Method for controlling the offgas scrubber of a melamine plant

Abstract

A method for controlling an offgas scrubber of a high-pressure melamine plant, wherein a recirculated urea stream is cooled in a urea cooler, which is a shell and-tube heat exchanger external to the scrubber, prior to reintroduction in the scrubber, wherein the heat removed from the urea stream in the urea cooler is controlled by regulating the pressure of the steam produced in the shell side of said urea cooler.

Description

Method for controlling the offgas scrubber of a melamine plant DESCRIPTIONField of applicationThe invention is in the field of industrial production of melamine. The invention particularly relates to a method for controlling the operation of the offgas scrubber of a high-pressure melamine plant, wherein melamine offgas is washed using a urea melt as washing medium. Prior artMelamine is produced at an industrial scale starting from urea following either a non-catalytic high-pressure (HP) process or a low-pressure (LP) catalytic process. The non-catalytic high-pressure process is considered the most advantageous and is becoming predominant.In the high-pressure process, a urea melt is reacted at pressure which is generally above 7 MPa, typically 7 to 20 MPa, and a temperature of around 375 °C. The melamine synthesis section may include two reactors arranged in series. In the first reactor, a raw melamine melt is produced. The second reactor removes carbon dioxide from the raw melamine melt using a stripping agent, such as ammonia. This second reactor may be referred to as post-reactor or stripping reactor. The first reactor and the second reactors may be separate vessels or combined in a single apparatus. The product stream containing melamine undergoes additional treatments in a plant section operating at lower pressure than the melamine synthesis section. Said treatments may include quenching, purification, crystallization, solid-liquid separation, and drying, to convert the product stream into a solid melamine product with a desired purity. Melamine purification and crystallization are typically carried out in an alkaline environment. Ammonia or sodium hydroxide are the most common alkaline agents used for this purpose.The reaction of urea to melamine produces a gaseous stream termed offgas, which contains mainly ammonia and carbon dioxide. The offgas contains also some melamine carried by the gas and other minor components. The offgas produced during the synthesis of melamine, also termed “melamine offgas”, is usually recycled as a feed material for a tied-in urea plant. A combination of production of urea and melamine is attractive because urea is the reagent for the production of melamine, and the offgas released during the synthesis of melamine contains ammonia and carbon dioxide which are the starting products for the urea synthesis. However, recycling the melamine offgas to a urea plant requires a proper purification of the offgas and recovery of the melamine contained therein. A known technique to purify the melamine offgas is washing with urea melt in a suitable scrubber. A portion of the available urea melt feed can be used to this purpose, and the urea melt after washing can be sent to the melamine synthesis section. A known and widely adopted approach to scrubbing the offgas is described in US 7,311,759. The offgas is washed in a scrubber (“melamine offgas scrubber”) by contact with a urea melt. The urea melt used in the washing portion includes fresh urea melt and a recirculated urea melt taken from the same scrubber. More in detail, a portion of the urea melt withdrawn from the scrubber, after contact with the offgas, is cooled in an external heat exchanger (“urea melt cooler”) and reintroduced into the scrubber thanks to the action of a recirculation pump. Said heat exchanger is typically a shell-and-tube heat exchanger, with the urea melt flowing through the tube side and water being fed into the shell side to produce steam. Said urea melt cooler is a crucial item and poses a number of challenges. The urea melt withdrawn from the scrubber contains melamine precursors and ammonia as a consequence of the washing process and can cause undesired solid deposits on the inner surface of the tubes, leading to fouling and occlusion of the tubes. Fouling causes decrease of the heat exchange coefficient and increase of flow resistance (pressure drop). According to a common practice, the urea melt cooler is designed to ensure operation in the presence of fouling which means, however, that the urea stream may be overcooled when the tubes are relatively clean. Overcooling of the urea stream would cause precipitation of melamine cyanurate in the tubes with a sudden decrease of the heat exchange coefficient and increase of pressure drop.Furthermore, an excessive cooling in the urea melt cooler causes a lower temperature of the urea stream removed from the scrubbing process and sent to the melamine synthesis section. Although the heat removed from said urea stream can be used to produce steam, in the energy balance of the melamine synthesis process, it denotes an energy loss, since the urea melt exiting from the scrubber must be re-heated to a temperature of about 380° C in the melamine synthesis section. On the other hand, cooling as much as possible the recirculated urea melt for a given scrubber temperature (i.e. for a given heat exchanged in the urea melt cooler) results in less urea melt to be recirculated to obtain the same washing efficiency, thus limiting the operating and investment costs of the recirculation pump. The above-mentioned aspects and the experience show that the heat exchanged in this item (i.e. heat transferred from the urea stream in the tube side to the water / steam in the shell side) must be carefully controlled.US 2019 / 367464 discloses a high-pressure melamine plant. CN 111 943 897 discloses a melamine production system and a process thereof by a high-pressure synthesis low-pressure gas phase quenching method. It further discloses a step of process gas cooling and purification wherein the gas after separating the melamine crystals from a triamine trap enters a urea scrubbing tower, where it is mixed with molten urea at and flows downwards. The gas is washed and cooled by the urea. The melamine particles and unreacted substances in the gas enter the molten urea. The heat released by the cooling of the process gas is taken away by an evaporative heat exchanger in a urea scrubber, saturated water is circulated in the heat exchanger tube, and the saturated water evaporates to generate low-pressure steam. Hence, said CN ‘897 teaches a heat exchanger internal to the scrubber, arranged to cool a biphasic flow, where water is fed to the tube side of the heat exchanger.Summary of the inventionThe invention addresses the problem of how to control the operation of the urea melt cooler of the melamine offgas scrubber, as mentioned above. The invention addresses the problem of how to control the heat transferred from the hot urea stream to the water / steam mixture, i.e. the heat or thermal power exchanged in said urea melt cooler. An aim of the invention is to provide an acceptable interval between cleaning procedures, to reduce downtime of the plant. The aim is reached with a method according to the claims. The method includes that the heat removed from the urea stream in the urea melt cooler is controlled by regulating the pressure of the steam produced in the shell side of said cooler. The invention goes against the conventional wisdom of controlling shell-and-tube evaporators with shell-side evaporation by controlling the liquid level in the shell side. The applicant has realized that said method is ineffective under the very special conditions of the urea melt cooler which is considered here. For example, lowering the liquid level in the shell side would result in a highly different heat exchange between the portion of tubes which is still submerged and the portion of tubes above water. The submerged tubes would exchange more heat (due to higher heat transfer coefficient of boiling water) and overcool the urea melt with formation of deposits. In contrast, the invention controls the heat exchanged (or thermal power transferred from one fluid to another) by regulating the shell-side pressure, i.e. the pressure at which water evaporates in the shell side and, consequently, the temperature of the water / steam in the shell side and the difference of temperature (delta-T) under which heat is transferred. By doing so, the heat transfer process remains uniform, the control is reliable and the undesired or unpredicted precipitation of solid matter is reduced. The invention allows a suitable time between cleaning, such as typical around 8’000 hours. Notably, the present invention teaches to control the pressure of water / steam in a heat exchanger external to the scrubber, to control the temperature of the scrubber and to reduce fouling of the heat exchanger. Ingeniously, the invention teaches to control the temperature of the scrubber with a method that avoids also the detrimental fouling of the external heat exchanger.Description of the inventionIn the urea melt cooler, the urea stream is cooled preferably to a temperature not less than 180 °C. The applicant has found that keeping the temperature of the cooled recirculated urea melt above 180 °C reduces effectively the precipitation of undesired solid matter like melamine cyanurate.The invention may include measuring the temperature of the urea-containing stream at the bottom of the scrubber. Said temperature may be used, advantageously, as controlling variable to regulate / adjust the pressure of the steam generated in the shell side of the urea melt cooler.The steam produced in the shell side is preferably saturated steam. In a preferred embodiment, evaporation of water in the shell side is performed at a temperature of at least 160 °C, preferably at least 165 °C. The shell side pressure is preferably at least 5.2 bar, more preferably at least 6.0 bar. In particularly preferred embodiments, the shell side pressure is in the range 6.0 to 8.0 bar. In the description and claims, the pressures are given in bar gauge (barg).The inventive method may include that the level of liquid water in the shell side of said heat exchanger is controlled independently of said control of heat removed from the urea stream. Preferably, the level of liquid water in the shell side is controlled to keep the tubes fully submerged, or at least 90% submerged or at least 95% submerged, meaning that at least 90% or at least 95% of the outer surface of tubes is submerged.The recirculated urea stream, withdrawn from the scrubber, can be mixed with fresh urea melt before or after cooling in the urea melt cooler. In some embodiments the recirculated urea stream is mixed with fresh urea and the so obtained stream is cooled; in other embodiments the recirculated urea stream is cooled as such and fresh urea is added to the stream after cooling. The fresh urea may also be fed to the scrubber separately from the recirculated urea stream. The invention provides control of the temperature of the urea stream at the bottom of the scrubber (“bottom temperature” of the scrubber). Preferably, the method includes the step of measuring the temperature of the urea-containing stream collected from the scrubber, wherein at least a portion of said stream is sent to the urea cooler for recirculation, and providing a setpoint for a pressure controller of the shell side of the urea cooler, said setpoint being determined on the basis of said temperature.The urea stream withdrawn from the scrubber has preferably a temperature in the range 170 °C to 250 °C, more preferably 175 °C to 240 °C.Compared to a control system based on the regulation of the liquid level, the method of the invention avoids a reduced temperature of submerged tubes, thus avoiding the drawback of a local acceleration of the fouling process.An object of the invention is also an offgas scrubbing section of a high-pressure melamine plant, including at least an offgas scrubber where melamine offgas containing ammonia and carbon dioxide are washed with a urea-containing stream, and a shell-and-tube heat exchanger external to the scrubber, said heat exchanger being arranged to cool a recirculated urea-containing stream withdrawn from the scrubber, before reintroduction of said stream into the scrubber, and to evaporate water producing steam, said scrubbing section including a control system configured for controlling the heat removed from the urea stream in said heat exchanger with a method as described above. Another object of the invention is a high-pressure melamine plant including a reaction section where urea is reacted to produce melamine with formation of an offgas containing ammonia and carbon dioxide, and including a scrubbing section for the offgas as mentioned above. The term urea melt denotes a highly concentrated solution, which typically contains only a few percent water. A urea melt for the synthesis of melamine typically contains at least 96% urea (in weight), the balance being water and small amounts of impurities. The scrubber may be a single-stage scrubber or a two-stage scrubber. According to a preferred embodiment, the two purification stages work with a counter-current flow. In the second stage, the melamine offgas from the first stage is contacted counter-current with a fresh urea melt; in the first stage, the melamine offgas is contacted counter-current with the urea melt from the second stage and with the recirculated urea melt containing ammonia and melamine precursors. A preferred embodiment of a two-stage scrubber is described. The second purification stage is placed above the first purification stage; the melamine offgas flows upward in the first stage and then in the second stage; a urea melt is sprayed over the offgas from top of the second stage and flows downward through the second stage and the first stage; a urea melt loaded with ammonia and melamine precursors is collected at the bottom of the first stage and a portion thereof is recirculated to the same first stage after cooling. The melamine offgas is scrubbed in counter-current firstly with the urea melt from the second stage and the urea melt recirculated in the first stage; then with the fresh urea melt introduced in the second stage. The invention is applicable to all high-pressure melamine plants. The term high-pressure melamine plant denotes that melamine is produced at high pressure (above 7 MPa) without a catalyst, in contrast to the catalytic, low-pressure process. The synthesis of melamine may include a conversion step and a stripping step, wherein said conversion step includes reacting said urea melt feed stream under suitable melamine synthesis conditions to generate a raw melamine product, and said stripping step includes the stripping of said raw melamine product in the presence of gaseous ammonia, to remove carbon dioxide contained in the raw melamine.The conversion of urea into melamine is performed in a melamine synthesis section. In some embodiments, said melamine synthesis section includes a single reactor, from which both the raw melamine and the melamine offgas stream are withdrawn. In other embodiments, said melamine synthesis section includes a primary reactor where urea melt is reacted, followed by a secondary reactor where the melamine-containing effluent of the primary reactor is stripped with gaseous ammonia. In such embodiments, each of the primary reactor and the secondary reactor produce a respective stream of melamine offgas. Both melamine offgas streams emerging from the primary and from the secondary reactor are made predominantly of ammonia and carbon dioxide, although they may differ in composition. The melamine offgas subject to scrubbing with urea melt, in accordance with embodiments the invention, may include only the melamine offgas stream from the primary reactor or both melamine offgas streams from the primary reactor and secondary reactor, possibly combined into a single stream. In a further embodiment, a combined reactor performs the function of the primary reactor and secondary reactor; to this purpose, said combined reactor includes a primary reaction stage and a secondary reaction stage. According to an embodiment, the off-gas is introduced via an off-gas distributor above or below a liquid level of the urea melt containing ammonia and melamine precursors. A preferred embodiment of said offgas distributor is disclosed in US 7,311,759.According to another embodiment, a urea melt is divided into a plurality of sub-streams and introduced at multiple locations e.g. at multiple heights in the second stage of said scrubber.In a combined urea-melamine embodiment, ammonia and carbon dioxide are reacted to form a urea solution in a urea synthesis section, the urea solution is processed in at least one recovery section to obtain a purified urea solution and water is removed from the solution to form a urea melt. Said urea melt is used in the above-described process for synthesis of melamine. The melamine offgas generated during the synthesis of melamine is recycled to the production of urea. The melamine synthesis is performed at a synthesis pressure which is typically above 70 bar, such as 70 to 200 bar and preferably 75 to 200 bar. The purification of the melamine offgas is performed at a pressure up to the synthesis pressure, typically in the range 50 to 200 bar. Preferably, the purification of the melamine offgas is performed substantially at synthesis pressure. Particularly, the offgas purification process may be performed at a pressure slightly less than the melamine synthesis pressure, wherein the difference is not more than 20 bar or not more than 5 bar.ExamplesThe following example 1A is a comparative example of a prior-art control system wherein the bottom scrubber temperature is controlled by regulating the water level at the shell side of the urea melt cooler. The example 1B relates to a system wherein, in accordance with an embodiment of the invention, the bottom scrubber temperature is controlled by regulating the pressure of the steam generated at the shell side of the urea melt cooler.Example 1AReference is made to a melamine offgas scrubber having a urea melt cooler controlled to cool the circulating urea to 180° C. The setpoint of the scrubber bottom temperature is 205° C. The temperature of the saturated steam generated at the shell-side of the cooler is 165 °C, corresponding to a pressure of 6 bar. It is assumed that said temperature and pressure of the steam are the same for a condition of fouled cooler and for a condition of fully clean cooler. The following assumptions are made: the heat transfer coefficient of the fouled cooler is 0.7 times the heat transfer coefficient of clean cooler; the heat transferred is the same in both conditions (fouled / clean); the heat exchanged by the non-submerged tubes is negligible (i.e. heat is exchanged only by the submerged tubes). Under the above assumptions, it has been calculated that the heat exchange surface under the clean condition must be 17% lower than the heat exchange surface under the fouled condition; therefore, under the clean condition 83% of tubes are submerged. The urea which traverses the submerged tubes is overcooled to 174.8 °C, whereas the urea passing through the non-submerged tubes remains at the inlet temperature of 205 °C. The average temperature of the two streams is 180 °C, as maintained by the control system; however, the overcooled portion causes an increase of the precipitation of solids and accelerates the formation of the fouling. It has to be noted that a conventional system detects the outlet temperature of 180 °C, so that the overcooling of a portion of urea stream remains undetected. Example 1BIn the urea melt cooler of example 1A, the bottom stripper temperature is controlled by adjusting the steam pressure at the shell-side of the urea melt cooler, while keeping the whole tube bundle submerged. The steam pressure is raised from 6 bar to 7.3 bar. Consequently, the steam temperature is raised from 165°C to 171.8°C. In such a way, all the urea stream is homogeneously cooled from 205°C to 180°C without acceleration of fouling. The increased steam temperature causes a rise in tube wall temperature with an additional beneficial effect on slowing down the formation of solid deposits.Description of the figuresFig. 1 illustrates a high-pressure melamine synthesis section 17 and a melamine offgas purification section comprising a scrubber 5.The high-pressure melamine synthesis section 17 is supplied with a urea melt feed stream 24 and gaseous ammonia 22. In the melamine synthesis section 17 the fresh urea melt 24 is reacted under high-pressure synthesis conditions to generate a raw melamine product 18 and melamine offgas 1. Said melamine offgas 1 contains carbon dioxide, ammonia, some residual melamine and other minor components. Ammonia 22 is injected in the synthesis section 17 to act as a stripping agent to remove carbon dioxide from the raw melamine. The synthesis section 17 may comprise two separate reactors wherein in the first reactor raw melamine is synthesized and in the second reactor ammonia is used as a stripping agent to remove the carbon dioxide from the raw melamine. Alternatively, the synthesis of raw melamine and stripping with ammonia can be carried out in a single reactor. In a preferred embodiment, a single reactor has coaxial zones for synthesis and stripping. For example, the synthesis of melamine is carried out in a central zone of the reactor and stripping is carried out in an annular zone wrapped around said central zone.The melamine offgas 1 is sent to a scrubber 5. Said scrubber 5 may comprise a single purification stage, as shown in Figure 1, or two stages. The scrubber 5 receives, from bottom to top, a stream of gaseous carbon dioxide 23, the melamine offgas 1, a fresh urea melt feed 25 and a cooled recirculated urea melt 10, which is taken from bottom of the scrubber 5, pumped into a circulation pump 6 and cooled in a shell-and-tube urea melt cooler 9. The fresh urea melt 25 is a portion of a urea melt 2 coming from a tied-in urea plant (not shown).The melamine offgas 1 traverses the scrubber 5 upwards, in counter-current with the fresh urea melt 25 and the recirculated urea melt 10. The fresh urea melt 25 and the recirculated urea melt 10 can be fed separately to the scrubber 5 or mixed before or after cooling in the urea melt cooler 9. The scrubber 5 produces a purified offgas 3 which can be exported for a further use, typically recycled to a tied-in urea plant, such as the same urea plant that produces the urea melt 2. From bottom of the scrubber 5, a urea melt 4 containing ammonia and melamine precursors is withdrawn. The injection of carbon dioxide 23 in the scrubber 5 promotes the formation of the melamine precursors contained in the urea melt 4.Said urea melt 4 is separated into a first portion 26 and a second portion 7. The first portion 26 is sent to the tube side of the urea melt cooler 9 via the pump 6 and line 8. The cooled recirculated urea melt 10 leaves the urea melt cooler 9 at a temperature above 180 °C, preferably in the range 180 °C to 200 °C.The shell side of the urea melt cooler 9 produces steam 12 from water 11. Preferably, the temperature of said steam 12 is between 160 °C and 240 °C. Particularly preferably, said steam 12 is saturated steam at least 6.0 barg.A temperature controller 14 measures temperature at bottom of the scrubber 5 and provides a setpoint to a pressure controller 15. The pressure controller 15 regulates the pressure of the steam 120, for example by means of a valve 13. Additionally, in a preferred embodiment, a liquid level controller 27 controls the liquid level at the shell side of the urea melt cooler 9, for example by regulating a water inlet valve 16. Preferably the liquid level is controlled independently from the pressure in the shell side of the urea melt cooler 9.The second portion 7 of the urea melt 4 is sent to the synthesis section 17, where a raw melamine melt 18 is produced. The raw melamine melt 18 is processed in a low-pressure section 21 to obtain solid melamine 20 of a desired purity. Said section 21 preferably includes quenching, purification, crystallization, solid-liquid separation and drying.

Claims

1. A method for controlling an offgas scrubber of a high-pressure melamine plant, wherein: in the offgas scrubber, offgas liberated as byproduct of the synthesis of melamine is washed with a urea-containing stream, including urea recirculated from the offgas scrubber; wherein the recirculated urea stream is cooled in a urea cooler, which is a shell-and-tube heat exchanger external to the scrubber, prior to reintroduction in the scrubber; wherein the recirculated urea stream is fed to the tube side of said urea cooler while water is evaporated and steam is produced in the shell side with heat removed from the urea stream; wherein the method includes that the heat removed from the urea stream in the urea cooler is controlled by regulating the pressure of the steam produced in the shell side of said urea cooler.

2. A method according to claim 1 wherein the heat removed from the urea stream in the urea cooler is controlled so that, in the urea cooler, the urea stream is cooled to a temperature not less than 180 °C.

3. A method according to claim 1 wherein the steam produced in the shell side of said heat exchanger is saturated steam.

4. A method according to claim 3, wherein the pressure in the shell side of the urea cooler is controlled so that evaporation of water is performed at a temperature of at least 160 °C, preferably at least 165 °C.

5. A method according to any of the previous claims, further including that the liquid level of water in the shell side of the urea cooler is controlled independently of said control of heat removed from the urea stream.

6. A method according to any of the previous claims, further including that the liquid level of water in the shell side of said urea cooler is controlled to keep the tubes fully submerged.

7. A method according to any of the previous claims, including the step of measuring the temperature of the urea-containing stream collected from the scrubber, wherein at least a portion of said stream is sent to the urea cooler for recirculation, and providing a setpoint for a pressure controller of the shell side of the urea cooler, said setpoint being determined on the basis of said temperature.

8. A method according to any of the previous claims, wherein the recirculated urea stream, withdrawn from the scrubber, is mixed with fresh urea melt before or after cooling in said shell-and-tube heat exchanger.

9. A method according to any of the previous claims, wherein the urea-containing stream withdrawn from the scrubber has a temperature in the range 170 °C to 250 °C, preferably 175 °C to 240 °C.

10. An offgas scrubbing section of a high-pressure melamine plant, including at least an offgas scrubber where melamine offgas containing ammonia and carbon dioxide is washed with a urea-containing stream, and a shell-and-tube heat exchanger external to the scrubber, said heat exchanger being arranged to cool a recirculated urea-containing stream withdrawn from the scrubber, before reintroduction of said stream into the scrubber, and to evaporate water producing steam, said scrubbing section including a control system configured for controlling the heat removed from the urea stream in said heat exchanger with a method according to any of claims 1 to 9.

11. A high-pressure melamine plant including a reaction section where urea is reacted to produce melamine with formation of an offgas containing ammonia and carbon dioxide, and including a scrubbing section for the offgas according to claim 10. 

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