CHPR5501: Advanced Reaction Engineering and Catalysis

Winthrop Prof. Mike Johns

School of Mechanical and Chemical Engineering

University of Western Australia

Lecture 6-7: Heterogeneous Reaction Engineering

Additional complicating factors Heterogeneous Reaction Systems (i) Since more than one phase is present, the movement of material from phase to phase is important. Thus the reaction rate equation must be adapted. It will probably include mass transfer (e.g. diffusion) terms. (ii) The contacting pattern of the different phases is important. Each phase can present its own flow pattern (e.g. plug or well mixed) and the phase form (discontinuous (e.g. droplets) or continuous) is also crucial.

Qg = 1

=-kg(Cg-Cs) Qs =

1

=-ks(Cs)

Cs

Cg

Cs is unknown can eliminate from above equations

Qs =

Cs =

+Cg

Qs = =1

1

+1

g

What is rate limiting? (gas + Solid reaction)

Reaction Surface

Gas phase mass transfer (flux) reaction rate at surface (flux)

Heterogeneous Non-Catalytic Reactions

fluid products A (fluid) + B (solid) solid products fluid + solid products

Initial unreacted particle

Partially reacted particle

Completely reacted particle

Flaking ash or gaseous products cause shrinkage in size

Final particle is hard, firm, and unchanged in size

Particle shrinks with time, finally disappearing

Solid liquid/gas Systems

Examples (i) Solid does not appreciably change in size during reaction a) Roasting (or oxidation) of sulphide ores to yield metal oxides. For example, in the preparation of zinc oxide the sulphide ore is mined, crushed, separated by flotation and then roasted to form hard white zinc oxide particles according to the reaction 2ZnS(s) + 3O2(g) 2ZnO(s) + 2SO2(g) Similarly, iron pyrites react as follows 4FeS2(s) + 11O2(g) 2Fe2O3(s) + 8SO2(g)

The nitrogenation of calcium carbide to produce calcium cyanamide: CaC2(s) + N2(g) 2CaCN2(s) + C(amorphous)

reaction

zone

ash

Unreacted core Low conversion

Radial position

Concentration of

solid reactant

R 0 R R 0 R R 0 R

High conversion

time time

Shrinking (Unreacted) Core Model no solid particle size change

The above assumes an unreacted core alternative is a progressive conversion model with no well-defined interface.

Representation of concentrations of reactants and products for the reaction

A(g) + bB(s) rR(g) + sS(s)

For a particle of unchanging size:

Step1. Diffusion of gaseous reactant A through the film surrounding the particle

to the surface of the solid.

Step 2. Penetration and diffusion of A through the ash layer to the surface of the

unreacted core.

Step 3. Reaction of gaseous A with solid at the reaction surface.

Step 4. Diffusion of gaseous products R through the ash back to the exterior surface

of the solid.

Step 5. Diffusion of gaseous products R through the gas film back into the main body

of the fluid.

Note: Some steps may not exist. For example if no gaseous products are formed

Steps 4 and 5 will not be relevant.

Also the resistances of the different steps usually vary greatly from one another. In such cases we can consider the step with the highest resistance to be

rate-controlling.

Step 1 or 5 controls

Diffusion of gas through the film surrounding the particle to/from the surface

of the solid.

Concentration of

gas phase reactant

R 0 R

= 1

3

= rc radius of unreacted core t time for complete reaction of particle

Step 2 or 4 controls

Diffusion of gas through the Ash Layer control.

Concentration of

gas phase reactant

R 0 R

= 1 3

2

+ 2

3

= 1 3(1 )2/3+2 1

Step 3 controls

Chemical Reaction Controls

Concentration of

gas phase reactant

R 0 R

= 1

= 1 (1 )1/3

For a Shrinking Particle

Step 2 and 4 are irrelevant as rate controlling steps no Ash layer Step 3 (chemical reaction control) identical to non-shrinking particles

Step 1 or 5 controls Shrinking particle

Concentration of

gas phase reactant

R 0 R

R is now changing with time

= 1

0

2

= 1 1 2/3

Assumes Stokes Law Regime (diffusion only no flow affects) All of the above assumes isothermal conditions heats of reaction for fast reactions can create temperature gradients in the particles - These affect both reaction rates and diffusion coefficients

Plot of rc or R versus t Plot of 1-XB versus t

t/t 0 0

1

1

rc / R or R/R0

t/t 0 0

1

1

1-XB

Contacting Patterns for Gas Solid Reactors

Fluid-Fluid Reactions

Example: Preparation of Sodium Amide

NH3(g) + Na(l) (250C) NaNH2(s) +0.5H2

The following factors are important: (i) Overall Rate Expression: Generally both the mass transfer coefficient and the

chemical reaction rate are important (ii) Equilibrium Solubility: This will limit movement between phases. (iii) The Contacting Scheme: Counter-current schemes predominate, particularly

for gas-liquid systems.

Mass transfer resistance will exist in both the liquid and gas phases.

Contacting Patterns for GasLiquid (Fluid-Fluid) Reactors

The additional form of Heterogeneous Reaction Engineering is the use of Solid Catalysts. These reduce the potential energy barrier over which the reactants must pass to form products see diagram below Whilst a catalyst will speed up a reaction, it never affects the end-point of the reaction or the equilibrium this is determined by thermodynamics Since the solid surface is responsible for catalytic activity, a large readily available surface area and easily handled materials is desirable. By a variety of method, surface areas of many 10s of meters per gram of (very porous) catalyst can be achieved. A lot of catalysts are susceptible to poisoning. This is the chemisorption or physical adsorption of materials on the catalytically active sites reducing their catalytic effectiveness. A classic example is coke (carbon) laydown. We will return to the topic of Catalysis towards the end of the course.