In order to prepare Lithium nitride, air is passed gently over a reactor. Suppose that Nitrogen diffuse to a length 5cm before getting reacted with Lithium. Nitrogen reacts with lithium quickly so that partial pressure of Nitrogen at that point is 0. Reactor is a cylinder of length 10cm and diameter 2cm. Assume air to be a mixture of Nitrogen and Oxygen only. The temperature is 298K and total pressure 1atm. Diffusivity of N2 in O2 is 2*10-5m2/s. Calculate the Molar flux (mol/m2.s) of N2 at a distance of 2.5cm from the top with respect to an observer moving with twice the mass average velocity a direction towards the liquid surface. (R=8.205*10-5m3atmK-1mol-1). Assume z=0 at the top of the reactor.

Correct Answer: 4.242*10-4
Case: Diffusion of A through non-diffusing B N2 in air= 79% O2 in air= 21% Molar flux of N2, NA=0.025mol/m2.s Pressure of N2 at the point z=2.5cm= ( DAB*P ln⁡)/RTl=(DAB*P ln⁡)/RTl P2.5=0.542atm Velocity of N2= NART/P2.5=112.78*10-5m/s Molar flux of O2-=0 Velocity of O2=0 Average molar mass=0.542*28+0.458*32=29.832 Mass average velocity= (0.542*28*112.78*10-5)/29.832=57.37*10-5m/s Required molar flux at the distance of z=5, is CA(uA-2V)= NA-2PAV/RT=4.242*10-4mol/m2.s.