EA040581B1 - METHOD AND PLANT FOR THE PRODUCTION OF UREA - Google Patents

Description

The technical field to which the invention belongs

The present invention relates to the production of urea, in particular urea solutions, and more particularly to the production of purified urea solutions which are suitable for use in NOx abatement systems, optionally after dilution with water. The invention relates in particular to the production of fluids (DEF) for cleaning exhaust gases from diesel engines, i.e. urea solutions, which can be diluted with water to give DEF, and other high purity urea solutions.

State of the art

The present invention relates to a method and plant for the production of a carbamide solution. The finished urea solution, optionally after dilution with water, is preferably suitable for use in a NOx reduction catalytic system and is preferably a DEF solution. Accordingly, it is required to have a very high purity of the product. For example, DEF is used in diesel vehicles to reduce NOx emissions. The composition of DEF for vehicles is defined in ISO 22241-1:2006. DEF for vehicles contains approximately 32.5 wt.% urea (ie essentially a eutectic composition) and also contains a very small amount of impurities. In some embodiments, the prepared urea solution can also be used, for example, to reduce NOx emissions in industrial plants, ships and locomotives. For NOx reduction systems used in maritime and rail transport, according to ISO 186111-1:2014, approximately 40 wt.% urea solution is used. To reduce NOx emissions from (fossil fuel) power plants, a 50 wt% urea solution is commonly used. As used herein, the term DEF includes a urea solution that is suitable, adapted, and/or designed for use in NOx reduction systems, for example, according to any of the above specifications.

For DEF, the concentration of urea is of great importance, since it determines the exact dosing of liquid when applied to the catalyst. A low concentration of organic impurities is important to prevent fouling and coking of the catalyst. A very low concentration of inorganic impurities, especially heavy metals, is important because such impurities lead to poisoning of the SCR catalyst. Metals will accumulate on the catalyst and thus shorten its life. In the DEF specifications cited, the maximum allowable limits for metallic impurities are close to typical achievable concentrations in urea streams produced in a typical urea stripping plant. In particular, under process upset or start-up conditions, the amounts of metals in the urea solution at the outlet of the urea stripper exceed DEF specifications. In the case of a urea plant that is designed for DEF only and does not produce solid urea (for example, plants without a urea curing section), this can result in significant quantities of materials with characteristics that do not meet product specifications that cannot be processed. at a given facility (eg within the boundaries of the facility). ISO 222411:2006 defines a limit for alkalinity as NH3 of less than 0.2% by weight; some commercially available DEF solutions have alkalinities as NH ₃ as low as 200 parts per million (ppm) by weight. Low alkalinity is desirable to reduce the risk of corrosion of equipment in contact with the DEF solution.

A known method for producing DEF is to dissolve commercially available solid urea (eg, used as fertilizer, in the form of prills and granules) in pure water with heating. However, the disadvantage of this method is the need for purification from aldehydes, since the refinement of urea (for example, granulation) usually uses aldehyde as an facilitating curing agent.

WO 2016/010432 describes a process for preparing a urea product suitable for dilution with water to form DEF. The method includes flash crystallization at subatmospheric pressure and packaging of the crystallized urea powder.

WO 2016/153354 describes a process for the integrated production of DEF and fertilizer - urea-ammonia mixture (UAN).

US 2008/0286188 describes a process for producing a urea-containing aqueous stream that is suitable for use in a device for reducing oxides of nitrogen (NOx) in the exhaust gases of an internal combustion engine, wherein the urea-containing aqueous stream is separated directly from or after the regeneration section in the urea production process and then diluted with water to obtain a urea content in the urea-containing stream of 30-35 wt.%. In one embodiment, the urea-containing stream is separated after a decomposition step, which preferably includes stripping using steam. US 2008/0286188 does not provide any information on treating the gas stream from the stripping step using steam.

Urea plants typically include a high pressure urea synthesis section and a medium and/or low pressure regeneration section. At

- 1 040581 in the manufacture of a solid urea product, the plant also includes an evaporation section downstream of the regeneration section. An evaporation section, for example, including a vacuum evaporation stage for evaporating water, is used to produce a carbamide smelt with a high urea content. The urea melt is cured in a urea curing device such as a pelletizing device or a prilling tower.

Water vapor from the evaporation section is condensed and the resulting condensate (which usually contains urea as an impurity) is treated in a wastewater treatment section, which generally includes a desorber and usually a hydrolyser. The treated condensate can be used, for example, as boiler make-up water (BFW) or scrubber cleaning fluid, for example for scrubbers provided in the recovery section.

With regard to the high pressure synthesis section, three generations of plants are usually distinguished. Initially, urea was obtained from CO2 and _NH3 in once-through reactors without recirculation of _NH3 and CO2. These plants soon gave way to the carbamate recycle total recycling process, which today is often referred to as conventional total recycling plants to distinguish them from later third generation plants that use a high pressure stripping process in the synthesis section. In high pressure stripping (HP) processes, most of the recycled unconverted NH ₃ and CO 2 (including ammonium carbamate) in the high pressure reactor effluent flows in the gas phase using a high pressure stripper and a high pressure carbamate condenser.

The high pressure stripper uses, for example, some or all of the high pressure feedstock CO2 as stripping gas to produce a high pressure stripped carbamide solution and a gas mixture stream which is condensed in a high pressure condenser.

The importance of traditional methods quickly faded with the development of methods using high pressure strippers in the synthesis section (Ullmann's Encyclopaedia, 2012, vol. 37, Urea, page 665).

The present invention provides a process and plant for the production of purified urea solutions, in particular as described above, and in particular having sufficient purity for use as DEF, preferably using relatively simple equipment and at low capital cost.

Disclosure of invention

The present invention relates to a process for producing a purified urea solution which is preferably suitable for use in NOx reduction, in particular for use as a diesel engine exhaust gas cleaning fluid (DEF) or for dilution to form DEF, the method comprising stages where:

A) reacting between CO2 and NH ₃ under urea synthesis conditions in a high pressure urea synthesis reactor to produce a urea synthesis stream containing urea, NH3, CO2 and some carbamate,

B) expanding the urea synthesis stream in the regeneration section with decreasing its pressure, wherein the expanded urea synthesis stream contains at least 90 wt.% of the specified amount of carbamate, and heating at least a part of the expanded urea synthesis stream in one or more decomposition devices at an average and/or low pressure to produce an aqueous urea stream and a regeneration section steam containing NH ₃ and CO ₂ ,

C) at least a portion of the aqueous urea stream is purified in the purification section, wherein the purification is preferably carried out by stripping, more preferably by low pressure (LP) stripping, and/or wherein after purification, preferably a urea solution with an alkalinity based on NH ₃ less than 0.2 wt.% with a urea content of 32.5 wt.%, to remove (excess) ammonia to obtain a purified urea solution and an off gas of the purification section containing water and ammonia,

D) optionally diluting at least a portion of the purified urea solution and/or the aqueous urea stream to be purified with water to obtain a target concentration of urea,

E) wherein the purification section off-gas is condensed to form a purification section condensate, and said purification section condensate is recycled to said urea synthesis reactor.

The process typically does not include high pressure stripping of unconverted NH ₃ and CO ₂ , resulting in feature B whereby the expanded urea synthesis stream contains at least 90 wt% of the original carbamate in the urea synthesis stream. In urea plants with high pressure strippers, a significant portion of the carbamate is recovered and sent to the carbamate condenser before expansion of the urea-containing stream under reduced pressure.

The present invention also relates to a plant for the production of a purified urea solution, wherein the purified urea solution is preferably suitable for use in NOx reduction systems, in particular for use as a diesel engine exhaust aftertreatment fluid (DEF) or for dilution to form DEF, containing:

- 2 040581

A) a urea synthesis section containing a high pressure urea synthesis reactor having a CO2 inlet and an NH ₃ inlet, in which the reaction between CO2 and NH ₃ is carried out under urea synthesis conditions to obtain a urea synthesis stream containing urea, water and ammonium carbamate,

B) a regeneration section in which the urea synthesis stream is heated under reduced pressure to produce an aqueous urea stream and a regeneration section steam containing CO2 and NH ₃ , wherein said regeneration section vapor is preferably condensed in said regeneration section to produce carbamate and ammonia, preferably with separate condensers for ammonia and carbamate,

C) a purification section in which the aqueous urea stream is treated, preferably purified, more preferably stripped, in a purification section treatment device to remove (excess) ammonia to obtain a purified urea stream and an off gas of the purification section,

D) optionally a urea dilution section in which a purified urea stream and/or an aqueous urea stream is diluted with water, the plant comprising a recirculation line for supplying carbamate from the regeneration section to the urea synthesis section and preferably a separate second recirculation line for supplying ammonia from the regeneration section to the section synthesis of carbamide, wherein the purification section further comprises a condenser of the purification section for condensing the exhaust gas of the purification section into the condensate of the purification section, and the installation contains a liquid flow connection for supplying the condensate of the purification section from the specified condenser of the purification section to the specified urea synthesis reactor.

In some embodiments, the plant does not include a high pressure stripper in the synthesis section to remove unreacted NH ₃ and CO 2 .

Brief description of the drawings

In FIG. 1 shows an exemplary flow chart of the method according to the present invention, and FIG. 1a-1d show its enlarged parts.

In FIG. 2 is a block diagram of an example of a method in accordance with the present invention.

Implementation of the invention

The present invention is based in part on the unexpected effect that expansion of the urea synthesis stream to reduced pressure without stripping it at high pressure leads to an increase in the purity of the resulting urea solution. The invention is also partly based on the rationale that a low NH ₃ content in the purified urea solution can be achieved by using a purification step in which, in addition to the purified urea solution, an off-gas containing water and ammonia is also formed, in which the off-gas is condensed and the resulting condensate is recycled. to the urea synthesis zone. In particular, the condensate is recycled through the liquid stream, so that the liquid aqueous fraction of the condensate is fed into the urea synthesis zone and ultimately ends up in the finished urea solution. This eliminates the need for condensate treatment in the wastewater treatment section. At the same time, the ammonia contained in the condensate reacts in the urea synthesis reactor. In addition, it has surprisingly been found that it is possible to achieve a relatively high conversion to urea in the urea synthesis reactor despite water recirculation from the purification section by using a synthesis section preferably without a high pressure stripper. In the preferred embodiment, no passivating air is used, resulting in a reduction in recirculated water flow from the inert gas scrubbers in the urea plant.

As used herein in the context of urea solutions and process fluid streams, high pressure (HP) is, for example, 120 to 300 bar; the mean pressure (MP) is, for example, 10 to 70 bar (including an intermediate pressure of 30 to 70 bar), in particular 15 to 25 bar; and the low pressure (LP) is, for example, 0 to 10 bar, in particular 1 to 8 bar, or 2 to 5 bar, or 3 to 10 bar, or 3 to 5 bar.

Step A involves the synthesis of urea in a urea synthesis reactor by reaction between CO ₂ and NH ₃ to form ammonium carbamate, which is dehydrated to give urea and water. The reactor is operated at a pressure preferably from 150 to 250 bar, for example from 180 to 250 bar, and/or, for example, at a temperature above 190°C. The indicated pressures and temperatures are higher than those normally used in urea plants with high pressure strippers. The reactor typically receives feed NH3 and feed CO2, both at high pressure and usually through separate inlets. The feed CO2 optionally contains passivating air to protect the reactor from corrosion; however, in preferred embodiments, very small amounts are used (e.g. less than 0.2 vol.%, preferably less than 0.1 vol.% (e.g., less than 0.10 vol.%), e.g., less than 0.050 vol. %) of passivating air or this passivating air is not used at all. For the reactor, the NH ₃ : CO 2 molar ratio (N/C ratio) is usually 2.0-6.0, preferably 2.7-5.0 and, for example, 3.3 to 5.0, from 3, 5 to 5.0 or 4.0 to 5.0, the ratio given reflecting the composition of the so-called initial mixture before

- 3 040581 urea production, consisting only of NH ₃ , CO2 and H2O. The molar ratio H2O : CO2 is eg 0.10-2.0, eg 0.50-1.0, also for the feed mixture before the production of urea. For example, recycling water from a treatment section results in a relatively higher H2O: _CO2 molar ratio. The 3.5-5.0 N/C ratio is higher than the 3:1 N/C ratio commonly used for the synthesis section of the Stamicarbon CO ₂ stripping process. A higher N/C ratio than 3:1 can be advantageously exploited by shifting the pressure minimum (or temperature maximum) towards higher NH ₃ : CO ₂ ratios as the amount of solvent (water and urea) in the reactor increases.

The CO ₂ feed usually also contains inert gases (ie gases that do not react in the urea synthesis reactor), such as, for example, H 2 from the CO 2 production process or N 2 from passivating air. The reactor also receives the carbamate recycle stream from the regeneration section in the form of a carbamate solution, which also contains water. The reactor has an outlet for the urea synthesis stream, for example at the top of the reactor and for example being the only outlet from the reactor. The urea synthesis stream at the outlet of the urea synthesis reactor contains urea, water, as well as unreacted NH ₃ and CO ₂ , carbamate, and usually also some inert gas. The urea synthesis stream typically contains liquid and vapor.

The process of the present invention generally does not involve high pressure stripping of the urea synthesis stream. Thus, the synthesis section is usually of the type of the so-called conventional synthesis section for urea production plants with full recycle. In this document, the term traditional means that the high pressure stripper is not used. As used herein, the term traditional does not indicate that any one or more features of the present invention are known or apparent, either alone or in combination.

The process of the present invention generally does not involve heating and/or high pressure gas/liquid separation of the entire urea synthesis stream or all of its liquid portion, for example at reactor operating pressures of or greater than 120 bar, and does not involve high pressure stripping such as stripping ammonia, thermal stripping (self-stripping) and CO2 stripping. These three types of high pressure stripping are well known to those skilled in the art and are discussed, for example, in Ullmann's Encyclopaedia, chapter Urea. The thermal stripping synthesis section typically uses a urea synthesis reactor with a carbamate/NH ₃ mixture inlet connected to an actuated NH ₃ feed ejector to feed recycle carbamate to the reactor. In some embodiments, the process does not include feeding CO2 into the high pressure urea synthesis stream at a location that is both downstream of the reactor outlet and outside the reactor (in the present invention, feedstock CO2 is typically fed into the reactor). However, in some alternative embodiments, a portion of the urea synthesis stream is distilled at high pressure and a portion is not. In some embodiments, at least a portion of the urea synthesis stream is not subjected to high pressure stripping. In some other embodiments in which some particular features are included, such as those incorporating a cyclone separator, the entire urea synthesis stream is stripped at high pressure, however, for example, with very little stripping gas.

Preferably at least 85%, preferably at least 90%, more preferably at least 95% of the fresh CO2 is fed directly into the reactor, i. is introduced into the reactor in the form of a gaseous stream. The remainder of the fresh CO2, if present, is eg introduced downstream of the synthesis section, eg to adjust the N/C ratio for optimal condensation of the ammonia as carbamate. In this document, fresh refers to the flow coming from the boundaries of the installation.

In addition, especially for the installation of the present invention, the urea synthesis section usually does not contain a high pressure stripper, more specifically does not contain a high pressure CO2 stripper. High pressure strippers (not commonly used in the present plant), for example, are shell and tube heat exchangers configured to pass the urea synthesis stream through the tubes and the steam through the annulus.

Moreover, the urea synthesis section typically does not include a high pressure carbamate condenser, such as a U-bundle device, where the high pressure condenser has a liquid outlet connected by piping to the inlet of the urea synthesis reactor.

Step B involves reducing the pressure, typically using an expansion valve, thereby expanding the urea synthesis stream, or at least the portion of said stream that is not subjected to high pressure stripping. Usually the pressure is reduced by at least 10 bar or at least 20 bar and is usually reduced to medium pressure. The expanded urea synthesis stream typically contains at least 90% by weight of the carbamate present at the urea synthesis reactor outlet and typically contains all of the unreacted carbamate since the process preferably does not include separating the carbamate from the urea synthesis stream between said reactor outlet and said expansion stage.

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Step B further includes heating at least a portion of the expanded urea synthesis stream, typically after gas/liquid separation, to decompose the unreacted carbamate, in one or more decomposers. In general, heating in stage B is carried out by indirect heat exchange.

For example, the decomposition device includes a distillation column and a decomposition unit, where the decomposition unit contains a heat exchanger for indirect heat exchange with water vapor. The distillation column provides countercurrent contact between the steam from the decomposition unit and the colder urea solution (from an earlier stage of the process), which allows the water vapor to condense from the steam. The decomposition unit is located, for example, under the distillation column.

The digesters operate at medium pressure (MP) and/or at low pressure (LP). Preferably, a medium pressure decompressor and a downstream low pressure decompressor are used in series. The low pressure decomposition device preferably operates at a pressure of 3 to 7 bar. The heated urea synthesis stream undergoes gas/liquid separation. The result is an aqueous urea stream and regeneration section steam containing NH3 and CO2. In one embodiment, it is possible to use the heat of the steam from the recovery section (containing gaseous CO2, NH3 and water vapor) from the medium pressure decomposition device to provide at least part or all of the heat to the decomposition unit of the low pressure decomposition device, for example, by indirect heat exchange between steam from this medium pressure regeneration section and the urea solution supplied to the decomposition unit of the low pressure decomposition device. In particular, the condenser for the steam of the medium pressure recovery section can be combined with such a decomposition unit, so that the steam of the medium pressure recovery section is condensed in an indirect heat exchange contact with the decomposition of the carbamate in the low pressure decomposition device.

The decomposition device, or the very first downstream of the stage B decomposition devices installed in the process, has an inlet to receive (at least a portion of) the expanded urea synthesis stream. In some embodiments, the molar ratio of NH ₃ : CO 2 in the urea synthesis stream (including both liquid and any gas) at this inlet is substantially the same as at the outlet of the reactor, for example, with a deviation of less than 10% relative to the value of this ratio by outlet of the reactor, i.e. less than 10% in numerical terms, for example, with an N / C ratio at the outlet of 4.0, the ratio at this inlet is in the range of 4.4-3.6.

Each decomposition device is preferably configured for gas/liquid separation and preferably includes a gas outlet and a separate outlet for the liquid containing urea.

The process typically further comprises condensing the regeneration section vapor in a regeneration section condenser and gas/liquid separation to produce a recycle stream of liquid carbamate and non-condensed gases. This condenser operates at a lower pressure than the urea synthesis reactor and generally operates at the same pressure as the corresponding decomposition device, eg medium pressure or low pressure. The liquid carbamate recycle stream is fed to the urea synthesis reactor, usually by means of a pump.

Non-condensed gases usually contain inert gases as well as NH ₃ and CO ₂ . The non-condensed gases are typically scrubbed, usually in a countercurrent manner, with the scrubber scrubbing liquid in one or more scrubbers to produce a scrubber gas and a liquid stream. The scrubber-cleaned gas may optionally be vented to the atmosphere after further treatment. The carbamate-containing liquid stream is recycled, optionally as a wash stream, to the urea synthesis reactor, for example via a condenser.

The scrubber scrubbing liquid typically contains water, and typically at least one of the scrubbers in the recovery section is fed with make-up water. In prior art urea plants, make-up water is typically obtained as treated condensate from the wastewater treatment section. In the present invention, the make-up water, for example, is supplied from another plant, such as an upstream gasification plant and/or an ammonia plant. For the production of a purified urea solution with a urea content of 32.5-50 wt.%, in any case, additional water must be supplied from the boundaries of the installation, since in the reaction for obtaining urea a mixture is formed with a molar ratio of urea and water of 1: 1, i.e. containing 77 wt.% carbamide.

Make-up water used as (part of) the scrubber scrubbing fluid in the absorber can simply be used as a carbamate solvent in the condenser by directing the spent scrubber scrubbing fluid to the condenser (wash stream).

In a preferred embodiment, stage B includes medium pressure and low pressure decomposition devices connected in series. Thus, step B preferably comprises: B1) expanding the urea synthesis stream to medium pressure, optionally gas/liquid separation, and heating at least a portion of the urea synthesis stream at medium pressure to produce medium pressure regeneration section steam and medium pressure aqueous urea solution. - 5 040581, and B2) expanding the medium pressure aqueous urea solution to low pressure and heating the expanded aqueous urea solution at low pressure to produce low pressure regeneration section steam and a second low pressure aqueous urea solution.

In this case, the recycling of the carbamate in step B preferably comprises:

a) condensing said steam from the medium pressure recovery section at medium pressure in a medium pressure condenser to form a medium pressure carbamate stream, and

b) condensing said vapor from the low pressure regeneration section at low pressure in a low pressure condenser to form a low pressure carbamate stream.

In some embodiments, the plant comprises a carbamate recycle line and a separate ammonia recycle line connecting the regeneration section and the high pressure reactor, in particular the medium pressure condenser and the reactor. The uncondensed gas from the medium pressure condenser is preferably condensed in an ammonia condenser and sent to a high pressure reactor separate from the medium pressure carbamate stream. In this way it is possible to adapt to excess NH3 due to the relatively high N/C ratio in the reactor. The recycled carbamate may contain free NH3 and preferably has an N/C ratio of 2.0-2.5, eg 2.0-2.3. If the (medium pressure) condenser feed gas has a higher N/C ratio, the excess NH ₃ is preferably condensed separately and recycled to the urea synthesis zone.

In step B, an aqueous solution of urea is formed, which usually contains ammonia, for example, unreacted ammonia (especially in view of the relatively high N/C ratio) and/or ammonia formed during the decomposition of ammonium carbamate in step B. The aqueous urea solution formed in step B typically contains less than 2.0 wt.% or less than 1.0 wt.% ammonium carbamate and, for example, more than 0.20 wt.% residual ammonium carbamate, in particular, based on the total weight of the stream.

Step C is used to produce a purified urea solution, such as the desired DEF or a DEF precursor solution. The method includes the step C of purifying the aqueous urea stream obtained in step B in a purification section to obtain a purified urea solution stream and an off-gas of the purification section. During purification, ammonia is preferably removed from the urea solution. In particular, the purification preferably removes excess ammonia, i. e. ammonia in excess of the desired level, in particular in excess of the desired alkalinity in terms of NH ₃ . This removal ensures that a sufficiently low alkalinity of the purified solution is obtained. Low alkalinity is important to prevent the risk of corrosion of equipment that comes into contact with the DEF solution. The advantage of the lower NH ₃ content in the DEF solution is that the ammonia odor is reduced. The removal of ammonia means the transfer of NH ₃ from the liquid phase to the gas phase; NH ₃ in the liquid phase is formed, for example, at least partially, as a result of the decomposition of ammonium carbamate in the liquid phase in stages B and/or C and/or enters there in the form of unreacted ammonia from stage A.

Purification, preferably stripping, preferably provides an alkalinity in terms of NH ₃ from such purification of less than 0.20 wt.%, less than 1000 ppm, less than 500 ppm or less than 200 ppm , all based on weight, with a urea content of 32.5 wt.%, i.e. alkalinity, calculated based on the addition or removal of the required amount of water to obtain a urea content of 32.5 wt.%; in other words, the indicated alkalinity levels refer to a carbamide solution with a concentration of 32.5 wt.%. Lower alkalinity is preferred.

In some embodiments, the purification comprises reducing the alkalinity, as NH3, of the urea solution to be purified by at least 50%, or at least 90%, or at least 99%, based on a relative fraction of the original alkalinity, based on NH3, by transfer of the corresponding amount of ammonia (in any form of being in solution) into the gas phase. A purified solution with such a low alkalinity typically has a urea content of less than 95 wt.% or less than 90 wt.% (for example, is not a urea melt), and/or usually has a urea content of more than 20 wt.% or more than 30 wt. .%, for example, has a urea content in the range of 20-95 wt.%. In an embodiment where purification results in a urea solution with an alkalinity in terms of NH ₃ of less than 0.2 wt.%, the original urea solution may already have an alkalinity below a given level, in which case the purification further reduces the alkalinity in terms of NH 3 _NH3 .

In some embodiments, purification, such as stripping, in particular low pressure stripping, involves decomposing ammonium carbamate into NH ₃ and CO 2 and transferring the thus obtained NH 3 from the liquid phase to the gas phase to remove NH 3 from solution, usually together with the transfer of free NH 3 from the liquid phase to the gas phase. Typically, this decomposition reduces the alkalinity as well as the carbonate content (as CO2), for example, to a carbonate content (as _CO2 ) in said purified solution of less than 0.5 wt.%, or less than 0.2 wt.%, or to less than 1000 hours/million, or less than 500 hours/million, by weight, in terms of urea solution with a concentration of 32.5 wt.%.

Cleaning includes, for example, pressure reduction (i.e. absolute pressure reduction), heating,

- 6 040581 stripping, as well as their combinations. In some embodiments, cleaning includes heating and/or depressurization without stripping.

The cleaning preferably includes stripping at low pressure. Stripping allows the cleaning temperature to be lowered, thereby leading to an advantageous reduction in biuret formation. Stripping preferably involves bringing the urea solution into contact with the gaseous stream in a countercurrent manner. The gaseous stream usually has a lower partial vapor pressure of NH ₃ than the urea solution that is in contact with this gas. Stripping preferably includes steam stripping. You can also use air stripping. Steam stripping preferably involves bringing an aqueous urea solution into contact with steam in a countercurrent manner. The water vapor is preferably supplied from the boundaries of the installation. In an alternative embodiment, steam can be generated by evaporating water from the urea solution, eg downstream of the purification step, eg by boiling. The stripping is preferably carried out at a pressure of less than 3 bar. The advantage of steam stripping is to obtain a stripping effect due to the low partial pressure of ammonia vapor, preferably with the additional maintenance of a high water vapor pressure to reduce water evaporation. Some advantages of direct steam stripping with externally supplied steam are that dilution of the urea solution with water is useful for producing DEF and that downstream boiling of the urea solution which causes biuret formation can be avoided.

For example, the purge may be based on heating the solution, which causes the ammonia to evaporate, and preferably also includes reducing the partial pressure of NH3 in the gas phase by stripping and/or lowering the absolute pressure. The result of the heating and the preferential reduction of the (absolute) pressure is also the evaporation of the water. The off-gas of the cleaning section may contain predominantly water vapor, for example contain more than 50 wt.%, more than 70 wt.%, more than 90 wt.% or more than 99 wt.% water and, for example, from 0.5 and/or up to 5 wt.% NH ₃ . In some embodiments, the purification section off-gas and/or the purification section condensate contain NH ₃ in an amount corresponding to at least 0.010 wt% and/or a maximum of 5.0 wt% of the urea solution entering the purification section. The cleaning in the cleaning section is preferably based on direct heat exchange and preferably involves bringing into contact, for example mixing, the urea solution with a fluid stream having a higher temperature than the urea solution.

For example, cleaning includes steam stripping of an aqueous urea solution. Steam stripping typically involves direct injection of water vapor into an aqueous urea stream, usually in a counter-current manner, such as where the liquid flows from top to bottom and the water vapor rises from bottom to top. The steam stream preferably contains at least 90 wt% H2O, more preferably at least 95 wt% H2O. The water vapor is preferably supplied from the boundaries of the installation, for example from another installation, such as engineering structures. The pressure of the injected water vapor is, for example, 1 to 30 bar (absolute), preferably 2 to 15 and usually 2 to 6 bar (absolute). The cleaning step may be carried out by low pressure stripping and may include steam stripping. Stripping at low pressure is carried out at a pressure of less than 10 bar, or less than 5 bar, less than 3.0 bar, less than 2.0 bar or less than 1.5 bar (absolute) or at a pressure of 0.10-1, 1 bar (absolute), for example, at a pressure of less than 1.0 bar (absolute), for example, at a pressure of 0.010 to 0.50 bar or 0.4 to 0.5 bar (absolute). Such operating pressures are used in particular in a steam stripper. The method includes, for example, expanding the urea solution at a pressure of 3 to 7 bar, obtained from stage B, to the pressures of the stripping stage, for example, to a pressure of less than 1.5 bar (absolute). The steam stripper for step C is, for example, a vessel configured to supply steam and liquid in a countercurrent manner, having a liquid inlet at the top and a liquid outlet at the bottom, and a steam inlet at the bottom and an outlet for gas at the top. The vessel contains, for example, plates and/or nozzle.

The preferred low operating (absolute) pressure in the cleaning section advantageously allows cleaning by evaporating ammonia at a lower temperature, thereby reducing biuret formation. Thus, in a preferred embodiment, step C comprises expanding the aqueous urea solution obtained from step B, preferably the second aqueous urea solution obtained from low pressure decomposition step (B2), to the operating pressure of the purification section, and then stripping the expanded aqueous urea stream, wherein steam stripping involves direct injection of steam into the expanded urea aqueous stream.

The method includes condensing the waste gas of the purification section to the condensate of the purification section. The present invention proposes a simple method for disposing of the treatment section condensate that contains ammonia without using a wastewater treatment section by recycling the treatment section condensate back to the urea synthesis reactor, preferably by means of a liquid stream.

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Thus, ammonia does not get into the finished purified urea solution, but participates in the urea synthesis reaction in the synthesis section. Due to this recirculation, the aqueous fraction of the condensate is also fed into the urea synthesis reactor.

Recirculation of the purification section condensate to the urea synthesis section is preferably carried out by means of a liquid flow, i. e. by creating one or more fluid streams. Accordingly, the plant preferably comprises a fluid connection from the condenser of the purification section, either directly or indirectly, to the synthesis section.

The condensation step can be implemented as the absorption of ammonia in an aqueous stream, such as a make-up water stream, with a gas/liquid separation to produce a condensate stream and a second off-gas stream.

The method essentially includes condensing the regeneration section steam to form a carbamate stream in the regeneration section condenser and recirculating the carbamate stream to the urea synthesis reactor. The cleaning section condensate is preferably sent to a regeneration section condenser. The condensate recirculation stream of the purification section is thus combined with the carbamate recirculation stream of the regeneration section. Thus, the steam used in purification step C and the water used to absorb ammonia can easily provide at least a portion of the condensate water required for the regeneration section condensers to prevent carbamate crystallization. In some embodiments, the carbamate recycle stream is passed through a gas/liquid separator to separate the ammonia gas from the carbamate solution, where the ammonia gas can be separately recycled to the reactor, optionally through an NH3 feed compressor.

The purge section condenser may include a gas/liquid separation to provide a purge section condensate stream and a second off-gas stream. At least a portion of said second off-gas stream preferably enters the condenser of the regeneration section. The second off-gas stream is preferably fed into the scrubber, for example through a steam ejector.

The scrubber for the second off-gas stream is preferably a scrubber to which non-condensed gases are directed from at least one condenser of the steam recovery section from the decomposition device, optionally via an additional condenser. This scrubber is preferably used for scrubber cleaning fluid, such as make-up water, to produce scrubber gas (which is typically vented to the atmosphere) and scrubber waste fluid. The scrubber waste scrubbing liquid scrubbing stream, for example, is fed to an additional condenser and then to the vapor condenser from the decomposition device.

In an embodiment with medium pressure and low pressure decomposition devices connected in series, the second off-gas stream preferably enters the low pressure scrubber, more preferably with an additional low pressure condenser installed between the low pressure condenser for condensing the low pressure recovery section steam and said low pressure scrubber. The optional low pressure condenser receives non-condensed gases from the low pressure condenser and is provided with an outlet to carry the gas to the low pressure scrubber.

The purified urea solution is a urea solution with a urea content of, for example, 50-70% by weight. The purified urea solution from step C, in some embodiments, is obtained as a DEF precursor solution product that can be packaged and/or stored. In some embodiments, the water content of the purified urea solution obtained from step C is no more than 10 percentage points lower (arithmetic difference) than the water content accepted by the purification section, or no more than 5 percentage points lower, and / or exhaust gas the cleaning section contains water vapor in an amount of less than 10 wt.% or less than 5 wt.% of the aqueous urea solution received by the cleaning section. Thus, stage C usually does not include intensive evaporation of water.

The method optionally includes the step D of diluting the purified urea solution with water, i.e. by adding water, to the desired target concentration of urea, for example up to about 32.5 wt.% urea for DEF in accordance with ISO 22241-1:2006, for example up to 30-35 wt.% urea, up to about 40 wt.% urea solution in accordance with ISO 186111-1:2014, for example up to 33-45 wt.% urea or, for example, up to about 50 wt.% urea solution for reducing NOx emissions in industrial plants, for example up to 45-55 wt.% , or diluted to any other desired target concentration. An optional dilution step is carried out, for example, in a urea dilution section, which is, for example, a mixer, mixing vessel or pipe, or channel, in which water is added to the urea solution. In the dilution step, for example, a ready-made urea solution stream is obtained.

The dilution water is, for example, demineralized water (for example, deionized water) from engineering facilities or, for example, purified process condensate,

- 8 040581, for example, from another plant (such as an upstream gasification plant or an ammonia plant) or, for example, from a urea production process if the urea plant includes a wastewater treatment section. The dilution water can also be, for example, steam condensate from the plant's steam system or from other plant or engineering structures. In principle, dilution water can also (and alternatively) be added upstream in the process (at earlier stages of the process), i.e. in addition to dilution downstream of said purification stage or alternatively (instead of) such dilution downstream flow from said purification step. Dilution upstream of the purification stage can be useful to prevent crystallization of urea in the purification stage and to prevent crystallization between stage B and stage C, for example, in a urea storage tank installed between the regeneration section and the purification section. Thus, in some embodiments, the aqueous urea stream is diluted by adding water prior to the purification in step C, and this dilution can also be performed in the dilution section. The dilution section may include two dilution devices, for example one upstream of the cleaning section and one downstream of said section.

The purified solution from step C and optional step D has a low impurity content. Preferably the impurity content corresponds to the intended use as DEF or for dilution to DEF, in particular DEF according to the requirements of ISO 22241-1:2006 (i.e. with a urea content of 32.5 wt.%) and/or ISO 186111-1: 2014. Thus, in some embodiments, the content of biuret does not exceed 0.3 wt.% and/or the content of NH3 does not exceed 0.2 wt.%, for example, the alkalinity in terms of NH3 does not exceed 0.2 wt.%. The content of carbonate in terms of CO ₂ preferably does not exceed 0.2 wt.%. In addition, the content of aldehydes does not exceed 5 ppm (by mass) and/or the content of insoluble impurities does not exceed 20 ppm (by mass). The content of each of PO ₄ , Ca, Fe, Al, Mg, Na and K preferably does not exceed 0.5 ppm. The content of each of Cu, Zn, Cr and Ni preferably does not exceed 0.2 ppm (all ppm values are by weight). For the DEF precursor solution, the impurity levels are preferably such that the above impurity levels will be obtained after dilution by adding water to obtain a urea content in accordance with the specification for DEF (for example, with a urea content of 31.8 wt.%). Thus, in some embodiments, step C is a treatment, preferably stripping, to produce a urea solution with a biuret content of not more than 0.3 wt% and/or an NH3 content of not more than 0.2 wt%. In some embodiments, step C is a treatment, preferably stripping, to produce a urea solution with a biuret content of not more than 0.3 wt.% and/or an NH3 content of not more than 0.2 wt.% when the concentration of urea is adjusted to 32.5 wt. .%, i.e. when the urea content is set to 32.5 wt.% by adding or removing the required amount of water.

Surprisingly, it was found that when using the installation and method of the present invention, the levels of impurities (heavy) metals and biuret are significantly lower than in the installations of the prior art. Without wishing to be bound by any theory, the present inventors believe that although the reactor in the process of the present invention operates at a higher pressure and/or higher temperature than the reactors in a typical stripping urea plant, fewer high pressure and/or less surface area in contact with the corrosive carbamate solution results in significantly lower metal concentrations in the finished DEF solution. The lower biuret level is provided by the process conditions being different from those in the stripper, in particular the higher N/C ratio, so that the amount of NH3 in the recovery section is increased. A lower level of biuret, such as not more than 0.20 wt.%, or not more than 0.10 wt.%, or not more than 0.05 wt.% in a purified solution (for example, in terms of 32.5 wt.% urea solution) indicates a higher product quality, as more urea is available to reduce NOx emissions in the (catalyzed) reaction of urea with NOx.

A solution, optionally after a dilution step, such as DEF and/or a DEF precursor solution, is obtained as a product. Thus, the solution is removed as a finished product at the boundary of the plant. The resulting solution is, for example, stored in a storage container. The finished solution, for example, is dosed in batches, for example from a storage container. The batches are, for example, packaged, for example, in vessels or containers, which is preferable for keeping clean, and/or pumped into vehicles such as cars or ships. The finished solution, for example, contains from about 30 to about 50 wt.% urea, or from about 32.5 wt.% to about 40 wt.%, or from about 40 to about 50 wt.%. The plant is preferably provided with a container for storing the purified urea solution and a device for dosing the purified urea solution in a batch, and also preferably with a transport pipeline from the storage container to the product dispensing device for dispensing the finished purified urea solution into containers and / or into mobile vehicles, such as ship, train or

- 9 040581 car.

In an interesting embodiment, the urea plant does not include a urea curing device such as a prilling tower or a granulating device. In addition, in some embodiments, the urea plant does not include an evaporation section for concentrating urea by evaporating water, in particular does not include a vacuum evaporator for evaporating water from the urea solution obtained from the regeneration section. In known plants, the (vacuum) evaporation section is usually located between the regeneration section and the curing section, in order to provide a urea melt, for example containing more than 90 wt.% urea, by evaporation of water. In conventional urea plants, the water evaporated in the flash section is not recycled to the high pressure synthesis section, since the water is essentially a by-product of the urea synthesis. This is in contrast to the water vapor in the gas stream from the decomposition unit in the regeneration section, which is essentially recycled to the synthesis section as part of the carbamate recycle stream, which helps prevent carbamate crystallization.

Known urea plants typically include a wastewater treatment section which is used to treat the condensate resulting from the condensation of steam from the evaporation section. In one example of a known wastewater treatment section, the urea present in the condensate is hydrolyzed to NH ₃ and CO 2 which are steam-distilled off and the treated process condensate is blown off. In accordance with many environmental regulations, only purified condensate may be discharged into the environment.

In some embodiments of the present invention, the plant does not include a wastewater treatment (WWT) section. In some embodiments of the present invention, the installation does not include a wastewater treatment section containing a desorption device and/or a hydrolysis device. Water condensate of the purification section is recycled to the urea synthesis reactor.

The cleaning section preferably includes a cleaning section processing device and a condenser. The treatment device is preferably a steam stripper and more preferably a low pressure steam stripper. The steam stripper has a supply element and/or an inlet for water vapor, which is usually connected to a source of water vapor, for example, at the boundaries of the installation, such as engineering structures. The steam stripper preferably includes a stripping column containing a cyclone separator at the top, a liquid distribution tray and a packed bed. The packing bed preferably contains Pall rings and is typically placed below the liquid distribution tray. During operation, a two-phase fluid mixture of the expanded aqueous urea stream enters the cyclone separator; the gas exits through the top, the liquid flows down from the cyclone separator, for example onto a liquid distribution plate, which distributes the liquid over the packed bed. The gas rises upwards, for example through caps on a liquid distribution plate.

The cleaning section is preferably configured to operate at low pressure, for example up to 1.5 bar (absolute), and preferably below 1.0 bar (absolute). The cleaning section preferably comprises an ejector, a blower and/or a vacuum pump to maintain a low pressure, eg less than 1.0 bar (absolute), preferably an ejector.

The plant preferably includes a recirculation line for supplying carbamate from the regeneration section to the urea synthesis section and preferably includes a separate second recirculation line for supplying ammonia from the regeneration section to the urea synthesis section. A separate ammonia recycle line (for example, optionally via an ammonia receiver) can be used to recycle excess ammonia (compared to carbamate recycle) at a lower temperature than the carbamate recycle temperature. An excess of ammonia can be provided due to the relatively high N/C ratio in the synthesis section. The regeneration section preferably includes a condenser for condensing the steam of the regeneration section to produce carbamate, and preferably includes a second separate condenser for ammonia production. The second condenser preferably receives uncondensed gas from the condenser of the recovery section, in which the carbamate is formed.

The plant preferably comprises a pipeline for supplying a gas flow from the cleaning section to an off-gas condenser, a pipeline for supplying a liquid stream from an off-gas condenser to said regeneration section condenser having a steam inlet for the regeneration section, and a pipeline for supplying a liquid stream from said regeneration section condenser to said urea synthesis reactor. Thus, the ammonia from the waste gas of the purification section is preferably recycled to the reactor via the regeneration section.

The reactor is preferably a vertical type reactor. The reactor preferably has a feed CO2 inlet and preferably a compressed NH ₃ inlet, these inlets may optionally be combined.

The urea synthesis reactor preferably contains a ferritic-austenitic duplex stainless steel, for example, at least partially made of such steel, for example, as described

- 10 040581 in the application WO 95/00674, and, for example, is commercially available under the trade name Safurex®, and, for example, registered as ASME 2295-3 and, for example, as UNS S32906; or as described in the application WO

2017/014632.

For example, the ferritic-austenitic duplex steel has a chromium content of 28 to 35 wt.% and a nickel content of 3 to 10 wt.%, and preferably has the following composition: C: not more than 0.05 wt.%; Si: not more than 0.8 wt.%; Mn: 0.3-4.0% by weight; Cr: 28-35 wt%; Ni: 3-10 wt%; Mo: 1.0-4.0% by weight; N: 0.2-0.6% by weight; Cu: not more than 1.0 mass%; W: not more than 2.0 wt.%; S: not more than 0.01 wt.%; Ce: not more than 0.2 wt.%; the rest is Fe and the usual impurities and additives, for example, the rest is Fe and unavoidable impurities, and ferrite is from 30 to 70% by volume. Another suitable duplex stainless steel is DP28W (TM) which is also designated ASME 2496-1 and UNS S32808. Such duplex stainless steel preferably has the following composition (% by mass): C: 0.03 or less; Si: 0.5 or less; Mn: 2 or less; P: 0.04 or less; S: 0.003 or less, Cr: 26 or more but less than 28, Ni: 6-10, Mo: 0.2-1.7, W: more than 2 but not more than 3, N: more than 0 ,3, but not more than 0.4, the rest is Fe and impurities, and the content of Cu as impurities is not more than 0.3% and is preferably the steel described in patent EP 2801396.

The amount of passivating air, if used, is preferably 0-0.6 wt.%, preferably 0-0.3 wt.%, more preferably 0-0.1 wt.% of the amount of raw CO ₂ . If passivating air or oxygen is used, it can be introduced, for example, together with the NH3 feed and/or the CO ₂ feed. In some embodiments, no passivating air is added to the CO ₂ feed, especially if the reactor is made from corrosion resistant materials such as these steels. Such small amounts of passivating air allow for less inert gas to be present in the urea synthesis stream and may provide a reduction in scrubber load and, accordingly, relatively less spent scrubber scrubbing liquid can be fed to the urea synthesis reactor, for example, with streams 533 and 515 per fig. 1.

The present invention can be used to modify already existing installations, as well as for installations built from scratch (ie, newly built installations). The present invention can be used for urea production plants of any size, in attractive embodiments the plant and/or process is a small scale urea production plant and/or process, e.g. for example, with a capacity of 50-500 MTPD. In some embodiments, the present invention is implemented as total recycling plants without high pressure strippers with a capacity of 50-1000 MTPD or 50-500 MTPD. The present invention also relates to a method for constructing such a plant, preferably comprising building sections A, B and C from scratch. The plant of the present invention is characterized by relatively low capital costs and small building area requirements.

The urea plant usually receives feed NH3 from the ammonia plant, and the ammonia plant is preferably provided with an ammonia storage tank and a dosing device for finished ammonia for use as fertilizer in a batch, and preferably also with a transport pipeline for supplying ammonia from the tank for storing ammonia to a product dispensing device for dispensing the finished ammonia fertilizer into a container and/or into a mobile vehicle such as a ship, train or car. For a plant comprising an ammonia plant and a urea plant, where the urea plant is as described above, the ammonia plant preferably has a switchable ammonia transport line to the feed NH3 supply inlet of the urea plant, so that a ready the ammonia could be supplied to the finished ammonia dispenser and to the urea plant in a variable ratio, for example, varied so that 0 to 100% of finished ammonia could be supplied to the finished ammonia dispenser.

Advantageously, the synthesis section may comprise a single high pressure reactor, thereby reducing capital costs compared to a process using high pressure stripping. The absence of a high pressure stripper (and heating in such a stripper, in particular in ammonia strippers and thermal strippers, and also in the case of CO ₂ strippers) leads to a decrease in the content of biuret in the urea solution, which is especially important for DEF. Due to the exclusion of stripping (and high pressure carbamate condenser), the content of metals in the carbamide solution is reduced, and the process is carried out in less aggressive technological conditions from the point of view of corrosion. In particular, the intermediate product, ammonium carbamate, is extremely corrosive at high temperature, such as the temperature in a high pressure stripper. In particular, in a high-pressure stripping plant, the reactor itself may be operated at a temperature of, for example, approximately 185°C (compared to a temperature of, for example, 195°C for a conventional urea plant reactor), but the maximum temperatures at the top high pressure strippers reach, for example, 210-220°C. In addition, high pressure strippers are usually shell-and-tube heat exchangers in which the urea solution

- 11 040581 is supplied through pipes, and water vapor - through the annular space, and in the high-pressure stripper, the area of the metal surface in contact with the ammonium carbamate solution is large due to the large number of pipes. Metal reduction is particularly advantageous in embodiments of a plant of the present invention without a urea curing section and/or a plant specifically designed to produce pure urea solution only.

The reduction of metals in the urea synthesis stream and in the final urea solution is important for their use as DEF and as a reductant in NOx reduction systems, since these systems contain an (expensive) selective catalytic reduction catalyst. The content of metals in the carbamide solution leads to a reduction in the service life of the catalyst due to the accumulation of metals. Therefore, lowering the metal content (i.e., metal concentration) in the purified urea solution provides an increase in catalyst life.

In a particular embodiment, no or substantially no passivating air is used, such as less than 0.05 wt%, or less than 0.010 wt%, or even less than 0.0010 wt% oxygen in the CO2 feed. Known stripping (CO2) urea plants, even when using corrosion resistant materials such as Safurex®, for high pressure synthesis equipment, typically use some passivating air (e.g., approximately 0.3 wt% oxygen in feed CO ₂ ), given the aggressive conditions, especially in the high pressure stripper. The process fluid (containing ammonium carbamate) is generally more corrosive under high pressure stripper conditions than under high pressure fusion reactor conditions, eg due to higher temperature. The use of a synthesis section that does not include a high pressure stripper makes it possible to optionally eliminate the use of passivating air. This has the advantage of being able to eliminate the step of removing hydrogen from the feed CO2. Since the feed CO2 is usually produced from natural gas in a gasification plant, the stream contains H2, which must be removed when passivated air is used to reduce the risk of generating an explosive off-gas from the urea synthesis reactor containing oxygen and hydrogen. Thus, in embodiments without the use of passivating air, the feed CO2 may contain greater than 20 ppm or greater than 100 ppm by weight H2.

An additional benefit is the reduction of inert gases (including N2) since no passivating air is added, which reduces the load on the scrubbers.

The present invention also relates to a process for producing a purified urea solution, comprising the steps of:

A) reacting between CO2 and NH3 under urea synthesis conditions in a high pressure urea synthesis reactor to produce a urea synthesis stream containing urea, NH ₃ , CO2 and some carbamate, and also usually water, the urea synthesis being carried out in a high pressure synthesis section that does not contain a high pressure stripper and/or in a reactor fed with CO2 and NH3,

B) expanding the urea synthesis stream in the regeneration section with decreasing its pressure, while the expanded urea synthesis stream preferably contains at least 90 wt.% of the specified amount of carbamate, and heating at least a part of the expanded urea synthesis stream in one or more decomposition devices at medium and/or low pressure to obtain an aqueous stream of urea and steam regeneration section containing NH ₃ and CO ₂ ,

C) treating at least a portion of the urea aqueous stream in the treatment section to remove (excess) ammonia to obtain a purified urea solution and an off gas from the treatment section containing water and ammonia,

D) optionally diluting at least a portion of the purified urea solution and/or aqueous urea stream with water to obtain a target urea concentration,

E) wherein the purification section off-gas is condensed to form a purification section condensate, and said purification section condensate is recycled to said urea synthesis reactor; and further preferably having the features described above. In some embodiments, the step C purification is stripping and/or results in a urea solution with an alkalinity based on NH3 of less than 0.20 wt.% with a urea content of 32.5 wt.%; with additional preferred features regarding stripping, dilution and purification as described herein.

Detailed description of the drawings

In FIG. 1 shows an exemplary embodiment of the method and apparatus, which does not limit the present invention or its claims.

Liquid ammonia 109 is fed from the plant boundaries into the ammonia receiving vessel V106 through heater E100. By means of booster pump P101 ammonia 109X, 110A is pumped to high pressure ammonia pump P102 and compressed to a pressure of approximately 200 bar. Pump sealant water P102 may seep into the process stream, shown as stream 1204. Ammonia 111, 112 is then directed through preheater E103 to the mixing compartment of the high pressure reactor R201.

The carbon dioxide 103 is supplied from the plant boundaries, optionally together with a small amount of air 104, for example via blower S101, as gas 105 to the carbon dioxide compressor K102 and is compressed to a pressure of approximately 200 bar, optionally also producing an aqueous condensate 106 to remove water droplets and stripping stream 108 CO ₂ . A hydrogen converter R101 is also optionally provided, for example after the compressor K102. In this optional R101 converter, the hydrogen present in the carbon dioxide 117 is removed by catalytic combustion. Part of the air supplied is used for this catalytic combustion, and the remainder is used to passivate the synthesis section equipment and thereby prevent corrosion. Hydrogen-free carbon dioxide 118 is also fed into the mixing compartment of the high pressure reactor R201. In some embodiments, passivation air is not used and converter 101 is not present.

Dehydration of ammonium carbamate to urea and water is carried out in a high pressure R201 reactor. The maximum temperature in the reactor is maintained at approximately 200°C. The missing heat required to reach the specified maximum temperature is supplied by heating the ammonia in heater E103 before being fed into the mixing compartment.

The urea synthesis stream (202, 212) containing uncondensed vapors and urea solution leaving the urea synthesis reactor R201 at its top is expanded to an average pressure of approximately 20 bar (501, 511) using expansion valve X1. As a result of the expansion, some of the carbamate remaining in the solution decomposes and evaporates to form part of the vapor 505. The remaining urea solution 512 is distributed in the distillation column C501, for example, over a layer of Pall rings. From the bottom of the distillation column, the urea/carbamate solution 513 is sent to the decomposition device E501, where it is raised to about 160° C. to decompose the carbamate remaining in the urea solution. The heat required for this decomposition is provided by water vapor. In a separator located at the bottom of the distillation column, the gas phase 502 is separated from the liquid phase 514. The gases 502 are sent to the distillation part C501 of the distillation column, where the gases 503 are cooled by a colder urea/carbamate solution 512. This leads to the condensation of part of the water vapor contained in the gases. The gases 504, 505 exiting the distillation column are introduced (as gases 508X, 521X) into the lower part of the medium pressure carbamate condenser E503, where they are mostly condensed as carbamate. The heat of condensation is dissipated in the temperature-controlled cooling water system. The uncondensed high ammonia vapor 521 from the medium pressure carbamate condenser flash vessel V501 is fed to wash column C502. In this wash column C502, the ammonia is separated from the water/carbon dioxide mixture by adding ammonia-containing liquid (110B, 113 and 114) from vessel V106, for example as reflux. Liquid stream 515 exits wash column C502 at the bottom and is added to gas stream 505. Ammonia vapor 523, containing only trace amounts of carbon dioxide, is removed from wash column C502 at its top and condensed in ammonia condenser E101. The condensed ammonia 530 is fed into the ammonia receiver V106 and is thus recycled to the reactor R201 separately from the carbamate recycle stream 510.

From condenser E101, uncondensed gas 524, a mostly inert gas, enters atmospheric absorber C102 along with gas 109A from vessel V106 and is washed with demineralized water 1205. Uncondensed gas 526 is vented to the atmosphere. The liquid stream 531 from the bottom of the atmospheric absorber C102 is fed through pump P103 to the cooler E102, and the cooled stream 532 in its part 533A is used as a scrubber cleaning liquid in the atmospheric absorber C102, and the rest of the washing part 533 is used as a scrubber cleaning liquid in the wash column C502.

The urea solution 514 exiting the medium pressure decomposition device is expanded to a low pressure of approximately 4 bar by expansion valve X2. As a result of the expansion, some of the carbamate remaining in solution decomposes and evaporates to form a two-phase fluid stream 301, 311, which is fed to the top of the low pressure distillation column C303. The gas/liquid separation at the top of the low pressure distillation column C303 produces a urea solution 312 and part of the vapor 305.

The remaining liquid 312 is distributed in the low pressure distillation column C303, for example, over a bed of Pall rings. The urea/carbamate solution 313 from the bottom of the distillation column is fed to heater E302 where it is raised to about 135° C. to decompose the remaining carbamate. The heat required for this decomposition is provided by low pressure steam. In a separator installed at the bottom of the distillation column, the gas phase 302 is separated from the liquid phase 314. The gases 302 are fed into the distillation part of the distillation column, where the gas 303, 304 is cooled by a colder urea/carbamate solution 312. This leads to the condensation of part of the water vapor contained in the gases. The gases 304, 305 exiting the distillation column are introduced into the lower part of the low pressure carbamate condenser E303 where they are almost completely condensed. The heat of condensation is dissipated in the temperature-controlled cooling water system. Non-condensed gases 321 separate from solution 308 in vessel V301. The resulting carbamate solution 308, 310 is pumped to medium pressure carbamate condenser E503 in the medium pressure recirculation section using pump P302 and through wash column C502.

The urea solution 314 is withdrawn from the bottom of the distillation column, expanded adiabatically to atmospheric pressure (eg 1-2 bar) by valve X3, and discharged into atmospheric flash vessel S304. Due to adiabatic expansion and flash evaporation, part of the water and almost all of the ammonia and carbon dioxide are released from the liquid. The urea solution 319 leaving the flash vessel S304 has a higher concentration of urea and a lower temperature than the carbamide solution 314. The liberated vapor 701 is discharged from the atmospheric pressure flash vessel S304 through the top and together with the uncondensed vapor 321 from the low pressure carbamate condenser is fed into condenser E311 with atmospheric pressure evaporation tank, where they are almost completely condensed. The resulting lean carbamate condensate 722, 723 from condenser E311 with atmospheric pressure flash tank is pumped by pump P308 to low pressure carbamate condenser E303. Carbamide solution 319 is fed into the purification section.

The urea solution 319 contains approximately 59 wt.% urea and 1-2 wt.% ammonia and is at a pressure of approximately 1.1 bar. The urea solution 319 is pumped in with pump P350, expanded with expansion valve X4, and stripped with steam in stripper C350 by being brought into countercurrent contact with low pressure steam 922 at a pressure of approximately 0.47 bar(abs).

The off-gas 353 of the stripper C350, optionally with some leakage air 785, is sent to a vacuum condenser E701, which is also fed with, for example, demineralized water 1206. The condensate 712 from the condenser E701 is sent to the low pressure carbamate condenser E303 by means of a pump P701. Thus, the aqueous condensate 712, which also contains ammonia, is removed, in particular, by recycling water and ammonia to the reactor R201.

The C350 stripping column packed bed of metal Pall rings provides good liquid-gas contact between the carbamide solution 319 and the low pressure steam 922. Through the bottom of column C350, stripped liquid 352 flows into an optional DEF mixing vessel V350. The mixing vessel operates under atmospheric conditions, so in this example a static height difference of approximately 8 meters is used between the bottom of the C350 stripper and the mixing vessel to properly remove liquid from the column.

In mixing vessel V350, the stripped urea solution is optionally diluted with demineralized water 1203 to the desired concentration (eg, 32.5, 40, or about 50 wt.% urea). The amount of water is determined from the concentration of urea in the urea solution in the mixing vessel, which can be measured, for example. Mixing and cooling is carried out by circulating the dilute urea solution 354, 355 through the vessel V350 and circulation cooler E350 using the circulation pump P351 and outputting the purge stream of the finished solution 356. In this circulation cooler E350, cooling is carried out by cooling water, and the urea solution in the mixing section is cooled to approximately 30°C.

The non-condensed gas 705 from atmospheric flash condenser E311 is sent to wash column C305 where it is washed with demineralized water 1202 to produce purified gas 341 which is vented to the atmosphere and a liquid stream 342 from which a portion 344 is recirculated by pump P309 and cooler E312 and the remaining purge stream 343 is fed to condenser E311 with an atmospheric pressure flash tank. The non-condensed gas 703 from condenser E701 is fed by ejector J701 using low pressure steam 921 to wash column C305.

In the illustrated exemplary embodiment of column C350, the top of column C350 contains a gas-liquid cyclonic separator C350A that separates the two-phase expanded liquid stream. Via a cyclone separator, gas 353 is removed from the top of column C350 and liquid flows down to liquid distribution tray C350B. The C350B liquid distribution tray evenly distributes the process liquid over the C350C packed bed through the liquid holes. The liquid distribution tray contains, for example, caps to allow rising gas to rise from the bottom of the column to the top of the C350 column. From flash vessel V501, recycle carbamate 508 is fed to pump P501, from where compressed recycle carbamate 510 is fed to reactor R201. Flow 1201 indicates a possible water seal leak. For FIG. 1, some features are explicitly listed as optional, other features may also be optional.

In FIG. 2 shows an additional example of a process flow diagram that illustrates, but does not limit, the present invention or its claims. In the compression section A (optionally including the removal

-

Claims (17)

hydrogen) feed NH ₃ 1 and feed CO2 2 are compressed to a high pressure, and air 3 is optionally added to the feed CO2 for passivation. In some embodiments, passivation air 3 and hydrogen removal are not used. The urea is formed in section B of the high pressure synthesis, and the synthesis stream contains component 9 containing NH ₃ , CO ₂ , water and inert gases (gases not involved in the reactions of formation of urea), and component 10 containing an aqueous urea solution. The flow of the synthesis section, containing a mixture of components 9 and 10, is sent to section C of the medium pressure regeneration. In the medium pressure regeneration section C, part of the NH ₃ and CO ₂ is removed and sent as carbamate recycle stream 6 to the synthesis section B. The excess ammonia is provided as a recycle stream 7 to the synthesis section B via the compression section A, usually separate from the carbamate recycle stream 6. The inert gases 8 are vented to the atmosphere after scrubbing in sections C and D. The stream containing components 9 and 10 is sent to the low pressure regeneration section D. The recirculating carbamate 6 is sent from section D to section C. The aqueous urea solution 10 is purified in the purification section E, for example by steam stripping, where steam from the water 4 is used. The carbamide solution 10 is optionally diluted with water in section E to obtain purified carbamide solution 5. Stream 9 from section E to section D contains some NH ₃ and usually consists mainly of water. Water 4 is supplied from an external source, usually to section E and usually also to the scrubbers in sections C and D. In this document, the terms usually, in general and in particular indicate features that are not essential to the invention. Numerical references to the drawings are for convenience and do not limit the present invention or its claims. The described method is preferably carried out on a plant corresponding to the present description. The described plant is preferably configured to implement the method according to the present description. CLAIM 1. A method for the production of a purified urea solution, which includes the steps in which: A) reacting CO ₂ and NH ₃ under urea synthesis conditions in a high pressure urea synthesis reactor to produce a urea synthesis stream containing urea, NH3, CO2 and some carbamate, B) expanding the urea synthesis stream in the regeneration section with decreasing its pressure, wherein the expanded urea synthesis stream contains at least 90 wt.% of the specified amount of carbamate, and heating at least a part of the expanded urea synthesis stream in one or more decomposition devices at an average and / or low pressure to obtain an aqueous stream of urea and steam from the regeneration section containing NH3 and CO2, C) treating at least a portion of the urea aqueous stream in the scrubbing section at or below an absolute pressure of 1.5 bar, the scrubbing comprising heating, stripping, and/or lowering the absolute pressure to remove ammonia to produce a purified urea solution and scrubbing section off-gas containing water and ammonia E) the waste gas of the purification section is condensed to obtain a condensate of the purification section, and said condensate of the purification section is recycled to said urea synthesis reactor, wherein the high pressure is from 120 to 300 bar, the medium pressure is from 10 to 70 bar, and the low pressure is from 3 to 10 bar. 2. The process according to claim 1, wherein the resulting purified urea solution is suitable for use in NOx reduction, in particular for use as a diesel engine exhaust aftertreatment fluid (DEF) or for dilution to form DEF. 3. The method according to claim 1 or 2, in which, after the purification step, a purified urea solution is obtained with an alkalinity based on NH ₃ , which, when converted to a urea solution with a carbamide content of 32.5 wt.%, is less than 0.2 wt. %. 4. The method according to any one of claims 1 to 3, further comprising diluting at least a portion of the purified urea solution and/or the aqueous urea stream to be purified with water to obtain a target concentration of urea. 5. Process according to any one of claims 1 to 4, wherein the purification section condensate is fed as a liquid stream to the urea synthesis reactor and wherein the purification is low pressure steam stripping and comprises bringing the aqueous urea stream into contact with the steam stream according to the counterflow principle at a pressure of less than 3 bar. 6. The method according to any one of claims 1 to 5, including: a) condensing the vapor of the regeneration section to obtain a recirculating stream of liquid carbamate in the condenser of the regeneration section, said condenser operating at a lower pressure than said urea synthesis reactor, b) feeding the carbamate recycle stream to the urea synthesis reactor, preferably using a pump, - 15 040581 c) supplying said condensate of the cleaning section to said condenser of the regeneration section. 7. The method of claim 6, further comprising the step of supplying non-condensed gases from the regeneration section condenser, optionally via an additional condenser, to a scrubber step using scrubber scrubbing fluid to produce scrubber gas and scrubber waste scrubbing fluid, wherein step E) also includes receiving non-condensed gas as a second off-gas stream and the method further includes supplying said second off-gas stream from a cleaning section condenser to said scrubber and preferably feeding a portion of said scrubber spent scrubber liquid to said regeneration section condenser, optionally through said additional condenser. 8. The method according to any one of claims 1 to 7, wherein step B comprises: B1) expanding the urea synthesis stream to medium pressure, optionally gas/liquid separation and heating at least a portion of the urea synthesis stream at medium pressure to produce medium pressure regeneration section steam and medium pressure urea water stream, B2) expanding the medium pressure urea aqueous stream to low pressure and heating the expanded urea aqueous stream at low pressure to produce a low pressure regeneration section steam and a low pressure urea aqueous stream. 9. A process according to any one of claims 1 to 8, which is carried out in a plant in which the feed CO ₂ is supplied from the boundaries of said plant, wherein at least 85% of said feed CO 2 is fed directly to the urea synthesis reactor. 10. The method according to any one of claims 1 to 9, wherein said step C comprises expanding said aqueous urea stream to the operating pressure of the purification section, wherein the operating pressure of the purification section is preferably in the range of 0.010 to 1.5 bar (absolute) and step C involves steam stripping of the expanded urea aqueous stream, wherein the steam stripping involves direct injection of steam into the expanded urea aqueous stream. 11. The method according to claim 10, wherein the steam stripping is carried out at a pressure of 0.010 to 1.5 bar (absolute) in a stripping column with a cyclone separator at the top of the column, a liquid distribution tray and a packed bed in which the gas is separated from the expanded urea water flow in the cyclone separator, the gas exits through the outlet at the top, the liquid flows from the cyclone separator down to the liquid distribution tray, and the liquid distribution tray distributes the liquid over the packed bed. 12. Process according to any one of claims 1 to 11, wherein no passivating air is used in step A) and/or wherein the CO2 feed contains less than 0.05 wt% oxygen. 13. Installation for the production of purified carbamide solution, designed to implement the method according to any one of claims 1-12, containing: A) a urea synthesis section containing a high pressure urea synthesis reactor having a CO ₂ inlet and an NH 3 inlet, in which the reaction between CO ₂ and NH 3 is carried out under urea synthesis conditions to obtain a urea synthesis stream containing urea, water and ammonium carbamate, B) a regeneration section in which the urea synthesis stream is heated under reduced pressure to produce an aqueous urea stream and a regeneration section steam containing CO2 and NH3, and in which said regeneration section steam is condensed to produce carbamate and ammonia, C) a purification section in which the aqueous urea stream is stripped in the treatment section of the purification section to remove ammonia to obtain a purified urea solution and an off gas of the purification section, and the installation contains a recirculation pipeline for supplying carbamate from the regeneration section to the urea synthesis section and a separate second recirculation a pipeline for supplying ammonia from the regeneration section to the urea synthesis section, while the purification section additionally contains a condenser of the purification section for condensing the off-gas of the purification section into the condensate of the purification section, and the installation contains a liquid medium connection for supplying the condensate of the purification section from the specified condenser of the purification section to the specified urea synthesis reactor. 14. The plant according to claim 13, which is designed to produce a purified urea solution that is suitable for use in NOx emission reduction systems, in particular for use as a diesel engine exhaust gas cleaning fluid (DEF) or for dilution to obtain a DEF fluid. 15. Plant according to claim 13 or 14, wherein the recovery section contains separate condensers for ammonia and for carbamate. 16. Plant according to any one of claims 13 to 15, further comprising D) a urea dilution section in which the purified urea stream and/or the aqueous urea stream is diluted with water. 17. The plant according to any one of claims 13-16, in which the treatment section of the cleaning section contains a low pressure steam stripper, into which an aqueous urea stream enters, and the steam stripper has an inlet for water vapor, and the cleaning section contains an ejector for removing gas from the section cleaning. -