Chem Eng - Catalysis Flashcards
How does a catalyst change the rate of a reaction?
It offers a different molecular pathway (mechanism) for the reaction.
What’s homogenous and heterogenous catalysis?
Homogenous - one phase. Catalyst is in solution with at least one of the reactants. (easy)
Heterogenous - involves more than one phase (complex)
How may catalytic reactors be categorised?
By phase (2 or 3 phase)
- 2 phase: fixed bed, moving bed, or fluidised bed
- 3 phase: slurry, fixed bed (trickle bed, packed bubble, or moving bed)
By catalyst motion
- slow/no motion: fixed bed, trickle bed, moving bed
- high motion: fluidised bed, slurry reactor
List the characteristics, pros, and cons of fixed (and packed bed) reactors:
Characteristics:
- Packed with catalyst particles
- Down-flow
- Catalyst particles are 1-20 mm in diameter/length
- Used in biodiesel production
Pros:
- Low cost of construction, operation, and maintenance
- Effective at high temp and pressure
- More contact between reactant and catalyst than other reactor types
Cons: - Catalyst is difficult to replace - Hard to control temperature - Heat transfer to/from reactor is hard - Plugging -
List the characteristics, pros, and cons of fluidised bed reactors:
Characteristics:
- Mobile particles (50 - 100um)
- Gas flows up from bottom and, when the gas reaches a certain velocity, the solids move like a liquid
- Used in fluid catalytic cracking
Pros:
- Catalyst easily replaced or regenerated
- Allows for continuous, automatically controlled operation
- More efficient contacting of gas and solid catalyst than other reactors
Cons:
- Expensive
- Regeneration equipment is expensive
- Catalyst particles may be deactivated and broken
- large pressure drop
What occurs in a 3-phase catalytic reactor?
Solid, liquid and gas all present.
The reaction occurs between a dissolved gas and a liquid phase in the presence of a solid catalyst.
List the characteristics, pros, and cons of moving bed reactors:
The solid catalyst is continuously moving in and out of the reactor.
Pros:
- Easy to regenerate catalyst
- Plug flow
- High conversion rate
- Good selectivity
Cons:
- Hard to maintain the flow of solids
- Fluid reactants may bypass catalyst bed
- Catalyst particles may be deactivated and broken
How do trickle bed and bubble column reactors differ?
Trickle bed has a continuous gas phase and dispersed liquid phase. Liquid always flow down and gas can flow up or down.
Bubble column has a continuous liquid phase and dispersed gas phase.
How do slurry and fixed bed reactors compare?
Fixed bed reactors are comparable to plug flow reactors, with high conversion.
Regarding heat transfer, they’re less efficient than slurry reactors.
Regarding catalyst handling, problems with bed plugging arise.
Slurry reactors are comparable to CSTRs with low conversion.
Heat transfer is more efficient and easier to control. Large liquid hold up helps avoid hot spots.
There are difficulties in continuous operation (due to filtration). It is easier in processes where frequent catalyst removal is necessary.
How do trickle bed and packed bubble bed reactors compare?
Trickle bed reactors:
- part of the catalyst may not be wetted when operating at low liquid rates
- hot spot formation
- temperature control is difficult
- poorer utilisation of catalyst
- easier in processes where frequent catalyst removal and replacement is necessary
- low liquid hold-up, lower contribution of homogeneous side reactions
Packed bubble bed:
- catalyst is completely wetted
- the pressure drop in down-flow operations less than that in up-flow
- removal of deposited tarry or polymeric substances from the surface of catalyst due to high liquid velocity
- better heat transfer characteristics (due to larger liquid hold-up and velocity
What are the 3 types of slurry reactor?
Agitated slurry
Bubble column
Fluidised bed
How do the 3 types of slurry reactor compare?
(regarding heat and mass transfer, catalyst attrition, design, catalyst separation, power requirement, and catalyst distribution)
Agitated slurry: Heat/mass transfer: Higher Catalyst Attrition: Higher Design: Difficult (moving parts) Catalyst Separation: relatively easy Power requirement: Highest Catalyst Distribution: uniform
Bubble column: Heat/mass transfer: Lower Catalyst Attrition: Lower Design: Simpler Catalyst Separation: Relatively easy Power requirement: Lowest Catalyst Distribution: Non-uniform can exist
Fluidised Bed: Heat/mass transfer: Lower Catalyst Attrition: Lower Design:Simpler Catalyst Separation: Easier Power requirement: Lower Catalyst Distribution: Non-uniform can exist
How many steps are there to a catalytic process?
What are the steps?
1) Transport of reactant from bulk to the catalyst external service
2) Transport through the pores to the internal surface
3*) Adsorption onto internal pore surface
4*) Chemical reaction. A to B
5*) Desorption of product
6) Transport of product from internal pore to external catalyst surface
7) Transport of product from external surface to the bulk
(* are the steps where the actual catalytic reaction occur)
What are the 3 key assumptions made for Langmuir - Hinshelwood kinetics?
The reaction occurs between 2 adsorbed molecules at the catalyst surface
All steps are reversible
The surface reaction is rate limiting, and all other steps are in equilibrium
What are the 4 main (reversible) reactions considered in the Langmuir - Hinshelwood model?
A + B ⇌ AB
A + S ⇌ AS
B + S ⇌ BS
AS + BS ⇌ ABS + S
ABS ⇌ AB + S
Where S is an active site on the catalyst surface and A and B are reactants.
How do the Langmuir-Hinshelwood and Eley-Rideal mechanism assumptions differ?
- Langmuir-Hinshelwood assumes two reactants need to be adsorbed
- Eley-Rideal assumes direct reaction between an adsorbed molecule and a gas molecule
What is the Biot number?
What does it show?
Bi = kc*dp/D
= Internal diffusion resistance / External diffusion resistance
Where:
kc - mass transfer coefficient
dp - particle diameter
De - effectuve diffusivity
A large Bi means that internal diffusion resistance is much more important than external
What are the key parts of a rate expression?
Rate = Kinetic term * Driving force term / Adsorption (Inhibition) term
What is effective diffusivity a function of?
Porosity
Tortuosity
Pore constriction
De = Da * Porosity * Pore constriction / tortuosity
Where Da is the overall diffusivity (considering molecular and Knudsen) and De is the effective diffusivity
What is the definition of the square of the Thiele Modulus?
φ^2 = surface reaction / max rate of diffusion
How does the effectiveness factor (n) vary with the Thiele modulus?
When φ is very small, n = 1
When φ is very large, n = 1 / φ
n can be greater than 1 for non-isothermal conditions
What are the 3 main states of a catalytic reactor under non-isothermal conditions?
Extinguished steady state (almost nothing happens)
Unstable intermediate steady state (less control)
Ignited steady state (high effectiveness factor)
Why can the external surface area of the catalyst pellet be neglected when deriving the overall effectiveness factor?
It is significantly smaller than the internal surface area
If the external mass transfer rate is limiting, what is the overall rate of reaction?
-r”A = -Ωr”Ab
What do effectiveness factor, n, and overall effectiveness factor, Ω, account for?
n: internal diffusion and surface reaction only
Ω: internal diffusion, external diffusion, and surface reaction
How does a catalytic process for a 3-phase reactor differ between the following?
i) A(g) + B(l) ⇌ P(l)
ii) A(g) + B(l) ⇌ P(g)
In the first case, A is absorbed into the liquid phase, travels via external diffusion to the catalyst surface, travels via internal diffusion through the pores, and then undergoes a reaction (after adsorption onto the active sites).
The product then diffuses (internal) to the S-L interface before diffusing (external) into the liquid.
In case 2, the product is a gas. A similar process to the above occurs. However, the product must then be absorbed and diffused back into a gas bubble. A gas film and liquid film will exist (when considering mass transfers)
What are the 2 main cases that dictate how fast a catalytic reaction can occur?
i) Rate of mass transfer (diffusion)»_space; Rate of reaction
Here, the reaction is controlled by the chemical reaction steps (and conc’ at active sites ≃ conc’ in bulk liquid)
ii) Rate of reaction»_space; rate of mass transfer
Here, the mass transfer will affect the overall rate of reaction
What are the main mechanisms for modelling the rate of catalytic reactions?
Langmuir-Hinshelwood
Dual site Langmuir-Hinshelwood
Eley-Rideal
How is an equilibrium constant defined?
As the ratio of the rate of forward reaction to the rate of the backward reaction.
K.eq = k.f / k.b
Why are the concentrations of molecules bound to active sites (e.g. C.AS, C.BS, C.ABS etc) replaced in rate expression derivations?
They are unknown and hard to measure.
We make the assumption that the rate of surface reaction is the rate limiting step, and that all other steps (e.g. adsorption and desorption) are fast and at steady state.
Hence, by rearranging their rate laws, they can be replaced.
E.g. C.AS = K.A * C.A * C.S
Comparing the two rate expression driving force terms below, what can we determine?
i) (C.aC.b - C.ab/K)
ii) (C.aC.b)
Reaction 1 is reversible
Regarding rate expressions, if we know that a certain reactant or product is weakly adsorbing, how many we simplify the rate expression?
Their values of Ki*Pi would be very small, and may be considered negligible.
How may Langmuir – Hinshelwood kinetics be described, considering rate of reaction with increasing partial pressure of A?
(Consider the reaction A + B → AB)
• Initially, rate increases with pressure because this increases the surface concentration of A
• Then, the surface gets equally saturated with
A and B
• With further increase of PA, surface becomes mainly occupied by A, leaving no spaces for B.
This decreases reaction rate.
How does the dual site model differ from the Langmuir-Hinshelwood mechanism?
The reactants adsorb to different types of active sites.
E.g.
A + S ⇌ AS
B + S* ⇌ BS*
The rest of the modelling process is similar to the LH mechanism, and the rate limiting step is the rate of surface reaction.
How does the Eley-Rideal mechanism differ from the L-H?
The reaction occurs directly between an adsorbed molecule and one which is still in the gaseous phase.
AS + B(g) ⇌ CS
In the case above, B would not bind to an active site, S.
What are the 3 main diffusion mechanisms within catalyst pores?
Molecular
Knudsen
Surface
What is molecular diffusion?
What formula is used to calculate this?
Molecular diffusion is the result of molecular encounters (collisions) in the void space (pores) of the particle.
The Chapman-Enskog formula is used to find D.ab, using: temperature, pressure, molecular weight, collision diameter (σ), and the collision integral (Ω)
What does molecular diffusion (in catalyst pores) depend on?
Temperature Pressure Molecular weight Collision diameter (σ) Collision integral (Ω)
