Continuous process for the production of ethylene terephthalate polyesters

Claim 1

A continuous process for the production of ethylene terephthalate polyesters which comprises passing a polyester-forming fusion melt predominantly comprised of bis-2-hydroxethyl terephthalate and/or its low polymers sequentially through a series of reduced pressure polymerization vessles while controlling the degree of polymerization of the product by a controlling system, characterized in that, the viscosity of material leaving each of said vessels except for the first vessel is controlled in cascade by the steps of: measuring melt viscosities of material entering and leaving a downstream vessel at predetermined time intervals; storing the values so measured in a memory and calculating a resultant sum wherein said resultant sum is calculated by adding a value which corresponds to a difference between a set point and a measured value for the melt viscosity of the material leaving said downstream vessel at one of said time intervals to a value which corresponds to a difference between a set point and a measured value for the melt viscosity of material entering said downstream vessel at a predetermined time interval prior to said one time interval; resetting the set point for pressure control of a melt viscosity of the material in said downstream vessel in response to the resultant sum; and controlling the melt viscosity by adjusting pressure of the preceding upstream vessel by automatically resetting, for the preceding upstream vessel, the set point for a melt viscosity of the material leaving said upstream vessel, when the reset set point for said downstream vessel falls outside a predetermined range.