RU2237016C2 - Method of recovering unreacted ammonia from stream flowing out of reactor - Google Patents

Abstract

FIELD: industrial organic synthesis.

SUBSTANCE: invention relates to improved method for reduction and recovery of unreacted ammonia from flowing out stream containing acrylonitrile or methacrylonitrile and coming out from reaction zone, where oxygen, ammonia, and hydrocarbon selected from propane and isobutene are brought into contact in reactor in presence of fluidized ammoxidation catalyst bed at elevated temperature to produce corresponding unsaturated nitrile by cooling flowing out stream containing corresponding nitrile and unreacted ammonia with first aqueous ammonium phosphate solution, wherein ratio of ammonium ions to phosphate ions (PO $\genfrac{}{}{0ex}{}{\text{3-}}{\text{4}}$ ) varies between 0.7 and about 1.3, to absorb essentially all unreacted ammonia present in flowing out stream to form second aqueous ammonium phosphate solution more rich in ammonium ions than the first solution. Second solution is then heated to temperature high enough to reduce amount of ammonium ions in the second solution to essentially the same level as in the first solution and to form ammonia-containing vaporous stream, which is returned into reactor.

EFFECT: avoided formation of ammonium salt-containing waste, achieved complete capture of acrolein, and reduced content of organic carbon and polymers in organic dregs.

18 cl, 2 dwg

Description

Technical field

The present invention relates to a method for recovering and recovering ammonia contained in an effluent obtained from a reaction zone, where ammonia and oxygen are reacted with paraffin to obtain the corresponding aliphatic nitrile. In particular, the present invention relates to the recovery and regeneration of unreacted ammonia contained in an effluent from a reaction zone, wherein ammonia and oxygen are reacted with (1) propane to produce acrylonitrile or (2) isobutane to produce methacrylonitrile.

Each of US patents 3936360 and 3649179 indicates a method for producing acrylonitrile using propylene, oxygen and ammonia as reagents. These gases pass through the catalyst in a fluidized bed reactor to form acrylonitrile, which flows from the reactor to the recovery and purification section. Unreacted ammonia is also present in this reaction, which is usually removed from the process by treatment with acid in a quench column. The '179 patent discloses that the cooling acid may be either sulfuric, hydrochloric, phosphoric, or nitric acid. The '360 patent discloses the use of sulfuric acid in quenching to remove unreacted ammonia. In the production of acrylonitrile using propylene as a hydrocarbon feed, in preferred embodiments, sulfuric acid with a final formation of ammonium sulfate is unconditionally used. Usually, ammonium sulfate is either reduced and sold as a co-product (fertilizer), or it can be combined with other high molecular weight organic substances obtained in the process and carefully sedimented with their further removal so as not to clog the environment.

UK patent 222587 is devoted to the extraction of ammonia from an ammonia-containing gas mixture using an aqueous solution of phosphoric acid, an aqueous solution of acid ammonium orthophosphate ((NH ₄ ) H ₂ PO ₄ ) or mixtures thereof. Ammonium is recovered by thermal decomposition and dissolution of the resulting residue in water to regenerate the ammonium-containing reduced phosphate solution. This method is designed to extract ammonia from coal gas or coke oven gases at temperatures from 50 to 70 ° C.

US patents 2797148 and 3718731 are devoted to the extraction of ammonium from the process stream used in the production of HCN. To do this, use a solution of ammonium phosphate to trap ammonium, and then apply distillation using steam to regenerate ammonia from a solution of ammonium phosphate. Typically, the method is implemented by contacting an ammonia-containing gas with a content of from 25 to 35% by weight of a solution of ammonium phosphate having a pH of about 6, at a temperature of from 55 to 90 ° C. Ammonia is regenerated by contacting the resulting ammonium phosphate solution with steam. The methods in each of these patents disclose that the ratio of ammonium ion / phosphate ion is at least 1.2 or higher.

The method according to the present invention has several advantages compared with previous practice used in the ammoxidation of propylene, as this avoids the formation of a waste stream containing an ammonium salt, which must either be (1) treated to restore the ammonium salt, or (2) used safely environmentally friendly way. Preferably, the method of the present invention extracts ammonia and regenerates a cooling solution of ammonium phosphate by subjecting the cooling solution to elevated temperature and pressure to decompose the ammonium phosphate salt. The cooling system of the present invention leads to additional unexpected advantages in the ammoxidation of propane compared with propylene ammoxidation to acrylonitrile. Among these advantages are: (1) complete capture of the acrolein by-product, which improves product recovery by minimizing product losses through, for example, the reaction of acrolein with HCN during product separation and recovery, (2) lower TOC (total organic content) carbon) in cooling sludge, (3) a higher percentage of organic matter in cooling sludge present as distilled / recoverable monomers instead of non-recoverable waste polymers and (4) in the possibility of using a less stringent treatment of organic waste (for example, wet oxidation) due to the presence of lower TOC and polymers in the solution of cooling sludge. An additional significant advantage of the method of the present invention is that all water waste streams can be easily controlled by standard bioprocessing processes as opposed to waste streams associated with ammoxidation of propylene to produce acrylonitrile.

SUMMARY OF THE INVENTION

The main objective of the present invention is the recovery or recovery of ammonia contained in the effluent from the reactor zone, where ammonia, oxygen and propane / isobutane interact to produce acrylonitrile / methacrylonitrile.

Another objective of the present invention is to prevent the need to remove excess ammonia, which is the result of ammoxidation of propane to acrylonitrile.

Another objective of the present invention is the extraction of ammonia from the reaction of ammoxidation of propane without any significant loss of ammonia.

Other objectives, as well as other aspects, features and advantages of the present invention, will become apparent from consideration of the description with the accompanying drawings and claims.

To achieve the above objectives and in accordance with the purpose of the present invention, embodied and fully disclosed in the description, a method of extracting unreacted ammonia from a stream obtained from a reactor zone, where oxygen, ammonia and a hydrocarbon selected from the group consisting of propane and isobutane are reacted in the presence of an ammoxidation catalyst at elevated temperature to obtain the corresponding nitrile, includes (1) quenching the effluent from the fluidized bed reactor containing the corresponding reducing nitrile and unreacted ammonia, the first cooling aqueous solution of ammonium phosphate, in which the ratio of ammonium ions (NH $\genfrac{}{}{0ex}{}{\text{+}}{\text{4}}$ ) to phosphate ions (PO $\genfrac{}{}{0ex}{}{\text{-3}}{\text{4}}$ ) is at least about 0.7, but not more than about 1.3, thereby absorbing ammonia to form a second aqueous solution of ammonium phosphate, richer in ammonium ions (NH $\genfrac{}{}{0ex}{}{\text{+}}{\text{4}}$ ) than the first solution, (2) heating the second solution to an elevated temperature to reduce the amount of ammonium ions in the second solution back to essentially the same level as in the first solution, and the formation of a vapor stream containing ammonia, the return of the specified vapor stream containing ammonia to the ammoxidation reaction zone.

In a preferred embodiment of the method according to the present invention, the second aqueous ammonium phosphate solution is treated with a stripping gas to remove substantially all of the acrylonitrile and other beneficial co-products from the second solution before heating the solution to reduce the NH content $\genfrac{}{}{0ex}{}{\text{+}}{\text{4}}$ and she.

In another preferred embodiment of the present invention, the gas containing acrylonitrile is returned to recover and purify the acrylonitrile.

In another preferred embodiment of the present invention, the second solution, after distillation and removal of ammonia, is transferred to the oxidation reactor in a humid oxygen atmosphere, whereby the solution undergoes said oxidation at elevated temperature and pressure to remove any high molecular weight organic compounds contained in the cooling solution.

In yet another embodiment of the present invention, the second solution, after removing ammonia, is transferred to the evaporator to remove excess water from the ammonium phosphate solution, which is then returned for use in cooling.

Preferably, the temperature of the first solution is between 40 and 80 ° C, particularly preferably between 50 and 65 ° C.

Typically, the first cooling solution has an ammonium / phosphate ratio between 0.7 and about 1.3, preferably between about 0.9 to about 1.2, most preferably between about 1.0 and about 1.2. The resulting pH of the first cooling solution is between 2.8 and about 6. The concentration of phosphate ions in the first cooling solution can be up to 40% by weight, preferably up to about 35% by weight.

Brief Description of the Drawings

Figure 1 is a flow diagram of a preferred embodiment of the present invention.

Figure 2 is a flow chart of another preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The following is a detailed description of preferred embodiments of the present invention.

The present invention is directed to a method for quenching an effluent obtained from a propane ammoxidation reaction zone. Preferably, the reaction proceeds in a fluidized bed reactor, although other types of reactors, such as transport line reactors, are considered to be suitable in the practice of the invention. Fluidized bed ammoxidation reaction conditions and a fluidized catalyst bed, useful in propane ammoxidation, are known in the art as proven by US Pat. No. 4,746,641, are intended to complement this application, and are hereby incorporated by reference. The new method according to the present invention contains a sharp cooling of the reactor effluent obtained by the interaction of ammonia, oxygen and propane in the reaction zone (for example, a fluidized bed reactor) to produce acrylonitrile with the first aqueous solution of ammonium phosphate having a ratio of ammonium ions to phosphate ions of at least at least 0.7, but not more than 1.3, thereby absorbing ammonia to enrich the second solution of ammonium phosphate with ammonium ions as compared with the first solution, heating the second solution to a higher temperature to reduce the content of ammonium ions in the second solution back to essentially the same content of ammonium ion as in the first solution, and the formation of a vapor stream containing ammonia and water, an increase in the molar concentration of ammonia in this vapor stream and the return of the vapor stream, containing ammonia to a fluidized bed reactor.

Preferably, the ratio of ammonium ion / phosphate ion is between about 0.9 and about 1.2, particularly preferably is from about 1.0 to 1.2. The temperature of the first cooling solution is usually between 40 to 80 ° C, preferably between about 50 to 65 ° C, particularly preferably between 55 to 60 ° C.

The pH of the first (initial) cooling solution will be maintained between about 2.8 to not higher than 6.0, preferably between 2.8 to about 5.8.

Preferably, the starting cooling solution contains a mixture of primary ammonium phosphate / aqueous phosphoric acid solution (90% primary ammonium phosphate — 10% H ₃ PO ₄ ), although a weak solution of an aqueous solution of primary ammonium phosphate is also considered suitable in the practice of the present invention. When using an aqueous solution of primary ammonium phosphate present in the effluent from the reactor, unreacted ammonium is absorbed to convert the primary ammonium phosphate to secondary phosphate.

During the quench procedure, products (acrylonitrile, acetonitrile, and HCN) are removed as top products and are substantially free of ammonia. The remainder of the cooling solution containing secondary ammonium phosphate also contains residual monomers (e.g., acrylonitrile) in small amounts. These monomers are preferably distilled off and returned to the cooling solution for further reduction and purification. Conventional flue gases to remove residual monomers from the cooling residues comprise propane, nitrogen, carbon dioxide and carbon monoxide, or a mixture thereof.

The cooling residual solutions, purified from useful monomers, are then regenerated at elevated temperatures and pressures to convert the secondary ammonium phosphate back to primary ammonium phosphate to release ammonia. Ammonia is captured as a stream of steam that contains water. This ammonia-saturated steam stream is heated to remove almost all of the water, and the ammonia then returns back to the reactor. Ammonium primary phosphate is reduced and returned back to the quench column.

In a further preferred embodiment of the present invention, the distilled cooling residue containing secondary ammonium phosphate passes through a wet oxidation reactor, where it is processed under normal wet oxidation conditions to remove any polymers formed during the ammoxidation process.

In another preferred embodiment of the present invention, the distilled cooling residue containing non-recoverable monomers and secondary ammonium phosphate is separately treated in a phosphate decomposing apparatus that separates the secondary ammonium phosphate from the residual monomers. Secondary phosphate is then regenerated back to primary ammonium phosphate in a separator unit, while the remaining polymers are transferred to a wet oxidation unit for wet oxidation at normal temperature and pressure to produce harmless by-products such as carbon dioxide and water.

1 and 2 depict preferred embodiments of the method according to the present invention in relation to ammoxidation of propane.

1, a reactor effluent obtained directly by the reaction of propane, ammonia and oxygen in a fluidized bed reactor (not shown) through a fluidized ammoxidation catalyst passes through a pipe 1 to a cooling column 3. In a cooling column 3, a reactor effluent containing the obtained acrylonitrile and unreacted ammonia is contacted with a depleted ammonium phosphate cooling solution, which adsorbs unreacted ammonia from the effluent, to obtain ammonia-free upper stream containing crude acrylonitrile. The crude acrylonitrile is passed through pipe 5 from above to standard recovery and purification sections (not shown) for the subsequent recovery of commercially pure acrylonitrile, crude acetonitrile and hydrogen cyanide. Examples of standard recovery and purification procedures can be found in US Pat. No. 3,936,360, incorporated herein by reference. The cooling residues leave the cooling column 3 through line 7 and are introduced into the cooling stripping section 9. A stripping gas containing a return stream containing a mixture of propane, carbon monoxide, carbon dioxide and nitrogen passes through conduit 13 to the stripping section 9 to remove any residual acrylonitrile , acetonitrile or hydrogen cyanide contained in the cooling residues. The top-off stripping gas entering the stripping section 9 through a pipe 13 containing these residual monomers is returned back to the cooling column 3 via a pipe 11 for further recovery (recovery) of useful products. The distilled chilled residues pass from stripping section 9 through line 15 to a wet oxidation reactor 17, where oxygen passes through line 25, and standard catalytic wet oxidation takes place to remove unwanted impurities such as polymers. In addition, the ammonium secondary phosphate contained in the cooling residues of the stripping section is heated to release ammonia and to convert the secondary ammonium phosphate in solution to the primary ammonium phosphate. The solution of primary ammonium phosphate passes from the reactor 17 through a pipe 27 to the evaporator 19, where the excess water is removed from the solution. This excess water flows from the evaporator 19 through a pipe 21 for recycling or liquidation. A weakly concentrated solution of primary ammonium phosphate passes from the evaporator 19 through line 23 to return to the cooling column 3. The decomposition of secondary ammonium phosphate takes place in a wet oxidation reactor 17 by heat treatment in the presence of oxygen, and then from the reactor 17 through a pipe 29 returns directly to the reactor with a fluidized bed (not shown).

Typical wet oxidation conditions are used to decompose undesired polymers obtained during the process. The usual catalysts for wet oxidation are soluble salts of copper and iron, oxides of copper, zinc, manganese, cerium and noble metals, well known previously. See, for example, Ind .. Eng. Chem. Res., 1995, v. 34, pp. 2-48, incorporated by reference. The wet oxidation reaction is designed to proceed normally. Typically, wet oxidation takes place at a pressure between about 42.19 to 210.9 kg / cm ² and a temperature of 200 to 650 ° C.

With reference to FIG. 2, another preferred embodiment of the present invention is described. The method shown in FIG. 2 is basically the same as in FIG. 1, except that the decomposition of phosphate takes place in a separate unit, followed by wet oxidation in another unit. The reactor effluent obtained by direct ammoxidation of propane, oxygen, and ammonia in a fluidized bed reactor (not shown) passes through the fluidized bed reactor through line 2 to cooler 4. The reactor effluent containing crude acrylonitrile and unreacted ammonia is contacted in cooler 4 with an aqueous solution of primary ammonium phosphate, which enters cooler 4 through line 40. The phosphate solution removes unreacted ammonia from the reactor effluent OK, allowing ammonia-free products (crude acrylonitrile) to flow upward from cooler 4 through line 6. The crude acrylonitrile, flowing upstream of line 6, is routed to a standard recovery and purification section to recover commercially pure acrylonitrile, crude acetonitrile and HCN. The cooling residues pass through cooler 4 through line 8 to the stripping section 10, where the stripping gas (having the same composition as described above) is introduced in small portions into the lower part of the stripping section 10 and passes up through the cooling residues to distill the cooling slots from any useful monomers present in sludge, such as acrylonitrile, acetonitrile and hydrogen cyanide. The gas of the stripping section containing useful monomers then exits from the stripping section 10 from the top through line 12 to cooler 4 for further recovery and purification. The distilled cooling sludge is transported from the stripping section 10 through line 16 to the phosphate decomposition section 18. In the phosphate decomposition section 18, the secondary ammonium phosphate present in the distilled cooling sludge is converted to free ammonium and primary ammonium phosphate by heating to an elevated temperature (from 100 to 300 ° C.). Typically, the pressure is between 1 to 5 atmospheres (from atmospheric pressure to 5.273 kg / cm ² ). Oxygen may be present but not required. The resulting solution of primary ammonium phosphate passes from decomposition section 18 through line 34 to be returned through line 40 to cooler 4. Free ammonia formed during the conversion of phosphate in reactor 18 passes to ammonia rectification section 22, where free ammonia is purified and fed to section 28 ammonia distillation via line 26 in order to recover ammonia for return to a reactor (not shown) to produce acrylonitrile. Water is recovered from the ammonia stripping section 28 and passes through conduit 32 for return or disposal. A weak solution of primary ammonium phosphate passing from the decomposition section 18 through line 34 can be sent through the wet oxidation section 38 through line 36 to remove the polymers and convert these undesirable substances into harmless by-products such as hydrogen, carbon monoxide and carbon dioxide. As described previously, wet oxidation can be carried out under known standard conditions.

Despite the fact that the invention has been disclosed in relation to the features of its embodiment, it is obvious that many alternatives, modifications and variations will be obvious to specialists from the above description. Accordingly, it is intended to cover all such alternatives and modifications in light of the appended claims.

Claims (18)

1. A method of recovering unreacted ammonia from an effluent obtained from a reaction zone, where oxygen, ammonia and a hydrocarbon selected from the group consisting of propane and isobutane are reacted in a reactor in the presence of a fluidized bed of ammoxidation catalyst at an elevated temperature to produce the corresponding unsaturated nitrile by cooling the effluent flow from a fluidized bed reactor containing the corresponding nitrile and unreacted ammonia with a first aqueous solution of ammonium phosphate, in which ammonium ions (NH $\genfrac{}{}{0ex}{}{\text{+}}{\text{4}}$ ) to phosphate ions (PO $\genfrac{}{}{0ex}{}{\text{-3}}{\text{4}}$ ) is approximately 0.7-1.3, to absorb essentially all of the unreacted ammonia present in the reactor effluent to form a second aqueous solution of ammonium phosphate, richer in ammonium ions than the first solution, heating the second aqueous solution of ammonium phosphate to an elevated temperature sufficient to reduce the amount of ammonium ions in the second solution to substantially the same level as in the first solution to form a vaporous stream containing ammonia, and return the flow of steam containing ammonia into the fluidized bed reactor. 2. The method according to claim 1, characterized in that the hydrocarbon is propane. 3. The method according to claim 1, characterized in that the ratio of ammonium ion to phosphate ion in the first solution is approximately in the range of 0.9-1.2. 4. The method according to claim 3, characterized in that the ratio of ammonium ion to phosphate ion in the first solution is approximately in the range of 1.0 - 1.2. 5. The method according to claim 1, characterized in that the stripping gas passes through a second aqueous solution of ammonium phosphate to remove essentially all of the residual nitrile from the second solution before heating the solution to an elevated temperature. 6. The method according to claim 5, characterized in that the hydrocarbon is propane. 7. The method according to claim 1, characterized in that the second solution containing substantially the same level of ammonium ions that is present in the first solution is returned for use to the cooling step. 8. The method according to claim 7, characterized in that the second solution is subjected to oxidation in the atmosphere of moist oxygen before returning to remove any undesirable impurities. 9. The method according to claim 5, characterized in that the second solution is heated to an elevated temperature in an oxidation reactor in a humid oxygen atmosphere to simultaneously remove unwanted impurities from the second solution and reduce the concentration of ammonium ion in the second solution to a substantially the same level of presence, as in the first solution. 10. The method according to claim 5, characterized in that the ratio of ammonium ion to phosphate ion in the first solution is in the range of about 0.9-1.2. 11. The method according to claim 10, characterized in that the ratio of ammonium ion to phosphate ion in the first solution is in the range of about 1.0 to 1.2. 12. The method according to claim 1, characterized in that the pH of the first solution is in the range of about 2.8 to 6.0. 13. The method according to p. 12, characterized in that the pH of the first solution is in the range of about 2.8 to 5.8. 14. The method according to claim 5, characterized in that the pH of the first solution is in the range of about 2.8 to 6.0. 15. The method according to 14, characterized in that the pH of the first solution is in the range of about 2.8 to 5.8. 16. The method according to claim 1, characterized in that the temperature of the first solution is from 40 to 80 ° C. 17. The method according to claim 1, characterized in that the first solution contains an aqueous solution containing primary ammonium phosphate and phosphoric acid. 18. The method according to claim 5, characterized in that the first solution contains an aqueous solution containing primary ammonium phosphate and phosphoric acid.

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