Urea plant with improved condenser and improved reactor internals

Abstract

The present disclosure relates to improvements in a stripping type urea plant by replacing the falling film type HP carbamate condenser with a horizontal submerged condenser combined with improvements in the internals of the reactor allowing to maintain a gravitational flow in the improved plant.

Description

Technical Field

[0001] This invention relates to the production of urea. Background Technology

[0002] A commonly used type of urea production process is the so-called stripping process, in which the high-pressure urea synthesis section includes a reaction zone, a stripping tower, and a condenser. Background reference for this type of method is Ullmann's Encyclopaedia of Industrial Chemistry, chapter Urea, 2010. This document mentions that some processes use a purely gravity-based recirculation flow, i.e., the main recirculation flow in the high-pressure (HP) loop maintaining the synthesis section, which is formed by flows from the stripping tower to the condenser, from the condenser to the reactor, and from the reactor back to the stripping tower.

[0003] This invention particularly relates to a CO2 stripping urea production process, wherein a urea solution from a reactor is stripped in a stripping tower, including a high-pressure section, by a feed CO2 gas.

[0004] Kaasenbrood and Chermin, in “The Urea Stripping Process,” Fertiliser Society, 1977, described an early embodiment of a CO2 stripping urea production process and apparatus. In this embodiment, gas from the top of the HP stripping tower flows to the head of a carbamate condenser. In the condenser tubes, the gas is partially condensed. The liquid-gas mixture exiting the condenser is introduced to the bottom of the reactor. The reactor, stripping tower, and condenser are spatially arranged such that the driving flow through these vessels is provided by a phase change and thus by a change in fluid density caused by heat supply in the stripping tower and heat removal in the condenser. The top of the stripping tower is approximately 10 m below the bottom of the reactor and condenser. The reactor operates adiabatably, achieved through the condensation of the gas mixture from the condenser. Figure 7 of Kaasenbrood 1977 shows that the condenser in this embodiment is a vertical shell-and-tube heat exchanger, with vapor rising in the shell, gas from the stripping tower supplied to the top of the tube bundle, and an outlet at the bottom for supplying fluid to the bottom of the vertical reactor. The bottoms of the reactor and condenser are at equal heights. Liquid is drawn from the top of the reactor and supplied to the liquid inlet of the upper chamber of the stripper.

[0005] An improved type of high-pressure condenser is described in EP0155735A1. Here, the gas to be condensed is introduced into the shell side of a shell-and-tube heat exchanger with horizontal tube bundles, particularly U-shaped tube bundles, where a continuous liquid phase is maintained in the shell-side compartments during operation. Thus, the condenser operates as a horizontally submerged condenser. The bottom of the vertical reactor is shown at substantially the same height as the fluid outlet of the pool condenser, which is located at the top of the pool condenser.

[0006] It is often desirable to increase the capacity of existing urea plants by maximizing the use of existing equipment and making minimal modifications. Such projects are referred to in the art as "modifications." Furthermore, many existing CO2 stripping urea plants with falling film high-pressure urethane condensers are still in operation. One aspect of this invention aims to provide a method for modifying such urea plants.

[0007] EP1036787A1 discusses a modification method in which the falling film condenser of the HP synthesis section is improved into a standing submerged condenser. Further reference to this method is US20080242890A1.

[0008] US2015343409A1 mentions replacing the "HPCC" (high-pressure urethane condenser) with a pool-type condenser as a known method of increasing the reactor volume of an existing device. This reportedly requires a complex equipment design. The document suggests increasing the volume by installing an additional reactor.

[0009] Figure 4B of US20030088126A1 shows an apparatus with a pool-type condenser, in which fluid from the condenser is supplied to the bottom of the upper compartment of a vertical combined reactor. Gas is released from the bottom compartment of the vertical combined reactor into the upper compartment. An ejector is used to supply a portion of the gas from the stripping tower to the bottom compartment of the vertical combined reactor.

[0010] EP0329215A1 illustrates a urea process utilizing an HP CO2 stripper, a vertical reactor, and a condenser. Liquid from the condenser is supplied to the bottom of the reactor, and gas from the condenser is supplied to the HP scrubber. A portion of the gas from the stripper is supplied to the bottom of the reactor using an ejector driven by a portion of the ammonia feed. Gas from the top of the reactor is supplied to the condenser. The document explains that if the piping connecting the condenser and the reactor, as well as the piping from the reactor to the stripper, is filled with liquid, these components act as connecting containers, allowing the liquid to be transported by gravity from the condensation zone through the reaction zone to the stripping zone. Non-condensable gas from a horizontal urethane condenser is supplied to the HP scrubber. The condenser is a horizontally mounted urethane condenser located 20 meters above ground level, employing a submerged condenser design with a diameter of approximately 3 meters. The reactor is placed on the ground at a height of approximately 18 meters. The stripper is placed on the ground at a height of approximately 11 meters.

[0011] US20110160486A1 describes a urea production process in which the process operates entirely by gravity flow, with the primary recycling of unconverted ammonia and carbon dioxide in the high-pressure synthesis section of the urea production method, making the use of energy-consuming pumps, compressors, or injectors redundant. Summary of the Invention

[0012] The present invention provides a method for modifying an existing urea plant, wherein the existing urea plant includes a high-pressure synthesis section comprising: a vertical urea reactor; a stripping tower having an inlet for gaseous CO2 feed; a high-pressure carbamate condenser configured to receive gas from the stripping tower, wherein the high-pressure carbamate condenser is a falling film shell-and-tube heat exchanger having a bottom outlet from the tube connected to a bottom inlet of the reactor, wherein the reactor includes one or more feed pipes for fluid from the condenser, the one or more feed pipes having an outlet at a first vertical level in the reactor; wherein the method comprises: replacing the high-pressure carbamate condenser with a replacement carbamate condenser, the replacement carbamate condenser being a horizontally submerged condenser, the horizontally submerged condenser being a shell-and-tube heat exchanger that condenses in a shell-side space, preferably wherein the replacement condenser is a pool condenser; and modifying at least one of the one or more feed pipes of the reactor to have an outlet at a second vertical level above the first vertical level. Preferably, the reactor includes a first feed pipe that receives gas from the condenser before and after the modification, and a second feed pipe that receives liquid from the condenser, and the method includes replacing the urethane condenser with a horizontally submerged condenser, and extending the first feed pipe for the gas to have an outlet at the second vertical level. In an alternative embodiment, the existing reactor (before the modification) does not include a feed pipe, and the modification involves adding one or more such feed pipes for the fluid from the condenser. The preference for the modified feed pipe, particularly the second vertical level, also applies to the addition of one or more feed pipes.

[0013] The second vertical level is preferably at least 1.0 m or at least 2.0 m higher than the first vertical level. The extended feed pipe preferably extends at least 1.0 m or at least 2.0 m. The first vertical level is, for example, less than 0.50 m from the bottom of the reactor.

[0014] A urea plant is also provided, comprising a high-pressure synthesis section, the high-pressure synthesis section comprising: a vertical urea reactor including a first feed pipe and a second feed pipe; a stripping tower having an inlet for gaseous CO2 feed; a high-pressure (HP) carbamate condenser configured to receive gas from the stripping tower, wherein the high-pressure (HP) carbamate condenser is a horizontally submerged shell-and-tube heat exchanger with horizontal tube bundles, wherein the gas from the stripping tower is condensed in the shell, preferably wherein the HP carbamate condenser is a pool condenser, and separate gas flow lines for the condensate from the first feed pipe to the reactor and for the second feed pipe from the condenser to the reactor; wherein the vertical distance between the outlet of the first feed pipe and / or the outlet of the second feed pipe to the bottom of the reactor is preferably at least equal to the height (i.e., diameter) of the tube bundle, or more preferably at least equal to the height or diameter of the shell of the pool condenser.

[0015] The device is preferably obtained through inventive improvements to existing devices.

[0016] A urea production process is also provided in the apparatus according to the invention, the process comprising supplying CO2 feed to the stripping tower and NH3 to the high-pressure carbamate condenser, and operating a high-pressure synthesis section at a pressure of at least 100 bar to form urea. Attached Figure Description

[0017] Figure 1 An exemplary existing urea device is illustrated schematically.

[0018] Figure 2 An exemplary urea device of the present invention is illustrated schematically.

[0019] Figure 3 A portion of an exemplary reactor of the urea plant of the present invention is shown schematically.

[0020] Any embodiments shown in the accompanying drawings are merely examples and do not limit the invention. Detailed Implementation

[0021] In one aspect, the present invention advantageously provides a method for improving existing urea plants by introducing a horizontally submerged condenser, while advantageously maintaining gravity flow in the synthesis section. The inventors believe that, in a preferred embodiment of the invention, this can be achieved by appropriately modifying the feed pipe in the reactor.

[0022] This invention provides a method for modifying an existing urea plant in its embodiments. In short, the existing urea plant is of the CO2 stripping type and includes a high-pressure synthesis section comprising a reactor, a condenser, and a stripping tower. The existing condenser is a falling film HP carbamate condenser (HPCC). The plant is improved by replacing the falling film HPCC with a horizontally submerged condenser (preferably a pool-type condenser). The pool-type condenser includes one or more U-shaped tube bundles. An example design of a pool-type condenser can be found in Nitrogen No. 222 (July-August 1996), page 29. Optionally, the pool-type condenser includes an extended interval between the bend of the U-shaped tube bundle and the shell to provide additional reactor volume. In operation, the U-shaped tube bundle is submerged in a liquid present and formed in the shell-side space. The U-shaped tube bundle has horizontal legs. The horizontally submerged condenser includes, for example, a distributor for distributing gas from the stripping tower in the shell-side space. Horizontal submerged condensers are typically shell-and-tube heat exchangers with tube bundles, a shell, and a shell-side space. They are configured to receive gas from a stripping tower in the shell-side space and cooling fluid, such as boiler feedwater, in the tubes.

[0023] Compared to falling film HPCC, pool-type condensers offer the advantage of increasing urea production capacity by allowing for a longer residence time of the urethane solution in which condensate forms. Therefore, urea production (tons of urea per hour) can be increased without modifying the HP stripping tower.

[0024] The degree of condensation in a pool-type condenser can be adjusted as needed, for example, by regulating the operating pressure of the steam generated in the tube bundle. Therefore, sufficient non-condensable gases can be retained to provide heat to the reactor.

[0025] Unlike falling film HPCCs, the liquid layer is maintained on the process side of the pool condenser, i.e., in the shell-side space of the pool condenser. In a pool condenser, gas from the stripper is supplied to the shell from the bottom, and the fluid outlet of the shell is at the top, resulting in a pressure drop in the liquid layer. Therefore, with a fixed gas pressure from the stripper, the gas and liquid from the pool condenser have lower pressures. The present invention advantageously provides for maintaining the fluid flow from the condenser to the reactor and the liquid flow from the reactor to the stripper, particularly advantageously based on gravity flow. In conventional apparatuses, the liquid flow from the condenser through the reactor to the stripper is maintained by "gravity flow," i.e., by utilizing the condenser's position above the top of the stripper in the apparatus, without using pumps or ejectors. Furthermore, in conventional apparatuses, non-condensable gas from the condenser is supplied to the bottom of the reactor. This provides the advantage that CO2 and NH3 contained in the non-condensable gas are condensed into carbamates in the reactor to provide heat to drive the endothermic urea formation reaction (carbamate dehydration) in the reactor. The advantage of this invention is that it supplies non-condensable gas from a pool condenser to the reactor in the improved apparatus to provide heat to the reactor. This invention is particularly advantageous in providing gravity flow in the improved apparatus.

[0026] In existing equipment, the urea reactor is a vertical urea reactor with a feed inlet at the bottom and a downpipe for drawing urea solution from the top of the reactor. The downpipe is also called an overflow pipe or drain pipe. The upper end of the downpipe acts as an overflow weir during operation, where the liquid level in the reactor is maintained. During operation, the liquid column is held within the downpipe.

[0027] The downpipe typically has a funnel at its upper end for degassing the urea solution. During operation, the urea level in the unit is maintained at the height of the upper end of the downpipe. The reactor has a separate gas outlet at the top. Improved reactors preferably also possess these features.

[0028] In this invention, supplying non-condensable gas from a horizontally submerged condenser to the reactor offers advantages, such as providing heat to the reactor by directly supplying a portion of the feed CO2 gas to the reactor instead of to a stripper, compared to an exemplary reference process. This reference process has a relatively low HP stripper efficiency (low stripping efficiency) and may result in excess NH3 in the condenser. The reference process involving directly supplying a portion of the gas from the stripper to the reactor presents challenges in terms of gas separation, which can be detrimental to gravity flow if the gas condenses at a low altitude in the reactor, and may require the use of ejectors given the large liquid column in the reactor.

[0029] In existing systems, the synthesis section includes, for example, a high-pressure scrubber. In the HP scrubber, exhaust gases from the top gas outlet of the reactor, including NH3, CO2, and inert substances, are scrubbed to recover CO2 and NH3 as a urethane solution. The inert components typically include passivating air and include non-condensable gases. The aqueous scrubbing liquid in the HP scrubber is typically a recycled urethane solution from the recovery section, and the water contained therein is also used as a solvent in the HP urethane condenser. For example, urethane solution from the HP scrubber is supplied to a falling film urethane condenser in existing systems using ammonia-driven injectors, and is also supplied, for example, in improved systems. For example, existing systems operate with an oxygen content of about 0.6% (e.g., at least 0.3% by volume O2 in the CO2 feed) to adequately passivate the high-pressure synthesis section, such as any components made of urea-grade stainless steel. For example, improved systems also operate with passivating air added to the CO2 feed at an amount of at least 0.3% by volume O2 relative to the CO2 feed.

[0030] In the prior art urea production process shown in Figure 22 of Ullmann's Urea 2010, the so-called Avancore® process, only the carbamate solution from the pool condenser is supplied to the bottom of the reactor, which is placed on the ground, with a liquid column in the flow line. Gas from the condenser is supplied to a medium-pressure scrubber. A portion of the CO2 feed is supplied to the bottom of the vertical reactor. Furthermore, in the prior art urea production process shown in Figure 28 of Ullmann's Urea 2010, namely the ACES21 process, a vertical submerged condenser is used, in which the carbamate solution is supplied to the bottom of the reactor via an ejector using feed NH3, and gas from the condenser is supplied to a medium-pressure absorption unit. A portion of the CO2 feed is supplied to the reactor.

[0031] In a preferred embodiment of the invention, the method involves improvements to the interior of the reactor, advantageously without any sealing of a portion of the reactor.

[0032] Preferably, prior to the improvement, the vertical urea reactor includes one or more feed pipes for conveying gas and liquid from the HP urethane condenser into the reactor space at a first vertical level within the reactor. Throughout this document, the term "feed pipe" is used extensively and can include pipes and conduits, and may also be referred to as a supply device. In this embodiment, as part of the improvement, the existing feed pipe has been modified.

[0033] In an alternative embodiment, the existing reactor has one or more inlets for gas and liquid from an existing falling film HP urethane condenser, but no feed pipe inside the reactor, and one or more feed pipes are added as part of the improvement. Preferred embodiments of the improved feed pipes also apply to the added feed pipes. Preferably, any added gas feed pipe has a length of at least 1.0 m or at least 2.0 m. This alternative embodiment can be conveniently used in cases where an existing vertical urea reactor has an inlet opening at the bottom for fluid from an HP urethane condenser but no (vertically) feed pipe for such fluid extending into the reactor space.

[0034] Preferably, the reactor includes a feed pipe for liquid from the HP urethane condenser and a (separate) feed pipe for gas (especially before and after the improvement); preferably, the apparatus includes flow lines for separating the gas and liquid from the condenser to the reactor (especially before and after the improvement), and preferably, the condenser before and after the improvement includes a gas / liquid separator. The gas / liquid separator is, for example, an overflow weir, provided, for example, by an outlet opening in the wall of a vertically upward outlet pipe mounted on the upper side of the pool condenser, which has a suitable gas outlet opening at the top.

[0035] For example, the feed pipe for liquids is a straight, vertical pipe with an upper outlet end. For example, the feed pipe for gases is a pipe with a downwardly curved outlet end (gooseneck pipe).

[0036] The feed tube for liquids can be relatively short, extending only slightly from the bottom wall of the reactor (e.g., the hemispherical head).

[0037] In a highly preferred improvement, at least the gas feed pipe is modified to introduce gas from the HP urethane condenser at a second vertical level within the reactor, where the second vertical level is higher than the first vertical level. Here, the vertical level refers to the distance between the bottom of the reactor and the fluid inlet point. Therefore, the vertical distance between the outlet end of the gas feed pipe and the bottom of the reactor is increased by this modification. The second vertical level is typically provided (defined) by the open end of the gas feed pipe. Preferably, the gas feed pipe is extended such that the outlet is at a higher level within the reactor, for example, extending at least 1.0 m or at least 2.0 m.

[0038] The inventors recognized that in the improved apparatus, the gas is introduced from an extended gas feed pipe into a shorter liquid column in the reactor, i.e., at a shorter vertical distance to the liquid level (held at the top of the reactor at the inlet of the drain pipe), and is therefore at a lower pressure compared to when it is introduced at the first vertical level (as before the improvement), and this pressure reduction can very advantageously help maintain gravity flow operation in the improved apparatus by compensating for the pressure of the liquid level maintained in the horizontally submerged condenser.

[0039] Preferably, the feed pipe for liquids is also extended. Preferably, the feed pipe for gases and / or liquids is modified to introduce gas and liquid from the HP urethane condenser at a second vertical level within the reactor. Introducing gas and liquid at the same vertical level within the reactor can provide a more robust process design.

[0040] Compared to the less preferred method involving sealing off the bottom of the reactor, this embodiment has the advantage that the bottom of the reactor can also be used in operation, i.e., the portion of the reactor below the second vertical level, since the bottom of the reactor will be filled with the reaction medium, particularly a liquid, during operation.

[0041] The improved reactor bottom is the section between the reactor bottom and the second vertical level.

[0042] If gas and liquid are supplied from a horizontally submerged condenser (e.g., a pool condenser) to a reactor with separate flow lines and feed pipes, the two feed pipes preferably have outlets at the same vertical level (i.e., a second vertical level). This preferred feature ensures that the pressures at the respective inlets of the flow lines in the horizontally submerged condenser (e.g., a pool condenser) are equal. In another embodiment, the outlet of the gas feed pipe is modified to be at a higher vertical level than the outlet of the liquid feed pipe.

[0043] The height of the bottom (and the distance between the bottom of the reactor and the second vertical level) is typically substantially the same as the diameter or height of the pool condenser, for example, about 3 meters. In a preferred exemplary embodiment, the gas feed pipe (and optionally the liquid feed pipe) extends the height of the pool condenser in the vertical direction. In particular, the vertical distance between the first and second vertical levels is the same as the diameter of the horizontal cylindrical form of the pool condenser. This preferred configuration is particularly suitable if the cylindrical horizontal vessel of the pool condenser is filled with liquid as a continuous phase during operation and the liquid level is maintained in a dome mounted on top of the horizontal cylindrical vessel during operation. These preferred heights also apply to any added feed pipes in embodiments involving the addition of feed pipes.

[0044] Preferably, the horizontally submerged shell-and-tube heat exchanger comprises a covered horizontal cylindrical container holding a horizontal tube bundle. The container may be capped, for example, with a half-cap at one end and a tube sheet at the other. Preferably, the vertical distance (h2) between the outlet of the second gas feed pipe and the bottom of the reactor is equal to or greater than the diameter of the horizontal cylindrical container. This embodiment is preferably used with a dome for maintaining the liquid level on the upper side of the horizontal container, the dome having a liquid outlet and a separate gas outlet connected to the liquid feed pipe and gas feed pipe, respectively. The gas outlet and liquid outlet of the dome are preferably vertically spaced apart from each other.

[0045] It was observed that, particularly for horizontal pool condensers with a length greater than the reactor diameter, the reactor volume provided by the pool condenser was greater than the reactor volume of the bottom portion.

[0046] In a possible implementation where the feed pipe is modified to introduce fluid at a second vertical level, the portion of the reactor below the second vertical level is lined with duplex stainless steel (i.e., duplex ferritic austenitic steel, such as, but not limited to, UNSS32906 steel). This can be useful because the liquid in the bottom portion may be stagnant. Furthermore, the liquid in the bottom portion will contain urethanes at relatively high temperatures and may therefore be highly corrosive.

[0047] In a more preferred embodiment, a fluid containing a passivating agent, such as a fluid containing O2, is introduced into the bottom portion. This preferred embodiment is applicable to urea production processes; accordingly, the urea production apparatus of the present invention preferably includes a reactor having a bottom portion having an inlet for a fluid containing O2, and more specifically, a suitable flow line for the fluid containing O2 to enter the bottom portion of the reactor.

[0048] In existing urea plants, passivation with oxidizing agents (typically O2) is frequently used to prevent corrosion of equipment in the high-pressure synthesis section that comes into contact with urethane-containing liquids during operation. Typically, some air is mixed into the CO2 gaseous feed for these purposes. In CO2 stripping type urea plants, this is used for passivation of the stripping tower and condenser, and also for passivation of the reactor by supplying gas from the condenser to the reactor, or by supplying a portion of the CO2 feed or a portion of the gas from the stripping tower to the reactor.

[0049] Passivating agents are fluids containing O2, such as gases containing O2 or liquids containing dissolved O2.

[0050] In a preferred embodiment, the passivating agent introduced into the bottom portion is therefore an O2-containing fluid, is gaseous, and is, for example, part of air, or a CO2 feed (containing O2), or part of gas from a stripping tower. Introducing gas from the stripping tower into the reactor typically requires an ejector.

[0051] In another embodiment, the passivating agent is a liquid containing dissolved O2, such as a urethane solution from a high-pressure scrubber. The apparatus preferably includes an HP scrubber connected to receive exhaust gas and an aqueous scrubbing liquid from the reactor (top outlet of the reactor). The scrubber is preferably positioned above the reactor, and the scrubbing process involves condensing the gas into a urethane solution during operation. The exhaust gas from the reactor includes NH3, CO2, and inert substances, including O2. The liquid containing dissolved O2 is preferably introduced into the bottom portion of the reactor via a liquid distributor (e.g., an injector) for horizontal gas distribution. Introducing the liquid containing dissolved O2 at the bottom of the reactor helps prevent corrosion from stagnant liquid urethane solution in the bottom compartment.

[0052] In a possible implementation, the bottom portion of the reactor is lined with duplex stainless steel and combined with the introduction of fluid containing a passivating agent into the bottom portion of the reactor.

[0053] In a preferred embodiment, the method of improving the urea plant includes providing an inlet of oxygenated fluid to the reactor in a bottom reactor compartment located between the bottom of the reactor and a second vertical level.

[0054] In a preferred embodiment, the reactor is improved by providing an additional inlet for an additional fluid and preferably by providing an internal supply device (e.g., a pipe, tube, or conduit) for introducing the additional fluid into the reactor at a third vertical level below the second vertical level (in methods for improving existing urea plants). Thus, the third vertical level is located between the bottom of the reactor and the second vertical level. The internal supply device is inside the reactor. The first and third vertical levels may be the same or different. Preferably, the third vertical level is close to the bottom of the reactor. Therefore, the improved reactor includes an internal supply device for vertically introducing the additional fluid into the reactor between the bottom of the reactor and the point where gas and liquid are introduced from the pool condenser. This additional fluid contains a passivating agent, such as O2, and can be used to passivate the bottom portion of the reactor. This is particularly advantageous if the reactor wall surface exposed to the urethane solution is composed of austenitic steel (e.g., 316L steel, also known as EN 1.4404 and UNS S31603) (e.g., as a lining). The urea production apparatus of the present invention preferably includes a reactor with such an inlet and internal supply device, and the urea production process is preferably carried out in an apparatus having such a reactor.

[0055] Furthermore, preferably, the additional fluid is an airflow containing CO2, and the reaction of the gaseous CO2 with NH3 to form urethane occurs in the bottom part of the reactor, which also provides heat for the urea formation reaction in the bottom part of the reactor.

[0056] It is known in the art that stagnant urethane-containing liquids have an increased risk of corrosion due to passivation loss caused by the depletion of passivating air.

[0057] In a possible implementation, the liquid at the bottom of the reactor is discharged, for example, to a stripping tower or a scrubber injector, with or without a passivating agent introduced into the bottom section. By discharging the liquid, i.e., draining it, stagnant liquid in the bottom section is avoided, thereby reducing the risk of corrosion. However, the conversion rate in the drained bottom section is not optimal.

[0058] In this implementation, the existing piping in the reactor is modified to accommodate the discharge of fluid flow from the bottom of the reactor.

[0059] For example, the existing drain pipe of the reactor is modified to have small perforations in the bottom section of the reactor. Due to the density difference of the fluid in the drain pipe and the bottom section of the reactor, the liquid will flow from the bottom section into the drain pipe.

[0060] In one implementation, the reactor is improved by adding a pipe for discharging fluid flow from the bottom portion of the reactor.

[0061] In a further preferred embodiment, in a urea production process carried out in a urea plant including a reactor, liquid is moved from the liquid feed pipe outlet (located at a second vertical level in this embodiment) to a vertical level below the second vertical level in the bottom section. This provides the advantage of better utilization of the reaction volume provided by the bottom section compared to liquid that is substantially stagnant in the bottom section. The reactor is preferably configured accordingly. For example, the bottom reactor section is provided with a vertical divider, preferably a tubular or annular vertical divider, to create a siphon flow to create two vertical compartments in the bottom section that are substantially separated by the vertical divider, particularly a first compartment inside the vertical divider and a second compartment between the vertical divider and the reactor wall. The vertical divider separates the two compartments at a height that is at least 80% or at least 90% of the height of the bottom compartment (i.e., the length of the divider is at least 80% or at least 90% of the distance between the reactor bottom and the second vertical level h2). During operation, the siphon flow descends from the liquid feed pipe outlet through the second vertical compartment into the bottom section, and ascends through the first vertical compartment (i.e., inside the tube, i.e., inside the tubular or annular vertical divider).

[0062] Preferably, gas is preferentially introduced into the bottom of the first vertical compartment (e.g., into the bottom of a tube) to create a density difference in the fluids in the first and second compartments to drive a siphon flow. Therefore, the first compartment preferably includes a supply device for the gas flow, such as a feed pipe, in its bottom portion, the gas advantageously comprising O2 for passivation. Thus, the gas is the preferred additional fluid and is referred to as the additional gas flow. The vertical separator is preferably a vertical annular separator, more preferably a vertical tubular separator (e.g., a vertical tube), having a liquid passage through (with an opening in the tubular wall) and / or below the bottom portion of the separator for liquid to flow from the bottom of the second compartment to the bottom of the first compartment. The vertical separator is, for example, a tube, and has, for example, a length of at least 0.5 m to provide a favorable degree of siphon flow.

[0063] The first compartment is, for example, inside a tubular separator, and the second compartment is between the tubular separator and the reactor wall. The first compartment is, for example, eccentrically located in the middle of the bottom. The second vertical compartment is typically an annular space surrounding the first vertical compartment, i.e., the annular space between the tubular separator and the vertical wall of the reactor. Advantageously, the feed pipe for the liquid is a straight vertical pipe passing through the second compartment. Preferably, the outlet of the feed pipe for the liquid is outside the vertical protrusion of the first vertical compartment. The horizontal spacing between the first vertical compartment and the outlet of the liquid feed pipe provides the advantage that the liquid can easily flow downward in the second compartment.

[0064] Preferably, the gas feed pipe extends through the tubular separator, such that the first compartment is an annular space between the gas feed pipe and the tubular separator. Therefore, preferably, the supply device for the additional fluid (e.g., for the additional gas) is configured to surround the annular space of the gas feed pipe. In this way, it is advantageously unnecessary to create additional openings in the reactor wall / shell for improvement. In practice, the inner tube can be inserted through the existing gas feed pipe.

[0065] Optionally, the improved reactor includes a tray above a second vertical level, i.e., above the outlet of the gas feed line (and preferably also the liquid feed line), for example, at most 1.0 m above the outlet, or more preferably at most 0.50 m above the outlet. Optionally, the improvement involves removing the tray as needed to create tubing for one or more extended feed lines.

[0066] The tray preferably comprises a horizontal plate. Preferably, the tray includes a downwardly extending edge that forms the periphery of a region of the tray (plate), and the plate includes an opening in that region. Preferably, the opening is small enough that a gas layer forms below the plate in that region during operation, bounded by the downward edge, and the edge extends into the liquid during operation. In this way, the upward-flowing liquid is transferred horizontally and radially outward toward the reactor wall below the tray. Preferably, the tray and the reactor wall provide a liquid passage for upward flow of liquid beyond the tray, for example, through the gap between the tray and the wall. The horizontal movement of liquid toward the reactor wall also facilitates the flow of liquid into a second vertical compartment.

[0067] Advantageously, in embodiments where a passivating gas is supplied to the bottom of the reactor, more preferably in combination with a siphon flow to prevent stagnant liquid at the bottom of the reactor, an existing reactor made of urea-grade austenitic steel can be used without the need for duplex stainless steel components (although the use of duplex stainless steel is possible). The process is carried out, for example, with 0.3-1.0 vol% O2 in the CO2 feed for passivation, and thus at approximately the same O2 level as before the modification.

[0068] In a possible implementation, the bottom reactor section is provided with a vertical divider to generate a siphon flow, creating two vertical compartments in the bottom section that are substantially separated by the vertical divider, specifically a first compartment inside the vertical divider and a second compartment between the vertical divider and the reactor wall. The vertical divider separates the two compartments over a portion of the reactor length, i.e., a portion of the reactor height. In operation, the siphon flow enters the bottom section from the liquid feed pipe outlet downward through the second vertical compartment and upward through the first vertical compartment (i.e., inside the vertical divider, which is preferably a tubular divider).

[0069] Preferably, gas is introduced by a gas feed pipe into the vertical middle section of an annular or tubular vertical divider that extends vertically above and below the outlet of the gas feed pipe, and this vertical divider allows liquid to flow from the second compartment into the first compartment, for example through an opening in a liquid vertical divider or an annular opening between the bottom of the vertical divider and the bottom of the reactor. Preferably, a gas distributor, such as a horizontal tray, is arranged in the first compartment (in the tubular divider) above the gas feed pipe and close to the gas feed pipe.

[0070] In embodiments where only the gas feed pipe extends to have an outlet at a higher vertical level, the liquid feed pipe outlet can be kept at a low height (a small vertical distance to the bottom of the reactor) and can be used to supply oxygenated liquid (from the condenser) to the bottom portion of the reactor.

[0071] In conjunction with the embodiments and preferred features discussed above, one aspect of the present invention relates to a method for modifying an existing urea plant.

[0072] Figure 1 A non-limiting example of an existing device improved in this method is illustrated schematically. In the following discussion, references to... Figure 1 This is for reference only, and is not limited to this implementation method.

[0073] Existing urea plants include a high-pressure (HP) synthesis section and at least one recovery section. The HP synthesis section operates under high pressure, i.e., at an absolute pressure of at least 100 bar, preferably at least 120 bar, and is used to react NH3 and CO2 to form urea. In the synthesis section, NH3 and CO2 react under urea formation conditions (at least 160°C and at a high pressure of at least 100 bar, HP) to form a urea synthesis solution comprising urea, water, ammonium carbamate (hereinafter: carbamate), and NH3 and CO2.

[0074] The HP synthesis section comprises a reactor (R), a stripping tower (S), and an HP carbamate condenser (C). The reactor is a vertical urea reactor, typically with a half-head bottom and a half-head top (not shown) and a cylindrical shell. The urea reactor is equipped with a downcomer (D) to draw urea solution (4) from the top of the reactor and allow the liquid to flow downwards in a column. The downcomer is, for example, a vertical pipe with a degassing funnel at the top. The downcomer passes through the bottom, for example, through the half-head bottom. During operation, a liquid level is maintained in the reactor, and the funnel of the downcomer acts as an overflow weir, where the liquid column is held in the downcomer. Thus, during operation, non-condensable gases are separated from the liquid at the top of the reactor.

[0075] In one embodiment, the drain pipe has perforations in its bottom portion to discharge liquid from a portion of the reactor (not shown).

[0076] The reactor has at least one fluid inlet (F1, F2) at the bottom and a gas outlet (7) at the top. Preferably, the reactor has a liquid inlet (F1) and a separate gas inlet (F2) at the bottom. The liquid inlet and the gas inlet preferably each have a feed pipe extending into the reactor. The gas inlet feed pipe is provided with, for example, a gooseneck, such as a 180-degree pipe, so that the gas flows downward in a downstream branch of the gooseneck pipe.

[0077] The vertical position of the outlet opening of the gas feed pipe defines the first vertical height (h1).

[0078] For practical reasons and for proper flow, the height h1 is, for example, 0.10 to 1.0 m, typically 0.10 to 0.50 m.

[0079] The openings of the feed pipes are preferably at the same height (h1) in the reactor, that is, the outlets of the gas feed pipe and the liquid feed pipe have the same vertical level, and the outlets are at the same vertical distance from the top opening of the drain pipe.

[0080] Vertical urea reactors preferably include trays, such as horizontal trays with perforations. Preferably, the bottom tray is arranged vertically above the gas and liquid feed pipes.

[0081] Existing (and improved) apparatuses include one or more HP stripping towers, which are falling film heat exchangers with vertical tube bundles, configured to receive urea solution (4) from a reactor (particularly from a downpipe) at a top inlet into the tube bundle. The urea solution (4) contains unconverted carbamates. The stripping tower has a gas top outlet (1), a liquid outlet (6) for the urea solution from the bottom of the tube bundle, and an inlet (5) for receiving CO2 at the bottom of the tube bundle as stripping gas. In existing apparatuses, preferably, all gaseous CO2 feed from the synthesis section is used as stripping gas in the stripping tower. A heating fluid such as steam is used on the shell side. Heating and stripping gas promote the dissociation of carbamates in the urea solution in the tubes and the release of NH3 and CO2 from the gas stream (1). The liquid inlets of the stripper are vertically spaced below the bottom of the reactor (i.e., there is a vertical gap between the liquid inlets at the bottom of the reactor and the top of the stripper), and during operation, a liquid column (h3) is maintained in the liquid flow line for the urea solution from the drain pipe to the stripper. This liquid column facilitates the gravity flow of the urea solution (4) into the stripper. The stripper is properly placed on the ground of the unit.

[0082] The liquid outlet of the stripping tower is connected to the recovery section, specifically the decomposer that supplies urea solution to the recovery section. Gas from the decomposer is condensed in a condenser to form a carbamate solution, which is recycled back to the synthesis section, preferably back to the condenser, and more preferably through a scrubber.

[0083] An HP urethane condenser is connected to receive gas (1) from a stripping tower, preferably all gas from the stripping tower in an existing unit; this gas contains NH3 and CO2. In existing units, the HP urethane condenser is a falling film condenser, i.e., a shell-and-tube heat exchanger with a vertical tube bundle, the gas to be condensed is supplied to the top of the tube bundle, and the liquid and uncondensed gas are drawn from the bottom of the tube bundle. The liquid (2), i.e., the condensate, is the urethane feed stream and is supplied to the reactor. In existing units, the vertical tube bundle is typically a straight tube bundle. Cooling fluids are disposed in the shell, such as steam rising in the shell. In existing units, non-condensable gas (3) typically from the bottom of the condenser is also supplied to the reactor inlet, including passivated air contained in the CO2 feed used as stripping gas in the stripping tower. Gas (3) contains NH3 and CO2, and its condensation in the reactor provides heat to drive the endothermic urea formation reaction in the reactor (R).

[0084] In the shell-side space of the HP urethane condenser, steam (13) rises from the boiler feedwater.

[0085] The bottom of the condenser is positioned vertically above the liquid inlet of the stripping tower, with sufficient vertical spacing to provide the driving force for gravity flow.

[0086] The synthesis section of the existing apparatus preferably includes an HP scrubber (SC) that receives exhaust gas (7) from the gas outlet at the top of the reactor. This exhaust gas contains NH3, CO2, and inert substances; the inert substances include at least O2 for passivation in the synthesis section, and typically also N2 contained in the air used for passivation. In operation, passivating air is added to the CO2 feed, particularly to the CO2 feed from the stripping tower. Existing apparatuses operate, for example, by adding air to the CO2 feed in an amount equivalent to at least 0.3 vol% O2 in the CO2 feed. Furthermore, the NH3 feed and CO2 feed may also contain inert components such as Ar and N2. The scrubber also receives a scrubbing liquid (8), particularly an aqueous scrubbing liquid, and more particularly a urethane solution from the recovery section. The scrubber is configured for gas / liquid contact to scrub CO2 and NH3 from the gas to form the urethane solution, and has a gas outlet (9) and a liquid outlet (10) for the urethane solution.

[0087] The existing apparatus also includes a supply line (11) for NH3 feed to the synthesis section, particularly to the existing condenser, preferably via an ejector (Ej). In the improved apparatus, this supply line is modified to connect to a replacement condenser. The ejector, driven by the NH3 feed, is used to convey the carbamate solution (10) from the scrubber to the condenser. For example, an ejector is used if the scrubber is located at a height below the top of the reactor.

[0088] The combined flow (12) from the ejector is supplied to the condenser. It can be observed that the gas pressure at the top of the reactor is lower than the gas pressure in the condenser due to the liquid column in the process side of the reactor. It is also observed that the ejector (Ej) is a liquid-liquid ejector for the relatively small carbamate flow from the scrubber and is not part of the main loop of the synthesis section. The main loop of the synthesis section is provided by the reactor, condenser and stripper and the gas line (1) from the stripper to the condenser, the liquid and gas flow lines (2, 3) from the condenser to the reactor and the liquid flow line (4) from the reactor to the stripper.

[0089] In this embodiment, liquid is discharged from the bottom portion of the reactor and supplied to the ejector (not shown).

[0090] like Figure 1 As shown, with a fixed pressure p0 at the gas outlet (1) of the stripper (which is also the pressure at the condenser outlet), the pressure of the urea solution at the inlet (4) is higher (h3) because the bottom of the reactor is higher than the top of the stripper, and the liquid column is maintained in the flow line from the bottom outlet of the reactor to the inlet of the stripper (as shown). Sufficient height h3 of the liquid column and sufficient vertical spacing between the stripper and the reactor are used to compensate for dynamic losses in the main fluid loop of the synthesis section. The bottom of the condenser is located at approximately the same height as the bottom of the reactor. Sufficient vertical distance is maintained between the top of the stripper and the bottom of the condenser to ensure gravity flow of liquid in the synthesis section from the condenser through the reactor to the stripper.

[0091] Figure 2 A non-limiting example of the improved device is shown schematically. Unless otherwise stated, the reference numerals are consistent with... Figure 1 Same. The following discussion will use the term "same". Figure 2 This invention relates to, but is not limited to, this embodiment. Therefore, the present invention relates to methods for improving existing apparatus to obtain improved apparatus, and to the improved apparatus itself (e.g., also as a new apparatus), and to a urea production process performed in the improved apparatus.

[0092] In the improved apparatus, the falling film HP urethane condenser is replaced with a replacement HP urethane condenser (PC), which is a horizontally submerged urethane condenser, preferably a pool-type condenser. The replacement condenser is a shell-and-tube heat exchanger having a tube bundle (201) and a shell (203). The tube bundle is a horizontal tube bundle, preferably a U-shaped tube bundle. The replacement condenser operates with the condensation of gas from the stripping tower in the shell-side space of the shell (203) and the cooling fluid in the tubes. The shell (203) is typically a horizontal vessel, such as a cylinder, having a half-head and tube sheets at horizontally opposite ends, having a horizontal length (i.e., from the half-head to the tube sheet, along the tube bundle) and a diameter; the length is typically greater than the diameter.

[0093] The cooling fluid is, for example, boiler feedwater used to rise the steam (13) inside the tubes. Two or more types of cooling fluids may also be used in the corresponding tube bundle.

[0094] The urethane condenser comprises a tube bundle (201), preferably a U-shaped tube bundle, in the case of a U-shaped tube bundle having horizontal legs of the tubes. The diameter of the condenser (container) is, for example, at least 2 m. During operation, the liquid layer in the condenser is, for example, in the range of 2 to 4 m. The condenser container is typically cylindrical.

[0095] The device includes, for example, a steam drum connected to the tube bundle for separating steam from the liquid. The liquid, namely boiler feedwater, is typically returned to the tube bundle.

[0096] The tube bundle is submerged in liquid within the shell-side space during operation. In other words, the liquid level in the shell-side space of the urethane condenser is above the upper tubes of the tube bundle. Therefore, all tubes of the tube bundle are submerged during operation. In the case of a horizontal U-shaped tube bundle, all straight legs (arranged horizontally) are submerged in liquid during operation.

[0097] The liquid level can be maintained in the horizontal cylindrical container of the HP urethane condenser, or in the dome (204) set at the top of the container.

[0098] Gas (1) from the stripper is introduced from the bottom into the pool condenser, typically using a distributor (205) for horizontal gas distribution. The distributor is at least partially mounted (located) below the tube bundle. Thus, the replacement condenser includes a distributor in the shell-side space, particularly at its bottom, where the distributor receives gas from the stripper. The distributor is typically a manifold with numerous outlet openings for gas at the top, distributed in the horizontal plane and along the length of the vessel. Optionally, in the case of a U-shaped tube bundle, the distributor extends beyond the bend of the tube bundle. This allows for better utilization of the volume of the shell-side space of the replacement condenser to provide residence time, and urea formation can be achieved in the replacement HP urethane condenser.

[0099] Typically, the urethane solution (12) from the ejector is introduced separately into the condenser. Typically, the urethane solution from the urethane condenser in the unit’s recovery section, either directly or indirectly, is supplied to the shell-side space, for example, through a scrubber and ejector.

[0100] Typically, a portion or all of the NH3 feed is also supplied as a liquid to the shell-side space, for example, in combination with a carbamate solution. A second distributor in the shell-side space can be used to distribute the NH3 feed. This second distributor preferably extends only below the tube bundle.

[0101] Horizontal submerged condensers are typically so-called pool condensers. The condenser has fluid outlets at the top, specifically a gas outlet and a liquid outlet. The liquid outlet is arranged vertically above the tube bundle. In operation, a liquid layer with a height h4 is maintained in the horizontal submerged condenser, the height of which is greater than the diameter (specifically the height) of the tube bundle, typically at least the diameter of the condenser. Therefore, the pressure of gas (2) at the gas outlet is (p0 - p...). h4 The condenser operates when the liquid level in the condenser is lower than that at the gas outlet.

[0102] Gas can be supplied to the reactor by extending the gas feed pipe to a height h2, for example, to the same height h4, and maintaining the liquid level at the top of the reactor at a constant vertical position. In other words, the outlet of the gas feed pipe is located in the reactor where the liquid pressure is (p0 - p) h4 The position below. Here, a larger height h2 indicates a smaller vertical distance to the top of the drain pipe compared to existing devices. In the embodiment, only the gas feed pipe is extended. Preferably, the liquid feed pipe is also extended (as shown) to have a robust design in which the liquid and gas are introduced into the reactor from the corresponding feed pipe at the same vertical level in the reactor and therefore at the same pressure.

[0103] Preferably, the location of the reactor remains unchanged during the modification, particularly the vertical and horizontal dimensions of the bottom of the reactor remain unchanged.

[0104] Gas (2) is released from the second feed pipe (F2) at a height h2 from the gooseneck into the liquid phase in the reactor, and thus into the liquid in the reactor.

[0105] The diameter of the reactor is typically the same as that of the horizontal condenser (e.g., 3m), and the volume of the horizontal condenser (PC) is larger than the bottom portion of the reactor defined by the height h2.

[0106] In the synthesis section of the improved apparatus, a scrubber is preferably used, along with an ejector for conveying the carbamate solution from the scrubber to the condenser.

[0107] In the improved apparatus, the reactor, for example, has a drain outlet for urea solution at the bottom, which is connected to the urea solution inlet of the stripping tower, as described above. In the improved apparatus, the bottom compartment of the reactor is, for example, lined with duplex stainless steel.

[0108] In a preferred embodiment, the gas feed pipe extends in the improved device, and it is also preferred that the liquid feed pipe extends in the improved device.

[0109] Preferably, the distance from the second vertical level (h2) to the bottom of the reactor is at least equal to (or greater than) the height of the tube bundle. Note that the height of the tube bundle is less than the height of the container.

[0110] Figure 3 The accompanying drawings illustrate a non-limiting example of a preferred embodiment of the improved device. Unless otherwise stated, reference numerals are used interchangeably with... Figure 2 Same. Dashed lines represent fluid flow patterns. The following discussion will use the term "same". Figure 3 This is for reference only, and is not limited to this implementation method.

[0111] Preferably, the reactor of the improved device includes an inlet (301) for fluid in the bottom portion, particularly a third inlet. Preferably, this inlet is provided with a distributor (302) for horizontally distributing the fluid. Preferably, the fluid at the third inlet is gas. Preferably, the third fluid includes O2 and serves to passivate the bottom compartment (303) between the bottom of the reactor (typically half-head) and the outlet opening of the gas feed pipe (F2) at a second height h2. Therefore, supplying this bottom compartment with a fluid containing O2, preferably gas, helps prevent corrosion of the bottom compartment.

[0112] As another preferred embodiment, the bottom compartment of the reactor includes a vertical divider (304) below the second vertical level h2, which restricts the upward flow of gas from the third inlet (301), particularly from the distributor (302). As discussed, the vertical divider specifically separates the first compartment (305) inside the vertical divider from the second compartment (305) between the vertical divider and the reactor wall (307) or shell.

[0113] Therefore, the fluid inside the vertical separator, i.e., the second compartment (306), has a relatively low density. The vertical separator provides orifices for liquid inlet, such as orifices between the distributor and the vertical separator, or openings within the vertical separator. Thus, the relatively low-density fluid inside the vertical separator draws in liquid, causing it to flow downwards from the outlet of the first feed pipe into the bottom end of the vertical separator. This provides movement of the liquid in the second compartment (306) of the bottom compartment, which ensures passivation of the reactor walls (exposed to the second compartment) and helps prevent corrosion from stagnant urethane-containing liquid.

[0114] Preferably, the vertical divider is arranged such that substantially all gas from the third inlet flows into the first compartment (305), for example, the first compartment (305) is located directly above the distributor (302) for gas from the third inlet. Preferably, a gap is provided between the bottom of the vertical divider and the partition, allowing liquid to selectively enter the first compartment from the second compartment at the bottom of the vertical divider; preferably, the vertical divider is a tube without perforations in its wall.

[0115] Preferably, the feed pipe extends through the first compartment, and more preferably a gas feed pipe, to provide a relatively high gas content and low density in the fluid within the first compartment. For example, the first compartment is a relatively thin ring surrounding the gas feed pipe.

[0116] A conduit (301) and distributor (302) providing an inlet (301) for the fluid in the bottom section can also be used to drain liquid from the bottom section of the reactor. The liquid can be supplied to, for example, an ejector (Ej). In possible embodiments, the liquid in the bottom section of the reactor is drained, for example, to an ejector in a stripping tower or scrubber, with or without the introduction of a passivating agent in the bottom section. Draining helps prevent the formation of stagnant, oxygen-deficient urethane-containing liquid in the bottom section of the reactor.

[0117] The preferred method for improving existing urea equipment is from Figure 1 The existing apparatus is modified by adding a replacement HP urethane condenser, which is a horizontally submerged condenser, preferably a pool-type condenser, as discussed earlier herein, and by extending the gas feed pipe, preferably two feed pipes, to improve the reactor. Preferably, a third inlet is added to the reactor, and preferably a vertical divider is also installed in the modification of the existing apparatus. Suitably, the HP scrubber is not modified during the modification.

[0118] The present invention also provides a urea production process carried out in the urea plant of the present invention, the process comprising supplying a CO2 feed to a stripping tower and supplying NH3 to a high-pressure urethane condenser, which is a horizontally submerged condenser, preferably a pool condenser. The process includes operating a high-pressure synthesis section at a pressure of at least 100 bar to form urea, typically at a temperature of at least 160°C. In this process, the liquid flow from the condenser to the reactor and from the reactor to the stripping tower is a gravity flow, i.e., no pumps or ejectors are used. Preferably, at least 20% of the total urea formed in the synthesis section is formed in the pool condenser. Suitably, passivating air is added to the CO2 feed at an amount of at least 0.3 vol% O2 relative to the CO2 feed.

[0119] As an implementation, this disclosure also provides a method for improving an existing urea production unit by changing the vertical position (height) of the reactor. An existing urea unit (A) includes a high-pressure (HP) synthesis section comprising: a vertical urea reactor; a stripping tower having an inlet for gaseous CO2 feed; and a falling film high-pressure carbamate condenser configured to receive gas from the stripping tower, wherein the falling film high-pressure carbamate condenser is a falling film shell-and-tube heat exchanger having a bottom outlet from the tube connected to a bottom inlet of the reactor, wherein the reactor optionally includes one or more feed pipes for fluid from the condenser, the one or more feed pipes having an outlet at a first vertical level within the reactor. In the existing unit, the reactor has a fluid inlet at the bottom, and the falling film carbamate condenser has a fluid outlet at the bottom, and the two bottoms are positioned at (substantially) the same height (vertical level). The improvements include replacing the existing falling film HP carbamate condenser with a horizontally submerged condenser, moving the entire vertical reactor upwards, incorporating the installation of a pool-type condenser, and extending the flow lines upwards from the reactor to the upstream stripper. In this way, a lower pressure can be maintained at the top of the reactor, i.e., at the gas-liquid interface at the top of the reactor, while urea solution can still be supplied from the bottom of the reactor to the HP stripper by gravity flow. However, arranging the vertical reactor at a higher level in the unit requires significant improvements to the unit, such as in the reactor's support structure and its connections, and this method of improving the unit is a less preferred embodiment. The details and preferred embodiments of the existing unit and the horizontally submerged condenser are the same as those of the improved feed pipe embodiment within the vertical urea reactor.

[0120] As an embodiment, this disclosure also provides a method for retrofitting an existing urea production unit. Details of the existing unit are given in Existing Unit (A). The method includes improving the unit by cutting off the bottom of the reactor, installing a new base plate approximately vertically horizontally above the top of a horizontally submerged condenser, and supplying gas and liquid from the horizontally submerged condenser to the new base plate; and replacing the existing falling film HP carbamate condenser with a horizontally submerged condenser. This embodiment is less preferred because it involves a loss of reactor volume and the method involves significant modifications to the reactor structure. The details and preferred embodiments of the existing unit and the horizontally submerged condenser are the same as those of the embodiment involving improvements to the feed pipe within a vertical urea reactor.

[0121] As an embodiment, this disclosure also provides a method for modifying an existing urea production plant. Details of the existing plant are given in Existing Plant (A). The method includes, for example, sealing the bottom portion of an existing vertical urea reactor with a plate, replacing the existing HP falling film urethane condenser with a horizontal submerged condenser, and providing flow lines for gas and liquid from the horizontal submerged condenser to supply these fluids to the bottom of the upper part of the reactor. This embodiment is less preferred because it involves a loss of effective reactor volume, and independently, the method can involve significant modifications to the reactor structure. The details and preferred embodiments of the existing plant and the horizontal submerged condenser are the same as those of the embodiment involving modifications to the feed pipe within the vertical urea reactor.

[0122] The method of improving an existing apparatus preferably produces the apparatus of the present invention. The process of the present invention is preferably carried out within the apparatus of the present invention. All preferred embodiments and details discussed in conjunction with the method of improving an existing apparatus are equally applicable to the apparatus of the present invention, and all preferred embodiments and details of the apparatus of the present invention are equally applicable to the improved apparatus obtained by the method.

[0123] As used in this article, carbamate refers to ammonium carbamate, as the term is used in the urea production field.

[0124] In summary, this disclosure relates to improvements to stripped urea plants that allow gravity flow to be maintained in the improved plant by replacing the falling film HP urethane condenser with a horizontal submerged condenser and by combining improvements to the reactor internal components.

Claims

1. A method for improving an existing urea plant, wherein the existing urea plant includes a high-pressure (HP) synthesis section, the high-pressure synthesis section comprising: - Vertical urea reactor (R); - A stripping tower (S) having an inlet for gaseous CO2 feed; - A falling film high-pressure urethane condenser (C) configured to receive gas (1) from a stripping tower, wherein the falling film high-pressure urethane condenser is a falling film shell-and-tube heat exchanger having a bottom outlet (2, 3) from the tube connected to the bottom inlet of the reactor, wherein the reactor includes one or more feed pipes (F1, F2) for fluid from the condenser, the one or more feed pipes having an outlet at a first vertical level (h1) in the reactor; The method includes: - Replace the high-pressure urethane condenser with a replacement urethane condenser (PC), said replacement urethane condenser being a horizontally submerged condenser, which is a shell-and-tube heat exchanger having tube bundles and a shell-side space, configured for condensation in the shell-side space, preferably wherein the replacement condenser is a pool-type condenser; and - Improve at least one of one or more feed pipes (F1, F2) of the reactor to have an outlet at a second vertical level (h2), the second vertical level being higher than the first vertical level, preferably improve one or more feed pipes (F1, F2) of the reactor to have an outlet at the second vertical level (h2), the second vertical level being higher than the first vertical level.

2. The method of claim 1, wherein the improved apparatus has a liquid flow line (2) for condensate from the condenser to the reactor via a first feed pipe (F1) and a separate gas flow line (3) from the condenser to the reactor via a second feed pipe (F2), and the method includes improving the second feed pipe to have an outlet at a second vertical level (h2) above the first vertical level.

3. The method according to claim 1 or 2, wherein the replacement urethane condenser has a horizontal tube bundle (7), preferably a U-shaped tube bundle, and wherein the distance from the second vertical horizontal (h2) to the bottom of the reactor is at least equal to (equal to or greater than) the height of the tube bundle.

4. The method according to any one of the preceding claims, wherein the method includes providing an inlet (301) for an oxygenated fluid in a bottom reactor compartment (303) disposed between the bottom of the reactor and the second vertical level.

5. The method of claim 4, wherein the method includes providing a vertical divider (304) for the reactor in the bottom reactor compartment, wherein the vertical divider (304) separates a first compartment (305) from a second compartment (306) between the vertical divider and the reactor wall, and wherein the vertical divider is arranged to receive the oxygenated fluid and liquid from the second compartment (306) into the first compartment (305) at the bottom of the vertical divider.

6. The method of claim 5, wherein the vertical divider is a vertical tubular divider.

7. The method according to claim 5 or 6, wherein the feed pipe extends through the first compartment (305), preferably wherein the second feed pipe extends through the vertical tubular divider (304).

8. The method according to any one of the preceding claims, wherein the first vertical level is the same as the vertical level of the bottom outlet of the falling film HP urethane condenser.

9. The method according to any one of the preceding claims, wherein the first vertical level is the same as the bottom of the shell-side space of the replacement urethane condenser (PC).

10. The method according to any one of the preceding claims, wherein the second vertical horizontal plane (h2) is at a height higher than the upper tube of the tube bundle of the horizontally submerged condenser.

11. A urea production unit, comprising a high-pressure synthesis section, the high-pressure synthesis section comprising: - A vertical urea reactor (R) comprising a first feed pipe and a second feed pipe (F1, F2); - A stripping tower with an inlet for gaseous CO2 feed; - A high-pressure urethane condenser (PC) configured to receive gas (1) from the stripping tower, wherein the high-pressure urethane condenser (PC) is a horizontally submerged shell-and-tube heat exchanger that condenses the gas from the stripping tower in a shell, the horizontally submerged shell-and-tube heat exchanger having horizontal tube bundles (7), preferably, wherein the high-pressure urethane condenser is a pool condenser. - and a liquid flow line (2) for the condensate from the first feed pipe (F1) from the condenser to the reactor and a separate gas flow line (3) for the second feed pipe (F2) from the condenser to the reactor; wherein the vertical distance (h2) between the outlet of the second feed pipe for the gas and the bottom of the reactor is at least equal to (equal to or greater than) the height or diameter of the tube bundle, preferably obtained by the method of any one of claims 1-7.

12. The urea apparatus of claim 11, wherein the outlet of the second feed pipe is located vertically at the same height as or above the liquid outlet of the high-pressure urethane condenser; and preferably the outlet of the first feed pipe is also located at the same height.

13. The urea plant according to claim 11 or 12, wherein the bottom of the high-pressure carbamate condenser (PC) and the bottom of the reactor are located at the same vertical level of the urea plant.

14. The urea apparatus according to any one of claims 11-13, wherein the reactor includes an inlet (301) for an oxygenated fluid in a bottom reactor compartment (303) disposed between the bottom of the reactor and the second vertical horizontal plane.

15. The urea plant according to any one of claims 11-14, wherein the reactor includes an internal supply device for vertically introducing additional fluid into the reactor between the bottom of the reactor and the gas and liquid inlet points from the pool condenser.

16. The urea apparatus according to any one of claims 11 to 15, wherein the reactor includes a vertical divider (304) in the bottom reactor compartment (303), wherein the vertical divider (304) separates a first compartment (305) from a second compartment (306) between the vertical divider and the reactor wall, and wherein the vertical divider is arranged to receive the oxygenated fluid and liquid from the second compartment (306) into the first compartment (305) at the bottom of the vertical divider; preferably, wherein the vertical divider is a vertical tubular divider.

17. The urea apparatus of claim 16, wherein the feed pipe extends through the first compartment (305), preferably wherein the second feed pipe extends through the first compartment (305), preferably wherein the first compartment (305) is provided by the internal space of a vertical tubular divider (304).

18. The urea production apparatus according to any one of claims 11-17, wherein the horizontal submerged shell-and-tube heat exchanger comprises a covered horizontal cylindrical container that holds a horizontal tube bundle, and the vertical distance (h2) between the outlet of the second feed pipe for gas and the bottom of the reactor is equal to or greater than the diameter of the horizontal cylindrical container.

19. A method for producing urea in an apparatus according to any one of claims 11-18, the method comprising supplying CO2 feed to the stripping tower and NH3 to the high-pressure urethane condenser, and operating the high-pressure synthesis section at a pressure of at least 100 bar to form urea.

20. The urea production method according to claim 19, wherein the liquid flow from the condenser to the reactor and from the reactor to the stripping tower is a gravity flow.

21. The urea production method according to claim 19 or 20, wherein passivating air is added to the CO2 feed in an amount of at least 0.3 vol% O2 relative to the CO2 feed.

22. The urea production method according to any one of claims 19-21, wherein the vertical urea reactor includes a bottom portion between the bottom of the reactor and the outlets of the first feed pipe and the second feed pipe, wherein the method includes introducing a fluid containing a passivating agent, such as a fluid containing O2, into the bottom portion, preferably via a supply device for introducing the fluid containing O2 into the bottom portion of the reactor.

23. The urea production method according to any one of claims 19-22, wherein the second vertical level (h2) is at the same height as the liquid level maintained in the horizontal submerged condenser.

24. The urea production method according to any one of claims 19-23, wherein the vertical urea reactor includes a bottom portion between the bottom of the reactor and the outlets of the first feed pipe and the second feed pipe, wherein the height of the bottom portion is the same as the height of the liquid volume maintained in the replacement carbamate condenser (PC).

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