Continuous manufacture of poly(trimethylene terephthalate)

1. A continuous process for the production of poly(trimethylene terephthalate) comprising the steps of:

(a) continuously producing poly(trimethylene terephthalate) oligomers comprising 1,3-trimethylene and terephthalate repeating units and having a degree of polymerization of from about 1.9 to about 3.5 by (i) ester exchange reaction of dimethyl terephthalate with excess 1,3-propanediol at an elevated temperature or (ii) direct esterification reaction of terephthalic acid with excess 1,3-propanediol at an elevated temperature;

(b) continuously precondensing the poly(trimethylene terephthalate) oligomers to form a poly(trimethylene terephthalate) prepolymer having an intrinsic viscosity of at least about 0.23 dl/g and gaseous by-products comprising volatilized by-product 1,3-propanediol; and

(c) continuously polymerizing the poly(trimethylene terephthalate) prepolymer to form higher molecular weight poly(trimethylene terephthalate) having an intrinsic viscosity of at least about 0.55 dl/g and additional gaseous by-products comprising volatilized by-product 1,3-propanediol,

wherein:

(i) the gaseous by-products are condensed in at least one spray condenser to form condensed by-product 1,3-propanediol comprising 1,3-propanediol and by-product solids comprising trimethylene terephthalate cyclic dimer and, optionally, poly(trimethylene terephthalate), which is then collected in at least one hotwell;

(ii) the total amount of trimethylene terephthalate cyclic dimer and poly(trimethylene terephthalate) in the condensed by-product 1,3-propanediol is raised at least about 0.2 wt % based on the weight of condensed by-product 1,3-propanediol;

(iii) a portion of the condensed by-product 1,3-propanediol from the hotwell is cooled in at least one heat exchanger and then sprayed in the at least one spray condenser to condense the gaseous by-products; and

(iv) a portion of the condensed by-product 1,3-propanediol from the hotwell, without purification, is fed back into the ester exchange or direct esterification reactions at one or more locations where the temperature is about 150° C. or higher.

2. The process of claim 1 , wherein the total amount of trimethylene terephthalate cyclic dimer and poly(trimethylene terephthalate) in the condensed by-product 1,3-propanediol is raised by withdrawing a portion of the 1,3-propanediol out of the condensed by-product 1,3-propanediol to raise the total percentage of trimethylene terephthalate cyclic dimer and poly(trimethylene terephthalate) in the condensed by-product 1,3-propanediol.

3. The process of claim 1 , wherein the total amount of trimethylene terephthalate cyclic dimer and poly(trimethylene terephthalate) in the condensed by-product 1,3-propanediol is raised by adding at least one of poly(trimethylene terephthalate) and trimethylene terephthalate cyclic dimer to the condensed by-product 1,3-propanediol.

4. The process of claim 1 , wherein the temperature of the condensed by-product 1,3-propanediol entering the at least one hotwell is about 50° C., or lower.

5. The process of claim 1 , wherein the higher molecular weight poly(trimethylene terephthalate) has a relative viscosity of at least about 0.91 dl/g.

6. The process of claim 1 , wherein the gaseous by-products and the additional gaseous by-products are condensed in at least two spray condensers to form condensed by-product 1,3-propanediol comprising 1,3-propanediol and by-product solids comprising trimethylene terephthalate cyclic dimer, which is then collected in at least one hotwell and wherein a portion of the condensed by-product 1,3-propanediol is cooled in at least two heat exchangers and then sprayed in the at least two spray condensers to condense the gaseous by-products and additional gaseous by-products.

7. The process of claim 1 , wherein the additional gaseous by-products are condensed in at least one spray condenser to form second condensed by-product 1,3-propanediol comprising 1,3-propanediol and by-product solids comprising trimethylene terephthalate cyclic dimer which is then collected in at least one hotwell and wherein a portion of the condensed by-product 1,3-propanediol is cooled in at least one heat exchanger and then sprayed in the at least one spray condenser; the total amount of trimethylene terephthalate cyclic dimer and poly(trimethylene terephthalate) in the second condensed by-product 1,3-propanediol is greater than the total amount of trimethylene terephthalate cyclic dimer and poly(trimethylene terephthalate) in the first condensed by-product 1,3-propanediol; and the total amount of trimethylene terephthalate cyclic dimer and poly(trimethylene terephthalate) in the first condensed by-product 1,3-propanediol is raised by adding some or all of the second condensed by-product 1,3-propanediol to the first condensed by-product 1,3-propanediol.

8. The process of claim 1 , wherein a direct esterification reaction is used in step (a).

9. The process of claim 1 , wherein an ester exchange reaction is used in step (a).

10. The process of claim 2 , wherein a direct esterification reaction is used in step (a).

11. The process of claim 2 , wherein an ester exchange reaction is used in step (a).

12. The process of claim 3 , wherein a direct esterification reaction is used in step (a).

13. The process of claim 3 , wherein an ester exchange reaction is used in step (a).

14. The process of claim 4 , wherein a direct esterification reaction is used in step (a).

15. The process of claim 4 , wherein an ester exchange reaction is used in step (a).

16. The process of claim 5 , wherein a direct esterification reaction is used in step (a).

17. The process of claim 5 , wherein an ester exchange reaction is used in step (a).

18. The process of claim 7 , wherein a direct esterification reaction is used in step (a).

19. The process of claim 7 , wherein an ester exchange reaction is used in step (a).