HK1152267B - Method for reducing aerosol emissions in a urea granulation plant - Google Patents

Description

Method for reducing aerosol emissions in a urea granulation plant

The present invention relates to a method for reducing the emission of aerosols in a urea granulation plant by recovering the resulting scrubber effluent. The process describes the production of urea granules in a known granulator that is connected to a urea isomerization unit that removes ammonium isocyanate from the gas stream. The ammonium isocyanate in the granulator effluent is further partially hydrolysed and partially isomerised to urea. The isomerized urea is passed into the liquid phase and then returned to the dedusting system or the scrubbing system. The invention also relates to a plant for producing urea granules using a corresponding process.

Urea is usually produced by crystallization of a concentrated urea melt. The melt is introduced into a granulator which performs granulation at elevated temperature and evaporates the water in the melt. The resulting granulated particles are usually obtained in a shape which makes them suitable for the desired application. Due to the high temperature, a portion of the urea is converted to ammonium isocyanate according to a reversible reaction. The corresponding chemical equation is:

this reaction causes a portion of the urea to decompose and produce ammonium isocyanate which enters the gas stream exiting the granulator. The reaction is reversible, with the result that ammonium isocyanate, which is an isomer of urea, can be isomerized to the desired product urea. The ammonium isocyanate further decomposes with water to ammonium carbonate, which at elevated temperatures eventually leads to the formation of carbon dioxide, ammonia and water:

however, the final decomposition products, carbon dioxide and ammonia, cannot be isomerized to urea, but can be recycled to the urea production process.

In order to maintain the economics of the overall process, it is preferred to separate the urea from the by-products and reintroduce them into the production process. Ammonium isocyanate is typically obtained as an aerosol and therefore separation of the aerosol allows isomerization of the ammonium isocyanate to urea. The gaseous byproducts carbon dioxide and ammonia are left and can be reintroduced into the production process. In addition, isocyanates are often emitted into the atmosphere, which may be harmful to the environment. There is thus a need for a process that reduces or eliminates the emission of isocyanate to the environment.

It is known to recover ammonia from exhaust gases from urea production. WO2005/032696a1 describes a method for removing ammonia and dust from exhaust gases produced in the production of fertilizers, preferably urea. The off-gas is introduced into a first scrubber where it is washed with scrubbing water. In order to be able to reduce the pollution caused by the exhaust gases considerably, both the exhaust gases from the granulator and the cooler gas from the granulator are passed through at least one droplet eliminator before exiting the scrubber. However, the invention does not teach how to recover the ammonium isocyanate, a common by-product of granulation.

It is therefore desirable to find a process which traps the by-products ammonium isocyanate, ammonia and water and separates the ammonium isocyanate, which is usually obtained in the form of separable aerosols or fine particles, from the by-products of urea granulation. In addition, the desired process should isomerize the ammonium isocyanate to urea and re-enter the production process. The desired process would also provide a scrubbing system for the gas and a means for recovering the heat of reaction.

A process for the recovery of ammonia, carbon dioxide and aerosols from the granulation of urea is described. The aerosols contain the major part of the ammonium isocyanate which is separated off in the first step and then hydrolyzed and isomerized to urea in the second step and subsequently recycled back into the process. The recirculation step of the process may introduce the urea solution into the dust scrubber. The dust scrubber normally also uses a urea solution, however, it is also possible to use a urea solution of reduced concentration, which is normally obtained from a recycling step. Contaminated urea can also be used for this purpose. The gaseous by-products carbon dioxide, ammonia and water are reintroduced into the urea production process.

The invention is also claimed in a suitable apparatus for recovering the heat of reaction provided by the process. The invention also provides suitable equipment for scrubbing and cleaning the resulting exhaust gas. The invention claims these methods and the devices to perform the methods. Also claimed is a particulate urea product produced by the claimed process.

The invention particularly claims a method for reducing the emission of suspended particles from a urea granulation plant by treating the resulting scrubber effluent, using

● A granulator for producing urea from a concentrated urea solution and evaporation of the water contained therein, producing urea granules and a waste material consisting of dust, ammonia and ammonium isocyanate, and

● a subsequent dust washing or dusting section, and

●, a first stream consisting of waste material consisting of air and ammonia, and a second stream consisting of ammonium isocyanate and water are discharged,

it is characterized in that

● the second stream consisting of ammonium isocyanate, some urea and water is passed to an isomerization unit where the ammonium isocyanate component is isomerized back to urea and the isomerization of ammonium isocyanate to urea is carried out in a stripper fed at the bottom with low pressure steam, and

● the urea produced in the stripper is brought into the liquid phase and the remaining ammonia and carbon dioxide are discharged overhead.

By using the invention it is possible to include a heat exchanger or cooler into the process at any desired location. In particular, it is possible to include the heat exchanger in the process after the separation of ammonium isocyanate. In particular, it is possible to heat the cold effluent stream from the suspended particle scrubber before entering the stripper. Higher process temperatures in this step will increase the isomerization rate. Therefore, the temperature should be 100-200 ℃. For the economy of the overall process it is therefore advantageous to pass the separated aerosols from the aerosol stage through a heat exchanger. A suitable heat exchanger heat source is the urea-containing liquid recovery phase from the isomerization unit. It is also possible to use a conventional heat source if a heat exchanger cannot be installed.

Typically, after granulation, residual gas and dust from granulation are directed to a dust stage. This section separates most of the dust from the gas from the production of gases, such as ammonia and usually consisting of ammonium isocyanate and residual urea. Air is then introduced into the aerosol stage to separate out the fine particles and aerosol, including the majority of the ammonium isocyanate and a portion of the very fine urea desublimation. The aerosol stage is preferably equipped with specially designed sprayers and collection pads, which allow for proper separation of the aerosol. It is also possible to direct the dust-laden exhaust gas directly into the aerosol stage without an intermediate dust separation stage. In this case, however, the dust content in the aerosol-separating section is relatively high, so that in a preferred embodiment of the invention a dust-separating section is used.

The residual gas from the aerosol separation stage still comprises air and residual ammonia. In a typical embodiment of the process, the residual gas still contains a minor amount of ammonia, which can be removed by an acidic scrubbing system. The gas may be scrubbed by any conventional means. The gas may also be scrubbed by a physical or chemical liquid. Suitable washing systems are known from the prior art.

For the recovery of urea, the isomerization of ammonium isocyanate is carried out in a stripper. The stripper column may be of any desired configuration. In a preferred configuration, it is possible to construct a column having a top which discharges gas and a bottom which discharges liquid. The column may have multiple trays to effectively separate the water vapor, ammonia and carbon dioxide overhead and the dilute urea solution at the bottom of the column. The column may be equipped with all suitable heating or cooling equipment. The recovery unit may also be equipped with all devices that generate pressure or vacuum. In a preferred configuration, the stripper is heated by low pressure steam to maintain temperature and pressure and discharges a gas consisting essentially of ammonia, carbon dioxide and water overhead. The ammonium isocyanate enters the column under heat and isomerizes to urea in the presence of steam and water.

The urea solution obtained is generally in a concentration of 40-85% by mass. The urea forms a hot liquid solution with water and is usually reintroduced into the dust separation section. Optionally, the concentration of the resulting urea solution may be used. However, it is also possible to reintroduce the obtained urea solution into the granulation process or for non-process purposes.

The process of the invention is not only suitable for the production of granules. Likewise, it can also be used for the production of urea powders, solutions, aggregate materials or prills.

The gas phase obtained in the isomerization unit, still consisting of ammonia, carbon dioxide and water, generally enters the low-pressure carbamate condenser of the urea synthesis unit. In one embodiment of the invention, the residual carbamate left free from carbon dioxide, ammonia and water is introduced into a condenser, where it is depressurized to a low pressure and a lower temperature steam is introduced. The result is a better condensation of the residual ammonium carbamate. The concentrated carbamate stream is then recycled to the urea synthesis unit.

The process conditions of the granulation step are those generally used for the granulation of urea. The typical depth of the urea solution as the granulation material is 90 to 99 mass%. The feed concentration to the dedusting section can be a lower concentration. Thus, the urea solution going to the dust scrubber may have a relatively small concentration of typically 40-85 mass%. The concentration step can be used at any stage of the process. The granulation usually takes place at a temperature of 110 to 130 ℃. A typical process for granulation of urea is given in WO2005/075383A 1.

The operating conditions of the isomerization unit are generally different from the granulation step. The isomerization unit comprises a stripper which is typically operated at a temperature of from 100 to 180 ℃ and a pressure of from 2.0 to 9.0 bar. Depending on the recovery conditions, they may vary with the product conditions.

The present patent application also relates to an apparatus for performing the method. The apparatus of this patent generally comprises a plant consisting of a urea granulation unit, a dust separation unit for separating dust and an aerosol separation unit for separating aerosols, which consist essentially of ammonium isocyanate and a portion of very fine urea desublimation. Optionally, the apparatus also includes a scrubbing unit that cleans the off-gas from the acidic component. Related devices are prior art and are described elsewhere.

The plant also comprises an isomerization unit consisting of a heating element and a stripping column. The stripping column may be a stripping column of the prior art. The stripper typically includes heating, cooling and vacuum pumping devices. Occasionally, it may contain a pressurizing device. The stripper contains the necessary means to discharge liquid or gas. It may also contain a reboiler or steam inlet. The latter may also be a low pressure steam inlet.

The invention claims a device for producing urea granules, which is characterized by comprising the following components

● granulator for granulation of urea solutions, and

● subsequent scrubber for dust removal, and

●, a subsequent aerosol section for removing aerosols, the section having a spray system and a collection pad designed,

it is characterized in that

● the suspended particles, which consist essentially of ammonium isocyanate and a portion of very fine urea desublimation, can be passed to an isomerization unit

● the isomerization unit is comprised of a stripper column having an overhead gas take off and a bottoms liquid take off.

The invention is also claimed in a plant for the production of urea granules, characterized in that it comprises a heat exchanger installed between the suspended particle section and the stripper. The heat exchanger will typically be indirect to avoid contamination of the product. However, it may also be of the direct type if the cooling water type and product quality permit. This reduces operating and capital costs. The invention is also claimed in a plant for the production of urea granules, wherein the plant comprises a low-pressure carbamate condenser for collecting the gaseous waste from the urea recovery system and for concentrating and condensing and disposing of the residual carbamate.

The invention is described herein by way of the accompanying drawings which illustrate the invention, but do not limit the scope of the invention. Which serve as illustrative examples.

FIG. 1 shows a flow diagram of the process of this patent, starting from the pelletizer on the left. Dry air and a urea melt having a concentration of 90 to 99 mass% are used as starting materials for a granulator for producing urea granulate products. The dry air leaves the granulator as air containing dust, ammonia, ammonium isocyanate and water and enters the dust scrubber. The dust scrubber removes coarser dust particles from the air and receives a feed of urea solution that is typically 40-85 mass% less concentrated. After the added water has evaporated, the urea solution from the dust scrubber produces a granulated urea product. The dust scrubber also discharges air containing ammonia, carbon dioxide, ammonium isocyanate and water, which then enters the aerosol separation section. The effluent from the suspended particles section, which contains most of the ammonium isocyanate and a portion of the very fine urea desublimation, enters a heat exchanger and then enters a stripper column as an isomerization unit. The ammonium isocyanate is isomerized to urea and leaves the stripper as a hot solution. The hot solution leaves the column and heats the incoming liquid through a heat exchanger. At the top of the stripper, a gas containing ammonia, carbon dioxide and water is discharged. This gas can be reintroduced into the urea synthesis process. The urea liquid from the stripper is reintroduced into the dust scrubber. The stripper may be fed with low pressure steam. In addition to the ammonium isocyanate aerosol, the aerosol stage also discharges air containing a residual portion of ammonia and acid gases. The air is introduced into an acid scrubber which removes residual ammonia and discharges a clean off-gas free of ammonia and acid gases into the atmosphere.

List of reference numerals

1 drying air

2 melt

3 Urea solution with poor concentration degree

4 clean water

5 granulator

6 products

7 air containing dust, ammonia, isocyanate

8 dust scrubber

9 Evaporation

10 air containing ammonia and isocyanate

11 aerosol separation section

12 air containing ammonia

13 acid scrubber

14 tail gas to atmosphere

15 isomerization unit

16 heat exchanger

17 stripping tower

18 low pressure steam

19 ammonia, carbon dioxide and water

Claims (13)

1. Method for reducing the emission of aerosols from a urea granulation plant by treating the resulting scrubber effluent, with

● A granulator for producing urea from a concentrated urea solution and evaporation of the water contained therein, producing urea granules and a waste material consisting of dust, ammonia and ammonium isocyanate, and

● a subsequent dust washing or dusting section, and

●, a first stream consisting of waste material consisting of air and ammonia and a second stream consisting of ammonium isocyanate and water are discharged, using specially designed sprayers and collection pads,

it is characterized in that

● passing the second stream consisting of ammonium isocyanate and water to an isomerization unit where the ammonium isocyanate component is isomerized back to urea and the isomerization of ammonium isocyanate to urea is carried out in a stripper fed at the bottom with low pressure steam, and

● the urea produced in the stripper is brought into the liquid phase and the remaining ammonia and carbon dioxide are discharged overhead.

2. The method according to claim 1, characterized in that the liquid phase containing urea from the stripper is concentrated in a concentration step.

3. A method according to claim 1 or 2, characterized in that the urea-containing liquid phase from the stripper is recycled to the granulator, to a dust separation unit or used for non-processing purposes.

4. The process according to claim 1, characterized in that the waste material consisting of ammonium isocyanate and water is fed to a heat exchanger before being fed to the isomerization unit.

5. The process according to claim 1, characterized in that the gaseous phase consisting of ammonia, carbon dioxide and water produced by the isomerization unit is passed to a carbamate condenser which condenses the carbamate into liquid and returns it to the urea synthesis.

6. The process of claim 1, wherein the stripper utilizes low pressure steam to maintain temperature and pressure in the isomerization unit.

7. The method of claim 1, wherein the gaseous waste from the aerosol stage enters an acid scrubber where dust and fines and residual ammonia are removed from the gaseous waste.

8. The method according to claim 1, wherein the operating conditions in the stripping column are set to 100 to 180 ℃ and the pressure is 2.0 to 9.0 bar.

9. The method according to claim 1, characterized in that a concentrated urea solution having a concentration of 90-99 mass% is fed to the granulator.

10. A plant for the production of urea granules, characterized in that it consists of

● granulator for granulation of urea solutions, and

● subsequent scrubber for dust removal, and

●, a subsequent aerosol section for removing aerosols, the section having a spray system and a collection pad designed,

it is characterized in that

● the suspended particles consisting essentially of ammonium isocyanate can pass into the isomerization unit

● the isomerization unit is comprised of a stripper column having an overhead gas take off and a bottoms liquid take off.

11. Plant for the production of urea granules according to claim 10, characterized in that it comprises a heat exchanger installed between said suspended particulate section and said stripper.

12. Plant for the production of urea granules according to claim 10 or 11, characterized in that said isomerization unit is connected to a low-pressure steam feed that provides low-pressure steam to a stripper of said isomerization unit.

13. Plant for the production of urea granules according to claim 10 or 11, characterized in that it comprises a low-pressure carbamate condenser where the gaseous waste from said isomerization unit is collected and the residual carbamate is condensed and disposed.