APPARATUS FOR HEAT TREATMENT OF POLYESTER PARTICLE AND METHOD OF MULTISTAGE SOLID-PHASE POLYCONDENSATION OF POLYESTER PARTICLE

1 - 47 . (canceled)

48 . A method of producing a polyethylene terephthalate, comprising performing a heat treatment of a polyethylene terephthalate prepolymer having an intrinsic viscosity of 0.18 dL/g or more and 0.40 dL/g or less in a solid state to provide an intrinsic viscosity of 0.70 dL/g or more,

wherein the heat treatment in the solid state comprises a first-stage solid-phase polycondensation step, a temperature increase step, and a second-stage solid-phase polycondensation step in the stated order, and the respective steps satisfy the following conditions (1) to (3):

(1) the first-stage solid-phase polycondensation step is a step of treating the polyethylene terephthalate prepolymer with heat under an inert gas atmosphere or reduced pressure, and a temperature (T 1 c ) of the heat treatment is 190° C. or higher and 225° C. or lower;

(2) the temperature increase step is a step of increasing a temperature of the polyethylene terephthalate prepolymer after the first-stage solid-phase polycondensation step under an inert gas atmosphere or reduced pressure from a temperature equal to or lower than the temperature (T 1 c ) of the heat treatment of the first-stage solid-phase polycondensation step to a temperature (T 2 c ), the temperature of the polyethylene terephthalate prepolymer is increased from the temperature T 1 c (° C.) to (T 1 c +15)° C. within 30 minutes, and the temperatures T 1 c (° C.) and T 2 c (° C.) satisfy the following (Eq. 1c) and (Eq. 2c):

T 1 c +15≦ T 2 c   (Eq. 1c), and

205° C.≦ T 2 c≦ 240° C.  (Eq. 2c); and

(3) the second-stage solid-phase polycondensation step is a step of treating the polyethylene terephthalate prepolymer after the first-stage solid-phase polycondensation step and the temperature increase step with heat under an inert gas atmosphere or reduced pressure, and a temperature (T 3 c ) of the heat treatment is 190° C. or higher and 240° C. or lower.

49 . The method of producing a polyethylene terephthalate according to claim 48 , wherein the heat treatment further includes a crystallization step prior to the first-stage solid-phase polycondensation step, and the crystallization step comprises a step of treating the polyethylene terephthalate prepolymer with heat at a temperature (Tx) of 110° C. or higher and 200° C. or lower.

50 . The method of producing a polyethylene terephthalate according to claim 48 , wherein the polyethylene terephthalate prepolymer to be subjected to the second-stage solid-phase polycondensation step has an intrinsic viscosity of 0.35 dL/g or more.

51 . The method of producing a polyethylene terephthalate according to claim 48 , wherein the heat treatment in the solid state is continuously performed.

52 . The method of producing a polyethylene terephthalate according to claim 48 , wherein the first-stage solid-phase polycondensation step and/or the second-stage solid-phase polycondensation step is performed in a continuous moving bed.

53 . The method of producing a polyethylene terephthalate according to claim 49 , wherein the crystallization step is performed in a fluidized bed.

54 . The method of producing a polyethylene terephthalate according to claim 48 , wherein the temperature increase step is performed in a fluidized bed.

55 . The method of producing a polyethylene terephthalate according to claim 48 , wherein the polyethylene terephthalate prepolymer is comprised of particles having an average mass of 0.1 mg/particle or more and 30 mg/particle or less.

56 . The method of producing a polyethylene terephthalate according to claim 48 , wherein the polyethylene terephthalate prepolymer has a terminal carboxyl group concentration of 100 equivalents/ton or less.

57 . The method of producing a polyethylene terephthalate according to claim 48 , wherein the polyethylene terephthalate comprises a titanium compound.