Ammonium carbamate is a chemical salt resulting from the reaction of ammonia and carbon dioxide. The structure is: H2N-C(=O)-O(-)(+)NH4. Ammonium carbamate can be formed without any intermediates by passing ammonia gas over solid carbon dioxide (dry ice).

In the real world, there is often water present and this complicates matters. Carbon dioxide and water can react to form carbonic acid, H2CO3. Carbonic acid is a diprotic acid and can react with 2 equivalents of base, so, when carbonic acid reacts with ammonia, ammonium bicarbonate begins to form; HO-C(=O)-O(-)(+)NH4. As more ammonia is added, ammomium carbonate begins to form; H4N(+)(-)O-C(=O)-O(-)(+)NH4. These are equilibrium reactions.

As the concentration of ammonium carbonate increases, there is a conversion of some of the ammonium carbonate to ammonium carbamate. This is a dehydration reaction.

An industrially important chemical that is derived from ammonia and carbon dioxide is urea, which is used as fertilizer. Urea is ammonium carbonate that has given up two molecules of water; H2N-C(=O)-NH2. The fertilizer plants that manufacture urea have severe corrosion problems due to the ammonium carbamate intermediate. The corrosion is so severe that carbon steel cannot be used. Some parts of the process will corrode 316 stainless steel. Those heated sections are manufactured from special alloys.

Why does iron corrode in the presence of ammonium carbamate? Ammomium carbamate is hygroscopic and will attract moisture. Wetted ammonium carbamate will become an equilibrium mixture of ammonium carbamate and ammonium carbonate. This combination of salts and moisture attacks the protective oxide layer on the surface of steel with chelation-type mechanism. This exposes the underlying base metal for further attack.

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ABSTRACTCorrosion in alkanolamine plants can be caused by a number of factors. Guidelines are given to maintain solvent cleanliness, acid gas loadings, solvent velocity and metallurgy required to minimize the effects of corrosion. Also, sources and ways to minimize the corrosive effects of CO, heat stable amine salts (HSAS), oxygen and ammonia are presented.

BACKGROUNDThere have been several books written that provide an excellent resource on the broad topic of gas treating.~4 Each give the reader extensive data covering everything from the design of a plant to the analytical methods required to analyze plant operating solutions. The topic of corrosion in alkanolamine plants learned over the past 45+ years is included in each of the

above books. For example, Kohl and Neilsen devote more than halfofa chapter to amine plant corrosion (reference I, pp188-224). The Gas Conditioning Fact Book devotes about one-third of a chapter to corrosion in amine systems (reference 2, pp 148-174). Reports reviewing the general topic of corrosion in alkanolamine plants dating back to the late 1950's have also been published (for example, see references 5-12). Many more papers have been published on more specific topics of the causes of corrosion in alkanolamine plants (i.e. effect of heat stable salts, acid gas loadings, amine degradation).

Clearly, the topic of corrosion plays an important role in the design and successful operation of a commercial gas plant. With this in mind, this paper is presented as a practical guide to identifying causes of corrosion and ways to minimize corrosion in alkanolamine plants.

DISCUSSIONAmine Concentration and LoadingsTable 1 gives guidelines for amine concentration and acid gas loadings that are commonly accepted in the industry to minimize carbon steel corrosion] 3 In mixed CO2/H2S acid gas service where the ratio of H2S to CO 2 is above about 1/19, the total acid gas loadings are somewhat higher due to the formation of a protective iron sulfide (FeS) film formed via the reaction of H2S with the iron in carbon steel. ~4 It is further pointed out by Nielsen et alJ that the acid gas should contain at least 5 vol.% H2S and that the H2S partial pressure should be greater than 5.1 kPa (0.74 psia) before the higher acid gas loading guidelines should be used. Although diglycolamine is listed at 70wt.% maximum, more recent reports suggest that diglycolamine gets much more corrosive with CO2 when the amine concentration exceeds about 50wt.%.

Plant Temperature Guidelines

According to DuPart et al., 7 the maximum bottoms absorber temperature is 180°F (82°C) and the maximum stripper bottoms temperature should be less than 255°F (124°C). Chakma and Meisen Z6 recommend a slightly lower reboiler temperature (248°F, 120°C) based upon their work with MDEA degradation. To minimize thermal degradation of the amine, the reboiler heat source should not exceed 300°F (149°C). 5'7

Plant Metallurgy

Carbon steel, 304 SS, 304 L SS, 316 SS and 316 L SS are all recommended for general u s e . 7 Avoid copper, brass or other copper bearing alloys due to known amine / copper complexation ~7 which may result in accelerated corrosion. Martensitic 410 SS is not recommended for amine service. DuPart showed one example of severe pitting towards 410 SS in a plant solution using 15wt.% DEA. 7 Work performed in this lab using liquid and vapor coupons with MDEA Blend A [loaded to only 0.010wt.% CO2 and heated to 260°F (126.7°C) for28 days in an autoclave] showed very little corrosion protection of the 410 S S compared to carbon steel whereas the 304L and 316L SS showed excellent resistance to corrosion (see Table 2).

Carbon steel is generally ace

A urea plant, operating on ammonia and carbon dioxide (CO2) gases, had to be shutdown due to corrosion in the intercooler and aftercooler of its CO

duplex stainless steel tubes, caused the shutdown of the fertiliser plant within 6 months. Investigations of the corrosion products by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) techniques showed the presence of carbon and ammonia based compounds, thus suggesting the role of ammonia and CO

their reactions, in the corrosion of type 304 stainless steel. Electrochemical polarisation studies showed that duplex stainless steel possessed a more positive open circuit potential and a nobler critical pitting potential than type 304 stainless steel thus confirming that the corrosion of type 304 stainless steel was caused by the galvanic action with the

duplex stainless steel heat transfer tubes. Hence, it was recommended that (i) the same material (type 304 stainless steel) be used for all parts of the intercooler and aftercooler to avoid galvanic corrosion, (ii) condense water carried over by CO

modification of the process to add up to 0.8% oxygen in the CO2 gas before entry into the intercooler, which will help in retaining/formation of an effective passive film on type 304 stainless steel.

Author Keywords: Chemical-plant failures; Galvanic corrosion; Stainless steel; Corrosion products; Corrosion protection

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ABSTRACTAustenitic stainless steel is a major construction material for urea plants. Since 1986, we have been faced several failures of austenitic stainless steel piping and vessels. In this article five cases of failure are reported . Investigations show that all cracks started from outside ( non process side ) under the insulation and the reason of failure is stress corrosion cracking (SCC) due to concentration of chloride in insulation which is impregnated in the prevailing ( marine ) environment of the site.

INTRODUCTIONMany new petrochemical plants were made after World War II . General purpose austenitic stainless steels of the 300 series are the common corrosion resistant material of construction in new plants. Approximately, 90 % of construction material in urea plants is austenitic stainless steel type 300 [1].

More than five vessels failed during last 15 years. All of this equipment was insulated for thermal efficiency. The cracks were started under the insulation and propagated towards the inside of equipment. Analysis of insulation showed that the chloride content in the insulation was remarkably high. This phenomena could be prevented by application of a suitable system such as aluminum foil or painting from an economical point of view. protective coating

The recovery of ammonia and carbon dioxide from leaching solutions in a nickel refinery contributes to the economics of the process and to reducing the environmental impact of waste streams. The ammonia and carbon dioxide recovery process involves the counter current contact between steam and the process and tailings streams in steam stripping columns. The recovered stream consists of a mixture of water vapour, carbon dioxide and ammonia at a pressure of 125 kPa and having a temperature range of 50 to

90°C. Energy recovery heat exchangers are part of the ammonia and carbon dioxide recovery plant. The plant is constructed of a 304L and 316L stainless steel and carbon steel.

Thermodynamic modeling has been used to identify locations in the piping and vessels where carbamate forms. The model needed to describe the equilibrium between the gas and liquid phases and estimate the activity coefficients of the solution components, ammonium carbamate and ammonium carbonate in the close-to-saturated concentrated solutions which form.

The model was first used to provide a rational explanation for a limited number of locations where carbon steel had failed and to provide guidance for selecting locations to inspect through out the plant.

INTRODUCTION

The recovery of ammonia and carbon dioxide from leaching solutions in a nickel refinery contributes to the economics of the process and to reducing the environmental impact of waste streams. The ammonia and carbon dioxide recovery process involves the counter current contact between steam and the process and tailings streams in steam stripping columns. The recovered streams consist of mixtures of water vapour, carbon dioxide and ammonia at a pressure of 125 kPa and having temperatures in the range of 50 to 90°C. The streams from a number of stripping columns are combined and fed to gas cooler condensers for recovery of the carbon dioxide and ammonia. Heat recovery exchangers are also part of the ammonia and carbon dioxide recovery plant. The plant is constructed of a 304L and 316L stainless steel and carbon steel. A flow chart illustrating the process is shown in Figure 1. (available in full paper)

FIGURE 1: Flow chart showing the stripping columns recovering ammonia and carbon dioxide from product (Nickel carbonate) and tailings streams. Reference should be made to Table 1 for the composition of the various numbered streams (1 to 7) and to the material used for the piping, either carbon steel, 304L or 316L stainless steel. A corrosion failure had occurred where stream 7 flowing in 304L stainless steel met with stream 1 flowing in carbon steel. The stripping columns were fabricated from carbon steel while the heat exchangers were 304L or 316L stainless and the Gas Cooler Condensers were titanium.

CORROSION FAILURE

A corrosion failure had occurred in carbon steel piping immediately down stream of a "tee" where the gas stream (1 in Figure 1) mixed with a gas liquid

stream which was derived from a heat recovery exchanger (stream 7 in Figure 1) which cooled the CO2 - NH3 - H2O stream from 85 to 52°C.

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Claims:1. Process for the production of urea from ammonia and carbon dioxide in a urea plant containing a high-pressure synthesis section comprising at least one reactor section, a stripper and a condenser wherein all the high-pressure equipment is placed in a low position, characterized in that the height of the high-pressure section is less than 35 m from ground level and at least one of the reactor sections comprises means for the separate distribution of ammonia over the volume of the reactor section.

2. Process according to claim 1, characterized in that the height of the high-pressure synthesis section is less than 30 m from ground level.

3. Process according to claim 1, characterized in that the means for the separate distribution of ammonia is a sparger.

4. Process according to claim 1, characterized in that the flow of the synthesis solution from the reactor section to the stripper, the flow of the mixed gas stream from the stripper to the condenser and of the condensate from the condenser to the reactor section is a gravity flow.

5. Process according to claim 1, characterized in that the stripper and the reactor section are located at ground level.

6. Process according to claim 1, characterized in that the reactor section comprising the means for the separate distribution of ammonia is a submerged condenser.

7. Process according to claim 1, characterized in that the reactor section is a horizontally placed combination of a submerged condenser and a reactor section, wherein the means for the separate distribution of ammonia is placed in the condensation section.

8. Process according to claim 6, characterized in that the submerged condenser is placed horizontally.

9. Process according to claim 7, characterized in that the means for the separate distribution of

ammonia is placed in the condensation section, and is extended into the reaction section.

10. Process according to any of the claims 1 to 9, where at least parts of the reactor and stripping sections are made of a an austenitic-ferritic duplex steel with a chromium content of between 26 and 35 wt.% and a nickel content of between 3 and 10 wt%.

Description:

The invention is directed to a process for the production of urea from ammonia and carbon dioxide in a urea plant containing a high-pressure synthesis section comprising at least one reactor section, a stripper and a condenser wherein all the high-pressure equipment is placed in a low position.

Stripping processes for the production of urea wherein all high-pressure equipment is placed on ground level are known in the art. An example of such a process is described in

GB-1188051 . In this patent publication is described that such a process can be obtained by using an ejector in the main recycle flow in the high-pressure part of the equipment. In the process described according to

GB-1188051 a process with all equipment on ground level can be achieved by using an ammonia-driven ejector for the transport of a carbamate stream from the condenser to the reactor.

A disadvantage of the use of an ejector in the main recycle flow is that all ammonia is needed as driving fluid in the ejector and can thus not be supplied at other places in the high-pressure synthesis section. Moreover, in the discharge of the ejector a combined stream of ammonia and carbamate will always be obtained. For the reasons mentioned above the urea process is not flexible and no easy adaptation of process conditions is possible.

Another disadvantage of the use of an ejector is an increase of energy consumption. The ammonia used as driving agent in the ejector has to be supplied to the motive fluid inlet of the ejector at a pressure substantially above the main synthesis pressure. This implies that the energy consumption of the pump that is used to supply the ammonia to the urea synthesis section has to be increased considerably.

The object of the invention is to overcome these disadvantages.

The invention is characterized in that the height of the high-pressure section is less than 35 m from ground level and at least one of the reactor sections comprises means for the separate distribution of ammonia over the volume of the reactor section.

Preferably, the height of the high-pressure section is less than 30 m from ground level.

This has the advantage that a better distribution of ammonia in the reactor section can be obtained and therefore a better conversion to urea in the reactor section is possible. This has the result that in a smaller reactor section the same amount of conversion can be obtained, so that a reactor section with a smaller volume is sufficient. Smaller equipment is cheaper and thus the process for the production of urea can be built more cost efficiently.

Alternatively, the advantage of a better distribution of ammonia in the reaction section could be used to obtain a better conversion in the same volume. In such a case, the amount of non-converted carbamate is reduced, such that the energy used by the equipment involved in the recycle of this non-converted material is reduced.

A combination of these two advantages is also possible, resulting in reduction of cost, as well as reduction in energy consumption. According to the invention ammonia is distributed in an optimized way over at least one reactor section of the high-pressure synthesis section. Optimized in this context means that ammonia is supplied at multiple locations over the volume of at least one reactor section in the urea production process. Such distribution of ammonia in practice can be achieved by many provisions. A very cost effective way of doing this is by means of a so called sparger, that consists out of one or more pipes or pipe sections, containing holes for the outflow of ammonia along these pipes. By choosing the diameter and location of these outflow holes, the distribution of ammonia over the volume of the reactor section can be optimized, either towards minimum volume, or towards maximum conversion, or towards a combination of minimum volume and maximum conversion.

In the process according to the invention preferably the flow of the synthesis solution from the reactor section to the stripper, the flow of the mixed gas stream from the stripper to the condenser and of the condensate from the condenser to the reactor section is a gravity flow. This means that no ejector, compressor or pump is present for rising the fluid pressure in the main recycle flow in the high-pressure section. For this reason the complete amount of ammonia that is fed to the high-pressure synthesis section is available for purposive distribution via a sparger to the reactor section.

A process for the production of urea contains a high-pressure synthesis section and one or more recovery sections at lower pressure. The high-pressure section comprises a reactor section in which the urea synthesis solution is prepared, a stripper in which the urea synthesis solution is stripped and a condenser in which the gases released in the stripping zone are condensed.

The synthesis can be carried out in more than one reactor section. A reactor section is herewith defined as a section wherein at least 20 wt% of the total amount of urea in the synthesis section is formed.

The reactor sections can be placed in serial order or parallel and can be two separate vessels or two reactor sections placed in one vessel. A reactor section can also be combined with a condenser section in one vessel. When the condenser is a submerged condenser and the residence time in the condenser section is long enough, more than 20 wt% of the total amount of urea is formed in the condenser and it thus functions as a reactor section.

Ammonia and carbon dioxide are fed to the reactor section either directly or indirectly. Ammonia and carbon dioxide can be introduced to the process for the production of urea at various places in the high-pressure synthesis section or in the recovery sections.

Preferably, carbon dioxide is mainly used as a counter-current gas stream during stripping of the urea synthesis solution. A part of the carbon dioxide can be fed to the reactor section.

Preferably, ammonia is fed to the condenser.

In the stripper the urea synthesis solution is stripped counter-current with carbon dioxide with the supply of heat. It is also possible to use thermal stripping. Thermal stripping means that ammonium carbamate in the urea synthesis solution is decomposed and the ammonia and carbon dioxide present are removed from the urea solution exclusively by means of the supply of heat. Stripping may also be effected in two or more steps. The gas stream containing ammonia and carbon dioxide that is released from the stripper is sent to a high-pressure condenser. The gas mixture obtained in the stripper is condensed under the removal of heat and absorbed in the high-pressure condenser, following which the resulting ammonium carbamate is transferred to the reactor section for the formation of urea.

The high-pressure condenser can for example be a falling-film condenser or a so-called submerged condenser as described in

NL-A-8400839 . The submerged condenser can be placed horizontally or vertically.

Several combinations of condenser sections and reaction sections are possible according to the invention:

Combination of a condenser section with a reaction section in so called submerged or poolcondensers. The submerged or poolcondenser is preferably placed horizontally.

Combination of the condenser with a reaction section into a single vessel, called poolreactor.

In case such a combination of the condenser with a reaction section is applied, it is of particular importance to obtain an optimized distribution of the liquid ammonia in the condenser using means for the distribution of ammonia, since the composition of the content of the condenser changes considerably along the condensation path, because urea formation takes place together with the condensation of the mixed gas coming from the stripper. This formation of urea, and thus also water, along the condensation path results in a change of the optimal NH 3 /CO 2 ratio along the condensation path. Optimal here is defined as the ratio resulting in the highest possible temperature, which is desirable to increase the reaction speed, as well as to maximize the available temperature difference for heat-exchange. In this way, optimizing the NH 3 /CO 2 ratio along the condensation path both reduces the required area for heat transfer, as well as reduces the required condensation volume for the ammonium carbamate dehydration reaction.

As the condenser is a submerged condenser and the residence time in the condenser section is long enough, more than 20 wt% of the total amount of urea is formed in the condenser and it thus functions as a reactor section.

Thus, preferably the reactor section comprising the means for distribution of ammonia is a submerged condenser that is, more preferably, placed horizontally.

In cases where the condenser and the first and second part of the reaction section are combined in one vessel, it may even be advantageous to extend the means for distribution of ammonia into the reaction section. In this way also in the reaction section the NH 3 /CO 2 ratio along the reaction path can be optimized, whereby higher temperatures and consequently a smaller reaction volume are obtained in the reaction section.

Preferably, the reactor section according to the invention is a horizontally placed combination of a submerged condenser and a reactor section, wherein the means for distribution of ammonia is placed in the condensation section and extends, more preferably, into the reaction section.

In the high-pressure synthesis section the pressure is substantially equal to the urea synthesis pressure in the reactor sections, which is the pressure at which urea formation takes place. The urea synthesis pressure is usually a pressure between 11-40 MPa, preferably 12.5-19 MPa. The pressure in the rest of the high-pressure section is substantially equal to the pressure in the reactor section. Substantially equal means that the pressure in the rest of the high-pressure section is less than 0.5 MPa higher or lower than in the reactor section.

In a prefered embodiment of the present invention, the stripper as well as the second reaction section are located on ground level in the plant. In this way, two heavy pieces of equipment are located at a very low elevation in the plant, which results in a considerable reduction of the required investment costs of the structure that has to carry these heavy pieces of equipment. The low location of these pieces of equipment further simplifies the operation and maintenance activities that are required on these equipment items. Also, from a safety point of view, low elevation of heavy pieces of equipment is prefered, since it minimizes the activities of human beings at high level and optimizes safety during construction and operation of the plant.

An oxidizing agent is added to the process for the production of urea in order to protect the materials of construction against corrosion. An oxide skin is formed on the metal parts, which protects against corrosion. This process is known as passivation. The passivating agent may be oxygen or an oxygen-releasing compound as described in for example

US-A-2.727.069 . Oxygen can be added, for instance, in the form of air or as a peroxide.

The corrosion sensitive parts in the high-pressure section in the process for the production of urea can be made of a an austenitic-ferritic duplex steel with a chromium content of between 26 and 35 wt.% and a nickel content of between 3 and 10 wt%. This type of steel is less corrosion sensitive. When this type of steel is used for the construction of the reactor section and the stripper it is possible to reduce or omit the introduction of an oxidizing agent to the process for the production of urea. Preferably, the chromium content of the austenitic-ferritic duplex steel is

between 26-30 wt %. In the high-pressure section preferably the reactor section and the stripper are made of the austenitic-ferritic duplex steel.

In the recovery section ammonia and carbon dioxide that were not removed from the urea synthesis solution in the stripper are recovered from the urea-comprising stream, produced in the high-pressure synthesis section, in order to be recycled to the high-pressure section. In the recovery section the pressure is lower than in the high-pressure synthesis section. In the process for the production of urea according to the present invention at least one low-pressure recovery section is present. When more than one recovery section is present at least one of the recovery sections is operated at medium pressure and one at low pressure. Medium pressure is a pressure between 1.0 and 8.0 MPa, preferably between 1.2 and 3.0 MPa. Low pressure is a pressure between 0.2 and 0.8 MPa, preferably between 0.3 and 0.5 MPa.

The synthesis gas that has not reacted in the reactor section can be removed from the reactor section and can be sent to a scrubber, wherein ammonia and carbon dioxide present in the gas flow are removed from the gas flow by absorption in a low-pressure carbamate stream. This carbamate stream is recycled from the low-pressure recovery section of the process for the production of urea. The scrubber can be operated at high-pressure or at medium-pressure. Preferably a medium-pressure scrubber is applied, because a medium-pressure apparatus is cheaper to construct. The scrubbing process in the scrubber can be stimulated by using a heat exchanger that extracts heat from the process. The carbamate stream from the medium-pressure or high-pressure scrubber can be returned to the reactor section, optionally via the high-pressure carbamate condenser.

The invention will hereafter be explained in more detail in the examples without being limited thereto.

Example I An example of a process according to the invention is given in figure 1. The high-pressure part of the process for the production of urea according to figure 1 comprised a reactor section (R), a CO 2 stripper (S) and a submerged condenser/ reactor section (C) that was placed horizontally. Further the process comprised a medium-pressure absorber (MA) and a low-pressure recovery section where the urea stream (U) was further purified.

A small amount of carbon dioxide was fed to the reactor section (R). In the reactor section a urea synthesis solution (USS) was formed which was sent to stripper (S) and stripped by the addition of heat and with carbon dioxide as a stripping gas. During stripping a mixed gas stream (SG) was obtained that was, together with reaction gases (RG) coming from the top of the reactor section (R) fed, via a sparger, to the condenser/ reactor section. To the condenser/ reactor section also a carbamate stream (MC) coming from the medium-pressure absorber (MA) was fed via a separate sparger. Also fresh ammonia was fed to the condenser/reactor section (C) via a separate sparger. The sparger for the distribution of ammonia and the sparger for distribution of the mixed gas (SG) were placed in the condenser section, but also extended into the reactor section of submerged condenser/reactor (C). The urea solution (CS) formed was sent to the reactor section

(R) and the gases that had not been condensed (CG) were sent to the medium-pressure absorber (MA). In the medium-pressure absorber the gases were absorbed in a low-pressure carbamate stream (LC) and condensed to form a carbamate stream (MC). The gases that had not been absorbed (MG) were sent to the low-pressure recovery section.

The flow from the USS, SG, and CS was a complete gravity flow. No pumps or ejectors were used to move the fluid or gases.

Both reactor (R) and stripper (S) were placed on ground level.

The height of the high-pressure synthesis section was 26 m from ground level.

What is claimed is:

1. A process for the recovery of ammonia from a reactor effluent stream comprising: contacting a gaseous reactor effluent stream containing ammonia with a first aqueous ammonium phosphate solution, in a quench zone, to absorb substantially all of the ammonia present in the reactor effluent stream to form a second aqueous ammonium phosphate solution richer in ammonium ions than said first aqueous ammonium phosphate solution; contacting said second aqueous ammonium phosphate solution with a stripping gas, substantially free of carbon dioxide, to remove volatile impurities contained in said second aqueous ammonium phosphate solution; heating said stripped second ammonium phosphate solution to an elevated temperature sufficient to reduce the amount of ammonium ions in said second aqueous ammonium phosphate solution back to substantially the same level present in said first aqueous ammonium phosphate solution to thereby generate a vapor stream comprising ammonia and an aqueous stream.

2. The process of claim 1, wherein the heating of said stripped second ammonium phosphate solution to an elevated temperature takes place in a wet oxidation reactor at wet oxidation conditions to simultaneously remove unwanted impurities from said second aqueous ammonium phosphate solution and reduce the ammonium ion concentration to substantially the same level present in said first aqueous ammonium phosphate solution, a caustic material being added to said wet oxidation reactor to convert any ammonium carbamate formed to an insoluble carbonate.

3. The process of claim 1, wherein the heating of said stripped second ammonium phosphate solution to an elevated temperature takes place in a wet oxidation reactor at wet oxidation conditions to simultaneously remove unwanted impurities from said second aqueous ammonium phosphate solution and reduce the ammonium ion concentration to substantially the same level present in said first aqueous ammonium phosphate solution, and wherein said vapor stream comprising ammonia is recycled to the reactor through a line, the interior wall of which is maintained at temperature above the condensation temperature of the vapor stream.

4. The process of claim 1, wherein the heating of said stripped second ammonium phosphate solution to an elevated temperature takes place in a wet oxidation reactor at wet oxidation conditions to simultaneously remove unwanted impurities from said second aqueous ammonium phosphate solution and reduce the ammonium ion concentration to substantially the same level present in said first aqueous ammonium phosphate solution, and wherein said vapor stream comprising ammonia is recycled to the reactor through a line, said wet oxidation reactor and said line being constructed of a material that is not susceptible to corrosion by ammonium carbamate.

5. The process of claim 1, wherein the heating of said stripped second ammonium phosphate solution to an elevated temperature takes place in a wet oxidation reactor at wet oxidation conditions to simultaneously remove unwanted impurities from said second aqueous ammonium phosphate solution and reduce the ammonium ion concentration to substantially the same level present in said first aqueous ammonium phosphate solution, and wherein the so-generated vapor stream comprising ammonia is treated to reduce the concentration of any ammonium carbamate therein.

6. The process of claim 1, wherein said aqueous stream is recycled to said quench zone.

7. The process of claim 6, wherein at least a portion of said aqueous stream is subjected to a wet oxidation reaction at wet oxidation conditions to remove unwanted impurities from said aqueous stream prior to recycle to said quench zone.

8. The process of claim 1, wherein said vapor stream comprising ammonia is recycled to said reactor, said vapor stream having been contacted with caustic material to convert any ammonium carbamate to a carbonate.

9. The process of claim 1, wherein said vapor stream comprising ammonia is recycled to said reactor through ammonia purification equipment and wherein the temperature of the ammonia purification equipment is maintained at a temperature above the condensation temperature of the vapor stream.

10. The process of claim 1, wherein said vapor stream comprising ammonia is recycled to said reactor through ammonia purification equipment and wherein the ammonia purification equipment is constructed of a material that is not susceptible to corrosion by ammonium carbonate.

11. The process of claim 1, wherein said first aqueous ammonium phosphate solution has a pH of 3.5 or less.

12. A process for the recovery of ammonia from a reactor effluent stream comprising: contacting a gaseous reactor effluent stream containing ammonia with a first aqueous ammonium phosphate solution, in a quench zone, to absorb substantially all of the ammonia present in the reactor effluent stream to form a second aqueous ammonium phosphate solution richer in ammonium ions than said first aqueous ammonium phosphate solution; heating said second aqueous ammonium phosphate solution in a stripping zone to remove volatile impurities contained in said second aqueous ammonium phosphate solution and to form a stripped second ammonium

phosphate solution; heating said stripped second ammonium phosphate solution, in a decomposition zone, to an elevated temperature sufficient to reduce the amount of ammonium ions in said second aqueous ammonium phosphate solution back to substantially the same level present in said first aqueous ammonium phosphate solution to thereby generate a vapor stream comprising ammonia and an aqueous stream.

13. The process of claim 12, wherein the heating of said stripped second ammonium phosphate solution to an elevated temperature takes place in a wet oxidation reactor at wet oxidation conditions to simultaneously remove unwanted impurities from said second aqueous ammonium phosphate solution and reduce the ammonium ion concentration to substantially the same level present in said first aqueous ammonium phosphate solution, a caustic material being added to said wet oxidation reactor to convert any ammonium carbamate formed to an insoluble carbonate.

14. The process of claim 12, wherein the heating of said stripped second ammonium phosphate solution to an elevated temperature takes place in a wet oxidation reactor at wet oxidation conditions to simultaneously remove unwanted impurities from said second aqueous ammonium phosphate solution and reduce the ammonium ion concentration to substantially the same level present in said first aqueous ammonium phosphate solution, and wherein said vapor stream comprising ammonia is recycled to the reactor through a line, the interior wall of which is maintained at temperature above the condensation temperature of the vapor stream.

15. The process of claim 12, wherein the heating of said stripped second ammonium phosphate solution to an elevated temperature takes place in a wet oxidation reactor at wet oxidation conditions to simultaneously remove unwanted impurities from said second aqueous ammonium phosphate solution and reduce the ammonium ion concentration to substantially the same level present in said first aqueous ammonium phosphate solution, and wherein said vapor stream comprising ammonia is recycled to the reactor through a line, said wet oxidation reactor and said line being constructed of a material that is not susceptible to corrosion by ammonium carbamate.

16. The process of claim 12, wherein the heating of said stripped second ammonium phosphate solution to an elevated temperature takes place in a wet oxidation reactor at wet oxidation conditions to simultaneously remove unwanted impurities from said second aqueous ammonium phosphate solution and reduce the ammonium ion concentration to substantially the same level present in said first aqueous ammonium phosphate solution, and wherein the so-generated vapor stream comprising ammonia is treated to reduce the concentration of any ammonium carbamate therein.

17. The process of claim 12, wherein said aqueous stream is recycled to said quench zone.

18. The process of claim 17, wherein at least a portion of said aqueous stream is subjected to a wet oxidation reaction at wet oxidation conditions to remove unwanted impurities from said aqueous stream prior to recycle to said quench zone.

19. The process of claim 12, wherein said vapor stream comprising ammonia is recycled to said reactor, said vapor stream having been contacted with caustic material to convert any ammonium

carbamate to a carbonate.

20. The process of claim 12, wherein said vapor stream comprising ammonia is recycled to said reactor through ammonia purification equipment and wherein the temperature of the ammonia purification equipment is maintained at a temperature above the condensation temperature of the vapor stream.

21. The process of claim 12, wherein said vapor stream comprising ammonia is recycled to said reactor through ammonia purification equipment and wherein the ammonia purification equipment is constructed of a material that is not susceptible to corrosion by ammonium carbonate.

22. The process of claim 12, wherein said first aqueous ammonium phosphate solution has a pH of 3.5 or less.

Description:[0001] The present invention relates to an improved process for the recovery and regeneration of ammonia, e.g., ammonia contained in the effluent obtained from a reaction zone where ammonia and oxygen are reacted with a paraffin to produce the corresponding aliphatic nitrile. In particular, the present invention relates to minimization of ammonium carbamate formation, and minimization of contamination in downstream processes resulting from the presence of ammonium carbamate such as, for example, in a process for the recovery and regeneration of unreacted ammonia contained in the effluent passing from a reaction zone wherein ammonia and oxygen are reacted with propane to produce acrylonitrile, or isobutane to produce methacrylonitrile.

[0002] U.S. Pat. Nos. 3,936,360 and 3,649,179 are each directed to a process for the manufacture of acrylonitrile utilizing propylene, oxygen and ammonia as the reactants. These gases are passed over a catalyst in a fluid bed reactor to produce acrylonitrile which passes from the reactor to a recovery and purification section. This reaction also has some unreacted ammonia which is typically removed from the process by treatment in the quench column with an acid. The '179 patent discloses that the quench acid may be either sulfuric, hydrochloric, phosphoric or nitric acid. The '360 patent discloses the use of sulfuric acid in the quench to remove the unreacted ammonia. In the manufacture of acrylonitrile using propylene as the hydrocarbon source, the preferred embodiments clearly utilize sulfuric acid with the resulting formation of ammonium sulfate. Typically, the ammonium sulfate is either recovered and sold as a co-product (fertilizer) or combined with other heavy organics produced in the process and deep-welled for environmentally safe disposal.

[0003] British Patent 222,587 is directed to ammonia recovery from an ammonia-containing gas mixture utilizing an aqueous phosphoric acid solution, an aqueous solution of ammonium hydrogen phosphate, or mixtures thereof. The ammonia is recovered by heat decomposition and dissolving the resulting residue in water to regenerate the ammonia recovery phosphate solution. This ammonia recovery process is directed to the recovery of ammonia from coal gas or coke ovens at temperatures of 50° C. to 70° C.

[0004] U.S. Pat. Nos. 2,797,148 and 3,718,731 are directed to the recovery of ammonia from a process stream used in the production of HCN. The process of recovery uses an ammonium phosphate solution to capture the ammonia and then uses steam stripping to regenerate the ammonia from the ammonium phosphate solution. Typically, the process is operated by contacting the ammonia-containing gas with a 25% to 35% by weight ammonium phosphate solution having a pH of about 6 at a temperature of between 55° C. to 90° C. The processes in each of these patents disclose that the ammonium ion/phosphate ion ratio is at least 1.2 or greater.

[0005] U.S. Pat. No. 5,895,635 discloses a process for the recovery or regeneration of ammonia contained in the effluent from a reactor zone where ammonia, oxygen and propane/isobutane are reacted to produce acrylonitrile/methacrylonitrile. The process of recovery uses an ammonium phosphate quench solution to capture the ammonia and then regenerates the ammonium phosphate quench solution by subjecting the quench solution to elevated temperatures and pressure in order to decompose the ammonium phosphate salt. The disclosed process provides several advantages in propane ammoxidation compared to propylene ammoxidation to acrylonitrile including: (1) complete capture of by-product acrolein, thus enhancing product recovery efficiency by minimizing loss of product through, for example, reaction of acrolein with HCN in the product separation and recovery train of the process, (2) lower TOC (Total Organic Carbon) in the quench bottoms, (3) higher percentage of organics present in the quench bottoms are present as strippable/recoverable monomers instead of unrecoverable waste polymers, and (4) the ability to use a lower severity waste organic treatment (e.g., wet oxidation) because of the presence of lower TOC and polymers in the quench bottoms solution. An additional disclosed feature of the process is that all the waste water streams may be readily handled by conventional biotreatment processes unlike the waste streams associated with propylene ammoxidation to manufacture acrylonitrile.

[0006] Despite the aforementioned advantages, a significant drawback to ammonia recovery processes exists. For example, the operation of the process as disclosed in U.S. Pat. No. 5,895,635 will lead to the formation of significant amounts of ammonium carbamate, a corrosive agent which will aggressively attack piping and equipment in the ammonia recovery process. Ammonium carbamate can also accumulate in the ammonia purification equipment, such as distillation columns, until a critical threshold concentration is achieved, at which point the carbamate is forcefully ejected from the column, creating transient pressure surges which interrupt steady-state operation and cause wide fluctuations in ammonia purity. Additionally, the presence of ammonium carbamate can lead to significant contamination of other systems, such as acrylonitrile reaction systems, which receive recycle ammonia from this process. Therefore, the industry would welcome the discovery of a process that maintains the aforementioned benefits while improving upon the prior art by minimizing the formation of ammonium carbamate in the ammonia recovery process and avoiding as much as possible process upsets and the transfer of contaminants (resulting from the presence of ammonium carbamate) to other systems, such as acrylonitrile reaction systems, which receive recycle ammonia from this process.

[0007] Accordingly, one object of the present invention is to provide an improved process for the recovery and/or regeneration of ammonia contained in the effluent from a reactor zone where an ammonia, oxygen and propane/isobutane are reacted to produce acrylonitrile/methacrylonitrile.

[0008] Another object of the present invention is to provide a process which minimizes the formation of ammonium carbamate in an ammonia recovery process.

[0009] Yet another object of the present invention is to provide a process which minimizes the accumulation of ammonium carbamate, thereby simultaneously minimizing down stream process upsets which used to result from said accumulation.

[0010] Still another object of the present invention is to provide a process which minimizes the transfer of contaminants, resulting from the presence of ammonium carbamate, to down stream processes—such as acrylonitrile reaction systems, which receive recycle ammonia from this process.

[0011] These and other objects, as well as other aspects and features and advantages of the present invention, will become apparent for those skilled in the art from consideration of the specification including the drawings and appended claims.

[0012] To achieve the foregoing objects, the present invention provides a novel process of recovering unreacted ammonia from a reactor effluent, e.g., from the effluent of a reactor wherein oxygen, ammonia and a hydrocarbon are reacted in the presence of a catalyst. This process comprises at least the following steps: (1) quenching a fluid bed reactor effluent containing unreacted ammonia with a first aqueous ammonium phosphate quench solution, thereby absorbing ammonia to form a second aqueous ammonium phosphate solution richer in ammonium ions than the first solution and (2) heating the second solution to an elevated temperature to reduce the amount of ammonium ions present to substantially the same level present in the first solution and generate a vaporous stream containing ammonia.

[0013] In one embodiment, the second aqueous ammonium phosphate solution is treated by means of a stripping gas which is substantially free of CO 2 to remove substantially all of the acrylonitrile and other useful co-products from the second solution, without increasing the CO 2

content of the second solution, prior to heating the solution to decrease the NH 4 + ion content.

[0014] In another embodiment, the second aqueous ammonium phosphate solution is heated in a stripping zone to remove substantially all of the acrylonitrile and other useful co-products from the second solution prior to heating the solution to decrease the NH 4

+ ion content.

[0015] Preferably, the temperature of the first solution is between 40° C. and 80° C., and more preferably between 50° C. and 65° C.

[0016] Typically, the first quench solution has an ammonium/phosphate ratio of 1.0 or lower, preferably between 0.2 and 0.95, and more preferably between 0.6 and 0.95. The resulting pH of the first quench solution is typically between 0.9 and 3.5 The phosphate ion concentration in the first quench solution can be up to 40% by weight, preferably up to about 35% by weight.

[0017] In a still further embodiment, iron contamination of a reaction system, which receives ammonia from an ammonia recovery process, is avoided by one or more of the following techniques:

[0018] a. Physically separating iron oxide, iron containing colloidal particles, and liquid droplets from the gas stream;

[0019] b. Preventing ammonium carbamate from depositing on piping and equipment by minimizing condensation, thereby preventing corrosion of the piping; or

[0020] c. Installing piping and equipment that is not susceptible to corrosive attack by ammonium carbamate, thereby eliminating the source of iron contamination.

[0021] The present invention will be further understood by the following detailed description and reference to the accompanying drawings briefly described below:

[0022] FIG. 1 is a flow diagram of one embodiment of the present invention.

[0023] FIG. 2 is a flow diagram of another preferred embodiment of the present invention.

[0024] FIG. 3 is a plot of pH vs. N:P ratio at 60° C.

[0025] FIG. 4 is a simplified drawing of one possible apparatus which can be used for separating liquids, colloids, and particulates from a gas stream.

[0026] Ammonia reacts with carbon dioxide to yield ammonium carbamate (AC, eq. 1). 1

[0027] AC can dissolve into liquid that condenses on the inside wall of process piping and equipment where it can react with the iron in carbon steel to produce iron oxide (Eq. 2). Iron oxide is abrasive and particularly damaging to rotating equipment, such as ammonia compression equipment, and at high temperatures can also catalyze the decomposition of ammonia, lowering yields in processes such as acrylonitrile reaction systems. In many processes, particulate filters are installed to trap iron oxide. However, it has been discovered that, if cyanide is also present in the gas stream, iron oxide will react with cyanide to yield iron hexacyano complexes (IHC) that exist as colloidal suspensions of the corresponding ammonium salts (Eq. 3). Such colloidal suspensions are not removed by particulate filtration and pass to downstream processes where the iron hexacyano complexes can be converted back into iron oxide by reaction

with oxygen in the presence of heat (Eq. 4). 2

[0028] These problems are minimized by the present invention. For example, the present invention, among other things, (1) minimizes the absorption of CO 2 in recovery processes, (2) removes IHC from process streams, (3) prevents condensation on the walls of the piping and equipment which will dissolve AC from gas streams, and/or (4) uses equipment and piping that is constructed of material that is not susceptible to corrosion by AC.

[0029] The present invention is directed to an improved process of quenching the effluent obtained from a propane ammoxidation reaction zone wherein the aforementioned problems associated with AC and its corrosion products are minimized. For example, in an ammoxidation reaction, the reaction typically takes place in a fluid bed reactor, although other types of reactors such as transport line reactors are envisioned as suitable when practicing the present invention. Fluid bed propane ammoxidation reaction conditions and fluid bed catalyst useful in propane ammoxidation are known in the art as evidenced by U.S. Pat. No. 4,746,641 herein incorporated by reference. The novel process of the present invention comprises quenching the reactor effluent obtained from the reaction of ammonia, oxygen and a hydrocarbon (e.g., propane and/or isobutane) in a reaction zone (e.g. fluid bed reactor) to produce a reaction product (e.g., acrylonitrile) with a first aqueous ammonium phosphate solution having a pH of about 3.5 or less, with a resulting ratio of ammonium ions to phosphate ions of not more than 1.0 (refer to FIG. 3 ). In such a process, ammonia is absorbed to produce a second ammonium phosphate solution richer in ammonium ions than the first solution but preventing significant absorption of CO 2 . Then, the second solution is heated to an elevated temperature to reduce the ammonium ion content therein to substantially the same ammonium ion content present in the first solution and generate a vaporous stream comprising ammonia, water, and CO 2 . Thereafter, the molar concentration of ammonia in this vapor stream is increased. The vaporous stream containing ammonia can then be recycled to a fluid bed reactor, in a manner which minimizes iron oxide contamination of the reactor.

[0030] Preferably the pH of the first quench solution is between 1.5 and 3.3. This results in an ammonium ion/phosphate ion ratio (N:P ratio) of between about 0.3 and about 0.95. More preferably, the pH of the first quench solution is between about 1.9 and 3.0. This results in an N:P ratio of about 0.5 to 0.90.

[0031] The temperature of the first quench solution is usually between about 40° C. and about 80° C., preferably between about 50° C. and about 65° C., and more preferably between about 55° C. and about 60° C.

[0032] Given the objective of minimizing CO 2 absorption in the ammonium phosphate quench solution, one of average skill might conclude that increasing the temperature of the quench solution and perhaps lowering the operating pressure of the quench column would suffice; while it is true that such changes will reduce CO 2 absorption, they will also tend to reduce absorption of all other components in the gas stream as well, including acrolein. As it is an objective of the present invention to maintain the benefits of the prior art, such a reduction in acrolein absorption would not be desirable in the chemical production industry.

[0033] In the present invention, the utilization of a first quench solution of low pH (e.g., a pH ranging from about 3.5 or less) serves to block absorption of all weak acidic species. Since acrolein is not an acid compound, it is unaffected by pH changes. Because carbonic acid (the aqueous form of CO 2 ) has a pH of about 3.2, its tendency to absorb in the low pH solution is greatly reduced. Thus, the method of the present invention allows the temperature of the first solution to be kept low, maintaining the efficacy of acrolein absorption, while simultaneously minimizing absorption of CO 2 . It should also be noted that an additional benefit of the present invention is that the absorption of HCN (pH of about 4.7), another undesirable occurrence, is

also minimized when the pH of the first solution is low. FIG. 3 is a graphical representation of the relationship between pH and the N:P Ratio of a phosphate solution. The data shown in FIG. 3 are for a typical aqueous phosphate solution comprising 30% H 3 PO 4 at 60° C. and 40 psia. From FIG. 3 , it can be seen that the pH of the phosphate solution increases with increasing N:P ratio. For example, at an N:P ratio of 0.0, the phosphate solution will have a pH of about 0.7; at an N:P ratio of 1.0, the solution will have a pH of about 3.6; and, at an N:P ratio of 2.0, the solution will have a pH of about 7.6.

[0034] Similar data may be obtained for phosphate solutions at different H 3 PO 4 concentrations, temperatures and/or pressures, as is within the ability of one of ordinary skill in the art. In general, the trend of FIG. 3 (increasing pH with increasing N:P ratio) remains the same with changing H 3 PO 4 concentration, although the specific pH at a given N:P ratio decreases slightly with increasing H 3 PO 4 concentration. For purposes of comparison, it should be noted that an aqueous phosphate solution comprising 20% H 3 PO 4 (not shown in the figure) will have a pH of about 3.9 at an N:P ratio of 1.0, while an aqueous phosphate solution comprising 40% H 3 PO 4

(not shown in the figure) will have a pH of about 3.3 at an N:P ratio of 1.0.

[0035] With the benefit of this disclosure and pH vs. N:P ratio data such as that shown in FIG. 3 , one of ordinary skill in the art can adjust the relative concentration of ammonium ions to phosphate ions (N:P ratio) in the phosphate solution to achieve pH values in accordance with the method of the present invention.

[0036] In one embodiment of the present invention, the low pH of the first solution is maintained by purging at least a portion of the monoammonium phosphate and adding fresh, make-up phosphoric acid, in a quantity sufficient to achieve the desired pH, prior to introduction to the quench column.

[0037] In another embodiment of the present invention, the low pH of the first solution is maintained by thermally decomposing at least a portion of the monoammonium phosphate solution to generate free ammonia and phosphoric acid, in a quantity sufficient to achieve the desired pH, prior to introduction to the quench column.

[0038] In still another embodiment of the present invention, the low pH of the first solution is maintained by oxidizing at least a portion of the monoammonium phosphate in a wet oxidation process to generate oxides of nitrogen and phosphoric acid, in a quantity sufficient to achieve the desired pH, prior to introduction to the quench column.

[0039] When the aqueous monoammonium phosphate solution is utilized, the unreacted ammonia present in the reactor effluent converts the monoammonium phosphate to diammonium phosphate. During the quench procedure, the products (e.g, acrylonitrile, acetonitrile and/or HCN) are removed as overheads and are substantially free of ammonia. The quench solution bottom containing the diammonium phosphate also contains residual monomers (e.g. acrylonitrile) in small quantities. These monomers are, preferably, stripped and returned to the quench for further recovery and purification. Typical stripping gases for removal of the residual monomers from the quench bottoms comprise propane, nitrogen, and carbon monoxide or mixtures thereof; however, it is necessary that whatever gas is employed be substantially free of

CO 2 content to avoid absorbing additional CO 2 into the solution. By substantially free it is meant that the gas contains less than 10% CO 2 , preferably less than 5% CO 2 , most preferably less than 1% CO 2 . In a preferred embodiment, the stripping gas is a recycle stream, derived from the effluent of the acrylonitrile product purification system; CO 2 removal methods known in the art, such as adsorption or membrane separations, are used to purify the effluent such that the gas is substantially free of CO 2 before being used for stripping. (Alternatively, the residual monomers may be stripped by heating the quench bottoms so as to drive the residual monomers out of the solution without the need for a stripping gas.) The quench bottoms solution stripped of useful monomers are then regenerated at an elevated temperature and pressure to convert the diammonium phosphate back to monoammonium phosphate with the release of ammonia. The monoammonium phosphate is recovered and recycled back into the quench column. The ammonia is captured as a vapor stream which contains water and CO 2 . This ammonia-rich vapor stream is heated to remove substantially all the water and the ammonia is then recycled back to the reactor. However, ammonia and CO 2 can react in the ammonia purification step as described above to form AC.

[0040] In a preferred embodiment, a caustic material is added in this purification step to convert any AC to an insoluble carbonate. Suitable caustic materials include NaOH, KOH, MgOH, CaOH and the like, as well as mixtures thereof.

[0041] In one embodiment of a typical prior art process, the stripped quench bottom containing the diammonium phosphate is passed through a wet oxidation reactor where it is treated under typical wet oxidation conditions to remove any polymers formed during the ammoxidation process. In so doing, however, the prior art process creates CO 2 within the process that will lead to the formation of ammonium carbamate. In an embodiment of the present invention wherein this prior art step is utilized, a caustic material is added to this step to convert the AC to an insoluble carbonate. Suitable caustic materials include NaOH, KOH, MgOH, CaOH and the like, as well as mixtures thereof.

[0042] In a further preferred embodiment of the present invention, the stripped quench bottom containing unrecoverable monomers and diammonium phosphate is separately treated in a phosphate decomposing unit which separates the diammonium phosphate from the residual monomers. The diammonium phosphate is then regenerated back to the monoammonium phosphate in a separate unit while the residual polymers are transferred to a wet oxidation unit for wet oxidation under conventional temperatures and pressure to produce harmless by-products such as carbon dioxide and water.

[0043] Reference will now be made to FIGS. 1 through 4 , which are illustrative of some embodiments of the present invention wherein the process is applied to propane ammoxidation.

[0044] Referring to FIG. 1 , reactor effluent obtained by the direct reaction of propane, ammonia and oxygen in the fluid bed reactor (not shown) over a fluid bed ammoxidation catalyst is passed via line 1 into quench column 3 . In quench column 3 , the reactor effluent containing product acrylonitrile and unreacted ammonia is contacted with a lean ammonium/phosphate quench solution of pH 3.5 or less which strips unreacted ammonia from the effluent without absorbing significant CO 2 , producing an ammonia-free product overhead stream containing crude

acrylonitrile. The crude acrylonitrile passes overhead via line 5 into conventional recovery and purification sections (not shown) for subsequent recovery of commercially pure acrylonitrile, crude acetonitrile and hydrogen cyanide. Examples of conventional recovery and purification procedures can be found in U.S. Pat. No. 3,936,360 incorporated by reference herein. The quench bottoms leave quench column 3 via line 7 and enter a quench stripper 9 . A stripping gas, substantially free of CO 2 , comprising a recycle stream comprising a mixture of propane, carbon monoxide, and nitrogen is passed via line 13 into stripper 9 to remove any residual volatile impurities, such as, for example, acrylonitrile, acetonitrile or hydrogen cyanide contained in the quench bottoms. (Volatile is used in a comparative sense as to the ammonia chemically bound in the ammonium phosphate solution.) Alternatively, the quench bottoms may be fed to stripper 9 where they are heated so as to drive off any of the residual volatile impurities albeit at a temperature lower than that used to cause decomposition of the diammonium phosphate present in the quench bottoms. The overhead stripper gas 13 containing these residual monomers is recycled back into quench column 3 via line 11 for further recovery of useful products. The stripped quench bottoms are passed from stripper 9 via line 15 into a wet oxidation reactor 17 wherein oxygen is passed via line 25 and a conventional catalytic wet oxidation take place to remove unwanted impurities such as polymers. In addition, the diammonium phosphate contained in the quench stripper bottoms is heated to free the ammonia and convert the diammonium phosphate in solution to monoammonium phosphate. An optional caustic material is added to wet oxidation reactor 17 to convert ammonium carbamate to an insoluble carbonate. Suitable caustic materials include NaOH, KOH, MgOH, CaOH and the like, as well as mixtures thereof.

[0045] The monoammonium phosphate solution is passed from reactor 17 via line 27 into evaporator 19 where excess water is removed from the solution. This excess water is passed from evaporator 19 via line 21 for recycle or disposal. The concentrated weak monoammonium phosphate solution is passed from evaporator 19 via line 23 for recycle into quench column 3 . The ammonia released during the heat treatment in wet oxidation reactor 17 is passed from reactor 17 via line 29 for recycle directly into the fluid bed reactor (not shown). Any CO 2

produced in the wet oxidation process can react with the ammonia according to reaction 1 to form AC. If condensation occurs on the inside wall of line 29 , dissolved AC can corrosively attack the piping material. In one embodiment of the present invention, the temperature of line 29 is maintained high enough to prevent condensation on the inside of the line. The temperature of the line may be maintained by heating the line with steam or electrical tracing or by jacketing. Insulation may also be present. In one embodiment of the present invention, the temperature of the line is maintained above the condensation temperature of the gas and below about 350° C. More preferably, the temperature of the line is maintained in the range from about 70° C. to about 200° C.

[0046] In still another embodiment of the present invention, line 29 and reactor 17 are constructed of a material that is not susceptible to corrosion by AC. In one embodiment of the present invention, line 29 and reactor 17 are constructed from a metal that has a lower iron content than carbon steel. Preferred material include stainless steel, L series stainless steel, Duplex 2205, Hastelloys, Inconels, and Zirconium. In one embodiment of the present invention, line 29 and reactor 17 are constructed from Type 316L stainless steel. In an alternative embodiment of the present invention, the inside wall of line 29 and reactor 17 are lined with a

non-metallic such as Teflon or glass. It is further contemplated that in some situations it may be advantageous to construct line 29 and reactor 17 from different corrosion-resistant materials, and further that equipment, such as the reactor itself, may employ more than one material of construction—for example, the reactor wall may be lined/clad with a non-metallic material such as glass or resin and the internals of the reactor may be of corrosion resistant metal.

[0047] Typical wet oxidation conditions are utilized for the destruction of the unwanted polymers obtained during the process. Typical catalysts for wet oxidation are soluble salts of copper and iron, oxides of copper, zinc, manganese and cerium and noble metals and are well known in the prior art. See, for example, Ind. Eng. Chem. Res., 1995 Vol. 34, Pages 2-48, incorporated by reference herein. The wet oxidation reaction is designed for normal operation. Typically, wet oxidation is run at a pressure of between about 600 to 3000 psia and a temperature of 200° C. to 650° C.

[0048] In an alternate embodiment, the ammonia containing gas in line 29 is treated to provide a gas with reduced ammonium carbamate (AC) concentration through one or more of the following methods:

[0049] contacting the gas with a scrubbing solution to convert AC to carbonate, wherein the scrubbing solution comprises a caustic material (suitable caustic materials are as previously described)

[0050] contacting the gas with a basic Ion Exchange Resin to remove AC

[0051] contacting the gas with an adsorbent to remove free CO2 and/or AC

[0052] condensing the gas, contacting the gas with sacrificial iron, such as a carbon steel mesh, to react away the AC, and then heating the condensate sufficiently to revaporize the ammonia

[0053] condensing the gas, contacting the condensate with caustic material to convert AC to carbonate (suitable caustic materials are as previously described), and then heating the condensate sufficiently to revaporize the ammonia

[0054] condensing the gas, contacting the condensate with an electrode to electrolytically decompose the AC, and then heating the condensate sufficiently to revaporize the ammonia

[0055] subjecting the gas to elevated temperature and pressure sufficient to convert the AC to urea, and then removing the urea.

[0056] With reference to FIG. 2 a further preferred embodiment of the present invention is described. The process illustrated in FIG. 2 is substantially the same as that of FIG. 1 except that the phosphate decomposition takes place in a separate unit followed by wet oxidation in a different unit. The reactor effluent obtained by the direct ammoxidation of propane, oxygen and ammonia in a fluid bed reactor (not shown) is passed from the fluid bed reactor via line 2 into quench 4 . The reactor effluent containing crude acrylonitrile and unreacted ammonia is contacted in quench 4 with an aqueous monoammonium phosphate solution of pH 3.5 or less

which enters quench 4 via line 40 . The phosphate solution removes the unreacted ammonia from the reactor effluent without absorbing significant CO 2 , allowing the ammonia-free products (crude acrylonitrile) to pass overhead from quench 4 via line 6 . The crude acrylonitrile passing overhead via line 6 is directed to a conventional recovery and purification section for recovery of commercially pure acrylonitrile, crude acetonitrile and HCN. Quench bottoms are passed from quench 4 via line 8 into quench stripper 10 where a stripping gas (having the same composition as described above) enters the lower portion of the bottom stripper 10 via line 14 and passes upward through the quench bottoms to strip the quench bottoms of any useful monomers present in the bottoms such as acrylonitrile, acetonitrile and hydrogen cyanide. The stripper gas containing useful monomers is then passed from stripper 10 overhead via line 12 into quench 4 for further recovery and purification. As in the previous embodiment, these useful monomers can also be recovered by merely heating the quench bottoms in quench stripper 10 . The stripped quench bottoms move from stripper 10 via line 16 to phosphate decomposer 18 . In phosphate decomposer 18 , the diammonium phosphate present in the stripped quench bottom is converted to free ammonium and monoammonium phosphate by heating to an elevated temperature (100° C. to 300° C.). Typically, the pressure is between 1 to 5 atmospheres (atmospheric to 75 psia). Oxygen may be present but is not required. The resulting monoammonium phosphate solution is passed from decomposer 18 via line 34 for recycle via line 40 into quench 4 . The free ammonia generated during phosphate conversion in reactor 18 is passed via overhead line 20 into an ammonia rectification unit 22 wherein the free ammonia is purified and passed on to ammonia stripper 28 via line 26 to recover the ammonia for recycle into the reactor (not shown) for manufacture of acrylonitrile or may be recycled via line 24 to rectification unit 22 prior to going to ammonia stripper 28 . Water is recovered from ammonia stripper unit 28 and passed via line 32 for recycle or disposal. Caustic material (not shown) is added to ammonia stripper 28 directly, or optionally, indirectly via lines 26 or 39 , to convert AC to insoluble carbonate. Caustic material may optionally be added to rectification unit 22 as well. Suitable caustic materials include NaOH, KOH, MgOH, CaOH and the like, as well as mixtures thereof.

[0057] Because Ammonia and CO 2 are present in this part of the process, AC will form according to reaction 1 , above, and Lines 24 , 26 , 39 , and 30 , as well as stripper 28 , its condenser, and the condenser on column 22 (here forward known as the “NH 3 purification equipment”) will be exposed to AC.

[0058] If condensation occurs on the inside wall(s) of the NH 3 purification equipment, the dissolved AC can corrosively attack this equipment. In one embodiment of the present invention, the temperature of line 30 is maintained high enough to prevent condensation on the inside of the line. The temperature of the “NH 3 purification equipment” may be maintained by heating with steam or electrical tracing or by jacketing. Insulation may also be present. In one embodiment of the present invention, the temperature of the “NH 3 purification equipment”is maintained above the condensation temperature of the gas and below about 350° C. More preferably, the temperature of the line is maintained in the range from about 70° C. to about 200° C.

[0059] In still another embodiment of the present invention, the “NH 3 purification equipment”is constructed of a material that is not susceptible to corrosion by AC. In one embodiment of the present invention, the “NH 3 purification equipment”is constructed from a metal that has a lower iron content than carbon steel. Preferred material include stainless steel, L series stainless steel,

Duplex 2205, Hastelloys, Inconels, and Zirconium. In one embodiment of the present invention, the “NH 3 purification equipment”is constructed from Type 316L stainless steel. In an alternative embodiment of the present invention, the inside wall(s) of the “NH3 purification equipment” are lined with a non-metallic such as Teflon or glass. It is further contemplated that in some situations it may be advantageous to construct different components of the “NH 3 purification equipment” from different corrosion-resistant materials, and further that equipment, such as columns 22 and 28 and their respective condensers, may employ more than one material of construction—for example, in the case of the condenser of stripper 28 , the condenser tubesheet may be lined/clad with a non-metallic material such as glass or resin and the tubes may be of corrosion-resistant metal.

[0060] The NH 3 is passed from stripper 28 via line 30 for recycle or is passed via line 39 to stripper 28 for processing prior to entry into line 30 for recycle. In one embodiment of the present invention, gas exiting via line 30 is transferred to optional compressor 42 . In one embodiment of the present invention, compressor 42 is constructed from materials that are resistant to corrosion by AC. Suitable materials are as listed above. In an especially preferred embodiment, the compressor is operated at an elevated temperature, such that the gas is discharged at a temperature between about 80° C. and 350° C. In an alternative embodiment of the present invention, optional compressor 42 is absent and lines 30 and 44 are contiguous.

[0061] In one aspect of the present invention, lines 30 and 44 are constructed of a material that is not susceptible to corrosion by AC. Suitable material are as listed above. In one embodiment of the present invention, lines 30 and 44 are constructed from 316L stainless. In an alternative embodiment of the present invention, the inside walls of lines 30 and 44 are lined with a non-metal material, preferably Teflon or glass.

[0062] In another embodiment of the present invention, the temperature of the gas inside lines 30 and 44 is maintained high enough to prevent condensation in these lines or in related equipment. In one embodiment of the present invention, lines 30 and 44 as well as any intervening equipment are heated with steam or electrical tracing to prevent condensation on the inside of the lines. Alternatively, lines 30 and 44 and any intervening equipment are heated with jacketing. Insulation may also be present. In these embodiments, the lines and equipment are maintained above the condensation temperature of the gas and below about 350° C., more preferably between 70° C. and 200° C.

[0063] In another embodiment of the present invention, the gas in lines 30 and 44 is passed through at least one heat exchanger to elevate and maintain the temperature of the gas above its condensation temperature and below about 350° C. More preferably, the temperature of the gas is maintained in the range from about 70° C. to about 200° C.

[0064] Condensation can also be minimized by operating ammonia stripper column 28 and its condenser such that the concentration of water in the purified gas stream entering line 30 is minimized. The concentration of ammonia in the gas stream entering line 30 is preferably greater than 75%, more preferably greater than 90%, and even more preferably greater than 95%.

[0065] In another embodiment of the present invention, gas in line 44 passes through a zone 46 where impurities are removed from the gas stream. The zone comprises a first component that separates colloidal particles and liquid droplets from the gas stream and a second component that separates particulate matter from the gas stream. In one embodiment of the present invention, the two components are combined into one apparatus. Referring to FIG. 4 , in one embodiment of the present invention, the gas in line 44 is directed into a chamber, wherein a vector change in the gas stream causes the colloidal material and liquid droplets entrained in the gas to impact internal structures, such as baffles, impingement plates, and (as shown here) the piping elbow, as well as the sides of the chamber. The colloid- and liquid-free gas then passes through particulate filtering media which is off the line of the impinging gas stream before exiting the chamber. In an alternative embodiment of the present invention, the zone in which impurities are separated from the gas stream may comprise one or more cyclones or impingement separators to physically remove droplets and colloidal materials from the gas stream followed by one or more filters to remove particulate from the gas stream.

[0066] In an alternate embodiment, the ammonia containing gas in line 30 is treated to provide a gas with reduced ammonium carbamate (AC) concentration through one or more of the following methods:

[0067] contacting the gas with a scrubbing solution to convert AC to carbonate, wherein the scrubbing solution comprises a caustic material (suitable caustic materials are as previously described)

[0068] contacting the gas with a basic Ion Exchange Resin to remove AC

[0069] contacting the gas with an adsorbent to remove free CO2 and/or AC

[0070] condensing the gas, contacting the gas with sacrificial iron, such as a carbon steel mesh, to react away the AC, and then heating the condensate sufficiently to revaporize the ammonia

[0071] condensing the gas, contacting the condensate with caustic material to convert AC to carbonate (suitable caustic materials are as previously described), and then heating the condensate sufficiently to revaporize the ammonia

[0072] condensing the gas, contacting the condensate with an electrode to electrolytically decompose the AC, and then heating the condensate sufficiently to revaporize the ammonia

[0073] subjecting the gas to elevated temperature and pressure sufficient to convert the AC to urea, and then removing the urea.

[0074] At least a portion of the weak monoammonium phosphate solution passed from decomposer 18 via line 34 may be sent through wet oxidation unit 38 via line 36 for removal of polymers and conversion of these unwanted materials into harmless by-products such as hydrogen, carbon monoxide and carbon dioxide.

[0075] As described previously, the wet oxidation may be performed under conventional conditions known in the art.

[0076] While the invention has been described in conjunction with specific embodiments herein, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly it is intended to embrace all such alternatives and modifications in variations as for within the spirit and broad scope of the appended claims.