HK1096327B - Method for removing ammonia and dust from a waste gas that results during the production of fertilizers - Google Patents

Description

Method for removing ammonia and dust from waste gas generated in fertilizer preparation

Technical Field

The invention relates to a method for removing ammonia and dust from exhaust gases produced in the production of fertilizers, preferably urea, in which method the exhaust gases are conducted into a first scrubber and cooling gas into a second scrubber and added water is conducted into the first scrubber and the aqueous solution produced in the first scrubber is conducted into the second scrubber, where both the exhaust gases and the cooling gas are passed through at least one droplet separator before being discharged from the respective scrubber.

Background

In the production of ammonia-containing fertilizers or, in the case of fertilizers which decompose ammonia, such as urea-containing fertilizers, ammonia-and dust-containing waste gas streams are produced in different process stages, which have to be purified before being discharged into the environment or conveyed back into the process. Such offgas is formed in particular during granulation and cooling.

For removing dust from the exhaust gases flowing out of the granulation process and from the particle cooling gas, the applicant is aware of a suitable type of process. In order to carry out this method, two scrubbers are provided, each of which has at least one droplet separator (demister) in the upper region. The offgas here formed from the granulation process is conducted to a first scrubber and the cooling gas is conducted to a second scrubber. For purification, added water, preferably purified or unpurified process water, is introduced into the second scrubber below the droplet separator in countercurrent to the cooling gas. The aqueous solution flowing out of the second scrubber is then also conducted into the first scrubber in countercurrent to the exhaust gas to be purified.

In practice, it has proven possible to improve this known method. Since the aqueous solution leaving the first scrubber must also be further processed or further used, there is a need to adjust the urea concentration in the aqueous solution as high as possible in order to keep the energy consumption for evaporating the exiting aqueous solution as low as possible. However, there is a limit to this maximum concentration in the processes to date. The urea concentration in the aqueous solution in the first scrubber has hitherto been a maximum of about 30-45%, higher concentrations not being possible because, despite the droplet separator, it cannot be completely avoided that droplets, which are correspondingly charged with urea, remain in the outflowing exhaust gas and cause a comparatively high urea concentration therein.

Disclosure of Invention

The object of the invention is therefore to develop a suitable type of method in such a way that the exhaust gas burden can be significantly reduced.

This object is achieved according to the invention by a method of the type mentioned at the outset in that the added water is initially introduced into a fine scrubbing zone of the first scrubber and sprayed onto at least one droplet separator, wherein the upper side of the fine scrubbing zone is delimited by the droplet separator and the lower side is delimited by a liquid-tight separating plate, and the aqueous solution produced in the fine scrubbing zone is subsequently introduced into a second scrubber, wherein the exhaust gas to be cleaned initially enters a main scrubbing zone fitted with sieve plates and then passes upward through the separating plate to the fine scrubbing zone, wherein dilution and reduction of the droplets adsorbed on the exhaust gas takes place by mixing with the added water, and the exhaust gas is subsequently discharged from the top of the first scrubber after passing through the droplet separator.

Thus, in contrast to the known methods, the added water is initially introduced completely into an additional fine scrubbing zone provided in the first scrubber, into which the exhaust gas laden with droplets enters before passing through the droplet separator. In this case, a strong dilution is carried out in the fine scrubbing zone by adding water, so that the urea concentration of the droplets drops significantly. At the same time, the droplet separator is additionally cleaned. By means of the intensive dilution of the droplets, the urea concentration in the aqueous solution can be increased significantly in the actual main scrubbing region of the scrubber, so that the energy consumption for the evaporation of the aqueous solution below can be reduced considerably. In addition, the method can be usedTo achieve a dust load in the exhaust gas of from about 50mg/m hitherto achievable³Can be reduced to 20mg/m³。

The aqueous solution flowing out of the second scrubber is conducted in a known manner into the first scrubber, naturally into the main scrubbing zone of the first scrubber, which is arranged below the separating plate, into which zone the exhaust gases also enter.

In order to separate the fine washing zone and the main washing zone of the first scrubber, preferably a bubble tray is applied. In principle, other separating plates which are not permeable to liquid but permeable to gas can also be used.

In order to reduce the ammonia concentration in the exhaust gas, it is provided in a further advantageous embodiment that the acid is introduced into the fine scrubbing zone of the first scrubber. For example, sulfuric acid or nitric acid may be used. Such acid treatments are basically known, as known from EP 0440932B 1.

In order to optimize the further processing of the aqueous solution flowing out of the first scrubber in terms of energy, it is preferably provided that the urea concentration is adjusted to 40 to 60%, preferably 55%, in the main scrubbing zone of the first scrubber. The energy consumption for evaporation can thus be reduced considerably, while such very high urea concentrations in the aqueous solution do not present problems in the purification of exhaust gases, since, as described above, a strong dilution of the droplets entering the fine scrubbing zone takes place in this zone.

Drawings

The invention is explained more clearly below with reference to the drawings. Wherein

FIG. 1 shows a schematic diagram of a process for carrying out the process and

fig. 2 shows a detailed illustration of fig. 1 in a particular arrangement.

Detailed Description

The apparatus for carrying out the process has first a first scrubber 1 and a second scrubber 2. A first scrubber 1 is preceded by a pre-cleaning stage 3. In the upper region of the first scrubber 1, a droplet separator 4 (demister) is arranged, as is a droplet separator 5 on the second scrubber 2. The first scrubber 1 is divided into two scrubbing zones, where below the droplet separator 4 a separating plate 12 (e.g. a bubble tray) through which liquid cannot pass but gas can pass and an outlet 11 are provided in order to form a fine scrubbing zone 14. Below the separation plate 12 is a main wash zone 21 of the first scrubber 1.

The above-mentioned apparatus part is preferably part of an apparatus for preparing fertilizer, preferably urea, and is connected to a granulator and a cooler, not shown. The off-gases laden with ammonia and dust are fed from a granulator, not shown, and are first fed into the pre-purification stage 3, which is indicated by the arrow 6. The exhaust gases pass through the pre-purification stage 3 and are conducted into the main scrubbing zone 21 of the first scrubber 1. The likewise loaded cooling gas is fed directly into the second scrubber 2, which is indicated by arrow 7.

The added water, preferably purified or unpurified process water, is fed directly into the fine scrubbing zone 14 of the first scrubber 1, where the water lines are indicated by arrows 8, 9. The water line opens with the nozzle head 10 arranged upwards into the interior of the scrubber 1 below the droplet separator 4 in such a way that the added water is sprayed against the droplet separator 4 and is purified thereby. The added water mixes with the droplets passing through the separating plate 12 and causes a strong dilution of the urea concentration of the droplets, so that even at a urea concentration of 55-60% in the main scrubbing zone 21, the droplets have, for example, only a urea concentration of 1-4%. The water is thereby enriched and flows out as an aqueous solution through an outlet 11, which outlet 11 opens into a line 13, which leads into the second scrubber 2, so that the aqueous solution is conducted into the second scrubber 2.

The exhaust gas to be cleaned, after passing through the preliminary cleaning stage 3, therefore first enters the main cleaning zone 21 of the first scrubber 1, where a screen 22 or the like is installed, and then passes through the separating plate 12 to the fine cleaning zone 14, where a strong dilution and reduction of the liquid droplets adsorbed on the exhaust gas takes place by mixing with the added water. The exhaust gas is then discharged through the droplet separator 4 and then purified at the top of the first scrubber 1 (arrow 15).

The cooling gas to be purified enters the lower region of the second scrubber 2 (arrow 7), where a sieve plate 23 is also arranged, in countercurrent through the introduced aqueous solution and subsequently through the droplet separator 5 to be purified out at the top of the second scrubber (arrow 16).

The bottom products in each of the two scrubbers 1 and 2 will be recycled in the usual manner, which is indicated by the respective recycle 17 or 18. From which the aqueous solution is branched off from the cycle 18 and sent to the prepurification stage 3 via line 19. Whereby the aqueous solution and the flue gases pass from the pre-cleaning stage 3 into the main scrubbing zone 21 of the first scrubber 1.

By means of the pronounced dilution or purification effect in the fine washing zone 14, it is possible to adjust the urea concentration in the aqueous solution to about 60% in the main washing zone 21 of the first scrubber 1, i.e. the aqueous solution (line 24) flowing out of the scrubber 1 has a urea concentration of 60%, so that this aqueous solution can be evaporated for further utilization with much less energy consumption than in the prior art. Despite this high urea concentration in the main wash zone 21, the urea concentration in the fine wash zone 14 itself can reach the order of 1-4% due to the implementation of the method of introducing the addition of water to the fine wash zone 14. The urea concentration in the second scrubber 2 is about 10%.

As shown in fig. 2, it is preferably additionally designed that an acid is introduced into the fine scrubbing zone 14 to reduce the ammonia load in the exhaust gas, which is indicated by arrow 20. In order to introduce the acid into the fine scrubbing zone 14, a portion of the aqueous solution flowing out of the outlet 11 of the first scrubber 1 is conveyed back into the circuit by the line 13 via the pump 25. Examples of acids which can be used are sulfuric acid or nitric acid. One such acid treatment is substantially as known from EP 0440932B 1. The addition of acid (stream 20) is preferably performed with a corrosion resistant, self-priming nozzle (e.g., a spray nozzle) after the pump, the inflow of which can be adjusted. The pressure line of the pump is used here completely or partially as a propulsive jet.

The method is also suitable in principle for use alternatively in scrubbers in which a plurality of droplet separators are arranged. The added water is then introduced in a corresponding manner first into the fine scrubbing zone of the scrubber for scrubbing the offgas produced during granulation.

Claims (7)

1. A method for removing ammonia and dust from exhaust gases produced in the production of fertilizers, in which method the exhaust gases are conducted into a first scrubber, cooling gases into a second scrubber, and water is conducted into the first scrubber, and an aqueous solution produced in the first scrubber is conducted into the second scrubber, where not only the exhaust gases but also the cooling gases are passed through at least one droplet separator before being discharged from the respective scrubber, characterized in that the water is first conducted into a fine scrubbing zone of the first scrubber, the upper side of which is delimited by the droplet separator and the lower side by a separating plate through which no liquid can flow, and is subsequently conducted into the second scrubber, in which the exhaust gases to be cleaned are first conducted into a main scrubbing zone fitted with a screen plate, and then up through the separator plate to a fine scrubbing zone where dilution and reduction of liquid droplets adsorbed on the off-gas takes place by mixing with added water, whereafter the off-gas passes through a droplet separator before being discharged from the top of the first scrubber.

2. The method of claim 1, wherein the fertilizer is urea.

3. Method according to claim 1, characterized in that the aqueous solution flowing out of the second scrubber is conducted into the main scrubbing zone of the first scrubber, which is arranged below the separation plate.

4. A method according to any one of claims 1-3, characterized in that a bubble tray is used as the separating plate.

5. A method according to any of claims 1-3, characterized in that an acid is introduced into the fine scrubbing zone of the first scrubber.

6. A method according to claim 2 or 3, characterized in that the urea concentration in the main scrubbing zone of the first scrubber is adjusted to 40-60%.

7. Method according to claim 6, characterized in that the urea concentration is adjusted to 55%.