L10/11/12- Kinetics & Reactor design Flashcards
What type of solutions are preferred and why?
Aqueous solutions preferred for cheapness & safety
Effect of increasing reagent in 1st order reaction
No effect on time taken for given conversion
2nd order reaction concentration v time profile
Long, inconvenient tail
Using excess of one of reactants to reduce tail
Why use one reactant in excess?
Reduce time needed to complete reaction to acceptable level
Ensure an expensive reactant is completely consumed
Cheaper or more easily separated used in excess
What is pre-equilibria?
When a reactive species is involved in equilibrium that influences its concentration
Equilibrium reaction reactant takes part in, seperate from desired reaction
Ex) Rate of second order
r = -k2[D][B]
Ex) Equation for dissociation, Ka and pKa
Dissociation: [BH+] -> [B] + [H+]
Ka = ([B][H+]/[BH+])
pKa = -log10[Ka]
Ex) Rate expression for [X] from mass balance of material (total material, i.e. [X]tot)
Ex) pH - pKa expression and values for [B]/[B]T
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
Ex) Rate expression and kobs
rate = k2[B][D] = k2[B]T*[D]/{1+{[H+]/Ka}
kobs = k2/{1+{[H+]/Ka}
-> rate = kobs[D][B]T
What is key and needed to control in Competing reactions & Types of competing reactions
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
What to vary in order to influence the yield?
Vary temperature, concentration or order of reactant addition
Minimising side reactions if reactant A unstable
Add A to other reactants to minimise standing concentration of A and maximise selectivity
Minimising side reactions if both reactants unstable
Separate simultaneous feeds of reactants to reactor
Minimising side reactions if product intrinsically unstable
Change to low residence time PFR
Continuously remove by distillation, evaporation
Minimising side reactions if product further reacts with one reactant
Require one in excess and recycle (depends on speed of reactions)
Manipulate pre-post reaction equilibria
Minimising side reactions if selectivity requires a minimum concentration of one reactant
Ensure adequate reactant supply
Divert reactive intermediate/product
Mixing effect in pseud-homogeneous systems problem & reason
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
Half life: 1st order
t1/2 = ln2/k
Half life: pseudo-1st order & pseudo-1st order definition
t1/2 = ln2/(k’*c)
pseudo-1st order: dependent on one reactant concentration only (simplified second order but follow 1st order kinetics)
Half life: 2nd order
t1/2 = 1/(k*c)
Mixing regime time constant
10mins-hr = independent of mixing
secs-mins = macromixing
ms = micromixing
Kolmogoroff length microscale - define and equation
Size of smallest eddies that are still tubulent
lamda k = [(v^3)/epsilon]^(1/4)
Time scale - define and equation
Lifetime of eddy
tow k = 12*[v/epsilon]^(1/2)
Rate of increase in reaction zone volume, Vr
dVr/dt = E*Vr
E is engulfment rate
Engulfment rate
E = ln2/tow k
When will selectivity be determined by micromixing rather than kinetics?
1/E = tow k/ln2»_space; t1/2
Mean energy dissipation rate for stirred tank reactor
epsilon mean = P/Vp
How to avoid micromixing problem?
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
