US2734080A - Recovery of phthalic acids - Google Patents

Description

Feb. 7, 1956 H. J. AROYAN ET AL 2,734,080

RECOVERY OF PHTHALIC ACIDS Filed July 12, 1954 5 Sheets-Sheet 1 0 2 o I N Z 9 2 LL.

o o w 10 v m N o D 92 .LV GHUQSVBW Hd lNVENTORS HARRY J. AROVAN JOHN B. W/LKES BY I'D- M ATT %EYS Feb. 7, 1956 H. J. AROYAN ET AL 2,734,080

RECOVERY OF PI-ITI-IALIC ACIDS Filed July 12, 1954 5 Sheets-Sheet 2 /FIRST STAGE HYDROLYSIS INITIAL PH "'3 T0 3.5

fsecowo STAGE HYDROLYSIS L INITIAL PH 3 0.1

/THIRD STAGE HYDROLYSIS INITIAL PH '3 0 2O 4O 6O REACTION TIM E MINUTES 119-3 INVENTORS HARRY J. AROI AN JOHN B. WILKES Feb. 7, 1956 H. J. AROYAN ETAL 2,734,080

RECOVERY OF PHTHALIC ACIDS Filed July 12, 1954 3 Sheets-Sheet 3 FIG. 3

. u) INVENTORS HARRY J. AROVAN JOHN B. WLKES BY 4% N rm 8- [K50 l I ATTO EYS United States Patent O RECOVERY OF PHTHALIC ACIDS Harry J. Aroyan, ElCerrito, and John B. Wilkes, Albany, Califl, assignors to California Research Corporation, San Francisco, Calif., a corporation of Delaware Application July 12, 1954, Serial No. 442,694 19 Claims. (Cl. 260-625) This invention relates to a process for recovering phthalic acids from mixtures of the ammonium salts and amides of these acids.

This application is a continuation-in-part of our copending application Serial No. 257,434, filed November 20, 1951, now abandoned.

Copending application of William G. T oland, Ir., Serial No. 371,209, filed July 30, 1953, now United States Patent No. 2,722,549, describes a process for oxidizing alkyl aromatic hydrocarbons such as' the three xylene isomers, 1,3'-dimethyI-S-tertiary-butyI benzene, toluene, and the like, by heating the alkyl aromatic hydrocarbons with ammonium sulfate, a sulfide'such as hydrogen sulfide or ammonium sulfide and water to a temperature from about 550 F. to the critical temperature of Water under a superatmospheric pressure sufficient to maintain a part of the water in liquid phase. In a representative operation, 1 mol of xylene, 1.83 mols of. ammonium sulfate, 0.18 mol of ammonium sulfide and 30 mols of water are heated to about 620 F. under a' pressure of 2800 p. s. i. g; for approximately 2 hours. 97% conversion of the xylene is achieved and a yield of phthalic acid derivatives amounting to 91% of theory is obtained. The reaction product produced in this oxidation process is a mixture of ammoniumphthalates and phthalic acid amides. Three types of compound are present having the formulas:

(COONH4)2,. (CONH2)2 and NHZOCCOONH4 Thus, the reaction product contains diammoni'um phthalate; diamide' of phthalic acid, and the half salt-half amide of phthalic acid. In the reaction product from about 60 to 80% of the total potential acid groups is in the: form: of the ammonium salt and the remainder is in the form ofthc amide, i. e., 60 to 80% of the side chains of the reaction product is ammonium carboxylate --COONH4, while the remainder is amide CONH2. Throughout the specification and claims the phrase mixtures of ammonium salts and amides of phthalic acids is'intended to describe mixtures of the" three compounds shown above.

While the oxidation process above referred to is characterized by'high conversions of xylene and high yields ofphthalic acid values, the recovery of phthalic acid having the degree of purity required in most commercial uses fromtheicrude oxidation reaction product is difficult'. Aphthalic' acid product satisfactory for virtually" any commercial use must have a nitrogencontent calculated as ammoniabel'ow about 0.1% by weight and for a great many uses, for example, in the production: of alkyd resins, it is necessary that the nitrogen content calculated as ammonia be below ;03% by weight. These low nitrogen contents are not easily attained. They can be reached by saponifying the ammonium salt-amide product with caustic. as disclosed. in our copendin'g application Serial No. 257,434, filed November 20, 1951,10. producea solu tion of sodium phthalates from which phthalic acids. are precipitated by the addition of acid. This methodinvolves 2,734,080 Patented Feb. 7 1056 considerably increases the ultimate cost of the phthalic acid product. It has been found also that the hydrolysis can be accomplished in a strongly acid medium, i. e., 2 mols of sulfuric acid per mol of phthalic acid contained in the salt-amide mixture at high temperature. This method, however, is attended by extraordinarily high corrosion rates with the most corrosion resistant materials now available requiring frequent replacement of expensive equipment with attendant reduction in the operating factor of the process.

It has now been found that mixtures of ammonium salts and amides of phthalic acids can be hydrolyzed to produce phthalic acids of low nitrogen content by acidifying an aqueous solution of the mixture with a mineral acid such as sulfuric, hydrochloric,- nitric and phosphoric acids, as disclosed in the aforesaid copending application to a pH in the range from about 2.5 to about 3.5 (this pH range is attained by adding approximately 1 equivalent of sulfuric acid to the solution of the mixture for each equivalent of ammonium carboxylatecontained in the mixture), heating the mixture so acidified to a temperature above 300 F. and desirably in the range from 400 F. to 500 F and preferably in the range from 430 F. to 500 F., under a superatmospheric pressure sutficient to maintain a substantial part of the water in liquid phase for a period of about 10 to 60 minutes and then cooling the mixture to a temperature to below about 250 F., and filtering the cooled mixture to recover a filter cake consisting essentially of phthalic acids; It is desirable to reacidifythe cooled mixture to pH 3 to-3.5 with a strong mineral acid prior to filtration. A single-stage treatment in this manner serves to reduce the nitrogen content of the phthalic acid product, determined as ammonia, from an initial value ordinarily in the range from 6 to 10% to a value in the range from about 0.6% to 1%. Further reduction in the nitrogen content maybe obtained by slurrying the phthalic acid filter cake with water to produce a slurry having a solids content from about 2% to about byweight, preferably from 10% to 25% by weight, and heating the slurry to a temperature above 300 F. and preferably in the range from about 430 F. to 500 F. For certain uses of the phthalic acid requiring extremely high purity, a third-hydrolysis stage may be conducted in a similar manner.

Pursuantto another embodiment of the invention, the phthalic acid filter cake obtained in the first hydrolysis stage is slurried with a quantity of an aqueous solution of a bisulfate selected from the group consisting of ammonium bisulfate and alkali metal bisulfates suflicient to produce a slurry having a' solids content in the range from about 2% to about 50% by weight, preferablyfrom about 10% to about 25 by weight, and characterized v by a pH in the range from about 1.0to about 1.8. This substantial consumption of caustic and'sulfuric acid which 7 slurry is then' heated to a temperature above 300 F. and preferably in the range from about 350 F. to 525 F., and preferably from 400 F. to 500 F.', under a superatmospheric pressure sufiicient to maintain the water in liquid phase, cooled, and filtered to recover a filter cake consisting essentially of phthalic acid. The desired pH range, i. e., 1.2 to 1.8, in thisemb'o'ditnent of the invention, is obtained by controlling" thebisulfate content of the slurry so that it contains approximately 1 mol of bisulfate per mol of nitrogen, determined as ammonia, which is contained in the filter cake: obtained in the first stage of hydrolysis andby the addition of some excess neutral sulfates which help to butter the mixture.

Figure 1 of the appended. drawings is a graphical illustration-ofthe change in. pH which occurs as sulfuric acid is addedto asolution of a mixture of ammonium salts and amidesof phthalic acids. Figure 2 of. the appended drawings is a graphical illustration of the reduction in the nitrogen content of a mixture of ammonium salts and ayrsapso amides of phthalic acids which is obtained in three hydrolysis stages carried out in the presence of any mineral acid. Figure 3 of the appended drawings is a diagrammatic illustration of arrangement of apparatus and process flowsuitable for the practice of the invention.

A typical ammonium phthalate-phthalic acid amide mixture produced pursuant to the process described in the above-mentioned Toland application is titrated with 19 normal sulfuric acid. The titration curve is shown in Figure 1. The phthalic acid salt-amide solution had a calculated phthalic acid content of 15.5 g. per 100 g. of solution, the phthalic acid values in the solution being a mixture of isophthalic acid and terephthalic acid and containing about 95% isophthalic acid. From the titration curve it will be observed that a rapid drop in pH occurs as the total acid added to the ammonium saltamide solution rose from 20 to 24 cc. This drop in pH occurs when approximately one equivalent of sulfuric acid has been added for each equivalent of ammonium carboxylate contained in the ammonium salt-amide mixture. After the addition of this quantity of sulfuric acid, substantially all of the ammonium carboxylate groups contained in the solution have undergone a replacement of the ammonium radical by hydrogen and exist as carboxyl groups. When this quantity of acid has been added, i. e., when the pH of the ammonium salt-amide solution has been adjusted to a value in the range from about 2.5 to 3.5, the solution is in proper condition for heating to a temperature above 300 F., and preferably in the range from 400 to 500 F., to cause hydrolysis of the amide groups. When the calculated phthalic acid content of the salt-amide solution is in the range from about 8 to 25% by weight, it is desirable to add the sulfuric acid at an elevated temperature, for example, a temperature in the range from about 150 F. to 250 F., to avoid the production of a viscous slurry as phthalic acids and amides are precipitated as a result of the acid addition. While the actual acid addition is desirably made at these higher temperatures, the pH values employed in controlling the acid addition are determined at about 70 F. Throughout the specification and claims it should be understood that the pH values recited are pH values determined at about 70 F. and that the actual pH existing at the higher temperatures may differ considerably from the value measured at 70 F.

Figure 2 of the appended drawings shows the reduction in nitrogen content of a mixture of ammonium salts and amides of phthalic acids in three stages of hydrolysis. In each of the three hydrolysis stages the acidified salt-amide mixtures were heated to 465 F. under a pressure of 500 p. s. i. g. for 60 minutes and samples were taken for analysis as thehydrolysis progressed. In all stages the initial pH of the mixtures was in the range from 3 to 4. During the 60 minute hydrolysis period the pH rose to a level in the range from 4.7 to 5.2 as amide groups were hydrolyzed to ammonium carboxylate groups and the ammonia liberated from the amide groups utilized hydrogen ions to form ammonium radicals. At the end of the first stage the reaction mixture was cooled to 180 F., acidified to pH below 4.0, and filtered. The nitrogen content of the first-stage filter cake expressed as ammonia was 0.74%. The firstastage filter cake was reslurried with water and this slurry was then heated to cause further hydrolysis. The second stage filter cake, obtained by filtering after adjusting the pH of the second-stage product to a value of 3 to 4, had a nitrogen content of 0.05 expressed as ammonia. The third hydrolysis stage was conducted by reslurrying the second stage hydrolysis filter cake in water, and heating to cause further hydrolysis. The final phthalic acid producthad a nitrogen content of 0.013% expressed as ammonia. For many uses of the isophthalic acid'and terephthalic acid hydrolysis product, nitrogen contents below about 0.06% by weight expressed as ammonia are satisfactory, while a phthalic acid product having a nitrogen content below 0.03% expressed as ammonia appears to be satisfactory in the manufacture of alkyd resins where purity requirements are most severe.

Corrosion rates of stainless steels were determined under the conditions of the first-stage hydrolysis illustrated in Figure 2. The initial corrosion rate during the first hydrolysis stage when the pH is about 3 was 26 mils per year for type 304 stainless steel and 29 mils per year for type 316 stainless steel. The corrosion rates for both of these stainless steels were below 20 .mils per year after the first few minutes of the hydrolysis treatment when the pH of the hydrolysis reaction mixture had risen to 4.0.

Isophthalic acid, terephthalic acid, and mixtures of these acids, having nitrogen contents below 0.03%, determined as ammonia, can readily be obtained employing two hydrolysis stages. The first hydrolysis stage is conducted as illustrated in Figure 2 of the drawings by adjusting the pH of an aqueous solution of a mixture of ammonium salts and amides of isoand terephthalic acids to a value in the range from 2.5 to 3.5 and heating the acidified mixture to 450 to 500 F. under a pressure of about 500 p. s. i. g. for 10 to 60 minutes. The hydrolysis reaction mixture is then cooled to a temperature below about 250 F. and sulfuric acid is added to adjust the pH of the hydrolysis reaction product to a value in the range from 2.5 to 3.5 (pH determined at F.). The cooled reaction product is then filtered and the filter cake is slurried with an aqueous bisulfate solution and heated to 350 F. to 500 F. under superatmospheric pressure to complete the hydrolysis. The second hydrolysis stage employing aqueous bisulfate solution is illustrated by the following examples.

Example The filter cake obtained in the first-stage hydrolysis illustrated in Figure 2, the first-stage hydrolysis product having been acidified to a pH of 3 prior to filtration, was slurried with water containing 13.2 g. of sodium sulfate per liter and 4.65 g. of sulfuric acid per liter. The calculated isophthalic acid content of the slurry was g. per liter. This slurry had a pH of 1.45 measured at 70 F. The slurry was heated to 465 F. under a pressure of about 500 p. s.i. g. Samples were withdrawn and analyzed for nitrogen as the hydrolysis progressed. At the end of 30 minutes the nitrogen content of ,the isophthalic acid product expressed as ammonia had been reduced to 0.036% by weight. At the end of 60 minutes the nitrogen content similarly expressed was reduced to 0.022% by weight. At the end of the 60 minute period, the pH of the hydrolysis reaction product measured at 70 F. had risen to 2.7.

Example 2 Example 1 was repeated, slurrying the first-stage isophthalic acid filter cake with water containing 26 g. per liter of sodium sulfate and 9.5 g. per liter of sulfuric acid. The isophthalic acid content of the slurry was again 170 g. per liter of water. The pH of the initial slurry at 70 F. was 1.27. At then end of 30 minutes heating at 465 F., the nitrogen content of the isophthalic acid expressed as ammonia was 0.03% by weight, and at the end of 60 minutes the nitrogen content,-similarly expressed, had been reduced to 0.018% by Weight. -At the end of 60 minutes the pH of the hydrolysis reaction product had risen to 1.67. The initial corrosion rate for type 304 stainless steel under the conditions of this example was 31 mils per year, but this rate decreases as the acidity decreases during hydrolysis. It appears that this low corrosion rate must be attributed to the fact that sodium bisulfate ion has a much lower dissociation constant at high temperature than at low temperature. For example, the constant for the ionization of bisulfate ion to hydrogen ion and sulfate ion is 2 10- at room temperature, while at 400 F. the

ionization constant is estimated to be approximately lXl- This unusual property of bisulfate ion enables it. to provide a reservoir of hydrogen ion for carrying out thehydrolysis of the amide groups without increasing the acidity of the hydrolysis reaction mixture to high levels at which corrosion rates become prohibitively high.

Figure 3 of the appended drawings illustrates an arrangement of apparatus and process flow suitable for the practice of a two-stage hydrolysis of mixtures of ammonium salts and amides of isophthalic and terephthalic acids. A crude reaction product produced by oxidizing 95% meta-xylene pursuant to Toland application Serial No. 371,209, mentioned above, is passed through line 1 into vessel 2. 96% sulfuric acid is passed into line 1 through line 3 where it mixes with the aqueous mixture of. ammonium salts and amides of isophthalic acid. The quantity of sulfuric acid introduced through line 3 is adjusted so that the mixture produced in vessel 2 has a pH. in the range from 2.5 to 3.5. A typical feed stock introduced through line 1 will have a calculated isophthalic acid content approximately of the total feed. Minor amounts of orthophthalic acid, benzoic acid and toluic acid will be contained in the feed as a result of the presence of ortho-xylene and ethylbenzene in the meta-xylene subjected to oxidation and to partial oxidation of a very small proportion of the metaxylene. Approximately one-half pound of sulfuric acid per calculated pound of phthalic acids contained in the feed is introduced into vessel 2. Vessel 2 is operated at about 200 to 250 F. and the contents of the vessel are strongly agitated to insure good mixing of the sulfuric acid and the feed. The acidified feed. having a pH of 2.5 to 3.5 is withdrawn from vessel 2 via line 4, passed through heat exchanger 5 into hydrolyzer 6. In heat exchanger 5 the acidified feed is heated to a temperature of about 400 to 450 F. Live steam is introduced into hydrolysis vessel 6 through line 7 to agitate the hydrolysis mixture and to raise its temperature to a level in therange from 450 to 500 F. The hydrolysis reactionv mixture flows from hydrolyzcr 6 through line 8 into a second hydrolyzer 9. Sulfuric acid or ammonium bisulfate solutions may be introduced into line- 8 through line 10 in amount sufficient to drop the pH of the liquid in line 8 from approximately 5 to below about 3.5. The residence time of the reaction mixture in the hydrolysis vessels is ordinarily from 10 minutes to 1 hour, being usually from about to minutes. Cold filtrate recovered as described hereinafter is introduced into the lower portion of hydrolysis vessel 9 through line 11, the temperature and amount of this stream being i adjusted to cool the hydrolysis reaction mixture in the lower portion of hydrolysis vessel 9 to about 250 F. or below. The hydrolysis vessels are operated at 500 to 700 p. s. i. g. to maintain the water component of the hydrolysis reaction mixture in liquid phase. The cooled hydrolysis reaction mixture is withdrawn from hydrolysis vessel 9 through line 12 and is passed into cooler 13. Sulfuric acid is introduced into cooler 13 through line 14 in amount sufficient to bring the pH of the hydrolysis reaction product to a value in the range from 2.5 to 3.5. A portion of the Water contained in the hydrolysis reaction product is evaporated in cooler 13 and the vapors are withdrawn through line 15. The cooled slurry produced in cooler 13 is withdrawn through line 16 and passed to filter 17 which may be either a conventional type filter or a centrifugal filter. The filtrate produced at filter 17 is-passed through line 18 into filtrate storage tank 19. A part of the filtrate is withdrawn from tank 19 through line 20 and is passed into cooler 21. A portion of the filtrate is withdrawn from the hydrolysis system: through line 22. Benzoic and toluic acids are extracted from this stream (line 22) and the. residual aqueous solution, which is. principally aqueous ammonium sulfate. can be returned to the oxidation unit in which meta-xylene is subjected to oxidation. where it. is. employed as the oxidizing agent. The liquid in cooler 21 is cooled ordinarily to a temperature in the range from 70 to F. and the cold filtrate is withdrawn from cooler 21 and passed into the lower portion of hydrolysis vessel 9 through line 11 as described above. The filter cake produced at filter 17 is passed through lines 23 and 24 into repulp vessel 25 where it is slurried with water containing about 2.5% byweight of sodium sulfate and about 0.8 to 1.0% by weight of sulfuric acid. The water containing sodium sulfate and sulfuric acid, i. e., sodium bisulfate with excess sodium sulfate, is passed through line 27 into line 24 and repulp vessel 25. The slurry produced in vessel 25 is withdrawnfrom the vessel through line 28 and is passed-into manifold line 29. Vessels 30, 31 and 32 are batch hydrolysis vessels, having connecting lines so arranged that simultaneously one vessel is being filled from manifold line 29 and heated to hydrolysis temperature, the second. vessel is being held at 450 to 525 F. under a pressure from about 500 to 700 p. s. i. g. to accomplish the hydrolysis, and the third vessel containing hydrolyzed slurry is being emptied. Live steam is passed into manifold line 33 from which it can be passed into the vessel which is being filled and heated. Steam from the hydrolysis vessels is withdrawn through manifold line 34 and line 35. During the cycle illustrated in the drawing, vessel 30 is being filled with slurry from manifold line 29 and heated with steam from steam manifold line 33. The filling and heating are continued until the vessel is filled to working capacity and the stock has been heated to a temperature from 350 to 500 F. In the illustrated cycle, vessel 31 is being held under pressure at a temperature in the range from 350 to 500 F. to hydrolize the amide contained in the slurry. In the illustrated cycle, vessel 32 is cooled from hydrolysis temperature to about 250 F. by evaporation of water which is removed through manifold line 34 and line 35. The cooled hydrolysis product produced in vessel 32 is withdrawn through line 36 and passed. into slurry tank 37. Slurry is withdrawn from tank 37 through line 38 and passed to filter 39. Filter cake is withdrawn through line 40 and filtrate through line 41.

In the cycle succeeding the illustrated cycle, tank 32 will be filling and heating, the contents of tank 30 will be held at hydrolysis temperature and pressure, and the contents of tank 31 will be cooled and withdrawn.

Isophthalic acid and terephthalic acid produced from mixtures of their ammonium salts and amides, in the manner described with reference to- Figure 3 of the drawings have nitrogen contents well below 0.03% detcrmined as ammonia. When these products are substituted for phthalic anhydride in conventional alkyd resin cooks, the resin is free of tar and sulfide precipitates and shows Gardner colors in the range from 4 to 6. Similarly, orthophthalic acid, tertiary-butyl isophthalic acid and tertiary-butyl ortho-phthalicv acid having extremely low nitrogen contents can be produced from mixtures of their ammonium salts and amidesv pursuant to the invention. Mixtures of the ammonium salts and amides of tertiary-butyl isophthalic acid and tertiary-butyl orthophthalic acid are produced. when LS-dimethyi-S-tertiarybutyl' benzene and l,2-dimethyl-4-tertiary butyl benzene, respectively, are subjected to oxidation in the manner described in the Toland application above referred to. While the process. of. the invention is especially well adapted to recovering pure phthalic acids from mixtures of the ammonium salts and amides of phthalic acids, which are characterized by limited water solubility, it may also be employed to recover other aromatic acids such as benzoic acid and toluic acid from mixtures of their. ammonium salts and amides. I The term phthalic acids as employed in the appended claims isintended to. comprehendv not only the. three isomeric phthalic acids themselves, but also the tertiary-butylesubstituted isophthalic and ortho-phthalic acids which appear as ars igoso 7 mixtures of their ammonium salts and amides When tertiary-butyl xylenes are subjected to the above-described oxidation process.

The second hydrolysis stage employing an aqueous mixture of sodium bisulfate and sodium sulfate may be carried out in a continuous manner paralleling the firststage operation, if desired. However, the batch hydrolysis illustrated in the drawing has the advantage of providing time for phthalic acid crystal growth during the period when a batch reaction vessel is being ed and emptied. By cooling the hot hydrolysis product at a rate not exceeding about 15 Fahrenheit degrees per minute, the major proportion of the solid acid is larger than 200 mesh size. Crystals of this size are readily filtered from the hydrolysis reaction product and are adaptable to use in alkyd resin manufacture as such. in continuous operation, smaller crystals are obtained which must be subjected to a flaking treatment in order to increase the particle size of the solid acid sufiiciently to make it readily usable in conventional alkyd cooking.

We claim:

1. A process for recovering phthalic acids from mixtures of ammonium salts and amides of these acids which comprises acidifying an aqueous solution of said mixture with sulfuric acid to a pH in the range from about 2.5 to about 3.5, heating the acidified mixture to a temperature in the range from 300 to about 500 F. under superatmospheric pressure sufficient to maintain a substantial part of the water in liquid phase, cooling the mixture to a temperature below about 250 F., adding a quantity of sulfuric acid to the cooled mixture sufficient to lower the pH of the mixture to a value in the range from about 2.5 to about 3.5, filtering the resultant mixture to separate a filter cake consisting essentially of phthalic acid, mixing the filter cake with sufficient water to form a slurry having a solids content from about 2% to about 50% by weight, heating the slurry to a temperature in the range from 430 to 500 F. under a superatmospheric pressure sufiicient to maintain a substantial part of the water in liquid phase, cooling the heated slurry to a temperature below about 250 F. and filtering the cooled slurry to recover a filter cake consisting essentially of phthalic acids.

2. A process for recovering phthalic acids from mixtures of ammonium salts and amides of these acids which comprises adding to an aqueous solution of said mixture approximately one equivalent of sulfuric acid per equivalent of ammonium carboxylate contained in said mixture, heating the resultant mixture to a temperature in the range from 300 to 500 F. under a superatmospheric pressure suflicient to maintain a substantial part of the water in liquid phase, cooling the mixture and adding to the cooled mixture approximately one equivalent'of sulfuric acid for each equivalent of ammonium carboxylate contained in said cooled mixture, filtering the resultant mixture to separate a filter cake consisting essentially of phthalic acid, slurrying the filter cake with a quantity of water suflicient to produce a slurry having a solids content in the range from about 2% to about 50% by weight, heating the slurry to a temperature in the range from about 430 F. to about 500 F., cooling the hot mixture and filtering the cooled mixture to separate a filter cake consisting essentially of phthalic acids.

3. A process for recovering phthalic acids from mix tures of ammonium salts and amides of these acids which comprises acidifying an aqueous solution of said mixture with sulfuric acid to a pH in the range from about 2.5 to about 3.5, heating the acidified mixtureto a temperature in the range from 300 F. to about 500 F. under a superatmospheric pressure sufiicient to maintain a substantial part of the water in liquid phase, cooling the mixture and filteringthe cooled mixture to recover a filter calie consisting essentially of phthalic acid, slurrying the filter cake with a quantity of an aqueous solution of a bisulfate selected'frorn the group consisting of ammonium bisulfate and alkali metal bisulfates suificient to produce a slurry having a solids content in the range from about 2% to about 50% by weight, and characterized'by a pH in the range from about 1.2 to about 1.8, heating the slurry to a temperature in the range from about 350 F. to 500 F., cooling the hot mixture and filtering it to recover a filter cake consisting essentially of phthalic acids.

4. A process for recovering phthalic acids from mixtures of ammonium salts and amides of these acids which comprises acidifying an aqueous solution of said mixture with sunurr'c acid to a pH in the range from about 2.5 to about 3.5, heating the acidified mixture to a temperature in the range from 300 F. to about 500 F. under a superetmospheric pressure sufiicient to maintain a substantial part of the water in liquid phase, cooling the mixture, acidifying the cooled mixture with sulfuric acid to a pH in the range from about 2.5 to about 3.5, and filtering the acidified mixture to recover a filter cake consisting essentially of phthalic acid, slurrying the filter cake with a quantity of an aqueous solution of a bisulfate selected from the group consisting of ammonium bisulfate and alkali metal bisulfates sufiicient to produce a slurry having a solids content in the range from about 2% to about 50% by weight, and characterized by a pH in the range from about 1.2 to about 1.8, heating the slurry to a temperature in the range from about 350 F. to 500 F, cooling the hot mixture and filtering it to recover a filter cake consisting essentially of phthalic acids.

5. A process for recovering phthalic acids from mixtures of ammonium salts and amides of these acids which comprises adding to an aqueous solution of said mixture approximately one equivalent of sulfuric acid per equivalent of ammonium carboxylate contained in said mixture, heating the resultant mixture to a temperature in the range from about 400 F. to 500 F. under a superatmospheric pressure sufiicient to maintain a substantial part of the water in liquid phase, cooling the mixture, adding to the cooled mixture approximately one equivalent of sulfuric acid per equivalent of ammonium carboxylate contained in said mixture, and filtering the resultant mixture to recover a filter cake consisting essentially of phthalic acid, slurrying the filter cake with an aqueous bisulfate soiutio-n, said bisulfate solution being a solution of a bisulfate selected from the group consisting of ammonium bisulfate and alkali metal bisulfates, the volume of said bisulfate solution being such that the resultant slurry has a solids content in the range from about 2% to about 50% by weight and the bisulfate content of the bisulfate solution employed in forming the slurry being approximately one mol of bisulfate per mol of nitrogen deter: mined as ammonia contained in said filter cake, heating said slurry to a temperature in the range from about 350 F. to about 500 F., cooling the heated slurry and filtering it to recover a filter cake consisting essentially of phthalic acids.

6. A process for recovering highly purified aromatic polycarboxylic acids characterized by low solubility in water, from the amides of these acids, which comprises digesting the amide with an aoueous solution of an inorganic hydrolyst to liberate the free acid, filtering the reaction product to recover a filter cake comprising the free acid, digesting the filter cake with water at a temperature above 300 F. and under a superatrnospheric pres sure sul'licient to maintain water in liquid phase and filtering the digestion product to recover the purified acid.

7. A process for recovering highly purified aromatic polycarbox-ylic acids characterized by low solubility in water, from the amides of theseacids, which comprises hydrolyzing the amide to separate the free acid, filtering the reaction product to recover a filter cake comprising the free acid, digesting the filter cake with liquid water at a temperature in the range 300 F. to 500 F. and filtering the digested product to recover the purified acid.

'8. A process for recovering highly purified isophthalic acid and terephthalic acid from the amides of these-acids,-

9 which comprises digesting the amides with an aqueous solution of a strong mineral acid to liberate the free organic acid, filtering the reaction product to recover the filter cake comprising the free acid, digesting the filter cake with liquid water at a temperature above 300 F. and under a superatmospheric pressure suificient to maintain the water in liquid phase, the quantity of water employed being substantially less than the amount required to dissolve the acid, and filtering the digestion product to recover the purified acid.

9. A process for recovering highly purified isophthalic acid and terephthalic acid from the reaction product obtained by oxidizing meta-xylene and para-xylene with ammonium sulfate, water and a water-soluble sulfide, which comprises digesting the reaction product with an aqueous solution of a strong base to hydrolyze ammonium salts and amides and form phthalic acid salts, acidifying the resulting mixture to precipitate free phthalic acids, filtering the acidified product to recover a filter cake comprising free phthalic acids, slurrying the filter cake with suflicient liquid water to form a slurry containing from 10 to 40 per cent by weight of solid phthalic acids, heating the slurry to a temperature from about 300 F. to 525 F. under a superatmospheric pressure sufiicient to maintain the water in liquid phase and filtering the slurry to recover purified phthalic acids.

10. A process for recovering highly purified isophthalic and terephthalic acids from the reaction product obtained by oxidizing meta-xylene and para-xylene .with ammonium sulfate, water and a water-soluble sulfide, which comprises digesting the reaction product with aqueous sulfuric acid to liberate free phthalic acids, filtering the resultant mixture to recover a filter cake comprising free phthalic acids, slurrying the filter cake with water to form a slurry containing from about 10 to 40 per cent by weight of solid phthalic acids, digesting the slurry at a temperature from about 300 F. to 525 F. under a superatmospheric pressure suflicient to maintain water in liquid phase and filtering the digestion product to recover purified phthalic acids.

11. A process for recovering highly purified isophthalic acid and terephthalic acid from the amides of these acids which comprises digesting the amides with an aqueous solution of a strong mineral acid to hydrolyze the amide, filtering the reaction product to recover a free acid product, digesting the filter cake with suflicient liquid water to form a slurry containing from about 5 to about by weight solids at a temperature above 300 F. and under a superatmospheric pressure suificient to maintain the water in liquid phase, the quantity of water employed being substantially less than the amount required to dissolve all of the acid, and filtering the digestion product to recover purified phthalic acid.

12. A process for recovering highly purified aromatic polycarboxylic acids characterized by low solubility in water, from the amides of these acids, which comprises hydrolyzing the amides to separate the free acids, filtering the reaction product to recover a filter cake comprising the free acids, digesting the filter cake with a liquid aqueous medium selected from the group consisting of water,

a solution of mineral acid, a solution of ammonium bisulfate and a solution of an alkali metal bisnlfate at a temperature in the range of about 300 F. to about 500 F., and under sufiicient superatmospheric pressure to maintain water in the liquid phase, and filtering the digested product to recover the purified polycarboxylic acids.

13. The process of claim 12 wherein the amides are bydrolyzed with sulfuric acid.

14. The process of claim 12 wherein the amides are hydrolyzed with phosphoric acid.

15. The process of claim 12 wherein the filter cake is digested with aqueous sulfuric acid.

16. The process of claim 12 wherein the filter cake is digested with an aqueous solution of bisulfate.

17. The process of claim 12 wherein the amides are mixed with a mineral acid to form a mixture having a pH below about 4.0 and the amides are hydrolyzed by digesting the mixture at a temperature above about 300 F. and under a superatmospheric pressure suflicient to maintain water in liquid phase.

18. The process of claim 17 wherein the acid is sulfuric acid.

19. The process of claim 12 wherein the amides comprise isoand tere-phthalic acid amides.

No references cited.

Claims (1)

1. A PROCESS FOR THE RECOVERING PHTHALIC ACIDS FROM MIXTURES OF AMMONIUM SALTS AND AMIDES OF THESE ACIDS WHICH COMPRISES ACIDIFYING AN AQUEOUS SOLUTION OF SAID MIXTURE WITH SULFURIC ACID TO A PH IN THE RANGE FROM ABOUT 2.5 TO ABOUT 3.5, HEATING THE ACIDIFIED MIXTURE TO A TEMPERATURE IN THE RANGE FROM 300 TO ABOUT 500* F. UNDER SUPERATMOSPHERIC PRESSURE SUFFICIENT TO MAINTAIN A SUBSTANTIAL PART OF THE WATER IN LIQUID PHASE, COOLING THE MIXTURE TO A TEMPERATURE BELOW ABOUT 250* F., ADDING A QUANTITY OF SULFURIC ACID TO THE COOLED MIXTURE SUFFICIENT TO LOWER THE PH OF THE MIXTURE TO A VALUE IN THE RANGE FROM ABOUT 2.5 TO 3.5, FILTERING THE RESULTANT MIXTURE TO SEPARATE A FILTER CAKE CONSISTING ESSENTIALLY OF PHTHALIC ACID, MIXING THE FILTER CAKE WITH SUFFICIENT WATER TO FORM A SLURRY HAVING A SOLIDS CONTENT FROM ABOUT 2% TO ABOUT 50% BY WEIGHT, HEATING THE SLURRY TO A TEMPERATURE IN THE RANGE FROM 430 TO 500* F. UNDER A SUPERATMOSPHERIC PRESSURE SUFFICIENT TO MAINTAIN A SUBSTANTIAL PART OF THE WATER IN LIQUID PHASE, COOLING THE HEATED SLURRY TO A TEMPERATURE BELOW ABOUT 250* F. AND FILTERING THE COOLED SLURRY TO RECOVER A FILTER CAKE CONSISTING ESSENTIALLY OF PHTHALIC ACIDS.

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