Method of purifying crude 2,6-naphthalenedicarboxylic acid dimethyl ester

In a method of obtaining purified 2,6-naphthalenedicarboxylic acid dimethyl ester by vacuum distilling crude 2,6-naphthalenedicarboxylic acid dimethyl ester under conditions of application of heat, the improvement which comprises vacuum distilling in a batch or continuous operation, a crude 2,6-naphthalenedicarboxylic acid dimethyl ester having an acid value (A.sub.f), as expressed in milligrams of KOH required for neutralizing the carboxyl groups contained in one gram of said crude 2,6-naphthalenedicarboxylic acid dimethyl ester, of from 1.0 to 50, at a bottom liquid phase temperature T.sub.b of from 190.degree. C. to 340.degree. C., and at a residual rate (W/F) of said dimethyl ester in the distillation zone which satisfies the relationship

• 0.9 .gtoreq. W/F .gtoreq. 10.sup.(log A.sbsp.f .sup.- 1/160.T.sbsp.b.sup.) .gtoreq. 0.0075
• wherein
• W/f is the residual rate;
• W is the rate of withdrawal of the bottoms liquid phase at kg/hr in the case of continuous operation, or the amount of the bottoms liquid phase in kg in the case of batch operation;
• F is the rate at which the crude 2,6-naphthalenedicarboxylic acid dimethyl ester is fed into the distillation zone at kg/hr in the case of continuous operation, or the amount charged of said ester in kg in the case of batch operation;
• A.sub.f is the acid value of the crude 2,6-naphthalenedicarboxylic acid dimethyl ester fed to the distillation zone at a numerical value as indicated by the KOH,mg/g of crude 2,6-naphthalenedicarboxylic acid dimethyl ester;
• and
• T.sub.b is the temperature of the bottoms liquid phase as a numerical value as indicated by .degree. C and recovering the distilled-off purified 2,6-naphthalenedicarboxylic acid dimethyl ester as distillate and removing any free dicarboxylic acids and mono-methyl esters contained in the crude 2,6-naphthalenedicarboxylic acid dimethyl ester as bottoms liquid phase;
• the operation of said method within the limits of said relationship being effective to substantially obviate formation of scales within the bottom of said distillation zone, so as to enable recovery of a product ester of high product reproducibility.