Urea granulation process and method of revamping

Abstract

The present invention relates to a urea granulation process comprising: scrubbing a polluted air stream (7) – produced by said granulation process and containing ammonia (NH3) and ammonium cyanate (NH4OCN) – with a formaldehyde (H2CO)-containing solution (16) to obtain an ammonia- and ammonium cyanate-depleted air stream (27) and a spent scrubber solution (29); reacting said formaldehyde with ammonium cyanate to obtain cyanuric acid (HOCN); recycling at least part of said cyanuric acid to said scrubbing of the polluted air stream (7).

Description

[0001] “Urea granulation process and method of revamping

[0002] DESCRIPTION

[0003] Field of the invention

[0004] The present invention relates to a urea granulation process, in particular for producing solid urea granules from a liquid such as urea melt or concentrated urea solution.

[0005] Prior art

[0006] Granulation processes are known in the art and are frequently used in the manufacture, for example, of fertilizers in granular form.

[0007] In urea fluid bed granulation (i.e. granulation performed in a fluidized bed), formaldehyde (H2CO) is typically used as an additive that is mixed to concentrated urea solution or liquid urea melt and serves both as a granulation aid and as an anti-caking agent for the solid urea granules.

[0008] Formaldehyde is mixed with the urea melt in the form of a formaldehyde aqueous solution (formalin) or of urea-formaldehyde (UF) condensate. E.g., UF80 or UF85 contain 56-60% by weight (wt%) of formaldehyde, 24-25 wt% of urea, the balance being water. The concentration of formaldehyde in the urea granules for fertilizer grade is comprised from 0.2 wt% and 0.7 wt%, typically 0.45 wt%.

[0009] Concentrated urea solution or liquid urea melt contains an amount - typically 550 ppm - of free ammonia, present as ammonia (NH3), ammonium bicarbonate (NH4HCO3) and ammonia carbonate ((NF ^COs), and an amount - typically 2000 ppm - of combined ammonia present as ammonium-cyanate (NH4OCN). All those ammonia products evaporate to great extent during spraying of the urea solution in the fluid bed granulator and are extracted from the granulator by the air flow.

[0010] A problem with making granules from liquid source materials is that a certain amount of dust is generated in the granulator and in the cooler of the solid granules. Dust is entrained by fluidization air drawn from the granulator and by cooling air drawn from the cooler of the solid granules. This entrained dust and residual ammonia need to be removed from air before air can be discharged into the atmosphere.

[0011] Many alternative processes have been proposed in order to clean the fluidization air and the cooling air, but the current processes still suffer from some drawbacks.

[0012] It has been proposed to feed a UF condensate solution in a warm and urea dust- loaden air of the granulator, but such solution proved to be not satisfactory due to demister clogging by urea-formaldehyde polymers.

[0013] WO 2013 / 167245 A1 discloses a urea granulation process with scrubbing system including a gaseous waste stream for removal of dust and ammonia whereby this waste stream is processed first through process step (a) and then through process step (b): (a) washing the dust and ammonia laden stream with an aqueous urea solution whereby a dust-laden liquid stream and a dust-reduced stream is generated, and (b) reacting the dust-reduced stream with formaldehyde to form a stream comprising hexamethylene tetramine and urea-formaldehyde and clean off-gas.

[0014] The process disclosed in WO 2013 / 167245 A1 has a drawback that formaldehyde is injected at the end of the scrubbing system, so that it incurs risks of formaldehyde entrainment to the stack.

[0015] To eliminate such formaldehyde entrainment, it would be possible to use under- stoichiometric amounts of formaldehyde in step (b) than requested by the reaction with ammonia but, in this case, a final acid scrubbing stage of ammonia with sulphuric acid becomes compulsory. Formation of ammonium sulphate in such scrubbing stage poses limitations when ammonia should be recovered from the scrubbing solution for recycling to the process, because ammonium sulphate is accepted only in limited amounts within such process. In this case, ammonia is captured in part by formaldehyde and in part by the sulphuric acid.

[0016] US 5 686 647 A discloses a process for the preparation of urea according to the known art. The Applicant, after long and intensive research and development, has developed a process that provides an adequate response to the existing limitations, drawbacks, and problems.

[0017] In particular, the present process is the result of the inventor’s judicious insight that ammonium cyanate entrained by the polluted air stream can be captured and used as a reaction partner for formaldehyde to give cyanuric acid, and that such cyanuric acid can be recycled for improving scrubbing of the polluted air stream.

[0018] In fact, the present process provides a double action on ammonia capture: a direct action is obtained by contacting formaldehyde and ammonia, and an indirect action is achieved by recycling cyanuric acid that can react with ammonia during scrubbing of the polluted air stream.

[0019] In view of the above, the subject matter of the present invention is a urea granulation process.

[0020] Another object of the present invention is a method of revamping a urea granulation process.

[0021] Summary of the invention

[0022] The purpose of the invention is to solve the aforementioned drawbacks of the known art.

[0023] The above aims are achieved by a urea granulation process comprising: scrubbing a polluted air stream - produced by said granulation process and containing ammonia (NH3) and ammonium cyanate (NH4OCN) - with a formaldehyde (H2CO)-containing solution to obtain an ammonia- and ammonium cyanate-depleted air stream and a spent scrubber solution; reacting said formaldehyde with ammonium cyanate to obtain cyanuric acid (HOCN); recycling at least part of said cyanuric acid to said scrubbing of the polluted air stream.

[0024] Moreover, the above aims are achieved by a method of revamping a urea granulation process; wherein said urea granulation process to be revamped:

[0025] - is performed in a fluid bed granulator fed with fluidization air and producing a polluted air stream containing ammonia and ammonium cyanate;

[0026] - comprises a scrubbing step of said polluted air stream with a formaldehyde (H2CO)-containing solution; wherein the method of revamping comprises:

[0027] (I) adding over-stoichiometric formaldehyde to the H2CO-containing solution with respect to the ammonia contained in said polluted air stream;

[0028] (II) providing a reaction environment and reacting said over-stoichiometric formaldehyde with said ammonium cyanate to obtain cyanuric acid; and

[0029] (III) if not already available, installing a line for recycling at least part of said cyanuric acid to said scrubbing step.

[0030] Advantageous effects of the invention

[0031] The invention has the following advantages.

[0032] Advantageously, the acid-base reaction according to the present process is carried out without addition of any foreign acid because the process itself produces ammonium cyanate.

[0033] Advantageously, the process of the present invention provides additional ammonium cyanate that is a reagent in reaction (II).

[0034] Advantageously, use of free formaldehyde in the present process avoids plugging of the demisters which is a lamented problem in existing plants that make use of urea formaldehyde polymers or formalin instead of free formaldehyde. Advantageously, the present process reduces or eliminates the risk that formaldehyde may escape from the stack: formaldehyde is a reaction partner for both ammonia and ammonium cyanate, so that it can be added in a stoichiometric excess with respect to the amount of ammonia in the polluted air stream to be treated. The over-stoichiometric amount of formaldehyde reacts with ammonium cyanate to give cyanuric acid that reduces the pH value of the spent scrubber solution, so as to capture further ammonia with an acid-base reaction when such spent scrubber solution is recirculated to the ultra-fine particle scrubbing.

[0035] Furthermore, any eventual further excess of formaldehyde in free form can also react with the ultra-fine particles of urea in the spent scrubber solution given the long residence time to form methylol urea which is not volatile and remains in such solution. The presence of urea in the washing solution enables this reaction and as such the recovery of any eventual remaining free formaldehyde, whereas a scrubbing with a liquid that does not contain urea will not recover the free formaldehyde.

[0036] According to a further advantage, the present process avoids to work with under- stoichiometric amounts of formaldehyde which traditionally imposes to provide a final acid scrubbing stage with additional acid and to manage the reaction products (typically salts such as ammonium sulphate) of ammonia with such additional acid.

[0037] Advantageously, in the present process any unreacted formaldehyde in free form is captured efficiently in the ultra-fine particle scrubbing together with the ultrafine particles of ammonium cyanate.

[0038] Advantageously, the pH value of the spent scrubber solution can be monitored easily, e.g., in the scrubber solution tank, and can be adjusted by regulating an amount of free formaldehyde in the formaldehyde-containing solution.

[0039] Advantageously, also ammonia capture in the cooler scrubber is made more efficient by the process of the present invention because of the acidic nature of the scrubbing liquid, that comprises cyanuric acid transported to the cooler scrubber by the spent stream.

[0040] Advantageously, in the process of the present invention there is no contact between formaldehyde in free form and urea under conditions (e.g., in presence of temperature conditions of > 80 °C) that could lead to urea-formaldehyde polymers in undesired steps or apparatuses of the process.

[0041] Advantageously, the process of the present invention has been designed so that an amount of ammonium cyanate in the spent scrubber solution is maintained higher than any eventual unreacted free formaldehyde, so that chances that free formaldehyde may escape to the stack are very low or null.

[0042] Advantageously, the wet granulator scrubber of the present invention is capable of recovering the ultra-fine particles comprising ammonium cyanate. This results in a substantial amount of ammonium cyanate in the compartment where the ultra-fine particle scrubbing step is performed. This salt cannot evaporate since it is not able to split into ammonia and formaldehyde at the relatively low temperature of such compartment.

[0043] Advantageously, stoichiometry of reaction (III) reduces the requested amount of formaldehyde to be used in reactions (I), (I. A), (I.B) and (II) since reaction (III) also binds I consumes ammonia.

[0044] Advantageously, formed hexamethylene tetramine has similar or same properties and effects as formaldehyde when it comes to serving as granulation additive. Hence, hexamethylene tetramine is an alternative to formaldehyde, or can be used together with formaldehyde, in the urea granulation process.

[0045] Advantageously, any cyanuric acid that has not reacted with ammonia according to reaction (III) will be converted into ammonium cyanate by the free ammonia present in the urea melt or concentrated urea solution. In fact, ammonium cyanate is an isomer of urea and is always present in hot urea melt or concentrated urea solution. Advantageously, any ammonium cyanate will evaporate in the final evaporation, end up in recovered condensates, and then sent to the normal ammonia water treatment.

[0046] Advantageously, also any amount of hexamethylene tetramine and / or of conjugated formaldehyde are within the normal limits since only a small part of the total formaldehyde is used for the ammonia abatement.

[0047] Preferred embodiments

[0048] Preferably, said urea granulation process is performed in a fluid bed granulator 3 fed with fluidization air 4 and producing said polluted air stream 7.

[0049] More preferably, said scrubbing is performed at least in part (e.g. all) in a wet granulator scrubber 10. Said wet granulator scrubber 10 may be a horizontal wet granulator scrubber 10.

[0050] According to an embodiment, formaldehyde contained in the formaldehyde- containing solution 16 is over-stoichiometric with respect to the ammonia contained in said polluted air stream 7.

[0051] Preferably said process comprises: reacting said formaldehyde with ammonia to obtain hexamethylene tetramine ((CH2)6N4), and reacting over-stoichiometric formaldehyde with said ammonium cyanate to obtain said cyanuric acid.

[0052] According to a preferred embodiment, said cyanuric acid is obtained in situ in the spent scrubber solution 29.

[0053] Preferably, at least part of the spent scrubber solution 29 with a lowered pH value (due to formation of cyanuric acid) is at least partially recycled to said scrubbing for an acid removal of ammonia contained in the polluted air stream 7 with formation of additional ammonium cyanate. According to an embodiment, an amount of ammonium cyanate in the spent scrubber solution 29 is higher than any unreacted free formaldehyde in said spent scrubber solution 29.

[0054] According to another embodiment, said scrubbing comprises an ammonia scrubbing step 9 and a scrubbing step 11 of ultra-fine particles comprising ammonium cyanate and optionally urea.

[0055] “Ultra-fine” means that the average particle size of the particles scrubbed in this step 11 is comprised from 100 nm to 1 pm, preferably comprised from 150 nm to 0.5 pm, more preferably comprised from 200 nm to 0.4 pm. Such average particle size is determined by laser diffraction.

[0056] According to still another embodiment, during the scrubbing step 11 , said ultrafine particles of urea are transferred into the spent scrubber solution 29 where the urea ultra-fine particles react with any residual formaldehyde to obtain methylol urea.

[0057] According to a first embodiment, said ammonia scrubbing step 9 and said ultrafine particle scrubbing step 11 are performed in sequence, preferably ammonia scrubbing step 9 being performed before the ultra-fine particle scrubbing step 11 .

[0058] According to a second embodiment, said ammonia scrubbing step 9 and said ultra-fine particle scrubbing step 11 are performed concurrently, preferably in a same compartment of the wet granulator scrubber 10.

[0059] According to a preferred embodiment, said scrubbing of the polluted air stream 7 comprises a urea dust scrubbing step 8 to obtain a de-dusted air stream (15), said de-dusted air stream 15 being contacted with said solution 16 containing formaldehyde.

[0060] According to another preferred embodiment, in the urea dust scrubbing step 8 said polluted air stream 7 is contacted with an aqueous scrubbing solution 13 that washes out at least part of the urea dust contained in the polluted air stream 7, so as to obtain a urea dust-enriched solution 14 and a de-dusted air stream 15. Preferably, said aqueous scrubbing solution 13 is a concentrated aqueous urea solution. ’’Concentrated” means that an amount of urea in such aqueous urea solution is comprised from 40 wt% to 60 wt%, more preferably comprised from 45 wt% to 65 wt%, with respect to an overall weight of said solution.

[0061] Said urea dust scrubbing step 8 has a given operating temperature. Such operating temperature, expressed in absolute terms, is preferably comprised from 40 °C to 45 °C.

[0062] Preferably, an operating temperature of said ultra-fine particle scrubbing step 11 (preferably of a scrubbing solution 26 used in said step 11 ) is at least 5 °C lower, preferably at least 8 °C lower, e.g., from 8 °C and 10 °C lower, than said operating temperature of the urea dust scrubbing 8.

[0063] According to an embodiment, said spent scrubber solution 29 is cooled by heat exchange before being at least partially recycled as scrubbing solution 26.

[0064] According to different embodiments, such heat exchange may be an indirect heat exchange 32 or a direct heat exchange in a cooler scrubber 40.

[0065] The urea granulation process preferably comprises steps of: providing a paraformaldehyde- and / or methylol urea-containing stream 17 wherein formaldehyde is in conjugated form, and mixing said paraformaldehyde- and / or methylol urea-containing stream 17 and a stream of hot process condensate 18 having a temperature > 90 °C to obtain said formaldehyde-containing solution 16 wherein formaldehyde is in free, unconjugated form.

[0066] The urea granulation further comprising a step of adjusting a flow rate of said paraformaldehyde- and / or methylol urea-containing stream 17 and / or a flow rate of said stream of process condensate 18, so as to obtain a desired pH value in the spent scrubber solution 29. Accordingly, the present process preferably comprises measuring a pH value in the spent scrubber solution 29 and adjusting said flow rate(s) so as to obtain the desired pH value. Preferably, said desired pH value is > 4.5, preferably comprised from 4.5 to 6.5. In this range of pH values, hydrolysis of NH4OCN into NH3 is substantially absent.

[0067] More preferably, the process comprises adding sodium hydroxide to said paraformaldehyde- and / or methylol urea-containing stream 17, or to said stream of process condensate 18, or to a mixture thereof, in order to adjust the desired pH value. Adding sodium hydroxide may be advantageous for increasing a reaction speed of paraformaldehyde to obtain said formaldehyde in free, unconjugated form. E.g., said sodium hydroxide may be in the form of caustic soda containing an amount of sodium hydroxide > 20% by weight, preferably > 40% by weight, even more preferably > 70% by weight.

[0068] According to an embodiment, said paraformaldehyde- and / or methylol urea- containing stream 17 is a split portion of a stream of granulation aid 24 added to a urea melt or concentrated urea solution 1 upstream of said granulation process (e.g. upstream of said fluid bed granulator 3). Preferably, said split portion is comprised from 30% to 70% by volume, preferably comprised from 40% to 60%, more preferably comprised from 45% to 55%, even more preferably comprised from 48% to 52%, with respect to said stream of granulation aid 24.

[0069] According to an embodiment, said fluid bed granulator 3 produces solid urea granules 5 subjected to a cooling step 6 with a stream of cooling air 38 to obtain thermally treated solid granules 50 and a second polluted air stream 39.

[0070] Said second polluted air stream 39 is preferably sent to a cooler scrubber 40 to obtain a second purified air stream 45.

[0071] Preferably, part of a scrubbing liquid 47 used in said cooler scrubber 40 is made by said spent scrubber solution 29.

[0072] More preferably, a portion 51 of said scrubbing liquid 47 is used for scrubbing of said polluted air stream 7.

[0073] In said method of revamping, said reaction environment preferably is a spent scrubber solution obtained in said scrubbing step.

[0074] Preferably, the method of revamping comprises: (VI) installing a scrubbing stage 11 of ultra-fine particles comprising ammonium cyanate and optionally urea; said ultra-fine particles having an average particle size comprised from 100 nm to 1 pm determined by laser diffraction.

[0075] According to a first embodiment, said scrubbing stage 11 is installed downstream an existing ammonia scrubbing stage.

[0076] According to a second embodiment, said scrubbing stage 11 is installed within an existing ammonia scrubbing stage. “Within” means that the scrubbing stage 11 and the existing ammonia scrubbing stage may be merged in a single stage.

[0077] Preferably, the method of revamping comprises:

[0078] (V) providing a mixing environment between a paraformaldehyde- and / or methylol urea-containing stream 17 and a stream of hot process condensate 18 having a temperature > 90 °C to obtain said H2CO-containing solution wherein formaldehyde is in free, unconjugated form.

[0079] According to different embodiments, said mixing environment may be or comprise a mixing vessel or a tube.

[0080] The advantages of the invention will be even more evident from the following detailed description made on the basis of the enclosed figures, which are provided as a non-limiting example.

[0081] Description of the figures

[0082] Fig. 1 : process of the present invention according to a first embodiment;

[0083] Fig. 2: enlargement of the wet granulator scrubber of Fig. 1 according to another embodiment;

[0084] Fig. 3: process of the present invention according to a second embodiment;

[0085] Fig. 4, Fig. 6: schematizations of processes to be revamped, according to different embodiments; Fig. 5, Fig. 7, Fig. 8: schematizations of processes revamped with the method of the present invention, according to different embodiments.

[0086] Detailed description of the invention according to a preferred embodiment

[0087] Fig. 1 shows a first preferred embodiment of the process according to the present invention.

[0088] A urea melt or a concentrated urea solution 1 and seeds 2 are fed to an inlet side of a fluid bed granulator 3. A stream of granulation aid 24, such as aqueous formalin and / or methylol urea, may be added to said urea melt or concentrated urea solution 1 upstream of the fluid bed granulator 3.

[0089] Fluidization air 4 is blown from the bottom of the fluid bed granulator 3 so as to maintain the bed in a fluidized state. Said urea melt or concentrated urea solution 1 is sprayed on said seeds 2, and then on granules formed by said seeds, which grow by increasing in size / diameter as the process continues. During such granulation process, said growing granules are gradually moved to an outlet side - opposite to the inlet side - of the fluid bed granulator 3.

[0090] Solid urea granules 5 having desired size / diameter and residual moisture content are withdrawn from said fluid bed granulator 3. Said residual moisture content is at most 0,3 wt%. Said solid urea granules 5 are then transferred to a cooling section 6 to obtain thermally treated solid urea granules 50.

[0091] A polluted air stream 7 containing, inter alia, ammonia (NH3), entrained urea dust and entrained ammonium cyanate (NH4OCN), is drawn from the fluid bed granulator 3 and is sent for scrubbing in a wet granulator scrubber 10. Said scrubbing comprises - in sequence - a urea dust scrubbing step 8, an ammonia scrubbing step 9, an ultra-fine particle scrubbing step 11 , and an optional extra ammonia scrubbing step 12. At least one demister 46 is arranged between adjacent steps to remove condensates from the air stream passing from one step to the other. In the urea dust scrubbing step 8 said polluted air stream 7 is contacted with an aqueous scrubbing solution 13, such as a concentrated aqueous urea solution, that washes out at least part of the urea dust contained in the polluted air stream 7, so as to obtain a urea dust-enriched solution 14 and a de-dusted air stream 15. Said de-dusted air stream 15 is water-saturated.

[0092] The de-dusted air stream 15 is supplied to the subsequent ammonia scrubbing step 9. The urea dust-enriched solution 14 is partially recirculated as aqueous scrubbing solution 13 to the urea dust scrubbing step 8. Another part 44 of the urea dust-enriched solution 14 may be sent to an evaporation section 19 for recycling an evaporated solution 20 - having a reduced water content with respect to the urea dust-enriched solution 14 - to the fluid bed granulator 3. Hence, said evaporation section 19 is configured for reducing appropriately a water content of said part 44 of the urea dust-enriched solution 14, so that the resulting evaporated solution 20 can be recycled to the fluid bed granulator 3. Said evaporation section 19 may be part of a urea synthesis plant or of a melamine synthesis section, or may be a dedicated evaporation section.

[0093] In the ammonia scrubbing step 9, the de-dusted air stream 15 is contacted with a formaldehyde-containing solution 16, wherein formaldehyde (H2CO) is in free form, to obtain an ammonia-depleted air stream 21 and a first washing solution 22.

[0094] In the ammonia scrubbing step 9 formaldehyde reacts with the ammonia present in the de-dusted air stream 15 to give hexamethylene tetramine ((CH2)eN4) and water (H2O) according to the following reaction (I):

[0095] 6 H2CO + 4 NH3(CH2)6N4 + 6 H2O (I)

[0096] Hexamethylene tetramine and water remain in the first washing solution 22 that is collected in a scrubber solution tank 25. Reaction (I) relates to free ammonia. However, ammonia may be present as ammonium hydroxide (NH4OH; “ammonia water”) and / or as ammonium hydrogen carbonate (NH4HCO3; ammonium bicarbonate) in the de-dusted air stream 15. In these cases, hexamethylene tetramine is formed according to the following reactions (I. A) and / or (LB):

[0097] 6 H2CO + 4 NH4OH (CH2)6N4 + 10 H2O (LA)

[0098] 6 H2CO + 4 NH4HCO3 (CH2)6N4 + 6 H2O + 4 CO2 (LB)

[0099] According to reaction (I.B), carbon dioxide (CO2) is also formed, and preferably carried away by the ammonia-depleted air stream 21 .

[0100] The formaldehyde-containing solution 16 is obtained by mixing a paraformaldehyde- and / or methylol urea-containing stream 17 and a stream of hot process condensate 18.

[0101] “Hot” means that the process condensate 18 has a temperature > 90 °C.

[0102] Such stream of hot process condensate 18 may originate from the urea synthesis plant, e.g., from steam users thereof.

[0103] The paraformaldehyde- and / or methylol urea-containing stream 17 may be a split portion of said stream of granulation aid 24 or, as an alternative, may originate from a separate source. E.g. the paraformaldehyde-containing stream 17 may be a stream of aqueous formalin.

[0104] Formaldehyde is in a conjugated form in both paraformaldehyde and methylol urea: methylol urea is a product of a condensation reaction between formaldehyde and urea, and paraformaldehyde is a product of a selfpolymerization of formaldehyde. A typical degree of polymerization of paraformaldehyde is comprised from 8 to 100 units of formaldehyde. Through said mixing of the paraformaldehyde- and / or methylol urea-containing stream 17 and of said stream of hot process condensate 18, that may be performed in a mixing vessel 23 or by feeding said streams to a common tube, formaldehyde is reacted from its conjugated form(s) into its free, unconjugated form, because the stream of hot process condensate 18 creates the conditions for such reaction(s). Without being bound by theory, paraformaldehyde depolymerizes to formaldehyde solution by water in presence of heat, and methylol urea undergo a hydrolysis reaction to give urea and formaldehyde in presence of water and heat. Optionally, an amount of sodium hydroxide may be added to said paraformaldehyde- and / or methylol urea-containing stream 17 and / or to said stream of process condensate 18, for obtaining said formaldehyde in free, unconjugated form more rapidly.

[0105] The ammonia-depleted air stream 21 is passed to the ultra-fine particle scrubbing step 11 wherein the ammonia-depleted air stream 21 is scrubbed with a cold scrubbing solution 26 to obtain an ammonia- and ammonium cyanate-depleted air stream 27 and a second washing solution 28.

[0106] “Cold” means that the temperature of the scrubbing solution 26 is at least 5 °C (e.g., from 8 °C and 10 °C) lower than an operating temperature of the urea dust scrubbing step 8. Expressed in absolute terms, the cold scrubbing solution 26 may have a temperature comprised from 30 °C to 45 °C, preferably comprised from 35 °C to 40 °C.

[0107] The ammonia-depleted air stream 21 entering the ultra-fine particle scrubbing step 11 is still water-saturated, so that a sudden drop in temperature obtained by contact with the cold scrubbing solution 26 leads to a quantitative condensation: the ultra-fine particles act as condensation nuclei, resulting in an increase in particle size, thus allowing capture of said ultra-fine particles of ammonium cyanate in the second washing solution 28.

[0108] The second washing solution 28 is also collected in the scrubber solution tank 25 thereby mixing with the first washing solution 22 and so obtaining a spent scrubber solution 29. Hence, the spent scrubber solution 29 contains formaldehyde in free form (contained in the first washing solution 22) and ammonium cyanate (contained in the second washing solution 28).

[0109] Formaldehyde and ammonium cyanate can thus react in the scrubber solution tank 25 to obtain hexamethylene tetramine, water and cyanuric acid (HOCN) according to the following reaction (II):

[0110] 6 H2CO + 4 NH4OCN (CH2)6N4 + 6 H2O + 4 HOCN (II)

[0111] Formation of cyanuric acid in the spent scrubber solution 29 makes such solution acidic.

[0112] The spent scrubber solution 29 is drawn from the scrubber solution tank 25 as spent stream 30, and is at least partially recirculated as cold scrubbing solution 26 to the ultra-fine particle scrubbing step 11 . Said spent stream 30 is subjected to cooling in an indirect heat exchanger 32 before scrubbing the ammonia- depleted air stream 21 .

[0113] As a consequence of the spent scrubber solution 29 being acidic, scrubbing in the ultra-fine particle scrubbing step 11 results in a more efficient removal of any ammonia remaining in the ammonia-depleted air stream 21 due to an acid-base reaction (III) between cyanuric acid and ammonia to give ammonium cyanate according to the following reaction:

[0114] HOCN + NH3NH4OCN (III)

[0115] If the ammonia content in the ammonia- and ammonium cyanate-depleted air stream 27 has reached a desired concentration, said ammonia- and ammonium cyanate-depleted air stream 27 may be vented from a stack 53.

[0116] However, if such desired concentration (e.g. a maximum concentration required by environmental regulations) has not been reached, the ammonia- and ammonium cyanate-depleted air stream 27 is fed to the extra ammonia scrubbing step 12 wherein such air stream 27 is contacted with a liquid acid stream 33 to obtain a first purified air stream 34 and an acid-depleted solution 35.

[0117] The first purified air stream 34 may be vented from a stack 53. The acid-depleted solution 35 is recycled within an acid abatement loop 37.

[0118] Preferably, the liquid acid stream 33 is an aqueous solution of a foreign acid. “Foreign” means that such acid is different from the cyanuric acid produced within the process. E.g. said a foreign acid may be sulphuric acid.

[0119] The acid stream 33 may be provided by an acid source 36. Said acid source 36 may provide additional acid to the acid abatement loop 37 before the acid stream 33 is returned to the extra ammonia scrubbing step 12.

[0120] Fig. 2 shows an enlargement of the wet granulator scrubber of Fig. 1 according to another embodiment wherein the ammonia scrubbing step and the ultra-fine particle scrubbing step are carried out concurrently.

[0121] In this embodiment no demister 46 is arranged between these steps, so that the de-dusted air stream 15 is scrubbed by the formaldehyde-containing solution 16 and by the cold scrubbing solution 26 substantially at the same time, and eventually in a same compartment of the wet granulator scrubber 10, thus obtaining the ammonia- and ammonium cyanate-depleted air stream 27. The first washing solution 22 and the second washing solution 28 are fed jointly (i.e., a single stream) to the scrubber solution tank 25.

[0122] Fig. 3 shows a second preferred embodiment of the process according to the present invention.

[0123] As a difference with respect to the embodiment of Fig. 1 , in Fig. 2 the spent stream 30 feeds a cooler scrubber 40, as explained in more detail herein after. In such embodiment no indirect heat exchanger 32 is provided.

[0124] Any ammonia that did not evaporate in the fluid bed granulator 3 is still contained in the solid urea granules 5. In the cooling section 6 of Fig. 2, such solid urea granules 5 are cooled to the thermally treated solid granules 50 with a stream of cooling air 38. A second polluted air stream 39 is drawn from the cooling section 6 and sent to said cooler scrubber 40. Pollutants of said stream are mainly ammonia and urea dust.

[0125] Said cooler scrubber 40 comprises at least a cooler scrubbing step 31 wherein said second polluted air stream 39 is contacted with a scrubbing liquid 47, thereby obtaining a pollutants-enriched solution 41 and a pollutants-depleted air stream 42.

[0126] The pollutants-enriched solution 41 is collected in a cooler solution tank 48. The pollutants-depleted air stream 42 is fed to a dry room 43, and then exists said dry room 43 as second purified air stream 45 that may be vented from a stack 54.

[0127] The cooler solution tank 48 is fed also by the spent stream 30 and, optionally, by a stream of makeup water 49 to cope with evaporation of water in the process. Hence, the scrubbing liquid 47 is made by the pollutants-enriched solution 41 , the spent stream 30, and the stream of makeup water 49 (if any).

[0128] A first portion 51 of the scrubbing liquid 47 is sent to the ultra-fine particle scrubbing step 11 as cold scrubbing solution 26. A second portion 52 of the scrubbing liquid 47 may be sent to the urea dust scrubbing step 8 to mix with the urea dust-enriched solution 14 and thus adjust (e.g., dilute) concentration of such solution.

[0129] Fig. 4 and Fig. 6 are schematizations of processes to be revamped, according to different embodiments. Fig. 5, Fig. 7, Fig. 8 are schematizations of processes revamped with the method of the present invention, according to different embodiments. Numbering of figures from Fig. 4 to Fig. 8 corresponds to the list of the reference signs. Hence, reference to this description is made for here unspecified features of these processes.

[0130] The process of Fig. 4 already comprises a scrubbing stage 11 of ultra-fine particles. From a comparison between Fig. 4 and Fig. 5 it can be noted that in Fig. 5 a mixing environment 55 is provided, wherein a paraformaldehyde- and / or methylol urea-containing stream 17 and a stream of hot process condensate 18 are mixed to obtain the FhCO-containing solution wherein formaldehyde is in free, unconjugated form.

[0131] If the process of Fig. 4 does not already comprise a recycle line (see dotted arrow of Fig. 4), the method of revamping comprises installing a line for recycling at least part of said cyanuric acid to an existing scrubbing step.

[0132] In Fig. 5 a first line 56 is installed for feeding the FhCO-containing solution to the ammonia scrubbing stage 9 and / or to the ultra-fine particle scrubbing stage 11 .

[0133] The process of Fig. 6 differs from the embodiment of Fig. 4 in that it does not comprise a scrubbing stage of ultra-fine particles. Consequently, the method of revamping schematized in Fig. 7 and Fig. 8 comprises installation of such scrubbing stage 11 of ultra-fine particles.

[0134] The embodiment of Fig. 7 shows the scrubbing stage 11 of ultra-fine particles as a separate stage (e.g. apparatus) with respect to the ammonia scrubbing stage.

[0135] The embodiment of Fig. 8 differs from the embodiment of Fig. 7 in that the scrubbing stage 11 of ultra-fine particles is installed within an existing ammonia scrubbing stage. This is possible if the existing ammonia scrubbing stage so allows. For example, if the structure and the volume of the existing stage permit(s) to operate scrubbing of ultra-fine particles together - substantially at the same time, and eventually in the same compartment - with scrubbing of ammonia.

[0136] The invention will now be elucidated based on the following non-limiting example.

[0137] EXAMPLE: in reaction

[0138] For recovering 68 ppm of ammonia reaction (I) requires 180 ppm of formaldehyde. A standard free ammonia content of a urea solution 96-98% is +500 ppm which would in theory request:

[0139] 180 ppm x 500 ppm 168 ppm = 1323 ppm of formaldehyde to recover all the ammonia, considering the availability of free formaldehyde.

[0140] The more free formaldehyde is available, the more efficient the interaction between formaldehyde and ammonia will be. The amount of free formaldehyde required to recover the ammonia that is present in the gas flow is significantly lower than the standard amount of formaldehyde required for urea granulation (3.000 to 5.000 ppm).

[0141] However, as already mentioned before in relation to WO 2013 / 167245 A1 , injecting excess formaldehyde at the end of the scrubbing section is not desirable because of the risks of formaldehyde entrainment to the stack.

[0142] An additional amount of formaldehyde (i.e., additional with respect to the injected amount) - approx, from 1 .500 ppm to 3.500 ppm by weight - is to be injected in the urea melt feed to the granulator in the standard way to serve as granulation process.

[0143] Hence, formaldehyde injection is a relevant issue in the process of the present invention.

[0144] Even if not specified above, a person skilled in the art may envisage, using the expertise typical of this technological field, varying or replacing some of the above features with other technically equivalent elements.

[0145] These variations or replacements also fall within the scope defined by the following claims. Furthermore, each alternative illustrated in connection with a particular embodiment can be realised independently of the other embodiments here described.

[0146] LIST OF THE REFERENCE SIGNS urea melt or a concentrated urea solution seeds fluid bed granulator fluidization air solid urea granules cooling section polluted air stream urea dust scrubbing step or stage ammonia scrubbing step or stage wet granulator scrubber ultra-fine particle scrubbing step or stage extra ammonia abatement step or stage aqueous scrubbing solution, preferably concentrated aqueous urea solution urea dust-enriched solution de-dusted air stream formaldehyde-containing solution paraformaldehyde- and / or methylol urea-containing stream

[0147] (hot) process condensate

[0148] (dedicated) evaporation section evaporated solution ammonia-depleted air stream first washing solution, preferably first diluted washing solution mixing vessel stream of granulation aid scrubber solution tank

[0149] (cold) scrubbing solution ammonia- and ammonium cyanate-depleted air stream second washing solution, preferably second diluted washing solution spent scrubber solution spent stream cooler scrubbing step or stage indirect heat exchanger liquid acid stream first purified air stream acid-depleted solution acid source acid abatement loop stream of cooling air second polluted air stream cooler scrubber ammonia-enriched solution pollutants-depleted air stream dry room part of the urea dust-enriched solution second purified air stream demister scrubbing liquid cooler solution tank stream of makeup water thermally treated solid granules first portion of the scrubbing liquid second portion of the scrubbing liquid stack stack mixing environment first line export scrubber solution

Claims

CLAIMS1 . A urea granulation process comprising: scrubbing a polluted air stream (7) - produced by said granulation process and containing ammonia (NH3) and ammonium cyanate (NH4OCN) - with a formaldehyde (H2CO)-containing solution (16) to obtain an ammonia- and ammonium cyanate-depleted air stream (27) and a spent scrubber solution (29); reacting said formaldehyde with ammonium cyanate to obtain cyanuric acid (HOCN); wherein formaldehyde contained in said hhCO-containing solution (16) is over-stoichiometric with respect to the ammonia contained in said polluted air stream (7); recycling at least part of said cyanuric acid to said scrubbing of the polluted air stream (7).

2. The urea granulation process according to claim 1 , wherein said process comprises: reacting said formaldehyde with ammonia to obtain hexamethylene tetramine ((CH2)eN4), and reacting said over-stoichiometric formaldehyde with said ammonium cyanate to obtain said cyanuric acid.

3. The urea granulation process according to any of the previous claims, wherein said cyanuric acid is obtained in situ in the spent scrubber solution (29), at least part of said spent scrubber solution (29) with a lowered pH value being at least partially recycled to said scrubbing for an acid removal of ammonia contained in the polluted air stream (7) with formation of additional ammonium cyanate.

4. The urea granulation process according to any of the previous claims, wherein said scrubbing comprises an ammonia scrubbing step (9) and a scrubbing step (11 ) of ultra-fine particles comprising ammonium cyanateand optionally urea; said ultra-fine particles having an average particle size comprised from 100 nm to 1 pm determined by laser diffraction.

5. The urea granulation process according to the previous claim, wherein - during the scrubbing step (11 ) - said ultra-fine particles of urea are transferred into the spent scrubber solution (29) where the urea ultra-fine particles react with any residual formaldehyde to obtain methylol urea.

6. The urea granulation process according any of claims 4-5, wherein said ammonia scrubbing step (9) and said ultra-fine particle scrubbing step (11 ) are performed in sequence or concurrently.

7. The urea granulation process according to any of the previous claims, wherein said scrubbing of the polluted air stream (7) comprises a urea dust scrubbing step (8) to obtain a de-dusted air stream (15), said dedusted air stream (15) being contacted with said solution (16) containing formaldehyde.

8. The urea granulation process according to claims 4 and 7, wherein said urea dust scrubbing step (8) has a given operating temperature, and wherein an operating temperature of said ultra-fine particle scrubbing step (11 ) is at least 5 °C lower, e.g., from 8 °C and 10 °C lower, than said operating temperature of the urea dust scrubbing (8).

9. The urea granulation process according to the previous claim, wherein said spent scrubber solution (29) is cooled by heat exchange before being at least partially recycled as scrubbing solution (26).

10. The urea granulation process according to any of the previous claims, comprising steps of: providing a paraformaldehyde- and / or methylol urea-containing stream (17) wherein formaldehyde is in conjugated form, and mixing said paraformaldehyde- and / or methylol urea-containing stream (17) and a stream of hot process condensate (18) having a temperature> 90 °C to obtain said formaldehyde-containing solution (16) wherein formaldehyde is in free, unconjugated form.

11. The urea granulation process according to the previous claim, further comprising a step of adjusting a flow rate of said paraformaldehyde- and / or methylol urea-containing stream (17) and / or a flow rate of said stream of process condensate (18), so as to obtain a desired pH value in the spent scrubber solution (29).

12. The urea granulation process according to claim 10 or 11 , wherein said paraformaldehyde- and / or methylol urea-containing stream (17) is a split portion of a stream of granulation aid (24) added to a urea melt or concentrated urea solution (1 ) upstream of said granulation process.

13. The urea granulation process according to any of the previous claims, wherein said urea granulation process is performed in a fluid bed granulator (3) fed with fluidization air (4) and producing said polluted air stream (7).

14. The urea granulation process according to the previous claim, wherein said fluid bed granulator (3) produces solid urea granules (5) subjected to a cooling step (6) with a stream of cooling air (38) to obtain thermally treated solid granules (50) and a second polluted air stream (39); said second polluted air stream (39) being sent to a cooler scrubber (40) to obtain a purified air stream (45).

15. The urea granulation process according to the previous claim, wherein part of a scrubbing liquid (47) used in said cooler scrubber (40) is made by said spent scrubber solution (29), and wherein a portion (51 ) of said scrubbing liquid (47) is used for scrubbing of said polluted air stream (7).

16. A method of revamping a urea granulation process; wherein said urea granulation process to be revamped:- is performed in a fluid bed granulator fed with fluidization air and producing a polluted air stream containing ammonia and ammonium cyanate;- comprises a scrubbing step of said polluted air stream with a formaldehyde (H2CO)-containing solution; wherein the method of revamping comprises:(I) adding over-stoichiometric formaldehyde to the H2CO-containing solution with respect to the ammonia contained in said polluted air stream;(II) providing a reaction environment and reacting said over- stoichiometric formaldehyde with said ammonium cyanate to obtain cyanuric acid; and(III) if not already available, installing a line for recycling at least part of said cyanuric acid to said scrubbing step.

17. The method of revamping according to claim 16, wherein said reaction environment is a spent scrubber solution obtained in said scrubbing step.

18. The method of revamping according to any of claims 16-17, comprising:(IV) installing a scrubbing stage (11 ) of ultra-fine particles comprising ammonium cyanate and optionally urea; said ultra-fine particles having an average particle size comprised from 100 nm to 1 pm determined by laser diffraction.

19. The method of revamping according to claim 18, wherein said scrubbing stage (11 ) is installed downstream an existing ammonia scrubbing stage.

20. The method of revamping according to claim 18, wherein said scrubbing stage (11 ) is installed within an existing ammonia scrubbing stage.

21. The method of revamping according to any of claims 16-20, comprising:(V) providing a mixing environment between a paraformaldehyde- and / or methylol urea-containing stream (17) and a stream of hotprocess condensate (18) having a temperature > 90 °C to obtain said H2CO-containing solution wherein formaldehyde is in free, unconjugated form.

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