Chemical Reaction Engineering
The reactions of high molecularity are rare, because

Of requirement of very large concentration for such reactions
Of low probability of many body collisions
Many body collisions are not favoured energetically
Of very large activation energy of many bodies

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Chemical Reaction Engineering
In case of staged packed bed reactors carrying out exothermic reaction, use

All of these
Plug flow for dilute liquid requiring no large preheating of feed
Cold shot operations for a dilute solution requiring large preheating to bring the stream upto the reaction temperature
High recycle for pure gas

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Chemical Reaction Engineering
Semibatch reactor is preferred, when a/an

A gas is to be reacted with liquid (e.g. hydrogenation of fat)
A highly exothermic reaction is to be controlled
Undersirable side reaction (at high concentration of one of the reactants) is to be avoided
All of these

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