JP4186132B2 - Process for recovering and recycling ammonia from an acrylonitrile reactor effluent stream using an ammonium phosphate quench system. - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a process for recovering and regenerating ammonia contained in an effluent obtained from a reaction zone in which ammonia and oxygen react with paraffin to produce the corresponding aliphatic nitrile. In particular, the present invention provides that ammonia and oxygen are contained in the effluent that has passed through a reaction zone that (1) reacts with propane to produce acrylonitrile or (2) reacts with isobutane to produce methacrylonitrile. It relates to the recovery and regeneration of reactive ammonia.
[0002]
[Prior art]
US Pat. Nos. 3,936,360 and 3,649,179 each relate to a process for the production of acrylonitrile using propylene, oxygen and ammonia as reactants. These gases are passed over the catalyst in the fluidized bed reactor to produce acrylonitrile, which is sent from the reactor to the recovery and purification section. The reaction system also has some unreacted ammonia, which is typically removed from the process by treatment with a quench column with acid. The '179 patent discloses that the quenching acid can be either sulfuric acid, hydrochloric acid, phosphoric acid or nitric acid. The '360 patent teaches the use of sulfuric acid in the quench to remove unreacted ammonia. In the production of acrylonitrile using propylene as the hydrocarbon source, the preferred embodiment clearly uses sulfuric acid, resulting in the formation of ammonium sulfate. Typically, this ammonium sulfate can be recovered and sold as a co-product (fertilizer) or combined with other heavy organics produced in this process and buried deep underground for environmentally safe disposal Either.
[0003]
GB 222,587 relates to recovering ammonium from an ammonia-containing gas mixture using an aqueous phosphoric acid solution, an aqueous solution of ammonium hydrogen phosphate ((NH ₄ ) H ₂ PO ₄ ), or mixtures thereof. The ammonia is recovered by pyrolysis and regenerating the ammonium recovery phosphate solution by dissolving the resulting residue in water. This ammonia recovery process relates to ammonia recovery from coal gas or coke ovens at temperatures between 50 ° C and 70 ° C.
[0004]
U.S. Pat. Nos. 2,797,148 and 3,718,731 relate to the recovery of ammonia from a process stream used to produce HCN. This recovery process uses an ammonium phosphate solution to capture ammonia and then uses steam stripping to regenerate the ammonia from the ammonium phosphate solution. Typically, this process is operated by contacting an ammonia-containing gas with a 25 wt% to 35 wt% ammonium phosphate solution having a pH of about 6 at a temperature between 55 ° C and 90 ° C. The regeneration of ammonia is performed by bringing the obtained ammonium phosphate solution into contact with steam. The process of each of these patents discloses that the ammonium ion / phosphate ion ratio is at least 1.2.
[0005]
[Problems to be solved by the invention]
The primary object of the present invention is to provide a process for recovering or regenerating ammonia contained in the effluent from the reaction zone where ammonia, oxygen and propane / isobutane react to produce acrylonitrile / methacrylonitrile. .
[0006]
Another object of the present invention is to avoid the need to discard excess ammonia resulting from ammoxidation of propane to acrylonitrile.
[0007]
Yet another object of the present invention is to recover ammonia from propane ammoxidation without significant loss of ammonia.
[0008]
Other objects and other aspects and features and advantages of the present invention will become apparent to those skilled in the art in view of the specification, including the drawings and claims.
[0009]
[Means for Solving the Problems]
To achieve the foregoing objectives and in accordance with the objectives of the invention as embodied and broadly described herein, oxygen, ammonia, and a hydrocarbon selected from the group consisting of propane and isobutane are ammoxidated. The process of recovering unreacted ammonia from the effluent obtained from the reaction zone that reacts at high temperature in the presence of a catalyst to produce the corresponding nitrile includes the following steps: (1) The corresponding nitrile and unreacted The fluidized bed reactor effluent containing the reaction ammonia is quenched with a first aqueous ammonium phosphate quench solution, thereby absorbing the ammonia and enriching the ammonium (NH ₄ ⁺ ) ions richer than the first solution. and forming a second ammonium phosphate solution, first ammonium in ammonium phosphate solution ion (NH ₄ ⁺⁾ and phosphorus San'i Although the ratio of the emission (PO ₄ ^-3) is at least about 0.7 is about 1.3 or less, step, (2) heating the second solution to a high temperature, the amount of ammonium ions present in the second solution Reducing to substantially the same level as present in the first solution and generating an ammonia-containing vapor stream and recycling the ammonia-containing vapor stream to the ammoxidation reaction zone.
[0010]
In a preferred embodiment of the process of the present invention, prior to heating the second aqueous ammonium phosphate solution to reduce the NH ₄ ⁺ ion content, the solution is treated with a stripping gas and substantially free from the second solution. All acrylonitrile and other useful coproducts are removed.
[0011]
In another preferred embodiment of the invention, the acrylonitrile-containing stripper gas is recycled to recover and purify acrylonitrile.
[0012]
In a further preferred embodiment of the invention, the second solution after stripping and ammonia removal is transferred to a wet oxidation reactor, whereby the solution is subjected to wet oxidation at high temperature and pressure to remove heavy organics contained in the quench solution. To do.
[0013]
In a further preferred embodiment of the invention, the second solution after removal of ammonia is transferred to an evaporator to remove excess water from the ammonium phosphate solution, which is then recycled for use in the quench section.
[0014]
Preferably, the temperature of this first solution is between 40 ° C. and 80 ° C., particularly preferably 50 ° C. to 65 ° C.
[0015]
Typically, the ammonium / phosphate ratio of the first quench solution is between 0.7 and about 1.3, preferably between about 0.9 and about 1.2, particularly preferably between about 1.0 and about 1.2. The pH of the resulting first quench solution is between 2.8 and about 6. The phosphate ion concentration in the first quench solution can be up to 40% by weight, preferably up to about 35% by weight.
[0016]
From the reaction zone of the present invention, oxygen, ammonia, and a hydrocarbon selected from the group consisting of propane and isobutane react at high temperatures in the presence of an ammoxidation catalyst to produce the corresponding unsaturated nitrile. The process of recovering unreacted ammonia from the resulting effluent is: quenching the fluidized bed reactor effluent containing the corresponding nitrile and unreacted ammonia with the first aqueous ammonium phosphate solution into the reactor effluent. Absorbing substantially all unreacted ammonia present to form a second aqueous ammonium phosphate solution that is richer in ammonium ions than the first solution, wherein the ammonium ions in the first aqueous ammonium phosphate solution The ratio of (NH ₄ ⁺ ) to phosphate ions (PO ₄ ^-3 ) is between about 0.7 and about 1.3; the second solution is added to the second solution in the second solution; Heating the ammonia-containing vapor stream to a temperature sufficient to reduce the amount of ammonium ions to substantially the same level as that present in the first solution and to generate an ammonia-containing vapor stream; and Recycling to the fluidized bed reactor.
[0017]
In one embodiment, the hydrocarbon is propane.
[0018]
In another embodiment, the ratio of ammonium ions to phosphate ions in the first solution is between about 0.9 and about 1.2, preferably between about 1.0 and 1.2.
[0019]
In yet another embodiment, prior to heating the second ammonium phosphate solution to an elevated temperature, a stripper gas is passed through the second solution to remove substantially all corresponding residual nitrile from the second solution. Preferably, the hydrocarbon is propane. Preferably the ratio of ammonium ions to phosphate ions in the first solution is between about 0.9 and about 1.2, more preferably between about 1.0 and about 1.2. Also preferably, the pH of the first solution is between about 2.8 and about 6.0, more preferably between about 2.8 and about 5.8. Preferably, the first solution contains an aqueous solution containing monoammonium phosphate and phosphoric acid.
[0020]
In a preferred embodiment, the high temperature heating of the second solution is carried out in a wet oxidation reactor under wet oxidation conditions to remove unwanted impurities from the second solution and at the same time ammonium ions in the second solution. The concentration is reduced to substantially the same level as that present in the first solution.
[0021]
In yet other embodiments, a second solution containing substantially the same level of ammonium ions as present in the first solution is recycled for use in the quench step. Preferably, the second solution before recirculation is subjected to wet oxidation to remove unnecessary impurities.
[0022]
In yet other embodiments, the pH of the first solution is between about 2.8 and 6.0, preferably between about 2.8 and about 5.8.
[0023]
In yet another embodiment, the temperature of the first solution is between 40 ° C and 80 ° C.
[0024]
In yet another embodiment, the first solution contains an aqueous solution comprising monoammonium phosphate and phosphoric acid.
[0025]
The process of the present invention must be carried out with an ammonium salt-containing waste stream that either (1) is treated to recover its ammonium salt or (2) is disposed of in an environmentally safe manner. ) Is avoided over conventional processes used in the ammoxidation of propylene. Rather, according to the process of the present invention, ammonia is recovered and the quench solution is regenerated by subjecting the ammonium phosphate quench solution to high temperature and pressure to decompose the ammonium phosphate salt. The quench system of the present invention provides a further unexpected advantage in the ammoxidation of propane compared to the ammoxidation of propylene to acrylonitrile. Among these advantages are: (1) Acrolein, a byproduct, is completely trapped, so, for example, the reaction of acrolein with HCN in the process of product separation and recovery Product recovery efficiency is increased by minimizing product loss through the process; (2) Less total TOC (total organic carbon) in quench bottoms; (3) Organic matter present in quench bottoms A higher percentage of them are present as strippable / recoverable monomers rather than non-recoverable waste polymers; and (4) because there is less TOC and polymer present in the quench bottoms solution Use less severe organic waste treatment (eg wet oxidation). A further important and advantageous feature of the process of the present invention is that all wastewater streams can be easily handled by conventional bioprocessing processes, unlike the waste streams associated with propylene ammoxidation to produce acrylonitrile. It is in.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
Preferred embodiments of the invention will now be described in detail.
[0027]
The present invention relates to a process for quenching effluent from a propane ammoxidation reaction zone. Preferably, this reaction is carried out in a fluidized bed reactor, although other types of reactors (eg, transport line reactors) are envisioned as suitable in practicing the present invention. Reaction conditions for propane ammoxidation in a fluidized bed, and fluidized bed catalysts useful for propane ammoxidation, are assigned U.S. Pat.No. 4,746,641 (incorporated herein by reference). As is apparent from the art. The novel process of the present invention involves quenching the reactor effluent resulting from the reaction of ammonia, oxygen and propane with a first aqueous ammonium phosphate solution in a reaction zone that produces acrylonitrile (e.g., fluidized bed reactor), A step of absorbing ammonia thereby to produce a second ammonium phosphate solution richer in ammonium ions than the first solution (where the first aqueous ammonium phosphate solution is a ratio of ammonium ions to phosphate ions). Is at least 0.7 but less than 1.3), heating the second solution to an elevated temperature to reduce the ammonium ion content in the second solution to substantially the same ammonium ion content as present in the first solution. And generating a vapor stream containing ammonia and water, increasing the molar concentration of ammonia in the vapor stream And recirculating the ammonia-containing vapor stream to the fluidized bed reactor.
[0028]
Preferably, the ammonium ion / phosphate ion ratio is between about 0.9 and about 1.2, particularly preferably between about 1.0 and 1.2. The temperature of the first quench solution is usually between 40 ° C. and about 80 ° C., preferably between about 50 ° C. and 65 ° C., particularly preferably between about 55 ° C. and about 60 ° C.
[0029]
The pH of the first (initial) quench solution should be maintained between about 2.8 and below 6.0, preferably between 2.8 and about 5.8.
[0030]
Preferably, the initial quench solution contains a mixture of monoammonium phosphate / aqueous phosphoric acid (90% monoammonium phosphate-10% H ₃ PO ₄ ), but also a lean monoammonium phosphate aqueous solution. It is also assumed that the present invention is suitable for carrying out the present invention. When this aqueous solution of monoammonium phosphate is used, unreacted ammonium present in the reactor effluent is absorbed and monoammonium phosphate is converted to diammonium phosphate.
[0031]
During the quench process, the products (acrylonitrile, acetonitrile and HCN) are removed as overhead and it is substantially free of ammonia. Quench solution bottoms containing diammonium phosphate also contain a small amount of residual monomer (eg, acrylonitrile). These monomers are preferably stripped and returned to the quench section for further recovery and purification. Typical stripping gases for removing residual monomer from the quench bottoms include propane, nitrogen, carbon dioxide and carbon monoxide or mixtures thereof.
[0032]
The quenched bottoms solution stripped of useful monomers is then regenerated at high temperature and pressure to release diammonium phosphate to monoammonium phosphate by releasing ammonia. Ammonia is captured as a vapor stream containing water. This rich ammonia vapor stream is heated to remove substantially all of the water and then the ammonia is recycled to the reactor. Monoammonium phosphate is recovered and recycled to the quench column.
[0033]
In a further preferred embodiment of the invention, the stripped quench bottoms containing diammonium phosphate are passed through a wet oxidation reactor where they are treated under typical wet oxidation conditions during the ammoxidation process. The polymer formed in is removed.
[0034]
In a further preferred embodiment of the invention, stripped quench bottoms containing unrecoverable monomers and diammonium phosphate are treated separately in a phosphate decomposition unit that separates diammonium phosphate from residual monomers. To do. The diammonium phosphate is then regenerated to monoammonium phosphate in a separation unit, while the residual polymer is transferred to a wet oxidizer for wet oxidation, which is a harmless byproduct under normal temperature and pressure Product (eg, carbon dioxide and water).
[0035]
Reference is now made to FIGS. These figures illustrate a preferred embodiment of the process of the present invention applied to propane ammoxidation.
[0036]
Referring to FIG. 1, the reactor effluent obtained by direct reaction of propane, ammonia and oxygen with a fluidized bed ammoxidation catalyst in a fluidized bed reactor (not shown) is passed through line 1 to quench column 3. In quench column 3, the reactor effluent containing the product acrylonitrile and unreacted ammonia is contacted with a dilute ammonium / phosphate quench solution, which strips unreacted ammonia from the effluent to remove crude acrylonitrile. A product overhead stream containing no ammonia is produced. In order to subsequently recover industrially pure acrylonitrile, crude acetonitrile and hydrogen cyanide, this crude acrylonitrile enters the normal recovery and purification section (not shown) via line 5 from the top. An example of a conventional recovery and purification process can be found in US Pat. No. 3,936,360, the contents of which are incorporated herein by reference. The quench bottoms exit the quench column 3 and enter the quench stripper 9 via line 7. A stripping gas containing a recycle stream containing a mixture of propane, carbon monoxide, carbon dioxide and nitrogen is passed through line 13 to stripper 9 to remove residual acrylonitrile, acetonitrile or hydrogen cyanide contained in the quench bottoms. To do. The overhead stripper gas 13 containing these residual monomers is recycled to the quench column 3 via line 11 to further recover useful products. The stripped quench bottoms is passed from stripper 9 via line 15 to wet oxidation reactor 17, where oxygen is passed via line 25 and normal wet catalytic oxidation takes place, resulting in unwanted impurities (eg heavy ions). Coalescence) is removed. Further, the diammonium phosphate contained in the quench stripper bottoms is heated to release ammonia, and the diammonium phosphate in the solution is converted to monoammonium phosphate. This monoammonium phosphate solution is passed from reactor 17 via line 27 to evaporator 19, where excess water is removed from this solution. This excess water is routed from the evaporator 19 via line 21 for recycling or disposal. The concentrated weak monoammonium phosphate solution is passed from the evaporator 19 via line 23 to the quench column 3 for recirculation. Ammonia released during the heat treatment in the wet oxidation reactor 17 is passed directly from the reactor 17 via line 29 to a fluidized bed reactor (not shown) for recirculation.
[0037]
Typical wet oxidation conditions are used to decompose the unwanted polymer obtained during this process. Typical catalysts for wet oxidation include copper and iron soluble salts, copper, zinc, manganese and cerium oxides, and noble metals, which are well known in the prior art. See, for example, Ind. Eng. Chem. Res., 1995 Vol 34, pages 2-48, the contents of which are incorporated herein by reference. The wet oxidation reaction is designed for normal operation. Typically, wet oxidation is performed at a pressure of about 600 to 3000 psia and a temperature of 200 ° C to 650 ° C.
[0038]
With reference to FIG. 2, a further preferred embodiment of the present invention will be described. The process illustrated in FIG. 2 is substantially the same as the process of FIG. 1 except that the decomposition of the phosphate is performed in another unit followed by wet oxidation in a different unit. The reactor effluent obtained by direct ammoxidation of propane, oxygen and ammonia in a fluidized bed reactor (not shown) is passed from the fluidized bed reactor via line 2 to quench section 4. In the quench unit 4, the reactor effluent containing crude acrylonitrile and unreacted ammonia is brought into contact with the aqueous monoammonium phosphate solution that has entered the quench unit 4 via the line 40. This phosphate solution removes unreacted ammonia from the reactor effluent so that the ammonia free product (crude acrylonitrile) is passed through line 6 from the top of quench section 4. The crude acrylonitrile passed through line 6 from the top goes to the normal recovery and purification section for the recovery of industrially pure acrylonitrile, crude acetonitrile and HCN. Quench bottoms passes from quench section 4 via line 8 through quench stripper 10 where stripping gas (which has the same composition as above) enters the bottom of this bottom stripper 10 and this Passing through the quench bottoms, useful monomers (eg, acrylonitrile, acetonitrile and hydrogen cyanide) present in the bottoms are stripped from the quench bottoms. The stripper gas containing useful monomers is then passed from the top of stripper 10 via line 12 to quench section 4 for further recovery and purification. The stripped quench bottoms are transferred from stripper 10 via line 16 to phosphate decomposer 18. In the phosphate decomposer 18, the diammonium phosphate present in the stripped quench bottoms is converted to free ammonium and monoammonium phosphate by heating to a high temperature (100 ° C. to 300 ° C.). Typically, the pressure is between 1 and 5 atmospheres (atmospheric pressure to 75 psia). Oxygen may be present but is not required. The obtained monoammonium phosphate solution is passed from the decomposer 18 through the line 34 (for recirculation), through the line 40 and into the quench unit 4. Free ammonia generated during phosphate conversion in reactor 18 is passed through ammonia rectification unit 22 (where the free ammonia is purified) and through line 26 to ammonia stripper 28 to provide acrylonitrile. Ammonia is recovered for recycling to the production reactor (not shown). Water is collected from the ammonia stripper unit 28 and passed through line 32 for recirculation or disposal. Weak monoammonium phosphate solution passed from cracker 18 to line 34 removes the polymer and converts these unwanted materials into harmless by-products (e.g., hydrogen, carbon monoxide and carbon dioxide) Therefore, it may be sent to the wet oxidation unit 38 via the line 36. As described above, this wet oxidation can be performed under normal conditions known in the art.
[0039]
Although the invention is described herein with reference to specific embodiments thereof, it will be understood that various changes, modifications, and variations thereof will be apparent to those skilled in the art upon reading this specification. Should. Accordingly, it is to be understood that the invention disclosed herein is intended to include changes and modifications in these variations as falling within the scope of the appended claims.
[0040]
【The invention's effect】
A process for recovering unreacted ammonia from a reactor effluent obtained from a reaction zone used to produce acrylonitrile or methacrylonitrile, wherein the reactor effluent is quenched with an aqueous ammonium phosphate solution Wherein the ratio of ammonium ions to phosphate ions in the solution is between about 0.7 and about 1.3, preferably between about 0.9 and 1.2.
[0041]
The present invention provides a process for recovering or regenerating ammonia contained in the effluent from the reaction zone where ammonia, oxygen and propane / isobutane react to produce acrylonitrile / methacrylonitrile.
[0042]
The present invention also avoids the need to discard excess ammonia resulting from ammoxidation of propane to acrylonitrile.
[0043]
Furthermore, according to the present invention, ammonia can be recovered from the propane ammoxidation reaction without significant loss of ammonia.
[Brief description of the drawings]
FIG. 1 is a flow diagram of a preferred embodiment of the present invention.
FIG. 2 is a flow diagram of another preferred embodiment of the present invention.
[Explanation of symbols]
3, 4 Quench part 9, 10 Quench stripper
17 Wet oxidation reactor
18 Phosphate decomposer
19 Evaporator
22 Ammonia rectification unit
28 Ammonia stripper
38 Wet oxidation unit

Claims (16)

Oxygen, ammonia, and a hydrocarbon selected from the group consisting of propane and isobutane react at high temperatures in the presence of an ammoxidation catalyst,
A process for recovering unreacted ammonia from an effluent obtained from a reaction zone that produces the corresponding unsaturated nitrile:
Quenching the fluidized bed reactor effluent containing the corresponding nitrile and unreacted ammonia with a first aqueous ammonium phosphate solution to absorb substantially all unreacted ammonia present in the reactor effluent; Forming a second aqueous ammonium phosphate solution that is richer in ammonium ions than the first solution,
Wherein the ratio of ammonium ions (NH ₄ ⁺ ) to phosphate ions (PO ₄ ⁻³ ) in the first solution is between about 0.9 and about 1.2 ;
The second solution is brought to a temperature sufficient to reduce the amount of ammonium ions in the second solution to substantially the same level as that present in the first solution and to generate an ammonia-containing vapor stream. Heating; and recycling the ammonia-containing vapor stream to the fluidized bed reactor;
Including the process.   The process of claim 1, wherein the hydrocarbon is propane. The process of claim 1 , wherein the ratio of ammonium ions to phosphate ions in the first solution is between about 1.0 and 1.2. Before heating the second ammonium phosphate solution to a high temperature,
The process of claim 1 wherein a stripper gas is passed through the second solution to remove substantially all corresponding residual nitrile from the second solution. The process of claim 4 , wherein the hydrocarbon is propane.   The process of claim 1, wherein the second solution containing substantially the same level of ammonium ions as present in the first solution is recycled for use in the quench step. The process according to claim 6 , wherein the second solution before recycling is subjected to wet oxidation to remove unnecessary impurities. High temperature heating of the second solution is performed in a wet oxidation reactor under wet oxidation conditions to remove unnecessary impurities from the second solution, and at the same time, the concentration of ammonium ions in the second solution is set to the first solution. 5. The process of claim 4 , wherein the process is reduced to substantially the same level as that present therein. The process of claim 4 , wherein the ratio of ammonium ions to phosphate ions in the first solution is between about 1.0 and about 1.2.   The process of claim 1, wherein the pH of the first solution is between about 2.8 and 6.0. The pH of the first solution is between about 2.8 to about 5.8, The process of claim 1 0. The process of claim 4 , wherein the pH of the first solution is between about 2.8 and about 6.0. The pH of the first solution is between about 2.8 to about 5.8, The process of claim 1 2.   The process of claim 1, wherein the temperature of the first solution is between 40 ° C. and 80 ° C.   The process of claim 1, wherein the first solution comprises an aqueous solution comprising monoammonium phosphate and phosphoric acid. The process of claim 4 , wherein the first solution comprises an aqueous solution comprising monoammonium phosphate and phosphoric acid.

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