Abstract: A distance measuring device includes a measuring member holding a length measuring instrument and at least one rail member (base member) on which the measuring member is movably disposed. When a straight line parallel to a direction in which the measuring member disposed on the base member moves is defined as a reference line, an angle formed by a straight line parallel to an axial length direction of a reaction tube and the reference line, which are on an identical plane, is constant for the plurality of reaction tubes disposed side by side along the reference line. A measurement direction of the length measuring instrument is parallel to the axial length direction of the reaction tube in a state where the measuring member is disposed on the base member. The measuring member is disposed on the base member to be able to sequentially move.

Abstract: This invention provides a method for producing acrylic acid by catalytic gas-phase oxidation, which method makes it possible to carry out a continuous operation steadily for a long period of time while a high yield is maintained. This method is characterized by comprising filling each of reaction tubes of a fixed-bed multitubular reactor with at least two species of catalysts each of which essentially comprises, as catalytically active components, oxide of molybdenum and oxide of vanadium and/or composite oxide of the same, said at least two species of catalysts being different in the ratio of D1/D2, D1 denoting the proportion of the total pore volume of pores whose pore diameter falls within the range of at least 0.03 ?m and less than 0.4 ?m to the total pore volume of the whole pores, and D2 denoting the proportion of the total pore volume of pores whose pore diameter fails within the range of at least 0.

Abstract: This invention provides a method for producing acrylic acid by catalytic gas-phase oxidation, which method makes it possible to carry out a continuous operation steadily for a long period of time while a high yield is maintained. This method is characterized by comprising filling each of reaction tubes of a fixed-bed multitubular reactor with at least two species of catalysts each of which essentially comprises, as catalytically active components, oxide of molybdenum and oxide of vanadium and/or composite oxide of the same, said at least two species of catalysts being different in the ratio of D1/D2, D1 denoting the proportion of the total pore volume of pores whose pore diameter falls within the range of at least 0.03 ?m and less than 0.4 ?m to the total pore volume of the whole pores, and D2 denoting the proportion of the total pore volume of pores whose pore diameter fails within the range of at least 0.

Abstract: Provided is a catalyst for producing unsaturated carboxylic acid, which excels in mechanical strength and attrition loss and is capable of producing the object product at a high yield. This catalyst is formed of a catalytically active component comprising molybdenum and vanadium as the essential ingredients and inorganic fibers, which are supported on an inert carrier, said catalyst being characterized in that said inorganic fibers comprise at least an inorganic fiber having an average diameter less than 1.0 ?m and another inorganic fiber having an average diameter ranging from 1.5 to 7 ?m.

Abstract: Provided is a catalyst for producing unsaturated carboxylic acid, which excels in mechanical strength and attrition loss and is capable of producing the object product at a high yield. This catalyst is formed of a catalytically active component comprising molybdenum and vanadium as the essential ingredients and inorganic fibers, which are supported on an inert carrier, said catalyst being characterized in that said inorganic fibers comprise at least an inorganic fiber having an average diameter less than 1.0 ?m and another inorganic fiber having an average diameter ranging from 1.5 to 7 ?m.

Abstract: The invention provides a catalyst for producing acrylic acid at high yield for a long time, in a method for producing acrylic acid by catalytic gas phase oxidation of propane and/or acrolein in the presence of molecular oxygen or a molecular oxygen-containing gas. This catalyst comprises a complex oxide containing molybdenum, vanadium and X component (here the X component is at least one element selected from antimony, niobium and tin) as the essential components, and is characterized in that its main peak as measured by X-ray diffractiometry using K? ray of Cu, d=4.00±0.1 angstrom, and in that the particle size of the X component in the catalyst does not exceed 20 ?m.

Abstract: The invention provides a catalyst for producing acrylic acid at high yield for a long time, in a method for producing acrylic acid by catalytic gas phase oxidation of propane and/or acrolein in the presence of molecular oxygen or a molecular oxygen-containing gas. This catalyst comprises a complex oxide containing molybdenum, vanadium and X component (here the X component is at least one element selected from antimony, niobium and tin) as the essential components, and is characterized in that its main peak as measured by X-ray diffractiometry using K? ray of Cu, d=4.00±0.1 angstrom, and in that the particle size of the X component in the catalyst does not exceed 20 ?m.