Lecture 3

Question 1

why can the volume element be taken as the whole reactor in batch reactors and CSTR ?

Answer 1

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

Question 2

what happens if the concentrations and temperature change as you move down the reactor

Answer 2

the mass balance takes the form of a differential equation

Question 3

is batch process steady state?

Answer 3

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

Question 4

what do we expect to happen to reactant concentration over time

Answer 4

the reactant concentration is normally expected to fall off as a function of time

Question 5

when do we assume a homogenous system in batch reactors?

Answer 5

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

Question 6

derive the mass balances on isothermal batch reactors

Answer 6

in notes

Question 7

what does the derivation for batch reactors tell us about the relationship between k and t

Answer 7

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

Question 8

what is the input into the CSTR

Answer 8

MA.nA0 in kg/hr

Question 9

what is the output of the CSTR

Answer 9

MA.nA in kg/hr

Question 10

what is the consumption in the CSTR

Answer 10

MA.rA.VR

Question 11

what is the design equation for the CSTR

Answer 11

MA.nA0-MA.nA-MA.rA.VR=0

Question 12

what is the molar rA

Answer 12

rA=k.CA=nA/VT molar flowrate/volumetric flowrate

Question 13

what is the design volume for CSTR

Answer 13

VR=nA0-nA/nA0

Question 14

what is the design volume for CSTR in termsn of conversion

Answer 14

VR=VT/k . (xA/1-xA)

Question 15

what is the average residence time for a CSTR

Answer 15

tau= VR/VT

Question 16

what do we know about the properties (composition, temperature, pressure) of the reaction in CSTR

Answer 16

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

Question 17

where do we evaluate the rate of reaction in CSTR?

Answer 17

the rate of reaction is constant throughout the vessel and should be evauluatd at the temperature, pressure and composition of the product stream

Question 18

why is the reaction rate rA now time independent in CSTR?

Answer 18

because in steady state operation, the properties of the system will not change with time

Question 19

what is nA0

Answer 19

molar flow rate of A into the reactor in mole/time

Question 20

what is nA

Answer 20

molar flow rate of A out of the reactor in moles/time

Question 21

what is rA

Answer 21

the rate of reaction in moles per volume per time moles/ volume * time

Question 22

why is CSTR approximate to perfect mixing

Answer 22

perfect mixing is close to possible in CSTR provided the fluid phase isn’t too viscous. a dye experiment can be done to show that the CSTR can be well mixed before residence time

Question 23

in CSTR and PFR would the mass balance equations change if the system is not in the steady state

Answer 23

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

Question 24

can the residence time in a PFR be compared to the reaction time in a batch reactor

Answer 24

Residence time in PFR is exactly equal to the reaction time in batch reactor.

Question 25

what is the design equation for an ideal batch reactor

Answer 25

𝑟𝐴=−1/𝑉𝑅 * 𝑑𝑁𝐴/𝑑𝑡

Question 26

what do we know about the concentration in a CSTR

Answer 26

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.

Question 27

why do CSTRs always operate at outlet conditions (concentration, temperature etc.)

Answer 27

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.

Question 28

what is the design equation for an ideal CSTR

Answer 28

𝑉𝑅=𝑛𝐴,0−𝑛𝐴/ 𝑟𝐴

Question 29

what is the design equation for an ideal PFR

Answer 29

𝑟𝐴=−𝑑𝑛𝐴/ 𝑑𝑉𝑅