Revamp of a stripping-type urea plant; stripper; urea plant

Abstract

The disclosure pertains to a method of modifying an existing urea production, the method comprising changing the position of the stripper (5), e.g. by installing a replacement stripper (5b), such that the stripper in the modified plant has a lower bottom elevation and a lower elevation of the liquid inlet than in the existing plant. Also provided are a stripper and a urea plant.

Description

P137664PC00 Title: REVAMP OF A STRIPPING-TYPE UREA PLANT; STRIPPER; UREA PLANT Field

[0001] The invention pertains to the production of urea in plants wherein thehigh pressure synthesis section contains a stripper. In embodiments, a method is provided of modifying an existing urea production plant. The invention also pertains to a high pressure stripper and a urea production plant. Introduction

[0002] Various types of urea production processes and plants are described inUllmann’s Encyclopedia of Industrial Chemistry, chapter Urea, 2010 (“Ullmann’s Urea”). A frequently used type of plant is the so-called stripping type plant wherein the high pressure (HP) synthesis section comprises a stripper, a reaction zone, and a condensation zone. Generally, two types of HP stripping are used: CO2 stripping or thermal stripping (the latter also known as self-stripping and including NH3 stripping). Self-stripping plants often include an ammonia-driven ejector to transport the carbamate-containing liquid stream from the condenser via the reactor to the liquid inlet of the stripper in the synthesis section. For CO2 stripping, either an ejector can be used for this transport of liquid, or gravity-flow (e.g. in the Stamicarbon process, see Ullmann’s Urea p.14 -15). As described in Kaasenbrood and Chermin, “The Urea Stripping Process”, Fertiliser Society, 1977, for gravity flow, the bottom of the condenser is located about 10 meters above the top of the stripper, giving a driving force of about 1 bar. Originally, a vertical falling-film condenser was used and a vertical reactor having a bottom at the same elevation as the condenser. In newer plants, a horizontal submerged condenser is used (pool condenser). Optionally the reaction zone and condensation zone are provided by a combined horizontal vessel (pool reactor). Ullmann’s Urea, Fig. 20 and 21 furthermore illustrate the vertical arrangement of the equipment in the synthesis section. As discussed in Ullmann’s Urea p.19, it is also possible to place the reactor on ground level while maintaining gravity flow by maintaining the condensation zone. As illustrated the pool condenser is maintained at a certain height above the stripper.

[0003] Ullmann’s Urea p.31 discusses that it is often desired to modify existingurea production plants to increase their capacity.

[0004] HP strippers need to be replaced relatively frequently (e.g. every twentyyears, or more frequently) due to severe corrosive conditions in the stripper tubes in particular, with passive corrosion often causing a yearly corrosion rate of, e.g. about 0.05 mm per year of operation. The other equipment parts (reactor, condenser) often need not be replaced simultaneously. So far, stripper replacement has not been identified as an opportunity for carrying out a revamp.

[0005] Also provided is a urea plant. The plant can be a grassroots plant (newlybuilt plant) or a modified plant. Also provided is a high pressure CO2 stripper for a urea plant. Summary

[0006] The invention pertains in a first aspect to a method of modifying anexisting urea production plant to give a modified urea production plant, the existing urea production plant comprising: a high pressure synthesis section comprising a reaction zone (e.g. a reactor), a condensation zone (e.g. a condenser, optionally part of a pool reactor or a combination reactor), and a stripper, wherein said stripper has a bottom with a bottom elevation and comprises an upper chamber, a lower chamber and a tube bundle, and has a liquid inlet in said upper chamber and a liquid outlet in said lower chamber, wherein the synthesis section is configured (in the existing plant) for gravity flow of liquid from the condensation zone through the reaction zone to the stripper, and wherein the synthesis section comprises a liquid flow line for urea solution from the reactor to the liquid inlet of the stripper; and a liquid flow connection for stripped urea solution from the liquid outlet of the lower chamber of the stripper to a pressure-reducing valve, wherein the method comprises: changing the position of the stripper, such that the stripper in the modified plant has a lower bottom elevation and a lower elevation of the liquid inlet than in the existing plant, or installing a replacement stripper wherein that the replacement stripper in the modified plant has a lower bottom elevation and a lower elevation of the liquid inlet than the stripper in the existing plant; modifying the liquid flow connection for stripped urea solution to permit the lower bottom elevation of the stripper; and modifying the liquid flow line from the reactor to the liquid inlet of the stripper such that the outlet of the liquid flow line is at a lower elevation.

[0007] Also provided is ahigh pressure (HP) stripper for a urea plant, whereinthe stripper is a shell-and-tube heat exchanger with a vertical tube bundle,configured to operate with a falling film of urea solution in the tubes of the vertical tube bundle, wherein the HP stripper comprises the vertical tube bundle, an upper tubesheet and a lower tubesheet, an upper chamber above the upper tubesheet, a lower chamber below the lower tubesheet, and wherein the stripper further comprises a liquid inlet in said upper chamber and a liquid outlet in said lower chamber, a gas outlet in the upper chamber, and a gas inlet for stripping gas in the lower chamber, wherein the lower chamber comprises a hemi-head and a cylindrical chamber arranged below the hemi-head; wherein the HP stripper comprises a manhole in the hemi-head of the lower chamber; wherein the HP stripper is preferably a replacement stripper as preferably used in the method of modifying an existing urea plant of the invention.

[0008] Also provided is a urea production plant comprising a HP synthesissection comprising a reaction zone (e.g. a reactor), a condensation zone (condenser, optionally part of a pool reactor or a combination reactor) and a HP CO2 stripper according to the invention, i.e. with a manhole in the hemi-head of the lower chamber, wherein the plant comprises, in the HP synthesis section, a gas flow line from the stripper to the condensation zone, a liquid flow connection from the condensation zone to the reaction zone, and a liquid flow line from the reaction zone to the inlet for urea solution of the HP stripper. The plant is preferably configured for gravity flow in the synthesis section for gas from the stripper to the condenser, for liquid from the condenser to the reactor, and for urea solution from the reactor to the stripper.

[0009] The invention also provides a urea production process carried out in theinventive urea production plant, wherein the process comprises condensing NH3 and CO2 in the condensation zone to form carbamate and converting the carbamate into urea and water in the reaction zone to give a urea synthesis solution, and subjecting at least a part of the urea synthesis solution to HP CO2 stripping in the stripper, wherein the stripper is the inventive stripper. The stripped urea solution is supplied e.g. to a recovery section as is customary in the art.

[0010] The present disclosure generally pertains to a method of modifying anexisting urea production, the method comprising changing the position of the stripper (5), e.g. by installing a replacement stripper (5b), such that the stripper in the modified plant has a lower bottom elevation and a lower elevation of the liquid inlet than in the existing plant. Also provided are a stripper and a urea plant.Brief description of the drawings

[0011] Fig. 1 schematically illustrates an example modification of a urea plantaccording to the invention. Fig. 2 shows an example inventive HP stripper for a urea plant. Figure 3 schematically illustrates a detail of the inventive HP stripper. Figure 4 illustrates a configuration of a lower chamber (206) of the inventive HP stripper. Figure 5 illustrates a configuration of the inventive HP stripper. Figure 6 illustrates a configuration of a lower chamber (206) of the inventive HP stripper. Figure 7 illustrates the preferred relative vertical position of the valve (8) to the liquid flow connection (6) for stripped urea solution in the modified plant. Figure 8: Fig.8A shows the existing plant, as illustrated in Fig. 1; Fig.8B shows the modified plant, again as illustrated in Fig.1

[0012] Any embodiments illustrated in the figures are examples only and do notlimit the invention. Detailed description

[0013] The invention provides, in an aspect, a method of modifying an existingurea plant (a so-called revamp). The existing urea plant comprises a synthesis section comprising a reaction zone (e.g. a reactor), a condensation zone (e.g. a HP carbamate condenser), and an existing HP stripper, in particular a HP CO2 stripper.

[0014] The existing HP CO2 stripper is a shell-and-tube heat exchanger with avertical tube bundle, and comprises an upper chamber and a bottom chamber. The bottom chamber is arranged to receive urea-solution from the tube bundle and has a liquid outlet for urea solution connected by a liquid flow connection to a pressure- reducing valve. In operation, the pressure of the urea solution is reduced in said pressure-reducing valve by at least 30 bar, e.g. from above 100 bar to below 70 bar.

[0015] The modification involves modifying the elevation of the stripper, e.g. byreplacing the stripper with a replacement stripper. Hence, modifying the elevation of the stripper involves changing the position of the existing stripper, or installing a replacement stripper. All preferences and details for the embodiment of the method of modifying the existing plant involving changing the position of theexisting stripper, apply also to the embodiment of the method with installing a replacement stripper.

[0016] In the existing plant, the stripper has a bottom with a first elevation; inthe modified plant the (replacement) stripper has a lower elevation of the bottom than in the existing plant.

[0017] Accordingly, the liquid flow connection for urea solution between thestripper and the pressure-reducing valve is modified in the method to permit the lower placing of the stripper.

[0018] It is very advantageous that the modification of the elevation is donewhen replacing the stripper; in principle also the elevation of a stripper can be changed without replacing the stripper, i.e. by relocating the existing stripper.

[0019] A background reference to the elevation of the stripper is the paper“Safurex, it was not a dream”, 2009, Figure 3, which shows the plant layout of a pool condenser plant, where the stripper bottom is arranged well above ground level (grade level). Ullmann’s Urea, Fig. 20 also shows the stripper at some elevation above ground level. As also shown therein, the HP carbamate condenser is placed at high elevation, at a higher elevation than the top of the stripper, and is supported by a support structure.

[0020] By the modification, the liquid inlet of the stripper (in the upper chamberof the stripper) also has a lower elevation in the modified plant than in the existing plant, without a need to decrease the length of the stripper. It must be noted that reducing the length of the stripper tubes is often not practical and is determined by process constraints to obtain sufficient purification of the urea solution in the stripper.

[0021] The method furthermore involves modifying the liquid flow line for ureasolution from the liquid outlet of the reactor to the liquid inlet of the stripper to connect to the liquid inlet of the (replacement) stripper at a lower elevation. The liquid flow line is provided e.g. by piping. Hence, the liquid flow line is modified to have its outlet at a lower elevation. In this way, the lower elevation of the liquid inlet of the stripper in the modified plant is accommodated and is still connected to said outlet of said liquid flow line. For example in embodiments wherein the stripper is replaced, the liquid flow line for urea solution from the reactor is modified to connect to the liquid inlet of the replacement stripper.

[0022] Preferably, the elevation of the reactor is not changed in the revamp andthe inlet of the liquid flow line has an elevation that is not changed by the revamp. For example, in case of a vertical urea reactor, the inlet of the liquid flow line is provided by a weir (arranged in an upper part of the vertical urea reactor) that has an unchanged elevation; e.g. the position of the reactor is not changed in the revamp. Furthermore, in an example embodiment, the volume of the reactor is not changed. Furthermore, in an example embodiment, no additional reactor volume is installed in the synthesis loop in the revamp.

[0023] In an example embodiment, the method involves elongating the liquidflow line, typically piping, for urea solution from the liquid outlet of the reactor to the liquid inlet of the stripper at a downstream end of the existing liquid flow line; e.g. by adding additional piping.

[0024] Preferably, in the revamp, the elevation of the liquid outlet of the HPcarbamate condenser is not changed. Thereby, the lower elevation of the liquid inlet of the stripper in the modified plant compared to the existing plant causes a greater driving force for the gravity flow in the modified plant, i.e. larger liquid head. The present disclosure provides the judicious insight that moving the stripper to a lower elevation (respectively placing the replacement stripper at a lower elevation) thereby can be used to operate the synthesis section with a higher production rate. The greater driving force for gravity flow can overcome the higher dynamic pressure drop that is caused by higher flow rates in the synthesis section (both in the individual units, such as in a vertical urea reactor, and in the connecting pipes). In the existing and modified plant, the liquid outlet of the HP carbamate condenser is at a higher elevation than the liquid inlet of the stripper.

[0025] The present application thereby provides the judicious insight ofmodifying an existing urea production plant operating with gravity flow of fluids in the synthesis section by giving the HP CO2 stripper a lower elevation (by modifying the position of the stripper or installing a replacement stripper), in particular with a lower elevation of both the top and the bottom of the stripper, to thereby permit operation of the plant with higher production rate, without upsetting the gravity flow.

[0026] It was found that by providing the (replacement) stripper with a bottomelevation that is lower than in the existing plant, the vertical distance between the bottom of the condenser and the liquid inlet of the stripper can increase by about20% (e.g. from 8.0 m to 9.6 m). In this way, the static head increases (for the liquid flow from the condenser, through the reaction zone, to the stripper), and the increase in the static head affords or permits an increase in the dynamic pressure drop (for said liquid flow) . The increase in dynamic pressure drop enables higher liquid flow rates in the synthesis section and, hence, higher urea production rates.

[0027] Furthermore, the modification of the liquid flow connection for strippedurea solution from the stripper to the pressure-reducing valve typically also involves a reduction in the length of the liquid flow connection, e.g. by at least 1.0 m, such as a reduction by approximately 2 m. This length reduction provides an advantage in terms of reduced residence time, and hence reduced biuret formation in the urea solution. The high temperature and low NH3 level of the stripped urea solution from a CO2 stripper, in the liquid flow connection upstream of the pressure-reducing valve, poses a particular risk of undesired biuret formation.

[0028] A further advantage of the reduced length of the liquid flow connectionfor stripped urea solution is the relatively low dynamic pressure drop, which depends in part on the length of liquid flow connection (e.g. piping). It was found that very advantageously this may at least in part offset an increase in dynamic pressure drop caused by having sharper curved or even straight-angle bends in the liquid flow connection in the modified plant. In this way, the risk of flashing of the stripped urea solution in the liquid flow connection is reduced. The method typically also involves modifying the gas flow line for gas from the stripper to the inlet of the condensation section, since typically the gas outlet of the stripper at the modified position and of the replacement stripper is at a lower elevation.

[0029] This modification, i.e. the lower elevation of the stripper, can be combinedwith other modifications.

[0030] In the existing plant, the stripped urea solution is for instance supplied,through the pressure-reducing valve, to a low pressure (LP) recovery section, in particular to an LP decomposer of the LP recovery section. The LP decomposer is a heater, in particular a shell-and-tube heat exchanger, used for decomposing carbamate contained in the urea solution. The LP decomposer has a gas outlet and a liquid outlet. The gas outlet is typically connected to an LP carbamate condenser, which is typically a shell-and-tube heat exchanger used for cooling. The LP carbamate condenser typically has a gas outlet and a liquid outlet for carbamate solution. The existing plant typically comprises a liquid flow line to supplycarbamate solution from the LP recovery section back to the HP synthesis section. Preferably, the modified plant also has such an LP recovery section.

[0031] In an embodiment, the method also comprises increasing the capacity ofthe LP recovery section, e.g. by installing an additional LP decomposer.

[0032] In a preferred embodiment, the method comprises installing areplacement stripper, wherein the replacement stripper has more stripper tubes (has a larger number of stripper tubes) in the tube bundle than the replaced stripper of the existing plant. Thereby, advantageously, the stripping capacity (kg urea solution / hr) increases.

[0033] In particularly preferred embodiment, the replacement stripper has asmaller pitch (distance) between the tubes than the replaced stripper, and also has a larger number of stripper tubes. Thereby the replacement stripper can have more tubes without an increase of the diameter of the tube bundle, and without a corresponding increase in the height of the hemi-head of the upper and lower chamber of the stripper.

[0034] In an example embodiment, in particular wherein no replacementstripper is installed but the position of the stripper is changed, the method comprises adding a medium pressure (MP) treatment section to the plant which is connected to receive urea solution from the reactor. The MP treatment section is configured to receive urea solution from the reactor, bypassing the HP stripper, and to purify it. Accordingly, the MP treatment section typically comprises an MP decomposer, which is typically a shell-and-tube heat exchanger used for heating to decompose carbamate contained in the urea solution. The MP decomposer has a liquid outlet for urea solution that is connected, directly or indirectly, to the LP recovery section. Gas from the MP decomposer is typically supplied to an MP carbamate condenser. Carbamate solution from the MP carbamate condenser is typically supplied to the synthesis section. The MP treatment section can be used to bypass the HP stripper. In this way, higher urea production rates can be achieved without the HP stripper becoming a bottleneck.

[0035] In an example embodiment, the method involves increasing thecondensation capacity of the condensation zone of the synthesis section, e.g. by installing an additional carbamate condenser in the synthesis section. The additional carbamate condenser is e.g. a shell-and-tube heat exchanger used forcooling, having an inlet to receive gas from the stripper. The additional carbamate condenser is arranged e.g. upstream of the carbamate condenser of the existing plant, i.e. preferably has an outlet (for at least condensate) to the carbamate condenser of the existing plant.

[0036] If in the existing plant the condensation section is a falling-film HPcarbamate condenser, the method involves for example replacing this carbamate condenser with a pool condenser, i.e. with a shell-and-tube heat exchanger with a U-shaped horizontal tube bundle configured for receiving gas from the stripper in the shell (shell side space) and cooling liquid (e.g. water) in the tubes. Other modifications of the HP carbamate condenser of the existing plant are also possible.

[0037] The method may involve increasing the reactor volume of the synthesissection, e.g. by installing an additional urea reactor. However, in some embodiments, the reactor capacity is not increased. For example, a higher throughput is obtained by increasing the liquid velocity in the reactor. The increased dynamic pressure drop over the reactor may be mitigated by the lower elevation of the stripper.

[0038] In some embodiments, the percentage conversion of the reactants intourea per pass is lower in the modified plant, e.g. due to the smaller residence time in the reaction zone. In some embodiments, this is addressed by operating the HP stripper with more severe stripping (e.g. more severe heating), to achieve a higher stripping efficiency.

[0039] In a further embodiment, the method involves adding a medium pressuretreatment section to the plant which is connected to receive urea solution from the reactor or stripped urea solution from the stripper. The medium pressure treatment section comprises, e.g. a MP urea-processing unit having a liquid inlet, a gas outlet, and a liquid outlet; and an MP carbamate condenser receiving gas from the MP urea-processing unit. The MP urea-processing unit comprises e.g. a heater for heating the urea solution or an adiabatic flash vessel for flashing the urea solution, or a combination of a flash vessel followed by a heater. The heater is e.g. a shell-and-tube heat exchanger. Directly treating in this MP treatment section a part of the urea solution from the reactor, said part of the urea solution bypassing the HP stripper, can help debottleneck the HP stripper. Treating stripped urea solution can permit operating with lower steam consumption in the HP stripper.The treatment of the urea solution in the MP treatment section involves carbamate decomposition from the urea solution.

[0040] The MP carbamate condenser typically is connected to receive carbamatesolution from the LP carbamate condenser and typically has a liquid outlet connected to supply carbamate solution to the synthesis section.

[0041] The existing plant comprises a pressure-reducing valve, which ispreferably an angle valve. The pressure-reducing valve is preferably a level control valve, with a control system coupled to the level gauge of the HP stripper. A level control valve is, according to standard engineering practice, designed with an area suitable for flow of a liquid stream. The presence of gas bubbles in the liquid, especially of relatively large size, can cause operational issues for the valve. Using a level control valve as the pressure-reducing valve provides the advantage of having a liquid seal between the HP synthesis section and the downstream lower pressure section.

[0042] A specific aspect of the stripped urea solution, after the heating in the HPstripper, is that it is boiling and contains a significant fraction of ammonium carbamate that is liable to decomposition on pressure reduction. Therefore, there is a risk of flashing, i.e. gas formation, in the liquid flow connection for stripped urea solution from the liquid outlet of the stripper to the pressure-reducing valve. Flashing upstream the pressure-reducing valve is undesirable, in particular for reasons of process control.

[0043] Hence, preferably, the pressure-reducing valve is provided at a lowerelevation than the liquid outlet of the stripper, with a sufficient difference in height to maintain a liquid head that compensates for the dynamic pressure drop in the liquid flow connection for stripped urea solution.

[0044] The present disclosure also provides the insight that a reduction of theelevation of the pressure-reducing valve can be beneficial for permitting a lower elevation of the stripper.

[0045] In many existing urea production plants, the pressure-reducing valve isan angle valve, e.g. an angle-globe valve, with a stem and a plug. In the angle valve, the inlet and outlet ports are at 90º. In the existing plant, the valve can be arranged with the inlet port at the bottom, the outlet port to the side, and with the stem and actuator on top. The actuator can be a relatively heavy piece ofequipment. The valve typically is essentially box-shaped, having a top surface, a bottom surface, and four sides.

[0046] The actuator is e.g. a pneumatic actuator, for example a pneumaticdiaphragm actuator. Usually, the valve comprises a valve positioner that increases or decreases the air load pressure on the actuator, placing the valve in the correct position.

[0047] It was found that arranging the angle valve with the inlet port on a sidesurface, and with the actuator on the opposed side surface, provides an advantage for reducing the stripper bottom elevation. In such a configuration, the outlet port can also be to a side surface or on the top surface (the outlet port on the bottom surface is not practical when maintaining low elevations). In particular, if in the existing plant a relatively large bent pipe is used to supply the stripped urea solution from a horizontal pipe part to a vertical pipe part, the vertical pipe part being connected to the bottom inlet port of the valve, the valve can be placed significantly lower with the inlet on a side surface, permitting the direct connection of the inlet to the horizontal pipe part in the modified plant (and in the inventive plant). Thereby also the stripper bottom elevation can be reduced without increasing the risk of flashing of the urea solution.

[0048] Hence, in a preferred embodiment, the method includes changing theorientation of the pressure-reducing valve from having, in the existing plant, the inlet at the bottom, the actuator on top, and the valve stem vertically, to having the inlet and the actuator at a side and the valve stem horizontally in the modified plant. It should be noted that changing the orientation or elevation of the valve may include replacing the valve and arranging the replacement valve at the new elevation and preferably new orientation, or rearranging the existing valve.

[0049] In a preferred embodiment, the method comprises changing the positionof the pressure-reducing valve such that the pressure-reducing valve is at a lower elevation in the modified plant than in the existing plant, preferably at least 1 m lower. Lowering the position of the pressure-reducing valve enables a lower position of the stripper. Furthermore, for a given position of the stripper outlet, the static liquid pressure (based on the liquid head in the liquid flow connection for stripped urea solution) is higher at the inlet of the valve for a lower position of the valve, which advantageously helps to reduce the risk of flashing.

[0050] In a preferred embodiment, the pressure-reducing valve has an elevationin the modified plant, or in the inventive urea production plant, above a baseline of the liquid flow connection for stripped urea solution, and this elevation is less than 4 times the diameter of the liquid flow connection, preferably less than 3 times said diameter; for instance the valve is at the baseline elevation. Herein, the diameter is the inner diameter of the pipe upstream of the valve. The baseline is the lowest elevation of the liquid flow connection for stripped urea solution between the stripper and the pressure-reducing valve. The elevation is the vertical difference between the valve and the baseline of the liquid flow connection. The vertical position of the valve can be taken to be the vertical position of the centre of the valve. The vertical position of the baseline of the liquid flow connection can be taken to be the centerline of the liquid flow connection for the stripped urea solution, at the position along the flow of the stripped urea solution where that centerline has the lowest elevation (vertical position).

[0051] In the existing plant, the pressure-reducing valve has, e.g., an elevationabove a baseline of the liquid flow connection for stripped urea solution of at least 4 times the diameter of the liquid flow connection.

[0052] In the method, the liquid flow connection for urea solution between thestripper and the pressure-reducing valve is modified in the method to permit the lower placing of the stripper.

[0053] For example, the stripper (both existing and replacement stripper, if therevamp involves replacing the stripper) has a bottom outlet and the liquid flow connection comprises a section where the direction of flow is changed from substantially vertically downward to substantially horizontal, e.g. by at least 60º and / or up to 120º from vertical to horizontal, preferably by (substantially) 90º. Such a section can be referred to as ‘a bend’ or ‘elbow’ or as a flow direction changing device. The section is not limited to bent pipes and can also be provided, for example, by forged or machined parts, or by angle valves, or by a tee pipe fitting with one end closed, in the modified plant and for the inventive HP stripper.

[0054] In an example embodiment, the existing plant uses a bent pipe, with aradius of curvature of at least 4 times the inner diameter of the pipe, i.e. a 4D bent pipe.

[0055] In an example embodiment, in the modified plant, a flow directionchanging device is used in the liquid flow connection, e.g. a forged or machined unit (or piece). This flow direction changing device, i.e. device configured to change the direction of the flow of the fluid, e.g. the forged or machined unit, comprises a channel with a channel height of max. 2 times the diameter of the inlet of the device (e.g. 2D bend), or max.1 times said diameter (i.e. equal to or smaller than said diameter); wherein the channel height is the vertical distance between the center of the outlet and the center of the liquid inlet. The device contains a channel for fluid from said inlet to said outlet, wherein the direction of fluid flow at said outlet is different than at said inlet. For instance, the direction of fluid flow at the inlet is vertical and the direction of fluid flow at said outlet is horizontal. For example, the fluid flow direction changes by 90° between said inlet and said outlet. Hence, the fluid flow channel is bent. Preferably, the device has a liquid inlet on top and a liquid outlet on a side. The total height of the device, including the wall thickness, will be larger than the channel height, but is less relevant for the elevation of the stripper. Preferably, the liquid inlet opening is horizontal, e.g. is the bottom end of a vertical pipe or vertical part of a pipe connecting the flow direction changing device to the liquid outlet of the stripper. The liquid inlet opening being horizontal indicates a vertical flow of the liquid at said inlet. The vertical pipe or vertical pipe part is for example used with a liquid outlet of the HP stripper in a bottom plate of the cylindrical chamber. It should be noted that the channel height can be even e.g. 0.5 times the inlet diameter (0.5D), e.g. if the flow is changed by 90º in a chamber of a device with a height equal to the inlet diameter as the channel height is based on the center of the outlet opening. In some embodiments, the inlet diameter is larger than the outlet diameter, and the channel height is also max. 2 times the diameter of the outlet or max.1 times said diameter. The preference for an inlet diameter that is larger than the outer diameter of the device, applies in particular in embodiments with a vertical pipe between said liquid inlet of the device and a liquid outlet for stripped urea solution in a bottom plate of the HP stripper, more in particular in embodiments wherein said vertical pipe is used for degassing of the urea solution.

[0056] It was found that even though a 4D bent pipe may have a lower K factorfor dynamic pressure drop than such a flow direction changing device (with channel height 0.5D), e.g., K = 0.11 for the 4D bent pipe and K = 1.25 for the flow direction changing device, the corresponding reduction in length of the liquid flowchannel (piping) may at least in part offset this. Herein, ^P = K * rho * v², wherein delta P is the pressure loss in terms of liquid head, K is the K factor, v is the velocity, and rho is the density of the fluid.

[0057] However, other configurations are also possible. In an exampleembodiment, the existing stripper has an outlet in the bottom wall of the cylindrical chamber of the lower chamber.

[0058] In an example embodiment, the replacement stripper has a liquid outletthat is provided horizontally to the side, e.g. in the side wall of the cylindrical chamber that is present at the bottom of the lower chamber of the stripper. In operation, the liquid level in the cylindrical chamber is maintained above the liquid outlet opening in the side wall, such that gas escape into the liquid outlet is (largely) prevented.

[0059] In an example embodiment, the replacement stripper uses a siphon outletfor withdrawing urea solution from the cylindrical chamber of the lower chamber, said siphon outlet extending vertically upward from the cylindrical chamber into the part of the lower chamber provided by the hemi-head and passing, with a curved part, through the hemi-head wall. A background reference for a siphon type urea solution outlet of a HP stripper is siphon 11 of the stripper shown in US 2006 / 0032620A1 and the pipe 11 of the stripper illustrated in EP 1195194A1. EP 1195194A1 also shows the support structure for mounting the stripper at some distance above ground level.

[0060] A hemi-head as used herein refers to a hemispherical head and mayinclude a semi-elliptical head. The head is attached to the cylindrical shell of the stripper.

[0061] In a possible embodiment wherein a HP stripper with an outlet through abottom plate in the existing plant is replaced by a HP stripper with a siphon outlet, preferably also the elevation of the pressure-reducing valve is reduced and preferably, if the pressure-reducing valve is an angle valve, the orientation of the pressure-reducing valve is changed from having the inlet at the bottom to having an inlet at the side.

[0062] In a preferred embodiment, the method involves replacing the stripper,and the replacement stripper is provided with a manhole in the hemi-head of the lower chamber. Preferably, in the existing plant, the stripper has a manhole in thebottom plate of the cylindrical chamber of the lower chamber, e.g. with the bottom plate attached with bolts to (nuts in the wall of the) the cylindrical chamber. By having the manhole in the hemi-head, the bottom plate of the lower chamber no longer needs to be accessible and removable (to provide manhole access maintenance), as is typically the case in the existing plant. Hence, the bottom of the stripper can be placed at a lower elevation.

[0063] A manhole is a hole dimensioned to provide access for a human into thelower chamber, and e.g. has a diameter of at least 60 cm, typically 80 to 100 cm. The replacement stripper comprises the manhole and the liquid outlet as separate and distinct elements.

[0064] An aspect of the invention also pertains to this stripper with a manholein the hemi-head of the lower chamber, suitable for modifying existing plants and for newly built plants, as will be discussed hereinafter.

[0065] Moreover, a valve can be placed directly below the stripper, in particularan angle valve, which can provide for a 90º bend from the bottom outlet of the stripper (or degassing pipe) to horizontal with low elevation of the stripper. Specifically, no flanges are necessary between the valve and the stripper.

[0066] In an example configuration with three valves in series (manual valve,on / off valve, and the pressure-reducing valve, preferably in series a manual valve, an on / off valve, and a pressure-reducing valve), this may provide for, e.g.: a vertical to horizontal bend (in the first valve), a bend in the horizontal plane (in the second valve), and a bend from horizontal to vertical in the third valve, which is e.g. the pressure-reducing valve.

[0067] Furthermore, advantageously, the wall of the cylindrical chamber can bethinner in the replacement stripper, as it does not need to receive bolts.

[0068] In the replacement stripper with a manhole in the hemi-head, the flowdirection changing device is e.g. welded directly to the bottom plate of the stripper.

[0069] In the existing plant, and also in the modified plant, and also for theinventive stripper, the stripper is a vertical shell-and-tube heat exchanger configured for flow of a falling film of urea solution in the tubes. The stripper comprises a vertical tube bundle, an upper chamber above an upper tubesheet and a lower chamber below a lower tubesheet.

[0070] The stripper tubes are individually provided with a liquid divider orferrule in the upper chamber, usually after installing the stripper in the urea production plant.

[0071] A liquid divider is a tube, with apertures in the wall for urea solution toenter the tubes and form a falling film. Each liquid divider may comprise a ferrule and a gas riser tube, the ferrule having the apertures.

[0072] In operation, a layer of urea solution is maintained in the upper chamber.The upper outlet of the gas riser tube is located above the layer of urea solution in operation. Urea solution flows from the liquid layer, through an aperture of a liquid divider (or ferrule) as a falling film into a stripper tube.

[0073] The upper chamber comprises a liquid inlet and a gas outlet. The gasoutlet is connected to the inlet of the condensation zone for gas to be condensed.

[0074] The shell compartment of the stripper has an inlet for steam and a steamcondensate outlet at a lower part of that compartment, said outlet being arranged above the lower tubesheet.

[0075] The lower chamber comprises a liquid outlet for stripped urea solutionand an inlet for CO2 used as strip gas. The lower chamber also comprises, typically, a gas distribution device for the strip gas. The lower chamber is arranged below the lower tubesheet.

[0076] The lower chamber typically comprises a hemi-head and a cylindricalchamber that is provided at the apex of the hemi-head, i.e. at the lowermost part of the lower chamber. The cylindrical chamber is used for maintaining a volume of liquid for control reasons.

[0077] The lower chamber comprises, at the bottom, a bottom plate, inparticular a horizontal bottom wall (i.e. horizontal relative to the vertical tube bundle). In the existing stripper, this horizontal bottom wall is provided, e.g., by a manhole, i.e. by a detachable cover plate. The cover plate is for instance attached to the cylindrical chamber wall by releasable fastening means such as nuts and bolts.

[0078] In the replacement stripper, preferably a manhole is provided in thehemi-head of the lower chamber. By having the manhole in the hemi-head, the bottom plate of the lower chamber no longer needs to be accessible and removable (to provide manhole access maintenance), as is typically the case in the existing plant. Hence, the bottom of the stripper can be placed at a lower elevation,advantageously during the modification of the plant. In the replacement stripper, the bottom plate is e.g. welded to the cylindrical chamber. It is noted that the present disclosure also pertains to a stripper having the manhole in the hemi-head as an inventive stripper.

[0079] The stripper (existing and replacement stripper) typically comprises alevel gauge for measuring the liquid level in the cylindrical chamber of the lower chamber. In operation, a liquid level is maintained in the cylindrical chamber of the lower chamber of the stripper.

[0080] The stripper comprises a fluid passage configured for withdrawing liquidfrom the cylindrical chamber. Preferably, in the existing stripper, the bottom wall comprises a liquid outlet.

[0081] When withdrawing liquid from the cylindrical chamber, in particularthrough the bottom wall, there is a risk of vortex formation which could cause gas entrainment in the liquid at the outlet. Accordingly, the stripper preferably comprises a vortex breaker in the cylindrical chamber.

[0082] In the existing plant, the liquid outlet of the stripper in the horizontalbottom wall (e.g. cover plate) of the cylindrical chamber is preferably connected to a vertical downward pipe. This pipe can provide for degassing of any entrained gas.

[0083] In the existing plant, the downward end of the vertical degassing pipe isconnected typically to a 90º bent pipe segment, having a curvature of at least 3D or 4D (D = inner diameter of the pipe). The bent pipe is e.g. made of duplex stainless steel.

[0084] In the existing plant and in the modified plant, the steam condensateoutlet of the stripper (shell compartment, i.e. shell side space) is connected to a steam drum. The steam drum is a relatively large vessel and is placed at grade level (the average ground level on the plant site). It is advantageous if the liquid condensate outlet is at a higher elevation than the top of the steam drum, e.g. at least 1.0 m higher or at least 2.0 m higher. In the existing plant, the steam condensate outlet of the stripper is usually at an elevation at least 2.0 m higher than the steam drum; in the modified plant this is preferably also the case.

[0085] The reaction zone preferably comprises a plurality of compartments inseries. The synthesis section comprises for instance a vertical urea reactor that is provided with trays.

[0086] If the existing plant comprises a vertical urea reactor, this verticalreactor is for example provided at an elevation above the condensation zone. However, the vertical reactor can also be located at lower elevations, including grade level.

[0087] The condensation zone is provided, in the existing plant and preferablyalso in the modified plant, for example by a HP carbamate condenser that is e.g. a horizontal shell-and-tube heat exchanger that receives gas from the stripper in the shell side space and with one or more cooling liquids in the tubes, with a horizontal tube bundle that is in operation submerged by liquid in the shell, e.g. with one or more U-shaped tube bundles. However, other types of condensers are also possible for the condensation zone, for example, a falling-film shell-and-tube heat exchanger, or a shell-and-tube heat exchanger with a vertical U-shaped tube bundle with condensation in the shell, or a shell-and-tube heat exchanger with a vertical tube bundle with condensation in the tubes and a gas inlet at the bottom (i.e., the gas inlet supplying the gas to the lower ends of the tubes of the tube bundle).

[0088] The HP stripper and the condensation zone are provided by differentpieces of equipment, such that a change of the elevation of the stripper during the modification of the plant does not affect the elevation of the condensation zone. The condensation zone is located at a higher elevation than the liquid inlet of the stripper; thereby the condensation of the gas at the higher elevation enables the gravity flow of liquid from the condensation zone, through the reaction zone, to the liquid inlet of the stripper.

[0089] In an embodiment, the HP carbamate condenser comprises a reactionzone and at least a part of the urea is formed already in the carbamate condenser.

[0090] In an embodiment, the synthesis section comprises e.g. a horizontalvessel with baffles, with a tube bundle for condensation extending over a part of the length of the vessel, with a gas distributor at the bottom, and with a liquid outlet for condensate from a baffle compartment opposite the tube bundle. An example is the pool reactor described in US 5,767,313.

[0091] The synthesis section comprises a liquid flow line for urea solution fromthe reaction zone to the liquid inlet of the stripper. In the method of modifying the urea plant, the liquid flow line is elongated, e.g. by adding piping, to reach the inletof the stripper at a lower elevation, i.e. at a larger vertical distance with the outlet of the reactor. This provides for a larger liquid head in said liquid flow line. In operation, a liquid column is maintained in said liquid flow line for urea solution. Preferably, the reactor has a separate gas outlet. Preferably, the liquid flow line includes a downcomer for urea solution, the downcomer being located in the reactor but not a part of the reaction zone, i.e. the urea solution is withdrawn from the reaction zone by the top inlet of the downcomer. Hence, the outlet of the reaction zone can be an inlet of a downcomer, said inlet being arranged inside a vertical urea reactor. The downcomer is a vertical pipe inside the urea reactor, with an inlet for urea solution at the top, arranged in an upper part of the urea reactor, and connected to an aperture in the reactor shell to provide an outlet for the urea solution from the downcomer.

[0092] The outlet of the reaction zone can also be an outlet of a reactor. Theliquid flow line for the urea solution from the outlet of the reaction zone to the liquid inlet of the stripper spans a vertical distance.

[0093] Preferably, in the modified plant, the liquid flow connection for strippedurea solution is provided, for the wetted parts, by duplex stainless steel parts, and for example also in the existing plant. For example, the stripper tubes of the stripper in the existing plant are made of duplex stainless steel. Preferably, the stripper tubes of the stripper in the modified plant are also made of duplex stainless steel. Preferably, in the existing plant and in the modified plant, at least the lower chamber of the stripper is provided, on the inside, with a layer of duplex stainless steel, e.g. as lining or overlay welding. This preference applies also to the upper chamber. An example of a suitable type of duplex stainless steel is the steel grade UNS S32906.

[0094] The liquid flow connection from the outlet of the stripper for strippedurea solution to the pressure-reducing valve comprises, in the existing plant and / or in the modified plant, e.g., a manual valve and an on / off valve, in this order. The manual valve is preferably an angle valve. The on / off valve is preferably an angle valve.

[0095] The liquid flow connection in the existing plant and / or in the modifiedplant may comprise a thermowell and a temperature sensor. In a preferred embodiment, the thermowell is included in the preferably forged unit. This provides an advantage of a reduced pipe length compared to a separate thermowell.

[0096] The synthesis section, in the existing plant and / or in the modified plant,comprises a flow line for gas from the stripper to the condensation section; a flow line for fluid (i.e. at least liquid condensate) from the condenser to the reactor if these are separate units; and a liquid flow line for urea solution from the reaction zone to the stripper. In embodiments with a vertical urea reactor having a bottom inlet arranged at a higher elevation than the outlet of the condenser, both gas and liquid are preferably supplied from the condenser to the reactor. In embodiments wherein only liquid is supplied from the condenser to the reactor, the reactor preferably also receives a part of the gaseous CO2 feed and / or a part of the gas from the stripper. The condensation of gas in the reactor provides heat for the endothermic reaction of carbamate into urea and water. The existing plant, the modified plant, or both, are configured for transport of liquid from the condenser, through the reactor, to the stripper, by gravity flow, i.e. without the use of pumps or ejectors for said transport. Optionally the synthesis section comprises a scrubber for scrubbing gas from the reactor and / or condenser, and an ejector for transport of carbamate solution from a scrubber to the condenser.

[0097] In the existing plant, the stripped urea solution is supplied from thepressure-reducing valve to a recovery section, for example directly to an LP recovery section, in particular to a carbamate decomposer of such a section.

[0098] In the following discussion of the method, references are to Figure 1without the invention or claims being limited to the embodiment shown in that figure.

[0099] The urea production plant (1) comprises a high pressure (HP) synthesissection (2). The HP synthesis section is operated at a pressure above 100 bara, preferably above 120 bara, typically below 400 bara. The HP synthesis section comprises a reaction zone (3), a condensation zone (4), and stripper, in the existing plant stripper (5a) and in the modified plant the replacement stripper (5b) which can also be the repositioned stripper. The reaction zone and condensation zone can be provided by a combined unit or (as illustrated) by separate vessels. The reaction zone is a separate unit from the stripper, and the condensation zone is also a separate unit from the stripper.

[0100] In the existing plant, the stripper (5a) has a liquid flow connection (6a)for stripped urea solution from the liquid outlet (7a) of the stripper to a pressure- reducing valve (8a). The liquid flow connection (6a) comprises a 5D bent pipesegment (14a) providing a 90º bend from vertical to horizontal, i.e. with a radius of curvature of 5 times the inner diameter of the pipe. The existing plant comprises a further bent pipe segment (15a) transitioning the liquid flow connection from horizontal to vertical upward, to the liquid inlet at the bottom of the pressure- reducing valve (8a).

[0101] In the modified plant, the liquid flow connection (6b) comprises a flowdirection changing device (10) that is provided by a forged or machined equipment and that provides, in this example, a 90º bend in a chamber having a channel height equal to the inlet diameter, i.e. corresponding to a radius of curvature of 0.5 D.

[0102] In the modified plant, the stripper (5b) is placed at a lower elevation thanin the existing plant, giving a difference in elevation (11)with the elevation of the stripper in the existing plant. In particular, the stripper (5b) in the modified plant has a lower bottom elevation (elevation of the bottom of the stripper) and a lower elevation of the liquid inlet than in the existing plant

[0103] Furthermore, the elevation of the stripper is changed independently ofthe elevation of the condenser (4), i.e. the elevation of the condenser is not changed. Hence, the vertical difference between the stripper and the condenser increased by the modification of the plant.

[0104] The method preferably involves decreasing the elevation of the pressure-reducing valve. Hence, in the modified plant, the pressure-reducing valve (8b) is preferably located at a lower elevation than the pressure-reducing valve (8a) in the existing plant, as illustrated in Fig.1. For example, in the modified plant, the pressure-reducing valve (8b) is preferably located at the same height as the flow direction changing device (10).Furthermore, in a preferred embodiment, in the existing plant, the liquid inlet (16) of the pressure-reducing valve (8a) is at the bottom, the actuator (17) at the top, and the liquid outlet (18) to the side. In the modified plant, preferably, the liquid inlet (16) of the pressure-reducing valve (8b) is at a side, and the actuator (17) is at a side; and the liquid outlet (18) is at the top, bottom, or side (at the top is illustrated). Furthermore, in this embodiment, in the valve (8a) of the existing plant, the valve stem (19) is vertical and in valve (8b) of the modified plant, the valve stem is horizontal.

[0105] Accordingly, in the modified plant, the liquid flow line (9) for ureasolution from the reactor to the inlet of the stripper is also elongated in vertical direction, as part of the method of modifying the plant, by a corresponding distance (11a) to reach the lower elevation of the stripper inlet. Hence, additional piping (9a) is added when modifying the plant. Accordingly, in operation, a longer vertical liquid column is maintained in said liquid flow line.

[0106] The synthesis section preferably is configured for gravity flow for fluidfrom the condensation zone to the reaction zone through a fluid flow connection (12) (e.g. for liquid and gas), for urea solution from the reaction zone to the stripper through liquid flow line (9), and for gas from the stripper to the condensation zone through gas flow line (13); all connections are for a part or all of said streams. The driving force for the gravity flow is the heat supplied to the stripper to effect carbamate decomposition into NH3 and CO2, released as gas that is supplied to the condensation zone. The condensation zone is provided at a vertical distance above the gas outlet of the stripper. Liquid condensate formed in the condensation zone flows through the reaction zone to the liquid inlet of the stripper by gravity. The modified and existing plant hence comprise the fluid flow connection from the condensation zone to the reaction zone (which fluid flow connection may be internal in a vessel, such as in case of a pool reactor), the liquid flow connection for urea solution from the reaction zone to the stripper, and the gas flow connection from the stripper to the condensation zone.

[0107] The invention also provides a HP stripper for a urea plant, and a ureaproduction plant comprising the HP stripper. The plant can be a modified plant or a newly built plant. The HP stripper is very useful as the replacement HP stripper as used in the method of modifying an existing urea production plant, and is also useful as a stripper used in, or suitable for, a newly built urea production plant.

[0108] The HP stripper is a shell-and-tube heat exchanger with a vertical tubebundle, configured to operate with a falling film of urea solution in the tubes of the vertical tube bundle, in particular with HP urea solution in the tubes. The HP stripper comprises an upper and a lower tubesheet, an upper chamber above the upper tubesheet, a lower chamber below the bottom tubesheet, and has a liquid inlet in said upper chamber and a liquid outlet in said lower chamber, a gas outlet in the upper chamber, and a gas inlet for stripping gas in the lower chamber. The liquid outlet is preferably an aperture with a diameter of max. 40 cm. The strippercomprises the manhole and the liquid outlet as separate and distinct elements. The tubes are made e.g. of duplex stainless steel. The stripper has an inlet for steam and an outlet for steam condensate in the shell side. The lower chamber comprises a hemi-head and a cylindrical chamber arranged below the hemi-head, i.e. at the apex of the hemi-head. The HP stripper comprises a manhole in the hemi-head.

[0109] Preferably, the cylindrical chamber comprises a vertical cylindricalsidewall and a bottom plate. The bottom plate is horizontal. The sidewall of the cylindrical chamber is straight in a vertical cross-section (with the vertical direction defined by the direction of the length of the tubes of the stripper). The sidewall is circular in a top view or horizontal cross section.

[0110] It is noted that the wall of curved in both the vertical cross section andthe horizontal cross section.

[0111] Preferably, the bottom plate is welded to the side wall. Preferably thecylindrical chamber has an upper end and is open at said upper end thereof and is configured to receive urea solution from the hemi-head at said upper end. In particular, the cylindrical chamber is configured for holding a volume of urea solution in it. Optionally, the bottom plate is entirely closed. In other embodiments, the bottom plate comprises an aperture providing a liquid outlet.

[0112] Placing the manhole in the hemi-head provides as an advantage that thestripper bottom can be located at a lower elevation compared to having the manhole access by a removable bottom plate.

[0113] Preferably, the manhole comprises a releasable cover plate, wherein thecover plate is attached in a releasable manner to the hemi-head, e.g. with nuts and bolts. Further preferences and details for the manhole are as described for the replacement stripper.

[0114] In an embodiment, the inventive HP stripper has a liquid outlet throughthe side wall of the cylindrical chamber, i.e. has a horizontal liquid outlet. The preferences of this embodiment are the same as for the corresponding replacement stripper.

[0115] In an embodiment, the HP stripper has a siphon type liquid outlet, with abent pipe comprising a pipe part protruding through the hemi-head of the lower chamber. Preferences are the same as for the corresponding replacement stripper.

[0116] In an embodiment, the HP stripper has a liquid outlet through thebottom plate of the cylindrical chamber, and preferably comprises a vertical pipe, e.g. a degassing pipe, joined (e.g. welded) to the bottom plate, and preferably comprises a unit (in particular, the flow direction changing device) having a liquid inlet on top and a liquid outlet on the side. Preferably, the unit (in particular, the flow direction changing device) is welded to the vertical pipe (e.g. degassing pipe) that is welded at one end to the bottom plate and at another end to the unit, wherein preferably said unit is a forged unit or a machined unit. Hence, the preferably used vertical pipe is at one end welded to the flow direction changing device and at the other end to the bottom plate.

[0117] Preferences are the same as discussed in connection with the method ofmodifying the plant and the replacement stripper.

[0118] References to ‘upper’ and ‘lower’ for the HP stripper should be understoodby references to the vertical tube bundle and the position of the HP stripper when installed in a urea plant.

[0119] The use of such a HP stripper in a grassroots urea plant provides anadvantage of a lower elevation of the liquid inlet of the stripper and a corresponding lower elevation of the reactor and condenser. This in turn reduces the costs of the support structures for mounting the HP equipment at the desired elevation. The use of the inventive HP stripper in a revamping method are as discussed before and pertain to a higher urea production rate.

[0120] Details and preferences for the inventive HP stripper and inventive plantare as described in connection with the method of modifying a urea plant. Details and preferences for the inventive HP stripper also apply to the replacement stripper of the method of modifying a urea plant.

[0121] The stripper typically comprises a level gauge for measuring the liquidlevel in the cylindrical chamber of the lower chamber. In operation, a liquid level is maintained in the cylindrical chamber of the lower chamber of the stripper.

[0122] The manhole preferably comprises a flange for the cover plate, with theflange joined to the hemi-head. For example, the flange is inserted in an opening of the hemi-head and welded to the hemi-head.

[0123] The stripper usually also comprises a CO2 strip gas supply pipe,extending through the hemi-head in the lower chamber.

[0124] The upper chamber usually also comprises a manhole, for instance at theapex of a hemi-head.

[0125] Typically, the tubes protrude through the tubesheet, and the tube endsare located respectively in the upper and lower chamber.

[0126] Typically, the upper and lower chamber are provided with a surfaceexposed to process fluid made of corrosion resistant steel, e.g. duplex stainless steel, e.g. provided as overlay welding or welding.

[0127] The invention also provides a urea production plant comprising a HPsynthesis section comprising a reaction zone, a condensation zone and the inventive HP CO2 stripper, and a gas flow line from the stripper to the condensation zone, a liquid flow connection (inside a unit or between units) from the condensation zone to the reaction zone, and a liquid flow line from the reaction zone to the inlet for urea solution of the HP stripper. The plant usually also comprises at least an LP recovery section receiving stripped urea solution from the HP stripper, and the plant comprises a pressure-reducing valve in the liquid flow connection from the urea solution outlet of the HP stripper. The plant comprises a feed line for NH3 to the condensation section, in particular from an ammonia pump, and comprises a feed line for CO2 to the HP stripper, in particular from a CO2 compressor.

[0128] Details for the reaction zone, e.g. vertical urea reactor or pool reactor,and condensation zone (e.g. pool condenser or pool reactor) are as described before. The synthesis section is preferably arranged for gravity flow of liquid from the condensation zone through the reaction zone to the stripper.

[0129] The plant comprises e.g. a pool reactor having an undivided vesselcomprising a condensation zone (with a tube bundle) and a reaction zone, with the liquid flow connection between said zones being inside the pool reactor.

[0130] In a preferred embodiment of the inventive urea production plant, thepressure-reducing valve has an elevation above a baseline of the liquid flow connection for stripped urea solution, and this elevation is less than 4 times the diameter of the liquid flow connection, preferably less than 3 times said diameter; for instance the valve is at the baseline elevation. Details and further preferences for this embodiment are the same as for the modified plant. The feature of theelevation of the pressure-reduction valve can also be used independently of the position of the feature of the manhole in the lower chamber of the stripper.

[0131] The invention also provides a urea production process carried out in theinventive urea production plant, the process comprises condensing NH3 and CO2 in the condensation zone to form carbamate and converting the carbamate into urea and water in the reaction zone to give a urea synthesis solution, and subjecting at least a part of the urea synthesis solution to HP CO2 stripping in the stripper, wherein the stripper is the inventive stripper. The stripped urea solution is supplied e.g. to a recovery section as is customary in the art. The urea production process preferably involves gravity flow, i.e. without the use of pumps and ejectors, for flow of liquid from the condenser, through the reactor, to the urea solution inlet of the stripper.

[0132] Fig. 2 shows an example inventive HP stripper for a urea plant, whereinthe stripper (201) is a shell-and-tube heat exchanger with a vertical tube bundle (202). The stripper is configured to operate with a falling film of urea solution in the tubes of the vertical tube bundle. The HP stripper comprises an upper tubesheet (203) and a lower tubesheet (204), an upper chamber (205) above the upper tubesheet, and a lower chamber (206) below the bottom tubesheet. The stripper has a liquid inlet (207) in said upper chamber and a liquid outlet (208) in said lower chamber, a gas outlet (209) in the upper chamber, and a gas inlet (210) for stripping gas in the lower chamber. The lower chamber comprises a hemi- head (211) and a cylindrical chamber (212) arranged below the hemi-head. The cylindrical chamber comprises a bottom plate (214) and a sidewall (215). The HP stripper comprises a manhole (213) in the hemi-head of the lower chamber. Thereby the HP stripper is different from known HP strippers wherein the manhole is in the bottom plate of the cylindrical chamber (212). The position of the manhole contributes to advantageous low elevation of the HP stripper since access space below the liquid outlet (208), i.e. between the ground and the bottom plate in which the liquid outlet (208) is provided, is no longer necessary. Hence, the clearance between the ground / grade level and the bottom plate can be smaller. Moreover, the connection from the liquid outlet (208) does not need to be a removable spool piece, which is necessary in known plants where the bottom plate is removable to provide manhole access to the lower chamber. The inventive HP stripper is preferably a replacement stripper as preferably used in the method ofthe invention. As an example, the liquid outlet (208) can be provided in the bottom plate (214).

[0133] Figure 3 schematically illustrates a detail of the inventive HP stripper ofFig. 2, with the liquid outlet (208) of the HP stripper located in the bottom plate (214). In operation, a liquid level (301) is maintained in the cylindrical chamber (212). The bottom plate provides an obstruction for the flow of the liquid. The aperture in the bottom plate providing the liquid outlet is for instance max. 30 % of the surface of the bottom plate. The gas inlet (210) is not shown. It is noted that the manhole and the liquid outlet are schematically illustrated, with the relative sizes in practice being different. Illustrated is a vertical cross-section, showing that the sidewall (215) of the cylindrical chamber (212) is straight in vertical cross-section and the hemi-head (211) is curved in vertical cross-section. Also shown is the horizontal-cross section A-A through the cylindrical chamber (212).

[0134] Figure 4 illustrates another configuration of a lower chamber (206) ofthe inventive HP stripper. In this configuration, the liquid outlet (408) is provided by an aperture in the sidewall (215). The bottom plate (214) can be entirely closed, sealing off the bottom of the cylindrical chamber.

[0135] Figure 5 illustrates another configuration of the inventive HP stripper.The lower chamber (206) is the same as in Fig.3. Additionally, the HP stripper comprises a flow direction changing device (517) having a liquid inlet (518) on top and a liquid outlet (519) on a side. Further preferably, the HP stripper comprises a vertical pipe (516) extending between said liquid outlet (208) in the bottom plate (214) and said liquid inlet, wherein the vertical pipe (516) is welded at a first end to the bottom plate (214) and at a second end to said flow direction changing device (517). The vertical pipe can be a degassing pipe, with a relatively wide diameter, or a simple connection piece.

[0136] As illustrated, the channel height (520) of the flow direction changingdevice (517), being the vertical distance between the center of the liquid inlet (518) and the center of the liquid outlet (519) can be less than 2 times the diameter of the liquid inlet (518).

[0137] Figure 6 illustrates another configuration of a lower chamber (206) ofthe inventive HP stripper. In this configuration, the liquid outlet (208) is providedby an aperture in the hemi-head (211) and a bent pipe (601). The pipe contains, from the pipe inlet to the pipe outlet, a vertical upward part and the curved part.

[0138] Figure 7 shows the preferred relative vertical position of the valve (8) tothe liquid flow connection (6) for stripped urea solution in the modified plant. Reference signs are the same as in Figure 1. The pressure-reducing valve (8) has an elevation (701) above a baseline (702) of the liquid flow connection (6) for stripped urea solution, which elevation (701) is less than 4 times the diameter (703) of the (tube providing the) liquid flow connection (6). As illustrated, the elevation (701) is two times the diameter (703). The elevation indicates the difference in vertical position. The vertical position of the valve (8) is based on the center (704) of the outlet opening (18) of the valve. The vertical position of the baseline (702) of the liquid flow connection (6) is taken as the centerline of the tube providing the liquid flow connection for the stripped urea solution.

[0139] It may be observed that also in Fig. 1, the elevation of the valve (8b) tothe lowest position centerline of the liquid flow connection (6b) for the stripped urea solution is suitably small, namely nil.

[0140] Figure 8: Fig. 8A shows the existing plant, as illustrated in Fig. 1;Fig. 8B shows the modified plant, again as illustrated in Fig. 1.

[0141] HP indicates high pressure, e.g. is at least 100 bar, or at least 120 bar,typically less than 400 bar or less than 200 bar. MP indicates medium pressure, and is e.g.10 to 80 bar, e.g. 10 to 40 bar. LP indicates low pressure and is e.g. max. 10 bar, e.g. 1 to 10 bar. These ranges apply to process streams and not necessarily to steam. Pressures are absolute pressure (in bar, i.e. bara) unless otherwise indicated.

[0142] Carbamate refers to ammonium carbamate, as that term is used in theart. Carbamate decomposition refers to the decomposition of carbamate into NH3 and CO2.

Claims

Claims 1. A method of modifying an existing urea production plant (1) to give a modified urea production plant, the existing urea production plant comprising: -a high pressure synthesis section (2) comprising a reaction zone (3), acondensation zone (4), and a stripper (5), wherein said stripper has a bottom with a bottom elevation and comprises an upper chamber, a lower chamber and a tube bundle, and has a liquid inlet in said upper chamber and a liquid outlet in said lower chamber, wherein the synthesis section is configured for gravity flow of liquid from the condensation zone through the reaction zone to the stripper, and wherein the synthesis section comprises a liquid flow line (9) for urea solution from the reactor to the liquid inlet of the stripper; -and a liquid flow connection (6) for stripped urea solution from the liquidoutlet (7) of the lower chamber of the stripper to a pressure-reducing valve (8), wherein the method comprises:- changing the position of the stripper (5), such that the stripper in the modifiedplant has a lower bottom elevation and a lower elevation of the liquid inlet than in the existing plant, or installing a replacement stripper (5b), such that the replacement stripper (5b) in the modified plant has a lower bottom elevation and a lower elevation of the liquid inlet than the stripper in the existing plant;- modifying the liquid flow connection (6) for stripped urea solution to permit thelower bottom elevation of the stripper; and- modifying the liquid flow line (9) from the reactor to the liquid inlet of thestripper such that the outlet of the liquid flow line is at a lower elevation.

2. The method according to claim 1, further comprising changing the position of the pressure-reducing valve (8) such that the pressure-reducing valve (8) is at a lower elevation in the modified plant than in the existing plant.

3. The method according to claim 2, wherein the pressure-reducing valve (8) is an angle valve with a valve stem (19) and with an actuator (17) and an inlet (16), with the actuator and the inlet at opposed sides, wherein the method comprises: changing the orientation of the pressure-reducing valve (8) from having theinlet (16) at the bottom, the actuator (17) on top, and the valve stem vertically, to having the inlet (16) and the actuator (17) at a side and the valve stem horizontally.

4. The method according to any of claims 1-3, wherein in the modified plant the pressure-reducing valve (8) has an elevation (701) above a baseline of the liquid flow connection (6) for stripped urea solution of less than 4 times the diameter (703) of the liquid flow connection (6).

5. The method according to any of claims 1-4, wherein in the modified plant the liquid flow connection (6) for stripped urea solution comprises a device (517) that is a forged unit or machined unit having a liquid inlet (518) on top and a liquid outlet (519) on a side and having a channel height of maximum 2 times the diameter of said liquid inlet.

6. A high pressure (HP) stripper for a urea plant, wherein the HP stripper (201) is a shell-and-tube heat exchanger with a vertical tube bundle (202), configured to operate with a falling film of urea solution in the tubes of the vertical tube bundle, wherein the HP stripper comprises: - said vertical tube bundle (202), - an upper tubesheet (203), - a lower tubesheet (204), - an upper chamber (205) above the upper tubesheet, and - a lower chamber (206) below the lower tubesheet (204), wherein the HP stripper further comprises: -a liquid inlet (207) in said upper chamber,- a liquid outlet (208) in said lower chamber,- a gas outlet (209) in the upper chamber,- and a gas inlet (210) for stripping gas in the lower chamber,wherein the lower chamber comprises a hemi-head (211) and a cylindrical chamber (212) arranged below the hemi-head; wherein the HP stripper comprises a manhole (213) in the hemi-head of the lower chamber.

7. The HP stripper according to claim 6, wherein the cylindrical chamber (212) comprises a bottom plate (214) and a vertical cylindrical sidewall (215) , and wherein the cylindrical chamber (212) is open at an upper end thereof and is configured to receive urea solution from the hemi-head (211) at said upper end.

8. The HP stripper according to claim 7, wherein the bottom plate (214) is welded to the sidewall (215).

9. The HP stripper according to claim 7 or 8, wherein said liquid outlet (408) is provided in said sidewall (215) of said cylindrical chamber (212).

10. The HP stripper according to claim 7 or 8, wherein the said liquid outlet (208) of said stripper is provided in the bottom plate (214) of the cylindrical chamber (212), and wherein the HP stripper further comprises a flow direction changing device (517) having a liquid inlet (518) on top and a liquid outlet (519) on a side, and wherein the HP stripper comprises a vertical pipe (516) extending between said liquid outlet (208) in the bottom plate (214) and said liquid inlet, wherein the vertical pipe (516) is welded at a first end to the bottom plate (214) and at a second end to said flow direction changing device (517). 11 The HP stripper according to claim 10, wherein said flow direction changing device (517) is a forged or machined unit.

12. The HP stripper according to claim 6, 7 or 8, wherein the HP stripper has a siphon type liquid outlet (208) comprising a bent pipe (601) comprising a pipe part protruding through the hemi-head (211) of the lower chamber.

13. A urea production plant comprising a HP synthesis section comprising a reaction zone (3), a condensation zone (4) and a HP CO2 stripper (5b), wherein the HP CO2 stripper is according to any of claims 6-12, and wherein the plant further comprises, in the HP synthesis section, a gas flow line (13) from the stripper (5b) to the condensation zone (4), a liquid flow connection (12) from the condensation zone (4) to the reaction zone (3), and a liquid flow line (9) from the reaction zone (3) to the inlet for urea solution of the HP stripper (5b).

14. A urea production process carried out in the urea production plant of claim 13, wherein the process comprises: - condensing NH3 and CO2 in the condensation zone to form carbamate, - converting the carbamate into urea and water in the reaction zone to give a urea synthesis solution, and - subjecting at least a part of the urea synthesis solution to HP CO2 stripping in the HP CO2 stripper.

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