Degassing system in particular for urea plant

Abstract

The disclosure pertains to a degassing apparatus with a confinement comprising a bottom part, a degassing zone is provided by the bottom part. Liquid is redistributed horizontally away from a liquid outlet in embodiments to provide residence time in the degassing zone. The degassing apparatus can be a part of a decomposer of a urea plant.

Description

P136271PC00Title: DEGASSING SYSTEM IN PARTICULAR FOR UREA PLANT Field

[0001] The invention pertains to an apparatus for degassing a liquid stream which comprises (small) gas bubbles. The apparatus can be used, for instance, in a dissociator (or decomposer) of a urea plant which receives a flashed (pressure reduced) urea solution stream comprising, in the liquid phase, urea, water, and ammonium carbamate, wherein the dissociator comprises an fluid inlet for the urea solution at a top part, a gas outlet for flashed gas released from the urea solution, preferably a gas / liquid contacting zone below the fluid inlet, a shell-and-tube heat exchanger with a vertical tube bundle, a liquid flow line from a liquid outlet arranged below the fluid inlet and above the vertical tube bundle to a bottom inlet of the tube bundle, and with a liquid outlet for urea solution exiting the tube bundle at an upper end of the tube bundle and a gas flow line from the upper end of the tube bundle to the degassing apparatus, in particular to the supply gas from the tube bundle to the gas / liquid contacting zone. Introduction

[0002] The invention pertains to a degassing apparatus, configured for degassing a liquid stream by release of gas remaining in the liquid stream, wherein the gas is in particular present in the liquid as relatively small gas bubbles. The degassing system may comprise an initial gas / liquid separation zone for initial release of gas from the liquid to give a gas stream and the liquid stream, with some remaining gas that is to be degassed. Degassing of the liquid, i.e. achieving a desirably nearly complete removal of gas entrained in the liquid, may be useful for downstream processing of the liquid, e.g. for supplying the liquid to a tube bundle.

[0003] The degassing system is particularly useful for a dissociator (decomposer) of a urea plant. The invention therefore also pertains to a urea plant and a urea production process, and to a dissociator (decomposer) comprising the degassing system, without being limited thereto. A dissociator (decomposer) is a device, usually a heater, used in urea plants to decompose carbamate comprised in urea solution from a urea synthesis section into CO2 and NH3 to purify the urea solution. In the urea synthesis section of the urea plant, CO2 and NH3 are reacted to form carbamate that is subsequently reacted to form urea and water.

[0004] In the urea production plant and process, NH3 and CO2 are reacted under urea-forming conditions (at least 160ºC and a high pressure, HP, of at least 100 bar) to form a urea synthesis solution comprising urea, water, ammonium carbamate (hereinafter: carbamate) and NH3 and CO2. An important aspect of urea plants is the purification of the urea synthesis solution by decomposition of carbamate into NH3 and CO2 in one or more decomposers by heating, the release of the gaseous NH3 and CO2, usually with recovery of the NH3andCO2 by condensation into carbamate solution that is recycled to the synthesis section, and water evaporation to obtain a urea melt. The synthesis section may consist of one or more reactors, and may comprise a so-called high pressure stripper and high pressure carbamate condenser. These types of urea plants and processes (with or without HP stripper) are known as total recycle plants in the art and involve the condensation of the gas stream from a decomposer, comprising NH3 and CO2 into a carbamate solution. A background reference of urea production plants is Ullmann’s Encyclopedia of Industrial Chemistry, chapter Urea, 2010.

[0005] The article “Urea synthesis – a status report”, Nitrogen, No. 185, May-June 1990, shows on p.24 a process sheet of a urea production process of the Stamicarbon CO2 stripping type with a rectifier on top of a heater in a vessel, with a bypass loop from the top of the heater to the bottom of the heater. The article explains that the urea solution from the HP stripper is let down from 140 bar and 170ºC to about 4 bar and in the top of the rectification column, NH3, CO2 and some water flash off under adiabatic conditions, cooling the temperature of the urea solution to about 110ºC. The liquid then falls through a packed section in which it is stripped of NH3 and CO2 with vapors produced by reheating the urea solution to 135ºC in the base of the column. This stripping limits the H2O content of the vapors leaving the top of the column so as to unduly dilute the solution in the carbamate condenser to which the gas is passed. The urea solution enters the heater from the bottom, and exits the heater at the top, beneath the rectifier at 135ºC. It has also been reported that the rectifying column provides that the gas leaving the column approaches equilibrium with the incoming cold flashed urea solution and has relatively low partial pressure of water (Slack and Blouin, Urea Technology: a critical review, 1969; available at https: / / ureaknowhow.com / 1969-slack-tva-urea-technology-a-critical-review / )

[0006] EP3707121B1 shows a similar decomposer / rectifier C501 / E513 for a urea production plant wherein no HP stripper is used.

[0007] The article “Capacity Increase of Urea plants”, 24th AFA Int. Technical Fertilizers Conference, 2011, Amman, Jordan, shows a figure with a MP section of a urea plant with an MP rectifying column with a heater, a rectifying unit on top, and a bypass line from the rectifying unit to the bottom of the heater.

[0008] It is desired to operate the decomposer with gravity flow. Moreover, a small footprint is desired and a simple equipment. It is desired to improve the design of the known decomposers in this respect, in particular of decomposer with a rectifier on top.

[0009] It is also generally desired to provide efficient degassing systems with a small footprint and simple design and construction. Summary

[0010] The invention provides in an embodiment, a degassing apparatus (or degassing unit) which comprises a confinement; the confinement comprising: a fluid inlet for a fluid stream comprising liquid in an upper part of the confinement; a liquid outlet at a lower part of the confinement, and a gas outlet arranged vertically higher than said liquid outlet, and wherein the confinement comprises a horizontally extending bottom part, preferably a bottom plate, for holding a layer of liquid on said bottom part in a degassing zone in said confinement. The degassing sone is located in said confinement, on said bottom part.

[0011] Provided is a method of degassing a liquid, in particular a flashed urea solution comprising carbamate, the method comprising supplying a fluid stream comprising a liquid to such a degassing apparatus wherein, in the method, the liquid is supplied onto the bottom part with an inhomogeneous distribution over said bottom part that is shifted horizontally away from said liquid outlet.

[0012] Preferably, the degassing apparatus comprises a liquid redistribution element arranged in said confinement vertically between said bottom part and said fluid inlet, wherein the liquid redistribution element comprises one or more parts, extends in a horizontal direction, and is arranged to receive liquid from said fluid inlet on an upper side of said liquid redistribution element and that provides one or more liquid passageways allowing liquid to flow from the upper side onto the bottom part, e.g. as an aperture in said element, or as apertures between parts of said element, and / or as an aperture between said element and said confinement; preferably suitable for a method according to the invention.

[0013] Also provided is a method of operating the degassing apparatus comprising the liquid redistribution element, the method comprising supplying a fluid stream comprising liquid and gas (typically, a small amount of gas) to said fluid inlet of the degassing apparatus, such that the liquid flows through the confinement to the degassing zone, wherein at least a part of the liquid from said fluid inlet is received on (e.g., impinges on) the upper side (upper surface) of said liquid redistribution element. For this method, it is optionally specified that the liquid is supplied onto the bottom part with an inhomogeneous distribution over said bottom part that is shifted horizontally away from said liquid outlet.

[0014] Preferably, said liquid redistribution element in operation increases ( is configured to increase) the average horizontal distance from said liquid outlet of liquid provided from said element onto said bottom part.

[0015] Preferably, said liquid redistribution element comprises a baffle plate and the upper side of the baffle plate is sloped with a higher part horizontally proximate to said liquid outlet and a lower part horizontally distant from said liquid outlet; preferably wherein the baffle plate is inclined.

[0016] Preferably, said liquid redistribution element comprises a baffle plate that is provided with a rim extending upward from the plate around a part of the perimeter of the baffle plate, preferably wherein the rim is provided at a higher part of the sloped or inclined baffle plate.

[0017] Also provided is a decomposer comprising the degassing apparatus. All preferences discussed in connection with the decomposer for parts of the decomposer that are also part of the degassing apparatus, apply also to the degassing apparatus.

[0018] Further embodiments are as defined in the claims and as described hereinafter.

[0019] The disclosure pertains to a degassing apparatus with a confinement comprising a bottom part, a degassing zone is provided by the bottom part. Liquid is redistributed horizontally away from a liquid outlet in embodiments to provide residence time in the degassing zone. The degassing apparatus can be a part of a decomposer of a urea plant. Brief description of the drawings

[0020] Figure 1 schematically illustrates an example degassing apparatus according to the invention.

[0021] Figure 2 schematically illustrates a further example degassing apparatus according to the invention. Figures 2A and 2B show two different views.

[0022] Figure 3 schematically illustrates an example decomposer according to the invention.

[0023] Figure 4 schematically illustrates an example urea plant according to the invention.

[0024] Any embodiments illustrated in the figures are examples only and do not limit the invention. Detailed description

[0025] The invention is broadly based on using degassing apparatus, in particular a degassing part of a decomposer, wherein the degassing apparatus comprises a confinement, and using the bottom part of the confinement as a degassing zone. The degassing zone is hence the zone of the confinement above the bottom part. Accordingly, a liquid layer is provided on the bottom part with a relatively low liquid velocity. Moreover, the liquid is provided on the bottom part of the confinement with a distribution that is shifted away horizontally from the liquid outlet, so as to increase the residence time of the liquid in the horizontal liquid layer on the bottom part compared to a set-up wherein the bottom part is homogenously wetted. In other words, compared to a reference embodiment wherein liquid is supplied from higher parts of the confinement to each part of the bottom plate equally (e.g. same m³ liquid per hour per cm² for each part of the bottom part), in the invention the average horizontal distance of the liquid that is supplied to the bottom part is increased.

[0026] This approach radically departs from the existing approach wherein the bottom part is steeply inclined (conical bottom plate with surface inclined steeply downward in radially outward direction) for efficient drainage of liquid to the liquid outlet and with a degassing boot outside the confinement, in particular with a degassing boot arranged in the liquid flow line between the liquid outlet and the bottom tube bundle inlet.

[0027] Advantageously, good degassing is obtained with an advantageous compact design, i.e. compact in terms of horizontal footprint (permitting a practical vessel design on top of the shell-and-tube heat exchanger), and vertical height (again advantageous for compact vessel design). A further advantage is that a degassing boot in the liquid flow line to the tube bundle is no longer necessary. In the invention, degassing can be achieved internally inside the decomposer. These advantages are especially useful in the context of decomposers for urea plants, but also provide benefits for degassing liquids more broadly.

[0028] The decomposer is configured for decomposing carbamate from a urea solution comprising carbamate. Carbamate is thereby decomposed into CO2 and NH3 by heating.

[0029] The decomposer preferably comprises a shell-and-tube heat exchanger with the urea solution to be heated (to decompose carbamate) in the tubes and with heating fluid (e.g. steam, hot steam condensate, or a condensing gas mixture comprising CO2 and NH3) in the shell. Preferably, the tube bundle is vertical, and preferably, urea solution is supplied to the vertical tube bundle at the bottom. This provides for co-current flow of gas and liquid (upward) in the tube bundle. An advantage of a vertical tube bundle with liquid supply a the bottom, compared to a falling-film heat exchanger, is a relatively simple design with a smaller heat exchange area and without a need for a large number of liquid distributors (also known as ferrules) for forming a liquid film in each tube.

[0030] It is desired to transport the urea solution from the flashing zone to the tube bundle bottom by gravity flow, i.e., without the use of pumps. On the other hand, the height of the decomposer (including the flashing zone, preferably a rectification zone, and the shell-and-tube heat exchanger with liquid exit zone) is preferably kept to a minimum. The flashing zone is the part of the decomposer receiving the two-phase fluid.

[0031] The inventors found that in order to ensure that liquid is properly fed to the bottom of a tube bundle, which tube bundle is arranged at a lower part of a decomposer of the type discussed hereinbefore for a urea plant, with a rectifying column by gravity flow, i.e. by liquid head in the liquid flow line (pipe) from the liquid outlet of the confinement to the tube bundle bottom, a sufficient degassing of the urea solution is important. This flow line is preferably a pipe and is preferably filled with liquid. The flow line is, structurally, typically arranged around (besides, i.e. external to) the vertical shell-and-tube heat exchanger (heater part) of the decomposer.

[0032] In a process and plant known to the inventors, without admitting published prior art, the flow line comprises a so-called degassing boot to achieve such degassing, the degassing boot in located in an upper part of the flow line. This degassing boot is a part (e.g. vertical pipe part) with a sufficiently large diameter to decrease the liquid velocity to max. 0.25 m / s in it; and with a length / height of approximately 1 m. With greater capacity, the degassing boot diameter becomes larger and the liquid outlet nozzle to the vertical degassing boot also becomes larger to be self-venting. The degassing boot receives fluid from an outlet nozzle having a large diameter to achieve similar low liquid velocity and to permit gas escape from the degassing boot.

[0033] This degassing boot is located essentially outside the column of the decomposer / rectifier and in the flow line around and besides the heater, and has a bottom that is located typically vertically higher than the upper end of the tube bundle (i.e. well above ground level), so as to have a liquid column (liquid head) between the degassing boot outlet and tube bundle inlet sufficient to provide a driving force, by gravity, for fluid transport through the tube bundle. Calculations of the present inventors showed that such a degassing boot reducing the velocity to max. 0.25 m / s enables the removal of gas bubbles with a size of min. 600 µm. It was found by the inventors that such degassing enables satisfactory operation of the decomposer by gravity flow and that insufficient degassing may lead to flooding of the decomposer as the presence of gas decreases the density of the liquid column between the degassing boot and the tube bundle inlet.

[0034] Without wishing to be bound by way of theory, the flashing of stripped urea solution comprising carbamate (pressure reduction from above 100 bar to e.g. about 15 bar or about 4 bar) results in the formation of gas, as small bubbles, throughout the urea solution. At any rate, experience of the inventors shows that a quite large diameter degassing boot is necessary in practice to obtain sufficient degassing for proper operation, with gravity flow, with existing designs of the decomposer. In particular, not only the larger gas bubbles must be removed, but also the small bubbles which will be dragged downward with a fast moving urea solution in the downward vertical flow line around the shell-and-tube heat exchanger.

[0035] In addition, in existing decomposers, the liquid outlet of the confinement, i.e. from the chamber used for flashing and gas / liquid contacting, has a large diameter nozzle so as to effect degassing. This increases constructional complexity.

[0036] The invention provides the advantage that sufficient degassing is achieved inside the vessel of the decomposer and that the degassing boot can be omitted. To this end, the bottom part of the confinement is adapted into a degassing zone providing residence time for a layer of liquid with low velocity, and liquid is supplied onto the bottom part in a horizontally inhomogeneous way.

[0037] It is noted that in existing decomposers, often a rectification zone is used, e.g. with a packing, which packing distributes the liquid over the horizontal plane and serves to optimize gas / liquid contact. Usually, a liquid distributor is provided on top of the packing to distribute the liquid, in the horizonal plane, over the entire packing. In such a case, a bottom plate directly below the packing would be homogenously wetted by liquid percolating down from the packing. Such a rectification zone, in particular packing and liquid distributor, is also used in preferred embodiments of the invention.

[0038] More broadly, the invention advantageously provides for a degassing zone inside the confinement, yet with a compact design of the degassing system or apparatus.

[0039] In the inventive method, liquid is supplied onto the bottom part (e.g. bottom plate) of the confinement in a inhomogeneous manner, in particular horizontally shifted away from the liquid outlet. Various approaches can be used to achieve such a liquid distribution.

[0040] In embodiment, a baffle plate is arranged above and spaced apart from the bottom plate, which is inclined with a decreasing upper surface away from the liquid outlet, e.g. with the liquid outlet in a side wall of the confinement.

[0041] In a further embodiment, the baffle plate (inclined or level) is provided with an upward rim over a part of the perimeter to guide the liquid horizontally away from the liquid outlet.

[0042] In yet a further embodiment, the baffle plate is provided with holes distributed in such a manner (in the horizontal plane) that liquid is guided horizontally away from the liquid outlet, for instance with larger apertures (or generally a larger void fraction) of the baffle plate horizontally near the liquid outlet. In operation, a liquid layer can be formed selectively on a part of the baffle plate horizontally close to the liquid outlet, that enables liquid transport on the baffle plate horizontally away from the liquid outlet.

[0043] Similarly, the baffle may be provided by a plurality of baffle elements that are spaced apart from each other, with increasing spacing in the direction horizontally away from the liquid outlet.

[0044] In yet a further embodiment, the baffle plate is located above the liquid outlet and obstructs vertical liquid flow in the vicinity of the liquid outlet; liquid flows horizontally on the upper surface of the baffle plate away from the liquid outlet towards and over an edge of the baffle plate that is horizontally removed from the liquid outlet onto the bottom part.

[0045] The baffle plate is e.g. provided with apertures and / or with spacing between the baffle plate and the confinement (in particular, the vertical vessel wall). In yet another embodiment, the apparatus contains a liquid flow connection to transport liquid from the upper side of the baffle plate to the degassing zone that is provided vertically between the baffle plate and the lower bottom part, to introduce liquid into the degassing zone at a position horizontally away from the liquid outlet.

[0046] Providing liquid horizontally away from the liquid outlet to the bottom part indicates that the average horizontal distance of the liquid received by the bottom part (or by the degassing zone) is larger than in the case of homogenous liquid supply (homogenous in the horizontal plane, e.g. in top view) over the bottom part (respectively degassing zone). Herein the average horizontal distance is based on the amount of liquid (e.g. m³ per hour) received at each position (e.g. per cm²), assuming equal composition of the liquid, and the horizontal distance between that position and the liquid outlet, or, in case of multiple liquid outlet, the minimum horizontal distance to a liquid outlet. Horizontal distance indicates the spacing in the horizontal plane. The skilled person understands that the horizontally redistributed liquid, e.g. on the liquid redistribution element, contains gas.

[0047] Further configurations to redistribute liquid horizontally away from the liquid outlet are also possible. The preferably used liquid redistribution element is arranged, for liquid flow, upstream of the degassing zone.

[0048] The liquid layer height in the degassing zone is preferably not too high, to ensure good gas escape from the liquid at the gas / liquid interface. On the other hand, for a given flow rate (m³ liquid per hour) and given width of the degassing zone (respectively bottom part) (width perpendicular to the flow direction), a larger height of the liquid layer decreases the liquid velocity, and increases residence time in the degassing zone. For instance, the liquid layer height is 5 to 20 cm, preferably about 10 cm. It was found that such a liquid layer is advantageous to provide for a self-venting liquid outlet (Froude number <0.3 approximately). Note, the liquid layer height in the liquid exit chamber (i.e. downstream of the tube bundle for liquid) can be larger than in the degassing zone (which is upstream of the tube bundle).

[0049] In a preferred embodiment, the liquid outlet is positioned in a side wall of the confinement, adjacent to (flush with) the bottom part (e.g. bottom plate) of the confinement. In principle, a plurality of liquid outlet openings in the confinement could be used, e.g. a plurality of liquid outlet openings in the side wall of the confinement. The side wall is the vertical wall of the confinement and is typically provided by the shell. In an example embodiment, the liquid redistribution element is baffle plate with a centre hole and a plurality of side wall liquid openings provide the liquid outlet. However, this embodiment is less desired. Preferably the one or more liquid outlet openings are arranged such that the bottom part comprises a location with a minimum horizontal distance to a liquid outlet opening that is at least 70% or at least 90% of the diameter of the bottom part (for a cylindrical vessel), e.g. by using one liquid outlet opening or a plurality of liquid outlet openings close to each other. The liquid outlet opening in the side wall is preferably sized such as to be self-venting.

[0050] It was found that the liquid outlet opening in the side wall can be much smaller in the invention than in existing designs, which also advantageously reduces height of the vessel.

[0051] In a further embodiment, the liquid outlet or a plurality of liquid outlet openings is provided in the bottom part (e.g. bottom plate), with a liquid conduit from the liquid outlet opening, typically through the shell wall, to the liquid flow line to the tube bundle inlet. The liquid outlet opening is preferably located eccentrically in the bottom part so as to increase residence time in the degassing zone by having the liquid supply to the degassing zone at the opposite side.

[0052] Various embodiments of the invention include the following, using reference signs as in the drawings (Fig. 1-4) for convenience and without limiting the embodiments in any way.

[0053] Provided is, in an embodiment, a degassing apparatus (100) (or degassing unit) (A1) which comprises a confinement (1). The confinement comprises: a fluid inlet (3) for a fluid stream comprising liquid in an upper part of the confinement; a liquid outlet (4) at a lower part of the confinement, and a gas outlet (2) arranged vertically higher than said liquid outlet (4), and wherein the confinement (1) comprises a horizontally extending bottom part (5), preferably a bottom plate, for holding a layer of liquid on said bottom part (5) in a degassing zone (12) in said confinement (1). Horizontally extending means that the part extends in the horizontal direction, e.g. a horizontal or inclined plate. Preferably the part is a horizontal or substantially plate. Preferably, the confinement is provided by a shell and a bottom plate, with the shell having vertical side walls and a top part (e.g. dome-like top part). The shell is e.g. pressure-bearing. The inlet and the outlets are in fluid connection with each other.

[0054] The confinement provides a chamber (15), having said inlets and outlets, and comprising preferably the liquid distribution element (6) and preferably also a flashing zone, a gas / liquid contacting zone, and the degassing zone (12).

[0055] The chamber (15) is preferably a vertical cylindrical vessel, having a circular horizontal cross section. Preferably, the bottom part (5) is a circular plate, preferably the degassing zone (12) is a layer on the bottom plate.

[0056] Preferably, the degassing apparatus is configured for supplying liquid onto the bottom part (5) with an inhomogeneous distribution over said bottom part (5) wherein the distribution is shifted horizontally away (i.e. away in the horizontal plane) from said liquid outlet (4).

[0057] The degassing zone (12) is configured for flow of liquid in a horizontal direction toward the liquid outlet. The gas outlet is for instance provided in an upper part of the confinement, for instance vertically higher than the fluid inlet. The gas outlet and liquid outlet are vertically spaced apart. The bottom part may comprise a gas inlet opening with a gas riser tube. Preferably the degassing zone covers at least 50% or at least 80% or at least 90% of the surface of the bottom part. The operation of the degassing apparatus is based on gravity and the horizontal and vertical direction can be defined accordingly, alternatively in embodiments wherein the bottom part is a flat plate, vertical is the direction perpendicular to the bottom plate. Preferably, the width of the degassing zone (dimension perpendicular to the flow direction in the horizontal plane) is over at least a part, at least 50% or at least 70% of the diameter of the bottom part or of the confinement (i.e. inner shell diameter). Preferably, the length of the degassing zone (dimension in flow direction) is for all liquid received by it, i.e. the minimum flow length, at least 30% or at least 50% of said diameter of the bottom part or of the confinement.

[0058] Preferably (A2), the degassing apparatus (A1) comprises a liquid redistribution element (6) that is arranged in said confinement (1) vertically between said bottom part (5) and said fluid inlet (3), and vertically higher than the degassing zone. The liquid redistribution element (6) comprises one or more parts, and extends in a horizontal direction and typically at least partially obstructs vertical fluid flow through the confinement, in particular typically at least partially covers the degassing zone. The element is arranged to receive liquid from said fluid inlet (3) on an upper side (10) (or upper surface) of said element (6); in operation at least some liquid flows over the horizontally extending upper side (10). Hence, in operation at least some liquid impinges on the upper surface (uppers side) of the liquid redistribution element (6) and is thereby diverted in the horizontal plane (i.e. in top view; the upper surface can be inclined). The liquid redistribution element (6) is provided, e.g., by a sloped plate or surface.

[0059] The element provides one or more liquid passageways (8) that allow liquid to flow from the upper side (10) onto the bottom part (5), inside the confinement; and that also permit gas flow. The liquid passageways (8) are provided e.g. as an aperture in said element, or as apertures between parts of said element, and / or as an aperture (8) between said element and said confinement, for instance as annular aperture between a baffle plate and the side wall of the confinement. Preferably, the liquid redistribution element (6) increases the average horizontal distance from said liquid outlet (4) of liquid provided from said element (6) onto said bottom part (5). Suitable measures to provide such liquid redistribution include, e.g., the location of the baffle plate relative to the liquid outlet, distribution of liquid passageways in the horizontal plane, and inclination of the baffle plate, and combinations thereof.

[0060] Preferably, (A3) as illustrated in Figure 1, the liquid redistribution element (6) (A2) comprises a baffle plate and the upper side (10) (or surface) of the baffle plate is sloped (slope in a vertical cross-section) with a higher part (10A) horizontally proximate to said liquid outlet (4) and a lower part horizontally distant from said liquid outlet (4) (i.e. proximate and distant in top view). Preferably the baffle plate is inclined (in a vertical cross-section); alternatively or in addition, the thickness of baffle plate (in vertical direction) may vary to provide the sloped top surface. Preferably the baffle plate is inclined with the horizontal at angle in the range of 1º to 30º, preferably 3º to 10º, more preferably approximately 5º; in particular with a downward inclination in vertical view from above in a direction away from the liquid outlet and / or with a downward inclination of the upper surface in at least one vertical cross-section through the element and the liquid outlet.

[0061] Preferably, (A4), the liquid redistribution element (6) (A2 or A3) comprises a baffle plate with a rim extending upward from the upper surface of the plate along a (first) part of the perimeter of baffle plate, in particular the perimeter adjacent to the one or more liquid passageways. For instance, the total perimeter of the baffle, provided e.g. by apertures in the plate and / or clear space between an edge of the plate and the confinement, comprises a first perimeter part provided with an upward rim, and a second perimeter part with no rim or with a rim with lower height than the rim at said first part. For instance, the first part with the rim is at least 30%, or at least 50% and / or max. 70 % of the perimeter. In operation, liquid is prevented from flowing into the liquid passageways at the position of the rim, e.g. at the first of the perimeter, and permitted to flow into a liquid passageway at the second part. Said first part is horizontally proximate to the liquid outlet opening and said second part is horizontally distant to the liquid outlet opening. In this way, liquid is redistributed on the element horizontally away from the liquid outlet opening. Preferably, all liquid is redistributed to the half of the bottom part that is remote from the liquid outlet opening, especially with the liquid outlet opening in the side wall. The upper surface of the plate with such can be horizontal or sloped (A3); the combination of a sloped surface and the rim is preferred. The rim is preferably provided at the perimeter of the higher parts of the sloped surface.

[0062] Preferably, (A5), the liquid redistribution element (6), preferably the baffle plate (A2, A3 or A4), obstructs vertical flow over, and / or extends over, at least 50%, or at least 70%, or at least 90% of the bottom part or bottom plate (5), in a horizontal plane (i.e. in vertical view); in particular these percentages are by surface area. Preferably, the liquid redistribution element, preferably the baffle plate (A2, A3 or A4), has a horizontal surface area that is at least at least 50%, or at least 70%, or at least 90% of the horizontal surface area of the bottom part or bottom plate, respectively of the degassing zone.

[0063] Some degassing may also occur advantageously on the liquid redistribution element, in particular during the flow of liquid over the upper surface of it, in particular in case of a baffle plate.

[0064] The liquid redistribution element is preferably mounted in the shell, e.g. by appropriate support means, and preferably can be removed from the chamber through a manhole.

[0065] The degassing apparatus (100) (any of A1-A5) preferably (A6) comprises a gas / liquid contacting zone (13) in the chamber provided by the confinement, preferably arranged vertically between said fluid inlet (3) and said liquid redistribution element (6), and more preferably below a flashing zone (14). The gas / liquid contacting zone (13) is preferably a rectification zone comprising a packing (packed bed), e.g. with Pall rings. The gas / liquid contacting zone (13) contacting zone preferably receives, in the embodiment of a decomposer, the (hot) gas from the tube bundle and relatively colder incoming liquid in operation, in particular liquid that is still to be supplied to the tube bundle (heater), and typically provides for counter-current contact between the gas and the liquid.

[0066] The gas / liquid contacting zone (13) is preferably arranged vertically lower than the gas outlet (2). Preferably, in operation, the gas stream from the gas / liquid contacting zone (13) flows to the gas outlet (2).

[0067] The degassing apparatus preferably comprises a flashing zone (14) in the chamber provided by the confinement, this zone is configured for release of gas from the incoming fluid stream (which is preferably a mixture of gas and liquid). The flashing zone (14) is preferably arranged vertically lower than the gas outlet (2).

[0068] The optionally used rectification zone comprises for example a liquid distributer arranged above (e.g., on top of) the packed bed to distribute the liquid in the horizontal plane.

[0069] The packed bed of the rectification zone is e.g. a random packed bed or a comprises a structured packing.

[0070] It is observed that the packing in the gas / liquid contacting zone (13) provides for homogeneous wetting of the liquid redistribution element (6) by liquid (urea solution) from the packing. In other words, the liquid (urea solution) is supplied to the liquid redistribution element (6) in a horizontally distributed manner. In particular, typically the entire bottom surface of the packing is wetted by the liquid (urea solution) in operation and liquid (urea solution) drips from the packing, especially from the entire horizontal bottom surface of the packing.

[0071] Preferably, the packing has a circular horizontal cross-section, especially in case of vertical cylindrical vessel. In this way, the packing can be conveniently flush with the vessel wall, avoiding gas slip.

[0072] Preferably, the liquid redistribution element (6) obstructs at least 80 % or at least 90 % of the bottom surface of the packing in vertical projection. Hence, in vertical projection, the liquid redistribution element covers preferably at least 80% or at least 90% of (the bottom of) the packing. This ensures that a relatively large part of the liquid from the bottom of the packing is received by the liquid redistribution element (6), in particular is contacted with the upper side of the liquid redistribution element (6), and is thereby redistributed as desired. Conveniently, the liquid redistribution element has, e.g., a circular shape in a vertical view, i.e. a view in vertical direction, especially if the liquid redistribution element (6) also has a circular shape in vertical view. However, other shapes are also possible for the liquid redistribution element, such as hexagonal.

[0073] Preferably (A7), the degassing apparatus, in particular the flashing zone (14) comprises a pipe (16) extending from the fluid inlet horizontally into the chamber, to introduce the fluid into the chamber. Preferably, the as / liquid contacting zone (13), e.g. packed bed, is located vertically below the pipe, and preferably the apparatus comprises a droplet removal zone (17) above the pipe (16), or at least above the fluid inlet, to remove droplets entrained in the gas stream. Gas can flow around the pipe from the packed bed to the gas outlet through the droplet removal zone. The minimum diameter of the chamber is typically defined by the droplet removal zone (17) to provide sufficiently low gas velocity in it.

[0074] Preferably (A8), the bottom part is substantially horizontal, e.g. has an average angle with horizontal of less than 5º. Preferably, the bottom part is a bottom plate, more preferably a horizontal plate. This may contribute to a low liquid velocity. The horizontal liquid flow in the degassing zone also provides for more efficient degassing and for a lower pressure drop.

[0075] The degassing apparatus is preferably used in combination with a pressure deducing valve in the fluid flow line to the fluid inlet. Hence, also provided is an apparatus comprising a pressure reducing valve, configured to receive a liquid, and the degassing apparatus, and a fluid flow line from the pressure reducing valve to the fluid inlet of the degassing apparatus. The valve is e.g. a control valve, e.g. a valve with an adjustable size of the flow passage, e.g. an angle valve.

[0076] Also provided is a decomposer (101) (B1), in particular a carbamate dissociator, comprising a degassing apparatus as described (A1 with preferably any of the features A2 to A8; with all further preferences), and further comprising a shell-and-tube heat exchanger (102) with a vertical tube bundle (103), and a liquid exit chamber (104), that is arranged at an upper end of said tube bundle, and that comprises a liquid outlet (105) of the decomposer, also called second liquid outlet (105). In operation, the degassed liquid is treated in the vertical tube bundle, and the treated liquid flows through the liquid exit chamber and leaves the decomposer at said second liquid exit. The degassing apparatus (100) is arranged above, e.g. on top of, the liquid exit chamber. Correspondingly, the liquid exit chamber is arranged below the degassing apparatus, preferably below the bottom part of the degassing apparatus. Preferably, the liquid exit chamber has a gas outlet on an upper part of the said chamber, that is arranged for venting gas from the liquid exit chamber into the degassing apparatus.

[0077] The decomposer comprises a liquid flow line (105) for liquid from said liquid outlet (4) to a bottom inlet side of the tube bundle (103).

[0078] The decomposer comprises a gas flow connection (106) from the liquid exit chamber into the degassing apparatus, such that the gas can be used or treated in the gas / liquid contacting zone, in particular in the rectification zone. The gas flow connection is preferably internal to the decomposer, e.g. is located inside the shell of the decomposer. The gas flow connection is provided, e.g., by a riser extending from the gas outlet of the liquid exit chamber through the degassing zone into a space of the degassing apparatus, such that in operation the gas exits the riser above the liquid level in the degassing zone. Accordingly, the gas from the liquid exit chamber in operation preferably contacts only liquid upstream, for the liquid, of the degassing zone.

[0079] The tube bundle preferably comprises straight tubes, e.g. at least 100 tubes. The shell may comprise one, two, or even more compartments, for instance for different heating fluids. The liquid exit chamber (104) has a bottom with the tube bundle upper end openings, and a gas outlet and a separate liquid outlet; with the gas outlet vertically higher than the second liquid outlet (105). The chamber is used for gas / liquid separation in operation. A further degassing may be performed on the liquid from the second liquid outlet (105), e.g. in a separate unit.

[0080] For example, the bottom part (5) of the confinement comprises (provides) a gas outlet (106) of the liquid exit chamber, for venting gas from the liquid exit chamber (104) into the degassing apparatus (100), preferably with a gas riser (riser tube) (107) extending upward from the gas venting hole (106) in particular through the degassing zone. The top of the gas riser tube is above the liquid level in the degassing zone in operation. The gas outlet and the riser tube provide a gas flow connection from the liquid exit chamber to the degassing apparatus. The gas flows from the riser (107) to the gas / liquid contacting zone (13) and to the gas outlet (2).

[0081] The liquid exit chamber preferably (B4) has, at least at the height of the second liquid outlet, a side wall with preferably the same diameter as the bottom plate of the confinement (which is arranged at the top or above the liquid exit chamber). Preferably, the liquid exit chamber comprises a bottom flange with a smaller diameter as the bottom plate, with the flange coupled to an upper tube sheet of the shell-and-tube heat exchanger. Preferably the liquid exit chamber comprises a bottom part extending from the flange to the side wall, to effect the diameter increase, in particular below the second liquid outlet. Preferably, the second liquid exit is provided in a side wall of the liquid exit chamber having a diameter, preferably over the entire height of the second liquid outlet, that is in the range 100% to 200%, e.g. 120 – 180%, of the diameter of the shell-and-tube heat exchanger (internal shell diameter) and / or of the tube bundle diameter (outer tube limit). In this way, the liquid velocity in the liquid exit chamber is relatively low and the second liquid outlet can have a nozzle with an advantageously relatively small nozzle, especially a small nozzle (e.g. more than 30% smaller diameter) compared to liquid exit chambers with a conical bottom part.

[0082] Also provided is a method of degassing a liquid, carried out in a degassing apparatus as provided ( A1 with preferably any of features A8), and optionally with a decomposer as described (B1, with preferably any of features B2 – B4). The method comprises supplying a fluid stream comprising a liquid to the fluid inlet of the degassing operates; the fluid stream at the inlet may comprise gas and / or the liquid partly vaporizes in the flashing zone. In the method, gas is withdrawn from the gas outlet and liquid from the liquid outlet and degassing is performed in the degassing zone. In the method, preferably, the liquid is supplied onto the bottom part (5) with an inhomogeneous distribution over said bottom part (5) that is shifted horizontally away from said liquid outlet (4). Hence, the average distance of the liquid received on the bottom part in the horizontal direction is greater than in case of homogeneous wetting of the bottom part.

[0083] Preferably, the degassing apparatus comprises the liquid redistribution element and the liquid redistribution element (6) increases the average horizontal distance from said liquid outlet (4) of liquid provided from said element (6) onto said bottom part (5) in the method.

[0084] Preferably, the method involve a pressure reduction of the liquid in the pressure reduction valve by at least 5 bar or at least 10 bar. Preferably, the method involves flashing of the liquid in the flashing zone, i.e. the release of gas from the liquid. Preferably, further gas released from the liquid in the degassing zone also flows to the gas outlet.

[0085] Optionally, the confinement is operated at a pressure of at least 1 bar, e.g. above 2 bara, e.g. in the range from 2 bara to 100 bara (bar absolute). Preferably, the confinement, tube bundle, and liquid exit zone are operated at substantially the same pressure. Preferably, the liquid is transported from the liquid outlet of the degassing apparatus to the liquid exit chamber by gravity flow (including the effects of heating in the tube bundle on the density). Preferably, the method involves heating the liquid in the tube bundle.

[0086] The invention also provides a method of operating the inventive decomposer, the method comprising: supplying a fluid stream comprising liquid and gas to the fluid inlet of the degassing apparatus comprised in said decomposer, such that at least a part of the liquid is received on said upper side of said liquid redistribution element and the liquid flows through the degassing zone thereby becoming the degassed liquid; supplying the degassed liquid from the liquid outlet through the liquid flow line to the vertical tube bundle of the shell-and-tube heat exchanger; and heating said degassed liquid in said vertical tube bundle of the shell-and-tube heat exchanger.

[0087] The heating of the degassed liquid in particular causes a component of the degassed liquid to decompose. For instance, the degassed liquid is a urea solution comprising carbamate and the heating causes decomposition of the carbamate into CO2 and NH3.

[0088] Preferably, the method further comprises flashing a urea solution also comprising carbamate and typically also ammonia, i.e. reducing the pressure, e.g. with a pressure reducing valve, to give a flashed urea solution also comprising gas, and supplying said flashed urea solution comprising gas as said fluid stream to said fluid inlet. Herein, the flashing refers to the reduction of the pressure of the urea solution, causing a part of the liquid to vaporize, forming fluid stream comprising gas and liquid; wherein both the gas and the liquid are supplied to the fluid inlet of the decomposer. Hence, the term flashing does not imply gas / liquid separation upstream of the decomposer.

[0089] Preferably, the liquid degassed in the inventive method of degassing a liquid, is a urea solution comprising urea, water, and ammonium carbamate (with the term carbamate as used in the art, including carbonate species). Preferably, such a liquid is supplied to the fluid inlet. Preferably, the gas at the gas outlet contains NH3, CO2, and H2O. Preferably, the heating in the tubes results in decomposition of carbamate into NH3 and CO2, and some water evaporation. Heating fluid in the shell of the heater part may be provided e.g. by steam, or a condensing gas comprising NH3 and CO2; the pressure in the shell can be lower or higher (or the same) as in the tubes.

[0090] Preferably, the degassing apparatus is operated at low pressure (LP) of e.g. 1.0 – 10 bara, preferably 2 -8 bara, or at medium pressure (MP) of e.g. 10 – 70 bara, preferably 10 – 40 bara. These pressures are advantageous for urea recovery sections comprising a decomposer and a carbamate condenser receiving gas from the gas outlet of the decomposer. The carbamate condenser is, for example, provided in a first compartment of heat exchanger used for heating a urea solution in a second compartment, typically with urea solution in the second compartment at a lower pressure than the gas in the first compartment of the heat exchanger.

[0091] The method involves for instance supplying to the pressure reducing valve a urea solution and reducing the pressure of the urea solution by at least 2 bara, or at least 10 bara, or at least 20 bara, e.g. from a high pressure (HP) of at least 100 bara to a pressure of max. 70 bara, max. 40 bara, or directly to an LP pressure of max. 10 bara; or supplying an MP urea solution of e.g. at least 20 bara and reducing the pressure to LP pressure, e.g. max. 10 bara or max. 6 bara.

[0092] For example, the urea solution supplied to the pressure reducing valve (expansion valve) comprises about 45 wt.% urea and about 40 wt.% NH3 and CO2 in total (including as carbamate). Flashing from HP (about 140 – 150 bar) to MP (20-25 bar) may result in the formation of, for example, 3 to 4 m³ gas per m³ of urea solution before the flashing (gas volume at actual temperature and pressure). The urea solution after flashing comprises e.g. about 20 wt.% NH3 and CO2 in total (including as carbamate. The temperature of the flashed urea solution is e.g. about 20ºC lower. Flashing directly from HP to LP results in the formation of even more gas.

[0093] The method preferably involves heating urea solution in the tube bundle thereby decomposing carbamate and releasing a gas stream, and contacting flashed urea solution with said gas stream from the tube bundle in said gas / liquid contacting zone, preferably with counter-current contact of gas and liquid in said zone. Hence, the relatively cold flashed urea solution is contacted with the gas stream from the tube bundle, where carbamate is decomposed by heating, said gas stream comprising CO2, NH3 and H2O. The gas / liquid contacting causes a part of the H2O to condense and be absorbed in the liquid, thereby reducing the water content of the gas, which provides for a water content of the carbamate recycle to the synthesis section in the urea plant.

[0094] The method preferably comprises supplying liquid from said liquid outlet (4) through the liquid flow line (105) through said tube bundle (103) by gravity flow.

[0095] The invention also pertains to a urea production plant (200) comprising a (high pressure) urea synthesis section (201) and a recovery section which comprises a decomposer (101) as described (B1) and a carbamate condenser (202); the decomposer preferably having any of the features B2-B4, and comprising a degassing unit (A1), preferably with any of the features A2-A8. The carbamate condenser has an inlet for gas connected to the gas outlet (2) of the decomposer and a liquid outlet for carbamate solution in liquid connection with an inlet the urea synthesis section. The plant comprises a urea solution flow line (203) from an outlet for urea solution of the urea synthesis section, in particular from a HP stripper, to the fluid inlet of the decomposer, wherein said urea solution flow line comprises a pressure reducing valve (204) directly upstream the fluid inlet of the decomposer. The valve is preferably a control valve, e.g. an angle valve.

[0096] The urea plant comprises a recycle flow line for carbamate solution from the recovery section to the synthesis section. The synthesis section comprises a reaction zone, e.g. provided at least in part as a vertical urea reactor, with inlet(s) at the bottom and an outlet for withdrawing urea solution from an upper part, and preferably with trays. The reaction zone may also be provided e.g. as a part of a horizontal combined reactor / condenser, e.g. as a part of a pool reactor.

[0097] The plant comprises a CO2 feed line and an NH3 feed line to the synthesis section, e.g. a CO2 supply line from a compressor to the synthesis section. The synthesis section may comprise a HP stripper and a HP carbamate condensation zone, e.g. HP carbamate condenser. The HP stripper is e.g. a CO2 stripper. The HP stripper is generally a shell-and-tube heat exchanger with a falling-film of urea solution in the tube bundle; receiving urea solution from the reaction zone. The HP carbamate condenser is typically a shell-and-tube heat exchanger, receiving gas from the stripper top outlet, and having carbamate solution outlet connected to the reaction zone. The HP carbamate condenser operates e.g. with gas to be condensed in the tubes or in the shell side space, and with a horizontal or vertical tube bundle.

[0098] An advantage of HP CO2 stripping is the relatively low N / C ratio of the resulting stripped solution (molar ratio of NH3 to CO2 including as carbamate) which is beneficial for operating the degassing apparatus.

[0099] The decomposer is for example comprised in recovery section in an MP recovery section directly receiving urea solution from the synthesis section, or in an LP recovery section downstream of an a HP CO2 stripper, or downstream of an MP treatment section, e.g. downstream of an MP recovery section, or e.g. downstream of an MP adiabatic flash vessel. The MP recovery section optionally further comprises an ammonia condenser having a separate ammonia recycle flow line to the synthesis section.

[0100] The decomposer design with internal degassing apparatus of the invention is advantageous both for modifying existing urea plants and for newly built plants.

[0101] The invention also provides a urea production process carried out in a urea production plant according to the invention, wherein the plant comprises the inventive decomposer (embodiment B1, preferably with features B2-B6). The process comprises reaction NH3 and CO2 to form a urea solution comprising urea water, and carbamate, in the urea synthesis section, in particular in the urea reaction zone, e.g. at a pressure above 100 bar (i.e., at high pressure) and at least 160ºC; and supplying the urea solution directly or indirectly to the fluid inlet of the decomposer, after pressure reduction to a pressure of max. 70 bar, preferably to a pressure in the range 2 – 10 bara or 10 – 40 bara, and flashing of the urea solution in the decomposer. The process furthermore involves conducting the inventive degassing method in the decomposer; preferably with gas / liquid contacting in the rectification zone.

[0102] Preferably, the inventive urea production process, carried out in the inventive urea production plant, comprises: reacting CO2 and NH3 in said urea synthesis section to form a urea solution also comprising carbamate, and typically also ammonia; expanding said urea solution through said pressure reducing valve to give flashed urea solution also containing gas, supplying the flashed urea solution through said fluid inlet of said degassing apparatus, comprised in said decomposer. Thereby, at least a part of the liquid is received on the upper side (surface) of the liquid redistribution element and the liquid flows through the degassing zone thereby becoming the degassed liquid, wherein the degassed liquid is a degassed urea solution also containing carbamate. The method further comprises: supplying the degassed liquid from the liquid outlet through the liquid flow line to the vertical tube bundle of the shell-and-tube heat exchanger; and heating said degassed liquid in said vertical tube bundle of the shell-and-tube heat exchanger thereby decomposing at least a part of the carbamate comprised in said degassed liquid into gaseous CO2 and NH3 , the resulting gas is preferably supplied through the gas outlet of the liquid exit chamber, preferably is supplied through the gas flow connection into the degassing apparatus, and preferably flows to the gas / liquid contacting zone.

[0103]

[0104] Figure 1 schematically illustrates an example degassing apparatus (100) with a confinement (1) defining a chamber (15) and provided with a fluid inlet (3) in an upper part of the confinement, a liquid outlet (4) at a lower part of the confinement, and a gas outlet (2) arranged vertically higher than said liquid outlet (4). The confinement (1) comprises a horizontally extending bottom part (5), e.g. bottom plate, for holding a layer of liquid in a degassing zone (12) that is located in said confinement (1). Hence, internal degassing is provided. The degassing apparatus comprises a liquid redistribution element (6) arranged in said chamber of said confinement (1) above the bottom part (5) and below the fluid inlet (3). The liquid redistribution element (6) comprises one or more parts and extends in a horizontal direction. It is arranged to receive liquid from the fluid inlet (3) on an upper side (10) (or surface) of it. The element provides one or more liquid passageways (8) allowing liquid to flow from the upper side (10) onto the bottom part (5), e.g. as an aperture in said element, or as apertures between parts of said element, and / or as an aperture (8) between said element and said confinement. The aperture is e.g. an annular aperture. Gas can also flow through the liquid passageways or apertures. The liquid redistribution element (6) comprises a baffle plate and the upper side (10) of the baffle plate is sloped with a higher part (10A) horizontally proximate to said liquid outlet (4) and a lower part (10B) horizontally distant from said liquid outlet (4). Liquid flows over the edge (11) onto the bottom plate in operation. The baffle plate comprises a perimeter (7) and a bottom side (9).

[0105] Figure 2 schematically illustrates an example degassing apparatus (100) having generally the same design as in Fig. 1, and reference numbers are the same as for Fig. 1 and as in the general description. The baffle plate (6) is provided with a rim (13) extending upward from the plate around (or over) a part (7A) of the perimeter (7) of the baffle plate. The rim (13) is provided at a higher part (10A) of the sloped or inclined baffle plate. Figure 2A shows a cross section through A-A (side view). Figure 2B shows a top view from B-B downwards.

[0106] As independent further features, the apparatus, in particular the chamber (15) provided by the confinement (1), also comprises a rectification zone (13) (a packed bed, allowing for fluid flow through it) and a flashing zone (14). These zones are especially preferred for the decomposer. The confinement (1) is, in this example, a vertical cylinder (cylinder with vertical acis)

[0107] In Figure 2A, the fluid flows in operation are furthermore illustrated, with liquid as the long dashed arrows (blue) and gas as the arrows with short dashes (green). Generally, gas flows up (with some gas flowing down with the liquid at the outlet of the pipe (16), liquid generally flows down or horizontal. The liquid contains gas bubbles until the degassing zone (12) where the gas escapes from the liquid.

[0108] Gas and liquid separate in the flashing zone (14), with liquid percolating down, preferably through the packed bed (13), and it is contacted with upward flowing gas. The liquid flows down and falls on the baffle plate, at various positions in the horizontal plane, and flows to the right side and over the lower perimeter part (7B) (also shown in Figure 2B) onto the bottom plate (5) and then, horizontally, at low speed, through the degassing zone (12) to the liquid outlet (4). It is noted that even liquid that falls o the bottom plate at the edge (13A) of the rim, travels through the degassing zone at least over the horizontal distance between the outlet and that edge (13A) Gas is released from the liquid in the degassing zone, flows around the baffle plate through the annular spacing (8) and to the rectification zone and gas outlet (2).

[0109] The confinement (1) has a circular horizontal cross section, e.g. is a cylindrical vertical vessel, and the bottom plate (5) is also a circular plate. The liquid passageway (8) is provided by an annulus.

[0110] Figure 3 schematically illustrates a decomposer (101) comprising the degassing apparatus (100) (similar as in Figures 1 and 2) and a shell-and-tube heat exchanger (102) as a heating part. The heat exchanger has a vertical tube bundle (103) and a shell (109). The shell (109) receives a heating fluid, e.g. steam, in operation.

[0111] The decomposer comprises a liquid exit chamber (104) at an upper end of the tube bundle. The liquid exit chamber comprises a second liquid outlet (105). The degassing apparatus (100) is arranged or mounted on top of the liquid exit chamber. The decomposer may comprise a pressure-bearing shell (108) housing both the degassing apparatus (100) and the liquid exit chamber (104) as compartments that are divided, for liquid flow, by the bottom plate (5). The pressure-bearing shell (108) is mounted on and supported by the shell-and-tube heat exchanger (102). The pressure-bearing shell (108) is supported by the shell-and-tube heat exchanger (102).

[0112] The shell (109) of the shell-and-tube heat exchanger (102) is a vertical cylindrical vessel. In practice, a (circular) tube plate can provide the bottom of the liquid exit chamber (104) and the essentially cylindrical pressure-bearing shell (108) is attached to the tube plate with a flange. For example, the horizontal cross-section of the shell and the tube plate are matching (circular).

[0113] However, the small horizontal footprint of the pressure-bearing shell (108) (which houses the degassing apparatus (100)) is an advantage for the decomposer in general, namely permitting the mounting on top of the shell-and-tube heat exchanger, and a constraint for the degassing in the degassing zone.

[0114] The decomposer comprises a liquid flow line (105) for liquid from the (first) liquid outlet (4) of the degassing apparatus to a bottom inlet side of the tube bundle (103). The decomposer comprises a gas flow connection (106) for supplying gas from the liquid exit chamber (104) into the degassing apparatus, preferably with a gas riser tube (107) extending from the bottom plate (5) through the liquid layer in the degassing zone in operation and under the baffle plate (6). The degassing apparatus (100) comprises a gas / liquid contacting zone (13) arranged vertically between said fluid inlet (3) and said liquid redistribution element (6), and in particular below said flashing zone (14). The gas / liquid contacting zone (13) is a rectification zone comprising a packing.

[0115] The flows of liquid (blue) and gas (green) are indicated with arrows (arrows with open tips: gas; arrows with closed tips: liquid). Dashed green lines indicate gas bubbles in the liquid.

[0116] Figure 4 schematically illustrates a urea plant (200) comprising a high pressure urea synthesis section (201) and a recovery section. The recovery section comprises a decomposer (101) (generally as shown in Figure 3), and a carbamate condenser (202). The carbamate condenser has an inlet for gas connected to the gas outlet (2) of the decomposer and a liquid outlet in liquid connection with the urea synthesis section. The plant comprises a liquid flow line (203) for urea solution from an outlet for urea solution of the urea synthesis section to the fluid inlet of the decomposer, wherein said liquid flow line comprises a pressure reducing valve (204) directly upstream the fluid inlet of the decomposer (101).

[0117] The inventive degassing method is preferably carried out in in the inventive degassing apparatus. The inventive urea production process is preferably carried out in the inventive urea plant.

[0118] Preferences and details discussed in connection with the degassing apparatus apply also for the equipment comprising the degassing apparatus, in particular for the inventive decomposer and the inventive urea production plant, and for the methods carried out with the degassing apparatus, in particular, the method of operating the decomposer, the method of degassing a liquid, preferably flashed urea solution, with the degassing apparatus, and the urea production process.

[0119] Carbamate as used herein refers to ammonium carbamate, as that term is used in the field of urea production.

[0120] As used herein, high pressure (HP) indicates at least 100 bara, medium pressure (MP) indicates 10 – 70 bara, low pressure (LP) indicates 1 – 10 bara, preferably 2 – 6 bara; and bara indicates absolute pressure in bar. Example

[0121] The invention will now be further illustrated by the following non-limiting example. The example does not limit the invention and does not limit the claims. Example 1

[0122] Calculations show that with a degassing unit as illustrated in Figure 2, i.e. with an inclined circular baffle plate with a rim around about 50% of the perimeter, gas bubbles down to 400 µm (or even lower) are degassed from a flashed urea solution in an inventive decomposer of a urea plant (carbamate decomposer, generally as in Figure 3); thereby exceeding the performance of a reference decomposer with an external degassing boot and advantageously reducing the liquid outlet nozzle diameter by about 40%. The flashed urea solution is obtained by flashing of a urea solution from a HP CO2 stripper operated at about 140 bar to medium pressure of about 20 to 25 bar in the decomposer. The diameter of the chamber was 1.2 m and this was the same as in the reference decomposer with a (external) degassing boot, i.e. this diameter was judiciously sufficient to accommodate degassing on a horizontal bottom plate, internal to the decomposer, in the inventive configuration, while maintaining the small footprint of the decomposer.

[0123] The inventive decomposer and the reference decomposer both used a horizontal inlet pipe and a droplet removal zone above the horizontal pipe.

Claims

1. A method of degassing a flashed urea solution also comprising carbamate, the method comprising supplying a fluid stream comprising the flashed urea solution comprising carbamate to a degassing apparatus (100) which comprises:- a confinement (1) comprising:- a fluid inlet (3) receiving said fluid stream comprising the flashed urea solution also comprising carbamate in an upper part of the confinement,- a liquid outlet (4) at a lower part of the confinement,- a gas outlet (2) arranged vertically higher than said liquid outlet (4),wherein the confinement (1) further comprises a horizontally extending bottom part (5) for holding a layer of liquid on said bottom part (5) in a degassing zone (12) in said confinement (1); wherein the flashed urea solution comprising carbamate is supplied onto the bottom part (5) with an inhomogeneous distribution over said bottom part (5) that is shifted horizontally away from said liquid outlet (4). 2. The method of claim 1, wherein the method involves reducing the pressure of a urea solution comprising carbamate and, in a flashing zone (14) in the confinement, flashing of gas from the urea solution to give the flashed urea solution. 3. The method of claim 2, wherein the degassing apparatus comprises a liquid redistribution element (6) arranged in said confinement (1) vertically between said bottom part (5) and said fluid inlet (3), wherein the liquid redistribution element (6) comprises one or more parts, extends in a horizontal direction, and is arranged to receive liquid from said fluid inlet (3) on an upper side (10) of said element (6) and that provides one or more liquid passageways (8) allowing liquid to flow from the upper side (10) onto the bottom part (5), e.g. as an aperture in said element, or as apertures between parts of said element, and / or as an aperture (8) between said element and said confinement, wherein said liquid redistribution element (6) increases the average horizontal distance from said liquid outlet (4) of liquid provided from said element (6) onto said bottom part (5). 4. The method of claim 3, carried out in a decomposer (101) comprising the degassing apparatus (100) and a shell-and-tube heat exchanger (102) with a vertical tube bundle (103), a liquid exit chamber (104) at an upper end of said tube bundle with a second liquid outlet (105) and the degassing apparatus (100) on top of said liquid exit chamber, and comprising a liquid flow line (105) for liquid from said liquid outlet (4) to a bottom inlet side of the tube bundle (103), wherein the decomposer comprises a gas flow connection (106) for supplying gas from the liquid exit chamber (104) into the degassing apparatus, and wherein the degassing apparatus (100) comprises a gas / liquid contacting zone (13) arranged vertically between said fluid inlet (3) and said liquid redistribution element (6), and preferably below said flashing zone (14);wherein the method involves:- heating urea solution in the tube bundle (103) thereby decomposing carbamate and releasing a gas stream;- contacting flashed urea solution with said gas stream from the tube bundle in said gas / liquid contacting zone (13);- supplying liquid from said liquid outlet (4) through the liquid flow line (105) through said tube bundle (103) by gravity flow. 5. The method of claim 4, wherein said liquid redistribution element (6) comprises a baffle plate and the upper side (10) of the baffle plate is sloped with a higher part horizontally proximate to said liquid outlet (4) and a lower part horizontally distant from said liquid outlet (4); preferably wherein the baffle plate is inclined. 6. The method of claim 4 or 5, wherein said liquid redistribution element (6) comprises a baffle plate that is provided with a rim (13) extending upward from the plate around a part (7A) of the perimeter (7) of the baffle plate, preferably wherein the rim (13) is provided at a higher part (10A) of the sloped or inclined baffle plate. 7. The method according to claim 4, 5 or 6, wherein said gas / liquid contacting zone (13) is a rectification zone comprising a packing. 8. The method according to claim 7, wherein the packing is arranged above the liquid redistribution element (6) and liquid flows from the packing to the liquid redistribution element (6). 9. The method according to any of the preceding claims, wherein said element (6), preferably the baffle plate, obstructs vertical flow over, and extends over, at least 50% of the surface area of the bottom part in a horizontal plane.

10. A decomposer (101) comprising a degassing apparatus (100), wherein the degassing apparatus (100) comprises a confinement (1) comprising:- a fluid inlet (3) for receiving a fluid stream comprising liquid in an upper part of the confinement,- a liquid outlet (4) at a lower part of the confinement for degassed liquid,- a gas outlet (2) arranged vertically higher than said liquid outlet (4),wherein the confinement (1) comprises a horizontally extending bottom part (5) for holding a layer of liquid on said bottom part (5) in a degassing zone (12) in said confinement (1); wherein the degassing apparatus further comprises a liquid redistribution element (6) arranged in said confinement (1) vertically between said bottom part (5) and said fluid inlet (3), wherein the liquid redistribution element (6) comprises one or more parts, extends in a horizontal direction, and is arranged to receive liquid from said fluid inlet (3) on an upper side (10) of said liquid redistribution element (6), and the degassing apparatus provides one or more liquid passageways (8) allowing liquid to flow from the upper side (10) of the liquid redistribution element (6) onto the bottom part (5), preferably wherein said one or more liquid passageways (8) are provided as an aperture in said element, or as apertures between parts of said element, and / or as an aperture (8) between said element and said confinement; wherein the degassing apparatus (100) comprises a gas / liquid contacting zone (13) that is arranged vertically between said fluid inlet (3) and said liquid redistribution element (6),wherein the decomposer further comprises:- a shell-and-tube heat exchanger (102) with a vertical tube bundle (103), - a liquid exit chamber (104) arranged at an upper end of said vertical tube bundle and comprising a second liquid outlet (105), wherein the degassing apparatus (100) is arranged above said liquid exit chamber, - a liquid flow line (105) for degassed liquid from said liquid outlet (4) for degassed liquid of the degassing apparatus to a bottom inlet side of the tube bundle (103), and- a gas flow connection (106) for supplying gas from the liquid exit chamber (104) into the degassing apparatus, wherein the gas flow connection (106) is preferably internal to the decomposer; preferably the decomposer is suitable for a method according to claim 1 or 2; and preferably wherein the degassing apparatus has the features specified in any of claims 3-9. 11. A decomposer (101) according to claim 10, wherein said liquid redistribution element (6) in operation increases the average horizontal distance from said liquid outlet (4) of liquid provided from said element (6) onto said bottom part (5). 12. A decomposer (101) according to claim 10 or 11, wherein said liquid redistribution element (6) comprises a baffle plate and the upper side (10) of the baffle plate is sloped with a higher part horizontally proximate to said liquid outlet (4) and a lower part horizontally distant from said liquid outlet (4); preferably wherein the baffle plate is inclined. 13. A decomposer (101) according to any of claims 10-12, wherein said liquid redistribution element (6) comprises a baffle plate that is provided with a rim (13) extending upward from the plate around a part (7A) of the perimeter (7) of the baffle plate, preferably wherein the rim (13) is provided at a higher part (10A) of the sloped or inclined baffle plate. 14. A decomposer (101) according to any of claims 10-13, wherein the gas / liquid contacting zone (13) is arranged below said flashing zone (14). 15. A decomposer (101) according to claim 14, wherein said gas / liquid contacting zone (13) is a rectification zone comprising a packing. 16. A method of operating a decomposer according to any of claims 10-15, the method comprising:- supplying a fluid stream comprising liquid and gas to the fluid inlet (3) of the degassing apparatus comprised in said decomposer, such that at least a part of the liquid is received on said upper side (10) of said liquid redistribution element (6) and the liquid flows through the degassing zone (12) thereby becoming the degassed liquid; - supplying the degassed liquid from the liquid outlet (4) through the liquid flow line (105) to the vertical tube bundle (103) of the shell-and-tube heat exchanger (102); and - heating said degassed liquid in said vertical tube bundle (103) of the shell-and-tube heat exchanger (102). 17. A method according to claim 16, wherein the liquid is a urea solution also containing carbamate, the degassed liquid is a urea solution containing carbamate, and the heating of the degassed liquid in the vertical tube bundle (103) causes at least a part of the carbamate comprised in the degassed liquid to decompose into CO2 and NH3. 18. A method according to claim 16 or 17, further comprising flashing a urea solution also comprising carbamate with a pressure reducing valve to give a flashed urea solution also comprising gas, and supplying said flashed urea solution comprising gas as said fluid stream to said fluid inlet (3) of the decomposer. 19. The method according to any of claims 1-9, carried out in a decomposer according to any of claims 10-15. 20. A urea production plant (200) comprising a high pressure urea synthesis section (201) and a recovery section, the recovery section comprising:- a decomposer (101) according to any of claims 10-15, and - a carbamate condenser (202) having an inlet for gas connected to the gas outlet (2) of the decomposer and a liquid outlet in liquid connection with the urea synthesis section, wherein the plant comprises a urea solution flow line (203) from an outlet for urea solution of the urea synthesis section to the fluid inlet of the decomposer, wherein said urea solution flow line comprises a pressure reducing valve (204) directly upstream the fluid inlet of the decomposer. 21. A urea production plant according to claim 20, wherein the urea synthesis section comprises a reaction zone, a high pressure CO2 stripper, and a condensation zone, and wherein the gas / liquid contacting zone (13) of the decomposer is a rectification zone comprising a packing. 22. A urea production process carried out in a urea production plant according to claim 20 or 21, the process comprising:- reacting CO2 and NH3 in said urea synthesis section to form a urea solution also comprising carbamate,- expanding said urea solution through said pressure reducing valve to give flashed urea solution also containing gas,- supplying the flashed urea solution through said fluid inlet of said degassing apparatus, comprised in said decomposer, such that at least a part of the liquid is received on said upper side (10) of said liquid redistribution element (6) and the liquid flows through the degassing zone (12) thereby becoming the degassed liquid, wherein the degassed liquid is a degassed urea solution also containing carbamate; - supplying the degassed liquid from the liquid outlet (4) through the liquid flow line (105) to the vertical tube bundle (103) of the shell-and-tube heat exchanger (102); and- heating said degassed liquid in said vertical tube bundle (103) of the shell-and-tube heat exchanger (102) thereby decomposing carbamate comprised in said degassed liquid into gaseous CO2 and NH3 that is supplied through the gas flow connection (106) into the degassing apparatus. 

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