Distillation column system and plant for production of oxygen by cryogenic fractionation of air

A distillation column system and a plant are for production of oxygen by cryogenic fractionation of air. The distillation column system has a high-pressure column and a low-pressure column, a main condenser, and an argon column with an argon column top condenser. The low-pressure column comprises an upper mass transfer region, a lower mass transfer region and a middle mass transfer region. The argon column top condenser is arranged within the low-pressure column between the upper and middle mass transfer regions and is configured as a forced-flow evaporator.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from European Application EP 15002477.6-160 filed on Aug. 20, 2015.

BACKGROUND OF THE INVENTION

The invention relates to a distillation column system for production of oxygen by cryogenic fractionation of air comprisinga high-pressure column and a low-pressure column,a main condenser configured as a condenser-evaporator, the liquefaction space of the main condenser being in flow connection with the top of the high-pressure column,and comprising an argon column whichis in flow connection with an intermediate point in the low-pressure column, andhas means of drawing off an argon-enriched stream andan argon column top condenser which is configured as a condenser-evaporator and is in flow connection with the top of the argon column,the low-pressure column has an upper mass transfer region, a lower mass transfer region and a middle mass transfer region,the middle mass transfer region has at least one first mass transfer space which is open in the upward direction toward the upper mass transfer region and in the downward direction toward the lower mass transfer region.

The basics of cryogenic fractionation of air in general and specifically the construction of two-column plants are described in the monograph “Tieftemperaturtechnik” [Cryogenic Technology] by Hausen/Linde (2nd edition, 1985) and in an article by Latimer in Chemical Engineering Progress (vol. 63, No. 2, 1967, page 35). The heat exchange relationship between high-pressure column and low-pressure column of a twin column is generally accomplished by means of a main condenser in which top gas from the high-pressure column is liquefied against evaporating bottoms liquid from the low-pressure column.

The distillation column system of the invention can in principle be configured as a conventional two-column system with high-pressure column and low-pressure column. In addition to the two separation columns for nitrogen-oxygen separation, it may additionally include further devices for obtaining other air components, especially noble gases, for example a krypton-xenon recovery.

The main condenser in the invention is referred to as condenser-evaporator. A “condenser-evaporator” refers to a heat exchanger in which a first condensing fluid stream enters into indirect heat exchange with a second evaporating fluid stream. Every condenser evaporator has a liquefaction space and an evaporation space consisting of liquefaction passages and evaporation passages respectively. In the liquefaction space the condensation (liquefaction) of the first fluid stream is conducted, and in the evaporation space the evaporation of the second fluid stream. The evaporation space and liquefaction space are formed by groups of passages that are in a heat exchange relationship with one another.

Typically, the main condenser is configured as a bath evaporator, especially as a cascade evaporator (for example as described in EP 1287302 B1=U.S. Pat. No. 6,748,763 B2). It may be formed by a single heat exchanger block or else by a plurality of heat exchanger blocks arranged in a common pressure vessel.

An “argon discharge column” refers here to a separation column for argon-oxygen separation which does not serve to recover a pure argon product, but serves to discharge argon from the air which is being fractionated in the high-pressure column and low-pressure column. The way in which it is connected differs only slightly from that of a conventional crude argon column, which generally has 70 to 180 theoretical plates; however, it contains far fewer theoretical plates, namely fewer than 40, especially between 15 and 35. Like a crude argon column, the bottom region of an argon discharge column is connected to an intermediate point in the low-pressure column, and the argon discharge column is typically cooled by a top condenser wherein expanded bottoms liquid from the high-pressure column is introduced on the evaporation side; an argon discharge column generally does not have a reboiler.

The expression “argon column” is used here as an umbrella term for argon discharge columns, full-scope crude argon columns and all intermediate stages in between.

The distillation column system of any fractionation plant is arranged in one or more coldboxes. A “coldbox” is understood here to mean an insulating shell encompassing a heat-insulated interior entirely with outer walls; arranged within the interior are plant components to be insulated, for example one or more separation columns and/or heat exchangers. The insulating effect can be brought about by appropriate configuration of the outer walls and/or by the filling of the intermediate space between plant components and outer walls with an insulating material. In the latter variant, preference is given to using a pulverulent material, for example perlite. Both the distillation column system for nitrogen-oxygen separation of a cryogenic air fractionation plant and the main heat exchanger, and further cold plant components have to be enclosed by one or more coldboxes. The outer dimensions of the coldbox typically determine the transport dimensions of the package in the case of prefabricated plants.

A “main heat exchanger” serves to cool down feed air in indirect heat exchange with return flows from the distillation column system. It may be formed from one single or more than one parallel- and/or series-connected heat exchanger sections, for example from one or more plate heat exchanger blocks. Separate heat exchangers which serve specifically for evaporation or pseudo-evaporation of a single liquid or supercritical fluid, without partial heating and/or evaporation of a further fluid, do not form part of the main heat exchanger. Such a separate heat exchanger may be formed, for example, by a secondary condenser or by a separate heat exchanger for evaporation or pseudo-evaporation of a liquid stream at elevated pressure. Some air fractionation plants contain, for example, in addition to the main exchanger, a secondary condenser or a high-pressure exchanger for evaporation or pseudo-evaporation of product which has been pressurized in liquid form against a high-pressure air stream which is formed by a portion of the feed air.

The relative spatial terms “top”, “bottom”, “over”, “under”, “above”, “below”, “vertical”, “horizontal”, etc. relate to the spatial alignment of the apparatuses in normal operation.

A distillation column system of the type specified at the outset is known from U.S. Pat. No. 5,235,816. Plants of this kind, in production, are regularly prefabricated as far as possible, and the prefabricated components are transported to the construction site and finally connected to one another there. According to the size of the plant, for example, the entire double column may be transported together with its coldbox. If the size of the plant does not permit this, the double column is transported—in two parts if appropriate—without the coldbox and piping. An additional column such as the argon column causes additional complexity with a separate coldbox. This column is brought separately to the construction site and connected there to the rest of the plant on site with a relatively high level of complexity. In order to avoid an additional cryogenic pump, this column (in its own coldbox) is positioned on a complex frame. This frame causes, inter alia, an increased footprint for the entire plant.

FIG. 1 of EP 1108965 A1 discloses an argon column which has been installed in the low-pressure column and which has a top condenser arranged outside the low-pressure column.

SUMMARY OF THE INVENTION

It is an object of the invention to configure a distillation column system of the type specified at the outset with maximum compactness, to simplify its construction and to find a particularly operationally reliable control method.

This object is achieved by a distillation column system for obtaining oxygen by cryogenic fractionation of air, comprisinga high-pressure column and a low-pressure column,a main condenser configured as a condenser-evaporator, the liquefaction space of the main condenser being in flow connection with the top of the high-pressure column,and comprising an argon column whichis in flow connection with an intermediate point in the low-pressure column, andhas means of drawing off an argon-enriched stream andan argon column top condenser which is configured as a condenser-evaporator and is in flow connection with the top of the argon column,the low-pressure column has an upper mass transfer region, a lower mass transfer region and a middle mass transfer region,the middle mass transfer region has at least one first mass transfer space which is open in the upward direction toward the upper mass transfer region and in the downward direction toward the lower mass transfer region, characterized in thatthe upper mass transfer region has a liquid collector at its bottom end,the first mass transfer space has a liquid distributor at its top,the argon column top condenser is arranged within the low-pressure column between the upper and middle mass transfer regions, and in thatthe argon column top condenser is configured as a forced-flow evaporator, having an evaporation space which has an inlet at its bottom end and an outlet at its top end, and the outlet being connected to the liquid distributor of the first mass transfer space, the system further includingmeans of introducing liquid from the liquid collector beneath the upper mass transfer region into the inlet of the evaporation space of the argon column top condenser, anda vessel, a two-phase conduit which is connected to the outlet of the evaporation space of the argon condenser and to the inlet of the vessel, a gas conduit for drawing off gas from the vessel and containing a control valve, and a liquid conduit for introducing liquid from the vessel into the liquid distributor at the top of the middle mass transfer section.

According to this, the argon column top condenser is arranged within the low-pressure column. The argon discharge top condenser is configured as a forced-flow (once-through) evaporator; at the upper end thereof, the evaporation space is connected to the interior of the low-pressure column, such that the gas produced therein can flow into the upper mass transfer region. The argon column top condenser, in the invention, need not be arranged in the middle above the argon column (if the argon column is wholly or partly installed in the low-pressure column); instead, it is possible to utilize the entire cross section of the low-pressure column.

In a forced-flow evaporator, a liquid stream is forced through the evaporation space under its own pressure and partially evaporated therein. This pressure is generated, for example, by means of a liquid column in the inlet conduit to the evaporation space. The height of this liquid column corresponds to the pressure drop in the evaporation space. The gas-liquid mixture leaving the evaporation space, separated by phases, is guided directly onward to the next method step and, more particularly, is not introduced into a liquid bath of the condenser-evaporator from which the proportion remaining in liquid form is aspirated again (“once through”).

A liquid is partly evaporated in the evaporation space of the forced-flow evaporator. The biphasic mixture flowing out of the outlet is preferably introduced into a liquid distributor at the top of the middle mass transfer region. The evaporated fraction flows upward into the upper mass transfer region; the fraction remaining in liquid form forms at least part of the reflux for at least part of the middle mass transfer region, which especially forms the argon section of the low-pressure column.

In principle, the forced-flow evaporator could, as in standard argon methods, be operated exclusively with the crude oxygen from the high-pressure column. In the context of the invention, however, it has been found to be more favourable to charge the evaporation space of the argon column top condenser with a liquid which comes from the upper mass transfer region of the low-pressure column. For this purpose, the liquid collector is connected below the upper mass transfer region to means of introducing liquid from the liquid collector via the inlet into the evaporation space of the argon column top condenser. Liquid running off from the upper mass transfer section is combined in the liquid collector and introduced, for example, via a conduit into the evaporation space of the argon column top condenser. The liquid thus serves to cool the top of the argon column. It is more oxygen-rich than the crude oxygen from the high-pressure column and hence enables a smaller temperature differential and correspondingly smaller thermodynamic losses in the argon column top condenser.

According to the invention (“control method 3”), the biphasic mixture from the evaporation space of the argon condenser is introduced into a vessel which acts as phase separation unit and liquid buffer. The liquid separated out in the vessel is guided into the liquid distributor beneath. The liquid volume is controlled by means of a fixed diaphragm or corresponding hole(s) in the base of the vessel or by means of a control valve in the liquid conduit. Gas is drawn off from the vessel via a gas conduit. The conduit contains a control valve, by means of which the pressure in the evaporation space is adjusted, and hence the temperature differential in the argon condenser and its performance.

In principle, it would be possible, rather than the forced-flow condenser, to use a falling-film evaporator as well, in which case all or almost all the liquid that flows downward in the upper mass transfer section likewise flows through the evaporation space of said falling-film evaporator.

DE 1272322 B discloses installing a crude argon column into the low-pressure column by means of a cylindrical dividing wall; the top condenser is configured as a conventional bath evaporator and a first portion thereof is arranged in the low-pressure column. In addition, a further vessel is utilized here for the second portion of the top condenser.

Preferably, in the invention, the argon condenser is configured such that it produces the entire reflux stream for the argon column. There is thus no further argon condenser that would be arranged outside the low-pressure column.

In general, the argon column is configured as an argon discharge column. If an argon product is required, however, it can also be configured as a crude argon column where an oxygen-depleted or oxygen-free crude argon product is obtained at the top. The crude argon product is either conducted away or sent to further workup in a pure argon column.

In a further development of the invention, the argon column or a portion thereof is also arranged within the low-pressure column, specifically in the middle mass transfer region. For this purpose, the latter is configured as a dividing wall section, meaning that it contains a vertical dividing wall which divides the argon section of the low-pressure column (“first mass transfer space”) from the argon column (“second mass transfer space”). The first mass transfer space is open in the upward direction toward the upper mass transfer region and in the downward direction toward the lower mass transfer region. This means that ascending gas can flow without significant hindrance into the first mass transfer space at the bottom and out of the first mass transfer space at the top.

The second mass transfer space is sealed in a gas-tight manner in the upward direction toward the upper mass transfer region. The gas flowing in at the bottom from the lower mass transfer region is thus, after the rectification in the second mass transfer space (in the argon column), not introduced back into the low-pressure column but guided onward via one or more specific gas conduits and/or introduced into the liquefaction space of the argon column top condenser.

If only part of the argon column is arranged within the low-pressure column, the argon column also has a separate crude argon column outside the low-pressure column.

In one embodiment of the invention, the second mass transfer space is open in the downward direction toward the lower mass transfer region. The ascending gas from the lower mass transfer region thus flows into the second mass transfer and is subjected to an argon-oxygen separation therein.

Alternatively, the second mass transfer space is closed in the downward direction toward the lower mass transfer region, such that a different concentration can exist in the lower region of the second mass transfer space than at the upper end of the lower mass transfer region. Thus, the “upper” portion, viewed in terms of rectification, of an argon column may be incorporated into the dividing wall section, while the rest of the argon column, which is connected to the low-pressure column at the lower end, is implemented separately.

For full-scope argon production, it is possible to add a separate crude argon column. In that case, the argon column consists of the combination of crude argon column and second mass transfer space, it being possible to connect the second mass transfer space, in terms of rectification, to the upper or lower end of the crude argon column. In either case, the top of the argon column is in flow connection with the liquefaction space of the argon column top condenser. If the low-pressure column does not contain a dividing wall section, the argon column is formed exclusively by a separate crude argon column. In that case, this is connected in a customary manner, in that the head of the argon column is in flow connection with the liquefaction space of the argon column top condenser and the bottom of the argon column is in flow connection with an intermediate region of the low-pressure column, especially with the region between the middle and lower mass transfer region.

It is also advantageous when the means of introducing liquid from the liquid collector into the evaporation space of the argon column top condenser are configured for introduction of at least 80 mol %, preferably at least 90 mol %, of the volume of liquid that flows into the liquid collector in normal operation into the evaporation space of the argon column top condenser.

In the context of the invention, in normal operation of the plant, as close as possible to 100% of the liquid from the liquid collector should be introduced into the evaporation space.

Preferably, a crude oxygen conduit is provided for introduction of crude oxygen from the bottom of the high-pressure column into the upper mass transfer region of the low-pressure column; alternatively, the crude oxygen can be fed directly into the liquid collector upstream of the evaporation space. In the case of introduction into the low-pressure column, this introduction—which is customary per se—of bottoms liquid from the high-pressure column into the low-pressure column is not conducted via the argon column top condenser but directly into the upper mass transfer region. The liquid which is introduced into the evaporation space of the argon column top condenser is thus oxygen-richer than in the conventional method because the liquid collected below the upper section is being used here.

In one embodiment, the distillation column system has a bypass conduit for introduction of liquid from the liquid collector arranged below the upper mass transfer section into the liquid distributor at the top of the lower mass transfer section, with a control valve disposed in the bypass conduit.

By means of this bypass conduit, outside the scope of the invention, the performance of the argon column top condenser can be controlled. If appropriate, the control valve is opened, and a small amount of relatively nitrogen-rich liquid flows directly into the distributor and hence bypasses the middle mass transfer section. As a result, the nitrogen content in the liquefaction space of the argon top condenser (or in the biphasic mixture at the outlet) is increased, the mean condensation temperature drops and the performance of the condenser is reduced as a result of reduction in the driving temperature difference (control method 1).

As an alternative to the control according to the invention, it would also be possible to control the conversion in the crude argon column with the aid of a valve in the gas flow upstream of the crude argon condenser. In this case, a gas inlet is utilized for introduction of gas from the argon column into the liquefaction space of the argon column top condenser, and contains a control valve (control method 2).

The gas inlet immediately downstream of the control valve can be connected to a start-up conduit configured for controlled removal of gas from the low-pressure column.

The start-up conduit is connected to the gas inlet outside the vessel wall and is used only when the plant is cold-started. It contains a control valve which is closed in steady-state operation. It is necessary here, on start-up, to make sure that the mass transfer spaces are cooled equally on either side of the dividing wall. Large temperature differences between these two sections should be avoided in order thus to minimize the load on the dividing wall through thermally induced stresses. The start-up conduit is either open to the air or is connected to an impure nitrogen conduit upstream of the main heat exchanger. According to the temperature to the right and left of the dividing wall, the control valve is opened to a greater or lesser degree on start-up. It is advantageous that no separate stub has to be provided on the column here for the start-up conduit; instead, the start-up conduit is incorporated directly into the gas inlet downstream of the control valve for the argon column top condenser—i.e. outside the column. This start-up technique can be utilized not just in the invention, but in principle in the case of a dividing wall column section with a condenser above it.

The invention also relates to a plant for production of nitrogen by low-temperature fractionation of air comprising a main air compressor, an air precooling unit, an air cleaning unit and the main heat exchanger, and comprising two of the above-described distillation column systems, both of which receive feed air from the common main heat exchanger.

The plant for production of oxygen by cryogenic fractionation of air, comprisesa main air compressor for compression of feed air,an air precooling unit for precooling of the feed air compressed in the main air compressor,an air cleaning unit for cleaning of the precooled feed air,a main heat exchanger for cooling of cleaned feed air,a first distillation column system for obtaining oxygen by cryogenic fractionation of air, comprisinga high-pressure column and a low-pressure column,a main condenser configured as a condenser-evaporator, the liquefaction space of the main condenser being in flow connection with the top of the high-pressure column,and comprising an argon column whichis in flow connection with an intermediate point in the low-pressure column, andhas means of drawing off an argon-enriched stream andan argon column top condenser which is configured as a condenser-evaporator and is in flow connection with the top of the argon column,the low-pressure column has an upper mass transfer region, a lower mass transfer region and a middle mass transfer region,the middle mass transfer region has at least one first mass transfer space which is open in the upward direction toward the upper mass transfer region and in the downward direction toward the lower mass transfer region, characterized in thatthe upper mass transfer region has a liquid collector at its bottom end,the first mass transfer space has a liquid distributor at its top,the argon column top condenser is arranged within the low-pressure column between the upper and middle mass transfer regions, and in thatthe argon column top condenser is configured as a forced-flow evaporator, having an evaporation space which has an inlet at its bottom end and an outlet at its top end, and the outlet being connected to the liquid distributor of the first mass transfer space, the system further includingmeans of introducing liquid from the liquid collector beneath the upper mass transfer region into the inlet of the evaporation space of the argon column top condenser, anda vessel, a two-phase conduit which is connected to the outlet of the evaporation space of the argon condenser and to the inlet of the vessel, a gas conduit for drawing off gas from the vessel and containing a control valve, and a liquid conduit for introducing liquid from the vessel into the liquid distributor at the top of the middle mass transfer section;a second distillation column system configured according to the first distillation column system,a first compressed air substream conduit for introducing cooled feed air from the main heat exchanger into the high-pressure column of the first distillation column system and comprisinga second compressed air substream conduit for introducing cooled feed air from the main heat exchanger into the high-pressure column of the second distillation column system.
In this case, at least a portion of the feed air for the two distillation column systems can be cooled together in the main heat exchanger and be drawn off from the main heat exchanger in a combined compressed air conduit. The combined compressed air conduit is then branched into the first compressed air substream conduit to the first distillation column system, and the second compressed air substream conduit to the second distillation column system. Alternatively, the two compressed air substream conduits are connected directly to the main heat exchanger.

If a plant according to the invention has, in addition to the main heat exchanger, a high-pressure exchanger, the latter is likewise utilized for both distillation column systems, meaning that the cold compressed air at high pressure from the high-pressure exchanger is distributed between the two distillation column systems and the product stream destined for the high-pressure exchanger is withdrawn in liquid form from the two distillation column systems, combined and sent to the high-pressure exchanger.

For manufacturing reasons, the main heat exchanger generally consists of a plurality of blocks connected in parallel in any case. In that case, it is advisable to divide the blocks into two symmetric groups in order to be able to better control the main heat exchanger. The air to be fractionated in the first distillation column system and the corresponding stream of impure nitrogen from the same distillation column system are conducted here through the first exchanger group. The corresponding streams for the and/or from the second distillation column system flow through the second group. The residual streams (product and turbine streams) are distributed homogeneously between the blocks of both groups.

U.S. Pat. No. 612,892 does disclose operating two double columns connected in parallel alongside one another in a common coldbox; however, the aim of this document is to configure the two double columns differently. The person skilled in the art would not consult this publication in the search for a way of maximizing the capacity of a plant. In any case, he does not receive any suggestion as to how a multistrand system could be altered in the manner of the object described above.

The apparatuses upstream and downstream of the two distillation column systems can especially be formed by a single precooling operation, a single air cleaning operation and/or a single heat exchanger.

It is favourable when, in the plant, the first distillation column system and the second distillation column system are of the same installation size and, more particularly, the high-pressure column, low-pressure column and argon column have equal dimensions. The “same installation size” is understood here to mean that the corresponding column heights and diameters differ from one another by not more than 10%, especially not more than 5%. The comparison relates, pair by pair, to the corresponding sections of the first and second high-pressure columns, the first and second low-pressure columns and the argon columns.

The two distillation column systems may each be accommodated in a separate coldbox. Alternatively, the first and second distillation column systems are arranged in a common coldbox.

In both cases, the two distillation column systems are operated independently of one another. The hot plant components and the main heat exchanger and optionally a high-pressure exchanger are utilized together, for example. For this purpose, one, more than one or all withdrawal conduit(s) for products from the two distillation column systems, if they are not intended for direct liquid product withdrawal, are combined pair by pair to a combined conduit which is connected to the cold end of the main heat exchanger, and then guided in a common conduit to the main heat exchanger or optionally to the high-pressure exchanger. Alternatively, each of the two distillation column systems has its own main heat exchanger and optionally its own high-pressure heat exchanger.

For independent operation, each of the two distillation column systems has a separate subcooling countercurrent heat exchanger which can be operated independently of the subcooling countercurrent heat exchanger of the other distillation column system and, more particularly, is not connected to pipelines from or to the other distillation column system.

More particularly, this means that the two distillation column systems are operable independently of one another.

The invention also relates to a method of obtaining oxygen by cryogenic fractionation of air. The method according to the invention can be supplemented by method features corresponding to the features of individual, several or all dependent apparatus claims.

The method of obtaining oxygen by cryogenic fractionation of air with a distillation column system comprisesa high-pressure column and a low-pressure column,a main condenser configured as a condenser-evaporator, the liquefaction space of the main condenser being in flow connection with the top of the high-pressure column,and comprising an argon column whichis in flow connection with an intermediate point in the low-pressure column, andhas means of drawing off an argon-enriched stream andan argon column top condenser which is configured as a condenser-evaporator and is in flow connection with the top of the argon column, whereinfeed air is introduced into the high-pressure column andan oxygen product stream is drawn off from the low-pressure column,the low-pressure column has an upper mass transfer region, a lower mass transfer region and a middle mass transfer region,the middle mass transfer region has at least one first mass transfer space which is open at the top toward the upper mass transfer region and at the bottom toward the lower mass transfer region, characterized in thatthe upper mass transfer region has a liquid collector at its bottom end,the first mass transfer space has a liquid distributor at its top,the argon column top condenser is arranged within the low-pressure column between the upper and middle mass transfer regions,the argon column top condenser is configured as a forced-flow evaporator, having an evaporation space which has an inlet at its bottom end and an outlet at its top end, and the outlet being connected to the liquid distributor of the first mass transfer space,an argon-enriched fraction is drawn off from the liquefaction space of the argon top condenser, liquid from the liquid collector arranged beneath the upper mass transfer region is introduced into the evaporation space of the argon column top condenser), and in thatthe argon column top condenser is controlled by withdrawing a biphasic mixture from the evaporation space of the argon condenser and introducing it into a vessel, drawing off a gas stream from the vessel via a control valve and drawing off a liquid stream from the vessel and introducing it into the liquid distributor at the top of the middle mass transfer section.

The advantages of the invention are manifested especially in particularly large plants having a multistrand configuration.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows a plant with a single distillation column system. The construction of the low-pressure column of this distillation column system is shown in detail inFIG. 6(some of the reference signs mentioned below are shown only therein). The distillation column system of the working example ofFIG. 1has a high-pressure column101, a low-pressure column102, a main condenser103and an argon column152.

The main condenser103is formed in the example by a three stage cascade evaporator, i.e. a multilevel pocket evaporator. The column pair101/102is arranged in the form of a double column. The argon column152is disposed in a middle mass transfer region130of the low-pressure column102. The argon column top condenser155is inside the low-pressure column102above the middle mass transfer region130. The low-pressure column102also has an upper mass transfer region131and a lower mass transfer region132(seeFIG. 6in particular).

The plant shown inFIG. 1has an entry filter302for atmospheric air (AIR), a main air compressor303, an air precooling unit304, an air cleaning unit305(typically formed by a pair of molecular sieve adsorbers), a booster air compressor306(BAC) with downstream cooler307and a main heat exchanger308. The main heat exchanger308is accommodated in a dedicated coldbox which is separate from the coldbox around the distillation column system. A combined compressed air stream100from the cold end of the main heat exchanger308is introduced into the high-pressure column101.

The air boosted to its final pressure in the booster compressor306is liquefied in the main heat exchanger308(or—if its pressure is supercritical—pseudo-liquefied) and fed via conduits311/111to the distillation column system.

A nitrogen gas stream104,114from the high-pressure column101is introduced into the liquefaction space of the main condenser103. In the liquefaction space of the main condenser103, liquid nitrogen115is produced therefrom and at least a first portion thereof is guided as the first liquid nitrogen stream105to the high-pressure column101.

A liquid oxygen stream from the low-pressure column102flows away from the lower end of the lowermost mass transfer layer107of the low-pressure column102and hence is introduced into the evaporation space of the main condenser103. Gaseous oxygen is formed in the evaporation space of the main condenser103. At least a first portion thereof is introduced into the low-pressure column102, in that it flows upward into the lowermost mass transfer layer107of the low-pressure column102; a second portion can be obtained directly, if required, as gaseous oxygen product and warmed in the main heat exchanger308(not implemented in this working example).

The reflux liquid109for the low-pressure column102is formed by a nitrogen-enriched liquid120which is drawn off from the high-pressure column101from an intermediate point (or alternatively directly from the top) and cooled down in a subcooling countercurrent heat exchanger123. Impure nitrogen110is drawn off from the top of the low-pressure column102and guided as residual gas through the subcooling countercurrent heat exchanger123and through the conduit32to the main heat exchanger308.

An oxygen-enriched bottoms liquid stream151is drawn off from the high-pressure column101and cooled down in the subcooling countercurrent heat exchanger123. In the example, the entire cooled bottoms liquid153is fed to the upper mass transfer region of the low-pressure column102. It flows together with the reflux liquid coming from above into the lowermost section of the upper mass transfer region. The liquid running off from this section is collected by a liquid collector133and introduced into the evaporation space of the argon column top condenser155. The argon column top condenser155here is configured in accordance with the invention as a forced-flow evaporator. The fraction evaporated in the top condenser155flows back into the upper mass transfer region131and the fraction157remaining in liquid form is fed into the middle mass transfer region130of the low-pressure column102. The argon-enriched “product”163of the argon column is removed in gaseous form from the argon column152or the top condenser155thereof and guided through the main heat exchanger308via conduit164through a separate passage group.

Alternatively, it would be possible to mix the argon-enriched fraction163with the impure nitrogen and guide the mixture through the main heat exchanger.

The liquid air111from the main heat exchanger is fed via the conduit111to the high-pressure column101at an intermediate point. At least a portion127is withdrawn again immediately and introduced via the subcooler123and via the conduit128into the upper mass transfer region of the low-pressure column102, and specifically above the feed of the bottoms fraction153. Via conduit129, gaseous air from an air injection turbine137is additionally introduced into the low-pressure column102, at the same level as the crude oxygen153.

The main product drawn off from the distillation column systems is liquid oxygen141from the evaporation space of the main condenser103, and it is fed via conduit14at least partly to an internal compression. This involves pumping the liquid oxygen14by means of a pump15to a high product pressure, evaporating it or (if its pressure is supercritical) pseudo-evaporating it in the main heat exchanger308under this high product pressure, warming it to about ambient temperature and finally drawing off GOXIC as the gaseous compressed oxygen product. This is the main product of the plant of the working example.

A further product from the plant is compressed nitrogen, which is drawn off directly from the top of the high-pressure column101(conduits104,142), conducted via conduit42to the main heat exchanger308, warmed therein and finally obtained as gaseous compressed nitrogen product MPGAN. A portion thereof can be used as seal gas. In addition, a portion143of the liquid nitrogen produced in the main condenser103can be fed via conduit43to an internal compression (pump16) and obtained as gaseous high-pressure nitrogen product GANIC. The plant can also supply liquid products LOX, LIN.

In a specific example, the mass transfer elements in the low-pressure column102are formed exclusively by structured packing. The oxygen section107of the low-pressure column102is equipped with a structured packing having a specific surface area of 750 m2/m3or alternatively 1200 m2/m3; in the other sections, the packing has a specific surface area of 750 or 500 m2/m3. In addition, the low-pressure column102may have a nitrogen section above the mass transfer regions shown in the drawing; this may likewise be equipped with a particularly dense packing (for example having a specific surface area of 1200 m2/m3for the purpose of reducing the column height). In a departure from this, it is possible to combine structured packing of different specific surface area within any of the sections mentioned. The argon column152, in the working example, contains exclusively packing having a specific surface area of 1200 m2/m3or alternatively 750 m2/m3.

In the high-pressure column101, the mass transfer elements are formed exclusively by structured packing having a specific surface area of 1200 m2/m3or 750 m2/m3. Alternatively, at least a portion of the mass transfer elements in the high-pressure column101could be formed by conventional distillation trays, for example by sieve trays.

The system ofFIG. 1is configured as a two-turbine method with a medium-pressure turbine138and an air injection turbine137.

The working example ofFIG. 2differs fromFIG. 1in that it is configured as a one-turbine system. It has only one air injection turbine, and no medium-pressure turbine.

FIG. 3is almost identical toFIG. 2, but instead of the air injection turbine has a compressed nitrogen turbine337. It is operated with a portion342of the compressed nitrogen142which is drawn off in gaseous form from the top of the high-pressure column101.

InFIG. 4, the turbine stream442is instead drawn off from an intermediate point in the high-pressure column101and expanded to perform work in an impure nitrogen turbine437.

FIG. 5shows a plant having two distillation column systems which is configured in accordance with the invention.

The first distillation column system of the working example ofFIG. 5has a first high-pressure column101, a first low-pressure column102, a first main condenser103and a first argon column152. A second high-pressure column201, a second low-pressure column202, a second main condenser203and a second argon column252form part of the second distillation column system in the plant shown inFIG. 1.

Each of the main condensers103,203is formed in the example by a three-stage cascade evaporator. The column pairs101/102,201/202are arranged in the form of two double columns. The argon columns152/252are arranged in a middle mass transfer region of the low-pressure columns102,202. The argon top column condensers155,255are inside the respective low-pressure columns102,202above the middle mass transfer region113,213and are configured in accordance with the invention as forced-flow evaporators. The low-pressure columns is102,202also each have an upper mass transfer region above their argon column top condenser155,255and a lower mass transfer region below their argon column152/252or the middle mass transfer region113,213. The arrangement of the mass transfer regions in the low-pressure columns is apparent fromFIG. 6in particular.

Each of the two distillation column systems is controlled independently. The pressure in the low-pressure columns can, for example, be set and controlled separately. This decoupling also lessens the overall closed-loop control complexity and allows any manufacturing tolerances in the two double columns to be better compensated for.

The plant shown inFIG. 5has an entry filter302for atmospheric air (AIR), a main air compressor303, an air precooling unit304, an air cleaning unit305(typically formed by a pair of molecular sieve adsorbers), a booster air compressor306(BAC) with downstream cooler307and a main heat exchanger308. The main heat exchanger308is accommodated in a dedicated coldbox which is separate from the coldbox(es) around the distillation column systems. A combined compressed air stream99from the cold end of the main heat exchanger308is branched into a first compressed air substream100and a second compressed air substream200. The first compressed air substream100is introduced into the first high-pressure column101, and the second compressed air substream200into the second high-pressure column201.

The air boosted to its final pressure in the booster compressor306is liquefied (or—if its pressure is supercritical—pseudo-liquefied) in the main heat exchanger308and fed via conduit311to the distillation column systems and branched therein into the streams111and112.

A first nitrogen gas stream104,114from the first high-pressure column101is introduced into the liquefaction space of the first main condenser103. Liquid nitrogen115is produced in the liquefaction space of the first main condenser103, and at least a first portion thereof is guided as a first liquid nitrogen stream105to the first high-pressure column101.

A second nitrogen gas stream204,214from the second high-pressure column201is introduced into the liquefaction space of the second main condenser203. Liquid nitrogen215is produced in the liquefaction space of the second main condenser203, and at least one first portion thereof is guided as a second liquid nitrogen stream205to the second high-pressure column201.

A first liquid oxygen stream from the first low-pressure column102flows away from the lower end of the lowermost mass transfer layer107of the first low-pressure column102and hence is introduced into the evaporation space of the first main condenser103. Gaseous oxygen is formed in the evaporation space of the first main condenser103. At least a first portion thereof is introduced as first oxygen gas stream into the first low-pressure column102, in that it flows from below into the lowermost mass transfer layer107of the first low-pressure column102; a second portion can, if required, be obtained directly as gaseous oxygen product and warmed in the main heat exchanger308.

A second liquid oxygen stream from the second low-pressure column202flows away from the lower end of the lowermost mass transfer layer207of the second low-pressure column202and hence is introduced into the evaporation space of the second main condenser203. Gaseous oxygen is formed in the evaporation space of the second main condenser203. At least a first portion thereof is introduced as second oxygen gas stream into the second low-pressure column202, in that it flows from the bottom into the lowermost mass transfer layer207of the second low-pressure column202; a second portion can, if required, be obtained directly as gaseous oxygen product and warmed in the main heat exchanger308(not shown).

The reflux liquids109,209for the two low-pressure columns102,202are each formed by an nitrogen-enriched liquid120,220which is drawn off in both high-pressure columns101,201from an intermediate point (or alternatively directly from the top) and cooled down in subcooling countercurrent heat exchangers123,223. Impure nitrogen110,210is drawn off from the top of both low-pressure columns102,202and guided as residual gas through one subcooling countercurrent heat exchanger123,223in each case and via the common conduit32to the main heat exchanger308.

One oxygen-enriched bottoms liquid stream151,251is drawn off from each of the two high-pressure columns101,201and cooled down in the respective subcooling countercurrent heat exchanger123,223. In the example, the entire cooled bottoms liquid153,253is fed to the upper mass transfer region of the low-pressure columns102,202. It flows together with the reflux liquid coming from above into the lowermost section of the upper mass transfer region. The liquid running downward from this section is collected by a liquid collector133,233and introduced into the evaporation space of the argon column top condenser155,255. The argon column top condenser155,255here is configured in accordance with the invention as a forced-flow evaporator. The fraction which has evaporated in the top condenser155,255flows back into the upper mass transfer region, and the fraction remaining in liquid form157,257is fed into the middle mass transfer region130of the low-pressure column102,202. The argon-enriched “product”163,263of the argon columns is withdrawn in gaseous form from the argon column152,252or the top condenser thereof155,255and guided through the main heat exchanger308via conduit164through a separate passage group.

Alternatively, it would be possible to mix the argon-enriched fractions163,263with the impure nitrogen110,210and conduct the mixture through the main heat exchanger.

The liquid or supercritical air311from the main heat exchanger is fed via conduits111,211to the high-pressure columns101,201at an intermediate point. At least a portion127,227is withdrawn again immediately and introduced through the subcoolers123,323and via the conduit128,228into the upper mass transfer region of the low-pressure columns102,202, above the feed of the bottoms fraction153,253. Gaseous air from an air injection turbine137is also introduced via conduit129,229into the low-pressure columns102,202, at the same level as the crude oxygen153,253.

The main product drawn off from the distillation column systems is liquid oxygen141,241from the evaporation space of the main condensers103,203, and it is combined and fed via conduit14at least partly to an internal compression. This involves pumping the liquid oxygen14by means of a pump15to a high product pressure, evaporating it or (if its pressure is supercritical) pseudo-evaporating it in the main heat exchanger308under this high product pressure, warming to about ambient temperature and finally drawing off GOXIC as the gaseous compressed oxygen product. This is the main product of the plant of this working example.

A further product from the plant is compressed nitrogen, which is drawn off directly from the top of the high-pressure columns101,201(conduits104,142and204,242), conducted together via conduit42to the main heat exchanger308, warmed therein and finally obtained as gaseous compressed nitrogen product MPGAN. A portion thereof can be used as seal gas. In addition, a portion143,243of the liquid nitrogen produced in the main condensers103,203can be fed via conduit43to an internal compression (pump16) and obtained as gaseous high-pressure nitrogen product GANIC.

The plant can also supply liquid products LOX, LIN. These can be removed separately as shown from each distillation column system.

In a specific example, the mass transfer elements in the two low-pressure columns102,202are formed exclusively by structured packing. The oxygen sections107,207of the two low-pressure columns102,202are equipped with a structured packing having a specific surface area of 750 m2/m3or alternatively 1200 m2/m3; in the other sections, the packing has a specific surface area of 750 or 500 m2/m3. In addition, the two low-pressure columns102,202may have a nitrogen section above the mass transfer regions shown in the drawing; this may likewise be equipped with a particularly dense packing (for example having a specific surface area of 1200 m2/m3for the purpose of reducing the column height). In a departure from this, it is possible to combine structured packing of different specific surface area within any of the sections mentioned. The argon columns152,252, in the working example, contain exclusively packing having a specific surface area of 1200 m2/m3or alternatively 750 m2/m3.

In the high-pressure columns101,201, the mass transfer elements are formed exclusively by structured packing having a specific surface area of 1200 m2/m3or 750 m2/m3. Alternatively, at least a portion of the mass transfer elements in the two high-pressure columns101,201could be formed by conventional distillation trays, for example by sieve trays.

The system ofFIG. 5is configured analogously toFIG. 1as a two-turbine method with a medium-pressure turbine138and an air injection turbine137. Alternatively, in the system ofFIG. 5having two distillation column systems, it would also be possible to use the turbine configurations ofFIG. 2, 3 or 4.

Each of the two distillation column systems is controlled independently. The pressure in the low-pressure columns can, for example, be set and controlled separately. This decoupling also lessens the overall closed-loop control complexity and allows any manufacturing tolerances in the two double columns to be better compensated for.

With reference to the detailed drawing ofFIG. 6, the exact function of argon column and argon column top condenser and the closed-loop control thereof will now be elucidated. This detail can be applied to any of the preceding working examples.

FIG. 6shows just a section of the double column101,102, which extends from the upper end of the high-pressure column101to the second packing layer of the upper mass transfer region131of the low-pressure column, and more particularly contains the argon column152and the argon column top condenser155. It will be appreciated that the working example ofFIG. 6can also be used in other two-column systems, for example those having an arrangement of the low-pressure column alongside the high-pressure column and/or with arrangement of the main condenser outside the low-pressure column.

In the main condenser103, liquid oxygen is evaporated, which runs down from the lower mass transfer region132or is sucked in from the bath in the bottom of the low-pressure column; in contrast to this, gaseous nitrogen from the top of the high-pressure column101is evaporated. (The nitrogen conduits are not shown inFIG. 6.)

The liquid collectors and distributors are not shown inFIG. 6apart from the collector133between the upper mass transfer region131and the argon column top condenser155, the two liquid distributors44,420at the top of the first and second mass transfer space134,135and the liquid distributor45at the top of the lower mass transfer section132. For the rest as well,FIG. 6is very schematic and should generally not be regarded as being to scale.

The middle mass transfer region130of the low-pressure column is subdivided by a vertical flat dividing wall136in a gas-tight manner into first mass transfer space134and a second mass transfer space135. The first mass transfer space134is open in the upward direction toward the upper mass transfer region131and in the downward direction toward the lower mass transfer region132, meaning that gas from the lower mass transfer region132can flow into the first mass transfer space134of the middle mass transfer region131, and gas from the first mass transfer space134can flow away upward into the upper mass transfer region of the low-pressure column. The first mass transfer space134fulfils the function of the argon section of the low-pressure column, i.e. of that mass transfer region which, in a conventional plant, is immediately above the argon transition, through which an argon-containing fraction would be passed to an external crude argon column or argon column.

The second mass transfer space135, which forms the argon column152, is likewise open in the downward direction toward the lower mass transfer region132; ascending gas flows out of the lower mass transfer region132of the low-pressure column into the second mass transfer space135in this way. At its upper end, the second mass transfer space135, however, is sealed in a gas-tight manner from the upper mass transfer region131. The seal in the upward direction is brought about by a horizontal plate36which—apart from the conduits37,62,41conducted through it—is gas tight. Between the upper131and middle130mass transfer regions is the argon column top condenser155, which is configured as a condenser-evaporator, here in accordance with the invention as a forced-flow evaporator. In this working example, it consists of a single plate heat exchanger block. Alternatively, it could also be formed by two or more plate heat exchanger blocks arranged in parallel. The liquefaction space of the argon column top condenser155is in flow connection with the top of the argon column152via the gas conduit37and the liquid conduits62,41. In this case, top tops gas from the argon column152flows via the gas conduit37from the upper end of the second mass transfer space135into the liquefaction space and is at least partly liquefied there. The liquid produced is drawn off via conduit62, recycled via conduit41into the second mass transfer space135and distributed by means of a liquid distributor420as reflux liquid to the argon column over the cross section of the second mass transfer space135. The proportion163remaining in gaseous form is drawn off from the vessel of the low-pressure column102and treated further as shown inFIGS. 1 to 5.

The liquid flowing away from the two mass transfer spaces134,135of the middle mass transfer region130is collected in a liquid collector (not shown). The liquid flows onward to the liquid distributor45, which distributes it over the entire column cross section and applies it to the lower mass transfer region132.

The crude oxygen153from the bottom of the high-pressure column101is—similarly toFIG. 1—introduced between two packing sections of the upper mass transfer region131. At the same point, an air stream129is introduced, which has previously been expanded to about low-pressure column pressure so as to perform work (see air injection turbines137inFIGS. 1, 2 and 5).

In addition, liquid air128is introduced into the upper mass transfer region131. Virtually all the liquid from the upper mass transfer region131is collected in the liquid collector133and introduced via the conduit71into the evaporation space of the argon column top condenser155. This has two advantages:The amount of liquid which flows via conduit71through the evaporation space is particularly large. In the argon column top condenser, preferably 35% to 55%, for example about 45%, of this amount of liquid is evaporated.This liquid has a relatively high oxygen content and hence a comparatively high evaporation temperature. This allows a particularly small temperature differential to be achieved; in three specific examples, it is 0.8 K, 1.0 K or 1.5 K. This allows the thermodynamic losses in the condenser to be kept particularly small.

The high liquid excess is thus of considerable significance for the efficiency of the forced-flow evaporator.

A biphasic mixture emerges via conduit73from the evaporation space of the condenser155. The liquid component L flows into the liquid distributor44at the top of the first mass transfer space134. The evaporated component V flows back upward into the upper mass transfer section131.

The closed-loop control of the argon column top condenser155is effected in the working example ofFIG. 6by a closed-loop control method 1 which requires a bypass conduit49/50and a closed-loop control valve48. In this way, the performance of the argon column top condenser155is controlled.

A small amount of relatively nitrogen-rich liquid flows into the distributor45and increases the nitrogen content in the vapour ascending out of the lower section132and hence also in the overall argon column152and additionally in the liquefaction space of the argon column top condenser155. Thus, this control conduit and the valve arranged therein enable a controlled reduction in the performance of the condenser. The relatively nitrogen-rich liquid, in the working example, comes from the collector133at the lower end of the upper mass transfer region131.

The closed-loop control valve48is closed in steady-state operation, or only a very small amount of liquid flows through it. In the event of deviations from steady-state operation, for example in the event of a change in load, generally less than 5% of the overall liquid71/49from the liquid chamber133flows through the bypass conduit, and in any case less than 15%.

Alternatively, other closed-loop control methods can be employed, one of which is described in detail hereinafter.

FIG. 7shows an alternative closed-loop control method 2 with a closed-loop control valve700in the gas inlet37to the liquefaction space of the argon column top condenser155. This valve can be used to adjust the condensation pressure with appropriate condensation temperature. This directly influences the driving temperature differential in the condenser155and correspondingly also the condenser performance or the conversion in the argon column152. The valve can be controlled via the pressure differential in the argon column (PDIC=pressure difference indication and control, not shown).

The only difference inFIG. 8with respect toFIG. 7is the lack of a mass transfer region between the introduction of the liquid crude oxygen153from the bottom of the high-pressure column and the liquid collector133at the lower end of the upper mass transfer region131. In other words, the liquid crude oxygen is introduced directly into the liquid collector133and hence into the evaporation space of the condenser155.

A closed-loop control method 3 is shown inFIG. 12. Here, the biphasic mixture from the evaporation space of the argon condenser155is introduced into an additional vessel1250. Via conduit1251, the gaseous component V is returned to the low-pressure column, such that it is available as ascending vapour in the upper mass transfer section131. The liquid component L is introduced via conduit1254into the liquid distributor44at the top of the first mass transfer space134(of the argon section). By means of a closed-loop control valve1252, the pressure in the evaporation space of the argon condenser155and hence the performance thereof can be adjusted.

The liquid conduit1254may likewise have a closed-loop control valve. Alternatively, the liquid flow is controlled by a fixed diaphragm, for example in the form of an opening in the base of the vessel1250. The dimensions of this have to be such that the liquid level in the vessel, according to the pressure in the vessel, will vary between the upper and lower vessel limits.

FIG. 9is based onFIG. 2, but has a complete recovery of argon, in which the oxygen content in the top product963from the argon column is reduced, for example, to 0.1 to 100 ppm. The substantially oxygen-free argon gas963is subsequently fed to a pure argon column in which an argon-nitrogen separation is undertaken. For the low oxygen content necessary for this purpose, the few theoretical plates in the dividing wall section135are insufficient. Therefore, a crude argon column900of almost standard length is used, utilizing the second mass transfer space135in the dividing wall section of the low-pressure column102as the uppermost mass transfer region of the crude argon rectification. For this purpose, the second mass transfer space135has to be sealed gas-tight at its lower end, for example by a semicircular plate. Below this plate, argon-containing gas901is drawn off from the low-pressure column102and fed to the bottom of the crude argon column900. The bottoms liquid902from the crude argon column900is conducted in the opposite direction at the same point in the low-pressure column102. The top of the crude argon column is in flow connection via conduits903(for gas) and904(for liquid) with the lower end of the second mass transfer space135. As known fromFIGS. 1 to 7, the upper end thereof is connected to argon column top condenser155.

In the working example ofFIG. 10, the second mass transfer space135is open at the bottom end and in this respect is operated analogously toFIGS. 1 to 5. However, the top thereof is not connected directly to the argon top condenser155, but connected via conduits905and906to the bottom of the crude argon column900. The top of the crude argon column is in flow connection via conduits907and908with the liquefaction space of the argon column top condenser.

FIG. 11shows a working example without a dividing wall section in the low-pressure column. The argon column here consists exclusively of the separate crude argon column900, the top of which, analogously toFIG. 10, is connected (907,908) to the argon column top condenser155. The bottom of the crude argon column900ofFIG. 11, analogously toFIG. 9, is connected (901,902) to an appropriate intermediate point in the low-pressure column102.