Method for removing ammonia and dust from a waste gas that results during the production of urea

Translation of description of equivalent WO 2005032696 A1

"A process for removing ammonia and dust from an exhaust gas obtained during the production of fertilizers"

The invention relates to a process for removing ammonia and dust from an exhaust gas in the production of fertilizers, preferably urea, is obtained, in which the exhaust gas are introduced into a first washer and a cooling gas in a second scrubber and into a scrubber makeup water and in the other, an aqueous scrubber solution is introduced, both the exhaust gas and the cooling gas prior to exit from the respective scrubber passes through at least a droplet.

In the production of ammonium-containing fertilizers and in fertilizers that can split off ammonia, containing eg urea fertilizer, falling in various process stages ammonia and dust-laden exhaust air streams, which have to be cleaned before discharge into the environment or the recycling to the process. Such exhaust gases arise in particular in the granulation and cooling.

To remove dust from escaping from the granulation and exhaust gas from the granulate cooling gas a generic process the applicant is known. To carry out this method, two washers are provided, each in the upper region with at least one equipped eliminators (demisters). Here in the first scrubber originating from the granulation exhaust gas is introduced, while in the second scrubber the cooling gas is introduced. For cleaning up water, preferably purified or unpurified process water is introduced into the second washer below the mist in countercurrent to the cooling gas. The air leaving the second scrubber aqueous solution is then also introduced into the first scrubber in counterflow to the waste gas to be cleaned.

In practice it has been found that this known method is still in need of improvement. Since emerging from the first scrubber aqueous solution must be further processed or used further, there is the desire, in the aqueous solution to adjust the urea concentration as high as possible to keep the energy required for evaporation of the exiting aqueous solution as low as possible. In the previous procedure, however, this maximum concentration is limited. The previous maximum value of Harnstoffkonzen- concentration in the aqueous solution in the first washer is about 30 to 45%, higher concentrations are not possible, because in spite of the drop is not allowed completely avoid that accordingly charged with urea drops remain in the off-gas and in cause this a correspondingly high urea concentration.

The object of the invention is, therefore, a generic process to further develop that the exhaust emissions can be reduced significantly.

This object is achieved in a method of the type described in the present invention that the addition of water initially introduced into a top side limited by the drop separator and bottom sides by a liquid-separating base fine-washing area of ​​the first washer and the at least a droplet is sprayed, and the resulting in fine-washing area aqueous solution is then passed into the second washer.

Unlike the known methods thus the additional water is first fully introduced into the measures provided for in the first scrubber additional fine-washing area, in which enters the drip-laden exhaust gas prior to passage through the mist collector. In fine-washing area this is done by the addition of water, a strong dilution, so that the urea concentration of the droplets may be significantly reduced. Simultaneously, an additional cleaning of the drop follows. Due to the strong dilution of the droplets, it is possible that fkonzentration the aqueous solution in the actual main washing area of ​​the washer to increase Harnsto significantly so that the energy required for the subsequent evaporation of the aqueous solution can be greatly reduced. In addition, can be achieved by means of this procedure, that the dust in the exhaust gas values ​​previously unattainable from about 50 mg / m³ to 20 mg / m³ can be reduced.

The emerging from the second scrubber aqueous solution is passed in a known manner into the first washer, and although naturally enters the provided below the separating tray main washing area of ​​the first washer, in the exhaust gas also.

For separating the fine-washing region and the main washing region of the first washer, a bubble cap tray is preferably used. In principle, other separation trays are used, which are liquid impermeable but gas permeable.

In order to reduce the ammonia concentration in the exhaust gas, is provided in a further advantageous embodiment that the fine-washing area of ​​the first washer, an acid is introduced. For example, sulfuric or nitric peter acid are used. Such an acid treatment is fundamentally known, for example from EP 0440932 Bl.

In order to optimize the processing of the emerging from the first washer aqueous solution energetically, it is preferably provided that in the main washing region of the first washer, a 40 - 60% strength, preferably 55% urea concentration is set. The energy required for evaporation can be significantly reduced without this very high urea concentration results in the aqueous solution to problems in the purification of exhaust gas, because, as mentioned above, in the fine-washing area occurs a strong dilution of the entering into this area drops.

The invention is explained below by way of example with reference to the drawing. This shows in

Fig. 1 is a schematic diagram for carrying out the method and in

Fig. 2 shows a detail of Fig. 1 in a special embodiment.

A plant for carrying out the method has to- next a first scrubber 1 and a second washer 2. The first scrubber 1 is preceded by a pre-purification stage. 3 In the upper area of ​​the first washer 1, a mist eliminator 4 (demister) is arranged, as in the second scrubber 2 a mist 5. The first scrubber 1 is divided into two washing areas, where 14 is a liquid-impermeable, below the mist 4 to form a fine-washing area, but gas-permeable partition bottom 12 (eg bell bottom) and a drain 11 are arranged. Below the separating tray 12 is the main washing area 21 of the first washer. 1

The above-described system components are preferably part of a plant for the production of fertilizers, preferably urea, and communicate with a granulator is not shown and a cooler connected. laden with ammonia and dust exhaust gas is supplied from the granulator is not shown, first in the pre-purification stage 3, which is indicated by an arrow. 6 The exhaust gas passes through the pre-cleaning stage 3 therethrough and is introduced into the main washing region of the first washer 21 first Also loaded cooling gas is fed directly to the second washer 2, this is indicated by an arrow. 7 Additional water, preferably purified or non-purified process water is answerable directly to the fine-washing area 14 of the first washer 1 is supplied with the water supply is indicated by arrows 8,. 9 The water supply line opens inside the scrubber 1, below the mist 4 in upward spray heads 10, such that the additional water is sprayed against the mist collector 4 and this cleans characterized. The additional water is mixed with the air passing through the dividing plate 12 drops and leads to a strong urea concentration dilution of the droplets, so that the drops have, for example, only a urea concentration of 1 to 4%, even when the urea concentration is in the main wash zone 21 at 55 to 60% , The additional water accumulates thereby and exits as an aqueous solution through the outlet 11, to which a line 13 is connected, which opens into the second washer 2, whereby the aqueous solution is passed into the second washer. 2

The exhaust gas to be cleaned thus occurs after passage through the first Vorreinigunsstufe 3 in the main washing region 21 of the first washer 1 in which sieve trays 22 or the like. are arranged, and then passes through the dividing plate 12 and into the fine-washing area 14, where by mixing with the addition of water, a strong ke dilution and reduction of adhering to the exhaust drop takes place. Subsequently the exhaust gas passes through the mist collector 4 through and then enters at the top of the first washer 1 purified from (arrow 15).

To be purified coolant gas into the second washer 2 enters in the lower region (arrow 7), in which trays are also 23 are arranged to exit through the introduced aqueous solution and then purified by the droplet separator 5 at the top of the second scrubber in countercurrent (arrow 16).

The respective bottoms in the two washers 1 and 2 is circulated in the usual way, which is indicated by respective circuits 17 and 18 respectively. The aqueous solution is from the circuit 18 and diverted through a line 19 of the pre-purification stage 3_, supplied. From the pre-purification stage 3 thus occurs aqueous solution and exhaust gas into the main washing area 21 of the first washer. 1

Our substantial dilution or cleaning effect in the fine-washing area 14, it is possible to adjust the main washing area 21 of the first washer 1, a urea concentration in the aqueous solution of about 60%, ie exiting the scrubber 1 aqueous solution (Line 24) then has a urea concentration of 60%, so that this aqueous solution can be evaporated with respect to the prior art considerably less expenditure of energy in order to further use. Despite this high concentration of urea in the main washing area 21, it is possible due to the process control with the introduction of additional water into the fine-washing area 14, to achieve self-urea concentrations in the order of 1 to 4% of the fine-washing area 14th The urea concentration in the second scrubber 2 is about 10%.

As shown in FIG. 2, is preferably additionally provided that, in the fine-washing region 14 for reducing the ammonia load of the exhaust gas, an acid is initiated, which is indicated by an arrow 20. For this purpose a part of the exiting from the outlet 11 of the first washer 1 aqueous solution from line 13 in the circuit via a pump 25 for introducing the acid is recycled into the fine-washing area 14th As the acid, for example sulfuric or nitric acid can be used. Such acid treatment is fundamentally known for example from EP 0440932 Bl. The addition of acid (stream 20) is preferably carried out in a corrosion-resistant, self-priming nozzle after the pump (for example, jet nozzle) whose flow is controlled. Here, the pressure line of Pump are completely or partially recognized as driving jet stream.

The process is basically alternatively for a scrubber, in which a plurality of droplet separators are arranged vertically. The additional water is then first passed in a corresponding manner in a fine-washing area of ​​the washer for the coming of the granulation exhaust.