1. Gas- Liquid and Gas –Liquid –Solid Reactions
Basic Concepts

2. Proper Approach to Gas-Liquid Reactions
References
Mass Transfer theories
Gas-liquid reaction regimes
Multiphase reactors and selection criterion
Film model: Governing equations, problem complexities
Examples and Illustrative Results
Solution Algorithm (computational concepts)

3. Theories for Analysis of Transport Effects in Gas-Liquid Reactions
Two-film theory
1. W.G. Whitman, Chem. & Met. Eng., (1923).
2. W. K. Lewis & W. G. Whitman, Ind. Eng. Chem., 16, 215 (1924).
Penetration theory
P. V. Danckwerts, Trans. Faraday Soc., (1950).
P. V. Danckwerts, Trans. Faraday Soc., (1951).
P. V. Danckwerts, Gas-Liquid Reactions, McGraw-Hill, NY (1970).
R. Higbie, Trans. Am. Inst. Chem. Engrs., (1935).
Surface renewal theory
P. V. Danckwerts, Ind. Eng. Chem., (1951).
Rigorous multicomponent diffusion theory
R. Taylor and R. Krishna, Multicomponent Mass Transfer,
Wiley, New York, 1993.

4. Two-film Theory Assumptions
1. A stagnant layer exists in both the gas and the liquid phases.
2. The stagnant layers or films have negligible capacitance and hence a local steady-state exists.
3. Concentration gradients in the film are one-dimensional.
4. Local equilibrium exists between the the gas and liquid phases as the gas-liquid interface
5. Local concentration gradients beyond the films are absent due to turbulence.

5. Two-Film Theory Concept W. G. Whitman, Chem. & Met. EngpA
pAi
pAi = HA CAi •
Bulk Gas
Gas Film
Liquid Film
Bulk Liquid
CAi •
CAb x
x + x L
x = G
x = 0
x = L

6. Two-Film Theory - Single Reaction in the Liquid Film -
A (g) + b B (liq) P (liq)
B & P are nonvolatile
Closed form solutions only possible for linear kinetics
or when linear approximations are introduced

7. Gas-Liquid Reaction Regimes
Instantaneous
Fast (m, n)
Rapid pseudo
1st or mth order
Instantaneous & Surface
General (m,n) or Intermediate
Slow Diffusional
Very Slow

8. Characteristic Diffusion & Reaction Times
Diffusion time
Reaction time
Mass transfer time

9. Reaction-Diffusion Regimes Defined by Characteristic Times
Slow reaction regime tD<<tR kL=kL0
Slow reaction-diffusion regime: tD<<tR<<tM
Slow reaction kinetic regime: tD<<tM<<tR
Fast reaction regime: tD>>tR kL=EA kL0>kL0
Instantaneous reaction regime: kL= EA kL0

10. S30

11. Comparison Between Theories
Film theory:
kL D,  - film thickness
Penetration theory:
kL D1/2
Higbie model
t* - life of surface liquid element
Danckwerts model
s - average rate of surface renewal = = =

12. Gas Absorption Accompanied by Reaction in the Liquid
Assume:
- 2nd order rate
Hatta Number :
Ei Number:
Enhancement Factor:
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13. S32

14. S33

15. Eight (A – H) regimes can be distinguished:
A. Instantaneous reaction occurs in the liquid film
B. Instantaneous reaction occurs at gas-liquid interface
High gas-liquid interfacial area desired
Non-isothermal effects likely
S34

16. D. Pseudo first order reaction in film; same Ha number range as C.
C. Rapid second order reaction in the film. No unreacted A penetrates into bulk liquid
D. Pseudo first order reaction in film; same Ha number range as C.
Absorption rate proportional to gas-liquid area. Non-isothermal effects still possible.
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17. S36

18. Temperature difference for liquid film with reaction
Maximum temperature difference across film develops at complete mass transfer limitations
Temperature difference for liquid film with reaction
Trial and error required. Nonisothermality severe for fast reactions.
e.g. Chlorination of toluene
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19. - Summary - Limiting Reaction-Diffusion Regimes
Slow reaction kinetic regime
Rate proportional to liquid holdup and reaction rate and influenced by the overall concentration driving force
Rate independent of klaB and overall concentration driving force
Slow reaction-diffusion regime
Rate proportional to klaB and overall concentration driving force
Rate independent of liquid holdup and often of reaction rate
Fast reaction regime
Rate proportional to aB,square root of reaction rate and driving force to the power (n+1)/2 (nth order reaction)
Rate independent of kl and liquid holdup
Instantaneous reaction regime
Rate proportional to kL and aB
Rate independent of liquid holdup, reaction rate and is a week function of the solubility of the gas reactant

20. S29

21. S45

22. Gas Limiting Reactant (Completely Wetted Catalyst)
Gas – Liquid Solid Catalyzed Reaction A(g)+B(l)=P(l)
Gas Limiting Reactant (Completely Wetted Catalyst)
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23. Our task in catalytic reactor selection, scale-up and design is to either maximize volumetric productivity, selectivity or product concentration or an objective function of all of the above. The key to our success is the catalyst. For each reactor type considered we can plot feasible operating points on a plot of volumetric productivity versus catalyst concentration.
Clearly is determined by transport limitations and by reactor type and flow regime. Improving only improves if we are not already transport limited.
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24. S39

25. Comparison Between Gas-Solid and Gas-Liquid-Solid Catalytic Converters
Category
Gas-Solid Catalytic
Gas-Liquid-Solid Catalytic
Design and engineering
Simple
More elaborate
Material
Often expensive material can be used
Corrosion problems can be critical
Catalyst
Possible poisoning by non-volatile byproducts
Resistance to corrosion is required
Thermal control
Low thermal stability and low heat capacity require internal heat exchange or low conversion
Better stability and higher heat capacity; partial vaporization is possible; better heat exchange coefficient
Reactant recycling
Often important
Stoichiometric ratio can generally be achieved; hydrodynamics can require gas recycling
Safety
Temperature run-away and ignition can occur. Gas mixture must lie outside the explosive range
Better stability
Operation within the inflammability or explosion limits sometimes possible
Dissipated power
Higher pressure drop
Low pressure drop but sometimes stirring is required
Reactant preheating
Always important
Less important or unnecessary
Heat recovery
Generally at a high level but low heat transfer rate
At a lower level but high heat transfer rate; high efficiency

26. Key Multiphase Reactor Types
• Mechanically agitated tanks
• Multistage agitated columns
• Bubble columns
• Draft-tube reactors
• Loop reactors
Soluble catalysts
&
Powdered catalysts
• Packed columns
• Trickle-beds
• Packed bubble columns
• Ebullated-bed reactors
Soluble catalysts
&
Tableted catalysts

27. Classification of Multiphase Gas-Liquid-Solid Catalyzed Reactors
1. Slurry Reactors
Catalyst powder is suspended in the liquid
phase to form a slurry.
2. Fixed-Bed Reactors
Catalyst pellets are maintained in place as
a fixed-bed or packed-bed.
K. Ostergaard, Adv. Chem. Engng., Vol. 7 (1968)

28. Modification of the Classification for Gas-Liquid Soluble Catalyst Reactors
1. Catalyst complex is dissolved in the liquid
phase to form a homogeneous phase.
2. Random inert or structured packing, if used,
provides interfacial area for gas-liquid contacting.

29. Multiphase Reactor Types for Chemical, Specialty, and Petroleum Processes

30. Multiphase Reactor Types at a Glance
Middleton (1992)

31. Key Multiphase Reactor

32. Comparison Between Slurry and Fixed-Bed Gas-Liquid-Solid Catalytic Converters
Category
Slurry Reactors
Trickle-Bed Reactors
Specific reaction rate
High or fast reactions
Rel. high for slow reactions
Catalyst
Highly active
Supported; high crushing strength, good thermal stability and long working life needed
Homogeneous side reactions
Poor selectivity
Good selectivity
Residence time distribution
Perfect mixing
Plug flow
Pressure drop
Low or medium
Low except for small particles
Temperature control
Isothermal operation
Adiabatic operation
Heat recovery
Easy
Less easy
Catalyst handling
Technical difficulties
None
Maximum volume
50 m3
300 m3
Maximum working pressure
100 bar
high pressure possible
Process flexibility
Batch or continuous
Continuous
Investment costs
High
Low
Operating costs
Reactor design and extrapolation
Well known
difficult

33. Bubble Column in different modes

34. Slurry and Fixed Bed Three Phase Catalytic Reactors
Typical Properties
Slurry
Trickle-bed
Flooded bed
Catalyst loading
0.01
0.5
Liquid hold-up
0.8
0.4
Gas hold-up
0.2
0.1
Particle diameter
0.1 mm
1 – 5 mm
External catalyst area
500 m-1
1000 m-1
Catalyst effectiveness 1
<1
G/L Interfacial area
400 m-1
200 m-1
Dissipated power
1000 Wm-3
100 Wm-3

35. Key Multiphase Reactor Parameters
Trambouze P. et al., “Chemical Reactors – From Design to Operation”, Technip publications, (2004)

36. S39

37. 2-10
40-100
10-100
10-50
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38. S41