The behavior of spray tower loop reactors has been examined in details, also by reviewing previous works, with the aim of suggesting useful criteria for the design and simulation of these devices. Different possible situations have been considered and quantitatively interpreted. For gas-liquid systems, for example, physical absorption in the absence of a reaction, absorption with a moderately fast reaction, and absorption with an extremely fast reaction have been studied. Moreover, an attempt to extend the use of spray tower loop reactors to a gas-liquid-solid system has been made with satisfactory results.