Design Standards for Chemical Reactors

Chemical Reactor should handle corrosive chemicals, withstand temperature variations and perhaps pressure variations. Pressure vessel as it sounds, has only one parameter specified. These definitions may change with respect to what kind of chemicals are to be handled or what is under pressure in the pressure vessel. Ask specifically to those are in you line of industry for real application.

"Pressure" is the key word here. In the chemical industry, a chemical reactor can be a simple open tub with an agitator (imagine a bath tub and a boat paddle), or a vessel with a man-way, gasket and sealable lid. Such a reactor would have pressure release vents with disks designed to rupture at pre-determined pressure. The agitator shaft would also be pressure-rated. It could routinely be subject to 10-20 psig, pressurized by N2 for the process, then vented through the condensor. The agitator would stir the reaction mix to distribute the catalysts, as you put it.

A pressure vessel would be designed to much higher standards to withstand pressures of at least 100 psig, possibly up to a couple thousand psig. It may have an agitator with pressure-rated bearings, or be on a shaker. Pressure vessels are normally situated in rooms with thick blast walls, remote controls, and a flimsy roof that will blow up and away in event of an accident. Not sure about your ASME question. Ask an engineer.

Depending on application a reactor can be a pressure vessel. We have chemical reactors that are designed to 375 MAWP. They have agitators, circulation loops and feed spargers.

I have also worked with reactors designed at much lower pressures, still a pressure vessel though. These reactors were glass lined and only had a circulation loop used for heating the material. The MAWP for this vessel or reactor was only 10 psig and had a vacuum breaker as it wasn't rated for full vacuum. The glass lining was in there to handle the corrosive nature of the reactants.

As stated too you can have an open reactor rated only to atmospheric pressure so it wouldn't need to be a pressure vessel.

I am no expert here but think you would design the reactor to a Maximum Allowable Working Pressure, being a pressure vessel making sure it can handle any corrosion and the temperatures of reaction.