Patent ID: 8692029

Claim:
A method for producing acrolein, wherein acrolein is produced by allowing supercritical water to act on glycerin separated from a mixture containing glycerin and an ester of monohydric alcohol and fatty acid, wherein the mixture is obtained via transesterification of monohydric alcohol and waste fat or oil containing as a main component an ester of fatty acid and glycerin in the presence of an alkali catalyst, the method comprising: a determination step of determining the hydrogen ion concentration in glycerin; an acid addition step of adding to the glycerin an acid at an amount calculated based on the results of the determination step so as to make the glycerin acidic; a moisture adjustment step of cooling glycerin and/or removing moisture so as to achieve the moisture content in after the acid addition step to be lower than the stoichiometric quantity of water that allows hydration of the total amount of the salt in the glycerin; a supercritical water treatment step of allowing supercritical water to act on the glycerin after the acid addition so as to produce acrolein from the glycerin; and a precipitate removal step of causing a precipitate of a salt formed from the alkali catalyst and the acid in the glycerin after glycerin neutralization in the acid addition step or after the end of the step and before the start of the supercritical water treatment step so as to separate and remove the precipitate, wherein the alkali catalyst is an oxide or hydroxide of an alkali metal or alkaline-earth metal, and the acid is an inorganic acid.