Removal of impurities during the recovery of crude phthalic anhydride

Claim 2

IN A CYCLICAL PROCESS FOR RECOVERING SOLID PHTHALIC ANHYDRIDE IN A SEPARATOR FROM A GAS CONTAINING PHTHALIC ANHYDRIDE VAPOR OBTAINED BY CATALYTIC AIR OXIDATION OF O-XYLENE IN CONTACT WITH A V2O5 CONTAINING CATALYST, COMPRISING CONTINUOUSLY REPEATING IN SEQUENCE THE STEP OF: (A) PASSING SAID GAS CONTAINING PHTHALIC ANHYDRIDE VAPOR OVER COOLED HEAT EXCHANGE TUBES TO PRECIPITATE SOLID PHTHALIC ANHYDRIDE ON THE SURFACES THEREOF; (B) PASSING A HOT FLUID HAVING A TEMPERATURE OF ABOUT 145*-190*C THROUGH SAID HEAT EXCHANGE TUBES TO HEAT THE SURFACES THEREOF TO AT LEAST 130*C AND MELT OFF RESULTANT PRECIPITATED PHTHALIC ANHYDRIDE; (C) COLLECTING RESULTANT MOLTEN PHTHALIC ANHYDRIDE FROM SAID HEAT EXCHANGE TUBE SURFACES; AND (D) COOLING SAID HEAT EXCHANGE TUBE SURFACES BY PASSING A FLUID HAVING A TEMPERATURE OF ABOUT 45*-55*C THERE THROUGH; THE IMPROVEMENT COMPRISING FLUSING SAID HEAT EXCHANGE TUBE SURFACES AT THE HEAT TRANSFER SURFACES THEREOF PROXIMATE TO THE GAS OUTLET SIDE OF SAID SEPARATOR TOWARDS THE END OF AT LEAST ONE STEP (B) WITH EXTERNALLY ADDED MOLTEN PHTHALIC ANHYDRIDE HAVING A TEMPERATURE OF 130*-280*C, WHEREBY THERE IS SUBSTANTIALLY PREVENTED A BUILDUP IN DIFFICULT-TO-REMOVE, FIRMLY ADHERING DEPOSITS OF IMPURITIES WHICH OTHERWISE WOULD CAUSE A SHUT-DOWN IN THE PROCESS.

Claim 3

A process according to claim 1, wherein said externally added molten phthalic anhydride has a temperature of 130*-175*C.

Claim 4

A process according to claim 1, wherein said flushing is accomplished By spraying said externally added molten phthalic anhydride onto the surface of said heat exchange tubes.

Claim 5

A process as defined by claim 1, wherein said heat exchange tubes are flushed at the coldest portions thereof proximate to the gas outlet side of said separator.

Claim 6

A process according to claim 1, wherein the end of said melting-off period is the last 0.5 percent of said period.

Claim 7

A process according to claim 5, wherein said flushing is conducted once every 20 to 50 cycles of step (b).

Claim 8

IN A CYCLICAL PROCESS OF RECOVERING SOLID PHTHALIC ANHYDRIDE IN A SEPARATOR FROM A GAS CONTAINING PHTHALIC ANHYDRIDE VAPOR OBTAINED BY CATALYTIC AIR OXIDATION OF NAPHTHALENE IN CONTACT WITH A V2O5 CONTAINING CATALYST, COMPRISING CONTINUOUSLY REPEATING IN SEQUENCE OF STEPS OF: (A) PASSING SAID GAS CONTAINING PHTHALIC ANHYDRIDE VAPOR OVER COOLED HEAT EXCHANGE TUBES TO PRECIPITATE SOLID PHTHALIC ANHYDRIDE ON THE SURFACES THEREOF; (B) PASSING A HOT FLUID HAVING A TEMPERATURE OF ABOUT 145*-190*C THROUGH SAID HEAT EXCHANGE TUBES TO HEAT THE SURFACES THEREOF TO AT LEAST 1300C AND MELT OFF RESULTANT PRECIPITATED PHTHALIC ANHYDRIDE; (C) COLLECTING RESULTANT MOLTEN PHTHALIC ANHYDRIDE FROM SAID HEAT EXCHANGE TUBE SURFACES; AND (D) COOLING SAID HEAT EXCHANGE TUBE SURFACES BY PASSING A FLUID HAVING A TEMPERATURE OF ABOUT 45*-55*C THERETHROUGH; THE IMPROVEMENT COMPRISING FLUSHING SAID HEAT EXCHANGE TUBE SURFACES AT THE HEAT TRANSFER SURFACES THEREOF PROXIMATE TO THE GAS INLET SIDE OF SAID SEPARATOR TOWARDS THE END OF AT LEAST ONE STEP (B) WITH EXTERNALLY ADDED MOLTEN PHTHALIC ANHYDRIDE HAVING A TEMPERATURE OF 130*-280*C, WHEREBY THERE IS SUBSTANTIALLY PREVENTED A BUILDUP IN DIFFICULT-TO-REMOVE, FIRMLY ADHERING DEPOSITS OF IMPURITIES WHICH OTHERWISE WOULD CAUSE A SHUT-DOWN IN THE PROCESS.

Claim 9

IN A CYCLICAL PROCESS OF RECOVERING SOLID PHTHALIC ANHYDRIDE IN A SEPARATOR FROM A GAS CONTAINING PHTHALIC ANHYDRIDE VAPOR OBTAINED BY CATALYTIC AIR OXIDATION OF NAPHTHALENE IN CONTACT WITH A V2O5 CONTAINING CATALYST, COMPRISING CONTINUOUSLY REPEATING IN SEQUENCE OF STEPS OF: (A) PASSING SAID GAS CONTAINING PHTHALIC ANHYDRIDE VAPOR OVER COOLED HEAT EXCHANGE TUBES TO PRECIPITATE SOLID PHTHALIC ANHYDRIDE ON THE SURFACES THEREOF; (B) PASSING A HOT FLUID HAVING A TEMPERATURE OF ABOUT 145*-190*C THROUGH SAID HEAT EXCHANGE TUBES TO HEAT THE SURFACES THEREOF TO AT LEAST 1300C AND MELT OFF RESULTANT PRECIPITATED PHTHALIC ANHYDRIDE; (C) COLLECTING RESULTANT MOLTEN PHTHALIC ANHYDRIDE FROM SAID HEAT EXCHANGE TUBE SURFACES; AND (D) COOLING SAID HEAT EXCHANGE TUBE SURFACES BY PASSING A FLUID HAVING A TEMPERATURE OF ABOUT 45*-55*C THERETHROUGH; THE IMPROVEMENT COMPRISING FLUSHING SAID HEAT EXCHANGE TUBE SURFACES AT THE HEAT TRANSFER SURFACES THEREOF PROXIMATE TO THE GAS INLET SIDE OF SAID SEPARATOR TOWARDS THE END OF AT LEAST ONE STEP (B) WITH EXTERNALLY ADDED MOLTEN PHTHALIC ANHYDRIDE HAVING A TEMPERATURE OF 130*-280*C, WHEREBY THERE IS SUBSTANTIALLY PREVENTED A BUILDUP IN DIFFICULT-TO-REMOVE, FIRMLY ADHERING DEPOSITS OF IMPURITIES WHICH OTHERWISE WOULD CAUSE A SHUT-DOWN IN THE PROCESS.

Claim 10

A process according to claim 7, wherein said externally added molten phthalic anhydride has a temperature of 130*-175*C.

Claim 11

A process according to claim 7, wherein said flushing is accomplished by spraying said externally added molten phthalic anhydride onto the surface of said heat exchange tubes.

Claim 12

A process as defined by claim 7, wherein said heat transfer surfaces are flushed at the warmest portions thereof proximate to the portion of said separator where the gas first contacts said heat exchange tubes.

Claim 13

A process according to claim 7, wherein the end of said melting-off period is the last 0.5 percent of said period.

Claim 14

A process according to claim 7, wherein said difficult-to-melt deposits are removed once every 20 to 50 cycles of step (b).