Questions $4.$ Depict the molecular diffusion in the $x$ direction through area $S$ and write the rate of transfer of species i through species $j$ by molecular diffusion in the $x$ direction. How to increase the rate of transfer of i molecules through $j$ molecules from one location to another by molecular diffusion?

Questions $5.$ Draw a shell and tube heat$-$exchanger: one tube pass, on shell pass. Summarize the steps for determining the heat transfer area of the heat exchanger and give ideals to design a heat exchanger in chemical process engineering