L10/11/12- Kinetics & Reactor design

Question 1

What type of solutions are preferred and why?

Answer 1

Aqueous solutions preferred for cheapness & safety

Question 2

Effect of increasing reagent in 1st order reaction

Answer 2

No effect on time taken for given conversion

Question 3

2nd order reaction concentration v time profile

Answer 3

Long, inconvenient tail

Using excess of one of reactants to reduce tail

Question 4

Why use one reactant in excess?

Answer 4

Reduce time needed to complete reaction to acceptable level

Ensure an expensive reactant is completely consumed

Cheaper or more easily separated used in excess

Question 5

What is pre-equilibria?

Answer 5

When a reactive species is involved in equilibrium that influences its concentration

Equilibrium reaction reactant takes part in, seperate from desired reaction

Question 6

Ex) Rate of second order

Answer 6

r = -k2[D][B]

Question 7

Ex) Equation for dissociation, Ka and pKa

Answer 7

Dissociation: [BH+] -> [B] + [H+]

Ka = ([B][H+]/[BH+])

pKa = -log10[Ka]

Question 8

Ex) Rate expression for [X] from mass balance of material (total material, i.e. [X]tot)

Answer 8

[B]total = [B]T = [B] + [BH+]

= [B] + ([B][H+]/Ka)

= B

Therefore, [B] = [B]T/(1+{[H+]/Ka})

Question 9

Ex) pH - pKa expression and values for [B]/[B]T

Answer 9

pH - pKa = -log10{[H+]/Ka} = log10{Ka/[H+]}

-> Ka/[H+] = 10^(pH-pKa)

Assume pH-pKa = 1:
[H+]/Ka = 0.1 and [B]/[B]T = 0.91

Question 10

Ex) Rate expression and kobs

Answer 10

rate = k2[B][D] = k2[B]T*[D]/{1+{[H+]/Ka}

kobs = k2/{1+{[H+]/Ka}

-> rate = kobs[D][B]T

Question 11

What is key and needed to control in Competing reactions & Types of competing reactions

Answer 11

Good yield key -> need to control processes that compete with desired reactions

Types: Parallel, rearrangement, reactant + solvent, further reaction, isomer, intermediate reacts with wrong reactant

Question 12

What to vary in order to influence the yield?

Answer 12

Vary temperature, concentration or order of reactant addition

Question 13

Minimising side reactions if reactant A unstable

Answer 13

Add A to other reactants to minimise standing concentration of A and maximise selectivity

Question 14

Minimising side reactions if both reactants unstable

Answer 14

Separate simultaneous feeds of reactants to reactor

Question 15

Minimising side reactions if product intrinsically unstable

Answer 15

Change to low residence time PFR
Continuously remove by distillation, evaporation

Question 16

Minimising side reactions if product further reacts with one reactant

Answer 16

Require one in excess and recycle (depends on speed of reactions)

Manipulate pre-post reaction equilibria

Question 17

Minimising side reactions if selectivity requires a minimum concentration of one reactant

Answer 17

Ensure adequate reactant supply
Divert reactive intermediate/product

Question 18

Mixing effect in pseud-homogeneous systems problem & reason

Answer 18

Physical mixing regime rather than chemical reaction regime- cause counterintuitive selectivities.
i.e tiny isolated reactors/areas within vessel that don’t interact properly -> effect conc.

Diffusional transfer of product away from reaction zone not fast enough to prevent consecutive reaction taking place.
Rate constant faster-> expect more product of that, but observed is opposite

Question 19

Half life: 1st order

Answer 19

t1/2 = ln2/k

Question 20

Half life: pseudo-1st order & pseudo-1st order definition

Answer 20

t1/2 = ln2/(k’*c)

pseudo-1st order: dependent on one reactant concentration only (simplified second order but follow 1st order kinetics)

Question 21

Half life: 2nd order

Answer 21

t1/2 = 1/(k*c)

Question 22

Mixing regime time constant

Answer 22

10mins-hr = independent of mixing

secs-mins = macromixing

ms = micromixing

Question 23

Kolmogoroff length microscale - define and equation

Answer 23

Size of smallest eddies that are still tubulent

lamda k = [(v^3)/epsilon]^(1/4)

Question 24

Time scale - define and equation

Answer 24

Lifetime of eddy

tow k = 12*[v/epsilon]^(1/2)

Question 25

Rate of increase in reaction zone volume, Vr

Answer 25

dVr/dt = E*Vr

E is engulfment rate

Question 26

Engulfment rate

Answer 26

E = ln2/tow k

Question 27

When will selectivity be determined by micromixing rather than kinetics?

Answer 27

1/E = tow k/ln2&raquo_space; t1/2

Question 28

Mean energy dissipation rate for stirred tank reactor

Answer 28

epsilon mean = P/Vp

Question 29

How to avoid micromixing problem?

Answer 29

Changing initial concentration: decreasing CA0 will increase t1/2 and reduce mixing problems

tow k the mixing length can be decreased, increasing E by increasing the specific energy input from the agitator