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

p0001The invention relates to a process for the removal of ammonia and dust from an exhaust gas which is produced in a production process in the production of urea, in which the exhaust gas is introduced into a first scrubber and a cooling gas originating from the production process is introduced into a second scrubber A scrubber purified or unpurified process water, and an aqueous solution is introduced into the other scrubber, wherein both the flue gas and the cooling gas pass through at least one droplet separator before exiting the respective scrubber.

p0002In the production of ammonium-containing fertilizers or in fertilizers which can split off ammonia, for example urea-containing fertilizers, ammonia- and dust-laden exhaust air streams are produced in various stages of the process, which must be cleaned before being released into the environment or recycled into the process. Such exhaust gases are generated, in particular, during granulation and cooling.

p0003A method of the generic type is known for the removal of dust from exhaust gas emerging from the granulation and from the granulate cooling gas (<nplcit id="ncit0001" npl-type="b"><text>International Fertilizer Industry Association, IFA Technical Conference, Marrakech Morocco, 28 September to 1 October 1998: "Saskferco urea plant revamping" by Pan Orphanides and Peter Orphanides</text></nplcit>). For carrying out this method, two scrubbers are provided, each equipped with at least one droplet separator (demister) in the upper region. The exhaust gas from the granulation is introduced into the first scrubber while the cooling gas is introduced into the second scrubber. For cleaning, additional water, preferably purified or unpurified process water, is introduced into the second scrubber below the droplet separator in counterflow to the cooling gas. The aqueous solution emerging from the second scrubber is then also introduced into the first scrubber in counterflow to the flue gas to be purified.

p0004In practice it has been found that this known process is still to be improved. Since the aqueous solution emerging from the first scrubber has to be further processed or further used, there is an endeavor to adjust the urea concentration in the aqueous solution as high as possible in order to keep the energy expenditure for the evaporation of the emerging aqueous solution as low as possible. In the previous method, however, these maximum concentration limits are set. The previous maximum value of the urea concentration in the aqueous solution in the first scrubber is about 30 to 45%; higher concentrations are not possible since, despite the droplet separator, it can not be completely avoided that urea-laden drops remain in the exiting exhaust gas, A correspondingly high urea concentration.

p0005Out <patcit id="pcit0001" dnum="DE3306664A1"><text>DE 33 06 664 A1</text></patcit> A two-stage embodiment of a single exhaust gas scrubber with a main wash zone and a fine wash zone is generally known.

p0006The object of the invention is therefore to further develop a generic method so that the exhaust gas load can be significantly reduced.

p0007According to the invention, this object is achieved by the fact that the additional water is first introduced into a fine wash zone of the first washer, which is delimited on the upper side by the drop separator and on the underside by a liquid-impermeable separating pan, and is sprayed onto the at least one droplet separator, The aqueous solution emerging from the second scrubber being introduced into the main scrubbing zone of the first scrubber, which is provided below the separating floor and into which the exhaust gas also enters.

p0008In contrast to the known method, the additional water is first of all introduced completely into the additional fine wash zone provided in the first scrubber, in which the drop-laden exhaust gas enters before passing through the droplet separator. In the fine-washing area, the dilution water is a strong dilution so that the urea concentration of the drops is significantly reduced. Simultaneously, a cleaning of the droplet separator follows.

p0009Due to the strong dilution of the droplets, it is possible to significantly increase the urea concentration of the aqueous solution in the actual main washing range of the scrubber so that the energy expenditure for the subsequent evaporation of the aqueous solution can be greatly reduced. In addition, it can be achieved by means of this process control that the dust load in the exhaust gas reaches from previously achievable values ​​of approximately 50 mg / m³ To 20 mg / m³ Can be reduced.

p0010The aqueous solution emerging from the second scrubber is passed into the first scrubber, and, of course, into the main washing area of ​​the first scrubber, which is provided below the separating floor and into which the exhaust gas also enters.

p0011A bell bottom is preferably used for separating the fine washing area and the main washing area of ​​the first washer. In principle, other separating bottoms which are liquid-impermeable but gas-permeable can also be used.

p0012In order to reduce the ammonia concentration in the exhaust gas, a further advantageous embodiment provides for an acid to be introduced into the fine washing area of ​​the first scrubber. For example, sulfuric or nitric acid can be used. Such an acid treatment is known in principle, for example from<patcit id="pcit0002" dnum="EP0440932B1"><text>EP 0 440 932 B1</text></patcit>.

p0013In order to optimally optimize the further processing of the aqueous solution emerging from the first scrubber, it is preferably provided that a 40-60%, preferably 55%, urea concentration is set in the main washing area of ​​the first scrubber. The energy expenditure for the evaporation can thereby be significantly reduced without this very high urea concentration in the aqueous solution leading to problems in the purification of the exhaust gas since, as mentioned above, a strong dilution of the droplets entering this region takes place in the fine washing region.

p0014The invention is explained in more detail below by way of example with reference to the drawing. This shows in<dl id="dl0001"><dt>FIG</dt><dd>A principle scheme for carrying out the method and in</dd><dt>FIG</dt><dd>A detail of the <figref idrefs="f0001">FIG</figref> In a special design.</dd></dl>

p0015An installation for carrying out the method first comprises a first scrubber 1 and a second scrubber 2. The first scrubber 1 is preceded by a preliminary cleaning stage 3. A droplet separator 4 (demister) is arranged in the upper region of the first washer 1, just as in the second scrubber 2 a droplet separator 5. The first scrubber 1 is divided into two washing regions, whereby a liquid-impermeable, Gas-permeable separating bottom 12 (eg bell bottom) and an outlet 11 are arranged. Below the separating floor 12 is the main washing area 21 of the first washing machine 1.

p0016The above-described plant components are preferably part of a plant for the production of fertilizers, preferably urea, and are connected to a granulator (not shown) and a cooler. Exhaust gas charged with ammonia and dust is fed from the granulator (not shown), namely first into the preliminary cleaning stage 3, which is indicated by an arrow 6. The exhaust gas passes through the preliminary cleaning stage 3 and is introduced into the main washing area 21 of the first scrubber 1. Charged cooling gas is likewise fed directly to the second scrubber 2, which is indicated by an arrow 7.

p0017Additional water, preferably purified or unpurified process water, is fed directly to the fine washing section 14 of the first washer 1, the water supply line being indicated by arrows 8, The water supply line opens into the upwardly directed spray heads 10 within the scrubber 1 below the droplet separator 4 in such a way that the additional water is sprayed against the droplet separator 4 and thereby cleans the latter. The additional water mixes with the droplets passing through the separating bottom 12 and leads to a strong urea concentration dilution of the droplets, so that the droplets have, for example, only a urea concentration of 1 to 4% even when the urea concentration in the main wash region 21 is 55 to 60% . The additional water accumulates thereby and emerges as an aqueous solution through the outlet 11, to which is connected a line 13, which opens into the second scrubber 2, whereby the aqueous solution is passed into the second scrubber 2.

p0018The exhaust gas to be purified thus enters into the main washing area 21 of the first washer 1, in which sieve plates 22 or the like are arranged, and then passes through the separating plate 12 into the fine washing area 14, The mixing with the additional water results in a strong dilution and reduction of the droplets adhering to the exhaust gas. Subsequently, the exhaust gas passes through the droplet separator 4 and then emerges cleaned (arrow 15) at the head of the first washer 1.

p0019The cooling gas to be purified enters the second scrubber 2 in the lower region (arrow 7), in which screen trays 23 are also arranged in order to emerge countercurrently through the introduced aqueous solution and subsequently through the mist eliminators 5 at the head of the second scrubber (arrow 16).

p0020The respective bottom product in the two scrubbers 1 and 2 is recirculated in a conventional manner, which is indicated by corresponding circuits 17 and 18, respectively. The aqueous solution is branched off from the circuit 18 and fed to the pre-purification stage 3 via a line 19. Aqueous solution and exhaust gas enter the main wash area 21 of the first washer 1 from the pre-cleaning stage 3.

p0021As a result of the considerable dilution or cleaning effect in the fine washing region 14, it is possible to set a urea concentration in the aqueous solution of about 60% in the main washing region 21 of the first scrubber 1, ie the aqueous solution (line 24) emerging from the scrubber 1 then has one Urea concentration of 60% so that this aqueous solution can be evaporated with a considerably lower energy expenditure compared to the prior art for further use. Despite this high urea concentration in the main washing area 21, it is possible to achieve urea concentrations in the fine washing area 14 in an order of magnitude of 1 to 4% by virtue of the process control with the introduction of the additional water into the fine washing section 14. The urea concentration in the second scrubber 2 is about 10%.

p0022As <figref idrefs="f0002">FIG</figref> It is also preferably provided that an acid is introduced into the fine wash zone 14 for reducing the ammonia load of the exhaust gas, which is indicated by an arrow 20. For this purpose, a part of the aqueous solution emerging from the outlet 11 of the first scrubber 1 is recirculated in the circuit from the line 13 via a pump 25 for introducing the acid into the fine washing area 14. Sulfuric acid or nitric acid, for example, can be used as the acid. Such an acid treatment is in principle, for example, from<patcit id="pcit0003" dnum="EP0440932B1"><text>EP 0 440 932 B1</text></patcit> known. The addition of acid (stream 20) is preferably carried out in a corrosion-resistant, self-priming nozzle downstream of the pump (eg jet nozzle), the flow of which is regulated. In this case, the pressure line of the pump can be used as a jet jet stream in whole or in part.

p0023The method is also suitable, in principle, alternatively for a scrubber in which several drop separators are arranged standing. The additional water is then conducted in a corresponding manner first into a fine wash zone of the scrubber for the exhaust gas coming from the granulation.