Process for preparing and recovering acrylic acid

A process for preparing acrylic acid comprising:

• (a) catalytic vapor phase oxidation of an olefinic compound of the general formula CH.sub.2 .dbd.CHX wherein X represents at least one group selected from the group consisting of CH.sub.3 and CHO with a molecular oxygen containing gas to yield a reaction product gas comprising acrylic acid and acetic acid,
• (b) contacting said reaction product gas with water in the presence of hydroquinone and obtaining an aqueous solution of said reaction product and hydroquinone,
• (c) separating the acrylic acid from the aqueous solution, which comprises the steps of
• (i) separating the organic components containing said reaction product and hydroquinone from the aqueous solution by extraction.
• (ii) separating acetic acid from the organic components to obtain the crude acrylic acid by distillation, and
• (iii) separating the acrylic acid from the crude acrylic acid and obtaining a bottom liquor containing acrylic acid, acrylic acid dimer, hydroquinone and high boiling substances by distillation,
• (d) decomposition evaporating said bottom liquor under reduced pressure of 20-500 mmHg at a temperature of 120.degree.-220.degree. C. to provide a distillate and an evaporation residue having the composition of 1-25 wt % of acrylic acid, 9-49 wt % of acrylic acid dimer, wherein the total amount of the acrylic acid and its dimer is in the range of 10-50 wt % and correspondingly 90-50 wt % of the other components, said distillate being recycled to step c, and
• (e) subjecting said evaporation residue to water extraction to separate said acrylic acid, acrylic acid dimer, and hydroquinone as aqueous solution from high boiling water insoluble substances, said aqueous solution being recycled to the step b or step c-i.