Sorry if this seems like a silly question but it has me confused a lot. So I have just started taking Chemical Reaction Engineering, and was studying about continuous-stirred tank reactor (CSTR).

So in my textbook it's stated that "It is normally operated at steady state and is assumed to be perfectly mixed; consequently, there is no time dependence or position dependence of the temperature, concentration, or reaction rate inside the CSTR. That is, every variable is the same at every point inside the reactor. Because the temperature and concentration are identical everywhere within the reaction vessel, they are the same at the exit point as they are elsewhere in the tank. Thus, the temperature and concentration in the exit stream are modeled as being the same as those inside the reactor"

This has me confused a lot, if concentration isn't changing with time, then does the chemical reaction has to occur instantaneously?

So the reactant is instantaneously and uniformly mixed throughout the reactor upon entry, but what about it's conversion into a product?. Because if that's taking time then the statement "concentration isn't changing with time" doesn't make any sense to me.

Can someone please clarify?

  • 1
    $\begingroup$ The "steady state" is used a lot in chemistry because it yields a condition which has a unique, defined and calculable answer. Obviously the reaction doesn't happen "instantly" but rather in a time frame which is short compared to how long the material takes to go from the inlet to the outlet. $\endgroup$
    – MaxW
    May 15, 2021 at 4:54
  • $\begingroup$ The mean residence time in the reactor is the tank holdup divided by the mass flow rate. So the amount of reaction that occurs is the reaction rate times the mean residence time times the holdup volume. $\endgroup$ May 23, 2021 at 12:29


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