JP7583017B2 - Reducing biuret formation in urea production. - Google Patents

Description

The present invention relates to a granular urea production apparatus having a synthesis unit, an evaporator unit and a granulation unit, in which product lines having different diameters or cross sections are used in the region between the evaporator unit and the granulation unit to suppress the formation of biuret under partial load operation.

Methods for the production of granular fertilizers have been described extensively in the technical and patent literature, in which US Pat. No. 6,203,730, German Patent No. 2825039 or US Pat. No. 4,947,308 may be mentioned as examples. Conventionally, within these processes, the product stream in the form of a melt or solution coming from the synthesis stage is fed to an evaporator to adjust the water content and subsequently sent to a granulation unit. In some processes of the prior art, mixing with a certain proportion of solid fine particles, generally recycled granules, is carried out before introduction into the granulator.

Urea is currently one of the most frequently used nitrogen fertilizers worldwide due to its high biologically available nitrogen content of about 46%. Additional advantages of urea consist of a lower risk potential compared to, for example, potassium nitrate or ammonium nitrate, as well as the fact that urea can be produced on an industrial scale from economical starting materials, namely ammonia and carbon dioxide.

The production of urea takes place in two reaction steps, which are carried out at high temperature and pressure. In the first reaction step, which is fast and exothermic, the conversion of two parts of ammonia and one part of carbon dioxide to ammonium carbamate occurs (2NH ₃ +CO ₂ →[NH ₂ COO][NH ₄ ]). From this, in the second step, which is slow and endothermic, urea is obtained by removing water ([NH ₂ COO][NH ₄ ] →H ₂ O+NH ₂ -CO-NH ₂ ). As the second step is a slow reaction, it is mostly carried out in a separate vessel with a relatively long residence time to ensure the most complete conversion possible.

Furthermore, in order to obtain a solid product, the process water formed during the reaction must be substantially removed from the reaction mixture (i.e., typically to a residual water content of about 3%). For this purpose, water is extracted from the hot reaction mixture to form a concentrated urea solution, which can then be further processed to form a granular product. Unfortunately, it is inevitable that as a result of the processing, the formation of urea condensates also occurs in hot, concentrated urea solutions. It is known, for example, from German Patent 19744404, that depending on the temperature and residence time in the product solution, polymers and condensates of urea are formed, which have no bioactive effect and thus reduce the active ingredient concentration in the microparticles.

In the production of urea, one particularly problematic by-product is biuret, which can be formed by the removal of ammonia from two urea molecules ( _2NH2 -CO- _NH2 → _NH3 + _NH2 -CO-NH-CO- _NH2 ). Biuret is not only poorly active as a nitrogen delivery agent, but also has strong phytotoxic effects. For example, it has been observed that significant amounts of biuret reduce plant growth, resulting in exactly the opposite of the purpose of fertilization. Most plants can tolerate a percentage of biuret in urea fertilizer of about 1% or less, but some plants, for example citrus trees, when fertilized with urea having a biuret content of only 0.5%, are visibly damaged by the formation of yellow leaves that do not completely return to their original green color even after a long time without exposure to biuret (see R.L. Mikkelsen, Better Crops 2007, Vol. 91, No. 3, p. 6/7). In addition to these confirmed adverse effects, there are further unresolved questions regarding possible health risks associated with biuret.

With regard to the occurrence of biuret in urea solutions, it has been observed that biuret formation increases with increasing temperature and residence time of the concentrated urea in the production system. The biuret formation relationship has been known for more than 30 years, for example as described in Austrian Patent No. 285621, Swiss Patent No. 617672 or British Patent No. 1404098.

In this case, a significant proportion of biuret is formed during and immediately after concentration by evaporation (i.e. removal of water) from the initially formed urea solution, since the solution must be heated to a temperature of about 110-150° to remove the necessary amount of water from the product solution within a suitable time. If the urea is introduced to the granulation unit in liquid form, this high temperature also spreads to the subsequent pipelines, since granulation of urea can be carried out with relatively little energy consumption. Thus, an amount of biuret relevant to the final quality of the product can be formed in the pipeline between the evaporator unit and the granulation unit, especially if the pipeline, which is generally heated by steam, is further heated.

To suppress biuret formation, German Patent No. 19744404 discloses a process in which urea crystallization is inhibited by adding dicyandiamide, so that it can be carried out at temperatures of 70-90°C. At these temperatures, biuret formation is significantly reduced. However, a disadvantage of this process is the high requirement of biologically inactive dicyandiamide, which, according to the examples described in German Patent No. 19744404, must be added in amounts of more than 5% by weight. Adding additives that are not active in the end effect further requires a correspondingly larger construction volume and increases the operating costs of the process to a considerable extent.

Since the evaporator has been identified as a significant source of biuret formation, Japanese Patent No. 57171956 proposes a special concentration of the solution, in which the evaporator is configured as a spray tower, resulting in a very fast and effective water extraction due to the very large liquid surface area created in the product solution. However, the drawback of this solution is the very sophisticated evaporator design, which requires very complex control and regulation techniques. Moreover, this method does not guarantee that 90-95% of the particles will have a particle size of 2.0-4.0 mm, which is typical for conventional urea products, which is an essential product characteristic of urea granules.

In GB 1404098 a method is proposed in which the urea solution is passed through an ion exchanger to separate the biuret prior to granulation. However, this method requires additional process components and therefore involves relatively high operating costs due to the periodically required regeneration of the ion exchange material.

In order to reduce the formation of biuret, EP 1711447 proposes the use of an automatically regulating pump after the evaporator unit and before the granulation unit, which conveys the product flow in the direction of the granulation unit. This measure has the consequence that the biuret formation between the evaporator and the granulation can be reduced to some extent, and therefore the proportion of free ammonia in the off-gas can be significantly reduced.

A common feature of the above solution approaches is that they aim to reduce biuret formation during normal operation of the system. However, during operation, there are times when the system cannot be operated at full load, for example because one of the required starting materials is not available in sufficient quantities to fully load the system, since the system is currently being ramped up or ramped down, or because production is seasonally reduced (e.g., in winter) due to low demand for urea fertilizer. It has been observed that urea produced during periods when the system is not operated under full load often has a higher biuret content than urea produced during periods when the system is operating under full load. This can result in the urea produced during these times no longer meeting the fertilizer specification requirements and therefore having to be disposed of at high cost by the producer. Even if the biuret content does not increase to such an extent that the material can no longer be used as a fertilizer, this material must be sold with a different specification and therefore at a relatively low price, since the biuret content may exceed the maximum content guaranteed by the supplier.

U.S. Patent No. 6,203,730 German Patent No. 2825039 U.S. Pat. No. 4,947,308 German Patent No. 19744404 Austrian Patent No. 285621 Swiss Patent No. 617672 British Patent No. 1404098 Japanese Patent No. 57171956 European Patent No. 1711447

R. L. Mikkelsen, Better Crops 2007, Vol. 91, No. 3, p. 6/7

Against this background, there is a need for an apparatus and method that can ensure consistent product quality even when the urea production system is not operating at the full load intended for the system. The present invention addresses this need.

In the context of this invention, the term "full load" refers to the average throughput of urea in the system for which the system is designed. For example, a "1000 t/day" system is designed to produce 1000 t of urea per day under normal operating conditions. Thus, such a system produces 1000 t of urea per day under full load operation.

During "part load" operation, less urea is produced in the same system than would be possible under normal operating conditions. For example, at a 50% part load in the system, only 50% of the daily production under normal operating conditions ("full load") is achieved. In a "1000 t/day" system, this corresponds to an amount of 500 t/day.

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.

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

The invention is based on the surprising discovery that the use of an apparatus with multiple lines of different diameters between the evaporator unit and the granulation unit allows the production of urea with a highly uniform product quality in an existing system. When the system is not operated at full load, such a configuration allows the urea solution or urea melt to be sent through smaller lines than in full load operation. As a result, the extended residence times that would occur due to reduced product throughput in lines designed for full load operation are avoided and increased biuret generation is suppressed.

According to a first aspect, the invention relates to an apparatus for the production of granular urea, essentially comprising a synthesis unit 3, an evaporator unit 5, a delivery pump, a granulation unit 7 and a number of lines 6 with different diameters, capable of conveying urea from the evaporator unit to the granulation unit. In this case, the diameters of the lines are dimensioned such that a first line ensures an optimal residence time of urea in the line under full load and a second line ensures an optimal residence time of urea in the line under partial load of the apparatus. The "optimum residence time" of urea in the line is based on the amount of urea conveyed through the line, and since the amount of urea under full load is greater than under partial load, the diameter of the line used under full load is greater than the diameter of the line intended to ensure an optimal residence time of urea in the line under partial load.

The "optimum residence time" of urea should be understood as the residence time of urea in the line that is necessary for technical process reasons (e.g. due to the structure of the system and the required residence time of the additive). Residence times longer than the minimum generally increase the formation of ammonia and biuret due to thermal conditions.

The term "essentially" in relation to the urea production apparatus, intended to comprise a synthesis unit 3, an evaporator unit 5, a delivery pump, a granulation unit 7 and a number of lines 6 with different diameters, should be understood in that the area of the system in which the production of urea takes place does not have any components that essentially impede the production of this granule. However, this does not exclude, for example, components or devices provided for washing or treating the by-products of the urea production or lines with which usable intermediate products can be recycled to the process.

The multiple lines 6 are preferably arranged parallel to one another within the device. This also applies to the device embodiments described below.

The device according to the invention is shown diagrammatically in FIG. 1A, where 1 and 2 represent the supply lines of ammonia and carbon dioxide to the synthesis unit 3, 4 represents the line between the synthesis unit 3 and the evaporator unit 5, and 8 represents the removal of granular urea from the device. FIG. 1B diagrammatically represents the operation of the system under full load, during which urea is conveyed from the evaporator unit into the granulation unit through the supply line with the larger diameter, whereas the supply line with the smaller diameter is closed. Correspondingly, FIG. 1C represents the operation of the system under partial load, during which urea is conveyed from the evaporator unit into the granulation unit through the supply line with the smaller diameter, whereas the supply line with the larger diameter is closed.

The device according to the above-mentioned embodiment can be advantageously improved by the diameters of a plurality of lines 6 capable of conveying urea from the evaporator unit to the granulation unit, the diameters of the plurality of lines 6 being adjusted so as not to overlap, so as to ensure an optimal residence time of urea in the lines under partial loads with an interval of about 20% between each other. Additionally or alternatively, such a device has at least three, preferably three to five, lines 6 capable of conveying urea from the evaporator unit 5 to the granulation unit 7. In one particularly advantageous configuration, the device has at least three lines 6 capable of conveying urea from the evaporator unit to the granulation unit, of which the diameter of the second line ensures an optimal residence time of urea in the lines under a partial load of the device of about 80%, and the third line ensures an optimal residence time of urea in the lines under a partial load of the device of about 60%. In a further preferred configuration, the apparatus has at least three lines 6 capable of transporting urea from the evaporator unit to the granulation unit, of which the diameter of the second line ensures an optimal residence time of urea in the line at a partial load of the apparatus of about 60%, and the third line ensures an optimal residence time of urea in the line at a partial load of the apparatus of about 20%. This design allows optimal residence times to be ensured under loads of 100%, 80%, 60% and 20%.

In addition to this, the apparatus can have an additional line 6 with a line diameter configured to ensure optimal residence time of the urea in the line under loads of more than 100%, for example 110%. Such relatively high loads can occur, for example, when processing a urea solution after cleaning a granulator.

In the synthesis unit 3 of the apparatus according to the invention, in a first stage, ammonium carbonate is produced from ammonia and carbon dioxide, which is conveyed to the synthesis unit 3 through the respective feed lines 1 and 2. In the second stage of the synthesis unit, the ammonium carbamate is then converted into urea and water. In the first and second stages, a pressure in the range of 120 to 200 bar, in particular 140 to 175 bar, is preferably set. The temperature during the second stage is generally in the range of 170 to 200 °C, in particular 185 to 190 °C.

The reaction mixture obtained from the synthesis stage consists essentially of urea and water, but may be contaminated by small amounts of ammonium carbonate and small residues of excess ammonia. A typical composition obtained from the synthesis unit contains about 54% by weight of urea, 26% by weight of water, and residual portions of ammonium carbamate, carbon dioxide and ammonia.

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

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 converted to ammonia and carbon dioxide under process conditions. Under vacuum conditions, this ammonia and carbon dioxide are then primarily transferred to the gas stream exiting the evaporator unit. This gas stream may also contain small amounts of excess ammonia that are released under vacuum conditions.

If the evaporator unit includes multiple evaporators, it may be expedient for the lines along which the urea is conveyed from one evaporator unit to the next to be in the form of multiple lines having different diameters as well (similar to what is described above for line 6).

The granulation unit 7 may comprise a fluid bed granulator, drum granulator or pan granulator, or similar known granulation equipment. The primary function of the granulation unit is to convert the urea melt into a stream of solidified particles. These solidified particles, also called granules, are the primary product of the urea production system.

In the solidification of urea, it is necessary to remove the heat of crystallization that is released when urea changes from the liquid phase to the solid phase. Moreover, additional heat is generally extracted from the solidified urea particles to cool them to a temperature that allows safe storage and transportation of the final product. The removal of heat in the granulation is usually achieved in two ways:
(i) By evaporation of water, which reaches the granulation unit as part of the urea melt and 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 released by cooling with air. For this purpose, an amount of air equivalent to 3-30 kg of air per kg of finished solidified product is generally required.

The air is inevitably contaminated by a certain percentage of urea dust due to the direct contact with the urea melt and the solidified urea particles in the granulation unit. Depending on the method of carrying out the granulation, the amount of dust in the air is between 0.05 and 10% (expressed in terms of the output flow of the final product). To recover the urea and purify the air used for cooling, the device according to the invention may advantageously be equipped with suitable recovery and purification devices, for example as described in WO 2013/165245.

The delivery pump may advantageously be a self-regulating rotary pump as described in EP 1 711 447. Suitable rotary pumps are described, for example, in AU 281 609 or AU 291 003. In the context of the present invention, it is further advantageous for the evaporator to be installed in the same plane as the granulator and for the rotary pump to be arranged slightly below, since such an arrangement allows the supply lines to be shortened. As a result, the residence time of the urea in the line path is correspondingly shortened. Corresponding structures are likewise described in detail in EP 1 711 447.

A second advantageous aspect of the invention relates to an apparatus having 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 by which urea can be conveyed from the evaporator unit to the granulation unit. The cross-sectional areas of the lines 6 are in this case dimensioned such that an optimal residence time of urea in the lines is ensured during the flow of urea through two or more of the multiple lines under full load. In other words, the described apparatus is designed for normal operation by using two or more lines conveying urea from the evaporator unit to the granulation unit, whereas according to an embodiment of the first aspect of the invention, only one line conveying urea from the evaporator unit to the granulation unit is used.

For the definition of "optimum residence time", reference is made to the above commentary, which may be applied to the cross-sectional area as well. For the term "essentially", the corresponding commentary in the context of the first aspect is also valid.

For the cross-sectional areas of two or more lines, it is preferred that the smallest cross-sectional area of the line is at least 10%, in particular at least 20%, of the largest cross-sectional area of the line.

In an embodiment according to the second aspect, the cross-sectional area of the lines may be advantageously dimensioned such that an optimal residence time of urea in the lines is ensured during urea flow through all of the lines under full load. For example, the apparatus may have two lines, a first line ensuring an optimal residence time of urea under 60% load of the system and a second line ensuring an optimal residence time of urea under 40% load of the system. Under full load, both lines are opened so that both lines have a transport capacity that ensures an optimal residence time of urea in the lines under full load of the apparatus.

In a particularly advantageous embodiment of the above embodiment, the apparatus has three lines capable of conveying urea from the evaporator unit to the granulation unit. It is particularly preferred that the cross-sectional areas of these lines are adapted such that the first, second and third lines have cross-sectional areas that ensure an optimal residence time of urea in the lines under about 50%, about 33% or about 17% partial load of the apparatus, respectively. In this embodiment, if all the lines are open, the normal residence time of urea in the lines can be ensured under full load of the apparatus. Furthermore, the combination of the first and second lines, or the first and third lines, can ensure an optimal residence time of urea under 83% or 67% partial load of the apparatus, respectively, if the third or second line is closed at the same time. Overall, with this apparatus having only three lines, an optimal residence time can be ensured under 100%, 83%, 77%, 50%, 33% and 17% partial load of the apparatus.

In an alternative preferred embodiment, the apparatus has three lines capable of conveying urea from the evaporator unit to the granulation unit, the cross-sectional areas of these three lines being adapted such that the first, second and third lines have cross-sectional areas that ensure optimal residence times of urea in the lines under partial loads of about 60%, about 40% or about 20% of the apparatus, respectively. This embodiment has the advantage over the above-mentioned embodiment that only two lines need to be open under full load, but nevertheless allows adaptation in stages of about 20% with only three lines.

The above-mentioned apparatus according to the first and second aspects can be advantageously improved by having means, preferably in the form of a valve, by which the line 6, by which the urea can be conveyed from the evaporator unit to the granulation unit, can be closed in the direction of the evaporator unit. Preferably, each of the lines has a corresponding means. The apparatus can be further advantageously improved by having means, preferably in the form of a valve, by which the line 6 can be flushed by a suitable working medium when the apparatus is offline. In particular, water or steam can be envisaged as suitable working medium. By flushing, crystallization of urea in the line can be avoided or blockages in the line caused, for example, by urea crystallization can be removed.

The invention also includes hybrids of the above-mentioned embodiments, in which one of the lines 6 has a cross-sectional area dimensioned such that the optimal residence time of the urea in the line is ensured during the flow of urea through this line under full load, and the cross-sectional areas of a number of additional lines of the lines 6 are dimensioned such that the optimal residence time of the urea in the line is ensured during the further flow of urea through two or more of the additional lines under partial load. Thus, a corresponding device may have, for example, one line 6 that ensures the optimal residence time of the urea in the line under full load and two lines 6 that ensure the optimal residence time of the urea in the line under 40% partial load. If these two lines are opened while the "full load" line is closed, the optimal residence time of the urea in the two lines under 80% partial load can be ensured.

The third aspect of the invention relates to the use of the apparatus according to the first aspect above for the production of granular urea with a low biuret content, in which the apparatus is operated under partial load, the urea is conveyed from the evaporator unit 5 to the granulation unit 7 through a line 6, the diameter of the line 6 being adapted to the product throughput, the diameter of the line being smaller than the maximum diameter of the lines in the apparatus. This comes into play when the apparatus is not operated under full load, since, as can be seen above, under full load the line with the maximum diameter of the lines must be used.

According to a fourth aspect of the invention, the invention relates to the use of the apparatus according to the second aspect above for the production of granular urea with a low biuret content, in which 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 one or more lines 6 being adapted to the product throughput, the total cross-sectional area of the lines being smaller than the cross-sectional area required for the optimal residence time of urea in the line or lines under full load. This use therefore only comes into play when the apparatus is not operated under full load.

Finally, a fifth aspect of the invention relates to a method for the production of granular urea, comprising the operation of an apparatus according to the first and second aspects described above, in which the product is removed in liquid form from the synthesis unit 3 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 lines being adapted to the product throughput. In the scope of the described method, it is preferred that the product throughput is less than the product throughput of the system under full load.

The following table illustrates, by way of example, the avoidance of biuret during operation of a urea system under 60% part load by the inventive use of multiple lines, where a line designed for approximately 60% throughput is used, in this case based on a mass flow rate of 155 metric tons/hour of urea melt at 135° C.

The use of a line designed for 60% partial load instead of a line designed for full load between the evaporator unit and the granulation unit results in a shorter residence time of urea in the line due to the relatively smaller line diameter. The rate of biuret formation therefore corresponded to the specified biuret content of 107 kg/h for full load.

1 Supply line for ammonia 2 Supply line for carbon dioxide 3 Synthesis unit 4 Supply line for raw urea to the evaporator unit 5 Evaporator unit 6 Urea solution/melt line to the granulation unit 7 Granulation unit 8 Urea removal from the granulation unit

Claims (14)

An apparatus for the production of granular urea, essentially comprising a synthesis unit (3), an evaporator unit (5), a delivery pump, a granulation unit (7) and a number of lines (6) with different diameters capable of conveying the urea from the evaporator unit (5) to the granulation unit, the diameters of the lines (6) being dimensioned such that a first line ensures an optimal residence time of the urea in the line under full load and a second line ensures an optimal residence time of the urea in the line under partial load of the apparatus. The device according to claim 1, characterized in that the diameters of the lines (6) capable of conveying the urea from the evaporator unit (5) to the granulation unit (7) are adjusted so as not to overlap, so as to ensure an optimal residence time of the urea in the lines under partial loads with an interval of 20% each. The device according to claim 1 or 2, characterized in that it has at least three lines (6) capable of conveying the urea from the evaporator unit (5) to the granulation unit (7). The device according to claim 1 or 2, characterized in that it has three to five lines (6) capable of conveying the urea from the evaporator unit (5) to the granulation unit (7). The apparatus according to any one of claims 1 to 4, characterized in that it has at least three lines (6) capable of conveying the urea from the evaporator unit (5) to the granulation unit (7), of which the diameter of the second line ensures an optimal residence time of the urea in the line at a partial load of the apparatus of 80% and the third line ensures an optimal residence time of the urea in the line at a partial load of the apparatus of 60%. The device according to any one of claims 1 to 5, characterized in that it has means for closing the line (6) in the direction of the evaporator unit (7) by which the urea can be conveyed from the evaporator unit (5) to the granulation unit (7). The device according to any one of claims 1 to 5, characterized in that it has means in the form of a valve capable of closing the line (6) capable of conveying the urea from the evaporator unit (5) to the granulation unit (7) in the direction of the evaporator unit (7). The device according to any one of claims 1 to 5, characterized in that it has means for flushing the line (6) with a suitable working medium when the device is offline. The device according to any one of claims 1 to 5, characterized in that it has means for flushing the lines (6) with water or steam when the device is off-line. The device according to any one of claims 1 to 5, characterized in that it has means in the form of a valve by which the line (6) can be flushed with a suitable working medium when the device is offline. Use of the apparatus according to any one of claims 1 to 6 for the production of granular urea with a low biuret content, the apparatus being operated under partial load, the urea being conveyed from the evaporator unit (5) to the granulation unit (7) through lines, the diameters of the lines being adapted to the product throughput, and the diameters of the lines being smaller than the maximum diameter of the plurality of lines (6) in the apparatus. Use of the apparatus according to claim 6 for the production of granular urea with a low biuret content, the apparatus being operated under partial load, the urea being 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, the total cross-sectional area of the line(s) being smaller than the cross-sectional area required for the optimal residence time of the urea in the line(s) under full load. A method for the production of granular urea, comprising the operation of an apparatus according to any one of claims 1 to 6, in which 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 one or more lines (6) being adapted to the product throughput. The method of claim 12, characterized in that the product throughput is less than the product throughput of the system under full load.

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