Continuous hydrolysis process for the preparation of 2-hydroxy-4-methylthiobutanoic acid

Abstract

A continuous process for the preparation of 2-hydroxy-4-methylthiobutanoic acid which includes introducing sulfuric acid into a first reactor including a continuous stirred tank reactor and introducing 2-hydroxy-4-methylthiobutanenitrile into the first reactor. 2-hydroxy-4-methylthiobutanenitrile is continually hydrolyzed within the first reactor to produce an intermediate aqueous hydrolysis solution containing 2-hydroxy-4-methylthiobutanamide. The intermediate aqueous hydrolysis solution is continuously introduced into a plug flow reactor. 2-hydroxy-4-methylthiobutanamide is continually hydrolyzed within the plug flow reactor to produce an aqueous hydrolyzate product solution containing 2-hydroxy-4-methylthiobutanoic acid. 2-hydroxy-4-methylthiobutanoic acid is recovered from the aqueous hydrolyzate product solution.

We claim:  
     
         1 . A process for the preparation of 2-hydroxy-4-methylthiobutanoic acid comprising: 
 introducing sulfuric acid into a first reactor comprising a continuous stirred tank reactor;    introducing 2-hydroxy-4-methylthiobutanenitrile into said first reactor;    continuously hydrolyzing 2-hydroxy-4-methyl-thiobutyronitrile within said first reactor to produce an intermediate aqueous hydrolysis solution containing 2-hydroxy-4-methylthiobutanamide;    continuously introducing water and the intermediate aqueous hydrolysis solution into a plug flow reactor; and    continuously hydrolyzing 2-hydroxy-4-methylthiobutanamide within said plug flow reactor to produce an aqueous hydrolyzate product solution containing 2-hydroxy-4-methylthiobutanoic acid.    
     
     
         2 . The process as set forth in  claim 1  wherein sulfuric acid is introduced into said first reactor in an acid stream having a strength of between about 50% by weight and about 70% by weight sulfuric acid.  
     
     
         3 . The process as set forth in  claim 1  wherein sulfuric acid is introduced into said first reactor in an acid stream having a strength of between about 70% by weight and about 99% by weight sulfuric acid, and the acid stream is continuously introduced to the first reactor concurrently with a water stream to form sulfuric acid having a strength of between about 50% by weight and about 70% by weight on an organic-free basis within the first reactor.  
     
     
         4 . The process as set forth in  claim 1  wherein at least about 90% of 2-hydroxy-4-methylthiobutanenitrile is converted to 2-hydroxy-4-methylthiobutanamide within the first reactor.  
     
     
         5 . The process as set forth in  claim 1  wherein the molar ratio of sulfuric acid to 2-hydroxy-4-methylthiobutanenitrile introduced into the first reactor is between about 0.7 and about 1.5.  
     
     
         6 . The process as set forth in  claim 1  wherein the molar ratio of sulfuric acid to 2-hydroxy-4-methylthiobutanenitrile introduced into the first reactor is between about 0.9 and about 1.2.  
     
     
         7 . The process as set forth in  claim 5  wherein the molar ratio of sulfuric acid to 2-hydroxy-4-methylthiobutanenitrile introduced into the first reactor is between about 1.0 and about 2.0 during the period between start up of the process until steady state conditions are established in the plug flow reactor, and thereafter said molar ratio of sulfuric acid to 2-hydrxy-4-methylthiobutanenitrile is between about 0.7 and about 1.5.  
     
     
         8 . The process as set forth in  claim 6  wherein the molar ratio of sulfuric acid to 2-hydroxy-4-methylthiobutanenitrile introduced into the first reactor is between about 1.0 and about 1.5 during the period between start up of the process until steady state conditions are established in the plug flow reactor, and thereafter said molar ratio of sulfuric acid to 2-hydroxy-4-methylthiobutanenitrile is between about 0.9 and about 1.2.  
     
     
         9 . The process as set forth in  claim 1  wherein the ratio of the rate of sulfuric acid flow into said plug flow reactor to the rates of 2-hydroxy-4-methylthiobutanamide and 2-hydroxy-4-methylthiobutanenitrile flow into said plug flow reactor is controlled to provide an excess of at least 5% by weight sulfuric acid than is stoichiometrically equivalent to 2-hydroxy -4-methylthiobutanamide and 2-hydroxy-4-methylthiobutanenitrile introduced into the plug flow reactor.  
     
     
         10 . The process as set forth in  claim 9  wherein sulfuric acid and 2-hydroxy-4-methylthiobutanenitrile are introduced into said first reactor at relative rates effective to provide said excess in said plug flow reactor.  
     
     
         11 . The process as set forth in  claim 1  wherein the intermediate aqueous hydrolysis solution comprises up to about 11 wt. % 2-hydroxy-4-methylthiobutanoic acid, up to about 8 wt. % ammonium bisulfate, at least about 10 wt. % water, at least about 35 wt. % amide and up to about 2 wt. % nitrile.  
     
     
         12 . The process as set forth in  claim 1  wherein the aqueous hydrolyzate product solution produced under steady state conditions at the exit of the plug flow reactor comprises at least about 36 wt. % 2-hydroxy-4-methylthiobutanoic acid, at least about 30 wt. % ammonium bisulfate, at least about 25 wt. % water, up to about 0.05 wt. % amide and up to about 0.05 wt. % nitrile.  
     
     
         13 . The process as set forth in  claim 12  wherein the aqueous hydrolyzate product solution produced upon start up of the process comprises up to about 0.05 wt. % amide and up to about 0.05 wt. % nitrile.  
     
     
         14 . The process as set forth in  claim 1  wherein the water and the intermediate aqueous hydrolysis solution are continuously introduced into a mixer to form a diluted intermediate hydrolysis solution, and the diluted intermediate hydrolysis solution is continuously introduced into the plug flow reactor such that the hydrolysis of 2-hydroxy-4-methyl-thiobutyramide is completed as the diluted intermediate hydrolysis solution flows through the plug flow reactor.  
     
     
         15 . The process as set forth in  claim 14  wherein the molar ratio of sulfuric acid to 2-hydroxy-4-methylthiobutanenitrile added to the first reactor is between about 0.7 and about 1.5, and the flow of the diluted intermediate hydrolysis solution through said plug flow reactor is turbulent.  
     
     
         16 . The process as set forth in  claim 14  wherein the water stream is heated before being introduced into the mixer to prevent liquid phase separation and precipitation of ammonium bisulfate in said plug flow reactor.  
     
     
         17 . The process as set forth in  claim 14  wherein said plug flow reactor comprises a packed column reactor and the diluted intermediate aqueous hydrolysis solution flows through the packed column reactor at or above the threshold velocity of the packed column reactor.  
     
     
         18 . The process as set forth in  claim 14  wherein said plug flow reactor comprises a pipeline reactor and the diluted intermediate aqueous hydrolysis solution moves through the pipeline reactor in turbulent flow.  
     
     
         19 . The process as set forth in  claim 18  wherein said plug flow reactor is operated at a Reynolds number greater than about 3,000.  
     
     
         20 . The process as set forth in  claim 18  wherein said plug flow reactor is operated at a Reynolds number greater than about 5,000.  
     
     
         21 . The process as set forth in  claim 1  wherein the plug flow reactor is operated at a Peclet number of at least 50, a peak temperature of about 90 to about 120° C. and a residence time between about 30 and about 90 minutes.  
     
     
         22 . The process as set forth in  claim 1  wherein the plug flow reactor operates substantially adiabatically.  
     
     
         23 . The process as set forth in  claim 1  wherein the plug flow reactor operates substantially isothermally.  
     
     
         24 . The process as set forth in  claim 1  wherein the plug flow reactor operates adiabatically and autothermally.  
     
     
         25 . The process as set forth in  claim 1  further including recovering 2-hydroxy-4-methylthiobutanoic acid from the aqueous hydrolyzate product solution.  
     
     
         26 . The process as set forth in  claim 1  wherein 2-hydroxy-4-methylthiobutanoic acid is recovered by extracting 2-hydroxy-4-methylthiobutanoic acid from the aqueous hydrolyzate product solution.  
     
     
         27 . The process as set forth in  claim 1  wherein 2-hydroxy -4-methylthiobutanoic acid is recovered by neutralizing the aqueous hydrolyzate product solution to form an organic phase containing 2-hydroxy-4-methylthiobutanoic acid and an aqueous phase, and separating the organic phase and the aqueous phase to recover 2-hydroxy-4-methylthiobutanoic acid.  
     
     
         28 . The process as set forth in  claim 1  wherein vapor emissions from the process are not greater than about 0.5 scf per 1000 lbs. product 2-hydroxy-4-methylthiobutanoic acid.  
     
     
         29 . The process as set forth in  claim 28  wherein vapor emissions from the process,are not greater than about 0.3 scf per 1000 lbs. 2-hydroxy-4-methylthiobutanoic acid.  
     
     
         30 . A process for the preparation of 2-hydroxy-4-methlthiobutanoic acid comprising: 
 introducing sulfuric acid into a first reactor comprising a continuous stirred tank reactor;    introducing 2-hydroxy-4-methylthiobutanenitrile into said first reactor;    continuously hydrolyzing 2-hydroxy-4-methyl-thiobutanenitrile within said first reactor to produce an intermediate aqueous hydrolysis solution containing 2-hydroxy-4-methylthiobutanamide;    continuously introducing the intermediate aqueous hydrolysis solution exiting said first reactor and a water stream into a second continuous stirred tank reactor such that a substantial portion of 2-hydroxy-4-methylthiobutanamide contained in said intermediate solution is hydrolyzed in the second continuous stirred tank reactor to form a finishing reaction solution;    continuously introducing the finishing reaction solution into a plug flow reactor; and    continuously hydrolyzing 2-hydroxy-4-methylthiobutanamide within said plug flow reactor to produce an aqueous hydrolyzate product solution containing 2-hydroxy-4-methylthiobutanoic acid.    
     
     
         31 . The process as set forth in  claim 30  wherein sulfuric acid is introduced into said first reactor in an acid stream having a strength of between about 50% by weight and about 70% by weight sulfuric acid.  
     
     
         32 . The process as set forth in  claim 30  wherein sulfuric acid is introduced into said first reactor in an acid stream having a strength of between about 70% by weight and about 99% by weight sulfuric acid, and the acid stream is continuously introduced to the first reactor concurrently with a water stream to form sulfuric acid having a strength of between about 50% by weight and about 70% by weight on an organic-free basis within the first reactor.  
     
     
         33 . The process as set forth in  claim 30  wherein at least about 90% of 2-hydroxy-4-methylthiobutanenitrile is converted to 2-hydroxy-4-methylthiobutanamide within the first reactor.  
     
     
         34 . The process as set forth in  claim 30  wherein the molar ratio of sulfuric acid to 2-hydroxy-4-methylthiobutanenitrile introduced into the first reactor is between about 0.7 and about 1.5.  
     
     
         35 . The process as set forth in  claim 30  wherein the molar ratio of sulfuric acid to 2-hydroxy-4-methylthiobutanenitrile introduced into the first reactor is between about 0.9 and about 1.2.  
     
     
         36 . The process as set forth in  claim 34  wherein the molar ratio of sulfuric acid to 2-hydroxy-4-methylthiobutanenitrile introduced into the first reactor is between about 1.0 and about 2.0 during the period between start up of the process until steady state conditions are established in the plug flow reactor, and thereafter said molar ratio of sulfuric acid to 2-hydroxy-4-methylthiobutanenitrile is between about 0.7 and about 1.5.  
     
     
         37 . The process as set forth in  claim 35  wherein the molar ratio of sulfuric acid to 2-hydroxy-4-methylthiobutanenitrile introduced into the first reactor is between about 1.0 and about 1.5 during the period between start up of the process until steady state conditions are established in the plug flow reactor, and thereafter said molar ratio of sulfuric acid to 2-hydroxy-4-methylthiobutanenitrile is between about 0.9 and about 1.2.  
     
     
         38 . The process as set forth in  claim 30  wherein the ratio of the rate of sulfuric acid flow into said plug flow reactor to the rates of 2-hydroxy-4-methylthiobutanamide and 2-hydroxy-4-methylthiobutanenitrile flow into said plug flow reactor is controlled to provide an excess of at least 5% by weight sulfuric acid than is stoichiometrically equivalent to 2-hydroxy-4-methylthiobutanamide and 2-hydroxy-4-methylthiobutanenitrile introduced into the plug flow reactor.  
     
     
         39 . The process as set forth in  claim 38  wherein sulfuric acid and 2-hydroxy-4-methylthiobutanenitrile are introduced into said first reactor at relative rates effective to provide said excess in said plug flow reactor.  
     
     
         40 . The process as set forth in  claim 30  wherein the intermediate aqueous hydrolysis solution comprises up to about 11 wt. % 2-hydroxy-4-methylthiobutanoic acid, up to about 8 wt. % ammonium bisulfate, at least about 10 wt. % water, at least about 35 wt. % amide and up to about 2 wt. % nitrile.  
     
     
         41 . The process as set forth in  claim 30  wherein the finishing reaction solution comprises at least about 32 wt. % 2-hydroxy-4-methylthiobutanoic acid, at least about 25 wt. % ammonium bisulfate, at least about 25 wt. % water, up to about 5 wt. % amide and up to about 1 wt. % nitrile.  
     
     
         42 . The process as set forth in  claim 30  wherein the aqueous hydrolyzate product solution produced under steady state conditions at the exit of the plug flow reactor comprises at least about 36 wt. % 2-hydroxy-4-methylthiobutanoic acid, at least about 30 wt. % ammonium bisulfate, at least about 25 wt. % water, up to about 0.05 wt. % amide and up to about 0.05 wt. % nitrile.  
     
     
         43 . The process as set forth in  claim 42  wherein the aqueous hydrolyzate product solution produced upon start up of the process comprises up to about 0.05 wt. % amide and up to about 0.05 wt. % nitrile.  
     
     
         44 . The process as set forth in  claim 30  wherein at least about 80% of 2-hydroxy-4-methylthiobutanamide formed in said first reactor is converted to 2-hydroxy-4-methylthio-butyric acid within the second continuous stirred tank reactor.  
     
     
         45 . The process as set forth in  claim 30  wherein the molar ratio of sulfuric acid to 2-hydroxy-4-methylthiobutanenitrile added to the first reactor ranges from about 0.7 to about 1.5, and the second continuous stirred tank reactor is operated at a temperature ranging from about 70° C. to about 120° C.  
     
     
         46 . The process as set forth in  claim 45  wherein said plug flow reactor comprises a packed column reactor and the finishing reaction solution flows through the packed column reactor at or above the threshold velocity of the packed column reactor.  
     
     
         47 . The process as set forth in  claim 45  wherein said plug flow reactor comprises a pipeline reactor and the finishing reaction solution moves through the pipeline reactor in turbulent flow.  
     
     
         48 . The process as set forth in  claim 47  wherein said plug flow reactor is operated at a Reynolds number greater than about 3,000.  
     
     
         49 . The process as set forth in  claim 47  wherein said plug flow reactor is operated at a Reynolds number greater than about 5,000.  
     
     
         50 . The process as set forth in  claim 30  wherein the plug flow reactor is operated at a Peclet number of at least 50, a peak temperature of about 90 to about 120° C. and a residence time between about 30 and about 90 minutes.  
     
     
         51 . The process as set forth in  claim 30  wherein the plug flow reactor operates substantially adiabatically.  
     
     
         52 . The process as set forth in  claim 30  wherein the plug flow reactor operates isothermally.  
     
     
         53 . The process as set forth in  claim 30  wherein the plug flow reactor operates adiabatically and autothermally.  
     
     
         54 . The process as set forth in  claim 30  wherein the molar ratio of sulfuric acid to 2-hydroxy-4-methylthiobutanenitrile added to the first reactor is between about 0.7 and about 1.5, and the flow of the finishing reaction solution through said plug flow reactor is turbulent.  
     
     
         55 . The process as set forth in  claim 30  further including recovering 2-hydroxy-4-methylthiobutanoic acid from the aqueous hydrolyzate product solution.  
     
     
         56 . The process as set forth in  claim 30  wherein 2-hydroxy-4-methylthiobutanoic acid is recovered by extracting 2-hydroxy-4-methylthiobutanoic acid from the aqueous hydrolyzate product solution.  
     
     
         57 . The process as set forth in  claim 30  wherein 2-hydroxy-4-methylthiobutanoic acid is recovered by neutralizing the aqueous hydrolyzate product solution to form an organic phase containing 2-hydroxy-4-methylthiobutanoic acid and an aqueous phase, and separating the organic phase and the aqueous phase to recover 2-hydroxy-4-methylthiobutanoic acid.  
     
     
         58 . The process as set forth in  claim 30  wherein vapor emissions from the process are not greater than about 0.5 scf per 1000 lbs. product 2-hydroxy-4-methylthiobutanoic acid.  
     
     
         59 . A process for the preparation of 2-hydroxy-4-methylthiobutanoic acid comprising: 
 concurrently introducing 2-hydroxy-4-methylthiobutyanenitrile, concentrated sulfuric acid stream having a strength of between about 70% by weight and about 99% by weight, and water into a vessel in which 2-hydroxy-4-methylthiobutanenitrile is hydrolyzed;    hydrolyzing 2-hydroxy-4-methylthiobutanenitrile within said vessel to produce an intermediate aqueous hydrolysis solution containing 2-hydroxy-4-methylthiobutanamide; and    hydrolyzing 2-hydroxy-4-methylthiobutanamide to produce an aqueous hydrolyzate product solution containing 2-hydroxy-4-methylthiobutanoic acid.    
     
     
         60 . The process as set forth in  claim 59  wherein the vessel is a first reactor comprising a continuous stirred tank reactor, 2-hydroxy-4-methylthiobutanenitrile is continuously hydrolyzed within the first reactor, the intermediate aqueous hydrolysis solution is introduced into a plug flow reactor, and 2-hydroxy-4-methylthiobutanamide is continuously hydrolyzed within the plug flow reactor.  
     
     
         61 . The process as set forth in  claim 59  wherein the vessel is a first reactor comprising a continuous stirred tank reactor, 2-hydroxy-4-methylthiobutanenitrile is continuously hydrolyzed within the first reactor, the intermediate aqueous hydrolysis solution is introduced into a second continuous stirred tank reactor, 2-hydroxy-4-methylthiobutanamide is continuously hydrolyzed within the second continuous stirred tank reactor to form a finishing reaction solution, the finishing reaction solution is introduced into a plug flow reactor, the hydrolysis of 2-hydroxy-4-methylthiobutanamide is completed as the finishing reaction solution flows through the plug flow reactor.  
     
     
         62 . The process as set forth in  claim 59  further including recovering 2-hydroxy-4-methylthiobutanoic acid from the aqueous hydrolyzate product solution.  
     
     
         63 . An apparatus for use in a process for the preparation of 2-hydroxy-4-methylthiobutanoic acid, comprising a first continuous stirred tank reactor for the continuous hydrolysis of 2-hydroxy-4-methylthiobutanenitrile in the presence of sulfuric acid to produce an intermediate aqueous hydrolysis solution containing 2-hydroxy-4-methylthiobutanamide, and a plug flow reactor for the continuous hydrolysis of 2-hydroxy-4-methylthiobutanamide with sulfuric acid to produce an aqueous hydrolyzate product solution containing 2-hydroxy-4-methylthiobutanoic acid.  
     
     
         64 . The apparatus as set forth in  claim 63  further including a second continuous stirred tank reactor for receiving water and the intermediate aqueous hydrolysis solution exiting the first continuous stirred tank reactor, such that a substantial portion of 2-hydroxy-4-methyl-thiobutyramide contained in the intermediate hydrolysis solution is hydrolyzed in the second continuous stirred tank reactor to form a finishing reaction solution, and the hydrolysis of 2-hydroxy-4-methylthiobutanamide is completed as the finishing reaction solution flows through the plug flow reactor.  
     
     
         65 . The apparatus as set forth in  claim 63  further including a mixer for mixing water and the intermediate aqueous hydrolysis solution exiting the first continuous stirred tank reactor to form a diluted intermediate hydrolysis solution, and discharging the diluted intermediate hydrolysis solution mixture to the plug flow reactor such that the hydrolysis of 2-hydroxy-4-methylthiobutanamide is completed as the diluted intermediate hydrolysis solution flows through the plug flow reactor.  
     
     
         66 . The apparatus as set forth in  claim 63  wherein the plug flow reactor is insulated for adiabatic operation.  
     
     
         67 . The apparatus as set forth in  claim 63  wherein said first reactor comprises an inlet for 2-hydroxy-4-methylthiobutanenitrile, an inlet for concentrated sulfuric acid, an inlet for water, and means within the reactor for mixing 2-hydroxy-4-methylthiobutanenitrile, concentrated sulfuric acid and water in proportions suited for hydrolysis of 2-hydroxy-4-methylthiobutanenitrile to 2-hydroxy-4-methylthiobutanamide.  
     
     
         68 . The apparatus as set forth in  claim 67  further including means for removing heat generated by dilution of sulfuric acid and reaction of 2-hydroxy-4-methylthiobutanenitrile. and water in order to maintain a reaction temperature for hydrolysis of 2-hydroxy-4-methylthiobutanenitrile.