Production method of diesel engine waste gas liquid urea

Abstract

Disclosed is a method for producing an aqueous urea solution suitable for use as a diesel tail gas treatment fluid (DEF), in which a tail gas condensate obtained by further purifying the aqueous urea solution in a treatment section suitable for the production of DEF is sent to an LP dissociation section, thereby removing ammonia therefrom. This enables water obtained from this condensate to be recycled to the DEF purification section and prevents unwanted water accumulation recirculation to urea synthesis.

Description

TECHNICAL FIELD

[0001] The present invention is in the field of urea production, in particular a method for producing a urea solution suitable for use in the abatement of NOx in exhaust gases of internal combustion engines, such as exhaust gases of diesel engines (DEF: Diesel Exhaust Fluid). The present invention also relates to an apparatus for carrying out the method, and a method for improving an apparatus. BACKGROUND

[0002] Urea is typically produced from ammonia and carbon dioxide. It can be made by introducing an excess of ammonia together with carbon dioxide into a high pressure (HP) urea synthesis section at a pressure of 12-40 MPa and a temperature of 150-250 °C. A typical urea production plant also comprises one or more recovery sections operating in a medium pressure (MP) range, typically 1-10 MPa, and / or a low pressure (LP) range, typically 0.1-1 MPa (1-10 bar). In the recovery section(s), an aqueous urea solution obtained from the synthesis section is subjected to one or more downstream steps in order to recover and recycle unreacted ammonia and carbon dioxide. If there is more than one MP and / or LP recovery section, these recovery sections can be in series, in parallel or in series and in parallel.

[0003] Recovery of ammonia and carbon dioxide in the MP and / or LP typically involves dissociation of ammonium carbamate, resulting in a further purified urea solution and a gas stream. The gas stream is typically condensed in a carbamate condenser of the applicable recovery section, resulting in an aqueous carbamate solution which is typically recycled to the urea synthesis section.

[0004] Downstream of the recovery section there is typically an evaporation section. Therein, the urea concentration in the further purified urea solution is further increased by evaporation of water, resulting in a highly concentrated solution, often referred to as urea melt. The urea melt is typically sent to a finishing section where it is made into the desired solid particulate form, often involving techniques such as prilling, granulation or spheronization.

[0005] In the evaporation section, the amount of ammonia remaining in its process condensate, in particular the amount of ammonia generated by entrainment of urea, is still too high to be released directly into the atmosphere, typically 2-3 wt%. This requires proper treatment in a waste water treatment unit, for example involving hydrolysis of urea and desorption of ammonia.

[0006] The urea product of interest is a solution for NOx abatement, for example for selective reduction, which can be a non-catalytic thermal process or a selective catalytic reduction (SCR) process. An example of an SCR solution is Diesel Exhaust Fluid (DEF), a term used in the specification to generically refer to a urea solution for NOx abatement.

[0007] DEF for cars and trucks is a purified water (usually demineralized) solution of 32.5 wt% urea, with a maximum of 0.3 wt% biuret and 0.2 wt% alkalinity as ammonia. For marine applications, DEF is 40 wt% concentration. Optionally, DEF is produced at higher concentrations, e.g. 50 wt% or higher, for transport and off-site dilution. DEF is marketed under the (commercial) trademarks Ad-Blue ® , Air1 ® , Arla 32 and AUS-32 and is injected into the exhaust gases of internal combustion engines to capture NOx, preventing it from escaping into the atmosphere. The purpose of DEF is to convert NOx into a harmless form of nitrogen. In fact, urea is a safe way to provide ammonia as a reactant in the process of converting NOx. In this process, urea is initially converted into ammonia and carbon dioxide, which reacts with NOx to produce the inert molecule nitrogen (N2) and water. Since NOx is one of the main environmental pollutants of global warming, such as global warming potential (GWP), tropospheric ozone formation potential (TOFP) and ozone depletion potential (ODP), the reduction of NOx in internal combustion engines has widespread applications.

[0008] An early method of producing diesel exhaust fluid (DEF) was to dissolve a solid urea product in demineralized water. An improved method is disclosed in WO 2006 / 096048, which uses an aqueous urea solution obtained directly from the recovery section of a urea production plant or after the recovery section and diluting the aqueous urea solution with water to obtain the desired solution. That is, with reference to a conventional urea melt plant, the aqueous urea solution used to produce DEF is obtained at an upstream location, otherwise the solution would be subjected to further concentration of the urea produced and the steps to obtain a solid product would be completed.

[0009] The foregoing method reduces the amount of water to be evaporated and sent to the corresponding waste water treatment. However, it is also understood that the aqueous urea solution thus obtained can contain relatively high levels of ammonia, which exceeds the specifications of the final DEF product.

[0010] To this end, WO 2006 / 096048 discloses that the ammonia level in solution (as free ammonia or in the form of ammonium carbamate) can be reduced by dissociating the aqueous urea solution, for example by heating or by reducing the pressure, optionally adding a stripping medium or a combination of the above. In various references dealing with different aspects of DEF urea production, including WO 2019 / 93891 and WO 2023 / 158303, similar treatments in the DEF purification section are disclosed for producing aqueous urea solutions meeting the stringent requirements of DEF. Typically, such DEF purification treatments produce a purified solution and a gas stream comprising NH3and CO2. With particular reference to WO 2019 / 93891 and WO 2023 / 158303, the latter gas stream is typically sent to a dedicated LP carbamate condenser of the DEF purification section, although it can also be sent to a different LP carbamate condenser.

[0011] The condensate, i.e. the aqueous carbamate solution, is sent to a low pressure carbamate condenser (typically referred to as LPCC) of the recovery section of the urea production process. Sending the DEF purification condensate to the LPCC is typically used to optimize the overall carbamate cycle of the urea synthesis.

[0012] The aforementioned process allows the production of DEF in a urea production plant that is also capable of producing finished solid urea products, such as granules, prills or prills, or urea melt for melamine production, or urea solution that can be used to make urea ammonium nitrate (UAN). Alternatively, it allows the provision of a dedicated production plant for DEF that does not include a finishing section. The production of DEF can also be combined with the production of other urea products, such as urea ammonium nitrate (UAN), in a single plant or in a joint plant, as disclosed in WO 2016 / 153354.

[0013] DEF must meet very stringent purity specifications. Preferably, this complies with standards, such as ISO 22241-1 :2006 for vehicular applications, or ISO 186111-1 :2014 for railway and marine applications, in particular with respect to biuret and ammonia. Therefore, a typical DEF purification treatment requires further dissociation of the unconverted ammonium carbamate that is still present. However, the conditions for such dissociation are limited, as higher temperatures promote the formation of biuret. Therefore, such dissociation is typically carried out at a low or reduced pressure level, for example 0.1 to 2 absolute bar, preferably below atmospheric pressure.

[0014] Considering process economics and environmental safety, it is generally desirable to recycle any recovered ammonia and carbon dioxide as feed to the urea synthesis in the form of a carbamate solution. However, at lower pressures, and thus at lower condensation temperatures, a relatively large amount of water needs to be present in the carbamate solution in order to prevent crystallization of the dissolved ammonium carbamate. However, in view of the adverse effect of the presence of water on the reaction equilibrium of the conversion of ammonium carbamate to urea and water, the additional water is not needed for recycling to the urea synthesis accordingly.

[0015] DEF is essentially a dilute aqueous solution of urea, which requires a large amount of water as part of the product. It will be appreciated that any dilution with water should not result in the introduction of more impurities in the DEF product. Therefore, such dilution is typically carried out with demineralized or distilled water.

[0016] Accordingly, a challenge in the field is to provide for an efficient management of the water streams involved in the production of DEF. This applies to dedicated DEF plants as well as to plants combining the production of DEF with the production of solid urea and / or combined urea products. SUMMARY

[0017] To better meet one or more of the aforementioned needs, the present invention provides in one aspect a process for producing an aqueous urea solution suitable for use as a diesel exhaust fluid (DEF), the process comprising reacting ammonia and carbon dioxide under urea forming conditions in a high pressure (HP) synthesis section, thereby obtaining a urea synthesis solution; subjecting the urea synthesis solution to recovery treatment for removing remaining ammonia and carbon dioxide therefrom, the treatment comprising dissociation in a low pressure (LP) dissociation section, resulting in an aqueous urea product stream and a LP dissociation off-gas; condensing the LP dissociation off-gas, thereby obtaining a LP condensate; recycling the LP condensate as feed to the urea synthesis; subjecting the aqueous urea product stream to further ammonia removal in a DEF purification section, thereby obtaining a purified urea solution and a DEF purification off-gas; condensing the DEF purification off-gas, thereby obtaining a DEF purification condensate; adjusting the water content of the purified urea solution, thereby obtaining a DEF product solution; wherein the process comprises subjecting at least part of the DEF purification condensate to ammonia removal in the LP dissociation section.

[0018] In another aspect, the present application provides a plant for producing an aqueous urea solution suitable for use as a diesel exhaust fluid (DEF), the plant comprising a high pressure (HP) synthesis section configured to cause ammonia and carbon dioxide to react under urea forming conditions, thereby obtaining a urea synthesis solution; downstream and in fluid communication with the HP synthesis section, a low pressure (LP) recovery section configured to recover residual ammonia and carbon dioxide contained in the passing urea solution, the LP recovery section comprising a LP dissociation section and a LP condensation section, the LP dissociation section comprising an inlet for the urea solution, optionally in fluid communication with an outlet for the urea synthesis solution from the HP synthesis section via the MP section, an outlet for an aqueous urea product stream, and an outlet for a dissociation off-gas in fluid communication with a gas inlet of the LP condensation section, the LP condensation section having an outlet for a LP condensate in fluid communication with an inlet of the HP synthesis section; the plant further comprising a DEF purification section configured to perform a further ammonia removal from the aqueous urea product stream, the DEF purification section having an inlet for a liquid urea stream in fluid communication with the outlet for the aqueous urea product stream of the LP dissociation section, an outlet for a purified urea solution, and an outlet for a DEF purification off-gas in fluid communication with an inlet of a DEF purification condensation section configured to condense the DEF purification off-gas, thereby obtaining a DEF purification condensate, the DEF purification condensation section having an outlet for the DEF purification condensate in fluid communication with the inlet of the LP dissociation section.

[0019] In another aspect, the present application relates to a method for improving an existing plant for producing an aqueous urea solution suitable for use as a diesel exhaust fluid (DEF), the existing plant comprising a high pressure (HP) synthesis section configured to cause ammonia and carbon dioxide to react under urea forming conditions, thereby obtaining a urea synthesis solution; downstream of and in fluid communication with the HP synthesis section, a low pressure (LP) recovery section configured to cause the urea solution passing therethrough to undergo recovery of residual ammonia and carbon dioxide contained therein, the LP recovery section comprising a LP dissociation section and a LP condensation section, the LP dissociation section comprising an inlet for the urea solution, an outlet for an aqueous urea product stream and an outlet for a dissociation off-gas, the outlet for the dissociation off-gas being in fluid communication with a gas inlet of the LP condensation section, the LP condensation section having an outlet for a LP condensate, the outlet being in fluid communication with an inlet of the HP synthesis section; the plant further comprising a DEF purification section configured to cause the aqueous urea product stream to undergo further ammonia removal, the DEF purification section having an inlet for a liquid urea stream, an outlet for a purified urea solution and an outlet for a DEF purification off-gas, the inlet being in fluid communication with the outlet for the aqueous urea product stream of the LP dissociation section, the outlet for the DEF purification off-gas being in fluid communication with an inlet of a DEF purification condensation section configured to cause condensation of the DEF purification off-gas, thereby obtaining a DEF purification condensate, the DEF purification condensation section having an outlet for the DEF purification condensate, the outlet being in fluid communication with the LP recovery section, the method comprising providing a fluid connection between the outlet for the DEF purification condensate of the DEF purification condensation section and the inlet for the urea solution of the LP dissociation section. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is a schematic representation of the method of the present application carried out in a plant according to the present application. DETAILED DESCRIPTION

[0021] The present application is based on the wise recognition that the LP dissociation section, which obtains the aqueous urea product stream on which the DEF purification is carried out, can also be used for the purpose of removing ammonia from the condensate obtained from the DEF purification. Indeed, by conveying at least part of the DEF purification condensate to the LP dissociation section, ammonia will be removed therefrom together with the LP dissociation off-gas.

[0022] As ammonia is thus preferentially removed in the LP dissociation section, the larger part of the water contained in the DEF purification condensate does not end up in the LP condensation section and thus also not in the urea synthesis. This reflects an improvement compared to the prior art process in which the DEF purification condensate is sent to the LP carbamate condenser. In addition, as a synergistic benefit, this prevents water from residing where it is actually detrimental, namely where it is actually needed, i.e. as part of the aqueous urea product stream in the DEF purification section, which is purified and further diluted to obtain DEF in form.

[0023] When referring to "DEF purification", this refers to the treatment of an aqueous urea stream by which the final amount of ammonia still contained therein is removed, so that the urea solution thus obtained, when diluted to a concentration suitable as DEF, has an ammonia content of no more than the corresponding specification.

[0024] DEF is used in diesel engine vehicles to reduce NOx emissions. In ISO 22241-1 :2006, the composition of vehicle DEF is standardized. Vehicle DEF is about 32.5 wt% urea (i.e. essentially a eutectic composition) and also has very low impurity concentrations. The product urea solution can also be used for NOx abatement in e.g. plants, ships and trains. For NOx abatement used in railway and marine applications, according to ISO 186111-1 :2014 standard, about 40 wt% urea solution is used. For NOx abatement of (fossil fuel) power plants, typically 50 wt% urea solution is used. The term "DEF" is used in the present disclosure, also for the present invention, in particular to refer to a urea solution suitable, fit and / or determined for use in NOx abatement, such as a urea solution according to any of the specifications, more in particular a urea solution according to ISO 22241-1 :2006.

[0025] When manufacturing DEF, the concentration of urea is important in order to allow a precise dosing of the liquid to the SCR catalyst. Low concentrations of organic impurities in DEF are very important to avoid plugging of the catalyst surface and formation of coke. Very low concentrations of inorganic impurities, in particular heavy metals, in DEF are important, as these impurities can cause poisoning of the SCR catalyst. The metals will accumulate on the catalyst, thus reducing the lifetime of the catalyst.

[0026] The DEF purification section will be performed in a DEF purification section, as further discussed below. The DEF purification section is typically part of a urea production plant, but can also be connected to a urea plant as a stand-alone DEF production plant. The urea plant can be a dedicated DEF production plant, or it can be a plant that is also capable of producing other urea products, such as urea melt, a final solidified urea product, such as prills, granules or nuggets, and / or a combined urea fertilizer product, such as urea ammonium nitrate or urea ammonium sulfate.

[0027] The first step of the process of the present invention comprises reacting ammonia and carbon dioxide in a high pressure (HP) synthesis section under urea forming conditions. Suitable urea synthesis techniques are known in the art, as reflected in, for example, the chapter on urea in Ullmann’s Encyclopedia of Industrial Chemistry (2012). In view of the involvement of the LP dissociation section in the process of the present invention, it is understood that the urea is synthesized by a full recycle type of process, in which substantially all of the unconverted ammonia and carbon dioxide is recycled into the urea synthesis. In particular, this can be a so-called conventional recirculation process or a stripping process. These processes are well known to the skilled person, as are the corresponding equipment. For the synthesis of DEF in a stripping plant, reference is made to, for example, WO 2006 / 096048 or WO 2023 / 158303. For the synthesis of DEF in a conventional urea plant, reference is made to WO 2019 / 93891.

[0028] In a stripping plant, the synthesis section comprises a reactor zone, a stripping column and a condensation zone, forming a high pressure loop. The output of the reactor is a reactor effluent, which is essentially an aqueous urea solution containing unreacted ammonia and carbon dioxide, mainly in the form of ammonium carbamate. The reactor effluent is heated in the stripping column, possibly with the aid of a gaseous stripping agent, to remove a gaseous stream containing ammonia and carbon dioxide. The gaseous stream coming out of the stripping column is condensed in a condenser, possibly with the aid of a solution recovered from a recovery section. The condensate thus obtained is recycled to the reactor. The Stamicarbon CO2 stripping process uses gaseous CO2 as stripping agent. Another stripping process uses gaseous ammonia as stripping agent. The present invention is not limited to any particular urea full recycle production process, such as the HEC process developed by Casale Urea Corporation, the ACES process developed by Toyo Engineering Corporation and the process developed by Snamprogetti. All of these and other processes can be used in the process of the present invention.

[0029] The synthesis section is typically operated at high pressure (HP), i.e. at a pressure in the range of 12-40 MPa and a temperature in the range of 150-250 °C. In a preferred embodiment, the reaction is carried out at a synthesis pressure in the range of 130-150 bar absolute. In another embodiment, the entire HP synthesis section is operated at a pressure in the range of 130-150 bar absolute.

[0030] As mentioned above, the process of the application comprises a recovery treatment of the urea synthesis solution obtained from the synthesis section, in order to remove remaining ammonia and carbon dioxide therefrom. This remaining ammonia and carbon dioxide can be present in the form of unconverted carbamate and it can include ammonia and carbon dioxide originating from one or both of the unreacted feed reactants and the dissociation section of the unconverted carbamate. The recovery treatment can comprise one or more recovery stages at a recovery pressure lower than the synthesis pressure, in order to remove this residual ammonia and carbon dioxide to a large extent from the urea synthesis solution and to obtain an aqueous urea solution consisting essentially of urea and water, however with some residual reactants inevitably still present. The recovery stages comprise for example heating the solution to obtain dissociation of the carbamate and condensing the off-gas thus obtained into a recycle solution containing carbamate. This solution can be recycled to the synthesis section, for example to the condenser of the synthesis loop.

[0031] The recovery optionally comprises one or more medium pressure (MP) stages, in which the carbamate is dissociated, optionally accompanied by stripping, at medium pressure (MP), i.e. at a pressure in the range of 1-10 MPa. The one or more MP recovery stages can be carried out in an MP recovery section comprising for example an MP scrubber or MP flasher and condenser, or an MP heater integrated with an MP CO2 stripper. The MP recovery stages can be in series and / or in parallel. Downstream of the one or more MP recovery stages, there follow one or more low pressure (LP) recovery stages, typically involving a pressure in the range of 0.1 MPa to 1 MPa, i.e. 1-10 bar absolute.

[0032] As a final stage of the recovery, the process of the present invention comprises a dissociation in a low pressure (LP) dissociation section. This section comprises at least one LP dissociator, typically a heat exchanger, such as a shell and tube heat exchanger, and possibly a combination of two or more, identical or different dissociators. The LP dissociation section as referred to in the present disclosure is typically a single section, and is typically a section already present in a urea plant. However, it is conceivable that two or more such LP dissociation sections are operated in parallel. In other words, in terms of the plant, for example, one LP dissociator can be used to obtain an aqueous urea product stream to be sent to DEF purification, another, parallel, LP dissociator can be used to receive a DEF purification condensate, and recycle water obtained therefrom to the DEF purification section. The same applies to one or more low pressure condensation sections, which are optionally present, all being part of the same carbamate recovery and recycle loop. Furthermore, it is conceivable that more than one carbamate recycle loop is applied, for example when retrofitting an existing plant according to the present invention.

[0033] The LP dissociation yields an aqueous urea product stream and a LP dissociation offgas. This offgas comprises residual reactants ammonia and carbon dioxide, which are eventually recycled to the urea synthesis. To this end, the process comprises a condensation of the LP dissociation offgas. This condensation takes place in a low pressure condensation section, which comprises at least one low pressure condenser, and possibly two or more, for example in parallel. The condenser is typically a heat exchanger, such as a shell and tube heat exchanger. As a result of this low pressure condensation, a LP condensate is obtained, which can eventually be recycled as reactant feed to the urea synthesis.

[0034] The liquid part resulting from the LP dissociation is a relatively pure aqueous urea product stream, but it is not yet suitable to meet the required purity for DEF, let alone the required purity according to a DEF specification, such as the one mentioned above. This particularly relates to the presence of undesirable ammonia. In general, depending on any evaporation steps that can still be included, the aqueous urea product stream will have a urea concentration of 20-95 wt.%, preferably 50-90 wt.%, more preferably 60-85 wt.%. In the case of a dedicated DEF plant, which is not also used to produce a urea melt or a similar urea product such as UAN, an evaporation section does not need to be present in the urea plant. If such a section is not present, or the aqueous urea product stream is obtained from a position upstream of such an evaporation section, or bypasses such a section, the urea concentration will typically not exceed 77 wt.%.

[0035] It will be understood that in case the urea concentration is lower than the required DEF concentration (for example 32.5 wt.% for vehicular applications, or for example 40 wt.% for marine and / or rail applications), the step of adjusting the water content of the purified urea solution will comprise removing water, typically by evaporation, rather than adding demineralized or distilled water.

[0036] As a general preference, the urea content in the aqueous urea stream is in the range of 60 to 77 wt.-%.

[0037] For the production of DEF, the aqueous urea product stream is subjected to further ammonia removal in a DEF purification section. This section serves to dissociate the remaining carbamic acid and to remove the ammonia thus produced, usually together with carbon dioxide.

[0038] DEF purification can involve washing with carbon dioxide, as disclosed in WO 2008 / 092647. Generally, it is preferred that the DEF purification section does not comprise heating to above 135°C, preferably not more than 100°C, to prevent the formation of biuret, which otherwise has to be removed in view of the applicable DEF specifications. Removal of biuret is mentioned, for example, in WO 2021 / 156024, for which reverse osmosis is applied. Thus, generally, DEF purification will involve further expansion of the aqueous urea product stream obtained from the LP dissociation section. Preferably, the DEF purification section is operated at a pressure of up to 3 bar absolute, preferably 0.1-2 bar absolute, such as 0.1-1.0 bar absolute, preferably below atmospheric pressure, more preferably 0.2-0.9 bar absolute, still more preferably 0.3-0.6 bar absolute.

[0039] In particular, purification preferably removes excess ammonia, i.e. ammonia above the desired level, in particular above the desired alkalinity as NH3 level. This removal ensures that the purified solution has a sufficiently low alkalinity. Low alkalinity is important to avoid corrosion of equipment in contact with the DEF solution. Lower NH3 content in the DEF solution advantageously reduces the ammonia odor. Removal of ammonia generally involves transfer of NH3 from the liquid phase into the gas phase.

[0040] Purification is preferably stripping, preferably such that the NH3 alkalinity of the purified solution obtained by the purification is less than 0.20 wt.-%, less than 1000 ppm, less than 500 ppm or less than 200 ppm by weight, all when at 32.5 wt.-% urea, i.e. alkalinity based on water added or removed as needed, to have 32.5 wt.-% urea, in other words, this alkalinity level is based on a 32.5 wt.-% urea solution. Lower alkalinity is preferred.

[0041] In some embodiments, purification comprises reducing the NH3 alkalinity of the urea solution to be purified by at least 50% or at least 90% or at least 99% in relative percentage of the initial alkalinity in NH3 values, by transferring the corresponding amount of ammonia (in whatever form in the solution) into the gas phase. In embodiments where purification results in a urea solution with NH3 alkalinity less than 0.2 wt.-%, the initial urea solution can already have an alkalinity below the prescribed level, in which case the alkalinity as NH3 is further reduced by purification.

[0042] In some embodiments, the purification (e.g. stripping) involves dissociation of the ammonium carbamate into NH3 and CO2 and transfer of this formed NH3 from the liquid phase to the gas phase to remove NH3 from the solution, typically at the same time as free NH3 is transferred from the liquid phase to the gas phase. Typically, with this dissociation, the alkalinity as well as the carbonate content (as CO2) decreases, for example the carbonate content (as CO2) of the purified solution is below 0.5 wt% or less than 0.2 wt% or less than 1000 ppm or less than 500 ppm by weight based on the 32.5 wt% urea solution.

[0043] The purification for example comprises a reduction of the pressure (i.e. a reduction of the absolute pressure), heating, stripping and combinations of these. In some embodiments, the purification comprises heating and / or a reduction of the pressure without stripping.

[0044] The purification preferably comprises stripping. Stripping allows to reduce the purification temperature, thereby advantageously reducing the formation of biuret. The stripping preferably comprises contacting the countercurrently flowing urea solution with a gas stream. The gaseous stream typically has a lower NH3 partial vapor pressure than the urea solution in contact with the gas. The stripping preferably comprises steam stripping. Air stripping can also be used. The stripping preferably comprises contacting the aqueous urea solution with steam countercurrently. The steam can be supplied from a boundary, for example from another installation, such as a utility installation. But preferably the steam is low pressure steam generated in a condenser of the high pressure synthesis section (often referred to as HPCC). Alternatively, the steam can be generated by evaporation of water from the urea solution, for example downstream of the purification step, for example by reboiling. The stripping is preferably performed at a pressure of less than 3 bar (absolute). One advantage of steam stripping is the stripping effect by the low partial ammonia vapor pressure, preferably in addition maintaining a high vapor pressure of water to reduce water evaporation. Some advantages of direct stripping with externally supplied steam are that dilution of the urea solution with water is useful for making DEF, and downstream reboiling of the urea solution to cause biuret formation can be avoided.

[0045] The purification section for example is based on heating the solution to cause ammonia evaporation, and preferably also comprises reducing the NH3 partial pressure of the gas phase by stripping and / or reducing the absolute pressure. Due to the heating and preferably the (absolute) pressure reduction, also water is evaporated. The DEF purification off-gas can mainly comprise water vapor, for example more than 50 wt%, more than 70 wt%, more than 90 wt% or more than 99 wt% water, and for example from 0.5 wt% and / or at most 5 wt% NH3. In some embodiments, the DEF purification off-gas and / or the DEF purification condensate comprises at least 0.010 wt% and / or at most 5.0 wt% of the NH3 corresponding to the aqueous urea stream received by the DEF purification section.

[0046] Purification for example comprises stripping, e.g. with aqueous urea. Steam stripping typically comprises injecting steam directly into the aqueous urea stream, typically in counter-current fashion, e.g. liquid flowing downwards and steam upwards. The steam stream preferably comprises at least 90 wt% H2O, more preferably at least 95 wt% H2O. The injected steam pressure is for example 1 to 30 bar absolute, preferably 2 to 15 bar absolute, typically 2 to 6 bar absolute. The purification step is preferably stripping, more preferably steam stripping, preferably at less than 3.0 bar absolute, less than 2.0 bar absolute or less than 1.5 bar absolute, more preferably 0.10 to 1.1 bar absolute, e.g. less than 1.0 bar absolute, e.g. 0.010 to 0.50 bar, or 0.4 to 0.5 bar absolute. Such operating pressures are especially used for steam stripping columns. The process comprises for example expanding a 3 to 7 bar urea solution obtained from LP recovery to the pressure of the stripping step, e.g. less than 1.5 bar absolute. A steam stripping column for DEF purification is for example a vessel configured for counter-current flow of steam and liquid, having a liquid inlet at the top and a liquid outlet at the bottom, and a steam inlet at the bottom and a gas outlet at the top. The vessel for example comprises trays and / or packing.

[0047] Preferably, the DEF purification section typically comprises at least one of a steam stripping column, a heater, and a combination of flash dissoiver and heater. The choice is at least partially based on the degree of fluctuation of the ammonia percentage in the aqueous urea product stream to be treated. For example, if the amount of ammonia varies too much, a heater is preferred over a stripping column due to its flexibility.

[0048] In case a heater is used, especially in a dissoiver section at sub-atmospheric pressure, a relatively large amount of water is evaporated and ends up in the DEF purification condensate. Therefore, the present invention sends at least part of the DEF purification condensate to the LP dissoiver section, with greater benefit in case the DEF purification section comprises a DEF dissoiver section operated based on a heater. It is further preferred that in the production of DEF, heating can advantageously be provided by direct steam injection, as water is anyway required in the product. The present invention is especially beneficial in terms of steam condensate resulting in additional water in the DEF purification condensate. Because, considering the route of the DEF purification condensate via the LP dissoiver section, this additional water is effectively allowed to return to the DEF treatment section, instead of affecting the water content in the carbamate cycle returning to the urea synthesis.

[0049] In another embodiment, purification is performed in a rectification column operated at reduced pressure, with or without stripping. The rectification column comprises a heater, preferably a shell-and-tube heat exchanger, at the bottom, and a rectification section comprising structured packing or trays.

[0050] DEF purification results in obtaining a purified urea solution suitable to meet the requirements of the purity of the DEF. For the production of DEF, the purified urea solution is further diluted to the required urea concentration, typically 30-35%, usually 32-33%, preferably 32.5%. It will be appreciated that this dilution is performed with water which does not result in the introduction of new impurities, typically demineralized or distilled water, or advantageously purified process condensate. The latter can be particularly useful in the case of a urea production plant comprising a waste water treatment section, in which the DEF is produced.

[0051] The DEF purification section produces a DEF purification off-gas. This is condensed, thereby obtaining a DEF purification condensate. As mentioned above, according to the present application, the process comprises sending the DEF purification condensate to the LP dissociation section. This provides the advantageous option of being able to reduce the amount of water returned to the urea synthesis through the LP condensation section, and it recycles water to a stage of the process where, eventually, the aqueous urea solution from which the DEF is produced will be diluted anyway, thereby reducing the amount of demineralized or distilled water required.

[0052] It will be appreciated that the presence of water in the low pressure condensation section cannot be completely avoided. As known to the skilled person, in order to prevent the crystallization of ammonium carbamate, the presence of a sufficient amount of water is required. This can typically be provided by the waste water treatment section of the urea plant, for example by means of a lean carbamate solution (i.e. an aqueous ammonium carbamate solution, typically containing less than 30 wt% of ammonium carbamate, preferably more than 8 wt%, more preferably between 15 and 25 wt% of ammonium carbamate), obtained as a condensate of gaseous components obtained from the waste water treatment. In the prior art processes referred to above in relation to the treatment of the DEF purification section, the amount of water required is included in the water sent to the LP condensation section in the form of the DEF purification condensate. If required, in the present application, the amount of water required in the LP condensation section can still be provided by a portion of the DEF purification condensate. In this case, according to the present application, the latter condensate will be split into a portion to be sent to the low pressure dissociation section, and a portion to be sent to the low pressure condensation section. Thus, in the process of the present application, at least part of the DEF purification condensate is sent to the LP dissociation section, and possibly all of it. Preferably, at least 20-30 wt% of the DEF purification condensate is sent to the LP dissociation section. The split ratio between the stream sent to the LP dissociation section and the stream sent to the LP condensation section is determined taking into account the amount of water required to prevent crystallization in the low pressure condensation section. In particular, under the preferred conditions of the low pressure condensation section, there should be approximately 32-35 wt% of water in the section. If the amount of water is lower than this value, additional sources of process water can be conventionally provided to the low pressure condensation section in order to deliver a portion of the DEF purification condensate to the low pressure dissociation section.

[0053] In another aspect, the present application also relates to a plant for producing DEF. The plant is for producing urea and DEF therefrom, optionally in addition to other urea products. To produce urea, the plant comprises a high pressure (HP) synthesis section configured to react ammonia and carbon dioxide under urea forming conditions, thereby obtaining a urea synthesis solution. To provide for the recycling of ammonium carbamate, the plant comprises, downstream of and in fluid communication with the HP synthesis section, a low pressure (LP) recovery section. The section is configured to recover the urea solution passed therethrough of residual ammonia and carbon dioxide contained therein. The LP recovery section comprises a low pressure dissociation section and a low pressure condensation section. To receive the urea solution to be dissociated of the ammonium carbamate contained therein, the LP dissociation section comprises an inlet for the urea solution in fluid communication with an outlet for the urea synthesis solution from the HP synthesis section. Such fluid communication can be directly from the HP synthesis section, or optionally via one or more other sections located between the HP synthesis section and the LP dissociation section, such as a MP section, in particular a MP recovery section, which can comprise a MP dissociator, optionally a MP stripper, and from which a urea synthesis solution is obtained in already partially purified form, and a MP ammonium carbonate condensate which can be directly or indirectly recycled into the urea synthesis. The optional presence of the MP section and its possible units and components do not require further explanation to the skilled person.

[0054] The LP dissociation section comprises an outlet for the aqueous urea product stream and an outlet for the dissociation off-gas. To allow for recycling of the off-gas as liquid carbamate, the outlet for the dissociation off-gas is in fluid communication with a gas inlet of the LP condensation section, which in turn has an outlet for the LP condensate which is in direct or indirect fluid communication with an inlet of the HP synthesis section.

[0055] The plant further comprises a DEF purification section. The section has an inlet for a liquid urea stream in fluid communication with the outlet for the aqueous urea product stream of the LP dissociation section. As discussed amply above, the DEF purification section is configured to further remove ammonia from the aqueous urea product stream.

[0056] The DEF purification section thus has an outlet for a purified urea solution which can be further processed, typically diluted, to obtain DEF at the desired concentration. In particular, the DEF purification section has an outlet for a DEF purification off-gas in fluid communication with an inlet of a DEF purification section condensation section. The latter section is configured to subject the DEF purification off-gas to condensation, thereby obtaining a DEF purification condensate. According to the present application, the DEF purification condensation section is connected to the LP dissociation section.

[0057] To this end, the DEF purification condensation section has an outlet for the DEF purification condensate in fluid communication with the inlet of the LP dissociation section. It will be understood that this refers to the inlet of a location which allows at least part of the DEF purification condensate to undergo removal of ammonia in the LP dissociation section. This can be the same inlet as the urea synthesis solution being conveyed to the low pressure dissociation section, or it can be a different inlet. The inlet of the DEF purification condensate into the LP dissociation section can be parallel to the inlet of the urea synthesis solution, downstream thereof or upstream thereof (thus allowing the DEF purification condensate to be combined with the urea synthesis solution first, e.g. by a T-connection).

[0058] In another aspect, the present application relates to a method of modifying an existing plant for the production of an aqueous urea solution suitable for use as a diesel exhaust fluid (DEF). The pre-existing plant will typically comprise a HP synthesis section, optionally an MP recovery section and a LP recovery section as described above, as is customary in the art. As a plant for the production of DEF, the existing plant will further comprise a DEF purification section. The latter section is configured to receive an aqueous urea product stream from the LP dissociation section, which will have an outlet for the purification of the urea solution and an outlet for a DEF purification off-gas. The latter outlet is in fluid communication with an inlet of a DEF purification condensation section configured to subject the DEF purification off-gas to condensation, thereby obtaining a DEF purification condensate. In the pre-existing plant, the DEF purification section can have an outlet for the DEF purification condensate in fluid communication with the LP recovery section, i.e. to its low pressure condensation section. The modification method of the present application comprises providing a fluid connection of the outlet of the DEF purification condensate from the DEF purification condensation section with the inlet of the urea solution into the LP dissociation section.

[0059] The present application is also capable of modifying an existing urea production plant which is not a plant for the production of DEF. This refers to a urea production plant comprising a high pressure (HP) synthesis section configured to subject ammonia and carbon dioxide to a reaction under urea forming conditions, thereby obtaining a urea synthesis solution; downstream thereof and in fluid communication therewith a low pressure (LP) recovery section configured to subject a passing urea solution to recovery of remaining ammonia and carbon dioxide contained therein, the LP recovery section comprising a LP dissociation section comprising an inlet for the urea solution, optionally in fluid communication with an outlet of the urea synthesis solution from the HP synthesis section via an MP section, an outlet of an aqueous urea product stream and an outlet of a dissociation off-gas, the outlet of the dissociation off-gas being in fluid communication with a gas inlet of a LP condensation section, the LP condensation section having an outlet of a LP condensate in fluid communication with an inlet of the HP synthesis section.

[0060] An improvement in accordance with the present application includes the addition of a DEF purification section, as discussed generally above. The section is configured to subject the aqueous urea product stream to further ammonia removal, the DEF purification section having an inlet for the liquid urea stream in fluid communication with the outlet of the LP carbamate condensate section, the outlet of the LP carbamate condensate section being in fluid communication with the inlet of the LP dissociation section, the outlet of the purified urea solution, and the outlet for the DEF purification offgas, the outlet for the DEF purification offgas being in fluid communication with the inlet of the DEF purification condensation section, the DEF purification condensation section being configured to subject the DEF purification offgas to condensation, thereby obtaining a DEF purification condensate, the DEF purification condensation section having an outlet for the DEF purification condensate, the outlet being in fluid communication with the inlet of the LP dissociation section.

[0061] Various general preferences and specific embodiments are discussed with reference to the process aspects of the present application. Unless otherwise indicated, these preferences and embodiments will apply similarly to the equipment aspects of the present application, as well as to aspects of modifying existing equipment.

[0062] The present application is further illustrated with reference to the accompanying drawings. It is to be understood that the drawings do not limit the application. For example, the application is not limited to the particular types of equipment and particular equipment systems shown. The figures show equipment components and process flows relevant to embodiments of the application.

[0063] Reference is made to Figure 1 CO2and NH3react under urea synthesis conditions in a urea synthesis section, which is operated at high pressure to produce a urea synthesis stream (1). An exemplary urea synthesis section includes a high pressure reaction zone (2) and a high pressure stripping column (3).

[0064] The urea synthesis stream (1) is then sent to a low pressure (LP) recovery section (4), where unconverted ammonia and carbon dioxide are recovered from the urea synthesis stream (1). By expansion to low pressure, a portion of the carbamate remaining in the urea synthesis stream (1) dissociates and vaporizes. The remaining liquid is heated in a LP dissociation section (5), or subjected to a dissociation step (such as by flashing or stripping) to further dissociate the carbamate. Vapor from the LP dissociation section (5) is condensed in a LP carbamate condensation section (6), and recycled back to the urea synthesis section as a condensate (7) containing LP carbamate. An aqueous urea stream obtained from the LP dissociation section (8) is sent to a DEF production section (9).

[0065] The DEF production section (9) comprises a preferably sub-atmospheric pressure dissociation section (LLPD) (10) in which ammonia is dissociated from the aqueous urea stream (8) to obtain a purified urea solution (1 1 ) and a DEF purification offgas (12). The purified urea solution (1 1 ) is then diluted with a stream of demineralized or distilled water (16) to the appropriate DEF concentration. The DEF purification offgas (12) is sent to a DEF carbamate condensation section (13) where it is condensed to obtain a lean carbamate stream (14), i.e. a DEF purification condensate. In the exemplary embodiment shown, a part of the lean carbamate stream (14a) is sent to the LP dissociation section (5) and the remaining part (14b) is sent to the LPCC (6).

[0066] While the application has been illustrated and described in detail in the drawings and foregoing description, such illustration and description is to be considered illustrative or exemplary and not restrictive; the application is not limited to the disclosed embodiments.

[0067] For example, the application can be operated in an embodiment wherein more than one water stream is dissociated from or after the recovery section to obtain an aqueous urea solution.

[0068] Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed application, from a study of the drawings, the disclosure, and the appended claims. In the claims, the term "comprising" does not exclude other elements or steps, the term "a" or "an" does not exclude a plurality, and the term "one" or "said" does not exclude a plurality. The mere fact that certain features are recited in mutually different dependent claims does not indicate that a combination of these features cannot be used to advantage.

[0069] In summary, a method of producing an aqueous urea solution suitable for use as a diesel exhaust fluid (DEF) is disclosed, wherein an exhaust condensate obtained from an aqueous urea stream further purified in a treatment section suitable for producing DEF is sent to a LP dissociation section, whereby ammonia is removed therefrom. This enables water obtained from such a condensate to be recycled to the DEF purification section and prevents undesired build-up of water in the urea synthesis.

Claims

1. A method for producing an aqueous urea solution suitable for use as a diesel exhaust fluid (DEF), the method comprising reacting ammonia and carbon dioxide in a high-pressure (HP) synthesis section under urea formation conditions to obtain a urea synthesis solution; recovering the urea synthesis solution to remove residual ammonia and carbon dioxide, the recovery comprising dissociating it in a low-pressure (LP) dissociation section to produce an aqueous urea product stream and LP dissociation exhaust gas; condensing the LP dissociation exhaust gas to obtain LP condensate; and recycling the LP condensate as feedstock for urea synthesis. In the DEF purification section, the aqueous urea product stream is further ammonia removed to obtain a purified urea solution and DEF purification waste gas; the DEF purification waste gas is condensed to obtain DEF purification condensate. Adjust the water content of the purified urea solution to obtain a DEF product solution; The method includes removing ammonia from at least a portion of the DEF purification condensate in the LP dissociation section.

2. The method of claim 1, wherein the DEF purification section is operated at a pressure of 0.1 bar to 2 bar, preferably 0.1 bar to 1.0 bar.

3. The method according to claim 1 or 2, wherein the DEF purification section operates at a pressure below atmospheric pressure.

4. The method of claim 3, wherein the pressure is in the range of 0.3 bar to 0.6 bar.

5. The method according to any one of the preceding claims, wherein adjusting the water content of the purified urea solution comprises diluting it with water selected from softened water, distilled water, and mixtures thereof.

6. The method according to any one of the preceding claims, wherein adjusting the water content of the purified urea solution comprises diluting it with purification process condensate.

7. The method according to any one of the preceding claims, wherein the aqueous urea product stream has a urea concentration of 60-85% by weight, preferably 60-77% by weight.

8. The method according to any one of the preceding claims, wherein further ammonia removal of the aqueous urea product stream in the DEF purification stage comprises heating to a temperature below 135°C, preferably not exceeding 100°C.

9. The method of claim 8, wherein the heating comprises injecting steam into an aqueous urea product stream.

10. The method according to any one of the preceding claims, wherein the DEF purification condensate is divided into two parts, one part is sent to the LP dissociation section, and the other part is sent to the LP condensation section.

11. The method according to any one of the preceding claims, wherein at least 20% by weight of the DEF purified condensate, preferably at least 30% by weight, is fed to the LP dissociation stage.

12. An apparatus for producing an aqueous urea solution suitable for use as a diesel exhaust fluid (DEF), the apparatus comprising a high-pressure (HP) synthesis section configured to react ammonia and carbon dioxide under urea-forming conditions to obtain a urea synthesis solution; downstream and in fluid communication therewith, a low-pressure (LP) recovery section configured to recover residual ammonia and carbon dioxide contained in the passing urea solution, the LP recovery section comprising an LP dissociation section and an LP condensation section, the LP dissociation section comprising an inlet for the urea solution, an outlet for an aqueous urea product stream, and an outlet for dissociated exhaust gas, the inlet optionally being in fluid communication via an MP section to the outlet of the urea synthesis solution from the HP synthesis section, the outlet of the dissociated exhaust gas being in fluid communication to the gas inlet of the LP condensation section. The LP condensation section has an outlet for LP condensate, which is in fluid communication with the inlet of the HP synthesis section. The equipment also includes a DEF purification section configured to subject the aqueous urea product stream to further ammonia removal. The DEF purification section has an inlet for liquid urea, an outlet for purified urea solution, and an outlet for DEF purified waste gas. The inlet is in fluid communication with the outlet of the aqueous urea product stream from the LP dissociation section. The outlet for the DEF purified waste gas is in fluid communication with the inlet of the DEF purification condensation section. The DEF purification condensation section is configured to condense the DEF purified waste gas to obtain DEF purified condensate. The DEF purification condensation section has an outlet for the DEF purified condensate, which is in fluid communication with the inlet of the LP dissociation section.

13. A method for improving an existing apparatus for producing an aqueous urea solution suitable for use as a diesel exhaust fluid (DEF), the existing apparatus comprising a high-pressure (HP) synthesis section configured to react ammonia and carbon dioxide under urea-forming conditions to obtain a urea synthesis solution; downstream and in fluid communication therewith, a low-pressure (LP) recovery section configured to recover residual ammonia and carbon dioxide contained in the passing urea solution, the LP recovery section comprising an LP dissociation section and an LP condensation section, the LP dissociation section comprising an inlet of urea solution, an outlet of an aqueous urea product stream and an outlet of dissociated exhaust gas, the inlet being in fluid communication with the outlet of the urea synthesis solution from the HP synthesis section, the outlet of the dissociated exhaust gas being in fluid communication with the gas inlet of the LP condensation section, the LP condensation section having an outlet of LP condensate, the outlet being in fluid communication with the... The equipment includes a DEF purification section, configured to subject the aqueous urea product stream to further ammonia removal. The DEF purification section has an inlet for a liquid urea stream, an outlet for a purified urea solution, and an outlet for DEF purified waste gas. The inlet is fluidly connected to the outlet of the aqueous urea product stream from the LP dissociation section. The outlet for the DEF purified waste gas is fluidly connected to the inlet of a DEF purification condensation section, configured to condense the DEF purified waste gas to obtain DEF purified condensate. The DEF purification condensation section has an outlet for the DEF purified condensate, which is fluidly connected to the LP recovery section. The method includes providing a fluid connection between the outlet of the DEF purified condensate from the DEF purification condensation section and the inlet of the urea solution from the LP dissociation section.

14. A method for improving an existing urea production facility, the facility comprising a high-pressure (HP) synthesis section configured to react ammonia and carbon dioxide under urea-forming conditions to obtain a urea synthesis solution; downstream and in fluid communication therewith, a low-pressure (LP) recovery section configured to recover residual ammonia and carbon dioxide contained in the passing urea solution, the LP recovery section comprising an LP dissociation section and an LP condensation section, the LP dissociation section comprising an inlet of urea solution, an outlet of an aqueous urea product stream, and an outlet of dissociated waste gas, the inlet optionally being in fluid communication via an MP section to the outlet of the urea synthesis solution from the HP synthesis section, the outlet of the dissociated waste gas being in fluid communication to the gas inlet of the LP condensation section, the LP condensation section having… The method includes adding a DEF purification section, configured to further remove ammonia from an aqueous urea product stream, having an inlet for a liquid urea stream, an outlet for a purified urea solution, and an outlet for DEF purified waste gas. The inlet is fluidly connected to the outlet of the aqueous urea product stream from the LP dissociation section. The outlet of the DEF purified waste gas is fluidly connected to the inlet of a DEF purification condensation section, configured to condense the DEF purification waste gas to obtain DEF purified condensate. The DEF purification condensation section has an outlet for the DEF purified condensate, which is fluidly connected to the inlet of the LP dissociation section.

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