Process for preparing polyethylene terephthalate and its low molecular weight oligomers

A process for preparing BHET in a reactor provided with a rectifying column which comprises:

• (i) suspending or dissolving terephthalic acid in a molten liquid of BHET at a temperature above the dew point of ethylene glycol;
• (ii) feeding gaseous or liquid ethylene glycol into the suspension or the solution in said reactor to react it at a temperature of about 197.degree. to about 300.degree. C. with the terephthalic acid, wherein the feeding of ethylene glycol to the reactor is controlled so that the temperature inside of and at a position at the top of the rectifying column is maintained at about 100.degree. to about 120.degree. C. at which more than about 3% of the ethylene glycol does not distill off from the rectifying column and the partial pressure of the ethylene glycol is maximized, said control being maintained by
• (iii) discontinuing feeding of the ethylene glycol when the temperature at a position about 1/3 to about 1/5 below the top of the rectifying column rises to a temperature within the range of about 110.degree. C. to about 160.degree. C.;
• and
• (iv) continuing feeding of the ethylene glycol when the temperature at said position about 1/3 to about 1/5 below the top of the rectifying column falls to 0.degree. to about 5.degree. C. lower than the temperature point in step (iii).