System and method for producing urea ammonium nitrate (UAN)

Abstract

A system for producing urea ammonium nitrate (UAN) is disclosed that is particularly suited for processing smaller ammonium nitrate waste streams into UANs. The system includes a concentration section for ammonium nitrate, a treatment section that allows for the recovery and recycle of nitrogen compounds entrained in the steam from the concentration section, and a pH control section that allows for the pH adjustment of the treated ammonium nitrate waste stream to a desired degree.

Description

Technical Field

[0001] This invention relates to the preparation of urea ammonium nitrate aqueous solution (UAN). In particular, this invention relates to systems and methods for processing ammonium nitrate (AN) waste streams into UAN. Background Technology

[0002] Urea ammonium nitrate (UAN) is an aqueous solution of urea and ammonium nitrate and is used as a fertilizer. Methods for producing UAN typically involve providing an aqueous solution of ammonium nitrate (AN), providing an aqueous solution of urea, and mixing the ammonium nitrate and urea solutions in a UAN production unit to produce urea ammonium nitrate. Ammonium nitrate and urea can be provided, for example, by production in separate ammonium nitrate and urea units or in corresponding sections of combined or integrated units. Furthermore, it has been proposed that the ammonium nitrate portion be provided as a byproduct of an acidic ammonia washing unit.

[0003] Background art concerning the sending of all or part of the washing liquid used in an acidic ammonia scrubber to a UAN production unit is known from WO2017 / 111588 or WO2017 / 111585. The apparatus disclosed herein is limited to integration with urea production. In both documents, additional ammonium nitrate from a separate ammonium nitrate production section is supplied to the UAN production unit. It is desirable to provide a system for producing UAN that can be applied to most of any ammonium nitrate waste stream. Therefore, a further desire is to reduce emissions, particularly ammonia, nitric acid, and ammonium nitrate emissions, and to minimize waste stream and equipment costs. Furthermore, it is desirable to further improve the efficiency of systems such as those disclosed in the aforementioned background art.

[0004] The present invention also relates to the treatment of acidic urea ammonium nitrate aqueous solution, said urea ammonium nitrate aqueous solution being derived from the acid scrubbing of ammonia-containing waste gas, such as the acid scrubbing of waste gas (waste gas flow) from the finishing section of a urea plant.

[0005] Various urea production facilities and processes are described in the chapter on "Urea" in Ullmann's *Encyclopedia of Industrial Chemistry* (2010). In many urea production processes, at least a portion of the formed urea melt undergoes a finishing step, in which it is converted into solid urea product using a cooling gas stream (such as a cooling air stream), for example, by granulation or tower granulation. The resulting waste gas, i.e., the waste gas stream, includes urea dust and NH3, as well as air. Urea dust and ammonia can be removed from the waste gas stream by washing. Many methods use a combination of urea dust washing and acid washing to obtain an aqueous solution containing urea and ammonium salts. The use of this solution as a liquid fertilizer has been proposed. Summary of the Invention

[0006] To better meet one or more of the above requirements, in one aspect the present invention provides a system for producing urea ammonium nitrate (UAN), the system comprising an inlet for aqueous ammonium nitrate, an inlet for liquid urea, and an outlet for UAN, the system comprising a concentration section configured to evaporate the aqueous ammonium nitrate to provide concentrated ammonium nitrate, and a production section located downstream of and in fluid communication with the concentration section configured to mix the concentrated ammonium nitrate and liquid urea to produce UAN, wherein the concentration section has a gas outlet for water vapor, the gas outlet preferably in fluid communication with a processing section configured to allow the water vapor from the concentration section to evaporate. The received water vapor is used for washing. The processing section has an inlet for washing liquid and an outlet for used washing liquid, wherein the outlet for used washing liquid is preferably in fluid communication with the inlet of the concentration section, and wherein the system includes a pH control section such that the inlet for aqueous ammonium nitrate is in fluid communication with the inlet of the pH control section, wherein the pH control section is configured to subject the aqueous ammonium nitrate to pH control, thereby providing a pH-controlled flow of aqueous ammonium nitrate, wherein the pH control section has an outlet for the pH-controlled flow of aqueous ammonium nitrate, the outlet being in fluid communication with the inlet of the concentration section, particularly the evaporation section.

[0007] In another aspect, a method for producing urea ammonium nitrate (UAN) is provided, the method comprising obtaining a used scrubbing liquid produced by contacting ammonia-containing waste gas with nitric acid in a scrubber; pH controlling the used scrubbing liquid, the pH control comprising determining the pH and adjusting the pH to a desired level within the range of 2.0 to 4.5 to provide a pH-controlled liquid; evaporating the pH-controlled liquid to obtain a concentrated ammonium nitrate solution; and combining the concentrated ammonium nitrate solution with an aqueous urea solution and / or urea melt to obtain a urea ammonium nitrate solution. This method is advantageously, but not exclusively, carried out in the system of the present invention.

[0008] In another aspect, a method is provided for improving a previously existing chemical treatment unit having an aqueous ammonium nitrate waste outlet, the method comprising providing a system for producing urea ammonium nitrate (UAN) as described above, and connecting said system to said chemical treatment unit such that the aqueous ammonium nitrate waste outlet of the chemical treatment unit is in fluid communication with the aqueous ammonium nitrate inlet of the UAN production system. Attached Figure Description

[0009] Figure 1 This is a general schematic diagram of a system for producing urea ammonium nitrate (UAN) according to the present invention.

[0010] Figure 2 and Figure 3 All of them show process flow diagrams of non-limiting examples of the UAN device of the present invention.

[0011] Figure 4 A process flow diagram of a system for producing urea ammonium nitrate (UAN) according to the present invention is shown, wherein the process is integrated with the urea production process. Detailed Implementation

[0012] Urea ammonium nitrate (UAN) is a fertilizer, typically used in the form of an aqueous solution of urea and ammonium nitrate. Ammonium nitrate is produced by reacting ammonia with a concentrated nitric acid solution while maintaining the pH of the solution within a narrow range. The resulting solution is then mixed with an aqueous urea solution to obtain UAN. A typical UAN product contains 28% to 32% by weight of total nitrogen. The UAN product generally comprises 29% to 38% by weight of urea and 36% to 48% by weight of ammonium nitrate, with the remainder being water. Preferably, the UAN contains a maximum of 35% by weight of water, more preferably 20-30% by weight. The UAN production method of the present invention preferably produces this type of UAN. Therefore, the water content of the UAN is preferably not too high. This is particularly advantageous for UAN used as a fertilizer.

[0013] This invention is based on the insightful observation that UAN is produced on an acidic aqueous ammonium nitrate stream, which is a used washing liquid, and its pH is controlled before evaporation and mixing with urea. Preferably, this is combined with a treatment step based on washing the vapor from the evaporation, and more preferably with a recycling step from the treated liquid.

[0014] The system disclosed herein can be used with and can be part of any apparatus capable of producing an acidic liquid stream containing ammonium nitrate (as a byproduct or waste stream). Specifically, this refers to any such stream generated by contacting ammonia-containing waste gas with nitric acid (NA) in a scrubber. Therefore, the acidic aqueous ammonium nitrate stream in question is a used scrubbing liquid, and this stream can also be referred to as "used scrubbing liquid." Given that the waste gas is a byproduct of another chemical reaction, it is understood that the acidic aqueous ammonium nitrate stream is typically a small waste stream. The present invention specifically seeks to provide a system designed to process such a small ammonium nitrate waste stream into UAN. Typically, and in exemplary embodiments of the invention, this small waste stream is in the range of 1-4 tons / hour, for example 2 tons / hour, containing 5-15% by weight of ammonium nitrate, typically 10% by weight, a small amount of nitric acid, typically 0.3-1.0% by weight, and the remainder being water. In one exemplary embodiment, in the case of a urea plant producing urea melt, the AN waste stream is typically less than 5% by weight of the total urea melt produced by the plant, more typically less than 3% by weight. In one embodiment of the method and system, the aqueous ammonium nitrate stream comprises 5-15 wt% ammonium nitrate, typically 10 wt%, a small amount of nitric acid, typically 0.3-1.0 wt%, the remainder being water, and at most 5 wt% or at most 1.0 wt% urea; the application of this typical composition is independent of flow rate. Low levels of urea (e.g., only trace amounts) are particularly suitable if dust washing is used upstream of acid washing.

[0015] This system can be integrated with chemical treatment units that generate a stream of aqueous ammonium nitrate waste, typically derived from an acidic ammonia scrubber, such as ammonia plants, ammonium nitrate plants, manure treatment units, composting units, waste treatment units, coke manufacturing units, or urea melting / granulation units. That is, the system can be built into a base unit that includes an acidic ammonia scrubber. The system can also be integrated into a pre-existing unit. More advantageously, the system can be provided as a standalone unit that can be connected to any unit seeking a useful application for the byproduct or waste stream generated from subjecting ammonia-containing waste gas to acidic scrubbing.

[0016] This versatility of the system of the present invention is ensured by the presence of a properly arranged pH control section. This pH control section is arranged such that the inlet of the aqueous ammonium nitrate is in fluid communication with the inlet of the pH control section. That is, regardless of the presence of any other sections, valves, pumps, or other types of equipment between the inlet of the aqueous ammonium nitrate and the system inlet, the pH control section is located downstream of the inlet and upstream of the evaporation section. The latter is fixed with reference to the pH control section, which has an outlet for the aqueous ammonium nitrate flow used for pH control, and this outlet is in fluid communication with the inlet of the evaporation section.

[0017] The pH control section is configured to determine and / or regulate the pH of the flow subject to such control. Determining the pH value may involve pH measurement or calculation based on process parameters. Depending on the source of the acidic aqueous ammonium nitrate flow, this calculation can be performed in advance or in situ, for example, by receiving input of relevant data related to the washing process via a data processing unit, i.e., the acidic aqueous ammonium nitrate flow (i.e.,...). Figure 1 The first wash solution used in the process (from the washing process) is used for the washing. Actual pH measurements of the acidic aqueous ammonium nitrate stream can be taken anywhere from inside the washer up to the pH control unit and including the area inside the pH control unit.

[0018] The pH control ensures that the controlled liquid downstream of the pH control section has a controlled pH within the desired range or at the desired value. It should be understood that pH control does not necessarily involve pH adjustment if the pH of the flow is already determined to be at the desired value. In the pH control section, the pH of the aqueous ammonium nitrate liquid is typically controlled within a weakly acidic to neutral range, typically between pH 2.0 and 7.0.

[0019] Preferably, to prevent the presence of potentially excessive ammonia, the pH is maintained at a slightly acidic level, preferably in the range of 2.0 to 4.5. More preferably, it is in the range of 2.5 to 3.5, for example, 3.0. This preference applies to both the system and the method. In this way, a high UAN yield is obtained, while low NH3 emissions are achieved from the pH-controlled downstream concentrate to the concentrated AN solution.

[0020] The system includes a concentration section, which is preferably an evaporation section, more preferably provided by a heater, such as a heat exchanger. For example, evaporation is used, which is heated by heat exchange with steam.

[0021] The concentration section is configured to evaporate the liquid ammonium nitrate stream from the pH control section to provide concentrated ammonium nitrate and water vapor, i.e., a separate vapor stream containing water vapor and entrained ammonium nitrate and nitric acid. The concentration section has an inlet for the aqueous ammonium nitrate for pH control, an outlet for the concentrated ammonium nitrate, and an outlet for the water vapor.

[0022] The system of the present invention includes a treatment section, which is optional for the method. The treatment section allows for the recovery and recycling of nitrogen compounds entrained in steam generated by evaporating and concentrating a pH-controlled stream of aqueous ammonium nitrate. This feature serves to prevent the undesirable release of nitrogen compounds (particularly those derived from nitric acid and ammonia entrainments) into the atmosphere. In one embodiment, the concentration section is in fluid communication with an outlet (e.g., a stack) for steam, where purified steam treated by the treatment section is discharged.

[0023] The vapor typically contains entrained ammonium nitrate (e.g., 0.05-0.2 wt% AN) and nitric acid (e.g., 0.05-0.2 wt% Na); the nitric acid is usually in a gaseous state (vapor).

[0024] In one embodiment, a treatment section is also useful, in which the treated water vapor is condensed, and at least a portion of the resulting condensate can be recirculated within the unit or otherwise transferred instead of being directly discharged. Advantageously, this recirculation can arrive at the treatment section as a treatment liquid used to wash away nitric acid and ammonium nitrate entrainments. It should be understood that, considering condensation and recirculation, the presence of nitrogen compounds in the vapor is generally undesirable.

[0025] The processing section is configured to wash the water vapor received from the concentration section (e.g., the evaporation section) with a processing liquid, also known as a washing liquid. Figure 1 and Figure 2 The first washing liquid in the process. Therefore, the treatment section has an inlet for the treatment liquid (washing liquid) and an outlet for the used treatment liquid. It is understood that the treatment liquid is typically an aqueous stream, such as water, provided as a condensate, such as a vapor condensate, and has a pH of at least 6, such as a pH of 6 to 8. The amount of the treatment liquid is, for example, at least 10% by weight of an acidic aqueous ammonium nitrate stream, in the range of 10-30% by weight.

[0026] The treatment section advantageously allows the recovered nitrogen (particularly ammonium nitrate and nitric acid) to be recycled back into the UAN production process. For this purpose, the treatment section preferably has an outlet for the used treated liquid, which is in fluid communication with the inlet of the concentration section. Therefore, it is preferable to add the recovered nitrogen as an aqueous used wash liquid to the pH-controlled ammonium nitrate stream upstream of the concentration section. The treatment section can be referred to as a scrubber, for example, a tray scrubber; other types of scrubbers are also possible.

[0027] The treatment section differs from and is separate from any acid scrubber used to wash the exhaust gas (waste gas) from the urea unit's finishing section. In a preferred embodiment, the urea unit's finishing section provides an acidic aqueous ammonium nitrate stream; and the composition of the gas stream is different. Furthermore, preferably, the treatment section uses a non-acidic treatment solution, preferably having a pH in the range of 6 to 8.

[0028] The processing section also has an outlet for purified steam. The system may include a condenser connected to receive the purified steam. The condenser includes an outlet for uncondensed gas and an outlet for condensate. The system may include a liquid flow line to supply at least partially the condensate to the inlet of the processing section.

[0029] As an advantage, the presence of a pH control section, particularly its proper arrangement upstream of the concentration section, provides a beneficial synergy with the presence of a treatment section, especially in the preferred embodiment, where used treated liquid from the treatment section is recycled back to the inlet of the concentration section. This recycling typically leads to acid buildup in one or both of the treatment and evaporation sections. By allowing pH control of the liquid to be evaporated upstream of the concentration section, the presence of acid at any given time can be offset by allowing the recycled treated liquid to be absorbed into a less acidic aqueous ammonium nitrate stream.

[0030] pH adjustment also advantageously avoids a decrease in the thermal stability of ammonium nitrate, thus preventing its easy decomposition at lower temperatures, which significantly increases the safety of the method. pH adjustment, especially increasing the pH to a less acidic level, provides the advantage of reducing the risk of AN decomposition during heating in the concentration section.

[0031] The advantage of the pH control step can be obtained in both embodiments where the used treatment liquid is supplied to the inlet of the concentration section and embodiments where the used treatment liquid is supplied directly to the UAN production section.

[0032] The used aqueous treatment solution, comprising water, ammonium nitrate, and nitric acid, and originating from wash water vapor in the treatment section (i.e., steam stream from the concentration section), is preferably combined with a pH-controlled ammonium nitrate stream, or with a concentrated ammonium nitrate stream, i.e., downstream of the pH-controlled section (for liquids). Preferably, the aqueous treatment solution is recycled back to the inlet of the concentration section (which is preferably the evaporation section).

[0033] The advantage of recycling the treatment liquid back to the inlet of the concentration section is that, with the favorable relatively low temperature in the evaporation section and the favorable relatively high amount of treatment liquid in the treatment section, a proper water balance for the preparation of UAN can be achieved, i.e., a sufficiently low water content in UAN.

[0034] Typically, in situations where pH control necessitates pH adjustment, this adjustment is beneficial in preventing the aforementioned acid accumulation, but it is not limited to this purpose. Therefore, such pH adjustment involves the addition of a base. Considering the production of UAN, the base is usually ammonia.

[0035] As is generally preferred, pH control includes adjusting the pH, particularly by adding a base, especially by adding ammonia to increase the pH. Adding ammonia as a pH adjuster provides the advantages discussed above regarding the thermal stability of ammonium nitrate, the advantages during the evaporation process, and helps the UAN solution to have a desired pH close to 7.

[0036] Therefore, in a preferred embodiment, pH control includes the addition of an alkali, more preferably ammonia, and even more preferably an aqueous ammonia solution. Preferably, the pH control step includes the introduction of an aqueous ammonia solution, preferably a dilute aqueous ammonia solution containing ammonia in an amount of 5% by weight or less. Preferably, a pH control section is configured for such a step. Preferably, the pH control section includes an inlet and a mixing zone for introducing such a solution (which may include a static mixer and / or an active mixer, and is, for example, a tie-in point). As described, the system of the present invention is preferably suitable for converting any byproduct or waste stream containing ammonium nitrate, particularly aqueous ammonium nitrate. This typically involves relatively small streams, or in any case, aqueous ammonium nitrate streams with relatively low ammonium nitrate concentrations. In an exemplary embodiment, the aqueous ammonium nitrate stream typically contains 5-20% by weight of ammonium nitrate, more typically 5-15% by weight of ammonium nitrate, and water, for example, at least 80% by weight of water. The aqueous ammonium nitrate stream typically also contains nitric acid, for example, 0.1-0.5% by weight of nitric acid relative to the total stream. The aqueous ammonium nitrate stream preferably contains less than 5% by weight of urea, more preferably less than 1.0% by weight of urea, for example, only trace amounts of urea. Trace amounts of urea in the exhaust gas may remain in the process, which involves dust scrubbing of the exhaust gas from urea finishing upstream of the acid scrubbing exhaust gas stream. The compositions of these examples are also applicable to the methods of the present invention.

[0037] Such a stream of aqueous ammonium nitrate can, for example, originate from the acid washing of an NH3-containing waste gas stream from the urea finishing section with nitric acid, in which cooling air is used to solidify the urea melt into a solid urea product (e.g., a granulator or granulation tower); other sources are also possible.

[0038] The nitrogen (especially ammonium nitrate and nitric acid) entrained in the water vapor generated by evaporating this small stream of aqueous ammonium nitrate (particularly by heating) will have a correspondingly low concentration in the water vapor. However, the resulting impact can be significant. Even a small increase in nitric acid content can have a substantial effect on pH, and continued processing of small amounts of nitric acid could lead to undesirable acid buildup. Furthermore, the safety implications are significant, considering the aforementioned safety hazards caused by the reduced decomposition temperature of ammonium nitrate. However, in absolute terms, the amount of nitric acid to be neutralized is low, at least relative to the total stream of aqueous ammonium nitrate, especially in the pH control section. This presents additional process challenges, as described below.

[0039] Therefore, the amount of ammonia added in the pH control section to adjust the pH of the aqueous ammonium nitrate stream is usually small. For example, 2.3 kg / h of ammonia (i.e., 2.3 kg / h NH3) needs to be added to a 2000 kg / h stream of 10% by weight aqueous ammonium nitrate solution containing 8 kg / h of nitric acid.

[0040] When the required amount of alkali is relatively low, the flow rate of alkali added during pH adjustment is too low to be regulated using standard industrial valves. While this can be addressed by using small-scale laboratory-type valves, this has drawbacks. For example, the valves need to be housed in specially designed cabinets to protect against harsh environments. Furthermore, the valves need to be positioned very close to the pipeline delivering the aqueous ammonium nitrate, which is more difficult to achieve in practice with non-industrial type valves (typically small valves). Therefore, in the system of this invention, it is desirable to use conventional industrial valves, such as valves at least 1 inch in size.

[0041] Preferably, the pH-control section is configured to introduce a dilute solution of ammonia solution, preferably containing up to 5% by weight of ammonia, more preferably 0.5-5% by weight of ammonia, and most preferably 0.5-2% by weight of ammonia, into the aqueous ammonium nitrate stream. This typically provides the advantage that the ammonia solution has a relatively large volume and can be more easily combined with, and particularly mixed with, the aqueous ammonium nitrate stream to ensure optimal and uniform neutralization of unreacted nitric acid in the ammonia solution, for example, using conventional industrial valves.

[0042] Referring to the pH control section described above, it should be understood that pH control is implemented, for example, as a feedback control system or a feedforward system. In a feedforward system, process data and mass balance are used to determine the pH value, and necessary adjustments are made based on the determined pH value. Then, preferably, a dilute ammonia solution is added downstream of the pH determination point. In a feedback system, the pH can be measured downstream of the pH control section, and the information is fed back to the control section to adjust the pH as needed. Here, the introduction of dilute ammonia is actually upstream of the pH determination point.

[0043] The system includes a production section configured to mix concentrated ammonium nitrate and liquid urea to produce UAN, namely the UAN production section. The UAN production section is connected, for example, to the acidic ammonia scrubber and pH control section of the chemical treatment unit, serving as the sole source of ammonium nitrate in the produced UAN.

[0044] On the other hand, the present invention provides a method for producing urea ammonium nitrate (UAN). This method is carried out, for example, but not limited to, the system of the present invention.

[0045] As the starting material for this method, an acidic aqueous ammonium nitrate stream is provided, which is generated by washing ammonia-containing waste gas (preferably containing air) with nitric acid. The acidic aqueous ammonium nitrate stream can also be referred to as the used washing liquid, and corresponds to... Figure 1 The first wash solution used in the process.

[0046] Alternatively, the method can be described as a method for converting an acidic aqueous ammonium nitrate stream into UAN, said acidic aqueous ammonium nitrate stream being obtained by washing ammonia-containing waste gas with nitric acid. The method preferably includes the step of washing the ammonia-containing waste gas with nitric acid; the method can also be described as a method of acid washing the ammonia-containing waste gas stream with a nitric acid washing solution, wherein the acidic ammonium nitrate solution obtained from said acid washing produces UAN.

[0047] In nitric acid scrubbing, the resulting purified exhaust gas and acidic aqueous ammonium nitrate stream are provided as separate streams. Therefore, the acid scrubber has an outlet for steam (purified exhaust gas) and a separate liquid outlet for the acidic aqueous ammonium nitrate stream. The acid scrubber is configured for gas / liquid separation, for example, by placing the gas outlet at the top and the liquid outlet at the bottom. Acid scrubbing is based on countercurrent contact between the gas and the liquid containing nitric acid, typically with the gas flowing upwards and the liquid flowing downwards, for example, with exhaust gas containing air and NH3.

[0048] This acidic aqueous ammonium nitrate stream is different from the used treatment liquid from the treatment section; these are two different streams with different compositions.

[0049] The method of the present invention includes obtaining an acidic aqueous ammonium nitrate stream as a used washing liquid, said used washing liquid being obtained by contacting ammonia-containing waste gas with nitric acid in a scrubber to form ammonium nitrate.

[0050] In one embodiment, the method includes providing ammonia-containing waste gas and scrubbing the waste gas with nitric acid in a scrubber, thereby forming an acidic aqueous ammonium nitrate stream and a purified gas stream, particularly in an acid scrubber. The acid scrubber operates, for example, at a pressure below 1.5 bar absolute pressure, such as at a slightly under-pressure (0.8-1.0 bar absolute pressure).

[0051] In one embodiment, the method includes solidifying urea melt in a finishing section (e.g., a granulator or granulation tower), using cooling air to provide solid urea product and a waste gas stream containing urea dust and NH3, and preferably supplying the waste gas stream as at least a portion of the waste gas treated in the scrubber after dust washing.

[0052] In a preferred embodiment, the method includes dust scrubbing the exhaust gas stream to remove at least a portion, for example, at least 90% by weight, of urea dust, and supplying the exhaust gas stream from the dust scrubber to the acid scrubber. Dust scrubbing uses, for example, a circulating urea solution and an aqueous replenishing scrubbing liquid. Therefore, preferably, the method includes dust scrubbing the exhaust gas stream in a dust scrubber to remove urea dust, and acid scrubbing the dust-scrubbed exhaust gas stream with nitric acid in a separate acid scrubber to obtain an acidic aqueous ammonium nitrate stream with a low urea content. The present invention incorporates a urea solution in the UAN production step. Therefore, compared to methods using combined dust and acid scrubbing, the risk of urea degradation by nitric acid is wisely reduced, wherein the scrubbing liquid used contains urea, ammonium salts, and unreacted acid.

[0053] The acidic aqueous ammonium nitrate stream comprises, for example, 5-50% by weight of ammonium nitrate, such as 5-20% by weight, preferably 5-15% by weight, and water, such as at least 80% by weight; preferably less than 5% by weight of urea, or even less than 1.0% by weight of urea, such as only trace amounts of urea. The acidic aqueous ammonium nitrate stream also contains nitric acid, and typically also contains nitric acid, for example, 0.1-0.5% by weight relative to the total stream. This composition is used, for example, at the inlet of a pH control step.

[0054] The pH of the acidic aqueous ammonium nitrate stream is, for example, less than 3.0, or less than 2, or less than 1.5. Acid washing at low pH can advantageously provide lower gaseous NH3 emissions from the exhaust stream.

[0055] For example, supplying an acidic aqueous ammonium nitrate stream at atmospheric pressure (1.0 bar absolute pressure).

[0056] It should be understood that the acidic aqueous ammonium nitrate stream contains ammonium nitrate, but it is not directly applicable to the production of UAN. For example, given the above-described example composition of the aqueous ammonium nitrate stream and the UAN product in terms of water content, and even assuming urea is supplied as urea melt to the UAN production section, at least in embodiments that produce UAN without adding additional AN, it is still necessary to concentrate the aqueous ammonium nitrate stream, i.e., reduce its water content. These embodiments are advantageous because a separate production section is not required, i.e., an AN neutralizer is not needed.

[0057] According to one or more requirements sought to be addressed by the present invention, the used washing liquid used as an acidic aqueous ammonium nitrate stream can come from any apparatus or process that includes, at some point, washing the ammonia-containing waste gas with nitric acid. As described, examples of the invention are ammonia plants, ammonium nitrate plants, manure treatment units, composting units, waste treatment plants, coke manufacturing plants, and urea production plants (e.g., urea melting / granulation plants), but this summary is not limiting. Urea production plants may include a urea finishing section; the urea finishing section is, for example, a granulator.

[0058] In a preferred embodiment of the system, the inlet of the acidic aqueous ammonium nitrate is in fluid communication, for example, via a liquid flow line, with the outlet of an aqueous ammonium nitrate waste stream (e.g., a liquid stream) of a chemical treatment unit, which includes an acidic ammonia scrubber that generates the aqueous ammonium nitrate waste stream. The acidic ammonia scrubber is configured to scrub the gas stream with a scrubbing liquid containing nitric acid and water.

[0059] In one embodiment, the system includes a chemical treatment unit, particularly an acidic ammonia scrubber. In an exemplary embodiment, the chemical treatment unit is a urea finishing section, such as a granulator or granulation tower for solidifying urea melt. For example, in the case of a granulator or granulation tower, a cooling gas (air) stream is used to solidify the urea melt into a solid urea product, generating an exhaust gas stream containing gaseous NH3 and urea dust; the NH3 originates at least partially from a side reaction in the finishing step. The urea dust is removed from the exhaust gas stream, for example, in a dust scrubbing step, and the gas from the dust scrubbing step is sent to an acid scrubbing step using dilute nitric acid as the scrubbing liquid. The acid scrubbing typically also uses an aqueous makeup liquid. This acid scrubbing step produces an AN solution with unreacted nitric acid, for example, having a low pH and a relatively low AN content, which is an acidic aqueous ammonium nitrate stream. The method of the present invention may include a corresponding step of providing an acidic aqueous ammonium nitrate stream from such a chemical treatment unit.

[0060] In one embodiment, the system includes a chemical processing unit comprising a urea production unit and a finishing section, wherein the finishing section includes an acidic ammonia scrubber.

[0061] In the method of the present invention, the acidic aqueous ammonium nitrate stream is preferably concentrated by evaporating water from a heated solution. The step of concentrating such an acidic stream will inevitably introduce corrosiveness, especially if concentration is carried out by heating and evaporation, which may be harmful to the apparatus (e.g., equipment) or require investment in expensive corrosion-resistant equipment. Furthermore, as previously mentioned, the low pH of the solution accelerates the potential thermal decomposition of ammonium nitrate, posing safety concerns.

[0062] The method of the present invention includes pH control of an acidic aqueous ammonium nitrate stream, said pH control including determining the pH and adjusting the pH to a desired degree within the range of 2 to 4.5 to provide a pH-controlled liquid.

[0063] A moderately acidic pH in the liquid is beneficial in reducing or preventing the emission of NH3 vapors during the concentration step.

[0064] In a preferred embodiment, pH control is pH adjustment, particularly pH increase, and includes adding an ammonia solution to the acidic ammonium nitrate stream to increase the pH, thereby obtaining a pH-adjusted liquid as the pH-controlled liquid. For example, in the case of pH changes in the acidic aqueous ammonium nitrate stream, the pH increase step can be applied intermittently or intermittently. pH adjustment can also be performed continuously.

[0065] For example, based on the nitric acid content before pH adjustment, the pH of an acidic aqueous ammonium nitrate stream can be in the range of, for example, 0.5 to 1.8, and after pH adjustment, for example, above 2.0.

[0066] The method involves evaporating the pH-controlled liquid to obtain a concentrated ammonium nitrate solution; preferably, evaporation is performed by heating the solution. The concentrated solution typically contains at least 60% by weight of ammonium nitrate, preferably 65 to 85% by weight, more preferably 70 to 80% by weight; and typically contains up to 40% by weight of water, or up to 20% by weight of water. Evaporation is carried out, for example, in the concentration section of the system. Evaporation is accomplished, for example, by heating with steam in a heat exchanger, preferably low-pressure steam at 3-5 bar. The use of low-pressure steam is advantageous for process efficiency and safety reasons regarding the thermal stability of the ammonium nitrate (AN).

[0067] In one exemplary embodiment, the ammonium nitrate concentration of the ammonium nitrate stream is increased from an initial concentration of 5-20% by weight to a concentration of 50-90% by weight, preferably 65-85% by weight, more preferably 70-80% by weight, after evaporation by removing water, preferably by heating evaporation.

[0068] Therefore, in the actual production of UAN, the method involves combining the concentrated ammonium nitrate solution with a urea-containing liquid stream, preferably an aqueous urea solution and / or a urea melt, to obtain a urea ammonium nitrate solution (UAN). Advantageously, by combining the concentrated AN solution with a liquid stream having a sufficiently high urea content, the resulting AN solution can have the desired low water content.

[0069] Generally, the pH range of the formed UAN is 6.5-7.2 (measured at 20ºC). A pH of 6.5 to 7.2 is typical for UAN-32, and this pH range is beneficial for the storage of the UAN solution. Therefore, as generally preferred, when the pH control step is to obtain a pH control liquid with a pH of 2-4.5 and then concentrate it, the UAN production step includes further raising the pH to at least 6.5, more preferably to 6.5-7.2. For example, the pH can be raised by adding a separate ammonia-containing stream (e.g., an aqueous ammonia solution) or by using an ammonia-containing liquid stream containing urea.

[0070] Furthermore, preferably, the liquid stream containing urea contains some NH3, for example, through the evaporation section from the urea unit, which can facilitate complete neutralization and concentration of AN (e.g., when the pH control step is pH 2-4.5) and facilitate the production of UAN products with the desired pH (e.g., 6.5-7.2) at 20ºC.

[0071] Preferably, at least 90% by weight of the ammonium nitrate in the UAN originates from an acidic aqueous ammonium nitrate stream; more preferably, at least 90% by weight of the ammonium nitrate in the UAN originates from a pH-controlled liquid; more preferably, all or substantially all of the ammonium nitrate originates from said stream. Therefore, it is advantageous that no separate ammonium nitrate (AN) needs to be added, thus eliminating the need for a separate AN neutralizer. Preferably, at least 90% of the urea in the UAN, preferably all, is added downstream of the evaporation step (or concentration section), for example, in the UAN production step. This provides the advantage that the urea is not affected by corrosive conditions in the AN concentration step (low pH, high temperature), where urea may be degraded and biuret formation may occur.

[0072] Urea solutions and / or urea melts used in UAN production typically originate from urea production processes carried out in urea production units. Urea is usually produced in this process from ammonia and carbon dioxide. It can be prepared by introducing excess ammonia along with carbon dioxide into the urea synthesis section of the urea production unit at a pressure of 12-40 MPa and a temperature of 150-250°C. Typical urea production units also include a recovery section and a finishing section. In the recovery section, unconverted ammonia and carbon dioxide are recovered and recycled back to the synthesis section. The recovery section is typically followed by an evaporation section. Here, the urea concentration is further increased by the evaporation of water, resulting in a highly concentrated solution, commonly referred to as urea melt. In the finishing section, the urea melt is typically shaped into the desired solid particulate form, usually involving techniques such as granulation, pelletizing, or pelletizing.

[0073] This solid urea can be melted and / or diluted to obtain the desired urea melt or solution. Preferably, the system of the present invention is connected to a urea production unit such that the inlet of liquid urea is in fluid communication with the outlet of liquid urea from the urea unit. This can be from different stages, producing different urea concentrations.

[0074] In a preferred embodiment of the system and method, a first portion of the urea melt formed in the urea production unit is used to produce solid urea product, and a second portion is used to produce the UAN, wherein the UAN production uses AN, and at least a portion (e.g., at least 90% by weight) of the AN originates from the ammonia-containing waste gas from urea production by acid washing with nitric acid, for example, from the acid washing waste gas stream generated in the finishing section using cooling air (e.g., a granulator or granulation tower) when the first portion of the urea melt is solidified into the solid urea product.

[0075] In one exemplary embodiment, the system or UAN production step receives urea-containing liquid from the evaporation section of the urea plant. In this method, a concentrated ammonium nitrate solution is combined with an aqueous urea solution and / or urea melt; the aqueous urea solution and urea melt may, for example, originate from the evaporation section.

[0076] The UAN production unit is typically located at a distance from the urea melting device, for example, at a distance from its evaporation section. In a preferred embodiment of the method, a portion of the urea melt containing at least 90% by weight urea (including biuret) or at least 95% by weight urea is diluted by adding an aqueous stream to form a diluted urea liquid stream, which is then combined with a concentrated AN solution to prepare UAN. Preferably, the diluted urea liquid stream is conveyed to the UAN production unit, for example, via a delivery line to UAN production for at least 10 m, or at least 50 m. Preferably, the diluted urea liquid stream has a concentration of 60-85% by weight urea (including biuret), more preferably 70-85% by weight urea, or even 75-85% by weight, particularly during delivery to UAN production. Preferably, the stream is conveyed to the UAN production unit at a temperature below 135ºC or below 130ºC. The relatively low temperature reduces the formation of undesirable biuret in the delivery line.

[0077] It should be noted that the diluted urea liquid stream should have a sufficiently high concentration to allow the production of UAN with the target urea concentration when mixed with a concentrated ammonium nitrate solution, preferably when combined only with a concentrated ammonium nitrate solution. Furthermore, it is desirable that the evaporation step in the AN concentration section be carried out at a moderate temperature. Therefore, the diluted urea liquid stream preferably contains at least 70% by weight or at least 75% by weight urea. Additionally, the diluted urea liquid stream originating from the evaporation section typically contains some NH3, which is beneficial for optimizing the pH of the UAN product.

[0078] Typically, a urea production unit includes a recovery section and a downstream evaporation section from which a urea solution is obtained. This urea solution, for example, has 70-90% by weight urea and 10-30% by weight water. The solution is concentrated in the evaporation section. This concentration is preferably achieved to the extent that a highly concentrated urea solution is formed (e.g., having more than 90% by weight urea, such as more than 95% by weight urea, such as more than 99% by weight urea). For highly concentrated urea solutions, the term "urea melt" is used in the art. Typically, the urea solution is concentrated in the evaporation section to a urea melt with a final water content of 0.03-5.0% by weight.

[0079] In a preferred embodiment of the method, the step of obtaining an acidic aqueous ammonium nitrate stream includes: Urea melt is produced in a urea production unit that includes a synthesis section, a recovery section, an evaporation section, a condensation section, and a wastewater treatment section, particularly through: In the synthesis section, CO2 and NH3 react to form a urea solution; The urea solution from the synthesis section is processed in the recovery section to increase the urea content and remove at least a portion of the unreacted ammonia and CO2 from the urea solution; The urea solution from the recovery section is concentrated by evaporating water in the evaporation section to obtain urea melt containing urea and ammonia and water vapor. The water vapor is condensed in the condensation section to obtain a process condensate containing water, urea, and ammonia.

[0080] In this embodiment, the method further includes: Preferably, the first part of the urea melt is solidified in a finishing section (e.g., a granulator or granulation tower) that uses cooling air to obtain a solid urea product and a waste gas stream containing urea and NH3 as waste gas. Preferably, urea dust is removed from the exhaust gas via dust scrubbing upstream of the acid washing process. In an acid scrubber, nitric acid is used to scrub the exhaust gas from the urea production unit, preferably at least partly from the finishing section, and preferably after dust scrubbing of the exhaust gas, to produce an acidic ammonium nitrate stream and purified exhaust gas.

[0081] Preferably, the method includes: providing a second portion of urea melt to UAN production.

[0082] In a preferred embodiment, acid washing is used for both exhaust gases from the finishing section after dust washing and exhaust gases from urea production units (e.g., from the synthesis and recovery sections).

[0083] Preferably, the method includes: in the wastewater treatment (WWT) section, treating the process condensate by supplying at least a portion of the process condensate through a first desorption tower, a hydrolysis tower, and a second desorption tower, wherein the units are connected in series in the aforementioned order via liquid flow lines.

[0084] In a preferred embodiment of the system, the system includes these units and connectors corresponding to the steps described above.

[0085] As a general preference for this process and system, the ammonia solution used in the pH control (pH adjustment) step of the process is an untreated process condensate stream, or a partially treated process condensate stream from the wastewater treatment section of the urea plant, or a combination thereof, for example, comprising 0.1-1.0 wt% ammonia, 0.5-1.5 wt% urea, up to 0.1 wt% CO2, and at least 90% water, preferably the remainder being water. This provides various advantages, including easier infeeding in the pH control section, and lower loading of the WWT, and more particularly lower loading of the hydrolysis tower, which is an energy-intensive unit using (medium-pressure) steam.

[0086] Preferably, in the embodiment of the process having a step of treating process condensate in the wastewater treatment (WWT) stage, the process includes: Ammonia solution is discharged from one or more liquid flow lines selected from the group consisting of: liquid flow lines from the condensation section to the first desorption tower, liquid flow lines from the first desorption tower to the hydrolysis tower, and flow lines from the hydrolysis tower to the desorption tower, wherein the ammonia solution is untreated or partially treated process condensate; preferably at least partially or entirely from the liquid flow line from the first desorption tower to the hydrolysis tower; and The ammonia solution is used in the pH control step, particularly in the pH adjustment step; wherein the ammonia solution preferably contains 0.1-1.0% by weight of ammonia, 0.5-1.5% by weight of urea, up to 0.1% by weight of CO2 and at least 90% by weight of water.

[0087] Here, the hydrolysis tower is used to hydrolyze urea to very low levels (ppm) using fresh steam, typically medium-pressure steam (15 to 25 bar).

[0088] The system preferably includes an ammonia solution supply line for the ammonia solution from the tap point to the pH control section, wherein the tap point is located in a flow line selected from the group consisting of: a liquid flow line from the condensation section to the first desorption tower, a liquid flow line from the first desorption tower to the hydrolysis tower, and a flow line from the hydrolysis tower to the desorption tower. At the tap point, a portion of the ammonia solution in the flow line is drawn into the ammonia solution supply line.

[0089] It should be understood that in the method of the present invention, the evaporation of the pH-controlled liquid (preferably by heating) will result in the formation of water vapor, such as a vapor stream containing water vapor and entrained ammonium nitrate.

[0090] In a preferred embodiment of the method, these vapors are processed as described above with reference to the system of the present invention. Therefore, the processing section of this system can be used in this preferred embodiment of the method.

[0091] Therefore, a preferred embodiment of the method includes subjecting water vapor generated by the evaporation of a pH-controlled liquid to washing, particularly with water. More preferably, in this preferred embodiment, the method is carried out in a system substantially as described above and / or has the characteristics described in the combined treatment section.

[0092] The present invention also provides a method for improving an existing chemical treatment unit having an outlet for an acidic aqueous ammonium nitrate waste stream. The method includes providing a system for producing urea ammonium nitrate (UAN) as discussed above, and connecting the outlet of the chemical treatment unit such that the outlet for the acidic aqueous ammonium nitrate waste stream is connected to the inlet of the acidic aqueous ammonium nitrate in the system for producing UAN. Thus, in the improved apparatus, the acidic aqueous ammonium nitrate stream is supplied from the outlet to the system, particularly to the pH control unit. Existing chemical treatment units preferably include an acid scrubber as described above in conjunction with the system.

[0093] In summary, a system and method for producing urea ammonium nitrate (UAN) are disclosed, particularly suitable for processing smaller streams of ammonium nitrate waste into UAN. The system includes an ammonium nitrate concentration section, a treatment section allowing the recovery and recycling of nitrogen compounds entrained in the steam from the concentration section, and a pH control section allowing the pH of the treated ammonium nitrate waste stream to be adjusted to a desired level.

[0094] The invention is illustrated below with reference to the accompanying drawings and the following non-limiting discussion.

[0095] Figure 1 This is a general schematic diagram of a system for producing urea ammonium nitrate (UAN) according to the present invention. The system can be added to any device or pipeline having an outlet for acidic aqueous ammonium nitrate, particularly in the form of used acidic washing liquid (also known as used first washing liquid). The system (1) includes an inlet (101) for aqueous ammonium nitrate (121), an inlet (102) for liquid urea (122), and an outlet (103) for UAN (123). The aqueous ammonium nitrate (121) typically also contains nitric acid. The acidic aqueous ammonium nitrate stream (121) is, for example, used first washing liquid.

[0096] The system includes a concentration section (104) configured to evaporate aqueous ammonium nitrate. The system is configured such that the concentrated ammonium nitrate (125) produced by the evaporation can be directed to a UAN production section (105) configured to mix the concentrated ammonium nitrate and liquid urea to produce UAN.

[0097] The concentration section has a gas outlet (106) for water vapor (126), for example for a vapor stream (gas stream) containing water vapor. The vapor stream may also contain nitric acid and entrained ammonium nitrate. The gas outlet is in fluid communication with the treatment section (107). The treatment section is configured to scrub the water vapor received from the concentration section (126) to produce a purified vapor stream (127). For this purpose, the treatment section has an inlet (108) for the treated liquid (128) and an outlet (109) for the used treated liquid (129). The latter outlet (109) is preferably in fluid communication with the inlet (110) of the concentration section (104), particularly via a liquid flow line.

[0098] In another embodiment, the outlet (109) of the used treatment liquid (129) is in fluid communication with the inlet of the UAN production section (105), particularly via a liquid flow line. For example, the used treatment liquid (129) is supplied directly to the UAN production section (105). However, the advantage of supplying the used treatment liquid to the inlet of the concentration section is that the concentration section needs to evaporate water to a smaller extent, i.e., at the outlet of the concentration section, the ammonium nitrate can contain more water (by weight), which is advantageous (energy efficiency, safety). Furthermore, the amount of treatment liquid (128) can be relatively large compared to the concentrated ammonium nitrate (125). In one embodiment, the amount of water in the treatment liquid (128) and the used treatment liquid (129) is approximately 10 times the amount of water in the concentrated ammonium nitrate (125). Adding such a large amount of water downstream of the concentration section would disrupt the water balance of UAN production.

[0099] Alternatively, a portion of the used treatment fluid (129) can be supplied to the concentration section (104), and another portion can be directly supplied to the UAN production section (105). The used treatment fluid (129) corresponds to Figure 2 The second wash solution used in the middle (214).

[0100] According to the invention, the system includes a pH control section (111) such that an inlet for aqueous ammonium nitrate is in fluid communication with an inlet (112) of the pH control section. The pH control section is configured to perform pH control on the aqueous ammonium nitrate. This control is preferably a feedforward or feedback system for determining the pH of the aqueous ammonium nitrate that has entered the section through the inlet (112) and for adjusting the pH as needed to provide a pH-controlled flow of aqueous ammonium nitrate (124). The pH control section has an outlet (113) for the pH-controlled flow of aqueous ammonium nitrate, which is in fluid communication with an inlet (110) of a concentration section. The pH control section (111) preferably has another inlet for an ammonia solution and a region for mixing or combining the flows, for example by active or static mixing.

[0101] In a preferred embodiment, two liquid streams are fed to a concentration section: used treated liquid (129) from the treatment section and a pH-controlled aqueous ammonium nitrate stream (124). These liquid streams enter the concentration section (104) for example through two different inlets. Although this is in Figure 1 It is not shown, but it should be understood that, in the case where the two liquid flows involved do not merge until within the concentration section, this is still based on the preferred feature of fluid communication between the inlet (110) of the concentration section (through which the pH-controlled aqueous ammonium nitrate flow enters the concentration section) and the outlet (109) for the used treated liquid from the treatment section, since the flows will merge within the concentration section, thus implying fluid communication between the inlet of either fluid entering the concentration section and the corresponding outlet from the respective sections upstream of the concentration section (i.e., the pH control section for the pH-controlled aqueous ammonium nitrate flow and the treatment section for the used treated liquid from there).

[0102] In another implementation scheme ( Figure 1 (Not shown in the diagram), the outlet (109) of the used processed liquid is directly connected to a liquid flow line leading to the inlet of the UAN production section (105), thus bypassing the concentration section (104). The processing unit (107) is optional for this method.

[0103] In the method of the present invention, the processing unit (107) is optional.

[0104] Figure 2A process flow diagram of a non-limiting embodiment of a UAN apparatus consistent with one or more embodiments of this disclosure is shown. It illustrates an exemplary embodiment of a system of the present invention for producing urea ammonium nitrate. The UAN apparatus (i.e., the system) can receive an acidic aqueous ammonium nitrate stream, particularly in the form of a used first wash liquid stream (205), and process the used first wash liquid stream (205) into a UAN product stream (211). In this embodiment, the used first wash liquid stream (205) originates from an acidic ammonia scrubber in a urea melt and granulation unit, wherein an ammonia-containing gas stream (exhaust gas or waste gas stream containing NH3) is scrubbed with a first wash liquid containing nitric acid.

[0105] The used first wash stream (205) has a flow rate of approximately 2000 kg / h and a composition of 10% by weight ammonium nitrate, 0.5% by weight nitric acid, and a balanced amount of water. The UAN device (226) includes a pH control section (228), which may include a tie-in point at which ammonia needs to be added to the used first wash stream (205) to adjust (i.e. increase) its pH to approximately 3.0. For this purpose, as a non-limiting example, a small amount (2.3 kg / h) of NH3 is injected via a line (206a) through a valve (231) into the used first wash stream (205) (aqueous ammonium nitrate stream) to obtain a pH-controlled (in this embodiment, pH-adjusted) used first wash stream (207) (i.e., a pH-controlled aqueous ammonium nitrate stream) with a temperature of approximately 37°C. If pure ammonia in gaseous or liquid form is to be injected into the used first wash stream (205), then the valve (231) should be very small, i.e., a laboratory-scale valve. In this case, the valve (231) must be placed in a specially designed cabinet to prevent the effects of harsh environments. Furthermore, the valve typically needs to be positioned very close to the pipeline transporting the used first wash stream (205), which introduces complexity in the case of a laboratory-scale valve. To avoid this complexity, and to generally allow the use of conventional industrial valves, the ammonia solution can be injected into the used first wash stream (205) via pipeline (206a) through valve (231), where the ammonia solution may have an ammonia content of 0.5% by weight, allowing the required 2.3 kg / h NH3 to be supplied at a higher flow rate. Therefore, valve (231) can be a normal-sized industrial valve.

[0106] In this embodiment (e.g.) Figure 4As shown in more detail below), and as generally preferred, the ammonia solution used for pH control (pH adjustment) is a condensate stream from a portion of the wastewater treatment section of the urea plant, for example, composed of 0.1-1.0 wt% ammonia, 0.5-1.5 wt% urea, and a maximum of 0.1 wt% CO2, with the remainder being water. A non-limiting exemplary composition is 0.5 wt% ammonia, 0.8 wt% urea, 0.09 wt% CO2, and a balance of water. Utilizing a stream with this composition, instead of injecting a very small amount of pure ammonia, eliminates the need to design specialized equipment such as laboratory-scale valves. Specifically, when using an ammonia solution with an ammonia content of 0.5 wt% by injecting 460 kg / h of this ammonia solution, the required amount of NH3 for pH adjustment of 2.3 kg / h is met. This means that the flow rate processed by valve (231) is 460 kg / h, instead of 2.3 kg / h. This advantage is achieved by using an ammonia solution containing 0.1–1.0 wt% ammonia, 0.5–1.5 wt% urea, and up to 0.1 wt% CO2, with the remainder being water. This ammonia solution is very well provided by the condensate stream from the wastewater treatment section of the urea plant.

[0107] The used first wash liquid stream (207), after pH adjustment, is then sent to a concentration section including an evaporator (230). In this example, the evaporator (230) is a vertical single-pass (VOP) type evaporator, which can operate under approximately atmospheric conditions or slightly under vacuum, and is heated to a temperature of approximately 128°C by pressure steam at 3.5 bar. Therefore, the evaporator is a heat exchanger. The effluent from the evaporator (230) is flash-distilled into a first vapor stream (209) and a concentrated ammonium nitrate solution stream (208). As a non-limiting example, the concentrated ammonium nitrate solution stream (208) has a concentration of 76% by weight and a temperature of 128°C. ° The temperature is ℃ and the pH is 2.0. The concentrated ammonium nitrate solution stream is sent to the UAN production section (232). The UAN production section includes a UAN mixing tank (236), a urea stream (210a), particularly a diluted urea melt stream (210a), which is mixed with the concentrated ammonium nitrate solution stream (208) in the mixing tank, and a circulating cooler (238) downstream of and in fluid communication with the mixing tank, wherein the resulting UAN is cooled to 40℃. The urea stream (210a) is preferably obtained by combining the urea melt (210) with a water stream (242) (e.g., process condensate), as described below.

[0108] The first vapor stream (209) contains 220 ppm HNO3 and 210 ppm AN (molar ppm), which are considered undesirable entrainments and must be separated from the first vapor stream (209) before it is discharged or condensed and reused.

[0109] As a non-limiting example, the processing unit (234) has a diameter of 1m and a diameter of 0.5m. 3 / m 2 A tray scrubbing column with a specific liquid loading of h, wherein a condensate stream (213), essentially pure water, is used as the scrubbing liquid (second scrubbing liquid), which contacts the first vapor stream (209) on the tray of the treatment unit (234). Based on the tray diameter and specific liquid loading, the condensate (213) is fed to the top of the treatment unit (234) at a flow rate of approximately 400 kg / h, and during its downward flow, entrained HNO3 and AN are washed away from the first vapor stream (209), thereby obtaining a purified first vapor stream (212) and a used treatment liquid (referred to herein as used second scrubbing liquid (214)). Here, the used second scrubbing liquid (214) contains entrainments of nitric acid and ammonium nitrate and has a temperature of approximately 100°C. In the illustrated embodiment, the used second scrubbing liquid (214) is recycled back to the inlet of the evaporator (230) and mixed upstream of the evaporator (230) with the aforementioned pH-adjusted first scrubbing liquid (207). This will adjust the temperature of the first wash solution from 37°C. ° ℃ rises to 55 ° ℃, thereby recovering heat.

[0110] In this embodiment ( Figure 2 In this process, a urea melt stream (210) with a concentration of 96% by weight and a temperature of 135°C is drawn from the first-stage evaporator in the urea melt unit and then diluted to a concentration of 79% by weight by adding process condensate to it via a process condensate stream (242). The diluted urea melt stream (210a) is then sent to the UAN production section (232) to be mixed with a concentrated ammonium nitrate stream (208). In this embodiment, the urea melt pipeline transporting the urea melt from the urea melt unit to the UAN unit is a 100-meter-long pipeline with a small diameter of 1 inch (2.5 cm). If urea melts at high temperatures and the concentrate is transported in such a long pipeline, there is a high chance of biuret formation. Furthermore, the urea melt at 135°C... ° The crystallization process is carried out at ℃, while at a concentration of 96 wt%, crystallization occurs at 130℃, thus presenting a high chance of crystallization over such a long production line. By adding water, for example as a process condensate, the urea melt is diluted to 79 wt%, and its temperature is also lowered, thereby eliminating the risk of crystallization. The added process condensate is, for example, process condensate taken from the condensation section of the urea plant, i.e., before being treated in the WWT; this untreated condensate may advantageously contain some NH3.

[0111] Figure 3 based on Figure 2The flowchart, in one embodiment, includes the following additional elements: The purified first vapor stream (312) is sent to the condensation unit (313), and at least a portion of the resulting condensate can be recycled back to the processing unit (234) as washing liquid. For example, at least a portion of the condensate is not added to the used first washing liquid stream in the flow line from the acid scrubber; thus, the used first washing liquid is concentrated in the concentration section.

[0112] Figure 4 A process flow diagram is shown for a system according to the invention for producing urea ammonium nitrate (UAN), wherein the process is integrated with a urea production process. The corresponding system includes a system (439) according to the invention for producing urea ammonium nitrate (UAN). [The last sentence appears to be incomplete and possibly refers to a different system.] Figure 1 Similar to the system described above, the system includes: (401) Inlet of aqueous ammonium nitrate; (402) Inlet of liquid urea; (403) Exports of UAN; (404) Concentration section; (405) UAN production section; (406) Gas outlet of water vapor (106); (407) Configured as a treatment section for washing water vapor; (408) Inlet of the treatment fluid; (409) The outlet of the used treatment fluid; (410) The inlet of the concentration section; (411) pH control section; (412) Inlet of pH control section; (413) Inlet for a dilute ammonia solution used for pH control.

[0113] In a non-limiting example of the method, the first waste gas stream (414) is an ammonia-containing waste gas stream generated from the production of urea solution in the urea production unit (435); the second waste gas stream (415) is an ammonia-containing waste gas generated from the solidification of urea solution in the finishing section (437). The first and second waste gas streams can also be used separately.

[0114] Ammonia in the ammonia-containing waste gas stream is reacted with nitric acid in an acidic ammonia scrubber (433) to form ammonium nitrate, which remains dissolved in water (AN solution), while residual nitric acid is also present. For example, as one embodiment of the system of the invention, the acidic ammonia scrubber (433) typically includes, for example, a packed bed or tray, and ammonia can react with nitric acid on the packed bed of the acidic ammonia scrubber. The AN solution is discharged from the acidic ammonia scrubber, generating a used first scrubbing liquid stream, namely an acidic aqueous ammonium nitrate stream (418) also containing nitric acid. For example, as one embodiment of the system of the invention, a scrubbing unit with two different compartments can be used, wherein dust is removed from the waste gas stream in one compartment, and then the dust-washed waste gas is sent to another compartment, which is constructed similarly to the acidic ammonia scrubber (433), wherein the ammonia content is reduced by using a nitric acid solution. For example, a typical washing unit in a urea production / curing plant may include a lower dust washing compartment and an upper acid washing compartment; in the lower compartment (dust scrubber), a urea solution of, for example, about 45% by weight is circulated to remove urea dust from the exhaust gas. After leaving the lower compartment, the exhaust gas is introduced into the upper compartment, where the ammonia content is reduced by passing a nitric acid solution. Ammonia reacts with nitric acid to produce ammonium nitrate. The ammonium nitrate solution is circulated on the ammonia scrubber before being sent to the boundary area. Circulation is maintained by an ammonia scrubber circulation pump. Fresh nitric acid (416) is provided to maintain the acidity of the circulating solution. In one embodiment, the used first washing liquid (418) may have a flow rate of about 2000 kg / h and may contain 5-20% by weight of ammonium nitrate, 0.1-2% by weight of nitric acid, 0.1-1.0% by weight of urea, and a measured amount of water. Based on its nitric acid content, the pH of the used first washing liquid (418) may be in the range of 0.5 to 1.8.

[0115] The aqueous ammonium nitrate stream, i.e., the used first washing liquid (418), which is typically a small waste stream, is sent to the UAN unit (439) (or UAN production section), i.e., the system according to the invention, which is designed to process the small ammonium nitrate waste stream into UAN. In the UAN unit (439), the pH of the aqueous ammonium nitrate stream (418) is first adjusted by adding an ammonia solution (419a) to the aqueous ammonium nitrate stream (418) in a pH control section, particularly a pH adjustment section (411). The ammonia solution (419a) may comprise, for example, an aqueous ammonia solution with a concentration in the range of 0.5 to 5% by weight. The pH adjustment section (PAS) (411) may include, for example, a static mixer, in which the ammonia solution (419a) can be mixed with the aqueous ammonium nitrate stream (418). Alternatively, the pH adjustment section (411) may be a connection point for the injection of the ammonia solution (419a) into the aqueous ammonium nitrate stream (418). The ammonia solution (419a) is, for example, an ammonia-containing stream with an ammonia concentration of, for example, up to 10% by weight. For example, untreated condensate (431) from the urea production unit (435) or partially treated condensate streams (419, 453) from the wastewater treatment section (449) can be used, as described below. In another embodiment, the ammonia solution (419a) can be a dilute urea solution with an ammonia content of up to 10% by weight.

[0116] The pH of the aqueous ammonium nitrate stream (418) can be adjusted (i.e. increased) to a slightly acidic range in the pH adjustment section (411) to obtain a pH-adjusted aqueous ammonium nitrate stream (420). The pH of this stream is typically in the range of 2.0 to 4.5. The flow rate ratio of the ammonia solution (419a) to the aqueous ammonium nitrate stream (418) can be controlled in a feedforward control loop.

[0117] The pH-adjusted aqueous ammonium nitrate stream (420) is evaporated in a concentration section (404) to obtain a concentrated ammonium nitrate solution (421) and a first vapor stream (422). Considering safety concerns associated with higher concentrations of ammonium nitrate solution, the concentrated ammonium nitrate solution (421) preferably has a concentration of up to 75% by weight or up to 80% by weight.

[0118] The concentration section (404) typically includes an evaporator. This can be a vertical single-pass (VOP) evaporator and can operate under approximately atmospheric conditions or slightly under vacuum and at a temperature of approximately 125-140°C. The heating medium can be low-pressure saturated steam with a pressure of approximately 3.5 bar.

[0119] The concentrated ammonium nitrate solution (421) with a concentration of 75-80% by weight is then combined with an aqueous urea solution and / or urea melt (423a) in the UAN production section (405) to obtain a urea ammonium nitrate solution (424). For example, the concentrated ammonium nitrate solution (421) can be combined with a urea solution or a diluted urea melt stream (423a) in the UAN production section (405). As an embodiment of a general implementation of the invention, such a diluted urea melt stream (423a) can be obtained by diluting the urea melt stream (423) from the evaporation stage in the urea production unit (435). For example, the urea melt stream from the evaporation stage of the urea production unit (435), for example, a urea melt stream (423) with a concentration of 90-96% by weight, is diluted to about 79% by weight by adding water, for example as process condensate (455), to the urea melt stream (423) upstream of the UN production section (405); particularly at a location near the evaporation section of the urea production unit and at a relatively far distance from the UAN production section, for example at least 50 m. Figure 4 (This is a schematic diagram). This dilution can be performed to reduce residence time, minimize biuret formation, and reduce the chance of crystallization within the melt pipeline.

[0120] The diluted urea melt stream (423a) is obtained, for example, by adding water to a urea melt stream with a urea concentration in the range of 70-98.5% by weight. The diluted urea melt stream (423a) preferably contains 70-85% by weight of urea.

[0121] In addition to the synthesis and recovery sections, the urea production unit (435) also includes an evaporation section. The evaporation section may, for example, include two evaporation stages. In this case, the urea melt stream (423) may originate from the first-stage evaporator, such that the urea melt concentration is at most between 90% and 96% by weight. In practice, the aforementioned process condensate (455) is added to the urea melt stream (423) near the urea melt pump. The melt pipeline between the urea production unit (435) and the UAN production section (405) may be, for example, a long pipeline approximately 100 meters long, thus posing a risk of biuret formation within the melt pipeline. By adding the process condensate (455), the temperature is lowered, thereby minimizing biuret formation.

[0122] The urea ammonium nitrate solution (424) can be further cooled to, for example, 40°C. °C, and then pumped to the boundary area via UAN output pump. Generally, as one embodiment of the invention, the desired UAN grade can be obtained by maintaining the AN / urea ratio in the mixing tank of the UAN production section (405) at a given value calculated based on the UAN grade. For example, in the case of UAN-32, the AN / urea ratio in the mixing tank of the UAN production section (435) can be maintained at about 1.33 (mass ratio), and the condensate is adjusted to have 20% by weight water in the final urea ammonium nitrate solution (424). However, other UAN grades, such as UAN-28 or UAN-30, can also be produced by adjusting the AN / urea ratio and the cooled steam condensate.

[0123] The first vapor stream (422) from the evaporator (404) is processed in the processing unit (407) to obtain a purified first vapor stream (425) by separating entrained ammonium nitrate and nitric acid from the first vapor stream (422), the first vapor stream (422) containing a certain amount of HNO3 entrainment and 200-300 ppm-mol of AN. As a general embodiment of the invention, the processing unit (407) may include, for example, a scrubber that allows the first vapor stream (422) to be washed with a condensate stream (426) as a processing liquid to obtain the purified first vapor stream (425) and a used processing liquid (427), also referred to as a used second wash liquid stream, which contains ammonium nitrate and nitric acid separated from the first vapor stream (422) containing entrained ammonium nitrate and nitric acid. For example, the processing unit (407) may include a scrubber and / or a demister equipped with trays or packing. The flow rate of the condensate stream (426) may be determined based on the specific liquid load of the trays and the diameter of the scrubber. In one embodiment, the processing unit (407) may include a diameter of 1 m and a specific liquid load of 0.5 m. 3 / m 2 The scrubbing tower is described above. Here, the flow rate of the condensate stream (426) entering the top of the processing unit (407) can be, for example, about 400 kg / h. This condensate stream (426) washes away the retained ammonium nitrate and nitric acid from the first vapor stream (422). Then, the used processing liquid stream (427) containing nitric acid and ammonium nitrate from the bottom of the scrubber is recycled, for example, to the inlet of the condensation section (404). Preferably, the used second processing liquid stream (427) is separated from the outlet of the concentrated ammonium nitrate solution (421) by separating the outlet of the used second processing liquid stream (427) from the outlet of the concentrated ammonium nitrate solution (421) (also in an embodiment where the processing unit 407 is directly mounted on top of the evaporator 404) to prevent dilution of the concentrated ammonium nitrate solution (421) that will be introduced into the UAN production section (405).

[0124] In an embodiment, the ammonia solution (419a) may comprise an ammonia-containing stream having a composition of 0.5% by weight of NH3 at a temperature of 140°C. The ammonia solution (419a) may be supplied from a wastewater treatment section (449), operatively connected to a production unit, such as a urea production unit (435), to receive an untreated ammonia-containing condensate stream (431). The NH3 concentration in the untreated ammonia-containing condensate stream (431) may be approximately 5% by weight; the condensate stream (431) may also contain urea. In the wastewater treatment section (449), the untreated ammonia-containing condensate stream (431) may be desorbed by a desorption tower (451) to obtain a partially treated condensate stream (419) containing at least 0.5% by weight of ammonia and which may be considered a dilute ammonia solution. A small portion of the partially treated condensate stream (419) may then be conveyed as the ammonia solution (419a) to the UAN unit (439). Alternatively, at least a portion of the untreated ammonia-containing condensate stream (431) may be sent to the UAN unit (439) as an ammonia solution (419a).

[0125] Wastewater treatment section 449 may include, for example, a desorption unit consisting of a first desorption tower 451 and a second desorption tower 452, and a hydrolysis tower 454. In one embodiment, the first desorption tower (451) may operate at, for example, about 3.6 bar, and most of the ammonia in the untreated ammonia-containing condensate stream (431) may be stripped off by the overhead steam from the second desorption tower (452) and the hydrolysis tower (454). The partially treated condensate stream (419) is actually the bottom effluent from the first desorption tower (451), which is pumped to the top of the hydrolysis tower (454) via a heat exchanger, wherein the partially treated condensate stream (419) is heated from about 140°C to 190°C. In the hydrolysis tower (454), the urea in the partially treated condensate stream (419) decomposes into ammonia and carbon dioxide, while being heated to about 200-210°C by fresh high-pressure steam. To achieve a very low urea concentration (approximately 1 ppm by weight) in the hydrolysis tower effluent (453), a portion of the treated condensate stream (419) is contacted countercurrently with fresh steam. The hydrolysis tower (454) operates at approximately 16 bar. The hydrolysis tower effluent (453) containing trace amounts of urea passes through the hydrolysis tower heat exchanger to the top of the second desorption tower (452). The top steam from the hydrolysis tower is sent to the bottom of the first desorption tower (451). After being cooled to approximately 149°C in the hydrolysis tower heat exchanger, the hydrolysis tower effluent (453) is sent to the top of the second desorption tower (451). Here, the remaining ammonia and carbon dioxide are stripped off with fresh low-pressure steam. The process condensate leaving the desorption tower is cooled in the desorption tower heat exchanger. It contains very small amounts of urea and ammonia (about 1 ppm-weight ammonia and about 1 ppm-weight urea) and can be used for a variety of purposes, namely for boiler feedwater or cooling water supplementation. As previously mentioned, untreated condensate stream (431), partially treated condensate stream (419) and / or hydrolysis tower effluent (453) can be used as ammonia solution stream (419a).

[0126] In an embodiment of the method, the method includes scrubbing exhaust gas in a scrubber (433) to obtain acidic aqueous ammonium nitrate (418), supplying it to a pH adjustment section (411) by adding an ammonia solution, evaporating it in an evaporator (404), and producing UAN (405), with further steps and units being optional. In this method, treatment (407) of the steam (422) from the evaporator (404) is preferred. As a further preferred embodiment of the method, the used treated liquid (427) is supplied to the inlet of the evaporator (404). As an independent preferred embodiment of the method, the ammonia solution (419a) for pH adjustment originates from a liquid flow line (419) between the first desorption tower (451) and the hydrolysis tower (454). As another independent preferred embodiment of the method, the UAN production (405) receives a urea solution (402) diluted by adding water (455) to the urea melt (423).

[0127] The term "may" indicates an example, optional, or preferred feature; other configurations besides the specified configuration are also possible.

[0128] The system's preferences and details also apply to this process, and vice versa. In particular, the composition and pH levels discussed in the system context also apply to this method. The method of the invention can be carried out, for example, but not limited to, the system of the invention. This system is preferably suitable for implementing the method.

[0129] The "system" of this invention may also be referred to as "device" or "apparatus".

[0130] When referring to a system that includes unit A in fluid communication with unit B or with the inlet or outlet of unit B, the system preferably includes units A and B.

[0131] Fluid connectivity at a gas inlet or outlet indicates a gas flow line, i.e., a line used for the flow of gases. Fluid connectivity at a liquid inlet or outlet indicates a liquid flow line, i.e., a line used for the flow of liquids.

Claims

1. A system for producing urea ammonium nitrate (UAN), the system comprising an inlet for an aqueous ammonium nitrate stream, preferably an acidic aqueous ammonium nitrate stream, an inlet for liquid urea, and an outlet for UAN, the system comprising a concentration section and a production section, the concentration section being configured to evaporate the aqueous ammonium nitrate, preferably by heating, to provide concentrated ammonium nitrate, the production section being located downstream of and in fluid communication with the concentration section, the production section being configured to mix the concentrated ammonium nitrate and liquid urea to produce UAN, wherein the concentration section has a gas outlet for water vapor, the gas outlet being in fluid communication with a processing section, the processing section... A treatment section is configured to wash water vapor received from a concentration section, the treatment section having an inlet for a washing liquid and an outlet for a used washing liquid, wherein the outlet for the used washing liquid is in fluid communication with the inlet of the concentration section, and wherein the system includes a pH control section such that an inlet for aqueous ammonium nitrate is in fluid communication with the inlet of the pH control section, wherein the pH control section is configured to subject the aqueous ammonium nitrate to pH control, thereby providing a pH-controlled flow of aqueous ammonium nitrate, wherein the pH control section has an outlet for the pH-controlled flow of aqueous ammonium nitrate, the outlet being in fluid communication with the inlet of the concentration section.

2. The system according to claim 1, wherein the pH control section is configured to control the pH of the aqueous ammonium nitrate liquid to a value in the range of 2.0 to 4.5, preferably 2.5 to 3.

5.

3. The system according to claim 1 or 2, wherein the pH control section is configured to introduce an aqueous ammonia solution, preferably a dilute solution containing up to 5% by weight of ammonia.

4. The system according to any one of the preceding claims, wherein the inlet for the aqueous ammonium nitrate is in fluid communication with the outlet of the aqueous ammonium nitrate waste liquid of the chemical treatment unit, the chemical treatment unit comprising an acidic ammonia scrubber for producing the aqueous ammonium nitrate waste liquid.

5. The system according to claim 4, wherein the chemical processing unit is selected from the group consisting of an ammonia production unit, an ammonium nitrate production unit, a manure treatment unit, a composting unit, a waste treatment unit, a coke manufacturing unit, and a urea production unit.

6. A method for producing urea ammonium nitrate (UAN), the method comprising obtaining an acidic aqueous ammonium nitrate stream as a used scrubbing liquid by contacting ammonia-containing waste gas with nitric acid in a scrubber; subjecting the acidic aqueous ammonium nitrate stream to a pH control step, the pH control step comprising determining a pH and adjusting the pH to a desired level within the range of 2 to 4.5 to provide a pH-controlled liquid; evaporating the pH-controlled liquid to obtain a concentrated ammonium nitrate solution; and combining the concentrated ammonium nitrate solution with an aqueous urea solution and / or urea melt to obtain a urea ammonium nitrate solution.

7. The method of claim 6, wherein the pH control step comprises adding an aqueous ammonia solution containing up to 5% by weight of ammonia, preferably 0.5-5% by weight of ammonia.

8. The method according to claim 6 or 7, wherein the concentration of the concentrated ammonium nitrate solution is 75% to 80% by weight.

9. The method according to any one of claims 6 to 8, comprising subjecting water vapor generated by the evaporation of the pH-controlled liquid to washing, particularly with water.

10. The method according to any one of claims 6 to 9, comprising carrying out the method in the system according to any one of claims 1 to 5; wherein the pH-controlled liquid is evaporated in a concentration section, and the concentrated ammonium nitrate solution and urea aqueous solution and / or urea melt are combined in a production section.

11. The method according to any one of claims 6 to 10, wherein the acidic aqueous ammonium nitrate stream comprises: -5-20% by weight of ammonium nitrate, preferably 5-15% by weight of ammonium nitrate. - Water, for example, at least 80% by weight; - Less than 5% by weight of urea, preferably less than 1.0% by weight of urea, and -0.1–0.5% by weight of nitric acid.

12. The method according to any one of claims 6 to 11, wherein the exhaust gas contains air, and the exhaust gas is in countercurrent contact with a nitric acid-containing liquid in the scrubber.

13. The method according to any one of claims 6 to 12, wherein the step of obtaining the acidic aqueous ammonium nitrate stream comprises: In the finishing stage, such as a granulator or granulation tower, the urea melt is solidified, cooling air is used to provide solid urea product and a waste gas stream containing urea dust and NH3, and after dust washing, the waste gas stream is supplied as at least a portion of the waste gas treated in the scrubber.

14. The method according to any one of claims 6 to 13, wherein at least 90% by weight of the ammonium nitrate in the UAN originates from an acidic aqueous ammonium nitrate stream.

15. The method according to any one of claims 6 to 14, wherein at least 90% by weight of urea contained in UAN is added to a concentrated ammonium nitrate solution.

16. The method according to any one of claims 6 to 15, wherein the urea aqueous solution and / or urea melt originate from the evaporation section of the urea plant and contain NH3.

17. The method according to any one of claims 6 to 16, wherein a portion of a urea melt containing at least 90% by weight of urea including biuret is diluted by adding an aqueous stream to form a diluted urea liquid stream having a urea concentration of 60-85% by weight including biuret, and the resulting diluted urea liquid stream is combined with a concentrated AN solution to prepare UAN.

18. The method according to any one of claims 6 to 17, wherein the step of obtaining the acidic aqueous ammonium nitrate stream comprises: - Urea melt is produced in a urea production unit comprising a synthesis section, a recovery section, an evaporation section, a condensation section, and a wastewater treatment section through the following steps: ○ In the synthesis section, CO2 and NH3 react to form a urea solution; ○ The urea solution from the synthesis section is processed in the recovery section to increase the urea content and remove at least a portion of the unreacted ammonia and CO2 from the urea solution; The urea solution from the recovery section is concentrated by evaporating water in the evaporation section to obtain a urea melt containing urea and ammonia and water vapor. ○ Preferably, the water vapor is condensed in the condensation section to obtain a process condensate containing water, urea, and ammonia. ○ Preferably, the first part of the urea melt is solidified in a finishing section using cooling air, such as in a granulator or granulation tower, to obtain a solid urea product and an exhaust gas containing urea and NH3 as waste gas. and ○ Preferably, urea dust is removed from the exhaust gas via dust scrubbing upstream of the acid washing process. - In an acid scrubber, nitric acid is used to treat the waste gas from the urea production unit, preferably at least partially from the finishing section, and preferably after dust washing, to produce an acidic aqueous ammonium nitrate stream and purified waste gas. - A second portion of the urea melt, preferably as part of a diluted urea-containing liquid, is supplied to the UAN production process.

19. The method of claim 18, comprising: - In the wastewater treatment (WWT) section, the process condensate is treated by supplying at least a portion through a first desorption tower, a hydrolysis tower, and a second desorption tower, wherein the units are connected in series in the aforementioned sequence via liquid flow lines. The method includes: Ammonia solution is discharged from one or more liquid flow lines selected from the group consisting of: liquid flow lines from the condensation section to the first desorption tower, liquid flow lines from the first desorption tower to the hydrolysis tower, and flow lines from the hydrolysis tower to the desorption tower, wherein the ammonia solution is untreated or partially treated process condensate; preferably at least partly or entirely from the liquid flow lines from the first desorption tower to the hydrolysis tower; and The ammonia solution is used in the pH control step, particularly in the pH adjustment step; The ammonia solution preferably contains 0.1-1.0% by weight of ammonia, 0.5-1.5% by weight of urea, up to 0.1% by weight of CO2, and at least 90% by weight of water.

20. A method for improving an existing chemical treatment unit having an outlet for an aqueous ammonium nitrate waste stream, the method comprising providing a system for producing urea ammonium nitrate (UAN) according to any one of claims 1 to 5, and connecting the system to the chemical treatment unit such that the outlet of the aqueous ammonium nitrate waste stream of the chemical treatment unit is in fluid communication with the inlet of the aqueous ammonium nitrate of the system for producing UAN.

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