Production and recovery of acetic acid by plural distillation

Claim 1

PROCESS FOR THE PRODUCTION OF ACETIC ACID WHICH COMPRISES OXIDIZING PROPYLENE FEEDSTOCKS WITH MOLECULAR OXYGEN IN A SOLVENT SELECTED FROM THE GROUP CONSISTING OF FULLY ESTERIFIED POLYACYL ESTERS OF POLYHYDROXYALKANES, POLYHYDROXYCYCLOALKANES, POLYGLYCOLS AND MIXTURES THEREOF, WHERE SAID ESTERS CONTAIN FROM 1 TO 18 CARBON ATOMS IN EACH ACYL MOIETY AND FROM 2 TO 18 CARBON ATOMS IN EACH ALKYLENE AND CYCLOALKYLENE MOIETY, UNDER TEMPERATURES AND PRESSURES SUFFICIENT TO CAUSE THE REACTION TO PROCEED IN THE LIQUID PHASE AND RECOVERING SAID ACETIC ACID BY: (A) DIRECTING EFFLUENT STREAM OF THE REACTION MIXTURE FROM A REACTION ZONE THROUGH A COMBINATION LETDOWN DISTILLATION ZONE COMPRISING A FLASHING ZONE FOLLOWED BY A STRIPPING ZONE, SAID FLASHING ZONE AND STRIPPING ZONE BEING MAINTAINED AT PRESSURES SUBSTANTIALLY LOWER THAN IN EACH PRECEDING ZONE AND AT TEMPERATURES NECESSARY TO SEPARATE SUBSTANTIALLY ALL OF THE ACETIC ACID AND LOWER BOILING PRODUCTS OVERHEAD AS GAS PHASE AND HIGHER BOILING COMPONENTS INCLUDING THE BULK OF THE SOLVENT AND RESIDUE WHICH ARE REMOVED AS BOTTOMS FROM SAID STRIPPING ZONE, (B) PASSING SAID OVERHEAD GAS PHASE TO CONDENSING ZONES, FROM WHENCE UNCONDENSED GASES ARE DIRECTED TO AN ABSORBING ZONE INTO WHICH THE BOTTOMS STREAM FROM SAID STRIPPING ZONE IS ALSO PASSED TO ABSORB UNCONDENSED PROPYLENE, PROPANE AND MINOR AMOUNTS OF OXYGENATED COMPONENTS; REMOVING VENT GASES OVERHEAD FROM SAID ABSORBER, WHILE FEEDING THE BOTTOMS STREAM FROM SAID ABSORBING ZONE BACK TO SAID REACTION ZONE AS RECYCLE SOLVENT, (C) DIRECTING A SIDE-STREAM OF SAID BOTTOMS FROM SAID STRIPPING ZONE THROUGH A RESIDUE REMOVAL DISTILLATION ZONE WHEREIN THE CONCENTRATION OF RESIDUES OF POLYMERIC REACTION PRODUCTS HAVING BOILING POINTS ABOVE THAT OF SAID SOLVENT IN SAID RECYCLE SOLVENT IS PURGE REGULATED TO ADMIT SUFFICIENT RESIDUE INTO SAID REACTION ZONE AS TO MAINTAIN A RESIDUE CONCENTRATION THEREIN NOT EXCEEDING THAT LEVEL ABOVE WHICH ADDITIONAL RESIDUE IS INCOMPLETELY OXIDIZED, AND WHEREIN EXCESS RESIDUE IS REMOVED AS BOTTOMS AND SOLVENT IS DISTILLED OVERHEAD AND RECOMBINED WITH THE SOLVENT BOTTOMS FROM SAID STRIPPING ZONE AND FED TO SAID ABSORBING ZONE, (D) FEEDING A COMBINED STREAM OF CONDENSED LIQUIDS FROM SAID CONDENSING ZONES INTO A PRIMARY PRODUCTS DISTILLATION SPLITTING ZONE FROM WHICH AN OVERHEAD STREAM CONTAINING UNREACTED PROPYLENE, PROPANE AND SOME LOW BOILERS IS REMOVED TO A PROPYLENE-PROPANE REMOVAL ZONE FROM WHICH SAID LOW BOILERS ARE REMOVED AS BOTTOMS WHILE PROPYLENE AND PROPANE ARE REMOVED OVERHEAD AND FED TO A DISTILLATION SPLITTER FOR THESE COMPONENTS AND WHEREIN PROPANE IS REMOVED AS BOTTOMS AND PROPYLENE IS REMOVED OVERHEAD AND RECYCLED TO SAID REACTION ZONE, (E) FEEDING THE BOTTOMS FROM SAID PRIMARY PRODUCTS DISTILLATION SPLITTING ZONE IN STEP (D) TO AN ACIDSOLVENT DISTILLATION SPLITTING ZONE WHERE MOST OF THE RESIDUAL SOLVENT FROM SAID REACTION ZONE NOT REMOVED IN SAID FLASHING AND STRIPPING ZONES AND HIGHER BOILING COMPONENTS ARE REMOVED AS BOTTOMS AND RECYCLED TO SAID ABSORBER, (F) FEEDING THE OVERHEAD FROM SAID ACID-SOLVENT DISTILLATION SPLITTING ZONE CONTAINING RESIDUAL SOLVENT, ACID VALUES, WATER AND LOW BOILERS TO AN ACIDS-LOW BOILERS DISTILLATION SEPARATION ZONE WHEREIN THE LOW BOILERS AND SOME WATER ARE RECOVERED OVERHEAD, WHILE DIRECTING THE BOTTOMS FROM SAID ACIDS-LOW BOILERS SEPARATION ZONE TO AN AZEOTROPIC DISTILLATION COLUMN USING BENZENE AS AN AZEOTROPE-FORMER FOR WATER AND FORMIC ACID, (G) REMOVING FROM SAID AZEOTROPIC DISTILLATION ZONE AN OVERHEAD STREAM CONTAINING A MIXTURE OF BENZENEWATER AND BENZENE-FORMIC ACID AZEOTROPES TO A CONDENSING ZONE WHEREIN BENZENE IS SEPARATED FROM WATER AND FORMIC ACID AND FEEDING THESE THREE COMPONENTS TO A COLLECTING ZONE IN WHICH BENZENE FORMS AN UPPER PHASE FROM WHICH BENZENE IS RETURNED TO SAID AZEOTROPIC DISTILLATION ZONE, WHILE WATER AND FORMIC ACID ARE REMOVED AS A LOWER PHASE, AND (H) REMOVING FROM SAID AZEOTROPIC DISTILLATION ZONE A BOTTOMS STREAM CONTAINING PRIMARILY ACETIC ACID AND A SMALL AMOUNT OF RESIDUAL SOLVENT TO A REFINING ZONE WHEREIN PURIFIED ACETIC ACID IS RECOVERED OVERHEAD AND SAID RESIDUAL SOLVENT I REMOVED AS BOTTOMS AND RECYCLED TO SAID ABSORBER.