Thermal stripping with HP-MP heat exchange

Abstract

The disclosure pertains to a thermal stripping type urea plant having a HP thermal stripper, a first HP carbamate condenser that is a kettle-type boiler, and a first MP decomposer for stripped urea solution, a second MP decomposer receiving a part of the urea solution from the synthesis section, and having a second HP carbamate condenser for gas from the stripper and / or a part of the CO2 feed upstream of the kettle-type boiler, wherein said second MP decomposer and second HP carbamate condenser are provided as compartments of a process-process heat exchanger.

Description

P137909PC00 Title: THERMAL STRIPPING WITH HP-MP HEAT EXCHANGE Field

[0001] The invention pertains to the production of urea, in particular to a ureaplant and process using a high pressure stripper of the thermal stripping type. The invention also pertains to a method of modifying an existent urea production plant of the thermal stripping type. Introduction

[0002] Ullmann’s Encyclopedia of Industrial Chemistry, chapter Urea, 2010,provides a general discussion of plants and processes for producing urea. One type of urea plant uses a thermal stripper, such as the “Snamprogetti” type of urea production process. A stripper of the thermal type allows for recycling non- converted NH3 and CO2 by heating a high pressure urea synthesis solution to decompose ammonium carbamate present in said high pressure urea synthesis solution and transfer of NH3 and CO2 into the gas phase by a falling film heat exchanger with urea solution in the tubes.

[0003] There is an increasing environmental awareness and a desire to provide aurea plant and process allowing for improved energy efficiency, thereby offering a urea production process with a reduced carbon footprint and a reduced cost of producing urea.

[0004] There is also desire in the field for urea plants with an increased capacity,and for a method of modifying an existing plant to increase its production capacity.

[0005] US 2022 / 0274916 describes a urea production plant and process using athermal stripper in which a reaction mixture from the reactor is separated in into a first part supplied to the thermal stripper and to a the medium pressure (MP) recovery section and a second part that bypasses the thermal stripper and is supplied to an additional MP recovery section.

[0006] There remains a desire for improved urea production plants and processes.For instance, it is desired to provide urea production plants and processes with an improved production capacity and high energy efficiency (low steam consumption).Summary

[0007] The invention provides, in an aspect, a urea production plant comprising ahigh pressure (HP) synthesis section, a medium pressure (MP) recovery section, and an MP treatment section, wherein the HP synthesis section comprises an HP reactor, an HP thermal stripper, and a first HP carbamate condenser that is preferably a kettle-type condenser; wherein the HP synthesis section has an outlet for urea solution connected by a liquid flow line with an inlet of the MP treatment section such that the MP treatment section receives, in operation, a part of the urea solution from the HP synthesis section is supplied, at medium pressure, to the MP treatment section.

[0008] Preferably, the HP reactor has one or more outlets for a urea synthesissolution and is connected with a first flow line for supplying a first part of the urea synthesis solution to the thermal stripper and with a second flow line for supplying a second part of the urea synthesis solution to the MP treatment section bypassing the thermal stripper; and / or the outlet of the HP thermal stripper for stripped urea solution is connected to supply a first part of the stripped urea solution to the MP treatment section and a second part of the stripped urea solution to the MP recovery section.

[0009] In the inventive urea production plant, the synthesis section furthercomprises a second HP carbamate condenser, the second HP carbamate condenser is connected to receive an HP gas stream from the thermal stripper and / or a part of the CO2 feed, and the second HP carbamate condenser has an outlet connected to an inlet of the first HP carbamate condenser to supply a liquid stream containing carbamate, and optionally also gas, from the second HP carbamate condenser to the first HP carbamate condenser. In the inventive urea production plant, the MP recovery section comprises a first MP decomposer connected to receive a part or all of the stripped urea solution from the thermal stripper, and the MP treatment section comprises, in series for an MP urea solution, a first gas / liquid MP separator, a second MP decomposer, and a second gas / liquid MP separator; and the second HP carbamate condenser and the second MP decomposer are provided as heat exchanging compartments of a process-process heat exchanger.

[0010] The invention provides, in an embodiment, a urea production plantcomprising a high pressure (HP) synthesis section, a medium pressure (MP)recovery section, and an MP treatment section, wherein the HP synthesis section comprises an HP reactor, an HP thermal stripper, and a first HP carbamate condenser; wherein the HP reactor has one or more outlets for a urea synthesis solution and is connected with a first flow line for supplying a first part of the urea synthesis solution to the thermal stripper and with a second flow line for supplying a second part of the urea synthesis solution to the MP treatment section bypassing the thermal stripper; wherein the synthesis section further comprises a second HP carbamate condenser connected to receive an HP gas stream from the thermal stripper and having an outlet connected to an inlet of the first HP carbamate condenser; wherein the MP recovery section comprises a first MP decomposer connected to receive stripped urea solution from the thermal stripper; wherein the MP treatment section comprises, in series for an MP urea solution (i.e. for the second part of the urea synthesis solution), a first gas / liquid MP separator, a second MP decomposer, and a second gas / liquid MP separator; and wherein the second HP carbamate condenser and the second MP decomposer are provided as heat exchanging compartments of a process-process heat exchanger.

[0011] The invention also provides urea production process carried out in theinventive urea production plant. The process comprises: reacting CO2 and NH3 in the HP reactor to give a urea synthesis solution and, optionally, dividing said urea synthesis solution into a first part of the urea synthesis solution and a second part of the urea synthesis solution; stripping urea synthesis solution, e.g. all or the first part of the urea synthesis solution, in the HP thermal stripper to give a stripped urea solution and an HP gas stream; supplying urea solution from the synthesis section in part to the first MP decomposer, preferably wherein the stripped urea solution is supplied all or in part to the first MP decomposer; subjecting a first gas stream comprising CO2 and preferably NH3, for example the HP gas stream, to carbamate condensation in the second HP carbamate condenser to yield a condensed liquid (carbamate solution) and uncondensed gas; supplying the condensed liquid to the first HP carbamate condenser and preferably also the uncondensed gas, e.g. as a fluid stream comprising said condensed liquid and said uncondensed gas; and supplying a second gas stream comprising CO2 and NH3, preferably the uncondensed gas stream, to the first HP carbamate condenser;

[0012] supplying a part of the urea solution from the synthesis section to the firstgas / liquid MP separator of the MP treatment section thereby obtaining a flashedMP urea solution (preferably by expanding the second part of the urea synthesis solution to a medium pressure and supplying said second part to the first gas / liquid MP separator of the MP treatment section bypassing the thermal stripper to give a flashed MP urea solution; and / or expanding a first part of the stripped urea solution to medium pressure and supplying said urea solution to the first gas / liquid MP separator); heating the flashed MP urea solution in the second MP decomposer thereby decomposing carbamate in the flashed MP urea solution to give an MP biphasic stream; and separating the MP biphasic stream in the second gas / liquid MP separator; wherein the second HP carbamate condenser and the second MP decomposer are provided as heat exchanging compartments of a process-process heat exchanger and exchange heat via a heat exchanging wall of said process- process heat exchanger or wherein the first HP carbamate condenser and the second MP decomposer are provided as heat exchanging compartments of a process-process heat exchanger and exchange heat via a heat exchanging wall of said process-process heat exchanger.

[0013] In an embodiment, the process comprises: reacting CO2 and NH3 in the HPreactor to give a urea synthesis solution and dividing said urea synthesis solution into a first part of the urea synthesis solution and a second part of the urea synthesis solution; stripping the first part of the urea synthesis solution in the HP thermal stripper to give a stripped urea solution and an HP gas stream, wherein the stripped urea solution is supplied to the first MP decomposer; condensing the HP gas stream in the second HP carbamate condenser to yield a partially condensed stream; supplying the partially condensed stream to the first HP carbamate condenser; expanding the second part of the urea synthesis solution to a medium pressure and supplying said second part to the first gas / liquid MP separator of the MP treatment section bypassing the thermal stripper to give a flashed MP urea solution; heating the flashed MP urea solution in the second MP decomposer thereby decomposing carbamate in the flashed MP urea solution to give an MP biphasic stream; separating the MP biphasic stream in the second gas / liquid MP separator; wherein the second HP carbamate condenser and the second MP decomposer are provided as heat exchanging compartments of a process-process heat exchanger and exchange heat via a heat exchanging wall of said process-process heat exchanger.

[0014] The invention also provides a method of modifying an existing urea plant ofthe thermal stripping type, wherein the existing plant comprises a high pressure (HP) synthesis section, a medium pressure (MP) recovery section; wherein the HP synthesis section comprises a HP reactor, a HP thermal stripper, a first HP carbamate condenser that is preferably a kettle-type condenser; wherein the MP recovery section comprises a first MP decomposer connected to receive a part or all of the stripped urea solution from the thermal stripper; wherein the method comprises: adding a process-process heat exchanger comprising a first compartment and a second compartment that are in heat exchanging contact through a wall, wherein the first compartment provides a second MP decomposer and the second compartment provides a second HP carbamate condenser; adding an MP expansion valve, first MP gas / liquid separator, a second MP gas / liquid separator, and a second MP carbamate condenser; connecting the second MP decomposer to receive a part of the urea synthesis solution from the HP reactor through a liquid flow line that bypasses the thermal stripper and goes through MP expansion valve and the first MP gas / liquid separator and / or to receive a part of the stripped urea solution from the HP thermal stripper; connecting the second gas / liquid MP separator to receive fluid from the second MP carbamate decomposer and connecting the second MP carbamate condenser to receive gas from the second gas / liquid MP separator; connecting the second HP carbamate condenser to receive gas, preferably all gas, from the thermal stripper and / or a part of the CO2 feed, and to supply a a liquid stream containing carbamate, and optionally also gas, to the first HP carbamate condenser. Preferably in the method, the process-process heat exchanger (PPHE) is a shell-and-tube heat exchanger, with the tube bundle as the second MP decomposer and the shell side space as the second HP carbamate condenser. Preferably the modified plant is in accordance with the inventive plant.

[0015] In an embodiment of said method, the existing plant comprises an HPreactor, an HP thermal stripper, a first HP carbamate condenser; wherein the existing plant further comprising an MP recovery section comprising a first MP decomposer connected to receive a stripped urea solution from the thermal stripper; and the method in the embodiment comprises: adding a process-process heat exchanger comprising a first compartment and a second compartment that are in heat exchanging contact through a wall, wherein the first compartment provides a second MP decomposer and the second compartment provides a second HP carbamate condenser; adding an MP expansion valve, first MP gas / liquidseparator, a MP second gas / liquid separator, and a second MP carbamate condenser; connecting the second MP decomposer to receive a part of the urea synthesis solution from the HP reactor through a liquid flow line that bypasses the thermal stripper and goes through MP expansion valve and the first MP gas / liquid separator; connecting the second gas / liquid MP separator to receive fluid from the second MP carbamate decomposer and connecting the second MP carbamate condenser to receive gas from the second gas / liquid MP separator; connecting the second HP carbamate condenser to receive gas, preferably all gas, from the thermal stripper and to supply a partially condensed stream to the first HP carbamate condenser; wherein the modified plant is preferably an inventive plant.

[0016] The disclosure generally pertains to a thermal stripping type urea planthaving a HP thermal stripper, a first HP carbamate condenser that is preferably a kettle-type boiler (i.e., the carbamate condensation occurs in the tube bundle of the kettle-type boiler), and a first MP decomposer for stripped urea solution, and a second MP decomposer receiving a part of the urea solution from the synthesis section, e.g. urea solution that is not stripped in the thermal stripper, and having a second HP carbamate condenser for gas from the stripper upstream of the kettle- type boiler, wherein said MP decomposer and second HP carbamate condenser are provided as compartments of a process-process heat exchanger. Brief description of the drawings

[0017] Figure 1 schematically illustrates an example of a urea production plant ofthe thermal stripping type not according to the invention. Figure 2 schematically illustrates a reference example urea production plant of the thermal stripping type not according to the invention. Figure 3 schematically illustrates an example of a urea production plant according to the invention. Figure 4 schematically illustrates an example of a medium pressure recovery section of the urea production plant according to the invention. Figure 5 schematically illustrates a further example process scheme according to a further embodiment of the invention

[0018] Any embodiments illustrated in the figures are examples only and do notlimit the invention.Detailed description

[0019] The inventors found that by introducing an additional shell-and-tube heatexchanger in the synthesis section of a thermal stripping type urea plant, which receives (directly or indirectly) a gas stream originating from the thermal stripper and / or a part of the CO2 feed on the shell side, and a medium pressure flashed carbamate-containing urea solution corresponding to a part of the urea production in the tube bundle, preferably as non-stripped urea solution, an advantageously low energy consumption of the plant can be achieved by utilizing process-to-process heat exchange in said heat exchanger, in particular for effecting a desired heating of said urea solution to promote carbamate decomposition thereby purifying said urea solution. This process-process heat exchanger shell side compartment provides, in an embodiment, a second HP carbamate condenser, in addition to the first HP carbamate condenser of the plant that is a kettle-type condenser (with gas to be condensed in the tube bundle and with steam raised in the shell side space).

[0020] In the invention, the liquid containing carbamate from the second HPcarbamate condenser is supplied to the first HP carbamate condenser and optionally also the gas from the second HP carbamate condenser is supplied to the first HP carbamate condenser. The second HP carbamate condenser provides residence time for a part of the carbamate to convert into urea and water, which are low volatility / high boiling temperature components of the liquid phase transported from the second to the first HP carbamate condenser. Thereby, the condensation temperature in the first HP carbamate condenser advantageously increases, which advantageously permits for raising steam at a higher pressure in the shell of the kettle-type boiler used as the first HP carbamate condenser.

[0021] In a further embodiment, the second HP carbamate condenser is a kettle-type boiler and the shell side compartment of the process-process heat exchanger is the first HP carbamate condenser, and liquid containing carbamate from the second HP carbamate condenser is supplied is supplied from the kettle-type boiler (second condenser) to the shell side compartment (first HP carbamate condenser).

[0022] In particular, the inventors observed that subjecting to carbamatecondensation of the gas stream originating from the thermal stripper in the shell side space of said heat exchanger, which is an exothermic reaction (carbamate formation), not only generates a sufficient amount of heat to decompose a desiredamount of carbamate in the medium pressure urea solution (which is an endothermic reaction) supplied in the tube bundle of the heat exchanger but also that the temperature in the shell side compartment is high enough, by virtue of the composition of gas and liquid phase therein, such that in particular efficient heat exchange is achieved even with a relatively small heat exchanging surface area, and to achieve significant carbamate decomposition in the MP urea solution, i.e. the flashed MP urea solution, in the tube bundle. Even more, only partial condensation needs to be achieved in the shell side compartment, such that the partially condensed stream (i.e., gas and liquid) can be supplied to the kettle-type carbamate condenser (first HP carbamate condenser) of the thermal stripping urea plant. These advantages are further illustrated in Example 1, Plant 3.

[0023] Thereby, the medium pressure decomposer in the medium pressuretreatment section does not need steam as a heating fluid, unlike in a known plant without the process-process heat exchange, which in turn reduces steam consumption of the urea plant.

[0024] Additionally, because a part of the gas stream originating from the thermalstripper is in a preferred embodiment already partially condensed at the shell side of the heat exchanger, the first HP carbamate condenser, e.g. a kettle-type condenser, can be advantageously used to raise higher quality steam, i.e. steam having a higher pressure, than is normally obtained in said unit in a thermal stripping urea production process. In addition, the shell side of the additional heat exchanger provides additional reaction volume such that the urea formation reaction already in part takes place there. This advantageously can be used to increase the capacity of an existing plant in a so-called revamp method and provides advantages for the condensation in the shell side, the heat exchange in the process-process heat exchanger, and the operation of the downstream kettle- type carbamate condenser.

[0025] US 2015 / 0119603 describes decomposing MP urea solution in a first tubebundle of a pool condenser of a urea plant also having a second tube bundle used for raising steam. The HP stripper uses CO2. In the figure, the stripped urea solution is supplied at MP to the first tube bundle.

[0026] In the invention, the second HP carbamate condenser receives a part or allof the gas from the stripper. In an embodiment, all of the gas stream from the thermal stripper is supplied to the second HP carbamate condenser, which isoperated with partial condensation, and non-condensed gas and liquid from the HP carbamate condenser are supplied to the first HP carbamate condenser, where the gas is subjected to further carbamate condensation. In another embodiment, a first part of the gas stream from the thermal stripper is supplied to the second HP carbamate condenser, only liquid from the second HP carbamate condenser is supplied to the first HP carbamate condenser, and a second part of the gas stream from the thermal stripper is supplied directly to the first HP carbamate condenser.

[0027] In an embodiment, the second MP decomposer receives a part of thestripped urea solution from the thermal stripper and / or a part of the urea solution from the reactor bypassing the thermal stripper. Generally, the first MP decomposer receives a first part of the urea solution from the synthesis section and the second MP decomposer receives a second part of the urea solution from the synthesis section. Hence, the first and second MP decomposer are arranged in parallel for urea solution.

[0028] The HP synthesis section comprises an HP reactor (e.g. a vertical ureareactor), an HP thermal stripper, and a first HP carbamate condenser, and typically an ejector and a carbamate separator. The stripper, first HP carbamate condenser, ejector and vertical urea reactor are e.g. placed on ground level. The reactor has a fluid outlet connected to the stripper, the stripper has a gas outlet connected directly or indirectly to the first HP carbamate condenser, and the first HP carbamate condenser has an outlet for a liquid comprising carbamate connected to an inlet of the reactor, typically through the ejector.

[0029] In the HP reactor, which can be one or more reactors in series and / or inparallel, CO2 and NH3 are reacted under urea forming conditions to yield the HP synthesis solution comprising urea, water, carbamate, and unconverted NH3. Typically, the HP reactor is operated at a temperature in the range of 180 to 200ºC, at a pressure in the range of 140 to 170 bar. Typically, the synthesis section is operated at an N / C ratio in the range between 3.0 and 3.6, e.g. 3.2 and 3.4. These ranges are advantageous for operating the plant with thermal stripping.

[0030] The HP reactor is typically a vertical vessel with a plurality of perforatedtrays, with one or more inlets at the bottom and an outlet at the top, usually only one outlet. The reactor is operated as a bubbling column reactor. The reactor typically receives CO2 gas and, separately, a stream comprising feed NH3 and carbamate from the ejector.

[0031] The first HP carbamate condenser is preferably a kettle-type boiler, i.e. ashell-and-tube heat exchanger with a horizontal U-shaped tube bundle (with the legs of the tubes in the horizontal direction), operated with carbamate condensation in the tubes and with steam being raised from boiler feed water in the shell side compartment.

[0032] The HP reactor has one or more outlets for a urea synthesis solution and isconnected with a first flow line for supplying a first part of the urea synthesis solution to the thermal stripper and preferably also with a second flow line for supplying a second part of the urea synthesis solution to the MP treatment section; the preferred second flow line bypasses the thermal stripper. Hence, the MP treatment section preferably receives urea solution from the reactor that has not been stripped in the thermal stripper and that is reduced in pressure from HP to MP. The flow lines are liquid flow lines. More generally, the MP recovery section and the MP treatment section each receive a part of the urea solution from the urea synthesis section, which urea solution also contains carbamate. The above- mentioned first and second part of the urea synthesis solution also both contain urea-containing solution, which solution also comprises carbamate. The fluid in the first and optional second flow line may include gas in addition to the urea solution. For example, the reactor has two outlets each connected to a respective flow line, or a single outlet and the plant comprises a branch of the flow line into two different liquid flow lines.

[0033] Optionally, the second part of the urea solution is degassed before beingsupplied to the MP treatment section, e.g. using an HP separator comprising a funnel structure as shown in Fig. 3 or Fig. 4 of US 2022 / 0274916. Optionally, the MP treatment section receives a degassed non-stripped urea synthesis solution. Degassing is helpful for determining the N / C ratio of the urea solution.

[0034] The second HP carbamate condenser and the second MP decomposer areprovided in the invention as heat exchanging compartments of a process-process heat exchanger. Preferably, the process-process heat exchanger is provided as a single unit comprising two compartments that are in heat exchange contact with each other through a heat exchanging wall, the first compartment providing the second HP carbamate condenser and the second compartment providing the second MP decomposer. Thereby, process-process heat exchange is provided, i.e. heat exchange is directly through the wall between said two compartments. The wall isimpermeable for fluids, i.e. separates the compartments operating at MP and HP respectively, and permits heat exchange. The wall is e.g. provided by the walls of the tubes of the tube bundle. Compared to heat exchange by a steam network, process-process heat exchange is more efficient.

[0035] Thereby, condensation of the HP gas stream from the thermal stripper,and / or a part of the CO2 feed in the second HP carbamate condenser advantageously generates heat which is efficiently used in the second MP decomposer to decompose carbamate in the flashed MP urea solution.

[0036] The term ‘carbamate condenser’ includes an embodiment wherein a CO2 gasstream is contacted with ammonia-containing liquid in that unit, with the formation of carbamate in the liquid, and with the liquid being cooled.

[0037] In the second HP carbamate condenser, CO2 and NH3 react to formcarbamate, the carbamate is present in the liquid phase, with at least the CO2 provided by a HP gas stream supplied to the second HP carbamate condenser. This gas stream is e.g. a part of the gaseous CO2 feed or a part of the gas from the stripper, and is supplied to the second HP carbamate condenser.

[0038] The NH3 that is reacted in the carbamate formation in the carbamatecondenser may also be provided by said HP gas stream or by another HP gas stream or by a liquid stream that is supplied to the second HP carbamate condenser, e.g. the liquid is an ammonia-rich carbamate solution from the MP recovery section (as recycle stream and / or as part of the NH3 feed), t, or by a combination of a gas stream and a liquid stream.

[0039] Preferably, the second part is an amount of 10 – 30 wt.% of the ureasynthesis solution, more preferably 15 – 25 wt.% of the urea solution (e.g., on the basis of the urea); preferably the MP treatment section receives such part of the total urea, in particular the second MP decomposer receives 10 – 30 wt.% of the urea produced, more preferably 15 – 25 wt.%.

[0040] Preferably the first part is 70 – 90 wt.% of the urea synthesis solution.Hence, e.g. about 80% of the urea solution, in particular the urea therein, from the reactor is supplied to the thermal stripper as the first part and about 20% is supplied to the second MP decomposer bypassing the thermal stripper, in an embodiment. Such split ratio advantageously allows for not only increasing urea production capacity of the urea production plant in this embodiment by processingthe second part of the urea synthesis solution in the MP treatment section bypassing the thermal stripper, but also allows for providing a suitable (not too large) amount of carbamate in the second part of the urea synthesis solution, such as the flashed MP urea solution, that can be decomposed by the amount of heat provided by carbamate condensation of the HP gas stream, (preferably originating from HP stripping of the first part of the urea synthesis solution), in the second HP carbamate condenser. This in turn allows for operating the second MP decomposer exclusively with the heat supplied by the heat exchanging contact (direct through a wall) with the second HP carbamate condenser instead of utilizing steam in said second MP decomposer.

[0041] Furthermore, preferably the second MP decomposer receives 10 – 30 wt.% ofthe urea produced in the HP synthesis section, more preferably 15 – 25 wt.%. This allows for operating the second MP decomposer exclusively with the heat supplied by the heat exchanging contact (direct through a wall) with the second HP carbamate condenser instead of utilizing steam in said second MP decomposer.

[0042] Preferably, in the process, the molar ratio of CO2 in the flashed MP ureasolution at the inlet of the second MP decomposer, including CO2 present as carbamate, to CO2 in the gas stream at the inlet of the second HP carbamate condenser, is in the range of at least 0.1 and / or max. 0.3, or max. 0.25, or max. 0.20, e.g.0.1 – 0.25, or 0.1 – 0.20. The ratio being not too high ensures that sufficient gas condenses relative to carbamate to be decomposes in the process-process heat exchanger. The ratio being not too low ensures useful operation of the MP treatment section and capacity increase in case of a revamp. The ratio can be adjusted by the split ratio of the urea synthesis solution and the stripping efficiency of the thermal stripper, i.e. the stripping efficiency known as “alpha” in the field.

[0043] The first part of the urea synthesis solution is at least in part, such asentirely, e.g. for at least 90 wt.% of the liquid phase, sent to the HP stripper of the thermal stripping type. Preferably, the HP stripper receives 70 – 90 wt.% of the urea produced in the synthesis section.

[0044] The reactor usually operates at slightly higher pressure than the stripper.The HP synthesis section preferably comprises a valve for reducing a pressure of the first part of the urea synthesis solution that is supplied to a HP thermalstripper, more preferably for decreasing of a pressure of the HP synthesis solution, e.g. by 5-20 bar.

[0045] Thermal stripping is also known as self-stripping in the art. The HPthermal stripper may be provided as a vertical shell-and-tube sheet exchanger of the falling film type, wherein the urea synthesis solution is provided in the tube bundle. Typically, the HP thermal stripper does not comprise an inlet for CO2 feed and it is operated as a thermal stripper. The HP thermal stripper comprises an inlet for the first part of the urea synthesis solution and an outlet for an HP gas in an upper chamber connected to the tube bundle, and an outlet for a stripped urea solution at a lower chamber connected to the bottom end of the vertical tube bundle. Steam is supplied to the shell side space for heating. The tube bundle comprises e.g. bimetallic tubes, suitable for the high temperatures.

[0046] The HP thermal stripper is preferably operated at a temperature in therange of 200 to 210ºC and at a pressure in the range of 140 to 160 bar (temperature and pressure measured at the outlet for a stripped urea solution).

[0047] The HP gas stream from the stripper is supplied, preferably entirely, to theinlet of the second HP carbamate condenser.

[0048] Optionally, some HP CO2 gas is supplied to the bottom of the thermalstripper, generally maximum 20 % or max. 10% or even max.5 % of the total CO2 feed. Hence, the thermal stripping is not based on HP CO2 stripping. The process may involve MP CO2 stripping in the MP treatment section, typically adiabatic MP CO2 stripping.

[0049] The HP gas stream is subjected to carbamate condensed in the second HPcarbamate condenser to yield a biphasic fluid stream comprising gas and a solution comprising carbamate, e.g. a partially condensed stream. The biphasic fluid stream, preferably the partially condensed stream, comprises a gas and a liquid phase. At least the liquid phase of the biphasic fluid stream (e.g. the partially condensed stream) is supplied to the first HP carbamate condenser, optionally also the gas phase. Hence, the first HP carbamate condenser is downstream of the second HP carbamate condenser for liquid, in particular for the solution comprising carbamate.

[0050] In operation, the second HP carbamate condenser is operated at HPpressure, e.g.140 - 150 bara, typically 2 – 10 bar lower than the reactor, and e.g. with an outlet temperature of 170 – 190ºC

[0051] In an embodiment, the process-process heat exchanger is preferably a shell-and-tube heat exchanger with a tube bundle and a shell side space, wherein the shell side space provides the second HP carbamate condenser and the tube bundle provides the second MP decomposer. In operation, the HP gas stream is preferably supplied to the shell side space of the second HP carbamate condenser and the tube bundle of the second MP decomposer receives the flashed MP urea solution.

[0052] More preferably, in operation at least 20 wt. % of the total urea formed inthe synthesis section is formed in second HP carbamate condenser, in embodiments with condensation in the shell side space, and e.g. up to 50 wt.%, such as 25 – 40 wt.% of the total urea.

[0053] Preferably, the second HP carbamate condenser provides a reaction volumethat is at least 30% or at least 40% and / or up to 70% or up to 60% of the reaction volume of the reactor of the reactor, e.g. 30 – 70 %, or 40 – 60 %, in particular with condensation in the shell-side compartment of process-process shell-and-tube heat exchanger.

[0054] The reaction volume is provided e.g. by increasing the space between theouter tube limit of the tube bundle and the shell of the process-process heat exchanger, i.e. over the diameter of the heat exchanger, and / or by using a U-shaped tube bundle and a free space / clearance, in the length of the heat exchanger, i.e. in the direction parallel to the straight tube parts, between the bend of the U-shaped tube bundle and the shell of the heat exchanger. For a vertical heat exchanger, preferably the reaction volume is provided by increasing the space over the diameter.

[0055] Thereby, the shell side advantageously provides an additional reactionvolume where both carbamate condensation and urea formation may take place due to an extended residence time in the second HP carbamate condenser. This additional reaction volume may advantageously increase capacity of the urea plant, in particular of an existing urea plant. Furthermore, due to urea formation in the second HP carbamate condenser (on the shell side), the composition of a liquid phase in the second HP carbamate condenser changes which leads to an increase ofboiling temperature of said liquid due to the presence of urea. Increased boiling temperature of the resulting partially condensed stream from the second HP carbamate condenser advantageously allows for further condensing said stream at a higher temperature compared to a typical condensation temperature in the first HP carbamate condenser which in turn allows for raising steam with higher temperature and therefore higher pressure on the shell side of the first HP carbamate condenser.

[0056] Preferably, the second HP carbamate condenser is a submerged carbamatecondenser. Preferably, the shell side space of the process-process heat exchanger, i.e. the second HP carbamate condenser, is operated with liquid as the continuous phase in operation, for example the shell side space is filled with condensate in operation, with condensing gas bubbles. This leads to better condensation compared to e.g. surface condensers.

[0057] Preferably, the shell side space of the process-process heat exchanger hasonly one outlet arranged at an upper part of the shell side space, or has an overflow weir in case of a horizontal tube bundle.

[0058] For example, the process-process heat exchanger has a horizontal tubebundle, preferably a U-shaped tube bundle. An advantage of a horizontal tube bundle is a relatively low dynamic pressure drop on the shell side.

[0059] In an embodiment, the process-process heat exchanger has a vertical tubebundle, for instance a vertical U-shaped tube bundle. In case of a vertical U-shaped tube bundle, the tube ends can be at the bottom or at the top. This embodiment provides the advantage of a relatively small footprint, which is especially useful in case of modifying an existing plant.

[0060] The process-process heat exchanger can be placed at ground level.

[0061] Preferably, the shell side space of the process-process heat exchangercomprises a plurality of compartments; preferably the compartments are separated by baffles and / or trays; especially if the process-process heat exchanger has a vertical tube bundle. The compartments are arranged in series for the liquid.

[0062] This contributes to urea formation by ensuring proper residence time.Thereby the invention provides for using a shell-and-tube heat exchanger with a compartmentalized shell space and having process streams in the tube bundle and in the shell side space, also with a vertical tube bundle.

[0063] The second carbamate condenser preferably comprises a gas distributor onthe shell side to distribute the gas into the liquid. In case of a horizontal tube bundle, a sparger can be used. In case of a vertical tube bundle, a tray or a gas divider can be used.

[0064] The shell-and-tube heat exchanger comprises corrosive process media in thetube bundle and in the shell. The heat exchange for example comprises a redistribution chamber connected to the tube bundle as described in US 2020 / 0306663.

[0065] Also possible is to use a tube sheet manufactured as described inUS 2015 / 0086440 using a method wherein sleeves are inserted in bore holes through the tube sheet such that the sleeves extend through the tube sheet.

[0066] In a particularly advantageous embodiment, the plant comprises a liquidflow connection from an outlet for a first MP carbamate solution of the MP recovery section to the inlet of the second HP carbamate condenser and / or a liquid flow connection from an outlet for the second MP carbamate solution of the MP treatment section connected to an inlet of the second HP carbamate condenser; more preferably both liquid flow lines. Preferably the second HP carbamate condenser receives both the first and second MP carbamate solution. Preferably the first and second MP carbamate solution are supplied entirely to the second HP carbamate condenser.

[0067] In an embodiment, the modified plant preferably comprises the abovementioned liquid flow connections, such that the method of modifying an existing plant comprises adding a liquid flow line for a first MP carbamate solution originating from the MP recovery section connected to the inlet of the second HP carbamate condenser and / or adding a liquid flow line for a second MP carbamate solution originating from the MP treatment section connected to the inlet of the second HP carbamate condenser; more preferably adding both liquid flow line. It is observed that the existing plant usually already comprises a liquid flow line for a first MP carbamate solution originating from the MP recovery section to the synthesis section, but to the kettle-type boiler.

[0068] Thereby, preferably, the first MP carbamate solution from the MP recoverysection is supplied to the second HP carbamate condenser and / or the second MP carbamate solution from the MP treatment section is supplied to the second HPcarbamate condenser, more preferably both. This contributes to good condensation and urea formation in the second HP carbamate condenser.

[0069] Preferably, the first MP carbamate solution from the MP recovery sectionhas an N / C ratio of 3.0 to 5.0, e.g. in the range 3.5 – 4.5.

[0070] Preferably, the second MP carbamate solution from the MP treatmentsection has N / C ratio between 2.2 and 2.7, more preferably 2.25 – 2.55, or even more preferably 2.35 – 2.45. This advantageous N / C ratio is achieved by the optimized condensation in the second MP carbamate condenser.

[0071] An advantage of supplying the first and / or the second MP carbamatesolution in the second HP carbamate condenser is that carbamate present in said MP carbamate streams can be converted to urea, particularly when the first and / or the second MP carbamate solution is supplied to the shell space of the second HP carbamate condenser, more preferably both. The skilled person realizes that formation of urea from carbamate requires a sufficient reaction volume and residence time, and that the reaction volume and residence time at the tube side is not sufficient for a slow endothermic formation of urea. Thereby, the second HP carbamate condenser, in particular its shell space, provides an additional reaction volume which in turn increases urea production capacity.

[0072] Furthermore, presence of urea in the partially condensed stream from thesecond HP carbamate condenser, resulting also from urea formation of the carbamate present in the first and / or second MP carbamate solution, more preferably both, advantageously increases boiling temperature even further, compared to supplying only the HP gas steam in the second HP carbamate condenser, which contributes to a possibility of raising higher pressure steam in the first HP carbamate condenser. It may be observed that also in the second HP carbamate condenser a higher condensation temperature by the urea formation in it is beneficial.

[0073] The partially condensed stream is supplied to the first HP carbamatecondenser. The first HP carbamate condenser may receive a part or an entire first and / or second MP carbamate solution. In particularly preferable embodiment, the first and the second MP carbamate solution is supplied to the second HP carbamate condenser.

[0074] An ammonia recycle flow line is also provided from the MP recovery sectionto the HP synthesis section, in particular from the ammonia condenser in the MP recovery section to the ejector comprised in the HP synthesis section. The ammonia condenser receives gas from the carbamate separation column.

[0075] Preferably, the first MP carbamate solution from the first MP carbamatecondenser passes through the MP ammonia-carbamate separation column (that is part of the MP recovery section) and is supplied from said separation column to the second HP carbamate condenser, and the first and second MP carbamate condenser are combined downstream of the MP ammonia-carbamate separation column. This provides the advantage of a relatively low water recycle by virtue of the different compositions (N / C ratio) and where applicable different temperatures of the MP carbamate streams. For example, the first and second MP carbamate solution are pumped to HP using separate HP pumps and only thereafter combined, e.g. at the inlet of the second HP carbamate condenser. For example, the MP ammonia- carbamate separation column receives a part of the fresh NH3 feed.

[0076] In an embodiment, the first HP carbamate condenser is a kettle-typecondenser, and the plant further comprises an ejector upstream the HP reactor.

[0077] The first HP carbamate condenser is, e.g. provided as the kettle-type boiler,typically comprising a horizontal shell and an U-shaped horizontal tube bundle. Typically, the kettle-type condenser receives a stream to be condensed, e.g. the partially condensed stream, on a tube side and a feed water to be converted into steam on the shell side. The first HP carbamate condenser has, at the process side an outlet for a HP carbamate stream comprising carbamate, and in the invention, urea. This stream is obtained at the tube bundle outlet of the kettle-type boiler.

[0078] Preferably the ejector is an ammonia-driven ejector configured to receiveammonia stream from the MP recovery section, in particular from the MP ammonia receiver, and the HP carbamate solution from the HP carbamate separator.

[0079] Preferably, the plant further comprises a HP carbamate separator, wherethe HP carbamate stream, e.g. a biphasic fluid comprising carbamate and optionally urea in a liquid phase and NH3 and inerts in a gas phase, is separated into a gas stream comprising inerts and a HP carbamate solution. The HP carbamate solution is recycled to the HP reactor, in particular after increasing itspressure to the pressure of the HP reactor using the ejector. The gas stream can be supplied to the first MP decomposer. The HP carbamate separator may be a separate gas / liquid separation vessel with an inlet, a top gas outlet and a bottom liquid outlet.

[0080] The stripped urea solution from the thermal stripper is at least in part, suchas entirely, such as for at least 90 wt.%, expanded to MP and sent to the MP recovery section.

[0081] The MP recovery section preferably comprises, a first MP decomposer, afirst MP carbamate condenser and an MP ammonia condenser, and preferably also an ammonia-carbamate separation column and an MP ammonia receiver. The ammonia condenser and the MP carbamate condenser are different units operating at different temperatures.

[0082] The first MP decomposer receives the stripped urea solution and has a gasoutlet connected to the first MP carbamate condenser and a liquid outlet connected to an LP decomposer of a LP recovery section. The first MP carbamate condenser, which can be provided by one or more condensers in series, has an outlet connected to an MP ammonia-carbamate separation column. The separation column also receives a part of the NH3 feed from the ammonia receiver and has bottom outlet for the first MP carbamate solution and a gas outlet connected to the MP ammonia condenser. The ammonia condenser has an outlet for a condensed ammonia stream that is combined with a part of the NH3 feed in the ammonia receiver and supplied, through a HP ammonia pump and the ejector, to the reactor.

[0083] The MP recovery section preferably has separate recycle flow connectionsfor the condensed ammonia stream and for the first MP carbamate solution, for recycling these streams separately to the HP section. The MP recovery section comprises an outlet for the first MP carbamate condensate, in particular for carbamate obtained in the first MP carbamate condenser which is separated in the downstream the ammonia-carbamate separation column to yield is the first MP carbamate solution. The first MP carbamate solution may be supplied to the first HP carbamate condenser. The first MP carbamate solution is preferably supplied to the second HP carbamate condenser in the invention to assist in the condensation of the gas stream from the stripper and to benefit from the urea formation in said second HP carbamate condenser.

[0084] The second part of the urea synthesis solution is supplied to the MPtreatment section, suitably after expansion from HP to medium pressure, e.g. using an expansion valve (control valve) giving MP urea solution, bypassing the HP thermal stripper. In particular, the second part of the urea synthesis solution, such as an expanded urea solution, is supplied as non-stripped MP urea solution to a first gas / liquid MP separator to give a flashed MP urea solution still containing carbamate. The first gas / liquid MP separator is in particular a flash vessel, more preferably an adiabatic flash vessel. The flash vessel allows the gas formed by the expansion of the ammonia and carbamate-containing urea synthesis solution to escape from the liquid. The second gas / liquid MP separator is preferably a separation vessel or drum, and is more preferably adiabatic, allowing gas formed in the second MP decomposer to escape from the liquid. The vessel preferably has an inlet and a gas outlet at the top and a liquid outlet at the bottom.

[0085] The MP treatment section suitably comprises, in series for the MP ureasolution, the first gas / liquid MP separator, the second MP decomposer, and the second gas / liquid MP separator, and also comprises a second MP condensation section connected to receive gas from the second gas / liquid MP separator. The MP treatment section may further comprise an MP CO2 stripper connected to receive urea solution from the second gas / liquid separator. The MP treatment section advantageously operates independently of the MP recovery section. This allows for using optimal process conditions in each section.

[0086] The second part of the urea synthesis solution bypasses the HP stripper andis sent at least in part, such as entirely, to the MP treatment section, preferably the second stream is entirely or substantially entirely (e.g. at least 95 wt.%) sent to the MP treatment section. Advantageously, the capacity of the HP stripper and the HP carbamate condenser can therefore be small relative to the capacity of the urea plant; this advantage applies for grassroots plants. By the same token, the bypass permits for modifying an existing urea plant to increase its capacity without replacement or modification of the existing thermal stripper.

[0087] Preferably, the MP treatment section receives at least 95 wt.% or at least 99wt.% of the second part of the urea synthesis solution as provided by the flow splitter.

[0088] Preferably, the first gas / liquid MP separator is an MP adiabatic flash unit.The MP adiabatic flash unit is preferably not actively heated.

[0089] Preferably, the second part of the urea synthesis solution is subjected toflashing, preferably adiabatic flashing, and the formed flash vapor is supplied directly or indirectly to the ammonia condenser (of the MP recovery section) to be included in the ammonia condensate which is recycled to the HP synthesis section separately from the carbamate recycle. Thereby advantageously essentially water- free recycle of the ammonia condensate is achieved while at the same time a low water content of the carbamate recycle stream from MP treatment section is obtained.

[0090] The first gas / liquid MP separator for example operates at the same or at ahigher pressure than the second MP decomposer, preferably max. 30 bar higher.

[0091] The first gas / liquid MP separator can be arranged vertically higher than thehighest part of the tube bundle of the process-process heat exchanger, e.g. with a vertical distance at least 2 m; such that a liquid column is maintained between the liquid outlet of the separator and the tube bundle. This may enable gravity flow of the MP urea solution.

[0092] The MP adiabatic flash unit preferably operates at a pressure of at least 15bar (absolute), for instance at a pressure in the range of 15 to 30 bar (absolute), e.g. 15 to 20 bara. The pressure of the MP adiabatic flash step can advantageously be used to adjust the composition of the flashed urea solution.

[0093] In an example, the flashed MP urea solution at the outlet of the firstgas / liquid MP separator and inlet of the second MP carbamate decomposer has, for example 20-30 wt.% NH3 (including NH3 present as carbamate), 10-20 wt.% CO2 (including CO2 present as carbamate), 30-50 wt.% urea and 10-25 wt.% water.

[0094] The flashed MP urea solution is supplied to the second MP decomposer. Inthe second MP decomposer, carbamate present in the flashed MP urea solution is at least partially, e.g. at least 50 wt.% or at least 70 wt.% or even at least 80 wt.%, decomposed into CO2 and NH3 to give a biphasic (gas and liquid) MP stream; in particular is this amount of carbamate decomposition achieved using process- process heat exchange

[0095] The second MP decomposer is provided preferably as a shell-and-tube heatexchanger having a tube bundle and a shell side space, wherein the tube bundle, preferably a U-shaped tube bundle, provides the second MP decomposer and receives the flashed MP urea solution and the shell side space provides the secondHP carbamate condenser receiving the HP gas stream from the HP thermal stripper.

[0096] The second MP decomposer is preferably operated at a pressure of 10 to 30bar, more preferably 15 – 20 bar; and / or at 140 – 170ºC, more preferably at 150 – 160ºC (outlet temperature).

[0097] The first and second MP decomposers are optionally operated at the same orat different pressures.

[0098] The biphasic MP stream is separated in the second gas / liquid MP separatorto a third MP gas stream and a second MP urea solution.

[0099] The second gas / liquid MP separator is e.g. a vessel with an inlet, and a gasoutlet at the top and a liquid outlet at the bottom.

[0100] The third MP gas stream is preferably condensed in the second MPcarbamate condenser to yield a second MP carbamate solution, and the second MP carbamate solution is recycled to the HP synthesis section via the first or the second HP carbamate condenser, preferably via the second HP carbamate condenser.

[0101] The second MP urea solution is directly or indirectly (via the MP CO2stripper) processed in the LP recovery section, e.g. via an adiabatic MP CO2 stripper. The second MP urea solution need not be heated between the second gas / liquid separation and the LP decomposer; in particular no carbamate decomposer needs to be present between the MP treatment section and the LP recovery section.

[0102] The use of the MP decomposer for urea solution bypassing the thermalstripper provides the advantage of a capacity increase in case of a revamp of an existing plant and of a relatively small thermal stripper (in particular, number of tubes) compared to the urea production capacity for grassroots plants.

[0103] The LP recovery section gives an LP urea solution which is for instancefurther concentrated in an evaporation section to form a urea melt, which is then supplied to a finishing section, comprising e.g. a granulator or a prilling tower to form a solid urea product or an aqueous urea solution for diesel exhaust fluid. The LP recovery section comprises an LP decomposer receiving the urea solution and having a gas outlet and a liquid outlet, and an LP carbamate condenser receiving gas from the LP decomposer.

[0104] The preferred adiabatic MP CO2 stripper is used for contacting the ureasolution with fresh CO2 gas at MP, in particular counter-current contacting, and without active heating, with the gas from the MP CO2 stripper supplied to the second MP carbamate condenser. This can be done to adjust the N / C ratio of the gas and the free N / C ratio the urea solution and to remove some NH3 from the urea solution. This improves the condensation in the second MP carbamate condenser and the condensation in the LP carbamate condenser. The MP CO2 stripper for example comprises a packed bed adapted for the counter-current contacting of gas and liquid.

[0105] In the invention, the gas from the stripper is supplied to the process-processheat exchanger first and thereafter to a kettle-type boiler. Thereby the kettle-type boiler condensation is improved as well. Furthermore, in the invention, the gas from the stripper is condensed in heat exchange with a part, not all, of the urea solution, by using an MP treatment section in parallel with the thermal stripping. Furthermore, the second MP decomposer operates at MP, e.g. at least 10 bar and below 30 bar, preferably max. 25 bar or max 20 bar, e.g. 18 bar. For instance, the first and second MP decomposer are operated at substantially the same pressure, e.g. less than 5 bar difference. Furthermore, the gas from the second MP decomposer is condensed in the second MP carbamate condenser that operates preferably at substantially the same pressure as the second MP decomposer, e.g. max. 5 bar lower. Operating the second MP decomposer at a pressure below 30 bar, preferably max. 25 bar or max 20 bar, contributes to efficient carbamate decomposition, such that the urea solution from the second MP decomposer after gas / liquid separation has sufficiently low carbamate and NH3 content to be supplied to the LP decomposer without a further heating step in between. The composition of the urea solution from the second MP decomposer after second gas / liquid separation may be comparable to the MP urea solution obtained by the conventional MP recovery section of a thermal stripping plant.

[0106] The invention also provides method of modifying an existing urea plant ofthe thermal stripping type, preferably into an inventive plant. The existing plant comprises an HP reactor, an HP thermal stripper, and a (first) HP carbamate condenser. The existing plant further comprising an MP recovery section comprising a first MP decomposer connected to receive a stripped urea solution from the thermal stripper (HPS). The method comprises (i) adding a process-process heat exchanger comprising a first compartment and a second compartment that are in heat exchanging contact through a wall, wherein the first compartment provides a second MP decomposer and the second compartment provides a second HP carbamate condenser, and (ii) adding an MP expansion valve, first MP gas / liquid separator, a MP second gas / liquid separator, and a second MP carbamate condenser.

[0107] The method further involves: connecting the second MP decomposer toreceive a part of the urea synthesis solution from the HP reactor through an (added) liquid flow line that bypasses the thermal stripper and goes through MP expansion valve and the first MP gas / liquid separator; connecting the second gas / liquid MP separator to receive fluid from the second MP carbamate decomposer and connecting the second MP carbamate condenser to receive gas from the second gas / liquid MP separator (by adding suitable fluid flow connections), and connecting the second HP carbamate condenser to receive gas, preferably all gas, from the thermal stripper and to supply a partially condensed stream to the first HP carbamate condenser; by modifying the gas flow connection from the striper to be connected to the process-process heat exchanger instead of the existing first HP carbamate condenser and by adding a liquid connection from the shell side of the process-process heat exchanger to the existing first HP carbamate condenser (to the tube bundle of the kettle-type boiler).

[0108] Preferably the method comprises: modifying the liquid flow line for a firstMP carbamate solution (8) from the MP recovery section (MPRS) of the existing plant to be connected to the inlet of the second HP carbamate condenser (HPCC-2) (instead of to the first HP carbamate condenser); and adding a liquid flow line for a second MP carbamate solution (16) from the second MP carbamate condenser to the inlet of the second HP carbamate condenser (HPCC-2).

[0109] Preferably, the added process-process heat exchanger (PPHE) is a shell-and-tube heat exchanger, with the tube bundle as the second MP decomposer and the shell side space as the second HP carbamate condenser.

[0110] All preferences and details for the inventive plant apply also for themodified plant. The modification of the existing plant provides for capacity increase while improving or at least maintaining energy efficiency.

[0111] Figure 1 schematically illustrates an example process scheme not accordingto the invention. The process shown in Fig. 1 refers to a urea production of the thermal stripping type. An example of such a process is also known as the Snamprogetti process and is described e.g. in Ullmann’s Encyclopedia of Industrial Chemistry, chapter Urea, 2010, Fig.25.

[0112] The comparative urea production plant (UPP) of Fig. 1 comprising an HPsynthesis section (HPSS), an MP recovery section (MPRS), and an LP recovery section (LPRS). The HP synthesis section (HPSS) comprises an HP reactor (HPR), an HP thermal stripper (HPS), and a first HP carbamate condenser (HPCC-1), a carbamate separator (CSp), an ejector (EJ). The HP reactor (HPR) comprises e.g. one or more HP reactors. The HP reactor is typically a vertical vessel with a plurality of trays, one or more inlets (usually two or more inlets) for feed streams at the bottom, and an outlet for an HP synthesis solution (1) at the top.

[0113] In the HP thermal stripper (HPS), the HP synthesis solution (1) is typicallyprovided at the top and it is stripped to yield an HP gas stream (2) and the stripped urea solution (7). The stripper operates with a falling film urea solution in the tube bundle and MP steam as heating fluid in the shell.

[0114] The stripped urea solution (7) is supplied to the MP recoverysection (MPRS), in particular to a first MP decomposer (MPD-1), as shown in Fig. 4, described in more detail below, to give a first MP carbamate solution (8) and a first MP urea solution (18). The urea is typically processed in the LP recovery section (LPRS) to give an LP urea solution which is for instance further concentrated in an evaporation section to form a urea melt, which is then supplied e.g. to a finishing section, e.g. a granulator or a prilling tower, to form a solid urea product.

[0115] The HP gas (2) from the stripper is condensed in a kettle-type boiler (tubeside), i.e. in the first HP carbamate condenser (HPCC-1), after combination with the first MP carbamate solution (8), to form a HP carbamate stream(4) while raising LP steam on the shell side. The HP carbamate stream(4) is supplied to a carbamate separator (CSp), where said stream (4) is separated to give a gaseous stream comprising inerts and ammonia (top outlet) and an HP carbamate solution (5). The HP carbamate solution (5) is recycled to the HP reactor (HPR), typically after increasing its pressure to the pressure of the HP reactor (HPR)using an ejector (EJ), such as an ammonia driven ejector, together with an ammonia stream (9) to give an HP recycle stream (6).

[0116] Figure 4 schematically illustrates an example the MP recoverysection (MPRS) of the urea plants according to Figure 1, 2, and 3.

[0117] The first MP decomposer (MPD-1) receives the stripped urea solution (7)which also contains carbamate, after expansion to MP and release of the formed gas, to heat it to decompose carbamate and to boil out NH3. For instance, the first MP decomposer (MPD-1) comprises an inlet for stripped urea solution (7) at the top and an outlet for the first MP gas (17) at the top, a packing, and a vertical shell- and-tube heat exchanging part, wherein the stripped urea solution (7) is supplied to the tube bundle, and a first MP urea solution (18) outlet at the bottom, and optionally an inlet, preferably at the bottom, for receiving a gas stream from the HP carbamate separator (CSp) (not shown). The first MP urea solution (18) is supplied to a low pressure recovery section (LPRS) (not shown).

[0118] The first MP carbamate condenser (MPCC-1) has an inlet for receivinggas (17) from the first MP decomposer (MPD-1) and preferably an inlet for a second MP gas (11) (from the MP treatment section in Fig. 2), and is configured for at least in part condensing CO2 and NH3 comprised in said gas (17) into an MP condensate (19), typically after also receiving an LP carbamate solution from the LP recovery section (no shown). The relatively high water content of LP carbamate solution can be used to avoid crystallization of the MP condensate (19). The condensation can be carried out using one or more heat exchanging units and using as cooling fluid e.g. cooling water and / or a urea solution that needs to be heated. The first MP carbamate condenser (MPCC-1) is e.g. provided, as a MP condenser / pre-evaporator for indirect heat exchange against urea solution with a pressure below 4 bar to be heated, optionally with a further downstream MP condenser for condensing at least a part of the non-condensed gases from the first MP carbamate condenser (MPCC- 1). The downstream MP condenser operates e.g. with cooling water. The MP condenser / pre-evaporator is for instance a shell-and-tube heat exchanger with gas to be condensed in the shell and urea solution to be heated in the tubes. The urea solution to be heated originates e.g. from the LP recovery section.

[0119] The MP condensate (19) together with an uncondensed gas is supplied to anammonia-carbamate separation column (MPSC). The uncondensed gas is for instance refluxed with relatively cold liquid NH3 feed (22) in the column (MPSC) inorder to remove CO2 from the gas. The column (MPSC) is for instance used as a distillation column. In a preferred embodiment, the MP condensate (19) is supplied to the bottom of the column and heated, and the feed liquid NH3 (22) is supplied to the top of the column. Suitable also an aqueous stream, e.g. from an absorber, is supplied to the top of the column. Preferably, the column comprises trays. Advantageously, a separated uncondensed gas stream (20) from the top of the separation column contains essentially only NH3 and inerts. This gas stream (20) is preferably supplied to the ammonia condenser (AC). The gas stream (20) is condensed as relatively pure liquid ammonia in an ammonia condenser (AC), e.g. MP ammonia condenser. An ammonia condensate (21) from the ammonia condenser (AC) contains (almost) no carbamate and has low water content. The ammonia condensate (21) is preferably substantially pure ammonia, e.g. at least 95 wt.% pure NH3, or at least 98 wt.%, or at least 99 wt.%, or at least 99.5 wt.%.

[0120] The bottom product of the ammonia-carbamate separation column (MPSC)is the first MP carbamate solution (8) which also contains a relatively high amount of NH3 and water. In other words, because of the N / C ratio and temperature of the carbamate from the separation column, the water content is relatively high. The first MP carbamate solution (8) is generally supplied directly or indirectly to the HP synthesis section as carbamate recycle solution using a pump.

[0121] The non-condensed gas stream from the first MP carbamate condenser(MPCC-1) is preferably after purification to purer ammonia gas in the ammonia- carbamate separation column (MPSC), supplied to a (first) ammonia condenser (AC), which is a heat exchanger preferably using cooling water as cooling fluid. The ammonia condenser (AC) is preferably operated at a temperature in the range of 20 to 50ºC.

[0122] The condensed ammonia (9) is recycled to the HP synthesis section (HPSS),separately from the carbamate streams, preferably after the inert gaseous components are separated from the feed liquid NH3 (22) in an ammonia receiver (AR) wherein liquid NH3 feed is also received. The liquid ammonia from the ammonia receiver (AR) is in part (9) pumped to the HP section and in part (22) used in the MP separation column (MPSC).

[0123] The inerts gas stream is for instance supplied to a scrubber (not shown)where it is scrubbed with an aqueous stream, e.g. steam condensate, from whichliquid is typically recycled to the HP synthesis section, e.g. by supplying it to the ammonia-carbamate separation column (MPSC).

[0124] Figure 2 schematically illustrates a process scheme not according to theinvention. The process shown in Fig. 2 refers to a urea production of the thermal stripping type, modified to let a part of the urea synthesis solution from the reactor bypass the thermal stripper and be supplied to a MP treatment section in parallel to the MP recovery section that receives stripped urea solution.

[0125] The urea plant shown in Fig. 2 comprises the HP synthesis section (HPSS)as described in Fig. 1 and the MP recovery section (MPRS) as described in Fig. 4. The HP synthesis section (HPSS) further comprises a flow splitter (FS), configured to divide the urea synthesis solution (1) into a first part (1-A) and the second part (1-B) of the urea synthesis solution; both parts contain the urea synthesis solution, i.e. the liquid phase, and hence contain urea, carbamate, NH3, CO2 and water.

[0126] The flow splitter (FS) is provided as e.g. a branching of the piping for theurea synthesis solution (1) or as a high pressure separator unit comprising a funnel structure to degas the second part of the urea synthesis solution.

[0127] The first part of the urea synthesis solution (1-A) is supplied to the HPstripper (HPS) and processed as described for Fig. 1.

[0128] The urea production plant (UPP) further comprises an MP treatmentsection (MPTS) receiving the second part (1-B) of the urea synthesis solution, i.e. non-stripped urea solution, after it is expanded to medium pressure using, e.g. an expansion valve (V) to give an expanded urea solution (10). The expanded urea solution(10) is supplied to a first gas / liquid MP separator (S-1). The first gas / liquid MP separator (S-1) is, e.g. provided as an adiabatic flash vessel, and has an inlet for the expanded urea solution (10), an outlet for a second MP gas (11), that is rich in NH3 and that is supplied to the first MP carbamate condenser (MPCC-1), and an outlet for the flashed MP urea solution (12) supplied to the second MP decomposer (MPD-2).

[0129] The second MP decomposer (MPD-2) is provided, e.g. as a shell-and-tubeheat exchanger, wherein a tube bundle receives the flashed MP urea solution (12) and a shell side receives LP steam, e.g. 8 bar steam. In the second MP decomposer (MPD-2), carbamate in the flashed MP urea solution (12) is decomposed for a large part to CO2 and NH3 to give a biphasic MP stream (13). The biphasic MP stream(13) is then separated in a second gas / liquid MP separator (S-2) to a third MP gas stream (14) and a second MP urea solution (15).

[0130] The second gas / liquid MP separator (S-2) is e.g. provided as a vessel with agas outlet at the top and a liquid outlet at the bottom.

[0131] The third MP gas stream (14) is then condensed in a second MPcondensation section (MPCC-2) to yield a second MP carbamate solution (16), recycled to the HP synthesis section (HPSS) via the first HP carbamate condenser (HPCC-1).

[0132] The second MP urea solution (15) is processed in the LP recovery section toto give an LP urea solution which is for instance further concentrated in an evaporation section to form a urea melt, which is then supplied to a finishing section, comprising e.g. a granulator or a prilling tower to form a solid urea product.

[0133] Figure 3 schematically illustrates an example process scheme according toan embodiment of the invention which does not limit the invention and does not limit the claims.

[0134] In the inventive plant according to Fig. 3, the high pressure synthesissection (HPSS), the MP recovery section (MPRS) and the MP treatment section (MPTS) are operated as described for Fig. 1 and Fig. 2, and with the MP recovery section (MPRS) as shown in Fig. 4, with differences discussed below.

[0135] The HP gas steam (2) from the HP stripper is supplied to a second HPcarbamate condenser (HPCC-2) (e.g. added in a revamp or included in a grassroots plant) instead of to the first HP carbamate condenser (HPCC-1). Furthermore, in the illustrated preferred embodiment, the second HP carbamate condenser (HPCC- 2) also receives the first MP carbamate solution (8) and the second MP carbamate solution (16), instead of supplying streams (8) and (16) to the first HP carbamate condenser (HPCC-1). Furthermore, the second MP decomposer (MPD-2) no longer utilizes steam as a heating medium.

[0136] In the urea plant according to Fig. 3, the second HP carbamate condenser(HPCC-2) and the second MP decomposer (MPD-2) are provided as heat exchanging compartments of a process-process heat exchanger (PPHE). In particular, the process-process heat exchanger (PPHE) is a shell-and-tube heat exchanger with a tube bundle which provides the second MP decomposer (MPD-2)and a shell side space which provides the second HP carbamate condenser (HPCC-2). Accordingly, the process-process heat exchanger (PPHE) preferably comprises an inlet for the HP gas steam (2), an inlet for the first MP carbamate solution (8), an inlet for the second MP carbamate solution (16), and an outlet for a partially condensed stream (3) on the shell side. The process-process heat exchanger (PPHE) also comprises an inlet for the flashed MP urea solution (12) and an outlet for the biphasic fluid stream (13) on the tube side. Preferably, the process-process heat exchanger (PPHE) is operated with a submerged tube bundle and with condensation in the shell.

[0137] It is a judicious insight of the present invention that the heat generated bycondensation of the HP gas (2) advantageously allows for heating the flashed MP urea solution (12) to provide for significant decomposition of carbamate present in said stream (12). Thereby, it is possible to substitute the use of steam in the second MP decomposer (MPD-2) with a heat exchanging contacting between the second MP decomposer (MPD-2) and the second HP carbamate condenser (HPCC-2), e.g. by a heat exchanging wall (HW), such as by the tubes of a shell-and-tube heat exchanger.

[0138] Advantageously, the shell side used for condensation provides additionalreaction volume where urea formation may take place which in turn increases the condensation temperature, in the second HP carbamate condenser and in the downstream first HP carbamate condenser. It is observed that both the first MP carbamate solution (8) and the second MP carbamate solution (16) benefit from this advantage when supplied to the said second HP carbamate condenser (HPCC-2). Moreover, the presence of both MP carbamate solutions contributes to condensation conditions in the process-process heat exchanger.

[0139] The plant of Fig. 1 can be modified into the inventive plant of Fig. 3 byadding the units PPHE, S-1, S-2, MPCC-2, FS, V; with their connections, and to connected flow line 8 to the unit PPHE instead of to HPCC-1. The invention also pertains to such a method of modifying an existing plant.

[0140] Figure 5 schematically illustrates a further example process schemeaccording to a further embodiment of the invention which does not limit the invention and does not limit the claims.

[0141] Figure 5 is based on Figure 3 and reference numbers are the same as inFigure 3 unless specified otherwise. In Fig. 5, a part (7a) of the stripped urea solution (7) is supplied to the second MP decomposer (MPD-2) through the expansion valve (V) and the first gas / liquid separator. Furthermore a part (2a) of the gas (2) from the HP thermal stripper (HPS) is supplied to the first HP carbamate condenser (HPCC-1) (kettle-type), in particular is added to the liquid supplied from the second HP carbamate condenser to the first HP carbamate condenser. Furthermore, the effluent from the second HP carbamate condenser (HPCC-2) is subjected to gas / liquid separation (S-3) with the gas supplied e.g. to an MP carbamate condenser (top outlet) an the liquid (3) supplied to the first HP carbamate condenser (HPCC-1).

[0142] The term ‘carbamate’, as used herein, refers to ammonium carbamate, asthat term is used in the field of urea production. In aqueous carbamate streams, the component can be present as carbonate species; hence an aqueous carbamate solution can also be referred to as a carbonate solution. Amounts of NH3 and CO2 for aqueous streams, as is customary in the field, include the amounts present as carbonate species.

[0143] As used herein, high pressure (HP) is at least 100 bara, e.g. 110 to 160 bara,in particular 140-170 bar; medium pressure (MP) is e.g. 10 to 60 bara, preferably 10 to 30 bara, more preferably 15 – 20 bara; and low pressure (LP) is max. 10 bara, in particular 0 to 10 bara e.g. 4 to 10 bara; these pressure ranges are for process solutions and not necessarily the same for steam and heating fluids. All pressures are absolute pressures in bar (bara).

[0144] The term ‘typical’ and ‘in particular’ are used to indicate features that canbe used in some embodiments but that are not mandatory. Also preferred features are not mandatory.

[0145] The N / C ratio for gas streams indicates the molar ratio of NH3 to CO2. TheN / C ratio as used herein for the urea synthesis section reflects the composition of the so-called initial mixture before urea production, consisting only of NH3, CO2 and H2O, as used in the art of urea plants, and is the molar ratio NH3 to CO2. The ‘free N / C ratio’ of a urea solution is the molar ratio of NH3 and CO2 on the basis of NH3, CO2 and carbamate, excluding urea. The N / C ratio of a carbamate solution indicates the molar ratio of the corresponding amounts of NH3 to CO2. The N / Cratio for a carbamate condenser refers to the N / C ratio of the carbamate solution at the liquid outlet.

[0146] The term ‘first’ as used herein for a unit or step permits the presence offurther, upstream, instances of such unit or step.

[0147] A liquid flow line, as used herein, indicates a flow line for transport of liquidstreams, the fluid being in the liquid state through the entire liquid flow line, and does not include flow lines for transport of gaseous stream. A liquid flow line may also be referred to as a liquid flow connection, and may pass through a unit, in particular from a liquid inlet to a liquid outlet of a unit.

[0148] All preferences and details discussed in connection with the process, applyalso for the plant, and vice versa. All preferences and details described in connection with the plant, apply also for the method of modifying a plant. The inventive process is preferably carried out in the inventive plant. The inventive plant is preferably suitable for the inventive process. Examples

[0149] The invention will now be further illustrated by the following non-limitingexample. These examples do not limit the invention and do not limit the claims. Example 1

[0150] Three urea plants of the thermal stripping type were simulated:comparative Plant 1 (all urea solution to thermal stripper), comparative Plant 2 (approx. 20% of the urea solution bypassing the stripper), and inventive Plant 3 (also approx.20% of the urea solution from the reactor bypassing the stripper). Plant 2 and 3 differ in the heat integration of the MP decomposer for the bypassed urea solution; the HP thermal stripper apparatus is the same in both plants. Plant 2 is included as a reference plant, for comparison, without admitting prior art.

[0151] Plant 1 is a comparative urea plant of the thermal stripping type, as shownin Fig.1. Plant 1 has a capacity of 2200 ton urea / day. In Plant 1, the HP gas stream (2) from the stripper is condensed only in the first HP carbamate condenser (HPCC-1), which is a kettle-type boiler, together with the first MP carbamate solution (8) to raise LP steam and to give the HP carbamate stream (4). In the MP recovery section (MPRS) of the reference plant 1 (this section is in linewith Fig. 4) all stripped urea solution (7) is decomposed in the first MP decomposer (MPD-1) to give the first decomposed urea solution (18) and the first MP gas (17), which after condensation in the first MP condenser and separation in the MP separation column (MPSC) gives the first MP carbamate solution (8).

[0152] Plant 2 is a comparative urea plant of the thermal stripping type, as shownin Fig. 2. Plant 2 has a capacity of 2800 ton urea / day. Plant 2 has two vertical urea reactors in series (inside unit HPR); the second reactor provides approx.30% of the total reaction volume. Plant 2 uses an MP CO2 stripper in the MP treatment section (not shown in Fig. 2).

[0153] In Plant 2, the urea synthesis solution (1) is separated into a first part (1A)(approx. 78 wt.% of the urea solution) and a second part (1B) (approx. 22 wt.% of the urea solution). The first part (1A) is supplied to the thermal stripper (HPS) and processed in the MP recovery section (MPRS), similar as in Plant 1. The gas (2) from the stripper is condensed in the tube bundle of the kettle-type boiler, i.e. in the only / first HP carbamate condenser (HPCC-1) which is operated for substantially complete condensation. LP steam is raised on the shell side of the kettle-type boiler. The HP carbamate stream (4) from the HP carbamate condenser tube bundle consists essentially of carbamate, water and NH3, i.e. without significant urea.

[0154] The second part of the urea synthesis solution (1B) from the reactorbypasses the thermal stripper and is supplied, through the expansion valve (V), to the first gas / liquid MP separator (S-1) in the MP treatment section (MPTS) to give the flashed MP urea solution (12) comprising 216 kmol / h of carbamate (assuming that substantially the entire amount of CO2 is in the form of carbamate), with a N / C ratio of 2.25 (N / C ratio without urea), at 131ºC and 18.7 bar. The expansion valve and the gas / liquid separator together provide for adiabatic flashing of the urea solution.

[0155] Carbamate present in the flashed MP urea solution (12) is decomposed inthe second MP decomposer (MPD-2) using steam to give a biphasic fluid (13) having a temperature of 160ºC and consisting of a liquid phase comprising 36 kmol / h of carbamate (assuming that substantially the entire amount of CO2 is in the form of carbamate), having a free N / C ratio of 2.25, and a gas phase. The second MP decomposer (MPD-2) utilizes 18.5 ton / h steam at a pressure of 8 bar, which indicates that 10.8 MW power is required to achieve this amount ofcarbamate decomposition in stream (12). In Plant 2, the second MP decomposer (MPD-2) is a heat exchanger with the flashed MP urea solution (12) in the tubes and with steam provided on the shell side. Further details of Plant 2 are summarized in Table 1 below.

[0156] Plant 3 is a urea plant of the thermal stripping type according to theinvention, and is as shown in Fig.3, with a capacity of 2800 metric ton urea / day.

[0157] In Plant 3, the HP urea synthesis solution (1) is separated into the firstpart (1-A) (approx.80 wt.% of the urea solution) and the second part (1-B) (first part 2225 ton urea, second part 575 ton urea).

[0158] The first part of the urea synthesis solution (1-A) is supplied to the thermalstripper (HPS). The stripped urea solution (7) is processed in the MP recovery section (MPRS). The steam consumption of the MP recovery section (MPRS) is per ton urea essentially the same as for Plant 2.

[0159] The HP gas stream (2) from the thermal stripper (HPS), is mixed with thefirst and the second MP carbamate solutions (8) and (16) supplied to the second HP carbamate condenser (HPCC-2), which is a compartment of the process-process heat exchanger (PPHE), to give a partially condensed biphasic stream (3).

[0160] After said mixing, the vapor at the inlet of the second HP carbamatecondenser is 127 ton / h with molar N / C ratio of 3.12 (NH3 to CO2) and with 55 ton / h CO2, i.e.1250 kmol / h CO2. The liquid at said inlet, after mixing, is 22 ton / h with N / C ratio 2.2 and with essentially no urea in the liquid. The second HP carbamate condenser (HPCC-2) is operated at 150 bar with 185ºC at the outlet and with 79 ton / h vapor and 176 ton / h liquid at the outlet, with the liquid at the outlet including 39 ton / h urea. The second HP carbamate condenser (HPCC-2) is hence operated with partial condensation, and provides a reaction volume that is about 50% of the volume of the vertical urea reactor (HPR), i.e. approx. 30% of the reaction volume of the synthesis section is provided by the second HP carbamate condenser (HPCC-2). The liquid is supplied to the tube bundle of the kettle-type boiler, used as the first HP carbamate condenser (HPCC-1). Therein condensation is substantially completed. On the shell side of the kettle-type boiler, 58.5 ton of 5.9 bar LP steam / ton urea is raised. In Plant 3, pressure of the steam from the kettle- type boiler is higher by virtue of the lower amount of carbamate being condensed in the tube side of the first HP carbamate condenser (HPCC-1). It is observed that theHP thermal stripper operation and flow split ratio can be adjusted to obtain a desirable amount of LP steam.

[0161] The second part of the urea synthesis solution (1B) is supplied to the firstgas / liquid MP separator (S1) in the MP treatment section (MPTS), where it is flashed (i.e. reduced in pressure with liberation of gas and gas / liquid separation) to give the flashed MP urea solution (12) comprising 227 kmol / h of carbamate (on the basis of CO2 present as carbamate), with further details in Table 1. Carbamate present in the flashed MP urea solution (12) is decomposed for a large part (approx. 85%) in the second MP decomposer (MPD-2) to give the biphasic fluid (13) having a liquid phase comprising 29 kmol / h of carbamate (on the basis of all CO2 present as carbamate). Hence, the amount of carbamate in the liquid at the outlet of the second MP decomposer is lower in Plant 3 than in Plant 2; i.e. a more complete carbamate decomposition is achieved in Plant 3, even without using steam. Further details regarding the operation of Plant 3 are summarized in Table 1 below.

[0162] In Plant 3, the second MP decomposer (MPD-2) does not utilize steam toprovide heat for decomposition of carbamate in said flashed MP urea solution (12), contrary to Plant 2. Instead, the second MP decomposer (MPD-2) utilizes the latent heat generated during carbamate condensation in the second HP carbamate condenser (HPCC-2), which is provided in process-process heat exchanging contact with the second MP decomposer (MPD-2). Condensation of CO2 and NH3 in the second HP carbamate condenser (HPCC-2) release heat at a high enough temperature and with power supply (MW) that is sufficient. Thereby, it is evident that the amount of heat generated in the second HP carbamate condenser (HPCC- 2) is sufficient, and of sufficient quality (temperature) for decomposition of a large part of the carbamate present in the flashed MP urea solution (12). It is observed that the split ratio of the urea synthesis solution ensures that sufficient gas is condensed relative to the amount of MP urea solution. The temperature delta over the two heat exchanging compartments is provided by a judicious combination of the composition and pressures in both compartments, while still permitting the ammonia condenser to operate at MP against cooling water and with low water recycle to the synthesis section.

[0163] Accordingly, the process-process heat exchanger successfully improvesenergy efficiency compared to a second MP decomposer (MPD-2) which is operated using steam as heating fluid.

[0164] Table 1 Plant 2 Plant 3 (comparative) (inventive) Urea production 2800 2800 (metric ton per day) Split ratio 1-A / 1-B 2175 / 625=3.48 2225 / 575=3.87Steam consumption18.5 ton steam (8 bar) / h 0in MPD-2 Steam consumption 860 kg steam (340ºC / 25754 kg steam (340ºC / 25 bar) / ton urea bar) / ton urea Vapor at inletN / A 127 ton / hourHPCC-2 N / C = 3.12 mol / mol CO2 in vapor = 55 ton / h Combined streamN / A 149 ton / hourreceived by HPCC- N / C (liquid) = 2.2 2 (steams 2, 8, 16) (urea = 0 ton / h in liquid) HPCC-2 operatingN / A 150 barpressure HPCC-2N / A 181ºCcondensation temperature Partially condensedN / A 185ºC ;carbamate stream (urea = 39 ton / h in liquid) (3) from HPCC-2 Vapor out=79 ton / h outlet: Liquid out=176 ton / h Reaction volume ofN / A Approx. 30%HPCC-2 to total reaction volume HP carbamate166 ºC 174ºCstream (4) at outlet of HPCC-1 LP steam 73.5 ton steam (4.7 bar) / 58.5 steam (5.9 bar) / ton production in ton urea urea HPCC-1 MPD-2 18.7 bar 18.7 barFlashed MP urea T= 131ºC T = 133ºC solution (12) at 62 ton / h 64 ton / h MPD-2 inlet Composition: Composition: Urea = 41 wt. %, urea = 40 wt. %, NH3* = 19 wt.%, NH3* = 23 wt.%, CO2* = 16 wt.% CO2* = 16 wt.% = 216 kmol / h = 227 kmol / h H2O = 24 wt.%, H2O = 21 wt.%, N / C = 2.25** N / C (free) = 2.25** *including present as *including present as carbamate carbamate ** free / excluding urea ** free / excluding ureaMP biphasic T = 160ºC T = 160ºC stream (13) at Gas = 19.5 ton / h Gas= 20.5 ton / h MPD-2 outlet Liquid = 43.4 ton / h Liquid = 37.5 ton / h Gas at MPD-2 Composition (gas): Composition (gas): outlet NH3 = 44 wt.% NH3 = 48 wt.% CO2 = 42 wt.% CO2 = 39 wt.% H2O = 14 wt.% H2O = 13 wt.% Liquid at MPD-2 Urea = 59 wt. % Urea = 64 wt. % outlet NH3* = 8 wt.% NH3* = 8 wt.% CO2* = 4 wt.% = 36 kmol / h, CO2* = 4 wt.% = 29 kmol / h, H2O = 28 wt.% H2O = 25 wt.% N / C = 2.25** N / C = 2.25** * including as carbamate * including as carbamate ** free / excluding urea ** free / excluding urea Heat exchanging350 m² 285 m²surface area of MPD-2

Claims

Claims 1. A urea production plant (UPP) comprising a high pressure (HP) synthesis section (HPSS), a medium pressure (MP) recovery section (MPRS), and an MP treatment section (MPTS); wherein the HP synthesis section (HPSS) comprises an HP reactor (HPR), an HP thermal stripper (HPS), and a first HP carbamate condenser (HPCC-1) that is preferably a kettle-type condenser; wherein the HP synthesis section has an outlet for urea solution connected by a liquid flow line with an inlet of the MP treatment section such that the MP treatment section receives, in operation, a part of the urea solution from the HP synthesis section supplied, at medium pressure, to the MP treatment section; preferably wherein: -the HP reactor (HPR) has one or more outlets for a urea synthesissolution (1) and is connected with a first flow line for supplying a first part of the urea synthesis solution (1-A) to the thermal stripper (HPS) and with a second flow line for supplying a second part of the urea synthesis solution (1-B) to the MP treatment section (MPTS) bypassing the thermal stripper (HPS); and / or -the outlet of the HP thermal stripper for stripped urea solution isconnected to supply a first part of the stripped urea solution to the MP treatment section and a second part of the stripped urea solution to the MP recovery section; wherein the synthesis section (HPSS) further comprises a second HP carbamate condenser (HPCC-2); wherein the second HP carbamate condenser (HPCC-2) is connected to receive an HP gas stream (2) from the thermal stripper (HPS) and / or a part of the CO2 feed; wherein the second HP carbamate condenser (HPCC-2) has an outlet (3) connected to an inlet of the first HP carbamate condenser (HPCC-1) to supply a liquid stream containing carbamate, and optionally also gas, from the second HP carbamate condenser (HPCC-2) to the first HP carbamate condenser (HPCC-1); wherein the MP recovery section (MPRS) comprises a first MP decomposer (MPD-1) connected to receive a part or all of the stripped urea solution (7) from the thermal stripper (HPS);wherein the MP treatment section (MPTS) comprises, in series for an MP urea solution, a first gas / liquid MP separator (S-1), a second MP decomposer (MPD-2), and a second gas / liquid MP separator (S-2); and wherein the second HP carbamate condenser (HPCC-2) and the second MP decomposer (MPD-2) are provided as heat exchanging compartments of a process- process heat exchanger (PPHE).

2. A urea production plant according to claim 1, wherein the process-process heat exchanger (PPHE) is a shell-and-tube heat exchanger with a tube bundle and a shell side space, wherein the shell side space provides the second HP carbamate condenser (HPCC-2) and the tube bundle provides the second MP decomposer (MPD-2).

3. A urea production plant according to claim 2, wherein the second HP carbamate condenser (HPCC-2) is a submerged carbamate condenser.

4. A urea production plant according to claim 3, wherein the process-process heat exchanger (PPHE) is a vertical shell-and-tube heat exchanger.

5. A urea production plant according to any one of the preceding claims, wherein the first HP carbamate condenser (HPCC-1) is a kettle-type condenser, and wherein the plant comprises an ejector (EJ) upstream the HP reactor (HPR), preferably wherein the ejector (EJ) is an ammonia-driven ejector configured to receive an ammonia stream (9) and a carbamate solution from the first HP carbamate condenser (HPCC-1).

6. A urea production plant according to claim 1, wherein the shell side space of the process-process heat exchanger (PPHE) comprises a plurality of compartments separated by baffles.

7. A urea production plant according to any one of the preceding claims, wherein the plant comprises a liquid flow line from an outlet for a first MP carbamate solution (8) from the MP recovery section (MPRS) to an inlet of the second HP carbamate condenser (HPCC-2) and / or a liquid flow line from an outletfor a second MP carbamate solution (16) from the MP treatment section (MPTS) to an inlet of the second HP carbamate condenser (HPCC-2).

8. A urea production plant according to any one of claims 2-7, wherein the second HP carbamate condenser provides a reaction volume that is at least 30% or at least 40% and / or up to 70% or up to 60% of the reaction volume of the reactor of the reactor, e.g. 30 – 70 %, or 40 – 60 %, in particular with condensation in the shell-side compartment of process-process shell-and-tube heat exchanger.

9. The urea production plant according to any of claim 1 – 8, wherein the HP reactor (HPR) has one or more outlets for a urea synthesis solution (1) and is connected with a first flow line for supplying a first part of the urea synthesis solution (1-A) to the thermal stripper (HPS) and with a second flow line for supplying a second part of the urea synthesis solution (1-B) to the MP treatment section (MPTS) bypassing the thermal stripper (HPS); and wherein the second HP carbamate condenser (HPCC-2) is connected to receive an HP gas stream (2) from the thermal stripper (HPS) 10. The plant according to any of claims 1 - 8, wherein the liquid flow line from the outlet (3) of the second HP carbamate condenser (HPCC-2) to the first HP carbamate condenser (HPCC-1) comprises a gas / liquid separator having a gas outlet and a liquid outlet for carbamate solution, wherein the liquid outlet is connected to the inlet of the first HP carbamate condenser (HPCC-1), and wherein the plant comprises a flow line for gas from the HP thermal stripper to the first HP carbamate condenser (HPCC-1).

11. A urea production process carried out in a urea production plant according to claim 1, preferably having the features of any of claims 2 to 10, the process comprising:– reacting CO2 and NH3 in the HP reactor (HPR) to give a urea synthesissolution (1) and, optionally, dividing said urea synthesis solution (1) into a first part of the urea synthesis solution (1-A) and a second part of the urea synthesis solution (1-B);– stripping urea synthesis solution, e.g. all or the first part of the urea synthesissolution (1-A), in the HP thermal stripper (HPS) to give a stripped urea solution (7) and an HP gas stream (2),– - supplying urea solution from the synthesis section in part to the first MPdecomposer, preferably wherein the stripped urea solution (7) is supplied to the first MP decomposer (MPD-1);– condensing a first gas stream comprising CO2 and preferably also NH3preferably the HP gas stream (2), in the second HP carbamate condenser (HPCC-2) to yield a condensed liquid (3) and uncondensed gas;– supplying the condensed liquid (3) to the first HP carbamatecondenser (HPCC-1) and preferably also the uncondensed gas, e.g. as a fluid stream comprising said condensed liquid and said uncondensed gas; and– supplying a second gas stream comprising CO2 and NH3, preferably theuncondensed gas stream, to the first HP carbamate condenser (HPCC-1);– supplying a part of the urea solution from the synthesis section to the firstgas / liquid MP separator (S-1) of the MP treatment section (MPTS) thereby obtaining a flashed MP urea solution (12), preferably by: oexpanding the second part of the urea synthesis solution (1-B) to amedium pressure and supplying said second part to the first gas / liquid MP separator (S-1) of the MP treatment section (MPTS) bypassing the thermal stripper (HPS) to give a flashed MP urea solution (12); and / or oexpanding a first part of the stripped urea solution to mediumpressure and supplying said urea solution to the first gas / liquid MP separator (S-1);– heating the flashed MP urea solution (12) in the second MP decomposer (MPD-2) thereby decomposing carbamate in the flashed MP urea solution (12) to give an MP biphasic stream (13); and– separating the MP biphasic stream (13) in the second gas / liquid MPseparator (S2); wherein the second HP carbamate condenser (HPCC-2) and the second MP decomposer (MPD-2) are provided as heat exchanging compartments of a process- process heat exchanger (PPHE) and exchange heat via a heat exchanging wall of said process-process heat exchanger (PPHE).

12. A process according to claim 11, wherein the second MP decomposer (MPD- 2), is operated at a pressure of 10 to 30 bar, more preferably 15 – 20 bar.

13. A process according to any of claims 11-12, wherein the HP gas stream is supplied to the shell side space of the second HP carbamate condenser and the tube bundle of the second MP decomposer receives the flashed MP urea solution.

14. A process according to claim 13, wherein at least 20 wt. % of the total urea formed in the synthesis section is formed in second HP carbamate condenser.

15. A process according to any of claims 11-14, wherein the molar ratio of CO2 in the flashed MP urea solution at the inlet of the second MP decomposer, including CO2 present as carbamate, to CO2 in the gas stream at the inlet of the second HP carbamate condenser, is in the range 0.1 – 0.

3.

16. A process according to any of claims 11-15, wherein a first MP carbamate solution (8) from the MP recovery section (MPRS) and a second MP carbamate solution (16) from the MP treatment section (MPTS) are both supplied to the second HP carbamate condenser (HPCC-2); preferably wherein the first MP carbamate solution from the first MP carbamate condenser passes through a MP ammonia-carbamate separation column comprised in the MP recovery section and is supplied from said separation column to the second HP carbamate condenser, and the first and second MP carbamate condenser are combined downstream of the MP ammonia-carbamate separation column to form a carbamate stream supplied to the second HP carbamate condenser (HPCC- 2).

17. A process according to any of claims 11-16, wherein the second part is 10 – 30 wt.% of the total urea synthesis solution, more preferably 15 – 25 wt.% of the urea solution.

18. A process according to any of claims 11-17, comprising: - said urea synthesis solution (1) is divided into a first part of the urea synthesis solution (1-A) and a second part of the urea synthesis solution (1-B);- the first part of the urea synthesis solution (1-A) is stripped in the HP thermal stripper (HPS); - the stripped urea solution (7) is supplied to the first MP decomposer (MPD- 1); - the second part of the urea synthesis solution (1-B) is expanded to a medium pressure and said second part is supplied to the first gas / liquid MP separator (S-1) of the MP treatment section (MPTS) bypassing the thermal stripper (HPS) to give a flashed MP urea solution (12).

19. A method of modifying an existing urea plant of the thermal stripping type, wherein the existing plant comprises a high pressure (HP) synthesis section (HPSS), and a medium pressure (MP) recovery section (MPRS); wherein the HP synthesis section (HPSS) comprises a HP reactor, a HP thermal stripper, a first HP carbamate condenser that is preferably a kettle-type condenser; wherein the MP recovery section comprises a first MP decomposer connected to receive a part or all of the stripped urea solution (7) from the thermal stripper (HPS); the method comprising:– adding a process-process heat exchanger comprising a first compartment and asecond compartment that are in heat exchanging contact through a wall, wherein the first compartment provides a second MP decomposer and the second compartment provides a second HP carbamate condenser;– adding an MP expansion valve, first MP gas / liquid separator, a second MPgas / liquid separator, and a second MP carbamate condenser;– connecting the second MP decomposer to receive a part of the urea synthesissolution from the HP reactor through a liquid flow line that bypasses the thermal stripper and goes through MP expansion valve and the first MP gas / liquid separator and / or to receive a part of the stripped urea solution from the HP thermal stripper;– connecting the second gas / liquid MP separator to receive fluid from the secondMP carbamate decomposer and connecting the second MP carbamate condenser to receive gas from the second gas / liquid MP separator;– connecting the second HP carbamate condenser to receive gas, preferably allgas, from the thermal stripper and / or a part of the CO2 feed, and to supply a a liquid stream containing carbamate, and optionally also gas, to the first HP carbamate condenser;preferably wherein the process-process heat exchanger (PPHE) is a shell-and-tube heat exchanger, with the tube bundle as the second MP decomposer and the shell side space as the second HP carbamate condenser; wherein the modified plant is preferably as defined in any of claims 1-10, more preferably in accordance with claim 9.

20. A method according to claim 19, the method comprising:– modifying a liquid flow line for a first MP carbamate solution (8) from the MPrecovery section (MPRS) to be connected to the inlet of the second HP carbamate condenser (HPCC-2); and– adding a liquid flow line for a second MP carbamate solution (16) from thesecond MP carbamate condenser to the inlet of the second HP carbamate condenser (HPCC-2).

Images