PROCEDURE FOR PURIFICATION OF EXHAUST AIR FROM A GRANULATION PLANT FOR THE PRODUCTION OF A GRANULE CONTAINING UREA.

Description

[0001] The present invention relates to a method for cleaning the exhaust air of a granulation plant for the production of urea-containing granules in which a gas stream containing urea-containing dust and ammonia is brought into contact with a sulfuric acid solution or nitric acid solution in a washing process.

[0002] The exhaust air from a granulation plant producing urea-containing granules contains urea dust and ammonia, necessitating purification of the exhaust air before its release into the environment. Known methods for scrubbing exhaust air in ammonia-containing gases produce an ammonium sulfate solution with a low pH (approximately pH 2-4), which cannot be directly processed in this acidic form. Therefore, increasing the pH is required to prevent corrosion damage in downstream process stages.

[0003] From EP 2 477 961 B1, a process for recovering urea dust and ammonia from a gas stream is known, in which the gas stream is contacted with a sulfuric acid solution to form an acidic solution of ammonium sulfate and urea, to which ammonia is added. In this known process, a gas stream containing air, urea, and ammonia is brought into contact with an aqueous sulfuric acid solution in a scrubbing device, thereby forming an acidic solution of urea and ammonium sulfate. This acidic solution is subsequently neutralized by adding ammonia in a neutralizing device and then concentrated in a further device.In a mixing device, the ratio of ammonium sulfate to urea can then be increased to a desired value by adding ammonium sulfate, and finally the concentrated melt is processed into a solid by granulation or spray crystallization, so that a solid mixture of urea and ammonium sulfate is obtained as the final product.

[0004] EP 2 192 099 A1 discloses a urea granulation process which includes acidic exhaust air treatment. The ammonium sulfate produced in the acidic exhaust air treatment can be recycled back into the granulation process.

[0005] WO 2016 / 074813 A1 discloses a urea granulation process comprising an exhaust air treatment with a dust scrub and an acidic exhaust air scrub.

[0006] From JP-A-2000-1466 a process for the recovery and utilization of urea dust and ammonia in exhaust gases of a urea granulation plant is known, in which the exhaust gases generated in a urea granulation unit are first fed to a dust scrubbing tower in which an aqueous urea solution circulates, and subsequently fed to an acidic exhaust air scrubber in which water containing acid circulates to recover ammonia, wherein the scrubbing solutions obtained in this process are fed to the feedstock for the urea granulation unit.

[0007] The object of the present invention is to provide a method for cleaning the exhaust air of a granulation plant for the production of a urea-containing granulate with the features of the aforementioned type, which does not require an external post-neutralization as a separate process step.

[0008] The solution to the aforementioned problem provides a method for cleaning the exhaust air of a granulation plant for the production of a urea-containing granulate of the type mentioned at the outset, with the features of claim 1.

[0009] According to the invention, the washing process comprises at least three separate, successive washing stages. In a first washing stage, the gas stream is washed with a first weakly acidic washing solution. In a second washing stage, the gas stream exiting the first washing stage is washed with a second washing solution that has a lower pH than the first weakly acidic washing solution. The first weakly acidic washing solution is a solution containing urea and ammonium sulfate or ammonium nitrate with a pH in the range of approximately 3.5 to approximately 6.1. The invention provides that the first washing stage is preceded by a pre-wash of the gas stream in a pre-wash stage, the purpose of which is to significantly reduce the amount of dust contained in the exhaust air stream. For this purpose, the gas stream can preferably be washed with a dilute urea-containing solution in the pre-wash stage.This process yields a solution with an increased urea content, which can then be concentrated by evaporation. The recovered urea can be reused in the granulation process to produce urea-containing granules. This pre-wash has the advantage that the dust content is already significantly reduced in the subsequent two acidic washing stages. Separating the dust washing (pre-wash stage) from the actual acid washing (first and second washing stages) allows the urea contained in the gas stream to be recycled back into the granulator and the ammonium sulfate produced in the acid washing (first and second washing stages) to be separated. The ammonium sulfate can preferably be integrated into the granulation process as described in WO2012 / 034650 A1, for example in

[0018] -

[0022] and

[0036] -

[0040] .

[0010] Furthermore, urea releases isocyanates and ammonia (NH₃) upon contact with acidic washing solutions. In addition to their known toxic properties, isocyanates exhibit a tendency to form aerosols, which are difficult and costly to wash out and are easily recognizable in exhaust air due to their brownish color. Examples of aerosol separation can be found in WO2014 / 094987 A1. Another side reaction involves the conversion of urea to mono- or diurea sulfate with sulfuric acid, which cannot be readily recycled.

[0011] A preferred embodiment of the inventive solution provides that an acidic washing solution produced in the second washing stage is recycled to the first washing stage and used there as the first weakly acidic washing solution. In particular, the first washing solution can be fed in crossflow to the gas stream to be washed in the first washing stage. Thus, in this preferred embodiment of the inventive method, an important step in achieving separate pH correction of the acidic washing solution within the multi-stage system for washing the exhaust air is the countercurrent flow of the acidic washing solution from a second washing device to a first washing device.

[0012] The first washing solution used in the first washing stage is only weakly acidic and therefore has a higher pH value than the more acidic second washing solution used in the second washing stage. For example, the first weakly acidic washing solution is a urea- and ammonium sulfate- or ammonium nitrate-containing solution with a pH value in the range of approximately 4.3 to approximately 5.3.

[0013] According to the inventive process, the neutralized ammonium sulfate solution or ammonium nitrate solution from the first washing stage is fed into the third washing stage (pre-wash stage), thus enabling further use of the generated ammonium sulfate solution.

[0014] Preferably, the second washing solution has a pH value of less than about 2. For example, concentrated sulfuric acid or concentrated nitric acid can be added on the one hand, and condensate from the concentration device for the resulting urea-containing and ammonium sulfate-containing solutions can be added to produce the second washing solution with a pH value of preferably less than 2.0, with which the gas stream leaving the first washing stage is washed again in the second washing stage.

[0015] Preferably, the first washing solution in the first washing stage is fed in a crossflow to the gas stream to be washed.

[0016] The solution obtained from the washing process in the first washing stage typically contains a certain amount of ammonium sulfate or ammonium nitrate, as well as traces of urea, and is preferably discharged from the first washing stage as a solution with an approximately neutral or slightly alkaline pH. Thus, the solution discharged from the first washing stage after the washing process is preferably an ammonium sulfate- or ammonium nitrate-containing solution with a pH greater than 7. This neutral or nearly neutral pH of the ammonium sulfate or ammonium nitrate solution has the advantage that the solution is no longer corrosive and can be processed further immediately, for example, concentrated by evaporating the water content. Post-neutralization outside the washing system is no longer necessary.

[0017] Preferably, in the pre-wash stage, the gas stream is washed with a diluted urea solution. This solution can preferably be recirculated in the pre-wash stage and subsequently evaporated and returned to the granulator.

[0018] In the inventive method, the above-mentioned pH value correction is thus achieved within the washing system through suitable process control, and an external post-neutralization as a separate process step can therefore be dispensed with.

[0019] According to a preferred embodiment of the invention, the more acidic washing solution used in the second washing stage can be produced by supplying the second washing device with concentrated sulfuric acid or concentrated nitric acid on the one hand and a condensate from the concentration device for the resulting urea-containing and ammonium sulfate-containing or ammonium nitrate-containing solutions on the other.

[0020] In a further preferred embodiment of the process according to the invention, the neutralized ammonium sulfate solution or ammonium nitrate solution is removed from the first washing stage and, for example, sent for separate use or disposal.

[0021] Preferably, condensate from a concentration device for the resulting urea-containing and ammonium sulfate- or ammonium nitrate-containing solutions is fed into the second washing stage. Dilution and adjustment of the solution is possible via the condensate feed.

[0022] In a preferred embodiment of the process according to the invention, condensate from the concentration device for the resulting urea-containing and ammonium sulfate-containing or ammonium nitrate-containing solutions is also supplied to the pre-wash stage, wherein the urea-containing dust is preferably washed out with a urea-containing solution in this pre-wash stage. The solution can be diluted via the condensate supply. During this pre-wash, the urea content of the solution increases because the washing solution absorbs urea-containing dust from the exhaust air stream to be washed.

[0023] The present invention further relates to a device for cleaning the exhaust air of a granulation plant for the production of urea-containing granules for use in a process of the type described above, which comprises at least a first washing device in which a gas stream containing urea-containing dust and ammonia from the granulation plant for the production of urea-containing granules is washed by means of an acidic washing solution, and which further comprises at least a second washing device connected to the first washing device in which a gas stream leaving the first washing device is washed with a further acidic washing solution which has a lower pH value than the first acidic washing solution.The concept according to the invention thus provides for at least two successive washing processes, each with acidic washing solutions in separate washing devices, wherein washing is first carried out with a weakly acidic first washing solution and subsequently with a more strongly acidic second washing solution. The invention provides that the device includes at least a third washing device, which is positioned upstream of the first washing device in the flow path and in which a pre-wash takes place to remove dust from the gas stream. This significantly reduces the amount of dust to be collected in the two subsequent acidic washing stages.

[0024] The device according to the invention comprises at least one return line by means of which a weakly acidic solution produced in the second washing device can be returned to the first washing device for use as the first acidic washing solution. In this embodiment, it is therefore particularly advantageous that the weakly acidic washing solution used in the first washing device is generated by the washing process in the second washing device and can then be returned from the second washing device to the first washing device.

[0025] The device according to the invention for cleaning the exhaust air of a granulation plant for the production of urea-containing granules provides that it comprises an exhaust air line leading from a granulation plant for the production of urea-containing granules, which leads to the third washing device, wherein a gas line is further provided on the outlet side of the third washing device for exhaust air cleaned in the third washing device, which leads from the third washing device to the first washing device, and wherein a gas line is further provided on the outlet side of the first washing device for exhaust air cleaned in the first washing device, which leads from the first washing device to the second washing device.

[0026] The device for cleaning the exhaust air of a granulation plant is designed to guide neutralized ammonium sulfate solution or ammonium nitrate solution from the first washing device to the third washing device.

[0027] The present invention further relates to a device comprising the previously described device for cleaning the exhaust air of a granulation plant for the production of urea-containing granules, the granulation plant and a concentration device, in which at least one return line for a urea-containing solution is provided from the third washing device, which leads to the concentration device in which this urea-containing solution is concentrated, wherein means are further provided to return the urea obtained in this process to the granulation plant for the production of urea-containing granules.

[0028] The present invention will now be explained in more detail using an exemplary embodiment with reference to the accompanying drawing. The drawing shows: Figure 1 A schematically simplified representation of an exemplary device according to the invention for cleaning the exhaust air of a granulation plant for the production of urea-containing granules.

[0029] The following refers to Figure 1An embodiment of the present invention is explained in more detail below. The illustration shows a simplified process diagram, depicting only the plant components relevant to the present invention. The exhaust air to be cleaned from a granulation plant for the production of urea-containing granules enters a washing device 11 via an inlet line 10. This washing device serves as a pre-wash to remove the majority of the urea-containing dust from the exhaust air stream. This pre-wash is also referred to in the present application as the third washing device 11. Condensate from the concentration device for the resulting urea-containing and ammonium sulfate solutions can be supplied to the washing device 11 via a line 12. A urea-containing solution, supplied to the washing device via the inlet line 13, is used as the washing medium in this washing device 11.The washing process produces a urea-enriched solution, which may contain, for example, urea at a concentration of 45%. This solution is discharged from the washing device 11 via the outlet line 14 and fed to a concentration device in which most of the water contained in the solution is evaporated, so that the highly concentrated urea-containing solution can then be returned to the granulation plant for the production of urea-containing granules.

[0030] After the washing process in the pre-wash 11, the gas stream, largely free of dust, leaves the washing device 11 via the gas line 15 and is then fed to a washing device 16, in which the gas stream is washed with a weakly acidic solution, this washing device 16 being referred to in the present application as the first washing device. The weakly acidic washing solution used in this first washing device, which, for example, has a pH of 4.8 and is essentially a weakly acidic aqueous solution of urea and ammonium sulfate or ammonium nitrate, is fed to the washing device 16 via the inlet line 17. In the washing device 16, the weakly acidic washing solution is fed in cross-flow to the gas stream introduced via line 15. This inlet line 17 for feeding the washing solution into the first washing device 16 is, as can be seen, Figure 1An output line can be taken from a second washing device 19, the significance of which is explained below.

[0031] The gas stream, which has been scrubbed in the first scrubbing unit 16, leaves this unit via the gas line 18 and is then introduced into the second scrubbing unit 19, where it is scrubbed again with an acidic scrubbing solution that is more acidic than the weakly acidic scrubbing solution used in the first scrubbing unit 16. For this scrubbing process in the second scrubbing unit 19, concentrated sulfuric acid or nitric acid is introduced into the second scrubbing unit 19 via an inlet line 20. Additionally, condensate from the process can be fed into the second scrubbing unit 19 via the inlet line 21. In the second scrubbing unit 19, the acidic scrubbing process is carried out, for example, with a scrubbing solution that has a pH value in the range of approximately 1.5.After this second washing process in the second washing device 19, the cleaned exhaust air stream is discharged via the outlet line 22 and can in principle be released into the environment, as it is almost dust-free and contains almost no ammonia.

[0032] The washing process of the gas stream in the second washing device 19 produces ammonium sulfate or ammonium nitrate through reaction with the sulfuric or nitric acid introduced via line 20, since the gas stream introduced into this second washing device still contains ammonia. Because partial neutralization with the ammonia from the gas stream takes place in the second washing device, the washing process generates a solution containing ammonium sulfate or ammonium nitrate and residual urea, exhibiting a weakly acidic pH value of, for example, 4.8. This weakly acidic urea / ammonium sulfate or urea / ammonium nitrate solution is then returned to the first washing device 16 via the outlet line 17 of the second washing device 19 and used there as a washing solution. This solution is fed in a crossflow to the gas stream introduced via line 15 and serves for gas scrubbing in the first washing device 16.Line 17 thus simultaneously serves as the inlet line for the washing solution of the first washing device 16, which is the first part of the gas stream to flow through. A first gas scrubbing operation therefore takes place in the first washing device 16 with a weakly acidic solution, followed by a second gas scrubbing operation in the second washing device 19 with a more acidic washing solution.

[0033] From the first washing device 16, since a further neutralization process takes place there by reaction of the weakly acidic washing solution with ammonia contained in the gas stream, an essentially neutralized aqueous solution can be discharged via the outlet line 23, which contains a proportion of dissolved ammonium sulfate or ammonium nitrate, contains traces of urea and has a pH value greater than 7.

[0034] According to the invention, the neutralized ammonium sulfate solution or ammonium nitrate solution is also fed via line 24 from the first washing stage 16 into a third washing stage (pre-wash stage 11).

[0035] The inventive design thus comprises three washing stages for cleaning the exhaust air released from the granulator of dust (pre-wash stage) and two subsequent stages for removing ammonia and simultaneously raising the pH value to a less corrosive range for the pipes in the first and second washing stages. Reference symbol list

[0036] 10 Inlet line 11 Pre-wash (third washing device) 12 Condensate line 13 Inlet line for urea-containing washing solution 14 Outlet line for urea-containing solution 15 Gas guide 16 First washing device 17 Inlet line for washing solution 18 Gas guide 19 Second washing device 20 Inlet line for sulfuric acid or nitric acid 21 Condensate line 22 Outlet line for purified exhaust air stream 23 Outlet line for neutralized ammonium sulfate solution or neutralized ammonium nitrate solution 24 Outlet line for neutralized ammonium sulfate solution or neutralized ammonium nitrate solution

Claims

1. A process for purifying the exhaust air from a granulation plant for producing a urea-containing granulate, in which in a scrubbing process, a gas stream containing a urea-containing dust and ammonia is brought into contact with a sulfuric acid solution or nitric acid solution, wherein in a first scrubbing stage (16), the gas stream is scrubbed with a first weakly acidic scrubbing solution, and in a pre-scrubbing stage (11), a pre-scrubbing of the gas stream is arranged upstream of the first scrubbing stage (16) and in the pre-scrubbing stage (11), dust is scrubbed out of the gas stream, characterized in that the scrubbing process comprises at least two separate successive acidic scrubbing stages (16, 19) and in a second scrubbing stage (19), the gas stream leaving the first scrubbing stage (16) is scrubbed with a second scrubbing solution, which has a lower pH than the first weakly acidic scrubbing solution, wherein the first weakly acidic scrubbing solution is a urea and ammonium sulfate containing solution (17) or ammonium nitrate containing solution (17) having a pH in the range from about 3.5 to about 6.1, and wherein the acidic scrubbing solution arising in the second scrubbing stage (19) is recycled into the first scrubbing stage (16) and used there as the first weakly acidic scrubbing solution, wherein the neutralized ammonium sulfate solution or ammonium nitrate solution (24) from the first scrubbing stage (16) is passed into the third scrubbing stage, i.e. pre-scrubbing stage 11.

2. The process for purifying the exhaust air from a granulation plant for producing a urea-containing granulate as claimed in claim 1, characterized in that the first weakly acidic scrubbing solution is a urea-containing and ammonium sulfate-containing solution (17) or ammonium nitrate-containing solution (17) having a pH in the range from about 4.3 to about 5.3.

3. The process for purifying the exhaust air from a granulation plant for producing a urea-containing granulate as claimed in one of claims 1 or 2, characterized in that the scrubbing solution for the second scrubbing stage (19) has a pH of less than about 2.

4. The process for purifying the exhaust air from a granulation plant for producing a urea-containing granulate as claimed in one of claims 1 to 3, characterized in that the first scrubbing solution in the first scrubbing stage (16) is run in crosscurrent with the gas stream to be scrubbed.

5. The process for purifying the exhaust air from a granulation plant for producing a urea-containing granulate as claimed in one of claims 1 to 4, characterized in that an ammonium sulfate-containing solution or an ammonium nitrate-containing solution having a pH in the range of more than 7 is discharged from the first scrubbing stage (16).

6. The process for purifying the exhaust air from a granulation plant for producing a urea-containing granulate as claimed in one of claims 1 to 5, characterized in that in the pre-scrubbing stage (11), the gas stream is scrubbed with a dilute urea solution.

7. The process for purifying the exhaust air from a granulation plant for producing a urea-containing granulate as claimed in one of claims 1 to 6, characterized in that the neutralized ammonium sulfate solution or ammonium nitrate solution (23) from the first scrubbing stage (16) is discharged.

8. The process for purifying the exhaust air from a granulation plant for producing a urea-containing granulate as claimed in one of claims 1 to 6 characterized in that at least the second scrubbing stage (19) is supplied with a condensate from a concentration apparatus for the arising urea-containing and ammonium sulfate-containing or ammonium nitrate-containing solutions.

9. The process for purifying the exhaust air from a granulation plant for producing a urea-containing granulate as claimed in one of claims 1 to 7, characterized in that the pre-scrubbing stage (11) is supplied with a condensate from a concentration apparatus for the arising urea-containing and ammonium sulfate-containing solutions.

10. An installation for purifying the exhaust air from a granulation plant for producing a urea-containing granulate for use in a process as claimed in one of claims 1 to 9, comprising an exhaust air conduit (10) for a gas stream containing a urea-containing dust and ammonia from the granulation plant for producing a urea-containing granulate, at least one third scrubbing apparatus (11), an outlet-side gas conduit (15), and at least one first scrubbing apparatus, wherein - the exhaust air conduit (10) leads into the at least one third scrubbing apparatus (11), - the at least one third scrubbing apparatus (11) is arranged upstream of the at least one first scrubbing apparatus (16) in the flow path and in the at least one third scrubbing apparatus (16), a pre-scrubbing for scrubbing out dust from the gas stream takes place, - the outlet-side gas conduit (15) is provided on the outlet side of the at least one third scrubbing apparatus (11) for exhaust air purified in the at least one third scrubbing apparatus, which leads from the at least one third scrubbing apparatus (11) to the at least one first scrubbing apparatus (16), and - in the at least one first scrubbing apparatus (16), the gas stream is scrubbed by means of an acidic scrubbing solution, and characterized in that the installation comprises at least one second scrubbing apparatus (19) in operative connection with the at least one first scrubbing apparatus (16), in which a gas stream leaving the at least one first scrubbing apparatus (16) is scrubbed with a further acidic scrubbing solution, which has a lower pH than the first acidic scrubbing solution, furthermore, an outlet-side gas conduit (18) on the outlet side of the at least one first scrubbing apparatus (16) is provided for exhaust air purified in the at least one first scrubbing apparatus (16), which leads from the at least one first scrubbing apparatus (16) to the second scrubbing apparatus (19), and the installation comprises at least one recycling conduit (17), which is configured to recycle a weakly acidic solution arising in the second scrubbing apparatus (19) into the at least one first scrubbing apparatus (16) for use as first acidic scrubbing solution, wherein it is configured to pass neutralized ammonium sulfate solution or ammonium nitrate solution from the first scrubbing apparatus (16) into the third scrubbing apparatus (11).

11. An installation with the installation for purifying the exhaust air from a granulation plant for producing a urea-containing granulate as claimed in claim 10, the granulation plant and a concentration apparatus, characterized in that at least one recycling conduit (14) proceeding from the at least one third scrubbing apparatus (11) is provided for a urea-containing solution, which leads to the concentration apparatus, in which this urea-containing solution is concentrated, wherein means are furthermore provided in order to recycle the concentrated urea-containing solution thereby obtained into the granulation plant for producing a urea-containing granulate.