Question

a) What are the two primary mechanisms of mass transfer (molecular transport)?

b) If ??_?? is positive, is species ?? being consumed or generated by the reaction?

c) Are gas-phase diffusivities smaller or larger than liquid-phase diffusivities, and why?

d) What is a heterogeneous reaction? Describe the difference between a reaction-limited and mass-transfer-limited heterogeneous reaction.

e) If the diffusivity is increased, with all else held constant in a tubular heterogeneous reactor, would the diffusion boundary-layer thickness increase or decrease?

Answer 1

a. The two primary mechanism of molecule transport are advection ( or convective mass transfer due to the fluid flow) and diffusion.

b. More context needed

c. gas phase diffusivities are generally larger than liquid phase diffusivities as liquid molecules are held together by greater force of attraction than gas phase molecules.

d. Heterogeneous reactions have reactants in 2 or more different phases. In reaction rate limited rxns the transport of molecules to the reaction site doesnt effect the overall rate of reaction rather the reaction that takes place is inheretently slow.

In mass transfer limited reaction the transport of molecules to the reaction zone effects the rate of the reaction. It means that the reaction takes place at a fast rate but is limited due to the transport of molecules for the reaction.

e. The diffusion boundary layer thickness will decrease as due to the increase in duffisity the rate of diffusion will increase.