Abstract: A method for producing acrylic acid from propylene at high efficiency by two-stage catalytic oxidation using a single fixed bed shell-and-tube heat exchanger type reactor is provided. The method comprises dividing the shell space of said reactor into an upper space and lower space with a partition plate, allowing a heating medium to circulate in each of the spaces substantially independently of each other, and carrying out the vapor phase oxidation under specific conditions. Said specific conditions including providing a first stage catalyst layer at lower portion of each of the reaction tubes, a second stage catalyst layer at upper portion thereof and an inert substance layer therebetween, and making void ratio of the inert substance layer 40-99.5%.

Abstract: A process for removal of solid organic matters formed and deposited in the reaction system when an unsaturated aldehyde or an unsaturated aidehyde-containing gas is subjected to catalytic gaseous oxidation in the presence of an oxidizing catalyst to prepare an unsaturated carboxylic acid corresponding thereto, which process comprises contacting the solid organic matters with a mixed gas containing at least 3 volume % of molecular oxygen and at least 0.1 volume % of steam at a temperature of 260.degree. to 450.degree. C.