Shell and tube oxidation reactor with improved resistance to fouling

A process of making acrylic acid comprising:

• a) providing a mixed feed gas comprising propylene to a first shell-and-tube reaction stage, wherein the first shell-and-tube reaction stage comprises a mixed metal oxide catalyst;
• b) oxidizing the propylene in the first shell-and-tube reaction stage to produce a process gas comprising acrolein;
• c) cooling the process gas to a temperature ranging from 240° C. to 280° C. in an interstage heat exchanger;
• d) passing the cooled process gas through an open interstage region;
• e) passing the process gas to a second shell-and-tube reaction stage, wherein the tubes of the second shell-and-tube reaction stage have an internal diameter greater than 22.3 mm and wherein the second shell-and-tube reaction stage comprises a mixed metal oxide catalyst;
• f) oxidizing the acrolein in the second shell-and-tube reaction stage to produce a product gas comprising acrylic acid;
• and
• g) transferring the product gas between a reactor outlet and downstream collection and purification equipment using large diameter exit piping having an outlet diameter ratio, Ko, of 0.08 or more, the outlet diameter ratio Ko being the ratio of the exit piping diameter (DP) to a diameter of an outlet reactor head (DR) (Ko=DP/DR).