Reactors

Question 1

Give 3 properties of a batch reactors.

Answer 1

1) All reactants supplied at outset.
2) No addition/removal of reactants during the reaction.
3) Perfectly mixed so no T or C gradient - equal composition at any spatial point at any point in time.

Question 2

Give 1 benefit of a batch reactor & an example of a product that is batch produced.

Answer 2

Different products can be made in the same equipment - flexible. eg. API

Question 3

How do you calculate the mass fraction of A at time t?

Answer 3

xA = (NA0 - NA)/NA0

Question 4

How do you calculate the rate of reaction for a batch reactor?

Answer 4

rA = -1/V x dNA/dt

Question 5

If 40% of the reactants are left about 2 hours, what is the value of xA?

Answer 5

xA = 60% = 1 - NA = 0.6

Question 6

If 40% of the reactants are left after 2 hours, what is the value of NA?

Answer 6

NA = 40% = 1 - xA = 0.4

Question 7

How do you calculate the time taken for xA conversion in a batch reactor?

Answer 7

[1] (t,0) = [NA0/V x dxA/rA] (x,0)

Question 8

Give 3 properties of plug-flow reactors (PFR).

Answer 8

1) Do not have to be linear - are a function of volume.
2) Perfect radial mixing across cross-section (is axial variation)
3) No axial mixing along tube. (each ‘plug’ doesn’t mix)

Question 9

Give 1 benefit of a PFR and give an example of a product from a PFR.

Answer 9

Can be any shape - materials balance not based on inlet/outlet but on differential volume. eg. gas-phase reactions FT reactor

Question 10

How do you calculate reaction rate, rA in a PFR?

Answer 10

rA = - dnA/dV

Question 11

How do you calculate the volume occupied in a PFR with respect to conversion?

Answer 11

[1] (V,0) = [nA0/rA x dxA] (xA,0)

Question 12

How do you calculate the volume occupied in a PFR with respect to moles in the system?

Answer 12

[1] (V,0) = - [dnA/rA] (nA,nA0)

Question 13

How do you calculate concentration?

Answer 13

C = n/V = n/Q
Q = volumetric flow

Question 14

Give 3 properties of a CSTR.

Answer 14

1) Continuous flow in and out - outlet composition = reactor composition
2) Continuously stirred - perfect mixing = concentration doesn’t change with time.
3) Operates at steady state - reaction rate not dependent on time.

Question 15

Give 1 benefit of a CSTR and give an example of a product produced in a CSTR.

Answer 15

Concentration doesn’t vary with time or space so not a differential. eg. fermentation.

Question 16

How do you calculate reaction rate for a CSTR?

Answer 16

rA = (nA0 - nA)/V

Question 17

How do you calculate the volume occupied in a CSTR?

Answer 17

V = (xA x nA0)/rA

Question 18

What is the value of CA in a CSTR when xA amount has been converted and there is an initial concentration CA0?

Answer 18

CA = CA0 (1 - xA)

Question 19

How do you calculate the volume occupied in a CSTR graphically?

Answer 19

Plot xA vs 1/rA (x,y)

At desired conversion of xA & the value of 1/rA with it, the rectangular area under the curve = V/nA0

Question 20

How do you calculate the volume occupied in a PFR graphically?

Answer 20

Plot xA vs 1/rA (x,y) At desired conversion xA, the area directly under the curve = V/nA0

Question 21

For a negative order reaction, does a PFR or CSTR require a larger volume for the same conversion & flow rate?

Answer 21

PFR

Question 22

For a positive order reaction, does a PFR or CSTR require a larger volume for the same conversion & flow rate?

Answer 22

CSTR - isothermal

Question 23

For a zero order reaction, does a PFR or CSTR require a larger volume for the same conversion & flow rate?

Answer 23

CSTR/PFR require same volume - not dependent on concentration/conversion.

Question 24

What is the value of nA in a CSTR when xA amount has been converted and there is an initial value nA0?

Answer 24

nA = nA0 (1 - xA)
1 - xA = nA0 (1 - xA)
nA0 = 1

Question 25

For a gas phase reaction in a batch reactor, state the values of NA, NB & NC at a given point.
vA A -> vB B + vC C

Answer 25

NA = NA0 - NA0 xA
NB = NB0 + vB/vA NA0 xA
NC = NC0 + vC/vA NA0 xA