Purification of p-aminophenol compositions and direct conversion to N-acetyl-p-aminophenol
This application is a continuation of application Ser. No. 199,531, filed May 27, 1988, abandoned which is a continuation of application Ser. No. 053,097, filed May 22, 1987, abandoned, which is a continuation of application Ser. No. 898,054, filed Aug. 19, 1986, abandoned, which is a continuation of application Ser. No. 809,940, filed Dec. 17, 1985, abandoned, which is a continuation of parent application Ser. No. 575,000, filed Jan. 30, 1984, abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to the manufacture of N-acetyl-p-aminophenol and more particularly to a improved process in which N-acetyl-p-aminophenol is prepared by acetylation of p-aminophenol that has been produced by catalytic hydrogenation of nitrobenzene in acidic aqueous medium. 
     N-acetyl-p-aminophenol (&#34;APAP&#34;) is an analgesic more commonly known as acetaminophen. Conventionally, APAP is produced by acetylation of p-aminophenol (&#34;PAP&#34;) with acetic anyhydride. In one variation of this process, the PAP used is produced by catalytic hydrogenation of nitrobenzene in an acidic aqueous medium, for example, in approximately 10% to 15% by weight solution of sulfuric acid. 
     According to conventional practice, the PAP is first isolated from the hydrogenation reaction medium through precipitation with a base, and the precipitate taken up in a solvent prior to acetylation. Precipitation and redissolution of the PAP has been considered necessary in order to exclude impurities from the acetylation reaction medium. Among the impurities typically present in the PAP reaction liquor are aniline, o-aminophenol, 4,4&#39;-diaminodiphenylether, and other aromatic amines at trace or low levels. Purification of the crude PAP system is necessary to provide for satisfactory manufacture of U.S.P. quality APAP. 
     In the co-pending and co-assigned application of Clingan et al., Ser. No. 251,461, filed Apr. 20, 1981, U.S. Pat. No. 4,440,954, a process is disclosed for removing 4,4&#39;-diaminodiphenylether from the PAP hydrogenation liquor by extraction with a mixture of aniline and toluene. While this process is quite effective for removing the 4,4&#39;-diaminodiphenylether, residual aniline content in the extract is typically in the range of 25 g/L to 45 g/L. In implementation of the Clingan et al. process, therefore, it has still been considered essential to precipitate PAP from the raffinate by treatment with base, separate the PAP by centrifugation or filtration, and wash the filter cake prior to acetylation. 
     SUMMARY OF THE INVENTION 
     Among the several objects of the invention therefore, may be noted the the provision of a process for removal of undesirable impurities, including aniline, from the reaction liquor obtained in the catalytic hydrogenation of nitrobenzene; the provision of an improved process for the preparation of APAP; the provision of a process for the preparation of APAP by acetylation of PAP in a purified catalytic hydrogenation liquor; the provision of a process for producing APAP of high quality; the provision of a process for producing APAP with high yields on PAP and other raw materials; and the provision of a process which simplifies handling of process materials and minimizes the number of process operations in the acetylation of PAP to APAP. 
     Briefly, therefore, the present invention is directed to an improvement in a process for the preparation of APAP by acetylation of PAP. In the process, PAP is initially prepared by catalytic hydrogenation of nitrobenzene in an aqueous acid medium to produce an aqueous acidic reaction mixture containing PAP, and prior to the conversion of PAP to APAP, by-product 4,4&#39;-diaminodiphenylether is removed from an aqueous feed mixture comprising the aqueous reaction mixture by a primary extraction with an extractant comprising aniline to produce a primary aqueous raffinate containing the PAP reaction product. According to the improvement, the primary raffinate is purified by contacting it at a pH of between about 5.5 and about 8.0 with toluene to extract aniline therefrom and produce a purified raffinate containing PAP dissolved therein. 
     The invention is further directed to a process for removing impurities from an aqueous feed mixture comprising a reaction solution produced by catalytic hydrogentation of nitrobenzene in an acid aqueous medium. The process comprises contacting the aqueous feed mixture with an extractant comprising aniline for extraction from the feed mixture of 4,4&#39;-diaminodiphenylether, thereby providing a primary raffinate that contains PAP and is contaminated with aniline. The primary raffinate is contacted at a pH of between about 5.5 and about 8.0 with a purifying solvent, the purifying solvent comprising an organic solvent effective for extraction of aniline from the primary raffinate. A purified raffinate comprising PAP is thereby produced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic flow sheet illustrating an embodiment of the process of the invention; and 
     FIG. 2 is a schematic flow sheet illustrating a particularly preferred embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In accordance with the present invention, an improved process has been discovered for purifying the PAP reaction liquor obtained upon the catalytic hydrogenation of nitrobenzene in acidic aqueous medium. In a particularly preferred application of the process of the invention, PAP is acetylated in the purified raffinate without prior separation of solid PAP from the raffinate. This unified process reduces the number of process steps and simplifies plant operations, yet provides a product of high quality with favorable yield on PAP and acetic anhydride. 
     In its preferred embodiments, the process of the invention represents a significant improvement over the process of Clingan et al., Ser. No. 251,461 U.S. Pat. No. 4,440,954. While the Clingan et al. process is effective for removal of 4,4&#39;-diaminodiphenylether and other basic amines from the PAP aqueous reaction liquor by extraction with a mixture of aniline and toluene, the improved process of the invention provides for purification of the aqueous raffinate by extraction to remove residual aniline. It further provides more efficient removal of all impurities in the primary extraction step. The purification step utilizes a solvent which is effective for reducing the aniline content to 2 g/L or less, and thus provides a purified raffinate suitable for direct acetylation. 
     In the embodiment of the invention illustrated in FIG. 1, PAP liquor obtained by catalytic hydrogenation of nitrobenzene is adjusted to a pH of 4.0 to 8.0 as, for example, by addition thereto of ammonia or ammonium hydroxide, and the resultant aqueous mixture is extracted with a mixture of aniline and toluene for removal of 4,4&#39;-diaminodiphenylether. As disclosed in the aforesaid Clingan et al. application, the aniline/toluene extractant contains between about 0.2 parts by volume and about 4 parts by volume toluene per part by volume aniline, preferably about 1 part by volume toluene per part by volume aniline. In this extractant mixture, aniline functions as the extracting agent while toluene functions as an anti-solvent and diluent. Extraction of the 4,4&#39;-diaminodiphenylether from the aqueous feed mixture may be carried out on either a batch or on a continuous basis. Preferably, the extraction is carried out in a countercurrent extraction column such as a Karr reciprocating plate extraction column or other similar columns known to those skilled in the art. Equipment such as a centrifugal countercurrent extractor, pumper-decanter, pulsed column, or a mixer/settler apparatus may also be employed. Using a Karr countercurrent extraction column, for example, it has been found that the 4,4&#39;-diaminodiphenylether content of the aqueous PAP phase may be reduced to below 50 ppm and usually below 10 ppm with four to six theoretical extraction stages, using a 50%--50% aniline/toluene mixture with a pH of 4.8. Alternatively, aniline alone may be utilized as the extractant for removal of 4,4&#39;-diaminodiphenylether, but aniline alone results in high PAP losses and phase separation problems. Preferably the volumetric ratio of aniline or aniline/toluene to aqueous feed solution is between about 0.05 and about 1:1, with a ratio of approximately 0.2:1 being preferred. 
     As illustrated in FIG. 1, the organic extract emanating from the primary extraction step is preferably back-extracted for recovery of the PAP contained therein. Back-extraction is effected by contacting the extract with an aqueous back-extraction solution containing ammonium sulfate, aniline sulfate, or sulfuric acid. Where the primary extraction is carried out in a continuous countercurrent extraction system such as the Karr reciprocating column or other vertical column, back-extraction may be carried out by adding the back extraction solution to the column at a point above the feed point at which the aqueous PAP feed liquor is charged to the column, and withdrawing the back-extracted organic phase from a point above the feed point. 
     Primary raffinate exiting from the primary extraction step is preferably treated with an adsorbent such as activated carbon for the removal of residual impurities and color. This treatment may be accomplished, for example, by passing the primary raffinate through a fixed bed comprised of the adsorbent. 
     In accordance with the improved process the primary raffinate, whether treated by adsorption or not, is contacted with a purifying solvent in a liquid-liquid extraction system for extraction of residual aniline to produce a purified raffinate containing PAP dissolved therein. A variety of solvents may be used as the purifying solvent including, for example, halogenated solvents such as ethylene dichloride and methylene dichloride, hydrocarbon solvents such as benzene, toluene, and xylene, as well as various esters of lower alcohols and lower molecular weight alkanoic acids. Hydrocarbon solvents are preferred, with toluene being particularly preferred. 
     Prior to purification the pH of the primary raffinate is preferably adjusted to between about 5.5 and about 8.0, preferably 6.5 to 7.5, by addition of a base such as ammonia. As pH is increased PAP precipitates, in which case the primary raffinate is in the form of a slurry. Where the purification step is operated at a pH in the preferred range, a portion of the secondary extract is preferably recycled to the pH adjustment point so that pH adjustment takes place in the presence of an organic solvent phase. This stratagem promotes removal of impurities from the aqueous phase simultaneously with precipitation, thereby minimizing occlusions of impurities in the PAP precipitate. 
     In the purification extraction step, the purifying solvent is preferably mixed with the primary raffinate in a ratio of at least 0.05 part by volume solvent per part by volume aqueous phase, more preferably about 0.1 part by volume solvent per part by volume aqueous phase. Although the purification extraction can be carried out at any temperature between the crystallizing point of the aqueous phase and the boiling point of the solvent at the operating pressure, it is preferred that the extraction be carried out at a temperature of at least about 80° C. to enhance the distribution coefficents and mass transfer rates. A temperature in the range of between about 80° C. and about 85° C. is particularly preferred for operation at atmospheric pressure. Even higher temperatures are advantageous for their favorable effect on the efficiency of extraction, but require the use of pressurized systems. 
     As in the case of the primary extraction, the purification may be conducted in a batch mixer/settler system, a centrifugal extractor, or a vertical column, with a Karr reciprocating plate column being especially preferred. In such a countercurrent extraction system, approximately six to eight stages are required for lowering the aniline content of the aqueous phase to the extent of producing a purified raffinate containing not more than about 2 g/L of aniline. By carrying out the purifying extraction at relatively high pH, i.e., from about 6.5 to about 7.5, the aniline content of the raffinate can be reduced to as low as about 0.2 g/L. Distribution coefficients between solvent and aqueous phase are more favorable at high pH for both aniline and other aromatic organic amine impurities. Where the primary raffinate contains between about 15 g/L and about 45 g/L of aniline, the secondary extract exiting the purification step typically contains 50 g/L to 300 g/L aniline. Even when the purification is conducted at relatively high pH, PAP losses can be limited to significantly less than 1% by weight where the extract is back-extracted as described above. 
     Where toluene is used as the purifying solvent, the secondary extract may be recycled and used in the primary extraction step for removal of 4,4&#39;-diaminodiphenylether from the aqueous PAP hydrogenation liquor. Any portion of the secondary extract that is not recycled may be combined with the back-extracted organic phase from the primary extraction step and subjected to distillation for separation of toluene and aniline. The toluene separated in this distillation step may be used as the purifying solvent in the purification extraction step, provided that the aniline content of this toluene fraction has been reduced to no greater than about 0.2% by weight. 
     Purified raffinate from the purification step can be directly acetylated in the manner outlined in FIG. 1 and described in further detail hereinbelow. 
     FIG. 2 illustrates an alternative and preferred embodiment of the invention in which both the primary and purification extraction operations are carried out in a continuous countercurrent extraction system, preferably a single vertical column, advantageously a Karr reciprocating plate column. As further illustrated in FIG. 2, it is also preferred that the back-extraction be carried out in the same column. In this system, the primary extraction is conducted in a section of the column below the feed point of aqueous PAP liquor and above the point of introduction of aniline or aniline/toluene mixture. Toluene or other purifying solvent is introduced into the column at a point below the point of aniline introduction and the purified raffinate is withdrawn from the column at a point below the point of aniline introduction. Thus, with respect to the flow of the aqueous phase, the purifying solvent is introduced downstream of the point of introduction of aniline. Inasmuch as the purifying solvent necessarily passes through the primary extraction zone of the column, an aniline/toluene primary extractant mixture is present in the primary extraction zone irrespective of whether aniline alone or an aniline/toluene mixture is injected at the point of aniline introduction. 
     For the back-extraction, an aqueous solution of aniline sulfate, ammonium sulfate, or sulfuric acid is introduced above the PAP liquor feed point and the organic extract is withdrawn above the PAP liquor feed point. The preferred conditions for the primary and purification extraction steps are essentially the same in the process of FIG. 2 as they are in that of FIG. 1, except, of course, that the entire purification extract is necessarily recycled through the primary extraction zone. Prior to extraction, the pH of the aqueous feed liquor is preferably adjusted to at least about 4.0 by addition of a base such as ammonia. Before it passes through the purification extraction zone, the aqueous phase should be adjusted to pH of between about 5.5 and about 8.0, preferably between about 6.5 and about 7.5. In a continuous countercurrent extraction pH adjustment may most easily be carried out prior to feeding the aqueous mixture into the primary extraction zone. However, it may be advantageous to distribute the injection of base over the purification extraction zone, primary extraction zone, or both, in order to provide a pH gradient with a relatively low pH at a point within the zones downstream of the feed point with respect to the flow of aqueous phase. This arrangement minimizes the solubility of PAP in the extract stream as it leaves the primary extraction zone. 
     Where the pH of the aqueous phase is adjusted to the preferred range of 6.5 to 7.5, PAP contained in the aqueous phase is typically precipitated. Thus, the primary raffinate is in the form of a slurry as it is contacted with the purifying solvent for extraction of aniline in the purification extraction step. If pH adjustment is carried out prior to delivery of the aqueous feed mixture to the extraction column, as is conveniently the case in the system of FIG. 2, a slurry is present in the primary extraction step as well. At a temperature of 80° C. and a pH of about 7, the solids content of the slurry is typically 2% to 7% by weight, preferably 5% to 6% by weight. In accordance with the invention, it has been discovered that each of the liquid/liquid extraction steps can be carried out with high efficiency even where the PAP has been precipitated and is slurried in the aqueous phase. It has further been found that acetic anhydride can be mixed directly with such a slurry and reacted with p-aminophenol contained in the slurry to produce APAP. 
     As illustrated in FIG. 2, the purified raffinate is preferably subjected to carbon treatment for removal of residual impurities prior to acetylation. Where the primary raffinate is in the form of a slurry, it is acidified to dissolve PAP in the aqueous phase, and the acidified purified raffinate treated with carbon or another adsorbent for removal of impurities. Repurified acidic raffinate exiting the carbon treatment step is preferably treated with a base such as ammonia to reprecipitate the PAP and produce a repurified raffinate slurry in which the acetylation reaction may thereafter be conducted. 
     Because the acetylation reaction has been found to proceed satisfactorily in a PAP slurry, further gains in productivity may be realized where the slurry is concentrated by partial dewatering prior to introducing acetic anhydride for reaction with the PAP. As shown in FIG. 2, this concentration step is preferably carried out using a solid bowl centrifuge. While this step concentrates the slurry, it does not remove PAP from the aqueous phase within which it is contained. In this fashion, very high payloads of APAP can be obtained. 
     Irrespective of the pH at which the purification extraction step is carried out, the pH of the purified raffinate or raffinate slurry is preferably adjusted to between about 6 and about 7, preferably between about 6.5 and about 7, prior to acetylation. In the process of the invention, this pH range has been found to provide maximium conversion of PAP to APAP without formation of excessive amounts of impurities. To carry out the acetylation reaction, acetic anhydride is added to the purified or repurified raffinate and the reaction carried out at a temperature of at least about 80° C., preferably between about 85° C. and about 95° C. An approximately 5% to 10% excess of acetic anhydride is preferably used for the acetylation. Substantially quantitative conversion of PAP to APAP is achieved. 
     After the acetylation reaction is complete, the reaction mixture is preferably treated with an adsorbent for removal of impurities, typically by passing it through a fixed bed of activated carbon. The carbon-treated reaction mixture is then cooled for crystallization of APAP therefrom. Preferably, the mixture is cooled to a temperature of about 15° C., after which the slurry produced upon crystallization is filtered or centrifuged for recovery of the crystalline product. The filter cake is preferably washed with deionized water. 
     All steps of the processes of both FIG. 1 and FIG. 2 should be maintained under an inert gas blanket, preferably a nitrogen blanket. 
     In accordance with the invention, it has been found that high quality APAP may be produced in a unified system which does not require recovery of a PAP precipitate for the exclusion of impurities. Good yields are obtained on both PAP and acetic anhydride. In fact, recovery of APAP in the crystallization step is enhanced because solubility of APAP in the crude mother liquor is lower where PAP is acetylated without prior removal from the aqueous phase derived from nitrobenzene hydrogenation. This apparently results from a salting out effect caused by the presence of (NH 4 ) 2  SO 4  in the aqueous phase. Additionally, process operations are simplified as compared to the commercial processes previously known to the art. 
     The following examples illustrate the invention. 
     EXAMPLE 1 
     An aqueous PAP liquor produced by the catalytic hydrogenation of nitrobenzene in dilute sulfuric acid solution was subjected to primary extraction in a plant reciprocating plate column for substantial removal of 4,4&#39;-diaminodiphenylether. The raffinate, containing 77.3 g/L PAP, was subjected to further primary extraction (for removal of OAP and other organic amines) and purification extraction (for removal of aniline and other organic amines) in a crosscurrent laboratory extraction system simulating an extraction scheme of the type illustrated in FIG. 1. Prior to laboratory extraction, the PAP liquor at a temperature of 80° C. was passed through a column containing activated carbon and the pH of the liquor was then adjusted from 4.3 to 7.0 by addition of concentrated ammonium hydroxide. 
     Cross-current extraction was first carried out at 80° C. in five equilibrium stages (primary extraction), using an extractant comprising 50% by volume aniline and 50% by volume toluene. In this extraction system the volumetric ratio of organic-to-aqueous phase was 0.2. 
     In the second extraction step, primary raffinate emanating from the first step was extracted in five equilibrium stages using toluene as the purifying solvent. The ratio of organic-to-aqueous phase in the second extraction step was 0.1. The aqueous phase contained PAP solids. 
     All extractions were conducted under a nitrogen blanket. Upon adjustment of pH to 7.0, PAP solids precipatated and were thereafter present as a suspension in the aqueous phase. 
     The aqueous feed stream and the aqueous and organic phases exiting each of the extraction stages were analyzed for PAP, OAP, aniline, and 4,4&#39;-diaminodiphenylether (ODA). Results of these analyses for the aqueous feed, primary extract, primary raffinate, secondary extract, and purified raffinate are set forth below in Table I. 
     
                       TABLE I                                                     
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                                       Other                              
                                       Aromatic                           
                                       Organic                            
           PAP     OAP     Aniline                                        
                                 ODA   Impurities                         
SAMPLE     (g/L)   (g/L)   (g/L) (g/L) (g/L)                              
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Aqueous Feed                                                              
           77.3    1.54    35.3  N.D.  0.43                               
Primary Extract                                                           
           50.6    4.36    506   0.12  4.5                                
Primary Raffinate                                                         
           27.6*   N.D.    15.3  N.D.  N.D.                               
Secondary  6.7     N.D.    176   N.D.  1.05                               
Extract                                                                   
Purified Raffinate                                                        
           25.1*   N.D.    0.81  N.D.  N.D.                               
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 *Reflects analysis for PAP soluble in aqueous phase; solids content of   
 raffinate slurry is not included.                                        
 
    
     EXAMPLE 2 
     A PAP liquor obtained by catalytic hydrogenation of nitrobenzene in dilute sulfuric acid was subjected to laboratory extraction simulating a process of the type illustrated in FIG. 2. Concentrated ammonia was added to the PAP liquor at a temperature of 80° C. to adjust the pH of the liquor from 1.2 to 7.0, as a result of which PAP precipitated in the liquor, forming a slurry. 
     In the first step of the cross-current extraction simulating the primary extraction zone in the extraction column of FIG. 2, the PAP slurry was contacted with an extractant containing 50% by volume aniline and 50% by volume toluene at 80° C. in five equilibrium stages. The organic-to-aqueous volumetric ratio in this step was 0.2. Thereafter the primary raffinate obtained from the first step was subjected to extraction with toluene in six equilibrium stages at an organic-to-aqueous volumetric ratio of 0.1, thus simulating the extraction carried out in the purification zone of the extraction column of FIG. 2. 
     Samples were taken from the organic and aqueous phases exiting each of the cross-current extraction steps. These samples; were analyzed for PAP, OAP, aniline, and ODA. The results of these analyses of the aqueous feed liqour, primary extract, primary raffinate, secondary extract, and purified raffinate are set forth in Table II. 
     
                       TABLE II                                                    
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                                       Other                              
                                       Aromatic                           
                                       Organic                            
           PAP     OAP     Aniline                                        
                                 ODA   Impurities                         
SAMPLE     (g/L)   (g/L)   (g/L) (g/L) (g/L)                              
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Aqueous Feed                                                              
           79.7    1.81    28.0  2.06  6.5                                
Primary Extract                                                           
           50.5    5.13    481   8.48  29.24                              
Primary Raffinate                                                         
           28.1*   N.D.    15.6  N.D.  N.D.                               
Secondary  8.82    0.58    222   0.09   1.30                              
Extract                                                                   
Purified Raffinate                                                        
           24.7*   N.D.    0.62  N.D.  N.D.                               
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 *Reflects analysis for PAP soluble in aqueous phase; solids content of   
 raffinate slurry is not included.                                        
 
    
     EXAMPLE 3 
     A portion of the purified raffinate obtained from Example 2 was adjusted to pH 6.5, cooled to between about 15° C. and about 20° C., and concentrated to 240 g/L PAP. Acetic anhydride (110% of stoichiometric equivalent) was added to the concentrated slurry, and the PAP content acetylated at approximately 60° C. The acetylation reaction mixture was treated with activated carbon at between about 95° C. and about 100° C. for removal of residual impurities and then cooled to crystallize the product APAP therefrom. The crystalline product was separated from the mother liquor by filtration and the filter cake analyzed for the presence of various impurities. The results of this analysis are set forth in Table III. 
     Another portion of the purified raffinate was adjusted to pH 7.0 by addition of ammonia and then concentrated and acetylated as described above. Product APAP was recovered by crystallization and filtration as described above. Analysis of the cake is also set forth in Table III. 
     
                       TABLE III                                                   
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CRUDE CAKE COMPOSITION                                                    
                                   Other Aromatic                         
                                   Organic                                
Acetylation                                                               
        APAP      H.sub.2 O PAP    Impurities                             
(pH)    (weight %)                                                        
                  (weight %)                                              
                            (ppm)  (ppm)                                  
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6.5     99.3      0.63      80     3300                                   
7.0     99.2      0.72      60     4000                                   
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     U.S.P. quality APAP is prepared by recrystallization of the crude APAP. 
     In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained. 
     As various changes could be made in the above methods and processes without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.