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There is provided a novel process for producing acrylic acid by which acrylic acid can be obtained from a raw material independent of petroleum. The process for producing acrylic acid according to the present invention comprises the steps of: applying a dehydration reaction to glycerol as a raw mate

There is provided a novel process for producing acrylic acid by which acrylic acid can be obtained from a raw material independent of petroleum. The process for producing acrylic acid according to the present invention comprises the steps of: applying a dehydration reaction to glycerol as a raw material in a gas phase; and then applying a gas phase oxidation reaction to a gaseous reaction product formed by the dehydration reaction.

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1. A process for producing acrylic acid, comprising the steps of: a) vaporizing a raw material comprising an aqueous glycerol solution to generate a first gas, wherein said aqueous glycerol solution has a water content of not more than 50% by weight;b) applying a dehydration reaction to glycerol in

1. A process for producing acrylic acid, comprising the steps of: a) vaporizing a raw material comprising an aqueous glycerol solution to generate a first gas, wherein said aqueous glycerol solution has a water content of not more than 50% by weight;b) applying a dehydration reaction to glycerol in a gas phase that includes said first gas; thenc) applying a gas phase oxidation reaction to a gaseous reaction product formed by the dehydration reaction to obtain said acrylic acid; andd) wherein the dehydration reaction and the gas phase oxidation reaction are conducted in a tandem-type reactor comprising the two reaction tubes which are linked to each other, where the two reaction tubes are filled with a catalyst for the dehydration reaction and a catalyst for the gas phase oxidation reaction respectively, and where the dehydration reaction and the gas phase oxidation reaction are separately conducted in their respective tubes. 2. The process for producing acrylic acid according to claim 1, wherein oxygen is added to a gas which is transferred from the dehydration reaction to the gas phase oxidation reaction. 3. A process for producing acrylic acid, comprising the steps of: a) vaporizing a raw material comprising an aqueous glycerol solution to generate a first gas, wherein said aqueous glycerol solution has a water content of not more than 50% by weight;b) applying a dehydration reaction to glycerol in a gas phase that includes said first gas; thenc) applying a gas phase oxidation reaction to a gaseous reaction product formed by the dehydration reaction to obtain said acrylic acid; andd) wherein the dehydration reaction and the gas phase oxidation reaction are conducted in a single-type reactor comprising one reaction tube, where said one reaction tube is filled with a catalyst for the gas phase oxidation on the reaction gas outlet side and with a catalyst for the dehydration reaction of glycerol on the reaction gas inlet side, thus conducting in said one reaction tube the dehydration reaction followed by the gas phase oxidation reaction. 4. The process for producing acrylic acid according to claim 1 or 3, and further comprising the steps of: a) adding an inert gas to the first gas to obtain a resultant mixed gas, wherein the inert gas is selected from the group consisting of nitrogen gas, carbon dioxide gas and rare gas; andb) controlling a concentration of said inert gas in said resultant mixed gas to a concentration of not less than 50% by volume. 5. The process for producing acrylic acid according to claim 1 or 3, wherein the gas phase in which the dehydration reaction is conducted is a mixed gas comprising glycerol, water vapor and oxygen. 6. The process for producing acrylic acid according to claim 5, wherein the amount of the water vapor relative to the glycerol in the mixed gas is not larger than 1.2 times by mol. 7. The process for producing acrylic acid according to claim 1 or 3, wherein the acrylic acid is used to produce a water-absorbent resin. 8. The process for producing acrylic acid according to claim 1 or 3, further comprising the step of collecting the resultant acrylic acid as a solution by using water or a solvent. 9. The process for producing acrylic acid according to claim 8, further comprising a distillation step for removing low- and high-boiling-point materials from the resultant solution containing acrylic acid. 10. The process for producing acrylic acid according to claim 8, further comprising a crystallization step for purifying acrylic acid by crystallizing it. 11. A petroleum independent process for producing acrylic acid, comprising the steps of: a) obtaining glycerol from one of i) hydrolysis of oils-and-fats, ii) waste fluids in soap production, and iii) a by-product in production of a bio-diesel fuel as a renewable fuel;b) vaporizing a raw material comprising an aqueous glycerol solution having said glycerol to generate a first gas, wherein said aqueous glycerol solution has a water content of not more than 50% by weight;c) adding an inert gas to the first gas to obtain a resultant mixed gas, wherein the inert gas is selected from the group consisting of nitrogen gas, carbon dioxide gas and rare gas, and controlling a concentration of said inert gas in said resultant mixed gas to a concentration of not less than 50% by volume;d) applying a dehydration reaction to glycerol in a gas phase that includes said resultant mixed gas, wherein the gas phase in which said dehydration reaction is conducted comprises glycerol, water vapor and oxygen, and wherein the amount of the water vapor relative to the glycerol in the mixed gas is not larger than 1.2 times by mol; thene) applying a gas phase oxidation reaction to a gaseous reaction product formed by the dehydration reaction to obtain said acrylic acid; andf) wherein the dehydration reaction and the gas phase oxidation reaction are conducted in a tandem-type reactor comprising two reaction tubes which are linked to each other, where the two reaction tubes are filled with a catalyst for the dehydration reaction and a catalyst for the gas phase oxidation reaction respectively, and where the dehydration reaction and the gas phase oxidation reaction are separately conducted in their respective tubes. 12. A process for producing a water-absorbent resin from glycerol as a raw material, comprising the steps of: producing acrylic acid from glycerol as a raw material by the process according to any one of claims 1 and 11; and then polymerizing said acrylic acid to obtain a water-absorbent resin. 13. A process for producing a water-absorbent resin from acrylic acid as a raw material, comprising the step of polymerizing acrylic acid as a raw material to obtain a water-absorbent resin, wherein said acrylic acid is acrylic acid obtained by the process according to any one of claims 1 and 11. 14. A petroleum independent process for producing acrylic acid, comprising the steps of: a) obtaining glycerol from one of i) hydrolysis of oils-and-fats, ii) waste fluids in soap production, and iii) a by-product in production of a bio-diesel fuel as a renewable fuel;b) vaporizing a raw material comprising an aqueous glycerol solution having said glycerol to generate a first gas, wherein said aqueous glycerol solution has a water content of not more than 50% by weight;c) adding an inert gas to the first gas to obtain a resultant mixed gas, wherein the inert gas is selected from the group consisting of nitrogen gas, carbon dioxide gas and rare gas, and controlling a concentration of said inert gas in said resultant mixed gas to a concentration of not less than 50% by volume;d) applying a dehydration reaction to glycerol in a gas phase that includes said resultant mixed gas, wherein the gas phase in which said dehydration reaction is conducted comprises glycerol, water vapor and oxygen, and wherein the amount of the water vapor relative to the glycerol in the mixed gas is not larger than 1.2 times by mol; thene) applying a gas phase oxidation reaction to a gaseous reaction product formed by the dehydration reaction to obtain said acrylic acid; andf) wherein the dehydration reaction and the gas phase oxidation reaction are conducted in a single-type reactor comprising one reaction tube, where said one reaction tube is filled with a catalyst for the gas phase oxidation on the reaction gas outlet side and with a catalyst for the dehydration reaction of glycerol on the reaction gas inlet side, thus conducting in said one reaction tube the dehydration reaction followed by the gas phase oxidation reaction. 15. A process for producing a water-absorbent resin from glycerol as a raw material, comprising the steps of: producing acrylic acid from glycerol as a raw material by the process according to any one of claims 3 and 14; and then polymerizing said acrylic acid to obtain a water-absorbent resin. 16. A process for producing a water-absorbent resin from acrylic acid as a raw material, comprising the step of polymerizing acrylic acid as a raw material to obtain a water-absorbent resin, wherein said acrylic acid is acrylic acid obtained by the process according to any one of claims 3 and 14. 17. The process for producing acrylic acid according to any one of claims 1, 3, 11, and 14, wherein said aqueous glycerol solution has a water content of not more than 20% by weight.