PROCESS FOR AMMONIA PRODUCTION

Abstract

A process for the synthesis of ammonia from a make-up gas containing hydrogen and nitrogen, comprising at least two steps for the synthesis of ammonia, wherein: said reactive steps are performed in series and each of said reactive steps provides an ammonia-containing product gas; the second and any subsequent reactive step receives, as a feed stream, at least a portion of the product gas of the previous reactive step; an intermediate adsorptive step of ammonia is performed between consecutive reactive steps, so that the product gas of each step is depleted of ammonia prior to the subsequent reactive step of said series.

Claims

1-16. (canceled)

17. A process for synthesis of ammonia from a make-up gas containing hydrogen and nitrogen, the process comprising: at least two reactive steps for the synthesis of ammonia, wherein: said at least two reactive steps are performed in series, and each of said at least two reactive steps provides an ammonia-containing product gas; the second and any subsequent one of the at least two reactive steps receives, as a feed stream, at least a portion of the ammonia-containing product gas of the previous one of the at least two reactive steps; an intermediate adsorptive step of ammonia is performed between consecutive ones of the at least two reactive steps, so that the ammonia-containing product gas of each of the at least two reactive steps is depleted of ammonia prior to the subsequent one of the at least two reactive steps of said series; wherein said at least two reactive steps are carried out in one or more catalyst beds, said one or more catalyst beds being arranged in a single reactor vessel or in different reactor vessels and each of said at least two reactive steps being carried out in a catalyst volume substantially equal to or greater than the catalyst volume of the subsequent one of the at least two reactive steps.

18. The process of claim 17, wherein said at least two reactive steps are carried out adiabatically or pseudo isothermally.

19. The process of claim 17, wherein the intermediate adsorptive step is carried out in at least one adsorption unit, the at least one adsorption unit including at least two vessels in series, each of the at least two vessels containing a solid adsorbent suitable to selectively adsorb ammonia and alternately carrying out the following steps: (a) an adsorption step including contacting the ammonia-containing product gas provided by a corresponding reactive step with the solid adsorbent and adsorption of ammonia from said product gas, providing an ammonia-loaded adsorbent; and (b) an adsorbent regeneration step, wherein ammonia is desorbed, providing said ammonia-depleted product gas and an ammonia containing-output stream.

20. The process of claim 19, wherein the adsorbent regeneration step (b) is carried out by depressurization and/or heating of the adsorbent.

21. The process of claim 19, wherein said intermediate adsorptive step includes at least two adsorptive steps and are performed in a single adsorption unit, wherein the adsorption step (a) and the adsorbent regeneration step (b) are scheduled to ensure that each of said adsorptive step is carried out continuously.

22. The process of claim 19, wherein said ammonia containing-output stream from the regeneration step is recycled to a further process in gaseous phase for the synthesis of nitric acid.

23. The process of claim 17, wherein the intermediate adsorptive step is performed in a dedicated adsorption unit.

24. The process of claim 17, wherein the intermediate adsorptive step is carried out at a temperature lower than the at least two reactive steps and higher than a dew point of the ammonia-containing product gas leaving the previous one of the at least two reactive steps.

25. The process of claim 24 wherein the temperature is in a range of 200 C. to 400 C.

26. The process of claim 17, wherein said ammonia-containing product gas provided by at least two reactive steps is cooled before being at least partially subjected to a subsequent adsorptive step.

27. The process of claim 26, wherein said ammonia-containing product gas is used to pre-heat the feed stream of a reactive step or to generate steam.

28. The process of claim 17, wherein said ammonia-depleted product gas provided by at least one adsorptive step is heated or cooled before being at least partially directed to the subsequent reactive step.

29. The process of claim 17, wherein the intermediate adsorptive step adsorbs at least 30% of the ammonia contained in the ammonia-containing product gas provided by the previous reactive step.

30. The process of claim 29, wherein the intermediate adsorptive step adsorbs more than 60% of the ammonia.

31. The process of claim 29, wherein the intermediate adsorptive step adsorbs more than 90% of the ammonia.

32. The process of claim 17, further comprising an adsorptive step of ammonia after the last reactive step, in order to adsorb ammonia contained in the effluent of said last reactive step.

33. A plant for the synthesis of ammonia from a make-up gas containing hydrogen and nitrogen, the plant comprising: at least two units for the synthesis of ammonia, said at least two units being arranged in series and each of said at least two units containing one or more reactor vessels, wherein each of the one or more reactor vessels contains one or more catalyst beds, each of said at least two units having a catalyst volume substantially equal to or greater than the catalyst volume of the subsequent one of the at least two units; and at least one adsorption unit located between consecutive synthesis units, said at least one adsorption unit including at least two vessels arranged in series and each of the one or more reactor vessels containing a solid adsorbent suitable to selectively adsorb ammonia.

34. A method of revamping of an ammonia synthesis loop including at least a first synthesis unit and a second synthesis unit, arranged in series and containing one or more catalyst beds each, the method comprising: installing an adsorption unit between said first and second synthesis unit; and wherein the total catalyst volume of said first synthesis unit is substantially equal to or greater than the second synthesis unit.

35. The method of claim 34, further comprising replacing at least one catalyst bed of the second synthesis unit with at least two catalyst beds of smaller size so that the total catalyst volume of said newly installed beds is substantially equal to or smaller than the catalyst volume of the original catalyst bed of the second synthesis unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0074] FIG. 1 shows a block scheme of a process for the synthesis of ammonia according to the prior art.

[0075] FIG. 2 shows a diagram of ammonia molar fraction (yNH.sub.3) versus temperature (T) of the prior art process according to FIG. 1.

[0076] FIG. 3 shows a block scheme of a process for the synthesis of ammonia according to a first embodiment of the invention.

[0077] FIG. 4 shows a diagram of ammonia molar fraction (yNH.sub.3) versus temperature (T) according to a first mode of implementing the process of FIG. 3.

[0078] FIG. 4a shows a diagram T-yNH.sub.3 according to another mode of implementing the process of FIG. 3.

[0079] FIG. 5 shows the performance of the process according to FIG. 3 over a T-yNH.sub.3 diagram of a corresponding process of the prior art without intermediate ammonia adsorption.

[0080] FIG. 6 shows a block scheme of a process for the synthesis of ammonia according to a second embodiment of the invention.

[0081] FIG. 7 shows the performance of the process according to FIG. 6 over a T-yNH.sub.3 diagram of a corresponding process of the prior art without intermediate ammonia adsorption.

[0082] FIG. 8 shows a block scheme of a process for the synthesis of ammonia according to a third embodiment of the invention.

[0083] FIG. 9 shows a block scheme of a process for the synthesis of ammonia according to a forth embodiment of the invention.

DETAILED DESCRIPTION

[0084] A prior art multi-bed reactor 100 is shown in FIG. 1. Said reactor 100 comprises three adiabatic catalytic beds 101, 102, 103 arranged in series, a first inter-bed heat exchanger 104 and a second inter-bed heat exchanger 105.

[0085] The operation of the reactor 100 is illustrated below.

[0086] A fresh make-up gas 106 passes through the second heat exchanger 105, wherein it is preheated by the effluent of the second bed 102, and subsequently through the first heat exchanger 104, wherein it is preheated by the effluent of the first bed 101. The so pre-heated make-up gas 106 enters the first bed 102, where it partially reacts to provide a first ammonia-containing stream 107. Said stream 107 is cooled inside the first heat exchanger 104 and the cooled effluent enters the second bed 102, where it further reacts to provide a second ammonia-containing stream 108. Similarly, said stream 108 is cooled in the second heat exchanger 105 before entering the third bed 103, which provides an ammonia product 109. Said product 109 leaves the reactor 101.

[0087] The performance of said multi-bed reactor 100 is illustrated in the plot temperature (T) vs ammonia molar fraction (yNH.sub.3) of FIG. 2, in comparison with an equilibrium curve. The equilibrium curve indicates the equilibrium ammonia concentration at any temperature for the following conditions: P=135 bar, H/N ratio=3, inlet molar concentration of inerts (i.e. methane and/or argon and/or helium)=7% mol. The equilibrium concentration is the maximum concentration achievable at any temperature.

[0088] The performance of the adiabatic beds 101, 102, 103 are represented as straight lines between the inlet and outlet points, A and B respectively.

[0089] The distance between the outlet point B and the equilibrium curve is determined by the volume of catalyst. In this example the catalyst volume amounts to 0.05 m.sup.3 per daily ton of ammonia, which is conventional for the above conditions of pressure and inlet reactor composition. A greater volume of catalyst would entail a higher conversion until the equilibrium curve is reached, and any further volume of catalyst beyond that value would be ineffective.

[0090] According to FIG. 2, the fresh make-up gas 106 enters the first adiabatic bed 101 (at point A) at a temperature Ta, where it reacts adiabatically to generate the first ammonia-containing stream 107 at a temperature Tb. Said stream 107 is cooled from Tb to Tc in the first inter-bed heat exchanger 104 and is fed to the second bed 102 where it reacts adiabatically to provide the second ammonia-containing stream 108. Within the second bed 102, the reaction temperature raises from Tc to Td. The second stream 108 leaving the second bed 102 is subsequently cooled in the second inter-bed heat exchanger 105 from Td to Te and fed to the third bed 103, where it further reacts adiabatically to provide a third gaseous stream 109 (at point B) with a temperature increase from Te to Tf.

[0091] In view of the above plot, it can be appreciated that an ammonia process performed in a reactor containing three beds in series allows achieving an outlet ammonia concentration of 18.5% mol, which corresponds to a conversion per pass of 31.4%, as shown in table 1 below.

[0092] FIG. 3 is a block scheme of an ammonia synthesis process which is carried out in a two-bed ammonia reactor 1. Said reactor 1 comprises two adiabatic catalyst beds 2 and 3 with an intermediate adsorption unit 4. Said two catalyst beds carry out a first and a second reactive step of conversion of a make-up gas into ammonia, respectively. The embodiment of the figure also includes three inter-bed heat exchangers 5, 6 and 7 and a bottom heat exchanger 8. The process shown in FIG. 3 is as follows.

[0093] A fresh make-up gas 9 is pre-heated in a first heat exchanger 5 by an effluent 10 of the first adiabatic bed 2. The so pre-heated gas enters said first adiabatic bed 2, wherein it reacts to provide a first ammonia-containing stream 10. Said stream 10 is cooled in the first heat exchanger 5 by means of the fresh make-up gas 9 and is further cooled in a second heat exchanger 6, providing a stream 10A. The cooled stream 10A is subsequently fed to the adsorption unit 4, wherein about 90% of the ammonia contained in the stream 10 leaving the first adiabatic bed 2 and less than 1% of the H.sub.2 and N.sub.2 contained therein are removed.

[0094] Said adsorption unit 4 contains at least two vessels (not shown), which alternately carry out an adsorption step and an adsorbent regeneration step, thus providing an ammonia-containing stream 11 and an ammonia-depleted stream 12. Said two steps are scheduled such that while one vessel performs the adsorption step the other vessel performs the adsorbent regeneration step, hence such that the adsorption of ammonia from the ammonia-containing stream 10A is carried out continuously.

[0095] Said ammonia-depleted stream 12 is subsequently pre-heated in a third heat-exchanger 7 by means of an effluent 13 of the second adiabatic bed 3, providing a stream 12A. Then, the pre-heated stream 12A enters the second adiabatic bed 3, where it reacts to provide an ammonia product gas 13. Said product gas 13 is cooled in said third heat exchanger 7 and further cooled in the bottom heat exchanger 8, providing a stream 13A which is exported from the reactor 1.

[0096] The ammonia-containing stream 11 and the product gas 13A contain the total amount of ammonia obtained through this process.

[0097] Preferably, the ammonia-containing stream 11 is suitably treated, for example compressed, condensed and purified, in order to recover an ammonia product stream which can be stored or further used. Alternatively, said stream 11 can be directly recycled to a process, where ammonia is used in gaseous phase, e.g. nitric acid production.

[0098] The heat recovered by cooling down the first ammonia-containing stream 10 in the second heat exchanger 6 and by cooling down the ammonia product gas 13 in the bottom heat exchanger 8 can be advantageously used to pre-heat the stream 9 and/or the stream 12 or for generating steam.

[0099] FIG. 4 is a representation of the performance of the ammonia synthesis process according to FIG. 3.

[0100] The fresh make-up gas 9 enters the first adiabatic bed 2 (at point A) at a temperature T.sub.1 of 350 C. where it reacts evolving heat and generating the first ammonia-containing stream 10 (at point B) at a temperature T.sub.2 of 500 C. Said first stream 10 is cooled e.g. from T.sub.2 to T.sub.1 in the first and second heat exchangers 5, 6 to provide the stream 10A. Said stream 10A is supplied to the adsorption unit 4, which removes about 90% of the ammonia contained in the stream 10A (at point C). The ammonia-depleted stream 12 is subsequently pre-heated in said third heat-exchanger 7 to provide stream 12A, which is fed to the second bed 3 where it reacts adiabatically to provide the ammonia product gas 13 (at point D) with a temperature increase e.g. from T.sub.1 to T.sub.2.

[0101] In a specific embodiment, point C coincides with point A and point D coincides with point B. Accordingly, the process is represented substantially as a triangle in a T-yNH.sub.3 plot (FIG. 4a).

[0102] For comparative purposes, the T-yNH.sub.3 plot of FIG. 5 shows as point D the ammonia concentration calculated on the basis of both the ammonia-containing stream 11 and the ammonia product gas 13A of the process of FIG. 3. In other words, point D represents the molar fraction of an output stream that would result from mixing said stream 11 and product gas 13A.

[0103] FIG. 5 also shows the equilibrium curve of a reference prior art process comprising two adiabatic reactive steps and an intermediate cooling (without the intermediate adsorptive step and separation of stream 11). This reference prior art process is illustrated by the straight dashed lines of FIG. 5.

[0104] As evident from the figure, point D falls beyond the equilibrium curve of the prior art process. This means that the ammonia concentration obtained with the process of FIG. 3 is beyond the ammonia concentration achievable with the prior art process, thanks to the fact that stream 11 is separated from the adsorption unit 4 and only stream 12A is subjected to the second reactive step in the second catalyst bed 3.

[0105] In view of the representation of FIG. 5, it can be appreciated that said process obtains an outlet ammonia molar fraction of 20.5%, which corresponds to a conversion per pass of 34.8%, as shown in table 1 below.

[0106] Depending on the temperature of the make-up gas 9, one the following heat exchange configurations is adopted.

[0107] If the temperature of the make-up gas 9 is lower than a given value, i.e. 200 C. in the example, the heat recovery in the second heat exchanger 6 upstream the adsorption unit 4 can be avoided, and the third heat exchanger 7 upstream the second bed 3 can be used to control the pre-heating temperature of the second bed itself.

[0108] On the other, if the temperature is higher than the above value, heat is advantageously recovered in the second heat exchanger 6 upstream the adsorption unit 4, and the third heat exchanger 7 upstream the second bed 3 can be avoided.

[0109] If the temperature is controlled to a precise value, i.e. 200 C. for the example, e.g. by means of an external exchanger, both the heat recovery in the second heat exchanger 6 upstream the adsorption unit 4 and the third heat exchanger 7 upstream the second bed 3 can be avoided, thereby simplifying the heat exchange network.

[0110] FIG. 6 is a block scheme of an ammonia synthesis process which is carried out in a four-bed ammonia reactor 20. Said reactor 20 comprises two pairs of consecutive catalyst beds 21 and 22 with an intermediate ammonia adsorption unit 23. A first pair 21 comprises two adiabatic catalyst beds 24, 25 performing a first reactive step, and a second pair 22 comprises two adiabatic catalyst beds 26, 27 performing a second reactive step.

[0111] The catalyst beds 24, 25 forming the first pair 21 of catalyst beds can be comprised within a single reactor vessel or within separate reactor vessels. Similarly, the catalyst beds 26, 27 forming the first pair 22 of catalyst beds can be comprised within a single reactor vessel or within separate reactor vessels.

[0112] The embodiment of the figure also includes three inter-bed heat exchangers 28, 29 and 30 and a bottom heat exchanger 31.

[0113] For the sake of brevity, the catalyst beds 24 to 27 will be referred to as first to fourth catalyst beds and the inter-bed heat exchangers 28 to 30 will be referred to as first to third heat exchanger.

[0114] The process shown in FIG. 6 is as follows. A fresh make-up gas 32 is pre-heated in the first heat exchanger 28 by an effluent 33 of the first adiabatic bed 24 and fed into said bed 24, wherein it reacts to provide a partial reacted stream 33. Said stream 33 is cooled in the first heat exchanger 28 and subsequently fed into the second adiabatic bed 25, wherein it reacts to provide an ammonia-containing stream 34 which is cooled in the second heat exchanger 29. The resulting cooled stream 34 is introduced into the adsorption unit 23, which provides an ammonia-containing stream 35 and an ammonia-depleted stream 36. The ammonia ammonia-depleted stream 36 is subsequently pre-heated in the third heat-exchanger 30 by means of an effluent 37 of the third adiabatic bed 26 and fed into the bed 26 itself, where it reacts to provide an ammonia-containing stream 37. Said stream 37 is cooled in said third heat exchanger 30 and subsequently fed into the fourth adiabatic bed 27, wherein it reacts to provide an ammonia product gas 38 which is cooled in the bottom heat exchanger 31 to provide stream 38A and exported.

[0115] Similarly to FIG. 5, the T-yNH.sub.3 plot of FIG. 7 shows as point D the ammonia concentration calculated on the basis of both the ammonia-containing stream 35 separated from the adsorption unit 23 and the ammonia product gas 38A separated from the second pair 22 of catalyst beds, according to the process of FIG. 6.

[0116] FIG. 7 also shows the equilibrium curve of a reference prior art process comprising four adiabatic reactive steps with intermediate cooling (without the intermediate adsorptive step and separation of stream 35). This reference prior art process is illustrated by the straight dashed lines of FIG. 7.

[0117] Also in this case, point D falls beyond the equilibrium curve of the prior art process, which means that the ammonia concentration obtained with the process of FIG. 6 is significantly greater than the concentration achievable with the prior art process.

[0118] From the representation of FIG. 7, it can be appreciated that the process according to FIG. 6 obtains an outlet ammonia molar fraction of 30.7%, which corresponds to a conversion per pass of 48.8%, as shown in table 1 below.

[0119] FIG. 8 shows a block scheme of the ammonia synthesis process shown in FIG. 5, wherein a portion 36a of the ammonia-depleted stream 36 leaving the adsorption unit 23 is joint, via a compressor 39, with the partial reacted stream 33 leaving the first adiabatic bed 24 and subsequently fed to the second adiabatic bed 25. The advantage is that the stream 33 is more reactive due to a lower content in ammonia.

[0120] FIG. 9 shows a block scheme of the ammonia synthesis process shown in FIG. 6, wherein a portion 36b of the ammonia-depleted stream leaving the adsorption unit 23 is joint with the stream 37 leaving the first adiabatic bed 26 of the first pair 22 of beds. Similarly to the process of FIG. 7, the stream 37 is more reactive due to a lower content in ammonia.

EXPERIMENTAL DATA

[0121] Table 1 below summarizes the performance of the first embodiment of the invention according to FIG. 3 and the second embodiment according to FIG. 5, compared with the embodiment of the prior art according to FIG. 1.

[0122] The evaluation is at the same pressure of 135 bar, a H/N ratio of 3, an inlet molar concentration of inerts of 7% mol for all cases. For the embodiments of FIGS. 3 and 5, an ammonia concentration of 1.5% mol at the inlet of the second reactive step and an ammonia separation of 90% within the adsorption unit have been considered.

[0123] The first embodiment of the invention (FIG. 3) requires 40% of the catalyst volume of the 3-beds configuration of the prior art (FIG. 1) and achieves much higher conversion per pass of 34.8%. The equivalent outlet ammonia concentration achieved by the first embodiment of the invention is 20.5% mol.

[0124] The second embodiment of the invention (FIG. 5) reaches a conversion per pass of 48.8%, which corresponds to an outlet ammonia concentration of 30.7% mol. Said values are obtained by using the same total catalyst volume of the 3-beds configuration of the prior art (FIG. 1), but it can be appreciated that the conversion per pass achieved by this embodiment is dramatically higher than that of the prior art.

[0125] A conversion per pass of 31.4% (FIG. 1) means that more than of the reactants are not converted in a single reactor pass and must therefore be recycled to the reactor together with fresh gas. On the other hand, a conversion per pass of 48.8% (FIG. 5) means that only half of the reactants are not converted in a single reactor pass.

[0126] A further embodiment of the invention, wherein the ammonia synthesis process is carried out in a reactor comprising three adiabatic beds in series with two intermediate adsorption units, allows achieving a conversion pass of 48.5%, which is similar to that obtained with the second embodiment of the invention (FIG. 5), while requiring 60% of the catalyst volume of the 3-beds configuration of the prior art (FIG. 1), which is much lower that the catalyst volume required for the embodiment of FIG. 5.

TABLE-US-00001 TABLE 1 Embodiment FIG. 1 FIG. 3 FIG. 5 Number of adiabatic catalytic beds 3 1 + 1 2 + 2 Total catalyst volume, relative to 3-beds 100% .sup.40% 100% configuration of FIG. 1 Outlet ammonia molar fraction 18.5% 20.5% 30.7% Ammonia conversion per pass 31.4% 34.8% 48.8% Conversion relative to 3-beds 111% 155% configuration of FIG. 1

[0127] Another remarkable benefit of the invention is the possibility to reach an extremely high conversion per pass even at much lower operating pressures.

[0128] Table 2 below compares the conversion achieved by the second embodiment of the invention (FIG. 5) at 100 bar with that obtained by the embodiment of the prior art (FIG. 1) at 135 bar. Despite the drastically lower pressure and the same catalyst volume, the embodiment of the invention achieves a conversion per pass of 42%, which corresponds to an outlet ammonia concentration of 25.7% mol.

TABLE-US-00002 TABLE 2 Embodiment FIG. 1 FIG. 5 Number of adiabatic catalytic beds 3 2 + 2 Total catalyst volume, relative to 3-beds 100% 100% configuration of FIG. 1 Reactor pressure [bar] 135 100 Outlet ammonia mole fraction 18.5% 25.7% Ammonia conversion per pass 31.4% 42.0% Conversion relative to 3-beds 134% configuration of FIG. 1