CRYOGENIC AIR RECTIFICATION SYSTEM, CONTROL UNIT, AIR SEPARATION UNIT AND METHOD OF CRYOGENICALLY SEPARATING AIR

Abstract

A cryogenic air rectification system comprising a high pressure column, a low pressure column and an argon removal unit coupled to a condenser evaporator, wherein the system is configured to pass gas from a position above an oxygen section of the low pressure column as an argon removal feed gas to a lower region of the argon removal unit, wherein the system is configured to condense gas from an upper region of the argon removal unit in the condenser evaporator to form a condensate, wherein the system is configured to pass further gas from the top of the upper region of the argon removal unit out of the system, and wherein the system is configured to pass at least a part of the condensate as a reflux to the upper region of the argon removal unit.

Claims

1. A cryogenic air rectification system comprising a high pressure column, a low pressure column and an argon removal unit coupled to a condenser evaporator, wherein the system is configured to pass gas from a position above an oxygen section of the low pressure column as an argon removal feed gas to a lower region of the argon removal unit, the system is configured to condense gas from an upper region of the argon removal unit in the condenser evaporator to form a condensate, the system is configured to pass further gas from the upper region of the argon removal unit out of the system, the system is configured to pass at least a part of the condensate as a reflux to the upper region of the argon removal unit, the system comprises a control unit configured to control an oxygen content of the argon removal feed gas as a controlled variable using a flow of the further gas from the upper region of the argon removal unit being passed out of the system as a manipulated variable on the basis of a oxygen content determined in the argon removal feed gas using a feedback control structure, the condenser evaporator is configured to at least partly evaporate a coolant forming an evaporated gas, and the system is configured to pass the evaporated gas, or a part thereof, uncontrolledly into the low pressure column.

2. The system according to claim 1, wherein the feedback control structure is a cascade control structure including an analysis controller as a primary controller and a flow controller or hand controller as a second controller.

3. The system according to claim 2, wherein the primary controller is configured to control the oxygen content of the argon removal feed gas as the controlled variable and wherein the secondary controller is configured to adjust the flow of further gas from the upper region of the argon removal unit being passed out of the system using a flow set point for the secondary controller as the manipulated value.

4. The system according to claim 3, wherein the control unit is adapted to perform a trim control using including a ramping of the flow set point of the flow controller or including a ramping of a valve stroke of the hand controller.

5. The system according to claim 1, wherein the low pressure column comprises a lower column region, an intermediate column region arranged above the lower column region, and an upper column region arranged above the intermediate column region, the lower column region including the oxygen section and the intermediate column region including a rectification section of the argon removal unit.

6. The system according to claim 5, wherein the lower region of the argon removal unit comprises a bottom open with respect to an upper region of the oxygen section to allow an entry of a part of a gas flow raising in the oxygen section as the argon removal feed gas.

7. The system according to claim 6, wherein the rectification section of the argon removal unit is at least in part arranged in a common space with an argon section of the low pressure column which comprises a bottom open with respect to the upper region of the oxygen section to allow an entry of a further part of the gas flow raising in the oxygen section.

8. The system according to claim 5, wherein the condenser evaporator is arranged above the rectification section of the argon removal unit in the intermediate column region and/or the system is configured to pass the evaporated gas, or the part thereof passed uncontrolledly into the low pressure column into the low pressure column via releasing the same into the intermediate column region.

9. The system according to claim 1, wherein the condenser evaporator is arranged above the argon removal column to form a rectification unit arranged besides the low pressure column and the system is configured to pass the evaporated gas, or the part thereof passed uncontrolledly into the low pressure column into the low pressure column via a tubing ending at a feed position into the low pressure column.

10. The system according to claim 1, wherein the condenser evaporator is a forced-flow condenser evaporator.

11. The system according to claim 1, wherein the condenser evaporator is configured to at least partly evaporate, as the coolant, a liquid collected from the intermediate column region and/or wherein the condenser evaporator is configured to at least partly evaporate, as the coolant, a liquid collected in a lower region of the high pressure column.

12. A control unit configured to control an air rectification system, the air rectification system comprising a high a pressure column, a low pressure column and an argon removal unit coupled to a condenser evaporator, wherein the system is configured to pass gas from a position above an oxygen section of the low pressure column as an argon removal feed gas to a lower region of the argon removal unit, the system is configured to condense gas from an upper region of the argon removal unit in the condenser evaporator to form a condensate, the system is configured to pass further gas from the upper region of the argon removal unit out of the system, the system is configured to pass at least a part of the condensate as a reflux to the upper region of the argon removal unit, the control unit is configured to control an oxygen content of the argon removal feed gas as a controlled variable using a flow of the further gas from the upper region of the argon removal unit being passed out of the system as a manipulated variable on the basis of a oxygen content determined in the argon removal feed gas using a feedback control structure the condenser evaporator is configured to at least partly evaporate a coolant forming an evaporated gas, and the system is configured to pass the evaporated gas, or a part thereof, uncontrolledly into the low pressure column.

13. An air separation unit adapted to cryogenically separate feed air, wherein the air separation unit comprises a system according to claim 1.

14. A method for cryogenically separating feed air using an air rectification system comprising high a pressure column, a low pressure column and an argon removal unit coupled to a condenser evaporator, wherein gas from a position above an oxygen section of the low pressure column is passed to a lower region of the argon removal unit as an argon removal feed gas, gas from an upper region of the argon removal unit is condensed in the condenser evaporator to form a condensate, further gas from the upper region of the argon removal unit is passed out of the system, at least a part of the condensate is passed as a reflux to the upper region of the argon removal unit, a control unit controlling an oxygen content of the argon removal feed gas as a controlled variable using a flow of the further gas from the upper region of the argon removal unit being passed out of the system as a manipulated variable on the basis of a oxygen content determined in the argon removal feed gas using a feedback control structure is used a coolant is at least partly evaporated in the condenser evaporator, forming an evaporated gas, and the evaporated gas, or a part thereof, is passed uncontrolledly into the low pressure column.

15. The method according to claim 14, wherein a cryogenic air rectification system comprising a high pressure column, a low pressure column and an argon removal unit coupled to a condenser evaporator, wherein the system is configured to pass gas from a position above an oxygen section of the low pressure column as an argon removal feed gas to a lower region of the argon removal unit, the system is configured to condense gas from an upper region of the argon removal unit in the condenser evaporator to form a condensate, the system is configured to pass further gas from the upper region of the argon removal unit out of the system, the system is configured to pass at least a part of the condensate as a reflux to the upper region of the argon removal unit, the system comprises a control unit configured to control an oxygen content of the argon removal feed gas as a controlled variable using a flow of the further gas from the upper region of the argon removal unit being passed out of the system as a manipulated variable on the basis of a oxygen content determined in the argon removal feed gas using a feedback control structure, the condenser evaporator is configured to at least partly evaporate a coolant forming an evaporated gas, and the system is configured to pass the evaporated gas, or a part thereof, uncontrolled into the low pressure column, is used.

Description

SHORT DESCRIPTION OF THE FIGURES

[0053] FIG. 1 illustrates an air separation unit.

[0054] FIGS. 2 and 3 are detail views of an air separation unit.

[0055] FIG. 4 is a detail view of an argon removal column.

[0056] FIGS. 5 to 12 are diagrams illustrating aspects of embodiments of the invention.

[0057] In the Figures, components with comparable or identical function are indicated with like reference numerals. A repeated explanation is omitted for reasons of conciseness only.

EMBODIMENTS OF THE INVENTION

[0058] FIG. 1 shows an air separation unit which may form the basis of an embodiment of the present invention in the form of a simplified, schematic process flow diagram. The air separation unit is indicated with 100.

[0059] In a compression unit 1 of the air separation unit 100, which may include a main air compressor as generally known in the field of air separation and which may comprise different compressor units or compressor stages with aftercoolers, respectively, an amount of feed air aspirated via a filter from the atmosphere is compressed to form a feed air stream a. The feed air stream a is cooled in a direct contact cooling unit 2 with water as also generally known in the field of cryogenic air separation and, still indicated a, supplied to a purification unit 3 which, in the embodiment illustrated, comprises two adsorber lines each containing two adsorption vessels. The feed air stream a is purified in parallel streams in the purification unit 3 as also known per se.

[0060] The purified feed air stream, still indicated a, is subdivided into partial streams b, c and d. Partial stream b is, without further compression, passed from the warm end to the cold end through the main heat exchanger 4 and then into the high pressure column 11 of a rectification column system 10 comprising the high pressure column 11, a low pressure column 12 and an argon removal unit 13 with a condenser evaporator 13.1 arranged in the low pressure column 12. More specifically, the low pressure column 12 comprises a lower column region 12.1, an intermediate column region 12.2 arranged above the lower column region 12.1, and an upper column region 12.3 arranged above the intermediate column region 12.2, the lower column region 12.1 including an oxygen section 12.4 of the low pressure column and the intermediate column region 12.2 including a rectification section 13.2 of the argon removal unit 13. Besides the rectification section 13.2 of the argon removal unit 13, there is arranged an argon section 12.5 of the low pressure column in the intermediate column region 12.2.

[0061] Partial stream c is further compressed in a booster air compressor 5 of the air separation unit 100 and thereafter, as a Joule Thomson stream, likewise passed from the warm end to the cold end through the main heat exchanger 4 and then expanded, e.g. using an expansion arrangement 6 comprising a valve and a dense liquid expander, into the high pressure column 11. Partial stream d is, in the example shown, self boostered in a turbine booster arrangement 7 and then expanded into the low pressure column as illustrated by connection d.

[0062] Enriched liquid from the sump of the high pressure column is, as illustrated with e, passed through a subcooler 8 and thereafter expanded into the low pressure column 12. In a manner known per se, nitrogen-rich gas is withdrawn from the top of the high pressure column 11. A first part thereof, illustrated with f, is heated in gaseous stated in the main heat exchanger 4 and withdrawn from the air separation unit 100 as a gaseous nitrogen product. The rest of the nitrogen-rich gas withdrawn from the top of the high pressure column 11 is, in the example illustrated, mostly condensed in a main condenser 19 interconnecting the high and low pressure columns 11, 12. A part of the condensate thus formed, which is indicated g, is refluxed to the high pressure column 11 while a further part, indicated h, is internally compressed and a yet further part, indicated i, is subcooled in subcooler 8 and provided as a liquid nitrogen product. An intermediate stream k is passed through subcooler 8 and expanded into the low pressure column, as is a liquid m withdrawn at the feed point of stream c.

[0063] An oxygen product is produced by internally compressing sump liquid n withdrawn from the low pressure column 12. Waste nitrogen o is withdrawn from the top of the low pressure column 12 while waste argon p is withdrawn from the condenser evaporator 13.1 of the argon removal unit as further illustrated below.

[0064] FIG. 2 is a detail view of an air separation unit such as the air separation unit 100 according to FIG. 1 including the lower part of the upper column region 12.3, the intermediate column region 12.2 and the lower column region 12.1 and an upper end of the high pressure column 11. All elements and streams are indicated with like reference numerals as before and reference is made to the explanations above. As shown, the lower region of the argon removal unit 13, i.e. its rectification section 13.2, comprises a bottom or underside open with respect to an upper region of the oxygen section 12.4 to allow an entry of a part of a gas flow raising in the oxygen section 12.4 as an argon removal feed gas and the rectification section 13.2 of the argon removal unit 13 is arranged in a common space with an argon section 12.5 of the low pressure column 12 which comprises a bottom or underside open with respect to the upper region of the oxygen section 12.4 to allow an entry of a further part of the gas flow raising in the oxygen section 12.4.

[0065] The condenser evaporator 13.1 is in this example arranged above the rectification section 13.2 of the argon removal unit 13 in the intermediate column region 12.2 and the condenser evaporator 13.1 is provided as a forced-flow condenser evaporator 13.1 configured to at partly evaporate a liquid containing about 70% oxygen collected from the intermediate column region 12.2 at a collector tray 12.9 and forcedly passed to the condenser evaporator 13.1 in the form of a liquid stream q. By said partial evaporation, a gas stream r to be passed to the upper column region 12.3 and a liquid stream s to be passed to the argon section 12.5 are provided. That is, the condenser evaporator 13.1 is configured to at least partly evaporate a coolant in form of the liquid stream q forming an evaporated gas r, and the evaporated gas r is uncontrolledly passed into the low pressure column 12 by simply releasing the same.

[0066] Gas raising from the rectification section 13.2 of the argon removal unit 13 is collected in the form of a stream t and partly condensed in the condenser evaporator 13.1 to form a biphasic stream. Using e.g. a syphon, a gas phase and a liquid phase contained in the biphasic stream are at least in part separated from each other and, in the general example shown, the liquid phase is refluxed to the rectification section 13.2 of the argon removal unit in the form of a stream u. That is, a part of the gas raising from the rectification section 13.2 of the argon removal unit 13 is, in form of the condensate, refluxed to the rectification section 13.2 of the argon removal unit As illustrated as before with p, but not explicitly illustrated in FIG. 2, a further part of the gas raising from the rectification section 13.2 of the argon removal unit 13 may, in the form of the gas phase of the biphasic stream downstream of the condenser evaporator 13.1, i.e. in the form of the part not condensed in the condenser evaporator 13.1 and thus not refluxed to the rectification section 13.2 of the argon removal unit 13, passed out of the system 10 and the air separation unit 100. Reference is made to FIG. 1, for example. As mentioned, a corresponding gas stream can, however, can also be formed from gas withdrawn upstream of the condenser evaporator 13.1.

[0067] FIG. 3 is a detail view of an air separation unit such as the air separation unit 100 according to FIG. 1 wherein also the components shown in FIG. 2 are partially illustrated. Therefore, as to these components, reference is made to the explanations above. As a focus is placed here to the intermediate column region 12.2 and the components therein, the upper and lower column regions 12.3 and 12.1 are shown in reduced detail only. Gas raising from oxygen section 12.4, or, more precisely, parts thereof passed to the argon section 12.4 and the rectification section 13.2 of the argon removal unit 13 are indicated with v1 and v2. Gas raising in the region of the condenser evaporator 13.1 is indicated w. As mentioned before and illustrated in FIG. 3, a stream p may be formed from the gas raising from the rectification section 13.2 of the argon removal unit 13 by using the, or a part of the gas phase of the biphasic stream downstream of the condenser evaporator 13.1, i.e. in the form of the part not condensed in the condenser evaporator 13.1 and thus not refluxed to the rectification section 13.2 of the argon removal unit 13. Reference is made to the explanations above.

[0068] A flow of the stream p is adjusted by using a valve 13.3. A control unit 20 is provided which is configured to control an oxygen content of the feed gas to the argon removal unit 13.1, i.e. stream v2 (and v1) as a controlled variable using a flow of stream p, i.e. the further gas from the top of the argon removal unit 13.1 being withdrawn as a manipulated variable, on the basis of an oxygen content determined in the feed gas to the argon removal unit 13.1 using a feedback control structure including an analysis (indicating) controller AC as illustrated.

[0069] FIG. 4 is a detail view of an argon removal unit provided, according to an embodiment of the present invention, as an argon removal column 13.0 external to the low pressure column 12 whose condenser evaporator 13.1 is cooled by liquid from the sump of the high pressure column 11 in the form of a stream e provided in essentially the same manner as the stream e according to FIG. 1. Other coolants may likewise be used. A part of the stream e not used for cooling is passed directly to the low pressure column in the form of a stream e.

[0070] As before, a stream of gas raising in the rectification section 13.2 of the argon removal unit 13 is indicated t, a part of the condensate refluxed to the rectification section 13.2 of the argon removal unit 13 is indicated u, and a waste argon stream is indicated p. Gaseous and liquid streams withdrawn from an evaporation space of condenser evaporator 13.1 are passed as streams r and s to the low pressure column 12. As indicated by a crossed-out valve in the stream r, such a valve can be omitted by using the control strategy according to an embodiment of the present invention. A valve in stream p (and s) is generally always available and preferably provided as a warm valve outside a coldbox. As illustrated in FIG. 4, therefore, the condenser evaporator 13.1 is configured to at least partly evaporate a coolant forming an evaporated gas, and the overall system 10 is configured to pass the evaporated gas uncontrolledly into the low pressure column 12, i.e. without the restriction of a flow restricting device.

[0071] FIGS. 5 to 12 are diagrams illustrating certain aspects. In all diagrams, a time in seconds is indicated on the horizontal axis and the other values discussed below are indicated on the vertical axis.

[0072] Two case studies have been conducted using a digital twin based on the concept of Kender et al., Development of a Digital Twin for a Flexible Air Separation Unit Using a Pressure-Driven Simulation Approach, Computers & Chemical Engineering, 151, 107349, 2021 using simulation models described in DE 10 2020 000 464 A1. These studies include dynamic simulations for a plant disturbance scenario as well as a state-of-the-art load change procedure to evaluate the proposed control concept. In addition, the plant disturbance scenario may be simulated with a legacy concept (blanketing of heat transfer area) to compare the results to the proposed concept.

[0073] As a plant disturbance scenario, the amount of pressurized gaseous nitrogen product such as stream f according to FIG. 1 (also referred to as PGAN) is changed with a rate of 8 percent per minute to impact the main condenser duty, as visualized in the diagram of FIG. 5. Simulations were based on 20000 normalized cubic metres per hour of the gaseous nitrogen (i.e. PGAN, 41500 to 21500 normalized cubic metres per hour at 5 bar absolute pressure from the high pressure column). In the diagram shown in FIG. 5, a flow is shown in moles per second on the vertical axis. Dashed lines in the diagram represent the start and end times of the set point change.

[0074] As load change, a 100 to 70 percent state-of-the-art turn-down scenario using automatic load change was considered to evaluate the control concept. In detail, the products were changed as follows with a load change rate of 1 percent per minute: 4500 to 3150 standard cubic meters per hour internally compressed nitrogen, such as stream h according to FIG. 1, 80000 to 56000 standard cubic meters per hour internally compressed oxygen, such as stream n according to FIG. 1, and 41500 to 29050 standard cubic meters per hour gaseous nitrogen withdrawn at 5 bar absolute pressure from the high pressure column, such as stream f according to FIG. 1 (i.e. PGAN). Flows are shown in moles per second on the vertical axis in diagrams A (internally compressed nitrogen), B (internally compressed oxygen) and C (pressurized nitrogen withdrawn from high pressure column, PGAN) of FIG. 6.

[0075] This study was used as an example to evaluate the behaviors of the proposed control concept to a disturbance in plant operation. The fast reduction of the nitrogen product withdrawn from the high pressure column (PGAN, 8 percent per minute) leads to a swift increase of the main condenser duty and thus to an increase of the gas load in the low pressure column.

[0076] FIG. 7 shows the required set point changes for the analysis control loop controlling the oxygen content in the gas to the argon removal unit for the partial load case just described in diagram A (dimensionless) and argon passed out of the system in diagram B (in moles per second). The value for the oxygen content is pre-calculated in an additional steady-state simulation. The set point change is linear in nature. The black, dashed line in diagram A represents the set point change whereas the solid line is the actual graph of the oxygen content. The rapid decrease in the oxygen content can be counteracted using the amount of argon passed out of the system until the oxygen content converges to its desired set point. Thus, by adjusting the flow of argon passed out of the system accordingly, the proposed control loop is able to react to a plant disturbance in a reliable manner.

[0077] In FIG. 8, relevant aspects of the plant response are illustrated. In diagram A, the outgoing vapor (top) and liquid (bottom) flows of the uppermost theoretical tray of the argon removal unit are shown in moles per second on the vertical axis. Diagram B shows the oxygen molar fraction of the internally compressed oxygen product resulting on the vertical axis.

[0078] The vapor and liquid flows of the uppermost theoretical tray are representative for the load of the argon removal unit. Approximately 1 hour after the disturbance, stable flow conditions can be observed in this. This shows that the proposed control concept is able to establish a new stable column state in little time. In addition, the applied control ensured the changes in the flows due to the plant disturbance remained within a small interval. The oxygen content of the internally compressed oxygen product is shown in diagram B of FIG. 8, as mentioned. The graph of the product purity is similar to the oxygen content in the feed stream to the argon removal unit (see FIG. 7). Compared to the latter stream, the changes in oxygen content in the internally compressed oxygen product are visible with a temporal delay, dampened by the holdup of the oxygen section. Thus, the proposed control concept is beneficial for the adherence of the product purity constraints for the internally compressed oxygen product.

[0079] To visualize the functionality of the proposed control concept, the temperature on both sides of the forced flow condenser (diagram A) and the resulting temperature difference MTD at the forced flow condenser (diagram B) are displayed in FIG. 9. In Diagram A, as relevant plant response the dew point temperature of the condensate formed from the gas from the argon removal unit (condenser side) and the bubble point temperature of oxygen (at the evaporation side) of the forced flow condenser are illustrated while diagram B illustrates the MTD of these two streams.

[0080] The graph B of the MTD is identical to the behaviors of the integrated argon removal column load over time (see FIG. 8, diagram A). This proves that the proposed concept works as stated above. The argon column load is explicitly controlled via the manipulation of the forced flow condenser duty via the driving temperature difference MTD. In addition, diagram A of FIG. 9 shows that the adjustment of the waste argon stream (see FIG. 7, diagram B) and the resulting change of oxygen content in the feed stream to the argon removal unit (see FIG. 7, diagram A) influences both temperatures of the forced flow condenser. This study shows that the proposed control concept allows for a swift reaction on a plant disturbance (reduction of the pressurized nitrogen flow (PGAN) by half with 8 percent per minute as described above). A new stable plant state is established approximately 1 hour after the disturbance ends. Furthermore, the oxygen content of the feed stream to the argon removal unit is an early indication of the behaviors of the internally compressed oxygen product. Thus, controlling this oxygen content is additionally beneficial for plant operations.

[0081] To compare the proposed analysis indicating controller concept to a legacy concept, the outgoing vapor and liquid flows of the uppermost theoretical tray of the argon removal column for both concepts are shown in FIG. 10. The results obtained for the proposed concepts are indicated with solid lines while the results obtained for the legacy concept are indicated with dashed lines. The upper dashed and dotted line indicates a vapor and the lower dashed and dotted line indicates a liquid flow, each in moles per second.

[0082] The vapor and liquid flows of the uppermost theoretical tray are representative for the load of the argon removal unit. Both concepts show a similar behaviors of the column load during the plant disturbance and converge with the same end value. That is, the proposed concept can reproduce the behaviors of the field proven concept. However, the proposed concept has certain advantages which are discussed above.

[0083] Furthermore, a state-of-the-art load change procedure using automatic load change was simulated reducing the plant load from 100 to 70 percent. This study is used as an example to evaluate the applicability of the proposed control concept to regular plant operation. In-and outputs of the proposed control concept.

[0084] FIG. 11 shows the required set point changes for the proposed control loop (oxygen content in feed to argon removal unit) for the part load case in diagram A expressed as mole fraction and the waste argon flow in diagram B expressed in moles per second. The part load value for the oxygen content is pre-calculated in an additional steady-state simulation. The set point change is linear in nature (state-of-the-art automatic load change). The black, dashed line in diagram A represents the set point change whereas the solid line is the actual graph of the oxygen content. The applied control is able to correct the drop in the oxygen content, which is caused by the load change, very quickly. Afterwards, the graph of the oxygen content converges to its desired part load set point. The manipulated value of the proposed controller, the waste argon flow, is shown in diagram B of FIG. 11. The graph of the waste argon flow confirms an explicit correlation of these two quantities, emphasizing the reliable controllability using the proposed control concept.

[0085] In FIG. 12, relevant aspects of the plant response are illustrated. In diagram A, the outgoing vapor (top) and liquid (bottom) flows of the uppermost theoretical tray of the argon removal unit are visualized. Diagram B depicts the oxygen molar fraction of the internally compressed oxygen product.

[0086] The oxygen content of the internally compressed oxygen product is shown in diagram B of FIG. 12. The graph of the product purity is similar to the oxygen content in the feed gas to the argon removal unit (see FIG. 11). This is due to the fact that the changes in the oxygen content are visible in the feed gas to the argon removal unit first. Thus, the proposed control concept is additionally beneficial for the adherence of the internally compressed oxygen product purity constraints. Due to the applied control the decrease of product purity remains very small. The dynamic simulation studies show that the proposed control concept allows for the reliable reduction of plant load from 100 to 70% using automatic load change with a higher than state-of-the-art load change rate (1 percent per minute). A new stable plant state is established approximately 1 hour after the setpoint changes of the end of the automatic load change. Furthermore, the oxygen content of the feed gas stream to the argon removal unit is an early indication of the behaviors of the internally compressed oxygen product. Thus, controlling this oxygen content is additionally beneficial for plant operations.

[0087] To sum it up, the presented case studies revealed that the proposed control concept is reliable to react on plant disturbances as well as is applicable for state-of-the-art load change procedures. The results of dynamic simulations shall be considered in design, particularly regarding pipe and valve sizing for the waste argon stream. The proposed control concept has a lower complexity (one control loop instead of two), requires a smaller volume of the crude argon condenser (10 to 15 percent less) and allows for the omission of large liquid control valve (liquid flow is ca. 25% of the process air flow).