Reactor vs. Pressure Vessel

There are two basic types of vessels.

One is an "Atmospheric" vessel or tank - which is open by some means (no top or an open vent or other) to the atmosphere.

The other is a "Pressure" vessel - This is a closed container which can have various forms, functions and may or may not have internals.

Examples of "Pressure" vessels are:

• Closed "Tanks" for the storage of hydrocarbons such as gasoline(petrol) etc.
• Horizontal "Accumulators" which may or may not have a "Boot" and will act as a separator for the removal of impurities.
• "Fractionation" Towers - which have trays to strip and separate primary and secondary products from the main stream.
• "Reactors" which have some form of "Catalyst" inside which causes a change to the product stream with-out becoming a part of that product. This is normally a continuous process reactor. This "Reactor will normally have a large "Manhole" at the top of the vessel to load the catalyst and another manhole or nozzle at the bottom to remove the catalyst.
• "Reactors" which are used as a mix chamber where multiple ingredients are added and then mixed (heated or cooled) to form a finished product. This is normally a batch mode reactor. This type of "Reactor" may be fitted with: mixers, internal cleaning systems (CIP) , internal or external heating/cooling coils or other special features.
  

All five of these are considered a "Pressure" Vessel

Some of the chemical reactions need specific parameters like temperature & pressure. If so it may called as pressure vessel or reactor.

Many reactions does not need any pressure like DAP plants..

So a reactor can be called as pressure vessel if it operates above atmospheric pressure. .