HK1160097B - Process for the production of ammonia - Google Patents

Description

Process for producing ammonia

The present invention relates to a process for producing ammonia from a hydrocarbon feedstock with improved heat integration, wherein the hydrocarbon feedstock is first converted to synthesis gas by steam reforming, the synthesis gas being subsequently converted to ammonia. The invention also relates to a new steam superheater, particularly suitable for use in the process, especially for large ammonia plants having a production capacity of at least 2000 MTPD.

Conventional plants for the production of ammonia are generally divided into two main sections, a reforming section, in which a hydrocarbon feedstock, such as natural gas, is converted at a pressure in the range of 30 to 80 bar, typically 30 to 40 bar, into a synthesis gas comprising a mixture of hydrogen and nitrogen, and an ammonia synthesis section, in which a synthesis gas with the appropriate proportions of hydrogen and nitrogen (ammonia synthesis gas) is catalytically converted after compression to 120-200 bar into ammonia which is subsequently concentrated by cooling.

In the reforming section, when utilizing a conventional process arrangement with a primary, autothermal or secondary reformer, a synthesis gas containing hydrogen is produced at elevated temperatures, for example at a temperature of about 1000 ℃ or higher. The syngas produced in the reformer must be cooled, which is typically accomplished by passing the gas through a number of waste heat boilers or steam superheaters. These devices are expensive and highly complex heat exchangers that need to be carefully designed to minimize the risk of mechanical and material related failures related to metal dusting, hydrogen corrosion and stress corrosion. In particular, steam superheaters in the reforming section are expensive devices, where metal dusting is difficult to avoid even with careful construction of these units. The risk of metal dusting is in fact inherent when superheaters are employed in the reforming section.

In the ammonia synthesis section, ammonia is catalytically produced from a mixture of hydrogen and nitrogen contained in the synthesis gas. The conversion of ammonia takes place under heat generating conditions, which utilize a waste heat boiler and optionally a steam superheater to generate high pressure steam, which is further used to drive the compressor of the ammonia synthesis section. The waste heat boiler and steam superheater in the ammonia synthesis section are also expensive and highly complex heat exchangers that are specifically designed to minimize the risk of mechanical and material related failures related to hydrogen corrosion, nitriding and stress corrosion. Waste heat boilers are particularly subject to nitriding and stress corrosion, since these units are usually arranged downstream of the ammonia converter.

Metal dusting, stress corrosion and nitriding are catastrophic or at least severe forms of corrosion that must be avoided by proper design and material selection. Metal dusting typically occurs in the presence of carbon monoxide in the gas, the gas being in contact with the metal and when the temperature of the metal is relatively low, typically 400 ℃ to 800 ℃, more particularly 500 ℃ to 750 ℃, the interaction with the gas results in the decomposition of the metal into fine particles.

Metal nitriding occurs when nitrogen from the gas comes into contact with the metal, diffuses into the metal material and produces nitrides. This results in a hard surface layer which is prone to fracture and in the worst case fractures throughout the entire metal. As a result, materials subject to nitriding become more brittle instantaneously. The thickness of the nitrided layer depends on temperature, time, and metal alloy. It is generally believed that metal temperatures of about 380 ℃ or higher for thin metal sheets and 400 ℃ or higher for thick metal sheets significantly increase the metal nitriding tendency of low alloy carbon steels. At higher temperatures, materials such as stainless steel or Inconel (Inconel) are required.

Stress corrosion represents a risk when austenitic materials such as stainless steel are exposed to water, especially when the water contains impurities such as chlorine. The risk of stress corrosion is less when using low alloy carbon steel.

As the capacity of ammonia plants steadily increases as plants are designed to produce 2000, 3000, 5000MTPD or more ammonia, designing larger and larger steam superheaters has become a formidable challenge. The specification of steam superheaters in these large ammonia plants is a problem because the diameter and thickness of the superheater tube sheet becomes simply too large in standard design to make its production technically or economically feasible.

This trend to install large plants also raises the necessity to provide steam to drive the compressors in the plant. This requires a higher steam pressure and thus a higher steam temperature. As a result, expensive materials for steam superheaters, such as steel or inconel, which can cope with higher steam temperatures, have to be employed.

U.S. patent No.4,213,954 describes a process for producing ammonia that includes a reforming section and an ammonia synthesis section. The two sections share a common steam drum (steam drum) for use as a steam separation unit for the waste heat boiler in the reforming and ammonia synthesis sections of the plant. Steam, generated in the reforming section, is thus used in the ammonia synthesis section, while the process gas from the secondary reformer is cooled via an integrated system of waste heat boilers and superheaters. Steam is also used in the expander to recover energy.

U.S. patent No.4,545,976 describes a method for producing ammonia synthesis gas by steam reforming of hydrocarbons with reduced steam output, wherein a series of steam superheaters are used to cool the process gas from the secondary reformer.

Our EP- ｃA-1,610,081 discloses ｃA heat exchanger for use in the vicinity downstream of the steam reforming stage. The heat exchanger includes a first, cooler heating zone containing a bundle of low alloy carbon steel tubes and a second, hotter heating zone containing a bundle of tubes made of a high temperature and corrosion resistant alloy such as an austenitic nickel/chlorine/iron alloy. The steam passes through the tube side of the heat exchanger, while the reformate gas (syngas) passes through the shell side. The cooler and hotter heating zones are connected in series for both the steam flow and the reformate flow.

It is an object of the present invention to provide a process for producing ammonia with improved heat integration and with a reduced tendency to metal dusting, nitriding and stress corrosion in waste heat boilers and in particular in the steam superheaters of the plant.

It is another object of the present invention to provide a process for producing ammonia which has improved heat integration by reducing steam export and which is more cost effective than prior art processes.

It is also an object of the invention to provide a method which is stable and less sensitive to plant trips of the ammonia section.

It is a further object of the present invention to provide a steam superheater suitable for use in large ammonia plants, while being able to withstand corrosion, particularly nitriding and stress corrosion.

These and other objects are solved by the present invention.

In a first aspect we provide a process for the production of ammonia from a hydrocarbon feedstock, the process comprising the steps of:

(a) passing the hydrocarbon feedstock through a reforming section and recovering synthesis gas from said reforming section;

(b) passing said synthesis gas through one or more waste heat boilers without the use of steam superheaters, wherein the synthesis gas is subjected to indirect heat exchange with a water-steam mixture, recovering steam from said waste heat boilers and passing said steam to one or more steam drums;

(c) passing the thus cooled synthesis gas of step (b) through a shift conversion stage for converting carbon monoxide in the synthesis gas to hydrogen and subsequently removing remaining carbon dioxide, carbon monoxide and methane in the synthesis gas by a scrubbing process and recovering a synthesis gas comprising nitrogen and hydrogen;

(d) passing the synthesis gas produced in step (c) through an ammonia synthesis section comprising catalytically converting the synthesis gas to ammonia by passage through a catalytic bed in one or more ammonia converters and recovering process gas containing ammonia from the one or more catalytic beds;

(e) passing said process gas containing ammonia through one or more steam superheaters to superheat steam from the one or more drums of step (b) therein, and recovering a superheated steam stream from said one or more steam superheaters;

(f) passing the thus cooled process gas of step (e) through one or more waste heat boilers in which the process gas is in indirect heat exchange with a water-steam mixture, recovering steam from the one or more waste heat boilers and passing said steam to the one or more steam drums of step (b).

Thus, all the steam produced in the waste heat boilers of steps (b) and (f) is superheated in the superheater or superheaters of step (e). Whereby as much cooling as possible is performed in the ammonia synthesis section.

We have found that by introducing one or more superheaters downstream of the ammonia converter for cooling the process gas containing ammonia and superheating all steam produced in the waste heat boiler of the reforming section, it is possible to provide a simpler and cheaper construction to the waste heat boiler otherwise required in the reforming section of the plant and to the waste heat boiler, in particular the superheaters, in the ammonia synthesis section. The present invention therefore has the significant advantage of not requiring a steam superheater to heat the syngas (superheater to heat the process gas), or simply a steam superheater in the reforming section of the plant to cool the syngas produced. The cooling capacity of the superheater is, for its part, moved from the reforming section of the plant to the ammonia synthesis section. The risk of metal dusting, which is actually inherent when using steam superheaters in the reforming section, is thus completely eliminated.

Furthermore, the method enables cooling of the process gas from the ammonia converter in the steam superheater, preferably in the form of a U-tube heat exchanger, to below about 380 ℃ as much cooling as possible in the ammonia synthesis section, whereby nitriding of the waste heat boiler installed downstream is avoided. As mentioned above, metal temperatures above about 380 ℃ significantly increase the tendency for nitriding effects. The waste heat boiler now cooling the ammonia synthesis section with process gases below 380 c can be conveniently constructed as a U-tube heat exchanger, for example of alloyed carbon steel, thereby also eliminating the problems associated with stress corrosion of otherwise required austenitic materials. Other very expensive waste heat boilers and steam superheaters can thus be constructed from cheaper materials.

A significant advantage of the present invention is that the plant, including the reforming and ammonia sections, becomes more stable to plant trip conditions such as ammonia production from the ammonia synthesis section being stopped while the reforming section remains in operation. According to the conventional process design, steam generation in the reforming section is affected immediately when such plant tripping occurs in the ammonia section. To compensate for this effect, the waste heat boiler in the reforming section that cools the syngas downstream of the secondary reformer is typically significantly oversized. It is possible to reduce this effect on the steam generation in the reforming section by the process of the present invention. If there is a plant trip in the ammonia section, it is now possible to balance the steam generation in the reforming section and as a result the waste heat boiler downstream of the secondary reformer in this section does not need to be significantly oversized. Smaller and thus cheaper waste heat boilers can be used.

As is conventional in the art, the reforming section may comprise the reforming of a hydrocarbon feedstock in one or more steps. The hydrocarbon feed may thus be subjected to a pre-reforming step, for example, following primary and secondary reforming, or the hydrocarbon feed, for example natural gas, may be passed directly through an autothermal reforming step to produce hot synthesis gas. The synthesis gas from the autothermal or secondary reforming step at a temperature above 1000 c is recovered before being cooled in the case of high pressure steam produced in one or more waste heat boilers.

The term synthesis gas containing nitrogen and hydrogen as used herein means ammonia synthesis gas, e.g. synthesis gas having a suitable ratio of hydrogen and nitrogen as feed for an ammonia converter.

The terms secondary reforming and autothermal reforming are used interchangeably herein, as secondary reforming is typically carried out in an autothermal reformer (ATR). Strictly speaking, however, the term autothermal reforming has only the appropriate meaning in the absence of primary reforming.

The term primary reforming as used herein means reforming of a hydrocarbon feedstock in a conventional fired tubular reformer (radiant furnace).

It will also be appreciated in accordance with the present invention that the process gas exiting the catalytic ammonia converter is first passed through a steam superheater and then through a waste heat boiler. All steam produced in the waste heat boiler of the reforming section, as well as steam produced in the waste heat boiler of the ammonia synthesis section, is fed to a first steam superheater installed downstream of the catalytic ammonia converter. At least part of the steam from the steam superheater may be used as process steam in the reforming section of the plant, preferably as process steam in the waste heat section of the primary reforming stage.

In a preferred embodiment of the invention, step (a) involves passing the hydrocarbon feedstock through a reforming section and recovering synthesis gas from said reforming section and comprises the steps of: passing a hydrocarbon feedstock through a primary reforming step to produce a partially reformed gas, passing said partially reformed gas through a heat exchange reforming step and a secondary reforming step and recovering a resulting synthesis gas stream from said heat exchange reforming step, wherein the partially reformed gas passing through the heat exchange reforming step is indirectly heat exchanged reformed with the synthesis gas recovered from said secondary reforming step.

Heat exchange reforming enables the heat from the primary and secondary reforming steps to be further used to reform the gas rather than simply using the heat to generate steam. It is therefore also possible to significantly reduce the steam production and indeed to an amount that almost exactly meets the requirements of the ammonia synthesis section. As a result, improper steam export is avoided.

As noted above, it is generally accepted that the risk of metal dusting is highest when the metal temperature is in the range of 400 ℃ to 800 ℃, more specifically 500 ℃ to 750 ℃. It is therefore preferred that the synthesis gas stream recovered from the reforming section, and in particular the synthesis gas stream recovered from the heat exchange reforming stage, has a temperature of about 800 ℃ or more, which is high enough to reduce the risk of metal dusting in the heat exchanger itself and to avoid metal dusting in the waste heat boiler located downstream.

Preferably, the heat-exchange reforming is carried out in one or more heat-exchange reactors comprising double-tubes (double-tubes). A double tube is essentially an arrangement of two virtually concentric tubes. The space between the tube walls forms an annular chamber through which a heat exchange medium (synthesis gas recovered from the secondary reforming step) can flow. The solid catalyst in the bed may be arranged outside and/or inside the double tube.

Thus, in another embodiment the invention also comprises forming a gas mixture in one or more heat exchange reactors having a number of double tubes for carrying out said heat exchange reforming step, preferably at the bottom of the one or more heat exchange reactors by mixing the synthesis gas recovered from said secondary reforming step with the reformed gas leaving the catalytic bed arranged at least outside the double tubes of the one or more heat exchange reactors, and indirectly heating said catalytic bed by passing said gas mixture through the annular space of said double tubes. The resulting syngas stream is then recovered and passed through one or more waste heat boilers installed downstream of the reforming section.

Preferably, the solid catalytic particles of the catalytic bed of the heat exchange reactor or reactors are not only arranged outside the double tube, but also inside, for example also inside the inner tube of the double tube.

In yet another embodiment, the heat exchange reforming step is carried out in a bayonet reactor. In a particular embodiment of the bayonet tube reformer at least one of the reforming tubes in the reformer has an outer tube having an inlet end for the introduction of the process gas to be reformed and a closed outlet end, and an inner tube open at both ends and mounted coaxially within the outer tube and separating the outer tube space, the annular space between the outer and inner tubes being filled with reforming catalyst, the inner tube being adapted to recover the synthesis gas stream effluent, the outer tube being optionally concentrically surrounded by a sleeve separate from the outer tube space and adapted to place the outer tube in heat transfer relationship with the process gas (reactant feed) to be reformed in the outer tube by the introduction of the synthesis gas stream from the secondary reformer into the space between the sleeve and the outer tube. One particular embodiment of such ｃA bayonet reactor is disclosed, for example, in our EP- ｃA-0535505.

In yet another embodiment of the present invention, the hydrocarbon feedstock to be reformed in step (a) is passed in parallel through one or more heat exchange reforming steps and an autothermal or secondary reforming step, and wherein the hot synthesis gas recovered from the autothermal or secondary reforming stage is used as a heat exchange medium in the one or more heat exchange reforming steps, as described in our U.S. patent No.6,726,851.

In a second aspect of the invention we provide a steam superheater for use in the process, more particularly for use in accordance with step (e) of the process, i.e. downstream of the ammonia catalytic converter.

Accordingly, the present invention also includes a steam superheater 30 comprising:

first and second compartments 301, 302, wherein the first compartment has an outer shell 305, a tube sheet 303, a rear end 307, a tube bundle 309, a baffle 317 and a steam inlet 315 for the outer shell 305, and the second compartment 302 has an outer shell 306, a tube sheet 304, a rear end 308, a tube bundle 310, a baffle 317 and a steam outlet 316 for the outer shell 306;

a transition chamber 311 separating the first and second compartments, defined by the space between the tubesheets 303, 304;

a duct 312 through tubesheets 303, 304 and further through transition chamber 311, which extends from first compartment 301 to second compartment 302 along the long axis 320 of steam superheater 30;

a dividing wall 321 between the inlet chamber 318 and the outlet chamber 319;

said transition chamber 311 having a process fluid inlet 313 extending into an inlet chamber 318 of the transition chamber, the inlet chamber 318 being confined between the tube walls of the transport tubes 312, the panel walls of the tube sheet 303 on one side and the panel walls of the tube sheet 304 on the opposite side, wherein the tube bundle 309 of the first compartment 301 extends into the tube sheet 303 and the tube bundle 310 of the second compartment 302 extends into the tube sheet 304;

said transition chamber 311 having a process gas outlet 314 extending from an outlet chamber 319 of the transition chamber, the outlet chamber 319 being confined between the tube walls of the transport tubes 312, the panel walls of the tube sheet 303 on one side and the panel walls of the tube sheet 304 on the opposite side, wherein the tube bundle 309 of the first compartment 301 extends into the tube sheet 303 and the tube bundle 310 of the second compartment 302 extends into the tube sheet 304;

and wherein the first and second compartments 301, 302 are connected in series for the steam flow and in parallel for the process gas flow.

The steam passes through the shell side of the superheater, while the process gas from the ammonia converter passes through the tube side.

Preferably, the process gas inlet 313 and the process gas outlet 314 of the transition chamber 311 are arranged diametrically opposite each other in the shell 305, 306 of the steam superheater, and more preferably said process gas inlet and outlet 313, 314 are arranged diametrically opposite each other and at the same position along the long axis 320 of the steam superheater.

A dividing wall 321 between the inlet chamber 318 and the outlet chamber 319 preferably extends along and through the length of the delivery tube 312. The wall helps prevent process gases from the inlet chamber 318 from passing directly into the outlet chamber 319. Preferably, the tube bundles in the individual chambers of the superheater are U-shaped tube bundles.

The tube bundle extends into each tube sheet and is thus supported therein. This will be understood as the tube bundle passing through the tubesheet. The pipe is thus in fluid communication with the inlet chamber of the transition chamber receiving incoming hot process gas from the ammonia converter or with the outlet chamber of the transition chamber from which cooled process gas is recovered.

In a particular embodiment, the outlet chamber 319 further includes valves 322, 323 disposed therein and in direct fluid communication with the tube bundles 309, 310 of the first and second compartments 301, 302. The valve is preferably a throttle valve. Providing a valve in the outlet chamber enables a suitable proportion of the process gas from the ammonia converter to be supplied to the first (cold) and second (hot) compartments of the steam superheater and thus makes it possible to adjust the temperature of the steam leaving the steam superheater in a simple manner at the steam outlet 316. Preferably, 40 wt% of the process gas is passed through the first compartment and 60 wt% is passed through the second compartment. By adjusting the temperature of the steam outlet in the superheater, which may be about 375 ℃, it is also possible to adjust the final superheat temperature of the steam after it has passed through the boiling heat section of the primary reformer where it is further heated to a final superheat temperature of, for example, 515 ℃. The final steam temperature is in fact a temperature that needs to be adjusted, and such adjustment is now made possible by simply adjusting the temperature of the steam leaving the superheater at steam outlet 316. Unnecessary options to adjust this final superheat temperature, such as adding Boiler Feed Water (BFW) to quench the steam as it passes through the hot section of the primary reformer, may be avoided.

The process of the invention enables saturated steam to be introduced into the first compartment of the superheater at a relatively low temperature (323 c). The steam will contain some residue in the form of water droplets from the drum. This can lead to stress corrosion of the superheater internal metal parts if they are made of austenitic material, such as stainless steel. However, in the steam superheater of the present invention, the internal metal parts, mainly the tube bundle in the first compartment, are preferably made of low alloy steel. Since the first (cold) compartment can be kept below 380 ℃ because of the introduction of cooling steam (323 ℃), it is feasible to use low alloy steels, such as low alloy carbon steels, without the risk of nitriding effects. Due to the risk of nitriding, the internal metal parts in the second (hot) compartment, mainly the tube bundle, are made of stainless steel, since the temperature cannot be kept below 380 ℃ throughout the chamber. The risk of stress corrosion is no longer relevant in this chamber, since the water droplets remaining in the introduced steam are already heated in the first compartment through which they pass, and the steam is therefore dry.

Thus, according to a further embodiment of the invention, the tube bundle in the first compartment is made of a low alloy steel, such as ferritic iron, chromium, molybdenum and carbon steel, while the tube bundle in the second compartment is made of stainless steel. Preferably, the low alloy steel is a low alloy carbon steel.

In addition to addressing corrosion issues, the superheaters of the present invention are particularly beneficial for large ammonia plants that are too large in size for them to be manufactured simply under standard design. By means of the inventive superheater, the process gas stream from the ammonia converter enters the first and second compartments, respectively. In other words, only a portion of the process gas flow passes through each tube sheet and at the same time the tube sheets are supported by transport tubes extending from one compartment to the other along the long axis of the superheater. This results in a considerable reduction in the thickness of the tubesheet compared to the case where a conventional single tubesheet is used. The invention can thus also be of a simpler and cheaper construction. The superheater of the invention can be assembled in virtually any specialized workshop.

The term "large ammonia plant" as used herein means an ammonia plant having a capacity equal to or greater than 2000MTPD, e.g., 3000, 5000MTPD or even more.

Conveniently, the superheater is oriented generally horizontally with respect to the heavy tubesheet and the header is generally mounted near the rear end of the superheater. However, such horizontal directions may convey corrosion problems, particularly in metal parts installed in the middle section of the superheater. Particularly in the case when the metal parts of the steam superheater are started without being warmed up, water droplets containing impurities such as chlorine may accumulate therein and condense. Since such metal parts are not typically made of corrosion resistant materials, serious corrosion problems may arise.

By means of the invention, it is possible to further prevent such corrosion problems by simply installing the superheater longitudinally. This orientation is easier to achieve in the superheater of the invention, since the heavy metal parts, which mainly contain the tube bundle, are installed towards the middle of the plant. The potential impurity-containing water droplets accumulate and collect at the bottom of the superheater in the first or second compartment. The accumulated water is then simply recovered through an outlet conduit installed therein.

Thus, in a further embodiment of the invention, the orientation of the steam superheater is longitudinal and the first or second compartment further comprises a water outlet at its rear end for removing accumulated water. Preferably, the bottom part in such a longitudinal steam superheater is the second (heat) compartment.

FIG. 1 shows a block diagram of one embodiment of a process for an ammonia plant showing a reforming section I and an ammonia synthesis section II leading to a heat exchange reformer and a secondary reformer.

Figure 2 shows a schematic view of a superheater for the ammonia synthesis section of a plant according to the invention.

In fig. 1, a hydrocarbon feedstock 1, such as natural gas, is passed through a primary reforming step in a primary reformer 20, along with added steam. Partially reformed gas 2 is recovered from primary reformer 20 and is partially split into streams 3 and 4. Stream 3 is fed to the top of a heat exchange reformer 21 having a double tube with catalyst particles placed on the outside and inside of the double tube, while stream 4 passes through a secondary reformer 22. At the bottom of the heat exchange reformer 21, the hot effluent gas from the secondary reformer is mixed with the converted process gas exiting the catalytic bed at the bottom of the reformer in the heat exchange reformer. By passing said mixed gas upwardly in the reformer, the mixed gas is used for indirect heat exchange with the catalytic bed arranged therein. The mixed gas is cooled and exits as syngas stream 5 as it passes through the heat exchange reformer. The feed water 6 is then used in the waste heat boiler 23, the gas stream 5 is cooled and the synthesis gas is indirectly heat exchanged with steam therein. No steam superheater is employed in this section. The steam-water mixture from the waste heat boiler 23 is introduced into the steam drum 24. The synthesis gas stream cooled in the water gas shift section 25 is enriched in hydrogen and then passed through a scrubbing section to remove the remaining carbon monoxide, carbon dioxide and methane in the synthesis gas. And a synthesis ammonia stream 8 comprising hydrogen and nitrogen in appropriate proportions is produced and sent to the ammonia synthesis part of the plant, a catalytic ammonia converter 27 comprising a number of ammonia catalytic beds 28. The process gas 9 containing ammonia at 460 ℃ is recovered from the catalytic converter and cooled by the system passing through a steam superheater 30 and a waste heat boiler 29. After the steam superheater 30, the process gas is cooled to about 380 ℃. The generated superheated steam 10 exits at about 375 ℃ and can be used to drive the compressor in the plant, while the steam 11 from the waste heat boiler 29 is delivered to the steam drum 24. Boiler Feed Water (BFW) is added as stream 12, while stream 13 from drum 24 is used to generate steam in waste heat boiler 29. All the steam, which is generated in the waste heat boiler 23 of the reforming section and in the waste heat boiler 29 of the ammonia synthesis section, is superheated in the form of high pressure steam at a boiling point of 323 c via the steam stream 14 in the steam superheater 30 of the ammonia synthesis section. The cooled process gas containing ammonia is recovered as stream 15.

Turning now to FIG. 2, a schematic diagram of the steam superheater 30 of FIG. 1 is shown. The superheater includes a first (cold) compartment 301 and a second (hot) compartment 302, two tube sheets 303, 304, two shells 305, 306 having respective rear ends 307, 308, two U-shaped tube bundles 309, 310, and a transition chamber 311 and a transport tube 312. The duct extends from the first compartment 301 to the second compartment 302 along the long axis 320 of the superheater. The superheater further comprises a process gas inlet 313 and a process gas outlet 314 arranged as part of the transition chamber 311, as well as a steam inlet 315 arranged in the outer shell 305 of the first chamber 301 and a steam outlet 316 arranged in the outer shell 306 of the second chamber 302. Baffles 317 are disposed in the first and second compartments to deflect the steam flow and thereby improve heat transfer. The baffle also provides support for the tube bundle. The transition chamber 311 includes an inlet chamber 318 that extends directly to the process gas inlet 313 and is in fluid communication with the tube bundles 309, 310 that extend into the tubesheets 303, 304. The transition chamber 311 also includes an inlet chamber 319 extending directly into the process gas outlet 314, the inlet chamber 319 also being in fluid communication with the tube bundles 309, 310 extending into the tubesheets 303, 304. A dividing wall 321 extends along the delivery tube portion 312 to separate the inlet and outlet chambers 318, 319. Thus, the first and second compartments are connected in series for the vapor stream and in parallel for the process gas stream. Throttle valves 322 and 323 located in the outlet chamber 319 are used to control the amount of process gas passing through the first (cold) and second (hot) compartments and thus the vapor outlet temperature at the vapor outlet 316.

The process of the present invention is capable of providing saturated steam to the superheater, i.e. at a relatively low temperature (323 c). The steam entering at this temperature passes through the steam inlet 315 near the back end of the first (cold) compartment where it then flows through its shell side. The steam is thus superheated to 345 ℃ and at that temperature is sent via the transport pipe 312 to the second (hot) compartment of the steam superheater. The steam is further superheated and exits through steam outlet 316 as superheated steam at 375 deg.c. The process gas from the ammonia converter enters the superheater and enters the inlet chamber 318 of the transition chamber 311 through the process gas inlet 313 at 460 ℃. The process gas is split and sent to the first and second compartments via tube sheets 303, 304 into U-shaped tube bundles 309, 310. After passing through the U-tubes, the process gas enters the outlet chamber 319 via tubesheets 303, 304 through throttle valves 322, 323. The process gas from the first compartment enters the outlet chamber 319 at 373 deg.c, while the process gas from the second compartment enters at 403 deg.c. The mixed gas in the chamber reaches a temperature of 380 ℃ and exits through process gas outlet 314 for further cooling in a downstream waste heat boiler.

Claims (6)

1. The steam superheater (30) includes:

a first compartment (301) and a second compartment (302), wherein the first compartment (301) has an outer shell (305), a first tube sheet (303), a back end (307), a first tube bank (309), a baffle (317) and a steam inlet (315) adapted to the outer shell (305), and wherein the second compartment (302) has an outer shell (306), a second tube sheet (304), a back end (308), a second tube bank (310), a baffle (317) and a steam outlet (316) adapted to the outer shell (306);

a transition chamber (311) separating the first and second compartments, defined by a space between the first tube sheet (303) and the second tube sheet (304);

a duct (312) through the first tube sheet (303) and the second tube sheet 304) and thus through the transition chamber (311), which extends along the long axis (320) of the steam superheater (30) from the first compartment (301) to the second compartment (302);

a dividing wall (321) between the inlet chamber (318) and the outlet chamber (319);

said transition chamber (311) having a process fluid inlet (313) extending into an inlet chamber (318) of the transition chamber, the inlet chamber (318) being confined between the tube walls of the transport tube (312), the plate walls of the first tube sheet (303) on one side and the plate walls of the second tube sheet (304) on the opposite side, wherein the first tube bank (309) of the first compartment (301) extends into the first tube sheet (303) and the second tube bank (310) of the second compartment (302) extends into the second tube sheet (304);

said transition chamber (311) having a process gas outlet (314) extending from an outlet chamber (319) of the transition chamber, the outlet chamber (319) being confined between the walls of the duct (312), the walls of the first tube sheet (303) on one side and the walls of the second tube sheet (304) on the opposite side, wherein the first tube bundle (309) of the first compartment (301) extends into the first tube sheet (303) and the second tube bundle (310) of the second compartment (302) extends into the second tube sheet (304);

and wherein the first compartment (301) and the second compartment (302) are connected in series for the steam flow and in parallel for the process gas flow.

2. The steam superheater according to claim 1, wherein the outlet chamber (319) further comprises a first valve (322) and a second valve (323) disposed therein and in direct fluid communication with the first tube bank (309) of the first compartment (301) and the second tube bank (310) of the second compartment (302).

3. Steam superheater according to claim 1 or 2, wherein the first tube bundle in the first compartment is made of low alloy steel and the second tube bundle in the second compartment is made of stainless steel.

4. Steam superheater according to claim 1 or 2, wherein the direction of the steam superheater is longitudinal, and the first or second compartment further comprises a water outlet at its rear end for removing accumulated water.

5. A steam superheater according to claim 3, wherein the direction of the steam superheater is longitudinal, and said first or second compartment further comprises a water outlet at a rear end thereof for removing accumulated water.

6. Use of a steam superheater according to claim 1 in a process for producing ammonia from a hydrocarbon feedstock, said process comprising the steps of:

(a) passing the hydrocarbon feedstock through a reforming section and recovering synthesis gas from said reforming section;

(b) passing said synthesis gas through one or more waste heat boilers without the use of steam superheaters, wherein the synthesis gas is subjected to indirect heat exchange with a water-steam mixture, recovering steam from said waste heat boilers and passing said steam to one or more steam drums;

(c) passing the thus cooled synthesis gas of step (b) through a shift conversion stage for converting carbon monoxide in the synthesis gas to hydrogen and subsequently removing remaining carbon dioxide, carbon monoxide and methane in the synthesis gas by a scrubbing process and recovering a synthesis gas comprising nitrogen and hydrogen;

(d) passing the synthesis gas produced in step (c) through an ammonia synthesis section comprising catalytic conversion of the synthesis gas to ammonia by passage through one or more catalyst beds in an ammonia converter and recovering a process gas comprising ammonia from the one or more catalyst beds;

(e) passing said process gas containing ammonia through one or more steam superheaters in which steam from the one or more steam drums of step (b) is superheated, and recovering a superheated steam stream from said one or more steam superheaters;

(f) passing the thus cooled process gas of step (e) through one or more waste heat boilers in which the process gas is in indirect heat exchange with a water-steam mixture, recovering steam from the one or more waste heat boilers and passing said steam to the one or more steam drums of step (b),

wherein the step (a) comprises the steps of: passing a hydrocarbon feedstock through a primary reforming step to produce a partially reformed gas, passing the partially reformed gas through a heat exchange reforming step and a secondary reforming step and recovering a resulting synthesis gas stream from the heat exchange reforming step, wherein the partially reformed gas passed through the heat exchange reforming step is reformed by indirect heat exchange with the synthesis gas recovered from the secondary reforming step.