Polymer Engineering Department, Amirkabir University of Technology.

Synthesis and Mathematical Modelling of Polyethylene Terephthalate via Direct Esterification in a Laboratory Scale Unit

A B S T R A C T
Synthesis of polyethylene terephthalate (PET) in laboratory is a challenging task due to high reaction temperature up to 280ºC and high pressure in esterification step and low vacuum in polycondensation step. In this research, synthesis of PET, in a laboratory size unit was studied via direct esterification of terephthalic acid and ethylene glycol. Antimony oxide was added as catalyst for polycondensation.

Mathematical model of process is presented based on the mass balance of different species such as acid and hydroxyl end groups, water output, diester groups during both esterification and polycondensation steps. Derived governing equations were integrated numerically using Runge-Kutta method. Reacting mixture mass variation was included in the model. Comparison of experimental and simulation results shows promising and good agreement. Hence, the model could be used as a powerful tool for engineering process. The model was applied to study different aspects of polymerization process.

INTRODUCTION

Production of polyethylene terephthalate (PET) has one of the fastest growing rates among thermoplastics during the last decades. PET has been produced mostly via direct esterification of terephthalic acid and ethylene glycol for a couple of decades. In the first step, esterification of terephthalic acid and ethylene glycol under goes reaction producing bishydroxyl ethyl terephthalate (BHET) as the main monomer for polycondensation. Regarding a reversible reaction, water, as side product should be extracted in order to perform reaction up to high conversions. In the second step, that is called polycondensation, oligomers and polymer chains under go reaction to produce long polymer. The byproduct of polycondensation is ethylene glycol, which should be removed in order to increase the rate of polycondensation and chain length. To produce high molecular weight polymer and yarn, polymerization should be conducted in solid state [1-4]. Although PET has been produced for a long time, there are still different aspects such as modelling and mass transfer which need more research. Partial solubility of acid terephthalate (TPA) in ethylene glycol (EG) and diffusion of water and EG in polymer melt and mass transfer in liquid-gas phases are the main reasons of previous mentioned restrictions.

Figure 1 shows a schematic of PETSYN unit. Reaction unit consists of a 1 L stainless steel reactor, a condenser, two oil baths, and a cooling fan. Cooling section has a cooling bath down to -40ºC and vacuum pump. Digital control system has responsibility of transferring instruments signal to a computer (Pentium 4) and process control. Three PT100 temperature sensors were mounted inside reactor, in reactor jacket and oil bath. Reactor pressure was measured by a pressure transducer model 3248 from Tecsis Company. Ribbon type mixer was used in order to mix high viscous reactive mixture. A 1000 Wbar type electrical heater was mounted around the reactor. Cooling of reactor was performed by air flow in a tube around reactor. On top of the reactor, there are six connections. These connections were used to mount temperature and pressure sensors, apply pressure by nitrogen, and connect vacuum pump. Motor mixer is a magnetic type that can tolerate 100 atm pressure and having 1.5 HP power could have 2500 rpm. Required vacuum was applied by a vacuum pump type JB-85N- 250 from FastVac Company. All tasks of data acquisition were performed by computer via an I/O card model PCL-818L and corresponding terminal PCLD- 8115 from AdvanTech.

LABORATORY SCALE POLYMERIZATION UNIT

Synthesis and Mathematical Modelling of Polyethylene Terephthalate via Direct Esterification in a Laboratory Scale Unit

A B S T R A C T
Synthesis of polyethylene terephthalate (PET) in laboratory is a challenging task due to high reaction temperature up to 280ºC and high pressure in esterification step and low vacuum in polycondensation step. In this research, synthesis of PET, in a laboratory size unit was studied via direct esterification of terephthalic acid and ethylene glycol. Antimony oxide was added as catalyst for polycondensation.

Mathematical model of process is presented based on the mass balance of different species such as acid and hydroxyl end groups, water output, diester groups during both esterification and polycondensation steps. Derived governing equations were integrated numerically using Runge-Kutta method. Reacting mixture mass variation was included in the model. Comparison of experimental and simulation results shows promising and good agreement. Hence, the model could be used as a powerful tool for engineering process. The model was applied to study different aspects of polymerization process.

INTRODUCTION

Production of polyethylene terephthalate (PET) has one of the fastest growing rates among thermoplastics during the last decades. PET has been produced mostly via direct esterification of terephthalic acid and ethylene glycol for a couple of decades. In the first step, esterification of terephthalic acid and ethylene glycol under goes reaction producing bishydroxyl ethyl terephthalate (BHET) as the main monomer for polycondensation. Regarding a reversible reaction, water, as side product should be extracted in order to perform reaction up to high conversions. In the second step, that is called polycondensation, oligomers and polymer chains under go reaction to produce long polymer. The byproduct of polycondensation is ethylene glycol, which should be removed in order to increase the rate of polycondensation and chain length. To produce high molecular weight polymer and yarn, polymerization should be conducted in solid state [1-4]. Although PET has been produced for a long time, there are still different aspects such as modelling and mass transfer which need more research. Partial solubility of acid terephthalate (TPA) in ethylene glycol (EG) and diffusion of water and EG in polymer melt and mass transfer in liquid-gas phases are the main reasons of previous mentioned restrictions.

Figure 1 shows a schematic of PETSYN unit. Reaction unit consists of a 1 L stainless steel reactor, a condenser, two oil baths, and a cooling fan. Cooling section has a cooling bath down to -40ºC and vacuum pump. Digital control system has responsibility of transferring instruments signal to a computer (Pentium 4) and process control. Three PT100 temperature sensors were mounted inside reactor, in reactor jacket and oil bath. Reactor pressure was measured by a pressure transducer model 3248 from Tecsis Company. Ribbon type mixer was used in order to mix high viscous reactive mixture. A 1000 Wbar type electrical heater was mounted around the reactor. Cooling of reactor was performed by air flow in a tube around reactor. On top of the reactor, there are six connections. These connections were used to mount temperature and pressure sensors, apply pressure by nitrogen, and connect vacuum pump. Motor mixer is a magnetic type that can tolerate 100 atm pressure and having 1.5 HP power could have 2500 rpm. Required vacuum was applied by a vacuum pump type JB-85N- 250 from FastVac Company. All tasks of data acquisition were performed by computer via an I/O card model PCL-818L and corresponding terminal PCLD- 8115 from AdvanTech.

LABORATORY SCALE POLYMERIZATION UNIT