DE3032799C2 - Process and apparatus for producing ammonia synthesis gas mixtures - Google Patents

Description

The invention relates to a method for forming a hydrogen and nitrogen-containing gas mixture for ammonia synthesis in the pressure swing adsorption process, in which a feed gas mixture containing hydrogen and adsorbable impurities is broken down by selective adsorption of the impurities taking place alternately in at least two adsorber beds, in that the feed gas mixture is cyclically fed to the inlet end of a bed with the highest pressure in the pressure swing cycle and non-adsorbed, hydrogen-rich gas is discharged from the outlet end of one bed, then gas initially enclosed in the cavities of the adsorber bed is released at the outlet end of one bed and fed to the outlet end of another bed which has previously been cleaned of the impurities and is initially at a lower pressure until a pressure equalization has been established between the two beds, in that additional gas is also released from one bed to reduce the pressure to the lowest pressure in the pressure swing cycle and purge gas is fed to the outlet end of one bed. wherein impurities are desorbed and flushed out through the inlet end of one bed, and the flushed bed is repressurized to the highest pressure and the pressure change cycle is then repeated. The invention further relates to an apparatus for carrying out such a process with a plurality of adsorber beds, the inlet ends of which are connected to a feed gas line via inlet valves and the outlet ends of which are connected to a product gas line via product valves and to one another via further valves.

In the formation of hydrogen and nitrogen-containing gas mixtures for ammonia synthesis, the hydrogen which is a component of the synthesis gas mixture can be produced in a variety of ways, for example by steam reforming of natural gas or naphtha, by partial oxidation of hydrocarbons or by coal gasification. Regardless of the type of hydrogen production used in a particular case, the resulting hydrogen-containing stream typically contains a number of impurities, such as carbon dioxide, carbon monoxide, methane and water. The hydrogen-rich gas mixture from the hydrogen production stage is therefore usually subjected to further treatment stages, for example a carbon monoxide shift conversion to remove the carbon monoxide content of the mixture, a carbon dioxide removal by selective adsorption of carbon dioxide from the gas mixture by means of an appropriate liquid solvent in a scrubbing column, and a final purification of the gas mixture by selective adsorption to remove residual impurities and to deliver a hydrogen gas of high purity. The high purity hydrogen gas can then be mixed with compressed nitrogen from an external source to form the ammonia synthesis gas mixture containing hydrogen and nitrogen in the desired proportions, for example a stoichiometric molar ratio of hydrogen to nitrogen of 3:1.

In the last purification stage of the process explained above, in which residual impurities are removed from the hydrogen-containing gas stream by selective adsorption, it is expedient to use an adiabatic pressure swing adsorption process according to the preamble of claim 1, as is known from US Pat. No. 3,430,418 or US Pat. No. 3,986,849.

The feed gas mixture is understood to be the gas mixture to be cleaned and fed to the adsorber beds from an external source. The gas initially enclosed in the cavities of the adsorber bed, hereinafter also referred to as the initial gap volume gas, is gas that is present in the adsorber bed in the spaces between the adsorbent particles after the adsorption phase has ended. The pressure swing cycle is understood to be the sequence of different phases that each adsorber bed goes through one after the other. Purge gas is the gas that is introduced into the adsorber bed in a purge phase in order to desorb impurities from the adsorber bed that have accumulated in the adsorber bed during the adsorption phase. A repressurization phase is a phase during which an adsorber bed, in which the pressure has been reduced to a value below the highest pressure in the pressure swing cycle, is brought back to a higher pressure. The gas used to repressurize the adsorber bed is referred to as repressurization gas. In contrast, countercurrent pressure reducing gas is understood here to mean gas which leaves the adsorber bed when the pressure is reduced from the inlet end of the adsorber bed, i.e. a pressure reduction which results in a gas flow direction which is opposite to the gas flow direction during the adsorption phase.

One of the known processes (US-PS 34 30 418) uses four adsorbent beds; a hydrogen product gas having a purity of 99.9999% and having no detectable amounts of the above-mentioned impurities can be obtained with yields in the order of 75 to 80%. Another known process (US-PS 39 86 849) uses at least seven adsorbent beds, at least two of which receive feed gas during the pressure swing cycle. The pressure swing cycle comprises at least three pressure equalization phases. A hydrogen product gas having a purity of 99.9999% can be obtained with a hydrogen yield in the order of 85 to 90%.

Although the above-mentioned adiabatic pressure swing adsorption processes produce a hydrogen product of extremely high purity, a considerable part of the hydrogen contained in the feed gas mixture fed to the pressure swing process is lost in the exhaust gas, i.e. the countercurrent pressure reduction gas and purge gas, which is discharged from the process. A further disadvantage of the known pressure swing adsorption processes in such ammonia synthesis gas applications is that relatively large amounts of adsorbent are required in the adsorber beds of the adsorption plant.

Within the overall process of ammonia synthesis gas production using the process steps discussed above to produce a high purity hydrogen gas stream as the starting component of the synthesis gas mixture, the nitrogen gas which is combined with the high purity hydrogen gas to deliver the final synthesis gas mixture typically comes from an external source, such as a cryogenic or pressure swing adsorption air separation plant, liquid nitrogen evaporation plants and nitrogen gas pipelines, at relatively low pressures on the order of, for example, 69 to 690 kPa. Because the necessary pressure of the synthesis gas mixture is typically above 2760 kPa, a significant amount of compression energy must be expended to compress the low pressure nitrogen to the high pressure values necessary to form the product gas mixture.

The invention is based on the object of creating a method and a device for forming a hydrogen and nitrogen-containing gas mixture for ammonia synthesis in the pressure swing adsorption process, which are characterized by an increased percentage hydrogen yield in the pressure swing adsorption stage used for the final purification, manage with a comparatively small amount of adsorbent and require only a significantly reduced external compression for compressing low-pressure nitrogen gas to the final product pressure compared to known processes.

• a) das Adsorberbett während der Spülphase mit von einer externen Quelle kommendem Stickstoffgas gespült wird, und nach Abschluß der Spülphase in dem Bett verbleibendes Stickstoffgas in der anschließenden Wiederaufdrückphase des gespülten Bettes auf den höchsten Druck komprimiert und anschließend in dem nicht adsorbierten wasserstoffreichen Gas unter Bildung des wasserstoff- und stickstoffhaltigen Gasgemischs ausgetragen wird, und/oder
• b) das Adsorberbett während der Wiederaufdrückphase mit von einer externen Quelle kommendem Stickstoffgas wiederaufgedrückt wird, und das eingeleitete Stickstoffgas auf den höchsten Druck komprimiert und anschließend in dem nicht adsorbierten wasserstoffreichen Gas unter Bildung des wasserstoff- und stickstoffhaltigen Gasgemisches ausgetragen wird.

Starting from a method of the type mentioned at the outset, this object is achieved according to the invention in that

• a) the adsorber bed is flushed with nitrogen gas from an external source during the flushing phase, and after completion of the flushing phase, nitrogen gas remaining in the bed is compressed to the highest pressure in the subsequent repressurisation phase of the flushed bed and then discharged in the non-adsorbed hydrogen-rich gas to form the hydrogen and nitrogen-containing gas mixture, and/or
• b) the adsorber bed is repressurised with nitrogen gas from an external source during the repressurisation phase, and the nitrogen gas introduced is compressed to the highest pressure and then discharged into the non-adsorbed hydrogen-rich gas to form the hydrogen and nitrogen-containing gas mixture.

According to the invention, nitrogen is therefore introduced into the non-adsorbed hydrogen-rich gas produced by selective adsorption by using nitrogen gas from an external source as a flushing agent and/or as a repressurizing agent. This makes it possible to significantly reduce the plant's external compression energy requirements compared to known systems in which all of the nitrogen gas from an external source is compressed externally, i.e. compressed outside the adsorber beds, before this gas is mixed with the hydrogen-rich product gas from the adsorber beds to form the product synthesis gas mixture. By using the nitrogen gas from an external source to flush and/or at least partially repressurize the adsorber beds according to the present invention, this gas is internally compressed within the bed, so that after the bed has been flushed and fully repressurized, the gas is discharged together with the non-adsorbed hydrogen-rich gas at the high product pressure. In this way, the need for external compression energy for the nitrogen purge gas and/or nitrogen recompression gas coming from the external source is significantly reduced or even eliminated.

In order to ensure a favorable balance between the economics of operation and the efficiency of the process, the ratio between the pressure of the hydrogen-rich gas discharged and the pressure of the nitrogen gas coming from an external source is between 3 and 30.

When the process according to the invention is carried out in such a way that nitrogen gas coming from an external source is divided into two parts, the first part of which is used as purge gas for the purge phase, while the second part is externally compressed and then mixed with the discharged hydrogen-rich gas of the selective adsorption process to form a synthesis gas mixture having a molar ratio of hydrogen to nitrogen of about 3, the molar ratio between the total first part of the nitrogen gas introduced into the adsorber bed during the purge phase and the total second part of the low pressure nitrogen gas discharged from the bed mixed with the hydrogen-rich gas is preferably above 0.6 in order to provide a highly efficient and economical operation with regard to the utilization of the nitrogen gas coming from an external source and the associated need for external compression.

In the present case, nitrogen gas coming from an external source means nitrogen gas that comes from a source other than the feed gas mixture supplied to the selective adsorption system. The external source of nitrogen gas can be, for example, a cryogenic or pressure swing adsorption air separation plant, a nitrogen gas pipeline or an evaporation facility for cryogenic liquid nitrogen.

A device for carrying out the new process is characterized according to the invention by a nitrogen feed line connected to an external nitrogen gas source, which is connected to the adsorber beds via a group of valves.

Preferred embodiments of the invention are explained in more detail below with reference to the drawings.

Fig. 1 a simplified flow diagram of a plant with ten adsorber beds suitable for carrying out the purification of hydrogen-containing gas in the context of the process according to the invention,

Fig. 2 is a time diagram for the different phases of a selective adsorption cycle which takes place according to the process of the invention in the 10-bed plant of Fig. 1,

Fig. 3 is a simplified flow diagram of an adsorption plant with four adsorber beds suitable for carrying out the process according to the invention,

Fig. 4 shows a time program for the various phases of a modified embodiment of the adsorption process according to the invention, which is suitable for example when using the device according to Fig. 3, and

Fig. 5 is a time program of another embodiment of the selective adsorption process according to the invention, which can be carried out with the plant according to Fig. 1.

The plant according to Fig. 1 has ten adsorber beds 1 to 10 which are connected in parallel between a feed gas line F and a product gas line E via inlet valves 11 to 20 and product valves 21 to 30. The selective adsorption plant according to Fig. 1 is designed so that an adiabatic pressure swing adsorption can be carried out essentially according to the process known from US-PS 39 86 849, but nitrogen gas coming from an external source is used to flush the various adsorber beds of the pressure swing adsorption zone. In the known process according to US-PS 39 86 849, at least seven adsorber beds are used, each adsorber bed experiencing a pressure reduction in at least three pressure equalization stages during the adsorption cycle and at least two adsorber beds receiving gas mixture at any time during the cycle. The use of such a procedure, modified by the use of nitrogen gas from an external source for flushing and/or repressurizing the adsorber beds, is preferred in the present case because it makes it possible to achieve high product gas purity levels and because such a procedure is particularly suitable for feed gas mixtures in which hydrogen is the main component containing weakly adsorbed impurities such as methane and carbon monoxide, as is the case in the production of ammonia synthesis gas mixtures.

In the plant according to Fig. 1, the line 4 EQ for the fourth (lowest) pressure equalization stage is provided with valves 70 to 79. The exhaust line W is assigned exhaust valves 41 to 50. The line M for the first (highest) pressure equalization repressurization stage is equipped with valves 51 to 60 for the first and third equalization stages. Gas flow control valves 81 and 82 are located in the lines which connect the pressure equalization repressurization line M and the product gas line E. When the valve 81 is closed and the valve 82 is open, the first pressure equalization phase is carried out with even-numbered beds, while at the same time the third pressure equalization phase takes place between two odd-numbered beds. Conversely, when valve 81 is open and valve 82 is closed, the first pressure equalization takes place between odd-numbered beds, while the third pressure equalization phase takes place simultaneously between even-numbered beds.

A common collecting line for the second (middle) pressure equalization stage is not provided. Instead, individual lines are provided which connect the outlet ends of the adsorber beds to one another. For example, a line with a valve 61 is located between beds 2 and 7 . A line with a valve 62 connects beds 4 and 9 . Beds 1 and 6 are connected via a line to a valve 63 . A line in which a valve 64 is located connects beds 3 and 8 . Beds 5 and 10 are connected via a line to a valve 65 . The nitrogen gas coming from an external source is fed in via a line P and a group of valves 31 to 40 .

In order to facilitate the association between beds 1 to 10 and valves 11 to 60 and 70 to 79 , the valves are assigned reference symbols, the last digit of which corresponds to the reference symbol of the adsorber bed which is directly controlled by the valves in question. For example, the direct control of the operation of bed 3 is carried out via valves 13, 23, 33, 43, 53 and 73 . There is a single exception to the association of valves which directly influence the operation of bed 10. These are valves 20, 30, 40, 50, 60 and 70 .

Fig. 2 shows a preferred cycle program for the plant according to Fig. 1 for carrying out a selective adsorption according to a first embodiment. The order of the phases of the working cycle and their designations in the program are the following: adsorption (A) , first pressure equalization pressure reduction phase (E 1 D) , second pressure equalization pressure reduction phase (E 2 D) , third pressure equalization pressure reduction phase (E 3 D) , fourth pressure equalization pressure reduction phase (E 4 D) , countercurrent pressure reduction phase (BD) , countercurrent purge phase with externally supplied low pressure nitrogen purge gas (P) , fourth pressure equalization repressurization phase (E 4 R) , third pressure equalization repressurization phase (E 3 R) , second pressure equalization repressurization phase (E 2 R) , first pressure equalization repressurization phase (E 1 R) and final repressurization to the feed gas pressure by introducing product gas at the product outlet end (FR) .

It should be noted that at any time during the cycle, three adsorber beds take up feed gas mixture and release non-adsorbed product gas at the feed pressure. For example, in the seventh time unit, the feed gas mixture is processed in each of the adsorbers 2, 3 and 4 .

In operation of the plant according to Fig. 1, each of the adsorber beds 1 to 10 undergoes the following sequence of operations: adsorption, in which feed gas mixture is supplied to the inlet end of the bed at a first highest overpressure, while non-adsorbed hydrogen-rich gas (including residual nitrogen gas remaining in the bed at the end of the previously performed purging phase with nitrogen gas coming from an external source) is discharged from the outlet end of the bed; release of initial void volume gas at the outlet end of the bed and introduction of the initial gas thus released into the outlet end of a previously purged gas chamber of the adsorbable impurities released and initially at a lower pressure, until a pressure equalization between the two beds has been established at a higher intermediate pressure, wherein in the case of this plant the release of initial gap volume gas and the pressure equalization are carried out in four separate phases, namely a first pressure equalization of the adsorber bed which has completed its adsorption phase with another previously purged bed which is at least fourth higher and is initially at an intermediate pressure, so that the two beds finally reach a first equilibrium pressure, a second pressure equalization of the adsorber bed which is initially at the first equilibrium pressure with another previously purged bed which is at least fifth higher and is initially at a lower intermediate pressure, so that the two beds finally reach a second equilibrium pressure, a third pressure equalization of the adsorber bed which is initially at the second equilibrium pressure with another previously purged bed which is at least sixth higher and is initially at an even lower intermediate pressure, so that the two beds finally reach a third equilibrium pressure, and a fourth pressure equalisation of the adsorber bed initially at the third equilibrium pressure with another previously purged bed of at least seventh higher number initially at a lowest pressure so that the two beds finally assume a fourth equilibrium pressure; releasing additional gas from the bed to reduce the pressure to a lower pressure (countercurrent blowdown); introducing low pressure nitrogen purge gas from an external source into the outlet end of the bed for desorption of contaminants and purging through the inlet end of the bed; and repressurising the purged bed to the first highest gauge pressure in four successive pressure equalisation repressurisation phases and a final product repressurisation phase, after which the cycle is repeated.

Compared to the known process for producing ammonia synthesis gas using the selective adsorption process according to US Pat. No. 3,986,849, the present process makes it possible to achieve a significantly higher hydrogen yield, while reducing the cost of compressing external nitrogen and allowing a reduced amount of adsorbent to be used. It is surprising that the purging of the adsorber beds with nitrogen gas from an external source, as explained here, increases the hydrogen yield in the product gas leaving the adsorption plant compared to the known processes in which internal purging is carried out by feeding a gas depleted of impurities to the bed to be purged from another bed. On the contrary, based on general considerations, one would expect that the hydrogen yield that can be achieved with purging with the aid of a nitrogen gas from an external source is significantly lower than in known processes. This is because an adsorber bed which has completed the adsorption phase has a significant amount of unadsorbed hydrogen trapped in the adsorber bed interstitial spaces. This hydrogen-containing interstitial volume gas can be conveniently used to repressurize other adsorber beds in the system during one or more pressure equalization phases. However, after the last pressure equalization is completed, a significant amount of hydrogen still remains in the adsorber bed. In known systems, this hydrogen-containing gas in the adsorber bed is used to purge another bed. In contrast, in this embodiment of the adsorption process according to the invention, the hydrogen remaining in the adsorber bed after the last pressure equalization phase is removed from the bed during the subsequent countercurrent pressure reduction (blowdown) and purge phases and is not recovered. At the end of the purge phases, which are carried out with nitrogen gas from an external source, there is therefore essentially no hydrogen in the adsorber bed. Consequently, it would be expected that in this embodiment of the process according to the invention the hydrogen yield would be significantly lower than in the known process. Surprisingly, however, it has been found that the hydrogen yield values that can be achieved in this embodiment of the process according to the invention are actually higher than those of the known process in which internal purging is carried out, i.e. purging with gas coming from another adsorber bed.

According to a preferred embodiment, of which the cycle program of Fig. 2 is an example, at least nine adsorber beds are provided which are operated in overlapping, identical operating cycles such that during the initial part of the adsorption phase of the bed, the two immediately preceding, lower-numbered beds also go through the adsorption phase. During the middle part of the adsorption phase of the bed, the immediately preceding, lower-numbered bed and the immediately following, higher-numbered bed are in the adsorption phase. During the last part of the adsorption phase of the bed, the two immediately following, higher-numbered beds also go through their adsorption phase. For example, in the case of the cycle sequence according to Fig. 2, the adsorption phase of bed 1 comprises six time units, units 1 and 2 representing the initial period and units 5 and 6 the final period of the adsorption phase. During the initial period, the adsorption phase also takes place in beds 9 and 10 (the two beds immediately preceding bed 1 ). During the middle part, the immediately preceding bed 10 and the immediately following bed 2 go through the adsorption phase. During the final period, beds 2 and 3 (the two immediately following higher numbered beds) are also switched to adsorption.

According to a further preferred embodiment, which can be derived from the timing program according to Fig. 2, the release of initial gap volume gas and the associated pressure equalization take place in at least three separate phases. In this case, a pressure equalization is first carried out between an adsorber bed that has completed its adsorption phase and the fourth higher numbered adsorber bed, which is initially at a second equilibrium pressure, until finally both beds are at a first equilibrium pressure. The same adsorber bed in which a pressure reduction to the first equilibrium pressure has taken place, is now brought into a second pressure equalization with the fifth higher numbered adsorber bed, which is initially at the third equilibrium pressure, until the two beds finally assume the second equilibrium pressure. The same adsorber bed, the pressure of which has been reduced to the second equilibrium pressure, is then brought into a third pressure equalization with the sixth higher numbered adsorber bed, initially at the lowest process pressure, until a pressure equalization between these two beds has been established at the third equilibrium pressure. The adsorber bed, which has been relaxed to the third equilibrium pressure, then experiences a further pressure reduction either by countercurrent venting or by a further pressure equalization, pressure reduction stage and countercurrent venting until the lowest process pressure is reached at the end of the countercurrent venting phase. After completion of the countercurrent venting phase, during which the bed is finally relaxed by releasing gas at the inlet end of the bed, the bed is purged with low pressure nitrogen gas from an external source. The bed is then repressurized in successive pressure equalization repressurization phases and a final repressurization phase in which product gas at the first highest overpressure is introduced into the bed before the working cycle is repeated.

If one considers, from Fig. 2, the relationship between the bed from which initial void volume gas is released and the other beds which are thereby pressure-equalized (by repressurization), it can be seen that the first phase of release of initial void volume gas by bed 1 (E 1 D) takes place during time unit 7 and is carried out with bed 5 , the fourth-highest numbered adsorption bed, during its highest pressure-equalization repressurization phase (E 1 R) . The second phase (E 2 D) of release of initial void volume gas by bed 1 falls in time unit 8 and is carried out with bed 6 , the fifth-highest numbered adsorber bed, during its second pressure-equalization repressurization phase (E 2 R) . The third phase (E 3 D) of release of initial void volume gas by bed 1 occurs during time unit 9 in conjunction with bed 7 , the sixth higher numbered adsorber bed, during its pressure equalization repressurization phase (E 3 R) . The fourth phase (E 4 D) of release of initial void volume gas by bed 1 occurs during time unit 10 and occurs with bed 8 , the seventh higher numbered adsorber bed, during its lowest pressure equalization repressurization phase (E 4 R) . The final countercurrent pressure reduction (BD) occurs during time unit 11. This is followed by purging (P) of bed 1 with low pressure nitrogen gas from an external source during time units 12 to 15 .

In the relationship defined above, if the higher bed number calculated in this way exceeds the actual number of adsorber beds present in a given system, the actual number of beds must be subtracted from the calculated number to obtain the respective "higher numbered adsorber bed". For example, consider the third pressure equalization phase of bed 6 , which occurs during unit time 19 with the sixth higher numbered adsorber bed. Since there are ten adsorber beds in the embodiment according to Figs. 1 and 2, the bed provided for phase E 3 R is bed 12-10 , ie bed 2 .

Fig. 3 shows a further embodiment of an adsorption plant suitable for carrying out the present process, which is constructed similarly to the plant known from US-PS 34 30 418. There are four adsorber beds 1, 2, 3 and 4 , which are connected in parallel flow terms between a feed gas line 410 and a product gas line 411. The feed gas mixture is fed to bed 1 , bed 2 , bed 3 and bed 4 via automatic valves 11, 12, 13 and 14. The automatic valves 21, 22, 23 and 24 direct product gas from these beds into the product gas line 411 .

The adsorbed impurities which are separated in the countercurrent pressure reduction and purge phases leave the plant via an exhaust line 412 which is connected to the inlet ends of the adsorber beds via valves 41, 42, 43 and 44. Valves 51, 52, 53 and 54 are used for pressure equalization via line 423 and for the final repressurization with non-adsorbed product gas via line 427. The flow rate of the gas for the final repressurization phase is regulated by means of a valve 60' located in line 427 .

In the purge phase, nitrogen purge gas from an external source is fed in via a line 421 and valves 31, 32, 33 and 34 , while purge gas leaving the adsorber beds is discharged to the exhaust line 412 via valves 41, 42, 43 and 44. Gas leaving the blow-off (pressure reduction) phase enters line 413 via valves 61, 62, 63 and 64. The blow-off gas coming from line 413 can be recompressed and recycled; it can also be partially or completely introduced into the outgoing purge gas stream.

After the flushing phase of the adsorber bed 1 , which takes place at the lowest pressure, has ended, the valve 31 closes; the pressure equalization valves 51 and 53 are opened. Either simultaneously with or after pressure equalization is achieved, a portion of the product gas coming from the adsorber bed 4 is diverted in the product gas line 411 via the line 427 , the valve 60' , the line 423 and the valve 51 to the outlet end of the adsorber 1. This product gas flow continues until the adsorber bed 1 is repressurized to approximately the product pressure. The inlet valve 11 and the product valve 21 are closed during repressurization. Repressurization can also be carried out instead by using the feed gas mixture as the repressurization gas, for example during the last part of the repressurization phase, in order to bring the adsorber bed to the first, highest overpressure.

Fig. 4 shows a cycle program for the device according to Fig. 3. As can be seen from the cycle program, each of the four adsorber beds of the plant according to Fig. 3 passes through the phases of adsorption, pressure equalization with another bed, countercurrent pressure reduction, flushing with low-pressure nitrogen gas coming from an external source, repressurization by pressure equalization with a bed of the plant undergoing pressure reduction and a final repressurization with Product gas, after which the working cycle is repeated. The 4-bed system according to Fig. 3 and 4 is particularly suitable for systems by means of which smaller volumes of ammonia synthesis gas are to be produced compared to the polybed systems of Fig. 1 and 2.

Fig. 5 shows a cycle program for selective adsorption according to another embodiment of the present process. The cycle program according to Fig. 5 can be used in a 10-bed system according to Fig. 1. The operating cycle according to Fig. 5 differs from that of Fig. 2 in that a cocurrent purge phase C is provided immediately after the adsorption phase and before the pressure equalization pressure reduction phases. During this cocurrent purge phase, an externally supplied purge fluid is fed to the bed at the inlet end, while product gas continues to be withdrawn from the outlet end of the bed. For this purpose, the cocurrent purge medium is supplied with the first highest overpressure of the feed gas, so that the gas discharged from the adsorber bed during the cocurrent purge phase has the same pressure as the product gas that leaves the bed during the preceding adsorption phase. The cocurrent purge phase has the task of displacing the hydrogen gap volume gas remaining in the adsorber bed into the product gas line.

The externally supplied flushing fluid used for the cocurrent flushing phase of the process according to Fig. 5 can be nitrogen gas or another gas, for example methane or natural gas. Which specific flushing fluid is selected for the cocurrent flushing phase depends on various factors, including the amount of flushing gas required, the quality of the exhaust gas generated in the pressure swing adsorption plant and the quality of the feed gas mixture that is fed to the pressure swing adsorption plant. The quality of the exhaust gas leaving the pressure swing adsorption plant becomes important if the exhaust gas, that is, the desorbate gas blown off and flushed out in countercurrent, is to be used as fuel within the plant. The adsorbable components of the gas mixture decomposed in the pressure swing adsorption plant for ammonia synthesis can include carbon monoxide and methane, so that the exhaust gas from the pressure swing adsorption plant has a moderate calorific value and can be used conveniently as fuel gas. When the calorific value of the adsorption plant exhaust gas becomes a significant factor, it may be appropriate to use a fluid such as natural gas as the cocurrent purge medium. In this connection, the quality of the feed gas mixture entering the adsorption plant is important, namely if the feed gas mixture contains large amounts of carbon dioxide, the use of the exhaust gas as a fuel gas will be impaired or made impossible if nitrogen were used as the cocurrent purge medium. This is because at the end of the cocurrent pressure reduction-pressure equalization phases, the cocurrent purge gas occupies a large part of the adsorber bed interspaces, so that cocurrent purge gas is removed together with the desorbate impurities in the exhaust gas. Therefore, when the suitability of the exhaust gas as a fuel gas is important, a purge fluid such as natural gas is preferably used.

In the operating cycle of Fig. 5, the cocurrent purge phase is inserted between the adsorption phase and the subsequent pressure equalization phases. Each adsorber bed therefore passes through the following phases in sequence: adsorption, cocurrent purge, three consecutive pressure equalization pressure reduction phases, countercurrent blowdown to the lowest process pressure, purging with low-pressure nitrogen gas from an external source, three consecutive pressure equalization repressurization phases and final repressurization using non-adsorbed, hydrogen-rich product gas. In this operating cycle, as in the operating cycle according to Fig. 2, feed gas mixture is fed to three beds of the adsorption plant simultaneously during each time point of the operating cycle.

• (a) der Austrittsendteil des Adsorberbettes wird im Gegenstrom entspannt, was dazu beiträgt, am Austrittsende des Bettes eine niedrige Reinheitskonzentration aufrechtzuerhalten;
• (b) der Eintrittsendbereich des Adsorberbettes wird im Gleichstrom entspannt, was dazu beiträgt, eine maximale Menge an Wasserstoff aus dem Eintrittsende des Adsorberbettes zu verdrängen;
• (c) ein erheblicher Teil des in dem Lückenvolumengas des Adsorberbettes vorhandenen Kohlenmonoxids läßt sich zusammen mit Wasserstoff in dem Nebenstromdruckminderungsgas zurückgewinnen. Weil das einen erheblichen Anteil an Kohlenmonoxid enthaltende Nebenstromdruckminderungsgas zu dem Kohlenmonoxid-Verschiebekonverter der Anlage zurückgeleitet werden kann, wo sich aus dem zurückgeleiteten Kohlenmonoxid mehr Wasserstoff bilden läßt, ist es auf diese Weise möglich, die Wasserstoffausbeute im Vergleich zu einem Verfahren zu verbessern, bei dem die Druckminderung des gesamten Adsorberbettes auf den niedrigsten Druck durch Gegenstromabblasen erfolgt. Außerdem werden die Mengen an Kohlendioxid und Methan in dem Nebenstromdruckminderungsgas gegenüber dem Gegenstromdruckminderungsgas des Adsorberbettes erheblich vermindert. Die Nebenstromdruckminderung des Adsorberbettes auf den niedrigsten Druck ist daher vorzuziehen, um die Belastung des Kompressors für zurückgeleitetes Gas und des Kohlenmonoxid-Verschiebekonverters gegenüber einem Prozeß zu senken, bei dem im Gegenstrom abgeblasenes Gas verdichtet und als Rückführgas zu dem Verschiebekonverter zurückgeleitet wird.

It has been found that when, in certain embodiments, the bed is depressurized by discharging gas from the adsorber bed at a point between the inlet and outlet ends of the bed, the following advantages are achieved:

• (a) the outlet end of the adsorbent bed is countercurrently expanded, which helps to maintain a low purity concentration at the outlet end of the bed;
• (b) the inlet end region of the adsorber bed is expanded in cocurrent, which helps to displace a maximum amount of hydrogen from the inlet end of the adsorber bed;
• (c) a significant portion of the carbon monoxide present in the void volume gas of the adsorber bed can be recovered together with hydrogen in the bypass depressurizing gas. Because the bypass depressurizing gas containing a significant portion of carbon monoxide can be recycled to the plant's carbon monoxide shift converter where more hydrogen can be formed from the recycled carbon monoxide, it is thus possible to improve the hydrogen yield compared to a process in which the depressurization of the entire adsorber bed to the lowest pressure is carried out by countercurrent blowdown. In addition, the amounts of carbon dioxide and methane in the bypass depressurizing gas are significantly reduced compared to the countercurrent depressurizing gas of the adsorber bed. Bypass pressure reduction of the adsorber bed to the lowest pressure is therefore preferable to reduce the load on the recirculated gas compressor and the carbon monoxide shift converter compared to a process in which countercurrent vented gas is compressed and returned to the shift converter as recycle gas.

Example I

This example is based on a process for producing an ammonia synthesis gas mixture from hydrogen and nitrogen according to the state of the art, wherein the feed gas mixture undergoes pressure swing adsorption to separate carbon monoxide, carbon dioxide and methane and to obtain a hydrogen-rich gas to form the ammonia synthesis gas mixture. The selective adsorption is carried out by means of a 10-bed adsorption plant of the type known from Fig. 1 of US-PS 39 86 849. The plant is operated according to the cycle program of Fig. 2 of US-PS 39 86 849, wherein the adsorber bed after the adsorption phase runs through three successive pressure equalization pressure reduction phases immediately before a purge gas supply phase, during which hydrogen-containing gas is released from the outlet end of the bed and used to purge another adsorber bed at the lowest process pressure. Following the purge gas supply phase, the adsorber bed is depressurized in countercurrent to the lowest process pressure and in turn purged with hydrogen-containing gas from another bed of the adsorption plant which runs through the purge gas supply phase. After purging, the adsorber bed is repressurized in three successive pressure equalization repressurization phases, followed by a final repressurization with product gas. In this process, all low pressure nitrogen gas from an external source is compressed to the synthesis gas pressure of 3400 kPa before it is mixed with the hydrogen-rich gas which is the product gas of the pressure swing adsorption plant. The operating conditions and the performance characteristics are summarized in Table I below. Table I Operating conditions and performance characteristics of a process using a 10-bed pressure swing adsorption plant according to US Pat. No. 3,986,849 &udf53;vu10&udf54;&udf53;vz32&udf54;&udf53;vu10&udf54;

The data presented above in tabular form serve as a basis for comparing various embodiments of the present method with the known procedure in the examples below.

Example II

This example is based on a process for producing an ammonia synthesis gas mixture from hydrogen and nitrogen according to the embodiment of Fig. 1 and 2. In contrast to Example I, no internal purge gas supply phase is provided for in the selective adsorption. Instead, a first part of a low-pressure nitrogen gas from an external source in the form of a cryogenic air separation plant is used as purge gas, while a second part of the low-pressure nitrogen gas supplied from the external source is compressed externally and then mixed with the discharged, hydrogen-rich product gas of the selective adsorption plant to form the final product in the form of the synthesis gas mixture. Due to the elimination of the purge gas supply phase, a further, fourth pressure equalization phase can be used in this case, while the same number of adsorber beds is provided as in Example I. Alternatively, the elimination of the internal purge gas supply phase allows the total number of adsorbers of the adsorption plant when working with the same number of pressure equalization phases as in Comparative Example I. The second alternative of reducing the total number of adsorbers while maintaining the number of pressure equalization phases of the known process is discussed in detail in a later example.

The operating conditions and performance characteristics of the ammonia synthesis gas production process using a 10 bed pressure swing adsorption plant according to Figs. 1 and 2 are summarized in Table II below. Table II Operating conditions and performance characteristics of the ammonia synthesis gas production process using the 10 bed pressure swing adsorption plant of Figs. 1 and 2 &udf53;vu10&udf54;&udf53;vz32&udf54;&udf53;vu10&udf54;

Comparing the table data for this example with those in Table I of Example I, it can be seen that the present process leads to a significantly higher hydrogen yield. With the feed gas mixture of column 1, a hydrogen yield of 92.2% is achieved with the present process compared to a yield of only 87.9% for the known process of Example I. A further comparison of the values in column 1 of Tables I and II shows that in Example II the volume of adsorbent required per bed is only 75% of that of the known process according to Example I. Furthermore, in this embodiment of the present process, the external compression energy that must be used to compress the nitrogen gas added to form the synthesis gas mixture is reduced by 40% for the feed gas mixture of column 1 and by 35% for the feed gas mixture of column 2 compared to the energy expenditure of the known process.

Example III

In this example, a 10-bed adsorption system is used, with the pressure swing adsorption system being operated as shown in Fig. 5 as part of the ammonia synthesis gas production. In this embodiment, a cocurrent purge phase is provided immediately after the adsorption phase, which takes place at the same pressure as the adsorption phase in order to drive hydrogen gap volume gas present in the adsorber bed into the product gas line of the pressure swing adsorption system. As explained above, various cocurrent purge media, for example nitrogen or natural gas, can be used, the selection depending on various considerations, for example the required purge gas quantity and the quality of the feed gas mixture and the exhaust gas of the pressure swing adsorption system with regard to the delivery of an exhaust gas that has a sufficiently high calorific value to be used as fuel gas within the system. The operating conditions and performance characteristics for a 10-bed adsorption plant operating on the duty cycle of Fig. 5 and using natural gas at a pressure of 3450 kPa as the co-current purge fluid are given in column 1 of Table IV below, the feed gas mixture having the same composition as in the examples in column 1 of Tables I and II. Column 2 of Table III gives the corresponding values for a 10-bed adsorption plant operating as shown in Fig. 5 and using nitrogen gas from an external source compressed to a co-current purge pressure of 3450 kPa as the purge fluid. The feed gas mixture has the same composition as in the examples in column 2 of Tables I and II. Table III Operating conditions and performance characteristics of a process for the production of ammonia synthesis gas using a 10-bed pressure swing adsorption plant with the cycle sequence according to Fig. 5 &udf53;vu10&udf54;&udf53;vz43&udf54;&udf53;vu10&udf54;

If the values in column 1 of Table III, which are based on a high-pressure cocurrent purge with natural gas and subsequent countercurrent purge with nitrogen gas from an external source, are compared with the corresponding values in column 1 of Table II of Example II, where the only purge phase is carried out at low pressure using nitrogen gas from an external source, it is found that the yield in the first case is 0.9% higher than in the latter case, while all other parameters are essentially the same. In the first case of the present example, the quality (calorific value) of the exhaust gas leaving the adsorption plant is significantly improved because natural gas is used as the purge medium. If the values in column 2 of Table III are compared with the corresponding values in column 2 of Table II, it is found that in the above example the hydrogen yield is 93.8% compared to the yield of 91.6% for the embodiment according to Table II. Column 2, where no cocurrent purging at high pressure is provided. The energy expenditure for the external compression of nitrogen gas has increased by 39% compared to the embodiment according to Table II, Column 2, which is due to the fact that in the embodiment of the present example, nitrogen gas coming from an external source must be externally compressed to the increased pressure of 3450 kPa required for cocurrent purging of the adsorber beds. In contrast to this embodiment of the present process, the known process according to Table I, Column 2, has a hydrogen yield of only 88.7% with a compression energy expenditure for the nitrogen gas to be added which is 11 to 64% higher than that of the embodiment of the present example. It follows that the high pressure cocurrent purging phase of this example leads to a significant improvement in yield over known processes with moderate to significant savings in terms of compression of the feed gas mixture which is introduced into the pressure swing adsorption plant.

Claims (12)

1. Process for forming a hydrogen and nitrogen-containing gas mixture for ammonia synthesis in the pressure swing adsorption process, in which a feed gas mixture containing hydrogen and adsorbable impurities is broken down by selective adsorption of the impurities taking place alternately in at least two adsorber beds, in that the feed gas mixture is cyclically fed to the inlet end of a bed with the highest pressure in the pressure swing cycle and non-adsorbed, hydrogen-rich gas is discharged from the outlet end of one bed, then gas initially enclosed in the cavities of the adsorber bed is released at the outlet end of one bed and fed to the outlet end of another bed previously cleaned of the impurities and initially at a lower pressure until a pressure equalization between the two beds has been established, in that additional gas is also released from one bed to reduce the pressure to the lowest pressure in the pressure swing cycle and purge gas is fed to the outlet end of one bed. wherein impurities are desorbed and flushed out through the inlet end of one bed, and by repressurising the flushed one bed to the highest pressure and then repeating the pressure swing cycle, characterized in that
a) the adsorber bed is flushed with nitrogen gas from an external source during the flushing phase, and after completion of the flushing phase, nitrogen gas remaining in the bed is compressed to the highest pressure in the subsequent repressurisation phase of the flushed bed and then discharged in the non-adsorbed hydrogen-rich gas to form the hydrogen and nitrogen-containing gas mixture, and/or b) the adsorber bed is repressurised with nitrogen gas from an external source during the repressurisation phase, and the nitrogen gas introduced is compressed to the highest pressure and then discharged into the non-adsorbed hydrogen-rich gas to form the hydrogen and nitrogen-containing gas mixture.
2. Process according to claim 1, characterized in that the flushed bed is repressurized during the repressurization phase with the feed mixture or with discharged, non-absorbed, hydrogen-rich gas. 3. Process according to one of the preceding claims, characterized in that the flushed bed is partially repressurized during the repressurization phase with nitrogen gas coming from an external source to an intermediate pressure which is between 0.10 and 0.30 times the highest pressure. 4. A process according to any one of the preceding claims, characterized in that nitrogen gas coming from an external source is divided into two parts and a first part is used as purge gas for the purge phase, while a second part is externally compressed and then mixed with the discharged hydrogen-rich gas to form a synthesis gas mixture having a molar ratio of hydrogen to nitrogen of about 3. 5. Process according to one of the preceding claims using at least four adsorber beds, characterized in that in order to repressurize the purged bed to the highest pressure, the purged bed is pressure equalized with another bed which has just completed its adsorption phase, and then a final repressurization to the first highest overpressure is carried out with the feed gas mixture. 6. A process according to claim 4, characterized in that the molar ratio between the total first portion of nitrogen gas introduced into the adsorber bed during the purging phase and the total second portion of low pressure nitrogen gas discharged from the bed mixed with the hydrogen-rich gas during the adsorption phase is greater than 0.6. 7. Process according to one of the preceding claims, characterized in that purge gas at the highest pressure coming from an external source is fed to the inlet end of the adsorber bed after completion of the adsorption phase and non-adsorbed hydrogen-rich gas initially enclosed in the cavities of the adsorber bed, displaced by the purge gas, is withdrawn from the outlet end of the bed as a further part of the discharged hydrogen-rich gas. 8. Method according to claim 7, characterized in that nitrogen, methane or natural gas is used as the purge gas coming from an external source. 9. A process according to any one of the preceding claims, characterized in that when additional gas is released to reduce the pressure of the adsorber bed to the lowest pressure, gas is discharged at a point between the inlet and outlet ends of the bed and the gas discharged from the bed is returned and processed to recover its hydrogen content, this hydrogen providing part of the hydrogen present in the feed mixture. 10. Apparatus for carrying out the process according to one of the preceding claims with a plurality of adsorber beds, the inlet ends of which are connected to a feed gas line via inlet valves and the outlet ends of which are connected to a product gas line via product valves and to one another via further valves, characterized by a nitrogen feed line connected to an external nitrogen gas source, which is connected to the adsorber beds via a group of valves. 11. Device according to claim 10, characterized in that the supply of nitrogen gas serving valves are connected between the outlet ends of the adsorber beds and the nitrogen feed line. 12. Device according to claim 10, characterized in that the valves serving to feed in nitrogen gas are connected between the inlet ends of the adsorber beds and the nitrogen feed line.