FI111187B - A process under normal pressure for producing oxygen or oxygen enriched air - Google Patents

Description

111187

NORMAL PRESSURE PROCESS FOR THE PRODUCTION OF OXYGEN OR OXYGEN enriched air

The invention relates to a process for producing oxygen or oxygen-enriched air, wherein the air entering the process is cooled near the dew point of the heat exchanger; air is supplied to a separation unit formed by two or more thermally coupled spaces 10, comprising A and B halves, where it is divided into two or more gaseous fractions using liquefaction and evaporation processes, and the gas fractions leaving the separation unit are recycled to the heat exchanger.

The cryogenic separation processes are based on a fractional distillation and enrichment process performed on two columns under different pressures, each of which must maintain a certain temperature difference between the bottom of the column and its top cooler. In the classical Linde-Fränkl process, this is accomplished by thermally coupling the high end of the high pressure column to the lower end of the low pressure column, but otherwise operating adiabatically. The temperature difference between the "free" ends of the columns, i.e. the lower end of the high pressure column and the upper end of the low pressure column, gives an idea of the process deviation. The difference is 20K in the Linde-Fränkl process.

US 5,592,832 and US 5,144,809 disclose the use of non-adiabatic, i.e., thermally coupled, full-length, distillation and enrichment columns for the separation of air constituents, for the former production of oxygen and the latter for nitrogen.

By thermally coupling the columns, the temperature difference between the columns is reduced and the energy consumption of the process 35 is slightly reduced.

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The achieved energy savings are comparatively small because here the temperature difference required for distillation is maintained here by utilizing the pressure difference between the process air pressurized to several bars and the "low pressure column" at lower than normal pressure 5, i.e. The process air.

For many reasons, the pressure of the multi-bar process used in the prior art compels, for economic reasons, to construct the separation apparatus 10 as small as possible, whereby the density of the heat flow to be transferred is high per unit volume. For this reason, the aforementioned publications and their claims limit themselves to treating plate heat exchangers exclusively, despite the high flow resistance provided by their narrow channels.

15

US 4,192,662 discloses a process for producing liquid oxygen and nitrogen, wherein the process is further cooled by liquid natural gas (LNG).

The object of the present invention is to provide an improved process for producing oxygen or oxygen enriched air. Characteristic features of the process of the invention are set forth in the appended claims.

The separation unit used in the process of this application operates in essentially the same way as the units used in the prior art, i.e. it consists of a double column, the halves of which are thermally connected to each other. As with prior art,. this method is based on the fact that, when the gaseous air and the liquid formed upon cooling thereof are in equilibrium with each other, oxygen is enriched in the liquid phase and nitrogen in the gas phase.

The process described in this application differs from the prior art in that the separation process is transferred to a pressure range below normal pressure. The thermal elevations required for the distillation process are now generated by utilizing the pressure difference between the normal-pressurized process air and the reduced-pressure oxygen fraction (and other possible vacuum-fractions), and the separation work is brought by supercharging said vacuum-free fractions to normal-5. The improvement in the thermodynamics of the separation process is manifested in several ways:. Instead of compressing the process air, only the oxygen concentrate separated in the process (and the fraction separated in its possible enrichment) having a total volume of about 40% of the process air volume has to be charged. As a result of the low pressure of the process, a lower pressure ratio than the prior art is sufficient to maintain the temperature difference required by the separation, so that the energy of supercharging the oxygen concentrate is further reduced. Em. In U.S. Patent No. 5,592,832, the process energy for supercharging the process air alone is 755 kJ per kg of oxygen, ideally with a compressor efficiency of 85% and the removal of all its oxygen from the air. In the Linde-Fränkl process, the work on supercharging alone is about 882 kJ per kg of oxygen.

.7: 20 - Possibility of energy-efficient separation processes that leave a substantial amount of its oxygen in the process air. Example: If the air is separated into 50% oxygen fraction and pure nitrogen, the theoretical separation work at 300 K is 83 kJ / kg oxygen. If 6.1% oxygen is left in the 25 nitrogen fractions, the separation work will be reduced to 58 kJ per kg of oxygen fraction, i.e. 30% energy savings.

- Possibility of utilizing the high oxygen enrichment coefficients at low liquefaction pressure, whereby the energy consuming oxygen fraction enrichment used in the prior art is not required or is less necessary to achieve a given oxygen concentration. For example, at a pressure of 5 bar, the liquid phase in equilibrium with the gaseous air contains 42% oxygen, i.e. the oxygen is enriched in the liquid phase by a factor of 2.0.

The liquid phase in equilibrium with 35 bar air contains 52% oxygen and has an enrichment factor of 2.48.

4 111187 - At low pressure, a lower temperature difference between the bottom of the "high pressure column" and the peak of the "low pressure column" is achieved. In this method, when producing both 50% oxygen concentrate and pure oxygen, this difference is 7.5 K, compared to 20 K in the 5 Linde-Fränkl method.

The invention further provides the following advantages:

It follows from the low pressure levels of the process that the heat input to or from a given 10 flow rate is substantially lower than in the prior art. For this reason, it is possible to use solutions in which the flow channels are looser than the plate heat exchangers discussed in the above publications, and the power losses caused by the flow resistance can be reduced. Examples of preferred solutions according to this method are shown in the Application Examples and the accompanying Figures 2-5.

When the process is carried out at and below normal pressure, the heavy and expensive pressure vessels required by the prior art are not required.

In most solutions, when the whole separation process is performed in a single double column, the required hardware is much simpler than in the prior art. This can be stated by comparison (Fig. 8 of US 5,592,832).

These solutions are described in the following examples with reference to the accompanying drawings.

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Figure 1 shows the composition of gaseous oxygen and nitrogen and the equilibrium liquid phase as a function of temperature and pressure,

Figure 2 schematically illustrates an apparatus according to the invention, 5111187

Figure 3 is a schematic view of another apparatus according to the invention,

Fig. 4 shows a third apparatus according to the invention. Fig. 5 shows an apparatus using the heat transfer circuit according to the invention.

Figure 1 shows the composition of gaseous nitrogen and oxygen mixtures and the equilibrium liquid phase as a function of temperature and pressure. The figure shows that the air at 1 bar (21% oxygen) has a dew point of 81.5 K and the liquid in equilibrium with it contains 52 mol% of oxygen. Similarly, at 5 bar, the corresponding value is 42% oxygen. The enrichment factor increases as the absorption pressure decreases.

15

In the process apparatus utilizing the invention shown in Fig. 2, oxygen separation from air takes place in a separating unit 10. Here, the separating unit 10 is divided into two parts by a cylindrical wall 13 which allows heat exchange between the first column 11 and the second column 12. On the heat transfer surfaces 13a and 13b, ribbed plates 38 and 39 can be used, which also control the gas flows. The first column 11 functions as a liquefaction section into which a fan (1 bar) J.1 'is carried. the air 25, cooled under pressure and heat exchanger 81.5 K, is introduced into column 11 from the air inlet 22 at the bottom thereof, and the air flows from the bottom of the column 11 upwardly cooled and partially fluidized to the heat transfer surface 13a. At the exit of the gas phase, at the top of the column 11, the nitrogen-rich exhaust air at 23 78 K, there remains only about 6%) of oxygen. An oxygen fraction of the heat transfer surface 13a of column 11; '; flows by gravity down the heat transfer surface 13a; and is collected at the bottom of the column 11 and led together through 25. In a fully reversible process, the liquid leaving the bottom of column 11 would contain about 52% oxygen. However, in use-35, about 50% is achieved. The liquid oxygen fraction is passed through a choke 37 which, in addition to the liquid lift height 6111187, reduces the pressure, further to the top of the evaporator portion of the separation unit 10, or column 12, from the liquid oxygen fraction inlet 42. From the top of the column 12, the oxygen-rich liquid flows downward, evaporating back to the gas phase from the first 5 column 11 under the effect of heat released by liquefaction of the oxygen fraction transmitted to the heat transfer surface 13b, interacting to cool the nitrogen fraction leaving column 11. In column 12, the pressure in the embodiment of Figure 1 is 0.4 bar and the temperature at the top of column 12 is 74 K.

10

In addition to the volatile liquid phase in column 12, the evaporated gas phase also flows downwardly, and the oxygen fraction leaving the bottom of the column 12 through the oxygen outlet 24 still contains about 50% oxygen at a temperature of 78.5 K and a pressure of 0.4 bar. The oxygen fraction from column 12 as well as the nitrogen fraction leaving the top of the column 11 under normal pressure from the air outlet 23 are used to cool the air entering the process in said heat exchanger. The oxygen fraction leaving the heat exchanger is compressed to the desired pressure according to its intended use.

20

The temperature difference between the columns 11 and 12 in this embodiment of the invention is 3-4 K, which is a typical value used in oxygen plants. When evaporation at lower temperature absorbs more heat than is released at higher temperature liquefaction, the process essentially produces the cooling power it needs.

The aforementioned temperature difference implies deviations from the reversibility, in addition to which losses occur in the separation unit 10 and the heat transfer unit 14. If the process is carried out at normal pressure, the supply of process air to the separation unit 10 requires a fan. Its need for energy is ignored here, but part of it is recovered at the higher pressure of the separated oxygen fraction. Other losses result in the pressure of the oxygen fraction remaining either lower than 0.4 bar or the composition of the fractions changing so that more oxygen remains in the oxygen fraction and / or more oxygen in the nitrogen fraction. Depending on the size of the separation unit 10 and the process details, it may additionally require external cooling power. On the A side, the pressure drop including the heat exchanger is not more than 0.3 bar, most preferably less than 0.2 bar.

5

As with other cryogenic processes, the energy consumption of this process consists mainly of gas supercharging, which can be used to compare the energy consumption of the various methods. In the classical Linde-Fränkl process, the process air 10 is compressed to 5.6 bar and the compressor work is 882 kJ / kg oxygen with a supercharger efficiency of 85%. In the process of US 5,592,832, the supercharging work is ideally 775 kJ / kg oxygen. These values are compared in the table below with the cases described in Examples 1 and 2 of the present invention, the theoretical pressure ratios are multiplied by 1.5 to cover the pressure losses and the efficiency of the supercharger is assumed to be 85%. In Example 2, the cold oxygen fraction pressurization shown in Figure 3 is not used.

Method Stacking / Oxygen kg

20 Linde-Fränkl 882 kJ

US 5,592,832> 755 kJ

This invention - Example 1 273 kJ (50% concentrate) This invention - Example 2 399 kJ (83%. * Concentrate) With these provisions, the initial energy consumption of the process consists of pressurizing the oxygen fraction to 1 bar, which in theory requires 115 kJ. / Nm3el 160 kJ / kg 02. The losses of the supercharger 21 can be covered by water injection. The heat transferred to gas during supercharging can also be utilized, for example, in the power plant cycle. If 30 different losses increase this value by about a factor of 1.5, the energy consumption per kg of oxygen would be only about 273 kJ / kg 02 (50% concentrate). The corresponding value for 83% concentrate is 399 kJ / kg 02. Thus, the total energy consumption is substantially lower than in the known processes mentioned above.

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Figure 3 is a schematic diagram of another embodiment of the process utilizing the process of the invention that provides a higher oxygen content than the previous embodiment. In this embodiment, the column 11 5 of the separation unit 10 functions in a similar manner to the embodiment shown in Figure 1. A portion of the downstream liquid oxygen fraction introduced into the column 12 of the separation unit 12 from the liquid oxygen fraction inlet 42 is evaporated in an enrichment section 27 at the top of the column 12. The pressure of the liquid fraction

If the liquid oxygen fraction contains 50% oxygen, then in the 0.4 15 bar reverse flow evaporated gas phase there is 83% nitrogen, so that the oxygen is enriched in the liquid phase. The liquid oxygen fraction leaving the concentrator is then evaporated to a final oxygen fraction in the evaporator portion 28 of the lower column 12. The evaporated gas from the concentrator 27 is used to cool the incoming air in the heat exchanger 20 and is discharged by a supercharger to cool the incoming air to the process, then remove it) with a supercharger. In column 11, the heat released in the production of the liquid oxygen fraction 25 is transferred from column 11 from the heat transfer surface 13b of wall 13 to column 12, the enrichment and evaporation portions 27 and 28 of which are separated.

The oxygen content of the oxygen fraction produced by the separation unit 10 30 according to the invention shown in Figure 3 can be adjusted by changing the proportion of enrichment and evaporation steps 27, 28 of the column 12 of the separation unit 10 from the evaporation process. The enrichment section 27 can be operated at 0.4 bar, but the evaporator section 28 must be selected with a choke 21 so that the dew point of the oxygen fraction is 35 lower than the dew point of incoming air at 81.5 K. Normal oxygen and its dew point are desired. 79 K, the pressure becomes 0.25 bar. At such a low pressure, the volume flow of the oxygen fraction is smaller than that of the air entering the process at normal pressure.

5 The pressure loss of the oxygen concentrate in the heat exchanger can be reduced, for example, by the arrangement of Figure 3. The concentrate from the evaporation section 28 of the column 12 is supercharged by the supercharger 30 while cooling in the heat exchangers 26 and 27 by the cold, oxygen-poor process air exiting the column 11 of the separation unit 10. This process air is then expanded in a cooling turbine 32, the vacuum of which is directed to a heat exchanger 14 together with a vacuum of nitrogen from the concentrator 27.

In the example of Figure 4a, the double column is constructed as a container 10,. having a plurality of tubes 33 having horizontal plates 34 on the outer surface, the tubes being divided into two groups X and Y arranged as shown in Figure 4b such that each X tube is surrounded by four Y tubes and each Y tube is surrounded by four X-20 tubes .

The X-tube plates coincide between the Y-tube plates so that each X-tube plate is in the center of the eight Y-tube plates in the quadrilateral and respectively the Y-tube plate 25 is in the middle of the eight X-tube plates in the quadrilateral. A regular three-dimensional maze is created between the tubes and the plates attached thereto, which increases the turbulence of the gas and liquid phases flowing therein and the exchange of material and heat between the phases. Due to the regularity of the structure and the loose flow channels, it is possible to maintain higher phase flow rates than the prior art columns. These structures thus provide regular, loose flow channels to enhance interphase material and heat transfer and to reduce pressure losses 35 in gas streams.

111187 In this example, the space between the tubes, the A-side of the double column, is a liquefaction unit, the bottom of which accumulates the liquid oxygen fraction through the throttle valve 37 to the B-side of the double column consisting of tubes 33 which serves as a evaporation unit. Inside the B hemisphere tubes are helical spring-shaped structures 36, which direct the liquid phase flowing in the tubes to the helix paths and also increase the turbulence of the gas phase flowing in the tubes, enhancing the interphase material and heat transfer.

This double column functions in the same way as the structure described in the first embodiment, i.e. the pre-cooled process air is led at normal pressure to the bottom of the A-hemisphere, from where it flows upwardly partially oxygenated 15 at about 0.4 bar and the resulting gaseous oxygen fraction exits the bottom of the unit from one to 24 heat exchangers. 1 2 3 4 5 6 30

Figure 5 illustrates an embodiment in which the major part of the heat flux released in liquefaction 2 is conveyed to evaporation using 3 heat transfer fluids, e.g., nitrogen or oxygen fractions or their constituents 4, which are circulated inside the ribs 35 by pump 20.

5

Due to the low viscosity of the liquid air constituents 6, this process provides an effective thermal coupling between the liquefaction and evaporation units. In this way, large flow cross-sections can be used in the liquefaction and evaporation unit and the shape and possible fillings of their flow channels can be optimized.

Claims (11)

A process for producing oxygen or air enriched with oxygen, according to which: the air entering the process is cooled to near the dew point of a heat exchanger (14), the air is conducted to a separation unit (10), consisting of two or more thermally coupled spaces, which comprise A and B sides (11, 12), where it is divided into two or more gaseous fractions by means of condensation and evaporation processes, and the gas fractions leaving the separation unit 15 (10) are led back to the heat exchanger (14), characterized in that the process air heats under normal pressure in addition to small (<0.3 bar) pressure differences for maintaining the air flow and optimizing the process, and 20 - when the process air flows through the A-side of the separation unit (11). the liquid oxygen concentration separated therein is evaporated in the B-side of the separation unit (12) at a pressure below the normal pressure, so that the oxygen concentration is reached compound and the condensation heat released in the A side (11) of the unit transfers to the B side of the unit (12) and binds there to the evaporation of the oxygen concentration, and leaving the A side (11) of the separating unit: oxygen-poor process air is conducted out of the process via the heat exchanger (14), and the oxygen concentration evaporated in the B-side (12) of the separating unit discharges under the relevant side, saving the normal pressure below pressure, either as the seed or via a compression and cooling process. to the heat exchanger (14) and further out of the process, and the condensation and evaporation processes are performed in said separation unit (10) under almost reversible conditions such that in both processes the gas phase present in each part of the separation unit is close thermodynamic balance with the liquid phase present in the same part. Process according to claim 1, characterized in that the oxygen fraction produced by the process, whether no such as the seed or or compressed to higher pressure, undergoes a new separation process described in claim 1 for raising its oxygen content. Process according to Claim 1, characterized in that the B-side (12) of the separating unit is divided into two parts, the first of which is a concentration part operating according to the countercurrent principle, in which the liquid state of the A-side (11) of the separating unit the oxygen concentration runs down while some of it evaporates under almost reversible conditions, such that the gas phase present at each height is close to thermodynamic balance with the liquid phase present at the same height, and at which concentration portion (27) it reaches a pressure below the normal pressure, such that in the portion 25 of the oxygen concentration which discharges as liquid from the bottom of said concentration portion, the desired composition is obtained, and the condensation heat released from the portion of the concentration portion (27) of the A-side (11) with said part in the heat exchange connection. ) is moved to said concentration portion (27) and is bound there at s evaporation of the desired concentration of the oxygen concentration, and the portion of the evaporated oxygen concentration (27) evaporating from the peak of the relevant portion under the said rescue pressure, which is lower than the normal pressure, via the heat exchanger to the compressor where it is compressed to the desired pressure. and the bottom of the concentration (27) remaining at the bottom of the concentration portion (27) is led to the second portion (27) of the B-side (12), from which it flows down-sealed and evaporates under a saturated pressure lower than the normal pressure, so that the condensation heat released in the portion (28) of the A-side (11) 10 (A) of the A-side (11) in the heat exchange connection is transferred to said second portion and binds therein to the desired evaporation of the oxygen concentration, and said evaporated the oxygen enrichment decides to no one as the seed or via a compression and cooling process to the heat exchanger (14) and further out of p. rocessen. Process according to any of claims 1-3, characterized in that the process conditions are adjusted so that a substantial part of the oxygen in the process air remains, thereby reducing the energy consumption of the process. Process according to any of claims 1-4, characterized in that the oxygen fraction leaving the separation unit is compressed to higher pressure, cooled by a separate cooling circuit and the cooled flow is conducted via the heat exchanger out of the process. Process according to Claim 5, characterized in that said separate cooling circuit consists of structures in which the nitrogen fraction emanating from the condensing part of the process is arranged to cool said oxygen fraction and said nitrogen fraction then expanded in a cooling turbine, whereupon the expander is heated. is switched on from the concentration portion of the unit to the heat exchanger 35 leaving under the underpressure flow. 18 111187 Process according to any of claims 1-6, characterized in that the heat exchange connection between the A and B sides of the separating unit is made so that the sides together form a container, in which there are a number of tubes, the space between the tubes being one of the units and the the inner space of the pipes is the other side of the unit. Process according to any of claims 1-7, characterized in that the 10. heat exchange connection between the A and B sides of the separating unit is made so that the sides are made up of a module consisting of two tubes within each other, which may be one or more, and in which the module space between the inner and outer tubes 15 can act as the A side and the inner space of the inner tube as the B side or vice versa. Process according to any of claims 1-8, characterized in that at least part of the heat flow released in the A-side of the separating unit is moved to the B-side of the unit by circulating suitable heat transfer fluid in the tubes located therein. Process according to claim 9, characterized in that the heat transfer fluid mentioned in claim 9 is liquid oxygen or nitrogen fraction or their constituents. Process according to any one of claims 1-10, characterized in that constructions are used in the separation unit which form regular, spacious flow channels for streamlining the material and heat transfer between the phases and reducing the pressure losses of the gas streams. 35