Multiple Reactions: Parallel & Series

Question 1

What is a parallel reaction and what is another word for it?

Answer 1

Where reactant A reacts simultaneously to form B, C and D, etc. It is also known as a competitive reaction.

Question 2

What is a series reaction?

Answer 2

A reaction where a secondary product is formed from a primary product.

Question 3

How can low activity be overcome?

Answer 3

Using recycle reactors.

Question 4

Which is preferred conversion rate or selectivity?

Answer 4

Selectivity - less waste.

Question 5

What is yield? Write the equation.

Answer 5

The fraction of the reactant feed converted to a specific product.
Y B/A = [(nB-nB0)/vB] x vA/nA0
nB0 is usually 0 if a product

Question 6

How can conversion of a reactant be calculated from yield?

Answer 6

xA = sum (product yields from A)

Question 7

What is selectivity? Write the equation.

Answer 7

The amount of product as a fraction of the amount of reactant converted.

S B/A = [(nB-nB0)/vB] x [vA/(nA0-nA)]

Question 8

How does selectivity relate to yield?

Answer 8

Y(B/A) = xA S(B/A)

Question 9

How do you calculate the selectivity at a given point in a PFR or Batch reactor?

Answer 9

If reaction orders are equal: S = k / sum(K)

If reaction orders are not equal: -vA/vB dnB/dnA

Question 10

Does selectivity depend on volume or conversion?

Answer 10

No - can be used to find molar flow rates at different residence times.

Question 11

If local selectivity is not constant, how is it calculated?

Answer 11

S = rA1/(rA1+rA2) = k1/(k1+k2)

Question 12

If the reaction order for the desired product is higher than the unwanted, what conditions should we run at?

Answer 12

Keep CA as high as possible. Run without dilution of inert gas or minimum solvent. Run at high P & use a PFR ot batch reactor.

Question 13

If the reaction order for the desired product is lower than the unwanted, what conditions should we run at?

Answer 13

Keep CA as low as possible. Run with dilution of inert gas or maximum solvent. Run at low P & use a CSTR.

Question 14

Write the equation for the ratio of rates of reaction between the desired and unwanted products.

Answer 14

-rD/-rU = kD/kU CA^(n-m)

Question 15

Write the equation for the ratio of k for the desired and unwanted product.

Answer 15

kD/kU = AD/AU exp [- (EA,D - EA,U)/RT]

Question 16

What temperature should the system be run at if the activation energy of the desired product is greater than the unwanted?

Answer 16

(EA,D - EA,U) > 0
exp < 1
kD/kU low
favour unwanted
High T required to get over activation energy.

Question 17

What temperature should the system be run at if the activation energy of the desired product is less than the unwanted?

Answer 17

(EA,D - EA,U) < 0
exp > 1
kD/kU high
favour desired
Low T

Question 18

For a series reaction, when is producing the intermediate product difficult & when is it easier?

Answer 18

Difficult - k1&raquo_space; k2

Easy - k1 &laquo_space;k2

Question 19

How does the values of k for a series reaction determine how long it will take?

Answer 19

It doesn’t! k only depends on temperature and the residence time only depends on k.

Question 20

What is the residence time for a PFR/batch reactor with series reactions?

Answer 20

t = ln(k1/k2)/(k1-k2)

Question 21

What is the residence time for a CSTR reactor with series reactions?

Answer 21

t = 1/sqrt(k1k2)

Question 22

Which type of reactor achieves the maximum production in a shorted time?

Answer 22

PFR

Question 23

Write the method for determining the yield for a series reaction

Answer 23

1. Balance on reactant in terms of nA.
2. Balance on desired intermediate product in terms of nB
3. Remember nB0 = 0
4. Substitute in nA into nB.
5. Divide nB by nA0
6. nB/nA0 = Y(B/A)

Question 24

How do you do a balance on the reactant for a series reaction where the intermediate product is desired?

Answer 24

rA = (nA0 - nA)/V

sum(reactions from A -> production) = (nA0 - nA)/V

Question 25

How do you do a balance on the intermediate product for a series reaction?

Answer 25

rB= (nB0 - nB)/V

formation(reactant to B) - sum(reactions from B -> production) = (nB0 - nB)/V

Question 26

Write the method for determining the residence time for a series reaction

Answer 26

1. Balance on reactant, A in terms of nA
2. Balance on product, B in terms of nB
3. Substitute nA into nB
4. Write in terms of nB
5. Differentiate dnB/dt
6. Equate to 0
7. Rearrange for t
8. Sub in values