Lecture 3 Flashcards
why can the volume element be taken as the whole reactor in batch reactors and CSTR ?
because the concentrations and the temperature are uniform throughout the reactor because the reactor is well stirred so the volume element can be taken as the whole reactor
what happens if the concentrations and temperature change as you move down the reactor
the mass balance takes the form of a differential equation
is batch process steady state?
no it is an unsteady state process where we expect all variables to change with time so there is an accumulation term in the mass balance
what do we expect to happen to reactant concentration over time
the reactant concentration is normally expected to fall off as a function of time
when do we assume a homogenous system in batch reactors?
we assume a homogenous chemcial reaction would be expected to proceed until equilibirum is reached or in the case of irreversible reactions until all the reactants are exhausted
derive the mass balances on isothermal batch reactors
in notes
what does the derivation for batch reactors tell us about the relationship between k and t
the bigger the value of k (rate constant), the samller the value of t (time). therefore, the faster the reaction, the shorter the reaction time
what is the input into the CSTR
MA.nA0 in kg/hr
what is the output of the CSTR
MA.nA in kg/hr
what is the consumption in the CSTR
MA.rA.VR
what is the design equation for the CSTR
MA.nA0-MA.nA-MA.rA.VR=0
what is the molar rA
rA=k.CA=nA/VT molar flowrate/volumetric flowrate
what is the design volume for CSTR
VR=nA0-nA/nA0
what is the design volume for CSTR in termsn of conversion
VR=VT/k . (xA/1-xA)
what is the average residence time for a CSTR
tau= VR/VT
what do we know about the properties (composition, temperature, pressure) of the reaction in CSTR
theese properties of the reaction mixture are uniform in all parts of the vessel but also these properties are identical to the properties of the reaction mixture in the exit stream
where do we evaluate the rate of reaction in CSTR?
the rate of reaction is constant throughout the vessel and should be evauluatd at the temperature, pressure and composition of the product stream
why is the reaction rate rA now time independent in CSTR?
because in steady state operation, the properties of the system will not change with time
what is nA0
molar flow rate of A into the reactor in mole/time
what is nA
molar flow rate of A out of the reactor in moles/time
what is rA
the rate of reaction in moles per volume per time moles/ volume * time
why is CSTR approximate to perfect mixing
perfect mixing is close to possible in CSTR provided the fluid phase isnβt too viscous. a dye experiment can be done to show that the CSTR can be well mixed before residence time
in CSTR and PFR would the mass balance equations change if the system is not in the steady state
the equation would be different because the composition would not be constant throughout the reaction and therefore the composition at the exit stream would not be constant
can the residence time in a PFR be compared to the reaction time in a batch reactor
Residence time in PFR is exactly equal to the reaction time in batch reactor.
what is the design equation for an ideal batch reactor
ππ΄=β1/ππ * πππ΄/ππ‘
what do we know about the concentration in a CSTR
in principle, the composition should change throughout the reactor. to simplify this we assume perfect mixing and the moment the feed enters the reactor, it is mixed instantly and reacts immediately, giving the tank a uniform concentration distribution.
why do CSTRs always operate at outlet conditions (concentration, temperature etc.)
because in principle the conditions should change throughout the reactor but we simplifiy the problem by assuming perfect mixing which give the tank uniform distributions for concentration, temperature etc.
what is the design equation for an ideal CSTR
ππ =ππ΄,0βππ΄/ ππ΄
what is the design equation for an ideal PFR
ππ΄=βπππ΄/ πππ
