DE3315930A1 - Process and apparatus for separating a gas mixture - Google Patents

Abstract

A gas mixture containing hydrogen and at least one further constituent, for example nitrogen and argon, is separated by absorption or cooling and partial condensation and/or rectification and/or scrubbing. Before this separation stage, at least a part of the hydrogen is separated off by diffusion on semipermeable membranes in the gas mixture. <IMAGE>

Description

Method and device for dismantling a

Gas mixture The invention relates to a method for breaking down a Gas mixture that contains hydrogen and at least one other component, by adsorption or by cooling and partial condensation and / or rectification and / or laundry, as well as a device for carrying out the method.

Such a gas mixture falls, for example, as purge gas in the Ammonia or methanol synthesis. Because the gas mixture is valuable, reusable Components, especially hydrogen, argon and methanol containing, are these recovered from the gas mixture and fed to a further use or undesired Components separated from the gas mixture.

If only the hydrogen is to be recovered, the rest will be Components of the gas mixture by condensation and / or adsorption of the hydrogen severed. If, on the other hand, other components are to be recovered, then will the gas mixture fed to one or more separation stages in which the individual Components through partial Condensation and / or rectification and / or laundry separated and according to their different physical Properties can be taken from the head or sump side.

External cooling is usually required to carry out the process, from a high-pressure refrigeration cycle, for example a nitrogen cycle, is delivered.

The disadvantage of this method is that it consumes considerable energy for the generation of the necessary cold must be provided or that the product streams can only be obtained under relatively low pressure. Moreover, conditionally a high-pressure refrigeration circuit, complex compressors, pipes and fittings.

The present invention is therefore based on the object of a method to develop the type mentioned at the beginning, which is energetically favorable and with little can be carried out with an outlay on equipment.

This object is achieved in that at least one Part of the hydrogen before cooling or

adsorption by diffusion on semipermeable membranes of the Gas mixture is separated.

The separation of hydrogen on semipermeable membranes is true known in principle, but it has the disadvantage that the hydrogen one suffers considerable pressure loss when passing through the membranes, so that the Process was therefore viewed as energetically unfavorable, especially because at an ammonia synthesis which requires hydrogen under high pressure. Only by the inventive combination of two process steps, namely a diffusion and an adsorption or a refrigeration process, it is possible to put every part of the system in to use the most favorable operating area for him, whereby an optimal times Adaptation to given boundary conditions is made possible.

The equipment and the energetic expenditure for the SefleX supply is thereby optimized.

The inventive method is particularly advantageous when the Components of the gas mixture to be broken down boil at temperatures of approx. 120 K and show below, because then the achievable economic advantages particularly are great.

In a preferred embodiment of the method according to the invention a maximum of 60% of the hydrogen is separated from the gas mixture during diffusion, while the remaining part of the hydrogen by partial condensation of the rest Components of the gas mixture is separated.

Preferably about 20-40%, especially about 30% of the hydrogen separated during diffusion. If necessary, the condensate is used to recover a Component, for example argon, then rectified.

Due to the relatively low yield demand on the low pressure fraction During diffusion, the required membrane exchange area becomes small and the pressure loss, which the low-pressure fraction suffers during diffusion is relatively low held. The low-hydrogen high-pressure fraction is in a subsequent adsorption or condensation process further decomposed. Since in this subsequent process the The amount of hydrogen in the gas is already relatively low, the Keep equipment costs low.

In another embodiment of the method according to the invention more than 60% of the hydrogen is separated from the gas mixture during diffusion and is at least one of the constituents contained in the remaining gas mixture share separated from the remaining components by rectification or washing.

In the case of low-temperature decomposition, the refrigeration requirement and thus the Energy input through the partial pressure of the condensable components of the gas mixture certainly. The partial pressure is higher, the lower the hydrogen content in the gas mixture. Therefore, according to the invention, before the low-temperature process most of the hydrogen is separated by diffusion. In diffusion There is indeed a loss of energy due to the pressure drop in the hydrogen during passage through the semipermeable membrane, but there is the gas mixture, which is then cooled and is fed to the cryogenic decomposition, largely freed from hydrogen is, there is far less energy to obtain the condensable components for the refrigeration than previously required, so that overall the process is energetically cheaper.

The inventive method also has the advantage that a medium pressure refrigeration circuit is sufficient for the refrigeration, so that the Process with smaller compressors than before.

In a preferred development of the method according to the invention is the gas mixture after separation of the hydrogen with simultaneous heating at least partially liquefied in a separation stage for breaking down the remaining gas mixture.

The low-hydrogen raw gas mixture condenses relatively easily and becomes with simultaneous heating of a separation stage in which two of the low-boiling ones Components are separated from each other, at least partially liquefied.

According to a preferred embodiment of the method according to the invention sump liquid from a separation stage znt decomposition of the remaining gas mixture evaporated at sub-atmospheric pressure.

In particular, it is advantageous if the evaporation is carried out by supplying heat from a refrigeration cycle, which is also used to cool the separation stage, he follows.

The pressure reduction can also lower the pressure of the refrigeration circuit to get voted. For example, nitrogen circulates in the refrigeration circuit, the is liquefied during this heat exchange. The negative pressure is, for example, in of the order of 0.1 to 0.5 bar.

In a further embodiment of the method according to the invention, it is proposed that that the fluid carried in the refrigeration circuit is part of the remaining gas mixture is.

It proves to be advantageous if, in a further development of the invention Process the sub-atmospheric pressure by jet compression by means of the gaseous Part of the partially liquefied remaining gas mixture produced as propellant steel will.

Before the jet compression, the gaseous part is warmed up. Of the In the case of jet compression, the negative pressure gas flow is recompressed to approximately atmospheric pressure.

In particular, it is advantageous if the circulating medium is fed to the separation stages at the head.

The liquefied circulating medium is used as the reflux liquid and / or used for indirect head cooling in the separation stages.

The inventive method is particularly useful when the Gas mixture is a purge gas from a gas synthesis. Examples of such gas syntheses are the ammonia synthesis or the methanol synthesis.

In the special case of ammonia synthesis, that contains the Gas mixture to be decomposed according to the invention as essential components in addition to hydrogen, methane, nitrogen and / or carbon monoxide and argon.

A device for carrying out the method according to the invention comprises a feed line for a gas mixture, the at least one heat exchange device or contains an adsorber and which opens into a separation column, and is characterized by that in front of the heat exchange device or the adsorber one selective for hydrogen acting diffusion device is arranged.

The invention, as well as further details of the invention, will become apparent with reference to of schematically illustrated embodiments explained in more detail.

Figures 1 to 3 show different embodiments of the method according to the invention using the example of the decomposition of a purge gas the synthesis of ammonia.

In the method according to FIG. 1, a purge gas 1 (590 Nm3 / h) is to be broken down that has a composition of for example 62% hydrogen, 20% nitrogen, 11% methane and 7% argon, a pressure of approx. 140 bar and a temperature of 350C having. In a separation device 2, which contains semipermeable membranes, the are permeable to hydrogen, the hydrogen 3 is separated from the purge gas. The pressure of the hydrogen 3, which is present with a purity of about 91%, is still approx. 24 bar.

The low-hydrogen raw gas 4 (7% hydrogen, 50% nitrogen, 27% Methane, 16% argon with approx. 138 bar and 35 ° c | is now a cryogenic decomposition fed. The amount of gas is only about 35% of the original amount of purge gas, so that the cryogenic system can be made much smaller and cheaper can.

In a heat exchanger 5, the raw gas is in heat exchange with a Nitrogen refrigeration cycle as well as residual gas from the low-temperature decomposition cooled about 150 K.

The cooled raw gas is then heated by heating the sump a first separation column 6 and in a heat exchanger 7 and by heating a second separation column 8 further cooled down to about 95 K. The cold gas is expanded and thereby partially liquefied. In a separator 9, the liquid portion separated from the gaseous and via a line 10 into the first separation column 6 relaxed. The separation column 6 is operated at a pressure of 2 bar.

In the separation column 6, the head of which is cooled by liquid nitrogen 22 is, the methane is separated on the sump side from the nitrogen and argon, which over the top of the separation column 6 can be removed in gaseous form (line 13). Methane 11 is together with the gaseous fraction 12 from the separator 9 in the heat exchangers 7 and 5 warmed up and removed from the system.

The nitrogen-argon mixture is fed to the separation column 8, the operated at a pressure of 1.3 bar.

Because the hydrogen has already been separated off in the separating device 2, the separation of the argon can already be carried out at a relatively high temperature will. The separation column 8 is filled with liquid nitrogen from a storage container 14 applied. The argon is made on the sump side via line 15 taken from the separation column 8 (47 Nm3 / h at approx. 91 K). Over the top of the separation column 8 nitrogen 16 is withdrawn and fed into a nitrogen refrigeration cycle, in the case of the nitrogen from the storage container 14 in a heat exchanger 21 against circulating nitrogen and warmed up in the heat exchanger 5 against the raw gas 4 and circulating nitrogen and is compressed in a three-stage compressor 18 to about 25 to 30 bar.

An amount of nitrogen corresponding to the nitrogen supplied via the purge gas is taken via line 17.

Medium pressure nitrogen 19 from the second and high pressure nitrogen from the third compressor stage are both cooled in the heat exchanger 5. The high pressure nitrogen 20 is used to heat the bottom of the first separation column 6 and is, after cooling in Eätmrzsudvhrt 21, relaxed in the storage container 14. The medium pressure nitrogen 19 is used to heat the bottom of the second separation column 8 and is also in the reservoir 14 relaxed.

FIG. 2 shows a modified method according to FIG. 1.

Purge gas 31 from an ammonia synthesis is in a separation device 32 by diffusion on semipermeable membranes in hydrogen 33 and a hydrogen-poor one Purge gas 34 decomposed (for example 2,400 Nin3 / h with a pressure greater than or equal to 40 bar), possible traces of ammonia and water are cleaned in a fine cleaner 35 away. The purge gas is cooled in a heat exchanger 36 and a condenser evaporator fed in the bottom of a first separation column 37, in which the purge gas is partially condensed. The liquefied portion 38 is fed to the separation column 37, which is at operated at a pressure of approx. 2 bar. The gaseous portion 39 is after cooling in a heat exchanger 40 in heat exchange with relaxed column sump liquid, in which part of the gas condenses out, fed to a separator 41 in which a liquid frac- tion 42, which is also in the separation column 37 is passed by a gaseous fraction 43, which is essentially residual hydrogen and containing nitrogen. Methane is converted from the separation column 37 on the sump side taken while over the top of the separation column 37 an essentially nitrogen and gas mixture containing argon is withdrawn (line 47).

The gas fraction 43, which has a pressure of about 35 bar, is warmed in part of the heat exchanger 36 to about 180 K and to drive one Jet compressor 44 used, the part of the methane 45 from the sump of the Separating column 37 is sucked in and recompressed to about atmospheric pressure. The methane will previously at a pressure of approx.

0.3 bar evaporated in a heat exchanger 46 against circulating nitrogen. The remaining methane 48 from the separation column 37 is in the propulsion jet behind the jet compressor 44 mixed in and fed together with this after heating in the heat exchanger 36.

The gas 47 from the top of the first separation column 37 becomes a second separation column 49 supplied, in the argon (sump side) and nitrogen (head side) from each other be separated.

The separation column 49 is filled with liquid nitrogen 50 from the nitrogen refrigeration cycle applied. If necessary, i.e.

if liquid argon is withdrawn, additional liquid nitrogen is used, for example from an air separation plant, supplied via line 51.

About 300 Nm3 / h of argon are liquid from the bottom of the separation column 49 taken (line 52). If the argon is required in gaseous form, the additional liquid nitrogen 51 is omitted.

Also in the embodiment, the cooling requirement for the Argon separation relatively low because of the previous separation of hydrogen.

Nitrogen 53 is withdrawn from the top of the separation column 49 and from the nitrogen refrigeration circuit supplied, in the case of nitrogen in a heat exchanger 54 against circulating nitrogen and warmed up against circulating nitrogen and purge gas in the heat exchanger 36, all in one Compressor 55 compressed to about 9 bar, after dissipating the heat of compression in the Heat exchanger 36 cooled, in the heat exchanger 46 against evaporating methane further cooled and liquefied and used to heat the bottom of the separation column 49 will. The liquid nitrogen is subcooled in the heat exchanger 54 and partially up abandoned the separation column 49, in which he ensures the necessary conversion, and for Part after relaxation to about 1.5 bar for indirect cooling of the head of the separation column 37 is used to generate the necessary return there. Nitrogen exhaust the circuit is drained via line 56.

Through the evaporation of the methane under negative pressure in the heat exchanger 46 it is sufficient to use the nitrogen, in contrast to the method described in FIG can only be compressed to a pressure of approx. 9 bar.

The cold losses of the system are solely due to the Joule-Tomson effect of the raw gas (expansion from approx. 40 to 1 bar) covered if the argon produced is obtained in gaseous form. The main advantage over the usual concept is that the usual nitrogen cycle because of the only low pressure of 9 bar (compared to previously at least 25 to 30 bar) with a comparatively cheap one Turbo or screw compressor can work instead of the otherwise expensive one Reciprocating compressor. The energy requirement is approx. 30% lower.

Figure 3 shows a method for recovering hydrogen from the raw gas of an ammonia synthesis. The raw gas 61 is passed into a scrubbing column 62, which is acted upon by water 63. The water wash is used to remove ammonia from the raw gas, which dissolved in the water, is discharged from the scrubbing column 62 via line 64 will. The crude gas leaves the scrubbing column 62 overhead and is passed through a dryer 65 passed, in which the remaining water is removed from the raw gas. The characteristic Data on the raw gas after the dryer 65 at point A are given in Table 1.

The raw gas is passed into a diffusion device 66, which consists of four units through which there is flow in parallel, the diffusion device 66 contains semi-permeable membranes that are permeable to hydrogen. On the low pressure side the diffusion device 66 is a hydrogen-rich fraction 67 with relative low pressure, on the pressure side of the diffusion device 66 a hydrogen-poor Fraction 68 won with relatively higher pressure. The characteristic data of the Gas flows 67, 68 at points B, F are compiled in Table 1.

The dryer 65 can, instead of before, also after the diffusion device 66 be arranged in the line 68 (dashed line). Because a large part of the water is separated off in the membrane process and, moreover, the amount of gas at this point is smaller, the dryer can be made much smaller with this arrangement be.

The gas stream 68 is cooled in a heat exchanger 70, in particular the relatively high-boiling components of the gas mixture apart from the hydrogen condensed will.

In a subsequent separator 71, the condensate is rich in hydrogen Separated steam. Both the condensate removed from the sump of the separator 71 72 and the portion 73 remaining in gaseous form are in countercurrent led to the gas stream 68 through the heat exchanger 70. The characteristic data of these two streams at points C, D are summarized in Table 1.

The hydrogen 73 becomes with the hydrogen 67 from the diffusion device 66, which has previously been brought to the required pressure by means of a compressor 74 is, mixed and the resulting mixture is compressed in a compressor 75 to 130 bar. The characteristic data at point E are shown in Table 1. The condensed one Gas stream 76 is finally with a gas stream 77 of the synthesis process, which in the contains substantial hydrogen and nitrogen, mixed. The gas stream 77 is previously in two compressors 78, 79 from approx. 25 to initially approx. 70 and then to 130 been compressed in cash. The resulting gas mixture 80 is used in an ammonia synthesis plant supplied, which is not shown in the drawing. With the invention Process gives a yield of 91.68.

The compressors 74 and 75 can be omitted if the gas 76 before Compressor 78 or before the compressor 79 is introduced into the gas flow 77.

The energy requirements for this procedure match that given in Table 1 Numerical values is 34.1 KW or 83.6 KW, depending on whether the compressors 74, 75 are included or not. Would the hydrogen recovery at comparable Hydrogen purity and hydrogen yield take place exclusively through diffusion, so the energy requirement would be about 126 KW.

Table 1 A B C D E F Nm3 / H 3821 2996 1342 1654 2479 825 bar 140.6 126.5 1.5 110.0 130.0 82.6 K 305 305 295 295 305 305 mole% H2 62.2 55.5 14.8 88.6 88.0 87.0 N2 20.9 24.7 43.8 9.1 8.5 7.2 CH4 10.7 12.6 27.4 0.7 1.7 3.7 Ar 6.1 7, 2 14.0 1.6 1.8 2.1

Claims (12)

Claims 1. A method for decomposing a gas mixture that Contains hydrogen and at least one other component by adsorption or by cooling and partial condensation and / or rectification and / or Laundry, characterized in that at least part of the hydrogen before Adsorption or cooling by diffusion on semipermeable membranes of the Gas mixture is separated. 2. The method according to claim 1, characterized in that in the Diffusion no more than 60% of the hydrogen is separated from the gas mixture, while the remaining part of the hydrogen by partial condensation of the rest Components of the gas mixture is separated. 3. The method according to claim 1, characterized in that in the Diffusion more than 608 of the hydrogen can be separated from the gas mixture and that at least one of the components contained in the remaining gas mixture through Rectification or washing is separated from the remaining components. 4. The method according to claim 3, characterized in that the remaining gas mixture after separation of the hydrogen with simultaneous heating at least partially liquefied in a separation stage for breaking down the remaining gas mixture will. 5. The method according to claim 3 or 4, characterized in that sump liquid from a separation stage for the decomposition of the remaining gas mixture in the case of subatmospheric Pressure is evaporated. 6. The method according to claim 5, characterized in that the evaporation by supplying heat from a cooling circuit, which is also used to cool the separation stage is used. 7. The method according to claim 5 or 6, characterized in that the Fluid guided in the refrigeration circuit is part of the remaining gas mixture. 8. The method according to any one of claims 5 to 7, characterized in that that the sub-atmospheric pressure by jet compression by means of the gaseous Part of the partially liquefied remaining gas mixture generated as a propulsion jet will. 9. The method according to any one of claims 5 to 8, characterized in that that the fluid guided in the refrigeration cycle is fed to the separation stages at the head. 10. The method according to any one of claims 1 to 9, characterized in, that the gas mixture is a purge gas from a gas synthesis. 11. The method according to any one of claims 1 to 10, characterized indicates that the gas mixture is high in addition to hydrogen Argon, methane and / or Contains nitrogen and / or carbon monoxide as further low-boiling components. 12. Apparatus for performing the method according to claim 1 with a supply line for a gas mixture, the at least one heat exchange device or contains an adsorber and which opens into a separating device, characterized in that that in front of the heat exchange device or the adsorber one selective for hydrogen acting diffusion device is arranged with semipermeable membranes.