CN206705995U - For producing the integrated system of urea and urea ammonium nitrate - Google Patents

Abstract

An integrated system for producing urea and ammonium nitrate urea is disclosed. The system comprises (i) a urea production unit comprising a urea synthesis section and a urea purification section downstream of and in fluid communication with the synthesis section, the urea synthesis sections comprising a urea synthesis section in fluid communication with each other to form a urea synthesis loop reactor, stripper and condenser, and (ii) a unit for the production of ammonium nitrate.

Description

Integrated systems for the production of urea and ammonium nitrate urea

technical field

The present invention relates to the field of urea production, in particular to the co-production of urea solutions and the production of urea ammonium nitrate solutions (UAN), suitable for reducing exhaust gases from internal combustion engines, such as those produced by diesel engines (DEF: Environmentally Friendly Urea for Automobiles) _NOx in . The invention also relates to a device for carrying out the method.

Background technique

Urea is usually produced from ammonia and carbon dioxide. It can be prepared by introducing excess ammonia together with carbon dioxide into the urea synthesis section at a pressure between 12 and 40 MPa and a temperature between 150°C and 250°C. A typical urea production plant further includes a recovery section and a finishing section. In the recovery section, unconverted ammonia and carbon dioxide are recovered and recycled to the synthesis section. The recovery section usually follows the evaporation section. Where the urea concentration is further increased by evaporation of the water, a highly concentrated solution is obtained, which is often referred to as a urea melt. In the finishing section, usually, the urea melt is made into the desired granular solid, usually involving techniques such as prilling, pelletizing or pelletizing.

In the evaporation part there is still considerable removal of CO2 and especially NH3. Ammonia is removed by treatment in a scrubber. It then goes to the wastewater treatment unit, which is a very costly and energy intensive operation.

Urea products of interest are solutions used for NOx reduction, eg in selective reduction, which can be a non-catalytic thermal process or a selective catalytic reduction (SCR) process. An example of a solution for SCR is Automotive Environmentally Friendly Urea (DEF), a term used in the specification generally referring to a urea solution for NOx reduction.

DEF is a solution of 32.5 wt.% urea in demineralized water, composed of a maximum of 0.3 wt.% biuret and a maximum of 0.2 wt.% alkalinity as ammonia. DEF is purchased in the market under the trade name Arla 32 and AUS-32, and are injected into the exhaust of internal combustion engines to trap _NOx , preventing it from leaking into the atmosphere. The purpose of DEF is to convert NO _x into harmless nitrogen and water through the following reaction: urea + NO _x → N ₂ + H ₂ O. The reduction of _NOx from internal combustion engines is widely practiced because _NOx is the main source of environmental pollution indicators for global warming, such as Global Warming Potential (GWP), Tropospheric Ozone Formation Potential (TOFP) and Ozone Destruction Potential (ODP).

The production of automotive environmentally friendly urea (DEF) is usually done by dissolving the solid urea product in demineralized water. A solid urea product, for example produced by one of the aforementioned finishing techniques, is combined with demineralized water, and the solution is mixed until the urea is completely dissolved. This method has the disadvantage that considerable investment is required in the urea finishing technology and the removal of by-products (such as additives produced during use or finishing) such as formaldehyde or biuret (which is a urea by-product) is required to obtain the desired specifications The product. The usual concentration of biuret in the finished product is 0.9-1.1 wt.%. This in turn leads to a biuret content exceeding 0.3 wt.% in the final DEF product, thus resulting in off-spec material. Currently the specification according to ISO22241 allows a maximum of 0.3%. Higher biuret concentrations lead to ineffective DEF solutions in capturing _NOx because less urea is available to capture _NOx . In addition, during the production process of granulation and granulation, a large amount of water is removed, which is added later in the dissolution step. This requires a lot of energy, resulting in additional costs.

An improved process disclosed in EP1856038A1 uses aqueous urea solution obtained directly from the recovery part of the urea melt plant or thereafter and dilutes said aqueous urea solution with water to obtain the desired solution. An aqueous urea solution may additionally be fed into the evaporation section to remove water from the urea melt to produce solid urea by fluidized bed granulation, pelletization or granulation. In this way, the need for evaporated water is eliminated, but the aqueous urea solution can contain relatively high levels of ammonia that exceed the specifications for the final DEF product. EP1856038A1 discloses that the level of ammonia in solution (as free ammonia or in the form of ammonium carbamate) can be reduced by dissociating an aqueous urea solution, for example by heating or reducing the pressure, optionally by adding a stripping medium or a combination of the foregoing . This step is practically the same as the evaporation step in the traditional finishing section of a urea plant and can be carried out with the same equipment as said evaporation step, so it is sometimes called "evaporation step" or "pre-evaporation step". The pre-evaporation step is designed to produce an aqueous urea solution after dilution with water, meeting the requirements for automotive environmentally friendly urea.

The main drawback is that the pre-evaporation step releases inert ammonia from the aqueous urea solution, which may need to be cleaned up to meet ammonia emission regulations, requiring a dedicated condensation/washing section and subsequent wastewater treatment section integrated with the urea melt plant to The released ammonia is recovered.

Urea Ammonium Nitrate (UAN) is a chemical fertilizer that is commonly used as an aqueous solution of urea and ammonium nitrate. Ammonium nitrate is produced by reacting ammonia with a solution of strong nitric acid while maintaining the pH of the solution within a narrow range. The resulting solution was then mixed with aqueous urea to obtain UAN. A typical UAN product includes 28 to 32 wt.% total nitrogen and typically 29 to 38 wt.% urea and from 36 to 48 wt.% ammonium nitrate, with the remainder being water.

The demand for UANs is often subject to strong seasonal fluctuations, which makes it desirable to find ways of operating UAN equipment that allow economically attractive applications during periods of low demand for UANs. It is therefore to be noted that aqueous urea produced in representative UAN plants used to produce final UAN products can often be used for the production of DEF. The ammonia concentration in aqueous urea (>2000 ppm wt.) is usually higher than the concentration allowed in DEF production (<0.2 wt.%). Also, since UAN products are liquids, transporting them over long distances is inconvenient and expensive.

It is desirable to combine the production of urea and UAN in such a way that the low demand for UAN can be compensated. It is particularly desirable to provide such an arrangement combining the production of urea and UAN in which an aqueous urea solution meeting the requirements for use as DEF is obtained. Additionally, it would be desirable to provide a more economical way to dispose of ammonia waste from the urea evaporation section.

As far background art, reference is made to US 4,174,379. Processes for the production of urea and UAN are disclosed therein. Urea is produced through a once-through urea production process. This type of urea production process is outdated and has fundamentally different characteristics from modern processes, which are basically a separate stripping process. For example, biuret is an unavoidable by-product from the stripping of the urea reaction solution while the once-through process inherently produces low levels of biuret. This is believed to be caused by the relatively high temperatures and concentrations prevailing in the urea stripper. As mentioned above, for DEF, biuret is only allowed to be present at very low levels (in particular as specified by applicable ISO and DIN standards). Therefore, producing a urea solution suitable for conversion to DEF is particularly challenging in the production of urea in a stripping process.

Contents of the invention

In order to better address one or more of the aforementioned needs, the present invention provides a process for producing ammonium nitrate urea, which comprises the following steps:

- reacting nitric acid and ammonia in the ammonium nitrate production unit under conditions enabling an aqueous solution of ammonium nitrate to be obtained;

- reacting carbon dioxide and ammonia under urea-forming conditions to obtain a reaction solution comprising water, urea, ammonium carbamate, and unreacted CO2 and NH3;

- stripping the reaction solution to obtain unreacted ammonia and carbon dioxide, and the stripped reaction solution;

recycling said recovered ammonia and carbon dioxide as starting materials for the formation of urea;

- separating the stripping reaction solution to obtain an aqueous urea stream and a gas stream comprising CO2 and NH3;

- feeding NH3 from said gas stream as a reactant to said ammonium nitrate production unit;

- mixing at least part of said aqueous urea stream with said aqueous ammonium nitrate solution to obtain an ammonium nitrate urea solution.

In another aspect, the present invention provides an integrated system for the production of urea and ammonium nitrate urea, said system comprising (i) a urea production unit comprising a urea synthesis section and a urea synthesis section downstream of and in fluid communication with A urea purification section, said urea synthesis section comprising a reactor, a stripper and a condenser in fluid communication with each other to be able to form a urea synthesis loop, said urea purification section being adapted to convert CO2 and NH3 from urea comprising said CO2 and NH3 and (ii) a unit for generating ammonium nitrate from ammonia and nitric acid; wherein the NH3 outlet of the purification part of the urea plant is connected to the NH3 inlet of the unit for the production of ammonium nitrate, and wherein the urea of the purified part Aqueous solution outlet and ammonium nitrate aqueous solution outlet of the ammonium nitrate production unit is connected to a unit for mixing said aqueous urea solution and said aqueous ammonium nitrate solution.

In another aspect, the invention is a method of modernizing a urea plant comprising a urea synthesis section and a urea purification section downstream of and in fluid communication with the synthesis section, the urea synthesis sections comprising fluid communication with each other to form A reactor, a stripper and a condenser of a urea synthesis loop, the urea purification section is adapted to separate CO2 and NH3 from an aqueous urea solution comprising said CO2 and NH3, said method comprising adding in the urea plant A unit for the production of ammonium nitrate from ammonia and nitric acid, in which the NH3 outlet of the purification part of the urea plant is connected to the NH3 inlet of the unit for the production of ammonium nitrate, and in which the outlet of the aqueous urea solution of the purification part and the aqueous ammonium nitrate solution of the ammonium nitrate production unit are connected The outlet is connected to a unit for mixing said aqueous urea solution and said aqueous ammonium nitrate solution.

In another aspect, the invention provides a method for modernizing a plant for the production of ammonium nitrate, said plant comprising a unit for the synthesis of ammonium nitrate from ammonia and nitric acid, said process being included in a plant for the production of ammonium nitrate A urea production unit is added, whereby the plant is integrated in the manner described in the previous paragraph.

Description of drawings

Figures 1 to 5 are schematic illustrations of plant components and process streams associated with embodiments of the present invention.

Figure 1 is a diagram showing one embodiment of feeding NH3 and CO2 to a urea synthesis zone. Where NH3 is fed to the urea synthesis reactor and CO2 is fed to the high pressure stripper. The resulting urea synthesis solution is led to the purification section. In which unreacted NH3 and CO2 are removed from the synthesis solution. Part of the resulting aqueous urea stream is used to obtain DEF and part is used for mixing with ammonium nitrate (not shown) to obtain UAN. The removed NH3, instead of being recycled to urea synthesis, is sent to a unit for the production of ammonium nitrate (not shown).

Figure 2 is a diagram showing another embodiment of feeding NH3 and CO2 to the urea synthesis zone. Here the aqueous urea solution is sent to the pool. Select a process from here to use aqueous urea for the production of UAN, for the production of DEF, or for both. This has the benefit of providing a buffered amount of aqueous urea, which increases process flexibility.

Figure 3 is a diagram showing steam stripping by using direct steam injection, and by sending tail gas to a unit for the production of ammonium nitrate by using LP steam injection.

Figure 4 is a graph showing DEF production using a thermal stripper. According to the integrated production of DEF and UAN of the present invention, the tail gas from the thermal stripper is sent to the AN neutralizer instead of being sent to the gas treatment and waste water treatment section for cleaning.

Figure 5 is a diagram showing an arrangement similar to Figure 4, whereby a condenser and steam ejector are used as an alternative route to send the tail gas to the AN section for neutralization.

detailed description

The present invention is based on the sensible insight of integrating DEF production and production of liquid UAN fertilizers. Due to the low number of devices, zero discharge and the possibility of eliminating the waste water treatment section, it offers an economically attractive, flexible adaptation to market requirements, as well as an environmentally and energy-friendly process. According to the invention, the ammonia off-gas from urea production is sent to the AN section of the UAN plant, where the ammonia off-gas (which can be either liquid ammonia or (possibly diluted) gaseous ammonia) is neutralized with nitric acid to form AN, by This mix is UAN. In this way the exhaust gas is not sent to the outside environment, but remains in the process.

In particular, when DEF production is integrated with the production of UAN liquid fertilizers, surprisingly low biuret concentrations between 0.01 wt.% and 0.3 wt.% can be achieved even in a process involving the production of urea through a stripping process . It is believed that this low biuret content can be obtained especially by having a recycle section with a relatively low pressure (LP). Typically, the LP recycle (recovery) section of a urea stripping plant comprises several plant components where high temperatures (eg around 135°C) are applied in some areas. The biuret content tends to increase due to the longer residence time in the LP fraction. The possibility of eliminating equipment is advantageous to reduce the residence time in the LP section (eg limited to a separate rectification column). This in turn provides the benefit of being able to reduce biuret formation.

Additionally, in the process of the present invention, steam from the recovery section is sent to the ammonium nitrate section. In a conventional process, these vapors are condensed to form a carbamate solution, which is then returned to the HP synthesis section. This actually results in recycling water to the synthesis section. In an embodiment of interest, urea is produced in a plant configured specifically for the co-production of DEF and UAN. In particular, urea is thus produced by a submerged condenser/reactor, which is a urea reactor integrated with a submerged carbamate condenser, such as a pool reactor, which is a horizontal submerged condenser/reactor device. There is no need to recycle water/carbamate to urea synthesis in this configuration. This is advantageous because it makes urea synthesis more efficient.

Further, since the present invention allows the production of urea exclusively for DEF, different process settings can be employed to minimize biuret production. For example, by reducing the temperature in the stripper. This can have considerable effect, since the biuret is mainly formed at its outlet. It is also possible to reduce the temperature and increase the pressure in the LP section to reduce ammonia content and biuret formation.

The result is that more urea is available in the DEF solution for NOx capture, resulting in a more efficient DEF end product in thermal deNOx applications. As a result, the efficiency of Selective Non-Catalytic Reaction (SNCR) technology can be increased to 20%. Through this route, less urea is converted to biuret, which means that more DEF end products can also be produced based on the present invention compared to existing solutions, while requiring approximately 1% less raw material per ton of DEF end product . Additionally, significant capital investment is eliminated as pre-evaporation, condensation and waste water treatment are no longer required. This can be achieved by using a pressurized urea solution directly from the recovery section.

The urea production process implemented in the present invention results in a water stream. Instead of evaporating the water stream which requires wastewater treatment, the method of the present invention provides for its flexible application. At least part of the stream is used for the production of UAN by combining it with ammonium nitrate. At the same time, the ammonia obtained from the separation step to obtain said aqueous urea stream is used for the production of ammonium nitrate.

Preferably, the amount of aqueous urea stream used for UAN production is at least equivalent to the amount of ammonium nitrate produced from the amount of ammonia separated in urea production.

This can be understood by referring to the fact that the ammonium nitrate urea solution comprises 29 wt.% to 38 wt.% urea and 36 wt.% to 46 wt.% ammonium nitrate. Therefore, preferably, the amount of the aqueous urea solution mixed with the ammonium nitrate solution is at least sufficient to provide the desired amount of ammonium nitrate obtainable from the amount of NH3 supplied to the ammonium production unit.

According to the invention, the option of further using an aqueous urea stream is given. This can be used to obtain DEF, and another part (or even all) of it can also be used for UAN production. It will be appreciated that if more urea is used for the production of UAN, a correspondingly higher amount of ammonium nitrate is also provided. This can be ammonium nitrate from an outside source, ammonium nitrate produced on-site from ammonia and nitric acid, or both.

In the method of the present invention, UAN is produced by mixing aqueous urea and aqueous said ammonium nitrate to obtain ammonium nitrate urea solution. Ammonium nitrate is produced by reacting ammonia and nitric acid.

The industrial production of ammonium nitrate involves the acid-base reaction of ammonia and nitric acid:

HNO3+NH3→NH4NO3

Ammonia is usually used in its anhydrous form gas and nitric acid is

Concentrated (typical concentration range: 40 to 80 wt.%, eg about 60 wt.%). Ammonium nitrate solutions are generally readily formed at concentrations of about 70% to 95%, for example 83% to 88%, by an exothermic reaction. In the manufacture of solid ammonium nitrate products, such as granules or pellets, excess water is evaporated to achieve 95% to 99.9% concentration of ammonium nitrate (AN) content, this step can be omitted when preparing ammonium nitrate urea solution. The UAN solution can be prepared by mixing aqueous urea and ammonium nitrate solutions.

Nitric acid for ammonium nitrate production can be obtained by external feeding. Preferably the nitric acid is produced on site as this is generally more attractive from an economic standpoint than relying on transport of nitric acid.

Preferably, the method of the invention is implemented in an integrated system for the production of urea and ammonium nitrate urea. The system comprises (i) a urea production unit and (ii) a unit for the production of ammonium nitrate from ammonia and nitric acid. The urea production unit comprises a urea synthesis section of the stripping type and, downstream and in fluid communication therewith, a urea purification section adapted to separate CO2 and NH3 from an aqueous urea solution comprising said CO2 and NH3 . According to the present invention, the NH3 outlet of the purification part of the urea production unit is connected to the NH3 inlet of the ammonium nitrate production unit, so that the NH3 separated from the urea production is supplied as the feedstock for the ammonium nitrate production. In addition, provision is made to mix the aqueous urea stream obtained from the urea production unit with the liquid ammonium nitrate obtained from the ammonium nitrate production unit. To this end, the aqueous urea outlet of the purification section of the urea production unit and the aqueous ammonium nitrate outlet of the ammonium nitrate production unit are connected to a unit for mixing said aqueous urea and said aqueous ammonium nitrate.

The invention also relates to such an integrated system for the production of urea and ammonium nitrate urea. The system of the present invention may or may not also include a unit for the production of nitric acid. The main industrial method of producing nitric acid is from ammonia.

Thus, anhydrous ammonia is oxidized to nitric oxide in the presence of a platinum or rhodium gauze catalyst at an elevated temperature of about 500 K and a pressure of 9 bar.

4NH3(g)+5O2(g)→4NO(g)+6H2O(g)

Nitric oxide then reacts with oxygen in the air to form nitrogen dioxide.

2NO(g)+O2(g)→2NO2(g)(ΔH=-114kJ/mol)

It is then absorbed into water to form nitric acid and nitric oxide.

3NO2(g)+H2O(l)→2HNO3(aq)+NO(gas) (ΔH=-117kJ/mol)

Nitric oxide cycles back to re-oxidize. Alternatively, the last steps can be implemented on air:

4NO2(g)+O2(g)+2H2O(l)→4HNO3(aq)

The resulting aqueous nitric acid can be concentrated by distillation, usually to about 68% by mass.

The benefit with the method and system of the present invention is that nitric acid can be produced on site with infrastructure for ammonia, as this can also be used as feedstock for urea production.

If the system of the invention includes a unit for producing nitric acid, the unit will typically be fed from an external source and will have an outlet for nitric acid in fluid communication with the nitric acid inlet of the unit for producing ammonium nitrate.

The urea production unit can be a traditional urea stripping facility, with an evaporation section, wastewater treatment section and finishing section comprehensively provided downstream of the synthesis and recovery section. However as an important benefit of the present invention, urea for DEF and UAN can be produced in a urea stripping production unit without evaporation section and waste water treatment section. Furthermore, conventional finishing parts (used to form solid urea forms such as granules, pellets and pellets) can be omitted. Because, in the process of the present invention, DEF can be obtained by diluting part of the urea water flow obtained from the separation of the urea reaction solution with demineralized water. This results in a solution suitable for use in NOx reduction units.

The method of the invention can also be practiced such that DEF is produced in solid form. For this purpose, flash crystallization can be performed on part of the aqueous urea stream separated from the urea reaction solution. This is used to form a solid urea powder suitable to be converted into a solution suitable for use in a NOx reduction unit by dilution with demineralized water. This is of great benefit as transporting solid urea powder is more economically attractive than the need to transport DEF as a solution which would require transporting large volumes of water at the EDF production site.

For the production of solid urea products by flash crystallization reference is made to WO2013/055219. The production disclosed therein is by applying mechanical forces in the crystallization of urea. This step can be omitted for the production of powders that can be diluted to produce DEF. In particular, a free-flowing urea powder can be obtained by obtaining an aqueous urea stream as described above, subjecting said stream to flash crystallization at subatmospheric pressure to obtain a product comprising solid crystalline urea and comprising water and ammonia wherein said solid crystalline urea product contains less than 0.2 wt.% of water; and preferably packaged under such conditions that the water content of the packaged product remains below 0.2 wt.%. Thus, flash crystallization is preferably carried out at a pressure below 15 kPa, more preferably from 1 to 10 kPa. In addition, the flashing is preferably performed as a dry flashing.

Flashing can be performed in flashing equipment, such as a dry flashing column. A dry flash column is characterized by the substantial conversion of a liquid stream to solids and vapor by crystallization and evaporation. The process conditions in the dry flash tower are selected so that the amount of residual liquid is basically zero. This allows a clean separation of gases and solids with little sticking and fouling. Another benefit of dry flash is that neither solid-liquid separation such as centrifugation nor re-melting is required since there is no slurry present. This allows for a very simple flow system. Such dry flash columns are vessels preferably operated at pressures between 1 and 15 kPa, more preferably between 2 and 10 kPa. In the dry flash tower, the urea solution is distributed through a liquid distributor. By expansion, urea and biuret crystallize spontaneously in the solid, and the residual components including water, ammonia and a small amount of carbon dioxide evaporate.

The particle size of the solid urea product obtained by adiabatic flash is in the range of 0.1 micron to 1000 micron, preferably 1 micron to 800 micron.

In this embodiment, the process of the present invention allows the production of a free-flowing powder that can be packaged in bags (20kg) or large bags (i.e. 500kg or 1000kg bags) that can be easily emptied on site to produce a DEF solution.

The urea powder produced by the process has a very low diurea content, typically less than 0.5%, preferably less than 0.4%, which translates to less than 0.20% or less than 0.15% by weight in the final DEF solution. This means that the amount of active urea in solution will then be higher at the same solids content. The biuret specification is a maximum of 0.3% by weight for the final DEF solution. When pellets are used to prepare solutions, the biuret content in the solution is usually between 0.27 and 0.36 wt%.

In this embodiment of the process of the invention, the obtained solid crystalline urea product is packaged under conditions such that the water content in the packaged product is kept below 0.2 wt.%. This low water content serves to ensure that the solid urea, which is inherently hygroscopic, remains a powder capable of being released from its packaging and reconstituted into solution. According to the invention, the water content of the packaged product is kept below the aforementioned level until the product is used to prepare a solution.

In practice this means packaging the obtained powder with the required low moisture content in a water-impermeable package (eg container or bag). Suitable packaging materials include high density polyethylene. Other plastics suitable for packaging hygroscopic materials are known in the art. The packaging material can be non-porous and hermetic. Also, the package can be formed from multiple layers of material.

It is planned to obtain DEF by dissolving the aforementioned powder in demineralized water for use. It should be understood that it can also be used to prepare UANs, which should be desirable eg in cases of high demand for UANs. In the latter case, the powder can also be dissolved in ordinary tap water.

In the process of the present invention, urea can be synthesized by any suitable urea stripping process. Such a process is usually carried out in a stripping plant. The urea synthesis section in the stripping plant includes at least one reactor, a stripping column and a condenser. These typically operate at high pressure (HP) and are generally referred to in the art as HP Reactor, HP Stripper, and HP Carbamate Condenser. The reactor, stripper and condenser are in fluid communication with each other to form a urea synthesis loop. This requires no further clarification for a person skilled in the art. Typically, the reactor can produce an aqueous urea synthesis solution. The solution will be stripped to produce a stripping solution and gases carbon dioxide and ammonia. The gaseous components will be condensed in the condenser to give ammonium carbamate which is recycled to the reactor. Depending on the type of condenser, a considerable amount of ammonium carbamate can also be converted to urea in the condenser. Furthermore, the condenser and the reactor can be integrated into a single device.

In the process of the present invention, the stripping solution is further separated to obtain an aqueous urea stream and a gas stream comprising CO2 and NH3. This separation usually takes place at a pressure lower than that of the synthesis section. Such low pressure may be medium pressure (MP), low pressure (LP), or may include both. MP and LP recycle sections (also called recovery sections) are well known to those skilled in the art. The prevailing pressure is usually of the order of 1-10 MPa for the MP section, more typically 1-5 MPa, and 0.1 to 1 MPa, more typically 0.2 to 0.6 MPa for the LP section. The plant of the invention and the plant retrofit method of the invention are adapted to carry out the described process by providing suitable MP and/or LP recirculation sections.

A frequently used urea stripping procedure is the carbon dioxide stripping procedure as described, for example, in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp. 333-350. In this scheme, a synthesis section is followed by one or more recovery sections. The synthesis section comprises reactor, stripper, condenser and scrubber, wherein the operating pressure is between 12 and 18 MPa and preferably between 13 and 16 MPa. In the synthesis section, the urea solution leaving the urea reactor is fed to a stripping column where large amounts of unconverted ammonia and carbon dioxide are separated from the aqueous urea solution. Such a stripper can be a shell and tube heat exchanger, where urea solution is fed to the top of the tube side and carbon dioxide for synthesis is fed to the bottom of the stripper. On the shell side, steam is added to heat the solution. The urea solution leaves the heat exchanger at the bottom while the vapor phase leaves the stripper at the top. The steam leaving the stripper includes ammonia, carbon dioxide and a small amount of water. The vapor is condensed in a falling film heat exchanger or submerged tube condenser which may be horizontal or vertical. Horizontal submersible heat exchangers are described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp. 333-350. The heat released by the carbamate condensation exothermic reaction in the condenser is generally used to generate steam which is used in the downstream urea treatment section for heating and concentrating the urea solution. Because of a certain liquid residence time in the submerged condenser, part of the urea reaction has already taken place in said condenser. The resulting solution comprising condensed ammonia, carbon dioxide, water and urea is sent to the reactor together with uncondensed ammonia, carbon dioxide and inert steam. The above mentioned reaction from carbamate to urea reaches equilibrium in the reactor. The molar ratio of ammonia to carbon dioxide in the urea solution leaving the reactor is usually between 2.5 and 4 mol/mol. It is also possible to combine the condenser and reactor into one unit. An example of an integrated device is described in Ullmann's Encyclopedia of Industrial Chemistry, Vol. A27, 1996, pp. 333-350. The formed urea solution leaving the urea reactor is fed to a stripper and the inert vapor including uncondensed ammonia and carbon dioxide is sent to a scrubbing section operating at a pressure similar to the reactor, or the vapor is sent directly to The ammonium nitrate portion is used for neutralization (ie reaction with nitric acid to form ammonium nitrate). In the scrubbing section, ammonia and carbon dioxide are scrubbed from inert steam. The formed carbamate solution from the downstream recovery system is used as absorbent in the wash section. The urea solution leaving the stripper in the synthesis section requires a urea concentration of at least 45 wt%, and preferably at least 50 wt%, for treatment in a separate recovery system downstream of the stripper. The recovery section includes heaters, liquid/gas separators and condensers. The pressure in the recovery section is between 200 and 600 kPa. In the heater in the recovery section, most of the ammonia and carbon dioxide are separated from the urea and water phase by heating the urea solution. Usually steam is used as heating agent. The urea and aqueous phase, containing a small amount of dissolved ammonia and carbon dioxide, leaves the recovery section and is sent downstream to the urea treatment section where the urea solution is concentrated by evaporating water from said solution.

The present invention is not limited to any particular urea production scheme. Other processes and equipment include those based on technologies such as full cycle plants, the HEC process developed by Urea Casale, the ACES process developed by Toyo Engineering Corporation, and the process developed by Snamprogetti. All of these and other schemes can be used in the methods of the invention.

In one embodiment, the urea purification section comprises a rectification column with a shell and tube heat exchanger and a gas/liquid separator, wherein aqueous urea flows through the tubes and steam is used on the shell side. In another embodiment stripping gas can also be used.

The purpose of the purification section is to remove CO2 and ammonia from the aqueous urea produced in the synthesis section. Aqueous urea usually includes urea, ammonium carbamate and unreacted CO2 and NH3. As known to those skilled in the art, CO2 and NH3 are in equilibrium with ammonium carbamate in aqueous urea solution. The formation of ammonium carbamate from CO2 and ammonia is an exothermic reaction. Removal of CO2 and NH3 from aqueous solutions is effected by heating and/or pressure reduction, optionally with stripping reagents. The conversion of CO2 and ammonia to the gas phase (evaporation) will change the equilibrium of the ammonium carbamate, thereby promoting the dissociation of ammonium carbamate, which in turn promotes the evaporation of CO2 and ammonia.

In urea plants, the equipment used for the purification step is usually named after the main function it is designed to perform. The terms carbamate recovery system, dissociator, stripper, (pre)evaporator are commonly used. The purification step can be carried out in the same type of equipment as used in the recovery section of a standard urea plant. For example, a rectification column can be used. Alternatively, a shell and tube heat exchanger may be used, optionally together with a gas/liquid separator, or a separate gas/liquid separator may be used on top of the vessel. Purification steps may be performed in one or more steps. Other configurations and devices are known to those skilled in the art. As an optional stripping agent, CO2, ammonia or steam can be used. In the standard recovery section, the raw ammonia and CO2 are recycled back to the urea synthesis section, so it is not desirable to use steam as a stripping agent as this would lead to an increase in water recycle to the urea synthesis section. The present invention avoids this limitation. If, as preferred, no ammonia is recycled to the synthesis section, the presence of water after the purification section is not a problem.

Part of the aqueous urea stream obtained from the purification section of the urea plant is used to produce DEF. DEF production units employ, for example, (thermal) stripping using pressurized steam based on direct injection (Fig. 3), or evaporation using pressurized steam in heat exchangers by means of shell-and-tube or plate heat exchangers (Fig. 4), to carry out product quality control on the ammonia content at the outlet of the urea synthesis part. Such stripping was employed to remove excess ammonia to meet the DEF final product specification of 0.2 wt.% ammonia.

If necessary, the off-gas from the hot stripper is condensed and pressurized in at least one stage with steam injectors and combined with the conventional synthesis off-gas to the neutralizer of the ammonium nitrate section. Alternatively, these tail gases are sent directly to the neutralizer (or after the neutralizer) section of the ammonium nitrate production unit (Figures 3 and 5). From this, the released ammonia is used to neutralize the nitric acid to produce ammonium nitrate. In this way, by using the ammonia from the thermal stripper, the conventional ammonia used for the neutralization reaction can be reduced, reducing the raw ammonia consumption of the ammonium nitrate plant. This arrangement can be used with any urea synthesis arrangement. This approach facilitates efficient small dedicated DEF production, or it can be combined with large scale DEF production with a production capacity similar to urea synthesis.

An alternative to the aforementioned stripping is to optimize the urea melt concentration at the stage of urea synthesis. In the case of ammonia or carbon dioxide stripping, the following variables are employed to obtain a urea melt solution suitable for DEF production:

- Higher stripping efficiency (0.70-0.95) in HP stripper

- Higher pressure (4-8bar) in the low pressure section

- Higher pressure (15-40bar) in the medium pressure recovery section

- Higher temperatures (90-145°C) employed by low pressure rectification heaters

- Condensation or increase in reactor efficiency.

The invention can also be used in methods for modernizing existing urea plants. Such a plant comprises a urea synthesis section downstream and in fluid communication therewith a urea purification section suitable for separating CO2 and NH3 from an aqueous urea solution comprising said CO2 and NH3. The method comprises adding to a urea plant a unit for the production of ammonium nitrate from ammonia and nitric acid. The NH3 outlet of the purification section of the urea plant is thus connected to the NH3 inlet of the unit for the production of ammonium nitrate. Also, the outlet of the aqueous urea solution of the purification section and the aqueous ammonium nitrate outlet of the unit for producing ammonium nitrate are connected to a unit for mixing the aqueous urea solution and the ammonium nitrate solution.

The benefit of the modern plant is that it offers the additional possibility of using the ammonia recovered from the urea plant, enabling waste water treatment to be dispensed with.

It should be understood that the invention is also suitable for setting up new urea plants integrated with ammonium nitrate production units. As discussed above, a great benefit is that new plants can be built without evaporation and waste water treatment sections. It is to be noted that if there is a requirement for more functional different urea products (including solid urea forms such as pellets or pellets) besides DEF and UAN, new plants with these parts can also be built. Also, in terms of producing dilutable DEF powders as discussed above, in the integrated system of the present invention a new urea production unit is built with a (flash) crystallization section, but, if required, without a shaping section because It may have traditionally been employed to produce solid urea in forms such as pellets or pellets.

The invention is further described with reference to the accompanying drawings. It should be understood that the drawings are not limiting of the invention, for example, the invention is not limited to the particular type of equipment and particular equipment systems shown. All figures show schematic representations of plant components and process streams relevant to embodiments of the invention. Components and streams are described in the figures with short key words.

In Figure 1 NH3 and CO2 are fed to the urea synthesis zone. An embodiment is shown wherein NH3 is fed to the urea synthesis reactor and CO2 is fed to the high pressure stripper. The resulting urea synthesis solution is led to the purification section. In which unreacted NH3 and CO2 are removed from the synthesis solution. Part of the resulting aqueous urea stream is used to obtain DEF and part is used for mixing with ammonium nitrate (not shown) to obtain UAN. The removed NH3, instead of being recycled to urea synthesis, is sent to a unit for the production of ammonium nitrate (not shown).

It should be noted that it is possible to use part of the NH3 to produce ammonium nitrate and recycle another part to urea synthesis. However, it is preferred that no ammonia is recycled in order to fully realize the benefits of the present invention.

Figure 2 is similar to Figure 1. Here the aqueous urea solution is sent to the pool. Select a process from here to use aqueous urea for the production of UAN, for the production of DEF, or for both. This has the benefit of providing a buffered amount of aqueous urea, which increases process flexibility.

Figure 3 shows steam stripping by using direct steam injection, and by sending tail gas to the ammonium nitrate production unit by using LP steam injection.

Figure 4 shows DEF production using a thermal stripper. According to the integrated production of DEF and UAN of the present invention, the tail gas from the thermal stripper is sent to the AN neutralizer instead of being sent to the gas treatment and waste water treatment section for cleaning.

Figure 5 shows a configuration similar to Figure 4, whereby the condenser and steam ejector are used as an alternative route to send the tail gas to the AN section for neutralization. While the invention has been illustrated and described in detail in the drawings and foregoing specification, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

For example, the invention can be operated in an embodiment wherein more than one water stream is separated from different places within or after the recovery section to obtain an aqueous urea solution.

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

In summary, the present invention includes a method for the integrated production of two different urea products. One is an aqueous urea solution suitable for reducing NOx (usually expressed as automotive environmental urea-DEF). The other is a solution used as a fertilizer, urea ammonium nitrate (UAN). The production of DEF and UAN is integrated in such a way that ammonia recovered from urea production is used as feedstock for ammonium nitrate production. At least part of the aqueous urea stream from urea production is mixed with ammonium nitrate to obtain UAN.

Claims (2)

1. An integrated system for producing urea and ammonium nitrate urea, said system comprising (i) a urea production unit comprising a urea synthesis section and urea downstream of the synthesis section and in fluid communication therewith A purification section, said urea synthesis section comprising a reactor, a stripper and a condenser in fluid communication with each other to form a urea synthesis loop, and (ii) a unit for generating ammonium nitrate; wherein the NH3 outlet of the purification section of the urea plant is connected to the NH3 inlet of the unit for the production of ammonium nitrate, and wherein the outlet of the aqueous urea solution of the purification part and the outlet of the aqueous ammonium nitrate solution of the unit for the production of ammonium nitrate are connected to the mixing of said aqueous urea solution with said aqueous ammonium nitrate solution unit. 2. The system of claim 1, further comprising a unit for producing nitric acid, the unit comprising an outlet for produced nitric acid in fluid communication with a nitric acid inlet of the unit for producing ammonium nitrate.