OA17151A - Urea granulation process with scrubbing system - Google Patents

Abstract

Urea granulation process with scrubbing system including at least one gaseous waste stream for removal of dust and ammonia whereby this waste stream is processed through a combination of the following process steps comprising: (a) washing the dust and ammonia laden stream (4) with water and/or an aqueous urea solution whereby a dust-laden liquid stream (26) and a dust-reduced stream (5) is generated, and (b) reacting the dust-reduced stream (5) with formaldeyhde (7) to form a stream (8) comprising hexamethylenetetramine and urea-formaldehyde and clean off-gas (6) wherein the gas stream is directed first through process step (a) and then through process step (b).

Description

Urea granulation process with scrubblng System

The Invention relates to a urea granulation process and to the apparatus suitable for operating such a process. The Invention intégrâtes a method for reducing ammonia émissions from a urea granulation plant which is currently emitted by a conventional urea production process by scrubblng the off-gas. The scrubbing system bears the advantage that the amount of ammonia in off-gas can be reduced and in addition the génération of ammonium salts can be reduced.

A common process for producing granules from a liquid composition is described ln US 5,779,945. The focus of patent US 5,779,945 is the treatment and sorting of generated granules with different sizes. Herein a gas/solids separating apparatus such as a cyclone or a scrubber is used to separate solid material from the off-gas stream of the apparatus. Advanced treatment of the off-gas stream is not taken into further account.

In US 4,370,198 the off-gas of the granulation unit is sent to a dust séparation cyclone followed by a continuous wet scrubber which both contributes to the scrubbing off said off-gas stream. The scrubbing liquid used is part of the solution or suspension to be proceeded and the scrubbing liquid leaving the wet scrubber is fed back directly into the granulation unit. Exemplarily, the described process can be achieved for the production of sodium chloride, urea, saccharose or ferrie oxide, respectively. Hereby the scrubbing liquor is part of the solution or suspension to be processed and is send directly back Into the granulation unit. This process can be only achieved for dust scrubbing but is not suitable for ammonia scrubbing.

A further example for an apparatus and a method for wet type simultaneous cleaning and dust-removing gas treatment in a horizontal cross-flow scrubber are disclosed in EP 0853971 A1. This invention performs the removal of pollutants and dust ln a packed tower.

ln a urea plant used air exiting a urea granulator that is equipped with a fluidized bed contains in addition to urea dust also ammonia. This ammonia contamination needs to be removed before the off-gas stream can be vented into the atmosphère.

Removing ammonia from an off-gas stream Is a well-known technology. Usually the off-gas stream Is treated with an acidic scrubbing solution. This scrubblng solution can be easily manufactured by adding an acid such as nitric acid or sulphuric acid to water. The ammonia Is removed from the gas stream by chemical absorption and converted to the corresponding ammonium sait. The use of nitric acid produces ammonium nitrate (AN), and the use of sulphuric acid produces ammonium sulphate (AS) respectively. These ammonium salt-containing solutions can be used for the production of ammonium sulphate fertilizer or NPK fertilizer, the technology for this is state of the art.

In a urea plant, ammonium salts do not occur In the process and cannot easily be processed at existing urea facilities. A conventional urea production facility therefore has only the following options to reduce gaseous ammonia émissions from the granulation plant: to discharge the diluted ammonium sait solution to a waste water stream, to concentrate the diluted ammonium sait solution up to a concentration which can be utilized by other plants, e.g. NPK, to produce UAS (urea / ammonium sulphate) fertilizer with a high sulphur content, to produce UAN (urea / ammonium nitrate) solution.

Ail of these alternatives require significant investments and changes to operating conditions or entai! changes of the product composition and characteristics. Ail above options resuit In new products that require additional facilities for transport and handling as well as energy utilities In expensive quantities. As a conséquence, nowadays, urea facilities are run without efficient ammonia removal causing severe environmental problems. Therefore, ammonia removal from a urea facility is a challenglng task that needs to be solved.

An alternative solution is described In WO 03/099721. This invention relates to a process for removing ammonia from an ammonia-containing gas stream by converting the ammonia in the ammonia-containing gas stream with an organic acid into an ammonium sait, whereas the obtained ammonium sait Is contacted, at elevated température, with peroxide. The ammonium sait is hereby converted into a NHj, CO₂and H₂O containing mixture in a décomposer and can readily be reprocessed In a urea synthesls unit. The peroxide Is supplementary to the common process and may relate to other négative accompanlments. Also, for the conversion of the ammonium sait into NH₃, CO₂ and H₂O a separate décomposer In addition to the normal plant layout is required. This emerging gas stream can not be reprocessed In a granulation unit but needs to be recycled in a urea synthesis unit.

Réductions of ammonia émissions are also described in M Potthoff, Nitrogen + Syngas, [online], July.August 2008, pages 39-41. In Fig. 1 a combined dust and acidic scrubber system is shown. The ammonia is absorbed in the acidic scrubbing section and converted into ammonium sulphate. The ammonium sulphate solution is added to the recycle flow going back to the évaporation section, ln this unit it is mixed with urea melt from the urea synthesis unit. The concentrated liquor stream from the évaporation Is conveyed into the urea granulator. The condensate coming out of the évaporation unit is utilised as makeup for the combined dust/ammonia scrubbing system. With this so called Ammonia ConvertTechnology ammonia in off-gas can be reduced to 30 mg/Nm³. The technology without acidic scrubbing as shown ln Brochure Urea, [online], 12-2007, pages 1-24 reduces ammonia in off-gas only to values of around 160 mg/m³.

The ammonia convert technology described in M Potthoff, Nitrogen + Syngas, [online], July. August 2008, pages 39-41 Implîcates stiil several disadvantages. First of ail, the water balance in this system Is a critical parameter. If disturbed, urea synthesis wili be contaminated with ammonium sulphate or altematively large amounts of waste water need to be treated. In addition, mixing of acidic solution with concentrated urea melt in the évaporation unit has adverse effects on granulation. Moreover, this technology implîcates the génération of large amounts of condensate contaminated with ammonium sulphate that needs to be distributed to various scrubbers, including dust and acidic scrubbing technology. Also the remaining ammonia concentration in the off-gas achieved with this technology is still not sufficient or satisfactory for modem urea granulation plants.

ln WO 2010/060535 A1 the ammonia convert technology described ln M Potthoff, Nitrogen + Syngas, [online], July. August 2008, pages 39-41 is improved in order to achieve ammonia concentrations in off-gas of 10 mg/Nm³. WO 2010/060535 A1 teaches that a scrubber dust stage, that is connected to process coolers, Is operated through an ammonium sait solution stream generated In a scrubber acid stage, which is connected to the urea granulator. Therefore the scrubbing system presented in WO

2010/060535 A1 represents an in itself complote closed system as described In the characteristic part of claim 1 of this invention. This technology avoids contamination of the urea melt generated in the urea synthesis unit by building such a complote closed scrubblng system. The disadvantage of this system is that It Is very complex in its performance.

In US 5686647 a process for preparing urea is described wherein an amount of formaldéhyde is added to an off-gas stream containing gaseous ammonia to form hexamethylenetetramine, which Is retumed into the process before the granulation step. This formaldéhyde addition can be performed before or during a washing step with liquid urea solution whereby this washlng step serves as dust scrubbing device. The disadvantage of this technology is the relatively high amount of ammonia In the off-gas of circa 90 mg/Nm³ in comparison to the technology presented in WO 2010/060535 A1.

The object of the invention Is to provide a process which intégrâtes and optimizes existing scrubbing technology of off-gas generated by the urea granulation process. The process should prevent problems related to conventional technologies as described above and should be easliy Intégrable in existing scrubbing Systems state of the art. It is aiso the object of the invention to provide the apparatus suitable to operate such a process.

This is achieved by a urea granulation process with scrubbing system Including at least one gaseous waste stream for removal of dust and ammonia whereby this waste stream Is processed through a combination of the following process steps comprising washing the dust and ammonia laden stream 4 with water and/or an aqueous urea solution whereby a dust-laden liquid stream 26 and a dust-reduced stream 5 is generated, and reacting the dust-reduced stream 5 with formaldéhyde 7 to form a stream comprising hexamethylenetetramine and ureaformaldehyde 8 and clean off-gas 6 wherein the gas stream is directed first through process step (a) and then through process step (b).

Surprisingly the sequence of process steps ln claim 1 allows to reduce further ammonia émissions form granulation plants in comparison to the technology described ln US 5686647 in which the order of process steps are vice versa. If process step (b) ls done before the dust scrubbing ln process step (a) the réaction ammonla-formaldehyde suffers from compétition with the standard urea-formaldehyde réaction which would prevaii ln the diiute urea solution obtained ln the scrubber. Therefore efficiency ln this process step ls lost and ammonia-reduction is limited.

Hereby the urea concentration of the dust-laden liquid stream 26 ls kept ln a range from 35 to 60 % wt, and preferably is kept in a range from 45 to 55 % wt and that dust laden liquid stream 26 is retumed Into the process before the granulation step.

Furthermore 70 to 90 wt% of ammonia ln relation to the total ammonia content of the dust-reduced stream 5 is reacted to hexamethylenetetramlne in the formaldéhyde stage 2.

Optionally, the stream comprising hexamethylenetetramine and ureaformaldehyde 8 is retumed Into the process before the granulation step. The hexamethylenetetramine comprises urea-formaldehyde solution and therefore replaces at least part of the urea/formaldehyde solution normally used as granulation additive.

In a further embodiment of the current process the dust-laden liquid stream 26 is mixed with the stream comprising hexamethylenetetramine and ureaformaldehyde 8 before retuming this mixture into the process before the granulation step.

ln a further embodiment of the invention an additional process step for removing ammonia ls implemented downstream of process step (b) wherein an ammonia-laden stream ls brought into contact with an acid 9 in liquid phase and thereby ammonia is scrubbed from that stream by the génération of an ammonium sait stream 10 in a scrubber acid stage 3.

The combination of these three process steps bears the advantage that the amount of ammonium sait generated ln the scrubber acid stage 3 is greatly reduced so that these salts do not disturb the granulation System or the urea synthesis System if recycled back in one of these Systems. Also the amount of ammonia reduced by this System can be Improved.

Hereby 94 to 99,9 % of ammonia in relation to the total ammonia content of the dust- and ammonia-laden stream 4 is eliminated through the combination of process steps (a) and (b) with a further acidic treatment.

In an embodiment of the invention the acid is selected from the group consisting of sulphuric acid, nitric acid, phosphoric add, citric acid, lactic acid and oxalic acid. Other acids can be used If they are non-volatüe. Preferably, sulphuric acid is used, as it is readily available and in addition, it supplies sulphur which is considered to be a highly demanded nutrient, Furthermore the ammonia sait concentration of the ammonium sait stream generated in the scrubber acid stage is kept < 40 % wt, and preferably ls kept In a range from 35 - 40 % wt.

The pH of the ammonia sait stream generated in the granulator scrubber acid stage is kept In a range from 2-6, and preferably is kept in a range from 3.5 - 5.0, and most preferably is kept in a range from 4.0 - 4.5.

In an optional embodiment a second gaseous dust- and ammonia-laden stream 14 drawn off from product coolers 13 is generated, which stream is send through a further scrubber dust stage 15 In which the ammonium sait stream 10 ofthefurtheracid treatment is usedto removethe ammonia from this second gaseous dust-and ammonia-ladden stream 14.

In a further optional embodiment of the current invention the scrubbing System being passed is in itself a complété closed System, whereby the ammonium sait stream 10 from the scrubber acid stage 3 is fed into said further scrubber dust stage 15, and the released solution 17 from said further scrubber dust stage 15 is send to a évaporation unit 16, the vapour stream 18 from the évaporation unit 16, which contains ammonia is given into a condenser unit 19, which releases a liquid process condensate 20. and said liquid process condensate 20 ls given Into the scrubber acid stage 3. and the concentrated liquor stream 21 generated In the évaporation unit 16, containing urea and ammonium sait, and a urea meit 22 is conveyed Into the urea granulator 1.

Hereby the scrubbing System In itself ls a complété closed system, and ls therefore totaily decoupied from urea synthesis. Thereby contaminations of the urea melt are totaily avoided.

With advantage the concentration of the urea melt 22 and concentrated liquor stream 21, containing urea and ammonium sait, for the urea granulator being kept in a range from 95 to 99.8 % wt, and being preferably kept In a range from 96 to 97.5 % wt.

Optionally a portion of urea melt 22 is fed into the évaporation unit 16.

Furthermore the clean off-gas 6 is released into the atmosphère and exhibits a concentration of NH₃ in the range of 5 - 30 mg/Nm³, and preferably exhibits a concentration of NH3 being < 10 mg/Nm³.

The current invention also comprises an apparatus with scrubbing system System Including at least one gaseous waste stream for the removal of dust and ammonia comprising a scrubber dust stage 11, in which dust ls washed off from a dust- and ammonia-ladden stream, and a formaldéhyde stage 2, In which part of the ammonia of the ammonialadden air 4 is reacted with formaldéhyde 7 to form hexamethylenetetramine, whereby the scrubber dust stage 11 is arranged upstream of the formaldéhyde stage 2.

Furthermore an additional scrubber acid stage 3 ls integrated into the scrubbing system downstream of the formaldéhyde stage 2.

Optionally the urea granulation apparatus with scrubbing system comprises also product coolers 13, in which a second gaseous ammonia-laden stream 14 is generated, and which product cooiers are connected with a further scrubber dust stage 15 which is connected with means for conveying the ammonium sait solution stream 10 from the scrubber acid stage 3 to said further scrubber dust stage 15.

ln a further embodiment of the urea granulation apparatus the apparatuses of the scrubblng system being connected in such a way that a complété ciosed system of waste streams Is buiit, comprising means for conveying the ammonium sait stream 10 from the scrubber acid stage 3 to the further scrubber dust stage 10, and means for conveying the solution 17 from said further scrubber dust stage 15 to an évaporation unit 16, means for conveying the steam vapour 18 of the évaporation unit 16 to a condenser unit 19, means for conveying the process condensate 20 from the condenser unit 19 to the granulator scrubber acid stage 3, and means for conveying urea melt 22 and a means for conveying a concentrated liquor stream 21, containing urea and ammonium sait into the urea granulator 1.

Furthermore the apparatus comprises means for conveying a portion of urea melt to the évaporation unit 16.

With advantage scrubbers used in the current technoiogy are horizontal scrubbers.

In the following, the Invention is described in more detail by way of example.

Fig. 1: Shown is a block diagram of the Inventive ammonia formaldéhyde convert process steps.

Fig. 2; Shown Is a block diagram of the inventive process steps including ammonia formaldéhyde convert process steps including a scrubber acid stage.

Fio. 3: Shown is a block diagram of the inventive process including an in itself closed scrubbing system.

Fig. 1 shows an urea granulator 1, which is supplied with urea melt or an aqueous urea solution 22. ln the urea granulator 1 urea granules are formed ln a fluidized bed, which ls fluidized by an air stream 27. A dust- and ammonia-laden stream 4 ls drawn off. It is first scrubbed ln the scrubber dust stage 11, where urea dust ls removed. A stream of process water or diluted urea solution 12 ls added to the scrubber dust stage 11 and the dust-laden stream 26 ls drawn-off from the scrubber dust stage 11. The dust-reduced stream 5 is then sent to the formaldéhyde stage 2. According to the Invention formaldéhyde 7 ls introduced In the formaldéhyde stage 2.

A hexamethylenetetramine and formaldéhyde containing stream 8 is drawnoff from the formaldéhyde stage 2. This hexamethylenetetramine can be retumed Into the granulation process before the granulation step. The clean off-gas 6 ls send Into the atmosphère.

Fig. 2 Includes In comparison to Fig. 1 an additional scrubber add stage 3 downstream of the formaldéhyde stage 2. The ammonia reduced stream 29 from the formaldéhyde stage 2 ls send Into the scrubber acid stage 3 where the rest of ammonia is removed, and the clean off-gas stream 6 can be drawn off. The scrubbing solution for the scrubber add stage 3 consists of process water and the acid 9 ln liquid phase. In the granulator scrubber add stage 3 the add solution reacts with ammonia producing an ammonium sait stream 10. This ammonium sait stream 10 can be further processed as shown ln Fig. 3 or can be drawn-off from the urea granulation system.

This Inventive process allows the réduction of ammonium salts generated in the scrubber add stage 3 but ls very effective ln redudng ammonia émissions from urea granulation plants. Ammonium salts are much undesired because they cause severe environmental problems and cause problems In urea granule quality If added to high concentrations to the granulation process.

Fig. 3 Includes in comparison to Fig. 2 a in itself closed scrubbing system Inciuding the Inventive process steps. ln addition to Fig. 2 product coolers 13 are shown, in which the hot granules 25 produced are conveyed. Air 28 cools the final product 25. The dust-laden air stream 14 is conveyed to a further scrubber dust stage 15, where the urea dust ls washed out while the air ls cooled down by évaporation of water ln the scrubber. The clean offgas 23 leaving the scrubber dust stage 15 is to the atmosphère.

The resulting solution from the scrubber dust stage 15, is combined with the dust-laden stream 26 from the granulator scrubber dust stage 11 and the resulting mixture is conveyed to the évaporation unit 16, where it is concentrated. The concentrated liquor stream 21 from the évaporation unit 16 Is fed to the urea granulator 1 to Integrate the generated ammonium sait into the granulation process. A portion of the urea melt 22 can be added to the évaporation unit 16 (not shown), in order to keep the urea concentration and the ammonium sulphate concentration of the concentrated liquor stream 21 in the right ratio. The steam vapour 18 drawn off from the évaporation unit 16 is conveyed to a condenser unit 19, where it is cooled by extemal cooling water. The liquid process condensate 20 generated during the condensation Is send into the scrubber acid stage 3. To close the scrubbing cycle the ammonium sait stream 10 drawn-off from the scrubber acid stage 3 is send to the scrubber dust stage 15.

Therefore a closed circle of waste streams is formed and ail waste streams are recycled. In addition the generated ammonium salts are integrated in the urea granulation process. Also extemal process water consumption is reduced to a minimum. Altogether, this combination is characterized by its environmental compatibility. Aiso the content of ammonium sait In the generated urea granules is reduced, which gets problematic If sulphuric acid Is used as acid 9 and the sulphur content of the granules Increase.

Example 1:

In example 1 a table is shown giving some typical figures conceming ammonïa In the urea granulation processes state of the art as described in Brochure Uhde, Urea, [online] 2011 compared with a formaldéhyde treatment as described in US5686647 implemented before or combined with a scrubber dust stage and the inventive technology:

In a urea granulation process with formaldéhyde scrubbing a formaldehydecontaining solution Is added to the ammonia-laden air or the formaldéhyde stage.

The formaldehyde-contalning solution used for scrubbing Is charged with hexamethytenetetramine and is partially relntroduced into the above described urea process. Basicaliy this mixture after being brought to the right pressure and température may be recycled in every phase of the process.

The amount of ammonia of 500 to 600 ppm by weight in the feed to the granulation unit is more or less unavoidable as it is the resuit of the equilibrium formed in an upstream to the granulation unit arranged évaporation unit, if the concentrated liquor stream generated in this évaporation unit shall be introduced into the granulator. About 90 ppm ammonia is added through biuret formation in the urea solution, which Is fed Into the granulator, so that in total about 590 to 690 ppm enters the granulation unit.

About 50 ppm of this ammonia is Included in the final product, whereby the rest leaves the granulation plant with the air flow from the granulation unit via stacks. This results ln a final concentration of approximately 130 to 160 mg/Nm³ for the technology state of the art as presented in Brochure Urea, [online], 2011. In the technology described by US 5686647 a final concentration of circa 86 mg/Nm³ can be reached. If formaldéhyde stage is put into practice after a dust scrubber where the urea is almost removed, as the current invention shown in Fig. 1 suggests, a final concentration of approximately 30 mg/Nm³ ammonia is found in a combined stack, The inventive technology in combination with a following acid scrubblng stage as shown in Fig. 2 can lead to ammonia concentrations of 10 mg/Nm³ with a minor amount of acid to be used. Therefore a drasticaliy Improvement can be achieved using this technology.

table 1 : technology state of the art ln comparison with current invention

	technolo gy state of the art (Brochur e Urea, 2011)	formaldeh yde treatment as described ln US568664 7 before a scrubber dust stage	formaldeh yde treatment ofthe current invention as shown in Fig. 1	Formaldeh yde treatment of the current invention as shown in Fig. 2
Free ammonia from évaporation unit	~ 500 to 600 ppm wt.
Ammonia

from biuret formation	« 90 ppm wt.
Total free ammonia at granulator Inlet	= 590 to 690 ppm wt.
Free ammonia ln final product	= 50 ppm wt.
Free ammonia released (based on urea solution)	« 540 to 640 ppm wt.
Ammonia formaldhyde stage	none	yes	yes	yes
Dose Formaldéhyde as UFC via 7 kg/ton		4	4	4
Typlcal ammonia concentration in comblned stack		= 0.40 kg/tonpnxjuct	= 0.14 kg/tonprodu cl	= 0.05 kg/tonpnxjud

Formaldéhyde efficiency		45,00%	75,00%	75,00%
Ammonium sulphate produced				= 0.35 kg/tonprodixt

The efficiency of formaldéhyde to abate ammonia ls strongly reduced to only 45% if the process is done before the dust scrubblng. If It ls done in combination with the dust scrubblng the formaldéhyde efficiency is 75%. The reaction ammonlaformaidehyde suffers from compétition with the standard urea-formaldehyde reaction which would prevail in the dilute urea solution obtained In the scrubber. Therefore the change in the sequence of process steps of the current invention in relation to the teaching of US5686647 has an enormous positive effect in respect to the ammonia content In off-gas. The combination shown in Fig. 2 of a scrubber acid stage downstream of the formaldéhyde stage has the advantage that the ammonium sait stream generated has a very low ammonium sait concentration if compared with the technology state of the art of W02010060535A1 (table 2) in which a formaldéhyde stage ls missing. Therefore this ammonium sait stream can be exported from the granulation system or can be further processed as shown in Fig. 3.

Example 2:

In example 2 a table ls shown glving some typical figures concemlng ammonia in the urea granulation processes state of the art as described in W02010060535A1, In which the ammonium sait stream generated is reintroduced into the granulation process, whereby a ln Itself complété closed system of scrubbing streams ls built, compared with the Inventive closed scrubber technology as shown ln figure 3:

In a urea granulation process with a scrubber system according to figure 3 a formaidehyde-containlng solution ls added via 7 to the formaldéhyde stage 2.

The formaldehyde-containing solution used for scrubbing in formaldéhyde stage 2 ls charged with hexamethylenetetramine and is partially relntroduced Into the standard urea process. Basically this mixture after being brought to the right pressure and température may be recycled in every phase of the process.

table 2: technology state of the art in comparison with current Invention as shown in Fig. 3:

	technology state of the art (Brochure Urea, 2011)	Ammonia convert technology W02010060 535A1	Inventive technology (Fig- 3)
Free ammonia from évaporation section	* 500 to 600 ppm wt.
Ammonia from biuret formation	« 90 ppm wt.
Total free ammonia at granulator Inlet	~ 590 to 690 ppm wt.
Free ammonia ln final product	« 50 ppm wt.
Free ammonia released (based on urea solution)	œ 540 to 640 ppm wt.
Ammonia formaldhyde stage	none	none	yes
Dose Formaldéhyde as UFC via 7 kg/ton			4
Formaldéhyde efficiency			75,00%
Acid scrubber stage	none	yes	yes
Typical ammonia concentration In combined stack

		= 0.05 kg/tonproduct	= 0.05 kg/tonproguct
	= 0.6 to 0.7 kg/tononxjuct
Sulphuric acid consumption		= 2.0 kg/ton^^	= 0.27 kg/tononxfuct
Ammonium sulphate produced		= 2.3 kg/ton^uct	= 0.35 kg/ton^uct

Thus, a solution is produced which shows ammonla concentrations In offgas that are comparable to those reached with the technology described in W02010060535A1. But in addition a very low ammonium sait concentration, which is approx. 8 times less then the technology described in W02010060535A1 is produced. Also the sulphuric acid consumption ls 8 times lower which ls a significant cost réduction. There is no significant change to the product spécification and quality by the addition of these small amounts of ammonium salts. The N content of the urea product stays above 46 % N, so that the product ls still a typical urea fertilizer.

The advantages of the proposed process are: Significant low ammonia émissions to the environment. Urea granule with very low ammonium sait concentration Cost benefits are achleved by reducing the ammonia and acid consumption A simple way is used to process ammonia-laden gas streams in existîng urea granulation plants.

A proven and low-cost technical process is used to remove ammonia from the offgas streams from the urea granulation plant with fluidized bed granulation.

As the recovered ammonia is Included In the product the urea production is increased, leading to a significant économie benefit.

A typical urea fertilizer grade product ls produced.

Key to referenced Items urea granulator formaldéhyde stage scrubber acid stage dust- and ammonia-laden stream dust-reduced stream clean off gas formaldéhyde stream comprising hexamethylenetetramine and urea-formaldehyde acid ammonium sait stream scrubber dust stage process water/urea solution product cooler gaseous ammonia-ladden stream further scrubber dust stage évaporation unit solution steam vapour condensor unit liquid process condensate concentrated liquor stream urea melt/ urea solution clean off-gas final product hot granules dust-ladden stream air ammonia-reduced stream

Claims (19)

1. Claims

   1. Urea granulation process with scrubblng System including at least one gaseous waste stream for removal of dust and ammonla whereby this waste stream Is processed through a combination of the following process steps comprising (a) washlng the dust and ammonla laden stream 4 with water and/or an aqueous urea solution whereby a dust-laden liquid stream 26 and a dust-reduced stream

   5 is generated, and (b) reacting the dust-reduced stream 5 with formaldeyhde 7 to form a stream 8 comprising hexamethylenetetramine and urea-formaldehyde and clean off-gas wherein the gas stream is directed first through process step (a) and then through process step (b).
2. 2. Process according to ciaim 1, characterized in that the urea concentration of the dust-laden liquid stream 26 is kept in a range from 35 to 60 % wt, and preferably Is kept in a range from 45 to 55 % wt and which dust laden liquid stream 26 is retumed into the process before the granulation step.
3. 3. Process according to claim 1 or 2, characterized ln that 70 to 90 wt% of ammonia in relation to the total ammonia content of the dust-reduced stream 5 is reacted to hexamethylenetetramine in the formaldéhyde stage 2.
4. 4. Process according to any preceding claim, characterized ln that the stream comprising hexamethylenetetramine and urea-formaldehyde 8 is retumed into the process before the granulation step.
5. 5. Process according to any preceding claim, characterized ln that the dust-laden liquid stream 26 is mixed with the stream comprising hexamethylenetetramine and urea-formaldehyde 8 before retuming this mixture into the process before the granulation step.
6. 6. Process according to any preceding claim, characterized ln that an additional process step for removing ammonia is Implemented downstream of process step (b) wherein an ammonla-laden stream is brought into contact with an acid 9 in liquid phase and thereby ammonia is scrubbed from that stream by the génération of an ammonium sait stream 10 in a scrubber acide stage 3.

   ιβ
7. 7. Process according to claim 6, characterized in that 94 to 99,9 % of ammonia in relation to the total ammonia content of the dust- and ammonia-laden stream 4 is elimînated through the combination of process steps (a) and (b) with a further acidic treatment.
8. 8. Process according to claim 6 or 7, characterized in that the acid is selected from the group consisting of sulphuric acid, nitric acid, phosphoric acid, citric acid, lactic acid and oxalic acid.
9. 9. Process according to claims 6 to 8, characterized in that the ammonia sait concentration of the ammonium sait stream 10 generated in the further acid treatment is kept < 40 % wt, and preferably is kept in a range from 35 - 40 % wt.
10. 10. Process according to claims 6 to 9, characterized in that the pH of the ammonia sait stream 10 generated in the further acid treatment is kept in a range from 2-6, and preferably ls kept in a range from 3.5 - 5.0, and most preferably is kept in a range from 4.0-4.5.
11. 11. Process according to claims 6 to 10, characterized in that a second gaseous dustand ammonia-laden stream 14 drawn off from product coolers 13 is generated, which stream is send through a further scrubber dust stage 15 in which the ammonium sait stream 10 of the further acid treatment ls used to remove the ammonia from this second gaseous dust-and ammonia-ladden stream 14.
12. 12. Process according to daim 6 to 11, characterized in that the scrubbing system being passed is in itself a complété closed system, whereby • the ammonium sait stream 10 from the scrubber acid stage 3 is fed Into said further scrubber dust stage 15, and • the released solution 17 from said further scrubber dust stage 15 is send to a évaporation unit 16, • the vapour stream 18 from the évaporation unit 16, which contains ammonia is given into a condenser unit 19, which releases a liquid process condensate 20, and said liquid process condensate 20 is given into the scrubber add stage 3, and • the concentrated liquor stream 21 generated in the évaporation unit 16, containing urea and ammonium sait, and a urea melt 22 Is conveyed Into the urea granulator 1.
13. 13. Process according to claim12, characterized in that the concentration of the urea melt 22 and concentrated liquor stream 21, containing urea and ammonium sait, for the urea granulator being kept ln a range from 95 to 99.8 % wt, and being preferably kept in a range from 96 to 97.5 % wt.
14. 14. Process according to claims 12 to 13, characterized in that a portion of urea melt 22 Is fed into the évaporation unit 16.
15. 15. Process according to any preceding claim, characterized in that the clean off-gas 6 is released Into the atmosphère and exhibits a concentration of NH₃ ln the range of 5 - 30 mg/Nm³, and preferably exhibits a concentration of NH₃ being <10 mg/Nm³.
16. 16. Urea granulation apparatus with scrubbing system including at least one gaseous waste stream for the removal of dust and ammonia comprising • a scrubber dust stage 11, in which dust is washed off from a dust- and ammonia-ladden stream, and • a formaldéhyde stage 2, ln which part of the ammonia of the ammonla-laden air 4 is reacted with formaldéhyde 7 to form hexamethylenetetramine, whereby the scrubber dust stage 111s arranged upstream of the formaldéhyde stage 2.
17. 17. Urea granulation apparatus with scrubbing system according to claim 16, characterized in that an additional scrubber acid stage 3 Is integrated Into the scrubbing system downstream of the formaldéhyde stage 2.
18. 18. Urea granulation apparatus with scrubbing system according to claims 16 or 17 comprising product coolers 13, in which a second gaseous ammonia-laden stream 14 is generated, and which product coolers 13 are connected with a further scrubber dust stage 15 which is connected with means for conveying .the ammonium sait solution stream 10 from the scrubber acid stage 3 to said further scrubber dust stage 15.
19. 19. Apparatus according to any of claims 16 to 18 comprising means for conveying a portion of urea melt 22 to a évaporation unit.