Process for preparing sorbic acid

The invention provides a process for making a sorbic acid. The process comprises: pyrolysis of acetic acid for producing high purity ketene with coke oven gas as a heat source, polymerization reaction of butenoic aldehyde and ketene gas to give a sorbic acid polyester and acid decomposition of sorbic acid polyester to obtain the sorbic acid, wherein the polymerization reaction of the butenoic aldehyde and the ketene gas, which is performed in a combined polymerization reactor comprising at least of a spray absorption reactor and a packed tower provided with a recycle pump outside continuously, comprises the steps that: fresh butenoic aldehyde and recycle feed from a tower bottom of the packed tower are delivered to the spray absorption reactor to form a sprayed material flow and to react with a low-temperature ketene gas absorbed by formed negative pressure into the reactor; the reaction materials flow into the tower bottom of the packed tower circularly so as to conduct further polymerization reaction of the unreacted ketene and the butenoic aldehyde contained in the material flow; one part of the materials from the tower bottom of the packed tower is used for the inner circulation of a reactor system, while the other part is recovered constantly for use in the next process, thereby realizing continuous production.