Abstract: A polygon in a tabular portion includes first and second axes of symmetry substantially orthogonal to each other. A top surface of the tabular portion has a swelling portion with an outline that is line-symmetric with respect to the first and second axes of symmetry. In a lateral half portion of the outline, a short-side side crest portion that is convex toward the polygonal outline in the direction of the first axis of symmetry, concave corner-side trough portions connected to each end of the short-side side crest portion and long-side side crest portions that are connected to each corner-side trough portion and convex toward the polygonal outline are formed. On one of a top surface and an undersurface of the swelling portion, a pair of lateral ribs or a pair of longitudinal ribs that spreads toward the polygonal outline and transversely crosses the swelling portion is formed.

Abstract: A polygon in a tabular portion includes first and second axes of symmetry substantially orthogonal to each other. A top surface of the tabular portion has a swelling portion with an outline that is line-symmetric with respect to the first and second axes of symmetry. In a lateral half portion of the outline, a short-side side crest portion that is convex toward the polygonal outline in the direction of the first axis of symmetry, concave corner-side trough portions connected to each end of the short-side side crest portion and long-side side crest portions that are connected to each corner-side trough portion and convex toward the polygonal outline are formed. On one of a top surface and an undersurface of the swelling portion, a pair of lateral ribs or a pair of longitudinal ribs that spreads toward the polygonal outline and transversely crosses the swelling portion is formed.

Abstract: In a process for producing acrylonitrile by the ammoxidation of propylene, comprising bringing, in a fluidized-bed reactor, a gas containing propylene, ammonia and molecular oxygen into contact with a highly-active ammoxidation catalyst (this catalyst being one which can attain a rate constant of 2 s.sup.-1 or more for the ammoxidation reaction of propylene, assuming that the reaction is a first-order reaction) which is in a state of being fluidized by the gas, the improvement comprising carrying out the ammoxidation under the following conditions:(a) the superficial gas velocity in the fluidized-bed reactor is maintained at 1 to 10 m/s under such an operation condition that the rate constant of the ammoxidation reaction of propylene is 2 s.sup.-1 or more, assuming that the reaction is a first-order reaction;(b) an area in which the density of fluidized solid matter is 100 kg/m.sup.