(a) Briefly explain the difference between mass transfer coefficient and diffusi

Question

(a) Briefly explain the difference between mass transfer coefficient and diffusion coefficient using a suitable diagram (only a brief sketch) of the diffusion layer model for a reaction between aqueous species and a solid (see lecture slides). (b) Assume that the mass transfer of A controls the rate of reaction between A and B at eaclh of the three interphases (i), (i) and (iii) shown below. Comment with reasons (one sentence only in each case) regarding the phase through which the transfer of A will control the reaction rate (solid, gas or aqueous). Solid (B) Gas (A) Organic (B) Aqueous (A) Aqueous (B) Aqueous (A) (c) Nickel dissolves from millerite (NiS) in an oxygenated sulfuric acid solution at 80 °C. List the important steps, which may control the leaching of nickel from the suspended Nis particles in this solution according to the reaction: N?S + 0.502 + 2H-= Ni2+ + S + H2O.

Explanation / Answer

a) By diffusion, Mass transfer rate = Diffusion constant* (Concentration difference between two point at infinitesimal distance)/(distance between two points)

By Mass transfer, Mass transfer rate = Mass transfer coeffient*(Concentration difference between two point at infinitesimal distance)

Therefore, If find Mass transfer using diffusion coeficient, Mass transfer rate is proportional to Concentration gradient. Whereas using Mass transfer coefficient, Mass transfer rate is proportional to Concentration difference only

b)

1) Aqueous A dissolves in solid B for reaction. Hence controlling Mass transfer of A happens in Aqueous phase

2) Gas A dissolves in Aqueous B for reaction. Hence controlling Mass transfer of A happens in Gas phase

3) Aqueous A dissolves in Organic B for reaction. Hence controlling Mass transfer of A happens in Aqueous A

c) Steps which control the leaching of Nickel

1) NIS dissolves in oxygenated sulphuric acid

2) Reaction of NiS with oxygenated sulphuric acid and production of Ni2+ salt

3) Diffusion of Ni2+ salt into oxygenated sulphuric acid

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