Fluid-Fluid Reaction Contacting Patterns and Models

Contacting Patterns in Fluid-Fluid Reactions

Batch Contacting (Dispersed Phase)

• Both fluids are mixed in a single vessel.
• No continuous flow; the reaction occurs over time.
• Suitable for small-scale or laboratory operations.
• Simple setup, but difficult to scale up.

Co-current Flow

• Both fluids flow in the same direction.
• Contact time is limited, but mixing can be effective.
• Used when reaction time is short or kinetics are fast.

Counter-current Flow

• Fluids flow in opposite directions.
• High mass transfer efficiency.
• Often used in packed columns or extraction towers.
• Better utilization of concentration gradients.

Crosscurrent Flow

• One fluid flows continuously while the other is introduced in stages.
• Common in multistage extraction processes.
• More control over the concentration of reactants.

Continuous Stirred Tank Reactor (CSTR)

• One or both phases are continuously fed and mixed in a stirred tank.
• Good mixing and uniform composition.
• Used when long contact time is required.

Packed Column

• One fluid flows as a continuous phase over packing material.
• The other is dispersed and flows counter-currently or co-currently.
• Enhances interfacial contact area.

Reaction Models

Progressive Conversion Model (PCM)

Shrinking Core Model (SCM)

Comparison of SCM and PCM

• Reaction Site: SCM occurs at the interface between the unreacted core and product layer; PCM occurs throughout the entire particle simultaneously.
• Core Dynamics: SCM features a distinct unreacted core that shrinks over time; PCM has no distinct core, with conversion progressing uniformly.
• Rate Expression: SCM leads to layer-by-layer rate equations; PCM is based on local reaction rates and diffusion inside the porous particle.
• Suitability: SCM is best for non-porous or dense solids; PCM is used for porous solids where fluid penetrates the interior.
• Modeling Approach: SCM assumes a sharp boundary with simpler geometry; PCM requires solving reaction-diffusion equations inside the particle.

True, Apparent, and Bulk Densities

• True Density: Mass of solid / True volume of solids (excluding pores).
• Apparent Density: Mass of solid / Volume excluding interparticle voids and pores.
• Bulk Density: Mass of solid / Total bulk volume (including voids in particles).

Catalytic Reaction Steps

• External Diffusion: Reactants diffuse from bulk fluid to the catalyst surface.
• Internal Diffusion: Reactants diffuse into the pores of the catalyst.
• Adsorption: Reactant molecules adsorb onto active sites on the catalyst surface.
• Surface Reaction: Chemical reaction occurs on the active sites.
• Desorption: Products desorb from the catalyst surface.
• Diffusion Out: Products diffuse out of the catalyst pores.
• External Transport: Products move from the catalyst surface to the bulk fluid.

Hatta Number (Ha)

1. The Hatta Number (Ha) is a dimensionless number used in gas–liquid reactions to compare the rate of reaction to the rate of diffusion of the solute into the liquid film.
2. Significance:
   • Ha < 0.3: Slow reaction (kinetically controlled); reaction occurs in bulk.
   • Ha > 3: Fast reaction (diffusion-controlled); reaction occurs within the film.
3. Application: Used to determine the enhancement factor (E) and identify rate-limiting steps in absorption with reaction.