Urea stripping process with liquid divider

Abstract

Disclosed is, in a process for the preparation of urea, providing ceramic liquid dividers for a high pressure stripper. The use of ceramic material for these liquid dividers presents a solution for the vulnerability of liquid dividers, particularly with reference to the holes in such liquid dividers, to the damaging effects of constant exposure to a hot ammonium carbamate solution. Further disclosed is a urea production unit, a stripper, and a method for modifying a urea stripping plant.

Description

P132625PC00Title:UREA STRIPPING PROCESS WITH LIQUID DIVIDER Field of the Invention  The invention is in the field of urea production by means of a stripping process. Particularly, the invention pertains to a liquid divider for a stripper in a urea plant.   Background of the Invention Urea generally is synthesized by reacting ammonia and carbon dioxide in a high-pressure urea synthesis zone of a urea production plant. The reaction can be considered as a two-step process in which ammonia and carbon dioxide form ammonium carbamate in a first step, and the ammonium carbamate is dehydrated in the second step to give urea and water. The resulting urea synthesis solution invariably comprises, in addition to urea and water, liquid ammonium carbamate in solution, as well as gaseous components ammonia and carbon dioxide. The high-pressure synthesis section of a urea stripping plant therefor comprises a stripping section, serving to remove carbamate from urea synthesis solution. Such stripping can be conducted by merely thermal stripping, also known as “self-stripping” in which said gaseous components are removed, naturally with further dissociation of ammonium carbamate into a new gas / liquid equilibrium. More typically, the stripping involves the action of a stripping medium, viz., ammonia or, more commonly, carbon dioxide. Accordingly, many urea production plants are of the type with a high-pressure stripper. The stripper is a vertical shell-and-tube heat exchanger with in operation a thin film of urea solution to be stripped on the inner tube walls and with heating fluid in the shell space. The strippers comprise tubes and a shell, a top end, and a bottom end. During normal use, the top end is situated at the top of the stripper and the bottom end is situated at the bottom of the stripper. At the top end, a urea / carbamate mixture is distributed into the tubes and a gas mixture comprising stripping gas and entrained NH3 and CO2 formed during carbamate decomposition leave the stripper. At the bottom end, the urea solution is collected. In the case of CO2 strippers, CO2 stripping gas is provided to the bottom end of the carbamate decomposer. Each tube is equipped at the top end with a liquid divider (liquid distributor, also known as ferrule in the art). The liquid divider is a tubular element, having a wall with at least one hole for liquid to enter into the tube and to wet the inner tube wall. Particularly in the event of carbon dioxide stripping, which in a CO2 stripping plant generally is fed as a reactant via the stripper , it is of paramount importance that the stripping action is predictable. Typically, reactant carbon dioxide is fed into a urea high-pressure synthesis section by allowing it to enter as a stripping medium. In order to have the optimal conditions in synthesis, particularly the optimal ratio of reactants, it is required that the stripping action is a stable operation. E.g., in the event that the flow of synthesis solution to be stripped would change, the flow of carbon dioxide stripping agent would have to be adjusted accordingly. The foregoing means that a not well-controlled change in flow of the synthesis solution, is undesirable. Such change would be difficult to accommodate, and as a result the efficiency of the urea synthesis section may well decline. The aforementioned liquid dividers thus play a key role in optimizing not only the stripping action, but ultimately also the synthesis efficiency. However, this involves a particularly challenging situation. As said, the synthesis solution is not just a hot aqueous urea solution, but essentially contains a significant amount of ammonium carbamate. Hot ammonium carbamate is a notoriously corrosive medium. In view hereof, all parts of a urea production plant today are customarily made of corrosion-resistant steel grades, typically duplex ferritic-austenitic stainless-steel type such as the Safurex® brand. Whilst these materials present today’s optimal solution for all general equipment in a urea synthesis section, the specific problem encountered with liquid dividers still requires a more satisfactory solution. This relates to the specific, highly sensitive action of the one or more small holes in the liquid dividers that serve as inlets into the stripper, for the solution to be stripped. Even small changes in the dimensions of these holes, will lead to serious perturbations of production. Such changes, over time, particularly involve a small, yet gradual, enlargement of said holes. It will be understood that any such enlargement, will result in a corresponding increase of flow. Whilst initially seemingly unnoticed, this translate into a thicker film of liquid in the stripper, which results in a lower stripper efficiency. A stripper contains typically several thousand tubes. It is very important that all the tubes receive the same amount of liquid. For this reason, the integrity of the liquid distributor over time is of high importance. The liquid distributor is therefore a precision instrument. In the event that too much liquid is entering the tubes, the film thickness will increase and the film surface becomes wavy (flooding), which is an unwanted situation. With too low an amount of liquid entering the tubes, the film will break and also this is an unwanted situation. So, the amount of liquid is important to have a stable liquid film, and should be the case simultaneously in all tubes of a stripper. The stripping efficiency generally is determined by the thickness of the liquid film running downwards the tubes and this should be equal for all the tubes in the stripper. By corrosion of the liquid divider holes, some tubes get more liquid than others, and these changes (too high liquid thickness) or too low liquid thickness will decrease the stripping efficiency. The lower stripper efficiency has still graver consequences than merely the efficiency of the stripping step. In a urea production plant, a urea synthesis section is fed with reactants originating from both an external feed and recirculation of unconverted ammonia and carbon dioxide. Particularly, the high-pressure stripper, notably a carbon dioxide stripper, comprised in such synthesis section, plays a role both as the typical source of entry for fresh carbon dioxide, viz., as a stripping medium, and as a source from which recirculating reactants are obtained, viz. the gaseous components that are stripped off from the urea synthesis solution that is subjected to stripping. Accordingly, changes in stripper efficiency are prone to ultimately result in a mismatch of reactants available for urea synthesis. Particularly, a reduced stripping efficiency results in overloading the downstream sections of a urea plant, and as a result a higher amount of recycle to the synthesis section. This will have a negative effect on the plant performance. A background reference addressing the corrosiveness affecting the holes of the liquid distributors made from corrosion-resistant duplex steel, is US 2018 / 0304224. The solution presented focuses on a specific form of corrosion, viz., cross-cut end attack. As taught in the reference, this specific type of corrosion is different from other types of corrosion. To this end, the reference teaches to manufacture the liquid distributor from such duplex steel by hot isostatic pressing (HIP). Although this presents quite an improvement, the manufacturing method is relatively complex. Moreover, the adverse action of the continuous flow of hot ammonium carbamate through a liquid divider, whilst specifically involving cross-cut end attack, is still more complex than relating to only cross-cut end attack. Without wishing to be bound by theory, the inventors believe that said continuous flow may lead to both corrosion (attack of the material chemically) and erosion (attack of the material due to friction). Another reason to prevent corrosion, is that it is important that the surface inside the hole is entirely smooth, Anther background reference disclosing liquid distributors is US2012 / 0097378. Herein zirconium and niobium are suggested as suitable materials, but these are generally too expensive to be acceptable in the field. Yet another reference relating to stainless steel stripper tubes and liquid dividers, is US2021 / 0187471. It is desired to provide liquid distributors that present an acceptable alternative to those made from steel. A further desire is to provide liquid distributors that still better withstand the adverse influence of being subjected to a continuously flowing hot ammonium carbamate stream.   Summary of the Invention In order to better address one or more of the foregoing desires, the invention presents, in one aspect,a process for the preparation of urea in a urea production plant, said plant comprising a synthesis section operating under a high pressure between 12 and 40 MPa and a recovery section operating under a pressure below 1 MPa, said synthesis section comprising, connected so as to be capable of forming a urea synthesis loop, a urea reactor, a stripper, a condenser and optionally a scrubber, the process comprising the steps of:a) reacting ammonia and CO2 under urea forming conditions in the urea reactor to obtain a urea synthesis solution;b) stripping the urea synthesis solution in the stripper to form a stripped urea solution and a stripper vapor comprising ammonia, CO2 and water;c) condensing the stripper vapor obtained in b) in the condenser to form an ammonium carbamate solution; d) recirculating said ammonium carbamate solution to the reactor;e) subjecting the stripped urea solution to recovery of ammonia, and optionally CO2, in the recovery section to obtain a urea product solution and a recovery vapor comprising the ammonia and the optional CO2;f) recycling the recovery vapor to the urea synthesis section; wherein the stripper comprises a plurality of tubes disposed within a shell, the process comprising distributing the urea synthesis solution into the stripper by means of liquid dividers that are fluidically connected to said tubes and positioned upstream thereof, said liquid dividers having a tubular inner wall comprising at least one hole, wherein said wall is made of a ceramic material. In another aspect, the invention provides a production unit for the synthesis of urea, said unit having a gas inlet configured to receive an ammonia feed, a gas inlet configured to receive a carbon dioxide feed, and a liquid outlet configured to obtain a urea production solution; said unit comprising a urea reactor, a stripper, and a condenser configured to form a urea synthesis loop, such that a liquid outlet for urea synthesis solution of the reactor is in fluid communication with a liquid inlet of the stripper; a gas outlet of the stripper is in fluid communication with a gas inlet of the condenser, and a liquid outlet of the condenser is in fluid communication with a liquid inlet of the reactor; said stripper being configured to allow a stripping gas stream to flow to the gas outlet of the stripper, and having a liquid outlet to obtain a stripped urea synthesis solution from the production unit; wherein the stripper is a vertical shell-and-tube heat exchanger comprising a tube bundle, the shell-side of which is in fluid communication with a stripping gas inlet and with the gas outlet of the stripper and wherein the liquid inlet of the stripper is in fluid communication with the tube side of the bundle, the stripper being configured for liquid obtained from the liquid inlet of the stripper to enter the tubes of said tube bundle via liquid dividers positioned on top of the tubes, said liquid dividers comprising an element having a tubular inner wall, said wall comprising at least one hole configured to allow said liquid to enter the tubes, wherein said wall is made of ceramic material. In a further aspect, the invention provides a shell-and-tube stripper for stripping a urea / carbamate mixture, the stripper comprising a plurality of tubes disposed within a shell; the tubes having a liquid entrance on an upstream end thereof and a liquid exit at a downstream end thereof, said ends optionally extending into corresponding tube sheets; wherein the upstream ends are each fluidically connected with a liquid divider positioned upstream of said upstream ends, said liquid divider having a tubular inner wall comprising at least one hole, wherein said wall is made of a ceramic material. In a still further aspect, the invention presents the use of a ceramic material to construct a liquid divider for a stripper in a urea plant.In yet another aspect, the invention provides a method of modifying a urea stripping plant comprising a high-pressure stripper, said stripper comprising a plurality of tubes disposed within a shell; the tubes having a liquid entrance on an upstream end thereof and a liquid exit at a downstream end thereof, said ends optionally extending into corresponding tube sheets; wherein the upstream ends are each fluidically connected with a liquid divider made of stainless steel, such as a duplex stainless steel, positioned upstream of said upstream ends, said liquid divider having a tubular inner wall comprising at least one hole, wherein the method comprises replacing the stainless steel liquid dividers by ceramic liquid dividers.   Detailed Description of the Invention In a general sense, the invention is based on the judicious insight to deviate from the use of duplex stainless steel or alternative corrosion-resistant metals, as a contact surface for a urea synthesis solution being divided into a stripper. Particularly, the invention is based on the judicious insight to use a ceramic material instead. A typical carbamate stripper comprises a plurality (several thousands) of tubes. Through the tubes, a liquid film runs downwards whilst stripping gas (typically CO2) runs upwards. Provisions are generally made to ensure that all tubes have the same load of liquid so as to have a flow of the liquid at the same speed. For, if the liquid does not flow through all of the tubes at the same speed, the efficiency of the stripper is reduced. Particularly, the tube load should be equal for all tubes to have optimally efficient stripping. The objective of said provisions is thus to have a liquid film in the tubes wherein the liquid film has substantially the same thickness in all the tubes. Thereby the same stripping efficiency per tube is achieved, which leads to an optimized stripping efficiency in the stripper. These provisions comprise a liquid divider, generally in the form of a cylinder with small holes in it. Liquid dividers are also known in the field as liquid distributors, or as ferrules. Without wishing to be bound by theory, the inventors have realized that the action of a liquid divider for a urea stripper is prone to lead not just to corrosion, but particularly also to erosion, as a result of a generally constant flow of a thin film of hot ammonium carbamate through the holes and against the walls of the liquid dividers. The inventors thereby uncovered a surprisingly good ability of ceramic materials to withstand the challenging combination of corrosion and erosion. The term ‘ceramic material’ as used herein refers to, and can be substituted by, a solid, rigid, material that comprises metal atoms and / or metalloid atoms (e.g. Si, B), that are bonded to atoms selected from the group consisting of oxygen, carbon, and nitrogen.The solid material is composed of crystalline grains. For instance, the term ceramic material includes a rigid material that consists of a three-dimensional network of sintered crystalline grains comprising metal atoms and / or metalloid atoms that are bonded to carbon, nitrogen or oxygen. The ceramic material is a solid, inorganic, non-metallic material, that comprises metal atoms and / or metalloid atoms, and / or that is subjected to high temperature during manufacture. Preferably, the ceramic material is selected from the group consisting of aluminium oxide, silicon nitride, silicon carbide, zirconium oxide, and combinations thereofOptionally, the ceramic material is a mixed ceramic, i.e. a material consisting of co-continuous interpenetrating networks of two or more metal carbides, nitrides or oxides.The invention thus relates to a urea production process of the stripping type. Urea is generally produced from ammonia and carbon dioxide. It can be prepared by introducing an ammonia excess together with carbon dioxide at a pressure between 12 and 40 MPa and at a temperature between 150°C and 250°C into a urea synthesis zone. The resulting urea formation can be presented best in the form of two consecutive reaction steps, in the first step ammonium carbamate being formed according to the exothermic reaction:2NH3 + CO2 → H2N – CO – ONH4 after which the ammonium carbamate formed is dehydrated in the second step to give urea according to the endothermic equilibrium reaction:H2N – CO – ONH4 ↔ H2N – CO – NH2 + H2OThe extent to which these reactions take place depends among other things on the temperature and the ammonia excess used. The reaction product obtained in a urea synthesis solution substantially consists of urea, water, unbound ammonia and ammonium carbamate. The ammonium carbamate and the ammonia are removed from the solution and are generally returned to the urea synthesis zone. In addition to the above-mentioned solution in the urea synthesis zone, a gas mixture is formed which consists of unconverted ammonia and carbon dioxide together with inert gases, the so called reactor off-gas. The urea synthesis section may comprise separate zones for the formation of ammonium carbamate and urea. These zones may also be combined in a single apparatus.The invention pertains to a process for the preparation of urea according to a stripping process, as conducted in a urea stripping plant. Such a process is described in, inter alia, Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. In a urea stripping plant, the decomposition of the ammonium carbamate that has not been converted into urea and the expulsion of the usual ammonia excess largely takes place at a pressure that is essentially almost equal to the pressure in the synthesis reactor. This decomposition and expulsion take place in one or more stripper(s) installed downstream of the reactor, possibly with the aid of a stripping gas such as, for example, carbon dioxide and / or ammonia, and with the addition of heat. It is also possible to apply thermal stripping. Thermal stripping means that use is made exclusively of the supply of heat to decompose ammonium carbamate and remove the ammonia and carbon dioxide present from the urea solution. The gas stream leaving a stripper contains ammonia and carbon dioxide which are condensed in a high-pressure condenser and then returned to the urea synthesis zone. In a urea stripping plant, the synthesis zone is operated at a temperature of 160 – 240 °C and preferably at a temperature of 170 – 220 °C. The pressure in the synthesis reactor is 12 – 21 MPa, preferably 12.5 – 20 MPa, more preferably 13 – 16 MPa. In the art, these ranges are generally considered to represent “high pressure” (as also used in connection with a conventional “High Pressure Carbamate Condenser”). The gross ammonia to carbon dioxide molar ratio (gross N / C ratio) in the urea synthesis zone of a stripping plant usually is in between 2.2 and 5 and preferably between 2.5 and 4.5 mol / mol. For completeness’ sake, it is noted that the synthesis zone will usually operate on the basis of both an external feed of the starting materials, ammonia and carbon dioxide, and recycled starting materials, generally comprising recycled ammonia and carbon dioxide in a free form as well as in the form of ammonium carbamate and / or biuret. The gross N / C ratio, which is a term having a customary meaning in the art, refers to a hypothetical mixture in which all starting materials are converted into free ammonia and carbon dioxide. The synthesis zone can comprise a single reactor or a plurality of reactors, arranged in parallel or in series. In addition to one or more reactors, the synthesis section comprises a stripper, a condenser and optionally a scrubber, all operating at substantially the same pressure. The synthesis zone is generally referred to as a High Pressure (HP) section.In the synthesis section of a urea stripping plant, the urea solution leaving the urea reactor is thus fed to a stripper in which a large amount of non-converted ammonia and carbon dioxide is separated from the aqueous urea solution. Such a stripper can be a shell and tube heat exchanger in which the urea solution is fed to the top part at the tube side and a carbon dioxide feed to the synthesis is added to the bottom part of the stripper. At the shell side, high pressure (HP) steam is added to heat the solution via indirect heat exchange. The urea solution leaves the heat exchanger at the bottom part, while the vapor phase leaves the stripper at the top part. The vapor leaving said stripper contains ammonia, carbon dioxide and a small amount of water. Said vapor is condensed in a falling film type heat exchanger or a submerged type of condenser that can be a horizontal type or a vertical type. A horizontal type submerged heat exchanger is described in the aforementioned Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp 333-350. After the stripping treatment, the pressure of the stripped urea solution is reduced in a urea recovery section. In the recovery section the non-converted ammonia and carbon dioxide in the urea solution are separated from the urea and water solution. A recovery section comprises usually a heater, a liquid / gas separation section and a condenser. The urea solution entering a recovery section is heated to vaporize the volatile components ammonia and carbon dioxide as well as water from that solution. The heating agent used in the heater is usually steam. The ammonium carbamate aqueous solution formed in a low pressure carbamate condenser in the recovery section, operated at a lower pressure than the pressure in the synthesis section, is preferably returned to the urea synthesis section operating at synthesis pressure. The recovery section is generally a single section or can be a plurality of recovery sections arranged in parallel or in series. The recovery section comprises a heater, a liquid / gas separator and a condenser. The pressure in this recovery section is generally between 200 to 600 kPa. This section is generally referred to as a low pressure (LP) recovery section (or recirculation section, the terms “recovery section” and “recirculation section” in this description are used interchangeably). In the heater of the recovery section the bulk of ammonia and carbon dioxide is separated from the urea and water phase by heating the urea solution. Usually low pressure (LP) steam is used as heating agent. The urea and water phase contains a small amount of dissolved ammonia and carbon dioxide that leaves the recovery section and is sent to a downstream urea processing section where the urea solution is concentrated by evaporating the water from said solution. This section is typically referred to as the evaporation section and it is typically comprised of one or two evaporators, whose vapors are condensed downstream and recycled back to the process.In some embodiments, in addition to the HP synthesis section and the LP recovery section, a medium pressure (MP) treatment section is present. E.g., WO 02 / 090323 discloses a urea process and plant of the carbon dioxide stripping type, wherein a MP treatment section is present parallel with the HP stripping section. A similar disclosure is found in EP 2 086 928. Processes also exist in which a MP treatment section is present in series, downstream of the urea synthesis section. In this respect reference can be made to, e.g., GB 1 542 371, and other disclosures of the Snamprogetti Ammonia and Self-Stripping processes. In another aspect, the invention provides a production unit for the synthesis of urea, as well as to a urea plant comprising such unit in fluid communication with one or more recovery sections as discussed above. Said unit has a liquid inlet configured to receive an ammonia feed, a gas inlet configured to receive a carbon dioxide feed, optionally one or more gas inlets configured to receive recycled gaseous reactants, and a liquid outlet configured to obtain a urea production solution. The unit comprises a urea reactor, a stripper, and a condenser configured to form a urea synthesis loop. This loop is formed such that a liquid outlet for urea synthesis solution of the reactor is in fluid communication with a liquid inlet of the stripper. It will be understood that the stripper has a gas outlet for stripped gases, particularly for used stripping gas having stripped the liquid of gaseous components. Preferably the stripper is a carbon dioxide stripper, in which case the carbon dioxide feed gas inlet of the unit is the inlet for stripping gas of the stripper. Said gas outlet of the stripper is in fluid communication with a gas inlet of the condenser. In the condenser condensed gases are sent to a liquid outlet of the condenser, which is in fluid communication with a liquid inlet of the reactor. The reactor, which can also be comprised in a reaction zone and the condenser, which can also be comprised in a condensation zone, can be provided as separate vessels (condenser and reactor) or as zones in a single vessel, e.g. as a pool reactor. The reactor is for example a vertical urea reactor with trays and with one or more inlets at the bottom and an outlet for withdrawing urea solution from the top. The condenser is for instance a shell-and-tube heat exchanger with gas to be condensed from the stripper received in the tubes or in the shell; and with e.g. a horizontal or vertical tube bundle. The stripper is configured to allow a stripping gas stream to flow to the gas outlet of the stripper, and has a liquid outlet to obtain a stripped urea synthesis solution from the production unit. The stripper is a vertical shell-and-tube heat exchanger comprising a tube bundle, the tube-side of which is in fluid communication with the stripping gas inlet and with the gas outlet of the stripper. The liquid inlet of the stripper is in fluid communication with the tube side of the bundle, and the stripper is configured for liquid obtained from the liquid inlet of the stripper to enter the tubes of said tube bundle via liquid dividers (also known in the art as liquid distributors or ferrules). These dividers are positioned on top of the vertical heat exchanger tubes. A liquid divider is an element comprising a tubular inner wall, wherein said wall comprises at least one hole, preferably two or three holes, that are configured for liquid to enter into the vertical heat exchanger tube and to wet the inner tube wall. The element as such can have any shape. E.g., it can be a block having a straight outside surface wall, and a tubular inner wall. It will be understood that for said hole or holes in the inner wall to be configured to receive liquid, said hole or holes will extend through the outside wall of the element. Preferably, the element just comprises a ceramic tube. More particularly, the element is a ceramic tube having at least one hole. Different designs exist for the dimensions and hole-configurations of such liquid dividers. This frequently concerns specific know-how, differing per urea manufacturer. Irrespective, however, of the specific design, the intended action of a liquid divider is always to provide an optimal surface area for the falling liquid film to be contacted with counter-currently flowing stripping medium (including gaseous components flowing up from self-stripping). According to the invention, the aforementioned tubular element comprises a wall made of ceramic material. Preferably this means that the liquid dividers as such are made of ceramic material. In an embodiment the liquid dividers consist essentially of ceramic material. Preferably, the liquid dividers consist of ceramic material. Typically, on top of the liquid dividers a gas riser is mounted. The gas riser is typically also tubular. Such gas riser extends beyond the holes in the liquid divider, in downstream direction with regard to the rising gas flow. The gas riser will generally be made from a metal, preferably a duplex stainless steel (such as having a composition in accordance with the Safurex® brand), but it can also be made of ceramic material, notably the same ceramic material from which the liquid dividers is made. The gas riser can be connected to the ceramic liquid divider in various way, e.g., by means of a screwed connection with a gasket, or by applying a bayonet mount. Preferably an adapter is fitted between the gas riser and the liquid divider, secured, typically, between the gas riser and the liquid divider by two O-rings. Preferably, the adapter comprises a ceramic material, preferably the same ceramic material as that of which the liquid divider is made. The gas riser and liquid divider also can be made out from a single ceramic material as one piece.The various embodiments of the discussed liquid dividers of the ceramic type, are applicable to all aspects of the invention. In one further such aspect, the invention provides a shell-and-tube stripper for stripping a urea / carbamate mixture. The stripper comprises a plurality of tubes disposed within a shell; the tubes having a liquid entrance on an upstream end thereof and a liquid exit at a downstream end thereof, said ends optionally extending into corresponding tube sheets; wherein the upstream ends are each fluidically connected with a liquid divider positioned upstream of said upstream ends, said liquid divider having a tubular inner wall comprising at least one hole, wherein said wall is made of a ceramic material. The invention is also particularly useful in modernizing existing urea production units and plants of the stripping type. Herein, the strippers including the aforementioned liquid dividers are customarily made of non-corrosive metal, most typically stainless steel, such as, e.g., austenitic stainless steels or duplex stainless steel. It will be understood that the liquid dividers require one or more times replacement during the lifetime of a high pressure stripper in a urea synthesis section. According to the invention, this replacement is done by providing liquid dividers of the ceramic type. An advantage of the present invention is that with ceramic liquid dividers being placed, the number of replacements thereafter required, will be reduced. In sum, a process for the preparation of urea is disclosed, providing ceramic liquid dividers for a high pressure stripper. The use of ceramic material for these liquid dividers presents a solution for the vulnerability of liquid dividers, particularly with reference to the holes in such liquid dividers, to the damaging effects of constant exposure to a hot ammonium carbamate solution. Further disclosed is a urea production unit, a stripper, and a method for modifying a urea stripping plant. The invention will now be further illustrated by the following non-limiting example(s).  Example 1 Two block shaped ceramic samples were tested with reference to the effect of exposure to hot liquid ammonium carbamate. This was done in a high pressure autoclave simulating the process conditions to which the liquid dividers are exposed to in the HP stripper; i.e. the hot ammonium-carbamate with similar concentration and temperature.These materials tested were silicon carbide and zirconia, at an operating temperatures of 210°C. The test environment composition in all cases was an aqueous liquid comprising ammonia and carbon dioxide (i.e. an aqueous solution of ammonium carbamate), urea, and water. The samples were exposed in zero oxygen conditions (i.e., no oxygen passivation occurring) for 24 hours. Taking weight-loss as the most representative parameter for resistance towards the test medium, the results are as follows: Material Start mass (g)End mass (g)Weight loss (g)Weight loss (%) Silicon carbide 20.598620.59330.00530.0257Zirconia74.476274.46570.01050.0141 Example 2 (comparative)  Two duplex stainless steels samples, of a material specifically designed for use in hot carbamate conditions in urea plants, have been exposed in a similar autoclave, under similar oxygen free conditions, as in Example 1. The operating temperature was lower (205°C) than in Example 1, which translates into a more corrosive solution in Example 1. The exposure time was longer (330 hours). The weight loss, corrected by a factor 24 / 330, was 0.05% and 0.04% for the tested samples. Accordingly, the ceramic materials strongly outperform the duplex stainless-steel materials.       

Claims

 1. A process for the preparation of urea in a urea production plant, said plant comprising a synthesis section operating under a high pressure between 12 and 40 MPa and a recovery section operating under a pressure below 1 MPa, said synthesis section comprising, connected so as to be capable of forming a urea synthesis loop, a urea reactor, a stripper, a condenser and optionally a scrubber, the process comprising the steps of:a) reacting ammonia and CO2 under urea forming conditions in the urea reactor to obtain a urea synthesis solution;b) stripping the urea synthesis solution in the stripper to form a stripped urea solution and a stripper vapor comprising ammonia, CO2 and water;c) condensing the stripper vapor obtained in b) in the condenser to form an ammonium carbamate solution; d) recirculating said ammonium carbamate solution to the reactor;e) subjecting the stripped urea solution to recovery of ammonia, and optionally CO2, in the recovery section to obtain a urea product solution and a recovery vapor comprising the ammonia and the optional CO2;f) recycling the recovery vapor to the urea synthesis section; wherein the stripper comprises a plurality of tubes disposed within a shell, the process comprising distributing the urea synthesis solution into the stripper by means of liquid dividers that are fluidically connected to said tubes and positioned upstream thereof, said liquid dividers comprising an element having a tubular inner wall, said wall comprising at least one hole, wherein said wall is made of a ceramic material.   2. A production unit for the synthesis of urea, said unit having a gas inlet configured to receive an ammonia feed, a gas inlet configured to receive a carbon dioxide feed, and a liquid outlet configured to obtain a urea production solution; said unit comprising a urea reactor, a stripper, and a condenser configured to form a urea synthesis loop, such that a liquid outlet for urea synthesis solution of the reactor is in fluid communication with a liquid inlet of the stripper; a gas outlet of the stripper is in fluid communication with a gas inlet of the condenser, and a liquid outlet of the condenser is in fluid communication with a liquid inlet of the reactor; said stripper being configured to allow a stripping gas stream to flow to the gas outlet of the stripper, and having a liquid outlet to obtain a stripped urea synthesis solution from the production unit; wherein the stripper is a vertical shell-and-tube heat exchanger comprising a tube bundle, the shell-side of which is in fluid communication with a stripping gas inlet and with the gas outlet of the stripper and wherein the liquid inlet of the stripper is in fluid communication with the tube side of the bundle, the stripper being configured for liquid obtained from the liquid inlet of the stripper to enter the tubes of said tube bundle via liquid dividers positioned on top of the tubes, said liquid dividers comprising an element having a tubular inner wall, said wall comprising at least one hole configured to allow said liquid to enter the tubes, wherein said inner wall is made of ceramic material.  3. A shell-and-tube stripper for stripping a urea / carbamate mixture, the stripper comprising a plurality of tubes disposed within a shell; the tubes having a liquid entrance on an upstream end thereof and a liquid exit at a downstream end thereof, said ends optionally extending into corresponding tube sheets; wherein the upstream ends are each fluidically connected with a liquid divider positioned upstream of said upstream ends, said liquid divider having a tubular inner wall comprising at least one hole, wherein said wall is made of a ceramic material.  4. The use of a ceramic material to construct a liquid divider for a stripper in a urea plant. 5. A method of modifying a urea stripping plant comprising a high-pressure stripper, said stripper comprising a plurality of tubes disposed within a shell; the tubes having a liquid entrance on an upstream end thereof and a liquid exit at a downstream end thereof, said ends optionally extending into corresponding tube sheets; wherein the upstream ends are each fluidically connected with a liquid divider made of stainless steel, such as duplex stainless steel, positioned upstream of said upstream ends, said liquid divider having a tubular inner wall comprising at least one hole, wherein the method comprises replacing the stainless steel liquid dividers by ceramic liquid dividers.   6. A process according to claim 1, a production unit according to claim 2, a stripper according to claim 3, a use according to claim 4, or a method according to claim 5, wherein the ceramic material is selected from the group consisting of aluminium oxide, silicon nitride, silicon carbide, zirconium oxide, and combinations thereof.  7. A process according to claim 1 or 6, a production unit according to claim 2 or 6, a stripper according to claim 3 or 6, a use according to claim 4 or 6, or a method according to claim 5 or 6, wherein the wall has two or three holes.  8. A process according to claim 1, 6, or 7, a production unit according to claim 2, 6 or 7, a stripper according to claim 3, 6 , or 7, a use according to claim 4, 6, or 7, or a method according to claim 5, 6, or 7, wherein the element comprises a ceramic tube.  9. A process according to any one of the claims 1 and 6 to 8, a production unit according to any one of the claims 2 and 6 to 8, a stripper according to any one of the claims 3 and 6 to 8, a use according to any one of the claims 4 and 6 to 8, or a method according to any one of the claims 5, and 6 to 8, wherein on top of the liquid dividers a gas riser is mounted.   10. A process, a production unit, a stripper, a use, or a method according to claim 9, wherein the gas riser is connected to the ceramic liquid divider via an adapter fitted between the gas riser and the liquid divider. 11.A process, a production unit, a stripper, a use, or a method according to claim 10, wherein the adapter comprises a ceramic material, preferably the same ceramic material as that of which the liquid divider is made.