Two-stage devolatilization process for making alkenyl-aromatic hydrocarbon-acrylonitrile copolymers

Claim 1

IN A PROCESS FOR COPOLYMERIZING AT LEAST ONE ALKENYLAROMATIC HYDROCARBON OF THE GROUP CONSISTING OF STYRENE AND ALPHA-METHYLSTYRENE WITH ACRYLONITRILE WHEREIN THE WEIGHT PROPORTION OF ACRYLONITRILE IS VARIE BETWEEN 20 AND 35 PERCENT OF THE COMONOMERS, AND THE WEIGHT PERCENT OF COMONOMERS RANGES BETWEEN 70 AND 85 PERCENT, THE BALANCE BEING AN AROMATIC DILUENT OF THE GROUP CONSISTING OF HYDROCARBONS AND AR-HALOGENATED HYDROCARBONS HAVING A BOILING RANGE BETWEEN ABOUT 120* AND 200* C., WHEREIN CONVERSION TO COPOLYMER IS CARRIED OUT TO BETWEEN 40 AND 70 WEIGHT PERCENT OF THE MONOMERS, THE IMPROVEMENT WHICH CONSISTS IN SUBJECTING THE REACTION MIXTURE CONTAINING SAID COPOLYMER, COMONOMERS AND DILUENT TO A TWO-STAGE DEVOLATIZATION PROCESS, THE FIRST STAGE OF WHICH IS CARRIED OUT AT A TEMPERATURE RANGING BETWEEN 120* AND 190*C. ACHIEVED BY HEAT TRANSFER SURFACES WHICH ARE BELOW 210*C. AND AT AN ABSOLUTE PRESSURE BETWEEN ABOUT 150 MM. AND 1000 MM. OF MERCURY SUCH THAT THE FREE ACRYLONITRILE CONTENT IS REDUCED BY AT LEAST 50 PERCENT, THEREAFTER SUBJECTING THE PRODUCT FROM THE FIRST STAGE DEVOLATILIZATION TO A SECOND STAGE DEVOLATILIZATION STEP CARRIED OUT AT A TEMPERATURE RANGING BETWEEN ABOUT 210* AND 260*C. AND AT AN ABSOLUTE PRESSURE OF NOT MORE THAN 150 MM. OF MERCURY TO REDUCE THE VOLATILES IN THE PRODUCT COMING FROM SAID SECOND STAGE DEVOLATILIZATION TO ABOUT ONE PERCENT AND TO INCREASE COPOLYMER CONTENT TO ABOUT 99 PERCENT, AND RECOVERING AN ALKENYLAROMATIC HYDROCARBON-ACRYLONTITRILE COPOLYMERIC PRODUCT WHICH IS SUBSTANTIALLY FREE OF ACRYLONITRILE.