Multi-product low shrinkage biuret urea production

Abstract

Embodiments relate to a urea plant and operation of the urea plant in a low load mode. In embodiments, two evaporators connected in parallel to a finishing section and a melamine plant, respectively, are used.

Description

TECHNICAL FIELD

[0001] The present invention relates to a method for the production of urea, and to a device suitable for use in such a method, which enables control of biuret formation even when the device is operated at a reduced capacity.

[0002] introduce

[0003] Various types of urea production methods are described in Ullmann’s Encyclopaedia, chapter Urea, 2010. As described therein, biuret is an unwanted by-product formed in urea plants.

[0004] US 2004 / 0116743 Al describes the turndown ratio of a urea plant as the lowest load at which the plant can be operated without the need to reduce turndown time. It is mentioned that it can be necessary to operate at the lowest load if the supply of raw materials is interrupted or substantially reduced.

[0005] US 2019 / 0210963 Al describes a method for controlling biuret formation in urea production by controlling the residence time of an aqueous urea stream treated in such a concentration section in a manner independent of the volumetric flow per time interval of said stream into said concentration section. The residence time can be controlled, for example, by providing an adjustable volume to the concentration section or by providing the addition of a gas to the urea stream to be treated.

[0006] One of the challenges in urea production relates to controlling the amount of biuret formed as a by-product and which is typically present in the urea product, such as prills or granules. Biuret is a dimer of urea and is formed during the ammonia release process. The amount of biuret is an indicator of the quality of urea that can be sold. Biuret in urea used as a fertilizer above a certain level is detrimental to at least various types of plants. Biuret levels in industrial grade urea and urea used for selective catalytic reduction of NOx must be even lower.

[0007] Typically, the international standard specification for biuret in urea product is below 1 wt.%. For example, for fertilizer use, the amount of biuret is typically below 0.9 wt.%. For other applications, such as the use of aqueous urea solutions in units for reducing NOx in diesel engine exhaust, in particular diesel exhaust treatment fluids, biuret content is required to be even lower.

[0008] DEF is standardised in ISO 22241-1 :2006 (version of 15 October 2006). DEF should have a low biuret content. DEF refers to an aqueous urea solution used for selective catalytic reduction of NOx in exhaust gases (SCR solution). ISO 22241-1 :2006 specifies a maximum of 0.3 wt.% biuret based on a 31.8 wt.% aqueous urea solution, whereas solid urea (which can be diluted by adding deionised water to a 31.8 wt.% urea solution) has a limit of 0.95 wt.% biuret. For solid urea products, especially for fertilizer grade products, a biuret level of less than 0.85 wt.% biuret is desirable. In view of the formation of biuret in other parts of the urea production process, the budget for biuret formation in the second vacuum evaporation stage for a urea melt used to make a solid urea product with an acceptably low biuret content is a maximum of 0.20 wt.% and is preferably lower.

[0009] One problem in the field of urea production is that it is more difficult to produce urea according to the required biuret specifications if the plant producing the urea is not running at full capacity. Typically, a plant running at full capacity can guarantee the biuret level. In practice, this means that if a manufacturer runs its plant at a reduced capacity, there is a risk that the product produced does not meet all end use specifications. It is desirable to provide a method of urea manufacturing and a plant suitable for use in this method which allows for control of biuret formation also in case the plant producing the urea is running at a reduced capacity.

[0010] In case of lower demand and / or a reduced supply of raw materials, such as a reduced supply of natural gas or coal for the production of hydrogen gas, the raw material for the synthesis of the urea plant, it can be necessary or desirable to run the urea plant at less than full capacity. SUMMARY

[0011] The present invention provides a plant for the production of urea which is capable of running in a low load mode, by which is meant running at less than full capacity in terms of urea production rate (ton / hr urea), while still producing a urea melt with a low biuret content. It is desirable to provide a plant which is capable of running in at least a first mode and a second mode, wherein the urea production rate in the second mode is lower than the urea production rate in the first mode. It is desirable that the plant is capable of switching from the first mode to the second mode and then back again. A corresponding method is also required, in particular a method of running a urea plant in a low load mode and a method of switching from a first mode of operation to a second mode of operation. Embodiments of the present invention provide such a plant and method.

[0012] In a first aspect, the present invention relates to a method of operating a urea plant in a low load mode, the method comprising: operating a high pressure urea synthesis section of the urea plant in a low load mode; subjecting a urea solution stream from the urea synthesis section to purification in a recovery section to obtain a purified urea solution; subjecting the purified urea solution to evaporation of water in a first vacuum evaporation stage to obtain a first concentrated urea solution; subjecting a first portion of the first concentrated urea solution to evaporation of water in a first evaporator of a second vacuum evaporation stage to form a first urea melt; subjecting a second portion of the first concentrated urea solution to evaporation of water in a second evaporator of the second vacuum evaporation stage to form a second urea melt. In the method, preferably, the first evaporator is operated at a first operating capacity ratio which is a percentage of the design capacity of the first evaporator, and wherein the second evaporator is operated at a second operating capacity ratio which is a percentage of the design capacity of the second evaporator, wherein the second operating capacity ratio is lower than the first operating capacity ratio, and wherein the second evaporator has an outlet for the second urea melt which is connected to an inlet of a melamine plant. Thus, the second evaporator is connected to the melamine plant, such that the second urea melt is supplied to the melamine plant.

[0013] The present invention further relates to a plant for the production of urea, the plant comprising: a urea production section comprising a synthesis section and a recovery section, a first vacuum evaporation stage receiving a urea solution from the urea production section, a second vacuum evaporation stage receiving a concentrated urea solution from the first vacuum evaporation stage, the second vacuum evaporation stage comprising a first evaporator and a second evaporator arranged in parallel, both receiving concentrated urea solution from the first vacuum evaporation stage, wherein the first evaporator has an outlet for a first urea melt which is connected to a finishing section and a melamine plant, and wherein the second evaporator has an outlet for a urea melt which is connected to said melamine plant.

[0014] The present invention further relates to a method of switching a urea plant, preferably a urea plant according to the present invention, from a first mode to a second mode, wherein the urea production rate in the first mode is higher than the urea production rate in the second mode, the method comprising reducing the flow rate of concentrated urea solution (6b) to the second evaporator (9); and keeping the flow rate of concentrated urea solution (6a) to the first evaporator (7) constant, or increasing the flow rate of concentrated urea solution to the first evaporator, or reducing the flow rate of concentrated urea solution to the first evaporator, the reduction being such that the reduction in the flow rate to the first evaporator relative to the flow rate of the first evaporator in the first mode is less than the reduction in the flow rate of concentrated urea solution to the second evaporator relative to the flow rate of the second evaporator in the first mode.

[0015] In summary, the embodiments relate to the operation of a urea plant in a low load mode. In the embodiments, an evaporation stage is used having two evaporation sections or evaporators in parallel, wherein this evaporation stage is connected to a finishing section and a melamine plant. In the embodiments, two evaporators in parallel are used, which are connected to the finishing section and the melamine plant, respectively. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 An exemplary urea plant according to the present application is schematically shown.

[0017] Any embodiment exhibited in the drawings is merely exemplary and does not limit the present application. DETAILED DESCRIPTION

[0018] It is advantageous in the present application that the biuret content in the first urea melt from the first evaporator of the second vacuum evaporation stage is relatively low, even if the plant is operated in a low load mode. In particular, the residence time in the first evaporator is relatively short, in particular shorter than the residence time in the parallel second evaporator of the second vacuum evaporation stage, which ensures a relatively low biuret content. Thus, the first urea melt can be used for the manufacture of a high purity urea product, such as for example a high purity solid urea product (e.g. in urea prills), or for the preparation of a diesel exhaust fluid (DEF), or for a urea product which can be diluted to a DEF by the addition of deionized water. Very ingeniously, the relatively high biuret content of the second urea melt from the second evaporator does not affect or disturb the melamine product, nor does it lead to any contamination of the melamine product. Typically, due to the higher temperature and lower NH3 level of the urea solution in the second vacuum evaporation stage compared to the upstream first vacuum evaporation stage, biuret formation is more pronounced in the second vacuum evaporation stage.

[0019] A high ammonia content in one or more stages of melamine production can lead to a biuret reaction to urea, which is subsequently processed in the melamine plant.

[0020] The present application relates to a method of operating a urea plant in a low load mode, in particular a low load operation mode. The method can also be described as a urea production method.

[0021] Low load operation mode indicates that the plant is operated at less than the design capacity of the plant in terms of urea production rate, for example at less than 95% of the design capacity, such as at less than 90% or less than 80% of the design capacity, typically above 40% or above 50%, or above 60%.

[0022] The low load mode of operation indicates that the apparatus (in particular the urea synthesis section) is operated at a urea production rate in a range between a minimum load rate (minimum production rate) and a maximum but lower than the design capacity of the apparatus (i.e. the urea synthesis section).

[0023] For convenience, reference will now be made to a reference Figure 1 Various aspects of the inventive method are discussed, the figure illustrates an exemplary method and apparatus of the invention; the figure and its references do not limit the claims or the invention.

[0024] The method comprises operating a high pressure synthesis section (1) of a urea plant (100) to react CO2 and NH3 to form urea. Typical reaction conditions are a pressure of 12 MPa to 40 MPa and a temperature between 150 °C and 250 °C.

[0025] The urea synthesis section is in particular operated at a low load mode (i.e. a urea production rate lower than the design capacity, for example a production rate of less than 95% or less than 90% or less than 80% of the design capacity, and for example at least 40% or at least 50% of the design capacity).

[0026] The urea synthesis section is for example a stripping type, wherein the synthesis section comprises a reaction zone, a stripping column and a condensation zone. The reaction zone and the condensation zone can be combined in a single vessel or can be provided as separate vessels. The reaction zone and the condensation zone can each be provided by multiple units. The stripping column uses for example hot stripping or CO2 stripping. The stripping column is typically a shell and tube heat exchanger configured for a falling film of urea solution in the tubes and a heating fluid in the shell, having an inlet for the urea solution at the top, an outlet for the stripped urea solution at the bottom and an outlet for the gas stream to the HP condenser at the top. The stripping column is for example a CO2 stripping column or a hot stripping column. The HP carbamate condenser providing the condensation zone is for example a shell and tube heat exchanger, wherein the gas to be condensed is in the shell or in the tubes. The reactor of the synthesis section is for example a vertical reactor with trays, having one or more inlets at the bottom and an outlet for the urea solution at the top. In an exemplary embodiment, the condensation zone and the reaction zone are combined in a single horizontal vessel, for example a pool reactor.

[0027] A urea synthesis section without a HP stripping column can also be used, which is referred to in the art as a conventional process.

[0028] The HP urea synthesis section can comprise two or more reaction zones, for example urea reactors, in parallel and / or in series. The use of two or more urea reactors in parallel can facilitate flexible operation at low load mode of operation.

[0029] The process involves subjecting a urea solution stream (2) from a urea synthesis section (1) to purification in a recovery section (3) to obtain a purified urea solution (4). The recovery section is for example operated at MP and / or LP. The recovery section comprises for example a MP section and a LP section in series, or a LP recovery section with an inlet for urea solution connected to an outlet for urea solution of a HP urea synthesis section. In the recovery section, unreacted carbamate is decomposed to CO2 and NH3, which are preferably condensed to a carbamate solution that is directly or indirectly partly or fully recovered to the synthesis section. The recovery section typically comprises a LP dissociator, and for example comprises two LP decomposition units (decomposers) in parallel.

[0030] The process involves subjecting the purified urea solution (4) to evaporation of water in a first vacuum evaporation stage (5) to obtain a first concentrated urea solution (6). The concentrated urea solution has for example a water content of 1.0 wt.% to 20 wt.%, preferably 1 wt.% to 5 wt.%, and / or preferably a urea content (including biuret) of for example 85 wt.% to 98 wt.%.

[0031] The first vacuum evaporation stage (5) is for example operated at a pressure of 0.10 bar to 0.50 bar absolute pressure.

[0032] The first vacuum evaporation stage (5) can comprise for example two or more evaporators, in particular in parallel. The term "first vacuum evaporation stage" does not imply that there are no vacuum evaporation units upstream of this stage. For example, there can be a pre-evaporation stage in the recovery section or between the recovery section and the first vacuum evaporation stage, which is operated at a pressure lower than 1 bar absolute pressure, and which is configured for heating the urea solution to effect evaporation of water.

[0033] In embodiments, the first concentrated urea solution (6) is divided into at least a first part and a second part. In other embodiments with parallel evaporators in the first vacuum evaporation stage (5), a first one of said parallel evaporators can provide the first concentrated urea solution of a first part (6a), while a second one of said evaporators can provide the first concentrated urea solution of a second part (6b). Optionally, a further third part of the concentrated urea solution (6) is supplied to a further finishing section (not shown in the figures) bypassing the second vacuum evaporation stage (101). The further finishing section is for example a prilling machine capable of using a concentrated urea solution with for example 92 wt.% to 98 wt.% urea as prilling liquid. Thus, in some embodiments, the apparatus comprises two finishing sections in parallel.

[0034] The process involves subjecting the first concentrated urea solution of the first portion (6a) to evaporation of water in a first evaporator (7) of the second vacuum evaporation stage (101) to form a first urea melt (8). The water content is reduced by at least 0.1 percentage points by weight, for example at least 1.0 percentage points by weight. The water content of the first urea melt is for example less than 0.50 wt.%. The first urea melt comprises for example at least 99.5 wt.% urea, including biuret. The first evaporator (7) is operated at a pressure of for example less than 0.20 bar absolute (for example between 0.01 bar and 0.015 bar absolute, or in case of a prilling tower as finishing section, for example less than 0.05 bar). The first urea melt is preferably suitable for solidification in the finishing section, more preferably for prilling.

[0035] The first evaporator is for example a shell and tube heat exchanger, for example with urea solution in the tubes, and preferably with an inlet for urea solution at the bottom of the tubes and with a gas / liquid separation at the upper end of the tubes. In the shell there is a heating fluid, for example steam, or for example gaseous streams comprising CO2 and NH3, which condense in the exothermic process of carbamate formation.

[0036] The process involves subjecting the first concentrated urea solution of the second portion (6b) to evaporation of water in a second evaporator (9) of the second vacuum evaporation stage (102). The preferences for configuration, pressure and water content as specified for the first evaporator also apply to the second evaporator (9). The second urea melt comprises for example at least 99.5 wt.% urea, including biuret.

[0037] The second evaporator has an outlet for the second urea melt (10) (i.e. a second urea melt stream) which is connected to an inlet of a melamine plant (11) which also has an outlet for a melamine melt (14). The process preferably involves supplying part or all of the second urea melt to the melamine plant; for example, at least 50 wt.% or at least 90 wt.% of the second urea melt is supplied to the melamine plant. In some embodiments, the process also involves using the second urea melt as part or all of the feedstock in the melamine plant to produce melamine. In some embodiments, the melamine plant also receives a urea melt from a further urea plant. The melamine production is based on pyrolysis of urea to melamine. The type of melamine plant is not particularly limited. Both high pressure (> 70 bar absolute) non-catalytic melamine processes and low pressure catalytic processes (< 70 bar absolute) can be used. Various suitable melamine processes are described in Ullmann’s Encyclopedia of Industrial Chemistry, Volume 21, Chapter Melamine and Guanamines, 2003. Further examples of particularly suitable melamine production plants are described in US20040162429A1, EP2385043A1 and EP3597641. However, other types of melamine plants can also be used. Off-gas from the melamine plant is preferably supplied to the urea production plant, optionally after condensation. Some exemplary configurations of the supply of off-gas to the urea plant are described in US20160318883A1.

[0038] In the process of the application, the first evaporator (7) is operated at a first operating capacity ratio which is a percentage of the design capacity of the first evaporator, and the second evaporator (9) is operated at a second operating capacity ratio which is a percentage of the design capacity of the second evaporator. The second operating capacity ratio is less than the first operating capacity ratio. For example, the second operating capacity ratio is at least 5 percentage points or at least 10 percentage points lower than the first operating capacity ratio. For example, the first evaporator is operated at 100% of its design capacity and the second evaporator is operated at 50% of the design capacity. By way of further example, in a low load mode the urea production is 80 ton / hr, with the first evaporator being 60 ton / hr and the second evaporator being 20 ton / hr. In a full load mode, the urea production is 100 ton / hr, with the first evaporator being 60 ton / hr and the second evaporator being 40 ton / hr. All of the urea from the second evaporator is supplied to the melamine plant. At least part of the urea from the first evaporator is transported to, for example, prilling. In the low load mode, the residence time of the urea solution in the second evaporator will be longer (in the given example, twice as long), resulting in relatively more biuret being formed, but this does not adversely affect the operation of the melamine plant. The residence time is increased due to the constant liquid volume in the evaporator and the lower flux. The residence time of the urea solution in the first evaporator is the same in the full load mode and the low load mode, so that the biuret content of the prills does not increase. Part of the urea melt from the first evaporator can be supplied to the melamine plant as required (for example, 20 ton / hr) to provide stable operation of the melamine plant. The design capacity of the first evaporator can be greater than, less than or the same as the design capacity of the second evaporator. The design capacity can also refer to the load of the unit in the full load operating mode of the plant.

[0039] For example, the process involves simultaneously supplying a first portion of the first urea melt (8) (i.e. a first urea melt stream) to the melamine plant and a second portion to the finishing section. The first portion of the urea melt and the second portion of the urea melt are separate streams, suitable for being transported through different flow lines, such as pipes and tubes.

[0040] In the low load mode, the residence time of the urea solution in the second evaporator is longer than the residence time in the first evaporator.

[0041] The biuret content of the second urea melt is, for example, at least 0.02 wt.% (percentage points) or at least 0.05 wt.% (percentage points) higher than the biuret content of the first urea melt. The biuret formation in the second evaporator is, for example, 0.20 wt.% to 0.30 wt.% and the biuret formation in the first evaporator is, for example, 0.10 wt.% to 0.20 wt.%. The biuret content in the second urea melt is, for example, higher than the biuret content allowed for prilling or other use of the first urea melt.

[0042] In exemplary embodiments, for each of the first evaporator and the second evaporator, the biuret formation in full load mode is in the range of 0.1 wt.% to 0.20 wt.% (relative to the amount of biuret formed in the urea melt at the outlet).

[0043] The flow rate of the urea solution to the first evaporator can be higher than, equal to, or lower than the flow rate to the second evaporator, depending on the respective design capacities of the evaporators.

[0044] In some embodiments, the first evaporator is operated at 90% to 100% of the design capacity of the first evaporator and the second evaporator is operated at 20% to 80% of the design capacity of the second evaporator. These ranges can be used in addition to or instead of the first and second operating capacity ratios.

[0045] Preferably, the first evaporator (7) is connected such that all or part of the first urea melt (8) is supplied to a urea finishing section (12) configured to form a solid urea product (13). The finishing section includes, for example, a priller, a prilling tower, or a prilling unit or a combination thereof.

[0046] Optionally, the first evaporator (7) is also connected such that part (15) of the first urea melt (8) is supplied to a melamine unit (11). The method involves supplying all or part (16) of the first urea melt (8) to the urea finishing section (12) and preferably supplying part (15) of the first urea melt (8) to the melamine unit (11). Thereby, the supply of urea to the melamine unit is kept stable even when the urea production of the urea synthesis section is reduced, which stabilizes the operation of the melamine unit.

[0047] In some embodiments, the method involves subjecting part or all of the first urea melt (8) to prilling in a prilling tower.

[0048] For the finishing section, prilling is preferred because prilling typically does not require additives such as formaldehyde. In some embodiments, the process involves purifying the exhaust gas from the prilling tower in a dust cleaner to remove urea dust therefrom, obtaining an aqueous urea solution as a spent scrubber liquor, and supplying the spent scrubber liquor to upstream of the inlet for the urea solution of the first vacuum evaporation stage and / or upstream of the inlet for the concentrated urea solution of the second vacuum evaporation stage.

[0049] In some embodiments, the purification of the spent scrubber liquor does not involve acid washing, and the spent scrubber liquor is free of ammonium salts of mineral acids, in particular free of ammonium nitrate and ammonium sulfate. Optionally, the gas stream from the dust cleaner is further subjected to acid washing in a separate downstream acid washing unit, obtaining a liquid stream comprising dissolved ammonium salts. This liquid stream can be processed as known in the art, for example to form ammonium sulfate solid products or urea ammonium nitrate solution. In some embodiments, the off-gas from the finishing section, such as the prilling tower or priller, is subjected to purification involving both dust and acid washing, resulting in a liquid stream comprising urea and ammonium salts. This liquid stream can be processed as known in the art, for example to form solid urea ammonium sulfate or liquid urea ammonium nitrate; using, for example, a dedicated evaporation stage separate from the first and second vacuum evaporation stages, to form a concentrated spent scrubber liquor which can be combined with the urea melt specifically supplied to the finishing section, i.e. downstream of the split between stream (15) and steam (16).

[0050] In further embodiments in which an additive such as formaldehyde is used in the finishing section, any spent scrubber liquor obtained from the scrubber of this finishing section is processed, for example in a dedicated evaporation unit separate from the first and second vacuum evaporation stages, and the concentrated spent scrubber liquor is supplied, for example, to the finishing section to solidify with the urea melt received by this finishing section.

[0051] In preferred embodiments, prilling does not involve the use of formaldehyde and the spent scrubber liquor is free of formaldehyde. In some embodiments, the second urea melt (10) at least partially supplied to the melamine plant (11) is free of formaldehyde; optionally, the second urea melt is also free of ammonium nitrate and ammonium sulfate.

[0052] Prilling of urea involves solidification of urea melt droplets during free fall from the top of the prilling tower. The top of the prilling tower is provided with a urea melt liquid forming unit such as a sparger or rotating basket. Cooling air is provided from the bottom of the prilling tower and exhaust gas is discharged from the top. Solid urea product is collected and discharged from the bottom of the prilling tower. Prilling provides the advantage that, since no additives are used in some embodiments, the granules can be used as a precursor to prepare DEF by dissolution in water, provided that the urea melt supplied to the prilling tower is of high purity, in particular with low biuret content.

[0053] The granulator is for example a spouted bed or fluid bed granulation unit. An exemplary granulator is a fluid bed granulation unit, in particular a unit with film nozzles.

[0054] The finishing section can also comprise for example flash crystallization, such as described in US 2017 / 0204054 Al. The finishing section can also comprise for example a granulator, for example a granulator comprising a rotating belt, such as described in US 2009 / 0084149.

[0055] For example, the finishing section requires a urea melt feed having less than 1.0 wt.% water, which is generally the case in case of granulation and some types of granulators.

[0056] Parallel combinations of two or more finishing sections are also possible.

[0057] Preferably, the method involves solidifying part or all of the first urea melt to form a solid urea product containing less than 0.95 wt.% biuret or less than 0.80 wt.% biuret, while the urea plant is operated in a low load mode. Thereby, the solid urea product can be dissolved by addition of (deionized) water to obtain a DEF solution complying with the biuret content specifications of ISO 22241-1 :2006. Preferably, the solidification is performed in the finishing section.

[0058] Preferably, at least 90 wt.% or at least 95 wt.% of the urea (urea melt) supply to the finishing section (12) originates from the first evaporator (7). Preferably, in the method, the second urea melt (10) is not supplied to the finishing section (12).

[0059] In some embodiments, the plant comprises a melamine plant, and is thus a plant for producing urea and melamine.

[0060] The preferences and details described for the apparatus in relation to the method also apply to the apparatus of the present application. In the second vacuum evaporation stage (101), the first evaporator (7) and the second evaporator (9) are arranged in parallel and both have an inlet connected to the outlet of the first vacuum evaporation stage (5) for the concentrated urea solution (6). The apparatus comprises a liquid flow line for the first urea melt from the first evaporator to a finishing section (12), a liquid flow line (15) for the first urea melt from the first evaporator to a melamine apparatus (11), and a liquid flow line for the second urea melt (10) from the second evaporator (9) to the melamine apparatus (11). The finishing section is preferably a prilling tower. It is preferred that the finishing section (12) is connected for exclusively receiving urea melt from the first evaporator. The apparatus preferably does not comprise a liquid flow line for urea melt from the second evaporator (9) to the finishing section. This helps to reduce the biuret content of the solid urea product. The liquid flow line can also be referred to as a urea melt transfer line.

[0061] The present application also provides a method of switching a urea plant, in particular a urea plant of the present application, from a first mode of operation to a second mode of operation. In the second mode of operation, the urea production rate, in particular in the urea synthesis section, is at least 10% lower than in the first mode of operation (in percentage of the production rate in the first mode of operation), and preferably at least 20% lower or even at least 30% lower, and for example not more than 50% lower.

[0062] The second mode of operation thus corresponds to a low load mode of operation. The first mode of operation can refer to operation at design capacity, or to operation at a urea production rate lower than or even higher than the design capacity of the plant, in particular of the urea synthesis section. The method involves reducing the flow rate of concentrated urea solution (6b) to the second evaporator (9) and keeping the flow rate of concentrated urea solution (6a) to the first evaporator constant, or increasing the flow rate of concentrated urea solution to the first evaporator, or reducing the flow rate of concentrated urea solution to the first evaporator by an amount such that the reduction in the flow rate of the first evaporator relative to the first mode is less than the reduction in the flow rate of the second evaporator relative to the first mode. In other words,

[0063] R 1,f / R 1,i >R 2,f / R 2,i ,

[0064] (R1,f / R1,i>R2,f / R2,i), wherein R is the flow rate of urea solution (metric tons / hour), the subscript 1 is the first evaporator, the subscript 2 is the second evaporator, the subscript f is the second mode of operation, and the subscript i is the first mode of operation. Preferably, R 1,f / R 1,i >1.2*(R 2,f / R 2,i )

[0065] (R1,f / R1,i>1.2*(R2,f / R2,i))).

[0066] This provides the advantage that the increase in residence time of the urea solution in the first evaporator caused by switching from the first to the second operating mode, if any, is smaller than the increase in residence time of the urea solution in the second evaporator. Thus, in the second operating mode, the increase in biuret formation in the second vacuum evaporation stage is limited to the second evaporator only and does not negatively affect the quality of the solid urea product.

[0067] Preferably, in the second mode, part or all of the second urea melt (10) from the second evaporator (9) is supplied to the melamine plant (11) and part or all of the first urea melt (8) from the first evaporator (7) is supplied to the finishing section (12). Preferably, in the second mode, a first part of the first urea melt (8) is supplied to the finishing section (12) and a second part of the first urea melt (8) is supplied to the melamine plant (11). Optionally, both the first and the second evaporator are used in the second operating mode. Optionally, only the first evaporator is used in the second operating mode, i.e. in some embodiments, R 2,f is zero.

[0068] Preferably, in the second mode, the urea melt flow rate from the first and second evaporator to the melamine plant (11) is at least 80% or at least 90% of the flow rate in the first mode, most preferably remains unchanged. Preferably, the decrease in urea production rate (as a percentage of the urea production rate in the first operating mode) is larger than the decrease in urea melt flow rate from the first and second evaporator to the melamine plant (11), if any, and larger than the decrease in urea melt flow rate from the second evaporator (9) to the finishing section, if any, all decreases from the first to the second operating mode and as a percentage of the flow rate in the first mode. This enables the melamine plant to be operated stably. The urea plant of the present invention is preferably capable of switching from the first to the second operating mode and vice versa.

[0069] Advantageously, during the switch from the first to the second mode, no mechanical modifications to the second vacuum evaporation stage are required, which facilitates an advantageous simple switch.

[0070] This method of switching from a first operating mode to a second operating mode of a urea plant, in particular of the urea plant of the present invention, is preferably performed in a plant for the production of urea comprising a urea production section (100) comprising a synthesis section (1) and a recovery section (2), a first vacuum evaporation stage (5) receiving a urea solution from the urea production section, a second vacuum evaporation stage (101) receiving a concentrated urea solution from the first vacuum evaporation stage, the second vacuum evaporation stage comprising a first evaporator (7) and a second evaporator (9) arranged in parallel, both receiving the concentrated urea solution from the first vacuum evaporation stage, wherein the first evaporator has an outlet (8) for a first urea melt connected to a finishing section (12) and a melamine plant (11), and wherein the second evaporator (9) has an outlet (10) for a second urea melt connected to the melamine plant (11). More preferably, the plant comprises a melamine plant and is also suitable for the production of melamine, and further comprises a liquid flow line for the second urea melt from the second evaporator (9) to the melamine plant (11).

[0071] As used herein, the term 'carbamate' when used in the context of urea production refers to ammonium carbamate. In an aqueous carbamate stream, this component can be present as carbonate species. The amount of NH3 and CO2 of the water stream includes the amount present as carbonate species.

[0072] As used herein, for process streams of a urea plant (i.e. not directed to a steam line nor to a melamine plant), high pressure (HP) is higher than 100 bar, such as 120 bar to 300 bar, such as 140 bar to 200 bar. Medium pressure (MP) is for example 10 bar to 80 bar (including intermediate pressures of 30 bar to 70 bar), in particular 15 bar to 30 bar, and low pressure (LP) is for example 0 bar to 10 bar, in particular 1 bar to 8 bar or 2 bar to 5 bar. All pressures are bar absolute (bar).

[0073] The terms 'typically','suitably' and 'in particular' and derivatives thereof are used to indicate features which can be used in some embodiments but are not mandatory. Preferred features are also not mandatory.

[0074] As used herein, the term'melamine off-gas' refers to off-gas from a melamine production section, and refers to a gas stream containing mainly NH3, CO2 and possibly H2O.

[0075] As used herein, the term 'first' for a unit or step allows for the presence of additional instances upstream of such unit or step.

[0076] Aspects of the present invention will now be further described by the following examples, which do not limit the present invention or the claims.

[0077] Example 1

[0078] As Figure 1 indicated in Table 1, the urea plant co-located with the melamine plant was operated in three configurations, A, B and C, in a second mode. The urea production rate in the first mode is higher than in the second mode. The plant can be switched from the first mode to the second mode and vice versa. T indicates metric tons.

[0079] Table 1

[0080]

[0081] (*) - in wt. % of urea; for 8 and 10, directly at the evaporator outlet.

Claims

1. A method for operating a urea plant (100) below its design capacity, the method comprising: - The high-pressure urea synthesis section of the urea plant operating below its designed capacity (1); - The urea solution stream (2) from the high-pressure urea synthesis section (1) is purified in the recovery section (3) to obtain a purified urea solution (4); - The purified urea solution (4) is subjected to water evaporation in the first vacuum evaporation stage (5) to obtain a first concentrated urea solution (6); - The first concentrated urea solution of the first part (6a) is subjected to water evaporation in the first evaporator (7) of the second vacuum evaporation stage (101) to form the first urea melt (8); - The first concentrated urea solution of the second part (6b) is subjected to water evaporation in the second evaporator (9) of the second vacuum evaporation stage (101) to form a second urea melt (10); The first evaporator (7) operates at a first operating capacity ratio that is a percentage of the design capacity of the first evaporator, and the second evaporator (9) operates at a second operating capacity ratio that is a percentage of the design capacity of the second evaporator, wherein the second operating capacity ratio is lower than the first operating capacity ratio, and wherein the second evaporator has an outlet for the second urea melt (10) connected to the inlet of the melamine unit (11).

2. The method of claim 1, wherein the first evaporator operates at 90% to 100% of its design capacity and the second evaporator operates at 20% to 80% of its design capacity.

3. The method according to claim 1 or 2, wherein the first evaporator (7) is connected to the urea refining section (12) to form a solid urea product (13).

4. The method according to claim 3, wherein the urea refining section (12) is a granulation tower.

5. The method according to claim 3, wherein at least 90 wt.% of the urea supply in the urea refining section (12) originates from the first evaporator (7).

6. The method according to claim 1, the method comprising supplying a portion of the first urea melt (8) to the melamine device (11).

7. The method according to claim 1, wherein the method comprises supplying part or all of the second urea melt (10) to the melamine device (11).

8. A urea production unit, the unit comprising: -A urea production section including a high-pressure urea synthesis section (1) and a recycling section (3), - Receive urea solution from the urea production section and provide a first vacuum evaporation stage (5) for a first concentrated urea solution. - A second vacuum evaporation stage (101) receiving a first concentrated urea solution from the first vacuum evaporation stage, the second vacuum evaporation stage comprising a first evaporator (7) and a second evaporator (9) arranged in parallel, wherein the first evaporator (7) receives a first portion of the first concentrated urea solution from the first vacuum evaporation stage; and wherein the second evaporator (9) receives a second portion of the first concentrated urea solution from the first vacuum evaporation stage, wherein the first evaporator has an outlet for a first urea melt (8) connected to the urea refining section (12) and the melamine unit (11), and wherein the second evaporator (9) has an outlet for a second urea melt (10) connected to the melamine unit (11).

9. The apparatus according to claim 8, wherein the urea refining section (12) is a granulation tower.

10. The apparatus of claim 9, wherein the urea refining section (12) is connected for specifically receiving molten urea from the first evaporator.

11. The apparatus according to any one of claims 8 to 10, suitable for the production of urea and melamine, the apparatus comprising the melamine unit and a liquid flow line for the second urea melt from the outlet of the second evaporator (9) to the melamine unit (11).

12. A method for switching a urea plant according to any one of claims 8 to 11 from a first mode to a second mode, wherein the urea production rate in the first mode is higher than the urea production rate in the second mode, the method comprising: -Reduce the flow rate of the second portion (6b) of the first concentrated urea solution to the second evaporator (9); - and maintain a constant flow rate of the first portion (6a) of the first concentrated urea solution to the first evaporator (7), or increase the flow rate of the first portion (6a) of the first concentrated urea solution to the first evaporator, or decrease the flow rate of the first portion (6a) of the first concentrated urea solution to the first evaporator, such that the decrease in the flow rate of the first portion (6a) of the first concentrated urea solution to the first evaporator relative to the first mode is less than the decrease in the flow rate of the second portion (6b) of the first concentrated urea solution to the second evaporator relative to the flow rate of the second portion (6b) of the first concentrated urea solution to the second evaporator relative to the first mode.

13. The method according to claim 12, wherein in the second mode, part or all of the second urea melt (10) from the second evaporator (9) is supplied to the melamine unit (11) in the second mode, and part or all of the first urea melt (8) from the first evaporator (7) is supplied to the urea refining section (12).

14. The method according to claim 13, wherein in the second mode, the first portion of the first urea melt (8) is supplied to the urea refining section (12), and the second portion of the first urea melt (8) is supplied to the melamine unit (11).

15. The method according to claim 14, wherein in the second mode the flow rate of the urea melt to the melamine device (11) is at least 80% of the flow rate of the urea melt to the melamine device in the first mode.

Images