WO2003099721A1

WO2003099721A1 - Process for removing ammonia from an ammonia-containing gas stream

Info

Publication number: WO2003099721A1
Application number: PCT/NL2003/000331
Authority: WO
Inventors: Andreas Johannes Biermans; Fredericus Henricus Maria Buittink
Original assignee: Dsm Ip Assets B.V.
Priority date: 2002-05-23
Filing date: 2003-05-06
Publication date: 2003-12-04
Also published as: CN1294080C; CA2483648C; AU2003224520A1; CN1656020A; NL1020665C2; MY142787A; AR040041A1; CA2483648A1

Abstract

Process for removing ammonia from an ammonia-containing gas stream by converting the ammonia in the ammonia-containing gas stream with an organic acid into an ammonium salt, characterized in that the obtained ammonium salt is contacted, at elevated temperature, with a peroxide.

Description

PROCESS FOR REMOVING AMMONIA FROM AN AMMONIA-CONTAINING GAS STREAM

The invention relates to a process for removing ammonia (NH₃) from an ammonia-containing gas stream by converting the ammonia in the ammonia- containing gas stream with an organic acid into an ammonium salt. Such a process is described in for example US-A-4424072. In column 10 of that patent specification is described that the ammonia is removed from the gas stream exiting from the top of a prilling tower in a urea plant by contacting the gas stream in a scrubber with a non-volatile, dilute acid solution whereby the NH₃ is absorbed. According to US-A-4424072, non-volatile acid solutions used in prilling towers in a urea plant include inorganic acids such as phosphoric acid, sulphuric acid and nitric acid as well as organic acids such as citric acid, oxalic acid and comparable non-volatile organic acids. According to US-A-4424072 the ammonia-containing gas stream is, vented to the air, but normally the ammonia-containing gas stream leaving the prilling tower is recycled to the urea production process. The drawback of using these acids is that contaminations by ammonium salts may occur in the final product, in this case urea, which contaminations are undesirable. For instance, ammonium salts in urea will usually render this urea unsuitable for the preparation of melamine. The separate processing of these ammonium salts (e.g. as a by-product) costs money and energy or often poses an environmental problem.

It has been found that the aforementioned drawbacks can be eliminated with a process whereby the obtained ammonium salt is contacted at elevated temperature with a peroxide.

In the present invention, an organic peroxide and/or hydrogen peroxide may be used as peroxide. Examples of organic peroxides that may be used are benzonyl peroxide, methyl ethyl ketone peroxide, peroxyesters, peroxydicarbonates and dialkyl peroxides. The peroxides can be used as such or as a solution in one or more solvents. Water can be used as a solvent. Mixtures of different peroxides can also be applied. Preferably hydrogen peroxide is used as the peroxide. The process according to the present invention is practiced at an elevated temperature. An elevated temperature here means a temperature of between 80°C and 400°C. The contacting of the ammonium salt with the peroxide is preferably carried out at a temperature of between 100°C and 300°C, in particular between 150°C and 250°C.

An organic acid is used to convert the ammonia in an ammonium salt. Various acids may be used as organic acid. Preferably formic acid, oxalic acid and/or citric acid is used, in particular oxalic acid.

The ammonium salts of these organic acids are converted with a peroxide into NH₃, CO₂ and H₂O-containing mixtures. The composition of these mixtures is determined in part by the acid used. An advantage of the process according to the invention is that the formed substances can readily be reprocessed in for example a urea process.

The invention may be applied in any process plant wherein ammonia- containing gas streams may develop. Examples of process plants in which such gas streams may develop are plants for the production of urea, melamine, and ammonia. A lot of these gas streams are waste gas streams. Various production processes are applied in practice for the production of urea, such as those described in Uhlmann's Encyclopedia of Industrial Chemistry Vol. A27 (1996) pages 333 - 354. Initially, urea was produced in so-called conventional high-pressure urea plants and these processes were supplanted in the late nineteen sixties by processes that are applied in so-called urea stripping plants. An embodiment often used for the production of urea by a stripping process is the Stamicarbon CO₂ stripping process described in Uhlmann's Encyclopedia of Industrial Chemistry Vol. A27 (1996) pages 344-346. The gas mixture obtained in the stripping treatment is largely condensed and absorbed, together with the ammonia needed for the process, in a high-pressure carbamate condenser whereupon the ammonia carbamate that has formed here is passed to the synthesis zone for the formation of urea.

The high-pressure carbamate condenser may be designed as for example a falling-film condenser or a so-called submerged condenser as described in NL-A-8400839. The submerged condenser may be placed in horizontal or vertical position.

In a urea plant employing the Stamicarbon CO₂ stripping process, the gas mixture developing in the reactor, consisting of inert gases, ammonia and carbon dioxide, is scrubbed in a high-pressure scrubber with an aqueous solution for the purpose of recovering especially ammonia. In this high-pressure scrubber there develops an inert gas stream that is still contaminated with traces of ammonia. This ammonia needs to be removed from the inert stream before the inert stream can be vented into the atmosphere. The removal of the ammonia from this inert stream can be effected with the process according to the invention. In other urea production processes, too, there is always a purge stream of non-condensable gases, which purge stream can be rid, by the present invention, of ammonia down to extremely low residual concentrations.

In the course of the process, the urea synthesis solution leaving the urea reactor is converted into a urea melt. Here, too, as gases are condensed and absorbed, there develop gas streams from which ammonia needs to be removed. The urea melt is converted into urea prills or granules in a cooling/drying unit with the aid of cooling/drying air. Examples of such cooling/drying units are prilling towers and granulators. The granulators are often fluid bed or spouted bed granulators. Drum-type or pan-type granulators are also applied. The cooling/drying air is discharged from the cooling/drying unit as a gas stream to a scrubber for removal of the traces of ammonia contained in this air. All these waste streams can be purified by the process of the invention before these waste streams are released in the environment. Another ammonia emission source is the stack to which the spring-loaded safety valves of plant items are connected. Spring-loaded safety valves serve to relieve pressure from plant items when too high a pressure threatens to occur in them, resulting in an unsafe situation.

Waste streams of a urea plant often contain ammonium formiate. This ammonium formiate is formed from for example ammonia and formaldehyde in the presence of oxygen. Formaldehyde is used in a urea plant as a granulation aid and serves to prevent or reduce dust formation during granulation.

This ammonium formiate can also be converted with peroxide into NH₃, CO₂ and H O-containing mixtures. Preferably hydrogen peroxide is used as peroxide. The formed NH₃, CO₂ and H₂O are returned to the urea production.

In the production of melamine use may be made of processes that operate at low pressure, the gas-phase processes, or at high pressure, the liquid- phase processes. An overview of current melamine processes is shown in Nitrogen No. 228, pages 43 - 51 , July - August 1997. In gas-phase and liquid-phase melamine processes, the purge stream of the desorption column in which ammonia is recovered is a source of possible ammonia discharge to the atmosphere. Here, too, the process of the invention may be very well applied. Waste streams in melamine plants often also contain ammonium formiate. This originates from the saponification of the HCN present with water at elevated temperature. Here, too, ammonium formiate can be converted with a peroxide into NH₃, CO₂ and H₂O-containing mixtures. Plants for the production of for example ammonia and ammonium nitrate also include waste streams from which ammonia needs to be removed before such streams can be discharged. The process of the invention can be advantageously be employed here too.

The process of the invention may also be applied in improving and optimizing existing plants.

The invention is explained in further detail with reference to the figures and the examples.

Figure 1 schematically shows a part of a new urea plant wherein the present invention is employed. Here, 1 represents a (fluid bed) granulator to which a urea melt 3 is supplied and converted with air stream 8 into urea granulate. This still- warm urea granulate is passed through 4 to a product cooler 2 and cooled further, again with air stream 8 . The cooled granulate passes through 5 to product screens 6 where the urea granulate is divided into three product streams. One stream 7 consisting of product of the desired size and streams consisting of oversize and undersize product. The coarse product stream is crushed and returned together with the fine product stream through 9 to the granulator. The off-gas from the granulator passes, together with the off-gas from the product cooler, to scrubber 10 where the off- gas is cleared of urea dust with the aid of water supplied through 23. The off-gas streams may also be treated in separate scrubbing units. The removed urea dust is returned to the process through line 22. The off-gas streams cleared of urea dust are returned to scrubber 11 where ammonia is removed from the off-gas and converted into ammonium oxalate with the aid of dilute oxalic acid solution supplied through 12. The ammonium oxalate-containing wash liquor obtained here is partly returned through 13 to 11 and partly transferred through 14 and through heat exchanger 15 to ammonium oxalate decomposer 16. The off-gas cleared of ammonia is removed from the process through 19. In ammonium oxalate decomposer 16, ammonium oxalate is converted, at elevated temperature, into NH₃, CO₂ and H₂O with the aid of hydrogen peroxide supplied through 17 and steam supplied through 18. The NH₃, CO₂ and H₂O- containing gas stream 20 is returned to the urea process. Waste water is removed from the plant through 21 after optional additional treatment in the waste-water section of the urea plant.

Figure 2 schematically shows a part of a new urea plant as in Figure 1. In Figure 2, however, a portion of the gas stream from scrubber 10 is recirculated through the urea melt feed to the granulator. In this way an ammonia level is built up in granulator 1 and scrubber 10 from which a purge stream with a relatively high NH₃ concentration is passed to scrubber 11 and removed from the process according to the present invention.

Figure 3 schematically shows a part of an existing urea plant where an existing plant is optimized by application of the present invention. In a urea plant, the reactor off-gas, consisting of non-converted NH₃ and CO₂ and inert gases, is scrubbed in a high-pressure scrubber with a dilute ammonium carbamate solution obtained in the urea recovery unit. An off-gas stream is obtained from this high- pressure scrubber which in an existing plant is supplied to an absorber 1 wherein ammonia is largely removed from the off-gas stream. In the present invention the inert purge stream obtained in 1 is passed through 2 to a newly installed off-gas scrubber 3 into which a dilute oxalic acid solution is metered through 5, with ammonia being converted into ammonium oxalate. The inert purge stream cleared of ammonia is removed from the process through 4. The dilute ammonium oxalate solution coming from 3 is partly returned to off-gas scrubber 3 where it is utilized as wash liquor. The rest of the ammonium oxalate solution coming from 3 passes through 6 to an existing urea decomposer 7 in the waste-water treatment system of the urea plant. Prior to being supplied to urea decomposer 7, the ammonium oxalate solution is mixed with hydrogen peroxide supplied through 8. The supplied hydrogen peroxide may also be supplied directly to urea decomposer 7. The NH₃, CO₂ and H₂O-containing gas stream from 7 is returned to the urea process through 9.

Example 1

An ammonia-containing gas stream originating from the fluid bed granulation unit in a urea plant was scrubbed at room temperature with an approximately 3 wt.% solution of oxalic acid in water. There was obtained a solution of 45 grams of ammonium oxalate in 1000 grams of water. 250 grams (= 4.5 wt.% ammonium oxalate in water) of this solution was transferred into a 500 ml autoclave. The contents of the autoclave were brought to a temperature of 200 °C whereupon a solution of 3 grams of hydrogen peroxide in 25 grams of water was introduced into the autoclave. The ammonium oxalate solution then decreased to 1.5 wt.%. Subsequently, a solution of 3 grams of hydrogen peroxide in 25 grams of water was again introduced into the autoclave. The ammonium oxalate concentration now decreased to 0.05 wt.% 3 grams of hydrogen in 25 grams of water were introduced into the autoclave for the third time. The ammonium oxalate concentration decreased to 0.0009 wt.% Example 2

An ammonia-containing gas stream originating from the inert purge of the absorber in a urea plant was scrubbed at room temperature with a solution of 50 ppm of formic acid in water. 250 ml of this solution was transferred to a 500 ml autoclave. The contents of the autoclave were brought to a temperature of 143 °C whereupon a solution of 27.5 mg of hydrogen peroxide in 25 grams of water was introduced into the autoclave. The ammonium formiate concentration decreased to 30 ppm. Subsequently, a solution of 27.5 mg of hydrogen peroxide in 25 grams of water was again introduced into the autoclave. The ammonium formiate concentration now decreased 9.3 ppm. 27.5 mg of hydrogen peroxide in 25 grams water were introduced into the autoclave for the third time. The ammonium formiate concentration decreased to 0.9 ppm.

Claims

1. Process for removing ammonia from an ammonia-containing gas stream by converting the ammonia in the ammonia-containing gas stream with an organic acid into an ammonium salt, characterized in that the obtained ammonium salt is contacted, at elevated temperature, with a peroxide.

2. Process according to Claim 1 , characterized in that an organic peroxide and/or hydrogen peroxide are used as peroxide.

3. Process according to Claim 1 , characterized in that hydrogen peroxide is used as peroxide.

4. Process according to any one of Claims 1-3, characterized in that the conversion of the ammonium salt is effected at a temperature of between 100°C and 300°C.

5. Process according to any one of Claims 1-3, characterized in that the conversion of the ammonium salt is effected at a temperature of between

150°C and 250°C.

6. Process according to any one of Claims 1-5, characterized in that formic acid, oxalic acid and/or citric acid is used as organic acid.

7. Process according to any one of Claims 1-3, characterized in that oxalic acid is used as organic acid.

8. Process according to any one of Claims 1 -7, characterized in that the ammonium salt is converted into a NH₃, CO₂ and H₂O-containing mixture.

9. Process according to any one of Claims 1-8, characterized in that the formed NH₃, CO₂ and H₂O are reprocessed in a urea process.

10. Process according to any one of Claims 1-9, characterized in that the ammonia-containing stream is an off-gas from a urea production unit.

11. Process according to Claim 10, characterized in that the ammonia-containing stream is a gas stream from a prilling tower or a granulation unit of a urea plant.

12. Process according to Claim 10, characterized in that the NH₃-containing stream is an off-gas of a urea scrubber.