CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from Italian Patent

Application No. 102017000090748 filed on August 4, 2017, the disclosure of which is incorporated by reference.

TECHNICAL FIELD

This invention relates to a urea production process and production plant using C02 produced by oxy-combustion of a carbon supply.

The requirements of environmental protection are increasingly felt and require inter alia a careful monitoring of C02 emissions.

Therefore, in various industrial sectors the adoption of solutions characterised by high energy efficiency and by the reduction in the emission of pollutants and of C02 is sought, for example through the recycling of the latter.

BACKGROUND ART

In the industrial urea production complexes, that normally include an ammonia plant and a urea plant, the C02 required for urea synthesis is recovered in the ammonia plant via process gas cleaning with known technologies and is sent to the urea plant. The quantity of C02 recoverable from the ammonia plant is a function of the capacity of the plant itself (in terms of produced ammonia) and of the supply composition. Especially in cases of supply with a high methane content gas (so-called "light" fuels) the quantity of produced C02 can prove to be limiting, with respect to the available ammonia, for the capacity of the urea plant. In these cases, the solutions adopted to increase the quantity of available C02 are basically two:

1. enlargement of the ammonia plant's process gas production section (that is the section designated to produce hydrogen starting from fossil fuel);

2. the capture of C02 from the exhausts of the reforming oven and/or from other chimneys (gas turbines, auxiliary boilers, etc.) typically following washing with amine or with another solvent and subsequent regeneration. The separation occurs by physical-chemical absorption of C02, that in order to be used has to be compressed at suitable pressures for the urea synthesis reaction.

However, these solutions may cause further problems themselves .

In particular, in the case of ammonia plants based on reforming or gasification, the enlargement of the ammonia plant's process gas production section in order to increase the production of C02 entails a corresponding increase of energy consumption; instead, in case of C02 recovery from chimneys the costs to be borne for the installation of a new unit and the operating costs for the regeneration and the reintegration of the solvent have to be considered.

On the other hand, with ammonia plants that are based on reforming or gasification of hydrocarbons (for example in case H2 and N2 are available from other sources external to the plants) it is not possible to convert the produced ammonia in urea due to the lack of C02 that should be made available from other sources.

A known technology for the production of C02 is based on oxy-combustion processes.

Briefly, oxy-combustion is a kind of combustion in which a fuel is burnt using oxygen as primary oxidant instead of air.

In general, since air's nitrogen is not present, the concentration of C02 in the oxy-combustion' s discharged exhausts increases. Indeed oxy-combustion mainly produces water vapour and concentrated carbon dioxide, simplifying the separation of C02 and/or its recycling. The discharged exhausts have a significantly lower nitrogen content compared with what can be obtained with the traditional combustion processes (and therefore also a definitely lower content of nitrogen oxides that are instead normally produced in the traditional air combustion processes and constitute particularly dangerous pollutants) and mostly contain C02 and water vapour. Therefore, by cooling the discharged gases in order to condense the water, C02 is recovered with minimal energy consumption. Moreover, the absence of nitrogen in the combustion process results in an improvement of the energy efficiency of the system since the heating of inert materials is avoided.

An example of the application of oxy-combustion for the recovery of C02 in industrial ammonia and urea production processes/plants is described in US2015/0183650.

In particular, US2015/0183650 describes the integration of an ammonia synthesis section with a standard oxy-combustion system. Such an ammonia synthesis section comprises an ammonia synthesis unit, where crude ammonia is produced starting from hydrogen and nitrogen; and a separation unit where the raw ammonia is condensed and separated from the unreacted nitrogen and hydrogen to produce a flow of purified ammonia. An oxy-combustion reactor, where the combustion of a fuel in the presence of oxygen coming from an air separation unit takes place, is used to generate hot water or steam, to be thermally integrated with the ammonia plant, in particular by thermal connection lines that connect the oxy-combustion reactor to the ammonia synthesis unit, and/or the air separation unit with the ammonia separation unit.

The integrated plant described in US2015/0183650 produces ammonia (using the hydrogen available from other sources external to the plant and nitrogen obtained from the air separation section) and C02 captured in the oxy- combustion process. In a specific application the conversion of the products themselves (ammonia and C02) into urea is also foreseen, in a dedicated urea plant.

In the US2015/0183650 solution, the plant for the production of ammonia is necessary both for the purification of the exhausts exiting from the standard oxy- combustion section (with a high NOx and SOx content if the fed supply contains nitrogen and sulphur) and for the subsequent urea synthesis, if present.

The plants and processes of the kind described in US2015 / 0183650 , like other essentially similar ones, may however prove to be not entirely satisfactory, at least for some applications.

For example, in the hypothesis described above of a urea production plant in which the C02 proves to be limiting with respect to the available ammonia, the US2015/0183650 solution is not applicable for the following reasons :

- additional ammonia synthesis and separation sections thermally integrated with the oxy-combustion unit are required. Therefore, it is not possible to produce only the C02 requested to close the material balance without producing ammonia at the same time; - H2 has to made available from another unit of the plant .

Moreover, the generic oxy-combustion does not enable to feed multiple supplies of whatever nature and it can be subject to the typical problems of any burner linked to variations in the fuel flow.

DISCLOSURE OF INVENTION

One of the purposes of this invention is that of supplying a urea process and a urea production plant that enable to overcome the highlighted drawbacks of the prior art .

In particular, one of the purposes of the invention is that of improving the efficiency of the known urea production processes/plants and to increase their flexibility in terms of kind and flow rate of the supply.

Therefore, this invention relates to a urea process and to a urea production plant as essentially defined in the annexed claims 1 and, respectively, 13.

Additional preferred features of the invention are specified in the dependent claims.

According to the invention, a part or all the carbon dioxide required for the urea synthesis is produced in an oxy-combustion process of a carbon supply carried out with specific modes: specifically, the oxy-combustion process is a flameless oxy-combustion process, preferably pressurised. In this way, the oxy-combustion process and the related oxy-combustion unit in which it is carried out are integrated in the urea process/production plant in a much more efficient and beneficial way compared with the prior art, in particular compared with the solution proposed by US2015 / 0183650 , allowing to manage the feeding of carbon supplies having a different physical state and a different composition and with variable flows inside the same combustor without having exhausts with a high content of NOx and SOx that require purification.

The oxy-combustion is carried out in a specific reactor, preferably pressurised, that can be integrated inside already existing or new urea plants.

The invention thereby gains the following main advantages :

- the capture of C02 is significantly simplified due to its high concentration in the exhausts and the low content of contaminants and inert materials;

- the wide flexibility of the carbon supply which can be fed enables the use of waste from adjacent plants, otherwise difficult to dispose of and/or to manage;

- a surplus of electric energy and/or steam can be exported and/or integrated inside the complex in which one is operating, increasing its efficiency;

- by carrying out pressurised flameless oxy- combustion, the pressurised C02 can be captured, reducing the compression costs for bringing it to the pressure required by the urea plant;

- by carrying out flameless and pressurised oxy- combustion one can considerably reduce the exhaust treatment section, in which, in particular, no washing with ammonia is necessary;

- it is possible to reduce (and in some particular cases, depending on the fed supply, even eliminate) the quantity of passivation air possibly required by the urea plant by exploiting the excess oxygen in the combustion exhausts ;

- it is possible to associate a urea plant with the ammonia plant also when the ammonia is produced with technologies which are different from reforming or gasification of hydrocarbons, for example starting from pure H2 and N2.

In short, the invention enables to increase the capacity of the existing urea production plants, assuring the quantity of required C02 with respect to the available ammonia .

Moreover, it is possible to use the C02 produced through the oxy-combustion process/plant in a new urea plant, irrespective of the urea production process and possibly having available ammonia from other sources. BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the present invention will become clear from the description of the following non-limiting embodiments thereof, with reference to the accompanying drawings, in which:

- Figure 1 is a block diagram showing in schematic and simplified form a urea production plant, equipped with an integrated oxy-combustion unit, in accordance with the invention;

- Figure 2 is a schematic view of a variation of the Figure 1 plant, also comprising an ammonia unit for the production of ammonia.

BEST MODE FOR CARRYING OUT THE INVENTION

In Figure 1 a urea production plant as a whole is indicated with 1 comprising a urea unit 2 for producing urea by reaction of ammonia and carbon dioxide, and an oxy- combustion unit 3 in which carbon dioxide (C02) is produced to be sent to the urea unit 2 for feeding the urea synthesis reaction from ammonia and carbon dioxide.

The urea unit 2 (that can also be an existing unit,

"enhanced" with the integration of the oxy-combustion unit 3) is per se essentially known and therefore not described nor shown in detail for simplicity.

The urea unit 2, as also the urea production process carried out in it, can be of various kinds. For example, but not necessarily, the urea unit 2 can be configured to carry out a traditional so-called "Snamprogetti™ Urea Technology" urea process, but it is understood that the invention is also applied to other urea production plants/processes.

In general, the urea unit 2 mainly comprises: a urea synthesis section, where the urea reaction synthesis from ammonia and carbon dioxide takes place; some recovery sections (for example a high pressure recovery section, a medium pressure recovery section and a low pressure recovery section) , in which a urea solution produced in the synthesis section gradually concentrates with the removal from it of unreacted ammonia and carbon dioxide and water and recirculation of the recovered components; a vacuum concentration section connected to a section for the treatment of process condensates (essentially water) ; a finishing/solidification section, comprising for example a granulation unit or a prilling tower.

The urea unit 2 receives C02 (to be used as reagent in the urea reaction synthesis) produced in the oxy- combustion unit 3.

The oxy-combustion unit 3 is fed, by a fuel supply line 4, with a carbon supply (fuel) and, by an oxygen supply line 5, with an oxygen stream (oxidant) .

The supply fed by the oxy-combustion unit 3 can be of any kind and physical state (for example low heating value gas, liquid or solid refinery residues, waste material, biomasses, coal, etc.) . If necessary, for example in the case of coal-fired power, the supply can be pre-treated, prior to being fed to the oxy-combustion unit 3, in a pre- treatment unit 6 positioned along the fuel supply line 4.

The oxygen that feeds the oxy-combustion unit 3, is produced in an oxygen generation unit 7, connected to the oxy-combustion unit 3 by the oxygen supply line 5.

The oxygen generation unit 7 is for example an air separation unit, per se essentially known, configured to separate the air in nitrogen and oxygen.

The separation of the air can be carried out with any known technology, for example by fractional distillation or cryogenic fractionation, membrane separation, adsorption on suitable materials (molecular sieves, zeolites, etc.), in particular by so-called pressure swing absorption techniques (Pressure Swing Adsorption, PSA) or vacuum swing absorption (Vacuum Swing Adsorption, VSA) or with hybrid solutions (Vacuum Pressure Swing Adsorption, VPSA) .

It is understood that the oxygen generation unit 7 can be of another kind, for example of the kind operating by electrolysis of aqueous solutions.

Advantageously, as shown in Figure 2, in the event that the urea production plant 1 includes or is adjacent to an ammonia unit 8 where ammonia is produced and the ammonia unit 8 is based on an autothermal reforming technology (AutoThermal Reforming, ATR) that makes use of a cryogenic air separation unit, the oxygen generation unit 7 is an air separation unit defined by said already existing cryogenic air separation unit, with a clear reduction of the investment and operating costs. The oxygen generation unit 7 (that is the air separation unit) is therefore connected, in addition to the oxy-combustion unit 3, also to the ammonia unit 8, by a nitrogen line 9 and an oxygen line 10 that feed nitrogen and oxygen to the ammonia unit 8.

The quantity and purity of the oxygen required by the oxy-combustion unit 3, are in any case such to make the air separation possible also with technologies alternative to cryogenic fractionation, more convenient in terms of investment, as the already mentioned techniques of adsorption or membrane separation that supply oxygen with 90-95%vol or lower content.

In general, the oxy-combustion unit 3 is fed by an oxygen stream containing at least about 80%vol, preferably at least 90%vol, of oxygen.

The oxy-combustion unit 3 is specifically a flameless oxy-combustion unit, in particular a flameless and pressurised oxy-combustion unit, configured so as to perform a flameless oxy-combustion, in particular a flameless and pressurised oxy-combust ion, of the fuel in the presence of oxygen.

In the flameless oxy-combust ion process (preferably pressurised) carried out in the oxy-combust ion unit 3, specifically in a combustor (combustion chamber) of the oxy-combust ion unit 3, the carbon supply (fuel) is burnt with the oxygen in such operating conditions that the combustion occurs without generating a flame.

According to the invention, the combustor of the oxy- combustion unit 3, is a flameless combustor, preferably pressurised and isothermic.

Preferably, the combustor 's operating pressure ranges between 0 and 40 bar g.

Preferably, the combustion temperature ranges between about 800 and about 1800°C, preferably between about 1000 and about 1500 °C.

As an example, the flameless oxy-combust ion process (preferably pressurised) is carried out with the modes and in a combustor of the kind described in one or more of the following documents: W02009071230 , W02009071238 ,

WO2009071239, W02014016235 , W02014016237 , W02015097001.

The oxy-combust ion process produces exhausts, containing in particular C02 and that exit from the oxy- combustion unit 3 by an exhaust line 11, and melted waste, that are solidified and inertised and then removed from the oxy-combustion unit 3 through a discharge line 12.

The oxy-combustion unit 3 is connected by the exhaust line 11 to an energy recovery unit 13.

The exhausts produced by the oxy-combustion process, indicatively at temperatures ranging between 1000 and 1500°C, are sent through the exhaust line 11 to the energy recovery unit 13 where the thermal energy is converted into steam and/or electric energy to support the energy consumption of plant 1.

The energy recovery unit 13 is therefore configured so as to recover heat from said exhausts produced in the oxy-combustion unit 3 and produce steam and/or electric energy .

For example, the energy recovery unit 13 comprises a boiler fed with water by a water line 14 and that produces steam, which is used as heating fluid to heat other process fluids in plant 1 and/or to generate electric energy by a turbine coupled with a generator.

In particular, steam and/or electric energy generated in the energy recovery unit 13 are used, for example, in a purification and compression unit 15 (described below) , that feeds the urea unit 2 with C02, or in the oxygen generation unit 7.

A possible excess of steam and/or electric energy is integrated with the existing network of plant 1 thereby improving the overall efficiency of the plant itself, or is exported (that is supplied to users that are external to plant 1) .

The steam can be produced in the energy recovery unit 13 at any wanted pressure level (for example by extracting steam from different stages of the steam turbine) so that it may be easily integrated with the existing plant.

Thus, when the oxy-combustion unit 3 is inserted in a pre-existing urea production plant 1 with the resulting increase in urea production capacity, part of the steam and/or energy produced in the energy recovery unit 13 can be sent to the urea unit 2 to support the greater consumption indeed due to the increase in the production capacity .

Another way to generate electric energy could be for example through a supercritical C02 cycle instead of a traditional steam cycle.

Part of the exhausts exiting from the energy recovery unit 13 is recircled, by an exhaust recirculation line 16 fitted with a blower 17, to the oxy-combustion unit 3 and possibly to the energy recovery unit 13.

In particular, the exhaust recirculation line 16 inserts itself in the oxygen supply line 5 with a first arm 16a and is optionally connected, by a second branch 16b, to the energy recovery unit 13. The remaining part of the exhausts exiting from the energy recovery unit 13 is treated in a C02 recovery section 20 configured so as to recover (separate) C02 from the exhausts with the C02 purity specification suitable for feeding the urea reaction synthesis carried out in the urea unit 2.

The energy recovery unit 13 is then connected by a portion 11a of the exhaust line 11 to the C02 recovery section 20.

For example, the C02 recovery section 20 comprises an exhaust treating unit 21, a condensation unit 22, and a purification and compression unit 15, connected to the energy recovery unit 13 and to each other, in series, by respective portions 11a, lib, 11c of the exhaust line 11.

The exhausts treatment is essentially needed in order to remove from the exhausts pollutants possibly present in the carbon supply fed by the oxy-combustion unit such as sulphur, chlorine, etc.

The kind of exhausts treatment depends on the composition of the carbon supply fed by the oxy-combustion unit and therefore on the pollutants that are present.

For example, if a carbon supply high in sulphur is fed, the exhaust treating unit 21 must be configured to remove the sulphur up to the specification required by the final user and/or by the subsequent treatments; amongst the various possibilities, for example, a lime-based treatment can be carried out. If chlorine is present in the fed supply, a treatment based on caustic soda can be effected, etc .

Clearly, the exhaust treating unit 21 can be configured to perform various different treatments; nevertheless, the treatments are in any case simpler and require a lower energy consumption compared with those downstream both in traditional combustion and in generic oxy-combustion .

In one embodiment, an excess of oxygen is present in the exhausts produced by the oxy-combustion process. The excess of oxygen in the exhausts is usefully maintained in the C02 stream fed to the urea unit 2, since it enables to reduce the quantity of passivation air possibly mixed with C02 to allow the passivation of the metal surfaces in the urea unit 2, with a further improvement of plant l's overall efficiency.

The exhausts treated in the exhaust treating unit 21 are then sent to the condensation unit 22, where they undergo condensation for the removal of the water which is present that is removed through a condensates recovery line 23; and then to the purification and compression unit 15, where the C02 is separated from the exhausts. The C02 stream separated from the exhausts is fed to the urea unit 2 by a C02 supply line 24, while the exhausts full of inert materials are discharged, for example to a chimney, by an exhaust discharge line 25.

In one embodiment, the gaseous stream coming from the oxy-combustion unit 3 and from the energy recovery unit 13 via the portion 11a of the exhaust line 11 already has the purity specifications required by the urea unit 2; in this case, usefully, the C02 recovery section 20 only comprises the condensation unit 22 (since the exhaust treating unit 21 is unnecessary) and a unit 15 reduced to only a compression unit (with no need for purification) .

The C02 exiting from the purification and compression unit 15 is sent to the urea unit 2 and introduced in the most suitable part of the urea unit 2, preferably already at the required pressure and possibly mixed with a C02 stream already available at plant 1.

In one embodiment, the oxy-combustion unit 3 and the subsequent C02 recovery section 20 supply all the carbon dioxide needed by the urea unit 2.

In this case, the C02 stream exiting from the purification and compression unit 15 is fed directly by the synthesis section of the urea unit 2, at a pressure of about 140-200 bar and at a temperature of about 90-150°C.

In other embodiments, the C02 stream coming from the purification and compression unit 15 is sent instead to the exit (discharge) of a C02 compressor of the urea unit 2 (at a pressure of about 160 bar g or higher) , at the entry (intake) of the same compressor (at a pressure of 0 and 2 bar g) or in one of the intermediate steps (at an intermediate pressure between 2 and 160 bar g) .

Finally, it is understood that further modifications and variations can be made to the process and to the plant described hereto that are not outside the scope of the annexed claims.