 s. The delay time of a fast gas chromatograph is typically 30 s.

[0009] According to a further aspect of the present invention, control actions are performed based on the predicted nitrogen content in the crude argon condenser. Preferably, the nitrogen concentration and the oxygen concentration in the argon-containing fraction are directly measured, and the nitrogen concentration and the oxygen concentration are predicted in the condenser and the argon concentration is calculated based on these values. The control actions can be one or more of the following.

[0010] - Correct the set value of the controller for the oxygen content in the argon-containing fraction.

[0011] - Initiate blowing of crude argon from the top of the crude argon column, from the crude argon top condenser, or from the crude argon product line to avoid instability of the crude argon top condenser.

[0012] - Adjust the crude argon condenser load to optimize the recovery rate, and such adjustment is preferably achieved by controlling the feed to the crude argon column.

[0013] As another aspect of the present invention, alternatively or in addition, in the case where the value of the nitrogen content measured in the argon-containing fraction is high, the control can trigger one or more of the following measures.

[0014] - Reduce the extraction of pressurized gaseous nitrogen from the high-pressure column, wherein the distillation system for oxygen-nitrogen separation further includes a high-pressure column that has a heat exchange relationship with the low-pressure column, and a pressurized gaseous nitrogen stream is extracted from the upper section of the high-pressure column and recovered as a pressurized gaseous nitrogen product.

[0015] - Reduce the extraction of oxygen product from the low-pressure column, wherein the oxygen product in gaseous form and / or liquid form is extracted from the lower section of the low-pressure column, and thus the control includes reducing the extraction of oxygen product from the lower section of the low-pressure column.

[0016] - Reduce the recycle stream, wherein the working fluid of the air separation is used as the recycle stream, and the recycle stream is heated, compressed, cooled, and turbo-expanded.

[0017] - Reduce the turbine stream, wherein the working fluid of the air separation is used as the turbine stream, and the turbine stream is turbo-expanded.

[0018] - Increase the amount of feed air entering the distillation system for oxygen-nitrogen separation.

[0019] "The value of the nitrogen content measured in the argon-rich fraction" means a value higher than a predefined threshold. This threshold can be the same for all of the above-mentioned controlled parameters, or can be different for at least two of them or for all of the controlled parameters. These thresholds can be constant for the operating conditions, can vary with the operating conditions, or can depend on other parameters. Specifically, the nitrogen concentration threshold can additionally depend on the load of the crude argon column, i.e., on the amount of argon-rich fraction withdrawn from the low-pressure column and introduced into the crude argon column; - In the case of reduced load, it is possible to increase the nitrogen content in the crude argon condenser.

[0020] The "turbine stream" and the "recycle stream" can be the same or can be different. If it is not a recycle turbine, the turboexpander can be an air turbine expanding into the high-pressure column or into the low-pressure column, or a nitrogen turbine expanding the gaseous nitrogen from the high-pressure column or from the low-pressure column.

[0021] In the present invention, the crude argon column can be used to recover the final product or intermediate product by withdrawing the argon-rich fraction from the crude argon column as the crude argon product. Such a crude argon product can be used directly or further purified in a pure argon column. Alternatively, the crude argon column can be used as an argon removal column for removing argon from the process without using it as the final product. In this case, the argon-rich fraction is either directly removed or heated in the main heat exchanger, either before or after being mixed with another waste gas.

[0022] In Figure 1In the method of the accompanying drawings, atmospheric air is drawn in via filter 2 by air compressor 3 and compressed in the air compressor to an absolute pressure of 5.0 bar to 7.0 bar, preferably about 5.5 bar, and then cooled in direct contact cooler 4 by direct heat exchange with cooling water 5, 6, where the cooling water on the one hand (5) comes from evaporative cooler 7 and on the other hand (6) is supplied by an external source. The compressed and cooled air 8 is purified in purification device 9, which has pairs of vessels filled with an adsorbent material, preferably molecular sieve. The purified air 10 is cooled to about its dew point in main heat exchanger system 11a, 11b, 11c. The cold air 12 is introduced into high-pressure column 13 of a distillation system for nitrogen-oxygen separation, which distillation system also has low-pressure column 14. High-pressure column 13 and low-pressure column 14 are designed as a classical Linde double column and are connected via main condenser 15 to have a heat exchange relationship. The operating pressure at the top of the high-pressure column is 4.5 bar to 6.5 bar, preferably about 5.0 bar, while the operating pressure at the top of the low-pressure column is 1.2 bar to 1.7 bar, preferably about 1.3 bar.

[0023] Liquid crude oxygen 16 is removed from the bottom of high-pressure column 13, cooled in subcooler 17, and further cooled to part 19 in bottom evaporator 21 of pure argon column 20. Another part 22 can bypass bottom evaporator 21. Subsequently, part 23 flows into the evaporation chamber of top condenser 24 of crude argon column 25, and another part flows into the evaporation chamber of top condenser 27 of pure argon column 20. The crude oxygen 28, 29 evaporated in top condensers 24, 27 is supplied to low-pressure column 14 at a first intermediate point via line 30. The fraction 31 remaining in liquid form in top condenser 24 of crude argon column 25 is also directed to the first intermediate point of low-pressure column 14. The part 32 remaining in liquid form in top condenser 27 of pure argon column 20 is fed to a second intermediate point of low-pressure column 14, which second intermediate point is located above the first intermediate point.

[0024] Gaseous nitrogen 33 from the head of high-pressure column 13 is fed to a first part 34 at the cold end of main heat exchanger 11a, heated in the main heat exchanger to about ambient temperature, and then divided into a pressure product stream 36 (GAN I) and a recycle stream 37. The recycle stream 37 is compressed in recycle compressor 38 with aftercooler 39 to a pressure of 25 bar to 60 bar, preferably about 35 bar, and cooled in main heat exchanger 11a. The intermediate-temperature part 40 of the high-pressure nitrogen is removed from the main heat exchanger and expanded in expansion turbine 41 to about the high-pressure column pressure. The expanded recycle stream 42 is added again to the cold pressure product stream 34. Any liquid present is pre-separated (43) and sent via line 44 to the top of low-pressure column 14. Another part 61 of the high-pressure nitrogen is directed to the cold end of main heat exchanger 11a and then sent to high-pressure column 13.

[0025] The remaining gaseous head nitrogen 45 of the high-pressure column 13 is at least partially condensed in the main condenser 15. The resulting liquid nitrogen 46 is fed in section 47 to the high-pressure column 13 as reflux liquid. Another part 48, 49 is led to the top of the low-pressure column 14 after being subcooled in the subcooler 17. There, part 50 can be removed as liquid nitrogen product (LIN).

[0026] Just above the bottom of the low-pressure column 14, gaseous oxygen 51 is withdrawn, heated in the main heat exchanger 11a, and withdrawn via line 52 as non-pressurized gaseous product (GOXIII). The liquid oxygen stream 53 from the bottom of the low-pressure column 14 is subcooled in the subcooler 17 and fed via line 54 to the liquid tank (LOX). At least part of the liquid oxygen is removed from the tank via line 55, pressurized in the pump 56 to the desired product pressure, for example 6 bar to 60 bar, preferably about 31 bar, and heated to ambient temperature by evaporation of high-pressure nitrogen (or pseudo-evaporation if at supercritical pressure) in the main heat exchanger 11a and finally withdrawn via line 57 as gaseous high-pressure product (GOX I). A part 58 of the high-pressure liquid is depressurized via the throttle valve 59 to an intermediate pressure of, for example, 6 bar to 25 bar, preferably about 15 bar, and evaporated at this lower pressure and withdrawn via line 60 as gaseous medium-pressure product (GOX II).

[0027] The gaseous nitrogen 62, 63, 64 from the top of the low-pressure column 14 and the gaseous impurity nitrogen 65, 66, 67 from the intermediate point of the low-pressure column 14 are each heated in the subcooler 17, heated in the main heat exchanger block 11c or 11b and optionally used, after heating 69, as regeneration gas for the cleaning device 9 via line 68, supplied via line 70 to the evaporative cooler 70 and / or directly supplied via line 71 to the purification device 9, supplied via line 70 to the evaporative cooler 70 and / or blown directly into the atmosphere via line 71.

[0028] At a third intermediate point arranged below the first intermediate point, the argon-containing fraction 72 is withdrawn from the low-pressure column 14 and fed to the crude argon column 25, just above the bottom. (In this embodiment, the crude argon column 25 is placed in a single container; alternatively, it can be split into two or more containers arranged side by side.) The bottom liquid 73 of the crude argon column is fed back to the low-pressure column via the pump 74 and line 75.

[0029] The top condenser 24 of the crude argon column 25 is designed as a reflux condenser. (Alternatively, it can be designed as a conventional condenser where the gas and liquid flow in the same direction.) The gas from the top of the crude argon column 25 flows into the return channel at the bottom of the column and is partially condensed there. The condensate thus formed flows downward countercurrently to the rising gas in the return channel and is used as the liquid return in the crude argon column 25. On the evaporation side, the top condenser 24 is designed as a bath condenser. The cooling fluid formed here from the liquid crude oxygen 23 flows into the evaporation channel at the bottom of the column via one or more lateral openings and is partially evaporated there. Due to the thermosyphon effect, the liquid is carried away and leaves together with the evaporated portion at the upper end of the evaporation channel and returns to the liquid bath. Thus, the top condenser is designed as a bath evaporator on the evaporation side. (Alternatively, it can be a once-through evaporator.)

[0030] From the upper end of the reflux channel, the gaseous crude argon stream 76 is taken out as an argon-rich fraction via the lateral manifold and fed to the pure argon column 20 at an intermediate point. In this embodiment, the top condenser of the pure argon column 20 is generally designed on the liquefaction side, i.e., the top gas 77 of the pure argon column 20 flows from the top to the bottom through the liquefaction channel. (Alternatively, the main condenser 15 and / or the top condenser 27 of the pure argon column 20 can also be formed as reflux condensers.) In this example, the residual gas stream 78 is extracted from the head condenser 27 and blown into the atmosphere. Alternatively, this residual gas stream can be returned via its own blower to the distillation column system for nitrogen-oxygen separation or to the front end of the air compressor 3.

[0031] The bottom liquid 79 of the pure argon column 20 is evaporated to a part 80 in the bottom evaporator 21, and the generated vapor 81 is used as the rising gas in the pure argon column 20. The remaining part is the liquid pure argon product stream 82.

[0032] According to the present invention, a nitrogen analyzer 100 for measuring the nitrogen content is arranged in the pipeline 72 for transporting the argon-containing fraction from the low-pressure column 14 to the crude argon column 25. The analyzer 100 (or a separate oxygen analyzer) can additionally measure the oxygen content in the argon-containing fraction 72. The measurement data is sent via the data connection 101 to a computer 110, which predicts the nitrogen content in the crude argon condenser based on the measured nitrogen content and optionally the argon content there. Another data pipeline controls at least one control device according to the predicted nitrogen content.

Claims

1. A method for producing an argon-rich fraction by cryogenic air separation, the method comprising - introducing feed air into a distillation system for oxygen-nitrogen separation including a low-pressure column, - withdrawing an argon-containing fraction from the low-pressure column and introducing it into a crude argon column, the crude argon column having a crude argon top condenser indirectly cooled by a cryogenic working fluid, - withdrawing an argon-rich fraction from the upper section of the crude argon column, - measuring the oxygen content in the argon-containing fraction, characterized in that - measuring the nitrogen content in the argon-containing fraction, - predicting the nitrogen content in the crude argon condenser based on the measured nitrogen content, and - controlling the operation of the crude argon column according to such predicted nitrogen content in the crude argon condenser.

2. The method according to claim 1, wherein a gas chromatograph, specifically a fast gas chromatograph, is used to measure the nitrogen content.

3. The method according to claim 1 or 2, wherein One or more of the following control actions are performed according to the predicted nitrogen content in the crude argon condenser: - correcting the set value of a controller for the oxygen content in the argon-containing fraction, - initiating the blowing of crude argon from the top of the crude argon column, from the crude argon top condenser and / or from a crude argon product line to avoid instability of the crude argon top condenser, - adjusting the crude argon condenser load to optimize the recovery rate, specifically by controlling the flow rate of the argon-containing fraction from the low-pressure column to the crude argon column.

4. The method according to any one of claims 1 to 3, wherein the distillation system for oxygen-nitrogen separation further includes a high-pressure column, the high-pressure column having a heat exchange relationship with the low-pressure column, whereby a pressurized gaseous nitrogen stream is withdrawn from the upper section of the high-pressure column and recovered as a pressurized gaseous nitrogen product, and the control includes reducing the withdrawal of the pressurized gaseous nitrogen when the value of the nitrogen content measured in the argon-containing fraction is high.

5. The method according to any one of claims 1 to 4, wherein an oxygen product is withdrawn from the lower section of the low-pressure column, whereby the control includes reducing the withdrawal of the oxygen product from the lower section of the low-pressure column when the value of the nitrogen content measured in the argon-containing fraction is high.

6. The method according to any one of claims 1 to 5, wherein When the value of the nitrogen content measured in the argon-containing fraction is high, the working fluid for air separation is used as a recycle stream, and the recycle stream is heated, compressed, cooled and turbo-expanded.

7. The method according to one of claims 1 to 6, wherein, The working fluid for air separation is used as a turbo stream, and the turbo stream is turbo-expanded, whereby when the value of the nitrogen content measured in the argon-containing fraction is high, the turbo stream is reduced.

8. The method according to any one of claims 1 to 7, whereby when the value of the nitrogen content measured in the argon-containing fraction is high, the amount of feed air introduced into the distillation system for oxygen-nitrogen separation is increased.

9. The method according to any one of the preceding claims, whereby the argon-rich fraction is withdrawn from the crude argon column as a crude argon product.

10. An apparatus for producing an argon-rich fraction by cryogenic air separation, the apparatus comprising - an air feed line for introducing feed air into a distillation system for oxygen-nitrogen separation including a low-pressure column, - An argon transition pipeline for withdrawing an argon-containing fraction from the low-pressure column and introducing it into a crude argon column, the crude argon column having a crude argon top condenser indirectly cooled by a cryogenic working fluid, - A crude argon product pipeline for withdrawing an argon-rich fraction from the upper section of the crude argon column, - An oxygen analyzer for measuring the oxygen content in the argon-containing fraction, characterized in that - A nitrogen analyzer for measuring the nitrogen content in the argon-containing fraction, - A computing device for predicting the nitrogen content in the crude argon condenser based on the measured nitrogen content, and - A control device for controlling the operation of the crude argon column according to such predicted nitrogen content in the crude argon condenser.

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