MULTI-BED AMMONIA CONVERTER

Abstract

A multi-bed ammonia converter comprising a plurality of catalytic beds for converting an input makeup gas into an ammonia-containing product gas comprising a recovery heat exchanger such as a steam superheater or a boiler which is integrated in the ammonia converter and can be partially accommodated in the cavity of an annular bed.

Claims

1-22. (canceled)

23. An ammonia converter, comprising: a plurality of catalytic beds for converting an input makeup gas into an ammonia-containing product gas; wherein: the plurality of catalytic beds have a cylindrical annular shape delimited by an outer cylindrical wall and an inner cylindrical wall; the plurality of catalytic beds are arranged in a pressure vessel of the ammonia converter sequentially from a first catalytic bed to a last catalytic bed according to a path of the gaseous flow from an inlet to an outlet of the ammonia converter, so that for each pair of consecutive beds the effluent gas of an upstream bed of the pair is further processed in the downstream bed of the pair; at least one integrated recovery heat exchanger having a first side arranged to be traversed by reacted process gas effluent from at least one of the catalytic beds and a second side arranged to be traversed by a heat exchange medium which is not a reactive stream directed to any of the catalytic beds of the ammonia converter; and a respective inter-bed heat exchanger or a quench system operatively placed between each pair of consecutive catalytic beds of the plurality of catalytic beds contained in the ammonia converter so that, for each pair, the inlet temperature of the downstream bed can be independently controlled; wherein said at least one integrated recovery heat exchanger and said inter-bed heat exchanger are arranged to be traversed in sequence by the hot process gas effluent from said catalytic bed, so that the effluent of said bed traverses the recovery heat exchanger (RHE) first, and then the inter-bed heat exchanger.

24. The ammonia converter according to claim 23, wherein said at least one integrated recovery heat exchanger includes a plurality of heat exchange elements and a shell around the plurality of heat exchange elements, said first side is a region around the plurality of heat exchange elements and said second side is an inside of the plurality of heat exchange elements.

25. The ammonia converter according to claim 24, wherein said at least one integrated recovery heat exchanger is a tube heat exchanger with a bundle of U-tubes or bayonet tubes connected to a tube sheet, the tube sheet being on top of the heat exchanger and the bundle of tubes extending downwards from the tube sheet.

26. The ammonia converter according to claim 25, wherein said tube sheet of the at least one integrated recovery heat exchanger is located above a top cover of the pressure vessel of the ammonia converter.

27. The ammonia converter according to claim 23, wherein the pressure vessel has a top cover that includes a shell of the at least one integrated recovery heat exchanger.

28. The ammonia converter according to claim 23, wherein said at least one integrated recovery heat exchanger is at least partially accommodated in the central cavity of at least one of the catalytic beds.

29. The ammonia converter according to claim 28, wherein the central cavity of at least one catalytic bed accommodates: at least a portion of said at least one integrated recovery heat exchanger; and an inter-bed heat exchanger arranged to transfer heat from the hot effluent of the catalytic bed to a reactant gas stream directed to the same or another catalytic bed.

30. The ammonia converter according to claim 23, wherein the at least one recovery heat exchanger includes a recovery heat exchanger which is located above the first catalytic bed of the ammonia converter and is arranged so that the first side of the recovery heat exchanger is traversed by the effluent of said first catalytic bed.

31. The ammonia converter according to claim 23, wherein the at least one integrated recovery heat exchanger includes a heat recovery exchanger that is placed downstream the second catalytic bed of the ammonia converter so that the first side of the at least one integrated recovery heat exchanger is traversed by the effluent of said second catalytic bed.

32. The ammonia converter according to claim 23, wherein said heat exchange medium is water or steam, and said at least one integrated recovery heat exchanger is arranged for connection to a steam system.

33. The ammonia converter according to claim 32, wherein said at least one integrated recovery heat exchanger includes a steam superheater.

34. The ammonia converter according to claim 32, wherein said at least one integrated recovery heat exchanger includes a boiler.

35. The ammonia converter according to claim 23, wherein said at least one integrated recovery heat exchanger has a shell having a diameter smaller than a diameter of the plurality of catalyst beds.

36. The ammonia converter according to claim 23, wherein said at least one integrated recovery heat exchanger is equipped with a level measurement to monitor a loss of the heat exchange medium inside the pressure vessel.

37. The ammonia converter according to claim 23, wherein the pressure of the heat exchange medium in the second side of said at least one integrated recovery heat exchanger is lower than the pressure of the reactant process stream.

38. The ammonia converter according to claim 23, wherein the pressure of the heat exchange medium in the second side of said at least one integrated recovery heat exchanger is higher than the pressure of the reactant process stream.

39. The ammonia converter according to claim 23, further comprising a control system, wherein the control system is configured to operate a shutdown of the ammonia converter according to a shutdown procedure, wherein in the shutdown procedure a pressure of the heat exchange medium traversing the at least one integrated recovery heat exchanger is maintained below a pressure of the reactant gas in the ammonia converter.

40. The ammonia converter according to claim 23, further comprising: wherein said heat exchanger, which is operated with an external fluid, is located between two consecutive catalytic beds; a heat exchanger or a quench system that is separate from said heat exchanger operated with an external fluid, and is arranged to adjust the temperature of the process stream entering the downstream bed of said two consecutive catalytic beds.

41. The ammonia converter according to claim 23, further comprising: wherein said heat exchanger, which is operated with an external fluid, is located between two consecutive catalytic beds; at least another heat exchanger, which is a gas pre-heater, arranged to transfer heat from a reacted gas withdrawn from at least one catalytic bed to a stream of a fresh reactive gas, the ammonia converter being internally arranged so that an input of fresh gas is passed over the internal surface of the pressure vessel of the ammonia converter before admission thereof to said gas pre-heater, so that the pressure vessel is cooled by the fresh gas.

42. A plant for the synthesis of ammonia, the plant comprising: the ammonia converter according to claim 23; and a steam system; wherein an input and an output of the second side of the at least one integrated recovery heat exchanger (RHE) integrated in the ammonia converter are connected to the steam system.

43. A process of synthesis of ammonia, the process comprising: passing a makeup gas containing hydrogen and nitrogen through a plurality of annular catalytic beds arranged inside a pressure vessel of an ammonia converter, including at least a first bed and a second bed, wherein said plurality of annular catalytic beds are arranged sequentially so that a partially reacted gas effluent from the first bed is further reacted in the second bed; transferring heat from the effluent of at least one of said catalytic beds to a heat exchange medium, wherein: said transfer of heat is performed in a recovery heat exchanger which is integrated in the ammonia converter according to claim 23; said heat exchange medium is not a reactive stream of the ammonia synthesis process.

44. The process according to claim 43, wherein said heat exchange medium is water or steam and the process includes the production of steam or of superheated steam in said at least one integrated recovery heat exchanger.

Description

DESCRIPTION OF THE FIGURES

[0068] FIG. 1 shows a scheme of an ammonia converter according to an embodiment.

[0069] FIG. 2 is a functional scheme of an ammonia converter according to a preferred application.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0070] FIG. 1 illustrates the following items:

[0071] R Vertical ammonia converter

[0072] A-A Axis of the converter R

[0073] 1 Pressure vessel of the converter R

[0074] C1 First catalytic bed

[0075] C2 Second catalytic bed

[0076] C3 Third catalytic bed

[0077] 2 Central cavity of the first catalytic bed C1

[0078] 3 Central cavity of the second catalytic bed C2

[0079] 4 Central cavity of the third catalytic bed C3

[0080] RHE Integrated recovery heat exchanger

[0081] HE1 First inter-bed exchanger of the converter, fitted in the central cavity

[0082] 2 of the first bed C1

[0083] HE2 Second inter-bed heat exchanger of the converter, fitted in the central cavity 3 of the second bed C2.

[0084] GI Gas inlet (reactants)

[0085] GO Gas outlet (products)

[0086] 30 bundle of u-tubes of the integrated exchanger RHE

[0087] 31 lower part of the u-bundle 30

[0088] 32 inlet of the tube side of the integrated exchanger RHE

[0089] 33 outlet of the tube side of the integrated exchanger RHE

[0090] 34 tubesheet of the u-bundle 30

[0091] 40 top flange of the pressure vessel 1

[0092] 41 top cover of the pressure vessel 1

[0093] 42 top cover of a cartridge containing the catalytic beds

[0094] S saturated steam line connected to the input 32 of the tube bundle 30

[0095] SH superheated steam line connected to the output 33 of the tube bundle 30

[0096] The catalytic beds C1, C2 and C3 and the inter-bed heat exchangers HE1, HE2 for example are part of a cartridge fitted in the pressure vessel 1. The cartridge may be removable from the pressure vessel.

[0097] The catalytic beds C1, C2 and C3 have a cylindrical annular shape. Each bed has a central cavity 2, 3 and 4 respectively.

[0098] The figure is schematic and the internals of the converter are not illustrated in detail.

[0099] The converter R is configured internally so that the process gas traverses the catalytic beds with a radial or axial radial flow. The flow is directed inwardly from the outer surface of the bed towards the axis A-A as indicated by the arrows of FIG. 1.

[0100] The input gas GI is directed to the first bed C1 and may be preheated in one or more of the heat exchangers of the converter, for example in the inter-bed exchangers HE2 and HE1. The input gas may also pass in the annular space between the pressure vessel 1 and a catalytic cartridge in order to cool the pressure vessel 1. The preheated gas may be mixed with a portion of cold gas to carefully adjust the inlet temperature of the first catalytic bed. The converter may include an additional input for said cold gas.

[0101] The partially reacted hot effluent of the bed C1 passes in the region around the tubes 30 of the recovery exchanger RHE and around the tubes of the inter-bed heat exchanger HE1. Each of said heat exchangers is basically a bundle of tubes traversed by a suitable medium. The hot effluent gas passes around the tubes and transfers heat to the medium inside the tubes.

[0102] It can be appreciated that the recovery exchanger RHE has a first side, which is represented by the space around the tubes, and a second side, which is formed by the inside of the tubes. Said first side and second side are not in a communication so that the medium in the first side (hot process gas) does not mix with the medium in the second side (e.g. water or steam).

[0103] The recovery exchanger RHE may be for example a steam superheater. The input 32 and output 33 of the tube side may be connected to a steam system of the ammonia plant. For example when the exchanger RHE works as a steam superheater, the input 32 is fed with saturated steam and a superheated steam is collected at the output 33. Alternatively for example the input 32 may be fed with water which is evaporated in the exchanger RHE and steam is collected at the output 33. FIG. 1 illustrates an example wherein the input 32 is connected to a steam line S and the output 33 feeds a superheated steam line 33.

[0104] The figure illustrates a preferred embodiment wherein the heat exchanger RHE has a bundle 30 of u-tubes. The u-bundle 30 includes a bottom portion 31 where the tubes are curved. The bottom portion of the heat exchanger RHE (e.g. bottom portion of the bundle 30) may be accommodated within the cavity 2 of the first catalytic bed.

[0105] The medium inside the tubes of the inter-bed exchanger HE1 may be the fresh gas which is preheated before entering the first bed.

[0106] FIG. 1 illustrates a preferred embodiment where the first bed C1 has a slim design thanks to a reduced radial width, compared to the subsequent beds C2 and C3. This design of the first bed C1 increases the size of the cavity 2 making said cavity wider in the radial direction and longer in the axial direction, and thus facilitates the accommodation of the lower part of the recovery exchanger RHE and of the inter-bed heat exchanger HE1 in the cavity 2 of the first bed C1.

[0107] The recovery exchanger RHE is installed on top of the vertical converter R and above the inter-bed exchanger HE1. This arrangement makes the exchanger RHE easy to access and, if necessary, easy to remove from the converter. Particularly, in a preferred embodiment the tubesheet 34 is above the cover 41 of the vessel, which makes possible the extraction of the whole tube bundle 30 without removing the cover 41.

[0108] In an embodiment the recovery exchanger RHE may have a tubesheet which is directly flanged to the cover of the pressure vessel.

[0109] The integration of the exchanger RHE in the pressure vessel 1 of the converter provides a better exploitation of the volume in the upper region of the vessel, particularly the region above the cover 42 of the catalytic cartridge.

[0110] After a passage around the tubes of said recovery heat exchanger RHE and around the tubes of the first inter-bed heat exchanger HE1, the effluent gas from the first bed C1 is redirected to the second bed C2 which is also traversed inwardly. Then the effluent of the second bed passes through the second inter-bed heat exchanger HE2 installed in the cavity 3 of the second bed C2. Said second inter-bed heat exchanger HE2 may also be a tube heat exchanger and the medium inside the tubes may be incoming gas to be preheated. For example the incoming gas may be preheated in the heat exchanger HE2 and then in the exchanger HE1.

[0111] After a passage through the heat exchanger HE2 the process gas is directed to the third bed C3 which is also traversed with inward radial flow. The effluent of the third bed is collected in the space 4 and represent the fully reacted outlet gas GO. A heat exchanger may optionally be installed also in the space 4.

[0112] The arrows in FIG. 1 indicate schematically the gas flow. Suitable internals of the converter provide the necessary distribution and collection of the gas.

[0113] FIG. 2 illustrates a process scheme that can be implemented with the converter R of FIG. 1. The numerals in FIG. 2 denote the following.

[0114] 11 partially reacted process gas effluent from the first bed C1 directed to the exchanger RHE

[0115] 12 process gas after passage through the exchanger RHE and directed to the first inter-bed exchanger HE1

[0116] 13 process gas after passage through the first inter-bed heat exchanger HE1 and directed to the inlet of the second bed C2

[0117] 14 process gas effluent from the second bed C2

[0118] 15 process gas at the inlet of the third bed C3

[0119] 16 fully reacted process gas (products) effluent from the third bed C3

[0120] 17 heat recovery heat exchanger

[0121] 18 effluent of the heat exchanger 17

[0122] 19 gas-gas heat exchanger

[0123] 20 fresh process gas (reactants)

[0124] 21 portion of gas 20 directed to the gas-gas heat exchanger 19

[0125] 22 portion of gas 20 bypassing the gas-gas heat exchanger 19, controlled by valve V1.

[0126] 23 cold fresh gas directed to the inlet of the first bed, controlled by valve V2

[0127] 24 fresh gas directed to the inter-bed heat exchanger HE2

[0128] 25 fresh gas bypassing the inter-bed heat exchanger HE2, controlled by valve V3

[0129] 26 pre-heated fresh gas directed to the inter-bed heat exchanger HE1

[0130] 27 fully pre-heated fresh gas effluent from exchanger HE1 and directed to the inlet of the first bed together with the gas 23.

[0131] The temperature of the process gas at the inlet of the beds is controlled via the valves V1, V2 and V3.

[0132] Particularly, the valve V2 controls the flow rate of the “cold shot” 23, i.e. a stream of fresh gas which is not preheated in the inter-bed exchangers HE2 and HE1. This cold gas 23 is mixed at the inlet of the first bed C1 with the fully preheated stream 27 effluent from the inter-bed exchanger HE1. The mixture of the stream 23 and stream 27 forms the inlet gas GI indicated in FIG. 1.

[0133] The partially reacted gas 11 from the first bed C1 is at elevated temperature (e.g.

[0134] above 500° C.) and transfers heat to the steam S flowing in the tubes of the integrated recovery exchanger RHE. The so obtained superheated steam SH may be used internally in the process as a heat source or to produce energy. For example the steam SH is sent to one or more steam users of the ammonia plant. The steam may also be exported from the ammonia plant if appropriate.

[0135] The effluent 12, still at a high temperature, transfers heat in the first inter-bed exchanger HE1 to the reactant stream 26. Said stream 26 is the result of mixing the stream 24 preheated in the exchanger HE2 with the bypass stream 25. Therefore the temperature of said stream 26 is controlled basically by the valve V3 which controls the bypass line of stream 25. This influences, in turn, the temperature of the process stream 13, i.e. the inlet temperature of the second bed.

[0136] Also, the temperature of the cold gas in lines 23 and 25 is controlled by the valve V1, as it is the result of mixing the effluent of the exchanger 19 with the gas 22 bypassing the same.

[0137] The product stream 16 leaving the third bed C3 may be cooled in the recovery exchanger 17. This exchanger 17 and also the gas-gas exchanger 19 may be installed in the annular cavity 4 of the third bed (i.e. inside the pressure vessel) or may be external. The gas 18 after cooling in the exchanger 19 represents the product gas.

[0138] It can be appreciated that the valves V1, V2 and V3 operates on streams of cold gas. No valve is required on hot lines such as lines 26 or 27. This is a considerable advantage because a valve operating on a hot stream at high pressure would be a critical and expensive item.

[0139] The invention achieves the aims enunciated above. Particularly, it does not require additional pressure vessels, other than the vessel of the ammonia converter; it does not require high-pressure piping for connection to an external vessel; it does not require valves on high-temperature and high-pressure lines to control the process. It can be appreciated that the invention provides an efficient recovery of the heat generated by the chemical reaction, particularly of the heat contained in the hot process streams 11, 14 and 15.