Multiple Reactions: Parallel & Series Flashcards
What is a parallel reaction and what is another word for it?
Where reactant A reacts simultaneously to form B, C and D, etc. It is also known as a competitive reaction.
What is a series reaction?
A reaction where a secondary product is formed from a primary product.
How can low activity be overcome?
Using recycle reactors.
Which is preferred conversion rate or selectivity?
Selectivity - less waste.
What is yield? Write the equation.
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
How can conversion of a reactant be calculated from yield?
xA = sum (product yields from A)
What is selectivity? Write the equation.
The amount of product as a fraction of the amount of reactant converted.
S B/A = [(nB-nB0)/vB] x [vA/(nA0-nA)]
How does selectivity relate to yield?
Y(B/A) = xA S(B/A)
How do you calculate the selectivity at a given point in a PFR or Batch reactor?
If reaction orders are equal: S = k / sum(K)
If reaction orders are not equal: -vA/vB dnB/dnA
Does selectivity depend on volume or conversion?
No - can be used to find molar flow rates at different residence times.
If local selectivity is not constant, how is it calculated?
S = rA1/(rA1+rA2) = k1/(k1+k2)
If the reaction order for the desired product is higher than the unwanted, what conditions should we run at?
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.
If the reaction order for the desired product is lower than the unwanted, what conditions should we run at?
Keep CA as low as possible. Run with dilution of inert gas or maximum solvent. Run at low P & use a CSTR.
Write the equation for the ratio of rates of reaction between the desired and unwanted products.
-rD/-rU = kD/kU CA^(n-m)
Write the equation for the ratio of k for the desired and unwanted product.
kD/kU = AD/AU exp [- (EA,D - EA,U)/RT]
What temperature should the system be run at if the activation energy of the desired product is greater than the unwanted?
(EA,D - EA,U) > 0 exp < 1 kD/kU low favour unwanted High T required to get over activation energy.
What temperature should the system be run at if the activation energy of the desired product is less than the unwanted?
(EA,D - EA,U) < 0 exp > 1 kD/kU high favour desired Low T
For a series reaction, when is producing the intermediate product difficult & when is it easier?
Difficult - k1»_space; k2
Easy - k1 «_space;k2
How does the values of k for a series reaction determine how long it will take?
It doesn’t! k only depends on temperature and the residence time only depends on k.
What is the residence time for a PFR/batch reactor with series reactions?
t = ln(k1/k2)/(k1-k2)
What is the residence time for a CSTR reactor with series reactions?
t = 1/sqrt(k1k2)
Which type of reactor achieves the maximum production in a shorted time?
PFR
Write the method for determining the yield for a series reaction
- Balance on reactant in terms of nA.
- Balance on desired intermediate product in terms of nB
- Remember nB0 = 0
- Substitute in nA into nB.
- Divide nB by nA0
- nB/nA0 = Y(B/A)
How do you do a balance on the reactant for a series reaction where the intermediate product is desired?
rA = (nA0 - nA)/V
sum(reactions from A -> production) = (nA0 - nA)/V
How do you do a balance on the intermediate product for a series reaction?
rB= (nB0 - nB)/V
formation(reactant to B) - sum(reactions from B -> production) = (nB0 - nB)/V
Write the method for determining the residence time for a series reaction
- Balance on reactant, A in terms of nA
- Balance on product, B in terms of nB
- Substitute nA into nB
- Write in terms of nB
- Differentiate dnB/dt
- Equate to 0
- Rearrange for t
- Sub in values
