Reducing biuret formation in urea formation

Abstract

To provide a formation apparatus for granular urea curbed in biuret formation; use of the apparatus; and a formation method.SOLUTION: Provided is an apparatus for forming granular urea low in a biuret content, the apparatus comprising: a synthesis unit 3, an evaporator unit 5, a supply pump, a granulation unit 7, and plural lines 6 capable of leading urea from the evaporator unit to the granulation unit and having different diameters or the same or different cross-section areas. The diameters or cross-section areas of the lines are dimension-determined so as to be capable of securing an optimal retention time of the urea in the lines by using one or plural lines in a case of full load or partial load.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a granular urea production process, comprising a synthesis unit, an evaporator unit and a granulation unit. The apparatus further comprises a refrigerant supplying unit for reducing the formation of biuret under partial load operation. Product lines with different diameters or cross sections in the area between the feed and granulation units are used. The present invention relates to an apparatus for producing granular urea. [Background technology]

[0002] Methods for producing granular fertilizers have been extensively described in the technical and patent literature. No. 6,203,730, German Patent No. 2825039 or U.S. Pat. No. 4,947,308 may be mentioned as an example. The product stream in the form of a melt or solution coming from the stage is subjected to an evaporator to adjust the water content. The pellets are then fed to a pelletizing unit. The granules are mixed with a certain proportion of solid particles prior to introduction into the granulator.

[0003] Urea is currently the most commonly used fertilizer worldwide due to its high biologically available nitrogen content of approximately 46%. It is one of the most commonly used nitrogen fertilizers. The added benefit of urea is that it can be used in combination with e.g. potassium nitrate or urea has a lower risk potential compared to ammonium nitrate and is an economical starting point. The fact that it can be produced on an industrial scale from the materials, i.e. ammonia and carbon dioxide Or it will be.

[0004] The production of urea occurs in two reaction steps that are carried out at high temperature and pressure. In the first reaction step, 2 parts ammonia and 1 part ammonium carbamate are reacted. The conversion of carbon dioxide occurs (2NH3 + CO2 → [NH2COO][NH4]). Thus, in a slow, endothermic second step, water is removed to give urea. ([NH2COO][NH4] → H2O + NH2-CO-NH2). The second step is Because this is a complex reaction, it is recommended to use a relatively long residence time to ensure maximum complete conversion. Most of the mixing is done in separate vessels.

[0005] In addition, to obtain a solid product, the process water generated during the reaction must be removed from the reaction mixture. It is necessary to remove it substantially (i.e., to a residual water content of usually about 3%). To do this, water is extracted from the hot reaction mixture to produce a concentrated urea solution, which is then It can be further processed to form a granular product. As a result of the processing, the urea condensate forms Unfortunately, it is inevitable that formation occurs even in high-concentration urea solutions at high temperatures. From Patent No. 19744404, it is known that depending on the temperature and residence time in the product solution, Polymers of urea and urea, which have no effect on the active ingredient concentration in the microparticles, It is known that condensates form.

[0006] In the production of urea, one particularly problematic by-product is biuret, which is It can be formed by removing ammonia from two urea molecules (2NH2-CO- NH2→NH3+NH2-CO-NH-CO-NH2). Biuret is used as a nitrogen delivery agent. Not only is it less active in plants, it also has a strong phytotoxic effect. It has been observed that fertilization reduces the growth of plants, resulting in the exact opposite of the purpose of fertilization. For most plants, the percentage of biuret in urea fertilizer is about 1% or less. However, some plants, such as citrus trees, contain only 0.5% biuret. Fertilization with urea containing high amounts of biuret resulted in the original growth even after a long time without exposure to biuret. The damage is visible due to the formation of yellow leaves that do not fully return to their green color (RLM ikkelsen,Better Crops 2007,Vol.91,No.3,p In addition to these identified adverse effects, there are several possible side effects associated with biuret. Further questions remain regarding the health risks posed by the substance.

[0007] In relation to the occurrence of biuret in urea solutions, biuret formation occurs due to the concentration It has been observed that the urea concentration increases with increasing temperature and residence time. The compositional relationship is shown, for example, in Austrian Patent No. 285621 and Swiss Patent No. 617672. or in British Patent No. 1404098 and which has been known for over 30 years .

[0008] In this case, the solution is heated to a temperature of about 110-150°C and the product solution is cooled to room temperature for a suitable period of time. The required amount of water must be removed from the initially formed urea solution. During and immediately after concentration by distillation (i.e., removal of water), a significant proportion of biuret is formed. When urea is introduced into the granulation unit in liquid form, the production of urea is achieved with relatively little energy consumption. This high temperature can then be propagated to the following pipelines. If the pipeline, which is generally heated by steam, is further heated, the final product A quality-related amount of biuret is pumped through the pipeline between the evaporator unit and the granulation unit. may be formed within

[0009] To suppress biuret formation, German Patent No. 19744404 uses dicyandia The addition of amide inhibits the crystallization of urea, and as a result, the temperature is set at 70-90°C. At these temperatures, biuret formation is greatly reduced. However, the disadvantages of this method are illustrated by the example given in German Patent No. 19744404. According to the FDA, the biologically inactive dicyandiamide must be added at more than 5% by weight. Adding additives that are not active in the final product is correspondingly more costly. This additionally requires a large construction volume and increases the operating costs of the process to a considerable extent.

[0010] Because the evaporator has been identified as a significant source of biuret formation, Japanese Patent No. In No. 71956, the evaporator is configured as a spray tower, resulting in the formation of The extremely large liquid surface area provided allows for very rapid and effective water extraction. A special concentration of the solution has been proposed. However, the drawback of this solution is the significantly more complicated control. This is an extremely sophisticated evaporator design that requires control and adjustment techniques. The method is to obtain a 2.0% urea content, which is an essential product characteristic of urea granules and is normal for conventional urea products. It is not guaranteed that 90-95% of the particles will be ~4.0mm in size.

[0011] In UK Patent No. 1404098, a urea solution was used to separate the biuret prior to granulation. A method has been proposed in which the water is sent through an ion exchanger, but this requires additional processing. ion exchange material. This involves relatively high operating costs.

[0012] In European Patent No. 1711447, the evaporator unit is modified to reduce the formation of biuret. An automatically regulating pump that conveys the product flow in the direction of the granulation unit after the granulation unit and before the granulation unit The use of a filter has been proposed. This measure prevents biuret formation between the evaporator and the granules. This can reduce the amount of free ammonia in the off-gas considerably. This results in the following result.

[0013] The common feature of the above solution approaches is that they aim to reduce biuret formation during normal operation of the system. However, during operation, for example, the system may be started at that time. Since the system is being turned on or off, one of the required starting materials is not fully loading the system. urea fertilizer is not available in sufficient quantities in the region or due to low demand for urea fertilizer seasonally (e.g. Due to reduced production (winter), there are times when the system cannot be operated at full load. Urea produced during periods when the system is not operating under full load is often has a higher biuret content than the urea produced during operation under full load. This means that the urea produced during these times is no longer sufficient for fertilizer use. does not meet the requirements and must therefore be disposed of at high cost by its generator. The biuret content can increase to such an extent that the material can no longer be used as a fertilizer. Even if the amount does not increase, the biuret content is greater than the maximum guaranteed by the supplier. content, this material is available in different specifications and therefore at a relatively low price. must be sold at price. [Prior art documents] [Patent documents]

[0014] [Patent Document 1] U.S. Patent No. 6,203,730 [Patent Document 2] German Patent No. 2825039 [Patent Document 3] U.S. Patent No. 4,947,308 [Patent Document 4] German Patent No. 19744404 [Patent Document 5] Austrian Patent No. 285621 [Patent Document 6] Swiss Patent No. 617672 [Patent Document 7] British Patent No. 1404098 [Patent Document 8] Japanese Patent No. 57171956 [Patent Document 9] European Patent No. 1711447 [Non-patent literature]

[0015] [Non-Patent Document 1] RLMikkelsen,Better Crops 2007,Vol.91,No.3,p.6 / 7 Summary of the Invention [Problem to be solved by the invention]

[0016] Against this background, the urea generation system is operated at the full load intended for the system. There is a need for an apparatus and method that can ensure consistent product quality even when the The present invention addresses this need. [Brief description of the drawings]

[0017] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] In the context of the present invention, the term "full load" refers to the average throughput of urea in the system. However, the system is designed for an average throughput of urea. For example, The "t / day" system is designed to produce 1,000 t of urea per day under normal operating conditions. Such a system is therefore designed to operate at full load for 10 00t of urea will be produced.

[0019] During "part load" operation, less urea is added to the same system than is possible under normal operating conditions. For example, under 50% partial load in the system, Only 50% of the daily production volume under "full load" is achieved. In the case of the system, this corresponds to a volume of 500 t / day.

[0020] The term "diameter" in relation to line 6 should be understood as "average diameter." Thus, for example, the diameter of a line having an elliptical cross section corresponds to the average diameter of this line. do.

[0021] The "cross-sectional area" of a line is its cross-sectional area perpendicular to its respective flow direction.

[0022] The present invention provides a method for manufacturing a granulator unit by using multiple lines with different diameters between the evaporator unit and the granulator unit. The use of equipment with this technology allows for the production of urea with highly consistent product quality within existing systems. The system is not operated at full load. Such a structure allows the urea solution or urea melt to be pumped through a line thinner than that used for full load operation. This allows for greater product throughput in a line designed for full load operation. This avoids the extended residence time that would otherwise occur due to reduced input, and reduces the increase in biuret generation. will be done.

[0023] According to a first aspect, the present invention provides a method for producing a refrigerant comprising: a synthesis unit 3; an evaporator unit 5; and a delivery pump. and a granulation unit 7, which can transport urea from the evaporator unit to the granulation unit. , and a plurality of lines 6 having different diameters, In this case, the diameter of the line is determined based on the optimum flow rate of urea in the first line under full load. The second line ensures optimal residence time of urea in the line under partial load of the unit. The "optimum residence time" of the urea in the line is determined by The amount of urea transported through the sump is based on the amount of urea transported through the sump, and the amount of urea under full load is greater than under partial load. Therefore, the diameter of the line used under full load is determined by the optimum retention of urea in the line under partial load. Larger than the diameter of the line intended to provide residence time.

[0024] The "optimum residence time" of urea depends on (e.g., the structure of the system and the required residence time of the additive). time) residence time of urea in the line, necessary for technical process reasons Residence times longer than the minimum should generally be understood as requiring more Increases the formation of monia and biuret.

[0025] The synthesis unit 3, the evaporator unit 5, the delivery pump, the granulation unit 7, and the different series and a plurality of lines 6 having a diameter The term "essentially" refers to the area of ​​the system in which the production of urea takes place that is essentially conducive to the production of this particulate. It should be understood that the present invention does not have any components that interfere with the For example, components or components provided for cleaning or treating the by-products of urea production. or equipment by which usable intermediate products can be recycled back into the process. This does not exclude lines that can be

[0026] The lines 6 are advantageously arranged parallel to one another within the device. The same is true for the embodiment of the device.

[0027] The apparatus according to the invention is shown diagrammatically in FIG. 1A, where 1 and 2 are the synthesis 4 represents the ammonia and carbon dioxide supply lines to unit 3, and 4 represents the synthesis unit 3 and the evaporator unit 5, and 8 indicates the removal of granular urea from the device. FIG. 1B is a schematic representation of the operation of the system under full load. During operation, urea is pumped from the evaporator unit through the feed line with the larger diameter to the granulator unit. The feed line with the smaller diameter is closed while the feed line with the smaller diameter is conveyed into the knit. Correspondingly, Figure 1C shows the operation of the system under partial load. During operation of the system at 100°C, urea is pumped through the smaller diameter feed line to the evaporator unit. The feed line with the larger diameter is closed, while the feed line with the larger diameter is closed. It is believed that.

[0028] The device according to the above embodiment transports urea from the evaporator unit to the granulation unit. The diameter of the plurality of lines 6 can be advantageously improved by To ensure optimal residence time of urea in the line under partial loads with intervals of approximately 20% each Additionally or alternatively, such devices may be arranged so that they do not overlap. , at least three capable of conveying urea from the evaporator unit 5 to the granulation unit 7; Preferably, there are 3 to 5 lines 6. In one particularly advantageous configuration, the device is At least three lines 6 capable of conveying the evaporator unit to the granulation unit are provided. The diameter of the second line is approximately 80% of the diameter of the device, which is the diameter of the urine in the line under partial load. The third line ensures optimal residence time of the raw material, and the third line is designed to operate at a partial load of the equipment of about 60%. In a further preferred configuration, the device includes a steam generator for steaming the urea. At least three lines 6 capable of conveying the mixture from the granulator unit to the granulator unit. The diameter of the second line is approximately 60% of the urea in the line at partial load of the device. The third line ensures optimal residence time for the equipment, and the third line operates under partial load of the equipment, about 20%. This design ensures optimal residence time of urea in the tank. and optimal residence time can be ensured under 20% load.

[0029] In addition, the device may also detect urine in the line under a load of more than 100%, e.g., 110%. The additional line 6 has a line diameter configured to ensure optimal residence time of the source material. Such relatively high loads can be achieved, for example, by washing the granulator with urine. This can occur when processing aqueous solutions.

[0030] In the synthesis unit 3 of the apparatus according to the invention, in a first stage, ammonia and dioxide are Ammonium carbonate is produced from the carbon, which flows through the respective supply lines 1 and 2. In the second stage of the synthesis unit, ammonium carbamate is The urea is then converted to urea and water. 0 bar, in particular in the range of 140 to 175 bar. The temperature in the floor is generally in the range of 170 to 200°C, in particular 185 to 190°C.

[0031] The reaction mixture obtained from the synthesis step consists essentially of urea and water, with small amounts of carbonate. may be contaminated with ammonium and small residues of excess ammonia. A typical composition obtained from the synthesis unit is about 54% urea and 26% water by weight. and a remaining portion of ammonium carbamate, carbon dioxide and ammonia.

[0032] In the evaporator unit 5, most of the water formed during the reaction is removed. For this purpose, The solution withdrawn from the synthesis stage is advantageously converted into a (liquid) concentrated urea melt and a gas stream. These are discharged from the evaporator unit. Typically, the urea melt is and concentrated to a residual moisture content of about 0.2-5% by weight.

[0033] The evaporator unit 5 is operated under vacuum conditions and may have one or more evaporators in series. Small amounts of excess ammonium carbamate that may be contained in the evaporated stream are Under vacuum conditions, the ammonia and carbon dioxide are converted The carbon dioxide and the benzene are then primarily transferred to the gas stream exiting the evaporator unit. The gas stream may also contain small amounts of excess ammonia that is released under vacuum conditions.

[0034] If the evaporator unit contains multiple evaporators, the urea is transferred from one evaporator unit to the next. The lines conveyed to the filter unit may also be in the form of a number of lines having different diameters. It may be advantageous to do so (similar to what is described above for line 6).

[0035] The granulation unit 7 may be a fluidized bed granulator, drum granulator, or pan granulator, or a similar known granulator. The main function of the granulation unit is to convert the urea melt into a solidified stream of particles. These solidified particles, also called granules, are the main components of the urea production system. It is a product of.

[0036] In the solidification range of urea, the heat of crystallization released when urea changes from the liquid phase to the solid phase is removed. In addition, additional heat is generally extracted from the solidified urea particles to The urea granules are cooled to a temperature that allows safe storage and transportation of the final product. Removal of heat within the range is usually accomplished in two ways: (i) By evaporation of water, which reaches the granulation unit as part of the urea melt; It may be sprayed at suitable locations within the granulation process to reduce the air inlet temperature; (ii) By cooling with air. Usually, most of the heat of crystallization and cooling is absorbed by air. For this purpose, 3 kg of finished solidified product is A quantity of air equivalent to .about.30 kg of air is typically required.

[0037] Air is in direct contact with the urea melt and solidified urea particles in the granulation unit and must be Depending on the method used to carry out the granulation, the airborne particles are naturally contaminated with a certain proportion of urea dust. The amount of dust is between 0.05 and 10% (expressed with respect to the final product flow). To recover the oxygen and purify the air used for cooling, the device according to the invention may comprise, for example: Suitable recovery devices and purification as described in WO 2013 / 165245 The device may be conveniently provided.

[0038] The delivery pump is conveniently an automatic pump such as that described in European Patent No. 1711447. It may be a regulated rotary pump. A suitable rotary pump is, for example, that described in Austrian Patent No. 281 609 or Austrian Patent No. 291003. The evaporator is installed in the same plane as the granulator, and the rotary pump is placed slightly below. This is even more advantageous since such a configuration allows for shorter supply lines. As a result, the residence time of the urea in the line is correspondingly reduced. This is described in detail in Patent No. 1711447.

[0039] A second advantageous embodiment of the present invention comprises a synthesis unit 3, an evaporator unit 5 and a delivery pump. and a granulation unit 7, which can transport urea from the evaporator unit to the granulation unit. , and a plurality of lines 6 having different or equal cross-sectional areas. The cross-sectional area of ​​the line is then determined so that the optimum residence time of urea in the line is determined by the cross-sectional area of ​​the line under full load. The urea is dimensioned to ensure that it flows through at least two of them. The described apparatus comprises two or more evaporators which convey urea from an evaporator unit to a granulation unit. The line is designed for normal operation, but the implementation of the first aspect of the invention According to the configuration, only one line is used to transport urea from the evaporator unit to the granulation unit. It is used.

[0040] For the definition of "optimum residence time", please refer to the above commentary, which also applies to the cross-sectional area. The same applies to the term "essentially" as used herein. is also valid.

[0041] For two or more line cross-sectional areas, the smallest cross-sectional area of ​​the line shall be less than the largest cross-sectional area of ​​the line. It is preferably at least 10%, particularly preferably at least 20%.

[0042] In an embodiment according to the second aspect, the cross-sectional area of ​​the line is determined based on the optimal residence time of urea in the line. is dimensioned to ensure that urea flows through all of the lines under full load. For example, a device may have two lines, The first line ensures optimal residence time of urea at 60% load in the system, and the second line Line 2 ensures optimal residence time of urea at 40% load in the system. In this case, both lines are opened, so that both lines are the maximum flow rate of urea in the lines under full load of the unit. It has a transport capacity that ensures an appropriate residence time.

[0043] In one particularly advantageous embodiment of the above embodiment, the device comprises: There are three lines that can transport the material to the granulation unit. The cross-sectional area of ​​these lines is , the first, second and third lines respectively account for about 50%, about 33% and about 17% of the devices. It is adapted to have a cross-sectional area that ensures optimal residence time of urea in the line under partial load In this embodiment, when the entire line is open, urine in the line is The normal residence time of the element can be ensured under full load of the device. Furthermore, the first and second lines or a combination of the first and third lines, the third or second lines simultaneously When closed, the optimum residence time of urea at 83% or 67% partial load of the unit, respectively Overall, in this device with only three lines, Optimal residence time under 100%, 83%, 77%, 50%, 33% and 17% partial loads It can be secured.

[0044] In an alternative preferred embodiment, the apparatus comprises: The cross-sectional areas of the three lines are: The second and third lines are at approximately 60%, 40% or 20% partial load of the equipment, respectively. The cross-sectional area is adapted to ensure optimal residence time of the urea in the line below. The embodiment is such that only two lines need to be open under full load, but The above-mentioned experiment shows that only three lines are required for a 20% degree of conformity. This has advantages over the above embodiment.

[0045] The above-mentioned apparatus according to the first and second aspects transports urea from the evaporator unit to the granulation unit. The line 6 which can convey the can be advantageously improved by having a means in the form of a valve. Each has a corresponding means for connecting the device to a preferred working medium when the device is offline. Thus, the line 6 can be flushed, preferably by having a means in the form of a valve. In particular, water or steam is envisaged as a suitable working medium. By flushing, crystallization of urea in the line can be avoided or, for example, Blockages in the lines caused by elemental crystallization can be eliminated.

[0046] The present invention also relates to a hybrid of the above embodiment, in which one of the lines 6 is Optimal residence time of urea in the inlet is ensured during urea flow through this line under full load. and a cross-sectional area of ​​a plurality of additional lines of the line 6 is dimensioned to be However, the optimum residence time of urea in the line is determined by passing through two or more of the additional lines under partial load. The hybrid is dimensioned so that the urea is secured during further flow. Thus, a corresponding device must be installed, for example, to ensure an optimal residence time of the urea in the line under full load. One line 6 that ensures optimal residence time of urea in the line under 40% partial load and two lines 7 that ensure optimal residence time of urea in the line under 40% partial load. The "full load" line may have two lines 6 and 7. These two lines may be closed while the "full load" line is closed. When the valve is opened, the optimum residence time of urea in the two lines is ensured under 80% partial load. It is possible.

[0047] A third aspect of the present invention relates to a process for producing granular urea with a low biuret content, comprising the steps of: Use of the apparatus according to the embodiment, the apparatus being operated under partial load and urea being fed through line 6 From the evaporator unit 5 to the pelletizing unit 7, the diameter of the line 6 is and the diameter of the line is smaller than the maximum diameter of the lines in the device. As can be seen above, under full load, the line with the largest diameter is used. must be used, this comes into play if the equipment is not operating under full load. Can.

[0048] According to a fourth aspect of the present invention, the present invention provides a method for producing granular urea with a low biuret content. Use of the device according to the second aspect for the treatment of urea, the device being operated under partial load and The evaporator unit 5 is conveyed through one or more lines 6 to a granulation unit 7, The total cross-sectional area of ​​the lines 6 is adapted to the product throughput, and the total cross-sectional area of ​​the lines is less than the cross-sectional area required for the optimum residence time of urea in one or more lines under full load. This use therefore applies when the equipment is not operating under full load. It only begins to act on

[0049] Finally, a fifth aspect of the present invention relates to a method for the production of granular urea, comprising the steps of the first and second methods described above. and operating the apparatus according to the second embodiment, wherein the product is extracted from the synthesis unit 3 in an aqueous solution or in a soluble form. The product stream is removed in liquid form as a melt, and downstream of the evaporator unit 5, one or more The product is conveyed to the pelletizing unit 7 through a number of lines 6, the diameter or total cross-sectional area of ​​which is The present invention relates to a method adapted to the throughput. Within the scope of the described method, the product throughput Preferably, the throughput is less than the product throughput of the system under full load.

[0050] The following table shows, by way of example, the results of the use of several lines according to the invention under 60% partial load: describes the avoidance of biuret during operation of a urea system at approximately 60% throughput. In this case, a line designed for 155 metric tons per hour at 135°C is used. Based on the mass flow rate of the urea melt. [Table 1]

[0051] Between the evaporator unit and the granulation unit, instead of a line designed for full load, 60 %Using lines designed for partial loads can reduce line loads due to the relatively small line diameter. The residence time of urea in the urea column is shorter. Therefore, the rate of biuret formation is This corresponded to a specified biuret content of 107 kg / h. [Explanation of symbols]

[0052] 1. Ammonia supply line 2. Carbon dioxide supply line 3. Composite Unit 4. Raw urea supply line to the evaporator unit 5 Evaporator unit 6 Urea solution / melt line to granulation unit 7 Granulation unit 8. Removal of urea from the granulation unit

Claims

1. An apparatus for the production of granular urea, comprising a synthesis unit (3), an evaporator unit (5), a delivery pump, a granulation unit (7) and a number of lines (6) with different or equal cross-sectional areas capable of conveying the urea from the evaporator unit (5) to the granulation unit (7), the lines (6) being adapted such that the cross-sectional areas of the lines (6) ensure an optimal residence time of the urea in the lines during the flow of the urea through two or more of the lines under full load.

2. 2. The apparatus according to claim 1, characterized in that the cross-sectional areas of the lines (6) are dimensioned to ensure an optimal residence time of the urea in the lines during the flow of the urea through all of the lines under full load.

3. 3. Apparatus according to claim 2, characterized in that the apparatus has three lines (6) capable of conveying the urea from the evaporator unit (5) to the granulation unit (7), the cross-sectional areas of the lines (6) being adapted so that the first, second and third lines have cross-sectional areas that ensure an optimal residence time of the urea in the lines under partial loads of 50%, 33% or 17% of the apparatus, respectively.

4. 2. Apparatus according to claim 1, characterized in that it has three lines (6) capable of conveying the urea from the evaporator unit (5) to the granulation unit (7), the cross-sectional areas of the lines (6) being adapted so that the first, second and third lines have cross-sectional areas that ensure an optimal residence time of the urea in the lines under partial loads of 60%, about 40% or 20% of the apparatus, respectively.

5. 5. Apparatus according to any one of claims 1 to 4, characterized in that it comprises means capable of closing, in the direction of the evaporator unit (7), the line (6) capable of conveying the urea from the evaporator unit (5) to the granulation unit (7).

6. 5. Apparatus according to any one of claims 1 to 4, characterized in that it comprises means in the form of a valve capable of closing, in the direction of the evaporator unit (7), the line (6) capable of conveying the urea from the evaporator unit (5) to the granulation unit (7).

7. 5. Apparatus according to any one of claims 1 to 4, characterized in that it comprises means capable of flushing the lines (6) with a suitable working medium, preferably water or steam, when the apparatus is off-line.

8. 5. Apparatus according to any one of claims 1 to 4, characterized in that it comprises means capable of flushing the lines (6) with water or steam when the apparatus is off-line.

9. 5. Apparatus according to any one of claims 1 to 4, characterized in that it comprises means in the form of a valve allowing the line (6) to be flushed with a suitable working medium when the apparatus is off-line.

10. 7. Use of the apparatus according to any one of claims 1 to 6 for the production of granular urea with a low biuret content, wherein the apparatus is operated under partial load, the urea is conveyed from the evaporator unit (5) to the granulation unit (7) through one or more lines (6), the total cross-sectional area of ​​the line(s) being adapted to the product throughput, and the total cross-sectional area of ​​the line(s) being smaller than the cross-sectional area required for an optimal residence time of the urea in the line(s) under full load.

11. A process for the production of granular urea, comprising the operation of an apparatus according to any one of claims 1 to 6, wherein the product from the synthesis unit (3) leaves the latter in liquid form as an aqueous solution or melt, and the product stream is conveyed downstream of the evaporator unit (5) through one or more lines (6) to the granulation unit (7), the diameter or total cross-sectional area of ​​the line or lines (6) being adapted to the product throughput.

12. 12. The method of claim 11, wherein the product throughput is less than the product throughput of the system under full load.

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