Chemical Reaction Engineering
For a tubular reactor with space time 'τ' and residence time 'θ', the following statement holds good.

τ = θ , for a non-isothermal reactor
τ = θ , when the fluid density changes in the reactor
τ and θ are always equal
τ = θ , for an isothermic tubular reactor in which the density of the process fluid is constant

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

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

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