Continuous processes for the highly selective conversion of sugars to propylene glycol or mixtures of propylene glycol and ethylene glycol

A continuous process for converting carbohydrate-containing feed, which feed contains at least one of aldopentose and ketose-yielding carbohydrate, said ketose comprising at least one of hexaketose and pentaketose, to propylene glycol or a mixture of propylene glycol and ethylene glycol comprising:

• a. continuously or intermittently passing said carbohydrate feed into a reaction zone having an aqueous-hydrogenation medium containing retro-aldol catalyst, hydrogen and hydrogenation catalyst;
• b. maintaining the aqueous-hydrogenation medium in the reaction zone at hydrogenation conditions to provide a product solution comprising propylene glycol and alkitol comprising at least one of pentitol and hexitol, and optionally, ethylene glycol, said hydrogenation conditions comprising a temperature in the range of between about 225° C. to 300° C., a ratio of retro-aldol catalyst to hydrogenation catalyst, and hydrogen partial pressure that, in combination, are sufficient to convert at least about 95 percent of the carbohydrate;
• and
• c. continuously or intermittently withdrawing product solution from said reaction zone,
• wherein said carbohydrate feed is at least partially hydrated and is under a pressure sufficient to maintain partial hydration;
• wherein said carbohydrate feed is below a temperature of about 170° C.;
• and wherein said carbohydrate feed is heated to above 225° C. immediately prior to or in the reaction zone and the rate of heating of the carbohydrate feed from below 170° C. to above 225° C. is sufficient to provide a product solution having a mass ratio of total propylene glycol and ethylene glycol to said alkitol greater than about 10:1.