The present invention relates to a continuous process for the production of poly(trimethylene terephthalate), which is also commonly referred to as poly(1,3-propylene terephthalate).  The process of the invention can be used as part of a four-vessel process, the first vessel being either an ester exchanger for producing a mixture of bis-3-hydroxypropyl terephthalate and low molecular weight polymers of 1,3-propanediol and terephthalic acid having an average degree of polymerization of 15 or less from dimethylterephthalate and 1,3-propanediol or a reactor for producing the starting material from terephthalic acid and 1,3-propanediol.  The second vessel is a flasher, the third vessel is a prepolymerizer, and the fourth vessel is a final polymerizer or finisher. 
Continuous, four vessel processes for the production of poly(ethylene terephthalate) are known.  For example, Sheller, U.S. Pat.  No.  3,438,942 discloses a process for the continuous production of poly(ethylene terephthalate) comprising ester exchange followed by three polycondensation steps. 
Also known are batch processes for the production of poly(trimethylene terephthalate).  For example, Doerr et al.,  U.S. Pat.  No.  5,340,909 discloses the production of poly(trimethylene terephthalate) using either an ester exchange reaction starting with lower dialkyl terephthalate ester or direct esterification of terephthalic acid followed by a polycondensation reaction, both of which are carried out in batches using an autoclave. 
It would be highly desirable to provide a continuous, four-vessel process for the production of poly(trimethylene terephthalate).  It would also be desirable to provide a continuous process for the production of poly(trimethylene terephthalate) in which the production of by-products, such as acrolein and allyl alcohol, is minimized, and in which the molecular weight of the final poly(trimethylene terephthalate) polymer is maximized.  The present invention provides such a process. 
The present invention comprises a continuous process for the production of poly(trimethylene terephthalate) comprising the steps of:
(a) continuously feeding a liquid feed mixture to a flasher, the liquid feed mixture comprising a catalyst and at least one of bis-3-hydroxypropyl terephthalate and low molecular weight polyesters of 1,3-propanediol and terephthalic acid, and the liquid feed mixture having a mole ratio of propylene groups to terephthalate groups of 1.1 to 2.2;
(b) continuously vaporizing and removing a first stream of gaseous by-products from the flasher, and continuously withdrawing a liquid flasher reaction product having a mole ratio of propylene groups to terephthalate groups of less than about 1.5 from the flasher;
(c) continuously feeding the liquid flasher reaction product to a prepolymerizer, and continuously polymerizing the flasher reaction product in the prepolymerizer to form a poly(trimethylene terephthalate) prepolymer and a second stream of gaseous by-products;
(d) continuously withdrawing the poly(trimethylene terephthalate) prepolymer from the prepolymerizer, the prepolymer having a relative viscosity of at least about 5;
(e) continuously feeding the poly(trimethylene terephthalate) prepolymer to a final polymerizer, and continuously polymerizing the poly(trimethylene terephthalate) prepolymer to form a higher molecular weight poly(trimethylene terephthalate) and a third stream of gaseous by-products; and
(f) continuously withdrawing the higher molecular weight poly(trimethylene terephthalate) from the final polymerizer, the higher molecular weight poly(trimethylene terephthalate) having a relative viscosity of at least about 17.